Low voltage VFD closed transition synchronous transfer to mains available yet?

[bentov]

My search continues, just checking to see if anyone knows of something new available that might work (other than medium voltage) . . . anybody tried a synchronizing relay designed for other stuff without destroying a VFD?

thread237-369367

 

[jraef]

ABB has released a unit for their ACS800 series drives that does this, it's called an RSYC-01Synchronizing Unit. I am looking at one now to use with another brand of VFD, because this unit appears to be stand-alone anyway; the interface to the VFDs is all hard wired and analog. The only thing is that the instruction manual provides you with all of the parameters within the ABB ACS 800 drives that you use with it, so all I am doing is converting those parameters to equivalents in the drives I have.

Haven't tested it yet and my end-user is in India, so I'm not going to be able to see it directly. I'll just hear about it, one way or the other...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[bentov]

Good Morning Jeff,

From the RSYC-01 Operation description:

BUFOUT signal reaches 5 volts indicating “synchronic” status for the drive application program.
 RSYC unit gives a control pulse for the contactor control circuitry.
 Drive control logic switches the inverter modulation (IGBT control pulses) off.
 Contactor control circuit opens contactor K1, disconnecting the motor from drive.
 Contactor control circuit closes contactor K2, connecting the motor to the supply line.

Still open transition though, right? Or are you going to try something brave, let K1 connect the motor and switch off the IGBTs at the same (exciting!) moment, then K2 disconnect the drive contactor? Sure would be cool if that could work!

 

[jraef]

I saw that probably at about the same time you did I think. As I said, I was just handed this project to work on. It appears as though they are using a reactor in series with the across-the-line connection during the transition, likely to dampen the transition spike when the motor slows down. That's fine, but you are right, this in NOT a closed transition system as was reported to me by my end user in India. I just let them know that last night. We have been doing open transition transfers for years, what we do is over speed the drive just prior to the transition so that when the speed decays while disconnected, it is closer to the operating speed. But the tricky part, as you probably know, it predicting that rate of decay, which is usually not consistent.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Jeff;
When using the "over-speed and slowdown" method do you use a sync-check relay to avoid possible current and torque spikes?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill,
That's essentially what this device is from ABB, but in reality, it's somewhat pointless because regardless of what it tells you, it's never going to be in perfect synch when the transfer takes place. The trick is to make sure the open transition is long enough for the motor flux to decay so that when re-connected, it is not generating. That rate of flux decay is highly variable from motor to motor and with size of the motor, so having a synch check would be helpful in that regard for sure.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[bentov]

But they (Robicon, Eaton, Toshiba) do it with large HP MV motors right? Synch the VFD & mains, then close the mains to the motor AND the load side of the VFD, then disconnect the VFD?

Maybe there's just not enough perceived market for same on LV. As you mentioned in that old thread, around here (Central California) the grid is so taxed that sometimes the only way to start up a new high hp irrigation well motor is with a VFD for current limiting. Not being able to transfer to mains at full speed imposes the energy penalty of drive losses and cooling - 2-3% extra on the power bill for a 500hp is a pretty big deal.

Reading about large motor starting with VFDs on weak grids, I came across this (starting 17.5Khp 12KV motors with 10khp 4160V VFDs and step up transformers):

https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6963463

My other interest is in a better test panel for my motor repair shop - looks like I'm going to use a VFD for rotor mass acceleration (that I don't have the current capacity for otherwise), then a soft start (into the spinning motor) to get back to bypass/mains only for standard measurements we need to take during full speed/full voltage/no load run tests.

It will be interesting to pursue possibly using that same VFD with a step up transformer to also test run MV motors . . .

 

[itsmoked]

Jeff; So what kind of times are we talking about for the field to collapse?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

One advantage of using a step-up transformer on the VFD output is the filtering that it provides.
I've seen this done on SAGD (Steam Assisted Gravity Drainage, a tar production method0 well pads.
The length of the conductors to the submersible pumps makes filters mandatory with VFDs, except where a step up transformer is used.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

In an installation of a bank of 10 HP propeller fans the SOP was to stop the fans by pulling the disconnect switches.
Each disconnect fed a contactor and two fan motors.
When the switch was pulled the back EMF would hold the contactor in for about 3 seconds.
This was consistent with these motors.
Results may vary with other motors.
Back to you, Jeff.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I was (almost two decades ago now) on a team designing stand-alone DC Injection Brakes. Because that HAD to be an open transition and any more than simple residual magnetism would smoke our SCRs, we had to make sure the motor fields were collapsed before turning on the DC. We tried coming up with a circuit to measure it but it was not really reliable enough. So we combined it with empirical evidence from testing. I wish I still had access to that research (but they stopped paying me when I left). As I recall it though, we found that at 10HP and under it was no more than a second, but the time increased with size (we conjectured it was based on mass) and at 500HP it was as much as 3 seconds on some motors with the data plot being roughly parabolic. We were only looking for worst case scenarios of course. I think that’s one of the problems with this issue though, the variability of motor designs.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden