The Basics of Partial Discharge Testing

Partial discharge (PD) is evidence of a degrading insulation system, which could lead to very costly repairs and can predictively lead to an electrical breakdown of high voltage apparatus. The phenomenon is of great practical interest in the electric power industry, as the presence and magnitude of PD are important criteria to measure for the early detection of degrading insulation quality and the assessment of manufactured, installed, or repaired product quality. One of the most difficult tasks is interpreting the PD data and determining the time an insulation system may remain in service before damaging and costly failure can occur. HV TECHNOLOGIES, Inc. has many years of experience in the field of PD testing and supplies equipment with different measurement technologies for different PD testing applications.









The most common circuit for measuring PD is the top left in the above image, in which the measuring impedance Zm is in series with the coupling capacitor Ck, both of which are in parallel to the DUT. Detection sensitivity can be increased if the measuring impedance is placed in series with the ground of the DUT (top right). However, in this configuration the measuring impedance could be seriously damaged from an unexpected breakdown. Even higher measurement sensitivity can be achieved by utilizing a bridge impedance (lower left) by connecting to the low voltage side of the DUT and coupling capacitor. This set-up significantly helps to reduce external electromagnetic influences. In transformer testing, if capacitively-graded bushings are available, these can be used as the coupling capacitor for the PD measurement circuit (bottom right).



When measuring PD via galvanic coupling to the terminals of the test object according to the circuits recommended in IEC 60270, high frequency attenuation and distortion of the PD pulse as the signal propagates from the source to the measurement location will cause the pulse peak, fall time, and pulse duration to vary over a wide range. However, since the signal is being integrated over time, this will always result in the same measured charge regardless of the shape of the pulse. Therefore, pulse charge is an extremely important and useful parameter to measure.

A typical wideband PD measurement according to IEC 60270 is characterized by a transfer impedance having fixed values of the lower and upper limit frequencies of 100 and 500 kHz, respectively (Note: an amendment to IEC 60270 in 2015 has allowed new frequency parameters to be 100 kHz and 1 MHz, respectively). In this frequency range a quasi-integration is conducted to determine the pulse charge.

Considering that a PD source is generally not directly accessible in HV apparatus, unless PD couplers are installed at time of manufacturing or "free space" PD detection (detection of the electromagnetic waves) is conducted, this means that the charge from the discharge cannot be measured directly. As a result, the PD measuring circuit needs to be "calibrated" in order to be able to measure apparent charge, which IEC defines as:

 

    "Apparent charge q of a PD pulse is the charge which, if injected within a very short time between the terminals of the test object in a specified test circuit, would give the same reading on the measuring instrument as the PD current pulse itself. The apparent charge is usually expressed in picocoulombs (pC)."








Typical phase resolved PD patterns for void or surface discharges.

Corona discharges, which IEC 60270 defines as "a form of partial discharge that occurs in gaseous media around conductors which are remote from solid or liquid insulation", typically have discharges that occur at the maximum of the AC voltage sinewave. There is also a polarity effect that when a sharp protrusion is at high voltage potential, the discharges will superimpose on the maximum of the negative sinewave (270° phase). When the sharp protrusion is at ground potential, the discharges will superimpose on the maximum of the positive sinewave (90° phase).



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