Blocking mode" which selects the blocking mode. The BLOCK input can becontrolled by a binary input, a horizontal communication input or an internal signalof the IED program. The influence of the BLOCK signal activation is preselectedwith the global setting Blocking mode.The Blocking mode setting has three blocking methods. In the "Freeze timers"mode, the operation timer is frozen to the prevailing value. In the "Block all"mode, the whole function is blocked and the timers are reset. In the "BlockOPERATE output" mode, the function operates normally but the OPERATE outputis not activated.4.5.5.5 ApplicationPSPTUV can be applied for protecting a power station used for embeddedgeneration when network faults like short circuits or phase-to-earth faults in atransmission or a distribution line cause a potentially dangerous situations for thepower station. A network fault can be dangerous for the power station for variousreasons. The operation of the protection can cause an islanding condition, alsocalled a loss-of-mains condition, in which a part of the network, that is, an islandfed by the power station, is isolated from the rest of the network. There is then arisk of an autoreclosure taking place when the voltages of different parts of thenetwork do not synchronize, which is a straining incident for the power station.Another risk is that the generator can lose synchronism during the network fault. Asufficiently fast trip of the utility circuit breaker of the power station can avoidthese risks.The lower the three-phase symmetrical voltage of the network is, the higher is theprobability that the generator loses the synchronism. The positive-sequence voltageis also available during asymmetrical faults. It is a more appropriate criterion fordetecting the risk of loss of synchronism than, for example, the lowest phase-to-phase voltage.Analyzing the loss of synchronism of a generator is rather complicated and requiresa model of the generator with its prime mover and controllers. The generator canbe able to operate synchronously even if the voltage drops by a few tens of percentfor some hundreds of milliseconds. The setting of PSPTUV is thus determined bythe need to protect the power station from the risks of the islanding conditionssince that requires a higher setting value.The loss of synchronism of a generator means that the generator is unable tooperate as a generator with the network frequency but enters into an unstablecondition in which it operates by turns as a generator and a motor. Such a conditionstresses the generator thermally and mechanically. This kind of loss ofsynchronism should not be mixed with the one between an island and the utilitynetwork. In the islanding situation, the condition of the generator itself is normalbut the phase angle and the frequency of the phase-to-phase voltage can bedifferent from the corresponding voltage in the rest of the network. The island canhave a frequency of its own relatively fast when fed by a small power station with alow inertia.1YHT530004D05 D Section 4Protection functions615 series 491Technical Manual
