IEEE 519 - 1992

[racobb]

In reviewing the standard I notice that the Isc used for the calcs appears to be infinite source. For those of you that have done the TDD calcs, do you use the infinite source or use the actual when it is available?

Just wondering what the intent of the standard is in this regard.

Thanks

Alan

 

[magoo2]

I thought the available sc was based on some of the higher breaker interrupting ratings rather than an infinite source. I hate to say - but I could be wrong!

 

[mayanees]

I just jumped on here to research a topic, and was drawn to this post due to my past familiarity with IEEE 519-1992.
I don't recognize TDD, and expect you mean THD or Total Harmonic Distortion, and I know that calc requires the use of the actual Utility source contribution. A system can tolerate much more distortion if it's a strong source.
John M

 

[rbulsara]

What mayanees said.

Once you have a intervening transformer of known impedance, the variation in THD calcs due to utility side SCC are minimized, but can still be a factor, if the numbers are border line.

Assuming an infinite bus when it is not would give lower THD, than actual and could be misleading.

Rafiq Bulsara

http://www.srengineersct.com

 

[racobb]

Thanks for the responses.....TDD is total demand distortion, which is a measure of actual current to Isc (actual distortion to source size).

I agree that using infinite source when it is not would make the source seem stiffer than it actually is, hence my question.

The standard is quite misleading, at least to me, in that what they call "actual" source Z is calculated as infinite bus.

Alan

 

[MAGTiger]

Not to pick nits, but 519 is not a standard. I it s recommended practice. The source transformer impedance determines the ISC and thus the limits on TDD based on table 10.3. The idea is that with lower Isc (essentially a higher impedance transformer) the Ithd will cause more Vthd and thus more problems so the limits for low Isc systems are more limiting than high Isc systems. Isc/Il <20 tdd= max 5%, Isc/Il>100 give a tdd =15%max. It's a round about way of determining how big the harmonic load is compared to the total load. Typically end users specify a max of 5% thd not realizing the limit is on tdd and that they may be able to get by with 12-15%tdd and much lower cost. But it's simply easier to specify the most limiting number and not worry about the associated costs.
And yest it is a misleading/confusing document, frequently misused at the expense of end users.

Neil

 

[racobb]

Thanks MAGTiger, my question is what are people that are applying the "document" are using for Isc. The document seems to talk about actual, but then calculates infinite source.

I understand the concept and understand that Isc/Iload will be greatly affected by that choice. Just trying to get a feel for how others are applying it and if anyone knows the actual intent of the committee.

It seems to me that if actual is known it should be used, but then you could have engineers doing it both ways. As rafiq says, using infinite when we know it is not will give you higher margins for TDD.

Hence my question.

Thanks

Alan

 

[mayanees]

... my experience with this is that engineers (of which I'm guilty of being, but not guilty of what I'm about to write...) may specify that a system meet the criteria of 519-1992 with the MCC being the Point of Common Coupling (PCC). Which, even though 519 was not created as a design requirement, it becomes one. So any supplier who tries to comply with that verbiage prices an active drive that produces no harmonics - and subsequently doesn't get the job. Then the firm that's responsible for the Power Study and the subsequent measurements for verification of spec compliance (me) has to explain that using the MCC as the PCC was not the intent of the IEEE 519-1992, and that the system does meet the spec as intended.
Has anyone ever seen a Utility come back on a power user with numbers showing violation of IEEE 519-1992?? I might expect th at an obvious violator that's causing tripounts in the neighborhood might get flagged, but certainly not small to medium sized service customers.
John M

 

[racobb]

Thanks, yes there are those that figure if the harmonics are stopped at each piece of equipment, then there can be none on the system. Don't think that was ever the intent of 519.

My experience has been that in most cases the customer is causing their own problems with current distortion, but the supply voltage is still well within the THD recommendations of 519. No other customers on the service. We end up telling them that as long as they can live with it, so can we.

Alan

 

[jghrist]

I don't have a copy of IEEE 519 with me, but I have no recollection of using infinite bus to calculate Isc. Isc is not used in calculating TDD, the load harmonic and fundamental currents are used for this calculation. Isc comes into play in determining the limit of TDD, as Neil said.

This Isc is the short circuit current at the Point of Common Coupling (PCC) between the utility and the customer, the point accessible to both, usually the service or metering point.

It isn't proper to specify that a piece of equipment meet IEEE-519 based on the load harmonic and fundamental current in that piece of equipment. For one thing, the equipment is probably not the only source of harmonic distortion and the IEEE-519 limits are for the entire plant. You also have to know the Isc at the PCC to determine the limits.

 

[racobb]

I had actually read some narratives on 519 before I was able to get back to my copy. I don't see that either in 519, just some folks presenting it incorrectly and hence my question.....should have gone back to the source first!

Now that I have gotten a chance to review the document, it does not refenence infinite source as some, not here, have stated.

I agree that it is not proper to spec the equipment that way, but some do!

Thanks for all the input.

Alan

 

[rcw retired EE]

One client specifies each piece of equipment to meet 5% THD for both current and voltage distortion at the switchgear feeding the equipment. The big harmonic producing equipment (a 7 MW drive) runs for about an hour a day five days a week and has three 20 second periods when THD for current exceeds 5% and hits a peak of 8.2% for one second. Voltage THD on the bus is below 3% at all times.

With the high Isc at this bus the current TDD limit would be 12%, not 5%, if the 519 recommendations were applicable to this bus and equipment. As John M, said the client is creating his own problems.

I did calculations per ANSI standards to derate the oversized upstream equipment for harmonic heating effects and determined that the load of 54% of transformer rating is actually equivalent to 56% load for those 20 second periods. No other equipment has been affected.

Together with vendors, we spent over $500K adding notch filters and active equipment on the circuits to cure the non-problem. But, when the drive’s feeder breaker opened, the filter capacitors discharged through the drive transformer creating surges that eventually blew up the drive input electronics, just prior to warranty completion, twice. My solution of turning off the filters was not accepted, except as a stop gap measure.

More design changes are in progress to solve problems caused by solutions to a non-problem.

Moral: make sure 519 is properly applied or you will waste $$ and resources with no benefit to the client.

 

[dpc]

I've always used the actual system impedance. I don't have the standard at hand, but I'm pretty sure the intent is to use the actual system data.



David Castor

http://www.cvoes.com

 

[racobb]

That is what I expected and is what I intend to do now that I have gotten ny hands on the real thing and gotten some very good comments....Thanks Again

Alan