I needto know the full conditions of parallel operation of transformers including details about vector groups.
I am putting 1 QA for reference. Question If I parallel three 2MVA 22/11kV three phase 50Hz Dyn11 transformers of the same voltages, vector groups and impedances, do I connect all three star points to earth or only one star point to earth? If I want to put a resistance in the neutral connection to earth (NER), do I only connect one which is common to all three or a seperate NER for each? Answer Dear Dario, I will start with the imepdance: every transformer shall have its own because it is there to protect against internal faults as well. You may install only one impedance but that will depend on your protection devices and what they measure, i.e. where you put your CTs and VTs. These impedances must have a disconnect switch. For the first part, it is very general to me but i figure your load is large Medium Voltage motors (a mill), because 2MVA seems little for distribution and a Y secondary in MV is tricky. All neutrals shall be interconnected and connected individually to earth. This is for the case where the only link you have in mind is gone, the whole system will go float; so again, for the first part: all three star points shall be connected. If you're already protected against lightning, then you don't need limitation in surge voltage, i advise not to install a resistor and not connect N to E, i.e. go for an IT system which provides you with two benefits: fault currents are small and it take more than one fault to trip and cause discontinuity to the service,
