At present, the most common heating method for the insulation press of transformers in domestic transformer plants is steam heating. Steam heating systems generally include high-temperature saturated steam systems, steam valves, thermocouples, hot platens, piping, and control systems. The hot press is the key equipment in the production process of transformer insulation materials. The heating temperature and its control accuracy determine the effect of curing and polymerization of the glue between the layers. It directly affects the adhesion between the insulation material and the board Various properties are an important factor in determining the quality of the insulating material.
The heating and pressing process of an insulating material is a heat transfer process in which heat is transferred from a high-temperature object to a low-temperature object. In this process, the temperature rise rate of the object determines the time difference between the temperature change of the object and the target temperature. When the high-temperature saturated steam parameters are stable, this time difference determines the accuracy of the temperature control. Therefore, how to accurately select the temperature control point, shorten the time difference, and control the temperature in a timely and accurate manner within the target range is the difficult point that the heating press heating system needs to solve.
The arrangement of the hot platen heating channels and the way of duct ventilation directly affect the uniformity of the heating temperature of the insulating material. By weighing the advantages and disadvantages of the traditional heating plate steam circulation method, a heating plate using a dual-channel S-type parallel convection heating method is designed. The steam flows relatively, which effectively overcomes the problem of excessive temperature difference between the head and the tail of the heating channel caused by heat conduction. . At the same time, by optimizing the design of the pipeline layout, the temperature difference between the surface of the board is minimized. The hot-pressing plate with the heating channel arranged in this way has uniform temperature rise, high heat utilization rate, good uniformity of the surface temperature, and is easy to accurately control. 2.2 Heating control system.
As mentioned earlier, the heating system designed this time uses a multi-layer, single-plate temperature control heating system. Compared with the traditional open-loop temperature control circuit, this system mainly improves the steam valve and the temperature control system, as shown in Figure 1: First, the electromagnetic control master control valve is arranged on the main pipe of the heating system; Each layer of heating plate is equipped with an electromagnetic on-off valve that can be controlled independently, so that the on-off of the solenoid valve can be controlled quickly and in real time according to the temperature of the heating plate. Install two temperature sensors.
In the heating system mentioned in this article, it is assumed that C1 and C2 are the measured values of the temperature sensor used on the No. 1 heating plate, C3 and C4 are the measured values of the temperature sensor used on the No. 2 heating plate, and C2m- [, C2m is the n The measured values of the temperature sensors used in the heating plate, C, C2, ........ ∈ (a, b), a and b are the upper and lower limits of the heating temperature set by the control program, and C is the average value. Then
C ∈ (a, b), and IC-C21 ≤ (b- -a), IC2m-x-C2n-, 1 ≤ (b-a).
Suppose a and b are based on d, and the left and right offsets are e and f, respectively.
C2m∈ (d- -e, d + f), then
C∈ (d-e, d + f), and IC-Cz, I ≤e + f, IC2n-x-C2m- | ≤e + f.
Suppose Ci≥C2, C3≥C4, .... C = C2m, C '= (C + ..... + Cm-1) In, then C'≥C and C' E (de, d + f ),vice versa.
For C ∈ (de, d + f), as long as the temperature of each heating plate is controlled within the range (d- -e, d + f), the total average temperature is (d- e, .d + f )within;
For C ′ ∈ (d-e, d + f), as long as the maximum temperature of each heating plate is measured and the average value is calculated from it, the total average temperature controlled will be more realistic and accurate.
For ICnm-x-C2ms1≤e + f, the absolute error value of the sensor measurement value on each heating plate should be ≤e + f, otherwise the measurement value of one sensor is abnormal;
For IC-C2m_1≤e + f, the absolute value of the sensor measurement value and the total average temperature error on each heating plate should be ≤e + f, otherwise the sensor measurement value is abnormal;
Among them, d is the set heating temperature, e is the lower limit temperature difference of the heating plate temperature, and f is the upper limit temperature difference of the heating plate temperature.
The control system uses LabVIEW to program the host computer software to communicate with Siemens S7-200PLC. The PLC controls the operation of the entire system. There are two types of operation modes: automatic and manual. The automatic operation is--key type. You only need to set the pressing mode and the pressing parameters, and then start. The control device operates according to the established mode. The control system collects the data fed back by the temperature sensor, calculates and logically judges the data, and then controls the on-off of the temperature control valves of the heating plates in each layer to form a closed-loop temperature control loop.
According to the process characteristics of the insulating material, the heating system uses 4 pressing modes, namely: no preloading without preheating, preloading without preheating, no preloading with preheating, preloading with preheating, as shown in Figure.
By setting the relevant parameters in the host computer software and selecting the corresponding pressing mode, the control program can control the equipment to perform pressure heating according to the set parameter values.
This article designs a heating system with a single board temperature control method. This heating method is characterized by comparing and judging the temperature acquisition value of each heating plate and the specified current pressing temperature value.