One of the key features of all low voltage VFDs is their ability to softly charge the DC bus capacitors. This feature is not a readily available on the Siemens/Robicon Perfect Harmony MV VFDs. The Perfect Harmony VFD can be easily modified to include this feature. Soft (or Pre) charging the bus capacitors limits the inrush current upon power up, and prevents damage to transformers, power devices, and other connected devices.
Within the Siemens Robicon Perfect Harmony medium voltage VFD the problem is compounded by the fact Siemens/Robicon medium voltage VFDs have phase shifting transformers to provide harmonic cancellation. This, in addition to the bus capacitors can draw large currents when power is first applied. The inrush current on large transformers upon power up can be up to 12 times the FLA of the transformer for roughly a quarter cycle. The combination of the bus capacitor inrush current and the inrush current of the transformer can stress the phase shifting transformer in the Siemens/Robicon MV VFDs. This can result in premature transformer failure when power is cycled to the medium voltage VFD.
It’s possible to add a a pre-charge circuit to the Siemens Robicon Perfect Harmony VFD. One simple way is by adding a contactor and start duty rated low to medium voltage transformer. This can pre-charge the capacitors and then open the contactor once the pre-charging is complete. This method is similar to how many pre-charge circuits work on low voltage VFDs. You must very carefully interlock the medium voltage contactor coming from the mains on the Robicon Harmony VFD. This method is inexpensive, and takes care of the pre-charging of the bus capacitors on the power cells of the Robicon Harmony VFD. However it still leaves the problem of the surge current on the transformer during initial power up.
The most effective wayto address both the instantaneous current on the transformer and capacitors is adding a pre-charge consisting of an inductor and a contactor in front of the phase shifting transformer. This, if done correctly will reduce the inrush current to 1.5 times the FLA of the transformer on the Robicon Perfect Harmony MV VFD, hence extending the life of both the capacitors and the transformers. EMA has successfully done this for customers running the Siemens/Robicon Perfect Harmony MV VFDs to prevent transformer failure due to inrush current.
We would welcome an opportunity to help prevent premature capacitor and phase shifting transformer failure on your Siemens/Robicon Harmony MV VFDs. Call us at 770-448-4644 or use any of the contact methods on this page and site. No One, Anywhere, is Better at Drives than we are.