Application of PLC Soft Redundancy System in Urban Sewage Treatment Project

introduction

With the acceleration of China's economic development and urbanization, the amount of sewage in small and medium-sized cities has been increasing year by year. In order to reduce the investment in the construction of sewage collection pipe networks, sewage treatment facilities in small and medium-sized cities are scattered and small in scale. Affected by the level of economic development, treatment requirements, operation and management experience and level, large-scale sewage treatment control systems are difficult to adapt to the construction requirements of these small and medium-sized sewage treatment projects. It is very important to find a sewage treatment control system suitable for medium and small urban sewage treatment plants. This article introduces the successful application of the Siemens S7-300 PLC soft redundancy system in the Wuwei County Sewage Treatment Plant in Anhui. The system not only ensures the reliability, stability and real-time performance of the system, but also reduces the cost of engineering construction.

1 System Introduction

Anhui Wuwei County Wastewater Treatment Plant is an urban wastewater treatment plant financed and constructed by Anhui Modern Wastewater Treatment Co., Ltd. in a BOT (Build-Operate-Transfer) manner. The treatment scale of the first phase is 40,000 m3/d. The plant is located on the west side of Chenjia Village in the northeast of Wuwei County, covering an area of ​​about 3.34 hectares. Wuwei Wastewater Treatment Plant mainly treats domestic sewage and industrial wastewater from about 130,000 people in the northeast of the county. It adopts A/A/O process. The main structures include coarse screen and water inlet pump room, fine screen and cyclone sedimentation tank, biological reactor, secondary sedimentation tank water distribution well and sludge pump room, secondary sedimentation tank, ultraviolet disinfection channel and sludge storage tank, etc. Its flow chart is shown in Figure 1. The design requires that the effluent water quality meets the national Class I B emission standard of GB18918-2002. The sludge generated during the sewage treatment process is concentrated and dehydrated and then transported to the landfill for landfill.

The control system of Wuwei Sewage Treatment Plant is equipped with 1 monitoring center (central control room) and 3 on-site PLC stations (analog screen station PLC01, transformer room station PLC02, sludge dewatering room station PLC03). Among them, the transformer room station is the most important, mainly controlling the automatic operation of the lifting pump station, screen well, sedimentation tank, oxidation ditch, secondary sedimentation tank, return pump station, residual sludge pump station, and sludge storage tank. The key control objects are the oxygen content in the oxidation ditch and the amount of return sludge to ensure the maintenance of the living environment of microorganisms and achieve the purpose of removing pollutants in the original sewage.

2 Selection Comparison

For the selection of substation control system, similar projects in the past mostly used AB's ControlLogix series or Siemens S7-400 series hardware redundancy systems, which have advantages such as good stability and fast switching speed, but are relatively expensive, and are the preferred choice for large-scale domestic sewage treatment control systems. At present, many sewage projects in small and medium-sized cities are BOT projects, and the requirements for control system switching time are not too high, so the selection of control systems requires both performance and cost reduction. According to the information provided by Siemens, the price of the hardware dual-machine redundant system is about 40% higher than that of the soft redundant system, but the availability rate is only increased by 0.08%. Most foreign sewage treatment plants of the same size do not use dual-machine hardware redundant systems, so it is necessary to have a deeper understanding of the control system technology to avoid copying and imitating in design selection. Through in-depth analysis and performance comparison of Siemens hardware redundant and software redundant systems, it was finally decided to use a relatively cheap and stable soft redundant system.

3 Soft Redundancy System Structure and Principle

Common forms of control system redundancy are central processor redundancy, I/O redundancy, and communication redundancy. Central processor redundancy means that when the main processor fails, the backup processor automatically starts running and takes over control. In terms of the interactive mode of control rights, it is divided into hardware redundancy and software redundancy. Hardware redundancy uses hardware to switch. In addition to using the central processor in pairs, a dedicated hot standby module is required to detect the processor. Once the main processor fails, the system control will be immediately handed over to the backup processor. Hardware redundancy uses optical fiber communication, which has fast communication speed, stable system, shorter switching time, but high cost. Software redundancy only requires paired processors, and uses software programming to switch processors. The composition is relatively economical and very flexible, but program processing takes a certain amount of time. For occasions where clock synchronization and switching time requirements are not very strict, the use of software redundancy is very economical and effective.

3.1 System Structure

The basic structure of the soft redundancy system is shown in Figure 2.

Siemens S7-300 PLC software redundancy system consists of two sets of PLC control stations, A and B. At the beginning, station A is the main station and station B is the backup station. When any component in the main station A fails, the control task will automatically switch to the backup station B for execution. At this time, station B is the main station and station A is the backup station. This switching process includes the overall switching of power supply, CPU, communication cable and IMl53 interface module. During the operation of the system, even if no component fails, the operator can set the control word to realize manual master-slave system switching. This manual switching function is very useful for the adjustment, replacement and expansion of the software and hardware of the control system.

3.2 System Working Principle

When the soft redundant system is working, the A and B control stations (processor, communication, I/O) run independently, and the PLC of the master station controls the I/O in the ET200 slave station. The PLC programs in stations A and B consist of non-duplicated user program segments and redundant back-up user program segments. The master system PLC executes all user programs, and the backup system PLC only executes non-duplicated user program segments and skips redundant user program segments. The internal program running process of the PLC in the soft redundant system is shown in Figure 3.

When a failure occurs in the main control station, the switching time of the main and standby control stations = fault diagnosis detection time + synchronous data transmission time + DP slave switching time. If the CPU failure is shutdown or power failure, the fault diagnosis is about 100 to 1,000 ms. The time required for data synchronization depends on the amount of synchronized data and the network method used for synchronization. Ethernet network is currently widely used. Taking CPU 315-2DP as an example, the time required to synchronize 1000B of data is about 200 to 300 ms. The switching time of 2 DP slaves is about 70 ms. No matter where the control program scans in a loop, the currently activated system (that is, the main system) will receive and process alarms at any time, but the alarms generated during the switching process between the main system A and the standby system B may be lost.

4 Hardware selection

The soft redundancy system of the substation of the sewage treatment project in Wuwei County uses SIMATIC S7-300 series PLC, the central processor is CPU315-2DP, and the programming software is Step7 V5.2. The communication between the PLC body and the remote I/O is the serial communication of the PROFIBUS protocol, and the PLC and the computer use industrial Ethernet, and the industrial Ethernet is used as the redundant network. The PLC module selection is as follows:

(1) Two power modules 6ES7307-1EA00-OAA0 provide power to the CPU;

(2) Two CPU315-2DP 6ES7315-2AGl00AB0 CPUs, 512 KB working memory;

(3) Two communication modules 6GK7343-1EX20-0XE0 communicate with the computer over the network and perform redundant communication between the two CPUs;

(4) Two interface modules 6ES7153-2AA02-0XBO for redundant communication between PROFIBUS remote I/O and systems;

(5) Two power modules 6EP1333-2AA00 provide power to ET200;

(6) DI digital input module 6ES7321-1BL00-0AA0 collects the start, stop, operation and fault status of field equipment;

(7) Three DO digital output modules 6ES7322-1BL00-0AA0 control the start and stop of field equipment;

(8) AI analog input module 6ES7331-7KF02-0ABO collects the liquid level, pressure, flow, PH/T, turbidity and frequency signals of the fan and dosing pump on site;

(9) The A0 analog output module 6ES7332-5HFOO-OABO automatically adjusts the frequency of the fan and the dosing device through the program's PID control.

5 System test results

After the S7-300 soft redundancy system of the substation was put into operation, the system worked normally. During the engineering commissioning, the technicians set the control word to manually switch the master and standby systems. During the test, there was no interruption when the master and slave systems switched, and all controls were normal during the switch, including the control relay would not be disconnected, the analog output control inverter speed would not be lost, etc. In addition, the PROFIBUS bus transmission was normal during the test, and no module dropped. This shows that the Siemens S7-300 soft redundancy system is completely feasible for small and medium-sized sewage treatment control systems that do not require high master and standby system switching time.

6 Conclusion

The system was put into use in September 2006 and has been running smoothly. Users are very satisfied with the choice of the soft redundancy system. At present, the system has been successfully promoted to sewage treatment systems in Fuyang, Anqing, Huainan and Suzhou, Anhui Province, and users have responded very well. Through the above projects, it is confirmed that the PLC soft redundancy system can fully meet the technical requirements of small and medium-sized sewage treatment control systems and basically achieve worry-free switching. The actual use of the PLC soft redundancy system in production has brought benefits to the enterprise, and it is believed that it also provides a certain reference value for the widespread promotion of this redundant system to other industries.