Research on parallel active power filter based on DSP

With the rapid development of power electronics technology, nonlinear loads in power systems have increased significantly, and various nonlinear and time-varying electronic devices such as inverters, rectifiers and various switching power supplies have been used more and more widely, resulting in increasingly serious harmonic and reactive power problems. Using power filter devices to absorb harmonics and reactive currents generated by nonlinear loads is an effective measure to suppress harmonic and reactive pollution. At present, parallel passive power filters (PPFs) are widely used to suppress harmonics. PPFs have the advantages of low investment, high efficiency, simple structure, reliable operation and convenient maintenance, but their inherent defects limit their development. Compared with PPFs, active power filters (APFs) have high controllability and fast response. They are characterized by not only being able to compensate for each harmonic, but also suppressing flicker and compensating reactive power; they are not affected by the system impedance characteristics, and can eliminate the risk of parallel resonance with the system impedance; they have adaptive capabilities and can automatically track and compensate for changing harmonics. This paper mainly studies parallel APFs.

1 Working Principle

The composition principle of the active power filter system is shown in Figure 1.

In the figure, Vs represents the AC power supply, and the load is a nonlinear harmonic source, which generates harmonics and consumes reactive power. The active power filter system consists of two major parts, namely the command current operation circuit and the compensation current generation circuit. Among them, the main function of the command current operation circuit (i.e., the harmonic and reactive current detection circuit) is to extract the required harmonic and reactive current components from the current of the compensation object. The compensation current generation circuit consists of three parts: the current tracking control circuit, the drive circuit and the main circuit. Its function is to derive the command signal of the compensation current according to the command current operation circuit and construct the actual compensation current. The main circuit currently uses PWM converters, which mainly work as inverters when generating compensation currents.

The basic working principle of APF shown in Figure 1 is: real-time detection of the voltage and current of the compensation object, and calculation of the command signal of the compensation current (harmonics and reactive current) by the command current operation circuit. The signal is amplified by the compensation current generation circuit to obtain the compensation current. The compensation current is equal to the harmonics and reactive current to be compensated in the load current, and the phase is opposite, and they cancel each other out. Finally, the desired grid current is obtained, making the grid current a sine wave with the same phase as the voltage, thereby achieving the purpose of suppressing harmonics and compensating reactive power. The above principle can be described by a set of formulas:

2 Hardware Circuit Design

The core of the command current operation circuit is DSP, which uses the extended dq algorithm to detect the harmonic and reactive components in the load current, and obtains the command signal of the compensation current according to the compensation purpose of the reactive compensation and harmonic suppression device. The compensation current generation circuit generates a compensation current that tracks the command current to achieve the purpose of compensating harmonics and reactive power. The hardware of the control system mainly includes DSP control chip, D/A and A/D circuits, sampling period signal generation circuit, nonlinear load current detection and conditioning circuit, triangle wave comparison circuit, drive circuit and DC side voltage control and voltage equalization control circuit. As shown in Figure 2, the system detects the current iLa, iLb and iLc of the nonlinear load through the current sensor, and sends them to the A/D port of DSPTMS320F2812 after current signal conditioning. The drive circuit receives the PWM signal from the DSP and drives the switch tube of the main circuit after isolation and amplification to control the main circuit current to follow the change of the command current. Two voltage sensors detect the total voltage of the converter DC side and the upper capacitor voltage respectively, and send them to DSP after the voltage signal conditioning circuit. Through reasonable control, the DC side voltage stability and the balance of the upper and lower capacitor voltages are adjusted. The startup, shutdown and protection module controls the startup and shutdown of the device according to a certain timing, and provides the device with overcurrent, overvoltage, overheating, phase loss and other fault protection functions.

[page]

3 Software Design System

The full digital control of the system has high requirements for real-time performance and must also consider control accuracy. These two points are related to the performance of the entire system. Therefore, shortening the program running time and ensuring calculation accuracy are the starting points of system software design.

The system operates in one sampling cycle. In each operation cycle, data sampling must be completed, instantaneous harmonic and reactive current component values ​​must be calculated, and 6 PWM signals must be generated to control the switch states of 6 IGBT tubes. These steps should be completed in one operation cycle, otherwise real-time performance is difficult to guarantee. The system software mainly includes the main program, A/D conversion subroutine, harmonic and reactive current calculation subroutine, PWM signal output subroutine, serial communication subroutine, etc.

The system software composition block diagram is shown in Figure 3.

4 Experimental results and analysis

In order to verify the effectiveness of the above harmonic detection and control scheme and whether the DSP-based parallel active power filter can compensate harmonics and reactive current well, this paper conducted an experiment. A resistive load was used as the load of the three-phase uncontrolled bridge rectifier. In the experiment, a 2 Ω resistor was connected to the load side. The experimental waveform is given below using phase A as an example. Figure 4 shows the load current waveform before and after compensation and the spectrum of the load current before and after compensation.

As can be seen from Figure 4, the A-phase power supply current waveform is seriously distorted and is a peak wave when the APF is not added. After adding this experimental device, the waveform of the power supply current has been significantly improved and is very close to a sine wave. At the same time, it can be seen from the spectrum that the power supply current distortion rate is very small after compensation, and the power supply current presents a relatively standard sine wave, that is, the harmonics and reactive components in the grid current are well compensated, effectively suppressing harmonics and compensating reactive components. This shows that the APF test device is effective and verifies the correctness of the algorithm and main circuit design proposed in this paper.

When the load current contains high-order harmonics and reactive current, the total harmonic distortion rate of the load current THD=63.86%, and the content of each harmonic current in the load current is shown in Table 1; the total harmonic distortion rate of the power supply current after compensation THD=5.35%, and the content of each harmonic current in the power supply current is shown in Table 2.

5 Conclusion

This paper takes the parallel active power filter as the research object, and makes a relatively systematic study on its topological structure, compensation component detection algorithm, control strategy and other issues. On this basis, a design of a parallel active power filter based on DSP is introduced. The simulation experiment shows that the designed active filter has good harmonic compensation characteristics and adaptive compensation capabilities.