Parallel Transformer Protection

If a transformer is connected in parallel with another transformer which is already energized, magnetizing inrush will occur in both transformers.  The dc component of the inrush associated with the switched transformer creates a voltage drop across the line resistance between the source and the transformer.  This voltage causes an inrush in the opposite direction in the transformer that was already connected.  After a time the two currents become substantially equal and since they flow in opposite directions in the transmission line they cancel and produce no more voltage drop in the line resistance.  The two currents then become a single circulating current flowing around the loop circuit made up of the two transformers in series - the rate of decay being determined by the R/L ratio of the transformer.

Parallel transformers are typically protected by directional overcurrent and earthfault protection on the LV side set to look back into the transformers. Where an LV bus section exists the directional relays can be replaced by non-directional relays, with the addition of a non-directional overcurrent and earthfault relay at the bus-section.

Protection are:

1. Directional Over current

2. Directional Earth fault