Analysis And Design For Load Frequency Control Systems With Nonlinearities

In power systems,frequency stability is an important index of power quality.Any sudden load perturbation can cause the deviation of tie-line exchanges and the frequency fluctuations.Therefore,to ensure the power quality,a load frequency control(LFC)system is needed.The goal of LFC is to return the frequency to its nominal value and minimize the unscheduled tie-line power flows between interconnected control areas.This paper studies load frequency control of power system with nonlinearities and proposes some anti-windup schemes to compensate the nonlinearities.Main contents of the dissertation include:1)For LFC systems with governor dead band(GDB),the effect of GDB is first analyzed.It is shown that GDB may result in limit cycle for LFC systems.To maintain the performance of the original LFC controller,two anti-GDB schemes are proposed.One is based on the observer structure of the LFC controller,which utilizes the output of the governor to reconstruct the input to the system in order to correctly estimate the controller states;The other is based on the error between the realistic output of the governor and its ideal value which is fed back and added to the output of the original LFC controller.Simulation results show that the two anti-GDB schemes are effective in overcoming the adverse effects of GDB in that the performances are close to that of the original controller.There are some limitations in observer-based scheme and the error-compensation-based scheme is better in performance and implementation.2)For LFC systems with governor dead zone(GDZ),the effect of GDZ is first analyzed.It is shown that GDZ may not affect the stability of the system,but can reduce the performance.Then an error-compensation based anti-GDZ scheme is proposed,with the idea to add the error between the realistic output of the governor and its ideal value to the output of the controller.Simulation results show that the performance of the control system can be recovered after adopting the anti-GDZ scheme,in that the performance of the LFC system with anti-GDZ scheme is close to that without GDZ.3)For LFC systems with generation rate constraint(GRC),the effect of GRC is first analyzed.It is shown that GRC affects the dynamic performance of the LFC system greatly,even causes instability.To reduce the effect of GRC,an anti-GRC scheme based on error compensation is proposed for observer-based LFC controllers.The error between the ideal and the realistic output of the turbine will be fed back to the observer of the controller,which is then used in the observer to estimate and eliminate.Simulation reulsts show that the performance of the control system can be recovered after adopting the anti-GRC scheme.4)For load frequency control of multi-area power systems,by utilizing decentralized control idea,the anti-windup schemes for single-area power systems are extended to multi-area power systems.Then for power systems with multiple generation units,anti-GDB and anti-GRC schemes can be combined to compensate the effects of GDB and GRC.Simulation results show that the performance of the LFC system with both GDB and GRC can be recovered after adopting the proposed scheme.