8 THEORY OF OPERATION
The conventional approach does not take into account the elliptical shape of the distribution of uncertainty that arises from
separate uncertainty parameters in the magnitude and the phase angle of a current measurement, but rather assumes a
circular distribution. In order to be secure, the diameter of the circle in the conventional approach must be at least as large
as the major axis of the adaptive ellipse. This means that with the conventional restraint characteristic, the power system is
unprotected 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 RESTRAINT
The dynamic behavior of fault detection is controlled by filtering the severity quantity, yielding an inverse square dynamic
response, with response times that vary inversely with the fault severity. Transient response time is 2 cycles for a fault that
is twice as large as the restraint, going down to 0.5 cycle for a fault that is ten times as large as the restraint.
Synchronization of data sampling clocks is needed in a digital differential protection scheme, because measurements must
be made at the same time. Synchronization errors show up as phase angle and transient errors in phasor measurements at
the 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 rather
than 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 tracking
algorithm keeps the measurements at all relays within a plus or minus 25 microsecond error during normal conditions for a
2 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 to
automatically adapt the restraint region to compensate. Frequency tracking is provided that will accommodate any fre-
quency shift normally encountered in power systems.
GE Power Management
L90 Line Differential Relay
8.1.8 CLOCK SYNCHRONIZATION