The document discusses islanding in power networks, where a portion of the grid becomes isolated, posing risks to utility workers and equipment. It reviews two papers: one focuses on preventing islanding in grid-connected PV systems using a twelve pulse line-commutated converter, while the other presents a hybrid method for islanding detection in micro-grids. Both papers emphasize the importance of effective detection mechanisms to ensure safety and reliability in power systems.

1. ISLANDING
PREPARED BY :
(16MEEEPV009)
Soni Divyangkumar R

2. INTRODUCTION
 What is islanding ?
Consider the power network as shown in fig.1
• Now if we disconnect the line AB from
the infinite transmission grid there will be
an isolated region . The D1, D2 are power
sources (eg : inverter , solar power cells ).
The power generated in this region is fed
to the island only.
• We see that there no longer is any control
over the island voltage at the bus X . Also
there is no mechanism here for control of
frequency.
• This state is referred to as islanding.

3. PAPERS
 PAPER-1: Prevention of Islanding in Grid Connected PV System Using Twelve
Pulse Line.
AUTHORS: 1. Dhanshree A. Diyewar
2. Jyoti M. Kumbhare
 PAPER-2 : Micro-grid Islanding Detection Based on PQ Active method.
AUTHORS: 1. Zhu Yipeng
2.Teng Yun

4. PAPER.1
PREVENTION OF ISLANDING IN GRID CONNECTED PV
SYSTEM USING TWELVE PULSE LINE.
 Outline :
• Abstract
• Introduction
• 12-Pulse Line commutated converter (LCC)
• Proposed Scheme
• Photovoltaic Characteristics
• Simulation results
• Conclusion

5. ABSTRACT
 One of the major drawback of connecting PV systems to the grid is unintentional
islanding condition.
 Islanding can be dangerous for utility workers and damage utility equipment so
anti islanding is a crucial subject for grid connected PV systems.
 For this reason inverter in the PV system must detect islanding and stop supplying
power if the grid is down.
 In this paper 12 pulse LCC is used in inversion mode for the grid connection of PV
system.
 A 12 pulse LCC converter needs commutating voltage of grid to operate.
 The LCC do not require maintaining synchronism between grid and converter and
having an ability to suppress all harmonics below 11 th order.
 The simulation results is carried out in Matlab/Simulink.

6. INTRODUCTION
 PV generation have been come into prominence all over the world.
 Solar energy is the most developed energy sources and is receiving wide attention
now a days because the everlasting solar energy is the best alternative to
conventional energy sources.
 Grid connected PV systems is well recognized all over the world despite the fact
that there have been some drawbacks about connecting them to the electrical grid.
 One of the major drawback of connecting PV systems to the grid is unintentional
islanding condition.
 Islanding can be dangerous for utility workers and damage utility equipment so anti
islanding is a important subject for grid connected PV systems.
 Islanding detection methods
1. Active methods
2. Passive methods

7. Passive methods Active methods
 Based on measurement of the natural
effects of islanding
 The active methods use intentional
transients or harmonic effects
 passive methods fail due to the small
natural effects of islanding.
 The passive methods have a non
detection zone (NDZ).
 The active methods can reduce the NDZ
size. However, these methods reduce the
grid power quality
Many islanding detection algorithm based on VSI inverter based PV- grid interfacing .
For high power conversion line commutated converter (LCC) is widely used.
In HVDC at receiving end LCC operate in an inversion mode.
Commutation of thyristor takes place due to ac grid voltage LCC based grid connected PV
system does not require extra synchronizing controller like VSI when connected to ac grid.

8. LINE COMMUTATED CONVERTER
(LCC)
 In this paper, twelve pulse LCC operates in an inversion mode for a firing angle greater than
90°.
 Twelve pulse LCC is capable of suppressing all harmonics below 11 th order which improves
power quality of the grid.
 Twelve pulse LCC has come into anti-islanding feature since islanded operation will not
occur as the converter needs commutating voltage of grid to operate.
 The configuration of proposed scheme is shown in fig. 1
Fig. 1. Grid connected PV system using twelve pulse line commutated inverter for islanding prevention

9.  In a twelve pulse LCC, two six pulse
converter is connected in series.
 There is a phase difference of 30°.
 It is obtained by lower bridge lags that
of upper by 30°.
 The upper bridge connected through
star-star transformer and lower bridge
through star-delta transformer.
LINE COMMUTATED CONVERTER
(LCC)
Fig. 2. Twelve pulse line commutated converter

10. PROPOSED SCHEME
 Figure 1 shows, the schematic of proposed scheme for grid connected PV system
using twelve pulse line commutated inverter.
 PV system is connected to grid through twelve pulse line commutated inverter
with transformer and breaker.
 PV generates dc and using inverter it convert in to ac and fed to the grid. The dc
source must be connected such that positive polarity is connected to the common
anode of the bridge and negative polarity is connected to the common cathode so
that power is transfer from the DC to AC side and converter operates in an inverter
mode by making firing angle greater than 90°.
 If islanding condition is occur then photovoltaic system continues to energies the
load after the system has been disconnected from the grid. In islanding condition
twelve pulse LCC in PV system will not operate and stop supplying power to the
load.

11. PHOTOVOLTAIC CHARACTERISTICS
 In the proposed scheme PV is
connected to LCC.
 For the operation of converter as
inverter the DC voltage must be
greater than the supply voltage then in
that condition power is transfer from
dc side to ac side and converter
operates in inversion mode.
 For this purpose PV voltage is 1000V
and supply voltage is 400V. The
simulation result of PV characteristic
is shown in Fig.3.
 Boost voltage of dc-dc converter is
1000V which is applied to LCC.
Fig. 3. Photovoltaic array voltage

12. SIMULATION RESULTS
Fig. 4. Waveform of twelve pulse LCC for
150 degree
Fig. 5. Grid side current at 33 kV
The nature of current wave is almost square wave contains harmonics from 11 th order
and above which can be eliminated using filters.

13. SIMULATION RESULTS
 In proposed scheme, simulation is done by considering islanding during 0.5 to 0.8
seconds. The islanding is shown in Fig. 6, from 0.5 to 0.8 seconds for which 33 kV
grid voltage is zero.
Fig. 6. Voltage during islanding condition at 33 kV Grid side

14. SIMULATION RESULTS
Fig. 7. Voltage of transformer across 440 V side Fig. 8. Dc link voltage of twelve pulse LCC during
islanding

15. CONCLUSION
 In this paper, islanding is prevented using twelve pulse LCC.
 During islanding dc link voltage of twelve pulse LCC is zero thus converter will
not operate and will not supply to any local loads and workers working on
converter side is prevented due to dangerous hazards.
 One major advantage of LCC is, it does not require any synchronizing circuits and
is robust topology for high power application

16. REFERENCES
I. John K. Pedersen and Soeren Baekhoej Kjaer, "A Review of SinglePhase Grid-
Connected Inverters for Photovoltaic Modules" iEEE Transactions On industry
Applications, vol. 41 ,no. 5, SEP/OCT 2005,
II. T. R. Sims, R. A. Jones and A. F. Imece, 'Investigation of potential islanding
problems of a line-commutated static power converter in photovoltaic systems',
iEEE Tr
III. S. Samerchur, S. Premrudeepreechacharn Y. Kumsuwun, and K. Higuchi , "Power
Control of Single-PhaseVoltage Source Inverter for Grid-Connected Photovoltaic
Systems", iEEE , pp. 01,2011
IV. C.Boonmee and Y. Kumsuwan, "Modified Maximum Power Point Tracking
Based-on Ripple Correlation Control Application for SinglePhase VSI Grid-
Connected PV Systems", IEEE. Department of Electrical Engineering, pp. 01,
2013. [ansactions on Energy Conversion" , pp. 429-435.1990

17. PAPER.2
MICRO-GRID ISLANDING DETECTION BASED ON
PQ ACTIVE METHOD.
 Outline :
• Abstract
• Introduction
• Islanding Detection Method
• Inverter modeling
• Relationship of Frequency with Islanding Operation
• Simulation results
• Conclusion

18. ABSTRACT
 Micro-grids are being developed as a building block for future smart grid system.
 Islanding detection is a great challenge in connecting distributed generation (DG)
in electrical power systems.
 Islanding detection methods are categorized into two groups, i.e., communication
based methods, active, and passive methods.
 Communication based methods are established upon communication between
utilities and DGs and have the inconvenience of being expensive and are limited in
use.
 Active methods perturb the system with a disturbance and the system response to
the disturbance is analyzed.
 This detection circuit model of the micro-grid islanding is simulated by the
software of MATLAB/Simulink, and the simulation results show that the islanding
detection method does achieve the effect of the micro-grid islanding detection.

19. INTRODUCTION
 Renewable energy and the development of green power has become a hot topic in
the field of electrical .
 With the increasing penetration of renewable energy and the application of large
area, the concentration of power production in the form of more decentralized
development.
 Therefore, the micro network as a part of the main network, provides a lot of
auxiliary services to network.
 It is a difficult task to guarantee the stability and reliability of micro grid, especially
in the isolated island mode.
 The micro-grid is aimed to provide electricity for small communities (buildings,
schools, industry), which is a small scale grid.
 The fossil fuels (diesel, gas turbine) and renewable energy (photovoltaic, wind
turbine) is its main source of electricity.
 The micro grid architecture is shown in Fig.1.The power is supplied by DG.

20.  Distributed storage systems store energy when they produce more than they
consume, and provide energy when consumption is greater than production.
 In the micro-grid, the consumer of power is the load.
 Electrical connection between the micro grid and the main power grid through the
point of common coupling (PCC).
 Islanding is a major issue in penetration of DGs in the power system . Equivalent
islands Circuit is shown in Fig. 2.
Fig. 1 Architecture of a micro-grid
Fig. 2 Equivalent Islands Circuit

21.  the islanding condition happens when “a portion of the utility system that contains
both load and distributed resources remains energized while it is isolated from the
remainder of the utility system”.
 Unintentional islanding may lead to adverse consequences such as uncoordinated
protection, inadequate grounding, and safety aspects.
 Current standards such as IEEE 929-1988 and IEEE 1547-2003 requires the
disconnection of the DG immediately and with delay of 2 seconds, respectively.
 If the DG is allowed to work autonomously , fast islanding detection is required to
make appropriate decisions to control the DG in the autonomous mode.
 Hence, detecting islanding correctly and as fast as possible is essential in
connecting DGs to the utility system.

22. ISLANDING DETECTION METHODS
 Islanding detection methods can be divided
into two main groups.
(1) DG resident techniques
(2) Communication based techniques
 Communication based methods : These
methods are based on communication
between the DG and the utilities.
 These methods have a zero Non Detection
Zone (NDZ) but due to their high costs are
rarely used .
 Islands Circuit is shown in Fig. 3.
 DG resident methods: (a) Passive
(b) Active
(c) Hybrid methods
Fig. 3 Islands Circuit

23. (a). Passive methods
 In passive methods certain system parameters such as frequency, voltage, phase angle, and
total harmonic distortion are monitored continuously and islanding detection is performed
from the variation of these parameters
 The main drawback of these techniques is their large NDZ.
 Particularly, when the power which the DG generates is equal to the power absorbed by the
loads, no power is exchanged between the grid and the DG.
 The grid parameter changes are negligible and may not be detected by the islanding detection
technique.
(b). Active methods
 Active techniques reduce the NDZ of passive methods.
 These techniques perturb the system with a periodic or transient disturbance and estimate the
systems response to detect islanding.

24.  In one way, active methods could be classified in two subgroups.
(i). Impedance measurement
(ii). Detection
 One setback of these methods is that they enter a disturbance to the grid and may
degrade the power quality of the system.
(c). Hybrid methods
 Hybrid islanding detection techniques combine the principles of active and passive
techniques.
 In a hybrid islanding detection method which uses total harmonic distortion and
continuous feedback and selection is presented.
 In a covariance index is used as the passive method, to activate adaptive reactive
power shift action.

25.  In this paper a hybrid islanding
detection method which is on the
principles of VU and HF impedance
is presented.
 The imbalance of PCC voltage is
measured, if it is above the selected
threshold, a HF voltage is injected
to the system and the impedance of
the DG is estimated at the injected
frequency.
 The propose method can detect
islanding in 32(ms) and discriminate
islanding from other system
disturbances.
Fig.4 Islanding test circuit

26. INVERTER MODELING
 Inverters are the main interference sources of the micro-grid . In this paper, two
control strategies are used to control the inverters installed in the micro-grid.
(1). PQ inverter control.
(2). Voltage source inverter (VSI) control.
Fig .5 Basic structure of the PQ inverter control scheme
(1). PQ inverter control
Fig.5 Pref in Fig. 2 represents the active power
which is produced by the micro-source, which is
connected to the Micro-grid by that inverter.
Qref represents the amount of reactive power
injected into or absorbed from the Micro-grid at the
inverter’s bus.
In this model, all PQ inverters operate at unity
power factor ,which means there is no reactive power
conversion between the PQ inverter and the Micro-
grid system.
In PQ inverters were worked at unity power factor
to reduce the ratings of those inverters and the cost.

27. Fig. 6 Voltage source inverter control model.
 This kind of inverter is used to feed the Micro-
grid with predefined values of voltage and
frequency subsequent to islanding occurrence.
 VSI in this paper is used to interface the
storage device (flywheel) to the Micro-grid
and represents the reference bus (slack bus) for
each Micro-grid during islanding mode.
 The VSI emulates the behavior of a
synchronous machine in conventional power
systems.
 The VSI is controlled through droops with the
magnitude and frequency of the output voltage,
as described by the following functional
relation:
(2). Voltage source inverter (VSI) control
 P and Q are the inverter active and reactive output powers
respectively.
PK and Q K are the F and V droop slopes respectively.
 f0 and V0 are the idle values of the frequency and voltages
A 3-ph model of a VSI implementing the droop concepts
described by Equation no. (1)
In this model, the amount of P and Q powers injected into
or absorbed from the Micro-grid will control the V and
frequency of the Micro-grid.

28. RELATIONSHIP OF FREQUENCY
WITH ISLANDING OPERATION
 Fig.4 shows the power flow chart of
inverter ,which is based on DG (IBDG)
and connected load in the presence of the
utility.
 Under the islanding operation, the power
flow can be described as:
The load is replaced by equivalent RLC
elements and is connected in parallel, PCC
voltage across R and parallel LC are the
same. The load power can be calculated as
 From Eqs.(2) and (3), we can get
If IBDG output power Pinv +jQinv is equal or
approximate to the rated load power Load
Load Pload +jQload, the PCC voltage frequency
will under the threshold at the moment, at the
instant the utility is disconnected, the
islanding detection fails.

29. SIMULATION
 In this part, simulation results are shown to verify the effectiveness of the proposed islanding
detection algorithm.
 The difference between inverter output capacity and rated load capacity is described as:
In this case, the islanding frequency will go beyond the allowable range (49.5 Hz—50.5 Hz)
immediately, and the islanding can be confirmed
The simulation parameters of the system are shown in Tab.I.
PARAMETERS VALUE
Voltage ; Frequency 220V,50Hz
Pload0 5kW,9.68Ω
QL 10KVar, 15.4mH
QC 5KVar, 328.8µH
fmax=wmax/ 2pi ; fmin =wmin/ 2pi 50.3Hz;49.7Hz

30. Fig.8 Simulation result of
Case
(the islanding frequency
exceeds the lower frequency
limit for 3 times within 0.1
s. )
Fig.7. ∆P= -1%Pload0 and the
utility is disconnected at t=0.1s
Fig.9 ∆P= 1%Pload0 , the
islanding frequency should
also shift continuously
when the proposed
algorithm is applied.

31. CONCLUSION
 In this paper a hybrid islanding detection method using VU and HF impedance is presented.
 The voltage unbalance is measured at PCC and if it overreaches the threshold settings, the
situation is suspicious of islanding.
 Therefore, a HF voltage is injected in the DG control loop and the PCC voltages and current
are measured to estimate impedance at the injected frequency.
 This hybrid method merges the advantages of active and passive methods.
 Furthermore, the method does not lead to system instability since the active method does not
change any parameters of the system.
 The test system is simulated in PSCAD/EMTDC to investigate the credibility of the method.
The detection method can detect islanding in 32ms (3 cycles) which is supported by standard.
 In conclusion , compared to other existing algorithms, the non-detection region of this
algorithm is very small.
 at the same time, it also provides an excellent detection speed of the island. The effectiveness
of the algorithm is proved by the simulation results.

32. REFERENCES
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Systems, IEEE Std. 1547-2003, 2003.
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during and subsequent to islanding.