15-02-2013, 12:09 PM
RECENT TRENDS IN ULTRA LARGE-CAPACITY THREE-PHASE TRANSFORMER TECHNOLOGY
RECENT TRENDS.doc (Size: 1.51 MB / Downloads: 39)
ABSTRACT:
Large capacity transformers have evolved to meet the changing needs of electric power companies. For example, the need to reduce the transportation cost to the site and the trend towards smaller installation spaces have led to the introduction of on-site assembly technology. In addition, plans to double the voltage rating will increase considering future long-distance transmission, and the need for a routine supervisory system will grow as transformer operating conditions become severe due to extended
operating life of equipment, overload operation, etc. Hitachi is meeting these needs with its computer analysis technology and verification testing using trial production. The result of our efforts is highly efficient large capacity transformers. In particular, we have just completed delivery of a 281.25-kV (525-kV) 1,060-MVA double-rating voltage transformer for the Hitachinaka Power Station of Tokyo Electric Power Co., Inc., as well as Hitachi’s first-ever on-site assembly of transformers (500 kV 1,000 MVA) for the Chizu Substation of the Chugoku Electric Power Co., Inc. Hitachi uses the newest
diagnostic technology for its apparatus supervisory system and provides rational maintenance and support.
INTRODUCTION:
THE market for large capacity transformers is highly dependent on the needs of the electric power companies to which they are supplied. Consequently, with cost-effectiveness as the driving concern there has been a doubling of the voltage specification and extension of the operating voltage range for the future long-distance transmission. Moreover, there are transportation restrictions especially regarding substations in mountainous areas, etc., for which the introduction of an on-site assembly system is seen as a solution to this problem while also minimizing installation space. Of equal importance is the need for stable operation over long period. Thus preventive maintenance is a necessary aspect of controlling the investment in the equipment. This paper reviews Hitachi’s efforts to meet the challenges described above. In particular, we discuss our 281.25-kV (525-kV) 1,060-MVA double-rating voltage transformer for
power stations and our 500-kV 1,000-MVA site-assembled transformer for substations. We also discuss the latest apparatus supervisory system we provide, which can support preventive maintenance systems.
HISTORY OF HITACHI’S LARGE-CAPACITY TRANSFORMERS
The historical trend in Hitachi’s transformer technology is shown in Fig. 2. Hitachi established 500- kV transformer insulation design technology in the 1970s. This eventually led to a UHV (ultra-high voltage) insulation design technology and to a practical UHV transformer in 1993. Moreover, the evolution of computer analysis technology has led to significant efficiencies. For instance, the optimum core design can be found by magnetic flux distribution analysis and a high-accuracy stray loss evaluation is also possible through detailed magnetic field analyses. The result is the present low-noise, high-efficiency transformer technology. Development of a double-rating voltage transformer started with the 250-/154-(77-)/22-kV 300-MVA transformer in 1973. Larger
capacity ones, the 225-kV (520-kV) 730-MVA transformer and 220-kV (500-kV) 250-MVA transformer, were supplied in 1994 for the Reihoku Power Station of Kyushu Electric Power Co., Inc. On-site assembled transformers for substation began in 1989 with the 220-kV 250-MVA site-assembly transformer. The technique used in 1989 is similar to the present one. The 500-kV 1,000-MVA site-assembled transformer was completed in 2000.
STRUCTURE
The high voltage winding consists of two windings. Two high-voltage windings are used in parallel for the 281.25-kV connection and are used in series for the 525-kV connection. By changing the internal lead line, the connection can be changed. The composition of the windings is shown in Fig. 3.
CORE
The three-phase five-leg core of the conventional large capacity transformer was adopted. The optimum joint structure was used to ensure the magnetic flux would be uniform and the local.
loss would not become concentrated. The cross-sectional ratio of the up-and-down yoke and side yoke was also optimized
WINDING
The windings are arranged in the order of tap winding, high-voltage winding 2, low-voltage winding, and high-voltage winding 1 from the inner side. In order to make the short-circuit impedance the same for both operating voltages, the current distribution needs to be made the same. For the parallel 281.25-kV connection of the high-voltage winding 1 and 2, the low-voltage windings among the high-voltage windings 1 and 2 are connected so that current distribution ratio can be about 50%. Furthermore, analysis of the main gaps ensured that the impedance
CONCLUSIONS
A large-capacity double rating voltage transformer and a 500-kV site-assembled transformer were
described as examples of transformers reflecting the latest needs of electric power companies. The need to control equipment investment is a serious concern for electric power companies. Hitachi is committed to improving the design, manufacture, and maintenance resume of our transformers, which we believe to meet our customer needs.