Quality Power Electrical Equipments, Sangli
 
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.
 
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.
 
Earthing transformers: Earthing transformers are used to create a neutral point in a three-phase system, which provides possibility for neutral earthing. The earthing can be through an arc-suppression reactor, a neutral earthing reactor or resistor or directly. The design can be a transformer with just one winding, which is zigzag connected. The zero sequence impedance of such a winding is normally quite low, but it can be increased if the purpose is to limit the current through the transformer in case of an earth fault somewhere in the system. Figure shows the connection diagram.          
           
During undisturbed system operation with balanced (symmetrical) voltages the current through the earthing transformer is small & of the same size as the magnetizing current. Unbalanced voltages will cause some higher currents flowing through the earthing transformer, which it must be capable to carry.
 
An alternative connection to the zigzag is star/delta connection where the delta connected winding will compensate the zero sequence magnetic fields so it will be confined to a leakage field between the star and the delta winding and make the zero sequence impedance of the transformer relatively small. However, if it is desired to increase the zero sequence impedance, this can be achieved by opening the delta connection and insert a reactor or resistor.
 
Earthing transformers are usually oil immersed and may be installed outdoor. In cases where a separate reactor is connected between the transformer neutral and earth, the reactor and the transformer can be incorporated in the same tank. When the earthing transformer is going to be used together with an arc-suppression reactor, the rated current (and its duration) of the earthing transformer will be determined by the data for thearc-suppression reactor.
 
If the earthing transformer is used for directly earthing or through a current limiting reactor, the neutral current through the transformer will be high but the duration is limited to a few seconds. Earthing transformer must be designed to withstand the thermal and mechanical effects of the rated neutral current. The characteristic of earthing transformer should be selected to match the property of the system. Most of the time in service the loading of earthing transformers is very low. It is the randomly occurring short – duration currents that cause any heating of significance. Ageing of the cellulose materials is then not a matter of concern. Regarding acceptable temperatures there are two aspects to consider. The temperature during currents of say 10 seconds duration must not cause softening of the winding conductor material, say 250o C for copper and 200 o C for aluminium.
 
Contact:
 
Quality Power Electrical Equipments Pvt. Ltd.
# L-61 , Kupwad MIDC,
Sangli 416 436. Maharashtra India  
Tel: +91-233-2645432 , 2645435
Fax: +91-233-2645433
Email: bharanidharan84@gmail.com