CC BY
9ТЕХНИЧЕСКИЕ НАУКИ
Khvalin D.I.
candidate of sciences (engineering), scientific secretary Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine
Dovydkov S.A. head of department
Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine
INFLUENCE OF CONSTRUCTIVE FACTORS ON THE TEMPERATURE DISTRIBUTION IN END
ZONE OF POWERFUL ELECTRICAL MACHINE
DOI: 10.31618/ESSA.2782-1994.2022.1.78.259 Abstract. Analysis of methods and means reducing the heat of a stator core end zone of powerful generators with gas cooling according to results of a review the technical literature and scientific works is made. The main drawbacks of these methods and means are revealed. Various constructive factors influencing of electromagnetic field and temperature distribution in end zone are analyzed. Taking into account the large unevenness of electromagnetic and heat load the constructive units of a generator, especially the stator and rotor end parts tends to increase with the rise of generators unit capacity, these methods solve a limited range of problems is shown. The methods for reducing the tangential unevenness and heat of stator core end packet teeth of a powerful turbogenerator are proposed.
Keywords: powerful electrical machine, end zone, stator core, electromagnetic field, temperature.
At one time, the desire to reduce operating and capital costs of power units led to the building a capacity limit generators of 300, 500, 800-1200 MW. Primarily, this is was due to economic efficiency, since for more powerful turbogenerators decreases the construction and installation works, reduces the operating costs and increases thermal efficiency [1].
However, an increase of generators unit capacity is a complex engineering and design task that can be seen from the expression (in kilovolt-amperes)
P = 1,11 A2 knASBk 110~2
where D1 is the diameter of stator boring, m; l1 is the active length of stator, m; n is the nominal frequency of rotation, rpm; AS1 is the linear load of stator, A/cm; Bg is the magnetic flux density (of first harmonic) in the air gap, T; ky1 is the coefficient of stator winding step reduction.
For constant n and ky1 the power will be
determined by the volume of stator boring () and
the scale of electromagnetic use the active materials (AS:Bs). Thus, the extension of machine power can be achieved either by increasing the geometric scales of a machine, or by rising the electromagnetic loads. Possibilities of increasing the linear sizes D1 and l1 in generators are limited by the requirements of the rotor and bandage rings mechanical strength and, ultimately, the capabilities of metallurgical industry, in particular a problem of building the heavy forgings of large dimensions with high mechanical characteristics and transport conditions. The increase of magnetic flux density in air gap is limited the stator and rotor magnetic condition. The use of electrotechnical cold-rolled steel allows some to increase the magnetic flux density in stator teeth, but the rapid saturation of rotor tooth zone of a generator with not changed magnetic properties of materials does not to give increase
considerably the magnetic flux density for extension of machine power.
In other words, when building a powerful generator the geometric scales remain almost unchanged, and the stator linear load is constantly rising. The increase of linear loads at constant geometric scales leads to intensive rise the leakage flux of winding frontal parts. These flows excite eddy currents in constructive parts of end zone that cause additional losses in these elements. Additional losses distribution in the elements of end zone design is quite uneven leads to increase and sometimes unacceptable local heat of separate units.
Therefore, the increase of stator linear load raises the probability of breakdown a generator due to overheating and failure of some units in end zone. It is necessary to build a working reliably machines in the required operating conditions, since the cost of downtime a powerful generator exceeds the cost of construction.
In direct cooling generators due to a sharp increase the linear load AS1 rise additional losses of short circuit. Additional losses on the surface of rotor and stator teeth caused by higher harmonics not increase, since the magnitude of leakage flux density in the gap are limited at increase AS1 by raising the gap length. At the same time, leakage flux losses in stator winding frontal parts increases because almost unchanged the magnetic conductivity in generator end parts and the magnetize strength of leakage flux increases proportional to AS1. These losses causes increased heat of the stator core end packets, press plates and other structural elements located near the winding frontal parts.
In generator designs with high using materials provides the special means to reduce additional losses.
The end packets stator teeth are made with slits for reduce the eddy currents losses caused the end magnetic field.
A very good mean for reducing the leakage flux in end parts is installation the damping shields of thick copper sheets placed on the outer side of press plate or between it and end packet. In the presence of a copper shield between press plate and end packet the leakage flux additional losses entering into packets end zone are reduced. It is advisable to provide artificial cooled copper shield (for example, water).
In some designs instead of the copper shield in stator winding frontal parts placed the circular core of electrical steel sheets in order to prevent the penetration of the leakage flux in massive parts of press plate and stator housing.
In more modern generators in order to reduce the losses and heat of stator end packets used the magnetic shunts that are packets of electrotechnical steel similar to packets of the main core, but less thickness (20 mm) and with considerably short teeth (or completely without them). Thus, these packets not have the main flux, but close the axial leakage flux of stator winding frontal parts penetrating through the press plate and shield. For better ventilation the magnetic shunts are separated from the main core by press fingers (Fig. 1). Magnetic shunts reduce the heat of core end packets compared to the usual design and allow operating generators in underexcitation modes without a high heat of core end packets
Fig. 1 Stator core end zone of a generator type TVV-1000-2U3: 1 - the winding frontal part; 2 - the press
plate;
3 - the electromagnetic shield; 4 - the press finger; 5 - core packets; 6 - magnetic shunt.
At the revolution of rotor the end leakage flux of stator and rotor winding frontal parts crosses the massive units placed in this zone of generator and thereby causes losses in them. Usually all fastening units of core and stator winding in order to reduce losses are made from non-magnetic materials. The rotor bandage rings of non-magnetic material also help to reduce end flux. The press plate of stator core is made of non-magnetic steel (^e = 1,05) with the high electric resistance p = 0,8 Ohmmm2/m.
The non-magnetic press plate is also an electric shield.
The thicker shield will better shielding the constructive units. So usually try to work in strong shielding zone, that is, in zone with a large thickness of a shield taking into account the economic and constructive factors.
It is necessary to note the unwanted side of the action of electric shield related with placing in the circuit excites a flux. The shield currents cause not only the weakening of general flux penetrates into shield, but also the distortion of magnetic field. So part of flux is displaced in the space near shield together with the shielding of constructive units located directly behind shield. Therefore the losses in some units may increase.
For example, the electric shield protecting the core back displaces the partial end flux in machine tooth zone.
Transposition of wires in slot for one full turn along the length eliminates the potential difference between conductors due to their different position in height of slot. At the same time, however, there is potentials difference due to uneven position the conductors in section of frontal parts. As a result currents causes the additional losses in bars with a normal transposition on 360° in slot will circulate.
The transposition conductors on one and a half turns (540°) in large generators with a high current density in the winding are appropriate. This is partially compensates for the absence of transposition in the frontal parts and minimizes the magnitude of circulate current within a half-bar.
The increase of unit capacity and current loads generators made task the research and development of new insulating materials and technological processes that meet the high requirements in this field. As a result the researches a series of insulation types have different kinds the initial materials and manufacturing method, but very similar physical properties. Such insulation is called thermo-reactive.
However, the mounting of winding bars with thermo-reactive isolation requires great precision and care because of inelasticity it prevents deformation.
Usually for direct cooling machines the more parallel branches in the cooling system the more even heat of winding. For direct cooling machines the coolant temperature flows inside of winding ducts is also of great importance.
It is necessary to take into account constructive factors influences on magnetic field distribution in end zone in order to achieve the most efficient cooling of electrical machine end zone (including turbogenerator). The analysis of theoretical studies showed for magnetic field determination in end zone it is important to consider the following:
1. Location of teeth the most loaded end magnetic flux and the values of flux on their surfaces depend on the shift direction of stator winding layers at a given the revolution direction of rotor. It is advisable to perform the stator winding so that the lower layer is shifted relative to the upper against the rotation of field for the goal of more even the end magnetic flux distribution between the generator teeth and decrease its concentration.
The value of axial magnetic flux through the surface of teeth located at the phase zone junction of stator winding is greatest [2-5]. In this case there are two possible variants, namely: at the junctions of the lower layer if it is displaced relative to the upper in the revolution direction of field; at the junctions of both the lower and upper layers if the lower layer is displaced relative to the upper against the revolution direction of field. In the second variant a load unevenness of separate teeth is less as compared with the first variant.
One of the effective methods of reducing the heat tangential unevenness is the redistribution of cooling hydrogen flow between the press plate and end packet with an increase its volume towards on cooling the interphase zones by increasing the appropriate apertures in the gas-distributive cylinder. Thus the hydrogen flow in interphase zone (correspondingly, and the heat transfer coefficient) can be increased in 1,5-2 times without the cooling deterioration other elements and units stator of a generator.
In particular, in [6] the increase in 1,5 times the heat transfer of tooth end surface of interphase zone in the ventilation duct between the press plate and packet was considered. According to the obtained results, such cooling intensification of interphase zone allows reduced the maximum temperature of packet at 7 %.
Another promising direction of reducing the total heat level of stator core end packet is increase the thermal conductivity of laminated iron across the sheets. One of the technologies allows to increase the transverse thermal conductivity is glue together the end zone packets, for example, with help an epoxy paste EP-3. On the one hand, this increases the packets mechanical stiffness in the changeable load modes and the underexcitation modes of a generator. On the other hand, studies [7] show the thermal conductivity of glued together packet an average is 3,4 W/(mK), that is almost twice above a similar indices in the absence
of glue together 1,9 W/(m-K)). Addition in paste the dispersive insulating fillers with high thermal conductivity (> 5-10 W/(m K)) allows bring the equivalent thermal conductivity of the packet up to 56 W/(mK).
The temperature maximum level of packet steel teeth decreases by 7 % according to the calculating results of end packet heat at the transverse thermal conductivity 3,4 W/(mK) [6]. Simultaneous introduction of intensification the interphase zone cooling and increase of transverse thermal conductivity of steel allows reducing the tooth maximal heat to 14 %. In this case the margin of packet teeth maximum temperature can be achieved allows extending the generator load diapason in under-excitation modes with respect to permissible temperature of stator core.
2. Influence of structural modifications (the shape of chamfer the end packets, the ratio of length the stator core and rotor, the geometric shape of stator and rotor winding frontal parts, for example, the scale of frontal parts, the angle of bend the stator winding frontal junctions etc., relative values of width the teeth and slot stator, the presence of slits and stepped form the end packets teeth etc.) on the end magnetic field. Using the chamfer of stator core end packets provides a reduction of magnetic flux concentration in chamfer zone, but causes an increase of magnetic flux density on the vertical surface area adjoined with chamfer. Thus the shape of chamfer affects the magnetic flux density on the surface end packets bevelled and located above the chamfer. It is necessary to consider the permissible heat of end packet under-slot zone and the stator core mechanical stiffness when increasing the chamfer almost on the entire height of tooth. The magnitude and distribution of magnetic flux along the bevelled surface depend on the shape of chamfer, as a rule, inappropriately make the chamfer beginning with an angle of slope to the horizontal more than 20°. Using a chamfer convex to air gap side instead of a uniform one allows reducing the maximum values of magnetic flux density on the surface located over chamfer. Changing the rotor length in respect of the stator core length affects the equivalent values of magnetic flux density on the separate areas of end surface at different configurations. It is necessary to note with an increasing of chamfer rises the beneficial effect area of rotor diminution. But too much rotor diminution may be inappropriate, for example, in a turbogenerators of asynchronized type for operate in deep under-excitation modes recommends the rotor core length to take a little longer than the stator core length. It is necessary to take into account when designing a generator the implementation of stator core with wider and smaller slots allows weakening the influence of leakage slot magnetic field on the end magnetic field.
Constructive feature of stator core end packets unlike the active zone packets may be the slits (for the monolithic of teeth the slits are oblique, angled to the radial direction and the lamination is performed with the overlapping of segments for the cross-overlap of slits), longitudinal slots and ducts in the teeth. This is done in order to reduce the eddy current losses created
in the sheets when entering the axial flux in it. In addition, an increase of slots and apertures total surface in teeth end part leads to more intensive cooling of end packets. The splitting on the entire height of tooth reduces the teeth end packets losses of a 300 MW generator by 35% under rated load conditions and by 44 % when a power factor cos 9 ~ 1,0 is shown in [8, 9].
The diameter of end packets boring increases with approach to the machine end zone, in other words, the stepped form of end packets teeth is used. This is achieved the round of active iron lugs in end zone of a generator and reducing the stator core end packets losses. Although the axial component leakage flux of frontal parts is greatly reduced with that much share of one enters the stator core along the height of laminated sheets tooth zone. With this always a compromise between the need of reduction the axial component leakage flux and preservation the core compression force in the tooth zone by the use of press fingers is found. The end packets are made monolithic, that is specially pressed and baked and to some extent takes the function of additional press plates.
The presence of sharp angles and ferromagnetic parts lugs also affects the field distribution causing its local concentrations in certain areas of end zone. Thus, the research carried out with help simulation method by Holly [10] showed the magnetic field in zone of the ferromagnetic press plate lower edge with a profile under the angle 90° increases by 40-50 % as compared with the field in edge zone with a round profile.
3. As a rule, all stator packets have the same width. However the end packets with additional losses due to the end leakage flux for the goal of more intensive cooling are performed less width than in central part. The narrower ventilation ducts allow reduces the packet width and increases the air speed.
4. In four-polar generators the values of magnetic flux density are lower than in two-polar ones.
5. As a rule the appropriate constructive decisions should be different for generators with the main load conditions are nominal and under-excitation when a power factor cos 9 ~ - 0,95 and generators for operate in deep under-excitation modes.
6. According to results of analysis the magnetic field components of separate sources on the end packets surface and it interaction in different load conditions [11-15] for the goal of facilitate the choice of suitable variants the end zone design, the most significant are magnetic flux density components because of displacement the magnetic field from the air gap and stator core slots and eddy currents of press plate with shield in the under-slot zone, also because of motive-magnetic forces the stator and rotor winding frontal parts. The relative value of magnetic flux density of separate sources in the result magnetic field depends on the end zone configuration.
Provided a number of practical measures obvious in physical substantiation and aimed at improving the conditions of machine end parts when constructing new series generators. The basic ones are: reduction of total level temperature the main parts; introduction of the
automatic control cooling system ensures a decrease of windings and gas temperature variations when changes a load conditions of a generator; reduction of relative length the active zone and increase the mechanical stiffness of rotor, stator core and stator winding fastening elements.
Conclusions
The end zone shielding, using the end stepped packets of stator core, the chamfer of teeth end packets of stator core, the longitudinal slits of teeth end packets, using the magnetic shunts and others are the main constructive methods for reduction the eddy currents losses and improvement the cooling of a powerful generators end zone. Taking into account the considerable unevenness of electromagnetic and heat load of generator constructive units especially the stator and rotor end parts tends to rise with increase of generators unit capacity, these methods solve a limited field of tasks the general problem.
Literature
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2. Danilevich Ya.B., Pipko R.M. Magnetic field in the stator core end zone of alternating current electrical machine taking into account the influence of leakage slot. Elektrotekhnika. 1982. № 9. P. 36-39. (Rus.)
3. Kostyaev B.V., Ptashkin A.V. About heat the stator core end packets of large turbogenerators at the condition of reactive power consumption. Elektricheskiye stantsii. 1979. № 5. P. 40-44. (Rus.)
4. Smorodin V.I., Karatsuba A.S., Rudenko L.N. et al Some features of electromagnetic processes on the turbogenerator stator end. Tekhnichna Elektrodynamika. 1983. № 3. P. 65-72. (Rus.)
5. Titko A.I., Fedorenko G.M., Livshits A.L., Kobzar K.A. Tangential unevenness of electromagnetic and thermal characteristics in the turbogenerator end packets for changes load. Gidroenergetika Ukrainy. 2012. № 3. Pp. 48-58. (Rus.)
6. Kensytskyi O.G., Hvalin D.I., Sorokina N.L. Reduction of heat unevenness for stator core end packet of powerful turbogenerator. Pratsi Instytutu elektrodynamiky Natsionalnoi Akademii Nauk Ukrainy. 2018. № 49. P. 27-32. (Ukr.)
7. Schastlivyi G.G., Fedorenko G.M., Tereshonkov V.A., Vygovskiy V.I. Electric machines with a liquid cooling. Kiev: Naukova dumka, 1985. 288 p. (Rus.)
8. Postnikov I.M., Stanislavskyi L.Ya., Schastlivyi G.G. Electromagnetic and thermal processes in the end parts of powerful turbogenerators. Kiev: Naukova dumka, 1971. 360 p. (Rus.)
9. Smorodin V.I. Experimental researches and analysis of electromagnetic processes features in the stators end zones of powerful turbogenerators. Prepr. № 709. Kiev: Instytut elektrodynamiky AN USSR, 1991. 28 p. (Rus.)
10. Hawley R. End-region analysis for large generators. Electrical Times. 1967. Vol. 152. № 20. P. 771-774.
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623.526.6
Шеманаева Л.И.
кандидат технических наук, доцент Федеральное государственное бюджетное образовательное учреждение высшего образования "Ковровская государственная технологическая академия имени В.А. Дегтярева".
601910, Владимирская область, город Ковров, ул. Маяковского, д.19
МОДЕЛИРОВАНИЕ БЫСТРОТЕКУЩИХ ТЕПЛОВЫХ ПРОЦЕССОВ.
L.I. Shemanaeva
Candidate of technical sciences, associate professor Federal State Budgetary Educational Institution of higher education "The Kovrov State Technological
Academy named after V.A. Degtyarev". 601910, Vladimir region, city of Kovrov, st. Mayakovsky, 19
SIMULATION OF FAST THERMAL PROCESSES.
DOI: 10.31618/ESSA.2782-1994.2022.1.78.257 Аннотация. Рассматриваются варианты упрочнения внутренней поверхности ствола с использованием специальных методов моделирования.
Annotation. Options for strengthening the inner surface of the barrel using special modeling methods are considered.
Ключевые слова: Разгар, азотирование, кварц, COMSOL. Keywords: Glow, Nitriding, Quartz, COMSOL.
Введение
В процессе выстрела, кроме воздействия давления и температуры на канал ствола оказывает действие пороховых газов (разгар). Первые признаки разгара - матовые пятна на полированной поверхности канала, преимущественно в зарядной каморе, далее появление сетки тонких и неглубоких трещин в металле их постепенный рост в глубину и длину.
При достаточном развитии этих продольных трещин пороховые газы и несгоревшие частички пороха начинают прорываться между ведущим пояском снаряда и выгоревшим участком поверхности канала. Далее получается вырывание металла и срывание полей нарезов.
Разгара ствола сначала протекает медленно, но по мере увеличения числа выстрелов скорость выгорания увеличивается. Принято считать, что если начальная скорость снаряда снизилась за счет разгара ствола на 10%, то такой ствол непригоден для дальнейшей стрельбы.
Цель исследования
Анализ методов упрочения внутренней поверхности канала ствола, подтверждение выдвинутых гипотез.
Материал и методы исследования
Анализ разгара ствола выявляет следующие причины:
1. Действие теплоты на поверхностный слой металла. Пороховые газы успевают во время выстрела нагреть до высокой температуры тонкий поверхностный слой стенок канала ствола. Этот слой стремится расшириться, чему препятствуют близлежащие холодные слои металла. В нагретом слое возникают напряжения, в результате которых на внутренней поверхности стенок появляются трещины.
2. Механическое воздействие пороховых газов на стенки канала ствола (эрозия). Износ канала ствола увеличивается с ростом давления пороховых газов, и наиболее сильное выгорание канала происходит у начала нарезов, т. е. в том месте, в котором ведущий поясок еще не успевает полностью войти в нарезы и где, следовательно, легче происходит прорыв газов.
