%THD Violation Question 1

[ThePunisher]

Hi all,

I have 5 VFDs each with manufacturer test reported %THD content of 4.3% at 88% loading.

IEEE std 519 requires total harmonic distortion of 8% and I am getting 8.6% THD at 80% VFD loading and 11% at 100% loading at the 600V MCC bus where all these VFDs are connected and this is the only MCC bus I can connect them.

The upstream supply transformer primary is less than 3% THD.

Is it applicable to apply TOTAL 8% THD criteria to a bus where all my VFDs are connected?

Or can I justify if the resulting voltage and current harmonics total are within the voltage and current rating of the bus and main cables?

Regards

 

[Kiribanda]

IEEE 519 THD(V) & TDD(I) limits are applied to the Point of Common Coupling (PCC). Since your installation is an industry,
the PCC is the high side of your dedicated transformer feeding the industry. Therefore, you have to either measure or calculate by modeling
(1)THD (Voltage)
(2)TDD (Current)
when all VFDs are running at their full loading conditions.
I have a question to you.
You have mentioned THD (not TDD) and at the same time you have mentioned it depends on VFD loading.
Are you talking about THD or TDD since TDD depends on the loading?

 

[ThePunisher]

Hi Kiribanda, thank you for your prompt response.

I was referring to THD but will also consider TDD. ETAP have THD(I) which they have not changed by definition to TDD.

I agree with you on the reference PCC.

Based on my understanding, I am aligned with you on the premise that the TDD and THD recommended limits must be at the PCC.

So my question is that, if the primary of my main power transformer have TDD and THD results within the recommended limits, what would be the assessment method to determinine if the MCC bus (way downstream)where the VFDs are directly connected are ok or not (because it will be impractical to use the IEEE 519 recommendations)?

 

[Kiribanda]

The assessment method be either by,
1) Actual measurement using an Harmonic analyzer at the MCC
2) During operation of the plant you may encounter issues such as unexpected fuse blowing, conductor heating etc. etc.

 

[ThePunisher]

Is it correct to make the following assumptions:

1. An 8.5% THD (volts) would mean 1.85% fundamental nominal voltage?

2. A 8.5% TDD (or THD (current) would mean 1.6% total maximum fundamental demand current

Will the distribution equipment(say the MCC) pass rated maximum voltage and continuous current rating exceeds 1.8% plus 1.25% contingency?

 

[LionelHutz]

In the bigger picture, IEEE-519 is intended to put limits on distortion so you don't distort the utility and cause issues with your neighbors. No-one cares what happens in your facility except you. You could use total RMS current to determine if the install is OK.

 

[Gr8blu]

If you have a distribution bus location within the facility where the THD (or TD) is higher than the limits set in IEEE 519, then you might want to take a hard look at the connected equipment. Most are sensitive to excessive harmonic content - so even though IEEE 519 doesn't care what happens down at say the 480 V bus, you might still need to introduce some mitigation to have everything work correctly. Remember, 519 is intended to protect the UTILITY, not the facility.

Converting energy to motion for more than half a century

 

[ThePunisher]

LionelHutz, can I compare to the total RMS Current (fundamental + harmonics) against derated cable ampacity (from temp, grouping plus 125% contingency) and breaker AT?

Gr8blu,

Will the specification of equipment or devices specify what or how much harmonic voltage and current they can tolerate?

Thanks and appreciate your patience

 

[Gr8blu]

ThePunisher: In general, rotating machines should be a bit more forgiving of harmonic content as they see the distorted voltage and current as sources for additional thermal stress. Some specific machines might take it hard though - particularly where the rate-of-change of current is high (steep slope for a large harmonic, for example). Power electronics on the other hand are usually EXTREMELY sensitive to harmonic content on the supply side, since they tend to use zero crossings as timing references. With harmonics, there are a LOT of "extra" zero crossings possible which can trigger all sorts of unexpected results.

For example - several common Uninterruptible Power Supply (UPS) devices will tend to switch to battery when they lose a steady 50 or 60 Hz input waveform ... which ultimately leads to depletion of the battery backup faster than the typical trickle charge will restore it.

Most equipment should have the harmonic capability listed somewhere int he documentation - might be a bit of effort to find it. A call to the equipment manufacturer might be a faster way to get what you need in terms of compatibility. And if you're thinking about putting new hardware onto the system, be sure to include the harmonic content value so that you get something that will work correctly "out of the box".

Converting energy to motion for more than half a century