DESIGN OF AN ACTIVE DC FILTER FOR A HVDC SYSTEM: INTRODUCTION(part 5)

INTRODUCTION(part 5)

The PWM converter is responsible for power processing in synthesizing the compensating current that should be drawn from the power system. The active filter controller is responsible for signal processing in determining the instantaneous compensating current reference, which is continuously passed to te PWM converter. The shunt active filter controller works in a closed loop manner, continuously sending the load current and calculating the instantaneous values of the compensating current reference for the PWM converter. In ideal case the PWM converter may considered as a linear power amplifier where the compensating current track correctly its reference. The PWM converter should have high switching frequency(fPWM) in order to reproduce accurately the compensating current. Normally fPWM > 10*fhPWM . where fhPWM is the frequency of the highest order of the harmonic that is to be compensated. The dc capacitor and IGBT with anti parallel diode are used to indicate a shunt active filter that is build up from a voltage source converter. In fact voltage source converter or current source converter can be used in shunt active filters. Now a days VSC are most widely used in the shunt active filters. IGBT modules are the most suitable for the VSC because freewheeling diode is connected antiparallel to each device, that means it need not provide the capability of reverse voltage blocking in itself thus bringing more flexibility to device design in a compromise between conducting and switching losses and short circuit capability than the reverse blocking IGBT

Active filter controller is a control algorithm implemented in the controller of the shunt active filter determines the compensation characteristics of the shunt active filter. There are many ways to design that algorithm one of them is the selective band pass filters in parallel.

Active filters are the type of filter which is using a transformer in series with the passive part. Basically the active filter is a combination of the passive and the active part. The active part is nothing but the source used to supply the transformer. The load current is taken as a feedback to a bank of selective band pass filter in which the specific harmonics those have to be eliminated is selected within the band. Then the signal is fed to the amplifier there by given to the pulse generator which will give corresponding triggering pulses to a single or multi phase inverter depending upon the application required. The inverter is giving supply to the transformer which will be in series with the passive part supplying a negative value of the same current flowing in the shunt branch, there by nullifying the effect of harmonics.

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