W. Mack Grady

Understanding Power System Hannonics

Harmonics – Past to Present Power systems are designed to operate at frequencies of 50 or 60Hz. However, certain types of loads produce currents and voltages with frequencies that are integer multiples of the 50 or 60 Hz fundamental frequency. These higher frequencies are a form of electrical pollution known as power system harmonics. Power system harmonics are not a new phenomenon. Concern over harmonic distortion has ebbed and flowed during the history of electric power systems. Steinmetz published a book in 1916 that devoted considerable attention to the study of harmonics in three-phase power systems. His main concern was third harmonic currents caused by saturated iron in transformers and machines, and he was the first to propose delta connections for blocking third harmonic currents. Later, with the advent of rural electrification and telephone service, power and telephone circuits were often placed on common rights-of-way. Harmonic currents produced by transformer magnetizing currents caused inductive interference with open-wire telephone systems. The interference was so severe at times that voice communication was impossible. This problem was studied and alleviated by filtering and by placing design limits on transformer magnetizing currents. Today, the most common sources of harmonics are power electronic loads such as adjustable-speed drives (ASDs) and switch-mode power supplies. These loads use diodes, silicon-controlled rectifiers (SCRs), power transistors, and other electronic switches to chop waveforms to control power or to convert 50/60Hz AC to DC. In the case of ASDs, the DC is then converted to variable-frequency AC to control motor speed. Example uses of ASDs include chillers and pumps. Due to tremendous advantages in efficiency and controllability, power electronic loads are proliferating and can be found at all power levels – from low voltage appliances to high voltage converters. Hence, power systems harmonics are once again an important problem.

Implementation and application of an independent Texas synchrophasor network

Synchronized phasor measurements are becoming widely used in wide-area networks (WANs) around the world for real-time control, monitoring, and post-disturbance analysis. While a few utilities in Texas presently employ synchrophasors in their service territories to increase overall service reliability, many challenges have prevented the development of a large-scale synchrophasor measurement network spanning the Electric Reliability Council of Texas (ERCOT) interconnection. New comprehensive analyses of data gathered from measurement units strategically placed throughout ERCOT will ultimately allow these utilities to make more informed control decisions. This paper discusses the ongoing development of a synchrophasor network by the University of Texas and the network applications, including modal analysis of measured angle differences between measurement locations. Understanding how high penetration levels of wind generation in West Texas affect overall system stability is of key interest. Phasor measurements in this network are taken from conventional 120 V wall outlets. Most data are sent to Austin through the public Internet.

Developing an upper-level undergraduate course on renewable energy and power systems

In keeping up with advances in renewable energy conversion technologies and the continued growth in the renewable energy area along with its impacts on electrical power systems, we believe it is important and timely to develop an upper-level undergraduate course on renewable energy and power systems. Therefore, it is the objective of this paper to describe our efforts in developing the course and present the course outline (including subjects, projects, and references). The course outline can be used as a starting point for other instructors considering offering a similar course. Our renewable energy and power systems course was first offered in fall 2004 at The University of Texas at Austin.

Adaptive power-line disturbance detection scheme using a prediction error filter and a stop-and-go CA CFAR detector

This paper presents a new power-line disturbance detection algorithm. The utilized recursive least square (RLS) prediction error filter extracts the power-line disturbance signal from recorded data, and the modified stop-and-go cell average constant false alarm rate (CA CFAR) detector makes a decision based on the squared output of the previous stage. The detection performance of the proposed algorithm is determined by simulations, and actual high voltage transmission line data is utilized to demonstrate the performance of the proposed algorithm.

Power Electronic Transient Load Model for Use in Stability Studies of Electric Power Grids

This paper develops a new large-signal transient load model to represent the composite power electronic load at a network bus. Traditional load models do not account for the transient responses of power electronic loads which occur at the onset and clearing of voltage sags. The new model is supported with actual utility fault response data and laboratory controlled tests. This paper shows that power electronic loads have a unique impact on generator rotor angle swings compared to those using conventional load models.

Impact of high-penetration PV on distribution feeders

This paper predicts the effects that high levels of photovoltaic penetration will have on the voltage magnitude and power flow on five actual North American feeders. The studies are based on EPRI test feeders, and simulations are conducted using EPRIs OpenDSS computer simulation program. Results for the five feeders studied, using PV penetration levels of 50% of total load, show that the voltage flicker due to shadow movement will be approximately in the range of 0.5% to 2%.

Image processing methods for predicting the time of cloud shadow arrivals to photovoltaic systems

This document presents a combination of image processing tools used for detecting clouds from sky images and for predicting the arrival time of cloud shadows to an area of interest, for example an area of solar panels. Solar panels are used to produce energy from the sun in areas without access to electricity from central power plants and are increasingly being used by people who want to have the flexibility of being disconnected from the main power grid. Unfortunately, the volatility of cloud motion makes photovoltaic (PV) power unreliable for those who want a steady production of electricity. Knowledge about expected drops in power generation due to cloud shadows in PV systems can improve the performance of microgids by granting users the flexibility to turn on alternative forms of power generation, such as fuel cells, or power storage devices such as batteries, to keep the same loads working uninterruptedly. The solution presented was designed to address the need to improve the predictability of solar power generation for possible PV users.

IEEE Power Engineering Society

Harmonic distortion is not a new phenomenon. Concern over harmonic distortion emerged during the early history of AC power systems. Widespread applications of power electronic-based loads continue to increase concerns over harmonic distortion. Harmonic problems have sparked research that has led to much of the present-day understanding of power quality problems. The current-drawn by electronic loads can be made virtually distortion-free (i.e., perfectly sinusoidal), but the cost of doing this is significant and is debate between equipment manufacturers and electric utility companies in standards-making activities. This paper discusses the history of harmonics in power systems. It then defines harmonic distortion and discusses harmonic sources. System response characteristics to harmonics and the impact of harmonics are also discussed. Control of harmonics is also discussed.

Evaluating the impact of wind turbine shadows on an integrated wind and solar farm

This paper analyzes the possibility of having an integrated wind and solar farm to optimize the use of land resources and capital investment by evaluating the effect that turbine shadows have on the area surrounding the turbine. Two methods are used to predict shadow impact. The first method is based on traditional textbook “Clear Sky” equations, which have maximum sensitivity to shadows because every day is a perfect day. The second method uses measured global-horizontal and diffuse-horizontal solar radiation in units of W/m2, which take into account the true variety of daily conditions. The calculations are performed for 1 square meter surfaces, over the 1 square kilometer area, for every second of the day. For purposes of shadow calculations, the tip-top height (i.e., tower height plus blade length) is used.

Problems in the use of Norton equivalent models for single-phase nonlinear loads

This paper illustrates the problems associated with using Norton equivalent models to represent single-phase nonlinear loads. The study is conducted with a specially-developed harmonics testing station that uses feedback to drive the load voltage to a desired target waveform. We apply various voltage waveshapes with 5% THDv to several types of single-phase nonlinear loads which allow us to compute the Norton parameters. The test results show that the Norton equivalent model for a nonlinear load is not unique and varies significantly with applied voltage waveshape.