I'll try to explain this as best as I can with what I remember Zach Stone's Electrical PE Review course taught about 2-wattmeter method.
In a 2-wattmeter method, the two wattmeters are ALWAYS connected such that:
- They share the same common negative terminal connection point.
Here, the common negative terminal point is the B phase.
- They have different positive terminal connection points.
Wattmeter W1's positive terminal is the C phase, and Wattmeter W2's negative terminal is the A phase.
For each wattmeter's connection going from the positive terminal to the negative terminal:
- The wattmeter that is connected in the positive or ABC sequence (AB, BC, or CA) will have its real power formula as: P = VLL * IL * cos(theta + 30).
In this problem, Wattmeter 2 is connected from A to B, which is the positive or ABC sequence.
So: P W2 = VLL * IL * cos(theta + 30)
- The wattmeter that is connected in the negative or CBA sequence (CB, BA, or AC) will have its real power formulas as: P = VLL * IL * cos(theta - 30)
In this problem, Wattmeter 1 is connected from C to B, which is the negative or CBA sequence.
So: P W1 = VLL * IL * cos(theta - 30)
The 2 wattmeters will have the same real power readings when theta = 0, which corresponds to pf = 1. The reason is that the cosine function is symmetrical about the origin theta = 0 (it is an even or evenly symmetrical function about theta = 0). So cos(-x) = cos(x).
When theta = 0:
- cos(0 - 30) = cos(0 + 30)
- cos(-30) = cos(+30) = 0.866
- Power factor: pf = cos(0) = 1
I hope that this explanation helps. I also included a few illustrations to try to reinforce my explanation.