What Is This Partial Discharge Thing?

PD is no longer only a factor in high-voltage systems.

By Howard W. Penrose, PhD, CMRP, MotorDoc

Partial Discharge (PD) is an electrical effect that, in the past, was only a concern for high-voltage system engineers and technicians. In systems of more than 6,000 V, rapid electrical discharges, such as tiny lightning storms, can occur across surfaces, between surfaces, within insulation systems, and even between cables or cables to ground. When these discharges occur across surfaces or on high-voltage transmission lines, they are often referred to as corona.

In the 1990s, with the advent of pulse-width modulated variable-frequency drives, the conditions that allowed PD to occur dropped to voltages from 200 to 300 VAC. This is related to how fast each pulse increases to the peak voltage (dV/dt), the impedance of the conductors and motor, the temperature of the insulation system, and any power harmonics on drive output. The PD occurs in small spaces and between conductors where they are, usually, spaced less than a millimeter apart without insulating varnish between them, or in high-potential crossovers.

The result of PD, in all cases, is chemical etching that results when ozone discharges into the air. The ozone degrades insulating materials and other dielectrics, such as paint, leaving an etched appearance that is often misidentified as a powdery substance.  In low-voltage, random-wound motors, the weakness tends to occur where stray conductors exist in the end turns or where a conductor crosses several other conductors in the same or a different coil.

The stress that causes extreme forms of this type of PD generally occurs when the drives are not set correctly and/or specific distances are met between the motor and drive. In some cases, it is very important to keep connection-box conductors from being pressed against the inside of the connection box, which can result in a grounded condition from both PD and vibration.

Ozone attacks the electrical insulation system, gradually removing material and changing it chemically.  This breakdown will eventually allow a short to ground or between conductors, depending on where the defect occurs. In medium voltage (>1,000 V) and higher cables, if it is carrying high harmonic content or associated with a VFD, if leaning against the inside of a cabinet or transformer enclosure, you may notice discoloration on the housing or cable where it is in contact. This can also occur in splices when they are not set up correctly or become contaminated.

Cable discharge is occurring on a 4,160-V cable feeding a VFD.

In motor systems exceeding 6 kV, PD (internal) and corona (surface) occur due to power, temperature, load, and harmonic content. Small voids in the coils between conductors or in the tapes and varnishes that make up the coil outer insulation, and then between the coils and stator frame, become points for PD. Dirt, contamination, and even humidity generate corona effects on the surfaces of the insulation system.

This screen capture shows Visual Acoustic testing for partial discharge and corona.

The types of methods and patterns associated with PD and corona are identified in IEEE and IEC standards that include IEEE 1434 for machines, IEEE 2862 for cables, IEC 60270 for cables and switchgear, as well as several others in both IEEE and IEC standards. Most of the detection systems include amplitude modulated antennae to detectors (RTDs and capacitors), corona camera, ultrasound instruments including visual acoustic cameras, and electrical signature analysis.

In this slot PD of a 13.8-kV generator, the red arrow indicates conductive tape (black) and the blue arrow indicates missing conductive tape from slot partial discharge. The white tape is the remaining armor tape.

PD and corona are long-term failure processes usually measured in years. Changes in PD signatures and values are trend-able for determining risk of failure with limits depending on the types of insulation systems involved. When detected, some mitigation is possible, usually involving power correction and/or cleaning components. However, most instances require more aggressive repair such as insulation replacement.

Howard W. Penrose, PhD, CMRP, is president of MotorDoc LLC, Lombard, IL (motordoc.com). He chairs the wind-power standards and government relations participation for American Clean Power (ACP/AWEA), is the USA representative to CIGRE for high voltage electric machinery, holds various IEEE standards positions, and is a past chair of SMRP.  He holds certifications from SMRP as a Certified Maintenance and Reliability Professional and the Association of Energy Engineers (AEE) as a Certified Energy Manager. Reach him at info@motordoc.com.