A Fuzzy Approach Used in Expert System for Optimal Neutral Grounding Toader Dumitru, Lustrea Bucur * , Blaj Constantin, Borlea Ioan * , Haragus Stefan Department of Physical Foundation of Engineering, * Department of Power Engineering “Politehnica” University of Timioara Bd.Vasile Pârvan Nr.2 300223 Timioara ROMANIA constantin.blaj@et.upt.ro http://www.et.upt.ro Abstract: - In this paper an expert system for selecting the appropriate neutral grounding method in medium voltage distribution network is presented. The grounding method should provide the best response in the case of a single- grounding fault. To fulfill this requirement, the fuzzy rules are based on a simple mathematical model of the fault, in conjunction with operation experience in medium voltage networks. The expert system is very easy to use even by medium level education operators. Key-Words: - Neutral Grounding Method, Medium Voltage Distribution Network, Fuzzy Rules. 1 Introduction Medium voltage power networks may have the neutral isolated, or grounded, via a resistor or a reactor. If a nonsymmetrical fault occurs, the zero-sequence voltage on the transformer terminals, the voltages on the healthy lines and the fault current greatly depend on the adopted solution for grounding the network’s neutral. For an isolated neutral network, in the case of a single- grounding fault, the voltage on a healthy phase raises to the line voltage, stressing the phase isolation. Consequently, the single-grounding fault may develop into double-grounding fault, which is very dangerous for the power system. On the other end, if the neutral is zero-impedance grounded then a single-grounding fault does not affect the voltages on the healthy lines. In what concerns the modification of the phase voltages during a single-grounding fault, the grounding method via a reactor or a resistor is situated between those two methods before mentioned [1, 2, 3, 4, 5, 6].In this paper an expert system is presented - Neutral Treatment Strategy Expert System (NTSES). This system allows the choosing of the optimal solution for grounding the neutral of a medium voltage power network based on the three classical methods, namely: isolated neutral, neutral grounded via a compensation reactor, respectively via a resistor. The main advantages offered by NTSES are the following: -it is easily transferable to various locations; -the recommended solution has a logical base; -it is easy to use as a medium level knowledge for the people working in the field is necessary; -it has low operation costs. The NTSES utility resides from the following considerations: -NTSES offers the necessary information needed to take the correct decision for the neutral grounding method; -using the results provided by the NTSES the operation personnel may appropriately interfere with the power system, if necessary; -during the design process of a medium voltage network, NTSES allows the selection of the optimal neutral grounding solution; -NTSES helps to discriminate the situations in which modifications of the network structure, impose changing the way in which the network’s neutral is grounded. The data provided by NTSES facilitates the maintenance activities scheduling, as the isolation stressing during a single-grounding fault do depend not only on the neutral grounding method, but as well on the specific fault conditions. 2 Basic Specification 2.1 General Specification For choosing the appropriate neutral grounding method the following aspects should be taken into account [4, 7, 8, 9, 10]: -the number of customer disconnections, due to grounding faults, should be as low as possible; -the line types (overhead or underground); -the fault nature (atmospheric discharge, internal overvoltage, etc.); -the geophysical characteristics of the landscape (residential zone, rural zone, etc.) -the length of the distribution lines; -overvoltages on the healthy lines for single-grounding faults, which may lead to the occurrence of dangerous multiple-grounding faults; -the value of the fault current for overhead lines; -the existent protective devices for detecting and removal of single-grounding faults. Proceedings of the 10th WSEAS International Conference on FUZZY SYSTEMS ISSN: 1790-5109 122 ISBN: 978-960-474-066-6