Weird VFD impact on feeder bus

[patm72]

Hi,
We have a pair of KEB VFD’s (319 kVA) installed in the field, driving a pair of turbo fans (250hp) within the same enclosure.
The power devices in order are MCCB – IR – VFD – Sine Filter – PMSM Motor. As mentioned, two sets are installed in the same enclosure, but they are fed from two different MCC cells. There is no other significant load, only a 500VA transformer for each.
The customer where it is installed reports to us that as soon as one of the MCCB’s is switched on, the line to ground voltage on the whole bus changes to:
L1-Ground ~130 volts
L2-Ground ~385 volts
L3-Ground ~365 volts

When both MCCB’s are OFF, voltage to ground is as you would expect: +/- 277VAC for each phase.
Note that the substation feeding the system has a delta-wye transformer (2.4kV to 480 VAC), with the wye grounded through HRG. They have also tried to ground our system to their building steel, with the same results.
Line to line voltages are stable at 479VAC. This situation is observed with the blowers NOT running.
I just can’t quite understand why the voltage to ground is affected that way, although the problem is tied to the VFD it seems. Any ideas?
Thanks.

 

[LionelHutz]

The VFD causing the issue has a relatively low impedance between L1 and ground. You will likely have to go find it.

 

[jraef]

And or the VFDs have line side surge protective devices (usually MIVs) that are configured in a Wye with a reference to ground, which in an HRG system presents a lower resistance path for the entire system. Most drives will give you instruction in the installation manual on special actions you must take to use them on HRG or Felta power sources, or will have a statement saying they can ONLY be used on solidly grounded systems. RTFM.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[GroovyGuy]

hi Pat,
1) Just curious, but did the VFD OEM know that the power supply was High-Resistance-Grounded? I know that some OEMs don't like a HRG system and have to tweek their drives to make it work.
2) Does the voltage imbalance (wrt ground) occur on both of the drives? If so;
a) Is it the same phase (ie A) on both drives that goes to ground?
b) Is the magnitude of Phase-A to ground the same for both drives?
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

Are the 500VA transformers connected phase to ground?
Can you roll the transformer connections and see if the low voltage to ground follows the transformers?

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

 

[patm72]

Groovy Guy, see my comments below:

1) Just curious, but did the VFD OEM know that the power supply was High-Resistance-Grounded? I know that some OEMs don't like a HRG system and have to tweek their drives to make it work. No. It seems HRG's are also a problem for them, although nothing was explicitly written in their manual.
2) Does the voltage imbalance (wrt ground) occur on both of the drives? Yes. And echoes right up to the MCC as well. That, regardless which VFD is powered.If so;
a) Is it the same phase (ie A) on both drives that goes to ground? Phase A has lower voltage (130 +/- 15VAC) but is not "to ground"..
b) Is the magnitude of Phase-A to ground the same for both drives? More or less yes.

Thanks.

 

[GroovyGuy]

Hi Pat,
Have you contacted KEB wrt this issue? If so, what did they say?
There is definitely something tying phase-A to ground. It is obviously not a dead short or phase-A would be at 0V and Phase B & phase-C would both be at 480V.
What is the size of your NGR (ie 1 or 2 amperes)? IS your transformer secondary a wye-type connection with a NGR, or is it a delta connection with a zig-zag grounding transformer and a NGR?
Regards,
GG
ps Do you have a complete OEM schematic for the drive (with all the bits & pieces shown)? If so, please post.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[patm72]

GG,
KEB has been in from the start, but what they say is grounding on the mains side is probably the culprit.. But otherwise, the system has been running fine despite this situation (remember all three phase to phase voltages are 480V and constant despite the phase to ground voltages being inconsistent).
I don’t have the specifics of the NGR. The one-line diagram I have shows a Delta-WYE xfo 2.4kV-480Y/277.
Below is a link to data on the KEB drive:

https://www.keb.de/fileadmin/media/Manuals/f5/power/w/00f50ebkw00.pdf

Thanks for your input.

 

[jraef]

What's wrong with their assertion that it's a grounding issue on the line side, is that you said the issue only shows up when one of the drives is connected by closing the breaker feeding it. So either the problem exists in BOTH sets of cables feeding the drives (because you said they have separate feeds), or its in the front-end of both drives. Assuming the cables were meggered after installation, I'm not buying their assertion that it is not something in their drive.

Glancing at the installation instructions, it clearly states (to those familiar with IEC transformer designations) that the drive is suitable for grounded wye secondaries of various types, nothing (that I could decipher) indicates that an HRG system is acceptable, nor do they detail any instructions to make it so.

But I will say this. KEB is German and when I worked for Siemens, they too struggled with their drive front-end designs being incompatible with North American delta and HRG wye systems. So Siemens USA had to come up with their own separate add-on instruction sheet on adapting their drives, by disassembling the front end and removing a ground reference connection strap. I suspect the same is true here, but KEB US sells so few drives here, their people have not yet learned this lesson. I suspect that in all likelihood, you have already blown the MOVs on one or both of the drives, it may have happened the instant they were first connected. Then because the failure mode of MOVs is often vaporization, your front end system on each unit is coated with the material and tracking, which would be resistive, but in parallel with the HRG resistor, so it is presenting a slightly lower resistance to ground, which to a meter shows up as a lower ground reference voltage on whichever line was closest to the MOVs. In this case I'd be willing to bet that the MOVs were mounted next to L1, explaining why it would preset the same way on both drives. All kinds of strange effects can happen once you get vapor deposition going on in a closed space.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[GroovyGuy]

Howdy Pat,

I scanned the manual (at the link you provided) and could not find any warning wrt treatment of HRG neutrals. I would be personally quite chocked with KEB if their drives do not play well with a HRG neutral, and they did not explicitly spell this out in their manual.

One thing that I did find interesting is that this drive can be configured as a 6-pulse or a 12-pulse drive. Seeing that you have two (2) transformers supplying the drive cabinet; Is there any chance that theses drives have been connected as a 12-pulse drive? I am not sure if there is anything here wrt your grounding issue, but I think it is worth confirming. (ie if this is true then the transformer secondaries are tied together (indirectly) thru the DC-caps.

Regards,
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

Basic troubleshooting. Roll the connections on one of the drives.
Is the supply HRG or floating ground?


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