An Analytical Procedure to Identify a Global Earthing System

Global Earthing System (GES) is defined by international standards IEC 61936-1 and EN 50522 as an equivalent Earthing System (ES) created by the interconnection of local ESs. Thanks to this interconnection, just a percentage of the total fault current is injected to ground in a single ES, with a significantly reduction of touch voltages in case of fault. If a GES is officially certified, the procedure to verify the effectiveness of an ES can be simplified, with advantages in terms of time and money. Unfortunately, Standards do not provide any practical guidelines to identify a GES. In this work, a methodology is proposed for MV network with the neutral point isolated from ground. A practical example is provided

Fault Current Detection and Dangerous Voltages in DC Urban Rail Traction Systems

In this paper, the electrical safety of dc urban traction systems is analyzed, with particular focus on fault current detection and on dangerous voltages which could arise in case of fault. For the discussion, the tram network of Turin, Italy, is used as a case study. First, the structure of the dc traction power supply is described, analyzing in detail the different components; then, the safety of the system is analyzed, examining possible types of fault. In particular, ground faults inside the substation and ground faults along the line are analyzed in detail. Fault currents and dangerous voltages are calculated, thanks to a simplified steady-state circuital model of the traction system. Finally, the consequent risks for the people are examined and some conclusions and possible solutions are presented

MV ground fault current distribution: An analytical formulation of the reduction factor

Global Earthing Systems (GESs) are defined by international standards IEC 61936-1 and EN 50522 as an equivalent Earthing System (ES) created by the interconnection of local ESs. Thanks to this interconnection, just a percentage of the total fault current is injected to ground in a single ES, reducing the risk of electrocution. However, even if several experiments and models proved this effect, the identification and official certification is already a difficult task. If dangerous scenarios caused by a single line to ground fault can be easily evaluated for a specific MV feeder by measurement or analytic models (quite cumbersome to use), operative procedures valid for all the scenarios are not still available. In this work, a simplified formula to compute the reduction factor is presented, as well as its rationale. The proposed formula is easy to use and the results provided are sufficiently accurate, taking into account a desired safety margin. For this reason, it could be a valid tool for Distributor System Operators (DSO) and Certification Bodies and a step forward for the GES identification

Rail Potential Calculation: Impact of the Chosen Model on the Safety Analysis

In Traction Electrification Systems (TESs), a current flows into the rails both in normal operation and fault conditions. Therefore, in both cases, a voltage between rails and earth, called Rail Potential (RP), occurs. The international Standard EN 50122-1 requires to evaluate the RP on the basis of the voltage drop in the return circuit. In this work, this approach is named Voltage Drop Method (VDM). Usually, in this approach, the rails are considered isolated from ground, the type of interconnection between the negative pole of the converter and the grounding system of the TPS is not taken into account, and the RP in a generic point of the railway is computed multiplying the current flowing in the return path and the longitudinal resistance of the rails up to the Traction Power Substation (TPS). If the RP exceeds the maximum permissible effective touch voltages, function of time, indicated by EN 50122-1, provisions to reduce the electrocution risk shall be applied. Even if the VDM generally provides conservative values for the RP, it cannot be considered completely faithful, due to the simplifying assumptions usually adopted. Therefore, the decision process to evaluate if some measures to reduce the RP shall be adopted can lead to wrong results. In this work, a faithful circuital model of the railways was used to compute the RP for several scenarios; a comparison with the results computed by VDM was carried out. The goal is to evaluate the trustworthiness of the VDM, highlighting the differences with a more faithful model

A Comparative Review of the Methodologies to Identify a Global Earthing System

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. Despite that, standards do not provide any official practical guidelines for its identification. The official classification of GES areas would lead to a simplification of the design and verification procedures of medium voltage/low voltage (MV/LV) substations grounding systems with associated economical savings for both distribution system operators and MV users. To overcome this regulatory vacuum, several teams of researchers proposed methods to identify the presence of a GES. In this paper, the main methods developed to identify a GES are presented. The different methodologies are applied to a real urban scenario and compared

Impact of Wind and Solar Generation on the Italian Zonal Electricity Price

This paper assesses the impact of increasing wind and solar power generation on zonal market prices in the Italian electricity market from 2015 to 2019, employing a multivariate regression model. A significant aspect to be considered is how the additional wind and solar generation brings changes in the inter-zonal export and import flows. We constructed a zonal dataset consisting of electricity price, demand, wind and solar generation, net input flow, and gas price. In the first and second steps of this study, the impact of additional wind and solar generation that is distributed across zonal borders is calculated separately based on an empirical approach. Then, the Merit Order Effect of the intermittent renewable energy sources is quantified in every six geographical zones of the Italian day-ahead market. The results generated by the multivariate regression model reveal that increasing wind and solar generation decreases the daily zonal electricity price. Therefore, the Merit Order Effect in each zonal market is confirmed. These findings also suggest that the Italian electricity market operator can reduce the National Single Price by accelerating wind and solar generation development. Moreover, these results allow to generate knowledge advantageous for decision-makers and market planners to predict the future market structure

Dangerous touch voltages in buildings: The impact of extraneous conductive parts in risk mitigation

International (IEC) European (CENELEC) and American (NEC) Standards require, in each building, the connection of extraneous conductive parts (i.e. metal water or gas pipes) to the main grounding terminal. There are two good reasons for this: the voltage between extraneous conductive parts and exposed conductive parts is zeroed and extraneous conductive parts can contribute to the leakage of fault current into the ground. There is however a third advantage in the bonding connection: the entire structure (floors and walls of the building), together with the exposed and the extraneous metallic parts, forms a quasi-equipotential system, with the consequent strong reduction of touch voltages. Metallic pipes and reinforcement of reinforced concrete have a particular relevance thanks to their large widespread through buildings. However, in some practical cases, it is not possible to connect all extraneous conductive parts to the protective equipotential bonding because they are not accessible. In the paper, the reduction of touch voltages in buildings, when these extraneous conductive parts are present but not connected to the protective equipotential bonding is quantified. Different building models are created and solved by the finite element method in order to calculate touch voltages in different scenarios. The results show that the mere presence of widespread metallic parts in buildings helps to reduce touch voltages, but not enough to ensure safety against indirect contacts. The electrical installation safety performance is greatly improved in reinforced concrete buildings if at least some easily accessible parts, like water or central heating pipes, are connected to the main grounding terminal. Also in brick buildings, they provide a certain reduction of GPR, maximum and mean touch voltages

Optimization of Digital Overcurrent Protection Settings in DC Urban Light Railway Systems

DC urban light railway systems are used for public transportation in many towns worldwide. In these systems, short circuit currents are often similar, both in steady state magnitude and in rate-of-rise, to normal operation currents. In order to properly set the protection relays, to obtain short circuit discrimination and to avoid nuisance trippings, it is important to analyze short circuit and normal operation current patterns. It is also important to highlight that normal operation current patterns are not only those related to rolling stock acceleration, but also to zone commutation. This paper presents the results of several measurement campaigns, performed for this purpose, on the tram network of Turin, Italy. The measurements results are then used to propose optimized settings for the installed overcurrent protections

Distinguishing short circuit and normal operation currents in DC urban light railway systems

DC urban light railway systems are used as public transportation systems in many towns worldwide. In these systems, short circuit currents are often similar, both in steady state magnitude and in rate-of-rise, to normal operation currents. In order to properly set the protection relays, to obtain short circuit discrimination and to avoid nuisance trippings, it is important to analyse short circuit and normal operation current patterns. This paper presents the results of measurement campaigns performed for this purpose on the tram network of Turin, in Northern Italy

Impact of MV Ground Fault Current Distribution on Global Earthing Systems

Global earthing systems (GESs), created by the interconnection of local earthing systems, should guarantee the absence of dangerous touch voltages. One of the reasons for this safety characteristic of GESs is the fault current distribution between grounding electrodes and medium-voltage (MV) cable sheaths: Only a small portion of the fault current is injected into the ground by the ground grid of the faulty substation. In systems with isolated neutral or with resonant earthing, this effect may be sufficient to provide safety from electric shock. In this paper, a model describing the behavior of the MV distribution system with interconnected grounding electrodes during a ground fault is built. It is then used to analyze the impact of different factors on the fault current distribution. A sensitivity analysis is performed, varying the main parameters, and the results are used to draw some conclusions on the current distribution influence on GESs