Design of Chemical Monitoring System for Power Plant Based on S7-200

1 Introduction
The chemical monitoring system of a power plant can automatically monitor the water quality parameters of boiler feed water, boiler water, steam and condensate in a thermal power plant in real time. Based on continuous online instrument monitoring, a design of a chemical monitoring system for a power plant is proposed. The system transmits monitoring data to a computer, controls automatic signal collection, realizes automatic control of feed water and boiler water dosing in the power plant, and configures a certain margin for subsequent expansion of the system. The system can also be used for online monitoring of chemical dosing systems.

2 System composition and control scheme
2.1 System composition
The host computer of the entire system (including data acquisition and dosing control) can be completed by an industrial computer (IPC), which can realize data processing and management and be connected to the MIS system. Figure 1 is a diagram of the online monitoring and diagnosis system.

2.2 Control scheme
The system collects various real-time data from chemical instruments and thermal instruments scattered at various sites, timely and accurately reflects the water vapor quality of the thermal system, chemical operating conditions, automatic control of feed water and boiler water dosing, and the switch status of each dosing pump during the operation of the unit, records the main operating parameters and events, and stores them at a certain time interval, which is convenient for trend chart display, report preparation, and expert diagnosis, treatment opinions and other collected data. The system extracts 4-20 mA signals from the online analyzer (pH meter), performs PID calculations according to the operating process parameters, and outputs them to the frequency converter to control the dosing pump to achieve automatic closed-loop adjustment of dosing. Figure 2 shows the control system flow.

3 System hardware design
3.1 Upper monitoring part
The upper monitoring part of the system is completed by industrial computers. The monitoring staff monitors the system's operating status in real time through CRT, sets or modifies the system's operating parameters, and remotely controls the system through CRT. The upper computer processes and manages data, connects to the MIS system, and configures the controller, including: controller network address, controller time, select control algorithm, set algorithm parameters, set control quantity set point, select algorithm input, output channel, etc.

3.2 Lower-level monitoring part
Considering that the controlled scale of the chemical monitoring system is small and most of them are analog input points, the lower-level monitoring part of the system (I/O module) uses the S7-200 series products of Siemens, which are currently widely used in the field of industrial control. The controller has strong environmental adaptability and is suitable for field control. By selecting different I/O modules, it can receive and send various standard signals, and realize two-way data communication with the upper industrial computer through the RS-485 communication interface. The controller is a specially designed hardware product, or the control program is directly transferred to the S7-200 system to enable the controller to have better hardware support and less hardware failure rate, and realize more functions. S7-200 is a PC-based programmable controller that can independently complete data acquisition and control. It can operate reliably in harsh industrial environments and also has general programming functions, allowing the running of applications compiled in high-level languages ​​such as C or C++. The system uses imported frequency converters as the regulating equipment, which controls the dosage by adjusting the speed of the AC motor of the dosing pump. The control system uses the combination of upper computer software WinCC + Siemens PLC. The PLC system is connected to the host computer WinCC through the PorfiBus bus. This configuration has strong scalability and good openness, which is convenient for realizing the entire chemical monitoring system. Figure 3 shows the structure of the boiler water monitoring system.

3.3 Controller selection
The control system CPU module has digital input/output, and no separate D/A conversion module is required. Siemens CPU214 is used as the core of the controller (CPU module). It has a rich instruction system and can perform various complex logical operations and arithmetic operations as well as various function operations, such as signal scale conversion, signal filtering, PID operation, etc. Inside the CPU module, the microprocessor connects the memory, interface and I/O unit through the data bus, address bus, control bus and auxiliary circuit. With the help of the programmer, the user program and data entered are received; the user program is read and interpreted and executed; the input status is received according to the specified timing, the output status is refreshed, and information is exchanged with external devices; the hardware status of the PC is diagnosed, etc. The A/D conversion module uses EM235, which is a high-speed 12-bit analog module that can convert analog quantities into corresponding digital quantities within 149 ms. Input voltage signal or current signal. Generally speaking, pH meters have 4~20mA signal output, so 4~20mA standard signal input is used.

4 System Software Design
4.1 Upper Computer Control Software
The upper computer adopts WinCC system. Windows NT32 has the characteristics of preemptive multi-tasking, which ensures rapid response to process events and provides a variety of protections against data loss. WinCC is an industrial technology neutral system based on Windows NT32 operating system to solve visualization and control tasks in production and process automation. It provides functional modules for industrial graphic display, message, archiving and reporting. High-performance process coupling, fast screen update, and reliable data make it highly practical.
In addition to these system functions, WinCC also provides an open interface for user solutions. This makes it possible for WinCC to integrate complex and extensive automatic control solutions, integrating archive data access in ODBC and SQL, as well as object and document links through OLE2.0 and ActiveX controls.
4.2 Lower Computer Control Software
STEP 7-Micro/WIN provides 3 editors to create programs: ladder diagram (LAD), statement list (STL), and function block diagram (FBD). Programs written in any program editor can be browsed and read in another program editor, but some input rules must be followed. The STEP 7-Micro/WIN project window provides a convenient workspace for creating control programs. The toolbar provides buttons for shortcuts to commonly used menu commands; the operation bar provides a set of icons for accessing different program components of STEP 7-Micro/WIN; the instruction tree displays all project objects and instructions required to create control programs; the program editor includes program logic and local variable tables, in which symbolic names can be defined for temporary local variables. There are tabs for subroutines and interrupt service programs at the bottom of the program editor. Click these tabs to switch between the main program, subroutine, and interrupt service program.
4.3 Lower computer control software programming example
4.3.1 Sampling signal software filtering
During on-site debugging, it was found that the system had a serious signal interference problem. Problem finding and hardware isolation measures were ineffective. Due to the short construction period and urgent time, software filtering was used to deal with it, which is also a common treatment method in engineering.

4.3.2 Proportional/Integral/Derivative (PLD) Loop Control Instruction Application
PID loop instructions (including proportional, integral, and differential loops) are used to perform PID operations. The PID loop of S7-200 does not set the control mode. As long as the PID block is valid, the PID operation can be performed. In this sense, there is an "automatic" operation mode for PID operation. When the PID operation is not executed, it is called "manual" mode. Similar to the counter instruction, the PID instruction has an enable bit. When the enable bit detects a positive jump of a signal (from 0 to 1), the PID instruction performs a series of actions to switch the PID instruction from manual mode to automatic mode without disturbance. In order to achieve a disturbance-free switch, the current output value must be manually filled in the Mn column in the loop table before switching to automatic control.

5 Conclusion
The chemical monitoring system of power plants based on Siemens S7-200 programmable controller uses computer network technology to realize the control and management of production processes and improve the efficiency and reliability of equipment operation. The system has been successfully applied to a 4x300 WM generator set. Practice has proved that the system not only meets production needs, improves the level of chemical monitoring diagnosis and management, but also creates higher economic benefits for enterprises.