Showing papers in "Iet Power Electronics in 2015"

Survey on non-isolated high-voltage step-up dc–dc topologies based on the boost converter

Abstract: The major consideration in dc-dc conversion is often associated with high efficiency, reduced stresses involving semiconductors, low cost, simplicity and robustness of the involved topologies. In the last few years, high-step-up non-isolated dc-dc converters have become quite popular because of its wide applicability, especially considering that dc-ac converters must be typically supplied with high dc voltages. The conventional non-isolated boost converter is the most popular topology for this purpose, although the conversion efficiency is limited at high duty cycle values. In order to overcome such limitation and improve the conversion ratio, derived topologies can be found in numerous publications as possible solutions for the aforementioned applications. Within this context, this work intends to classify and review some of the most important non-isolated boost-based dc-dc converters. While many structures exist, they can be basically classified as converters with and without wide conversion ratio. Some of the main advantages and drawbacks regarding the existing approaches are also discussed. Finally, a proper comparison is established among the most significant converters regarding the voltage stress across the semiconductor elements, number of components and static gain.

Cell capacitor sizing in multilevel converters: Cases of the modular multilevel converter and alternate arm converter

Abstract: Multilevel converters, such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), rely on charged capacitors in their cells to generate their AC voltage waveform. Since the cell capacitors are physically large and occupy approximately half the cell volume, their capacitance must be kept minimal while limiting the voltage fluctuation caused by the current passing periodically through these capacitors. This study proposes a mathematical model which estimates the energy deviation for the stacks of both the MMC and the AAC during steady-state operation under any power factor and for AC voltage magnitude fluctuation of up to ±10%. The analysis is then used to calculate the minimum size for the cell capacitors in order to keep their voltage fluctuation within set boundaries for both topologies. The results show that the MMC requires 39 kJ/MVA of capacitive energy storage under sinusoidal modulation but this reduces with triplen injection modulation. The AAC has a lower requirement for storage in its cells of 11 kJ/MVA but the AAC has a six-pulse DC current ripple which requires a filter estimated to have a further 33% capacitive storage.

Robust sliding-mode control of dc/dc boost converter feeding a constant power load

Abstract: Tightly regulated power electronic converters show negative impedance characteristics and behave as a constant power load (CPL) which sink constant power from their input bus. This incremental negative impedance characteristics of tightly regulated point-of-load converters in multi-converter power systems have a destabilising effect on source converters and may destabilise the whole system. Similar phenomena also occur in many situations like dc microgrid, vehicular power system. Here, the authors present a robust pulse-width modulation-based sliding-mode controller for a dc/dc boost converter feeding the CPL in a typical dc microgrid scenario. A non-linear surface is proposed which ensures constant power to be delivered to the load. The existence of sliding mode and stability of the sliding surface are proved. The proposed controller is implemented using OPAL-RT real-time digital simulator on a laboratory prototype of dc/dc boost converter system. The effectiveness of the proposed sliding-mode controller is validated through simulation and experimental results under different operating conditions.

Single phase three-level neutral-point-clamped quasi-Z-source inverter

Abstract: This study presents a novel three-level neutral-point-clamped quasi-Z-source inverter in the single-stage buck-boost multilevel inverter family. The topology was derived by combining the properties of the quasi-Z-source network with those of a three-level neutral point clamped inverter. It features such advantages as low voltage stress of the switches, single-stage buck-boost power conversion, continuous input current, short-circuit withstandability and low total harmonic distortion of the output voltage and current. The authors present a steady state analysis of the topology along with a special modulation technique to distribute shoot-through states during the whole fundamental period. Component design guidelines for a single-phase case study system are described. All the findings have been confirmed by simulations and experiments. The topology could be recommended for applications requiring continuous input current, high input voltage gain and enhanced quality of the output voltage.

Interleaved high step-up DC–DC converter based on three-winding high-frequency coupled inductor and voltage multiplier cell

Abstract: This study presents an interleaved high step-up DC–DC converter based on three-winding high-frequency coupled inductor and voltage multiplier cell (VMC) techniques. The primary and secondary windings of each coupled inductor are inserted in the same phase and the third winding is inserted in the other phase. The VMC in each phase consists of two diodes, two capacitors, the secondary winding of the same phase coupled inductor and the third winding of the other phase coupled inductor. The voltage gain is increased and the output voltage is clamped across the capacitors of the VMCs. Then, the voltage across the power metal oxide semiconductor field effect transistors (MOSFETs) is decreased. The leakage inductance of the coupled inductors controls the output diode falling rate, which alleviates reverse recovery problems. The power MOSFETs are turned-on under zero current switching that helps to conversion efficiency improvement. Three modes of operation named as continuous conduction mode, discontinuous conduction mode and boundary conduction mode are investigated for the proposed converter. The carried mathematical analysis and satisfying operation of the proposed converter are verified via experimental results of an 870 W 60 V-input to 590 V-output laboratory prototype with 95.2% conversion efficiency.

Mitigation of power quality problems using distribution static synchronous compensator: a comprehensive review

Abstract: Electric power quality (PQ) in distribution system has become increasingly significant subject both for end users and power suppliers with the deregulation of the electric power market. The inadequate performance of conventional compensation devices to mitigate PQ problems has revealed the use of advanced power electronics based compensation devices. Distribution static synchronous compensator (DSTATCOM) is one of the shunt connected custom power devices used to improve PQ, voltage and reactive power support and to increase the capability of the auxiliary service for utility grid. This study presents a comprehensive review of the various DSTATCOM configurations for single-phase (two-wire) and three-phase (three or four-wire) systems and control strategies for the compensation of different PQ problems in distribution systems. In addition, comprehensive explanation, comparison and discussion on DSTATCOM technology are performed. Furthermore, latest trends, practical consideration and some future research fields on DSTATCOMs are discussed in detail. This is intended to present a broad perspective on the status of DSTATCOM technology to researchers dealing with compensation of PQ problems in distribution systems.

Observer-based open transistor fault diagnosis and fault-tolerant control of five-phase permanent magnet motor drive for application in electric vehicles

Abstract: To meet increasing demand for higher reliability in power electronics converters applicable in electric vehicles, fault detection (FD) is an important part of the control algorithm. In this study, a model-based open transistor fault diagnsosis method is presented for a voltage-source inverter (VSI) supplying a five-phase permanent magnet motor drive. To realise this goal, a model-based observer is designed to estimate model parameters. After that, the estimated parameters are used to design a sliding mode observer in order to estimate the phase current in an ideal model. Subsequently, the proposed FD technique measures the similarity between the estimated current and real current using cross-correlation factor. This factor is used for the first time in this study to define a FD index in VSI. The presented FD scheme is simple and fast; also, it is able to detect multiple open switch or open phase faults in contrast to conventional methods. On the other side, in order to track reference current of the motor, the estimated parameters are used to design a proportional resonant controller. The FD technique is used to operate a multiphase fault-tolerant brushless direct current (BLDC) motor drive. Experimental results on a five-phase BLDC motor with in-wheel outer rotor applicable in electrical vehicles are conducted to validate the theory.

H6-type transformerless single-phase inverter for grid-tied photovoltaic system

Abstract: There has been an increasing interest in transformerless inverter for grid-tied photovoltaic (PV) system because of the benefits of lower cost, smaller volume as well as higher efficiency compared with the ones with transformer. However, one of the technical challenges of the transformerless inverter is the safety issue of leakage current which needs to be addressed carefully. In addition, according to the international regulations, transformerless inverter should be capable of handling a certain amount of reactive power. In this study, a new H6-type transformerless inverter for grid-tied PV system is proposed that can eliminate the threat of leakage current. The proposed topology has also the capability to inject reactive power into the utility grid. Three-level output voltage employing unipolar sinusoidal pulse-width modulation can be achieved with the proposed topology. The proposed topology structure and detail operation principle with reactive power control are investigated. The relationship among the existing topologies and their reactive power control capability are also discussed. The proposed topology is simulated in MATLAB/Simulink software to initially verify the accuracy of theoretical explanations. Finally, a universal prototype rated 1 kW has been built and tested. The experimental results validate the theoretical analysis and simulation results.

High step-up DC–DC converter based on the switched-coupled-inductor boost converter and diode-capacitor multiplier: steady state and dynamics

Abstract: In this study a new scheme of a step-up converter with very high voltage gain is proposed. The scheme is based on a natural combination of the switched-coupled-inductor boost converter and the diode-capacitor multiplier. The study proposes a special scheme of their mutual use for attaining very high voltage gain. An important advantage of the proposed circuit is the avoidance of the current spikes through the transistor and diodes because of the leakage inductance of the coupled inductors. The scheme provides soft commutation of the switch and the diodes. The study analyses the modes of operation and obtains the basic fundamental relations in steady state; an expression for voltage stress across the switch is derived. A new method for dynamic analysis is proposed. The corresponding analytical expressions and curves of the transient behaviour are also obtained. Modelling of the proposed structure and the experimental results are in full agreement regarding the expected efficiency and correctness of the theoretical analysis. A 100 W laboratory prototype was built and verified.

High-voltage gain dc–dc boost converter with coupled inductors for photovoltaic systems

Abstract: This study presents the analysis, design and experimental evaluation of a high-voltage gain dc–dc converter applied to photovoltaic (PV) systems. A PV module rated at 17 V is connected to the input side, whereas the converter is responsible for stepping the voltage up to 311 V with the achievement of maximum power point tracking (MPPT). An experimental prototype rated at 100 W is implemented, which does not employ electrolytic capacitors, thus increasing the useful life of the arrangement and also allowing the incorporation of the converter to the PV module. Prominent advantages of the topology are low cost and simplicity in terms of the MPPT algorithm and the system implementation, which are mandatory characteristics for renewable energy applications.

Evaluation and analysis of transformerless photovoltaic inverter topology for efficiency improvement and reduction of leakage current

Abstract: In transfomerless photovoltaic (PV) grid-connected inverter application, to reduce leakage current and to increase efficiency, many inverter topologies have been proposed. The method for increasing efficiency and reducing leakage current is evaluated and analysed in the present study. The operation of transfomerless PV inverter topologies with high-performance such as full-bridge, H5, H6, HERIC and paralleled-buck topology is analysed to calculate switching losses, conduction losses and free-wheeling losses. Device total losses for inverter topology are calculated according to the switching frequency and the output power. Also, a novel high-frequency model of inverter topology is proposed using the leg voltage and high-frequency equivalent circuit. The relation between the parasitic capacitor voltage and its leakage current is explained using the proposed high-frequency model. Also, the magnitude of the high-frequency voltage in the parasitic capacitor is derived mathematically. Efficiency of the several inverter topology according to the output power is compared and the leakage current is analysed using a novel high-frequency model. Finally, the efficiency and leakage current analysis are verified by simulation tool and 3 kW prototype.

Hybrid topology of symmetrical multilevel inverter using less number of devices

Abstract: The interest in development of newer topologies of multilevel inverter has been increasing rapidly in past few years. Recently introduced topologies achieve higher number of output voltage steps with reduced number of switches, DC voltage sources, voltage stress across switches and losses as compared with the conventional topologies. In this study, a new structure of symmetrical multilevel inverter is proposed. The proposed structure offers reduced number of controlled switches, power diodes and DC sources as compared with classical and recently proposed topologies in the literature. Reduction of switch count and DC voltage sources reduces the size, cost, complexity and enhances overall performance. Proposed topology is capable of producing 7, 9 and 11 levels of output voltage with seven switches only. Moreover, significant reduction in voltage stress across the switches can be achieved. A comparative analysis of proposed topology with the conventional topology and recently published topologies has been made in terms of controlled switches, power diodes, driver circuit requirement, DC voltage sources and blocking voltage. Multi-carrier pulse-width modulation strategy is adopted for generating the switching pulses. Simulation study of the proposed topology has been carried out using Matlab/Simulink and feasibility of topology has been validated experimentally.

Improved passivity-based control method and its robustness analysis for single-phase uninterruptible power supply inverters

Abstract: An improved passivity-based control (IPBC) method is proposed for single-phase uninterruptible power supply inverters. The proposed IPBC method is based on energy shaping and damping injection idea which is performed for regulating the energy flow of inverter to a desired level and assure global asymptotic stability, respectively. It is shown that the control of output voltage can be accomplished indirectly provided that the inductor current tracks its reference. Since the estimated parameters do not match with the actual parameters in practice, a perfect tracking without any output voltage error is not possible. To reduce the influence of parameter mismatch on the output voltage, an outer voltage loop is added into the feedback path of conventional passivity-based control (PBC) method. The robustness of both PBC methods has been investigated and analytical expressions in terms of estimated and actual parameters are derived for the output voltage. The effectiveness of the IPBC method in terms of both robustness and harmonic distortion is verified by the simulations and experiments under resistive and diode rectifier loads. The results demonstrate that the IPBC method not only leads to good quality output voltage with a reasonably low harmonic distortion, but also offers a global asymptotic stable operation with a strong robustness to wide range of parameter mismatch.

High-voltage-gain quadratic boost converter with voltage multiplier

Abstract: In this study, a novel high-gain quadratic boost converter with a voltage multiplier circuit is presented to give an alternative power electronic circuit for high-voltage conversion and low-to-medium power applications. The proposed converter combines the traditional quadratic boost converter and a coupled-inductor-based voltage multiplier circuit. Compared with the traditional quadratic boost converter, the proposed converter can obtain a much higher output voltage under the same duty cycle and input voltage, and can also reduce the voltage stresses in the power devices. Moreover, the serious input current ripple in the converter with coupled-inductor is reduced. Consequently, the efficiency and the reliability can be improved by using semiconductors with low-voltage level and high performance. The theoretical analysis of the proposed converter is verified by the experimental results.

Design, implementation issues and performance of an inductive power transfer system for electric vehicle chargers with series–series compensation

Abstract: The design, construction and evaluation of a contactless battery charger for electric vehicles (EVs) based on inductive power transfer (IPT) is presented in this study. The design of such systems entails a high degree of complexity because of the large number of design parameters involved and, consequently, trade-offs in selecting the key design parameters have to be established. The design process and selection of the IPT system parameters is detailed in this study, considering the most common specifications of EV chargers and the practical issues of the implementation. Regarding the compensation scheme, which is one of the main issues in the design, series compensation in both the primary and secondary has been adopted because of the advantages identified after a comprehensive analysis. A laboratory prototype has been built and tested, providing extensive results of the system performance in terms of efficiency and power transfer capability, considering load power variations, as well as changes in the air gap between coils. A detailed analysis of the efficiency of each stage in the IPT system and their contribution to the overall efficiency is also provided.

Class of high boost inverters based on switched-inductor structure

Abstract: This study deals with a class of the switched-inductor boost inverters (SLBIs) including the DC-linked type, continuous, discontinuous and ripple input current topologies. Compared with the conventional switched boost inverters, the SLBIs add only three diodes and one inductor to obtain a strong boost factor. In comparison with the conventional switched-inductor Z-source inverters (ZSIs) for the same voltage gain at the same modulation indexes, the proposed SLBIs use one more active switch and less passive components which results in reducing the size and cost. Moreover, the efficiency of the proposed inverters is improved in comparison with the conventional switched-inductor ZSIs. Analysis and operating principles are presented. A laboratory prototype was built with 24, 36, 48 V DC input and 127 Vrms output voltage to test high boost ability of the proposed inverters.

Symmetric and asymmetric transformer based cascaded multilevel inverter with minimum number of components

Abstract: In this study, a novel transformer based cascaded multilevel inverter is presented. The proposed inverter can operate in both symmetric and asymmetric topologies. The presented inverter benefits from the advantages such as reduced number of power switches and reduced total peak inverse voltage of the switching components. The numbers of insulated gate driver circuits are also decreased with respect to the power switches. Furthermore, the presented topology requires just a single DC source. In addition, the numbers of on-state switches in the current paths are reduced. Therefore the voltage drops across the switches are mitigated and as a result the efficiency of the presented inverter is improved. The mentioned advantages cause the implementation cost to be reduced. The operation of the converter is discussed thoroughly for both symmetric and asymmetric operations. The feasibility of the presented inverter topology is validated using the simulation results. Experimental results under 1.5 kW are also added to justify the theoretical analyses.

Coupled‐inductor boost integrated flyback converter with high‐voltage gain and ripple‐free input current

Abstract: DC–DC converters with high-voltage gain and low-input current ripple have attracted much attention in photovoltaic, fuel cells and other renewable energy system applications. Conventional boost–flyback converter can achieve high-voltage set-up ratio; however, its input current is pulsing and the voltage stress across output diode of flyback-cell is high. In this study, by incorporating coupled-inductor into the boost-cell of boost–flyback converter, the voltage stress across the output diode is effectively reduced. Passive snubber circuit is utilised to suppress the voltage spike across power switch, low-voltage-rated metal–oxide semiconductor field effect transistor (MOSFET) with low R ds_on can thus be used to reduce the conduction loss of power MOSFET. In addition, ripple-free input current can be achieved, which makes the design of electromagnetic interference filter easy. Steady-state characteristics of the proposed converter are analysed, and experimental results are given to verify the analysis results.

Non-isolated high step-up DC–DC converter based on coupled inductor with reduced voltage stress

Abstract: In this study, a novel topology for high step-up DC–DC converters is proposed based on coupled inductors. The proposed converter has many features, such as high conversion ratio, recycling the leakage energy of the coupled inductor, minimisation of voltage stress of components, simplicity of control and high efficiency. An optimisation process is employed to optimise coupled inductor turn ratios which minimise voltage stresses of the diodes and the switch. Minimisation of the switch voltage stress results in the use of switch with smaller on-resistance and improvement in the overall efficiency. The operation principles and steady-state analysis of continuous, discontinuous and boundary conduction modes are discussed in detail. The performance of the proposed converter is compared with the similar converters and it is concluded that new topology has considerable advantages over them. The simulation results verify the performance of the proposed circuit. A 250 W prototype with 400 V output voltage is implemented and the experimental results are in a good agreement with the simulation results.

Model predictive-based shunt active power filter with a new reference current estimation strategy

Abstract: This study presents a new reference current estimation method using proposed robust extended complex Kalman filter (RECKF) together with model predictive current (MPC) control strategy in the development of a three-phase shunt active power filter (SAPF). A new exponential function embedded into the RECKF algorithm helps in the estimation of in phase fundamental component of voltage ( v h ) at the point of common coupling considering grid perturbations such as distorted voltage, measurement noise and phase angle jump and also for the estimation of fundamental amplitude of the load current ( i h ). The estimation of these two variables ( v h , i h ) is used to generate reference signals for MPC. The proposed RECKF-MPC needs less number of voltage sensors and resolves the difficulty of gain tuning of proportional-integral (PI) controller. The proposed RECKF-MPC approach is implemented using MATLAB/SIMULINK and also Opal-RT was used to obtain the real-time results. The results obtained using the proposed RECKF together with different variants of Kalman filters (Kalman filter (KF), extended KF (EKF) and extended complex KF (ECKF)) and PI controller are analysed both in the steady state as well as transient state conditions. From the above experimentation, it was observed that the proposed RECKF-MPC control strategy outperforms over PI controller and other variants of Kalman filtering approaches in terms of reference tracking error, power factor distortion and percentage total harmonic distortion in the SAPF system.

Design and implementation of 15-level cascaded multi-level voltage source inverter with harmonics elimination pulse-width modulation using differential evolution method

Abstract: This study describes the application of a soft computing method - known as differential evolution (DE) to implement the harmonics elimination pulse-width modulation (HEPWM) for a 15-level, multi-level voltage source inverter (MVSI). The main feature of this technique is the wide range of fundamental output voltage that can be achieved, while maintaining a very low total harmonics distortion (below 6%). The fundamental voltage can be controlled very precisely (very small step size) and the trajectories of the HEPWM angles are continuous over the whole range of modulation index - making it very suitable for utility and variable speed drives application. The study details the procedures to obtain the HEPWM angles trajectories, which include the formulation of objective functions, appropriate selection of the DE evolutionary parameters and stopping criterions. The viability of the proposed method is simulated using MATLAB/Simulink and verified by a three-phase, cascaded-type MVSI test rig. Experimentally, The HEPWM waveforms are implemented using the Altera field programmable gate array, whereas the 15-level MVSI is built using metal-oxide semiconductor field effect transistor H-bridge. Selected results are presented; the simulation and hardware results are found to be in very close agreement with the theoretical predictions.

High step-up Z-source DC–DC converter with coupled inductors and switched capacitor cell

Abstract: Z-source converter has several advantages such as high-voltage gain, clamped switch voltage and positive output voltage polarity. This study presents a new non-isolated high step-up DC–DC converter which is derived from Z-source converter and provides higher-voltage gain compared with its conventional counterpart. Owing to its high-voltage conversion ratio, the proposed converter is a proper choice for photovoltaic applications. Furthermore, reverse-recovery problems caused by the output diode are reduced in the proposed converter which reduces the switching losses. In addition, the leakage energy is recycled so the conversion efficiency is improved. Analysis and operating principles of the proposed converter are discussed and design guidelines are presented. Moreover, the effect of non-ideal elements on the proposed converter performance is analysed. A 100 W laboratory prototype to convert 20–300 V is implemented and experimental results are presented to verify theoretical analysis.

Family of quadratic step-up dc–dc converters based on non-cascading structures

Abstract: The interconnection of basic switching converters to increase power handling capabilities in dc applications has been widely reported in technical literature. This study presents a family of switching step-up dc-dc converters based on the principle of reduced redundant power processing ( R 2 P 2 ). This principle states that the power transfer from input port and output port on interconnected converters is reduced when a non-cascading connection is used. As shown in this study, the principle R 2 P 2 is useful for developing new converters. These have wide conversion ratios and quadratic dependence with respect to the duty ratio. In this study, the voltage conversion ratios, average models and steady-state operating conditions for proposed converters are derived, which are verified by experimental results.

Sliding mode harmonic compensation strategy for power quality improvement of a grid-connected inverter under distorted grid condition

Abstract: A novel sliding mode harmonic compensation (SMHC) scheme is proposed for the enhanced power quality in distributed generation systems under distorted grid condition. The harmonic pollution caused by non-linear loads in electrical networks brings about distorted grid voltage, power losses and heating in electrical equipments. The proposed SMHC scheme is composed of a harmonic detector and a sliding mode harmonic current controller based on the integral sliding mode control. By using the fourth order band pass filter, the proposed harmonic detector can effectively extract harmonic components without phase delay. These harmonic components can be notably suppressed by adopting the sliding mode harmonic current controller with fast dynamic response. Whereas the conventional sliding mode schemes have been developed to control the entire current value, the proposed SMHC scheme controls only the harmonic components by dividing inverter voltage model into the fundamental and harmonic models. Since the fundamental component in charge of power flow is controlled by proportional-integral controller, the chattering can be quite reduced. The proposed scheme is a non-selective harmonic compensation, which reduces the computational burden than the conventional selective schemes. The validity of the proposed scheme is demonstrated through simulations and experiments using 2 kVA laboratory prototype grid-connected inverter.

Generalised approach for predictive control with common-mode voltage mitigation in multilevel diode-clamped converters

Abstract: This study proposes a generalised approach based on model predictive strategy for the current control, dc-link capacitor voltages balancing, switching frequency reduction and common-mode voltage mitigation in multilevel diode-clamped converters. A generalised discrete-time model of the converters is presented, where all the control objectives are formulated in terms of the switching states. The control goals are expressed as a cost function, and with the help of suitable weighting factors these goals are met simultaneously. The cost function minimisation is used as criteria for choosing the best switching state which would be applied to the converter during next sampling interval. The real-time digital control issues such as computational burden and delay compensation are also discussed. The feasibility of the proposed method is verified by simulations in three- to six-level converters, and by experiments in three- and four-level converters.

Hyper-plane sliding mode control of the DC–DC buck/boost converter in continuous and discontinuous conduction modes of operation

Abstract: In this study, a new proportional–integral-type hyper-plane sliding mode controller has been designed for output voltage control of the DC–DC buck/boost converter for its continuous and discontinuous conduction modes of operating conditions. The proposed controller is robust and stable against parameters uncertainties, load disturbance and variations of the converter input voltage. In addition, it is capable of cancelling the non-minimum phase nature effect of the converter so that the designed controller does not need to know the inductor reference current. Moreover, the coefficients of the controller have been designed so that the steady-state error of the converter asymptotically converges to zero. The controller is designed based on fixed-frequency equivalent control approach. Using MATLAB/SIMULINK toolbox and digital signal processor (TMS320F2810) from Texas Instruments, some simulation and practical results are presented to verify the capability and effectiveness of the proposed control approach.

Analysis and modelling of circulating current in two parallel-connected inverters

Abstract: Parallel-connected inverters are gaining attention for high power applications because of the limited power handling capability of the power modules. Moreover, the parallel-connected inverters may have low total harmonic distortion of the ac current if they are operated with the interleaved pulse-width modulation (PWM). However, the interleaved PWM causes a circulating current between the inverters, which in turn causes additional losses. A model describing the dynamics of the circulating current is presented in this study which shows that the circulating current depends on the common-mode voltage. Using this model, the circulating current between two parallel-connected inverters is analysed in this study. The peak and root mean square (rms) values of the normalised circulating current are calculated for different PWM methods, which makes this analysis a valuable tool to design a filter for the circulating current. The peak and rms values of the circulating current are plotted for different PWM methods, and a discontinuous PWM is identified which offers the minimum peak and rms value of the circulating current. Experimental results are presented to verify the analysis.

Design of adaptive H ∞ filter for implementing on state-of-charge estimation based on battery state-of-charge-varying modelling

Abstract: This study suggests a new method for modelling lithium-ion battery types and state-of-charge (SOC) estimation using adaptive H∞ filter (AHF). First, a universal linear model with some free parameters is considered for dynamical behaviour of the battery. The battery voltage and SOC are used as states of the model. Then for every period in the charge/discharge process the free parameters of the model are identified. Each period of process is associated with a specific SOC value, hence the parameters can be regarded as functions of SOC in the entire process. The functions are determined based on polynomial approximation and least squares method. The proposed SOC-varying model is incorporated in AHF for SOC estimation. Moreover, a new method for adjusting the tuning parameters of the filter is suggested. The proposed method is verified by experimental tests on a lithium-ion battery and is compared with adaptive extended Kalman filter and square-root unscented Kalman filter

Sigma-Z-source inverters

Abstract: To increase the boost capability with minimum passive components, existing transformer-based Z-source inverter (ZSI) topologies require the turn ratio to be increased. This results in larger transformer windings to be used in high dc–ac voltage gain applications. This shortcoming is addressed in this study by introducing a sigma-ZSI (ΣZSI) that improves the dc–ac voltage gain by reducing the turn ratio leading to a lower winding transformer. The proposed topology is compared with the TZSI as they used the same number of transformers and capacitors configured in an X-shaped network. The comparison shows that the ΣZSI will have a higher dc–ac voltage gain at a turn ratio lower than 1.618 for a given shoot-through duty cycle allowing the use of smaller transformers. Simulation and experimental results have validated the effectiveness of the proposed ΣZSI.

Robust optimal current control for grid-connected three-phase pulse-width modulated converters

Abstract: This study deals with the current control of three-phase inverters connected to the grid by means of LCL filters. The control action is given by the feedback of the filter states, in coordinates αβ0, and of the internal states of resonant controllers. The control gains are computed by means of an optimal linear quadratic regulator that is robust to uncertain and time-varying parameters related to the grid impedance at the point of common coupling. As contributions, one has the detailed proof of a robust discrete linear quadratic control, showing its applicability also for the time-varying case, the experimental validation of the results in terms of the total harmonic distortion and harmonic limits for the grid injected current, according to the IEEE 1547 standard, and the analysis of the ℋ∞ norm of the closed-loop system for several values of grid inductance, indicating a value of grid inductance for which one has best performance for the time-invariant case. For comparison, a conventional state feedback controller is designed and implemented, showing the limitation of the non-robust strategy.