Abstract
Contents
- Introduction
- 1. Theme urgency
- 2. Goal and tasks of the research
- 3. Main content of work
- Conclusion
- References
Introduction
Single-phase short circuit on the earth is the most frequent type of damage to three-phase electric networks of all classes of tension.
In electric networks 6–35 kV of Ukraine work, as a rule, with the isolated or compensated neutral, values of currents of single-phase short circuit on the earth (OZZ) are insignificant, they don't exceed 20–30 A. Poetomu of a network of these classes of tension traditionally call networks with small current of short circuit on the earth. However OZZ constitute big danger to the equipment of electric networks and to OZZ which were near a place of people and animals. In this regard Rules of technical operation of power plants and networks demand to disconnect quickly automatically in one cases OZZ, and in others — immediately to start accession definition with OZZ and then to disconnect it.
To create selective (selective) and highly sensitive protection against OZZ, suitable for any kinds of networks with small current of short circuit on the earth, so far anybody didn't manage. It is valid, difficult to create universal protection against OZZ for such different types of electroinstallations, as air and cable lines, generators and electric motors, for such different modes of grounding of neutral points of a network, as also isolated neutral, resonant grounded or neutral resistive grounded neutral (the neutral grounded through limiting active resistance — the resistor).
Special difficulties when performing selective protection against OZZ arise in networks 6 (10) of kV with the resonant grounded neutral where current of damage of industrial frequency is completely compensated by current of the dugogasyashchy reactor (DGR) and therefore can't be used as information source for protection. Additional difficulties arise in need of selective definition of accession with OZZ in electric networks of a difficult configuration, at absence on accession of the cable insert necessary for installation of the transformer of current of zero sequence, at often changing primary scheme of a protected network and in other cases [1].
Reduction of probability of transitions of OZZ in multiphase KZ and respectively increase of reliability of work of networks 6–10 kV is connected, first of all, with a problem of improvement of modes of grounding of their neutral. In turn, the mode of grounding of a neutral of a network defines the principles of performance of protection against OZZ and ways of its action (on a signal or on shutdown). Considering dokstignuty now in Ukraine levels of reservation and automation of electric networks 6–10 kV and technological processes at the enterprises, it is necessary to expect that the preservation requirement in work (during limited time) the element having OZZ in the short term for the majority of networks of 6-10 kV, won't lose the relevance as effective and economically the cheapest way of ensuring demanded reliability of power supply.
Possibility of network functioning with OZZ during limited time, sufficient damage for elimination without shutdown of consumers, in principle is provided by some options of a mode of grounding of a neutral (the isolated neutral, resonant grounding of a neutral, high-resistance grounding of a neutral via the resistor), each of which has the scope. In certain cases more effective solution of the problem of increase of reliability of network functioning of 6-10 kV can be provided with application of a mode of grounding of the neutral which isn't allowing long network functioning with OZZ (the low-impedance grounding of a neutral via the resistor) and performance of protection against OZZ with action on shutdown [2].
1. Theme urgency
Short circuit currents on the earth, besides capacitor, inductive and active components of industrial frequency, may contain the highest harmonious components which part appears only during a transition period at short circuit on the earth and is defined by parameters of a loop of current of short circuit on the earth. Other part of currents of the highest harmonicas continues to proceed through an arch during the whole time of its burning. These currents are caused by existence of harmonious components in of page of generators, and also the electromotive forces of various harmonicas owing to saturation of magnetic conductors of power transformers and fast-variable uneven loadings when in technology of consumers arc processes (mercury выпрямительные installations of the industrial enterprises and electrodraft, electrosteel-smelting furnaces etc.) are used.
In short circuit current on the earth the fifth harmonious component most often prevails. At the same time with it there is sometimes the seventh harmonious component, but smaller in size [3].
At quickly variable uneven loadings in windings of network transformers arise unbalanced a self-induction of the highest frequencies under the influence of which there are charges and categories of capacities of phases concerning the earth and therefore, current course through a damage place.
As a result of the resonant phenomena currents of frequency of burning of a grounding arch of f1 are usually allocated. The currents caused this unbalanced, often represent the eleventh or thirteenth harmonious components or both together. Being summarized, they create current of the twelfth harmonious component which frequency of biyeniye is equal to industrial frequency.
Before short circuit on the earth, besides loading currents, in the damaged phase the capacitor current of i'c advancing phase tension of three-phase symmetric system proceeds. Short circuit on the earth represents a cut of tension, as a result of which of page of interinduction of windings of the transformers feeding a network, change the directions to the size of a cut of tension.
On a network neutral Uo tension kolebatelno is established, the return in the direction to tension of the damaged phase, and tension on the nepovrezhdeshchshkh phases is got by values of linear tension.
Capacitor current of i'c passes into short circuit current to the earth which in the established mode has the return relatively i'c the direction if the network works with the isolated neutral or with undercompensation, or has the same direction, as i'c at network functioning in an overcompensation mode.
The first half wave of a transitional component of current of short circuit on the earth has purely capacitor character and therefore always is directed aside, coinciding with the direction of tension of a cut on the damaged phase and advances on 90 hail of fluctuation of tension. On a neutral and on the intact phases which direction also coincides with the direction of a cut of tension on the damaged phase.
Fig. 1. Equivalent circuits of the network working with the isolated neutral and with compensation of capacitor currents. and — three-phase; — zero sequence.
The assessment of the main components of currents of short circuit on the earth (capacitor tc and inductive ik), arising in a transition period at short circuit on the earth of one phase of a network, can be made according to the equivalent circuits shown in fig. 1. The three-phase scheme (fig. 1, a) is transformed to the simplified scheme of zero sequence (fig. 1, b), representing parallel inclusion on tension of zero sequence of parameters of a network R, L, C and the concentrated inductance of the dugogasyashchy device LK. The main ways of currents are shown by thick lines.
Directly from the moment of emergence of short circuit on the earth before emergence of current of fluctuations of iкол through grounding arch the current caused by the wave category of the damaged phase and establishment of the general initial tension on interphase capacities of Cm and capacities of intact phases on the earth proceeds [4].
At the time of phase short circuit on the earth there is a transient at which currents and tension of zero sequence change under the difficult law and can exceed the established values considerably. In these conditions false operations are possible are sewn up. Therefore, at justification are sewn up from single-phase short circuits on the earth it is necessary to consider and consider currents and tension of zero sequence in transitional modes.
Fig. 2. The scheme of calculation of currents of a zero poslekdovatelnost at transients.
Now electric networks of 6-10 kV, in the light of the concept of creation of an intellectual power supply system, also are of great importance when using them as a link of Micro Grid of technologies. Due to their broad application at the industrial enterprises, in system of own needs of power plants, in city networks they will define substantially reliability of functioning of Micro Grid of objects with nonconventional power sources. Known lack of these networks are frequent damages of electric equipment because of high level of retension at arc short circuits on the earth, and also difficulty of ensuring selective operation of relay protection, especially at low level of currents of short circuit of a phase on the earth. The attention as is constantly paid to increase of reliability of work of these networks from the research and operating organizations and electric equipment manufacturing plants. So there is an introduction of high-resistance and low-impedance grounding of a neutral of a network, modern microprocessor protection, limiters of retension like OPN. Works on research of transients in these networks as with use of modern digital registrars, and methods of mathematical modeling are carried out. However, despite it, reliability of work of the specified networks remains insufficient.
Therefore development of the principles of creation of systems of automatic control of objects on the basis of the concept of Smart Grid and increase of reliability of work of networks of 6-10 kV at OZZ due to improvement of methods of mathematical modeling of these networks, resistive grounding of a neutral, restriction of retension and increase of sensitivity and selectivity of action of relay protection is an actual task [3].
2. Goal and tasks of the research
- To define all factors influencing work of relay protection for optimum work of protection against OZZ;
- To improve mathematical model of relay protection against OZZ for not directed protection;
- Creation of algorithms by means of programming of mathematical model in the MathCad and MatLab programs;
- To obtain experimental data of work of relay protection against OZZ.
The received experimental will be compared to similar types of relay protection by their data and their characteristics.
3. Main content of work
Effective protection against OZZ allows to reduce dangerous influence on the equipment of networks of 6-35 kV of those influences which arise at OZZ. It, in turn, will increase reliability of operation of engines, cables and other elements of a network and will allow to prolong the term of their operation. Safety for people and animals who can appear in a zone of falling of a wire of the air high voltage line increases also.
In certain cases necessary efficiency can be provided by means of not directed current protection of zero sequence.
First of all it belongs to the resistive grounded networks and installations with small capacitor currents. If, for example, it is a question of protection of a cable network of own needs of power plant in which the grounding resistor is installed, and in a place of OZZ active current of the resistor about 35-40 proceeds And and capacitor currents of separate accessions don't exceed several amperes, protection much of known not directed current devices here can be successfully used.
The second similar case is intra shop networks at the enterprises where the dugogasyashchy reactor can be established, but there is a large number of accessions with small capacitor current. In that case difficult to execute effective selective protection without grounding resistor. Having installed such resistor with current 10-15 A, we have an opportunity on the majority of accessions to establish simple current protection of zero sequence.
in the Ukrainian market can get Now the following devices of not directed current protection of zero sequence.
The protection constructed on domestic RTZ-51 relays, blocks of microprocessor protection of BMRZ (for example, BMRZ-KL-11, BMRZ-KL-36, BMRZ-KL-42, BMRZ-KL-51) scientific and technological center the Mechanotronics, terminals of protection of S20 SEPAM (the ANSI 50N/51N code or 50G/51G) Schneider Electric firms, protection of the SPACOM series, for example, SPAC-800 of Relay Cheboksary ABB firm, the MiCOM P121, P122 Compact, P123 structure of AREVA firm (the former ALSTOM), protection like SIEMENS firm SIPROTEC 4 7SJ61, etc.
The devices most suitable for the concrete enterprise have to get out taking into account local conditions and opportunities.
Current of operation of not directed current protection should be built up from own capacitor current of protected accession therefore on accessions with big capacitor currents such protection often are tolerant.
Not directed current protection of the zero sequence based on "relative measurement", for example, the device microcontroller MKZZP-6 of ESTRA firm represent essential interest. This device is started at OZZ on the basis of emergence of tension of zero sequence and compares currents of zero sequence in all accessions of protected system or section of combined tires. That accession where the main harmonica of current of zero sequence more, is considered damaged. Such protection can't be used in the presence in a network of the dugogasyashchy reactor or at small number of accessions to combined tires (for example, at two accessions). At big quantity accessions protection works very effectively[7].
Not directed protection registering current of zero sequence of industrial frequency and urged to disconnect the damaged accession, at arc short circuits and small currents are, as a rule, not selective. Current of operation of not directed current protection should be built up from own capacitor current of protected accession therefore they will difficult be coordinated on operation settings, especially if own capacitor currents of OZZ of separate accessions strongly differ among themselves. For increase of sensitivity and selectivity of action of not directed protection against OZZ. It is offered to carry out compensation of own capacitor current of accession at external OZZ. However, it demands connection of an additional winding in number of 2-5 rounds, reeled up on the core of the cable transformer of the current of zero sequence (CZS). Such execution of protection in the conditions of real industrial facilities causes difficulty.
The directed current protection, currents reacting to the main harmonious components and tension of zero sequence, are built up from own capacitor currents of protected accessions in the direction therefore such protection manage to be executed more sensitive and effective. However high sensitivity of the directed protection in actual practice operation (at a network there are any not balances, big errors of domestic cable transformers of current of TNP, ferrorezonansny processes) is often the reason of loss of an orientation of action of protection and false to their operations. Therefore more reliable directed protection which assumes installation on each accession of consistently two TNP with a podmagnichivaniye was offered, the scheme of inclusion of secondary main and which additional windings provides increase of stability of work and reliability of protection. However, the main lack of this decision is need of installation of the second TNP on a feeder that in actual practice operation practically isn't feasible, and this decision is accepted only in case of its realization at a stage electroinstallation design [8].
In work for elimination described above shortcomings authors offer to use one cable TNP on accession to which secondary windings the device consisting of two special dividing transformers with various direction of a podmagnichivaniye is connected. Signals of these transformers after straightening are compared and, thereby, the orientation of action of protection and selective shutdown of accession is provided at OZZ on it.
Work of this protection was modelled with use of the MatLab program of the Simulink appendix. Results of modeling are given in figure 8.
Fig. 8. – Oscillograms of currents on executive body: a) at internal and b) external OZZ Oscillograms of the currents on the executive board: a) domestic and b) external PTG (Animation: 5 frames, 3 repetition, 146 kilobytes)
Conclusion
The advanced directed protection against OZZ is developed, allowing without changes in schemes of connection and installation of cable TNP to realize the simple and reliable device for timely detection of OZZ on accession and, if necessary, its shutdowns.
At the time of paper writing work on tests of new protection wasn't carried out. Work expiration date in this direction December, 2014
List of references
- Шабад М. А., Защита от однофазных замыканий на землю в сетях 6–35 кB, Москва, НТФ
Энергопрогресс
,Энергетик
, 2007. - Шуин А.В.,Гусенков А.В., Защита от замыканий на землю в электрических сетях 6-10 кВ, Москва, НТФ
Энергопрогресс
,Энергетик
, 2001. - Отчет
о научно-иследовательской работе д-7-12
Автоматизация и управление элетрическими системами с нетрадиционными источниками энергии
, 30.12.2013. - Лихачев Ф.А., Замыкания на землю в сетях с изолированной нейтралью и с компенсацией емкостного тока,
Энергия
, 1971г. - Правила устройства электроустановок. — М.:
Энергоатомиздат
, 1985. - Цапенко Е.Ф., Замыкания на землю в сетях 6-35кВ, Москва
Энергоатомиздат
, 1986. - Шалин А.И., Релейная защита от замыканий на землю в сетях с резистивным заземлением нейтрали. ООО «ПНП БОЛИД», г. Новосибирск, Россия.
- Сивокобыленко В.Ф., Лебедев В.К., Ковязин О.В., Математическое моделирование работы ограничителя напряжения в сетях 6-10 кВ с изолированной нейтралью.