Keywords: power factor; reactive power compensation; microcontroller
With the development of modern industry, more and more inductive loads are used in the power grid, such as induction motors and transformers. These devices not only consume active power during operation, but also require the grid to provide corresponding reactive power, resulting in a low power factor of the grid. Shunt capacitors in the power grid can reduce the reactive power provided by the grid to inductive loads, thereby reducing the transmission loss caused by the transmission of reactive power and improving the operating conditions of the power grid. Therefore, power factor compensation controllers have always had a broad application market. The power factor compensation controller introduced in this article complies with the JB / T9663-1999 national standard, and its main functions are:
(1) Automatic identification of phase sequence (2) Sampling and display of voltage, current and power factor (3) Over-voltage release and under-current blocking to protect capacitors and avoid cyclic switching (4) First cut first, first cut The principle of the first input, the cyclic switching of the compensation capacitor (5) All operating parameters can be set by the panel buttons, including the input threshold, cut-off threshold, overvoltage protection threshold, undercurrent blocking threshold, switching delay time
1. Working principle Sampling the phase difference between the first-line current (such as line A) and the other two-line voltages (such as BC line) in the three-phase power supply, and through certain calculations, the real-time power factor of the current power grid can be obtained. This power factor is compared with the set input threshold and cut-off threshold. During the entire switching delay time, if it is within the switching threshold, it will not act; if it is less than the input threshold, another group of capacitors will be input; When the threshold or the power factor is found to be negative, a set of capacitors that have been put in are cut off. After the switching delay time, the comparison and switching are repeated until the current power factor reaches the switching threshold. During the switching process, if it is found that the detected voltage is greater than the set overvoltage protection threshold, all the capacitors that have been put in are cut off in groups; when the detected voltage exceeds 10% of the set overvoltage protection threshold, then Cut off all the input capacitors at once to protect the capacitors. If it is found that the detected current is less than the undercurrent blocking threshold during switching, the switching operation is stopped to avoid cyclic switching in the system.
Because there are different wiring methods in three-phase power supply, and different wiring methods have different algorithms for power factor, we stipulate that the current of the ARC series power factor automatic compensation controller is taken from the A line in the three-phase power supply, and the voltage is taken from The line voltage between BC, in order to reduce the complexity of field wiring, we automatically discriminate the phase in the program.
In the three-phase power supply, we assume that the phase voltages of the three phases are Ua, Ub, and Uc, respectively, and the A line current is Ia
Then Ua = Usin (ωt), Ub = Usin (ωt + 120º), Uc = Usin (ωt + 240º),
Thus, the line voltage between BC is Ubc = Ub-Uc = Usin (ωt-90º)
If the load of line A is purely resistive, the current Ia of line A is in phase with the voltage Ua of line A, and the angle of Ia leading Ubc is 90º;
If the load on the A line is inductive, the A line current Ia lags the A line voltage Ua at an angle of φ (0º≤φ≤90º), and Ia leads Ubc at an angle of 90º-φ;
If the load of line A is capacitive, the current of line A leads the voltage of line A, the angle of line Ua is φ (0º≤φ≤90º), and the angle of Ia leads Ubc by 90º + φ
In our ARC power factor automatic compensation controller, for the convenience of calculation, our current phase sampling is the second cycle of voltage sampling, that is, if there is no phase difference, Ia lags Ua by 360º. In the actual detection, suppose we detect that Ia lags behind Ubc at an angle of α. According to the above analysis, we know:
If 180º <α <270º, the circuit is a capacitive load, COSφ = COS (270º-α)
If α = 270º, the circuit is an inductive load, COSφ = 1
If 270º <α <360º, the circuit is inductive load COSφ = COS (α-270º)
In order to facilitate the user's wiring, if the user connects the voltage Ubc to Ucb, or reverses the input of Ia, according to the above inference, we can also get:
If 0º <α <90º, the circuit is a capacitive load, COSφ = COS (90º-α)
If α = 90º, the circuit is an inductive load, COSφ = 1
If 90º <α <180º, the circuit is an inductive load COSφ = COS (α-90º)
Figure 1 Voltage and current vector
2. Design of hardware The CPU of the controller uses ATMEL's ATMEGA16-8L. This single-chip microcomputer has a wide operating voltage range (2.7-5.5V) and a maximum operating frequency of 8MHz; the chip has 16k bytes of Flash program memory, 512 bytes EEPROM, 1K bytes of on-chip SRAM; 8-way 10-bit ADC; a programmable serial USART, programmable watchdog timer with independent on-chip oscillator; two with independent prescaler and comparison 8-bit timer / counter with timer function; a 16-bit timer / counter with prescaler, compare function and capture function. The display chip adopts the keyboard and special display chip CH451S produced by Nanjing Qinheng Company. The CH451S can drive up to 8 digital tubes, and can directly drive the LED digital tubes without additional drivers, which greatly reduces the size of the printed board. The single chip uses SPI Mode, only 3 lines (chip select CS, clock CLK, data input DIN), because this system does not use the keyboard function of CH451S, so the DOUT pin of CH451S is not used. The voltage signal of Ubc enters the 2mA / 2mA isolation converter through the resistance current limit and is divided into two channels. One channel enters the analog absolute value processing circuit and is sent to the A / D conversion port ADC0 of the microcontroller as a voltage display signal. The other channel undergoes zero comparison Then enter the interrupt interface INT0 of the single-chip microcomputer; similarly, the current signal of Ia is divided into two channels after the isolation converter of 5A / 5mA, all the way into the analog absolute value processing circuit and sent to the A / D conversion port ADC1 of the single-chip microcomputer as the current display signal All the way to the ICP pin of the single-chip timer gate control terminal after zero comparison.
Figure 2 ATMEGA16 external pins
Figure 3 Input signal processing
Third, the design of the software because the entire system does not have high requirements for the accuracy of voltage and current sampling, we directly use the 10-bit A / D of the CPU to perform A / D conversion on the voltage and current signals. On the one hand, the conversion results are for display needs. , On the other hand, as a comparison signal of overvoltage and undercurrent. We set INT0 to a rising edge to generate an asynchronous interrupt, and ICP to a rising edge to trigger input capture. When INT0 generates an interrupt, the 16-bit counter starts counting at a constant internal frequency until the next interrupt is generated. While counting, when there is a rising edge pulse on TCP, the data counted by the 16-bit counter is put into the capture register. When a sampling cycle ends, the data (N) in the counter is the base of a cycle of the external AC signal. The data (n) in the capture register is the base of the current Ia lagging the voltage Ubc, and (n / N) * 360º is the angle According to the above principle, it can be judged whether the voltage leads the current or the current leads the voltage in the same cycle. At the same time, the angle of lead or lag can be obtained, and the power factor can be obtained by looking up the table.
In order to avoid frequent switching of a certain group of capacitors, balance the working time of each group of capacitors and extend the service life of the entire system. We adopt the principle of prioritized removal of the first input of the capacitor, and the principle of prioritized input of the first input. We have created a space in the RAM of the single chip microcomputer to record the input and removal time of each group of capacitors, and then sort them The one with the longest working time is regarded as the priority removal target, and the one with the longest removal time is regarded as the priority input target.
When the current of the three-phase AC load circuit is very small, the phenomenon of switching oscillation will occur. That is to say, if a group of capacitors is put into the control system, an over-investment will occur. If a group of capacitors are cut off, an under-investment will occur, and the controller will produce repeated switching. In order to avoid this imagination, we have set undercurrent lock. When the current value is less than this value, the system will stop the switching operation of the capacitor to maintain the operation of the capacitor that has been put in.
In the working process, if the sampled voltage data is greater than the set overvoltage protection value, the controller will gradually cut off the capacitor that has been put in. If it is found that it exceeds 10% of the set protection value, all the The input capacitor protects the capacitor.
The above technology has been applied to the company's ARC power factor automatic compensation controller. After testing and operation, the system works stably, and various indicators have reached the requirements of national standards.
The article comes from: "Low-Voltage Electrical Appliances" 2006, Issue 10.
references
[1] ATMEL Corporation. Chinese translation of ATmega16 technical manual, 2466G–AVR–10 / 03
About the Author:
Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system
Hybrid Stepper Motor,Stepper Motor,Standard Hybrid Stepping Motor,Linear Hybrid Stepping Motor
Changzhou Sherry International Trading Co., Ltd. , https://www.sherry-motor.com