Transformer LV Side Overcurrent and Earthfault Relay Protection Schemes While Transformer Operating in Parallel.

               For transformer operating in parallel we can have different protection schemes as shown below along with respective comments.

This is the most preferred scheme where we have directional as well as non-directional relays (or separately assigned contacts of same relay) on LV side. LV side Directional relay provides selective tripping of transformer along with or in case of non-operation of Differential (Main) protection. Thus operation of this relay (output contact) operates HV side as well as LV side breaker of the transformer. Whereas LV side Non-Directional relay provides simple means for protection of transformer against un-cleared fault by feeder breaker. Hence operation of this relay (or output contact) operates only LV side breaker of the transformer. Transformer HV side protection shall always be simple as a second defence level hence it is non-directional.

In some cases for LV side relay it is not possible to configure output contacts separately for Non-Directional characteristics. Instead we may configure output contacts for Forward (Towards Transformer) and Reverse (Towards Feeder) direction separately. Thus In such case we may use protection scheme as shown here.

                  

It is also possible that we may not have Directional relay on LV side even though may need to operate the transformer in parallel. In such case Non-directional relay provided on transformer LV side may serve the basic purpose of providing necessary protection to transformer against through fault. However in case of fault in transformer differential zone we can’t expect selective tripping by overcurrent earth fault relay. But the chances to render this duty by overcurrent earth fault relay are very remote. Thus we may use the protection scheme as shown here as compromised protection scheme.

Though the scheme as shown here is most economical and sufficient it is not recommended because protection of transformer against through fault has to be rendered by transformer HV side relay only. This may be risky decision considering cost of the transformer.