PGCIL Field Engineer & DT key terms

The cosine of the angle between voltage and current in an AC circuit; it equals real power divided by apparent power and indicates how effectively current is converted into useful work.

The portion of AC power that oscillates between source and reactive elements without doing net work, measured in volt-amperes reactive (var) and needed to sustain magnetic fields in inductors and transformers.

The product of RMS voltage and RMS current in an AC circuit, expressed in volt-amperes; it is the vector sum of real power and reactive power.

A method of expressing electrical quantities as fractions of chosen base values, so that transformers of different ratings can be handled uniformly and calculations become independent of voltage level.

A technique that resolves an unbalanced three-phase set of phasors into three balanced sets called positive, negative and zero sequence, enabling fault and unbalance analysis using single-phase equivalent circuits.

The balanced component set whose phase rotation opposes the normal phase order; it appears during unbalanced faults or unbalanced loads and causes rotor heating and torque pulsation in machines.

A steady-state network calculation that determines bus voltage magnitudes and angles and the real and reactive power flows on each branch for specified generation and demand conditions.

A graphical stability test in which the accelerating area of a synchronous machine's power-angle curve must be matched by an available decelerating area for the system to remain transiently stable after a disturbance.

The longest fault duration for which a power system can retain synchronism after the fault is cleared; clearing faster than this value keeps the machine transiently stable.

The differential equation relating rotor angular acceleration of a synchronous machine to the imbalance between mechanical input power and electrical output power, scaled by inertia constant.

A protection relay that estimates the impedance from the relay location to a fault by dividing measured voltage by measured current, then trips when the impedance falls inside a set zone.

A scheme that compares currents entering and leaving a protected zone and trips when their difference exceeds a threshold, giving fast, selective protection for transformers, generators and busbars.

A sensitive earth-fault scheme in which a core-balance or neutral current transformer measures only the residual current inside a defined zone, so faults outside the zone do not operate it.

An instrument transformer that reproduces a large primary current as a proportional, phase-related smaller secondary current for measurement and protection relays, while isolating the secondary circuit from high voltage.

A voltage measuring device that uses a capacitive divider, often with a carrier-access unit, to produce a low secondary voltage for metering and protection and to couple power-line carrier signals.

A switching device capable of making, carrying and interrupting fault currents, with an arc-quenching medium such as SF6, vacuum or air, and a mechanism that reseals the circuit after interruption.

A mechanical switching device that provides a visible isolating gap for safe maintenance but has no rated fault-interrupting capability, so it must be operated only after the circuit breaker has cleared current.

An automatic scheme that re-closes a circuit breaker after a trip to restore service following transient faults, often with single-pole or three-pole operation and a dead time before reclosure.

A logic arrangement that prevents unsafe sequences of switching or operation, for example blocking an isolator from opening while its associated circuit breaker is still carrying load current.

The selection of insulation withstand levels and protective device characteristics so that insulation is protected against overvoltages while cost and clearance are kept reasonable.

The standard lightning-impulse withstand voltage that equipment insulation must tolerate, used as a reference for coordinating insulation and protective margins across a system voltage class.

A protective device that presents high impedance at normal voltage but becomes conducting during overvoltage, diverting surge current to earth and then resealing to protect insulation.

Ionization of air around a conductor when the surface electric field exceeds the breakdown strength of air, causing power loss, audible noise, radio interference and possible insulation damage.

Two or more sub-conductors per phase spaced by small spacers, which reduces corona and inductive reactance while increasing the effective diameter and current-carrying capability of the phase.

Insertion of capacitors in series with a transmission line to cancel part of its inductive reactance, raising power transfer capability and improving voltage profile, with protection against sub-synchronous effects.

The rise of receiving-end voltage above sending-end voltage on a lightly loaded or open long AC line, caused by the line's capacitive charging current flowing through its series inductance.

An inductor connected from line or bus to earth to absorb capacitive reactive power, limiting overvoltage on light load and long lines and aiding voltage control.

The tendency of alternating current to concentrate near the surface of a conductor at higher frequencies, which raises effective resistance and reduces usable cross-section.

The redistribution of alternating current in a conductor caused by the magnetic field of nearby current-carrying conductors, increasing effective resistance in windings and cables.

The change in terminal voltage of a machine or transformer between no load and full load at a stated power factor, usually expressed as a percentage of rated voltage.

The difference between synchronous speed and actual rotor speed of an induction machine, expressed as a fraction of synchronous speed; it determines rotor frequency and developed torque.

A variable-speed drive technique for induction motors that keeps the voltage-to-frequency ratio nearly constant so that magnetic flux and hence torque capability remain approximately unchanged as speed varies.

A switching method that synthesizes a variable-voltage, variable-frequency waveform by rapidly chopping a DC supply and varying the on-time of pulses, shifting harmonics to higher frequencies.

The ratio, usually expressed as a percentage, of the RMS value of all harmonic components to the RMS value of the fundamental component of a voltage or current waveform.

A power-electronic converter that injects compensating currents or voltages to cancel harmonics and correct reactive power, adapting in real time to changing load conditions.

Supervisory control and data acquisition: a system of remote terminal units, communication links and a master station that lets operators monitor and control substation and plant equipment from a control centre.

The maximum prospective short-circuit current or apparent power that a network can deliver at a given point, used to select switchgear ratings and check protection settings.