Multiple earth referance points.

[dubstep]

i need to investigate the disadvantages of having multiple earth referance points on a single bus system. The system comprises of 2 x 6MW TA's identicle and 3 x 1.5MW DG's identicle, either the TA'a will run or the DG's. they all have high resistance NER's connected. with this system in place would one need to be concerned about the amount of third harmonic circulating current that could posibbly cause heating on the windings? With two NER's operating in paralell i understand the impedance of the network would be halfed and the earth fault much higher?