Tripping of Generators for Downstream Feeder Earth faultsCase of Nirmex, Nimbol, Rajasthan
Nirmex is one of the Grey Cement Production unit owned by Nirma Limited in Nimbol Village, Jaitaran Dist., Rajasthan. Plant has in-house thermal based generator of 33MW and a WHRS of 8MW Generating Power at 11kV. Plant is also having the local utility power support at 132kV from RSEB. Plant was facing problem of frequent blackouts mainly for 11kV network earthfaults in downstream network.Client approached EEPL with their above problem. EEPL suggested to carry out the Power System Study and Relay Coordination of the plant. EEPL was awarded this job in September 2017.
EEPL team conducted the detailed power system study on etap software based on the inputs they collected from the site. Star point of Both Generators and 132/11kV Grid TR were grounded through 100A, NGR each. However, at a time only one NGR of 33MW Generator was kept ON. During Study it was observed that plant 11kV network had many large size feeders in order of more than 1000 Amp up to 2500 Amp. Desired sensitivity of earthfault current was not achieved in such large size feeders. Time grading was also missing between downstream relays and generator protection relay. As a result it was finally tripping the generator in event of earthfaults in such feeders. EEPL suggested new operating philosophy of NGR which ensured utmost reliability and availability of main generator power. EEPL engineered the solution to enhance the sensitivity of earthfault protection of downstream 11kV feeders to a desired value. Time grading was provided in EEPL recommended settings.
In annual shutdown of plant in May 2018, EEPL team implemented the suggested scheme at site to achieve the desired earthfault protection scheme as per solution worked out by EEPL engineering team. Recommended relay settings also were implemented in entire complex by EEPL Execution team and all protective relays were tested and their tripping and annunciation scheme were validated.
Plant have witnessed two similar 11kV network earthfaults in their system post above implementations at site, however, end result now got changed drastically and is now more favourable to plant. Now CPP is not getting tripped, fault gets cleared faster and get restricted to faulty part of the network only. As a result effective plant down time has reduced and plant production loss is prevented. The most importantly this gives complete peace of mind to the operation staff and electrical team in particular.