Sharif Digital Repository

This article proposes two novel approaches to improve the superimposed frequency droop scheme for the control of dc microgrids (MGs). Conventional voltage-based control strategies suffer from issues such as undesirable voltage regulations, poor power sharing among the sources, and negative effects of line resistances on the equivalent droop characteristics. To overcome these challenges, a superimposed frequency droop scheme has been introduced. However, this method suffers from three major issues: 1) instability in terms of load variation, which is due to the location of system dominant poles; 2) limitation in system loading due to the limitation in the transferred reactive power; and 3)... 

This paper presents an experimental evaluation of the effectiveness of the Master/Checker (M/C) architecture in a 32-bit Pentium® processor system using both power-supply disturbance (PSD) fault injection and software-implemented fault injection (SWIFI) methods. A total of 6000 faults were injected in the Master processor to measure the error detection coverage of the Checker processor. The results of the experiments with PSD fault injection show that the error detection coverage of the M/C architecture is about 66.13%, which is not quite effective. This low coverage depends on the high rate of Master processor hangs because of voltage fluctuation. The coverage increased to about 99.73% when... 

The high penetration of residential air conditioners (RACs) poses a growing concern in emerging fault-induceddelayed voltage recovery (FIDVR) in power systems. FIDVR is expected to be a significant threat to the stability and reliability offuture power grids. Hence, the system planners should carry out appropriate simulation studies to mitigate the severeconsequences of FIDVR events. A key question to implement these studies is how to determine the worst condition whichresults in FIDVR. Most of the actual FIDVR events have been experienced in the hot hours of the day and this issue makes onecurious to think about the relationship between the FIDVR events and the ambient temperature. In this... 

Control of three phase inverters is of paramount importance in the realm of control applications. In this regard, this paper proposes active disturbance rejection control (ADRC) scheme as a new method for voltage control of three phase inverters in UPS applications. The main advantage of ADRC refers to the fact that it is model independent. In this paper, the controller is composed of a double loop structure including an inner current loop and outer voltage loop. This structure is used in a three phase inverter in order for implementation of voltage control. To draw a comparison between ADRC and PI as the common control method, a case study has been simulated in SIMULINK. Simulation results... 

With the rapid proliferation of residential rooftop photovoltaic (PV) systems, current and voltage unbalance issues have become a matter of great concern in low voltage (LV) distribution feeders. To overcome the issues, this study proposes a model to optimally rephase customers and PVs among the three phases via static transfer switches (STSs). The optimal STS placement is also considered in the model to achieve a cost effective solution with optimum number and location of STSs. The objective is to minimise total energy losses caused by current unbalance, minimise the number of STSs, and keep voltage unbalance along the feeder within the acceptable range. The model is solved via a... 

We present a nonlinear geometric approach to fault detection and isolation (FDI) in a grid-connected inverter system. Open-switch faults in inverter power transistors together with faults in grid voltages and source current sensors are detected and isolated in the presence of disturbance. The detection process is based on dynamic model of an inverter, including a dc-bus capacitor and an output RL filter. The proposed FDI system does not rely on balanced or zero-sum conditions in the grid point and is functional under nonsinusoidal voltages and currents. IEEE  

This paper presents a method for the optimal allocation of Distributed generation in distribution systems. In this paper, our aim would be optimal distributed generation allocation for voltage profile improvement and loss reduction in distribution network. Genetic Algorithm (GA) was used as the solving tool, which referring two determined aim; the problem is defined and objective function is introduced. Considering to fitness values sensitivity in genetic algorithm process, there is needed to apply load flow for decision-making. Load flow algorithm is combined appropriately with GA, till access to acceptable results of this operation. We used MATPOWER package for load flow algorithm and... 

Management of reactive power resources is vital for stable and secure operation of power systems in the view point of voltage stability. This paper deals with the management of on-load tap changers (OLTCs) and dynamic VAR sources (including synchronous generators, synchronous condensers, and shunt reactive power compensators) to improve voltage stability margin (VSM) of power systems. This problem is usually called optimal reactive power dispatch (ORD) in the literature. The main contribution of the paper is to introduce a new objective function for the ORD problem. The proposed objective function is derived based on a local voltage stability index, called DSY, and has a strong correlation... 

Increase of nonlinear loads has led to the rise of harmonic currents, voltage distortion and imbalance in power systems. IEEE Std. 1459 proposes new definitions for electric power in sinusoidal, nonsinusoidal, balanced and unbalanced conditions which can be used for compensation of nonsinusoidal and unbalanced systems. Among different definitions, IEEE Std. 1459 proposes the new concept of effective apparent power and power factor which is more related to the real losses in a power system. In this paper, by expansion of definitions of IEEE Std. 1459, instantaneous forms of effective parameters are defined. Then, a new control strategy for a Shunt Active Power Filters (SAPFs) is defined based... 

With the increasing penetration of renewable energies as distributed generation (DG) in power systems, technical problems arise in both distribution and transmission system level. Voltage rise is the main barrier for connection of DGs in rural areas, while excessive reactive power demand from transmission system is the major concern for Transmission System Operators (TSO). Here, a centralized control scheme based on the Smart Grid is proposed to reduce the impact on the transmission system voltages and improve its stability, while mitigating the voltage rise issue in the distribution network. Reactive power output of DGs, substation capacitors and tap setting of transmission transformer are... 

Low energy consumption and fault tolerance are often key objectives in the design of real-time embedded systems. However, these objectives are at odds, and there is a trade-off between them. Real-time systems usually use system level energy reduction methods, i.e., dynamic voltage scaling (DVS) and dynamic power management (DPM). Also hard real-time systems often use replication to achieve fault tolerance. In this paper, we investigate the impact of system level energy reduction methods on both the reliability and energy consumption of hard real-time systems which use replication for fault tolerance. In this analysis, we have considered four various existing energy management methods: 1)... 

One of the underlying requirements in the present energy management systems (EMSs) is to have a complete understanding of the system status. This feature is realized via state estimation (SE) engine. This paper presents a new and efficient SE approach which leads to a desirable outcome using a non-iterative calculation. The proposed model is based on a new AC power flow formulation designated as the line flow based (LFB) model. The objective function is to minimize the weighted least square of measurement residuals. The developed method adopts the line flows and square of voltage magnitudes as the problem state variables and incorporates both active and reactive power quantities. The... 

Checkpointing with rollback recovery is a well-established technique to tolerate transient faults. However, it incurs significant time and energy overheads, which go wasted in fault-free execution states and may not even be feasible in hard real-time systems. This paper presents a low-overhead two-state checkpointing (TsCp) scheme for fault-tolerant hard real-time systems. It differentiates between the fault-free and faulty execution states and leverages two types of checkpoint intervals for these two different states. The first type is nonuniform intervals that are used while no fault has occurred. These intervals are determined based on postponing checkpoint insertions in fault-free... 

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... 

This paper proposes a new framework for corrective voltage control (CVC) of power systems. It ensures a desired loading margin (LM) after encountering severe contingencies while minimizing the corresponding control costs. The framework is divided into primary corrective voltage control (PCVC) and secondary CVC (SCVC) stages for restoration of voltage stability and ensuring a desired LM. These stages are based on the sequence and quickness of the control actions required in post-contingency state of the system. The PCVC sub problem deals with the condition faced by a power system subject to voltage instability as the result of severe contingencies. Such control is merely devised to restore... 

In this paper, a new approach based on hybrid Particle Swarm-Based-Simulated Annealing Optimization technique (PSO-B-SA) is proposed for solving under-voltage load shedding (UVLS) problem. Under-voltage load shedding (UVLS) is one of the most important tools for avoiding voltage instability. In this paper, the UVLS problem is formulated based on the concept of the static voltage stability margin and its sensitivity at the maximum loading point or the collapse point. The voltage stability criterion is modeled directly into the load-shedding scheme. In any UVLS scheme finding the global point is very important for having cost effective economy. The proposed PSO-B-SA methodology is implemented... 

This paper proposes a new framework for corrective voltage control (CVC) of power systems. It ensures a desired loading margin (LM) after encountering severe contingencies while minimizing the corresponding control costs. The framework is divided into primary CVC (PCVC) and secondary CVC (SCVC) stages for restoration of voltage stability and ensuring a desired LM. These stages are based on the sequence and quickness of the control actions required in post-contingency state of the system. The PCVC sub problem deals with the condition faced by a power system subject to voltage instability as the result of severe contingencies. Such control is merely devised to restore system stability. Next,... 

In modern multi-core Mixed-Criticality (MC) systems, a rise in peak power consumption due to parallel execution of tasks with maximum frequency, specially in the overload situation, may lead to thermal issues, which may affect the reliability and timeliness of MC systems. Therefore, managing peak power consumption has become imperative in multi-core MC systems. In this regard, we propose an online peak power and thermal management heuristic for multi-core MC systems. This heuristic reduces the peak power consumption of the system as much as possible during runtime by exploiting dynamic slack and per-cluster Dynamic Voltage and Frequency Scaling (DVFS). Specifically, our approach examines... 

Application of Fractional Order PID (FOPID) controller to an Automatic Voltage Regulator (AVR) is presented and studied in this paper. An FOPID is a PID whose derivative and integral orders are fractional numbers rather than integers. Design stage of such a controller consists of determining five parameters. This paper employs Particle Swarm Optimization (PSO) algorithm to carry out the aforementioned design procedure. A novel cost function is defined to facilitate the control strategy over both the time-domain and the frequencydomain specifications. Comparisons are made with a PID controller from standpoints of transient response, robustness and disturbance rejection characteristics. It is... 

We consider a DVS-enabled single-processor firm real-time (FRT) system with Poisson arrival jobs having exponential execution times and generally distributed relative deadlines. The queue size of the system bounds the number of jobs which may be available therein. Further, the processor speed depends on the number of jobs in the system which varies because of the job arrivals, service completions, and dead-line misses. Thus, the processor power consumption, includling both the dynamic and leakage powers, depends on the stochastic nature of the system. More specifically, the instantaneous dynamic power consumption lonely depends on the number of jobs at that moment. However, the instantaneous...