Transformer temperature intelligent monitoring instrument

Introduction: This paper presents a temperature intelligent monitoring instrument with ATMET's 89C51 single-chip microcomputer as the core. The monitoring instrument has the functions of automatically recording the three-phase temperature, phase, historical maximum temperature and other data before power failure and black box function.

1 DOWN Introduction

The intelligent transformer temperature monitor consists of a sensor, a single-chip temperature controller and corresponding output relays. The temperature of the transformer is measured by a platinum resistor, and after comparing it with the preset temperature inside the temperature monitor, the output controls the opening and closing of the fan relay contacts, the over-temperature alarm relay and the over-temperature trip relay contacts, so as to monitor the transformer winding temperature, prevent the transformer from being damaged due to overheating, and ensure the designed service life of the transformer.

2 Hardware System Design

2.1 Hardware system composition of the monitoring instrument

The 89C51 microcontroller is an 8-bit Flash microcontroller from ATMET. Its biggest feature is that it contains 4KB reprogrammable Flash memory, 128×8B internal RAM and two 16-bit timers/counters. It is easy to modify the program during the development process and is compatible with MCS-51. For the LED control driver, PS7219 is selected. Its interface uses a synchronous serial peripheral interface (SPI), which can drive 8-bit digital tubes (using 5 of them) at the same time. Because PS7219 has a 15×8RAM function control register inside, it is easy to address, and each digit can be refreshed separately. The display brightness can be digitally controlled, and each digit has a flashing enable control and full brightness function test. This can simplify the design of the hardware circuit, save CPU port lines and reduce the time occupied by the CPU.

Because the keys for setting working status and parameters are in the form of a composite keyboard, there are fewer keys. The keyboard is designed as an independent key interface. By detecting the level of the input line, it is easy to determine which key is pressed. The communication with the upper meter uses the MAX232 chip. This chip has a dual set of drivers/receivers. It contains a capacitive voltage generator that can provide EIA/TIA-232-E level when powered by a single power supply. Each receiver converts the EIA/TIA-232-E level input to 5V TTL/COMS level. There are also some special data that need to be saved in the instrument. The circuit design is equipped with EEPROM and power supply monitoring circuit to achieve the above functions. The output drive circuit uses the ULN2003 driver. This chip has 7 Darlington tubes, and its collector can collect a maximum current of 500mA. Because there is a freewheeling diode inside, it can drive the relay of the inductive load. Its overall block diagram is shown in Figure 1.

2.2 Sensors

Platinum resistance has stable physical and chemical properties, strong antioxidant ability, high sensitivity, easy material purification, good processability, easy production, good product consistency, and very good interchangeability. According to the provisions of the new international temperature scale (ITS-90 for short) approved by the 77th International Committee for Weights and Measures in July 1989, the entire temperature scale is divided into 4 temperature ranges, of which 13.803~961.78℃ and its corresponding standard instrument uses a platinum resistance thermometer. The standard for resistance thermometers for transformers stipulates that the sensor uses Pt100. The transmission lead uses a three-wire system, as shown in Figure 2. The advantage of the three-wire system is that it can reduce the measurement error caused by the change in the resistance value of the connecting wire. Use an unbalanced bridge to detect the change in output voltage caused by temperature changes, and draw a temperature signal detection circuit diagram for one of the channels.

2.3 Pre-signal processing circuit

CD4052 is a dual 4-to-1 multi-channel analog switch. Its 4 input groups are 3-way 3-phase winding temperature signals and ambient temperature signals (optional). The switching of these signals is controlled by the program through the port line of the single-chip microcomputer. Because the detected temperature signal is a slowly changing signal. In order to effectively improve the anti-interference ability of the instrument, RC low-pass filtering is performed. In order to improve the input impedance and high common mode rejection ratio, the first stage of the preamplifier adopts a differential connection method. The two pairs of resistors R4, R5, R6, R7, R8, and R9 should be carefully selected, and the amplifier uses a chopper-type automatic zeroing op amp TIL7650 with micro-drift, low offset, high gain and high common mode rejection ratio. The gain of the second-stage inverting amplifier is adjusted by the RW2 multi-turn potentiometer.

2.4 A/D conversion using V/F converter

In single-chip microcomputer measurement and control systems, the reading of external data and signals is mostly achieved through A/D methods. For some non-fast A/D processes, V/F conversion technology is very popular. The interface between V/F converter and single-chip microcomputer has the following characteristics:

(1) The interface is simple and occupies less hardware resources of the microcontroller. The frequency signal can be input into any I/O line of the microcomputer or used as an interrupt source and counting input, etc.

(2) Good anti-interference performance. V/F conversion itself is an integration process, and using a V/F converter to achieve A/D conversion is a frequency counting process, which is equivalent to integrating the frequency signal within the counting time, so it has strong anti-interference ability.

(3) Convenient for long-distance transmission. The V/F converter needs to be used in conjunction with a frequency meter to achieve A/D conversion. The circuit block diagram is shown in Figure 3.

The principle is as follows: start the frequency counter and timer at the same time, the frequency counter uses the frequency signal f output by the V/F converter as the counting pulse, and the timer uses the reference pulse as the timing pulse. When the timing ends, the timer generates an output signal to stop the frequency counter from counting. The relationship between the count value and the frequency is:

Where fs is the reference frequency

Ds - initial value of timing counter

Therefore, , as long as the D value is known, the output frequency of the V/F converter can be calculated, and the voltage value can be obtained according to the relationship between V and F. The instrument uses the universal V/F converter LM331.

3 Software Design

3.1 Software composition

The resistance thermometer software consists of a main program and an interrupt service program. The main program completes the initialization of the timer, counter, EEPROM, display control driver PS7219, etc., keyboard operation management, data acquisition processing, control output and display. The interrupt service program mainly handles data retention during sudden power outages, such as three-phase temperature and phase.

3.2 Basic functions

(1) Circuit/top conversion function

Circuit measurement: Circuit measurement and display of A, B, C three-phase temperature in sequence; Highest phase temperature measurement: Circuit measurement of A, B, C three-phase temperature and display of the highest phase temperature. This key is in circuit state when the power is turned on.

(2) Black box function

In case of sudden power outage, the three-phase temperature and phase before the power outage can be automatically retained.

(3) Data retention and processing functions

The historical highest temperature data can be automatically retained (not lost after power failure), and the historical highest temperature can be cleared. The retained temperature control setting value can be easily modified (satisfying T4>T3>T2>T1. Among them, the fan start temperature T2=100℃, adjustable range ±20℃; fan stop temperature T1=80℃, adjustable range ±20℃; over-temperature alarm temperature T3=130℃, adjustable range ±20℃; over-temperature trip temperature T4=150℃, adjustable range ±20℃).

(4) Fault detection function

If the sensor is open or short-circuited, it will automatically alarm (fault contact closes) and display.

(5) “Fan” automatic start and stop function

When the measured temperature is higher than the set temperature T2, the "fan" starts automatically and the "fan" start indicator light comes on; when the three-phase measured temperature is lower than T1, the "fan" stops automatically and the "fan" start indicator light goes out.

(6) Over-temperature alarm function

When the measured temperature is higher than the set temperature T3, the over-temperature contact is automatically closed, the over-temperature indicator light is on, and the buzzer sounds an intermittent alarm; when the three-phase measured temperature is lower than T3-0.6℃, the over-temperature contact is disconnected, the over-temperature indicator light is off, and the buzzer stops sounding an alarm.

(7) Over-temperature trip alarm function

When the measured temperature is higher than the set temperature T4, the trip contact is automatically closed, the trip indicator light is on, and the buzzer emits a long alarm; when the three-phase measured temperature is lower than T4-0.6℃, the trip contact is disconnected, the trip indicator light is off, and the buzzer stops the long alarm.

(8) Warning Return

When the thermostat overheats or trips the buzzer, press this key once and the alarm stops for about 1 hour. Press it again to resume the alarm.

(9) Security alarm

The thermostat accepts external long-open contact input. If the contact is closed (when the distribution room door is open), the buzzer will emit a long alarm.

(10) Self-test function

The thermostat can self-check all output functions except tripping.