Equipment Protection, Such As For Motors, Generators, Reactors And Transformers; Equipment Protection, Capacitors; Power Supply Quality; High Impedance Grounded Systems - ABB REB670 Applications Manual

Section 8
Voltage protection
8.3.3.1
8.3.3.2
8.3.3.3
8.3.3.4
240
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

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.

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.

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
1MRK 505 337-UUS -
Application manual