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_
(Equation 193)
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 about system disturbances and their possible causes, it can also detect
problem conditions throughout the system before they cause customer complaints,
equipment malfunctions and even equipment damage or failure. Power quality
problems are not limited to the utility side of the system. In fact, the majority of
power quality problems are localized within customer facilities. Thus, power quality
1MRS757644 H
Power quality measurement functions
620 series
Technical Manual
1083
