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1MRK 505 370-UEN D
8.3.2
Application
Two step residual overvoltage protection ROV2PTOV is primarily used in high impedance
earthed distribution networks, mainly as a backup for the primary earth-fault protection of the
feeders and the transformer. To increase the security for 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.
8.3.3
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.
8.3.3.1
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.
8.3.3.2
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 winding. The setting
must be above the highest occurring "normal" residual voltage and below the highest
acceptable residual voltage for the capacitor.
8.3.3.3
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.
Busbar protection REB670
Application manual
Section 8
Voltage protection
M13809-3 v8
M13853-3 v8
M13853-9 v8
M13853-12 v4
M13853-15 v3
213
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