In this paper, a new topology of two-stage cascaded switched-diode (CSD) multilevel inverter is proposed for medium voltage renewable energy integration. First, it aims to reduce the number of switches along with its gate drivers. Thus, the installation space and cost of a multilevel inverter are reduced. The spike removal switch added in the first stage of the inverter provides a flowing path for the reverse load current, and as a result, high voltage spikes occurring at the base of the stepped output voltage based upon conventional CSD multilevel inverter topologies are removed. Moreover, to resolve the problems related to dc source fluctuations of multilevel inverter used for renewable energy integration, the clock phase-shifting (CPS) one-cycle control (OCC) is developed to control the two-stage CSD multilevel inverter. By shifting the clock pulse phase of every cascaded unit, the staircase-like output voltage waveforms are obtained and a strong suppression ability against fluctuations in dc sources is achieved. Simulation and experimental results are discussed to verify the feasibility and performances of the two-stage CSD multilevel inverter controlled by the CPS OCC method.
1. Novel cascaded multilevel inverter
3. One-cycle control
Fig. 1. Renewable energy generation system with multilevel inverter.
EXPECTED SIMULATION RESULTS:
Fig. 2. The output voltage and current of the first stage converter of the 5-level
simulation prototype. (a) Output voltage ug ; (b) Output current ig .
Fig. 3. The output voltage and inductor current of the 5-level simulation
prototype. (a) Output voltage uCD ; (b) Output voltage after filter uo ; (c) Inductor current il
Fig. 4. The output voltage and current of the first stage converter of the
9-level simulation prototype. (a) Output voltage ug ; (b) Output current ig .
Fig. 5. The output voltage and inductor current of the 9-level simulation
prototype. (a) Output voltage uCD ; (b) Output voltage after filter uo ; (c) Inductor current il .
Fig. 6. The simulation results of the 5-level prototype: DC source with basic
unit 1 contains a 10 Hz ripple with amplitude 16 V. (a) uo using CPS OCC; (b) uo using CPS SPWM.
Fig. 7. The simulation results of the 5-level prototype: DC source with each
basic unit contains a 10 Hz ripple with amplitude 8 V. (a) uo using CPS OCC; (b) uo using CPS SPWM.
A new topology of two-stage CSD multilevel inverter has been proposed in this paper. n cascaded basic units and one spike removal switch form the first stage. Then by adding a full-bridge inverter as the second-stage converter, both of the positive and negative output voltage levels are generated. Since the one full-bridge converter in the output side leads to the restriction on high-voltage applications, the proposed topology is suitable for medium-voltage renewable energy integration. The comparisons with the CHB and cascaded half-bridge topologies show that the CSD topology requires less switches and related gate drivers for realizing Nlevel output voltage. As a result, the installation space and cost of the multilevel inverter are reduced. Meanwhile, the spike removal switch added in the first stage provides a flowing path for the reverse load current under R-L loads, thus, the high voltage spikes, due to the collapsing magnetic field in a very short time interval, are removed. The CPS OCC method, which is composed by n similar but dependent OCC controllers, has been designed and implemented to control the CSD multilevel inverter. Simulation and experimental results demonstrate that, by shifting the clock pulse phase of each cascaded unit, the staircase-like voltage waveforms are obtained. Moreover, to evaluate the performance of CPS OCC, in both the simulation and experiment, the DC sources mixed with low frequency ripples are implemented to simulate the DC supply from renewable energy generations, and the comparative results between CPS OCC and CPS SPWM reveal that CPS OCC possesses a superior ability in suppressing the unbalance or low frequency ripples in DC sources. These results demonstrate that the CPS OCC method can be a substitute for conventional controllers to control multilevel inverters for renewable energy integration with improved control performances.
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