Need for a Consistent Vocabulary
The term power quality is applied to a wide variety of electromagnetic phenomena on the power system. The increasing application of electronic equipment and distributed generation has heightened the interest in power quality in recent years, and this has been accompanied by the development of a special terminology to describe the phenomena. Unfortunately, this terminology has not been consistent across different segments of the industry. This has caused a considerable amount of confusion as both vendors and end users have struggled to understand why electrical equipment is not working as expected. Likewise, it is confusing to wade through the vendor jargon and differentiate between a myriad of proposed solutions.
Many ambiguous words have been used that have multiple or unclear meanings. For example, surge is used to describe a wide variety of disturbances that cause equipment failures or misoperation. A surge suppressor can suppress some of these but will have absolutely no effect on others. Terms like glitch and blink that have no technical meaning at all have crept into the vocabulary. Unscrupulous marketers take advantage of the ignorance of the general public, selling overpriced gadgets with near-miraculous claims for improving the power quality. Of course, this all comes with a money-back guarantee. Readers can protect themselves by obtaining a better understanding of power quality vocabulary and insisting on technical explanations of how a gadget works. Our basic rule: If they won’t tell you what is in the box and how it works, don’t buy it!
This chapter describes a consistent terminology that can be used to describe power quality variations. We also explain why some commonly used terminology is inappropriate in power quality discussions.
General Classes of Power Quality Problems
The terminology presented here reflects recent U.S. and international efforts to standardize definitions of power quality terms. The IEEE Standards Coordinating Committee 22 (IEEE SCC22) has led the main effort in the United States to coordinate power quality standards. It has the responsibilities across several societies of the IEEE, principally the Industry Applications Society and the Power Engineering Society. It coordinates with international efforts through liaisons with the IEC and the Congress Internationale des Grand Réseaux Électriques a Haute Tension (CIGRE; in English, International Conference on Large High-Voltage Electric Systems).
U.S. power industry efforts to develop recommended practices for monitoring electric power quality have added a few terms to the IEC terminology. Sag is used as a synonym to the IEC term dip. The category short-duration variations is used to refer to voltage dips and short interruptions. The term swell is introduced as an inverse to sag (dip). The category long-duration variation has been added to deal with American National Standards Institute (ANSI) C84.1 limits. The category noise has been added to deal with broadband conducted phenomena. The category waveform distortion is used as a container category for the IEC harmonics, interharmonics, and dc in ac networks phenomena as well as an additional phenomenon from IEEE Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, called notching.
The categories and their descriptions are important in order to be able to classify measurement results and to describe electromagnetic phenomena which can cause power quality problems.
Transients
The term transients has long been used in the analysis of power systemvariations to denote an event that is undesirable and momentary in nature. The notion of a damped oscillatory transient due to an RLC network is probably what most power engineers think of when they hear the word transient.
Other definitions in common use are broad in scope and simply state that a transient is “that part of the change in a variable that disappears during transition from one steady state operating condition to another. Unfortunately, this definition could be used to describe just about anything unusual that happens on the power system.
Another word in common usage that is often considered synonymous with transient is surge. A utility engineer may think of a surge as the transient resulting from a lightning stroke for which a surge arrester is used for protection. End users frequently use the word indiscriminantly to describe anything unusual that might be observed on the power supply ranging from sags to swells to interruptions. Because there are many potential ambiguities with this word in the power quality field, we will generally avoid using it unless we have specifically defined what it refers to.
Broadly speaking, transients can be classified into two categories, impulsive and oscillatory. These terms reflect the waveshape of a current or voltage transient. We will describe these two categories in more detail.
Impulsive transient
An impulsive transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both that is unidirectional in polarity (primarily either positive or negative).
Impulsive transients are normally characterized by their rise and decay times, which can also be revealed by their spectral content. For example, a 1.2 x 50-µs 2000-volt (V) impulsive transient nominally rises from zero to its peak value of 2000 V in 1.2 µs and then decays to half its peak value in 50 µs. The most common cause of impulsive transients is lightning.
Figure 2.1 illustrates a typical current impulsive transient caused by lightning. Because of the high frequencies involved, the shape of impulsive transients can be changed quickly by circuit components and may have significantly different characteristics when viewed from different parts of the power system. They are generally not conducted far from the source of where they enter the power system, although they may, in some cases, be conducted for quite some distance along utility lines. Impulsive transients can excite the natural frequency of power system circuits and produce oscillatory transients.
Oscillatory transient
An oscillatory transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both, that includes both positive and negative polarity values. An oscillatory transient consists of a voltage or current whose instantaneous value changes polarity rapidly. It is described by its spectral content (predominate frequency), duration, and magnitude. The spectral content subclasses defined in Table 2.2 are high, medium, and low frequency. The frequency ranges for these classifications are chosen to coincide with common types of power system oscillatory transient phenomena.
Oscillatory transients with a primary frequency component greater than 500 kHz and a typical duration measured in microseconds (or several cycles of the principal frequency) are considered high-frequency transients. These transients are often the result of a local system response to an impulsive transient.
A transient with a primary frequency component between 5 and 500 kHz with duration measured in the tens of microseconds (or several cycles of the principal frequency) is termed a medium-frequency transient. Back-to-back capacitor energization results in oscillatory transient currents in the tens of kilohertz as illustrated in Fig. 2.2. Cable switching results in oscillatory voltage transients in the same frequency range. Medium-frequency transients can also be the result of a system response to an impulsive transient.
A transient with a primary frequency component less than 5 kHz, and a duration from 0.3 to 50 ms, is considered a low-frequency transient. This category of phenomena is frequently encountered on utility subtransmission and distribution systems and is caused by many types of events. The most frequent is capacitor bank energization, which typically results in an oscillatory voltage transient with a primary frequency between 300 and 900 Hz. The peak magnitude can approach 2.0 pu, but is typically 1.3 to 1.5 pu with a duration of between 0.5 and 3 cycles depending on the system damping (Fig. 2.3).
Oscillatory transients with principal frequencies less than 300 Hz can also be found on the distribution system. These are generally associated with ferroresonance and transformer energization (Fig. 2.4). Transients involving series capacitors could also fall into this category. They occur when the system responds by resonating with low-frequency components in the transformer inrush current (second and third harmonic) or when unusual conditions result in ferroresonance.
It is also possible to categorize transients (and other disturbances) according to their mode. Basically, a transient in a three-phase system with a separate neutral conductor can be either common mode or normal mode, depending on whether it appears between line or neutral and ground, or between line and neutral.
Long-Duration Voltage Variations
Long-duration variations encompass root-mean-square (rms) deviations at power frequencies for longer than 1 min. ANSI C84.1 specifies the steady-state voltage tolerances expected on a power system. A volt age variation is considered to be long duration when the ANSI limits are exceeded for greater than 1 min.
Long-duration variations can be either overvoltages or undervoltages. Overvoltages and undervoltages generally are not the result of system faults, but are caused by load variations on the system and system switching operations. Such variations are typically displayed as plots of rms voltage versus time.
Overvoltage
An overvoltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for a duration longer than 1 min. Overvoltages are usually the result of load switching (e.g., switching off a large load or energizing a capacitor bank). The overvoltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate. Incorrect tap settings on transformers can also result in system overvoltages.
Undervoltage
An undervoltage is a decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than 1 min. Undervoltages are the result of switching events that are the opposite of the events that cause overvoltages. A load switching on or a capacitor bank switching off can cause an undervoltage until voltage regulation equipment on the system can bring the voltage back to within tolerances. Overloaded circuits can result in undervoltages also. The term brownout is often used to describe sustained periods of undervoltage initiated as a specific utility dispatch strategy to reduce power demand. Because there is no formal definition for brownout and it is not as clear as the term undervoltage when trying to characterize a disturbance, the term brownout should be avoided.
Sustained interruptions
When the supply voltage has been zero for a period of time in excess of 1 min, the long-duration voltage variation is considered a sustained interruption. Voltage interruptions longer than 1 min are often permanent and require human intervention to repair the system for restoration. The term sustained interruption refers to specific power system phenomena and, in general, has no relation to the usage of the term outage. Utilities use outage or interruption to describe phenomena of similar nature for reliability reporting purposes. However, this causes confusion for end users who think of an outage as any interruption of power that shuts down a process. This could be as little as one-half of a cycle. Outage, as defined in IEEE Standard 100, does not refer to a specific phenomenon, but rather to the state of a componentin a system that has failed to function as expected. Also, use of the term interruption in the context of power quality monitoring has no relation to reliability or other continuity of service statistics. Thus, this term has been defined to be more specific regarding the absence of voltage for long periods.
Short-Duration Voltage Variations
This category encompasses the IEC category of voltage dips and short interruptions. Each type of variation can be designated as instantaneous, momentary, or temporary, depending on its duration as defined in Table 2.2.
Short-duration voltage variations are caused by fault conditions, the energization of large loads which require high starting currents, or intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either tempo rary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions). The fault condition can be close to or remote from the point of interest. In either case, the impact on the voltage during the actual fault condition is of the short-duration variation until protective devices operate to clear the fault.
Interruption
An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment failures, and control malfunctions. The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices. Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption. The duration of an interruption due to equipment malfunctions or loose connections can be irregular.
Some interruptions may be preceded by a voltage sag when these interruptions are due to faults on the source system. The voltage sag occurs between the time a fault initiates and the protective device operates. Figure 2.5 shows such a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles and then drops to zero for about 1.8 s until the recloser closes back in.
Sags (dips)
A sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min.
The power quality community has used the term sag for many years to describe a short-duration voltage decrease. Although the term has not been formally defined, it has been increasingly accepted and used by utilities, manufacturers, and end users. The IEC definition for this phenomenon is dip. The two terms are considered interchangeable, with sag being the preferred synonym in the U.S. power quality community.
Terminology used to describe the magnitude of a voltage sag is often confusing. A “20 percent sag” can refer to a sag which results in a voltage of 0.8 or 0.2 pu. The preferred terminology would be one that leaves no doubt as to the resulting voltage level: “a sag to 0.8 pu” or “a sagwhose magnitude was 20 percent.” When not specified otherwise, a 20 percent sag will be considered an event during which the rms voltage decreased by 20 percent to 0.8 pu. The nominal, or base, voltage level should also be specified.
Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors. Figure 2.6 shows a typical voltage sag that can be associated with a single-line-to-ground (SLG) fault on another feeder from the same substation. An 80 percent sag exists for about 3 cycles until the substation breaker is able to interrupt the fault current. Typical fault clearing times range from 3 to 30 cycles, depending on the fault current magnitude and the type of overcurrent protection. Figure 2.7 illustrates the effect of a large motor starting. An induction motor will draw 6 to 10 times its full load current during start-up. If the current magnitude is large relative to the available fault current in the system at that point, the resulting voltage sag can be significant. In this case, the voltage sags immediately to 80 percent and then gradually returns to normal in about 3 s. Note the difference in time frame between this and sags due to utility system faults.
Until recent efforts, the duration of sag events has not been clearly defined. Typical sag duration is defined in some publications as ranging from 2 ms (about one-tenth of a cycle) to a couple of minutes. Undervoltages that last less than one-half cycle cannot be characterized effectively by a change in the rms value of the fundamental frequency value. Therefore, these events are considered transients. Undervoltages that last longer than 1 min can typically be controlled by voltage regulation equipment and may be associated with causes other than system faults. Therefore, these are classified as long-duration variations.
Sag durations are subdivided here into three categories—instantaneous, momentary, and temporary—which coincide with the three categories of interruptions and swells. These durations are intended to correspond to typical utility protective device operation times as well as duration divisions recommended by international technical organizations.
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.