5 SENSITIVE EARTH FAULT PROTECTION
With some earth faults, the fault current flowing to earth is limited by either intentional resistance (as is the case
with some HV systems) or unintentional r
esistance (e.g. in very dry conditions and where the substrate is high
resistance, such as sand or rock).
To provide protection in such cases, it is necessary to provide an earth fault protection system with a setting that is
considerably lower than for normal line protection. Such sensitivity cannot be provided with conventional CTs,
therefore the SEF input would normally be fed from a core balance current transformer (CBCT) mounted around
the three phases of the feeder cable. The SEF transformer should be a special measurement class transformer.
5.1 SEF PROTECTION IMPLEMENTATION
The product provides four stages of SEF protection with independent time delay characteristics.
S
tages 1, 2 provide a choice of operate and reset characteristics, where you can select between:
● A range of IDMT (Inverse Definite Minimum Time) curves
● DT (Definite Time)
This is achieved using the cells
● ISEF>(n) Function for the overcurrent operate characteristic
● ISEF>(n) Reset Chr for the overcurrent reset characteristic
where (n) is the number of the stage.
Stages 1 and 2 also provide a Timer Hold facility. This is configured using the cells ISEF>(n) tReset.
Stages 3 and 4 have definite time characteristics only.
Each stage can be individually inhibited with a DDB signal Inhibit ISEF>(n), where n is the stage number.
5.2 EPATR B CURVE
The EPATR B curve is commonly used for time-delayed Sensitive Earth Fault protection in certain markets. This
cur
ve is only available in the Sensitive Earth Fault protection stages 1 and 2. It is based on primary current settings,
employing a SEF CT ratio of 100:1 A.
The EPATR_B curve has 3 separate segments defined in terms of the primary current. It is defined as follows:
Segment Primary Current Range Based on 100A:1A CT Ratio Current/Time Characteristic
1 ISEF = 0.5A to 6.0A t = 432 x TMS/ISEF 0.655 secs
2 ISEF = 6.0A to 200A t = 800 x TMS/ISEF secs
3 ISEF above 200A t = 4 x TMS secs
where TMS (time multiplier setting) is 0.025 - 1.2 in steps of 0.025.
P54A/B/C/E Chapter 9 - Current Protection Functions
P54xMED-TM-EN-1 207
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