[Abstract]: This article mainly describes the linkage control system between the heating network and heat source (high-temperature water boilers) of a heating company. This system can comprehensively predict the heating load and transmit the predicted load to the boiler control center and dispatch monitoring center through the communication network. The boiler control center automatically adjusts the boiler combustion meter and the number of boilers in operation based on the predicted load and meteorological data, thereby realizing automated energy-saving operation control of the entire network. This article is based on the author's on-site investigation and learning at Yongli Heating Company in Binhai New Area, and takes the plant's six 80T high-temperature hot water boilers as an example for detailed introduction.
[Keywords]: Load forecasting, boiler linkage control system, controller, communication network, primary monitoring center, secondary monitoring center, multi-level network, unattended heating station, GPRS communication, Profibus, industrial Ethernet, gigabit Ethernet, SCADA monitoring
I. Overview:
With my country's accession to the World Trade Organization, advanced foreign technologies and equipment have sprung up and been widely applied in various industries in my country. The overall level of industry and manufacturing is developing towards automation control, energy conservation and consumption reduction, and increased output. my country's entire industry has entered the era of automation control.
my country is a major country for winter heating, with enormous energy consumption (electricity and coal) required for heating. Reducing operating costs and saving energy while maintaining acceptable indoor temperatures for residents is a key focus for heating companies. Currently, each heating company employs different strategies based on its specific hardware configuration. However, for heating companies with large heat sources and direct heat users, a comprehensive load forecasting and boiler linkage control system represents the most advanced, scientific, and energy-efficient operating method.
The heating network full load prediction and boiler linkage control system refers to the system that predicts the total load of the entire heating network through the heating network load prediction program, and automatically adjusts the combustion ratio of the heat source boilers and the number of boilers put into operation to meet the needs of the heating network. The unattended heat exchange stations at all levels will automatically measure the outdoor temperature and user temperature according to the characteristics of their area to optimize heating, and transmit the operating status and various measurement and control data to the remote control center and boiler control center. The entire system is composed of GPRS, Internet, industrial Ethernet and Profibus bus, realizing the full network automated integrated control.
II. System Components:
This system is a large-scale enterprise-level SCADA monitoring system, consisting of an external network access layer, a heating company monitoring layer, a boiler room control layer, and unattended heat exchange stations. Each part performs the following functions:
2.1 External Network Access Layer: Accessed via a 10M fiber optic leased line with a fixed IP address, this layer is responsible for receiving and sending data. It implements three functions:
2.1.1 Transmitting GPRS data: Receives monitoring data from various unattended heat exchange stations and downloads operational decisions and setting data to the heat exchange stations.
2.1.2 Receiving meteorological data: The system receives real-time meteorological information from the meteorological bureau and transmits the meteorological data to the load forecasting program.
2.1.3 External Network Monitoring and Publication: The system's operating site will be published on the global Internet. Any user authorized by the heating company can view the system from anywhere in the world, and mobile browsing is also supported.
2.2 Heating Company Monitoring Layer: This layer is located in the heating company's office building and consists of three parts: the Operation Support Department, the Dispatch and Operation Management Center, and the Manager's Monitoring Room. The heating company's monitoring layer is the first-level monitoring center.
2.2.1 Operation Support Department: Composed of two computers, the department's technical personnel can monitor the operation of the boiler room and unattended stations in real time, view and print curves and reports, read the system's fault information, and provide guiding maintenance plans to help maintenance engineers maintain the system.
2.2.2 Dispatch and Operation Management Center: Composed of 6 computers and a large-screen video wall, the dispatch and operation management center is the core layer of this SCADA monitoring system, possessing the highest operational authority. Computer 1 is responsible for web data publishing; Computer 2 is the monitoring layer server host; Computer 3 is the large-screen control server host; Computer 4 is the boiler monitoring direct connection monitoring host; Computer 5 is the unattended heat exchange station monitoring host; and Computer 6 is the integrated monitoring host, capable of monitoring and implementing the functions of Computers 1-5 respectively, and also serves as a hot standby machine. The large-screen video wall is controlled in real time by Computer 3, allowing for scrolling and switching of various monitoring screens, including (3D simulation images, curves, bar charts, data analysis tables, operation reports, load forecast data, unattended station remote transmission data, and boiler operation data).
2.2.3 Manager's Monitoring Room: This station consists of a computer. The same interface and data as the monitoring center can be viewed on this station, but control is not supported. On this station , you can view (3D simulation images, curves, bar charts, data analysis tables, operation reports, load forecast data, remote transmission data of unattended stations, boiler data), and print them.
2.3 Boiler Room Control Layer: This layer is located in the Heat Source and Power Department (boiler room) of the heating company, and consists of the boiler room main monitoring room, the boiler monitoring main server, monitoring clients for boilers one through six, and the overall load forecasting host. The boiler room monitoring center is the secondary monitoring center of this system.
2.3.1 Boiler Room Supervisor Monitoring Room: Consists of two computers, located in the boiler room manager's office and the boiler operator supervisor's office. This workstation can only read data from unattended heat exchange stations and has no control permissions, but it has full access to monitor boilers one through six. At this station, the boiler room supervisor can directly set operating data, participate in boiler operating parameter adjustments, boiler start-up control, load adjustment, and other functions. After adjustments are made at this station, the data will be synchronized to the boiler room monitoring main platform within 10ms and updated to the primary monitoring center (heating company monitoring layer) monitoring layer within 100ms.
2.3.2 Boiler Monitoring Main Server: Responsible for communicating with six S7300 controllers. The host computer monitoring software uses WinCC 6.0 . This station is the only station directly connected to the S7300 boiler controller. All control commands sent to the boiler must be gathered at this station and sent to each boiler controller by WinCC. This station is the core host for boiler control.
2.3.3 Boiler Monitoring Client: The client consists of six computers, with one computer operating station corresponding to each boiler . The operation and viewing are convenient and secure. All monitoring screens and data come from the boiler monitoring main server. These six clients cannot work when the server is shut down; they are just diskless workstations. This design reduces the amount of system maintenance and improves the system's security level.
2.3.4 Network Load Forecasting Host: This host receives data from each heat exchange station, receives meteorological data from the meteorological station, measures the outdoor temperature at each point, detects and calculates the total calorific value of the boiler, and predicts the maximum heat required by the heating network in the next 5-24 hours.
2.4 Diagram of the heating network full load prediction and boiler linkage control system.
System Flowchart
III. Multi-level and multi-layered network communication system
The heating network load forecasting and boiler linkage control system is a large-scale control system with extremely high real-time requirements and a relatively complex communication network. This system consists of four layers: unattended stations (first-layer wireless network), field control layer (second-layer Profibus-DP network), boiler room monitoring layer (third-layer industrial Ethernet), and heating company monitoring layer (fourth-layer gigabit Ethernet).
3.1 First-layer wireless network: Composed of 100 Hongdian GPRS industrial wireless data communication modules. The GPRS serial port establishes a communication connection with the PLC. The communication protocol is the standard Modbus RTU format. The unattended heat exchange station is the lowest level slave station. Data request and setting are completed by the monitoring center's multi-task scheduling program through inspection calls. When the master station does not call the slave station, it does not occupy communication resources, which can reduce the GPRS wireless traffic cost during operation. The GPRS wireless signal is processed by the mobile data center and forwarded to computer No. 5 in the first-level monitoring center (a dedicated computer for unattended heat exchange stations). Computer No. 5 is responsible for establishing communication forwarding and issuing commands to complete the unified scheduling and management of 100 heat exchange stations.
3.2 Second Layer Profibus-DP Network: The second layer network is the core low-level network for heat source and boiler control. This network has the highest real-time performance and reliability. This network adopts Siemens' most advanced Profibus-DP network. All equipment is provided by Siemens. The boiler controller consists of 6 S7300-315DP PLCs. Each boiler has an independent controller and corresponding power distribution, sensor, and drive system, which are independent of each other. These 6 PLCs are connected to the boiler control main server via the Profibus-DP network at a speed of 12M through a CP5611 card. The boiler control main server runs 512K Siemens WinCC configuration software.
3.3 Layer 3 Industrial Ethernet Network: Industrial Ethernet refers to a high real-time network with a 10/100M adaptive transmission rate, redundant 24VDC power supply, a maximum network reconfiguration time of 0.3 seconds, communication fault diagnosis and disconnection diagnosis functions, and the ability to reconfigure the network in real time. This network segment uses Siemens ProfiNET network equipment, network switches, and professional network cables. The Layer 3 network mainly realizes communication and synchronous monitoring between 6 client operator stations and the boiler control main server, as well as monitoring and synchronous monitoring by the boiler room management team.
3.4 Layer 4 Gigabit Ethernet: Gigabit Ethernet is the backbone network in the entire heating company office building. It consists of two-level gigabit D-Link switches, enterprise-level routers, and hardware firewalls. The two-level network switches (heating company office building) and the third-level industrial Ethernet switches (heat source power boiler room) are connected by fiber optic cables. The fourth-level network has the largest data exchange volume. In addition to internal use in the monitoring center, this network segment is responsible for web publishing, supports remote monitoring by mobile phones, and receives data from unattended heat exchange stations via the mobile center's CPRS.
3.5 System Network Structure Diagram: (See last page)
IV. Methods for Network Load Forecasting
The whole heating network heat load forecast refers to the scientific statistics and filing of the heat consumption of the area supplied by the heating company, and the plotting of the total heat demand in the heating area using a scientific and economic curve, so as to predict the total heat demand data of the heating network, rationally allocate the heat produced by the heat source power boilers, and the boiler DCS control system can adjust the boiler load and number of boilers to achieve coordinated, energy-saving and economical operation of the entire system.
4.1 Total Heat Statistics: Each heat exchange station calculates its total heat supply for 24 hours daily and transmits this total heat to the heating company's dispatch center via GPRS signal at midnight on the same day. The heat demand statistics software in computer No. 5 of the dispatch center summarizes the data and draws a 24-hour heat flow diagram for each station, compiling a total heat flow diagram for all heat exchange stations. The total heat consumption of all heat exchange stations within 24 hours is calculated, and the calculated data is simultaneously transmitted to the secondary network load forecast host.
4.2 Receiving meteorological information from the meteorological bureau: Computer No. 5 of the heating network dispatch and management center can obtain local meteorological data from the National Meteorological Bureau in real time for the current period and the next 10 days. Computer No. 5 stores the meteorological temperature, draws real-time meteorological curves and future temperature change curve reports, and transmits them synchronously to the secondary network load forecast host.
4.3 Acquiring Current Outdoor Temperature: The heating network dispatch and management center acquires three outdoor temperature sensors installed at the heat source power boiler room and at locations 2 meters and 40 meters away from the heating company office building, respectively. The center stores the values measured by these three outdoor temperature sensors, plots them as historical curves and real-time curves, and transmits them synchronously to the secondary network load forecasting host.
4.4 Confirm the total heat required by the heating network in the next 5-24 hours: The secondary network load forecasting computer calculates the total heat required by the heating company in the next 5-24 hours. The calculation software extracts the total load data of the heating network from the previous day, the corresponding meteorological data, and the average outdoor temperature from the database. The basic heat demand is determined by the temperature extraction algorithm. The calculation program extracts the heat compensation table corresponding to the meteorological data for the next 24 hours and the heat compensation table corresponding to the current outdoor temperature. The forecasting program performs optimization calculations to finally confirm the total heat demand in the next 5-24 hours. This amount is then sent to the heat source power boiler control center and transmitted to the primary monitoring center heating network dispatch center.
V. Operation and Management of Unmanned Heat Exchange Stations
An unattended heat exchange station refers to a station that operates automatically 24 hours a day. The control system can automatically adjust the heat load, regulate the water supply flow and pressure, automatically replenish the system with water, automatically collect parameters from various points of the heat exchange station, and transmit the parameters to the central monitoring and dispatch center of the heating company through the GPRS communication network. The station can also receive control commands from the central monitoring and dispatch center and operates around the clock without personnel on duty.
5.1 Composition of the unattended heating station automatic control system: The unattended station control system consists of an S7200 series small PLC, GPRS communication equipment, a 10-inch true color human-machine interface, heat regulation electric valve, overpressure safety protection valve, circulating water pump frequency converter cabinet, makeup water pump frequency converter cabinet, and sensors for on-site temperature, pressure, flow, etc., forming an independent automatic control system to complete the automatic temperature and pressure regulation of this station.
5.2 Monitoring parameters for unattended heating stations
1) Primary water supply temperature, pressure, and flow rate (heat);
2) Primary network return water temperature and pressure;
3) Electric regulating valve for primary water supply;
4) Temperature and pressure of the secondary water supply network;
5) Secondary network return water temperature and pressure;
6) Outdoor temperature;
7) Start-up/stopping of circulating water pump and makeup water pump;
8) Operating status of circulating pump and makeup water pump;
9) Frequency control and feedback of circulating pump and makeup water pump;
10) Leakage detection alarm at heating stations.
5.3 Secondary Network Water Supply Temperature Control: The secondary network water supply temperature (hot water supplied to residential users) control system integrates three temperature regulation methods: the first is a preset temperature curve regulation method; the second is a fixed water supply temperature regulation method; and the third is a time-segmented constant temperature control method, with an independent manual operation mechanism. In actual use, these three programs can be selected remotely by the monitoring center according to different heat users, or they can be selected locally.
5.3.1 Temperature Curve Adjustment Method: The monitoring center calculates 16 curves corresponding to outdoor temperature and water supply temperature based on load forecasts and meteorological data for the next day. The calculated results are remotely set and downloaded to some heat exchange stations. The heat exchange station controllers monitor the secondary network water supply temperature and outdoor temperature, automatically adjusting the valve opening of the primary network to regulate the secondary network water supply temperature, ensuring the secondary network temperature operates within the adjustment curve range for optimal energy saving. The heat exchange station controllers automatically receive the unified outdoor temperature from the monitoring center. If the controller does not receive the latest outdoor temperature within one hour, it will automatically switch to collecting data from the local outdoor temperature sensor.
Temperature curve adjustment method
5.3.2 Fixed Water Supply Temperature: Fixed water supply temperature regulation refers to a heating method where a fixed target secondary water supply temperature is issued to the heating station controller by the remote control center. The controller automatically adjusts the actual secondary water supply temperature to within the set fixed temperature range. In this method, the water supply temperature remains constant and does not change according to outdoor temperature variations. Our company has over a dozen heat exchange stations using fixed water supply temperature regulation. The requirement for fixed water supply temperature control in these heat exchange stations is determined by the heating company's operations department based on customer characteristics. For example, fixed water supply temperature operation is more suitable for office buildings and large public buildings.
5.3.3 Time - segmented constant temperature: This method supports adjusting the fixed water supply temperature and curve at different time periods, so that the control system will generate a more energy-efficient operating curve. Time-segmented constant temperature can also operate independently. Six time periods can be set every day, each time period corresponding to a different temperature. When the clock reaches the time zone, the water supply temperature in this time zone will become the target secondary water supply temperature. Some heat exchange stations adopt the time-segmented constant temperature heating method, most of which are schools.
Time-segmented constant temperature curve
5.4 Circulating Water Pump Control: The circulating water pumps can be started and stopped at the heating station or a remote monitoring center, and support remote fault reset and setting of target water supply pressure. In this system, the circulating pumps at the heating station are configured in one-on-one standby, two-on-one standby, and three-on-one standby modes. When an electrical fault or frequency converter failure occurs in the running circulating pump, the standby frequency converter pump will automatically start. The circulating pumps use PID closed-loop control to determine the secondary water supply pressure.
5.5 Makeup Water Pump Control: The makeup water pumps can be started and stopped independently at the remote monitoring center of the heating station, and remote fault reset and target makeup water pressure setting are supported. In this system, the heating station's makeup water pumps are configured with one in operation and one on standby. When an electrical fault or frequency converter failure occurs in the operating makeup water pump, the standby frequency converter pump will automatically start operation. The makeup water pumps use PID closed-loop control to determine the secondary return water pressure.
5.6 Emergency Manual Control: Each heating station's temperature control valve, circulating water pump, and makeup water pump are equipped with electrical manual control buttons, allowing manual control of the equipment in case of automatic failure.
VI. Boiler Interlocking Control System
The boiler linkage control system refers to a set of control systems that use the secondary monitoring center of the boiler room and the DCS controller to adjust the boiler combustion rate and automatically start and stop the boiler based on the total heat data predicted by the heat load of the entire network. This system is also an independent boiler DCS control system. This system can reduce the number of operators in the boiler room, adjust the total heat output according to the load forecast, and achieve economical, efficient and reliable operation.
6.1 Composition of the control system: The control system consists of a large-scale DCS monitoring system comprising 6 boiler combustion DCS control cabinets, 6 sets of S7300315-2DPCPU, 1 WinCC server station, 6 client operator stations, 6 375KW induced draft fan frequency converter control cabinets, 6 160KW blower frequency converter control cabinets, 12 7.5KW grate motor frequency converter control cabinets, 18 75KW frequency converter circulating water pumps, and 100 temperature and pressure sensors.
6.2 Basic Monitoring Parameters
The following points are measured for each boiler using an S7300 PLC.
6.3 Boiler load regulation: The start-up and stop control of the boiler can be operated on the boiler monitoring main server in the secondary monitoring center, or on operator terminals 1-6. The operation of the boiler consists of two programs: independent regulation and load prediction linkage regulation.
6.3.1 Independent Adjustment Program: Each boiler is not subject to any conditions and can be operated directly on the boiler monitoring server. The DCS controller will automatically adjust the boiler 's forced draft volume, induced draft volume, and grate speed, and automatically adjust the matching air volume and coal feed ratio to achieve optimized fuel combustion, ensuring that the boiler operates under the best combustion conditions. The system can maintain the boiler outlet water temperature within ±3 degrees Celsius of the set value.
6.3.2 Load Prediction and Linkage Regulation Program: When this program is selected, boilers one to three will automatically operate at full load, with the best combustion ratio and the best maximum heat production efficiency, without participating in regulation, and each boiler will operate continuously for 24 hours.
Boilers No. 4 and No. 5 are peak-shaving boilers (boilers are in hot-start mode). When the total heat demand is calculated by the network load forecasting program, if boilers No. 1 to No. 3 cannot meet the network demand, boiler No. 4 will automatically start and run, automatically set the target water supply temperature, and automatically adjust the air-coal ratio. After boiler No. 4 starts, it can make up for the total heat shortage in a short time. When the system calculates that the total heat demand cannot be met by starting boiler No. 4 alone, the system will automatically start boiler No. 5 30 minutes after boiler No. 4 starts.
When the total calorific value of the peak-shaving boiler exceeds the predicted total calorific value, the peak-shaving boiler will automatically lower the set temperature, reduce the amount of air and coal supplied, and reduce the amount of combustion. The total calorific value of the peak-shaving boiler will decrease. When the peak-shaving boiler is operating at 20% of the total load, it will automatically shut down. The system will automatically increase the circulation speed of the boiler room circulating water pump to achieve energy-saving operation.
6.4 Fault Transfer Control
6.4.1 Grate system mutual switching: When the main grate control electrical system or frequency converter fails, the backup grate control system will automatically start operation in less than 5 seconds.
6.4.2 Boiler Interoperability: When a critical equipment failure occurs in the blower, induced draft fan, or sensor of a boiler in operation, the No. 6 standby boiler will automatically be put into operation after manual confirmation.
6.4.3 Interoperability and timed rotation of circulating water pumps: Each boiler is equipped with three circulating pumps, with two in operation and one on standby. The system rotates the standby pump every 24 hours to ensure that all three pumps can operate at all times. If a circulating pump in operation fails, the standby pump can be put into operation within 3 seconds.
VII. Conclusion
The heating network load forecasting and boiler linkage control system is a large-scale automated SCADA monitoring system with a very comprehensive scope and a long construction period. It took three years from initial construction to expansion to over 100 heating stations. However, with the system's operation, its operational effectiveness and economic benefits are particularly outstanding. It has saved 350 personnel on duty at 100 heating stations; maintenance personnel at regional heating stations have been reduced from 50 to 20 through consolidation; data reporting, recording, and statistical analysis personnel (originally 30) have been replaced by computers; boiler combustion is computer-controlled, reducing the workforce by more than 20 workers; and the system has reduced personnel costs and positions by 430. Due to the load forecasting and linkage control system, the heat source power boiler room can more accurately and effectively adjust the heating supply and the number of boilers in operation according to load requirements. Preliminary statistics show savings of approximately 20% in electricity and coal consumption. The data above shows that the heating network load forecasting and boiler linkage control system is economically efficient. With the acceleration of urban and rural unification in my country, small heating companies are gradually being replaced, while large heating enterprises are becoming more and more common. The heating network load forecasting and boiler linkage control system will have broader application value.
(Continued from page 3, network structure diagram of this system):
Communication network architecture of this system
