John F. Hauer

Making Prony analysis more accurate using multiple signals

Prony analysis has proven to be a valuable tool in estimating the modal content of power oscillations from measured ringdowns. The accuracy of the mode estimates is limited by the noise content always found in field measured signals. Current Prony analysis methods assume the system to be single output, and individual signals are analyzed independently often resulting in conflicting frequency and damping estimates (due to noise effects). This paper considers a simple extension to Prony analysis that allows multiple signals to be analyzed simultaneously resulting in one set of mode estimates. Examples are used to show that this extension improves the accuracy of modal estimates and simplifies the analysis steps. The first example uses a Monte Carlo type simulation model and the second analyzes field measured data from the western North American power system.

Performance of Three Mode-Meter Block-Processing Algorithms for Automated Dynamic Stability Assessment

The frequency and damping of electromechanical modes offer considerable insight into the dynamic stability properties of a power system. The performance properties of three mode-estimation block-processing algorithms from the perspective of near real-time automated stability assessment are demonstrated and examined. The algorithms are: the extended modified Yule Walker (YW); extended modified Yule Walker with spectral analysis (YWS); and sub-space system identification (N4SID). The YW and N4SID have been introduced in previous publications while the YWS is introduced here. Issues addressed include: stability assessment requirements; automated subset selecting identified modes; using algorithms in an automated format; data assumptions and quality; and expected algorithm estimation performance.

Use of the WECC WAMS in Wide-Area Probing Tests for Validation of System Performance and Modeling

During 2005 and 2006, the western electricity coordinating council (WECC) performed three major tests of western system dynamics. These tests used a wide-area measurement system (WAMS) based primarily on phasor measurement units (PMUs) to determine response to events including the insertion of the 1400-MW Chief Joseph braking resistor, probing signals, and ambient events. Test security was reinforced through real-time analysis of wide-area effects, and high-quality data provided dynamic profiles for interarea modes across the entire western interconnection. The tests established that low-level optimized pseudo-random plusmn20 -MW probing with the pacific DC intertie (PDCI) roughly doubles the apparent noise that is natural to the power system, providing sharp dynamic information with negligible interference to system operations. Such probing is an effective alternative to use of the 1400-MW Chief Joseph dynamic brake, and it is under consideration as a standard means for assessing dynamic security.

Current Status and Experience of WAMS Implementation in North America

The 15 years of successful implementation of wide-area measurement systems (WAMS) in the WECC power grid have shown significant value of WAMS data in system dynamic modeling and validation, FACTS control validation and pilot implementations of wide area protection schemes. The August 14 2003 blackout in the eastern interconnection of the North America revealed the urgent need for wide-area information acquisition for better power grid operations. The Eastern interconnection phasor project (EIPP) was launched in 2003 to deploy a WAMS system in the eastern interconnection. Development of IEEE C37.118, a standard for phasor data acquisition and transmission, will aid in deployment of phasor measurement systems for WAMS applications. Technologies of phasor measurement units (PMUs) with high precision time synchronization and phasor data concentrators (PDCs) for phasor data aggregation and event recording are key to the success of WAMS implementation. This paper reviews the WAMS development in the North America and presents current and potential WAMS applications including dynamic modeling and validation and wide-area control. Past experience shows a promising future of WAMS in improving power system planning, operation and control. However, there remain challenges to make phasor measurement consistent and to meet both slow and fast data application needs

Probing Signal Design for Power System Identification

This paper investigates the design of effective input signals for low-level probing of power systems. In 2005, 2006, and 2008 the Western Electricity Coordinating Council (WECC) conducted four large-scale system-wide tests of the western interconnected power system where probing signals were injected by modulating the control signal at the Celilo end of the Pacific DC intertie. A major objective of these tests is the accurate estimation of the inter-area electromechanical modes. A key aspect of any such test is the design of an effective probing signal that leads to measured outputs rich in information about the modes. This paper specifically studies low-level probing signal design for power-system identification. The paper describes the design methodology and the advantages of this new probing signal which was successfully applied during these tests. This probing input is a multi-sine signal with its frequency content focused in the range of the inter-area modes. The period of the signal is over 2 min providing high-frequency resolution. Up to 15 cycles of the signal are injected resulting in a processing gain of 15. The resulting system response is studied in the time and frequency domains. Because of the new probing signal characteristics, these results show significant improvement in the output SNR compared to previous tests.

Large-scale hybrid dynamic simulation employing field measurements

Simulation and measurements are two primary ways for power engineers to gain understanding of system behaviors and thus accomplish tasks in system planning and operation. Many well-developed simulation tools are available in todays market. On the other hand, large amount of measured data can be obtained from traditional SCADA systems and currently fast growing phasor networks. However, simulation and measurement are still two separate worlds. There is a need to combine the advantages of simulation and measurements. In view of this, This work proposes the concept of hybrid dynamic simulation which opens up traditional simulation by providing entries for measurements. A method is presented to implement hybrid simulation with PSLF/PSDS. Test studies show the validity of the proposed hybrid simulation method. Applications of such hybrid simulation include system event playback, model validation, and software validation.

Evaluation of PMU Dynamic Performance in Both Lab Environments and under Field Operating Conditions

Capturing system dynamics is one important feature of phasor measurements. To ensure PMU accurately reflect system dynamic behavior, one must evaluate the dynamic performance of a PMU. PMU dynamic performance evaluation includes three aspects: PMU modeling studies, laboratory testing and field evaluation. This paper briefly reviews the general PMU model structure, and then continues on PMU dynamic performance evaluation from actual field measurements. Reasons for field evaluation include: (1) inappropriate field settings of a PMU would generate unexpected phasor measurements; and (2) many conditions can not be easily produced in a lab environment. PMU field evaluation includes aspects like time synchronization, timing inconsistency due to filtering, frequency calculation issues, parasitic oscillations/processing artifacts, etc. Actual WECC measurement examples will be presented. Dynamic PMU testing in a lab environment is explored with a special focus on PMU filtering characteristics. How the phasor quality would impact derived system dynamic characteristics is addressed in the later part of this paper.

Improving small signal stability through operating point adjustment

ModeMeter techniques for real-time small-signal stability monitoring continue to mature, and more and more phasor measurements are available in power systems. It has come to the stage to bring modal information into real-time power system operation. This paper proposes to establish a procedure for Modal Analysis for Grid Operations (MANGO). Complementary to PSS and other traditional modulation-based control, MANGO aims to provide suggestions such as redispatching generation for operators to mitigate low-frequency oscillations. Load would normally not be reduced except as a last resort. Different from modulation-based control, the MANGO procedure proactively maintains adequate damping at all times, rather than reacting to disturbances when they occur. The effect of operating points on small-signal stability is presented in this paper. Implementation with existing operating procedures is discussed. Several approaches for modal sensitivity estimation are investigated to associate modal damping and operating parameters. The effectiveness of the MANGO procedure is confirmed through simulation studies of several test systems.

PMU Testing and Installation Considerations at the Bonneville Power Administration

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.

MANGO – Modal Analysis for Grid Operation: A Method for Damping Improvement through Operating Point Adjustment

Small signal stability problems are one of the major threats to grid stability and reliability in the U.S. power grid. An undamped mode can cause large-amplitude oscillations and may result in system breakups and large-scale blackouts. There have been several incidents of system-wide oscillations. Of those incidents, the most notable is the August 10, 1996 western system breakup, a result of undamped system-wide oscillations. Significant efforts have been devoted to monitoring system oscillatory behaviors from measurements in the past 20 years. The deployment of phasor measurement units (PMU) provides high-precision, time-synchronized data needed for detecting oscillation modes. Measurement-based modal analysis, also known as ModeMeter, uses real-time phasor measurements to identify system oscillation modes and their damping. Low damping indicates potential system stability issues. Modal analysis has been demonstrated with phasor measurements to have the capability of estimating system modes from both oscillation signals and ambient data. With more and more phasor measurements available and ModeMeter techniques maturing, there is yet a need for methods to bring modal analysis from monitoring to actions. The methods should be able to associate low damping with grid operating conditions, so operators or automated operation schemes can respond when low damping is observed. The work presented in this report aims to develop such a method and establish a Modal Analysis for Grid Operation (MANGO) procedure to aid grid operation decision making to increase inter-area modal damping. The procedure can provide operation suggestions (such as increasing generation or decreasing load) for mitigating inter-area oscillations.