Total Harmonic Distortion (Vthd and Ithd)

[aelectricalman]

If I am trying to monitor the total harmonic distortion of a system (12.47kV) but I am not able to measure at the meter point (due to external factors), can I measure the sum of its parts and at a stepped down voltage (480V)? Also, can the Vthd and Ithd be measured accurately by summation of all the parts, given whatever factor needed to be considered for the ratios of the transformers? The example is that I have 4 services that are 480V. I want to place capacitors on the primary 12.47kV but have no access to measure the 12.47kV, just the 480V. Since there are 4 services each with 4 pole mounted step down transformer, I will have to measure at each of the 4 gear. My understanding is that the transformers will filter a lot the harmonics. I dont know the ratios but any help or direction would be appreciated.

 

[Besoeker]

If I am trying to monitor the total harmonic distortion of a system (12.47kV) but I am not able to measure at the meter point (due to external factors), can I measure the sum of its parts and at a stepped down voltage (480V)? Also, can the Vthd and Ithd be measured accurately by summation of all the parts, given whatever factor needed to be considered for the ratios of the transformers? The example is that I have 4 services that are 480V. I want to place capacitors on the primary 12.47kV but have no access to measure the 12.47kV, just the 480V. Since there are 4 services each with 4 pole mounted step down transformer, I will have to measure at each of the 4 gear. My understanding is that the transformers will filter a lot the harmonics. I dont know the ratios but any help or direction would be appreciated.

To get total distortion you have to take the individual harmonics (we use hn2 to hn50), square them, add them together, and take the square root of the result.
It's easier and more succinct in mathematical symbols but difficult to type that here.

 

[aelectricalman]

In particular Im looking for the fifth harmonic across 4 different services. I'm not particularly interested in the other range of harmonics. I just want to look at the fifth harmonic on 4 different services and combine them for a total distortion of the fifth harmonic. Is that just a simple summation. Also how will running harmonics through the transformer affect the overall measurement.

 

[Besoeker]

An attempt at it.

God bless Microsoft Equation Editor, Paint Shop Pro, and Photobucket...........

 

[aelectricalman]

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[Besoeker]

In particular Im looking for the fifth harmonic across 4 different services. I'm not particularly interested in the other range of harmonics. I just want to look at the fifth harmonic on 4 different services and combine them 4 a total distortion of the fifth harmonic. Is that just a simple some Asian. Also how will running harmonics through the transformer affect the overall measurement.

OK. I assume all the services are from the same 3-phase source and are thus in phase?
This is the wording from a standard we use.
"For each harmonic order up to including the fifth, for all triplens and for the harmonic order having the highest measured value (if not so included), it must be assumed that on some occasions the background and load related emissions will be in phase and therefore they will add arithmetically."

From that, I would add the magnitudes of the fifth measurements to get the total for the fifth. If you want the THD, then what I posted above is applicable.

 

[aelectricalman]

Yes they are all derived from the same service and are in phase. Thank you very much . really appreciate the good response!

 

[aelectricalman]

Since I really want to measure on the primary side but we are forced to measure the secondary , what affects what does step down transformer have on each service for the sake of measurement. Keep in mind I will be measuring the 480 volt side of the service with hopes of understanding the total harmonics from the fifth harmonic at the primary side. There are 4 primary to secondary step down transformers that feed 4 sets of gear on this property?

 

[Besoeker]

Since I really want to measure on the primary side but we are forced to measure the secondary , what affects what does step down transformer have on each service for the sake of measurement. Keep in mind I will be measuring the 480 volt side of the service with hopes of understanding the total harmonics from the fifth harmonic at the primary side. There are 4 primary to secondary step down transformers that feed 4 sets of gear on this property?

The transformers obviously put impedance in the circuit which makes the voltage distortion on the secondary side greater if the secondary side has harmonic generating loads which, these days is fairly common. Imagine, for example, if the 12.47kV was infinitely stiff, you would could not distort it at all. And the 12.47kV is likely to be much stiffer than the 480V.

It is common here for there to metering of the primary (11kV in our case) with a voltage transformer, usually 11kV to 110V and loaded only with the instrumentation. That's normally where I make the measurements. At the metering panel.

 

[Besoeker]

Since I really want to measure on the primary side but we are forced to measure the secondary , what affects what does step down transformer have on each service for the sake of measurement. Keep in mind I will be measuring the 480 volt side of the service with hopes of understanding the total harmonics from the fifth harmonic at the primary side. There are 4 primary to secondary step down transformers that feed 4 sets of gear on this property?

Just another thought on this.
If you can get one of the transformers off load it would give you a reasonable representation of the primary side voltage.

 

[Haji]

Just another thought on this.
If you can get one of the transformers off load it would give you a reasonable representation of the primary side voltage.

I am not able to believe that loads on the secondary side of a relatively large transformer could distort significantly its voltage wave form. Is it possible for you to produce evidence? Thanks.

 

[Besoeker]

I am not able to believe that loads on the secondary side of a relatively large transformer could distort significantly its voltage wave form. Is it possible for you to produce evidence? Thanks.

Information recorded from an industrial site:

Measured voltage at the secondary of a 1500kVA transformer. The flattened tops show the proliferation of third harmonic loads:

A not so nice current:

And another:

A nasty voltage:

Except for the last, these are all measurements I personally made.
That enough evidence for you?

 

[Haji]

Information recorded from an industrial site:

Measured voltage at the secondary of a 1500kVA transformer. The flattened tops show the proliferation of third harmonic loads:

A not so nice current:

And another:

A nasty voltage:

Except for the last, these are all measurements I personally made.
That enough evidence for you?

Thanks for your reply .
The second figure is the secondary voltage wave shape of the transformer on no load. What is the effect of harmonic loads you displayed in other figures on it, as the last figure is not related to other figures per what you said?

 

[Besoeker]

Thanks for your reply .

YW

The second figure is the secondary voltage wave shape of the transformer on no load.

It was not at no load.

What is the effect of harmonic loads you displayed in other figures on it, as the last figure is not related to other figures per what you said?

All the measurements are independent.

The last figure was sent to me by a guy from a different division of our company. He was having some difficulties with some equipment on a commercial site. I mostly deal with industrial and generally in power electronics - variable speed drives, high current rectifiers etc.

The LV distribution in UK (which is where I'm from) is 400V four wire giving 230V single phase which is the voltage used in domestic and commercial installations. The last figure is just that. I don't have numerical data for it so I can't give a harmonic analysis of it. However, what it clearly does demonstrate is significant distortion on the secondary of the transformer feeding the site. Some of that may be present on the primary but, in my experience, it has the hallmarks of distortion resulting from secondary distortion.

I have seen worse. I mostly deal with industrial and generally in power electronics - variable speed drives, high current rectifiers etc. In one industrial installation I measured over 15% total voltage distortion on the 400V system with nothing of great significance on the 11kV primary of the site supply transformers.

I don't just believe that loads on the secondary side of a relatively large transformer can distort significantly its voltage wave form. I've seen it, measured it, and recorded it.

It happens.

 

[robbietan]

The transformers obviously put impedance in the circuit which makes the voltage distortion on the secondary side greater if the secondary side has harmonic generating loads which, these days is fairly common. Imagine, for example, if the 12.47kV was infinitely stiff, you would could not distort it at all. And the 12.47kV is likely to be much stiffer than the 480V.

It is common here for there to metering of the primary (11kV in our case) with a voltage transformer, usually 11kV to 110V and loaded only with the instrumentation. That's normally where I make the measurements. At the metering panel.

if measuring at the metering panel is not possible I would suggest that the transformer with the highest VTHD measurement be used as the guide for all the services.