Current-controlled Inverter Research Articles

Modeling of Grid Tied PV Inverter to Improve its Performance During Unbalanced Load and Unbalance Fault

Integrating solar through inverter brings about various challenges in the microgrids and the inverter itself. This paper aims to the study of problems in the operation of inverter during unbalance load and unbalance fault condition along with the possible control measures to mitigate those problems to ensure the safe and reliable operation of the inverter. A grid connected PV inverter is modeled using hysteresis band controller and implementing the MPPT of the PV system with the buck boost converter. During unbalance load and unbalance fault condition, the negative and zero sequence components of current exists. All the zero-sequence component of current is delivered by the grid. The negative sequence component of current also flows through the inverter resulting in high value of current and loss of stability. Also, the voltage across the dc link capacitor increases during fault. The high value of current flowing through the inverter and high voltage across dc link capacitor may damages the inverter and capacitor respectively. An inverter dual current controller is introduced replacing the hysteresis band control such that the negative sequence component of current flowing through the inverter is almost mitigated from 600A to 10A and positive sequence current is also reduced relatively to a tolerable value 150A from 550A. The total current flowing through the inverter is limited to 200A from 950A and voltage across dc link capacitor is limited to 700V from 1050V during LG fault at the inverter side thus ensuring safe operation of the inverter during voltage dip condition. A further study can be done on limiting the positive sequence component of current to a rated value and improving the performance while operating in islanded mode.

Control and mathematical design of PV-based renewable energy systems for LCL-SR filter-based power quality improvement

Power quality challenges arise when photovoltaic (PV) systems are integrated into power networks. This research work examines the control and mathematical design of an LCL-SR (series resistance) filter for power quality improvement in PV-based sustainable energy systems to meet this difficulty. Power quality may be improved via an LCL filter in grid-connected systems. A non-linear load-connected PV system incorporates a variety of power electronics controllers, including an MPPT controller, battery controller, and inverter current controller, which introduce harmonics into the grid. A passive filter, LCL-SR, is suggested for PV applications that interact with voltage source inverters and the utility grid or electric load to reduce harmonics. A detailed mathematical modeling is presented to design an LCL-SR filter, and its performance is evaluated under different load conditions in terms of THD (Total Harmonics Distortion) and efficiency. LCL-SR filter has 4.32% THD but in the case of LCL filter with parallel R is less THD, which is 3.61%. In terms of efficiency and THD LCL-SR is best among all compared filters which have THD is 4.32% along with an efficiency is 75.95%. The results are compared without a filter, and different configurations of LCL-SR are simulated to show the effectiveness of the proposed filter for PV & residential applications.

Passivity-Based Design for LCL-Filtered Grid-Connected Inverters with Inverter Current Control and Capacitor-Current Active Damping

Passivity-Based Design for LCL-Filtered Grid-Connected Inverters with Inverter Current Control and Capacitor-Current Active Damping

Controlling the output current of a SEPIC H-bridge inverter

The rapid depletion of fossil fuels and humanity's increasing dependence on electricity drive us to adopt renewable energy-based power generation. However, using renewable energy requires a system to maximize the results of converting renewable energy into electrical energy. An inverter is a DC-to-AC power conversion device. Inverters have two types: buck and boost. In closed-loop control, it can be controlled using current or voltage control. Current-controlled inverters sent to the grid have constraints, such as the inverter input voltage value must exceed the voltage on the grid. This study suggests using an H-bridge inverter and an AC-AC SEPIC to generate a voltage value more significant than the input voltage value. The proposed converter uses a single DC source and bases grid current injection on the regulated output current approach. The proposed converter satisfies the IEEE Std 519-2014 requirement of less than 5% and has a Total Harmonic Distortion (THD) of 4.6%.

Quick Search Algorithm-Based Direct Model Predictive Control of Grid-Connected 289-Level Multilevel Inverter

Multilevel inverters, known for their low switching loss and suitability for medium- to high-power applications, often create a heavy computational overhead for the controller. This paper addresses the aforementioned limitation by presenting a novel approach to Direct Model Predictive Control (DMPC) for a grid-tied 289-level ladder multilevel inverter (LMLI). The primary objective is to achieve perfect inverter current control without enumeration. The proposed control method provides a single best solution without complete exploration of the search space. This generalized method can be applied to any multilevel inverter (MLI), enabling them to be used in the grid-tied mode without the computational burden due to a large number of switching states. The DMPC of LMLI with 289-level output and corresponding 289 control inputs, utilizes a discrete model to predict the future state of the state variable. In order to alleviate the enumeration burden, virtual sectors on a linear scale are introduced, and a general formula is provided to identify the single best state among the 289 states, reducing the time required to find the best optimal state per sampling period. Moreover, the proposed control scheme is independent of objective evaluation.

Small Signal Stability of Phase Locked Loop Based Current-Controlled Inverter in 100% Inverter-Based System

Present-day inverter control mainly employs phase locked loop (PLL) based current-controlled strategies. With growing penetration of inverter-based resources in power systems, stability issues associated with inverter control have been reported in weak grid scenarios. A natural and fundamental question to answer is whether this instability is inherent to PLL based current-controlled strategies or it can be resolved with proper frequency and voltage control loop design. This paper addresses this question by analyzing the small signal stability of the current-controlled inverter in island and weak grid operation scenarios. Torque analysis, which is well known for synchronous machine stability analysis, is adapted to uncover the stability enhancement effect of properly designed and parameterized outer-loop controls. Different types of loads are considered and investigated in the study. Electromagnetic transient (EMT) simulations are conducted on a test network to validate the analytical results. The results from this study indicate that with appropriate outer-loop voltage and frequency control design, the PLL based current-controlled inverter can be small signal stable with high inverter penetration, even in a 100% inverter-based system.

Comparative Analysis of Three-Phase PV Grid Connected Inverter Current Control Schemes in Unbalanced Grid Conditions

Recently, the regulation of photovoltaic inverters, effectively under imbalanced voltages on the grid, has been crucial for the operation of grid-connected solar systems. In this regard, determining the output current reference is an integral aspect of managing a solar inverter with an unbalanced voltage. Based on evaluations of Instantaneous Active-reactive Control (IARC), Positive Negative Sequence Control (PNSC), Balanced Positive Sequence Control (BPSC), and Average Active-reactive Control (AARC), this paper proposes a novel variable-current-t-reference calculation method for minimizing power fluctuations and current harmonics. The controller is employed to regulate both constructive and destructive sequences inside a static framework, therefore enhancing dynamic performance and facilitating the selection of suitable controls in the presence of significant network defects. This study also suggests comparing the four MPPT methodologies examined (fuzzy logic, current only, Incremental Conductance, and Perturb & Observe) to maximize energy output. The simulation results efficiently validate the suggested computation approach that is presented in the current reference.

A comparative analysis of PI & FLC based grid connected PV system with power backup to improve power quality

In this article, a fuzzy logiccontroller (FLC) tuned DC-link controller is presented. This controller is utilized to enhance the power quality characteristic of grid-integrated PV systems with power backup in comparison to the PI controller, without any filter at PCC (point of common coupling). The primary purpose of these controllers is to maintain power to the DC/AC sides by maintaining the DC-link voltage under transient conditions. The proposed system is tested with a non-linear load; hence, the proposed system requires power quality enhancements on PCC. By using both controller power flow control, reactive power compensation and harmonic mitigation is done on PCC, after that grid current becomes sinusoidal. For the power quality improvement, no filter/shunt active power filter (SAPF) is connected to PCC, only proposed inverter current controller improves the power quality at PCC by using these controllers. Using the MPPT (maximum power point tracking) Controller, the maximum power extraction feature is also included in this paper to extract maximum power from the PV module under varying atmospheric circumstances. The power backup unit is also connected with this system, power backup unit includes a battery controller which works as a boost and buck converter according to system requirements, along with a battery. On the MATLAB/Simulink platform, the control scheme and functioning of both controllers are tested, and the results are validated as satisfactory. According to IEEE 519, the grid current THD is below 5% when the suggested control system is implemented.

Optimized servo-speed control of wind turbine coupled to doubly fed induction generator

<p>Optimal control of any variable speed wind turbine needs maximum power point tracking (MPPT) coupled to doubly fed induction generator (DFIG) for better power generation. This paper offers a novel direct power servo-speed control of wind turbine. This latter is based on DFIG optimal hysteresis MPPT inverter current control combined with space voltage modulation (SVM) inverter voltage technique, thus providing a stable and continuous energy flow to power grid. In this design, the asynchronous machine stator is directly connected to the grid. Bidirectional power converter, acting as frequency converter, is rotor circuit located. Rectifier supplies rotor windings with voltages and reference frequency resulting from control procedure of the power exchange between the stator and grid. Inverter is directly controlled by means of SVM technique to maintain direct current (DC) bus voltage constant. Simulation results show that the proposed configuration improves power converters efficiency due that rotor circuit needs less power than stator circuit which is injected into the grid.</p>

A Novel Method to Reduce Low-Frequency Output Current Ripple of PWM Inverters

The output current of PWM inverters may contain low-frequency harmonics due to nonidealities, such as fluctuating dc voltage and dead-time. Low-frequency harmonics are detrimental to the inverter's current controller performance as well as its operation. They increase losses, cause synchronization problems in a grid-connected inverter, and cause torque pulsations in motors. Reducing low-frequency harmonics through conventional means require a big and expensive filter. In this article, a method to reduce low-frequency current harmonics is proposed. This method uses a virtual impedance, which is a controller block that mimics the behavior of a real impedance. Because it is virtual, it does not have physical drawbacks to the system, such as volume, weight, and losses. It can be designed only to affect the disturbance signals and to assume characteristics unlike a real impedance. It can also be added as a plug-in block to an already existing controller, saving a significant amount of design effort. Simulation and experimental results show a decrease of current THD to almost 1/3 of the original THD when no virtual impedance is used, demonstrating the effectiveness of the proposed method.

Comparative Study of DC/AC Inverter Control Techniques for Three Phase Grid Connected PV System

The goal of this project is to develop and analyze a three-phase grid-connected photovoltaic (PV) system with a 250KW power capacity with expandable property. The PI, Slide, and MPC methodologies are used to test three distinct inverter current control techniques. The dynamic performances of the three inverter control techniques are virtually comparable under grid connected working conditions, although the dynamic performances of the traditional PI control technique are slightly better and smoother than the other two inverter control techniques. The unity power factor criterion was reached by all techniques. The output power levels obtained using the sliding mode inverter control method are higher than those obtained using the PI and MPC methods. MPC's DC link voltage readings are also consistent and match the intended voltage levels.

Research on Modeling, Stability and Dynamic Characteristics of Voltage-controlled Grid-connected Energy Storage Inverters Under High Penetration

With the increasing penetration of renewable energy, the power grid is characterised by weak inertia and weak voltage support. Some current-controlled inverters have been modified to voltage-controlled inverters and are gradually being used in distributed systems, thus constituting a multi-inverter hybrid operation mode system, which brings more severe challenges to the system's stable operation. In this paper, a framework consisting of three main parts of this particular voltage-controlled energy storage inverter is built. Each part's small-signal transfer function matrices are established by their control and structure. Based on this, the sensitivity of the SCR (short-circuit ratio) variation and droop coefficients of grid-connected inverter systems to stability is analysed in both dq domain and pos-neg sequence domain. The coupling of the inverter output active and reactive power and the effect of grid voltage disturbances are analysed under SCR variations in dq domain. Finally, the accuracy of the proposed model, the stability and dynamic response are verified by simulation and experimental platform of a 20 kW energy storage inverter system.

Research on the influence of switching frequency on the stability of large-scale distributed generation system

Wind power generation is generally a large-capacity low-switching frequency power station, and photovoltaic power generation is generally a small-capacity high-switching frequency power station. Large-scale access of different types of new energy power stations increased risk of grid voltage fluctuations. Therefore, in order to analyze the variation law of the stability of different types of new energy sources connected to the weak grid, it is very important to analyze the stability of inverters with different switching frequencies, but there are few studies on this issue. In this paper, the detailed sequence impedance model of the current-controlled inverter is firstly established, and its correctness is verified by frequency sweeping. Secondly, considering the control bandwidth, ripple suppression and filtering performance, the filtering parameters of inverters with different switching frequencies are designed using a unified standard. On this basis, the stability criterion based on impedance ratio is used to analyze the stability variation law of inverters with different switching frequencies under strong and weak grid conditions. Finally, the correctness and accuracy of the theoretical analysis are verified by simulation experiments.

Harmonic Voltage Control in Distributed Generation Systems Using Optimal Switching Vector Strategy

With increased penetration of renewable power and the nonlinear loads in the distributed generation (DG) systems, increased power quality concerns are exhibited in the active distribution networks, especially the challenges associated with the current and voltage harmonics in the system. Various conventional harmonic compensation techniques are developed for voltage-controlled DG inverters in past, majority involve either multiple proportional-integral (PI) or proportional-resonant (PR) controllers in eliminating grid current harmonics. The current-controlled inverters, on the other hand, are not preferred in industrial applications, accounting to their wide variations in the switching frequency. A novel and adaptive harmonic voltage control is developed here, for voltage-controlled DG inverters, which neither uses any PI regulators nor imposes stability issues associated with nonideal implementation of infinite gains of PR controllers. Interestingly, the developed control logic can be used for DG inverters, both in grid-connected and off-grid operational modes. Furthermore, this strategy allows a network operator to use this as an additional supplement that can be enabled/disabled as per the network requirement. The control logic exploits the property of an optimal-switching-vector controller, i.e., accurate output voltage tracking. Simulations results demonstrate the effectiveness of the controller, to suppress grid current harmonics and load voltage harmonics in grid-interfaced (GI) and off-grid modes, respectively, ultimately satisfying the mandatory IEEE standard-1547. Experimental results verify the viability of the controller for practical applications.

Guest Editorial: Enhancing hosting capability for renewable energy generation in active distribution networks

Guest Editorial: Enhancing hosting capability for renewable energy generation in active distribution networks

Control and capacity planning for energy storage systems to enhance the stability of renewable generation under weak grids

Current-controlled inverters (CCIs), often used in renewable power generation, are prone to harmonic instability under weak grids with a low short-circuit ratio (SCR). This paper proposes that this type of instability can be prevented by configuring part of the energy storage system (ESS) converters in the distribution network as voltage-controlled inverters (VCIs). The main contributions of this paper are as follows: (1) A model of the CCI/VCI hybrid grid-connected system is established, which can characterize not only the plant-level stability, but also the unit-level stability. (2) A rapid power control method for VCIs is proposed, which can improve the power control bandwidth of VCIs to tens of Hz to meet the rapidity requirements for ESS power control. (3) A capacity planning method is proposed, which can give the required minimum VCI/ESS capacity for a specific stability margin and SCR range. Analysis shows that as long as the ESS converters with a capacity of no less than 6% of the CCIs are configured as VCIs, large-scale CCIs that are originally critically stable at SCR = 1.9 can operate stably under the very weak grid with SCR = 1.2. Finally, the proposed method is verified by simulations and experiments.

Step response of Rectifier DC Current Controller and Inverter DC Current-Voltage-Extinction Angle Controllers in a Current Source Converter Based HVDC Transmission Link Connected to a Strong Inverter Side AC system

Abstract: Transmission of electrical energy with High Voltage Direct Current (HVDC) has provided the electric power industry with a dominant means to transmit huge quantity of electricity over very long distances. To investigate the performance, a welldeveloped current source converter based HVDC transmission system model is projected, in which the AC system represented as damped LLR equivalent and is equipped with double tuned harmonic filter to mitigate the AC-DC harmonics and the DC system is secured with rectifier current control, inverter current control, voltage control and inverter extinction angle control. The MATLAB/Simulink based simulation results validate the step response of HVDC transmission system with various controllers. Keywords: HVDC transmission systems, double tuned filter, Rectifier DC Current Control, Inverter DC Current-Voltage - Extinction Angle Control, Step response.

A Hybrid Voltage/Current Control Scheme With Low-Communication Burden for Grid-Connected Series-Type Inverters in Decentralized Manner

In this article, a novel hybrid voltage/current control scheme with low-communication burden is proposed for series-type inverters in a decentralized manner. All the inverter units are controlled by their individual local controllers without a real-time central global-communication. Among them, a few inverters serve as the current controlled inverters (CCIs) to regulate the grid current, and the rest inverters work as the voltage controlled inverters (VCIs). Under this control architecture, five main control targets are achieved: 1) frequency self-synchronization of each VCI inverter based on local current phase-angle signals; 2) fault-tolerant capability of all inverters without one-to-all-failure; 3) high disturbance-rejection-capability to grid variations; 4) adjustable grid power factor; and 5) independent power control for inverters with unbalanced power sources. As the central high-bandwidth communication is avoided, it has the features of decreased cost, increased scalability, and reduced geographic restrictions of inverters. Compared with conventional decentralized approaches, only a few CCI units acquire the grid phase-angle signal to achieve grid synchronization by communication, and the most VCI units are local self-synchronization via the local line-current-phase signal rather than real-time grid-voltage-phase communication signal. Thus, the proposed control can be realized with a very low-communication burden, which is more cost-effective and higher reliability in terms of communication faults. The feasibility of the proposed method is verified by experimental results of eight different cases, such as source power change, large power gap, grid voltage/frequency deviation, grid harmonics, and one-fault redundancy.

Repetitive Predictive Control for Current Control of Grid-Connected Inverter Under Distorted Voltage Conditions

A repetitive predictive control for grid-connected inverter current control scheme is presented in this paper under voltage harmonic distortion in the stationary reference frame. Predictive control is an approach that uses a receding horizon to achieve the optimal track for the reference. In this approach, the repetitive controller behavior is added to the predictive control to increase its performance under distorted voltage conditions. In addition, to apply the controller in the generation system, controller is designed from the perspectives of the power system. The controller is designed considering the state-space model in the stationary reference frame. The controller performance was verified using a laboratory experimental setup under distorted grid voltage condition. Experimental results confirm that the proposed controller can effectively suppress harmonics under both normal operation and distorted grid voltage conditions, and also satisfy the requirements stipulated in IEEE Std. 1547.2-2008.

A Photo-Voltaic Invert-er Cluster Resonance Suppression Method Based on Capacitor Voltage Feed-Forward

Photo-voltaic inverts usually use LCL filter for filtering, and LCL filter will bring resonance problems to the system. This paper focuses on invert-er side current feedback control (invert-er side current feedback, ICF). This paper proposes a method for suppressing resonance of photo-voltaic invert-er clusters based on capacitor voltage feed-forward. First, establish the ICF mathematical model based on HPF and CVF under the weak grid, qualitatively analyze the resonance mechanism of the cluster invert-er, then analyze the system control structure and stability according to the proposed method, and finally verify the effectiveness of the proposed method through simulation and experiment sexuality and correctness.