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Multi-input-multi-output robust controllers recently designed for the megawatt output/throttle pressure control in a coal-fired power plant boiler/turbine unit have demonstrated performance robustness noticeably superior to that of the currently employed nonlinear PID-based controller. These controllers, however, have been designed only for the range of 150–185MW around the 185MW nominal operating point, exhibiting a significant loss of performance in the lower range of 120–150MW. Through system identification, the reason for this performance loss is demonstrated in the current work to be a pronounced dependence of the boiler/turbine unit steady state gains on the operating point. This problem is addressed via a hybrid control law consisting of two robust controllers and a robust switch between them activated by the set point change. The controllers are designed to cover the corresponding half-ranges of the full operating range. This permits attainment of the desired overall performance as well as reduction of modeling uncertainty induced by the operating point change to approximately 25% of that associated with the previous designs. Robust switching is accomplished through a novel hybrid mode of behavior—robustly controlled discrete transition. The latter mode is produced through realizing that the off-line transfer speedup suggested by Zaccarian and Teel (2005, “The L2(l2) bumpless Transfer Problem for Linear Parts: Its Definition and Solution,” Automatica, 41, pp. 1273–1280) can be taken to the limit and incorporating the result into a robust bumpless transfer technique recently developed by the authors. As demonstrated by simulation results, the proposed strategy provides an adequate solution to the problem of robust boiler/turbine unit performance over the full operating range. This fact combined with numerical algorithm tractability, relative ease of its design, its insensitivity to implementation nonidealities, and accompanying identification methodology for nominal model generation makes it a viable candidate for industrial acceptance.

Linear quadratic (LQ) bumpless transfer design introduced recently by Turner and Walker [17] gives a very convenient and straightforward computational procedure for the steady-state bumpless transfer operator synthesis. It is, however, found to be incapable of providing convergence of the output of the offline controller to that of the online controller in several industrial applications, producing bumps in the plant output in the wake of controller transfer. An examination of this phenomenon reveals that the applications in question are characterized by a significant mismatch, further referred to as controller uncertainty, between the dynamics of the implemented controllers and their models used in the transfer operator computation. To address this problem, while retaining the convenience of the Turner and Walker design, a novel state/output feedback bumpless transfer topology is introduced that employs the nominal state of the offline controller and, through the use of an additional controller/model mismatch compensator, also the offline controller output. A corresponding steady-state bumpless transfer design procedure along with the supporting theory is developed for a large class of systems. The new technique is shown to be capable of eliminating the online/offline controller output tracking errors under significant controller uncertainty, while preserving fast convergence of Turner and Walker design. Due to these features, it is demonstrated to solve a long-standing problem of high-quality steady-state bumpless transfer from the industry standard low-order nonlinear multiloop PID-based controllers to the modern multiinput-multioutput (MIMO) robust controllers in the megawatt/throttle pressure control of a typical coal-fired boiler/turbine unit.

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SUMMARYThis paper presents an application of H and -synthesis controller design methods to a coal-"red power generation unit and compares the closed-loop performance and robustness of H and -synthesis control laws with those of an H control law. The model which relates "ring rate and turbine valve position inputs to throttle pressure and megawatt outputs presented by Ollat and Smoak in an earlier work is modi"ed to match the test data from a Tennessee Valley Authority (TVA) power generation unit. All three controller synthesis procedures are applied to a two-input two-output plant model which has time delay, di!erential part, colored noise output disturbance, and sensor noise disturbance. Application of the procedures to the model shows that when the shape of the closed loop control signals of all three designs is closely matched, in the low-frequency range the -synthesis and H control laws have robustness much better than that of H control law, while providing adequate robustness in the high-frequency range. H control law gives the best performance, and H } the worst of the three designs, exhibiting the largest overshoot. The balancing procedure permits signi"cant reduction of the order of the controllers without degradation in performance and robustness. The comparative evaluation of three designs shows that in power plant control problem H and -synthesis designs provide much more consistent and convenient performance/robustness trade-o! than H design.

This paper presents an application of H∞ and μ‐synthesis controller design methods to a coal‐fired power generation unit and compares the closed‐loop performance and robustness of H∞ and μ‐synthesis control laws with those of an H2 control law. The model which relates firing rate and turbine valve position inputs to throttle pressure and megawatt outputs presented by Ollat and Smoak in an earlier work is modified to match the test data from a Tennessee Valley Authority (TVA) power generation unit. All three controller synthesis procedures are applied to a two‐input two‐output plant model which has time delay, differential part, colored noise output disturbance, and sensor noise disturbance. Application of the procedures to the model shows that when the shape of the closed loop control signals of all three designs is closely matched, in the low‐frequency range the μ‐synthesis and H∞ control laws have robustness much better than that of H2 control law, while providing adequate robustness in the high‐frequency range. H∞ control law gives the best performance, and H2 – the worst of the three designs, exhibiting the largest overshoot. The balancing procedure permits significant reduction of the order of the controllers without degradation in performance and robustness. The comparative evaluation of three designs shows that in power plant control problem H∞ and μ‐synthesis designs provide much more consistent and convenient performance/robustness trade‐off than H2 design. Copyright © 1999 John Wiley & Sons, Ltd.

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A complete design and implementation methodology for the steady state bumpless controller transfer under controller uncertainty is presented. The off-line transfer speedup idea of Zaccarian and Teel is formalized to yield an implementation technique that completely eliminates the need for any performance/robustness trade-off in the transfer operator design and provides arbitrarily fast convergence of the offline controller output to that of the online one, removing a key obstacle to acceptance by practitioners of the technique proposed.