High Voltage Equipment
The University of Canterbury impulse generator is an inverted Marx multi-stage unit. It has 14, variable 0 - 100 kV stages with a maximum peak voltage of 1.4 MV. It can be used to generate standard lightning impulses of the form 1.2 µs rise time with a 50 µs decay to 50% of the peak voltage. By placing a resistor across the object under test, the generator can also be used to provide non-standard impulses. Two smaller impulse units, with outputs of 20 kV and 2 kV respectively, provide a wider range of pulse lengths. Actual values depend on loading.
The HVac transformer has a dual rated output voltage, either 150 kV or 300 kV at the secondary. The power rating of the transformer is 100 kVA on a continuous basis and 200 kVA on a 5 min on, 15 min off duty cycle. The basic unit is a fixed turn power frequency (50 Hz) transformer. The design of this unit is similar to power transformers except that the insulation is graded and the windings are designed to withstand forces associated with overcurrents as a result of flashovers under test. A voltage regulator supplies the HVac transformer so that voltages from 0 up to rated value can be applied to a test object. A number of smaller distribution and testing transformers allow for testing over a variety of lower power and lower voltage levels.
By the use of a diode rectifier circuit rated to 200 kV, the output of the HVac testing transformer can be used to generate either +ve or -ve HVdc. This is necessary for testing HVdc equipment.
A 100 kVA, 3 phase motor speed controller has been modified to produce a spectrum of harmonics for a single phase output. This can be put into transformers to give high voltage harmonics up to about 1 kHz. Also, the motor controller can be used to drive a ripple frequency generator to give a single frequency output up to about 1 kHz.
A Tesla coil is a radio frequency, quarter-wavelength antenna. The lab's Tesla coil operates at 150 kV. It is generally used as a demonstration of high frequency, high voltage arcing.
Measurement of high voltage
There are a number of different measuring transducers available in the laboratory.
For a given sphere diameter, the breakdown electric field strength of air is known. The separation of the gap is a direct indication of the peak voltage of the applied waveform eg. HVac, HVdc and impulses. The sphere gaps are accurate to within" 3%.
The standard capacitor can be converted into a HVac divider, using low loss (high quality) capacitors. This gives an output that is independent of frequency.
Research at the University has produced a range of highly accurate voltage transducers, suitable for both HVac and HVdc measurements. These dividers are portable and can be used at a customer's network. They can be attached under live-line conditions.
HV probes with high bandwidth can be used directly with oscilloscopes and spectrum analyzers and associated recording devices to provide performance estimation of a customer's appliance or HV generating device. Measurement of waveform parameters, distortion, power and energy can be undertaken.