09-05-2013, 03:38 PM
Problems arising from power-system harmonics
[attachment=53657]
INTRODUCTION
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.
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.
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.
[attachment=53657]
INTRODUCTION
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.
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.
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.