*Article* **Multi-Level Multi-Input Converter for Hybrid Renewable Energy Generators**

**Salvatore Foti 1, Antonio Testa 1,\*, Salvatore De Caro 1, Luigi Danilo Tornello 2, Giacomo Scelba <sup>2</sup> and Mario Cacciato <sup>2</sup>**


**Abstract:** A three-phase multi-level multi-input power converter topology is presented for gridconnected applications. It encompasses a three-phase transformer that is operated on the primary side in an open-end winding configuration. Thus, the primary winding is supplied on one side by a three-phase N-level neutral point clamped inverter and, on the other side, by an auxiliary two-level inverter. A key feature of the proposed approach is that the N-level inverter is able to perform independent management of *N* − 1 input power sources, thus avoiding the need for additional dc/dc power converters in hybrid multi-source systems. Moreover, it can manage an energy storage system connected to the dc-bus of the two-level inverter. The N-level inverter operates at a low switching frequency and can be equipped with very low on-state voltage drop Insulated-Gate Bipolar Transistor (IGBT) devices, while the auxiliary inverter is instead operated at low voltage according to a conventional high-frequency two-level Pulse Width Modulation (PWM) technique and can be equipped with very low on-state resistance Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) devices. Simulations and experimental results confirm the effectiveness of the proposed approach and its good performance in terms of grid current harmonic content and overall efficiency.

**Keywords:** multi-level inverter; multi-input converter; renewable energy sources; maximum power point tracking; solar power; wind power; open-end winding

### **1. Introduction**

The number of electricity generators powered by renewable energy sources (RESs) is continuously increasing because of concerns about environmental pollution and the limited reserves of fossil energy sources such as oil, coal, and gas [1]. Grid-connected photovoltaic (PV) and wind turbine (WT) generators are the most widely diffused types of RES power plants and their specific cost is continuously decreasing [2–4]. However, available solar and wind energy are affected by some factors, such as season cycle, daily cycle, temperature, and weather conditions, which make them intermittent and stochastic. Therefore, a power plant relying only on a single form of RES and without an energy storage capability can hardly cope with the requirements for a reliable electric power generation unit. Hybrid renewable energy systems (HRESs) combining more than one energy source are a viable solution to this problem [5] because they are effective not only in enhancing the reliability of power supply but also in reducing the size of energy storage systems [6,7]. However, in HRESs, a specific dc–dc power converter is normally used to manage each input power source, leading to a quite complex and expensive structure [8,9].

The multiple input power converter (MIPC) concept is a possible alternative to HRESs having to cope with sources with different power capacity and/or voltage levels, providing a well-regulated dc output voltage. Both isolated and non-isolated dc/dc multi-input converters find application in hybrid vehicles [10], the aerospace industry, and RES power

**Citation:** Foti, S.; Testa, A.; De Caro, S.; Tornello, L.D.; Scelba, G.; Cacciato, M. Multi-Level Multi-Input Converter for Hybrid Renewable Energy Generators. *Energies* **2021**, *14*, 1764. https://doi.org/10.3390/ en14061764

Academic Editors: Athanasios I. Papadopoulos and Massimiliano Luna

Received: 24 February 2021 Accepted: 19 March 2021 Published: 22 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

plants. A non-isolated double input dc/dc converter is proposed in [11] combining buck and buck-boost single-input topologies, while an *n*-input buck-boost topology is presented in [12], which however could not supply the load simultaneously from different sources. A bidirectional multi-input dc/dc converter was also developed in [13], which is burdened by high conduction losses. The efficiency of an MIPC can be increased by exploitation of zero voltage switching approaches, as in [14,15]. Some MIPC topologies have been purposely developed for application in HRESs [16]. Among them, three-port dc/dc converters are of major interest. They feature an input port, an output one, and a storage port, enabling a bidirectional power flow towards/from an energy storage system (ESS). Some non-isolated three-port converters are discussed in [17–21].

A different approach is proposed in this paper, where a particular kind of multipleinput multi-level converter (MMC) is exploited to connect photovoltaic and wind generators to an energy storage system and a three-phase ac grid. It is based on an open-end winding configuration, the asymmetrical hybrid multi-level inverter (AHMLI), whose applications on both motor drives and grid-connected generators are discussed in [22,23] and which has also been successfully exploited to reduce the overvoltage caused by long cables in PWM motor drives [24] as well as to realize a high-speed Gen-set [25,26]. In the present HRES application, the AHMLI topology encompasses an open-end winding three-phase transformer (OWT) whose primary winding operates in an open-end configuration. The primary winding is, in fact, supplied on one side by a three-phase neutral point clamped (NPC) multi-level inverter (MLI) and, on the other side, by a conventional two-level inverter (TLI). The MLI operates at a low switching frequency (<1 kHz), thus featuring very low switching power losses. It is tasked to control the active power supplied to the grid, while also managing *N* − 1 unidirectional input power flows, being *N* the number of the output voltage levels. Thus, *N* − 1 energy sources (ESs) such as photovoltaic (PV) strings or wind turbines (WTs) can be managed without the introduction of additional dc/dc power converters. Moreover, it can also accomplish a multi-channel maximum power point tracking (MPPT) function at the string level on PV arrays. Compared with the MLI, the TLI operates at a high switching frequency, but at a lower dc bus voltage. It is tasked to control the grid current and to compensate for low-order current harmonics and unbalanced components. Moreover, its dc-bus can act as the storage port of a three-port dc–dc converter, enabling the connection of an energy storage system (ESS) to the HRES. In other words, energy sources connected to the MLI are managed by the TLI, avoiding the introduction of dc/dc power converters.

The paper is organized as follows. Section 2 presents the proposed approach and its application to 5LI + TLI and 3LI + TLI topologies. In Section 3, the operations of these topologies are discussed. Simulation and experimental results are presented in Sections 4 and 5, respectively. Finally, Sections 7 and 8 concern the discussion and conclusion.

#### **2. The Proposed MMC Topology**

The proposed MMC for HRESs, tailored around the AHMLI topology, is shown in Figure 1. The primary winding of the three-phase transformer is connected to an *N*-level NPC inverter on one side and to an auxiliary TLI on the other side. The secondary winding is instead connected to a three-phase ac grid. The NPC inverter, which acts as an *N*-level multi-input converter, encompasses *N* − 1 DC-bus capacitors *Cj*, each one connected to a dc power source of voltage *Vck* (*k* = 1, 2, 3, *N* − 1). According to the AHMLI topology and assuming that the two inverters are supplied by two independent power sources, *VDC* and *VDC*, a phase voltage *Vpj* (*j* = 1,2,3,) of the primary winding of the OWT is given by:

 $V\_{pj} = V\_{NPCj} - V\_{TLIj} - V\_{O'O''} = \frac{2l'-2}{4}$   $V\_{DC}' - (2l'' - 1)V\_{DC}'/2 - V\_{O'O''} \\ \tag{1}$   $l' = 0, 1, 2$   $l'' = 0, 1$ 

where *VNPCj* is the NPC output *j*-phase voltage referred to the mid-point *O* (Equation (2)), *VTLIj* is the TLI output *j*-phase voltage referred to the mid-point *O* (Equation (3)), *VDC* is

the dc-bus voltage of the TLI, *VDC* is the total dc-bus voltage of the NPC, and *VO <sup>O</sup>* is the voltage between the mid points *O* and *O* of the dc-buses of the two inverters (Equation (4)).

$$V\_{\rm NPCj} = \frac{2l'-2}{4} \,\, V\_{\rm DC}' \,\, l' = 0, 1, 2 \tag{2}$$

$$V\_{TLIj} = \frac{2l^{\nu} - 1}{2} \; V\_{DC}'' \; l^{\nu} = 0, 1 \tag{3}$$

$$V\_{O'O''} = \frac{1}{3} \sum\_{j=1}^{3} \left( V\_{MLIj} - V\_{TLIj} \right) \tag{4}$$

**Figure 1.** Proposed multi-level converter (MMC) configuration.

As *VNPCj* may take *N* levels, while *VTLIj* may take two, the transformer primary phase voltage *Vpj* may take *2N* levels, whose amplitude is a function of *VDC* and *VDC*. Table 1 shows that in terms of phase voltage levels, the proposed configuration is equivalent to a conventional multi-level inverter with a larger number of power devices. From another point of view, a lower phase voltage THD (Total Harmonic Distortion) is obtained with the same number of switches.

**Table 1.** Basic multi-level inverter (MLI) topologies vs. the asymmetrical hybrid multi-level inverter (AHMLI).


As an example of an HRES application of the AHMLI topology, a six-level MMC is shown in Figure 2. PV strings, or groups of strings, are directly connected to the NPC's dcbus capacitors while the permanent magnet synchronous generators of the wind turbines are connected through a three-phase controlled rectifier or a diode rectifier and an output dc-link capacitor. All dc sources must have about the same rated output voltage in order to prevent largely unbalanced dc-bus voltages. However, the TLI is able to compensate a NPC DC-bus capacitor voltage variation Δ*Vc* provided that it is lower than *VDC*, thus achieving a sinusoidal grid current. The dc-buses of the NPC and auxiliary inverters are isolated between them in order to prevent the circulation of zero-sequence currents [22]. Moreover, the TLI dc-bus is supplied through a floating capacitor; thus, an additional power source is not required. Another example is shown in Figure 3, where a five-level NPC inverter is used. In this case, an ESS is connected to the TLI dc-bus, and a bidirectional power flow

can be managed towards/from the ESS. In both examples, the NPC provides the active power flow to the grid, while the TLI works as an active power filter, while also regulating the output current and the NPC dc-bus capacitor voltages *Vck*.

**Figure 2.** Six-level MMC (three-level inverter (3LI) + two-level inverter (TLI)) with two energy sources.

**Figure 3.** Ten-level MMC (5LI + TLI) with four energy sources and an energy storage device.

#### **3. Proposed MMC Topology Operation**

In order to manage multiple input sources while controlling the main power flow towards the grid, a suitable control system has been developed that is divided into two main parts: an MLI control subsystem and a TLI control subsystem.
