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CIGRE 2012
CONTROL AND PROTECTION STRATEGIES FOR MULTI-TERMINAL HVDC
TRANSMISSION SYSTEMS BASED ON VOLTAGE SOURCE CONVERTERS
Chengyong Zhao*, Jing Hu, Liu Yang, Xiaodong Yang
State Key Laboratory for Alternate Electrical Power System with Renewable Energy
Sources (North China Electric Power University),
Beijing, 102206, P.R. China
SUMMARY
Multi-terminal HVDC transmission system (MTDC) has better efficiency and flexibility than
conventional two-terminal HVDC system, with many potential applications, such as
distributed generation, wind power generation, city centre feeds, etc. So far, the MTDC
projects which have already been put into operation are all based on current source converters
(CSC-MTDC). However, MTDC systems based on voltage source converters (VSC-MTDC)
are still in theoretical research. VSC–HVDC system has many advantages. For example, it
can feed power into passive network due to adoption of full-controllable power electronic
devices; DC voltage polarity do not need to be changed when reversing the real power flow; it
enables fast and independent control for active and reactive power. Therefore, VSC-MTDC,
which consists of multi- VSC converters, is a very promising structure for multi-terminal
HVDC system in future.
VSC-MTDC system has the ability to operate more flexibly and economically, compared with
two-terminal VSC-HVDC system, however it requires more complex strategy for control and
protection. At present, most of the literatures about VSC-MTDC system mainly focus on the
system architecture of VSC-MTDC, control schemes and its power flow calculations. While,
the researches on the VSC-MTDC protection are not investigated comprehensively. As for the
two-terminal HVDC system, it will probably block when a serious fault occurs at DC side or
AC side. However, the MTDC system will react in a better way, compared to the twoterminal system. When a fault happens, the faulted line or device needs to be located and
isolated rapidly, to ensure that the remaining DC network resume power supply service.
Firstly, the model of three-terminal DC system based on VSC is presented in this paper.
Based on this model, the operating principles of single-point DC voltage and multi-point DC
voltage control strategy for VSC-MTDC are analyzed. The one with multi-point DC voltage
control scheme can improve DC voltage performance, increase stability and efficiency, and
make master controller more valuable. Based on the dq0 reference frame, the corresponding
controllers for different operation modes of VSC-MTDC system are designed.
* North China Electric Power University, 2 Beinong Road, Beijing, China
e-mail: chengyongzhao@ncepu.edu.cn
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Secondly, the mechanisms and features under AC system faults in multi-terminal VSCHVDC system are illustrated. Taking the more common single-phase to ground fault as an
example, the dynamic performances under AC faults at receiving terminal and voltage
balance terminal are studied respectively in a three-terminal VSC-HVDC system without any
protection actions, by full electromagnetic transient simulation in PSCAD/EMTDC. The
simulation results show that, if small disturbances occur at constant power control terminal,
multi-terminal VSC-HVDC system can suppress over-voltage and over-current by controllers
and will not significantly impact the other two terminals.
Thirdly, two protection strategies under AC system faults in VSC-MTDC system are
proposed. One is soft shut-off control, which has the self-protection function and can act as
the spinning reserve power source. The other one is control mode switch, which combines the
multi-point DC voltage control and soft shut-off control together. The simulation results show
that the proposed protection strategies can improve the system performances under different
faults.
Finally, the complexity of control and protection strategies for VSC-MTDC system is
presented. The control and protection strategies in the paper are primary researches for multiterminal VSC-HVDC system. Thus, more detailed control and protection strategies under
different operation modes need to be addressed and investigated deeply in future.
KEYWORDS
Multi-terminal VSC-HVDC (VSC-MTDC), Control and protection strategy, AC system faults
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1. INTRODUCTION
Multi-terminal HVDC transmission systems (MTDC), with better efficiency and flexibility than
conventional two-terminal HVDC systems, whose potential applications are: distributed generation,
wind power generation and city center feeds[1,2]. So far, the MTDC projects which have already been
put into commercial operation are all based on current source converters (CSC-MTDC). MTDC
systems based on voltage source converters (VSC-MTDC) are still in theoretical research. VSC has a
lot of merits as[3]: VSC–HVDC system can feed power into passive networks with no local power
generation; voltage polarity of DC voltage is unaltered when system power flow reverses; it enables
fast control of active and reactive power independent of each other. Therefore, VSC-MTDC is a very
appropriate structure for multi-terminal HVDC system.
VSC-MTDC system presents more economical and flexible operation than two-terminal system,
however, it also faces more complex situation in control and protection. With two converters at both
ends and DC line in between, two-terminal HVDC system blocks as a complete part of power system
when a permanent fault occurs on it. Compared to two-terminal systems, MTDC system reacts in a
different way. When a fault happens, the fault needs to be located and isolated at once, so that the
remainder of the DC network can resume service[4,5]. In this paper, control and protection strategies
dealing with AC system faults in VSC-MTDC Systems are proposed. The effectiveness and feasibility
of the strategies are verified by simulation.
2. VSC-MTDC CONTROL SCHEMES AND CONTROLLER DESIGN
2.1 Mathematical Model of Three-Terminal DC System Based on VSC
The model of three-terminal DC system based on VSC is shown in Fig. 1.
E1a
L1
E1b
L1
E1c
L1
Pc1,Qc1
R1 i1a V
1a
Pd1
Id1
Ld1
I1
Pd2
Id2
Ld2
I2 Rd2
2C
2C
R1 i1b V
1b
R1 i1c
Rd1
V1
Vd1
2C Rd1
V2
Ld1
Vd2
Ld2
Rd2
L2
E2a
V2b i2b
R2
L2
E2b
V2c i2c
R2
L2
E2c
R3
L3
E3a
R3
V3b i3b
L3
E3b
R3
L3
E3c
2C
V1c
VSC2
VSC1
Ps1,Qs1
Pc2,Qc2
V2a i2a R2
I3
Rd3
Ld3
V3a i3a
2C
V3
Vd3
Ld3 2C
Rd3
Ps2,Qs2
Pc3,Qc3
Pd3
Id3
V3c i3c
VSC3
Ps3,Qs3
Figure 1. Model of VSC-MTDC
The model of the AC side in the VSC based HVDC system in abc coordinates can be expressed as the
following formulars:
Ln
dI abcn
  Rn I abcn  U abcn  Vabcn 
dt
(1)
Where, I abcn  ian , ibn , icn  ; U abcn  usan , usbn , uscn T ; Vabcn  Van ,Vbn ,Vcn T ; n=(1,2,3). Transform (1) into dq
coordinates:
T
d idn  1   Rn
 

dt iqn  Ln  Ln
1  E  usqn 

  

 Rn  iqn  Ln  usqn 
 Ln  idn 
(2)
Where n=(1,2,3). Pcn is the active power flowing from AC side into VSC and Pdn is the active power
flowing from VSC into DC side, they can be expressed in dq coordinates as follows:
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pcn  Vn  idn cos  n  iqn sin  n 
2
(3) ; pdn  udn  I dn
(4)
Ignor the losses of the VSC converter, Pcn is equal to Pdn
3
3
Vn  idn cos  n  iqn sin  n   udn  I dn
2
(5)
Equations (6)~(9) signify the relationship among the DC sides of the VSC-based three-terminal system:
Vn  Vdn  Ldn
dI n
 I n Rdn
dt
(6); V1  V2  V3 (7); I n  I dn  C
dVdn
dt
(8); I1  I 2  I 3  0 (9)
Equation (2), (5), (6), (7) (8) and (9)are the mathematical expressions of the three-terminal DC system
based on VSC.
2.2 Control Strategy of VSC-MTDC
The VSC based multi-terminal DC system has merits over the two-terminal HVDC system in the
aspects of control flexibility, reliability and economy. The converter station can be equivalent to a
controlled current source in the MTDC system, and the station which is operated by DC voltage plays
a role of active power balanced node.
As illustrated in Figure 2(a), when the MTDC system works, station 1 transmits power to station 2 and
3. With large adjustable range of capacity, station 1 operates with the constant DC voltage control.
Station 2 and 3 which are connected with active networks operate with constant active power control.
When the three-terminal HVDC system adopts single-point DC voltage control only, the system may
lose the ability to adjust the DC voltage and not function properly if station 1 blocks because of fault
or limit of capacity intrusion. It means that in the three-terminal HVDC system at least two converter
stations should be able to control the DC voltage.
When the three-terminal DC transmission system operates with the multi-point DC voltage control
method, DC voltage control method is adopted in Station 1 and Station 2. When Station 1 is out of
operation due to the fault, the power of DC network can not be kept balance. The input power of the
DC network falls down and the DC voltage drops, so the system operating point is changed. Since
Station 2 adopts the DC voltage error controller, its control mode changes from DC voltage control to
active power control, in order to ensure that the system can maintain the balance of active power and
finally achieve a new stable DC voltage at the new operating point. The situation is shown in Figure
2(b).
(a) Single DC Voltage Control Method
(b) Multi-Point DC Voltage Control Method
Figure 2. The Output Power Characteristics of Three-Terminal HVDC System
2.3 Controller Design of Three-Terminal DC System
The controller structure and system characteristics of the three-terminal DC system is different from
that of the two-terminal DC system. A high-performance station controller should be designed to make
sure that the multi-terminal HVDC system can operate stably.
The station controller of the three-terminal DC system adopts direct current control, which has fast
current response and good current limiting capability.
Stable DC voltage is essential for stable operation of the three-terminal DC system. Therefore,
constant DC voltage control must be used at one terminal and the appropriate control methods used at
the other two terminals according to the type of AC networks being connected.
2.3.1 Constant DC voltage controller design of three-terminal DC system
For the three-terminal DC transmission system, the converter station with constant DC voltage control
acts as an active power balanced node to maintain the operating reliability. In the three-terminal
HVDC system model, it can be seen that the active current is proportional to the active power of
transmission system. The DC voltage controller is shown in Figure 3.
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2.3.2 The receiving end controller design of three-terminal DC system
In the VSC based DC system, the receiving end converter always adopts constant active power control
and constant reactive power control, or constant AC voltage control when it is connected to the active
network；when the receiving end converter supplies power to the passive network or weak AC system,
constant AC voltage controller with constant active power control will provide a stable AC voltage.
The controller shown in Figure 4 is designed, which can be used in both cases above.
L
L
Figure 3. Constant DC Voltage Controller
uc1aref
uc1bref
uc1cref
L
L
uc 2 aref
uc 2bref
uc 2 cref
Figure 4. Receiving End Controller
2.3.3 A multi-point DC voltage control strategy based on DC voltage error method
The controller design of multi-point DC voltage control is shown in Figure 5. After using this
controller in three-terminal VSC-HVDC converter station, the DC voltage will be increased or
decreased when the previous constant
DC voltage controlled converter station
is out of operating because of faults. The
station using this controller will detect
uc 2 aref
the system DC voltage at any time, and
uc 2bref
change its control mode from constant
uc 2 cref
active power control to constant DC
voltage control if the DC voltage is out
of the limit, so that the system can keep
running.
Figure 5. Multi-Point DC Voltage Controller
3. VSC-MTDC PROTECTION STRATEGIES FOR AC SYSTEM FAULTS
For small disturbance or non-serious temporary fault, VSC-HVDC system can restrain overvoltage or
overcurrent by control. For serious temporary fault or permanent fault, the protection device operates,
and then VSC is blocked and stopped. After the faulted VSC terminal stopped, the control modes at
remainder terminals of VSC-MTDC need to be switched.
3.1 Types and Features of AC System Faults
Converter of VSC-HVDC connects with AC system through a connection transformer, which transfers
active power and reactive power from AC grid to HVDC system. Hence, the power exchange between
converter and AC system, as well as the operating performance of VSC-HVDC system, will be
influenced by AC system faults. VSC-HVDC system should be running chronically and reliably,
which means that the devices of itself shall be not compromised by various faults in AC system, and
provide timely support for faulted AC system on occasion.
There are various types of faults in AC system linked to HVDC: AC line faults; overvoltage caused by
lightning, load shedding or fault clearing; undervoltage caused by generator tripping and reactive
power compensation device breakdown. The operation experience of the power system shows that
single phase ground fault accounts for a bigger chunk than other AC faults, so this paper takes single
phase ground fault as a typical AC fault to analyse protection strategies of AC faults for multi-terminal
VSC-HVDC.
A three-terminal VSC-HVDC system studied is shown in Figure 6, and a solid single phase to ground
fault happens at the point F1, on the AC bus next to primary side of transformer. The discharging path
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of the fault current is illustrated by the dotted lines through the ground from the neutral point of the
DC-link capacitors to the fault point. The DC-link capacitors discharge through the loop rapidly,
which brings about the over-current through valves that could damage IGBTs and diodes. Further
more, the loop provides a path for the zero sequence currents, causing a large current flow on the
terminal bus. Fortunately, the zero-sequence component caused by the unsymmetrical faults can be
prevented from pouring into the internal AC line at converter side by using transformer with star/delta
(Y/Δ) connection form. In addition, the negative sequence component generates the fluctuating
voltages and currents at twice of the AC fundamental frequency on the DC side.
Figure 6. Sketch Map of AC System Faults in a Three-Terminal VSC-HVDC
3.2 Simulation Research under Faults Conditions
3.2.1 Modeling of three-terminal VSC-HVDC
The model of VSC-MTDC connected three active AC networks shown in Figure 6 is established by
PSCAD/EMTDC simulation tool. To simplify the modeling, ideal voltage source and inductive
impedance are adopted to simulate the AC systems, and the primary parameters of rectifier and
inverter are designed. The base voltage of system and the transformer ratio are the same in system 1
and 2: 420 kV and 420/230 kV, the ones of system 3 are 500 kV and 500/230kV. The commutation
inductance is 72.40 mH, the value of two DC capacitors is 300 μF respectively. T line model is
adopted for DC transmission line. The VSC 1 operates in the constant DC voltage and constant AC
voltage mode, and the DC voltage is set to 400 kV. The VSC 2 and VSC 3 operate in the constant
active power and constant AC voltage mode, the active power setting is 200 MW in VSC 2 and 200MW in VSC 3.
3.2.2 Single-phase ground fault at receiving terminal
A solid single-phase to ground fault takes place at 5.5 s on AC bus of line side connected to VSC 3, at
the inverter station, as the shown in Fig. 6. And the fault lasts for 1.0 s. Without any protection
operations, the dynamic performances of three terminal after fault happening are shown as Figure 7.
(a) DC Voltage of VSC 1
(b) Active and Reactive Power of VSC 2
6
(c) Phase Voltage of VSC 3
3.0
(d) DC Voltage of VSC 3
VSC3 current
2.0
1.0
0.0
-1.0
-2.0
-3.0
5.450
5.500
5.550
5.600
5.650
Time (s)
(e) AC Current of VSC 3
(f) Active and Reactive Power of VSC 3
Figure 7. Simulation Waveforms of A-Phase to Ground Fault at VSC 3 Terminal
Simulation results show that a single-phase to ground AC fault at one terminal with constant power
control in the VSC-MTDC system will not significantly impact on the other terminals. The DC voltage
can be controlled well in VSC 1 after the fault occurs. However, when the fault is cleared, about 1.25
p.u. overvoltage appears which may damage dc devices, as is shown in Fig. 7(a). The active power and
reactive power of VSC 2 is able to remain at pre-fault state (in Fig.7 (b)). Along with fault-phase
voltage falling to zero, the other two non-fault-phase voltages increase a little (shown as Figure 7(c)).
And the dc voltage of VSC 3 can remain at the primary value basically (in Fig.7 (d)). The serious issue
is VSC 3 ac current, as shown in Fig. 7(e), which will cause overcurrent exceeding the protection
limitation and endangering the converters. Because of the falling of ac line voltage RMS, VSC provide
amounts of reactive power to AC system and absorb active power from system simultaneously, and
recover after fault clearing, as shown in Fig. 7(f).
3.2.3 Single-phase ground fault at voltage balance terminal
In the sumulation, a solid single-phase to ground fault takes place at 5.5 s on AC bus of line side
connected to VSC 1, at the voltage balance station. And the fault lasts for 1.0 s, the dynamic
performances of faulted end without any protection operations are shown in Figure 9(a)(c)(e). Because
VSC 1 is the only dc voltage balance station, the DC voltage of the three-terminal system can not be
fixed any longer.
3.3 Protection Strategies
3.3.1 Soft shut-off
When a AC short-circuit fault at AC grid side of converter transformer happens, though no serious
adverse effects onto the other two terminals, it brings about overcurrent at AC side and fault clearing
overvoltage at DC side. To avoid damage to devices in converter station, the normal actions are
blocking the IGBTs of converter at dozens of microseconds after fault, and then tripping the breaker of
AC side about 100ms later. But this method may have some terrible subsequences, such as overcurrent
beyond the blocking setting. A kind of protection strategy, soft shut-off control of VSC-HVDC, is
considered to be suitable for MTDC system, which hase the function of self-protection and can make
MTDC system act as the spinning reserve electric power source.
A simulation is taken for AC fault at receiving terminal as referred to at 3.3.2, and the two protection
methods, the blocking one and soft shut-off one, are adopted in the simulation respectively. We name
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them Method 1 and Method 2 simply. The protection effects are compared by simulation as shown in
Figure 8.
(b) DC Voltage of VSC 1 using Method 2
(c) AC Current of VSC 3 using Method 1
(d) AC Current of VSC 3 using Method 2
P (MW), Q (Mvar)
(a) DC Voltage of VSC 1 using Method 1
(e) P and Q of VSC 3 using Method 1
(f) P and Q of VSC 3 using Method 2
Figure 8. Simulation Waveforms of Soft Shut-off Strategy
 Method 1 : blocking at 5.52 s, triping at 5.60 s, shown in Fig.8 (a)(c)(e).
 Method 2 : gradually reducing the reference values of reactive power and AC voltage to zero
from 5.510 s to 5.526 s, triping at 5.60 s, shown in Fig.8 (b)(d)(f).
From Fig.8, it is obvious that Method 2 gains better current and voltage response during the fault, and
the remaining two-terminal system can operate more steadily by the Method 2 after the transient sleep
of faulted end.
3.3.2 Control mode switch
To deal with the problem intruded in chapter 3.2.3, a multi-point controller shown in Figure 5 is
applied in both VSC1 and VSC 2 stations. When the voltage balance station VSC 1 suffers an ac fault,
a dc voltage detector make a judgement whether the dc voltage exceeds the limit, then the control
mode switch starts. Control mode of VSC 1 is switched from constant DC control to constant active
power control which the reference power should set to zero, in the same time, control mode of VSC 2
is switched from constant active power control to constant DC control which determine a new voltage
balance operation point of system.
The simulation results in Figure 9 show that a new DC voltage level is established and fixed steadily
(Fig9(b)). Moreover, the overcurrent due to ac fault is reduced(Fig9(d)) and power fluction of AC
system due to fault clearing is eliminated(Fig9(f)).
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(a) DC Voltage of VSC 1 without protection
(b) DC Voltage of VSC 1 with protection
VSC1 current
Current (kA)
4.0
3.0
2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
5.00
(c) AC Current of VSC 1 without protection
(e) P and Q of VSC 1 without protection
5.50
6.00
6.50
Time (s)
7.00
7.50
8.00
(d) AC Current of VSC 1 with protection
(f) P and Q of VSC 1 with protection
Figure 9. Simulation Waveforms of Control Mode Switch Strategy
4. CONCLUSION
The various AC systems and different faults make the control and protection strategies of VSC-MTDC
system very complex. The control strategy as muti-point voltage and protection strategy as soft shutoff for multi-terminal system presented in the paper are all the basic strategies, and more complete
strategies in various kinds of conditions should be researched in the future work.
5. BIBLIOGRAPHY
[1]
[2]
[3]
[4]
[5]
Zhao Wanjun, “HVDC transmission engineering recbnology” (China Electric Power Press,
Beijing, China, Auguest 2004).
Jiebei Zhu, Campbell Booth. “Future multi-terminal HVDC transmission systems using Voltage
source converters” (UPEC 2010, pages 1-6).
Chunyi, G., Z. Chengyong. “Supply of an Entirely Passive AC Network Through a DoubleInfeed HVDC System” (IEEE transactions on Power Electronics, November 2010, pages 28352841).
Lianxiang Tang and Boon-Teck Ooi. “Locating And Isolating DC Fauits in Muiti-terminal DC
Systems Protection of VSC Multi-terminal HVDC Against DC Faults” (IEEE Trasactions on
Power Delivery, July 2007, pages 1877-1884).
Jin Yang, John E. Fletcher, John O’Reilly. “Multi-terminal DC wind farm collection and
transmission system internal fault analysis” (IEEE 2010, pages 2437-2442).
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