Protection and Relays
What ANSI device function is used for transformer differential protection?
87T (Transformer Differential)
What does PI (Polarization Index) represent in insulation resistance testing?
The ratio of 10-minute IR to 1-minute IR, used to indicate insulation condition (e.g., moisture/contamination effects and polarization behavior)
What is the field verification step called that confirms equipment is de-energized before applying grounds?
Test-before-touch / prove dead (verify absence of voltage)
A relay pickup is set to 0.30 A secondary on a 600:1 CT. What is the primary pickup current?
180 A
In transformer DGA, which key gas is most indicative of high-energy arcing?
Acetylene (C₂H₂)
A 30 MVA, 34.5 kV transformer has Z = 8% on its own base. Ignoring other sources, estimate the 3-phase symmetrical fault current at the 34.5 kV bus.
~6.275 kA
A biased differential uses:
Iop = |I1 - I2|; Ires = (|I1| + |I2|)/2
Trip criterion: Iop > slope * Ires
Assuming above minimum pickup, at what slope will the relay trip if I1 is 1.2 A and I2 is 0.8 A?
Slope < 0.4
In PF / tan-delta testing, which is typically a bigger concern: a single “acceptable” value, or trend behavior over time? Explain briefly.
Trend behavior (rate of change vs. baseline) is often more concerning; a deteriorating trend can indicate progressive insulation degradation even if the absolute value remains within limits.
A 30 MVA, 34.5 kV transformer has Z = 8%, estimate the available 3-phase fault level (MVA) at the 34.5kV bus (transformer-limited).
375 MVA