Reliability Improvement Strategies for HVDC Transmission System

doi:10.4236/epe.2013.53B011

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Energy and Power Engineering, 2013, 5, 52-56

doi:10.4236/epe.2013.53B011 Published Online May 2013 (http://www.scirp.org/journal/epe)

Reliability Improvement Strategies for HVDC

Transmission System

Yidong Hong, Tao Yu

Institute of Power, South China University of Technology, Guangzhou, China

Email: [email protected]

Received 2013

ABSTRACT

This paper analyses reliability data of HVDC systems of Southern China Power Grid. The weak links of HVDC sys-

tems’ operational reliability are DC control and protection, valve hall and v alve cooling system and transmission lines.

Some improvement measures and HVDC system reliability technology are proposed in this paper.

Keywords: HVDC; Transmission; Operation; Reliability

1. Introduction

HVDC Transmission has obvious advantages in the re-

spect of long distance transmission on a large scale and

interconnection of power systems. It's significant for

west-east power transmission and national interconnec-

tion of power systems in China. The reliability in HVDC

systems indicates the amount of power transmitted in

specified time, conditions and environment. [1] Data

shows that the availability rate of operating HVDC sys-

tems is over 90 percent. It has been proposed in Inter-

Mountain HVDC system that reliability level of bipolar

HVDC should be higher than double-circuit AC lines.

More requirements are raised for the reliab ility of HVDC

systems with the technology of HVDC developing and

more real projects go into oper ation. The improv ement of

reliability would bring great benefits to the safe, reliable

and economic operation of the systems. Thus, reinforcing

the work of analysis about reliability indices and man-

agement of reliability is good for searching the weak

links and influencing factors of reliability scientifically.

Taking measures to improve HVDC systems reliability

can guarantee the secure and available operation of HVDC

systems.

2. Analysis of Reliability Indices of HVDC

Systems

Statistics of HVDC transmission systems include con-

verter transformers, converter valves, AC filters, capaci-

tor, and DC filers, smoothing reactor, transmission lines

and control protection devices. While failure types in

HVDC systems are various and complex, they can be

classified into six types: AC and its device, Converter

Valve, DC control and protection, primary equipments,

transmission lines and others. [2-3]

Failures in HVDC systems have a great influence on

operating reliability so that reliability indices are influ-

enced directly or indirectly. Through statistics and analy-

sis of reliability indices of Tianguang, Gaozhao, Xingan

and Chusui HVDC systems during 2005 to 2011, it's ob-

vious that DC block have a great impact on safe and re-

liable operation of HVDC systems. So the reasons that

may cause DC blocks are analyzed and classified spe-

cifically.

It's shown in the statistics and classification of causes

of DC blocks that monopole block and bipolar blocks are

both mainly caused by failures in primary equipments,

control and protection system, valve hall and valve cool-

ing system, transmission lines and AC protection. Ac-

cording to statistics, there are 110 DC blocks during

2005 to 2011 in these four HVDC systems. The causes

leading to DC blocks are 6 times of primary equipments,

48 times of DC control and protection, 18 times of valve

cooling systems,37 times of transmission lines and 1 time

of AC protection. They account for 5.45 percent, 43.64

percent, 16.36 percent, 33.64 percent, and 0.91 percent

separately as is shown in Figure 1.

From Table 1 and Figure 1, it's shown that measures

should be taken to improve operating reliability of

HVDC system. According to the distribu tion of causes of

DC blocks in daily operation, this paper focuses on DC

control protection and valve hall and valve cooling sys-

tem to improve operation reliability o f HVDC systems.

Y. D. HONG, T. YU 53

Table 1. Statistics and Classification of Causes of HVDC Systems’ Blocks.

HVDC System Block Reasons 20052006200720082009 2010 2011Sum

Monopole Block（Times） 5 7 10 5 1 3 2 33

Bipolar Block（Times） 1 1 3 0 0 0 0 5

Primary Equipment 1 0 0 0 0 0 0 1

Control and Protection E q ui pment 1 1 10 4 1 1 1 19

Valve Hall and its Cooling System 1 4 0 1 0 0 1 7

Transmission Lines 3 2 3 0 0 2 0 10

Tianguang HVDC System

AC Protection 0 1 0 0 0 0 0 1

Monopole Block（Times） 6 8 5 7 5 3 4 38

Bipolar Block（Times） 0 1 1 0 0 0 0 2

Primary Equipment 0 1 1 0 0 1 0 3

Control and Protection E q ui pment 2 6 2 2 2 0 2 16

Valve Hall and its Cooling System 3 1 0 1 1 0 1 7

Transmission Lines 1 1 3 4 2 2 1 14

Gaozhao HVDC System

AC Protection 0 0 0 0 0 0 0 0

Monopole Block（Times） --- --- --- 8 5 6 1 20

Bipolar Block（Times） --- --- --- 4 0 0 0 4

Primary Equipment --- --- --- 1 0 1 0 2

Control and Protection Equipment --- --- --- 4 4 2 2 12

Valve Hall and its Cooling System --- --- --- 1 0 1 1 3

Transmission Lines --- --- --- 6 1 2 1 10

Xingan HVDC System

AC Protection --- --- --- 0 0 0 0 0

Monopole Block（Times） --- --- --- --- --- 2 3 5

Bipolar Block（Times） --- --- --- --- --- 0 0 0

Primary Equipment --- --- --- --- --- 0 0 0

Control and Protection Equipment --- --- --- --- --- 1 0 1

Valve Hall and its Cooling System --- --- --- --- --- 0 1 1

Transmission Lines --- --- --- --- --- 1 2 3

Chusui HVDC system

AC Protection --- --- --- --- --- 0 0 0

Figure 1. Number of Causes of DC Blocks in HVDC Systems.

3. Reliability Strategies of DC control and

Protection

3.1. High Resistance Fault Detection Using

Broadband Fault Signal in HVDC

Transmission Lines

The acts of rejection of transmission line primary protec-

tion and fault location device caused by failure of high

resistence happen occasionally. For example, a high re-

sistance grounded failure happened in pole two in Gaoz-

hao HVDC systems in July 8th, 2004, which led to the

block of pole two. Another high resistence grounded

failure happened in pole two in Tianguang HVDC sys-

tem in May 15th, 2005.The fault location device didn't

act in both two failures. [4,5]

Y. D. HONG, T. YU

When high resistence fault happens in transmission

lines, the reason why protection and location devices act

insensitively or even reject action is that the signature

information reflecting high resistence grounded fault is

not obvious. Sampling frequency in existing transmission

lines protection device is not more than 10 kHz, which is

0 to 5 kHz in fact. And it's difficult to extract transient

signature information because only simple signal proc-

essing algorithms like differential and integral are used.

On the other hand, in order to guarantee enough high

resolution of ranging, the sampling frequency in existing

travelling wave location dev ice is more than 1 MHz. But

travelling wave location device limits the bandwidth of

analog input signal to eliminate the disturbance of high

frequency. The passband of analog filter circuit is 5 to

100 kHz. So it's difficult for the existing transmission

lines protection device and travelling wave location de-

vice to effectively detect high resistence fault. [6-8]

To solve the problem of high resistence fault detection,

the most effective way is to extract the subtle transient

feature information. HVDC transmission lines high re-

sistence fault detection uses broadband fault information

which is 0 to 500 kHz and requires that the sampling

frequency of the device should be high enough and a

fault transient feature extraction algorithm should be

constructed. [9]On account of the problem of rejection of

transmission lines protection and travelling location de-

vice when high resistence fault happens, some solutions

are proposed as follow.

1) For a certain resolution of location device, the

broader the frequency band is, the less the location result

is affected by tansition resistence. When location device

resolution and frequency band is rather high, the impact

of transiton resistence on ranging result is eliminated

basically, which effectively solve the problem of big error

in location under the condition of high resistence fault.

2) Based on the broadband and high resolution, it can

be known that wave velo city is only related to fault posi-

tion. According to different fault positions, adopting dif-

ferent wave velocity which corresponds to different fre-

quency component to calculate distance is an effective

way to solve the problem of impact of fault position on

wave velocity.

3) According to different features of maximum of noisy

singal and fault wave singular point under the wavelet

transform, the accurate detection of fault wave head can

be realized. According to different resolution and different

features of fault waveform, singular point can be calcu-

lated accurately by corresponding methods.

4) The broadband fault information and DC travelling

wave location based on wave velo city optimization algo-

rithm can be applicated into monopole single end, mono-

pole double end, bipolar single end and bipolar double

end fault location. It can improve the location accurance's

adaptability for different fault distance and robustness of

transient resistence.

3.2. High Voltage Direct Current Filter Modeling

and Fault Simulation Based on

PSCAD-EMTDC

Since Gaozhao and Xingan HVDC system went into op-

eration, there were problems that DC filter quited opera-

tion or even caused system block.The DC filter protection

of Anshun converter station has acted 10 times since 2004,

where capacitor C1 unbalance protection acted 8 times.

A model of DC filter is constructed by electric mag-

netic simulation program PACAD-EMTDC. It could be

used to detect fault, simulate faults in DC filter and ana-

lyze fault features. The model construction is showed as

Figrure 2.

Figure 2. Construction of HVDC DC Filter Mode based on PSCAD-EMTDC.

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.

Y. D. HONG, T. YU

Figure 3. Time of DC Blocks in HVDC System During 2005

to 2011.

6. Acknowledgements

The authors gratefully acknowledge the support of Na-

tional High-tech R&D Program (863 Program) (2012A

A050209) and China Southern Power Grid EHV Trans-

mission Company Science and Technology Project.

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