Giovanni Valtorta

The ADDRESS project: Developing Active Demand in smart power systems integrating renewables

The ADDRESS European project aims to develop a comprehensive commercial and technical framework for the development of “Active Demand” and the market-based exploitation of its benefits. In ADDRESS, “Active Demand” (AD) means the active participation of domestic and small commercial consumers in the electricity markets and in the provision of services to the other electricity system participants. This paper gives an overview of the results obtained so far in the project and more specifically the services that AD can provide to the different regulated and deregulated electricity system participants, the aggregation of demand flexibility, the technical validation performed by distribution and transmission system operators, the overall technical and commercial architecture. The paper also describes how Active Demand can be used to support the integration of Renewables and more generally Distributed Generation.

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.

The Global Grounding System: Definitions and guidelines

The present paper presents the preliminary results of the ongoing Italian METERGLOB project on the contribution given by the exposed conductive parts to a Global Grounding System. One of the expected results of METERGLOB is to carry out guidelines for the identification of a Global Grounding System. These guidelines must be defined on the basis of the definitions and methods present in the current international standards on grounding and safety. In the paper some definitions and elements to be taken into account for the identification of a Global Grounding System are given.

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.

Current and voltage behaviour during a fault in a HV/MV system: Methods and measurements

When a single line to ground fault happens on the MV side of a HV/MV system, only a small portion of the fault current is injected into the ground by the ground-grid of the faulty substation. In fact the fault current is distributed between grounding electrodes and MV cables sheaths. In systems with isolated neutral or with resonant earthing this may be sufficient to provide safety from electric shock. Experimental measurements were performed on a real MV distribution network: a real single line to ground fault was made and fault currents were measured in the faulty substation and in four neighbouring substations. In this paper the problem of fault current distribution is introduced, the test system is described and the measurements results are presented.

A practical method to test the safety of HV/MV substation Grounding Systems

The adequacy of a Grounding System (GS) to the safety conditions has to be periodically tested by measurements. The test methods and techniques used to verify the electrical characteristics of the GS include the measurements of step and touch voltages. The goal of the test is to verify that touch voltage and step voltage remain below a safe value in all the zones of the installation. The measurements can present some operational difficulties. The purpose of this paper is to present the procedure, step-by-step, of a practical method of measuring touch/step voltages in grounding systems located in urban or industrial areas with reduced accessibility. The suggested method uses auxiliary current electrodes located at short distances. This paper demonstrates by test measurements done in a real case that the method provides conservative results.

Influence of LV neutral grounding on global Earthing Systems

International Standards define a Global Earthing System as an earthing net created interconnecting local Earthing Systems (generally through the shield of MV cables and/or bare buried conductors). In Italy, the regulatory authority for electricity and gas requires distributors to guarantee the electrical continuity of LV neutral conductor. This requirement has led to the standard practice of realizing “reinforcement groundings” along the LV neutral conductor path and at users’ delivery cabinet. Moreover, in urban high-load scenarios (prime candidates to be part of a Global Earthing System), it is common that LV distribution scheme creates, through neutral conductors, an effective connection between grounding systems of MV/LV substations, modifying Global Earthing System consistency. The aim of this paper is to evaluate the effect, in terms of electrical safety, of the aforementioned LV neutral distribution scheme when an MV-side fault to ground occurs. For this purpose, simulations are carried out on a realistic urban test case and suitable evaluation indexes are proposed.

Currents Distribution During a Fault in an MV Network: Methods and Measurements

When a single line to ground fault (SLGF) happens on the MV side of an HV/MV system, only a small portion of the fault current is injected into the ground by the ground grid of the faulty substation. In fact, the fault current is distributed between grounding electrodes and MV cables sheaths. In systems with isolated neutral or with resonant earthing, this may be sufficient to provide safety from electric shock. Experimental measurements were performed on a real MV distribution network: a real SLGF was made and fault currents were measured in the faulty substation and in four neighboring substations. In this paper, the problem of fault current distribution is introduced, the test system is described and the measurements results are presented.

Global Earthing Systems: Characterization of Buried Metallic Parts

International Standards IEC 61936-1 and EN 50522 define a Global Earthing System (GES) as the earthing network, created by the interconnection of local earthing systems, that should guarantee the absence of dangerous touch voltages. This is achieved through two effects: the division of the earth fault current between many earthing systems and the creation of a quasi equipotential surface. The second effect can be enhanced by the presence of buried metallic parts, such as light poles and water/gas pipelines, that can modify the earth surface potential profile. In order to characterize these buried conductors, an extensive measurement campaign was organized; in order to determine the resistance to earth of these buried conductors a simplified measurement protocol has been applied to more than 800 metallic objects. In this paper, the measurement set-up, the results and their analysis are reported.

DSOs and active demand: Address project outcomes and perspectives

ADDRESS (“Active Distribution networks with full integration of Demand and distributed energy RESourceS”) is a 5-year large-scale R&D project [1] launched in June 2008 and ended in May 2013, receiving funding from the European Communitys Seventh Framework Program (FP7/2007-2013) under grant agreement n° 207643. This project aims to develop a comprehensive commercial and technical framework for the development of “Active Demand” and the market-based exploitation of its benefits. In ADDRESS, “Active Demand” (AD) means the active participation of domestic and small commercial consumers in the electricity markets and in the provision of services to the other electricity system participants. The paper deals with the architecture and the algorithms developed as well as the validation field test and the AD perspectives being, at least from the technical point of view, a reality for the smart grid of the (next) future.