Corona resistance performance evaluation of high-temperature polyimide (PI) films is essential for verifying the long-term electrical reliability of insulation systems in traction motors, EV drives, wind turbines, and aerospace power equipment. This technical article explains the sample types, testing items, test methods, and standards-based procedures used to assess the corona endurance of PI films under elevated temperature and high-frequency electrical stress. Understanding these evaluation practices helps engineers select the right insulation film and predict service life in demanding partial discharge environments.
Technical Overview
Polyimide (PI) film is a high-performance engineering plastic film widely used as slot insulation, turn-to-turn insulation, and barrier insulation in rotating machines and power electronics. It offers an exceptional combination of thermal stability (continuous service temperature up to 200–260 °C, short-term tolerance above 300 °C), high dielectric strength (typically 150–300 kV/mm), excellent mechanical toughness, and strong resistance to most chemicals and radiation. These properties make PI film the material of choice in traction motors, electric vehicle (EV) drive motors, wind turbine generators, and aerospace electrical systems.
However, standard PI film has a well-known weakness: limited resistance to partial discharge, commonly known as corona. In modern inverter-fed drive systems, fast-switching insulated gate bipolar transistors (IGBTs) and silicon carbide (SiC) devices generate high-frequency PWM voltage pulses with steep rising edges. These pulses excite resonance overvoltages and intensify partial discharge activity in motor windings. The discharge bombardment erodes the organic polymer surface, causing rapid dielectric breakdown long before thermal aging alone would cause failure. The problem is aggravated at elevated operating temperatures, where the polymer matrix softens and degradation kinetics accelerate.
To address this limitation, corona-resistant (CR) PI films have been developed by dispersing inorganic nanoparticles, such as nano-silica (SiO2), nano-alumina (Al2O3), or layered silicates, into the polyimide matrix. These particles act as physical barriers that interrupt discharge erosion paths and substantially extend voltage endurance life—often by 10 to 100 times compared with standard film. Evaluating and quantifying the corona resistance of these films at realistic operating temperatures is therefore a critical step in insulation system qualification, material selection, and life prediction.
Testing Samples
Standard Polyimide Film (Reference Material)
Commercial-grade standard PI film, typically 25 μm, 50 μm, 75 μm, or 100 μm in nominal thickness, is tested as the baseline material. These samples establish reference endurance data against which corona-resistant grades are compared. Samples are cut into specified dimensions (commonly 100 mm × 100 mm squares), visually inspected for defects, pinholes, creases, or contamination, and conditioned in a standard atmosphere (23 ± 2 °C, 50 ± 5 % relative humidity) for a minimum of 24 hours before testing.
Corona-Resistant PI Film
Corona-resistant PI film grades incorporate inorganic nano-fillers distributed through the polymer matrix. Testing laboratories evaluate these materials in the same thickness range as the standard film to enable direct comparison. Key sample attributes recorded before testing include nominal thickness, measured thickness at the test point, filler type and loading level declared by the manufacturer, and film surface condition. Any protective coating or corona shield layer, if present, must be documented because it significantly influences test outcomes.
High-Temperature Specialty Grades
Some applications require films designed specifically for combined thermal and electrical stress, such as films rated for continuous operation at 220 °C or higher, thermally conductive filled films, or films with enhanced partial discharge endurance for SiC-based converter systems. These specialty grades are tested under both room-temperature and elevated-temperature conditions to characterize the temperature dependence of their corona endurance.
Sample Preparation Requirements
Proper sample preparation is essential for reproducible results. The following practices apply to all sample types:
- Clean sample surfaces with lint-free wipes and anhydrous ethanol when necessary; avoid solvent contact that could swell or degrade the film.
- Measure and record the minimum thickness at the active electrode region using a precision micrometer or optical gauge.
- Condition all samples in a controlled environment (23 ± 2 °C, 50 ± 5 % RH) for at least 24 hours before the test.
- Prepare a sufficient number of specimens (typically 10–20 per stress level) to support Weibull statistical analysis.
- Reject specimens with visible defects, edge damage from cutting, or thickness deviation exceeding the tolerance stated in the material specification.
Testing Items
A comprehensive corona resistance evaluation of PI high-temperature film covers the following test items:
- Corona endurance life (time to breakdown): The primary metric, defined as the elapsed time from voltage application to dielectric breakdown under a specified surface or partial discharge stress, field strength, frequency, and temperature.
- Voltage endurance characteristics: Life curves plotting failure time versus applied voltage (or electric field) across multiple stress levels, used to derive the voltage endurance coefficient (VEC) n in the inverse power law relationship t = C·V^(−n).
- Temperature-dependent endurance: Corona life measured at multiple ambient temperatures (for example, 23 °C, 105 °C, 155 °C, 180 °C, and 200 °C) to quantify thermal acceleration and activation behavior.
- Dielectric strength before and after corona exposure: AC breakdown strength measured on unaged specimens and on specimens subjected to defined corona exposure durations, revealing progressive degradation.
- Partial discharge inception and extinction voltage (PDIV/PDEV): Determination of the voltage threshold at which sustained partial discharge activity begins, important for insulation coordination in converter-fed systems.
- Erosion depth and morphology analysis: Measurement of surface erosion pits using profilometry, optical microscopy, or scanning electron microscopy (SEM) to characterize degradation mechanisms.
- Weight loss and chemical degradation: Gravimetric analysis and FTIR spectroscopy to detect chain scission, imide ring opening, and oxidation products after discharge exposure.
- High-frequency endurance under PWM-like waveforms: Endurance testing under repetitive impulse voltages at frequencies from several hundred hertz to 20 kHz, simulating inverter duty.
- Comparative tracking and surface resistance changes: Supplementary measurements that help interpret failure modes.
Testing Methods
Test Standards and References
Corona resistance evaluation of PI film is commonly performed with reference to the following standards and industry practices:
- IEC 60343 – Recommended test methods for determining the relative resistance of insulating materials to breakdown by surface discharges.
- ASTM D2275 – Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface.
- IEC 60243-1 / ASTM D149 – Electric strength of insulating materials, used for breakdown strength verification.
- IEC 60270 – High-voltage test techniques for partial discharge measurement, applied to PDIV determination.
- IEC 60034-18-21 and IEC 60034-25 – Thermal endurance and insulation system considerations for rotating machines fed from converters.
- IEEE 1044 and related guidance on corona-resistant magnet wire and film evaluation for inverter-duty motors.
Surface Discharge (Rod–Rod or IEC Electrode) Method
In the most widely used configuration, the film specimen is placed on a grounded flat electrode, and a high-voltage electrode—typically a pair of cylindrical stainless-steel rods or the IEC 60343 electrode assembly—is applied to the top surface. A specified test voltage (commonly 1.5 kV to 5 kV, 50/60 Hz, selected according to film thickness and class) is applied, creating intense surface partial discharges in the electrode region. The time to breakdown is recorded for each specimen. Failure is detected by the operation of an overcurrent trip circuit or a fuse in series with the specimen.
Elevated Temperature Testing Procedure
To evaluate high-temperature performance, the complete electrode assembly is installed inside a forced-convection oven or temperature chamber capable of maintaining the target temperature within ±2 °C. The chamber is allowed to reach thermal equilibrium before energization. Care must be taken to use high-temperature cable insulation, prevent air currents from distorting the discharge region, and allow electrodes to thermally expand without shifting position. Typical evaluation temperatures include 155 °C (Class F), 180 °C (Class H), and 200 °C or above for specialty grades. Corona endurance life is then plotted versus temperature to quantify the thermal acceleration factor.
High-Frequency and Impulse Voltage Method
For inverter-duty qualification, specimens are energized with repetitive square-wave or bipolar impulse voltages generated by a power semiconductor-based pulse generator. Test variables include pulse frequency (e.g., 1 kHz, 5 kHz, 10 kHz, 20 kHz), duty cycle, rise time (typically 50 ns to 500 ns), peak-to-peak voltage, and temperature. The test records the number of voltage pulses to failure, which is converted to time-to-failure for comparison with power-frequency results. This method realistically reproduces the accelerated degradation observed in converter-fed motor insulation.
PDIV Measurement Method
Partial discharge inception voltage is measured using a coupling capacitor and PD detector in accordance with IEC 60270. The applied voltage is raised gradually until sustained discharges exceeding a defined magnitude (commonly 10 pC) appear, then lowered to determine the extinction voltage. Testing may be repeated across the temperature range to capture how PDIV shifts with thermal condition.
Statistical Data Analysis
Corona endurance data scatter significantly and must be treated statistically:
- Failure times at each stress level are analyzed using two-parameter Weibull statistics; the scale parameter (63.2 % failure time) and shape parameter are reported.
- Voltage endurance curves are constructed on log–log coordinates, and the endurance coefficient n is determined from the slope.
- Arrhenius-type or multi-factor models may be applied to extrapolate high-temperature results toward service conditions.
- A minimum of 10 valid breakdowns per condition is recommended; suspended (censored) tests must be handled with appropriate statistical methods.
Post-Test Examination
After failure, each specimen is examined to confirm that breakdown occurred within the active discharge region. Erosion patterns, carbonized tracks, and electrode deposits are documented photographically. SEM and energy-dispersive X-ray (EDX) analysis of the eroded zone reveals whether nanoparticles concentrate at the eroding surface, which is the characteristic protection mechanism of corona-resistant grades.
Applications
The corona resistance evaluation of PI high-temperature film directly supports the design, qualification, and reliability assurance of insulation in demanding electrical equipment:
- Traction motors for railways: Traction insulation systems experience high thermal class requirements combined with converter-induced voltage stress; evaluated CR PI films ensure dependable turn and slot insulation over 20–30 year service lives.
- Electric and hybrid vehicle drive motors: High power density, compact windings, and SiC inverters with very fast switching create severe partial discharge environments; film endurance data guides insulation coordination and derating decisions.
- Wind turbine generators: Long cable runs between converter and nacelle cause voltage doubling at the machine terminals, making verified corona endurance essential for maintenance planning and gearbox-to-grid reliability.
- Aerospace and aviation electrical systems: More-electric aircraft use high-altitude, low-pressure environments where partial discharge inception occurs at lower voltages; evaluated PI films support high-reliability insulation design for actuators, starters, and auxiliary power units.
- High-voltage transformers and reactors: Barrier and layer insulation made from corona-resistant PI film extends life in locations with unavoidable discharge activity.
- Flexible printed circuits and power modules: Substrate and interlayer films in high-density power electronics benefit from documented discharge endurance when operated near PD thresholds.
- Material development and quality control: Film manufacturers use standardized corona endurance testing for new grade development, batch release, and continuous quality assurance.
FAQ
What is corona resistance and why does it matter for PI film?
Corona resistance is the ability of an insulating material to withstand partial discharge bombardment without rapid erosion and breakdown. It matters because standard organic films, including conventional PI, degrade quickly under sustained discharge, while corona-resistant PI film with inorganic nanofillers can last orders of magnitude longer under identical stress.
Why is the evaluation performed at high temperature?
Insulation in traction, EV, wind, and aerospace machines routinely operates at Class F (155 °C), Class H (180 °C), or higher temperatures. Elevated temperature reduces polymer mechanical strength, accelerates discharge-induced chemical degradation, and shortens endurance life. Testing at realistic temperatures provides conservative, application-relevant life data that room-temperature tests would overestimate.
How much longer does corona-resistant PI film last than standard PI film?
Under identical surface discharge stress at power frequency, corona-resistant PI films typically achieve 10 to 100 times longer endurance life than standard PI film, with some nanocomposite grades exceeding two orders of magnitude. The exact ratio depends on filler type, loading, film thickness, field strength, and test temperature.
Which standards govern corona resistance testing of insulating films?
IEC 60343 and ASTM D2275 are the primary standards for surface discharge endurance testing. Supplementary measurements follow IEC 60243-1/ASTM D149 for breakdown strength and IEC 60270 for partial discharge detection. Converter-duty evaluation draws on IEC 60034-18-21 and IEC 60034-25 methodologies adapted to repetitive impulse waveforms.
How are scattered breakdown times handled in reporting?
Because corona endurance data follow a statistical distribution, results are analyzed using two-parameter Weibull statistics. Reports present the Weibull scale parameter, shape parameter, and confidence bounds, along with B10 or median life values, instead of single-specimen measurements.
Does higher test frequency shorten corona life?
Yes. Each voltage transition can trigger partial discharge events, so raising the repetition frequency from 50/60 Hz to several kilohertz proportionally increases the discharge activity per unit time. Endurance testing under PWM-like high-frequency pulses is therefore essential for inverter-fed applications and typically reveals substantially shorter lifetimes than power-frequency testing at the same peak voltage.
What sample information should accompany a test request?
A complete submission should include material grade and manufacturer, nominal and measured thickness, filler type if applicable, conditioning history, desired test voltage and frequency, target test temperatures, applicable standard, required number of specimens per stress level, and any special reporting requirements such as Weibull analysis or SEM examination.
Can corona endurance data be used to predict service life?
Yes, with appropriate modeling. By combining voltage endurance curves (inverse power law), thermal acceleration factors (Arrhenius analysis), and the actual electrical stress spectrum of the application, engineers can extrapolate laboratory data to estimate insulation life. Safety margins are always applied because real duty cycles, humidity, vibration, and manufacturing variability introduce additional stresses not captured in laboratory tests.