ABB REB670 - Application; Setting Guidelines; Equipment Protection, such as for Motors, Generators, Reactors and Transformersequipment Protection for Transformers

8.3.2 Application
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Two step residual overvoltage protection ROV2PT
OV (59N) is primarily used in high
impedance grounded distribution networks, mainly as a backup for the primary ground
fault protection of the feeders and the transformer. To increase the security for different
ground fault related functions, the residual overvoltage signal can be used as a release
signal. The residual voltage can be 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.
8.3.3 Setting guidelines
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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.
8.3.3.1 Equipment protection, such as for motors, generators, reactors and
transformersEquipment protection for transformers
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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
1MRK 505 370-UUS A Section 8
Voltage protection
Busbar protection REB670 2.2 ANSI 251
Application manual