Sel 411L - Page 192

P.3.84
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
87XFMRM
This setting allows the relay to optimize internal scaling of transmitted
currents and to maximize the usage of the communications bandwidth and
accuracy. This setting specifies the lowest ratio between the full load current
of the transformer winding associated with the relay current input terminal,
and the CT primary current of the same current input terminal, among all the
87L zone currents. This setting is normally below 1 as the CTs are typically
rated to carry the full load current without exceeding their nominal ratings,
and is identical in all relays of the same 87L scheme.
Consider the example shown in Figure 3.44. The full load current of the
240 kV wye-connected winding is 200/(sqrt(3) • 240) = 0.4811 kA. The full
load current of the 138 kV delta-connected winding is 200/(sqrt(3) • 138) =
0.8367 kA. The five CTs that bound the differential zone have the primary
currents of (1 kA, 1 kA, 1.2 kA, 1.2 kA, 1 kA). The ratios to consider
therefore are (0.4811/1, 0.4811/1, 0.4811/1.2, 0.4811/1.2, 0.8367/1), with the
minimum value being 0.4811/1.2 or 0.4009. A value for 87XFMRM of 0.4
should be used at Relays 1, 2 and 3 in this example.
6 Zigzag-connected transformer winding, virtual double delta CT connection
7 Wye-connected transformer winding, virtual delta CT connection
8 Zigzag-connected transformer winding, virtual double delta CT connection
9 Wye-connected transformer winding, virtual delta CT connection
10 Zigzag-connected transformer winding, virtual double delta CT connection
11 Wye-connected transformer winding, virtual delta CT connection
12 Zigzag-connected transformer winding, virtual double delta CT connection
Table 3.31 Transformer Winding Compensation Matrices (Sheet 2 of 2)
87CTCnL
Setting
Compensation Matrix Typical Application
1
3
---
–2 1 1
1 –2 1
1 1 –2
1
3
-------
–1 1 0
0 –1 1
1 0 –1
1
3
---
–1 2 –1
–1 –1 2
2 –1 –1
1
3
-------
0 1 –1
–1 0 1
1 –1 0
1
3
---
1 1 –2
–2 1 1
1 –2 1
1
3
-------
1 0 –1
–1 1 0
0 –1 1
1
3
---
2 –1 –1
–1 2 –1
–1 –1 2