Showing papers on "Power-flow study published in 2018"

PyPSA: Python for Power System Analysis

Abstract: Python for Power System Analysis (PyPSA) is a free software toolbox for simulating and optimising modern electrical power systems over multiple periods. PyPSA includes models for conventional generators with unit commitment, variable renewable generation, storage units, coupling to other energy sectors, and mixed alternating and direct current networks. It is designed to be easily extensible and to scale well with large networks and long time series. In this paper the basic functionality of PyPSA is described, including the formulation of the full power flow equations and the multi-period optimisation of operation and investment with linear power flow equations. PyPSA is positioned in the existing free software landscape as a bridge between traditional power flow analysis tools for steady-state analysis and full multi-period energy system models. The functionality is demonstrated on two open datasets of the transmission system in Germany (based on SciGRID) and Europe (based on GridKit). Funding statement: This research was conducted as part of the CoNDyNet project, which is supported by the German Federal Ministry of Education and Research under grant no. 03SF0472C. The responsibility for the contents lies solely with the authors

A Unified Control and Power Management Scheme for PV-Battery-Based Hybrid Microgrids for Both Grid-Connected and Islanded Modes

Abstract: Battery storage is usually employed in photovoltaic (PV) system to mitigate the power fluctuations due to the characteristics of PV panels and solar irradiance. Control schemes for PV-battery systems must be able to stabilize the bus voltages as well as to control the power flows flexibly. This paper proposes a comprehensive control and power management system (CAPMS) for PV-battery-based hybrid microgrids with both ac and dc buses, for both grid-connected and islanded modes. The proposed CAPMS is successful in regulating the dc and ac bus voltages and frequency stably, controlling the voltage and power of each unit flexibly, and balancing the power flows in the systems automatically under different operating circumstances, regardless of disturbances from switching operating modes, fluctuations of irradiance and temperature, and change of loads. Both simulation and experimental case studies are carried out to verify the performance of the proposed method.

Runtime Semantic Security Analysis to Detect and Mitigate Control-Related Attacks in Power Grids

Abstract: In this paper, we analyze control-related attacks in supervisory control and data acquisition systems for power grids. This class of attacks introduces a serious threat to power systems, because attackers can directly change the system’s physical configuration using malicious control commands crafted in a legitimate format. To detect such attacks, we propose a semantic analysis framework that integrates network intrusion detection systems with a power flow analysis capable of estimating the execution consequences of control commands. To balance detection accuracy and latency, the parameters of the power flow analysis algorithm are dynamically adapted according to real-time system dynamics. Our experiments on IEEE 24-bus, 30-bus, and 39-bus systems and a 2736-bus system demonstrate that by opening three transmission lines, an attacker can put the tested system into an insecure state, and the semantic analysis can complete detection in 200 ms for the large-scale 2736-bus system with about 0.78% false positives and 0.01% false negatives, which allow for timely responses to intrusions.

A Generalized Approach to the Load Flow Analysis of AC–DC Hybrid Distribution Systems

Abstract: This paper proposes a unified load flow (LF) model for AC–DC hybrid distribution systems (DSs). The proposed model can be applied in hybrid DSs with mixed configurations for AC/DC buses and AC/DC lines. A new classification of DS buses is also introduced for LF analysis. A set of generic LF equations has been derived based on comprehensive analysis of the possible AC–DC hybrid system configurations. Three binary matrices, which are used as a means of describing the configuration of the AC and DC buses and lines, have been employed in the construction of the unified power equations. These matrices enable a single configuration at a time to be activated in the power equations. The proposed LF model is generic and can be used for both grid-connected and isolated hybrid DSs. The new model has been tested using several case studies of hybrid DSs that include different operational modes for the AC and DC distributed generators. As a means of evaluating the effectiveness and accuracy of the proposed model, the LF solution was compared to the solution produced by PSCAD/EMTDC. A comparison of the results reveals the efficacy of the proposed model.

Optimal capacitor placement in radial distribution systems using flower pollination algorithm

Abstract: In this paper, a new metaheuristic algorithm that mimics the pollination process of flowers and is known as the flower pollination algorithm (FPA) has been proposed for the solution of the optimal capacitor placement (OCP) problem in a radial distribution system (RDS). The objective of this problem is to minimize the total power loss and cost of capacitor installation by optimal location and sizing of the capacitors. OCP also improves the voltage profile. In this work, the power flow and losses in the network are obtained with the help of load flow analysis using data structures. The proposed FPA approach has been applied to solve 33-, 34-, 69- and 85 bus RDSs. The results obtained are compared with those of the analytical method, fuzzy real coded genetic algorithm, two stage fuzzy approach and teaching learning based optimization algorithms. The comparison shows that FPA is a highly suitable optimization approach for solving capacitor placement problems.

Benchmarking of Constant Power Generation Strategies for Single-Phase Grid-Connected Photovoltaic Systems

Abstract: With a still increase of grid-connected photovoltaic (PV) systems, challenges have been imposed on the grid due to the continuous injection of a large amount of fluctuating PV power, like overloading the grid infrastructure (e.g., transformers) during peak power production periods. Hence, advanced active power control methods are required. As a cost-effective solution to avoid overloading, a constant power generation (CPG) control scheme by limiting the feed-in power has been introduced into the currently active grid regulations. In order to achieve a CPG operation, this paper presents three CPG strategies based on a power control method (P-CPG), a current limit method (I-CPG), and the perturb and observe algorithm (P&O-CPG). However, the operational mode changes (e.g., from the maximum power point tracking to a CPG operation) will affect the entire system performance. Thus, a benchmarking of the presented CPG strategies is also conducted on a 3-kW single-phase grid-connected PV system. Comparisons reveal that either the P-CPG or I-CPG strategies can achieve fast dynamics and satisfactory steady-state performance. In contrast, the P&O-CPG algorithm is the most suitable solution in terms of high robustness, but it presents poor dynamic performance.

Steady-State Power Flow Model of Energy Router Embedded AC Network and Its Application in Optimizing Power System Operation

Abstract: The energy router is an emerging device concept that is based on an advanced power electronic technique. It is able to realize flexible and dynamic electric power distribution in power systems analogous to the function of information routers in the Internet. It is of great interest to investigate how the energy router can be used to optimize power system operation. This paper formulates the steady-state power flow model of the energy router embedded system network and the related optimal power flow formulation. The role of the energy router in providing extra flexibility to optimize the system operation is studied. Case studies are carried out on a modified IEEE RTS-79 system and a modified IEEE 118 bus system with the energy router. The results show that the energy router is able to optimize the operation of the power system through controlling the power injections and voltage of ports of the energy router. Operating objective such as adjusting branch power flow, improving bus voltage, and reducing active power losses of the grid can be reached under different objective functions.

Analysis of Main Topologies of Shunt Active Power Filters Applied to Four-Wire Systems

Abstract: This paper presents a complete comparative study about the five most common configurations of shunt active power filter applied to four-wire distribution systems. In order to evaluate the applicability of the studied topologies, two scenarios of four-wire loads are investigated differing by level of zero-sequence harmonic contents. The analysis of simulation steady-state results comprise dc-link capacitor voltage and current stresses, harmonic distortion in grid currents, and semiconductor losses. The results show that structures with fourth inductor generate higher number of levels at output converter voltage. However, when they operate by compensating high levels of zero-sequence harmonics, they can present higher dc-link voltage and lower efficiency. Furthermore, system models, control strategy, and experimental results are presented as well.

Robust Dispatch of High Wind Power-Penetrated Power Systems Against Transient Instability

Abstract: High-level wind power integration can dramatically affect a power system's dynamic performance and introduce significant uncertainties to system's operation. This paper proposes a robust dispatch method to optimize the power system's operation state while sustaining its transient stability with highly variable and stochastic wind power generation. The problem is first modeled as an augmented optimal power flow model with uncertain variables and differential-algebraic equations. Then, the stability constraints are converted to approximately-equivalent algebraic equations based on one-machine-infinite-bus equivalence technique and trajectory sensitivity analysis. Next, the uncertain wind power generation is represented by a small number of strategically selected testing scenarios. Finally, a decomposition-based computation strategy is developed to divide the original model into a master problem and a series of slave problems which are solved iteratively. Using industry-grade system dynamic models and simulation software, the proposed method is tested on the New England 39-bus system and Nordic32 system, showing high performance on economic optimality, stability robustness, and computational efficiency.

A Two-Stage Approach for Network Constrained Unit Commitment Problem With Demand Response

Abstract: This paper proposes a two-stage formulation for the day-ahead energy scheduling problem with demand response (DR). The first stage solves a network-constrained unit commitment problem with DR, to determine the hourly net demand changes (i.e., difference between final and initial demand values) happening at each DR bus 1 along with the unit commitment schedule and ac load flow solution. Here, the objective is to maximize the social welfare which is expressed as the total utility of the demand side minus the total generation cost. The second stage solves an incentive or penalty minimization problem to determine the demand shifting and demand curtailment across the 24-h period at each DR bus, offering DR, based on the hourly net demand changes obtained during the first stage. The proposed formulation shows how demand shift and demand curtailment happening at different DR buses can be traced back to the hourly net demand changes occurring at the system level. The results, presented for a six-bus system and IEEE 118 bus system, show the benefits of including DR into the network-constrained unit commitment problem according to the proposed formulation. 1 Any bus which is capable of offering DR will be referred to as DR bus. It should not be mistaken for PQ bus referred in the load flow analysis.

Impact of Plug-in Electric Vehicles Integrated into Power Distribution System Based on Voltage-Dependent Power Flow Analysis

Abstract: This paper proposes the impact of plug-in electric vehicles (PEVs) integrated into a power distribution system based on voltage-dependent control. The gasolinegate situation has many people turning to electric vehicles as a more environmentally friendly option, especially in smart community areas. The advantage of PEVs is modern vehicles that can use several types of fuel cells and batteries as energy sources. The proposed PEVs model was developed as a static load model in power distribution systems under balanced load conditions. The power flow analysis was determined by using certain parameters of the proposed electrical network. The main research objective was to determine the voltage magnitude profiles, the load voltage deviation, and total power losses of the electrical power system by using the new proposed methodology. Furthermore, it investigated the effects of the constant power load, the constant current load, the constant impedance load, and the plug-in electric vehicles load model. The IEEE 33 bus system was selected as the test system. The proposed methodology assigned the balanced load types in a steady state condition and used the new methodology to solve the power flow problem. The simulation results showed that increasing the plug-in electric vehicles load had an impact on the grids when compared with the other four load types. The lowest increased value for the plug-in electric vehicles load had an effect on the load voltage deviation (0.062), the total active power loss (120 kW) and the total reactive power loss (80 kVar), respectively. Therefore, this study verified that the load of PEVs can affect the electrical power system according to the time charging and charger position. Therefore, future work could examine the difference caused when PEVs are attached to the electrical power system by means of the conventional or complex load type.

An Interval Power Flow Analysis Through Optimizing-Scenarios Method

Abstract: Interval power flow (IPF) is a special kind of uncertain power flow problem whose demand load and active power generation are regarded as interval uncertainties. A novel optimizing-scenarios method (OSM) is envisaged in this paper to solve this problem to obtain more accurate results. The OSM includes two kinds of approaches, namely the minimum and maximum programming models, in which the interval uncertainties are regarded as variables with varying bounds, and the objective function under study is set to determine these unknown variables. By solving these nonlinear programming models through the interior point method, the changing variables of the IPF problems are determined. Performance of the proposed approach is compared with that of previously established methods, including the affine arithmetic-based method as well as Monte Carlo simulation method, and the overall simulation results demonstrate the effectiveness and robustness of the proposed OSM.

A Generic Modeling and Power-Flow Analysis Approach for Isochronous and Droop-Controlled Microgrids

Abstract: This paper proposes a generic steady-state modeling and power-flow analysis approach for droop- and isochronously controlled microgrids. The proposed framework adopts symmetrical sequence component models, rather than phase-coordinate models, of microgrid elements. Such approach immensely reduces the power-flow execution time, as it breaks down the system model into independent equation sets with considerably reduced sizes. To render the proposed approach practical and generic, it integrates different types and control schemes of distributed generation (DG), including synchronous generator-based DG (SGDG) and electronically interfaced DG units. Furthermore, it incorporates unbalanced loads and feeders, transformer connections, different load characteristics, and configurations, as well as microgrid droop features. A novel power-flow algorithm based on a modified Newton–Raphson method is proposed to solve for the microgrid steady-state voltage magnitudes, angles, and frequency. The accuracy of the models and algorithm is verified through comparison with detailed time-domain simulations in MATALB/Simulink. Additionally, the proposed approach is shown to outperform the reported Newton-Trust Region approach in generality, accuracy, and performance. Two case studies, incorporating IEEE 123-node test microgrid, are further performed to examine the effectiveness of the proposed approach in solving complicated droop-controlled microgrids, and to examine the behavior of droop-controlled DGs in isochronous microgrids.

Electrical modelling of a DC railway system with multiple trains

Abstract: Electrical modelling of rail tracks with multiple running trains is complex due to the difficulties in solving the power flow. The train positions, speed and acceleration are constantly varying resulting in a nonlinear system. In this work, a method is proposed for modelling DC electric railways to support power flow analysis of a simulated metro train service. The method exploits the MathWorks simulation tool Simscape, using it to model the mechanical and electrical characteristics of the rail track system. The model can be simulated to provide voltages at any position in the track and additionally, the voltages seen by any train. The model includes regenerative braking on trains, this is demonstrated to cause overvoltage in the feeding line if it is higher than the power demand of the other trains at that time. Braking resistors are used to protect the network from overvoltage by burning the excess energy. Through the implementation of Energy Storage Systems (ESSs), it will be possible to improve the energy efficiency and remove timetabling restrictions of electric railways by effectively controlling the rail track voltage. The paper proposes several methods to validate the model.

Power system state estimation via feasible point pursuit: Algorithms and Cramér-rao bound

Abstract: Accurately monitoring the system's operating point is central to the reliable and economic operation of an autonomous energy grid. Power system state estimation (PSSE) aims to obtain complete voltage magnitude and angle information at each bus given a number of system variables at selected buses and lines. Power flow analysis amounts to solving a set of noise-free power flow equations, and is cast as a special case of PSSE. Physical laws dictate quadratic relationships between available quantities and unknown voltages, rendering general instances of power flow and PSSE nonconvex and NP-hard. Past approaches are largely based on gradient-type iterative procedures or semidefinite relaxation (SDR). Due to nonconvexity, the solution obtained via gradient-type schemes depends on initialization, while SDR methods do not perform as desired in challenging scenarios. This paper puts forth novel feasible point pursuit (FPP)-based solvers for power flow analysis and PSSE, which iteratively seek feasible solutions for a nonconvex quadratically constrained quadratic programming reformulation of the weighted least-squares (WLS). Relative to the prior art, the developed solvers offer superior numerical performance at the cost of higher computational complexity. Furthermore, they converge to a stationary point of the WLS problem. As a baseline for comparing different estimators, the Cramer-Rao lower bound is derived for the fundamental PSSE problem in this paper. Judicious numerical tests on several IEEE benchmark systems showcase markedly improved performance of our FPP-based solvers for both power flow and PSSE tasks over popular WLS-based Gauss-Newton iterations and SDR approaches.

Uncertain Power Flow Analysis Based on Evidence Theory and Affine Arithmetic

Abstract: A novel uncertain power flow analysis method is proposed in this letter. It utilizes evidence theory to fuse probabilistic, possibilistic, and interval uncertainties. Besides, the extended affine arithmetic is applied to handle variable dependencies existed in power flow. The final analysis results are represented as probability boxes (p-boxes). The comparisons with Monte Carlo simulation are given to demonstrate the validity of the proposed method.

Generalized Microgrid Power Flow

Abstract: Power flow analysis for islanded microgrid is a challenging problem due to the lack of means to incorporate the hierarchical control effect. This letter bridges the gap by devising a generalized microgrid power flow (GMPF). The novelty of GMPF includes: 1) it introduces the generalized distributed generator (DG) bus and the adaptive swing bus to model the DGs’ behaviors; 2) the droop-based power flow is used to initialize the secondary control adjustment; and 3) three types of secondary control modes are developed within a double loop framework. Test results validate the effectiveness and excellent convergence performance of GMPF.

A Fully Parallel Stochastic Multiarea Power System Operation Considering Large-Scale Wind Power Integration

Abstract: Multiarea power system operation/coordination is used to increase the reliability of the interconnected power grids and maintain the consistency of the price across the integrated power systems. However, the implementation of this coordinated operation is facing challenges due to increase in size and complexity of the modern power systems, high penetration of the volatile renewable energy, and interdependence issues among various power systems. To address these concerns, this paper presents a fully parallel decision-making approach for the day-ahead scheduling of interconnected power systems with large-scale wind power integration while respecting the information privacy between different systems/areas. In the proposed parallel approach, each system/area solves its day-ahead scheduling problem along with its local subproblems for different wind generations’ scenarios, and sends its equivalent (or processed) boundary information to other systems/areas. The proposed inter-regional coordination among systems/areas and intra-regional coordination between scenarios in each system/area will continue until the tie-line power flows and the generation outputs of generating units get converged. The modified IEEE 118-bus testing system is used in this paper to show the effectiveness of the proposed approach.

Probabilistic Power Flow Analysis Using Multidimensional Holomorphic Embedding and Generalized Cumulants

Abstract: This paper proposes a new analytical probabilistic power flow (PPF) approach for power systems with high penetration of distributed energy resources. The approach solves probability distributions of system variables about operating conditions. Unlike existing analytical PPF algorithms in literature, this new approach preserves nonlinearities of ac power flow equations and retain more accurate tail effects of the probability distributions. The approach first employs a multidimensional holomorphic embedding method to obtain an analytical nonlinear ac power flow solution for concerned outputs such as bus voltages and line flows. The embedded symbolic variables in the analytical solution are the inputs such as power injections. Then, the approach derives cumulants of the outputs by a generalized cumulant method, and recovers their distributions by Gram–Charlier expansions. This PPF approach can accept both parametric and nonparametric distributions of random inputs and their covariances. Case studies on the IEEE 30-bus system validate the effectiveness of the proposed approach.

Achieving Multiple Functions of Three-Phase Electric Springs in Unbalanced Three-Phase Power Systems Using the Instantaneous Power Theory

Abstract: Three-phase electric spring (3-ph ES) has recently been proposed as a fast demand response technology for applications in unbalanced power systems fed with a mixture of conventional and renewable power generation. Using the instantaneous power theory as the theoretical framework, this paper presents the criteria and conditions for minimizing the average and oscillating power of the 3-ph ES for the first time. A detailed analysis of the use of 3-ph ES is included for providing multiple control objectives of voltage regulation and power balancing of the 3-ph power system, and minimization of the average and oscillating ac power of the ES. A corresponding control scheme implementable in a single controller is included and explained. The control scheme has been practically verified with experiments.

Joint Load Scheduling and Voltage Regulation in the Distribution System With Renewable Generators

Abstract: By equipping with the advanced smart meters and two-way communications infrastructure, smart grids, as a key component of future smart cities, are able to improve the energy efficiency and reduce the energy cost through real-time monitoring and customer load scheduling. However, the high penetration of intermittent renewable energy such as solar power may cause frequent overvoltage and undervoltage problems at certain buses, making the load scheduling face new challenges on voltage regulation. In this paper, we investigate the impact of voltage constraints on load scheduling by power flow analysis in a power distribution system with renewable generators. A voltage regulator (VR) is introduced to regulate the voltage of buses in the distribution system and assist load scheduling. To jointly minimize the cost and stabilize the voltages of the distribution system, we propose a grid-customer coordinated load scheduling strategy, which simultaneously determines the tap changes of the VR and scheduling of customer electricity loads in each time slot. Finally, we evaluate the performance of the proposed strategy based on realistic power demand and renewable energy generation datasets. Extensive numerical results demonstrate that the proposed strategy can remarkably reduce the energy cost and stabilize the voltage fluctuation of distribution systems.

Optimal Reactive Power Flow of Interconnected Power System Based on Static Equivalent Method Using Border PMU Measurements

Abstract: A new static equivalent method using border PMU measurements is proposed to effectively retain various voltage and reactive power support characteristics of external network without real-time synchronous information of the entire external system. Based on the proposed method, a set of equivalent criteria is developed to determine whether the simplest and the most widely used PV/PQ equivalence is still appropriate. Based on the proposed equivalent method and criteria, we establish a new equivalent optimal reactive power flow model, which can achieve high calculation accuracy for interconnected power systems. The simulation results of the IEEE 39-bus system and an actual 661-bus system demonstrate the accuracy and effectiveness of the proposed method, criteria, and model.

Lightning-Induced Voltages on Complex Power Systems by Using CiLIV: The Effects of Channel Tortuosity

Abstract: Recently, Andreotti et al. (2015) have presented a new tool for lightning induced voltage calculations on power networks. This tool, named Circuit for Lightning Induced Voltage (CiLIV), has been initially developed for straight and vertical lightning channels; afterwards, the tool has been extended to evaluate induced voltages produced by tortuous lightning channels. However, in the aforementioned studies, only simple power network configurations have been examined. Therefore, aim of this paper is twofold: 1) to apply CiLIV to more complex network configurations, that is, complex networks equipped with transformers, surge arresters, groundings, and so on; 2) to investigate the effects of channel tortuosity on the voltages induced in these complex networks.

Robust Convergence of Power Flow Using TX Stepping Method with Equivalent Circuit Formulation

Abstract: Robust solving of critical large power flow cases (with 50k or greater buses) forms the backbone of planning and operation of any large connected power grid. At present, reliable convergence with applications of existing power flow tools to large power systems is contingent upon a good initial guess for the system state. To enable robust convergence for large scale systems starting with an arbitrary initial guess, we extend our equivalent circuit formulation for power flow analysis to include a novel continuation method based on transmission line (‘Tx’) stepping. While various continuation methods have been proposed for use with the traditional ‘PQV’ power flow formulation, these methods have either failed to completely solve the problem or have resulted in convergence to a low voltage solution. The proposed “Tx Stepping” method in this paper demonstrates robust convergence to the high voltage solution from an arbitrary initial guess. Example systems, including 75k+ bus test cases representing different loading and operating conditions for Eastern Interconnection of the U.S. power grid, are solved from arbitrary initial guesses.

Virtual Multi-Slack Droop Control of Stand-Alone Microgrid With High Renewable Penetration Based on Power Sensitivity Analysis

Abstract: Recently, microgrids have faced a great challenge for the stable operation of converter-dominant system with very high renewable penetration. The converter-based-generator reduces inertia constant. In this case, very small power imbalance can cause large frequency variation. Therefore, the accurate load sharing between generators is important for the stable operation. This paper proposes the virtual multi-slack (VMS) droop control based on the power sensitivity analysis. In other words, one physical slack generator directly controls the magnitude and phase angle of its bus voltage. At the same time, the other generators indirectly control the magnitudes and phase angles of their bus voltages by using the VMS droop control. This results in the proper load sharing between the generators in the stand-alone microgrid. That is, all nonslack generators operate as virtual slacks so that they solve the power imbalances together with one physical slack generator. The proposed VMS control is realized with the multi-slack power flow analysis, which enables adaptive droop based power allocation between the generators. Then, its effectiveness is verified with several case studies on the practical stand-alone microgrid in South Korea by using the electromagnetic transient program.