Urgency

Urgency of the theme. Single-phase short circuits on the ground are the most widespread kind of damage to electric networks of middle class of a voltage (within the limits of up to 85-90% from the general number of infringements of normal work of networks depending on their purpose and a design). In conditions of a strong deterioration of isolation, they develop in interphase short circuits or many-placed breakdowns of isolation with group failure of electric equipment. Therefore, the core the direction of struggle for increases of reliability of work of electric networks is struggle against single-phase short circuits on the ground.

In work the research problem over voltage in electric network of Dokuchaevsk ferrodolomite plant (FDP) by the voltage SS 6-35 kV "Rudnichnaja" was put. At absence of reliable means of protection of electric equipment from consequences arc over voltage, the effective decision of a problem can be found in optimization and management of the mode of a neutral of a network. In addition, the method of the computer modeling applied in given work, differs in great volume of the received results at low material inputs and high accuracy of calculations.

Purpose and tasks of work

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. To develop based on the received results the list of recommendations on elimination of negative consequences. According to it the primary goals are:

1. Development and introduction of ways of increase of reliability of electrosupply of Dokuchaevsk FDP at single-phase short circuits on the ground in network SS 6-35 kV "Rudnichnaja".
2. Studying active system of electrosupply of Dokuchaevsk FDP. The analysis of the reasons not selective work of relay protection.
3. Development of algorithm, the program and calculations on the computer of processes at short circuits of a phase on the ground, and development of principles of performance of improvement ferroresonance processes.
4. Research of samples of devices in laboratory and real conditions

Scientific novelty

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

Practical value

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.

Basic part

THE MAINTENANCE OF work

1 MODERN CONDITION OF THE PROBLEM AND WAYS OF ITS DECISION

1.1              Work of networks 6-35 kV now

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 own needs of power stations, agricultural consumers, the enterprises of the mining industry, objects of a municipal services, etc.

On a design distributive networks 6-35 kV in overwhelming majority of cases within the limits of up to 15% from the general extent are executed air, up to 2-5% - the mixed air-cable networks and the most part from them represent cable networks.

Distributive networks 6-35 kV work in a mode with the isolated neutral terminal [1, 2]. The main advantage of a network with the isolated neutral terminal is that single-phase short circuits, the most frequent kind of damage, are not here emergency operation and a network, during till four o'clock, can work in such mode that provides high reliability of electrosupply of consumers at reduction in charges on reservation. However in a mode of single-phase short circuit on the ground isolation of the intact phases can is long to be under a linear voltage and through a place of damage the current of short circuit on the ground proceeds. It can lead to destruction of isolation of a healthy phase and will lead to biphase short circuit that will transfer a network in an emergency condition.

Now on the foreground the problem of maintenance at a sufficient operational level of working capacity of networks 6-10 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.

1.2 The Basic theories used for the analysis over voltage at single-phase short circuit on the ground

The question of studying of processes at single-phase short circuits on the ground in distributive networks experts of all technically developed countries traditionally gave enhanced attention. For some decades the big scientific and experimental material is saved up, measures on protection of networks against a consequence of single-phase short circuits on the ground are offered. However, complexity of transients, variety of influencing factors and rapid development of electric networks constantly generate new problems on which decision the level of operational reliability entirely depends.

At studying a physical picture of processes in a three-phase electric network at single-phase short circuit on the ground it is established, that frequency rate arc over voltage is appreciably determined by a free component of a voltage in a transitive mode [6, 8, and 9].

Free fluctuations, being imposed on the established value of voltage of phases of a network, lead to occurrence over voltage on which size the big influence renders character of burning of an arch in a place of short circuit of a phase on the ground. In networks with the isolated neutral terminal at breakdown of isolation of one of phases on the ground depending on size of a current of short circuit three modes of burning grounded arches are possible.

1. At rather big current the arch burns steadily.
2. At small enough current the arch burns unstably.
3. At intermediate value of a current are observed repeated starvation and ignitions of an arch.

The founder of these researches over voltage was German engineer Petersen who in 1916 has developed the theory explaining physical essence of process of occurrence maximal over voltage. According to this classical theory in a basis of consideration of process of arc alternating short circuit of a phase on the ground the assumption about starvation arches is necessary at passage through zero of a current of high-frequency fluctuations and new  it  ignitions at a maximum of a voltage on the damaged  phase.

In 1923 Peters and Slepjan have offered another essentially distinct from before the generation examined the theory arc over voltage.

Later these theories were supplemented with various authors on the basis of theoretical and laboratory researches concerning levels maximal over voltage and forms of their development.

Extensive researches arc over voltage in networks 6-35 kV with the isolated neutral are spent by N.N.Belyakovym and C.M.Dzhuvarly which have shown, that the essential role in the mechanism of development arc over voltage is played with character of restoration of electric durability of an arc interval after clearing an arch [10,11].

Thus, in the developed conditions of constantly worsening technical condition of networks because of absence of means for duly replacement and qualitative restoration of the damaged electric equipment the problem of maintenance of electroinstallations at a sufficient operational level appreciably will depend on correct understanding of conditions of its job in networks with the weakened isolation and, knowledge of laws of dynamics of change of parameters and characteristics of networks.

1.3 Review of existing methods of restriction over voltage in networks 6-35 kV кВ

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 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 a 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 a small current of short circuit on the ground.

 

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

2.1 Equivalent circuit of the network

For mathematical modeling scheme of Dokuchaevsk FDP SS 6-35 kV "Rudnichnaja" (fig.2.1) it is used its equivalent circuit (fig. 2.2).

 

Fig.2.1- Scheme of Dokuchaevsk FDP SS 6-35 kV "Rudnichnaja"

Fig.2.2 - Equivalent scheme circuit of Dokuchaevsk FDP SS 6-35 kV "Rudnichnaja"

 

On an equivalent circuit the working transformer, being the power supply, is presented by phase inductance of dispersion L, by active resistance R and source EMP (Electro-motive power) of E. Network is reflected concentrated phase Си and interphase  Sm in capacities, active resistance of isolation Rи and Rм. The transformer of a voltage entered for the account of influence ferroresonance of processes in a network, is presented by phase value of active resistance Rт1 and nonlinear dependence of phase inductance of dispersion Lт1 on size of a proceeding current. Two connections (feeders) are presented in an equivalent circuit by active resistance Rпр and capacities Спр. Short circuit of a phase on the ground is simulated by the active resistance of an arch included instead of active resistance of isolation in any place of an equivalent circuit.

2.2 Differential equations of mathematical model

Fig. 2.3 - Columns of a circuit of Dokuchaevsk FDP SS 6-35 kV "Rudnichnaja"

 

Using constructed for an equivalent circuit of columns of a circuit (fig. 2.3) is made following system of the differential equations concerning unknown planimetric currents:

І_х=[і₁,i₂,i₃, і₄,i₅,i₆, і₇,i₈,i₉, і₁₀,i₁₁,i₁₂,];

i₂₂=i₁₉=і₁+i₂+i₃+і₄+i₅+i₆+і₇+i₈+i₉+і₁₀+i₁₁;
i₂₃=-i₂-i₃+i₅+i₆+i₈+i₉+і₁₀+i₁₁+i₁₂;
i₂₄=-i₃+i₆+i₉+i₁₁+і₁₂;
i₂₅=і₁+i₂+i₃;

Contour: 1) 1,19,22,25;  2) 2,19,22,-23,25;  3) 3,19,22,-23,-24,25;  4) 4,-19,-22,13;  5) 5,14,-19,-22,23;  6) 6,15,-19,-22,23,24;  7) 7,19,-19,-22;  8) 8,17,-19,-22,23;  9) 9,18,-19,-22,23,24;  10) 10,-19,20,-22,23;  11) 11,-19,21,-22,23,24;  12) 12,23,24.

For voltage Uk, Un, Up we accept voltage in branches 23, 24, 12 accordingly between inductance and capacity.

List of unresolved problems

On the basis of great volume of the researches executed with use of mathematical and physical models of a network and separate experiences in a real network, essentially new phenomena demanding the new approach to a choice and arrangement of means of protection from over voltage are found out. During the analysis of results of research it is established, that modes of grounding of a neutral of distributive networks traditionally applied in our country 6-35 kV in the developed conditions do not meet the requirements of a modern condition of isolation of an electric equipment and consequently should be improved, however is shown, that the unequivocal decision of this problem does not exist.

Current and planned results on a theme

The basic results of work:

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

Conclusions

CONCLUSIONS

The task of the further researches consists in perfection of structure of mathematical model of a network 6-35 kV and development on the basis of modern microprocessor techniques and computer ways and devices of automatic search of the damaged connection with the purpose of localization of failure and the fastest restoration of a normal operating mode of a network for increase of reliability of functioning of systems of electrosupply of consumers of electric energy.