CONTROL OF SUPERHEATED STEAM TEMPERATURE UNDER VARIABLE LOADS OF THE BOILER UNIT

G. D. Kitaeva, A. A. Gurchenok

(Presented by the scientific seminar of the Department of Automation of Thermal-Power Processes of Industrial Enterprises)

A modern boiler unit is a complex thermal-power object controlled by a set of automatic control systems (ACS). Obtaining the maximum positive effect from the use of automation is possible only with correct adjustment of the controllers in accordance with the dynamic properties of the controlled sections of the boiler unit, which must be carried out over the full range of load variation.

For the safe operation of the unit, the temperature of the superheated steam ahead of the turbines must be maintained with only slight deviations from the specified value. This is achieved by the automatic control system.

Changes in load are characteristic of boiler-unit operation (Fig. 1). Under such conditions, the required quality of control is not always ensured.

Fig. 1. Steam output of the PK-10 boiler during the winter period

control. A change in load causes a change in the parameters of the dynamic characteristic of the controlled object: the gain coefficient \(K_{\mathrm{ob}}\), the time constant \(T_{\mathrm{ob}}\), and the lag time \(\varepsilon\) change. This leads to the fact that high-quality control of superheated steam temperature by means of a controller optimally adjusted only for one of the boiler-unit loads becomes impossible at other loads. Figure 2 shows the response curves of the main section of the boiler-unit superheater at different loads:

Fig. 2. Run-up curves of the main section of the superheater under a disturbance of 5 t/h and at loads: dashed line — 170; dash-dotted line — 200; solid line — 230 t/h.

Axis labels: \(h\,[\mathrm{mV}]\); \(t,\ \mathrm{s}\).

Visible annotations on the graph: \(2.09\ \mathrm{mV}\), \(2.6\ \mathrm{mV}\), \(1.5\ \mathrm{mV}\).

230; 200 and 170 t/h. The values of the parameters of the dynamic characteristics are given in Table 1.

In order to establish the dependence of the quality of regulation on the change in load, the tuning parameters of the proportional-integral controller and the differentiator were determined according to the method of Aleksandrova [1, 2]. Then transient processes were constructed for the minimum and maximum loads with optimal parameters for the average load of 200 t/h. The choice of comparing precisely these transient processes is explained by the fact that, in operation, the tuning parameters on the controllers are set for the average load of the boiler unit. From the plotted graphs of the transient control processes, an assessment of their quality was made (Fig. 3). It was established that for

Table 1

Table 2

off-design operating modes of the control system, a larger dynamic error is observed and the regulation time increases.

In Fig. 4, graphs are plotted of the optimal tuning parameters of the proportional-integral controller and the differentiator as a function of the boiler-unit load. From the graphs it is evident that, on average, the tuning parameters differ by a significant amount, although a small range of variation of the boiler-unit load was selected for the analysis.

The design of standard controllers used for regulating boiler units does not make it possible to change the values of the assigned tuning parameters. Therefore, in practice, the above-described reduction in the quality of the regulation process is observed. To eliminate this shortcoming, in the automatic control system one may compensate for the nonlinearity of the controlled object by choosing a regulating element with a corresponding, likewise nonlinear, flow characteristic.

Thus, constancy at all loads of some one quality criterion of the system can be ensured. For example, for the case under consideration it is necessary, for each load, to select such values of the gain coefficient of the regulating element that the transfer gain of the open-loop control system will not depend on the load. Under this condition, the stability margin of the system will be the same and the quality of the transient process in the control system at all loads will remain optimal [3, 4]. Assigning the value $K_{\mathrm{ro}}$ to the object, we obtain the relation

\[ K_{\mathrm{ob}}\cdot K_{\mathrm{ro}}=C. \]

Fig. 3. Transient processes of the superheated-steam temperature control system:
1 — load 170 t/h, optimal controller tuning;
2 — load 170 t/h, optimal controller tuning for a load of 200 t/h;
3 — load 230 t/h, optimal controller tuning;
4 — load 230 t/h, optimal tuning for a load of 200 t/h.

Ordinate: $\dfrac{\delta T^m}{\delta T_{\max}}$; abscissa: $t$, s.

Fig. 4. Values of the optimal settings of the controller and differentiator as a function of the boiler-unit load

Table 2 gives the required values of \(K_{\mathrm{ro}}\), at which the specified condition is satisfied. From the values obtained, the flow characteristic of the regulating element can be determined and its cross section profiled.

REFERENCES

1. N. D. Aleksandrova. Calculation of the parameters of dynamic tuning of a controller (ACS) for steam temperature with an anticipatory velocity signal. Teploenergetika, 1965, No. 4.

2. N. D. Aleksandrova. Automation of Thermal Power Stations. Moscow, Gosenergoizdat, 1959.

3. V. V. Solodovnikov. Frequency Methods for Constructing Transient Processes. Moscow, Mashgiz, 1959.

4. M. A. Seleznev. Regulating Elements in Control Systems. Moscow, Moscow Power Engineering Institute, 1969.