Classification of Hazardous Areas Produced by Maintenance Interventions on N.G. Distribution Networks and in Presence of Open Surface of Flammable Liquid

The safety and protection of workers is a duty of their employer. In case of the presence of hazardous areas due to the risk of explosion, an area classification has to be performed to identify the shape and size of the locations where an explosion may happen. Two typical cases that can produce hazardous areas are gas emissions from a containment system, because of normal operation or because of a failure, and vapor emissions from an open surface pool of flammable liquid. In this paper, two studies are presented: the first deals with the problem of natural gas releases during maintenance work on the gas distribution network, and the second with vapor emissions from a pool of flammable liquid. In the first case, experimental measures have been performed to easily calculate the size of the hazardous area; in the second case, computer simulations are used to derive a simplified model to determine it. The results of the two studies presented are examined and commented in the light of the International and national Standard

Lightning protection of PV systems

Lightning strikes can affect photovoltaic (PV) generators and their installations, involving also the inverter's electronics. It is therefore necessary to evaluate the risk connected to lightning strikes in order to adopt the correct protective measures for the system. The Standard IEC (EN) 62305-2 reports the procedures for the risk calculation and for the choice of proper lightning protection systems. Usually the technical guidelines suggest protecting with SPDs (surge protective devices) both DC and AC sides of the PV installation. The paper estimates overvoltages due to lightning discharges and evaluates the actual need of lightning protection measures on the basis of the results of the risk analysis and of the protection costs. The paper in the first part presents the procedure for the evaluation of the risk connected to lightning strikes according to the Standard IEC EN 62305-2; then it applies the procedure to typical PV installations, analyzing risks and risk components which have to be kept into account. In the second part the paper studies the surge overcurrents to be expected on LV systems, induced voltages caused by direct flashes and by flashes near the PV installation. Approximated equations for the calculation of induced voltages and currents are given for different types of LPS (lightning protection systems) and lightning flashes. In the last part of the paper the methodology is applied as an example to a practical case and some conclusions are give

A topological reconfiguration procedure for maximising local consumption of renewable energy in (Italian) active distribution networks

Distribution networks (DNs) are facing great changes, due to the strong increase in distributed generation (DG), often driven by renewable energy sources. Designed to deliver electrical power from the transmission system to the final consumers, they are now becoming active and may inject power into the transmission network. In case of large DN, a portion of the system can be absorbing power from the transmission grid, while another portion injects power into it. In order to satisfy the power balance as much as possible at the local level, the distribution system operators are interested in the minimisation of the power exchange with the transmission network, maximising the local consumption of DG energy. This paper presents a topological reconfiguration procedure, based on the branch exchange technique, for the maximisation of the local consumption of renewable energy. A case study is presented, based on a real DN located in northern Italy

Evaluation of the Electromagnetic Environment Around Underground HVDC Lines

This paper analyses the magnetic-field emissions of a high-voltage dc transmission line constituted by two couples of underground cables laid along a highway. The transmission system, including all its components (transformers, converters filters, and line), is modeled through a circuital approach, which provides the distribution of the current harmonics along the line length. The magnetic field produced in the environment is then estimated by a hybrid finite element/boundary element method. The electromagnetic interferences with existing appliances and the human exposure to magnetic fields are investigated considering different laying configurations, conductor dispositions, and supply conditions. Compliance with regulations limiting human exposure and technical standards ensuring electromagnetic compatibility of appliances and devices are assessed

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

Area classification for explosive atmospheres: comparison between European and North American approaches

The object of this paper is to review various methods of determining the extent of hazardous areas in industrial facilities where explosive gas or vapor atmospheres may be present. Three different approaches are analyzed and compared. The first one is recommended in North American Standards, such as API500, API505 and NFPA 497. The second is one of the proposals for the second edition of the International Standard IEC 60079-10-1 (adopted as European standard EN 60079-10-1). The third approach had been previously worked out with the authors‘ contribution and had been adopted by the Italian Guide CEI 31-35 since 2001. The last two approaches are analytical, meanwhile the first one is prescriptive. In the second part of the paper both analytical approaches are applied to the releases which are analyzed in NFPA 497 as practical examples. Resulting hazardous area extents are compared and the differences among the three methods are discusse

Hazardous areas extension in explosive atmospheres caused by free gas jets

This paper regards the validation procedure of the Italian Guide CEI 31-35 formula, used to calculate the hazardous areas extensions in places where explosive gas atmospheres may be present. In industrial activity, a typical event which cause explosive atmosphere consists of damaging and leakage from unions, gaskets, valves of pipes and vessels. At this purpose, in this work it has been taken into account the accidental discharge of flammable gas into a quiescent atmosphere through an orifice. Validation has been performed by comparing calculated values with experimental data. Two gases have been taken into account: methane and hydrogen. Different scenarios have been analyzed, each one differing from the others in the gas release cross section and in the vessel pressure. Results show that the formula fits well not catastrophic industrial accident situation

Large N.G. explosion and fire involving several buried utility networks

This paper describes an accident (explosion and natural gas fire) that occurred in Turin (Italy), in which power distribution cables, tramway network feeding cables and a gas pipe were involved. The described accident is particularly interesting because it occurred in the town centre and lasted several hours, producing a very high risk for the population. Fortunately, nobody was injured, but 120 people were evacuated for 24 h. The sequence of events is described, the involved facilities are examined and the physical processes which led to the different top events are discussed. Actually, starting from a modest event (600 V electric cable loss of insulation), which most likely lasted for months, the aforementioned accident was reached in a crescendo of domino effects. This sequence has been represented by an ISD in which the failure of the different protection systems is highlighted. These protection systems were mostly based upon the strict respect of procedures both in the installation and in the following maintenance of the different utilities. These aspects have been also briefly devised in the light of Italian and foreign regulations concerning the problem of the coexistence of buried utilities

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

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