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The large penetration of distributed energy resources (DERs), especially photovoltaic (PV), in low-voltage (LV) distribution grids has raised the concern of unintentional islanding operations. The probability of unintentional islanding is increased mainly due to the newly introduced standards with extended frequency and voltage thresholds and the additional grid support functions, such as active and reactive power regulation using P/f and Q/V droop characteristics. Moreover, islanding operation due to faulty events at the medium-voltage (MV) distribution level may be dangerous in the presence of automatic reclosing procedure leading to out-of-phase reconnections. In this paper, an analysis of this phenomena is provided using simplified generator and load models. Simulation results based on MATLAB and DIgSILENT tools on a simplified LV distribution network are reported together with experimental results on a lab-scale prototype.

The ever growing penetration of Distributed Energy Resources (DERs), involving both conventional and renewable technologies, is changing the power system face. International Standards are being revised for including new requirements for DERs such as wider voltage amplitude and frequency protection thresholds and new local P=f and Q=V power regulation. This scenario has led to an increasing concern about the problem of uncontrolled islanding on distribution networks. The purpose of this paper is to describe a hardware in the loop (HIL) test-bed, where the effects of the DER inverter control actions and of the physical interface protection systems (IPSs) on the unintentional islanding can be addressed. The analysis has shown that the frequency measurement under transient conditions is very important, influencing the P=f DER control action and the IPS tripping. Thus, the preliminary results on the HIL test-bed suggest the need of standardization for the frequency measurement. This solution would be of fundamental importance to predict system behavior of distribution network having large DER diffusion and its protection system during unintentional islanding and other transient conditions.

National and internationals grid codes define the connection rules for passive and active users, guaranteeing the electrical power system stability and safety. Recently, the increasing diffusion of Distributed Generators from Renewable Energy Sources, mainly interfaced through electronic converters, has called for a drastic review of traditional approaches to distribution network operation. As a consequence, new regulating functions, aiming to support network stability by regulating local units power flows, have been defined, and others are under investigation. The paper discusses the role of existing and proposed stabilizing actions in terms of unintentional islanding identification, according to the Low Voltage interface protection characteristics.

Distributed Generation in Low Voltage grids has raised the concern of unintentional islanding and its probability has increased due to the newly introduced standards for generators, which in particular impose wider frequency and voltage ranges and active and reactive power support capabilities using of P/f and Q/V droop characteristics. Anti-islanding protections that each inverter is equipped with, may fail to detect the grid transition and so uncontrolled islanding operation may appear. This operation may be dangerous especially in presence of automatic reclosing procedure, because of possible out of phase reconnections. In this paper, the temporary unintentional islanding operation (e.g. below 600 ms) is studied considering the effects of P/f and Q/V droop characteristics of generators and their response times. A potential increase of such phenomenon will be shown with simulations and experimental results.

The number of small power distributed generators connected to the low voltage distribution networks has increased the possibility of unintentional islanding operations. In this paper, the influence of the Synchronous Reference Frame — Phase Locked Loop (SRF-PLL) for single-phase and three-phase inverter, combined with the wider frequency and voltage protection ranges stated by the Standards, has been considered as causes of increasing possibility of unintentional islanding events. The paper is focused on the enlargement of the protection system Non Detection Zone, due to the single phase PLL on a current- controlled Inverter, as typically used in PhotoVoltaic (PV) or Renewable Energy Source (RES) applications.

In many countries different technical rules have been introduced in order to integrate the DG units in the medium voltage (MV) and low voltage (LV) distribution network. New standards state requirements for all generators, included inverter based ones, imposing a Fault Ride Through (FRT) philosophy and new active and reactive power regulation actions with the aim of maintaining the transmission system stability at the level it was with traditional rotating generators, or even increasing it. These technical rules may not be fully compliant with the automatic reclosing cycle usually performed from the CB at the beginning of MV feeders and/or with the automatic procedure of faulty section selection and supply restore to healthy ones. In this paper the effects of DG units connected at MV and LV voltage level in relation to different fault conditions are analyzed. The possibility of temporary uncontrolled islanded operations are investigated in order to evaluate the role of new Standards or Regulatory or Law requirements in the automatic reclosing procedure and MV network automation.

The analysis of unintentional islanding in distribution grids with a large penetration of renewable energy sources is of increasing interest due to the recently introduced requirements of P/f and Q/V supporting capabilities of Distributed Energy Resources (DERs) and the wider frequency and voltage ranges, DERs should be able to operate with, before protection tripping. DER anti-islanding protections, if present, may fail to detect this grid transition and so uncontrolled islanding operation may appear. Since this operating mode may be dangerous for the electric network operation and may cause damages for the grid equipment, in this paper the increase of its risk due to P/f and Q/V droop characteristics of generators is addressed. Analytic results are initially derived for the steady-state unintentional islanding operation and then they are validated on a laboratory-scale grid setup. The proposed investigation will show that the risk of permanent unintentional islanding increases introducing P/f or Q/V droop regulation and that such risk is maximum including both the P/f and Q/V droop regulations, rather than only one of them.

The large penetration of Distributed Energy Re- sources (DERs), especially PhotoVoltaic (PV), in Low Voltage (LV) distribution grids has raised the concern of unintentional islanding operations. The probability of unintentional islanding is increased due to the newly introduced standards with extended frequency and voltage thresholds and additional grid support functions, such as active and reactive power regulation using P/f and Q/V droop characteristics, which can make ineffective the anti-islanding provisions each PV is equipped with. Moreover, islanding operation due to faulty events at the Medium Voltage (MV) distribution level may be dangerous in presence of auto- matic reclosing procedure leading to out of phase reconnections. In this paper, an analysis of this phenomena is provided using a simplified characterization of the generators and the loads. Simulation results based on Matlab and DIgSILENT tools on a simplified LV distribution network are reported together with preliminary experimental results on a lab-scale prototype.

The interest of unintentional islanding operation has recently grown in Low Voltage (LV) distribution grids because of the large penetration of Distributed Energy Resources (DERs), especially the ones based on the PhotoVoltaic (PV) energy source. The probability of unintentional islanding is increased also due to the newly introduced standards with extended frequency and voltage limits before the PV protection trips and due to the large number of PV generators, which can make ineffective the anti-islanding provisions each PV is equipped with. Moreover, interconnection standards of DERs require some additional grid support functions, such as active and reactive power regulation using P/f and Q/V droop characteristics, which can further increase the probability of unintentional islanding. In this paper, an analysis of this phenomena is provided using a simplified static and dynamic characterization of the generators and the loads. Simulation results based on Matlab and DIgSILENT tools on a simplified LV distribution network are reported together with preliminary experimental results on a lab-scale prototype.

In the last twenty years the Distributed Generation (DG) increased till a high level that introduced several issues to the classical management system of distribution networks. In many countries different technical rules have been introduced in order to integrate the DG units in the medium voltage (MV) and low voltage (LV) distribution network. New standards state requirements for all generators, included inverter based ones, imposing a Fault Ride Through (FRT) philosophy and new active and reactive power regulation actions with the aim of maintaining the transmission system stability at the level it was with traditional rotating generators, or even increasing it. These technical rules may not be fully compliant with the automatic reclosing cycle usually performed from the CB at the beginning of MV feeders and/or with the automatic selection procedure of faulty section selection and supply restore to healthy ones. In this paper the effects of DG units connected at MV and LV voltage level in relation to different fault conditions are analysed. The possibility of temporary uncontrolled islanded operations are investigated in order to evaluate the role of new Standards or Regulatory or Law requirements in the automatic reclosing procedure and MV network automation.

The increasing number of inverter-based Distributed Generator (DG) units connected to both MV and LV distribution networks can be used as a resource in order to overcome some emergency conditions. The presence of different inverter-based DG units could introduce issues regarding the management of intentional islanded networks, therefore reliable inverter control strategies for islanded operation assume an essential role. In this paper an inverter operating scheme has been implemented suitable in presence of networks supplied by a single DG or multiple DGs and for both grid tie and grid off operation. This inverter control scheme has been modeled and tested through detailed dynamic simulations in LV and MV islanded networks. Moreover the dynamic behavior of the proposed inverter control strategy has been studied and analyzed in light of the recently issued Italian connection grid codes for active users. The results show that the control scheme allows operation of intentional islanded networks supplied by single or multiple DG units. In case of multiple DG units, simulations show that, grid connection rules influence the dynamic of islanding operations depending on the power balance existing before the islanding forming.

The increasing penetration of Distributed Generation (DG) in distribution electrical systems may cause alteration of voltage profiles on the lines. Traditionally, distribution networks with radial configuration are designed as a passive top-down architecture where voltage regulation is mainly performed by an On-Load Tap Changer (OLTC) transformer located at the Primary Substation. Therefore it becomes rather difficult to compensate lines radiating out from the same bus-bar where some of these are subject to overvoltages due to the power injection by DGs connected whereas others are subject to voltage drops due to the presence of passive loads only. A first step in regulation and managing the voltage levels, in order to be consistent with the contingent needs of the network, may be a decentralized local control strategy performed by the DG units, however better results will be obtained with a coordinated OLTC intervention. In this paper an OLTC control strategy, based on only few remote measurements, is proposed. This procedure allows the tap changer controller to infer the state of network node voltages and to act accordingly. The performance of the proposed control method is demonstrated through simulations on a realistic MV distribution network.

The number of small power Distributed Generation (DG) units connected to the distribution networks has increased particularly for photovoltaic (PV) applications. According to recently issued Italian standards, each DG unit is allowed to participate in the active and reactive power regulation, using specifically defined P-f and Q-V droop characteristics. Furthermore, these standards have introduced wider frequency and voltage thresholds allowed during normal operation of DG units. Therefore the risk of unintentional islanding operations is expected to increase, due to the possibility of the DG units to supply the loads with a wider voltage and frequency range without the protections interventions. In this paper, the impact of the loads' voltage and frequency dependence on the isolated grid's frequency and voltage deviation, during unintentional islanding operations, has been considered. The system has been modeled and analyzed through detailed dynamics simulations. The results show that, using a classical grid-connected inverter control system, the loads' voltage and frequency dependence may play a crucial role in determining the unintentional islanding operation and possible failure of the protections intervention.