Multiple Generators in Parallel

The sequence of events is unclear but it sounds like the prime mover has tripped and the generator breaker(s) failed to open. A one-line diagram would help but without it we have to assume that the Stuck-Breaker/backup protective relaying scheme would initiate tripping of all individual generator breakers and the next zone out; i.e., the high-side of the generator step-up transformer, so as to isolate the remainder of the switchyard from the failed generation. Until the fault clears the generator on the failed prime mover will draw real and reactive power from the remaining generation and the grid as the answer from Arishkdhar stated. (Editorial remark: I'm not sure why his answer was flagged as "off-topic'.)

The NGR scheme would play no part in any of this unless OP had specified that there was a ground fault somewhere in the first protective zone. If this was an interview question it was designed to to test the depth of the interviewee's specific knowledge of generation and switchyard protective relaying, there is no way to fake your way through this one.

Dear Sir

I am repeating your query for easy understanding.

There are 3 DG sets namely DG1,DG2 and DG3.

Whenever you are paralleling these 3 sets you have to make the first set to be Master.In this case let us say DG1.

The neutral of the Master DG set (DG1) will be connected to the System Neutral.The neutral of other two DG sets DG2 and DG3 will be floating.

Under such circumstances if the DG1 trips or stops, There will not be any neutral for the system, and any single phase loads connected to the system will be affected/ damaged.

It is ,therefore, advisable to open other two DG sets from the system immediately on failure of Master Set .

This is more common on 3 phase 4 wire LT systems.The system should not operate with out any neutral even for a second other wise single phase loads will be affected and get damaged due to over voltage.