Chuck Yung
EASA Senior Technical Support Specialist

Concerns about partial discharge (PD) used to be limited to repairers and users of machines rated over 7 kV. PD is a common consideration for machines rated 11 kV and higher. This article will describe PD, explain how to detect it, and offer repair solutions.

What is partial discharge?
Air, like Nomex^®, Mylar^®, mica and Dacron^®, is an insulator. Like any insulation, it will break down electrically if subject to too high an im­pressed voltage. Air is capable of with­standing approximately 75 volts/mil (3000 volts/mm), as compared to mod­ern insulating materials rated many times higher. (See Table 1.)

Figure 1 illustrates the Northern Lights, a vivid example of partial discharge occurring. It’s a beautiful, sometimes haunting sight. The PD that occurs in windings is on a much smaller scale, of course, but not some­thing that a motor owner wants to have present in a winding.

Air as an insulator
The high-tension transmission lines crisscrossing developed countries use tall towers with robust insulators to suspend bare cables for the purpose of power transmission. The conduc­tors – thousands of miles (km) of them – would be far more expensive if in­sulated conductors were used, so the bare conductors are strung many yards (meters) apart. That separation between the conductors is the only insulation needed between phases – a great dis­tance of air.

When sufficient­ly high voltage is impressed across air, the air begins to break down electri­cally. As that hap­pens, it arcs, and the arcing etches or burns away the adjacent insulating ma­terial. At the same time, the air ionizes, releasing ozone. The ozone chemically attacks the insulation. 

In a form-wound electric motor, even though each form coil is wrapped with insulation, the air between coils becomes part of the insulation system. If the voltage stress across the gap between any coils is sufficiently high, the air be­tween those coils will break down electrically. When that happens, arcing etches the adjacent in­sulation, and ozone released by the arc­ing chemically attacks the insulation. Over time, the damage progressively worsens and may lead to turn-to-turn or phase-to-phase shorting and wind­ing failure.

In a well-designed winding, most insulation materials – Nomex, mica, epoxy and the like – are capable of withstanding the voltage stress im­pressed upon them. As with most insulating materials, the presence of moisture will reduce the voltage-withstand capability of air.

Phase voltage equals line voltage for a delta-connected motor, or 58% of line voltage for a winding that is con­nected wye. Thus, the voltage potential between coils of different phases is much higher than the voltage stress between adjacent coils within the same phase group. Visual inspection should focus on the area between groups (see Figure 2).

Winding-to-ground potential equals phase voltage for a winding operating on a properly grounded system. For form coil windings rated above 5 kV, there is a greater potential for partial discharge between coils and the slots. Machines rated over 7 kV should use special tapes to control the risk. Con­ductive tape (usually black in color) is used along the straight section of each coil, to discharge voltage that builds along the coil surface, back to ground. Slightly overlapping the end of the con­ductive tape should be semi-conductive tape (usually gray in color) to conduct surface voltage back to the conductive tape and – ultimately – to ground.

If the conductive and semi-con­ductive tapes do not make complete contact, the voltage stress at the edge of the conductive tape will increase, and erosion of the insulation immediately at the edge of the conductive tape will occur. (See the August 2007 issue of Currents, “Voltage Stress: Not as Simple as it Sounds.”)

Can we fix it?
When there is visible damage from PD, that does not mean the winding is in imminent dan­ger of failing. Steps to reverse or slow down the damage are available. First, if damage from PD is obvious on the coil extensions (see Fig­ure 2), it is practical to clean the wind­ing, and then use a stiff brush or scrub­bing pad to abrade the damaged area. Follow by blowing out the loose mate­rial with dry compressed air, and then use a high quality, 2-part epoxy resin to paint the damaged area. In cases where the erosion is severe, some winders use powdered mica mixed into the epoxy.

Detecting partial discharge
An industry goal for decades has been to evaluate how severe the dam­age from PD really is. If we can quan­tify it, at what point is it “too severe” to remain in service. How long can such a machine operate before it must be rewound? There are companies who specialize in detecting, measuring and evaluating the extent of winding dam­age caused by PD.

For a low-tech approach to detect­ing the presence of PD, drape the suspect winding with a dark tarp or black plastic. Use a surge tester to en­ergize the winding to at least 120% of line voltage, and visually inspect the winding for arcing. At the same time, listen for audible sounds of arcing that might occur out of your line-of-sight. The voltage stress is higher between coils of adjacent phases than between coils within the same group, so look closely at the phase coils. 

Monitoring PD activity
Adwel; Doble; Electrical Diagnostic Innovations; IRIS Power, a Qualitrol Company; ndb Technologie and simi­lar companies offer instrumentation to monitor PD activity on-line. While it is not practical to offer a hard-and-fast PD cutoff value above which a motor should be rewound, trending the PD al­lows an end user to judge how quickly the damage is progressing. Compar­ing the PD activity level of identical motors is useful; trying to compare significantly different designs is less so.

There are other, less sophisticated ways to detect PD in service. One is to measure the ozone in the atmosphere inside the stator. 

A few years ago, a member called me to report that they could smell the ozone when they walked into the building where a particular 12 kV motor was running. That’s alarming. Another method is to use an ultrasonic listening device to listen for the arc­ing that accompanies PD. If you have the ultrasonic listening devices com­monly used for checking compressed air leaks, those are a useful tool for detecting PD. They can also be used to reveal arcing contacts or other arcing problems that might not otherwise be obvious. Thermal imaging can be used to pinpoint the source of such damage.

A member in Australia described using an AM radio to search for PD. If you’ve ever experienced a noisy radio as you passed a source of electrical noise, you know just what he meant.

PD in random windings
PD is not limited to high-voltage machines; it is a very real problem in random wound motors operating from variable frequency drives (VFD). Figure 3 shows a random wound stator with a winding failure that resulted from PD activity due to operation from a VFD. 

Recall that the breakdown voltage of air is approximately 75 volts/mil; when a 460v winding operates from a VFD, the voltage overshoots can be as high as 1500v to 2000v. On a 575v winding, the voltage spikes are proportionally higher. Where random wires are close together, the volts/mil limit can be exceeded. Partial discharge damages the film coating and resin on the wire, and a turn-turn fault is inevitable. Inverter duty wire uses an oxide coating to effectively shield the wire and better resist PD. 

Ideally, there are two things to consider when a random wind­ing operates from a VFD. First, and most obvious to us as re­pairers, is the voltage at which the winding will be damaged by the VFD. That volt­age is termed the “PD inception voltage” of the winding. Second is the “PD inception voltage” produced by the VFD. The same winding procedures you use with great success on many VFD applications may fail if matched up with a VFD that causes higher voltage overshoots. Or a VFD in an application where there is a long cable run between the motor and the drive. Not only that, the type of cable has an effect on the severity of PD.

Compounding the difficulty of cor­rectly identifying a winding damaged by PD is the sophistication of most VFDs. The VFD can detect the turn-turn short, often before the damage is visible to the naked eye. Members often report that a motor, sent in for a winding fail­ure, will still operate fine on sinusoidal power in their service center. That is a good clue that the motor is indeed operating from a VFD. The best test at that point is to surge test the winding and look for incipient faults.

Categories: Stators/windings, Design/theory/application, Troubleshooting & failure analysis

Tags: Partial discharge

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