Y. D. HONG, T. YU 55
Table 2. Degee of Unbalance of C1 Capacitor for different
number of capacitor faulted.
Number of
Faulted Capacitor Degree of
Unbalance Number of Faulted
Capacitor Degree of
Unbalance
1 0.016 8 0.197
2 0.034 9 0.24
3 0.053 10 0.291
4 0.075 11 0.351
5 0.1 12 0.426
6 0.128 13 0.518
7 0.16
It’s proposed that instantaneous value IR11/IT1 which
directly reflects capacitor unbalance could be used as a
criterion for C1 unbalance protection This improvement
can effectively avoid abnormal jump of C1 capacitor
unbalance protection on differential current. When C1
capacitor breaks down, the C1 protection can act right
aiming at different types of faults. It has better reliability
and sensitivity.
3.3. DC Melting Ice Using Two Ends Back to
Back Operation Mode
The icing of transmission lines in winter is a nature dis-
aster to power system, which threats the safe operation of
power system. It brings mechanical damage to wires,
towers and other metallic devices, which may breaks
lines, pushes down towers and causes power failure. [10]
In 2008 disastrous weather of low temperature, rain,
snow and ice impacted on southern area of China, caus-
ing huge economic losses that transmission lines broke
down for long time on a large scale in Guizhou, Guang-
dong, Yunnan, Guangxi province. According to statistics,
there are 36740 lines broken, 2018 substations outage
and 8381 towers pushed down during this period. It's
necessary to add operating mode of icing to control and
protection system in HVDC systems which are vulner-
able to icing weather.
When the load is low in dry season or the system
comes to a stop as planned, the direct current is low so
that problems of icing appear on transmission lines. [11]
Designing icing lines protection function of DC two ends
back to back operation mode in DC control protection
system can satisfy the needs of lines protection in the
period of low load in dry season.
DC two ends back to back operation mode means that
in the period of icing weather, while a pole operates in
rectifier mode transferring positive power, another pole
operates in inverter mode transferring negative mode. [12]
In this mode the power transferring direction of two
poles is opposite while two poles can operates with a
relatively larege power.The total transferring power is
relatively low which is fit for the conditio n of low lo ad in
dry season.Loop cu rrent would be formed to produce DC
loss to melt the ice.
The alternating system is affected less in DC two ends
back to back operation mode that it's not necessary for
alternating system to provide source for large load and
reactive-load compensation equipments in converter sta-
tions can be in balance with the ones in alternating sys-
tem.[13]
4. Reliability Technique for Weak Links of
Valve Cooling System
According to the statistics of fault, exceptions and ex-
perience of maintainence since Guangzhou, Zhaoqing
and Baoan converter stations was in operation, there are
some weak links in valve cooling systems in these sta-
tions, which may cause unbanlance of DC system or
even DC blocks.
No redundancy for valve cooling system sensor is an
important reason why valve system may be blocked by
error. Exceptions of single configuration may cause data
which are sent to valve cooling control system changes
so that DC system is blocked by error.Some measures are
proposed in this papaer as follow to avoid this situation.
1) It's better to add an expansion water gauge for water
tank to create redundancy and compare the values of two
sensors to choose a better one. The function of stopping
primary pump should be cancelled and the water gaouge
added shoud be linked with the other one so that opera-
tion workers can observe water level of expansion water
tank when patrolling and judge whether the control data
transferred to sensor is abnormal or not.
2) A water supply sensor can be added to create re-
dundancy which can be an electronic or thermoelectric
one to make it easy for operators to realize the actual
temperature of internal cooling water.The values of two
sensors should be compared and hot water temperature
can be a useful criterion to avoid mistakes.
3) The differential pressure of two adjacent valve tow-
ers can be used to create redundancy that it alarms when
one sensor's differential pressure is low and it trips when
the two sensors' differential pressure are both low.
5. Conclusions
This paper count the reliability data in late years and
proposes some measures to improve operation reliability
according to some faults happen in HVDC systems lately.
Xingan HVDC system went into operation in 2008
while Chusui HVDC system went into operation in
2010.So DC block times rose in according year. It’s ob-
vious that after many years' experience of operation in
HVDC systems and measures of improving reliability are
taken, time of DC blocks descents year by year and op-
eration reliability in HVDC systems are improved.
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