Why is the Theoretical Calulated Three-Phase Apparent Power Values (ΣVA and ΣS) Different From the Measured Values?

This is due to measurement and calculation error, or differences in calculation methods.
 
On the WT, the three-phase apparent power (ΣS) equation is calculated under the assumption of a balanced condition (the absolute value of each phase is the same, and the phase angle is 120°). When balanced, the line to line voltages U1, U2, and U3 in three-phase three-wire or 3V3A configuration result in phase voltages of √3 times UR, US, and UT. Considering the three-phase apparent power, three-phase four wire is simply the sum of each phase:
                        Three-phase four-wire ΣS = S1+S2+S3

For three-phase three-wire and three-phase three-wire (3V3A), the apparent power is, in princicple, not measured. Therefore when balanced, a coefficient is used such that the same value as three-phase four-wire ΣS, and the following equations are used:
                     Three-phase three-wire ΣS = √3/2(S1+S3)
                     3V3AΣS = √3/3(S1+S2+S3).
 
Consequently, when unbalanced, since the phase-to-phase phase angle is not 120°, and the votlage value of each phase is of a different size, the line to line voltages U1, U2, and U3 above do not result in phase voltages of √3 times UR, US, and UT. Therefore, ΣS calculated for three-phase three-wire or 3V3A can differ from the value of ΣS determined in a three-phase four-wire configuration.

Related Products & Solutions

Precision Power Analyzer WT3000

With 0.02% accuracy and 1MHz bandwidth, the WT3000 delivers where the highest precision measurements are required. It is the industry standard for R&D work on inverters, motor drives, lighting systems and electronic ballasts, UPS systems, aircraft power, transformer testing, and other power conversion devices.

PZ4000 Power Analyzer

The PZ4000 is designed for High Frequency Power Measurements as well as Transient Power Measurements. Its real-time acquisition system can perform cycle-by-cycle power measurements. One to Four input elements offer Basic Power Accuracy of 0.2% with 2 MHz Bandwidth and 5MS/sec digitizing rates.

WT1600 Digital Power Meter

The High-end WT1600 is designed for a wide range of applications, from energy-saving to large load applications. The WT1600 has a six element input capability to provide maximum flexibility, and Basic Power Accuracy of 0.1% with 1 MHz Bandwidth.

WT1800 High Performance Power Analyzer

The WT1800 Power Analyzer offers maximum flexibility with up to six wattmeter elements, high bandwidths, simultaneous high speed digitizing, and wide voltage and current ranges. Although this model is still available, the newer WT1800E offers higher accuracy.

Power Analyzers and Power Meters

Measure characteristics of devices that generate, transform or consume electricity. Also called power meters or wattmeters, these devices measure parameters such as true power (watts), power factor, harmonics, and efficiency.

Top