7.2.4.2 Settings of protection functionOperation: The operation of the high impedance differential function can be switchedOn or Off.U>Alarm: Set the alarm level. The sensitivity can roughly be calculated as a certainpercentage of the selected Trip level. A typical setting is 10% of U>Trip This alarmstage can be used for scheme CT supervision.tAlarm: Set the time delay for the alarm. A typical setting is 2-3 seconds.U>Trip: Set the trip level according to the calculations (see examples below for aguidance). The level is selected with margin to the calculated required voltage toachieve stability. Values can be within 20V - 400V range dependent on theapplication.SeriesResistor: Set the value of the used stabilizing series resistor. Calculate the valueaccording to the examples for each application. Adjust the resistor as close as possibleto the calculated value. Measure the value achieved and set this value for thisparameter.The value shall always be high impedance. This means for example,for 1A circuits say bigger than 400 ohms (400 VA) and for 5 A circuitssay bigger than 100 ohms (2500 VA). This ensures that the currentwill circulate and not go through the differential circuit at throughfaults.That the settings of U>Alarm, U>Trip and SeriesResistor must bechosen such that both U>Alarm/SeriesResistor and U>Trip/SeriesResistor are >4% of IRated of the used current input. Normallythe settings shall also be such that U>Alarm/SeriesResistor andU>Trip/SeriesResistor both gives a value <4*IRated of the usedcurrent input. If not, the limitation in how long time the actual currentis allowed to persist not to overload the current input must beconsidered especially during the secondary testing.7.2.4.3 T-feeder protectionIn many busbar arrangements such as one-and a half breaker, ring breaker, meshcorner, there will be a T-feeder from the current transformer at the breakers up to thecurrent transformers in the feeder circuit (for example, in the transformer bushings).It is often required to separate the protection zones that the feeder is protected with onescheme while the T-zone is protected with a separate differential protection scheme.The 1Ph high impedance differential HZPDIF function in the IED allows this to bedone efficiently, see Figure 63.1MRK 502 071-UEN - Section 7Differential protectionGenerator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 159Application manual
