Redundant VFD package - Safety issue?

[elec_eng]

I have a redundant VFD package for 100hp motor, basically the second VFD is connected to the bypass of the first VFD so if the first VFD fails, it will automatically transferred to the second VFDs. The idea is no service interruption should any one of VFDs fails.

The question is can we replace the failed VFD from the safety standpoint while the second VFD is still running? Since two VFD outputs are connected together at the motor, could the output of the first VFD conductors be still hot? Will that be an issue? Should there be another isolation disconnect at the output of the VFD so that VFD is totally isolated while it is replaced? Any thoughts?

 

[Ingenieur]

yes it is an issue
I would not trust contactors
use a manual visible disconnect switch on each side of the vfd

 

[Besoeker]

yes it is an issue
I would not trust contactors
use a manual visible disconnect switch on each side of the vfd

I agree. In fact we wouldn't be allowed to touch it unless it was isolated and locked off.
We generally used fuse switches ABB OESA or similar.

 

[petersonra]

there should be some kind of switch or CB on both the line and load side of each VFD. That way you can open them both on one VFD and safely replace it if need be.

 

[Ingenieur]

we use a std knife disconnect with a window
in most cases it has a limit switch that opens before the switch breaks and drops a uv cb, no breaking under load
if it does'nt have this the window will will have a hinged cover for arc flash

it can be fused based on application

 

[elec_eng]

That's what I thought. The redundant VFD package assembly offered by the Vendor do not have the output disconnects. They only include the input disconnect with contactor at the output. I guess either I have to ask the vendor them to include the output disc in the package or install one in the field. Thank you for your comments.

 

[petersonra]

That's what I thought. The redundant VFD package assembly offered by the Vendor do not have the output disconnects. They only include the input disconnect with contactor at the output. I guess either I have to ask the vendor them to include the output disc in the package or install one in the field. Thank you for your comments.

Be careful what you actually ask for if both be at these are in the same enclosure chances are he won't be allowed to work on it live anyway they will need to be in separate enclosures

 

[Jraef]

What you are describing sounds like an "N-1" hot backup drive arrangement, and having an output disconnect would likely not be possible anyway as, per what petersonra said, they are in the same enclosure so working live is an unlikely option anyway. In an N-1 arrangement, they often use one rectifier and two inverters or the DC busses are tied together, so you can't power down only half of it anyway. The intent is that the load continues running, then you can remove the dead inverter at the next opportunity for a normal shutdown, not WHILE the load is in operation.

 

[RumRunner]

I have a redundant VFD package for 100hp motor, basically the second VFD is connected to the bypass of the first VFD so if the first VFD fails, it will automatically transferred to the second VFDs. The idea is no service interruption should any one of VFDs fails.
The question is can we replace the failed VFD from the safety standpoint while the second VFD is still running? Since two VFD outputs are connected together at the motor, could the output of the first VFD conductors be still hot? Will that be an issue? Should there be another isolation disconnect at the output of the VFD so that VFD is totally isolated while it is replaced? Any thoughts?

The arrangement you described is akin to (UPS) uninterruptible power supply. If this is your utmost criteria, a simplistic approach may not be simple as it seems.

The two VFDs share a common bus which if installed in one cabinet will still have hot terminals even though either VFD is de-energized. Personnel working on this would be exposed to hazard which might raise eyebrows with OSHA.

That is one aspect.

Another is: in the event of failure of either VFD. . . the instantaneous power transfer that you require will involve more than meets the eye.
The VFD at rest that is suddenly subjected to both input and output stresses may react unfavorably. It needs time for the capacitors to charge in order to achieve the desired output that will match the needed power to the prevailing demand.
For most application similar that you cited, this UPS style for large power units are so called Dynamic Uninterruptible Power Supplies (DUPS)-- are sometimes used.

A synchronous motor or alternator is connected at the source via a choke along with a (mechanical) flywheel that stores energy and therefore allows the VFDs to stabilize before initiating the transfer.
Flywheel design should be based on how long it will take for the capacitors to get charged.
Charge time should not be longer than what the flywheel can substantially store the energy needed while the transition is taking place.

One can argue that capacitors are pre-charged which is true, but they also lose charge with inactivity.

 

[Jraef]

The arrangement you described is akin to (UPS) uninterruptible power supply. If this is your utmost criteria, a simplistic approach may not be simple as it seems.

The two VFDs share a common bus which if installed in one cabinet will still have hot terminals even though either VFD is de-energized. Personnel working on this would be exposed to hazard which might raise eyebrows with OSHA.

That is one aspect.

Another is: in the event of failure of either VFD. . . the instantaneous power transfer that you require will involve more than meets the eye.
The VFD at rest that is suddenly subjected to both input and output stresses may react unfavorably. It needs time for the capacitors to charge in order to achieve the desired output that will match the needed power to the prevailing demand.
For most application similar that you cited, this UPS style for large power units are so called Dynamic Uninterruptible Power Supplies (DUPS)-- are sometimes used.

A synchronous motor or alternator is connected at the source via a choke along with a (mechanical) flywheel that stores energy and therefore allows the VFDs to stabilize before initiating the transfer.
Flywheel design should be based on how long it will take for the capacitors to get charged.
Charge time should not be longer than what the flywheel can substantially store the energy needed while the transition is taking place.

One can argue that capacitors are pre-charged which is true, but they also lose charge with inactivity.

There are numerous high-end VFDs on the market that offer this (N-1 operation) as a standard option. In normal operation, BOTH inverters are connected in parallel and are sharing the load between them, fed from a common DC bus and synchronized internally via fiber optic comms. When one inverter faults, the other one simply takes on the entire load and since it was already connected to it, there is no transfer function involved. Optionally, you can have two inverters sized for the entire load, or two inverters sized for a portion of it, then when one faults, the speed is reduced to whatever the capacity of the single inverter is.

 

[RumRunner]

There are numerous high-end VFDs on the market that offer this (N-1 operation) as a standard option. In normal operation, BOTH inverters are connected in parallel and are sharing the load between them, fed from a common DC bus and synchronized internally via fiber optic comms. When one inverter faults, the other one simply takes on the entire load and since it was already connected to it, there is no transfer function involved. Optionally, you can have two inverters sized for the entire load, or two inverters sized for a portion of it, then when one faults, the speed is reduced to whatever the capacity of the single inverter is.

OK, I got the concept.
However, it seems like it is outside OPs goal of having a stand-by unit that will supplant the other in the event one gets out of commission.

If both are running all the time, would it not be possible for both to fail simultaneously?

What you cited is certainly a viable option, but OPs situation is different.

 

[Jraef]

OK, I got the concept.
However, it seems like it is outside OPs goal of having a stand-by unit that will supplant the other in the event one gets out of commission.

If both are running all the time, would it not be possible for both to fail simultaneously?

What you cited is certainly a viable option, but OPs situation is different.

Maybe, or maybe he is INTERPRETING is that way because he has never seen an N-1 VFD arrangement before so he is relating it to something he has seen, a VFD with a bypass. We won't know unless he checks in again.

 

[GoldDigger]

OK, I got the concept.
However, it seems like it is outside OPs goal of having a stand-by unit that will supplant the other in the event one gets out of commission.
...
If both are running all the time, would it not be possible for both to fail simultaneously?

1. If the goal is to prevent an outage, then either a redundant system always on or an automatically switched backup system will deliver. But the redundant system has at least two advantages: You know that the "spare" is usable! And you have less stress on the "running" system.
2. Yes, both could fail concurrently. But you need to ask whether you are worried about spontaneous failure or an event that causes one to fail. If you are worried about load shorts or line input transients that might cause a single VFD to fail, it might well cause both elements of a redundant system to fail at the same time. But if a short would take out the running system then as soon as the backup switched in it would be gone too.

 

[topgone]

1. If the goal is to prevent an outage, then either a redundant system always on or an automatically switched backup system will deliver. But the redundant system has at least two advantages: You know that the "spare" is usable! And you have less stress on the "running" system.
2. Yes, both could fail concurrently. But you need to ask whether you are worried about spontaneous failure or an event that causes one to fail. If you are worried about load shorts or line input transients that might cause a single VFD to fail, it might well cause both elements of a redundant system to fail at the same time. But if a short would take out the running system then as soon as the backup switched in it would be gone too.

Good points! But if one wants a fast replacement of a damaged VFD, a backup VFD, installed on site could help.

 

[elec_eng]

Thanks for all good comments. Actually, I didn’t know about N-1 hot backup drive arrangement. The intent was to prevent an outage and fast replacement. So what I am hearing is I would need two separate enclosures for each VFD disconnects for this system to work as it was intended.

 

[RumRunner]

1. If the goal is to prevent an outage, then either a redundant system always on or an automatically switched backup system will deliver. But the redundant system has at least two advantages: You know that the "spare" is usable! And you have less stress on the "running" system.
2. Yes, both could fail concurrently. But you need to ask whether you are worried about spontaneous failure or an event that causes one to fail. If you are worried about load shorts or line input transients that might cause a single VFD to fail, it might well cause both elements of a redundant system to fail at the same time. But if a short would take out the running system then as soon as the backup switched in it would be gone too.

Components that comprise a VFD, that have hundreds of tiny parts in a circuit board. . . it would be unproductive to worry about something that would fail .

The subsequent failure may not be related to the first one that occurred . Load shorts, heat and transients are ever present cause of failure in all electronic components that are in operation. So, if both are connected 24/7, the chances of them failing concurrently is much greater as opposed to having a spare that is just sitting idle and not connected.

Heat being the nemesis of any electronic part would cause degradation and eventual destruction.
Keeping them always energized increases the odds of failing.

This reasoning follows the “Theorem of Statistics and Probabilities”
Putting it in a different perspective: A person standing next to a body of water has a chance of drowning as opposed to someone standing on a high ground away from water.

Worrying about a component amid a whole bunch of electronic gizmos to fail-- ie ICs , transistor, resistor or crystal oscillator is like worrying whether your tire will blow out, connecting rods would shoot out through the cylinder head etc., would make life miserable.

And to paraphrase the pragmatic Mr. Murphy: All things will fail….....

But to pinpoint which one is going to fail at any given moment is like telling that the world will end on June 12, 2018.