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2-28
Specifications
Date Code 20000421
SEL-251, -2, -3 Instruction Manual
Figure 2.11: Distribution Transformer Bank Protected by High-Side Fuses
What Happens When a High-Side Fuse Blows?
When a transformer high-side fuse operates, the low-side phase-to-phase voltage magnitudes
drop. One phase-to-phase voltage magnitude goes to zero, and the remaining two drop to 0.87
per unit of nominal voltage. If two high-side fuses operate, the low-side phase-to-phase voltages
all go to zero.
If a VT secondary fuse blows while the transformer bank is otherwise operating normally, two of
the phase-to-phase voltages presented to the relay drop to 0.58 per unit of nominal. If two VT
secondary fuses operate, one phase-to-phase voltage measured by the relay goes to zero while the
other two drop to 0.58 per unit.
With these facts in mind, the logic described below is easy to understand.
How Does the SEL-251 Relay Detect Transformer Fuse Operations?
When the relay setting 27C = 4, the
27
phase-to-phase undervoltage logic detects high-side fuse
operations. To use the relay undervoltage logic in this application, make the following relay set-
ting calculations:
27L = 0.40 • Vnom
27H = 0.72 • Vnom
27C = 4
where:
Vnom =
Nominal Phase-to-Phase Voltage, V secondary