Showing papers on "Power-flow study published in 2015"

Impedance-Source Networks for Electric Power Conversion Part I: A Topological Review

Abstract: Impedance networks cover the entire of electric power conversion from dc (converter, rectifier), ac (inverter), to phase and frequency conversion (ac-ac) in a wide range of applications. Various converter topologies have been reported in the literature to overcome the limitations and problems of the traditional voltage source, current source as well as various classical buck-boost, unidirectional, and bidirectional converter topologies. Proper implementation of the impedance-source network with appropriate switching configurations and topologies reduces the number of power conversion stages in the system power chain, which may improve the reliability and performance of the power system. The first part of this paper provides a comprehensive review of the various impedance-source-networks-based power converters and discusses the main topologies from an application point of view. This review paper is the first of its kind with the aim of providing a “one-stop” information source and a selection guide on impedance-source networks for power conversion for researchers, designers, and application engineers. A comprehensive review of various modeling, control, and modulation techniques for the impedance-source converters/inverters will be presented in Part II.

Distributed Reactive Power Feedback Control for Voltage Regulation and Loss Minimization

Abstract: We consider the problem of exploiting the microgenerators dispersed in the power distribution network in order to provide distributed reactive power compensation for power losses minimization and voltage regulation. In the proposed strategy, microgenerators are smart agents that can measure their phasorial voltage, share these data with the other agents on a cyber layer, and adjust the amount of reactive power injected into the grid, according to a feedback control law that descends from duality-based methods applied to the optimal reactive power flow problem. Convergence to the configuration of minimum losses and feasible voltages is proved analytically for both a synchronous and an asynchronous version of the algorithm, where agents update their state independently one from the other. Simulations are provided in order to illustrate the performance and the robustness of the algorithm, and the innovative feedback nature of such strategy is discussed.

Hierarchical Control of Hybrid Energy Storage System in DC Microgrids

Abstract: Hybridization of energy storages (ESs) with different characteristics takes advantages of all ESs. Centralized control with high-/low-pass filter (LPF) for system net power decomposition and ESs' power dispatch is usually implemented in hybrid ES system (HESS). In this paper, hierarchical control of HESS, composed of both centralized and distributed control, is proposed to enhance system reliability. The conventional HESS centralized control is refined with implementations of online iteration, secondary voltage regulation, and autonomous state-of-charge (SoC) recovery. ESs' power references are iteratively generated to maximize the utilization of ES ramp rates and power capacities. Secondary voltage regulation and autonomous SoC recovery are applied to minimize bus voltage deviation and limit slack terminal SoC variation, respectively. In case of communication failure, a novel algorithm for HESS distributed control is proposed to retain system operation. Bus voltage is regarded as the global indicator for system power balance, and droop relationships are imposed for ES control. System net power decomposition and ESs' power dispatch are realized with localized LPFs. SoC recovery in distributed control is implemented by tuning the threshold voltage of a slack terminal. A laboratory-scale dc microgrid is developed to verify the proposed hierarchical control of HESS.

Robust Optimal Power Management System for a Hybrid AC/DC Micro-Grid

Abstract: Hybrid ac/dc micro-grid is a new concept decoupling dc sources with dc loads and ac sources with ac loads, while power is exchanged between both sides using a bidirectional converter/inverter. This necessitates a supervisory control system to split power between its different resources, which has sparked attention on the development of power management systems (PMSs). In this paper, a robust optimal PMS (ROPMS) is developed for a hybrid ac/dc micro-grid, where the power flow in the micro-grid is supervised based on solving an optimization problem. Satisfying demanded power with maximum utilization of renewable resources, minimum usage of fuel-based generator, extending batteries lifetime, and limited utilization of the main power converter between the ac and dc micro-grids are important factors that are considered in this approach. Uncertainties in the resources output power and generation forecast errors, along with static and dynamic constraints of the resources, are taken into account. Furthermore, since uncertainties in the resources output power may result in fluctuations in the dc bus voltage, a two-level controller is used to regulate charge/discharge power of the battery banks. Effectiveness of the proposed supervisory system is evaluated through extensive simulation runs based on dynamical models of the power resources.

Cascading Failure Analysis With DC Power Flow Model and Transient Stability Analysis

Abstract: When the modern electrical infrastructure is undergoing a migration to the Smart Grid, vulnerability and security concerns have also been raised regarding the cascading failure threats in this interconnected transmission system with complex communication and control challenge. The DC power flow-based model has been a popular model to study the cascading failure problem due to its efficiency, simplicity and scalability in simulations of such failures. However, due to the complex nature of the power system and cascading failures, the underlying assumptions in DC power flow-based cascading failure simulators (CFS) may fail to hold during the development of cascading failures. This paper compares the validity of a typical DC power flow-based CFS in cascading failure analysis with a new numerical metric defined as the critical moment (CM). The adopted CFS is first implemented to simulate system behavior after initial contingencies and to evaluate the utility of DC-CFS in cascading failure analysis. Then the DC-CFS is compared against another classic, more precise power system stability methodology, i.e., the transient stability analysis (TSA). The CM is introduced with a case study to assess the utilization of these two models for cascading failure analysis. Comparative simulations on the IEEE 39-bus and 68-bus benchmark reveal important consistency and discrepancy between these two approaches. Some suggestions are provided for using these two models in the power grid cascading failure analysis.

Decoupled Active and Reactive Power Control for Large-Scale Grid-Connected Photovoltaic Systems Using Cascaded Modular Multilevel Converters

Abstract: Large-scale grid-connected photovoltaic (PV) systems significantly contribute to worldwide renewable energy growth and penetration, which has inspired the application of cascaded modular multilevel converters due to their unique features such as modular structures, enhanced energy harvesting capability, scalability and so on However, power distribution and control in the cascaded PV system faces tough challenge on output voltage overmodulation when considering the varied and nonuniform solar energy on segmented PV arrays This paper addresses this issue and proposes a decoupled active and reactive power control strategy to enhance system operation performance The relationship between output voltage components of each module and power generation is analyzed with the help of a newly derived vector diagram which illustrates the proposed power distribution principle On top of this, an effective control system including active and reactive components extraction, voltage distribution and synthesization, is developed to achieve independent active and reactive power distribution and mitigate the aforementioned issue Finally, a 3-MW, 12-kV PV system with the proposed control strategy is modeled and simulated in MATLAB and PSIM cosimulation platform A downscaled PV system including two cascaded 5-kW converters with proposed control strategy is also implemented in the laboratory Simulation and experimental results are provided to demonstrate the effectiveness of the proposed control strategy for large-scale grid-connected cascaded PV systems

Fast power system analysis via implicit linearization of the power flow manifold

Abstract: In this paper, we consider the manifold that describes all feasible power flows in a power system as an implicit algebraic relation between nodal voltages (in polar coordinates) and nodal power injections (in rectangular coordinates). We derive the best linear approximant of such a relation around a generic solution of the power flow equations. Our linear approximant is sparse, computationally attractive, and preserves the structure of the power flow. Thanks to the full generality of this approach, the proposed linear implicit model can be used to obtain a fast approximate solution of a possibly unbalanced three phase power system, with either radial or meshed topology, and with general bus models. We demonstrate how our approximant includes standard existing linearizations, we validate the quality of the approximation via simulations on a standard testbed, and we illustrate its applicability with case studies in scenario-based optimization and cascading failures.

Convex Optimization of Power Systems

Abstract: Optimization is ubiquitous in power system engineering. Drawing on powerful, modern tools from convex optimization, this rigorous exposition introduces essential techniques for formulating linear, second-order cone, and semidefinite programming approximations to the canonical optimal power flow problem, which lies at the heart of many different power system optimizations. Convex models in each optimization class are then developed in parallel for a variety of practical applications like unit commitment, generation and transmission planning, and nodal pricing. Presenting classical approximations and modern convex relaxations side-by-side, and a selection of problems and worked examples, this is an invaluable resource for students and researchers from industry and academia in power systems, optimization, and control.

Reactive Power Compensation and Optimization Strategy for Grid-Interactive Cascaded Photovoltaic Systems

Abstract: Cascaded multilevel converter structure can be appealing for high-power solar photovoltaic (PV) systems thanks to its modularity, scalability, and distributed maximum power point tracking (MPPT). However, the power mismatch from cascaded individual PV converter modules can bring in voltage and system operation issues. This paper addresses these issues, explores the effects of reactive power compensation and optimization on system reliability and power quality, and proposes coordinated active and reactive power distribution to mitigate this issue. A vector method is first developed to illustrate the principle of power distribution. Accordingly, the relationship between power and voltage is analyzed with a wide operation range. Then, an optimized reactive power compensation algorithm (RPCA) is proposed to improve the system operation stability and reliability, and facilitate MPPT implementation for each converter module simultaneously. Furthermore, a comprehensive control system with the RPCA is designed to achieve effective power distribution and dynamic voltage regulation. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed reactive power compensation approach in grid-interactive cascaded PV systems.

Conceptual Study for Open Energy Systems: Distributed Energy Network Using Interconnected DC Nanogrids

Abstract: We describe the general concept and practical feasibility of a dc-based open energy system (OES) that proposes an alternative way of exchanging intermittent energy between houses in a local community. Each house is equipped with a dc nanogrid, including photovoltaic panels and batteries. We extend these nanogrids with a bidirectional dc–dc converter and a network controller so that power can be exchanged between houses over an external dc power bus. In this way, demand-response fluctuations are absorbed not only by the local battery, but can be spread over all batteries in the system. By using a combination of voltage and current controlled units, we implemented a higher-level control software independent from the physical process. A further software layer for autonomous control handles power exchange based on a distributed multiagent system, using a peer-to-peer like architecture. In parallel to the software, we made a physical model of a four-node OES on which different power exchange strategies can be simulated and compared. First results show an improved solar replacement ratio, and thus a reduction of ac grid consumption thanks to power interchange. The concept’s feasibility has been demonstrated on the first three houses of a full-scale OES platform in Okinawa.

A Single-Objective Predictive Control Method for a Multivariable Single-Phase Three-Level NPC Converter-Based Active Power Filter

Abstract: A single-objective predictive control method that deals with four main control objectives applied to a multivariable single-phase three-level neutral-point-clamped converter operating as an active power filter is proposed in this paper. The four control objectives are to self-support the dc-bus voltage under load variations, to compensate the reactive power and the current harmonics, and to balance the dc capacitor voltages by using a predefined combination of the redundant switching states of the converter. The main contribution of the proposed method is that these objectives are accomplished without using weighting factors in the cost function, which eliminates problems such as multiobjective optimization or additional empirical procedures for determination of these factors. As a result, the method is easy to implement and rapidly selects the optimal voltage to improve the dynamic-state performance. Experimental results from a 2-kVA prototype are presented to prove that the method is valid for single-phase compensation. The well-known effect of model parameter errors' issue, which is inherent in predictive control methods, is also tested to confirm that the harmonic distortion in the grid current is below 5% even when the predictive model has a 25% error between actual and theoretically estimated grid impedance values.

Pushing the Limits: Europe's New Grid: Innovative Tools to Combat Transmission Bottlenecks and Reduced Inertia

Abstract: In the future a growing amount of power electronics will lead to a transition of the power system to a structure with very low synchronous generation. Due to large transit power flows and uncertainties, transmission systems are being operated under increasingly stressed conditions and are close to their stability limits. Together with the integration of large amounts of renewable generation with power electronic interfaces and the addition of high-voltage direct current (HVdc) links into the power system, these challenges will necessitate a review of the operation and control of transmission networks. This article will demonstrate the need for R&D performed by network operators and explain a set of challenges, focusing on three main areas: transmission grid operation in a new power system environment, the need to increase overhead line (OHL) utilization, and the impact of reduced inertia on power system frequency.

Networked-Based Hybrid Distributed Power Sharing and Control for Islanded Microgrid Systems

Abstract: Distributed generation (DG) microgrid systems are forming the building blocks for smart distribution grids. Enhanced networked-based control structure is needed not only to eliminate the frequency deviations, power-sharing errors, and stability concerns associated with conventional droop control in microgrids but also to yield: 1) improved microgrid dynamic performance, 2) minimized active/reactive power-sharing errors under unknown line impedances, and 3) high reliability and robustness against network failures or communication delays. This paper proposes a new hybrid distributed networked-based power control scheme that addresses the aforementioned problems in a distributed manner. The new method consists of a set of distributed power regulators that are located at each DG unit to ensure perfect tracking of the optimized set points assigned by the centralized energy management unit (EMU). The average power measurements are transmitted to the EMU to calculate the share of each unit of the total power demand based on real-time optimization criteria; therefore, a low-bandwidth communication system can be used. In the proposed method, the distributed nature of the power regulators allows them to adopt the delay-free local power measurements as the required feedback signals. Therefore, the proposed structure provides great robustness against communication delays. Further, this paper presents a generalized and computationally efficient modeling approach that captures the dominant dynamics of a microgrid system. The model can be used to study the impact of power-sharing controllers and delays in microgrid stability. Comparative simulation and experimental results are presented to show the validity and effectiveness of the proposed controller.

Dynamic Reactive Power Compensation of Large-Scale Wind Integrated Power System

Abstract: Due to progressive displacement of conventional power plants by wind turbines, dynamic security of large-scale wind integrated power systems is significantly compromised. In this paper we first highlight the importance of dynamic reactive power support/voltage security in large-scale wind integrated power systems with least presence of conventional power plants. Then we propose a mixed integer dynamic optimization based method for optimal dynamic reactive power allocation in large-scale wind integrated power systems. One of the important aspect of the proposed methodology is that unlike static optimal power flow based approaches, the proposed method considers detailed system dynamics and wind turbine grid code compliance while optimizing the allocation of dynamic reactive power sources. We also advocate that in large-scale wind integrated power systems, 1) better utilization of existing wind turbines especially wind farms with additional grid support functionalities like dynamic support (e.g., dynamic reactive power support, etc.) and 2) refurbishment of existing conventional central power plants to synchronous condensers could be one of the efficient, reliable and cost-effective option to address not only the issue of dynamic voltage security but also to strengthen other dynamic capabilities of the system including system inertia, etc. that are also significant challenges in large-scale wind penetrated power system. The proposed methodology is applied to the detailed model of the western Danish power system which is characterized by large-scale wind integration and least presence of central power plants.

Assessment of Robustness of Power Systems From a Network Perspective

Abstract: In this paper, we study the robustness assessment of power systems from a network perspective. Based on Kirchhoff's laws and the properties of network elements, and combining with a complex network structure, we propose a model that generates power flow information given the electricity consumption and generation information. It has been widely known that large scale blackouts are the result of a series of cascading failures triggered by the malfunctioning of specific critical components. Power systems could be more robust if there were fewer such critical components or the network configuration was suitably designed. The percentage of unserved nodes (PUN) caused by a failed component and the percentage of noncritical links (PNL) that will not cause severe damage are used to provide quantitative indication of a power system's robustness. We assess robustness of the IEEE 118 Bus, Northern European Grid and some synthesized networks. The influence of network structure and location of generators are explored. Simulation results show that the connection with short average shortest path length can significantly reduce a power system's robustness, and that the system with lower generator resistance has better robustness with a given network structure. We also propose a new metric based on node-generator distance (DG) for measuring the accessibility of generators in a power network which is shown to affect robustness significantly.

Table-Based Direct Power Control for Three-Phase AC/DC Converters Under Unbalanced Grid Voltages

Abstract: This paper proposes a simple but effective direct power control (DPC) strategy for three-phase ac/dc converters operating under unbalanced grid voltage conditions. An extended instantaneous power theory is adapted and applied in the proposed DPC, where the reactive power is expressed in the form of dot product of grid currents and delayed grid voltages. Neither complicated positive/negative sequence extraction of grid voltage/current nor power compensation is required. The switching table suitable to control both active power and reactive power is constructed by analyzing the influence of each converter voltage vector on power slopes. Compared to conventional table-based DPC using original pq theory, the proposed DPC achieves constant active power and reactive power as well as sinusoidal grid currents, while maintaining the simplicity and robustness as much as possible. The presented simulation and experimental results confirm the theoretical study and the effectiveness of the proposed method.

Particle Swarm Optimization Based Optimal Reactive Power Dispatch

Abstract: Optimal reactive power dispatch is one of the major and important optimization problem in electrical power system operation and control. This is nothing but multi objectives, nonlinear, minimization problem of power system optimization. This paper focuses on this sub problem of optimal power flow. The particle swarm optimization (PSO) is one of the best population based intelligent technique of optimization. The basic PSO algorithm is used for optimal reactive power dispatch. This approach applies to the IEEE-14 bus, IEEE-30 bus, IEEE-57 bus and IEEE-39 New England bus test systems for minimization of active power loss. Simulation results are compared with the other optimization algorithm.

Analysis of the Load Flow Problem in Power System Planning Studies

Abstract: Load flow is an important tool used by power engineers for planning, to determine the best operation for a power system and exchange of power between utility companies. In order to have an efficient operating power system, it is necessary to determine which method is suitable and efficient for the system’s load flow analysis. A power flow analysis method may take a long time and therefore prevent achieving an accurate result to a power flow solution because of continuous changes in power demand and generations. This paper presents analysis of the load flow problem in power system planning studies. The numerical methods: Gauss-Seidel, Newton-Raphson and Fast Decoupled methods were compared for a power flow analysis solution. Simulation is carried out using Matlab for test cases of IEEE 9-Bus, IEEE 30-Bus and IEEE 57-Bus system. The simulation results were compared for number of iteration, computational time, tolerance value and convergence. The compared results show that Newton-Raphson is the most reliable method because it has the least number of iteration and converges faster.

Decentralized Communication and Control Systems for Power System Operation

Abstract: Due to the rapid deployment of phasor measurement units (PMUs) on large power grids, the system operators now have access to high speed high resolution data. A new class of monitoring and control applications are made possible with the PMUs. Although PMU based monitoring systems have been well developed, implementations of PMU based fast acting closed loop wide area control systems are relatively rare. To meet the stringent latency requirements of a wide area controller the communication and power infrastructures have to collaborate strongly. In this paper, a combined process for design and simulation of both communication network and power network has been presented with the objective of damping interarea oscillations. A method to determine the optimal location of data routing hubs so as to minimize the volume of communications is also proposed. The IEEE 118 bus system is used to study the performance of communication system and the wide area power damping control system on both centralized and decentralized topologies, and the results are discussed. One of the conclusions of the paper is that the decentralized communication architectures involving data routing hubs are better suited for control applications requiring fast control actions.

Cost-Optimized Battery Capacity and Short-Term Power Dispatch Control for Wind Farm

Abstract: Utilizing the optimal capacity of a battery in wind–battery hybrid power systems is crucial to minimize costs. In this paper, we modify the min–max dispatch method to effectively integrate wind power into the grid. In line with the dispatch principle, we define a lifetime cost function, which indicates the battery energy storage system cost of dispatching 1 kWh of wind energy, to determine the optimal battery capacity. By using the optimal battery capacity, the operation costs are minimized, and the hybrid system is able to dispatch the scheduled power at any dispatching time. Moreover, the short-term power dispatch control is also considered; we smooth the transient power between two consecutive dispatching intervals and control the state of charge of the battery by an online control algorithm. To evaluate the performance of the proposed optimization method and the short-term power dispatch control, we perform several numerical studies with a 3-MW wind turbine generator and real wind speed data.

Introducing a Novel DC Power Flow Method With Reactive Power Considerations

Abstract: The DC power flow model is in widespread utilization in electricity-market applications and contingency analysis. The presented versions of this model can be classified into two categories: state-dependent, or Hot-Start, models and state-independent, or Cold-Start, models. A reasonable accuracy is reported in the literature regarding Hot-Start models as they take into account branch losses and bus voltages by using available base point. On the contrary, due to the absence of base point in Cold-Start models, branch losses must be either neglected or guessed (which is an uncertain precautionary measure), or evaluated by a cumbersome iteration process. In addition, the bus voltage profiles are inevitably considered to be flat. Hence, the accuracy of available Cold-Start models in different circumstances remains of great concern. This paper addresses this concern and unveils a new Cold-Start model that does not rely on a risky assumption. In other words, there will be no lossless or flat voltage profile assumption in the presented approach whereas the equations remain linear. Besides, the exact effect of the net reactive loads on phase angles is considered and, consequently, the reactive power balance equations are reflected in the model for the first time.

Long-Term Stability Analysis of Power Systems With Wind Power Based on Stochastic Differential Equations: Model Development and Foundations

Abstract: In this paper, the variable wind power is incorporated into the dynamic model for long-term stability analysis. A theory-based method is proposed for power systems with wind power to conduct long-term stability analysis, which is able to provide accurate stability assessments with fast simulation speed. Particularly, the theoretical foundation for the proposed approximation approach is presented. The accuracy and efficiency of the method are illustrated by several numerical examples.

A centralized reactive power compensation system for LV distribution networks

Abstract: A centralized reactive power compensation system is proposed for low voltage (LV) distribution networks. It can be connected with any bus which needs reactive power. The current industry practice is to locally install reactive power compensation system to maintain the local bus voltage and power factor. By centralizing capacitor banks together, it can help to maintain bus voltages and power factors as well as reduce the power cable losses. Besides, the centralized reactive power system can be easily expanded to meet any future load increase. A reasonably sized centralized reactive power compensation system will be capable of meeting the requirements of the network and the optimization algorithm proposed in this paper can help to find this optimal size by minimizing the expected total cost (ETCH). Different load situations and their respective probabilities are also considered in the proposed algorithm. The concept of the centralized reactive power compensation system is applied to a local shipyard power system to verify its effectiveness. The results show that an optimally sized centralized reactive power system exists and is capable of maintaining bus voltages as well as reducing the power losses in the distribution network. A significant power loss reduction can be obtained at the optimal capacity of the centralized reactive power compensation system in the case study.

Power decoupling techniques for single-phase power electronics systems — An overview

Abstract: The well-known double line frequency ripple power is a critical issue for single-phase power electronics systems, because it may lead to reduced system reliability and efficiency in many applications, e.g. photovoltaic (PV), light-emitting-diode (LED), fuel cell, and battery charger systems. In this paper, recently proposed state-of-the-art power decoupling techniques for ripple power reduction in these systems are presented and classified into different groups for performance comparison. The pros and cons of these techniques are discussed and identified, and the conclusions drawn will be useful for choosing suitable power decoupling topologies and control algorithms according to specific applications from a practical point of view. The future development trends and potentials on this research topic are also discussed.

Islanding and Scheduling of Power Distribution Systems With Distributed Generation

Abstract: This paper presents a short-term power regulation mechanism for post-segmentation power distribution systems in the presence of severe fault or large disturbance. Firstly, load priority is introduced into system separation and centroids of load clusters are allocated at nodes with distributed sources, respecting power balance and network connectedness. Secondly, fast power flow calculation is performed on the primarily formed islands to resolve load interruption for operational feasibility of partitions from power quality perspective. Then the effect of distributed generation fluctuations on durable operation of subsystems with various optimization objectives is examined. The load response is instructed by the proposed cost-based pricing scheme contemplating consumers' willingness. In addition, the influence of network congestion and load controllability on results of economic scheduling is investigated. Numerical results from the PG&E-69 distribution system are used to show the effectiveness of the developed model in radial structure load clusters. Illustration on the IEEE 118-bus case further proves its robustness and practicability for other distribution systems applications. This property is also useful for transmission switching and micro-grid applications.

Decision Trees-Aided Active Power Reduction of a Virtual Power Plant for Power System Over-Frequency Mitigation

Abstract: The incorporation of distributed generation (DG) under the virtual power plant (VPP) paradigm allows the coherent central control, and the coordinated market integration of several and widely dispersed electric power sources. That way, VPPs can participate in frequency control by regulating in a coordinated manner their output power for the sake of system stability. This study analyzes a decision tree (DT)-based methodology that dispatches a requested reduction of active power within a VPP (among the sources it consists of), so as to support the mitigation of over-frequency. The presented control con-cept finds its application concerning grid support in case of over-frequency, and is also meant as a response to related reports and studies asking for the increased contribution of DG to overcome such phenomena.