Antonenko Leonid

Faculty:Electrotechnical

Speciality:Electrical stations

Theme of Master's work:

Transient processes calculation-experimental investigation in electro supplying systems 6-10kV

Scientific adviser: Lebedev Viktor

Abstract

Introduction

        Single-phase ground short circuits are the most widespread voltage middle class electric networks damage kind (about 85-90 % from the general number of networks normal work infringements depending on their appointment and design). According to isolation strong deterioration conditions in overwhelming majority of cases pole-to-pole faults or many-placed isolation breakdowns with group electric equipment failure are developed, being accompanied by the big material damage and energy undersupply to consumers. Therefore fairly it is considered, that the basic struggle direction for electric networks work reliability increases is struggle against single-phase ground short circuits. For this reason in the technically developed countries majority, including Ukraine, works on electric equipment functioning condition perfection in distributive networks now are intensively conducted. Thus various researchers offer approach ways rather contradicting each other to the specified problem decision. So one specify in existing directions perfection necessity with use compensating ground-fault neutralizer, and others suggest to transfer considered networks to other neutral grounding modes a etc. All it specifies in an insufficient processes scrutiny level at arc ground short circuits of a phase and offered decisions practical realisation problems misconception. The specified problem optimum decision can be found only on the basis of transients course character electrophysical bases deep understanding at arc ground short circuits with the account all influencing factors.

        Proceeding from stated in considered work the research arc revoltage problem in different to destination and to design electric networks 6-10 kV was put. Presented more low these researches results are executed by voltage with physical and mathematical models use for networks with different neutral grounding modes at a variation in a electric networks parametres wide range and phase ground short circuit places. Simultaneously with it at researches carrying out all the most significant for arc revoltage factors limiting frequency rates reception, such as: arch burning character; resistance in a short circuit place of a phase on the earth; pole-to-pole capacities, network loading character etc. are considered.

        By the spent researches results in work the circuit decisions which practical realisation will allow to lower essentially all influencing voltage on an electric equipment with the weakened isolation are offered and, thus, to raise network work reliability for the electric equipment service life prolongation account with the old isolation.

Master's Thesis actuality

        Electric networks 6-35 kV have received a wide circulation and they concern to the networks working with the isolated neutral. The most frequent damage kind to these networks are metal or arc single-phase ground short circuits which are accompanied by revoltage; ferroresonant processes; transition ОЗЗ in pole-to-pole short circuits; voltage transformers damages and not selective consumers switching-off by relay protection. Last years introduction attempts in neutral grounding operation practice through current-limiting resistor established in a special transformer neutral that should lead to decrease in revoltage level and increase in protection functioning reliability against phase ground short circuits are undertaken.

The work purpose

        Resistors installation from the part, connected in the opened triangle, neutral based transformer lowest voltage windings, possibility substantiation.

• Dokuchaevsky FDI electrosupply reliability increase ways working out and introduction at single-phase ground short circuits in network substation 35/6 kV "Rudnichnaya".
• Dokuchaevsky FDI electrosupply operating system studying. The relay protection not selective work reasons analysis.
• The revoltage arising analysis at single-phase ground short circuits .
• The transients calculation basic algorithm working out.
• Devices samples research in laboratory and real conditions.

Scientific novelty

        New decisions on revoltage level restriction in networks with the isolated neutral and protection against phase ground short circuits in electric networks performance way improvement are offered.

Practical value

        Circuit decisions offered in work will find wide practical realisation for electric systems with isolated and resonant earthed neutral work reliability increase.

The work maintenance

        For networks with the isolated neutral work reliability increase, depending on network voltage value, at ground short circuit capacitor currents from above 10-30А neutral grounding through inductive resistance is applied to capacitor currents indemnification [1]. For networks with less specified short circuit currents, recently apply network neutral grounding through active resistance.

        So, for example, in a auxiliaries network 6 kV thermal (ТЭС) and nuclear (atomic power stations) power plants according to [2] network neutral grounding through low-value resistor in value 100 Ohm which is connected to a special transformer TSNZ-63-10 with capacity 63 kVА neutral is required. Such resistor installation according to [2] increases ground short circuit currents that provides relay protection selective work which operates on damaged unit switching-off. However for the arch thermal action decrease purpose and resistor thermal stability increase, in [3] high-resistance resistors in value 1000-2000 Ohm application is recommended. resistors installation positive sides concern revoltage level decrease to (2,2-2,5) Uф, occurrence prevention ferroresonant processes and relay protection action clearness increase. Originally as grounding resistors were used glass-epoxy betel resistors which lacks are design bulkiness and the limited thermal firmness that did not allow to use widely them in responsible consumer electrosupply systems. Now by the Novosibirsk enterprise «PNP Bolid» are developed resistors from a material "Ecom" (an electrospending composite material - ceramics with electrospending additives) which have the raised thermal stability are issued and can longly (to 6) remain in work at single-phase ground short circuits . However a such resistors lack is high cost, design bulkiness, installation and adjustment complexity. In connection with, actual ground short circuit active component current increase way further perfection, not demanding high-voltage resistors installation is stated.

Researches results

        In electric networks 6-35 kV which can to work for a long time with phase ground short circuit, neutral based transformer is chosen with the nominal phase primary windings voltage equal to linear network voltage, and a higher voltage windings neutral is earthed through the resistor (fig. 1).

                                    Figure 1. The resistor connection to a transformer windings neutral scheme

        At the questions decision connected with resistor connection with the neutral based transformer help, it is necessary to choose the corresponding transformer, resistor value, its capacity and connection way. Now by the Moscow electrofactory for networks 6 kV the special dry transformer type ТСНЗ-63/10 for neutral grounding with rated voltage 10 kV and capacity 63 kVА, with windings connection group - star with a zero conclusion-triangle serially is issued. Thus the lowest voltage windings linear ends do not deduce, as the transformer is not calculated on loading connection, but only on resistor connection to a the higher voltage windings neutral. It is necessary to notice, that the given transformer is used, basically, only in power stations auxiliaries networks 6 kV for neutral grounding through the resistor in value about 100 Ohm. For other electrosupply systems it is necessary to define demanded capacity neutral based transformer taking into account real network and connected resistor parametres. For these purposes can be used, at insignificant reconstruction, usual dry or oil serially let out transformers, which higher voltage windings neutral has no contact. Transformers reconstruction consists of contact for higher voltage windings neutral which is earthed through the resistor is in addition established.

        At arc short circuit occurrence in a network with the isolated neutral and arch extinction at the high-frequency component transition through "zero" moment, the charged network capacities have no discharging way, as is the revoltage in value to (3–4) Uф occurrence reason at repeated arch ignitions. Resistor inclusion in a neutral discharges network capacity that prevents charges process accumulation on it and big revoltage occurrence possibility.

        The high-voltage resistor should prevent also ferroresonant process occurrence in a network at alternating arch burning character. These processes can arise at high voltage transformers magnet core magnetisation degree having the earthed neutral through which windings inductance flows down capacitor charge after arch extinction. Transformers magnet core saturation can arise, if the energy reserved in network capacities by the arch extinction moment ( W_c), it will appear more than electromagnetic energy ( W_L), reserved in inductance specified above transformers. In other words, if on capacity there is a charge which value is sufficient for high transformer magnet core saturation degree at the capacity discharge on the earth through one winding in a network arises ferroresonant process:

W_c>W_L

        Magnet core saturation can be prevented, if the capacity discharge after arch extinction proceeds in time less then a network industrial frequency half-cycle (0,01с). Knowing, that capacity attenuation process through the resistor should occur for 3,6 capacity discharge time constants, the resistor value can be defined as:

0.01·(3.6·C)^-1≥R

        The [ower on which the resistor should be calculated, together with the transformer, we find from a condition:

P≥R·(3·U_ф·w·C)²

        Thus it is necessary to mean, that an active ground short circuit current component , caused by resistor resistance, should not exceed network capacitor current value. Revoltage level in a network that above, than is more resistor resistance and transformer power. Therefore it is not necessary to accept transformer power too overestimated.

        The same level in a primary network, as well as in the scheme on fig. 1 A, it is possible to receive zero sequence currents if to connect the resistor to the transformer low-voltage windings connected in the opened triangle, and higher voltage windings neutral to earth (fig. 1 B).Scheme use (fig. 1B) does not demand bulky high-voltage resistor installation, and allows to use serially let out wire fehral resistors having rather low cost, small dimensions and carrying out same functions, as the high-voltage resistor in the scheme (fig. 1А). Serially let out transformers reconstruction thus consists in an additional higher voltage windings star zero connection conclusion. Lowest voltage windings are coalesced in the opened triangle, using the same conclusions.

NETWORK Dokuchaevsky FDI SUBSTATION 35/6 kV "Rudnichnaya"MATHEMATICAL MODEL WORKING out ON the COMPUTER

        In Master's Thesis the mathematical model is presented, allowing to analyze transients at phase ground short circuit in networks 6 – 35 kV. On fig. 2 the network equivalent circuit is established. In this scheme the power supply is presented by phase dispersion inductance L and resistance R. The network is reflected by concentrated phase C and pole-to-pole Cм capacities. Isolation is presented by active phase Rи and pole-to-pole Rм resistance. Units in an equivalent circuit are presented in the resistance Rпр and capacities Спр form. As overvoltage clippers aerial fuses are established. On fig. 3 the network graph is established, where tree chord and branches are designated. At transients at phase ground short circuit research in networks 6 – 35 kV the electric network analysis formalized topological methods, such as a planimetric currents method and Euler's implicit method are used. On fig. 4 phase ground short circuit calculation results are established.

                                    Figure 2. A network equivalent circuit

                                    Figure 3. The network graph

Figure 3. The phase ground short circuit calculation results.
Shots quantity in animation - 7.
Cycles quantity - 6
The size - 129 КБ

The basic results

• The typical network Dokuchaevsky FDI 6 kV substation 35/6 kV "Rudnichnaya" with the isolated neutral mathematical model is developed.
• The Dokuchaevsky FDI electrosupply operating system is studied. The relay protection not selective work reasons are analysed.
• The transients mathematical model on the PC is investigated and the received results analysis is made.
• Processes at phase ground short circuits algorithm, programs and calculations on the PC are developed, and also ferroresonant processes improvement performance principles.
• Devices samples in laboratory and real conditions are investigated .

The conclusion

        On the researches great volume basis executed with network mathematical and physical models and separate experiences in a real network use, essentially new phenomena demanding the new approach to revoltage protection frames choice and arrangement are found. In the research results analysis course it is established, that distributive networks traditionally applied in our country 6-10kV neutral grounding modes in the developed conditions do not meet electric equipment isolation modern condition requirements and consequently should be improved, however is shown, that the unequivocal this problem decision does not exist.

        At a neutral mode choice for each concrete network its specific features should be considered, in particular: its parametres, an isolation condition, a consumers category, protection frames from ground short circuits presence, the electrosecurity requirement etc. For the neutral mode influence decrease on equipment working conditions purpose are offered the circuit decisions which practical realisation allows to exclude single-phase short circuits in pole-to-pole short circuits possibility, electric equipment group failure and many-placed isolation breakdowns on the damaged phase that will essentially raise distributive networks work reliability in developed conditions.

Literature