Power Transmission and distribution networks are designed to operate with sinusoidal voltage and current having constant frequency. However there are number of non -linear loads, such as thyristor drives and converters, that generate harmonics on the network, causing distortion in the voltage and current waveforms. These harmonic filters are used when the main objective is not the reactive power compensation at the fundamental frequency, but to reduce the harmonic distortion in the supply system.
Active Harmonic Filter
Product Features: For real - time compensation of harmonic current and reactive power. There are several electrical equipment with non-linear voltage-current characteristics connected to the network. Harmonic currents produced by this cause harmonic voltages in network impedance, which add to the fundamental system voltage resulting in voltage distortion.
Benefits to Customers:
Tuned harmonic filter are used when it is necessary to reduce the harmonic voltage and current distortion in the supply system. Technical characteristics:
|Rated Voltage||230 V, 400 V, 440 V, 660 V|
|Network Frquency||50 Hz (60 Hz)|
|Filter Tuning||5th, 7th, 11th, 13th|
|Regulation||By current or by harmonics|
|Degree of protection||IP20|
|Standards||IEC 60831, IEC 60439, IEC 61642|
Detuned filter are used, in supply networks having a high level of harmonic distortion, when the final objective is reactive power compensation at the fundamental frequency. Their purpose is to avoid the harmonic currents overload to the capacitors by diverting them to the mains. Protection filters are made by connecting reactors in series with capacitors, in such a way that the tuning frequency of the whole unit is set at a value between the fundamental frequency and the frequency of the lowest present harmonics, which is usually the 5th order harmonic. In this the filter has a high inductive impedance for all the harmonic frequencies.
Connection of a reactor in series with a power capacitor, makes the capacitor to work at a voltage higher than the supply voltage. Because of this, capacitors need to be connected to protection reactors and need to be designed to work at higher voltages than standard capacitors. The choice of the tuning point of the filters is to balance between the quantity of harmonics rejected by the filter and the voltage increase produced in the capacitor at the fundamental frequency.
It is to be also kept in mind that reactive power supplied by the filter at rated frequency (50 or 60 Hz), is different to the one that the capacitor would supply without the reactor. Taking all the above into account, the reactor is normally chosen in such a way that its impedance is about 7% of the impedance of the capacitor that protects. This will give a tuning frequency, for example at 50 Hz, of 189 Hz. Other tuning frequencies are also available.
Third harmonic filter is basically passive filters with a parallel combination of inductance and condenser. The advantages of third harmonic filter are:
|Phase-neutral tension||Up to 750 V|
|Frequency||50 / 60Hz|
|(IR)Nominal current (IR)||6-100A|
|Current max. transitory||(1 min.of each 10 min.)|
|Degree of protection||IP 00/IP21|
|Color/Colour||Gris/Grey RAL 7035|
|Temperature range||-10º C + 50ºC|
Desciption: In installations with 3rd harmonic generation loads, the current through the neutral cable can be higher than the one in the phases, even in case of balanced loads. The most proper solution for these problems is to use a specific equipment for the 3rd harmonic filtering, consisting basically in a three-phase isolating transformer, delta-star connected, that filters the 3rd harmonic current, plus a passive filter which reduces the 5th harmonic.