Specific Design of Converter Transformers for HVDC Transmission System

What are Converter Transformers?

"Converter transformers used in High Voltage Direct Current (HVDC) system are specially designed power transformers of high MVA rating connected between AC bus bar and converter valves."

The windings of the converter transformer on the AC line side are called “line winding” and the windings on the converter valve side are called “valve winding”. These transformers may be a 1-phase unit with two or three windings or may be a 3-phase two winding unit.

Main Difference between Conventional Power Transformer and Converter Transformer:

The main difference between the conventional power transformer and the converter transformer is the DC voltage appearing on the valve winding, harmonics and commutating short circuit pulse currents of rectangular waveform flowing through the windings of the converter transformer.  The harmonic content in the current produced because of converter operation causes additional leakage flux in the transformer. It produces eddy current losses and hot spots in the winding and the transformer tank. Thus, a converter transformer is subjected to higher electrical and thermal stresses, and hence the insulation issue is more prominent.

Due to the said limitations and requirements, certain special features are necessary in the design of a converter transformer. They are:

·         Higher short circuit strength,

·         Special design for windings and insulation,

·         Special design of magnetic circuit,

·         Specially designed on-load tap changers,

·         Close control of leakage flux.

In this transformer, the gap between winding and the core is larger, resulting in higher leakage flux and higher eddy current losses. The end turns of the windings near core need special design with respect to cooling, hot spots and insulation.

During commutation process the valve side windings are short circuited for a short interval. The role of commutating reactance is very significant in limiting the short circuit (S.C) currents. The inductive reactance of the converter transformer controls the short circuit current and its rate of change. The reactance of the converter transformer should be finely matched with the requirements of commutation process so as to have better commutation, lesser harmonics and lesser transients. This is achieved by precise design of core winding and leakage paths.

The windings of converter transformer experience axial and radial forces because of the flow of S.C. currents. Therefore, the windings should be strong enough to withstand the forces thus produced, in addition to the thermal stresses. The cross section of the core is so selected to avoid magnetic saturation in extreme conditions because of DC and AC component flux.

The on-load tap changers (OLTC) for converter transformers are also specially designed to give a wide range of regulation (say +17.5% to -12.5%) in multi-steps of 24 steps or so. These transformers are normally forced cooled i.e. Oil Forced Air Forced (OFAF).