Types, applicat ions and connect ions of Overcurrent relay (on photo: T ransmission linesf rom Gillam to Churchill)

http://electrical-engineering-portal.com/types-and-applications-of-overcurrent- relay-1 Februray 5, 2013

Types and Applications Of Overcurrent Relay (part 1)

IndexTypes of protection:

1. Unit Type Protection

2. Non-unit schemes

1. Overcurrent protection

2. Earth f ault protection

Various types of Line Faults

Overcurrent Relay Purpose and Ratings

Primary requirement of Overcurrent protection

Purpose of overcurrent Protection

Overcurrent Relay Ratings

Dif f erence between Overcurrent and Overload protection

Types of Overcurrent Relay:

1. Instantaneous Overcurrent relay (Def ine Current)

2. Def inite 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 protectionProtection schemes can be divided into two major groupings:

1. Unit schemes

2. Non-unit schemes

1. Unit Type Protection

Unit type schemes protect a specif ic area of the system, i.e., a transformer,transmission line, generator or bus bar.

The unit protection schemes is based on Kirchhof f ’s Current Law – the sum of thecurrents entering an area of the system must be zero.

Any deviation f rom this must indicate an abnormal current path. In these schemes, theef f ects of any disturbance or operating condition outside the area of interest aretotally ignored and the protection must be designed to be stable above the maximumpossible f ault current that could f low through the protected area.

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2. Non unit type protection

The non-unit schemes, while also intended to protect specif ic areas, have no fixedboundaries. As well as protecting their own designated areas, the protective zones

can overlap into other areas. While this can be very benef icial f or backup purposes,there can be a tendency f or too great an area to be isolated if a f ault is detected bydif f erent non unit schemes.

The most simple of these schemes measures current and incorporates an inverse timecharacteristic into the protection operation to allow protection nearer to the f ault tooperate f irst.

The non unit type protection system includes following schemes:

1. Time graded overcurrent protection

2. Current graded overcurrent protection

3. Distance or Impedance Protection

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2.1 Overcurrent protection

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

It owes its application f rom the f act that in the event of f ault the current wouldincrease to a value several times greater than maximum load current. It has alimitation that it can be applied only to simple and non costly equipments.

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2.2 Earth fault protection

The general practice is to employ a set of two or three overcurrent relays and aseparate overcurrent relay f or single line to ground f ault. Separate earth f ault relayprovided makes earth fault protection faster and more sensitive.

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

Therefore, relay connected f or earth f ault protection is dif f erent f rom those f or phaseto phase f ault protection.

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Various types of Line Faults

No Type of Fault Operation of Relay

1 Phase to Ground f ault (Earth Earth Fault Relay

Fault)

2 Phase to Phase f ault Not withGround

Related Phase Overcurrentrelays

3 Double phase to Ground f ault Related Phase Overcurrentrelays and Earth Fault relays

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Overcurrent Relay Purpose and RatingsA relay that operates or picks up when it’s current exceeds a predetermined value(setting value) is called Overcurrent Relay.

Overcurrent protection protects electrical power systems against excessive currentswhich are caused by short circuits, ground f aults, etc. Overcurrent relays can be usedto protect practically any power system elements, i.e. transmission lines,transformers, generators, or motors.

For f eeder protection, there would be more than one overcurrent relay to protectdif f erent sections of the f eeder. These overcurrent relays need to coordinate witheach other such that the relay nearest f ault operates f irst.

Use time, current and a combination of both time and current are three ways todiscriminate adjacent overcurrent relays.

OverCurrent Relay gives protection against:

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

1. Phase f aults

2. Earth f aults

3. Winding f aults

Short-circuit currents are generally several times (5 to 20) full load current. Hence f astf ault clearance is always desirable on short circuits.

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Primary requirement of Overcurrent protection

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

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Purpose of overcurrent Protection

These are the most important purposes of overcurrent relay:

Detect abnormal conditions

Isolate f aulty part of the system

Speed Fast operation to minimize damage and danger

Discrimination Isolate only the f aulty section

Dependability / reliability

Security / stability

Cost of protection / against cost of potential hazards

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Overcurrent Relay Ratings

In order f or an overcurrent protective device to operate properly, overcurrentprotective 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 f or equipment andpersonnel 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 atleast equal to or greater than the circuit voltage. The overcurrent protective devicerating can be higher than the system voltage but never lower.

Ampere Rating: The ampere rating of a overcurrent protecting device normally shouldnot exceed the current carrying capacity of the conductors As a general rule, theampere rating of a overcurrent protecting device is selected at 125% of thecontinuous load current.

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Difference between Overcurrent and Overload protection

Overcurrent protection protects against excessive currents or currents beyond theacceptable current ratings, which are resulting f rom short circuits, ground f aults andoverload conditions.

While, the overload protection protects against the situation where overload currentcauses overheating of the protected equipment.

The overcurrent protection is a bigger concept So that the overload protection can beconsidered as a subset of overcurrent protection.

The overcurrent relay can be used as overload (thermal) protection when protects theresistive loads, etc., however, f or motor loads, the overcurrent relay cannot serve asoverload protection Overload relays usually have a longer time setting than theovercurrent relays.

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Types of Overcurrent RelayThese are the types of overcurrent relay:

1. Instantaneous Overcurrent (Define Current) Relay

2. Def ine Time Overcurrent Relay

3. Inverse Time Overcurrent Relay (IDMT Relay)

Moderately Inverse

Very Inverse Time

Extremely Inverse

Directional overcurrent Relay

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1. Instantaneous Overcurrent relay (Define Current)

Def inite current relay operate instantaneously when the current reaches apredetermined value.

Instantaneous Overcurrent Relay -Def inite Current

Def inite t ime of overcurrent relay

Operates in a def inite time when currentexceeds its Pick-up value.

Its operation criterion is only currentmagnitude (without time delay).

Operating time is constant.

There is no intentional time delay.

Coordination of def inite-current relays isbased on the f act that the f ault currentvaries with the position of the f aultbecause of the dif f erence in theimpedance between the f ault and thesource

The relay located f urthest f rom the source operate f or a low current value

The operating currents are progressively increased f or the other relays whenmoving towards the source.

It operates in 0.1s or less

Application: This type is applied to the outgoing f eeders.

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2. Definite Time Overcurrent Relays

In this type, two conditions must be satisf ied f or operation (tripping), current mustexceed the setting value and the f ault must be continuous at least a time equal to timesetting of the relay.

Modern relays may contain more than one stage ofprotection each stage includes each own currentand time setting.

1. For Operation of Def inite Time OvercurrentRelay operating time is constant

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

3. It has pick-up and time dial settings, desiredtime delay can be set with the help of an intentional time delay mechanism.

4. Easy to coordinate.

5. Constant tripping time independent of in f eed variation and f ault location.

Drawback of Relay:

1. The continuity in the supply cannot be maintained at the load end in the event off ault.

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

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

4. It is not suitable f or long distance transmission lines where rapid f ault clearanceis necessary f or stability.

5. Relay have dif f iculties in distinguishing between Fault currents at one point oranother when f ault impedances between these points are small, thus poordiscrimination.

Application:

Definite time overcurrent relay is used as:

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

2. Back up protection to dif f erential relay of power transformer with time delay.

3. Main protection to outgoing f eeders and bus couplers with adjustable time delaysetting.

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3. Inverse Time Overcurrent Relays (IDMT Relay)

In this type of relays, operating time is inversely changed with current. So, high currentwill operate overcurrent relay f aster than lower ones. There are standard inverse, veryinverse and extremely inverse types.

Discrimination by both ‘Time’ and ‘Current’. The relay operation time is inverselyproportional to the fault current.

Inverse Time relays are also referred to as Inverse Def inite Minimum Time (IDMT)relay.

Inverse Def inite Minimum T ime (IDMT)

Inverse types

The operating time of an overcurrent relaycan be moved up (made slower) byadjusting the ‘time dial setting’. The lowesttime dial setting (fastest operating time) isgenerally 0.5 and the slowest is 10.

Operates when current exceeds itspick-up value.

Operating time depends on themagnitude of current.

It gives inverse time currentcharacteristics at lower values of f ault current and def inite time characteristics athigher values

An inverse characteristic is obtained if the value of plug setting multiplier is below10, f or values between 10 and 20 characteristics tend towards def inite timecharacteristics.

Widely used f or the protection of distribution lines.

Based on the inverseness it has three different types:

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3.1. Normal Inverse Time Overcurrent Relay

The accuracy of the operating time may range f rom 5to 7.5% of the nominal operating time as specif ied inthe relevant norms. The uncertainty of the operatingtime and the necessary operating time may require agrading margin of 0.4 to 0.5 seconds.

It’s used when Fault Current is dependent ongeneration of fault not f ault location.

Normal inverse time Overcurrent Relay is relativelysmall change in time per unit of change of current.

Application:

Most f requently used in utility and industrial circuits.especially applicable where the f ault magnitude ismainly dependent on the system generating capacityat the time of f ault.

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3.2. Very Inverse Time Overcurrent Relay

Gives more inverse characteristics than that of IDMT.

Used where there is a reduction in f ault current, as the distance f rom sourceincreases.

Particularly ef f ective with ground f aults because of their steep characteristics.

Suitable if there is a substantial reduction of f ault current as the f ault distancef rom the power source increases.

Very inverse overcurrent relays are particularly suitable if the short-circuit currentdrops rapidly with the distance f rom the substation.

The grading margin may be reduced to a value in the range f rom 0.3 to 0.4seconds when overcurrent relays with very inverse characteristics are used.

Used when Fault Current is dependent on f ault location.

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

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3.3. Extremely Inverse Time Overcurrent Relay

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

Suitable f or 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 thecurrent

The use of extremely inverse overcurrent relays makes it possible to use a shorttime delay in spite of high switching-in currents.

Used when Fault current is dependent on f ault location

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

Application:

Suitable f or protection of distribution f eeders with peak currents on switching in(ref rigerators, pumps, water heaters and so on).

Particular suitable f or grading and coordinates with f uses and re closes

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

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3.4. Long Time Inverse Overcurrent Relay

The main application of long time overcurrent relays is as backup earth f aultprotection.

4. Directional Overcurrent Relays

When the power system is not radial (source on one side of the line), an overcurrentrelay may not be able to provide adequate protection. This type of relay operates inon direction of current f low and blocks in the opposite direction.

Three conditions must be satisf ied f or its operation: current magnitude, time delay anddirectionality. The directionality of current f low can be identif ied using voltage as areference of direction.

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Application of Overcurrent RelayMotor Protection:

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

Inverse time and instantaneous overcurrent phase and ground

Overcurrent relays used f or motors above 1000 kW.

Transformer Protection:

Used only when the cost of overcurrent relays are not justif ied.

Extensively also at power-transformer locations f or external-f ault back-upprotection.

Line Protection:

On some sub transmission lines where the cost of distance relaying cannot bejustif ied.

primary ground-fault protection on most transmission lines where distance relaysare used f or phase f aults.

For ground back-up protection on most lines having pilot relaying f or primaryprotection.

Distribution Protection:

Overcurrent relaying is very well suited to distribution system protection f or thefollowing reasons:

It is basically simple and inexpensive.

Very of ten the relays do not need to be directional and hence no PT supply isrequired.

It is possible to use a set of two O/C relays f or protection against inter-phasefaults and a separate Overcurrent relay f or ground f aults.

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