9 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.
9.1 NEGATIVE SEQUENCE OVERCURRENT PROTECTION IMPLEMENTATION
Negative Sequence Overcurrent Protection is implemented in the NEG SEQ O/C column of the r
elevant 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
● A range of User-defined curves
● DT (Definite Time)
For stage 1, this is achieved using the following settings:
● I2>1 Function for the overcurrent operate characteristic
● I2>1 Reset Char for the overcurrent reset characteristic
● I2>1 Usr RstChar for the reset characteristic for user-defined curves
The setting names for other stages follow the same principles.
The IDMT-equipped stages, (1 and 2) also provide a Timer Hold facility. Stage 1, for example, is configured using the
cells I2>1 tRESET. This setting is not applicable for curves based on the IEEE standard.
Stages 3 and 4 have definite time characteristics only.
Chapter 6 - Current Protection Functions P14x
116 P14xEd1-TM-EN-1
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