Iskra MT440 - 5.5 Calculation and display of measurements; 5.6 Present values

Measurements
30 User’sManual
5.5 Calculation and display of measurements
This chapter deals with capture, calculation and display of all supported quantities of
measurement. Only the most important equations are described; however, all of them are shown
in chapter Equations on page 53 with additional descriptions and explanations.
Note
Calculation and display of measurements depend on used connection. For
more detailed information see chapters Survey of supported measurements
regarding Connection mode on page 27.
5.6 Present values
All values are calculated as an average of number of periods set in General settings/average
interval.
Voltage
Instrument measures true RMS values of all phase voltages (U1, U2, U3), connected to the
meter. Phase-to-phase voltages (U12, U23, U31), average phase voltage (Uf) and average phase-
to-phase voltage (Ua) are calculated from measured phase voltages (U1, U2, U3).
N
u
=
U
2
n
N
1=n
f
∑
()
N
2
ynxn
N
1=n
xy
uu
=
U
−
∑
All voltage measurements are available via communication.
Current
Instrument measures true RMS values of phase currents, connected to current inputs. Neutral
current (I
n
), average current (I
a
) and a sum of all phase currents (I
t
) are calculated from phase
currents.
N
i
=
2
n
I
N
1=n
RMS
∑
All current measurements are available via communication.
Active, reactive and apparent power
Active power is calculated from instantaneous phase voltages and currents.
Two different principles of reactive power calculation are used:
Standard method:
With this method a reactive power is calculated based on assumption that all power that is not
active is reactive.
Q
2
= S
2
– P
2
This means also that all higher harmonics will be measured as reactive power.
Delayed current method:
With this method, reactive power (energy) is calculated by multiplication of voltage samples and
delayed current samples (see chapter
Equations on page 53):
Q = U × I|
+90°