Design and construction of a 20 kA 10/350 µs current impulse generator

By T Gora, S Muzoka, H Hunt and K Nixon, IR Jandrell, University of the Witwatersrand

 

Published in:

Electricity+Control, January 2014 (pages 22 - 25)

Enquiries: ken.nixon@wits.ac.za

 

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The use of surge protective devices (SPDs) as a means of protecting power and communication systems from direct and induced lightning surges is an established practice. Understanding the behaviour of these devices and the protection they can offer under different energy conditions is vital to the installation of a device that will provide the necessary protection.

The use of Metal Oxide Varistors (MOV) as SPDs has become common. The arrestor has the ability to clamp the voltage waveform to a range that can be handled by the downstream protection. MOVs may be contaminated by dust, moisture or manufacturing defects. This results in an inhomogeneous surface of the arrester tube thus affecting the MOV. SPDs therefore need to be tested to ensure they function as per the required standards and provide the required protection.

Current impulse generators may be used to simulate the effects lightning surges. The challenge in implementing current impulse generators in High Voltage Laboratories is generating the necessary energy to correctly simulate the effects of lightning. For this reason, and in the interests of establishing a methodical approach to lightning protection, the effects of a direct lightning attachment are differentiated from the induced effects from a nearby attachment. The practical implication of this is that differing amounts of energy need to be generated to simulate either a direct lightning surge or the induced effects.

A 20 kA 10/350 s current impulse generator has been configured and is housed in the School of Electrical and Information Engineering’s High Voltage Laboratory at the University of the Witwatersrand. The generator is designed to meet the specifications required for Class 1 current impulse tests in the IEC 61643-1. It is able to perform Class 1 current impulse testing of SPDs - the design utilises a capacitor bank (a parallel connection of eight 62,5 F capacitors) to store the required energy. The generator is fired using a solenoid with brass electrodes. A bifilar wire wound resistor is used to obtain a low inductance wire with a resistance of 1  and an inductance of 0,3 H. The current impulse generator produces an 18,8 kA 12/406 s current impulse waveform with a specific energy of 111,7 kJ at a charging voltage of 20 kV. These results fall within the specified tolerances of the IEC 61643-1 and recommendations are suggested to improve this performance.

 

Take note:

  • Current impulse testing sometimes requires the use of a 10/350 V waveform.
  • Current impulse generators may be used to simulate the effects lightning surges
  • A 20 kA 10/350 s current impulse generator has been configured to perform Class 1 current impulse testing of SPDs.