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This article introduces a signal reformation-based direct phase-angle detection (DPD-SR) technique for three-phase inverters supporting asymmetrical grids. Asymmetries in three-phase systems may happen because of unbalanced three-phase loads, which can lead to the voltage asymmetry at the terminals of grid-tied inverters. The proposed DPD-SR technique can detect the voltage phase angle under asymmetrical conditions. The DPD-SR technique also offers precise and rapid phase-angle detection through signal reformation and trigonometric functions. In essence, the phase-angle detection is directly derived from the trigonometric properties of the line–line voltages, while signal reformation is implemented to handle asymmetrical voltages. The proposed method measures two line–line voltages and detects the phase angle of a three-phase system under both symmetrical and asymmetrical conditions. Unlike classical phase-locked loop (PLL) techniques, DPD-SR does not need any closed-loop proportional-integral (PI) controller, which significantly reduces the complexity and the delay in detecting the phase angle. In this article, the efficacy of the proposed technique is examined in comparison with the state-of-the-art PLLs under asymmetrical conditions through a set of laboratory experiments. The performance of the phase-angle detection is also tested as part of an inverter feeding an asymmetrical grid.

An adaptive virtual-inductance feedforward scheme is presented in this article to enhance the stable operation of smart inverters in weak grids. In the developed scheme, the virtual inductance is updated in real time according to an adaptation law to ensure that the inverter follows the dynamic response of a robust reference model, thereby ensuring stability without the need for any grid impedance estimation. In this article, the developed scheme is validated with experimental results obtained from testing a small-scale two-level, 208-V, 2-kW inverter.

Traditionally, a grid-interactive inverter providing ancillary services is called a smart inverter. However, broader features will be required for the next generation of inverters that can be categorized as self-governing, self-adapting, self-security, and self-healing. For grid-interactive inverters, the self-governing feature can be identified as the capability of inverters to operate in grid-following and grid-forming control modes, where the self-adapting is referred to as more flexibility realized by adaptive controllers for stable dynamics of inverters under various grid conditions. Moreover, for supervisory control and economic dispatch in a grid with high-penetration of inverter-based power generators, a minimum communication might be necessary, but it can place grid-interactive inverters in danger of being hacked when self-security becomes essential to identify malicious setpoints. Furthermore, the self-healing is defined as fault-tolerance and stress reduction under abnormal conditions. It suggests that after realizing these features, an inverter is called a smart inverter. In this paper, the advancements toward achieving these features for grid-interactive inverters are reviewed.

DC stacked topologies have gained popularity for interfacing individual photovoltaic panels to a high-voltage DC bus. A similar approach can be used to interface individual battery packs to the same DC bus, which allows active balancing of battery substrings and the use of lower voltage switches. Any imbalance in battery current, however, can cause the converter output voltages to diverge. This paper describes the source of voltage divergence and demonstrates a solution that allows seamless stacking of multiple battery packs to be interfaced to a high voltage DC bus. To prevent the voltage divergence phenomenon and allow asymmetrical battery charging for efficient battery state of charge management, the PI-based decentralized voltage control enabling both capacitor voltage sharing and asymmetrical capacitor voltage control is proposed. The effectiveness of the decentralized voltage control strategy is demonstrated through controller hardware-in-the-loop test results.

The growth in load and generation sources at the edge of the grid is driving the innovation in power electronic (PE) grid interfaces at the consumer end to improve the grid resiliency, reliability, security, and cost of the future infrastructure. Novel PE technologies are key enablers for the future grid infrastructure to handle the issues that arise because of the projected growth. This paper introduces a novel fundamental building block (FBB) architecture with plug-and play features. An example framework that enables co-ordination of multiple FBBs hierarchically to provide various grid functions is also presented. The FBBs are designed to be equipped with advanced features like online health monitoring, embedded intelligence, and decision-making capability for enhancing the metrics of consumer end plug and play interfaces. The paper elaborates on the proposed architecture and the developed framework i.e., the controls, communication, protection, and the corresponding timing requirements for the building blocks. The architecture and the framework have been validated through simulations and the communication framework has been validated with a control hardware in the loop platform.

Grid-interactive inverters allow distributed generation units to provide various ancillary services in microgrids. As the linear modulation region of inverters is restricted by the dc-bus voltage, providing ancillary services may drive the fundamental positive-sequence inverter voltage to the overmodulation region. Therefore, to operate in the linear modulation region the maximum active- and reactive-power of the inverter may have to be reduced unless pulsewidth modulation (PWM) references are adjusted. In this paper, an atypical PWM method is proposed for maximizing dc-bus utilization of grid-interactive two-level voltage source inverters (2L-VSI) providing ancillary services. Two important ancillary services, i.e., negative-sequence compensation and harmonic compensation are considered in this paper. The proposed method increases the maximum active- and reactive-power that a 2L-VSI can deliver when providing ancillary services by injecting a common-mode component in the references computed based on instantaneous reference magnitudes. The validity of the proposed technique is verified through simulation, as well as experimental data obtained using a 208-V three phase grid-connected 2L-VSI.

In this paper, a step-ahead direct power control (DPC) scheme with reduced sensor count for grid-connected active rectifiers with LCL filter is formulated. Compared to classical model predictive control (MPC) schemes, which implement an L filter, the proposed controller is designed to operate with an LCL filter and thus benefit from higher harmonic attenuation and lower component size and weight. In this work, an expression is derived to allow the prediction of the grid-side current by only measuring the dc-bus voltage, grid voltages, and grid-side line currents. As a result, only 6 sensors are required as opposed to 9 or 12 sensors. An extensive digital computer simulation using Matlab/Simulink is used to demonstrate the validity and performance of the proposed step-ahead DPC scheme. The widely used classical proportional integral (PI)-based power controller is used as a reference to benchmark the steady-state and dynamic performance of the proposed control scheme. The results validate the proposed controller and show its sound steady-state performance consisting of high quality grid-side currents with low Total Harmonic Distortion and unity power factor. In addition, robust and rapid dynamic performance is demonstrated when compared to the classical PI-based controller.

An adaptive stabilization technique is presented in this paper to enable stable operation of inverters operating under weak grids. The large impedance associated with a weak grid can deteriorate the stability of inverters in grid-feeding, grid-supporting, and grid-forming modes of operation. In this paper, the gain of the cross-coupling current terms are adaptively varied using a direct model reference adaptive control (MRAC) approach to enhance the stability of inverters in weak grids. The direct MRAC approach does not require a parameter estimation stage, instead, the parameters are updated according to an adaptation law derived using the Lyapunov stability criterion to ensure that the response of the system follows the response of a stable reference model. The inner current loops are present in a controller operating in a synchronously rotating frame, irrespective of the operating mode of the inverter, making the presented technique suitable for various modes of inverter operation. The efficacy of the developed scheme is verified for various weak grid scenarios through simulation studies carried out in the MATLAB/Simulink environment.

Recent success in the fabrication of 1.2 kV high-performance GaN HEMTs has made this technology an interesting alternative for high power applications. In this paper, for the first time, the newly available 1200V/15A GaN HEMTs is experimentally studied. The switching behavior of the device is investigated precisely to provide a better perspective on this family of switches. A test converter circuit is built with the GaN HEMT devices, which has been tested under different voltages and frequencies. Various challenges faced throughout the designing and testing stages are identified and reported in this paper for further investigations in the future.

This paper presents a second order model for stability analysis of grid-connected voltage source inverters (VSIs). VSIs are commonly modeled as ideal voltage sources for analysis. However, this approach cannot guarantee accuracy since it does not take into consideration the dominant states of the inverter. In this paper, a reduced-order model based on balanced truncation method is derived from a comprehensive twelfth-order model for a grid-connected VSI. The developed second-order model is computationally modest while providing an accurate representation of the system characteristic. The efficacy of the developed model is verified through time-domain simulations and root locus studies of the model eigenvalues against its full-order counterpart.

This paper presents the stability analysis of voltage source inverters (VSIs) in the presence of asymmetrical weak grids. Large grid impedances associated with weak grids can make grid-connected VSIs unstable. State-of-the-art research on the stability in weak grids focuses solely on symmetrical grid impedances, with little attention paid to the asymmetrical grid impedance case. The stability analysis of VSIs in asymmetrical weak grids is studied in this paper based on a developed state-space model of a grid-connected VSI considering asymmetrical grid impedances. Root locus studies of the eigenvalues of the model show that an asymmetrical increase in the grid impedance could result in instability for an otherwise stable system in the balanced case. The results are verified through simulations and experiments for a three-phase 208V grid-connected hardware setup for both symmetrical and asymmetrical weak grid cases.

In this paper, a virtual inductance control scheme is proposed to improve the stability of voltage source inverters (VSIs) in weak grids. The large grid impedance in weak grids can make grid-tied VSIs unstable. To keep VSIs stable in weak grids, control and circuit parameters can be designed to expand the stable operating region of VSIs. In this paper, a closed-loop state-space model is developed for PQ-controlled VSIs with the virtual inductance term in the controller. Using the root locus analysis, impacts of the LCL filter inductance, as well as the virtual inductance feedforward controller on the stability of VSIs are studied. The performance of the virtual inductance is then verified for both symmetrical and asymmetrical grid impedance setups through hardware tests carried out on a three-phase grid-tied VSI in the laboratory.

This paper presents a high power factor and sinusoidal input current based electronic ballast using a AC-DC Cuk converter. Unlike traditional Cuk converters it can take AC voltages as inputs and by converting the input into high enough values it can also operate as a starter. The converter along with an input and output filter completes the proposed ballast circuit. Signals provided by a microcontroller or a SMPS at the gate of the switching device, with optimum duty cycles determined by sliding mode control for both the starting and running conditions, can ensure proper operation as both starter and ballast. The principle of operation, control scheme of the signal at the gate and the simulation results of the proposed circuit are included in the paper.