METHOD FOR SETTING PARAMETERS OF LOAD FEEDFORWARD CONTROLLER FOR SUPERHEATED STEAM TEMPERATURE CONTROL

Abstract

Disclosed is a method for setting parameters of a load feedforward controller for superheated steam temperature control, which belongs to the technical field of thermal automatic control. This method adds a load feedforward controller to the conventional boiler superheated steam temperature spray desuperheating cascade control system. The application provides a structure of the load feedforward controller, and a method for designing the parameters of the load feedforward controller according to the dynamic characteristics of the superheated steam temperature related to feed coal flow disturbance, feedwater flow disturbance and desuperheating water spray disturbance. The method of the application could effectively reduce the superheated steam temperature deviation in the process of unit load rise or drop, and the design method is simple, effective and easy to realize in engineering.

Claims

1. A method for setting parameters of a load feedforward controller for superheated steam temperature control, wherein the load feedforward controller is added to a superheated steam temperature spray desuperheating cascade control system, and the load feedforward controller adopts the following transfer function: $G_F\left(s\right)=-\frac{K_{F}s\left(1+T_{1}s\right)}{\left(1+T_{2}s\right)},$ where K.sub.F, T.sub.1, and T.sub.2 are parameters of the load feedforward controller; an output Ne′ of a load processing module which processes a unit power Ne is used as an input of the load feedforward controller, and an output of the load feedforward controller and an output of the secondary controller of the superheated steam temperature spray desuperheating cascade control system are added up; the load processing module calculates the output of the module according to the following method: ${\mathrm{Ne}}^{\prime }=\frac{\mathrm{Ne}-\mathrm{Ne}\mathrm{\_min}}{\mathrm{Ne}\mathrm{\_max}-\mathrm{Ne}\mathrm{\_min}},$ where Ne_max, Ne_min are the maximum and minimum load of the unit respectively; parameters of the load feedforward controller G.sub.F(s) are set according to following steps: S1: for a unit, switching its unit load control system to manual, its superheated steam temperature control system and its reheat steam temperature control system to manual, and its boiler combustion control system to automatic, and making the unit in a stable state; S2: under the stable state in S1, step reducing of the total feed coal flow of the unit by 1% of the rated total feed coal flow, and collecting the variation values of the superheated steam temperature with T seconds as the sampling period to obtain the unit step response data ΔT.sub.1(k) of the superheated steam temperature, where k=1, 2, . . . , N, N is the number of sampling data; S3: making the unit in the stable state in S1, step reducing of the feedwater flow of the unit by 1% of the rated feedwater flow, and taking T seconds as the sampling period, collecting the variation values of the superheated steam temperature to obtain the unit step response data ΔT.sub.2(k) of the superheated steam temperature; S4: calculating out a data sequence ΔT(k), ΔT(k)=ΔT.sub.1(k)+ΔT.sub.2(k) and searching the maximum value of the data sequence ΔT(k) and a sampling time corresponding to the maximum value, which are respectively recorded as K.sub.0 and T.sub.0; calculating out the data sequence DT(k), k=1, 2, . . . , N−1, DT(k)=ΔT(k+1)−ΔT(k), finding out the sampling time corresponding to the maximum value in the data sequence DT(k), and recording the sampling time as Tq; S5: keeping the unit in the stable state in S1, step reducing of the opening of the superheated steam spray desuperheating valve by 5%, taking T seconds as the sampling period, collecting the variation values of the superheated steam temperature, obtaining the unit step response curve of the superheated steam temperature, and calculating out the characteristic parameters τ, T.sub.p and K.sub.p of the step response curve; wherein τ is a lag time, the value of which is the intersection point value of the tangent at the inflection point on the step response curve and the abscissa axis; T.sub.p is a time constant whose value is the time required to change from the inflection point value to the final equilibrium value at the maximum speed on the step response curve; K.sub.p is a steady-state gain, and its value is the ratio of the steady-state value of the variation values of the superheated steam temperature and the variation value of the opening of the superheated steam spray desuperheating valve; S6: based on the calculation results of S4 and S5, calculating the parameter K.sub.F of the load feedforward controller G.sub.F(s) as follows: $\{\begin{array}{c}{K}_F=\frac{K_1}{K_p}\\ K_1=\frac{K_{0}T}{e^{-\left(n-1\right)}}\frac{\left(n-1\right)!}{{\left(n-1\right)}^{n-1}}\\ n={\left(\frac{T_0}{T_0-T_q}\right)}^2+1\end{array};$ calculating parameters T.sub.1 and T.sub.2 of the load feedforward controller G.sub.F(s) as follows: $\{\begin{array}{c}{T}_1=T_2+T_p+\tau -\left(T_0+x\right)\\ \begin{array}{c}{T}_2=\frac{2\tau T_p+{\left(\tau -T_0-x\right)}^2+\left(x-2T_p\right)\left(T_0+x\right)}{2\left(T_p+\tau -T_0-x\right)}\\ x=\frac{{\left(T_0-T_q\right)}^2}{T_0}\end{array}\end{array},$ wherein x represents an intermediate variable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a block diagram of superheated steam temperature control principle.

[0025] FIG. 2 shows a process step response curve and its characteristic parameters.

[0026] FIG. 3 is a flow chart of parameter setting steps of a load feedforward controller.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] In order to better explain the technical scheme disclosed by the application, the following description will be further elaborated with reference to the attached drawings and specific implementation cases.

[0028] The present application adds a load feedforward controller to the conventional boiler superheated steam temperature spray desuperheating cascade control system, as shown in FIG. 1. In the figure, G.sub.2(s) and G.sub.1(s) are the transfer functions of the superheated steam temperature process in leading zone and inert zone respectively, G.sub.c1(s) and G.sub.c2(s) are the primary controller and the secondary controller of the cascade control system respectively, Y is superheated steam temperature, R is set value of the superheated steam temperature and Y′ is the leading steam temperature. For the load feedforward controller G.sub.F(s), the following transfer function is adopted:

[00005]$G_F\left(s\right)=-\frac{K_{F}s\left(1+T_{1}s\right)}{\left(1+T_{2}s\right)},$

[0029] where K.sub.F, T.sub.1 and T.sub.2 are parameters of the load feedforward controller; the output Ne of the load processing module which processes the unit power Ne is used as the input of the load feedforward controller, and the output of the load feedforward controller and the output of the secondary controller of the cascade control system are added up.

[0030] The load processing module calculates the output of the module according to the following method:

[00006]${\mathrm{Ne}}^{\prime }=\frac{\mathrm{Ne}-\mathrm{Ne}\mathrm{\_min}}{\mathrm{Ne}\mathrm{\_max}-\mathrm{Ne}\mathrm{\_min}},$

[0031] where Ne_max, Ne_min are the maximum and minimum loads of the unit respectively.

[0032] As shown in FIG. 3, the parameters of the load feedforward controller G.sub.F(s) are determined by the following methods and steps:

[0033] S1: switching a unit load control system to manual, a superheated steam temperature control system and a reheat steam temperature control system to manual, and a boiler combustion control system to automatic, and making the unit in a stable state;

[0034] S2: under the stable state in S1, step reducing of the total feed coal flow of the unit by 1% of the rated total feed coal flow, and collecting the variation values of the superheated steam temperature with T seconds as the sampling period to obtain the unit step response data ΔT.sub.1(k) of the superheated steam temperature, where k=1, 2, . . . , N, N is the number of the sampling data;

[0035] S3: making the unit in the stable state in S1, step reducing of the feedwater flow of the unit by 1% of the rated feedwater flow, and taking T seconds as the sampling period, collecting the variation values of the superheated steam temperature to obtain the unit step response data ΔT.sub.2(k) of the superheated steam temperature, k=1, 2, . . . , N;

[0036] S4: calculating a data sequence ΔT(k), k=1, 2, . . . , N, ΔT(k)=ΔT.sub.1(k)+ΔT.sub.2(k) and searching the maximum value of the data sequence ΔT(k) and the sampling time corresponding to the maximum value, which are respectively recorded as K.sub.0 and T.sub.0; calculating out the data sequence DT(k), k=1, 2, . . . , N−1, DT(k)=ΔT(k+1)−ΔT(k), finding out the sampling time corresponding to the maximum value in the data sequence DT(k), and recording it as Tq;

[0037] S5: keeping the unit in the stable state in S1, step reducing the opening of the superheated steam spray desuperheating valve by 5%, taking T seconds as the sampling period, collecting the variation values of the superheated steam temperature, obtaining the unit step response curve of the superheated steam temperature, and calculating out the characteristic parameters τ, T.sub.p and K.sub.p of the step response curve; as shown in FIG. 2, τ is the lag time, the value of which is the intersection point value of the tangent at the inflection point on the step response curve and the abscissa axis; T.sub.p is a time constant whose value is the time required to change from the inflection point value to the final equilibrium value at the maximum speed on the step response curve; K.sub.p is the steady-state gain, and its value is the ratio of the steady-state value of the variation values of the superheated steam temperature and the variation value of the opening of the superheated steam spray desuperheating valve;

[0038] S6: based on the calculation results of S4 and S5, calculating the parameter K.sub.F of the load feedforward controller G.sub.F(s) as follows:

[00007]$\{\begin{array}{c}{K}_F=\frac{K_1}{K_p}\\ K_1=\frac{K_{0}T}{e^{-\left(n-1\right)}}\frac{\left(n-1\right)!}{{\left(n-1\right)}^{n-1}}\\ n={\left(\frac{T_0}{T_0-T_q}\right)}^2+1\end{array};$

[0039] calculating parameters T.sub.1 and T.sub.2 of the load feedforward controller G.sub.F(s) as follows:

[00008]$\{\begin{array}{c}{T}_1=T_2+T_p+\tau -\left(T_0+x\right)\\ \begin{array}{c}{T}_2=\frac{2\tau T_p+{\left(\tau -T_0-x\right)}^2+\left(x-2T_p\right)\left(T_0+x\right)}{2\left(T_p+\tau -T_0-x\right)}\\ x=\frac{{\left(T_0-T_q\right)}^2}{T_0}\end{array}\end{array},$

[0040] where x represents an intermediate variable.

[0041] The following is a detailed description of the summary of the present application by taking a 1000 MW supercritical unit of a power generation limited liability company which adopts the load feedforward controller of the present application for example. Based on this application scenario, the above related parameters are selected as follows:

[0042] the maximum and minimum load of the load processing module is taken as: Ne_max=1000 MW, Ne_min=0 MW;

[0043] step response curves of S2, S3 and S5 are all obtained under 800 MW load, the sampling period is T=5 s, and the number of samples is N=200;

[0044] in S4, the calculated parameters are K.sub.0=0.26, T.sub.0=235 s and Tq=125 s;

[0045] in S5, the characteristic parameters of the step response curve are τ=149.5 s, T.sub.p=81.2 s and K.sub.p=0.622;

[0046] the load feedforward controller parameters calculated in step S6 are: K.sub.F=10.2, T.sub.1=63.2 and T.sub.2=118.3.

[0047] Testing with the method of the application shows that the maximum dynamic deviation of the superheated steam temperature of the unit decreases from 12° C. to 4.5° C. when the load of the unit increases from 800 MW to 900 MW at a rate of 20 MW/min, which suggests that the method of the application can effectively improve the safety and cut down costs of the unit operation.

[0048] The above is only the preferred embodiment of the present application, and it should be pointed out that for ordinary technicians in the technical field, without departing from the principle of the present application, improvements and modifications could be made, which should also fall in the protection scope of the present application.