Overcurrent Protection Relays

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EE 3043

Power Systems III

Power System ProtectionRelay Coordination

by

Dr. Lidula N. Widangama Arachchige

Department of Electrical EngineeringUniversity of Moratuwa, Sri Lanka

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Outline

Relay coordination

Types of Over-current relays

Discrimination by time, current, time and current

Inverse characteristics

Discriminative grading

Earth fault detection

Theory and operation of directional over current

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Over-current Protection

Over-current Relays: Most common form of protection

Use to deal with excessive currents due to faults in the

power system

Use the current level to detect faults

Primarily intended to operate under faults

Not designed to protect systems against overloads

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Types of Over-current relays

Instantaneous

Definite Time

Inverse Time

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Instantaneous Over-current Relay

Named as Definite current relay as well

Operate instantaneously when the current reaches a

predetermined value

Characteristic curve

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t

iiPickup

Coordination of Instantaneous Relays

Based on the fact:

Fault current varies with the position of the fault

Calculate the fault currents at Bus-bars A, B, C and D

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Vs/3

j 0.485 j 0.24 j 0.04 j 2.12

CBA D


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Coordination of Instantaneous Relays

Relay furthest away from the source will operate for

a low current value

Relay operating currents are progressively increased

towards the source

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t

i

D CB A

Drawbacks of having only Instantaneous Protection

Little selectivity at high values of fault currents

Difficult to distinguish between the fault currents at

two points when the impedance between the two

points is small compared to the impedance back to

the source

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Definite-Time Over-current Relay

Allows adjustment of operating time according to the

current level

Relay can be adjusted to trip the breaker nearest to

the fault in the shortest time

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i

t

iPickup

tSet

Different Settings of Definite-Time Over-current Relays

Current Setting (Pickup Setting, Plug Setting, Tap

Setting)

To select the value of current at which the relay will start

responding

Time-dial Setting

To obtain the exact timing of the relay operation

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Coordination of Definite-Time Over-current Relays

The time interval between each relay should be long

enough to ensure the upstream relays do not

operate before the fault is cleared by the nearest

circuit breaker to the fault

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t

i

D CB A

Discrimination

margin

Drawbacks of Definite-Time Over-current Relays

Longest fault clearance time occurs for the section

closest to the source, where the fault current is the

highest

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Inverse-Time Relay

Relay operating time is inversely proportional to the

fault current

Also referred to as Inverse Definite Minimum Time

(IDMT) relays

For very high currents, much shorter tripping times

can be achieved without the risk of protectionselectivity

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Categorization of IDMT Relays

Inverse

Very Inverse

Extremely Inverse

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Coordination of IDMT Relays

For a fault beyond A, Relay at A should operate

before the Relay at B operates

Relay characteristic curves can be adjusted by

selecting two parameters

Pick-up current or plug setting Time dial or time multiplier setting

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B A

Pickup Setting

Referred to as plug setting or tap setting as well

Defines the pick-up current of the relay

Plug Multiplier Setting (PMS)

Fault current is expressed as multiples of the pickup setting

in the characteristic curves

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currentup-Pick

ansformercurrent trtheofsecondaryin thecurrentFaultPMS


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Time Dial Setting

Adjust the time delay before the relay operates

whenever the current reaches a value equal or

greater than the relay current setting

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Costruction of an Induction Disc Relay

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Inverse Current Relay Characteritics

For a given relay, a set ofcharacteristic curves canbe generated for differenttime multiplier settings(TMS)

Current axis is given asmultiples of the pick-upcurrent

In most relays, continuous

adjustment of TMS ispossible

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Relay Characteristics for Proper Coordination

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B A


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Inverse-Current Relay: Standard Characteristics

IEC Standard

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Inverse-Current Relay: Standard Characteristics

ANSI/IEEE Standards

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Connection of Phase and Earth Fault Over-current Relays

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Setting Over-current Relays

Over-current relays are normally supplied with both

Instantaneous element

Time delay element

Setting of the relay involves

Selecting parameters that defines the required time/currentcharacteristics

The process has to be carried out twice for:

Phase relaysusing 3 phase short circuit fault currents

Earth fault relaysusing phase-to-earth fault currents

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Coordination of Over-Current Relays

Selection of the correct characteristics

Choice of relay current settings

Choice of grading margins

Selection of time settings

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Setting Instantaneous Over-Current Units

Instantaneous units are useful for:

Reducing the operating time of the relays for severe

system faults

Avoiding loss of selectivity in a protection system with

relays having different characteristics

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Criteria for setting Instantaneous Units

Criteria differ depending on the

location

Type of system element being protected

Criteria is defined separately for

Lines between substations

Transformers

Distribution lines

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Setting Instantaneous Over-current Units

Lines between substations Iset = 125% of the maximum symmetrical fault current at the next

downstream substation

Setting is started from the substation furthest from the sourceand proceeded backward towards the source

The 25% margin avoids overlapping with the downstreaminstantaneous unit when a dc off set is present

In HV systems operating at 220kV or above, higher margin

should be usedHigh X/R ratio result in higher dc offsets

When the coordination of time delay units is lost due to crossingof characteristics, the instantaneous unit of the downstreamsubstation is set at a lower value to avoid the loss ofcoordination

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Distribution lines supplying MV/LV transformers

One of the following criteria can be used

50% of the maximum short circuit current at the point of CT

supplying the relay

Between 6 to 10 times the maximum circuit rating

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Instantaneous over-current relay installed at the primary(HV) side of a transformer

125-150% of the short circuit current at the bus bar on the LVside, referred to the HV sideHigher margin is required to avoid lack of coordination with the high

inrush currents

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Setting Time-delay Over-current Relays

Definitetime relays and inverse time relays can be

adjusted by selecting the pick-up setting and time

dial or time multiplier setting

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Pick-up Setting of Phase Over-current Relays

Where,

OLF = overload factor

CTR = CT ratio

Recommended OLF values

Motors = 1.05

Lines, Transformers, Generators = 1.251.5

Distribution feeders = up to 2

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Pick-up Setting of Earth Over-current Relays

Where,

UBF = unbalanced factor

Recommended UBF values

Typical = 0.2

HV lines = 0.1 Distribution feeders = 0.3

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Procedure for calculating the time dial setting

Step-1: Determine the required operating time t1of

the relay furthest from the source

Use the lowest time dial setting

Consider the pick-up current of the instantaneous relay

Raise the time dial setting if the following conditions apply

Necessary to coordinate with devices (eg. Fuses) installed

downstream

Load current during re-energizing (cold load pick-up) is high

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Step-2: Determine the operating time of relay at thenext substation towards the source

t2a=t1+tmargin Use the fault current used to determine the timing t1of

the previous relay for this calculation

Step-3: With the same fault current as in theprevious step, knowing t2aand the pick-up currentfor relay 2, calculate the time dial setting for relay 2

Use the closest available time dial setting whosecharacteristics is above the calculated value

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Step-4: Determine the operating time t2bof relay 2

but now using the fault current just before the

operation of instantaneous unit

Step-5: Continue the procedure with the sequence

starting from the second step

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Time Discrimination Margin

Typical values: 0.25 s

0.4 s

This value avoids loss of selectivity due to one or more ofthe following: Breaker opening time

Relay overrun time after the fault has been cleared

In numerical relays there is no overrun and margin could beselected as low as 0.2s

Variations in fault levels, deviations from the characteristicscurves and errors in the current transformers

Single phase faults on star side of a Dy transformer arenot seen on the Delta side. When setting earth fault relays, the lowest available time dial

setting can be applied to the relays on the Delta side

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Overcurrent Protection Relays

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