2 IC:1BC PCC

Dear All,

Am facing a problem with 2 IC:1BC PCC which am unable to rectify Request Please help.

1] Autochange over from BUS A is normal. (IC-1 trip on Undervoltage Buscoupler close). 2] Autochange over from BUS B is normal (with only 2 Motor running).(IC-2 trip on Undervoltage Buscoupler close). 3] If more than 3 Motors are running Bus transfer from IC-1 to BC is normal while IC-2 to BC is creating problem. Buscoupler close & open frequently. Please note UV in Motor relays are disable. None of the relays are registering any alarm or fault. SEL-751 is used for IC & BC Feeder Protection. 4] I had checked the scheme , Relay Logic & Protections settings & found in line. 5] Some ACB open on surges without any fault on relays. 6] Am in opinion that Motor Residual Voltage is causing this problem but stuck up for the cause of BC tripping.Am attaching BC tripping scheme for your reference, There are two trip commands, a] BC relay trip logic is 50P1T or 51P1T or 50N1T or 51N1T or G1T only. There is no other source for tripping. Any Suggestions. the BC.