CHAPTER 4: SETPOINTS PROTECTION
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 4–195
Negative Sequence
Instantaneous
Overcurrent
Protection (50_2)
The 845 relay is equipped with the Negative Sequence Instantaneous Overcurrent
protection element. The Negative Sequence Instantaneous Overcurrent element may be
used to determine and clear unbalance in the system. The input for computing negative
sequence current is the fundamental phasor value. The 845
computes the negative
sequence current magnitude |I_2| using the following formula:
|I_2|=1/3*|Ia+Ib*(1∠240º)+Ic*(1∠120 º)|
The element responds to the negative-sequence current and applies a positive sequence
restraint for better performance: a small portion (12.5%) of the positive sequence current
magnitude is subtracted from the negative sequence current magnitude when forming the
operating quantity:
Iop = |I_2| - K * |I_1|
where K = 1/8 and |I_1| = 1/3*|Ia+Ib*(1∠120º)+Ic*(1∠240 º)|
The positive sequence restraint allows for more sensitive settings by counterbalancing
spurious negative-sequence currents resulting from:
• system unbalances under heavy load conditions
• current transformer (CT) transformation errors during three-phase faults
• fault inception and switch-off transients during three-phase faults.
The positive sequence restraint must be considered when testing for Pickup accuracy and
response time (multiple of Pickup). The operating quantity depends on the way the test
currents are injected into the relay (single-phase injection: Iop = 0.2917 * I_injected; three-
phase injection, opposite rotation: Iop = I_injected).
The settings of this function are applied to the calculated negative sequence current to
produce Pickup and Trip flags. The Negative Sequence IOC Pickup flag is asserted, when
the negative sequence current is above the PKP value. The Negative Sequence IOC
Operate flag is asserted if the element stays picked up for the time defined by the
Negative Sequence IOC PKP Delay setting. If the Pickup time delay is set to 0.000 seconds,
the Pickup and Operate flags are asserted at the same time. The element drops from
Pickup without operation if the negative sequence current drops below 97 to 98% of the
Pickup value.
Path: Setpoints > Protection > Group 1(6) > Current > Negative Sequence IOC 1(X)
FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
PICKUP
Range: 0.050 to 30.000 x CT in steps of 0.001 x CT
Default: 1.000 x CT
PICKUP DELAY
Range: 0.000 to 6000.000 s in steps of 0.001 s
Default: 0.000 s
DROPOUT DELAY
Range: 0.000 to 6000.000 s in steps of 0.001 s
Default: 0.000 s
BLOCK
Range: Off, Any FlexLogic operand
Default: Off
OUTPUT RELAY X
For details see Common Setpoints
.
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