1MAC309294-MB F Section 4Protection functionsRER620 203Technical ManualBlocking logicThere are three operation modes in the blocking functionality. The operation modes arecontrolled by the BLOCK input and the global setting Configuration > System > Blockingmode which selects the blocking mode. The BLOCK input can be controlled by a binaryinput, a horizontal communication input or an internal signal of the relay program. Theinfluence of the BLOCK input signal activation is preselected with the global Blockingmode setting.The Blocking mode setting has three blocking methods. In the “Freeze timers” mode, theoperate timer is frozen to the prevailing value. In the “Block all” mode, the whole functionis blocked and the timers are reset. In the “Block TRIP output” mode, the function operatesnormally but the TRIP output is not activated.4.3.2.5 Timer characteristicsThe operating curve types supported by 27 are:Table 210: Supported IDMT operate curve types4.3.2.6 Application27 is applied to power system elements, such as generators, transformers, motors andpower lines, to detect low voltage conditions. Low voltage conditions are caused byabnormal operation or a fault in the power system. 27 can be used in combination withovercurrent protections. Other applications are the detection of a no-voltage condition, forexample before the energization of a high voltage line, or an automatic breaker trip in caseof a blackout. 27 is also used to initiate voltage correction measures, such as insertion ofshunt capacitor banks, to compensate for a reactive load and thereby to increase thevoltage.27 can be used to disconnect from the network devices, such as electric motors, which aredamaged when subjected to service under low voltage conditions. 27 deals with lowvoltage conditions at power system frequency. Low voltage conditions can be caused by:• Malfunctioning of a voltage regulator or incorrect settings under manual control(symmetrical voltage decrease)• Overload (symmetrical voltage decrease)• Short circuits, often as phase-to-ground faults (unsymmetrical voltage increase).The “Freeze timers” mode of blocking has no effect during the “Inversereset” mode.Operating curve type(5) ANSI Def. Time(15) IEC Def. Time(21) Inv. Curve A(22) Inv. Curve B(23) Programmable
