how can 3phase motor speed can be controlled without vvf drive?

Dear Sirs, I am a technical persons.In one of our Electrical Sub- Station, We have installed three numbers of 1250KVA ,11KV/0.415KV Transformer to feed the Factory load. We have found some abnormality. In one of our Transformers the leakeage current from transformer body to earth through earth continuity wire is almost 86 Amp but in other transformer it is (5-10)Amp. where the load current on three phase and neutral are respectively 1360Amp,1400Amp,1370 Amp & 176Amp. So please any technical person help me about the reasons of this situation and how I can rectify the fault. Moreover,I have measured the earth resistance of the Earth continuity wire which is less than 1 Ohm. Thanks Tapas Barua

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