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5.6 S4 ELEMENTS
The basic percent differential operating principle for two-winding transformers is illustrated by the following equations:
where
I
= per-phase maximum of the currents after phase, ratio, and zero-sequence correction
restraint
I
= per-phase vector sum of currents after phase, ratio, and zero-sequence correction
differential
In the above equations, the 180° phase shift due to the wiring connections is taken into account, hence the + sign
to obtain the differential current.
NOTE
5
The base for the percent differential setpoints is the
point value. The percent differential setpoints are explained below.
•
PERCENT DIFFERENTIAL PICKUP: Enter the minimum differential current required for operation. This setting is cho-
sen based on the amount of differential current that might be seen under normal operating conditions.
•
PERCENT DIFFERENTIAL SLOPE 1: Enter the Slope 1 percentage (of differential current to restraint current) for the
dual-slope percent differential element. The Slope 1 setting is applicable for restraint currents of zero to the kneepoint,
and defines the ratio of differential to restraint current above which the element will operate. This slope is set to ensure
sensitivity to internal faults at normal operating current levels. The criteria for setting this slope are
1. To allow for mismatch when operating at the limit of the transformer's onload tap-changer range.
2. To accommodate for CT errors.
•
PERCENT DIFFERENTIAL KNEEPOINT: Enter the kneepoint for the dual-slope percent differential element. This is
the transition point between Slopes 1 and 2, in terms of restraint current, in units of relay nominal current. Set the knee-
point just above the maximum operating current level of the transformer between the maximum forced-cooled rated
current and the maximum emergency overload current level.
•
PERCENT DIFFERENTIAL SLOPE 2: Enter the Slope 2 percentage (of differential current to restraint current) for the
dual-slope percent differential element. This setting is applicable for restraint currents above the kneepoint and is set
to ensure stability under heavy through fault conditions which could lead to high differential currents as a result of CT
saturation.
Since
I
restraint'
current. Because of this enhancement, the
(in rare cases no operation) in the following situations:
NOTE
1.
PERCENT DIFFERENTIAL SLOPE 2
2. The source is connected to one winding only.
Therefore, the
the Slope 2 settings should be less than 98%
5-42
Courtesy of NationalSwitchgear.com
(
I
I
max I
=
=
r
restraint
1
%slope
I
(x CT)
differential
OPERATE
REGION
1.00
PICKUP 0.30
0.05
Figure 5–7: PERCENT DIFFERENTIAL DUAL-SLOPE CHARACTERISTIC
(
,
,
)
max I
I
I
, the differential current is not always greater than 100% of the restraint
=
1
2
3
PERCENT DIFFERENTIAL SLOPE 2
745 Transformer Management Relay
,
)
I
; I
I
I
=
=
+
2
d
differential
1
I
d
--- -
×
100%
=
I
r
SLOPE 2
100%
100%
SLOPE 1
RESTRAINT
25%
REGION
15%
2.0
KNEEPOINT
S2 SYSTEM SETUP
WINDING 1
PERCENT DIFFERENTIAL SLOPE 2
is set above 100%.
value cannot be greater than 100%. To increase dependability,
5 SETPOINTS
I
2
200%
50%
I
(x CT)
restraint
WINDING 1 PHASE CT PRIMARY
settings may cause slow operation
(EQ 5.5)
set-
GE Multilin
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