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Medium Voltage Direct Current (MVDC) technology can be very helpful in meshing traditional AC distribution grids by means of multi-terminal DC links; this way, power flows among different feeders can be redistributed, as similar as transmission lines. Moreover, MVDC converters can easily interface renewable generators (e.g. photovoltaic farms) with AC, handling at the same time reverse power flows. This paper presents an overview of such technology, from initial concept and state-of-the-art to some innovative projects, including advanced protection schemes between MVDC links and existing distribution grid. The examples include active current limiter devices, hybrid circuit breakers and a communication layer for the coordination of these protection devices and power converters (logic selectivity), making MVDC a valuable support for AC grid improvement.

The boosting progress of HVDC (High Voltage Direct Current) technology developed for transmission grid applications can be exploited for distribution network needs as well. This has made raise a growing interest on the development of MVDC (Medium Voltage Direct Current) technology. The present paper provides an overview of the ongoing developments related to HVDC (High Voltage Direct Current) and MVDC (Medium Voltage Direct Current) in Italy. Particular attention is paid to HVDC projects and to MVDC studies that have been conducted in the latest years in Italy.

This paper assesses the development of a novel solid-state MVDC breaker and current limiter. Matlab/Simulink simulations are used as benchmark, then Real Time Control Hardware In the Loop (CHIL) simulations are performed. Same control algorithm is implemented into a microcontroller by using two different software languages: C and MicroPython. In both cases, high performances are achieved by using fast features like hardware interrupts and low level assembler code. Results are then compared in terms of simplicity, ease and speed of implementation and code readability, very important in fast prototyping activity.

This work presents the optimization process of real time control equipment for a Medium Voltage Direct Current (MVDC) static switch-limiter device. Description of the control strategy and its implementation into a microcontroller programmable board are described. System performances have been analyzed by real time Control Hardware in The Loop (CHIL) simulations. Initial results did not match expected performances. Control has been optimized by hardware and software features, like interrupt routines and low-level programming. Final tests showed a substantial improvement of system performances, like speed response and current limitation accuracy.

This work aims to investigate about integration of a Dual Active Bridge converter (DAB) developed for MV distribution lines into a traction system of an urban metro railway. Equipment is fed by 1500 V DC catenary and supplies active/passive loads at low voltage (750 V DC), providing galvanic insulation for safety reasons. Operation at very maximum voltage limits (up to 2 kV DC) and in case of MV drops and fluctuations have been simulated, focusing on system stability, control and proper sizing of passive filters to interface the converter with a storage device.

The design of an interface device for interconnecting a SMES (Superconducting Magnetic Energy Storage) to a LV distribution grid is carried out, in order to demonstrate the feasibility of a SMES for achieving, with the same device, power quality and critical load protection functions. The work is developed inside a three-year research project, called DRYSMES4GRID, funded by the Italian Minister of Economic Development and sees the cooperation of five partners (companies, universities, and research institutes). The aim of the project is to demonstrate the feasibility of the SMES (500 kJ/200 kW) in the short/medium term at competitive cost based on Magnesium Diboride (MgB2). In particular, in this article, after a brief state of art of the SMES applications for the grid, the design and the control logics of the DC/DC chopper for the power modulation of the energy stored in the SMES and of the threephase AC/DC inverter are shown. The activities are mainly based on digital simulations to evaluate the performances of this device in a LV distribution grid in presence of linear and non-linear loads, considering in particular the peak shaving function, while the device is connected to the mains, and the islanding operation condition after the detection of a voltage dip.

A three-year research project, called DRYSMES4GRID, was recently funded by the Italian Minister of Economic Development, Italy. The project, which involves five Italian partners (companies, universities, and research institutes), is aimed to demonstrate the feasibility of cost-competitive SMES based on magnesium diboride (MgB2) with a cryogen-free cooling by means of the manufacturing and the testing of a demonstrator with an objective rating of 500 kJ/200 kW. This rating is deemed suitable for disclosing the critical technological aspects of all components in view of practical applications. A further goal of the project is the assessment of the technical and economic benefits that the SMES can bring to real-world electric grids. The general outline and technical objectives of the project are presented in this paper. Preliminary design assumptions and results concerning the MgB2 coil and the power condition system are also discussed. © 2018 IEEE.

This paper presents a protection scheme for a medium voltage dc (MVDC) link, that meshes three different MVAC distribution grids through ac/dc power converters. The proposed scheme includes active current limiters, hybrid circuit breakers and a communication layer for the coordination of these protection devices and power converters (logic selectivity). Active current limiters, installed just after dc bulk capacitors of each power converter, ensure fault current limitation in order to protect the power converters until the fault clearance by dc grid protections. The coordination logic among the protection devices is described in this paper, together with notes on the design of those devices. In case of communication failure, the proposed scheme could not guarantee logic selectivity, but it is anyway able to clear the fault. The effectiveness of the proposed protection strategy is validated in a fast transients simulation environment, using detailed model of power electronic converters.

The paper is focused on the possible influence that some network characteristics (i.e. network configuration, type of neutral operation, geographical area, lines and cable length...) and environmental phenomena (lighting) may have on voltage dip performance in Medium Voltage (MV) networks. The investigation has been performed by applying both correlation techniques and Multiple Linear Regression analysis (MLR) on the voltage dips collected along several years of monitoring by the Italian MV network monitoring system QuEEN. As to the network characteristics and environmental factors used in the performed analysis they are referred to the data supplied in the past for the monitoring system and to the lighting data provided by the Italian lightning location system SIRF. The percentage contribution of each network and environmental factor to the voltage dips statistic, irrespective of the other parameters, have been evaluated by multiple linear regression methods. The results of these analysis confirm that the overhead line length has got a significant influence on the phenomena while cable lines length has, as expected, only a negligible effect. The influence of lightning seems to be not so critical as expected and this happen also with the neutral type of operation while voltage dips performances depend, in a non-negligible way, on the belonging to a geographical area.

Local control of Distributed Energy Resources (DER) in microgrids usually relies on droop algorithm in order to ensure stability to the system in islanded operation. Conventional and reverse droop control are applied respectively to Voltage Control Mode (VCM) inverters and to Current Control Mode inverters (CCM). In low-voltage microgrid, traditional P-ω and Q-V droop characteristic is subject to the power coupling and could lead to unstable operating conditions due to dependence of load on the voltage and on the frequency. Therefore P-V and Q-ω droop characteristic, named resistive droop, is used in simulations and in experimental tests to show its effectiveness for a stable and decoupled regulation of islanded LV microgrid.

Reporting and analysis procedures play an important role in power quality monitoring systems especially for long term surveys. As far as voltage dips are concerned, the network voltage dip performance can be effectively described in many different ways depending on the aims of the data presentations at both site and system level. The paper deals with network dip performance different representation methods focussing in particular on the way of presentation proposed by the working group UIE/CIGRE’/CIRED C4.110. This method has been applied to the Italian 2009 voltage dips statistic as acquired by the Italian power quality monitoring system named QuEEN. Contour charts have been drawn for both different percentage of the monitoring sites and voltage dips types.

This paper evaluates the economic sustainability of different premium power park (PPP) solutions – derived from two prototypes and a theoretical solution – and completes a previous technical analysis of the same PPP solutions. All of these are utility-provided solutions that improve the quality of the electricity delivered at medium voltage (MV) level. We calibrate the study on the potential customers of a PPP, which are MV users sensitive to power quality (PQ) events and badly served. As to PQ events, accidental interruptions and voltage dips are concerned. We evaluate the direct costs caused by PQ events through original and simple cost models developed using different approaches. Even though until today PPPs have not proved to be a winning idea, the results prospect possible future applications of this idea as a consequence of the increasing penetration of dispersed generation, maturation of renewables and developments of energy storage technologies.

One of the main issues concerning the Inverter based Distributed Generators (DGs) is the possibility that inverters could feed parts of the public grid, even when the grid is disconnected from the main power system. Such a condition, that is called unwanted island, is potentially critical both for safety and for consumers' operations. In order to avoid unwanted islands, it's mandatory equipping the generating plant with an Interface Protection (IP) which has to detect the islanding condition and, in this case, to disconnect the generator from the public grid. Several methods for identifying island condition have been proposed, both passive and active, each one characterized by its pros and cons. The standard IEC 62116 was promulgated with the aim of regulating a test procedure to evaluate the IP effectiveness of PhotoVoltaic (PV) inverters independently from the island detection method implemented. The paper discusses the test procedure proposed by the standard IEC 62116 and presents some experimental activities developed by RSE spa, aimed at the assessment of test feasibility and effectiveness.

The subject addressed is the assessment of the origin (High Voltage HV / Medium Voltage MV / Low Voltage LV) of voltage dips monitored on networks and its effect on both voltage dip statistics and indices evaluation at both national and macro area level from the point of view of regulation. The method applied to assess the HV origin of the events refers to both the signal coming from the distance protection of HV lines and to the correlation of the occurrence time of those events which have been monitored on MV bus bars belonging to different HV/MV Primary Substations but underlying a common part of the HV grid. The MV bus bars monitored in this research are those of the Italian MV network monitoring system QuEEN. This method has been partially extended also to the events propagation from the MV to the LV networks thanks to the late installation of a small number of Measurements Units (MU) at specific Secondary Substations in order to realize a sort of “disturbed network tree”. The estimation of the percentage of MV events with a probable HV origin at macro area level, for the 2010÷2011 period, has shown a range of variation of about 6%÷15% in comparison with the 37% evaluated at national level. As to the voltage dips performances, indices N2a and N3b, their evaluation at national level for 2011 is affected respectively by the 18,6% and 14,7% by voltage dips with a HV origin. As to the events propagation from the MV to the LV networks a good correspondence between the events monitored at the two voltage levels has been observed. (4 pages)

New problems concerning the distribution grid protection system arise from the large diffusion of installed distributed energy resources. In particular, unwanted trips, unwanted island operations and worsening of power quality are the main issues coming from the interactions among active networks and grid protections. A possible new protection scheme, which can be employed in active networks, is proposed and discussed in the paper. Logic selectivity is implemented introducing communications among grid and distributed generator systems protections. Digital simulations confirm the suitability of the proposed solution for a large set of grid faults.

The paper presents three different approaches for the control of grid connected inverters for Permanent Magnet Wind Generators. Their performances in facing grid unbalances are compared through digital simulations. For each control scheme, three alternative Fault Ride Through (FRT) strategies are considered and tested. From simulations, it emerges that, in addition to control expedients, some hardware devices are required for satisfying FRT requirements.