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peaks have raised higher performance demands for the control system design of
supercritical
units.
Due
to
complex
dynamic
characteristics,
the
Ultra-supercritical units have the following control diculties[2,3,4]:
(1) Coupling between turbine and boiler. Without the buffering of steam
drum, dynamic characteristics of the ultra-supercritical boiler are influenced by
the terminal resistance. The turbine valve opening on the one hand controlled the
turbine power, and on the other hand it influenced directly the characteristics of
terminal resistance of the boiler. This is different from the situation of the drum
boiler.
(2) Strong non-linearity. Ultra-supercritical units have complex controlled
characteristics. As the load changes, the dynamic characteristics parameters of
units change greatly. Most ultra-supercritical units are operated in voltage trans-
formation, even sometimes units will be operated under subcritical pressure.
With the large difference between the steam properties of supercritical and sub-
critical and migration of evaporation zone (phase transformation points) under
different combustion ratios, strong non-linearity and variable parameter charac-
teristics are presented in ultra-supercritical units, which makes it more dicult
to be controlled than normal units.
(3) Delay in dynamic characteristics. One of the important points for ultra-
supercritical units control lies in the control of main steam and reheated steam
temperature. Steam temperature is generally controlled by the coarse tuning
of ratio of fuel to water and the first and second level desuperheating water.
Because of its delayed response to the changes of ratio of fuel to water, the
outlet temperature cannot be used as feedback quantity of ratio of fuel to water
regulation. In order to increase the response speed and precision of ratio of
fuel to water, intermediate point temperature or enthalpy is usually used as
feedback variable of control loop to realize the regulation and control of main
steam temperature.
3 Application of Multivariable Model Predictive Control
The main steam temperature control of ultra-supercritical unit is a typical object
with large time delay and dynamic characteristics which change greatly along
with change of load. Currently, most controllers used for main steam tempera-
ture control of ultra-supercritical unit are still PID controllers or adaptive PID
controllers with adaptive measures added on original controllers. The nature of
delay of PID regulation decides the existence of various disadvantages in main
steam tempearture control system based on PID. However, Model predictive
control (MPC) is suitable to solve the time delay and multivariable control prob-
lems. MPC was firstly used in the slow process like chemical industry. Richalet
proposed MHPC (model heuristic predictive control) or MAC (model algorithm
control) in 1976 and 1978 and applied it in process control [5,6,7]. Cutler et al.
presented DMC (dynamic matrix control) In 1980 [8] and Garcia et al presented
QDMC (quadratic dynamic matrix control)[9] in 1986. Later in 1987, Clarke
et al. proposed the GPC (generalized predictive control)[10,11] and Yuan Pu
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