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This paper proposes an indirect current control algorithm for seamless transfer of three-phase utility-interactive voltage source inverters. With the proposed method the inverter is able to provide critical loads with a stable and seamless voltage during the whole transition period including both clearing time and control mode change. The LCL filter design which is suitable for the indirect current control is also proposed to meet the harmonic limits. The proposed control method is validated through simulation and experiment. [ABSTRACT FROM AUTHOR]

This paper describes a hybrid active filter intended for mitigating the line-side harmonic currents of a three-phase 12-pulse diode rectifier used as the front end of a medium-voltage high-power motor drive. This hybrid filter is characterized by series connection of a simple LC filter and a small-rated active filter. This circuit configuration brings low cost, small size, and light weight to the hybrid filter. A three-phase experimental system rated at 400 V and 15 kW is designed, constructed, and tested, which is a downscaled model of the medium-voltage motor drive system. In this experiment, the LC filter is tuned to the 11th-harmonic frequency, and the active filter is based on a three-level neutral-point-clamped pulsewidth modulation converter (NPC PWM) with a dc capacitor voltage as low as 28 V. This hybrid filter is connected on either first or fourth winding of a line-frequency transformer with a first Δ-winding voltage of 400 V in the primary, and a second Δ-winding voltage of 220 V, a third Y-winding voltage of 220 V, and a fourth Δ-winding voltage of 400 V in the secondary. Experimental results show that the hybrid filter performs satisfactory filtering in a range from no-load to full-load conditions. [ABSTRACT FROM AUTHOR]

This paper analyzes the current reference calculation for the control of grid-connected voltage source converters meant to operate under voltage unbalanced sags produced by grid faults. The well-known reference calculation method that allows to control the active power ripple produced by the existence of negative-sequence components in the grid voltage is extensively analyzed. A type of voltage sag that produces unfeasible current reference values is identified and a possible workaround is proposed. Also, the need to compensate the power ripple produced by the filter inductance of the converter is demonstrated and an extension of the calculation method to compensate for this is introduced. The theoretical results are confirmed using simulation tools and experimental tests. [ABSTRACT FROM AUTHOR]

Grid-interfacing converter systems with enhanced voltage quality are proposed for microgrid applications in this paper. By adapting the conventional series-parallel structure, a group of grid-interfacing system topologies are proposed for the purpose of interfacing local generation/microgrid to the grid, or interconnecting microgrids. The functionality of the proposed systems is also reconfigured in order to ease the control design and to improve overall system performance, differing from existing series-parallel structure-based systems. Experiments with a concrete laboratory system are given to detail the proposed concepts and to demonstrate the practical implementations. Two three-phase four-leg inverters, together with dc microsources and nonlinear loads, are employed to construct a general series-parallel grid-interfacing system. Through the introduction of multilevel control objectives, it is illustrated that the proposed system could ride through voltage disturbances and continue the power transfer between the local generation and the grid, while a high-quality voltage is maintained for the local loads. The system also shows the possibility to achieve auxiliary functions such as voltage unbalance correction and harmonic current compensation. The main design aspects of the controllers are specified, and the entire system is effectively validated on a laboratory setup. [ABSTRACT FROM AUTHOR]

This paper comprehensively investigates and compares different multiloop linear control schemes for single-phase pulsewidth modulation inverters, both in stationary and synchronous (d -q) frames, by focusing on their steady-state error under different loading conditions. Specifically, it is shown how proportional plus resonant (P + R) control and load current feedback (LCF) control can, respectively, improve the steady-state and transient performance of the inverter, leading to the proposal of a PID + R + LCF control scheme. Furthermore, the LCF control and capacitive current feedback control schemes are shown to be subject to stability issues under second and higher order filter loads. Additionally, the equivalence between the stationary frame and d -q frame controllers is discussed depending on the orthogonal term generation method, and a d-q frame voltage control strategy is proposed eliminating the need for the generation of this orthogonal component. This is achieved while retaining all the advantages of operating in the synchronous d-q frame, i.e., zero steady-state error and ease of implementation. All theoretical findings are validated experimentally using a 1.5 kW laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper proposes a current control strategy with zero steady-state tracking error for grid-connected boost inverters. Two boost converters are controlled separately to achieve both output current tracking and balanced output voltages. Based on cascading control and feedback linearization, the inner- and outer-loop controllers are as the combination of the inversion of boost converter model and the output error compensation. After the boost converter model is linearized, proportional and resonant controllers are derived from the internal model principle to achieve zero tracking error. Experimental results confirm that the proposed control achieves good current tracking to meet stringent grid-connection harmonics standards. [ABSTRACT FROM AUTHOR]

This paper presents a dc microgrid including a switched-reluctance generator (SRG) with isolated boost dc–dc converter, a battery energy storage system, and a single-phase three-wire (1P3W) load inverter for generating ac 220/110 V 60-Hz outputs. The 400-V dc grid is established by the SRG with dc 48-V output followed by a current-fed push–pull (CFPP) dc–dc converter. The robust commutation and dynamic control schemes are developed for the SRG to have excellent output performances. For providing the domestic load power sources, a 1P3W transformerless 220/110 V inverter is established. A master and slave robust control schemes are proposed to yield balanced sinusoidal output voltage waveforms in both voltage outputs under varying load conditions. To improve the dc grid power supporting reliability, a battery energy storage system with bidirectional buck–boost interfacing converter is established. It can support the common dc bus voltage immediately when the main power source fault occurs. Conversely, the battery bank can be charged from the common dc bus through the same converter. Particularly, an auxiliary charger formed by a flyback switch-mode rectifier is equipped to allow the battery to be charged from the plug-in utility power as the long duration of microgrid fault occurs. Normal operation and good operating performance of the established microgrid are verified experimentally. [ABSTRACT FROM AUTHOR]

This paper presents a control strategy for input-parallel–output-parallel (IPOP) connected modular inverter systems. Each modular inverter is composed of a high-frequency isolated dc/dc converter followed by a dc–ac inverter. The control objective of the configuration is to achieve output current sharing among the constituent modules and also can suppress the circulating current among them successfully. The control of all the dc–dc converters in the proposed configuration is based on open-loop control by sharing a fixed and common duty cycle for them. For the control of dc–ac inverters, a three-loop control strategy, consisting of common output voltage regulation (OVR) loop, individual circulating current suppression (CCS) loops, and individual inner current tracking (ICT) loops, is proposed. The ICT loops are implemented by hysteresis current control. Based on hysteresis current control for all ICT loops, the relationship between the OVR loop and CCS loops is analyzed with revealing that both of them are decoupled. The compensator designs of the OVR loop and individual CCS loops are also presented. The effectiveness of the proposed control strategy is verified by simulation and experimental results of a 1100-W IPOP two-module inverter system with each module consisting of two-transistor forward dc–dc converter followed by a full-bridge dc–ac inverter. [ABSTRACT FROM AUTHOR]

This paper proposes a new high step-up dc–dc converter designed especially for regulating the dc interface between various microsources and a dc–ac inverter to electricity grid. The figuration of the proposed converter is a quadratic boost converter with the coupled inductor in the second boost converter. The converter achieves high step-up voltage gain with appropriate duty ratio and low voltage stress on the power switch. Additionally, the energy stored in the leakage inductor of the coupled inductor can be recycled to the output capacitor. The operating principles and steady-state analyses of continuous-conduction mode and boundary-conduction mode are discussed in detail. To verify the performance of the proposed converter, a 280-W prototype sample is implemented with an input voltage range of 20–40 V and an output voltage of up to 400 V. The upmost efficiency of 93.3% is reached with high-line input; on the other hand, the full-load efficiency remains at 89.3% during low-line input. [ABSTRACT FROM AUTHOR]

This paper presents a new active anti-islanding detection method for distributed power generation systems. This method is based on introducing a disturbance at the inverter output and observing the behavior of the voltage at the point of common coupling (PCC), which depends on the impedance connected to the PCC in an islanding situation. The islanding detection is based on the Goertzel algorithm. [ABSTRACT FROM AUTHOR]

In this paper, a cascade controller is designed and analyzed for a boost converter. The fast inner current loop uses sliding-mode control. The slow outer voltage loop uses the proportional–integral (PI) control. Stability analysis and selection of PI gains are based on the nonlinear closed-loop error dynamics. It is proven that the closed-loop system has a nonminimum phase behavior. The voltage transients and reference voltage are predictable. The current ripple and system sensitivity are studied. The controller is validated by a simulation circuit with nonideal circuit parameters, different circuit parameters, and various maximum switching frequencies. The simulation results show that the reference output voltage is well tracked under parametric changes, system uncertainties, or external disturbances with fast dynamic transients, confirming the validity of the proposed controller. [ABSTRACT FROM AUTHOR]

A novel H-bridge multilevel pulsewidth modulation converter topology based on a series connection of a high-voltage diode-clamped inverter and a low-voltage conventional inverter is proposed in this paper. A dc link voltage arrangement for the new hybrid and asymmetric solution is presented to have a maximum number of output voltage levels by preserving the adjacent switching vectors between voltage levels. Hence, a 15-level hybrid converter can be attained with a minimum number of power components. A comparative study has been carried out to present high performance of the proposed configuration to approach a very low total harmonic distortion of voltage and current, which leads to the possible elimination of the output filter. Regarding the proposed configuration, a new cascade inverter is verified by cascading an asymmetrical diode-clamped inverter, in which 19 levels can be synthesized in output voltage with the same number of components. To balance the dc link capacitor voltages for the maximum output voltage resolution as well as synthesize asymmetrical dc link combination, a new multi-output boost converter is utilized at the dc link voltage of a seven-level H-bridge diode-clamped inverter. Simulation and hardware results based on different modulations are presented to confirm the validity of the proposed approach to achieve a high-quality output voltage. [ABSTRACT FROM PUBLISHER]

This paper presents a three-stage 18-level inverter design with a novel control method. The inverter consists of a series-connected main high-voltage, medium-voltage, and low-voltage stages. The high-voltage stage is made of a three-phase, six-switch conventional inverter. The medium- and low-voltage stages are made of three-level inverters constructed by H-bridge units. The proposed control strategy assumes a reference-input voltage vector and aims to operate the inverter in one state per sampling time to produce the nearest vector to that reference. The control concept is based on representing the reference voltage in 60^\circ-spaced two-axis coordinate system. In this system, the inverter vectors’ dimensions are integer multiples of the inverter's dc voltage, and the expression of the inverter's vectors in terms of its switching variables is straightforward. Consequently, the switching signals can be obtained by simple fixed-point calculations. The approach of the proposed control strategy has been presented, the transformed inverter vectors and their relation to the switching variables have been defined, and the implementation process has been described. The test results verify the effectiveness of the proposed strategy in terms of computational efficiency as well as the capability of the inverter to produce very low distorted voltage with low-switching losses. [ABSTRACT FROM PUBLISHER]

The output voltage quality of some of the single-phase multilevel inverters can be improved when their dc-link voltages are regulated asymmetrically. Symmetrical and asymmetrical multilevel diode-clamped inverters have the problem of dc-link capacitor voltage balancing, especially when power factor of the load is close to unity. In this paper, a new single-inductor multi-output dc/dc converter is proposed that can control the dc-link voltages of a single-phase diode-clamped inverter asymmetrically to achieve voltage quality enhancement. The circuit of the presented converter is explained and the main equations are developed. A control strategy is proposed and explained in details. To validate the versatility of the proposed combination of the suggested dc–dc converter and the asymmetrical four-level diode-clamped inverter (ADCI), simulations and experiments have been directed. It is concluded that the proposed combination of introduced multioutput dc–dc converter and single-phase ADCI is a good candidate for power conversion in residential photovoltaic (PV) utilization. [ABSTRACT FROM PUBLISHER]

In this paper, the transient operation of a four-leg inverter equipped with an innovative control strategy under unbalanced load conditions is investigated. The inverter is proposed for transformerless hybrid power system applications, in order to provide simultaneous supply of three-phase and single-phase ac loads with balanced voltage and constant frequency. The four-leg inverter is controlled to ensure balanced voltage by means of a control strategy based on the decomposition of the supply three-phase voltage and current into instantaneous positive, negative, and homopolar sequence components using phasor representation. These three sequences are controlled independently in their own reference frames as dc signals. The implementation derived for the controller design is also described. The transient operation performance of the proposed control strategy has been tested in simulations with an average model and experimentally using a laboratory prototype. [ABSTRACT FROM AUTHOR]