Hello Every one

I am working in a petrochemical company. We have got number of substations in our company. Recently couple of months back we faced some unusual problem.. Our main feeder breaker (VCB) 13.8kV is feeding to 4 VCB in different substations its like a loop these are connected with Bus Tie Bus A & Bus B. If any unabnormal in the main feeder the down stream feeders get switched to Bus B. That day sudden switching was taken place but unfortunately Main feeder breaker showed any sign of abnormality all protective relays were safe Differential relay, U/V,OC..etc but in the downstream in one of the substation UV relay flag was set and only that VCB did not switched to Bus B automatically.Other substations VCB were switched to Bus B and also no sign of any abnormality in any relays in remaining S/S. After troubleshooting in one of our flag S/S we found the loose connection at the back of the VCB which it was not transfered to Bus B. But still why this switching took place still we are unable to find out. No ground fault relay, no signs of any relays alram. Today again we faced the same problem but in different S/S this time same UV flag was set and transfered stop automatic. Could anyone help us any giving the right solutions what exactly is happening in our Main feeder. We assume main feeder VCB needs to calibrate as it was recently calibrated along with others.Help me Enginners.