Principles of Relay Application

Protection Zones: The overall relay based protection system is divided into different protection zones according to the equipment and available circuit breakers. Seven possible protection zones are listed below:

–- Generator & generator transformer unit protection

–- Transformer protection

–- Busbar protection

–- Lines (Transmission, sub-transmission, and distribution)

–- Feeder protection

–- Utilization equipment

–- Capacitor or reactor banks

•

•The power system is divided into protection zones defined by the equipment and available circuit breakers. Six possible protection zones are listed below:

–- Generators and generator-transformer units

–- Transformers

–- Buses

–- Lines (Transmission, sub-transmission, and distribution or Feeders)

–- Utilization equipment

–- Capacitor or reactor banks

• •

Short-circuit protection includes two protection systems: primary and backup protection. Primary protection is the first line of defense. The figure shows the one-line diagram of a power system section. We may observe that we use breakers to connect adjacent system elements. Using the breakers in this manner permits the protection system to completely isolate a faulted element. An exception is the case of the generator-transformer units. Generators have dedicated step-up transformers in this arrangement, and we may omit the breaker between them.

•The zones indicated with dotted lines are the primary protection zones. The significance of these zones is that a fault inside a zone implies the tripping of all the breakers belonging to that zone. Protective relays define these zones. Adjacent protection zones overlap to provide full primary protection coverage in the power system. A fault in the overlapping areas produces the tripping of more breakers than the breakers needed to isolate the fault. We need the overlapping areas to be as small as possible.

•Primary protection operation should be as fast as possible, preferably instantaneous, for stability and

 power quality reasons.

•Protective relays define the primary protection zones. Relays use system currents and voltages as

 input signals. We will see during the course that current information is instrumental for the relays

 in determining fault location. Then, current transformer location defines the limits of the primary

 protection zones in many cases.

•In lower-voltage systems, we use bushing-type current transformers installed inside breaker and

 transformer bushings. In this case, protection zones overlap around the breaker, and the breaker

 lies in the ovelapping zone. A breaker fault produces the tripping of all breakers at both zones.

•In higher-voltage installations, we use multiwinding current transformers. We use different

 secondary windings for the relays of the two protection zones. The overlapping zone is inside 

the current transformer. The probability of an overlapping-zone fault is very low. The price we pay

 for this arrangement is that it could be 

necessary to trip some Zone B breakers with Zone A relays to completely disconnect some Zone B 

faults.