Source of ignition? 1

[Marke]

In a recent enquiry into a fatal coal mine explosion, it was suggested that one possible source of ignition of the methane gas, was an electrical arc from the stray voltages cuased by the common mode voltages from VFDs and motors installed underground.

Has anyone experienced electrical discharges from common mode voltages that could be an ignition source?
Any documentation that supports this suggestion?
Are there any regulations minimising the potential of VFDs providing an ignition source?

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[Marke]

http://pikeriver.royalcommission.govt.nz/vwluResources/13-February-Final/$file/13-February-Final.pdf

Note: by "harmonics", the witness is referring to all emissions from the VFDs, not just the line side harmonics.

Mark Empson
Advanced Motor Control Ltd

 

[MiketheEngineer]

From my point of view - just about ANYTHING electrical can spark - even cell phones. That's why you turn yours off when re-fueling.

Again FMPOV - that's a no-brainer and probably what happened.

Might not have been the motor or VFDs - but some dumb ass calling his wife!!

 

[dpc]

Motors can be a potential spark source even without a VFD, unless specifically designed otherwise, but I'm sure you are aware of that. Most VFDs some type of relay or switch that is capable of producing arcs. I don't see how a VFD could even be installed in a hazardous location. But maybe I'm missing something.

Natural gas plants do not allow workers to even carry cell phones in the plant.

I've not seen any documentation specifically related to common mode voltages. I'd like to know how they could zero in on that as a cause after an explosion.

We see so many fires in the US blamed on faulty wiring or described as "of electrical origin" when there is often not much evidence to support it. If they can't come up with anything else, they blame on the wiring. Similar to airplane crashes, if you can't find a cause, blame it on the pilot.

 

[djs]

The high frequency components of the VFD output will cause a common mode current to flow to ground. If a VFD motor has faulty grounding, then sparks will fly.

 

[OhioAviator]

Marke,

I can't directly speak to how common mode voltages generated by a VFD could become an ignition source for methane gas. However, there is a more fundamental issue to be explored, that being how did the so-called common mode voltages get into contact with methane gas in the first place, and were the equipment (VFDs and motors) rated for use in a gassy atmosphere.

I have more than 20 years experience in underground coal mining, both in the USA and in Australia, and from my point of view, whenever the so-called 'experts' start focusing on esoteric things such as 'common mode voltages', that generally means they have no idea what caused the explosion and are extremely reluctant to point to human error as the cause.

For example, in Australian coal mines in the 1990's (maybe still is) it was illegal to have aluminum (aluminium in the British lexicon), in an underground coal mine because someone somewhere determined that a spark from a piece of aluminum caused a fatal explosion in a Queensland underground coal mine. Pure rubbish. Coal mines in the USA are full of aluminium control panels in the hazardous (inbye) areas of the mine and I can guarantee you that if someone were to take a 20 lb sledge hammer and beat on the aluminium enclosures for an hour in an 8% methane atmosphere, the only thing that would happen is that person would get really, really tired.

If the VFDs and motors were in properly maintained flameproof enclosures, the methane ignition would never have propagated outside the enclosure. If the VFDs and motors were in the outbye (non-hazardous) areas of the mine, then how did the methane concentration in those areas get into the dangerous zone of 5 to 15 percent? There's no way that would have happened if the ventilation system (fans, stoppings, curtains, etc.) were in proper order.

I'll get off my soapbox now. Please accept my apologies. It's just that I'm pretty passionate about this particular subject.

 

[LiteYear]

Mark, my coworkers and I have been keeping a close eye on this enquiry. We've been consulting on the hazards of touch potentials due to VFDs for some time. To answer your questions directly:

1. Not personally, only anecdotally.
2. Only our internal documentation, public lecture material, and internally developed simulations and models. We believe we're one of the few groups to have formally documented the causes.
3. Currently lacking, in our opinion.

Since I'm not directly involved in the internal project, to elaborate would quickly go beyond my personal expertise. If you'd like to pursue this professionally, please contact us at ResTech

For the sake of discussion, djs is on the money. There's another important factor though - the high frequency current is usually contained within the VSD-motor circuit. If the ground path within this circuit is poor, any current paths (other cables, buried pipes) between the motor and the VSD become susceptible to induced harmonic currents.

dpc, the bit you're missing is that the VSDs installed underground are usually inside a flameproof enclosure. What gets complicated is that ultimately, the power cables cannot be entirely contained within the enclosure, so there are considerations for what gets carried by those cables.

MikeTheEngineer, this was an underground mine. Sources of ignition are tightly controlled. No one would ever flippantly call their wife underground!

 

[PersonalProfile]

A bit off topic, though may be of interest:

http://www.dpi.nsw.gov.au/__data/as...4-Electrical-hazards-associated-with-vsds.pdf

http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0020/318710/19---Tim-Wylie.pdf

http://www.dpi.nsw.gov.au/__data/as...rives-and-Electrical-Protection-Tim-Wylie.pdf

Regards,
Lyle

 

[dup18]

Mark,
I have worked as an electrical engineer in the underground (coal)mining industry for over 30 years, and VFD type controllers (or any other type of electrical equipment) are 1) housed in an explosion proof enclosure, or 2) installed where there is no possibility of methane gas accumulation.
The damage that was done to the ventilation system does not appear consistent with the damage that would be caused by a "normal" equipment malfunction.
Without knowing all of the specifics, anything else would be a guess.
If you or anyone else in this forum has questions regarding mining electrical systems, I will be following this forum, and will try to address them.
Dave

 

[Marke]

Hello lyle
Goodd information, thank you.

Hello dup18
Thewre were a number of VFDs, all 690V except one 3.3KV. These were all housed underground in a safe area.
Cables then ran into the mine to the pump and fan motors.
The pumps pump water throughout the working area of the mine, (unsafe areas where methane can accumulate) and there was a fan underground for ventilating the gas (unsafe area) that was driven by a VFD.
The proposal is that the frame of the motor can become live due to common mode voltages (from the VFD outputs) coupled into the frame of the motor. These are then coupled to the pump (non isolated) and then into the pipework travelling through the mine.
The common mode voltage is a high frequency voltage which could spark to other earthed metalwork.
Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

 

[burnt2x]

Mark,
Could be hammers hitting hard stones, or compromised explosion-proof switches, or hotwork being done away from the forward panels but methane crept towards the area (by ventilation action of joy fans), or electrical equipment not conforming to the required classification (as you mentioned - a spark from the live parts to the ground).

OTOH, we all know any source of sparks will ignite an "explosive gas mixture". I always see this propensity of investigators to pick the electrical aspect in most fire/ explosion incidents but I'd rather point them to the presence of explosive gas rather than the ignition source. IDK if that coal mine you are investigating ever had gas monitors in place that will automatically shutdown all electrical equipment if your sensors register more than 1% of methane level. Although the concentration of methane gas will never be uniform inside mines, it is a requirement to provide miners with personal gas monitors for them to manually shutdown electrical equipment and other powered tools should they hear there gas monitors beeping. I have brushes with these types of investigators who find it easier to just blame electrical if these accidents happen.

 

[purplepete]

We had a three-phase 415 volt 355 HP 4-pole electric motor driving a compressor. The drive end bearing failed within 1 year due to substantial current flow through it. We tong-tested the phase and earth conductors after the replacing the motor. Phase currents approx 400 Amps, earth current approx 90 Amps. The phase conductors were XLPE/PVC singles run in trefoil about 150 metres. The earth conductor was positioned perfectly alongside and against the same phase conductor the whole route. After separation the about 600 mm all was well, minimal induced earth current. Others have had similar experiences.
I assume the references (sorry, haven't time to read them) talk about the need for insulated bearings on larger VSD-driven motors due to harmonics in the drive shaft also generating unacceptable voltages (then circulating currents hazardous to the bearings). I guess these voltages will still be present along the rotor and drive shaft after the insulating sleeve is installed.

 

[dup18]

Mark,
Referring to your comment to me (dup18), if you are concerned about common mode voltages raising the frame voltage, and this subsequently causing an explosion hazard, would you also be concerned if it were a across the line starter and the motor had a ground fault? If so, there are other problems. If not, why not? - These questions are to help me better understand the problem.

 

[Marke]

dup18

Yes, the common mode output from a VFD is capacitively coupled to the frame of the motor and can result in very high voltages on the motor frame.
An insulation breakdown within the motor can also cause transient voltages on the frame of the motor.

Mark Empson
Advanced Motor Control Ltd

 

[dup18]

Marke,
What I was getting at was that if a ground fault on a conventionally started motor would have the same result as a VFD powered motor, there was something wrong with the installation.

If the theory is that the frame voltage were coupled through piping (for example) into a hazardous location where methane gas was present, then there should have been a section of insulated pipe installed before the piping enterd into this hazardous location.

Another solution would have been to continuously monitor the atmosphere in that area and de-energize the equipment before the methane concentration could have built up to an explosive level.

When coal is actively being mined, a certified person must make methane examinations before the machinery goes "inby the last open crosscut" and every 20 minutes afterwards. "Inby the last open crosscut" refers to an area where there is a possibility that the air may contain methane gas, because of the freshly exposed coal.

Methane - air mixtures are explosive when the methane concentration is between 5 and 15 percent.

The actual machine the cuts the coal (called a continuous miner) is equipped with a methane monitor that continuously monitors for methane and alarms the machine operator when the methane concentration reaches 1 percent. At that point, the operator is supposed to back the machine out of the area and de-energize the machine. At a 2 percent (or lower)concentration,the methane monitor automatically de-energizes the machine.

I have repaired many problems on mining machinery over the years involving grounded leads or motors that occured inby the last open crosscut, but if the machine is properly grounded and if the safety devices work properly, there should not be any catasphrophic results.

Maybe VFD drives used in these applications need looked at more seriously because of these common mode voltage peculiarities, but nothing replaces proper installation and maintenance practices.

Dave

Dave

 

[PersonalProfile]

Although again slightly off topic, this may also be of interest:

http://www05.abb.com/global/scot/sc...3cca8e/$file/abb_technical_guide_no5_revc.pdf

Regards,
Lyle