Optimizing Power Quality and Reliability Initiatives
On Aug. 14, 2003, 22 million customers lost power in the Northeast Blackout, an event that brought worldwide attention to the issues of infrastructure security and reliability. Cost estimates for this event are on the order of US$6 to $8 billion.
What many people don't realize is that 500,000 customers in the United States alone lose power every day for an average of about two hours. The number of customers affected by momentary interruptions and voltage sags is even greater. These shorter duration events are most important for industrial customers. Power disturbances ranging from milliseconds (voltage sags) to several seconds (momentary interruptions) impact industrial processes. A 0.1-second event, which literally is the blink of an eye, can cause a refinery to shut down or a semiconductor processing plant to stop production. Once down, it often takes hours to bring a line back to normal production. On any given day, power disturbances such as these impact approximately 30,000 industrial customers.
Finding solutions to improve the performance of the electric infrastructure must be considered in combination with the requirements and designs of end-use equipment and processes.
The Consortium for Electric Infrastructure to Support a Digital Society (CEIDS; Palo Alto, California, U.S.), an initiative established by EPRI and its affiliate Electricity Innovation Institute E21, has launched efforts to better understand the economics of improving quality and reliability at different levels of the system. The objective is to improve the compatibility between the electric infrastructure quality and reliability, and the design of end-use technologies and processes. This compatibility will be achieved by: developing new advanced technologies to improve performance of the supply system; and by identifying market and regulatory structures that can facilitate flexible and tailored quality and reliability levels as a function of system, customer types and contractual arrangements.
The quality and reliability of the electricity supply will continue to grow in importance. But how will we define the actual quality and reliability requirements for the grid of the future and how will these levels of quality and reliability be achieved? These are complex tasks that depend on many factors, including regulatory environment, standardized indices for assessing performance, economics, technologies, standards, and the role of new automation and communications system infrastructures.
This article describes a three-part approach to assure that system reliability and quality levels meet the requirements of the digital economy.
Infrastructure and Technologies for Flexible Quality and Reliability. New technologies and system-design strategies will be required to provide improved and flexible reliability and power quality (PQ), based on end-user requirements in the digital economy.
Assessment of Regulatory and Market Structures to Support Flexible Quality and Reliability. Regulatory and market structures have an important impact on the quality and reliability characteristics of the supply system. What should the market and regulatory structures look like to support flexible and cost-justified quality and reliability supply-system characteristics?
Optimizing the Overall Economics of Reliability and Quality Levels. We still need to understand the true costs associated with quality and reliability problems. These costs should be the basis of optimizing the quality and reliability offerings as a function of customer requirements.
Defining PQ and Reliability
PQ and reliability have many different definitions, and they are often confused by end users. Electric utilities have specific definitions and indices for measuring reliability, but many customers consider any event that disrupts their processes as a reliability problem. It is important to understand the terminology and different components of the quality and reliability of the electric service.
Reliability
The term reliability is used to indicate the ability of a system to continue to perform its intended function. Power-system reliability is measured in terms of the ability of the power system to provide electricity to end users. It is usually measured at the service to the end users, and tracked by average frequency and duration indices that are reported to regulators and commissions.
Outages are typically defined as lasting less than 5 minutes, and momentary problems, such as voltage sags, are not considered in reliability indices. However, these shorter events can affect the reliability of end users.
Power Quality
While reliability measures the availability of electric service to end users, PQ measures a wide range of power-supply characteristics that also can influence the performance of equipment and processes. In other words, the reliability of end-use processes is dependent on both the reliability and quality of the electric service.
It is important to note that many PQ characteristics are a function of both the supply system and end-user system and equipment characteristics. For instance, customer nonlinear loads that draw distorted currents and interact with the supply-system impedance typically cause harmonic distortion. This influences the need to achieve compatibility between the characteristics of the supply system and end-user equipment characteristics. It is not sufficient to just evaluate the performance of the supply system.
The steady-state PQ characteristics of the supply voltage include frequency variations, voltage variations, unbalances in the three-phase voltages, flicker and harmonic distortion.
The definition of PQ characteristics can be found in IEEE Standard 1159 and in IEC Standard 61000-2-2. The characteristics fall into two major categories: steady-state PQ variations and disturbances. “Compatibility levels” for these characteristics should be identified. These compatibility levels are designed such that end-use equipment will operate properly and are feasible for the system to achieve. Most of these PQ characteristics require close coordination between the operation of end-use facilities and the performance of the power system. IEC has defined compatibility levels for these characteristics, and these compatibility levels are the basis of actual requirements for the power-system performance in Europe (EN 50160).
Limits for steady-state voltage characteristics can be specified in definite terms. However, these characteristics are continually varying, and they are characterized with trends and probability distributions. The limits also must be specified, taking into account the probabilistic nature of the variations (Fig. 1).
Limits for variations in the steady-state PQ characteristics are developed based on the concept of compliance for a percentage of the observation time, such as 95% of the observations in any period of one week (see table). Future research into optimizing system performance and compatibility should address the relationship between the performance levels for different supply systems and the immunity characteristics of end-use equipment.
PQ disturbances also can affect the operation of end-use equipment and processes. These include transient voltages, voltage sags and momentary interruptions. Voltage sags (Fig. 2) and momentary interruptions are probably the most important. Customers experience many more voltage sags than actual interruptions. If the customer facility includes sensitive equipment, these voltage sags can be as disruptive as actual interruptions.
Confusion between electric utilities and end users arises when electric utilities think of reliability in terms of 5-minute outages, and customers think of reliability problems as any event that interrupts their processes. When this includes voltage sags, the difference in perspective can be difficult to reconcile.
It is important that the supply infrastructure provide the quality needed for sensitive loads in the digital economy. Voltage sags will continue to be an important issue. Providing service relatively free of voltage sags will improve the productivity of many industries and may be critical to a much wider range of customers as automation reaches the home. The economics of improving the performance of the supply infrastructure must be weighed against improving the immunity of equipment to voltage sags.
Electric utilities worked together with the semiconductor industry (both customers and manufacturers) to develop new standards for equipment performance based on the quality of power supplied by typical power systems (SEMI F47 Standard). This model may be important for many other industries as they seek to improve productivity and compatibility with the supply. Research is still needed to evaluate the relative economics of the different approaches on a system-wide scale, based on a range of technologies available for improving performance on both sides of the meter.
Benchmarking the System
Understanding system performance is a prerequisite to setting any standards for system performance. Benchmarking efforts will provide a more complete picture of the performance of the supply system as a function of important parameters:
Urban versus rural systems
Underground versus overhead systems
System topologies
Voltage levels, source strength and the number of circuits from a common bus
Lightning activity and other causes of system disturbances
Investment in maintenance and equipment.
As the cost of system monitoring continues to decrease, we must evaluate system performance in terms of quality and reliability characteristics on a continuous basis. This information provides the basis for evaluating the economics of system improvements versus investments in end-user technologies to improve their immunity to disturbances.
As the expected performance of the supply system is better understood, information can be used to prioritize maintenance and expenditures on system-improvement technologies. For instance, United Illuminating monitors at all distribution substations and tracks voltage-sag performance monthly. The company uses the chart in Fig. 3 to track performance at individual substations. The chart shows voltage sag performance at each substation over the past five years compared with performance for the last year. It also divides sags into events that are caused by transmission faults and events caused by distribution faults. This helps focus on areas where performance improvement can be expected with investment in the distribution system.
End-User Compatibility
Regulators are grappling with setting standards for re-liability based on historical performance. It is not clear that these historical performance levels are in any way related to the optimum performance potential for end-use equipment and different types of customers.
Some equipment manufacturers are doing a better job of describing equipment performance when subjected to power-supply variations. The ITI curve, developed by the Information Technology Industry Council, defines the expected performance of PCs, printers and monitors. This concept was taken up by the semiconductor industry, in cooperation with utilities, and a new standard for semiconductor production equipment was developed that will result in more reliable performance for typical power systems. The SEMI ride-through curve (Fig. 4) specifies that these tools should ride through sags with a minimum voltage as low as 50% that last up to 200 msec.
| Power-Quality Category | Limits |
|---|---|
| Voltage Regulation | +/- 5% of nominal for normal conditions +/- 10% of nominal for unusual conditions |
| Voltage Unbalance | 2% negative sequence |
| Voltage Distortion | 5% total harmonic distortion 3% individual harmonic components |
| Voltage Flicker | Pst* less than 1.0 individual step changes less than 4% |
| * Note: Pst is a measure of flicker where a value of 1.0 indicates that 50% of the people are likely to be able to notice flicker in a 60-W incandescent lamp. Measurement procedures are defined in IEC Standard 61000-4-15 and are being adopted by IEEE (Standard 1453). | |
System Improvement Economics
As described previously, the costs associated with power outages can be tremendous. A single outage can cost a manufacturing facility anywhere from $10,000 to millions of dollars. Costs to banks, data centers and customer-service centers can be just as high if not higher. Unfortunately, these facilities can be sensitive to a wider range of PQ disturbances than outages counted in utility-reliability statistics. Momentary interruptions or voltage sags lasting less than 100 msec can have the same impact as an outage lasting many minutes.
Commercial facilities have access to a variety of technologies for equipment protection and improving PQ. Utilities must evaluate the need to improve quality of supply if large numbers of customers are impacted by PQ variations.
Improving facility performance during PQ variations can result in significant savings and can be a competitive advantage. Therefore, it is important for customers and suppliers to work together in identifying the best alternative for achieving the required level of performance.
Facility managers and utility engineers must evaluate the economic impacts of the PQ variations against the costs of improving performance for the different alternatives. The best choice will depend on the cost of the problem and the total operating cost of each possible solution.
The ultimate objective of the economic analysis is to find the optimum amount of investment in reliability and quality for both sides of the meter. This is a societal problem as well as an individual facility problem. Figure 5 illustrates the optimization problem from a societal perspective. The challenge for researchers, regulators, government, utilities and customers is to develop regulatory and market structures that encourage the investments to be made where they are the most beneficial.
CEIDS Research Initiative
A new research initiative is taking a system perspective to the reliability and quality performance problem. The research initiative is described in three areas: infrastructure and technologies, regulatory and market strategies, and optimizing economics. These areas are being addressed in a coordinated effort that characterizes previous and ongoing research, and provides a road map for achieving the optimum compatibility between supply-system performance and specific end-user system requirements.
This effort will result in indices, benchmarking results, technologies, infrastructure designs, and regulatory/market structures to facilitate optimum compatibility between the supply system and customer systems in the digital economy. As research results are turned into real systems, higher levels of productivity will be unleashed throughout the digital economy.
Mark McGranaghan is the vice president of Consulting Services at EPRI PEAC Corp. (Knoxville, Tennessee, U.S.). His current services include research projects, seminars, monitoring services, power-system analysis, testing, failure analysis, and designing solutions to improve system performance. His past efforts have included working with electric utilities in the areas of power-quality monitoring, development of power-quality programs, analysis of distribution systems, customer services and analysis software applications. He received BSEE and MSEE degrees from the University of Toledo and a MBA from the University of Pittsburgh.
mmcgranaghan@epri-peac.com
John Blevins is manager of Power Quality Services at Salt River Project, a public power and water company serving the metropolitan Phoenix area and portions of eastern Arizona. He is a member of IEEE, president of the Arizona Chapter of the 7×24 Exchange, and serves on the CEIDS Steering Committee.
jdblevin@srpnet.com
Marek Samotyj is program director of the Consortium for Electric Infrastructure to Support a Digital Society (CEIDS), an initiative established by EPRI and the Electricity Innovation Institute in 2001 to ensure an adequate supply of high-quality, reliable electricity to a digital economy and integrate energy users and markets. Before joining CEIDS, Samotyj was the business line manager responsible for EPRI Retail Sector's Technical and Business Services. He received a MSEE degree from Silesian Technical University in Poland and a MS degree engineering-economic systems from Stanford University.
msamotyj@epri.com
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