Schweitzer Engineering Laboratories SEL-421-4 - Figure 5.14 Frequency Element Logic

5.20
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Over- and Underfrequency Elements
Each frequency element can operate as an overfrequency or as an underfrequency
element, depending on its pickup setting. If the element pickup setting (81DnP,
n = 1–6) is less than the nominal system frequency setting, NFREQ, the element
operates as an underfrequency element, picking up if measured frequency is less
than the set point. If the pickup setting is greater than NFREQ, the element oper-
ates as an overfrequency element, picking up if measured frequency is greater
than the set point.
Note that Relay Word bit 27B81 controls all six frequency elements. This under-
voltage supervision control prevents erroneous frequency element operations
during system faults.
Over- and Underfrequency Element Settings
E81 (Enable 81 Elements)
Set E81 to enable as many as six over- and underfrequency elements. When
E81 = N, the relay disables the frequency elements and hides corresponding set-
tings; you do not need to enter these hidden settings.
81UVSP (81 Element Undervoltage Supervision)
NOTE: See Undervoltage
Supervision Logic on page 5.17
for a
discussion on the 81UVSP setting.
This setting applies to all six frequency elements. If the instantaneous alpha volt-
age falls below the 81UVSP setting, all frequency elements are disabled.
Figure 5.14 Frequency Element Logic
+
FREQP
81DnP
+
81DnP NFREQ
81DnP NFREQ
81Dn
81DnT
81DnOVR
81DnUDR
Frequency Timer n
FREQOK
81DnD
0
0
1 cyc
FREQFZ
27B81
Setting Prompt Range Default Category
E81 Enable Frequency Elements N, 1–6 N Group
Setting Prompt Range Default Category
81UVSP 81 Element Under Voltage Super 20.00–200 V, sec 85 Group