Short circuit protection in VSD

K

Thread Starter

K C Oh

i,

I have a 132kw vector controlled VSD which was damaged when the motor it was driving at half load had a phase to phase short. Motor was burnt. I checked the specification of the inverter it has 4 times overcurrent protection.

Assuming all settings in the inverter is done correctly. It should have protected the VSD and motor from damages. But 2 out of 6 pieces of IGBT blown and 4 out of 6 fuses had blown.

My questions
1. VSD has overcurrent protection, does it include phase to phase short?

2. what about phase to earth short?

3. is this a VSD failure to protect the motor and itself?
 
M
Not really an answer - just an age old observation: The most expensive component will protect the fuse and burn out before it or
together with it. (Murphy law MLXXXIV)
Sorry to be sarcastic :)
Meir Saggie
 
A

Anthony Kerstens

Overcurrent protection is not the same thing as short-circuit protection. I suggest you read your drive manual, or call your sales rep.
 
D

Donald Pittendrigh

Hi All

I suspect the overcurrent protection is designed to protect the load and not the VSD or motor. I am not a drive specialist however as I recall it
there should be and I sq Dt setting in the VSD to protect the motor and there should be some form of earth leakage detection done by the drive, I know this is the case with Siemens Masterdrives and have seen cases of all 3 of the above protections doing their job.

Perhaps the VSD was not correctly commissioned??

Donald P
 
A

Alan Garwood

Ordinary fuses never blow fast enough to protect solid state power control devices against short circuit currents. I found this out after a heater destroyed an expensive SCR proportional controller. The solution is to use high speed solid state fuses.
 
What do you mean 4 times overcurrent protection, is the drive oversised for the motor. The largest I've seen is 200%.

The drive looks at an overcurrent as 400% of the drives continueus max current not the motor current. After all an overcurrent is a problem bad motor, wiring or locked rotor. Motor overload (I sq T), ground fault and phase loss protects the motor.

Sounds to me if the drive has (had) ground fault protection or motor phase protection it was not turned on. Look back at your records of when it was started up to see if they were turned on. They should have been. Ground fault protection should not only detect current going to earth but an inbalance of current in the motor legs.

What you needed was short circuit protection. This detects shorted motor leads, shorted IGBTs and shorted motors.

It sounds to me like the motor was being overloaded for a long time causing the phase to phase. Are you sure it was running at half load or was it half speed. Was the overload protection turned on (I sq T). Or did it have an external overload. I've also seen the situation where the drive trips and someone resets it so many times it shorts.

Good luck hope I helped not confused.

Also let us know what make and model it was.

Mike
ISG
 
hi,

I went through the manual, it states that a overcurrent protection trip limit is 4*I(rated) instantaneously.

The manual has printed possible causes for overcurrent:

1. sudden load increase
2. short circuit in motor cables
3. unsuitable motor

In a short circuit, it is basically an instantaneous inrush current. I thought it should be covered in the overcurrent protection?

Questions:

1. Does protections means reducing damages to a minimum? Or immune from such a disaster?

2. According to their sales rep, when in such a situation, the warranty claim is void as the cause is external, ironically it seems the VSD was unable to protect itself.

What are your comments?
 
Generally speaking protection built into the drive is only to protect the drive. You would have to check with your drive manufacturer for full details.
If you wish to protect the motor, you have to provide additional protection.
 
The Normal VSD will protect the motor from overloads.
During short circuit the current rises so rapidly exceeding the di/dt rating of the IGBT and thus destroying it.No fuse or overload or MCCB will prevent this.
In every power semiconductor like thyristor or transistor once there is a trigger from the gate, the expected output current will reach immedietly.But the full conduction gradually increases to the full surface area of the semiconductor in nano seconds. At the beginning of this time the current density nearer to the gate is very high.How much the device can tolerate depend on this rate of rise.
In case of short cicuit the current density on the portion of the semiconductor nearer to the gate is so huge as to melt it. Generally this can be prevented by adding additional reactors in series with the VSD output so that the Di/Dt will be within limits. Some VSD manufaturerd like Danfoss have built in reactors and test certificates to show this short circuit with standing capability can be got from them. The short circuit current is determined by the total line impedance from the transformer to your distribution point until motor and that of the supply transformer rating itself.If your VSD or Softstarter is connected to very near the distribution system as will be with your 132 Kw VSD you will need one at the supply side also to limit the Di/Dt. I have tested two VSD from two different suppliers by shorting the output with contactors and only one withstood this test.
Hope this explains.
regards,
Sekar
 
H
I want to correct a common mistake. Overcurrent protection and short-circuit protection are completely different in VFD's. Short circuit protection reacts in microseconds, whereas overcurrent protection can vary from msec to 10's of msecs depending on the overload factor. Overcurrent protection can be adjusted by the user, however short circuit cannot. Overcurrent protection is determined by the control electronics of the VFD, but short circuit is handled by the IGBT's.

Nowadays all IGBT's are produced with integrated short circuit protection (phase to phase or phase to earth), therefore the behaviour of VFD in your case is extraordinary.

This seems to be a more complicated case than just a simple phase-to-phase short circuit, because it is not expected that both motor and VFD fail. If you can, check the followings:
1. High voltage (by lightning, by other devices, etc) on motor-VFD cable
2. Rewired motor instead of an original one
3. Sabotage?
4. Triple phase-to-phase plus phase to earth short circuit if earth connections at VFD and motor are not proper.

Otherwise we can conclude that there is problem with the VFD.

I do not think that this is a commissioning mistake, since short circuit protections cannot be parameterized by the user, i.e. user is not allowed to make changes that may result to any damage to the VFD or the motor. This is a general rule for ALL VFD's, regardless of the manufacturer.
 
F

FACTORY AUTOMATION SYSTEMS LTD.

I once had a similar problem.
Drive and motor were new.
Wiring was tested and found okay.
Drive was protected with both Semi conductor fuses and a surge absorber.
Drive was powered and performed superbly .
Then a week later, drive went off. Even the display could not power up.
Semiconductors fuses were tested and found alright.
Motor winding tested and found alright.
Cabling was tested and found alright.
I opened the drive and found the power module (a combination of three phase rectifier and IGBTs) was burnt.
Everything else looked okay.
Replaced this module and three years now , the drive is still running with same motor , same cabling.
My conclusion was that the power module could have a manufacturing defect right from the start and it is only time that made it yield.
Rgds,
Gabriel
 
almost all modern ac drives have decent short circuit protection.

One quick check is to see if the transistors firing circuit protection (usually a capacitor, diode and resistor) are soldered directly to the transistor, or the transistor is soldiered directly to a pc board with the protective devices mounted very close. This is essential because any kind of wire length or non soldered connection will cause too much inductance and the transistors will take longer to turn off.

Short circuits do put a great deal of heat stress on the transistors. Most drives are designed to survive a dead short every 5 to 20 minutes, depending on how good their heat sinks are, the ambient temperature of the drive, etc... Most drives fail because operators restart them after a fault, which is not the fault of the operator, but the controls engineer that gave the operator the ability to clear drive fault.

If your drive is more then a decade old, i would expect it to loose a tranie on a short circuit.

Open core output reactors will reduce your di/dt, and are generally a good thing.

Going to a current sourcing inverter, may be an option on larger hp drives. They need a motor, or a dead short on their output to operate, but they don't like open circuits on their outputs, and tend to blow their tranies if their load opens when they are running.
 
P

Pascal Poirier

The same thing happened to me with a 20hp 1336PLUS II AC drive. The power module burned three days after initial start-up. Everything besides the drive has been tested OK. So we replaced the drive and, six months later, it is still up and runnnig. So i guess this is not an uncommon problem...

Pascal
 
I would like to have more information about the problems with short-circuit protection in VFDs.

All the VFD has protection against Short circuit?

If I energize a VFD and then short circuit the motor terminals, the VFD will suffer some damage?
 
O

Omer H. Yilmaz

Hi,

I guess I'm considerably late about the subject, but neverthless I feel like replying.

The consideration does not emphasize the VSD but the protection principles:

1- The overcurrent protection is basically the same as short circuit protection and called instantenous overcurrent protection. Short circuit refers to the event while overcurrent refers to the result which should be sensed.

2- The neutral treatment system important in considering protection. If the system is earthed neutral then the single phase short circuit current is bigger than the phase to phase fault current.There is a whole theory about the neutral treatment system. The magnitude of the current is important when designing overcurrent protection.

3- Therefore it is expected that the VSD is capable of enduring the short circuit current at least as long as the time to clear the fault current in the worst case conditions.

4- 4 times the rated current of the VSD may not mean that it is bigger than the short circuit current.

5- The fuses operated correctly against the phase to phase fault.

Regards,
Omer H. Yilmaz
 
Dear you

Here many techniques to protect the IGBT in several VSD products:

1. By measuring first the Currents i.e to motor, to -DC bus, in AC inputs, etc. Problem is the speed of protecting is too low compares to your IGBT work, and in some case also IGBT out puts it`s doesnt good Symetries.

2. In many inverters, not only the currents read by CPU but also the drop at DC bus, so with this combination protections work better.

3. If you have ASIC inside Power board, and work serial so this will be also decrease the protection work.

4. I have the better experince for protecting the IGBT techniques, if you need just contact me at my HP 62 817 812 344.

good luck.
 
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