Converter transformers

Converting systems are usually supplied by means of a transformer, the parameters of which are adjusted to the converter requirements. Converting transformers operate in extremely hard conditions. Their secondary currents comprise the whole range of harmonics. The qualitative and quantitative content of the current harmonics depends on the converter system in which the transformer operate. 

Converters systems with neutral conductor. Here the current on the transformer secondary side takes the shape of unidirectional rectangular impulses. This causes a DC component of core magnetization which is a negative effect from the viewpoint of the transformer. Hence these converting transformers are usually larger and heavier than typical supply transformers because, by design, they must have lower induction to take account of the high harmonic content in the secondary.

In addition to adjusting the voltage level to the load requirement, the converting transformer provides protection to the converter thyristors. The converting transformers have considerable leakage inductance which limits current during thyristor commutation. The are also equipped with reinforced winding insulation. These properties cause that the dimensions and price of converting transformers to be greater comparing to traditional ones.

Converting transformers are configured in a variety of ways to suit the converting system. One of the simplest configurations is a transformer for 3-pulse converters systems. Here the primary windings are connected in delta, while the secondary windings are star connected with a neutral terminal (Dyn group):

Diagram of 3-pulse converter with neutral conductor.

The transformers intended for 6-pulse converters systems are applied much more widely. Such a system is supplied via a transformer or line chokes. The transformer is used when it is necessary to adjust the converter output voltage to the required receiver voltage. The converter bridge system does not require directing the neutral conductor on the transformer secondary side and its
windings may form the following combination systems: Yy, Yd, Dy, Dd.

Another constructional solution is a special transformer for supplying the six-phase. The primary windings of the transformer are connected in delta and the secondary ones are combined into 6-phase system with an external neutral terminal:

Diagram of 6-pulse converter with directed neutral conductor.

Multi-pulse complex converting systems are another group requiring special transformers.

Diagram of 12-pulse complex converter