System harmonics

[defreitas]

Hi everyone...

While trying to identify the level of total harmonic distortion in a HV system (11kV), my client wants to know if it is possible to find out if the harmonics are being generated on site or if they are part of the input voltage coming into the site.

The trick is that I can't fully isolate any part of site distribution system at any time.

The system set up is as follows:

2x 66kV to 11kV Dyn11 transformers supplying the site where (66kV side of transformers share a common supply - parallel) but the secondaries are isolated (bustie open).

The HV SWBD has 2 incomers and 4 ring feeders with a bustie in the middle, which is open at all times, making the system "look like" 2 systems containing 1 incomer and 2 ring feeders each.

During a previous power analysis I have found out that Incomer 2 does not have significant THD (anywhere between 1 to 2.5%), but Incomer 1 has anywhere between 17 to 47%...

Showing that the harmonics are not generated within the site, but now I have to find where the THD in being generated.

After looking at the previous power analysis, I have noticed the phase angle and power of all harmonic components have been recorded and I am wondering if that information gives me any clue on how I can find where the problem is.

Maybe by switching the open point of the ring and finding the angle of the harmonic components change or maybe some other way that I am not thinking about.

Any ideas will be appreciated...

If it makes it easier I can put the results of the power analysis on here so you guys can see it.

Thank you.

Dan de Freitas
Field Services

 

[bgustave]

Are there any power-factor correction banks on incomer 1?

If so it's possible that they are establishing a resonance condition at or near one of the harmonics (3rd, 5th, 7th, etc) with the system impedance and acting as a low-impedance sink for harmonic current (i.e. unnecessarily drawing in distortion current from the system).

If there are automatic pfc banks (where the reactive support occurs in stages which are switched on and off to get the required power factor) then it is easy to determine if they are the culprit. By changing the amount of reactive power supplied (ie (de)activating more steps) the resonance will shift. If the harmonic current levels change when a pfc step is activated then the pfc bank is likely the culprit.

I know there's also a small equation to quickly estimate the harmonic order of the resonance based on the short-circuit MVA and pfc reactive supply level but I don't remember it right now.

 

[defreitas]

Thank you bgustave, for the response, the system does have PFC banks (PCF 1 at Incomer 1 and PFC 2 at Incomer 2), but they have been switched off for months, as they were affecting the ripple frequency of the local supply authority (hot water and Air Con switching frequency).

So I guess they are out of the picture.

Thank you once again.

Dan de Freitas
Field Services

 

[Pafi]

I think will be better to detail a little:
- where did you make the measurements (suppose 11kV, on each transformer)
- which THD did you measure - voltage or current, I suppose current
- if there are not capacitor banks installed on either of the 2 suppplies, then I would say that the current harmonics on the second supply are generated on site (in fact you can have the information by simply investigate which type of loads you have on this second supply - power electronic, arc furnaces, etc. loads produce harmonics)
- do you have access to measure the 66kV supply voltage? If yes, then you shall have the answer to your question because if there are not capacitor banks in service the 47% THD has no other way to exist but to be generated on site

 

[defreitas]

Thanks Pafi...

OK...

The measurement of the THD was done at the incomers 1 & 2 on site (on VTs and CTs circuits)...

My understanding is that the "valuable information is the Voltage component of the harmonics, but I have both readings...

As I mentioned both incomers on site have PFC banks, but they have been off the as long as I can remember...

But now comes the tricky part of the whole job: The site I am trying to investigate / prove the "generation" of harmonics is somewhat classified (military base), which means that I have no chance in trying to find out what sort of load the field transformers are supplying. Which is why I want to find if I can tell where the harmonics are "generated" based on the phase angle of all the components (2nd, 3rd... up to 21st)...

The power analyser I have put on site shows me what the PF is for every components, but I am not sure if I can use that info to show me anything concrete about it...

About the 66kV supply, it belongs to the local supply authority, so again, no chance of installing power analysers there either... But I assume that there is not a significant level of THD on that line, otherwise my Incomer 2 would be showing whatever level doesn't get "filtered" by the delta winding of the transformer and as per my first post those levels are only between 1 and 2.5%...

I do realise that I am sort of trying to do some magic, but once trying to convince my client that we show fault find by switching distributions transformers ON and OFF, I got a big "NO" as answer...

Thank you very much for your time and input, guys...

IT'S REALLY APPRECIATED...

Dan de Freitas
Field Services

 

[locals]

Copied from Dranetz site:

Harmonic Power Flow Direction
Rich Bingham
--------------------------------------------------------------------------------

Most people in the power engineering field have encountered the question about which direction the harmonic power is flowing, from the source to the load, or, from the load to the source. While this is still a controversial topic for some people, the most commonly accepted practice for determining this is to look at the harmonic watts phase angle, or the relationship between the voltage and current for a particular harmonic. The same rules that would be applied to a pure sine wave of voltage and current (which has only a fundamental frequency component) would be applied here.
The phase relationship between voltage and current with a pure resistive load is zero degrees, or a power factor of 1. If the load is a pure inductor, then the current lags the voltage by 90 degrees, which is normally displayed as a +90 degrees. If the load is pure capacitor, the current leads the voltage by 90 degrees, so the phase angle is called -90 degrees. This is so that the power factor of an inductive and resistive load would be a positive number between 0 and 1, while a capacitive and resistive load would be a negative number.

If the phase angle between the voltage and current is more than 90 degrees apart, this usually means that the current probe used with a power/harmonic meter or analyzer is placed in opposite direction of the assumed power flow. Most current probes have an arrow that should be pointing in the direction from the source to the load, which is the normal direction of power flow. When the phase angle of the harmonic voltage and current is between 90 degrees and 270 degrees (270 is also referred to as -90 degrees) on a properly installed CT, then it is assumed that this harmonic power flow is in the opposite direction of the fundamental power flow, or from load to source.

In some Dranetz products, this is indicated by the words SOURCE or LOAD next to the printout of each harmonic watt. In other products, you have to look at the phase angle of the harmonic watts to determine where it falls. The user must be cautioned that in a number of measurements, the harmonic current and voltage levels are so low that the harmonic watts number is so small that it may be meaningless, as would be the direction of power flow information. For example, if on a 120V/30A circuit, there is a 5th harmonic voltage of 0.05 V and harmonic current of 0.2 A, 0.01W is really insignificant and the accuracy of the direction is very low.

 

[defreitas]

Thank you so much, locals...

I think that has just sroted me out...

Now I just need to convince my client that I want the power analysers back on and I should be able to study this data and get to a conclusion.

Thank you once again, bgustave, pafi and locals...

Dan de Freitas

Lifelong learning is the way to the future...