Protection schemes and zones

-Zones of protection

1. Protection schemes

2. Zone of protection Power system is protected in zones, meaning that the whole system is divided into various sections. For each zone, there is one or more protective schemes, which are coordinated with the overall protection with the following characteristic of boundaries set by CT locations. a) The zones are arranged to overlap so that no part of the system remains unprotected as in Fig.1 b) Circuit breakers are located in the overlapped region.

3. Attributes of relaying Reliability: assurance that the relay will perform correctly. Reliability has two aspects: Dependability & security Dependability is defined as the degree of certainty that a relay system will operate correctly when required. On the other hand, Security is the ability of the protection relay to avoid unnecessary operation (malfunction) during day after day operation and for faults outside the designated zone of protection. Such faults are termed External or Through faults. In other words, dependability indicates the ability of the protection system to perform correctly when required, while security is its ability to avoid unnecessary operation for external faults. Hence the protective relaying must be ready to function, reliable and correct in-operation at all times under any kind of fault and abnormal conditions of the power system for which it has been designed.

4. Selectivity: the relay scheme should be selective i.e., it should have the ability to distinguish between the faults within its own zone of operation and the faults outside the zone . The selective scheme ensures maximum continuity of service with minimum system disconnection. Relays have an assigned area known as the primary protection zone. They must promptly operate for any fault within that zone. However, relays may also operate for a fault outside the zone. In these cases they provide backup protection for the areas outside their primary zone; called the over-reached area. Selectivity is the process of applying and setting the protective relays in a way that they operate with fast speed for any fault within its primary zone of protection and are operated with time lag for faults in their secondary zones for which they are to back up. This is necessary to permit primary relays designed to this backup or over reached area a sufficient time to operate. Otherwise, both sets of relays may operate for faults in this over reached area. It is very important that backup protection should only operate if the primary protection of that area fails to clear the fault.  

5. Speed : It minimizes fault duration and consequent equipment damage. Speedy isolation of faulty portion of a power system ensures: Improved power system stability Decreased amount of damage incurred Less annoyance to electric power consumers Less likely the development of one type of fault into other more severe type Relay aid re-closure of circuit breakers to restore service to customer.  clearing time “is the sum of the operating time of the protective relaying and breaker interrupting time where:

6. “The operating time of the protective relaying” is the time elapsed from the instant of the fault occurrence up to the closure of contact in the trip circuit of the circuit breaker and  “The breaker interrupting time” is the time elapsed from the instant of the closing of trip circuit up to the instant when the current is interrupted after the arc has extinct between the open contacts of the circuit breaker . Modern high speed circuit protective relaying has operating time of about 1.0 to 2.0 cycles i.e., 0.02 to 0.04 sec. whereas modern high speed circuit breakers have interrupting time of about 2.5 to 3.0 cycles i.e . 0.05 to 0.06 sc. Therefore the clearing time may be about 0.07 to 0.1 sec.