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The problem of degrees of freedom needs to be considered in steady opti-
mization. When the degrees of freedom (degrees of freedom=effective free MV
number-controlled CV number) is insucient, how to choose to release and relax
the soft constraints and hard constraints is an important problem [6].
Two kinds of constraints corresponding to setpoint control and zone control
are used in the multiple models MCPC. Setpoint control requires controlled
variables or manipulated variables to maintain a given value. Its form is as
follows:
CV i = kc i
MV j = km j .
(8)
The optimization process emphasizes the concept of degree of freedom. When a
variable is constrained by setpoint control, the degree of freedom will be reduced.
Therefore, CV zone control is often used to replace setpoint control. Zone control
of CV and MV requires that corresponding CV or MV should be maintained
within certain limits. Its form is as follows:
kc min ,i
CV i
kc max ,i
(9)
km min ,j
MV j
km max ,j .
Dynamic optimization is based on steady optimization value and use DMC to
accomplish the rolling optimization process with constraints:
2
Q +
2
R +
2
V
min
ΔU k
J k =
W k
Y k
ΔU k
U k
U IRV,k
(10)
s.t.CΔU k
b,
in which W k is the expected value of CV, Y k the value of CV, ΔU k the incremental
control vector, U k the value of MV and U IRV , K the MV steady-state optimal
value which constitutes the soft constraint of MV variable. Matrix V is the
soft constraint weighted matrix. Matrix C and vector b integrate all the hard
constraints.
When the Ultra-supercritical load differs by 10%, models of Ultra-supercritical
unit have a large deviation. Multiple models policy was used to deal with the
nonlinearity of models. The piecewise step response models of Ultra-supercritical
unit at different load point are built at the load work point 600MW, 700MW,
800MW, 900MW, 1000MW respectively. They are shown in figure 3:
In the multiple models policy, only outputs of two piecewise models which are
adjacent to the current load were weighted for final output of MCPC controller.
The weight is a linear function of the distance from the current load to the
selected model's load. Other models which are not near the current load are
ignored.
4
Simulation Results
In simulation, the nonlinear model of 1000MW Ultra-supercritical unit in [1]
was used. The multiple models MCPC is used to control the model in real-time
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