The Effect Of Harmonics In A Power Distribution System; Harmonic Mitigation Standards And Requirements; Application-Specific Requirements - Danfoss VLT AHF 005 Design Manual

Introduction to Harmonics a...
h 2
I
max
∑
h
TDD = × 100 %
I
L
h = 2
Another term often encountered is the partial weighted
harmonic distortion (PWHD). PWHD is a weighted
harmonic distortion that contains only the harmonics
between the 14 th and the 40 th
definition.
2
I
40
∑
h
PWHD = × 100 %
I
1
h = 14
2.1.3 The Effect of Harmonics in a Power
Distribution System
In Illustration 2.4, 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 and feeds several loads. PPC 2 is the point of common
xfr
coupling where all loads are connected. Each load is
connected through cables that have an impedance Z
Z .
3
Illustration 2.4 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 harmonic
performance of the load. To predict the distortion in the
PCC, the configuration of the distribution system and
relevant impedances must be known.
MG80C502
Design Guide
, as shown in the following
, Z ,
1 2
Danfoss A/S © 10/2016 All rights reserved.
A commonly used term for describing the impedance of a
grid is the short-circuit ratio R
the ratio between the short circuit apparent power of the
supply at the PCC (S ) and the rated apparent power of
sc
the load (S ).
equ
S
ce
R =
sce S
equ
2
U
where and S = U × I
S =
equ
sc Z
supply
The negative effect of harmonics is twofold
•
Harmonic currents contribute to system losses (in
cabling and transformer).
•
Harmonic voltage distortion causes disturbance
to and increases losses in other loads.
Non-linear
System
Current
Impedance
Contribution to
system losses
Illustration 2.5 The Negative Effect of Harmonics
2.2 Harmonic Mitigation Standards and
Requirements
The requirements for harmonic limitation can be:
•

Application-specific requirements.

•
Requirements from standards that have to be
observed.
2.2.1 Application-specific Requirements
The application-specific requirements are related to a
specific installation where there are technical reasons for
limiting the harmonics.
Example
Two 110 kW motors are connected to a 250 kVA
transformer. One motor is connected direct online, and the
other is supplied through a frequency converter. If the
direct online motor should also be supplied through a
frequency converter, the transformer is, in this case,
undersized. To retrofit without changing the transformer,
mitigate the harmonic distortion from the 2 frequency
®
converters by using VLT Advanced Harmonic Filters
AHF 005/AHF 010.
. This ratio is defined as
sce
equ
Voltage
Disturbance to
other users
11
2 2