S4 Elements
745
Transformer Management Relay
Se
tpoints
http://www.GEindustrial.com/multilin
5–43
GE Multilin
(EQ 5.5)
where
I
restraint
= per-phase
maximum
of the currents after phase, ratio, and
zero-sequence correction
I
differential
= per-phase
vector sum
of currents after phase, ratio, and zero-
sequence correction
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.
FIGURE 5–7: Percent Differential Dual-Slope Characteristic
The base for the percent differential setpoints is the
S2 SYSTEM SETUP
!"
WINDING 1
!"
WINDING 1 PHASE CT PRIMARY
setpoint value. The percent differential setpoints are
explained below.
•
PERCENT DIFFERENTIAL PICKUP
: Enter the minimum differential current
required for operation. This setting is chosen 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:
–
To allow for mismatch when operating at the limit of the transformer’s
onload tap-changer range.
–
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
kneepoint 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
, the differential current is not always greater
than 100% of the restraint current. Because of this enhancement, the
PERCENT
DIFFERENTIAL SLOPE 2
setting may cause slow operation (in rare cases no operation) in
the following situations:
I
r
I
restraint
max
I
1
I
2
,
(
)
=
=
;
I
d
I
differential
I
1
I
2
+
=
=
%slope
I
d
I
r
-----
100%
×
=
NOTE
100%
OPERATE
REGION
RESTRAINT
REGION
SLOPE 1
25%
SLOPE 2
100%
200%
50%
I
(x CT)
restraint
I
(x CT)
differential
15%
0.05
1.00
PICKUP 0.30
2.0
KNEEPOINT
NOTE
I
restraint
'
max
I
1
I
2
I
3
,
,
(
)
=
Summary of Contents for Multilin 745
Page 2: ......
