Pre-charging of Siemens Robicon Perfect Harmony VSD

Pre-charging of Siemens Robicon Perfect Harmony VSD

One of the key features of all low voltage VSDs is their ability to soft charge the DC bus capacitors. Siemens Robicon Perfect Harmony MV VSD does not yet have this function. Perfect Harmony VSD can be easily modified to include this function. Soft (or pre-)charging the bus capacitors will limit the inrush current at power-up and prevent damage to the transformer, power equipment and other connected equipment.

In the Siemens Robicon Perfect Harmony medium voltage VSD, the fact that the Siemens/Robicon medium voltage VSD has a phase shift transformer to eliminate harmonics makes the problem more complicated. In addition to the bus capacitor, this will also absorb a large current when it is first powered up. After power-up, the inrush current on a large transformer can be as high as 12 times the transformer FLA in about a quarter of a cycle. The combination of bus capacitor surge current and transformer surge current will stress the phase shifting transformer in Siemens/Robicon MV VSD. When the power is cycled to the medium voltage VSD, this may cause premature failure of the transformer.

You can add a pre-charging circuit for Siemens Robicon Perfect Harmony VSD. A simple method is to add a contactor and activate the duty cycle rated for low and medium voltage transformers. This allows the capacitor to be precharged and then the contactor is opened after the precharge is completed. This method is similar to how many pre-charge circuits work on a low-voltage VSD. You must be very careful to interlock the medium voltage contactors from the Robicon Harmony VSD power supply. This method is cheap and can protect the pre-charging of the bus capacitor on the Robicon Harmony VSD battery. However, there is still the problem of inrush current on the transformer during the initial power-up.

The most effective way to solve the instantaneous current on transformers and capacitors is to add a pre-charge composed of inductors and contactors in front of the phase-shifting transformer. If the operation is correct, the surge current will be reduced to 1.5 times the FLA of the transformer on Robicon Perfect Harmony MV VSD, thereby extending the life of the capacitor and transformer. EMA has successfully completed this operation for customers running Siemens Robicon Perfect Harmony MV VSD to prevent transformer failure due to inrush current.

We welcome your opportunity to help prevent premature failure of capacitors on Siemens Robicon Harmony MV VSD and failure of phase shift transformers. Call us or use any contact information on this page and website. Thank you!