I. INTRODUCTION In the urban central heating system, the thermal station is used as the thermal network system to face the final thermal regulation unit of the system thermal user. The control effect of the thermal station directly determines the thermal user's heating effect. The urban heat network under the jurisdiction of the Taili Thermal Power Co., Ltd. contains more than 400 heat stations. The heat supply area covers 60% of the total heating area of ​​Taiyuan City. All heat stations use inter-connected heat exchange stations. Intermittent heat station automatic control system according to the type of equipment can be divided into: temperature, pressure transmitter, flow meter, electric control valve, circulating pump and make up water pump; according to the control circuit, it can be divided into: a network flow control Loop, secondary net loop control loop, secondary net constant pressure loop. 2, secondary network circulation pump control: About the Author Truck Refrigeration Unit,Refrigeration Unit For Cargo Truck,Truck Cooling Unit,Refrigeration Unit For Truck Freezer XINXIANG HUATAI REFRIGERATION CO., LTD , https://www.trustht.com
In the heating station of the Interconnection Heating Network, the pressure, temperature, and flow rate of water supply and return of the secondary network are all important factors affecting the heating effect. The adjustment of the secondary network's supply parameters is mainly based on the secondary network circulating pump and water supply. Pump control. In traditional thermal station control, the circulating pump and make-up pump generally use industrial frequency pumps. The system has determined the main parameters of the secondary network of the system when designing and selecting the type, but relative, the adaptability, expandability and various parameters of the system. The precise adjustments are very limited.
Since 1999, Taiyuan Thermal Power Co., Ltd. has gradually started to automatically control and transform various heat stations in Taiyuan Central Heating and Heating Network. For the existing thermal stations, the automatic control instruments, PLCs, and frequency conversion equipment were uniformly added; for the new thermal stations, automatic control equipment was introduced on the basis of the process system at the time of design. The automatic control system assists in the precise control of the thermal station, combines the control network full network balancing system and the communication network system in the thermal network, and evenly adjusts the entire network to achieve a better control effect. This article focuses on the application of automatic control systems and frequency converters in thermal station control.
In the automatic control system of the thermal station, a network flow control loop is mainly realized by adjusting a return control valve. The adjustment loop of the secondary net is achieved by adjusting the secondary net circulating pump and the feed pump speed. The control instructions of the primary network are mainly issued by the thermal network dispatching center according to the whole network balancing algorithm, and the rotational speeds of the secondary network circulating pump and the supplemental pump are calculated by the PLC system in the station according to the actual situation of the heat network carried by each thermal station. .
Thermal station automatic control system structure as shown below. 
Figure 1 Typical heat station system structure
In the district heating project, because the user's building area, radiator performance and house insulation quality are different, it is difficult to determine a typical set of indoor temperature as the directly controlled quantity, and the average temperature of the supply and return water is overall. Reflects the average temperature of each user's radiator, so the general heating system is based on the outdoor ambient temperature and different heating periods to control the average temperature of the supply and return water to indirectly control the user's room temperature.
In the control of various heat networks in Taiyuan, the heat network dispatching system was also adjusted due to the automatic control of the heat station. At present, the thermal network dispatching centers in various thermal power plants in Taiyuan have added a whole-network balancing system. The dispatching center communicates with each thermal station to obtain the heat network data, and the heating effect of the whole network heat station is evenly distributed according to the outdoor temperature conditions. Adjustment.
Each thermal station obtains the corresponding secondary net supply and return water average temperature from the control center. The station system divides the independent control loop into the secondary net supply and return water average temperature control loop and the primary net flow control loop, and determines once according to the average temperature deviation. The setting value of the net flow, then adjust the valve opening so that the flow reaches the set value.
The control system in the station also regulates the secondary network circulating pump and the supplementary pump according to the actual situation of the thermal station.
Based on the temperature difference between the average temperature of the secondary network and the return water, the system automatically adjusts the rotational speed of the circulating pump through the inverter to realize the adjustment of the total flow and temperature of the system. To make the circulating pump output power in accordance with the actual load, reduce unnecessary power loss, and realize the operation mode of small flow and large temperature difference. Through this move, the flow rate and head can be timely adjusted to the required value, eliminating unnecessary power consumption and achieving a good energy saving effect. Usually the thermal system will design two frequency conversion pumps, not only for system backup, but also to prevent system overshoot. If the load is not enough, the pump speed will increase, and if it does not meet the requirement when it reaches 100%, the second pump will be started. At the same time, the system can automatically switch each circulation pump according to the running time, and also provide low pressure protection and chaining function.
The pressure and water return pressure of the secondary network of the control system is an important parameter for the safe operation of the thermal network. The high water supply pressure may cause the hot water pipe and the user's radiator to break; the supply and return water pressure is too low, so that some hot users cannot get enough heat. The best solution for constant pressure control is to perform variable frequency speed control on the make-up pump. However, considering that the pressure stability requirement is not high here, as long as the pressure does not exceed a certain range, the switch make-up water control scheme can also be adopted.
Fourth, the realization of the control system of thermal station
1, a network of loop control:
The primary network loop control of the thermal station is mainly the thermal load control. The flow of the primary hot water flowing through the heat station is regulated by controlling the electric regulating valve on the primary net circuit. In the control system of the whole network, the control center of the whole network calculates the degree of opening instructions or secondary control of the control valves of the primary power network of each heat station based on the current outdoor temperature conditions, referring to the operation of the heat source and the secondary network operation data fed back by each heat station. The net target controls the temperature. The thermal power station system adjusts the net flow control valve according to the instruction issued by the entire network control center so as to achieve even distribution of the thermal resources of the full heat network.
The main reference object in the primary network loop control is the temperature of the return and supply water in the first and second network of the thermal power station; the object controlled by one network is a network control valve; the purpose of the control is to provide the necessary heating energy for the thermal station.
Heat station system secondary network circulating pump is through the inverter to speed.
Traditional heat station system circulating pump usually adopts industrial frequency pump. After the circulating pump is selected, the flow of the secondary network of the heat station cannot be adjusted, resulting in that the heat station system cannot be adjusted according to the outdoor temperature and the actual heating demand, resulting in heat and electricity. The waste of resources. And high-powered industrial frequency pumps will impact the power grid when starting and stopping.
At present, in the automatic control of thermal systems, variable frequency control is generally used for circulating pumps of more than 15KW. The general circulation pump adopts the pressure difference control method, that is, the rotational speed of the circulating pump is adjusted by the pressure difference of the secondary net supply and return water. Differential pressure control can reduce waste by adjusting the speed of the circulating pump and adjusting the flow of the second network to meet the heating demand.
In the thermal station circulation pump control, we use the supply and return water temperature difference in combination with the supply and return pressure differential control method.
The thermal station control system sets a target value for the return pressure difference based on the actual conditions of each system. Set this supply and return pressure difference to meet the heating water cycle of the secondary pipe network. On this basis, the thermal station PLC system corrects the circulating pump by measuring the temperature difference of the secondary network supply and return water. When the temperature difference between the two networks of supply and return water is too large, it is necessary to increase the speed of the circulating pump, increase the flow of the second network, increase the return temperature of the second network, and improve the heating effect; when the temperature difference between the second network supply and return water is too small, the cycle needs to be properly reduced. Pump speed, reduce the flow of the secondary net, realize the operation mode of small flow and large temperature difference. This adjustment can have the effect of saving electric energy and heat energy. In large heating networks, this energy-saving method can achieve considerable results.
3, two network constant pressure water control:
The replenishment control of the secondary net adopts the constant pressure control, and the traditional thermal power station often uses the pressure gauge electrical node control. With the development of urban central heating, the heat load of the system is getting larger and larger. The heating area of ​​the heat station system is relatively large, and the conditions of the heating network are different. The hydraulic loss of the second network system is relatively large. Severe hydraulic loss makes the pressure of the secondary net's replenishment system increase, and frequent replenishment. The frequent starting and stopping of the conventional frequency-frequency water-supplying pumps may easily cause pressure fluctuations in the secondary pipe network.
In the system with large heat load, we use the frequency conversion control of the make-up pump to precisely fine-tune the make-up water system. When the system loses water, the pressure of the second network drops, and the system controls the make-up pump through the inverter to make up the water at a certain speed. The speed of the make-up pump is evenly adjusted according to the difference between the current pressure and the target pressure, so as to prevent the make-up pump from starting and stopping. Impact on the secondary network system.
4. On-site man-machine interface In the on-site man-machine interface, various operating states required by the system can be arbitrarily adjusted through the operation panel, for example, temperature and temperature difference of the secondary network supply and return water, the maximum and minimum operating frequency of the inverter, etc. And can always check the past run records. According to user requirements, current parameters can be displayed on the screen in the form of screens, curves, and reports.
V. Advantages of the automatic control system of the thermal station The inverter and programmable controller are used in the thermal station to give full play to the advantages of speed control and energy saving of the inverter and the advantages of flexible configuration of the programmable controller, reliable control, and convenient programming, so that the entire The stability of the system has a reliable guarantee.
Through the commissioning of the automatic control system of the thermal station, the control methods that mainly rely on manual adjustment have been completely improved in the past. The operation of the heating network has been reasonably controlled, the misalignment phenomenon has been effectively solved, and the uneven heating and cooling of the stations in the heating network have been eliminated. The phenomenon. On-demand heating, energy saving and consumption reduction have changed the unreasonable small temperature difference and large flow operation mode, which not only ensures the remote customer's heating needs but also avoids the near-end user's overheating and directly improves the heating effect of the heating network.
Yan Huidong is currently working at the Materials and Equipment Department of the Thermal Power Company in Taiyuan, Shanxi Province, where he is responsible for technical work and concurrently serves as a special electrical expert for the Shanxi Provincial Urban Heating Association.
references [1] Wang Yizhao, Liu Xiong, Heating Engineering, Mechanical Industry Press, August 2007;