Operation Of Voltage Source Inverters Research Articles

Comparative Evaluation of Three-Phase Inverter Topologies Based on Voltage Boosting Features

Voltage source inverter (VSI) is commonly used in industrial due to its stable operation and low cost. However, VSI needs to operate with an extra converter stage which is a DC-DC converter for voltage boosting purposes. In contrast, current source inverter (CSI) inherits voltage boosting features may become an alternative option to VSI. Yet, there were minimal research on CSI that dedicates to the voltage boosting features. This research focuses on comparing the voltage boosting features of CSI and VSI in both open-loop and closed-loop conditions. The performance of VSI and CSI are simulated using MATLAB/Simulink. Under open-loop operation, CSI produces a voltage boosting capability at approximately 55% higher than VSI. Yet, CSI suffers high THD percentage as compared to VSI for the same switching frequency. This high THD shortcoming can be easily resolved by using a simple CL filter. For closed-loop operation, VSI and CSI with voltage-controlled synchronous frame PI control systems are proven to have good reference tracking and harmonic rejection and are suitable to be implemented for household applications or for a standalone system. Interestingly, CSI closed-loop system can achieve a wider range of output due to the voltage boosting capability and provide a better quality of output waveform as compared to VSI.

DER integrated BTB-VSI based DSTATCOM for PQ enhancement

ABSTRACT This article demonstrates the performance of back-to-back connected voltage source inverter (BTB-VSI)-based distribution static compensator (DSTATCOM). In the proposed system, a three-phase three wire electrical utility system (EUS) is integrated with distributed energy resources (DER) through BTB-VSI. It has drawn more attention due to enhanced active power filtering, simple design procedure and performs reliable operation. The objectives of the research work is to provide clean power for weak EUS with improved power quality (PQ) such as, better voltage regulation, improved power factor (P.F), reduced total harmonic distortion (THD), balanced voltage at point of common coupling (PCC) and better optimal active power flow control (OAPFC), and so on. An artificial neurocontrol technique Kernel Hebbian Least Mean Square (KHLMS) is chosen to determine the accurate reference source current for operation of voltage source inverter (VSI). The BTB-VSI performance is investigated on both shunt compensation and OAPFC. The proposed system is converted to a simulation model and hardware unit using MATLAB/Simulink software and SPARTAN−6 field programmable gate arrays (FPGA) controller. Finally, the Simulation and experimental results illustrate that the shunt compensating ability with OAPFC of BTB-VSI performs better over VSI as per IEEE Std 1159–2019 grid code.

A Hybrid PWM Technique to Improve the Performance of Voltage Source Inverters

Due to the rapid advancement of power semiconductor devices, the use of voltage source inverters (VSIs) has gained widespread acceptance. As a consequence, the performance of the voltage source inverter has emerged as a critical aspect that is highly reliant on the modulation strategy. The pulse width modulation (PWM) technique is the most widely utilized method of controlling power semiconductor switches of VSI. Power quality is always considered as an industrial concern for VSI-based power system such as grid-connected renewable energy systems and industrial motor drives which largely depends on the PWM technique used for switching. However, the existing PWM schemes for VSIs suffer from high total harmonic distortion (THD) and power loss problems. To mitigate the THD and power loss of VSI, a hybrid PWM technique has been proposed in this paper. The proposed hybrid PWM technique introduces a modified modulating signal along with newly shaped carrier signal. A two-level VSI is used to evaluate the performance of the proposed hybrid PWM technique both for with and without filter conditions against RL load. The proposed hybrid PWM technique offers 0.89% filtered voltage THD and 0.69% filtered current THD which are lower than that of existing PWM techniques. On the contrary, at without filter condition, the proposed hybrid PWM technique shows THDs of 45.77% and 12.50% for inverter output voltage and current, respectively which are also lower than those of existing PWM techniques. Apart from these, the proposed hybrid PWM technique reduces the switching and conduction power losses of the VSI as compared to existing PWM techniques. The simulation works are carried out in MATLAB/Simulink environment and a reduced scale experiment is conducted in laboratory to evaluate the performance of the proposed hybrid PWM technique.

A Harmonic Suppression SVPWM Strategy for the Asymmetric Six-Phase Motor Fed by Two-Level Six-Phase VSI Operating in the Overmodulation Region

For the asymmetric six-phase motor fed by a two-level six-phase voltage source inverter (VSI), when the modulation index is less than 0.5774, the motor operates in the linear region, and the harmonics in the Z1 − Z2 subspace can be suppressed to zero. When the modulation index is beyond 0.5774 and less than 0.6221, the motor operates in overmodulation regions, and the harmonics in the Z1 − Z2 subspace cannot be suppressed to zero. To minimize the harmonics in the Z1 − Z2 subspace for these regions, a harmonic suppression strategy, namely, harmonic suppression overmodulation strategy (HSOS), is proposed in this paper. The lower-order harmonics of the Z1 − Z2 subspace, namely, the 5th, 7th, 17th and 19th, are considered. Compared with the traditional four-vector overmodulation strategy (TFOS), the content of the 5th harmonic is reduced by about 20%, and the total harmonic distortion (THDZ1Z2) of these four kinds of harmonics is reduced by about 21% in the proposed HSOS. Finally, the simulation and experiment are carried out to verify the effectiveness of the proposed strategy.

Research and Development of Adjustable Discontinuous Pulse Width Modulation Method for Three-Phase Voltage Source Inverter

Continuous pulse width modulation (CPWM) and discontinuous pulse width modulation (DPWM) strategies are used to control voltage source inverter operation. CPWM strategies allow for the reduction of total harmonic distortion values, while DPWM strategies provide a more effective reduction in inverter switching losses. The paper is devoted to the problem of improving the three-phase voltage source inverter efficiency by a controlled transition from CPWM to DPWM. The article proposes an adjustable discontinuous pulse width modulation (ADPWM) method, which means a transition from space vector PWM (refers to CPWM strategies) to DPWM in all inverter phases when the switches’ temperatures exceed the allowable value in at least one of the inverter phases. The method flowchart is presented and an algorithm for modifying the envelope curve within one sector is given. In order to test the ADPWM method a Simulink-model of a three-phase voltage source inverter and its control system were developed. A study of the proposed ADPWM method’s efficiency in comparison with CPWM, PCDPWM and NCDPWM methods was carried out using a Simulink-model. It was established that the proposed ADPWM method provides dynamic losses reduction in the inverter switches by 2.86 times compared to CPWM and by 1.89 times compared to PCDPWM and NCDPWM methods. Power supply systems for medium and high-power AC motors provide a promising area for application of the proposed ADPWM method.

Switching Frequency Formulation for Predictive Current Control in Grid-Connected VSI

In this paper, a switching frequency has been formulated for grid-connected voltage source inverter (VSI) operated using the Continuous Control Set-Predictive Current Control (CCS-PCC) technique. An instantaneous expression of switching frequency has been derived for the VSI operated using the predictive current control technique. The effect of sampling frequency has been analyzed on the switching frequency of the converter operation. The effect of sampling time has a direct impact on the switching frequency as the cost function of the PCC depends on the sampling frequency. The analysis provides a generalized mathematical expression of switching frequency for the VSI operating in current control mode using the predictive current control method. With the help of this generalized expression, the maximum switching frequency and minimum load current error have been obtained. Simulation has been performed for verification of the analytical results through a Typhoon HIL-402 kit for real-time simulation and experimental validation.

A Novel H∞ Robust Control Strategy for Voltage Source Inverter in Microgrid

The voltage control performance of the voltage source inverter (VSI) in a microgrid may change under different load conditions. However, in the case of traditional control strategies, the robustness of VSI is insufficient. In response to the above problems, a novel robust control scheme for VSI in the microgrid based on H∞ hybrid sensitivity is proposed in this study. The grid-side interference during the VSI operation is taken as the variable, and the sensitivity function is designed to build a H∞ robust voltage controller for VSI. In addition, an adaptive virtual impedance group is designed to further improve the voltage control robustness under a variety of operation conditions. Finally, comparative simulation experiments are carried out to verify the anti-interference ability of the proposed control strategy under different working conditions.

Load Sharing Scheme Incorporating Power Security Margins for Parallel Operation of Voltage Source Inverters

The interconnection of distributed energy resources (DERs) in microgrids (MGs) operating in both islanded and grid-connected modes require coordinated control strategies. DERs are interfaced with voltage source inverters (VSIs) enabling interconnection. This paper proposes a load demand sharing scheme for the parallel operation of VSIs in an islanded voltage source inverter-based microgrid (VSI-MG). The ride-through capability of a heavily loaded VSI-MG, where some of the VSIs are fully loaded due to the occurrence of an event is investigated. In developing analytical equations to model the VSI, the concept of virtual synchronous machines (VSM) is applied to enable the VSI mimic the inertia effect of synchronous machines. A power frame transformation (PFT) that takes the line ratios of the MG network into account is also incorporated to yield satisfactory transient responses of both network frequency and bus voltages in the MG network. A Jacobian-based method is then developed to take into account the operational capacity of each VSI in the VSI-MG. The resulting amendable droop control constrains the VSIs within their power capabilities when an event occurs. Simulation results presented within demonstrate the effectiveness of the proposed procedure which has great potential to facilitate efforts in maintaining system reliability and resiliency.

A Three-Phase Resonant Boost Inverter Fed Brushless DC Motor Drive for Electric Vehicles

The present article proposes a three-phase resonant boost inverter (TPRBI) to feed a permanent magnet brushless DC (PMBLDC) motor at the requested torque with low ripples due to the sinusoidal current injected into the PMBLDC motor. PMBLDC motors have the highest torque-to-weight ratio compared to other motors and are the best choice for electric vehicle applications. Conventionally, these motors are driven by voltage source inverters (VSI) with trapezoidal current injection, introducing unwanted torque ripples. Moreover, due to the buck operation of VSI, an extra power conversion stage is required to elevate the battery voltage level to desired DC-link voltage. This extra stage increases the number of components used, complexity of control and decreases the efficiency and reliability of the overall system. TPRBI injects sinusoidal current in the PMBLDC motor in the proposed method, thus minimizing the torque ripples. The proposed inverter also has an inherent voltage boost characteristic, thus eliminating the extra power conversion stage. The single-stage conversion from DC to boosted sinusoidal AC enhances the system reliability and efficiency and minimizes the cost and weight of the system. A MATLAB/Simulink model is presented along with simulation results and mathematical validation. A comparative evaluation of the proposed system with the conventional VSI-fed PMBLDC motor is presented in terms of induced torque ripples.

DVR-based power quality enhancement using adaptive particle swarm optimisation technique

This paper proposes a heuristic control of the series active power filter for power quality enhancement. In the context of power quality, the series active filter is better utilised as a voltage source controller contrary to its conventional usage as variable impedance. The present day utility system as a linear model is unsatisfactory and the steps are laid down to discuss utility systems as a nonlinear model. This paper deals with comparative analysis of particle swarm optimisation (PSO) and its contemporary modified PSO algorithms modified particle swarm optimisation (MPSO), and adaptive particle swarm optimisation (APSO). The harmonic reduction in the source current and sags/swells mitigation in the load voltage is carried out with the optimal tuning of the PI controller. The series active power filter as a harmonic suppressor with a specific reference controlled strategy is discussed in this paper. The synchronous reference frame (SRF) theory and its modified versions are used to generate a compensating signal. The hysteresis band current controller (HBCC) is used to perform the switching operation of voltage source inverter. Simulations are carried out in the MATLAB/Simulink environment.

Safe Bayesian Optimization for Data-Driven Power Electronics Control Design in Microgrids: From Simulations to Real-World Experiments

Micro- and smart grids (MSG) play an important role both for integrating renewable energy sources in electricity grids and for providing power supply in remote areas. Modern MSGs are largely driven by power electronic converters due to their high efficiency and flexibility. Controlling MSGs is a challenging task due to requirements of power availability, safety and voltage quality within a wide range of different MSG topologies resulting in a demand for comprehensive testing of new control concepts during their development phase. This applies, in particular, to data-driven control approaches such as reinforcement learning, of which the stability and operating behavior can hardly be evaluated on an analytical basis. Therefore, the OpenModelica Microgrid Gym (OMG) package, an open-source software toolbox for the simulation and control optimization of MSGs, is proposed. It is capable of modeling and simulating arbitrary MSG topologies and offers a Python-based interface for plug & play controller testing. In particular, the standardized OpenAI Gym interface allows for easy data-driven control optimization. The usage and benefits of OMG for designing and testing data-driven controllers are demonstrated utilizing Bayesian optimization. Both the current and voltage control loops of a voltage source inverter operating in standalone, grid-forming mode for a remote MSG are automatically tuned given an uncertain application environment. Finally, the transfer to real-world laboratory experiments is successfully demonstrated.

Pre‐filter phase‐locked loop: principles and effects with inter‐harmonic perturbation

The phase-locked loop (PLL) is an available synchronisation technique that can extract the fundamental frequency and phase of the grid. Benefited from the filter properties, pre-filter PLL is widely applied in voltage source inverters (VSIs) to enhance the accuracy of phase extraction. With the consideration of inter-harmonic perturbation, the principles between pre-filter PLL and VSI performance as well as the effects of pre-filter PLL on VSI are investigated in this study. Based on the conventional PLL, the small signal of pre-filter PLL is proposed. To fully represent the pre-filter PLL effects, the simplified VSI model is established and the current performance of VSI could be presented by the pre-filter PLL. From the angles of current total harmonic distortion, the effects of pre-filter PLL under inter-harmonic perturbation are clarified. The effects of pre-filter PLL on VSI are validated under the inter-harmonic perturbation based on the experiment platform. The results indicate that the current performance of VSI could be improved with the decreased bandwidth of both pre-filter and PLL, which shows a valuable application reference for the optimised design of VSI.

Impedance-Based Analysis of Digital Control Delay in Grid-Tied Voltage Source Inverters

Digital control delay is studied as one of the leading factors causing instability in grid-tied voltage source inverter (VSI) systems. However, how this delay affects the small-signal output impedance of the VSI and how the change in impedance shape further influences the system stability are still not clear. This article, for the first time, thoroughly analyzes the impact of delay on the dq-frame output impedance of the VSI operating in both the single current loop control mode and the dual-loop control mode. It is found that when delay is considered, a distinct magnitude drop and a phase rise always exist at the high-frequency area of the impedance, which is named the “impedance trough.” By studying the dq-frame output impedance of the VSI, the origin of the impedance trough, and its analytical relationships with the current loop dynamics, voltage loop dynamics, and delay values are unveiled. Meanwhile, the influence of the impedance trough on the stability of the grid-tied VSI system under these two control modes is evaluated through d-d channel impedance matching and generalized Nyquist criterion (GNC). It is demonstrated that the joint effects of delay and control parameters result in the impedance mismatch between the grid and the VSI, which is the root cause of high-frequency oscillations in the grid-tied system. Finally, experimental results verify the analysis.

Modelling and Controlling of Induction Heating Unit for Induction Cooking Application

A study on the control methodology of voltage source inverter (VSI) fed series resonant circuit for induction heating has been done in this work with the goal that converter can be worked at about unity power factor. In this work, phase locked loop (PLL) has been accustomed to bring the converter’s switching frequency near to the natural frequency of series coil and capacitor branch of induction heater. Using this control method, the effectiveness of induction heater can be improved and necessity of input power is turned out to be less when compare to the fixed frequency operation. This control technique gives reliable operation of voltage source inverter fed circuit for varying load. Controlled phase locked loop gives increment or decrement in resonance frequency corresponding to the phase difference in voltage and current of resonant circuit. The coil of the induction heater is utilized as load in the resonant circuit of inverter. This power control procedure enables the inverter to operate near to the natural frequency for controlling power at all level. In this work, ZVS (zero-voltage switching) and ZCS (zero-current switching) condition has been obtained, and the switching losses are minimized. Pulse density modulated (PDM) power control method has been simulated to control the output power of a half bridge VSI fed induction heater.

A Five-Level Space Vector Modulation Scheme for Parallel Operated Three-Level Inverters With Reduced Line Current Distortion

Parallel operation of voltage source inverters is useful to achieve larger power capacity and improve the system reliability, which has become the trend of power system design. However, the current harmonic of the parallel system is the same as that of every inverter when synchronized pulses are used for the paralleled inverters. The interleaved operation is helpful to reduce the current distortion, though the space left for improvement is limited, and high-frequency circulating current inevitably emerges. A novel parallel operation scheme based on five-level space vector modulation applied on two parallel three-level inverters is proposed in this article, which can provide reduced line current distortion, smaller zero-sequence circulating current, and less switching transitions. Theoretical analysis reveals that it is viable to operate the parallel three-level inverters as a single five-level system, and specific criteria of switching sequences design are presented to ensure the simple implementation. A neutral point balancing algorithm is also developed based on the specific switching sequences of the proposed modulation process, which is essential to the normal function of the system. The simulation and experimental results verify that the proposed scheme has superior performance compared with the synchronized and the interleaved operation.

Comparative Evaluation of Three-phase Voltage and Current Source Inverter in Renewable Energy System

In the renewable energy power generation system, voltage source inverters (VSIs) are commonly used due to its stable operation, high efficiency and low cost. However, a dc-dc boost converter is necessary for VSI to operate has increased the system complexity. Thus, current source inverters (CSIs) has become an alternative of VSI due to its ability of voltage boosting and can provide a better output waveform. This paper presents a comparative evaluation of VSI and CSI focusing on topological circuits and basic operational parameters of these inverters. The performances of VSI and CSI are analyzed in MATLAB/Simulink focusing on two operational characteristics which are modulation index, ma and switching frequency, fsw. The performance of VSI and CSI are dependent on the modulation index, increasing ma increase the inverter efficiency. In term of switching frequency, it does not affect much on the efficiency of inverters, yet a normalized switching frequency of odd multiple of three provides better fundamental output and harmonic minimization. Furthermore, CSI presents its voltage boosting capability which can boost 6-27% within ma of 0.05-1.0 and using the same modulation technique, VSI and CSI have present duality relationship in term of output voltage and current wave shapes.

One-Cycle Controlled Bridgeless SEPIC with Coupled Inductors for PAM Control-Based BLDC Drive

This paper presents a novel approach for the speed control of BLDC motor for residential air conditioning application, using pulse amplitude modulation (PAM) control of voltage source inverter (VSI). PAM control of VSI is accomplished by using a bridgeless SEPIC converter embedded with coupled inductors at the front end and adopting one-cycle control (OCC) technique in the inner voltage control loop. The DC reference voltage required for inner voltage control loop is obtained using a PI controller in the outer speed control loop and speed feedback signal. The PAM control (DC supply voltage control) of VSI reduces switching losses by allowing the operation of VSI at fundamental frequency. Bridgeless SEPIC with coupled inductors is designed to enable PAM control for VSI and is operated in discontinuous conduction mode (DCM) for the complete range of DC link voltage. DCM operation simplifies power factor correction control scheme to a simple voltage follower approach, since it has inherent input current shaping feature. The introduction of coupled inductors in the bridgeless SEPIC converter lowers the overall count of components, allows better integration and lowers the requirement of inductance, compared to conventional bridgeless SEPIC. OCC which is a nonlinear control technique, used in the voltage control loop, enhances the performance with improved startup and transient state response. It also improves the quality of supply current drawn by reducing the distortion compared to PI control technique. The proposed BLDC motor drive is modelled and simulated using MATLAB/Simulink. The performance of proposed system is evaluated for a wide range of speed control. The experimental prototype for bridgeless SEPIC with coupled inductors is implemented. The inherent power factor correction for supply voltage variations is validated using the results. The bridgeless operation of the converter with coupled inductor configuration is also described with experimental waveforms at rated supply voltage of 220 V.

Analysis of Equivalent Inductance of Three-Phase Induction Motors in the Switching Frequency Range

The equivalent inductance of three-phase induction motors is experimentally investigated in this paper, with particular reference to the frequency range from 1 kHz to 20 kHz, typical for the switching frequency in inverter-fed electrical drives. The equivalent inductance is a basic parameter when determining the inverter-motor current distortion introduced by switching modulation, such as rms of current ripple, peak-to-peak current ripple amplitude, total harmonic distortion (THD), and synthesis of the optimal PWM strategy to minimize the THD itself. In case of squirrel-cage rotors, the experimental evidence shows that the equivalent inductance cannot be considered constant in the frequency range up to 20 kHz, and it considerably differs from the value measured at 50 Hz. This frequency-dependent behaviour can be justified mainly by the skin effect in rotor bars affecting the rotor leakage inductance in the considered frequency range. Experimental results are presented for a set of squirrel-cage induction motors with different rated power and one wound-rotor motor in order to emphasize the aforesaid phenomenon. The measurements were carried out by a three-phase sinusoidal generator with the maximum operating frequency of 5 kHz and a voltage source inverter operating in the six-step mode with the frequency up to 20 kHz.

DAVIC: A New Distributed Adaptive Virtual Impedance Control for Parallel-Connected Voltage Source Inverters in Modular UPS System

In this paper, an average active power sharing control strategy based on the distributed concept for the parallel operation of voltage source inverters is proposed to be applied to the modular uninterruptible power supply (UPS) systems. The presented method is named distributed adaptive virtual impedance control (DAVIC), which is coordinated with the droop control method. Low bandwidth CAN-based communication is used for the requirement of data sharing of the proposed method in the real modular UPS system. Unlike the conventional virtual impedance control techniques, the virtual impedance of a converter module is adjusted automatically by using global information when DAVIC is applied, further to tune the output impedance of the power modules. The adaptive virtual impedance is calculated by using the difference between the active power of a local module and the average active power of all the modules in a modular UPS. The DAVIC overcomes the drawback of the conventional virtual impedance control since an accurate value of the real output impedances of different converter modules is not required. Simulations using PLECS and experiments on a real commercial modular UPS are developed to verify the effectiveness of the proposed control methodology. These results shown a superior power sharing performance is obtained when using the proposed method.

Analysis of the Effect of Clock Drifts on Frequency Regulation and Power Sharing in Inverter-Based Islanded Microgrids

Local hardware clocks in physically distributed computation devices hardly ever agree because clocks drift apart and the drift can be different for each device. This paper analyzes the effect that local clock drifts have in the parallel operation of voltage source inverters (VSIs) in islanded microgrids (MG). The state-of-the-art control policies for frequency regulation and active power sharing in VSIs-based MGs are reviewed and selected, and prototype policies are then reformulated in terms of clock drifts. Next, steady-state properties for these policies are analyzed. For each of the policies, analytical expressions are developed to provide exact quantification of the impact that drifts have on frequency and active power equilibrium points. In addition, a closed-loop model that accommodates all the policies is derived, and the stability of the equilibrium points is characterized in terms of the clock drifts. Finally, the implementation of the analyzed policies in a laboratory MG provides experimental results that confirm the theoretical analysis.