The purpose of the ship transformer is to isolate the different parts of the electric power distribution system into several partitions, normally in order to obtain different voltage levels and sometimes also for phase shift.
Voltage Distortion on Ship Power Network
Phase shifting transformers can be used to feed frequency converters, e.g. for variable speed propulsion drives, in order to reduce the injection of distorted currents into the electric power network on ship by canceling the most dominant harmonic currents. This reduces the voltage distortion for ship generators and other consumers.
The ship transformers also have a damping effect of high frequency conductor emitted noise, especially if the transformer is equipped with a grounded copper shield between primary and secondary windings.
Design and Types of Ship Transformers
There are numerous different transformer designs in use, and the most common types are; air insulated dry type, resin insulated (cast or wound), or oil/fluid insulated.
Regulations, ambient conditions, and user’s, yard’s, or supplier’s preferences govern the selection of type, material, and design of the ship transformer.
Physically, the transformer is normally built as three-phase units, with three-phase primary coils and three-phase secondary coils around a common magnetic core. The magnetic iron core constitutes a closed path for magnetic flux, normally with three vertical legs and two horizontal yokes; one in bottom and one at top. The inner winding constitutes the low voltage or secondary windings, and the outer is the primary or high voltage winding. The ratio of primary to secondary windings gives the transformation ratio.
The coils may be connected as a Y-connection or D-connection (also called D-connection). The connection may be different on primary and secondary sides, and in such transformers, not only the voltage amplitude will be converted, but there will also be introduced a phase shift between the primary and secondary voltages. The phase shift can also be adjusted by use of Z-connected
windings, normally in the primary, where the phase shift angle can be accurately determined by the ratio of turns in the segments of the Z-windings.
Three – or four winding transformers with multiple secondary windings are also in use, e.g. for multi-pulse drive applications.
A transformer on ship with D-connected primary and Y-connected secondary is called a Dy type transformer. The first and capital letter describes the primary winding, and the second and small letter describe the secondary winding.
The letter n is used to describe if the common point in a Y-connection is grounded, e.g. Dyn or Ynyn.
Transformers may be designed according to IEC standards.
Ship Converter Transformers
For converter transformers, it is essential that the design accounts for the additional thermal losses due to the high content of harmonic currents. IEC also gives design rules and guidelines for such applications.
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