A quick rundown is that this has occurred twice now on board a ship where a 3Ph 480v 200hp VFD for a refrigeration compressor suffers a catastrophic failure resulting in the entire power system tripping off line. The power plant consists of multiple 480v generators fed into the power distribution panel the failure trips both breakers of the generators currently feeding the distribution panel that are online at the time as well as overloading the generators causing them to completely shut down. From the distribution panel there is another breaker that feeds power to the refrigeration panel and then a third breaker that supplies the VFD power. All of these breakers trip when the failure occurs. My concern is that obviously the breaker in the refrigeration panel for the VFD and the breaker that supplies the are not adequately protecting the rest of the power distribution system. I suspect that the somewhere along the line the distribution is improperly set up/installed and was hoping you could help point me in the right direction. I am sure that there is further information you will need and I will certainly provide it I will just need a nudge in the right direction here and there to properly track this down and correct this system.
VFD Failure Tripping Generators Off Line
- Thread starter miscro
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Sounds like you need to look at trip curves of devices in question and make certain you have branch circuit device that will trip at lower level in the curve than any upstream devices have. Keep in mind the amount of fault current the generator can put out may not be same level you would see if on utility power and must also be considered, low fault current will increase the time component in the trip curve for the same device.
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
When you say that the VFD suffers a catastrophic failure, can you provide more details?
Is the input rectifier failing, in a fashion that is somehow dumping DC across your power distribution system?
-Jon
Is the input rectifier failing, in a fashion that is somehow dumping DC across your power distribution system?
-Jon
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
You'll need a unpdated one line drawing, gather information on all the OCPDs with time current curves, then do a coordination study as Kwired pointed out. The generator SSC issue is an interesting one so please post back how this all works out, we like to hear back so we can learn
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
As others have said, I see two problems here;
- Coordination of the trip settings on the circuit breakers appears to be wrong or non-existent. You need a thorough/detailed coordination study to be preformed by someone. But you may want to consider high speed semiconductor type fuses for protecting the VFDs, most VFDs require them.
- Two VFDs having "catastrophic" failures that cause breakers to trip is not something I would accept as "normal", so something is wrong here. In many cases, shipboard generated power systems are Delta and many VFDs do not do well with Delta ungrounded sources. There are things you can do about it, but if the installer was unaware of these issues, it can lead to failures like this.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
What is the issue with VFDs and ungrounded deltas, is it the voltage has no ground reference and tends to vary?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Many VFDs have ground referenced surge protection on the input. If the shipboard power system is an ungrounded delta, then the SPD might be trying to act as the ground reference for the entire ship. Or everything might be fine until some other device has a ground fault, lifting the other two phases to 480V relative to ground. The SPD is expecting 277V to ground....
The other issue is the DC rail voltage relative to ground. The rectified output of a wye supply is roughly the same +- relative to ground (with some 3x AC ripple on it). The rectified output of an ungrounded delta is similar, but the DC rectified from a corner grounded delta shows much greater AC relative to ground (the DC bus voltage + to - is exactly the same, but the voltage relative to ground is quite different).
-Jon
The generators are connected as a grounded wye. I have considered the coordination issue, correct me if I'm wrong but if that were the case wouldn't that manifest with the breakers down the line remaining closed? In this case all breakers have opened in the entire circuit all the way back to the main distribution panel.
As far as the two VFD failures I should add that these have not been back to back. The previous instance was over a year ago. However it had the same impact of shutting down the entire plant when it failed. That being said I am certainly not considering the failures to be "normal", however my primary focus as of now is ensuring that such a failure does not result in the plant shutting down and complete loss of power (not good on a ship while at sea). I would absolutely like to prevent future VFDs from failing in this manner. A visual inspection of the recently failed drive shows that the most damaged component is a thyristor that has melted/blown apart. I will gladly consider suggestions on protecting the drives from future failures though some quick research has shown me that large VFDS can be prone to failure and though we do our best a ship will never be an ideal environment for electronics.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
anything can be prone to failure. I have not seen that larger VFDs are especially prone to die.
I'd want to look at the alarm logs for the generators and see what happened and in what order. For instance, did the CBs trip because of an overload or did a protection relay open and trip them. You also mentioned the generators shut down. What tripped them? Surely there must be some record of what happened.
I'd want to look at the alarm logs for the generators and see what happened and in what order. For instance, did the CBs trip because of an overload or did a protection relay open and trip them. You also mentioned the generators shut down. What tripped them? Surely there must be some record of what happened.
And what is the failure mode of the VFD? Considering multiple generators with an SCR front end for instance it is subject to self commutation and MOVs are an absolute must.
Also consider if you need additional impedance on the line side. If the line kVA (generators) exceeds the VFD kVA more than 10 times you definition need 3% line reactors and maybe consider extra external surge protection as well. All that generator switching is rough. Also is it ungrounded delta feed? If so double my comments about surge protection.
Also consider if you need additional impedance on the line side. If the line kVA (generators) exceeds the VFD kVA more than 10 times you definition need 3% line reactors and maybe consider extra external surge protection as well. All that generator switching is rough. Also is it ungrounded delta feed? If so double my comments about surge protection.
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