NEUTRAL POINT CONNECTIONS IN MV POWER NETWORKS WITH GROUNDING ZIGZAG TRANSFORMERS – ANALYSIS AND SIMULATIONS

Treatment of transformer neutral point in middle-voltage (MV) networks become an important issue with increasing proportion of MV cables in power networks. As consequence, overall capacitance of MV network is increased and moreover earth fault currents magnitudes. In MV networks with feeding transformer winding in delta connection (isolated networks), that earth fault current increase requires forming of artificial ground point – a neutral connection point on a three-phase ungrounded power system. Grounding transformer use, in zigzag or delty-wye connection, is common, well-known solution for constructing neutral connection in power systems. Physical characteristics of grounding transformers, protection principles, short-circuit calculations with symmetrical components and simulation techniques are presented in this paper. Characteristical operational modalities of MV power networks are also revieved on practical examples