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CASE STUDY 1/21/08 We recently arrived at a jobsite to support the customer in a trouble shooting quest to find why it was that one generator could not synchronize or load share with the other two generators. All generators was 175kW and it was a 480V 3p/3w system, 60Hz. We spend a few minutes inspecting the paralleling system, and making sure the entire paralleling and power management system was functional. All was in order except for generator power got the middle generator to the control system. Second we went to the generator switchboard where the troubled generator was being controller from. Here we found the following. 1) When the generator was running no voltage was displayed. 2) When the generator was stopped there was still faults displayed We now knew to first investigate the missing voltage input to the generator control modules from the generator. Here we found that the secondary 10A protection breaker that feeds the control module voltage from the generator had tripped. Before we closed it again we needed to figure why it had tripped. Max consumption was 1A, yet a 10A breaker had tripped. No apparent reason was found, in tracing wires, and looking for loose strands. The isolation inside the control module was intact also, so all we could do was to reclose the 10A breaker again to see if we would be tripped again. With all the safety precautions we could get we reclosed the breaker. Result— a 10A breaker feeding another control module tripped. These two breaker was completely unrelated to each other by the sheer fact they was feeding power from each their own generators to each their own control modules. The only related point was some selector relays for the synchroscope with which the synchroscope would be supplied from either generator set. First thing first was to eliminate those relays, by simple disconnection. Second we reclosed the new tripped breaker, and all came alive without re-trips. With an ohmmeter we located three relay contacts on one relay that had welded shut, and by virtue of the welding, the two generators would be paralleled through the relay when one was online and the other was just started, and our secondary 10A breakers had saved the panel from severe damage. New relays was inserted, and all was cleared for faults. We thoroughly tested the system on all generators, and found all to be working flawlessly. We later found authorized personnel admitting to opening the panel and seeing the test buttons on the relay, and when pressing it, blue arc’s was seen and the breaker had tripped. The selector switch for the synchroscope is wired to a PLC and from the PLC the synchroscope relays was energized so the synchroscope would be powered from the correct generator. To prevent this situation again where two generators could be paralleled through these relays, we selected to add program lines to the PLC. The program we wrote on the spot first of all disconnected any generator voltage from the synchroscope when the generator breaker was closed. Second we selected the put in a 500mS delay when a generator was selected to prevent a relay de-energizing and another energizing at the same time. RESULT—no further faults has been reported, and customer reports flawless functionality with the system. To discuss a case study please contact us using this link Copyright 1995-2008 SELCO USA, Inc. |
