C I R E D 18 th International Conference on Electricity Distribution Turin, 6-9 June 2005 CIRED2005 Session No 2 LOW VOLTAGE NETWORKS OPERATIONAL NEUTRAL INTERRUPTION IMPACT ASSESSMENT Gianfranco CHICCO*, Petru POSTOLACHE**, Mircea SCUTARIU*** and Cornel TOADER** *Dipartimento di Ingegneria Elettrica, Politecnico di Torino, Torino, Italy **Universitatea Politehnica din Bucuresti, Power Engineering Faculty, Bucuresti, Romania ***Electrica Muntenia Sud Distribution and Supply Company, Bucuresti, Romania Email gianfranco.chicco@polito.it , roberto.napoli@polito.it, federico.piglione@polito.it, mscut@sdb.ro , petru.postolache@k.ro SUMMARY Phenomena usually associated to neutral conductor interruption in low voltage networks touch a sensitive aspect of distribution business clients: the safety of utilisation devices. Therefore a tool to allow assessments of risks associated to this behaviour is needed. This papers aims to propose a mathematical model to realistically describe the behaviour of low voltage networks subject to operational neutral conductor interruptions. 1. INTRODUCTION Distribution operators customers satisfaction plays a major role within the assessment of distributors activity under current conditions in most of the distribution and transmission structures across the world. Under this scenario deterioration of clients utilisation devices due to events happened in distribution operators assets represent a phenomenon to be contained via organisational and technical means. For four-wire low voltage networks overvoltages caused by interruption of the operational neutral conductor result in major disruption of the utilisation equipment including irreversible damage. In order to prevent these things to happen an in-depth knowledge of the phenomena implied in development of such behaviour is valuable. Although a significant share of published works go into the analysis of harmonically perturbed effects on the neutral loading, such as Verde et al. [1], Arthur and Shanahan [2], Bompard et al. [3], Desmet et al. [4] and Toader et al. [6], little effort has been deployed to find accurate explanations of the phenomena usually associated to neutral interruption. A mathematical model able to capture and represent as adequate as possible the distribution and utilisation networks behaviour when operational neutral interruption occurs is presented in the coming sections. This model is subsequently applied to determine effects of this event when it happens in different sections of a given distribution network with a typical layout. The mathematical model includes a detailed writing of the network operational equations where the impedances associated to various sections of the neutral conductor, given the hypothetic location of single-phase customers, are no longer negligible. The approach is not far from the one presented by Levey [7] but has some particularities that will be thoroughly explained in the coming sections. By accurate handling of these equations, according to various scenarios of interruption locations, the analytical formulae for voltages applied to each single-phased customer can be derived. The presented model is then applied to determine the effects this event may cause in various locations of a simplified electrical network, considered representative for such networks. Some comments on the effects detected via the theoretical model are inserted at the end of the paper. 2. ANALYTICAL DETERMINATION OF THE STEADY STATE OPERATION REGIME 2.1. Computation Hypothesis Generally speaking a simplifying hypothesis used in handling low voltage networks is to regard the supply voltage system to be symmetrical and balanced, i.e. at any time to hold this relationship: A A U U = , A 2 B U a U ⋅ = , A C U a U ⋅ = , 0 U N ≠ (1) where (A, B, C) are subscripts to depict the three phases of the system while N holds for the neutral. In completion of the data needed to describe the mathematical model one needs to have the material parameters of low voltage network elements, i.e.: - resistances for the active and neutral conductors; - both mutual and self – inductances of the electrical network components; - capacities of all conductors. It has been also considered that the low voltage distribution network does not benefit of the presence of multi-grounded neutral, a situation quite often encountered in old structures. 2.2. The Case of Three Single-Phased Loads The steady-state equations for the simplified situation presented in figure 1, before the neutral conductor interruption, are as follows:      + ⋅ + ⋅ = + ⋅ + ⋅ = + ⋅ + ⋅ = c C C C LC C b B B B LB B a A A A LA A U I Z I Z U U I Z I Z U U I Z I Z U (2) where the quantities involved represent: - C B A Z ; Z ; Z load impedances associated to each single- phased customer under steady-state conditions; - c b a U ; U ; U voltage-to-remote ground for points a, b and c on the neutral conductor;