## 5 Discussion  
From Table 7, it was observed that the simulation results followed the calculated results. Also, the results obtained from the MC-HAPF showed a 600 kHz maximum HAPF switching frequency for a 50 kW MC with 6 kHz switching frequency. The implementation of the above active part of the hybrid filter will involve the use of high-switching transistors. An example will be paralleling two IPW60R120P7 or R6030 KN24 power MOSFETs for this purpose. Insulated gate bipolar transistors, IGBT's, are the most used transistors for inverter design for low frequencies (20 to 100) kHz and higher power applications, while metal-oxide-semiconductor field effect transistors, MOSFETs, are most suitable for low power and higher frequency (in MHz) range applications. However, there is the possibility of paralleling power MOSFETs for high power and frequency applications. From the above simulations, the following deductions were observed:  
1. The converter rated power is directly proportional to the maximum attainable active filter hysteresis switching frequency. 2. Again, the maximum hysteresis switching frequency increases with the decrease in the hysteresis bandwidth current. 3. Reference signals with rate of change close to the supraharmonic signal frequency reduces the maximum hysteresis switching frequency of the VSC. With these observations, the following recommendations are suggested. For higher-frequency applications (6-10 kHz), power MOSFETs are recommended for hysteresis active filter-switching applications. However, to prevent switching loses, it is recommended to maintain a converter rated power below 50 kW. In applications with higher power (above 50 kW) \begin{table}
\begin{tabular}{l r r r r} \hline \hline  & **Novel Equation** & \multicolumn{3}{c}{**Simulation Results**} \\ \hline MC power, \(\mathsf{S_{MC}}\) & 1 kW & 10 kW & 1 kW & 10 kW \\ Current ref, \(\mathsf{I_{f}}/\mathsf{A}\) & 1.44 & 14.44 & 14.4 & 14.4 \\ Trapezoidal & 131.9 kHz & 1.31 MHz & 140 kHz & 1.3 MHz \\ MC-HAPF & 136.74 kHz & 712 kHz & 130 kHz & 610 kHz \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Simulation results at hysteresis error current of 1 A.  
Figure 10: MC-HCC simulation results. and relatively lower converter switching frequencies (below 6 kHz), IGBTs are recommended. IGBTs are not recommended for higher frequency applications but are most recommended for lower switching frequency converters. Also, for EMI and oscillations control, the gate resistor of MOSFETS, irrespective of the connected load, needs to be carefully selected as the size of the gate resistors mostly affects the switching frequency of the power MOSFET. Table 8 shows an IGBT and MOSFET data sheet from Infineon and the ST catalog. Further information about the MOSFET switching frequencies is provided in Appendix A. The harmonic distribution of the MC coupled load was analyzed, and Table 9 shows the MC input current harmonic distribution obtained after the simulation without any filter, passive or hybrid, at a sampling time of \(4\times 10^{-7}\) with 50,000 samples per cycle. Without any filter, fast Fourier transform analysis (FFT) of the input current shows the total harmonic distortion of 57.49%, as seen in Figure 11a, with the dominant higher frequency harmonics occurring at the 155 th harmonic level, which is close to the MC switching frequency. This further proved the influence of the switching frequency of the MC on the grid. The introduction of the passive L-C filter, as the interface to the MC, reduced the THD of the MC input current to 20.82% with the dominant h155 harmonic reducing to 10% of the fundamental. The passive filter could be designed to achieve more attenuation, but this will increase the size and weight of the filter as well as the reactive power burden and possibly lead to resonance conditions. Figure 11b shows the FFT analysis of the input current when a passive L-C filter was employed.