Low-impedance bus differential relays are so named because the differential relay current inputs have a low-impedance to the flow of CT secondary current. This means that the low-impedance bus differential relays can share the CTs with other relays, meters, transducers, etc. The low-impedance bus differential scheme typically has one set of current inputs for each phase from every set of CTs in the scheme.

The Busbar protection (BBP) operates according to the principle of a combined differential current measurement with operation and restraint features and a phase comparison function. In a healthy condition and according to Kirchhoff’s first law, all the currents flowing towards a busbar section must leave it again. The busbar protection scheme is based on a measurement algorithm, which compares the amplitudes of the feeder currents and derives a restraint criterion. The algorithm is executed independently for each protection zone and phase.

A. Bay Unit

The bay unit is the interface between the protection and the primary system process comprising the

• Current Transformer

• Bus isolators status

• Circuit Breaker status

and performs the associated data acquisition, pre-processing, control functions and bay level protection functions. It also provides the electrical insulation between the primary system and the internal electronics of the protection.

The bay unit is provided with local intelligence and performs Local Protection as

• Breaker failure

• End fault

• Breaker pole discrepancy

Also providing Bay Protection as back-up bay protections as well as the event and disturbance recording.

Very Important Note

In the event that the central unit is out of operation, or the optical fiber communication is disrupted an alarm is generated, the bay unit will continue to operate, and all local and bay protection as well as the recorders (event and disturbance) will remain fully functional (stand-alone operation).

B. Central Unit

The central unit is the system manager, it configures the system, contains the busbar replica, assigns bays within the system, manages the sets of operating parameters, acts as a process bus controller, assures synchronization of the system and controls communication with the station control system.

The variables for the busbar protection function are derived dynamically from the process data provided by the bay units. The process data are transferred to the central processor via a star coupler module.

As shown below, the complete single line diagram is drawn in the Central unit, each bay receives the current measurements, isolators and CB statuses from its associated Bay unit that is connected and configured with the central unit with a unique IP address and ID name.

The central unit contains the Busbar differential protection function and based on the data retrieved from all bay units, the central unit start to calculate for the differential and restrain quantities. Once a fault is detected in any Bus Zone, the central unit is not responsible for issuing a trip command to those circuit breakers, however it sends through the fiber communications tripping signals to the bay units associated with the faulty bus zone that will issue the trip command to the local circuit breaker.