 ## DESIGN OF AN ACTIVE DC FILTER FOR A HVDC SYSTEM: INTRODUCTION(part 2)

4. Harmonics

Harmonics are generated both in AC side as well as DC side of the converter. As we can see that getting pure DC is difficult. Whatever DC we are getting that can be obtained by using a converter, which can be 6,12,24 or 32 pulse. The more the no of pulse, the less is the ripple content but at the same time it will be more complex.

As per the analysis made the harmonic content of a particular converter in the DC side will be np, where

n=the no of harmonics and p=no of pulses

That means there is the chance of occurrence of even harmonics in the output of the converter which has to be removed.

4.1 Causes of Harmonics

• It can be caused by any electrical equipment that uses switched mode power supply.

• Due to improper switching of the converter switches, on linear loads, faults, etc.

4.2 Results of Harmonics

• Harmonic current can cause overheating of the distribution system.

• It will cause voltage distortion reducing the amount of voltage to the load circuit.

• It will add to the loss component of the system.

• Interference in telephone lines.

4.3 Harmonic Current Measurement

The load current is measured with the help of a current sensor made up of a Rogowski Coil mounted inside the aluminum case which provides the shielding and means for the connection to the line. The measured signal is digitized and transmitted to the control equipment by fiber optics link with a normal frequency range up to 3000Hz.

4.4 FFT Method (Fast Fourier Transform)

As we know that the Fourier transform method is used to separate out the dc and the ac components in a particular signal. The FFT method reduces the duplicate terms in the mathematical algorithm to reduce the mathematical operation required. It also reduces the computation factor thereby saving the time and more accurate. where
Id(t) = dc component of the signal selected Iload (t) = load component of the signal Ih = harmonic component of the signal
Hence in this method after integrating and subtracting the dc component from the load value we can find out the harmonic component of the signal. 