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MPC2000D relay 'phase unbalance' alarm

MPC2000D relay 'phase unbalance' alarm

MPC2000D relay 'phase unbalance' alarm

(OP)
Hi Friends:
 
I have recently experienced some power dips, on two ocassions at our steel making plant. However, this has not tripped the incomer feeders. I enquired from the supply authorities if they had experienced some power dips, but they say they haven't. It was found out that every time this occurs, all the MPC2000D motor protection relays on the 3,3kV busbars give an alarm indication of "phase unbalance". I then tried to measure all the motor phases after that and they looked fine. What I cannot establish properly is; the SPAM 150C motor relays on the 11kV busbars have not picked up these dips or show any unbalance alarm as one would expect if dips are coming from the supply or  at point of common coupling (PCC). What could be the cause of these dips?, can it be harmonics?
 
 
Thanks
 
Nduvho

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