With the desire for energy conservation and lower costs, the application of pulse-width modulated (PWM) voltage source converter (VSC) drives has grown at an exponential rate. However, due to their high switching frequency and high dv/dt, increased dielectric stresses and thermal stresses are applied to the insulation system of the motors, which may lead to the failure of the insulation. In order to test the performance of the motor insulation under the above complex stress conditions, an IGBT-based pulse voltage generator which can produce high voltage square wave and PWM waveforms has been successfully developed in this research. The generator consists of IGBT switches and other wave shaping components. The special cascade connection circuit design enables the generator to produce the stable high voltage square wave and PWM waveforms. A microcontroller-based trigger signal generator is used to trigger the power electronic switches in the generator. In order to avoid false triggering from electromagnetic interference (EMI), optical fibre cables are used to connect the trigger signal generator to the switches which are located in a high electric potential area. The generator can produce square wave and PWM waveforms with a peak voltage up to 15 kV and with a switching frequency of 600 Hz to 6 kHz. The fundamental frequency of the PWM waveform is 20 Hz to 1200 Hz, the rise time is less than 200 ns, and the pulse width can be varied up to several milliseconds. A 4 kVrms form wound model stator coil was tested under different voltage waveforms: power frequency, exponential decay pulse, square wave, and sinusoidal pulse-width modulated (SPWM) waveform. Infrared images and the maximum temperature rise of the coil under different electrical stresses were recorded. The results show that both the square and SPWM voltage waveforms cause a significantly higher temperature rise than the power frequency and exponential decay pulse voltage waveforms. Since the actual VSCs generate transients similar to those of the square and PWM voltage waveforms, it is recommended that the stator coil insulation be analyzed using PWM voltage waveforms in order to simulate actual conditions.

During the fabrication of solid electrical insulation, small cavities known as micro voids may form in the material. As electrical stress increases in this micro void, the breakdown probability also increases. This type of electrical breakdown is commonly known as partial discharge. Magnitudes of partial discharge currents are typically small but enough to cause degradation of the electrical insulation. To study degradation for fast-rise time voltage square pulse train, partial discharge measurement is needed. In current studies, partial discharge pulse widths have been measured in the range of nanoseconds. The best approach for measurement at ultra wide band frequencies is a bridge type measurement system, to reduce external noise and improve sensitivity to PD currents. The bridge configuration can be used with samples instead of one sample and one coupling capacitor. Identically created samples will have a close match for impedance and frequency response. This type of bridge also helps to reduce other sources of measured current such as the high displacement currents due to fast rise time square pulse voltage on the samples. Further improvement includes simultaneous measurements using a “linked” bridge configuration, where bridges share a common sample. A directly connected measurement current shunt should be used for high sensitivity with a uniform ultra wide band frequency response. Post-measurement digital signal processing (DSP) algorithms will perform the task of pulse discrimination and time delay from the pulse front. This research presents a method to improve the measurement of partial discharge when applied voltage is non-sinusoidal, with high frequency components. The improvements are apparent when square pulse voltage rise times are less than 50 ns. Ultra wide band measurements of physical samples will be performed for short time duration with a digital storage oscilloscope. A DSP algorithm is used to filter residual noise from the partial discharge current. The presented measurement technique for samples for this study is an original approach. Sample results demonstrate the effectiveness of the technique.

It is common practice to perform accelerated aging with 60 Hz ac to determine the lifetime characteristics of insulation used in the machine. Comparable breakdown measurements are performed at different voltage levels and temperatures for the polyimide insulated machine winding under normal operating conditions of 60 Hz ac. The result shows that the time to failure can be represented by the inverse power law and the Arrhenius equation with respect to test voltage and temperature respectively. However, the use of power electronic devices causes harmonics, and spikes that electrically degrade the machine winding insulation. Applied high frequency pulse voltages can be used to study the impact of electrical degradation of the machine winding insulation that exists in electrical machines. Evaluation of change in dielectric strength, partial discharge and breakdown voltage is monitored versus aging caused by high frequency pulse voltage at 90% of operating temperature.