I have created this report for my final semester seminar at Poornima college of engineering Jaipur, electrical department. This report covers various chapters and other contents. feel free to download. Note: some minor editsin format and a quick spelling check might be needed. **content source is wikipedia and internet** any thing you would like to suggest please let me know in the comments.

1. [2]
A
SEMINAR REPORT
ON
“VSC-HVDC Application to Improve the Long-Term Voltage
Stability”
Submitted in partial fulfillment of the requirements for the award of Degree of
Bachelor of Technology
In
ELECTRICAL ENGINEERING
Affiliated to
Rajasthan Technical University, Kota
(Session 2017-18)
GUIDED BY: SUBMITTED BY:
Mr. Pravin Kumar Swapnil Solanki
Assistant Professor PCE2/EE/14/056
Dept. of Electrical Engg. 4th year, EE (D)
Roll. No: 44
DEPARTMENT OFELECTRICAL ENGINEERING
POORNIMA COLLEGE OF ENGINEERING
ISI-06, RIICO INSTITUTIONAL AREA
SITAPURA, JAIPUR-302022 (Raj.)

2. [3]
A
SEMINAR REPORT
ON
“VSC-HVDC Application to Improve the Long-Term Voltage
Stability”
Submitted in partial fulfillment of the requirements for the award of Degree of
Bachelor of Technology
In
ELECTRICAL ENGINEERING
Affiliated to
Rajasthan Technical University, Kota
(Session 2017-18)
GUIDED BY: SUBMITTED BY:
Mr. Pravin Kumar Swapnil Solanki
Assistant Professor PCE2/EE/14/056
Dept. of Electrical Engg. 4th year, EE (D)
Roll. No: 44
DEPARTMENT OFELECTRICAL ENGINEERING
POORNIMA COLLEGE OF ENGINEERING
ISI-06, RIICO INSTITUTIONAL AREA
SITAPURA, JAIPUR-302022 (Raj.)

3. [4]
ACKNOWLEDGEMENT
I would like to acknowledge my indebtedness and render my warmest thanks to my guide,
Asst. Professor Mr. Pravin Kumar, who made this work possible. His friendly guidance and
expert advice have been invaluable throughout all stages of the work. I would also wish to
express my gratitude to Asst. Professor Mr. Gaurav Srivastva for extended discussions and
valuable suggestions which have contributed greatly to the improvement of the report. The
report has also benefited from comments and suggestions made by Dr. Deepika Chauhan
and who have read through the manuscript. I take this opportunity to thank them. Special
thanks are due to my friend, Sourabh Kumar, for his continuous support and understanding,
but also for more concrete thinks like commenting on earlier versions of the report, helping
with the figures and the final preparation of the manuscript. This report has been written
during my final semester at the Department of Electrical Engineering of Poornima
College of Engineering. I would like to thank the college for providing excellent working
conditions and for its support.
Swapnil Solanki
(PCE2/EE/14/056)
IV YEAR
PCE, Jaipur

4. [5]
TABLE OF CONTENTS
FRONT PAGE i
CERTIFICATE ii
ACKNOWLEDGMENT iii
TABLE OF CONTENTS iv
FIGURE INDEX vi
ABRIVATIONS vii
ABSTRACT 1
CHAPTERS Page-No
1. Introduction 2-4
1.1 High voltage transmission 2
1.2 Voltage-source converters (VSC) 2
1.3 Conversion process 2
1.4 Configurations 2
1.4.1 Monopole 3
1.4.2 Symmetrical monopole 3
1.4.3 Bipolar 3
1.4.4 Multi-terminal
2. Literature Review 5-11
3. Methodologyusedin literature review 12-20
3.1 High voltage transmission 12
3.2 Voltage-source converters (VSC) 13
3.3 Conversion process 13
3.3.1 Line-commutated converters (LCC) 14
3.3.2 Voltage-sourced converters 16
3.4 Configurations 17
3.4.1 Monopole 17
3.4.2 Symmetrical monopole 17
3.4.3 Bipolar 17

5. [6]
3.4.4 Back to back 19
3.4.4 Multi-terminal 19
3.4.5 Tripole 20
4. Comparative analysis of Methodologies 21-24
4.1. Cable systems 21
4.2. Overhead line systems 22
4.3. Asynchronous connections 23
4.4. High-voltage DC circuit breaker 23
5. Strength and weakness 25-27
5.1 Comparison of HVDC and AC 25
5.1.1 Advantages 25
5.1.2 Disadvantage 26
5.2 Corona discharge 27
6. Future scope 28
 Conclusion
 References
 Paper Published and certificate

6. [7]
FIGURE TABLE
Fig. No. Figure Name Page
No.
1 HVDC System 2
2 High voltage DC transmission 12
3 Voltage-source converters (VSC) 13
4 Line-commutated converters (LCC) 14
5 Different HVDC configuration 20
6 HVDC cables 21
7 Overhead cable system 22
8 High-voltage DC circuit breaker 23
9 Corona discharge 27

7. [8]
ABRIVATIONS
HVDC – High Voltage Direct Current
GIL – Gas Insulated Transmission Line
DC CTL – Direct Current Compact Transmission Line
AC GIL – Alternating Current Gas Insulated Transmission Line
DC GIL – Direct Current Gas Insulated Transmission Line
GIS - Gas Insulated Switchgear
OHL – Overhead Line
UG – Underground
EHV – Extra High Voltage
MMC – Multilevel Converters
FSW – Friction Stir Welding
TIG - Tungsten Inert-Gas welding
PD – Partial Discharge
ED – Extruded Dielectric
HPFF – High-Pressure Fluid-Filled Cables
HPGF – High-Pressure Gas-Filled Cables
SCFF – Self-Contained Fluid-Filled Cables

8. [9]
ABSTRACT
This paper concerns the VSC-HVDC application to improve the long-term voltage stability.
An appropriate control strategy for VSC-HVDC is proposed in the paper. The goal is to
reach the best condition from long-term voltage stability perspective with respect to system
configuration and also the VSC-HVDC capability curve. To show the capability of the
proposed control methodology, two long-term voltage instability scenarios are considered.
VSC-HVDC is considered as new flexible control technologies to improve power system
performance. Active and reactive powers of the VSC-HVDC can change rapidly within its
capability curve and be controlled independently. This is a very interesting feature of this
technology to modulate the power flow in the system and support the power system with the
best mixture of active and reactive powers during stressed conditions to improve voltage
stability.

9. [10]
CHAPTER-I
Introduction
High voltage transmission
A high-voltage, direct current (HVDC) electric power transmission system (also called a power
superhighway or an electrical superhighway) uses direct current for the bulk transmission of electrical power, in
contrast with the more common alternating current (AC) systems. For long-distance transmission, HVDC
systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids
the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the
higher cost of DC conversion equipment compared to an AC system may still be justified, due to other benefits
of direct current links.
HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow
through an HVDC link can be controlled independently of the phase angle between source and load, it can
stabilize a network against disturbances due to rapid changes in power. HVDC also allows transfer of power
between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and
economy of each grid, by allowing exchange of power between incompatible networks.
Fig. 1 HVDC System
Voltage-source converters (VSC)
Development of higher rated insulated-gate bipolar transistors (IGBTs), gate turn-off thyristors (GTOs)
and integrated gate-commutated thyristors (IGCTs),has made smaller HVDC systems economical. There are
several different variants of VSC technology: most installations built until 2012 use pulse-width modulation in a
circuit that is effectively an ultrahigh-voltage motor drive. Current installations, including HVDC PLUS and
HVDC MaxSine, are based on variants of a converter called a Modular Multilevel Converter (MMC).
Multilevel converters have the advantage that they allow harmonic filtering equipment to be reduced or
eliminated altogether. By way of comparison, AC harmonic filters of typical line-commutated converter stations

10. [11]
cover nearly half of the converter station area.With time, voltage-source converter systems will probably replace
all installed simple thyristor-based systems, including the highest DC power transmission applications.
Conversion process
At the heart of an HVDC converter station, the equipment which performs the conversion between AC
and DC is referred to as the converter. Almost all HVDC converters are inherently capable of converting from
AC to DC (rectification) and from DC to AC (inversion), although in many HVDC systems, the system as a
whole is optimized for power flow in only one direction. Irrespective of how the converter itself is designed, the
station that is operating (at a given time) with power flow from AC to DC is referred to as the rectifier and the
station that is operating with power flow from DC to AC is referred to as the inverter.
Early HVDC systems used electromechanical conversion (the Thury system) but all HVDC systems built since
the 1940s have used electronic (static) converters. Electronic converters for HVDC are divided into two main
categories:
 Line-commutated converters (LCC)
 Voltage-sourced converters, or current-source converters.
Configurations
Monopole
In a monopole configuration one of the terminals of the rectifier is connected to earth ground. The
other terminal, at high voltage relative to ground, is connected to a transmission line. The earthed terminal may
be connected to the corresponding connection at the inverting station by means of a second conductor.
Symmetrical monopole
An alternative is to use two high-voltage conductors, operating at ± half of the DC voltage, with only a
single converter at each end. In this arrangement, known as the symmetrical monopole, the converters are
earthed only via a high impedance and there is no earth current. The symmetrical monopole arrangement is
uncommon with line-commutated converters (the NorNed interconnection being a rare example) but is very
common with Voltage Sourced Converters when cables are used.
Bipolar
An In bipolar transmission a pair of conductors is used, each at a high potential with respect to
ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line
costis higher than a monopole with a return conductor. However, there area number of advantages to bipolar
transmission which can make it an attractive option.
Multi-terminal
The most common configuration of an HVDC link consists of two converter stations connected by an
overhead power line or undersea cable. Multi-terminal HVDC links, connecting more than two points, are rare.
The configuration of multiple terminals can be series, parallel, or hybrid (a mixture of series and parallel).

11. [12]
Parallel configuration tends to be used for large capacity stations, and series for lower capacity stations. An
example is the 2,000 MW Quebec - New England Transmission system opened in 1992, which is currently the
largest multi-terminal HVDC systemin the world.
Multi-terminal systems are difficult to realize using line commutated converters because reversals of power are
effected by reversing the polarity of DC voltage, which affects all converters connected to the system. With
Voltage Sourced Converters, power reversal is achieved instead by reversing the direction of current, making
parallel-connected multi-terminals systems much easier to control. For this reason, multi-terminal systems are
expected to become much more common in the near future.

12. [13]
CHAPTER-II
Literature Review
1. M. Reza Safari et. al. (2010)
The VSC-HVDC application to improve the long-term voltage stability is investigated
in this paper. An appropriate control strategy is proposed to control the power flow of
the VSC-HVDC link to improve the AC system stability as much as possible. The
NORDIC32 test system is utilized to investigate the performance of VSC-HVDC link.
Two longterm voltage instability scenarios are considered to show the capability of
the VSC-HVDC link to enhance the AC power system performance and improve the
long-term voltage stability. Simulation results show that the optimal control of VSC-
HVDC link leads to voltage collapse avoidance in the system. Many other voltage
instability scenarios have been conducted to verify the proposed control method but
are not shown in the paper due to space limit. In all other cases, having VSC-HVDC
with optimal control leads to long-term voltage stability improvement.
2. Jared Candelaria et. al. (2014)
If VSC-HVDC is ever to effectively compete with classical HVDC, protective devices
must be implemented in the system. This paper has provided an overview of the
existing proposed methods. Presently AC protection devices are widely in use for
protection. AC side protection appears to be a good solution on two-terminal systems,
but may cause unnecessary outages in multi-terminal systems. Converter embedded
devices provide better and more versatile protection than AC side protection, but still
cause complete converter shutdown in the event of a permanent fault. DC device
protection provides the best form of non-active protection. DC devices operate faster
than AC devices under fault conditions and allow for more flexibility in MT-VSC-
HVDC systems. Controllers can provide good protection in terms of allowing the
system to continue to operate under fault conditions. Ultimately, the best form of
protection appears to be a combination of active controllers and DC devices. The
controllers could allow for continuous system operation under temporary faults and
the DC devices would take over isolating the fault if a permanent fault occurs. While
it may not be the most economical and it increases the system complexity, it does
provide better system protection. If by increasing the number of systems this

13. [14]
combination of protective devices can be implemented, then the full potential of VSC-
HVDC could be utilized resulting in a stronger and more reliable grid.
3. Christian Feltes et. al. (2014)
In this paper a new FRT approach has been introduced for WFs connected through a
VSC-based HVDC link. This approach allows a very fast power reduction during
fault and deals with a reliable protection of the HVDC system against DC
overvoltages. In this way, the costs for a full-rated DC chopper can be saved without
downgrading the safety and reliability of the system. The proposed method can easily
be implemented, since it only requires small changes in the control software of the
SEC and the WT converter. It works for all types of WTs and can also be applied to
different VSC-HVDC topologies.
4. Wenyuan Wan et. al. (2009)
The frequency-domain responses and time-domain simulations reveal the fundamental
stability and robustness issues of the active power control for converters in rectifier
operations. The controllability limitations imposed by the current control bandwidth,
the ac system strength, and the power operating point on the active power loop have
been evaluated analytically. The results can be used to establish a degree of initial
guidelines for controller structure design and bandwidth setting for power control and
its outer loops. Limitations imposed by the active power control systems on the
stability and performance of the outer voltage droop control have been analyzed. For
rectifiers with a weak ac system, a fast feedforward power loop could imply high-gain
instability and restricted stability margins for the voltage droop control. Highgain
droop control is not recommended from the perspective of robustness and transient
performance. Better disturbance rejection performance of the voltage droop control
can be achieved by employing a faster power controller. Generally, the feedback
approach is recommended due to its superior stability and robustness. However, when
fast control of active power is required, the feedforward design may be preferred for
the VSCs normally operating as inverters, provided a relatively strong ac network
exists. Furthermore, this paper provides a framework to analyze more complex plant
and power-electronics controller systems

14. [15]
5. Dr RL Sellick et. al. (2013)
The East-West Interconnector, which uses ABB’s 2-Level HVDC Light® Generation
3 VSC technology, is the first commercial VSC-HVDC project connected to the
United Kingdom, and to date it is the world’s most powerful VSC project. The 2-level
VSC-HVDC technology uses a significantly smaller site area than an equivalent-rated
LCC-HVDC project. This is at the expense of increased converter building size and
higher losses. The use of a cascaded two-level VSC-HVDC converter presents
significant benefits over an equivalently rated 2-level VSC-HVDC converter or a
LCC-HVDC scheme. The CTL arrangement offers a smaller overall site footprint and
a lower building height than both the 2-level and LCC-HVDC alternatives. However,
this is at the expense of increased converter building size. The losses for the latest
generation of CTL VSC-HVDC technology are now comparable with those of the
LCCHVDC technology.
6. J.C. Clare et. al. (2010)
New modes of STATCOM operation have been identified for a recently published
and practical VSC topology for HVDC power transmission. These new modes allow
the converter to remain connected to the AC network and additionally operate as a
STATCOM in the event of a DC network fault. Thus, enabling fault-ride through
without the need to open the AC side circuit breakers, a problem traditionally
associated with VSCs. This results in the elimination of post fault down time,
reduction in AC and DC side fault currents and AC network voltage stabilisation
during the disturbance. Simulation results have shown the fault ride through capability
of the converter based on 20MW demonstration equipment. Additionally, these have
verified the operation of the STATCOM modes presented here. Successful transition
between STATCOM modes has been further demonstrated; this has been done while
still maintaining independent control of the bulk energy stored in each of the capacitor
limbs.
7. Houlei Gao et. al. (2009)
In this paper, a MPC based EVCS is developed to optimize voltage control within
VSC-HVDC connected OWFs, which can regulate the voltages while taking into
account economical operation of the OWFs. The predictive model of WFVSC with a
typical cascaded control structure is derived in details. An analytical sensitivity
coefficient calculation method is adopted to improve computational efficiency. In the

15. [16]
MPC-EVCS, two control modes are designed for different operating conditions. The
case studies show that all the three different optimization control methods OPC,
MPC-Q and MPC-EVCS show good control performance in different scenarios. In
comparison, the overall performance of the MPC-EVCS is better than the MPC-Q and
OPC. Of course, more work is required for further improvement. A nonlinear model
of the system will be investigated to more accurately capture the complex dynamics
of the systems and improve the control performance in the future work.
8. Daniel Adeuy et. al. (2013)
A communication-free alternative coordinated control (ACC) scheme is developed to
prevent further frequency drop on disturbed AC grids during wind turbine recovery
period. Also, the proposed ACC scheme allows correct operation of fast frequency
response from MTDC-connected wind farms during multiple power imbalances on
different AC grids, due to the linear relationship between the MTDC voltage and the
onshore AC grid frequencies. The ACC scheme, which gives priority to the frequency
versus active power droop on VSCs connected to disturbed AC grids, is compared
with a coordinated control (CC) scheme presented in the literature, which uses a
frequency versus DC voltage droop. During a single power imbalance in one AC grid,
fast frequency response from MTDC-connected wind farms (equipped with the ACC
and CC scheme) limits the RoCoF on disturbed AC grids and additional active power
transferred from another AC system contains the system frequency deviation. This
will prevent unintended tripping of protection relays on power systems and enhance
the frequency control requirements of AC grids.
9. Sotirios I. Nano et. al. (2011)
In this paper, an augmented PSC scheme was introduced for island VSC-HVDC links,
to achieve droop-type and inertia frequency response solely via modulation of the
operating frequency of the island VSC. Such functionality is essential in order to
comply with the latest grid code requirements for HVDC systems, as well as to
guarantee the dynamic security of the island system during severe N-1 contingencies.
The frequency domain analysis illustrates that the combined droop and inertia control
scheme proposed in this work does not compromise the stability of the PSC loop,
provided that the washout filter of the INEC path is properly tuned. Time-domain

16. [17]
simulations were performed to assess the frequency response of the island system
following severe N-1 contingency events, simulated by the sudden loss of the external
ac interconnector, then followed by tripping of the local thermal station. It was shown
that the island VSC seamlessly transitions from grid-connected to islanded operation,
while the proposed INEC loop effectively contains frequency transients and induced
ROCOF values.
10. M. Haileselassie et. al. (2010)
In this paper a three terminal MTDC connecting offshore wind farm, oil & gas
platform load and onshore grid was proposed. Equivalent circuit of VSC in d-q
reference frame was established and used for developing control strategy for the
multi-terminal VSC. This together with voltage margin method was used to control
the MTDC system for a stable steady state and dynamic performance. Simulation
results have confirmed that the proposed control results in a stable steady state and
dynamic state operation and is also capable of restoring operation during loss of DC
voltage regulating terminal without the need of communication between terminals.
11. Grain Philip Adam et. al. (2014)
This paper presented a new generation VSC-HVDC transmission system based on a
hybrid multilevel converter with ac-side cascaded H-bridge cells. The main
advantages of the proposed HVDC system are: potential small footprint and lower
semiconductor losses compared to present HVDC systems. low filtering requirements
on the ac sides and presents high-quality voltage to the converter transformer. does
not compromise the advantages of VSC-HVDC systems such as four-quadrant
operation; voltage support capability; and black-start capability, which is vital for
connection of weak ac networks with no generation and wind farms. modular design
and converter fault management (inclusion of redundant cells in each phase may
allow the system to operate normally during failure of a few H-bridge cells; whence a
cell bypass mechanism is required) resilient to ac side faults (symmetrical and
asymmetrical).inherent dc fault reverse blocking capability that allows converter
stations to block the power paths between the ac and dc sides during dc side faults
(active power between ac and dc sides, and reactive power exchange between a

17. [18]
converter station and ac networks), hence eliminating any grid contribution to the dc
fault current.
12. Guanjun Ding et. al. (2015)
This paper introduced some new technologies of VSC, i.e., modular multilevel VSC,
and its relevant characteristics. The output ac voltages can be adjusted in very fine
increments. It minimizes the generated harmonics and in most cases completely
eliminates the need for ac filters. Furthermore, the small and relatively shallow
voltage steps cause very little radiant or conducted high-frequency interference. The
improved modulation technology makes the individual semiconductors have much
lower switching frequency (less than 3 times of fundamental frequency) resulting in
lower switching losses compared to 2 or 3-level VSC. Total system losses are
therefore relatively small and efficiency is consequently increased. So the conclusion
is drawn that these new technologies have great potentiality in application of VSC-
HVDC projects in future.
13. Pinaki Mitra et. al. (2013)
In this paper, the application of a recently invented power-synchronization control is
proposed for integrating a doubly fed induction generator (DFIG)-based offshore
wind farm to a weak ac grid through a voltage-source converter (VSC)-based high-
voltage dc link. The control strategy, along with the antiwindup techniques and the
bumpless transfer between two different control modes, is elaborately discussed. Two
different fault cases, namely, onshore and offshore faults are considered and the fault-
ridethrough techniques are presented. In case of the onshore fault, both with-chopper
and without-chopper solutions are investigated. For an offshore fault, a coordinated
fault-ridethrough scheme is proposed when the offshore HVDC converter and the
wind farm are in voltage-control modes. The entire study is carried out in a real-time
digital simulator (RTDS) platform.
14.Chunyi Gu et. al. (2008)
This paper develops a comprehensive small-signal model of a hybrid LCC-VSC
HVDC system. Based on the eigen-analysis and PSCAD-based time-domain
simulation studies, the accuracy of the deduced smallsignal model is validated, and
the overall system dynamic behavior is investigated. The results conclude: The eigen-

18. [19]
analysis results show that the parameters of DC-current controller of LCC, DC-
voltage controller of VSC, DC-side capacitor of VSC and smoothing reactor of LCC
can noticeably impact the dampings of the oscillatory modes and even result in
instability. Larger proportional and integral gains, within acceptable range, in the
outer loop of the DC voltage control of VSC are preferable since provide higher
dampings for dominant modes. However, smaller proportional and integral gains of
DC current controller of LCC are preferable. Larger DC-side capacitor and smoothing
reactor of VSC, and smaller smoothing reactor of LCC may lead to lower dampings
of dominant modes for the hybrid HVDC system.
15.Benish Peiny et. al. (2013)
HVDC system is discussed in this paper. The dq-axis voltage and current signals
which are computed using Park trans 10rmation from tne source 1 side,are utilized as
input parameter in tne FIE to identity tne fault types. Different types 01 fault
occurrence in tne MVDC system are represented using a term Fault Index . Fault
simulation studies 01 tne VSC — MVDC system tor various power transfer capacity
nave been conducted in tne MATEAB/Simulink platform and the performance
evaluation 01 the developed FIE has been carried out. Simulation results prove that
the developed FIE identifies the ac faults occurring in the source 1 side and DC fault
successfully. The developed FIE identifies different faults of the HVDC system based
on the measured dq - axis voltage and current from the source 1 side. However, it
could not identify the line m which fault has occurred. Hence, to classify the faults
occurring m either AC side of the HVDC system, the FIE has to be expanded with
appropriate data input. Development of a FIE which identifies different types of fault
and the corresponding line where the fault occurs, anywhere in the HVDC system is
proposed as future work to this paper.

19. [20]
CHAPTER-III
Methodology Used in Literature review
Circuit Breaker Origins
An early form of circuit breaker was described by Thomas Edison in an 1879 patent
application, although his commercial power distribution system used fuses.[1] Its purpose
was to protect lighting circuit wiring from accidental short circuits and overloads. A modern
miniature circuit breaker similar to the ones now in use was patented by Brown, Boveri &
Cie in 1924. Hugo Stotz, an engineer who had sold his company to BBC, was credited as the
inventor on DRP (Deutsches Reichspatent) 458392.[2] Stotz's invention was the forerunner

20. [21]
of the modern thermal-magnetic breaker commonly used in household load centers to this
day.
Interconnection of multiple generator sources into an electrical grid required development of
circuit breakers with increasing voltage ratings and increased ability to safely interrupt the
increasing short-circuit currents produced by networks. Simple air-break manual switches
produced hazardous arcs when interrupting high voltages; these gave way to oil-enclosed
contacts, and various forms using directed flow of pressurized air, or of pressurized oil, to
cool and interrupt the arc. By 1935, the specially constructed circuit breakers used at the
Boulder Dam project use eight series breaks and pressurized oil flow to interrupt faults of up
to 2,500 MVA, in three cycles of the AC power frequency.
Fig 1 Circuit Breaker
Operations
All circuit breaker systems have common features in their operation, but details vary
substantially depending on the voltage class, current rating and type of the circuit breaker.
The circuit breaker must first detect a fault condition. In small mains and low voltage circuit
breakers, this is usually done within the device itself. Typically, the heating or magnetic
effects of electric current are employed. Circuit breakers for large currents or high voltages
are usually arranged with protective relay pilot devices to sense a fault condition and to

21. [22]
operate the opening mechanism. These typically require a separate power source, such as a
battery, although some high-voltage circuit breakers are self-contained with current
transformers, protective relays, and an internal control power source.
Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; This is
commonly done using mechanically stored energy contained within the breaker, such as a
spring or compressed air to separate the contacts. Circuit breakers may also use the higher
current caused by the fault to separate the contacts, such as thermal expansion or a magnetic
field. Small circuit breakers typically have a manual control lever to switch off the load or
reset a tripped breaker, while larger units use solenoids to trip the mechanism, and electric
motors to restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive heating, and must
also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts
are made of copper or copper alloys, silver alloys and other highly conductive materials.
Service life of the contacts is limited by the erosion of contact material due to arcing while
interrupting the current. Miniature and molded-case circuit breakers are usually discarded
when the contacts have worn, but power circuit breakers and high-voltage circuit breakers
have replaceable contacts.
When a high current or voltage is interrupted, an arc is generated. The length of the arc is
generally proportional to the voltage while the intensity (or heat) is proportional to the
current. This arc must be contained, cooled and extinguished in a controlled way, so that the
gap between the contacts can again withstand the voltage in the circuit. Different circuit
breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in. Different
techniques are used to extinguish the arc including:
Lengthening or deflecting the arc
Intensive cooling (in jet chambers)
Division into partial arcs
Zero point quenching (contacts open at the zero current time crossing of the AC waveform,
effectively breaking no load current at the time of opening. The zero crossing occurs at twice
the line frequency; i.e., 100 times per second for 50 Hz and 120 times per second for 60 Hz
AC.)Connecting capacitors in parallel with contacts in DC circuits.Finally, once the fault

22. [23]
condition has been cleared, the contacts must again be closed to restore power to the
interrupted circuit.
Fig 3 Tow pole MCB
Arc interruption
At Low-voltage miniature circuit breakers (MCB) use air alone to extinguish the arc.
These circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel
metal plates which divide and cool the arc. By splitting the arc into smaller arcs the arc is
cooled down while the arc voltage is increased and serves as an additional impedance which
limits the current through the circuit breaker. The current-carrying parts near the contacts
provide easy deflection of the arc into the arc chutes by a magnetic force of a current path,
although magnetic blowout coils or permanent magnets could also deflect the arc into the arc
chute (used on circuit breakers for higher ratings).So The number of plates in the arc chute is
dependent on the short-circuit rating and nominal voltage of the circuit breaker.In larger
ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil
through the arc.[4]
Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic
field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the
stretched arc.

23. [24]
Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the
contact material).the arc quenches when it is stretched a very small amount (less than 2–3
mm (0.079–0.118 in)). Vacuum circuit breakers are frequently used in modern medium-
voltage switchgear to 38,000 volts.Air circuit breakers may use compressed air to blow out
the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the
escaping of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current very quickly: typically the arc is
extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending
upon age and construction of the device. The maximum current value and let-through energy
determine the quality of the circuit breakers.i
Short-circuit
Circuit breakers are rated both by the normal current that they are expected to carry,
and the maximum short-circuit current that they can safely interrupt. This latter figure is the
ampere interrupting capacity (AIC) of the breaker.Under short-circuit conditions, the
calculated maximum prospective short-circuit current may be many times the normal, rated
current of the circuit. When electrical contacts open to interrupt a large current, there is a
tendency for an arc to form between the opened contacts, which would allow the current to
continue. This condition can create conductive ionized gases and molten or vaporized metal,
which can cause further continuation of the arc, or creation of additional short circuits,
potentially resulting in the explosion of the circuit breaker and the equipment that it is
installed in. Therefore, circuit breakers must incorporate various features to divide and
extinguish the arc.
The maximum short-circuit current that a breaker can interrupt is determined by testing.
Application of a breaker in a circuit with a prospective short-circuit current higher than the
breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt
a fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to
explode when reset.Typical domestic panel circuit breakers are rated to interrupt 10 kA
(10000 A) short-circuit current.Miniature circuit breakers used to protect control circuits or
small appliances may not have sufficient interrupting capacity to use at a panel board; these
circuit breakers are called "supplemental circuit protectors" to distinguish them from
distribution-type circuit breakers.

24. [25]
Types of circuit breakers
Low-voltage circuit breakers
Low-voltage (less than 1,000 VAC) types are common in domestic, commercial and
industrial application, and include: Miniature circuit breaker (MCB)—rated current not more
than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic
operation. Breakers illustrated above are in this category.
Molded Case Circuit Breaker (MCCB)—rated current up to 2,500 A. Thermal or thermal-
magnetic operation. Trip current may be adjustable in larger ratings. Low-voltage power
circuit breakers can be mounted in multi-tiers in low-voltage switchboards or switchgear
cabinets.
The characteristics of low-voltage circuit breakers are given by international standards such
as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow
removal and interchange without dismantling the switchgear.
Large low-voltage molded case and power circuit breakers may have electric motor operators
so they can open and close under remote control. These may form part of an automatic
transfer switch system for standby power.
Low-voltage circuit breakers are also made for direct-current (DC) applications, such as DC
for subway lines. Direct current requires special breakers because the arc is continuous—
unlike an AC arc, which tends to go out on each half cycle. A direct current circuit breaker
has blow-out coils that generate a magnetic field that rapidly stretches the arc. Small circuit
breakers are either installed directly in equipment, or are arranged in a breaker panel. Inside
of a circuit breaker The DIN rail-mounted thermal-magnetic miniature circuit breaker is the
most common style in modern domestic consumer units and commercial electrical
distribution boards throughout Europe. The design includes the following components:
 Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the
status of the circuit breaker (On or Off/tripped). Most breakers are designed so they
can still trip even if the lever is held or locked in the "on" position. This is sometimes
referred to as "free trip" or "positive trip" operation.
 Actuator mechanism - forces the contacts together or apart.

25. [26]
 Contacts - allow current when touching and break the current when moved apart.
 Terminals Bi metallic strip - separates contacts in response to smaller, longer-term
over-currents
 Calibration screw - allows the manufacturer to precisely adjust the trip current of the
device after assembly.
 Solenoid - separates contacts rapidly in response to high over-currents
 Arc divider/extinguisher complex.
Magnetic circuit breaker
Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force
increases with the current. Certain designs utilize electromagnetic forces in addition to those
of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the
solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the
latch, which lets the contacts open by spring action used.
Thermal magnetic circuit breakers
Thermal magnetic circuit breakers, which are the type found in most distribution
boards, incorporate both techniques with the electromagnet responding instantaneously to
large surges in current (short circuits) and the bimetallic strip responding to less extreme but
longer-term over-current conditions. The thermal portion of the circuit breaker provides a
time response, that trips the circuit breaker sooner for larger over-currents but allows smaller
overloads to persist for a longer time. This allows short current spikes such as are produced
when a motor or other non-resistive load is switched on. With very large over-currents during
short-circuit, the magnetic element trips the circuit breaker with no intentional additional
delay.
Multi-terminal systems
The most common configuration of an HVDC link consists of two converter
stations connected by an overhead power line or undersea cable. Multi-terminal HVDC links,
connecting more than two points, are rare. The configuration of multiple terminals can be
series, parallel, or hybrid (a mixture of series and parallel). Parallel configuration tends to be
used for large capacity stations, and series for lower capacity stations. An example is the
2,000 MW Quebec - New England Transmission system opened in 1992, which is currently
the largest multi-terminal HVDC system in the world.
Multiterminal systems are difficult to realize using line commutated converters because
reversals of power are effected by reversing the polarity of DC voltage, which affects all

26. [27]
converters connected to the system. With Voltage Sourced Converters, power reversal is
achieved instead by reversing the direction of current, making parallel-connected
multiterminals systems much easier to control. For this reason, multiterminal systems are
expected to become much more common in the near future.
Medium-voltage circuit breakers
Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into
metal-enclosed switchgear line ups for indoor use, or may be individual components
installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units
for indoor applications, but are now themselves being replaced by vacuum circuit
breakers (up to about 40.5 kV). Like the high voltage circuit breakers described
below, these are also operated by current sensing protective relays operated
through current transformers. The characteristics of MV breakers are given by
international standards such as IEC 62271. Medium-voltage circuit breakers nearly
always use separate current sensors and protective relays, instead of relying on
built-in thermal or magnetic overcurrent sensors.
Medium-voltage circuit breakers can be classified by the medium used to extinguish
the arc:
 Vacuum circuit breakers—With rated current up to 6,300 A, and higher for
generator circuit breakers. These breakers interrupt the current by creating and
extinguishing the arc in a vacuum container - aka "bottle". Long life bellows are
designed to travel the 6–10 mm the contacts must part. These are generally
applied for voltages up to about 40,500 V,[9] which corresponds roughly to the
medium-voltage range of power systems. Vacuum circuit breakers tend to have
longer life expectancies between overhaul than do air circuit breakers.
 Air circuit breakers—Rated current up to 6,300 A and higher for generator circuit
breakers. Trip characteristics are often fully adjustable including configurable trip
thresholds and delays. Usually electronically controlled, though some models
are microprocessor controlled via an integral electronic trip unit. Often used for
main power distribution in large industrial plant, where the breakers are arranged
in draw-out enclosures for ease of maintenance.
 SF6 circuit breakers extinguish the arc in a chamber filled with sulfur
hexafluoride gas.

27. [28]
Medium-voltage circuit breakers may be connected into the circuit by bolted
connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage
circuit breakers in switchgear line-ups are often built with draw-out construction,
allowing breaker removal without disturbing power circuit connections, using a
motor-operated or hand-cranked mechanism to separate the breaker from its
enclosure. Some important manufacturer of VCB from 3.3 kV to 38 kV are ABB,
Eaton, Siemens, HHI(Hyundai Heavy Industry), S&C Electric Company, Jyoti and
BHEL.
FIG 5. Medium voltage circuit breaker

28. [29]
Fig 8. High-voltage DC circuit breaker
Initially, all switches are closed (on). Because the high-voltage semiconductor switch has much
greater resistance than the mechanical switch plus the low-voltage semiconductor switch, current
flowthroughitis low.Todisconnect,firstthe low-voltagesemiconductor switch opens. This diverts
the current throughthe high-voltagesemiconductorswitch.Because of itsrelativelyhighresistance,
it begins heating very rapidly. Then the high-speed mechanical switch is opened. Unlike the low-
voltage semiconductor switch, which is capable of standing off only the voltage drop of the closed
high-voltagesemiconductor switch, this one is capable of standing off the full voltage. Because no
current is flowing through this switch when it opens, it is not damaged by arcing. Then, the high-
voltage semiconductorswitchisopened.Thisactuallycutsthe power. However, it is not quite 100%
off. A final low-speed mechanical switch disconnects the residual current.

29. [30]
CHAPTER-IV
Comparative analysis of Methodology
4.1. HVDC Breaker
High-voltage cables have a high electrical capacitance compared with overhead
transmission lines, since the live conductors within the cable are surrounded by a relatively
thin layer of insulation (the dielectric), and a metal sheath. The geometry is that of a long
coaxial capacitor. The total capacitance increases with the length of the cable. This
capacitance is in a parallel circuit with the load. Where alternating current is used for cable
transmission, additional current must flow in the cable to charge this cable capacitance. This
extra current flow causes added energy loss via dissipation of heat in the conductors of the
cable, raising its temperature. Additional energy losses also occur as a result of dielectric
losses in the cable insulation. However, if direct current is used, the cable capacitance is
charged only when the cable is first energized or if the voltage level changes; there is no
additional current required. For a sufficiently long AC cable, the entire current-carrying
ability of the conductor would be needed to supply the charging current alone. This cable
capacitance issue limits the length and power carrying ability of AC powered cables. DC
powered cables are limited only by their temperature rise and Ohm's Law. Although some
leakage current flows through the dielectric insulator, this is small compared to the cable's
rated current.
Fig 6. HVDC Breaker

30. [31]
4.2. Hybrid Breakersystems
The capacitive effect of long underground or undersea cables in AC transmission
applications also applies to AC overhead lines, although to a much lesser extent.
Nevertheless, for a long AC overhead transmission line, the current flowing just to charge the
line capacitance can be significant, and this reduces the capability of the line to carry useful
current to the load at the remote end. Another factor that reduces the useful current carrying
ability of AC lines is the skin effect, which causes a nonuniform distribution of current over
the cross-sectional area of the conductor. Transmission line conductors operating with direct
current do not suffer from either of these constraints. Therefore, for the same conductor
losses (or heating effect), a given conductor can carry more current to the load when
operating with HVDC than AC.
Finally, depending upon the environmental conditions and the performance of overhead line
insulation operating with HVDC, it may be possible for a given transmission line to operate
with a constant HVDC voltage that is approximately the same as the peak AC voltage for
which it is designed and insulated. The power delivered in an AC system is defined by the
root mean square (RMS) of an AC voltage, but RMS is only about 71% of the peak voltage.
Therefore, if the HVDC line can operate continuously with an HVDC voltage that is the same
as the peak voltage of the AC equivalent line, then for a given current (where HVDC current
is the same as the RMS current in the AC line), the power transmission capability when
operating with HVDC is approximately 40% higher than the capability when operating with
AC.
4.3. Smart circuit Breaker
HVDC allows power transmission between unsynchronized AC distribution
systems, it can help increase system stability, by preventing cascading failures from
propagating from one part of a wider power transmission grid to another. Changes in load
that would cause portions of an AC network to become unsynchronized and to separate,
would not similarly affect a DC link, and the power flow through the DC link would tend to
stabilize the AC network. The magnitude and direction of power flow through a DC link can
be directly controlled, and changed as needed to support the AC networks at either end of the
DC link. This has caused many power system operators to contemplate wider use of HVDC
technology for its stability benefits alone.

31. [32]
4.4. High-voltage DC circuit breaker
The breaker contains four switching elements, two mechanical (one high-speed and
one low-speed) and two semiconductor (one high-voltage and one low-voltage). Normally,
power flows through the low-speed mechanical switch, the high-speed mechanical switch and
the low-voltage semiconductor switch. The last two switches run parallel with the high-
voltage semiconductor switch.
Fig 8. High-voltage DC circuit breaker
Initially, all switches are closed (on). Because the high-voltage semiconductor switch has
much greater resistance than the mechanical switch plus the low-voltage semiconductor
switch, current flow through it is low. To disconnect, first the low-voltage semiconductor
switch opens. This diverts the current through the high-voltage semiconductor switch.
Because of its relatively high resistance, it begins heating very rapidly. Then the high-speed
mechanical switch is opened. Unlike the low-voltage semiconductor switch, which is capable
of standing off only the voltage drop of the closed high-voltage semiconductor switch, this
one is capable of standing off the full voltage. Because no current is flowing through this
switch when it opens, it is not damaged by arcing. Then, the high-voltage semiconductor
switch is opened. This actually cuts the power. However, it is not quite 100% off. A final
low-speed mechanical switch disconnects the residual current.

32. [33]
CHAPTER-V
Strength and Weakness
Comparisonof HVDC and AC
A long distance point to point HVDC transmission scheme generally has lower overall
investment cost and lower losses than an equivalent AC transmission scheme. HVDC
conversion equipment at the terminal stations is costly, but the total DC transmission line
costs over long distances are lower than AC line of the same distance. HVDC requires less
conductor per unit distance than an AC line, as there is no need to support three phases and
there is no skin effect.
Strength
Depending on voltage level and construction details, HVDC transmission losses are quoted as
less than 3% per 1,000 km, which are 30 to 40% less than with AC lines, at the same voltage
levels. This is because direct current transfers only active power and thus causes lower losses
than alternating current, which transfers both active and reactive power. HVDC transmission
may also be selected for other technical benefits. HVDC can transfer power between separate
AC networks. HVDC powerflow between separate AC systems can be automatically
controlled to support either network during transient conditions, but without the risk that a
major power system collapse in one network will lead to a collapse in the second. HVDC
improves on system controllability, with at least one HVDC link embedded in an AC grid—
in the deregulated environment, the controllability feature is particularly useful where control
of energy trading is needed.
The combined economic and technical benefits of HVDC transmission can make it a suitable
choice for connecting electricity sources that are located far away from the main users.
Specific applications where HVDC transmission technology provides benefits include:
 Undersea cables transmission schemes.
 Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps',
usually to connect a remote generating plant to the main grid.
 Increasing the capacity of an existing power grid in situations where additional wires
are difficult or expensive to install.

33. [34]
 Power transmission and stabilization between unsynchronized AC networks, with the
extreme example being an ability to transfer power between countries that use AC at
different frequencies. Since such transfer can occur in either direction, it increases the
stability of both networks by allowing them to draw on each other in emergencies and
failures.
 Stabilizing a predominantly AC power grid, without increasing fault levels
(prospective short-circuit current).
 Integration of renewable resources such as wind into the main transmission grid.
HVDC overhead lines for onshore wind integration projects and HVDC cables for
offshore projects have been proposed in North America and Europe for both technical
and economic reasons. DC grids with multiple voltage-source converters (VSCs) are
one of the technical solutions for pooling offshore wind energy and transmitting it to
load centers located far away onshore.
Weakness
The disadvantages of HVDC are in conversion, switching, control, availability, and
maintenance. HVDC is less reliable and has lower availability than alternating current (AC)
systems, mainly due to the extra conversion equipment. Single-pole systems have availability
of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault-tolerant
bipole systems provide high availability for 50% of the link capacity, but availability of the
full capacity is about 97% to 98%.
 The required converter stations are expensive and have limited overload capacity. At
smaller transmission distances, the losses in the converter stations may be bigger than
in an AC transmission line for the same distance. The cost of the converters may not
be offset by reductions in line construction cost and lower line loss.
 Operating a HVDC scheme requires many spare parts to be kept, often exclusively for
one system, as HVDC systems are less standardized than AC systems and technology
changes faster.
 In contrast to AC systems, realizing multi-terminal systems is complex (especially
with line commutated converters), as is expanding existing schemes to multi-terminal
systems. Controlling power flow in a multi-terminal DC system requires good
communication between all the terminals; power flow must be actively regulated by

34. [35]
the converter control system instead of relying on the inherent impedance and phase
angle properties of an AC transmission line.
Corona discharge
Corona discharge is the creation of ions in a fluid (such as air) by the presence of a
strong electric field. Electrons are torn from neutral air, and either the positive ions or the
electrons are attracted to the conductor, while the charged particles drift. This effect can
cause considerable power loss, create audible and radio-frequency interference, generate
toxic compounds such as oxides of nitrogen and ozone, and bring forth arcing.
Both AC and DC transmission lines can generate coronas, in the former case in the form of
oscillating particles, in the latter a constant wind. Due to the space charge formed around the
conductors, an HVDC system may have about half the loss per unit length of a high voltage
AC system carrying the same amount of power. With monopolar transmission the choice of
polarity of the energized conductor leads to a degree of control over the corona discharge. In
particular, the polarity of the ions emitted can be controlled, which may have an
environmental impact on ozone creation. Negative coronas generate considerably more ozone
than positive coronas, and generate it further downwind of the power line, creating the
potential for health effects. The use of a positive voltage will reduce the ozone impacts of
monopole HVDC power lines.
Fig 9. Corona Discharge

35. [36]
CHAPTER-VI
Future Scope
This paper introduces different types of the DC circuit breaker, including mechanical circuit
breaker, static circuit breaker and the hybrid circuit breaker. The hybrid DC circuit breaker as
a representative topology has been detail described including its breaking characteristics and
commutation mode. Different arc models including the Cassie arc model and the Mayr arc
model have also been studied in this paper. Finally this paper concludes the major technical
difficulties and research points of the DC circuit breaker. New technologies, i.e., modular
multilevel and its relevant characteristics. The output ac voltages can be adjusted in very fine
increments. It minimizes the generated harmonics and in most cases completely eliminates
the need for ac filters. Furthermore, the small and relatively shallow voltage steps cause very
little radiant or conducted high-frequency interference. The improved modulation technology
makes the individual semiconductors have much lower switching frequency (less than 3 times
of fundamental frequency) resulting in lower switching losses compared to 2 or 3-level VSC.
Total system losses are therefore relatively small and efficiency is consequently increased. So
the conclusion is drawn that these new technologies have great potentiality in application of
HVDC projects in future.

36. [37]
CONCLUSION
The VSC-HVDC application to improve the long-term voltage stability is investigated in this
paper. An appropriate control strategy is proposed to control the power flow of the VSC-
HVDC link to improve the AC system stability as much as possible. The NORDIC32 test
system is utilized to investigate the performance of VSC-HVDC link. Two longterm voltage
instability scenarios are considered to show the capability of the VSC-HVDC link to enhance
the AC power system performance and improve the long-term voltage stability. Simulation
results show that the optimal control of VSC-HVDC link leads to voltage collapse avoidance
in the system. Many other voltage instability scenarios have been conducted to verify the
proposed control method but are not shown in the paper due to space limit.In all other cases,
having VSC-HVDC with optimal control leads to long-term voltage stability improvement.

37. [38]
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