Research on the Fault Location Principle for the Earthing Electrode Line of the UHVDC Transmission System

doi:10.4236/epe.2013.54B227

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Energy and Power Engineering, 2013, 5, 1198-1201

doi:10.4236/epe.2013.54B227 Published Online July 2013 (http://www.scirp.org/journal/epe)

Research on the Fault Location Principle for the Earthing

Electrode Line of the UHVDC Transmission System

Xu Peng, Yu Jiang

CSG Power Dispatching Control Center, Guangzhou, China

Email: [email protected]

Received February, 2013

ABSTRACT

The UHVDC transmission line transports large-capacity power, and its safety operation has very important rule on the

safe operation of grid. Currently, in the DC transmission system, the earthing electrode line has th e functions of ground

potential fixing and bipolar imbalance current circulation during the operation of bipolar balance. Therefore the moni-

toring to the earthing electrode line operation is very essential. This article firstly introduced the curren t situation of the

research on the earthing electrod e line fault locatio n method, then bu ilt the relevant PSCAD/EMTDC model for simula-

tion verification of the current using Time Domain Reflectometry for the UHVDC earthing electrode line, and at last

analyzed the reasons for the formation of the blind monitoring zones.

Keywords: Earthing Electrode Line; Pulse Reflection Method

1. Introduction

The earthing electrode system of the DC transmission

system could be divided into three parts: the neutral bus

of the converter station, the earthing electrode line, and

the earthing electrode. Currently, UHVDC systems put

into operatio are majority DC double-end transmission

system, which has only one transmitting end and one

receiving end. Figure 1 shows the structure of the earth-

ing electrode system of the double-end UHVDC trans-

mission project [1].

The line connection modes of the UHVDC transmis-

sion system are:①The bipolar (BP) mode;②The mono-

pole ground return (GR) mode;③The monopole metallic

return (MR) mode; and ④ The open line test (OLT)

mode. Among which, the mode ③ is only single-point

earthing, and there is no current in the ground, so the

earthing electrode only has the function of clamping the

Figure 1. Structure of the earthing electrode system.

neutral point potential, while in the mode ①, ②, and

④ the earthing electrode not only clamps the neutral

point potential, but also provides a pathway for the DC

current. Therefore the characteristics of the earthing

electrode system in the ways ①, ②, and ④ need to be

investigated in particu larly.

The GR and OLT modes form a circuit with the

ground via a high voltage DC line, and therefore the cur-

rent flowing in the earthing electrode and the earthing

electrode line is equal to the curren t on the line.

The BP mode is the normal operation way of the

UHVDC, the earthing electrode line only has the func-

tions of ground potential fixing and bipolar imbalance

current circulation, but the fault of the earthing electrode

line would still h ave great impact on the safe and reliable

system operation. One is if the earthing electrode line has

fault or the working status is not clear, then the system

could not convert fro m the balance oper ation mode to the

unbalanced operation mode in a safe and reliable manner.

The other is the fault of the earthing electrode line might

also cause communication disturbance to the nearby

communication facilities, and result in personnel injuries

or corrosion of the metal equipment at the fault lo cations.

So, when the DC transmission line running with BP

mode, the monitoring of the earthing electrode line is

very essential. [2]

2. Research on the Fault Location Principle

of the Earthing Electrode Line

X. PENG, Y. JIANG 1199

2.1. The Research on the Fault Location

Principle of the Earthing Electrode Line

When the DC system runs in BP mode, the imbalance

current flowed through the earthing electrode line is very

few, which is usually no more than 1% of the system

rated current; also the voltage at the outlet end of the

earthing electrode line (end of the converter station) is

only a dozens of volts, which makes it difficult for the

fault monitoring of the earthing electrode line running

under the BP mode.

Currently, th e fault location principles in the applica-

tion of the HVDC and UHVDC earthing electrode line

have the impedance method (ABB) [3] and the Time

Domain Reflectometry (SIMENS) [4]. As the impedance

method is unable to determine the fault type and fault

location, have the disadvantages of unwanted action or

refusing action and the settin g valu es are great in fluen ced

by the earthing transition resistance, so the existing

UHVDC transmission system is widely using the Time

Domain Reflectometry.

2.2. Time Domain Reflectometry

Based on the theory of long distance transmission line,

the Time Domain Reflectometry [5] is a type of line

monitoring method to determine the fault location, ac-

cording to the amplitude, polarity, arrival time, or other

electrical signals of the reflected pulse on the conductor

which detected by inputting a high frequency pulse sig-

nal to one end of the conductor [6-8]. Figure 2 shows

different characteristics between normal and abnormal.

The basic principle of the Time Domain Reflectometry

[9-10] which applied to the earthing electrode line is as

follows: apply the first voltage pulse at the side of the

end of the converter station of the earthing electrode line

and on the earthing electrode line at the same time, the

pulse goes through the earthing electrode line and is re-

flected when reaching the earthing electrode. Take the

record of the voltage wave on the line at the applying

position of the pulse, and when recording the reflected

wave at the end of the earthing electrode line, apply the

Figure 2. The Time Domain Reflectometry principle.

second pulse. Repeat the above processes until the fault

of the line and receiving the “extra” reflected wave. If the

earthing electrode line has fault, the fault poin t will form

a wave impedance singularity, which is a reflection point

for the pulse in addition to the earthing electrode, and by

the reflection of the pulse at this location, a measurable

reflected pulse in addition is generated. After receiving

this reflected pulse, stop to apply the monitoring pulse.

The reflected pulse at the fault point arrives at the pulse

applying point earlier than the reflected pulse at the end

of the earthing electrode line, and according to the return

time of the reflected pulse at the fault point and wave

velocity, the distance between the fault point and the

monitoring point is determined as

t

(1)

3. Simulation Verification

In order to verify the correctness and the validity of the

Time Domain Reflectometry proposed in this article, the

simulation model of the ±800kV UHVDC transmission

system is established by using the PSCAD/EMTDC

electromagnetic transient simulation software, which is

shown in Figure 3.

Figure 4 shows the line parameters of the earthing

electrode line of UHVDC transmission.

Figure 3. Simulation model of the ±500 kV HVDC trans-

mission system.

Figure 4. Model of the e arthing electrode line.

X. PENG, Y. JIANG

1200

Normal line Single-line earthing short circuit fault

Double-line earthing short circuit fault Double-line short circuit fault

Single-line disconnection fault Double-line disconnection fault

Figure 5. Results of the PSCAD/EMTDC simulation for

various fault types.

Transmit a pulse with amplitude U = 150 V and pulse

width tp= 30 µs at the start side of the earthing electrode

line at 1s (2s as a cycle). Figure 5 shows the PSCAD/

EMTDC simulation results of the pulse on the earthing

electrode line when the different fault types occured at

the neutral point of the UHVDC earthing electrode line.

It is conclued from Figure 5: polarities of the reflec-

tion pulse and the transmission pulse under disconnection

fault is same, while under earthing faul is opposite, and

the fault type of the earthing electrode line could be de-

duced based on the polarity of the reflection pulse.

For the metallic short circuits of the earthing electrode

line happed at different locations and different types, this

article also made the PSCAD/EMTDC simulation, take

the sampling frequency fs = 100 kHz, v = 2.97×108 m/s.

The fault location is shown in Table 1.

The simulation result in Table 1 shows, in the case of

metallic faults, there are certain blind detection zones at

the start side (transmission side) and the end side of the

line; and the longer the fault distance, the bigger the fault

location error sh ows. Within the range of 25% to 75% of

the line, the maximum actual error and the maximum

relative error of the fault location are 2.042 km and

1.442% in separately.

Reasons for the occurred detection blind zones:

1) As shown in Figure 6, the transmission pulse can

not separated from the reflection pulse at the start side

fault. The reason is, the transmission pulse itself occupies

a certain period of time, when the fault point is very

close by, the reflection pulse shows almost simultane-

ously with the transmission pu lse, as the amp litud e of the

transmission pulse is large and also exist when the line

has no short circuit, and therefore the transmission pulse

can not separated from the reflection pulse by fault near

by the start side, also the fault location of the start side

could not be identifi e d.

As shown in Figure 7, the reflection pulse at the end

side fault can not separated from the fixed reflection

pulse of the earthing electrode resistance. This is because

when the fault point at the end side is very close to the

earthing electrode resistance, the reflection pulse by fault

also shows up almost simultaneously with the fixed re-

flection pulse and with almost equal amplitude, the two

reflection pulses are superimposed, and therefore the

reflection pulse at the end side fault can not separated

from the fixed reflection pulse of the earthing electrode

resistance, also the fault location of the end side could

not be identified.

Table 1. Results of fault location under metallic faults (km).

Method of the article

Fault typeFault

location Simulation

result Actual error Relative error

2% Can not

identified Can not

identified

25% 47.52 0.309 0.655%

50% 93.555 -0.867 0.918%

75% 139.59 -2.042 1.442%

Single-line

short cir-

cuit

earthing

98% Can not

identified Can not

identified

2% Can not

identified Can not

identified

25% 47.52 0.309 0.655%

50% 93.555 -0.867 0.918%

75% 139.59 -2.042 1.442%

Dou-

ble-line

short cir-

cuit

earthing

98% Can not

identified Can not

identified

Figure 6. Single-line short circuit earthing (lF=2%l).

X. PENG, Y. JIANG

1201

Figure 7. Double-line short circuit earthing (lF = 98%l).

4. Summary

Aiming at the current situation that the Time Domain

Reflectometry is used for fault location of the earthing

electrode line of the UHVDC system, this article de-

scribed the principle of the Time Domain Reflectometry,

established corresponding PSCAD/EMTDC model for

simulation verification, analyzed the error precision of

the Time Domain Reflectometry, pointed out the reasons

for the formation of the monitoring blind zones, and con-

structed the foundation for the further study of the fault

location principle of the earthing electrode line of the

UHVDC transmission system.

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