Darko Ostojic

A New Multilevel Conversion Structure for Grid-Connected PV Applications

A novel scheme for three-phase grid-connected photovoltaic (PV) generation systems is presented in this paper. The scheme is based on two insulated strings of PV panels, each one feeding the dc bus of a standard two-level three-phase voltage-source inverter (VSI). The inverters are connected to the grid by a three-phase transformer having open-end windings on the inverter side. The resulting conversion structure performs as a multilevel power active filter (equivalent to a three-level inverter), doubling the power capability of a single VSI with given voltage and current ratings. The multilevel voltage waveforms are generated by an improved space-vector-modulation algorithm, suitable for the implementation in industrial digital signal processors. An original control method has been introduced to regulate the dc-link voltages of each VSI, according to the voltage reference given by a single maximum power point tracking controller. The proposed regulation system has been verified by numerical simulations and experimental tests with reference to different operating conditions.

Multi-phase multi-level AC motor drive based on four three-phase two-level inverters

A novel multi-phase multi-level ac motor drive is analyzed in this paper. The proposed scheme is based on four conventional 2-level three-phase voltage source inverters (VSIs) supplying the open-end windings of a dual three-phase motor (asymmetric six-phase machine), quadrupling the power capability of a single VSI with given voltage and current ratings. The proposed control algorithm is able to generate multi-level voltage waveforms, equivalent to the ones of a 3-level inverter, and to share the total motor power among the four dc sources within each switching period. The proposed ac motor drive has been numerically implemented and a complete set of simulation results is given to prove the effectiveness of the whole scheme.

Control strategy for a multilevel inverter in grid-connected photovoltaic applications

A novel conversion structure for photovoltaic (PV) grid connection is proposed in this paper. The conversion system is aimed to be relatively simple and effective in medium and high power range. The topology utilizes a dual two-level voltage source inverter (VSI), fed by two insulated PV fields, supplying a grid transformer with open-end connection. Use of proposed scheme doubles the output power range of the conversion system with respect to the rated power of a single inverter, without the need of series/parallel connections of power switches. Furthermore, the dual inverter structure allows multilevel voltage waveforms, reducing grid current harmonics and mitigating output voltage derivatives. An original control method has been introduced to regulate the dc-link voltages for each VSI. The proposed algorithm has been verified by numerical tests with reference to different operating conditions.

Dual inverter space vector modulation with power balancing capability

Dual two-level inverter topology is a simple structure capable to produce a multilevel voltage output equivalent to a 3-level inverter. Its modular structure consists of two standard two-level three-phase voltage source inverter (VSI) supplied by one or two dc sources. Known modulations cannot provide arbitrary power sharing between inverters in order to balance their dc voltages in case of two separate dc supplies. This paper proposes a new space vector modulation (SVM) technique for a dual two-level inverter which can provide load control of the two dc sources as well as multilevel voltage output. An original modulation algorithm has been introduced to regulate the dc-link voltages of each VSI according with the requirement of a control system, by means of a specialized space vector modulation. The proposed algorithm has been verified by experimental tests.

Quad-inverter configuration for multi-phase multi-level AC motor drives

A novel quad-inverter configuration for multiphase multi-level ac motor drives is analyzed in this paper. The proposed scheme is based on four conventional 2-level three-phase voltage source inverters (VSIs) supplying the open-end windings of a dual three-phase motor (asymmetric six-phase machine), quadrupling the power capability of a single VSI with given voltage and current ratings. The proposed control algorithm is able to generate multi-level voltage waveforms, equivalent to the ones of a 3-level inverter, and to share the total motor power among the four dc sources within each switching period. The whole ac motor drive has been numerically implemented and a complete set of simulation results is given.

Dual-inverter-based MPPT algorithm for grid-connected photovoltaic systems

A novel MPPT algorithm for three-phase grid-connected photovoltaic generation systems is presented in this paper. Reference is made to a conversion scheme consisting in two balanced arrays of PV modules, each one feeding a standard two-level three-phase voltage source inverter (VSI). The dc-link voltages of each VSI are regulated according with the requirement of the proposed MPPT algorithm, based on the comparison of the operating points of the two PV arrays. Inverters are connected to grid by a three-phase transformer with open-ends winding configuration on inverters side. The resulting conversion structure performs as a power active filter, doubling the power capability of a single VSI, with the additional benefit of voltage multilevel waveforms. The proposed MPPT algorithm has been successfully verified by numerical simulation and experimental tests.

Dual inverter configuration for grid-connected photovoltaic generation systems

The design and control issues associated with the development of a novel three-phase grid-connected photovoltaic generation system are discussed in this paper. The scheme is based on two insulated strings of panels, each one feeding a standard two-level three-phase voltage source inverter (VSI). Inverters are connected to grid by a three-phase transformer with the open winding configuration on primary side. The resulting conversion structure performs as a multilevel power active filter, doubling the power capability of a single VSI with given current rating. An original control method has been introduced to regulate the dc-link voltages for each VSI. The proposed algorithm has been verified by numerical tests with reference to different operating conditions. The simulation results confirm the effectiveness of the whole conversion scheme.

Multilevel power conditioner for grid-connected photovoltaic applications

A novel scheme for three-phase grid-connected photovoltaic (PV) generation systems is presented in this paper. The proposed scheme is based on two insulated strings of PV panels, each one feeding a standard two-level three-phase voltage source inverter (VSI). Inverters are connected to grid by a three-phase transformer with the open winding configuration on inverters side. The resulting conversion structure performs as a multilevel power active filter, doubling the power capability of a single VSI with given current rating. An original control method has been introduced to regulate the dc-link voltages of each VSI. The proposed algorithm has been verified by numerical simulations with reference to different operating conditions. The results confirm the effectiveness of the whole conversion structure.

Carrier-based discontinuous modulation for Dual three-phase two-level inverters

Dual two-level inverter topology is a simple structure able to produce multilevel output voltage waveforms equivalent to a 3-level inverter. Its modular structure consists of two standard two-level three-phase voltage source inverters (VSI) supplied by two dc supplies. In case of two separate dc supplies an independent modulation of the two inverters can provide arbitrary power sharing between inverters in order to balance their dc voltages. However, proper multilevel voltage waveforms can be obtained only by relatively complex space vector modulation (SVM). This paper proposes two carrier-based modulation techniques for a dual two-level inverter with power sharing capability and proper multilevel voltage waveforms. Their main advantage is a simpler implementation compared to SVM. The proposed algorithms have been verified by numerical tests and preliminary implementation in an industrial DSP.

Experimental tests on a multilevel converter for grid-connected photovoltaic systems

A complete set of experimental tests on a novel conversion scheme for three-phase grid-connected photovoltaic (PV) generation systems is presented in this paper. The proposed scheme is based on two insulated strings of PV panels, each one feeding a standard two-level three-phase voltage source inverter (VSI). The inverters are connected to grid by a three-phase transformer with the open winding configuration on inverters side. The resulting conversion structure acts as a multilevel converter, doubling the power capability of a single VSI with given current rating. An original control method has been introduced to regulate the dc-link voltages of each VSI, allowing the whole converter to inject the PV energy into the grid and to perform as a power active filter.