The Effect Of Harmonics In A Power Distribution System; Harmonic Limitation Standards And Requirements - Danfoss AHF005 Design Manual

Introduction to Harmonics a...
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3.1.3 The Effect of Harmonics in a Power
Distribution System
The figure below shows an example of a small distribution
system. 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
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
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 harmon-
ic 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 , defined as the ratio between the
sce
short circuit apparent power of the supply at the PCC (S
the rated apparent power of the load (S
S ce
R sce
=
S equ
2
U
S sc = S equ =
where and
Z supply
AHF005/010 Design Guide
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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
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 250 kVA transformer with two 110 kW
motors connected. One is connected direct on-line and the
other one is supplied through a frequency converter. If the
direct on-line motor should also be supplied through a
frequency converter the transformer will, in this case, be
undersized. In order to retrofit, without changing the
transformer, the harmonic distortion from the two drives has
to be mitigated using AHF filters.
There are various harmonic mitigation standards, regulations
and recommendations. Different standards apply in different
geographical areas and industries. The following four
commonly encountered standards will be presented:
•
IEC61000-3-2
•
IEC61000-3-12
•
IEC61000-3-4
•
IEEE 519
•
G5/4
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