Very challenging problem ( Urgent solution required)

Please find the problem I am facing with a RING MAIN UNIT last few days.

My 11 KV RMU breaker connected to a transformer primary is behaving very abnormally. The breaker is of Merlin Gerin and the relay used is VIP200. The protections are over current and earth fault. CT ratio is

400/5. Relay has no provision to identify whether the fault is over current or earth fault. There is no provision in the breaker to have the test position and rack out =96 only, Close, Open and Earthing. Already its in a ring and once you close the breaker means, you are closing it by supplying 11KV. Isolation of the unit from the other feeders is not permitted as its in a ring. Have a look at the incidents.

1. First day the breaker tripped and was not able to re close. There was no indication on the relay and the relay flag also was not there. The breaker is in lockout.

1. On removing the cable from the transformer end, the breaker is closing and opening properly. Not tripping. Means no problem in the cable.
2. Removed all the cabled from the transformer secondary and connected back the primary cables. Now, on closing the breaker, first time its holding without tripping. Then the problem is, on opening it manually, it is going to the lockout position. Not possible to re close the breaker.
3. Steps 2 and 3 are carried out for 2 - 3 times. Same result.
4. Now, suspecting the problem on the transformer, meggering, ratio test and a test with an oscilloscope ( Don=92t know what is the test) are carried out on the transformer. All the tests are passed. Also, up to 230 V, 1Ph is applied to secondary winding phase to neutral and measured the leakage current. The readings are found to be OK (Up to 2 A). That means transformer is OK. Winding resistance in the primary is

0.5 ohm and in the secondary is 0.25 ohm across a single winding.

1. Now, suspecting the problem on the breaker mechanism, the breaker experts came from MG and they certified that the breaker is healthy and no problem.
2. Suspecting the problem on the relay, secondary injunction is carried out and found the relay is proper. Still, the relay is replaced with a new one and the result is same.
3. Next, the relay setting is increased to very high range and closed the breaker. This time, the main breaker tripped. (The breaker prior to the feeder breaker-lead to a full shutdown). That means, the fault is there in actual and not any mal operation of the equipment.

What is the next step I can go ahead? The body earthing of the system is common for 6 transformers and all other transformers are running without any problem. The feeder cable armor is earthed with the transformer body and in the breaker side. The transformer is ONAN without conservator. Breathers are there and no buckholz. Tap changer is off load. All the cases the meggered values are more than 20 G ohm HV to LV, HT to ground and LV to ground.

I urgently need an expert advice on the same and hope some body can help me.