Definitions  Each field of engineering has its own 'buzz' words, Power Engineering is no exception. What becomes evident is that there are many terms often relating to the same thing and we'll attempt to name them here. CREST FACTOR CVT DC COMPONENT DIP DISTORTION DPF FLICKER FORM FACTOR FREQUENCY HARMONICS HRC INRUSH INRUSH FACTOR LEAKAGE NETWORK PU Pu POWER FACTOR RCD RMS ROCOF SAG SINE WAVE SPIKE SURGE SWELL THD %THD-F %THD-R THC TRANSIENT VA VAR W (watts) WAVE IMBALANCE NETWORK:In full this would be referred to as the 'electricity supply network' and comprises all of the components from generation to wall sockets, including all distribution, transformer, and cable systems in between. SINE WAVE:The wave created by a magnet being rotated within the field of a coil (see diagram). FREQUENCY:The number of such waves generated per second. The two standards are 50 and 60 cycles per second, or otherwise known as Hertz (Hz). RMS:The Root of the Mean of the Squares (usually shortened to Root Mean Square) - The equivalent DC value of an AC waveform. An example would be the equivalent DC voltage that would cause a lamp to shine at the same brilliance as the applied AC voltage. This value is the square root of the mean (average) of the squared values of points along the curve of the waveform. For a sine wave the result is 0.70717 and (for interest) a triangle wave is 0.645 and a square wave is 1.0 (k)W:(kilo-) Watts - With one watt equalling one Joule/second, this is the measurement of Real or True Power dissipated in an electrical circuit i.e. combines both that which is the intended output (e.g. torque from a motor shaft) as well as losses (usually in the form of heat). (k)VA:(kilo-) Volt-Amperes - VA is the product of the RMS values of voltage and current i.e. apparent power. It is not a measure of true power (as some seem to portray) but a convenient means to indicate the capacity of a source or the demand of a load on that source. Being the product of voltage and current, it allows the most important element to be extrapolated when sizing a system - the current involved. (k)VAR:(kilo-) Volt-Amps Reactive. - VARs are the reactive component of VA (Apparent Power), caused by a phase shift between AC current and voltage in inductors (coils) and capacitors. In inductors, current lags voltage (in time), while in capacitors, current leads voltage. VARs are typically first present in a distribution system as a result of inductive loads such as motors, reactors and transformers. VARs are then used in sizing power factor correction capacitors, which are used to offset the effects of these inductive loads. POWER FACTOR:The apparent power vs the true power consumed by a load expressed as a factor and can be calculated as the COS of the angle of lead or lag of the current vs voltage curves. Although COS can never be negative a minus sign is used to depict a lagging current (inductive load) and plus (or no sign) indicating a leading current (capacitive load). DISPLACEMENT POWER FACTOR:DPF, also known as Fundamental Power Factor, is used to measure the voltage-current phase relationship of a load whose current is drawn as a linear load (i.e. as a sine wave) or the fundamental (i.e. the 50/60Hz) component of a complex load. If the load were to be purely inductive (e.g. motor or transformer) or capacitive, then the DPF and PF would be identical. As DPF does not include harmonic content it can be vastly different to the true PF, especially in loads which are mainly hi-tech. However, DPF will always contribute to the true PF. FORM FACTOR:This is the older version of crest factor but in this case is the difference between the peak value and the average value of a waveform. It was used when instruments were not capable of measuring true RMS and one had to rely on average and peak readings. INRUSH:The current demanded by a load when first applied to the power. Inrush is expressed in absolute current, preferably peak current but often as RMS (as it usually appears less demanding). INRUSH FACTOR:For a load, this is expressed as ratio of maximum inrush current over normal running current, or, for a supply, as the ratio of maximum available current over normal running current. DISTORTION:The degree by which the waveform is mis-shapen with respect to what is desired, this figure usually quoted in percent. Distortion, however, is not a word heard very often in Power Quality. It is more common to single out the various aspects that cause or describe the distortion of the waveform. HARMONICS:Direct multiples of the main frequency e.g. 100Hz, 150Hz, etc., on a 50Hz system. Although the first harmonic is actually 100Hz, and the second 150Hz, it is extremely difficult for the human to comprehend and therefore calculate this, especially when refering to 'odd' harmonics etc. It is now accepted that the harmonic order is not referred to but rather the multiplication factor of the harmonic. The 'second harmonic' now refers to the 'second multiplication of the main frequency' e.g. 2 x 50 = 100Hz and the 'third' being the 'third multiplication' therefore 150Hz. WAVE IMBALANCE:When the shape of the waveform differs between the positive and negative portions. DC COMPONENT:It is possible to feed AC waveforms on top of a DC carrier hence resulting in a DC voltage being measured (after filtering out the AC) on the line. However, wave imbalance can also create an apparent DC component. This is as a result of the mean of the positive portion of the wave being different from the mean of the negative portion. SPIKE:The exceptionally sharp deviation from and then return to normal (total disturbance < 50µs) of the main waveform, usually experienced during lighting strikes, and can be up to many times the peak voltage of the main waveform. They have, however, such a high frequency component that transformers, power supply filters, and even long and cumbersome wiring will minimise the effects. Spikes are usually non-consequential disturbances. TRANSIENT:A relatively short-term, overpowering disturbance superimposed on a waveform that returns to normal within a maximum of a half-cycle. Transients, owing to their duration and magnitude, are consequential disturbances i.e. there is likely to be equipment damage. SURGE/SWELL/SAG/DIP:A rise (surge or swell) or fall (sag or dip) in voltage for a period of a half-cycle or longer. Usually the waveform, although different in magnitude, is not drastically distorted. The word 'surge' is an emotive one and has, through this, been erroneously used to describe a damaging transient. It is accepted, in layman situations only, to use the term 'surge' to refer to a significant increase in the RMS voltage owing to a large impulse. FLICKER:The perceived or real change in brilliance of lighting as a direct result of sudden changes in incoming voltage. This is usually expressed in Perception Units (Pu). A level of 1Pu is where flicker becomes annoying, or worse still can induce epileptic fits in those who suffer the condition. The section on "Flicker" expands on this. Pu (Perception Units):Also known as Units of Perception and is mostly used to relate the severity of flicker experienced, but can be used to determine any annoyances resulting from bad power (although there appears to only be one standard, this being flicker). The section on "Flicker" shows this. PU (Per Unit):A term used so often by electrical engineers it was incorporated into IEEE "definitions"! This must not be confused with Pu being Perception Units whose 'u' is lower case. PU is the result of dividing the actual voltage by the nominal regardless of the voltage itself. Example: A sag to 0.94PU on a 230V feed would be a sag to 216V with a swell to 1.1PU being 253V. This same sag or swell could be on the 11kV feeder into a suburb and the figures would therefore be 10.34kV and 12.1kV respectively. It is up to the engineer whether this may be a fixed or sliding nominal (sliding being where if a slow change occurs over an extended period then the new value is the nominal), but fixed is the usual. LEAKAGE:The current that originates from but does not return to the source on the primary current carrying conductors (i.e. Live(s) and Neutral). Such return usually occurs via the Protective Earth or Ground and is primarily due to capacitance between the current carrying conductors and Earth/Ground (e.g. filter components). RCD:A Residual Current Device, also known as an 'Earth Leakage Breaker', is a device for detecting leakage and, if above the limit, to trip and thus remove power to the circuit on which such leakage exists. HRC:High Rupturing Capacity/Current - A characteristic defining a protection device will withstand a large fault current (usually specified in amps) without blowing up in one's face (literally!). It does not define how the device will operate, merely that it will do so safely when it operates (up to the fault current, if specified). CVT:Those involved in light current electricity will think this refers to a Constant Voltage Transformer - it does not. It means Capacitive Voltage Transformer, and the difference is significant. ROCOF:Rate Of Change Of Frequency - A very important parameter when dealing with the output of UPS systems that are designed to synchronise (phase-lock) to the incoming mains. Any frequency sensitive equipment (motors, clock circuits, etc.) may be upset if the UPS decides to adjust the output frequency too quickly should it lose synchronisation because of mains fluctuations (e.g. generator) or failure. Organizations  >> | | Ask a Question | © 14.06.03
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