Assessing insulator withstand for non standard waveshapes 1

[Power0020]

The subject looks a bit academic but it is realistic that the power system gets non standard waveshapes during switching operations and also due to the propagation of lightning impulses. For example, the case of unloaded transformer being switched by a vacuum CB through a MV cable (say 13.8 kV), the switching may well end up with a current chopping and some re-ignition, the voltage received at cable terminations and transformer bushings peak value may reach about 3.4 p.u. (typical waveshape attached).

How can the withstand cabaility of a transformer, motor, insulator...etc be estimated based on this non-standard waveshape? note that in voltage less than 245 kV, no switching withstand level is defined.

Note that in these cases, without knowing the prospective withstand, we will always end up with a requirement for surge arrestors and compare the residual voltage with system BIL (which is always less!)

Clues?

 

[prc]

Simply transformers or insulators cannot withstand such over voltages. Please see IEEE std C57.142-2010 "Guide to describe the occurrence and mitigation of switching surges induced by transformers ,switching device and system interaction" and CIGRE Technical Brochure 577 A &B of 2014 "Electrical transient interaction between transformers and the Power system A-Expertise &B-Case Studies. VCB manufacturers (ABB ,Siemens etc) normally give mitigation measures required (surge arrester, snubber circuits etc) with VCBs for various applications.