Eaton PowerXL DC1, Manual

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2 Engineering
2.2 Electrical power network
32
DC1 Frequency Inverter 10/12 MN04020003Z-EN www.eaton.com
If this condition is not fulfilled, or symmetry at the connection location is not
known, the use of an assigned main choke is recommended (see „Appen-
dix“, section “Mains chokes”, Page 180).
2.2.4 Total Harmonic Distortion (THD)
Non-linear consumers (loads) in an AC supply system produce harmonic volt-
ages that again result in harmonic currents. These harmonic currents at the
inductive and capacitive reactances of a mains supply system produce addi-
tional voltage drops with different values which are then overlaid on the sinu-
soidal mains voltage and result in distortions. In supply systems, this form of
"noise" can give rise to problems in an installation if the sum of the harmonics
exceeds certain limit values.
Non-linear consumers (harmonics producers) include for example:
• Induction and arc furnaces, welding devices,
• Current converters, rectifiers and inverters, soft starters, frequency
inverters,
• Switched-mode power supply units (computers, monitors, lighting),
uninterruptible power supplies (UPS).
The THD value (THD = Total Harmonic Distortion) is defined in standard
IEC/EN 61800-3 as the ratio of the rms value of all harmonic components to
the rms value of the fundamental frequency.
For example, the THD for a current is:
Where I
1
is the rms value of the fundamental frequency current and n is the
order of a harmonic with its own frequency, which is an integer multiple of
the fundamental frequency (Fourier analysis).
Example: 5th harmonic of a mains frequency of 50 Hz : 5 x 50 Hz = 250 Hz.
The THD value of the harmonic distortion is stated in relation to the rms
value of the total signal as a percentage. On a frequency inverter, the total
harmonic distortion is around 120 %. A mains choke (such as 4 % u
k
) on the
supply side of a frequency inverter enables the THD value with a single-phase
supply (B2 diode rectifier bridge) to be reduced to around 80 % and with a
three-phase supply (B6 diode rectifier bridge) to around 50 %.
The supply quality is thus improved and the mains supply distortion is
reduced. The power factor is also improved.
￫
In the project planning for the connection of three-phase sup-
plied frequency inverters (DC1-3…), consider only AC supply
systems that handle permitted asymmetric divergences in the
mains voltage
≦ +3 %.
THD
n 2
xx
=
I
1
----------------------- =
∞
n=2
Σ I
n
2