# A Current Source Buck-Boost Inverter for Active and Reactive Power Injection to the Low Voltage Grid

Christoforos Tsounis, Emmanuel Tatakis, Panagis Vovos

► To cite this version:

Christoforos Tsounis, Emmanuel Tatakis, Panagis Vovos. A Current Source Buck-Boost Inverter for Active and Reactive Power Injection to the Low Voltage Grid. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0250. hal-05087890

HAL Id: hal-05087890

https://utc.hal.science/hal-05087890v1

Submitted on 28 May 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# A Current Source Buck-Boost Inverter for Active and Reactive Power Injection to the Low Voltage Grid

Christoforos Tsounis, Dr.-Ing. Emmanuel Tatakis, Dr. Panagis Vovos

UNIVERSITY OF PATRAS

Department Of Electrical and Computer Engineering

University Campus of Patras, GR26504 Rion-Patras, Greece

Tel.: +30/2610-996412

E-Mail: xristoftsounis@gmail.com, e.c.tatakis@ece.upatras.gr, panagis@upatras.gr

URL: http://www.lemec.ece.upatras.gr

## Keywords

«Current Source Inverter (CSI)», «Grid-Connected Inverter», «Buck-Boost Converter», «Reactive Power», «Experimental Testing».

## Abstract

With the ever-rising dependence and demand for Renewable Energy Sources, the development of reliable power converters that correspond to the modern-day grid requirements has become more necessary now than ever. In this paper a Bidirectional Buck-Boost inverter acting as a current source is proposed. This is realized by designing a Buck-Boost converter to operate only in the DCM mode, followed by an unfolding inverter to connect to the low voltage network. This arrangement offers complete freedom of choice both for the output current amplitude as well as its phase relative to the grid voltage. So, it is possible to precisely select the output power factor and inject both active and reactive power into the low voltage grid, by designing judiciously the converter control unit. Simulation and experimental results are used to validate the converters' design and operation.

## I. Introduction

In recent years, the rise in demand for Renewable Energy Sources (RESs) has been decisive and immediate. According to the European Commission, in 2018 a binding energy target was set for European Union countries in which, of all energy production, 32% must come from Renewable Energy Sources by 2030, such as photovoltaics and wind turbines [1]. That target has since been revised and raised to 42.5% in 2021, as part of the Renewable Energy Directive [2]. Thus, the need for evolution and optimization

for Power Electronics topologies used in such applications is also imperative.

This paper aims to analyze and discuss a two-stage inverter connected to the low voltage grid, in which a Bidirectional Buck-Boost converter and a single-phase full bridge Inverter are used. The DC-DC converter will operate in the Discontinuous Conduction Mode (DCM) behaving as a constant current source. So, the direct current control of the output current is possible. The DC-AC converter will be an Unfolding Inverter, responsible for reversing its input voltage and current polarity when necessary.

This topology has been proposed and analyzed in previously published works [3]-[9], but with the significant difference that the Buck-Boost converter was designed to operate in Continuous Conduction Mode (CCM). Moreover, the control unit was designed to inject only active power into the grid. Changing the operation mode introduces a significant advantage to the function of the circuit, since the duty cycle is now dependent on the reference output current, making the direct current control possible and, thus, allowing the precise control of the output apparent power. This means that the Buck-Boost converter behaves as a current source instead of a voltage one.

Essentially, the primary purpose of this work is to achieve the capability of both active and reactive power injection into the low voltage grid, not only either active or reactive [10]. Although operation with unity power factor was the preferred power control practice imposed upon Distributed Generators by the Distribution System Operators, recent studies have shown that reactive power interchange provides auxiliary service to the grid in the form of voltage support [11], especially during voltage drops, short-circuits or dynamic instabilities. This contributes to probably the most
<figure><img src="image_2.png" /><figcaption>Fig. 1: Bidirectional Twisted Single-Phase Buck-Boost Unfolding Inverter</figcaption></figure>

important requirement for the grid, which is the reliable and stable voltage level throughout the day. In this application the desired active and reactive power is implemented by imposing a phase shift between the grid voltage and the desired output current.

After the initial theoretical analysis of the converters and their desired operation (Section II), the application was tested with the use of a simulation model. Utilizing the reliable Matlab/Simulink software, the proper operation of the topology was confirmed along with the mathematical equations describing the model (Section III). Subsequently, an experimental PCB prototype was developed in the lab, with the aim of further testing the correct operation of the proposed topology as well as the accompanying control unit, under realistic conditions and varying parameters. Finally, the experimental results are presented and discussed along with the methodology with which they were achieved (Section IV).

## II. Theoretical Analysis

In this section, the operation of the two converters is discussed along with the method with which the Pulsed Width Modulation (PWM) technique is applied. Also, the rated operating conditions are presented as well as the calculations for the passive components of the configuration.

### The Bidirectional Buck-Boost Converter

The bidirectional Buck-Boost converter consists of the two MOSFET transistors $S_1$ and $S_2$ which have a built-in monolithic body diode. Two capacitors at the input and output, $C_{IN}$ and $C_{OUT}$ respectively, are used along with the power inductor $L_F$, responsible for storing and releasing magnetic energy in the form of current. The two capacitors are used as both part of the necessary

filters for the input - output current and as a way to stabilize the voltage.

Before continuing on to the detailed theoretical operation of the DC-DC converter, it is important to understand its desired operation purpose. Fig. 2a depicts an ideal theoretical example of the DC-DC converter output current and voltage as well as the grid voltage and reference current (Fig. 2b) during inductive load operation. The voltage at the output of the Buck-Boost converter $V_{O,BB}$ is the fully rectified grid voltage and the current must be such that, when it is reversed by the inverter, it creates the desired sinusoidal waveform.

<figure><img src="image_3.png" /><figcaption>Fig. 2: (a) Buck-Boost converter output Voltage $V_{O,BB}$ and Current $I_{O,BB}$ (b) Grid Voltage $V_{GRID}$ and output Current reference $I_{O,REF}$</figcaption></figure>

Equations (1) and (2) describe the grid voltage and the output reference current respectively, considering the grid voltage phase as the reference. Also, equation (3) describes the voltage at the output of the Buck-Boost converter.
$$V_{GRID} = V_{GRID,PEAK} \cdot \sin(2\pi \cdot f \cdot t) \quad (1)$$

$$I_{O,REF} = I_{O,PEAK} \cdot \sin(2\pi \cdot f \cdot t + \varphi) \quad (2)$$

$$V_{O,BB} = V_{GRID,PEAK} \cdot |\sin(2\pi \cdot f \cdot t)| \quad (3)$$

where $\varphi$ the desired phase difference between the grid voltage and the output current, $f = 50Hz$ the grid frequency, $V_{GRID,PEAK} = \sqrt{2} \cdot 230V$ the peak grid voltage value and $I_{O,PEAK} = \sqrt{2} \cdot I_{O,RMS}$ the desired peak current value.

The output current of the Buck-Boost converter is defined as the mean value of the current that flows through the transistor $S_2$ per switching period, either when it operates as a switch or as a diode. In order to achieve a sinusoidal waveform for the output current, the Sinusoidal Pulse Width Modulation (SPWM) technique is used to trigger the transistors $S_1$ and $S_2$. This method requires the use of a reference signal which depicts the waveform that the output current should follow. In this application, the reference signal is calculated using the mathematical equations that characterize the DC-DC converter.

Moving onto the operation of the Buck-Boost converter, it is important that its two transistors must never be ON at the same time to avoid short-circuiting the input with the output. This means it is of the utmost importance to clarify when, during the period, one must behave as a switch and when as a diode. The decision is made according to the grid voltage and the output reference current. When the two quantities have the same sign (i.e. their product is a positive number), transistor $S_1$ is the switch while transistor $S_2$ acts only as a diode. On the other hand, when the two quantities have the opposite sign (i.e. their product is a negative number), transistor $S_1$ acts as a diode with transistor $S_2$ being the switch.

As previously noted, the converter operates only in DCM, behaving as a current source. This means that the power inductor current is raised and decreased linearly, since the input and output voltages are considered stable for the duration of a switching period, according to the characteristic equation (4).

$$-V_{LF} = L_F \cdot \frac{dI_{LF}}{dt} \quad (4)$$

After the transistor $S_1$ turns off, the power inductor current will decline and stay at zero, until the next time the $S_1$ switch turns on again. This

means that, for a switching period, the inductor $L_F$ current and voltage waveforms will look like the ones shown in Fig 3.

<figure><img src="image_4.png" /><figcaption>Fig. 3: Voltage and Current of the Inductor $L_F$</figcaption></figure>

The power inductor is being both charged and discharged during a switching period, which results in equation (5) being true. Also, the output current of the Buck-Boost converter is the mean value of the current that flows through the transistor $S_2$ (either when it turns on or through its diode $D_2$).

$$\delta_1 \cdot T_S \cdot V_{in} = D_1 \cdot T_S \cdot v_{O,BB} \quad (5)$$

Using equation (5) as well as the formula for the mean value, equation (6) is produced by solving with respect to the duty cycle of transistor $S_1$.

$$\delta_{S1} = \sqrt{\frac{2 \cdot f_S \cdot L_f \cdot |i_{O,REF}| \cdot v_{O,BB}}{V_{in}^2}} \quad (6)$$

where $f_S$ is the switching frequency of the transistor, $L_F$ is the inductance, $i_{O,REF}$ is the instantaneous value of the output current reference, $v_{O,BB}$ is the instantaneous value of the output voltage and $V_{IN}$ is the input voltage.

Equation (6) is especially important for the operation of the converter, since this will be the method used to calculate the duty cycle and implement the SPWM on the Buck-Boost converter. It is noted that the output voltage of the DC-DC converter will be the rectified grid voltage, imposed by the grid through the unfolding inverter. This also eliminates the need for a high capacitance at the output since the voltage is not a fixed value.
A similar approach is taken in order to calculate the duty cycle for the transistor S<sub>2</sub>. This topology of the Buck-Boost converter is symmetrical which makes the understanding of the overall operation easier. It is important to note that, during the reverse energy flow, the current of the power inductor is negative, meaning it has the opposite direction than before. Since the roles of the output and input voltage are reversed, with the transistor S<sub>1</sub> acting only as a diode, equation (7) is calculated for the duty cycle of the transistor S<sub>2</sub>.

$$ \delta_{S2} = \sqrt{\frac{2 \cdot f_S \cdot L_f \cdot |i_{O,REF}|}{v_{O,BB}}} \quad (7) $$

It should be noted that, when the grid voltage value is very low, i.e. when the DC-DC output voltage is very low, the duty cycle reaches abnormally high values. In practice, that should not be possible since, by definition, the duty cycle must be between 0 and 1. For this reason, a saturation limit of 95% is implemented so that the duty cycle never surpasses this value.

## The Unfolding Inverter

The Unfolding Inverter used in this application consists of four MOSFET transistors which have a built-in monolithic body diode. The operation of this DC-AC converter is simple. Its purpose is to reverse the polarity of the input voltage and current every half period in order to match the polarity of the grid voltage and achieve a proper connection to the low voltage grid. This means that transistors S<sub>3</sub>-S<sub>6</sub> have a switching frequency of just 50Hz, the same as the grid voltage, thus very low switching losses can be encountered. More specifically S<sub>3</sub> and S<sub>4</sub> are turned ON simultaneously only when the grid voltage is positive, while S<sub>5</sub> and S<sub>6</sub> are behaving as diodes. On the contrary when the grid voltage is negative, only transistors S<sub>5</sub> and S<sub>6</sub> allow current flow. In summary the transistors operate according to the logic shown in Fig. 4.

<figure><img src="image_5.png" /><figcaption>Fig. 4: Control Pulses of the Unfolding Inverter</figcaption></figure>

It is worth mentioning that care should be given so that two transistors of the same branch never

allow current flow at the same time. Otherwise, a short circuit at the inverter input would be created, resulting in severe equipment damage. The solution is to add a short dead time of a few microseconds that does not affect the overall operation of the circuit, so that there is enough time for the transistor to completely turn OFF before the other turns ON. Based on the theoretical analysis of the two converters, it is evident that, with this method of operation, the switching losses of the transistors are minimized. The losses on the inverter can be considered negligible since the switching frequency is so small and, on the Buck-Boost, only one transistor at a time is responsible for the SPWM. Also, the function state of all the transistors of the topology is summarized in Table I during a period of the grid's waveform, according to the sign of grid voltage and the reference for the output grid current.

<table><caption>Table I: Operation States of Transistors</caption><thead><tr><th></th><th>S<sub>1</sub></th><th>S<sub>2</sub></th><th>S<sub>3</sub>, S<sub>4</sub></th><th>S<sub>5</sub>, S<sub>6</sub></th></tr></thead><tbody><tr><td>V<sub>GRID</sub> > 0<br>I<sub>O,REF</sub> > 0</td><td>SWITCH</td><td>DIODE</td><td>SWITCH</td><td>DIODE</td></tr><tr><td>V<sub>GRID</sub> > 0<br>I<sub>O,REF</sub> < 0</td><td>DIODE</td><td>SWITCH</td><td>SWITCH</td><td>DIODE</td></tr><tr><td>V<sub>GRID</sub> < 0<br>I<sub>O,REF</sub> > 0</td><td>DIODE</td><td>SWITCH</td><td>DIODE</td><td>SWITCH</td></tr><tr><td>V<sub>GRID</sub> < 0<br>I<sub>O,REF</sub> < 0</td><td>SWITCH</td><td>DIODE</td><td>DIODE</td><td>SWITCH</td></tr></tbody></table>

## Topology Design

Firstly, the rated operation circumstances must be specified. The grid voltage is considered stable at 230V RMS and a desired output of 350VA is assumed. That sets the rated current at 1.52Amps RMS with the rated input voltage being 150V DC. Next, the switching frequency is set at 50kHz and the maximum duty cycle is calculated at 0.68 using formula (8). It is noted that equation (8) is true only during CCM operation and, in this case, it is used to calculate the maximum boundary duty cycle between CCM and DCM.

$$ \delta = \frac{V_{OUT}}{V_{OUT} + V_{IN}} = \frac{|V_{GRID}|}{|V_{GRID}| + V_{DC}} \quad (8) $$

An especially important element of the Buck-Boost converter is the power inductor and the choice of its inductance value. For the Buck -
Boost to operate in the DCM state under all circumstances, care should be taken to choose the correct inductance. It needs to be sufficiently small so that the current has a big enough slope to increase and decrease fast enough. The mathematical equation for ensuring the DCM operation is the following:

$$ I_{O,MAX} \le I_{O,B} = > \\ = > L_F \le \frac{V_{OUT}}{I_{O,MAX} \cdot f_S} \cdot \frac{(1 - \delta)^2}{2} \quad (9) $$

where $I_{O,MAX}$ is the maximum output current and $I_{O,B}$ is the boundary current between CCM and DCM.

Using the values mentioned previously, it is found that: $L_F \le 238.9$ uH. Therefore, the inductor was chosen at $L_F=100$uH so that the DCM state is always and securely achieved.

For the proper operation of the circuit, the use of low-pass filters both at the input and the output is mandatory as shown in Fig. 1. The two filters chosen are passive low pass second order LC filters. Their purpose is to eliminate the higher order harmonics from the current waveform so that the undesirable reactive power is greatly reduced. The values of the components are set by considering the desired cut-off frequency, the frequency of the signal and the available lab equipment.

For the input filter, the capacitor $C_{IN}$ is chosen at 1.16mF and the inductor $L_{IN}$ at 14mH, which sets the cut-off frequency at 39.5Hz. Such a low cut-off frequency is desired since it is optimal for the input current to fluctuate as little as possible from its mean value. As for the filter at the grid's side, the capacitor $C_{OUT}$ is equal to 1.5µF and the inductor $L_{OUT}$ to 7.2mH, accomplishing a cut-off frequency of 1.53kHz. This value is satisfactory since the primary purpose is to filter out the harmonics with frequencies higher than 50kHz and leave the fundamental harmonic at 50Hz unaltered.

The values of the components discussed in this section will be the ones used for the rest of this study, for both the simulation and the experimental prototype. They are summarized in Table II.

### III. Simulation Analysis

In this section, a simulation model for the topology is developed to test, analyze and confirm its proper operation. The simulation environment chosen is the Matlab/Simulink

software, known for its reliability and accurate results.

<table><caption>**Table II: Rated Values and Values of the Topology Components**</caption><thead><tr><td>Quantity</td><td>Symbol</td><td>Value</td></tr></thead><tbody><tr><td>Apparent Power</td><td>S</td><td>350 VA</td></tr><tr><td>Grid Voltage</td><td>V<sub>GRID</sub></td><td>230V RMS</td></tr><tr><td>Input Voltage</td><td>V<sub>IN</sub></td><td>150V DC</td></tr><tr><td>Switching Frequency</td><td>f<sub>S</sub></td><td>50 kHz</td></tr><tr><td>Power Inductor</td><td>L<sub>F</sub></td><td>100 uH</td></tr><tr><td>Input Capacitor</td><td>C<sub>IN</sub></td><td>1.16 mF</td></tr><tr><td>Input Inductor</td><td>L<sub>IN</sub></td><td>14 mH</td></tr><tr><td>Output Capacitor</td><td>C<sub>OUT</sub></td><td>1.5 uF</td></tr><tr><td>Output Inductor</td><td>L<sub>OUT</sub></td><td>7.2 mH</td></tr></tbody></table>

### Connection to the Grid and start of Operation.

Before highlighting and discussing the results, it is important to elaborate on the method with which a safe connection to the voltage grid is achieved and a smooth start of operation is attained. If those two conditions are not met, there is a significant risk for overcurrents and/or overvoltages to appear, which could compromise the validity of the measured results or even damage the experimental prototype.

First of all, the voltage grid is connected to the output of the inverter. This way the output capacitor $C_{OUT}$ is charged using the freewheeling diodes of the inverter and matches the grid voltage. This is possible because of its low capacitance, which does not cause an overcurrent. At the same time, the grid voltage is measured and, with the use of a Phase Locked Loop (PLL), the grid's frequency and phase are continuously calculated. The SOGI PLL that is used satisfies the requirements of the application and accurately detects the grid frequency and phase [12].

The next step is the creation of the pulses for the Unfolding Inverter. As mentioned above, the inverter's pulses are synchronized with the voltage grid so that it reverses the input polarity every half period. It is necessary for the inverter to start operating before the Buck-Boost converter so that the current of the power inductor has a way be diffused. Otherwise, the generated current will reach exceedingly high values.
<figure><img src="image_6.png" /><figcaption>Fig. 5: Simulation Results. Top waveforms depict the voltage grid with the respective produced current. Bottom waveforms depict the duty cycle for the two transistors of the Buck-Boost along with the power inductor current. (a)-(b): 0° voltage-current phase shift. (c)-(d): -40° voltage-current phase shift [inductive] (e)-(f): +40° voltage-current phase shift [capacitive]</figcaption></figure>

Finally, the Buck-Boost converter starts operating when the input voltage surpasses a certain threshold. That threshold is set by the user depending on the variables and the parameters of the experiment, such as the desired output power. A better study of equation (6), which describes the duty cycle of transistor $S_1$, explains this methodology. When the input voltage is exceptionally low, the duty cycle reaches inappropriately high values, which results in distorted output current.

## Simulation Results

The simulation results for the rated parameters of operation are presented in Fig. 5. Three different simulations are showcased, all of which were conducted with the same parameters, the only difference being the 40° phase shift between the voltage grid and the output reference current. The desired phase shift was created by simply adding or subtracting the required degrees from the phase calculated by the PLL and setting its response as the phase for the output reference current.

The results appear to match the theoretical expectations. Fig. 5(a) confirms that the current is in-phase with the grid voltage when a unity power factor is desired. It is noted that, of the two transistors of the Buck-Boost, only $S_1$ is working as a switch during one period while $S_2$ never conducts and acts as a diode. Also, by observing the power inductor current in fig. 5(b), it is

confirmed that the Buck-Boost converter functions only in the DCM state during the period and that the SPWM technique works as expected, since the current has a rectified sinusoidal shape before being filtered.

Fig. 5(c) and 5(d) depict the operation of the topology when injection of both reactive (inductive) and active power is needed. The chosen phase shift was selected to be 40°, so that it is clear from the presented waveforms that this function is achieved. However, closely observing the output current, a small dip can be seen at the zero-crossing voltage of the grid. This happens exactly due to the low grid voltage. More specifically, at that moment, the Buck-Boost converter begins its reverse energy flow and the voltage grid acts as the source instead of the load. However, the voltage value is exceptionally low for the desired current, so the current momentarily sags. This is compensated by the high duty cycle value of the $S_2$ transistor, which reaches the set saturation of 95% as shown in fig. 5(d). Because of this, the current waveform suffers minor distortion and a 5.33% THD is reached. It should be noted that when the desired phase shift is increased, so does this dip but the resulting THD is not significantly worse. Finally, as expected, the power inductor current of the Buck-Boost converter is flowing in reverse when transistor $S_2$ acts as a switch and $S_1$ as a diode. Remarkably similar observations are also true for the capacitive reactive power injection as shown
in fig. 5(e) and 5(f). This time a 3.36% THD is achieved for the output current waveform, slightly better than the previous occasion. Evidently, the simulation results are quite satisfactory and encouraging. The topology functions as theoretically expected and the values of the parameters in section II appear to have been calculated correctly. With these results in mind, an experimental prototype has been created so that the performance of the topology is tested in real life.

# IV. Experimental Testing

In this section the design and development of the experimental prototype is discussed and the produced results are described. The setup is mainly comprised of two Printed Circuit Boards (PCBs): one being the Bidirectional Buck-Boost and the other being the Unfolding Inverter.

## Experimental Setup

Fig. 6 shows the experimental bench and the complete setup. Following is a description of the hardware used in this topology.

The input power supply was the Chroma Programmable AC source Model 61602, which was configured to function as the desired DC voltage source. Also, the FN2030-16-06 choke inductor was used to filter the input current. In order to reduce the reactive power at the input two 5mH inductors were connected in series with the

<figure><img src="image_7.png" /><figcaption>Fig. 6: Experimental Bench</figcaption></figure>

choke inductor. The input capacitor is composed of three electrolytic capacitors connected in parallel, two of which are rated at 470 uF and one at 220uF.

The output filter is comprised of two inductors made in-lab rated at 3.6mH each connected in series, achieving the mentioned 7.2mH. The

output capacitor used is a polyester WIMA MKS4 rated at 1.5uF and 630V. Furthermore, a solid-state circuit breaker was implemented with the function of opening when an overcurrent is detected. This was of the upmost necessity because it protected the topology of possible mistakes that happened during the development stage. Finally, a 1:1 isolation transformer was used between the circuit breaker and the low voltage grid in order to provide galvanic isolation. The Full Bridge Unfolding Inverter PCB is comprised only of four transistors and their respective drivers. The MOSFETs used are the IXFK24N100 with a rated voltage of 1kV and current of 24A. The transistors are also supported by passive snubber circuits, comprised of a 10Ω resistor and a 10nF capacitor.

The bidirectional Buck-Boost PCB was designed to support both the power circuit and the microcontroller along with the required censors. The transistors used were Infineon's IGBTs IHW30N160R5, each one equipped with a monolithic diode and rated at 1.6kV and 30A current. As for the power inductor, two of the 7443763540470, produced at Wurth Electronics, were used after being connected in series.

Because of the DCM function state, the inductor current reaches exceptionally high values at its peak, making the saturation current a very important deciding parameter. The chosen power inductor is rated at 31A saturation current and 47uH inductance, which is the reason two of these are used.

Additionally, four sensors were needed. Two voltage sensors, in order to measure the input and the grid voltage, and two current sensors, to measure the input and the grid side current. The voltage sensors are based on the ACPL-C79A integrated circuit produced by Avago Technologies, with the purpose of "translating" the high voltage into values the ADC pins of the microcontroller can read. Similarly, the current sensors utilized the ACS712 integrated circuit produced by Allegro.

Finally, the microcontroller used is the Delfino TMS320F28377D control card by Texas Instruments. As mentioned, four analog-to-digital pins were used as well as four pulse width modulation pins (PWM) for switching the transistors. Also, a general-purpose output pin controlled the circuit breaker. Finally, the code for the processor was created using the DSP hardware support package by Matlab / Simulink, which translated the discrete-time simulation logic into executable code.
<figure><img src="image_8.png" /><figcaption>Fig. 7: Experimental Results. Top waveform depicts the grid voltage and the bottom depicts the respective produced current. (a): 0° voltage-current phase shift. (b): -40° voltage-current phase shift [inductive] (c): +40° voltage-current phase shift [capacitive]</figcaption></figure>

## Experimental Results

The results of the experiments that were conducted are depicted in fig. 7. The same parameters and variables were used for the simulation and the experimental analysis, so that a comparison between them is possible.

Fig. 7(a) shows the case where a unity power factor is desired. It is evident that the two waveforms of the output voltage and current are in-phase, which means that only active power is produced. Also, the oscilloscope calculates the RMS of the output current at 1.43A which means that the output power is 329W. As mentioned in section II, the rated output power was set at 350W, meaning that the actual operation is very close to the expected value.

Figure 7(b) corresponds to -40° phase shift between the grid voltage and the output reference current with the phase of the grid voltage considered as zero. It is possible to calculate the exact phase shift generated using the cursor of the oscilloscope. By measuring the time difference between the positive zero-crossing of the two waveforms, the phase shift can be calculated by the definition of the angular frequency where f is assumed 50Hz for both the waveforms:

$$ \omega = \frac{d\varphi}{dt} \Rightarrow d\varphi = 2 \cdot \pi \cdot f \cdot dt \cdot \frac{180}{\pi} \quad (10) $$

Substituting dt = 2.6ms, the calculated phase shift is 46°, very close to the desired reference of 40°. The final experiment shown in fig. 7(c) is the capacitive reactive power injection with positive phase shift between voltage grid and reference current. Using the same methodology as above, the resulting phase shift is 36°, again very close to the reference value of 40°. Also, the current

sag, which was discussed in the simulation section, is more clearly evident this time. Finally, it is noted that, although the phase shift reference is the same for both experiments, for the inductive operation the phase shift is a bit larger. This is an expected result, considering the relatively large inductor at the output of the inverter.

## V. Conclusion

The suggested operation method of the Current Source Buck-Boost Unfolding Inverter appears to be appropriate for Renewable Energy Source applications. It offers a wide range of choices as far as the active and reactive power output is concerned, since it is possible to set the phase shift between the low voltage grid and the output current. This way the topology corresponds effectively to one of the most important modern energy requirements being flexible and adaptable. Both the simulation and the experimental results correspond to the theoretical expected outcome as well as previous work conducted with this topology. The wide variety of achievable power factors renders this operation method very interesting and worth further studying for low voltage grid-connected applications.

## References

[1]. European Commission: "Renewable Energy Targets", 2018

[2]. European Commission: "Renewable Energy Directive", 2023

[3]. O. Husev, O. Matiushkin, C. Roncero-Clemente, D. Vinnikov, V. Chopyk, "Bidirectional Twisted Single-Stage Single-
Phase Buck-Boost DC-AC Converter”,
MDPI Energies 2019, 12(18).

[4]. D. L. Caiza, S. Kouro, F. Flores-Bahamonde, R. Hernandez, "Unfolding PV Microinverter Current Control: Rectified Sinusoidal vs Sinusoidal Reference Waveform", 2018 IEEE Energy Conversion Congress and Exposition (ECCE), pp. 7094-7100, September 2018.

[5]. O. Husev, O. Matiushkin, D. Vinnikov, C. Roncero, E. Romero-Cadaval, L. Kutt, "Buck-Boost Unfolder Inverter as a Novel Solution for Single-Phase PV systems", IECON 2018 - 44th Annual Conference of the IEEE Industrial Electronics Society, pp. 6116-6121, October 2018.

[6]. O. Matiushkin, O. Husev, J. Rodriguez, H. Young, I. Roasto, "Feasibility Study of Model Predictive Control for Grid-Connected Twisted Buck-Boost Inverter", IEEE Transactions on Industrial Electronics, vol. 69, no. 3, pp. 2488-2499, March 2022.

[7]. O. Matiushkin, O. Husev, R. Strzelecki, S. Ivanets, A. Fesenko, "Novel Single-Stage Buck-Boost Inverter with Unfolding Circuit", 2017 IEEE First Ukraine Conference on Electrical and Computer Engineering (UKRCON), pp. 538-543, May 2017.

[8]. O. Matiushkin, O. Husev, D. Vinnikov, V. Gordienko, "Grid-Connected Buck-Boost Inverter Based on Unfolding Circuit", 2018 IEEE 59th International Scientific Conference on Power and Electrical Engineering of Riga Technical University (RTUCON), pp. 1-6, November 2018.

[9]. O. Matiushkin, O. Husev, D. Vinnikov, C. Roncero-Clemente, "Model Predictive Control for Buck-Boost Inverter Based on Unfolding Circuit", 2019 IEEE 2nd Ukraine Conference on Electrical and Computer Engineering (UKRCON), pp. 431-436, July 2019.

[10]. C. Roncero-Clemente, O. Husev, O. Matiushkin, D. Vinnikov, F. Blaabjerk, "Reactive Power Injection Capability of Buck-Boost Inverter with Unfolding Circuit", IEEE Trans. Power Electron., vol. 37, no. 10, pp. 11876-11886, October 2022.

[11]. Y. Yang, F. Blaabjerk, H. Wang, M. C. Simoes, "Power control flexibilities for grid-connected multi-functional photovoltaic inverters", IET Renewable Power Generation, vol. 10, no.4, pp. 504-513, 2016.

[12]. F. Blaabjerk, R. Teodorescu, M. Liserre, A. V. Timbus, "Overview of Control and Grid Synchronization for Distributed Power Generation Systems", IEEE Trans. Power Electron., vol. 53, no. 5, pp. 1398-1409, October 2006.

[13]. Christidis G, Nanakos A, Tatakis E. "Optimal Design of a Flyback Microinverter Operating under Discontinuous-Boundary Conduction Mode (DBCM)", Energies. 2021; 14(22):7480.