Insulation test generators can be used for a multitude of applications. The 1.2/50µs “Lightning” impulse is an industry standard for testing everything from transformers to semiconductors. The following are some sample applications.
Protection Automation and Control equipment in the power grid
Testing of components in the power distribution network is an essential element for maintaining serviceability. Equipment would be typically located in power stations, medium voltage (MV) and high-voltage (HV) substations (air insulated (AIS) and gas insulated (GIS) substations). Apart from quality assurance during the production process, overvoltage testing of measuring relays and communication systems ensures reliability of the power distribution grid.
Transformer parameters tested include insulation breakdown between windings and between winding layers. The impulse stresses winding insulation by producing a high voltage differential between turns. Impulse amplitudes are typically many times the operating voltage, so a three phase transformer for 690V should be tested at 2.5kV for overvoltage category I. Protection and control equipment has a rated impulse voltage for a specific overvoltage category based on installation location and the equipment ability to withstand transients. Parameters such as clearance and creepage distances are evaluated using the 0.5Joule impulse.
Specific faults are detected by changes to the impulse waveform. A failure will result in voltage breakdown and distortion of the wave shape. It is common practice to compare wave shapes before and after a test to check for problems. This can be performed manually or automatically.
Power electronics in general and renewable energy sources in particular are continually increasing in voltage and current for both AC and DC applications. There is an increasing need for specialist components to separate low voltage control from the high voltage power circuits and ensure safety and functionality. Semiconductor opto or digital isolators can be used to provide an insulation barrier between low voltage control and high level circuits. Impulse tests are one method used to verify the insulation barrier breakdown capability. Test voltages many magnitudes higher than the operating voltage are required. From IEC 60664-1 the impulse withstand voltage rating used for 1,500Vdc ranges from 6kV for overvoltage category I up to 15kV for overvoltage category IV.
Electricity Meter Testing
Electromechanical and static electricity meters must be tested to verify their safe operation and to protect the user from possible electrical shock. This is done by insulation testing to ensure the dielectric properties of materials used. Tests are performed on each circuit that is electrically isolated from other circuits in the meter. Impulse voltages are applied between the electric circuits and earth. The 1.2/50µs impulse with 0.5Joules energy is used to assess clearance and creepage distances between circuits. During the test, no flashover, disruptive discharge or puncture shall occur. The meter shall not suffer mechanical damage and must still be able to operate without change to the percentage error for energy measurements.
Insulation Tester 1.2/50 up to 48 kV and 8/20 up to 3kA
Impulse Insulation Testers from 500 V to 30 kV
Insulation tester 1.2/50 with two source impedances: 48 Ohm and 500 Ohm. Up
Overvoltage impulse tester 1.2/50us with 0.5J and 500Ohm from 500V to 8kV