1. Introduction
Multilevel inverters (MIs) are attractive devices in many industrial applications. These devices can reduce the total harmonic distortion (THD), electromagnetic interference (EMI), dv/dt, switching frequency and voltage stress. One of the most regarded applications of MIs is PV application. The neutral point clamped converter (NPC) and cascaded multilevel inverter (CMI) are two types of multilevel inverters, which are popular in PV applications [
1,
2]. Between the two topologies, the CMI stands out for its modularity and high magnitude of the output voltage. However, this topology requires several isolated dc sources. This drawback not only calls for a complex control system, but also it gives rise to inter module leakage currents in grid-tied PV applications. The inter-module leakage currents result from differential-mode voltage (DMV) and common mode voltage (CMV) variations. In order to tackle the issue, several topologies are suggested in the literature [
3,
4,
5]. One solution is using only one dc-source along with some passive components. Single-source CMIs are categorized into three types. (i) Topologies which use low frequency transformers instead of several isolated dc sources. These topologies are referred to as cascaded transformers multilevel inverters (CTMIs) [
6,
7,
8,
9]. (ii) Topologies which provide the isolated dc sources by adopting a high-frequency link and a single dc-source (HFLMI) [
10,
11]. (iii) A switched-capacitor (SC) based cascaded multilevel inverter (SC-CMI) [
12,
13]. The main advantage of CTMIs is their ability to provide galvanic isolation between the dc source and the load/grid. This is also the case when applying HFLMIs in PV applications, where the leakage current issue is addressed. On the contrary, CTMIs need several bulky and inefficient transformers. Although the transformer size in HFLMIs is reduced due to the use of a high frequency link, many rectifiers are required to convert the isolated high frequency voltages to the desired dc voltages. Thus, the reliability decreases and the cost increase in this topology. Alternatively, SC-CMIs employ several capacitors instead of the isolated dc sources. Therefore, the SC-CMI topologies have a compact size and lower cost. However, these kinds of multilevel inverters lack galvanic isolation.
Moreover, many attempts have been made to use isolated PV arrays as the isolated dc sources in grid-tied CMIs [
14]. However, as illustrated in [
15], the main constraint of these configurations is the capacitive leakage currents between the H-bridge (HB) cells and grid. Even using an interfacing transformer cannot address the mentioned problem, because inter-module leakage currents appear and circulate between the cascaded HB cells. In ref. [
15], the mentioned problem was addressed by equipping each HB cell with additional ac and dc side filters. Apart from limiting various leakage currents, these filters are deemed to eliminate the EMI; however, equipping each cell with several filters increases the volume and cost of the inverter. In ref. [
16], several level-double networks (LDN) are used as the auxiliary blocks to enhance the quality of the output voltage. This topology can also offer a common ground between the PV module and the grid, which results in the elimination of the leakage current. Although the suggested topology can eliminate the leakage current in PV applications, balancing of the capacitor voltage in the auxiliary cell is challenging. In another attempt, a two-stage inverter was suggested in [
17], which can be regarded as a combination of the H5 and Highly Efficient and Reliable Inverter Concept (HERIC) topologies. When the output voltage is higher than the grid voltage, the inverter operates in the H5 mode; when the dc link voltage decreases, the inverter is switched to the two-stage HERIC mode. This inverter can properly deal with voltage variation. However, it uses a complicated structure and control approach. Moreover, a charge pump circuit was employed to eliminate the leakage in [
18]. The topology is simple and compact, but it imposes a non-continuous current to the input side. Notably, in ref. [
19] a comprehensive study was conducted to investigate the state-of-the-art inverters for grid-tied PV applications.
In light of the above, a single-source asymmetric CMI is proposed in this paper, which provides a common ground for ac and dc sides. This topology uses capacitors instead of the isolated dc sources in the HB cells. Each capacitor is independently charged through a charging switch. Since there is a common ground for ac and dc sides, the common mode voltage is zero; hence, this topology can totally eliminate the leakage current in grid-tied PV applications. Another merit of the proposed topology is the capability to boost the input dc voltage; this is also an advantage in many applications such as grid-tied transformerless PV and fuel cell systems. In addition to the mentioned features, the three-phase configuration of the proposed topology draws a continuous input current, which makes it feasible in battery, un-interruptible power supply, and PV applications. The proposed topology can exchange reactive power with the load and the grid as well. Furthermore, it can smoothly charge the capacitors, facilitate the protection, and avoid bulky and expensive transformers in the grid-tied mode. As mentioned, the main issue of the conventional CMI in PV applications is the inter-module leakage currents. However, the proposed topology can address this problem properly and effectively. Compared to the transformer-based single-source multilevel inverters, the proposed topology is smaller in size, lower in cost, and higher in efficiency. Additionally, considering that the SC-based single-source MIs mostly suffer from inrush currents, the proposed topology is, however, an inrush-current free CMI, being a promising converter in many industrial applications.
The rest of the paper is organized as follows: In
Section 2, the structure and operation principle of the proposed topology are illustrated. In
Section 3, the proposed MI is compared with state-of-the-art MI topology. In
Section 4 the performance of the proposed topology in off-grid and grid-tied modes is investigated through simulations. Experimental tests are provided in
Section 5, where a fifteen-level 0.55 kVA prototype is adopted to demonstrate the off-grid performance of the proposed topology. Moreover, a seven-level 1.5 kVA prototype is used to extract the grid-tied results. Finally, the overall work is concluded in
Section 6.
3. Benchmarking with Prior-Art Inverters
Several efforts have been done to make the CMI compatible with grid-tied PV applications [
22,
23,
24,
25,
26,
27,
28,
29]. The main difficulties with the CMI in PV applications are the leakage current and complicated MPPT [
14]. Single-source CMIs facilitate the MPPT, but the leakage current problem remains. A transformer can solve the problem, however, transformers are not recommended in grid-tied PV applications due to extra power losses and additional costs. Therefore, as stated previously, the SC-based CMI can fulfill many requirements. The state-of-the-art PV MI topologies are compared with the proposed MI topology in this section to assess its pros and cons.
Table 2 lists the main features of the considered MI topologies.
In table,
Nsw,
Nd,
Nc,
G, and
TSV are the number of switches, diodes, capacitors, voltage gain, and the total standing voltage of the switches. The TSV is calculated as
The proposed topology and the topology in [
29] can be scaled up to obtain higher voltage gains and levels. However, this is not the case for the other topologies. Since the proposed MI topology is a common-ground-type inverter and the topology in [
24] is a mid-point-grounded topology, these two topologies can limit the leakage current in grid-tied PV applications. In this regard, the other topologies listed in
Table 2 encounter serious problems. The main disadvantage of the MI topologies in [
24] and [
25] is that they require a complicated control approach to balance the voltages across the capacitors. The voltage balancing system of these topologies should sense the direction of the ac current and the capacitor voltage magnitude, and then the sensed values are processed though the processor to execute the right switching pattern to balance the voltage of the capacitors. However, this does not happen in the other topologies and the proposed one. Notably, the topologies in [
23,
25,
27,
28] suffer from high inrush currents in the charging stage of the capacitor. Owing to the controlled voltage balancing of the capacitors, the inrush current does not appear in the topologies in [
24,
25]. In the proposed topology and the topology in [
29], the inrush current is limited through the charging inductor. As it is seen in
Table 2, the proposed inverter has a fairly low TSV, high voltage gain and fewer components.
6. Conclusions
In this paper, a single-source high step-up asymmetric power converter topology is proposed. The proposed topology offers several advantages in many industrial applications such as PV, fuel cell, etc. It is synthesized with two parts, namely, the main and charging parts. The main part is the same as the conventional asymmetric CMI with certain capacitors instead of the isolated dc sources. The charging part, however, consists of charging switches, a charging inductor, a freewheeling diode, and one dc source. The main feature of the proposed topology is to provide a common-ground for ac and dc sides, which eliminates the leakage current in grid-tied PV applications. It also has the ability to boost the input voltage. Thus, in the grid-tied PV applications, bulky and expensive transformers can be avoided. Moreover, it uses only one dc source, at the expense of using many switches, employing many switches can be considered as the main drawback of the proposed inverter. Simulations and experiments were performed to verify the effectiveness of the proposed topology. Through simulations, the performances of the suggested topology with single- and three-phase configuration, in both off-grid and grid-tied conditions, were assessed. In the experimental tests, the performance of the proposed topology was studied in the presence of a 550-VA load. Moreover, using a 1.5 kVA seven-level prototype the grid-tied results were provided. Both results have demonstrated the feasibility of the proposed multilevel inverter in terms of the ability to develop a boosted voltage of high quality using only one dc source.