Danfoss VLT AHF005, Руководство по конструкции

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In non-linear systems the relationship between true power
factor and displacement power factor is:
PF =
DPF
1 +
THD 2
The power factor is decreased by reactive power and
harmonic loads. Low power factor results in a high RMS
current that produces higher losses in the supply cables and
transformers.
In the power quality context, the total demand distortion
(TDD) term is often encountered. The TDD does not charac-
terize the load, but it is a system parameter. TDD expresses
the current harmonic distortion in percentage of the
maximum demand current I
L
.
TDD =
∑
h=2
hmax
(
Ih
IL
)
2
× 100 %
Another term often encountered in literature is the partial
weighted harmonic distortion (PWHD). PWHD represents a
weighted harmonic distortion that contains only the
harmonics between the 14th and the 40th, as shown in the
following definition.
PWHD =
∑
h=14
40
(
Ih
I1
)
2
× 100 %
3.1.3 The Effect of Harmonics in a Power
Distribution System
In Illustration 3.1 a transformer is connected on the primary
side to a point of common coupling PCC1, on the medium
voltage supply. The transformer has an impedance Z
xfr
and
feeds a number of loads. The point of common coupling
where all loads are connected together is PCC2. Each load is
connected through cables that have an impedance Z
1
, Z
2
,
Z
3
.
Illustration 3.1 Small Distribution System
Harmonic currents drawn by non-linear loads cause
distortion of the voltage because of the voltage drop on the
impedances of the distribution system. Higher impedances
result in higher levels of voltage distortion.
Current distortion relates to apparatus performance and it
relates to the individual load. Voltage distortion relates to
system performance. It is not possible to determine the
voltage distortion in the PCC knowing only the load’s
harmonic performance. In order to predict the distortion in
the PCC the configuration of the distribution system and
relevant impedances must be known.
A commonly used term for describing the impedance of a
grid is the short circuit ratio R
sce
, defined as the ratio
between the short circuit apparent power of the supply at
the PCC (S
sc
) and the rated apparent power of the load
(S
equ
).
Rsce =
Sce
Sequ
where
Ssc =
U 2
Zsupply
and
Sequ = U × Iequ
The negative effect of harmonics is twofold
•
Harmonic currents contribute to system losses (in
cabling, transformer)
•
Harmonic voltage distortion causes disturbance to
other loads and increase losses in other loads
Non-linear
Current Voltage
System
Impedance
Disturbance to
other users
Contribution to
system losses
130BB541.10
3.2 Harmonic Limitation Standards and
Requirements
The requirements for harmonic limitation can be:
•
Application specific requirements
•
Requirements from standards that have to be
observed
The application specific requirements are related to a specific
installation where there are technical reasons for limiting the
harmonics.
For example on a 250kVA transformer with two 110kW
motors connected. One is connected direct on-line and the
other one is supplied through a frequency converter. If the
Introduction to Harmonics a... AHF005/010 Design Guide
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