Power Electronics Application: HVDC

Course: Distribution Generation and Smart Grid
Prof. (Dr.) Pravat Kumar Rout
Sunita S. Biswal
Department of EEE, ITER,
Siksha ‘O’Anusandhan (Deemed to be University),
Bhubaneswar, Odisha, India
Class-12: Power Electronics
Application: HVDC

HVDCTransmission
 HVDC POWER transmission systems and technologies
associated with the flexible ac transmission system (FACTS)
continue to advance as they make their way to commercial
applications.
 Both HVDC and FACTS systems underwent research and
development for many years, and they were based initially on
thyristor technology and more recently on fully controlled
semiconductors and voltage-source converter (VSC) topologies.
 The ever increasing penetration of the power electronics
technologies into the power systems is mainly due to the
continuous progress of the high-voltage high power fully
controlled semiconductors.

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 The fully controlled semiconductor devices available today for
high-voltage high-power converters can be based on either
thyristor or transistor technology.
 These devices can be used for a VSC with pulse-width modulation
(PWM) operating at frequencies higher than the line frequency.
These devices are all self-commuted via a gate pulse.

WhyVSC?
 Typically, it is desirable that a VSC application generates PWM waveforms of
higher frequency when compared to the thyristor-based systems.
 However, the operating frequency of these devices is also determined by the
switching losses and the design of the heat sink, both of which are related to
the power through the component.
 Switching losses, which are directly linked to high-frequency PWM
operation, are one of the most serious and challenging issues that need to be
dealt with inVSC-based high-power applications.
 Other significant disadvantages that occur by operating a VSC at high
frequency are the electromagnetic compatibility/electromagnetic interference
(EMC/EMI), transformer insulation stresses, and high frequency oscillations,
which require additional filters.

Two distinct technology
 Type-1: Line-commutated current-source converters (CSCs) that use
thyristors (CSC-HVdc):
 This technology is well established for high power, typically around 1000 MW, with
the largest project being the Itaipu system in Brazil at 6300 MW power level.
 The longest power transmission in the world will transmit 6400 MW power from the
Xiangjiaba hydropower plant to Shanghai.
 The 2071 km line will use 800 kV HVdc and 1000 kV ultrahigh-voltage ac
transmission technology.

• Line commutated current source
converters (CSCs) that use
thyristors.
•This technology is well established
for high power around 1000 MW
• A matured technology today and
they have been used extensively in
present power system.
•The CSCs have the natural ability
to withstand short circuits as the dc
inductors can assist the limiting of
the currents during faulty operating
conditions.

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 Second: Forced-commutated VSCs that use gate turn-off
thyristors (GTOs) or in most industrial cases insulated gate bipolar
transistors (IGBTs).
 (VSC-HVdc): It is well-established technology for medium power
levels, thus far, with recent projects ranging around 300–400 MW
power level.

• UseVSCs with pulse width modulation (PWM) operating at frequencies higher than the
line frequencies
•These devices are all self-commutated via a gate pulse
• Operating frequency is higher in-comparison to thyristor-based systems
•This technology around 300-400 MW (till 2009)
• Recently developed
•The world’s firstVSC-based PWM-controlled HVdc system using IGBTs was installed in
March 1997 (Hellsj¨on project, Sweden, 3 MW, 10 km distance, ±10 kV).

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• The VSCs are more vulnerable to line faults, and therefore,
cables are more attractive forVSC-HVdc applications.
• Faults on the dc side of VSC-HVdc systems can also be
addressed through the use of dc circuit breakers (CBs) .
• In the event of the loss of a VSC in a multi-terminal HVdc, the
excess of power can be restricted by the advanced dc voltage
controller.

How does HVDC transmission system work?
 In generating substation, AC power is generated which can be
converted into DC by using a rectifier.
 In HVDC substation or converter substation rectifiers and
inverters are placed at both the ends of a line.
 The rectifier terminal changes the AC to DC, while the
inverter terminal converts DC to AC.
 The DC is flowing with the overhead lines and at the user
end again DC is converted into AC by using inverters, which
are placed in converter substation.
 The power remains the same at the sending and receiving
ends of the line. DC is transmitted over long distances
because it decreases the losses and improves the efficiency.

 A system having more than two converter stations and one transmission
line is called a‘two terminal DC system’ or a‘point-to-point system’.
 Similarly, if substation has more than two converter stations and
interconnecting DC terminal lines, it is called multi-terminal DC
substation.

Core HVDCTechnologies
Two basic converter technologies are used in modern
HVDC transmission systems.These are:
1. Conventional line-commutated current source
converters (CSCs)
2. Self-commutated voltage source converters (VSCs).

Advantages of VSC as opposed to a line-
communicated CSC
 Avoidance of commutation failures due to disturbances in the AC
network.
 Independent control of the reactive and active power consumed or
generated by the converter
 Possibility to connect the VSC-HVDC system to a weak ac network or
even to one where no generation sources is available, naturally, the
short circuit level is very low.
 Faster dynamic response due to higher PWM than the fundamental
switching frequency (phase-controlled) operation, which further
results in reduced need of filtering, and hence small filter size.
 No need of transformers to assist the commutation process of the
converters fully controlled semiconductors

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 Advanced power flow control capability, which allows a rapid
switch of power flow direction by reverse the current direction
but not the voltage polarity;
 Capability of multi-terminal interconnection;
 Capability to independently control active and reactive flows at
each terminal by the converters;
 Possibility to connect the VSC-HVDC system to a “weak” ac
network;
 Capability of paralleled operation of DC network on regional AC
grid; and
 Reduced construction and commissioning time of a HVDC
system.

Summary of fully controlled high power
semiconductors
Acronym Type Full Name
IGBT Transistor Insulated Gate BipolarTransistor
IEGT Transistor Injection Enhanced GateTransistor
GTO Thyristor GateTurn-offThyristor
IGCT Thyristor Integrated Gate CommutatedThyristor
GCT Thyristor Gate CommutatedTurn-offThyristor

Comparison between AC and DC
Transmission
 Investment Cost:
DC transmission requires fewer conductors
than AC transmission - 2 conductors per
DC circuit whereas three conductors per
3 phase AC circuit. HVDC allows line
supporting towers to be smaller and,
hence, requires lesser right-of-way. Thus,
clearly, HVDC transmission line would
cost lesser than an HVAC line. However,
the terminal converter stations in HVDC
are much more expensive which are not
required for HVAC transmission. Over a
specific distance, called as break-even
distance, HVDC line becomes cheaper
than HVAC. The break-even distance for
overhead lines is around 600 km and for
submarine lines it is around 50 km.

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 Losses:
Skin effect s absent in DC. Also, corona losses are significantly
lower in the case of DC. An HVDC line has considerably lower
losses compared to HVAC over longer distances
 Controllability:
Due to the absence of inductance in DC, an HVDC line offers better
voltage regulation. Also, HVDC offers greater controllability
compared to HVAC.

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 Asynchronous interconnection:
AC power grids are standardized for 50 Hz in some countries
and 60 Hz in other. It is impossible to interconnect two
power grids working at different frequencies with the help of
anAC interconnection.An HVDC link makes this possible.
 Interference with nearby communication lines:
Interference with nearby communication lines is lesser in the
case of HVDC overhead line than that for an HVAC line.
 Short circuit current:
In longer distance HVAC transmission, short circuit current
level in the receiving system is high.An HVDC system does
not contribute to the short circuit current of the
interconnectedAC system.

CSC-HVDC System Configurations
Depending upon the function and location of the converter stations, various
configurations of HVdc systems can be identified. The ones presented in this
section involve CSC-HVdc configurations but similar types of configurations
exist forVSCHVdc with or without transformers.
 Back-to-Back CSC-HVDC System
 Monopolar CSC-HVDC System
 Bipolar CSC-HVDC System
 Multiterminal CSC-HVDC System

Back-to-Back CSC-HVDC System
 In this case, the two converter stations are located at the same site and
there is no transmission of power with a dc link over a long distance.
 A block diagram of a back-to-back CSCHVdc system with 12-pulse
converters is shown in Fig.
 The two ac systems interconnected may have the same or different
frequency (asynchronous interconnection).

Monopolar CSC-HVDC System
 In this configuration, two converters are used that are separated by a single
pole line, and a positive or a negative dc voltage is used.
 Many of the cable transmissions with submarine connections use a
monopolar system.
 The ground is used to return current.
 Fig. shows a block diagram of a monopolar CSC-HVdc system with 12-
pulse converters.

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 This is the most commonly used configuration of a CSCHVdc system
in applications where overhead lines are used to transmit power.
 The bipolar system is two monopolar systems.The advantage of such
system is that one pole can continue to transmit power in case the
other one is out of service for whatever reason. In other words, each
system can operate on its own as an independent system with the
earth return.
 Since one is positive and one is negative, in case that both poles have
equal currents, the ground current is zero theoretically, or, in
practice, within a difference of 1%.
 The 12-pulse-based bipolar CSC-HVdc system is depicted in Fig.

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 In this configuration, there are more than two sets of converters.
 A multi-terminal CSC-HVdc system with 12-pulse converters per pole
is shown in Fig. In this case, converters 1 and 3 can operate as rectifiers
while converter 2 operates as an inverter.
 Working in the other order, converter 2 can operate as a rectifier and
converters 1 and 3 as inverters.
 By mechanically switching the connections of a given converter, other
combinations can be achieved.

VSC-HVDC FUNDAMENTAL CONCEPTS
 A basicVSC-HVdc system comprises of two converter
stations built withVSC topologies.
 The simplestVSC topology is the conventional two-level
three-phase bridge shown in Fig.

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 Typically, many series-connected IGBTs are used for each
semiconductor shown in Fig. in order to deliver a higher blocking
voltage capability for the converter, and therefore in-crease the dc
bus voltage level of the HVdc system.
 It should be noted that an anti-parallel diode is also needed in order
to ensure the four-quadrant operation of the converter.
 The dc bus capacitor provides the required storage of the energy so
that the power flow can be controlled and offers filtering for the dc
harmonics.
 TheVSC-HVdc system can also be built with otherVSC topologies.

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 The converter is typically
controlled through sinusoidal
PWM (SPWM), and the
harmonics are directly
associated with the switching
frequency of each converter leg.
 Fig. presents the basic
waveforms associated with
SPWM and the line-to-neutral
voltage waveform of the two-
level converter.
 Each phase leg of the converter
is connected through a reactor
to the ac system.
Two-level sinusoidal PWM method:
reference (sinusoidal) and carrier
(triangular) signals and line-to-neutral
voltage waveform.

VSC-HVDC multilevel topologies
 Multilevel converters extend the well-known advantages of low- and
medium-power PWM converter technology into the high-power
applications suitable for high-voltage high-power adjustable-speed
drives and large converters for power systems through VSC-based
FACTS and HVdc power transmission.
 There are numerous multilevel solid-state converter topologies
reported in the technical literature. However, there are two distinct
topologies, namely, the diode-clamped neutral-point-clamped
(NPC) converter and the flying capacitor (FC) converter.

Five-level FCVSC phase leg
Five-level PWM line-to-neutral voltage waveform

Neutral-point-clamped (NPC) phase leg
Three-level PWM line-to-neutral voltage waveform

Perspective Applications
 Submarine and Underground CableTransmission
 Long-Distance Bulk-Transmission
 Asynchronous Interconnection
 OffshoreTransmission of Renewable Energy
 Infeed Large UrbanAreas
 DC Segmented Grid
 WeakAC Network Connections

EmergingApplications
VSC-HVDC can be effectively used in a number of key areas as
follows:
 small, isolated remote loads
 power supply to islands
 infeed to city centers
 remote small scale generation
 off-shore generation and deep sea crossings
 multi-terminal systems

Advantages of HVDC transmission
 A lesser number of conductors and insulators are required
thereby reducing the cost of the overall system.
 It requires less phase to phase and ground to ground clearance.
 Their towers are less costly and cheaper.
 Lesser corona loss is less as compared to HVAC transmission lines
of similar power.
 Power loss is reduced with DC because fewer numbers of lines are
required for power transmission.

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 The HVDC system uses earth return. If any fault occurs in one
pole, the other pole with ‘earth returns’ behaves like an
independent circuit.This results in a more flexible system.
 The HVDC has the asynchronous connection between two AC
stations connected through an HVDC link; i.e., the transmission
of power is independent of sending frequencies to receiving end
frequencies. Hence, it interconnects two substations with different
frequencies.
 Due to the absence of frequency in the HVDC line, losses like skin
effect and proximity effect does not occur in the system.
 It does not generate or absorb any reactive power. So, there is no
need for reactive power compensation.
 The very accurate and lossless power flows through DC link.

Disadvantages of HVDC transmission
 Converter substations are placed at both the sending and the
receiving end of the transmission lines, which result in increasing
the cost.
 Inverter and rectifier terminals generate harmonics which can be
reduced by using active filters which are also very expensive.
 If a fault occurs in the AC substation, it may result in a power
failure for the HVDC substation placed near to it
 Inverter used in converter substations have limited overload
capacity.
 Circuit breakers are used in HVDC for circuit breaking, which is
also very expensive.
 It does not have transformers for changing the voltage levels.
 Heat loss occurs in converter substation, which has to be reduced
by using the active cooling system.
 HVDC link itself is also very complicated.

Types of Converters
There are basically two configuration
types of three phase converters
possible for this conversion process.
1. Current source converter (CSC)
2. Voltage source converter (VSC)

Comparison: on AC side
CSC VSC
 Acts as a constant voltage
source
 Requires a capacitor as its
energy storing device
 Requires large ac filters for
harmonic elimination
 Requires reactive power
supply for power factor
correction
 Acts as a constant current
source
 Requires an inductor as its
 Requires a small ac filters
for higher harmonic
elimination
 Reactive power supply is
not required as converter
can operate in any
quadrant.

Comparison: on DC side
CSC VSC
 Acts as a constant current
source
 Requires an inductor as its
 Requires DC filters
 Provides inherent fault
current limiting features
 Acts as a constant voltage
source
 Requires a capacitor as its
 Energy storage capacitors
provide DC filtering
capability at no extra cost
 Problematic for DC line
side faults since the
charged capacitor will
discharge into the fault

Comparison: Switches
CSC VSC
 Line commutated or forced
commutated with a series
capacitor.
 Switching occurs at line
frequency i.e only single
pulsing per cycle.
 Lower switching losses.
 Self commutated
 Switching occurs at high
frequency i.e. Multiple
pulsing within one cycle
 Higher switching losses

Comparison: Rating range
CSC VSC
 0-550 MW per converter
 up to 600 KV
 0-1000 MW
 up to 450KV

References
 Flourentzou, N., Agelidis, V. G., & Demetriades, G. D. (2009).
VSC-based HVDC power transmission systems: An overview. IEEE
Transactions on power electronics, 24(3), 592-602.
 Jacobson, B., Karlsson, P., Asplund, G., Harnefors, L., & Jonsson,
T. (2010, August). VSC-HVDC transmission with cascaded two-
level converters. In Cigré session (pp. B4-B110).
 Jacobson, B., Karlsson, P., Asplund, G., Harnefors, L., & Jonsson,
T. (2010, August). VSC-HVDC transmission with cascaded two-
level converters. In Cigré session (pp. B4-B110).

Questions
 Differentiate between CSC andVSC based converter.
 What are the major CSC-HVDC based System
configurations?
 What are the advantages and disadvantages of HVDC system?
 Give the comparison between AC and DC high voltage
transmission.

Power Electronics Application: HVDC

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