In the earlier Case Study, we saw how we solved a Transformer mal-tripping issue during our Routine Maintenance and Testing activities. We are presenting herewith another case which demonstrates why these activities should be entrusted to experts, who can make such routine jobs more meaningful and save assets.
We were appointed to carry out the routine maintenance and testing of the Switchgears, Relays, CTs & PTs in one of the Cement Plants in India, having its own Power Plant. This Power Plant combined with Grid Supply catered to the plant load.
Usually, at generating stations, we have TIE Feeders which connects Generator and Grid to maintain availability of Power in case of various Power-Flow scenarios. In most of the cases, TIE Feeder is a Synchronising Feeder, where Generator and Grid supplies are synchronised to cater the plant load. It is THE most important Feeder of any Plant, any problem in this can cause the entire plant black-out.
It was the time to test the TIE Feeder. Necessary shut-down activities were carried out and permits obtained.
A team was carrying out Relay Testing and another team was performing CT Testing. These 6.6 kV CTs were having a ratio of 2500/1 A.
As we have seen in earlier Case Study that Primary Injection testing of CT is having many limitations, plus it does not usually provide the Diagnostics of the CTs. Moreover, injection and measurements of such high currents also pose various challenges.
So, usually, we employ a state-of-the-art CT Testing Instrument, CT Analyzer of Omicron Make, which uses IEC 61869-2 standard and performs model-based testing which is far more accurate and conclusive.
During this Testing, Protection Core (5P10) of a CT was having a very high secondary resistance, the instrument displayed “Secondary Winding resistance is too high”. CT Analyzer determines winding resistance by applying DC current into Secondary winding and checks voltage drop across Secondary Terminals.