Based on instantaneous reactive power theory and three-phase full-bridge Delta inverter

0 Introduction

Improving power quality and saving energy are major issues that need to be solved in today's power system. In recent years, a new technology for power electronics applications has emerged, namely, Delta inverter technology. This new technology is the basis of series, parallel and series-parallel structure compensation circuits that are being widely studied internationally. It can effectively improve power quality and has attracted increasing attention from the industry.

The improved method of the detection principle of the instantaneous reactive power theory (first proposed by Japanese scholar Yasufumi Akagi in 1983) is the dq method based on synchronous rotation Park transformation, which is the main method for real-time calculation of harmonics. The characteristics of this method are that it not only simplifies the current increment detection under symmetrical and distortion-free conditions, but is also suitable for asymmetrical and distorted mains voltage detection. Therefore, this article studies the use of the dq method to realize the three-phase half-bridge Delta inverter AC purification regulated power supply, and gives simulation and experimental results and analysis.

1 Delta Inverter and Instantaneous Reactive Power Theory

Delta inverter is a special way and special purpose inverter that uses voltage increment (voltage fluctuation value ±Vu and harmonic component uh) or current increment (reactive current iq and harmonic current ih) as modulation wave or reference signal to control high switching frequency PWM inverter. Its function is to compensate for voltage increment or current increment. Delta inverter works in high switching frequency linear PWM state, and can reproduce the value and waveform of voltage or current increment in proportion without distortion; it can meet the requirements of positive and negative compensation for the AC voltage fluctuation value ±Vu; it is mainly used in AC purification stabilizer, reactive compensation and power active filter, power quality comprehensive compensator and series-parallel compensation online UPS and other fields.

The three-phase instantaneous reactive power theory was first proposed by Japanese scholar Yasufumi Akagi in 1983, and has been gradually improved through continuous research. It now includes the pq method, ip-iq method and dq method. The pq method was first applied and is only applicable to symmetrical three-phase and non-distorted mains power grids; the ip-iq method is not only effective for power supply voltage distortion, but also suitable for the detection of asymmetrical three-phase mains power grids; the dq method based on synchronous rotating Park transformation not only simplifies the current increment detection under symmetrical and non-distorted conditions, but is also applicable to the detection of asymmetrical and distorted mains power grids.

The dq method is currently the main method for real-time harmonic calculation. The characteristics of this method are that it not only simplifies the current increment detection under symmetrical and distortion-free conditions, but is also applicable to the detection of asymmetrical and distorted mains voltage. Its basic principle is shown in Figure 1.

The instantaneous three-phase current or voltage is transformed to the dq coordinates through the following transformation:

In the above formula, is the d-axis current DC component, corresponding to the active power of the load; is the q-axis current DC component, corresponding to the reactive power of the load fundamental phase shift; the d-axis AC component and the 0-axis component i0 correspond to the load fundamental asymmetry and higher harmonic reactive power.

In Figure 1, ia, ib, and iv are the three-phase input currents, iaf, ibf, and icf are the calculated fundamental output currents, and their differences are the three-phase harmonic currents iah, ibh, and ich.

The dq transformation is to transform the phasor in the stationary coordinate system into a coordinate system rotating at the fundamental angular velocity. The frequency of the transformed signal differs from the original signal by a fundamental frequency, i.e. 50Hz. If the signal is a typical three-phase characteristic harmonic 1th (fundamental), 5th, 7th, etc., they correspond to DC, 4th, 6th, etc. in the dq coordinate system. After the low-pass filter removes all AC harmonics, the DC component can be used to obtain the fundamental current through the dq inverse transformation (CT).

2 Circuit structure

The circuit is shown in Figure 2. It consists of two parts: the main circuit and the detection and control circuit. The main circuit is divided into two parts: a three-phase switching rectifier and a Delta inverter. The main function of the switching rectifier is to provide a rectified DC power supply for the Delta inverter and keep the voltage Ud on the DC capacitor Cd constant. The DC capacitor plays a filtering and energy storage role. There are two purposes for using a switching rectifier. One is to keep the mains input power factor COSφ=1 and make the input current waveform close to a sine wave to reduce the pollution to the mains; the other is to make the electric energy flow in both directions. The Delta inverter part consists of three single-phase Delta full-bridge inverters and their output transformers, and its function is to compensate for the fluctuation value Vu, harmonic component uh and three-phase asymmetry of the mains voltage. The Delta inverter part uses three single-phase Delta inverters and their output transformers for two reasons: first, the load carried by the three-phase four-wire mains system is asymmetric in most cases, and must be compensated independently by unconnected single-phase Delta inverters; second, it can also improve the reliability of the three-phase four-wire power supply. In case one phase fails, the other two phases can continue to supply power. The three single-phase Delta inverters can all work in two-way four-quadrants to meet the positive and negative compensation of the mains voltage fluctuation value ±Vu.

The three-phase reference sinusoidal voltages uar, ubr, and ucr in Figure 2 are transformed by dq to obtain and u0r. The harmonic components are filtered out by a low-pass filter (as shown in Figure 3), and the active and reactive components corresponding to the fundamental wave in the dq coordinate system, and the zero-sequence component u0r, and the mains voltages ua, ub, and uc are also transformed by dq . After subtraction and addition operations on these two groups of signals, it can be obtained ; ;. Among them. reflects the fluctuation value of the fundamental voltage, reflects the harmonic component, and 2u0r-u0 reflects the asymmetry of the three-phase voltage. Therefore, udf, uqf, and u0f are transformed by dq inverse to obtain the modulated wave voltage that reflects the fluctuation change of the load voltage fundamental wave, the harmonic component, and the asymmetry of the three-phase voltage. Then, the corresponding SPWM control circuit is used for control to make the voltage on the load a stable and pure sinusoidal voltage.

Figure 3. Third-order Chebyshev analog low-pass filter

The low-pass filter in Figure 3 generally uses a third-order Chebyshev analog low-pass filter. According to the parameters given in the figure, its cut-off frequency is 22Hz and the error is less than 2.5%. The transfer function of the filter is:

The simulation waveforms are shown in the figure. Figure 4 shows that the amplitude of the three-phase voltage is unbalanced before compensation, and the amplitude of the three-phase voltage is basically balanced after compensation. Figure 5 shows that the three-phase voltage is unbalanced and contains harmonics before compensation, and the three-phase voltage is basically balanced after compensation, and the harmonic content is greatly reduced.

3 Conclusion

Today's power grid conditions are very complex and harsh. The complex load properties in the power grid cause the power grid to have low power factor, waveform distortion, surge, phase loss and other unfavorable conditions. Research and simulation results show that when the dq detection method and PWM control method in the transient reactive power theory are used, when the mains voltage is unbalanced and contains harmonics, after compensation by the transient voltage compensator, the three-phase voltage on the load can be balanced and become a stable and pure sine wave voltage.