Outer Voltage Control Loop Research Articles

Research on the Nonlinear Control Strategy of Three-Phase Bridgeless Rectifier under Unbalanced Grids

The three-phase Y-connected bridgeless rectifier is essentially a nonlinear system, and it is difficult to obtain superior dynamic performance under the action of traditional linear controller. Under the condition of unbalanced power grids, this paper has established a mathematical model based on Euler–Lagrange (EL) equations with line voltage and line current as state variables. Furthermore, it then designed a passivity-based controller in inner current loop based on the mathematical model. The hybrid nonlinear control strategy consisting of active disturbance rejection controller (ADRC) in the outer voltage loop and passivity-based controller (PBC) in the inner current loop is adopted to control the system, which does not need to consider the positive and negative sequence components. The control structure is simple and can improve the steady-state accuracy, dynamic performance and anti-interference ability. The feasibility of the proposed control strategy is verified by computer simulation, which has a guiding significance for the application of three-phase bridgeless rectifier in practical engineering.

A Fast Converging Hybrid MPPT Algorithm Based on ABC and P&O Techniques for a Partially Shaded PV System

Among all the conventional maximum power point tracking (MPPT) techniques for a photovoltaic (PV) system that have been proposed, incremental conductance (INC) and perturb and observe (P&O) are the most popular because of their simplicity and ease of implementation. However, under partial shading conditions (PSCs), these MPPT algorithms fail to track the global maximum power point (GMPP) and instead converge into local maximum power points (LMPPs), resulting in considerable PV power loss. This paper presents a new hybrid MPPT technique combining the artificial bee colony (ABC) and P&O algorithms named ABC-P&O. The P&O technique is used to track the MPP under uniform irradiance, and only during irradiance variations is the ABC algorithm employed. The effectiveness of the proposed hybrid algorithm at tracking the GMPP, under both uniform and nonuniform irradiance conditions, was assessed by hardware-in-the-loop (HIL) tests employed by a dc–dc boost converter. Then, the ABC-P&O strategy was applied to obtain the voltage reference for the outer PI control loop, which provided the current reference to the discrete-time sliding-mode current control. The ABC-P&O algorithm has a reasonable computational cost, allowing the use of a commercial, low-priced digital signal controller (DSC) with outer voltage and inner current control loops. Many challenging tests validated that the proposed ABC-P&O technique converges fast to the GMPP with high efficiency and superior performance under different PSCs.

Design and Real-Time Implementation of Cascaded Model Reference Adaptive Controllers for a Three-Phase Grid-Connected PV System

This article proposes the development of a model reference adaptive controller (MRAC) for grid integration of a single-stage three-phase grid-connected photovoltaic system (GCPVS). The performance of a GCPVS is influenced by uncertainties such as measurement noise, solar irradiance, photovoltaic (PV) operating point, aging of the dc-link capacitor, and variation in the filter impedance. In order to achieve the desired tracking performance under both nominal and parametric uncertainties, MRACs are designed for both the outer PV voltage control loop and the inner grid current control loop. The proposed control scheme comprises a reference model and a Lyapunov-based parameter adaptation scheme, which provides the nominal tracking performance despite uncertainties in the GCPVS dynamics. As a result, improved performance of the GCPVS is achieved in terms of low dc-link voltage ripple, superior power extraction, and improved grid power quality. The performance of the proposed adaptive controller is verified by pursuing simulation in MATLAB/Simulink followed by experimentation on a 2.5-kW GCPVS. The control algorithm is implemented in real time on the SPARTAN-6 FPGA scheme. The efficiency of maximum power point tracking (MPPT) is obtained as 99.92%. The total harmonic distortion of grid currents and voltages at the point of common coupling is found to be less than 3%, which is well within the limit specified by the IEEE-1547 standard.

Indirect Effective Controlled Split Source Inverter-Based Parallel Active Power Filter for Enhancing Power Quality

The existing solutions for reducing total harmonic distortion (THD) using different control algorithms in shunt active power filters (SAPFs) are complex. This work proposes a split source inverter (SSI)-based SAPF for improving the power quality in a nonlinear load system. The advantage of the SSI topology is that it is of a single stage boost inverter with an inductor and capacitor where the conventional two stages with an intermediate DC-DC conversion stage is discarded. This research proposes inventive control schemes for SAPF having two control loops; the outer control loop regulates the DC link voltage whereas the inner current loop shapes the source current profile. The control mechanism implemented here is an effective, less complex, indirect scheme compared to the existing time domain control algorithms. Here, an intelligent fuzzy logic control regulates the DC link voltage which facilitates reference current generation for the current control scheme. The simulation of the said system was carried out in a MATLAB/Simulink environment. The simulations were carried out for different load conditions (RL and RC) using a fuzzy logic controller (FLC) and PI controllers in the outer loop (voltage control) and hysteresis current controller (HCC) and sinusoidal pulse width modulation (SPWM) in the inner loop (current control). The simulation results were extracted for dynamic load conditions and the results demonstrated that the THD can be reduced to 0.76% using a combination of SPWM and FLC. Therefore, the proposed system proved to be effective and viable for reducing THD. This system would be highly applicable for renewable energy power generation such as Photovoltaic (PV) and Fuel cell (FC).

Multifunctional Cascade Control of Voltage-Source Converters Equipped With an LC Filter

This article proposes a multifunctional cascade controller structure for voltage-source converters. The proposed structure contains a decoupling loop between the outer voltage control loop and the inner current control loop, and operation in either voltage or current control mode is possible. In voltage control mode, the current controller can be made completely transparent. In the case of faults, the proposed structure enables inherent overcurrent protection by a seamless transition from voltage to current control mode, wherein the current controller is fully operational. Seamless transitions between the control modes can also be triggered with an external signal to adapt the converter to different operating conditions. The proposed structure allows for integration of simple, accurate, and flexible overcurrent protection to state-of-the-art single-loop voltage controllers without affecting voltage control properties under normal operation. The properties of the proposed controller structure are validated experimentally on a 10-kVA converter system.

A Two-Layer Control Scheme Based on P – V Droop Characteristic for Accurate Power Sharing and Voltage Regulation in DC Microgrids

The increasing penetration of dc distributed energy resources (DERs) and dc loads has motivated the utilization of dc microgrids (MGs). In this paper, a new two-level control scheme is proposed for accurate power sharing and appropriate voltage regulation in dc MGs during islanded operation mode. In the primary control level, a P-· V droop method is proposed to eliminate the dependency of power sharing among DERs on line resistances. Since the P-· V droop deviates the voltage derivative to a nonzero value, a voltage derivative restoration mechanism is adopted in the secondary control level to provide voltage stability. The secondary control level also compensates the voltage deviations by cascading an outer voltage control loop with the inner voltage derivative restoration loop. The secondary control signal is broadcasted to each DER via a unidirectional low-bandwidth communication link. Small signal stability of the proposed scheme is analyzed by studying the locus of the system eigenvalues. To verify the efficacy of the proposed method and compare it with the conventional scheme, real-time simulations in OPAL-RT lab setup are provided.

An improved control strategy of grid‐tied solar photovoltaic system using active current detection method

SummaryThis paper presents an active current detection method (ACDM) for three‐phase grid‐tied solar photovoltaic (PV) system. The proposed control utilizes the character triangular function (CTF) for fundamental component extraction and minimizes the computational burden. This shows the effectiveness of proposed ACDM under variety of operating conditions. Moreover, the design of control algorithm allows superior voltage regulation in outer voltage control loop, which reduces the peak overshoot during transients. This will also help in regulating grid current quickly, and it accurately follows the reference current. Further, the proposed control algorithm effectively deals with several power quality (PQ) issues and maintains the continuous power flow to satisfy the load demand. Finally, the withstand capability of the proposed ACDM controller is verified by simulation studies under MATLAB®/Simulink environment and experimentally validated by dSPACE‐1104 based downscaled laboratory prototype.

Experimental Validation of a Nested Control System to Balance the Cell Capacitor Voltages in Hybrid MMCs

In a hybrid modular multilevel converter (MMC), capacitor voltage balance between the Full-Bridge Sub-Modules (FBSMs) and Half-Bridge Sub-Modules (HBSMs) is only possible when the arm currents are bipolar. For a grid-connected MMC, operating at unity power factor, this is typically only achievable when the modulation index is less than 2. Previous control methodologies, based on open-loop feed-forward compensating currents, have been proposed to operate an MMC with a higher modulation index. However, these solutions do not minimize the compensating currents; they cannot compensate entirely for both the variations in the operating conditions and the parameters typically encountered in a real implementation; and they do not consider the actual capacitor voltage imbalance between the FBSM and HBSMs. In this paper, a new nested closed-loop control algorithm based on an outer voltage control loop with an inner current loop is proposed and experimentally validated. Feed-forward currents are still utilised in the inner loop, but they are calculated using a new optimising algorithm which minimises the required compensating currents. Moreover, to the best of our knowledge, this is the first work where explicit algebraic equations to calculate these compensating currents are provided. Experimental results to validate the approach, obtained with an 18-cell hybrid MMC, are presented and discussed in the paper.

Modeling and control of DC/AC converters for photovoltaic grid-tie micro-inverter application

• Modeling and control for a Photovoltaic-based microinverter system. • An efficient maximum power point tracking algorithm is implemented here. • Both state-space modeling and small-signal analysis are carried out. • Enhancement of transient and dynamic performance by using a cascaded controller. • Finally, a 500 Watts, 110 V, 50 Hz microinverter prototype is fabricated and tested. This paper is devoted to the modelling and control for a low cost, high-power quality single-phase voltage source inverter (VSI) for a grid-tied PV-based micro-inverter system. The first stage includes a high-efficiency isolated boost dual half-bridge dc-dc converter topology which interfaces to the PV panel and produces a dc-link voltage. The second stage is comprised of a single-phase VSI circuit. A state-space average model for a 1-Φ VSI with low-pass LCL filter topology has been developed. The steady-state and dynamic performance of the system can be improved by using three cascaded control loops. The fast inner current controller is adopted to regulate the inductor current of the inverter thereby improved dynamic response of the system. The small-signal analysis is conducted to assess the robustness of the proposed controller. An outer dc bus voltage control loop is employed to keep the constant voltage across the dc-bus capacitor. Next, the grid-side current loop is employed to regulate the grid current with a low total harmonic distortion level. The Bode-diagrams are plotted to assess the stability of each control loops. Finally, a 500 Watts, 110 V, 50 Hz micro-inverter prototype is fabricated and tested. The simulation and experimental results are given to show the effectiveness of the proposed control scheme.

Fuzzy Linear Active Disturbance Rejection Control of Injection Hybrid Active Power Filter for Medium and High Voltage Distribution Network

In order to reduce the capacity of the active power filter (APF), improve its dynamic tracking speed and anti-disturbance ability of harmonic currents, so as to better solve the problem of harmonic pollution in the medium and high voltage distribution network, this article is suitable for medium and high voltage distribution networks. The three-phase three-wire injection hybrid active power filter (IHAPF) of high voltage distribution network is the research object, and a fuzzy linear active disturbance rejection controller (Fuzzy-LADRC) suitable for IHAPF voltage outer loop control is proposed. The controller is composed of a fuzzy proportional controller, a linear extended state observer (LESO), and a total disturbance compensation link. The introduction of fuzzy logic control solves the difficult problem of controller parameter tuning, and uses Lyapunov stability definition to prove the stability of the system. Finally, the control performance of IHAPF under the control of Fuzzy-LADRC is simulated and verified by MATLAB&SIMULINK simulation platform, and compared with the traditional PI controller. The results show that the Fuzzy-LADRC controller is better than the traditional PI controller and has good tracking and anti-disturbance capabilities.

Model Predictive Virtual Synchronous Control of Permanent Magnet Synchronous Generator-Based Wind Power System

As much wind power is integrated into the power grid through power electronic equipment, the use of wind power is increased rapidly. Wind power system makes the power grid lack inertia and damping, thereby reducing power grid stability; in severe cases, it may even be disconnected. virtual synchronous generator (VSG) has been put forward to enhance the anti-disturbance performance of power grid. However, conventional VSG adopts an outer power loop and inner-current loop control. The inner-current loop control needs a pulse width modulation (PWM) module and proportion integration (PI) parameter settings. In order to reduce the parameter settings and simplify control structures, in this study, model predictive control (MPC) is used instead of inner-current loop control. At the same time—for the overall stability and control flexibility of the back-to-back system—we further propose to use outer-voltage loop control (OVLC) and MPC to stabilize direct current (DC) voltage on the machine-side and to employ model predictive virtual synchronous controls to provide inertia and damping for the power grid. The proposed control method was simulated in Matlab/Simulink (MathWorks, Massachusetts, MA, 2016) and verified by experimental results.

An Active Disturbance Rejection Control-Based Voltage Control Strategy of Single-Phase Cascaded H-Bridge Rectifiers

In the traditional control strategy for single-phase PWM converter applications, the proportional-integral (PI) controller has the role of voltage tracking in the outer loop voltage controller, which has limited capability to provide acceptable dynamic performance and disturbance rejection. With regard to this, this article proposes a voltage control strategy based on the active disturbance rejection control (ADRC) for single-phase cascaded H-bridge rectifiers, aiming at achieving satisfactory dynamic performance and strong robustness to the disturbances. First, the extended state observer is used for the state and disturbance estimations. Then, the nonlinear state error feedback deals with the system error and makes the dynamic compensation to the disturbances. More specifically, an improved continuous nonlinear function is adopted to mitigate the jitter behavior of the traditional ADRC and provide immunity to large disturbance errors. Furthermore, the performance and stability analysis of the proposed strategy are implemented. A parameter tuning guideline of the proposed strategy is detailed. The effectiveness of the proposed ADRC-based voltage control strategy is verified through extensive experimental tests and is compared to that of the conventional PI-based voltage 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.

Robust backstepping output voltage controller for standalone voltage‐sourced converters

This work presents a linear state-feedback controller using a backstepping design approach for output voltage regulation of 3 ϕ voltage-sourced converters feeding to customers' loads in a stand-alone AC microgrid system. Irrespective of load type and its variations, parameter uncertainties, and other disturbances, the controller is robust enough to achieve a regulated voltage magnitude within the prescribed bounds and exact frequency tracking. The criterion for gain selection using Lyapunov analysis is given. With global exponential convergence feature and better coordination among inner current and outer voltage control loops, improved transient response is achieved. Controller features are examined by eigenvalue analysis and extensive simulation studies. Experimental results demonstrate the feasibility of the proposed controller implementation.

DSP-Based SiC-MOSFET SAPF With 100-kHz Sampling and Switching Frequency for Wideband Harmonic Suppression

In this article, 100-kHz sampling and switching frequency shunt active power filter (SAPF), which is based on a dual-core DSP TMS320F28377D, is designed for the potential market of high-frequency harmonic suppression in more electric aircraft (MEA) or high-speed train (HST). By utilizing the multithreading and multiprocessing method, different computation segments, such as analog-to-digital conversion, recursive discrete Fourier transform (RDFT), and inner loop current and outer loop voltage control, can be rearranged as several parallel operation groups. Compared with the traditional single pipeline structure of the mostly 10–20 kHz switching frequency commercial SAPF, this parallel execution speeds up the signal processing so that the highly increased sampling frequency makes the current control loop reaches wider bandwidth even with simple proportional integral (PI) regulator. Meanwhile, for wideband harmonic suppression, a multiresolution RDFT strategy is proposed to extend the range of the selected harmonic without sacrificing its extraction accuracy. After raising the sampling frequency to 100 kHz, the proposed SAPF with much easier current controller could suppress wider frequency harmonics than other 10–20 kHz SAPF. A 10- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {kV}\cdot \text {A}$ </tex-math></inline-formula> prototype with CREE SiC power module CCS050M12CM2 is set up to verify its effectiveness. The result complies with the IEEE standard 519-2014.

Linear Active Disturbance Rejection Control for DC Bus Voltage Under Low-Voltage Ride-Through at the Grid-Side of Energy Storage System

The energy storage inverter system has the characteristics of nonlinearity, strong coupling, variable parameters, and flexible mode switching between parallel and off grid. In order to improve the control performance of the grid-side inverter of the energy storage system, an improved Linear Active Disturbance Rejection Control (LADRC) based on proportional differentiation is proposed to replace the traditional LADRC in the voltage outer loop control. In this paper, the observation gain coefficient of the sum of the disturbances of the traditional Linear Extended State Observer (LESO) is improved to a proportional differentiation link, which effectively reduces the degree of the disturbance observation amplitude drop and the phase lag, and increases the observation bandwidth of LESO. Compared with traditional LADRC, it not only improves the observation accuracy of LESO for disturbance, but also improves the anti-interference performance of LADRC. Finally, the control effects of improved LADRC and traditional LADRC on low-voltage ride-through at different degrees are analyzed and compared through simulation, which proves the rationality of the controller designed in this paper.

Dual Closed-Loop Linear Active Disturbance Rejection Control of Grid-Side Converter of Permanent Magnet Direct-Drive Wind Turbine

In the permanent magnet direct-drive wind power grid-connected system, in order to solve the coupling problem between d -axis and q -axis currents and to improve the disturbance rejection performance of direct current (DC) bus voltage under grid faults, a new dual closed-loop structure based on linear active disturbance rejection control (LADRC) is proposed. This new dual closed-loop control includes current inner loop decoupling control and DC bus voltage outer loop control with first-order LADRC. As the LADRC has the advantages of decoupling and disturbances rejection, it is applied to the control of wind power grid-connected inverter. Through analysis, it is demonstrated that the current decoupling control is simpler than proportional integral (PI) control algorithm, the dynamic response speed is faster, and the DC bus voltage control has better anti-disturbance. Finally, a 1.5 MW direct-drive permanent magnet wind power system was established through digital simulation, and the control effects of the two control modes under different working conditions are compared. The simulation results verify that the proposed dual closed-loop control based on first-order LADRC is superior to PI double closed-loop control in terms of decoupling performance and disturbance rejection performance under grid faults.

Super-Twisting direct torque optimal L2 -gain control for self-excitation induction generation system

The use of self-excitation induction generation system (SEIGs) was identified by many researchers as an important way to construct the high power density isolated power supply system (IPSS). However, for the wide variation range of the prime mover speed and the load power, the majority of the researchers consider it’s important to improve the power supply performance of the SEIGs. This research contributes to propose a new Super-Twisting direct torque optimal L2-gain control (ST-OP-L2-DTC) method for the SEIGs. In order to enhance the torque response speed, the Super-Twisting space vector direct torque control method is adopted in the voltage-linkage outer loop subsystem, comparing with sinusoidal angular modulation, which can improve 13.5 percent of the voltage usage ratio and raise the DC rectifier voltage. For minimizing disturbance influence, the optimal L2-gain control method is used in the current inner loop subsystem, which can minimize the impact of disturbance by solving the Linear Matrix Inequality (LMI) to obtain the optimal control law against the worst disturbance. Results of simulation and experiment show that, under the load power and the speed varying condition, comparing with traditional voltage outer loop-current inner loop (VOL-CIL) control method, ST-OP-L2-DTC method can enhance the DC output voltage transient stability, realize the derivative convergence of the sliding mode surface, shorten the stability time of the electromagnetic torque.

Experimental validation of multi-loop controllers for two-level cascaded positive output boost converter

In this paper, an investigation of multi-loop controllers (MLCs) for two-level cascaded positive output boost converter (TLCPOBC) operating in the continuous conduction mode (CCM) was carried out. The TLCPOBC is a recently designed DC–DC converter that can offer a higher voltage transfer ratio, good efficiency, and minimized capacitor voltage and inductance current ripples when compared to traditional DC–DC converters. The dynamic characteristics of the TLCPOBC are nonlinear in nature due to its switching mode operation. During line and load variations, conventional controllers are not able to overcome the dynamic characteristics. Hence, to control the output voltage and inductor current of the TLCPOBC, MLCs are suggested in this paper. The control structure of the TLCPOBC consists of two loops. Here, a fuzzy logic controller (FLC) and a proportional double integral controller (PDIC) act as an outer loop voltage controller for voltage regulation. Meanwhile, a proportional controller (PC) acts as an inner current loop for the inductor current regulation of the converter. The PC and PDIC parameters were obtained by the state-space equations of the TLCPOBC using the Ziegler–Nicholas tuning method. The rules of the FLC are derived from the characteristics of the TLCPOBC without a mathematical model. The performance of the TLCPOBC using MLCs was verified at different states by developing MATLAB/SIMULINK simulations and prototype models. Simulation and experimental results are validated to show the importance of the TLCPOBC with the designed controllers.

Closed-Loop Analytic Filtering Scheme of Capacitor Voltage Ripple in Multilevel Cascaded H-Bridge Converters

This article proposes a closed-loop analytic filtering scheme for the low-frequency capacitor voltage ripple in cascaded H-bridge converters. Although this technique can be applied to a number of cascaded converter applications, this article focuses particularly on the low-capacitance cascaded H-bridge static compensator. Filtering of capacitor voltage signals is particularly important in this application, due to the presence of large low-frequency harmonic voltage ripple. The main novelty of the proposed algorithm in this article is the estimation of the capacitance value of each H-bridge. The closed-loop capacitance estimation method reduces steady-state error in estimated voltage ripple magnitude and phase, which is otherwise present in the available open-loop filters in the literature. Furthermore, fast dynamic response is achieved compared to filtering schemes based on low-pass and bandstop filters. Therefore, the proposed solution optimizes the tradeoff between filtering accuracy and transient response. It also mitigates parametric uncertainties, time delays, and harmonic contamination in the outer voltage control loop. Furthermore, the estimated capacitance is also useful to track the state of health of the dc-link capacitors. Experimental results on a seven-level 1-kVA cascaded H-bridge prototype are presented to demonstrate the comparative performance of the proposed filtering scheme and traditional approaches.