جستجوی مقالات مرتبط با کلیدواژه « hierarchical control » در نشریات گروه « برق »

This paper presents an efficient method based on the adaptive virtual impedance and distributed communication link with a hierarchical control system in the resistive AC islanding micrigrids for accurate power sharing and circulating current reduction. In existing methods, the adaptive virtual resistance can take negative values and violate the assumption of feeders’ resistive dominance based on which the droop controller is designed, and as a result, deteriorate its performance. Besides, the negative virtual resistance, with a reduction in the system overall damping, can reduce the stability margin and lead to side effects on the closed-loop system performance, especially during transients. In the proposed method, the problems associated with the negative virtual resistance are removed via the intelligent implementation of a new distributed communication link among microgrid inverters. The advantages of the proposed method include: circulating current elimination, accurate power sharing among distributed generators proportional to their rated capacities, prevention of voltage and frequency deviations from their reference values in point of power coupling (PCC) bus, guarantee of the resistive or inductive dominance of the feeder impedance in various operating points, decoupling between active and reactive powers, and as a result, guarantee of a desirable performance for droop controller in different operating points, performance and stability improvement, and finally using a simple, one-sided and a low bandwidth communication link instead of the complex, two-sided, and centralized communication system. Simulation results in MATLAB/SIMULINK environment demonstrate that the proposed control strategy has obviated effectively the shortcomings of the conventional droop and adaptive virtual impedance controllers.

In this paper, an online optimal coordinated solution for the voltage regulation of a distribution smart grids including an on-load tap changer (OLTC) and multiple distributed generators (DG) is presented. The proposed strategy sets reactive power points of distributed generators as well as tap position of the OLTC hierarchically. In the first stage, a distributed sub-gradient method in order to find the optimal reactive powers for the DGs is implemented to minimize the power losses subject to voltage limits. If the first stage could not regulate voltage profile inside acceptable limits, in the second stage tap position of the OLTC will change optimally. The proposed solution is verified by studying suddenly load change and DGs’ active power output. An IEEE 123-bus unbalanced test system is used for the verification of the proposed method. Fast response, responsibility to different suddenly changes in the operating condition, and reduction in the number of needed taps are the results of this study. Moreover, analogous steady state results in comparison with another centralized method i.e. interior point algorithm demonstrate the ability and efficiency of the proposed solution.

Microgrids are considered as a platform for the use of distributed energy resources (DERs). The traditional AC distribution systems result in the emergence of AC grids. In order to reduce costs and losses and increase reliability, recently, DC distribution systems and subsequently DC microgrids have been considered. In this paper, the quality of DC voltages during the islanded mode is studied. In this mode, DERs are usually controlled by droop characteristics. In the use of the droop characteristics, the micro grid voltage deviation occurs from a nominal value. For voltage restoration, low band width communications as a part of secondary control are used. This control level, provides voltage restoration with one of two centralized and distributed approaches. In this paper, the voltage restoration with these two approaches is studied and analyzed, and then a comparison is presented. It is shown that for the centralized approach, the point of common coupling has desirable voltage quality and for the distributed approach, DERs buses have desirable voltage quality. For verification, the time domain simulation of a DC microgrid is used.

In this paper, an optimal integrated inner controller is designed for the microgrid primary control level. The main task of the primary control level is to maintain stability and proper power sharing in microgrids. Non-optimal controllers have been generally used to design the inner controller in this level in the majority of researches. On the other hand, accurate and complete models of microgrids can be obtained. Therefore, it is an advantage to use the model based optimal control approaches. In this paper, an optimal inner controller is designed to simultaneously control of the microgrids voltage and current using a technique called linear quadratic tracking controller (LQT). In order to evaluate the performance of the proposed controller, it is applied to a microgrid including two distributed generators (DGs) in MATLAB/Simulink. Results of the simulations illustrate the proper performance of the controller in comparison with conventional methods.

With increasing the capacity and number of distributed energy resources (DERs), it is necessary to keep them connected to the microgrid during the disturbances. Two of the most important measures for satisfying this requirement are to provide the high quality voltage for feeding the critical loads and to limit the electronically-coupled DERs current and voltage. This paper investigates the performance of hierarchical control structure of islanded microgrids during and after the fault conditions. The primary control level of this control structure is equipped with the hybrid reference frame limiting strategy to limit the voltage-sourced converter (VSC) current and voltage. The effect of employing the concept of independent control of natural reference frame in both primary and secondary control levels and the influence of using the conditional integration method in the voltage controller of secondary control level on the microgrid performance during and after fault condition are investigated. The study results are demonstrated through several time-domain simulations of symmetrical and asymmetrical faults.

In this paper, a multi-loop hierarchical controller for a renewable DG based microgrid system is proposed in order to control the voltage of point of common coupling and also to perform accurate active and reactive power sharing. Inner current and voltage loops that are designed based on the dynamic model of microgrid system are used to obtain appropriate switching functions for interfaced inverters and the reference values for currents of DG units. Also, a modified droop controller is introduced to enhance the performance of designed voltage control loop. In order to improve the dynamic operation of the proposed controller in supplying demanded power for harmonically distorted loads, the extraction of harmonic components of PCC voltages is accomplished. Upper and lower limits of voltage amplitude and frequency droops are analyzed using capacity curves of each DG unit. The effect of renewable power generation uncertainty on DC link voltage variations are also compensated using an additional control loop in hierarchical structure. The main contributions of this paper are the above-mentioned multi-objective control system along with a droop control based on capacity curves. The MATLAB/SIMULINK simulation shows a proper steady state and transient performance during changes in generation and nonlinear harmonically distorted loads.

To achieve accurate reactive power and harmonic currents sharing among inverter interfaced distributed generation (DG) units in islanded microgrids¡ a hierarchal control scheme consisting of primary and secondary levels based on instantaneous circulating currents is proposed in this paper. Firstly¡ fundamental and main harmonic components of each inverter output current are extracted at the primary level and transmitted to the secondary controller. Then¡ using this information¡ instantaneous circulating currents at different frequencies are calculated and to generate proper control signals are applied to the primary controller. Consequently¡ these signals are inserted as voltage references after passing control blocks. In contrast to the conventional virtual impedance schemes¡ where reactive power and harmonic currents sharing are realized at the expense of introducing additional voltages drop and harmonic distortions¡ the proposed control strategy effect on the amplitude and waveform quality of DGs’ voltage is negligible. Meanwhile¡ it is able to provide accurate harmonic currents sharing even if nonlinear load(s) is directly connected at the terminal of DG unit(s). Control system design is described in detail and simulation results for four voltage-controlled DG units are provided to demonstrate the effectiveness of the proposed control method.

In this paper، a hierarchical control scheme consisting of primary and secondary levels is proposed for compensation of voltage harmonics in islanded microgrids. The primary control level comprises Distributed Generators (DGs) local controllers. Each of these controllers includes a harmonic virtual impedance loop which is considered to improve nonlinear load sharing among the DGs، but، at the expense of increasing the voltage harmonic distortion. The secondary control level manages the compensation of voltage harmonics by sending proper control signals to the primary level. Since، the microgrid may supply sensitive loads; the compensation is performed to reduce load bus main voltage harmonics. The procedure of setting control system parameters is presented and simulation results are provided in order to demonstrate the effectiveness of the proposed control scheme.