Energy and Power Engineering, 2013, 5, 1230-1234
doi:10.4236/epe.2013.54B233 Published Online July 2013 (http://www.scirp.org/journal/epe)
Influences of ±800 kV Yunnan-Guangdong HVDC System
on Security and Stability of China Southern Power Grid
Baorong Zhou1, Xia om in g Jin1, Chao Hong1, Pei Zhang2,Wu Zhigang3
1Electric Power Research Institute of China Southern Power Grid, Guangzhou, China
2Tianjin University, Tianjin, China
3South China University of Technology, Guangzhou, China
Received April, 2013
ABSTRACT
The interaction mechanism between AC and DC systems in a hybrid AC-DC transmission grid is discussed with PSS/E
software. Analysis shows that receiving-end AC faults may cause much more damage on the HVDC system operation
than the sending-end AC faults in a multi-infeed HVDC system, and the damage severity depends on the power recov-
ering rate of the HVDC systems. For HVDC systems with slow power recovering rate, the receiving-end AC faults may
probably be a critical factor to constrain power transfer limits. Larger capacity of HVDC system means not only h igher
power transfer-limit of th e parallel connected AC-DC transmission grid, but also more expensive stabilizing cost.
Keywords: HVDC; Security; Stability; AC/DC; Power System; Interaction
1. Introduction
Yunnan-Guangdong HVDC (YG HVDC) with 5GW
capacity is the first ±800kV ultra HVDC system in the
world. Its capacity is equal to about two thirds of total
Yunnan export power in 2010, or one third of total Yun-
nan grid load in 2010, or one quarter of CSG west to east
tie-line scheduled capacity in 2010 . Thus it has important
influences on the stability o f China Southern Power Grid
(CSG). This paper focuses on the mutual influences of
YG HVDC and the AC systems based on 2010 CSG op-
eration mode [1-8].
Mutual influences of HVDC and AC systems include
the following two aspects:
(1) AC system faults effects on HVDC power trans-
fer capability and thus degrades system stability.
(2) Huge power redispatches from HVDC system to
AC transmission network caused by HVDC system faults
and degrade system stability further.
Therefore, the kernel of this kind of study is to probe
mutual effect of HVDC and AC system in a hybrid
HVDC and AC parallel connected system. In this paper,
related techniques in PSS/E is introduced in section 2,
and then both the two mentioned stability characteristics
of CSG is investigated by PSS/E software in detail in
section 3 and 4. Finally the paper concludes in section 5.
2. Simulation Model
The Siemens PTI PSS/E software package is widely rec-
ognized as one of the best commercial programs avail-
able for power systems analysis. The most prominent
characteristics of CSG power system is that five HVDCs
including YG HVDC parallel connected with several AC
transmission lines indeed into Guangdo ng power system,
which is shown in Figure 1. Therefore, the H V D C model
is the most important part to simulate dynamics of CSG
accurately.
Both response model and detailed model of HVDC are
built in PSS/E software. The response models of HVDC
such as CDC6T suppose that HVDC has fast response,
and thus its dominant transient dynamics in DC circuit
and DC control system could be neglected. In addition,
steady equation of converter is used for response model
of HVDC to calculate real and reactive power injecting
into AC system. The PSS/E detailed models of HVDC
such as CASEA1 and CDCRL consider transient dy-
namics of DC circuit and DC valve control system; it is
Figure 1. CSG grid structure framework in 2010.

B. R. ZHOU ET AL. 1231
solved with much smaller integration steps compared to
AC system. Since the response model of HVDC in
PSS/E is simple and flexible and can simulate various
dynamics of HVDC conveniently and simply, it is easy
to implement sensitivity analysis about the effect of DC
system dynamic characteristics on stability and security
of AC system. Therefore, CDC6T response model of
HVDC in PSS/E is used in this paper, which is shown in
Figure 2[13]. HVDC control structure consists of substa-
tion control, polar control and valve control with differ-
ent function respectively. Substation control can realize
reactive power control and DC control; valve control can
produce trigger pulse control; and polar control is just in
between the previous two controls and it is the key con-
trol part to produce DC system voltage and current order
and can transfer it into trigger angle order to valve con-
trol. The polar control rectifier side has fixed DC current
control, fixed DC voltage control and minimum trigger
angle control, the inverter side has fixed DC voltage con-
trol, fixed DC current control, fixed extinction angle
control and current difference control. Under normal
operation, the rectifier side employs fixed current control;
the inverter side utilizes fixed DC voltage control. In
transient process, various control variables are compared
in DC rectifier side, and control switch is based on min-
imum priority rules; various control variables are com-
pared in the inverter side, and control switch is based on
maximum priority rules.
Restoring rate of HVDC power has great impacts on
multi-infeed HVDC system. The following parameters
can be set to simulate various restoring rate of HVDC in
PSS/E software, all of them can be found in Figure 2.
Tvdc and Tidc: Measurement time constant of voltage
and current;
Vramp and Cramp: Ramping rate after communication
failure of v o l t age and cu r r ent.
According to response curves of HVDC simulated by
BPA software and actual curves simulated by RTDS for
dynamic performance test of Guizhou-Guangdong II
HVDC, Tvdc and Tidc are set as 0 .02 second, and Vramp
and Cramp are set as 8 p.u./s for all HVDC in CSG,
which is abbreviated as paramater A.
Figure 2. Diagram of DC model CDC6T.
BPA software is widely employed by CSG planning
and operation engineers to simulate dynamics in the sys-
tem, and BPA format data for 2010 plan ning CSG power
system are built. All the devices of CSG power system
are converted from BPA model into PSS/E model ac-
cording to the same mathematics model. Computation
result shows that power flow obtained with PSS/E agrees
with the counterpart from BPA, and the difference of real
power through each branch is no more than 1 MW and
the difference of voltage at each bus is no more than 1
kV. Moreover, transient stability results from PSS/E are
also consistent with BPA.
3. Influences of YG HVDC Faults on AC
System
Fault on each HVDC pole may redispatch the power
from DC transmission lines to AC transmission lines in
hybrid AC-DC transmission systems, while larger capac-
ity of HVDC means more power flow is redispatched to
AC transmission system from the blocked pole of HVDC,
and then more reactive power will be required to support
the requirement. Under such case, an AC transmission
system may lose stability if dynamic VAR compensatio n
is not enough to support voltage magnitude around its
nominal rating. Effects of YG HVDC block patterns on
system stability are analyzed as following [9-14].
3.1. Influences of YG HVDC Mono-polar Block
on System Stability
In hybrid AC-DC transmission systems, HVDCs with
small capacity have little effects on system stability if
mono-polar block happens. With HVDC capacity in-
creasing, mono-polar block has much more impacts on
system stability and will develop into a critical fault that
restricts the power transfer limit. However, HVDC
mono-polar block becoming as a critical fault does not
mean that power transfer limit will decrease with HVDC
capacity increasing, the larger capacity of HVDC may
also bring higher power transfer limit. Computation re-
sult in CSG 2010 power system shows that Yunnan
power transfer limits are 8100 MW, 9550 MW and
10700 MW when YG HVDC transmits 2500 MW,3750
MW and 5000 MW respectively, which sh ows that larger
capacity of YG HVDC means more power transfer limit
of Yunnan power grid.
The above results can be explained as follows. In a
hybrid AC-DC system, AC transmission system must
reserve some power transfer capability to carry on the
power flow redispatched from blocked pole of YG
HVDC. However, the required reserved power transfer
capability of AC transmission system will always be less
than the capacity of blocked pole of YG HVDC, so total
transfer capability of the hybrid AC-DC transmission

B. R. ZHOU ET AL.
1232
system is still increased when the capacity of both pole
of YG HVDC increased.
3.2. Influences of YG HVDC Bipolar Block on
System Stability
In peak load period during 2010 summer season, any
bipolar block except YG HVDC will not destabilize the
CSG system and repeated restart attempts are allowed to
reduce their forced outage times remarkably. Bipolar
block of YG HVDC will destabilize CSG system and
some generators are required to be tripped to keep system
stable. The amount of tripping generators after YG
HVDC bipolar block will increase noticeably with export
power of Y u nnan AC system increasing.
When Yunnan exporting 7800 MW scheduled power,
2125 MW hydro units in Yunnan power system will be
tripped in 200 milliseconds after an YG HVDC bipolar
block happens. If one restart attempt is considered after
the last blocked pole, 430 MW additional hydro units in
Yunnan power system need to be tripped. If two restart
Table 1. Effect on Yunnan output power limit by different
operation pattern of YG HVDC.
Operation Pattern Constrained Fault Yunnan Output
Limit（MW）
YG HVDC operation with
bi-polar trans fer 5000MW YG HVDC mono-polar block 10700
YG HVDC operation with
bi-polar trans fer 3750MW YG HVDC mono-polar block 9550
YG HVDC operation with
mono-polar transfer
2500MW
YG HVDC
mono-polar block 8100
Table 2. Stability analysis of YG HVDC blocks and restart
pattern if Yunnan transfer 7800 MW out in 2010.
Order Fault Pattern Stability and
corresponding action
1 YG HVDC bi-polar
lock System stable af ter generation shedding 2125MW
2 YG HVDC mono-polar block, another p olar DC
line fault restart success Stable
3 YG HVDC mono-polar
block, another po lar DC
line fault restart failure
System stable af ter generation
shedding 2555MW
4 YG HVDC mono-polar
block, another polar DC line
fault second-restart success Stable
5 YG HVDC mono-polar block, another polar DC line
fault second-restart failure
Generation shedding4225MW,
and split Yunnan northwestern
area grid to stabilize the whole
system
Table 3. Stability analysis of YG HVDC blocks and restart
pattern when Yunnan output 10700MW.
Order Fault Pattern Generation
shedding(MW)
1 HVDC bi-polar block System stable after genera-
tion shedding 6525 MW
2 HVDC mono-polar
block, the other polar DC
line fault restart failure
System unstable after split-
ting northwestern area of
Yunnan and generati o n
shedding 6525MW
attempts are considered after last blocked pole, 4225
MW hydro units need to be tripped and western Yunnan
regional power grid is disconnected with the main Yun-
nan power grid after restart attempt fails.
When power export of Yunnan power grid reaches the
power transfer limit 10700 MW, 6525 MW hydro units
in Yunnan power system will be tripped in 200 millisec-
onds after YG HVDC bipolar block happens. If one re-
start attempt is considered after last blocked pole, CSG
system can not keep stability even if 6525 MW hydro
units are tripped and the western Yunnan regional power
grid is disconnected with the main Yunnan power grid.
Therefore, only one restart attempts are suggested for
YG HVDC line fault for sake of simplying strategy for
stability control system. Otherwise, stabilizing cost is
expensive and more strong dependency of CSG system
on stability control system may be indu ced.
4. Influences of AC System Faults on Hvdc
Systems
HVDC sending end and receiving end faults can have
different effect on HVDC system. HVAC fault near
HVDC rectifier side result in low converter bus voltage
and HVDC VDCL start and lower down HVDC power;
HVDC fault near HVDC inverter side can result in
commutation failure and reduce the HVDC power to zero.
AC system faults in CSG 500kV grid are scanned. The
results show that AC system faults in Yunnan and
Guizhou power grid may cause one or two HVDCs de-
creasing active power and do not influence other HVDCs
whose converter are far away from the fault point. How-
ever, AC faults in 500 kV power gr id of Guan gdong Chu
Chiang Delta may cause five HVDC to be commutation
failure due to five HVDCs feed into Guangdong Chu
Chiang Delta area and the electrical distance is too close
among each converter station. The system may lose sta-
bility if multiple HVDCs can not restore power quickly
after commutation failure.
It is predicted that restoring rate of converter bus volt-
age and HVDC power after a short circuit fault will be-
come slower if proportion of motor load in Guangdong
Chu Chiang Delta power grid increases . In ord er to indi-

B. R. ZHOU ET AL. 1233
vidually analyze the influences of slow restoring rate of
HVDC on system stability, Tvdc and Tidc are set as 0 .05
second, and Vramp and Cramp are set as 4 p.u./s, which
is abbreviated as parameter B while ZIP load model still
kept unchanged.
Comparing with simulation results for parameter A,
simulation results for parameter B shows that AC system
faults in 500 kV power grid of Guangdong Chu Chiang
Delta have more severe influence on system stability and
restoring rate of HVDCs will become slower. 200 milli-
seconds are required for HVDC to restore eighty percent
of rated power, which are 100 milliseconds more than
that of parameter A. (Shown in Figure 3). In addition,
power transfer limit of Yunnan power grid decrease
about 1306 MW than that of parameter A, AC fault oc-
curing in Wuzhou-Luodong 500kV transmission line
turns into the critical fault restricting power transfer limit,
instead of mono-polar block of YG HVDC for parameter
A.
Analysis above shows that restoring rate of HVDCs
determines power transfer limits and critical fault. Sub-
stantially, no matter the critical fault is mono-polar block
of YG HVDC or AC system fault of Guangd ong system,
the stability mechan ism is the same, that is, partial or full
power of HVDC redispatches to AC transmission system
when HVDC operation is disturbed or destroyed. YG
HVDC mono-polar block will permanently redispatch
full power of one pole of YG HVDC and its influences
on system stability depend on the capacity of YG HVDC
and the strength of th e AC transmission system. Whereas
influences on system stability of AC faults of Gu angdong
system depend on the number of disturbed HVDC and
strength of AC transmission system as well as restoring
rate of disturbed HVDCs. Therefore the fault restricting
Time(sec.) 0.40.20
MW
2,500
2,000
1,500
1,000
500
0
parameter Aparam eter B
power recovery of YG HVDC one pole
(
Figure 3. YG HVDC mono-polar power restoration
comparison curve under parameters A and B.
power transfer limit may be AC system faults on
Guangdong system when the restoring rate of HVDC
becomes slow.
If Guizhou-Guangdong AC transmission lines are
strengthened and all CSG HVDC is modeled by parame-
ter B, power transfer limit exporting from Yunnan power
grid can be restricted again by mono-polar block of YG
HVDC. It is explained that power angle between Guizhou
and Guangdong does not enlarge because the strength-
ened Guizhou-Guangdong AC transmission lines can
carried on more power diverted from
Guizhou-Guangdong HVDC during the period of com-
mutation failure. AC system faults in Guandong power
grid do not restrict the power transfer limit of Yunnan
power grid because their influence on system stability is
reduced with the strengthening of AC transmission lines.
5. Conclusions
The conclusions can be drawn after calculation and
analysis from PSS/E:
(1) Although large capacity of YG HVDC increases
power transfer limit of CSG remarkably, it could also
bring CSG tremendous security risks. If YG HVDC re-
start is not considered after HVDC line fault, the security
and stability control policy tripping generation or shed-
ding load cou ld be redu ced, b ut HVDC availability could
decreased. If DC restart is considered after YG HVDC
fault, the stabilizing cost is expensive and the policy is
also much more complicated.
2) In CSG system, the AC system faults in receiving
end system (Guangdong) can have much more severe
damage on stability of the system than those in sending
end system and may redispatchav power transfer of all
HVDCs of CSG. Power transfer limit of CSG may be
restricted by AC system faults in receiving system when
the restoring rate of HVDCs is slow.
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