Power Systems equipment, directly or in-directly connected, is expensive and needs protection against both external factors and normal every day power system operation. Protection assessment of these equipment, especially outdoor, against voltage surges is almost mandatory due to their exposure to lightning and general switching operations. Insulation material’s strenght is usually expressed using Basic Insulation Level (BIL) in kV that indicates voltage withstand capability of the material before it breaks down, providing a path for fault current. In order to assess equipment performance to a real environment, actual equipment or prototypes are tested to Lightning and switching surge. This is where surge generators come handy that generator desired surge wave fronts and tails.
Various standards (IEEE, IEC, EN etc.) provide rules and guidelines on how testing is to be performed. Typical surge-wave shapes recommended by standards is a double exponential curve functions with rise time to reach the peak and then gradual decay fall time. Irrespective of the equations that define this curve, what’s of real importance is the voltage peak rise reached at the end of rise time and the total time (also the fall time) it takes to reach half the peak value during its decay. There are two variations in surge reference curve depending on how the rise time is defined, while the fall time remains the same.
- Type 1, where the rise time is the time reached by the surge peak to 90% of its value. Also shown in Figure.
- Type 2, where the rise time is the time reached by the surge peak to 100% of its value.
As I understand, Type 1 is used for fast-front surge waveshapes (e.g. Lightning), while Type 2 is used for slow-front surge waveshapes (e.g. Switching).
Now you know the basics, here’s a simple Linear Lightning Surge Generator Model in PSCAD based on [1]. Download. If you are looking for a double-exponential surge generator model, there’s one in PSCAD examples folder in c:\program files for MS Windows.
Connect the current source output to the point of surge injection in the test system. It’s simple to use: Enter the current peak (say 50kA), surge front time (say 20μs) and tail time (say 80μs), initial time (say 0.1s) when you want the surge in the simulation and you are done.
An opportunity to test equipment to lightning and switching surges without frying ourselves up.
Have fun! ^__^
References:
- TVS Diode Application Note , Semtech Corp., 2001. Link: http://www.semtech.com/pc/downloadDocument.do?id=535. Accessed: Aug 2009.
- P. Chowdhuri, Electromagnetic Transients in Power Systems, Edition 1. Exeter: Research Studies Press LTD, 1996, pp 4-6.
Posted by scvegunta 
