Robust hybrid control strategy for the boiler/turbine unit full operating range in a coal-fired power plant

Multi-input-multi-output (MIMO) robust controllers recently designed for the megawatt output/throttle pressure control in a coal-fired power plant boiler/turbine unit have demonstrated performance and robustness noticeably superior to that of the currently employed nonlinear PID-based controller. These controllers, however, have been designed only for the range of 150-185 M W around the 185 M W nominal operating point, exhibiting a significant loss of performance in the lower range of 120-150 MW. Through system identification, the reason for the latter performance drop has been found in the current work to be a pronounced dependence of the boiler/turbine unit steady state gains on the operating point. This dependence induces significant modeling uncertainty that degrades system performance. This problem is addressed in the present work via two control strategies. The first one employs the single robust controller based on the middle (150MW) operating point in place of the 185 M W one. This effectively reduces the modeling uncertainty over the entire range due to the operating point change to approximately 50% of that associated with the previous design. The second one uses hybrid controller synthesized through partitioning the full operating range into two sub-ranges and designing a robust controller for each of them. This permits attainment of the desired overall performance, while further reducing the modeling uncertainty due to the operating point change to approximately 25% of that associated with the previous designs. Fast, smooth, and reliable switching between two controllers, as dictated by the setpoint change, is performed by a recently developed robust bumpless transfer algorithm. The single controller strategy features simplicity and ease of use, while the hybrid controller approach provides better robustness in face of uncertainty due to plant aging, soot deposits, etc. As demonstrated by simulation results, each strategy provides an adequate closed loop performance. This offers a robust performance/complexity trade-off between two strategies for robust control of a boiler/turbine unit over the full operating range.