The demand for renewable energy has spurred dramatic growth in the number of wind farms in North America and around the world. For more than a decade, Xcel Energy has played a pivotal role in the commercialization of wind energy and has upheld its commitment to provide clean, renewable energy at a reasonable cost to its consumers. While the environmental and economic advantages are exciting, wind technology does present interesting engineering challenges, as well. As the mix of generation sources shifts, it is critical to thoroughly understand potential impacts of the interactions between the variety of generation sources and the bulk electric power system, especially during system disturbances.
Transmission system operators already adhere to interconnection guidelines and North American Electric Reliability Corp. reliability standards, which promote stability of the system as a whole. However, wind generation increases the complexity of reliability issues. The various types of wind turbine generators behave differently during system disturbances, and a deep knowledge of system interconnections is necessary to protect against possible transients that can result from these interactions.
Between 2004 and 2008, several hundred megawatts of wind generation were installed in the Buffalo Ridge area of Minnesota. This resulted in significant improvements to the transmission system in Minnesota and South Dakota. To limit the additional loading of constrained interfaces and direct power flow toward the Twin Cities of Minneapolis and St. Paul, Xcel Energy installed series compensation on a 54-mile (87-km) 345-kV line in southern Minnesota. One end of this series-compensated line had 150 MW of installed wind generation and six combustion turbine generators totaling 639 MW.
A Subharmonic Challenge
During a planned switching procedure in October 2007, system engineers discovered the potential threat from subharmonics was actually very real. As equipment was brought on-line, a flashover occurred, triggering protection schemes at the generating station. Subsequent analysis revealed that fault recorders operating at the time had captured the presence of high levels of subharmonic oscillations, which had gone undetected by the protective system in place. Such subharmonics, if not detected and protected, can damage equipment and cause system instability.
A Subharmonic Solution
After analyzing these experiences, system engineers realized the need for accurate detection of subharmonic oscillations on the system. In addition, they required protection schemes to isolate disturbances when they occur. By isolating the healthy grid from subharmonic generation sources, damage to the electrical interconnection could be prevented. An electromagnetic transient program (EMTP) study was performed. The EMTP study confirmed transient oscillations resulting from the wind turbine interactions with series capacitors and gas turbine generation sources.
Because a suitable subharmonic protection relay did not exist on the market, Xcel Energy approached protection equipment manufacturers to design a solution using a new algorithm to detect subharmonic frequencies. After a process of securing funding and structuring contracts to gain commitment to objectives and timelines, teams from Xcel Energy and ERLPhase Power Technologies collaborated to specify, design and build a new type of relay capable of detecting and isolating subharmonic oscillations in the range between 5 Hz and 25 Hz. After significant planning, just over six months of intensive development and rigorous testing, the new subharmonic relay was installed and commissioned in October 2010.
Based on this experience, recommendations begin with ensuring fault recorders are in place to capture system dynamics. The analysis of events is critical. It is equally important to cultivate relationships with manufacturers who supply substation equipment. In this case, there were strong working relationships at many levels between the utility and manufacturer. In fact, much of the development began with a simple request to assist Xcel Energy with a fuller understanding of the fault recorder data: “Here is what we are seeing. What is your analysis of this?” The discussion grew into the effort to develop a relay to respond to the subharmonics that were revealed.
When joint development is being considered, some keys set the stage for success. In this case, significant effort was invested at the outset to get a firm cost and schedule approved by high-level management at Xcel Energy. This enabled the technical team to move ahead freely with a sourcing solution within those parameters.
Next, time was devoted to getting firm, clear, written commitments from all involved parties. The time spent on contracts and timelines paved the way for efficient development to come. Once the project was underway, communication was less frequent, mostly status updates and impromptu technical or usage questions as they arose.
When embarking on development efforts such as this, utilities must be willing not only to invest their time and expertise, but also to be the test case for the equipment being developed. In this case, the timeline was one of the most pressing constraints; this project took just over six months to complete. Additionally, by having a dedicated expert resource to manage the development, surprises were avoided and daily efforts were kept on track. Questions were dealt with promptly, and if a discussion was needed to explore the best approach, that dialogue was held in a timely manner, enabling development to continue forward. In this case, there was already good communication from years of working together. This project benefitted from that rapport.
Even with simulation studies, subharmonic disturbances are difficult to predict. Not only is it vital to monitor and visualize these subharmonic phenomena, protection schemes are needed that act appropriately to protect the system if and when they do occur. The event captured at Xcel Energy led to the development of a new microprocessor-based subharmonic protection technique and device.
With the increased use of wind generators feeding utility networks through series-compensated lines, it is necessary to ensure subharmonic oscillations are monitored and the electrical grid is protected from any resulting detrimental effects. In cases where commercial technology does not offer a solution, success can be found working cooperatively with manufacturers. Sharply focusing efforts on solving a specific real-world technical need can be mutually rewarding, and the outcome benefits the industry as a whole.
Adi Mulawarman (firstname.lastname@example.org) received his BSEE and MEE degrees from the University of Minnesota. He has worked for Xcel Energy since 1997 and currently serves as a consultant's sponsor for the substation design and engineering department. He is a Senior IEEE member and actively involved in the IEEE Power & Energy Society's Power System Relaying Committee working group.
Pratap Mysore (email@example.com) is a national senior relay and protection engineer with HDR Engineering Inc. in Minneapolis, Minnesota, U.S. Prior to joining HDR in January 2011, he was with Xcel Energy in the substations engineering group since 1987. Mysore is actively involved in IEEE Power System Relaying Committee activities. He is a registered professional engineer in the state of Minnesota.
Editor's note: The authors wish to acknowledge the contribution of ERLPhase Power Technologies to this article and for its collaboration to design the subharmonic protection relay employed in this application.
The Challenge of Integrating Wind
Wind farms are generally located away from load centers. The power they generate must be transferred over transmission lines to the load centers. Series compensation is commonly used to effectively shorten the electrical length of a transmission line to allow more power to be delivered over it. The series capacitor can be switched in or out or bypassed as needed.
Electrical systems and transmission lines are represented by resistive, inductive and capacitive elements. Whenever such a system has energy abruptly added or removed, which happens when a transmission line is switched, the energy stored in the reactive elements transfers back and forth, causing an oscillation. It is the electric version of a spring being stretched and released.
In these systems, the frequency of the subharmonic oscillations typically ranges from 5 Hz to 40 Hz. System configuration and parameters, as well as load conditions, will determine whether the oscillations are damped and die out, or whether they are undamped and grow until a system element is damaged or fails.
Under special conditions, the presence of subharmonic oscillations in induction generators can cause them to present an apparent negative resistance. The system and load present a resistance that is positive. If the magnitude of the apparent negative resistance is less than the resistance of the system and load, the subharmonic oscillations will likely be damped and die out without any ill effect. However, if the magnitude of the apparent negative resistance of the generator is greater than the resistance of the system and load, the oscillations will continue and may grow until a system element fails.
ERLPhase Power Technologies www.erlphase.com
Xcel Energy www.xcelenergy.com