Intelligent temperature controller for dry-type transformer based on ATmega16 microcontroller

Introduction: This paper introduces a dry-type transformer intelligent controller composed of ATmega16 single-chip microcomputer. The controller can detect 4-way temperature at the same time, with an error of less than ±0.5℃ and stable operation on site; users can control the start and stop of the transformer cooling fan and set the alarm and tripping thresholds through the human-machine interface.
　　

1 Working Principle

The temperature controller consists of three parts: temperature monitoring, signal processing, and output control. The system block diagram is shown in Figure 1. It obtains the winding temperature value through three platinum resistance sensors embedded in the three-phase winding of the transformer, and directly sends it to the A/D conversion input of the controller after being processed by the signal conditioning circuit. The microcontroller performs calculations and processing according to the signal data and various control parameters set, and according to the embedded software control rules, it automatically displays the temperature value of the transformer winding, outputs the corresponding control signal, controls the start and stop of the fan, and outputs normal, alarm and trip signals according to the current status, etc. At the same time, various data are transmitted to the host computer through RS-485 for centralized monitoring.

The temperature controller uses ATmegal6 microcontroller as the control core, which is a low-power CMOS 8-bit microcontroller based on AVRRISC. It can execute one instruction in one clock cycle and achieve 1MIPS/MHz performance, so it has real-time performance. It has 16KB FLASH and 512B E2PROM on the chip, which can temporarily store faults and over-temperature upper limit temperature values.

1.1 Analog conversion circuit

The analog conversion control circuit is used to convert the temperature analog quantity into an electrical signal that can be recognized by the single chip. The conversion principle is shown in Figure 2. When the temperature changes, the resistance value of PT100 will change linearly with the temperature change. Its voltage division value is compared with a certain fixed circuit voltage division value, and the result is sent to the operational amplifier and converted into an analog quantity within the A/D conversion range.

The A/D conversion accuracy in ATmega16 is 10 bits. Since the reference voltage is 5V, the analog signal must be converted into a voltage of 0 to 5V. Therefore, when designing this circuit, the parameters of each component are designed according to this requirement. At the same time, its linearization must also be considered. In order to make the calculation in the software design linear, when designing the hardware, the temperature must be linearly changed with the digital quantity converted to the microcontroller. From the circuit, we can see that:

It can be seen from the formula that the A/D conversion voltage is not proportional to Rw and does not meet the linear requirements. If R3 ≥ RW is satisfied, the conversion voltage is approximately proportional to Rw and also approximately proportional to the temperature. In this way, the temperature of any point can be calculated by linear calculation. However, using linearization to calculate this approximately linear graph will also bring about slight errors, which can be solved in software design.

1.2 Output Circuit

The output circuit is the embodiment of the calculation and control result of the analog-to-digital conversion by the single-chip microcomputer, as shown in Figure 3. The control quantity output by the single-chip microcomputer is input to the JK port. If this signal is low level, the optocoupler device is turned on, making the CMOS transistor turned on, so that the relay is energized, the normally open contact is closed, and the 220V voltage is output; otherwise, the 0V voltage is output.

In the actual circuit, the four similar circuits mentioned above monitor the motor, fault alarm, over-temperature alarm and over-temperature trip respectively. For example, when the temperature exceeds the upper temperature limit of the fan, the single-chip microcomputer will set the JK terminal to a low level through software, thereby turning on the CMOS, so that a 12V voltage will be added to the relay, so that the fan is powered on and turned on. If the temperature is higher and reaches the upper temperature limit of the over-temperature alarm, an over-temperature alarm will sound; if the temperature is high enough to reach the upper temperature limit of the over-temperature trip, an over-temperature trip will occur. In this way, the purpose of real-time monitoring of the controlled object is achieved.

2 Software Implementation

The software adopts a modular structure, including 1 main module and 5 submodules (button processing submodule, digital quantum module for setting upper limit temperature and collecting boundary points, communication submodule, fault output processing submodule and display submodule). The main module completes the initialization of each submodule and calls the fault output processing submodule and display submodule. The button processing submodule, digital quantum module for setting upper limit temperature and collecting boundary points, and communication module work in interrupt mode, and the main module communicates with them through a shared RAM area. Since the analog input signal of the single-chip microcomputer application system contains various noises and interferences, this program uses digital filtering technology to filter. In addition, for the linearization problem mentioned above, we divided 0~200℃ into four areas and performed linearization calculations in each area. This is much more accurate than directly calculating in the 0~200℃ area, and can achieve an accuracy of 0.1℃.

The functions of each sub-module are as follows:

(1) The button processing submodule requests an interrupt from ATmega16 when a key is pressed and modifies the pre-set flag in the interrupt subroutine.

(2) Setting the upper limit temperature and collecting boundary points The digital quantum module can enter the interface for modifying the upper limit temperature by entering a password when the key is pressed for a long time. The digital quantities corresponding to 0℃, 50℃, 100℃, 150℃, and 200℃ are collected through the button and the results are stored in the E2PROM. This data is used as the boundary point to calculate any temperature between 0 and 200℃.

(3) The communication submodule can connect to the remote controlled object for RS485 communication through the LBC184 (converts RS232 signals into RS485 signals) chip and the microcontroller.

(4) The fault output submodule can determine whether an abnormal situation occurs on site by comparing the actual temperature with the upper limit temperature. At the same time, the flag bit is set to determine whether to perform A/D conversion and whether to display.

(5) The display submodule converts the linear calculation result from binary to BCD code and sends it to the 5-digit LED display for display.

3 Application of anti-interference technology in temperature controller

(1) To solve the AC power interference in the temperature controller, a power filter is connected in series at the incoming end of the AC power supply, that is, the lower end of the power transformer. This can effectively suppress the intrusion of high-frequency interference (Figure 4).

(2) Use optocoupler devices in the fault output circuit to ensure that the output has higher electrical isolation and anti-interference capabilities.

(3) Use varistors at both ends of the temperature sensor in the analog conversion circuit and other places to absorb overvoltages of different polarities.

(4) Conduct electromagnetic interference tests at the dry-type transformer operation site, conduct probability statistical analysis on the test results, and minimize the electromagnetic interference generated by the interference source by carefully selecting components and using hardware anti-interference technology and software anti-interference technology.

The temperature controller has low power consumption, advanced technology, complete functions, simple operation, reliable performance, and can work stably for a long time in extremely harsh electromagnetic interference or high temperature environment. It is an ideal monitoring device for dry-type transformers.