chapter-7

J.M. PANG & SEAH PTE LTD 1

On-Line Partial Discharge Measurement on High Voltage Equipment Using the Continuous Mode Transient Earth Voltage (TEV) Technique

he measurement of partial discharge has become less expensive, easier to use and hence more

popular as a means to determine the condition of the electrical insulation system in high voltage

equipment like switchgears and transformers. The most common unit for quantifying partial

discharge magnitude is the pico-coulomb (pC), which is the product of voltage and capacitance. One is

most concerned with the maximum value of pC because it will indicate the maximum amount of damage

being inflicted on the electrical insulation system. The larger the value of pC the more rapid the rate of

deterioration of the electrical insulation system. The actual pC value at the location of the partial discharge

cannot be directly measured because the location of the partial discharge is always embedded inside the

electrical insulation system, and hence there will be no direct access for measurement of the actual pC value

due to the partial discharge. However IEC 270 has established the calibration method to determine th pC

equivalent at the measurable voltage of the phase conductor of the electrical equipment due to the partial

discharge. This will require the injection of a known quantity of pC into the phase conductor and to

measure the peak magnitude of the resulting voltage at the phase conductor. The ratio of the known

injected pC at the phase conductor is divided by the measured value of low voltage at the phase conductor

to obtain the pC/millivolts relationship. This derived value of pC at the phase conductor is called the

apparent pC, and is directly proportional to the actual pC value at the location of the partial discharge.

The main disadvantage of the pC method of partial discharge measurement is that the measured

electrical equipment need to be de-energized. For any partial discharge measurement to be widely used, it is

crucial for the measurement to be done without any need to de-energize the electrical equipment. The

transient earth voltage (TEV) technique will provide this feature. This article will share my experience with

the measurement of partial discharge activity using the continuous mode TEV technique with equipment

from EA Technology of UK. The electrical equipment measured are high voltage air-insulated switchgears

and transformers.

Transient Earth Voltage (TEV) Partial discharge activity in any electrical equipment will produce electromagnetic waves in a

T

Chapter

7


very wide frequency spectrum that will radiate in all directions away from the location of the partial

discharge. The higher frequency components of the radiated electromagnetic waves will be more

attentuated by the air medium than the lower frequency components. These lower frequency

electromagnetic waves will hit the inner surface of the earthed metal cladding of switchgears. At any

available openings, such as joints or air vents, the electromagnetic waves will escape from the switchgear.

This will cause a transient rise in the voltage of the earthed metal cladding of the switchgear. This TEV has

rise time in the range of nano seconds with an amplitude in the millivolts range. The magnitude of the

TEV is a function of the amplitude of the partial discharge and the attentuation in the propagation path

along the air medium and earthed metal cladding of the switchgear. The TEV is measured using a capacitive

probe placed at the earthed metal cladding of the switchgear. The measued value of TEV will be displayed

in dB as explained in the given Equation (1).

Measured TEV Where dB = 20 log10 ( 1 millivolts )

dB Measured TEV in millivolts 0 1 10 3 52 398

The detection circuit of the continuous mode TEV measurement set has a wide bandwidth of 70

MHz. The measured TEV is from a peak detection circuit, and so only the maximum value of TEV in any

measurement period will be recorded.

Partial discharge activity is often intermittent in nature. This means that the partial discharge may

be dormant for long periods and become active when initiated by changes in switching, loading, temperature

and humidity. In order to detect intermittent partial discharge, it is important to monitor over a period of

time. A minimum of 24 hours is recommended to study the effects due to changes in switching, loading,

temperature and humidity.

The continuous mode TEV measurement set has 12 channels, of which 8 channels are connected to

8 capacitive probes placed at the earthed metal cladding of the switchgears. These capacitive probes are

required for the measurement of the TEV magnitude at different locations. The remaining 4 channels are

connected to 4 aerial probes, whose purpose is to detect the presence of interference from external

electromagnetic source. Figure 7.1 shows a typical setup. External electromagnetic waves will first trigger

the aerial probe before the capacitive probe. This time delay is used to screen out the external interference.

The resolution time of the TEV measurement set is about 7 nanoseconds and so the minimum separation

distance between the aerial probe and the capacitive probe will be the product of the speed of the


electromagnetic wave (about 3 x 108 m/s) and the 7 nanoseconds resolution time. This works out to

2100mm, which is good enough to locate the source of the partial discharge to individual switchgear.

The continuous mode TEV measurement set will provide the following information:

maximum TEV magnitude in dB

average TEV magnitude in dB

number of pulses

number of pulses in any one channel expressed as a percentage of the total number of pulses in

the measurement period.

percentage of the time in which the pulses were active during the measurement period.

number of pulses per cycle.

short term severity.

maximum short term severity.

long term severity

Short Term Severity It is defined as (maximum pulse amplitude x number

of pulses per cycle).

Maximum Short Term Severity It is defined as the maximum value of (pulse

amplitude x number of pulses per cycle) for every

measurement period. The two short term severity numbers

take into consideration the information of “how bad” and

“for how long” the damage to the electrical insulation due to

the maximum magnitude of partial discharge.

Long Term Severity

It is defined as the (average maximum pulse

amplitude x average number of pulses per cycle x duration of when pulses were detected). This number is

more sensitive to detect partial discharge that is continuous or nearly continuous in nature.

Number of Pulses

All pulses detected by the continuous mode TEV measurement set are counted in the “total number

of pulses” of the summary table, and at the same time allocated to the channel that first detected the pulse.

If the pulse is detected by two channels within the 7 nanoseconds resolution time, then the pulse is deemed

FIGURE 7.1 : Typical Setup


to arrive at both channels at the same time, and the associated counter for both channels will count up by

one. In a 5 minutes measurement period, the number of cycles for 50 hertz supply will be (5 x 60 x

50) or 15,000 cycles.

Numberof Passes Duration in Seconds Remarks

12 12 passes x 12 channels x 2 seconds = 288

13 13 passes x 12 channels x 2 seconds = 312

Each channel is measured for 2

seconds

13 passes over the 12 channels is not possible because the 312 seconds will exceed the 5 minutes or 300

seconds measurement period. Hence the maximum number of passes will be 12.

288The maximum number of cycles will be ( 300 x 15,000 ) = 14,400 cycles with a

5 minute measurement period. The total number of pulses detected in the 5 minutes period will be divided

by 14,400 cycles to obtain the number of pulses per cycle. Hence if the partial discharge is continuous over

5 minutes, the total number of pulses will be 14,400 if we assume one partial discharge in each cycle. For a

partial discharge of intermittent nature, the number of pulses over the 5 minutes may be less than 14,400.

For example, the number of pulse per cycle for an intermittent partial discharge of 5,000 pulses will be

(5,000 pulse/14,400 cycle) = 0.35 pulse per cycle.

Therefore for an intermittent partial discharge within a 5 minute measurement period, the number

of pulse per cycle will be smaller than 1, and in EA Technology practice, values of activity greater than 0.05

pulse per cycles [1] or 1 pulse per 20 cycle will require further investigation.

Guidelines for Partial Discharge Activity that will require further investigation [1]

short term severity with capacitive probes > 0

or

number of pulses per cycle with capacitive probes > 0.05

Case History for Switchgears Table 7.1 is a summary of the continuous mode TEV measurement on a 24kV rated air insulated

switchgear with vacuum circuit breaker. The aerial probes were connected to channel 1, 2, 11 and 12. The

capacitive probes were connected to channel 3, 4, 5, 6, 7, 8, 9 and 10.

We only need to apply the guidelines to the capacitive probes. All the respective short term severity

values are zero and all the respective number of pulses per cycle are zero. It was concluded that

there was no detectable magnitude of partial discharge.


Case History for Transformer Table 7.2 is a summary of the continuous mode TEV measurement on a 22kV/400 volts, 2MVA

cast resin transformer. The aerial probes were connected to channel 1, 2, 11 and 12. The capacitive

probes were connected to channel 3, 4, 5, 6, 7, 8, 9 and 10.

In this measurement, the number of pulses per cycle is not zero at channel 3, 8 and 10. This means

that there is partial discharge activity detected at the three channels. Channel 3 will be of more interest

because of the relatively large 25dB TEV magnitude. During one of the 5 minutes measurement interval,

of which there are 288 such intervals, the total number of pulses detected were (0.02 pulse per cycle x

14,400 cycles) or 288 pulses. Figure 7.2 shows the dB magnitude of the pulses measured over the

measurement period for Channel 3. Figure 7.3 shows the number of pulses over the same measurement

period for Channel 3. With all these information, we have the following conclusion for the maximum and

intermittent partial discharge activity at channels 3, 6, 8 and 10.

Intermittent partial discharge was detected in one of the 5 minutes measurement interval

because the number of pulses per cycle was not zero and equal to 0.02 pulse per cycle.

LEVEL NUMBER OF PULSES

Ch MaxLevel

dB

No. of Pulses per

cycle

Av Level

dB

Short Term

Severity

No. of Pulses

%Pulses

MaxPulses

per cycle

Assoc Level

dB

%Time

Long Term

Severity

MaximumShort Term

Severity 1 28 0.021 1 1 1398 58 0.021 28 44 0 1 2 28 0.026 1 1 844 35 0.026 28 44 0 1 3 19 0.000 0 0 282 12 0.000 0 38 0 0 4 0 0.000 0 0 0 0 0.000 0 0 0 0 5 22 0.000 1 0 177 7 0.001 0 32 0 0 6 0 0.000 0 0 34 1 0.000 0 7 0 0 7 0 0.000 0 0 4 0 0.000 0 1 0 0 8 0 0.000 0 0 5 0 0.000 0 2 0 0 9 16 0.000 0 0 11 0 0.000 16 1 0 0 10 0 0.000 0 0 3 0 0.000 0 1 0 0 11 0 0.000 0 0 0 0 0.000 0 0 0 0 12 0 0.000 0 0 0 0 0.000 0 0 0 0

Total number of pulses = 2423

Total number of sets of data = 282 Capacitive Probes

12 channels connected, 14400 cycles per 5 minutes Aerial Probes

Start Time : 10/03/2003 14:10:09

Finish Time : 11/03/2003 13:35:00

Channel 3

Channel 4

Channel 5

Channel 6

Channel 7

Channel 8

Channel 9

Channel 10

24kV VCB

24kV VCB

24kV VCB

24kV VCB

24kV VCB

24kV VCB

24kV VCB

24kV VCB

Channel 1

Channel 2 Channel 12

Channel 11

TABLE 7.1 : Summary of Continuous Mode TEV Measurement for Switchgears


However, only a value greater than 0.05 pulse per cycle [1] will require further investigation.

The short term severity of zero indicated that the largest magnitude partial discharge was not

large enough nor this same partial discharge occurred very long enough to cause problem.

In addition, a near continuous partial discharge was measured at channel 6 because of the 90% value

in the “% time” column. Figures 7.4 and 7.5 show the detailed information. A long term severity value of

zero indicated that this near continuous partial discharge was not large enough to cause problems to the

electrical insulation system.

Limitation of the TEV Measurement In general, the partial discharge magnitude in the electrical equipment must be above 100pC [2]

before reliable TEV can be measured. Lower magnitude partial discharge are better detected

using ultrasonic sensors.

TEV measurements are not suitable for gas insulated switchgears (GIS) because the totally

enclosed construction will not allow the escape of any TEV within the GIS to appear at the

outside of the GIS. Again ultrasonic sensors of the contact type will be more suitable for GIS.

LEVEL NUMBER OF PULSES

Ch Max Level

dB

No. of Pulses per

cycle

Av Level

dB

Short Term

Severity

No. of Pulses

%Pulses

MaxPulses

percycle

Assoc Level

dB

%Time

LongTerm

Severity

Maximum Short Term

Severity

1 31 0.002 0 0 804 11 0.033 13 43 0 0 2 0 0.000 0 0 0 0 0.000 0 0 0 0 3 25 0.020 0 0 324 4 0.020 25 4 0 0 4 0 0.000 0 0 73 1 0.001 0 20 0 0 5 0 0.000 0 0 43 1 0.001 0 3 0 0 6 13 0.000 0 0 780 11 0.004 10 90 0 0 7 10 0.000 0 0 71 1 0.002 0 8 0 0 8 16 0.020 1 0 1943 26 0.020 16 25 0 0 9 0 0.000 0 0 45 1 0.002 0 2 0 0

10 13 0.006 0 0 303 4 0.006 13 28 0 0 11 40 0.006 2 1 3330 45 0.016 13 33 0 1 12 0 0.000 0 0 0 0 0.000 0 0 0 0

Total number of pulses = 7351

Total number of sets of data = 288 Capacitive Probes

12 channels connected, 14400 cycles per 5 minutes Aerial Probes

Start Time : 18/03/2003 15:06:00

Finish Time : 19/03/2003 14:26:00

Channel 2 Channel 12

Channel 1

Transformer Inside

Enclosure 1 Channel 5 Channel 6 Channel 10

Transformer Inside

Enclosure 2 Channel 9

Channel 4 Channel 8

Channel 3 Channel 7

Channel 11

TABLE 7.2 : Summary of Continuous Mode TEV Measurement for Transformer


ConclusionCompared to any form of snapshot partial discharge measurement, the continuous mode TEV

measurement will provide a higher confidence level to detect intermittent partial discharge. In addition,

the TEV technique does not need the de-energization of the electrical equipment, or the removal of any

protective cover panel. The on-line and non-intrusive nature of the TEV technique makes this as a very

powerful preventive maintenance tool in detecting partial discharges.

Amplitude of pulses arriving first at each channel

0

5

10

15

20

25

30

18/0

3/20

03 1

5:06

:00

18/0

3/20

03 1

5:46

:00

18/0

3/20

03 1

6:26

:00

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3/20

03 1

7:06

:00

18/0

3/20

03 1

7:46

:00

18/0

3/20

03 1

8:26

:00

18/0

3/20

03 1

9:06

:00

18/0

3/20

03 1

9:46

:00

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3/20

03 2

0:26

:00

18/0

3/20

03 2

1:06

:00

18/0

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03 2

1:46

:00

18/0

3/20

03 2

2:26

:00

18/0

3/20

03 2

3:06

:00

18/0

3/20

03 2

3:46

:00

19/0

3/20

03 0

0:26

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1:06

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03 0

1:46

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03 0

2:26

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3:06

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3:46

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4:26

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5:06

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5:46

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3/20

03 0

6:26

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03 0

7:06

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03 0

7:46

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8:26

:00

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03 0

9:06

:00

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3/20

03 0

9:46

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03 1

0:26

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1:06

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1:46

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2:26

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03 1

3:06

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3/20

03 1

3:46

:00

19/0

3/20

03 1

4:26

:00

Date/Time

Am

plitu

de d

B

CH3 Amplitude

FIGURE 7.2 : Magnitude of Pulses for Channel 3

Number of pulses arriving first at each channel

0

50

100

150

200

250

300

350

18/0

3/20

03 1

5:06

:00

18/0

3/20

03 1

5:51

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18/0

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03 1

6:36

:00

18/0

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03 1

7:21

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8:06

:00

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3/20

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8:51

:00

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03 1

9:36

:00

18/0

3/20

03 2

0:21

:00

18/0

3/20

03 2

1:06

:00

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3/20

03 2

1:51

:00

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3/20

03 2

2:36

:00

18/0

3/20

03 2

3:21

:00

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3/20

03 0

0:06

:00

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3/20

03 0

0:51

:00

19/0

3/20

03 0

1:36

:00

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03 0

2:21

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3/20

03 0

3:06

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03 0

3:51

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03 0

4:36

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3/20

03 0

5:21

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3/20

03 0

6:06

:00

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3/20

03 0

6:51

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03 0

7:36

:00

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3/20

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8:21

:00

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03 0

9:06

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9:51

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0:36

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1:21

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2:06

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2:51

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3:36

:00

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3/20

03 1

4:21

:00

Date/Time

Num

ber o

f Pul

ses

CH3 Number of Pulses

FIGURE 7.3 : Number of Pulses for Channel 4


-- END --

Amplitude of pulses arriving first at each channel

0

2

4

6

8

10

12

14

18/0

3/20

03 1

5:06

:00

18/0

3/20

03 1

5:46

:00

18/0

3/20

03 1

6:26

:00

18/0

3/20

03 1

7:06

:00

18/0

3/20

03 1

7:46

:00

18/0

3/20

03 1

8:26

:00

18/0

3/20

03 1

9:06

:00

18/0

3/20

03 1

9:46

:00

18/0

3/20

03 2

0:26

:00

18/0

3/20

03 2

1:06

:00

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3/20

03 2

1:46

:00

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3/20

03 2

2:26

:00

18/0

3/20

03 2

3:06

:00

18/0

3/20

03 2

3:46

:00

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3/20

03 0

0:26

:00

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03 0

1:06

:00

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1:46

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2:26

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3:06

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3:46

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4:26

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5:06

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5:46

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03 0

6:26

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3/20

03 0

7:06

:00

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3/20

03 0

7:46

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3/20

03 0

8:26

:00

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3/20

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9:06

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9:46

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0:26

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1:06

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1:46

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2:26

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3:06

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03 1

3:46

:00

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3/20

03 1

4:26

:00

Date/Time

Am

plitu

de d

B

CH6 Amplitude

FIGURE 7.4 : Magnitude of Pulses for Channel 6

Number of pulses arriving first at each channel

0

10

20

30

40

50

60

18/0

3/20

03 1

5:06

:00

18/0

3/20

03 1

5:51

:00

18/0

3/20

03 1

6:36

:00

18/0

3/20

03 1

7:21

:00

18/0

3/20

03 1

8:06

:00

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3/20

03 1

8:51

:00

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3/20

03 1

9:36

:00

18/0

3/20

03 2

0:21

:00

18/0

3/20

03 2

1:06

:00

18/0

3/20

03 2

1:51

:00

18/0

3/20

03 2

2:36

:00

18/0

3/20

03 2

3:21

:00

19/0

3/20

03 0

0:06

:00

19/0

3/20

03 0

0:51

:00

19/0

3/20

03 0

1:36

:00

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3/20

03 0

2:21

:00

19/0

3/20

03 0

3:06

:00

19/0

3/20

03 0

3:51

:00

19/0

3/20

03 0

4:36

:00

19/0

3/20

03 0

5:21

:00

19/0

3/20

03 0

6:06

:00

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3/20

03 0

6:51

:00

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3/20

03 0

7:36

:00

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3/20

03 0

8:21

:00

19/0

3/20

03 0

9:06

:00

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3/20

03 0

9:51

:00

19/0

3/20

03 1

0:36

:00

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03 1

1:21

:00

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2:06

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03 1

2:51

:00

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3/20

03 1

3:36

:00

19/0

3/20

03 1

4:21

:00

Date/Time

Num

ber o

f Pul

ses

CH6 Number of Pulses

FIGURE 7.5 : Number of Pulses for Channel 6

chapter-7

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