Dual-loop Control Research Articles

Current Control of LCL-Type Shunt APFs: Damping Characteristics, Stability Analysis, and Robust Design Against Grid Impedance Variation

This article is focused on a dual-loop current control (grid current loop and inverter-side current loop) with the point of common coupling (PCC) voltage feedforward control for digitally controlled LCL-type shunt active power filters (APFs). However, both inverter-side current feedback and PCC voltage feedforward introduce active damping (AD) effects on different LCL elements, leading to uncertain system damping characteristics and robustness against grid impedance variation. To address this issue, the accurate system damping characteristics are evaluated through open-loop pole analysis, and the benchmarking of system stability and robustness is revealed with respect to various control and system parameters. Moreover, the system instability may occur under a large grid impedance due to high-frequency resonant units of current controllers, and an improved PCC voltage feedforward control is presented to avoid this problem. Based on these findings, a robust design procedure is proposed to obtain proper control parameters and enhance system stability and robustness. Experimental results on a 30-kVA APF prototype have finally validated the correctness and effectiveness of the proposed analysis and method.

A Novel Hybrid PFM/IAPWM Control Strategy and Optimal Design for Single-Stage Interleaved Boost-LLC AC–DC Converter With Quasi-Constant Bus Voltage

Single-stage boost-LLC ac-dc converters have been a popular soft-switched topology because of the high-efficiency and being cost-effective. However, low power capacity is its main disadvantage. In this article, a novel single-stage interleaved boost-LLC ac-dc converter with higher power capacity is presented while achieving the low input current ripple. However, it still suffers from the wide and high bus voltage for universal operation, as a result of the conventional pulse frequency modulation (PFM) with the duty cycle of 0.5. Furthermore, this wide bus voltage will bring the difficulties in designing the LLC resonant cell. To solve these issues, a dual-loop feedback control strategy based on PFM/interleaved asymmetric pulsewidth modulation hybrid modulation and power balance based optimal design are proposed. With the proposed control strategy and optimal design, the bus voltage of the studied converter is not only significantly reduced, but also zero voltage switching for the primary switches and zero current switching for the secondary diodes are still maintained. Thus, the higher efficiency can be achieved. Operation principles and control scheme for the studied converter are fully addressed. The optimal design based on power balance principle is also discussed in this article. A 300 W prototype is fabricated to validate the theoretical analysis and optimal design. The experimental results demonstrate the prototype can simultaneously obtain the universal operation with high power factor above 0.98 and the quasi-constant bus voltage below 500 V. The measured peak efficiency is 92.7% under 115 Vac input voltage at full load.

Lyapunov-Based Large-Signal Control of Three-Phase Stand-Alone Inverters With Inherent Dual Control Loops and Load Disturbance Adaptivity

In this article, existed Lyapunov-based control methods for stand-alone inverters either have a single control loop accompanied with steady-state errors or dual control loops at the sacrifice of negative definiteness of the derivative of the Lyapunov function. Besides, load-current sensors or observers are indispensable in these methods. Proposed Lyapunov-based control inherently has dual control loops that can rigorously guarantee the global large-signal stability of the system. Meanwhile, load disturbance is suppressed adaptively without any load-current sensors or observers. Stability analysis proves that the proposed method is valid both for a linear and nonlinear load. The proposed approach complies with the internal model principle, leading to the minimized steady-state error. An adaptive weighted Lyapunov function (V) is proposed to derive the dual-loop control law and adaptive law. The closed-loop system is inherently d-q decoupled. Three controller gains are tuned quantitatively via explicit formulas based on pole-placement strategy. Simulation and experimental results demonstrate that the proposed method has good steady-state and dynamic performance with great robustness against the parametric mismatch.

A Hybrid Control Strategy of LCC-S Compensated WPT System for Wide Output Voltage and ZVS Range With Minimized Reactive Current

To regulate the output voltage of inductor–capacitor–capacitor-series ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -S) compensated wireless power transfer (WPT) system, a hybrid control strategy of phase shift modulation (PSM) and a switch-controlled capacitor is presented in this article. In the proposed hybrid control strategy, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -S compensated WPT system can realize wide output voltage regulation with zero voltage switching (ZVS), and the reactive current in a resonant tank can be minimized in the whole voltage range. Besides, the current in the primary coil of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -S compensated WPT system can be regulated with PSM, which helps to reduce the power loss of the primary coil at light load. Compared with conventional dual-loop hybrid control, the proposed hybrid control can regulate two control variables with one feedback loop simultaneously. To verify the theoretical analysis, a 500-W prototype with 400-V input voltage, 100−250-V output voltage is built. The experimental results show that the inverter can realize ZVS within the whole voltage range and a peak efficiency of 94.7% can be obtained.

Analysis of an efficient interleaved ultra‐large gain DC–DC converter for DC microgrid applications

This study deals with steady-state analysis and control of a step-up interleaved winding cross-coupled inductor DC–DC converter with high voltage gain and reduced voltage stress across semiconductors using voltage multiplier cells (VMCs). Because of the interleaved scheme, the thermal stress is reduced and the input current ripple is minimised. The leakage energy is recycled by passive lossless clamp circuit to the output. Meanwhile, the voltage stress across the semiconductors is substantially low. Hence, MOSFETs with less ON-state resistances and diodes with a less forward voltage drop can be utilised that improves the circuit performance. The operation principle and the steady state are discussed. Small-signal modelling of the proposed converter is derived via state-space averaging technique and a dual loop controller for output voltage regulation is designed. The adopted strategy utilises a fast dynamical inner loop to control the input current and an outer loop for the output voltage regulation. Finally, a 1 kW prototype with 60 V–1 kV voltage conversion is fabricated and tested to probe the carried analysis.

Research of auxiliary frequency modulation in smart microgrid

In this paper, the component model of bidirectional converter and inverter is established, and the precise control of input and output power of the energy storage system is realized by using dual loop control strategy and PQ control strategy. the control strategy of the energy storage system is proposed to improve the performance of the primary frequency modulation process. The grid interconnection model of offshore oil platform is established. The frequency variation in primary and secondary frequency modulation is simulated to verify the effectiveness of the control strategy.

A Dual-Loop Control to Ensure Fast and Stable Fault-Tolerant Operation of Series Resonant DAB Converters

When a single-switch open-circuit fault occurs in the series resonant dual-active bridge (SRDAB) converter, the output dc voltage will drop by a half or rise by twice. To maintain a continuous power supply, a fault-tolerant control method based on the voltage single-loop control is proposed in this article, where the rectifier-side output square voltage is regulated. Nevertheless, it may excite the resonance between the resonant inductors and dc capacitors, leading to severe low-frequency oscillations, (appearing as the envelope of the high-frequency current). This may trigger the overcurrent protection and the SRDAB fails to ride through the fault. To address this issue, low-frequency equivalent models are proposed first for the bidirectional power-flow of the SRDAB, enabling frequency-domain analysis of the single-loop voltage control. The analysis reveals that the oscillation depends on the duty-cycle and control parameters, and it is more likely to occur when the converter operates in the boost mode. However, it is not possible to suppress the oscillations by the voltage single-loop control. Thus, a dual-loop fault-tolerant control method is developed. The proposed control strategy includes an outer-loop voltage control, an inner-loop current envelope control, and a nonlinear correction unit. Experimental tests on a 1-kW SRDAB are performed, which validate the effectiveness of the proposal in terms of oscillation suppression.

Voltage Stability and Transient Symmetrical Fault Current Control of Voltage-Controlled MMCs

This paper presents a design-oriented analysis on the voltage stability and the transient fault current limitation of voltage-controlled modular multilevel converter (MMC) with L-filters. First, a dual-loop voltage control based on the L-filter plant is systematically designed, where the upper limits of controller parameters to ensure the small-signal stability are identified. Then, considering the transient current limitation during faults, the lower boundaries of inner current controller parameters are derived. Within the region of allowed controller parameters, the optimization is further made to minimize the closed-loop output impedance of MMC, which enhances the output voltage stiffness against the load current disturbance. Lastly, time-domain simulations corroborate the theoretical analysis.

Unbalanced current analysis of three‐phase AC–DC converter with power factor correction function based on integrated transformer

Three-phase AC–DC converter based on integrated transformer is a novel converter which is capable of handling high power levels and can realise power factor correction function. Owing to different parameter tolerances of the primary windings, grid current could be unbalanced, which usually could not be solved through traditional control methods. To handle this problem, a current balancing method is adopted in this study to improve former current control loop. Meanwhile, based on the principle of instantaneous power balance, a dual loop control strategy with an outer voltage loop and an inner current loop is designed. Besides, load current feedforward control is introduced to enhance the dynamic response. Moreover, phase-shift control is employed as the modulation method, in which duty ratios change along with sinusoidal signals. Simulation and experimental results show that the control strategy could make the three-phase current of the converter sinusoidal and balanced.

Three‐phase interleaved high step‐up bidirectional DC–DCconverter

A three-phase interleaved high step-up bidirectional DC–DC converter is proposed, which can achieve three times the voltage gain of the traditional three-phase interleaved boost converter only by adding two capacitors. The proposed converter has the characteristics of low switch voltage and current stresses, low output voltage ripple, automatic voltage-balancing and current-sharing abilities. Compared with the traditional interleaved switching strategy, a 180° interleaved switching strategy is proposed, which is more suitable for the proposed converter by widening the working duty cycle range. Closed dual-loop control is used to regulate the output voltage of the converter. The parameter determining the method of the closed-loop controllers based on the Bode diagram is described in order to control the high-order converter. Operating principle, modelling, steady-state analysis, voltage-balancing and current-sharing ability analysis and closed-loop design of the converter are studied in more detail. Finally, experimental verification and efficiency measurements are implemented by a 300 W, 24 V input to 230 V DC bus prototype, which proved that the proposed converter is more suitable than the traditional three-phase interleaved boost converter for some applications where requiring about six to ten times the voltage conversion ratio.

Practical Fixed-Time Disturbance Rejection Control for Quadrotor Attitude Tracking

In this article, a practical fixed-time disturbance rejection controller is investigated for quadrotor attitude tracking. The disturbance rejection is achieved by applying fixed-time disturbance observers (FxTDOs), and the practically fixed-time convergence is proved via homogeneous theory and Lyapunov method. Then, a dual-loop fixed-time controller is designed for quadrotor attitude tracking, and the disturbances are estimated by FxTDO in the inner loop. Finally, the efficiency and robustness of the proposed method are illustrated by simulations and experiments in real flights.

Trajectory Tracking Control for Nonholonomic Wheeled Mobile Robots with External Disturbances and Parameter Uncertainties

Aiming at the trajectory tracking problem of nonholonomic wheeled mobile robots (WMR) with bounded external disturbances and parameter uncertainties, a dual-loop attitude tracking robust controller is proposed for WMR. Firstly, a kinematic controller is designed to generate the virtual velocity based on the kinematic error model. Secondly, the sliding mode control with a modified reaching law is adopted to ensure the actual velocity can converge to the virtual velocity in finite time based on the dynamic model. Finally, the stability of the controller is verified through the Lyapunov function. Numerical simulation shows the robustness and effectiveness of the proposed dual-loop tracking controller.

A Systematic Approach via IIR Filters for Enhancing the Robustness of LCL-Type Shunt Active Power Filters to Grid Impedance

A dual-loop current control method (grid current loop and inverter-side current loop) is promising for digitally controlled LCL -type shunt active power filters (APFs), due to its inherent active damping effect for LCL resonance. However, the damping region is only up to one-sixth of the sampling frequency $(f_{s}/ 6)$ , which leads to limited system robustness against grid impedance variation at the point of common coupling (PCC). Hence, in this study, the robustness of such control is discussed through mathematic derivations and diagram analysis, which reveals that both the system delays and PCC voltage feedforward affect the system robustness. On the basis of theoretical analysis, a systematic approach via infinite impulse response (IIR) filters has been proposed for enhancing the robustness of grid impedance. Thereof, a second-order IIR filter-based inverter-side current feedback is proposed to widen the damping region up to almost the Nyquist frequency $(f_{s}/ 2)$ , while a band-stop IIR filter-based PCC voltage feedforward is proposed to achieve the superior stability margins under a large grid impedance in comparison with unit feedforward. Finally, experimental results on a 30-kVA APF prototype have verified the effectiveness of the proposed approach.

A DC Hybrid Active Power Filter and Its Nonlinear Unified Controller Using Feedback Linearization

In the current power system, the conversion between dc and ac is widely existing and the dc-side harmonic problem is prominent. To suppress the second-harmonic current at the dc side of a single-stage single-phase inverter, a dc hybrid active power filter (DC-HAPF) structure is presented, which composes of a bidirectional dc-dc circuit-based active power filter (APF) and a CL passive filter. In this article, the CL passive filter is used to mitigate the high-frequency harmonics and the APF is applied to compensate the low-frequency harmonic current. Meanwhile, the influence of the filter parameters on the harmonic suppression is analyzed based on the average switching model. In addition, for the control of the DC-HAPF, a nonlinear unified controller via feedback linearization is proposed, where the voltage and current dual-loop control is converted to a single-loop control of energy. By analyzing the control system stability and DC-HAPF's performance, appropriate control parameters are selected. To verify the feasibility of the proposed topology and control strategy, a 500-W single-stage single-phase inverter with the DC-HAPF is built and a good performance of the dc-side harmonic suppression has been achieved.

Analysis and Demonstration of a Dynamic ZVS Angle Control Using a Tuning Capacitor in a Wireless Power Transfer System

In this article, a dynamic zero-voltage switching (ZVS) angle control scheme on the wireless power transfer (WPT) system inverter is proposed by implementing a tuning capacitor. High-frequency (HF) and high-power WPT systems are desired in battery charging applications to reduce the battery charging time. Consequently, the power semiconductor devices are subjected to increased voltage, current, and thermal stresses. In this article, the switching losses in the primary-side inverter of a WPT system are suppressed by realizing the ZVS operation during the battery charging process. A dual-loop control strategy is proposed, which involves traditional phase shift control between the inverter phase legs to maintain constant-current (CC)/constant-voltage (CV) at the output and a tuning capacitor to dynamically tune the ZVS angle. The dynamic tuning of the ZVS angle ensures a minimum reactive power requirement from the input source and, therefore, helps to improve the overall efficiency of the system. Closed-loop simulations of the proposed control strategy are carried out using the piecewise linear electrical circuit simulation (PLECS) simulation software. The proposed control strategy is verified on a 10-W WPT prototype by performing the open-loop experimental validations. The maximum efficiency of the proposed control strategy is found to be ~ 87 % during the CC/CV charging modes. The improvement of ~ 10 % in the dc-dc efficiency is achieved with the proposed ZVS control strategy.

A Complete HSS-Based Impedance Model of MMC Considering Grid Impedance Coupling

Harmonic state space (HSS) is seen as an effective impedance modeling method to precisely characterize the internal harmonic features of the modular multilevel converter (MMC). However, the existing MMC impedance models assume the ideal grid, ignoring the grid impedance, and they also do not incorporate the widely used dual-loop control and phase-locked loop (PLL). In this article, a complete MMC impedance model based on HSS is proposed to reveal the grid impedance coupling effect of MMC. The model analysis results demonstrate that the MMC impedance is coupled with the grid impedance due to the internal harmonics. This coupling causes MMC to be affected by the grid impedance and may cause instability. On the other hand, the proposed model not only consists of the ac current and the circulating current control, but also incorporates dc voltage outer loop and PLL with a clear physical meaning. Based on the proposed model, this article illustrates the factors that will enhance the coupling, which shows that the proposed model has to be used to improve the accuracy of the analysis. Finally, effectiveness of the proposed model is verified by simulation and experimental results.

Faulted terminal open circuit voltage controller for arc suppression in distribution network

Existing active arc suppression methods compensate fault current through controlling busbar voltage to zero. However, it may boost the fault point voltage in the case of considering load and feeder impedance, and fail to suppress arc. This study proposes a fault current suppression method by controlling the faulted terminal open-circuit voltage (FTOCV) and harmonic impedance of neutral-point. The effect of load and feeder impedance are taken into account. Depending on this, a faulted terminal open-circuit voltage controller (FTOCVC) is proposed for arc suppression, which consists of a reference voltage calculation module (RVCM) and a reference voltage tracking controller (RVTC). RVTC utilises a dual-loop voltage control method to adjust the neutral voltage for reducing FTOCV and providing a very low harmonic impedance for neutral-point. A reference voltage needed by RVTC can be figured out using RVCM accurately considering load impedance and fault distance. Thus, the proposed method can restrain the fault current to almost zero without influence by the load and feeder impedance. Comparison of several arc suppression methods was performed by simulation, and the results indicated the proposed method is more beneficial to extinguish fault arc.

Modelling and robust control of digitally controlled large‐scale PV plants

Large-scale photovoltaic (PV) plants may be destabilised by the large grid impedance introduced by transformers and long transmission lines. When considering the digital control delay of grid-connected inverters, the problem will become more serious. In this study, the equivalent impedance model of digitally controlled large-scale PV plants is established, considering that the values of the parameters of the LCL filter follow Gaussian distributions. Then, the system stability can be analysed conveniently, and the stability analysis shows that the digital control delay will make the system robust to large grid impedance worse. To enhance the stability-robustness of the system, an H ∞ robust controller is proposed in this study. By properly selecting the weighting functions, the desired tracking error performance and robustness are achieved on the basis of the mixed-sensitivity optimisation control design. Compared with the traditional dual current loop control strategy, the H ∞ robust controller improves significantly the system stability-robustness against large grid impedance. Finally, simulation and experimental tests are carried out in this study, and the results demonstrate the effectiveness of the proposed H ∞ robust controller.

Research on Dual Loop Control Strategy of Dynamic Wireless Charging for Electric Vehicle

This paper focuses on a dynamic wireless charging method of electric vehicles with segmented structure, which uses multiple transmitting coils at the transmitting end to realize wireless power supply for electric vehicles in motion. Based on the structure of the dynamic wireless charging system of the electric vehicle with magnetic coupling resonance, the system is modeled and analyzed by using the equivalent circuit theory, and a control strategy of the transmitting end is proposed, which can effectively solve the problem of coil deviation and dynamic characteristics when the receiving coil and the transmitting coil of the electric vehicle are in the charging process. A double loop controller is designed to adjust the power and current of the transmitting end, which allows the sequence and timely activation of the segmented transmitting coil. Under the condition of no-load and load, the current of the transmitting coil is controlled at the reference value to compensate the power transmission reduction caused by the lateral misalignment of the vehicle and prevent the overload at one time. The system is modeled and analyzed with Simulink software, and the feasibility of the control method is determined.

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.