The Two Watt-Hour Meter Method and How It Works

The two watt hour method is an alternative watt hour meter connection to measure the active power in a three phase power system using only 2 watt hour meters instead of three meters. The connection of a two watt meter is as follow;

For two watt hour meter to work properly, the two meter must have a common connection, in the above example, it is phase C.

The below figure is a phasor diagram illustration of how the three phase power is computed using two meters.

That is;
The power computed by Meter 1 is equal to: (The voltage measured) X (Current Measured) X (cosine(30 degree - theta))

And

The power computed by Meter 2 is equal to: (The voltage measured) X (Current Measured) X (cosine(30 degree + theta))

Where theta is the power factor angle (Arcosine of the power factor) in degrees.

Observations:
1. The electrical power measured by both meters will only be equal if the power factor is 1.0 or theta is zero.
2. The electrical power measured by meter 2 is always less than the measurement in meter 1 (for a lagging power factor) and zero if the power factor is 0.5 or theta is 60 degress.
3. And most importantly, electrical power measured by the two watt hour method is accurate if the three phase system is balance.

Keywords: Two watthour method, two watthour meter method, two watt-hour method, 2 watthour method, 2 watthour meter method, 2 watt-hour meter method

Filed in: Other Tools and Equipments -

How Capacitor Overcompensation in a Distribution System Can Induce Additional Losses »