Critical Thinking

A of the capacitors are measured by the

A 30MWsimulation model was designed to verify the effectiveness and demonstrate the working of MMC. The proposed MMC is simulated with the MATLAB software based on the diagram shown Fig.2. The results are obtained with the Sinusoidal pulse width modulation is presented in Fig.3.The parameters of the simulated system are presented in Table. I. Although HVDC transmission system are of several hundreds of volts and hundreds of megawatts, thus the simulation model was limited to lower power and voltage levels. In these results, the arm currents and output phase voltages are measured by the sensing devices, the individual voltages of the capacitors are measured by the local voltages sensors and the DC voltage is considered constant with 25kv and 15kv respectively.Fig.3 (a) shows the primary and secondary phase voltages to demonstrate the steady state performance operating with 450Hz. As can be seen, the observed top valve voltage and bottom valve voltages contain low harmonics with better amplitude shown in Fig.3 (b). According to the design specifications from Table .I the observed currents for primary side and secondary side of the transformer are shown in Fig.3. (c). On the other hand, Fig.3 (d) shows the top arm current and bottom currents. In addition, Fig.3 (d) shows the positive pole current and negative pole current. The comparison for 50Hz and 450 Hz is stated in Table II; cell capacitance for a 50 Hz model was defined as 7mF for HVDC1 and HVDC2. As the number of cell circulations was kept constant for both 50 and 450Hz ac frequencies, thus the cell capacitance for a 450Hz model is reduced to 5-7 times of 50Hz model. Next, the arm inductances are chosen arbitrarily in such a way to maintain the current control for the converters without additional switches by the cells. In this result, Fig.3 (e) demonstrates the   Fig.3.Voltages and currents for a 30MW system for a 450Hz ac link, for a half bridge MMC individual Leg currents.    effectiveness of the proposed converters by observing the average cell voltage, minimum cell voltage and maximum voltages. This performance is suitable to guarantee to the control in voltages, since the mean voltages are controlled.CONCLUSIONThe proposed MMC system is suitable for interconnection of HVDC networks or links operating with non-fundamental frequencies and at different voltage levels. This proposed MMC consists of tow front-to-front VSCs coupled by the transformer. Thus the ac link is internally dc/ac/dc system and therefore the switching frequency as well as operating frequency is chosen in such a way to tradeoff the volume of the transformer and cell capacitors.This case study considered half-bridge modular multilevel converter. When the same is   working with 50Hz, the cell capacitors require approximately 50% of the total cell volume with lower power losses. An increase in frequency results in reduces the capacitance required in converters with a penalty in losses.

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