PE Electrical Power flashcards

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

Per-unit impedance base

Z_base equals base voltage squared divided by base voltampere. Divide actual ohms by Z_base to get per-unit, which is unchanged across an ideal transformer regardless of side.

Balanced three-phase power

Total apparent power equals square root of three times line-to-line voltage times line current. Real power multiplies that product by the cosine of the power factor angle.

Series resonance

A series RLC circuit resonates when inductive and capacitive reactances are equal. Impedance becomes purely resistive, current is limited only by resistance, and reactive voltages can exceed the source.

Two-wattmeter method

On a balanced three-phase load, the two wattmeter readings add to give total real power, and root three times their difference gives total reactive power.

Current transformer open secondary

Opening a current transformer secondary while primary current flows removes the counter-flux, producing very high secondary voltage. Short the secondary before changing connected burdens.

Synchronous speed

The rotating field speed in revolutions per minute equals one hundred twenty times frequency divided by pole count. A sixty-hertz four-pole machine therefore runs at eighteen hundred rpm.

Slip

Slip is the difference between synchronous speed and rotor speed divided by synchronous speed. Rotor frequency equals slip times stator frequency, and rotor reactance falls with low slip.

Induction motor torque

Developed torque varies approximately with the square of terminal voltage. A ten percent voltage reduction therefore cuts torque about nineteen percent while lengthening acceleration and raising rotor heating.

Power angle equation

For a round-rotor synchronous machine, real power approximately equals internal voltage times terminal voltage divided by synchronous reactance, all multiplied by sine of the power angle.

Transformer impedance referral

Impedance on the secondary referred to the primary is multiplied by the square of the turns ratio. The per-unit impedance value is identical on both sides of the transformer.

Surge impedance

The characteristic impedance of a lossless overhead line is the square root of inductance divided by capacitance, typically a few hundred ohms. Cables have much lower surge impedance.

Ferranti effect

On a long, lightly loaded or open transmission line, receiving-end voltage exceeds sending-end voltage because charging current through series inductance adds voltage along the line.

Voltage regulation

Voltage regulation is the no-load receiving voltage minus full-load receiving voltage, divided by full-load receiving voltage, expressed as a percentage at a stated load power factor.

Bundle conductors

Two or more subconductors per phase reduce series inductive reactance and corona loss, and increase the effective geometric radius compared with a single conductor of the same total area.

Corona

Ionization of air at conductor surfaces above a threshold voltage gradient causes power loss, audible noise, and radio interference. Larger conductors and bundled phases raise the inception voltage.

Symmetrical components

Any unbalanced three-phase set separates into positive, negative, and zero sequence sets. The operator a is one at one hundred twenty degrees, and a squared is at two hundred forty degrees.

Zero-sequence path

Zero-sequence current needs a grounded return path. It flows through grounded-wye windings and grounded neutrals but cannot pass through an ungrounded delta or ungrounded neutral.

Per-unit base change

Converting per-unit impedance to a new base multiplies the old value by the old-to-new base voltage ratio squared and by the new-to-old base power ratio.

Swing equation

Rotor angle accelerates in proportion to the difference between mechanical and electrical power. The inertia constant and synchronous speed determine how rapidly the angle changes.

Equal-area criterion

Transient stability is assessed by comparing the accelerating area with the available decelerating area. Stability holds when decelerating area equals or exceeds the accelerating area.

Line-to-ground fault current

Single line-to-ground fault current equals three times the prefault positive-sequence voltage divided by the sum of positive, negative, and zero sequence impedances plus three times ground impedance.

Distance zone one reach

Zone one typically reaches eighty to eighty-five percent of the protected line with no intentional time delay, deliberately underreaching so it never operates for faults beyond the remote terminal.

Differential protection

Differential relaying compares currents entering and leaving the protected zone. Equal through-fault currents balance and produce no operation, while an internal fault creates a differential current.

Overcurrent coordination

Protective devices are graded so the device nearest a fault trips first. Time-current curves, pickup multiples, and the coordination time interval establish the margin between successive devices.

Loss-of-field relay

This element detects an under-excited or open-field generator condition, usually through impedance or admittance measurement, and trips to prevent machine overheating and system voltage collapse.

Reverse power relay

Applied to generators to detect motoring, when real power flows from the system into the machine. It typically operates after loss of the prime mover or turbine trip.

CT saturation effect

When a current transformer saturates, its secondary no longer reproduces the primary current, delaying relay pickup and potentially causing overreach or miscoordination between protective zones.

Controlled rectifier output

A single-phase full converter with firing angle alpha delivers an average direct voltage of twice the peak input voltage divided by pi, multiplied by cosine of alpha.

Buck converter duty ratio

An ideal buck converter produces an output voltage equal to duty ratio times input voltage. The switch conducts for fraction D of each switching period in continuous conduction.

Boost converter

An ideal boost converter steps voltage up by a factor of one divided by one minus the duty ratio, assuming continuous inductor current and ideal lossless components.

Twelve-pulse rectifier

Two six-pulse bridges fed from delta and wye transformer secondaries produce twelve-pulse operation, cancelling the fifth and seventh harmonic currents on the alternating-current side.

HVDC converter voltage

The direct voltage of a line-commutated converter equals its ideal no-load value times cosine of the firing angle, minus the drop caused by commutation reactance.