Sharif Digital Repository

This paper presents a power flow controller for a matrix converter as the power electronic interface between a high-speed micro-turbine generator and a utility distribution system. The matrix converter converts the high-frequency of a micro-turbine generator to a conventional frequency of the utility system, based on a novel switching strategy. The controller regulates magnitude and phase-angle of the converter output voltage to accommodate real and reactive power flow requirements of the utility system. Performance of the matrix converter based microturbine generation system including the power flow controller is evaluated based on digital time-domain simulation studies in the PSCAD/EMTDC... 

This paper presents an effective control strategy for autonomous operation of a multi-bus medium voltage (MV) microgrid (MG) consisting of several dispatchable distributed generation (DG) units. Each electronically-coupled DG unit supplies the loads which can be unbalanced due to the inclusion of single-phase loads. The proportional resonance (PR) and droop controllers are, respectively, used to regulate the load voltage and share the average powers among the DG units. The virtual negative-sequence impedance controller (VNSIC) is proposed to effectively compensate the negative-sequence currents of the unbalanced loads. Moreover, the VNSIC minimizes the negative-sequence currents in the MV... 

This paper presents an effective voltage control strategy for the autonomous operation of a medium voltage (MV) microgrid under nonlinear and unbalanced load conditions. The main objectives of this strategy are to effectively compensate the harmonic and negative-sequence currents of nonlinear and unbalanced loads using distributed generation (DG) units. The proposed control strategy consists of a multi-proportional resonant controller (MPRC) whose parameters are assigned with particle swarm optimization (PSO) algorithm. The optimization function is defined to minimize the tracking error at the specific harmonics considering the stability limitations. In this paper the performance of the... 

This paper presents a new decentralized control strategy for the islanded operation of a microgrid under unknown load conditions. In the islanded mode of operation, the microgrid should provide the load with a set of regulated balanced three-phase voltages. The load which is parametrically and topologically uncertain can also be unbalanced and/or nonlinear. Thus, the use of conventional control strategies results in the poor performance and even instability of the microgrid system. The proposed method assumes that the load current is a measurable disturbance signal. The robust optimal control approaches are used to design a controller to overcome the disturbances resulting from the unknown... 

Brushless Doubly Fed Induction Generator (BDFIG) has been recently proposed to be used in variable speed wind turbines. This paper intends to investigate the influence of certain model simplifications to obtain a reduced-order model of a wind turbine with BDFIG suitable for transient studies. To achieve this goal, small signal analysis is performed for a recently manufactured 250 kVA BDFIG to obtain system modes and their participation factors. Identifying highly damped modes, influences of neglecting dynamics of their participated states are studied through time-domain simulation in MATLAB/Simulink  

This paper presents a mathematical model for Brushless Doubly-Fed Machine (BDFM) based on Stator Flux Orientation (SFO) in positive and negative reference frames under unbalanced grid voltage conditions. A negative controller is designed to eliminate the effects of unbalanced conditions on electrical torque. The model and controller performance during unbalanced conditions is validated by time domain simulation in MATLAB/Simulink  

Thermal noise is one of the most important limiting factors on the performance of switched-capacitor (SC) circuit due to the aliasing effect of wide-band thermal noise. In this paper a new simple method for estimating the effect of thermal noise is presented. In the proposed technique only the discrete sampled noise is considered. HSPICE simulator and analytical analysis are used to estimate the sampled noise specification on each clock state. Next, using difference equations of the circuit, time domain simulation is done by MATLAB. Based on this method, a SC integrator is analyzed and results compared to the measured noise response  

A DC microgrid (DC-MG) provides an effective mean to integrate various sources, energy storage units and loads at a common dc-side. The droop-based, in the context of a decentralised control, has been widely used for the control of the DC-MG. However, the conventional droop control cannot achieve both accurate current sharing and desired voltage regulation. This study proposes a new adaptive control method for DC-MG applications which satisfies both accurate current sharing and acceptable voltage regulation depending on the loading condition. At light load conditions where the output currents of the DG units are well below the maximum limits, the accuracy of the current sharing process is... 

This study addresses the robust stability analysis for an islanded microgrid with droop-controlled inverter-based distributed generators (DGs). Owing to large load changes, microgrid structure reconfiguration, and higher-power demands, the low-frequency (LF) dominant modes of a microgrid stir toward unstable zone and make the system more oscillatory or even unstable. In this study, a robust two-degree-of-freedom (2DOF) decentralised droop controller, which is the combination of the conventional droop with a robust transient droop function, is utilised for each inverter-based DG unit. Unlike conventional tuning of 2DOF droop controllers, a new design procedure is proposed to robustly... 

In this paper, it is shown that non-reciprocity can be observed in time-varying media without employing spatiotemporal modulated permittivities. We show that by using only two one-dimensional Fabry–Perot slabs with time-periodic permittivities having quadrature-phase difference, it is possible to achieve considerable non-reciprocity in transmission at the incidence frequency. To analyze such a scenario, generalized transfer matrices are introduced to find the wave amplitudes of all harmonics in all space. The results are verified by in-house finite-difference time-domain simulations. Moreover, in order to have a simple model of such time-varying slab resonators, a time-perturbed coupled-mode... 

This paper presents an effective current injection method for a single phase grid-connected PV inverter with LCL filter. The main objective of the proposed control strategy is to compensate the resonance effect of the LCL filter. In the proposed control strategy, a resonance compensator is augmented to the conventional proportional resonance (PR) controller to attenuate current oscillations in resonance frequency of LCL filter. The proposed strategy robustly regulates the output current of grid connected inverter in various grid impedances and provides a high quality current injection capability for the PV inverter in harmonic polluted condition of the grid voltage. The performance of the... 

Control strategies of a microgrid which includes both synchronous generators and converter-based distribution generation (DG) units must be designed such that effective operation of the microgrid is achieved. The main objective of this paper is to develop a high performance control strategy for an islanded medium voltage (MV) microgrid consisting of inverter and non-inverter interfaced DG units. A new control method for the synchronous generator in an islanded microgrid is proposed based on a virtual droop scheme. The proposed strategy can effectively manage the real and reactive powers of the microgrid among the inverter and non-inverter based DG units. The steady state and dynamic... 

This paper presents a decentralized self-adjusting reactive power controller for the autonomous operation of a multi-bus medium voltage (MV) microgrid. The main objective of the proposed control strategy of each distributed generation (DG) unit is to compensate the reactive power of its local loads and to share the reactive power of the nonlocal loads among itself and other DG units. The proposed control strategy includes an improved droop controller whose parameters are adjusted according to the reactive power of the local loads. A virtual inductive impedance loop is augmented to the voltage controller to enhance the steady state and transient responses of the proposed reactive power... 

This paper proposes a control strategy for an islanded microgrid to effectively coordinate hybrid power source (HPS) units and to robustly control individual interfaced inverters under unbalanced and nonlinear load conditions. Cascaded H-bridge (CHB) multilevel inverters are flexibly deployed in order to enhance the power quality and redundancy. The HPS employs fuel cell (FC) as the main and supercapacitors (SC) as complementary power sources. Fast transient response; high performance; and high power density are the main characteristics of the proposed HPS system. The presented control strategy consists of a power management strategy for the HPS units and a voltage control strategy for the... 

The brushless doubly fed induction generator has been proposed for use in wind turbines as a more reliable alternative to the more commonly used doubly fed induction generator. To realize the commercial promises of the brushless doubly fed induction generator in this role, it must meet the existing grid connection codes, particularly with respect to low-voltage ride-through. In this article, the application of series dynamic resistors is proposed to enhance brushless doubly fed induction generator low-voltage ride-through capability, and the required analysis is provided. Three locations for series dynamic resistors are suggested in the brushless doubly fed induction generator circuit and... 

As the penetration of wind power in electrical power systems increases, it is required that wind turbines remain connected to the grid and actively contribute to the system stability during and after grid faults. This study proposes an efficient control strategy to improve the low-voltage ride-through (LVRT) capability in doubly fed induction generators (DFIGs). The proposed scheme consists of passive and active LVRT compensators. The passive compensator is based on a new crowbar arrangement located in series with stator windings. It considerably reduces the rotor inrush current at the instants of occurring and clearing the fault. The active LVRT compensator is realised through rotor voltage... 

This paper presents an adaptive delay compensator (ADC) on the basis of the latest development in wide-area measurement system (WAMS) to cater to varying latencies. The proposed compensator can effectively reciprocate the phase deviation resulting from varying delays to improve the performance of central wide-area damping controller (WADC). The greatest challenge to continuous delay compensation is the uncertainty regarding the inherent robustness of the central controller. The proposed technique provides an opportunity to epitomize the uncertainty retrieved from phase compensated delay. Next, taking the uncertainty into consideration, the mixed H2H∞ synthesis technique is adopted for the... 

In order to restrain ferroresonance in coupling capacitor voltage transformers (CCVT), two kinds of ferroresonance suppression circuits (FSCs) , are proposed in this paper: the resonant type, the fastsaturation reactor type which are connected to secondary side of CCVTs. Frequency domain analyses, for voltage ratio magnitude, are carried out for CCVTs to obtain preferable FSC. Furthermore, using surge suppressors such as metal oxide varistors (MOV) to improve the performance of the FSCs is investigated. Time domain simulations demonstrate that ferroresonance can be cleared in few cycles in presence of FSCs. The Electromagnetic Transients Program (EMTP) is used for modeling and fine-tuning... 

This study proposes a modified virtual inertial control (MVIC) scheme for doubly-fed induction generator (DFIG)based wind turbines (WTs), which both improves the frequency response of these renewable resources and enhances the power system oscillation damping capabilities. It is shown that the proposed control structure enables the WT to participate prudentially in system frequency regulation, which means the amount of WT kinetic energy released to the grid and its participation in system frequency support is alleviated as its stored energy decreases. The proposed control strategy is introduced conceptually, and its performance is verified analytically. Effects of wind speed variations on... 

Doubly-fed induction generator (DFIG) is the most commonly used technology for wind power generation due to the variable speed performance, decoupled control of active and reactive powers, and high efficiency. However, the DFIG originally cannot participate in damping of power system oscillations since it is not synchronously connected to the power system. This paper proposes an optimal and robust additional damping controller for the DFIG wind turbine to contribute it to damp power system oscillations. It is a fuzzy logic controller that its parameters are optimally tuned using the genetic algorithm (GA). The proposed controller modifies the DFIG active power output by using feedback from...