The Effect Of Harmonics In A Power Distribution System; Harmonic Limitation Standards And Requirements; Harmonic Mitigation - Danfoss VLT AutomationDrive FC 300 Design Manual

Electrical Installation
To ensure low harmonic currents, the frequency converter
is equipped with intermediate circuit coils as standard. DC-
coils reduce the total harmonic distortion (THD) to 40%.
7.9.1 The Effect of Harmonics in a Power
Distribution System
In Illustration 7.63, 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
common coupling where all loads are connected together
is PCC2. Each load is connected through cables that have
an impedance Z
7 7
Illustration 7.63 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 affects apparatus performance and the
individual load. Voltage distortion affects system
performance. It is not possible to determine the voltage
distortion in the PCC knowing only the load's harmonic
performance. 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
ratio between the short circuit apparent power of the
supply at the PCC (S
the load (S ).
equ
S ce
R sce =
S equ
216
VLT
and feeds a number of loads. The point of
, Z , Z .
1 2 3
). R is defined as the
sce sce
) and the rated apparent power of
sc
®
AutomationDrive FC 300 Design Guide 90-1200 kW
where
S sc =
The negative effect of harmonics is two-fold
•
Harmonic currents contribute to system losses in
cabling and the, transformer
•
Harmonic voltage distortion causes disturbance
to other loads and increases losses in other loads
7.9.2 Harmonic Limitation Standards and
Requirements
The requirements for harmonic limitation can be
•
Application specific
•
Standards that must be observed
The application specific requirements are related to a
specific installation where there are technical reasons for
limiting the harmonics.
Example: If one of the motors is connected directly on-line
and the other is supplied through a frequency converter, a
250 kVA transformer with 2 110 kW motors connected is
sufficient. However, the transformer will be undersized if
both motors are frequency converter supplied. Using
additional means of harmonic reduction within the instal-
lation or choosing low harmonic drive variants makes it
possible for both motors to run with frequency converters.
There are various harmonic mitigation standards,
regulations, and recommendations. The following
standards are the most common:
•
IEC61000-3-2
•
IEC61000-3-12
•
IEC61000-3-4
•
IEEE 519
•
G5/4
See Harmonic Filter AHF 005/010 for VLT 5000 for specific
details on each standard.

7.9.3 Harmonic Mitigation

Where additional harmonic suppression is required,
Danfoss offers the following mitigation equipment:
•
VLT 12-pulse drives
•
VLT AHF filters
•
VLT Low Harmonic Drives
•
VLT Active Filters
MG34S202 - Rev. 2013-08-19
U 2
and
S equ = U × I equ
Z supply