EFFECTS OF VOLTAGE SAGS IN PROCESS INDUSTRY APPLICATIONS Mark McGranaghan, Dave Mueller Electrotek Concepts, Inc. Knoxville, Tennessee Abstract This paper describes the causes of voltage sags in voltage sags include power conditioning or equipment affecting process industries, their impacts on equipment design modifications. Both of these options are described. operation, and possible solutions. The definition proposed focuses on system faults as the major cause of voltage sags. The sensitivity of different types of process industry equipment; including adjustable speed drive controls, programmable logic controllers, and motor contactors; is analyzed. Available methods of power conditioning for this sensitive equipment are described. INTRODUCTION A voltage sag is a momentary (i.e. 0.5-60 cycles) decrease in the rms voltage magnitude [1,2], usually caused by a remote fault somewhere on the power system (Figure 1). Voltage sags are the most important power quality problem facing many process industry customers. Equipment used Figure 1. Voltage sag waveform caused by a remote fault in modern industrial plants (process controllers, condition (7 cycles) programmable logic controllers, adjustable speed drives, robotics) is actually becoming more sensitive to voltage sags as the complexity of the equipment increases and the equipment is interconnected in sophisticated processes. CAUSES OF VOLTAGE SAGS Even relays and contactors in motor starters can be sensitive to voltage sags, resulting in shut down of a process when Voltage sags are typically caused by fault conditions. Motor they drop out. starting can also result in undervoltages, but these are typically longer in duration than 60 cycles and the It is important to understand the difference between an associated voltage magnitudes are not as low. Motor interruption (complete loss of voltage) and a voltage sag. starting voltage variations are often referred to as "voltage Interruptions occur when a protective device actually flicker", especially if the motor starting can occur interrupts the circuit serving a particular customer. This frequently. will normally only occur if there is a fault on that circuit. Voltage sags occur during the period of a fault for faults Faults resulting in voltage sags can occur within the plant over a wide part of the power system. Faults on parallel or on the utility system. The voltage sag condition lasts feeder circuits or on the transmission system will cause until the fault is cleared by a protective device. In the plant, voltage sags but will not result in actual interruptions. this will typically be a fuse or a plant feeder breaker. On Therefore, voltage sags are much more frequent than the utility system, the fault could be cleared by a branch fuse interruptions. If equipment is sensitive to these voltage or a substation breaker. If reclosing is used by the utility, sags, the frequency of problems will be much greater than if the voltage sag condition can occur multiple times, with the equipment was only sensitive to interruptions. varying durations (Figure 3). Also, faults on the distribution system can result in voltage sags that are This paper describes the voltage sag characteristics and the difficult to characterize with simple magnitude/duration sensitivity of equipment. With this information, the range information because the fault characteristics change with of fault locations on the power system that can cause time (Figure 4). problems can be estimated (area of vulnerability). Options for improving equipment performance in the presence of 1 Voltage (%) will not reinitiate after they have been cleared and the line is reclosed. Since faults (and, therefore, voltage sags) are inevitable, it is important for customers to make sure that critical equipment sensitive to voltage sags is adequately protected. PLANT VOLTAGE DURING SINGLE LINE- TO-GROUND FAULTS ON THE UTILITY SYSTEM Single line-to-ground faults (SLGFs) on the utility system are the most common cause of voltage sags in an industrial Figure 2. Typical distribution system one line diagram plant. The voltage on the faulted phase goes to zero at the illustrating types of protection devices. fault location. The voltage at the substation and on parallel feeders will depend on the distance of the fault from the Recloser Operating Sequence substation. On transmission systems, the faulted phase Contacts Closed voltage at a remote location depends on the overall network Fault Current impedances. Load Current Recloser Lockout 30 Cycles 5 Sec 30 Sec The important quantities for equipment sensitivity are the Fault voltages at the customer bus. These voltages will depend on Initiated Reclosing Intervals (Contacts Open) the transformer connections between the faulted system and the customer bus. For a distribution system fault, the worst Figure 3. Typical recloser operating sequence. case occurs when the fault is close to the substation bus. Effectively, this is the same as a fault near the customer RMS Variation transformer primary. The voltages on the customer bus will 120 then be a function of the customer transformer connections, Duration as indicated in Table 1. 100 0.700 Sec Min 80 Table 1. Transformer secondary voltages with 26.31 Ave a SLGF on the primary. 60 Transformer Phase to Phase Phase to Neutral Phasor 78.60 Connection Vab Vbc Vca Van Vbn Vcn Diagram Max 40 101.5 20 0.58 1.00 0.58 0.00 1.00 1.00 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Time (Seconds) Figure 4. Example of voltage sag caused by a distribution system fault. Faults on the transmission system can affect even more customers. Customers hundreds of miles from the fault location can still experience a voltage sag resulting in 0.58 1.00 0.58 0.33 0.88 0.88 equipment misoperation when the fault is on the transmission system. The large majority of faults on a utility system are single line-to-ground faults (SLGF). Three phase faults are more severe, but much less common. SLGFs often result from weather conditions such as lightning, wind, and ice. Contamination of insulators, animal contact, and accidents involving construction or transportation activities also cause faults. Although utilities go to great lengths to prevent faults on the system, they cannot be eliminated completely. Usually, these faults are temporary, which means that they 2 • Adjustable speed drive and other power electronic devices that use 3-phase power will be connected 0.33 0.88 0.88 ---- ---- ---- directly to the LV bus, or through an isolation transformer. • Lighting often utilizes single-phase connections from phase-to-neutral. • Control devices such as computers, contactors, and programmable logic controllers are often supplied 0.88 0.88 0.33 0.58 1.00 0.58 through a single phase control transformer. The voltages experienced during a voltage sag condition will depend on the equipment connection. Table 1 showed that the individual phase voltages and phase-to-phase voltages are quite different during a SLGF condition on the transformer primary. Some single phase loads will be unaffected and other single phase loads may drop out, even though their sensitivities to voltage sags may be identical The relationships in Table 1 are very important. One might think that a SLGF on the primary of a wye grounded/delta Voltage unbalance is also a concern for motor heating. transformer could result in zero voltage across one of the However, the durations of the unbalanced voltages during secondary windings. Instead, circulating fault current in fault conditions are so short that motor heating is not a the delta secondary windings results in a voltage on each significant concern. Adjustable speed drives, however, may winding. The magnitude of the lowest secondary voltage have controls that trip very quickly during unbalanced depends upon the relationship: conditions. X T α = X + X T S 0 < α < 1 Different categories of equipment and even different brands of equipment within a category (e.g. two different models of adjustable speed drives) have significantly different where: XT - Transformer short circuit reactance sensitivities to voltage sags. This makes it difficult to XS - Source equivalent reactance develop a single standard that defines the sensitivity of industrial process equipment. For industrial power distribution, the ratio α will usually be very close to unity and the relationships in Table 1 are for The closest document to a standard is the CBEMA curve this case. given in Figure 5, which was developed by the Computer Business Manufacturers Association [3]. This applies Even with a SLGF on the primary of the transformer, the primarily to data processing equipment. The curve shows voltage sag at the customer bus will be no lower than 33% that the load sensitivity is very dependent on the duration of normal value. These faults account for the great majority of the sag. Allowable sags range from 0% voltage for 1/2 faults on the power system. cycle to only 87% voltage for 30 cycles. SENSITIVITY OF EQUIPMENT TO VOLTAGE SAGS Process industry equipment can be particularly susceptible to problems with voltage sags because the equipment is interconnected and a trip of any component in the process can cause the whole plant to shut down. Examples of these industries include plastics, petrochemicals, textiles, paper, semiconductor, and rubber). Important loads that can be impacted include the following: • Motors, heating elements, and other 3-phase loads can be connected directly to the LV bus. 3 The sensitivity range for these types of equipment is shown in Figure 6 with the durations of fault induced voltage sags also indicated. The wide range of sensitivities underlines the importance of working with the manufacturer to make sure the equipment can work in the environment where it will be used and to develop specifications based on realistic power system conditions. It is important to recognize that the entire process in an industrial plant can depend on the sensitivity of a single piece of equipment. The overall process involves controls, drives, motor contactors, robotics, etc. that are all integral to the plant operation. This can also make it difficult to identify the sensitive piece of equipment after the entire process shuts down. Figure 5. CBEMA operating voltage envelope. While the CBEMA limits suggest a "standard" sensitivity to voltage sags, actual plant equipment has a variety of operational characteristics during voltage sags. A few examples are listed here. Motor Contactors and Electromechanical Relays One manufacturer has provided data that indicates their line of motor contactors will drop out at 50% voltage if the condition lasts for longer than one cycle. This data should be expected to vary among manufacturers, and some contactors can drop out at 70% normal voltage or even higher.[4] Figure 6. Range of equipment sensitivity to voltage sags. High-Intensity Discharge (HID) Lamps ESTIMATING THE PROBABILITY Mercury lamps are extinguished at around 80% normal OF A VOLTAGE SAG PROBLEM voltage and require time to restrike [5]. A voltage sag that extinguishes HID lighting is often mistaken as a longer Voltage sags and momentary interruptions are caused by outage by plant personnel. faults on the power system. Therefore, determining voltage sag performance characteristics involves calculation of fault Adjustable Speed Motor Drives (ASDs) performance characteristics for the power system supplying Some drives are designed to ride through voltage sags. The a particular customer of interest. Faults over a wide area of ride through time can be anywhere from 0.05 sec to 0.5 sec, the power system can affect the operation of a facility that obviously depending on the manufacturer and model. Some has sensitive equipment. Faults can occur on the models of one manufacturer monitor the ac line and trip transmission system or on the distribution system. For most after a voltage sag to 90% of normal voltage is detected for facilities, both cases need to be evaluated to estimate the 50 ms. overall performance expected. Figure 7 illustrates the breakdown of events that caused equipment disruption for Programmable Logic Controllers (PLC's) one process industry customer. This is an important category of equipment for industrial processes because the entire process is often under the control of these devices. The sensitivity to voltage sags varies greatly but portions of an overall PLC system have been found to be very sensitive. The remote I/O units, for instance, have been found to trip for voltages as high as 90% for a few cycles [8]. 4 Breakdown of Events Causing Equipment Disruptions at Industrial Plant Voltage Sag Analysis Flow Chart Faults on Transmission Faults on Own Feeder System 23% 31% Determine voltage sag performance due to transmission system faults Faults on Parallel Feeders Is customer connected at 46% Yes transmission level? Figure 7. Breakdown of utility fault events that caused equipment disruption at a MV customer. No For facilities that are supplied directly from the transmission level, only transmission faults usually need to Determine voltage sag be considered. performance due to distribution system faults The first task is estimating the expected voltage sag and momentary interruption performance. This analysis will result in information that describes the expected number of voltage sags per month where the voltage goes below a Determine equipment sensitivity to voltage sags specified threshold. Then this performance is compared to and momentary interruptions equipment to determine the expected performance of the process or the overall facility. Finally, methods for improving the performance can be evaluated at the different levels of the system. The overall flow chart for the Perform economic analysis of available solutions to improve evaluation is given in Figure 8. equipment performance Transmission System Performance Evaluation Figure 8. Voltage sag evaluation procedure. This evaluation must be performed, regardless of the end user facility location. For facilities supplied from MV A standardized procedure can be used to calculate expected systems, the performance of the transmission system performance. The result of the calculation is the expected determines the expected number of voltage sags and voltage sag performance at a selected bus on the system. interruptions due to transmission faults. This is measured as the voltage at the supplying substation. 1. Build and maintain a transmission line data table for reference. This table will include the historical performance information and expected performance for each line section in terms of number of faults expected per year for at least single line-to-ground and three phase faults. 2. Perform short circuit analyses to determine the Area of Vulnerability for different voltage sag severities. This gives the total circuit miles where a fault will result in a voltage sag below a specified threshold. This analysis must be performed for at least single line-to-ground and three phase fault conditions. 3. Convert the area of vulnerability data to actual expected events per month at the specified location. This is done using the area of exposure and the expected 5 performance for three phase and single line-to-ground up and restart, damaged product, reduced product quality, faults over that area. delays in delivery, and reduced customer satisfaction. The momentary interruption performance for an end Proper evaluation of alternatives to improve plant user due to transmission system faults should be equipment and the power distribution network requires a calculated if the customer is supplied as a tap from a cost vs. benefit comparison. For example, once the costs of switched transmission line. In this case, the expected retrofitting sensitive process equipment with some method number of momentary interruptions per year due to of improving voltage sag ride through are determined, the transmission events is the expected number of faults on benefits of recovering lost production, material, product that line. This should be calculated separately from the quality, and customer responsiveness must be determined. voltage sag performance. Experience by the industrial plant will provide data on production losses for a given occurrence following a voltage 4. Perform the above calculations for different voltage sag sag. There may even be a record of the number of severities and for momentary interruptions. The results disruptions due to voltage sags in the past calendar months can be presented as a histogram for use by the end use or years. If the necessary data exists, the cost of facility (Figure 9). implementing a solution can be evaluated against the expected cash flow of recovered production losses. Solutions can be implemented at different levels of the system for an end user that has equipment or a process that 40 is sensitive to voltage sags and momentary interruptions. 33.9 35 For instance, the individual sensitive equipment can be Fault 30 Location protected with power conditioning with ride through 23.3 46kV 25 support, a whole portion of the facility could be protected, Events 115kV Per 20 15.2 230kV or measures could be implemented on the utility system to Year 15 345kV 10.3 improve performance. The individual solutions must be 10 5.0 identified and a system perspective used to evaluate the 5 1.8 economics. The most economic alternative usually involves 0 protection closest to the sensitive equipment or within the <50% <60% <70% <80% <85% <90% Voltage Sags design of the equipment itself (Figure 10). (Percent Normal Voltage) Figure 9. Example of expected voltage sag performance at a customer site due to transmission INCREASING COST system faults. 4 - Utility Solutions 3 - Overall 2 - Controls 1 - Equipment Protection Protection Specifications Inside Plant Distribution System Performance Evaluation 1 Feeder or Utility For end users that are supplied from the distribution system, Group of Source Machines CONTROLS 2 the voltage sag and momentary interruption performance 3 due to distribution system events must be calculated in a 4 MOTORS similar manner. Faults on parallel feeders and fused branches will result in voltage sags while faults on the same OTHER LOADS part of the feeder as the end user will result in at least Sensitive Process Machine momentary interruptions. The total performance at the customer is a combination of the performance due to Figure 10. Economics of voltage sag ride through support transmission events and the performance due to distribution at different levels of the system. events. In the long run, the best solution to voltage sag problems will be to purchase equipment that has the necessary ride EVALUATING SOLUTIONS FOR through capability. As manufacturers become increasingly VOLTAGE SAG PROBLEMS aware of the need for this capability, it will become more and more standard in industrial process equipment. Even The interruption of an industrial process due to a voltage now, manufacturers offer new models or simple sag can result in very substantial costs to the operation. modifications that permit extended ride through capability. These costs include lost productivity, labor costs for clean- 6 Trip Voltage Percent Until equipment can handle voltage sags directly, it will CVTs will handle the majority of voltage sag conditions. If often be necessary to apply power conditioning equipment voltage sags which are too severe for CVTs or if the loads for particular sensitive loads. Most voltage sag conditions are too large for protection with CVTs, some type of energy can be handled by ferroresonant, or constant voltage, storage technology will have to be used for ride through transformers (CVTs). CVTs are especially attractive for support. Protection for extremely critical loads, such as life loads with relatively low power requirements and loads safety systems and critical data processing equipment, which are constant. Variable loads are more of a problem should include UPS systems or the equivalent for complete for CVTs because of the tuned circuit on the output. backup capability. New energy storage technologies that can provide short duration backup for large portions of a These power conditioners work similar to a transformer facility are now becoming available. These include being excited high on its saturation curve, so that the output superconducting magnetic energy storage [10], flywheels, voltage is not significantly affected by input voltage and advanced battery systems. variations. The actual design and construction is more complicated. A typical ferroresonant circuit is shown in CONCLUSIONS Figure 11. 1. Voltage sags are becoming an increasing concern for process industries due to increasing automation. Automated facilities are more difficult to restart, and the electronic controllers used are sometimes more sensitive to voltage sags than other loads. 2. Single-line-to-ground faults on the utility distribution or transmission system are often the cause of voltage sags. Lightning is a frequent cause. Evaluation of the fault performance of transmission and distribution lines can be used to predict the voltage sag performance at a customer facility. 3. A single-line-to-ground fault on the primary side of a Figure 11. Typical circuit for a ferroresonant transformer. distribution transformer will result in a voltage sag to no lower than 33% of normal voltage on any phase-to- Ferroresonant transformers output over 90% normal voltage phase connection. as long as the input voltage is above a minimum value, at which the output collapses to zero voltage. Voltage support 4. The sensitivity of industrial equipment to voltage sags during voltage sags can be very good if the CVT is varies greatly. The more sensitive equipment widens a oversized for the load. Figure 12 illustrates the sensitivity plant's area of vulnerability to disruptive voltage sags. of a chiller control with and without an oversized CVT for protection. With ride through down to almost 30% of 5. Constant Voltage Transformers can be applied normal voltage, the chiller should never trip due to remote economically at constant loads to handle the great single line-to-ground faults on the power system. majority of voltage sag conditions. If needed, increased protection for voltage sags or actual interruptions can Temperature Process Controller Ride Through Improvement be provided in the form of UPS systems or other energy 100 W/Out Ride- storage technologies. Through 80 REFERENCES 60 CBEMA W/ Ride- 40 Through [1] IEEE P1159, Draft 6, Recommended Practice on Monitoring Electric Power Quality, IEEE, December, 20 1994. 0 0.1 1 10 100 1000 [2] IEC Standard 1000-2-2, “Compatibility Levels for Low Sag Duration in Cycles Frequency Conducted Disturbances and Signalling in Public Low Voltage Power Supply Systems.” Figure 11. Voltage sag ride through for a process controller with and without a CVT. 7 [3] IEEE Std 446-1987, Recommended Practice for Emergency and Standby Power for Industrial and Commercial Applications. [4] D. M. Sauter, "Voltage Fluctuations on Power Systems," Westinghouse Electric Utility Engineering Reference Book, Distribution Systems, pg. 362, 1965. 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