Connection of wind farms to weak AC networks

671
Connection of wind farms
to weak AC networks
Working Group
B4.62
December 2016

Lead Authors
Nalin Pahalawaththa, Convenor AU
Sebastian Achilles, Secretary US
Katherine Elkington SE
Davor Vujatovic GB
Andrew Isaacs CA
Udaya Annakkage CA
Mark Davies AU
Babak Badrzadeh AU
Charlie Smith US
Contributing Authors
Marian Piekutowski AU Peeter Muttik AU Tony Morton AU
Xu Li Chao AU José Antonio Jardini BR Marcos Tiago Bassini BR
Marco A. Barbosa Horita BR Geethma Dissanayake NZ Victor Lo NZ
Bathiya Jayasekera CA Dharshana Muthumuni CA Hiranya Suriyaarachchi CA
Richard Gagnon CA Yongning Chi CN Junzheng Cao CN
Alvaro Jose Hernandez DE Aramis Schwanka Trevisan DE Jesper Hjerrild DK
John Bech DK Peter Christensen DK Jorge Martinez Garcia ES
Silvia Sanz ES Rafael Portales ES Jako Kilter EE
Antti Harjula FI Tuomas Rauhala FI Koji Temma JP
Hur Kyeon KR Afshin Pashaei GB Jun Liang GB
Tom Gallery IE Fred Huang US Narend Reddy US
Paul Marken US Saeed Kamalinia US Steve Saylors US
WG B4.62
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CONNECTION OF WIND FARMS
TO WEAK AC NETWORKS
ISBN : 978-2-85873-374-3

Connection of Wind Farms to Weak AC networks
Page 5
Connection of Wind Farms to
Weak AC networks
W G B 4 . 6 2
Table of Contents
GLOSSARY OF ABBREVIATIONS AND SPECIAL TERMS .................................... 8
EXECUTIVE SUMMARY............................................................................... 11
1 INTRODUCTION.................................................................................. 16
1.1 Purpose........................................................................................... 16
1.2 Background ..................................................................................... 16
1.3 Scope .............................................................................................. 16
1.4 Summary of the salient work completed by other working groups.... 17
1.5 Gap analysis .................................................................................... 19
1.6 Structure of the report..................................................................... 19
1.7 References ...................................................................................... 20
2 TECHNOLOGY SUMMARY .................................................................... 21
2.1 Introduction .................................................................................... 21
2.2 Types of Wind Turbines Based on Speed Control.............................. 21
2.3 Generator Technologies................................................................... 26
2.4 Wind Turbine Mechanical Control..................................................... 27
2.5 Power Electronic Converters for Wind Turbine Converters ................ 27
2.6 Complementing wind farm technologies.......................................... 30
2.7 Complementing Grid Connection Technologies................................ 31
2.8 References ...................................................................................... 31
3 ISSUES ASSOCIATED WITH WEAK SYSTEMS ........................................... 32
3.1 Introduction .................................................................................... 32
3.2 Overview of WPP Performance Requirements.................................... 32

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3.3 Technical Issues Identified in Weak Grids......................................... 34
3.4 Additional Challenges Identified in Weak Grids ................................ 36
3.5 Examples......................................................................................... 38
3.6 References ...................................................................................... 41
4 MODELLING SUMMARY ....................................................................... 42
4.1 Introduction .................................................................................... 42
4.2 Power-flow Analysis ........................................................................ 42
4.3 Short circuit Analysis ....................................................................... 42
4.4 Transient Stability Analysis .............................................................. 44
4.5 Small Signal Stability Analysis .......................................................... 46
4.6 Electromagnetic Transient Analysis.................................................. 47
4.7 Islanding Assessment ...................................................................... 48
4.8 Generic and Vendor Specific Models ................................................ 49
4.9 References ...................................................................................... 50
5 ASSESSMENT ...................................................................................... 52
5.1 Introduction .................................................................................... 52
5.2 Benchmark Model for Assessment ................................................... 53
5.3 Generic Model of Type 4 WTG for Fault Recovery Investigation......... 57
5.4 Impact of the Interaction between WPPs on Fault Ride-through........ 63
5.5 Small Signal Stability........................................................................ 65
5.6 Possible Mitigation Solutions ........................................................... 69
5.7 A weak AC connected WPP in parallel with VSC HVDC....................... 78
5.8 A weak AC system with a WPP connected via a LCC HVDC ................ 84
5.9 Summary......................................................................................... 87
5.10 References ...................................................................................... 88
6 QUANTIFICATION OF THE ISSUES ........................................................ 89
6.1 Introduction .................................................................................... 89
6.2 Short circuit ratio............................................................................. 89
6.3 X/R ratio ....................................................................................... 106
6.4 Voltage sensitivity ......................................................................... 106
6.5 Rate of change of frequency (RoCoF) ............................................. 113
6.6 Available Fault Level - Impact of nearby WPPs and HVDC links ....... 116
6.7 References .................................................................................... 120

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7 GUIDE TO WIND POWER PLANT CONNECTION .................................... 121
7.0 Introduction .................................................................................. 121
7.1 Identification / Assessment of System Capability ........................... 121
7.2 Issues to be Considered for the Identified Level of System Capability128
7.3 Performance Improvement Technology Selection ........................... 138
7.4 Planning Study Summary ............................................................... 142
7.5 References .................................................................................... 145
8 CONCLUSIONS.................................................................................. 146
9 BIBLIOGRAPHY.................................................................................. 148
APPENDIX A: GENERIC WIND TURBINE MODELS ........................................ 150
APPENDIX B: CALCULATION OF VOLTAGE SENSITIVITY INDICES ............... 158
B.1 Worked example............................................................................ 158
B.2 Relation to voltage angle displacement.......................................... 160
APPENDIX C: CASE STUDIES ................................................................... 163
C.1 ERCOT - Actual Operating Experience............................................ 163
C.2 GE - Operating Experience ............................................................ 170
C.3 Vestas - Weak Grid/Low SCR Interconnection Experience............... 173
C.4 Enercon - Application Experience .................................................. 175
C.5 State Grid Corporation of China - Experience................................. 182
C.6 WPP Connections in Tasmania, Australia - Experience.................... 190

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GLOSSARY OF ABBREVIATIONS AND SPECIAL TERMS
The table below lists the abbreviations used throughout this brochure.
Abbreviation Full Text
AC Alternating Current
AVR Automatic Voltage Regulator
BTB Back to Back
CSCR Composite Short Circuit Ratio
DC Direct Current
DFIG Doubly Fed Induction Generator
DSO Distribution System Operator
EirGrid The electric power transmission operator in Ireland
EMT Electromagnetic Transient
ENTSO-E The European Network of Transmission System
Operators,
ESCR Equivalent Circuit-based Short Circuit Ratio
ERCOT Electricity Reliability Council of Texas
FACTS Flexible AC Transmission Systems
FRRS Fast Responding Regulation Service
FRT Fault Ride-Through
GTO Gate Turn Off Thermistor
GW Giga Watt
H Inertia Constant
HV High Voltage
HVDC High Voltage Direct Current
HVRT High Voltage Ride-Through
Hz Hertz
IEEE Institute of Electrical and Electronic Engineers

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IGBT Insulated Gate Bipolar Transistor
LCC Line Commutated Converter
LV Low Voltage
ms millisecond
MSC Mechanically Switched Capacitor
MSCR Minimum Short Circuit Ratio
MV Medium Voltage
Mvar Mega Volt Ampere Reactive
MW Mega Watt
NSG Non-Synchronous Generator
PCC Point of Common Coupling
PI Proportional and Integral
PLL Phase Locked Loop
PMU Phasor Measurement Unit
POI Point of Interface / Interconnection
PSCAD© Power System Computer Aided Design
PWM Pulse Width Modulation
PU, pu Per Unit
PV Power - Voltage
QV, Q-V Reactive Power (Q) - Voltage
RES Renewable Energy Sources
RMS Root Mean Square
RoCoF Rate of Change of Frequency
SC Synchronous Compensator
SCADA Supervisory Control and Data Acquisition
SCC Short Circuit Current
SCIG Squirrel Cage Induction Generator
SCR Short Circuit Ratio
SNSP System Non-Synchronous Penetration
SSSC Static Synchronous Series Compensator

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STATCOM Static Synchronous Compensator
SVC Static Var Compensator
TCSC Thermistor Controlled Series Capacitor
THD Total Harmonic Distortion
TOV Temporary Over-Voltage
TSC Thermistor Switched Capacitor
TSO Transmission System Operator
Var Volt Ampere Reactive
VSC Voltage Source Converter
WG Working Group
WPIF Wind Plant Interaction Factor
WPP Wind Power Plant
WRIG Wound Rotor Induction Generator
WSCR Weighted Short Circuit Ratio
WTG Wind Turbine Generator

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EXECUTIVE SUMMARY
Background
In recent years wind generation has become the fastest growing energy generation sector worldwide. Wind farms,
or more formally wind power plants (WPP), with capacities of 100-1000 MW or more have been constructed both
onshore and offshore. Many of the most economic sites, in particular those onshore locations with favourable wind
speeds and good access to high capacity transmission systems have often been fully utilised. WPP developers must
now focus attention on the less favourable sites, in terms of capacity factor and accessibility to high capacity
transmission. In future, WPPs will more often connect to weaker parts of the power transmission networks, with
longer transmission distances, bringing greater challenges in the maintenance of system and network stability. In
search of suitable sites the developers are also now selecting to develop multiple wind power plants closer to each
other. This increases the risk of adverse interactions among the nearby plants and places added demands on
network operators to manage power system security and supply quality.
The size of individual wind turbines has also grown exponentially, with the new developments now concentrating on
turbine sizes above 5 MW. The use of permanent magnet direct drive generators and full energy conversion using
AC-DC-AC power electronic converters is now becoming common for grid connected wind power plants.
Objective of the Working Group
The use of full energy conversion within the wind turbines using AC-DC-AC power electronic converters now make
it possible to synergistically use the experiences and lessons learned in designing, developing and operating high
power HVDC converters for power transmission applications. In particular for addressing the stability issues
associated with connecting WPPs to weak AC systems and mitigating adverse interactions among nearby WPPs.
The objective of this working group has been to: identify the issues associated with connecting WPPs to weak AC
grids, understand the cause-effect relationships and propose solutions for mitigating the potential issues. In this
regard, the working group has also attempted to create a guide for connecting WPPs to weak grids, which highlights:
the assessment of AC network strength, screening for potential issues, and the available solutions for mitigating the
issues.
Quantification Framework
The “strength” of a power system is a metric used to describe the ability of a power system to maintain the core
characteristics through which it interacts with a connection, namely voltage and frequency, as steadily as possible,
under all operating conditions. The “strength” or “weakness” of a power system is a relative concept and needs to
be addressed both in terms of the system characteristics at a given connection point as well as the size of the WPP(s)
to be connected to the connection point. For example, a particular part or point in a power system may be considered
sufficiently strong to connect a WPP of capacity 200 MW but the same part of the power system would be considered
weak and incapable of handling the issues associated with connecting WPP(s) of capacity 2000MW. Hence the
working group has devoted a significant effort in understanding and reporting upon the avenues for quantifying the
“strength” of a power system, so that the issues and solutions can be discussed and presented in the correct context.
Quantification of the “strength” of a power system has been considered in terms of the:
 shared power system impedance seen from the connecting WPPs;
 ability of the power system to transfer power in steady state while maintaining an adequate level of supply
voltage and;

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 resilience of the power system to maintain the supply frequency.
Short Circuit Ratio (SCR) is a commonly used metric for quantifying the relative power system impedance seen from
a connection point. The SCR seen by a generator strongly influences its ability to operate satisfactorily both in steady
state and following system disturbances. While this is a very powerful and simple concept, extending its use to
describe the “shared” impedance seen by multiple WPPs connecting to the same part of a network, electrically close
to each other, or close to other power electronic plant such as HVDC converters, has not been unified across the
industry. The working group has collated and compared the approaches generally used in the industry for using
SCR to: anticipate potential issues, screen WPP technology, and to assess proposed solutions. Other related
indications of transmission system strength, the (X/R) ratio of the system impedance seen from the connection point,
and the concept of available fault level have also been described.
The ability to stably transfer power over a weak transmission system, from a WPP connecting point to stronger parts
of a network (where generally the load is) has been quantified by using the sensitivity of the connection point's voltage
to the active and reactive power outputs of the WPP. The maximum stable power transfer capability has been
derived, providing an insight for WPP designers of the potential issues to be anticipated when power transfer reaches
the maximum transfer limits.
The ability of a power system to maintain steady frequency, as far as possible, under all operating conditions is
characterised by the following metrics: rate of change of frequency (RoCoF), system inertia, provision of synchronous
spinning reserves, and level of penetration of non-synchronous generation.
Issues Associated with Weak Systems
The performance requirements of WPPs are defined by the respective regulatory bodies for power system operation
and are usually described in the form of a grid code. These codes usually define the expected operational ranges of
frequency and voltage, the requirements for reactive power/voltage control and active power/frequency control. The
performance requirements must be complied with under all operating conditions which include: operation and
isolation under islanding conditions, operation during peak/light load periods, and during generator/network outage
conditions. Compliance must be demonstrated both pre and post connection by using adequate simulation studies.
Post commissioning monitoring should also confirm the ability of the plant to operate satisfactorily during and post
disturbances (i.e. the ability to ride through disturbances) and to operate satisfactorily in harmony with the other
connected generators and dynamic plant installed in the network.
The working group has compiled a significant quantity of experiences associated with connection of WPPs to weak
AC grids from a number of utilities, manufacturers and consultants worldwide. The collected experiences confirm
that when connecting WPPs to weak AC systems almost all of the above performance requirements are adversely
impacted. The salient experiences include: failure to ride through disturbances, electro-mechanical instability, control
interaction and operating mode instability, and operation under islanding conditions.
The challenges have been to identify potential issues at the planning stage itself and then implement economical and
effective solutions. Robust planning simulations that: assess the network’s capacity to connect, anticipate potential
issues and verify proposed solutions have been identified as critical in successfully meeting these challenges.
Modelling
The working group has addressed modelling requirements as well as associated issues and challenges in detail.
An application to connect any generator to a high voltage network requires significant simulation effort to assess
generator performance with the connected network under all operating conditions. These simulation studies provide
assurances for the robustness of the investment decision as well as for power system security. The studies define

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the anticipated performance under common operating conditions as well as under conditions which are rare but
stable, where operation of the plant will have a significant bearing on the stable operation of the power system.
In operating WPPs connected to weak AC power systems, the operation of many dynamic components associated
with a WPP is likely to come into play in a significant manner and to closely interact with the other dynamic
phenomena driven by the network and other nearby connected plants. In this regard, the working group has
concluded that accurate, high resolution, time domain modelling of pertinent WPP dynamics is critical. The traditional
"RMS" assessment of plant and power system performance, being unable to provide adequate attention to the fast
acting power electronics converters associated with WPP, would be inadequate.
Hence the selection of the modelling platform and models to adequately and accurately represent the pertinent
dynamic phenomena associated with the WPP is critically important.
The appropriate modelling platforms are required to represent the time variations of electro-mechanical dynamics
and those of individual electrical phases (and not approximations such as RMS variations) and simulate the dynamic
interactions with high time resolution. Hence the preferred modelling platforms would be those commonly used for
electromagnetic transient (EMT) simulations.
Generic WPP models, with adequate representation of all the dynamic phenomena associated with WPP’s power
electronic converters, are adequate for initial assessment and screening studies, but the relevant WPP manufacturer
specific models should be used as soon as possible to confirm expected performance, issues and potential solutions.
Representation of WPPs and other dynamic plants, from different manufacturers, connected to the network in close
vicinity through detailed and accurate models will require significant planning, coordination and effort.
Assessment of Issues and Solutions
The working group has used simulation studies extensively to:
a) confirm the experienced or identified potential issues using appropriate modelling of the operating
conditions under which the issues were encountered;
b) unravel the limitations of the equipment or contributory operating circumstances which cause the
manifestation of the issues; and
c) develop the solutions which may effectively mitigate the issues.
A generic type 4, full AC-DC-AC conversion, WPP model has been used for the simulations. Attention has been
given to modelling and understanding the performance degradation as the SCR of the connection is reduced, toward
its theoretical minimum of unity.
The inability of the power electronic converters within a WPP to follow and work together with the rapidly changing
conditions at the interface with the power system, in particular the changes in power system frequency and the
voltage phase angles, has been found to be the cause of many issues. The resulting performance deteriorations
were seen to be exacerbated with the increasing “weakness” of the connecting power system.
The assessment also covered the identification of potential solutions for mitigating the above issues and confirmed
their effectiveness. The potential solutions considered included in high level, potential improvements to the WTG
controllers as well as the installation of ancillary devices for supporting and enhancing the WPP performance. The
following potential solutions were specifically modelled and assessed:
 Addition of ancillary supporting devices: Synchronous Compensators (SC) and Static Synchronous
Compensators (STATCOM)

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 Stabilising the operation of WTG converter operation by using remote synchronising reference and by
using locally synthesised synchronising reference
The potential for connecting the weak and remote regions of the AC power systems with a high penetration of WPPs
to other regions of the power system was also assessed. The inter-regional connection solutions investigated include
connection of the regions via a Voltage Sourced Converter (VSC) HVDC link in parallel with an AC link, and
connection via a Line Commutated Converter (LCC) HVDC link.
A Guide for Connecting to Weak AC Systems
The final chapter of the working group's technical brochure has been compiled as a guide. The guide summarises:
the issues to be anticipated, the potential solutions available, how to screen a connection for the expected
performance and issues, how to assess the potential issues and solutions, and how to proceed in developing a
connection in consultation with the WPP’s turbine manufacturer.
The following considerations are recommended in support of the thought process when undertaking power system
studies for planning and connection assessment of WPPs:
 For weaker WPP applications (e.g. 3 < SCR < 5):
o Comparison of RMS-type dynamic models against the detailed EMT-type models and confirmation
of the models through field measurements is advantageous;
o Detailed RMS-type models may be used as opposed to generic models; and
o Changes to the plant control system and/or installation of supplementary equipment, e.g.
synchronous condensers, is less likely.
 For very weak WPP applications (e.g. SCR < 3):
o Verification of RMS-type models against the detailed EMT-type models is necessary before
carrying out any detailed connection assessment studies;
o In the event that an acceptable correlation does not exist between the RMS-type and EMT-type
models, either RMS-type models need to be revised, or EMT-type models need to be used for the
connection assessment studies;
o Changes to the plant control system and/or installation of supplementary equipment, e.g.
synchronous condensers, may be necessary; and
o Any simulation models used for the studies must capture the application specific changes rather
than using an off-the-shelf standard simulation model.
Conclusion
The working group has made the following contributions and added to the body of knowledge in the industry:
 Collation of an almost exhaustive list of issues experienced and to be anticipated in connecting WPPs to
weak AC systems;
 Identification of the metrics suitable for screening of the potential issues in relation to the capacity of the
network considered for connection(s);

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 Definition of a modelling and simulation framework for assessing and confirming the potential issues and
solutions;
 Unravelling the limitations of the equipment or contributory operating circumstances which cause the
manifestation of the issues and development of solutions which may effectively mitigate the issues; and
 Compilation of a summary guide to assist the WPP developers for anticipating the potential issues and for
planning effective and economically efficient WPP connections to the networks.

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1 INTRODUCTION
1.1 Purpose
This brochure aims to increase the understanding of the issues which can arise when wind power plants (WPPs)
operate in weak AC systems and information on how to improve the performance of these systems. The results of
the assessment carried out by the working group are presented in the form of recommendations for connection
studies for WPPs connecting to weak AC networks.
1.2 Background
Many countries are experiencing a fast growth of renewable generation in general, and wind based generation in
particular, imposing (significant stresses on transmission grids. It is expected that future wind power developments
are likely to:
 utilise power electronic converters for converting either some or all of the power output from the
generators
 connect to remote and weak parts of the transmission and distribution grids
 load the transmission systems beyond their firm capacities and may be operated closer to short term
ratings of the transmission lines
Presently, there are concerns regarding reliable operation of power electronic driven wind generators in weak AC
networks. These concerns include the fast dynamic response of wind generator converter systems following system
disturbances, and the interactions between wind generator converter systems and any other power electronic driven
network assets (e.g. HVDC links and FACTS devices) in the vicinity. Wind farms connected through or in the vicinity
of series compensated transmission lines or HVDC lines may also be vulnerable to sub synchronous oscillations. For
this reason wind farm developers are looking for either classical (e.g. synchronous condenser) or FACTS based
solutions for addressing expected operational issues.
The Short Circuit Ratio (SCR), the ratio of the short circuit power at a given location in the network to the rating of
the generator connected to that location, is a common analytical indicator used in the industry to quantify system
strength. Low values of SCR indicate risk of insufficient system strength for reliable operation of the connected
generation and transmission plant. There is no industry consensus on the methodology for calculating SCR,
particularly for applications with several adjacent WPPs, or for WPPs adjacent to HVDC terminals.
There are also concerns on the veracity of dynamic models available and on the suitability of presently used tools
and methods, for simulating the performance of wind farms connecting to weak AC networks and assessing the
impacts.
1.3 Scope
This brochure aims to provide education on how wind plants operate in weak AC systems and the issues which can
arise in these systems.
Cigre has initiated a number of working groups which study various aspects of wind generation: WG B4.39, WG
B4.55, JWG C1/C2/C6.18, and WG C4.601. This brochure addresses issues which are not included in these, but
which are specifically related to wind power in weak power systems. Practical experience of such issues is also
related in the document.

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Sub-synchronous torsional interactions and sub-synchronous control interactions can occur with all types of
generation, and are not necessarily related to weak grids, however these issues are often a concern with wind farms,
and therefore references to other work in the area are included in the scope.
Issues which often arise in discussions about wind power connections are not dealt with in this brochure, which
focuses on issues related to both wind power and weak AC systems. Harmonic distortion is related to weak grids,
but not necessarily to wind power. Voltage control and coordination can be related to wind farms, but is considered
a system issue, and therefore out of the scope of this brochure.
The recommendations in this brochure are based on the grid integration of wind farms, but many of the ideas
presented here can also be applied to other types of generation connected via power electronics, such as photovoltaic
generation.
1.4 Summary of the salient work completed by other working
groups
A large quantity of information on the planning, connection, and performance of the WPPs, connecting to AC as well
as DC power systems are already available as a result of the work undertaken by many working groups and
taskforces driven by utilities, peak industry bodies, and professional organisations, such as Cigré and IEEE.
The effects of uncertainties related to wind power generation and similar technologies on power system planning
have been examined by the Cigre WG C1.3 [1]. The working group considered the uncertainty related to the
development of wind power projects, the variable nature of wind generation and technical performance of new
technologies in power systems.
The working group has made suggestions for managing the variability of wind generation over different time scales.
Examples are presented for how WPPs concentrated in small areas give rise to highly correlated wind generation,
whereas WPPs spread out over larger areas have less correlation, due to geo-diversity. Concentrated wind farms
give rise to reduced prediction confidence compared to their widely spread counterparts, because their total output
is more sensitive to wind speed changes. Suggestions are also made for managing technical issues such as voltage
tolerance, frequency tolerance, rate of change of frequency (RoCoF) tolerance, fault tolerance, voltage and frequency
control. The working group considered these can be handled by drawing up a set of grid codes which are suitable
for a particular area. WPPs need to be able to tolerate wider ranges of frequency and RoCoF in cases of small
synchronous networks.
The technical brochure completed by the Cigre working group WG C4.601 provides a detailed summary of the
popular wind turbine generator technologies, including the features of each design and a general discussion of their
controls and dynamics [2]. The conclusion drawn is that in order to comprehensively model characteristics and
performance of WPPs and to assess the technical issues related to connecting wind farms to transmission and
distribution systems, four types of WPPs are needed to be modelled:
 the conventional induction generator
 the wound rotor induction generator (WRIG) with variable rotor resistance
 doubly fed induction generator (DFIG) and
 full converter units
The brochure also includes recommendations on the appropriate level of modelling detail for power system analysis,
and the improvements necessary in existing models. Suitable methods to aggregate wind turbine generators in a
wind farm into a simpler model of the collector system are also described. Relevant issues discussed on the
transmission level are voltage-ride through, reactive power and power factor requirements, voltage control and
regulation, controls interaction, harmonic, power quality and frequency control. Issues for the distribution level include
voltage variations, flicker, power quality and harmonic emissions. Protection requirements are briefly discussed and
but fault level contribution is not dealt in this document.

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The Cigre working group WG B4.39 assessed the issues associated with integrating large scale wind generation with
particular emphasis on the use of HVDC and FACTS devices for improving the performance of the WPPs and the
networks [3].
The benefits of using power electronic components such as SVCs, STATCOMs are presented with regard to
transmission over long distances and, voltage stability and power quality. The benefits of energy storage are also
described. An overview of HVDC systems is also presented, including descriptions of LCC and VSC HVDC systems,
and their applications in connection of WPPs, in particular those sited offshore. Examples of actual wind power
projects using power electronics are provided, as well as a discussion of economic issues related to wind power
projects and associated transmission costs.
Cigré working group WG C6.08 has focused on the technical aspects of connecting wind generation to the grid [4].
It deals with issues of integrating large amounts of wind power in large scale power systems, such as:
 power flow and contingency management
 frequency control and operational reserve requirements
 voltage and transient stability
 reactive power and voltage control
 influence on conventional generation, and
 regulation and support strategies.
The brochure includes a review of different market schemes, as well as a list of FACTS devices which can help to
alleviate congestion. Frequency control in different time scales is presented, including a discussion on inertia in power
systems with wind power. Technical possibilities for frequency control from wind power plants are also described.
The operational reserve requirement is described, and case studies are presented. The effects of wind power on
voltage stability and the effects of different turbines on transient stability are described, and a review on the reactive
power and voltage requirements in different grid codes is presented. Methods to evaluate the influence of wind power
generation on conventional generation in the form of generation displacement are described, and finally a review of
the support strategies in different countries is presented.
Cigre joint working group, JWG C1/C2/C6.18 has surveyed the mechanisms employed for coping with limits for very
high penetrations of renewable energy from 18 countries regarding issues seen with variable non synchronous
renewable generation [5]. Based on these observations, general recommendations are made:
 Power system operators need to develop a broad understanding of the policy objectives that will materially
impact on the operation of the power system. These might include increasing levels of renewable energy,
the introduction of EVs or energy efficiency measures.
 Research examining the impact of high levels of renewable energy sources (RES) appears to be limited.
There is concern that many of the limits to RES integration will be caused by voltage stability, reactive power
and transient stability.
 A strict adherence to grid code provisions is required and enforcement is needed.
 The increasing levels of variable non synchronous renewable generation will fundamentally change the
characteristics of power systems across the world. In order to manage these changes, system operators will
need greater system performance.
 The design of markets needs to consider the technical requirements of the power system (such as flexibility,
ramping, frequency control and storage).
In addition to the above, the working groups also made the following observations:
 Offshore WPPs are becoming more prevalent. This is likely to lead to the development of significant HVDC
grids. The control and interaction of these grids with the power systems is an area that would benefit from
future studies.
 Challenges are emerging in the connection of non-synchronous generation in relatively weak parts of
electricity networks. This is increasing the need for detailed three phase electromagnetic transient studies.

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1.5 Gap analysis
While a large amount of material has been published regarding integration of wind power into power systems, the
salient issues associated with integration of large WPPs in weak AC networks have not received significant attention.
While mitigation of issues using supplementing primary plant, such as fast acting reactive power supply devices such
as SVCs or STATCOMs have been suggested for integration of WPPs into weak AC networks, the use of fast
controllability of the modern WPPs equipped with full converter units, has not been widely considered as a potential
mitigation measure.
Further, the measures used for quantifying the “weakness” of the AC grids has not been harmonised within the
industry, making it difficult to compare the effectiveness of various mitigation measures. The definition of a simple
and agreeable measure of the system “weakness” has been further compounded by the facts that connection of
more than several WPPs in the same vicinity would have to share the available system strength for providing an
adequate performance.
Similarly the issue of weak networks has been discussed extensively regarding the HVDC connections, but not wind
power connections. This brochure aims to fill in this gap.
1.6 Structure of the report
In order to describe the issues associated with connecting wind farms to weak AC networks, the brochure begins by
describing the types of wind farms which will be considered. The turbines which comprise the wind farms and their
controls are described as well as the controllers for the farms themselves. Supplementary equipment which may be
present to support a wind farm, such as FACTS, and connection alternatives such as HVDC, are also described.
In Chapter 1 – Introduction (this chapter), the previous works, by the Cigre working groups, associated with the
subject of WPP connections have been briefly reviewed and the gaps in relation to connection of WPPs to weak AC
system have been highlighted.
In Chapter 2 - Technology Summary, different wind turbine technologies control schemes and power electronic
components used in WPPs are described, for the purpose of understanding the ways in which wind turbines interact
with weak AC networks
In Chapter 3 – Issues Associated with Weak Systems, the experiences associated with connection of wind generators
onto weak AC networks are summarised and potential issues are identified. A distinction is made between the issues
associated with operation and dynamics of wind farm equipment, operation and dynamics of network, and dynamic
interaction of other nearby transmission equipment.
Chapter 4 - Modelling Summary, reviews the modelling and simulation requirements of wind power plants and power
system for examining the different types of issues identified in Chapter 3. The components of the WPPs which need
to be modelled for different studies, and their level of detail, are discussed with particular consideration of the
connection of WPPs to weak power systems.
In Chapter 5 - Assessment, modelling and simulation studies carried out for confirming the issues identified in Chapter
3 are reported. Potential solutions for mitigating the issues have been investigated, their effectiveness have been
assessed, and reported.
In Chapter 6 – Quantification of Issues, the indices suitable for quantifying the weakness of a power system with a
high penetration of wind power are presented. Indices are considered for the assessment of both local and system
wide issues. These indices include X/R ratio, short circuit ration (SCR) and its derivatives, system inertia and the rate
of change of frequency (RoCoF).
Chapter 7 – Guide to Wind Power Plant Connection, addresses the application of engineering considerations for
interconnection of a wind power plant to a weak AC system, by applying the information from the first six chapters in

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practical situations. The chapter deals with different aspects of system capability, issues to be considered, and
various technology options to address these issues, and examples are provided throughout.
Chapter 8 presents the results obtained in the working group in the form of recommendations, and potential future
work that could contribute to enhancing the knowledge in this subject has been identified.
Appendix A provides a detailed description of WPP models. The discussion on voltage sensitivity of connections in
section 6.4 is further extended and clarified in appendix B. Appendix C summarises the experiences of a number of
utilities and manufacturers in connection of WPPs to weak AC networks.
1.7 References
[1] WG C1.3, "Electric power system planning with the uncertainty of wind generation," Cigre Technical
Brochur3 293, April 2006.
[2] WG C4.601, "Modelling and dynamic behaviour of wind generation as it relates to power system control
and dynamic performance," Cigre Technical Brochure 328, August 2007.
[3] WG B4.39, "Integration of large scale wind generation using HVDC and power electronics," Cigre Technical
Brochure 370, February 2009.
[4] WG C6.08, "Grid Integration of wind generation," Cigre Technical Brochure 450, February 2011.
[5] WG C1/C2/C6.18, "Coping with Limits for Very High Penetrations of Renewable Energy," Cigre, 2013.

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2 TECHNOLOGY SUMMARY
2.1 Introduction
The technologies used for electricity generation and control within WPPs as well as complementing technologies
used within the wind-farm and in the connecting grid for supporting the operation of the WPPs have a significant
bearing on the ability to stably connect and operate WPPs in “weak” grids. In this chapter, the presently implemented
WPP technologies as well as the supporting technologies implemented within the wind-farm and connecting grids
are summarised.
There are two main types of wind turbines: horizontal and vertical axis. The wind flows over the turbine blades which
are connected to a shaft, causing rotation. The shaft turns the generator; depending on the turbine design, there may
be a gear box to adjust the rotational speed. The generator produces electricity and sends it into the power grid.
Horizontal shaft, three-blade turbines are the most common today. Figure 2.1 schematically shows the main
components of the modern horizontal axis wind turbine.
Figure 2.1 Horizontal Axis Wind Turbine Constituent Parts [1]
The modern wind turbines are also equipped with several key elements that support to improve their controllability
and efficiency. Inside the Nacelle (or head) is an anemometer, wind vane, and controller that read the speed and
direction of the wind. As the wind changes direction, a motor (yaw motor) turns the nacelle so that the blades are
always facing the wind. The power source also comes with a safety feature; in case of extreme winds, the turbine
has a brake that can be applied to inhibit any damage to the turbine.
2.2 Types of Wind Turbines Based on Speed Control

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In recent literature, the available wind turbine technologies are classified into five principal types mainly based on
their ability or the mechanisms used for their speed control [2]. While these are commonly referred to as Type 1 –
Type 5, they may be sometimes referenced by different names or abbreviations in the literature.
Type 1: Fixed Speed
Wind turbines constituted of squirrel-cage induction generators (SCIG) connected directly to grid via step-up
transformers are usually classified as a Type 1 WTG. Figure 2.2 shows a schematic representation of a Type 1 WTG.
Figure 2.2 Type 1 Wind Turbine
The turbine speed is fixed (or nearly fixed) to the electrical grid’s frequency, and generates active power (P) when
the turbine shaft rotates faster than the electrical grid frequency creating a negative slip. While there is a bit of
variability in output with the slip of the machine, Type 1 turbines typically operate at or very close to a rated speed.
A major drawback of the induction machine is the reactive power that it consumes for its excitation field and the large
currents the machine can draw when started. To mitigate these effects the turbine typically may employ a soft starter
and discrete steps of capacitor banks within the turbine.
Type 2: Limited Variable Speed
Type 2 WTGs generally refers to wound rotor induction generators (WRIG) whose stators are connected directly to
the grid via a step-up transformer in a fashion similar to Type 1 WTGs, but also include a mechanism for controlling
the speed of the machine, connected externally to the rotor via slip-rings. The speed control is achieved via control
of resistors (either mechanical and/or power electronic controlled) connected to the rotor windings. Alternatively, the
resistors and power electronics can be mounted on the rotor, eliminating the slip rings. The resistance connected to
the rotor circuit can be controlled and varied and hence in turn can control the rotor currents quite rapidly. Figure 2.3
shows a schematic representation of a Type 2 WTG.

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Similar to the Type 1 WTGs, the turbine speed is fixed (or nearly fixed) to the electrical grid’s frequency, and
generates active power (P) when the turbine shaft rotates faster than the electrical grid frequency creating a negative
slip.
However, by adding resistance to the rotor circuit, the “torque – speed” curve of the generator can be “stretched”
making the rotor rotate faster to create the same output power. This allows some ability to control the speed, together
with the blades’ pitching mechanisms and allows the turbines operation to move to a better tip speed ratio (ratio of
tip speed to the ambient wind speed) to achieve the best energy capture. It is typical that speed variations of up to
10% are possible, allowing for some degree of freedom in energy capture and self-protective torque control.
Ability to rapidly vary rotor current resistance makes it possible to control the power output at set values, even during
gusting conditions, and can influence the machine’s dynamic response during grid disturbances.
Type 3: Variable Speed with Partial Scale Power Electronics Converters
Type 3 WTGs generally represent the WTGs constituted of induction generators known commonly as the Doubly
Fed Induction Generator (DFIG) or Doubly Fed Asynchronous Generator (DFAG). While similar to Type 2 WTGs, in
Type 3 WTGs the wound rotor of the generator is connected to a variable frequency AC source (instead of simply
resistance). The additional rotor excitation is supplied via slip rings from a voltage-source converter which is in-turn
connected back-to-back with a grid side converter and exchanges power directly with the grid as required. Figure
2.4 shows a schematic representation of a Type 3 WTG.

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The voltage source converter connected to the rotor can adjust the rotor currents’ magnitude and frequency nearly
instantaneously. The ability to change the rotor current frequency, make the rotor speed not fixed to the stator
frequency (i.e. unlike Type 1 or Type 2 WTG), and hence provides a wider freedom for rotor speed control.
A small amount of power injected into the rotor circuit through the rotor-converter can effect a large control of power
in the stator circuit. This is a major advantage of the DFIG: a great deal of control of the output is available with the
presence of a set of converters that typically are only 30% of the rating of the machine. In addition to the active power
that is delivered to the grid from the generator’s stator circuit, power is delivered to the grid through the grid-connected
inverter when the generator is moving faster than synchronous speed. When the generator is moving slower than
synchronous speed, active power flows from the grid, through the converters, and from rotor to stator. These two
modes, made possible by the four-quadrant nature of the two converters, allow a much wider speed range, both
above and below synchronous speed by up to 50%, although narrower ranges are more common. The greatest
advantage of the DFIG, is that it offers the benefits of separate active and reactive power control, much like a
traditional synchronous generator, while being able to run asynchronously.
Type 4: Variable Speed with Full Scale PE Converters
Type 4 WTGs represent connection of rotating generators to the grid via AC-DC-AC converters. Figure 2.5 shows a
schematic representation of a Type 4 WTG.

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The Type 4 turbine offers a great deal of flexibility in design and operation as the speed of operation of the generator
is not coupled with the grid frequency. The turbine is allowed to rotate at its optimal aerodynamic speed. In addition,
the gearbox may be eliminated, such that the machine spins at the slow turbine speed and generates an electrical
frequency well below that of the grid. The rotating machines of this type have been constructed as wound rotor
synchronous machines, permanent magnet synchronous machines, or as squirrel cage induction machines.
Advances in power electronic devices and controls in the last decade have made the converters both responsive and
efficient. The converters in type 4 turbines also offer the possibility of reactive power supply to the grid, with or without
the generator in operation. However, the power electronic converters have to be sized to pass the full rating of the
rotating machine, plus any capacity to be used for reactive compensation.
Type 5: Variable speed with Mechanical Transmission
Type 5 WTGs represent the WTGs where a variable speed turbine is coupled to a fixed speed synchronous
generator, via a mechanical, variable ratio torque/ speed converter. Figure 2.6 shows a schematic representation of
a Type 5 WTG.

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The torque/ speed converter changes the variable speed of the turbine rotor shaft to a constant output shaft speed.
The closely coupled synchronous generator, operating at a fixed speed (corresponding to grid frequency), can then
be directly connected to the grid through a synchronizing circuit breaker. The synchronous generator can be designed
appropriately for any desired speed (typically 6 pole or 4 pole) and voltage (typically medium voltage for higher
capacities). This approach requires speed and torque control of the torque/ speed converter along with the typical
voltage regulator (AVR), synchronizing system, and generator protection system inherent with a grid-connected
synchronous generator.
2.3 Generator Technologies
Presently, the generator types used in the WTGs include induction generators with either squirrel cage or wound
rotor construction and synchronous generators with either wound-rotor or permanent magnet construction.
2.3.1 Squirrel Cage Induction Generator
The primary advantage of the squirrel cage induction generators is their rugged brushless construction without a
need for separate rotor field excitation. These machines are very economical, reliable, and are available in the ranges
of fractional horse power (FHP) to multi megawatt capacity. Also, unlike synchronous machines, induction machine
operating speeds can be varied. In many WPPs the squirrel cage induction generators are connected to and driven
by a wind turbine through a gear box.
A major drawback of the squirrel cage induction machine is the reactive power that it consumes. At the nominal
operating point, the reactive power consumption is typically in the region of 70% of the active power.
2.3.2 Wound Rotor Induction Generator
A wound rotor induction generator is equipped with a three phase rotor winding, whose terminals are brought out via
slip rings and brushes.
This makes it possible to alter the performance of the generator, either by simply adding resistors external to the
rotor or by controlling the rotor currents via power electronic devices. By changing the rotor circuit resistance or
controlling the rotor currents, the operating speed, power factor, starting current and torque can be controlled.
Increased rotor resistance, will allow operation at higher speeds, reduce reactive power demand and reduce the
starting current.
The doubly fed WTGs make use of the flexibility to change the generator performance characteristics by changing
the frequency, phase and magnitude of the rotor current through power electronics.
2.3.3 Wound Rotor Synchronous Generator
A wound rotor synchronous generator consists of a three phase stator winding and a DC exited rotor winding. While
the rotor field is fixed (with respect to the rotor), the rotor rotates synchronously at the synchronous speed of the
machine.
The rotor winding terminals are brought out of the rotor via slip rings and brushes and normally connected to an
external DC source.
The advantages of wound rotor synchronous machines are the controllability of the generation voltage and reactive
power output by changing the field current.

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2.3.4 Permanent Magnet Synchronous Generator
In permanent magnet synchronous generators, the DC excitation field is provided by permanent magnets mounted
on the rotor and hence require no field excitation supplied from an external voltage source.
They typically have a long air-gap, and hence show linear magnetic characteristics, low reactance, and are compact
and efficient, compared to wound rotor machines.
2.4 Wind Turbine Mechanical Control
For the purposes of this brochure, a three-bladed, horizontal-axis, pitch controlled wind turbine, as illustrated in Figure
2.1 is considered.
The aerodynamic torque on the wind turbine rotor results from the local action of wind on blades. The contribution of
each blade to the rotor torque depends on the rotor speed, the actual blade pitch, the yaw error, the drag error, and
any other motion due to elasticity of the wind turbine structure. Except for aeroelastic effects, each of the other
contributing inputs to aerodynamic torque (rotor speed, pitch, yaw and drag) may be monitored by specific control
systems. All wind turbines are equipped with yaw drives that monitor yaw error and with supplementary devices that
are used to modify rotor drag.
In the case of variable speed wind turbines, these installations can operate at different speeds or equivalently at
variable tip-speed ratios. Pitch-regulated wind turbines are controlled by modifying the blade orientation with respect
to the direction of incident wind.
2.5 Power Electronic Converters for Wind Turbine Converters
2.5.1 Two-Level Power Converter (2L-BTB)
Two-Level Power Converter (2L-BTB) with pulse width modulation-voltage source converter with two level output
voltage (2L-PWM-VSC) is the most frequently used three-phase power converter topology so far in WPPs. Figure
2.7 shows the schematic connection of the 2L-BTB converter.
This type of converter can be used as the power electronic converter controlling the rotor winding currents in type 3
WPPs or as the full scale power electronic converter for type 4 WPPs. As the interface between the generator and
grid in the WPP, two 2L-PWM-VSCs are usually configured as a back-to-back structure (2L-BTB) with a transformer
on the grid side, as shown in the Figure 2.7.

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Figure 2.7 Two level pulse width modulation -voltage source converter
A technical advantage of the 2L-BTB solution is the relatively simple structure and few components, which contributes
to a well-proven robust and reliable performance.
However, as the power and voltage range of the wind turbine are increasing, the 2L-BTB converter may suffer from
larger switching losses and lower efficiency at high power levels. The available switching devices also need to be
connected in parallel and/or in series in order to obtain the required power and voltage of WPPs, which may lead to
reduced simplicity and reliability of the power converter. Another problem in the 2L-BTB solution is the two-level
output voltage. Having only two voltage levels introduces relatively higher dv/dt stresses to the generator and
transformer. Bulky output filters may be needed to limit the voltage gradient and reduce the THD.
Multilevel Power Converters are used more and more as the power capacity of WPPs climbs, and it becomes more
and more difficult for a traditional 2L-BTB solution to achieve acceptable performance with the available switching
devices. Generally, multilevel converters can be classified into three categories: neutral-point diode clamped
structure, flying capacitor clamped structure, and cascaded converter cells structure. In order to get a cost-effective
design, multilevel converters are mainly used in the variable speed full-scale power converter wind turbines with the
power range of several MW. Several possible multilevel solutions are presented in the following figures.
2.5.2 Three-Level Neutral-Point Diode Clamped Back-To-Back Topology (3L-NPC
BTB)
Three-level neutral-point diode clamped topology is one of the most commercialized multilevel converters on the
market. Similar to the 2L-BTB, it is usually configured as a back-to-back structure in wind turbines, as shown in Figure
2.8 below, which is called 3L-NPC BTB for convenience.

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Figure 2.8 Three-level neutral-point diode clamped topology
It achieves one more output voltage level and less dv/dt stress compared to the 2L-BTB, thus the filter size is smaller.
The 3L-NPC BTB is also able to output the double the voltage amplitude compared to the two-level topology with
switching devices of the same voltage rating. The midpoint voltage fluctuation of the DC bus used to be a drawback
of the 3L-NPC BTB. However, this problem has been extensively researched and is considered improved by the
controlling of redundant switching status. However, it is found that the loss distribution is unequal between the outer
and inner switching devices in a switching arm, and this problem might lead to underutilised converter power capacity.
2.5.3 Three-Level H-Bridge Back-to-Back Topology (3L-HB BTB)
The 3L-HB BTB solution is composed of two H-bridge converters which are configured in a back-to-back structure,
as shown in Figure 2.9 below. It can achieve output performance similar to the 3L-NPC BTB solution, but the unequal
loss distribution and clamped diodes are eliminated. More efficient and equal usage of switching devices as well as
higher designed power capacity can be obtained.
Figure 2.9 Three-level H-bridge back-to-back topology
Moreover, as only half of the DC bus voltage is needed in 3L-HB BTB compared to the 3L-NPC BTB, there is less
series connection of capacitors and no midpoint DC bus, thus the size of DC link capacitors can be further reduced.

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However, a 3L-HB BTB solution needs an open-winding structure in the generator and transformer in order to achieve
isolation between each phase. This feature has both advantages and disadvantages: on one hand, an open-winding
structure enables relatively isolated operation of each phase, and a potential fault-tolerant ability is thereby obtained
if one or even two phases of the generator or the generator side converter are out of operation. On the other hand,
an open-winding structure requires double cable length and weight in order to connect with the generator and the
transformer. Extra cost, loss, and inductance in the cables can also be major drawbacks. The open-winding impacts
on the loss/weight of the generator and the transformer.
High level modelling of the power electronic converters in WPPs and their control strategies are discussed in
Chapter 4.
2.6 Complementing wind farm technologies
The complementing technologies, generally employed at the wind farms for improving the WPP performance include,
static and/or dynamic reactive power support devices such as, mechanically or thyristor switched capacitors, SVCs,
STATCOMs and synchronous condensers.
2.6.1 Thermistor Switched Capacitors (TSCs)
A TSC consists of a shunt capacitor in series with two thyristor switches in antiparallel. The thyristors are used only
for switching on or off the capacitors.
TSCs provide almost instantaneous controllability of the reactive power support provided, and are usually used as
an integral part of the SVCs.
2.6.2 Static Var Compensators (SVCs)
An SVC comprises a combination of shunt connected Thermistor Controlled Reactor(s) (TCR), filters and in some
cases Thermistor Switched Capacitors (TSC) or Mechanically Switched Capacitors (MSC). The technology, based
on TCR is mature.
The TCR consists of a shunt reactor in series with a thyristor controller with two thyristor switches in antiparallel.
Each of the two parallel thyristor switches conduct for a period of up to one half period of the reactor current. By
controlling the trigger instant of the thyristor switch, the reactor current, and thereby the reactive power absorbed
from the AC grid, can be controlled continuously between zero and rated power. In the simplest design, a capacitor
bank produces a fixed amount of reactive power and the TCR adjusts its absorption, thereby performing reactive
power control or voltage control at the connection point.
2.6.3 Static Compensator (STATCOM)
Static synchronous compensators (STATCOMs) are Voltage Sourced Converters (VSC) used solely for reactive
power absorption or generation. Unlike thyristor based solutions (TSC and SVC), VSC power electronic systems are
based on IGBT (Insulated Gate Bipolar Transistor) or GTO (Gate Turn Off Thermistor) technology, which intrinsically
enables a faster response than thyristors. The VSC uses the semi-conductors to switch a DC capacitor to the three
AC terminals at high speed, such that a sinusoidal fundamental frequency voltage (after filtering) appears to the AC
network behind a large reactor (the converter reactor). By controlling the phase angle between the converter AC
voltage source and the network voltage, the active power flow is limited to that which is required to keep the DC
capacitor voltage to a desired value. By controlling the amplitude of the converter AC voltage relative to the network
voltage, the flow of reactive power from the converter can be controlled. The reactive power flow is determined by
the difference between the two voltages and the impedance between the converter and the network.

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2.7 Complementing Grid Connection Technologies
The technologies that support the performance of WPPs in connecting to AC grids include:
 Reactive power devises such as capacitors, SVCs and STATCOMs installed in the network in electrical
proximity to the WPPs
 Series compensation of the AC transmission lines
 Connection of the windfarms via HVDC transmission lines
Series compensation of the AC transmission lines connecting the wind farms, improve the WPP performance by
providing an increasing level of reactive compensation when the power transferred through the line is increased, and
hence improving the ability for the control of the WPPs.
Connection of windfarms via HVDC transmission lines have been increasingly used for connecting the offshore wind
farms with the onshore grids. Long length of the submarine cables required for the connection makes the use of
HVDC to be the most economic and preferred technology for connection.
This brochure does not cover the connection technologies, series compensated transmission lines or HVDC
transmission lines, but the readers are referred to the relevant publications.
2.8 References
[1] North American Offshore Wind Project Information, http://offshorewind.net/
[2] IEEE PES Wind Plant Collector System Design Working Group: "Characteristics of Wind Turbine
Generators for Wind Power Plants", IEEE, 2009

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3 ISSUES ASSOCIATED WITH WEAK SYSTEMS
3.1 Introduction
This chapter is intended to provide an overview of typical requirements imposed by regulatory authorities upon all
generators, including power electronic based generation such as wind power generation. In light of these
requirements, a number of issues associated with weak systems that impede wind generation from performing as
required are presented, along with several examples of how these issues have impacted real wind interconnection
projects.
3.2 Overview of WPP Performance Requirements
Wind power plants (WPP) interconnected to high voltage transmission networks are expected to provide energy
without negatively impacting the ability of transmission providers to serve their load in a safe, reliable, and cost
effective fashion. In addition, they are increasingly expected to provide basic support functions which contribute to
the overall good performance of the electric system. To ensure that wind plants and other equipment to be connected
to power systems effectively support these two targets, network operators set performance requirements in their
network connection codes (or "grid codes" or "connection requirements"). As network connection codes are an
inherent part of the network planning criteria and practices, they may vary between synchronous areas and regions
within synchronous areas, dependent also upon historical approaches that have been used as bases for robust
network designs. Local phenomena affecting the transmission network performance (due to specific network
topologies and generation characteristics, for example) may significantly affect the content of the network connection
codes. Although connection codes may vary in specific details, on a more general level most codes address the
following:
 Frequency and voltage operation range
 Reactive power and voltage control
 Active power and frequency control
 Capability to ride through disturbances
In addition to these main areas of technical performance requirements, connection codes typically address areas
that are relevant for network planning and operation. For example, from a network planning perspective, essential
areas include modelling requirements and requirements related to documentation of technical information.
One of the inherent challenges related to requirements set for WPPs is that they are subjected to technology
development cycles which may be faster than the rate at which interconnection standards can be updated and
applied. Issues related to connection points with relatively low short circuit capacity are a prime examples of grid
connection related issues that may have been recognized in their full extent only late in the interconnection process,
either at the stage of highly detailed grid connection studies or even after the power plant has been connected to
network. In these cases interconnection standards may have been inadequate to predict the problems and correctly
plan for the interconnection implementation procedures.
The following sections provide examples of the technical performance requirements that have been recognized to
require special attention when the SCR at the connection point of the WPP is low. This is not a comprehensive list
of grid code requirements, but a selection of issues impacted by weak systems.
3.2.1 Capability to ride through disturbances

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Capability to ride through disturbances, or fault ride-through (FRT) requirements as commonly referred in the grid
codes, normally state in some form that:
1. The WPP should not be tripped in the event of normally cleared system faults. Fault ride through is a
requirement where wind generators are required to stay connected to the grid during and after the clearance
of a system fault. Following the clearance of the fault, the WPP should be able to provide active and reactive
power to the grid. This will assist to maintain angle and voltage stability of the system.
This requirement may be more critical in weak systems because of the following reasons:
 Local reactive power support to maintain system voltages is more critical in weak systems.
 Active power deficiencies as a result of WPP tripping may not be easily compensated by other
generators in the local area requiring power transfer over long distances.
 Tripping of a significant generator is more likely to result in undesirable poorly damped power oscillations
in weak system compared to a strong system.
2. The wind plant should be able to control active and reactive power injection during the fault recovery. A
typical requirement of a weak grid during a fault is curtailment of active power and boosting of reactive power
injection to support voltage. Power electronics based wind generation has this inherent capability. This is
achieved through fast control of active and reactive currents. However, active power and reactive power
injection should be coordinated within the equipment rating limitations. Grid code may require fast ramping
of active power in the fault recovery period. This requirement is difficult to meet in weak grids as reactive
power requirements to meet voltage recovery limits the active power ramping rate.
3. In situations where WPPs are required to absorb reactive power for maintaining the grid voltages below their
maximum allowable limits, a failure of an WPP to ride through a disturbance and subsequent disconnection
from the grid would likely to increase the grid voltage above permissible limits. Depending on high voltage
ride through (HVRT) capability of WPPs, operation of wind plant overvoltage relays may occur following a
grid disturbance, especially in regions dominated by inverter based generation, with very little load or
conventional generators. Loss of a wind plant through HVRT protection can potentially lead to overvoltage
cascading, as the system voltage rises further when generation trips.
3.2.2 Appropriate disconnection due to an unintentional electric island
Large generation facilities must quickly disconnect themselves from the system if they are inadvertently left in an
electric island together with load to prevent uncontrolled behavior and damage to equipment. This is normally a
natural occurrence, since an imbalance between the size of the load and the size of the generator causes the power
frequency to quickly rise or fall beyond protection thresholds. Likewise, system voltages often quickly deviate from
nominal values and the generators tend to protect themselves. However, care must be taken in cases where a
load/generation balance is close, and the plant is not quickly disconnected. The system in these cases may be left
in an uncontrolled state (lacking a system reference and any form of frequency control), and power electronic devices
may oscillate, drive voltages to damaging levels, or otherwise degrade power quality. In cases where inadvertent
islanding is of concern, direct transfer trip schemes are normally employed.
Although these disconnection concerns apply equally in weak systems and strong systems, cases lending
themselves to islanding tend to be more remote from the bulk electric system, and this issue often is raised along
with weak system concerns.
3.2.3 Stable coordination of dynamic controllers
Generators are expected to operate in a stable manner, and to avoid interfering with the controls of neighboring
equipment. Modern power electronics based wind generators are equipped with numerous control systems

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performing numerous control functions. These control functions can interact with nearby power electronic based
dynamic devices with comparable control system time constants. This can lead to detrimental dynamic oscillations.
The potential of such oscillations is greater when the devices are connected to a weak grid.
3.2.4 Stable performance during weak or outage conditions
Generators are expected to operate in a stable manner during varying system conditions, including following outages
which may significantly weaken the connection strength. For power electronic based generation, it may be
challenging to have a single set of control parameters to meet grid requirements under all conditions. Adaptive control
parameters may be required.
3.2.5 Sufficient contribution to network voltage support
Generators are expected to contribute to voltage support and control of the bulk electric system. This includes
reactive power available to regulate voltage (fast and slow support). For modern power electronic based generation
such as wind power generation, these types of support are typically achievable through special controls. Additional
reactive power devices may be required in some cases to meet steady state and dynamic performance requirements.
In strong systems, this may be achieved by using mechanically switched shunt devices. In weak systems, dynamic
devices such as SVCs and STATCOMs or even SCs (synchronous compensators) may be required to meet dynamic
performance criteria.
3.2.6 Frequency Support
Wind power plants in some cases are expected to contribute to control system-wide frequency. For wind this may
be accomplished over short time periods (up to several seconds) as a type of "inertial control" or over longer
timeframes depending on the ability of the plant to increase or reduce power based on its operating point and control
capabilities.
3.3 Technical Issues Identified in Weak Grids
The following issues have been identified in existing bulk electric systems:
3.3.1 Failure to ride through disturbances
As discussed in section 3.1.1 above, in weak systems, wind plants may be unable to adequately meet ride-through
criteria as defined by regulatory agencies. This can manifest in several ways, but failure of the plant to regulate its
terminal voltage adequately as the plant recovers its active power following a disturbance can cause the plant
protections to operate inappropriately and fail to "ride-through". Further, when connected to weak AC systems, the
ability of the controllers to adequately follow the connection point system frequency and phase immediately after a
fault, reduce significantly causing the disconnection of the WPPs from the system.
Over compensation for ride through during transmission faults, may lead to exceeding the connection point voltage
following the clearance of the fault. Depending on high voltage ride through (HVRT) capability of individual wind plant
designs compared to others, operation of wind plant overvoltage relays may occur upon outage conditions, especially
in regions dominated by inverter based generation types like wind, with very little load or conventional generators.
Loss of a wind plant through HVRT protection can relieve loading on the high voltage circuits exporting the power,
causing the voltage to rise further, and potentially leading to overvoltage cascading, as the system voltage rises

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further when generation trips. Overvoltage tripping can be minimized through a combination of system strength
enhancements and better HVRT capability of wind generation projects. System collapse caused by overvoltage
cascading presents a significant reliability risk and suggests a need for wind generation projects to comply with HVRT
requirements.
3.3.2 Electromechanical oscillatory stability
In power systems several phenomena are known to cause undamped or growing oscillations. One common mode of
instability is the oscillations of the generator rotors with respect to each other, with energy transfer form one group of
machine to another group of machines taking place over the electricity transmission system. The typical frequency
range of electromechanical oscillations is 0.2-0.8 Hz (i.e. period time of oscillation varies from 1.3s to 5s).
Although fast voltage control of synchronous machines, in particular those are equipped with high gain, fast voltage
controller may contribute the damping of electromechanical oscillations, wind generators do not often participate in
these electro-mechanical oscillations. The participation has been mostly limited to Type 1 and Type 2 WPPs.
It is worth recognizing, however, that both the wind power plants located to close to the units participating in the
oscillations and the units located along the long and often relatively weak tie-line connecting the two system may
affect the power transmission constraints, set due to consideration of the small signal stability limits.
3.3.3 Control interactions and instability
The possibility of interaction between devices is very broad. Power electronic based generators may interact with
each other, or they may interact with other power electronic devices such as HVDC ties, FACTS devices such as
SVCs or STATCOMs, or even with non-power electronic based devices such as series capacitors, switched shunt
devices and conventional generators.
Control instability can occur due to the interaction of the fast, high gain controllers of WPPs (in particular Type 3 and
Type 4) with the power system, with other nearby plants such as HVDC converters, SVCs or STATCOMs, or with
the other nearby WPPs.
The weaker the system is in relation to the controlled devices, the more impact each of the devices has on the others.
In general the open loop gain as experienced by the interacting controllers is higher when they are connected and
operated in weak AC systems, making them more susceptible to control instability.
The device controller interactions to be expected are similar to the HVDC and SVC controls interactions is explored
and reported by Cigre WG 14.28.
3.3.4 Cycling between turbine control modes
A problem which may be observed if the system is too weak to support a turbine, depending on control
implementation, is cycling between turbine control modes. Wind turbines often have dedicated controls intended to
govern performance during severe events (such as faults), and to refrain the plant from tripping to protect itself.
When these ride-through modes are operational, WPP level reactive power orders may be ignored to allow all the
reactive power capability to be used to support local turbine terminal conditions. Gains and time-constants may be
adjusted, PLL controls may be changed, and active power may be reduced as required by the application.
If the system is very weak, the WPP may have difficulty transitioning from a reduced-power ride-through mode into
its normal full power operation. The transient caused by the plant recovery and shift from control modes may be
sufficient to cause the plant to re-enter its ride through mode, causing a major transient to occur in a periodic manner.
This is generally unacceptable, as the transients can be severe and can ultimately lead to plant tripping.

Page 36
Although synchronous compensators are a valuable tool in providing system inertia to stabilize power electronic
converter controls, care must be taken to ensure that the synchronous compensator itself does not introduce new
instability concerns. These are more likely to occur in weakened systems separated by some distance from a strong
angular reference.
3.3.5 Islanding issues
The risk of a wind plant creating a local island is typically higher for wind power plants that operate with low SCRs
because their connection point is often remote from strong grid mesh points. Study must be undertaken to ensure:
 the anti-islanding protection (if fitted) functions correctly
 other network users are not exposed to insecure operating conditions
 the wind power plant's design ratings are not exceeded, i.e. the WPP remains within its safe operating limits
3.4 Additional Challenges Identified in Weak Grids
3.4.1 Initial estimation of feasibility of connection and screening of issues
Short circuit calculations (in simplified form as described in chapter 6) are currently being used to estimate the ability
of power electronic based generation to connect to a weak transmission system and operate correctly.
While this is a useful technique, incorrect application can result in serious underestimations of system strength
performance. In other words, using assumptions for wind turbine SCC calculation which are conservative for breaker
duty calculations may be optimistic for SCR calculations, disguising the requirement for system strength to be shared
among several wind plants.
Conventional powerflow studies are often used to evaluate VAR adequacy in periods following a fault, and these
studies often assume the full VAR range capability claimed by the turbine manufacturers. It is possible for wind
plants to support the voltage across a system, but in practice these controllers may not be configured to operate in
the fast timeframes required to prevent voltage collapse in the few seconds following a fault, and the controllers may
not be available if the wind plants are out of service, or the wind is not blowing. A mix of network based voltage
support and wind power plant voltage support is desirable, and special care is required in conventional planning to
ensure sufficient VARs are available in the immediate post-fault timeframes as well as the extended simulation
timeframes typically examined in power flow studies.
3.4.2 Limitations of simulation tools and models
Most of the presently available transient stability simulation tools are meant for simulating the phenomena which are
symmetrical and can be represented as slow varying dynamics (with respect to a synchronously rotating reference
frame). While they are capable of simulating phenomenon such as synchronous machine stability and fault ride-
through of the WPPs, they may not be used for modelling power electronic controls in the wind generators in sufficient
detail to represent their behavior under weak conditions (e.g. adequate modelling of operation of the PLL). This can
result in either over or underestimation of control stability leading to planners making inaccurate or uninformed
decisions based on system impact study results.
Conventional tools that could model transient stability alone are not suitable to perform wind integration studies under
weak system configurations. In very weak networks, transient stability studies usually need to be supplemented with
modelling of fast acting power electronic controllers and assessment of the stability of their operation, using a suitable
simulation platform, such as an electromagnetic transient simulation tool.

Page 37
The assessment of the stability of WPPs connected to weak transmission systems also requires accurate modelling
and representation of the behavior of the WPP.
In some rare cases, generic models may be used, but in most cases the generic models do not adequately and
accurately represent the detailed control and protection functions associated with the wind turbine.
The models which can accurately represent those functions are highly detailed and often include detail information
on proprietary controls, in some cases utilizing actual firmware or software code found in the real equipment. Because
the value of intellectual property embedded within these models are very high, equipment manufacturers are
understandably cautious and anxious to control model distribution. They may also be reluctant to release the models
without complete control of the disclosure and distribution (requiring time-consuming and sometimes onerous non-
disclosure negotiations and agreements).
A significant support is often required from manufacturers for understanding and familiarization of the proprietary
models and for interpretation of the simulation results depicting the performance of the WPP. This type of support
requires special expertise, which is limited in many organizations. Additionally, the end-users of these models also
require significant training and experience to use the models. Without these, the support burden on the model
developers becomes even more onerous.
The modelling of WPPs connected to weak transmission systems are further explored in chapter 4.
3.4.3 Challenges in transmission network planning
Integration of WPPs has presented new challenges to traditional transmission network planning processes for
reasons that may vary significantly between different jurisdictions and transmission networks.
Renewable generation in general has caused very different challenges for different network planning processes. In
those parts of the world where large scale integration of the wind power has been occurring, the main challenges
from planning perspective are related to adjustment of the technical requirements and the network planning practices
so that the wind integration is possible without endangering system security and reliability. As result the technical
requirements have evolved quickly, and they have taken even in some extent the role of the technology driver.
A prime example of a new challenge is the connection of WPPs to connection points with low SCR. After the initial
round of wind power integrations has utilized the optimal locations in the existing infrastructure, such as good access
roads and straightforward grid connections, the next round of connections would likely to be farther from the existing
infrastructure, and will require more attention. The decreasing short circuit ratio of the connection points brings new
issues to the tables of the transmission planners:
 The type of phenomenon to be simulated and studies required
 The adequacy and suitability of the available simulation tools
 The adequacy and suitability of the available models of the network, including the connected WPPs
 The criteria for accepting the acceptable level of performance
It is worth recognizing that although these questions were originally raised by the wind power plants located far from
the bulk transmission network, the same issues are starting to appear also in some parts of the network where the
SCR ratios have traditionally been high. This happens especially in areas where wind power penetration has become
relatively high compared to the connected synchronous generation. On those areas the SCR may significantly vary
depending on operational situations as under good wind conditions, the synchronous generation will be displaced by
the wind generation. This emphasizes not only the importance of relevant wind power plant models for the system
operational planning, but also the availability of knowledge and relevant models of existing power system equipment.

Page 38
The time required for engineering, procurement and construction of a WPP is comparatively short and hence the
planers face the additional pressure in identifying, analysis and resolving the above challenges within a short time
frame.
3.4.4 Grid-Code and Connection Agreement Challenges
Through the existing Grid Codes and Connection Agreements WPPs are asked to support the grid and to provide
ancillary services. Although these codes may differ from each other on some specific aspects depending on the area
they are covering, in general they are all addressing relevant performance requirements needed for the stable,
reliable and efficient operation of the system.
As described in the previous sections of this chapter, in the case of an interconnection to a weak system, a WPP
might be confronted with some challenges associated with these types of systems. Furthermore, some of these
challenges may only be recognized in their full extent late in the interconnection process.
In order to achieve successful grid integration considering all these challenges, the implementation of a non-standard
technical solution may be necessary. The non-standard solutions can represent relevant controller changes at WTG
level and/or at WPP level for normal and fault-ride-through operation, the need of extra equipment or a combination
of these, depending on the project. Although non-standard, specific solutions should still satisfy the original aim of
the grid-codes and connection agreements namely the stable, reliable and efficient operation of the system.
Grid Codes and Connection Agreements are normally not specifically addressing weak grids and not specifying the
minimum level of system strength at which they are valid. As a consequence, a potential issue may arise when the
standard requirements are not physically applicable in the case of weak systems. Furthermore, one may also face
the case where the designed non-standard technical solution does not conform to all standard requirements and/or
original connection agreements.
Hence, further efforts will be necessary in order to find acceptable technical, economical, and commercial solutions
for the identified issues. Therefore close cooperation between all parties involved in projects in weak systems is
mandatory in order to achieve successful grid integration.
3.4.5 Economic Challenges
Many of the issues identified above can be solved, either through careful analysis and control tuning, or through the
addition of supporting equipment or new transmission infrastructure, or through reduction of generator output.
However, as systems become very weak, the economics of enabling the generation to connect and perform
satisfactorily can become onerous, and can affect project feasibility. As the system becomes very weak, and
especially as the limits of technical feasibility are approached, the costs can rise exponentially.
3.5 Examples
3.5.1 Issues masked by limitations of the assessment tools
A large wind power plant connecting into a weak system in the Northeast USA was nearing completion of all required
studies. Transient stability analysis showed stable results, however a final check using EMT tools revealed that the
plant would be unlikely to operate due to control instability in the weak system. A synchronous compensator was
added late in the project development, however the severe delay and requirement for subsequent restudy (among
other factors) caused the project considerable expense, and the utility performing the studies was severely
inconvenienced. This project caused a change in utility practice to move to EMT type studies earlier in the study
process when warranted.

Page 39
3.5.2 Issues due to control incompatibilities and interactions
A small wind power plant connecting into a weak radial system in the Northeast USA was nearing completion of all
required studies. The project was located very near a second project which was larger in size and already in service
(different turbine manufacturer). Again, transient stability analysis showed stable results, however a check using
EMT tools revealed that a fault near the system source side of the radial connection would cause the smaller plant
to consistently trip and fail to ride through. Detailed investigation using EMT tools showed that the control
philosophies used for the WPPs were incompatible and the smaller WPP would usually trip during a fault. The
inability of one wind turbine manufacturer to look at the model of the other manufacturer made coordination of the
control issue nearly impossible, and the ultimate delay caused the smaller project application to be withdrawn, and
the project was not built. This highlights some of the serious issues surrounding the proprietary nature of EMT type
models outlined in section 3.4.3.
3.5.4 Solutions – augmentation of the transmission system with series capacitors
The transmission capability of Finnish transmission network is stability restricted when power is either imported or
exported to Scandinavia. To limit the stability related restrictions a number of different approaches to improve the
transfer capability has been taken by Fingrid, the Finnish transmission system operator, during last 15 years.
One of these approaches has been series compensation of the tie-lines connecting the main transmission paths
connecting South Finland to North Finland as well as the paths connecting Finland and Sweden (see Figure 3.1).
Series compensation has greatly improved the transfer capability of Finnish transmission network considering both
the voltage stability phenomena restricting the import capability from North Finland to South and the damping of
electromechanical oscillations restricting the export capability from South Finland to North and further down to South
Scandinavia via North Sweden. New series compensated lines are under construction to accommodate the new wind
power generation. [1]
Figure 3.1 Location of main wind power areas in Finnish transmission system

Page 40
As the main transmission network of Finland is strongly meshed and the target level of wind integration rather modest
as compared with the overall generation capacity, the identified issues with low SCR are related mainly to special
operational situations like windy summer nights when most of the minimum system load could be covered by wind
generation and HVDC import. Similarly, special outages as well as grid restoration have been determined as
situations, when the SCR on certain regions may become low or very low. Whereas the situations where low SCR is
prevailing under most or all the operational situation present obviously the main technical challenge, these situations
that may prevail only few hours each year present different challenge for system planning. It is not only the effort
required by system planning that increases significantly if the low SCR issues are properly addressed, but taking into
account these special operational conditions e.g. in tuning the power plant controllers may result into less optimal
control performance during the normal operation conditions prevailing most of the year. Tuning the controls to solve
the low SCR situation issues can lead in poor performance from perspective of transient voltage stability or damping
of electromechanical oscillations.
As shown in Figure 3.1 the wind power is located along the long and series compensated AC transmission path
between Finland and the rest of the Nordic synchronous system. Therefore also the effect of the wind power plant
control performance on the voltage stability and the damping of electromechanical oscillations as well as sub-
synchronous control interaction shall be taken into account in the system planning.
3.5.5 Solutions – tuning of controllers
The ERCOT Panhandle grid is remote from both synchronous generators and load centers. It requires long distance
power transfer from the Panhandle region to the load centers in ERCOT. Under weak grid conditions, voltage control
will be very difficult because of the high voltage sensitivity of dV/dQ, in other words, a small variation of reactive
support results in large voltage deviations.
Large amounts of wind generation with advanced power electronic devices are expected to be installed in Panhandle
grid and the dynamic response in the area will be dominated by power electronic devices (WPPs, SVC, etc.)
The effect of weak system strength on the WPP voltage control performance can be best demonstrated with a recent
event where an existing WPP connected to a weak system in ERCOT experienced undesirable un-damped voltage
oscillations under weak grid conditions.
The WPP is connected to the ERCOT grid through two transmission lines. When one line was taken out of service,
the WPP experienced un-damped voltage oscillations, which were recorded by Phasor Measurement Units (PMUs).
The investigation of the event showed that the key cause for the oscillatory response was the plant level voltage
control of the WPP was not suitable for a weak grid condition. The calculated SCR at POI after losing one line is
less than two. The event was simulated with the WPP represented with a detailed dynamic model to re-create the
oscillatory response; simulation results are presented in Figure 3.2 The voltage oscillation is effectively damped when
potential system strength improvements that increase the SCR are modelled, as shown in the purple color curve.
Tuning the voltage controller gains based on the lower SCR value also improved the oscillatory response as shown
in the green color curve.

Page 41
Figure 3.2 Voltage Response at WPP's Point of Interconnection
3.6 References
[1] Cigre WG 14.07, IEEE WG 15.05.05 “Guide for Planning DC Links Terminating at AC systems Locations
having Low Short-Circuit Capacities”, June 1992.

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