Filtering of zero sequence current:

The zero sequence current will flow through the transformer in case of external earth fault. This current will be seen as unbalance by differential protection if the protected power transformer cannot transform this zero sequence current to other side. This will happen in case of star connected transformers. Transformers of this type do not transform zero sequence current. This current will flow only in earthed star connected winding. In such cases, an external earth fault on star side of the transformer causes the zero-sequence current to flow on star side only and not on other side. This results in false differential / unbalance current consisting exclusive of zero-sequence current. This will lead to un-necessary tripping of power transformer for external earth fault. Therefore, this current needs to be filtered out from fundamental component of differential current to avoid un-necessary operation. In case of static and electromechanical relays zero sequence current are filtered out by using delta connected interposing CTs. For digital / numeric relays this is achieved through relay software. The zero sequence current entering the transformer having Delta connected winding will circulate in the delta winding itself. Hence, for delta connected windings of the transformers, zero sequence filtering is not required. Table No. – 1 summaries the phase compensation and zero sequence filtering requirements of various transformers as per their vector groups.