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Person# Konstantina Christakou

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Electric power distribution

Electric power distribution is the final stage in the delivery of electricity. Electricity is carried from the transmission system to individual consumers. Distribution substations connect to the tr

Demand response

Demand response is a change in the power consumption of an electric utility customer to better match the demand for power with the supply. Until the 21st century decrease in the cost of pumped stora

Content delivery network

A content delivery network, or content distribution network (CDN), is a geographically distributed network of proxy servers and their data centers. The goal is to provide high availability and perfo

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Konstantina Christakou, Xiang Gao, Mario Paolone, Fabrizio Sossan

Dispatching active distribution networks is expected to play an important role in the smart grid technologies. Voltage control is also starting to be widely proposed to avoid voltage violations in the electrical grid. The integration of the storage in the so called smart transformer (ST), which is a solid state transformer with control and communication functionalities, can help combine both the services. The added value of such a configuration is that it allows the full decoupling of the reactive power flows between the medium voltage (MV) and low voltage (LV) networks. We show the augmented flexibility of such a configuration by proposing a control strategy where, i, the storage system is used to achieve dispatched-by-design operation of the LV network active power flow, and, ii, the two ST power converters to control the voltage in both the MV and LV grid on a best effort basis. The control strategy is validated by simulations using the IEEE 34-node MV test feeder and the Cigre LV reference network. Moreover, control performance is benchmarked against a conventional network setup, where the BESS is connected to the LV network through a DC/AC power converter and the ST transformer is replaced by a conventional transformer.

2018Konstantina Christakou, Jean-Yves Le Boudec, Mario Paolone, Dan-Cristian Tomozei

The optimal power-flow problem (OPF) has always played a key role in the planning and operation of power systems. Due to the non-linear nature of the AC power-flow equations, the OPF problem is known to be non-convex, therefore hard to solve. During the last few years several methods for solving the OPF have been proposed. The majority of them rely on approximations, often applied to the network model, aiming at making OPF convex and yielding inexact solutions. Others, kept the non-convex nature of the OPF with consequent increase of the computational complexity, inadequateness for real time control applications and sub-optimality of the identified solution. Recently, Farivar and Low proposed a method that is claimed to be exact for the case of radial distribution systems under specific assumptions, despite no apparent approximations. In our work, we show that it is, in fact, not exact. On one hand, there is a misinterpretation of the physical network model related to the ampacity constraint of the lines’ current flows. On the other hand, the proof of the exactness of the proposed relaxation requires unrealistic assumptions and, in particular, (i) full controllability of loads and generation in the network and (ii) no upper-bound on the controllable loads. We also show that the extension of this approach to account for exact line models might provide physically infeasible solutions. In addition to the aforementioned convexification method, recently several contributions have proposed OPF algorithms that rely on the use of the alternating direction method of multipliers (ADMM). However, as we show in this work, there are cases for which the ADMM-based solution of the non-relaxed OPF problem fails to converge. To overcome the aforementioned limitations, we propose a specific algorithm for the solution of a non-approximated, non-convex OPF problem in radial distribution systems. In view of the complexity of the contribution, this work is divided in two parts. In this first part, we specifically discuss the limitations of both BFM and ADMM to solve the OPF problem.

Konstantina Christakou, Mario Paolone

Within the context of ancillary services for Active Distribution Networks (ADNs), application of intelligent control techniques is required in order to achieve specific operation objectives. Despite their differences, most control mechanisms proposed in the literature rely on the assumption that the Distribution Network Operator (DNO) has an accurate and upto- date model of the network topology and a complete knowledge of the line parameters, i.e., a correct network admittance matrix Y. However, this assumption does not always hold in reality due to both an incomplete knowledge of the grid asset and /or a physical change of the line parameters. In this work, we consider the problem of optimal voltage control in ADNs under uncertain, but bounded, line parameters with no assumptions on the parameters’ uncertainty distribution. In particular, availability of a monitoring infrastructure is assumed and the goal is to control the active and reactive power injections of a number of distributed generators connected to the network buses in coordination with the transformers on-load tap changers (OLTC). The optimal control problem is formulated as a mixedinteger linear problem by means of sensitivity coefficients and a robust optimization framework is used in order to account for the uncertainties in the network admittance matrix. In order to estimate the benefits of the proposed method, the evaluation of the algorithm is carried out by using both the IEEE 13-and the IEEE 34-nodes test feeder.

2018