Active Front End(AFE) VFD fault current contribution

[elec_eng]

I am reviewing the fault current study for a project and the engineer did not include any motor contributions for the short circuit calculation because there are VFDs on the motors.

My understanding is that the SCR front VFDs do not provide a motor (fault current) contribution to upstream of VFD when there is a fault at the downstream of the VFD. But Is this true for the AFE or Matrix VFDs? I would think the answer is NO, since the current can flow backward with AFE or Matrix VFD. Any thoughts?

 

[Jraef]

Fault contribution is not about a fault below the VFD, that is handle BY the VFD. Fault contribution is that if the motor is spinning and there is a fault UPSTREAM of the motor, the motor becomes a generator and adds it's kinetic energy to the fault, so the available fault current in the entire system is increased by the all of the connected spinning motors' contributions.

In any VFD, there is no contribution from the motor, despite the assertions otherwise when you have an active front end. In a standard drive the diode bridge is unidirectiinal. But even in an Active Front End drive, the drive is only allowing power flow to take place on direct command. The at rest state of the power devices is blocking both directions. There is a lot of misconception about this based mostly on conjecture. Side note though, I'm not sure about matrix drives, that topology is an odd duck and although I think the same may be true, I don't know for sure.

That said, most Active Front End drives must have a line filter of some sort, such as an LCL filter. That filter will contain capacitors, and those caps will contribute a small amount. How much will be totally dependent upon the size of the caps used. So if you are concerned, simply ask the drive mfr to outline the amount of potential fault current contribution. I just went through this exercise on some 4,000HP medium voltage active front end drives, the fault contribution was about 10 amps per drive.

 

[Electric-Light]

If the drive is not regenerative you can think of the power as being through a normal bicycle power train. The wheel doesn't force the pedals to turn when you stop pedaling.

 

[elec_eng]

Fault contribution is not about a fault below the VFD, that is handle BY the VFD. Fault contribution is that if the motor is spinning and there is a fault UPSTREAM of the motor, the motor becomes a generator and adds it's kinetic energy to the fault, so the available fault current in the entire system is increased by the all of the connected spinning motors' contributions.

You are correct. Thanks for correcting that. I was not thinking clearly.

In any VFD, there is no contribution from the motor, despite the assertions otherwise when you have an active front end. In a standard drive the diode bridge is unidirectiinal. But even in an Active Front End drive, the drive is only allowing power flow to take place on direct command. The at rest state of the power devices is blocking both directions. There is a lot of misconception about this based mostly on conjecture.

That is good to know. Thank you for clarifying that. I was one of them believing that.

Side note though, I'm not sure about matrix drives, that topology is an odd duck and although I think the same may be true, I don't know for sure.

That said, most Active Front End drives must have a line filter of some sort, such as an LCL filter. That filter will contain capacitors, and those caps will contribute a small amount. How much will be totally dependent upon the size of the caps used. So if you are concerned, simply ask the drive mfr to outline the amount of potential fault current contribution. I just went through this exercise on some 4,000HP medium voltage active front end drives, the fault contribution was about 10 amps per drive.

I will try to contact the manufacturer for more info. As always, surprised with your in-deep knowledge of the VFDs and thank you so much for sharing them in this forum.

 

[Jraef]

Just one small qualifier though, after a co-worker and I discussed this. I have been caught up lately in people using AFE drives for the purpose of mitigating harmonics. the OLDER reason for using AFE drives was to be capable of line regenerative BRAKING of a motor. So with an AFE drive in the midst of line regenerative braking at the exact moment of the up stream fault, you DO have to consider the motor contribution, because power is ALREADY flowing toward the fault. So since you cannot control when the fault may happen, you would need to take that possible contribution into effect.

So amending my earlier statement;

"In an Active Front End drive, the drive is only allowing power flow to take place on direct command. So if the AFE is used only for harmonic mitigation, there is no direct command for power to flow the other way and the drive does not allow the motor to contribute."