Schweitzer Engineering Laboratories SEL-311L, Instruction Manual, Page: 127 / 734

Date Code 20011112 Line Current Differential, Distance, Out-of-Step, Overcurrent, 3-73
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
Figure 3.52: Channel VS Voltage Elements
Voltage Element Operation
Note that the voltage elements in Table 3.12 and Figure 3.50 through Figure 3.52 are a
combination of “undervoltage” (Device 27) and “overvoltage” (Device 59) type elements.
Undervoltage elements (Device 27) assert when the operating voltage goes below the
corresponding pickup setting. Overvoltage elements (Device 59) assert when the operating
voltage goes above the corresponding pickup setting.
Undervoltage Element Operation Example
Refer to Figure 3.50 (top of the figure).
Pickup setting 27P is compared to the magnitudes of the individual phase voltages V
A
, V
B
, and
V
C
. The logic outputs in Figure 3.50 are the following Relay Word bits:
27A = 1 (logical 1), if V
A
< pickup setting 27P
= 0 (logical 0), if V
A
≥
pickup setting 27P
27B = 1 (logical 1), if V
B
< pickup setting 27P
= 0 (logical 0), if V
B
≥
pickup setting 27P
27C = 1 (logical 1), if V
C
< pickup setting 27P
= 0 (logical 0), if V
C
≥
pickup setting 27P
3P27 = 1 (logical 1), if all three Relay Word bits 27A, 27B, and 27C are asserted (27A =
1, 27B = 1, and 27C = 1)
= 0 (logical 0), if at least one of the Relay Word bits 27A, 27B, or 27C is deasserted
(e.g., 27A = 0)
Overvoltage Element Operation Example
Refer to Figure 3.50 (bottom of the figure).
Pickup setting 59P is compared to the magnitudes of the individual phase voltages V
A
, V
B
, and
V
C
. The logic outputs in Figure 3.50 are the following Relay Word bits:
59A = 1 (logical 1), if V
A
> pickup setting 59P
= 0 (logical 0), if V
A
≤
pickup setting 59P