Substation Generators Tripped -- Why?

We have in our substation 5 sources
3 generators each of them 6 mw , 6.6kv
2 feeders from national grid 10mw 6.6kv

our grounding system by using ground transformer zig-zag transformer with grounding resistance connected from it's star point to earth.

we have also capacitor banks for power factor correction .

all of these connected to main bus par by oil circuit breaker.
The load is 12mw where each source loaded 3mw with one feeder from national grid is standby.
Once a day the operator notice that the feeder which loaded was only two phases loaded with normal current while the third phase was loaded with very low current , so he opened the circuit breaker of that feeder , when he did that , all generators tripped except one generator stay loaded for a few seconds because of it's opening circuit failure , also both the ground transformer and the resistance were burned

it is working since 1970 so it is old ,

i hope it is clear for you

can i discuss this with you?