CoNvErTeR Transformer

The converter transformer can have different configurations (i) three phase, two winding, (ii) single phase, three winding ,(iii) single phase, two winding. The valves side windings are connected in star and delta with neutral point ungrounded. On the AC side, the transformers are connected in parallel with neutral grounded. The leakage reactance of the transformer is chosen to limit the short circuit currents.One problem that can arise is due to the DC magnetization of the core due to unsymmetric firing of valves. In back to back links, which are designed for low DC voltage levels, an extended delta configuration can result in identical transformers being used in  twelve pulse converter units. This results in the reduction of the spare capacity required. However, the performance of extended delta transformers in practice is still to be tested.

Filters :

          There are three types of filters used :

1. AC filters : These are passive circuits used to provide low impedance,shunt paths for AC harmonic currents. Both tuned and damped filter arrangements are used. 
2. DC Filters : These are similar to AC filters and are used for the filtering of DC harmonics.
3. High frequency (RF/PLC) filters: These are connected between the converter transformer and the station AC bus to suppress any high frequency currents. Some times such filters are provided on high -Voltage DC bus connected between the DC filter and DC line and also on the neutral side.

Reactive Power Source :

               converter stations require reactive power supply that is dependent on the active power loading (about 50 to 60% of the active power). Fortunately, part of this reactive power requirement is provided by AC filters. In addition , shunt (switched) capacitors, synchronous condensors and static var systems are used depending on the speed of control desired.

Smoothing Reactor : 

                                A sufficiently large series reactor is used on DC side to smooth DC current and also for protection. The reactor is designed as a linear reactor and is connected on the line side, netural side or at intermediate location. 

DC Switchgear :

                          This is usually a modified Ac equipment used to interrupt small DC currents (employed as disconnecting switches). DC breaks or metallic return transfer breakers (MRTB) are used, if required for interruption of rated load currents.

           In addition to the equipment described above, AC switchgear and associated equipment for protection and measurement are also part of the converter stat