High neutral currents is a common problem when running any sort of non-linear load, in this case, scads of switch-mode PSU's. All of them (not just ones from Bitmain) cause it. For powering large farms, using K-factor rated transformers which are designed to accommodate these loads is a must.
Ref this from Jefferson Electric.
Poking around the Jefferson Electric site I do see that they also
carry a line of Harmonic Suppression Systems to use with existing distribution transformers and gensets.