Research on harmonic control algorithm based on three-level SVPWM

Abstract: Aiming at the existing problems of harmonic control algorithm of active power filter (APF), a harmonic control algorithm based on three-level voltage space vector pulse width modulation (SVPWM) technology is proposed. The control algorithm has low power switching frequency, low output harmonic voltage content, strong anti-electromagnetic interference ability and good real-time effect. Matlab simulation study of the control algorithm was carried out, and its correctness was successfully verified in APF through experiments. The experimental results show that the control algorithm has the advantages of low harmonic voltage content, high voltage utilization, good compensation performance, etc., and at the same time proves the effectiveness and feasibility of the control algorithm.

Keywords: active power filter; three-level; space vector pulse width modulation; harmonic control

1 Introduction

Harmonic control algorithm is one of the most important parts of APF, which directly affects the compensation effect of APF. Among the three-level inverter PWM control methods, SVPWM is widely used due to its advantages such as easy digital implementation, high voltage utilization, less output harmonic components, and low pulsating torque. The topological structure characteristics of three-level SVPWM enable switching devices with the same withstand voltage level to be applied to large-capacity systems of medium and high voltage, thereby increasing the equivalent switching frequency of the device; the output of the three-level inverter is a three-level step wave, which is closer to a sine wave, reducing the harmonic content of the output voltage and output current.

Based on the in-depth analysis of the two-level topology, the harmonic control function of the three-level SVPWM in the APF is proposed. The principle of the harmonic control algorithm is introduced in detail, and the algorithm is

Matlab/Simulink simulation verification was used to complete the writing of control software using TMS320F2812 . A digital APF control system based on TMS320F2812+CPLD was designed, which effectively verified the correctness of the harmonic control algorithm and improved the real-time compensation effect of APF.

2 Working principle of three-level SVPWM

2.1 Topology of two-level SVPWM

The process of generating the two-level SVPWM compensation signal is as follows: perform α, β coordinate transformation according to the three-phase harmonic detected by the front end, perform sector judgment under α, β coordinates, calculate the action time Tx, Ty on the adjacent basic vector corresponding to the reference voltage vector Ur, and then determine the space vector switching points Tcm1, Tcm2, Tcm3, so that the waveform of the harmonic compensation signal is generated by turning on the corresponding switching devices through the space vector switching points. The main topology structure diagram of the two-level inverter is shown in Figure 1.

At the same time, the driving signals of the upper and lower switching devices of each bridge arm of the three-phase inverter are complementary. Assuming Sk=1 means that the upper bridge arm is turned on, and Sk=0 means that the lower bridge arm is turned on, the output voltage of each bridge arm Uk=SkUd (k=a, b, c) can be obtained, and the voltage space vector is:

The space voltage vector sector division is shown in Figure 2. Taking the first sector as an example, then:

Where: T is the sampling period.

Transform Tx, Ty into the stationary α, β two-phase coordinate system by 3/2 coordinate transformation, then:

This is explained and verified in detail in the literature.

2.2 Basic principle of three-level SVPWM

The three-level inverter is composed of two switching devices connected in series with a pair of neutral point clamping diodes. Its circuit topology is shown in Figure 3.

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Among them, the four main switch tubes of each phase bridge arm have three different on-off combinations, that is, 1 represents +Ud/2, 0 represents 0, and -1 represents -Ud/2. The 27 switch combination states correspond to 19 basic space voltage vectors, and the amplitudes 2Ud/3, ·Ud/3, Ud/3 and 0 correspond to large vectors, medium vectors, small vectors and zero vectors respectively. According to the two-level vector construction principle, the three-level vector diagram can be obtained as shown in Figure 4.

When Uβ>0 and , Ur is in the Ith sector (0<θ<π／3). Taking this sector as an example, it contains large vectors U13= and U0, medium vector U7= , small vector U1= , and zero vector U0. Connect the vertices of U1 and U2, and the large sector is divided into 4 regions A, B, C, and D. Assume that Ur is located in region A, and U0, U1, and U2 correspond to action times T0, T1, and T2 respectively. Ur is approximated by the time linear combination of U0, U1, and U2, and we can get:

U0T0+U1T1+U2T2=UrTs, T0+T1+T2=Ts (4)

Where: Ts is the modulation pulse period.

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Similarly, the vector action time in other small triangles can be calculated. When calculating the vector action time in the other five sectors, just replace the θ value in the above formula with θ-60°, θ-120°, θ-180°, θ-240° and θ-300° respectively.

3 Control Algorithm Simulation

3.1 Simulation of two-level SVPWM

Take three-phase a, b, c as the power supply containing only standard sine waves, and observe the modulation waveform of the space vector switching point Tcm1 through the simulation model. The phases of Tcm2 and Tcm3 differ by 120° and 240° respectively, and the waveform is the same as Tcm1. The waveform of Tcm1 is shown in Figure 5. It can be seen from the figure that the modulation waveform obtained by simulation meets the principle of opposite phase with the input waveform.

3.2 Simulation of three-level SVPWM

The three-level simulation input also uses a standard sine wave. The entire system mainly includes the judgment of each area, the judgment of the small triangle, the calculation of the vector synthesis time, the distribution of the trigger pulse, etc. The line voltage uab and line current iab waveforms of the inverter output are shown in Figure 6.

4 Software Design

According to the above three-level SVPWM basic algorithm principle, in the developed digital control system based on DSP+CPLD, due to the high integration of DSP chip, it is convenient for harmonic sampling and control calculation; and CPLD has fast speed and many I/O ports. The CPLD development platform adopts MAX+PLUS II platform, and the program is developed based on Verilog HDL hardware description language. The combination of the two can ensure the real-time synchronization of multiple groups of trigger pulses. In the control system, DSP performs sampling and control calculation of harmonic signals, and CPLD completes the expansion of ports and receives the time and PWM wave of DSP operation. The software algorithm flow is shown in Figure 7.

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5 Experiments

Here, an APF device with DSP ( TMS320F2812 ) + CPLD (EPM3256ATC144) as a digital control system is developed and designed , and the feasibility and practicality of the control algorithm are verified. In this device, the high-speed digital signal processing chip TMS320F2812 is the core main control device, the maximum voltage that can be charged on the DC side of the main circuit is 1 kV, and the inverter bridge switch device is IGBT, which is driven by the power module M57962L. Since the transformers in the power grid are mostly connected in a angular shape, 3N (N=1, 2, 3...) harmonics can be filtered out, so the experimental harmonic current uses the superposition of the 5th harmonic signal with the fundamental signal. In the experiment, the harmonic current is generated by a self-made harmonic source, and then the APF device is incorporated into the harmonic source system to form a laboratory simulated harmonic generation and compensation device. Set the output harmonic to 5 times, with a content of 20%, press the start switch, the harmonic power supply starts, and the output phase voltage is measured with a multimeter. The harmonic control algorithm based on three-level SVPWM is downloaded to DSP, and the grid waveforms before and after compensation are shown in Figure 8. It can be seen that the compensation effect of the control algorithm is obvious, which further proves the correctness and feasibility of the algorithm.

6 Conclusion

Aiming at the problems existing in the commonly used harmonic control algorithms of active power filters, a harmonic control algorithm based on three-level space vector pulse width modulation technology is proposed. The frequency of power switching of this control algorithm is low and the real-time effect is good. The control algorithm is simulated and experimentally studied to verify the correctness of the control algorithm and show that it has good practical application value.