JPS5953442B2 - First cut back control system for natural circulation boiler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a first cut back (PCB) control method for a natural circulation boiler that allows the power plant to shift to standalone operation within the power plant without stopping the power plant in the event of a power transmission system fault.

Figure 1 shows an outline of a natural circulation boiler plant.

The steam generated in the boiler 1 is sent to the turbine 2 and drives the generator 3, and then is returned to water in the condenser, passes from the condensate pump 4 to the deaerator 5, and is then sent to the turbine-driven feedwater pump 6 and the electric motor. The water is returned to the boiler 1 by the drive water pump 7.

8 is a water supply control valve. The electric power generated by the generator 3 is transmitted via the main transformer 9 and the main circuit breaker 11, and
Power is supplied to the station bus 12 via the station transformer 10 .

In such a natural circulation boiler plant, in the event of a power transmission system fault, the main circuit breaker 11 is opened, and the output of the generator 3 is transferred to be supplied only to the internal load of the power plant through the internal transformer 10.

This is commonly referred to as a first cut hack.
It is called Cut Back (FCB).

When a PCB occurs in this way, the generator output becomes a small value of about 5% of the rated output, so the boiler input must be controlled to a value commensurate with this output and within the boiler limit value. There is.

However, in many cases, the drum water level drops below the limit and the plant automatically shuts down, making PCB unsuccessful.

According to conventional test results, the reason why PCB is not successful in natural circulation boilers is that when PCB occurs, the drum water level falls below the negative limit value.

Analyzing this phenomenon from a control perspective, the rise in main steam pressure caused by the sudden drop in boiler load causes the bubbles in the boiler drum to collapse, causing the drum water level to drop rapidly. As shown by the thin line in Figure 2, the main steam flow rate 21. Since this is performed by a three-element controller 24 that receives the signals of the main water supply flow rate 22 and the drum water level 23 as inputs, a sudden decrease in the main steam flow rate signal 21 causes a large deviation between the main steam flow rate signal 22 and the main steam flow rate signal 22. This is because the water supply control operation end 25 is throttled too much.

Therefore, an object of the present invention is to switch the boiler feed water control from three-element control to single-element control of only the drum water level at the same time as a PCB occurs, thereby preventing the operation end of the water feed control from being narrowed down, and furthermore, by switching the boiler feed water control from three-element control to one-element control of only the drum water level, Since controllability is poor in low load areas, we automatically start the motor-driven water supply pump as needed and switch to control using the water supply control valve.
Even when PCB occurs, the water level in the drum is kept within the limit value, and PCB is prevented.
The aim is to make it successful.

In the water supply control system diagram of FIG. 2, according to the present invention, only the drum water level signal 23 is input to the one-element controller 25 shown in bold and the one-element controller 25 to which the control switch 26 is added, and the PCB When this occurs, the water supply control is switched from the three-element controller 24 to the one-element controller 25 by the control switch 26, and the water supply to the boiler 1 is controlled based on the output thereof.

Furthermore, if necessary, the motor-driven water supply pump 7 is automatically started at the same time as the PCB occurs, the opening and closing of the water supply control valve 8 is controlled by the output of the one-element controller 25, and the turbine-driven water supply pump 6 is stopped.

Because of this configuration, after the occurrence of PCB, boiler 1
Since the water supply flow rate to the drum is controlled only by the drum water level signal 23, the drum water level is maintained at a predetermined value, and the plant will not stop moving when the water level falls below the limit value.

FIG. 3 shows the results of controlling the drum water level after the occurrence of PCB, where A is the conventional method and B is the method of the present invention.

In the figure, the horizontal axis is the time after the occurrence of PCB, the vertical axis is the drum water level, L is the specified value, Lh is the positive limit value, Ll
indicates a negative limit value.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a natural circulation boiler plant, FIG. 2 is a block diagram showing the water supply control system, and FIG. 3 is a graph showing the drum water level control results. 1... Boiler, 6... Turbine driven water supply pump, 7... Electric motor driven water supply pump, 21
...Main steam flow rate signal, 22...Main water supply flow rate signal, 23...Drum water level signal, 24...
...Three-element controller, 26...Control switching device,
27...One-element controller.

Claims (1)

[Claims] 1. In a natural circulation boiler where the drum water level is controlled by three-element water supply control using a main steam flow rate signal, a main feedwater flow rate signal, and an Olam water level signal as inputs, only the drum water level signal is used when a first cutback occurs. A first cut-back control method for natural circulation boilers, which is characterized by controlling the drum water level through one-element water supply control. 2. The first natural circulation boiler according to claim 1, characterized in that when a first cutback occurs, an electric motor-driven water supply pump is started, and this is controlled by one-element water supply control using only a drum water level signal. Cut-back control method.