In interconnected Power System network , a good protection scheme is necessary for reliable operation. So, which qualities needed for good protective relay?
A protective relaying scheme should have certain important qualities such as;
- Speed and Time
- Selectivity and discrimination
These qualities must have protective relays for better protection of the power system network.
Let’s explain in detail about each quality.
It is the ability of a protective relays to operate under predetermined conditions. The reliability of the protection system depends on the reliability of various components like a circuit breaker, relays, current transformer (CT), potential transformer (PT), trip coils, connecting cables and many more.
There are many components to operate before the operation of the relay. And the operation of the relaying scheme is also depending on the component which plays a role in the operation of the relay.
The reliability of the system can be judged from the statistical data but cannot be expressed in terms of mathematical operation.
Proper maintenance also plays an important role in improving the reliable operation of the system. It is also depending on the design of the system.
Speed and Time
The protective scheme must disconnect the faulty part of the system as soon as possible. If the faulty part takes more time to disconnect,
It may damage the component which is carrying faulty current.
The system voltage will decrease. And low-voltage may affect the motors and generators running on the consumer side.
In worst condition, if the fault persists a long time, subsequently other faults may get generated and it results in a blackout.
Therefore, very high-speed protection relays are used in the power system network.
The relay time is the time taken from the instant of the fault and the instant of the closure of contact of the relay. The circuit breaker time is the time taken by a circuit breaker to operate and open the contacts and extinguish the arc.
The total time required between the instant of the fault and the instant of final arc interruption in the circuit breaker is called fault clearing time. It includes the relay time and circuit breaker time.
In high-speed operation of the relaying scheme, the fault clearing time is very small.
The protective scheme operates if the value of the actuating quantity is not in the predefined range. The sensitivity is the ability to operate with a low value of actuating quantity.
In a protected zone, the sensitivity indicates the smallest value of the actuating quantity at which the protection starts operating in relation to the minimum value of fault current.
The input of the relay coil is measure in volt-ampere. Smaller the value of volt-ampere input, the relay is more sensitive. Generally, there are two configurations 1 VA and 5 VA relay. Out of these two types, 1 VA relay is more sensitive than 5 VA.
Selectivity and Discrimination
Selectivity is the ability of the protective scheme to identify the fault part of the system. It must operate only for the faulty part without affecting the rest of a healthy system.
Discrimination is the ability of the protective scheme to identify between the normal condition and abnormal condition. The relaying scheme must operate under abnormal conditions only.
Therefore, the protective scheme selects the faulty part from the healthy system and disconnect in abnormal condition for faulty part only without disturbing the healthy system.
One more thing to mention here is that the relaying scheme must not operate beyond their protective zone.
If the system is not selective, it operates for the fault beyond its protective zone and unnecessary tripping occurs. By which it will disconnect large areas compared to a faulty area.
The power system network must remain stable under the condition of transient, disturbance, and faults.
The stability of power system is the quality of a protective scheme due to which the system remains inoperative and stable under abnormal conditions.
To achieve stable operation of the system, time delay, a filter circuit, mechanical and electrical bias are provided.
To maintain and install easy, the protective scheme must be simple in operation. The complex systems are difficult while maintenance and installation.
The simpler the system it will more reliable. Therefore, the protective scheme must simple.
The important quality of any system is the cost. The cost of a system is as low as possible. Sometimes, to make the system more secure, the cost of a protective scheme is more than the cost of the system which we want to secure.
So, to avoid this situation, the cost of a protective scheme is not more than 5 % of the total cost of equipment.
But if the equipment to be protected is very important, the economic constraint can be relaxed.
There are varieties of faults and disturbances that exist in the power system network. It may be impossible to provide protection against each and every abnormal condition. But the protective system must provide adequate protection for the element of the system.
The adequateness of the system can be evaluated by considering the following factors.
- Probability of abnormal condition
- Discontinuity of supply due to the failure of the equipment.