Abstract
Contents
- Introduction
- 1. Analysis of operating experience of electrical networks, their own needs Starobeshevo TPP
- 2. Present state of the arc voltage surge in the networks of CH CHP and justification of methodologies for research
- 3. A mathematical model for the study of transients in the network's own needs TPP
- 4. Results of the study of transients in the network's own needs for power arc faults in the earth
- Conclusions
- Source List
Introduction
Plant's own needs are very important part in the scheme of the reliability of the power plant. Network of their own needs determine the reliability of the station as a whole, and the power system, which issued its power. Modern thermal power plants were built on the TEC hub principle, in which each generating an object mounted in a separate unit. Each unit includes a boiler, turbine, generator, transformer.
Starobeshevo station was built on the same block principle, the power turbine generators now stands at 200 MW. At the power plant installed 10 such units, fueled by a low-grade coal - coal rubble.
The technological process of electricity generation in modern thermal power plants is fully mechanized and therefore there are a large number of mechanisms for their own needs (SN) as the main electrical service shops and stations. As the drive mechanism of the thermal power plants are mainly electric drive.
The basic requirements of their own needs, is to ensure the reliability and efficiency of the mechanisms. The first requirement is the most important, because disruption of the mechanisms of their own needs entails a disorder of complex technological cycle of production of electricity, disruption of basic equipment, and sometimes the whole plant development and leads to a system crash. Very important is also the requirement of efficiency. This is achieved by reducing power consumption and heat generation in the system of their own needs, improvement of primary and auxiliary equipment, reducing capital costs for the system of their own needs, management efficiency, etc.
For the drive mechanism of their own needs at the station uses three-phase asynchronous motors with short circuited rotor, three-phase synchronous motors, DC motors.
Asynchronous motors with squirrel-cage rotor are most commonly used for the drive mechanism of AE, due to its reliability, ease of start-up and operation. Their drawback - a large inrush current.
Induction motors with slip-ring motors more complex: the presence of brush apparatus of the rotor winding, regulating rheostat. They apply only to drive the lifting mechanisms.
Synchronous motors are set to drive sharobarabannyh mills.
Electric motors are much more expensive and harder to AC motors. They are used to drive the feeder dust, oil pumps turbine emergency oil pumps and emergency generator seals. Rated power of electric motor, which are used in networks of their own needs and the power range from 250 5000kVt. Together with the motors on the bus sections are connected transformers CH 6/0, 4 kV, the power of up to 1000kV.
The network's own needs 6kV operates in an isolated neutral. The main advantage of the regime network with isolated neutral is that the single-phase circuit - the most frequent type of injury, there is not an emergency mode and a network for up to four hours can be operated in this mode, which ensures high reliability of power supply to consumers while reducing the cost of redundancy. However, in the mode of single-phase earth fault (OZNZ) Isolation of intact phases may be a long time is at line voltage and the injury occurs through the fault current to earth. This can damage the insulation of healthy phases and two-phase short circuit, causing the network in the state of emergency.
1. Analysis of operating experience of electrical networks, their own needs Starobeshevo TPP
reliability of power plants is largely determined by the electrical safety of their own needs. Damage to the equipment entails a disorder of complex technological process of production of electricity, changing modes of operation of individual units or stations in general, accompanied by nedootpuskom electricity consumers. However, as the experience of operating a number of large power plants, operational reliability of networks auxiliary units of 200 MW and over remains at a fairly low level. Rather extensive material on the static damage in networks, SN presented in [1]. Some of the results, which for a number of large thermal power plants. From the analysis of Table
. 1 that year at thermal power plants damaged by about 6% of the total installed electric motors. At the same time, such as Uglegorsk TPP "Donbasenergo" an annual average of about 35 damaged by large electric motors (Table 2), working under severe conditions of contamination and moisture, under the impact of large electro-surge.
From the analysis of these tables, it follows that under the circumstances, damage to electrical networks SN is at a high enough level. Analysis of these results shows that almost 90% of the malfunction of the network begins with the closure phases on the ground, so the main focus of the struggle for improving the reliability of the network discounts sc is a single-phase ground fault. Statistical data on the damage to electrical equipment in the network's own needs Starobeshevo TPP.
seen from this figure that the damaged cables and mostly electric motors. Quite often there are many-group breakdown of insulation from the failure of electrical equipment.
2. Present state of the arc voltage surge in the networks of CH CHP and justification of research methodology
Among the numerous studies of communication surge arising from any kind of opening and closing electrical circuits, the greatest amount of research was devoted to a very common voltage spikes with arc ground faults in high voltage networks, working with isolated neutral.
founder of the surge of research was the German engineer, Petersen, who in 1916 developed a theory to explain the physical nature of the emergence of the maximum surge.
In 1923 two American engineers and Slepian Peterson offered another theory, fundamentally different from the theory of Petersen. Later, these theories have been supplemented by the results of research sovetskih NM Juvarli and NN Belyakov, who on the basis of theoretical and laboratory studies of the maximum surge levels and forms of development, have made their proposals.
In 1957 NN Belyakov was published by the theory of a surge in the arc ground faults in networks with isolated neutral.
It is known that the closure phase on the ground in networks with isolated neutral in the steady state voltage on the intact (healthy) phases increases to a linear value. However, the steady state is preceded by the transition process, the multiplicity of the surge in which both the healthy and the damaged phase can reach a much greater magnitude. The process is complicated by the fact that in most cases, ground fault occurs through an arc that occurs as a result of the overlap, or insulation breakdown. In this case the arc is not stable, and there are re-ignition and combustion of (intermittent arc), which lead to the development of transient oscillatory processes and increase the surge. The value of surge depends on terms of extinguishing the arc, and the nature of the process of restoring the strength of the electric arc gap after its extinction.
Since the ground-fault arc passes through the capacitive current operating frequency:
and current high-frequency oscillations. It can be assumed that the quenching of the arc occurs when passing through zero current high-frequency oscillations (theory Petersen) or when a current is passed through the operating frequency to zero (the theory of Slepian and Peters), and ignited at the maximum stress on the damaged phase. According to the theory
Petersen, the maximum voltage on the healthy phases in the transition regime can be defined by the formula:
where Uf - the amplitude of phase voltage;
- coefficient depending on the ratio of interphase and capacities in relation to the land of C0 to investigated the network;
- coefficient depending on the capacitance, inductance, and power supply resistance leakage through the insulation of the network;
- an expression that defines the attenuation of the amplitude of the transition process associated with the leakage of energy through the active resistance of the network. The maximum voltage on the affected phase thus can be estimated by the expression:
According to this theory surge in intact phases can rise to 7.5Uf, and the broken phase they reach 3.7 Uf. As Peters and Slepyanu extinction of the arc is a half-period after ignition, when the free oscillations are damped and the instantaneous voltage on the undamaged phase reaches its maximum value, and the displacement of the neutral:
the maximum overvoltages on healthy phases will be
and the voltage on the affected phase depending on the moment of breakdown is determined by the expression
Thus, according to the theory of Peters and Slepian, as a result of exchange capacities of wires in the ignition and extinction of the arc voltage on the wire reaches the proper value 3.5Uf, and the damaged wire - 2 Uf. These values ??agree well with the surge results of calculations for the healthy and damaged phases, taking into account the attenuation and line to line capacitance in real networks. According to the theory of NN
Belyakov for the occurrence of the maximum voltage is not required a number of re-ignition of the arc. It suffices to consider only one cycle of the ignition-extinction-ignition. Assumptions
N. Belyakov theory occupies an intermediate position between the theories and Peters Petersen and Slepian. If, for Petersen, the process must cease arcing at the first passage of current through zero vibration, and by Peters and Slepyanu - passing through the zero-frequency current, then the occurrence of the maximum surge of NN Belyakov need to match the two basic conditions in a single cycle of the ignition-extinction-ignition of the arc.
Studies have shown (N. Belyakov) in real conditions both possible behavior of the arc, but the multiplicity of the surge is determined not so much, at what point of the arc quenching occurs as the properties of the arc gap and the nature of the process of growth of its electric strength.
N. Belyakov into account the real physical processes taking place in the arc gap, proposed the following mathematical expressions to determine Uper and Uper.p.f.:
where Usm - offset neutral; the remaining quantities have the same meaning as before
According to the theory of NN Belyakov, in the three-phase system, taking into account the maximum attenuation of high frequency voltage on the healthy phases do not exceed the values ??(3.2-3.4) Uf, and the damaged Uper.p.f. - 2.2 Uf. Numerous experiments in real networks, the 6-10kV confirmed that the arc voltage during ground faults do not exceed specified values. Long-term surge of such an order for networks with isolated neutral dangerous for weak electrical insulation, which may be in the system. It should be noted that these overvoltages are dangerous not only for its amplitude and duration, and high-frequency nature of the process. In addition, they cover the whole network, which increases the probability of overlap of isolation that can occur not only at the point of closure, but also in remote locations. At the same time, as already noted, the existence of a long arc earth fault usually leads to a short circuit between phases, accompanied by the shutdown of the installation. Therefore, in cases when you can not rely on the spontaneous extinction of the arc requires a fast elimination of arc-fault ground, which can be achieved by limiting the current through the arc gap and reduce the rate of recovery voltage.
Thus, the arc voltage spikes with phase to earth faults have traditionally paid much attention by leading experts of World Energy. The studies were conducted in real networks, as well as on mathematical models and physical models of electrical networks. For more than half a century of accumulated considerable theoretical and experimental data, the implementation of which in practice has significantly improve the reliability of the electrical networks of the class tension. However, so far in the literature there are many conflicting and sometimes contradictory data obtained by different researchers on the problem. Such contradictions are due to the complexity and diversity of factors influencing the nature and magnitude of transient overvoltage in different settings and mode of neutral grounding of electrical networks.
currently in continuous deterioration of the insulation of electrical power systems own use of TPP due to lack of funds to replace worn-out recovery and quality of electric urgency of this problem is further increased, as shown earlier, they are a major cause of damage to electrical equipment. As a reliable means of protection against surges arc absent, the successful solution of the problem can only be found in the optimization of the neutral networks of their own needs in conjunction with various schematics.
most reliable results can be obtained through experiments in real networks, but the possibilities of this method is limited by a number of objective factors, the main ones are: the inability to identify the experimental conditions from experiment to experiment, the complexity of the registration of such fast and repetitive processes which occur in single-phase ground faults, the limited amount of research inevitably caused by the scrapping of expensive electrical equipment during a large number of experiments, etc. All this makes it possible to obtain the required amount of information enabling to give truthful answers to many questions facing the problem. Features
Mathematical modeling of transient processes in the unwieldy OZNZ limited substitution patterns in the case meet the requirements of accounting required elements of the network and the adequacy of the distribution of their parameters, the difficulty of determining the parameters of the equivalent circuit of the individual network elements, the extreme complexity of modeling ground arcs, a large amount of the payment and m . etc. The adoption of any assumptions in the preparation of schemes of substitution leads to a drastic decrease in efficiency of the research.
3. A mathematical model for the study of transients in the network's own needs TPP
For the analysis of transients in the network's own needs with CHP arc ground faults take the circuit as a basis for power sc TPP.
In contrast to the well-known mathematical models of power supply systems of this type will be considered:
1) ground fault in the stator windings of induction motors and consideration of their impact on the character of the processes depending on the distance from the point of closure of the findings of stator;
2) displacement of the neutral network in the pre-damage mode due to unbalanced load or different phases of the active-phase conduction and capacitive isolation between phases;
3) the presence of a special connecting the transformer for partial neutral grounding through a resistance or a current limiting reactor;
4) the presence of nonlinear surge arresters connected to the busbars 6 kV;
5) different conditions of the arc - the arc extinction in passing through the zero-frequency current or current high-frequency oscillations;
6) different values ??of the breakdown arc gap with repeated intermittent ignition of the arc.
In preparing the plan take into account the substitution of relatively small length of cable connections for the conditions of their own needs of power stations (0.5 km) can be taken for all elements of the study focused network settings. We also consider the network under study as a linear, ie saturation of the individual elements is neglected. Based on the above in shows the equivalent circuit of the studied networks, adopted on the basis of a mathematical model.
This equivalent circuit is represented phase AC power supply emf, the leakage inductance L and the resistance of R. In the equivalent circuit network is taken into account capacitances (Ca, Cb, Cc) and resistances (Rua, Rub, Ruc) phase to earth insulation, inductive-capacitive (M, D) between phases bonds, which has a capacity of leakage resistance RT. In the neutral of the transformer can be connected to current limiting resistor RD or arcing reactor LD. High-voltage induction motor is included in the equivalent circuit subtransient phase AC leakage inductance L1 and resistances R1. In one phase of the electric motor can change the location of a single-phase ground fault along the winding through the introduction of variable resistors R11, R12, and the leakage inductance L11, L12. The chain phase to earth fault in motor winding simulates its capacity Cz and the resistance of the arc Rz. Zinc-oxide surge suppressors (ARF), mounted on busbars and the findings of engines are taken into account the nonlinear dependence of the resistance of the current or voltage.
mathematical model is described by the following system of differential equations:
4. Results of the study of transients in the network's own needs for power arc ground faults
As a result, a large amount of studies that were carried out using a mathematical model for different parameters and mode of neutral grounding networks sc CHP found that the main factor that determines the nature and magnitude of transient voltage surge when OZNZ network with isolated neutral is the capacity of phase with respect to ground and line to line capacitance, inductance and transformers, power supply, the nature of the load resistance in place of the closure phase on the ground and etc. To limit the multiplicities of overvoltages in the network with the specified parameters have a decisive importance: the value of the instantaneous value of voltage on the injured at the time of the initial phase of the arc ignition, the moment of arc extinction and voltage during the second and subsequent ignition of the arc.
below shows the calculated waveforms of transients in networks, SN TPP with arc ground faults. The first and subsequent breakdowns occurred at the maximum voltage the faulted phase, and the extinction of the arc at the time of passage of the current of industrial frequency (Fig. 4) and total fault current (Fig. 5) passes through zero.
Figure 4 - Process for arc fault phase C on the ground in the theory of Peters and Slepian in a network with isolated neutral (phase fault current to earth - 30 A)
Figure 4 - Process for arc fault phase C on the ground in the theory of Peters and Slepian in a network with isolated neutral (phase fault current to earth - 30 A)
Studies have shown for the different parameters on the electrical networks of CH CHP maximum surge in advanced phase after the breakdown of insulation up to (2.4-2.5) Uf, and the subsequent surge in the breakout quantity of healthy phases increases to 3.5 and even 4Uf. Escalation (gradual increase) surge in the network by arcing in the second scenario, due to increased stress on the neutral in the process of repeated ignition and extinction of the arc fault current in the arc gap. For networks of CH CHP, with their characteristic parameters, the magnitude of surge could reach (3.2-3.5) Uf. When the network voltage unbalance in phases over-voltage can rise significantly, as studies have established that the multiplicity of the arc surges grows approximately proportional to the displacement of the neutral.
Under modern conditions, improve electrical network's own needs can be due to the transfer mode networks with isolated neutral mode with a resistive-grounded, ie, neutral grounding resistor by an active size of the order of 130 150Om.
limit for the multiplicity of the arc voltage surge resistive grounded neutral is due to the discharge capacity of healthy phases and the decrease in neutral voltage to a value that excludes the escalation surge in repeated breakdowns of the emergency phase, the weakened insulation. The calculated transient waveform in the network SN TPP RC-grounded is shown in Fig. 6.
Figure 6 - Processes for the closure phase C on the ground in a network with a resistive-grounded
Conclusions
As a result of the work was the analysis of the processes that take place in the networks of their own needs for thermal power plants arc ground faults. It should be noted that the studies were based on the current state of networks based on actual operating data.
main results are as follows:
1) assess the current state of network problems auxiliary thermal power plants, where we see that the immediate need to replace electrical equipment because of its isolation is almost worn out, or use the necessary measures to ensure the operation of the equipment under specific conditions;
2) was used to calculate the mathematical model of auxiliary thermal power plants was written in the programming language Fortran, taking into account the ability to play different modes of operation;
3) defined by the multiplicity of the arc voltage surge limiting the network's own needs for conditions Starobeshevo TPP;
4) the variant of a high-resistance neutral grounding resistor, and an assessment of the method of grounding.
Source List
1. Pereh³dn³ processes in systems elektropostachannya Vlasna needs elektrostants³y: Navch. pos³bnik / VF Sivokobilenko, VK Lebedºv - Donetsk: Donetsk National Technical University PBA, 2002. - 136 p.
2. Dolginov A. High Voltage Engineering in electrical engineering. - Moscow: "Energy", 1968. - 464.
3.Sivokobylenko VF Dergilev MP Modes neutral distribution networks 6-10 kV. - Sat scientific papers DonSTU. Series: Electrical and Power Engineering, Vol. 67: - Donetsk: Donetsk National Technical University, 2003. - S. 49 - 58.
4.Sivokobylenko VF Lebedev, VK, Mahinda Silva. Analysis of the processes arc earth fault in networks of their own needs and the CHP plant. - Sat scientific papers DonSTU. Series: Electrical and Power Engineering, Vol. 17: - Donetsk: Donetsk State Technical University, 2000. - S. 129 - 133.
5. Likhachev, FA Surge in the networks of their own needs / / Electric Station. - 1983. - ¹ 10. - C. 37 - 41.
6. Silberman VA, Epstein IM et al Influence of neutral grounding of auxiliary unit of 500 MW at the overvoltage protection relay and work. / / Electricity. - 1987. - ¹ 12. - S. 52 - 56.
7. Dergilev MP, VK Obabkov Irreducible multiplicity surge in the 6-35 kV network with resistive grounded neutral. / / Science, Engineering and Business in the energy sector. - Ekaterinburg. - 2002. - ¹ 5. - S. 10 - 14.
8. Neklepaev BN Hooks IP Electrical part of power plants and substations. - Moscow: Energoatomizdat, 1989. - 490 p.
9. Gindulin FA, Goldstein, VG, Dulzon AA, FA Khalilov Overvoltage in networks 6-35 kV. - Moscow: Energoatomizdat, 1989. - 234 p.
10. Sirota IM, Kislenko SN Mikhailov, AM Neutral modes of electrical networks. - Kiev.: Science. Dumka, 1985. - 190 p.
11. Likhachev, FW Earth fault in networks with isolated neutral, and with compensation of capacitive currents. - Moscow: Energiya, 1971. - 254 p.