## DESIGN OF AN ACTIVE DC FILTER FOR A HVDC SYSTEM: INTRODUCTION(part 6)

5.5 Limitations of the Active filter

• It is difficult to realize large VA rating PWM converter with rapid current response and low loss, which is the main circuit of the active filter

• The initial running cost is high as compared to the passive filters

• The injection current from the active filter may flow into the inductor and capacitors of the circuit.

This is one of the factor by which we can find out what amount of the reduction of harmonic content in a particular signal has occurred in percentage. The THD is always calculated taking reference to the fundamental component for ac as well as dc.

This can be given as

Where

Vh = harmonic voltage other than fundamental V1 = fundamental component of the voltage

Harmonic analysis is done w.r.t the fundamental component because

• The modulation and the demodulation of the same value has to be done in the conversion and the inversion process

• As Vh is represented in percentage of the fundamental component as(we are considering the harmonics in the multiples of fundamental) hence it will get cancelled out in the THD calculation.

• The average value of the fundamental component is zero for a full cycle.

(as the average value of the fundamental component is zero for a full cycle hence the fundamental component is not considered for the harmonic analysis of the signal)

5.6 Control Principle

• The dc circuit of the HVDC can be simplified into a circuit consisting of a converter harmonic EMF, smoothing reactor,dc filter with harmonic impedance, line impedances.[1]

• The output current will be sensed by a sensing element and fed to the feedback path containing the active filter consisting of a transformer , an amplifier and the control circuit.

• The control circuit contains a pwm generator and the pi controller (to provide proportional output)

• The active filter is represented as a voltage source .The line current generated by the active filter will give a negative current to the harmonic value there by it will cancel out the harmonic value.

• The FFT analysis of the signal without filter will give the amount of compensation required.

• As per that value the dc voltage to the control circuit and the no of turn is calculated.

Figure 1 : Block Diagram of the Active filter