BSTP EN HVDC PSSE Training, Day 1 - THEORY.pptx

ENERGY TECHNOLOGY AND GOVERNANCE PROGRAM
Black Sea Transmission System Planning Project (BSTP)
Workshop on Modeling High Voltage Direct Current
(HVDC) Converter Stations
This presentation is made possible by the support of the American people through the United States Agency for
International Development (USAID). The contents are the responsibility of the United States Energy Association and
do not necessarily reflect the views of USAID or the United States Government.
March 21-22, 2017
Tbilisi, Georgia

1. INTRODUCTION
2. LCC HVDC SYSTEM
3. VSC HVDC SYSTEM
4. COMPARISON
5. CONTROL OF HVDC SYTEMS
6. REACTIVE POWER BALANCE
7. SHORT CIRCUIT RATIO
8. ADDITIONAL BENEFITS OF HVDC SYSTEMS
9. ENTSO-E HVDC NETWORKC CODE
Contents

1. INTRODUCTION
2. LCC HVDC SYSTEM TECHNOLOGY
3. VSC HVDC SYSTEM TECHNOLOGY
4. TECHNOLOGIES COMPARED
9. ENTSO-E HVDC NETWORK CODE

 First HVDC System Commissioned in 1954, Gotland,
Sweden
 ±100kV 20MW 97 kilometers of submarine cable
 Longest Distance in Operation 1983, DR Congo
 ±500kV 560MW 1709 kilometers overhead-line
 Longest Submarine Cable 2008,
 Norway to Netherlands
 ±450kV, 700MW 583 km submarine cable
 Connection of asynchronous systems
 Highest Voltage in Operation 2010,
 Yunnan-Guangdong, China
 ±800kV, 5000 MW
 First Multi-Terminal HVDC System 1992,
 Quebec‐New England
 ±450kV 2000MW
Introduction

 Taken from a 3-phase AC network
 Converted to DC in a converter station
 Transmitted by DC line or cable (underground or submarine)
 Converted back to AC in another converter station
 Injected into AC network
Basics of HVDC Operation

 Inductive and capacitive elements of
overhead lines and cables put limits to
the transmission capacity and the
transmission distance
 Depending on the required
transmission capacity the achievable
transmission distance for an AC cable
will be in the range of 40 to 100 km. It
will mainly be limited by the charging
current
Disadvantages of HVAC Systems
 Direct connection between two AC systems with different frequencies is not
possible
 Direct connection between two AC systems with the same frequency or a new
connection within a meshed grid may be impossible because of system
instability, too high short-circuit levels or undesirable power flow scenarios

 Major advantage of flexibility in power exchange in
comparison with HVAC
 Fast control of power flow – practically independently from
frequency, voltage or angle at terminal buses
 Fast change of direction of transmitted power – due to inherent
properties of the electronic equipment in converters
 Controllable – power injected where needed, supplemental control,
frequency control
 Bypass congested circuits – no inadvertent flow
 Lower losses
 Reactive power demand limited to terminals independent
of distances
 Narrow Right-of-Way (RoW) –
 land coverage and the associated right-of-way cost for an HVDC
overhead transmission line is smaller,
 reduced visual impact
 higher power transmission capacity for same RoW
 no Electromagnetic field (EMF) constraints
 Cost Comparison HVDC vs. HVAC
 HVDC has a higher installation cost due to the converter stations
and filtering requirement
 The cost of an HVDC line is less than the cost of an AC line. Long AC
lines are more expensive due to shunt and series compensation
requirements
Advantages of HVDC Systems
Comparison 6000MW ‐ HVDC vs.
HVAC

HVDC Systems: Current- & Voltage- Link
CURRENT LINK
VOLTAGE LINK
Introduction

 three ways of achieving AC/DC/AC conversion in HVDC system:
1. Natural Commutated Converters:
 Most used in the HVDC systems as of today
 The component that enables this conversion process is the thyristor,
which is a controllable semiconductor
 Known as CSC – Classic or LCC – Line Commutated Converters
 Producers
 SIEMENS – HVDC CSC CLASSIC
 ABB – HVDC CSC CLASSIC
 ALSTOM – HVDC LCC
2. Capacitor Commutated Converters (CCC):
 Improvement in the thyristor-based commutation
 Characterized by the use of commutation capacitors inserted in
series between the converter transformers and the thyristor valves
 Improve the commutation failure performance of the converters
when connected to weak networks
Introduction

3. Forced Commutated Converters:
 The valves of these converters are built up with semiconductors
with the ability turn-on but also to turn-off.
 Two types of semiconductors are normally used GTO (Gate Turn-Off
Thyristor) or the IGBT (Insulated Gate Bipolar Transistor)
 Known as VSC – Voltage Source Converters
 Introduced a spectrum of advantages, e.g. feed of passive networks
(without generation), independent control of active and reactive
power, power quality…
 Producers:
 SIEMENS – HVDC PLUS
 ABB – HVDC LIGHT
 ALSTOM – HVDC VSC
Introduction

1. INTRODUCTION
2. LCC HVDC SYSTEM
3. VSC HVDC SYSTEM
4. COMPARISON

 AC power is fed to a converter operating as a rectifier. Output of this
rectifier is DC power, independent of AC supply frequency and
phase.
 DC power is transmitted through a conduction medium; overhead
line, a cable or a short length of bus bar
 Second converter is operated as inverter and allows the DC power to
flow into the receiving AC network.
 Converter requires alternating AC voltage (Vac) to operate as an
inverter. This is why the thyristor-based converter topology used in
HVDC is known as a line-commutated converter (LCC).
LCC HVDC Systems

 Conventional HVDC transmission utilizes line-commutated thyristor
technology.
 Thyristor - controllable semiconductor that can carry very high
currents (4000 A) and is able to block very high voltages (up to 10
kV).
 Thyristors used for LCC HVDC valves are amongst the largest
semiconductors of any type produced for any industry
 By means of connecting the thyristors in series it is possible to
build up a thyristor valve, which is able to operate at very high
voltages (figure shows 8.5 kV thyristor)
LCC HVDC Systems

 The required DC system voltages are achieved by a series
connection of a sufficient number of thyristors.
 A group of four valves in a single vertical stack is known as a
“quadrivalve”
 Three such quadrivalves being required at each end of each pole
 Since the voltage rating of thyristors is several kV, a 500 kV
quadrivalve may have hundreds of individual thyristors connected
in series groups of valve or thyristor modules
LCC HVDC Systems
1. Valve branch
2. Double Valve
3. Valve tower – Quadrivalve
4. 6-pulse bridge

 The thyristor valve is operated at net frequency (50 Hz or 60 Hz)
 By means of a control angle it is possible to change the DC voltage
level of the bridge.
 This ability is the way by which the transmitted power is controlled
rapidly and efficiently.
LCC HVDC Systems

 Standard graphical symbols for valves and bridges
 6 pulse convertor
 12 pulse convertor
LCC HVDC Systems

 Converter station is normally split into two areas:
 AC switchyard which incorporates the AC harmonic filters and HF filters
 “Converter island” which incorporates
the valve hall(s), control and
services building, converter
transformers and DC switchyard
CONVERTER STATION

 Valves associated with each twelve-pulse bridge are normally
contained within a purpose built building
 This enclosure provides a clean, controlled environment in which
the thyristor valves can safely operate without the risk of exposure
to pollution or outdoor conditions.
 Within the valve hall, the thyristor valves are typically suspended
from the roof of the building
 low voltage being closest to the roof
 high voltage being at the lowest point
 on the valve.
 An air gap between the bottom of the
valve and the valve hall floor provides
the high voltage insulation.
VALVE HALL

 AC side current waveform of a HVDC converter, is highly non-
sinusoidal, and, if allowed to flow in the connected AC network,
might produce unacceptable levels of distortion
 AC side filters are therefore required as part of the converter
station in order to reduce the harmonic distortion of the AC side
current and voltage to acceptably low levels
 Shunt-connected AC filters appear as capacitive sources of reactive
power at fundamental frequency, and normally are used to
compensate most or all of the reactive consumption of the
converter
 Design of the AC filters, therefore, normally has to satisfy these two
requirements of harmonic filtering and reactive power
compensation.
AC FILTERS

 Design influenced by a number of factors
 Specified harmonic limits
 AC system voltage conditions
 Switched filter size (dictated by voltage step limit, reactive power balance…)
 Two main filter types:
 Tuned filter or band-pass filter which is sharply tuned to one or several
harmonic frequencies (single (e.g. 11th) double (e.g. 11/13th) and triple (e.g.
3/11/13th) tuned types)
 Damped filter or high-pass filter offering a low impedance over a broad band
of frequencies i.e. designed to damp more than one harmonic. Filter tuned
at 24th harmonic will give low impedance for both 23rd and 25th harmonic
 Scheme with a 12-pulse converter, the largest characteristic
harmonics will be the following: 11th, 13th, 23rd, 25th, 35th, 37th,
47th, and 49th
. Level of the 11th and 13th harmonic are generally
twice as high as for the rest. Common practice is to provide:
 band-pass filters for the 11th and 13th harmonic
 high-pass filters for the higher harmonics.
AC FILTERS

 Possible low-order resonance between the AC network and the
filters and shunt banks
 When a big HVDC scheme is to be installed in a weak AC system, a
low-order harmonic filter (most often tuned to 3rd harmonic) may
be also needed.
 Each filter branch can have one to three tuning frequencies
 AC harmonic filters are typically composed of a high voltage
connected capacitor bank in series with a medium voltage circuit
comprising air-cored air-insulated reactors, resistors and capacitor
banks
 Connected directly to the converter bus bar or connected to a “filter
bus bar” which, in-turn, is connected to the converter bus bar.
 AC harmonic filters are automatically switched-on and off with
conventional AC circuit breakers when they are needed to meet
harmonic performance and reactive power performance limits.
AC FILTERS

 Interface between the HVDC converter and the AC system and
provide several functions
 Galvanic isolation between the AC and DC systems
 Correct voltage to the converters
 Limit effects of steady state AC voltage change on converter
operating conditions
 Fault-limiting impedance
 30° phase shift required for twelve-pulse operation via star and
delta windings
 Equipped with on-load tap-changers in order to provide the correct
valve voltage
 tap changer will adjust to keep the delay angle α at a rectifier at its
desired normal operating range
 at the inverter, tap changer will adjust to maintain the inverter
operation at its desired level of DC voltage or extinction angle γ
CONVERTER TRANSFORMERS

 The largest plant item to be shipped to site for an HVDC project
 12-pulse converter requires two 3-phase systems which are spaced
apart from each other by 30 or 150 electrical degrees. This is
achieved by installing a transformer on each network side in the
vector groups Yy0 and Yd5.
 Common transformer arrangements in HVDC schemes

It is important that the converter transformer be thermally designed
to take into consideration both the fundamental frequency load and
the AC harmonic currents that will flow from the converter through the
converter transformer to the AC harmonic filters.

 Essential component of the monopolar HVDC transmission
system, since they carry the operating current on a
continuous basis
 Contribute to lower cost costs for the earth electrodes are
significantly lower than the costs for a second conductor
(with half the nominal voltage)
 Earth electrodes are also found in all bipolar HVDC systems
 Since the direct currents in the two poles of the HVDC are
never absolutely equal, in spite of current balancing control,
a differential current flows continuously from the station
neutral point to ground.
 It is common practice to locate the grounding of the station
neutral point at some distance (10 to 50 kilometers) from the
HVDC station by means of special earth electrodes.
EARTH ELECTRODES

 Normally required for power transmission schemes; they are not
required for back-to-back schemes
 In general it is used to
 Reduce the DC current ripple on the overhead transmission line or
cable
 Limitation of the DC fault currents
 Prevention of resonance in the DC circuit
 Protect the thyristor valve from fast front transients originating on
the DC transmission line (for example a lightning strike)
 DC smoothing reactor is normally a large air-cored air-insulated
reactor
 DC SWITCHGEAR
 Switchgear on the DC side of the converter is typically limited to
disconnectors-switches and earth switches for scheme
reconfiguration and safe maintenance operation
DC SMOOTHING REACTOR

 Converter operation results in voltage harmonics being generated
at the DC terminals
 This AC harmonic component of voltage will result in AC harmonic
current flow in the DC circuit
 The field generated by this AC harmonic current flow can induce
harmonic current flow in open-wire telecommunication systems
 In a back-to-back scheme, these harmonics are contained within the
valve hall with adequate shielding
 With a cable scheme, the cable screen typically provides adequate
shielding
 With open-wire DC transmission it may be necessary to provide DC
filters to limit the amount of harmonic current flowing in the DC line
DC FILTER

 CCC is characterized by the use of
commutation capacitors in series,
between valve bridge and converter
transformer
 These capacitors provide reactive power
proportional to the loading of converter
 This eliminates the need for reactive power
compensation by shunt capacitors and large
filter banks
CCC HVDC Systems
 Commutation capacitors reduce the risk of commutation failures in the
converter
 Filters still needed to mitigate harmonics, but instead of high MVA filter
banks active DC filters and continuously tuned AC filters can be used
 Other effects of commutation capacitors
 Reduced converter transformer rating (reactive power flow through transformer
minimized)
 Reduced required area for the HVDC station due to elimination of switchable filter
banks
 Reduced valve short circuit currents due to voltage drop across capacitor varistors
used to protect capacitors from overvoltages

 Basic HVDC cable transmission scheme is a monopolar installation
that uses the earth and sea to return the current.
 To avoid potential problems associated with ground return current
a monopolar metallic return system is used - return current flows
through a conductor in the form of a medium-voltage cable
 A further development of the monopolar transmission scheme is
the bipolar configuration
 Bipolar configuration is actually two monopolar systems combined -
one at positive and one at negative polarity with respect to ground
Configurations

Monopolar HVDC System with Ground Return
 Consists of converter units at each end, a single conductor and
return through the earth or sea
 At each end of the line, it requires an electrode line and a ground or
sea electrode built for continuous operation
 It can be a cost-effective solution for a HVDC cable transmission
and/or the first stage of a bipolar scheme
 Most feasible solution for very long distances and in particular for
very long sea cable transmissions.
Configurations

Monopolar HVDC System with Metallic Return
 Consists of converter units at each end, one high voltage and one
medium voltage conductor
 Used when construction of electrode lines and ground electrodes
results in an uneconomical solution due to a short distance or high
value of earth resistivity
 In many cases, existing infrastructure or environmental constraints
prevent the use of electrodes and metallic return path is used in
spite of increased cost and losses
Configurations

Bipolar HVDC System
 Most commonly used configuration for a bipolar transmission
system - high degree of operational flexibility
 Operate in monopole configuration as needed
 Allows for maintenance or outage of one pole
 Up to half of rated capacity
 For power flow in the other direction, the two conductors reverse
their polarities
 Advantage over two monopoles is reduced cost due to one common
or no return path and lower losses
 Disadvantage is that unavailability of the return path with adjacent
components will affect both poles.
Configurations

Bipolar HVDC System with Ground Return
A Bipolar balanced operation (normal)
B Monopolar ground return operation (converter pole or OHL
outage)
 Upon a single-pole fault, the current of the sound pole will be
taken over by the ground return path and the faulty pole will be
isolated.
C Monopolar metallic return operation (converter pole outage)
 Following a pole outage caused by the converter, current can be
commutated from ground return path into a metallic return
path provided by the HVDC conductor of the faulty pole.
Configurations

Bipolar HVDC System with Dedicated Metallic Return
 Dedicated LVDC metallic return conductor can be considered as an
alternative to a ground return path with electrodes
 If there are restrictions even to temporary use of electrodes
 If the transmission distance is relatively short
Configurations

Bipolar HVDC System without Dedicated Metallic Return
 Scheme without electrodes or a dedicated metallic return path for
monopolar operation will give the lowest initial cost
 Monopolar operation is possible by means of bypass switches
during a converter pole outage, but not during an HVDC conductor
outage.
 A short bipolar outage will follow a converter pole outage before
the bypass operation can be established
Configurations

 LCC HVDC control system only has one degree of freedom – when to
turn on the thyristor.
 Thyristors can only be turned on (not off) by control action, and rely
on the external AC system to effect the turn-off process.
 This limits the usefulness of LCC HVDC in some circumstances.
 AC system to which the LCC HVDC converter is connected must
always contain synchronous machines in order to provide the
commutating voltage.
 LCC HVDC converter cannot feed power into a passive system.
 With some other types of semiconductor device such as the
insulated-gate bipolar transistor (IGBT), both turn-on and turn-off
can be controlled, giving a second degree of freedom.
VSC HVDC Systems

 As a result of turn-on and turn-off capability, IGBTs can be used to
make self-commutated converters.
 In such converters, the polarity of DC voltage is usually fixed and
being smoothed by a large capacitance, can be considered constant.
 For this reason, an HVDC converter using IGBTs is usually referred
to as a Voltage Source Converter VSC.
 The development of Insulated Gate Bipolar Transistors (IGBT) with
high voltage ratings have accelerated the development of voltage
sourced converters for HVDC applications.
VSC HVDC Systems

 The operation of the converter is achieved by Pulse Width
Modulation (PWM).
 With PWM it is possible to create any phase3 angle and/or
amplitude (up to a certain limit) by changing the PWM pattern,
which can be done almost instantaneously.
 PWM offers the possibility to control both active and reactive power
independently.
 From a transmission network viewpoint, it acts as a motor or
generator without mass that can control active and reactive power
almost instantaneously.
VSC HVDC Systems

 IGBT cells have a small size (around 1 cm2).
 Many IGBT cells are connected in parallel in IGBT chips and then in
modules capable to handle current up to 2.4 kA with blocking
voltage up to 6.5 kV.
 As for the thyristors, many modules are connected in series into
valves to withstand to the high voltage levels.
 So, a valve may comprise up to 20 billions of IGBT cells.
VSC HVDC Systems

 VSC converter consists of two level or multilevel converter, phase-
reactors and AC filters.
 VSC normally use the 6-pulse connection because the converter
produces much less harmonic distortion than LCC. 12-pulse
connection is unnecessary.
CONVERTER STATION

 The converter reactor is one of the key components in VSC.
 It permits continuous and independent control of active and
reactive power.
 The main purposes of the converter reactor are
 to limit the short circuit current at the IGBT valves and
 to provide a low-pass filter of the PWM pattern
 The harmonic currents related to the switching frequency and
generated by the converter are blocked by the converter reactor.
 There is one converter reactor per phase.
CONVERTER REACTOR

 Up to now, implemented VSC converters have been based on two or
three-level technology which enables switching two or three
different voltage levels to the AC terminal of the converter.
 Converter voltage created by PWM is far from the desired voltage. It
needs AC filters to achieve an acceptable waveform.
PWM MODULATION
Achieved voltage
Desired voltage

 Both, the size of voltage steps and the related voltage gradients can
be reduced or minimized if the AC voltage generated by the
converter can be selected in smaller increments than at two or
three levels only.
 The more steps that are used, the smaller is the proportion of
harmonics and the lower is the high-frequency noise.
 Converters with high number of steps are termed multilevel
converters.
 A new and different approach is Modular Multilevel Converter
(MMC) technology.
MMC MODULATION

 MMC consists of six converter arms.
 Each of them comprises a high number of power modules (PM) and
one converter reactor connected in series.
 The power modules contain:
 IGBT half bridge as a switching element
 DC capacitor unit for energy storage
MMC MODULATION

 It is possible to separately and selectively control each of the
individual power modules in all phase units.
 Two converter arms of each phase unit represent a controllable
voltage source.
 The total voltage of the two converter arms in each phase unit
equals the DC voltage.
 By adjusting the ratio of the converter arm voltages in one phase
unit, the desired sinusoidal voltage at the AC terminal is achieved.
MMC MODULATION

 Special cases of monopolar HVDC interconnections
 Back-to-back indicates there is no DC transmission line and both
converters are located at the same site i.e. station
 Valves for both converters may be located in one valve hall
 DC filters are not required, nor are electrodes or electrode lines, the
neutral connection being made within the valve hall.
Back-to-Back HVDC Systems

 Mainly used as interconnections between adjacent asynchronous
networks which can not be synchronized
 They can also be used within a meshed grid in order to achieve a
defined power flow
 Used in Japan for interconnections between power system networks
of different frequencies (50 and 60 Hz)
Back-to-Back HVDC Systems

 Three or more HVDC substations geographically separated with
interconnecting transmission lines or cables
 Parallel multi-terminal DC
 If all substations are connected to the Monopolar
configuration/Bipolar configuration same voltage
 Series multi-terminal DC
 If one or more converter bridges are added in series in one or both
poles
 A combination of parallel and series connections of converter
bridges is a hybrid multi-terminal system
Multiterminal HVDC Systems

1. INTRODUCTION
2. LCC HVDC SYSTEM
3. VSC HVDC SYSTEM
4. COMPARISON
5. CONTROL OF HVDC SYSTEMS

 Layout and Footprint of the
converter station.
 VSC converters are also
considerably more compact than
line-commutated converters
(mainly because much less
harmonic filtering is needed) e.g.
600MW LCC converter station
requires about 14000 m2 whereas a
VSC HVDC needs only 3000m2. This
requirement is very important on
offshore platforms.
LCC vs VSC
 Transformers
 The VSC controller allows the use of standard two-winding
transformers
 This gives more flexibility to build and design the offshore station
 Harmonics
 LCC require harmonic filters, VSC only simple high‐pass filter for high
order harmonics

 VSC converters are self-commutating, not requiring an external
voltage source for its operation i.e. do not rely on synchronous
machines in the AC system for its operation.
 Therefore the Possibility of the converters starting with a dead grid,
not needing any start-up mechanism (“Black-start” capability). They
can feed power to an AC network consisting only of passive loads,
something which is impossible with LCC HVDC.
 In contrast to LCC HVDC converters, VSC converters maintain a
constant polarity of DC voltage and power reversal is achieved
instead by reversing the direction of current. This makes voltage-
source converters much easier to connect into a Multi-terminal
HVDC system.
 For the same reason XLPE cables cannot be used with HVDC LCC.
LCC vs VSC

 Reactive power control and stability
 Improved voltage stability VSC controller can control the reactive power
and the voltage. Reactive power flow can be independently controlled at
each AC network and the reactive power control is independent of active
power control
 This gives a serious advantage to the VSC technology in fault through
capability and black start capability.
 The reactive control for the classical technology is done by capacitor bank
(slow switching scheme), thus the flexibility is not good and a continuous
control can not be done.
 Voltage stability problems may also be experienced at the terminals of
HVDC links used for either long distance or back-to-back applications.
They are usually associated with the unfavorable reactive power “load”
characteristics of the converters.
 Transmission capacity
 HVDC LCC up to 6400 MW and Udc=±800kV=
 HVDC VSC up to 1100 MW and Udc=±300kV=
LCC vs VSC

 HVDC Converter Development
LCC vs VSC

LCC HVDC VSC HVDC
Size single range converter 150 - 1500MW 50 – 1100MW
Semiconductor technology Thyristor IGBT
DC voltage ±800kV ±320kV
Converter technology Line commutated Self commutated
Control of reactive power No, only switching regulation yes, continuous control
Voltage control Limited Extensive
Fault ride through No Yes
Black start capability No Yes
Power reversal without interruption No Yes
Minimum ESCR 2 No required
Minimum DC power flow 5-10% of rated power No minimum required
Typical losses per convertor 0,80 % 2%
Operating experience >20 years 8 years
Operating experience offshore No Yes
Construction time 3 (2)* years 1 year
LCC vs VSC

Long distance
transmission over
land/sea
Interconnections of
asynchronous
networks
WPP connection
to network
Feed of isolated
loads
LCC or CCC HVDC with
OHL/Cables √ √
CCC Converters in
Back-to-Back √
VSC converters in Back-
to-Back √ √
VSC Converters with
land/sea cables √ √ √ √
LCC vs VSC

 Rectifier uses a current control (CC) and a α-limit control, which
includes minimum α-limit
 Goal of minimum firing angle limit is to make sure there is enough
voltage across valves before the firing takes place, otherwise the
commutation will fail
 In CC mode, firing angle is controlled and thereby DC voltage to
keep the DC current at the desired value
 If the measured DC current is smaller than a reference value, firing
angle delay will be decreased and vice versa
 Tap changer control of the converter transformer is used to keep α
within allowed range
Control of LCC HVDC

 Inverter is provided with a constant extinction angle control (CEA)
and a current control.
 In CEA mode a compromise is made between a low risk of
commutation failures (if gamma is to large) and a low reactive
consumption (if gamma is small) which result in value of gamma of
around 15
 In normal operation, rectifier is in current control mode and
inverter in CEA mode.
 If the AC voltage decreases, firing
angle in rectifier will also decrease
to keep the DC voltage up.
 When α hits minimum limit, rectifier
will switch to α-limit control and
inverter will take over current control
Control of LCC HVDC

 Idc=(Vrectifier-Vinverter)/Rdc
 Under steady-state operation, the inverter control system is
normally arranged to maintain the DC voltage at a certain point on
the HVDC link (known as the “compounding point”) at a target value
(typically 1.0 pu)
 The “compounding point” is usually at the rectifier DC terminal and
hence the inverter must calculate this voltage based on the DC
voltage at the inverter terminals, the DC current and the known
resistance of the transmission circuit
 The rectifier normally controls the DC current flowing in the circuit
and does this by adjusting its output DC voltage to give a current
flow as described by the above equation
Control of LCC HVDC

POWER CONTROL
 Power transferred between sending and receiving end of the HVDC
link is controlled to meet an operator-set value at some point,
known as the compounding point.
 Typically compounding point is at the rectifier DC terminal
 It can also be at the inverter DC terminal, the mid-point of the DC
transmission conductors (e.g. border between two countries), the
inverter AC terminal or the rectifier AC terminal
 If the power demand is changed then the power order will ramp to
the new power transfer level at a rate of change (“ramp rate”) pre-
selected by the operator.
Control of LCC HVDC

FREQUENCY CONTROL
 All HVDC lines have the ability to regulate the power by means of
frequency feed back loop acting on the HVDC line control system
 HVDC scheme can control the AC frequency of an AC system by
automatically adjusting the power being delivered into that AC
system in order to balance the load with the supply.
 Fast power control by HVDC reduces the under-frequency or over-
frequency which can result from a changing load in a small system
 For example receiving end AC system could have an upper
frequency limit to automatically stop further increases in the power
being delivered by the HVDC scheme.
 Equally, the receiving AC system can have a lower frequency limit
which, if reached, automatically increases the power being
delivered into the receiving AC system.
 This can normally be overridden by sending end minimum
frequency limit, sending end system will help out the receiving end
AC system as much as possible without risking a cascade failure.
Control of LCC HVDC

POWER-FREQUENCY CONTROL
 Usually, a combination of control modes is
used.
 Normally, the control signal that acts on the
power controller of the DC link is a scheduled
delivered power by the DC link. That signal
remains the same as long as the scheduled
predetermined frequency remains within the
limits (dead band).
 In case of violating the limits, the frequency
control system of the DC link will take over to
support the system frequency by modifing its
power output as needed (droop control).
 If the maximum capacity of DC line is
reached, then frequency control system turns
to be out of action.
Control of LCC HVDC

POWER MODULATION CONTROL
 Power being transferred through a HVDC link can be automatically modulated
to provide damping to low-frequency power oscillations within either, or both,
interconnected AC systems
 This is determined by studies during the design phase of the HVDC scheme
POWER DEMAND OVERRIDE
 In response to certain events, such as loss of an AC transmission line, loss of
an AC generator or loss of a major load, the HVDC interconnection can be
programmed to respond in a pre-defined manner.
Example: if the loss of a line may result in instability within the AC system, the
HVDC interconnection can be preprogrammed to reduce the power transfer at a
pre-determined ramp rate to a safe value as established by studies.
Control of LCC HVDC

 DC Voltage control makes sure of the power balance between
sending and receiving ends
 Active Power can be controlled by changing the phase angle of the
converter ac voltage with respect to the filter bus voltage
 Reactive power can be controlled by changing the magnitude of the
fundamental component of the converter ac voltage with respect to
the filter bus voltage
Control of VSC HVDC

 By controlling these two aspects of the converter voltage, operation
in all four quadrants is possible.
 This means that the converter can be operated in the middle of its
reactive power range near unity power factor to maintain dynamic
reactive power reserve for contingency voltage support similar to a
SVC
 It also means that P transfer can be
changed rapidly without altering
Q exchange with ac network or
waiting for switching of shunt
compensation.
 Reactive power control on both
sides is responsible for the
transformer secondary winding
AC voltage magnitude
Control of VSC HVDC

 VSC HVDC uses Pulse Width Modulation (PWM) control to give the
desired fundamental frequency voltage
 Control signal is needed to achieve the control of the switching of
the IGBT valves.
 Sinusoidal control signal Vcontrol is compared with a triangular
signal Vtri to decide which valve should be conducting.
Control of VSC HVDC

 The triangular wave form Vtri is at a switching frequency fs.
 The control signal Vcontrol is used to modulate the switch duty
ratio at a frequency f1(modulating frequency).
 In a 3-phase transmission, each “leg” of the converter is controlled
separately from the 2 others
Control of VSC HVDC

 Output voltages are similar than with one leg
 Due to 120 phase shift, line to line
rms voltage at fundamental
frequency is:
Control of VSC HVDC

 LCC HVDC converters consume substantial reactive power
 Large proportion of reactive power must normally be supplied
locally within the converter station
 Important design consideration of LCC HVDC scheme relates to
reactive power loading that a converter imposes on the network
 Main sources of capacitive (positive) reactive power in a HVDC
station are the AC harmonic filters
 reduce harmonics injected into AC system
 generate reactive power
 AC filter is composed of capacitances,
inductances and resistances
 At fundamental frequency the HV-connected
capacitor is main contributor to reactive
power generated.
Reactive power balance

 The reactive power consumption of an HVDC converter depends on
the active power, the transformer reactance and the control angle.
 The reactive power consumption increases with increasing active
power.
 A common requirement to a converter station is full compensation
or overcompensation at rated load.
 In addition, a reactive band for the load and voltage range and the
permitted voltage step during bank switching must be determined.
 These factors will determine the size and number of filter and shunt
capacitor banks.

 Converter operating power factor can be approximately calculated
from the overlap angle and the converter firing angle:
Rectifier: cosφ= ½ x [cos(α)+cos(α +µ)]
Inverter: cosφ= ½ x [cos(γ)+cos(γ+µ)]
 Power factor should be high as possible
 To avoid high reactive power consumption in the converters
 Keep rated power of the converter as high as possible
 Minimize losses
 In order to keep power factor high, α for rectifier and γ for inverter
must be kept low (however, not too low)
 α 15° - 20°
 γ minimum value 15°

 Based on power factor reactive power absorption is approximately
Qdc = tan [arccos(φ)] x Pdc
 This gives reactive power as a function of Load i.e. changing active
power P of an HVDC station
 Reactive power demand of a converter is presented under three
different control methods
1. Udc=const, α/γ=const
2. Udc=const, Uv=const
3. Idc=const

 In order to meet the AC harmonic performance and exchange of
reactive power with AC system at desirable level, each filter has to
be switched in at a certain DC power transmission level

Q
Id
1.
.13
.5
Filter
Unbalance
Converter
 Reactive power balance at the connection point

 Another requirement imposed on reactive power control is that of
not exceeding a specified AC voltage step change as a consequence
of switching a filter bank (or any reactive power element)
 Magnitude of a voltage step change as a consequence of switching a
filter can be approximated as

 The AC/DC system interactions can be extremely impacted by the
strength of the AC network relative to the HVDC link capacity.
 The weakness of AC system can be due to its high impedance or its
low inertia.
 The strength of the AC/DC system can be measured by its short
circuit ratio (SCR) which is the ratio between the short circuit MVA
of the AC system (Ssc) compared to the DC converter MW rating
(Pdc)
 The SCR presents inherently the strength of the AC system.
Short Circuit Ratio

 SCR gives just the AC system strength taking into account the DC
transmission
 Index called effective short circuit ratio (ESCR) is introduced to
measure the AC system strength taking into account the effects of
the HVDC equipment connected to the AC side
 ESCR is ratio between the short circuit level reduced by the reactive
power of the shunt capacitor banks and AC filters connected to the
AC bus at 1.0 per-unit voltage and the rated DC power
Short Circuit Ratio

 Lower ESCR means more pronounced interaction between the HVDC
substation and the AC network.
 AC networks can be classified in the following categories according
to strength:
 strong systems with high ESCR: ESCR > 3.0
 systems of low ESCR: 3.0 > ESCR > 2.0
 weak systems with very low ESCR: ESCR < 2.0
 In the case of high ESCR systems, changes in the active/reactive
power from the HVDC substation lead to small or moderate AC
voltage changes.
 Therefore, the additional transient voltage control at the bus bar is
not normally required.
Short Circuit Ratio

 In the case of low and very low ESCR systems, the changes in the AC
network or in the HVDC transmission power could lead to voltage
oscillations and a need for special control strategies
 Dynamic reactive power control at the AC bus at or near the HVDC
substation by some form of power electronic reactive power
controller SVC or STATCOM may be necessary
 Connecting the HVDC to the weak AC systems will cause the
following problems in AC system
 High dynamic over-voltages that come from the excessive reactive
power at the HVDC terminals after the DC power being interrupted
followed by zero absorption of reactive power.
 Voltage instability which is associated to the loading sensitivities of
the HVDC link.
 Harmonic resonance due to the parallel resonance between AC
capacitor filters and the AC system at lower harmonic.
 Voltage flickers due to the continuous switching of shunt
capacitors and reactors causing unacceptable transient voltage
flickers.
Short Circuit Ratio

 Static VAR Compensators (SVCs), Static Compensators (STATCOMs) or
synchronous compensators, may also be used to ensure that the
desired reactive balance is maintained within specified limits under
defined operational conditions.
 More advanced solution consists of a STATCOM
 STATCOM provides
 Necessary commutation voltage to the HVDC converter
 Continuous AC voltage control
 Fast reactive power compensation to the network under transient
conditions
 Removal of possible non-characteristic harmonic interactions
ADVANCED REACTIVE POWER CONTROL

 In CCC scheme reactive power is compensated by the series
capacitors installed in series between the converter valves and the
converter transformer.
 Elimination of switched reactive power compensation simplify the
AC switchyard and reduces area required for CCC HVDC station.
 With VSC converters there is no need to compensate any reactive
power consumed by the converter itself and the current harmonics
on the AC side are related directly to the PWM frequency.
 Therefore the amount of filters in this type of converters is reduced
dramatically compared with natural commutated converters.
Reactive power at CCC HVDC

 Objective to establish the necessary sub-bank rating and switching
sequence to meet the reactive power control requirements of the
scheme.
 Establish HVDC converter absorption under all extremes of
operating condition tolerances
 From this converter absorption, the total reactive power required,
allowing for the appropriate tolerance conditions, is established.
 Established switch points which keep the net reactive power
interchange of the converter plus AC reactive power banks with the
AC systems within the established limits.
Reactive power study

 Sub-synchronous oscillation damping.
 A steam turbine and electric generator can have mechanical sub-
synchronous oscillation modes between the various turbine stages
and the generator.
 If such a generator feeds into the rectifier of a DC link,
supplementary control may be required on the DC link to ensure the
sub-synchronous oscillation modes of concern are positively
damped to limit torsional stresses on the turbine shaft.
Sub-Synchronous Torsional Interaction
(SSTI) factor

 In view of the semiconductor and microprocessor based control
systems included, modern HVDC links can be operated remotely.
 Moreover, modern HVDC systems are designed to operate
unmanned. There are some existing installations in operation
completely unmanned.
 This feature is particularly important in situations or countries
where skilled people are few, and these few people can operate
several HVDC links from one central location.
 In recent years VSC HVDC transmission has been introduced. Whilst
sharing some commonality with LCC HVDC in terms of the
asynchronous nature of the interconnection and benefits it can
bring to AC system, technologies differ in several ways.
Overview HVDC System Benefits

 Direction of power flow can be changed very quickly (bi-directionality).
 HVDC link doesn´t increase the short-circuit power in the connecting point
(no need to uprate existing switching equipment).
 The need for RoW is much smaller for HVDC than for HVAC, for the same
transmitted power. The environmental impact is smaller.
 VSC technology allows controlling active and reactive power independently
without needs for extra compensating equipment.
 VSC technology gives a good opportunity to alternative energy sources to
be economically and technically efficient.
 HVDC transmissions have a high availability and reliability rate, shown by
more than 30 years of operation.
 No limits in transmitted distance, for
OH lines, sea or underground cables.
 HVDC can carry more power for a
given size of conductor.
 Very fast control of power flow, which
implies stability improvements.

 HVDC has a lot of other value added compared to conventional
HVAC transmission.
 Interconnecting two AC systems using AC tie lines require
automatic generation controllers of both systems to be coordinated
using the tie line power and frequency.
 However, the interconnected AC systems with control coordination
are still subjected to some operational problems such as
 Large oscillations which may lead to equipment’s tripping.
 Faults level problem (High short circuit levels).
 Transmission of disturbances from one system to the other.
 Using the DC line as a tie line would eliminate most of the
mentioned problems.
 DC line is insensitive to the frequency and it would connect two
asynchronous systems and isolate the system disturbances
(“firewall”).

 Strong control system for HVDC lines will enhance the dynamic
performance of the AC systems in several aspects:
 Damping electromechanical AC system oscillations
 To enhance the transient stability in the AC system
 Control of frequency and reactive power oscillation.
STABILITY IMPROVEMENT
 Control system provides accurate and fast control of the active
power flow - to increase the transient stability of the AC system.
 After a specific disturbance, HVDC link can be controlled in a
manner such that the DC power can be ramped up and down quickly
to restore the balance between generation and load in both sides of
the AC system.
 In some situation ramping up the power is necessary to assist
system stability and this can be done by means of the short term
overloading capabilities of the HVDC link.
 Controlling the HVDC converters so as to provide reactive power
and voltage support, can be useful to augment transient stability.

DYNAMIC STABILIZATION OF AC SYSTEMS
 A power system is stable if and after any disturbance it returns to
condition of equilibrium.
 HVDC link will dynamically support the system by means of
alleviating instability problems such as power swinging after the
disturbances.
 HVDC will contribute in damping process of the system during and
after any disturbance by a small signal modulation of its
transmitted active power.
 This DC power modulation is proportional to the frequency
difference between the inter-tied systems.
 Fast modulation of DC transmission power can be used to damp
power oscillations in an AC grid and thus improve the system
stability.

 The Network Code on High Voltage Direct Current Connections (NC
HVDC) specifies requirements for long distance DC connections, links
between different synchronous areas and DC-connected Power Park
Modules, such as offshore wind farms, which are becoming
increasingly prominent in the European electricity system.
 This is a relatively new area in which fewer standards or grid codes
exist, making a pan-European approach particularly beneficial.
Following on from the Network Code on Requirements for Generators
and the Demand Connection Code, the NC HVDC is build on the same
foundations, to create a consistent and complete set of connection
codes.
 Latest Status Update – EC Regulation 2016/1447 (EU) 2016/1447
https://www.entsoe.eu/major-projects/network-code-development/high-voltage-di
rect-current/Pages/default.aspx
ENTSO-E HVDC NETWORK CODE

 Sets requirements for HVDC connections and offshore DC connected
generation (PPM – Power Park Modules)
 Set of coherent requirements for generators (of all sizes) in order to
meet the future power system challenges
 Set requirements for new demand users and DSO connections and
to outline demand side response requirements related to system
frequency
HVDC and DC connected PPM

 Frequency ranges
 stay connected to the Network and remain operable within the Frequency ranges
and time periods (minutes in table)
 capable of automatic disconnection at specified frequencies
 Rate-of-change-of-Frequency withstand capability up to a 2.5 Hz/s
 Active power controllability (control range and ramping rate)
 adjust the transmitted active power within the HVDC System maximum output
following an Instruction from the Relevant TSO
 modifying the transmitted active power in accordance with pre-defined regulation
sequences in case of Disturbance in one of the connecting AC Networks
 fast active power reversal from the Maximum Capacity in one direction to the
Maximum Capacity in the other
 provide FCR-Frequency Containment Reserve and FRR-Frequency Restoration
Reserve linking various Control Areas or Synchronous Areas
 adjust the ramping rate of active power variations in accordance with
instructions sent by the Relevant TSO
 take automatic remedial actions (stopping the ramping, blocking FSM, LFSM-O,
LFSM-U or Frequency control)
ACTIVE POWER CONTROL
Frequency Range Time period for operation
47.0 Hz – 47.5 Hz 30 minutes
47.5 Hz – 51.5 Hz Unlimited
51.5 Hz – 52.0 Hz 30 minutes

 Synthetic inertia
 rapidly adjusting the active power injected to or withdrawn from the AC
network in order to limit the rate of change of Frequency
 emulating Synchronous Generator Performance
 Frequency Sensitive Mode (FSM)
 respond to Frequency deviations in each connected AC network by
adjusting the active power transmission (diagram, ranges)
 The Frequency Response Dead-band of Frequency deviation and Droop
are selected by the Relevant TSO
AUTOMATIC REMEDIAL ACTIONS
Parameters Ranges
Frequency response
deadband
0-±500 mHz
Droop s1 (upward
regulation)
Minimum 0,1 %
Droop s2 (downward
regulation)
Minimum 0,1 %
Frequency response
insensitivity
Maximum 30 mHz

 Limited Frequency Sensitive Mode Over-
frequency (LFSM-O)
 adjust Active Power transmission to
the AC Network(s) according to
diagram at a Frequency threshold
50.2 - 50.5 Hz with a Droop having a
minimum value of 0.1 %
 At over-frequencies where f is above
∆
f1 the HVDC System has to reduce
∆
Active Power according to the Droop
setting
 Limited Frequency Sensitive Mode
Under-frequency (LFSM-U)
 adjust the Active Power Frequency
Response to the AC Network(s)
according to diagram at a Frequency
threshold 49.8 Hz - 49.5 Hz with a
Droop having a minimum value of
0.1 %.
 At under-frequencies where f is
∆
below f1 the HVDC System has to
∆
increase Active Power output
according to the Droop S
AUTOMATIC REMEDIAL ACTIONS

Voltage ranges
 stay connected to the Network and capable of operating at the maximum output
of HVDC Converter Station within the ranges of the Network Voltage at the
Connection Point
 Reactive power exchanged with the Network
 ensure that the reactive power of its HVDC Converter Station exchanged with
the Network at the Connection Point is limited to values defined
 Reactive Power variation shall not result in a Voltage step exceeding the allowed
value at the Connection Point - Maximum Voltage step shall be specified by the
Relevant TSO)
 Short circuit contribution during faults requirements
 Reactive Short Circuit Current contribution at the Connection Points
 Provide at least 2/3 of the rated reactive current
REACTIVE POWER CONTROL
Voltage Range Time period for operation
0.85 pu – 1.118 pu Unlimited
1.118 pu – 1.15 pu To be established by each relevant
system operator, in coordination
with the relevant TSO but not less
than 20 minutes
<300 kV
Voltage Range Time period for operation
0.85 pu – 1.05 pu Unlimited
1.05 pu – 1.0875 pu To be specified by each TSO, but not
less than 60 minutes
1.0875 pu – 1.10 pu 60 minutes
300-400 kV

 Reactive power capability
 Reactive Power capability requirements in the context of varying Voltage:
U-Q/Pmax-profile, within the boundary of which the HVDC Converter Station
shall be capable of providing Reactive Power at its Maximum Capacity
 Reactive power control mode
 as a minimum operate in any of the following reactive-power-control modes at
perform respective control utilizing its capability:
- Voltage Control mode
- Reactive-Power Control mode
- Power-Factor Control mode
REACTIVE POWER CONTROL

 Voltage against-time-profile at the Connection Point(s)
 fault conditions under which the HVDC Converter Station shall stay connected
to the Network and continue stable operation after the power system has
recovered following fault Clearance
Uret - retained Voltage at the Connection Point(s) during a fault
Tclear - duration of the fault,
Urec, and trec specify a point of lower limits of Voltage recovery following fault
clearance.
Ublc - blocking Voltage at the connection point at time Tblc,
FRT – Fault Ride Through

 Auto-reclosure
 Transient faults on HVAC lines in the Network adjacent or close to HVDC Systems
shall not cause any of the equipment in the HVDC System to disconnect from the
Network due to auto-reclosure of lines in the Network
 HVDC Systems with overhead lines shall be capable of auto-reclosing for
transient faults within the HVDC System
 Converter energization and synchronization
 shall smooth any voltage transients to a steady-state level not exceeding 3% of
the pre-synchronization AC Voltage
 Power oscillation damping capability
 Sub-synchronous torsional interaction damping capability
 contribute to electrical damping of torsional frequencies
 TSO shall define the necessary extent of SSTI studies
 Harmonics
 HVDC System robustness
 Reconnection - reconnect after an incidental disconnection due to a Network
disturbance, the Relevant TSO shall define the conditions under which an HVDC
System shall be capable of reconnecting to the Network
 Black start Capability
 Isolated network operation
OTHER REQUIREMENTS

 Models are needed for
 Interconnection study
 Grid planning strudy
 Operations security assessment
 Future system assessment
 Power flow studies (Steady State Studies)
 Active/Reactive power capability
 Voltage control settings
 Collector system
 Short circuit Studies
 Connection point adequacy evaluation
 Sizing of equipment and configuration
 Dynamic Studies
 Transient stability analyses (reactive power and voltage control)
 Frequency stability analyses (power-frequency control)
 Voltage stability analyses (reactive power and voltage control)
 Small signal Stability analyses (parameter settings and power oscillation
damping)
 Other studies (cannot or partialy performed using PSS/E)
 Harmonics analyses
 SSTI – Sub-synchronous Interaction (SSTI studies
MODEL REQUIREMENTS

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