Improved Inner Approximation for Aggregating Power Flexibility in Active Distribution Networks and its Applications

Concise and reliable modeling for aggregating power flexibility of distributed energy resources in active distribution networks (ADNs) is a crucial technique for coordinating transmission and distribution networks. Our recent research has successfully derived an explicit expression for the exact aggregation model (EAM) of power flexibility at the substation level under linearized distribution network constraints. The EAM, however, is impractical for decision-making purposes due to its exponential complexity. In this paper, we propose an inner approximation method for aggregating flexibility in ADNs that utilizes the properties of the EAM to improve performance. Specifically, the geometric prototype of the inner approximation model is defined according to a subset of the coefficient vector set of the EAM, which enhances the accuracy. On the other hand, the computation efficiency of the inner approximation is also significantly improved by exploiting the regularity of coefficient vectors in the EAM in the parameter calculation process. The inner approximated flexibility model of ADNs is further incorporated into the security-constrained unit commitment problem as an application. Numerical simulations verify the effectiveness of the proposed method.Comment: 10 page

Improving Accuracy and Computational Efficiency of the Load Flow Computation of an Active/Passive Distribution Network

Over the last couple of decades, there has been a growing trend to make a paradigm shift from the passive distribution network to the active distribution network. With the rapid enlargement of network and installation of distributed generation (DG) units into distribution network, new technical challenges have arisen for load flow computation. The available techniques for the active distribution load flow calculation have limited scope of application and, sometimes, suffer from computational complexity. The complexity level of the distribution system power flow calculation is higher because of the issues of phase imbalance and high R/X ratios of feeder lines. The phase-imbalance increases computational complexity, whereas, the high R/X ratio makes time-consuming derivative based solver such as Newton-Raphson inviable for such large system. The motivation behind this work is to propose distinct mathematical approach for accurate modeling of network components, and loads to reduce computational time with improve accuracy. The applicability of an existing technique remains limited either by DG control modes, or by transformer configurations. The objective of this work is basically to develop an active distribution load flow (ADLF) algorithm with the following features. • Improved computational efficiency. • Applicability to any feeder network. • Accurate modeling of loads. • Applicability to different mode of operations of distributed generators (DGs). Typically, distributed generators are power-electronically interfaced sources that can be operated either in the current-balanced or in the voltage-balanced mode. The integration of DGs to the feeder network enables the distribution system to have bidirectional power exchange with the transmission grid. Which, also improve the voltage profile of the distribution network by providing additional sources of reactive power compensation. The contribution of the first work is to carry out the load flow analysis of a distribution network in the case of the dominant presence of induction motor loads. For a given operating condition, the load representation of an induction motor on the distribution network is made by analyzing its exact equivalent circuit. Thus, the induction motor is precisely represented as a voltage and frequency dependent load. The necessity of representing an induction motor by means of its precise load model is verified through a detailed case study. The convergence of the load flow solution with the precise modeling of induction motor loads is ensured by carrying out the load flow analysis over a complex distribution network containing several loops and distributed generations. The specific contribution of the second work is to improve the accuracy of the results obtained from the load flow analysis of a distribution network via forwardbackward sweeps. Specific attention is paid to the two-port modeling of a transformer with precise consideration for the zero sequence components of its port voltages. The zero sequence voltages at transformer ports are often ignored in the conventional load flow analyses. A new two-port network model is derived, which is generalized enough for the accurate representation of a transformer in the cascaded connection. Based upon the novel two-port representation made, a new set of iteration rules is established to carry out the forward-backward sweeps for solving the load flow results. All possible transformer configurations are taken into account. It is shown that the load flow analysis technique proposed is suitable for both active and passive distribution networks. The accuracy analysis of the load flow results is also carried out. For a given load flow result, by assessing the nodal current imbalances are evaluated based upon the admittance matrix representation of the network. Extensive case studies are performed to demonstrate the utility of the proposed load flow analysis technique. The contribution of the third work is to develop a computationally efficient and generalised algorithm for the load flow calculation in an active distribution network. The available techniques for the active distribution load flow calculation have limited scope of application and, sometimes, suffer from computational complexity. The applicability of an existing technique remains limited either by DG control modes or by transformer configurations. In this chapter, the load flow calculation is carried out by using the concept of Gauss-Zbus iterations, wherein the DG buses are modeled via the technique of power/current compensation. The specific distinctness of the proposed Gauss-Zbus formulation lies in overcoming the limitations imposed by DG control modes for the chosen DG bus modeling as well as in having optimized computational performance. The entire load flow calculation is carried out in the symmetrical component domain by decoupling all the sequence networks. Furthermore, a generalised network modeling is carried out to define decoupled and tap-invariant sequence networks along with maintaining the integrity of the zero sequence network under any transformer configurations.The computational efficiency and accuracy of the methodology proposed are verified through extensive case studies. The contribution of the fourth work is to identify and eliminate unnecessary itvii eration loops in the load flow analysis of an active distribution network so as to improve its overall computational efficiency. The number of iteration loops is minimized through the integrated modeling of a distributed generator (DG) and the associated coupling transformer. The DG bus is not preserved in the load flow calculation and the aforementioned DG-transformer assembly is represented in the form of a voltage dependent negative load at the point of connection to the main distribution network. Thus, the iteration stage that is involved in indirectly preserving the DG in the form of a voltage source or negative constant power load can be got rid of. This, in turn, eliminates the need for multiple rounds of forward-backward sweep iterations to determine the bus voltages. The power characteristics of the DG-transformer assembly are thoroughly investigated through a carefully performed case study so as to assess the potential convergence performance of the proposed

Distributed N-body Simulation on the Grid Using Dedicated Hardware

We present performance measurements of direct gravitational N -body simulation on the grid, with and without specialized (GRAPE-6) hardware. Our inter-continental virtual organization consists of three sites, one in Tokyo, one in Philadelphia and one in Amsterdam. We run simulations with up to 196608 particles for a variety of topologies. In many cases, high performance simulations over the entire planet are dominated by network bandwidth rather than latency. With this global grid of GRAPEs our calculation time remains dominated by communication over the entire range of N, which was limited due to the use of three sites. Increasing the number of particles will result in a more efficient execution. Based on these timings we construct and calibrate a model to predict the performance of our simulation on any grid infrastructure with or without GRAPE. We apply this model to predict the simulation performance on the Netherlands DAS-3 wide area computer. Equipping the DAS-3 with GRAPE-6Af hardware would achieve break-even between calculation and communication at a few million particles, resulting in a compute time of just over ten hours for 1 N -body time unit. Key words: high-performance computing, grid, N-body simulation, performance modellingComment: (in press) New Astronomy, 24 pages, 5 figure

Distributed SIR-Aware Opportunistic Access Control for D2D Underlaid Cellular Networks

In this paper, we propose a distributed interference and channel-aware opportunistic access control technique for D2D underlaid cellular networks, in which each potential D2D link is active whenever its estimated signal-to-interference ratio (SIR) is above a predetermined threshold so as to maximize the D2D area spectral efficiency. The objective of our SIR-aware opportunistic access scheme is to provide sufficient coverage probability and to increase the aggregate rate of D2D links by harnessing interference caused by dense underlaid D2D users using an adaptive decision activation threshold. We determine the optimum D2D activation probability and threshold, building on analytical expressions for the coverage probabilities and area spectral efficiency of D2D links derived using stochastic geometry. Specifically, we provide two expressions for the optimal SIR threshold, which can be applied in a decentralized way on each D2D link, so as to maximize the D2D area spectral efficiency derived using the unconditional and conditional D2D success probability respectively. Simulation results in different network settings show the performance gains of both SIR-aware threshold scheduling methods in terms of D2D link coverage probability, area spectral efficiency, and average sum rate compared to existing channel-aware access schemes.Comment: 6 pages, 6 figures, to be presented at IEEE GLOBECOM 201

An Iterative Quality-Based Localization Algorithm for Ad Hoc Networks

An iterative quality-based algorithm for location discovery is presented which can be used in wireless ad hoc sensor networks. The algorithm will take the reliability of measurements into account and will produce a reliability index for every estimated location using a statistical approach. The algorithm can also work in a hybrid network with different kinds of distance measuring techniques. It will use the reliability of each of these methods in the final result. Satisfactory results can be achieved with this approach

Power quality and electromagnetic compatibility: special report, session 2

The scope of Session 2 (S2) has been defined as follows by the Session Advisory Group and the Technical Committee: Power Quality (PQ), with the more general concept of electromagnetic compatibility (EMC) and with some related safety problems in electricity distribution systems. Special focus is put on voltage continuity (supply reliability, problem of outages) and voltage quality (voltage level, flicker, unbalance, harmonics). This session will also look at electromagnetic compatibility (mains frequency to 150 kHz), electromagnetic interferences and electric and magnetic fields issues. Also addressed in this session are electrical safety and immunity concerns (lightning issues, step, touch and transferred voltages). The aim of this special report is to present a synthesis of the present concerns in PQ&EMC, based on all selected papers of session 2 and related papers from other sessions, (152 papers in total). The report is divided in the following 4 blocks: Block 1: Electric and Magnetic Fields, EMC, Earthing systems Block 2: Harmonics Block 3: Voltage Variation Block 4: Power Quality Monitoring Two Round Tables will be organised: - Power quality and EMC in the Future Grid (CIGRE/CIRED WG C4.24, RT 13) - Reliability Benchmarking - why we should do it? What should be done in future? (RT 15