Power quality benchmarking is an important aspect in the overall structure of a power quality program. The benchmarking process begins with defining the metrics to be used for benchmarking and evaluating service quality. The EPRI Reliability Benchmarking Methodology project defined a set of PQ indices that serve as metrics for quantifying quality of service. These indices are calculated from data measured on the system by specialized instrumentation. Many utilities around the world have implemented permanent PQ monitoring systems for benchmarking power quality.
However, there are still considerably large gaps in coverage of the power system with PQ monitors. As part of the EPRI Reliability Benchmarking Methodology project, investigators explored the idea of estimating the voltages at locations without monitors given the data at only one monitor or a few monitors. This resulted in the development of the concept of the EPRI Power Quality State Estimator (PQSE), which uses feeder models and recorded data to estimate what would have been recorded on the customer side of the service transformer.
Because of sensitive customer loads, there is a need to define the quality of electricity provided in a common and succinct manner that can be evaluated by the electricity supplier as well as by consumers or equipment suppliers.
One of the basic tenets of solving power quality problems is that disturbances in the electric power system are not restricted by legal boundaries. Power suppliers, power consumers, and equipment suppliers must work together to solve many problems. Before they can do that, they must understand the electrical environment in which endues equipment operates. This is necessary to reduce the long-term economic impact of inevitable power quality variations and to identify system improvements that can mitigate power quality problems.
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The power quality indices was defined for the Electric Power Research Institute (EPRI) Reliability Benchmarking Methodology (RBM) project to serve as metrics for quantifying quality of service. The power quality indices are used to evaluate compatibility between the voltage as delivered by the electric utility and the sensitivity of the end user’s equipment.
A few of the indices have become popular, and software has been developed to compute them from measured data and estimate them from simulations. We will examine the definitions of some of the indices and then look at how they might be included in contracts and planning.
Power Quality Benchmarking Process:
Electric utilities throughout the world are embracing the concept of benchmarking service quality.
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The typical steps in the power quality benchmarking process are:
i. Select Benchmarking Metrics:
The EPRI RBM project defined several performance indices for evaluating the electric service quality.
ii. Collect Power Quality Data:
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Collect power quality data involves the placement of power quality monitors on the system and characterization of the performance of the system. A variety of instruments and monitoring systems have been recently developed to assist with this labor-intensive process.
iii. Select the Benchmark:
This could be based on past performance, a standard adopted by similar utilities, or a standard established by a professional or standards organization such as the IEEE, IEC, ANSI, or NEMA.
iv. Determine Target Performance Levels:
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These are targets that are appropriate and economically feasible. Target levels may be limited to specific customers or customer groups and may exceed the benchmark values.
The benchmarking process begins with selection of the metrics to be used for benchmarking and evaluating service quality. The metrics could simply be estimated from historical data such as average number of faults per mile of line and assuming the fault resulted in a certain number of sags and interruptions. However, electricity providers and consumers are increasingly interested in metrics that describe the actual performance for a given time period. The indices developed as part of the EPRI RBM project are calculated from data measured on the system by specialized instrumentation.
Electric utilities throughout the world are deploying power quality monitoring infrastructures that provide the data required for accurate benchmarking of the service quality provided to consumers. These are permanent monitoring systems due to the time needed to obtain accurate data and the importance of power quality to the end users where these systems are being installed. For most utilities and consumers, the most important power quality variation is the voltage sag due to short-circuit faults.
Finally, after the appropriate data have been acquired, the service provider must determine what levels of quality are appropriate and economically feasible. Increasingly, utilities are making these decisions in conjunction with individual customers or regulatory agencies. The economic law of diminishing returns applies to increasing the quality of electricity as it applies to most quality assurance programs.
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Most utilities have been benchmarking reliability for several decades. The metrics are defined in terms of system average or customer average indices regarding such things as the number of interruptions and the duration of interruption (SAIDI, SAIFI, etc.). However, the reliability indices do not capture the impact of loads tripping off-line for 70 percent voltage sags nor the loss of efficiency and premature equipment failure due to excessive harmonic distortion.
Indices were defined for:
i. Short-duration rms voltage variations. These are voltage sags, swells, and interruptions of less than 1 min.
ii. Harmonic distortion.
iii. Transient over-voltages. This category is largely capacitor-switching transients, but could also include lightning-induced transients.
iv. Steady-state voltage variations such as voltage regulation and phase balance.