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 measurement of the three-phase 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.
3.8.3.2 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 and time characteristic.
There is a very wide application area where general single input or residual
overvoltage functions are used. All voltage related settings are made as a percentage of
a settable base voltage, which can 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 single 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.
Equipment protection, such as for motors, generators, reactors and
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
Section 3 1MRK504116-UUS C
IED application
522
Application manual
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