Chapter 6 - Current Protection Functions
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 resistance (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.
SEF PROTECTION IMPLEMENTATION
The product provides four stages of SEF 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 IDMT (Inverse Definite Minimum Time) curves
A range of User-defined curves
DT (Definite Time)
This is achieved using the cells
ISEF>(n) Function for the overcurrent operate characteristic
ISEF>(n) Reset Char for the overcurrent reset characteristic
ISEF>(n) Usr RstChar for the reset characteristic for user -defined curves
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.
NON-DIRECTIONAL SEF LOGIC
I2 H Any Start
2H Blocks ISEF>1
ISEF>1 Timer Blk
AR Blk Main Prot
AR Blks ISEF>3
AR blocking available for DT-only stages
Figure 58: Non-directional SEF logic
The SEF current is compared with a set threshold (ISEF>(n) Current) for each stage. If it exceeds this threshold, a
Start signal is triggered, providing it is not blocked. This can be blocked by the second harmonic blocking function,
or an Inhibit SEF DDB signal.
Note: This diagram does not show all stages . Other stages follow similar
AR blocking is only available for stages 3 and 4