5. Filters

To reduce the harmonic content filters are used which are of two types such as
• Passive filter
• Active filters

5.1 Types of passive filters High pass filter

A high pass filter is the type of filter which pass all the frequency above the cutoff frequency specified. The filter characteristics equation can be H(s)=s CR/(1+sRC) Band pass filter

A band pass filter is the type of filter which pass all the frequency within the band of the cutoff frequency specified. It is the combination of a high pass and low pass filter. The filter characteristics equation can be the combination of a low pass and a high pass filter that is H(s)=Ks/(s2+Ks+w2).

Single tuned filter are the type of filter which has been tuned to a single frequency, Similarly Double tuned filters can be tuned simultaneously to two specific frequencies. Same as the multi-tuned filter.

5.2 Selection of the passive filter

Selection of passive filter is a trivial task for any system. The method involve the percentage of harmonics in the signal without the filter and the required value with the filter. Normally a HVDC system has smoothening reactor on both side(rectifier and the inverter side).Hence the formula used for the calculation of the filter impedance can be given as

Zf=Vo*Zs/(Vs-Vo) (2)

Zf = filter impedance
Zs = impedance of the smoothing reactor
Vo = output voltage
Vs = input/supply voltage

All the components can be calculated in the Laplace Transform form for better calculation. If the total value of the harmonic has to be removed then a LC filter can be used. But practically pure LC filter is not present some lumped resistance value will be there in the component. The formula holds good for the single harmonic content in the supply voltage but for more than one harmonic component in the supply voltage there will be interference in the filter output. Hence it has been observed that the selective harmonics are getting decreased but there will be generation of some more even as well as odd harmonics. Again the more no of filters means there will be chances of spikes in the output voltage as well as current waveform. Again it is also difficult to design for more than 2 harmonics because the calculation will be complicated and time consuming.

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