PE Electrical key terms

The ratio of real (average) power to apparent power in an AC circuit; for sinusoidal quantities it equals the cosine of the angle between voltage and current. Loads with low power factor draw more current for the same useful work.

The average load over a stated period divided by the peak load occurring in that period. A high load factor means equipment is used more evenly and fixed costs are spread over more energy sales.

The maximum demand of an installation divided by its total connected load, always one or less. It recognizes that not all connected equipment operates at the same time.

The sum of the individual maximum demands of two or more loads divided by the coincident maximum demand of the combined system; a value of one or greater.

The maximum continuous current a conductor can carry under specified installation conditions without exceeding its rated temperature, accounting for insulation type, ambient temperature, and the number of current-carrying conductors.

The reduction in voltage between the source and the load caused by current flowing through the impedance of conductors. Design limits usually cap total feeder plus branch-circuit drop at a small percentage of nominal voltage.

The maximum short-circuit current, at a stated voltage, that a protective device such as a circuit breaker or fuse can safely clear without damage or hazard to personnel.

Odd multiples of the third harmonic, such as the third, ninth, and fifteenth, that are in phase in all three phases and therefore add arithmetically in the neutral of a four-wire system.

A complex number that represents the magnitude and phase angle of a sinusoidal quantity at a single frequency, allowing steady-state AC circuits to be solved with ordinary complex arithmetic.

The total opposition to sinusoidal current flow, a complex quantity whose real part is resistance and whose imaginary part is reactance. It is the ratio of phasor voltage to phasor current.

The product of phasor voltage and the conjugate of phasor current, expressed in volt-amperes. Its real part is average power in watts and its imaginary part is reactive power in var.

A two-terminal network replaced by a single voltage source in series with an impedance, valid at the terminals for any external connection. The source is the open-circuit voltage and the impedance is seen looking back with sources dead.

A method for linear circuits in which the response to several independent sources is found by adding the responses computed with each source acting alone, with other voltage sources shorted and current sources opened.

The condition in an RLC circuit where inductive and capacitive reactances cancel, leaving only resistance, so current and voltage are in phase and impedance magnitude is minimum in series or maximum in parallel.

The interval after which a first-order RC or RL circuit's transient response changes by about 63 percent of the way toward its final value; roughly five time constants are treated as steady state.

The difference between synchronous speed and actual rotor speed of an induction machine, usually expressed as a fraction or percentage of synchronous speed. Slip is zero at synchronous speed and one at standstill.

The rotational speed of the rotating magnetic field produced by a polyphase winding, equal to 120 times the frequency divided by the number of poles.

The magnetomotive force produced by current flowing in the armature winding, which distorts or opposes the main field flux and shifts the neutral axis in a DC machine.

The direct current supplied to a synchronous machine's field winding, which sets the internal generated voltage and therefore controls reactive power output and terminal voltage.

The effective reactance of a synchronous machine in steady state, combining armature leakage reactance and the reactance representing armature reaction; it relates internal voltage to terminal voltage and current.

The current an induction motor draws when its rotor is held stationary and rated voltage is applied; typically five to eight times rated current and used for starting and protection studies.

The maximum torque an induction motor can develop without stalling, occurring at a slip greater than the slip at full load. Loads demanding more than this torque cause the motor to stall.

The voltage, as a percentage of rated voltage, that must be applied to one winding with the other short-circuited to circulate rated current; it sets short-circuit current magnitude and voltage regulation.

Power dissipated in the magnetic core of a transformer or machine by hysteresis and eddy currents; it depends on flux density and frequency and is essentially constant from no load to full load.

Aluminum conductor steel reinforced: a stranded overhead conductor whose outer aluminum strands carry current while a central steel core provides tensile strength for long spans.

The vertical distance between a conductor's support points and its lowest point; it depends on span, conductor weight, tension, and temperature and must keep clearances safe while limiting tension.

Ionization of air around a conductor when the surface voltage gradient exceeds the breakdown strength of air, causing power loss, radio noise, and audible hum, especially at high altitude or in rain.

Two or more subconductors per phase spaced a small distance apart. Bundling reduces corona loss and inductive reactance and increases line capacitance compared with a single conductor.

The rise in receiving-end voltage above sending-end voltage on a long, lightly loaded AC line, caused by the line's shunt capacitance charging current flowing through series inductance.

Basic impulse insulation level: the crest voltage a piece of equipment must withstand under a standard lightning impulse wave without flashover or breakdown; used to coordinate insulation with protective devices.

A protective device that presents high impedance at normal voltage but conducts surge current to ground above a threshold, then reseals, limiting overvoltage on equipment insulation.

A scaling method in which voltages, currents, powers, and impedances are divided by chosen base values so that transformer turns ratios disappear and results are dimensionless and easy to compare.

A technique that resolves unbalanced three-phase phasors into three balanced sets - positive, negative, and zero sequence - so unbalanced faults and loads can be analyzed with per-phase networks.

The positive-, negative-, or zero-sequence equivalent circuit of the system, interconnected according to fault type to compute sequence currents and voltages at an unbalanced fault.

A square matrix whose diagonal entries are the sum of admittances connected to each bus and whose off-diagonal entries are the negatives of the admittances between buses; the basis of load-flow and short-circuit calculations.

The iterative solution of network equations that finds bus voltage magnitudes and angles and the resulting real and reactive power flows for specified generation and load conditions.

The reference bus in a load flow, with voltage magnitude and angle fixed, that supplies whatever real and reactive power balances system losses against scheduled generation.

A graphical stability test in which the accelerating area under a synchronous machine's power-angle curve must not exceed the maximum decelerating area available after a disturbance for the system to remain stable.

A protective relay that measures the ratio of voltage to current at its location to estimate the impedance to a fault, then trips if that impedance falls inside a set zone.

The additional cost to produce one more megawatt from a generating unit, equal to the derivative of its cost curve; economic dispatch equalizes incremental costs among committed units within their limits.