GE Digital Energy L60, Инструкция по эксплуатации

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9-8 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
OVERVIEW CHAPTER 9: THEORY OF OPERATION
9
The following figure shows the relative phase positions of the outputs of a positive sequence network, a negative sequence
network, and a zero sequence network all referenced to phase A. The transfer functions of these three networks are given
by the following equations.
Eq. 9-1
Figure 9-6: Sequence network outputs
The phase positions of the sequence network outputs differ depending on the phase or phases that are faulted as well as
the type of fault. For example, while the positive, negative, and zero sequence components are all in phase for a single-
phase-A-to-ground fault, they are 120° out of phase with each other for phase-B-to-ground, and phase-C-to-ground
faults.
Observe from the table presented that positive sequence currents are available for all kinds of faults, negative sequence
currents are available for all but three-phase faults, and zero sequence currents are available only for faults involving
ground. Thus, it appears that if one single sequence component of current were to be selected for use to make the phase
comparison, then the positive sequence component suffices. Actually, this is not the case in many if not most of the
applications because of the presence of through-load current during the fault.
I
1
1
3
-- I
a
I
b
120
° ∠
I
c
120
°
–
∠
+ +
( )
=
I
2
1
3
-- I
a
I
b
120
–
° ∠
I
c
120
° ∠
+ +
( )
=
I
0
1
3
-- I
a
I
b
I
c
+ + ( ) =