Other protection functions like time delayed phase and zero sequence overcurrentfunction can be connected to ZCVPSOF function to increase the dependability in thescheme.6.7.3 Setting guidelinesThe parameters for Automatic switch onto fault logic, voltage and current based function(ZCVPSOF) are set via the local HMI or Protection and Control Manager PCM600.The distance protection zone used for instantaneous trip by ZCVPSOF function has to beset to cover the entire protected line with a safety margin of minimum 20%.GlobalBaseSel: Selects the global base value group used by the function to define (IBase),(VBase) and (SBase).Operation: The operation of Automatic switch onto fault logic, voltage and current basedfunction is by default set to Enabled. Set the parameter to Disabled if the function is notto be used.IphPickup is used to set current level for detection of dead line.IphPickup is by default setto 20% of IBase. It shall be set with sufficient margin (15 - 20%) under the minimumexpected load current. In many cases the minimum load current of a line is close to 0 andeven 0. The operate value must exceed the maximum charging current of an overhead line,when only one phase is disconnected (mutual coupling the other phases).UVPickup is used to set voltage level for detection of dead line. UVPickup is by default setto 70% of VBase. This is a suitable setting in most cases, but it is recommended tochecking the suitability in the actual application.Mode: The operation of ZCVPSOF has three modes for defining the criteria for trip. Thesetting of the Mode is by default set to VILevel, which means that the tripping criteria isbased on the setting of IPhPickup and UVPickup. The choice of VILevel gives faster andmore sensitive operation of the function, which is important to reduce the stress that mightoccur when energizing onto a fault. On the other hand the risk for over function might behigher due to that the voltage recovery in some systems can be slow given unwantedoperation at energizing the line if the timer tDuration is set too short.When Mode is set to Impedance, the operation criteria is based on the start of overeachingzone from impedance zone measurement. A non-directional output signal should be usedfrom an overreaching zone. The selection of Impedance mode gives increased security.In operation mode VILvl&Imp the condition for trip is an OR-gate between VILevel andVILvl&Imp.The setting of the timer for release of the VILevel, tDuration is by default set to 0.020 sec.In some cases, especially for series compensated lines, it might be needed to increase this1MRK 506 334-UUS A Section 6Impedance protection167Application manual
