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IEC 62068: Evaluating Electrical Insulation Endurance Under Repetitive Voltage Impulses
A technical guide to IEC 62068:2013 – screening tests, endurance testing, and partial discharge evaluation for insulation systems under impulse stress
IEC 62068:2013 provides a general test methodology for evaluating the electrical endurance of insulating materials and systems subjected to repetitive voltage impulses – a critical requirement for modern power electronics applications.
1. The Challenge of Repetitive Impulse Ageing
Traditional AC power-frequency voltage endurance testing is insufficient for equipment fed from electronic power supplies. Modern variable frequency drives, PWM inverters, and power converters generate repetitive voltage impulses with rise times as short as 40 ns and repetition rates up to 10 kHz. These impulses degrade insulation through fundamentally different processes: partial discharge activity, space charge injection/extraction, electromechanical fatigue, and dielectric heating from high-frequency components.
| Degradation Mechanism | Physical Process | Key Factor |
|---|---|---|
| Partial discharge (PD) | Discharge in voids or interfaces | Rise time, repetition rate |
| Space charge effects | Charge injection at electrodes | Polarity, voltage, temperature |
| Electromechanical fatigue | Stress from impulse currents | Current, capacitance |
| Dielectric heating | Heat from HF components | Repetition rate, dissipation |
2. Screening Test Method
The standard defines two evaluation methods. The screening test (Clause 4.3) applies a single test voltage to assess materials by comparison with a previously evaluated EIS. Specimens face repetitive impulses (rise time 0.04-1 microsecond, repetition rate up to 10 kHz). RPDIV and RPDEV are measured under impulse conditions, not conventional power-frequency, because PD behaviour differs fundamentally between impulse and sinusoidal waveforms.
A minimum of five test objects per voltage level is recommended. Time-to-failure data must be processed using the two-parameter Weibull distribution per IEC 62539 with 90% confidence intervals.
3. Endurance Test and Life Modelling
The endurance test (Clause 4.4) requires testing at least three voltage levels above service stress with consecutive levels differing by at least 10%. Results use an inverse power law (L = kU^-n) or exponential model (L = Ae^-hU). The voltage endurance coefficient n is critical – higher n indicates greater sensitivity to voltage stress. A reference EIS with known service experience provides comparative benchmarking.
Failure processes must not differ between test and operating conditions. A slope change in the log-log voltage versus life plot indicates a change in failure mechanism – e.g., transitioning from space-charge dominated (below RPDIV) to PD dominated (above RPDIV).
4. Engineering Design Insights
Annex A reviews key influencing factors. Temperature has complex effects: it can accelerate degradation by increasing dielectric loss, but in confined systems thermal expansion may close voids. Bipolar impulses generally produce more deterioration per impulse than unipolar of the same magnitude. Humidity alters air breakdown strength and surface conductivity affecting PD behaviour.
For inverter-fed motor windings, impulse rise time is the most critical parameter. Shorter rise times concentrate more voltage across the first few turns, potentially exceeding inter-turn insulation withstand. Designers should specify minimum rise time and test using Table 1 parameters.
5. Frequently Asked Questions
Q: What is RPDIV and how is it different from conventional PDIV?
A: RPDIV is measured under repetitive impulse voltage, representing the minimum peak-to-peak voltage where more than 5 PD pulses occur on 10 impulses.
A: RPDIV is measured under repetitive impulse voltage, representing the minimum peak-to-peak voltage where more than 5 PD pulses occur on 10 impulses.
Q: How many specimens are needed for a valid endurance test?
A: Minimum 5 per voltage level, sufficient to detect differences at the 10% significance level.
A: Minimum 5 per voltage level, sufficient to detect differences at the 10% significance level.
Q: Can IEC 62068 be applied to any type of electrical equipment?
A: Yes, it is equipment-agnostic. Examples include motor stators, power capacitors, transformers, power cables, and PCBs.
A: Yes, it is equipment-agnostic. Examples include motor stators, power capacitors, transformers, power cables, and PCBs.
Q: What is the significance of the voltage endurance coefficient (n)?
A: The VEC quantifies how rapidly insulation life decreases with increasing voltage. Higher n means greater sensitivity to overvoltage.
A: The VEC quantifies how rapidly insulation life decreases with increasing voltage. Higher n means greater sensitivity to overvoltage.
