GE L60 Instructions Manual page 658

OVERVIEW
In general, the heavier the fault current and the lighter the load current, the more suitable is the use of pure positive
sequence for phase comparison. Heavier line loadings and lower fault currents tend to make the scheme less apt to
function properly for internal faults. Thus, pure positive sequence phase comparison appears practical only in a minority of
the cases and so is not suitable for a scheme that is to be generally applicable.
Significant negative sequence currents are present only during faults, they are present in all but balanced three-phase
faults, and there is no significant negative sequence component of load current. All this combines to make pure negative
sequence ideal for phase comparison except that it does not operate for balanced three-phase faults. Similar comments
can be made regarding pure zero-sequence phase comparison with the additional limitation that it does not operate for
phase-to-phase faults. Thus, there does not appear to be a single sequence component or a single phase current that can
be used in a phase comparison scheme to protect against all types of faults.
There are several approaches that are possible to provide a complete scheme. The most obvious is to do the phase
comparison on each phase separately. This is undesirable because the cost is high given that three communication
channels are required. Another approach is to use two separate phase comparison measurements and communication
channels, one for pure positive and the other for pure negative sequence currents. The latter serves to protect against all
unbalanced faults while the former takes care of three phase faults and also provides a measure of back-up protection for
heavy unbalanced faults. Here again cost is an important factor.
As soon as consideration is given to the use of a separate positive and a separate negative phase sequence comparison,
the idea of switching from one to the other presents itself. Such schemes are available. They include detectors separate
from the phase comparison function that distinguish between three phase faults and all other types. For three-phase
faults, the negative sequence network is unbalanced so that it produces an output for positive sequence current as well as
for negative sequence current. The scheme operates normally to provide negative sequence phase comparison for all
unbalanced faults. When a three phase fault occurs, the three-phase detectors at both ends of the line operate to
automatically unbalance their respective negative sequence networks and make them sensitive to positive as well as
negative sequence currents. Since the fault is three phase, there is no negative sequence current produced so the phase
comparison is made on a pure positive sequence basis. This is all accomplished with a common communication channel
for both modes.
Another similar approach is to provide two separate sequence networks, one pure positive sequence and the other pure
negative sequence. Then use the three-phase detector to switch the logic so that only for three-phase faults the outputs of
the positive sequence networks at both ends of the line are compared but for all other faults the negative sequence
outputs are compared. Here again all of this is accomplished over a common channel. This approach has never been used
possibly because of the idea of using "Mixed Excitation." Mixed Excitation is a term used to describe a phase comparison
scheme that mixes the outputs of the different sequence networks in a given proportion and phase angle and then makes
a phase comparison for all faults based on this mix. Thus, all such schemes must include positive sequence plus negative
sequence and/or zero sequence in order to operate for all faults. The two main questions to be resolved are:
1. Which sequence components should be mixed with the positive sequence?
2. What percentages of the full magnitude of each sequence component of current should be used?
The left figure that follows illustrates a two-terminal line with an internal phase B-to-ground fault. The phasor diagrams
indicate the phase positions of the sequence currents at both ends of the line assuming current flow into the line and also
assuming a phase A reference as in equations (1), (2), and (3), previously shown.
At this point, recognize that the positive sequence component of current is made up of two parts, the load component
(I_1L) and the fault component (I_1F). By an analysis utilizing superposition, the load component (I_1L) can be established
as the current flowing just prior to the fault. The three fault components of current (I_1F, I_2F, and I_0F) are then calculated
using the voltage that existed at the point of fault just prior to the fault. Since the load component of current is equal to the
vector difference between Bus X and Bus Y voltages divided by the impedance of the line, and since the prefault voltage (at
the point of fault) has a phase position somewhere between that of X and Y voltages, the positive sequence component of
fault current is displaced from the load component by about 90° ± about 30°. The phasor diagrams at the top of the figure
9
assume that load current flow is from bus X to bus Y.
The first row of the table in the figure indicates that for the conditions assumed, the net positive sequence current entering
both ends of the line are about 120° displaced from each other. Heavier fault current and lighter load current reduces this
angle toward zero while the converse increases the angle toward 180°.
The second and third rows of the table indicate the relative phase positions of the positive plus negative, and positive plus
negative plus zero sequence components respectively. These appear to be more unsatisfactory. Rows 4 and 5 combine the
components differently and both appear to yield much better results.
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CHAPTER 9: THEORY OF OPERATION
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL