Laminated busbar insulation and partial discharge

What partial discharge is

Partial discharge is a small electrical breakdown inside or on the surface of an insulation system. It does not bridge the whole insulation, so it does not trip a hipot set. It occurs in a void, at a sharp edge or along a delaminated interface, where the local field is higher than the surrounding material can hold.

Each discharge erodes the material a little. Over time the erosion forms a conducting path, the insulation fails and the busbar shorts. The damage can take months or years, which is why a part can pass a production test and fail in service. The test is looking for a defect that will grow, not for a part that is already broken.

Laminated busbars are particularly exposed because they contain many interfaces. Each layer of film, each bond line and each cut edge is a place where a void can sit. The quality of the laminate, the cleanliness of the film and the geometry of the conductor edge all decide whether the part discharges at its working voltage.

Where defects sit in a laminate

A void in the bond line is the classic defect. Air has a lower dielectric strength than the polymer film, so the field concentrates in the void and it discharges first. The void may come from a bubble in the adhesive, a contaminant, a low bond pressure or a film that did not wet the conductor.

A sharp conductor edge is the second source. The electric field rises at a corner or a burr, and the film over it sees a higher stress than the flat face. A conductor that is punched or sheared has a burr on one side, and that burr can be the highest field point in the part.

A delamination at the edge or around a hole is the third. The layers separate, air enters and the interface becomes a discharge path. The edge is also where the creepage distance is shortest, so a defect there has less material to erode before it tracks.

Dielectric tests and what each proves

Insulation resistance shows the bulk leakage path. It catches moisture, contamination and a gross crack, but it does not find a small void. Hipot applies a high voltage for a short time and catches a part that breaks down. It is a go or no-go test, and it can damage a marginal part without showing why.

Partial discharge measurement finds the discharge itself. The test raises the voltage until discharge starts, records the inception voltage, then lowers the voltage until the discharge stops, which is the extinction voltage. The magnitude is measured in picocoulombs. A part with a low inception voltage has a defect that will age.

Capacitance and dissipation factor are supporting measurements. A change in capacitance can show a missing layer or a void, and a rise in dissipation factor can show moisture or a lossy bond. Neither replaces the PD measurement, but together they give a fuller picture of the laminate.

Running a PD test that means something

The test has to be run in a room with a low electrical noise floor. A PD set that is not screened will pick up corona from the leads, the fixture and the surrounding equipment, and the operator will not know whether the discharge is in the sample or in the room. A background check with a known defect-free sample is the first step.

The calibration should be done with a calibrator at the test setup, not on a previous day. The leads, the coupling capacitor and the sample capacitance all affect the reading. The test voltage sequence should be written down: raise to the pre-stress level, hold, raise to the test level, hold, lower to the extinction level and record.

The acceptance limit should state the voltage at which it applies. A limit of a few picocoulombs at the working voltage is a different requirement from the same limit at twice the working voltage. A part that is quiet at working voltage and discharges at twice working voltage may still be acceptable, depending on the insulation coordination and the transient overvoltage.

The test is also a check on the fixture. A sample that discharges at the terminal, rather than in the laminate, is a test problem. The connections should be smooth and the sample should be clean and dry.

Design rules that lower PD risk

Round the conductor edge. A radius on the edge reduces the field concentration and gives the film a smooth surface to cover. A punched edge with a burr is the worst case, so the drawing should call the burr direction and a maximum burr height, or specify a deburr before lamination.

Increase the edge margin. The margin is the distance from the conductor edge to the cut edge of the laminate. A larger margin puts more insulation between the conductor and the outside, which raises the inception voltage and lengthens the creepage path. The margin should be called on the drawing, not left to the laminator.

Control the bond. A void-free bond needs a clean film, a controlled adhesive weight, the right temperature and enough pressure. The process window is part of the design, and a change in adhesive or press cycle should trigger a re-test.

• Round or deburr the conductor edge before lamination.
• Call an edge margin and check it on the finished part.
• Specify the bond quality and the void acceptance.
• Keep the cut edge clean and free of delamination.
• Avoid sharp internal corners in the conductor profile.
• Re-test when the film, adhesive or press cycle changes.

Inspection and records

A production inspection plan for a laminated busbar usually combines a visual check, a dimensional check, an insulation resistance test and a hipot test. Partial discharge testing is often a type test or a sample test, because it needs a quiet room and a longer setup. The plan should say which test is applied to every part and which is applied to a sample.

The record should identify the sample, the test voltage, the PD level, the inception and extinction voltages, the background noise and the calibration date. A result without the background is hard to interpret. A result without the test voltage cannot be compared with the drawing.

For a critical part, a microsection of a production sample shows the bond line, the void content and the edge geometry. It is a destructive check, so it belongs on a sample or a first article, not on every part.

When to ask for PD testing

Ask for PD testing when the busbar works at a voltage where a small defect can discharge, when the insulation is thin, when the part is safety related, or when the service life is long and the part is difficult to replace. The test is also useful as a development tool, because it shows which design change moved the inception voltage.

Do not use a PD test as a substitute for a good laminate. A part with a large void will fail, and a part with a small void may pass at the test voltage and fail later. The test sets a limit; the process controls the population. The drawing, the laminate specification and the inspection plan have to work together.

If the application has transients, the test voltage should reflect them. A part that is quiet at the nominal working voltage can discharge during a switching transient, so the insulation coordination study should give the test level.