Synchronization of Low Voltage Grids Fed by Smart and Conventional Transformers

The Smart Transformer (ST) is a power electronicsbased transformer, which operates as grid-forming converter in the low voltage-fed grid. It synthesizes the voltage waveform with magnitude, phase and frequency independently from the main power system. If a meshed operation of the ST with a conventional transformer is required, to improve the power flow control and to control the voltage profile, the voltage waveforms between the two grids have to be synchronized. The switching under different voltage magnitude, phase or frequency, can lead to a large power in-rush. This work proposes a synchronization strategy that enables a seamless transition of the ST to parallel operations with conventional transformers. Differently from classical communication-based methods, this work addresses a more realistic implementation case with limited communication infrastructure. The ST relies only on local measurements and on its advanced control capability to determine the effective switch to parallel operations. The performance of the proposed strategy has been proved analytically and through simulations in a PLECS/Matlab environment, and validated experimentally by means of Power-Hardware-In-Loop (PHIL) evaluation

Synchronization of Low Voltage Grids Fed by Smart and Conventional Transformers

The Smart Transformer (ST) is a power electronicsbased transformer, which operates as grid-forming converter in the low voltage-fed grid. It synthesizes the voltage waveform with magnitude, phase and frequency independently from the main power system. If a meshed operation of the ST with a conventional transformer is required, to improve the power flow control and to control the voltage profile, the voltage waveforms between the two grids have to be synchronized. The switching under different voltage magnitude, phase or frequency, can lead to a large power in-rush. This work proposes a synchronization strategy that enables a seamless transition of the ST to parallel operations with conventional transformers. Differently from classical communication-based methods, this work addresses a more realistic implementation case with limited communication infrastructure. The ST relies only on local measurements and on its advanced control capability to determine the effective switch to parallel operations. The performance of the proposed strategy has been proved analytically and through simulations in a PLECS/Matlab environment, and validated experimentally by means of Power-Hardware-In-Loop (PHIL) evaluation

Frequency-based control of a micro-grid with multiple renewable energy sources

Stand-alone micro-grids need a proper management of the active power exchange. This work is focused on the parallel operation of multiple grid-connected converters in an island-grid system. The proposed solution features a master inverter which emulates the grid and multiple grid-connected converters operating in parallel. The current sharing and overload protection is achieved by small frequency variations of master inverter's output, that are detected by the grid-connected converters. This mechanism exploits the behavior of the derating characteristics embedded in grid-connected inverters, that must reduce the output power if the grid frequency increases. In this case, standard grid-connected equipment can be used to realize micro-grids without the need of digital communication between the power units. Two possible scenarios are analyzed: low-power microgrid with master/slave converters, and low voltage grid fed by a Smart Transformer (ST) which performs the frequency control

Restoration of an active MV distribution grid with a battery ESS: A real case study

In order to improve power system operation, Battery Energy Storage Systems (BESSs) have been installed in high voltage/medium voltage stations by Distribution System Operators (DSOs) around the world. Support for restoration of MV distribution networks after a blackout or HV interruption is among the possible new functionalities of BESSs. With the aim to improve quality of service, the present paper investigates whether a BESS, installed in the HV/MV substation, can improve the restoration process indicators of a distribution grid. As a case study, an actual active distribution network of e-distribuzione, the main Italian DSO, has been explored. The existing network is located in central Italy. It supplies two municipalities of approximately 10,000 inhabitants and includes renewable generation plants. Several configurations are considered, based on: the state of the grid at blackout time; the BESS state of charge; and the involvement of Dispersed Generation (DG) in the restoration process. Three restoration plans (RPs) have been defined, involving the BESS alone, or in coordination with DG. A MATLAB®/Simulink® program has been designed to simulate the restoration process in each configuration and restoration plan. The results show that the BESS improves restoration process quality indicators in different simulated configurations, allowing the operation in controlled island mode of parts of distribution grids, during interruptions or blackout conditions. The defined restoration plans set the priority and the sequence of controlled island operations of parts of the grid to ensure a safe and better restoration. In conclusion, the results demonstrate that a BESS can be a valuable element towards an improved restoration procedure

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

Ancillary Services in Hybrid AC/DC Low Voltage Distribution Networks

In the last decade, distribution systems are experiencing a drastic transformation with the advent of new technologies. In fact, distribution networks are no longer passive systems, considering the current integration rates of new agents such as distributed generation, electrical vehicles and energy storage, which are greatly influencing the way these systems are operated. In addition, the intrinsic DC nature of these components, interfaced to the AC system through power electronics converters, is unlocking the possibility for new distribution topologies based on AC/DC networks. This paper analyzes the evolution of AC distribution systems, the advantages of AC/DC hybrid arrangements and the active role that the new distributed agents may play in the upcoming decarbonized paradigm by providing different ancillary services.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea (Programa Horizonte 2020) 76409