INVERTER

S.NO | TITLES | ABSTARCTS | Year |

PEI-1 | Universal Integrated Synchronization and Control for Single-Phase DC/AC Converters | The universal integrated synchronization and control (UISC) is proposed to operate a single-phase dc/ac converter inboth grid-connected (GC) and stand-alone (SA) modes and offer seamless transition between these modes without any reconfiguration of control structure. As a matter of fact, the UISC does not need to use an islanding detection algorithm in the conventional sense. In GC mode, the UISC controls the real and reactive powers independently. In SA mode, it provides voltage and frequency support. The UISC is not based on direct current control or direct output voltage control. It adjusts the internal converter voltage through a nonlinear mechanism that amounts to the control of the current in GC mode and the voltage in SA mode. The UISC does not require a separate synchronization unit such as a phase-locked loop (PLL). In principle, the UISC is analogous to the combined governor and automatic voltage regulator controls in a synchronous machine (SM). In this sense, it may be considered among the converters that mimic the SMs such as the synchronverter. The proposed UISC is arguably the most unified method that can address different modes of operation of a converter within a micro grid environment without control switching actions. This paper presents the derivations, stability analysis, and numerical results to illustrate the performance of the proposed controller in a single phase situation. A mathematical analysis on the analogies and differences between the UISC and some similar methods is also presented. | 2015 |

PEI-2 | Predictive Control Method With Future Zero-Sequence Voltage to Reduce Switching Losses in Three-Phase Voltage Source Inverters | This paper proposes a predictive control method with zero-sequence voltage injection to efficiently reduce the switching losses of three-phase voltage source inverters (VSIs). In the proposed predictive control method, three-phase future voltage references modified by a zero-sequence voltage injection are generatedto clamp one of the three legs with the largest load current. Furthermore, the future zero-sequence voltage, which is produced online with the future voltage and current references in every sampling period, optimally adjusts the clamping duration on each leg, depending on the load angle. In addition, the proposed method selects the zero vector on the basis of the polarity of the future zero sequence voltage to reduce the switching losses. Using a predefined cost function, the proposed predictive control scheme chooses one optimal voltage state closest to the future voltage references modified by the zero-sequence voltage injection. Therefore, the proposed predictive control method can perform load current control and minimize the switching losses of the VSI under any load condition regardless of the load angle. | 2015 |

PEI-3 | An Adaptive Droop DC-Bus Voltage Controller for a Grid-Connected Voltage Source Inverter With LCL Filter | This paper presents a very fast dc-bus voltage controller for a single-phase grid-connected voltage-source inverter(VSI) with an LCL output filter used in renewable energy applications. In single-phase grid-connected inverters, the design of the dc-bus voltage control scheme is very challenging due to the presence of a second harmonic ripple across the dc-bus voltage. The proposed dc-bus voltage control scheme is able to address the difficulties introduced by the second-harmonic ripple. The dc-bus voltage controller is based on an adaptive droop control technique, which is able to provide a very fast transient response for the closed-loop system and ensures the optimal operation of the VSI during steady-state conditions. Also, the simple structure of the controller makes it very practical for grid-connected VSIs used in renewable energy power conditioning systems. Theoretical analysis and experimental results demonstrate the superior performance of the proposed control approach compared to conventional dc-bus voltage control schemes. | 2015 |

PEI-4 | Swinging Bus Operation of Inverters for Fuel Cell Applications With Small DC-Link Capacitance | For reliability reasons, the employment of small film capacitors instead of electrolytic ones is an interesting alternative for the dc-link in single-phase inverters for fuel cell applications. Due to the low capacitance that can be accomplished at an acceptable cost using this technology, there are large low-frequency voltage fluctuations (100/120 Hz and harmonics) in the dc-link caused by the double-frequency power transfer. By allowing these variations in the bus, the capacitor bank absorbs the current ripple from the inverter to avoid detrimental oscillations in the fuel cell. Traditional control strategies for inverters are usually designed to operate with nearly constant input voltage and are not able to effectively handle large (e.g., > 10%) low-frequency input voltage fluctuations. This paper introduces the analysis of a swinging bus in the context of fuel cell standalone applications (i.e., voltage–Source inverters) and proposes a nonlinear control approach to operate inverters with very large input voltage swing: the natural switching surface (NSS). Under the proposed scheme, the inverter presents excellent dynamic and steady-state characteristics, even at moderate switching frequency (e.g., 3.6 kHz). In order to illustrate the superior performance of the NSS, a comparison to a proportional-resonant (PR) controller is performed. Unlike the linear compensator, the NSS is able to reject the large bus voltage oscillations and achieve high-quality output voltage with low total harmonic distortion (THD). Simulation and experimental results are provided to illustrate the behavior of the swinging bus and to validate the NSS control scheme under the proposed demanding operating conditions. | 2015 |

PEI-5 | Power Controllability of a Three-Phase Converter With an Unbalanced AC Source | Three-phase dc–ac power converters suffer from power oscillation and over current problems in case of the unbalancedac source voltage that can be caused by grid/generator faults. Existing solutions to handle these problems are properly selecting and controlling the positive- and negative-sequence currents. In this paper, a new series of control strategies which utilize the zero sequence components are proposed to enhance the power control ability under this adverse condition. It is concluded that by introducing proper zero-sequence current controls and corresponding circuit configurations, the power converter can enable more flexible control targets, achieving better performances in the delivered power and the load current when suffering from the unbalanced ac voltage. | |

PEI-6 | A Novel Control Strategy of Suppressing DC Current Injection to the Grid for Single-Phase PV Inverter | Photovoltaic (PV) inverters without the isolation transformer become more attractive due to higher efficiency andlower weight. However, it may have dc offset current problem and is critical to the power system. In this paper, a novel control strategy of suppressing dc current injection to the grid for PV inverters is investigated. It is based on the idea of accurately sensing the dc offset voltage of PV inverter output. Since dc component of the inverter output can be eliminated, dc injection to the grid can be effectively suppressed. Finally, the control scheme is verified by the experiment. | 2015 |

PEI-7 | Switching State Vector Selection Strategies for Paralleled Multilevel Current-Fed Inverter Under Unequal DC-Link Currents Condition | A paralleled multilevel inverter topology consists of n three-phase three-level current-fed inverters (CFIs) connected in parallel on ac side. AC currents with (2n + 1) levels can be generated utilizing redundant switching states when all three-level inverters have equal dc-link currents. However, the multilevel space vector diagram gets modified, redundancy in the switching states is lost and the multilevel current pattern changes when the dclink current of all inverters is not the same. This introduces low frequency harmonics in output current, thereby deteriorating total harmonic distortion (THD). The presence of low-frequency components in the output current could be avoided by selecting suitable switching state vectors and ensuring proper time sharing among these vectors. Two methods to select such switching state vectors are proposed in this paper. In the first method, a reference current space vector is realized using the nearest switching state vectors. However, this method results in low-frequency pulsation in dc-link voltage of each inverter. In the second method, the switching state vectors are chosen to eliminate this low-frequency pulsation. Effectiveness of these methods is experimentally validated for a five-level CFI. Further, performance of these methods is compared based on efficiency, THD, and dc-link voltage ripple for various inequality ratios in dc-link currents. | 2015 |

PEI-8 | A High-Efficiency MOSFET Transformer less Inverter for Non isolated Micro inverter Applications | State-of-the-art low-power-level metal–oxide–semiconductor field-effect transistor (MOSFET)-based transformer lessphotovoltaic (PV) inverters can achieve high efficiency by using latest super junction MOSFETs. However, these
MOSFET-based inverter topologies suffer from one or more of these drawbacks: MOSFET failure risk from body diode reverse recovery, increased conduction losses due to more devices, or low magnetic utilization. By splitting the conventional MOSFET based phase leg with an optimized inductor, this paper proposes a novel MOSFET-based phase leg configuration to minimize these drawbacks. Based on the proposed phase leg configuration, a high efficiency single-phase MOSFET transformer less inverter is presented for the PV micro inverter applications. The pulse width modulation (PWM) modulation and circuit operation principle are then described. The common-mode and differential-mode voltage model is then presented and analyzed for circuit design. Experimental results of a 250Whardware prototype are shown to demonstrate the merits of the proposed transformer less inverter on non isolated two-stage PV micro inverter application. |
2015 |

PEI-9 | Frequency Adaptive Selective Harmonic Control for Grid-Connected Inverters | A frequency adaptive selective harmonic control (FA-SHC) scheme is proposed. The FA-SHC method is developed from a hybrid SHC scheme based on the internal model principle, which can be designed for grid-connected inverters to optimally mitigate feed-in current harmonics. The hybrid SHC scheme consists of multiple parallel recursive (nk ±m)-order (k=0, 1, 2, . . ., andm ≤ n/2) harmonic control modules with independent control gains, which can be optimally weighted in accordance with the harmonic distribution. The hybrid SHC, thus, offers an optimal tradeoff among cost, complexity, and also performance in terms of high accuracy, fast response, easy implementation, and compatible design. The analysis and synthesis of the hybrid SHC are addressed. More important, in order to deal with the harmonics in the presence of grid frequency variations, the hybrid SHC is transformed into the FA-SHC, being the proposed fractional order controller, when it is implemented with a fixed sampling rate. The FA-SHC is implemented by substituting the fractional order elements with the Lagrange-polynomial-based interpolation filters. The proposed FA-SHC scheme provides fast on-line computation and frequency adaptability to compensate harmonics in grid-connected applications, where the grid frequency is usually varying within a certain range (e.g., 50 ± 0.5 Hz). Experimentaltests have demonstrated the effectiveness of the proposed FA-SHC scheme in terms of accurate frequency adaptability and also fast transient response. | 2015 |

PEI-10 | An Extended Lyapunov-Function-Based Control Strategy for Single-Phase UPS Inverters | In this study, an extended Lyapunov-function-based control strategy that assures global asymptotic stability is proposed for single-phase UPS inverters. The Lyapunov function is formed from the energy stored in the inductor and capacitor due to the fact that the system states converge to the equilibrium point if the total energy is continuously dissipated. It is shown analytically that the classical Lyapunov-function-based control leads to a globally asymptotically stable system at the expense of steady-state errors in the output voltage, which exist due to the lack of outer voltage loop in the control input. Therefore, an extended Lyapunovfunction-based control strategy is proposed, which eliminates the steady-state error without destroying the global stability of the closed-loop system. The steady state and dynamic performance of the proposed control strategy has been tested by simulations and experiments under resistive and diode bridge rectifier loads. The results obtained from a 1-kW inverter demonstrate that the developed control strategy not only offers global stability, but also leads to good quality sinusoidal voltage with a reasonably low THD, almost zero steady-state error in the output voltage, and fast dynamicresponse under linear and nonlinear loads. | 2015 |