The effects of harmonics ( full page )
From Electrical Installation Guide
Problems arising from power-system harmonics
Equipment which uses power electronics components (variable-speed motor controllers, thyristor-controlled rectifiers, etc.) have considerably increased the problems caused by harmonics in power supply systems.
Harmonics have existed from the earliest days of the industry and were (and still are) caused by the non-linear magnetizing impedances of transformers, reactors, fluorescent lamp ballasts, etc.
Harmonics on symmetrical 3-phase power systems are generally odd-numbered: 3rd, 5th, 7th, 9th..., and the magnitude decreases as the order of the harmonic increases. A number of features may be used in various ways to reduce specific harmonics to negligible values - total elimination is not possible. In this section, practical means of reducing the influence of harmonics are recommended, with particular reference to capacitor banks.
Capacitors are especially sensitive to harmonic components of the supply voltage due to the fact that capacitive reactance decreases as the frequency increases.
In practice, this means that a relatively small percentage of harmonic voltage can cause a significant current to flow in the capacitor circuit.
The presence of harmonic components causes the (normally sinusoidal) wave form of voltage or current to be distorted; the greater the harmonic content, the greater the degree of distortion.
If the natural frequency of the capacitor bank/ power-system reactance combination is close to a particular harmonic, then partial resonance will occur, with amplified values of voltage and current at the harmonic frequency concerned. In this particular case, the elevated current will cause overheating of the capacitor, with degradation of the dielectric, which may result in its eventual failure.
Several solutions to these problems are available. This can be accomplished by
- Shunt connected harmonic filter and/or harmonic-suppression reactors or
- Active power filters or
- Hybrid filters
|Harmonics are taken into account mainly by oversizing capacitors and including harmonic-suppression reactors in series with them|
(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
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.
for a 50 Hz system, or for a 60 Hz system.
For example: 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 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.
(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
(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
Choosing the optimum solution
Figure L31 below shows the criteria that can be taken into account to select the most suitable technology depending on the application.
|Passive filter||Active filter||Hybrid filter|
| Applications |
… with total power of non linear loads
(variable speed drive, UPS, rectifier…)
| greater than|
| lower than|
| greater than |
|Power factor correction||No|
|Necessity of reducing the harmonic distorsion in voltage for sensitive loads|
|Necessity of reducing the harmonic distorsion in current to avoid cable overload|
|Necessity of being in accordance with strict limits of harmonic rejected||No|
Fig. L31: Selection of the most suitable technology depending on the application
For passive filter, a choice is made from the following parameters:
- Gh = the sum of the kVA ratings of all harmonic-generating devices (static converters, inverters, speed controllers, etc.) connected to the busbars from which the capacitor bank is supplied. If the ratings of some of these devices are quoted in kW only, assume an average power factor of 0.7 to obtain the kVA ratings
- Ssc = the 3-phase short-circuit level in kVA at the terminals of the capacitor bank
- Sn = the sum of the kVA ratings of all transformers supplying (i.e. directly connected to) the system level of which the busbars form a part
If a number of transformers are operating in parallel, the removal from service of one or more, will significantly change the values of Ssc and Sn. From these parameters, a choice of capacitor specification which will ensure an acceptable level of operation with the system harmonic voltages and currents, can be made, by reference to Figure L32.
|Standard capacitors||Capacitor voltage rating increased by 10% (except 230 V units)|| Capacitor voltage rating increased by 10% + harmonic-suppression |
|Gh > 0.60Sn|
|Standard capacitors||Capacitor voltage rating increased by 10% (except 230 V units)||Capacitor voltage rating increased by 10% + harmonic suppression reactor|| Filters|
Fig. L32: Choice of solutions for limiting harmonics associated with a LV capacitor bank supplied via transformer(s)