Voltage sag is by far the most serious power quality issue. It is defined as a short duration reduction in voltage magnitude. The principal cause of voltage sag.
AN OVERVIEW OF VOLTAGE SAG THEORY, EFFECTS AND EQUIPMENT COMPATIBILITY Alexis Polycarpou and Hassan Nouri, Terrence Davies and Rade Cinc Power Systems and Electronics Research Group, University of the West of England. Bristol, UK ABSTRACT
In the modern industrial world, many electronic and electrical control devices are part of automated processes, to increase energy efficiency and productivity. Due to the fact that most of these devices are designed with the use of modem power electronics, they are characterized by an extreme sensitivity on power quality variations. This paper describes an overview of the theory and effects of voltage sag. Reference to non-rectangular events, and the procedure to evaluate the power systems performance regarding voltage sag, through using particular indices to assess equipment compatibility issues is presented. Keywords: Voltage sag index, acceptability curves, quality standards
INTRODUCTION Voltage sag is by far the most serious power quality issue. It is defined as a short duration reduction in voltage magnitude. The principal cause of voltage sag is a short duration increase in current, usually appearing during motor starting, transformer energizing and faults. Voltage sags at equipment terminals can be caused by short circuit faults hundreds of kilometers away in the transmission system. The interest in voltage sag has been increasing rapidly in the last decade, due to problems induced on several types of equipment, such as adjustable speed drives, process control equipment and computers. Some equipment will trip as soon as the voltage drops to about 90% for only one or two cycles, something, which happens tens of times per year. Other equipment might be less sensitive to voltage sags and survive a worse event. The tripping can have devastating effects on the production and thus the financial condition of the associated plant. Thus the need arises for assessing equipment compatibility through indices describing voltage sag events.
THEORY To date, total prevention of voltage sag does not exist. It cannot be predicted when a fault will occur within hundreds of kilometers radius of the facility under investigation. An option is to mitigate the effects of sag when it happens, and to avoid human made circumstances, which could result in voltage sag. However, it is possible to predict the voltage sag due to individual faults by estimating the voltage drop at the critical load. This method requires knowledge of
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network impedance, fault impedance, and fault location relative to sensitive load, transformer connections and pre-sag voltages. Since the impact of the sag depends on thc associated equipment sensitivity, to evaluate compatibility of equipment, sag lines are plotted along with equipment sensitivity. Analysis of voltage sag requires knowledge of voltage sag characteristics, statistical information describing the likelihood of a voltage sag occurring, and information describing the sensitivity of important loads within the facility. Generally, there are two types of methods used for voltage sag analysis [I]. Sfatis/icn/ anal-vsis merl7ods observe the behavior of the system and caIculate averages from the observations. They require a number o f monitors to observe the system for s time period. To get accurate results a large number of monitors and a long observing time period are required. The major disadvantage of this type of method is that the results have limited application for other locations and for future prediction. Srochmric methods use a modei of the system to predict the stochastic properties of the system, like voltage sag frequency. Site indices can be calculated for all locations within the system and for any period in the past or in the future. The disadvantage is that the results are as accurate or inaccurate as the model used. Emphasis in the Iiterature is given to statisticaI methods. Stochastic methods have not yet received the attention they require. Factors, which impose extreme difficulty in voltage sag analysis, is the wide variation of fault types (L-L, L-G, 3-Ph), transformer windings (Y-D,Y-Y, D-D,
grounded or not) and load characteristics (dynamic, static. influence of XIR ratio). According to each factor. thc effect obtained would be different [2?3]. The process. of predicting the voltage sag frequcncy, is to identify thc apparatus, which cause unacceptable voltage sags in the system, when faulted, and the probability of each fault occurring. A good way to describe sag frequency is to plot the number of events versus sag voltage in percent ofnominal [2]. Accurate estimates of sag magnitude and duration probabilities help system designers to define appropriate equipment specifications for critical processes. In order to achieve a good estimate, reliability data, fault clearing device characteristics and the means of unbalanced current and voltage calculation, must be taken into consideration. Short circuit analysis programs allow thc users to accurately model the electrical network, apply short circuits around the network, and observe the resulting voltage at any bus of interest. Changes in equipment specification, based on studies, can reduce the number of nuisance outages from voltage sags. A rapid way to determine whether the sag is tolerable is to plot it on the power acceptability curves [4j. They are an empirical set of curves that represent the intensity and duratioii of bus voltage disturbances. Their shape and parameters are defined by the required quality standards. A typicaI acceptability curve is the CBEMA, established in the early 1980's and has since bccome a standard reference within the industry. The curve is shown in figure I below. Extensive study o f various acceptability curves is presented in part IV of this report.
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Take mitigation measures at the interface between system and load Redesign and improve load equipment, in terms of sensitivity.
EFFECTS Effects Of system Parameters on voltage sag: Various conditions and parameters of the system affect voltage sags. Many investigations in the past have referred to these, as parameters affecting the sag. The most important of these are briefly described below: Taking into account the influence of loading conditions, thc estimated sag is different. Higher deviations may be observed in realistic distribution networks where pre-fault bus voltages may be lower than rated[5]. The number of voltage sags per bus increases, as the system becomes more interconnected. Similarly, the area affected by voltage sags following a short circuit increases significantly if the network is more interconnected.
The influence of induction motor loads on the charactcrizarion of voltage sags is of great importance, as indicated by recent researchers [3,6,7]. They decelerate during thc short circuit, and for a time interval behave as generators, keeping a higher voltage supplied by its internal or back electromotive force (emQ Aftcr fault clearing the motor will accelerate again, drawing a high reactive current from the supply, until the steady-state speed is reached, causing a prolonged postfault voltage sag. The deceleration and acceleration of motors influences the duration and shape of voltage sags. Sensitive equipment. which was able to withstand original voltage sag would trip during the post sag period due to the induction motor effects. Figure 2 demonstrates the effect the motor has on the voltage sag [XI. w i t h o u t the
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Mitigation of the voltage sag effect on the system and its apparatus can be achieved in many ways. Generally the methods can be classified into the following categones: - Reducing the frequency of sag occurrence - Improve the power system
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Figure 2 Effect of motor load on Voltage sag Magnitude and duration The motor load limit fur each particular system should be determined based on the existing sensitive
equipment. The system recovery can seriously be affected by the induction motor presence. Recent research [7], has shown that sag currents can reach levels higher than the direct start values and the post sag overcurrent duration can last more than twice the normal start time period. The effect of distribution system protection on voltage sag could result into quality issue problems. Thus, protection should be dcsigned, while taking into consideration reliability indices regarding voltage sags in depth as well as in duration. Voltage sag lasts as long as the protection equipment allows current to flow. Every protection system has a minimum operation time. Time delays are also introduced for coordination and discrimination reasons. Furthermore, the autoreclosure operation can complicate things. Most voltage sags last for less than 0.2 s. Voltage sag ends when the clearing device interrupts the current flow at zero crossing. Therefore the rate at which the voltage retums to its rated value is great. Transformers, connected D-Y or Y-D will alter unbalanced voltage sags. Thus the importance of including the effect of transformers in the calculations arises. It also provides a tool for controlling the effect of voltage sags, where a particular connection could improve performance by reducing the problems. Effects of voltage sag on system apparatus Voltage sags can easily disrupt the operation of sensitive loads such as electronic adjustable speed drives (ASDs). If the sag is short, it could cause the ASD to introduce speed fluctuations, and damage the end product. In order to improve the ASD performance during a sag event, researchers have studied many approaches [9,10]. Amongst them, it is the integrated boost converter (IBC), design of ASD with active rectifiers and voltage sag ride-through capability. ASDs could trip either because the power supply of the control electronics of the drive also experiences a voltage sag, or because some processes can not tolerate the loss of precise speed or torque control. even for a small time interval.
The voltage sag magnitude and duration also influences non-utility generator’s shaft torque, because the transient electromagnetic torque is generated at the moment of fault occurring and its clearing by the protection devices, and continues to oscillate. Examples of other equipment, sensitive to voltage variations, are briefly described below:
Process controllers can be very sensitive to voltage sags They can cause the equipment they control, to
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trip off-line during voltage sag conditions. During extreme .voltage sags, enough units can trip, causing large momentarily losses. Electronic chip testers are also very sensitive to voltage variations and because o f their complexity, they require a large amount of time to restart. In addition the chips involved in the testing process can be damaged. Programmable logic controllers are very important because any industrial process is often under the control of such devices. Some have been found to trip for voltages as much as 90%, for a few cycles. Machine tools are can be very sensitive too. Often, machines are used for cutting, drilling and metal processing. Any variation in voltage can affect the quality of the machined part. Determining equipment sensitivity can be a difficult task when analyzing voltage sag concems. However it is essential to evaluate it correctly, in order to know the effective voltage sag levels and be able to implement mitigation with minimum cost.
EQUIPMENT COMPATIBILITY Equipment compatibility is defined, according to the specific site, based on the acceptability curve representing the required voltage level. Usually the compatibility is judged with the use of an index value, representing the quality of supplied voltage. In this case the SAFRI related index is used to demonstrate the quality assessment procedure. The SAFRl index (System Average RMS Variation Frequency Index) relates how often the magnitude of a voltage sag is below a specified threshold. It is a power quality index which provides a rate of incidents, in this case voltage sags, for a system [ 1 I]. SARFI-X corresponds to a count or rate of voltage sags, swell and interruptions below a voltage t threshold. It is used to assess short duration rms variation events only. SARFI-Curve corresponds to a rate of voltage sags below an equipment compatibility curve. For example SARFI-CBEMA considers voltage sags and interruptions that are not within .the compatible region of the CBEMA curve. Since curves like CBEMA do not limit the duration of a rms variation event to 60 seconds, the SARFI-CBEMA curve is valid for events with duration greater than % cycle. To demonstrate the use of this method the following table is assumed for a given site [ 121.
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Table 1. Voltage sag Event Characteristics Magnitude (pu) IDuration (s)l
ITIC Curve Scatter Plor The ITIC Curve describes an ac input voltage boundary that typically can be tolerated [15]. Events above the upper curve or below the lower curve are presumed to cause the misoperation of information technology equipment. The curve is not intended to serve as a design specification for products or ac distribution systems. In this case, the number o f events which are below the lower limit of the ITIC curve, in Fisure 4, is six, giving a SAFRI-ITIC of six events.
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The table above will be used with various typcs of scatter plots to illustrate the use of known curves in equipment compatibihty studies. Scatter plots - CBEMA curve Scatt’er Plot The CBEMA chart presents a scatter plot of the voltagc magnitude and event duration for each rms variation. The CBEMA group created the chart as a means to predict equipment misoperation due to rms variations. An rms variation event with a magnirude and duration that lies within the upper and lower limit of the CBEMA curve, has a high probability to cause misoperation of the equipment connected to the monitored source [I 3,141. Observing Figure 3, the number of events, which are below the lower limit of the CBEMA curve, IS seven, giving a SAFRI-CBEMA of seven events
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Figure 4 The lTlC curve Scatter Plot - SEMI Curve Scatter Plot In 1998, the Semiconductor Equipment and Materials International group, power quality and Equipment Ride Through Task force, recommended SEMI Standard F-47 Curve to predict voltage sag problems for semiconductor manufacturing equipment. Figure 5 below shows the application of the data o f Table I on the SEMI curve.
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[Is]. Figure 3 CBEMA curve Scatter Plot
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Voltage sag coordination chart-IEEE Std.493 and 1346
The chart contains the supply performance for a given site through a given period, and the tolerance of one or more devices. It illustrates the number of events as a function of event severity. Observing the graph shown in figure 6 , there are 5 events per year where the voltage drops below 40% of nominal for 0.1 s or longer [16]. Equally are there 5 events more severe than 70% magnitude and 250 ms duration.
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less than the given magnitude for longer than the given duration. EPRIiElectrotek mentions that each phase of each ms variation measurement may contain multiple components [ 131. Consequently, these phase rectangdar voltage sag measurements are easily characterized with respect to magnitude and duration. Approximately IO% of the events are non-rectangular. These events are much more difficult to characterize because no single magnitude-duration pair completely represent the phase measurement. The method suggested for calculating the indices used by electrotek is called the "Specified Voltage" method. This method designates the duration as the period of time that the rms voltage exceeds a specified threshold voltage level used to characterize the disturbance' The consequence of this method is that an event may have a different duration when being assessed at different voltage thresholds as shown in figure 7.
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