Comprehensive Modeling, Analysis, and Comparison of State-Space and Admittance Models of PLL-Based Grid-Following Inverters Considering Different Outer Control Modes

Microgrid (MG) control in off-grid mode of operation is one of the main challenges in MGs. Under different loading conditions, voltage and frequency deviate from their nominal limits. The droop control method that applied with the current control and voltage control loops can restore the MG voltage and frequency to their nominal limits. However, it can’t regulate these parameters to their specified values. Additionally, the droop control method can’t do well when the system supplies either nonlinear or unbalanced loads. This paper proposes a control technique for adjusting the voltage and frequency of an MG in islanded mode under different loading conditions. The proposed method is based on applying the H-infinity ($$H\infty$$) robust control as a secondary controller with the droop control method to improve its performance. The proposed method is composed of four stages including; droop control loop, voltage control loop, current control loop and control loop for inductance–capacitance–inductance (LCL) filter and coupling circuit. $$H\infty$$ control has an internal model that can control the frequency and improve the system power quality by decreasing the disturbance frequencies. Moreover, it has two weighted parameters $$\xi$$ and $$\mu$$ that must be correctly adjusted to improve the stability of the system. For a proper adjusting of these parameters, the Harmony Search (HS) optimization method is applied. To prove the effectiveness and superiority of the proposed method, it is applied on a test system with three different loading scenarios using MATLAB/Simulink program. The results are compared with those of droop controller and they confirm that the proposed control method is more effective in controlling the microgrids under islanded mode operation.

The design and implementation of a prototype hybrid power generation system, consisting of a ram air turbine (RAT) generator and an energy storage unit, is presented. The system is intended as a mechanism to supply additional or emergency power on-board an aircraft. Here we present the design, manufacture and testing of a synchronous rectifier required to interface an asynchronous generator to a dc bus. The dc bus connects to a battery of electrochemical cells through a bidirectional dc-dc converter. The dc-dc converter functions to regulate the dc bus voltage by utilising a double control loop strategy with an inner current control loop and outer voltage control loop. The power delivered to the dc bus from the generator is controlled as a function of the battery voltage, dc bus current and generator shaft speed. The control strategy used by the prototype circuit is presented in this paper along with simulation and experimental results.

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.

This paper presents the stability analysis for an inverter based Distributed Generation (DG) unit in an autonomous microgrid operation. The small-signal model of the controlled Voltage Source Inverter (VSI) system is developed in order to investigate the dynamic stability for the given operating point and under the proposed power controller. This model includes all the details of the proposed controller, while no switching actions are considered. System oscillatory modes and the sensitivity to the control parameters are the main performance indices which are considered, particularly when the microgrid is islanded or under the load change condition. In this work, the proposed power controller is composed of an inner current control loop and an outer power control loop, both based on a synchronous reference frame and conventional PI regulators. These controllers also utilize the Particle Swarm Optimization (PSO) for real-time self-tuning in order to improve the quality of the power supply. The complete small-signal model is linearized and used to define the system state matrix which is employed for eigenvalue analysis. The results prove that the stability analysis is fairly accurate and the controller offers reliable system's operation.

Autonomous Underwater Vehicles (AUVs) are widely used for the inspection of seabed pipelines. To address the issues of low trajectory tracking accuracy in AUV inspection processes due to uncertain ocean current disturbances, this paper designs a new dual-loop controller based on Model Predictive Control (MPC) and Variable Universe S-plane algorithms (S-VUD FLC, where VUD represents Variable Universe Discourse and FLC represents Fuzzy Logic Control) to achieve three-dimensional (3-D) trajectory tracking of an over-actuated AUV under uncertain ocean current disturbances. This paper uses MPC as the outer-loop position controller and S-VUD FLC as the inner-loop speed controller. The outer-loop controller generates desired speed instructions that are passed to the inner-loop speed controller, while the inner-loop speed controller generates control input and uses a direct logic thrust distribution method that approaches optimal energy consumption to distribute the thrust generated by the propellers to the over-actuated AUV, achieving closed-loop tracking of the entire trajectory. When designing the outer-loop MPC controller, the actual control input constraints of the system are considered, and control increments are introduced to reduce control model errors and the impact of uncertain external disturbances on the actual AUV model parameters. When designing the inner-loop S-VUD FLC, the strong robustness of the variable universe fuzzy controller and the easy construction characteristics of the S-plane algorithm are combined, and integral action is introduced to improve the system’s tracking accuracy. The stability of the outer loop controller is proven by the Lyapunov method, and the stability of the inner loop controller is verified by simulation. Finally, simulations show that the over-actuated AUV has fast tracking processes and high tracking result accuracy under uncertain ocean current disturbances, demonstrating the effectiveness of the designed dual-loop controller.

In this paper, a cascaded nonlinear control design for the position of a hydraulic servo cylinder is presented that is based on a control-oriented mathematical model of the test rig. The difference pressure is controlled in the inner loop, whereas the position control is addressed in the outer control loop. Additionally, the outer position control loop is extended by a combination of feedforward friction compensation and an observer-based disturbance compensation to improve the position tracking behaviour. Thereby, the benefits of both measures can be exploited: fast reaction and robustness w.r.t. remaining parameter uncertainties and inaccuracies in the friction model. As the system states are measurable, a reduced-order disturbance observer is designed. The efficiency of the overall control structure and the impact of the individual control actions are pointed out by experimental results obtained at a test rig at the Chair of Mechatronics, University of Rostock.

Quasi-Z Source Inverter based 3-Phase Grid-Tied Photovoltaic System with Dual Loop Shoot-Through Control using Discrete Time Sliding Mode Control

This paper presents a current sharing method to actively balance the output currents of a parallel dc–dc converters’ (PDCC) system regarding the demanded power. First, the operating principle of the PDCC system with parallel-connected bidirectional converters is studied. To regulate the output voltage in dc bus and share the output currents of the individual converters, a dual-loop control architecture comprising an outer voltage control loop and multiple inner current control loops is designed based on the automatic master–slave control scheme. Moreover, a feedback-type two-degree-of-freedom proportional-integral-derivative (FB2PID) controller is introduced to obtain the pulse-width modulation control signals for the converters. In order to improve the dynamic response and robustness of the active current-sharing control performances of the FB2PID controlled PDCC system, a bat algorithm (BA)-optimized FB2PID control system is further proposed to concurrently and dynamically optimize the control parameters of the FB2PID controller in the current control loop. Thus, the output current of each converter can be controlled to share the demand power equally in the presence of uncertainties. Finally, the simulation and experimental results reveal that the proposed BA-optimized FB2PID control system outperforms the conventional PID and FB2PID control systems with regard to the voltage regulation and current sharing performances under the time-varying electric load condition.

As a single controllable unit, microgrid can be grid-connected with utility grid and detached from grid to become a controllable autonomous system. When microgrid is switching between grid-connected mode and islanding mode, it will cause fluctuation of voltage, current and frequency, which will have a certain impact on the load. The stable control in both modes is the basic condition to ensure the reliable operation of microgrid. In this paper, control strategy of grid-connected and islanding mode switching based on the master-slave structure of microgrid system is mainly researched. The proposed method of PQ outer loop controller, V/f outer loop controller and current inner loop controller of the master inverter of microgrid is given. Based on the analysis of the structure of the controller, a mathematic model of the master inverter of the microgrid is established, and a current inner loop controller based on the LADRC (Linear Active Disturbance Rejection Control) technique is designed. The presented smooth switching control strategy makes the outer loop controller running in parallel, and the inner loop controller share the improved current inner loop controller. LADRC control structure has strong anti-interference ability, it can effectively inhibit the fluctuations of various parameters, has a good inhibitory effect on the uncertainty of the power grid. The design method is simple and feasible. The simulation results show that the switching range of voltage, current and frequency is small, and the switching process time is less than 20ms, which can achieve stable and smooth switching between microgrid and utility grid.

In voltage‐controlled voltage source inverters (VSIs)‐based microgrids (MGs), the inner control is of prime interest task for guaranteeing safe and stable operation. In this paper, an in‐depth investigation of the modelling, control design, and analysis of the voltage and current inner control loops intended for single‐phase voltage‐controlled VSIs is established. The main objective of this work is to provide a comprehensive study of the mathematical modelling, control design, and performance evaluation of the inner control's loops considering different proportional‐integral (PI) controller types with and without compensation, and to determine the optimal scheme that can offer better performance in terms of implementation simplicity, robustness, and transient and steady‐state responses. Thus, the mathematical closed‐loop models of designed outer voltage and inner current control schemes based on PI, P, and feedforward controllers with and without compensation are, first, derived. Following this, a systematic and effective control design for tuning the different PI controllers’ parameters is proposed. Furthermore, an analysis revealing the performance of the designed voltage and current control schemes is provided. This analysis enables us to choose a P controller and PI feedforward controller for the current control loop and the voltage control loop, respectively. The chosen P and PI controllers should be simple; meanwhile, they should offer a wide bandwidth. A simulation study is carried out in MATLAB/Simulink software to assess the performance of the adopted inner control scheme for both linear and non‐linear loads. In addition, an experimental setup, based on a TMS320F2837xD μ C, of a single‐phase VSI supplying linear and non‐linear loads is built to verify the effectiveness and the robustness of the adopted inner controller. The results demonstrated: (1) the necessity of introducing the compensation term, which is responsible for offering control improvement against voltage perturbation, (2) the high tracking performance of the chosen controller in terms of dynamic and steady‐state responses as well as its simplicity of implementation.

Distributed generators based on wind and solar energy will give prominent momentum in near future [1]. A microgrid consists of group of loads and distributed generators that operate as a single controllable system. The microgrid concept introduces hybrid AC/DC microgrid which reduces multiple reverse conversions in an individual AC or DC grid. This project aims at providing a reliable control system for integrating small renewable power generating units with main grid. The control system designed in this work consist of inner current loop and outer voltage control loop collectively known as Multiloop current voltage PI control system. In this work the performance of the hybrid AC/DC microgrid is analysed by using this control system along with d-q Transformation. This method is implemented for the converters to properly co-ordinate the AC sub-grid and DC sub-grid. The circuits with the proposed method are modeled and simulated by using the MATLAB/ SIMULINK. The results show that with this technique, phase and frequency of the converters are synchronised with that of the utility grid. Hence the reactive power in the transmission line is reduced‥ As the reactive power is reduced, we obtain a stabilized output power which is transmitted to the loads and utility grid by using the above proposed method.

The proposed topology is used to connect a single-phase and a three-phase renewable energy resources to the grid. The single-phase source is coupled to a single-phase PFC boost converter, which enhances the input PF utilizing two feedback loops: outer voltage loop control and inner current loop control. The basic hightlight is to study the PFC converter in microgrid application where renewables are integrated with the systems. The basic aim is to observe the overall performance of the converters with various disturbances such as load variations, etc. Here, the single-phase and three-phase stand-alone inverter is used to get the the output of the PFC boost converters. A symmetrical sinusoidal output voltage waveform should be produced and maintained by the inverter. The three-phase source is also coupled to a PFC buck converter, which enhances the input PF utilizing two feedback loops: outer voltage loop control and inner current loop control. The single-phase stand-alone inverter receives the output of the PFC buck converter. The transformer receives the outputs of both the inverters as it is a multi-winding high-frequency transformer and offers isolation between the grid and the source. The pulses for the switches in the single-phase inverter coupled stand-alone system were generated using a sinusoidal pulse width modulation approach. Both the PI controllers are implemented to maintain the regulations. The simulation results are achieved by varying the load, maintaining a constant voltage, and observing whether the current varies as the load changes. It also provides the efficacy of the study.

Control of shunt active power filter (SAPF) requires a cascaded control structure as inner current closed loop followed by outer voltage control loop. The inner current control loop is responsible for tracking of a reference current while the outer voltage loop regulates DC bus voltage to a fixed value. Dynamic performance of the DC bus voltage regulation suffers from the adverse effects such that harmonic currents produced by SAPFs lead to fluctuations in DC bus voltage at certain harmonic frequencies. This causes SAPF to produce additional harmonic components, since the outer voltage loop deals with only fundamental waves where the fluctuations should not appear. This study proposes a DC bus voltage regulation methodology based on a Second Order Generalized Integrator (SOGI) acting as a band-stop filter with the aim of increasing the voltage regulation performance in the steady state. The proposed filter does not affect the transient response thanks to that it only suppresses the harmonic-induced fluctuations in DC bus voltage. Through a simulation study, the proposed methodology is validated, and the results prove the performance and feasibility of the proposed filter.

The paper design a current control based active power filter (APF) for improving power quality by reactive power compensation and harmonic filtering. The designed APF is based on a voltage source inverter (VSI), where the VSI is controlled by two loops, the voltage control loop and the current control loop. The voltage control loop is used to regulate the DC link capacitor voltage of the VSI. Via triangular wave control, the current control loop is used to produce a tracking PWM signal to shape the harmonic current such that it is in-phase with the same shape as the input voltage. The major advantage of the proposed APF is to search the reference current for improving power quality by using dq0 detecting method. Simulation results are given to validate the proposed scheme. From the results, it shows that the reduction of %THD is better in power system networks.

To solve the trajectory tracking problem of underwater vehicles in the presence of unknown dynamics and external disturbances, a double-loop adaptive integral sliding mode controller that yields chattering-free performance is proposed in this paper. The designed double-loop controller consists of an outer-loop position controller (the kinematic controller) and an inner-loop velocity controller (the dynamic controller). The outer-loop controller is designed based on the desired position and current position and it serves as the virtual control input to achieve accurate position tracking. A saturation function is adopted to replace the conventional signum function to overcome the chattering problem of the outer-loop sliding mode. And then, the outer-loop controller is considered as the reference velocity for the inner loop, and the inner-loop controller (the real control input) is developed to make the actual velocity of the vehicle to precisely track the reference velocity. A continuous adaptive term is designed instead of the discontinuous switching term in the conventional sliding mode to estimate and compensate the unknown dynamics and external disturbances and to eliminate the chattering effects of the inner-loop sliding mode. The global asymptotical stability of the double-loop tracking system is analytically proven using Lyapunov stability theory and Barbalat's lemma. Numerical simulation performed on the dynamic model of ODIN AUV demonstrates the effectiveness of the proposed approach.