*4.2. Experimental Results*

The experimental PV system comprises of two 3L-TNPC power cells without isolation and were fed directly by one PV string. Each string was composed by two PV modules emulated with the Agilent E4360 solar array simulator, which enabled a total control of the temperature and irradiation parameters. Each simulator has two output channels connected in series to emulate the PV string generator. The parameters such as maximum power *Pm*, current at maximum power *imp*, short-circuit current *isc*, voltage at maximum power *vmp* and open-circuit voltage *voc* are listed in Table 2. The simplified layout of the experimental small-scale setup is depicted in Figure 11. The control algorithm is fully programmed in C code by using a dSPACE 1103 digital control platform running at 15 μs. The modulation stage and dead-time generation is implemented by using a FPGA Spartan3. To experimentally validate the proposed control scheme, three different operation points are evaluated. A steady-state operation, and two dynamic operation under an irradiation and a temperature step.

The first experimental results shown in Figures 12 and 13 are analyzed during steady-state operation. The grid-side variables i.e., grid current and voltage waveforms are presented in Figure 12, where the unitary power factor operation is achieved. Furthermore, Figure 13 captures the output voltage of each cell and the total voltage composed by a five-level waveform. Similar to the simulation results, the steady-state performance of the capacitor voltage balancing in Figure 14 is included.

**Figure 11.** Simplified diagram of implemented experimental setup.

**Figure 12.** Steady-state experimental results at grid-side variables: Grid current and voltage waveforms.

**Figure 13.** Steady-state experimental results at converter-side variables: Output voltage for each cell and total output voltage.

**Figure 14.** Steady-state experimental results of capacitor voltage across each 3L-TNPC cell.

The second set of experimental results during dynamic operation is shown in Figures 15 and 16. Firstly, an irradiation step from 1 kW/m<sup>2</sup> to 0.8 kW/m<sup>2</sup> was applied to the second PV cell (lower cell operating at reduced power), while the first array irradiation level was retained. After this irradiance step variation, a temperature step change was tested from 25 ◦C to 18 ◦C and applied to the first PV cell (upper cell operating at increased power). Under the above conditions, the input voltages generate a three-level waveform signal due to the use of the conventional P&O MPPT method. Note that under both scenarios, the coupling effects from one cell to each other is fully avoided, ensuring a decoupled operation between power cells.

**Figure 15.** Experimental dynamic operation of DC-link voltage under unbalanced power per string due to solar irradiation and temperature changes.

**Figure 16.** Experimental dynamic operation of DC power under unbalanced power per string due to solar irradiation and temperature changes.

#### *4.3. Brief Comparison with Other Five-Level T-type Converters*

A comprehensive comparison between three different five-level T-type converters have been presented in Table 3. The studied topologies are the proposed 5L-CTNPC, the hybrid version 5L-HTNPC and the conventional 5L-TNPC. Each topology presents the same features of the one described in simulation and experimental results. To evaluate the power efficiency of each converter, switching and conduction losses is required in respect to the power operation point for each cell. Semiconductor device losses are included with a thermal model library developed in PLECS, based on the manufacturer datasheet [30]. The resulting efficiency evaluation is depicted in Figure 17, where the obtained efficiency of the proposed configuration 5L-CTNPC is equal to the conventional 5L-TNPC. Due to the fact that there is a reduced number of switches at the second parallel leg in the 5L-HTNPC

a slightly higher efficiency was achieved. This analysis is corroborated by counting the number of semiconductor devices used for each evaluated topology, which is summarized in Table 3.

The symmetrical topology configuration and the multiple MPPT possibilities are the main advantages of the studied topology in respect to the rest power converters. Finally, in Figure 18 the current spectrum for each evaluated converter topology is computed. The current THD obtained with the proposed topology is similar to the conventional 5L-TNPC power topology, while the worst value (over 4.9%) was reached in the 5L-HTNPC configuration. In fact, the apparent switching frequency of this topology was equal to the carrier frequency, while in the proposed 5L-CTNPC and 5L-TNPC the apparent switching frequency was twice the switching frequency.


**Table 3.** Brief comparison between five-level T-type topologies.

**Figure 17.** Efficiency comparison between 5L-CTNPC, 5L-HTNPC and 5L-TNPC inverter topologies respect to the power operation.

**Figure 18.** Grid current FFT comparison between 5L-CTNPC, 5L-HTNPC and 5L-TNPC inverter topologies.
