The main problem related to cascaded H-bridge cells multilevel inverters (CHBs) is their using of a large number of components, such as switches and DC-sources. Therefore, minimization of components in these kinds of devices is of great importance. Cascaded transformers multilevel inverters (CTMIs) have completely eliminated the need for several DC-sources in CHBs. Thus, minimization of the other components in CTMIs can lead to obtain an optimized structure for multilevel inverters. The present paper introduces a simple and compact structure for transformer based multilevel inverters. Since the number of utilized components in the proposed structure is remarkably reduced, the cost, volume and complexity are minimized. The performance of the suggested inverter has been scrutinized through two different strategies. Firstly, it is tested under condition of supplying a local load, and secondly, employing sample based current control strategy, its performance is inspected when being connected to the grid. In the latter test the leakage inductances of the transformers are utilized to execute the sample based current control strategy, thus, the need for extra filter is eradicated. The feasibility of the suggested topology has been validated by using laboratory-built prototype along with a computer-aid simulated model.
- Multilevel inverter
- Component reduction
- Grid-connected converter
- Sample based current control
Fig. 1. Proposed topology
EXPECTED SIMULATION RESULTS
Fig. 2. Simulation results of the output voltage, load current, FFT analyses, and switches voltages & currents. (a) Output voltage in no load condition, (b) output voltage and load current when supplying the RL load (c) output voltage and load current when supplying the pure resistive load. (d) FFT analysis of the output voltage in no load condition. (e) FFT analysis of the output voltage when supplying the RL load. (f) FFT analysis of the output voltage when supplying the pure resistive load. (g) Voltages and currents of the switches of the two outer legs. (h) Voltages and currents of the switches of the two middle legs. (j) Load and capacitor currents when providing active an reactive power (resistive-inductive loading condition).
Fig.3. Simulation results of grid-tied model. (a) Reference current and injected current. (b) Injected current and grid voltage. (c) FFT analysis of the injected current
This paper put forth a novel nine-level topology for transformer based multilevel inverters. The proposed topology can deservedly reduce switches count of a nine-level single-phase multilevel inverter. To prove the feasibility of the suggested topology, by using a model simulated under Mathlab/Simulink environment and a laboratory-built prototype, it went under two different tests. Firstly, its performance was assessed when supplying a local load. The load was assumed to be either a pure resistive load or a resistive-inductive load. Secondly, employing sample based current control strategy, its performance was inspected under grid-tied condition. By employing a simulated model in the latter test, the proposed topology took the responsibility of delivering an assumed active power of 15kW, and 25kW to the grid. In the experimental test the power value injected to the grid was considered to be 800w. Since the CTMIs mostly use only one DC-source they are a competent candidate in microgrid and PV application. Being based on CTMIs, the proposed topology employs fewer numbers of components and offers the same advantages as the conventional topology does. When adopting the sample based current control strategy in grid-tie applications an inductive element is required to be located between the converter and the grid. In this paper the leakage inductances of the transformers were used as the inductive filter. This facilitated executing the sample based current control strategy and consequently the need for an extra filter is eradicated. The other advantage of using transformers was their providing a galvanic isolation. To sum up, the accomplished tests verified the feasibility and viability of the suggested topology.
- Z. Peng, J.W McKeever, D.J. Adams,―A power line conditioner using cascade multilevel inverters for distribution systems,‖ IEEE Trans. Ind. Appl., Vol. 34, no. 6, pp. 1293 – 1298, nov. 1998.
- M. Tolbert, F. Z. Peng, T.G. Habetler, ―Multilevel converters for large electric drives‖. IEEE Trans. Ind. Appl, Vol. 35, no. 1, pp. 36-44, Aug. 1999.
- Dixon, J. Pereda, C. Castillo, S. Bosch, ―Asymmetrical multilevel inverter for traction drives using only one DC supply,‖ IEEE Trans Veh, Techno, Vol. 59, no. 8, pp. 3736-3743, Oct. 2010.
- Poompavai, A. Chitra, C. Srinivas, K. Giridharan, ―A meticulous analysis of induction motor drive fed from a nine-level Cascade H-Bridge inverter with level shifted multi carrier PWM,‖ in Proc. IEEE Int. Conf. on Smart Structures and Systems (ICSSS), 2013, pp 6-12.
- Suresh, A.K. Panda, ―Research on a cascaded multilevel inverter by employing three-phase transformers,‖ IET Power Electron, Vol. 5, no. 5, pp. 561 – 570, Jun. 2012.