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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
1
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
5
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
6
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
7
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
9
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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Setting Instantaneous Over-current Units
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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Setting Instantaneous Over-current Units
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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Procedure for calculating the time dial setting
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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Procedure for calculating the time dial setting
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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