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Some digital meters calculate the individual phase power values one phase at a time.
This means the meter samples the voltage and current on one phase and calculates a
power value. Then it samples the second phase and calculates the power for the
second phase. Finally, it samples the third phase and calculates that phase power.
After sampling all three phases, the meter combines the three readings to create the
equivalent three-phase power value. Using mathematical averaging techniques, this
method can derive a quite accurate measurement of three-phase power.
More advanced meters actually sample all three phases of voltage and current
simultaneously and calculate the individual phase and three-phase power values. The
advantage of simultaneous sampling is the reduction of error introduced due to the
difference in time when the samples were taken.
Figure 1.6: Three-Phase Wye Load Illustrating Kirchhoff's Law and Blondell's Theorem
Blondell's Theorem is a derivation that results from Kirchhoff's Law. Kirchhoff's Law
states that the sum of the currents into a node is zero. Another way of stating the
same thing is that the current into a node (connection point) must equal the current
out of the node. The law can be applied to measuring three-phase loads. Figure 1.6
shows a typical connection of a three-phase load applied to a three-phase, four-wire
service. Krichhoff's Law holds that the sum of currents A, B, C and N must equal zero
or that the sum of currents into Node "n" must equal zero.
If we measure the currents in wires A, B and C, we then know the current in wire N by
Kirchhoff's Law and it is not necessary to measure it. This fact leads us to the
conclusion of Blondell's Theorem- that we only need to measure the power in three of
Electro Industries/GaugeTech
Electro Industries/GaugeTech
The Leader In Power Monitoring and Smart Grid Solutions
The Leader In Power Monitoring and Smart Grid Solutions
C
B
Phase B
Phase A
A
N
1: Three Phase Power Measurement
Phase C
Node "n"
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