ABB Relion 670 Series Applications Manual page 232

Section 8
Voltage protection
8.3.4 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.
8.3.4.1 Equipment protection, such as for motors, generators, reactors and
transformers Equipment 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.
8.3.4.2 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.
8.3.4.3 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.
8.3.4.4 High impedance grounded systems
In high impedance grounded systems, ground faults cause a neutral voltage in the feeding transformer
neutral. Two step residual overvoltage protection ROV2PTOV (59N) is used to trip the transformer, as a
backup protection for the feeder ground fault protection, and as a backup for the transformer primary
ground fault protection. The setting must be above the highest occurring "normal" residual voltage, and
below the lowest occurring residual voltage during the faults under consideration. A metallic single-phase
ground fault causes a transformer neutral to reach a voltage equal to the nominal phase-to-ground
voltage.
The voltage transformers measuring the phase-to-ground voltages measure zero voltage in the faulty
phase. The two healthy phases will measure full phase-to-phase voltage, as the faulty phase will be
connected to ground. The residual overvoltage will be three times the phase-to-ground voltage. See
figure 117.
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