ABB Relion REQ650, Applications Manual, Page: 168 / 340

7.3.3 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
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 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 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.
7.3.3.1 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.
7.3.3.2 High impedance earthed systems
In high impedance earthed systems, earth faults cause a neutral voltage in the feeding
transformer neutral. Two step residual overvoltage protection ROV2PTOV is used to
trip the transformer, as a backup protection for the feeder earth-fault protection, and
as a backup for the transformer primary earth-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 earth fault causes a transformer neutral to reach a voltage equal to the nominal
phase-to-earth voltage.
The voltage transformers measuring the phase-to-earth voltages measure zero voltage
in the faulty phase. The two healthy phases will measure full phase-to-phase voltage,
as the earth is available on the faulty phase and the neutral has a full phase-to-earth
voltage. The residual overvoltage will be three times the phase-to-earth voltage. See
Figure 74 .
Section 7
1MRK 505 291-UEN A
Voltage protection
162
Application manual