NEW TAP CHANGING SCHEME

Tap changers have not changed radically since their invention in the e arly part of this century. They are slow in operation, typically trave rsing 19 taps in around 100 seconds, and require frequent maintenance. Previous attempts to devise improved tap changers have concentrated u pon thyristor assisted or all solid-state units. For various reasons n one of these schemes has been adopted. A new GTO thyristor assisted ta p changer is introduced here which addresses the problems of previous schemes. It eliminates excessive conduction losses which are inherent in solid-state tap changers, and at the same time provides a fast resp onse speed by means of fast acting vacuum switches. Calculations show that it will be possible to traverse a 19 tap range in around 0.5s. A unit for a 240MVA transformer will require minimal maintenance as the vacuum switches used in the selector and diverter never make or break more than a few amperes. Test results are given for a 3kVA transformer and tap changer unit.