Inverter Output Research Articles

An adaptive phase‐locked loop‐less control strategy for LCL‐filtered grid‐connected inverter under complex grid conditions

SummaryThe phase‐locked loop, as a method to extract grid voltage phase angle, is commonly used for grid‐connected inverter to synchronize the injected grid current with the grid. However, the phase‐locked loop will also bring some problems. The current loop controller, the phase‐locked loop, and the grid impedance are coupled together under the weak grid. Then, the phase angle of the inverter output impedance will be reduced, causing the system to be unstable. Moreover, the output phase angle of the phase‐locked loop participates in the coordinate transformation under the unbalanced grid voltage condition, which degrades the quality of the injected grid current generated by the inverter with high harmonic content. Therefore, a vector current control strategy without phase‐locked loop derived from direct power control is used in this paper. An adaptive linear Kalman filter algorithm is also applied to observe the voltage information and avoid the harmonic problems caused by power calculation. Finally, simulations and experiments are conducted to verify that the proposed method is effective, and the results are consistent with the theoretical analysis.

Robust Intelligent Tracking Control Technique for Single-Phase SPWM Inverters

This paper presents a robust intelligent tracking-control technique which is subsequently applied to single-phase SPWM inverters. The proposed technique mixes advanced sliding mode control (ASMC) with grey-Markov model (GMM). The advanced sliding mode control allows the system state to converge quickly to the origin in a limited amount of time. Unfortunately, a chattering problem frequently occurs when the inverter suffers drastically from highly nonlinear or internal parameters changes. The large power losses and high harmonic distortion emerge in the inverter output. The role that the grey-Markov model plays is to reduce the chattering of the ASMC during system uncertainty overestimation, or to reduce the steady-state error caused by underestimation of system uncertainty. In conjunction with the GMM and the ASMC, fast transient response, low distortion of the sinusoidal output-voltage and reduced chattering can be obtained. Simulation results of the proposed single-phase SPWM inverter are carried out in MATLAB/Simulink. The experimental results have been validated through digital signal processors, enabling the single-phase SPWM inverter output to effectively improve the transient tracking speed and steady-state performance.

Coordinate Compression Strategy of Resistance and Reactance for MHz-Range Inverter System

This article proposes a coordinate impedance compression strategy for high frequency inverter systems. Based on a two parallel inverter structure, the strategy adopts two methodologies: 1) a passive <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> network to adjust the current and voltage seen by each inverter module, and 2) an active modulation of the inverter input voltage and output voltage phase to adjust the load current distribution. For the proposed strategy, the resistance and reactance can be compressed coordinately, which can help the inverter to operate in high efficiency load range. The Half-Bridge Class D inverter is taken as an example to be analyzed. A 10-MHz prototype is built and the experimental results verify the correctness and feasibility of the proposed strategy.

A New Stability Enhancement Method Using KF Estimation for the PWM-SMC-Based Grid-Tied Inverter Under Weak Grid Condition

A sliding-mode control (SMC) technology is widely used in inductor-capacitor-inductor (LCL)-filtered grid-tied inverters (GTIs) due to its strong robustness against parameter drift. However, under weak grid conditions, the steady performance of GTI using conventional pulsewidth-modulation-based SMC (PWM-SMC) degrades, resulting in increased harmonics in grid-injected currents. One reason is that the voltage feedforward term at the point of common coupling (PCC) in the SMC loop causes the phase margin of the inverter output impedance to decrease. Another reason is that the gain of conventional PWM-SMC-based systems is not high enough, especially under weak grid conditions. This article proposes a PCC feedforward voltage reconstruction method (named KF-PWM-SMC) for a PWM-SMC-based controller using Kalman filter (KF) estimation. As a result, the phase and magnitude of the inverter output impedance at the desired frequency can be significantly improved. Moreover, the proposed PCC feedforward voltage extraction method can save the PCC voltage control sensor, thereby improving the reliability of the whole system. A 3-kW experimental GTI based on the dSPACE DS1202 platform has been developed to verify the feasibility of the proposed control strategy.

Closed-Loop Voltage Control for Maximizing Inverter Output Voltage in the Field Weakening Region of Induction Machines

It is desirable in induction motors drives to maximize the inverter output voltage to increase the output torque and power in the field weakening (FW) region. Existing FW control methods produce high torque ripples and show high step reductions of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula> -axis current during the transient period from constant torque (CT) to FW. In addition, many proposed controllers are difficult to tune and require specific control algorithms to deal with parameter sensitivities. In this article, a closed-loop voltage control method is developed based on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula> -axis reference current to maximize the voltage extraction from dc-link voltage while minimizing the above disadvantages. This is achieved by applying the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula> -axis current, which compensates the difference between the hexagonal reference voltage and stator voltage for corresponding stator voltage vector position. Therefore, the output torque and power of the induction machine (IM) are maximized in the FW region. The proposed method is first presented analytically, and then, simulation and experimental results are included to verify the control method.

An advanced control and protection integration scheme for microgrids

The increasing trend in integrating intermittent distributed energy resources (DERs) into AC microgrids presents operational challenges in stability and protection. In islanded microgrids with power electronic interfaces, protection poses a major challenge due to the reduced level of short circuit currents resulting from inverter output capabilities. However, traditional protection schemes that are utilized in distribution systems are no longer appropriate to protect the microgrid in the presence of different levels of fault currents. This paper develops an integrated control and protection framework based on state observer and fault current limiter (FCL) devices. The state observer has been developed to detect and identify the faults that occur within multiple protection zones. In addition, controlled switches, consisting of FCLs, have been utilized to limit the fault currents and provide rapid switching during the faults, thereby improving system reliability. An adaptive control and protection integration scheme proposed in this paper has been applied to islanded microgrid configuration and is demonstrated to be an effective means to protect the system and maintain the voltage and frequency within an acceptable range with the capability of power continuity during both transient and persistent faults.

A Convolutional Neural Network-Based Method of Inverter Fault Diagnosis in a Ship’s DC Electrical System

Abstract Multi-energy hybrid ships are compatible with multiple forms of new energy, and have become one of the most important directions for future developments in this field. A propulsion inverter is an important component of a hybrid DC electrical system, and its reliability has great significance in terms of safe navigation of the ship. A fault diagnosis method based on one-dimensional convolutional neural network (CNN) is proposed that considers the mutual influence between an inverter fault and a limited ship power grid. A tiled voltage reduction method is used for one-to-one correspondence between the inverter output voltage and switching combinations, followed by a combination of a global average pooling layer and a fully connected layer to reduce the model overfitting problem. Finally, fault diagnosis is verified by a Softmax layer with good anti-interference performance and accuracy.

Investigation of Staggered PWM Scheme for AC Common Mode Current Minimization in SiC-Based Three-Phase Inverters

The advent of SiC devices which results in lower rise times and steep <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{d}v/\text{d}t$ </tex-math></inline-formula> at the inverter terminal can compromise the common mode (CM)-conducted emissions’ compliance, motor peak ac -CM currents, and leakage currents. Consequently, this results in a high ac and dc side CM filter weight. AC filters at the inverter output terminal are effective in reducing the slew rate of inverter voltage thereby reducing the peak CM current. However, a special case with high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{d}v/\text{d}t$ </tex-math></inline-formula> arising from low duty cycles is found to increase the peak ac-CM current significantly. Reduced CM voltage (CMV) schemes are popular pulsewidth modulation (PWM) methods to minimize the peak CMV but suffer from high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{d}v/\text{d}t$ </tex-math></inline-formula> under this special case as well. In this article, a staggered PWM scheme to minimize such peak ac -CM currents for a high-speed variable frequency drive (VFD) is evaluated using a three-level SiC-based T-type inverter as a demonstrator. From the analysis, the proposed switching pattern shows benefits over conventional sinusoidal PWM (SPWM) scheme and is found to be scalable to other PWM schemes and voltage levels. Simulation and experimental results are shown to validate the performance of the proposed switching pattern.

Design and analysis of robust backstepping controller for DC suppression of parallel UPS inverter system

AbstractHere, an improved control strategy is proposed, which adopts a non‐linear robust backstepping controller to eliminate DC circulating current in a parallel uninterruptible power supply (UPS) system. The traditional active and reactive power sharing control can ideally suppress the AC circulating current, but cannot effectively restrict the DC circulating current. Based on the traditional active power and reactive power sharing control (TAPRPSC) of the parallel UPS system, the proposed control strategy is used in each parallel inverter module to suppress the DC component disturbance in the inverter output voltage, which varies within a certain range. Hence the desirable steady‐state and dynamic performance of the system can be obtained. Moreover, the Lyapunov method is utilized to ensure the stability and robustness of the system. A parallel system of two UPS inverters with the proposed control strategy is built by Matlab/Simulink, and further experiments are carried out on the semi‐physical simulation platform of the Modelling Tech. Both the simulation and experimental results verify the effectiveness of the proposed control strategy for DC circulating current suppression and ideal output voltage tracking.

A New Transformer-Less Single-Phase Photovoltaic Inverter to Improve the Performance of Grid-Connected Solar Photovoltaic Systems

Photovoltaic (PV) energy systems have found diverse applications in fulfilling the increasing energy demand worldwide. Transformer-less PV inverters convert the DC energy from PV systems to AC energy and deliver it to the grid through a non-isolated connection. This paper proposes a new transformer-less grid-connected PV inverter. A closed-loop control scheme is presented for the proposed transformer-less inverter to connect it with the power grid. The proposed transformer-less inverter reduces extra leakage current and holds the common-mode voltage at a constant point. To eliminate extra leakage current, as well as achieve constant common-mode voltage, a midpoint clamping method is utilized to operate the inverter. The proposed transformer-less inverter is formed of seven insulated gate bipolar transistors (IGBTs) employing a unipolar sinusoidal pulse width modulation (SPWM) technique for switching purposes. An LCL filter is employed to reshape the two-level inverter output voltage and current to obtain closer sinusoidal waveforms. The output voltage and current total harmonic distortion (THD) of the proposed transformer-less inverter were found to be 1.25% and 0.94%, respectively, in the grid-connected mode. The leakage current elimination mechanism with the proposed transformer-less inverter is deeply analyzed in this paper. The performances of the proposed transformer-less inverter were evaluated with MATLAB/Simulink simulation and validated in a laboratory scale experiment.

Development of Voltage Control Algorithm for Improving System Stability in Korean Distribution Grids

Problems with overvoltage and reverse energy flow arise as the percentage of distribution energy resources (DERs) in distribution system rises. This paper implements a MATLAB/Simulink linked model that communicates with the OpenDSS engine. The point of common coupling (PCC) voltage typically mitigates using the inverter outputs control. However, case overvoltage issues occur in distribution networks and the influence of the volt–var control effect is insufficient to reduce PCC voltage to a stable range. To improve the reactive control effect, this paper proposes active power control and voltage error compensation methods that maximize the reactive power control effect. The DER management system (DERMS) platform was constructed to analyze the effect of changing DER output values. The DERMS data are organized based on real distribution line (D/L) system data where overvoltage occurred in Korea and the control effect of the real system is analyzed. This control method analysis can be used to develop an inverter control scheme for the over voltage issues described in this paper.

Photovoltaic system faults diagnosis using discrete wavelet transform based artificial neural networks

Introduction. This research work focuses on the design and experimental validation of fault detection techniques in grid-connected solar photovoltaic system operating under Maximum Power Point Tracking mode and subjected to various operating conditions. Purpose. Six fault scenarios are considered in this study including partial shading, open circuit in the photovoltaic array, complete failure of one of the six IGBTs of the inverter and some parametric faults that may appear in controller of the boost converter. Methods. The fault detection technique developed in this work is based on artificial neural networks and uses discrete wavelet transform to extract the features for the identification of the underlying faults. By applying discrete wavelet transform, the time domain inverter output current is decomposed into different frequency bands, and then the root mean square values at each frequency band are used to train the neural network. Results. The proposed fault diagnosis method has been extensively tested on the above faults scenarios and proved to be very effective and extremely accurate under large variations in the irradiance and temperature. Practical significance. The results obtained in the binary numerical system allow it to be used as a machine code and the simulation results has been validated by MATLAB / Simulink software.

A control strategy for a grid-connected virtual synchronous generator with virtual impedance

Virtual synchronous generator (VSG) can simulate synchronous generator turning and damping characteristics, which can further improve the penetration rate of renewable energy in the grid. However, the output impedance of virtual synchronous generator in medium and low voltage microgrids is dominated by resistance, which leads to the coupling of active/reactive power and affects the grid integration control effect, and the phase of VSG and grid voltage will be biased in the absence of a pre-synchronization link. For this purpose, a strategy of grid-connected control of VSG with virtual impedance is proposed. Firstly, the VSG mathematical model is established and virtual impedance is introduced into the VSG electrical portion to improve the grid-connected inverter output characteristics. Secondly, a secondary frequency and voltage regulation strategy to improve VSG control is proposed to restore the frequency and voltage of the VSG, and the SOGI phase-locked loop collects the grid voltage phase to complete the grid-connected pre-synchronization design. Finally, the effectiveness and correctness of the proposed strategy were verified by the simulation results based on MATLAB/ Simulink.

Pelatihan Pemasangan Panel Surya Sebagai Sumber Listrik Di Objek Wisata Alam Punthuk Ngepoh Brajan

For lovers of the outdoors and looking for tranquility, the Puthuk Ngepoh tourist attraction is located in Brajan Hamlet, Banjararum, Kalibawang Kulon Progo, which is in the Menoreh Hills. The location is 45 km from downtown Jogjakarta. Managed by a tourism awareness group (POKDARWIS), it can be a choice of destination to fill the weekend. One of the existing problems, is the constraint on electricity which is more economical. Therefore, the authors seek to Install Solar Panels (PLTS), as an alternative source of electricity by utilizing sunlight as a producer of electrical energy, which is environmentally friendly. This is expected to be a solution for the burden of lighting at tourist sites. Batteries are one option to store electrical energy, from solar panels. In observations, the battery is charged by solar panels that generate voltage by converting solar energy into electrical energy. The voltage generated from the solar panels ranges from 14.8 to 17.5 volts DC. The solar panels used are of the type Polycrystalline (Poly-crystalline) with a power of 100wp. The measurement results show that the solar panel current and voltage are ± 17V, the distribution to the battery is regulated by the solar charger controller (SCC), an average of 13.5V. Change the DC to AC voltage using an inverter. This shows that after the battery is charged for 2-3 hours, it can operate for 3 hours with an inverter output voltage of 220-176volt AC. Voltage and current will start to increase at 07.00 WIB, maximum at 10.00-13.00 WIB, starting to decrease until the afternoon.&#x0D;

A Control Strategy of LCL-Type Grid-Connected Inverters for Improving the Stability and Harmonic Suppression Capability

The conventional inverter-side current single-loop feedback control scheme is weak in suppressing the grid-side current harmonics, posing a challenge for an inverter to inject high-quality current under distorted grid voltage. With capacitor current compensation added, the control scheme achieves controllability of the grid-side current harmonics so that it can effectively suppress some specific harmonic components. However, due to the stability requirements, only a few low-order harmonic resonance controllers can be applied, which limits the mitigation of high-order harmonics. To tackle this problem, the grid-side current feedback control with inductor–capacitor–inductor (LCL) resonance damping is proposed in this paper. In this case, a higher LCL resonance frequency can be set compared to the inverter-side current single-loop feedback control scheme. Thereby, more resonance controllers can be applied to suppress high-order grid-side current harmonics. The active damping method of capacitor current proportional feedback plus capacitor voltage proportional feedback is adopted because of its high robustness to grid impedance variations. Furthermore, this paper reveals that the applied active damping method has a limitation in that it only considers a single inverter under inductive grid impedance, which cannot eliminate the risk of resonance caused by the interaction of multiple inverters and the grid. To address this issue, a phase lead compensator (PLC) is proposed, eliminating the resonance risk by removing the non-passive region of the inverter output admittance. To retain the advantage of the inverter-side current single-loop feedback control scheme, i.e., only a few measuring devices are required, a digital differentiator is used to calculate the capacitor current from the capacitor voltage. The difference between the measured inverter-side current and the calculated capacitor current is taken to approximate the grid-side current for the feedback control. The control performance is comparable to using the grid-side current for feedback. Simulation and experimental results demonstrate that the proposed control scheme endows the inverter with good stability and current quality without extra measurement devices.

Improved Modulation Methods of the Dual-inverter Fed Open-end Winding Three-phase Motor Drives with Equal DC-link Voltage

This paper investigates PWM method for the dual-inverter fed open end winding three-phase induction motor drives. This is aimed to improve the quality of the output voltage as well as to enhance the motor performance by solving the drawback of the PWM method. The proposed PWM method operates two inverters in different technique, i.e one inverter operates in square wave mode (slow switching) and one inverter in PWM mode (fast switching). Although the PWM method can improve the quality of the inverter output voltage over the two-level inverter output, the proposed PWM method causes an excessive ripple in both torque and stator current especially at the transient period. The drawback is solved by applying a new switching strategy that manage the inverter switches operate in turn between slow switching and fast switching in one circle of operation. The proposed method is verified via simulation. The result shows that the quality of output voltages improves for all modulation indices in comparison with two-level inverter. Furthermore, torque ripple is reduced, and current ripple are improved as the current waveform is enhanced to be more symmetrical.

A novel space vector modulation strategy for legs parallel three-level inverters to reduce common-mode voltage

In order to increase the converter’s rated power, the parallel connection of legs between inverters has been an effective way. The common-mode voltage is one of the main problems of this inverter. Based on the special connection mode of legs parallel inverter, the output voltage can be regarded as the output of a five-level inverter. Therefore, this paper proposes a space vector modulation based on the virtual five-level to suppress common-mode voltage. The traditional SVPWM algorithm contains a lot of trigonometric function calculation, which slows down the response speed of the system. This paper proposes a novel coordinate system division method, which avoids trigonometric function calculation and reduces the amount of calculation. The effectiveness of this method is verified by Matlab / Simulink simulation.

Multioutput Hybrid Solar Inverter With No Right Half-Plane Zero and Reduced Common-Mode Leakage Current

In this article, a multioutput-based transformerless hybrid solar inverter (TLHSI) with no right half-plane zero (RHPZ) and reduced common-mode leakage/ground current is presented for photovoltaic (PV) based microgrid applications. In the proposed multioutput TLHSI, two magnetically coupled inductor coils are placed between the input and dc output sides along with a dc-link capacitor to continuously provide the stored energy to the dc load so that the RHPZ is eliminated and, thus, makes it a minimum phase system. Furthermore, a hybrid unipolar sinusoidal pulsewidth modulation technique is established to nullify the high-frequency components of common-mode voltage across the PV to ground stray capacitor. Also, it normalizes both dc and ac loads at the same instant from a single dc input during each operating state of the proposed TLHSI. The shoot-through issue is resolved in the proposed TLHSI, which increases the system reliability. Furthermore, as the deadtime is not required in between the switching pulses, the quality of the inverter output voltage improves. The performance analysis of the proposed TLHSI is compared with the existing topologies. To validate the performance of TLHSI, a scaled down 340 W laboratory prototype is developed.

Selective harmonic mitigation based two-scale frequency control of cascaded modified packed U-Cell inverters

A new Modified Packed U-Cell (MPUC) converter architecture with cascading is proposed in this paper. To provide an output voltage of 25 levels, the proposed cascaded MPUC needs only twelve power switches and four power sources. The converter comprises two cascaded MPUCs with DC supply in a ratio of 5:1. One converter is operating at low frequency (LF) and the other at high frequency (HF) that leads to lower power losses and higher levels. Besides, a variable frequency method is anticipated to produce a 25-level output voltage which has low harmonic content (THD) with the help of Selective Harmonic Mitigation (SHM). The optimum switching angles for SHM are obtained through solving the SHM equations using the Genetic Algorithm (GA). The designed controller is efficient and suitable for applications that require low-frequency operation either in stand-alone or grid-tied. The proposed inverter and its operation procedure have been investigated using MATLAB®/Simulink software, and the findings demonstrate that the proposed inverter output voltage has reduced THD significantly. The simulation results are verified using the typhoon HIL-402 emulator. Also, the power loss analysis is done using PLECS. The maximum efficiency of the converter is found to be around 98.34%. The simulation results justified the efficiency and viability of low 25-level THD voltages.

Optimal LLC Inverter Design With SiC MOSFETs and Phase Shift Control for Induction Heating Applications

This article presents the analysis of a converter based on an LLC resonant output inverter and its optimal design used in induction heating applications. The new optimal design method improves several operating parameters that leads to an optimization of the dimensioning of the components of the converter. Additionally, this converter achieves an output power factor that can be considered optimal since it allows to minimize the reactive power of the resonant circuit components and reduces the rms values of the output current of the inverter and the current of its switching devices in relation to that found in traditional designs. A complete study of the circuit based on classic models is carried out to introduce simple rules for the design of this type of inverter for induction heating applications and to control its output power based on a phase shift (PS) system. Since the inverter is made with silicon carbide MOSFET transistors, an efficiency greater than 99% is reached. The experimental results were obtained from the test of a 12 kW 20 kHz converter for induction heating application.