ABB Relion 670 Series Applications Manual page 228

Section 9
Voltage protection
measured either at the transformer neutral or from a voltage transformer open delta connection.
The residual voltage can also be calculated internally, based on the measurement of the three
phase-to-ground voltages.
In high impedance grounded systems the residual voltage will increase in case of any fault
connected to ground. Depending on the type of fault and fault resistance the residual voltage will
reach different values. The highest residual voltage, equal to three times the phase-to-ground
voltage, is achieved for a single phase-to-ground fault. The residual voltage increases
approximately to the same level in the whole system and does not provide any guidance in finding
the faulted component. Therefore, ROV2PTOV (59N) is often used as a backup protection or as a
release signal for the feeder ground fault protection.
9.3.3 Setting guidelines
All the voltage conditions in the system where ROV2PTOV (59N) performs its functions should be
considered. The same also applies to the associated equipment, its voltage withstand capability
and time characteristic.
All voltage-related settings are made as a percentage of a settable base voltage, which shall be set
to the primary nominal voltage (phase-phase) level of the power system or the high-voltage
equipment under consideration.
The time delay for ROV2PTOV (59N) is seldom critical, since residual voltage is related to ground
faults in a high-impedance grounded system, and enough time must normally be given for the
primary protection to clear the fault. In some more specific situations, where the residual
overvoltage protection is used to protect some specific equipment, the time delay is shorter.
Some applications and related setting guidelines for the residual voltage level are given below.
9.3.3.1 Equipment protection, such as for motors, generators, reactors and
transformersEquipment protection for transformers
High residual voltage indicates ground fault in the system, perhaps in the component to which
two step residual overvoltage protection (ROV2PTOV, 59N) is connected. For selectivity reasons to
the primary protection for the faulted device, ROV2PTOV (59N) must trip the component with
some time delay. The setting must be above the highest occurring "normal" residual voltage and
below the highest acceptable residual voltage for the equipment.
9.3.3.2 Equipment protection, capacitors
High voltage will deteriorate the dielectric and the insulation. Two step residual overvoltage
protection (ROV2PTOV, 59N) has to be connected to a neutral or open delta winding. The setting
must be above the highest occurring "normal" residual voltage and below the highest acceptable
residual voltage for the capacitor.
9.3.3.3 Power supply quality
The setting must be above the highest occurring "normal" residual voltage and below the highest
acceptable residual voltage, due to regulation, good practice or other agreements.
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