This paper presents a generalized power-decoupling control scheme using a multiport isolated bidirectional converter for multilevel inverter, which has multiple DC-links inside. In the proposed method, a single power-decoupling capacitor is needed for all the DC-links in the multilevel inverter cell. First, a prototype of the power-decoupling concept of individual H-bridge cells in the multilevel inverter is proposed, using a separate power-decoupling circuit. Then, a more advanced one-step power-decoupling method is proposed. The lifetime and reliability of the multilevel inverter is improved as film capacitors replace the large capacitance electrolyte capacitors. A multi-input ports/single output voltage-fed dual half bridge converter (MDHB) is used for the power-decoupling circuit. Steady state analysis for the peak and root-mean-square of the MDHB current is carried out for the loss breakdown. The currents are functions of the switching frequency, phase-shift, leakage inductance, turn ratio, and output voltage, which make the multiport transformer design complex. A design methodology is proposed that takes into account the design of the copper and core losses as functions of the switching frequency and number of turns. Furthermore, a special winding method for the input port is illustrated to obtain identical leakage inductances for the uniform current distribution in the multiport transformer. The proposed MDHB employs a current-sensorless power-decoupling control that contributes to the spontaneous ripple rejection of all the DC-links without individual link-current information, as well as to the cost and size reduction. Hence, the ripple-rejection controller is independent of the control configuration of the multilevel inverter, and also available for universal applications of various inverter topologies. Since the primary-input ports of MDHB share a single magnetic core for interfacing the ripple power to the unified secondary ripple capacitor, the controller design becomes difficult in considering the dynamic interaction among the ports, along with the average voltage-control loop design. In this paper, the dynamic analysis and controller design procedure of the circuit is also presented. The power-decoupling is achieved even when the ripple frequency is other than the double frequency of the inverter output, since the single-pole transfer function of the small-signal model of the MDHB allows sufficient phase-margin, along with high bandwidth. The proposed power-decoupling method for the multilevel inverter is validated with the help of simulation and 1.2-kW hardware-prototype experimental results.
- Multilevel inverter
- Cascaded H-bridge inverter
- Multiport isolated converter
- Multiport transformer
- Active power-decoupling
Fig.1: The proposed single decoupling capacitor power-decoupling circuit for a cascaded H-bridge multi-level inverter.
EXPECTED SIMULATION RESULTS
Fig. 2 (a): Key waveforms of numerical analysis: DC-Link voltages of all the three cells of multilevel inverter (Vdc1, Vdc2, Vdc3), voltage across the power-decoupling capacitor (Vopd), and output voltage of the multilevel inverter (VO); and (b) Experimental key waveforms: DC-Link voltage, output voltage of the multilevel inverter, and Voltage across the power-decoupling Capacitor. Both results agree well with each other.
Fig. 3. Key waveforms (a) Numerical analysis: Transformer input port 1, IL1 current and output port current Isec, and (b) Hardware experiment: Transformer input port 1 current and the output port current
Fig. 4. Transient waveforms (a) Simulation under a load step of the proposed power-decoupling control scheme, and (b) Hardware experiment under a load step of the proposed power-decoupling control scheme
A generalized power-decoupling control scheme without current sensors is presented for a symmetric cascaded H-bridge multi-level inverter by using a multi-input port/single output DHB converter as a power-decoupling circuit. Film capacitors are implemented instead of the electrolytic ones, to improve the lifetime and reliability. With the proposed power-decoupling method, the double frequency ripple power at all the DC-links can be directed to a common magnetic circuit, and the total ripple power is able to link to a single power-decoupling capacitor. Hence, a multi-level inverter power-decoupling is achieved by one power-decoupling capacitor. This gives an optimized solution in terms of cost and size. In the multi-port dual half bridge converter, the multi-port transformer is one of the major concerns with regard to the efficiency of the converter. This paper proposed a design methodology for designing the multiport transformer. To design the transformer, expressions of the RMS currents through the transformer are obtained. The transformer’s currents are dependent on the switching frequency, leakage inductance, phase-shift, and the output voltage. The design methodology for the transformer is significant, due to the non-linear and complex behavior of the transformer currents. With the proposed design methodology, the efficiency of the hardware prototype is 95.67% at 1.2 kW. A generalized power-decoupling control for the multi-level inverter is proposed in this paper. The proposed control scheme is of the current-sensorless type, and the fast voltage dynamics based on the sensorless control gives the advantage of using a universal “zero” reference to eliminate the double frequency ripple voltage. Therefore, the proposed control scheme is independent of the multi-level inverter topology, architecture, or control strategies. The bandwidth of the power-decoupling controller can be increased without any inner feedback loop, due to the small capacitance and single-pole behavior of the multi-input/single output bidirectional DHB converter. The analysis and design guidelines of the proposed power-decoupling control scheme are presented. The control design shows the effect of the feedforward controller on the elimination of the DC-link ripple. The proposed power-decoupling method is able to reduce the voltage ripple at all the DC-links in the multi-level inverter to 10% at the DC-links with an equivalent film capacitor of about 100 μF for 1.2 kW. The results of experiments under the steady state and transient operation are obtained through 1.2-kW simulation and hardware tests. The proposed power-decoupling control method for the multi-level inverter by a multi-port DHB converter is verified, and shows a promising ability to provide universal power-decoupling for any type of multi-level inverter.
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