A commonly used term for describing the impedance of a
grid is the short circuit ratio (R
sce
). R
sce
is 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 =
U2
Zsupply
and
Sequ =U×Iequ
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 online
and the other is supplied through an adjustable frequency
drive, a 250 kVA transformer with two 110 kW motors
connected is sufficient. However, the transformer will be
undersized if both motors are supplied with an adjustable
frequency drive. Using additional means of harmonic
reduction within the installation or choosing low harmonic
drive variants makes it possible for both motors to run
with adjustable frequency drives.
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
In cases 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
Choosing the right solution depends on several factors:
•
The grid (background distortion, line power
unbalance, resonance and type of supply
(transformer/generator)
•
Application (load profile, number of loads, and
load size)
•
Local/national requirements/regulations (IEEE519,
IEC, G5/4, etc.)
•
Total cost of ownership (initial cost, efficiency,
and maintenance)
7.9.4 Harmonic Calculation
Use the free Danfoss MCT 31 calculation software to
determine the degree of voltage pollution on the grid and
needed precaution. The
VLT
®
Harmonic Calculation MCT 31
is available at
www.danfoss.com
.
7.10 Residual Current Device
Use RCD relays, multiple protective grounding, or
grounding as extra protection, provided they comply with
local safety regulations.
If a ground fault appears, a DC current may develop in the
faulty current. If RCD relays are used, local regulations
must be observed. Relays must be suitable for protection
of 3-phase equipment with a bridge rectifier and for a brief
discharge on power-up. See
for more details.
Electrical Installation
Design Guide
MG34S222
Danfoss A/S © Rev. 2014-02-10 All rights reserved.
239
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