VFD power cell failure

[zbang]

And, if it existed would, as Goldilocks says, it would cool the heat sensitive components.

We're barking up two different trees in the same grove- one is IGBT failure and the other is temp. related alarms.

 

[Sahib]

Thermal runaway from poor cooling would result in destruction of IGBT.

 

[Besoeker]

Thermal runaway from poor cooling would result in destruction of IGBT.

Yes, bust mostly the heat sinks are fitted with thermal protection these days.

 

[Sahib]

Yes, bust mostly the heat sinks are fitted with thermal protection these days.

So after shutdown ie IGBT is switched off, no thermal runaway could take place due to no cooling. So I suspect manufacturing defect unless there is repetition of failures.

 

[Besoeker]

So after shutdown ie IGBT is switched off, no thermal runaway could take place due to no cooling. So I suspect manufacturing defect unless there is repetition of failures.

I don't know what the shut down sequence is but that could be an issue.

 

[Ingenieur]

it could over temp
a transient surges thru the igbt and raises the temp as the cooling shuts down
at the point is could be above design temp for a period of time as is cools down

I doubt it's a manufacturing defect
they would not keep sending him defective components
this sounds like it has happened more than a few times

OP
is the damage random or the same cells?

 

[Ingenieur]

check
the fan rotation, airflow direction, they usually draw thru the enclosure
filters on inlet louvers
IR the components, discuss results with mfg

does the fan stop immediately upon a stop command? does the drive continue to run in a ramp down mode?

have you talked to the mfg? what feedback does he offer?

 

[Jraef]

The model is a Hyundai N5000-4000M6. The manual is downloadable from the internet.

The power cell is located at the inverter (output) going to the motor.

I was thinking that this is a "power cell" board control issue. But, what cause it? Sometimes, it says "W4, Cell abnormality, IGBT3 fault", I know that this IGBT is inside the power cell.

It was corrected by replacing the power cell boards - precharging board, power board, SCR snubbing circuit board, and the control board.

Say, if the error was IGBT3, what can caused such error or fault? Is it heat related? Or some other worst issue?

I have tried to look at the VFD parameters, they are using SLV control mode - sensorless vector control mode which was set by Hyundai.

Thanks

I was also thinking it could be a grounding issue.. As for heating, we have already corrected the cooling fans inside the panel.

So this is what's called a "Cascaded H Bridge" (CHB) multi-level inverter design. Those familiar with MV drives may have heard of the Robicon "Perfect harmony" drive design that stormed through the MV drive world a decade ago. Siemens has since bought Robicon, but the patent on the CHB design expired in around 2012, at which time a LOT of other manufacturers jumped on the CHB design train because of the advantages it offers.

One of the major advantages is that the MV output is comprised of multiple LV power cells that are each individually fed by a specific isolated winding in the complicated transformer ahead of the drive, then the LV PWM outputs are cascaded into the MV level going to the motor. In that design, a 4160V drive will be "24 pulse", meaning there are 4 sets of secondary windings, each separated by a 15 deg. phase shift so that the harmonics on the primary of that transformer are very effectively reduced. You said this has "6 windings", which would actually be 6 SETS of windings, typically used for 6.6kV motors, but the same drive COULD be told to output 4000V, it's just a waste. it appears though that Hyundai doesn't actually make a 24 pulse design, so it's 36 pulse.

So assuming that's true, then within each power cell the "line" voltage AC feeding it is going to be around 400V (sq. rt. of 4160 / number of series cells in each phase, in this case 6). So the transistors inside are really not experiencing any more voltage stress than any standard 400V VFD. The current in each cell is, by the way, very low. The total CUMULATIVE heat in the VFD is high, but on each individual cell, you are looking at maybe 1200W of heat rejection, divided across 12 IGBTs per cell is only around 100W of heat rejected by each IGBT. Thermal mass dissipation at an individual IGBT level at shut down becoming too slow because the fans shut down is not worth considering as a possibility IMHO...

So I think there are a few better possibilities here;

1. That cell had/has a bad IGBT inside of it from the beginning and has been limping along. You replaced everything BUT the bad IGBT, so the problem continues after replacing the firing boards. As a general rule I always recommend replacing an ENTIRE CELL when there is a problem, then send the bad CELL into the mfr for a thorough forensic analysis. Do not ASSume you know what the problem is...
2. Your firing circuit for that cell is picking up some noise somewhere and causing the cell to misfire, which eventually results in the death of the cell. The thing that points to this possibility for me is that the way these CHB designs work (and Hyundai states this in their literature) is that the drive can function fine with a bad cell, it just reduces the capacity of the drive, so your maximum speed / load is curtailed until you replace it. To facilitate that, each cell has a "bypass" that can automatically allow power to flow around that cell if there is a problem detected and the drive will bypass corresponding cells in the other phases to balance everything out. From the sounds of it, your cell is NOT going into bypass, it is just failing. That to me points to something being haywire in the cell's overall command and control system and in my experience if it repeats itself after replacing everything, that's usually noise related. Someone may have moved / re-routed a control signal cable at installation, thinking it was unimportant but on MV drives, routing is CRITICALLY important.
3. The transformer has a bad winding and despite the repairs, the power getting to that cell is always going to be bad. That's simple enough to check for a qualified technician, there should be safe test points for each individual cell input. On the CHB drives I've worked on (A-B, you can read each cell's individual incoming AC input on the display and you would know if a transformer winding was bad). I don't know if Hyundai allows for that, but they likely have something. So although that's not likely it, it's worth mentioning.

Count the mistakes already made as a learning experience and get a qualified service technician trained by Hyundai to come out and test the cell input to eliminate a bad transformer winding, then replace the ENTIRE cell. Send the bad cell to Hyundai and have them do a thorough Root Cause Failure Analysis on it. Speculation on failure causes in MV VFDs can become very expensive, very quickly.

 

[Besoeker]

Speculation on failure causes in MV VFDs can become very expensive, very quickly.

Totally agree. I made that point a few posts back. Replacement parts and the cost of it being out of service.

 

[Ingenieur]

... Speculation on failure causes in MV VFDs can become very expensive, very quickly...

totally agree, as I previously said
better to eliminate the obvious and get the mfg involved

 

[rian0201]

So I think there....

Thanks everybody for the reply. As of now, I haven't talked to my friend yet.

The IGBTs that failed were still intake. My friend told me that they haven't tested any of the IGBTs - but they were able to pinpoint this fault based on the indication the VFD's indication. They tried to swap the parts of an old spare power cell to the damaged power cell. And they confirmed that some failures were IGBTs, control boards, rectifier diodes and others through the swapping of this parts and test them by connecting them again.

The actual number of power cells are four and it displays the following voltages:

U = 893, 908, 913, 906
V = 893, 901, 912, 894
W = 889, 898, 914, 896

Is it right to add them like U or A/phase = 893+908+913+906 =3620 V and take the sqrt(3) [assuming the LL voltage) = 6270.03 V? But, the motor is rated 4160V? is this okay?

They tried to contact Hyundai representative and there were some proposal or solutions - but the problem still occurred. Among the solutions were - space heaters (since other heaters failed), Grounding re-routing, re-structured the shutdown sequence.

Also, the power cell controller is connected via optical links - is this still susceptible to noises on its surroundings?

 

[Sahib]

The IGBTs that failed were still intake. My friend told me that they haven't tested any of the IGBTs - but they were able to pinpoint this fault based on the indication the VFD's indication. They tried to swap the parts of an old spare power cell to the damaged power cell. And they confirmed that some failures were IGBTs, control boards, rectifier diodes and others through the swapping of this parts and test them by connecting them again.

Let us wait for the results.

The actual number of power cells are four and it displays the following voltages:

U = 893, 908, 913, 906
V = 893, 901, 912, 894
W = 889, 898, 914, 896

Is it right to add them like U or A/phase = 893+908+913+906 =3620 V and take the sqrt(3) [assuming the LL voltage) = 6270.03 V? But, the motor is rated 4160V? is this okay?

Arithmetic sum of voltages not always correct. Any output voltmeter for the power cell as a whole?

They tried to contact Hyundai representative and there were some proposal or solutions - but the problem still occurred. Among the solutions were - space heaters (since other heaters failed), Grounding re-routing, re-structured the shutdown sequence.

Carry out the proposal or solutions in consultation with the manufacturer.

Also, the power cell controller is connected via optical links - is this still susceptible to noises on its surroundings?

No.

 

[rian0201]

Hi all,

Here is an update. When there is a fault upstream like at the source, there will be no power at the auxilliaries or at the fans, the fans will not run.

We are still placing power quiality metera to monitors the quality of voltage that's coming in the powwr cell.

Also in the power cell there is a resistor rated around 150W that gets easily damaged. We are still monitoring the voltage and current. Will update soon.

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