Harmonics in Grid - How to measure
Harmonics in Grid - How to measure
(OP)
Measurement of harmonics is different from measuring the fundamental frequency currents and voltages.
Does it require any specially ordered CTs and PTs!
Does it require any specially ordered CTs and PTs!






RE: Harmonics in Grid - How to measure
For CTs, they're generally pretty good up to 25th to 32nd harmonic.
For VTs, oil-filled VTs are normally decent up to about 1 kHz.
RE: Harmonics in Grid - How to measure
We are supposed to be measuring up to 50th harmonic for computing the THD in 132kV grid.
RE: Harmonics in Grid - How to measure
RE: Harmonics in Grid - How to measure
Neil
RE: Harmonics in Grid - How to measure
RE: Harmonics in Grid - How to measure
If the measurements are to be used for THD calculation, I do not think that there is any practical reason to go beyond 41st harmonic. The contriubtion from higher harmonics will most likely be neglible or 'absorbed' in the total error budget.
Also, There is at least one transformer between the VFDs and the PCC. That transformer usually has a worse frequency response than the VTs and CTs that you use on the 132 kV side. So, the small contributions from 41+ harmonics will already be reduced when they reach the PCC.
And, finally, filters for 12 pulse loads are seldom designed for components higher than 35/37, and that is also quite unusual. Harmonics 23/25 is, in my experience, the highest one would need to filter. But, we do not know all circumstances. Are there good reasons to take the measurements and mitigation methods any higher?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Harmonics in Grid - How to measure
I wonder what methods are adopted in Industry when you need to measure harmonics beyond 20th order.
Could be that CT / VT are first calibrated at higher frequencies and the measured errors are accounted for, while actual meaurements are done at site!!
RE: Harmonics in Grid - How to measure
Capacitive VTs are used when better frequency response is needed.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Harmonics in Grid - How to measure
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Harmonics in Grid - How to measure
If you're speaking of capacitor voltage transformers (i.e. CVTs, CCVTs) they are the worst option for harmonic measurements, as they are specifically tuned for accuracy at the fundamental frequency.
There are devices that can be added to CCVTs to facilitate the measurement of harmonics, but they are relatively rare and somewhat new on the market.
Resistive or capacitive dividers are the most common method, but they are somewhat unique for transmission level voltages and can be rather expensive.
Optical CTs and VTs due offer very good frequency response. They are normally expensive and often have reliability issues.
Best way, in my option, is to use a conventional CT that had been tested for frequency response and a CCVT fitted with an added-on auxiliary device for harmonic measurements.
RE: Harmonics in Grid - How to measure
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Harmonics in Grid - How to measure
Thanks for summarising. But, you mentioned that the CVTs with added-in auxiliary devices are "relatively rare and some what new". Thta is my understanding too. So, any reason (could be some experiences etc.) why you feel that is the best way to go.
How do you feel about using Electromagnetic VTs!
These are OK in our system as coupling capacitors are not required (Bus VTs) and when it comes to cost, EM VTs are not costlier than CVTs for 132kV voltage level.
RE: Harmonics in Grid - How to measure
As for the aux-device for CCVTs, they are available now, just haven't been much in the past, so it can be assumed a CCVT installed in the field is not suitable to PQ measurements.
This is a link to such a device. When AREVA purchased the high-voltage designs from Ritz, they chose not to keep this offering. I originally wrote this brochure when at Ritz Waynesboro, which was a few yrs ago, so I'm not sure the current status of the offering. I would suggest contacting http://www.pqs-consulting.com/ for questions on this technology. I know we did install some at US utilities in 2006/2007 and they worked well.
http://www.pqs-consulting.com/ritzbrochure.pdf
RE: Harmonics in Grid - How to measure
With PQ sensor, the accuracy of measurement for harmonics upto 20kHz is possible which is way beyond the codal requirement of 50th order harmonic.
Still, the question about current harmonic measurement remains. This will have to be done using the conventional electromagnetic CTs only and I guess the deterioration in accuracy at higher frequencies may not be as bad as that with EMVTs.
RE: Harmonics in Grid - How to measure
Is anyone manufacturing Rogowski coils for HV applications? In principle they are ideal in terms of bandwidth, I'm just not sure which manufacturers are in the market at transmission and primary distribution voltage. We've used them at generation voltages with great success.
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If we learn from our mistakes I'm getting a great education!
RE: Harmonics in Grid - How to measure
The frequency response for CTs varies quite a bit, but in general, they're much better than for VTs, mainly because the insulation capacitance inter-winding capacitances really don't come into play like in a VT.
When I was with AREVA HV IT's, we did some testing on the Extended-Range Current Transformer design and it was very good up to 32nd harmonic. Don't remember how it performed above that. You could contact AREVA in Waynesboro, GA USA for more info (706-554-8800 and ask for the engineering manager).
ScottyUK-
There is always a lot of talk about Rogowski coils in HV and MV, but very little, if any, action. I think they're used more in Europe than in the US. In principle, the design of an HV CT or MV CT is the same with a normal core/coil or a Rogowski coil. The problem really lies with the design of the RC, as it's much more difficult to calculate the performance, as compared to a traditional core/coil and almost has to be manufactured using a trial and error process.
In my opinion, for HV applications, if you can't get the harmonic performance in a conventional CT that you need, then optical CTs probably are a better solution over Rogowski coils or linear couplers.
RE: Harmonics in Grid - How to measure
RE: Harmonics in Grid - How to measure
I guess that's why we never seem to hear much about them in HV. In the 11 - 25kV range we usually have so much distance around things like generator main terminals that air insulated Rogowski coils work just fine but I can imagine, though probably not fully understand, the difficulties in installing one in a conventional HV CT design.
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If we learn from our mistakes I'm getting a great education!