## DESIGN OF AN ACTIVE DC FILTER FOR A HVDC SYSTEM: INTRODUCTION(part 7)

Us    harmonic voltage output of the converter
Zf(s)    impedance of the passive filter
Ls    impedance of the smoothing reactor
n: 1    transformer ratio of the coupling transformer
(n can be any number including fraction)
G    Gain
Is    harmonic current output of the converter circuit
IF    harmonic current output of the filtering branch
IL    harmonic current output of the transmission line

5.7.1 Design of DC voltage

To cancel all the harmonics the Dc voltage requirement can be given by the formula

Udc = -Ih.Zfh

Where Ih = harmonic current of the order h induced by the converter Zfh = impedance of the associated passive filter

where Usi = the harmonic voltage output of the converter

5.7.2 Design of F(s)

F(s ) can be designed by the following equations F(s) = X Ki.HFi(s)

HFi = 2^wi.s/(s2+2^wi.s+wi2)

where

Ki = Control co efficient of each harmonic term Wi = Passband center angular frequency ^i = Parameter to adjust the pass band width

5.8 Circuit Diagram

5.8.1 Circuit of the active filter without load

Figure 2

5.8.2 Circuit without Filter (Bipolar HVDC line)

Figure 3

5.8.3 Circuit Active Filter (Bipolar HVDC line)

Figure 4

5.9 Waveforms and Table

5.9.2 For T Network

5.9.2.1 fft of the line voltage without filter

Without Filter

waveform of the line voltage(top) and current (bottom) without filter

5.9.2.2 (fft of the line voltage with adf)

With Active Filter

waveform of the line voltage(top) and current(below) after adf

5.9.2.3 Comparison of the THD among circuits without filter, with passive filter and with ADF

5.9.3 For PI Network

5.9.3.1 fft of the line voltage without filter

waveform of the line voltage(top) and current (below) without filter

waveform of the line voltage(top) and current(bottom) with adf

5.9.3.2 fft of the line voltage with adf

5.9.3.3 Comparison of the THD among circuits without filter, with passive filter and with ADF

5.10 Comparison of the percentage of the harmonics on dc line voltage

5.10.1 T Network

5.10.2 Pi Network

With ADF1&2=scope of the active filter on the rectifier & inverter side of pole 1 in Fig5.9 respectively.

With ADF3&4= scope of the active filter on the rectifier & inverter side of pole 2 in Fig5.9 respectively.

Without filter1&2= scope of the circuit on the rectifier & inverter side of pole 1 in Fig5.7 respectively.

Without filter3&4= scope of the circuit on the rectifier& inverter side of pole 2 in Fig5.7 respectively.

The analysis has been carry forward in Mat Lab and Simulink for DC line generated from a 150KV ,39,50Hz AC system having the line impedance of

0.0Ш and 1mH.The 12 pulse converter system is used to find out the DC output for 45 degree firing angle. The 250km transmission line has been done for both T and PI network having 0.0015Q line resistance, 0.0752H line inductance and 1.42p,F capacitance per km for T network and 0.0258Q line resistance, 0.0752H line inductance and 0.0123^F line capacitance per km.

The load parameters are calculated by varying the line parameters and thereby giving the load to the line so that the line can be able to draw the current. Because in ideal case the line current has to be negligible as compared to the voltage developed.

5.11 CONCLUSIONS

As per the analysis given we can see that the active filter is able to reduce the THD of the output waveform appreciably than the normal passive filter used. The passive filter used in series with the active filter with a switch to isolate the active part in case of any short circuit due to fault. Some figures are taken as the design parameters whose values cannot be disclosed and are used due to confidentiality mentioned by the company.

5.12 FUTURE WORK

The Active filter has minimum limitation other than the initial installation cost. The future analysis can be done by analyzing the behavior of the circuit with different type of loading by varying the torque angle of the supply. In case the power transfer is required then the HVDC line is also helping to make the load draw current from the line. The behavior of that current can be studied .