Study on Series-Parallel Compensation UPS Series Converter

　　The development of modern industry has higher and higher requirements for power quality. How to provide safe and reliable green power for power users is a hot topic in the current power supply field. As an uninterruptible power supply device, UPS is one of the important measures to improve power quality and an important guarantee for the normal operation of key equipment. At present, the structure of UPS includes several types such as backup type, online type, three-port online interactive type and dual converter series-parallel compensation type. Among them, the dual converter series-parallel compensation type can not only compensate for the reactive current and harmonic current in nonlinear loads, but also compensate for the harmonic and fundamental deviation of the power supply voltage. It has comprehensive power quality regulation capabilities and is a new type of UPS that has only recently appeared. At present, foreign APC companies have such physical products, and China is still in the theoretical research stage.

　　This paper introduces the working principle of the dual-converter series-parallel compensation type. On this basis, the control method of the series-parallel compensation UPS series converter is discussed, and the working characteristics of the system are verified by simulation. The results show the correctness of the proposed control strategy. The series converter part of the series-parallel compensation UPS studied can always ensure the total distortion rate of the grid input current is about 3%, and the input power factor is close to 1. The system performance confirms its effectiveness in improving the power quality on the grid side.

　　1 Working principle of dual converter series-parallel compensation UPS

　　Figure 1 shows the principle of a dual-converter series-parallel compensation UPS. In the figure, converters I and II are both bidirectional SPWM AC/DC converters, with the DC side connected to the battery. Converter I outputs a voltage △v (current Is) in series between the power supply voltage, vs, and the load voltage vL via inductor L1, capacitor C1 and transformer Ts. This is called a series compensation converter. The compensation voltage it outputs consists of two parts: △ v = △v 1 + △ vh, Uh is the harmonic compensation voltage, which is equal to the harmonic voltage vsh in the AC power supply, △ vh =vsh, but in opposite direction; △ v1 is the fundamental voltage compensation amount, which compensates for the deviation between the fundamental component of the power supply voltage vsl and the rated value of the load voltage vR. Therefore, the compensation voltage △ v provided by converter I not only offsets the harmonic vsh in the power supply voltage vs, but also compensates for the fundamental voltage vsl, making the load voltage vL a sinusoidal rated voltage in phase with the fundamental voltage vs of the power supply.

　　Figure 1 Dual-converter series-parallel compensation UPS

　　After being filtered by L2 and C2, the converter II is connected to both ends of the load or connected to the load through the output transformer Tp, which is called a parallel compensation converter. If the load is a nonlinear load, the load current iL consists of three parts: fundamental active current iLP, fundamental reactive current iLQ and harmonic current iLh. Real-time and appropriate control of converter II can make it output the voltage to the load as the sinusoidal rated voltage vR, and output the current i3 = i LQ + iLh + (iLP-is) to the load, where iLQ and iLh compensate for the reactive and harmonic currents of the load, so that the power supply only outputs the fundamental active current is to the load, and the active current iLP of the load is provided by the AC power supply (is) and converter II (i2d). The active current i2d = iLP-is output by converter II to the load. When there is a nonlinear load, the power supply voltage is higher or lower than the rated value vR and contains harmonic voltage, the system can compensate the load voltage vL to the rated sinusoidal voltage vR in phase with the power supply voltage through the joint action of this series-parallel compensation converter. At the same time, the AC power supply only inputs the fundamental active current is and the power factor is 1.

　　From the above analysis, it can be seen that this UPS overcomes the disadvantage of low input power factor caused by the input rectification part of the traditional double conversion online UPS. Usually, the fundamental voltage of the power supply deviates from the rated value by less than ± 15%, so the converter I only compensates for the rated voltage of v% ≤ 15V, and its capacity is only about 20% of the system capacity. Under normal circumstances, the mains and the double converters jointly supply power to the load, and the maximum power intensity of the two converters is only 20% of the load power. Compared with the traditional double conversion online UPS that always works at 100% load power, it not only has high overall efficiency, low power device loss, long life, and high reliability, but also has sufficient power margin to cope with special loads (impact load, instantaneous overload, etc.), so the output capacity is greatly enhanced, and the cost of products with the same capacity is also reduced.

　　2 Control of series converter under ideal grid voltage

　　When the grid voltage is constant, the rapid and effective control of the input current can control the speed and size of energy flow. At this time, the series converter can actually ignore the influence of the 0 axis and be regarded as a three-phase three-wire PWM rectifier. The ideal converter input current control effect can be obtained by using the dq axis cross decoupling control technology. Ignoring the power consumption of the converter, reactor and capacitor in Figure 1, if the battery is neither charged nor discharged, the active power Psdc of the power input should be equal to the active power PLdc of the load. When the series converter is made into a sinusoidal fundamental current source I s and cosθ = 1. 0, the active power P sdc is:

　　In the formula, Is is the current in phase with the fundamental voltage VS1 of the power supply, and this current should be selected as the power supply command current I*s:

　　The load current, load voltage and power supply voltage of the three-phase A, B, C system are detected, and the DC components V Ld, VLq, I Ld, I Lq, VSd, and VSq corresponding to the fundamental wave are obtained after coordinate transformation and low-pass filtering LPF. According to formula (2), I *s is calculated and this value is used as the current control instruction of the series converter. The output current Is on the sinusoidal current source converter is PWM controlled so that the power supply current Is tracks I *s. The control function of the series converter as a sinusoidal current source on the power supply current and the compensation (isolation) function of the power supply harmonic voltage can be realized, as shown in Figure 2.

　　Fig.2 Control block diagram of series converter under ideal power gridFig.3 　　Simulation waveform under ideal voltage

　　The system simulation parameters are as follows: AC grid input voltage rated amplitude VR = 100 V, frequency f = 50 Hz; load rated voltage amplitude VL = 100 V, load rated capacity 500 VA, cosθ = 0.8; three-phase combined series transformer rated capacity 500 VA, turn ratio N1 / N2 = 1/1.5; series converter input inductance L = 4 mH, inductance resistance R = 0.1Ω, output filter capacitor C1 = 1μF; parallel converter output filter inductance L = 1 mH, inductance resistance R = 0.1Ω; output filter capacitor C1 = 90μF. Battery pack E = 86 V, internal resistance R = 0.1Ω, DC bus capacitor Cdc = 6 800μF; converter switching frequency f = 9 kHz. The simulation waveforms are shown in Figures 3, 4, and 5.

　　Figure 3 Pure resistive load

　　Figure 4: Resistive-inductive load

　　Figure 5 Rectifier load

　　It can be seen from the simulation waveform that under the three-phase symmetrical ideal power grid, the control effect of the series converter is very good, the three-phase power grid input current is a balanced sinusoidal current, the ripple of the DC bus voltage is very small, and there is almost no second harmonic AC component fluctuation; the distortion rate of the power grid current is about 3%.　　4 Control strategy and simulation waveform under non-ideal voltage

　　When the input voltage of the series converter is asymmetric, if the PWM switching function contains harmonics, it will affect the generation of undesirable harmonics in the DC voltage, especially the 2nd harmonic, which makes the DC output voltage ripple serious. In turn, it also affects the input voltage of the bridge end of the series converter, causing the input voltage of the bridge end to contain 3rd, 5th, 9th harmonics, thereby increasing the total harmonic distortion rate of the input current.

　　The impact of the converter input voltage being three-phase symmetrical and containing a certain k-th harmonic is: the DC output voltage contains (k-1) and (k+1) harmonics, and thus the converter input current contains the k-th harmonic, that is, the harmonics of the input voltage are completely transmitted to the three-phase input current, thereby increasing the total distortion rate of the input current and increasing the difficulty of controlling the sinusoidal nature of the input current.

　　In Figure 6, the input current of the three-phase grid is seriously unbalanced, the current of phase B obviously exceeds the current amplitude of the other two phases, and the grid current of phases A and C has obvious phase shift with the input grid voltage, the input power factor is not completely 1, and the DC bus voltage obviously has fluctuations of the 2nd harmonic AC component. In Figure 7, the input current of the three-phase grid remains balanced, but the harmonic component of the input voltage greatly affects the sinusoidality of the input current, the 5th harmonic content is serious, and the total distortion rate is large; the DC bus voltage also fluctuates greatly, especially the 4th harmonic component.

　　Figure 6 Simulation waveform under unbalanced input grid voltage

　　Figure 7 Simulation waveform when the input grid voltage contains harmonics

　　The simulation waveforms in the previous article show that the results of applying the voltage and current double closed-loop control strategy under the dq axis decoupling control under the ideal power grid to the non-ideal power grid are not ideal. Therefore, for the non-ideal power grid, a control strategy that is more suitable for its special properties is sought, which is the dq+0 axis control with power supply voltage harmonic feedforward, so that the output voltage of the converter bridge terminal contains harmonic components of the same magnitude, and there is no harmonic current in the AC input current. Figure 8 is a block diagram of the dq+0 axis control system with power supply voltage harmonic feedforward, and Figure 9 is its simulation waveform.

　　Figure 8 Block diagram of dq+0 axis control system with power supply voltage harmonic feedforward

　　Figure 9. Simulation waveform of dq+0 axis control with harmonic feedforward and Isa harmonic analysis

　　It can be seen from the simulation waveform that the waveform and imbalance of the three-phase grid input current are well controlled, the total harmonic distortion rate of I sa is 1.74%, and the waveform distortion has been greatly improved; the DC voltage is stable, the ripple is small, and the harmonics on the DC side are also reduced.

　　5 Conclusion

　　The simulation results show that the dq+0 axis control method with power supply voltage harmonic feedforward can achieve excellent control effect. The series converter is controlled as a fundamental sinusoidal current source, the power supply current Is is a sinusoidal active current in phase with the power supply fundamental voltage, and the deviation between the harmonics and the fundamental in the power supply voltage is compensated (or isolated) by the series converter.