introduction protection

7/30/2019 Introduction Protection

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Relay coil

Trip coil

CBCT

PT

Protection


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Main Features of Good Protective System

Reliability: It is defined as the probability that the system will functioncorrectly 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:


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2 Security: security is a property used to characterize false

tripping on the relays. A relay is said to be secure if itdoes not trip when it is not expected to trip. It is the

degree of certainty that the relay will not operate

incorrectly:


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3 Sensitivity:

The protective system must have ability to detect the

smallest possible fault current. The smaller the currentthat 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 itdoes not occur in the normal operation.

For example, earth faults involve zero sequence current.

This provide a very sensitive method to detect earthfaults. Once, this signature is seen, abnormality is rightly

classified and hence appropriate action is initialized.


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4. Selectivity

It refers to the overall design of protective strategy wherein onlythose protective devices closest to a fault will operate to removethe faulty component. This implies a grading of protective device

threshold, timing or operating characteristics to obtain the desiredselective operation.

A relay should not confuse some peculiarities of an apparatus with afault. For example, transformer when energized can draw up to 20times rated current (inrush current) which can confuse, both overcurrent and transformer differential protection. Typically, inrush

currents are characterized by large second harmonic content.


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This discriminant is used to inhibit relay operationduring 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 detectfirst existence of fault in its vicinity in the system andthen take a decision whether it is in its jurisdiction.


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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 ofrelays 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:


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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

Zone of Protection


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Terms Connected with Relays:

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

relay.

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

Pick up: It is defined as the threshold value of current, voltage etc. above

which relay will close its contact.

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

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

Plug Setting Multiplier (PSM): It is the ratio of fault current in relay coil

to pick up current

PSM=Primary current

Relay Current setting CT ratio


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Classification of Relay

1. According to the function in the

protection scheme.(a) Main Relay: Responds to when any change in theactuating quantity .

(b) Auxiliary Relay: Controlled by other relay to performsome auxiliary function such as introduction of a delay,increasing number of contacts, passing a signal fromone 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 canalso actuate an audible alarm circuit.


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(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 torotate by the interaction of induced eddy currents and

the alternating magnetic field producing them.


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(c) Thermal Relays: Movement depends upon the actionof 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 varietyof functions can be achieved by mathematical

calculations based on algorithms.


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(3) According to applications:(a) Under / Over voltage, Under / Over current, under /

over freq. etc.

(b) Directional Relays: operation takes place when theapplied voltage and current assume a specific phase

displacement.

(c) Differential Relay: operation occurs at some specificphase and magnitude difference between two or more

electrical quantities.

(d) Distance Relay: Operation depends upon the ratio ofvoltage to current.


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(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 causesoperation.

(b) Definite time lag Relay: Operation takes place afterfixed time delay.

(c) Inverse time lag relay: Time of operation is inverselyproportional to magnitude of current or other quantitycausing 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 minimumtime as the value increases without limit.


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(5) According to Number of operating

Quantities:(a) Single quantity relay

(b) Two quantity relay

(c) Multi quantity relay

introduction protection

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