2.2 Long-term effects
Over and above mechanical fatigue due to produced (see fig. 3 ). Values of around 10-4
vibrations, the main long-term effect of may be cited for tanδ. The heat produced can harmonics is heating.
lead to dielectric breakdown.
Capacitor heating
The losses causing heating are due to two phenomena: conduction and dielectric
δ
hysteresis.
As a first approximation, they are proportional to P
Q
S
tanδ =
the square of the rms current.
Q
Capacitors are therefore sensitive to overloads, whether due to an excessively high fundamental or to the presence of voltage harmonics.
P
These losses are defined by the loss angle δ of Fig. 3 : triangle relating to the capacitor powers the capacitor, which is the angle whose tangent (active (P), reactive (Q), apparent (R)).
is the ratio of the losses to the reactive power Cahier Technique Schneider Electric no. 152 / p.6
Heating due to additional losses in machines shielding earthed at both ends of the cable, etc.
and transformers
Calculations for steady state can be carried out c additional losses in the stators (copper and as described in IEC 60287.
iron) and principally in the rotors (damping Generally speaking, all electrical equipment windings, magnetic circuits) of machines caused (electrical switchboards) subjected to voltage by the considerable differences in speed between harmonics or through which harmonic currents the harmonic inducing rotating fields and the flow, exhibit increased energy losses and should rotor. Note that rotor measurements (temperature, be derated if necessary.
induced currents) are difficult if not impossible, For example, a capacitor feeder cubicle should c supplementary losses in transformers due to be designed for a current equal to 1.3 times the the skin effect (increase in the resistance of reactive compensation current. This safety factor copper with frequency), hysteresis and eddy does not however take into account the
currents (in the magnetic circuit).
increased heating due to the skin effect in the conductors.
Heating of cables and equipment
Harmonic distortion of currents and voltages is Losses are increased in cables carrying
measured using spectrum analysers, providing harmonic currents, resulting in temperature rise.
the amplitude of each component.
The causes of the additional losses include: It is important to use current or voltage sensors c an increase in the rms value of the current for having a sufficient band width for the measured an equal active power consumed;
frequencies.
c an increase in the apparent resistance of the The rms value of the distorted current (or core with frequency, due to the skin effect; voltage) may be assessed in any of three ways: c an increase in dielectric losses in the insulation c measurement using a device designed to give with frequency, if the cable is subjected to non-the true rms value,
negligible voltage distortion;
c reconstitution on the basis of the spectrum c phenomena related to the proximity of
provided by spectral analysis,
conductors with respect to metal cladding and c estimation from an oscilloscope display.
Cahier Technique Schneider Electric no. 152 / p.7
3 Acceptable limits, recommendations and standards 3.1 General limits
c synchronous machines: permissible stator c power capacitors: current distortion = 83%, current distortion = 1.3 to 1.4%;
corresponding to an overload of 30% (1.3 times the rated current); overvoltages can reach up to 10%; c asynchronous machines: permissible stator current distortion = 1.5 to 3.5%;
c sensitive electronics: 5% voltage distortion with c cables: permissible core-shielding voltage a maximum individual harmonic percentage of distortion = 10%;
3% depending on the equipment.
3.2 Standardised limits
The series of standards (IEC 61000) for
c IEC 61000-2-4 which defines compatibility electromagnetic compatibility define certain limits levels in industrial networks.
concerning harmonics, mainly:
We would remind that compatibility level does c IEC 61000-3-2 which define the limits of not define an absolute limit. There remains some harmonic emissions for equipment consuming less probability to be slightly beyond the fixed level.
than 16 A per phase (except for certain category of equipment indicated in the standards).
Another standard, EN 50160, gives the
The case of equipment consuming over 16 A per characteristics of voltage supplied by a utility phase is examined in the technical spec. IEC/TS
network.
61000-3-4 and should finally be determined by In France, EDF proposes a contract for large the projected standards IEC 61000-3-12.
consumers called "Emeraude" which consists in c IEC 61000-2-2 which defines compatibility a reciprocal commitment: a quality commitment levels for harmonic voltages in public LV power by EDF against limitation of pollution due to the supply systems (see fig.4 ).
consumer.
Odd harmonics
Odd harmonics
Even harmonics
non multiples of 3
multiples of 3
Harmonic
Harmonic
Harmonic
Harmonic
Harmonic
Harmonic
order
voltage
order
voltage
order
voltage
n
%
n
%
n
%
5
6
3
5
2
2
7
5
9
1.5
4
1
11
3.5
15
0.3
6
0.5
13
3
21
0.2
8
0.5
17
2
>21
0.2
10
0.5
19
1.5
12
0.2
23
1.5
>12
0.2
25
1.5
>25
0.2+0.5x25/n
Fig. 4 : compatibility level for individual harmonic voltages in low voltage distribution networks (IEC 61000-2-2).
Cahier Technique Schneider Electric no. 152 / p.8
4 Harmonics generators In industrial applications, the main types of c lighting,
equipment that generate harmonics are:
c saturated reactors,
c static converters,
c other equipment, such as rotating machines c arc furnaces,
which generate slot harmonics (often negligible).
4.1 Static converters on 3-phase networks Rectifier bridges and, more generally, static a Graetz bridge requires a rectangular pulsed converters (made up of diodes and thyristors) AC current when the load is highly inductive generate harmonics.
(see fig. 5 ), or tips when the bridge is followed For instance, to deliver a perfect DC current, by a capacitor (see fig. 6 ).
Load
I
I
T
T
t
t
T/6
T/3
T/6
Rectifier supply phase current
Phase current upstream from a delta-star connected transformer supplying the rectifier Fig. 5 : alternating current upstream of a Graetz bridge rectifier delivering a perfect direct current on a highly inductive load.
i1
i1
u1
u1
i2
u2
C
Load
i3
u3
Fig. 6 : alternating current upstream of a Graetz bridge rectifier followed by a capacitor.
Cahier Technique Schneider Electric no. 152 / p.9