1. Introduction

Efficient power conversion and high energy utilization are of great importance in photovoltaic (PV) power systems. Efficient power converters and inverters with maximum power point tracking (MPPT) capability have been commercialized to extract as much energy from PV

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

panels as possible. However, even with such efficient converters, energy yield from PV panels are known to be significantly reduced due to partial shading. A standard PV panel comprising three substrings and its characteristics under a partially shaded condition are shown in Figure 1. A shaded substring, PV3, is less capable of generating current is bypassed by a parallel-connected bypass diode, and therefore, it no longer contributes to power generation, though it can potentially generate power to some extent. In addition, a partially shaded panel exhibits multiple power point maxima including global and local MPPs, which likely confuse ordinally MPPT tracking algorithms.

converters [1–3] and switched capacitor converters [4, 5], are used as DPP converters in the adjacent equalization architecture. With the substring-to-bus equalization architecture shown in Figure 2(b), respective bidirectional isolated flyback converter-based DPP converters transfer power between the bus and each substring [6–8]. The architectures in Figure 2(a) and (b) require multiple DPP converters in proportion to the number of substrings, likely increasing the system complexity and cost. The string-to-substring equalization architecture, on the other hand, can reduce the DPP converter count, as shown in Figure 2(c). Single-input multi-output converters, such as a multi-winding flyback converter [9], multi-stacked buck-boost converters [10, 11], and LLC resonant voltage multiplier [12], can be employed as a DPP converter in this architecture. Integrated converters having a DPP converter function have also been proposed [13, 14]. The simplified system and reduced cost of the integrated converters are appealing features, but the performance as a DPP converter cannot be optimized because two converters

Single-Switch Differential Power Processing PWM Converter to Enhance Energy Yield of Photovoltaic Panels…

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From the perspective of the performance and cost, string-to-substring DPP converters are considered the most viable solution to the partial shading issues. Representative circuit topologies of the string-to-substring DPP converters are shown in Figure 3. Although the multiwinding flyback converter [9] (Figure 3(a)) is very simple as it needs only one switch, the design difficulty of the multi-winding transformer is a top concern. The multi-stacked buckboost converter-based DPP converter is also a single-switch circuit [10, 11], but it may be bulky as multiple inductors are necessary. From the viewpoint of magnetic components, the DPP converter based on the LLC resonant voltage multiplier (VM) [12] (Figure 3(c)) would be the best solution, but the switch count is doubled compared to that of other string-to-substring

A string-to-substring single-switch DPP PWM converter based on the forward-flyback resonant inverter (FFRI) and VM is proposed in this chapter. In addition to the single-switch topology, the magnetic component count is also one, realizing the simple, easy-to-design, and miniaturized circuit. The circuit derivation and description of the proposed single-switch DPP converter are presented in Section 2, followed by the detailed operation analysis in Section 3. The current sensorless control strategy suitable for the proposed single-switch DPP converter

Figure 3. String-to-substring equalizers based on (a) multi-winding flyback converter, (b) multi-stacked buck-boost

are combined into a single unit to form the integrated converter.

DPP converters.

converters, and (c) LLC resonant voltage multiplier.

Partial shading issues originate from characteristic mismatch among series-connected substrings. With differential power processing (DPP) converters, power differences among substrings are transferred so that all substring characteristics are virtually unified, thus precluding the partial shading issues. Various kinds of DPP architectures have been proposed. The most straightforward architecture is the adjacent substring-to-substring equalization system, shown in Figure 2(a), in which adjacent substrings exchange power difference through a DPP converter, depending on shading conditions. Bidirectional converters, such as PWM

Figure 1. Characteristics of (a) substrings and (b) string under partial shading.

Figure 2. DPP architectures based on (a) adjacent substring-to-substring equalization, (b) substring-to-bus equalization, (c) string-to-substring equalization, and (d) integrated converter.

converters [1–3] and switched capacitor converters [4, 5], are used as DPP converters in the adjacent equalization architecture. With the substring-to-bus equalization architecture shown in Figure 2(b), respective bidirectional isolated flyback converter-based DPP converters transfer power between the bus and each substring [6–8]. The architectures in Figure 2(a) and (b) require multiple DPP converters in proportion to the number of substrings, likely increasing the system complexity and cost. The string-to-substring equalization architecture, on the other hand, can reduce the DPP converter count, as shown in Figure 2(c). Single-input multi-output converters, such as a multi-winding flyback converter [9], multi-stacked buck-boost converters [10, 11], and LLC resonant voltage multiplier [12], can be employed as a DPP converter in this architecture. Integrated converters having a DPP converter function have also been proposed [13, 14]. The simplified system and reduced cost of the integrated converters are appealing features, but the performance as a DPP converter cannot be optimized because two converters are combined into a single unit to form the integrated converter.

panels as possible. However, even with such efficient converters, energy yield from PV panels are known to be significantly reduced due to partial shading. A standard PV panel comprising three substrings and its characteristics under a partially shaded condition are shown in Figure 1. A shaded substring, PV3, is less capable of generating current is bypassed by a parallel-connected bypass diode, and therefore, it no longer contributes to power generation, though it can potentially generate power to some extent. In addition, a partially shaded panel exhibits multiple power point maxima including global and local MPPs, which likely confuse

Partial shading issues originate from characteristic mismatch among series-connected substrings. With differential power processing (DPP) converters, power differences among substrings are transferred so that all substring characteristics are virtually unified, thus precluding the partial shading issues. Various kinds of DPP architectures have been proposed. The most straightforward architecture is the adjacent substring-to-substring equalization system, shown in Figure 2(a), in which adjacent substrings exchange power difference through a DPP converter, depending on shading conditions. Bidirectional converters, such as PWM

Figure 2. DPP architectures based on (a) adjacent substring-to-substring equalization, (b) substring-to-bus equalization,

ordinally MPPT tracking algorithms.

132 Solar Panels and Photovoltaic Materials

Figure 1. Characteristics of (a) substrings and (b) string under partial shading.

(c) string-to-substring equalization, and (d) integrated converter.

From the perspective of the performance and cost, string-to-substring DPP converters are considered the most viable solution to the partial shading issues. Representative circuit topologies of the string-to-substring DPP converters are shown in Figure 3. Although the multiwinding flyback converter [9] (Figure 3(a)) is very simple as it needs only one switch, the design difficulty of the multi-winding transformer is a top concern. The multi-stacked buckboost converter-based DPP converter is also a single-switch circuit [10, 11], but it may be bulky as multiple inductors are necessary. From the viewpoint of magnetic components, the DPP converter based on the LLC resonant voltage multiplier (VM) [12] (Figure 3(c)) would be the best solution, but the switch count is doubled compared to that of other string-to-substring DPP converters.

A string-to-substring single-switch DPP PWM converter based on the forward-flyback resonant inverter (FFRI) and VM is proposed in this chapter. In addition to the single-switch topology, the magnetic component count is also one, realizing the simple, easy-to-design, and miniaturized circuit. The circuit derivation and description of the proposed single-switch DPP converter are presented in Section 2, followed by the detailed operation analysis in Section 3. The current sensorless control strategy suitable for the proposed single-switch DPP converter

Figure 3. String-to-substring equalizers based on (a) multi-winding flyback converter, (b) multi-stacked buck-boost converters, and (c) LLC resonant voltage multiplier.

will be discussed in Section 4. The experimental results of the laboratory and field testing for a standard 72-cell PV panel consisting of three substrings will be presented in Section 5.
