GLOSSARY OF TERMS
ALTERNATING CURRENT (AC)
A current which periodically reverses in direction and changes its magnitude as it flows through a conductor or electrical circuit. The magnitude of an alternating current rises from zero to maximum value in one direction, returns to zero, and then follows the same variation in the opposite direction. One complete alternation is one cycle or 360 electrical degrees. In the case of 50 cycle alternating current the cycle is completed 50 times per second.
AMBIENT TEMPERATURE
The air temperature of the surroundings in which the generating system operates. This may be expressed in degrees Celsius or Fahrenheit.
AMPERE (A)
The unit of measurement of electric flow. One ampere of current will flow when one volt is applied across a resistance of one ohm.
APPARENT POWER (kVA, VA)
A term used when the current and voltage are not in phase i.e. voltage and current do not reach corresponding values at the same instant. The resultant product of current and voltage is the apparent power and is expressed in kVA.
AUTOMATIC SYNCHRONIZER
This device in its simplest form is a magnetic type control relay which will automatically close the generator switch when the conditions for paralleling are satisfied.
CAPACITANCE (C)
If a voltage is applied to two conductors separated by an insulator, the insulator will take an electrical charge. Expressed in micro-farads (μf).
CIRCUIT BREAKER
A protective switching device capable of interrupting current flow at a pre-determined value.
CONTINUOUS LOAD
Any load up to and including full rated load that the generating set is capable of delivering for an indefinitely long period, except for shut down for normal preventive maintenance.
CONTINUOUS RATING
The load rating of an electric generating system which is capable of supplying without exceeding its specified maximum temperature rise limits.
CURRENT (I)
The rate of flow of electricity. DC flows from negative to positive. AC alternates in direction. The current flow theory is used conventionally in power and the current direction is positive to negative.
CYCLE
One complete reversal of an alternating current or voltage from zero to a positive maximum to zero to a negative maximum back to zero. The number of cycles per second is the frequency, expressed in Hertz (Hz).
DECIBEL (dB)
Unit used to define noise level.
DELTA CONNECTION
A three phase connection in which the start of each phase is connected to the end of the next phase, forming the Greek letter Delta (D). The load lines are connected to the corners of the delta. In some cases a centre tap is provided on each phase, but more often only on one leg, thus supplying a four wire output.
DIRECT CURRENT
An electric current which flows in one direction only for a given voltage and electrical resistance. A direct current is usually constant in magnitude for a given load.
EFFICIENCY
The efficiency of a generating set shall be defined as the ratio of its useful power output to its total power input expressed as a percentage.
FREQUENCY
The number of complete cycles of an alternating voltage or current per unit of time, usually per second. The unit for measurement is the Hertz (Hz) equivalent to 1 cycle per second (CPS).
FREQUENCY BAND
The permissible variation from a mean value under steady state conditions.
FREQUENCY DRIFT
Frequency drift is a gradual deviation of the mean governed frequency above or below the desired frequency under constant load.
FREQUENCY DROOP
The change in frequency between steady state no load and steady state full load which is a function of the engine and governing systems.
FULL LOAD CURRENT
The full load current of a machine or apparatus is the value of current in RMS or DC amperes which it carries when delivering its rate output under its rated conditions. Normally, the full load current is the "rated" current.
GENERATOR
A general name for a device for converting mechanical energy into electrical energy. The electrical energy may be direct current (DC) or alternating current (AC). An AC generator may be called an alternator.
HERTZ (Hz)
SEE FREQUENCY.
INDUCTANCE (L)
Any device with iron in the magnetic structure has what amounts to magnetic inertia. This inertia opposes any change in current. The characteristic of a circuit which causes this magnetic inertia is know as self inductance; it is measured in Henries and the symbol is "L". kVA - 1,000 Volt amperes (Apparent power). Equal to kW divided by the power factor. kW - 1,000 Watts (Real power). Equal to KVA multiplied by the power factor.
POWER
Rate of performing work, or energy per unit of time. Mechanical power is often measured in horsepower, electrical power in kilowatts.
POWER FACTOR
In AC circuits, the inductances and capacitances may cause the point at which the voltage wave passes through zero to differ from the point at which the current wave passes through zero. When the current wave precedes the voltage wave, a leading power factor results, as in the case of a capacitive load or over excited synchronous motors. When the voltage wave precedes the current wave, a lagging power factor results. This is generally the case. The power factor expresses the extent to which voltage zero differs from the current zero. Considering one full cycle to be 360 degrees, the difference between the zero point can then be expressed as an angle q. Power factor is calculated as the cosine of the q between zero points and is expressed as a decimal fraction (0.8) or as a percentage (80%). It can also be shown to be the ratio of kW, divided by kVA. In other words, kW= kVA x P.F.
PRIME POWER
That source of supply of electrical energy utilised by the user which is normally available continuously day and night, usually supplied by an electric utility company but sometimes by owner generation.
RATED CURRENT
The rated continuous current of a machine or apparatus is the value of current in RMS or DC amperes which it can carry continuously in normal service without exceeding the allowable temperature rises.
RATED POWER
The stated or guaranteed net electric output which is obtainable continuously from a generating set when it is functioning at rated conditions. If the set is equipped with additional power producing devices, then the stated or guaranteed net electric power must take into consideration that the auxiliaries are delivering their respective stated or guaranteed net output simultaneously, unless otherwise agreed to.
RATED SPEED
Revolutions per minute at which the set is designed to operate.
RATED VOLTAGE
The rated voltage of an engine generating set is the voltage at which it is designed to operate.
REACTANCE
The out of phase component of impedance that occurs in circuits containing inductance and/or capacitance.
REAL POWER
A term used to describe the product of current , voltage and power factor, expressed in kW.
RECTIFIER
A device that converts AC to DC.
ROOT MEAN SQUARE (RMS)
The conventional measurement of alternating current and voltage and represents a proportional value of the true sine wave.
SINGLE PHASE
An AC load or source of power normally having only two input terminals if a load, or two output terminals if a source.
STANDBY POWER
An independent reserve source of electrical energy which upon failure or outage of the normal source, provides electric power of acceptable quality and quantity so that the user's facilities may continue in satisfactory operation.
STAR CONNECTION
A method of interconnecting the phases of a three phase system to form a configuration resembling a star ( or the letter Y). A fourth or neutral wire can be connected to the centre point.
TELEPHONE INFLUENCE FACTOR (TIF)
The telephone influence factor of a synchronous generator is a
measure of the possible effect of harmonics in the generator voltage
wave on telephone circuits. TIF is measured
at the generator terminals on open circuit at rated voltage and frequency.
THREE PHASE
Three complete voltage/current sine waves, each of 360 electrical degrees in length, occurring 120 degrees apart. A three phase system may be either 3 wire or 4 wire ( 3 wires and a neutral).
UNINTERRUPTABLE POWER SUPPLY (UPS)
A system designed to provide power without delay or transients, during any period when the normal power supply is incapable of performing acceptably.
UNITY POWER FACTOR
A load whose power factor is 1.0 has no reactance's causing the voltage wave to lag or lead the current wave.
WATT
Unit of electrical power. In DC, it equals the volts times amperes. In AC, it equals the effective volts times the effective amps times power factor times a constant dependent on the number of phases.
