Partial discharge

In electrical engineering, partial discharge (PD) is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors. While a corona discharge is usually revealed by a relatively steady glow or brush discharge in air, partial discharges within solid insulation system are not visible.

PD can occur in a gaseous, liquid or solid insulating medium. It often starts within gas voids, such as voids in solid epoxy insulation or bubbles in transformer oil. Protracted partial discharge can erode solid insulation and eventually lead to breakdown of insulation.

Contents

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 considerably less than the surrounding dielectric, the electric field 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.

PD can also occur along the surface of solid insulating 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.

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.

When PD occurs, electromagnetic waves propagate away from the discharge site in all directions. Detection of the high-frequency pulses can identify the existence and location of partial discharge.

Discharge detection and measuring systems

With the partial discharge measurement, the dielectric condition of high voltage equipment can be evaluated, and treeing in the insulation can be detected.

Whilst tan delta measurement allows detection of water trees, partial discharge measurement is suitable for detection and location of electrical trees.

Data collected during the procedure is compared to measurement values of the same cable gathered during the acceptance-test.

This allows simple and quick classification of the dielectric condition (new, strongly aged, faulty) of the device under test and appropriate maintenance and repair measures may be planned and organized in advance.

Partial discharge measurement is applicable to cables and accessories with various insulation materials, such as polyethylene or lead-covered paper-insulated cable. Partial discharge measurement is routinely carried out to asses the condition of the insulation system of rotating machines (motors and generators), transformers and gas-insulated switchgear.

Partial discharge measurement system

A partial discharge measurement system basically consists of:

The principle of partial discharge measurement

A number of discharge detection schemes and partial discharge measurement methods 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 realm. On an oscilloscope, the discharges look like randomly occurring 'spikes' or pulses. The usual way of quantifying partial discharge magnitude is in picocoulombs. The intensity of partial discharge is displayed versus time.

An automatic analysis of the reflectograms collected during the partial discharge measurement – using a method referred to as time domain reflectometry TDR – allows the location of insulation irregularities. They are displayed in a partial discharge mapping format.

A phase-related depiction of the partial discharges provides additional information, useful for the evaluation of the device under test.

Calibration setup

The actual charge change that occurs due to a PD event is not directly measurable. Apparent charge is used instead. The apparent charge (q) of a PD event is the 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_b \Delta(V_c)

The apparent charge is not equal to the actual amount of changing charge at the PD site, but can be directly measured and calibrated. 'Apparent charge' is usually expressed in picocoulombs.

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 no voltage area. Calibrators are usually disconnected during the discharge testing.

Laboratory methods

In wideband detection, the 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.

Field testing methods

Field measurements preclude the use of a Faraday cage and the energising supply can also be a compromise from the ideal. Field measurements are therefore prone to noise and consequently less sensitive. [1][2]

Other methods have therefore been developed for field measurement which, while not as sensitive as an IEC measurement, are substantially more convenient. By necessity field measurements have to be quick, safe and simple if they are to widely applied by owners and operators of MV and HV assets

Transient Earth Voltages(TEVs) are induced voltage spikes on the surface of the surrounding metalwork. These occurs because the partial discharge creates current spikes in the conductor and hence also in the earthed metal surrounding the conductor. TEV pulses are full of high frequency components and hence the earthed metalwork presents a considerable impedance to ground. Therefore voltage spikes are generated. These will stay on the inner surface of surrounding metalwork (to a depth of approximately 0.5 microns in mild steel at 100 MHz) and loop around to the outer surface wherever there is an electrical discontinuity in the metalwork. There is a secondary effect whereby electromagnetic waves generated by the partial discharge also generate TEVs on the surrounding metalwork - the surrounding metalwork acting like an antenna. TEVs are a very convenient phenomenon for measuring and detecting Partial Discharges as they can be detected without making an electrical connection or removing any panels.

Ultrasonic measurement relies on fact that the partial discharge will emit sound waves. The frequency bandwidth for emissions tends to be centred on 40kHz but will stretch into the audible area for extremely bad discharges. Ultrasound will not be emitted by an Internal Discharge. The usefulness of Ultrasonic detection is therefore restricted to Surface Discharge and Corona Discharge.

Electro Magnetic Field detection picks up the radio waves generated by the partial discharge. As noted before the radio waves can generate TEVs on the surrounding metalwork. More sensitive measurement, particularly at higher voltages, can be achieved using in built UHF antennas or external antenna mounted on insulating spacers in the surrounding metalwork.

Effects of partial discharge in insulation systems

Once begun, PD causes progressive deterioration of insulating materials, ultimately leading to electrical breakdown. 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.

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.

International Standards

References

  1. ^ D. F. Warne Advances in high voltage engineering, Institution of Electrical Engineers, 2004 ISBN 0852961588 , page 166
  2. ^ "Testing Distribution Switchgear for Partial Discharge in the Laboratory and the Field". IEEE. 2008-06-12. http://ieeexplore.ieee.org/Xplore/login.jsp?url=http://ieeexplore.ieee.org/iel5/4544566/4570258/04570430.pdf%3Farnumber%3D4570430&authDecision=-203. 

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