A24
The accuracy of active power calculations can be calculated as follows, taking into account the phase
accuracy:
Example measurement conditions
Wiring: 3-phase/4-wire (3P4W)
Current sensor: Model CT7136
Current range: 50 A (power range: 150 kW)
"13.9 Range Breakdown and Combination Accuracy" (p.244)
Measured values: Active power of 30 kW, power factor lag 0.8
Accuracy
Active power accuracy for current sensor combination (Model CT7136 sensor, 50 A range):
±0.4% rdg.±0.22% f.s.
Internal circuit voltage of the instrument
- current phase difference: ±0.2865°
(Effect of power factor: 1.0% rdg. or less)
Phase accuracy of the CT7136: ±0.5°
"13.2 Input Specifications/Output Specifications/Mea
surement Specifications" (p.192)
"13.9 Range Breakdown and Combination Accuracy" (p.244)
Phase accuracy shown in “Specifications” of the CT7136 Instruction Manual
Power factor accuracy based on phase accuracy
Phase accuracy (in combination with current sensor) = Instrument internal circuit phase accuracy
(±0.2865°) + CT7136 phase accuracy (±0.5°) = ±0.7865°
Phase difference θ =
cos
−1
(power factor)= cos
−1
0.8= 36.87°
Power factor error range based on phase accuracy = cos(36.87°±0.7865°) = 0.7916 to 0.8082
Power factor accuracy based on phase accuracy
Active power accuracy = current sensor combined accuracy + power factor accuracy based on phase
accuracy
= ±0.5% rdg. ±0.22% f.s. ±1.05% rdg.
= ±1.55% rdg.±0.22% f.s.
Accuracy relative to measured values = active power 30 kW × ± 1.55% rdg. + 150 kW range × 0.22% f.s.
= ±0.795 kW
= ±0.795 kW/30 kW = ±2.65% rdg.
Appendix 8 Method for Calculating Active Power
Accuracy
Power factor accuracy based on phase accuracy (minimum) = % = -1.05%
Power factor accuracy based on phase accuracy (maximum) = % = +1.025%
Power factor accuracy based on phase accuracy: ±1.05% rdg.
0.7916 0.8–
0.8
---------------------------------
100×
0.8082 0.8–
0.8
---------------------------------
100×
The value, whichever is
worse, is specified to be
the phase accuracy.
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