Registered Electrical Engineer (G&T) key terms

A method of expressing voltages, currents, impedances and powers as fractions of chosen base values, so transformers and machines of different ratings can be combined in one network calculation without ratio conversions.

A technique that resolves an unbalanced three-phase set of phasors into positive-, negative- and zero-sequence balanced sets, allowing unbalanced faults and load flows to be solved with per-phase impedance networks.

The impedance offered to the zero-sequence component of current, which depends strongly on transformer winding connections and grounding, and which governs earth-fault current magnitude.

The apparent power that a source or network can deliver into a bolted three-phase fault, calculated as fault current times prefault voltage times root three; used to size switchgear and compare source strength.

The current that would flow at a point in an installation if a short circuit of negligible impedance occurred there, including contributions from all parallel paths and rotating machines as required by the standard.

The maximum prospective fault current a switching device can safely interrupt at a stated voltage without damage or danger, distinguishing it from making capacity and short-time withstand ratings.

Coordination of protective devices so that the device nearest a fault operates first, limiting supply interruption to the smallest healthy section of the network.

The total impedance of the path taken by fault current from the point of fault back to the source through the protective earth conductor and the earthed neutral, used to verify automatic disconnection of supply.

Deliberate connection of a system neutral to earth, either solidly, through resistance or reactance, or via a transformer, which fixes the earth-fault current level and insulation duty.

A device that compares currents in live conductors and trips when their vector sum indicates earth leakage above a set threshold, giving protection additional to overcurrent devices.

Intentional electrical connection of exposed conductive parts and extraneous conductive parts to keep them at substantially the same potential, reducing touch voltage during faults.

Potential differences appearing across a person's feet, or between hand and feet, near an earthed structure carrying fault current; substation earthing design must keep both within tolerable limits.

The resistance between a substation earthing grid and remote earth, measured by fall-of-potential or similar methods, determining how much the grid potential rises during an earth fault.

The ratio of voltage to current for a wave travelling along a lossless line, equal to the square root of inductance per unit length divided by capacitance per unit length, typically 250 to 400 ohms for overhead lines.

The power a lossless line delivers when its receiving-end load equals its surge impedance, a convenient reference for comparing line loading and reactive power behaviour.

Rise of receiving-end voltage above sending-end voltage on a lightly loaded or open-circuited line, caused by the line's shunt capacitance charging current flowing through its series inductance.

An inductive device connected from line or bus to earth to absorb capacitive reactive power, controlling voltage rise on long lightly loaded cables and lines and limiting switching overvoltages.

A thyristor-controlled shunt device that continuously adjusts reactive power injection or absorption to regulate voltage and damp power oscillations on transmission networks.

The ability of synchronous machines in an interconnected system to remain in synchronism after a disturbance, classified by rotor-angle, frequency and voltage stability phenomena.

The longest fault clearing time for which a generator or system remains transiently stable after a disturbance, a key output of transient stability studies used to set protection speeds.

A graphical method for judging transient stability of a single machine against an infinite bus by comparing accelerating and decelerating areas on the power-angle curve.

Numerical solution for steady-state bus voltages, line flows and losses in a network given generation, load and one slack bus, usually by Newton-Raphson or Gauss-Seidel iteration.

Allocation of real power output among committed generating units to meet demand at least cost, conventionally by equal incremental fuel cost subject to losses and limits.

Unloaded generation capacity that is already synchronised and can pick up load within minutes, required to cover sudden loss of a unit or load forecast error.

Automatic adjustment of generator setpoints in response to frequency deviation and tie-line power error, restoring balance between system generation and demand.

The steady-state relationship between frequency and generator output, expressed as percent speed change for full load change, which determines how load is shared between parallel machines.

The process of matching voltage magnitude, frequency, phase angle and phase sequence before closing a generator or tie breaker onto an energised bus to avoid damaging currents.

A designation of transformer winding connections and phase displacement between high- and low-voltage windings, which must be matched for parallel operation and affects earth-fault behaviour.

The condition in which a current transformer core cannot maintain flux proportional to primary current, distorting secondary current and potentially causing protection maloperation or refusal.

A scheme that trips when the vector difference between currents entering and leaving a protected zone exceeds a threshold, providing fast unit protection for transformers, generators and busbars.

Line protection that measures the impedance from the relay location to a fault and trips within set reach zones, giving fast clearing for in-zone faults and time-graded backup beyond.

A voltage and current disturbance propagating along a line at near light speed, reflected at discontinuities, which explains lightning and switching overvoltage behaviour and relay fault location methods.

A protective device that becomes conducting above a specified voltage and diverts surge energy to earth, then reseals, limiting overvoltage on equipment insulation.

The standard lightning impulse withstand voltage a piece of equipment insulation is designed to tolerate, used to coordinate insulation strength with protective device levels.

Selection of insulation withstand strengths and protective device characteristics so that protective levels lie below equipment withstand levels with adequate margin across the system.

The written standards of conduct binding a licensed engineer, covering competence, honesty in reports, avoidance of conflicts of interest, confidentiality and duty to public safety.

A situation where an engineer's personal, financial or other interest could improperly influence professional judgement; it must be disclosed and avoided or managed.

The licensed engineer's formal authentication of drawings and documents, accepting professional responsibility for their technical content and compliance with regulations.

Failure to exercise the degree of care and skill ordinarily expected of a competent engineer in similar circumstances, giving rise to liability for resulting loss.

A documented instruction changing the scope, schedule or price of contracted works, issued under the contract's variation clause and forming part of the agreed record.

Schedule technique identifying the longest chain of dependent activities, whose delay directly delays project completion and which therefore receives the tightest management attention.

Project control method comparing budgeted cost of work performed against actual cost and planned value to forecast cost and schedule performance objectively.