PE Electrical Power key terms

Any two-terminal linear network of resistors and sources can be replaced by one voltage source in series with one resistor, found from the open-circuit voltage and the short-circuit current.

In a linear circuit with several independent sources, the response in any branch equals the algebraic sum of the responses produced by each source acting alone with the others deactivated.

The cosine of the angle between sinusoidal voltage and current at a terminal; it equals real power divided by apparent power and indicates how much current does useful work.

The effective value of an alternating waveform, equal to the square root of the mean of the squared instantaneous values; a sine wave's RMS is its peak divided by the square root of two.

A measurement technique for balanced or unbalanced three-wire three-phase circuits in which total real power equals the sum of the two wattmeter readings, whose difference indicates reactive power.

The total impedance, expressed in volt-amperes at rated secondary current, that a current transformer must drive, including connected relays, meters and leads; excessive burden causes ratio and phase errors.

The ratio of primary to secondary voltage of a voltage transformer used to scale high line voltage down to meter and relay levels while preserving phase angle within accuracy limits.

The difference between synchronous speed and actual rotor speed of an induction machine, usually expressed as a fraction or percent of synchronous speed; it must be nonzero to produce torque.

The combined armature leakage reactance and armature-reaction reactance of a synchronous machine, used with terminal voltage and internal emf to compute steady-state current and power.

The angle by which the internal emf of a synchronous machine leads its terminal voltage; it governs the real power transferred and grows as mechanical load increases.

The magnetomotive force produced by current flowing in the armature winding, which distorts or shifts the main field flux and changes the machine's effective terminal voltage.

Plots of synchronous machine armature current against field current at constant load and voltage; they are U-shaped because armature current is minimum at unity power factor.

Power dissipated in laminated magnetic material by alternating flux, comprising hysteresis loss, which depends on frequency and flux density, and eddy-current loss, which rises with frequency squared.

The voltage, as a percentage of rated voltage, needed to circulate rated current through a short-circuited secondary winding; it sets fault current magnitude and voltage drop.

The process by which current in a coil is reversed as the coil passes the neutral axis during rotation, requiring brushes and interpoles to suppress sparking at the commutator.

The rise in receiving-end voltage above sending-end voltage that occurs on lightly loaded or open long high-voltage alternating-current lines because charging current flows through the line inductance.

The ratio of voltage to current for a wave traveling on a lossless line, equal to the square root of series inductance divided by shunt capacitance, typically several hundred ohms.

Ionization of air around a conductor when the surface voltage gradient exceeds the breakdown strength of air, producing audible noise, radio interference, light and power loss.

Two or more subconductors per phase spaced by small spacers; the arrangement reduces reactance, raises surge impedance loading, and lowers surface voltage gradient and corona loss.

Two-port network constants relating sending-end voltage and current to receiving-end quantities for a transmission line, convenient for cascading series, shunt and line sections.

The tendency of alternating current to concentrate near the outer surface of a conductor, raising effective resistance and reducing internal inductance as frequency increases.

A scaling scheme in which voltages, currents, powers and impedances are divided by chosen base values, so transformer turns ratios disappear and calculations across voltage levels simplify.

A transformation resolving an unbalanced set of three phasors into positive-, negative- and zero-sequence sets, enabling unbalanced fault and load analysis with decoupled sequence networks.

A numerical solution for steady-state voltages, angles, real power and reactive power at all buses of a network given loads, generation and network impedances.

A square complex matrix whose diagonal entries sum admittances connected to each bus and whose off-diagonal entries are the negative admittances of branches joining buses.

A differential equation equating accelerating torque to the product of inertia and rotor angular acceleration, used to study transient rotor-angle behavior after disturbances.

The path taken by currents that are equal in magnitude and phase in all three conductors, determined by grounding and transformer winding connections; it dominates single-line-to-ground fault currents.

The minimum current, voltage or other quantity at which a protective relay begins to operate; it is set above normal load to avoid false tripping.

The process of selecting relay settings so that the device nearest a fault operates first and upstream devices wait, using published time-current curves and coordination time intervals.

A scheme that compares currents entering and leaving a protected zone and trips when the difference exceeds a threshold, covering transformers, buses and generators with good sensitivity.

Settings based on measured impedance to a fault; a typical scheme uses three forward zones covering roughly 80 percent, 120 percent and 100 percent plus backup of adjacent lines, with time delays.

The condition in which the iron core of a current transformer cannot support further flux, so the secondary current no longer faithfully reproduces the primary, producing distorted relay inputs and possible misoperation.

The delay, measured in electrical degrees from the zero crossing of the supply voltage, at which a thyristor is turned on; it controls the average output of a phase-controlled converter.

A switching technique that controls the average output of a converter by varying the on-time duty ratio of fixed-frequency pulses, shifting harmonics to higher, easily filtered frequencies.

The ratio of the root-sum-square of harmonic components to the fundamental component of a waveform, expressed as a percentage, indicating distortion from nonlinear loads or converters.

A diode placed across an inductive load or switch to provide a path for stored current when the main switch turns off, protecting components and improving efficiency.