Russian DonNTU Master's port

          Borzenko Olga

          Theme of master's work:

          Investigation of the operating conditions of the circuits with insolated neutral in a circuit phase on the ground

          Leader of work: C.T.S., the lecturer of faculty "Power Station" Lebedev V.K.
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Abstract

         The relevance of the topic
        Electrical network 6-35 kV have been widely disseminated and relate to the networks, which work with an isolated neutral. The most frequent type of damage in these networks are metal or arc single earth fault, accompanied by overvoltage; ferrorezonanst processes; transformation of single earth fault into interphase short circuit; injuries of voltage transformers and nonselective disconnection of consumer by relay protection. There are the attempts of implantation in the practice of exploitation of neutral grounding using the current-limiting resistor, placed in the neutral of a special transformer, this activities have to reduce the overvoltage and increase the reliability of functioning of protection from the ground short circuit.

         The purpose of work:
        To develop mathematical model for research over voltage and the analysis of transients at single-phase short circuits in a network with the isolated neutral terminal, methods of increase of reliability of work of this network. It is necessary to develop based on the received results the list of recommendations on elimination of negative consequences.



         The primary goals are:
        

        Scientific novelty: new decisions on the restriction of overvoltage level in networks with the isolated neutral terminal and are offered to improve way of performance of protection against short circuits of the phase on the ground in electric networks.

        Practical value: circuit decisions offered in work will find wide practical realization for increase of reliability of job of electric systems with isolated and resonance-grounded neutral terminal.

CONTENTS OF WORK


        Distributive networks 6-35 kV are most long-distance with the heaviest operating mode of electric equipment. Therefore, on reliability of their work appreciably depends fail-safety electrosupply of consumers that is especially actual in conditions of constantly worsening technical condition of networks because of a strong deterioration of isolation of electric equipment. Distributive networks 6-35 kV provide electrosupply of consumers of large industrial targets, consumers of private needs of power stations, agricultural consumers, the enterprises of mining industry, objects of municipal services, etc.
        Distributive networks 6-35 kV work in a mode with the isolated neutral terminal.
        Now on the foreground the problem of maintenance at a sufficient operational level of working capacity of networks 6-35 kV and the maximal prolongation of their service life is put forward. One of successful decisions of the given problem is the way of grounding of a neutral.
        Networks of the same rated voltage at different ways of grounding of a neutral have a line of distinctions in technical and economic parameters. The way of grounding of a neutral at first influences size of a current of short circuit on the ground. Therefore PUE all electric networks, depending on size of a current, subdivides into networks with small and networks with greater current of short circuit on the ground. According to the norms of a network accepted in Ukraine 6-35 kV concern to networks with small current of short circuit on the ground.

DEVELOPMENTS ON the COMPUTER of MATHEMATICAL MODEL of NETWORK of Dokuchaevsk FDP SS 6-35 kV "Rudnichnaja"



        In the master’s work the mathematical model is submitted and allows to analyze the transitional transients at short circuit of a phase on the ground in networks of 6-35 kV. On fig. 1 the equivalent circuit of the network is resulted. In this circuit the power supply is submitted by the phase inductance of dispersion L and resistance R. The network is reflected by the concentrated phase C and inter-phase Ñì in capacities. The isolation is submitted by the active phase by Rè and interphase Rì resistance. The connections of the equivalent circuit are submitted as resistance Rpr and capacities Ñðr. Arresters are established as a terminators of the overvoltage. On fig. 2 the columns of the network are resulted, where branches of the tree and chords are designated. At the research of the transitional transients at short circuit of the phase on the ground in networks 6-35 kV the formalized topological methods of the analysis of electric circuits, such as the method of contour currents and Euler's implicit method are used. The values of the surge voltage are resulted in table 1. On fig. 3 results of calculation of short circuit of the phase on the ground are resulted.



Figyre 1 – Equivalent circuit of the network



Figyre 2 – The columns of the network



 

Figyre 3 - Results of calculation of short circuit of the phase on the ground.
The number of frames at animation - 8.

        The basic results of work:
  • The mathematical model of typical network of Dokuchaevsk FDP SS 6-35 kV "Rudnichnaja" with the isolated neutral terminal is developed.
  • The operating system of electrosupply of Dokuchaevsk FDP is studied.
  • The reasons of not selective work of relay protection are analysed.
  • The mathematical model of transients on the computer is investigated and the analysis of the received results is made.
  • Programs and calculations on the computer of processes an algorithm is developed, at short circuits of a phase on the ground, and principles of performance of improvement ferroresonance processes.
  • Samples of devices in laboratory and real conditions are investigated.
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