Analysis of Partial Discharge Principle

Analysis of Partial Discharge Principle (1)

Under the action of an electric field, in an insulation system, discharge occurs only in some regions and does not penetrate between the conductors with the applied voltage. This phenomenon is called partial discharge. If partial discharge occurs near a conductor surrounded by gas, it can also be called corona.

Partial discharge can occur not only at the edge of a conductor but also on the surface or inside an insulator. The discharge occurring on the surface is called surface partial discharge, and that occurring inside is called internal partial discharge. When discharge occurs in the air gap inside the insulator, the exchange and accumulation changes of charges in the air gap will inevitably be reflected in the charge changes of the electrodes (or conductors) at both ends of the insulator. The relationship between the two can be analyzed by means of an equivalent circuit.

Taking a cross - linked polyethylene cable as an example below to explain the development process of partial discharge. When there is a small air gap inside the cable insulation medium, its equivalent circuit is shown as follows:

In the figure, Ca is the air - gap capacitance, Cb is the solid dielectric capacitance in series with the air gap, and Cc is the capacitance of the remaining intact part of the dielectric. If the air gap is very small, then Cb is much smaller than Cc and Cb is much smaller than Ca. When an AC voltage with an instantaneous value of u is applied between the electrodes, the voltage ua across Ca is .

When ua increases with u to reach the discharge voltage U2 of the     air gap, the air gap starts to discharge. The space charges generated by the discharge will establish an electric field, causing the voltage across Ca to drop sharply to the residual voltage U1. At this point, the spark extinguishes, and one partial discharge cycle is completed.

During this process, a corresponding partial discharge current pulse appears. The discharge process is extremely short and can be regarded as completed instantaneously. Each time the air gap discharges, its voltage drops instantaneously by Δua = U2 - U1. As the applied voltage continues to rise, Ca recharges until ua reaches U2 again, and the air gap discharges for the second time.

The moment partial discharge occurs, the air gap generates voltage and current pulses, which in turn create moving electric and magnetic fields in the line. Partial discharge detection can be carried out based on these fields.

In actual detection, it is found that the magnitude of each discharge (i.e., the pulse height) is not equal, and discharges mostly occur in the phase of the rising stage of the absolute value of the applied voltage amplitude. Only when the discharge is extremely intense will it extend to the phase of the falling stage of the absolute value of the voltage. This is because in practical situations, there are often multiple air bubbles discharging simultaneously; or there is only one large air bubble, but each discharge does not cover the entire area of the bubble, only a local region.

Obviously, the charge quantity of each discharge is not necessarily the same, and there may even be reverse discharges, which may not neutralize the originally accumulated charges. Instead, both positive and negative charges accumulate near the bubble wall, causing surface discharge along the bubble wall. In addition, the space near the bubble wall is limited. During discharge, a narrow conductive channel forms inside the bubble, leading to the leakage of some space charges generated by the discharge.