Unipolar output voltage is achieved by disconnecting the PV array from the grid during the period of the zero voltage state, using a method called DC decoupling. dcCABT1T2T3T4dcCFilterT5Fig. 6. H5 bridge topology for PV converter .
To keep high efficiency and keep all the advantages given by the unipolar PWM, but still have the common-mode behavior as in case of the bipolar PWM, the H-Bridge topology may be modified , the Highly Efficient and Reliable Inverter Concept (HERIC), as shown in Fig. 7. The modification includes two extra switches (T5-T6) each connected in series with a diode. There will be no high frequency fluctuations present at the DC terminals of the PV array. The common-mode behavior of the HERIC topology is similar to the H-Bridge with bipolar PWM. The voltage to ground of the PV array terminals will only have a sinusoidal shape, while having the same high conversion efficiency as the H-Bridge with unipolar switching. Based on these results, it can be stated that the HERIC topology is suitable for transformer-less PV systems. dcCABT1T2T3T4dcCT5T6FilterFig. 7. HERIC topology for PV converter . The single phase three-level Neutral Point Clamped (NPC) topology can also be used in the photovoltaic application. The connection of the neutral line of the middle point of the DC-link fixes the potential of the PV array to the grounded neutral . Fig. 8 shows this topology. The NPC topology is suitable for transformer-less PV systems, since the voltage to ground is constant in the case of both terminals of the PV. The only drawback for the single-phase NPC topology is the higher DC-link voltage, which has more than twice the grid peak voltage and might reach voltages higher than the allowed maximum system voltage, therefore a boost stage may be needed before the inverter, which decreases the overall efficiency of the whole PV system. Fig. 8. 3L-NPC topology for PV converter . . III.RELIABILITY ISSUES FOR POWER ELECTRONICS IN WIND TURBINESThere are several significant trends for the development of wind power generation system in the last few decades: The penetration of wind power into the power grid is growing even expected to be 20% of the total electricity production at 2020 in Europe . Meanwhile, the power capacity of a single wind turbine is increasing continuously in order to reduce the price pr. produced kWh. Moreover the location of wind farms is moving from onshore to offshore because of land limits and potentially richer wind energy resources. Consequently, due to much more significant impacts to the power grid and higher cost to repair after failures, the wind power generation system is required to be more reliable and able to withstand extreme grid/environment disturbances.