This article mainly introduces the relevant knowledge about the design of single-phase power filter and its design process. I hope that through this article, you can have a deeper understanding of single-phase power filter.
Power filterThe power filter is an electrical equipment that effectively filters out the frequency point of a specific frequency in the power line or the frequency other than the frequency point. The function of the power filter is to obtain a power signal of a specific frequency by connecting the power filter to the power line, or to eliminate the power signal of a specific frequency.
Using this characteristic of the power supply filter, a square wave group or compound noise after passing the power supply filter can be turned into a sine wave of a specific frequency.
The commonly used filter circuits of power filters include passive filter and active filter. The main forms of passive filtering are capacitive filtering, inductive filtering and compound filtering (including inverted L-type, LC filtering, LCÏ€-type filtering and RCÏ€-type filtering, etc.). The main form of active filter is active RC filter, also known as electronic filter. The magnitude of the pulsation component in the direct current is represented by the pulsation coefficient S. The larger the value, the worse the filtering effect of the filter.
Ripple coefficient (S) = the maximum value of the fundamental wave of the AC component of the output voltage / the DC component of the output voltage
The principle of the power filter is an impedance adaptation network: the greater the impedance adaptation between the input and output sides of the power filter and the power and load sides, the more effective the attenuation of electromagnetic interference.
Single-phase power filter designThis article mainly takes single-phase active power filter as an example to discuss the design process of single-phase power filter.
Basic principles and system structure
Active power filter is a device that can compensate dynamically changing harmonic reactive power. It can actively track and compensate each harmonic. It calculates the command signal of the compensation current by sampling the load current and voltage, and calculates the command signal of the compensation current through the command current calculation circuit. After inverting, the signal is used to control the conduction of the inverter to obtain the compensation current, thereby canceling the corresponding current in the load, and realizing the dynamic Tracking compensation, and it can compensate both harmonics and reactive power [4]. Figure 1 shows the principle diagram of a single-phase parallel active power filter.
It can be seen from the figure that the active power filter and the non-linear load are connected in parallel on the single-phase power supply. The nonlinear load connected to the grid generates load current, and injects harmonic and reactive current into the grid. The system sends the collected load current to the harmonic current detection arithmetic circuit through the sensor, and the calculated command signal controls the compensation current The generating circuit (consisting of three parts: current tracking control circuit, drive circuit and inverter main circuit) works. The voltage difference between the output voltage of the inverter and the grid voltage acts on the filter inductor to generate a compensation current to be injected into the grid to compensate The harmonic reactive current is reduced, so that the grid current signal presents a standard sinusoidal waveform.
Figure 1 Schematic diagram of single-phase active power filter
2 Harmonic detection and control strategy
Harmonic detection is the key link of active power filter, and its speed and accuracy directly affect the output result of compensation current. This paper chooses the harmonic detection method based on the principle of phase detection. Taking the grid voltage signal as the reference signal, standard sine and cosine signals are generated through a phase-locked loop, the detected instantaneous current and the standard sine and cosine signal are linearly changed, and then the DC component is filtered out after passing through a low-pass filter. So as to get the harmonic and reactive current [5]. This method is efficient, simple and easy to implement in experiments. Figure 2 shows the harmonic detection process with DC side voltage control.
Figure 2 Harmonic detection process
The detected harmonic current is controlled by the closed-loop overall system. The outer loop of this system is the DC side voltage loop control, and the inner loop is the compensation current control output by the inverter. The DC side voltage of the active power filter is generally obtained by the power tube anti-parallel diode grid voltage rectification. Through the control voltage loop, the DC side and the power grid can be exchanged to ensure that the active power filter is DC at startup The side is charged to the expected voltage. During normal operation, the control voltage loop obtains active power from the grid and maintains the DC side voltage stability [6-7].
In the figure, Udc is the actual voltage on the DC side and Uref is the given voltage on the DC side. The difference between the two is adjusted by the PI regulator and superimposed on the DC component Ip of the active current detected by the harmonics, and then multiplied by the standard sine. Signal, the instantaneous active current is obtained. By subtracting the active current from the detected load current is(t), the sum of harmonics and reactive currents ih(t) can be obtained.
Figure 3 shows the system control diagram. This article selects the PI + triangle wave modulation method. It is obtained by inverting ih(t), which is the difference with the compensation current ic(t) output by the inverter. The error signal is adjusted by PI and compared with the triangular wave signal to generate the corresponding PWM signal to control the inverter to turn on .
Figure 3 System control diagram
3 System software and hardware design
The hardware system of the single-phase parallel active power filter is mainly composed of APF main circuit, DSP control board, signal sampling and conditioning circuit, MOSFET drive circuit, etc. The design system uses TMS320F28335 as the main control chip, and the system platform is built as shown in Figure 4. Shown.
Because the active power filter has relatively high requirements for real-time and high efficiency, the main control chip of this article has chosen TI's TMS320F28335. TMS320F28335 is a 32-bit DSP chip with floating-point core newly launched in TI's 2000 series, and it is also one of the most advanced processors in the industrial control field.
The main frequency is up to 150MHz (6.67nS clock cycle), the core voltage is 1.9V/1.8V, the IO port voltage is 3.3V; it has an IEEE-754 single-precision floating-point unit (FTU), 16×16 and 32×32 media access Control (MAC) operation, Harvard final calculation structure, fast interrupt response and processing; 16-bit or 32-bit external interface, which can handle more than 2M×16-bit address range; on-chip memory 256K×16-bit Flash memory, 34K×16-bit single-port random Memory (SARAM), 1K×16-bit one-time programmable (OTP) ROM[8].
Figure 4 System hardware architecture diagram
The system sends the collected signal to the sampling and conditioning circuit through the sensor, and the conditioned signal is input into the DSP control board for harmonic calculation. The calculated command signal controls the work of the APF main circuit, and the final output voltage of the main circuit is The voltage difference of the grid voltage acts on the filter inductance L, thereby generating a compensation current that is injected into the grid, compensating for harmonics and reactive currents, and making the grid current signal present a standard sinusoidal waveform. The system software includes the main program and the interrupt program. The main program is shown in Figure 5.
Figure 5 System main program flow chart
The main program mainly completes the initialization, defines the system variables and sets the system interrupt vector and pin initialization. It mainly includes CPU and peripheral clock settings, EPWM module initialization, ADC module initialization, GPIO initialization, and external interrupt configuration. After the initialization is completed, the program then enters the loop state, waiting for the arrival of the interrupt.
The interrupt program is mainly composed of external interrupt subroutine and timer period interrupt subroutine. As shown in Figures 6 and 7, respectively. The external interrupt subroutine is used to synchronize the grid voltage, reset the sine and cosine table, and start the timer. It initializes the system count value by continuously detecting the rising edge of the square wave signal, and then resets the sine and cosine table.
Figure 6 External interrupt main program flow chart
The timer period interrupt subroutine is responsible for all control processes, including AD sampling, sine table address shift, harmonic calculation, low-pass filtering, DC side voltage PI control, compensation current command control, PWM signal generation, etc. In the design, the sampling period is set to 6.4kHz, the timer triggers AD sampling, and then enters the harmonic calculation part, which sequentially performs digital low-pass filtering, DC side voltage control, and PI+triangular wave comparison to generate PWM signals.
Figure 7 Timer interrupt subroutine flowchart
4 Experimental results
According to the hardware structure shown in Figure 4, a 220VA single-phase parallel active power filter experimental device is designed. The switching frequency is selected as 6.4kHz, and the harmonic load is a single-phase uncontrolled rectifier circuit with inductive load. Experiment under the system. Collect and observe the grid voltage and load current before and after compensation as shown in Figure 8. Perform FFT analysis on the load current before and after compensation, and the result is shown in Figure 9.
Figure 8 Grid voltage and grid current before and after compensation
Figure 9 Comparison of current distortion rate before and after compensation
It can be seen from the figure that the current distortion rate has dropped from 29.5% before compensation to 3.82%, indicating that the active power filter of this design has good compensation performance.
How to judge the quality of single-phase AC power filter
Use a multimeter to measure resistance:
1.The input-output wires of the same wire color should be short-circuited separately.
2.The input wires and output wires of different wire colors should be opened separately.
3. If there is a third wire (ground wire), the input wires and output wires of different wire colors should be open to the ground wire.
ConclusionThis paper studies the single-phase parallel active power filter, introduces a new method of harmonic detection, and analyzes the control strategy. A prototype is built on the basis of TMS320F28335, and the discussed The theory has been verified by experiments, and the experimental results show that the active power filter has good compensation performance.
This article is intended for reference, and if there are any deficiencies, I would like to see Haihan.
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