ABB RELION REX640 Technical Manual page 557

1MRS759142 F
Voltage start value
Maximum
healthy-state
Uo
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
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
REX640
Technical Manual
Resonance Curve
Uo [%]
20
68 A
There are three MFADPSDE instances (stages) available. The sensitivity
of different stages should be selected differently. For example, stage
1 could have the highest
delay time . Thus it would fulfill the requirements for protection operation
speed by providing fast operation for low-ohmic earth faults. Stage 2
Voltage start value and longer Operate delay time
should have lower
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,
value should be selected based on the network's resonance curve (see
Figure 320
).
40 60 80
Voltage start value and the shortest Operate
Protection functions
Margin of
few %
lcoil: A
100 120
Voltage start
557