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10.2.3
Functionality
The voltage total harmonic distortion function VMHAI is used for monitoring the
voltage total harmonic distortion THD.
10.2.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The
corresponding parameter values are "On" and "Off".
The operation of VMHAI can be described with a module diagram. All the modules
in the diagram are explained in the next sections.
Distortion
measure-
ment
Demand
calculation
BLOCK
ALARM
U_A_AB
U_B_BC
U_C_CA
GUID-615D1A8A-621A-4AFA-ABB0-C681208AE62C V1 EN
Figure 359:
Functional module diagram
Distortion measurement
The distortion measurement module measures harmonics up to the 11th harmonic.
The total harmonic distortion THD for voltage is calculated from the measured
harmonic components with the formula
THD
U
U
k
k
N
=
=
∑
2
2
1
GUID-83A22E8C-5F4D-4332-A832-4E48B35550EF V1 EN
(Equation 76)
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.
1MRS758755 A
Section 10
Power quality measurement functions
REC615 and RER615
677
Technical Manual
Summary of Contents for RELION Series
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