ABB Relion 670 Series Applications Manual page 226

Section 9
Voltage protection
9.3.3.1 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.
9.3.3.2 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.
9.3.3.3 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.
9.3.3.4 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 faulty
phase will be connected to earth. The residual overvoltage will be three times the phase-to-
earth voltage. See figure 101.
IEC07000190 V2 EN-US
Figure 101: Earth fault in Non-effectively earthed systems
220
IEC07000190-2-en.vsd
© Copyright 2017 Hitachi Power Grids. All rights reserved
1MRK 511 401-UEN Rev. K
M13853-9 v8
M13853-12 v4
M13853-15 v3
M13853-18 v10
Bay control REC670
Application manual