224MVA 220/69/13.8kV Transformer
Shell Form History
Mitsubishi Electric Corporation began manufacturing shell-form transformers after signing a licensee agreement with Westinghouse Electric Corp. in 1923. Over the years, Mitsubishi Electric has made many improvements to the original shell-form design and today is proud to be the world’s leading manufacturer of large shell-form transformers.
Through vast amounts of production experience and research and development, Mitsubishi Electric Power Products Inc. (MEPPI) can supply shell-form transformers of the highest level of quality, reliability and performance. From very early on, thre-dimensional computer analysis of electric and magnetic fields inside the transformer have allowed Mitsubishi Electric Power Products' shell-form transformers to avoid certain local-heating problems that have been associated with other shell-form transformer manufacturers’ designs.
Advantages of Shell Form Transformers
MEPPI chooses to manufacture shell-form transformers for high voltage, large capacity applications due to the inherent flexibility and advantages that the design offers for these applications. These advantages can be summarized as follows:
· High short-circuit withstand capability. The shell-form transformer is a mechanically strong design for a transformer, with the coils arranged vertically and completely surrounded on all sides by the iron core. The tank of the shell-form transformer is a “form-fit design” that fits just over the core, creating a very strong reinforcement against possible coil movement. This helps the transformer to withstand the high mechanical forces that are experienced during an external short-circuit condition. Also, the coils are arranged in alternating groups of high voltage and low voltage windings, referred to as an interleaved arrangement. This allows the short-circuit forces of the high and low voltage windings to act in opposing directions, thus partially canceling each other out and further increasing the short-circuit withstand capability of the transformer. For a shell-form transformer, as the transformer capacity increases, the size of each coil is kept similar, but more numbers of high and low voltage coil groups are added, which reduces the ampere-turns per winding group. This keeps the short-circuit force inside the transformer relatively constant even for very large capacity units.
Core-form transformer designs cannot employ this technique as the mean turn length of the coils would become prohibitively large due to the winding arrangement.
· High mechanical strength. The strong design of the shell-form transformer also has other benefits besides short-circuit withstand strength. The shell-form transformer also has high seismic withstand strength. This is further expanded by MEPPI’s experience in supplying many transformers to areas of the world that have a very high probability of experiencing earthquakes, such as Japan, California, and other areas. MEPPI has applied much research and effort into building transformers with high seismic-withstand capabilities. Along with seismic strength, the shell-form transformer can also withstand the demands of transportation by various means, including ocean vessels and railroads. This allows the transformer to remain intact during shipping, without the need for internal shipping braces. Shell-form transformers can even be shipped laid on their sides for instances where the inland transportation dimensional limitations are severe.
· High dielectric strength. The MEPPI shell-form winding design can achieve a very high dielectric strength against impulse voltage due to the design of the windings and insulation structure. In general, the number of coils in a shell-form transformer is relatively low and the surface area of each coil is large. This creates a large capacitance between the coils and a low capacitance to ground. This allows for the surge distribution across the winding to be almost uniform, with no oscillations. The static plate located on the line end of each winding also enhances the surge voltage distribution and plays an important role in smoothing the steep front of the impulse wave. The oil-immersed transformer insulation system generally consists of special paper and pressboard materials immersed in oil that provide insulation for the copper conductors and other internal parts of the transformer. In shell-form transformers, these pressboard materials that surround the coils subdivide the oil insulation into many small gaps. The oil impregnated pressboard materials have a much higher dielectric strength than the oil gaps. This insulation design leads to a very high dielectric strength. In shell-form transformers, the coils and solid insulation are also easily arranged along equipotential lines, providing a high dielectric strength against insulation breakdown. Due to this arrangement, insulation breakdown cannot develop unless the solid insulation is punctured. And from the insulation structure of shell-form transformers, for a discharge to break down the insulation and cause an internal fault in the transformers it must puncture several layers of this pressboard material. This provides a highly reliable insulation system. For these reasons, shell-form transformers offer an advantage for extra high voltage applications.
· Excellent control of leakage magnetic flux. The shell-form transformer provides for excellent control of leakage magnetic flux to prevent against local heating inside the transformer. The coil windings are almost completely surrounded by the iron core, which absorbs most of the leakage flux from the windings. Additional tank shield areas are added to further eliminate leakage flux. Also, the wire size of each coil can be selected flexibly, depending on the leakage flux in the windings, to prevent from local heating in the winding itself. Similar core-form designs cannot freely select the wire size of each coil, due to their continuous winding construction. This may lead to the possibility of local heating in the winding of core-form transformers.
· Efficient cooling capability. Shell-form transformers have an efficient cooling capability due the vertically arranged pancake coils. This allows the oil to easily flow across the entire coil surface and cool the transformer. The form-fit design also directs almost all of the oil flow over the coil surfaces, eliminating oil flow to unnecessary areas inside the transformer. This efficient cooling allows for a lower hot spot temperature vs. the average winding temperature when compared with a core-form design. This allows for longer life of the insulation system and also provides greater ability to handle short term overloading.
· Flexible design. The shell-form interleaved high and low voltage winding group arrangement allows for flexible design of the transformer to meet virtually any requirement. The impedance between windings can easily be adjusted, which allows for easy parallel operation with any other transformer, even other core-form transformers. It is also easy to lead out taps from any winding, due to the interleaved arrangement of the coils, which makes it easier to design and manufacture transformers with tap changers. It is also easy to build coils of high current capacity, which makes shell-form designs especially suitable for transformers of ultra-large current capacity.
· Compact size. Due to the form-fit construction of the shell-form transformer, unnecessary oil volume in the transformer tank is eliminated. The oil volume of a shell-form transformer is generally approximately 40% less than a comparable core-form transformer. This also provides for a compact transformer with a compact footprint. Thus for cases where an existing shell-form transformer must be replaced with a new one, the new MEPPI shell-form transformer can easily fit to the existing foundation. Sometimes core-form transformers cannot easily fit onto existing shell-form transformer foundations and also, the increased oil volume of the core-form unit may require the existing oil containment facilities to be increased to handle the extra oil volume, which can greatly add to the cost, duration and complexity of the installation work.
· Highly Reliable Design. All of the above advantages combine to provide a transformer design with a very high degree of reliability and robustness, even in the most severe of operating environments. MEPPI’s reliability is well proven by our vast number of units in operation over many years for critical and demanding applications worldwide.