GL.6
SEL-751 Relay Instruction Manual Date Code 20170927
Glossary
Phase Rotation—Residual Current
Phase Rotation
The sequence of voltage or current phasors in a multi-phase electrical system.
In an ABC phase rotation system, the B-phase voltage lags the A-phase
voltage by 120 degrees, and the C-phase voltage lags B-phase voltage by 120
degrees. In an ACB phase rotation system, the C-phase voltage lags the A-
phase voltage by 120°, and the B-phase voltage lags the C-phase voltage by
120 degrees.
Pickup Time
The time measured from the application of an input signal until the output
signal asserts. The time can be settable, as in the case of a logic variable timer,
or can be a result of the characteristics of an element algorithm, as in the case
of an overcurrent element pickup time.
Pinout
The definition or assignment of each electrical connection at an interface.
Typically refers to a cable, connector, or jumper.
Power, P
Real part of the complex power (S) expressed in units of Watts (W), kilowatts
(kW), or megawatts (MW).
Power Factor
The cosine of the angle by which phase current lags phase voltage in an ac
electrical circuit. Power factor equals 1.0 for power flowing to a resistive load.
Power, Q
Reactive part of the complex power (S) expressed in units of Vars (W),
kilovars (kVar), or megavars (MVar).
Protection and Control
Processing
Processing interval is four times per power system cycle (except for math
variables and analog quantities, which are processed every 25 ms).
PT
Abbreviation for potential transformer. Also referred to as a voltage
transformer or VT.
RAM
Abbreviation for random-access memory. Volatile memory where the relay
stores intermediate calculation results, Relay Word bits, and other data that are
updated every processing interval.
Rate-of-Change-of-
Frequency Element
A protection element that causes the relay to trip when the measured electrical
system rate-of-change of frequency exceeds a settable rate.
Relay Word
The collection of relay element and logic results. Each element or result is
represented by a unique identifier, known as a Relay Word bit.
Relay Word Bit
A single relay element or logic result that the relay updates once each
processing interval. A Relay Word bit can be equal to either logical 1 or
logical 0. Logical 1 represents a true logic condition, picked up element, or
asserted contact input or contact output. Logical 0 represents a false logic
condition, dropped out element, or deasserted contact input or contact output.
You can use Relay Word bits in SEL
OGIC control equations to control relay
tripping, event triggering, and output contacts, as well as other functions.
Remote Bit
A Relay Word bit for which state is controlled by serial port commands,
including the CONTROL command, binary Fast Operate command, or
Modbus
command.
Residual Current
The sum of the measured phase currents. In normal, balanced operation, this
current is very small or zero. When a ground fault occurs, this current can be
large.
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