Energy and Power Engineering, 2013, 5, 941-944

doi:10.4236/epe.2013.54B180 Published Online July 2013 (http://www.scirp.org/journal/epe)

New Method of Measuring the Positive-sequence

Capacitance of T-connection Transmission Lines

Yuansen Zhang1, Demin Cui1, Qingtao Cao1, Yongqiang Chai1, Peng Liu1, Xiaobo Li 1,

Zhijian Hu2, Chuanqi Li2, Chengxue Zhang2

1Dezhou Power Supply Company, Shandong Electric Power Group Co., Dezhou, China

2School of Electrical Engineering, Wuhan University, Wuhan, China

Email: zhijian_hu@163.com

Received February, 2013

ABSTRACT

A novel method of measuring the positive-sequence capacitance of T-connection transmission lines is proposed. The

mathematical model of the new method is explained in detail. In order to obtain enough independent equations, three

independent operation modes of T-connection transmission lines during the line measurement are introduced. The digi-

tal simulation results and field measurement results are show n. The simulation and measurement results hav e validated

that the new method can meet the needs of measuring the positive-sequence capacitance of T-connection transmission

lines. This method has been implemented in the newly developed measurement instrument.

Keywords: T-connection Transmission Lines; Line Parameter; Positive-sequence Capacitance; Measurement

1. Introduction

With the rapid development of power systems, line cor-

ridors become more and more crowed. In order to reduce

synthetic cost of the line construction and limited objec-

tive conditions, T-connection transmission lines are al-

ways applied in HV/EHV. The reliable operation of

power systems depends on the correct operation of re-

laying protection devices [1].

It is well known that the computation for parameters of

T-connection transmission lines is derived from Carson

formula [2]. It is influenced by many practical factors,

such as the resistance of earth, the equivalent depth of the

lines and, etc. So the parameters should be practically

measured rather than theoretically calculated.

There are many related literatures discussing about

measuring the impedance parameters of T-connection

transmission lines [3-5]. The capacitance parameters are

always ignored. But the capacitance is very important for

transmission line protection. When it exceeds certain

length, because the numerical value of capacity current is

big, the current flow on both sides of the lines and phase

relation will change according with the increase of ca-

pacity current value. Especially, when load current and

short-circuit current are small, it can lead to the malop-

eration of high frequency protection [6, 7]. If we use the

traditional method to measure the capacitance of T-con-

nection transmission lines, it can only calculate parallel

values of three branches. When the parameters of three

branches are different, the traditional method no longer

works. So, new measurement method needs to be found

out for the positive-sequence capacitance of T-connec-

tion transmission lines.

In this paper, a new method of measuring positive-

sequence capacitance of T-connection lines is proposed.

The method has been applied to the newly developed

measurement instrument.

2. The Theory of the Measurement Method

Usually, the length of T-connection transmission lines is

less than 50 km. So it can use the lumped parameter

model. Each branch can be equivalent to T-form circuit.

The lumped parameter model of T-form equivalent cir-

cuit is shown in Figure 1.

When the length and impedance of transmission lines

are known, the susceptance can be calculated. Three

phases end is opening circuit, , the susceptance y

is as follows, 20I

1

2

/2z/2z

1

U

2

U

ygjb

Figure 1. The lumped parameter model of T-form equiva-

lent circuit.

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