Wen-Long Ming

Ripple Eliminator to Smooth DC-Bus Voltage and Reduce the Total Capacitance Required

Bulky electrolytic capacitors, which are often needed in dc systems to filter out voltage ripples, considerably reduce power density and system reliability. In this paper, a ripple eliminator, which is a bidirectional buck-boost converter terminated with an auxiliary capacitor, is adopted to replace bulky capacitors in dc systems. The voltage ripples on the terminals (i.e., the dc bus) can be transferred to the auxiliary capacitor, and the ripples on the auxiliary capacitor can vary in a wide range. Moreover, the average voltage of the auxiliary capacitor can be controlled either lower or higher than the dc-bus voltage, which offers a wide operational range for the ripple eliminator and also the possibility of further reducing the auxiliary capacitance. Hence, the total capacitance required can be much smaller than the originally needed. After proposing a control strategy to transfer the voltage ripples to the auxiliary capacitor, three control strategies are proposed to regulate the auxiliary-capacitor voltage to maintain proper operation. Intensive experimental results are presented to demonstrate the performance.

Reduction of DC-bus voltage ripples and capacitors for single-phase PWM-controlled rectifiers

The problem of voltage/current ripples is a primary concern for DC systems, e.g. those with fuel cells and batteries. It could seriously deteriorate the system performance on both the source side and the load side. In this paper, a single-phase PWM-controlled rectifier is taken as an example to analyse the ripple energy that causes the voltage ripples on the DC bus. Moreover, a ripple-current compensator is proposed to absorb/inject ripple energy from/to the DC bus so that the voltage ripples are reduced actively. The compensator is a boost/buck converter with an auxiliary capacitor having a voltage higher than the DC-bus voltage. A repetitive controller is then proposed to compensate the ripple energy on the DC bus instantaneously, with a fixed switching frequency. Simulation results are presented to demonstrate that the proposed strategy is able to reduce the voltage ripples on the DC bus considerably. As a result, the DC-bus capacitor can be reduced significantly in order to achieve the same level of voltage ripples.

Stabilization of a Cascaded DC Converter System via Adding a Virtual Adaptive Parallel Impedance to the Input of the Load Converter

Connecting converters in cascade is a basic configuration of dc distributed power systems (DPS). The impedance interaction between individually designed converters may make the cascaded system become unstable. The previous presented stabilization approaches not only need to know the information of the regulated converter, but also have to know the characteristics of the other converters in the system, which are contradictory to the modularization characteristic of dc DPS. This letter proposes an adaptive-input-impedance-regulation (AIIR) method, which connects an adaptive virtual impedance in parallel with the input impedance of the load converter, to stabilize the cascaded system. This virtual impedance can adaptively regulate its characteristic for different source converters. Therefore, with the AIIR method, all the load converters can be designed to a fixed standard module to stably adapt various source converters. In addition, at any cases, the AIIR approach only changes the load converters input impedance in a very small frequency range to keep the load converters original dynamic performance. The requirements on the AIIR method are derived and the control strategies to achieve the AIIR method are proposed. Finally, considering the worst stability problem that often occurs at the system whose source converter is an LC filter, a load converter cascaded with two different LC input filters is fabricated and tested to validate the effectiveness of the proposed AIIR control method.

A Single-Phase Four-Switch Rectifier With Significantly Reduced Capacitance

A single-phase four-switch rectifier with considerably reduced capacitance is investigated in this paper. The rectifier consists of one conventional rectification leg and one neutral leg linked with two capacitors that split the dc bus. The ripple energy in the rectifier is diverted into the lower split capacitor so that the voltage across the upper split capacitor, designed to be the dc output voltage, has very small ripples. The voltage across the lower capacitor is designed to have large ripples on purpose so that the total capacitance needed is significantly reduced and highly reliable film capacitors, instead of electrolytic capacitors, can be used. At the same time, the rectification leg is controlled independently from the neutral leg to regulate the input current to achieve unity power factor and also to maintain the dc-bus voltage. Experimental results are presented to validate the performance of the proposed strategy.

Stabilization of Cascaded DC/DC Converters via Adaptive Series-Virtual-Impedance Control of the Load Converter

It has been shown recently that a cascaded dc/dc converter system can be stabilized via amplitude compensation (SAC) or phase compensation (SPC) for the input impedance of the load converter. In this letter, it is shown that the cascaded system when adopting the SAC is unconditionally stable but conditionally stable when adopting the SPC, that is, SAC is more stable than SPC. Then, the comparison is carried out for the parallel-virtual-impedance (PVI) and series-virtual-impedance (SVI) control strategies that are adopted to implement the SAC, and it is found that only the SVI control strategy can achieve the SAC for the whole load and input voltage range of the load converter without limitation. Therefore, SVI is in general better than PVI when realizing SAC. Following on this, an adaptive mechanism is introduced to improve the traditional SVI control strategy so that the load converter can be stably connected to different source converters such as LC input filters and traditional dc/dc converters. Finally, a load converter cascaded with three different source converters is fabricated to validate the effectiveness of the proposed adaptive SVI control strategy.

A Single-Phase Rectifier Having Two Independent Voltage Outputs With Reduced Fundamental Frequency Voltage Ripples

Half-bridge rectifiers are able to provide two voltage outputs, which offers three voltage levels, but the two voltage outputs depend on each other and also on system parameters. Moreover, the two voltage outputs contain large ripples because the currents following through the split capacitors contain significant fundamental-frequency components. In this paper, after analyzing the drawbacks of half-bridge rectifiers in detail, an independently controlled neutral leg is added to conventional half-bridge rectifiers to address these drawbacks. Furthermore, the associated decoupling control strategies are proposed. The rectification leg from the conventional half-bridge rectifier is controlled to maintain the dc-bus voltage and to draw a clean sinusoidal current that is in phase with the supply voltage. The neutral leg is controlled with a PI-repetitive controller to regulate one voltage output and also to provide the current path for any dc and/or fundamental-frequency components. As a result, the two voltage outputs are regulated independently and are robust against system parameters. The output voltage ripples are also reduced, and hence, the required capacitance to achieve the same level of voltage ripples is reduced. Experimental results are provided to validate the performance of the proposed single-phase rectifiers having two independent voltage outputs.

A Virtual RLC Damper to Stabilize DC/DC Converters Having an LC Input Filter while Improving the Filter Performance

The LC filter at the input of a dc/dc converter may cause instability when the converter is controlled as a constant power load (CPL) and one of the effective solutions is to reduce the output impedance of the LC input filter with different stabilization dampers. In this letter, the impact of these dampers on the LC filter is analyzed with two-port network analysis and it is found that the existing dampers all degrade the performance of the original LC input filter to some extent. In order to overcome this drawback, an RLC damper is proposed to stabilize the whole system while improving the performance of the LC input filter. In addition, this RLC damper is also designed to achieve high robustness against the parameter variations of the LC input filter. Furthermore, in order to avoid the power loss when implementing the damper physically, a control strategy for the CPL is proposed to implement the RLC damper as a virtual RLC (VRLC) damper. The actual effectiveness of the VRLC damper and its impact on the CPL are fully evaluated via two-port network analysis as well. Finally, experimental results from a 100-W 48-24-V buck converter with an LC input filter are presented to demonstrate the proposed VRLC damper.

PLL-Less Nonlinear Current-Limiting Controller for Single-Phase Grid-Tied Inverters: Design, Stability Analysis, and Operation Under Grid Faults

A nonlinear controller for single-phase grid-tied inverters, that can operate under both a normal and a faulty grid with guaranteed closed-loop stability, is proposed. The proposed controller acts independently from the system parameters, does not require a phase-locked loop, and can achieve the desired real power regulation and unity power factor operation. Based on nonlinear input-to-state stability theory, it is analytically proven that the inverter current always remains below a given value, even during transients, independently from grid variations or faults (short circuit or voltage sag). The desired performance and stability of the closed-loop system are rigorously proven since the controller has a structure that does not require any switches, additional limiters, or monitoring devices for its implementation. Therefore, nonlinear stability of a grid-tied inverter with a given current-limiting property is proven for both normal and faulty grid conditions. The effectiveness of the proposed approach is experimentally verified under different operating conditions of the grid.

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In this paper, a single-phase converter consisting of two legs with four switches, called the θ-converter, is proposed. It has a common ac and dc ground, which reduces common mode currents and removes the need for an isolation transformer, and two capacitors: one across the whole dc bus and the other across the output. The dc bus capacitor provides a direct path for the double-frequency ripple current inherently existing in single-phase converters to return continuously so the output capacitor can be sized very small, only to filter out switching ripples. Moreover, the dc bus capacitor is intentionally designed to store the system ripple energy with large voltage ripples, which reduces its capacitance. Hence, the total capacitance needed and the output voltage ripples are reduced at the same time. This makes it cost-effective to use highly reliable film capacitors instead of bulky and vulnerable electrolytic capacitors. Because of the removed isolation transformers and bulky electrolytic capacitors, the power density and system reliability are improved. In order to properly operate the converter, two independent controllers are designed for the two legs, respectively, to achieve the desired functions and other normal objectives, such as the unity power factor. Experimental results are presented to demonstrate the high performance of the proposed converter.

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In this paper, a transformerless PV inverter is proposed, which is formed by adding a neutral leg into conventional half bridge inverter. It consists of one neutral leg and one inverter leg. The presence of the neutral leg enables the direct connection between the ground of PV panels and the ground. Two main benefits are then obtained. First, the common mode currents are completely eliminated because the stray capacitors between the PV panels and the grid neutral line are bypassed. Another benefit is the voltage of the PV is only required to be higher than the peak value of the grid voltage, which is same as that of conventional full bridge inverter. In addition, the synchronverter technology designed for three-phase inverters is extended to a single-phase case to design the controller of the inverter leg. As a result, the proposed inverter becomes more grid-friendly. The control of the two legs are independent with each other, which means the controller design much easier. The performance of the whole system is evaluated in detail with the provided real-time simulation results.