Converter Transformers

One of the main differences from other types of transformers is that the load currents contain higher harmonics due to the distorted waveform. The converter to the transformer causes the distorted current waveform. This has to be considered current leads to higher losses & temperatures in the transformer. Network regulations also require reduction of harmonic distortion. The most common use for converter transformers is found in applications such as: Variable speed drives (VSD) Aluminium electrolysis DC arc furnace Graphitizing furnaces Traction substations Copper refining VSD transformers are used in applications where a variable speed is required on the motor shaft. VSD are used in a wide variety of applications such as rolling mill drives, ship propulsion systems, mine hoist drives, wind tunnels drives etc. The figure below shows a basic circuit diagram for a VSD system configuration for a wind tunnel. The transformer is a 12-pulse unit that feed two 6-pulse rectifiers, which are connected to two inverters, which in turn is connected to a synchronous motor. The transformer is also equipped with a tertiary winding, which is to reduce the harmonic voltage distortion in the HV supply system as increasing the power factor of the load. One major converter transformer application is aluminium electrolysis. High secondary currents, secondary voltages up to 1500 V & a large voltage regulating range characterize such converter transformers. Due to the converter valves, the currents in the transformer windings will not be sinusoidal but contain harmonics, which must be considered when designing the transformer. Integrated transformer & converter units for DC currents up to more than 100 KA have been manufactured. Usually, four, five or six converter transformers are connected in parallel to feed one aluminium pot-line. Depending on the degree of disturbance tolerated on the network &/or on the DC current output, the transformers can be connected as 6-, 12-, 24-, 36- or 48-pulse system. The disturbance on the network & on the output DC current decreases with increasing pulse number. A 12-pulse system is made by two 6-pulse systems with 30 degrees phase shift between the two systems. This is achieved by connecting one 6-pulse system in delta & the other in star. A Diagram for a typical unit for this purpose is a 12-pulse transformer with phase shift winding as show in the figure below. A typical aluminium pot-line is built as a 48-pulse system with four rectifier transformers connected in parallel. In this case, four 12-pulse units with different phase shift windings build up the system. The following phase shift angles, 11, 25o, 3, 75o, 3, 75o and 11, 25o, can achieve a 48-pulse system.As mentioned, one of the characteristics of rectifier transformers for aluminium plants is a very large regulating voltage range, from zero volts up to several hundred volts. The magnitude of the secondary voltage depends on how many pots that are connected in series.When diodes are used, it is necessary to make a separate regulating transformer equipped with on-load tap changer in series with the rectifier transformer to regulate the secondary voltage. The regulating transformer is often auto-connected. In combination with diode rectifiers, transductors are normally used to regulate the voltage between the steps of the on-load tap changer. The regulating transformer that is feeding the rectifier transformer may be built in the same tank as the rectifier transformer or it may be made as a separate unit. Another possibility to regulate the secondary voltage is to use thyristor rectifiers, which may replace the regulating transformer and the transductors. When thyristor rectifiers are used, it may be sufficient to equip the rectifier transformer with an off-circuit tap changer (OCTC). At inquiry and order, the technical specification for the convertor transformer needs some additional information compared to power transformer specifications. The system designer or the purchaser should provide this information. Such information is valve winding current waveform, i.e. harmonic current spectrum, The harmonic content influences the transformer losses in general but, more importantly localized winding loss and hence potential hot spot temperatures When this information is known, the winding can be designed to avoid unacceptable temperatures. The distortion of the power system voltage caused by the harmonic current is another aspect to consider. Disturbance due the harmonic content of the currents can be reduced by increasing the pulse number, as mentioned above.There may also be special requirements regarding tolerances on the transformer short circuit impedance because this impedance influences the efficiency of the electrolysis process.