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Nonlinear control of grid-tied inverters for stabilising future smart grids

Department of Automatic Control and Systems Engineering

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Dr G Konstantopoulos Applications accepted all year round

About the Project

Power systems stability has been an active area of research for many decades. However, recently, the large number of renewable energy sources being integrated to the power network has started to significantly affect the stable and reliable operation of the grid. This has created essential requirements for the grid-connected units to contribute to the stability of the network by assisting in the regulation of the grid voltage and frequency via adjusting the injected real and reactive power. Hence, control design methods for grid-tied inverters will play a key role in the future smart grid architecture, which will be dominated by inverter-interfaced units.

Although several control methods have been developed for grid-tied inverters to achieve power regulation or provide ancillary services (contribute to the voltage and frequency regulation), the stability properties of the resulting closed-loop system have not been adequately exploited, especially for a system with multiple inverters, such as in the case of a smart grid. Most of the existing approaches to analyse stability are based on small-signal model and linearization methods. However, the inherent nonlinear dynamics of the power inverters together with the nonlinearities that arise from the power expressions in the control loop make nonlinear analysis essential to achieve global stability results.

This project will focus on the development of nonlinear control methods for grid-tied inverters that are based on a strong mathematical background and guarantee nonlinear closed-loop stability. These methods are required to: a) have simple structure, b) be independent of the system parameters and c) fulfil the technical requirements of the grid. Hardware-in-the-loop and experimental implementation of the developed methods are essential to verify the theoretical development.

Prospective applicants should have a good first degree (I or II.i) and/or Masters degree in a mathematical or engineering-related subject. A background in nonlinear control/systems theory, power systems and/or electrical engineering and knowledge of DPS programming is desirable. Competence in Matlab/Simulink is essential.

Funding Notes

Applicants can apply for a Scholarship from the University of Sheffield but should note that competition for these Scholarships is highly competitive. it will be possible to make Scholarship applications from the Autumn with a strict deadline in late January/early February. Specific information will appear:


Further Reading
J. W. Simpson-Porco, F. Dörfler, and F. Bullo, “Synchronization and power sharing for droop-controlled inverters in islanded microgrids,” Automatica, vol. 49, no. 9, pp. 2603–2611, 2013.
J. Schiffer, R. Ortega, A. Astolfi, J. Raisch, and T. Sezi, “Conditions for stability of droop-controlled inverter-based microgrids,” Automatica, vol. 50, no. 10, pp. 2457–2469, 2014.
M. Karimi-Ghartemani, “Universal integrated synchronization and control for single-phase dc/ac converters,” IEEE Trans. Power Electron., vol. 30, no. 3, pp. 1544–1557, Mar. 2015.
G. C. Konstantopoulos, Q.-C. Zhong, B. Ren, and M. Krstic, “Bounded droop controller for parallel operation of inverters,” Automatica, vol. 53, pp. 320 – 328, 2015.
G. C. Konstantopoulos, Q.-C. Zhong, B. Ren, and M. Krstic, “Bounded Integral Control of Input-to-State Practically Stable Non-linear Systems to Guarantee Closed-loop Stability,” IEEE Trans. on Automatic Control, 2016, to appear.

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