PDC极化去极化电流法测电缆绝缘

Insulation Detection and Analysis of XLPE Medium-Voltage Cables

Abstract:

The

polarization-depolarization

current

(PDC)

method

is

used

to

detect

the

insulation

status of medium -voltage cables and the testing data are analyzed. Thelow frequency component

of depolarization current, which is relatively less sensitive to the cable length, is used to overcome

the impact of cable length. Cable aging status is determined through analyzing the value range of

the depolarization current ’s low frequency components,the average discharge rate and the impact

of

different

polarization

time.

Finally,

this

method

is

applied

in

repaired

cables

and

it

is

verifiedthat the characteristics of the repaired cable is close to that of new cable with same length,

which shows that the method can be used for identification of the cable’s aging status.

Key

words:

medium- voltage

cable;

insulation

detection;

polarization

and

depolarization

(PDC)

method

0 introduction

Cross-linked

polyethylene

(XLPE

)

power

cable

has

been

widely

used

due

to

its

realiable

[1]

dielectric

and

mechanical

properties

.

The

insulation

of

the

cable

is

greatly

damaged

by

[2]

electricity tree and water tree generated in the long operation

.

Power outage caused by insulation

aging

could

cause

great

economic

losses,

which

requires

the

detection

of

the

cable

insulation

condition.

The PDC method is a method of detecting a large segment of the overall performance

[3-4]

of

the

cable

.

The

measuring

instrument

used

is

simple,

lightweight,

and

the

method

can

effectively detect the cable insulation performance in a variety of reasons

.

1 Fundamentals of Polarization - depolarization current method

Charging the tested cable by a constant voltage source,then a charging current

ich

is generated,

after a long charge time we can obtain a steady-state current

idc

. After a time of

t

, switch off the

voltage

power

to

discharge,

and

a

discharge

current

idisch

is

generated(

idisch

'

is

merely

the

discharge current with polarity reversed and shifted backward in timeline by

t

), and then after a

sufficiently

long

discharge,

the

current

will

be

reduced

to

know

the

charging

and

discharging

process

are

related

to

electrical

characteristics

of

the

power

as

well

as

the

supply

voltage, so we can analyze

ich

and

idisch

to determine the electrical characteristics of the cable,

and

then

to

determine

whether

the

tested

is

excessively

aging.

Current

variation

in

PDC

measurement shown in FIG. 1

2 Measuring circuit and measurement procedures

PDC measurement circuit structure shown in Figure 2

.Charging the tested cable by connected

the high- voltage interrupter switch to the voltage source. After a period of polarization switch to

connect KEYTHLEY 6485. We can use KEYTHLEY648 to observe and record depolarizing current,

then transmit the information to a computer for analysis.

Fig.1 Current variation

Fig.2 Circuit schematic

Fig.3 The depolarization currents