7.5% THD

[chris kennedy]

I was reviewing some data I collected for an EE at a manufacturing facility last week.This was collected from a 150kva POCO tranny 208y/120.
Along with the usual V,A,pf and hz I captured THD peaking at 7.5%.
Is that bad?

Thanks

 

[mayanees]

IEEE 519

IEEE 519

It's all relative Chris.
IEEE 519 gives V and I limits for the PCC based on the strength of the source.
Strong sources are more tolerant.
So you need to get the Utility contribution before passing judgment.

 

[Besoeker]

I was reviewing some data I collected for an EE at a manufacturing facility last week.This was collected from a 150kva POCO tranny 208y/120.
Along with the usual V,A,pf and hz I captured THD peaking at 7.5%.
Is that bad?

Thanks

Is your measured THD voltage or current?

 

[chris kennedy]

Is your measured THD voltage or current?

Good question. Have to revisit that tomorrow. I just did a quick review to make sure I had the relevant data before I sent it.

 

[Russs57]

Ideally, I’d like to be no more than 5% on both current and voltage. But consider the best ballast is close to 10% and computers are truly horrendous. So it depends on load diversity. VFD’s can be a source that can be easily mitigated with DC bus reactors as well as AC side line reactors.

Ideally you would want to move downstream and identify individual sources and attack them (IMHO).

 

[Sahib]

Is your measured THD voltage or current?

If it is voltage THD, OP need not worry about any penalty: it is utility caused.

 

[Besoeker]

If it is voltage THD, OP need not worry about any penalty: it is utility caused.

That would not accord with my experience.

 

[Sahib]

That would not accord with my experience.

Please elaborate.

 

[Sahib]

Please elaborate.

In my own case, we had POCO penalty for current THD in a telephone Exchange. I proved it was caused by voltage THD and convinced POCO to drop penal charge for current THD.

 

[Jraef]

If it is voltage THD, OP need not worry about any penalty: it is utility caused.

That statement lacks a lot of context and should not be taken as gospel. It can ONLY be true if you measure THD at the PCC (Point of Common Coupling) with ALL loads in your facility disconnected, in other words with your main service disconnect open.

Chris,
Current distortion (THD-I) in YOUR facility is what causes voltage distortion (THD-V) on the supply, and THD-V is what can "travel" to your neighbors via the utility lines. But THD-V that is already high at your PCC when it arrives from the utility will exacerbate any THD-I that your facility creates. That's why IEEE-519 has requirements for BOTH you and the utility to limit it. The utility must ensure that the THD-V at your PCC is no more than 5% with your service disconnected, YOU must ensure that your THD-I is low enough to not CAUSE more than 5% at the PCC when the service is connected.

The 5% THD-V rule applies in any case, unless you are a "critical system" like a hospital or air traffic control site, in which case it's 3%, or 10% for a site that is exclusively converter loads, such as a utility HV transmission converter station, radio transmitting tower or a radar site (because they are typically remote enough to mitigate the THD-V by the time it gets to someone else).

The thing that DOES vary by the size of your service (i.e. available short circuit current) vs the load you connect to it, referred to as the TDD (Total DEMAND Distortion), is the allowable amount of THD-I. That's why you were asked that question. 7-1/2% THD-I in a facility where the ratio of I_load to I_SC is high may be perfectly acceptable, because the system is so "stiff" that it has little effect on the THD-V at the PCC. but 7-1/2% THD-V at your PCC is in violation of IEEE-519. Then again, the ONLY place it matters is at the PCC, so if you are measuring that value at a panelboard somewhere way down stream of the service connection, it may be something that ends up mitigated by the time it gets to the PCC and not worth getting excited about.

So what all this means is, the devil is in the details here.

 

[Sahib]

That statement lacks a lot of context and should not be taken as gospel. It can ONLY be true if you measure THD at the PCC (Point of Common Coupling) with ALL loads in your facility disconnected, in other words with your main service disconnect open.

Chris,
Current distortion (THD-I) in YOUR facility is what causes voltage distortion (THD-V) on the supply, and THD-V is what can "travel" to your neighbors via the utility lines. But THD-V that is already high at your PCC when it arrives from the utility will exacerbate any THD-I that your facility creates. That's why IEEE-519 has requirements for BOTH you and the utility to limit it. The utility must ensure that the THD-V at your PCC is no more than 5% with your service disconnected, YOU must ensure that your THD-I is low enough to not CAUSE more than 5% at the PCC when the service is connected.

The 5% THD-V rule applies in any case, unless you are a "critical system" like a hospital or air traffic control site, in which case it's 3%, or 10% for a site that is exclusively converter loads, such as a utility HV transmission converter station, radio transmitting tower or a radar site (because they are typically remote enough to mitigate the THD-V by the time it gets to someone else).

The thing that DOES vary by the size of your service (i.e. available short circuit current) vs the load you connect to it, referred to as the TDD (Total DEMAND Distortion), is the allowable amount of THD-I. That's why you were asked that question. 7-1/2% THD-I in a facility where the ratio of I_load to I_SC is high may be perfectly acceptable, because the system is so "stiff" that it has little effect on the THD-V at the PCC. but 7-1/2% THD-V at your PCC is in violation of IEEE-519. Then again, the ONLY place it matters is at the PCC, so if you are measuring that value at a panelboard somewhere way down stream of the service connection, it may be something that ends up mitigated by the time it gets to the PCC and not worth getting excited about.

So what all this means is, the devil is in the details here.

If OP load increases voltage THD beyond 5%, similar load of other people connected to same source would also do the same and thus it is responsibility of POCO to make the supply source stiff so that it does not happen.

 

[Besoeker]

Please elaborate.

It was always a consideration for us at the outset of any project that included non-linear loads.
In short, it's what I did.

 

[Besoeker]

If OP load increases voltage THD beyond 5%, similar load of other people connected to same source would also do the same and thus it is responsibility of POCO to make the supply source stiff so that it does not happen.

It is the responsibility of the users that they don't design and conect equipment that does not comply.

 

[Sahib]

It is the responsibility of the users that they don't design and conect equipment that does not comply.

In respect of current distortion and not voltage distortion.

 

[Besoeker]

In respect of current distortion and not voltage distortion.

Have you been directly involved in these matters?
Have you ever had to agree to meeting design performance figures, agree to system testing that measures performance and face financial penalties if you get it wrong?

 

[Sahib]

Have you been directly involved in these matters?
Have you ever had to agree to meeting design performance figures, agree to system testing that measures performance and face financial penalties if you get it wrong?

Yes, sort of. See post#9.

 

[Besoeker]

Yes, sort of. See post#9.

Who caused the voltage distortion?

 

[Sahib]

Who caused the voltage distortion?

The POCO, of course.

 

[Besoeker]

The POCO, of course.

Oh, how silly of me not to realise that they might actually generate non-sinusoidal voltages....:roll:

 

[Sahib]

Oh, how silly of me not to realise that they might actually generate non-sinusoidal voltages....:roll:

Not a matter of non-sinusoidal voltage generation by POCO. It is a matter of supplying from relatively weak power source.:slaphead: