Riccardo Sgarbossa

Analysis of load-induced unintentional islanding in Low Voltage grids with PV generators

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.

Impact of non-simultaneous P/f and Q/V grid code requirements on PV inverters on unintentional islanding operation in distribution network

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.

A novel measurement-based procedure for load dynamic equivalent identification

The distribution network modeling is of great importance on power system analysis, considering both power quality and network stability. Computational effort reduction is one of the main targets for the network equivalent representation, even if results reliability remains a key point to be verified. In this paper an improved model is proposed, based on the most commonly used composite load models. Its main property is the generalization ability: the equivalent model can be applied to networks in various configurations, e.g. with different power absorption or different share of load types. A measurement-based procedure for load modeling is presented and discussed with simulations. Thanks to the combination of two optimization algorithms, namely the genetic algorithm and the grey-box approach, the procedure reaches high accuracy levels with a huge variety of load representations.

Temporary islanding operations of MV/LV active distribution networks under fault conditions

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.

Analysis of ΔP-ΔQ area of uncontrolled islanding in low voltage grids with PV generators

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 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 automatic 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.

A simplified algorithm for OLTC control in active distribution MV networks

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.

Analysis of interface protection system with Distributed Energy Resources compliant with the most recent grid codes: The unintentional islanding case

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.

Uncontrolled islanding operations of MV/LV active 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 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.

ΔP — ΔQ area assessment of temporary unintentional islanding with P/f and Q/V droop controlled PV generators in distribution networks

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.

Effects of P/f and Q/V regulations on anti-islanding detection methods in distribution networks

The diffusion of distributed energy resources (DERs) is continuously increasing and changing the structure of medium and low voltage (MV and LV) distribution networks. In this scenario the concern and the probability of unintentional islanding operations have increased due to the recently updated international standard for generator connection, which in particular require wider voltage and frequency protection thresholds and local P/f and Q/V to support the droop capabilities. Active ant-islanding detection methods have been widely used for grid-connected inverter-based DERs, such as frequency shift based protection systems. This work focuses on the performance of this active anti-islanding detection method combined with the DERs connection requirements stated by the most recent grid codes. Moreover, it describes a hardware in the loop (HIL) test-bed, where the effect of the active anti-islanding method and the droop curves on a physical interface protection system (IPS) have been tested. The obtained results emphasize the importance of having a coordination between the islanding protection system and the standard requirements for new certification procedure and connection rules for upcoming guidelines and standards.