Keywords

1 Introduction

The important issue of today’s society is increasing demand of electric energy and reduction of greenhouse gas emission, the depletion of fossil fuels and to protect the environment from pollution caused by conventional energy sources. Therefore, there is a need for application of renewable energy sources and storage devices based on distributed generation systems. Among these renewable energy sources, wind generators, solar panels and fuel cells are the most common and important. The powers generated from nonconventional energy sources such as solar, wind, etc. are not adequate to deliver the high power load demand. Therefore, use of the power electronics converters units becomes necessary in between the nonconventional energy sources and the load according to requirements of application area. The DC–DC boost chopper and conventional voltage source buck inverter are used in grid integration applications, which generates sinusoidal AC power output from the lower DC input and requires two-stage power conversion. For these applications, we may use single-stage boost inverter topology circuit in which generated sinusoidal AC output is greater than lower DC input voltage. The major advantages of using three-phase single-stage DC–AC boost inverter are reduced volume, weight and cost of the system and improved efficiency as compared to two-stage inverter topology configuration. In this paper, operating principle, mathematical modelling, MATLAB simulation analysis and advantages of single-stage three-phase DC–AC boost inverter are presented.

Significance of new inverter topologies enhances due to convenient and green energy conversion of renewable energy sources such as photovoltaics (PV), wind turbine systems, fuel cells, etc. Consequently, important aspects become overall reduction of converter size and passive element [1, 2]. For DC–AC conversion, most common converter topology is conventional two-level buck voltage source inverter [3], where the peak value of output AC voltage is always lower than the input DC voltage and output AC current peak is always greater than the input DC current. Due to the buck voltage nature of VSI, a DC–DC boost chopper is installed for voltage matching and maximum power point tracking (MPPT); consequently, total weight, volume, losses and hence cost of overall system will be increased. Single-stage buck and boost inverter topologies are investigated. Three-phase boost DC–AC inverters generate higher sinusoidal AC output voltages than the input one in single stage [4–6]. Due to differential output voltage, boost inverter topology has unique operating behaviour. The control of these boost inverters is usually done with the help of control scheme, which consists of inductor current control inner loop and output voltage control outer loop. To control the boost nature of the model around a particular operating point, small signal linear models have been analysed. The sliding mode controller describes variable operating point conditions with achievements of good steady-state results. However, the selection of controlling parameters related to variable switching frequency and inductance average current control is a major drawback of sliding mode control technique [7,8,9,10,11,12,13]. A new control strategy has been adopted for three-phase boost inverters in case of standalone distributed generation systems [14, 15]. Rule-based controller is proposed to reduce second-order harmonics current ripple component appears at the DC side of the inverter [16] Neuro-fuzzy controllers are also analysed for control of single-phase switched boost inverters [17]. So in this paper single-stage three-phase DC–AC boost inverter’s both boosting and inversion characteristics are analysed simultaneously from lower DC input voltage. Finally, sinusoidal AC output voltage is obtained which is the difference of outputs of any two boost converters. The three-phase DC–AC boost inverter operation method is examined by the MATLAB simulations.

2 Topology of Proposed Three-Phase DC–AC Boost Inverter

DC-biased sinusoidal AC output voltage is obtained from the proposed three-phase DC-to-AC boost inverter. It is a cascade connection of three symmetrical DC–DC boost converters with a common DC input voltage \(\left( {V_{\text{in}} } \right)\). Each DC–DC boost converters have DC-biased sinusoidal AC output voltage across the output capacitor. Due to same DC biasing voltage in each DC–DC converter, DC-biased voltage is cancelled to each other in resultant output voltage. Hence, resultant output voltage of any two converters is a pure sinusoidal waveform. The load is connected differentially across all the boost converters. The basic configuration of proposed three-phase boost inverter consists of three inductors and three capacitors which are used as energy storing elements, six power semiconductor switches (MOSFETs), DC supply voltage source as input and load (R, RL). The analysis for R and RL load is done. The proposed three-phase DC–AC boost voltage source inverter circuit diagram is shown in Fig. 1. The operation of proposed DC–AC boost inverter can be explained by considering the circuit operation in the following ways—Six power semiconductor switches (MOSFETs) have different switching sequences in six modes 1, 2, 3 4, 5, and 6 (180° mode of operation).

Fig. 1
figure 1

Circuit diagram of proposed single-stage three-phase DC–AC boost inverter

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In mode 1, only switches S5, S6, and S1 conduct simultaneously and remaining three switches are in off state. Similarly in Mode 2, switches S6, S1, and S2 conduct, in mode 3, switches S1, S2, and S3 conduct, in mode 4, switches S2, S3, and S4 conduct, in mode 5, switches S3, S4, and S5 conduct and in mode 6, switches S4, S5, and S6 conduct.

In each individual mode, one inductor magnetizes and stores energy in it through one switch from input supply, and consequently its current rises and remaining two inductors demagnetise and deliver energy to the capacitors and loads. Also, the remaining two capacitors discharge via the load resistance R and the capacitor. In this way, capacitors charge and output voltage develops across the load.

The following mathematical equations for boost inverter in case of continuous conduction mode are as follows:

$$V_{an} = K\,\sin \,\omega t + V_{\text{DC}}$$
(1)
$$V_{bn} = K\,\sin (\omega t - {{2\pi } \mathord{\left/ {\vphantom {{2\pi } 3}} \right. \kern-0pt} 3}) + V_{\text{DC}}$$
(2)
$$V_{cn} = K\,\sin (\omega t - {{4\pi } \mathord{\left/ {\vphantom {{4\pi } 3}} \right. \kern-0pt} 3}) + V_{DC}$$
(3)
$$\begin{aligned} V_{ab} = & V_{an} - V_{bn} = (K\,\sin \,(\omega t) + V_{\text{DC}} ) - (K\,\sin \,t(\omega t - 2\pi /3) + V_{\text{DC}} ) \\ = & \sqrt 3 K\,\sin \,(\omega t + \pi /6) \\ \end{aligned}$$
(4)
$$V_{bc} = V_{bn} - V_{cn} = \sqrt {3\,} K\,\sin (\omega t - {{5\pi } \mathord{\left/ {\vphantom {{5\pi } {6)}}} \right. \kern-0pt} {6)}}$$
(5)
$$V_{ca} = V_{cn} - V_{an} = \sqrt 3 \,K\,\sin (\omega t + 5{\pi \mathord{\left/ {\vphantom {\pi {6)}}} \right. \kern-0pt} {6)}}$$
(6)

\(V_{\text{in}}\) is the common DC input voltage of all three converters. \(V_{an} ,V_{bn} ,V_{cn}\) are the output phase voltages, and \(V_{ab} ,V_{bc} ,V_{ca}\) are the line-to-line output voltages across the capacitor of the DC–DC boost converter. \(V_{\text{DC}}\) is the DC-biased voltages. K is the amplitude of the desired sinusoidal voltages. The outputs of all three converters are connected with load in series. The desired boost inverter sinusoidal output phase voltages are as follows:

$$V_{aN} = K\,\sin \,\omega t,$$
(7)
$$V_{bN} = K\,\sin (\omega t - {{2\pi } \mathord{\left/ {\vphantom {{2\pi } 3}} \right. \kern-0pt} 3})$$
(8)
$$V_{cN} = K\,\sin (\omega t - {{4\pi } \mathord{\left/ {\vphantom {{4\pi } 3}} \right. \kern-0pt} 3})$$
(9)

To avoid zero level crossing, the minimum DC voltage required is \(V_{\text{DC}} \ge V_{\text{in}} + K\).

The system is analysed by two distinct sets of equations: first describing the input side and the other the output side. Both the input and output sides include DC and AC components. The generalized input and output equations are as follows:

$$V_{\text{in}} { = }Ri_{{L_{j} }} { + }L{\text{d}}i_{{L_{j} }} / {\text{dt + }}p_{j} V_{kn} ,\;j{ = }a ,\;b ,\;c\;\& \;k = A ,\;B ,\;C$$
(10)
$$\sum\limits_{j = a,\,b,\,c} {i_{{L_{j} }} } = I_{\text{in}}$$
(11)
$$\begin{aligned} i_{j} & = - C\frac{{{\text{d}}V_{\text{in}} }}{{{\text{d}}t}} + i_{{{\text{OUT}}_{j} }} \\ i_{j} & = - C\frac{{{\text{d}}V_{\text{in}} }}{{{\text{d}}t}} + p_{j} i_{{L_{j} }} \quad j = a,\;b,\;c\quad k = A,\;B,\;C \\ \end{aligned}$$
(12)
$$\sum\limits_{j = a,b,c} {i_{j} = 0}$$
(13)

where \(i_{{L_{a} }}\),\(i_{{L_{b} }}\) and \(i_{{L_{c} }}\) are the currents flowing through inductors \(L_{a} = L_{b} = L_{c} = L\) and \(R_{a} = R_{b} = R_{c} = R\) is the resistive part (not shown in Fig. 1) for each inductor. \(i_{a} ,\;i_{b} ,\;i_{c}\) are the currents flowing across the loads and the value of capacitance of each capacitor is C.

3 Simulation Results and Discussion

Circuit parameter specifications are as follows:

Vin = 100 V (input voltage); Vo = 200 V (output voltage); fs = 1 kHz, 5 kHz (switching frequency),

Load R = 4 Ω, L = 0.75 mH, La = Lb = Lc = 8 mH; Ca = Cb = Cc = 9 µF; fo = 50 Hz (output voltage frequency).

Figure 2 shows the MATLAB Simulink diagram of single-phase DC–AC boost inverter. Figure 3 shows the gate pulses for six power switches. Figures 4 and 5 show the output voltage waveform across the R and RL load; AC voltage is obtained with switching frequency of 5 and 1 kHz with 100 V DC source input and output frequency of 50 Hz is also obtained. Figures 6 and 7 show the output current waveform across the load. Figures 8 and 9 show the voltage across the capacitor, which is DC-biased sinusoidal in nature.

Fig. 2
figure 2

MATLAB Simulink diagram of proposed single-stage three-phase DC–AC boost inverter

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Fig. 3
figure 3

Gate pulse voltage across switches S1, S2, S3, S4, S5, and S6

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Fig. 4
figure 4

Output line voltages across RL load (for 50% duty cycle and 5 kHz switching frequency)

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Fig. 5
figure 5

Output line voltages across RL load (for 60% duty cycle and 1 kHz switching frequency)

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Fig. 6
figure 6

Output current across R = 4 Ω, L = 0.75 mH load (for 5 kHz switching frequency and 50% duty cycle)

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Fig. 7
figure 7

Output current across R = 4 Ω load (for 5 kHz switching frequency and 50% duty cycle)

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Fig. 8
figure 8

Voltage across capacitors Ca, Cb, Cc (for R = 4 Ω and L = 0.75 mH)

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Fig. 9
figure 9

Voltage across capacitors Ca, Cb, Cc (for resistive load R = 4 only, 5 kHz switching frequency, 50% duty cycle)

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Comparison with conventional buck VSI

Conventional buck VSI

Three-phase DC–AC boost inverter

1. It requires two-stage power conversion

It requires single-stage power conversion

2. It requires extra filter design for achievement of pure sinusoidal AC output voltage

It does not require extra filter design for achievement of pure sinusoidal AC output voltage

3. Its output is always less than input

Its output is greater than input

4. Overall size of system is bulky and hence efficiency will decrease

Overall size of system is less bulky than conventional buck VSI and hence efficiency will increase

4 Conclusion

In this paper, three-phase single-stage DC–AC boost inverter topology has been presented, which always produces sinusoidal AC output voltage whose average value is greater than the input DC voltage. This configuration and unique property of three-phase DC–AC boost inverter produces the differential of DC-biased sine wave output of three separate DC–DC boost converters and differentiates it from conventional three-phase buck voltage source inverters (VSI). It may be applied as power electronics converter in the field of grid integration of distributed generation system, storage devices, electric vehicles, hybrid electric vehicles to drive the power train of vehicles and interfacing with AC motor drives. Simultaneous boosting and inversion process in one stage can be analysed from the mathematical expression and simulation results of circuit configuration, which is a clear advantage over two-stage (DC–DC boost converter and DC–AC conventional buck voltage source inverter) power conversion. The common dangerous shoot-through problem in conventional three-phase VSI will not occur in this proposed three-phase boost inverter due to inductor in the input side. The future work will be oriented towards the controller design for higher duty cycle operation, output voltage regulation, grid integration and system stability issues of three-phase boost inverter for distributed generation system.