Understanding Reactive Power Measurement with Wattmeter Configuration

[mnada]

Does anybody understand how this wattmeter configuration is measuring reactive power?

Thanks

 

[subtech]

The wattmeter responds to current that is in phase with the voltage.
In your picture, The reference voltage is Y-B. (picture this vector as horizontal, the base of the delta
triangle). The current is from phase R. Its vector would be vertical or at 90 deg to the voltage vector.
If the power factor is 1.00, then the vector representing current would remain at 90 deg in relation to the voltage vector. As the power factor decreases however, the current vector rotates right or left (inductive or capacitive) and comes closer to being in phase with the voltage vector. As it does, the meter begins to respond. It will measure and display VAr. This is common and has practical application in the electrical power industry.

 

[mnada]

Thank you very much, your explanation is crystal clear, and helped me so.
Again, thank you.

 

[m.s.j]

That varmeter regarding to special connection is a wattmeter inherently, and we can write:

P = K₁. V_YB . I_R . COS ø (output of measurement device)

V_YB = √3.V_R< 90˚ (In three phase balance system)

P = K₁. √3.V_R . I_R . COS (ø - п/2) = K₂. V_R . I_R . Sin ø

If K₂ = 3 we can write:

P = 3. V_R . I_R . Sin ø = Q_3PH

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[mnada]

Another excellent Explanation. Thanks

 

[niko_bellic]

you need to multiply $$\sqrt{3}$$ to the wattmeter reading to get 3-phase reactive power...