Transformer Over current protection
Over current protection of a transformer is a backup protection to the differential protections when it is used and it also performs as a primary short circuit protection when the differential protection is not used for small transformers. It also acts for through faults for the transformer as a back up to protection relays on the load side of the transformer. In some cases impedance relays and direction relays are used instead of over current protection for better protection of transformer when there is a large difference between the maximum and minimum short fault MVA.
Causes of over current in transformer:
If any internal fault occurs like short circuit or ground fault there will be over current flow through the transformer windings from the source. Normally, if the differential relay used, it will detect the fault and it operates and safeguards the device. If it is not used then over current relay operates and safe gauds the device.
Over current may occur due to over loading of the transformer i.e. suddenly if an extra high rating motor is loaded on transformer secondary then there will high motor starting current which increases transformer secondary current. Due to this transformer primary current also increases which increases the heat and may damage the transformer.
If any short circuit occurs in the load side and if it is not cleared by the downstream protections then also over current may occur in the transformer windings.
Operation of over current relay
The connection diagram of over current relay for a three windings transformer with each phase consists of one over current relay. I.e. on each phase a current transformer is mounted, and the CT output is compared with the threshold setting value.
In transformers below 10MVA, normally fuses are used for over current protection. Time over current relays (50) or instantaneous relays (51) are used for over protection of transformers and in some high power applications both are used with instantaneous protection as back up to time over current with higher setting value. The setting of these fuses and over current relays depends on the thermal balance of the transformer core and tank. The relay settings will be chosen below the thermal balance of transformer and transformer magnetization current rating is also taken in to consideration.
The time over current (IDMT) relay is normally set for operation at about 150% of the rated current of the transformer. The time delay must be long enough to avoid tripping due to the magnetizing in rush current when the transformer is energized. Time selectively must also be attained between the relays on the primary and secondary side of the transformer.
The instantaneous element has to be set about 25% above the maximum through fault current and above the maximum in rush current with this setting, instantaneous tripping is only obtained for severe faults on the feeding side of the transformer. There can , for example be faults on the transformer winding close to the bushing, faults or on the circuits between the CTs and the transformer.