Capacitor Current Feedback Research Articles

Research on Adaptive Suppression of LCL Converter Resonance Grid-Connected System

LCL-type converters are widely used in grid-connected systems due to their small size and good filtering performance. However, the resonance suppression problem brought by the LCL filter cannot be ignored. The capacitive current feedback is a commonly used resonance suppression method. In applications, the grid impedance can cause LCL filter resonance. Thus, this paper presents an adaptive resonance suppression method based on the RBF network optimized by particle swarm optimization. This method optimizes the initial parameters of the RBF network through particle swarm optimization, identifies the parameters of the PI controller by RBF neural network’s own identification capability, and updates the active damping coefficient based on constraints such as stability margin, thereby realizing the LCL-type inverter to maintain the system stability when the grid impedance changes. The effectiveness of the method is verified by experiments.

Analysis and verification of harmonic interaction between DDWTs based on αβ stationary coordinate system

This study presents an analysis method of harmonic interaction characteristics between direct-drive wind turbines (DDWTs) based on αβ coordinates and the relative gain array (RGA). First, considering the digital delay of inverters, the capacitive current feedback active damping is applied to suppress the resonance of LCL filter, and the output current of DDWT is regulated by quasi-proportional resonant controller in the αβ coordinates and then combined with the topology of the direct-drive wind farm (DDWF), the transfer function matrix of the DDWF is established. Second, based on the RGA principle, the relationship between parameters and harmonic interaction characteristics is quantitatively analysed. The results show that with the proportional coefficient increasing, the degree of negative interaction between DDWTs is strengthened, which increases the harmonic content in the negative interaction frequency band. However, the increase of resonance coefficient and number of DDWTs will weaken the degree of negative interaction. In addition, with the increase of capacitive current feedback coefficient and grid inductance, the interaction frequency gradually decreases. Finally, the DDWF model is built both on Matlab/Simulink and StarSim + PXI hardware-in-the-loop platform to verify the analytical results. The harmonic interaction characteristics can be used to guide the parameter design of DDWTs, so as to optimise power quality of the wind farm.

Parameters optimization design for LCL-type STATCOMs under complex power grid

LCL filter parameters and control parameters are interdependent and inter-restricted. They can all affect the stability of a static synchronous compensator (STATCOM) under the effects of a complex power grid such as harmonic grid voltage and grid impedance variation. An advanced parameters optimization method integrated with LCL filter and control strategy is proposed. At first, the ABC-Pareto algorithm (Pareto multi-objective optimization of an artificial bee colony algorithm) is used to reasonably choose the LCL filter parameters. Under the premise of using capacitor current feedback active damping control and grid voltage feedforward control, the mathematical models of the STATCOM are derived. The constraints of control performances on the control parameters are obtained. According to these constraints, it is possible to construct a satisfactory 3D space. The control parameters can be chosen reasonably by finding the optimized space when the power grid is changed. Simulation and experimental results show the effectiveness and superiority of the proposed method.

Enhanced robustness with damping interval widening strategy of LCL-type converter under weak grid condition

Active damping methods are widely adopted to present a damping effect on the resonance phenomenon without the power loss of LCL-type converters connected to a grid. However, extra sensors are used to implement the damping strategy. In this paper, a sensorless damping strategy where only the grid current is sampled has been proposed. Furthermore, the inner relationship between the tradition capacitor current feedback strategy and the proposed strategy has been clarified. Considering the digital delay and a weak grid, a robust method that uses unit delay feedback to mitigate the phase lag and to widen the effective damping interval has been developed. With this simple measure, stability is enhanced during the Nyquist frequency and robustness is improved when the LCL parameters vary and grid impedance exists. Finally, simulation and experimental results are presented to verify the feasibility and engineering significance of the proposed strategy.

Floquet-Theory-Based Small-Signal Stability Analysis of Single-Phase Asymmetric Multilevel Inverters With SRF Voltage Control

This paper proposes a small-signal model and Floquet-theory-based method for analyzing small-signal stability of a single-phase asymmetric cascaded H-bridge multilevel inverter (ACHMI) operating in the stand-alone mode. The studied ACHMI system consists of the power stage and cascaded control loops, which includes the voltage loop in the synchronous reference frame (SRF), capacitor current feedback control, and hybrid modulation scheme. Due to the SRF voltage control, the derived small-signal model under linear and nonlinear load conditions are inherently periodically time-variant. Therefore, the Floquet theory is employed to analyze the stability regions of the dual-loop control parameters of the ACHMI. Furthermore, the loci and moduli of Floquet multipliers are calculated to accurately evaluate the respective effects of control parameters on the stability of the system. With the stability analysis based on the small-signal model and Floquet-theory-based method, an effective selection range of control parameters of the multilevel inverter can be obtained. Finally, the experimental results from a reduced-scale laboratory prototype ACHMI are presented to validate the theoretical analysis, and the effectiveness of proposed analysis method in high-power applications is verified by the simulation results from a 10-kV medium-voltage ACHMI.

Control delay compensation scheme based on non‐instantaneous loading and pulse‐width equivalence for active damping of LCL ‐type inverters

Active damping (generally by the feedback of capacitor current) is an effective solution to the resonance peak of LCL -type grid-connected inverters. However, the damping performance is significantly affected by the computation and pulse-width modulation delays in the digital control system, which results in poor system robustness. Moreover, the phase lag would be introduced by these control delays, leading to the constrained bandwidth and low-frequency gain of control loop. In this study, a novel compensation method based on non-instantaneous loading and pulse-width equivalence is proposed to completely eliminate the computation delay without any reliability problem. Moreover, compared to the traditional strategies, the proposal can be extended to most two-level converters because of its simplicity. Finally, the validity of the proposed method is confirmed by the experimental results from a 3 kW single-phase grid-connected inverter with an LCL filter.

Design of a 100 kHz wide bandgap inverter for motor applications with active damped sine wave filter

In this study, a three-phase motor inverter with sinusoidal output voltages based on the application of gallium nitride transistors and advanced control is analysed. In comparison to standard silicon-insulated gate bipolar transistors much higher feasible switching frequencies of 100 kHz and above are possible and reduce the output sine filter component size such that the filter can be directly included into the inverter. This considerably improves the electromagnetic interference (EMI) behaviour of the drive system as well as the acoustic noise, eases the inverter-to-motor wiring and protects the motor isolation against high d u /d t rates. The study describes the dimensioning and design of the used two-stage LC filter including motor current control based on proportional-integral-type phase current controllers. The LC filter damping is performed actively by capacitor current feedback. Using this active damping scheme avoids additional losses of conventional sine filters and guarantees high system efficiency up to 98%. Finally, experimental results of a laboratory prototype verify the proper behaviour of the proposed concept.

A Method for Improving Robustness of Grid-Connected Inverter System with Control Delay

In the grid-connected inverter system, due to the existence of digital control delay, the inverter presents the characteristic of negative damping. The traditional damping method is usually the form of capacitive current feedback, but the stability margin of the system is very low. Passive damping can improve the stability margin of the system, but it increases the loss of the system and reduces the high-frequency attenuation characteristics of the filter. Therefore, this paper considers the use of active + passive damping, while ensuring the system phase margin, reducing the loss as far as possible, improving the filter’s high-frequency filtering characteristics. Finally, the simulation model is built in Matlab/Simulink, and the correctness of the theoretical analysis is verified by simulation.

Observer-Based Capacitor Current Sensorless Control Applied to a Single-Phase Inverter System With Seamless Transfer

In this paper, capacitor current sensorless control by using an observer is applied in a seamless transfer single-phase inverter. The proposed control system is a multiloop control system. The control loop is composed of a capacitor voltage outer control loop and capacitor current inner control loop for stand-alone mode. For on-grid mode, the grid current outer loop controller is combined with the stand-alone control structure to form a simple controller structure for mode switching. The capacitor current feedback signal is calculated using the observer in the discrete-time domain and thus sensors are not required. Use of the proposed observer for the inverter with capacitor current control has many advantages, such as sensorless capacitor current calculation, one-sample-ahead data prediction for sampling delay compensation, and an average capacitor current without ripple components. Verification of the theoretical analysis is presented through simulation and experiment. A 5-kW single-phase inverter prototype combined with a Li-ion battery is set up with a digital controller using a TI TMS320F28377D microcontroller unit to confirm the feasibility of the system.

Resonance Mitigation and Performance Improvement in Distributed Generation based LCL Filtered Grid Connected Inverters

Resonance turns into a growing issue of paramount importance for stable operation of LCL filtered grid-connected inverters. Active damping algorithms are widely adopted to restrain the resonance peak associated with LCL filters. The focus of this paper is to develop an improved active damping solution based on filter capacitor current for better control performance of three-phase LCL grid-connected inverters. In the proposed solution, an improved compensator is included across the LCL filter system and capacitor current feedback. A damping loop is implemented with the proposed combination, which is further feedback at a reference voltage point of the three-phase inverter to damp the aroused resonance peak. The substantial features of the proposed configuration are wide damping range of resonance frequency and high control bandwidth, which results in faster dynamic response in comparison with conventional capacitor current proportionally feedback. Moreover, the stability of current loop is examined in detail by implementing the proposed damping method under the filter parametric variations. Finally, the efficacy of the proposed method is validated by illustrating the outcomes based on steady state and transient responses through simulations and experimental results of the laboratory prototype.

Whale Optimization Algorithm for Active Damping of LCL-Filter-based Grid-Connected Converters

Grid-connected converters which are equipped with LCL-filters have drawn more and more attention due to their capability to accomplish a better harmonic reduction in comparison with the L-filters based ones. However, due to the resonance problem of the LCL-filters, stability issues may emerge in the current loop in case of the resonance peak is not fairly damped. Active damping strategy is preferred to the passive one due to the power losses on passive resistor and its additional cost. The capacitor current feedback active damping control approach, which is based on voltage oriented of Proportional Integral (PI), can be adopted as a good candidate to outperform the resonance hazard . Since the selection of PI controllers’ parameters and capacitor current gains by iterative trials-errors based procedure becomes a time consuming and difficult task, an optimization problem under operating time-domain restriction s is modeled for tuning these decision variables. Such an optimization based-current feedback active damping control problem is managed by means of the advanced Whale Optimization Algorithm (WOA). To evaluate the effectiveness of the introduced WOA metaheuristic, an empirical comparison study with the homologous Water Cycle Algorithm (WCA), Grey Wolf Optimizer (GWO), Artificial Bee Colony (ABC ) and Particle Swarm Optimization methods is achieved. Statistical analysis is performed by using nonparametric Friedman’s rank and Bonferroni-Dunn’s test to check the significance of each algorithm. Demonstrative simulations highlight that the WOA method provides competitive solutions in terms of robustness and performance for the active damping controllers’ tuning problem .  Â

Passivity Based Damping Design for Grid-Connected Converter With Improved Stability

The impedance interactions between weak grid and LCL-filtered converter can bring severe harmonic resonance. Recently, frequency domain passivity theory has been applied to converter damping design. However, it is usually not easy to achieve full frequency passivity. What’s more, traditional passivity based damping design is not enough for system robustness. This paper first proposes a relaxed passivity based active damping design for capacitor current feedback. Then the system stability under grid impedance variation is also analyzed. It has been found that the wide variation of grid impedance can decrease system stability margin. As a result, critical oscillation can even happen in the passive region of converter output admittance, causing power quality problems. In order to further improve system stability, this paper proposes an enhanced passivity based impedance shaping method, which is based on hybrid damping and combined active damping. Effects of damping methods and damping parameters are also investigated. A step by step damping design procedure is formulated, which is easy to implement. Simulation and experimental results validate the effectiveness of proposed method.

Parallel‐connected buck converters controlled by capacitor current feedback based PT with current‐sharing ability

Parallel-connected power converters have been widely used in the application scenarios required high power to deal with the high load current. In this study, the pulse train (PT) controller with the advantages of fast response speed and simple structure is newly applied to the parallel-connected buck converters with current-sharing ability. However, as the converters operated in continuous conduction mode, undesirable low-frequency voltage oscillation will be brought about by the PT control method. To resolve this problem, a novel capacitor-current-feedback-based PT (CCF-PT) control method is proposed to eliminate the low-frequency oscillation of the parallel-connected buck converters. The approximate discrete time model of the CCF-PT-controlled buck converters is established, and the bifurcation diagrams of output voltage and inductor current changing along with the current scale factor, the capacitor-current-feedback coefficient and the load resistance are drawn to show the operation stability of the converters. Simulation and experimental results are obtained, which indicate that the proposed parallel-connected buck converters with CCF-PT control have excellent performance such as fast response, wide load range and effective suppression of low-frequency oscillation.

A Fixed-Frequency Sliding-Mode Controller for Fourth-Order Class-D Amplifier

Since the parasitic voltage ringing and switching power losses limit the operation of active devices at elevated frequencies; therefore, a higher-order inductor-capacitor (LC) filter is commonly used, which offers extended attenuation above the cutoff frequency and thus, improves the total harmonic distortion (THD) of the amplifier. This paper applies the concept of integral sliding-mode control to a fourth-order class-D amplifier. Two fixed-frequency double integral sliding-mode (FFDISM) controllers are proposed, where one uses the inductor current while the other involves the capacitor current feedback. Their equivalent control equations are derived, but from the realization viewpoint, the controller using the capacitor current feedback is advantageous and, therefore, is selected for final implementation. The performance of the proposed FFDISM controller for fourth-order GaN class-D amplifier is confirmed using simulation and experimental results.

Damping region extension for digitally controlled LCL‐type grid‐connected inverter with capacitor‐current feedback

This study investigates the extent to which the damping region of a digitally controlled LCL-type grid connected inverter is influenced by the ratio between sampling and switching frequencies, the grid impedance variation and changes in the LCL filter. Based on this analysis, an improved capacitor-current feedback is proposed to preserve stability by extending the damping region under the critical conditions identified. The attention is focused on the case when the switching frequency is higher than one-half of the sampling frequency, which is not covered in the existing literature. In this case, the undamped region is obviously expanded and a conventional capacitor-current feedback will not provide sufficient damping region. An improved capacitor-current feedback active damping method is proposed to increase the system critical frequency greatly and to obtain a wider damping region for all possible LCL resonances. With a reasonable parameter design, the extended system damping region is able to cover almost all possible resonance frequencies even when the switching frequency is the same as sampling frequency. A high robustness to grid-impedance variation is achieved in the system as a result of this improved feedback control. The effectiveness of the theoretical analysis and proposed method is verified by the experimental results.

UDE-based current control of grid-connected photovoltaic inverters

PurposeThe purpose of this study is to improve the control performance of grid-connected photovoltaic (PV) inverters with inductive-capacitive-inductive (LCL) filters by proposing a new robust current control based on uncertainty and disturbance estimator (UDE).Design/methodology/approachThe control strategy combines the capacitor current feedback with a UDE-based control to solve robust stability issues in the presence of parametric uncertainties and disturbances.FindingsThis paper provides guidelines for tuning the controller parameters where it is shown to be easy to implement by simply selecting the appropriate feedback coefficient, the reference model and an approximate lumped disturbance bandwidth. Simulation and experimental results demonstrate the effectiveness of the proposed controller in terms of resonance damping, tracking performance and robust stability under grid uncertainties and disturbances.Practical implicationsThis paper offers a new approach for designing implementable robust controllers for LCL-filtered grid-connected PV inverters.Originality/valueA new UDE-based current control is proposed to improve the stability performance of grid-connected PV inverters. The advantages of UDE-based control are its simple structure, easy tuning and robustness under parameter uncertainties and disturbances. Simulation and experimental results support the theoretical findings.

SRF Based Output Voltage Control of 3-Level 3-Phase 4-Leg AT-NPC Inverter

This paper proposes a synchronous reference frame (SRF) based high performance output voltage controller for the 3-level 3-phase 4-leg (3P4L) advanced T-type neutral point clamped (AT-NPC) inverter operated in stand-alone mode. 3-phase inverters for stand-alone operation are required to provide 3-phase balanced nominal voltage under different load types such as unbalanced linear and non-linear loads. 3P4L inverters working with these types of load allow controlling zero sequence voltage by additional fourth leg. The main contribution of this work is the control of the 3-level 3P4L AT-NPC inverter with an LC-type filter modeled based on the output voltage and capacitor current feedback in the synchronous coordinate system. According to obtained capacitor current decoupled model, double loop PI controller is adopted to control the output voltage of the inverter. An inner capacitor current feedback loop is employed to provide fast dynamic response and active damping of the capacitor current. Finally, transient and steady state operation performance of the controller have been tested with PSIM simulation studies considering different load types. Simulation results validate that the proposed SRF based double loop PI controller ensure high dynamic response and high quality output voltage with less than 3% total harmonic distortion (THD) value for the 3-level 3P4L AT-NPC inverter.

Direct Realization of Digital Differentiators in Discrete Domain for Active Damping of LCL -Type Grid-Connected Inverter

To damp the LCL-filter resonance in a grid-connected inverter, the feedback of capacitor current is usually adopted, and it can be replaced by the feedback of capacitor voltage as a low-cost solution, if an accurate digital differentiator can be made. The best way for realizing such a differentiator has so far proved to be an indirect nonideal generalized integrator (GI). As a simple alternative, this paper proposes two digital differentiators, which are directly developed in the discrete domain. They are a first-order differentiator based on backward Euler plus digital lead compensator and a second-order differentiator based on Tustin plus digital notch filter. The basic idea of the proposed methods is to correct their frequency responses to match the ideal differentiator with embedded digital filters. It is shown that the proposed differentiators exhibit the same derivative performance as the nonideal-GI differentiator, and they are more attractive for digital implementations due to their direct discrete natures, compact expressions, and easy algebraic manipulations. In particular, the proposed first-order differentiator is most competitive for its general representation and simplest implementation. Finally, a 12-kW prototype is built, and experiments are performed to verify the theoretical analysis.

Robust Active Damping Methods for LCL Filter-Based Grid-Connected Converters

Grid-connected converters (GcCs) employ LCL filters instead of simple L filters in order to meet new grid codes and their on-going changes in the near future. Active damping methods, without power losses, are preferred to passive ones for solving the resonance problems of LCL -filter-based GcCs ( LCL -GcCs). However, large changes in the grid inductance (typically under weak grid conditions and in rural areas) may compromise the system stability. Moreover, the delay of digital controllers will change system phase-frequency characteristics and consequently will affect the system stability. In view of this, this paper proposes a systematic design procedure for the active damping of voltage-oriented PI control for LCL- GcCs. The procedure considers active damping methods based on capacitor current feedback and is aimed to ensure stable operation under severe grid inductance variations while taking into account the influence of digital control delay and LCL filter parameters changes on the system stability. Simulation and experimental results are presented and discussed in order to validate the proposed design procedure.

Modeling and design of a multivariable control system for multi-paralleled grid-connected inverters with LCL filter

The quality of injected current in multi-paralleled grid-connected inverters is a matter of concern. The current controlled grid-connected inverters with LCL filter are widely used in the distributed generation (DG) systems due to their fast dynamic response and better power features. However, designing a reliable control system for grid-connected inverters with LCL filter is complicated. Firstly, overcoming to system resonances due to LCL filters is a challenging task, intrinsically. This could become worse as number of paralleled grid-connected inverters increased. In order to deal with resonances in the system, damping methods such as passive or active damping is necessary. Secondly and perhaps more importantly, paralleled grid-connected inverters in a microgrid are coupled due to grid impedance. Generally, the coupling effect is not taken into account when designing the control systems. In consequence, depending on the grid impedance and the number of paralleled inverters, the inverters installed in a microgrid do not behave as expected. In other words, with a proper control system, a single inverter is stable in grid-connected system, but goes toward instability with parallel connection of other inverters. Therefore, consideration of coupling effect in the multi-paralleled grid-connected inverters is vital. Designing control systems for multi-paralleled grid-connected inverters becomes much more difficult when the inverters have different characteristics such as LCL filters and rated powers. In this paper, the inverters with different characteristics in a microgrid are modeled as a multivariable system. The comprehensive analysis is carried out and the coupling effect is described. Also, the control system design for multi-paralleled grid-connected inverters with LCL filter is clarified and a dual-loop active damping control with capacitor current feedback is designed. Finally, the proposed multivariable control system for a microgrid with three-paralleled grid-connected inverters with LCL filter is validated by simulation.