Zian Qin

A Dual Voltage Control Strategy for Single-Phase PWM Converters With Power Decoupling Function

The inherent double line ripple power in singlephase systems is adverse to the converter performance, e.g. limited lifetime due to the requirement of large electrolytic capacitors and low voltage control bandwidth due to harmonic disturbance. In this paper, an active converter topology based on a symmetrical half bridge circuit is proposed to decouple the ripple power so that balanced instantaneous power flow is assured between source and load, and the required dc-link capacitance can be dramatically reduced. For proper closed-loop regulation, the small signal modeling of the proposed system is presented, and a dual voltage control strategy is then proposed, which comprises one voltage loop implemented in the synchronous reference frame for active power balancing, and another one implemented in the harmonic reference frame for ripple power compensation. Special attention is given to the bandwidth of voltage control loop because the variation of dc-link voltage should be kept within an acceptable range during load transients. This is particularly important for systems with reduced dc-link capacitance because they are of lower energy capacity and very sensitive to step load changes. Simulation results and experimental results are presented to show the effectiveness of the proposed circuit and control algorithm.

Benchmark of AC and DC Active Power Decoupling Circuits for Second-Order Harmonic Mitigation in Kilowatt-Scale Single-Phase Inverters

This paper presents the benchmark study of ac and dc active power decoupling (APD) circuits for the second-order harmonic mitigation in kilowatt-scale single-phase inverters. First, a brief comparison of recently reported APD circuits is given, the best solution that can achieve high efficiency and high-power density is identified and comprehensively studied, and the commercially available film capacitors, the circuit topologies, and the control strategies adopted for APD are all considered. Then, an adaptive decoupling voltage control method is proposed to further improve the performance of dc decoupling in terms of efficiency and reliability. The feasibility and superiority of the identified solution for APD together with the proposed adaptive decoupling voltage control method are finally verified by both the simulation and experimental results obtained on a 2-kW single-phase inverter.

Application Criteria for Nine-Switch Power Conversion Systems with Improved Thermal Performance

Nine-switch converter is a reduced-switch equivalence of the widely used 12-switch back-to-back converter. Like other reduced-switch topologies, the nine-switch converter experiences operating constraints, which may limit its efficiency. This is, however, not universal, meaning losses of the nine-switch converter can at times be lower than the 12-switch back-to-back converter. The scope of this paper is, thus, to identify application criteria for the nine-switch converter, which upon met, will lead to lower conduction and hence total losses. Loss distribution among switches of the nine-switch converter is also investigated, since it can greatly influence reliability of the overall converter. Experimental results obtained have promptly justified the analytical reasoning formulated.

An Improved Second-Order Generalized Integrator Based Quadrature Signal Generator

The second-order generalized integrator based quadrature signal generator (SOGI-QSG) is able to produce in-quadrature signals for many applications, such as frequency estimation, grid synchronization, and harmonic extraction. However, the SOGI-QSG is sensitive to input dc and harmonic components with unknown frequencies (e.g., interharmonics). To overcome the drawback, this letter begins by analyzing the dynamic response of SOGI-QSG from the first-order system (FOS) perspective. A second-order SOGI-QSG (SO-SOGI-QSG) with a fourth-order transfer function is then proposed, after referring to the relationship between standard FOS and second-order system. The proposed method is subsequently found to inherit the simplicity of the SOGI-QSG, while demonstrates better disturbance attenuation. Its parameter design procedure is also easy to understand, and can be followed step-by-step without difficulty. Performance of the proposed SO-SOGI-QSG is finally validated by experimental results presented in this letter.

A dual voltage control strategy for single-phase PWM converters with power decoupling function

The inherent double line ripple power in single-phase systems is adverse to the performance of power electronics converters, e.g., limited lifetime due to the requirement of large electrolytic capacitors and low voltage control bandwidth due to harmonic disturbance. In this paper, an active converter topology based on a symmetrical half-bridge circuit is proposed to decouple the ripple power so that balanced instantaneous power flow is assured between source and load, and the required dc-link capacitance can be dramatically reduced. For proper closed-loop regulation, the small signal modeling of the proposed system is presented, and a dual voltage control strategy is then proposed, which comprises one voltage loop implemented in the synchronous reference frame for active power balancing, and another one implemented in the stationary reference frame for ripple power compensation. Special attention is given to the bandwidth of voltage control loops because the variation of dc-link voltage should be kept within an acceptable range during load transients. This is particularly important for systems with reduced dc-link capacitance because they are of lower energy capacity, and the dc-link voltage is therefore very sensitive to step load changes. Comprehensive simulation and experimental results are presented to show the effectiveness of the proposed circuit and control algorithm.

Investigation into the Control Methods to Reduce the DC-Link Capacitor Ripple Current in a Back-to-Back Converter

Three-phase back-to-back converters have a wide range of applications (e.g. wind turbines) in which the reliability and cost-effectiveness are of great concern. Among other components and interconnections, DC-link capacitors are one of the weak links influenced by environmental stresses (e.g. ambient temperature, humidity, etc.) and operating stresses (e.g. voltage, ripple current). This paper serves to investigate the ways of reducing ripple current stresses of DC-link capacitors in back-to-back converters. The outcome could benefit to achieve either an extended lifetime for a designed DC-link or a reduced DC-link size for fulfilling a specified lifetime target. The proposed control strategies have been demonstrated on a study case of a 1.5 kW converter prototype. The experimental verifications are in well agreement with the theoretical analyses.

Evaluation of switch currents in nine-switch energy conversion systems

Converters with reduced switch counts usually face some performance tradeoffs, which make them suitable for some applications but not others. The same applies to the nine-switch converter, which is a reduced-switch version of the back-to-back twelve-switch converter. The nine-switch converter has since been shown to experience a higher voltage stress, which can be lowered in some cases. A corresponding evaluation of its current stress is however lacking, and is hence addressed now by computing its switch currents when used for ac-ac, ac-dc, dc-ac and dc-dc energy conversions. Relevant expressions, application requirements and simulation results are presented for identifying cases, where the nine-switch converter can have an improvement in performance despite its reduced switch count.

A Rotating Speed Controller Design Method for Power Leveling by Means of Inertia Energy in Wind Power Systems

Power fluctuation caused by wind speed variations may be harmful for the stability of the power system as well as the reliability of the wind power converter, since it may induce thermal excursions in the solder joints of the power modules. Using the wind turbine rotor inertia energy for power leveling has been studied before, but no quantified analysis or generic design method have been found. In this paper, the transfer functions from the wind speed to electrical power, electromagnetic torque, and rotating speed are built based on which the rotating speed controller is designed in the frequency domain for power leveling. Moreover, the impact of other parameters on power leveling, including the time constant of maximum power point tracking (MPPT) and the rotor inertia, are also studied. With the proposed optimal design, the power fluctuations are mitigated as much as possible, while the stability of the rotating speed is still guaranteed. Moreover, the oscillation of the electromagnetic torque is also reduced, and the performance of the MPPT is only weakened slightly.

Energy storage system by means of improved thermal performance of a 3 MW grid side wind power converter

Wind speed variations make the power of wind turbine system to fluctuate, which could increase the thermal stress of the power converter and reduce its lifetime. In order to relieve this problem, short-term energy storage technologies are applied to improve the thermal performance of a 3 MW grid side wind power converter. The cost, weight and cycle life of the energy storage technologies are evaluated based on a typical low speed high turbulence wind profile. In detail, a wind turbine system model is established and its control strategy is illustrated, which is followed by the power control method of the energy storage system. Then the conventional thermal evaluation approach is simplified for evaluation with long term wind profile. The case studies are done to address the optimal power size and capacity of the energy storage system by comparing the improvement of the thermal performance. Also, the two promising candidates, ultracapacitors and batteries, are compared.

Modulation Schemes With Enhanced Switch Thermal Distribution for Single-Phase AC–DC–AC Reduced-Switch Converters

Modulation schemes can significantly change the performance of a converter in the steady state. They have, thus, been extensively studied, but directed more at three-phase topologies, where triplen offsets are added. Single-phase systems, including ac-dc-ac, are less widely pursued, even though various topologies have since been proposed. The intention of this paper is, thus, to study two promising single-phase ac-dc-ac converters using reduced number of switches, and subsequently proposes modulation schemes for them. The proposed schemes help the converters to achieve better thermal spread among their switches, while keeping their dc-link voltages low. Single points of failure are thus minimized, allowing the converters to have longer lifetimes. Simulation and experimental results have demonstrated the intended performances, hence verifying the analyses presented in this paper.