Possible solutions for power-system harmonics
| Harmonics are taken into account mainly by oversizing capacitors and including harmonic-suppression reactors in series with them |
Passive filter
(see Fig. L28)
Countering the effects of harmonics
The presence of harmonics in the supply voltage results in abnormally high current levels through the capacitors. An allowance is made for this by designing for an r.m.s. value of current equal to 1.3 times the nominal rated current. All series elements, such as connections, fuses, switches, etc., associated with the capacitors are similarly oversized, between 1.3 to 1.5 times nominal rating.
Harmonic distortion of the voltage wave frequently produces a “peaky” wave form, in which the peak value of the normal sinusoidal wave is increased. This possibility, together with other overvoltage conditions likely to occur when countering the effects of resonance, as described below, are taken into account by increasing the insulation level above that of “standard” capacitors. In many instances, these two counter measures are all that is necessary to achieve satisfactory operation.
Fig. L28: Operation principle of passive filter
Countering the effects of resonance
Capacitors are linear reactive devices, and consequently do not generate harmonics. The installation of capacitors in a power system (in which the impedances are predominantly inductive) can, however, result in total or partial resonance occurring at one of the harmonic frequencies.
The harmonic order ho of the natural resonant frequency between the system inductance and the capacitor bank is given by
[math]\displaystyle{ h_o=\sqrt{\frac{Ssc}{Q}} }[/math]
where
Ssc = the level of system short-circuit kVA at the point of connection of the capacitor
Q = capacitor bank rating in kvar; and ho = the harmonic order of the natural frequency fo i.e.
[math]\displaystyle{ \frac{f_o}{50} }[/math] for a 50 Hz system, or [math]\displaystyle{ \frac{f_o}{60} }[/math] for a 60 Hz system.
For example: [math]\displaystyle{ h_o=\sqrt{\frac{Ssc}{Q}} }[/math] may give a value for hoof 2.93 which shows that the natural frequency of the capacitor/system-inductance combination is close to the 3rd harmonic frequency of the system. From [math]\displaystyle{ h_o=\frac{f_o}{50} }[/math] it can be seen that fo = 50 ho = 50 x 2.93 = 146.5 Hz
The closer a natural frequency approaches one of the harmonics present on the system, the greater will be the (undesirable) effect. In the above example, strong resonant conditions with the 3rd harmonic component of a distorted wave would certainly occur.
In such cases, steps are taken to change the natural frequency to a value which will not resonate with any of the harmonics known to be present. This is achieved by the addition of a harmonic-suppression inductor connected in series with the capacitor bank.
On 50 Hz systems, these reactors are often adjusted to bring the resonant frequency of the combination, i.e. the capacitor bank + reactors to 190 Hz. The reactors are adjusted to 228 Hz for a 60 Hz system. These frequencies correspond to a value for ho of 3.8 for a 50 Hz system, i.e. approximately mid-way between the 3rd and 5th harmonics.
In this arrangement, the presence of the reactor increases the fundamental frequency (50 Hz or 60 Hz) current by a small amount (7-8%) and therefore the voltage across the capacitor in the same proportion.
This feature is taken into account, for example, by using capacitors which are designed for 440 V operation on 400 V systems.
Active filter
(see Fig. L29)
Active filters are based on power electronic technology. They are generally installed in parallel with the non linear load.
Active filters analyse the harmonics drawn by the load and then inject the same harmonic current to the load with the appropriate phase. As a result, the harmonic currents are totally neutralised at the point considered. This means they no longer flow upstream and are no longer supplied by the source.
A main advantage of active conditioners is that they continue to guarantee efficient harmonic compensation even when changes are made to the installation. They are also exceptionally easy to use as they feature:
- Auto-configuration to harmonic loads whatever their order of magnitude
- Elimination of overload risks
- Compatibility with electrical generator sets
- Connection to any point of the electrical network
- Several conditioners can be used in the same installation to increase depollution efficiency (for example when a new machine is installed)
Active filters may provide also power factor correction.
Fig. L29: Operation principle of active filter
Hybrid filter
(see Fig. L30)
This type of filter combines advantages of passive and active filter. One frequency can be filtered by passive filter and all the other frequencies are filtered by active filter.
Fig. L30: Operation principle of hybrid filter
