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

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