In the urban central heating system, the thermal station is the thermal system's final adjustment unit for system thermal users. The thermal station's control effect directly determines the heating user's heating effect. The urban heat network under the jurisdiction of Taiyuan 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 are connected with heat exchange stations.
In the heating station of the Interconnection Heating Network, the pressure, temperature, and flow rate of the secondary net supply and return water 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 the 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 original thermal station, the automatic control instrument, PLC and frequency conversion equipment were uniformly added; for the new thermal station, the 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, combined with the control network full network balancing system and the communication network system of the thermal network, and performs uniform adjustment of 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.
Second, the composition of the thermal station automatic control system
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.
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. .
Third, the system control idea
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 amount, 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 achieve 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 and head can be adjusted to the required values ​​in a timely manner, 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 heating 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 frequency control of the make-up pump. However, considering the pressure stability requirement is not high here, as long as the pressure does not exceed a certain range, the switch make-up 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 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 a 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 first and second network supply and return water of the thermal power station; the object of the one network control is a primary net control valve; the purpose of the control is to provide necessary heating energy for the thermal power station.
2, secondary network circulation pump control:
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, frequency conversion 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 be achieved by adjusting the speed of the circulating pump to adjust the flow of the second network to meet the heating demand, thereby reducing waste.