A Four-Level T-Type Neutral Point Piloted Inverter for Solar Energy Applications
Multilevel inverters provide an output signal with low harmonic distortion and superior output voltages. This work proposes a new four-level T-type neutral point piloted (T-NPP) topology with higher efficiency and low total harmonic distortion (THD) and with the ability to withstand high voltage stresses, especially for high-power applications. The proposed topology is designed in such manner that the direct current (DC)-voltage stresses split over the components with strong possibilities to
... possibilities to increase the load current and switching frequency. However, the operation of the proposed topology is based on two essential principles. The first principle is that each upper and lower switch of each leg consists of two insulated gate bipolar transistors (IGBTs) connected in series in order to withstand high voltage stresses and make it split over the two IGBTs in each switch. The second principle is using the DC-link circuit (T1 & T2) to generate 2Vdc and 1Vdc by connecting the bidirectional switches of each leg to the DC-link's mid-point. Furthermore, the proposed four-level T-NPP inverter outperforms other converters by the high number of output voltage level, low number of components, simple structure and higher efficiency. Finally, the proposed T-NPP topology concept was validated via simulation, experiments and theoretical analysis. Energies 2018, 11, 1546 2 of 14 converter, and the three-level converter, such as low power losses and high-quality waveforms for the output voltages  . Furthermore, T-type converters can be used for medium-voltage applications [23, 24] by adding devices connected in series with the upper and lower switches in every single leg, in which special gate-derived units are needed for the voltage balancing and transient [25, 26] . For medium-voltage applications, T-type inverters are known to be neutral point piloted (NPP). The bidirectional switches of such inverters replace the clamping diodes that connect the points between the capacitors and the switches. The NPP structure is aimed at a medium voltage (3.3, 6.6, and 9.9 kV) and high power of up to 48 MW  . Furthermore, the series connection of the upper and lower switches allows this topology to operate under high frequencies and variable high-speed applications, and this topology is known as TCC  . In addition, the bidirectional switches in NPP topology are used to control the current path and to generate the zero-voltage level. However, this topology can only produce three-level output voltages using the capacitors which are connected to the input DC-link voltage. T-type converters can be used for low-voltage applications with better advantages compared to the other types of three-level topologies [28, 29] . Instead of a three-level NPC topology, an active bidirectional switch is employed to the DC-link voltage midpoint, thus serving as an alternative to increasingly complex three-level topologies      . Although a T-type converter comprises two switches connected in series, it exhibits very low switching losses and acceptable conduction losses because the bidirectional switches block only half of the DC-link voltage. In this paper, a T-NPP topology with less number of power components and zero-passive components such as capacitors to generate four levels of output voltages with low harmonic distortion and high efficiency is presented. The T-NPP topology is an extended version of the proposed four-level T-type topology presented in  which is originally proposed for low-voltage application. However, the T-NPP is proposed for high and medium voltage application and designed in such manner that the DC-voltage stresses split over the components with strong possibilities to increase the load current and switching frequency. In addition, the concept of T-NPP topology is based on the combination of the four-level T-type topology presented in  and separated DC-link which is designed to increase the output voltage level. In fact, the proposed topology can be used for high-power applications with less switching and conduction losses compared to the other topologies as presented in the following sections.