VFD Line Side Reactors

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nhee2

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I reviewed a set of vendor control panel drawings - panel has single power feed, tapped from the input distribution block to four individual VFDs within a Hoffman enclosure.

The design included line reactors installed on the line side of each VFD's overcurrent protective device - so there was no isolation or OCPD between the input distribution block (protected at 400A) and the individual reactors.

Is this a normal configuration? I'd assume the line reactor would typically be between the OCPD and drive. But I don't see many multi-VFD drive cabinets, maybe there is a reason they laid it out this way.
 
So like this?
multi-drive.jpg

For that to be valid, the conductors from the splitter block (TB) to the reactors and the reactors to the drives must be rated fro whatever the feeder is upstream of this. In addition, the reactors would have to be approved with that size OCPD as well. It MIGHT be legit, in that if for example these are all 1HP 480V drives, the minimum wire size will be 14ga even though each individual drive is less than 2 amps, so a 15A feeder ahead of the whole shebang would be totally OK.

If the panel can be considered "industrial machinery", not having a main disconnect would be a violation of NFPA 79, but that may not be enforceable.
 
Jraef said:
So like this?
View attachment 22354

For that to be valid, the conductors from the splitter block (TB) to the reactors and the reactors to the drives must be rated fro whatever the feeder is upstream of this.
Click to expand...
Not how I would arrange it. The CBs you have shown would be on the upstream side of the reactors and the reactors sized for the VSD they are feeding.
 
Besoeker3 said:
Not how I would arrange it. The CBs you have shown would be on the upstream side of the reactors and the reactors sized for the VSD they are feeding.
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Right, that would be the best way to do it. I was depicting how I was interpreting he said it is connected now, which is not optimal but MIGHT be technically allowable depending on the details, not enough of which were provided, i.e. sizes.

An alternative that I would have used, if those details made it within the realm of possibility, would be one large line reactor sized for the total VFD load (input amps). One large reactor would have the same benefits as 4 smaller ones in this instance.
Example:
Line-Reactor-Figure-2-1.jpg

But in this case it is already built and delivered, so whether or not it is acceptable is going to depend on those details.

PS: In re-reading the OP, I realize that if the only upstream OCPD is 400A, the likelihood of it being acceptable is very slim, because the conductors going into and out of each Line reactor would need to be rated for 400A, and the complex interactions of 430.131, 430.53 and 240.4 would ALL need to be satisfied.
 
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Jraef said:
Right, that would be the best way to do it. I was depicting how I was interpreting he said it is connected now,

.
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Yes. I should have paid better attention to what he posted.
My apologies.
 
Thanks for the replies
- The panel configuration is as shown in jraef's post.
- The reactor sizes are not suitable for 400A upstream protection.

My comments on the drawings were to move the reactors to the load side of the CBs. Just wasn't sure if there was some technical reason that they would have shown it the way they did. I did not consider a single reactor.

The panel is not delivered, these were customer drawings submitted to the client.

Regarding the (lack of) disconnect, their design includes a disconnect switch adjacent to the panel we are discussing.
 
nhee2 said:
Thanks for the replies
- The panel configuration is as shown in jraef's post.
- The reactor sizes are not suitable for 400A upstream protection.

My comments on the drawings were to move the reactors to the load side of the CBs. Just wasn't sure if there was some technical reason that they would have shown it the way they did. I did not consider a single reactor.

The panel is not delivered, these were customer drawings submitted to the client.

Regarding the (lack of) disconnect, their design includes a disconnect switch adjacent to the panel we are discussing.
Click to expand...


Hi
Reactors are meant to provide protection for both the VFD and the power supply that provide energy to the VFD and the motor. When I say protection for the power input—it means the primary purpose to limit harmonics at the supply side.


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Line side reactors are the common method of harmonics correction due to its low cost and reliability.

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Utilities require at least 5% impedance for a corrective measure for managing allowable harmonics.

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Reactors could be installed on the load side as opposed to line side
.

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This approach [load side] is preferred if the lines going to the motor is in excess of 100 ft. More harmonics management is needed as the distance is increased.​
As for why it should be connected without the OCPD protection. . . it is mandated by some utilities when installation is intended for large motors which is your case. . . a 400 amp draw is not a small motor.



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Their rationale is: this reactor should always be online as it will affect other nearby loads that are connected to a common TB in the event of a disconnection.


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Being disconnected, either intentionally or tripping has devastating effects on the power supply lines.. . . a sort of Scylla and Charybdis situation (caught between a rock and a hard place.) where the utility has to choose between two equally difficult issues.



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You could choose to install an OCPD or just a disconnect but you have to have POCO know about your reason why you should.

​
 
myspark said:
Hi
Reactors are meant to provide protection for both the VFD and the power supply that provide energy to the VFD and the motor. When I say protection for the power input—it means the primary purpose to limit harmonics at the supply side.​

​
Click to expand...

A common misconception.
 
myspark said:
Care to elaborate?
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I'll try.

Most of the 3-phase input VFDs have an uncontrolled 6-pulse rectifier as the input converter. The input current has 6n+/-1 harmonics so 5th, 7th, 11th 13th etc.
The magnitude of these harmonic currents is, at best, the reciprocal of their number so, with very a good DC link smoothing choke you'd get 20% 5th harmonic. Not many VFDs have that and more likely have 40% 5th harmonic. Line reactors won't come close to fixing that.
 
Besoeker3 said:
I'll try.

Most of the 3-phase input VFDs have an uncontrolled 6-pulse rectifier as the input converter. The input current has 6n+/-1 harmonics so 5th, 7th, 11th 13th etc.
The magnitude of these harmonic currents is, at best, the reciprocal of their number so, with very a good DC link smoothing choke you'd get 20% 5th harmonic. Not many VFDs have that and more likely have 40% 5th harmonic. Line reactors won't come close to fixing that.
Click to expand...
Unbuffered harmonics can cause high dv/dt and high peak voltages. Harnessing impedance as a function of REACTORS to correct mismatches-- can cause pulses to be reflected back to the source.


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As wire length in some installs increase (along with the unbuffered harmonics) the overshoot voltage (surge) also increases.​
This cause motor insulation degradation and eventual failure.
​
 
myspark said:
Unbuffered harmonics can cause high dv/dt and high peak voltages. Harnessing impedance as a function of REACTORS to correct mismatches-- can cause pulses to be reflected back to the source.


​
As wire length in some installs increase (along with the unbuffered harmonics) the overshoot voltage (surge) also increases.​
This cause motor insulation degradation and eventual failure.
​
Click to expand...


That's on the load side.
I've been called in to investigate that. Some submersibles had up to 3000V/us and had frequent failures. Reactors were the answer there.
 
Sahib said:
So what is your solution to the harmonics problem caused on the line side by VFD's?
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Depends on whether there actually needs to be a solution, the size of the drive, the size of the system, whether there are other loads (e.g. PFC) that could be affected whether the THD would exceed applicable limits......

If the THD was going to be an issue we would design and fit harmonic filters, usually 5th and 7th. On larger drives, we went 12 or 24-pulse at the design stage.

For PFC, we fitted de-tuning reactors to resonate at 177 Hz. Other designers went for 225Hz but, for me, that was too close to 5th Harmonic.
 
Sahib said:
Why do you think the line reactor in OP's case cannot serve as such ie a harmonic filter?
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They are not part of a circuit tuned to a harmonic frequency nor can they be large enough to attenuate the harmonics.
 
Sahib said:
Perhaps you were misled.
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Every time I did the measurements on every single drive system?
Numbers where we would have had swingeing financial penalties had we failed to meet the figures we committed to?
 
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