ABB A43, User Manual

Page: 92 / 295

Measurement Methods
A43/A44 92
2CMC48001M0201
User Manual Revision: C
by the load is the product of momentary voltages U1, U2 and U3 and the
currents I1, I2 and I3 integrated over the desired measuring time period.
Illustration The picture below shows a direct connected 3-element meter measuring the
active energy (E) consumed by a load.
I1
Load
L1
U1
Meter
L2
I3
L3
I2
N
U2
U3

   
  
dt t I t U t I t U t I t U E )) ( 3 ) ( 3 ) ( 2 ) ( 2 ) ( 1 ) ( 1 (
Calculating total In the case where no harmonics are present and the rms values of the voltages and
active power
currents are constant, the total active power can be expressed as:
Ptot = P1 + P2 + P3 = U1 x I1 x cos φ1 + U2 x I2 x cos φ2 + U3 x I3 x cos φ3
where φ1, φ2 and φ3 is the phase angles between the phase voltage and its
respective current.
3-element metering with the neutral disconnected
Sometimes it is desired to use a 3-element meter without having the neutral con-
nected. It can be done with both transformer connected and direct connected me-
ters.
This can for example be desired in cases where a voltage transformer without a
neutral is being used for the moment but where a change to a voltage transformer
with neutral will be made sometime in the future. To save the trouble of changing
the meter at that time a 3-element meter is used from the beginning.
Using a 3-element meter without having the neutral connected will decrease the
accuracy due to the fact that the floating neutral connection on the meter
(terminal 11) will lie at a different level than the true neutral (N) because of
impedance imbalance inside the meter, resulting in the phase voltages not being
correct. The imbalance error is usually however rather small (typically 0-2%)
and if the cur-rents are balanced the total error in the energy measurement will
be very small, as a too small energy measurement on one element will be
compensated by approx-imately opposite errors for the other phases.