“ELECTRIC POWER ENGINEERING & CONTROL SYSTEMS 2013” (EPECS-2013), 21–23 NOVEMBER 2013, LVIV, UKRAINE 72 Methods of improving the reliability of distribution networks 6-35 kV Fedor Shkrabets, Maryna Kyrychenko The Department of Renewable Energy Sources, State Higher Educational Institution “National Mining University”, UKRAINE, Dnipropetrovsk, K. Marksa Ave., 19, E-mail: maryna_kyrychenko@mail.ru Abstract – The causes of damage to distribution networks are considered and the main methods for insulation condition monitoring as well as fault detection and location are described. Кеу words – distribution network, insulation, ground fault, power supply systems, reliability. I. Introduction At present, requirements to the reliability and continuity of power supply for industrial enterprises are increasing. The reliability of power supply systems is largely determined by the failure-free operation of transmission lines, and the distribution networks 6-35 kV constitute a considerable part of them. It is known that most faults that occur in the power supply systems (about 80%) fall exactly on the distribution networks. The distribution networks fault analysis shows that up to 60- 90% of all failures are the ground faults. The purpose of this study is to analyse the up-to-date methods used to improve the reliability of power supply systems and electrical safety conditions of the maintenance personnel. II. Basic Data The main causes of damageability and insufficient security levels of power supply systems, of both service personnel and people and animals contacting with them are [1]:  imperfection of power supply schemes;  imperfection of rules of operation and their proper execution;  absence of protective signaling systems dealing with the single-phase short circuits on the ground;  absence of diagnostic systems for assessing insulation condition;  high levels of internal overvoltage;  use of equipment (switching equipment, cables) which exhausted its regulatory resources. The single-phase ground short circuits represent the most common type of faults in distribution networks. They are dangerous for both the electrical equipment and the staff due to the peculiarities of network and electrical equipment operation. The ground fault is an asymmetric type of damage and it is characterized by the appearance of zero sequence components in the network. The voltage and the current zero sequence parameters in the transient and steady-state modes depend on many factors, the principal of which is the ground fault and neutral point operation mode. The danger of single-phase short circuits is connected with a high voltage impact on the phase insulation, including the emergence of significant overvoltages, which may reach up to 1.73 from the phase voltage value during a zero resistance fault. The vector diagram (Fig. 1) clearly shows the phase voltages distribution for ground short circuits. By the nature of damage, ground faults are divided into metallic (zero resistance) and arc (across the intermittent arc and across the contact resistance at the points of damage). Fig. 1. The voltage vector diagram in a single-phase voltage ground fault Single-phase ground or enclosure faults result from the aging, mechanical damage or electrical breakdown of the insulation of one of the network phases related to the ground or the enclosure. Therefore the task of providing effective control over the insolation condition, as well as early detection and elimination of defects remains topical so far. The majority of insulation control devices signal that in the power network there is a decrease of resistance and they aren't able to detect the fault location selectively. Sometimes the problem of detecting the fault location in the insulation is solved by the serial electrical separation of the system elements with subsequent monitoring the insulation resistance of a disconnected element. By using this way of detecting a damaged element there arises a danger of the relay protection and automation malfunction and this requires large expenditures of time and highly qualified personnel. Depending on the line type (cable or overhead) various methods are used for detecting fault locations. Several classifications of these methods are offered in the literature. The most common methods of detecting fault locations for the underground cables are shown in the following diagram (Fig. 2). Fig. 2. The most common methods of detecting faults of underground cables Lviv Polytechnic National University Institutional Repository http://ena.lp.edu.ua