Insulation Performance of Insulated Coil Discs:High-Voltage Coil Withstand Voltage Testing Explained

Insulated coil discs are core components of windings in transformers,reactors and high-voltage inductors.Through the insulation base material and winding process,they achieve electrical isolation between turns,between layers and to ground.Insulation performance directly determines the reliability of high-voltage coil operation,and the withstand voltage test is the most common and critical method to verify the insulation capability of insulated coil discs.This article explains insulation performance indicators,the principle of the withstand voltage test,the test procedure,acceptance criteria and testing precautions for high-voltage coil R&D and testing personnel.

1.Core Insulation Performance Indicators of Insulated Coil Discs

The insulation system of an insulated coil disc consists of the conductor insulation layer,inter-layer insulation material,end insulation and the overall impregnated insulation structure.Several key indicators require attention.

Dielectric strength represents the highest voltage the insulation material can withstand without breakdown and is the core focus of the withstand voltage test.Insulation resistance reflects the overall leakage level of the insulation structure;the higher the insulation resistance,the lower the leakage current and the better the insulation condition.

Partial discharge characteristics:under high-voltage conditions,tiny bubbles and gaps inside the insulation can easily generate partial discharge,and long-term discharge gradually erodes the insulation material,causing insulation aging.Thermal class:different insulation materials tolerate different temperatures,and long-term over-temperature accelerates insulation embrittlement and cracking,causing continuous degradation of insulation performance.

Moisture resistance should also be considered.Humid environments greatly reduce insulation resistance and increase breakdown risk.Therefore,the drying and varnish-impregnation curing processes after coil disc forming are designed to stabilize insulation performance.

2.Basic Principle of the High-Voltage Coil Withstand Voltage Test

The withstand voltage test,also called the dielectric strength test,is divided into power-frequency withstand voltage testing and induced withstand voltage testing.For high-voltage coils composed of insulated coil discs,power-frequency testing is more commonly used.

The test principle is to apply a test voltage higher than the rated working voltage to the test terminals of the coil within a specified time,hold it for a period,and observe whether the insulation structure experiences breakdown,flashover,obvious heating or continuously increasing leakage current.

This is a type of acceptance test designed to expose latent defects inside the insulated coil disc,such as damaged insulation film,displaced insulation during winding,metallic impurities between layers,and air gaps caused by insufficient varnish impregnation.Note that the withstand voltage test is a short-time overvoltage assessment.It should not be held continuously at high voltage,or irreversible damage may occur to qualified insulation.

3.Preparation and Test Procedure

Pretreatment should be done first.The insulated coil disc coil needs to be dried and cooled according to process requirements,with a dry and clean surface free of dust,oil and moisture.Place the sample away from metal structures with sufficient safety clearance to avoid interference from external air breakdown.Also confirm the test plan,including test voltage,hold time and voltage ramp rate,matching the voltage class standard of the coil.

During connection,connect the high-voltage output of the withstand voltage tester to the tested coil winding,and reliably ground the low-voltage terminal and shield terminal.Non-tested windings should also be grounded.Check that connections are firm and set up isolation warnings in the test area.

Main test steps:

1.Ramp the voltage up slowly from zero,controlling the ramp rate according to the standard;never apply the test voltage directly at once.

2.After the voltage reaches the specified value,start timing and hold,with a typical hold time of 60 seconds.

3.During the hold period,continuously observe changes in leakage current and watch for abnormal phenomena such as noise,smoke,sparks or odor.

4.After the hold time ends,reduce the voltage steadily and uniformly to zero;do not cut power suddenly.

5.After the test,fully discharge the coil to release residual charge and prevent electric shock.

If breakdown,flashover or a sharp rise in leakage current occurs during the test,the sample is judged as failed.Reduce the voltage and cut power immediately.

4.Result Acceptance and Common Influencing Factors

Acceptance criteria:within the specified hold time,if there is no breakdown,no flashover,stable leakage current and no continuous rise,the sample passes the withstand voltage test.

Many factors affect test results.High ambient humidity increases conductance on the air and insulation surface,causing surface tracking.Insufficient drying leaves residual moisture inside the coil disc,greatly reducing the withstand voltage value.Scratches and wrinkles in the insulation layer during winding are the main causes of inter-turn breakdown.Poor electrode contact and burrs on lead wires can induce tip discharge and cause misjudgment.

5.Testing Safety and Usage Suggestions

The withstand voltage test is a high-voltage test.Operators must follow high-voltage testing specifications,and personnel must not approach the sample during the test.The same insulated coil disc should not undergo repeated withstand voltage tests,because repeated application of voltage accumulates insulation damage and shortens service life.

The withstand voltage test is only one item in insulation performance testing.Complete evaluation of an insulated coil disc also requires insulation resistance testing and partial discharge testing for multi-dimensional verification.

The insulation protection of an insulated coil disc originates from material selection,winding control and varnish-impregnation curing processes.As a quality checkpoint,the withstand voltage test can screen out defective workpieces in advance,avoid insulation failures during the operation of high-voltage coils,and ensure the stable operation of high-voltage electrical equipment.