Engineer Electricity key terms

At any node of a circuit, the algebraic sum of currents entering equals the sum leaving, because charge cannot accumulate at a point. It underpins nodal analysis and current-balance equations.

Around any closed loop in a circuit, the algebraic sum of voltage rises and drops is zero. It expresses energy conservation per unit charge and forms the basis of mesh analysis.

Any linear two-terminal network can be replaced by one voltage source in series with one resistance, measured at the terminals. It simplifies load calculations and maximum power transfer problems.

Any linear two-terminal network can be replaced by one current source in parallel with one resistance. The equivalent current is the short-circuit current and the resistance equals the Thevenin resistance.

A source delivers maximum power to a load when the load resistance equals the source's Thevenin resistance. Efficiency at that point is only fifty percent, so it suits signal rather than power circuits.

Condition in an AC circuit where inductive and capacitive reactances cancel, leaving impedance purely resistive. Series resonance gives minimum impedance and high current; parallel resonance gives maximum impedance.

Ratio of real power to apparent power in an AC circuit, equal to the cosine of the angle between voltage and current. Low values increase current and losses for the same delivered power.

Power that oscillates between source and reactive elements without doing net work, measured in var. It sustains magnetic fields in motors and transformers and electric fields in capacitors.

Adding capacitors, or active compensators, in parallel with an inductive load to raise power factor toward unity. It reduces supply current, cable losses, and utility reactive-power charges.

Method that resolves an unbalanced three-phase set of phasors into positive, negative, and zero sequence sets. Fault studies and relay settings rely on it for unbalanced conditions.

Tendency of alternating current to concentrate near the surface of a conductor as frequency rises, raising effective resistance. It drives the use of stranded, tubular, and bundled conductors.

Ionisation of air at conductor surfaces where the electric field exceeds breakdown strength, producing hiss, ozone, radio interference, and power loss. It is reduced by larger conductor diameter or bundling.

Power a lossless line delivers when terminated in its characteristic impedance, roughly the natural loading of the line. It indicates how heavily a line can be loaded before voltage problems appear.

Rise of receiving-end voltage above sending-end voltage on a lightly loaded or open long line, caused by capacitive charging current through line inductance. Shunt reactors limit it.

Code such as Dyn11 that states primary and secondary winding connections and the phase displacement between line voltages, in clock notation. It governs paralleling and protection coordination.

Ratio of primary to secondary winding turns, which sets the ideal voltage transformation and the inverse current transformation. It is confirmed on site by a turns-ratio test.

Large transient magnetising current drawn when a transformer is energised, caused by core saturation and offset flux. It decays within cycles to seconds and can trip instantaneous differential protection.

Difference between synchronous speed and actual rotor speed of an induction motor, expressed as a fraction of synchronous speed. It rises with load and must exist for torque production.

Rotational speed of the rotating magnetic field, equal to 120 times frequency divided by pole number. Induction rotors run slightly below it and synchronous machines run exactly at it.

Curve relating induction motor torque to slip, showing starting torque, maximum (breakdown) torque, and the near-linear normal operating region. It explains motor behaviour during starting and overload.

Method of expressing voltages, currents, impedances, and powers as fractions of chosen base values. It simplifies multi-voltage-level calculations and makes transformer ratios disappear from the model.

Allocation of generation among committed units to meet demand at minimum fuel cost while respecting limits and losses. It uses incremental cost curves and equal incremental-cost loading.

Numerical solution for bus voltages, angles, and line power flows in a network under specified generation and load. It checks thermal limits, voltage limits, and losses before operation or expansion.

Capacity of generators already synchronised and ready to pick up load within minutes to cover outages or demand surges. It is a key reliability measure in unit commitment.

Prospective short-circuit current, or the corresponding apparent power, a network can deliver at a point. Switchgear and cable ratings must exceed it for safety.

Scheme that compares currents entering and leaving a protected zone and trips when they differ beyond a threshold. It is fast and selective for transformers, generators, and busbars.

Line protection that measures impedance from voltage and current and trips when the fault lies inside a set reach. Zones with time delays provide selective backup.

Total impedance, expressed in volt-amperes at rated secondary current, connected to a current transformer secondary. Excessive burden causes ratio error and saturation.

Switching technique that varies the width of fixed-frequency pulses to control average output voltage. It shapes inverter and drive output and pushes switching harmonics to higher frequencies.

Converter that rectifies supply, then inverts it at adjustable frequency and voltage to control induction motor speed. It saves energy on pumps and fans but can inject harmonics.

Steady-state currents or voltages at integer multiples of fundamental frequency, produced by non-linear loads and switching converters. They cause extra heating, derating, and protection maloperation.

Controller that sums proportional, integral, and derivative actions on the error signal. Proportional action speeds response, integral removes steady-state offset, and derivative damps overshoot.

Scheme where measured output is fed back and compared with a reference, and the error drives the plant. It reduces sensitivity to disturbance and parameter variation at the cost of stability risk.

Ratio of output to input in the Laplace domain for a linear time-invariant system, with zero initial conditions. Poles and zeros of it reveal stability and dynamic response.

Standard designation, such as 0.2 or 5P10, stating the maximum permissible ratio and phase error over a specified current range. Metering and protection classes serve different purposes.

Classification of supply and exposed-conductive-part earthing arrangements that determines the fault path and required protective device. TN uses a direct metallic return, TT uses local earth electrodes, IT is isolated from earth.

Device that trips when the vector sum of live and neutral currents indicates earth leakage above its rated residual current. It protects people from electric shock and limits fire risk.

Deliberate connection of exposed conductive parts and extraneous conductive parts to a common earth so they sit at nearly the same potential. It removes dangerous touch voltages during faults.

Project control method comparing planned value, earned value, and actual cost to derive schedule and cost performance indices. It forecasts completion cost and flags variances early.

Scheduling technique that identifies the longest sequence of dependent activities, which sets minimum project duration. Delays on that path delay the whole project.

Sum of discounted future cash flows minus initial investment, evaluated at a chosen discount rate. A positive value indicates the project adds value.

Discount rate at which a project's net present value becomes zero. It is compared with the required hurdle rate to accept or reject an investment.