Energy and Power Engineering, 2013, 5, 15-19
doi:10.4236/epe.2013.53B004 Published Online May 2013 (http://www.scirp.org/journal/epe)
Simulation Study of Dynamic Fault Recovery
Performance of VSC-HVDC System
Zi-Xia Cheng, Xiao-Feng Wang*, Xi-Bo Wu, Gang Du, Fei-Yan Li
School of Electric Engineering, Zhengzhou University, Zhengzhou, China
Received 2013
ABSTRACT
The fault recovery of VSC-HVDC transmission system is often influenced by many factors, such as the reactive power
compensation characteristics of the inverter and the dynamic performance of DC controllers. In this paper, the PSCAD/
EMTDC simulation tool is used to study the dynamic recovery performance of VSC-HVDC system for several different
var compensating devices in VSC-HVDC inverter-Fixed capacitor (FC), Static Var compensator (SVC), and Static
synchronous compensator (STATCOM) when VSC-HVDC is subject to various faults, including three phase
groundings , single phase grounding and three phase br eakings. The result shows that the recover y process of the whole
system will be slowed down due to its negative influence on the strength of AC power system with the application of
SVC, while the STATCOM can improve VSC-HVDC recovery performance greatly for its advantages over other
compensating devices in areas such as voltage support ability and DC power recov ery.
Keywords: VSC-HVDC; PSCAD/EMTDC; Fau lt Recovery; Dynamic Characteristics
1. Introduction
Because of the unique technical and economic advan-
tages, the high voltage direct current (HVDC) transmis-
sion technology has made a very wide range of applica-
tions in the long-distance and high-capacity regional net-
working transmission [1]. However, it has caused some
problems in the applications [2-6]. For example, when it
comes into being large disturbance, the interact- tion
among ac/dc systems generally causes dynamic states
mutation of the system, which causes many prob- lems,
such as transient overvoltage, unstable harmonic and
unstabl e vo l tage etc.
In recent years, with the development of power elec-
tronics technology, especially, the study for all-control-
ling power electronic devices (Gate Turn off Thyristor
and Insulated Gate Bipolar Transistor) has made signifi-
cant breakthrough. The new DC transmission technology
(VSC-HVDC) has emerged. Compared with the tradi-
tional HVDC, the current source inverter is replaced with
the voltage source inverter [7,8].
At present, VSC-HVDC has been studied in mathe-
matical model, control strategy and protection methods,
and so on [9-13]. Zhang Gui-Bin has studied the steady
state mathematical model for the VSC-HVDC system.
Based on it, the control strategy for th e VSC-HVDC sys-
tem is proposed by using the inverse steady state model
controller to trace the operating point and using the two
decoupled controlling loops to eliminate the steady state
deviations [10]. Chen Qian has designed a steady-state
controller based on dqo-axis. The performances of such
VSC-HVDC have been analyzed, and finally the feasibil-
ity and advantage have been verified by the simulation
results [11]. Based on them, the research for VSC-HVDC
has developed rapidly. But the operating characteristics
of VSC-HVDC especially in fault cases have not drawn
much attention and the kind of the situation is common
in the high voltage DC system. In this paper, VSC-
HVDC system model would be established with PSCAD/
EMTDC in th e first place [14 ]. Based on this, sev eral com-
mon faults could be simulated at the AC side of VSC-
HVDC system, and the dynamic recovery performance
of VSC-HVDC system for several different var compen-
sating devices in VSC-HVDC inv er ter-Fixed capacitor (FC),
Static Var compensator (SVC), and Static synchronous
compensator (STATCOM) would be studied. It is helpful
to take appropriate measures to reduce the influence and
harm caused by the fault, and provides some reference
for the further study.
2. The Basic Structure and Principle of TSC
TSC is the most common compensation equipment in the
reactive power compensation. It is evolved from the
fixed capacitance (FC), which belongs to the parallel
*Corresponding author.

Z.-X. CHENG ET AL.
16
compensation device, and it is also a branch of static var
compensator (SVC) [15 ].
Single-phase TSC is composed of the capacitor, bidi-
rectional thyristor and the current-limiting reactor with a
low impedance value; it is shown in Figure 1. TSC is a
kind of reactive power compensation device which made
use of the thyristor as a non-contact switch, and it can
fast and smoothly go into or cut off the compensation
capacitors based on the accurate trigger characteristic of
the thyristor. TSC can track the mutation of the impact
load and give the closed-loop feedback to the best power
factor quickly. It can realize the dynamic reactive power
compensation and reduce the voltage fluctuation, so as to
achieve the purpose of saving energy and reducing con-
sumption.
3. The Operation Principle of VSC- HVDC
Figure 2 shows the main circuit structure of the double
terminal VSC-HVDC transmission system [16,17]. The
main parts of the voltage source converter include: full-
bridge rectifier, DC capacitor, AC converter transformer
or converter reactor and ac filter. Three-phase two level
topology is used for the full-bridge rectifier and each
bridge arm consists of the multiple IGBTs, the DC ca-
pacitor is used for providing the voltage support and
buffering the impulse current when the bridge arm is
turned off, at the same time, it can reduce the harmonic
of DC side; The converter transformer or converter reac-
tor of AC side is the link of the energy exchange between
VSC and AC system, and also has the filtering effect;
The AC filter is used for filtering the harmonic of DC
side. The double terminal voltage source inverters are
connected by the DC transmission line, and one terminal
runs in the rectifier state, another end runs in the inverter
state, so as to realize the active power exchange between
the two ends.
VSC-HVDC is developed from the voltage source con-
verter technology and the fully controlled devices, such
as IGBT. Figure 3 shows the single-phase circuit of
Voltage Source Converter composed of the high fre-
quency switch device IGBT. The working principle is
that the trigger signal i produces from comparing the
work frequency sine signal c U with triangular carrier
signal . It is shown in Figure 4.
U
tri
I t can be seen from Figures 3 and 4, when 2
U

is
triggered, the output voltage is ; when
/2
od
UU 2

is triggered, the output voltage is . The
/2
od
UU 2

and are not triggered at the same time. The sine
voltage
2
s
u
u
in the ac bus, acquired by the converter re-
actor and filter to eliminate the higher harmonic compo-
nent of o, has the same waveform with c. The action
frequency of switch is detemined by tri , the phase and
amplitude of output voltage o
u is detemined by c.
When the phase of is changed, it will change the
magnitude and direction of the active power; When the
amplitude of c is changed, it will change magnitude
and polarity (inductive or capacitive) of the reactive
power. Therefore, voltage source converter can adjust the
active power and reactive power ind ividually [18].
u
uu
c
u
u
Figure 1. Principle and structure of TSC.
Figure 2. Double terminal VSC-HVDC transmission system.
Figure 3. Single-phase voltage source converter composed
of the IGBT.
Figure 4. Working principle of VSC.

Z.-X. CHENG ET AL. 17
4. The Modeling and Fault Simulation of
VSC-HVDC System
4.1. The Model of Simulation System
At first, the mathematical model of the VSC-HVDC can
be established in the PSCAD/EMTDC. It is shown in
Figure 5. The main circuit parameters are: the reference
voltage of the ac system is 115 kV; the rated capacity of
the transformer is S = 100 MVA and the ratio is 115 kV /
62.5 kV. The dc capacitor is 500 uF. The simulation test
system uses the back-to-back operation mode. The con-
troll mode of the fixed active power and constant reactive
power is used by the rectifier, and the control mode of
the fixed dc voltage and constant reacti ve power is adopted
by the inverter. Based on this, the dynamic recovery
characteristics of the different system faults were studied
which respectively use the SVC and STATCOM instead
of the FC in the ac system.
4.2. The Results and Analysis of the Simulation
4.2.1. Three Phase Grounding Fault
The three phase grounding belongs to a typical symmet-
ric fault. The fault occurred at 1 s and disappeared at 1.05
s. The fault point was set in the ac system side of inverter.
The blue represents FC; The red stands for SVC and the
green represents SC (the following figures are the same
to this).
Figures 6 and 7 show the dynamic recovery charac-
teristics of DC power and DC voltage under the three
phase grounding fault. It can be seen that the recovery of
the DC power and DC voltage is the slowest when SVC
is used for compensation. Because the TSC repeatedly
switch during and after the fault, the system will appear
the oscillation during the recovery. The recovery of the
system is the fastest when SC is used. That is because SC
emits and absorbs reactive power by changing the volt-
age and current waveform of VSC besides it don’t need
the capacitor group and shunt reactor, and there is no
shortcoming of the SVC compensation runtime. The
main advantage is that SC not relies on the system volt-
age when it emits the capacitive reactive current, and the
ability is suitable for the occasion that the system needs
to support voltage during and after the fault especially
[19].
4.2.2. Si ng le Phase Gr ou nd ing Fault
The single phase grounding fault is a common fault in
the ac system, and also is a kind of typical asymmetric
fault. The fault occurred at 1 s and disappeared at 1.05 s.
The fault point was set in the ac system side of inverter.
Figures 8 and 9 show the dynamic recovery charac-
teristics of DC power and DC voltage under the single
phase grounding fault. They are similar with Figures 6
and 7. Among the different var compensating devices,
the recovery of the system is the fastest when SC is used,
while the recovery of the dc power and dc voltage is the
slowest when SVC is used for compensation, but the
overall the rate of recovery increases.
4.2.3. Three Phase Breaking Fault
Three-phase breaking fault is not a common fault in
practical engineering, but once it appears, it will be seri-
ously harmful to the system. The fault occurred at 2 s and
disappeared at 2.05 s.
Figure 5. Model of the VSC-HVDC.
Figure 6. Dynamic recovery characteristics of DC power
under the three phase grounding fault.
Figure 7. Dynamic recovery characteristics of DC voltage
under the three phase grounding fault.

Z.-X. CHENG ET AL.
18
Figure 8. Dynamic recovery characteristics of DC power
under the single phase grounding fault.
Figure 9. Dynamic recovery characteristics of DC voltage
under the single phase grounding fault.
Figures 10 and 11 show the dynamic recovery charac ter-
istics of DC power and DC voltage under the three phase
breaking fault. When FC is used for compensation, the
recovery of the dc power and dc voltage is the slowest
and when SVC is used, the system will appear a certain
overvoltage, but the system recovered faster from the
fault because SVC has the function of the voltage
regulation; When SC is used, it only will appear a very
small disturbance and the recovery of the system is the
fastest.
5. Conclusions
This paper analyzed the VSC - HVDC system recovery
characteristics through the power system simulation
analysis software PSCAD/EMTDC. The following con-
clusions can be drawn:
 When SVC is used for compensation, it will lead to
the ac system strength dropping further because of
the structure, and make the recovery characteristics
of the system worsen.
Figure 10. Dynamic recovery characteristics of DC power
under the three phase bre aking fault.
Figure 11. Dynamic recovery characteristics of DC voltage
under the three phase bre aking fault.
 When SC is used for compensation, it will increase
the short-circuit capacity of ac system. It not only
can provide the necessary reactive power, but also
makes the system recover from the fault rapidly.
 SVC and SC can rapidly and efficiently restrain the
overvoltage under the three phase breaking fault,
while FC has no the voltag e-control function, so the
capacitor and filter must be rapidly resected, in or-
der to avoid ap- pearing a higher overvoltage.
 Whether the suppression level of the transient
overvoltage or the recovery characteristics of the dc
power in the three kinds of reactive compensation
modes, we can conclude that SC has more obvious
advantages than other two compensations.
6. Acknowledgements
This work was financially supported by the Foundation
of Henan Educational Committee of China (12A470008).

Z.-X. CHENG ET AL.
19
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