GE Power ManagementL90 Line Differential Relay8-58 THEORY OF OPERATION 8.1 OVERVIEW8The conventional approach does not take into account the elliptical shape of the distribution of uncertainty that arises fromseparate uncertainty parameters in the magnitude and the phase angle of a current measurement, but rather assumes acircular distribution. In order to be secure, the diameter of the circle in the conventional approach must be at least as largeas the major axis of the adaptive ellipse. This means that with the conventional restraint characteristic, the power system isunprotected for fault current phasors that fall within the region between the circle and the ellipse shown in Figure 8–3.Figure 8–3: IMPROVED FAULT COVERAGE OF ADAPTIVE ELLIPTICAL RESTRAINTThe dynamic behavior of fault detection is controlled by filtering the severity quantity, yielding an inverse square dynamicresponse, with response times that vary inversely with the fault severity. Transient response time is 2 cycles for a fault thatis twice as large as the restraint, going down to 0.5 cycle for a fault that is ten times as large as the restraint.8.1.8 CLOCK SYNCHRONIZATIONSynchronization of data sampling clocks is needed in a digital differential protection scheme, because measurements mustbe made at the same time. Synchronization errors show up as phase angle and transient errors in phasor measurements atthe terminals. By phase angle errors, we mean that identical currents produce phasors with different phase angles. By tran-sient errors, we mean that when currents change at the same time, the effect is seen at different times at different measure-ment points. For best results, samples should be taken simultaneously at all terminals.In the case of peer to peer architecture, synchronization is accomplished by synchronizing the clocks to each other ratherthan to a master clock. Each relay compares the phase of its clock to the phase of the other clocks and compares the fre-quency of its clock to the power system frequency and makes appropriate adjustments. The frequency and phase trackingalgorithm keeps the measurements at all relays within a plus or minus 25 microsecond error during normal conditions for a2 or 3 terminal system. For 4 or more terminals the error may be somewhat higher, depending on the quality of the commu-nications channels. The algorithm is unconditionally stable. In the case of 2 and 3 terminal systems, asymmetric communi-cations channel delay is automatically compensated for. In all cases, an estimate of phase error is computed and used toautomatically adapt the restraint region to compensate. Frequency tracking is provided that will accommodate any fre-quency shift normally encountered in power systems.ImaginaryCurrentRealCurrent
