ABB REL650 series, Applications Manual

Page: 269 / 516

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.
8.3.3.1 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.
8.3.3.2 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
the ground is available on the faulty phase and the neutral has a full phase-to-ground
voltage. The residual overvoltage will be three times the phase-to-ground voltage. See
Figure 123 .
1MRK 506 334-UUS A
Section 8
Voltage protection
263
Application manual