Shortfalls of Existing Methods for Classification and Presentation of

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005

Shortfalls of Existing Methods for Classification and Presentation of Voltage Reduction Events ˇ Djokic´, Jovica V. Milanovic´, Senior Member, IEEE, David J. Chapman, and Saˇsa Z. Mark F. McGranaghan, Member, IEEE

Abstract—This paper discusses the theoretical and practical aspects of various methodologies currently used for definition and characterization of voltage sags and short interruptions. Existing power-quality standards and procedures that are recommended for classification and presentation of sags and interruptions are critically reviewed and their shortfalls are highlighted. Index Terms—Power quality, short interruptions, voltage sags classification and presentation.

I. INTRODUCTION

V

OLTAGE SAGS and short interruptions are probably two of the most serious power-quality problems, as they are frequent causes of malfunctioning electrical equipment in industrial and commercial installations. Although these two sectors on average comprise less than 10%–20% of all customers, the economic impact of sags and interruptions is so strong that the total losses caused are expressed in millions (or even billions) of funds per year [1]–[3]. Thus, the only meaningful way for description, characterization, and presentation of sags and interruptions is with regards to the consequences they have on various types of equipment. Existing sag definitions and methods for sag characterization are inadequate. Many of the recommended procedures for description and characterization of sags and interruptions provide only limited information that is insufficient for investigation of impacts on equipment. Furthermore, the usual ways of presenting voltage sags and short interruptions data (so called “sag density tables” and “one-point sag representations”) also cannot be used for full and precise assessment of equipment sensitivity. II. DEFINITION AND CHARACTERIZATION OF VOLTAGE REDUCTION EVENTS IN EXISTING STANDARDS Voltage sags and short interruptions can be generally described as brief voltage variation events, followed by restoration of the nominal supply conditions. In other words, a voltage

Manuscript received September 9, 2003; revised December 18, 2003. This work was supported in part by the U.K. Engineering and Physical Sciences Research Council (EPSRC) under Grant GR/R40265/01, in part by the Copper Development Association (U.K.), and in part by Electrotek Concepts Inc. (USA). Paper no. TPWRD-00460-2003. ˇ Djokic´ and J. V. Milanovic´ are with the School of Electrical and ElecS. Z. tronic Engineering, The University of Manchester, Manchester M60 1QD, U.K. D. J. Chapman is with the Copper Development Association, Hempstead HP2 7TE, U.K. M. F. McGranaghan is with the Electric Power Research Institute (EPRI) Power Electronics Applications Center (PEAC), Knoxville, TN 37932 USA. Digital Object Identifier 10.1109/TPWRD.2004.833880

variation event must have “short duration” and “voltage magnitude reduction” as two basic characteristics before it can be identified as a voltage sag or short interruption. However, not all voltage reduction events are sags or interruptions. Various standards define the limits of magnitude and duration that qualify events as voltage sags and short interruptions. A. Voltage Sags In existing power-quality standards [4]–[12], a voltage sag is defined as a short duration reduction of voltage magnitude in any or all of the phase voltages of a single-phase or a polyphase power supply at a point in the electrical system. In [10], a voltage sag is described as a “ two-dimensional (2-D) electromagnetic (EM) disturbance, the level of which is determined by both voltage and time (duration).” Other characteristics of voltage sag (e.g., phase shift, point on wave of sag initiation/ending, sag shape) as the possible additional sag “dimensions” were ignored. Standard [12] explicitly states that information about the phase shift and point on wave: “ are not typically available in the sag environment data. Therefore, for compatibility evaluation, it is recommended that phase shift and point of initiation should not be considered.” However, behavior of certain equipment is influenced by the phase shift and/or point on wave (e.g., [13]), and if these sag parameters are not known, sensitivity of equipment cannot be fully assessed. Recent advances in monitoring equipment allow recording of both root mean square (rms) and instantaneous voltage waveforms during the sags, from which information about all important sag characteristics can be easily extracted. Although there are slight differences between the various standards, a voltage sag is always expressed and referred to as an rms voltage reduction event. It is defined as: “ a sudden [18] or temporary [5] reduction or decrease [6] of the supply voltage to a value between 10% [6] or 1% [9] and 90% of the nominal (or declared, or rated) voltage, at the power frequency, followed by the voltage recovery after a short period of time.” Instead of using fixed values (i.e., nominal or declared voltage [9]), sag magnitude is also expressed in percentage of the actual presag voltage (so called “sliding reference voltage” [10]). Some standards (e.g., [5] and [11]) identify voltage sag using the term “threshold” instead of concrete numerical value. That way, voltage sag is defined as a reduction in the supply voltage that has a magnitude between the upper, “sag threshold” limit, and the lower, “interruption threshold” limit. Different thresholds, then, can be used for different purposes (e.g., sag threshold values for monitoring purposes are usually in the range of 85%–95% of the nominal voltage [14], but

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for contractual purposes, that value can be as low as 70% of the nominal voltage [15].) Also, some standards discriminate between the “sag start threshold” and the “sag end threshold” [5], [11]. Finally, all current standards assume that rms voltage magnitudes in all sagged phases are constant during the sag (i.e., that sag has a “rectangular shape”). This is an important assumption, because if sag magnitude is constant and not a function of time, further sag analysis can be performed with these two parameters separated, regarding each one individually and independently of the other. 1) Voltage Sag Magnitude: Definition of the sag magnitude is straightforward for rectangular single-phase sags (i.e., when only one phase experiences constant magnitude reduction below the sag threshold, and two other phases have magnitudes above it). However, in the case of “polyphase events” (i.e., sags, interruptions, or their combinations in two or three phases), different drops in magnitude below the sag threshold may occur in two or three phases. For such asymmetrical (or unbalanced) polyphase sags, it is recommended that the magnitude of the lowest phase voltage during the sag be used as the sag magnitude [5], [10], [11]. The main reason for this “phase aggregation” is to prevent counting of the same event more than once (e.g., simultaneous occurrence of sag in one phase and interruption in another as one voltage sag and one short interruption). However, that does not make much sense from the equipment point of view. Single-phase equipment will respond according to the actual voltage of the phase to which it is connected. Three-phase equipment, on the other hand, will respond with regards to all three phase voltages, not just to the minimum one. (The concept of aggregation of events is often extended further using “time aggregation” and “spatial aggregation” to characterize multiple sags that occur within a short period of time or at different locations. This approach would be appropriate for reclosing events or contractual purposes.) If only the magnitude of the phase with the minimum voltage is used for characterization of completely rectangular but asymmetrical polyphase sag, the magnitude of the sag will be overestimated for any other sagged phase. Furthermore, if sag shape is not rectangular in the phase with the minimum voltage, the magnitude of the sag will be overestimated even in that phase. Fig. 1 illustrates four examples of recorded rectangular and nonrectangular polyphase sags for which sag magnitude is overestimated in one, two or in all three phases, if phase with the minimum rms voltage magnitude is used for sag characterization. 2) Duration of Voltage Sag: The “short period of time” in the above sag definitions is the duration of the voltage sag. As with the sag magnitude, all power-quality standards define sag duration with regards to two limits. The upper limit is the longest allowed duration of a voltage reduction event for which it can be still identified as the voltage sag. It separates voltage sag events from undervoltage events. The lower limit is the shortest duration of a voltage disturbance that can be termed a sag, usually derived from the definition of the sag magnitude through the rms value of the phase voltage. This limit is related to the shortest possible duration for which the rms value of periodic quantity can be calculated—0.5 cycle (i.e., according to existing sag definitions, a voltage reduction event shorter than 0.5 cycles cannot be regarded as voltage sags). Therefore, the duration of a sag

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Fig. 1. Recorded voltage sags with different rms voltage magnitudes and different sag duration in different phases (a) is adopted from [16], and (b)–(d) are adopted from [17]). (a) Rectangular asymmetrical two-phase sag evolves into a rectangular symmetrical three-phase sag; (b) rectangular single-phase sag evolves into a rectangular symmetrical two-phase sag; (c) nonrectangular multistage asymmetrical three-phase sag with rectangular symmetrical three-phase inter-stage; (d) rectangular asymmetrical two-phase sag evolves into a nonrectangular asymmetrical two-phase sag.

in various standards is defined in the range from 0.5 cycle [6], [7] or 1 cycle [18] to a few seconds [4], or even minutes [6]. It should be noted though that voltage reduction events shorter than 0.5 cycles influence the sensitivity of some equipment. To resolve this, the term “undervoltage transients” is proposed in [13] for description of such very short voltage sags. Determination of the sag duration is straightforward in the case of single-phase sags. For polyphase sags, however, sag duration is usually defined as the time between the instant that the rms voltage of any phase drops below the sag (start) threshold, to

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the instant that the rms voltages of all sagged phases rise above the sag (end) threshold. Although such a definition of sag duration takes into account the duration of the entire event, it is inappropriate in situations where a polyphase sag has different durations in each of the phases (e.g., if a single-line fault develops into a three-phase fault, sag at the fault location initially starts as the single-phase voltage sag, and then evolves into the three-phase sag. If such a sag is characterized with the total sag duration (regarding the individual sags in all sagged phases), sag duration is overestimated for two phases in which the sag was not initiated. Also, if the sag in these two phases lasts longer than the sag in the initially sagged phase, sag duration will be overestimated for all three phases.) Examples of polyphase sags are shown in Fig. 1. In Fig. 1(a), the sag starts as a two-phase sag, but evolves into a three-phase sag, which means that the sag duration is overestimated in one phase. In Fig. 1(b), the sag starts as a single-phase sag, but evolves into a two-phase sag. In Fig. 1(c), the sag duration is overestimated in all three phases, and in Fig. 1(d), the sag duration is grossly overestimated in one phase. 3) Minimum Magnitude/Total Duration Approach: 2-D sag definition (advocated in all existing standards) usually involves characterization of a sag in terms of the minimum of all sagged phase rms voltages during the event and with the total duration of event, again regarding the durations of individual sags in all sagged phases. In this paper, such a method of sag characterization is denoted as the “minimum magnitude/total duration” approach. Although this approach provides conservative characterization, it may result in a substantial overestimation of the sag for many voltage reduction events. If a polyphase sag has different durations and different magnitudes in different phases, both the sag magnitude and sag duration would be overestimated in one or more phases (e.g., for the polyphase two-stage sag shown in Fig. 1(a), application of the minimum magnitude/total duration approach results in sag magnitude slightly below 60% of nominal voltage, and sag duration slightly shorter than 500 ms. Only the sag in one phase is close to that description. The magnitude (but not duration) of the sag in the second phase is overestimated, while for the sag in the third phase, both magnitude and duration are overestimated. The same is true for the sag shown in Fig. 1(b). For the multistage sag shown in Fig. 1(c), the application of this approach results in a sag magnitude of about 16% of nominal voltage and in a sag duration of about 110 ms. Both sag duration and sag magnitude are overestimated for all three phases. Finally, for the sag illustrated in Fig. 1(d), sag magnitudes in two sagged phases and sag duration in one sagged phase are substantially overestimated by the minimum magnitude/total duration approach.) Strictly speaking, the minimum magnitude/total duration approach for the description of voltage sags is accurate only for the following three types of rectangular sags: 1) symmetrical three-phase voltage sags, 2) symmetrical two-phase sags with nominal voltage in unsagged phase, and 3) single-phase sags with nominal voltage in two unsagged phases. For these sags, a single rms voltage value is sufficient for sag magnitude description and, where sag duration is the same in each sagged phase, one duration value will suffice. All other sag types (i.e., nonrectangular sags, rectangular but asymmetrical polyphase

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005

sags, and sags with different durations in different phases) can be described and their influence on equipment can be assessed only if pairs of magnitude/duration values are identified for each sagged phase individually. B. Short Interruptions In a similar way to a voltage sag, standards define a short interruption as: “ a disappearance [4], or complete loss, or reduction [5] of the supply voltage to a value less than 10% [6], [7] or 1% [9], or below some “interruption threshold” [11] of the nominal (or declared, or rated) voltage, for a time interval whose duration is between 0.5 cycle [6] or a few tenths of a second [4] and a few seconds [18], or even minutes [9].” Although some standards consider a short interruption as a particular type of voltage sag, they also introduce differences in their definitions. In [5] and [10], a voltage reduction event in a three-phase power supply system is regarded as a short interruption only if all three-phase voltages are reduced below the interruption threshold. Other standards (e.g., [6] and [18]) define a short interruption not as an exclusively three-phase event, but also as a single-phase, or two-phase event, assuming the nominal voltage in any uninterrupted phase. Regardless of the definition, all standards leave a “gap,” because they determine magnitude of the three-phase voltage sag as the lowest phase voltage above the interruption threshold. For example, if a three-phase voltage reduction event has one phase voltage below the sag threshold and, at the same time, another phase voltage below the interruption threshold, standards do not define the nature of that event: (single-phase) voltage sag with zero magnitude (interruption) in second and nominal voltage in third phase, or (single-phase) short interruption with sag in one and nominal voltage in another phase? An explanation offered by standard [5]: “The interruption of one or more phases on a polyphase system can be seen as an interruption of the supply to single-phase customers connected to that system,” does not help, because differentiation between the ‘interruption’ in two or more phases (for “two-phase” and “three-phase” customers), and interruption in one phase (for “single-phase” customers) is not explained. Basically, short interruptions should not be considered as the 2-D events, because the interruption threshold is very close to zero voltage magnitude (i.e., phase voltage magnitudes of interrupted phases are allowed to vary only in a narrow range, usually from 0% to maximum 10% of the nominal voltage). Practically, only one dimension (i.e., duration of interruption), is needed for discrimination of various short interruption events. Also, it should be noted that interruption could be defined for events shorter than 0.5 cycles, as long as the actual instantaneous voltage waveforms are used for event characterization, rather than rms calculation (i.e., assuming that interruptions are related to a complete disappearance of voltage). This suggests that the “time frame” of short interruption duration is established regarding the defined “time frame” of voltage sag duration. Finally, some of the “standard” sag parameters cannot be defined for short interruptions. For example, phase shift cannot be used as an interruption parameter (phase shift does not exist in interrupted phases, where the voltage completely ceases or degenerates close to the zero voltage).

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III. EXISTING METHODS FOR CLASSIFICATION OF VOLTAGE REDUCTION EVENTS

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TABLE I INTUITIVE CLASSIFICATION OF VOLTAGE SAGS

A. Voltage Sag Classifications Basically, there are two main approaches used for classification of voltage sags. The first one is related to the definition and description of various sag types in regards to their general three-phase nature. With this approach, sags can be divided according to the number of sagged phases and presence of asymmetries. Additionally, if complex phase voltages (phasors) are used instead of voltage magnitude values, sag types can be defined with regards to phase angles too. The minimum magnitude/total duration approach is the second sag classification approach. This approach eliminates any possibility for classification of sags with regards to their three-phase nature. With this approach, all sag types are reduced to one “typeless” sag, which is represented by the minimum of all phase rms voltages during the sag and the total duration of the sag in all sagged phases. In the second approach, the classification of sags is simply the categorization/separation of sags into several ranges, regarding the (minimum) sag magnitude, or (total) sag duration, or both. 1) Basic (“Three-Phase”) Classification: Strictly speaking, all voltage reduction events in a three-phase power supply system are three-phase events, regardless of the number of phases that actually experience reduction in voltage magnitude. Usually, all three phases are being measured and are available for characterizing events. Using this information, all sags can be classified in two (disproportional) categories: a) symmetrical three-phase voltage sags, with equal rms voltage magnitude in three simultaneously sagged phases, and b) asymmetrical three-phase voltage sags, with at least one sagged rms phase voltage different from the other two (sagged or unsagged) phase voltages. Symmetrical three-phase sags are originated only by the symmetrical three-phase faults in symmetrical three-phase power systems. Thus, they represent the rarest type of all voltage sags. With this classification, all other sag types will be put in only one category: “generalized” asymmetrical three-phase sags which, however, is not an effective way of sag classification. This classification is denoted as “Classification 1.” 2) Classification With Regards to Number of Sagged Phases: Other sag-type classification methods take into account the actual number of sagged phases, and divide voltage sags further into single-phase sags (one phase has rms voltage magnitude below the sag threshold, the other two have magnitudes above it), two-phase sags (two phases have rms voltage magnitudes below the sag threshold, the third phase has magnitude above it), and three-phase sags (all three phases have rms voltage magnitudes below the sag threshold). Polyphase sags then can be divided into symmetrical sags (which means that sagged phases have equal rms voltage magnitudes), and asymmetrical sags (when at least two sagged phases have different rms voltage magnitudes). This is a good example of “intuitive” sag classification, because differences between the various sag types are self-explanatory. This classification is implicitly incorporated in all power-quality standards, and widely used in a day-to-day practice. A short description of this classification is given in Table I. In this paper, this classification is denoted

as “Classification 2.” Although it is general (applicable to all possible sag types), Classification 2 is related only to rms phase voltages during the sag and it assumes that they do not change during the sag. It does not consider the phase angles of phase voltages, the origin, and propagation of the sag, nor duration of the sag. In the general case, three-phase voltage magnitudes are necessary for the description of sag types in Classification 2. If there are other, additional sag parameters of interest (e.g., sag shape, phase shift, point on wave, etc.), they also have to be specified for each phase separately. In that way, the full description of sag characteristics is related to the identification of the three corresponding per-phase sets of values. 3) Classification With Regards to Complex Phase Voltages: Another sag classification, based on the analysis of propagation and changes of four basic sag types due to the four general fault types in an idealized power systems, is proposed in [1] and given in Table II. Instead of using the rms phase voltage values, it uses complex phase voltages (magnitudes and phase angles). In this paper, this classification is denoted as “Classification 3.” Although Classification 3 is based on the origin and propagation of various sag types, it is, in essence, just a further elaboration of Classification 2 (i.e., inclusion of voltage phase angles). All sag types from Classification 3 have at least two-phase voltage magnitudes equal during the sag. As a direct consequence, sag type 4 from Classification 2 (asymmetrical twophase sag) [Fig. 1(a)] is excluded from Classification 3. Sag types A, B, and E from Classification 3 are the same as sag types 3, 1, and 2 from Classification 2, respectively. Sag type C is practically sag type 2 (symmetrical two-phase sag), but with the phase shift in two sagged phases. The three remaining sag types (D, F, and G) from Classification 3 correspond to general sag type 5 from Classification 2 (asymmetrical three-phase sag), except they all have phase shift in two phases with equal magnitudes. In fact, pairs of sag types D&F and C&G are so similar, that their distinction and identification from the recording in actual power systems is almost impossible without further knowledge about the fault types that caused them. Some of the

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TABLE II CLASSIFICATION OF VOLTAGE SAGS IN REGARDS TO COMPLEX PHASE VOLTAGES (PHASORS) PROPOSED IN [1]

Note 1: Sag types A, B, C, and E are four basic sag types, which means that they are caused by four general fault types, as they occur at the fault location. They correspond to three-phase, single-phase, two-phase, and two-phase to ground faults, respectively. Note 2: Sag type C may occur as a result of propagation of sag types B and D. Sag types D, F, and G are only the results of propagation of sags caused by single-phase and two-phase faults (for type D), or two-phase-to-ground fault (for types F and G). Only sag type A does not change in propagation. Note 3: Phase shift characterizes sag types C, D, F, and G. As given in Table II, the phase shift for sag type C is introduced either due to the nature of two-phase fault at the fault location, or due to the further propagation of sag types B (and D) through the system’s transformers. The phase shift for sag types D, F, and G is introduced only in propagation through the system’s transformers.

factors that are associated with the faults in the power system, which can make it difficult, if not impossible, to distinguish between different sag types are: unbalances in the fault resistances, load characteristics or system impedance characteristics, contributions of motors, coupling between the overhead transmission lines, changes in unfaulted voltages and phase shifts introduced by the differences in X/R ratios during the fault [1]. The main advantage of Classification 3 is that three complex phase voltages for all sag types in Table II can be reconstructed if the characteristic voltage and related sag type are known. Characteristic voltage is generally determined as the lowest of six phase-to-ground and phase-to-phase voltages, calculated in per-unit values and after extracting the zero-sequence compo-

nent from phase-to-ground voltages. Although Classification 3, as given in Table II, neglects the phase shifts due to the differences in X/R ratios introduced by the fault, (some values of) this parameter can be easily incorporated. This means that three per-phase values are not necessary for the full description of various sag types as defined in Classification 3, and that only one set can be provided instead. The assumption that magnitudes, phase angles, and phase shifts of phase voltages do not change in time means that sag duration is an independent parameter. However, if the sag duration is different in different phases, individual sag duration values should be provided as additional information, and related sag types, which now change during the sag, should be de-

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termined regarding the individual sag duration intervals. These different portions of an event with different sag types are sometimes called “subevents”. It is also important to consider sag propagation and sag change phenomena when the (expected) number of sags is calculated from the known fault rate data of system components. Depending on the number and type of transformers between the fault locations and buses of interest, single-line faults, for example, can produce either two-phase or single-phase sags. Related fault rates, expressed as the number of single-line faults per year, then should be carefully related to each sag type in order to obtain the precise number of their occurrences. Practically, this means that in Classification 3, the fault type should also be considered as an additional parameter for characterization of various sag types. Finally, information about the fault type can be useful in a more general context. For example, it is generally assumed that the point on wave of sag initiation is, in most faults, associated with the breaking of electrical insulation and flashovers/arcs, which are more likely to occur when voltage is near the maximum (90 or 270 in the voltage waveform), than when voltage is near zero. However, all characteristics of sags change in propagation and it may happen that at the bus of interest, point on wave values are completely random, or start to cluster around some other point on the voltage waveform. In that situation, “back-tracking” of the sags all of the way back to the fault locations at which the sags originate can provide explanation. Variations of the sag magnitude, phase angle, and duration values for defined sag types are not explicitly included in Classifications 1–3. These three classifications merely define different sag types—they do not provide any instructions as to how different sags of the same type, for example, symmetrical three-phase sags from Classification 1 (same as sags of type 5 from Classification 2, also same as sags of type A from Classification 3) with different magnitudes and/or duration should be counted and/or presented. It seems that the only practical way for presentation of different sags (i.e., sags with different magnitude/phase angle/duration values) of the same type is to provide data for each sag type separately. This approach, however, may cause additional complications for Classification 3, because different sag types can be discriminated only by the phase-angle values (of three phase voltages) introduced by the fault (types B, D, and F, as well as types C, E, and G, differ only in phase angles). The phase angle, however, is “sensed” by the single-phase equipment as the phase shift at the sag initiation. Consequently, if sensitivity of the single-phase equipment is not influenced by the phase shift at the sag initiation, Classification 3 can be reduced to only three sag types: type A, type B/D/F, and type C/E/G. 4) Classifications Based on Minimum Magnitude/Total Duration Approach: The minimum magnitude/total duration approach is the second most often used concept for sag classification. This is the simplest, but also possibly the least accurate way for sag description. Related classification methods in standards are established regarding: 1) only sag magnitude (i.e., magnitude of the phase with the minimum rms voltage during the sag); 2) only sag duration (i.e., total duration of the sag regarding the all sagged phases); and 3) both sag magnitude and sag dura-

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TABLE III EXAMPLE OF SAG CLASSIFICATION BASED ON BOTH SAG MAGNITUDE AND SAG DURATION VALUES [18]

Note: Different sag types, which correspond to one or more magnitude/ duration ranges, are marked with capital letters.

tion. These classification methods simply divide voltage sags in several magnitude and/or duration ranges. They are established as the means for comparing, benchmarking, and statistical reporting of system sag performances. Thus, they are usually assembled in tabular form, ready for inputting the numbers of occurrence of related sag types (i.e., sags with a particular minimum magnitude and particular total duration). As previously stated, these classification methods do not provide any information about the sag type, magnitude, and duration values for all sagged phases in the case of polyphase sags, phase shift during the sag, actual sag shape, or any other sag parameters. Tables III–V give examples of such classifications. Particular discrete (and wide) ranges for sag magnitude/duration values in Tables III–V are based on the recommendations in related standards. For example, Table V presents only those sag magnitude/duration values that are proposed for testing of equipment against voltage sags in [20]. Although the introduction of additional magnitude/duration ranges (e.g., in step of 5% for magnitude and 50 ms for duration) will substantially increase the amount of useful information about the system sag performance, the equipment sensitivity will be nevertheless overestimated due to the use of the minimum magnitude/total duration approach. When filled with results from statistical surveys, Tables III–V are also denoted as the “sag density tables,” because each cell in the table then corresponds to the number of voltage sags that have magnitude/duration values within the related ranges. That way, these tables can be used for representation of statistical sag data related to a particular location/site or to a (part of) particular system. 5) Classification Based on Duration: If none of the sag parameters change in time, sag duration can be considered independent of all other sag parameters. Additionally, if the duration of the sag is equal in all sagged phases, classification with regard to sag duration is straightforward. In other words, different duration ranges for counting and presenting the sags can be easily attached to each particular sag type. That is already done in Tables III–V. One example of sag classification with regard to duration ([6], [11]) is given in Table VI. If these duration ranges are used, sags of type A from Classification 3, for

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TABLE IV PROPOSED TABULAR FORMAT FOR CLASSIFICATION/COUNTING VARIOUS VOLTAGE SAGS [6], [19]

OF

TABLE VI EXAMPLE OF SAG CLASSIFICATION BASED ON SAG DURATION [6], [11]

TABLE VII EXAMPLE OF CLASSIFICATION OF SHORT INTERRUPTIONS BASED ON INTERRUPTION DURATION [6]

Note: The names/abbreviations in Table IV are [19]: system average rms variation frequency index (SARFI); system instantaneous average rms variation frequency index (SIARFI); system momentary average rms variation frequency index (SMARFI); system temporary average rms variation frequency index (STARFI).

PROPOSED

TABLE V TABULAR FORMAT FOR CLASSIFICATION VARIOUS VOLTAGE SAGS [10]

OF

Note: Contrary to previous Tables III and IV, no specific marks/names are given to sags related to different magnitude/duration ranges.

example, can be additionally grouped/classified as: (instan(momentary), and (temporary) sags. taneous), 6) Other Classification Methods: One alternative way for sag classification that is implicitly used in existing standards is based on the shape of the sag. All power-quality standards assume that reduction in voltage magnitude is constant during the sag (i.e., that sag has a “rectangular shape”). Other voltage sag “shapes,” for example, two-stage or multistage sags, or voltage sags caused by the starting of large motors, or their combinations—voltage sags in systems with large, directly connected motors (when, after the fault clearance, reacceleration of motors effectively prolongs the sag and has a strong postsag effect), are all categorized as “nonrectangular” sags. Finally, there are other methods that use some of the sag characteristics (usually sag magnitude and sag duration) to calculate additional parameters that may represent the potential severity of the sag. Examples include: “sag score” method [15], “loss of voltage” method [11], “loss of energy” method [11], “voltage sag energy” method [11], and “lost energy in sag event” method [21]. These “one-dimensional” sag characterization methods are proposed in order to allow simpler and more efficient comparison of various sags (sag severity). They are, in fact, alternative methods for sag description, not methods for sag classification and, therefore, will not be discussed here. B. Short Interruptions Classifications Short interruptions are defined as the voltage reduction events characterized by substantial reduction (below 1% or

10% of nominal voltage), or complete disappearance of phase voltage(s). As discussed earlier, this means that after the identification of an event such as the short interruption, the magnitude of interruption can be neglected, and classification can be performed using only the duration of interruption. The example of classification of short interruptions proposed in [6] is given in Table VII. Comparison of duration ranges between Tables IV and VII show that clear definition of events that have a sag in one phase and an interruption in another is additionally complicated. Such events may have an instantaneous sag in one phase and a momentary interruption in another, even if the durations of both are equal. IV. REPRESENTATION OF VOLTAGE REDUCTION EVENTS A. Representation of Individual Sag Events In all existing standards, voltage sag is characterized by using the rms value of the phase voltage, which is calculated from the instantaneous phase voltage. The plots showing the change in time of the rms phase voltage and instantaneous phase voltage waveform during the sag are two most frequently used representations of an individual sag event. Accordingly, all voltage sag characteristics/parameters therefore can be divided in three categories: 1) characteristics related to rms voltage values (primary characteristics); 2) characteristics related to instantaneous voltage values (secondary characteristics); and 3) other (tertiary) characteristics. In addition to difficulties related to the calculation of the rms values for different window sizes (0.5 cycle and 1 cycle are two most common values) and different updating rates of sampled (digitally measured) instantaneous voltage values [22], there are some inconsistencies between these two sag representations. Voltage sag magnitude, duration, shape, and type are the “rms sag characteristics.” They are directly related to rms voltage values and can be clearly seen and identified from the rms voltage plot. Phase shift and the point on wave at the sag initiation and ending are two sag characteristics that cannot be identified from the rms plot. They, together with the transients and complex voltages (phasors), are the “instantaneous sag characteristics” that can be identified on the instantaneous voltage waveform plot.

´ et al.: SHORTFALLS OF METHODS FOR CLASSIFICATION AND PRESENTATION OF VOLTAGE REDUCTION EVENTS DJOKIC

In Fig. 2(a) and (b), the same sag is represented by both the rms and the instantaneous voltage waveform plot. The instants of the sag initiation and ending are different in these two plots. This is because according to current definitions, sag starts when the rms voltage drops below the sag threshold, and ends when the rms voltages in all phases rise above it. As a consequence, point on wave and phase shift at the sag initiation are related not to the instant when at least one rms phase voltage drops below the sag threshold, but to some time before it. Similarly, point on wave and phase shift at the sag ending are related to some time before the instant when all rms phase voltages rise above the sag threshold. Thus, sag duration determined using the rms voltage value and adopted sag threshold is different from the sag duration determined using the point on wave/phase shift of sag initiation and sag ending. Fig. 2(a) and (b) shows that these differences can be significant (with the rms definition, sag duration is 62 ms; if the points on wave of sag initiation and ending are used, sag duration is 50 ms, which gives more than 20% difference). Generally, these differences in determining sag duration increase with the shortening of the event duration. The extreme case is undervoltage transients, which may not be registered at all if only the rms phase voltage values are used for the description of voltage reduction events. Full and exact assessment of the equipment sensitivity to voltage sags is possible only if all sag characteristics are known. The example in Fig. 2 clearly shows that for this purpose, both the rms and the instantaneous waveform plot should be available. If only one of these two plots is used for sag representation, some of the sag characteristics will be lost. Finally, there are some sag characteristics that cannot be seen or identified neither from the rms nor from the instantaneous phase voltage waveform plot. These “other sag characteristics” are: phase and magnitude unbalance values, missing voltage, harmonic distortion, and symmetrical (positive-, negative-, and zero-sequence) components. Additional calculations or processing of recorded rms and instantaneous waveforms/data are necessary before they can be determined. B. Representation of Site/System Sag Events versus Equipment Voltage-Tolerance Curves The 2-D definition of voltage sag concentrates only on sag magnitude and sag duration values, neglecting all other sag characteristics/parameters. In essence, the 2-D definition of sags is an approach that limits the number of parameters of influence. The minimum magnitude/total duration approach additionally simplifies analysis and reduces all sags, no matter how complex or simple they are, to one constant magnitude value and one duration value. The minimum magnitude/total duration approach limits both the allowed range and characteristics of two selected parameters of influence. With these two characteristics, all sags can be summarized and presented with a relatively small set of values, allowing simple and efficient processing of large statistical and measured sag data. This simplification is the reason for using these approaches in presentation of site/system sag performance. Graphical interpretation of the minimum magnitude/total duration approach is straightforward: if time (i.e., sag duration) is displayed along the abscissa (horizontal) axis, and voltage (i.e.,

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Fig. 2. Two most often used ways for representation of individual sag events (adopted from [17]; crosses mark starting/ending of sag according to rms sag definition). (a) Voltage sag represented by the rms values of phase voltages; (b) voltage sag represented by the instantaneous values of phase voltages.

sag magnitude) is displayed along the ordinate (vertical) axis, changes of voltage in time (i.e., sags and interruptions) can be illustrated graphically. The abscissa and ordinate axis form duration-magnitude plane, and each sag, being characterized with constant magnitude and one duration values, can be represented with one point. This point is determined by the intersection of the related sag duration value and related sag magnitude value. That procedure is commonly and widely used for representation of sag system performances and known as the “one-point sag representation” or “mag/dur sag representation” Fig. 3. Although simple and to some extent practical (at least for fast and rough illustration of site/system sag performance), the one point (mag/dur) representation of statistics/measured sag and interruption data cannot be used effectively for the assessment of equipment sensitivity. For example, single-phase sag with magnitude (retained voltage) of 30% of the nominal voltage will have a different effect on equipment than rectangular threephase symmetrical sag with the same magnitude and duration. Single-phase equipment will respond to the single-phase sag only if it is connected to the sagged phase. The response of threephase equipment will depend on the voltage magnitude values of all three phases but it may be very different for asymmetrical sags and for symmetrical sags affecting equally all three phases. If mag/dur representation is used, however, both sags

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005

voltage sags and short interruptions (“sag density tables” and “one-point sag representations”) may be useful as performance indicators for the overall (or part of) supply system, but they are not useful for the assessment of equipment sensitivity at any specific location. Work on the preparation of the proposal for new methods for classification and presentation of voltage sags and short interruptions is in progress. ACKNOWLEDGMENT The authors acknowledge contributions to this project by M. T. Aung, Dr. C. P. Gupta, Prof. D. Kirschen, and Prof. G. Strbac. REFERENCES

Fig. 3. Usual representations of statistics/measured data about voltage sags and short interruptions used for the assessment of equipment sensitivity. (a) Recorded sags and interruptions superimposed on SEMI F47 curve, adopted from [23]; (b) voltage sag coordination (conture) chart with superimposed voltage-tolerance curves of different equipment, adopted from [12].

would be represented with the same point in the magnitude -duration plane. The method of assessment of equipment sensitivity illustrated in Fig. 3 (i.e., superimposing equipment voltage-tolerance curves on the mag/dur sag representation) is likely to yield to an overestimation of the sag effect on equipment performance (i.e., number of equipment tripping), unless the more detailed sag characteristics are taken into account. The one-point (mag/dur) sag representation can be used for representation of rectangular symmetrical three-phase/twophase sags and rectangular single-phase sags (assuming equal sag duration in each sagged phase and nominal voltage in each unsagged phase) only if information about the sag type is provided. For rectangular but asymmetrical polyphase sags, magnitude/duration values should be provided for all sagged phases. V. CONCLUSION This paper critically reviews and discusses existing standards, methodologies, and procedures used for definition, characterization, classification, and presentation of voltage sags and short interruptions. Current (“two-dimensional”) definitions and descriptions of voltage sags are generally inadequate for assessment of both single-phase and three-phase equipment sensitivity. Usual ways for presenting the site/system data about the

[1] M. H. J. Bollen, Understanding Power Quality Problems: Voltage Sags and Interruptions, ser. series on Power Engineering. Piscataway, NJ: IEEE Press, 2000. [2] F. Pereira, O. Souto, J. de Oliveira, A. Vilaca, and P. Ribeiro, “An analysis of cost related to the loss of power quality,” in Proc. 8th Int. IEEE Conf. Harmonics Quality Power, vol. 2, Athens, Greece, Oct., 14–16 1998, pp. 777–782. [3] “The cost of power disturbances to industrial & digital economy companies,” EPRI Consortium for Electric Infrastructure for a Digital Society (CEIDS), Rep. 1 006 274, Jun. 2001. [4] International Electrotechnical Vocabulary (IEV), IEC Standard 60 050, 1999. [5] Electromagnetic Compatibility (EMC), Part 4: Testing and Measurement Techniques, Section 30: Power Quality Measurement Techniques, IEC Standard 61 000-4-30, 2003. [6] Recommended Practice for Monitoring Electric Power Quality, IEEE Standard 1159, 1995. [7] IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment (Emerald Book), IEEE Standard 1100, 1992. [8] Electricity Supply—Quality of Supply, Part 1: Overview of Implementation of Standards and Procedures, South African Bureau of Standards NRS 048-1, 1996, 2002. [9] Voltage Characteristics of the Electricity Supplied by Public Distribution Systems, European/British Standard EN (EuroNorms) BS/EN 50160, CLC, BTTF 68-6, Nov. 1994. [10] Electromagnetic Compatibility (EMC), Part 2: Environment, Section 8: Voltage Dips and Short Interruptions on Public Electric Power Supply Systems With Statistical Measurement, IEC Standard 61 000-2-8, 2000. [11] Voltage Sag Indices, [Online] Available: http://grouper.ieee.org/groups/ sag/IEEEP1564_01_15.doc. [12] IEEE Recommended Practice for Evaluating Electric Power System Compatibility With Electronic Process Equipment, IEEE Standard 1346, 1998. ˇ Djokic´ , J. V. Milanovic´ , and D. S. Kirschen, “Sensitivity of ac coil [13] S. Z. contactors to voltage sags, short interruptions and undervoltage transients,” IEEE Trans. Power Del., vol. 19, no. 3, pp. 1299–1307, Jul. 2004. [14] D. Dorr, M. Hughes, T. Gruzs, R. Jurwicz, and J. McClaine, “Interpreting recent power quality surveys to define the electrical environment,” IEEE Trans. Ind. Appl., vol. 33, no. 6, pp. 1480–1487, Nov./Dec. 1997. [15] D. D. Sabin, A. Dettloff, and F. Goodman, “Overview of Detroit Edison’s voltage dip performance agreements,” in Proc. 5th Int. Conf. Electrical Power Quality Utililization. Kraków, Poland, Sep. 1999, pp. 53–58. [16] Single-Site Power Quality Summary Rep. (2001, May). Available: www.pqmonitoring.com/pqdata/sample.pdf [Online] [17] D. Divan, G. Luckjiff, W. Brumsickle, J. Freeborg, and A. Bhadkamkar, “I-Grid: infrastructure for nationwide real-time power monitoring,” in Proc. Industry Applications Conf., Rec. 37th IAS Annu. Meeting Conf., vol. 3, Oct., 13–18 2002, [Online] Available: http://www.i-grid.com. [18] Electricity Supply—Quality of Supply, Part 2: Minimum Standards, South African Bureau Stds. NRS 048-2, 1996. 2002. [19] D. Sabin, T. Grebe, and A. Sundaram, “RMS voltage variation statistical analysis for a survey of distribution system power quality performance,” in Proc. IEEE Power Engineering Society Winter Meet., vol. 2, Jan./Feb. 1999, pp. 1235–1240.

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[20] Electromagnetic Compatibility (EMC), Part 4: Testing and Measurement Techniques, Section 11: Voltage Dips, Short Interruptions and Voltage Variations Immunity Tests, IEC Standard 61 000-4-11, 1994. [21] R. Thallam and G. Heydt, “Power acceptability and voltage sag indices in the three-phase sense,” in Proc. IEEE Power Engineering Soc. Summer Meet., vol. 2, Jul. 2000, pp. 905–910. [22] N. Kagan, E. Ferrari, N. Matsuo, S. Duarte, A. Sanommiya, J. Cavaretti, U. Castellano, and A. Tenorio, “Influence of rms variation measurement protocols on electrical system performance indices for voltage sags and swells,” in Proc. 9th Int. IEEE Conf. Harmonics Quality of Power, vol. 3, Oct., 1–4 2000, pp. 790–795. [23] Specification for Semiconductor Processing Equipment Voltage Sag Immunity, Semiconductor Equipment and Materials International Standard SEMI F47-0200, 1999/2000.

Jovica V. Milanovic´ (M’95–SM’98) received the Dipl.Ing. and M.Sc. degrees from the University of Belgrade, Belgrade, Yugoslavia, and the Ph.D. degree from the University of Newcastle, Newcastle, Australia. Currently, he is a Reader with the School of Electrical and Electronic Engineering, The University of Manchester, Manchester, U.K., where he has been since 1998.

ˇ Djokic´ received the Dipl.Ing. and M.Sc. degrees in electrical engiSaˇsa Z. neering from the University of Ni, Ni, Yugoslavia, and the Ph.D. degree from the University of Manchester Institute of Science and Technology (UMIST), Manchester, U.K. Currently, he is a Research Associate with the School of Electrical and Electronic Engineering, The University of Manchester, Manchester, U.K., where he has been since 2001.

Mark F. McGranaghan (M’77) received the B.S.E.E. and M.S.E.E. degrees from the University of Toledo, Toledo, OH. Currently, he is in charge of utility consulting services for the Electric Power Research Institute (EPRI) Power Electronics Applications Center (PEAC), Knoxville, TN. He has been involved in the development of various power-quality standards for the IEEE and International Electrotechnical Commission (IEC).

David J. Chapman received the B.Eng. degree in electrical engineering from the Polytechnic of the South Bank, South Bank, U.K. Currently, he is with Copper Development Association, Hempstead, U.K., where he is responsible for the power-quality (PQ) education program and preparation of the LPQI Power Quality Application Guide. He has worked in a variety of industrial research and development fields.