Allen-Bradley Powermonitor 3000 User Manual page 159

Allen-Bradley HMIs
increase the heat rise in a power transformer. The additional harmonic
heating may cause a transformer to exceed designed temperature
limits even though the RMS current is less than the transformer rating.
The K-Factor is used as justification to oversize a power transformer to
allow extra margin for harmonic losses or to select an appropriate
K-Factor rated transformer. A K-Factor rated transformer is the
preferred choice since it has known performance in the presence of
harmonics. The formula for K-Factor is as follows:
∞
⎛
Σ
H n
⎝
n = 1
---------------------------------------- -
K-Factor =
∞
(
Σ
n = 1
IEEE-519 TDD and IEEE-519 Pass/Fail
IEEE-519 is the IEEE standard for "Recommended Practices and
Requirements for Harmonic Control in Electrical Power Systems". The
1404-M6 refers to the 1992 version of this standard. IEEE-519 provides
recommended limits for the level of harmonic current injection at the
Point of Common Coupling (PCC) between the utility and the user.
The PCC is typically defined as the location in the power distribution
system where the utility meters are connected. The standard provides
recommended limits for individual harmonic components as well as a
limit for Total Demand Distortion (TDD). Total Demand Distortion is
defined as the root sum square of the current distortion expressed as a
percent of the maximum fundamental demand load current (based on
the maximum demand over the applicable demand interval). The
formula for computing TDD is the same as the IEEE THD formula
except the configured value for maximum fundamental load current is
substituted for the magnitude of the measured fundamental load
current.
∞
Σ
) 2
(
H
n
n = 2
------------------------ -
TDD =
H
1
Table 10.3 of the standard specifies the limits. The appropriate limits
are selected by computing the ratio of the available short circuit
current to the maximum fundamental demand load current. The row
of the table that corresponds to the ratio is then used to determine the
Where:
2 2 ⎞
•
n
⎠
• H
= magnitude of the n
n
harmonic (n ≤ 41 or 63).
2
)
H n
Where:
• H
= magnitude of the n
n
(n ≤ 41 or 63)
• H
= maximum fundamental load
1
current
Publication 1404-UM001D-EN-E - October 2004
Advanced Features 8-9
th
th
harmonic