2 UNDERVOLTAGE PROTECTIONUndervoltage conditions may occur on a power system for a variety of reasons, some of which are outlined below:● Undervoltage conditions can be related to increased loads, whereby the supply voltage will decrease inmagnitude. This situation would normally be rectified by voltage regulating equipment such as AVRs (AutoVoltage Regulators) or On Load Tap Changers. However, failure of this equipment to bring the systemvoltage back within permitted limits leaves the system with an undervoltage condition, which must becleared.● If the regulating equipment is unsuccessful in restoring healthy system voltage, then tripping by means ofan undervoltage element is required.● Faults occurring on the power system result in a reduction in voltage of the faulty phases. The proportion bywhich the voltage decreases is dependent on the type of fault, method of system earthing and its location.Consequently, co-ordination with other voltage and current-based protection devices is essential in order toachieve correct discrimination.● Complete loss of busbar voltage. This may occur due to fault conditions present on the incomer or busbaritself, resulting in total isolation of the incoming power supply. For this condition, it may be necessary toisolate each of the outgoing circuits, such that when supply voltage is restored, the load is not connected.Therefore, the automatic tripping of a feeder on detection of complete loss of voltage may be required. Thiscan be achieved by a three-phase undervoltage element.● Where outgoing feeders from a busbar are supplying induction motor loads, excessive dips in the supplymay cause the connected motors to stall, and should be tripped for voltage reductions that last longer thana pre-determined time.2.1 UNDERVOLTAGE PROTECTION IMPLEMENTATIONUndervoltage Protection is implemented in the VOLT PROTECTION column of the relevant settings group. TheUndervoltage parameters are contained within the sub-heading UNDERVOLTAGE.The product provides three stages of Undervoltage protection with independent time delay characteristics.Stages 1 and 3 provide a choice of operate characteristics, where you can select between:● An IDMT characteristic● A range of user-defined curves● DT (Definite Time)You set this using the V<1 Function and V<3 Function cells depending on the stage.The IDMT characteristic is defined by the following formula:t = K/( M-1)where:● K = Time multiplier setting● t = Operating time in seconds● M = Measured voltage / IED setting voltageThe undervoltage stages can be configured either as phase-to-neutral or phase-to-phase voltages in theV< Measure't mode cell.There is no Timer Hold facility for Undervoltage.Stage 2 can have definite time characteristics only. This is set in the V<2 status cell.Three stages are included in order to provide multiple output types, such as alarm and trip stages. Alternatively,different time settings may be required depending upon the severity of the voltage dip. For example, motor loadswill be able to cope with a small voltage dip for a longer time than a major one.Chapter 10 - Voltage Protection Functions P14D188 P14D-TM-EN-8
