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Converter Transformers
Our compact and low-weight transformers fully comply with the customers’ specifications. The high quality of our reliable products provides an outstanding capacity to withstand short circuits. Special type tests and quality control ensure safe operation while STS’s product support and service network guarantee free and easy use of the products.
Converter transformers are manufactured in dry or oil immersed types. The oil immersed types are manufactured as either hermetic or with breathing type. Additionally water forced version can be produced as well. (Oil Forced Water Forced, OFWF)
In dry types enclosures can be produced up to IP45 according to protection class. Besides, with the fan(AF) assembly temporary over load up to 50 % can be dragged.
The task of these transformers is to isolate the system from the network and preventing the converter unit to produce less harmonics by changing the number of phases upon demand. They can be produced for 6, 12, 18, 24 and 48 pulse systems. Transformers are designed specially according to the number of pulses, taking the harmonics into account. Moreover, a screen has been placed between the input and output windings.
The rectifiers used in low voltage are usually both 6- pulse or 12-pulse rectifiers, and they may be diode bridges or thyristor bridges.The 6-pulse rectifier is supplied by a two-winding transformer and the 12-pulse rectifier needs a three-winding transformer, having two separate secondary windings with 30° phase shift.
Especially with higher ratings, a higher converter pulse number is needed to limit the network harmonic distortion to an acceptable level. Pulse numbers higher than 12 require special phase shifting; multi-winding transformers or several transformers with phase shifts running in parallel.
Core:
The magnetic circuit is core-type with mitered joints. M5 grain oriented, 0,3 mm magnetic steel is most commonly used.
Windings:
The windings may either be copper, aluminium or mixed. Low voltage windings are always foil windings in these transformer sizes. Foil windings are mechanically extremely strong, which is important in transformers for convertor duty. An earthed screen is installed between the primary and secondary windings.
Testing:
Routine tests as per standards are conducted for each transformer unit produced. Extensive type testing programs can be performed for each design of the transformers to fulfill the requirements of the customers.
Standards:
The following standards govern variable speed drive transformers giving the rules for correct rating, specifying and testing of transformers of that type.
IEC 61378-1: 1997, Converter transformers, Part 1, Transformers for Industrial Applications.
IEEE Std, C57.18.10-1998, Practices for Semiconductor Power Rectifier Transformers
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