Isolation transformer needed for AC Drives?

[AmazingTrans 0]

Hello gurus, I have a question for my projects that it has been days I am trying to get an answer out from AB but no clue. I have a few AC /VFD Drives (Powerflex 40). and 2 huge powerflex 700S AC drive which is 300amps input that drives a 250hp motor. My Scenario is this, all these drives including smaller transformer that provide power to the plcs are going to sit on a transformer that either supply 480VAC 1500KVA or 2000KVA. Will I need an isolation transformer on the ingoing side of the powerflex 700s ac drive ? Below is a rough sketch on how the powerline is routed. transformer out plant | transformer in plant 13000V->480VAC -> | -> 480VAC -> 480VAC (1500/2000)KVA | -----> (isolation transformer "optional???") --> P700S Drives | ^-- -> 480->120 --> plc Figure above here shows the 13kV transform to 480V from the power line outside, and inside the building there is 2 more transformer , but for example we will take 1500kva transformer 480->480VAC, and then it branches to 2 way. One to the drives, and one to smaller transformer that provides power to plc. My concern is, will the AC drive dump back power to the AC line or create harmonics/voltage notch? if it does, should i put a isolation transformer? Hope this is not to complicated. Thanks

[drforsythe 15]

In my experience, I have not used isolation tranformers on drive applications. I have, however, used line reactors on the incoming supply to the drive. We have line fluctuations at my location that are smoothed by the line reactors. The drives are able to accept "normal" levels of line fluctuation, but if your plant voltage is on the higher end of the acceptable range, it does not take much of a spike to cause and overvoltage fault on the drive. One of the causes of incoming line fluctuations is when the power company (or your plant) switches the power correction capacitors in and out of service. We had one machine that would fault every morning at approximately 10:30 am like clockwork. The drives (PF70) would display overvoltage faults. Our bus voltage had a nominal of 490 volts, which is within the tolerance of the drive, but the caps switching caused spikes. The problems were eliminated through the use of a line reactor on the input side of each drive.

[paulengr 44]

DC drives have notoriously noisy front ends and pretty much require isolation transformers. I have 3 large servo systems with roughly 6,000 HP worth of DC servo motors on each one and the amount of harmonics and voltage sags/surges they cause is huge...we use active power factor regulation just to make them operate (6,000 worth of sync motors driving Ward Leonard loops when in turn run the DC motors), and even then, as the machines cycle, the voltage fluctuates close to +/-10%. That's with DC drives. With AC drives, the story is much different. Modern drives have active front ends that keep your power factor and harmonics tightly controlled. Ask the vendor for THD and harmonic data and look at what it will do to your power quality. These days, isolation transformers and line reactors are usually just a profit center for the drive manufacturer since frequently you don't even need this stuff any more. I don't buy that stuff as a general rule unless it is actually going to be a problem. That being said, if I ran a large AC system with several drives such as a glass bottle machine or a paper machine, I'd be concerned about interaction between the drives and might consider something more aggressive. Don't forget though that your PLC is going to see harmonics attenuated by the turns ratio of the transformer powering it so 120:480=0.25 attenuation ratio on the THD being generated.