Function of Busbars in a Power system & why they must be protected

Busbars form a critical component for a reliable power supply. Thus protection of busbars requires special consideration bearing in mind that the loss of a busbar following a busbar fault can result in subsequent loss of lines and transformers connected to the busbar.

Busbars form an important link between the incoming and outgoing circuits in generating stations and substations. A fault on a busbar as aforementioned can cause a loss of equipment and disruption of supply. To avoid this, a protection scheme needs to be in place to automatically isolate the faulty busbar as quick as possible.

The extent of a busbar area, for the target of protection includes busbars as well as the isolating switches, circuit breakers and the related connections. In an event of fault on part of the busbar, all the circuit equipment linked to that section trips out to give complete isolation.

Common Busbar Protection Schemes

The often employed protection schemes for busbars include:

• Differential protection.
• Fault bus protection.

Differential Protection of Busbars

With this scheme, currents entering and leaving the bus are totalized.  During a normal load condition, the sum of these currents is equal to zero. On occurrence of a fault, the fault current upsets the balance and produces a differential current to operate a relay. A typical differential protection scheme is illustrated below:

The busbar in this case is fed by a generator and supplies load to two lines.  The secondaries of current transformers (CT) in the generator lead, in line 1 and line 2 are all connected parallel. A protective relay is connected across this parallel connection. All current transformers must be of the same ratio in the scheme regardless of the capacities of the various currents.

Operation

Under normal load conditions or external fault conditions, the sum of the currents entering the bus is equal to those leaving it and no current flows through the protective relay. If a fault occurs within the protected zone, the currents entering the bus will no longer be equal to those leaving it. The difference of these currents will flow through the protective relay and cause the opening of the generator circuit breaker (CB) and each of the line circuit breakers.

Fault Bus Protection Scheme   

A power station can be constructed such that faults that develop are predominantly earth-faults. This can be accomplished by providing earthed metal barrier referred to as fault bus surrounding each conductor throughout its entire length in the bus structure.

With this structure, every fault that might happen must involve a connection between a conductor and an earthed metal component. By directing the flow of earth-fault current, it is possible to detect the faults and establish their locations. This protection scheme is referred to as fault bus protection. An illustration of fault bus protection structure is shown below:

With reference to the figure above, the metal supporting structure or fault bus is earthed through a current transformer (CT). A relay is connected across the secondary of this current transformer.

Operation

Under normal operating conditions, there is no current flow from fault bus to ground and the relay remains inoperative. An occurrence of a fault involving a connection between a conductor and earthed supporting structure results in current flow to ground through the fault bus, causing the relay to operate. This operation of relay trips all circuit breakers linking equipment to the busbar.

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