Automatically select the appropriate calculation for each data updating period
AC signals have waveforms that fluctuate repeatedly when viewed instantaneously.
Therefore, measuring the power values of AC signals requires averaging for each period in
a repeated interval, or averaging the data of several periods using a filtering process. The
WT3000 automatically selects the appropriate calculation method (one of the above two
methods) based on the data updating period. This approach ensures fast response and high
stability as suitable for the particular measurement objective.
When the data updating period is 50ms, 100ms, 5s, 10s, or 20s
Measurement values are determined by applying an Average for the Synchronous Source
Period (ASSP) calculation to the sample data within the data updating period. (Note that this
excludes power integrated values WP, as well as current integrated value q in DC mode).
With ASSP, a frequency measurement circuit is used to detect the input signal period set as
the synchronous source. Sample data corresponding to an interval which is an integer
multiple of the input period are used to perform the calculation. Based on its fundamental
principles, the ASSP method allows measurement values to be obtained simply by
averaging an interval corresponding to a single period, so it is useful in cases where the
data updating period is short or when measuring the efficiency of low-frequency signals.
This method will not provide correct measurement values unless the period of the set
synchronous source signal is accurately sensed. Therefore, it is necessary to check whether
the frequency of the synchronous source signal has been accurately measured and
displayed. See the user’s manual for notes on the synchronous source signal and frequency
filter settings.
When the data updating period is 250ms, 500ms, 1s, or 2s
Measurement values are determined by applying an Exponential Average for Measuring
Period (EAMP) calculation to the sample data within the data updating period. With EAMP,
the sample data are averaged by applying a digital filtering process. This method does not
require accurate detection of the input period. EAMP provides excellent measurement value
stability.
Selecting formulas for calculating apparent power and reactive power
There are several types of power––active power, reactive power, and apparent
power. Generally, the following equations are satisfied:
Active power P = UIcosØ (1)
Reactive power Q = UIsinØ (2)
Apparent power S = UI (3)
In addition, these power values are related to each other as follows:
(Apparent power S)2 = (Active power P)2+(Reactive power Q)2 (4)
U:
I:
Ø: |
Voltage RMS
Current RMS
Phase between current and voltage
Three-phase power is the sum of the power values in the individual phases.
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These defining equations are only valid for sinewaves. In recent years, there has
been an increase in measurements of distorted waveforms, and users are
measuring sinewave signals less frequently. Distorted waveform measurements
provide different measurement values for apparent power and reactive power
depending on which of the above defining equations is selected. In addition,
because there is no defining equation for power in a distorted wave, it is not
necessarily clear which equation is correct. Therefore, three different formulas for
calculating apparent power and reactive power are provided with the WT3000.
TYPE1 (method used in normal mode with older WT Series models)
With this method, the apparent power for each phase is calculated from equation (3),
and reactive power for each phase is calculated from equation (2). Next, the results
are added to calculate the power.
Active power for three-phase four-wire connection: PΣp=P1+P2+P3
Apparent power for three-phase four-wire connection: SΣ=S1+S2+S3(=U1xI1+U2xI2+U3xI3)
Reactive power for three-phase four-wire connection: QΣ=Q1+Q2+Q3
*S1, S2, and S3 are calculated with a
positive sign for the leading phase and a negative sign for the lagging phase.
TYPE2
The apparent power for each phase is calculated from equation (3), and the results are added together
to calculate the three-phase apparent power (same as in TYPE1). Three-phase reactive power is
calculated from three-phase apparent power and three-phase active power using equation (4).
Active power for three-phase four-wire connection: PΣ=P1+P2+P3
Apparent power for three-phase four-wire connection: SΣ=S1+S2+S3(=U1xI1+U2xI2+U3xI3)
Reactive power for three-phase four-wire connection:
TYPE3 (method used in harmonic measurement mode with WT1600 and PZ4000)
This is the only method in which the reactive power for each phase is directly calculated using
equation (2). Three-phase apparent power is calculated from equation (4).
Active power for three-phase four-wire connection: PΣ=P1+P2+P3
Apparent power for three-phase four-wire connection:
Reactive power for three-phase four-wire connection: QΣ=Q1+Q2+Q3
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