22 TRANSFORMERS FOR HVDC CONVERTERS

 TRANSFORMERS FOR HVDC CONVERTERS

With the increasing number, world-wide, of high-voltage direct current (HVDC) interconnections between high-voltage transmission networks such as, for example, that between the UK and France, the use of HVDC converter transformers is becoming more widespread. 

HVDC links may simply be back to-back schemes used for the interconnection of AC systems having incompatible characteristics, which usually means having different frequencies, or they may be used for EHV transmission over long distances.

In the former case the DC voltage need not be very high and can be optimised to suit the economics of the converter station. 

Since 25–30 per cent of these substation costs are determined by the cost of the converter transformers, transformer design considerations have an important bearing on the overall design of the interconnection. 

In the case of long distance transmission, where the requirements of the transmission line represent a signifi cant factor in the economic equation, it is often the case that the highest technically feasible voltage is selected for the DC system. 

In both cases, however, system interconnections are usually made at those points on the AC systems having the highest voltage level, so that the ‘AC windings’ of converter transformers are normally of 400 kV or higher.

Since all the windings of any transformer normally operate on AC, it is not very specifi c to refer to the AC windings of converter transformers. 

The windings which are directly connected to the AC system are normally termed the line windings. 

The windings connected to the converter are termed the valve windings. 

The other parameter unique to converter transformers is the commutating reactance which can usually be taken as the transformer reactance.

IEC 61378 Converter transformers is a three-part document that deals specifi cally with these equipments. 

Part 1 has the sub-title Transformers for industrial applications and Part 2 is the one that deals with HVDC converter
transformers. 

Part 3 is an application guide covering both industrial and HVDC converter transformers and sets out in narrative form much useful information concerning the differences between converter transformers and conventional power transformers designed to EN 60076. 

In early 2007 only Parts 1 and 2 have been accepted as European Norms.