PE Electrical flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Power triangle

Real power P, reactive power Q, and apparent power S form a right triangle with S = P + jQ, so S squared equals P squared plus Q squared and power factor equals P/S.

Power factor

Power factor is the cosine of the angle between sinusoidal voltage and current, equal to real power divided by apparent power; unity means voltage and current are in phase.

Per-unit system

A normalized calculation method in which actual quantities are divided by base values; for three-phase systems Z_base equals line-to-line base voltage squared divided by base power.

Load factor

The ratio of average load to peak load over a specified period, usually a day or year; it measures how fully equipment capacity is used.

Three-phase real power

For a balanced three-phase load, real power equals the square root of three times line-to-line voltage times line current times power factor.

Transformer turns ratio

The ratio of primary to secondary turns equals the ratio of primary to secondary voltages; current ratio is the inverse, and ideal power is unchanged.

Demand factor

The ratio of a system maximum demand to its total connected load; it is used to size feeders and transformers and is less than or equal to one.

Kirchhoff's current law

Kirchhoff's current law states that the algebraic sum of currents entering a node is zero, so current entering equals current leaving at every instant.

Kirchhoff's voltage law

Kirchhoff's voltage law states that the algebraic sum of voltages around any closed loop is zero; it follows from conservation of energy.

Thevenin equivalent

Any linear two-terminal network can be replaced by a voltage source V_th in series with R_th, where V_th is open-circuit voltage and R_th equals V_oc divided by I_sc.

Maximum power transfer

Maximum power is transferred from a source to a load when load resistance equals Thevenin resistance; the maximum power is V_th squared divided by four R_th.

Series resonance

In a series RLC circuit, resonance occurs at f0 = 1/(2π√(LC)), where inductive and capacitive reactances cancel and impedance is purely resistive and minimum.

RC time constant

For a first-order RC circuit, the time constant τ equals resistance times capacitance; after one τ a charging capacitor reaches about 63.2 percent of final voltage.

RMS value

The RMS value of a sinusoid equals peak value divided by the square root of two; it produces the same heating effect in a resistor as an equivalent DC value.

Synchronous speed

Synchronous speed in rpm equals 120 times frequency in hertz divided by the number of poles; it is the speed of the rotating magnetic field.

Slip

Slip equals synchronous speed minus rotor speed, divided by synchronous speed; it is one at standstill and zero at synchronous speed.

Transformer EMF equation

The RMS induced EMF in a transformer winding is 4.44 times frequency times turns times peak core flux, derived from Faraday's law for sinusoidal flux.

DC machine EMF

The generated armature EMF of a DC machine equals K times flux per pole times angular speed, where K depends on machine winding and pole count.

Induced conductor voltage

A conductor of length l moving at velocity v perpendicular to magnetic flux density B induces voltage e = Blv, with direction given by the right-hand rule.

Torque-power relation

Mechanical torque equals mechanical power divided by angular speed; in SI units, torque in newton-meters equals watts divided by radians per second.

Surge impedance

Surge impedance of a lossless line is the square root of inductance divided by capacitance; for overhead lines it typically ranges from about 250 to 400 ohms.

Surge impedance loading

Surge impedance loading equals line-to-line voltage squared divided by surge impedance; at SIL a lossless line has no net reactive power flow.

Short line model

For a short transmission line, sending-end voltage equals receiving-end voltage plus line series impedance times line current, ignoring shunt capacitance.

Ferranti effect

Ferranti effect is the rise of receiving-end voltage above sending-end voltage on a lightly loaded or open-circuit line due to charging current through series inductance.

Corona

Corona is ionization of air near high-voltage conductors when surface voltage gradient exceeds breakdown strength, causing power loss, audible noise, and radio interference.

Skin effect

Skin effect forces alternating current toward the conductor surface as frequency rises, increasing effective AC resistance compared with DC resistance.

Symmetrical components

Unbalanced three-phase phasors are resolved into positive, negative, and zero-sequence sets; the operator a equals one at 120 degrees.

Three-phase fault current

A bolted three-phase fault current equals the prefault voltage at the fault divided by the total positive-sequence impedance from the fault to the source.

Short-circuit MVA

Short-circuit MVA at a bus equals voltage squared divided by Thevenin impedance in ohms, or base MVA divided by per-unit impedance on the same base.

Load-flow bus types

Load-flow buses are classified as slack, PV, or PQ; the slack bus fixes voltage magnitude and angle, PV buses fix P and V, and PQ buses fix P and Q.

Swing equation

The swing equation relates rotor acceleration to mechanical minus electrical power, inertia constant H, and synchronous speed, and is used in transient stability studies.

Distance relay

A distance relay measures voltage-to-current ratio to estimate line impedance to a fault and trips when the measured impedance falls inside a set zone.