The protection engineer's cheat sheet

P / G / N

Phase, ground (residual or ground CT), neutral

Q

Negative-sequence quantity (e.g. 50Q, 32Q, 67Q)

V

Voltage-polarised (e.g. 32V zero-sequence impedance directional). Vendor nomenclature (SEL); other vendors name these elements differently

B / T / L / M

Bus, transformer, line, motor/machine zone

BF

Breaker failure: a suffix, not a base device (50BF, 62BF)

U

Unrestrained (87U high-set)

F / R

Forward / reverse looking (50QF, 50QR). Vendor nomenclature (SEL), not IEEE C37.2

NERC status checked October 2026. Standards are revised regularly and NERC requirements phase in on their own dates (BES and non-BES IBRs differ), so always confirm the current edition and the implementation plan dates that apply to your facility.

IBR

Inverter-based resource: solar PV, BESS and Type 3/4 wind, interfacing through power electronics.

POI / RPA

Point of interconnection; reference point of applicability where IEEE 2800 performance is measured.

Collector system

The MV (typically 34.5 kV) cable network gathering power from pad-mount transformers to the substation bus.

MPT / GSU

Main power transformer / generator step-up transformer between the collector and the transmission voltage.

COG

Coefficient of grounding: the highest sound-phase L–G voltage during a ground fault divided by the L–L voltage.

TOV

Temporary overvoltage: power-frequency overvoltage lasting cycles to seconds (e.g. during ground faults or islanding).

Weak infeed

A line terminal whose fault contribution is too small or unusual to operate conventional elements.

POTT / DCB

Permissive overreaching transfer trip / directional comparison blocking pilot schemes.

DTT

Direct transfer trip: an unsupervised trip command sent to a remote breaker.

CFIDS / VFIDS

Current- and voltage-based fault identification (faulted-phase selection) logic. CFIDS is SEL nomenclature; "VFIDS" is this course's label for the voltage-based equivalent.

Grid-forming (GFM)

An inverter controlled as a voltage source behind impedance (within its current limit), unlike the more common grid-following (current-source) IBR.

Grid-following (GFL)

An inverter that locks to the grid with a PLL and injects commanded current; most PV and BESS today.

Ferroresonance

Sustained overvoltage when VT magnetizing inductance resonates with system capacitance to ground; damped on broken-delta VTs with a loading resistor.

IT system

An LV system whose star point is not grounded; many utility-scale inverters require it and use an insulation monitor to detect the first ground fault.

PLL

Phase-locked loop: the inverter's estimate of grid voltage angle and frequency.

Momentary cessation

The inverter blocks current output during very low voltage without disconnecting.

CTI

Coordination time interval: the margin between upstream and downstream device operating times.

Remanence

Flux left in a CT core after a fault or DC test, which reduces headroom before saturation.