Voltage start value
Maximum
healthy-state
Uo
Margin
Resonance Curve
of
few %
Uo [%]
lcoil: A
20
68 A
40 60 80 100 120
Figure 320: Example of selection of setting Voltage start value to maximize the
earth-fault detection sensitivity based on the resonance curve calculated by the coil
controller
There are three MFADPSDE instances (stages) available. The sensitivity
of different stages should be selected differently. For example, stage
1 could have the highest
Voltage start value
and the shortest
Operate
delay time
. Thus it would fulfill the requirements for protection operation
speed by providing fast operation for low-ohmic earth faults. Stage 2
should have lower
Voltage start value
and longer
Operate delay time
compared with stage 1. This stage would fulfill the requirements for
protection sensitivity (in terms of fault resistance value). Stage 3 could
be used to detect high-ohmic earth faults and generate alarms for them.
To maximize the earth-fault detection sensitivity of stage 3,
Voltage start
value
should be selected based on the network's resonance curve (see
Figure 320
).
Such coordination of different protection stages enables:
• Indication (but no tripping) of transient, self-extinguishing earth faults, where
the zero-sequence voltage may temporarily rise to a high value and then decay
slowly away. This can be achieved by monitoring the START outputs. Delayed
operation times of stages 1 and 2 avoid unnecessary breaker operations during
self-extinguishing earth faults.
• Fast tripping of low-ohmic permanent earth faults regardless of their type
(continuous or intermittent), stage 1
• Delayed tripping of higher-ohmic permanent earth faults regardless of their type
(continuous or intermittent), stage 2
1MRS759142 F
Protection functions
REX640
Technical Manual
557
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