ABB REU 610 Technical Reference Manual

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ABB REU 610 Technical Reference Manual Manual pdf 27 page image

27A situation of this kind can be critical for different reasons. The power plantcan be left to feed an isolated network due to a trip caused by a fault. In thiscase, there is a risk that the isolated network, which is in asynchronous statecompared to the rest of the network, is reconnected to the network as a resultof an autoreclosure, for instance. In addition, the power plant can also fall intoan asynchronous state in a fault situation. These critical situations can beprevented by disconnecting the power plant from the network fast enough bytripping the connecting circuit breaker.The benefit of this function is that the voltage value measured during or aftera network fault is a good measure of how critical the fault is for a smallerpower plant. When the positive phase-sequence voltage value falls below thecritical limit, the power plant has to be disconnected from the network.REU 610 measuring the positive phase-sequence voltage complements othermethods used to disconnect smaller power plants. The application of thesemethods is based on frequency and voltage measurement. The undervoltageprotection based on calculated positive phase-sequence voltage requires thatthe relay is in three-phase use.When the calculated positive phase-sequence voltage value U 1 falls below theset pickup value of the element 27D, the element generates a pickup signalafter a ~ 50 ms’ pickup time. When the set operate time at definite-timecharacteristic or the calculated operate time at IDMT characteristic elapses,the element generates a trip signal. The element is reset in 70 ms after thecalculated positive phase-sequence voltage value has exceeded the set pickupvalue of the element.The positive phase-sequence voltage is calculated as follows:U1 = + × + ×U a U a Uab bc ca23 (2)a = 1∠ 120°a2 = 1∠ -120°On a symmetrical network, this formula scales the positive phase-sequencevalue to the same amplitude as the measured voltage signal.Element 27P-2 can be set to be blocked when the measured voltage valuesfall below 0.15 × U n (VT). In addition, the tripping of element 27P-1 can beset to be blocked by element 27P-2 pickup. The selection is made in SGF4.Element 27D has a fixed drop-off/pick-up ratio of 1.04.5.1.4.4. Residual overvoltage protectionThe residual overvoltage protection detects residual voltages caused byground faults.1MRS755972 Voltage RelayTechnical Reference Manual - ANSI VersionREU 610