Measurement Tools at Heart of Smart Grid Need Calibration to Ensure Reliability

The North American interconnections, or electric transmission grids, operate as a hugely complex machine.

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By Tyrone Foster

The North American interconnections, or electric transmission grids, operate as a hugely complex machine. Recent history shows that minor instability in one part of the grid can lead to catastrophic failure of large parts of the system. For that and many other reasons, efforts are underway to create a new, smart grid. The new grid will be modernized and intelligent, utilizing a number of advanced computing, networking and measurement technologies. It will enable efficient energy usage, increased reliability and integration of alternative energy sources such as wind and solar.

One of those new technologies is a measurement device known as a phasor measurement unit (PMU). Sometimes stand-alone but often built into a relay, PMUs will play a critical role in monitoring stability throughout the smart grid. The PMUs will provide the knowledge necessary to minimize and control power outages and avoid problems such as cascading blackouts. But, to fulfill this promise, PMUs must be extremely accurate, reliable and fully interchangeable from model to model. That means PMU calibration requirements are extremely important. Presently, PMUs can only be calibrated in a very small number of government and university laboratories. As more PMUs are deployed in larger and larger numbers, the power transmission and distribution industry will need a more practical calibration tool.

The U.S. National Institute of Standards and Technology (NIST) recently awarded a grant to Fluke Corp. to develop a commercial PMU calibration system. The goal of this project is to develop a calibration system that can test PMUs under steady state and dynamic conditions that mimic real-world applications with lower uncertainties than any other tool. Fluke also hopes to see this calibration system adopted as the de facto standard for testing and calibrating PMUs so they can be deployed reliably in smart grids worldwide. The availability of an economical, off-the-shelf, easy-to-use PMU calibrator will help advance the widespread implementation of PMUs and thereby the implementation of a modernized energy network.

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Monitoring the Smart Grid

The smart grid encompasses a wide range of technologies that are designed to jointly modernize the electrical transmission grid. One example is time-of-day monitoring that will make it possible to charge consumers and businesses more for electricity usage during high-demand periods and less during low-demand periods. Time-of-day monitoring is also important in promoting the use of renewable energy sources. Renewable energy sources tend to be intermittent, so the power infrastructure must be able to shed demand by increasing prices when, for example, the wind stops blowing.

A critical aspect of the smart grid is the ability to monitor differences in phase angles and magnitudes between different sections of the grid. These differences can cause problems such as tripping relays at the point where grid sections are interconnected. Problems can cascade across the network—potentially causing a cascading blackout like the one experienced in the 2003 Northeast Blackout. The U.S.-Canada team that investigated this blackout recommended that PMUs be installed across North America to understand real-time grid conditions including the amount and nature of stress on the system to provide an early warning of grid problems. PMUs measure voltage, current and frequency at speeds of typically 30 observations per second. The time when each measurement is taken is recorded with a high degree of accuracy so measurements taken by PMUs in different locations can be synchronized with each other to provide a comprehensive view of the entire grid. The investigation hypothesized that if such a system had been in operation the blackout could have been avoided by identifying, understanding and mitigating the conditions that were its root cause.

As the primary measurement and sensing tool in the smart grid, PMUs will play a broader role beyond avoiding blackouts. PMU data will also be used to manage real-time grid operations to improve transmission and distribution efficiency by increasing line throughput and reducing line losses. Southern California Edison is already using PMUs to drive the automated control of static volt-amperes reactive compensators for reactive power support, and the Bonneville Power Administration (BPA) uses PMUs for a real-time stability control system. BPA, American Electric Power Co. Inc. (AEP) and the Tennessee Valley Authority (TVA) are also working to incorporate PMU data to improve the accuracy and sampling rate of their state estimation tools. PMU data is also being used to calibrate simulation models to improve power system planning.

More than 200 PMUs are already installed in North America. In 2009, the U.S. government announced an investment of $3.4 billion in energy grid modernization. This investment will include the installation of more than 850 PMUs that will monitor the complete U.S. electric grid. According to the North American Synchrophasor Initiative (NASPI), PMUs will be installed at the following locations by 2014:

• Major transmission interconnections and interfaces

• All 500 kV and above substations and most 200 kV and above substations

• Generator switchyards and even on some individual generators in plants with 500 MW or higher capacity

• Major load centers

• Large wind generators, solar and storage locations

• Other locations to assure observability in areas with sparse PMU coverage

PMUs Must be Accurate, Reliable and Interchangeable

The accuracy, speed and reliability of PMUs are critical to fulfilling the promise of the smart grid. However, the current standard that governs PMU performance is quite broad, and NIST testing has shown that PMUs from various manufacturers can report dynamic conditions very differently. For example, the primary transducer that converts high line voltages to the smaller voltages used by the measuring system can introduce inaccuracies. From the primary transducer, signals pass through an anti-aliasing filter that also generates some level of error. Next, the signal goes through an analog/digital converter that can introduce magnitude or gain error and channel phase shift. Signal processing is carried out to evaluate the phasor magnitude and phase angle that might create additional error. The phase is reported with respect to the global time reference, so errors in the global positioning system (GPS) source and cable delays might introduce synchronization errors.

The Power Systems Relaying committee of the IEEE Power Engineering Society developed a standard that specifies the performance requirements of PMUs with respect to the amplitude, phase and frequency of the input signals, as well as interference signals such as harmonics and interharmonics. Published in December 2005, The Standard for Synchrophasors IEEE C37.118-2005 defines the uncertainty requirements for the PMUs in terms of the total vector error (TVE), which combines both magnitude and time synchronization errors. The standard specifies TVE errors under various operating conditions to be less than 1 percent for two levels of performance conditions. These conditions include various ranges of signal frequency, magnitude and phase angle, as well as levels of harmonic distortion and out-of-band frequency distortion. Some PMU manufacturers go even further by specifying that their PMUs measure power at an accuracy level of 0.25 percent or better. The IEEE is in the process of preparing a new standard for PMU measurement accuracy that is expected to be considerably more strict than the existing standard.

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Need for a Practical PMU Calibration Tool

The ability to trust and calibrate PMUs will be required by anyone who depends on the performance of these devices. The goal of PMU calibration is to ensure that all PMUs have consistent performance across the system, are interoperable regardless of their makes and models, and comply with system application requirements. A general rule of thumb is that a calibration system must have less than one-fourth of the total uncertainties of the system under calibration. The NASPI has prepared a PMU system testing and calibration guide to establish common methods for calibrating PMUs with the objective of making units from different manufacturers interchangeable within the electric power grid.

The Electric Power Research Institute (EPRI) summarized the calibration challenge as follows: "The reliable power sources, samplers and associated standards for PMU testing and calibration have become a major hurdle to the further development and implementation of PMU applications in power systems. Utilities need the guarantee of reliability and accuracy of PMUs and also the seamless interchangeability among PMUs from different vendors before they will invest heavily in them."

NIST developed a test system that calibrates PMUs for their ability to meet the performance requirements of the IEEE C37.118-2005 standard. The calibration system provides Coordinated Universal Time (UTC)-synchronized three-phase power signals to the PMU being calibrated. The PMU outputs a C37.118 standard formatted continuous data stream of waveform measurements at a minimum rate of 30 frames per second, each time-stamped. A commercial three-phase power simulator generates voltage and current signals. The time synchronization of the calibrator is maintained by triggering the waveform sampling with a one pulse per second signal from the clock.

One NIST calibration system performs static testing while another performs dynamic testing. Static tests hold the input signals to the PMU at various constant levels of magnitude, frequency and interference signals. Dynamic tests are intended to show the performance of PMUs under varying magnitude and frequency conditions typical of real operating power systems. The NIST calibration system has an uncertainty of less than 0.05 percent TVE. It covers all of the measurement conditions specified in the NIST standard with several hundred individual tests.

NIST developed its calibration system by repurposing equipment that already existed in its lab. The cost, time and expertise required to duplicate this system would be very high. NIST identified the need for a less expensive, commercially available, and easy-to-operate system that provides the high levels of accuracy needed for calibrating PMUs.

NIST Awards Commercial PMU Calibrator Grant

NIST awarded Fluke with a Measurement Science and Engineering Research Grant entitled Phasor Measurement Units Calibrator Development. The scope of the grant is to develop an instrument for calibrating measurements of the magnitude and phase of voltage and current signals in power systems. The 26-month project will be jointly funded by NIST and Fluke. The project has four key deliverables, with the final deliverable due 26 months after the start of the project:

1. A comprehensive requirements survey based on IEEE C37.118-2005, NASPI PMU testing guidelines and industry experts that understand real-world transient and dynamic PMU testing requirements.

2. Detailed product requirements specifications (PRS) for a PMU calibrator based on the requirements identified in the survey in the first deliverable. The PRS will define steady state testing of PMUs and an appropriate set of dynamic tests that allows the effective adoption of PMUs in the smart grid.

3. Design of a commercially available PMU calibrator system that implements the PRS described in the second deliverable.

4. An inter-comparison of PMU measurement capability with Fluke's Primary Laboratory, NIST and other laboratories which may include PMU manufacturers, universities and China's EPRI. The purpose of this inter-comparison is to bring further visibility of the need for standardized testing of PMUs and work towards a uniform, global standard for calibrations under both steady state and dynamic conditions.

Design Based on Existing Electrical Power Standard

Fluke has already embarked on the process of assembling experts from academia, industry and government who have extensive experience and knowledge of the working of PMUs and compliance with synchrophasor standards. Particular focus is being placed on understanding real-world applications of PMUs and translating the application requirements into specifications for a PMU calibration system. The next step will be to conduct a thorough examination of commercially available PMUs, their applications and test methodologies. This information will be used to specify calibration techniques across platforms. Fluke engineers will then architect a highly accurate and precise tool capable of testing and calibrating PMUs from any manufacturer. The PMU calibration system will be built around the Fluke 6105A Electrical Power Standard and the Fluke 910R GPS Controlled Frequency Standard. The 6105A is a very good platform for PMU calibration because of its 0.003 degree phase angle accuracy and ability to accurately generate the required distorted signals. The 6105A digital subsystem will be modified to facilitate locking to GPS time, among other changes.

The upcoming PMU calibrators will help utilities ensure the reliability and accuracy of PMUs and seamless interoperability among different vendors. Overcoming this hurdle should encourage utilities to invest more heavily in PMUs and speed the implementation of the smart grid. The end result will be fewer outages, improved power quality, increased energy efficiency and greater stability—even with a multitude of renewable energy sources on the grid.

About the author: Tyrone Foster is the marketing manager at Fluke Calibration and manages the program to develop a PMU calibration system for smart grid devices. Foster worked at HP for more than 15 years. His work at Fluke is focused on the power and energy industry and developing products that serve this market. Foster regularly works with customers to understand their needs and ensure Fluke's solutions meet those needs today and in the future.

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