Negative Sequence Overcurrent Protection - GE MiCOM P40 Agile Technical Manual

P543i/P545i
3

NEGATIVE SEQUENCE OVERCURRENT PROTECTION

When applying standard phase overcurrent protection, the overcurrent elements must be set significantly higher
than the maximum load current. This limits the element's sensitivity. Most protection schemes also use an earth
fault element operating from residual current, which improves sensitivity for earth faults. However, certain faults
may arise which can remain undetected by such schemes. Negative Phase Sequence Overcurrent elements can
help in such cases.
Any unbalanced fault condition will produce a negative sequence current component. Therefore, a negative phase
sequence overcurrent element can be used for both phase-to-phase and phase-to-earth faults. Negative Phase
Sequence Overcurrent protection offers the following advantages:
Negative phase sequence overcurrent elements are more sensitive to resistive phase-to-phase faults,
●
where phase overcurrent elements may not operate.
In certain applications, residual current may not be detected by an earth fault element due to the system
●
configuration. For example, an earth fault element applied on the delta side of a delta-star transformer is
unable to detect earth faults on the star side. However, negative sequence current will be present on both
sides of the transformer for any fault condition, irrespective of the transformer configuration. Therefore, a
negative phase sequence overcurrent element may be used to provide time-delayed back-up protection for
any uncleared asymmetrical faults downstream.
3.1 NEGATIVE SEQUENCE OVERCURRENT PROTECTION IMPLEMENTATION
Negative Sequence Overcurrent Protection is implemented in the NEG SEQ O/C column of the relevant settings
group.
The product provides four stages of negative sequence overcurrent protection with independent time delay
characteristics.
Stages 1, 2 provide a choice of operate and reset characteristics, where you can select between:
A range of standard IDMT (Inverse Definite Minimum Time) curves
●
● DT (Definite Time)
This is achieved using the cells
I2>(n) Function for the overcurrent operate characteristic
●
I2>(n) Reset Char for the overcurrent reset characteristic
●
where (n) is the number of the stage.
The IDMT-capable stages, (1 and 2) also provide a Timer Hold. This is configured using the cells I2>(n) tReset, where
(n) is the number of the stage. This is not applicable for curves based on the IEEE standard.
Stages 3 and 4 have definite time characteristics only.
3.2 DIRECTIONAL ELEMENT
Where negative phase sequence current may flow in either direction, directional control should be used.
Directionality is achieved by comparing the angle between the negative phase sequence voltage and the negative
phase sequence current. A directional element is available for all of the negative sequence overcurrent stages.
This is found in the I2> Direction cell for the relevant stage. It can be set to non-directional, directional forward, or
directional reverse.
A suitable characteristic angle setting (I2> Char Angle) is chosen to provide optimum performance. This setting
should be set equal to the phase angle of the negative sequence current with respect to the inverted negative
sequence voltage (–V2), in order to be at the centre of the directional characteristic.
P54x1i-TM-EN-1
Chapter 13 - Current Protection Functions
369