The circuit breakers used to disconnect any element of the system for repairing work, usual operation and maintenance requirements and also under abnormal conditions like short circuits.
Protective Zones established around each system element. The significance of protective zones is any fault that occurs in a given zone will cause tripping of relays. And it will isolate that part of the system by opening the circuit breaker.
The edges of protective zones decided by the locations of the current transformers.
In practice, these zones overlapped to ensure the complete safety of all elements of the system. The part of the system which is unprotected, that known as a dead spot or blind spot. Hence, to avoid the dead spot, use overlapping of zones.
Overlapping of Protective Zones
The above figure shows the overlapping of zones. The circuit breakers are located in the connections to each power system element. because of this, at the time of the fault, only the faulty element will disconnect.
Due to overlapping of the protective zone, if a fault occurs in the overlapped area, more circuit breaker than the minimum necessary for the disconnection of fault region. So, there is a chance of unnecessary tripping of breaker, then also for security reasons, the overlapping done in the practical power system network. And also, there are very fewer chances that the fault occurs in this area.
The protection provided by the protective relaying equipment can be categorized into two types
- Primary Protection
- Backup Protection
It is the first line of defense and is responsible to protect all the power system elements from all the types of faults.
When the primary protection made inoperative for any reason, the backup protection has to sense the fault and trip an appropriate circuit breaker.
There are many conditions when the primary protection may not operate due to many reasons like,
- Failure in circuit breaker: The breaker cannot operate due to jammed contacts,
- Failure in the protective relay: The relay setting is not properly done. Hence it cannot sense the fault.
- Loss of voltage and current supply to the relay: Protective equipment supplied by DC battery bank. If there is a fault in this supply, the relay cannot generate a trip command to the breaker.
- Failure in tripping circuit: The tripping circuit used to transfer a signal from relay to breaker. If this system is failed to operate, then the breaker will not open.
So, these are the possibilities for failing the primary protection scheme.
Backup protection is provided to protect the elements when the primary protection is failed to operate due to any reason.
Suppose, the back protection is not given to the element, in this case, if the primary protection failed to operate, it will create serious damage in the power system network. Therefore, it is necessary to implement the backup protection.
The arrangement of backup protection done in such a way that, the failure in primary protection should not cause the failure in backup protection. To avoid this, the backup protection is located at a different station from the primary protection.
Generally, the backup protection given to the costly equipment. For cost and economy consideration, the backup protection is employed only for the protection against short circuit and not for any other abnormal conditions.
Backup Relaying with Example
Let’s get more details about backup relaying,
From the above figure, C, D, G, E, and H are primary relays and A, B, I, F, and J are the backup relays.
For example, if the primary relay E fails to operate, the backup relay A and B have to trip as backup protection. If the fault is in the station K, then also the relay A and B acts as a backup relay.
But one thing should be noted that, whenever the backup relay operates, it will isolate the large portion of the system. Because it isolates some healthy parts also. So, it reduces the reliability of the system.
One more thing you should keep in mind that, the backup protection must operates after some time delay. So, primary relaying gets a chance to operate.
When a fault occurs, both the relays have to sense this fault, but the primary relay expected to trip first and backup will trip if the primary relay I failed to operate.
Methods of Backup Relaying
Relay backup protection
A single breaker used by both primary as well as backup protection but the two protective systems are different.
Breaker backup protection
In this scheme, separate breakers provided for primary and backup protection. Both types of breakers are in the same zone.
Remote backup protection
Separate breakers provided for primary and backup protection. The two types of breakers are at different stations and are completely isolated and independent of each other.
Centrally coordinated backup protection
In this method, primary protection is at various stations. There are a central control room and backup protection controlled for all stations from the central control room. And the control room used to inspect and monitor the entire system. The load flow analysis done from the control room.
The control room consists of digital computers that decide the proper switching action. The method also called controlled backup protection.
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