GE Multilin L90 Line Current Differential System 9-79 THEORY OF OPERATION 9.1 OVERVIEW9(PFLL) as shown on the diagram above. If GPS time reference is lost, the channel asymmetry compensation is notenabled, and the relay clock may start to drift and accumulate differential error. In this case, the 87L function has to beblocked. Refer to Chapter 9: Application of Settings for samples of how to program the relay.9.1.10 FREQUENCY DETECTIONEstimation of frequency deviation is done locally at each relay based on rotation of positive sequence current, or on rotationof positive sequence voltage, if it is available. The counter clockwise rotation rate is proportional to the difference betweenthe desired clock frequency and the actual clock frequency. With the peer to peer architecture, there is redundant frequencytracking, so it is not necessary that all terminals perform frequency detection.Normally each relay will detect frequency deviation, but if there is no current flowing nor voltage measurement available ata particular relay, it will not be able to detect frequency deviation. In that case, the frequency deviation input to the loop filteris set to zero and frequency tracking is still achieved because of phase locking to the other clocks. If frequency detection islost at all terminals because there is no current flowing then the clocks continue to operate at the frequency present at thetime of the loss of frequency detection. Tracking will resume as soon as there is current.The rotational rate of phasors is equal to the difference between the power system frequency and the ratio of the samplingfrequency divided by the number of samples per cycle. The correction is computed once per power system cycle at eachrelay. For conciseness, we use a phasor notation:(EQ 9.21)Each terminal computes positive sequence current:(EQ 9.22)Each relay computes a quantity derived from the positive sequence current that is indicative of the amount of rotation fromone cycle to the next, by computing the product of the positive sequence current times the complex conjugate of the posi-tive sequence current from the previous cycle:(EQ 9.23)The angle of the deviation phasor for each relay is proportional to the frequency deviation at that terminal. Since the clocksynchronization method maintains frequency synchronism, the frequency deviation is approximately the same for eachrelay. The clock deviation frequency is computed from the deviation phasor:(EQ 9.24)Note that a four quadrant arctangent can be computed by taking the imaginary and the real part of the deviation separatelyfor the two arguments of the four quadrant arctangent. Also note that the input to the loop filter is in radian frequency whichis two pi times the frequency in cycles per second; that is, .So the radian frequency deviation can be calculated simply as:(EQ 9.25)9.1.11 PHASE DETECTIONThere are two separate sources of clock phase information; exchange of time stamps over the communications channelsand the current measurements themselves (although voltage measurements can be used to provide frequency information,they cannot be used for phase detection). Current measurements can generally provide the most accurate information, butare not always available and may contain large errors during faults or switching transients. Time stamped messages areI n Re Phasor n j Im Phasor n+=I a k n I n= for phase a from the kth terminal at time step nI b k n I n= for phase b from the kth terminal at time step nI c k n I n= for phase c from the kth terminal at time step nI pos k n 13--- I a k n I b k n e j2S 3e I c k n e j2S 3e+ +=Deviation k n I pos k n I pos k n N–u=FrequencyDeviation f'f----- tan 1– Im Deviation Re Deviatione2S-------------------------------------------------------------------------------------------------= =Z' 2S f'=Z' f' tan 1– Im Deviation Re Deviatione=
