Partial discharge

In electrical engineering, a partial discharge (PD) is a localised dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high voltage stress. While a corona discharge is usually revealed by a relatively steady glow or brush discharge in air, partial discharges within an insulation system may or may not exhibit visible discharges, and discharge events tend to be more sporadic in nature than corona discharges.

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Discharge mechanism

PD usually begins within voids, cracks, or inclusions within a solid dielectric, at conductor-dielectric interfaces within solid or liquid dielectrics, or in bubbles within liquid dielectrics. Since discharges are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials.

Partial discharges within an insulating material are usually initiated within gas-filled voids within the dielectric. Because the dielectric constant of the void is consideraby less than the surrounding dielectric, the electric field (and the voltage stress) appearing across the void is significantly higher than across an equivalent distance of dielectric. If the voltage stress across the void is increased above the corona inception voltage (CIV) for the gas within the void, then PD activity will start within the void.

Once begun, PD causes progressive deterioration of insulating materials, ultimately leading to electrical breakdown. PD can be prevented through careful design and material selection. In critical high voltage equipment, the integrity of the insulation is confirmed using PD detection equipment during the manufacturing stage as well as periodically through the equipment's useful life. PD prevention and detection are essential to ensure reliable, long-term operation of high voltage equipment used by electric power utilities.

Partial discharge equivalent circuit

The equivalent circuit of a dielectric incorporating a cavity can be modeled as a capacitive voltage divider in parallel with another capacitor. The upper capacitor of the divider represents the parallel combination of the capacitances in series with the void and the lower capacitor represents the capacitance of the void. The parallel capacitor represents the remaining unvoided capacitance of the sample.

Partial discharge currents

When partial discharge is initiated, high frequency transient current pulses will appear and persist for nano-seconds to a micro-second, then disappear and reappear repeatedly. PD currents are difficult to measure because of their small magnitude and short duration. The event may be detected as a very small change in the current drawn by the sample under test. One method of measuring these currents is to put a small current-measuring resistor in series with the sample and then view the generated voltage on an oscilloscope via a matched coaxial cable.

Apparent Charge

The actual charge change that occurs due to a PD event is usually not directly measurable. Apparent charge is used instead. The apparent charge (q) of a PD event is not the actual amount of charge changing at the PD site. Instead, it is the change in charge that, if injected between the terminals of the device under test, would change the voltage across the terminals by an amount equivalent to the PD event. This can be modeled by the equation:

q = C_bΔ(V_c)

The apparent charge is not equal to the actual amount of changing charge at the PD site, but is more realistic than ΔV_a. 'Apparent charge' is usually expressed in picocoulombs. partial discharge is not electronic.

Discharge detection and measuring systems

A number of discharge detection schemes have been invented since the importance of PD was realised early in the last century. Partial discharge currents tend to be of short duration and have rise times in the nanosecond regime. On an oscilloscope, the discharges look like randomly occurring 'spikes' or pulses. The usual way of quantifying partial discharge magnitude is in picocoulombs.

Calibration setup

This is measured by calibrating the voltage of the spikes against the voltages obtained from a calibration unit discharged into the measuring instrument. The calibration unit is quite simple in operation and merely comprises a square wave generator in series with a capacitor connected across the sample. Usually these are triggered optically to enable calibration without entering a dangerous high voltage area. Calibrators are usually left connected during the discharge testing.

Laboratory methods

Wideband PD detection circuits

In wideband detection, the coupling impedance usually comprises a low Q parallel-resonant RLC circuit. This circuit tends to attenuate the exciting voltage (usually between 50 and 60 Hz) and to amplify the voltage generated due to the discharges.

Tuned (narrow band) detection circuits

Differential discharge bridge methods

Acoustic and Ultrasonic methods

Field testing methods

Clearly the field testing of plant and equipment in service by the above method, although it has been done, is not very convenient. Therefore field testing for PD activity has taken on a slightly different role where accuracy of measurement is less important than basic indication of discharge activity. To this end, a number of specialised instruments have been developed by many companies around the world.

Effects of partial discharge in insulation systems

PD are localised ionisation within electrical insulation that are caused by a high electrical field. They occur in part of the insulation system and are limited in extent, so they do not immediately cause full insulation breakdown.

PD can occur in a gaseous, liquid or solid insulating medium. It is often initiated within gas voids enclosed in solid insulation, or in bubbles within a liquid insulating material, such as voids in an epoxy insulator, or gas bubbles dissolved within transformer oil.

As the gas within the void has a dielectric constant much less than the surrounding material, it experiences a significantly higher electric field. When this becomes high enough to cause electrical breakdown in the gas, a partial discharge occurs. PD can also occur along the surface of solid insulting materials if the surface tangential electric field is high enough to cause a breakdown along the insulator surface. This phenomenon commonly manifests itself on overhead line insulators, particularly on contaminated insulators during days of high humidity. Overhead line insulators use air as their insulation medium.

The effects of PD within high voltage cables and equipment can be very serious, ultimately leading to complete failure. The cumulative effect of partial discharges within solid dielectrics is the formation of numerous, branching partially conducting discharge channels, a process called treeing. Repetitive discharge events cause irreversible mechanical and chemical deterioration of the insulating material. Damage is caused by the energy dissipated by high energy electrons or ions, ultraviolet light from the discharges, ozone attacking the void walls, and cracking as the chemical breakdown processes liberate gases at high pressure. The chemical transformation of the dielectric also tends to increase the electrical conductivity of the dielectric material surrounding the voids. This increases the electrical stress in the (thus far) unaffected gap region, accelerating the breakdown process. A number of inorganic dielectrics, including glass, porcelain, and mica, are significantly more resistant to PD damage than organic and polymer dielectrics.

In paper-insulated high-voltage cables, partial discharges begin as small pinholes penetrating the paper windings that are adjacent to the electrical conductor or outer sheath. As PD activity progresses, the repetitive discharges eventually cause permanent chemical changes within the affected paper layers and impregnating dielectric fluid. Over time, partially conducting carbonized trees are formed. This places greater stress on the remaining insulation, leading to further growth of the damaged region, resistive heating along the tree, and further charring (sometimes called tracking). This eventually culminates in the complete dielectric failure of the cable and, typically, an electrical explosion.

PD dissipate energy, generally in the form of heat, but sometimes in as sound and light as well, like the hissing and dim glowing from the overhead line insulators. Heat energy dissipation may cause thermal degradation of the insulation, although the level is generally low. For high voltage equipment, the integrity of the insulation can be confirmed by monitoring the PD activities that occur through the equipment's life. To ensure supply reliability and long-term operational sustainability, PD in high-voltage electrical equipment should be monitored closely with early warning signals for inspection and maintenance.

International Standards

• IEC 60270:2000/BS EN 60270:2001 "High-Voltage Test Techniques - Partial Discharge Measurements"
• IEEE 400-2001 "IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems"
• IEEE 1434-2000 "IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Machinery"

References

• High Voltage Engineering Fundamentals, E.Kuffel, W.S. Zaengl, pub. Pergamon Press. First edition, 1992 ISBN 0-08-024213-8
• Engineering Dielectrics, Volume IIA, Electrical Properties of Solid Insulating Materials: Molecular Structure and Electrical Behavior, R. Bartnikas, R. M Eichhorn, ASTM Special Technical Publication 783, ASTM, 1982
• Engineering Dielectrics, Volume I, Corona Measurement and Interpretation, R. Bartnikas, E. J. McMahon, ASTM Special Technical Publication 669, ASTM, 1979, ISBN 0-8031-0332-8

• Example of an Instrument for Partial Discharge Analysis