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Abstract—BPA has a real-time phasor measurement system

with more than 22 PMUs installed in substations feeding data to a control center. To assure the quality and accuracy of the measurement, BPA has developed an extensive test plan for PMUs. From simple beginnings of basic tests, these procedures have continually evolved with both the variety of PMU equipment and applications for which the data was used. Current test methods under development are aimed for more comprehensive characterization of PMU measurement and test process simplification. Basic test procedures assure steady state accuracy of magnitude, phase, and frequency measurement. Dynamic testing includes step changes to magnitude, phase and frequency as well as signals modulated on the power (60 Hz) waveform. These tests characterize speed of response, reproduction of measured values, and rejection of interference. All these effects have an analogy in the actual power system, so the results can be reasonably applied. Developing methods simplify the generation of test signals and analysis of results. A goal is to test for all measurement aspects that may create a problem and correct these before deployment. In addition, determining characteristics that can be used to adjust all measurements to the same basis will enable the performance of precision analysis regardless of the make or model of PMUs that are making the measurements.

Index Terms—IEEE 1344, IEEE C37.118, Phasor, PMU, PDC, Synchrophasor.

I. NOMENCLATURE

BPA – the Bonneville Power Administration CT – Current Transformer GPS – Global Position System PDC – Phasor Data Concentrator PMU – Phasor Measurement Unit PT – Potential Transformer

II. INTRODUCTION

ESTING Phasor Measurement Units (PMUs) at the Bonneville Power Administration (BPA) started in 1988

with the delivery of the first PMU prototypes. The first tests were oriented to determining the accuracy and measurement characteristics in comparison with analog transducers

Author affiliations are: K. E. Martin & T. J. Faris are with the Bonneville Power Administration,

PO Box 491, Vancouver, WA 98666 USA (e-mail: kemartin@ieee.org, ajfaris@bpa.gov ).

J. F. Hauer is with the Pacific Notrwest National Labs, Richland, WA, 99352 USA (e-mail: John.Hauer@pnl.gov ).

currently in service. Testing has continued and increased in scope and complexity as PMUs have evolved into an increasing variety of commercial products. BPAs primary testing objective is qualifying PMUs for BPA use. They need to reliably make an accurate measurement with known signal processing characteristics. They have to integrate into BPAs measurement system and meet substation installation requirements. A PMU uses complex mathematical algorithms to estimate the phasor equivalent and system frequency from data samples. Different manufacturers typically use different algorithms. This can result in values that differ from that expected for a particular condition, and differing results between vendors. Testing should reveal these differences and demonstrate overall compatibility. An eventual goal of testing is to derive a set of parameters that completely characterize the measurement. These parameters could be applied to the output from each PMU, adjusting all measurements to a common basis. This is currently being done with basic quantities like magnitude and phase angle, and could potentially be applied to dynamic and auxiliary characteristics as well.

III. BPA PHASOR MEASUREMENT SYSTEM

BPA has developed a phasor measurement system as shown in Figure 1. PMU measurements in the substations feed data into a data concentrator (PDC). The PDC correlates data from the PMUs by timetag to create system-wide, time synchronized measurements. It then sends the correlated data to other systems which include measurement and display applications as well as other utilities.

PDC

StreamReader Display & recording

Real-time controls:

Voltage & reactive stability;

inter-area angle limits Direct exchange

with utilities

Phasor Data Concentrator

(PDC) Correlates data,

feeds selected applications,

monitors system

SCADA - Voltage, angle & frequency

Data storage

V - I - F Measurement - substations

Data input & management

- control center

Operation monitors – display & alarms

System Controls

PMU

PMU

PMU

PMU

PMU

Fig 1. Real-time phasor measurement system used at the Bonneville Power Administration, typical of many real-time systems used by many utilities.

PMU Testing and Installation Considerations at the Bonneville Power Administration

Kenneth E. Martin, Senior Member, IEEE, John F. Hauer, Fellow, IEEE, Tony J. Faris, nonmember

T Displays – RTDMS, Power World, etc.

1-4244-1298-6/07/$25.00 ©2007 IEEE.

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A. Compatibility

Installation compatibility includes substation qualification, signal input, and data communication interface. The substation environment requires interference and surge protection as well as minimal temperature/humidity survivability. Signal input at BPA uses PT/CT secondary circuits (69V/5A). The BPA measurement system can accommodate both serial and network data communications in a variety of protocols. Testing includes assuring that the protocol implementation is correct and communication works reliably with the BPA system. Measurement output delay is also measured since these signals may be used in control applications.

B. Measurement

Typical PMUs sample AC waveforms with an A/D converter. Sample rates are in the range of a few kilohertz which sufficiently reproduces the waveform. The PMU estimates a phasor equivalent for the waveform relative to a time reference that is usually derived from GPS. It outputs the estimated values, often combined into a positive sequence equivalent, along with supporting information like a time stamp, frequency, and status. The input circuitry, sampling, and conversion algorithms will all affect the estimated values. Testing with various known inputs shows the effects of the measurement process and reveals errors. The measurement needs to meet basic requirements before it can be used in the BPA system.

IV. PMU TEST PLAN

PMU testing needs to include basic accuracy tests performed over a range of operating conditions. It also needs to include transient operating conditions, both those that might reasonably occur in operation and that serve to characterize the PMU measurement process. BPA has developed three types of tests to qualify and characterize PMUs. These are described as steady state, step, and structured signal tests. These provide basic characterization, but have not been developed sufficiently to provide full correction factors. BPA test methods are continuing refinement toward that goal.

A. Steady-state tests

Steady-state tests confirm measurement in constant conditions, particularly magnitude and phase angle accuracy. They include the following:

• Phasor magnitude • Phase angle (relative & absolute) • Phasors with unbalanced inputs • Frequency estimation • Measurement noise

These tests need to be performed over the entire ranges of interest and include a range of operating conditions including off nominal frequency. These are the primary measurement

qualification tests. PMUs that cannot meet accuracy and repeatability requirements cannot be used with the BPA system. Fortunately all products that BPA has decided to use have met these requirements, though several required one or more modifications.

B. Step tests

Step tests provide a simple and easily observed method of comparing the PMU response to a sudden input change. They also emulate what may be observed during load switching and faults. Step tests include:

• Magnitude steps • Phase angle steps • Frequency steps

Steps include positive and negative steps of various magnitudes. In some cases the step was initiated by a signal at a precise time, and others by manual control at a random time. The precise time initiation allows determining the absolute response time. Once established, manual initiation can be used to determine relative times between units under test. Timed events from test files, as described below, provide the simplicity of manual tests with the precision of timed tests.

C. Structured signal tests

Structured signals are built using Matlab formulations of specific signal types. These are primarily 60 Hz waveforms that are amplitude, phase, or frequency modulated with a sine wave. They could, however, be any kind of signal modulation or other combination that could be input to a PMU to determine a specific type of response characteristic. Presently these tests include:

• Amplitude modulation • Phase modulation • Frequency modulation

Modulation, frequency, or amplitude is varied over a range that will demonstrate the PMU response characteristics.

V. PMU TESTING

BPA testing has evolved from basic accuracy tests to full performance testing outlined in the previous section. Some of the tests are easy to perform and document, and others are tedious and time consuming. In an effort to simplify procedures, the structured signal methodology was expanded to include nearly all the test types.

A. New test methodology

This test method requires a template for producing a test file, including GUI interfaces to produce standard plots and statistics. The general procedure is: 1. Produce a playback file with signals that represent the

test settings 2. Run the playback file so it can be captured in a single

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data file 3. Process the data file using the original signal generator as

a template. To general test signals, a calibrated signal source that

exceeds the accuracy requirements is used. The signal source is synchronized to GPS and the file is started at an exact known time. Every element of the test file thus occurs at an exact, known time. We also use a digital output from the test set recorded by the PMU to check file and processing synchronization. The time accuracy of this recorded digital signal is not as precise as the data itself, but it is close enough to provide a marker for synchronization. Figure 2 shows a block diagram of this testing methodology.

Test system, possibly including both a signal generation unit and PC controller

Playback file

PMU under test

V & I test signals

Analysis comparing PMU measurement with signal model

Data file

Mathematical computer program that can create playback files and analyze results (Matlab)

Phasor based signal model

Formulas converting phasor model into 3-phase signals

Signal conversion into sampled waveforms (COMTRADE)

Fig 2. Testing methodology using recorded data.

B. Test results

Magnitude (amplitude) testing is one of the simplest tests to perform. It consists of a balanced 3-phase signal varied over a range of amplitudes. While simple to perform, recording and calculating the error is time consuming. With this new methodology, a file containing 40 points can be run and processed in a few minutes, documenting a wide response range as shown in Figure 3.

Fig 3. Error plot for an amplitude response test.

Phase angle measurements are made relative to a time

reference. All phasors measured by a single PMU should use the same time reference which should be linked to a universal source. There may be errors between phasors on the same

PMU as well as relative to the universal time input to the PMU. By using the capability of synchronizing the signals output to GPS, both these errors can be tested using recorded files. Two test signals at a fixed (30°) offset are moved from 0° to 360° to test both relative and absolute phase measurement (Figure 4). Sections of the file hold the angle at a fixed ± 90° to facilitate manual checking of the test set synchronization. A-phase will be at a zero crossing at ± 90°, which can be compared with a GPS synchronization pulse on an oscilloscope.

Fig 4. Error plot for absolute phase measurement test. .

PMU performance over a range of frequencies is critical.

The power system operates at a nominal 60 Hz in North America, but is usually not exactly at 60 and may deviate widely during extreme conditions. A scan of constant magnitude (Figure 5) and phase angle (Figure 6) over a reasonable range of frequencies shows how the measurement will deviate at off nominal frequency. These tests use a range of ± 5 Hz as recommended by IEEE C37.118.

52 54 56 58 60 62 64 66 680.85

0.9

0.95

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1.05

Input Frequency (Hz)

Pha

sor

Mag

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Phasor Magnitude vs. Frequency

Fig 5. Test constant amplitude over frequency. Includes PMUs with 1 & 4 cycle Fourier filters, frequency tracking 55-65 Hz, & narrowband filtering.

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All PMUs

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Fig 6. Test constant phase angle over frequency. Includes PMUs with 1 & 4 cycle Fourier filters & different windowing.

Step tests with several PMU responses on the same plot are

easy to compare visually (Figure 7). The user can analyze the generated plot to determine differences in the response of each PMU. However, PMU responses are typically much quicker than the reporting sample rate, so the observed response does not tell exactly how fast the PMU itself responds. Without a large number of tests initiated at random times, it is not possible to quantify the difference in PMU responses. This improved test uses a series of steps, each generated at a small time delay relative to a base reference. The procedure is to make a step at a precise time, recover to the initial value, make another step with a small sub-sample delay plus a major delay from the first step, and repeat. The PMU samples at a precise time based on the fixed data rate and UTC time, so this generates steps that “fill in” between PMU samples. The data processing utility knows exactly when these samples are generated and moves the responses back to fill in the response curve with many points where previously there were only a few. With this “slip-sampling” technique we can quantify the step response relative to the actual input pulse (figure 8).

Fig 7. 10% magnitude step response with 4 PMUs showing the differences due to windowing and filtering.

-2 -1.5 -1 -0.5 0 0.5 1 1.5 270

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Sample Point

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Phasor Magnitude vs. Time

Fig 8. 10% magnitude step response showing the actual response using extra samples at 8 points/cycle. Dashed line shows apparent response curve without extra sampling. PMU used center-of-window timing.

Structured signal tests as described earlier consisted of

amplitude, phase, and frequency modulation. These are difficult to generate with standard test instruments in a 3-phase voltage and current format suitable for power system instrumentation. These tests are particularly useful since they mimic power system phenomena like generator swings, load variations, and system oscillations. Originally wideband frequency scans showed PMU filtering characteristics which determine protection from aliasing. With increasing signal processing complexity such as tracking filters, these scans do not reveal filtering characteristics since they change with the fundamental frequency. Modulation keeps the fundamental at nominal (or wherever appropriate) and allows looking at PMU processing of the information riding on the power waveform. A structured signal test involves a sweep of modulation frequencies at a constant modulation level (Figure 9). The magnitude response in the PMU passband (frequencies under the Nyquist rate) illustrates what we can measure on the power system (Figure 10). The magnitude outside the passband is the amount of infiltration (aliasing) that the PMU will allow (Figure 11). The phase angle response in the passband is the group delay of measured signals which relates the generator modal responses (Figure 12).

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Fig 11. Amplitude modulation scan of four PMUs.

0 5 10 150

1

2

3

4

5

6

Modulation Frequency (Hz)

Mod

ulat

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Mag

nitu

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Vrm

s)

Magnitude Modulation vs. Frequency

Fig 10. The Nyquist rate is 15 Hz for a 30/s sample rate. Figure illustrates passband response for 1 cycle (top-lt blue) & 4 cycle (bottom-red) Fourier and bandpass (middle-green) filters.

15 20 25 30 35 40 45 50 55 60-60

-50

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Out of band rejection vs. Frequency

Fig 11. The Nyquist rate is 15 Hz for a 30/s sample rate. Figure illustrates rejection band for 3 PMUs (same as Fig 9).

Fig 12. The Nyquist rate is 15 Hz for a 30/s sample rate. Figure illustrates PMU phase response for 3 PMUs in passband.

VI. CONCLUSIONS

PMU testing at BPA started with the first PMU delivered in 1988. Data gathered from field installations demonstrated a high quality measurement, but it needed controlled laboratory tests to determine the quality of the measurements and the problems that existed. Subsequent tests have witnessed improvements in PMU technology. Expanded testing took on a new urgency when BPA purchased newly designed PMUs in 2001. With more than one PMU type in the same measurement system, it is necessary to have better measurement characterization to ensure agreement the measurement. As tests have become more complex, the time to perform the test and analyze the data has grown. BPA has moved to a semi-automated procedure using computer generated test files and GUI based processing to handle the increasing complexity and minimize analysis time. This procedure also allows implementing more complex tests, with an ultimate goal of complete measurement characterization. If the PMU measurement can be confirmed to be free of errors, it should be possible to apply a set of compensating characteristics to account for differences between PMU technologies. These could compensate for attributes like rise time, group delay, and windowing time offsets. These, in addition to those already used like magnitude scaling and phase angle offsets, will enable the full potential for use of phasor measurement data.

VII. ACKNOWLEDGMENT

The authors gratefully acknowledge the support of BPA and PNNL in continuing development of testing capability.

VIII. SELECTED BACKGROUND

The following references are not cited in the text but provide useful background in the subjects presented.

[1] IEEE Standard for Synchrophasors for Power Systems, IEEE Standard

C37.118-2005, March 2006.

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[2] K.E. Martin, T.J. Faris, J. F. Hauer, “Standardized Testing of Phasor Measurement Units”, Fault and Disturbance Analysis Conference, Georgia Tech, Atlanta, GA, May 2006.

[3] J. F. Hauer, W. A. Mittelstadt, K. E. Martin, J. W. Burns, and Harry Lee,

“Integrated Monitor Facilities for the Western Power System: WAMS

Analysis in 2005”, Interim report of the WECC Disturbance Monitoring

Work Group, December 2005.

[4] K.E. Martin, “Phasor Measurement Using GPS Timing,” 1990 BPA

Engineering Conference, Portland, OR, March 1990.

[5] K.E. Martin, “Phasor Measurement System Test,” 1992 BPA Engineering

Symposium, Vol. 2, pp. 689-704, Portland, OR, April 1992. [6] J. F. Hauer, "Validation of Phasor Calculation in the Macrodyne PMU for

California-Oregon Transmission Project Tests of March 1993,” IEEE Trans. Power Delivery, vol. 11, pp. 1224-1231, July 1996.

[7] U.S. Department of Energy, “Wide Area Measurements for Real-Time

Control and Operation of Large Electric Power Systems – Evaluation And Demonstration Of Technology For The New Power System”, prepared under BPA Contracts X5432-1, X9876-2; January 1999. Report and attachments available on CD.

[8] K.E. Martin, R. Kwee, “Phasor Measurement Unit Performance Tests,”

Precise Measurements in Power Systems Conference, Sponsored by The Center for Power Engineering, Virginia Tech, Arlington, VA, November 1995.

[9] J. F. Hauer, K.E. Martin, H. Lee, “Evaluating Dynamic Performance of Phasor Measurement Units: Experience in the Western Power System”, Interim Report of the WECC Disturbance Monitoring Work Group, partial draft of August 5, 2005.

[10] J. Depablos, V. Centano, A. G. Phadke, M. Ingram, “Comparative Testing

of Synchronized Phasor Measurement Units”, IEEE Power Engineering Society General Meeting, 2004, June 2004, pp 948-954 Vol 1.

[11] J. F. Hauer et. al, “PMU Performance During Stressed Operating Conditions: Harmonic Effects for Off-Nominal Frequencies”, Working Note for the WECC Disturbance Monitoring Work Group, partial draft of January 16, 2004.

Many of these or related documents are available at ftp.bpa.gov/pub/WAMS%20Information/.

IX. BIOGRAPHIES

Ken Martin (SM’ 1991) is a Principal Engineer at the Bonneville Power Administration. His primary responsibility is the development of Wide Area Measurement Systems (WAMS), particularly phasor measurement systems for high-speed dynamics measurements. Duties include system development, operation oversight, and coordination with other utilities. He is also responsible for the development of precise timing systems at BPA. He has worked primarily with instrumentation, communication, and power system protection systems at BPA. Mr. Martin holds a BSEE from Colorado State University and an MA from the University of Washington. Mr. Martin is a Senior Member of IEEE, a member of the Power System Relay Committee and the Relay Communications Sub-committee. He is the chair of the Synchrophasor Standard working group. He is a registered Professional Engineer in Washington State. Tony Faris is an engineer in the Measurements Systems group at the Bonneville Power Administration. He began working with phasor measurement systems in

2004 as a student at BPA. He holds a BSEE from the University of Portland and completed an MSEE at the University of Washington in 2006. John Hauer (F’90) started his engineering career with the General Electric Company in 1961. This was followed by industrial work at Boeing Aerospace, a Ph.D. at the University of Washington, and a faculty position at the University of Alberta. In 1975 he joined the Bonneville Power Administration and began a long involvement with identification, analysis, and control of power system dynamics. In 1994 he stepped down as BPA Principal Engineer for power system dynamics, and assumed technical leadership of the power systems group at the DOE’s Pacific Northwest National Laboratory in Richland, Washington. He is a Laboratory Fellow at PNNL, and a Life Fellow of the IEEE.