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
uenc
y of
th
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
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