GE P50 Agile P253, Technical Manual

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P50 Agile P253
6 Protection Functions
P253/EN M/C 6-21
2.8.1 Sensitive Earth Fault Protection (ordering option)
Sensitive Earth Fault protection is an ordering option and is implemented in the EARTH FAULT 1
column of the relevant settings group.
The product provides three stages of SEF protection with independent time delay characteristics.
Stages 1, 2 and 3 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
•
IN1>(n) Function for the overcurrent operate characteristic
•
IN1>(n) Reset Char for the overcurrent reset characteristic
where (n) is the number of the stage.
Stages 1, 2 and 3 also provide a Timer Hold facility. This is configured using the cells
IN1>(n) tRESET .
2.9 Restricted Earth Fault Protection
Winding-to-core faults are quite common due to insulation breakdown. Such faults can have very low
fault currents, but they are faults nevertheless and have to be picked up, as they can still severely
damage expensive equipment. Often the fault currents are even lower than the nominal load current.
Clearly, neither overcurrent nor percentage differential protection is sufficiently sensitive in this case.
We therefore require a different type of protection arrangement. Not only should the protection
arrangement be sensitive, but it must create a protection zone, which is limited to the
transformer/Motor/Generator windings. Restricted Earth Fault (REF) protection satisfies these
conditions. The high-impedance measurement principle is a sensitive method for detecting earth
faults. It can be applied to transformers, motors, generators and reactors when these are operated on
an earthed network.
Application of REF protection is explained by means of an example.
The figure below shows an REF protection arrangement for the star connected winding. The current
transformers measuring the currents in each phase are connected in parallel. In this arrangement the
phase currents and the zero sequence current in the neutral line is measured. We know that an
external unbalanced fault causes zero sequence current to flow through the neutral line, resulting in
uneven currents in the phases, which would cause the IED to maloperate. By measuring this zero
sequence current and placing the current transformer in parallel with the other three the currents are
balanced up resulting in stable operation.
Now only a fault inside the star connected winding can create an imbalance sufficient to cause the IED
to issue a trip command.