Multi-Input Converter with MPPT Feature for Wind-PV Power Generation System

Abstract

A multi-input converter (MIC) to process wind-PV power is proposed, designed, analyzed, simulated, and implemented. The MIC cannot only process solar energy but deal with wind power, of which structure is derived from forward-type DC/DC converter to step-down/up voltage for charger systems, DC distribution applications, or grid connection. The MIC comprises an upper modified double-ended forward, a lower modified double-ended forward, a common output inductor, and a DSP-based system controller. The two modified double-ended forwards can operate individually or simultaneously so as to accommodate the variation of the hybrid renewable energy under different atmospheric conditions. While the MIC operates at interleaving mode, better performance can be achieved and volume also is reduced. The proposed MIC is capable of recycling the energy stored in the leakage inductance and obtaining high step-up output voltage. In order to draw maximum power from wind turbine and PV panel, perturb-and-observe method is adopted to achieve maximum power point tracking (MPPT) feature. The MIC is constructed, analyzed, simulated, and tested. Simulations and hardware measurements have demonstrated the feasibility and functionality of the proposed multi-input converter.

1. Introduction

Conventionally, electric power is mainly generated from fossil fuels. However, this kind of energy resources is highly limited and will be exhausted in the near future. With the rapid requirement of electricity and the increase of worse and worse energy crisis, it is of great urgency to replace the fossil fuels with renewable energy. Among the renewable resources, solar energy and wind power attract a great deal of interest owing to their easy acquirement.

In photovoltaic (PV) or wind power generation system, a power converter is needed so as to process renewable energy. In literature [1–5] PV converters are presented while wind power converters are studied in [6–10]. However, these power converters only handle a single kind of renewable energy, that is, which cannot deal with multi-input power. Therefore, some researchers propose multi-input converters for solar-wind hybrid power generation system. As shown in Figure 1, a series double-boost converter is presented to process PV power and wind energy simultaneously [11], in which, as compared with single-boost configuration, power component only imposes one-half power rating. Even though this boost-type converter steps up the voltage and is suitable for a high voltage supply, it cannot be applied to galvanic isolated applications. Double-input buck-boost converter shown in Figure 2 is capable of processing high-/low-voltage sources [12]. Like boost-type converter, this type of configuration is still nonisolated electrically. Instead of combining renewable energy in electricity, the concept of magnetic flux additivity is proposed to design a multi-input isolated converter, as shown in Figure 3, but its structure is complex and control low is complicated [13]. In order to simplify power-stage configuration, the forward-derived configuration is proposed. However, it cannot trap the energy in the leakage inductor and is incapable of applying to high output voltage applications [14].

Figure 1 

An illustration of double-boost converter for wind-PV system.

Figure 2 

An illustration of double-input buck-boost converter.

Figure 3 

An illustration of multi-input converter based on magnetic flux additivity.

In this paper, a multi-input converter (MIC) is proposed, which can not only deal with PV power and wind-turbine energy simultaneously/individually but recycles the leakage-inductance energy and steps up the input voltage. As compared with the aforementioned isolated double-input converter, the proposed one has a much simpler structure. Furthermore, the proposed MIC removes the third winding from the conventional forward converter, which releases the energy stored in the magnetizing inductor to a capacitor through the second winding. As a result, its output voltage can be stepped up significantly and the efficiency is improved. Simulated and practical results have validated the proposed PV-wind MIC.

In this paper, system architecture of the proposed MIC is described in Section 2. Section 3 presents the operational principle of the converter, while simulations and practical measurements to verify the feasibility of the MIC are shown in Section 4. Finally, conclusion is given in Section 5.

2. Configuration of the Proposed Converter

Figure 4 illustrates the block diagram of the proposed MIC, which mainly includes PV arrays, a wind turbine, a high step-up multi-input converter, and a system controller. Figure 5 shows the corresponding schematics of the main power stage of the MIC. In Figure 5, the MIC is composed of an upper modified double-ended forward, a lower modified double-ended forward, a common output filter 𝐿_mic, and a system controller. The upper modified double-ended forward is in charge of dealing with wind-turbine energy while the lower one processes solar power. The both modified double-ended forwards can be operated independently, which expands the degree of control freedom. The active switches in the upper forward or in the lower one are switched synchronously so as to trap the leakage energy and to release the magnetizing inductance energy. The capacitors in the secondary windings, 𝐶_wind and 𝐶_pv, will absorb the energy of magnetizing inductance and then can boost the output voltage. The system controller determines the control signals to perform output power controlling and maximum power point tracking (MPPT). In this paper, perturb-and-observe method is adopted for MPPT.

Figure 4 

A block diagram to represent the configuration of the proposed MIC.

Figure 5 

The proposed multi-input converter to process PV-wind power.

3. Operation Principle of the Proposed Converter

In Figure 5 the two modified double-ended forwards can operate individually to deal with PV power and wind energy. To achieve better output performance and to lower the output filter volume, the switches SW_wind and SW_pv are turned on alternatively with a duty ratio less than 0.5 at the same switching frequency. Figure 5 shows conceptual key waveforms. According to the conduction status of the switches SW_wind and SW_pv, the operation of the MIC over one switching period can be mainly divided into four modes. Corresponding equivalents are presented in Figure 6. Each operation mode is described in the following.

Figure 6 

Conceptual key waveforms of the proposed MIC.

Mode 1 (Figure 7(a), 𝑡₀≤𝑡<𝑡₁). During the interval of Mode 1, the status of switches SW_wind is on but SW_pv off. While SW_wind conducts at 𝑡₀, this mode begins. The active switch SW_pv is in the off-state, and the magnetizing inductor of the lower modified forward discharges energy through the path of 𝑁_2,pv-𝐷_2,pv-𝐶_pv. Meanwhile, wind power is forwarded to output. Therefore, 𝑖_𝐿,mic is linearly built and can be described as 𝑖_𝐿,mic(𝑡)=2⋅𝑁_wind⋅𝑘_wind⋅𝑣_wind𝐿_mic⋅𝐷_wind⋅𝑇_𝑠2+𝑖_𝐿,mic𝑡₀, (1) where 𝑘_wind is the coupling coefficient of the transformer in the upper forward, 𝐷_wind stands for duty ratio of SW_wind, and 𝑇_𝑠 represents the switching period. The voltage stresses of the switch SW_pv and diode 𝐷_2,wind can be found by 𝑣_𝐷𝑆,pv=𝑣_pv,𝑣_𝐷2,wind=2⋅𝑁_wind⋅𝑘_wind⋅𝑣_wind. (2) As the current of magnetizing inductor 𝐿_𝑚,pv drops to zero, this mode ends.

(a)

(b)

(c)

(d)

Figure 7 

Equivalent circuits of the MIC corresponding to the four operation modes over one switching cycle: (a) Mode 1, (b) Mode 2, (c) Mode 3, and (d) Mode 4.

Mode 2 (Figure 7(b), 𝑡₁≤𝑡<𝑡₂). In Mode 2, all the active switches are in off-state. At time 𝑡₁, the switch SW_wind is turned off and SW_pv in the lower forward still stays in the off-state. The magnetizing inductor in the upper modified forward 𝐿_𝑚,wind discharges via the path of 𝑁_2,wind-𝐷_2,wind-𝐶_wind, while the energy of leakage inductor 𝐿_{𝐿𝐾,wind} is trapped. The voltages across 𝐶_wind and 𝐶_pv, 𝑣_𝑐,wind and 𝑣_𝑐,pv, are obtained by 𝑣_𝑐,wind=𝑁_wind⋅𝑘_wind⋅𝑣_wind,𝑣_𝑐,pv=𝑁_pv⋅𝑘_pv⋅𝑣_pv, (3) respectively, in which 𝑘_pv is the coupling coefficient of the transformer in the lower forward. In addition, the output inductor 𝐿_mic releases the stored energy to the load by the path of 𝐿_mic-𝐶_dc,link-𝐷_2,wind, of which the current is linearly decreased and is expressed as 𝑖_𝐿,mic(𝑡)=𝑖_𝐿,mic𝑡₁−𝑉_dc,link𝐿_mic⋅𝐷_wind⋅𝑇_𝑠2⋅1−𝐷_wind. (4)

Mode 3 (Figure 7(c), 𝑡₂≤𝑡<𝑡₃). During this mode, SW_pv is in the on-state but SW_wind in off-state. Since the SW_pv is turned on at 𝑡₂, thus PV energy is dealt with by the lower modified forward. The inductor current 𝑖_𝐿,mic increases linearly. The inductor 𝐿_𝑚,wind releases the energy to the capacitor 𝐶_wind through the path of 𝑁_2,wind-𝐷_2,wind-𝐶_wind. In this interval, the current of output inductor 𝐿_mic is linearly built and can be described as 𝑖_𝐿,mic(𝑡)=2⋅𝑁_pv⋅𝑘_pv⋅𝑣_pv𝐿_mic⋅𝐷_pv⋅𝑇_𝑠2+𝑖_𝐿,mic𝑡₂, (5) where 𝐷_pv denotes the duty ratio of SW_pv. The voltage stresses of the switch SW_pv and diode 𝐷_2,wind can be expressed as follows: 𝑣_{𝐷𝑆,wind}=𝑣_wind,𝑣_𝐷2,pv=2⋅𝑁_pv⋅𝑘_pv⋅𝑣_pv. (6) This mode ends at the moment the current flowing through 𝐿_𝑚,wind equals zero.

Mode 4 (Figure 7(d), 𝑡₃≤𝑡<𝑡₄). At time 𝑡₃, the switch SW_pv is turned off and the operation of the MIC enters into Mode 4. That is, in Mode 4 all active switches are off. During this mode, the magnetizing inductor 𝐿_𝑚,pv releases energy to capacitor 𝐶_pv via 𝑁_2,pv, 𝐷_2,pv, and 𝐶_pv. In addition, the energy stored in leakage inductance is recycled. Meanwhile, the output inductor discharges and the current 𝑖_𝐿,mic decreases linearly, which can be expressed as 𝑖_𝐿,mic(𝑡)=𝑖_𝐿,mic𝑡₃−𝑉_dc,link𝐿_mic⋅𝐷_pv⋅𝑇_𝑠2⋅1−𝐷_pv. (7) A complete switching cycle is terminated at 𝑡=𝑡₄, at which SW_wind is turned on again.

While the proposed MIC operates in continuous conduction mode (CCM), the corresponding waveform of output inductor current is illustrated in Figure 8. The 𝑇_𝑠 is the switching period and can be expressed as 𝑇_𝑠,wind+𝑇_𝑠,pv=𝑇_𝑠. (8) In (8), 𝑇_𝑠,wind stands for the intervals that the upper modified forward works, while 𝑇_𝑠,pv for the lower modified forward. In the interleaved operation, the following relationship holds: 𝑇_𝑠,pv=𝑇_𝑠,wind=𝑇_𝑠2. (9) Based on volt-second balance criterion, one can obtain the following identity: 2⋅𝑁_wind⋅𝑘_wind⋅𝑣_wind−𝑉_dc,link⋅𝐷_wind⋅𝑇_𝑠,wind+2⋅𝑁_pv⋅𝑘_pv⋅𝑣_pv−𝑉_dc,link⋅𝐷_pv⋅𝑇_𝑠,pv=1−𝐷_wind⋅𝑇_𝑠,wind⋅𝑉_dc,link+1−𝐷_pv⋅𝑇_𝑠,pv⋅𝑉_dc,link. (10) Rearranging (10) yields 𝑉_dc,link=𝑁_wind⋅𝑘_wind⋅𝑣_wind⋅𝐷_wind+𝑁_pv⋅𝑘_pv⋅𝑣_pv⋅𝐷_pv, (11) which reveals that the DC-link voltage can be controlled by the duty ratios of SW_wind and SW_pv.

Figure 8 

The waveform of output inductor current.

In the MIC, perturb-and-observe method is employed to draw maximum power from wind turbine and PV arrays since it is easy to carry out. The perturb-and-observe MPPT is realized by dsPIC30F4011. The related flowchart is presented in Figure 9.

Figure 9 

The flowchart of perturb-and-observe method to achieve MPPT feature.

4. Simulations and Practical Measurements

To demonstrate the feasibility of the proposed MIC, a prototype is constructed, simulated, and tested. Important parameters are listed as follows:(i) PV panel voltage: 𝑣_pv = 40–56 V,(ii) wind turbine voltage: 𝑣_wind = 22–48 V,(iii) dc-link voltage: 200 V,(iv) upper modified double-ended forward: 350 W,(v) lower modified double-ended forward: 350 W,(vi) switching frequency: 50 kHz for all active switches,(vii) output inductance: 𝐿_mic = 400 μH,(viii) output capacitance: 𝐶_dc,link = 400 μF,(ix) capacitance of upper forward: 𝐶_wind = 22 μF, (x) capacitance of lower forward: 𝐶_pv = 22 μF.

In the case of only wind turbine providing 350 W, the simulated active switch signals and corresponding output inductor current are shown in Figures 10 and 11, respectively, while Figures 12 and 13 are the practical measurements. With the perturb-and-observe method for maximum power point tracking, the measured result is shown in Figure 14, which has illustrated that the upper modified double-ended forward can draw the maximum power from wind turbine. If only 350 W PV power feeds the MIC, simulations of control signals for active switches and output inductor current are presented in Figures 15 and 16. In addition, Figures 17 and 18 are the measured results. For MPPT feature verification, Figure 19 is the relationship between PV power and the terminal voltage of PV panel, from which it can be found that the proposed MIC is able to draw maximum power from PV panel. From Figures 10–19, it has been demonstrated that the proposed MIC is capable of dealing with individual renewable power. As the solar power and wind energy feed the MIC simultaneously, Figures 20 and 21 show the simulations of control signals and output inductor current. Then, Figures 22 and 23 present the hardware measurements. In addition, switch currents are also shown in Figure 24, in which the upper trace and the lower trace are the drain-to-source currents of SW_wind and SW_pv, respectively. Figure 25 is the hardware measurements of the secondary currents 𝑖_𝑁2,wind and 𝑖_𝑁2,pv. All the experimental results correspond with the theoretical waveforms in Figure 6. From Figures 20–25, it is verified that the MIC not only can process hybrid wind-PV power but can operate in interleaved mode for current ripple suppression. Additionally, in Figures 21 and 23, the ripple of output inductor current is double the switch frequency, which results in lower volume requirement for output filter inductor. The measured efficiency of the MIC is shown in Figure 26. In the case of wind turbine shutting down from the hybrid power generation system, the output power variation of the MIC is shown in Figure 27. For converse condition, the related output power curve is shown in Figure 28.

Figure 10 

Simulated control signals while wind turbine provides 350 W.

Figure 11 

Simulated output inductor current while wind turbine provides 350 W.

Figure 12 

Practical measurements of control signals while wind turbine provides 350 W.

Figure 13 

Measured output inductor current while wind turbine provides 350 W.

Figure 14 

Measured result: drawn power from wind turbine with MPPT.

Figure 15 

Simulated control signals while PV panel provides 350 W.

Figure 16 

Simulated output inductor current while PV panel provides 350 W.

Figure 17 

Practical measurements of control signals while PV panel provides 350 W.

Figure 18 

Measured output inductor current while PV panel provides 350 W.

Figure 19 

Measured result: operation point of PV panel after MPPT.

Figure 20 

Simulated control signals while hybrid wind-PV power is 700 W.

Figure 21 

Simlated output inductor current while hybrid wind-PV power is 700 W.

Figure 22 

Measured control signals while hybrid wind-PV power is 700 W.

Figure 23 

Measured output inductor current while hybrid wind-PV power is 700 W.

Figure 24 

Measured waveforms of switch currents.

Figure 25 

Measured current waveforms of the secondary windings in the MIC.

Figure 26 

The measured efficiency of the proposed MIC.

Figure 27 

Output power variation while wind turbine shuts down from the hybrid generation system.

Figure 28 

Output power variation while wind turbine incorporates into the hybrid generation system.

5. Conclusions

This paper proposed a galvanic isolated multi-input converter to deal with wind turbine energy and solar power with MPPT feature. The converter integrates two forward converters and only uses one output inductor. Therefore, the structure of the proposed MIC can lower the volume of the converter. In addition, the MIC can operate in interleaved mode so that the output current ripple is suppressed significantly. The energy stored in leakage inductor can be recycled, which improves efficiency. In this paper, the proposed MIC is analyzed, simulated, and tested. Simulations and hardware measurements have validated the proposed MIC.