Partial Discharge Detection

Partial discharges

In electrical engineering, a partial discharge is a very localised breakdown of the dielectric strength of liquid or solid insulation. In contrast to the corona effect, which manifests itself in conductors or overhead switchgear in a more or less stable form, partial discharges are much more sporadic in nature.

Mechanism of discharges

Partial discharges ordinarily start in gaps, cracks or foreign elements in solid insulation, at interfaces between solid and liquid insulation (or between two insulating materials), or between conductor and insulation or in bubbles in liquid insulation. Partial discharges reduce the distance between live elements but only in the portion of the insulation affected.

Partial discharges in an insulating material usually initiate in gas-filled voids within the dielectric. Since the dielectric constant of the gap is considerably lower than that of the insulating material, the electric field is higher in the gap than at similar distances within the insulating material. If the voltage per metre within the gap increases above the corona voltage threshold, the partial discharges will become active.

Once the partial discharges have started, a progressive deterioration of the insulating materials occurs and may eventually lead to the failure of the insulation. Partial discharges are prevented by careful design and good materials. In high voltage equipment, the integrity of the insulation is verified by the use of partial discharge detection equipment both during the manufacturing process and periodically during the lifetime of the units. Partial discharge prevention and detection is critical to ensure long and reliable operation of high voltage utility equipment.

Equivalent circuit of partial discharges

The equivalent circuit of a dielectric with a cavity can be modelled as a capacitive voltage divider in parallel with another capacitor. The top capacitor of the divider represents the parallel of the series capacitances with the cavity and the bottom capacitor represents the capacitance of the gap. The parallel capacitor represents the remaining capacitance unaffected by the cavity.

Partial discharge currents

When partial discharge activity is initiated, transient pulses of high frequency current will appear with a duration between nanoseconds and microseconds, these pulses will reappear in a repetitive fashion. Partial discharge currents are difficult to measure because of their small magnitude and duration. The event can be detected as a minute change in the current drawn by the equipment under test. Another method of measuring these currents is to install a resistor in series with the equipment under test and analyse the voltage drop with an oscilloscope.

Apparent electrical load

The actual change in charge that occurs during partial discharge is not directly measurable. The concept of apparent charge is used. The apparent load (q) of a PD event does not represent the actual load of the equipment but represents the change in load which, if connected between the terminals of the equipment under test, would cause a change in voltage equivalent to the PD event. Mathematically it can be modelled by the equation:

The apparent electrical charge is usually measured in picocoulombs (pC).

Partial discharge measuring equipment

This new Amperis product line comprises several devices for partial discharge detection. The equipment detects partial discharges associated with the presence of insulation defects on cable joints and other cable accessories, in order to verify the safety and reliability of electrical networks. All our products are accurate, excellent for use as part of a preventive maintenance programme.

Our products detect partial discharges of XLPE/EPR type cable joints, circuit breakers, cable bends, cable terminations and distribution transformers with high accuracy. In addition, the ULD 40 allows the detection of ultrasound generated by corona effects and arcing on overhead power network components.