situations, where the residual overvoltage protection is used to protect somespecific equipment, the time delay is shorter.Some applications and related setting guidelines for the residual voltage level aregiven below.8.3.3.1 Equipment protection, such as for motors, generators, reactors andtransformersEquipment protection for transformers M13853-9 v8High residual voltage indicates earth-fault in the system, perhaps in the componentto which two step residual overvoltage protection (ROV2PTOV) is connected. Forselectivity reasons to the primary protection for the faulted device, ROV2PTOVmust trip the component with some time delay. The setting must be above thehighest occurring "normal" residual voltage and below the highest acceptableresidual voltage for the equipment.8.3.3.2 Equipment protection, capacitors M13853-12 v4High voltage will deteriorate the dielectric and the insulation. Two step residualovervoltage protection (ROV2PTOV) has to be connected to a neutral or open deltawinding. The setting must be above the highest occurring "normal" residual voltageand below the highest acceptable residual voltage for the capacitor.8.3.3.3 Power supply quality M13853-15 v3The setting must be above the highest occurring "normal" residual voltage andbelow the highest acceptable residual voltage, due to regulation, good practice orother agreements.8.3.3.4 High impedance earthed systems M13853-18 v10In high impedance earthed systems, earth faults cause a neutral voltage in thefeeding transformer neutral. Two step residual overvoltage protection ROV2PTOVis used to trip the transformer, as a backup protection for the feeder earth-faultprotection, and as a backup for the transformer primary earth-fault protection. Thesetting must be above the highest occurring "normal" residual voltage, and belowthe lowest occurring residual voltage during the faults under consideration. Ametallic single-phase earth fault causes a transformer neutral to reach a voltageequal to the nominal phase-to-earth voltage.The voltage transformers measuring the phase-to-earth voltages measure zerovoltage 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 residualovervoltage will be three times the phase-to-earth voltage. See figure 59.Section 8 1MRK 504 169-UEN AVoltage protection152 Transformer protection RET650 2.2 IECApplication manual
