Single vs. Double Inverter Bridge for large MV-VFD

[cuky2000]

We are in process selecting a VFD for an existing ID fan motor rated for 7500 HP at 4.16 kV. Some manufacturers are offering single inverter bridge and others two bridges in parallel. Appears that a double bridge VFD is oferred because their single bridge unit at 4.16 kV is rated under 7500 HP.

a) What are the pros and cons of a single vs. double inverter bridge in parallel regarding the VFD reliability and overall system performance? . Are more components pron to fail?

b) What issues we should be aware related with cable length from the overvoltage point of view for the input and output power to the VFD?

c) Does the control power require UPS from the reliability point of view in the event of voltage sag or high harmonic in the plant LV distribution system?

Thanks in advance for your comments.

 

[ScottyUK]

Big semiconductor switches are fairly slow, but by using multiple bridges operating with a phase displacement it is possible to apparent ripple frequency at the output appear to be higher than the carrier frequency, which allows smaller and lighter filter components to be used.

In terms of reliability you don't gain any redundancy by having the seond bridge, but do increase component count. All other things being equal the design with multiple bridges will be more likely to have a failure. The reality is that component count alone does not determine reliability - the design of the circuits plays a big part, as does thermal management and protection for the power stack.


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