Plug-and-play and coordinated control for bus-connected AC islanded microgrids

This paper presents a distributed control architecture for voltage and frequency stabilization in AC islanded microgrids. In the primary control layer, each generation unit is equipped with a local controller acting on the corresponding voltage-source converter. Following the plug-and-play design approach previously proposed by some of the authors, whenever the addition/removal of a distributed generation unit is required, feasibility of the operation is automatically checked by designing local controllers through convex optimization. The update of the voltage-control layer, when units plug -in/-out, is therefore automatized and stability of the microgrid is always preserved. Moreover, local control design is based only on the knowledge of parameters of power lines and it does not require to store a global microgrid model. In this work, we focus on bus-connected microgrid topologies and enhance the primary plug-and-play layer with local virtual impedance loops and secondary coordinated controllers ensuring bus voltage tracking and reactive power sharing. In particular, the secondary control architecture is distributed, hence mirroring the modularity of the primary control layer. We validate primary and secondary controllers by performing experiments with balanced, unbalanced and nonlinear loads, on a setup composed of three bus-connected distributed generation units. Most importantly, the stability of the microgrid after the addition/removal of distributed generation units is assessed. Overall, the experimental results show the feasibility of the proposed modular control design framework, where generation units can be added/removed on the fly, thus enabling the deployment of virtual power plants that can be resized over time

New Control Algorithms for the Robust Operation and Stabilization of Active Distribution Networks

The integration of renewable distributed generation units (DGs) alters distribution systems so that rather than having passive structures, with unidirectional power flow, they become active distribution networks (ADNs), with multi-directional power flow. While numerous technical, economic, and environmental benefits are associated with the shift toward ADNs, this transition also represents important control challenges from the perspective of both the supervisory and primary control of DGs. Voltage regulation is considered one of the main operational control challenges that accompany a high penetration of renewable DGs. The intermittent nature of renewable energy sources, such as wind and solar energy, can significantly change the voltage profile of the system and can interact negatively with conventional schemes for controlling on-load tap changers (OLTCs). Another factor is the growing penetration of plug-in electric vehicles (PEVs), which creates additional stress on voltage control devices due to their stochastic and concentrated power profiles. These combined generation and load power profiles can lead to overvoltages, undervoltages, increases in system losses, excessive tap operation, infeasible solutions (hunting) with respect to OLTCs, and/or limits on the penetration of either PEVs or DGs. With regard to the dynamic control level, DG interfaces are typically applied using power electronic converters, which lack physical inertia and are thus sensitive to variations and uncertainties in the system parameters. Grid impedance (or admittance), which has a substantial effect on the performance and stability of primary DG controllers, is nonlinear, time-varying, and not passive in nature. In addition, constant-power loads (CPLs), such as those interfaced through power electronic converters, are also characterized by inherited negative impedance that results in destabilizing effects, creating instability and damping issues. Motivated by these challenges, the research presented in this thesis was conducted with the primary goal of proposing new control algorithms for both the supervisory and primary control of DGs, and ultimately of developing robust and stable ADNs. Achieve this objective entailed the completion of four studies: Study#1: Development of a coordinated fuzzy-based voltage regulation scheme with reduced communication requirements Study#2: Integration of PEVs into the voltage regulation scheme through the implementation of a vehicle-to-grid reactive power (V2GQ) support strategy Study#3: Creation of an estimation tool for multivariable grid admittance that can be used to develop adaptive controllers for DGs Study#4: Development of self-tuning primary DG controllers based on the estimated grid admittance so that stable performance is guaranteed under time-varying DG operating points (dispatched by the schemes developed in Study#1 and Study#2) and under changing grid impedance (created by network reconfiguration and load variations). As the first research component, a coordinated fuzzy-based voltage regulation scheme for OLTCs and DGs has been proposed. The primary reason for applying fuzzy logic is that it provides the ability to address the challenges associated with imperfect information environments, and can thus reduce communication requirements. The proposed regulation scheme consists of three fuzzy-based control algorithms. The first control algorithm was designed to enable the OLTC to mitigate the effects of DGs on the voltage profile. The second algorithm was created to provide reactive power sharing among DGs, which will relax OLTC tap operation. The third algorithm is aimed at partially curtailing active power levels in DGs so as to restore a feasible solution that will satisfy OLTC requirements. The proposed fuzzy algorithms offer the advantage of effective voltage regulation with relaxed tap operation and with utilization of only the estimated minimum and maximum system voltages. Because no optimization algorithm is required, it also avoids the numerical instability and convergence problems associated with centralized approaches. OPAL real-time simulators (RTS) were employed to run test simulations in order to demonstrate the success of the proposed fuzzy algorithms in a typical distribution network. The second element, a V2GQ strategy, has been developed as a means of offering optimal coordinated voltage regulation in distribution networks with high DG and PEV penetration. The proposed algorithm employs PEVs, DGs, and OLTCs in order to satisfy the PEV charging demand and grid voltage requirements while maintaining relaxed tap operation and minimum curtailment of DG active power. The voltage regulation problem is formulated as nonlinear programming and consists of three consecutive stages, with each successive stage applying the output from the preceding stage as constraints. The task of the first stage is to maximize the energy delivered to PEVs in order to ensure PEV owner satisfaction. The second stage maximizes the active power extracted from the DGs, and the third stage minimizes any deviation of the voltage from its nominal value through the use of available PEV and DG reactive power. The primary implicit objective of the third stage problem is the relaxation of OLTC tap operation. This objective is addressed by replacing conventional OLTC control with a proposed centralized controller that utilizes the output of the third stage to set its tap position. The effectiveness of the proposed algorithm in a typical distribution network has been validated in real time using an OPAL RTS in a hardware-in-the loop (HiL) application. The third part of the research has resulted in the proposal of a new multivariable grid admittance identification algorithm with adaptive model order selection as an ancillary function to be applied in inverter-based DG controllers. Cross-coupling between the and grid admittance necessitates multivariable estimation. To ensure persistence of excitation (PE) for the grid admittance, sensitivity analysis is first employed as a means of determining the injection of controlled voltage pulses by the DG. Grid admittance is then estimated based on the processing of the extracted grid dynamics by the refined instrumental variable for continuous-time identification (RIVC) algorithm. Unlike nonparametric identification algorithms, the proposed RIVC algorithm provides a parametric multivariable model of grid admittance, which is essential for designing adaptive controllers for DGs. HiL applications using OPAL RTS have been utilized for validating the proposed algorithm for both grid-connected and isolated ADNs. The final section of the research is a proposed adaptive control algorithm for optimally reshaping DG output impedance so that system damping and bandwidth are maximized. Such adaptation is essential for managing variations in grid impedance and changes in DG operating conditions. The proposed algorithm is generic so that it can be applied for both grid-connected and islanded DGs. It involves three design stages. First, the multivariable DG output impedance is derived mathematically and verified using a frequency sweep identification method. The grid impedance is also estimated so that the impedance stability criteria can be formulated. In the second stage, multi-objective programming is formulated using the -constraint method in order to maximize system damping and bandwidth. As a final stage, the solutions provided by the optimization stage are employed for training an adaptation scheme based on a neural network (NN) that tunes the DG control parameters online. The proposed algorithm has been validated in both grid-connected and isolated distribution networks, with the use of OPAL RTS and HiL applications.1 yea

Direct Power Control based on Point of Common Coupling Voltage Modulation for Grid-Tied AC Microgrid PV Inverter

In this paper, a direct power control (DPC) approach is proposed for grid-tied AC MG’s photovoltaic (PV) voltage source inverter (VSI) to regulate directly active and reactive powers by modulating microgrid’s (MG) point of common coupling (PCC) voltage. The proposed PCC voltage modulated (PVM) theory-based DPC method (PVMT-DPC) is composed of nonlinear PVM, nonlinear damping, conventional feedforward, and feedback PI controllers. For grid synchronization rather than employing phase-locked-loop (PLL) technology, in this study, direct power calculation of the PCC voltage and current is adopted. Subsequently, at PCC, the computed real and reactive powers are compared with reference powers in order to generate the VSI’s control signals using sinusoidal pulse width modulation (SPWM). Because of the absence of the PLL and DPC method adoption, the suggested controller has a faster convergence rate compared to traditional VSI power controllers. Additionally, it displays nearly zero steady-state power oscillations, which assures that MG’s power quality is improved significantly. To validate the proposed PVMT-DPC method’s performance real-time simulations are conducted via a real-time digital simulator (RTDS) for a variety of cases. The results demonstrate that PV VSI using the suggested PVMT-DPC approach can track the reference power quicker (0.055 s) along with very low steady-state power oscillations, and lower total harmonic distortion (THD) of 1.697% at VSI output current.© The Authors. Published by IEEE. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. For more information, see https://creativecommons.org/licenses/by-nc-nd/4.0/This research has been supported by the University of Vaasa under the Centralized Intelligent and Resilient Protection Schemes for Future Grids Applying 5G (CIRP-5G) research project funded by Business Finland with Grant No. 6937/31/2021. Some parts of this work were done in the SolarX research project funded by Business Finland with Grant No. 6844/31/2018.fi=vertaisarvioitu|en=peerReviewed

Advanced Control Strategies for Voltage Source Converters in Microgrids and Traction Networks

Increasing concerns regarding global warming caused by greenhouse gases, which are mainly generated by conventional energy resources, e.g., fossil fuels, have created significant interest for the research and development in the field of renewable energies. Such interests are also intensified by the finitude availability of conventional energy resources. To take full benefit of renewable energy resources, e.g., wind and solar energy, interfacing power electronics devices are essential, which together with the energy resources form Distributed Generation (DG) units. If properly controlled and coordinated, the optimal and efficient operation of DG units, which are the main building block of rapidly emerging microgrid technologies, can be ensured. In fact, the optimal and efficient operation of any energy conversion systems, e.g., microgrids, traction networks, etc., necessitates some sorts of control strategies. Being structured into two main parts and exploiting two-level Voltage Source Converters (VSCs), this thesis introduces several control strategies in the context of microgrids and electrified traction networks. Although the proposed approaches of this thesis are mainly tailored for two-level VSCs, the methods are equally applicable to other converter technologies. In the first part, adopting an optimization-based loop shaping approach, a vector current control strategy for three-phase grid-tied VSCs is proposed. The proposed control strategy is able to independently regulate the direct and quadrature (dq)-components of the converter currents in a fully decoupled manner and shows very fast dynamic response similar to the existing methods. In order to extend the applicability of the proposed vector control method to single-phase systems, a countermeasure is also proposed. In single-phase systems, to form the orthogonal component of the current needed to create the dq-axes, the converter current is phase-shifted a quarter of a fundamental period. This phase-shift is the reason of strongly coupled dq-axes and oscillatory dynamic response in such systems. To obviate the need for the problematic phase-shifting, adopting a Fictive Axis Emulator (FAE), the orthogonal fictive current is created concurrent to the real one. In such a case, utilizing the proposed decoupled vector control strategy and the FAE, the dq-currents of single-phase converters are also regulated in a fully decoupled manner. Moreover, in this part, using a generalized version of the optimization-based loop shaping approach, three voltage control schemes are proposed for the voltage regulation of islanded microgrids. Since the dedicated loads of islanded microgrids are not fixed, the loop shaping is simultaneously carried out for various operating points of interests, i.e., for various combinations of the load parameters. Two single-stage control strategies and a cascade one are proposed: (i) a single-stage PI-based Multi-Input Multi-Output (MIMO) controller, (ii) a single-stage PI-based MIMO controller in conjunction with resonant terms, which is able to compensate for the adverse impacts of nonlinear loads, and (iii) a cascade PI-based MIMO controller. The cascade control scheme utilizes the proposed decoupled vector control strategy as its inner loop for regulating the converter current. In the second part, this thesis focuses on electrified traction networks and addresses a power quality problem in such networks, i.e., catenary voltage fluctuations. The Active Line-side Converter (ALC) of modern locomotives is utilized as STATic COMpensator (STATCOM) in order to inject reactive power to compensate for the adverse effects of catenary line voltage fluctuations. To determine the proper amount of reactive power, several control strategies belonging to the PI-controllers family are proposed: (i) a P-controller, (ii) a PI-controller, and (iii) a gain-scheduled PI-controller. Among the proposed approaches, the gain-scheduled strategy provides the best performance. The gain-scheduling is performed through identifying the catenary inductance at the connection point of the locomotive to that. The inductance identification is carried out by the injection of harmonic current through the ALC and monitoring its effect on the locomotive voltage. Despite its acceptable performance, the gain-scheduled approach shows several shortcomings. Therefore, utilizing the optimization-based loop shaping technique, a high-order voltage support scheme is also proposed. The proposed high-order scheme does not need any online tuning and/or modification while provides excellent performance for various operating points