ABB RELION REX640 Technical Manual page 1610

Section 10
Power quality measurement functions
1604
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 829
Total demand distortion
The total demand distortion TDD is calculated from the measured harmonic
components with the formula
2
n
I
h
 
TDD
=
∑
 
I
L
h  
2
=
GUID-7EF84895-E6E7-451C-A244-BE84339C7BFD V1 EN
I
th
h harmonic component
h
I Maximum demand load current set with
L
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
and the description for sliding values.
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.
and the description for sliding values.
Demand current setting
1MRS759142 C
(Equation 352)
Figure 829
REX640
Technical Manual