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Research Papers

Full Operating Range Robust Hybrid Control of a Coal-Fired Boiler/Turbine Unit

[+] Author and Article Information
Kai Zheng

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801kaizheng@uiuc.edu

Joseph Bentsman1

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801jbentsma@uiuc.edu

Cyrus W. Taft

 EPRI I&C Center, Kingston Fossil Plant, 714 Swan Pond Road, Harriman, TN 37748ctaft@epri.com

1

Corresponding author.

J. Dyn. Sys., Meas., Control 130(4), 041011 (Jun 10, 2008) (14 pages) doi:10.1115/1.2907367 History: Received August 07, 2006; Revised November 06, 2007; Published June 10, 2008

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 150185MW around the 185MW nominal operating point, exhibiting a significant loss of performance in the lower range of 120150MW. 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.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of a boiler/turbine power generation unit

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Figure 2

Block diagram of the closed-loop control of the boiler/turbine unit

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Figure 3

Full operating range control performance of the controller design for 185MW

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Figure 4

The augmented input sequences for identification

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Figure 5

Comparison of the magnitude Bode plots of H185(s) solid, H150(s) dash-dot, and H120(s) dashed

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Figure 6

Effect of modeling uncertainty on pole locations: filled diamond, the controller designed for 185MW operating on 185MW: circles, the controller designed for 185MW operating on 120MW

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Figure 7

The robust hybrid controller based on the two-dimensional state/output feedback bumpless transfer topology, with switching invoked by a set point change

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Figure 8

Nominal off-line controller subsystem in the state/output feedback topology

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Figure 9

Reduced nominal off-line controller subsystem in the state/output feedback topology

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Figure 10

Internal model based controller/model mismatch compensator for bumpless transfer in the steady state

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Figure 11

Off-line controller subsystem with uncertainty in G2 in the state/output feedback topology

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Figure 12

The dependence relationship between the online closed-loop and the off-line controller subsystem

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Figure 13

The dependence relationship between the online closed-loop and the off-line controller subsystem in the steady state

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Figure 14

Comparison of the convergent time for three cases: N=1, N=5, and N=800

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Figure 15

Performance of the state/output feedback bumpless transfer under the proposed implementation technique

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Figure 16

Steady-state performance comparison between controllers: plant output

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Figure 17

Tracking performance comparison between controllers: ramp from 185MWto150MW

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Figure 18

Tracking performance comparison between controllers: ramp from 150MWto120MW

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Figure 19

Performance comparison between controllers: megawatt output tracking/regulation error

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Figure 20

Performance comparison between the PID and an alternate hybrid controller: plant output

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Figure 21

Performance comparison between the PID and an alternate hybrid controller: megawatt output tracking/regulation error

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Figure 22

Block diagram of the augmented model

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