3
The Method of Measurement and Evaluation of Individual Vari-
ables
Measurement includes three continuously performed processes: frequency evaluation, sampling voltage and cur-
rent signals and evaluation of these sampled data.
Frequency of the fundamental harmonic voltage component
is continuously measured and evaluated
every 10 seconds. The measured signal is a line voltage of first phase signal modified with a low pass filter.
Frequency is assessed as a percentage of the number of full cycles of the network established within each 10
seconds and the cumulative duration of full cycles.
Voltages and currents
are evaluated continuously without gaps. Basic evaluation interval is 10/12 cycles
of the network (
∼
200
ms
for both 50
Hz
or 60
Hz
network). All channels are sampled at the frequency of 128
samples per network cycle. Sampling is controlled by the measured frequency in channel
U
1
. If the value of
the frequency is in measurable range it also controls the sampling — sampling is automatically adjusted to the
frequency change. Otherwise, the sampling runs according to the preset nominal frequency (50
Hz
or 60
Hz
).
RMS voltage and currents are evaluated from the sampled values for the measuring cycle according to equations:
3.1
Basic quantities (RMS)
Line-to-Neutral and Line-to-Line voltages, currents:
U
1
=
v
u
u
t
1
n
n
X
i
=1
U
2
1
i
, U
12
=
v
u
u
t
1
n
n
X
i
=1
(
U
1
i
−
U
2
i
)
2
, I
1
=
v
u
u
t
1
n
n
X
i
=1
I
2
1
i
where:
i
............................. sample index
n
............................ number of samples per cycle of measurement (128)
U
1
,i
,
U
2
,i
,
I
1
,i
........ individual samples of voltage and current
3.2
Powers and power factor (PF)
Active and reactive power (single phase, three phase)
P
1
=
1
n
n
X
i
=1
U
1
i
×
I
1
i
,
3
P
=
P
1
+
P
2
+
P
3
Q
1
=
N
X
k
=1
U
1
,k
×
I
1
,k
×
sin
4
ϕ
1
,k
,
3
Q
=
Q
1
+
Q
2
+
Q
3
6
The equations are valid for a three phase star connection.
27
