A Simplified Strategy for Smooth Grid Switching in Coal Mine Microgrids Using Enhanced Voltage Loop and LADRC Pre-Sync Control

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.

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.

The phase-locked loop (PLL)-based grid-following inverters have been widely used as the interface between renewable energy resources (e.g., wind and solar power) and the utility grid. In addition to the basic PLL and inner current control loop that are responsible for tracking the phase angle of the point of common coupling (PCC) voltage and regulating the current injection according to the estimated phase angle, respectively, these grid-following inverters are commonly also equipped with outer power and voltage control loops. Some examples are outer active power/dc-link voltage control loops and outer reactive power/PCC voltage control loops, which provide the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -axis current references for the inner current control loop. However, the existing literature generally focuses on specific outer control modes within a specific small-signal modeling framework, e.g., state-space or admittance modeling framework. In addition, different research groups usually repeatedly derive these small-signal models without a systematic modeling and analysis procedure. Furthermore, the systematic and comprehensive comparative modal and admittance analysis of these outer control modes is still missing in the literature. To avoid reinventing the wheel, provide the small-signal stability analysis community with handy both state-space and admittance models, and investigate the effects of various outer control loops on the small-signal models, this article presents a comprehensive and systematic small-signal modeling, analysis, and comparison framework of the PLL-based grid-following inverters, where the state-space and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance models of eleven control modes are derived step by step in a completely followable style. All of these derived small-signal models are verified by the time-domain step responses and frequency-domain <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance measurement. The insights into the effects of these outer control modes on the poles and residues of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance model are also explored, which help to understand how these outer control modes contribute to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance shaping.

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 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.

The toroidal magnetic field power supply for the DIII-D tokamak is of the 12 pulse line commutated variety. It consists of four individual modules and a main system control cabinet which are combined to deliver 127,000 A and 1000 V to the toroidal field (TF) coil. The modules are connected in a series-parallel configuration but can be run alone or two at a time as well. Normally on DIII-D experiments, the series-parallel configuration is required. The original design provided each individual module with its own voltage (E) and current (I) control loop and a main control loop. A problem with this design was that the individual control loops would cause a current sharing imbalance in the parallel modules if the calibrated loops drifted by the slightest amount. It was determined that individual control loops were not needed and a single phase lock firing circuit was employed in the system cabinet with fiber optic links to the modules for gate drive signals. Since all four modules have to be on line for DIII-D to operate, a problem in any of the five E&I control loops resulted in the supply, and, therefore, the tokamak, being idled. By reducing the number of control loops to one, the sharing problem was eliminated, as well as 4 out of 5 potential control failures. The original supply employed relay logic for sequence control and fault monitoring. There were over 130 relays in each module plus an additional 100 in the system cabinet. The combination of the number of relays with the required interconnecting wiring, the age of the supply, the vibrations of the cabinets and the harsh environment, resulted in a continuously escalating number of phantom, and often intermittent, faults. In some cases, a day of testing and searching was required to isolate the problem. The fault and sequence logic relays were replaced by a new programmable logic controller (PLC). All existing interconnect wire was removed and replaced with multiconductor cables that connect directly from fault sensors and input devices to the PLC. It is anticipated that an average of 12 h per year in control loop downtime will be eliminated. In addition, 100 man-hours of annual calibration time will he eliminated. New capabilities introduced by the PLC should provide for at least one additional plasma shot per day.

An integrated fuzzy logic stabilizing controller has been proposed in this paper for the stabilization of generators through the excitation and governor control. The proposed controller has three control loops: the first one is the voltage control loop which has the function of automatic voltage regulator (AVR), the second one is the damping control loop which has the function of power system stabilizer (PSS), and the last one is the speed control loop which has the function of governor (GOV). A simple fuzzy logic control scheme has been applied to all these three loops. The control scheme is simple enough so as not to require heavy computation for the controller, therefore, its real time application is feasible. The effectiveness has been demonstrated through nonlinear simulations using a one machine infinite bus system. Comparison studies have also been performed to show the advantages of the proposed controller with conventional excitation and speed governing control systems.

This study investigates modelling of multi‐voltage source converters (VSCs) in DC voltage control timescale for small‐signal stability analysis. Multi‐VSCs stability in a DC voltage timescale is impacted due to the coupling among their control systems and grid dynamics. With considering control loops interactions and grid dynamics, multi‐VSCs’ small‐signal model is proposed for stability analysis. Control loops contain DC voltage control loop, terminal voltage control loop and phase‐locked loop. Furthermore, VSC control dynamics and grid dynamics are separated. Besides, eigenvalues analysis are conducted to analysis multi‐VSCs stability in DC voltage control timescale. Impacts of grid strength and control loops dynamics on the stability of multi‐VSCs are verified.

Efficient adaptive controllers and recursive filters for centralized control scheme of islanded microgrid with linear and non-linear loads

This paper concern with the study of a single-phase rectifier circuit and its effects on the AC supply system. System compensation through the used of boost technique in wave shaping the supply current is proposed. Two control loops namely current control loop (CCL) and voltage control loop (VCL) employing TWM technique and fuzzy logic system respectively are used. This compensation strategy is analyzed using PSpice and MATLAB/Simulink simulation package.

The boost converter operates either in discontinuous conduction mode (DCM) or in continuous conduction mode (CCM). The operation mode is determined by the duty ratio, load and parameters of the boost converter. The plant model in DCM and CCM are different in the frequency domain response. Therefore, it will be difficult to design a controller with stable operation and fast transient response for both modes. Moreover, the boost converter will be unstable between DCM and CCM. In this paper, the proposed control strategy can transit between DCM and CCM seamlessly by adding a mode detector, the boost converter can be automatically operated by selecting of control loop in both operation modes. The proposed controller still has a voltage control loop in DCM and current/voltage control loops in CCM. Each controller is designed by considering the stability and response time. The proposed mode detector will be explained with a frequency domain analysis. Due to the wide range operation capability of the proposed control strategy, the proposed control strategy is suitable in the fuel cell portable applications. In addition, smooth operation of the converter will also be beneficial to the fuel cell stack reliability. A 20 W boost converter prototype is used to verify the performance of the proposed control scheme.

• Equivalent circuit of GFM-converter system is derived by the analogy-based method, which shows control loop interactions. • Control loop interaction and their impact on small-signal synchronization stability are visualized by equivalent circuits. • It uncovers that AC voltage control brings about negative damping and deteriorates small-signal synchronous stability. • Instructions for improvements of small-signal synchronous stability are given by parameter analyses of AC voltage control. For grid-forming (GFM) converter-based power grids, the impacts of AC voltage control (ACVC) loops on small-signal synchronization stability are rarely discussed in literature. To present the interactions between the synchronization control loop and the ACVC loop in an intuitive and clear approach, this paper establishes the equivalent circuit model of multi-GFM-converter based power systems, and analyzes the impacts of control loop interactions on small-signal synchronization stability. Firstly, the model of multi-GFM converter-based power grids has been established that incorporates the ACVC loop. Then, for the further visualization of control loop interactions, equivalent circuits of the whole systems are derived by the analogy-based method. This analysis allows us to assess the impacts of the ACVC on small-signal synchronization stability. This paper illustrates that ACVC can be equivalently represented as a negative impedance in parallel with the filter inductor. In this case, the ACVC brings about negative damping effects and deteriorates small-signal synchronization stability. Furthermore, the control parameters of the ACVC on small-signal synchronization stability are also quantitatively evaluated. Additionally, interactions between different GFM converters on small-signal synchronization stability are also investigated. Finally, the modified IEEE 10-GFM-converter 39-bus system is built in MATLAB/Simulink platform to validate analyzes and the proposed method.

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.

DC-link loop bandwidth selection strategy for grid-connected inverters considering power quality requirements

This article investigates a hybrid control scheme to grant multiple functions to a grid-connected PV inverter. This strategy guarantees constant energy supply independently of the intermittent nature of solar energy. Further, it contributes to the mitigation of power-quality issues such as harmonic cancellation and power-factor correction. The control scheme comprises a current control loop based on passivity theory, which improves the inverter power quality, while the operation under null or intermittent generation conditions is ensured by the proportional-integral-based voltage control loop. Therefore, the proposed strategy extends the inverter operation in conditions in which there is no power generation. These additional functions decrease the total harmonic distortion and improve the power factor of the active power demanded from and supplied to the grid. Simulation tests and experimental results are presented to verify the suggested control approach and validate the operation and effectiveness of the system with the control scheme.