In many applications several steps with different current pick up levels and time delaysare needed. PH4SPTOC(51) can have up to four different, individual settable, steps.The flexibility of each step of PH4SPTOC(51) function is great. The following optionsare possible:Choice of delay time characteristics: There are several types of time delaycharacteristics available such as definite time delay and different types of inverse timedelay characteristics. The selectivity between different overcurrent protections isnormally enabled by co-ordination between the function time delays of the differentprotections. To enable optimal co-ordination all overcurrent IEDs, to be co-ordinatedagainst each other, should have the same time delay characteristic. Therefore a widerange of standardised inverse time characteristics are available: IEC and ANSI. It isalso possible to programme a user defined inverse time characteristic.Normally it is required that the phase overcurrent function shall reset as fast as possiblewhen the current level gets lower than the operation level. In some cases some sort oftime delayed reset is required. Therefore three different kinds of reset timecharacteristics can be used.For some protection applications there can be a need to change the current pick-uplevel for some time. A typical case is when the protection will measure the current to alarge motor. At the start up sequence of a motor the start current can be significantlylarger then the rated current of the motor. Therefore there is a possibility to give asetting of a multiplication factor to the current pick-up level. This multiplication factoris activated from a binary input signal to the function.Power transformers can have a large inrush current, when being energized. Thisphenomenon is due to saturation of the transformer magnetic core during parts of theperiod. There is a risk that inrush current will reach levels above the pick-up current ofthe phase overcurrent protection. The inrush current has a large second harmoniccontent. This can be used to avoid unwanted operation of the protection. Therefore theFour step phase overcurrent protection (OC4PTOC, 51/67) function have a possibilityof second harmonic restrain if the level of this harmonic current reaches a value abovea set percentage of the fundamental current.7.2.3 Setting guidelines SEMOD127939-1 v1SEMOD127941-4 v3A typical starting time delay of 24ms is subtracted from the set triptime delay, so that the resulting trip time will take the internal IED starttime into consideration.The parameters for the four step phase overcurrent protection function (OC) are set viathe local HMI or Protection and Control IED Manager (PCM 600).Section 7 1MRK 505 370-UUS ACurrent protection188 Busbar protection REB670 2.2 ANSIApplication manual
