Showing papers by "Yushan Liu published in 2017"

Front-End Isolated Quasi-Z-Source DC–DC Converter Modules in Series for High-Power Photovoltaic Systems—Part I: Configuration, Operation, and Evaluation

Abstract: A quasi-Z-source modular cascaded converter (qZS-MCC) is proposed for dc integration of high-power photovoltaic (PV) systems. The qZS-MCC comprises series-connected front-end isolated qZS half-bridge (HB) dc–dc converter submodules (SMs). With the front-end isolation, the qZS-MCC achieves high-voltage dc capability, while maintaining modularity and PV panel grounded. The post-stage qZS-HB handles the PV voltage and power flows, dc-link voltage balance, and output-series power integration. Whereas, the front-end isolation converters of all SMs perform a constant duty cycle, lowing the control complexity. There is no double-line-frequency power flowing through the dc-side PV panels, qZS inductors, and qZS capacitors in the qZS-MCC, so small qZS impedance is possible compared to the existing qZS cascaded multilevel inverter. The configuration, operating principle, power loss evaluation, and passive components design of the proposed system are investigated in this part of the paper. The system control, modeling, and corresponding verifications are stated in Part II of this paper.

Front-End Isolated Quasi-Z-Source DC–DC Converter Modules in Series for High-Power Photovoltaic Systems—Part II: Control, Dynamic Model, and Downscaled Verification

Abstract: This paper is the continuation of Part I in which a quasi-Z-source modular cascaded converter (qZS-MCC), comprising front-end isolated qZS half-bridge dc–dc converter submodules (SMs), for dc integration of high-power photovoltaic (PV) systems is proposed. The qZS-MCC-based PV system features modular structure, high-voltage dc collection of PV power, simple control with a unified and constant duty cycle for the front-end isolation converter of all SMs, and low qZS impedance due to no double-line-frequency pulsating power. Here, control scheme of the qZS-MCC PV system integrated into the dc collection grid is investigated. Dynamic model of the system is established for controllers design and time-domain transient simulation. Experimental tests are carried out on the downscaled prototype as a proof-of-concept of the proposed control and modeling, demonstrating the validity of the proposed approaches.

Investigation on pulse-width amplitude modulation-based single-phase quasi-Z-source photovoltaic inverter

Abstract: The pulse-width amplitude modulation (PWAM) method was proposed for the single-phase quasi-Z-source inverter (qZSI)-based photovoltaic (PV) power system to reduce quasi-Z-source (qZS) impedance values while improving efficiency. The method modified sinusoidal pulse-width modulation (SPWM) of the qZSI by combining pulse-amplitude modulation (PAM) and a varied dc-link voltage envelope was produced. The SPWM worked at low dc-link voltage, lowering voltage stress and avoiding shoot-through switching. The PAM worked at the varied dc-link voltage, reducing the number of switching events. As a result, the power dissipation decreased compared to working at the constant dc-link voltage. This study further investigates the PWAM-based single-phase qZS PV inverter system. An improved topology with control strategy is proposed for its grid-connected and standalone operation. Design method of impedance parameters is detailed. The performance in boost and buck operation is discussed when the single-phase qZSI using SPWM and PWAM. Simulation and experimental results verify outstanding features of the PWAM for single-phase qZSI, and the proposed approach for dual-mode operation of the PWAM-based single-phase qZS PV power system.

PWAM controlled quasi-Z source motor drive

Abstract: This paper proposes a novel pulse-width-amplitude-modulation (PWAM) method for three-phase quasi-Z source inverter system in motor drive application. It is demonstrated that it operates at only 1/3 switching frequency of traditional PWM methods, with less harmonic distortion. As a result, switching actions and losses are also reduced significantly. With the proposed modulation, the required capacitance is reduced greatly, which makes a system of smaller volume and lighter weight. Compared to traditional PWM methods, the higher efficiency and better reliability are confirmed in PWAM controlled motor drive system. The motor drive with the proposed hybrid PWAM modulation method presents good performance in simulation. Theoretical analysis is provided to verify the inverter efficiency and design improvements.

Model predictive control for permanent magnet synchronous motor drives considering cross-saturation effects

Abstract: This paper presents a control strategy for permanent magnet synchronous motors (PMSMs) based on finite-state model predictive control (FS-MPC). The predictive model is enhanced by incorporating saturation and cross-coupling effects on the dq-axis flux linkages of the motor. Although a surface PMSM is used, the maximum torque per ampere (MTPA) trajectory is found dependent on both dq-axis currents and affected by geometrical saliencies. The dependence of the dq-axis inductances is inserted into the control law of the developed MPC routine, thus improving the model accuracy. The performance of the direct predictive torque controller is tested by controlling a PMSM drive supplied by a neutral-point clamped inverter. Implementation results have proven that the proposed control scheme can effectively satisfy the requirements that arise from both the PMSM and the inverter.

Pulse width amplitude modulation based single-phase quasi-Z-source photovoltaic inverter with energy storage battery

Abstract: The pulse width amplitude modulation (PWAM) was earlier proposed for the single-phase quasi-Z source inverter (qZSI) based photovoltaic (PV) power system to reduce qZS impedance values while improving efficiency The method resulted in highly fluctuated qZS inductor currents and capacitor voltages as well, which makes it difficult to integrate energy storage battery as a conventional qZSI This paper proposes an improved topology with energy storage elements for the PWAM-based single-phase qZS PV inverter, maintaining constant battery current Also, control strategy is disclosed for grid-connected and islanding operation Simulation and experimental results verify the proposed approach for dual-mode operation of the PWAM-based single-phase qZS PV power system

A model predictive control for low-frequency ripple power elimination of active power filter integrated single-phase quasi-Z-source inverter

Abstract: A model predictive control (MPC) is proposed for a buck-type active power filter (APF) integrated single-phase quasi-Z-source inverter (qZSI) to directly eliminate the dc-side low-frequency ripple power. The buck-type APF operates independently from the main circuit power devices, comparing to the coupled-type APF. The proposed MPC compensates the low-frequency ripple power directly by the APF capacitor power and achieves fast responses during load power changes. There is no requirement to preset the capacitor voltage, thus to easily handle the non-linear loads. With the proposed solution, the APF is able to freely divert the dc-side low frequency (not only double line frequency) ripple power to the APF's capacitor in the form of highly fluctuated voltage and current, so as to decrease qZS inductance and capacitance. The buck-type APF is analyzed and the proposed method is illustrated. Simulation results demonstrate the validity of the proposed approach.

Overview of double-line-frequency power decoupling techniques for single-phase Z-Source/Quasi-Z-Source inverter

Abstract: The power decoupling strategies of single-phase Z-source/quasi-Z-source inverter (ZSI/qZSI) to handle the inherent double-line-frequency (2ω) power are overviewed. They are categorized as passive and active power decoupling methods. The former suppresses the 2ω ripple by the impedance network, modified modulation methods, or closed-loop damping control. The latter diverts the 2ω ripple to the compensation capacitor of integrated active power filters, through properly operating a half-bridge phase leg. The pros and cons of those 2ω power decoupling methods are discussed and recommendations are given for future development of such single-phase inverters.

Optimizing control strategy of quasi-Z source indirect matrix converter for induction motor drives

Abstract: This paper proposes a novel optimized control strategy of quasi-Z source indirect matrix converter (QZS-IMC) for induction motor drives. The QZS-IMC provides boost function to overcome the output voltage limit of the traditional matrix converter, and achieves a voltage gain over 0.866. This paper proposes an indirect field oriented control (IFOC) technique based closed loop speed control of an induction motor fed by a QZS-IMC. By selecting the best optimized shoot-through duty ratio (D), modulation index (m i ;) of the rectification stage and modulation index (mo) of the inversion stage, the solution ensures effective control and stable system. The simulation results are conducted to validate the proposed control strategy.

Deadbeat current control of qZS based grid-connected multilevel inverter

Abstract: This paper proposes a Current Deadbeat Control (DBC) algorithm for a three-phase 9-cells qZS based Cascaded H-Bridge (CHB) inverter to ensure grid current supply with low THD. The studied topology is considered as a single-stage DC/AC topology having the capability of boosting the DC input voltage with high-quality multilevel AC voltage, independent DC-link voltage compensation, and control of the power transfer with high reliability. Moreover, a Repetitive Controller (RC) is connected to the inductor current loop with the aim to reject periodic disturbances (double line-frequency fluctuations). The employed RC has the capability of learning through iterations based on tracking error. During the control, the compensation method uses the error between the DC-link peak voltage reference and the actual value to produce the compensating inductor current reference. Theoretical analysis and simulation results shows that the proposed solution achieves high dynamic performance and reduced cyclical disturbances effects.

Model predictive control for maximum power point tracking of quasi-Z-source inverter based grid-tied photovoltaic power system

Abstract: Stochastic dynamic behavior of solar energy necessitates the use of robust controllers for photovoltaic (PV) power electronics interfaces. Such robust controller maximizes the energy harvest through continuous operation using a maximum power point tracker (MPPT). A model predictive control MPPT (MPC-MPPT) is proposed in this paper for a quasi-Z-source inverter (qZSI) based grid-connected PV power system. MPC is a robust suboptimal controller and is proposed in this paper as an elegant, embedded controller. Such controller has shown better dynamic performance than the conventional perturb and observe (P&O) technique, particularly under rapidly changing meteorological conditions. The qZSI is a single-stage topology that can guarantee MPPT and control the injected power to the grid simultaneously. The proposed method simulation results are presented in this paper.