Determining approximate THD

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ChargedUp

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For a utility connected service entrance switchgear with multiple inverters interconnected on load side of service entrance MCB for solar/battery back-feed, how could you estimate the total THD contribution back into the utility grid if you know the individual THD of each inverter?
 
ChargedUp said:
For a utility connected service entrance switchgear with multiple inverters interconnected on load side of service entrance MCB for solar/battery back-feed, how could you estimate the total THD contribution back into the utility grid if you know the individual THD of each inverter?
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I believe it is the arithmetic for third and upwards.
 
Believe that's correct as well... I should have clarified. I don't actual have an actual THD report, but rather a percentage from each of the inverter manufacturer cutsheets. So if there are multiple inverters (with THDs all below 5%), how could you approximate the total THD as a percentage?
 
ChargedUp said:
Just following up from my last post... can anyone shed additional insight to my question and follow up clarification?
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Third, 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th, and not a lot after that. We have used 24-pulse drives in paper mills. At that level we didn't use/need any harmonic correction.
Just my tuppence's worth.
 
If talking about VOLTAGE distortion, it’s not cumulative. So if all contributors are 5% or less, the total will not be above 5%. That’s why you see specs calling for that.
 
Jraef said:
If talking about VOLTAGE distortion, it’s not cumulative. So if all contributors are 5% or less, the total will not be above 5%. That’s why you see specs calling for that.
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I assumed that it was harmonic distortion (THD) and they would be additive.
 
The voltage THD will be a function of the utility source impedance, best measured by the available short circuit current. The higher the available SCC, the lower the voltage THD. A good resource on the topic is IEEE 519.
 
Its the utility that is asking for the THD contribution back into the grid from the customer side. We can't run a sweep on the system yet because its not installed. We also don't have available fault current from utility yet, but we can ask.

Even so, just wanted to know if its possible to estimate the THD from customer contribution at this time, and it seems Jraef's post makes it clear that yes you can estimate this, which would roughly be around the largest THD% from a given inverter. So if you have (3) inverters back-feeding a utility fed switchgear, with THDs of 3%, 4% and 5%, the estimated THD contribution from all (3) inverters into the utility grid would be around 5%, plus or minus.
 
Well... sort of. The problem is, buying a VFD that is only going to give 5% V-THD is all but impossible, unless you are willing to buy Active Front End VFDs for everything, and that would be really expensive.

CURRENT distortion is what causes voltage distortion. Voltage distortion is what the utility is concerned with, because it migrates back through their lines and transformers to their other customers, and IEEE519 applies to THEM too in that they cannot DELIVER more than 5% V-THD to their customers, yet they are not the ones creating it; other connected customers are.

So technically, if you cannot measure in advance, there are harmonic "estimators" out there that can do a decent job of looking at ALL of the contributing factors and giving you an idea of whether or not you will meet IEEE-519. Most of the people that sell harmonic filters have on-line calculators, some of the larger VFD mfrs have them available as well. But for that to work, you need a lot of details, such as size and impedance of any transformers, size and length of conductors, all of the loads, linear and non-linear and most importantly, the Available Fault Current at the utility line connection. If you calculate everything based on "infinite bus", you get some bad numbers that you may not like.

The quick and dirty thing to do: add up all of your non-linear loads (VFDs, UPS, Servos, large power supplies, HID lighting etc.), assume 35% I-THD, and buy an Active Harmonic Filter to install at the PCC (Point of Common Coupling) big enough to cover that I-THD current value. So for example if you have 1000A of non-linear load, that means 350A of harmonic current, so buy an AHF capable of at least 350A corrective current. It may end up as a bit of overkill, but it avoids the nasty surprises later.
 
Jraef said:
Well... sort of. The problem is, buying a VFD that is only going to give 5% V-THD is all but impossible, unless you are willing to buy Active Front End VFDs for everything, and that would be really expensive.

CURRENT distortion is what causes voltage distortion. Voltage distortion is what the utility is concerned with, because it migrates back through their lines and transformers to their other customers, and IEEE519 applies to THEM too in that they cannot DELIVER more than 5% V-THD to their customers, yet they are not the ones creating it; other connected customers are.

So technically, if you cannot measure in advance, there are harmonic "estimators" out there that can do a decent job of looking at ALL of the contributing factors and giving you an idea of whether or not you will meet IEEE-519. Most of the people that sell harmonic filters have on-line calculators, some of the larger VFD mfrs have them available as well. But for that to work, you need a lot of details, such as size and impedance of any transformers, size and length of conductors, all of the loads, linear and non-linear and most importantly, the Available Fault Current at the utility line connection. If you calculate everything based on "infinite bus", you get some bad numbers that you may not like.

The quick and dirty thing to do: add up all of your non-linear loads (VFDs, UPS, Servos, large power supplies, HID lighting etc.), assume 35% I-THD, and buy an Active Harmonic Filter to install at the PCC (Point of Common Coupling) big enough to cover that I-THD current value. So for example if you have 1000A of non-linear load, that means 350A of harmonic current, so buy an AHF capable of at least 350A corrective current. It may end up as a bit of overkill, but it avoids the nasty surprises later.
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Agree. Those surprises will be eliminated "if" you took an actual harmonics sweep of your systems. Imaginary power systems will have to rely on published claims of your load equipment manufacturers.
 
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