GE T35 Instruction Manual page 128

5.3 SYSTEM SETUP
The T35 automatically corrects for CT mismatch errors. All currents are magnitude compensated to be in units of the CTs of
one winding before the calculation of differential and restraint quantities.
The reference winding (w
currents will be in per unit of nominal of the CTs on the reference winding. This is important to know, because the settings of
the operate characteristic of the percent differential element (pickup, breakpoints 1 and 2) are entered in terms of the same
per unit of nominal.
The reference winding is chosen by the relay to be the winding which has the smallest margin of CT primary current with
respect to winding rated current, meaning that the CTs on the reference winding will most likely begin to saturate before
those on other windings with heavy through currents. The characteristics of the reference winding CTs determine how the
percent differential element operate characteristic should be set.
The T35 determines the reference winding as follows:
1. Calculate the rated current (I
Note: enter the self-cooled MVA rating for the P
2. Calculate the CT margin (I
3. Choose the winding with the lowest CT margin:
In our example, the reference winding is chosen as follows.
5
1. Calculate the rated current for windings 1 and 2:
P
rated
1 [ ]
I
----------------------------------- -
=
rated
×
3 V
nom
2. With these rated currents, calculate the CT margin for windings 1 and 2:
CT primary 1 [ ]
1 [ ]
I ------------------------------------- -
=
margin
2 [ ] I <
3. Since I
margin
The reference winding is shown in
.
REFERENCE WINDING
The unit for calculation of the differential and restraint currents and base for the differential restraint settings is the CT pri-
mary associated with the reference winding. In this example, the unit CT is 1500:5 on winding 2.
Magnitude compensation factors (M) are the scaling values by which each winding current is multiplied to refer it to the ref-
erence winding. The T35 calculates magnitude compensation factors for each winding as follows:
In our example, the magnitude compensation factors are calculated as follows:
The maximum allowed magnitude compensation factor (and hence the maximum allowed CT ratio mismatch) is 32.
5-50
) is the winding to which all currents are referred. This means that the differential and restraint
ref
) for each winding:
rated
[ ]
P
rated
[ ]
I w ------------------------------------ -
=
rated
×
3 V
nom
) for each winding:
margin
CT primary w
-------------------------------------- - , where w
I
=
margin [ ]
I w
rated
1 [ ]
100 MVA
226.4 A
--------------------------------
= =
1 [ ] ×
3 220 kV
500 A
-------------------- - 1.91
= =
1 [ ]
I 262.4 A
rated
1 [ ]
, the reference winding w
margin
!"
ACTUAL VALUES
[ ] V ×
I w
primary nom
[ ]
M w --------------------------------------------------------------------- - , where w
=
[ ] V ×
I w
primary ref nom
1 [ ] V ×
I
primary nom
M 1 [ ]
------------------------------------------------------- -
=
2 [ ] V ×
I
primary nom
2 [ ] V ×
I
primary nom
M 2 [ ]
------------------------------------------------------- -
=
2 [ ] V ×
I
primary nom
T35 Transformer Management Relay
w
1 2 ...w , ,
, where w
=
total
[ ]
w
setting.
rated
[ ]
1 2 ...w , ,
=
total
2 [ ]
P
rated
2 [ ]
I
----------------------------------- -
,
=
rated
×
3 V
nom
CT primary 2 [ ]
2 [ ]
I ------------------------------------- -
,
=
margin
2 [ ]
I
rated
is winding 2.
ref
!
METERING TRANSFORMER
[ ]
w
1 2 ...w , ,
=
[ ]
w
ref
1 [ ]
×
500 A 220 kV
1.0628
---------------------------------------- -
= =
2 [ ] ×
1500 A 69 kV
2 [ ]
×
1500 A 69 kV
---------------------------------------- - 1.0000
= =
2 [ ] ×
1500 A 69 kV
5 SETTINGS
100 MVA
836.7 A
---------------------------- -
= =
2 [ ] ×
3 69 kV
1500 A
-------------------- - 1.79
= =
836.7 A
!"
DIFFERENTIAL AND RESTRAINT
total
(EQ 5.7)
(EQ 5.8)
(EQ 5.9)
(EQ 5.10)
!"
(EQ 5.11)
(EQ 5.12)
(EQ 5.13)
GE Multilin