Low voltage (480 VAC) variable frequency drives, regardless of the manufacturer, almost all share the same design topology in their power sections. With medium voltage VFDs, the design architecture can vary considerably.
In particular, let’s consider the precharge circuitry. Pre-charge, in a Pulse Width Modulated (or PWM) VFD, refers to a current limiting circuit that slows the charge rate of the bus capacitors during power up. (for a discussion of bus capacitors see HERE ) Otherwise, the initial inrush would damage the input power devices. Almost all low voltage (480 volt) VFDs use some form of designated precharge circuit.
This is not the case with all medium voltage VFDs. Some medium voltage VFD manufacturers depend solely upon the impedance of the input phase shifting transformer to pre-charge the bus capacitors. (See image of a typical medium voltage phase shifting transformer)
One problem with this is that the magnetization current on large transformers used on the front of medium voltage VFDs typically runs about 10 to 12 times its nominal current which causes the initial inrush to be quite large.
This can have a ripple effect, in that upstream protective relays, such as the GE Multilin (see GE Multilin Repair), have to sized such that they don’t trip when these large magnitude currents (10 to 12 times normal) is present.
Another disadvantage to this method, is that the increased current comes with increased potential ground fault current danger. The severity of arc flash depends on the supplied current to the overall medium voltage VFD system.
Another problem with only using transformer impedance to limit inrush for medium voltage VFDs is that the magnetizing current actually causes the windings to vibrate, which can damage insulation and cause premature transformer failure.
One medium voltage VFD that uses a designated precharge reactor to limit inrush is the Toshiba T300MVi medium voltage VFD. The Toshiba T300MVi pre-charge circuit reduces inrush current to only 2.5 times the VFD frame rating. This is quite convenient for the power control engineers to set their protective relays and limits the potential of arc flash current. This is a safety advantage for the operators and users as well.
Want to know more about this topic? EMA offers seminars and webinars on medium voltage drive topologies to help you make more informed decisions. Contact us by and of the means on the right of this page OR, call 770-448-4644770-448-4644.
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