DIRECTIONAL COMPARISON BUS RELAYING

Occasionally, it is desirable to add bus protection to an older substation where additional current transformers and control cable are too costly to install. In this instance, the existing current transformer circuits used for line relaying can also be used to provide protection for bus faults.

A directional comparison bus protection system compares the direction of current flow in each circuit connected to the bus. If the currents in all the circuits flow into the bus, a bus fault exists.

If the current in one or more circuits is flowing away from the bus, an external fault exists. Fault detectors are used to initiate the system. This system can be used for either or both phase and ground bus fault protection.

The basic system requires directional relays, fault detectors, and a timer. Directional relays are used on each circuit connected to the bus. Fault detectors are used to indicate a fault within the vicinity of the bus.

Phase fault detectors are instantaneous overcurrent relays connected to a bus tie breaker or to one or more of the circuit connections. Ground fault detectors are instantaneous overcurrent relays connected in power transformer neutrals or to one or more of the circuit connections.

A timer is required to permit contact coordination of the directional relays. The contacts of the directional relays are connected in series with the timer contact to initiate a trip signal.

Current transformer saturation is usually not a problem when comparing direction of current rather than comparing magnitudes of current as is done in conventional bus relaying systems. The current transformers in each circuit do not require the same ratio and can be used for other forms of relaying and metering.

In some cases, the directional elements of relays used for normal circuit connection relaying can also be used for this bus protection system. The system is rather complex and requires rigorous maintenance due to the number of relay contacts.

The timer must be set for at least four cycles to assure contact coordination. Contact bounce must be avoided in the directional relays and in the overcurrent fault detector relays.

Contact bounce should be checked when high-magnitude fault currents are likely.

The relays and relay settings must be reviewed when system changes are made near the protected bus to make certain that selectivity between initiating and blocking relays exists. There is presently available a directional comparison system based on directional comparison of the current in all circuits connected to the bus in relation to the sum of all the currents.

This system employs solid-state relays. Intermediate current transformers are used, and main current transformer ratios and characteristics need not be the same on all circuits.