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VFD and grid in parallel 2

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electrolitic

electrolitic

Electrical
Dec 9, 2007
105
Sirs
The HTLC, a Converteam soft starter that uses VFD technology, permits the running in parallel with the grid before the soft starter be disconnected.
May I do the same using a standard VFD?
 
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How long do you want your VFD to last before the magic smoke escapes?
 
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In other words: NO!!!

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Thanks
The HTLC uses basically a MV3000 (VFD) and a special auto-transformer to connect it to the grid?
No smoke escapes.
Where is the magic?
 
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A VFD needs to be designed to do a closed line tranfer.
 
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If it was done before and no magical smoke appeared, I hope your standard VFD was designed to be connected to the grid via closed-transition and the VFD got off the grid before any transient hit the grid!
What is the rationale for this, if I may ask?
 
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Most VFDs run free and do not sync their frequency or phase angle to the grid. Running in parallel with the grid or doing a closed transition by-pass to the grid requires synchronizing circuits that may not be included in a VFD that is not specifically designed to sync with the grid.

Bill
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"Why not the best?"
Jimmy Carter
 
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That unit you describe appears to be offered for MV compressors and once you get to a MV VFD, a little extra fr the synchronous transfer capability is relatively insignificant to the overall cost. Converteam is the old Alstom drive group.

By the way, it is also done by several other large reputable manufacturers, Robicon (Siemens) and Allen Bradley are the two I am aware of. It's not that it cannot be done, it's that the economics make it unattractive for off-the-shelf low voltage VFDs that rarely need it.



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Thank you all
The HTLC is a short-term equipment just for starting purpose.
My idea of design is to use MV VFDs (2) to start sequentially heavy pumps and keep the last one connected to the VFD to get speed control.Adequate arrangement of VFDs and busbars can provide high flexibility.
 
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It's a novel idea. There would also be a lot of complexity in providing capability to switch each load to vfd or dedicated bypass as well as ability to align the same VFD to different motors.

Also I hope the last started pump that you are controlling speed of is not a running in parallel with the others at full speed (assuming we are talking centrifugal-like pumps). If it is you generally don't get any pumping out of the pump running at lower speed, just a lot of recirculation, vibration, lost energy, possibly damage.

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It's done all the time, it makes sense on MV equipment in particular.

Just make sure you factor in all of the necessary isolation components and consider the issue of servicing one piece of equipment while others continue to run if that's a requirement. The issue there is that vacuum contactors, what are typically used to switch power back and forth, cannot be relied upon as a safety disconnect for equipment servicing. You need isolation switches for that, even if they are not load-break rated.


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It can also be done in LV inverters. Siemens MasterDrives had an optional synchronization module that used fibre optics to syncronize two or more inverters. And I am sure the Sinamics has it too. But haven't used it.

Still, you cannot parallel inverters if they were not built to be parallelled. It is a sure way to destroy them.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Thanks jraef
I am not familiarized with american standards so, my questions are:1.Can a Vacuum circuit breaker at racked-out position and electrically/mechanically locked (for example VD4/ABB Unisafe switchgear) be considered as a safe "disconnect device"? 2.Is it mandatory a grounding switch after the circuit breaker?
 
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A racked out vacuum breaker is fine as a isolation means. This is why vacuum breakers are rackable. The racking-out is the service disconnect.

The components should be considered live until isolated (via racking) and checked and grounded. The grounding can be via a grounding switch or via grounding clamps installed on the bussing.

Depending on the number of pumps in question it can also become an operating and service nightmare. These systems seem simple when just thinking about them but once considering the operation, motor protection and servicing it becomes obvious they are much more complex than a simple starter per motor.
 
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What he said.

I only brought it up because I have seen people do this with non-draw out gear and in that case it would be unsafe unless you have another isolation switch.


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For the best use of Eng-Tips, please click here -> faq731-376
 
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