Showing papers on "Power-flow study published in 2013"

A Novel and Generalized Three-Phase Power Flow Algorithm for Islanded Microgrids Using a Newton Trust Region Method

Abstract: A new formulation is required to provide a proper power flow analysis in islanded microgrids taking into consideration their special philosophy of operation. In this paper, a novel and generic three-phase power flow algorithm is formulated for islanded microgrids. The algorithm is novel since it adapts the real characteristics of the islanded microgrid operation; i.e., 1) some of the distributed generation (DG) units are controlled using the droop control methods and their generated active and reactive power are dependent on the power flow variables; 2) the steady-state system frequency is considered as one of the power flow variables. The proposed algorithm is generic, where the features of distribution systems, i.e., three-phase feeder models, unbalanced loads and load models have been taken in consideration. Further, all possible operation modes of DG units (droop, PV, or PQ) have been considered. The problem has been formulated as a set of nonlinear equations. A globally convergent Newton-trust region method has been proposed to solve this set of nonlinear equations. The proposed algorithm is a helpful tool to perform accurate steady state studies of the islanded microgrid. Different case studies have been carried out to test the effectiveness and the robustness of the proposed algorithm.

International Industry Practice on Power System Load Modeling

Abstract: Power system load modeling is a mature and generally well researched area which, as many other in electrical power engineering at the present time, is going through a period of renewed interest in both industry and academia. This interest is fueled by the appearance of new non-conventional types of loads (power electronic-based, or interfaced through power electronics) and requirements to operate modern electric power systems with increased penetration of non-conventional and mostly intermittent types of generation in a safe and secure manner. As a response to this renewed interest, in February 2010 CIGRE established working group C4.605: “Modelling and aggregation of loads in flexible power networks”. One of the first tasks of the working group was to identify current international industry practice on load modeling for static and dynamic power system studies. For that purpose, a questionnaire was developed and distributed during the summer/autumn of 2010 to more than 160 utilities and system operators in over 50 countries on five continents. This paper summarizes some of the key findings from about 100 responses to the questionnaire received by September 2011 and identifies prevalent types of load models used as well as typical values of their parameters.

Decentralized Controls and Communications for Autonomous Distribution Networks in Smart Grid

Abstract: The traditional power grid system was constructed in a centralized and radial topology where power is generated and delivered from one end to the other. Conventional methods for unidirectional power flow analysis will no longer be effective to control renewable energy sources implemented at the consumption sector efficiently; new strategies are called for to facilitate the bidirectional flow incurred by power production of the distributed energy resource units. The transformation will require intelligent distribution automation by means of decentralized power management as well as information and communications technologies to actualize smart grid modernization. In this paper, we design autonomous distribution networks that take scalability into account by dividing the legacy distribution network into a set of subnetworks. We tackle the power flow and power balance issues individually in parallel to optimize the global distribution operation by our proposed power-control method. Meanwhile, we propose an overlay multi-tier communications infrastructure for the underlying power network to analyze the traffic of data information and control message required for associated power flow operation. Based on the proposed schemes, we show that we can potentially improve the utilization of renewable energy production and achieve data traffic reduction under decentralized operations as compared to legacy centralized management.

Probabilistic Power Flow Analysis With Generation Dispatch Including Photovoltaic Resources

Abstract: This paper proposes a novel probabilistic power flow (PPF) algorithm considering generation dispatch. The intent is to examine the influence of uncertainty due to photovoltaic (PV) generation resources and loads. PV generation uncertainty may have a significant impact on transmission systems since this resource is easily influenced by changing environmental conditions. However, it is prudent to include generation dispatch in the PPF algorithm since the dispatching strategy compensates for PV generation injections and influences the uncertainty in the results. The proposed PPF algorithm is based on the cumulant method. The Gram-Charlier expansion is applied to approximate the distribution of probabilistic variables. Furthermore, this paper also proposes a probabilistic optimal power dispatching strategy which considers uncertainty problems in the economic dispatch and optimizes the expected value of the total cost with the overload probability as a constraint. The Arizona area of the Western Electricity Coordinating Council (WECC) system is used to test the proposed PPF algorithm and compare results with Monte Carlo simulation (MCS).

Swarm Intelligence Approaches to Optimal Power Flow Problem With Distributed Generator Failures in Power Networks

Abstract: Distributed generation becomes more and more important in modern power systems. However, the increasing use of distributed generators causes the concerns on the increasing system risk due to their likely failure or uncontrollable power outputs based on such renewable energy sources as wind and the sun. This work for the first time formulates an optimal power flow problem by considering controllable and uncontrollable distributed generators in power networks. The problem for the cases of single and multiple generator failures is addressed as an example. The methods are presented to find its power output solution of controllable online generators via particle swarm optimization and group search optimizer for coping with the difficult scenarios in a power network. The proposed methods are tested on an IEEE 14-bus system, and several population initialization strategies are investigated and compared for the algorithms. The simulation results confirm their effectiveness for optimal power management and effective control of a power network.

Droop Control of Distributed Electric Springs for Stabilizing Future Power Grid

Abstract: This paper describes the droop control method for parallel operation of distributed electric springs for stabilizing ac power grid It provides a methodology that has the potential of allowing reactive power controllers to work in different locations of the distribution lines of an ac power supply and for these reactive power controllers to support and stabilize the ac mains voltage levels at their respective locations on the distribution lines The control scheme allows these reactive power controllers to have automatically adjustable voltage references according to the mains voltage levels at the locations of the distribution network The control method can be applied to reactive power controllers embedded in smart electric loads distributed across the power grid for stabilizing and supporting the ac power supply along the distribution network The proposed distributed deployment of electric springs is envisaged to become an emerging technology potentially useful for stabilizing power grids with substantial penetration of distributed and intermittent renewable power sources or weakly regulated ac power grid

A Range Arithmetic-Based Optimization Model for Power Flow Analysis Under Interval Uncertainty

Abstract: This paper presents a novel framework based on range arithmetic for solving power flow problems whose input data are specified within real compact intervals. Reliable interval bounds are computed for the power flow problem, which is represented as an optimization model with complementary constraints to properly represent generator bus voltage controls, including reactive power limits and voltage recovery processes. It is demonstrated that the lower and upper bounds of the power flow solutions can be obtained by solving two determinate optimization problems. Several numerical results are presented and discussed, demonstrating the effectiveness of the proposed methodology and comparing it to a previously proposed affine arithmetic based solution approach.

Enhancement of PV Penetration With DSTATCOM in Taipower Distribution System

Abstract: The PV penetration level of a distribution system is often limited by the violation of voltage variation caused by large intermittent power generation. This study investigates the use of a distribution static compensator (DSTATCOM) in reactive power compensation for system voltage control, during peak solar irradiation, in order to increase the PV installation capacity of a distribution feeder and avoid the voltage violation problem. PV power generation is simulated using hourly solar irradiation and temperature data provided by the weather bureau. The voltage variation at the point of common coupling (PCC) is also derived by executing the 3- φ load flow analysis to determine the maximum PV power injection without causing voltage violation. By applying the proposed voltage control scheme of DSTATCOM during high solar irradiation periods, the total power generation and the total energy delivered by the PV system over one year are determined according to the annual duration of solar irradiation. The annual sales of PV power, the system O&M cost, the cost of DSTATCOM installation and the initial capital investment for a PV system are then used to calculate the cash flow over the system life-cycle and the final net present value (NPV) of the PV project. With the proposed DSTATCOM voltage control to perform reactive power compensation, the optimal installation capacity of PV systems can be determined by maximizing the net present value of the system to ensure the best cost-effectiveness of the PV project and to better utilize solar energy.

Time Series Power Flow Analysis for Distribution Connected PV Generation

Abstract: Distributed photovoltaic (PV) projects must go through an interconnection study process before connecting to the distribution grid. These studies are intended to identify the likely impacts and mitigation alternatives. In the majority of the cases, system impacts can be ruled out or mitigation can be identified without an involved study, through a screening process or a simple supplemental review study. For some proposed projects, expensive and time-consuming interconnection studies are required. The challenges to performing the studies are twofold. First, every study scenario is potentially unique, as the studies are often highly specific to the amount of PV generation capacity that varies greatly from feeder to feeder and is often unevenly distributed along the same feeder. This can cause location-specific impacts and mitigations. The second challenge is the inherent variability in PV power output which can interact with feeder operation in complex ways, by affecting the operation of voltage regulation and protection devices. The typical simulation tools and methods in use today for distribution system planning are often not adequate to accurately assess these potential impacts. This report demonstrates how quasi-static time series (QSTS) simulation and high time-resolution data can be used to assess the potential impacts in a moremore » comprehensive manner. The QSTS simulations are applied to a set of sample feeders with high PV deployment to illustrate the usefulness of the approach. The report describes methods that can help determine how PV affects distribution system operations. The simulation results are focused on enhancing the understanding of the underlying technical issues. The examples also highlight the steps needed to perform QSTS simulation and describe the data needed to drive the simulations. The goal of this report is to make the methodology of time series power flow analysis readily accessible to utilities and others responsible for evaluating potential PV impacts.« less

Control Strategy for Power Flow Management in a PV System Supplying DC Loads

Abstract: The growing concern for energy saving has increased the usage of LED-based street lights, electronic chokes, compact fluorescent lamps, and inverter-fed drives. Hence, the load profile seen by the electrical grid is undergoing a notable change as these devices have to operate from a dc source. Photovoltaics (PV) being a major energy source, the aforementioned loads can be connected directly to the dc bus. A grid-connected PV system involves a power source (PV array), a power sink (load), and two power sources/sink (utility and battery), and hence, a power flow management system is required to balance the power flow among these sources. One such system is developed for selecting the operating mode of the bidirectional converter by sensing the battery voltage. The viability of the scheme has been ascertained by performing experimental studies on a laboratory prototype. The control strategy is digitally implemented on an Altera Cyclone II Field Programmable Gate Array (FPGA) board, and the algorithm is verified for different modes of operation by varying the load. Experimental results are presented to bring out the usefulness of the control strategy.

Coherency identification in interconnected power system - an independent component analysis approach

Abstract: Summary form only given. This paper presents a novel approach to the coherency identification technique in interconnected power system using independent component analysis (ICA). The ICA is applied to the generator speed and bus angle data to identify the coherent areas of the system. The results of the application of ICA using simulated data from 16-machine 68-bus system model and on data gathered through U.K. University-based Wide-Area Measurement System are presented. The approach is able to identify the cluster of generators and buses following a disturbance in the system. It is also demonstrated that the approach is robust in the presence of noise in measured signal, which is an important factor to be considered for assessing the effectiveness of any measurement-based technique.

Probabilistic Load Flow Modeling Comparing Maximum Entropy and Gram-Charlier Probability Density Function Reconstructions

Abstract: Probabilistic load flow (PLF) modeling is gaining renewed popularity as power grid complexity increases due to growth in intermittent renewable energy generation and unpredictable probabilistic loads such as plug-in hybrid electric vehicles (PEVs). In PLF analysis of grid design, operation and optimization, mathematically correct and accurate predictions of probability tail regions are required. In this paper, probability theory is used to solve electrical grid power load flow. The method applies two Maximum Entropy (ME) methods and a Gram-Charlier (GC) expansion to generate voltage magnitude, voltage angle and power flow probability density functions (PDFs) based on cumulant arithmetic treatment of linearized power flow equations. Systematic ME and GC parameter tuning effects on solution accuracy and performance is reported relative to converged deterministic Monte Carlo (MC) results. Comparing ME and GC results versus MC techniques demonstrates that ME methods are superior to the GC methods used in historical literature, and tens of thousands of MC iterations are required to reconstitute statistically accurate PDF tail regions. Direct probabilistic solution methods with ME PDF reconstructions are therefore proposed as mathematically correct, statistically accurate and computationally efficient methods that could be applied in the load flow analysis of large-scale networks.

Online Static Security Assessment Module Using Artificial Neural Networks

Abstract: Fast and accurate contingency selection and ranking method has become a key issue to ensure the secure operation of power systems. In this paper multi-layer feed forward artificial neural network (MLFFN) and radial basis function network (RBFN) are proposed to implement the online module for power system static security assessment. The security classification, contingency selection and ranking are done based on the composite security index which is capable of accurately differentiating the secure and non-secure cases. For each contingency case as well as for base case condition, the composite security index is computed using the full Newton Raphson load flow analysis. The proposed artificial neural network (ANN) models take loading condition and the probable contingencies as the input and assess the system security by screening the credible contingencies and ranking them in the order of severity based on composite security index. The numerical results of applying the proposed approach to IEEE 118-bus test system demonstrate its effectiveness for online power system static security assessment. The comparison of the ANN models with the model based on Newton Raphson load flow analysis in terms of accuracy and computational speed indicate that the proposed model is effective and reliable in the fast evaluation of the security level of power systems. The proposed online static security assessment (OSSA) module realized using the ANN models are found to be suited for online application.

Distributed Operation of Interlinked AC Microgrids with Dynamic Active and Reactive Power Tuning

Abstract: Microgrids are small grids formed by clustering modern generating sources, storage systems, and loads together. Being independent, the formed microgrids can, in principle, operate at their own preferred voltages and frequencies. Tying them to the mains grid or another microgrid would therefore require some interlinking power converters, whose control should preferably be autonomous without depending on fast communication links. Contributing to this theme of research, a distributed power management scheme has been proposed in this paper for interlinking two or more independent microgrids operating at different voltages and frequencies. The proposed scheme allows sources in the microgrids to concentrate more on active power harnessing, while the interlinking converters focus more on meeting the load reactive demand. If necessary, backup active power from an underloaded microgrid can also be transferred to an overloaded microgrid, allowing it to supply loads in excess of its rated capacity. The performance of the proposed scheme has already been tested in experiment.

An Adaptive Zone-Division-Based Automatic Voltage Control System With Applications in China

Abstract: The power industry in China has grown significantly over the past decade, spurring the adoption of system-wide automatic voltage control (AVC) technology to meet stricter requirements for security and economical power system operation. To cope with the rapidly developing and frequently changing Chinese power grid, an AVC scheme based on adaptive zone division is introduced in this paper. Logically, this type of system has a three-level hierarchical structure, but, here, both secondary voltage control (SVC) and tertiary voltage control (TVC) are implemented via software at the same control center. The control zones are no longer fixed but are reconfigured online and updated in accordance with variations in the grid structure. The technical details of these procedures are presented here. The implementation of TVC (which is based on an online reactive optimal power flow) and SVC are also discussed, together with some technical details on improvements to their reliability and robustness. Some results obtained from field-site applications based on similar-days testing rather than simulations are employed to evaluate the performance and improvements of the AVC system. This system has been installed at over 20 control centers in China.

Probabilistic security-constrained AC optimal power flow

Abstract: We propose a probabilistic framework for designing an N-1 secure dispatch for systems with fluctuating power sources This could be used in various optimal power flow related applications, however in this work, we demonstrate our approach for a day-ahead planning problem We extend our earlier work on probabilistic N-1 security, to incorporate recent results on convex AC optimal power flow relaxations The problem is formulated as a chance constrained convex program; to deal with the chance constraint we follow an algorithm based on a combination of randomized and robust optimization We also enhance the controllability of the system by introducing a corrective scheme that imposes post-contingency control of the Automatic Voltage Regulation (AVR) set-point This scheme allows us to inherit a priori probabilistic guarantees regarding the satisfaction of the system constraints, unlike the base case where the AVR set-points are constant To illustrate the performance of the proposed security-constrained AC optimal power flow we compare it against a DC power flow based formulation using Monte Carlo simulations, and show that it results to lower operational cost compared to the case where the AVR set-points are constant

Coordinated Predictive Control of a Wind/Battery Microgrid System

Abstract: This paper presents the operation and controller design of a microgrid consisting of a direct drive wind generator and a battery storage system A model predictive control strategy for the ac-dc-ac converter of wind system is derived and implemented to capture the maximum wind energy as well as provide desired reactive power A novel supervisory controller is presented and employed to coordinate the operation of wind farm and battery system in the microgrid for grid-connected and islanded operations The proposed coordinated controller can mitigate both active and reactive power disturbances that are caused by the intermittency of wind speed and load change Moreover, the control strategy ensures the maximum power extraction capability of wind turbine while regulating the point of common coupling bus voltage within acceptable range in both grid-connected and islanded operations The designed concept is verified through various simulation studies in EMTDC/PSCAD, and the results are presented and discussed

A Sufficient Condition for Power Flow Insolvability With Applications to Voltage Stability Margins

Abstract: For the nonlinear power flow problem specified with standard PQ, PV, and slack bus equality constraints, we present a sufficient condition under which the specified set of nonlinear algebraic equations has no solution. This sufficient condition is constructed in a framework of an associated feasible, convex optimization problem. The objective employed in this optimization problem yields a measure of distance (in a parameter set) to the power flow solution boundary. In practical terms, this distance is closely related to quantities that previous authors have proposed as voltage stability margins. A typical margin is expressed in terms of the parameters of system loading (injected powers); here we additionally introduce a new margin in terms of the parameters of regulated bus voltages.

Performance Investigation of Isolated Wind–Diesel Hybrid Power Systems With WECS Having PMIG

Abstract: This paper presents the automatic reactive power control of isolated wind-diesel hybrid power systems having a permanent-magnet induction generator for a wind energy conversion system and a synchronous generator for a diesel generator set. To minimize the gap between reactive power generation and demand, a variable source of reactive power is used such as a static synchronous compensator. The mathematical model of the system used for simulation is based on small-signal analysis. Three examples of the wind-diesel hybrid power system are considered with different wind power generation capacities to study the effect of the wind power generation on the system performance. This paper also shows the dynamic performance of the hybrid system with and without change in input wind power plus 1% step increase in reactive power load.

Energy Imbalance Management Using a Robust Pricing Scheme

Abstract: This paper focuses on the problem of energy imbalance management in a microgrid. The problem is investigated from the power market perspective. Unlike the traditional power grid, a microgrid can obtain extra energy from a renewable energy source (RES) such as a solar panel or a wind turbine. However, the stochastic input from the RES brings difficulty in balancing the energy supply and demand. In this study, a novel pricing scheme is proposed that provides robustness against such intermittent power input. The proposed scheme considers possible uncertainty in the marginal benefit and the marginal cost of the power market. It uses all available information on the power supply, power demand, and imbalanced energy. The parameters of the scheme are evaluated using an performance index. It is shown that the parameters can be obtained by solving a linear matrix inequality problem, which is efficiently solvable due to its convexity. Simulation examples are given to show the favorable performance of the proposed scheme in comparison with existing area control error pricing schemes.linear matrix inequality problem,area control error pricing schemes.

Dynamic Reactive Power Control of Islanded Microgrids

Abstract: Reactive power control is a fundamental issue in microgrids, especially during islanded mode operation with no support from the main grid. Lack of infinite bus, tightly coupled generation and consumption, and existence of nondispatchable intermittent renewable power sources reinforce the need for a new VVC scheme. This paper presents a new model predictive control (MPC)-based dynamic voltage and var control (VVC) scheme, which includes the dynamics of the microgrid in the VVC formulation. The MPC-based controller uses a simplified voltage prediction model to predict the voltage behavior of the system for a time horizon ahead. The advantage of this method is that it can avoid unstable voltage conditions in microgrids by prediction of the instability ahead of time. This method can also avoid voltage drops or swells in any of the phases of the system since the model can predict the voltage of each phase separately. Also, the presented method can be implemented online so it can efficiently use the time-variant reactive capabilities of the distributed generators to compensate for reactive power needs of the system. This controller is tested for different operating conditions of the microgrid and the simulation results confirm that the MPC controller successfully keeps the system stable and achieves a smooth voltage profile.

Harmonic and Unbalance Compensation Based on Direct Power Control for Electric Railway Systems

Abstract: This paper presents a general filtering and unbalance compensation scheme for electric traction systems using a direct power control-based algorithm. For a balanced three-phase three-wire system, the proposed method is able to control the power flow exchange between the grid and the load so that the instantaneous complex power is maintained constant. As a consequence, any nonlinear unbalanced load is seen by the three-phase supply as a balanced linear load. The proposed filter is evaluated on power substations with open delta (V-V) and Scott transformer feeders, and for two-level and dual-converter in the power stage. The scheme has been simulated and experimentally validated. The results from experimental and simulation tests show the controller advantages and the applicability of the proposed method in railway systems.

Voltage stability analysis of unbalanced distribution systems using backward/forward sweep load-flow analysis method with secant predictor

Abstract: This study presents a new method for tracing voltage power curves in unbalanced radial distribution power systems. The proposed method starts with the base case loading conditions and leading to the maximum loading point (MLP) whereas no ill-conditioning problems were detected. The calculation is performed by first-order polynomial secant predictor and the solution is corrected using the backward/forward radial power-flow method. Adaptive stepwise control is implemented to improve the overall solution process and reduce the number of the calculated points along the traced curve. The results calculated using the developed method are for both a 33-bus radial feeder and the unbalanced IEEE 13-node and IEEE123-node feeders. The results show that the developed method accurately traces the voltage power curves up to MLP. Comprehensive analysis on unbalanced scenarios shows that load unbalance greatly affects the stability limit of the study system.

Blind topology identification for power systems

Abstract: In this paper, the blind topology identification problem for power systems only using power injection data at each bus is considered. As metering becomes widespread in the smart grid, a natural question arising is how much information about the underlying infrastructure can be inferred from such data. The identifiability of the grid topology is studied, and an efficient learning algorithm to estimate the grid Laplacian matrix (i.e., the graph equivalent of the grid admittance matrix) is proposed. Finally, the performance of our algorithm for the IEEE-14 bus system is demonstrated, and the consistency of the recovered graph with the true graph associated with the underlying power grid is shown in simulations.

Dynamic Model Predictive-Based Energy Management of DG Integrated Distribution Systems

Abstract: This paper presents a new dynamic control strategy to control active and reactive power in distributed-generation integrated distribution systems. The distribution system comprises several microgrids that have a local energy-management system. The proposed method improves voltage and frequency profile in the distribution system by applying control action ahead of time. Moreover, the proposed method can operate with minimal topology information of the microgrid as it directly balances generation and consumption of power in the microgrid. Another advantage of the proposed method is that it can use fast and expensive sources, such as gas turbine generators, to balance power during transients and let slower and cheaper generators gradually take over after the transients are damped out. The proposed method can be implemented online. Therefore, it can efficiently use the time-variant reactive capabilities of the DGs to compensate reactive power needs of the system.