Mega MEg40+ - DETERMINATION OF VOLTAGES, CURRENTS AND POWERS

Měřící Energetické Aparáty
37
A similar relation as for voltages applies to currents. From the instantaneous current
values i(k) with the serial numbers k = 0, 1, 2, …, K – 1, the RMS current value is
calculated according to the formula
(2)
Using this formula, you calculate the RMS value I
1
of the current in phase 1 from the
group of instantaneous values i
1
(k) of the current in phase 1, the RMS value I
2
of the
current in phase 2 from the group of instantaneous values i
2
(k) of the current in phase 2
and the RMS value I
3
of the current in phase 3 from the group of instantaneous values
i
3
(k) of the current in phase 3.
DETECTION OF EVENTS
In case of this function, RMS values of all three voltages are used, always in the range
of the network period T
s
, and the interval T
s
gradually moves by 0.5 T
s
. Because the used
sampling rate is 32 times higher than the mains frequency, every interval T
s
contains 32
instantaneous values. For calculation, the formula (1) is used where K = 32. e obtained
RMS values of all three voltages are compared with the preset thresholds. If it is found
out that there is a voltage dip, swell or interruption, the extreme voltage value, duration
and time of occurrence are recorded.
DETERMINATION OF VOLTAGES, CURRENTS AND POWERS
Voltages and currents in a three-phase system are evaluated according to their RMS
values in measuring time intervals T
m
lasting 10 network periods, i.e. T
m
= 10T
s
.
ese intervals follow immediately one by one but they do not overlap. e sampling runs
continuously with a frequency, which is 32 times higher than the network frequency.
erefore 320 instantaneous values fall into each of the measuring time intervals T
m
for
each of the voltages and currents. To calculate the RMS voltage values, the formula (1)
is used where K = 320. So the RMS values of phase voltages U
1
, U
2
, U
3
or delta voltages
U
12
, U
23
, U
31
are obtained.
To calculate the RMS phase current values, the formula (2) is used where K = 320 to
receive RMS phase current values I
1
, I
2
, I
3
.
( )
ki
K
I
K
k
∑
−
=
=
1
0
2
1