U
k
k
th
harmonic component
U
1
the voltage fundamental component amplitude
Demand calculation
The demand value for THD is calculated separately for each phase. If any of the
calculated demand THD values is above the set alarm limit
THD alarm limit, the
ALARM
output is activated.
The demand calculation window is set with the
Demand interval setting. It has
seven window lengths from "1 minute" to "180 minutes". The window type can
be set with the
Demand window setting. The available options are "Sliding" and
"Non-sliding".
The activation of the
BLOCK
input blocks the
ALARM
output.
10.2.5
Application
VMHAI provides a method for monitoring the power quality by means of the voltage
waveform distortion. VMHAI provides a short-term three-second average and long-
term demand for THD.
10.2.6
Signals
Table 1036: VMHAI Input signals
Name
Type
Default
Description
U_A_AB
SIGNAL
0
Phase-to-earth volt-
age A or phase-to-
phase voltage AB
U_B_BC
SIGNAL
0
Phase-to-earth volt-
age B or phase-to-
phase voltage BC
U_C_CA
SIGNAL
0
Phase-to-earth volt-
age C or phase-to-
phase voltage CA
BLOCK
BOOLEAN
0=False
Block signal for all bi-
nary outputs
Table 1037: VMHAI Output signals
Name
Type
Description
ALARM
BOOLEAN
Alarm signal for THD
10.2.7
Settings
1MRS757644 H
Power quality measurement functions
620 series
Technical Manual
1087
