6.1.3.10 Instantaneous differential currentsThe instantaneous differential currents are calculated from the instantaneous values ofthe input currents in order to perform the harmonic analysis and waveform analysisupon each one of them (see section "Harmonic and waveform block criteria" for moreinformation).6.1.3.11 Harmonic and waveform block criteriaThe two blocking criteria are the harmonic restrain and the waveform restrain. Thesetwo criteria have the power to block a trip command by the restrained differentialprotection and sensitive negative sequence based turn-to-turn fault protection.Harmonic restrainThe harmonic restrain is the classical restrain method traditionally used with powertransformer differential protections. The goal is to prevent an unwanted trip commanddue to magnetizing inrush currents at switching operations, or due to magnetizingcurrents at over-voltages.The magnetizing currents of a power transformer flow only on one side of the powertransformer (one or the other) and are therefore always the cause of false differentialcurrents. The harmonic analysis (the 2nd and the 5th harmonic) is applied toinstantaneous differential currents. Typical instantaneous differential currents duringpower transformer energizing are shown in figure 36. The harmonic analysis is onlyapplied in those phases, where pickup signals have been set. For example, if thecontent of the 2nd harmonic in the instantaneous differential current of phase A isabove the setting I2/I1Ratio, then a block signal is set for that phase.Waveform restrainThe waveform restrain criterion is a good complement to the harmonic analysis. Thewaveform restrain is a pattern recognition algorithm, which looks for intervals withineach fundamental power system cycle with low instantaneous differential current. Thisinterval is often called current gap in protection literature. However, within differentialfunction this criterion actually searches for long-lasting intervals with low rate-of-change in instantaneous differential current, which are typical for the powertransformer inrush currents. Block signal BLKWAV is set in those phases where suchbehavior is detected. The algorithm does not require any end user settings. Thewaveform algorithm is automatically adapted dependent only on the power transformerrated data.1MRK 502 043-UUS B Section 6Differential protection101Technical Manual
