TSSPDCL Sub-Engineer 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. Used to write nodal equations for network analysis.

Around any closed loop of a circuit, the algebraic sum of all voltage drops and rises is zero. It follows from energy conservation and underlies mesh analysis.

Any linear two-terminal network can be replaced by a single voltage source in series with a single resistance. The source equals the open-circuit voltage and the resistance equals that seen with sources suppressed.

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

Condition in an AC circuit where inductive and capacitive reactances cancel, making the impedance purely resistive. In series resonance the current peaks at the resonant frequency.

Ratio of real power to apparent power in an AC circuit, equal to the cosine of the angle between voltage and current. A low value means more current for the same useful power.

Power dissipated as circulating currents induced in the magnetic core of a machine. It rises with the square of frequency and flux density and is reduced by laminating the core.

Difference between synchronous speed and actual rotor speed of an induction motor, usually expressed as a fraction or percentage of synchronous speed. It must be nonzero for torque production.

Speed of the rotating magnetic field in an AC machine, equal to 120 times frequency divided by the number of poles. The rotor of a synchronous machine turns at exactly this speed.

Voltage induced in the armature of a DC motor that opposes the applied supply voltage. It limits armature current and is proportional to speed and flux.

Distortion of the main magnetic field caused by current flowing in the armature winding. It shifts the neutral axis and can be corrected with interpoles or compensating windings.

Process in a DC machine during which a coil is short-circuited by the brush and its current reverses direction. Poor commutation causes sparking at the brush surface.

Laminated magnetic path that links the primary and secondary windings and carries the mutual flux. Laminations and silicon steel reduce eddy current and hysteresis losses.

Tendency of alternating current to concentrate near the surface of a conductor, raising its effective resistance. It becomes significant at high frequency and in large conductors.

Rise in voltage at the receiving end of a long lightly loaded transmission line, caused by the line charging capacitance. It is corrected by shunt reactors.

Ionisation of air around a high-voltage conductor when the surface voltage gradient exceeds the breakdown value. It causes power loss, audible noise and radio interference.

Power delivered by a lossless line when its load impedance equals its characteristic impedance, at which the receiving voltage equals the sending voltage. Often used to compare line loading.

Protection relay that measures line impedance from voltage and current and trips when the fault lies within a set reach. Zones of decreasing reach provide backup.

Scheme that compares currents entering and leaving a protected zone and trips when the difference exceeds a setting. It is fast and selective for transformers, generators and busbars.

Instrument transformer that steps down primary current to a standard low value for relays and meters. Its secondary must never be left open-circuited while primary current flows.

Instrument transformer that steps high voltage down to a low, safe value for meters and relays. Its secondary must not be short-circuited during operation.

Fraction of each switching period for which a power electronic switch remains on. Varying it controls the average output voltage of choppers and inverters.

DC-to-DC converter that switches a DC supply on and off rapidly to produce a variable average DC output. The average voltage is set by the duty cycle.

Technique of generating a train of variable-width pulses to control output voltage or to synthesise a sine wave. Widely used in inverters and variable-speed drives.

Power electronic converter that changes direct current into alternating current, often with adjustable frequency and voltage. Used in solar plants and AC motor drives.

Apparent power in volt-amperes that a current or potential transformer must supply to its connected meters and relays. Exceeding rated burden causes ratio and phase errors.

Four-arm resistive network used to measure an unknown resistance by balancing two voltage dividers. At balance the galvanometer reads zero and the arms satisfy a ratio relationship.

Instrument that applies a high DC voltage to measure insulation resistance of cables, windings and equipment. A low reading indicates moisture or insulation deterioration.

Integrating instrument that records electrical energy consumed over time, usually in kilowatt-hours. Its accuracy is checked against a standard meter or by a rotating-disc test.

Control loop that combines proportional, integral and derivative actions on the error signal. Proportional action gives speed of response, integral removes offset and derivative damps overshoot.

Device that converts one form of energy into another, typically a physical quantity into an electrical signal for measurement or control. Examples include thermocouples and strain gauges.

Control scheme in which the output is measured and fed back to be compared with a reference, so the system corrects its own error. It is more accurate than open-loop control.

Luminous flux produced by a lamp per unit of electrical power consumed, measured in lumens per watt. Higher values mean more efficient conversion of electricity into visible light.

Luminous flux falling on unit area of a surface, expressed in lux. It determines how brightly a surface appears lit and is the usual design quantity for interior lighting.

SI unit of luminous flux, describing the total visible light output of a source in all directions. Lamp ratings are commonly quoted in lumens rather than watts.

Basic digital circuit that performs a Boolean operation such as AND, OR or NOT on one or more binary inputs. Combinations of gates build adders, counters and processors.

Bistable digital storage element that holds one bit until clocked or set. Types include SR, JK, D and T, and they form registers and counters.

Mathematical system for manipulating binary variables using operations AND, OR and NOT. It is used to simplify logic expressions before implementing them with gates.

Deliberate connection of exposed metalwork or a circuit point to the general mass of earth. It limits touch voltage and provides a path for fault current to operate protection.

Switching device that can make, carry and break fault current automatically. Unlike a fuse it can be reset and reclosed after clearing a fault.

Ratio of average load over a period to the maximum demand in that period. A high load factor indicates efficient use of installed plant capacity.

Ratio of the sum of individual maximum demands to the maximum demand of the whole system. Values above one show that consumers peak at different times.

Greatest load drawn by a consumer or system during a specified period, usually measured in kilowatts or kVA. Tariffs often charge for it because it sizes supply equipment.