PARTIAL DISCHARGES
SCOPE
AND OBJECT
A partial
discharge in an insulating medium is a localized electrical discharge,
which does not bridge the electrodes of the insulation structure. The
field strenght of a weak part of the dielectric may exceed the dielectric
strenght, which causes a breakdown. It is, however, to be observed that
the weak parts mentioned may from a small portion of the insulation
structure only. The remaining whole gap can, therefore, withstand voltage
stresses corresponding even to the test voltage, and the breakdown remain
partial. The ionic discharge following the breakdown is called a partial
discharge for the above mentioned reasons
Resulting from a partial breakdown the voltage difference across the weak part of the dielectric decreases so much that the discharge current is interrupted. Due to the sinusoidal variation of the applied voltage the electrical field strength increases again after the discharge has been extinguished. When the field strength reaches its critical value, a new discharge occurs. Thus discharges take place repeatedly. (Fig. 14.1).
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Fig.
14.1 |
Uc
voltage strength across the cavity.
The situation is enlightened by the simple analogue circuit of a cavity (Fig. 14.2). Ca is the capacitance of the whole insulating gap, the spark-gap and the capacitance Cc represent the cavity and the capacitance Cb represents the dielectric in series with Cc.
When the voltage Uc across Cc has incrased enough, the spark-gap ignites. The capacitance Cc discharges and the voltage difference across the cavity vanishes within 1...1000 ns. The discharge magnitude or apparent charge q and the voltage Uc are related by the following equation:
| (14.1) | q = Cb . Uc |
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Fig.
14.2
Analogue circuit of a gas-filled cavity. |
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The partial discharges do not lead to an immediate breakdown. They have, however, other effects on the insulating medium:
- the surface of the dielectric is bombarded by iones, which cause temperature-rise and may result in degrading and chemical change in the insulating material
- chemical changes may give rise to material components, which speed up ageing. On the other hand the partial discharges may also be extinguished by the influence of some other degradation products.
- discharges cause high local field strengths near the discharge site.
These phenomena result in degradation of the dielectric properties of the insulating medium, and increase of losses.
The object of the partial discharge measurement is to reveal the above mentioned weak parts of the dielectric, which may cause destruction of the transformer in service.
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| Fig.
14.3 Partial discharges measurement. |
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| G1
feeding generator T1 transformer to be tested C1 charge capacitors E capacitance voltage divider |
P1
ammeters P2 voltmeter (peak value) P4 voltmeter TM partial discharge meter AKV Compling quadripole |
The measurement is based on observing and evaluating the apparent charge in accordance with the standard IEC 270. The measuring system is basically a wide-band system as shown on Fig. 14.3
The voltage is increased stepwise, first up to measuring voltage U2, when the occurence of discharge is checked. The test voltage is increased to the pre-stress voltage level U1 and help there for a duration according to standard. The pre-stress voltage is applied in order to ignite the discharges. Thereafter, the voltage is rapidly reduced to U2 and maintained at this value for the agreed duration of time tmes (Fig. 14.4). During this period the occurence of discharges is being checked at the terminals of the transformer. The voltage measurement is carried out at the high voltage side of the transformer to be tested (Fig. 14.3).
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| Fig.
14.4 Test voltage |
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| U1
pre-stress voltage U2 measuring voltage Ui partial discharge inception voltage Ue partial discharge extinction voltage |
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Liquid
filled transformers
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Dry-type transformers |
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The partial discharge measurement test report shows the test performance and the measuring values..