Types and Applications of Overcurrent Relay (Part 1) _ EEP

11/7/13 Types and Applications Of Overcurrent Relay (part 1) | EEP

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Types and Applications Of Overcurrent

Relay (1)

Types, applications and connections of Overcurrent relay (on photo: Transmission l ines from

Gillam to Churchil l)

IndexTypes of protection:

1. Unit Type Protection

2. Non-unit schemes

1. Overcurrent protection

2. Earth fault protection

Various types of Line Faults

Overcurrent Relay Purpose and Ratings

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Primary requirement of Overcurrent protection

Purpose of overcurrent Protection

Overcurrent Relay Ratings

Difference between Overcurrent and Overload protection

Types of Overcurrent Relay:

1. Instantaneous Overcurrent relay (Define Current)

2. Definite Time Overcurrent Relays

3. Inverse Time Overcurrent Relays (IDMT Relay)

1. Normal Inverse Time Overcurrent Relay

2. Very Inverse Time Overcurrent Relay

3. Extremely Inverse Time Overcurrent Relay

4. Directional Overcurrent Relays

Application of Overcurrent Relay

Types of protection

Protection schemes can be divided into two major groupings:

1. Unit schemes

2. Non-unit schemes

1. Unit Type Protection

Unit type schemes protect a specific area of the system, i.e ., a transformer, transmission line,

generator or bus bar.

The unit protection schemes is based on Kirchhoff’s Current Law – the sum of the currents

entering an area of the system must be zero.

Any deviation from this must indicate an abnormal current path. In these schemes, the effects of

any disturbance or operating condition outside the area of interest are total ly ignored and the

protection must be designed to be stable above the maximum possible fault current that could flow

through the protected area.

Go back to Index ↑

2. Non unit type protection

The non-unit schemes, while a lso intended to protect specific areas, have no fixed boundaries.

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As well as protecting their own designated areas, the protective zones can overlap into other areas.

While this can be very beneficial for backup purposes, there can be a tendency for too great an

area to be isolated if a fault is detected by different non unit schemes.

The most simple of these schemes measures current and incorporates an inverse t ime characteristic

into the protection operation to al low protection nearer to the fault to operate first.

The non unit type protection system includes following schemes:

1. Time graded overcurrent protection

2. Current graded overcurrent protection

3. Distance or Impedance Protection


2.1 Overcurrent protection

This is the simplest of the ways to protect a line and therefore widely used.

It owes its application from the fact that in the event of fault the current would increase to a

value severa l times greater than maximum load current. It has a limitation that i t can be applied

only to simple and non costly equipments.


2.2 Earth fault protection

The general practice is to employ a set of two or three overcurrent relays and a separate

overcurrent relay for single line to ground fault. Separate earth fault relay provided makes earth

fault protection faster and more sensitive.

Earth fault current is always less than phase fault current in magnitude.

Therefore, relay connected for earth fault protection is different from those for phase to phase

fault protection.


Various types of Line Faults

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No Type of Fault Operation of Relay

1 Phase to Ground fault (Earth Fault) Earth Fault Relay

2Phase to Phase fault Not with

GroundRelated Phase Overcurrent relays

3 Double phase to Ground faultRelated Phase Overcurrent relays

and Earth Fault relays


Overcurrent Relay Purpose and Ratings

A relay that operates or picks up when it ’s current exceeds a predetermined value (setting value) is

ca lled Overcurrent Relay.

Overcurrent protection protects electrical power systems against excessive currents which are

caused by short circuits, ground faults, etc. Overcurrent relays can be used to protect practically

any power system elements, i.e. transmission lines, transformers, generators, or motors.

For feeder protection, there would be more than one overcurrent relay to protect different sections

of the feeder. These overcurrent relays need to coordinate with each other such that the relay

nearest fault operates first.

Use time, current and a combination of both t ime and current are three ways to discriminate

adjacent overcurrent relays.

OverCurrent Relay gives protection against:

Overcurrent includes short-circuit protection, and short circuits can be:

1. Phase faults

2. Earth faults

3. Winding faults

Short-circuit currents are generally several times (5 to 20) full load current. Hence fast fault

clearance is a lways desirable on short circuits.


Primary requirement of Overcurrent protection

The protection should not operate for starting currents, permissible overcurrent, current surges. To

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achieve this, the time delay is provided (in case of inverse relays).

The protection should be co-ordinate with neighboring overcurrent protection.

Overcurrent relay is a basic element of overcurrent protection.


Purpose of overcurrent Protection

These are the most important purposes of overcurrent relay:

Detect abnormal conditions

Isolate faulty part of the system

Speed Fast operation to minimize damage and danger

Discrimination Isolate only the faulty section

Dependabil ity / reliabili ty

Security / stabili ty

Cost of protection / against cost of potential hazards


Overcurrent Relay Ratings

In order for an overcurrent protective device to operate properly, overcurrent protective device

ratings must be properly selected. These ratings include voltage, ampere and interrupting rating.

If the interrupting rating is not properly selected, a serious hazard for equipment and personnel will

exist.

Current limiting can be considered as another overcurrent protective device rating, although not all

overcurrent protective devices are required to have this characteristic

Voltage Rating: The voltage rating of the overcurrent protective device must be at least equal to

or greater than the circuit voltage. The overcurrent protective device rating can be higher than the

system voltage but never lower.

Ampere Rating: The ampere rating of a overcurrent protecting device normally should not exceed

the current carrying capacity of the conductors As a general rule, the ampere rating of a

overcurrent protecting device is selected at 125% of the continuous load current.




Difference between Overcurrent and Overload protection

Overcurrent protection protects against excessive currents or currents beyond the acceptable

current ratings, which are result ing from short circuits, ground faults and overload conditions.

While, the overload protection protects against the situation where overload current causes

overheating of the protected equipment.

The overcurrent protection is a bigger concept So that the overload protection can be considered

as a subset of overcurrent protection.

The overcurrent relay can be used as overload (thermal) protection when protects the resistive

loads, etc., however, for motor loads, the overcurrent relay cannot serve as overload protection

Overload relays usual ly have a longer time setting than the overcurrent relays.


Types of Overcurrent Relay

These are the types of overcurrent relay:

1. Instantaneous Overcurrent (Define Current) Relay

2. Define Time Overcurrent Relay

3. Inverse Time Overcurrent Relay (IDMT Relay)

Moderately Inverse

Very Inverse Time

Extremely Inverse

4. Directional overcurrent Relay


1. Instantaneous Overcurrent relay (Define Current)

Definite current relay operate instantaneously when the current reaches a predetermined value.

Instantaneous Overcurrent Relay -

Definite Current

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Operates in a definite time when current exceeds its

Pick-up value.

Its operation criterion is only current magnitude

(without time delay).

Operating time is constant.

There is no intentional t ime delay.

Coordination of definite-current relays is based on the

fact that the fault current varies with the position of

the fault because of the difference in the impedance

between the fault and the source

The relay located furthest from the source operate for

a low current value

The operating currents are progressively increased for the other relays when moving towards

the source.

It operates in 0 .1s or less

Application: This type is applied to the outgoing feeders.


2. Definite Time Overcurrent Relays

In this type, two conditions must be satisfied for operation (tripp ing) , current must exceed the

setting value and the fault must be continuous at least a time equal to time sett ing of the relay.

Definite time of overcurrent relay

Modern relays may conta in more than one stage of protection each stage includes each own current

and time setting.

1. For Operation of Definite Time Overcurrent Relay operating t ime is constant

2. Its operation is independent of the magnitude of current above the pick-up value.

3. It has pick-up and time dia l sett ings, desired t ime delay can be set with the help of an



intentional time delay mechanism.

4. Easy to coordinate.

5. Constant tripping time independent of in feed variation and fault location.

Drawback of Relay:

1. The continuity in the supply cannot be maintained at the load end in the event of fault .

2. Time lag is provided which is not desirable in on short circuits.

3. It is difficult to co-ordinate and requires changes with the addition of load.

4. It is not suitable for long distance transmission lines where rapid fault clearance is necessary

for stability.

5. Relay have difficulties in distinguishing between Fault currents at one point or another when

fault impedances between these points are small, thus poor discrimination.

Application:

Definite time overcurrent relay is used as:

1. Back up protection of distance relay of transmission line with t ime delay.

2. Back up protection to differentia l relay of power transformer with time delay.

3. Main protection to outgoing feeders and bus couplers with adjustable time delay setting.


3. Inverse Time Overcurrent Relays (IDMT Relay)

In this type of relays, operating time is inversely changed with current. So, high current will

operate overcurrent relay faster than lower ones. There are standard inverse, very inverse and

extremely inverse types.

Discrimination by both ‘Time’ and ‘Current’. The relay operation time is inversely

proportional to the fault current.

Inverse Time relays are a lso referred to as Inverse Definite Minimum Time (IDMT) relay.

Inverse Definite Minimum Time (IDMT)

The operating time of an overcurrent relay can be moved up (made slower) by adjusting the ‘ time

dial setting’. The lowest time dial setting (fastest operating time) is generally 0 .5 and the slowest is 10.

Operates when current exceeds its pick-up value.

Operating time depends on the magnitude of current.

It gives inverse time current characterist ics at lower values of fault current and definite time



characteristics at higher values

An inverse characteristic is obta ined if the value

of plug setting multipl ier is below 10, for values

between 10 and 20 characteristics tend towards

definite time characteristics.

Widely used for the protection of distribution

lines.

Based on the inverseness it has three different

types:

Inverse types


3.1. Normal Inverse Time Overcurrent Relay

The accuracy of the operating t ime may range from 5 to 7.5% of the nominal operating time as

specified in the relevant norms. The uncerta inty of the operating t ime and the necessary operating

time may require a grading margin of 0.4 to 0.5 seconds.

It ’s used when Fault Current is dependent on generation of fault not fault location.



Normal inverse time Overcurrent Relay is relatively small change in time per unit of change of

current.

Application:

Most frequently used in utili ty and industrial circuits. especially applicable where the fault

magnitude is mainly dependent on the system generating capacity at the time of fault.


3.2. Very Inverse Time Overcurrent Relay

Gives more inverse characterist ics than that of IDMT.

Used where there is a reduction in fault current, as the distance from source increases.

Particularly effective with ground faults because of their steep characterist ics.

Suitable if there is a substantial reduction of fault current as the fault distance from the

power source increases.

Very inverse overcurrent relays are particularly suitable if the short-circuit current drops

rapidly with the distance from the substation.

The grading margin may be reduced to a value in the range from 0.3 to 0.4 seconds when

overcurrent relays with very inverse characteristics are used.

Used when Fault Current is dependent on fault location.

Used when Fault Current independent of normal changes in generating capacity.


3.3. Extremely Inverse Time Overcurrent Relay

It has more inverse characteristics than that of IDMT and very inverse overcurrent relay.

Suitable for the protection of machines against overheating.

The operating time of a time overcurrent relay with an extremely inverse time-current

characteristic is approximately inversely proportional to the square of the current

The use of extremely inverse overcurrent relays makes it possible to use a short time delay in

spite of high switching-in currents.

Used when Fault current is dependent on fault location

Used when Fault current independent of normal changes in generating capacity.

Application:

Suitable for protection of distribution feeders with peak currents on switching in

(refrigerators, pumps, water heaters and so on).

Particular suitable for grading and coordinates with fuses and re closes



For the protection of alternators, transformers. Expensive cables, etc.


3.4. Long Time Inverse Overcurrent Relay

The main application of long time overcurrent relays is as backup earth fault protection.

4. Directional Overcurrent Relays

When the power system is not radial (source on one side of the line), an overcurrent relay may not

be able to provide adequate protection. This type of relay operates in on direction of current flow

and blocks in the opposite direction.

Three condit ions must be satisfied for its operation: current magnitude, time delay and

directionality. The directionality of current flow can be identified using voltage as a reference of

direction.


Application of Overcurrent Relay

Motor Protection:

Used against overloads and short-circuits in stator windings of motor.

Inverse time and instantaneous overcurrent phase and ground

Overcurrent relays used for motors above 1000 kW.

Transformer Protection:

Used only when the cost of overcurrent relays are not justified.

Extensively also at power-transformer locations for external-fault back-up protection.

Line Protection:

On some sub transmission lines where the cost of distance relaying cannot be justified.

primary ground-fault protection on most transmission lines where distance relays are used for

phase faults.

For ground back-up protection on most lines having pilot relaying for primary protection.

Distribution Protection:



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Overcurrent relaying is very well suited to distribution system protection for the fol lowing reasons:

It is basically simple and inexpensive.

Very often the relays do not need to be directional and hence no PT supply is required.

It is possible to use a set of two O/C relays for protection against inter-phase faults and a

separate Overcurrent relay for ground faults.


jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat

University. He is member of Institution of Engineers (MIE),India. Membership

No:M-1473586.He has more than 12 years experience in Transmission -

Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical

Projects (Planning-Designing-Technical Review-coordination -Execution). He is

Presently associate with one of the leading business group as a Assistant Manager

at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror",

"Electrica l India", "Lighting India", "Industria l Electrix"(Australian Power Publications) Magazines.

He is Freelancer Programmer of Advance Excel and design useful Excel base Electrica l Programs

as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi,

Gujarati, French languages. He wants to Share his experience & Knowledge and help technical

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Types and Applications of Overcurrent Relay (Part 1) _ EEP

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