Date Code 20080110 Protection Functions 3-13
SEL-387E Instruction Manual
1. Independent Harmonic Blocking (IHBL = Y) blocks the percentage differential
element for a particular phase if the harmonic (second or fifth) in that phase exceeds
the block threshold. No blocking occurs on other differential elements.
2. Common Harmonic Blocking (IHBL = N) blocks all of the percentage differential
elements if the harmonic magnitude of any one phase is greater than the blocking
threshold.
Common Harmonic Blocking is more secure but may slightly delay percentage differential
element operation because harmonics in all three phases must drop below the thresholds for the
three phases.
Setting Calculation
General Discussion of Connection Compensation
The general expression for current compensation is as follows:
()
[]
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
=
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
ICWn
IBWn
IAWn
•mCTC
ICWnC
IBWnC
IAWnC
where IAW
n
, etc., are the three-phase currents entering terminal “
n
” of the relay; IAW
n
C, etc.,
are the corresponding phase currents after compensation; and [CTC(
m
)] is the three-by-three
compensation matrix.
Setting W
n
CTC =
m
specifies which [CTC(
m
)] matrix the relay is to use. The setting values are
0, 1, 2, …, 11, 12. These are discrete values “
m
” can assume in [CTC(
m
)]; the values physically
represent the “
m
” number of increments of 30 degrees that a balanced set of currents with ABC
phase rotation will be rotated in a counterclockwise direction when multiplied by [CTC(
m
)]. If a
given set of such currents is multiplied by all 12 of the CTC matrices, the resulting compensated
values would seem to move completely around the circle in a counterclockwise direction,
returning to the original start position. This is the same as successively multiplying [CTC(1)]
times the original currents, then times each successive compensated result value, a total of 12
times.
If a balanced set of currents with ACB phase rotation
undergoes the same exercise, the rotations
by the [CTC(
m
)] matrices are in the clockwise direction. This is because the compensation
matrices, when performing phasor addition or subtraction involving B or C phases, will produce
“mirror image” shifts relative to Phase A, when ACB phase rotation is used instead of ABC. In
ACB phase rotation the three phases still rotate in a counterclockwise direction, but C-phase is in
the 120-degree lagging position and B-phase leads by 120 degrees, relative to A-phase.
The discussions below assume ABC phase rotation, unless mentioned otherwise.
The “0” setting value is intended to create no changes at all in the currents and merely multiplies
them by an identity matrix. Thus, for W
n
CTC = 0,
()
[]
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
=
100
010
001
0CTC
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