THD sliding mean value outputs are SMNTHD_A, SMNTHD_B and SMNTHD_C. Non-
sliding 10-minute mean values LMNTHD_A, LMNTHD_B and LMNTHD_C are seen
in the monitored data. These values are always shown as percentage of the
fundamental frequency current mean value. The 10-minute mean values follow 10
min ticks. See also
Figure 961
and the description for sliding values.
10.1.5.3 Total demand distortion
The total demand distortion TDD is calculated from the measured harmonic
components with the formula
(Equation 391)
I
h
h
th
harmonic component
I
L
Maximum demand load current set with
Demand current setting
n Number of harmonics
The used reference current is the absolute value of the
Demand current
setting. It is
typically set according to the maximum load current of the feeder. The TDD value is
calculated separately for each phase.
TDD sliding mean value outputs are SMNTDD_A, SMNTDD_B and SMNTDD_C. Non-
sliding 10-minute mean values LMNTDD_A, LMNTDD_B and LMNTDD_C are seen in
the monitored data. These values are always shown as percentage of the setting
Demand current
. The 10-minute mean values follow 10 min ticks. See also
Figure 961
and the description for sliding values.
10.1.5.4 Harmonic calculation
Harmonic mean values are calculated for each phase. Also, the mean values of the
nominal frequency (fundamental) and direct current component are calculated. The
fundamental frequency mean values are shown as percentage of the nominal phase
current.
The actual harmonic mean values are shown as percentage of the reference current.
By default, the fundamental frequency current is used as reference. This can
be selected by setting
Reference Cur Sel
to "fundamental". When amplitude of
fundamental frequency current is very low, below 0.02 xIn, calculation is blocked
internally. As a result, value zero is used in the mean value calculation. Extremely
high harmonic values are limited to 2.0 xIn. This is also used for fundamental mean
value, but not for reference.
The value of Demand current setting can be used as reference by setting
Reference
Cur Sel
to "absolute". Typically, it is set according to the maximum demand load
current. That is more typically used when TDD is preferred, but might also be found
useful when THD is monitored.
Settings
Reference Cur Sel
and
Demand current
have no effect in THD
calculation, but setting
Demand current
is used in TDD calculation.
Power quality measurement functions 1MRS759142 F
1810 REX640
Technical Manual
To Next Page
Loading...
