Consultant team of an industrial power network recommended installation of suitable interconnection between mentioned distributed generation unit and national grid for stability evaluation of industrial electrical system. They supply a 132/132 KV,100 MVA transformer with DD0 connection group for establish that interconnection between two 132 KV substations of two independent networks.
But Installation team didn't know which side of interconnection transformer shall be connected to national grid and they preferred a random selection. After the time that transformer faced a sever insulation failure due to a suddenly voltage fault.
How can you explain the reason of that serious failure?