**1. Introduction**

Rooftop photovoltaic (PV) energy conversion systems (less than 20 kW), have become a well-established technology in the industry. The most common configurations for single-phase grid-connected PV systems commercially found are the string, multistring and ac-module integrated topologies. Central and string inverters have been widely applied to manage and control PV energy systems [1]. Among the string topologies, the transformerless H5, H6, HERIC, neutral point clamped (NPC) and T-type NPC converters have been successfully commercialized [2]. In fact, multilevel inverters (MLI) are designed to produce a stepped voltage waveform by reducing the Total Harmonic Distortion (THD) and the voltage stress across semiconductor devices. Secondly, reduction of the

output filter size and power footprint also permit an important improvement in terms of costs, weight and efficiency [3]. These technical features have led to the massive adoption of MLI over the last thirty years for high-power medium voltage (MV) motor drive applications. In the last years, three-level neutral point clamped (3L-NPC) converters have been used for interfacing PV systems into the grid, where a higher PV incorporation has brought substantial concerns on power efficiency, power quality and grid code compliance [1] as well as power grid services [4].

T-type neutral point clamped inverters (3L-TNPC), also known as neutral point piloted converters (3L-NPP), [5] have gained a wide presence in the industry sector due to several advantages as symmetrical loss distribution, higher overall efficiency, small footprints [6] and low harmonic injection in relation to the conventional 3L-NPC [7]. In fact, many manufactures such as Fuji, On Semiconductor, Mitsubishi and Semikron have commercial T-type legs used in central PV inverters and motor drive applications [8–10]. For the three-level inverter, based on the T-type leg, was presented thirty-five years ago for motor drives, with the bidirectional medium switch being realized with thyristors and improved with GTO-thyristors [11]. After some years, many configurations based on the well-known three-level T-type NPC leg can be found in the literature [12]. In [13] a five-level TNPC (5L-TNPC) was introduced, which corresponds to the parallel connection of two 3L-TNPC legs [14,15]. Furthermore, a variation of this configuration with reduced switches, also known as five-level hybrid T-type NPC (5L-HTNPC), was presented in the recent literature [16]. This topological variation is built with a 3L-TNPC leg and a two-level leg inverter, forming a five-stepped voltage waveform in the AC terminals.

In the literature there are two main possibilities for increasing the number of levels in the power converter field, which is by increasing the internal DC capacitors connected to a single DC source or by connecting several converters in the series at the AC side, in which each converter has an independent DC source. Focusing on the second alternative, cascade MLI can be developed by using symmetrical or asymmetrical voltage levels and by using different type of topologies such as: Full H-Bridge, 3L-TNPC converters or by performing a hybrid configuration [17]. Note that symmetrical cascade configurations have had a more industrial presence as the case of Cascade H-Bridge (CHB) converters [3] due to modulation and control simplicity compared with asymmetrical configurations [18]. In fact, in [19] a symmetrical nine-level T-type converter (9L-TNPC) is presented for motor drive applications, which is based on the cascade connection of two 5L-TNPC converters. The same number of levels can be generated with advanced hybrid topologies as presented in [6,12].

Considering the advantages and features previously presented regarding the 3L-TNPC and symmetrical cascaded configurations, this paper described and validated the 5L-CTNPC topology for rooftop PV applications by using a cascaded connection of two 3L-TNPC legs which was firstly introduced in [20] as a cascade 3L-TNPC converter. Thus, the advantages of symmetrical cascade configurations with multistring inputs are merged. Each 3L-TNPC converter can interface a dedicated DC bus, and consequently two separate maximum power point tracking (MPPT) algorithms are allowed to obtain the maximum power of each PV string. Note that the PV string of each module can be sized to handle half the entire PV string in the conventional 3L-TNPC converter, providing better MPPT efficiency since less modules are combined in a series per string. The main contribution of this paper is the experimental validation of a simplified control scheme to alleviate the power unbalancing mismatch between 3L-TNPC modules and to compensate capacitor voltage variations per each converter, which was presented earlier in [20]. Furthermore, a brief comparison between five-level voltage waveform converters based on the conventional 3L-TNPC is performed as a second contribution in terms of the main electrical features.

The rest of the document is organized as follows. In Section 2, a hardware description of the proposed converter, switching states and implemented modulation is presented. In Section 3, a simple stationary reference-frame voltage-oriented control and a voltage control loop to compensate a possible power unbalance operation are included. Then, in Section 4, simulation results and experimental verification of the proposed multilevel converter and its control system behavior are

added. Furthermore, a brief comparison with the 5L-TNPC and 5L-HTNPC is performed to highlight the main advantages of the proposed configuration. Finally, in Section 5, the conclusions of the paper summarize the work done.

## **2. The 5L-CTNPC Converter Topology**

The power topology of the analyzed cascade 5L-CTNPC for a rooftop grid-connected PV system is depicted in Figure 1. The configuration is composed of a series connection of two 3L-TNPC legs, where each of them is built with two conventional IGBTs and one bidirectional switch. This bidirectional switch could be formed either by two conventional IGBTs in common-emitter or by a common-collector and reverse blocking IGBT connection. Actually, a classical IGBT semiconductor structure could be replaced by a reverse blocking MOSFETs for a high-voltage [21] and high-switching frequency operation [22]. Although more than two cells in a series connection are conceptually feasible, for the sake of simplicity, two-cell 3L-TNPC converters have been introduced as a proof-of-concept applied to conventional string rooftop PV applications.

**Figure 1.** Proposed 5L-CTNPC power topology.

Each 3L-TNPC leg operates as a string inverter connected to a single potential-induced degradation converter which is fed by one PV string. This potential-induced degradation stage can be designed to boost the DC voltage and perform the MPPT. Furthermore, it could be isolated [23] to avoid leakage currents due to the PV aluminium metallic frame grounded [1]. To reduce leakage currents paths and avoid high-frequency transformers there are three successful options well-documented in the literature: Changing the modulation stage to avoid switched common mode [16], by reducing surface conductivity of PV modules to avoid potential-induced degradation (PID) and by including extra switches between the PV array and the inverter, also well-known as transformerless inverters [24]. Although, a potential-induced degradation stage is desired to provide an independent DC voltage control, in this work, the validation of the proposed configuration does not integrate a potential-induced degradation stage, giving place to the worst case scenario under study where the MPPT control is fulfilled directly from each 3L-TNPC power cell, instead of using a high frequency galvanic isolated converter with its appropriate MPPT control. Thus, the overall control loops are more challenging since the voltage fluctuations in the PV panel is directly presented in the DC-side of each 3L-TNPC module, i.e., *vdck* = *vpvk*, where *k* = {1, 2} is given by the number of cell. Furthermore, in order to extract the maximum power from the PV panels, an integration of an external

MPPT algorithm is required so as to define the appropriate DC voltage reference in each cell. Note that the proposed topology is modeled without affecting the basic control objectives.

#### *2.1. Fundamental Principle of the 5L-CTNPC*

The 3L-TNPC provides three-output voltage levels: *vdck*/2, 0 and −*vdck*/2, where *k* is the cell or module number. These voltage steps are generated by connecting the AC terminals to the positive, neutral and negative pole of the DC-link terminals. Although, the 3L-TNPC configuration gives rise to four switching states, in order to avoid a short-circuit to the DC side there are only three of them possible. The cascade connection of two 3L-TNPC cells permits the generation of five voltage steps, where the zero level is combined into just one at the AC converter output voltage *vc*. According to the switching states presented in Table 1, the output voltage in the 5L-CTNPC can be modeled as:

$$v\_c = \underbrace{(S\_{11} + S\_{12} - 1)\frac{v\_{dc1}}{2}}\_{v\_{c1}} + \underbrace{(S\_{21} + S\_{22} - 1)\frac{v\_{dc2}}{2}}\_{v\_{c2}}\tag{1}$$

where *vc* is the addition of the converter voltages of both modules, *S*1*k* and *S*2*k* are the switching states of the *k*-th 3L-TNPC unit and *vdck*/2 is the total DC-link voltage of each cell. Furthermore, the dynamic model of the AC current in terms of the output voltage is governed by the next expression:

$$
\upsilon\_c = i\_s R\_s + L\_s \frac{di\_s}{dt} + \upsilon\_{s\star} \tag{2}
$$

with *vs* as the grid voltage measured at the point of common coupling (PCC), *is* as the grid current, *Ls* the grid filter inductance and *Rs* is the filter resistance included for modeling purposes. According to Table 1, the switching states are able to generate nine voltage levels in the output voltage *vc* where each state has an associated voltage level in function to the DC-link *vdc*1 and *vdc*2. Note that in this rooftop PV application both strings will be considered to work with similar DC-link voltages, i.e., *vdc*1 ≈ *vdc*2 = *vdc*. By doing this, the output voltage *vc* can be reduced just to ±*vdc*, ±*vdc*/2 and 0. This assumption leads to five switching states with similar output voltage steps between two consecutive levels [20]. The redundant switching states will be used to balance the voltage in the DC-link capacitors by adjusting the power mismatch between the converter cells. The computed peak amplitude of the converter output voltage *v*ˆ*c* is equal to *vpv*1/2 + *vpv*2/2, i.e., each power cell has a DC-link equal to the maximum level of the converter voltage. Therefore, for a proper grid current regulation, each PV string must be designed to satisfy *vpv*1 ≈ *vpv*2 > *vs*. In fact, this aspect is a practical advantage of cascaded configurations, since the overall DC-link voltage of the central configuration is split among power cells, thus reducing the string size.

**Table 1.** Switching states and output voltage in the AC terminals.


#### *2.2. Proposed Hybrid LS-PWM and PS-PWM Modulation Scheme for 5L-CTNPC Converter*

The proposed modulation scheme for the 5L-CTNPC is based on two well-known carriers based on the sinusoidal PWM methods. The first is the Sinusoidal Level-Shifted Pulse Width Modulator (LS-PWM), used in 3L-NPC three-phase converters [25] and the single 3L-TNPC legs [20]. This modulation strategy requires two carrier signals in phase, to generate the three voltage levels in the output terminals of each cell. One carrier signal has a positive polarity (0 to 1) and the other has a negative polarity (–1 to 0). Furthermore, the LS-PWM is merged with the Sinusoidal Phase-Shifted PWM (PS-PWM) conventionally used in cascaded H-bridge power converters [26]. In the PS-PWM modulation, a phase shift between the carrier signals of each series connected to a power cell is introduced to increase the number of voltage levels, giving rise to a five-level stepped voltage waveform. The operation principle of this hybrid modulation technique is illustrated in Figure 2, where *<sup>m</sup>*<sup>∗</sup>*ck* and *vck* are the modulation signal and the output voltage in the *k*-th cell, respectively. Note that each cell uses two carrier signals defined as *vcr*1 and *vcr*2. Thus, the stacked connection of both cells creates the converter voltage *vc*, which is commanded by its reference *v*∗*c* . The combination of both methods is simpler in respect to the space vector modulation (SVM) [27]. Finally, the implementation of this modulation technique is depicted in the block diagram of Figure 3, where simple comparators and two carrier signals are required to implement the proposed technique.

**Figure 2.** Proposed modulation scheme for 5L-CTNPC based on the hybrid LS-PWM and PS-PWM.

**Figure 3.** Straightforward implementation of hybrid LS-PWM and PS-PWM modulation for a 5L-CTNPC converter.
