Design of reactive power compensation controller based on single chip microcomputer

    Among the industrial and domestic electrical loads, resistive and inductive loads account for a large proportion. Asynchronous motors, transformers, fluorescent lamps, etc. are typical resistive and inductive loads. The reactive power consumed by asynchronous motors and transformers accounts for a high proportion of the reactive power provided by the power system. Reactors and overhead lines in the power system also consume some reactive power. Resistive and inductive loads must absorb reactive power to work properly, which is determined by their own working nature. The most reasonable method is to connect capacitor banks in parallel near these inductive devices and at appropriate locations on the line for reactive compensation. In the power system, the use of reactive compensation methods to improve power factor and reduce reactive loss is one of the important means to improve power supply quality and equipment utilization.

    1 Introduction to reactive power compensator based on ATT7022A

    The premise of reactive power compensation is to accurately measure the operation status of the power grid, mainly active power, reactive power, power factor, harmonic status, etc. ATT7022A is a high-precision three-phase power metering chip suitable for three-phase three-wire and three-phase four-wire applications.

    It can measure active power, reactive power, apparent power, active energy and reactive energy of each phase and combined phase, and can also measure parameters such as current, voltage RMS, phase angle, frequency, etc. of each phase, fully meeting the needs of power data collection. It also provides an SPI interface to facilitate the transmission of metering parameters with an external MCU, and all metering parameters can be read out through the SPI interface. It can accurately collect these parameters with high precision and simple software design. The internal block diagram is shown in Figure 1.

    
    The reactive power compensation controller introduced in this paper is based on the three-phase power metering chip ATT7022A. The control chip is the AVR microcontroller ATmega128 produced by atmel. Compared with ATme ga16, it is the highest configuration microcontroller of the AVR8-bit series microcontroller, and has 53 programmable IO ports, which is convenient for designing LCD display and drive circuits . The controller can analyze and determine whether to perform reactive power compensation based on the active power, reactive power and power factor provided by ATT7022A, automatically switch capacitors, realize automatic reactive power compensation of distribution network, and improve power transmission efficiency.

2 Hardware circuit design
    The hardware circuit design of the controller mainly consists of voltage and current detection circuit, power supply circuit, drive circuit, communication circuit and LCD display module. With Atmega128 as the control chip, these control functions can be easily completed. Its hardware structure block diagram is shown in the figure below. After the grid voltage and current pass through the transformer , the voltage and current signals are sent to ATT7022A. ATT7022A provides an SPI interface, through which all metering parameters can be read. The microcontroller determines the power factor based on these parameters, decides whether to perform reactive power compensation, and transmits the signal to the capacitor switching module.

    The detection circuit is shown in the figure. This module mainly collects the voltage and current signals of the power grid. The voltage input is UA, UB, UC, and UN, and the range is 0 to 220V. The current input is IA1, IA2, IB1, IB2, IC1, and IC2, and the range is 0 to 5 A. ATT7022A recommends that the sampling value on the chip voltage input pin is 0.2 to 0.5 V, and the sampling voltage on the chip current input pin is 0.1 V. Therefore, the 5 A current signal is converted into 2.5 mA current through the 5 A/2.5 mA current transformer. After passing through a 40 Ω resistor, the input voltage is 0.1 V. The 220 V voltage signal passes through a 120 K resistor, and then passes through a 2 mA/2 mA current type voltage transformer. The output current is about 2 mA, and then passes through a 250 Ω resistor. It is converted into a voltage of 0.5 V. ATT7022 integrates 7-channel 16-bit ADC, reference voltage circuit and digital signal processing circuits for all power, energy, effective value, power factor and frequency measurement. The measured values ​​are stored in the corresponding storage space respectively, and data is exchanged with the microcontroller through SPI. There is no need to program in the system software to obtain reactive power, which not only saves CPU space but also improves calculation efficiency.

    
    The power supply circuit design is shown in Figure 4. It uses a 24V switching power supply, which is a type of high-frequency inverter switching power supply. The switching power supply input is 200-240VAC, and the output is a positive 24V voltage for the driver chip MCl413. Then use the MC34063DC/DC converter to realize a boost or buck power converter, converting the 24 V voltage into a 5 V voltage. The voltage required by Atmega128, ATT7022, and the LCD module LCD is +5 V. Digital circuits and analog circuits are generally separated, and the last point is brought together. At this time, they can be connected with a 0-ohm resistor. The functions are: 1) The current path directly connected may be very large, and a 0-ohm resistor can obtain a very narrow current path, which can effectively limit the loop current and suppress noise. 2) When wiring, it is very troublesome to distinguish between digital ground and analog ground. Using a 0-ohm resistor to separate them can clearly distinguish between digital ground and analog ground.

    Capacitor switching unit. By analyzing the reactive power or power factor measured by ATT7022A, it is decided whether to perform reactive compensation, that is, switch the capacitor. This controller uses thyristors to control the capacitor for switching. Since the voltage across the capacitor cannot change suddenly, when the difference between the grid voltage and the capacitor voltage is large, triggering the thyristor will produce a large current impact. In order to prevent inrush current when switching the capacitor, the thyristor is switched on when the voltage across the thyristor is zero and removed when the current is zero, so as to compensate the reactive power dynamically in real time.

    The driver chip is MC1413 produced by Motorola, which is a high-current Darlington array reverse driver with high working voltage, sinking current up to 500mA, and can withstand 50 V voltage in the off state. As shown in the figure, when the input of MC1413 is 1, the output is 0, so that the transistor forms a voltage drop between the emitter, base and collector under the voltage division of the resistor, connects the emitter and collector, and outputs a voltage of +24 V. Through a pulse transformer, it can drive the thyristor to open, otherwise it will be closed.

    
    Thyristor-controlled capacitors, one capacitor is connected to every two thyristors. The figure shows a thyristor output. G1 and K1 are connected to the gate and cathode of the thyristor respectively. The controller uses two MC1413 chips, which can control 12 thyristors in 6 groups, and can control the switching of 6 groups of capacitors.
    Communication module. Its circuit uses a single power supply level conversion chip max232, which is used to communicate with the host computer. It is mainly used for data transmission of power grid operation parameters and system operation status, as well as calibration of system time, and accepts switching and other operation commands issued by the host computer.
    Human-machine interface unit. The human-machine interface unit includes two parts: keyboard input and LCD display. The keyboard part is used to set working parameters, switch display content and set clock time. It can also realize manual switching of capacitors under special circumstances. The keyboard is directly driven by I/O port. The LCD display function mainly displays current, voltage, active power, reactive power, power factor, operating status of capacitors and harmonic conditions.

3. Software flow design
    Using ATT7022A metering chip, the single-chip microcomputer does not need to perform any complex calculations, and the measurement data is provided directly, and it can provide four-quadrant power measurement parameters, and can accurately measure more than 21 harmonics. The CPU does not involve the processing of A/D sampling data, which greatly reduces the amount of CPU calculations and greatly simplifies the software program design, making the system operation more reliable.

    During power transmission, the power factor of the power system is generally greater than 0.95. If the power factor is less than 0.95, compensation must be performed. The power factor is the basis for switching. If the power factor is lower than 0.95, the capacitor is switched on. If it is greater than 1.0, the capacitor is switched off. The control strategy of the controller is to analyze whether the power factor of the power grid is lower than 0.95, whether the power grid is overvoltage or undervoltage, etc., based on the active power, reactive power and other parameters provided by the metering chip. By comparing with the set value, it is determined whether the capacitor is switched in phase or switched in three phases at the same time. When switching the capacitor, the principle of "first switch on, first switch on, first switch on" is adopted to prevent frequent switching of a group of capacitors, ensure the safety of the capacitors, and realize the normal operation of the power grid.

    4. How the controller measures high current and high voltage

    The voltage input range of the controller is 0-220 V, and the current input range is 0-5 A, so voltage-current conversion is required when measuring large current and voltage. For example, when measuring a load of 0.4 kV, 500 KVA, the rated current is about 700 A, so a current transformer of LMZJ1000/5 can be selected with a transformation ratio of 200. In the program, the current, active power, reactive power, and apparent power read are multiplied by the transformation ratio of 200, which is the measured electrical parameters.

    When measuring a 10 kV, 200 A load, the voltage and current cannot be measured directly. Voltage transformers and current transformers are required. You can use a voltage transformer with a transformation ratio of 100 and a current transformer with a transformation ratio of 50. Connect as shown in the figure below. In the program, multiply the voltage read by the transformation ratio of 100, and the current, active power, reactive power, and apparent power read by the transformation ratio of 50 to obtain the actual measurement parameters.

      5 Conclusion

    The reactive power compensation controller designed with ATmega128 microcontroller as CPU and ATT7022A as metering chip can accurately calculate the reactive power, active power and power factor of the power grid, and provide accurate data for reactive power compensation. The hardware structure is simple and reliable, while reducing the requirements for CPU, simplifying the software design, and ensuring good stability.

    This controller is only suitable for reactive power compensation of low-voltage distribution network. For the solution of measuring high voltage and high current, the article also gives the measurement method and wiring diagram. This controller has a good effect on improving the power factor of the power grid and reducing the power grid loss. It has a simple design, low cost and good market prospects.