App-24
IM WT310E-01EN
Error in Three-Phase Power
The error when measuring power of a three-phase, three-wire system using elements 1 and 3 of the
WT332E/WT333E is explained with the next example.
• Voltage measurement range: 150 V, measured voltage: 100V for U1, U3, and UΣ
• Current measurement range: 1 A, measured current: 0.8 A for I1, I3, and IΣ
• Measured power: P1 = 69.28 W, P3 = 69.28W, PΣ = 138.56 W
• 60 Hz sine wave for both voltage and current
• Phase difference between the voltage and current signals = 0°
• Phase angle between phases = 60°
Three-Phase Measurement Range
For the three-phase measurement range, refer to the table of Σ function equations on page App-2.
This table shows the expressions that the WT332E/WT333E uses to internally calculate the measured
values. This table also applies to how to think about the measurement range. In this example, we will
apply the three-phase, three-wire (3P3W) column.
Voltage and Current
Three-phase voltage (UΣ) measurement range = (U1 measurement range + U3 measurement range)/2
= (150 + 150)/2 = 150
Three-phase current (IΣ) measurement range = (I1 measurement range + I3 measurement range)/2 = (1
+ 1)/2 = 1
The reading error and range error included in the measured three-phase voltage (UΣ; 100.00 V) are as
follows:
• Reading error: 100.00 [V]×0.1% = 0.1 [V]
• Range error: 150 [V]×0.05% = 0.075 [V]
The error included in 100.00 [V] is the sum of the reading and range errors, which is ±0.175 [V]. This
corresponds to 0.175% of the displayed value. Because the measured values of U1 and U3 are the
same, UΣ also has the same error. The same calculation method applies for the currents.
Power
Referring to the table of Σ function equations on page App-2, the power range is as follows:
Three-phase power (PΣ) measurement range
= P1 measurement range + P3 measurement range
= (U1 measurement range × I1 measurement range) + (U3 measurement range × I3 measurement
range)
= (150 × 1) + (150 × 1)
= 300
The reading error and range error included in the measured three-phase power (PΣ; 138.56 W) are as
follows:
• Reading error: 138.56 [W]×0.1% = 0.13856 [W]
• Range error: 300 [W]×0.05% = 0.15 [W]
The error included in 138.56 [W] is the sum of the reading and range errors, which is ±0.28856 [W].
This corresponds to approximately 0.208% of the displayed value.
Appendix 5 Measurement Accuracy and Measurement Error
