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Measurement of Harmonics in 132kV system
2

Measurement of Harmonics in 132kV system

Measurement of Harmonics in 132kV system

(OP)
Capacitor Voltage Transformers are provided for voltage measurement in 132kV system. As we know these are tuned to fundamental frequency and cannot give dependable information if we are looking to measure the harmonic voltages.
One way is to bring portable electromagnetic voltage transformers and connect at the time of measurement.
Is there any other method to have dependable harmonic measurements!
Thanks.

RE: Measurement of Harmonics in 132kV system

2
VTs aren't perfect for measuring harmonics either. The frequency response varies dramatically from design to design and from voltage class to voltage class.

Up to what harmonic are you trying to measure? VTs at this voltage class will start to severely distort above approx. 600-1000 Hz.

Also, some CVTs manufacturers make a device that can be retrofitted on installed CVTs that will enable very good frequency response on a separate output.  

RE: Measurement of Harmonics in 132kV system

raghun,

You can make a pure capacitive divider by using the tap of your transformer capacittive bushing (if there exist).

Regards,

H. Bronzeado

RE: Measurement of Harmonics in 132kV system

(OP)
scottf,

Thanks.
You say Electromagnetic VTs also will have distortion at harmonic frequencies. This could happen for sub-harmonic frequencies (due to possible overfluxing) but will it be the case for higher order harmonics too??

Bronzeado,
Use of Trafo bushing tap in case of condenser bushings - It is a good point. Thanks. Appreciate if you could share any experiences / reports on this practice that you may have.

RE: Measurement of Harmonics in 132kV system

Raghun-

Remember the winding of a VT has a capacitance value (inter-turn and inter-layer) and the insulation of an oil-paper insulated VT also has a capacitance (in the area of 1000 pC or so). The frequency response of a VT is pretty good up to around 600 Hz.

See the following link of a brochure I wrote in a past life. Note the graph on page 2 of a Ritz 230 kV VT type OTEF245...that is RCF (ratio error) plotted against frequency.

http://www.pqs-consulting.com/ritzbrochure.pdf

The main point is that frequency reponse varies a lot between different designs and between manufacturers, even from one voltage class to the next. In other words, you can't make assumptions on a VTs capability to pass harmonics accurately, up past about the 5th or 6th.

RE: Measurement of Harmonics in 132kV system

(OP)
scottf,

Thanks for the informative document. I was looking for it in ABB / GE websites but to no avail.

The internal capacitance is responsible for signal distortions in electromagnetic VTs, so how about current transformers (guess this may not be an issue with CTs)!!

CVTs are provided generally with an LV terminal for use in Power Line Carrier Communication schemes to catch the high frequency signal and route it to the PLCC panels through LMU. Will this LV terminal be of any use when it comes to measuring harmonics, the way capacitor-resistor voltage divider can be!!

RE: Measurement of Harmonics in 132kV system

raghum-

For CTs, the problem is not nearly as pronounced. The tests I've been privy to have shown CTs behave pretty well up to the 36th harmonic or so.

The HF (or "carrier") terminal on CCVTs cannot generally be used for voltage measurement directly in the field:

1. This terminal provides ground reference for the CCVT, so you would never want to float the terminal in service, due to safety concerns, as well as it will affect the normal voltage output of the CCVT, normally used for metering or protection.

2. A voltage taken directly at the carrier terminal would not be reliable, as it is influenced by a number of factors.  You could please another capacitance between this point and ground to create a capacitive voltage divider, but you would still have the concerns listed above, plus it would be next to impossible to know the exact divider ratio if done in the field.

3. If you want to take harmonic measurements from a CCVT, then the best option is to use a device like is shown in the cut-sheet I attached.

 

RE: Measurement of Harmonics in 132kV system

(OP)
Thanks, scottf. It is clear. Appreciate your depth of knowledge on the subject.

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