In this paper, a transformer air-cooling system controller based on programmable logic controller (PLC) is proposed to realize the control and communication functions of transformer cooling system in an all-round way. The control strategy of the device adopts the combination of the top oil temperature of the transformer and the load of the transformer to control the switching and switching of the cooler. Based on the principle of balancing the cumulative running time of the cooler group, the circulating switching of the cooler is carried out. The trial operation shows that the device has not only high reliability and accuracy, but also remarkable energy saving and prolongation.
Effectiveness of service life of equipment. The operation condition of power transformer is one of the important factors affecting the service life and operation safety and reliability of transformer, and the temperature condition of transformer operation is the most important one among many factors. Therefore, it is very important to monitor and control the operation temperature of transformer.
So the cooling control system of transformer is very important. Continuous improvement and upgrading is the general trend. In the traditional transformer cooling control system, which is controlled by relay switches in the electrical control circuit, the cooling system starts frequently, coupled with the aging and corrosion of the relay switches, the probability of failure will be greatly increased, and the relay switches can not realize the communication function. In order to improve the stability of transformer operation and prolong its service life, a new type of air-cooling control system for transformer with PLC as the core control element is designed. The design of air-cooling control system of power transformer must strictly comply with the regulations of operation and cooling device of power transformer in operation of power transformer (DL/T572-95). The relevant regulations are as follows. The power transformer with strong oil circulation cooling should be controlled according to the change of load and temperature. Generally, the top oil temperature of oil-immersed transformer should not exceed the corresponding values listed in Table 1. When the cooling medium temperature is low, the top oil temperature will decrease accordingly. For natural circulation cooling power transformer, the top oil temperature should not exceed 85 C. When the forced oil circulation cooling power transformer is running, the cooler must be put into operation. When the power transformer is under light or no-load condition, it should not put too many coolers (no-load state can be short-term without cooling).
Under different loads, the corresponding number of coolers should be put in accordance with the requirements of the manufacturer. And ensure that the automatic control device of switching cooler can make correct switching decision according to the change of load and temperature. When the cooler of forced oil circulation air-cooled transformer is completely removed due to cooling system failure, the power transformer is allowed to run for 20 minutes with rated load. If the top oil temperature does not reach 75 C after 20 minutes of operation, it can continue to run, but the longest running time under this condition should not exceed 1 hour. In this transformer air-cooling control system, PLC is the core control element. The structure principle of this air-cooling control system is shown in Figure 1. The PLC logic control module collects information about transformer environment (including transformer load, oil temperature, etc.), power supply status and air-cooled device status, generates control decision by synthetically judging the collected information, and uploads the information of transformer, air-cooled system and air-cooled device to the upper computer through communication module, at the same time receives the command of the upper computer.
To achieve intelligent control of air cooling system. Because of the working characteristics of air-cooled device and can only be switched on and off in groups, in order to realize the function of automatic control of transformer oil temperature, intermittent negative feedback control mode is needed in the design. The automatic control system is shown in Fig. 2. In this control system, the controlled quantity is the top oil temperature of the transformer, the controlled object is the air-cooled device, the actuator is the solid-state relay, the controller is the PLC, and the temperature controller acts as the transmitter. The factors causing the oil temperature change (winding temperature, ambient temperature and load change) are taken as external disturbances. When transformer load or ambient temperature causes oil temperature change, the temperature controller compares the collected oil temperature with the required reference temperature and sends the data to the PLC. Then the programmable controller generates the control decision to control the switching of the air-cooled device according to the programmed control program, and carries out the switching decision through the solid-state relay. The switching cooler controls the oil temperature. The intermittent negative feedback control used in this control system is different from the general follow-up negative feedback control and constant negative feedback control. The characteristic of intermittent negative feedback control is that the control effect of the control system on the controlled object is not continuous. Correspondingly, when the controlled quantity changes due to external disturbance, thermostatic element the controlled quantity (transformer oil) is controlled by the control effect. Temperature) is eventually stabilized within a certain temperature range, rather than a fixed value.
Unlike relay control, switching is only an air-cooled device in the “auxiliary” operation state. By using negative temperature feedback, the distinction between “working”, “reserve” and “auxiliary” is cancelled.
Instead, the accumulative operation and stopping time of the cooling device are taken as the basis of judgment, so that the switching strategy with margin of difference produces a group of air-cooled devices to be switched on and off. Control decision-making. Then how to define the cumulative operation and stop time of the cooling device? Each time the cooling device is put into operation is the cumulative operation time. From the time when the cooling device is put into operation to the time when the cooling device is out of operation, the time when the cooling device is out of operation is cleared, and the time when the cooling device is put into operation is restarted after the next time.
Cumulative stopping time indicates the time when the cooling device exits each time. The time is counted from the start of the cooling device exiting operation. The time is cleared until the cooling device is put into operation, and the time is restarted after the next exit. The working flow of a group of cooling devices with difference margin is illustrated in Figure 3, which is synthesized by temperature control switching and accumulative time control switching. Firstly, the switching control strategy with difference margin threshold produces the output of switching control decision-making; secondly, it decides whether the switching control is allowed, because the switching control must be delayed for a period of time after the switching air-cooled device has just been operated, and then the switching action must be done after the oil temperature of transformer is stabilized, otherwise too much wind will be put in or removed in a short time. Cooling device may cause frequent switching of air cooling device.
If switching is allowed, switching is carried out; if switching is not allowed, the judgment of whether or not the accumulated running time of the air-cooled device is timed out is entered; if the accumulated running time of the air-cooled device is timed out, the switching is carried out, and the longest accumulated running time of the air-cooled device is removed; if there is no accumulated running time of the air-cooled device, the reverse is done. Back to the switch with margin margin threshold. The process of putting into the air-cooled device is to select the air-cooled device with the longest cumulative stopping time, which is controlled by solid-state relay. After the air-cooled device is put into operation, the cumulative running time begins to be counted. The process of removing the cooler is to select the air-cooled device with the longest cumulative running time, which is controlled by solid-state relay and starts to stop after the air-cooled device is removed.
No time. The control system adopts software design instead of relay control mode, removes the original complicated logic circuit, and organically combines network technology, computer technology and programmable logic controller (PLC), so that the control object is dispersed, the structure and function are stratified, and the huge hardware logic circuit is simplified and improved, which not only greatly improves the performance of the system.
It improves the stability of the system and reflects the running state of the system to the upper computer in time, thus realizing the remote monitoring function.