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Distortion measurement
The distortion measurement module measures harmonics up to the 11th harmonic.
The total demand distortion TDD is calculated from the measured harmonic
components with the formula
TDD
I
I
k
k
N
demand
=
=
∑
2
2
max_
GUID-9F532219-6991-4F61-8DB6-0D6A0AA9AC29 V1 EN
(Equation 75)
I
k
k
th
harmonic component
I
max_demand
The maximum demand current measured by CMMXU
If CMMXU is not available in the configuration or the measured maximum demand
current is less than the
Initial Dmd current
setting,
Initial Dmd current
is used for
I
max_demand
.
Demand calculation
The demand value for TDD is calculated separately for each phase. If any of the
calculated total demand distortion values is above the set alarm limit
TDD 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.
10.1.5
Application
In standards, the power quality is defined through the characteristics of the supply
voltage. Transients, short-duration and long-duration voltage variations, unbalance
and waveform distortions are the key characteristics describing power quality. Power
quality is, however, a customer-driven issue. It could be said that any power problem
concerning voltage or current that results in a failure or misoperation of customer
equipment is a power quality problem.
Harmonic distortion in a power system is caused by nonlinear devices. Electronic
power converter loads constitute the most important class of nonlinear loads in a
power system. The switch mode power supplies in a number of single-phase
electronic equipment, such as personal computers, printers and copiers, have a very
high third-harmonic content in the current. Three-phase electronic power converters,
that is, dc/ac drives, however, do not generate third-harmonic currents. Still, they can
be significant sources of harmonics.
Power quality monitoring is an essential service that utilities can provide for their
industrial and key customers. Not only can a monitoring system provide information
Section 10
1MRS758755 A
Power quality measurement functions
674
REC615 and RER615
Technical Manual
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