Four-leg Inverter Research Articles

Minimization of DC-Link Capacitance for a DC-Link-Based Variable Speed Constant Frequency Aircraft Power System

DC-link-based variable speed constant frequency (VSCF) ac power generating system is being considered as one of the popular power architectures in the new generation aircraft to provide backup power to the 115 V, 400 Hz aircraft loads. These systems directly convert the variable frequency output of the generator to the fixed frequency of 400 Hz using a voltage source inverter. These inverters use dc-link capacitors that are heavy and space consuming. In aircraft, the weight requirement is very critical; hence, minimizing the dc-link capacitance is one of the major research challenges to achieve high power density in the VSCF power converter. In this article, novel control strategies to minimize the dc-link capacitance of the ac–dc–ac VSCF power converter are presented. The proposed control strategies enable to achieve 95% reduction in dc-link capacitance value when compared with the conventional control strategy, while the control is robust enough to meet MIL-STD-704F transient specification at the inverter output. The proposed strategies are validated through controller hardware-in-loop testing of the VSCF system based on an 80 kVA brushless synchronous generator and a four-leg inverter. The transient and steady-state operation of the VSCF system with a 95% reduction in dc-link capacitance has been verified for rated power operation.

A Multilevel Converter Topology for a STATCOM System Based on Four-Leg Two-Level Inverters and Cascaded Scott Transformers

This paper presents a new power electronic topology for a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">STATCOM</i> allowing the generation of multilevel <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AC</i> voltages. This topology is characterized by a modular configuration since it uses two well known four-leg two-level voltage source inverters. A <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Scott</i> transformer with multiple secondary windings connects the two four-leg inverters to the grid. One of the important features of the proposed power topology is the capability to generate <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AC</i> voltages with high number of levels, in spite of using only two four-leg two-level voltage source inverters. To maximize the number of voltage levels a geometric progression ratio of 3 between the windings of the transformers connected in cascade is proposed. However, to reduce the turns-ratio span, alternatively a fractional geometric ratio of 1.5 is also proposed and analyzed. A control strategy adapted to the proposed power converter topology is also presented. Besides the control of the reactive power the regulation of the four-leg inverter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DC</i> voltage will also be considered. To support the theoretical studies and assumptions, simulations and experiments are presented. Experimental results were obtained using a low power laboratory prototype. The results shown give evidence to the characteristics and features of the proposed power converter topology and control system.

An Active Power Decoupling Topology for Three-phase Four-leg Inverters with Unbalanced Load

Traditional three-phase four-leg inverters are widely used in microgrid systems because of their ability to carry unbalanced loads, but unbalanced load currents will cause instantaneous power to contain double-frequency fluctuations, which will affect the normal operation of the microgrid system. In order to solve the above problems, this paper proposes an improved three-phase four-leg topology and its control strategy, so that the secondary pulsating power on the load side can be provided by the decoupling capacitor. Then the working principle of the proposed topology and its control strategy is analyzed, and the relationship between the current stress of the power device and various influencing factors is deduced. Finally, An experimental prototype was built to verify the effectiveness of the proposed solution.

Performance Comparisons of Three-Phase/Four-Wire Model Predictive Control-Based DC/AC Inverters Capable of Asymmetric Operation for Wave Energy Converters

A study on the capacity increase of a power converter according to the increase in the single capacity of wave energy converters and four-leg topology that can supply stable power even under unbalanced load conditions during independent operation is required. Therefore, in this paper, the performances of various four-leg inverters, from two-level inverters to three-level inverters, which are used as power converters for wave energy converters, are compared respectively. Since the four-leg converter has an unusual structure, the performance of each four-leg inverter was analyzed by applying the model predictive control that can easily and simply configure the controller. To verify the performance of each four-leg inverter, a comparison was performed under balanced load and unbalanced load conditions. Based on this, a suitable four-leg topology of the power converter for wave energy converters was confirmed.

Impedance Modeling and Stability Analysis of Three-Phase Four-Wire Inverter with Grid-Connected Operation

With the continuous penetration and development of renewable energy power generation, distributed grids and microgrids are becoming increasingly important in power systems. In the distribution networks and microgrids, the grid impedance is comparatively large and cannot be ignored. Usually, the parallel compensation is used to improve the grid quality. In these three-phase four-wire power systems, analyzing the impedance characteristics of the grid-connected inverter is vital to carry out the small-signal stability analysis. Thus, it is vital to consider the influence of the zero-sequence component in addition to the positive-sequence component and the negative-sequence component when it comes to analyzing system stability. In this paper, the impedances of three-phase four-wire split capacitor inverter and three-phase four-leg inverter are established. Based on the achieved impedance, the similarities and differences between the impedances of three-phase four-wire split capacitor inverter and impedance of three-phase four-leg inverter are studied. The main difference is reflected in zero-sequence impedance. Additionally, the zero-sequence impedance characteristics and the dominating factors deciding the zero-sequence impedance are analyzed. Then, the stability of the system considering the grid impedance and impedance of three-phase four-wire inverter is investigated by separately considering the stability of the positive–negative-sequence component and the stability of the zero-sequence component. Several cases of small-signal instability caused by the positive–negative-sequence component or zero-sequence component are revealed. The experimental results validate the theoretical analysis.

Minimization of Capacitor Voltage Difference for Four-Leg Inverter Dual-Parallel IM System

In the situations where multiple motors operate at the same speed, this article designs a dual-parallel induction motor (IM) system supplied by three-phase four-leg inverter for cost reduction and fault tolerance of the normal system. The dc offset and ac fluctuation of capacitor voltages exist in this system due to the motor phases connected to bus capacitors&#x2019; midpoint. It seriously affects voltage utilization and stable operational range. A novel synchronous control scheme with the ability of ac voltage fluctuation minimization (AVFM) and dc voltage offset elimination (DVOE) is proposed. The unified model of dual IMs is used to design the speed regulator, and the model predictive flux control method is combined with space vector modulation to realize torque and flux control of dual IMs. AVFM is realized by controlling the electrical angle difference of dual IMs as <inline-formula><tex-math notation="LaTeX">$\pi$</tex-math></inline-formula>, which is first presented in this scheme. An angle regulator is proposed to control the electrical angle by adjusting stator flux amplitude. DVOE is realized by injecting a compensatory voltage vector, which is simplified in this scheme to avoid the voltage filter or additional coordinate transformation in the conventional method. Experiment results verify the effectiveness of the proposed scheme.

Adjustable unbalanced current controller for tranformerless PV generation to suppress DC voltage ripples of inverter in low‐voltage ride‐through (LVRT) operation

Low voltage ride-trough (LVRT) feature is compulsory for each grid-connected inverter to support the grid by injecting reactive power in low voltage conditions. It is well known that the operation of an inverter in a normal grid condition is different from its operation in a faulty one. One of the detrimental effects of working in a faulty grid is ripples in the DC bus voltage due to the active power oscillation caused by unbalance voltages. Here, a new control system based on unbalance current injection is proposed to eliminate these ripples. For the analysis, a four-leg inverter is considered in a transformerless microgrid. Then, instantaneous power constraints, which are imposed by the capacity of the inverter, and low voltage ride-through necessities are considered to achieve the required reference currents. In addition, constant power strategy is used as a power control method and essential reference currents for this condition are presented. Moreover, the required reference currents are achieved to eliminate the ripples in the DC bus voltage when the constant power strategy cannot be performed because of current limitation restriction. Finally, simulation results are presented to prove the effectiveness of the proposed controller in different conditions.

A coordinated virtual impedance control scheme for three phase four leg inverters of electric vehicle to grid (V2G)

The three-phase four-leg (3p4L) inverter can be utilized to interface electric vehicles (EVs) with the distribution networks. Vehicle to grid (V2G) inverters are employed to charge EVs from the utility grid as well as compensating harmonics and unbalanced voltages & currents on demand. Motivated by this scenario, this paper proposes a coordinated virtual impedance control scheme for a vehicle to grid three phase four leg (V2G 3p4L) inverter. The proposed control scheme achieves simultaneously multifunctional objectives consisting of effective and good harmonic rejection capability, power sharing among V2G 3p4L inverter units with a smaller error, reactive power/voltage support during load power changes and balanced/unbalanced voltage sag conditions. An independently neutral current control approach is proposed for the fourth leg of the V2G 3p4L inverter to regulate the DC-link along with neutral current control capability. Besides, the proposed control scheme achieves grid connected mode (GCM) or standalone mode (SAM) operation capability as well as fulfilling smooth transition between operation modes without reconstructing control structure. Compared to well-known control schemes in the literature, synchronization during operation modes is also achieved without transients in voltage and current. Comprehensive tests under various scenarios validate the excellence of the proposed control scheme over existing methods.

Fault-tolerant control via four-leg inverter of a full-electric propulsion system for lightweight fixed-wing UAVs

The work deals with the development and the performance characterization of a novel control strategy for the detection, isolation and accommodation of coil faults in a three-phase Permanent Magnet Synchronous Motor (PMSM), used to drive the propeller of a modern lightweight fixed-wing UAV. The health-monitoring algorithms on motor currents (used to detect the open-circuit fault and to activate the control reconfiguration) are based on a slope method, associated to the evaluation of the current phasor trajectory in the Clarke plane. Actually, when an open-circuit fault occurs in PMSM driven by a standard three-leg converter, the typical circular trajectory of the current phasor in the Clarke plane collapses into a linear track and relevant torque ripples are generated. On the other hand, if the PMSM is driven by a four-leg converter, a control reconfiguration can be applied: the fourth leg of the power bridge is in stand-by when the system operates without faults, but it is enabled to regulate the current flowing at the central point of the Y connection of the 3-phase PMSM. The performances of the fault-tolerant algorithms are assessed via detailed nonlinear simulation of the propulsion system (including propeller loads, electrical faults, mechanical transmission compliance, digital signal processing and sensors errors). The results demonstrate that the health-monitoring algorithms and the fault-tolerant control strategies permit to obtain extremely small detection and isolation latencies, and negligible performance degradation in terms PMSM torque.

Space Vector Pulse-Width Modulation Control Strategy for Four-Leg Inverters Under Single Line-to-Ground Faults in Islanded Microgrids

In this paper, we propose a novel switching modulation scheme for three-phase four-leg inverters that enables the separation of the fault points occurring in AC distribution networks. The proposed method is aimed at continuously supplying normal voltage to the remaining healthy phases while applying zero voltage to any faulty phase when a single line-to-ground (SLG) fault occurs in the distribution network. This modulation scheme allows isolation without physically blocking the point of failure. This can prevent unnecessary continuous outages in the AC distribution network. To this end, we propose a novel coordinate transformation method to extract accurate values for the magnitude of unbalanced three-phase voltages. A novel space vector pulse-width modulation (SVPWM) method for switching schemes that are used under fault conditions is also presented. The proposed coordinate transformation considers a plane lying in a three-dimensional (3-D) space that describes the rotation of a three-phase voltage vector. This transformation converts a physical quantity that rotates in 3-D space into a value that moves in two-dimensional (2-D) space. Subsequently, the use of synchronous reference frame (SRF) transformation enables the detection and control of the voltage magnitude in case of an SLG fault. Additionally, switching vectors are selected to apply zero voltage to the faulty phase by analyzing the switching vector diagram of a four-leg inverter. Mathematical analyses conducted for the entire process are detailed herein. Further, the experimental results of four-leg inverters designed for the 10 kW class verify the proposed strategies.

Transient Operation of a Four-Leg Inverter in Rooftop Solar Connected to a Grid Using Optimized Constructive Neural Network

Transient Operation of a Four-Leg Inverter in Rooftop Solar Connected to a Grid Using Optimized Constructive Neural Network

Enhanced controller for a four-leg inverter operating in a stand-alone microgrid with unbalanced loads

&lt;span lang="EN-US"&gt;Stand-alone low voltage (LV) microgrids supplying small local loads far from the utility grid are becoming an increasingly popular alternative to a total reliance on the centralized utility grid. In most of LV microgrids, three-phase four-wire distribution systems are used to supply both single- and three-phase loads. Unequal distribution of loads can result in voltage unbalance problems. The use of the four-leg inverter is one of the best solutions for providing a neutral current path and compensating unbalanced load conditions in stand-alone LV microgrids. This paper proposes a fast control technique to compensate unbalanced voltage conditions for a four-leg inverter operating in a stand-alone LV microgrid. The suggested technique provides the current controller’s orthogonal component without introducing any additional dynamics or distortions. The major benefits of the recommended per-phase control technique over conventional orthogonal signal generation (OSG) methods are enhanced steady-state and dynamic performances as well as independency to the system parameters. Several simulation results are provided to confirm the superior performance of the suggested methods.&lt;/span&gt;

Frequency-Adaptive Repetitive Control for Three-Phase Four-Leg V2G Inverters

Three-phase four-leg inverters with an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter are employed as onboard vehicle to grid (V2G) inverters due to their advanced features—high utilization of the dc bus voltage, counteraction of unbalanced voltages, and so on. Repetitive control (RC) can exactly track periodic signals with known frequency, which offers a simple high-accuracy current tracking and harmonic suppression solution for three-phase four-leg inverters. However, the conventional RC (CRC) offers slow dynamic responses due to the embedded delay element, and grid frequency fluctuation would cause significant performance degradation of CRC controllers in V2G applications. Moreover, the three-order <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter often causes complexity in developing good performance feedback control schemes for the inverters, especially for complex three-phase four-leg ones. To address these problems, an easy-for-implementation hybrid control scheme that combines a frequency-adaptive RC (FARC) controller with a deadbeat (DB) controller is proposed to obtain fast and accurate control of feed-in grid current for the onboard three-phase four-leg V2G inverters with an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter in the presence of grid frequency variation and load changes. The fast-response DB control scheme is based on a simplified first-order model of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter. The FARC controller is immune to grid frequency fluctuation. Simulations and experiments have been done to prove that the DB plus FARC control strategy can provide an excellent current control solution for the three-phase four-leg V2G inverters.

Fault-Tolerant Control of a Three-Phase Permanent Magnet Synchronous Motor for Lightweight UAV Propellers via Central Point Drive

This paper deals with the development and the performance characterization of a novel Fault-Tolerant Control (FTC) aiming to the diagnosis and accommodation of electrical faults in a three-phase Permanent Magnet Synchronous Motor (PMSM) employed for the propulsion of a modern lightweight fixed-wing UAV. To implement the fault-tolerant capabilities, a four-leg inverter is used to drive the reference PMSM, so that a system reconfiguration can be applied in case of a motor phase fault or an inverter fault, by enabling the control of the central point of the three-phase connection. A crucial design point is to develop Fault-Detection and Isolation (FDI) algorithms capable of minimizing the system failure transients, which are typically characterized by high-amplitude high-frequency torque ripples. The proposed FTC is composed of two sections: in the first, a deterministic model-based FDI algorithm is used, based the evaluation of the current phasor trajectory in the Clarke’s plane; in the second, a novel technique for fault accommodation is implemented by applying a reference frame transformation to post-fault commands. The FTC effectiveness is assessed via detailed nonlinear simulation (including sensors errors, digital signal processing, mechanical transmission compliance, propeller loads and electrical faults model), by characterizing the FDI latency and the post-fault system performances when open circuit faults are injected. Compared with reports in the literature, the proposed FTC demonstrates relevant potentialities: the FDI section of the algorithm provides the smallest ratio between latency and monitoring samples per electrical period, while the accommodation section succeeds in both eliminating post-fault torque ripples and maintaining the mechanical power output with negligible efficiency degradation.

Experimental Verification for Active Power Filter based Four-Leg Inverter with SUI-PI Controller

Experimental Verification for Active Power Filter based Four-Leg Inverter with SUI-PI Controller

Digital Control of a Three-Phase Four-Leg Inverter to Feeds Non-Linear Loads

This paper presents a new control strategy for three-phase four-leg inverters, to meet the requirements of total and individual harmonic content imposed by IEC 61000-2-2 when the inverter feeds a non-linear reference load defined by IEC 62040-3. The analysis and design of the controllers are based on a discrete-time model, developed in the 0 reference frame. The proposed control strategy consists of an inner current control loop and an outer loop to control the phase voltages, using Proportional-Resonant controllers. The concept of harmonic impedance, proposed initially for single-phase inverters, is used to tune the controllers. Simulation and experimental results are presented to validate the harmonic impedance obtained and the design of the proposed control strategy when the inverter feeds non-linear loads

Prediction of DC-Link Voltage Switching Ripple in Three-Phase Four-Leg PWM Inverters

This paper presents a thorough prediction of DC-link voltage switching ripples in the three-phase four-leg inverters operating in balanced and unbalanced working conditions. The unbalanced modes examined here employ the highest degree of AC current imbalance while still preserving three-phase operation. This behavior can be found in many grid-connected or standalone grid-forming three-phase converters that supply “heavy” single-phase loads, comprising a recent trend in smart-grid, smart electric vehicle (EV)-charging applications. In this sense, for instance, the smart EV chargers might be employed in conditions when different power is drawn/injected from/to the grid, providing power conditioning services to the latter. The analysis of three-phase four-leg inverters is then extended to single-phase operations typical of home-charging or vehicle-to-home (V2H) applications. Their performances in terms of DC-link voltage switching ripple are demonstrated. Two of the most common carrier-based PWM modulation techniques are employed to drive the three-phase inverter—namely, sinusoidal PWM and centered PWM (carrier-based analogy of the space vector modulation). The derived mathematical expressions of peak-to-peak and RMS values of DC-link voltage switching ripple for balanced and unbalanced conditions are handy for designing the associated DC-link capacitor and estimating the overall efficiency of the converter. Extensive numerical simulations and experimental tests have been performed to validate the presented analytical developments.

Performance Enhancement of PPMIM Drives by Using Three 3-Phase Four-Leg Inverters

In this article, a multilevel inverter (MLI) scheme is proposed for 9-phase pole phase modulated induction motor (PPMIM) drives. The proposed inverter scheme is realized with the help of three 3-phase four-leg inverters. In 3-phase 12-pole mode (3PH-12PO), the proposed MLI supplies a 3-level voltage for each phase winding of the PPMIM drive with an improved harmonic profile, which results in better torque ripple. In this mode, the PPMIM drive performance is analyzed with a level shifted as well as phase shifted carrier based space vector pulse width modulation (SVPWM) techniques. On the other hand in 9-phase 4-pole mode (9PH-4PO), with help of phase grouping concept a 3-phase SVPWM implemented for enhancing the linear modulation of the 9-phase PPMIM drives by 15.4%. The fourth leg present in each 3-phase four-leg inverter is used effectively for suppressing the circulating currents in phase windings during 9PH-4PO operation. The proposed MLI is efficient to operate the PPMIM drive in both normal as well as fault conditions with rated power/torque. The proposed MLI-fed PPMIM drive performance with open circuit/short circuit of switch and source fault conditions are analyzed. The proposed MLI scheme is validated with the Ansys Maxwell finite element method (FEM) as well as laboratory testbed on a 5-hp 9-phase PPMIM drive under normal as well as open circuit switch/source fault conditions.

Disturbance observer-based composite voltage synchronisation control of three-phase four-leg inverter under load variation

The three-phase four-leg inverter is widely used in microgrid recently. The control strategy is critical for the performance of nonlinear four-leg inverter system owing to the problems of coupling, disturbances of nonlinear/unbalanced loads, variable operating condition, etc. In view of the above problems, a nonlinear composite controller is proposed for four-leg inverter system under reference frame by integrating a higher-order sliding mode observer (HOSMO) and finite-time feedback control technique. The HOSMO can observe the uncertainties and disturbances by means of the soft sensing manner for feedforward compensation, which economises some sensors for their states being inconvenient to measure. In addition, it is a novel way for the HOSMO to realise the decoupling of the inverter system under frame. The finite-time feedback controller works in coordination with the HOSMO which provides more rapid convergence rate and further improves anti-disturbance capacity of the system. Finally, the simulation results with PI comparison illustrate the feasibility of the proposed method for stabilizing system.

Reduced-Sensors-Based Predictive Controller for LC Filtered Four-Leg Inverters

The high number ofsensors, ten in total, required by the traditional finite control set model predictive control (FCS-MPC) strategy of a four-leg inverter with LC output filter increases the complexity and cost of the system and limits the practical application and commercialization. To simplify the implementation, reduce the cost and enhance the reliability, a novel current sensorless predictive load voltage control scheme, with only four voltage sensors and no current sensors, is proposed in this work. A new dynamic model of the LC filtered four-leg inverter is constructed in the stationary ( $\alpha \beta \gamma$ ) reference frame for the proposed load voltage and capacitor current observer. The dual-boundary-layer nonlinear feedback is combined with a linear feedback term to enhance the robustness while retaining the accuracy of the observer. Extensive experiments are performed to confirm the effectiveness of the proposed controller. The results validate that the proposed sensor-less FCS-MPC with only four voltage sensors achieves an equivalent performance to the conventional FCS-MPC with ten sensors.