7.6.7.1 Operating quantity within the functionIf the function is set to measure zero sequence, it uses Residual Current (3I0) for itsoperating quantity. The residual current can be:1. directly measured (when a dedicated CT input of the IED is connected in PCM600 tothe fourth analog input of the pre-processing block connected to EF4PTOC (51N/67N) function input I3P). This dedicated IED CT input can be for example, connectedto:• parallel connection of current instrument transformers in all three phases(Holm-Green connection).• one single core balance, current instrument transformer (cable CT).• one single current instrument transformer located between power system WYEpoint and ground (that is, current transformer located in the neutral groundingof a WYE connected transformer winding).• one single current instrument transformer located between two parts of aprotected object (that is, current transformer located between two WYE pointsof double WYE shunt capacitor bank).2. calculated from three-phase current input within the IED (when the fourth analoginput into the pre-processing block connected to EF4PTOC (51N/67N) functionAnalog Input I3P is not connected to a dedicated CT input of the IED in PCM600).In such case the pre-processing block will calculate 3I0 from the first three inputs intothe pre-processing block by using the following formula (will take I2 from sameSMAI AI3P connected to I3PDIR input (same SMAI AI3P connected to I3P input)):If zero sequence current is selected,opI 3 Io IA IB IC= × = + +EQUATION2011-ANSI V1 EN (Equation 47)where:IA, IB, IC are fundamental frequency phasors of three individual phase currents.The residual current is pre-processed by a discrete Fourier filter. Thus the phasor of thefundamental frequency component of the residual current is derived. The phasormagnitude is used within the EF4PTOC (51N/67N) protection to compare it with the setoperation current value of the four steps (Pickup1, Pickup2, Pickup3 or Pickup4). If theresidual current is larger than the set operation current and the step is used in non-directional mode a signal from the comparator for this step is set to true. This signal will,without delay, activate the output signal PU_STx (x=step 1-4) for this step and a commonPICKUP signal.1MRK 506 335-UUS A Section 7Current protection221Technical manual
