There is a method to detect the reactive power in three phase line, that use only two wattmeter (used to measure real part).

I have problem to understand this method and resultant expression of the reactive power.

I have attached a representation of this method: connection of two instruments and the formula valid for reactive power. In the figure “Rv” is the resistance added to the third line, his value equals the internal resistance of wattmeter.

I desired to ask you if you can describe me the derivation of the Q expression, involving phasors present in the line.