9-12
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
OVERVIEW
CHAPTER 9: THEORY OF OPERATION
9
•
Make the limits of application broad enough to render the scheme useful as a protection tool
In line with these considerations, the best overall results using mixed excitation are attained by using I_2 – KI_1, where K is
a constant that is adjustable within limits. While it is likely that the inclusion of zero sequence excitation is helpful for one
case or another, it is not generally employed because the problem of evaluating the overall performance of the scheme is
magnified considerably. This is true mainly because the current distribution in the zero sequence network is generally quite
different from that in the positive and negative sequence networks where the current distributions are approximately the
same. For any given fault on a transmission line, the ratio of I_1F / I_2F at any terminal is the same as at any other terminal
of that line. This is not true of either I_1F / I_0F or I_2F / I_0F. It is this that makes the use of zero sequence excitation
undesirable.
9.1.4.2 Mixed excitation
If the mixing network of the Single-Phase Comparison Blocking Scheme Principle figure were designed to produce an
output that is proportional to I_2 – KI_1, then this logic would be a simplified representation of a mixed excitation phase
comparison scheme. In such schemes, the pick-up setting of FDH must be high enough so that the KI_1 output from the
mixing network does not result in continuous phase comparison on load current (I_2 is normally zero during normal system
conditions). Also, it can be desirable to have FDL set to pick up at some level above full load so that channel is not keyed on
and off continuously during normal load conditions. Since FDH is set higher than FDL, this requirement results in a still
higher setting for FDH.
Because FDH controls tripping, this arrangement limits the applicability of the basic scheme to circuits where the minimum
three phase fault current is significantly higher than the maximum load current. The requirements for the satisfactory
performance of a mixed excitation scheme using overcurrent fault detectors (FDH and FDL) are:
•
Both the FDL and FDH fault detectors must be set above full load current
•
All internal faults regardless of type or the particular phases involved must produce enough I_2 - KI_1 to operate FDH
at all ends of the line
•
FDL must be set with a lower pick-up than FDH at the remote end(s) of the line for security during external faults
•
The phase angle difference between the I_2 – KI_1 quantities obtained at all terminals of the protected line during all
types of internal faults, and for any combination of phases, must be less than 115°
9.1.4.3 Zero-sequence excitation
With zero sequence excitation, the phase comparison portion of the overall scheme is not be capable of operating for
phase-to-phase and three-phase faults. For this reason, the overall protective scheme must include measurement
functions that can detect and operate for faults involving any two or more phases. Mho-type phase distance functions
have typically been employed for this protection.
Note that distance relays designed to operate for faults involving two or more phases do operate for double-phase-to-
ground faults and also for certain close-in single-phase-to-ground faults. Thus, it is reasonable to expect both phase
comparison and distance protection to be activated for many faults.
9.1.4.4 Negative-sequence excitation
Since negative-sequence phase comparison protects against all unbalanced faults, the directional comparison functions
are required only for three-phase fault protection. However, if these functions are designed to respond to all multi-phase
faults, then phase-to-phase and double phase-to-ground faults are protected by both modes while single-phase-to-
ground faults are protected by only the phase comparison mode and three phase faults only by directional comparison.
9.1.5 Blocking versus tripping schemes
9.1.5.1 Introduction
Further consideration of blocking versus tripping pilot schemes is outlined here. Figure 2C illustrates the comparer
integrator logic for a tripping scheme using an ON-OFF type of pilot channel. In order to trip, a receiver output is required to
be present during the half-cycle that the local current is positive. Figure 2D is representative of a blocking pilot scheme
where tripping takes place if there is no receiver output during the half cycle that the local current is positive.
