Landis+Gyr E650 Series 4 User Manual page 19

Device description
Instantaneous power
with sign
Calculation of reactive
power
U , I calculation
RMS RMS
Time measurement
D000062026 en a – E650 Series 4 – ZMD400AT/CT, ZFD400AT/CT – User Manual
I
Q
I
j
I
U
P
Figure 6. Active power calculation
If the meter is parameterised to calculate instantaneous power as signed
values, the following values of power are available:
Active P: + in QI and QIV, - in QII and QIII
Reactive P: + in QI and QII, - in QIII and QIV
There are two possibilities to calculate the instantaneous value of reactive
power (ZxD400Cx combimeters only):
Measured:
For the instantaneous value of reactive power q the instantaneous value of
voltage u must be rotated by 90° before multiplication (the reactive
component is the product of the voltage component with the current
component vertical to the voltage). Thereby no harmonics are measured
since only the fundamental wave is rotated through 90°.
I
Q
I
j
I
U
P
Figure 7. Reactive power calculation (method: measured)
Calculated vectorial (not recommended):
The instantaneous value of reactive power is calculated using the values of
active power and apparent power.
The reactive power is the square root of the square value of apparent
power minus the square value of active power. This method includes the
harmonics.
�� = √ �� 2 2
− ��
The square values of voltage and current are obtained by multiplying the
instantaneous values of voltage and current by themselves. From these
values the signal processor forms the corresponding single-phase RMS
values U and I .
RMS RMS
The network frequency can be calculated from the time measured between
two zero passages (change from negative to positive value of voltage U1).
The times between zero passage of the phase voltage U1 and those of the
other phase voltages U2 and U3 serves to determine the phase angle
between the voltages and of the rotating field.
Calculation per phase of
. . j
P = U I cos
Calculation per phase of
. . j
Q = U I sin
19/96
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