Power Loss Due to Harmonics

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Rho_HV

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How one should calculate the power loss due to harmonics? A typical 6 pulse VFD with diodes in rectifier module produces about %30 THD (Total Harmonics Distortion). An "Active Front End" VFD reduces the harmonics to %4 or lower. Is it safe to say that, if you are running a 1000HP motor, using an "Active Front End" VFD saves you about %24 of 1000HP in electric bills?
I am just looking or a ballpark estimate.
 
That's an interesting question.

Where are you trying to estimate the savings, I'm guessing the utility meter. Are there any transformers between the VFD and the utility meter? Sometimes transformers losses can be 3-4% higher with significant load harmonics (most transformer no-load losses are not significantly effected by harmonics). I'd have to put some further thought into kWH savings if there are no transformers in between the meter and the VFD.
 
ron said:
That's an interesting question.

Where are you trying to estimate the savings, I'm guessing the utility meter. Are there any transformers between the VFD and the utility meter? Sometimes transformers losses can be 3-4% higher with significant load harmonics (most transformer no-load losses are not significantly effected by harmonics). I'd have to put some further thought into kWH savings if there are no transformers in between the meter and the VFD.
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there is a transformer between meter and VFD feeding the motor. Transformer is 12.4k:600V.
in addition to transformer, there is a line-reactor upstream to VFD (between VFD and transformer).
I kind of see how my original thought (%30 power loss due to %30 THD) is nonsensical. I think I need to find the voltage drop due to higher frequency currents and find the power loss.
 
If I remember correctly from my college days, harmonics don't actually cause a power loss except for the additional voltage drop along the wiring.

The reason is that the current caused by the harmonic is twice the frequency as the voltage source. So half of the harmonic power is positive, and half is negative. It flows to the load on the first half of the cycle, and back to the source on the second half. (I believe the reasoning was similar for higher harmonics).

So the losses cancel out, and that's why your first guess was way too high. Your second guess looks much better. I think you need to find the voltage drop due to the harmonics along the wiring, and in the transformer windings. THis voltage drop will have the same frequency as the harmonic. Then you can multiply voltage x current, and find the power lost.
 
LMAO said:
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I kind of see how my original thought (0 power loss due to 0 THD) is nonsensical.
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After reading OP I was thinking [input] THD is not a direct indication of power loss. Some of the THD current is likely output and transmitted through the motor.
 
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LMAO said:
How one should calculate the power loss due to harmonics? A typical 6 pulse VFD with diodes in rectifier module produces about %30 THD (Total Harmonics Distortion). An "Active Front End" VFD reduces the harmonics to %4 or lower. Is it safe to say that, if you are running a 1000HP motor, using an "Active Front End" VFD saves you about %24 of 1000HP in electric bills?
I am just looking or a ballpark estimate.
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The 30% THD is the current waveform. The lowest order harmonic for this application is the 5th then 7th (6n?1 series).
Generally the magnitude of the harmonic varies roughly as the reciprocal of the harmonic number.
The non-linear current will increase the total RMS in the supply conductors a little above what they would be on a pure sine wave but in any case you would rate them for the VSD RMS input current.

To answer your original question no, the active front end won't save anything like 24% and it may actually consume more. The AFE will have more losses than a six-pulse plain rectifier input and my initial reaction is that this would likely exceed any losses saved in the supply.

One other thing to think about on losses. If there was 24% of the energy getting lost, something would be getting almighty hot!
 
Besoeker said:
The 30% THD is the current waveform. The lowest order harmonic for this application is the 5th then 7th (6n?1 series).
Generally the magnitude of the harmonic varies roughly as the reciprocal of the harmonic number.
The non-linear current will increase the total RMS in the supply conductors a little above what they would be on a pure sine wave but in any case you would rate them for the VSD RMS input current.

To answer your original question no, the active front end won't save anything like 24% and it may actually consume more. The AFE will have more losses than a six-pulse plain rectifier input and my initial reaction is that this would likely exceed any losses saved in the supply.

One other thing to think about on losses. If there was 24% of the energy getting lost, something would be getting almighty hot!
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read my next comments; I immediately found out how dumb my first assumption was! imagine %30 of 1000HP being wasted as heat; that's about 225kW heat!
 
Here's another way of thinking about it. In an Active Front End, harmonics are CREATED that cancel out the other harmonics, but they are not created from thin air. There is no net gain in energy efficiency by using an AFE for the purpose of harmonic mitigation. There is a SLIGHT improvement in overall throughput efficiency by virtue of lower losses in the distribution system feeding the VFD, but it is not in and of itself a reason to use it.

Some mfrs taut energy savings from using AFE drives, but that is for when there is a lot of regenerative energy available in the system where the AFE can be used to pump that regen energy back to the supply, which does have the effect of lowering your energy bills. Then in another sentence on the brochure the AFE suppliers talk about how an AFE drive can mitigate harmonics. What they DON'T tell you up front is that those two feature are essentially mutually exclusive, they go ahead and allow you to infer that you get both. You are EITHER going to recapture some energy when slowing down a load, OR you are going to mitigate harmonics, but not at the same time.
 
LMAO said:
read my next comments; I immediately found out how dumb my first assumption was! imagine %30 of 1000HP being wasted as heat; that's about 225kW heat!
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I wasn't casting aspersions on you intellect. If it came across that way, I apologise.
I was just commenting that sometimes a simple approach will stop you going down the wrong path.
The Occam's Razor idea.
 
Jraef said:
Here's another way of thinking about it. In an Active Front End, harmonics are CREATED that cancel out the other harmonics, but they are not created from thin air. There is no net gain in energy efficiency by using an AFE for the purpose of harmonic mitigation. There is a SLIGHT improvement in overall throughput efficiency by virtue of lower losses in the distribution system feeding the VFD, but it is not in and of itself a reason to use it.
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I'm inclined to think not. See post #8. The AFE is pretty much the same in power circuit terms as the output inverter. Both use PWM switching to take/produce sine wave current. The output switched voltage waveform results in roughly sine wave current as a result motor and, if fitted, inverter output inductors. The input section needs LCL filters between it and the supply. So lots more components. And lots more losses. About double that of a six-pulse unit. Maybe 5-6% compared to 2.5-3%.
But I take your point. You wouldn't use an Active Front End drive as a means of improving the system efficiency. Hugely more expensive and debatable if you'd get an efficiency improvement anyway.

Jraef said:
Then in another sentence on the brochure the AFE suppliers talk about how an AFE drive can mitigate harmonics.
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If they say that, it isn't quite true. The don't produce any harmonics to be mitigated.
And yes, you can have regen and low harmonics.
 
Besoeker said:
I wasn't casting aspersions on you intellect. If it came across that way, I apologise.
I was just commenting that sometimes a simple approach will stop you going down the wrong path.
The Occam's Razor idea.
Click to expand...

oh no, I never said you did such a thing. no need for apology at all and I appreciate your push in the right direction.
 
Besoeker said:
I'm inclined to think not. See post #8. The AFE is pretty much the same in power circuit terms as the output inverter. Both use PWM switching to take/produce sine wave current. The output switched voltage waveform results in roughly sine wave current as a result motor and, if fitted, inverter output inductors. The input section needs LCL filters between it and the supply. So lots more components. And lots more losses. About double that of a six-pulse unit. Maybe 5-6% compared to 2.5-3%.
But I take your point. You wouldn't use an Active Front End drive as a means of improving the system efficiency. Hugely more expensive and debatable if you'd get an efficiency improvement anyway.


If they say that, it isn't quite true. The don't produce any harmonics to be mitigated.
And yes, you can have regen and low harmonics.
Click to expand...

I ran a simulation with regular VFD compared to AFE, measured the input power for the same load and saw almost no difference. I too think AFE does nothing to improve power efficiency. Its only purpose is to reduce harmonics which can be harmful to other instruments on the same power line.
 
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