The terminology used in this blog relating to the power grid.
A note about units in physics. Every numerical quantity involves units. Often these units are the product or ratio of other units. For example, frequency is measured in Hertz, which in turn is the ratio of cycles / second (or in English phrasing, cycles per second). 
Alternating Current 
Power in which the voltage and current alternate direction, typically at a frequency of 50 Hz or 60 Hz. See also DC. 
Ampere, A, 
The unit of current. Measures the quantity of electrons passing through. 1 A = 1 "Avogadro's number", or 6.022 x 10e23 Uses standard metric prefixes: (k)ilo (M)ega, (G)iga, (m)illi, (μ)micro, and (n)ano For AC, we can refer to either peak current or the average current (RMS). 
Average Power 
See RMS. 
Capacity Factor 
Over an interval of time (often one year), the ratio of average generated power to the rated power, usually expressed as a percentage. In rare cases, may exceed 100%. 
Current 
See Ampere. 
Direct Current 
Current and voltage that do not alternate direction. I.e. not AC. 
Distribution Line 
A line or system bringing power to individual consumers, or feeding such lines from a transmission line. Operates at a lower voltage than a transmission line, but a higher voltage than consumers. 
Frequency (Hertz) 
How often something happens. In power distribution, usually refers to how fast one cycle of voltage or current happens (e.g. 50 Hz or 60 Hz power). See Hertz. 
Generator (watts) 
A producer of electrical power from an energy source. Depending on context it can mean: 
HVDC (kV) 
High Voltage DC. A transmission system using voltages in the range of hundreds of kilovolts to megavolts. Used for efficient longdistance power, and to isolate two AC grids from each other. 
Hertz 
Unit of measurement of frequency. 1 Hz = 1/second. Uses standard metric prefixes: (k)ilo (M)ega, (G)iga, (m)illi, (μ)micro, and (n)ano (Units smaller than Hz are seldom used, and are instead measured in the length of one cycle as an interval of time). 
IGBT 
A type of semiconductor switch that is replacing the earlier thyristor technology in grid usage. 
Inverter 
An electronic circuit which converts DC power to AC power.

Joule 
Unit of energy, or how much work is done. 1 J = 1 watt x 1 second Also measured in watthours. One WH = 3600J (i.e. 1 watt for 3600 seconds). 
Load (watts) 
The opposite of a generator. Any consumer of electrical energy. e.g. lights, motors, heaters. 
Mercuryvapor valve 
An early kind of electronic switch, now obsolete, used to switch large loads in converting AC to DC (rectifier) and DC to AC (inverter). Have been replaced by thyristors, and more recently by IGBTs. 
Motor (watts) 
The opposite of a rotating generator. Converts electricity into mechanical power. 
Phase (degrees) 
The relationship between the cycles of alternating voltage vs current. If the voltage rises before the current, we say it leads the current. If the voltage rises after the current, we say it lags the current. Phase is measured in degrees, with 0° indicating the voltage and current are in phase. The voltage and current rise together. 180° is also in phase, with the current going in the opposite direction. (If you turn around and look at the line from the other side, it becomes 0°). 90° and 90° are completely out of phase. Any other values are partly in, partly out of phase. 
The rate at which energy is produced, transferred, or consumed. In AC systems, it is important to distinguish from VA, because the current and voltage may not occur at the same moments. We get the average (RMS) power by multiplying the VA times the cosine of the phase angle. 1 watt = 1 volt * 1 ampere, or 1 joule / second. 

Power Factor 
The ratio of Reactive Power to Apparent Power. 1.0 means the voltage and current are in phase, 0.0 means they are 90° out of phase (either direction) and 1.0 means they are in phase in the opposite direction (i,.e. the real power is flowing in the opposite direction of the measurement). 
Reactive Power (VA) 
The portion of the power (VA) in a powerline that is not transferring real power. Instead, power is alternately being passed between the two ends, temporarily stored on each end as either electrical or magnetic fields (capacitance or inductance). This is undesirable as no useful energy is transferred, yet the current contributes to heating via resistance in the wires. This both wastes power and pushes the transmission line closer to its current limit. Even more importantly, it may prevent generators from pushing power to where it is needed, and is thus a critical parameter in grid operation standards. The reactive power equals the total VA times the sine of the phase angle. 
RootMeanSquare 
RootMeanSquare. The square root of the average of the squares of the values. For AC, this is the effective average. 
Synchronous Condenser 
A type of shortterm energy storage, used for very shortterm voltage and frequency support, but primarily for adjusting the phase relationship between voltage and current (reactive power). This is essentially a generator without a driver, alternately taking power from the grid and turning it into stored rotational energy, and restoring it back to the grid. 
ThreePhase Power 
Power using 3 lines to transfer power, with the voltages between each being 120° out of phase from each other. This allows efficient transfer with less redundancy than two phase. 
Thyristor 
A type of electronic switch used to rapidly switch power circuits. Thyristors replaced mercuryvapor valves in AC/DC conversion. See the Wikipedia article for more general information about the device. 
Transformer 
A device which uses windings of wire coupled magnetically to power between different voltages in an AC system. A higher voltage results in a proportionally smaller current, enabling efficient transmission over long distances. The side of the transformer that receives input power is called the primary side, and the output is called the secondary side. 
Transmission Line 
A line or system connecting a large generator to a system of distribution lines. These generally operate at very high voltages. 
Turbine 
A device which converts energy from a fluid under pressure into rotary motion. This can drive a generator. 
TwoPhase Power 
AC power transferred over two wires. Simpler but less efficient than threephase. 
Voltage (Volt) 
The electrical force (Electromotive Force, EMF) which drives current. Analogous to pressure differences in a pipe. 
Volt, V 
Unit of electromotive force (EMF). A measure of the total electrical field driving a current. It is measured between two points, often against the earth ground. Uses standard metric prefixes: (k)ilo (M)ega, (G)iga, (m)illi, (μ)micro, and (n)ano For AC, we can refer to either peak voltage or the average voltage (RMS). 
VoltAmpere, VA, 
Product of the RMS voltage times RMS current, without regard to phase. Only used for AC. If the phase angle is 0° or 180° (the voltage and current are inphase), equals the power transferred. To get the power transferred in watts, multiply the VA by the cosine of the phase angle. 
Watt, W, 
The unit of power. 1 volt * 1 ampere = 1 watt. Also, 1 joule / 1 second = 1 watt. Uses standard metric prefixes: (k)ilo (M)ega, (G)iga, (m)illi, (μ)micro, and (n)ano See also VoltAmpere and Reactive Power 
WattHour, WH 
An alternate unit of energy, more commonly used in power engineering than the joule. One watthour = 3600 joule. 
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