This part of GB/T 3859 specifies the characteristics wherein convertor transformers differ from ordinary power transformers.
It should be borne in mind that a rectifier transformer operates with non-sinusoidal current waveshape. In single-way connection, the current in each cell winding contains a d.c. component which calls for special attention in design and testing. In some cases, a special design is necessary when external short-circuits and cell failures would cause abnormal stress.
For certain types of transformers, the waveshape of the normal operating voltage is non-sinusoidal. The core loss of such equipment is to be determined by applying a sinusoidal voltage having the same half-cycle arithmetic mean value and the same fundamental frequency as the voltage applied in service.
This part is applicable to the convertor transformers and reactors used with power convertor equipment specified in GB/T 3859.1. In other respects, all rules specified for power transformers in the National Standard Power Transformers shall apply to convertor transformers if not in contradiction with this part.
2 Normative References
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 3859.1-2013 Semiconductor Converters — General Requirements and Line Commutated Converters — Part 1-1: Specification of Basic Requirements (IEC 60146-1-1:2009, MOD)
GB/T 3859.2-2013 Semiconductor Converters — General Requirements and Line Commutated Converters — Part 1-2: Application Guide (IEC 60146-1-2:2011, MOD)
3 Rated Values for Convertor Transformers
3.1 Rated current values
3.1.1 Single convertor or double convertor supplied from one common cell winding
The convertor transformer shall be capable of carrying current corresponding to rated direct current of the convertor continuously, followed by overload current (where specified) for the specified duration (See GB/T 3859.1-2013, 6.4.3 and 6.5), at maximum cooling temperatures without exceeding its thermal limit.
3.1.2 Double convertor where each thyristor assembly has separate cell windings
The convertor transformer shall be capable of carrying in each secondary group the appropriate current corresponding to rated direct current of the convertor continuously, followed by overload current for the specified duration (See GB/T 3859.1-2013, 6.4.3 and 6.5), at maximum cooling temperatures without exceeding its thermal limit.
Where common line windings are used for the two secondary windings, these should be rated as in 3.1.1.
3.2 Temperature limits of cooling media
3.2.1 Air cooled outdoor equipment
The convertor transformer shall be designed to operate at an ambient air temperature which does not exceed +40℃, does not average more than+ 20℃ over a year and does not average more than +30℃ for any 24 h period.
3.2.2 Air cooled indoor equipment
The convertor transformer shall be designed to operate in an ambient air temperature of +40℃.
3.2.3 Water cooled equipment
The convertor transformer shall be designed to operate with incoming cooling water temperature which does not exceed +25℃.
4 Losses and Voltage Drops in Transformers and Reactors
