DCS system can automatically adjust the control objects and control strategy according to the operation mode of two condensate pumps, Two kinds of control strategies are mainly included:
1. Variable frequency regulating According to the changes of deaerator water level under different load, regulate the rotating speed of the condensation pump and control the stability of deaerator water level.
2. Regulating of valve. As long as there is one pump in power frequency operation mode in the condensation water system , the system will be controlled by the valve regulating, at this moment, the high voltage frequency converter shall operate at high frequency (45 ~ 50 hz) section for fear that water does not flow.
The condensate pump has two kinds of operating modes of variable frequency and power frequency. Under normal circumstances, the condensate pump of variable frequency will operate for a long-term as the operating pump, and the condensate pump of power frequency is used as the standby pump. High voltage converters are divided into two kinds of local control and remote control. When it is in local control state, the command signal of rotating speed output from DCS tracks the feedback of rotating speed of high -voltage frequency converter, and is invalid to the remote control of high-voltage frequency converter.
The high voltage inverter is controlled by DCS in two ways of automatic and manual state. In annual state, the operator controls the rotating speed of high voltage frequency converter through changing the speed- control block on the screen. In automatic state, according the constant value of the deaerator water level set inside DC, the rotating speed of high- voltage frequency converter is automatically controlled. When the pump of variable frequency and the pump of power frequency are operating in parallel, the deaerator water level are automatically solved, then manually adjust the control valve C-1 for adjusting the valve C - 1. When the pump of variable frequency is operating, "High Voltage Frequency Converter Fault" interlocks the high-voltage switch of this pump( this interlocking is realized locally), the signal of "High-voltage switch has been tripped" ( namely the pump trips) is sent to DCS system in order to interlocking start the standby pump in power frequency mode.
The recirculation valve C - 2 of main condensate water is adjusted according to the outlet flow of condensate pump and pressure of main pipe. When the flow rate is less than 300T\H, it is adjusted by flow signal, C - 2 is opened to maintain minimum flow; When the flow rate is more than 300 t/H, it is adjusted by pressure signal to maintain the outlet pressure of condensate pump to be ≤ 2.6. When the pressure exceeds the set value, C - 2 is opened. The opening range is determined by the differential value. If the condensate pump is operating in power frequency mode, the logics related with the water level control of the deaerator, valves interlocking and the interlocking between pumps are still accomplished according to the logic functions before modification.
The rotating speed of condensate pump of variable frequency is changed to adjust the water level of the deaerator in order to lower the pressure of condensate water, however the pump of power frequency uses the control valve of the main condensate water to adjust the water level of deaerator, the pressure of condensate water is very high, during operation, in case the variable frequency mode of the condensate pump breaks down and after the standby pump of power frequency is interlocking started, the pressure and flow rate of condensate water will suddenly increase, thus causing very big impact on the water level control of the dearator and even the safety of the dearator. Special control logic is designed for solving this problem . When the high- voltage switch of the pump of the variable frequency trips in emergency and interlocking starts the standby pump of power frequency, the starting signal of the motor for B condensate pump is used as the trigger signal for quickly closing C-1, the control valve of water level of the deaerator is quickly interlocked and turned down. The target value of closing the valve has a certain function relationship with the load of the unit, then the operator shall manually the opening.
C - 1 the curve of the relationship between the target value of valve position and the unit load is shown in the following table.
During operation, the pressure of the condensate water decreases with the decreasing of the load, the logic of the original " if the pressure of the condensate water is low, the standby pump will be interlock started" , on the contrary, increases the instability of the system, so the set value is reduced to 0.18 Mpa, the logic circuit is constant, at the same time "Condensate Water Pressure alarms" is reserved. In order to guarantee good standby of the unit, during normal operation of the unit, the outlet electric valve of the pump of power frequency is in fully opened state. If the pressure of the condensate water is low and interlocks the low-pressure bypass valve protection: the original loop and set value are adopted. During application of frequency conversion at beginning of the unit starting, it is possible to close the deaerator C - 1 to a certain position in order to make the condensate have enough pressure and to ensure the normal operation of the low-pressure bypass. The condensate pump is protected for low water level, the original circuit and set value are adopted.
In order to improve the water level of deaerator and condenser water level, automatically control the regulating quality of the system and, improve the level of control, in this automatic control plan for revamping the frequency conversion device, the latest research results to control the water level of the dearator in this field, namely the intermittent control theories, are used, some advanced control thoughts(static incomplete forecast) are introduced. The main idea is to introduce the signal of rotating speed of the condensate pump of frequency conversation into the control system to balance with the water level deviation in order to ensure that the water level is relatively stable( to be controlled in a certain interval), and to introduce the main steam flow signal of the unit, in order to improve the load adaptability of the system and to compensate the deviation caused by the signal of the rotating speed of the condensate pump of frequency conversion, so that the system only acts during the process of load change of the unit and the process of spontaneous disturbance change of water level. After the signal of rotating speed of the condensate pump of frequency conversation is balanced with the change of the disturbance above mentioned, the system is in wait state, in order to adapt to the lagging of the thermal system and various uncertainties.
