The term dead time refers to a prime safety factor for most power electronic converter topologies, and it is included either in the control software or in the gate/base driver hardware, depending on the application as well as the control requirements. In this paper, the authors present a comprehensive numerical analysis of dead-time effects on the output voltage of a three-level neutral-pointclamped (NPC) inverter. To incorporate the dead-time effect in the output voltage, 3-D models of three-level carrier pulse width modulation (PWM) methods are modified for two dead-time implementations. Closed-form expressions of inverter phase voltage harmonics for phase opposition disposition (POD) PWM are derived based on the double Fourier series approach and modified contour plots. The harmonic spectra from numerical evaluations, simulations, and experiments for natural sampling (NS), symmetrical regular sampling (SRS), and asymmetrical regular sampling (ARS) are compared to validate the mathematical models. In addition, the fundamental voltage with respect to the dead time and the load phase angle is presented based on analytical results and simulation.
The present work proposes a simple cost effective multilevel topology for generating high quality sinusoidal AC waveform based onmulti-tapped multi-winding transformer switching technique. Multi-winding multi-tapped transformers are used to aid the multi-level switching process which guarantees a large number of intermediate switching levels. Each secondary tapping can act as a separate DC source derived from the single DC supply input to feed the second transformer. The proposed topology can generate 27 switching states by using only 8 switches and 3 full bridge diode rectifiers. The basic working principle is based on the selective addition and subtraction of magnetic flux in the transformer core. Although the mathematical modeling of multi-winding multi-tapped transformer is slightly complex, the resulting circuit complexity reduces when compared to the conventional topologies like diode clamped, capacitor clamped and cascaded multilevel inverters. The present work uses PSPICE and MATLABmodeling techniques to simulate the entire system using synthesized multi-winding multi-tapped transformer models. Also the proposed system has superior quality performance characteristics when compared to the conventional topologies, due to its ability to avoid major drawbacks like capacitor voltage unbalancing, common mode voltage stresses at the load end and the requirement of large filters to avoid the presence of harmonic frequencies at the output.
Distributed generation and smart grid integration of renewable energy sources introduce a lot of challenges for the enabling power electronic converter technology. Some of these challenges include wide controllability range, high power handling and good reliability. Three-level boost converter is one of the attractive solution for applications requiring voltage cross regulation such as three-level neutral point clamped inverter based grid integration of renewable sources. The present work shows the advantages and disadvantages of using discontinuous conduction mode of a Three-level boost converter for voltage cross regulation. The converter working principle, modes of operation and operating cases are listed briefly. The simulation results compare the DCM and CCM cross regulation effects. Based on these results, the controllability range of the converter is analyzed to understand the suitability of the converter for various applications.
When medium- or high-voltage power conversion is preferred for renewable energy sources, multilevel power converters have received much of the interest in this area as methods for enhancing the conversion efficiency and cost effectiveness. In such cases, multilevel, multi-input multi-output (MIMO) configurations of DC-DC converters come to the scenario for integrating several sources together, especially considering the stringent regulatory needs and the requirement of multistage power conversion systems. Considering the above facts, a three-level dual input dual output (DIDO) buck-boost converter, as the simplest form of MIMO converter, is proposed in this paper for DC-link voltage regulation. The capability of this converter for cross regulating the DC-link voltage is analyzed in detail to support a three-level neutral point clamped inverter-based grid connection in the future. The cross-regulation capability is examined under a new type of pulse delay control (PDC) strategy and later compared with a three-level boost converter (TLBC). Compared to conventional boost converters, the high-voltage three-level buck boost converter (TLBBC) with PDC exhibits a wide controllability range and cross regulation capability. These enhanced features are extremely important for better regulating variable output renewable energy sources such as solar, wind, wave, marine current, etc. The simulation and experimental results are provided to validate the claim.
Magnetocaloric effects of various materials are getting more and more interesting for the future, as they can significantly contribute towards improving the efficiency of many energy intensive applications such as refrigeration, heating, and air conditioning. Accurate characterization of magnetocaloric effects, exhibited by various materials, is an important process for further studies and development of the suitable magnetocaloric heating and cooling solutions. The conventional test facilities have plenty of limitations, as they focus only on the thermodynamic side and use magnetic machines with moving bed of magnetocaloric material or magnet. In this work an entirely new approach for characterization of the magnetocaloric materials is presented, with the main focus on a flexible and efficient power electronic based excitation and a completely static test platform. It can generate a periodically varying magnetic field using superposition of an ac and a dc magnetic field. The scale down prototype uses a customized single phase H-bridge inverter with essential protections and an electromagnet load as actuator. The preliminary simulation and experimental results show good agreement and support the usage of the power electronic test platform for characterizing magnetocaloric materials.
Highly random nature of input power from wave energy converters (WEC), especially from direct-driven point absorbers, demands customized power electronic converters for grid connection. In this paper, analysis and comparison of the DC-link stresses in the converter systems for two cases - a single and three collective units, of wave energy converters is given. The AC/DC/AC converter system includes a conventional uncontrolled three phase rectifier, a DC/DC converter to boost the DC-link voltage and an inverter with RL load. The system has been studied under two different controller actions for the DC/DC converter: with constant boost factor and with constant DC-link voltage. A Proportional Integral controller has been used to regulate the voltage in the latter case. Matlab/Simulink based system simulation has been done to compare the DC-link stress. The analysis shows the comparison in DC-link stresses and the requirements of the system for different cases, proving the advantages and the importance of having customized active power conversion methods for minimizing the DC-link stresses.
The cross regulation effect in multi-output DC/DC converters offers a reliable support for the grid integration of multilevel inverters by balancing the capacitor voltages. The capacitor voltage balancing by single input dual output boost converter is often realised by conventional three-level switching scheme. The three-level operation benefits lower inductor ripple current, but it limits the maximum possible compensation voltages. In this study, the entire operating modes of the boost converter is presented and all the possible cases which contribute to the voltage balancing are employed for balancing the capacitor voltages in a three-level neutral point clamped inverter. A proportional-integral controller based duty ratio control and pulse delay control are used for DC link voltage regulation and capacitor voltage balancing. Since the classical state-space averaging technique is not suitable for SIDO converters, inductor current ripple averaging technique is utilised for controller design. The circuit simulation is performed in Matlab/Simulink. The digital controller is realised using the Virtex-5FPGA in Labview/CompactRIO module. Both simulation and experimental results are presented to validate the controller performance.
This paper investigates the capacitor voltage balance scheme for a neutral point clamped inverter. A symmetrical space vector modulation is used to analyze the imbalance in neutral point voltage. This method overcomes the drawbacks of Nearest Three Vector modulation method. The redundancies of different switch configurations for the generation of the same output voltage are used to limit the voltage drift when modulation index is less than half. The deviation in the neutral voltage is further reduced by adjusting the duty cycle distribution variable. The voltage balance scheme is analyzed for linear and non linear loads. The performance of control strategy is also examined for a grid connected inverter.
This paper presents a synchronous current control method for a three-level neutral point clamped inverter. Synchronous reference frame control based on two decoupled proportional-integral (PI) controllers is used to control the current in direct and quadrature axes. A phase disposition pulse width modulation (PDPWM) method in regular symmetrical sampling is used for generating the inverter switching signals. To eliminate the harmonic content with no phase errors, two first-order low pass filters (LPFs) are used for the dq currents. The simulation of closed-loop control is done in Matlab/Simulink. The Vertex-5 field programmable gate array (FPGA) in Labview/CompactRio is used for the implementation of the control algorithm. The control and switch pulse generation are done in independent parallel loops. The synchronization of both loops is achieved by controlling the length of waiting time for each loop. The simulation results are validated with experiments. The results show that the control action is reliable and efficient for the load current control.
Direct-driven point absorber type tubular wave energy converters, reflect the random nature of wave energy input to its output voltage waveform. The better conditions in the generator output stage are the first step that allows the reduction in the need of complicated power conversion system. This work presents the preliminary analysis and comparison of two important interconnection strategies of WEC-rectifier units. Two different interconnection strategies are considered based on how they build up the DC-link, i. e. parallel (wellknown and used by many researchers in wave area) and cascaded (less considered) configuration of WEC-rectifier units. Conventional two-level AC/DC/AC power conversion system is used in both cases. Matlab/Simulink based system simulation is used to compare the different interconnection strategies. Various results are presented and discussed, together with the voltage ripple calculations of the DC-link voltage and output phase voltage and current, being it important for the sizing and the cost of the system.
The increased smart grid integration of renewable energy sources demands high power handling and wide controllability for the enabling power conversion technologies. The conventional energy conversion techniques are inadequate to efficiently handle the highly varying nature of renewable energy sources like wave, solar, tidal and wind. The present work examines the advantages of using a three-level buck-boost DC-DC converter to aid three-level neutral-point-clamped inverter based grid integration. There are two main reasons for using this converter. It can provide the conventional buck-boost capability at higher power levels for absorbing and conditioning the renewable source output. Besides, it can be used as a voltage balancing device to satisfy the input requirement for the three-level neutral-point-clamped inverter. The work includes complete operating range analysis of the converter for the combined buck-boost action and voltage balancing effects to understand its suitability for various applications. The converter switching modes of operation are also presented in detail along with essential example waveforms. The final results show good controllability bandwidth for the converter which makes it an attractive solution for smart grid integration of renewable energy sources.
The multilevel converters offer significant advantages for high power applications. The use of multilevel DC/DC converters provides improved efficiency for power conversion and transmission at high voltage. This paper investigates the control and implementation of a three level boost converter for regulating the load voltages. A PI controller based switch signal phase delay control (SSPDC) method is used for adjusting the load voltages at equal turn on and turn off time of the converter switches. The circuit simulation is done in Matlab/Simulink. The controller is realized by using the FPGA in Labview/Compact-Rio module. Software waiting loop length control technique is used for implementing the switch signal delay control. The hardware circuit is implemented and tested. The results show a validation of the controller for regulating the voltages. This method can easily be applied for voltage balancing in a three level neutral point clamped inverter where neutral voltage imbalance is always an issue.