E00873EVTVT I RL∆ILILI XL∆I XLθFigure 118: Transient voltage vector change q due to change in load current IDLThe voltage vector shift function is designed to respond within one to two full mains cycles when its threshold isexceeded. Discrimination between a loss of mains condition and a circuit fault is therefore achievable only byselecting the angle threshold to be above expected fault levels. This setting can be quantified by calculating theangular change due to islanding. However, this angular change depends on system topology, power flows andvery often also on the instant of the system faults. For example a bolted three phase short circuit which occursclose to the relay may cause a problem in that it inherently produces a vector shift angle at the instant of the faultwhich is bigger than any normal setting, independent of the mains condition. This kind of fault would cause therelay to trip shortly after the instant of its inception. Although this may seem to be a disadvantage of the vectorshift function, isolating the embedded generator at the instant of a bolted three phase fault is of advantage to thePES. This is because the mains short circuit capacity and consequently the energy feeding the short circuit islimited by the instant operation of the relay. The fast operation of this vector shift function renders it to operate atthe instant of a disturbance rather than during a gradual change caused by a gradual change of power flow.Operation can occur at the instant of inception of the fault, at fault clearance or following non-synchronizedreclosure, which affords additional protection to the embedded generator.P14D Chapter 10 - Voltage Protection FunctionsP14D-TM-EN-8 213
