Alberto Cerretti

Ground Fault Temporary Overvoltages in MV Networks: Evaluation and Experimental Tests

Single-phase-to-ground faults may cause substantial temporary overvoltages (TOVs) in large radial medium-voltage networks with isolated neutral, even over 3-p.u. phase to ground. Resonant neutral earthing limits these overvoltages to 1.8 p.u. but credible earthing apparatus failures might trigger TOVs up to 2.4 p.u. This paper presents the ground fault study of an Italian 20-kV ENEL Distribuzione network. Analytical evaluations in a wide parametric range of neutral earthing arrangements, include isolated neutral and ENEL resonant earthing with parallel resistance, as evidence of 2.4-p.u. TOVs with isolated neutral, 1.8 p.u. with resonant earthing, and more than 2.0 p.u. with partial compensation. Recordings of ground faults staged in the same network are presented, showing excellent agreement between analytical predictions and experimental test. The tests confirm TOVs of more than 2.3 p.u. with isolated neutral, sometimes evolving into cross-country faults (possibly explaining unforeseen cable fault rates), and the effectiveness of the ENEL neutral earthing practices in suppressing these TOVs.

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.

SDNO: Smart Distribution Network Operation Project

The increasing diffusion of dispersed generation (DG) offers many benefits to the electric system, such as the better exploitation of the renewable energy potentials and higher efficiency in conversion from primary energy sources. However, the technical challenges for a fully functional and efficient DG-based development of the electric system are still many. One of the main problems is related to the integration of large amounts of DG in the traditional passive distribution networks, designed for unidirectional power flows and where island operation is not possible. The main goal of the SDNO project is to define and develop an integrated protection, automation and control system that can meet several targets: to improve the performances of the existing system, to optimise the investment and the maintenance costs and to reduce the barriers to the integration of large amounts of DG. The project takes into account four macro-areas of intervention: the HV/MV substation, the MV network automation, the communication networks and the main SCADA system. The main development trends and the respective progresses of the project are here presented.

Temporary overvoltages due to ground faults in MV networks

The paper deals with the temporary overvoltages that build up in radial MV distribution networks following the inception of a 1-phase-to-ground fault (1-Φ -to-Gr). For extended cable/overhead MV distribution networks with ungrounded neutral, in case of low resistance faults at critical stretch of overhead lines, in [1] it has been evidenced that the temporary overvoltages on healthy phases can be very large, much higher than √3 p.u. (up to 3.5 p.u.). Fault currents can reach twice the value calculated with simplified methods, i.e. neglecting series impedances. In this paper the study is extended to MV networks with neutral grounded by both Petersen coil and compensating impedance (Petersen coil with a resistance in parallel), in normal operation and under contingency, i. e. in case of whole or partial loss of the compensating impedance. It is demonstrated that the presence of Petersen coil, stand alone or in parallel with a grounding resistance, drastically reduces the above temporary overvoltages at values not greater than 1.7-1.8 p.u. Application of simple derived formulas to the case of partial loss of the compensating neutral impedance show that overvoltages can be reduced at 1.8-5÷2.2 p.u., also in case of MV network having very high capacitive fault current (e.g. ≥300 A) and long overhead lines. An ATP case study on an existing 20kV large Enel-Distribuzione network reported in the paper confirm that the theorical predicted overvoltages are in the above mentioned range, and that the technical solutions adopted by Enel-Distribuzione [9-15] are able to limit in most cases the overvoltages at values not greater than 1.85 p.u.

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.

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.