Vidyut Sahayak (Pareshan) Contractual key terms

Any linear two-terminal network of voltage sources, current sources and resistors can be replaced, as seen from those terminals, by one voltage source in series with one resistor. Load current and voltage then follow from simple series-circuit calculation.

Ratio of real power in watts to apparent power in volt-amperes in an AC circuit. It equals cosine of the angle between voltage and current for sinusoidal waveforms, and a low value means larger current for the same useful power.

Condition in an RLC series circuit where inductive and capacitive reactances are equal and cancel, leaving only resistance. Impedance is minimum, current is maximum, and the circuit behaves as a pure resistor at the resonant frequency.

Effective value of an alternating quantity: the steady direct current that would produce the same heating in a resistor. For a pure sine wave it equals peak value divided by the square root of two.

Difference between synchronous speed and actual rotor speed of an induction motor, usually expressed as a percentage or fraction of synchronous speed. Slip is zero at no load and approaches one when the rotor is stalled.

Speed of the rotating magnetic field produced by a three-phase winding, equal to 120 times supply frequency divided by number of poles. A four-pole machine on 50 Hz supply therefore runs at 1500 rpm.

Gas- and oil-actuated protective device fitted in the pipe between a transformer tank and its conservator. It detects incipient faults by collecting gas or by tripping on rapid oil flow caused by a severe internal fault.

Effect of current flowing in the armature winding on the main magnetic field of a DC or synchronous machine. It distorts or weakens the field, shifting the magnetic neutral axis, so brushes need shifting or compensating windings are added.

Ratio of primary turns to secondary turns of a transformer. For an ideal transformer it equals the ratio of primary to secondary voltage, and the inverse of the ratio of primary to secondary current.

Change in secondary terminal voltage of a transformer, or terminal voltage of an alternator, when full load at a stated power factor is removed while primary or excitation conditions stay unchanged, expressed as a percentage of rated voltage.

Method of expressing voltage, current, power and impedance as fractions of chosen base values. Because base values are linked, transformer ratios disappear from calculations and results stay within a convenient numerical range.

Luminous violet glow with hissing sound and ozone smell around transmission line conductors when surface voltage gradient ionises surrounding air. It wastes power and causes radio interference, and is reduced by larger conductor diameter or bundled conductors.

Vertical drop of a transmission line conductor between two supports due to its own weight and tension. Sag must be kept within limits so ground clearance is maintained, and it increases with span length, weight and temperature.

Protection relay that measures voltage and current at its location and trips when the computed impedance, proportional to line distance to the fault, falls inside a set zone. It gives fast discrimination on transmission lines.

Average load over a period divided by the maximum load during that period. A high load factor means plant and equipment are used efficiently, and it influences tariff design and generation cost per unit.

Four-layer semiconductor power switch with anode, cathode and gate. Once triggered by a gate pulse it latches on and keeps conducting until anode current falls below holding current, so it is used in controlled rectifiers and phase control.

Technique of switching a power device on and off at high frequency and varying the on-time ratio to control average output voltage or current. It reduces harmonic content and is widely used in inverters and motor drives.

Static DC-to-DC converter that switches a DC supply on and off to give a variable average DC output. Step-down and step-up versions serve in battery chargers, DC drives and traction applications.

Induction motor speed control in which supply voltage is varied in proportion to frequency so that stator flux stays nearly constant. It keeps torque capability up to base speed and is the commonest method in variable frequency drives.

Diode connected across an inductive load to provide a path for stored energy when the main switch turns off. It protects the switch from voltage spikes and keeps load current continuous in converters and drives.

Instrument that integrates power over time and records electrical energy consumed, usually in kilowatt-hours. Induction types use a rotating aluminium disc; static types use electronic multipliers and are used for billing and net metering.

Load connected to the secondary of a current transformer, expressed in volt-amperes at rated secondary current. Excessive burden raises ratio and phase errors, so the connected relays and leads must stay within the rated burden.

Portable instrument that applies a high DC test voltage and measures insulation resistance of cables, windings and equipment. Readings in megohms indicate insulation condition; a very low value warns of moisture, dirt or damage.

Ratio of Laplace transform of output to Laplace transform of input of a linear time-invariant system with all initial conditions zero. It describes system dynamics and from it poles, zeros and stability can be found.

Feedback controller whose output combines a term proportional to error, a term proportional to integral of error and a term proportional to derivative of error. The three gains are tuned to reduce steady-state error and improve speed of response.

Amount by which open-loop gain can be increased before a closed-loop system becomes unstable, measured at the frequency where phase shift is 180 degrees. It is read from a Bode or Nyquist plot and stated in decibels.

Graph of magnitude in decibels and phase angle against logarithmic frequency for a system's sinusoidal frequency response. Straight-line approximations make it easy to judge stability margins, bandwidth and the effect of compensation.

Silicon diode designed to operate safely in reverse breakdown, where its voltage stays nearly constant over a range of current. It is used for voltage reference and simple shunt voltage regulation in electronic circuits.

Bistable digital storage element with two stable output states that holds one bit until a clock or input signal changes it. Types such as SR, JK and D are basic building blocks of registers and counters.

Deliberate connection of exposed metalwork or a circuit neutral point to earth through a conductor and electrode of low resistance. It limits touch voltage, gives a fault current path for protective devices and stabilises system voltage.

Miniature circuit breaker: a small automatic switch that trips on overload through thermal action and on short circuit through magnetic action. It is resettable, unlike a fuse, and is rated by current and breaking capacity.

Residual current circuit breaker that compares current in live and neutral conductors and trips when a small earth leakage is detected, protecting people from electric shock. It does not normally protect against overload or short circuit.

Method of running insulated wires inside metal or PVC pipes fixed on the surface or buried in walls. Conduits give mechanical protection, allow rewiring, and must be earthed when metallic and properly sized for wire fill.

Semiconductor device, usually silicon, that converts sunlight directly into direct current by the photovoltaic effect. Cells are connected in series and parallel to form modules and arrays rated in peak watts.

Systematic study of energy use in a building or plant that measures consumption, identifies losses and recommends cost-effective measures such as efficient equipment, insulation, power factor correction and better operating practices.

Gujarat Electricity Regulatory Commission, the state regulator that sets retail tariffs, approves power purchase arrangements and issues licences and standards for distribution and transmission utilities operating in Gujarat.

Form of logical reasoning in which two or more given statements, called premises, are assumed true and a conclusion is tested for validity. Questions usually ask which conclusion definitely follows, using terms such as all, some and none.