32 Products and technology

When less is moreVLF partial discharge measurement using the MPD 600

Partial discharge measurement plays an important part in the field of

high-voltage technology. With such data, precise statements can be made

regarding the condition of the insulation in high-voltage equipment such

as cables, generators, and transformers. Very low frequencies (VLF) in the

range of 0.1 Hz are often used to minimize power requirements during

measurement. However, since most partial discharge testing devices are

optimized for power system frequency or higher, low frequencies often

lead to measurement errors. With the new MPD 600, OMICRON offers a

device which provides optimum results at even the lowest frequencies

and which can also handle DC voltage.

"Electrical treeing": Close-up of the tree-like discharge structure in the insulation of a medium-voltage cable shortly before insulation breakdown.

Weakened insulation due to partial discharge

Partial discharge (PD) is a local discharge in an electri-

cal insulation system. Phenomena of this kind often

occur prior to complete breakdown of the insulation

layer and display a tree-like structure. Since this can

lead to serious and costly damage, and even cause

high-voltage systems to fail completely, equipment

should be tested for PD. The limits for partial dis-

charge are laid down for the various high-voltage

elements in dedicated IEC standards. PD measure-

ment not only indicates whether partial discharge has

occurred, but also allows the precise position of the

Source: Pepper: "Grundlagenuntersuchung zum Teilentladungsverhalten in kunst-stoffisolierten Mittelspannungskabeln bei Prüfspannungen mit variabler Frequenz und Kurvenform", Dissertationsschrift, Technische Universität Berlin, Germany, 2004

~ 0.3 m

Freq

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e te

st v

olta

ge

Device under test

Diagram of test set-up

OMICRON Magazine | Volume 2 Issue 2 2011

0.1 Hz

155 min

0.2 Hz

140 min

1 Hz

12 min

10 Hz

13 min

20 Hz

18 min

50 Hz

75 min

100 Hz

72 min

200 Hz

95 min

500 Hz

81 min

Products and technology

Coupling capacitor

Coupling device

Due to a smaller ground loop the noise level is significantly re duced in comparison to conven-tional PD measuring methods.

Point of PD measurement

MPD 600 + MPP 600

Transmission via optical fiber

up to 2 km

Interference

Products and technology 33

OMICRON Magazine | Volume 2 Issue 2 2011

damage to be determined. This localization aspect is particularly

important, especially since the high- and medium-voltage cables

to be tested often stretch over several kilometers. This allows

weakened cable sections to be identified quickly using an uncom-

plicated process thus enabling prompt replacement.

Typical test set-up

In a typical test set-up, the element to be tested is taken out of

operation. A test voltage is applied at one end and a coupling

capacitor connected in parallel. This capacitor allows PD pulses

(mV) to be isolated from the high voltage (kV), enabling them to

be processed by the highly sensitive measuring instrument.

Measurement of high-voltage equipment takes place at its instal-

lation site, which is why the voltage source is often mobile and

fitted to a truck. The laptop required is generally also set up in

this vehicle. The PD testing device is connected to the computer

via fiber-optic cable, which guarantees the greatest possible

safety for the test engineer. Any electromagnetical interference,

which can have an effect on the sensitive test device, is also elimi-

nated by this approach.

VLF partial discharge measurement

PD measurements on medium-voltage cables are often performed

with a low-frequency test voltage of 0.1 Hz. This allows the power

requirements to be significantly reduced. Compared with mea-

surement at power system frequency, the reactive power which

needs to be provided by the high-voltage source can be reduced

by a factor of 500. However, this low-frequency measurement

also has a disadvantage. PD testing devices that are optimized for

use at power system frequency have difficulty in processing the

measured data. This can lead to measurement errors.

Statistical determination of the partial discharge location (cable length: 1,061 m / 3,481 feet, fault location at 826 m / 2710 feet)

VLF-compatible

The OMICRON MPD 600 is fully compatible with VLF and can

even process DC voltage without any complications. Although

a full cycle at 0.1 Hz takes 10 seconds, the MPD 600 can still

measure the relevant factors with no problems whatsoever. Using

statistical TDR (time domain reflectography), partial discharge can

be precisely localized. However, most manufacturers of measur-

ing devices employ standard TDR, which limits the precision of

the results due to a lack of statistical back-up.

Special software solution

OMICRON's software solution compares the time differences of

the reflections relative to the original PD pulse while using statisti-

cal TDR. The software tool used in the MPD 600 measures several

thousand of these pulses and illustrates them in a graphic display

as a collection of dots. Where multiple dots overlap, the system

changes them from blue, through green, to yellow and red in the

display. Reflection points – particularly at the end of the cable

and PD fault locations – can be made visible, which in turn allows

them to be removed. The MPD 600 can be used to test many dif-

ferent components – such as transformers, cables and bushings.

Reflection point 1:10.328 µs, 826 m, 12%

Reflection point 2:13.266 µs, 1,061 m, 11%

You can find further information in the Customer Area on the

OMICRON website www.omicron.at

> Measuring PD with VLF test voltage

> VLF PD Measurements in Medium Voltage Cable Systems

> PD fault location on cable lengths