End Fault Protection in bus bar relays

Bus bar protection is generally differential protection for which zone of protection is defined by location of CTs. However, location of isolation is defined by CBs. The area between CB and CT is always an issue for protection schemes. 

In above image, CT location in Bay-1 is on line side and CT location in Bay-2 is on bus side. It has been done intentionally to discuss both cases. However, in actual practice mostly CTs are placed on line side. In old schemes, without end fault protection enabled, this area is taken care by Breker Failure Relays with a time delay around 200ms. For Numerical bus bar relays, end fault protection is an additional feature to take care of the area between CB and CT.

Bay-1: CT location on line side

Case 1.1 When CB is open: Current flowing through 1CT (feed from remote end) is not included in busbar differential relay calculations. Therefore, busbar relay will not operate for fault between 152CB and 1CT. End fault protection relay has one additional over current element, which will operate in this condition when current through 1CT is above set value. This will send direct trip command to remote end through communication channel so that fault is cleared without any time delay.

Case 1.2 When CB is closed: Current flowing through 1CT is included in busbar differential relay calculations. Therefore, busbar relay will operate for fault between 152CB and 1CT. Depending on scheme, Busbar protection will send direct trip command to remote end through communication channel so that fault is cleared without any time delay or end fault protection will operate and send direct trip after CB is open.

Bay-2: CT location on bus side

Case 2.1 When CB is open: Current flowing through 2CT (feed from local end) is not included in busbar differential relay calculations. This causes extension of bus bar protection zone upto CB. Therefore, busbar relay will operate for fault between 252CB and 2CT. 

Case 2.2 When CB is closed: Current flowing through 2CT is included in busbar differential relay calculations. Since area between 252CB and 2CT is outside differential zone, busbar relay will not operate for this fault. This fault will be detected by line protection. After opening of CB, End fault protection relay will come in picture as mentioned in Case 2.1.

Over current and earth fault protection

Over current protection and earth fault is simple protection. Being low cost protection it is widely used across all levels. For lower voltage levels it is main protection, and for higher voltage levels it is used as backup protection. The reason being this protection does not have clear boundaries of protection.

Over current protection is for phase current and earth fault is for neutral current (residual current). The characteristics for both protections are same.

This protection is of two types: definite time (DT) or inverse definite minimum time (IDMT).

Definite Time (DT): In this type, protection will start when current reaches a set value, and protection will operate after a set time. For example, we for a transformer having maximum through fault current of 100A we can set DT protection 110A with time delay 100ms. In this way protection will operate in case of faults inside transformer only. For faults on downstream current will be less than 100A and protection will not operate.

Inverse Definite Minimum Time (IDMT): In this type, protection will start when current reaches a set value, and operating time of protection will depend on the level of current. If current is very high compared to set value, protection will operate fast. If current is slightly high from set value, relay will take more time to operate. In this way, a curve can be drawn between current and time for each relay. There are standard curves for which relays are designed. User can select from these curves as per requirement. For mechanical relays these curves are fixed as per design. However, in case of static / numerical relay curve can be selected form settings of relay.

For example, we have set relay in IDMT standard inverse for a transformer. Full load current of the transformer is 100A, and we have set IDMT start at 120A. The relay will start when current increases beyond 120A. If the current is 1200A, which means 10 times of set value. For 10 times current, the operating time is 2s (from curve). There is another setting called Time multiplier setting (TMS), say it is set at 0.4 for this transformer. The operating time of relay will be 0.8s.

If fault current is 240A, which means double of set value. For double current the operating time is 10s. The operating time of relay will be 4s.

In this way, relay will operate in 800ms for fault current of 1200A and in 4 sec for fault current of 240A

The operating time can be calculated as per below (IEC):

Standard inverse: t = TMS x 0.14 / [(Ir^0.02) – 1]

Very inverse: t = TMS x 13.5 / [Ir – 1]

Extremely inverse: t = TMS x 80 / [(Ir^2) – 1]