- Passivity-based Decentralized Frequency Control for Electric Power Systems
In a large scale electric power grid, it is desirable to have decentralized control for many reasons: lack of infrastructure, unwillingness to share information between different owners, delay of communication, etc. Even at the design stage, usually the control is designed in a localized fashion. Therefore it is important to make sure that the overall system is stable with all these decentralized control design.
We propose a network modeling framework similar to that in process systems for large-scale power systems. The power network can be shown to satisfy the generalized Tellegen's Theorem. With the aid of proper storage function for each node of the system, we can choose proper local control and output variables so that individual subsystems are passive and stabilized with feedback control, and the overall system is also stable. Passivity-based decentralized control allows us to perform robust control design for large-scale power systems.
- Robust Control Design for Frequency Regulation in Power Systems with High Wind Penetration
The frequency of a power system should be maintained nearly constant at nominal frequency to ensure safe and reliable operation. System frequency indicates the balancing condition between generation and load. To maintain the frequency in an acceptable range, the generation must be adjusted in real time to meet deviations of the load from predicted values.
Increasing the proportion of wind power to traditional generators can degrade frequency performance. In traditional power systems, load variation is the main disturbance in the frequency control loop under normal operating conditions. When a significant amount of wind energy penetrates the power system, fluctuations in wind generation due to wind variations need to be compensated in addition to load variations, thus increasing the amount of control effort required to maintain system frequency. Short-term wind power fluctuations are characterized by frequency spectra. This information is used together with robust control, specifically the H-infinity method, to synthesize new governors for conventional plants in order to better attenuate frequency deviation caused by continuous wind power fluctuations. A reduced-order controller is obtained based on Hankel singular values. Simulation results show the effectiveness of the H-infinity controller.
- Saturation-induced Frequency Instability in Electric Power Systems
Due to the increasing demand for electricity, today's power systems are often operated near generation, transmission and control constraints. When operating limits are hit, it indicates to some extent that the electric power system is in an extreme condition and special attention needs to be paid to analyzing and understanding the impact of saturation on the viability of the system. If the limiting condition cannot be alleviated over a period of time, it is highly likely that one binding constraint will lead to another limit to being reached, thus causing a cascading effect that can ultimately result in system collapse.
It is shown through an example that when the system operates near its limits, saturation of controllers in particular the turbine/governor can lead to frequency instability that would not be evident from load-flow analysis. A linearized model with controller saturation nonlinearity is obtained and it preserves the saturation-induced instability phenomenon. Linear matrix inequalities (LMI) technique is used to find a region of attraction for the saturated linear system which gives an estimate of the largest disturbance the system can withstand when the saturation effect is taken into account. Simulation also shows a power system stabilizer (PSS) providing frequency feedback to the automatic voltage regulator (AVR) significantly enhances the system resistibility against large disturbance, while reducing the AVR gain has little effect.
Juhua Liu, Bruce H. Krogh, Marija D. Ilic. “Robust Control Design for Frequency Regulation in Power Systems with High Wind Penetration”, accepted by the 2010 American Control Conference, Baltimore, MD, June 2010
Le Xie, Pedro Carvalho, Luis M Ferreira, Juhua Liu, Bruce Krogh, Nipun Popli, Jeff Chapman. “Engineering and Economic Problems and Solutions in Support of High Wind Power Penetration”, to appear in the special issue of Proceedings of the IEEE, 2009
Juhua Liu, Bruce H. Krogh, Marija D. Ilic. “Saturation-Induced Instability in Electric Power Systems”, Proceedings of the 2008 American Control Conference, Seattle, WA, June 2008
Juhua Liu, Bruce H. Krogh, Marija D. Ilic. “Saturation-Induced Frequency Instability in Electric Power Systems”, Proceedings of the 2008 IEEE Power & Energy Society General Meeting, Pittsburgh, PA, July 2008
Juhua Liu, “Dynamics in Extreme Electric Power System Conditions: Effects of Control Limits on Long-term Stability”, Master Thesis, Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA, May 2007
Juhua Liu, Bruce H. Krogh, Marija D. Ilic, “Impact of Wind Fluctuations on Frequency Dynamics and Robust Control for Ensuring Frequency Specifications,” Fifth Annual Carnegie Mellon Conference on the Electricity Industry, Pittsburgh, PA, March 2009
Juhua Liu, Marija D. Ilic, Bruce H. Krogh, “Revisiting the Role and Control Logic of Governors for Stable Load Following,” Dynamic Monitoring and Decision Systems (DYMONDS) for Future Electric Energy Systems Consortium, Pittsburgh, PA, Dec. 2008
Juhua Liu, Bruce H. Krogh, Marija D. Ilic, “Saturation-Induced Frequency Instability in Electric Power Systems,” Fourth Annual Carnegie Mellon Conference on the Electricity Industry, Pittsburgh, PA, March 2008