YOKOGAWA wt3000, Руководство пользователя, Страница: 44 / 384

2-18
IM 760301-01E
Setting the Compensation for Two-Wattmeter Method «For procedures, see section
5.8.»
In the two-wattmeter method, an error results when current flows through the neutral line.
This function computes the current the flows through the neutral line for measurements
using the two-wattmeter method with a three-phase, three wire (3V3A) system and adds
the compensation value to the measured power. For details, see appendix 8. This
function can be set on models with the delta computation option.
Selecting the Computing Equation for Apparent Power and Reactive Power «For
procedures, see section 5.9.»
There are three types of powers: active power, reactive power, and apparent power.
In general, each type is defined by the following equations.
Active power P = UIcos φ ............................................................. (1)
Reactive power Q = UIsin φ ......................................................... (2)
Apparent power S = UI ............................................................... (3)
where U: rms voltage, I: rms current, and φ : Phase between voltage and current
For three-phase power, it is the sum of the powers of each phase.
In addition, these power values are related as follows:
(Apparent power S)
2
= (active power P)
2
+ (reactive power)
2
.....
(4)
These definitions apply only for sine waves. The measured values for apparent power
and reactive power vary for distorted waveform measurement depending on which
definitions above are combined for the computation. Because the equations for deriving
the power for distorted waveforms are not defined, none of the equations can be said to
be more correct than the other. Therefore, the WT3000 provides three equations for
determining the apparent power and reactive power.
Because active power is derived directly from the sampled data, errors resulting from
different definitions do not occur such as in the apparent power and reactive power.
• TYPE 1 (The Method Used by Normal Mode of Conventional WT Series Power
Meters)
The apparent power of each phase is calculated using equation (3), and the reactive
power of each phase is calculated using equation (2). The results are summed to
derive the power.
Active power for three-phase, four-wire system P Σ = P1 + P2 + P3
Apparent power for three-phase, four-wire system S Σ = S1 + S2 + S3 (= U1 × I1 +
U2 × I2 + U3 × I3)
Reactive power for three-phase, four-wire system Q Σ = Q1 + Q2 + Q3
(= s1 ×
(U1 × I1)
2
-P1
2
+s2 × (U2 × I2)
2
-P2
2
+s3 × (U3 × I3)
2
-P3
2
)
However, the sign for s1, s2, and s3 is negative when the current leads the voltage
and positive when the current lags the voltage.
• TYPE2
The apparent power of each phase is determined from equation (3), and the results
are added to derive the three-phase apparent power. The three-phase reactive power
is calculated from the three-phase apparent power and three-phase active power
using equation (4).
Active power for three-phase, four-wire system P
Σ
= P1 + P2 + P3
Apparent power for three-phase, four-wire system S Σ = S1 + S2 + S3 (= U1 × I1 +
U2
×
I2 + U3
×
I3)
Reactive power for three-phase, four-wire system
Q Σ = S Σ
2
– P Σ
2
2.5 Computation