ABB REG670 - Setting Guidelines

different earth-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-earth voltages.
In high impedance earthed systems the residual voltage will increase in case of any
fault connected to earth. 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-earth voltage, is achieved for a single phase-to-earth 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 is often used as a backup protection or as a release signal for the feeder
earth-fault protection.
10.3.3 Setting guidelines
All the voltage conditions in the system where ROV2PTOV 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 is seldom critical, since residual voltage is related to
earth faults in a high-impedance earthed 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.
10.3.3.1 Equipment protection, such as for motors, generators, reactors and
transformersEquipment protection for transformers
High residual voltage indicates earth-fault in the system, perhaps in the component to
which two step residual overvoltage protection (ROV2PTOV) is connected. For
selectivity reasons to the primary protection for the faulted device, ROV2PTOV 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.
10.3.3.2 Equipment protection, capacitors
High voltage will deteriorate the dielectric and the insulation. Two step residual
overvoltage protection (ROV2PTOV) has to be connected to a neutral or open delta
1MRK 502 071-UEN - Section 10
Voltage protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 411
Application manual