5.13
Date Code 20150130 Instruction Manual SEL-787 Relay
Metering and Monitoring
Through-Fault Event Monitoring
Through-Fault Event Monitoring
Figure 5.16 shows a fault that occurs outside the area of unit protection of the
transformer. Such through faults can last for several cycles, subjecting the
transformer windings to mechanical stress and the transformer winding
insulation to thermal stress. Monitor and document this through-fault activity
with the through-fault element in the SEL-787. The through-fault element also
calculates the cumulative mechanical stress on the transformer windings.
Figure 5.16 Transformer Bank Subjected to Through Fault
Figure 5.17 shows through-fault curves for Category IV transformers as
published in IEEE Standard C57.109-1993. These curves apply to
transformers that are covered by the IEEE standard or, in general, to
transformers that were built beginning in the early 1970s. For transformers
built prior to 1970, consult the manufacturer to obtain the transformer short-
circuit withstand capabilities.
Figure 5.17 Category IV Transformers Through-Fault Protection Curves
The curves in Figure 5.17 are a function of the transformer short-circuit
impedance, and is keyed to the maximum I
2
t of the worst-case mechanical
duty (maximum fault current for 2 seconds). Equation 5.1 through
Source
HV CT
HV
Breaker
Transformer
LV
Breaker
LV CT
Fault
% Transformer Impedance
Time (seconds)
2000
1000
500
200
100
50
20
10
5
2
1
0.5
0.2
0.1
Times Normal Base Current
21055020
12 10 8 7 6 5 4
For fault current from 50% to 100% of
maximum: I
2
t = K
where:
I = symmetrical fault current times
normal base current
K = constant determined at maximum
I with t = 2 seconds
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