VFD Output Phase Imbalance Protection

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oilandgas

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We have a new VFD on a 350HP pump that is miss-firing intermittently and have seen 420A on two of the output phases and 80A on the last phase (for a sustained 10 seconds or longer and read in the drive display), but the drive has not interrupted on a fault. The manufacturer tells us they don't have phase imbalance or loss protection on the output but they have it on the input. Is this normal? How come the input side is not detecting the output phase imbalance and interrupting?
 
oilandgas said:
We have a new VFD on a 350HP pump that is miss-firing intermittently and have seen 420A on two of the output phases and 80A on the last phase (for a sustained 10 seconds or longer and read in the drive display), but the drive has not interrupted on a fault. The manufacturer tells us they don't have phase imbalance or loss protection on the output but they have it on the input. Is this normal? How come the input side is not detecting the output phase imbalance and interrupting?
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To the drive, the input is just a raw material for making DC. So that explains why the input does not see the imbalance, in fact it may not even care if the input is single phase if you tell it to ignore that (meaning you understand the consequences, separate issue).

As to the drive not having output phase current imbalance detection, that seems very odd on a 350HP drive. The ability to accomplish this is essentially just software code, unless the drive is not really monitoring each phase current and is estimating the motor current off of the DC bus. That's something that is typically done on small cheap throwaway drives, but is a potentially risky cost cutting measure for something that large. Who's drive is the anyway (he asks, hoping that it is not his employer)?
 
Jraef said:
To the drive, the input is just a raw material for making DC. So that explains why the input does not see the imbalance, in fact it may not even care if the input is single phase if you tell it to ignore that (meaning you understand the consequences, separate issue).

As to the drive not having output phase current imbalance detection, that seems very odd on a 350HP drive. The ability to accomplish this is essentially just software code, unless the drive is not really monitoring each phase current and is estimating the motor current off of the DC bus. That's something that is typically done on small cheap throwaway drives, but is a potentially risky cost cutting measure for something that large. Who's drive is the anyway (he asks, hoping that it is not his employer)?
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Thanks for the reply. Confirms our thoughts exactly, and what we have been telling the manufacturer. No, the drive is not Rockwell or a cheap throwaway, it is a standard drive used for electric submersible pumps in oil wells, that is being used on a surface multi-stage centrifugal pump (so no long cable). It is a new generation Baker-Hughes drive (though we don't know if it is a re-packaged unit manufactured by someone else, though we understand it is their own design and manufacture). They have finally admitted to us they can install some after-market phase imbalance detection or shutdown, or that maybe they can program a trip (SURPRISE!!!). Very surprising they don't have fault detection or SD on a drive this large for a fault condition the drive itself can be causing (by miss-firing of the IGBT's).
 
oilandgas said:
Thanks for the reply. Confirms our thoughts exactly, and what we have been telling the manufacturer. No, the drive is not Rockwell or a cheap throwaway, it is a standard drive used for electric submersible pumps in oil wells, that is being used on a surface multi-stage centrifugal pump (so no long cable). It is a new generation Baker-Hughes drive (though we don't know if it is a re-packaged unit manufactured by someone else, though we understand it is their own design and manufacture). They have finally admitted to us they can install some after-market phase imbalance detection or shutdown, or that maybe they can program a trip (SURPRISE!!!). Very surprising they don't have fault detection or SD on a drive this large for a fault condition the drive itself can be causing (by miss-firing of the IGBT's).
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It's a long shot
The 420A on the two phases doesn't seem inordinately high for 350HP (261kW). If you assume 0.82pf, 93% efficiency and 460V it would be within the full load rating of the motor.
But the 80A on the third phase seems incompatible with that.

Would it be worth doing a test with a clip on ammeter to verify that the VFD is displaying the currents correctly?
 
oilandgas said:
Thanks for the reply. Confirms our thoughts exactly, and what we have been telling the manufacturer. No, the drive is not Rockwell or a cheap throwaway, it is a standard drive used for electric submersible pumps in oil wells, that is being used on a surface multi-stage centrifugal pump (so no long cable). It is a new generation Baker-Hughes drive (though we don't know if it is a re-packaged unit manufactured by someone else, though we understand it is their own design and manufacture). They have finally admitted to us they can install some after-market phase imbalance detection or shutdown, or that maybe they can program a trip (SURPRISE!!!). Very surprising they don't have fault detection or SD on a drive this large for a fault condition the drive itself can be causing (by miss-firing of the IGBT's).
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No, BH does indeed make their own drive, it's not a brand label of someone else' drive. Although rare now, this is still somewhat economically viable to do if you have access to a decent sized niche market for large HP drives, which can be well illustrated with BH as an example; very big in the Oil & Gas extraction and deep well ground water industry, almost a captive market in some areas. The "Hughs" in Baker-Hughs is Howard Hughs' original company, Hughs Tool, the people who made the original rotating head drilling bits and now complete driling rigs for deep wells (that's how HH first became so rich). They still have enough of a market just for their own rigs to make it viable for them to build their VFDs from scratch, have been doing that for a long time now. Still, it's surprising that this issue is described as being optional given that they ONLY make large HP drives mostly for ESPs (Electric Submersible Pumps), where this issue is much more critical. But then again, if it's their rig (and they often just lease them), it's likely more often their financial risk too.
 
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mike_kilroy said:
Important to know: is the motor rotating or enabled and at 0rpm when you see this?
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Ooh, interesting point! We're you thinking DC Injection Braking? That would possibly explain that actually. I know on deep well rod pumps, the rod twists during use and when shut down, it untwists the other direction, which can cause problems. A particularly annoying one of those problems is that during the untwist, the rod pump is essentially un-pumping, ie sending the oil back down the hole. In really deep wells, once that begins, the weight of the oil flowing back down the pump will be enough to KEEP it turning backward thus losing revenue, but not enough to start it turning. So they use DC injection braking when stopped to hold the shaft still for a few seconds until the shaft finishes untwisting, then they can let go and it stays still.

If one were to measure phase currents on the output of the drive during that process, I believe you would in fact read it as described; full current on two legs, a "ghost" current on the third if the motor is still turning while it is applied.
 
If it is not injection braking, I'd say you need to confirm the readings are accurate and/or that there is proper output voltage on all three lines, and there isn't something going on in the motor/motor conductors to cause a change in current.
 
oilandgas said:
Thanks for the reply. Confirms our thoughts exactly, and what we have been telling the manufacturer. No, the drive is not Rockwell or a cheap throwaway, it is a standard drive used for electric submersible pumps in oil wells, that is being used on a surface multi-stage centrifugal pump (so no long cable). It is a new generation Baker-Hughes drive (though we don't know if it is a re-packaged unit manufactured by someone else, though we understand it is their own design and manufacture). They have finally admitted to us they can install some after-market phase imbalance detection or shutdown, or that maybe they can program a trip (SURPRISE!!!). Very surprising they don't have fault detection or SD on a drive this large for a fault condition the drive itself can be causing (by miss-firing of the IGBT's).
Click to expand...

It is not repackaged; BH make their own drives; my friend used to be their R&D engineer. It is very bizarre that they don't have phase imbalance monitoring. You just a need a CT, not even 3, just one.

I used to work for BH competitor; submersible pumps are quite interesting.
 
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