Control of Ripple Eliminators to Improve the Power Quality of DC Systems and Reduce the Usage of Electrolytic Capacitors

The problem of voltage/current ripples has become a primary power quality issue for DC systems, which could seriously degrade the performance on both the source side and the load side and lead to reliability concerns. In this paper, a single-phase PWM-controlled rectifier is taken as an example to investigate how active control strategies can improve the power quality of DC systems, reduce voltage ripples and, at the same time, reduce the usage of electrolytic capacitors. The concept of ripple eliminators recently proposed in the literature is further developed and the ratio of capacitance reduction is quantified. With such ripple eliminators, this power quality problem is formulated as a control problem to actively divert the ripple current on the DC bus. The main focus of this paper is to investigate how advanced control strategies could improve the performance of ripple eliminators. An advanced controller on the basis of the repetitive control is proposed for one possible implementation of ripple eliminators in the continuous current mode (CCM). Experimental results are presented to verify the effectiveness of the strategy with comparison to another ripple eliminator operated in the discontinuous current mode (DCM). It has been shown that the proposed instantaneous ripple-current diversion in CCM leads to a nearly fourfold improvement of performance

Active Control of Voltage Ripples in Power Electronic Converters

Two major challenges, i.e., bulky electrolytic capacitors and isolation transformers, remain as critical obstacles for further improvement on reliability, power density and efficiency of power electronic converters, which are mainly used to reduce low-frequency voltage ripples and high-frequency common-mode voltage ripples, respectively. In order to overcome the two challenges, the most straightforward way is to simply combine existing solutions developed for each of them. However, this would considerably increase system complexity and cost, which should be avoided if possible. In this thesis, these two challenges are innovatively addressed in a holistic way by using active control techniques. This thesis first focuses on the reduction of low-frequency voltage ripples in conventional half-bridge converters, after adding an actively-controlled neutral leg. As a direct application of this strategy, a single-phase to three-phase conversion is then proposed. After that, a ρ-converter with only four switches is proposed to significantly reduce both low-frequency ripples and high-frequency common-mode ripples in a holistic way. It is found that the total capacitance can be reduced by more than 70 times compared to that in conventional full-bridge converters. As a result, there is no longer a need to use bulky electrolytic capacitors and isolation transformers. Then, the ρ-converter equipped with the synchronverter technology is operated as an inverter for PV applications. Another converter is also proposed for the same purpose but with reduced voltage stress. In order to further reduce the total capacitance and to reduce the neutral inductor in the ρ-converter, a new type of converter, called the θ-converter, is proposed. Finally, two actively-controlled ripple eliminators are proposed to reduce low-frequency ripples in general DC systems while the aforementioned research is focused on some specific topologies. Extensive experimental results are presented to validate most of the developed systems while the rest are validated with simulation results

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

Beijing converters: bridge converters with a capacitor added to reduce leakage currents, DC-bus voltage ripples, and total capacitance required

Abstract: Isolation transformers and bulky electrolytic capacitors are often used in power electronic converters to reduce leakage currents and voltage ripples but this leads to low power density and reduced reliability. In this paper, an auxiliary capacitor is added to the widely used conventional full-bridge converter to provide a path for, and hence significantly reduce, the leakage current. The operation of the full-bridge converter is split into the operation of a half-bridge converter and a dc-dc converter so that the ripple energy can be diverted from the dc-bus capacitor to the auxiliary capacitor. Hence, the dc-bus capacitor can be significantly reduced while maintaining very low voltage ripples on the dc bus because it is only required to filter out switching ripples. The auxiliary capacitor is designed to allow high voltage ripples because its voltage is not supplied to any load. Accordingly, the auxiliary capacitor can also be very small as well. As a result, the total required capacitance becomes very small. The reduction ratio of the total capacitance is significant, which makes it cost-effective to use film capacitors instead of electrolytic capacitors. The proposed converters can be also operated as an inverter without any restriction on power factor because the adopted four switches are all bidirectional in terms of power flow. Experimental results for both rectification and inversion modes are presented to demonstrate the performance of the proposed converter in reducing the ripples, the leakage currents, and the total capacitance needed, with comparison to the conventional bridge converter without the auxiliary capacitor

Dynamic Interactions of a Double-stage Photovoltaic Power Converter: Modelling and Control

Photovoltaic (PV) systems are a promising renewable source to achieve green energy targets and be part of the electricity generation. Lots of efforts have been devoted to increase the penetration level of PV systems and its share in the generated electricity. Power quality is one of the challenges that impact the penetration level of PV systems. It is important to ensure high power quality from PV systems to allow more installations to the grid. So, PV power quality issues have to be addressed properly. It was reported that the poor power quality of the PV systems might be caused by many reasons such as the large amount of PV power fluctuation, the low level of current from the PV system, and large populations of PV inverters. In addition to the aforementioned reasons, recently it was suggested that perturb and observe (P&O) controller is another source of harmonics which result in a deprived PV power quality. This newly reported problem is based on experimental observations without full understanding of the generation mechanism of these harmonics in the PV system, the relation between the P&O controller design and the generated harmonics, and the effect of these harmonics on the rest of the system. Thus, in-depth analysis of the harmonics in PV systems due to P&O controller and a solution to eliminate these harmonics are demanded. Therefore, in this research an investigation is carried out to explore P&O related harmonics in a double-stage grid-connected PV system. First, regarding the P&O related harmonics full explanation of how harmonics are generated due to the perturbing nature of the P&O controller is provided, a modelling approach is suggested to identify the frequency and the amplitude of the variations in the DC bus due to the P&O controller, the effect of different factors (e.g. weather conditions, system parameters, system operating point, and P&O architecture) on the induced harmonics are investigated. Secondly, regarding the effect of the P&O related harmonics on the rest of the system an intense simulation analysis is provided to explore the possible effect of the P&O related harmonics on increasing the interaction between the system power stages. This can help to set system design recommendations and guidelines such as sizing the dc-link capacitance and designing the system controllers. Finally, a novel mitigation solution is proposed to supress the P&O related harmonics. That can help to reduce the dynamic interaction between system power stages and improve the power quality of the PV system

Toward high-efficiency high power density single-phase DC-AC and AC-DC power conversion - architecture, topology and control

Power conversion between the single-phase AC grid and DC sources or loads plays an indispensable role in modern electrical energy system for both generation and consumption. The renewable resources and electrical energy storage are integrated to the grid through inverters. Telecoms, data centers and the rest of the digital world is powered by the grid through rectifiers. Existing and emerging applications all demand the DC-AC and AC-DC systems to be not only more efficient to reduce energy consumption, but also more compact to reduce cost and improve portability. Therefore, new AC-DC and DC-AC converter designs that improve the efficiency and power density of the system is a critical area of research and is the focus of this dissertation. The recent development of wide band-gap devices stimulates a new round of improvement on efficiency and power density of AC-DC converters. However, despite the new transistors used, the fundamental system architecture and topology remain relatively unchanged, which is becoming the bottleneck for further improvement. This dissertation explores new architecture, topology and control to overcome this bottleneck, targeting an order-of-magnitude improvement on power density and comparable efficiency to the conventional design. The proposed solutions build on two key innovations: the series-stacked buffer architecture for twice-line-frequency power pulsation decoupling in single-phase AC-DC and DC-AC conversion, and the flying capacitor multilevel topology for power transfer and waveform conversion between AC and DC. This work provides complete solutions for these ideas, including the theoretical development, design procedure, control method, hardware implementation and experimental characterization

Active power decoupling for current source converters:An overview scenario

For single-phase current source converters, there is an inherent limitation in DC-side low-frequency power oscillation, which is twice the grid fundamental frequency. In practice, it transfers to the DC side and results in the low-frequency DC-link ripple. One possible solution is to install excessively large DC-link inductance for attenuating the ripple. However, it is of bulky size and not cost-effective. Another method is to use the passive LC branch for bypassing the power decoupling, but this is still not cost-effective due to the low-frequency LC circuit. Recently, active power decoupling techniques for the current source converters have been sparsely reported in literature. However, there has been no attempt to classify and understand them in a systematic way so far. In order to fill this gap, an overview of the active power decoupling for single-phase current source converters is presented in this paper. Systematic classification and comparison are provided for researchers and engineers to select the appropriate solutions for their specific applications

Control of the offshore wind turbine and its grid integration

This thesis investigates the way to reduce the maintenance cost and increase the life cycle of the offshore wind turbines, as in the offshore case maintenance is highly difficult and expensive. Firstly, we study the possibility to replace the vulnerable and expensive DC link capacitor in wind power integration system by the virtual infinite capacitor (VIC), which is a power electronic circuit functioning as a large filtering capacitor. We propose a control algorithm for the VIC. Before applying it to the wind power system, we firstly test it in a simple power factor compensator (PFC) as the output filter capacitor. The simulation results show the effective filtering performance of VIC in low-frequency range. Then, we validate it experimentally by directly injecting the DC voltage together with a 50 Hz ripples to the VIC. The VIC successfully eliminates the ripple and extracts the DC voltage at the output terminals. Besides, the experiment performances are highly consistent with the corresponding simulations, which demonstrates the possibility to use VIC to replace the DC-link capacitor in wind power integration system and use it in other industrial systems. Since the VIC mainly filters the ripple in low frequency range while the DC-link voltage usually includes ripples in two distinct frequency ranges, we further develop it into the parallel virtual infinite capacitor (PVIC), aiming to suppress the voltage ripple in a wider frequency range. The PVIC is applied to replace the DC-link capacitor in wind turbine grid integration system. The simulations are conducted under different grid conditions with turbulent wind input. The results show that the PVIC provides much better voltage suppression performance than the equivalent DC-link capacitor, which facilitates the power generation control under normal operations and reduces the risks of converter failure under grid faults. In this way, the PVIC proves to be a great solution to substitute the vulnerable DC-link voltage in the offshore wind turbine power integration system. The wind power conversion system from the generator to the grid is composed of a DC-link capacitor and two back-to-back power converters. Though the application of PVIC removes the fragile DC-link capacitor in the power conversion system, the power converters are also fragile and expensive. In addition, the existence of power converters decouples the generator with the grid, which hinders the direct inertia support and frequency regulations from wind turbines. It would be desirable to completely remove the whole power conversion system. Hydrostatic wind turbine (HWT) may provide a suitable solution. The HWT is a wind turbine using hydrostatic transmission (HST) to replace the original heavy and fragile gearbox. The HST can provide the ‘continuously variable gearbox ratio’ , which allows HWT to be connected to a synchronous generator (SG) and then directly to the grid. We propose a coordinated control scheme for the HWT. The simulations are conducted with turbulent wind under variable system loads. The results indicate that with the proposed coordinated control system, the HWT (without power converters) provides efficient frequency support to the grid, which shows it is a promising solution for the future offshore wind power system. Finally, we consider to further reduce the maintenance cost and improve the performance of the HWT by using a new and novel control algorithm called model-free adaptive control (MFAC). It is applied to both torque control and pitch control of the HWT. Their control performances are compared to some of the existing algorithms. The simulation results demonstrate that the MFAC controller has much better tracking and disturbance rejection performances than the existing algorithms which can increase the fatigue life of the wind turbine and reduce the maintenance cost