Dual-DC-Port Asymmetrical Multi-Level Inverters with Reduced Conversion Stages D and Enhanced Conversion Efficiency
Abstract
A new concept of dual-DC-port asymmetrical multi-level inverter (DP-AMI), which is able to interface a low voltage DC port, a high voltage DC port, and an AC port simultaneously using only one topology, is presented in this paper. A systematic method to derive the DP-AMI is proposed. With the proposed DP-AMI, a low voltage DC source, e.g. PV modules or battery, can directly supply power to the AC load within single-stage power conversion. Therefore, in comparison with a traditional two-stage DC/AC power conversion system, the power conversion stages are reduced and the voltage/current stress of the front-end DC/DC converter can be significantly alleviated, which can improve the overall conversion efficiency dramatically. Furthermore, by using the low voltage DC source as a new voltage level, asymmetrical multiple voltage levels are generated by the proposed DP-AMI, which is benefit for reducing switching losses and the size/volume of the output filter. The topology derivation method of the DP-AMIs is presented. The operation principles, modulation schemes and characteristics of one of the proposed DP-AMIs are analyzed in detail. A 1-kW prototype is built and tested to verify the effectiveness and advantages of the proposed method.
EXISTING SYSTEM:
Taking the full-bridge inverter as an example, the DC input voltage is usually in the range of 380V-400V for the 220VAC applications. Therefore, a boost converter is usually employed as the front-end DC-DC converter in the conventional two-stage DC/AC power system if the voltage of DC energy sources cannot meet the requirement of the AC output voltage. As a result, the input power has to be processed by two cascade-connected power conversion stages, which will induce higher conduction and switching losses and reduce the overall conversion efficiency. In the case of a DC voltage lower than the peak amplitude of the AC output voltage, impedance-source inverter such as Z-source or qZ-source inverters with voltage step-up capability can be employed to achieve single-stage DC/AC power conversion. However, the limited voltage step-up capability and modulation index, high current/voltage stresses and complicated control make it difficult to achieve high efficiency with these inverters. This is the reason that voltage-fed inverters are more popular in applications, e.g. renewable energy and DC distribution power systems, where higher efficiency is one of the major concerns. Although topologies of inverters have evolved over the past years, the requirements of higher efficiency, lower voltage/current stresses and simpler control/design solutions are still significant driving force in the related research field.
PROPOSED SYSYETM:
The major contribution of this paper is to propose a new family of DP-AMIs and a novel DC/AC power system with reduced conversion stages and enhanced efficiency. The proposed topology and DC/AC power system has the following advantages: 1) the power conversion stages are reduced, which is benefit for improving conversion efficiency of the overall DC/AC power system; 2) the multi-voltage-level characteristic can help to reduce the size/volume of the output filter; 3) with the help of the proposed DP-AMI, the power loss, power rating and cost of the front-end DC/DC converter is reduced. This paper is organized as follows. The derivation principles of the DP-AMIs are presented in Section II. Operation principles of one of the proposed DP-AMIs The derivation of the proposed DP-AMIs is based on the three-level switching cells, i.e. the neutral-point-clamped (NPC) type and the T-type three-level switching cells shown in. The conventional three-level switching cells are operated in a symmetrical manner to generate three voltage levels: 0, V /2 and V H V V H H . The DC input voltage is only connected to V low voltage level V /2 is obtained using dividing capacitors. The proposed DP-AMIs are illustrated i, where it is seen that there are two DC input ports in the proposed DP-AMIs. In particular, one DC port is connected to a high voltage source V H while the other one is connected to a low voltage source V . It is obvious that a new power flow path between the low voltage source V and the AC output is built by the proposed DP-AMI. Therefore, partial input power from the low voltage source V L can be directly supplied to the inverter. It should be noted that the output filter in the DP-AMI can be either L-type, LC-type, or LCL type. When the proposed DP-AMIs are applied to the two-stage DC/AC power conversion system shown in Fig. 1(b), only a part of the input power needs to be processed by the front-end DC/DC stage. Hence, the stresses and power losses of 0278-0046 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. VI.
CONCLUSION
A new, straightforward and simple method for improving the efficiency of the conventional two-stage DC/AC power system has been proposed and verified in this paper. This method is based on a simple fact that a low voltage dc source can also directly supply power to the AC output side of an inverter, no matter the instantaneous value of the AC output voltage of the inverter is lower or higher than the low voltage dc source. This solution is realized by proposing a family of dual-DC-port asymmetrical multi-level inverter. Based on the proposed dual-DC-port asymmetrical multi-level inverter, a new power flow path between the low voltage source V and the inverter is built. As a result, part of the input power can be directly supplied to the inverter and will not be processed by the 0278-0046 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. L front-end DC/DC converter. Hence, the conversion stages, the current/power stresses of the front-end DC/DC converter can be reduced significantly. Furthermore, multiple voltage levels can be generated by the proposed inverter, and the voltage stresses on the switches can be reduced as well. Therefore, the conduction and switching losses are reduced and the conversion efficiency is improved. The experimental results on a 1kW prototype verify the effectiveness and feasibility of the proposed solution. The proposed asymmetrical multi-level inverter provides a good candidate for various DC/AC power systems, e.g., renewable energy systems, micro-grids, energy storage and electrical vehicles, etc.
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