Power Quality Terms

So that you will be better able to understand the material in this book, we have included the definitions of many common power quality terms that are relevant to the material in this book. For the most part, these definitions coincide with current industry efforts to define power quality terms. We have also included other terms relevant to the material in this book. Active filter Any of a number of sophisticated power electronic devices for eliminating harmonic distortion. See passive filter. CBEMA curve A set of curves representing the withstand capabilities of computers in terms of the magnitude and duration of the voltage disturbance.

Developed by the Computer Business Equipment Manufacturers Association (CBEMA), it had become the de facto standard for measuring the performance of all types of equipment and power systems and is commonly referred to by this name. CBEMA has been replaced by the Information Technology IndustryCouncil (ITI), and a new curve has been developed that is commonly referred to as the ITI curve. See ITI curve. 

common mode voltage The noise voltage that appears equally from current-carrying conductor to ground.

coupling A circuit element, or elements, or a network that may be considered common to the input mesh and the output mesh and through which energy may be transferred from one to another.

crest factor A value reported by many power quality monitoring instruments representing the ratio of the crest value of the measured waveform to the root mean square of the fundamental. For example, the crest factor of a sinusoidal wave is 1.414.

critical load Devices and equipment whose failure to operate satisfactorily jeopardizes the health or safety of personnel, and/or results in loss of function, financial loss, or damage to property deemed critical by the user.

current distortion Distortion in the ac line current. See distortion.

differential mode voltage The voltage between any two of a specified set of active conductors. dip See sag.

distortion Any deviation from the normal sine wave for an ac quantity.

fast tripping Refers to the common utility protective relaying practice in which the circuit breaker or line recloser operates faster than a fuse can blow. Also called fuse saving. Effective for clearing transient faults without a sustained interruption, but is somewhat controversial because industrial loads are subjected to a momentary or temporary interruption.

fault Generally refers to a short circuit on the power system. fault, transient A short circuit on the power system usually induced by lightning, tree branches, or animals, which can be cleared by momentarily interrupting the current.

ferroresonance An irregular, often chaotic type of resonance that involves the nonlinear characteristic of iron-core (ferrous) inductors. It is nearly always undesirable when it occurs in the power delivery system, but it is exploited in technologies such as constant-voltage transformers to improve the power quality.

flicker An impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time.

frequency deviation An increase or decrease in the power frequency. The duration of a frequency deviation can be from several cycles to several hours. 

frequency response In power quality usage, generally refers to the variation of impedance of the system, or a metering transducer, as a function of frequency.

fundamental (component) The component of order 1 (50 to 60 Hz) of the Fourier series of a periodic quantity.

ground A conducting connection, whether intentional or accidental, by which an electric circuit or electrical equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth. Note: It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground currents to and from earth (or the conducting body).

ground electrode Aconductor or group of conductors in intimate contact with the earth for the purpose of providing a connection with the ground.

ground grid A system of interconnected bare conductors arranged in a pattern over a specified area and on or buried below the surface of the earth. The primary purpose of the ground grid is to provide safety for workers by limiting potential differences within its perimeter to safe levels in case of high currents that could flow if the circuit being worked became energized for any reason or if an adjacent energized circuit faulted. Metallic surface mats and gratings are sometimes utilized for the same purpose. This is not necessarily the same as a signal reference grid.

ground loop A potentially detrimental loop formed when two or more points in an electrical system that are nominally at ground potential are connected by a conducting path such that either or both points are not at the same ground potential.

 

ground window The area through which all grounding conductors, including metallic raceways, enter a specific area. It is often used in communications systems through which the building grounding system is connected to an area that would otherwise have no grounding connection.

harmonic (component) A component of order greater than 1 of the Fourier series of a periodic quantity.

harmonic content The quantity obtained by subtracting the fundamental component from an alternating quantity.

harmonic distortion Periodic distortion of the sine wave. See distortion and total harmonic distortion (THD).

harmonic filter On power systems, a device for filtering one or more harmonics from the power system. Most are passive combinations of inductance, capacitance, and resistance. Newer technologies include active filters that can also address reactive power needs.

harmonic number The integral number given by the ratio of the frequency of a harmonic to the fundamental frequency.

harmonic resonance A condition in which the power system is resonating near one of the major harmonics being produced by nonlinear elements in the system, thus exacerbating the harmonic distortion. 

impulse A pulse that, for a given application, approximates a unit pulse or a Dirac function. When used in relation to monitoring power quality, it is preferable to use the term impulsive transient in place of impulse.

impulsive transient A sudden, nonpower frequency change in the steady-state condition of voltage or current that is unidirectional in polarity (primarily either positive or negative).

instantaneous When used to quantify the duration of a short-duration variation as a modifier, this term refers to a time range from one-half cycle to 30 cycles of the power frequency.

instantaneous reclosing A term commonly applied to reclosing of a utility breaker as quickly as possible after an interrupting fault current. Typical times are 18 to 30 cycles.

interharmonic (component) A frequency component of a periodic quantity that is not an integer multiple of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz).

interruption, momentary (electrical power systems) An interruption of a duration limited to the period required to restore service by automatic or supervisory-controlled switching operations or by manual switching at locations where an operator is immediately available. Note: Such switching operations must be completed in a specified time not to exceed 5 min.