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1–6
In modern digital meters, Blondell's Theorem is still applied to obtain proper metering.
difference in modern meters is that the digital meter measures each phase voltage and
current and calculates the single-phase power for each phase. The meter then sums the
three phase powers to a single three-phase reading.
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.
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 Laws hold
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.
C
B
A
N
FIGURE 1–6: Three-Phase Load Illustrating Kirchhoff's Law and Blondell's Theorem
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 the four wires if
they are connected by a common node. In the circuit of Figure 1-6 we must measure the
CHAPTER 1: THREE-PHASE POWER MEASUREMENT
Phase B
Phase A
EPM 6010 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE
The
Phase C
Node "n"
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