Sharif Digital Repository

This paper presents a phase-noise reduction technique for voltage-controlled oscillators (VCOs) using a harmonic tuned (HT) LC tank. The phase-noise suppression is achieved through almost rectangular-shaped VCO oscillating signal which effectively maximizes oscillating signal slope at zero crossing points resulting in-phase-noise degradation. In addition, by shortening down converted noise power around oscillating signal second harmonic, more phase-noise suppression has been achieved. A comprehensive analysis for frequency and amplitude deviations as high as 20% for third harmonic and its effect on output phase-noise suppression has been discussed. In the followings, a comprehensive analysis... 

Distribution systems management is becoming an increasingly complicated issue due to the introduction of new technologies, new energy trading strategies, and new deregulated environment. In the new deregulated energy market and considering the incentives coming from the technical and economical fields, it is reasonable to consider Distributed Generation (DG) as a viable option to solve the lacking electric power supply problem. This paper presents a mathematical distribution system planning model considering three planning options to system expansion and to meet the load growth requirements with a reasonable price as well as the system power quality problems. DG is introduced as an... 

This paper presents a robust dc-link voltage as well as a current control strategy for a bidirectional interlink converter (BIC) in a hybrid ac/dc microgrid. To enhance the dc-bus voltage control, conventional methods strive to measure and feedforward the load or source power in the dc-bus control scheme. However, the conventional feedforward-based approaches require remote measurement with communications. Moreover, conventional methods suffer from stability and performance issues, mainly due to the use of the small-signal-based control design method. To overcome these issues, in this paper, the power from DG units of the dc subgrid imposed on the BIC is considered an unmeasurable... 

Voltage stability is the ability of a power system to maintain acceptable voltages at all buses in the system under normal conditions and after being subjected to a disturbance. This paper employs the management of reactive power generation to improve the voltage stability margin (VSM), in the framework of optimal reactive power dispatch (ORD) problem. Scheduling of VAR sources is performed using the concept of relative value of VAR sources. The proposed methodology has been tested on the IEEE 14-bus test system. Simulation results show that after the optimal reactive power scheduling, VSM of the systems is increased considerably. In addition, it is observed that reactive power reserve is... 

Linear controllers have been widely used for controlling Active Power Filters (APFs). However, due to their limitations, a tradeoff between the bandwidth of the current control loop (CCL) and the voltage control loop (VCL) need to be made to remain stable in all conditions. This tradeoff leads to imperfect usage of the APF's capacity. Nonlinear controllers can eliminate such limitations and simultaneously remain stable in all conditions. In this paper, a new nonlinear controller for controlling the output current and the DC voltage (Vdc) of an APF is proposed. Feedback linearization method is applied on the CCL and the VCL is controlled via indirect method. In this method, an auxiliary... 

In trasformerless grid-connected photovoltaic (PV) systems, common-mode voltage (CMV) fluctuations cause leakage current flow through the stray capacitance of the PV panels. Shoot-through (SH) states in a quasi-Z-source inverter (q-ZSI), increase the amplitude of high order harmonics of CMV. In this paper, by using the modulation technique based on odd PWM (OPWM) and minor change in the Z network of the three-phase q-ZSI, the leakage current is blocked. No extra semiconductor element is added. By the proposed technique, CMV is kept nearly constant during switching cycles. The experimental results for CMV analysis in a 1kW prototype are presented to verify the theoretical analysis. IEEE  

This paper presents a robust dc-link voltage as well as a current control strategy for a bidirectional interlink converter (BIC) in a hybrid ac/dc microgrid. To enhance the dc-bus voltage control, conventional methods strive to measure and feedforward the load or source power in the dc-bus control scheme. However, the conventional feedforward-based approaches require remote measurement with communications. Moreover, conventional methods suffer from stability and performance issues, mainly due to the use of the small-signal-based control design method. To overcome these issues, in this paper, the power from DG units of the dc subgrid imposed on the BIC is considered an unmeasurable... 

Centralized or decentralized secondary controller is commonly employed to regulate the voltage drop raised by the primary controller. However, in the case of high capacity microgrids (MGs) and long feeders with much voltage drop on the line resistances, the conventional methods may not guarantee the voltage regulation on the load busses within a suitable range. Therefore, in addition to compensate the voltage drop of the primary controller, it is necessary to regulate the voltage of critical loads. In this paper, a new voltage regulation strategy is proposed to regulate the voltage of MG by employing the average voltage of identified critical busses, which are determined by the proposed... 

In this paper a novel droop approach for autonomous power management in low voltage dc microgrids based on a master-slave concept is presented. Conventional voltage-based droop approaches suffer from poor power sharing due to line resistance effects on a virtual resistance, which is solved a by introducing a communication system to increase the current sharing accuracy. In this paper, a virtual frequency is superimposed by the master units, and slave units determine their output power according to the corresponding frequency-based droop characteristics. Unlike the voltage-droop methods, the proposed virtual frequency-droop approach can be applied for proportional power management among the... 

Voltage unbalance and short circuits in distribution networks adversely affect the performance of grid-tied voltage source inverters (VSIs). Consequently, the dc-link voltage ripple may significantly increase leading to operation of VSI in an unsatisfactory manner. Conventionally in order to maintain the negative sequence current under unbalanced conditions, low-pass/trap filter in the current control loop are required which significantly reduce the controller bandwidth. In order to minimize the dc-link voltage ripple without impairing the controller bandwidth, this paper investigates the design and performance of Proportional-Integral-Resonant (PIR) controller in improving the performance...