Introduction Protection

Protection

CB

CT

Trip coil

PT Relay coil

Main Features of Good Protective System • Reliability: It is defined as the probability that the system will function correctly when required to act. It has two aspects (i) The system must operate in the presence of a fault that is within its zone of protection and (ii) It must refrain from operating unnecessarily for faults outside its  protective zone or in the absence of a fault. A quantitative measure for  reliability is defined as follows:

2 Security: security is a property used to characterize false

tripping on the relays. A relay is said to be secure if it does not trip when it is not expected to trip. It is the degree of certainty that the relay will not operate incorrectly:

3 Sensitivity: The protective system must have ability to detect the smallest possible fault current. The smaller the current that it can detect, the more sensitive it is. One way to improve sensitivity is to determine characteristic signature of a fault. It is unique to the fault type and it does not occur in the normal operation. For example, earth faults involve zero sequence current. This provide a very sensitive method to detect earth faults. Once, this signature is seen, abnormality is rightly classified and hence appropriate action is initialized.

4. Selectivity It refers to the overall design of protective strategy wherein only those protective devices closest to a fault will operate to remove the faulty component. This implies a grading of protective device threshold, timing or operating characteristics to obtain the desired selective operation. A relay should not confuse some peculiarities of an apparatus with a fault. For example, transformer when energized can draw up to 20 times rated current (inrush current) which can confuse, both over  current and transformer differential protection. Typically, inrush currents are characterized by large second harmonic content.

• This discriminant is used to inhibit relay operation during inrush, there by, improving selectivity in transformer protection. • Also, a relay should be smart enough, not just to identify a fault but also be able to decide whether  fault is in its jurisdiction or not. • For example, a relay for a feeder should be able to discriminate a fault on its own feeder from faults on adjacent feeders. This implies that it should detect first existence of fault in its vicinity in the system and then take a decision whether it is in its jurisdiction.

5.Dependability: A relay is said to be dependable if it trips only when it is expected to trip. This happens either when the fault is in it's primary jurisdiction or when it is called upon to provide the back-up protection. However, false tripping of  relays or tripping for faults that is either not within its jurisdiction, or  within its purview, compromises system operation. Power system may get unnecessarily stressed or else there can be loss of service. Dependability is the degree of certainty that the relay will operate correctly:

Zone of Protection A relay's zone of protection is a region defined by relay's jurisdiction. It is shown by demarcating the boundary. This demarcation for differential  protection is quite crisp and is defined by CT's location. On the other hand, such boundaries for overcurrent and distance relays are not very crisp. It is essential that primary zones of protection should always overlap to ascertain that no position of the system ever remains unprotected. This overlap also accounts for faults in the circuit breakers. To provide this overlap additional CTs are required

Terms Connected with Relays: •

Operating torque: It is the torque which tends to close the contacts of the relay.

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Restraining torque: It is the torque which opposes the operating torque.

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Pick up: It is defined as the threshold value of current, voltage etc. above which relay will close its contact.

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Reset: It is defined as the value of current, voltage etc. below which the relay will open its contacts and return to normal position.

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Flag or Target: It is a device which indicates the operation of a relay.

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Plug Setting Multiplier (PSM): It is the ratio of fault current in relay coil to pick up current Primary current PSM=

Relay Current setting× CT ratio

Classification of Relay 1. According to the function in the protection scheme. (a) Main Relay: Responds to when any change in the actuating quantity . (b) Auxiliary Relay: Controlled by other relay to perform some auxiliary function such as introduction of a delay, increasing number of contacts, passing a signal from one relay to another, energizing a signal or an alarm etc. (c) Signal relays: They function to register the operation of some relay by flag indication, simultaneously it can also actuate an audible alarm circuit.

(2) According to their construction and principle of operation: (a) Electromagnetic Attraction type: Operation depends on the movement of an armature under the influence of  attractive force due to magnetic field set up by current flowing through relay winding. (b) Electromagnetic Induction type: operation depends on the movement of a metallic disc or cylinder free to rotate by the interaction of induced eddy currents and the alternating magnetic field producing them.

(c) Thermal Relays: Movement depends upon the action of heat produced by the current flowing through the element of the relay . (d) Gas Operated Relays: It operates when a specified amount of gas has accumulated. (e) Static Relays: Semiconductor devices such as diodes, transistors, ICs etc. are used. (f) Microprocessor based Relays: These relays are built on the central processing unit like computers and variety of functions can be achieved by mathematical calculations based on algorithms.

(3) According to applications: (a) Under / Over voltage, Under / Over current, under / over freq. etc. (b) Directional Relays: operation takes place when the applied voltage and current assume a specific phase displacement. (c) Differential Relay: operation occurs at some specific phase and magnitude difference between two or more electrical quantities. (d) Distance Relay: Operation depends upon the ratio of  voltage to current.

(4) According to their time of operation (a) Instantaneous Relays: Operation occurs after a negligibly small interval of time from the incidence of  the current or other actuating quantity which causes operation. (b) Definite time lag Relay: Operation takes place after  fixed time delay. (c) Inverse time lag relay: Time of operation is inversely proportional to magnitude of current or other quantity causing operation. (d) Inverse Definite Minimum Time (IDMT) lag Relay : The time of operation is inversely proportional to smaller  values of actuating quantity and tends to a minimum time as the value increases without limit.

(5) According to Number of operating Quantities: (a) Single quantity relay (b) Two quantity relay (c) Multi quantity relay