8-26 L60 Line Phase Comparison System GE Multilin8.1 OVERVIEW 8 THEORY OF OPERATION8Two levels of fast overcurrent supervision are required: fault detection low (FDL) for keying and fault detection high (FDH)for tripping. These conditions are supervisory; therefore, they do not have to be very accurate. Instead, they shall be fastenough not to slow down the remainder of the 87PC algorithm.The fast magnitude is calculated as:(EQ 8.4)Figure 8–21: EXAMPLE OF MIXING CURRENT OPERATION (RELAY COMTRADE RECORD)The response of the overcurrent condition to switch off transients, including current reversal on parallel lines and heavilysaturated CTs, is important. The key design requirement is keep the FDL and FDH picked up and resetting in a way thatensures both dependability and security in both tripping and blocking arrangements.From this perspective, to boost the magnitude on heavily saturated CTs, the RMS component is calculated as follows (on asample-by-sample basis):(EQ 8.5)In the above equation, N1 represents the number of samples per cycle (64).The magnitude estimator combines the fast estimator for accuracy, the RMS value for dependability on CT saturation orother severe transients, and the waveform peak for speed:(EQ 8.6)The local operating current is converted into phase pulses. It is important to realize that the operation is nonlinear, erasingalmost all information contained in the magnitude of the signal and presenting exclusively the phase information by encod-ing the on/off pulses signifying polarity of the operating signal. This polarity is preserved with respect to the universal ana-log time. This is one of the key advantages of the phase comparison principle, even when implemented digitally: nosynchronization is required between the individual relays of the 87PC scheme.The raw LOC-al pulses (Positive and Negative polarity) are produced disregarding the FDL and FDH flags. The fault detec-tor flags are used in the dual-breaker, key and trip logic. The raw pulses are calculated as follows.For tripping schemes:(EQ 8.7)I FAST i MIX( )2 I MIX_Q( )2+=F1-IAF2-IBF2-IC87PC BKR1 current1) Internal AG fault tripped single-pole.2) Followed by the second intervalBG fault during the single-pole open condition831802A1.CDR3) Composite signal (negative-sequence) of the 87PC functionI RMS k( )2N1------- I MIX k p–( )( )2p 0=N1 1–=I AUX max I RMS I FAST 0.85 abs i MIX( )×, ,( )=LOC 1P_RAW i1_MIX 0.005 2 CT 1pu××>=LOC 1N_RAW i1_MIX 0.005 2 CT 1pu××–<=
