ICT Operation Overview

Overcurrent and Earthfault Relay Setting Methodology for ICT

Though the principally ICTs are same as that of transformer it’s HV side and IV side overcurrent and earthfault relay (collectively called as Backup Relays) setting methodology differs from that of transformer due to following reasons.

1)    Generally ICT operates in grid.

2)    Its tap changing arrangement on IV side.

3)    ICTs are provided with delta tertiary winding.

4)    ICTs are auto-transformers.

How these things matters is explained in next sections.

(As ICTs are three voltage levels general trend is to represent them as HV, IV and LV at in this documentation at some places IV is changed by LV)

ICTs operation in grid.

ICTs in transmission system generally connects two transmission networks operating at different voltage levels. Such type of operation of ICTs is called as grid operation. In following figure such grid operation of ICT is shown. Note the symbol used for ICT. This symbol represents auto transformer. For deciding setting of backup relays it does not matter whether a transformer is two winding transformer or auto transformer. However only to introduce the readers with symbolic representation of ICTs and to get reminded about the work area this symbol has been used.

Hence while ICTs operating in grid; whenever there is fault to either side of network; proportionate fault current get contributed by ICTs. For protecting the ICTs against these fault current; knowledge of fault current contributed by ICTs to its HV and IV bus fault is essential.