Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

bus impedance matrix

See Z-bus
equivalent impedance
the impedance of the windings of an electromagnetic machine reflected to one side (component) of the machine. For example, in a transformer, the equivalent impedance consists of the combined leakage reactances and resistances of the primary and secondary.
footing impedance
the electrical impedance between a steel tower and distant earth.
impedance
(1) electrical property of a network that measures its ability to conduct electrical AC current for a given AC voltage. Impedance is defined as the ratio of the AC voltage divided by the AC current at a given point in the network. In general, impedance has two parts: a real (resistive) part and an imaginary (inductive or capacitive "reactive") part. Unless the circuit is purely resistive (made up of resistors only), the value of impedance will change with frequency.

(2) in an antenna, usually defined at the input to an antenna, the impedance is the ratio of the applied (or induced) voltage to the current flowing into (or out of) the antenna input. More generally, it is defined as the ratio of the electric field to the magnetic field.
impedance relay
a protective relay that senses the operational impedance at a location, i.e., the ratio of voltage to current at any given time. During fault conditions on the protected line, the impedance relay will sense the impedance (distance in ohms) between the location of the relay and the fault.

Typical impedance relay characteristics are mho and reactance. Impedance relays are widely used in sensing phase faults on transmission lines. Ground impedance relays are available that measure the distance to a single phase to ground fault using a modified technique.
See distance relay
input impedance
(1) when a voltage is applied to a conducting material, a current will flow through the material. The ratio of the voltage to the current is known as the input impedance and is a complex number with magnitude measured in units of ohms. See also Ohm's Law.

(2) impedance seen when looking into the input terminals of an antenna. input layer a layer of neurons in a network
that receives inputs from outside the network. In feedforward networks, the set of weights connected directly to the input neurons is often also referred to as the input layer.
intrinsic impedance
(1) the impedance presented when a source is open-circuited.

(2) a characteristic parameter associated with a medium that is the ratio of the magnitudes of the transverse components of the electric field intensity and magnetic field intensity for a wave propagating in a given direction. It has units of ohms.
line impedance stabilization network (LISN)
a network designed to present a defined impedance at high frequency to a device under test, to filter any existing noise on the power mains, and to provide a 50 Ω impedance to the noise receiver.
negative-sequence impedance
the impedance offered by a circuit when negative-sequence currents alone flow through it, expressed in ohms. The impedance is complex, with its real part being the circuit resistance and imaginary part, which is a function of frequency and inductance referenced as negative-sequence reactance, also expressed in ohms.
output impedance
the ratio of the drop in voltage to the current drawn is known as the output impedance of the electric source and is measured in units of ohms.
percent impedance
the per-unit impedance expressed as a percentage on a certain MVA and voltage base.
subtransient impedance
the series impedance that a generator or motor exhibits during the subtransient period, typically the first few cycles of a fault. Subtransient impedances are generally used in calculating fault currents for determining instantaneous relay settings.
surge impedance
the ratio of voltage to current on that line for a high speed wave propagating down the line. The surge impedance of a line is a constant which depends on the line geometry and conductor characteristics. On power transmission lines, these waves are typically generated by lightning strokes, circuit breaker switching, etc.
See characteristic impedance
surge impedance loading (SIL)
of a transmission line, the characteristic impedance with resistance set to zero (resistance is assumed small compared to reactance). The power that flows in a lossless transmission line terminated in a resistive load equal to the line's surge impedance is denoted as the surge impedance loading of the line.
transient impedance
the series impedance that a generator or motor exhibits following the subtransient period but prior to the steady-state situation.
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