Author Topic: Two-Wattmeter, 3-Phase power measurement  (Read 2311 times)

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Offline 741Topic starter

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Two-Wattmeter, 3-Phase power measurement
« on: January 22, 2020, 10:31:06 am »
WRT a 'star' (4 wire) system, the textbook Further Electrical & Electronic Principles states that 3-phase power can be found by taking simultaneous measurements with just 2 watt-meters. Further:
   (a) Access to star point not required 
   (b) Power for either balanced or un-balanced loads is obtained

Calling the 3 phases r,b,g the picture in the book shows we measure
   Vrb and Ir, Vyb and Iy
That is, our common measurement node for voltage is the 'b' phase.

However, the proof then seems to assume a balanced load
Quote
Ir + Iy = -Ib
I looked this up online and found the following
   https://www.newtons4th.com/wp-content/uploads/2010/03/APP014-3-Phase-2-Wattmeter-Explained.pdf
Now this document states
Quote
Note: In a 4 wire system the Star Point/Neutral will be used as the Volts Lo common point
This would mean access to the star-point is required.

For a 4-wire 'star' 3-phase system with an un-balanced load and using the two-wattmeter method:
  • Can I derive total load power?
  • Is access to the Nuetral (Star) terminal needed?

Online coppice

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Re: Two-Wattmeter, 3-Phase power measurement
« Reply #1 on: January 22, 2020, 11:29:23 am »
The 2 watt meter approach does not work when you have a 4th wire (assuming the 4th wire is used for something more than instrumentation). The centre of the star has to be balanced. Most practical 3 phase watt meters measure the 3 phases separately, so they deal with any amount of imbalance. Its fairly unusual in the real world for a 3 phase load to be properly balanced. Even when the main load is something inherently balanced, like a large 3 phase machine, you'll find some lights and other accessories unbalancing the load.
 
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Offline SG-1

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Re: Two-Wattmeter, 3-Phase power measurement
« Reply #2 on: January 23, 2020, 05:51:27 pm »
Here are some pages from my Introductory Circuit Analysis Textbook.

In the medium voltage world nearly all three phase loads are balanced.  This is where you would find this type of metering.

See this explanation of 2 1/2 & 2 element watt meters:        https://www.novatechweb.com/news/novatech-news-bryan-2-12-element-configuration/

Advice is a dangerous gift, even from the wise to the wise.
 

Offline 741Topic starter

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Re: Two-Wattmeter, 3-Phase power measurement
« Reply #3 on: January 24, 2020, 09:49:44 am »
Thank you for the pdf excerpts. Since my interest is accurate measuring of 3-phase power with un-balanced loads, could I request the rest of section 21.13?

I'll need to draw some phasor plots to properly see how/why fig 21.30 works, but it does at least 'feel' logical at this point.


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