transformer engineering design and practice 2_phần 4

12 Recent Trends in Transformer Technology In the last one decade, rapid changes and developments are being witnessed in the transformer design, analysis, manufacturing and condition monitoring technologies. The technological leap is likely to continue for the forthcoming years with the simultaneous increase in the power rating and size of transformers. There is ongoing trend to go for higher system voltages for transmission which increase the voltage rating of the transformers. The phenomenal growth of power systems has put up tremendous responsibilities on the transformer industry to supply reliable and cost-effective transformers. Any failure of a transformer or its component will not only impair the system performance but it also has a serious social impact. The reliability of transformers is a major concern to users and manufacturers for ensuring a trouble-free performance during the service. The transformer as a system consists of several components such as core, windings, insulation, tank, accessories, etc. It is absolutely necessary that integrity of all these components individually and as a system is ensured for a long life of the transformer. The chapter identifies the recent trends in research and development in active materials, insulation systems, computational techniques, accessories, diagnostic techniques and life estimation/refurbishment. The challenges in design and manufacture of the transformers are also identified. 12.1 Magnetic Circuit There has been a steady development of core steel material in the last century from non oriented steels to scribed grain oriented steels. The trend of reduction in 437 Copyright © 2004 by Marcel Dekker, Inc. 438 Chapter 12 transformer core losses in the last few decades is related to a considerable increase in energy costs. One of the ways to reduce the core losses is to use better and thinner grades of core steels. Presently, the lowest thickness of commercially available steel is 0.23 mm. Although the loss is lower, the core-building time increases for the thinner grades. The price of the thinner grades is also higher. Despite these disadvantages, core materials with still lower thicknesses will be available and used in the future. The commercial materials can be divided into three distinct groups: non oriented, grain oriented and rapidly quenched alloys [1]. The amorphous magnetic alloys, typically available in thickness of 0.025 to 0.05 mm, are part of the third group. The loss of amorphous materials is quite low; about 30% of cold rolled grain oriented (CRGO) steel materials, because of their high resistivity and low thickness. Due to their non-crystalline nature (low anisotropy), the flux distribution is more uniform in them as compared to the CRGO materials. However, they are costlier and have low saturation magnetization (~1.58 T as compared to 2.0 T for CRGO). The maximum operating flux density for amorphous cores is therefore limited to about 1.35 T. Hence, although the core (no-load) loss is substantially low, the size and cost of the core increases, and the load loss is also higher. Therefore, the use of amorphous material is attractive when users specify a high no-load loss capitalization ($per kW). The space factor of the amorphous material is lower than the CRGO material. The amorphous materials are very sensitive to mechanical stresses; the core loss increases significantly with the stress. Also, they have a limited operating temperature range as compared to the CRGO materials. The properties of amorphous metals, viz. thinness, lower space factor, hardness and brittleness, pose design and manufacturing problems for the mass production of distribution transformers [2]. Distribution transformers up to 2.5 MVA have been made with amorphous core. Automation of core assembly process is desirable to make the amorphous core transformers cost-effective and to improve their performance. 12.2 Windings There has been no significant change in the type of winding conductors used in distribution and power transformers. The rectangular strip or bunch conductors and continuously transposed cable (CTC) conductors are used for windings of power transformers. Foils of either copper or aluminum may find preference for the LV winding of distribution transformers. The CTC conductor is preferably of epoxy bonded type for greater short circuit strength. There have been some attempts [3] to improve the winding space factor significantly by using a cable in which a number of parallel rectangular insulated conductors are bonded edge-to-edge with epoxy. It is reported that there is significant reduction in transformer losses and weight when this type of cable conductor is used. Recently, HV cable technology, used for power transmission and distribution, has been applied to transformers windings [4]. It results into a dry Copyright © 2004 by Marcel Dekker, Inc. Recent Trends in Transformer Technology 439 type transformer without oil with a current density lower than that of the conventional oil cooled transformer. The conductor consists of an innermost bundled conductor surrounded by a thin semi-conducting layer resulting into a more uniform field around the conductor. This semi-conductor layer is then surrounded by cross-linked polyethylene whose thickness depends on the voltage class. There is also an outermost semi-conducting layer which is earthed on each turn along the winding. Thus, the electric field is totally contained in the insulation. A special arrangement of forced air cooling is used. It is reported that the dielectric, mechanical and thermal design of windings can be done independently giving more flexibility for optimizing these functions. It is also claimed that the transformers manufactured by this technology will be more efficient, reliable and eco-friendly. The comparison of their cost with that of the conventional oil cooled transformers and their commercial viability are not yet reported. Superconducting transformers: Advent of high-temperature superconducting (HTS) materials has renewed interest in research and development of superconducting transformers. Previously developed low-temperature superconductors (LTS) required cooling by liquid helium to about 4°K, which was quite expensive. The development of technology based on liquid nitrogen (LN2) at temperatures up to 79°K has reduced the complexity and cost of the superconducting transformers [5]. Some of the most promising HTS materials are based on Bismuth compounds (BISCCO) and Yttrium compounds (YBCO). The principal advantages of HTS transformers are: much lower winding material content and losses (current density value of at least 10 times that of the conventional oil cooled transformers can be used), higher overload capacity up to about 2.0 per-unit current and possibility of coreless design [6]. Although HTS transformers have higher overload capacity, they have a very low through-fault sustaining capability due to small thermal mass. It is proposed in [7] that a conventional transformer can be operated in parallel with a HTS transformer. The HTS transformer is normally connected, and under through fault conditions it is disconnected and the conventional transformer is switched in immediately. The arrangement is shown to be more cost-effective (with lifetime costing) as compared to the parallel arrangement of two conventional transformers. In [8], it is suggested to use the HTS transformer as a current limiting device to limit the through-fault currents. During the fault conditions, the transition from the superconducting to normal conducting mode occurs increasing the resistance. Due to greatly reduced conductor dimensions, the strength of the superconducting winding against radial and axial short circuit forces is inherently quite low. The series capacitance also reduces due to reduction in winding dimensions whereas the ground capacitance is not significantly affected. This results into a very non-uniform voltage distribution. Special countermeasures (e.g., interleaving) need to be taken which increase the complexity of Copyright © 2004 by Marcel Dekker, Inc. 440 Chapter 12 construction. Although there is a possibility for optimization, certain minimum clearances between windings are required to get the specified leakage impedance. The main challenges of superconducting transformers are: short circuit withstand, through-fault recovery and withstand against high voltage tests (particularly the impulse test). For efficient cooling, it is desirable to have a direct contact between LN coolant and the conductor; hence in some designs the inter-turn insulation is arranged in such a way that the conductor edges are left as bare. Windings of each phase may be kept in a separate cryostat (made of fiberglass) and the tap winding is generally kept outside the cryostat to simplify the overall construction [5]. The tap winding and core may be cooled by forced gas cooling in which case it becomes oil-less, fire-hazard free and eco-friendly transformer. There is a considerable amount of research and development work currently being done to make the superconducting transformers commercially viable. A development of three-phase 100 kVA superconducting transformer with amorphous core has been reported in [9]. A design feasibility study for a 240 MVA HTS autotransformer has been reported in [5]. With the rapid development in technology, the availability of commercial units is certainly on the horizon. The prototype HTS transformers of rating 30/60 MVA are being developed [10] for their use by utilities in the year 2005. The commercial units may be available thereafter. 12.3 Insulation Low permittivity pressboard: If pressboard with low permittivity (around that of oil) is developed and if it is commercially made available, a more uniform electric stress distribution can be obtained opening avenues for insulation optimization as discussed in Chapter 8. Gas insulated transformers: There is considerable progress in the technology of gas immersed transformers in the last one decade. Unlike the oil-immersed transformers, they have SF6 gas for the insulation and cooling purposes. Initially, SF6 transformers were manufactured in small ratings (10 to 20 MVA). Now, the ratings as high as 275 kV, 300 MVA are quite common in some parts of the world. SF6 gas has excellent dielectric strength and thermal/mechanical stability. It is non-flammable and hence the main advantage of SF6 transformers is that they are fire-hazard free. Hence, these are suited for operation in the areas with a high fire risk. Due to lower specific gravity of SF6 gas, the gas insulated transformer is generally lighter than the oil insulated transformer. The dielectric strength of SF6 gas is about two to three times that of air at atmospheric pressure and is comparable to that of the oil at about two to three atmospheric pressure. But as the operating gas pressure is increased, a tank with higher strength is required increasing its weight and cost. Constructional features of SF6 transformer are not very much different than the Copyright © 2004 by Marcel Dekker, Inc. Recent Trends in Transformer Technology 441 oil-immersed transformer. The core of SF6 transformer is almost the same as that of the oil-immersed transformer. It usually has higher number of cooling ducts since the cooling is not as effective as that with the oil. Typical insulation over conductor is polyethylene terephthalate (PET). This material does not react with SF6 gas and permits higher temperature rise as compared to the oil-immersed transformer. The impulse strength ratio (strength for impulse test divided by strength for AC test) is lower for SF6 gas as compared to the oil-pressboard insulation system. Hence, the clearances in SF6 transformers get mostly decided by the impulse withstand considerations [11] and the methods have to be used which improve the series capacitance of windings. The ratio of the permittivity of SF6 to that of the solid insulation is lower than the corresponding ratio between the oil and solid insulation; this results into higher stress in SF6 gas than that in the oil. The duct spacers with lower permittivity may have to be used in the major insulation [12] to reduce the stress in the small SF6 gaps at the corners of winding conductors. The heat capacity of the gas is smaller than that of the oil and the thermal time constant is also smaller reducing the overload capacity of SF6 transformers as compared to the oil-immersed transformers. Due to the lower cooling ability of SF6 gas, a large volume of gas has to be circulated by gas blowers; this may increase the noise level of the transformers. For large capacity transformers, perflurocarbons may be used [13] for adequate cooling, and SF6 gas is used only as the insulating medium. But the construction becomes complicated; hence even for large capacity transformers, SF6 gas has been used as the insulation as well as the cooling medium [14]. Due to higher thermal stability of SF6 gas and quite a high value of temperature at which it decomposes, the dissolved gas analysis is not as easy as in the case of oil-immersed transformers to detect incipient faults [15]. The challenges which have to be overcome for the widespread use of SF6 transformers are viz. environmental concerns, sealing problems, lower cooling capability and present high cost of manufacture. 12.4 Challenges in Design and Manufacture of Transformers Stray loss control: There is continuous increase in ratings of generator transformers and autotransformers. Hence, one of the challenges is to accurately evaluate stray losses for their optimization (to have competitive/compact designs) and for elimination of hot spots. Advanced 3-D numerical techniques are being used to optimize stray losses in the windings and structural parts of large transformers. These techniques along with the stray loss control methods are described in Chapter 5. Even in small distribution transformers, the shielding methods are being adopted to reduce the stray losses [16]. Short circuit withstand: A steady increase in unit ratings of transformers and simultaneous growth of short circuit capacities of networks have made short circuit withstand as one of the most important aspects of the power transformer design. The short circuit test failure rate is high for large transformers. In fact, the Copyright © 2004 by Marcel Dekker, Inc. ... - tailieumienphi.vn