Electrical Protective devices such as circuit breakers, isolators, or disconnecting switches, sensing devices require a proper operation against fault current.
The operation of protective devices must meet several requirements and this requirement is essential.
7 fundamental requirements of protective relaying are,
- Selectivity.
- Sensitivity.
- Speed.
- Discrimination.
- Stability.
- Reliability.
- Economics.
To confirm the correct operation of all protective devices in electrical, It should meet these seven requirements. This may also consider the fundamental of a protective system.
These seven requirements must be fulfilled by protective devices. So engineers considered this as a fundamental requirement of protective devices.
In this article, we can explain each fundamental requirement with an example.
Selectivity of the protective devices (selectively isolate the faulty circuit)
Selectivity is the ability of protective devices to isolates the faulty network or faulty section from the healthy network.
The possibility of failure and failure of protective relays and the circuit breaker has to be considered in determining the selectivity.
Benefit of selectivity
Selectively, isolating the faulty circuit without affecting the healthy circuit will minimize the outage area, at the same time, it is possible to maintain the normal power supply to the remaining power system.
Categorization of selectivity
There are two types of selectivity in protective devices,
- Absolute selectivity
- Relative selectivity
What is called absolute selectivity?
The single electrical equipment protected by a single protective device is known as absolute selectivity.
The absolute selectivity of the power system can be achieved when we want to protect particular equipment or winding up the equipment at a small distance.
Differential protective relays are the best example of absolute selectivity.
For example, consider the situation if we want to protect the winding of the transformer, then we need to install two Current Transformers on each side of the transformer winding, and these two transformers should connect with one differential protective relay.
Whenever the transformer winding experiencing an electrical fault, then the differential relay should capable to operate.
What is called Relative selectivity?
Protecting electrical equipment by coordinating the operation of different relays is known as relative selectivity.
Relative selectivity is achieved when we want to obtain the selectivity by coordinating different protective devices.
Over-current relay and distant relays are the best examples of relative selectivity.
For example, if we need to protect the transmission lines which connect the three different buses [bus-A, bus-B, and bus-C] and they are protected by a distant relay [R1, R2, R3].
Now, we require selectivity from one bus of the relay with other buses of the relay, then selectivity can be achieved by coordinating the distance relay R1 and R2 using IEC standard procedure.
Sensitivity of the protective devices (High sensible to fault current)
Sensitivity is also one of the requirements of protective devices.
The ability of the protective device to sense any electrical fault correctly for isolating the faulty section from the healthy section means protective devices should sense and operate even for the minimum amount of electrical fault.
For example, Consider a transmission line that carries 200 continuously without any damage or any temperature rise. If a fault occurs on this transmission line and the magnitude of fault current lower than the rated current of electrical equipment that is 150ma.
Though the magnitude of fault current is lower than the full load current, it is harmful because it going to generate negative or zero sequence voltages and currents. Fault current also reduces the rated voltage.
So, the protective devices should be capable to sense the magnitude of that fault current at any value and operate properly.
Sensitivity factor
The sensitivity factor determines the fast detection of the electrical fault. Sensitivity factors usually relate to the full load current of the feeder or rated current of any device or equipment.
The sensitivity of a protective device can be expressed in apparent power such as 1VA, 3VA, 5VA.
The sensitivity of the protective relay depends on two parameters,
- The sensitivity of protective devices
- Operating condition of the power system or sensitivity of the whole system
Both sensitivities of the protective device and sensitivity of the whole system are different because the sensitivity of the whole system involves the sensitivity of the relay, instrument transformer, circuit breaker, and some other equipment.
Speed of the protective device (Operation should be fast)
The next requirement of a protective device is Speed. The speed of the protective device should minimize the duration of the fault clearing time by performing a quick disconnection of the faulted area.
Fault clearing time
Fault clearing time = Time of operation of protective relay + Time of operation of Circuit breaker
Benefits of the speed of the protective device
- Quick disconnection of the faulty area prevents damage to the electrical equipment.
- Quick isolation of the faulty area minimizes the power outage duration.
- The speed of the protective system improves the stability of the power system because the chance of loss of synchronism can also be reduced.
So, considering all these benefits we can say that speed is one of the important criteria for protective devices.
Discrimination of the protective device ( Recognizing the difference between fault condition and faultlessness condition)
The discrimination of the protective device is the ability of good judgment to the electrical fault.
Protective devices should not operate for the gradual increment of a current such as overload condition, starting current, and inrush current.
The protective devices should discriminate against electrical fault conditions, even when the minimum fault current is less than the rated full load current.
For example, the protective devices should recognize the difference between electrical fault and overloading conditions.
In the case of an electrical fault, there is a rapid increase in the current above the rated current and causes damage to the electrical equipment.
In the case of an overload phenomenon, there is a gradual increase in the current and causes no immediate damage to the electrical equipment.
Stability of the protective devices ( Well balance to all types of electrical fault)
The next requirement of the protective devices is stability. The word stability is also referred to as a state of being stable. The protective devices should provide stable operation, particularly for electrical faults.
The protective system or devices should remain stable for any mal-operation or misoperation.
Reliability of the protective devices
The other important criterion or requirement for protective devices is reliability. Basically, reliability is considered on two conditions 1) dependability and 2) Security.
Reliability is the ability of protection devices to operate properly during their whole life span, the period they are in service.
Dependability is the ability to respond against the fault condition.
Security is the ability of the protective devices to secure against electrical fault, maloperation, etc.
Economy of the protective devices
The final requirement of the protective devices is the economy. The features of the protective system should be correct to the requirement, and at the same time, the cost of the protective device should be minimum.
The number of features of protective devices will vary according to the requirement of protection, considering that requirements we need to select the protective devices.