% Volts % Volts Economics of Different Plant Ride-Through Improvement Solutions for Power System Problems Mark McGranaghan, Electrotek Concepts Chris Melhorn, Electrotek Concepts with each technology. The improved performance is then Introduction translated into economic benefits for customers based on the expected costs of the different types of power quality Many industrial and commercial electric customers now variations (in this case, voltage sags of different severity). require a higher level of power quality due to increasing With the costs of the different technologies and the sensitivity of sophisticated process controls and the expected benefits, benefit/cost ratios can be calculated to growing reliance on computers. These customers are compare the alternatives. especially sensitive to momentary voltage sags (Figure 1) caused by remote faults on the transmission system or on parallel feeder circuits. The Example System Determining the optimum supply system and customer The example system used for this evaluation involves a electric system characteristics for these sensitive customers plastics manufacturer supplied from a 13 kV distribution requires an economic evaluation of different alternatives. system. The plant has four step down transformers to Power quality can be improved through system-side supply various plant loads (see Figure 3). solutions, customer service entrance solutions, power conditioning for selected equipment within a facility, or The facility has a primary feeder and another feeder that improved specifications and equipment design. All of can be used as an alternate in the case of problems with the these alternatives have costs and associated benefits. primary feeder. There is one other feeder from the bus at each of the substations that supply the plant. This paper describes a procedure for performing economic evaluations of different power quality improvement The most critical loads in the plant are extruder process alternatives. The alternative technologies for improving machines used to produce plastic bags. Figure 2 is a power quality are identified and evaluated in terms of the simplified block diagram of the overall machine. expected performance improvements that can be obtained Extruder Drives Extruder Blowers (93 kW, 45 kW), (1.1 kW, 380 W), Bubble Cage Heaters Barrel Cooler Convey Blowers (380 W, 1.1 kW) 1449 August 19, 1994 at 06:43:01 PQNode Local (11.2 kW) (2 x 7.5 kW) Phase C Voltage Trigger RMS Variation 120 Pinch Blower 110 Duration (380 W), 100 0.267 Sec 90 Pinch Drive 80 Controls (1.6 kW), Min 37.00 70 Pinch Turn (400 W) 60 Ave 57.16 50 40 Max 98.27 30 Ref Cycle 0 0.1 0.2 0.3 0.4 0.5 0.6 Winder Drives 41405 Time (Seconds) (2 x 1.6 kW), Winder Hydraulic 100 (1.5 kW) 75 50 25 0 Figure 2. Diagram of Extruder Process Machines -25 -50 -75 -100 0 25 50 75 100 125 150 175 200 Time (mSeconds) BMI/Electrotek Figure 1. Example of Voltage Sag that will cause process to shut down 1 Transmission System Sub1 138 kV Sub2 69 kV Sub1 13 kV Sub2 13 kV N.O. Plastics Plant Figure 3. Simplified One Line Diagram of Example System faults on the transmission system but all actual Characterizing System interruptions to the plant are caused by distribution faults. Performance The first step in the evaluation is to characterize the Characterizing Equipment performance of the system. The most important Sensitivity disturbances affecting the facility are voltage sags and momentary interruptions which occur when there is a fault The voltage sags are not a concern unless they cause on the supplying power system. The voltage sags can be equipment to misoperate. This depends on the equipment caused by faults on the distribution system or the sensitivity to disturbances (ride through characteristics). transmission system. The dc drives, ac drives, and controls that make up these machines can be particularly sensitive to voltage sags. The The expected system performance is characterized through sensitivity can be determined by logging impacts to the monitoring efforts and calculations that can be made by the machines and correlating them with monitored voltage sag supplying utility using historical fault performance characteristics. Both the magnitude and duration can be information. Figure 4 is an example of the expected important, as indicated by this plot of events that caused performance broken down by severity of the sags and the extruders to trip at another plastics plant. cause of the voltage sag (transmission or distribution). This expected performance is in line with national average If voltage sags where the voltage goes below 80% cause statistics determined by EPRI over a two year monitoring equipment to misoperate, the example facility would period with approximately 300 sites across the country. experience 20 events per year that cause problems Note that in this case the less severe sags are dominated by according to the performance estimates in Figure 4. 2 Number of Events per Year Expected Voltage Sag Performance 70 60 Distribution Transmission 50 40 30 20 10 0 Interruptions < 50% < 60% < 70% < 80% < 90% Minimum Sag Voltage Figure 4. Expected Voltage Sag Performance. EXTRUDER SHUTDOWNS (Unprotected Lines) 100 90 80 70 60 PARTIAL Voltage 50 ALL % 40 30 20 10 0 1 10 100 Duration, Cycles Figure 5. Example of Extruder Sensitivity to Voltage Sags. 3 plant but it is sufficient to protect the extruder machines Power Quality Improvement themselves. Technologies A variety of different options for power quality improvement can be considered, ranging from power Static Supply 3 phase ac 3 phase ac Protected Switch Load conditioning at sensitive loads to energy storage technologies on the distribution system. The most important categories for the power quality improvement options are discussed briefly here. Inverter System System Controls and Monitoring End Use Equipment Power Conditioning It is almost always best to evaluate the potential to improve Energy Storage Technology (Batteries, the performance of the end use equipment itself first. This Charging Flywheel, SMES, System Capacitors) can be accomplished through the specification stage if ride through characteristics are considered before the equipment is purchased. After installation, various retrofit alternatives Figure 6. General configuration for standby energy can also be available. These may include protection of storage technologies. controls, PLCs, and starters with constant voltage transformers or other small ride through technologies. Sometimes, the modifications to the controls or relay settings are possible, such as adding a delay to prevent Technologies for Supply System tripping for very short voltage sags (e.g. less than a half Application second). A number of different supply-side options are available for This option involves understanding the design and improving the voltage sag and interruption performance at installation of each machine and its controls. There may be customer facilities over a portion of the distribution system. some significant engineering effort involved to identify specific loads for protection, size the protection, and The first and most obvious solution is to eliminate faults on coordinate with the overall process. However, this effort is the power system. Of course, it’s not feasible to usually very worthwhile and economically justified. completely eliminate faults. Measures that help include improving tower grounding, applying arresters, using Technologies for Service Entrance animal guards, tree trimming, and preventive maintenance practices. Application When system maintenance and protection practices have For plants that have critical loads making up a large portion been improved as much as possible and additional of the total plant load or when it is possible to segregate the performance improvement is needed, there are a few new loads in the plant so that all the critical loads can be technologies that can be employed. These include the supplied from a common service, service entrance distribution voltage restorer (DVR) which can be series protection may be appropriate. This takes advantage of the connected to compensate for voltage sags and distribution economies of scale associated with protection of larger static switches to instantaneously switch to a backup feeder loads. The most obvious choice for protection at the in the event of a disturbance. service entrance is still UPS systems. They can be obtained with individual unit sizes up to 1000 kVA and they can be For this example, a static switch is evaluated because there paralleled to obtain much larger installations. is an alternate feeder available that can be used for the backup. Although conventional UPS systems (static or rotary) are applicable for protecting large loads at the service entrance, many other options are available. They may have significant advantages in terms of lower operating costs, Economic Analysis of improved efficiency, and reduced maintenance. Usually these alternative technologies involve some type of energy Alternatives storage technology configured as a standby power supply. A static switch is used to switch over to the backup supply Weighting Factors for Different Power in the event of a disturbance. Figure 6 is a general block diagram for a wide range of these technologies. Quality Variations In this case, the economic evaluation involves a 2 MW The actual dollar impacts of the different types of energy storage system that can provide ride through support disturbances are often not known or may be confidential for for 10 seconds. This is not enough to protect the entire a customer operation. However, it is clear that power interruptions are generally more severe than momentary 4 voltage sags. It is often possible for a customer to estimate caused by transmission faults. Technologies like ride the economic impacts for a power interruption because all through support for controls on individual machines will unprotected equipment will trip and the impact to the not help the whole machine ride through actual process can be determined. Different costs will be interruptions associated with less severe voltage sags because not all unprotected equipment will trip (some equipment can ride Table 1. Assumed weighting values for voltage sags of through the voltage sags) and not all processes will be different severities and application of the weighting factors interrupted. The procedure developed here uses the to the expected system performance. concept of weighting factors for different power quality Equivalent Weighting for Expected Number Interruptions per variations. Category of Event Economic Analysis per Year Year The weighting factors are developed using the cost of a Interruption 100% 6 6.0 Sag below momentary interruption as the base. Usually, a momentary 50% 100% 0 0.0 interruption will cause a disruption to any load or process Sag between that is not specifically protected with some type of energy 50% and 70% 50% 6.5 3.3 storage technology. These base costs associated with a Sag between 70% and 80% 20% 8.5 1.7 momentary interruption will be designated as Ci. Voltage Sag between sags and other power quality variations will always have an 80% and 90% 10% 42.5 4.3 impact that is some portion of this total shutdown. The weighting factors used for the example case are given TOTAL 63.5 15.2 inTable 1 and they are used to calculate an equivalent number of momentary interruptions (from an economic . point of view) for the facility. In this case, the facility Table 2 compares four different ride-through improvement experiences the economic impact equivalent to 15.2 technologies in terms of the expected performance momentary interruptions per year. If one interruption event improvement for the overall process in the plant. The costs the plant $20,000, then the annual impact of these economics of these different alternatives are then compared disturbances is approximately $300,000. in Figure 7 assuming a cost of $20,000 for a momentary interruption. Note that the most benefit per dollar spent is achieved by improving the performance of individual Evaluating the Performance machines by protecting controls. However, the primary Improvement for Each Technology static switch is also potentially an economically attractive solution in this case and combining the static switch with Each individual technology needs to be evaluated in terms protection of machine controls is very attractive. of the performance improvement that can be achieved with the technology. For instance, a primary static switch will provide support for all events that are caused by distribution faults but will not help with the voltage sags Table 2. Performance Improvement Estimates for Different Technology Alternatives Base Reduction with Reduction with Reduction with Type of Condition Performance Reduction with Service Entrance Primary Static Static Switch and Affecting Customer Weighting (events/year) Controls Protection Energy Storage Switch Controls Protection Interruptions 1 6.0 0% 60% 100% 100% Sags below 50% 1 0.0 0% 90% 90% 90% Sags 50-70% 0.5 6.5 50% 90% 50% 70% Sags 70-80% 0.2 8.5 80% 95% 30% 90% Sags 80-90% 0.1 42.5 90% 95% 10% 92% TOTAL EVENTS AFFECTING PLANT 63.5 15.2 5.6 47.45 6.2 Total Events Weighted for Severity 15.2 8.4 3.0 6.6 1.5 5 $500,000 $450,000 $400,000 $350,000 $300,000 $250,000 Solution Costs PQ Costs $200,000 $150,000 $100,000 $50,000 $0 Base Case - No Primary static switch Service Entrance Protect Machine Combined static changes Energy Storage Controls and switch with (2 MVA) Winders controls protection Figure 7. Economic Comparison of Ride Through Improvement Alternatives (chart shows the total annual costs of the power disturbances plus the annual costs of the solution – lower total bar heights are better). Summary Authors The economic evaluation procedure described provides a Mark McGranaghan manages systematic method for evaluating a range of alternatives the Power Systems Engineering that could be used to improve the reliability of plant Group at Electrotek Concepts operations during power system disturbances. The (www.electrotek.com) in technologies can be applied at the end use equipment, at the Knoxville, TN. They provide customer service entrance, or on the utility supply system. consulting services, seminars, software, and research and The procedure is based on characterizing the expected development projects for the number of power quality variations in a number of different electric power industry, especially categories. The impacts of variations in each category are in the areas of distribution system characterized by a weighting factor that expresses the planning and operations and power economic costs associated with the variation in per unit of quality. Mark has been studying power system problems the costs associated with an interruption. The total impacts and solutions for 20 years. He has been involved in to a customer are determined by summing the costs developing advanced monitoring equipment and the most associated with the events in each category (number of advanced software for analyzing and managing power expected events times the weighting factor). quality available in the industry. He has recently been involved in defining the indices that can be used to The different technologies are then evaluated by estimating characterize system power quality performance. Along the improved performance that can be expected after the with Roger Dugan and others at Electrotek, Mark is a co- technology has been applied. The cost savings are author of the book Electrical Power Systems Quality. He calculated for each technology along with the costs of has authored numerous technical publications and applying the technology. The economics are compared in magazine articles over the years. He has been active in terms of the annual cost associated with the power quality IEEE and is currently the Chairman of IEEE 519Am a Task variations and the costs of the power conditioning Force developing a “Guide for Applying Harmonic Limits technology use to improve the performance. on the Power System.” 6 quality case studies for PG&E, Con Edison, Arkansas Chris Melhorn is the Manager of Power & Light. He is currently project manager for the Utility and Industrial Studies at power quality monitoring effort at Consolidated Edison Electrotek. Chris is responsible for Company of New York. Chris is also involved in software developing marketing strategies, development efforts at Electrotek. Chris is secretary for the coordinating industrial and IEEE P1100 working group, "The Emerald Book," a commercial proposals and studies, member of the IAS Power Quality Subcommittee, and a and developing and supporting member of IEEE P1346, "Electric Power System seminars. Since joining Electrotek Compatibility with Electronics Process Equipment". Chris in 1990, he has been involved in has written over 20 technical papers for organizations like numerous projects that have the IEEE, CIRED, PQ/PCIM, and PQA. He has presented involved monitoring, simulating, and analyzing power over 50 talks and seminars on power quality that include quality phenomena. Some major projects include the EPRI tutorials and workshops on power quality monitoring and Distribution Power Quality project (RP-3098-1), power analysis. 7