Single-Switch Bipolar Output DC-DC Converter for Photovoltaic Application

: Bipolar DC grids have become an adequate solution for high-power microgrids. This is mainly due to the fact that this conﬁguration has a greater power transmission capacity. In bipolar DC grids, any distributed generation system can be connected through DC-DC converters, which must have a monopolar input and a bipolar output. In this paper, a DC-DC converter based on the combination of single-ended primary-inductor converter (SEPIC) and ´Cuk converters is proposed, to connect a photovoltaic (PV) system to a bipolar DC grid. This topology has, as main advantages, a reduced number of components and a high e ﬃ ciency. Furthermore, it can contribute to regulate / balance voltage in bipolar DC grids. To control the proposed converter, any of the techniques described in the literature and applied to converters of a single input and single output can be used. An experimental prototype of with bipolar output based on the combination of SEPIC and ´Cuk converters was developed. On the other hand, a perturb and observe method (P and O) has been applied to control the converter and has allowed maximum power point tracking (MPPT). The combined converter was connected in island mode and in parallel with a bipolar DC microgrid. The obtained results have allowed to verify the behavior of the combined converter with the applied strategy.


Introduction
Nowadays, the use of renewable energies is a reality. According to recent articles of economic/business analysis and electric/electronic industry publications, in many countries, this type of energy represents a high percentage of the energy consumed. Nevertheless, international agreements against climate change suggest that the percentage of energy from renewable resources should increase in the coming years. Furthermore, it is necessary to increase the number of this type of facilities, and improve their efficiency [1,2]. A clear example is the imminent electrification and expansion of the transport sector that makes necessary direct links between renewable energy and electric vehicles (EVs), in grid power.
Within the renewable energies, photovoltaic (PV) energy is one of the most installed. To increase the installed power of this technology, it has been proposed to use more efficient technologies, and reduce the facilities' cost [3,4]. Technological advances in solar power include the increased efficiency [5], the reduction of overheads, reduced size of films, use of polymers, utilization of new materials for solar power cells, improvements of DC-AC converters for connection to the grid [6][7][8][9] and new architectures for power management.
There are several possible configurations for PV systems: (i) centralized configuration, where the PV panel can be connected together, and DC power is delivered to one converter; (ii) string configuration, when several PV panels are connected in rows, and each row has its own converter; and (iii) modular configuration, where each PV module is connected to one converter.
The most common PV plant topology consists of a specific module of PV panels connected in series, forming a string or array. The connection of several strings in parallel will configure the corresponding solar plant. There are several configurations for PV systems if we consider the DC bus connection [10]. A monopolar DC configuration uses a single wire; the return conductor may be land or metallic. Its main advantage is the use of a single cable. The PV plant is connected to a monopolar DC network through a DC-DC converter (Figure 1). The converter will adapt the voltage level between the DC bus and the PV modules, so that maximum power is transferred to the system with the adequate control strategy. The energy injected to the DC bus can be transferred to the utility through a DC-AC converter. A different topology for the DC bus is the bipolar DC configuration [11]. It consists of two wires, one with positive polarity and other with negative polarity. In normal operation, the current through ground is zero. It has two voltage levels, allowing, under fault conditions, a monopolar operation. The main advantage of the installation of a bipolar DC bus is the reduction in losses, which makes it possible to reduce the section of the wires, and therefore reduce the conductor volume or increase the transmission capacity. Finally, the DC bus can use a homopolar DC configuration. It has two or more wires with the same polarity, usually negative. The return may be by land or metallic. Its main advantage is the reduction in the costs of insulation; however, the return current is double.
This work proposes the use of a bipolar DC bus instead of the monopolar DC bus, for which a new DC-DC converter topology is proposed, which allows the connection of each photovoltaic string to this bus [12,13]. The connection of a distributed generation system with DC output to a bipolar network can be done in different ways. One of them is to include a DC-DC converter of one input and one bipolar output. When the appropriate control strategy is applied, this option allows setting the generator working point. Regarding the converters, different configurations have been proposed, both in isolated and non-isolated versions. Non-isolated versions are usually more advantageous than isolated versions, from the point of view of the efficiency, cost, volume and weight. The proposed DC-DC converter with one input and two outputs is based on the combination of two converters of one input and one output, such as the single-ended primary-inductor converter (SEPIC) and Ćuk [14][15][16][17]. It is a step-down/step-up converter. This topology is effective for this type of application, with high efficiency and with the use of only one controlled power switch. In addition, for the control of the maximum power point, any proposed technique for one input and one output converter can be used [18][19][20][21][22][23][24].
The combined SEPIC-Ćuk converter has been theoretically analyzed in [14,15]. However, in this paper, a design procedure is established for the use of these converters in the connection of photovoltaic systems to bipolar DC networks. The operation of the converter is analyzed for a system This work proposes the use of a bipolar DC bus instead of the monopolar DC bus, for which a new DC-DC converter topology is proposed, which allows the connection of each photovoltaic string to this bus [12,13].
The connection of a distributed generation system with DC output to a bipolar network can be done in different ways. One of them is to include a DC-DC converter of one input and one bipolar output. When the appropriate control strategy is applied, this option allows setting the generator working point. Regarding the converters, different configurations have been proposed, both in isolated and non-isolated versions. Non-isolated versions are usually more advantageous than isolated versions, from the point of view of the efficiency, cost, volume and weight. The proposed DC-DC converter with one input and two outputs is based on the combination of two converters of one input and one output, such as the single-ended primary-inductor converter (SEPIC) andĆuk [14][15][16][17]. It is a step-down/step-up converter. This topology is effective for this type of application, with high efficiency and with the use of only one controlled power switch. In addition, for the control of the maximum power point, any proposed technique for one input and one output converter can be used [18][19][20][21][22][23][24].
The combined SEPIC-Ćuk converter has been theoretically analyzed in [14,15]. However, in this paper, a design procedure is established for the use of these converters in the connection of photovoltaic systems to bipolar DC networks. The operation of the converter is analyzed for a system isolated Electronics 2020, 9, 1171 3 of 14 and connected to the network. On the other hand, a control strategy has been applied so that the photovoltaic system works at its maximum power point.
In this paper, a comparison between bipolar DC and monopolar DC grids is included in Section 2. In Section 3, the combined converter SEPIC-Ćuk topology is described. In Section 4, an experimental prototype is designed, and a control strategy applied so that the set operates at the point of maximum power transfer. Experimental results are also presented. Finally, Section 5 presents the conclusions of this work.

Monopolar Versus Bipolar DC Grid
A monopolar DC grid consists of two wires between which a DC voltage is defined. Any load can be connected to this network at the voltage V dc (Figure 2a). isolated and connected to the network. On the other hand, a control strategy has been applied so that the photovoltaic system works at its maximum power point.
In this paper, a comparison between bipolar DC and monopolar DC grids is included in Section 2. In Section 3, the combined converter SEPIC-Ćuk topology is described. In Section 4, an experimental prototype is designed, and a control strategy applied so that the set operates at the point of maximum power transfer. Experimental results are also presented. Finally, Section 5 presents the conclusions of this work.

Monopolar Versus Bipolar DC Grid
A monopolar DC grid consists of two wires between which a DC voltage is defined. Any load can be connected to this network at the voltage Vdc (Figure 2a).
On the other hand, the bipolar DC grid (Figure 2b) is formed by three wires, "+", "−" and "0". Each load can be connected to two different voltage levels: Vdc if the load is connected between "+" and "0" or between "0" and "−", and 2Vdc if the load is connected between "+" and "−". Another characteristic to highlight is when the loads connected between "+" and "0" and between "−" and "0" are balanced, the current through the wire "0" is null; therefore, if this criterion is taken in consideration when the loads are connected, the section of this "0" wire can be significantly reduced. In Figure 2a, RLM represents the resistance of the wires for a monopolar DC grid. In Figure 2b, RLB represents the wire resistances for the bipolar DC grid.
When the two topologies are compared, for the same power transmission conditions in both grids and wires of the same section, the power losses in the bipolar DC grid are a quarter of the losses that occur in the monopolar DC grid.
The comparison of the two topologies with the same losses and wires of the same section results that the bipolar DC grid can transmit twice as much power as in the monopolar DC grid.
Finally, when the power losses and the transmission power in both are equal, the relationship between line resistances in both topologies can be calculated. Therefore, the resistance of the bipolar DC grid is four times greater than the resistance of the monopolar DC grid. This means that the wire section in the bipolar grid is a quarter of the monopolar grid.
Therefore, from the point of view of the power losses, the transmission power and the section of the wires, bipolar DC grids have advantages with respect to monopolar DC grids, which are usually used.

Proposed Converter
The connection of a distributed generation system with DC output to a bipolar network can be done in different ways. One of them is to include a DC-DC converter of one input and one bipolar output. When the appropriate control strategy is applied, this option allows setting the generator working point. For the connection of a PV system to a bipolar grid, different solutions based on DC-DC converters have been analyzed [8,9]. Regarding the converters, different configurations have been proposed, both in isolated and non-isolated versions. Non-isolated versions are usually more advantageous than isolated versions, from the point of view of the efficiency, cost, volume and On the other hand, the bipolar DC grid (Figure 2b) is formed by three wires, "+", "−" and "0". Each load can be connected to two different voltage levels: V dc if the load is connected between "+" and "0" or between "0" and "−", and 2V dc if the load is connected between "+" and "−". Another characteristic to highlight is when the loads connected between "+" and "0" and between "−" and "0" are balanced, the current through the wire "0" is null; therefore, if this criterion is taken in consideration when the loads are connected, the section of this "0" wire can be significantly reduced.
In Figure 2a, R LM represents the resistance of the wires for a monopolar DC grid. In Figure 2b, R LB represents the wire resistances for the bipolar DC grid.
When the two topologies are compared, for the same power transmission conditions in both grids and wires of the same section, the power losses in the bipolar DC grid are a quarter of the losses that occur in the monopolar DC grid.
The comparison of the two topologies with the same losses and wires of the same section results that the bipolar DC grid can transmit twice as much power as in the monopolar DC grid.
Finally, when the power losses and the transmission power in both are equal, the relationship between line resistances in both topologies can be calculated. Therefore, the resistance of the bipolar DC grid is four times greater than the resistance of the monopolar DC grid. This means that the wire section in the bipolar grid is a quarter of the monopolar grid.
Therefore, from the point of view of the power losses, the transmission power and the section of the wires, bipolar DC grids have advantages with respect to monopolar DC grids, which are usually used.

Proposed Converter
The connection of a distributed generation system with DC output to a bipolar network can be done in different ways. One of them is to include a DC-DC converter of one input and one bipolar output. When the appropriate control strategy is applied, this option allows setting the generator working point. For the connection of a PV system to a bipolar grid, different solutions based on DC-DC converters have been analyzed [8,9]. Regarding the converters, different configurations have been proposed, both in isolated and non-isolated versions. Non-isolated versions are usually more advantageous than isolated versions, from the point of view of the efficiency, cost, volume and weight. In this sense, various configurations with bipolar output have been proposed in the literature, all of them have in common the use several power switches and several reactive components. In [11,25], a three-level boost converter was reported, to provide two bipolar output voltages. It required an inductor, two switches (one of them floating) and two diodes. In [26], a three-level boost converter with two inductors was shown. It provided two bipolar output voltages also. Three-level boost converters are widely used in low-voltage bipolar-type DC microgrids. In the same way, a three-level DC-DC converter with coupled inductors for high-power applications was proposed in [27]. A single-inductor power converter with buck-boost capability, two switches (one of them floating) and three diodes was presented in [28], where the switches operated in overlapping mode. A bipolar output DC-DC converter was proposed in [29], consisting of two basic DC-DC converters. The first one was a boost converter and the second was a DC-DC buck-boost converter. Both operated in a complementary way, in a series-combined boost and buck-boost converter. A dual-output DC-DC boost converter was reported in [30], where it employed two inductors, two diodes and two switches.
In this work, a converter based on the combination of SEPIC andĆuk converters is proposed. Figure 3 shows the configuration of this converter. This combination is based on the fact that both converters have the same input, one power switch and one input inductor. They have the same conversion ratio and the output of theĆuk converter is inverted with respect to that of the SEPIC. Therefore, its combination allows obtaining a bipolar output voltage from a monopolar input.
Electronics 2020, 9, x FOR PEER REVIEW 4 of 15 weight. In this sense, various configurations with bipolar output have been proposed in the literature, all of them have in common the use several power switches and several reactive components. In [11] and [25], a three-level boost converter was reported, to provide two bipolar output voltages. It required an inductor, two switches (one of them floating) and two diodes. In [26], a three-level boost converter with two inductors was shown. It provided two bipolar output voltages also. Three-level boost converters are widely used in low-voltage bipolar-type DC microgrids. In the same way, a three-level DC-DC converter with coupled inductors for high-power applications was proposed in [27]. A single-inductor power converter with buck-boost capability, two switches (one of them floating) and three diodes was presented in [28], where the switches operated in overlapping mode. A bipolar output DC-DC converter was proposed in [29], consisting of two basic DC-DC converters.
The first one was a boost converter and the second was a DC-DC buck-boost converter. Both operated in a complementary way, in a series-combined boost and buck-boost converter. A dual-output DC-DC boost converter was reported in [30], where it employed two inductors, two diodes and two switches.
In this work, a converter based on the combination of SEPIC and Ćuk converters is proposed. Figure 3 shows the configuration of this converter. This combination is based on the fact that both converters have the same input, one power switch and one input inductor. They have the same conversion ratio and the output of the Ćuk converter is inverted with respect to that of the SEPIC. Therefore, its combination allows obtaining a bipolar output voltage from a monopolar input. This configuration employs only one power switch, three inductors, two link capacitors and two diodes. It has an advantage over the other proposals in that it has a reduced number of components. Only one power switch is necessary, which has its gate referenced to ground what simplifies the design of the gate drivers. This makes both the power switch and the input inductor concentrate all the input current, and it is also possible to couple several power switches and several inductors in parallel to distribute the current, allowing a synchronous or overlapping mode (the same control signal) or interleaved mode (shifting phase the control signals of each switch).
The converter can work in two operating modes: continuous conduction mode (CCM) and discontinuous conduction mode (DCM). CCM means that the inductor current is continuous. On the other hand, in DCM, the current of the inductor is discontinuous, i.e., in a time interval of the switching period, this current is zero.
The operation of the converter, in steady state, can be analyzed through a simple model with controlled sources. Figure 4 shows the equivalent circuit. Rs and Vs represent the equivalent of the DC source. The relationship between input and output variables can be expressed as where the parameter k depends on the operating mode of the converter. If the converter works in CCM, k must be This configuration employs only one power switch, three inductors, two link capacitors and two diodes. It has an advantage over the other proposals in that it has a reduced number of components. Only one power switch is necessary, which has its gate referenced to ground what simplifies the design of the gate drivers. This makes both the power switch and the input inductor concentrate all the input current, and it is also possible to couple several power switches and several inductors in parallel to distribute the current, allowing a synchronous or overlapping mode (the same control signal) or interleaved mode (shifting phase the control signals of each switch).
The converter can work in two operating modes: continuous conduction mode (CCM) and discontinuous conduction mode (DCM). CCM means that the inductor current is continuous. On the other hand, in DCM, the current of the inductor is discontinuous, i.e., in a time interval of the switching period, this current is zero.
The operation of the converter, in steady state, can be analyzed through a simple model with controlled sources. Figure 4 shows the equivalent circuit. R s and V s represent the equivalent of the DC source. The relationship between input and output variables can be expressed as Electronics 2020, 9, 1171 5 of 14 where the parameter k depends on the operating mode of the converter. If the converter works in CCM, k must be where D is the duty cycle.
Electronics 2020, 9, x FOR PEER REVIEW 5 of 15 where D is the duty cycle.
In the case of DCM, the value of k is given by the expression Here, TS is the switching period and according to Figure 3, Ro and Leq are given by In PV applications, the converter works by following the maximum power point of the source. For this, the following condition must be fulfilled: Here, Rg is the equivalent resistance from the terminal of the controlled current source. A PV system in isolated operation can be represented by the circuit shown in Figure 4. At the input loop, the current is given by That can be expressed in the way of Therefore, according Equation (6), it must fulfill Equation (9) is useful when the system is operating in isolated mode, since it allows to determine the duty cycle under different load conditions when the system is transferring the maximum power.
In many applications, the PV system is connected to a bipolar grid in parallel with other sources. When the same model of controlled sources is considered, the equivalent circuit from the bipolar network is useful because it allows the analysis of the power flows and establishing the range of powers when the converter works in CCM mode. In this case, from the terminals of the bipolar grid, the equivalent circuit of the converter can be obtained. Figure 5 shows the equivalent voltage, which is kVS, and the value equivalent resistance, 2k 2 RS. In the case of DCM, the value of k is given by the expression Here, T S is the switching period and according to Figure 3, R o and L eq are given by In PV applications, the converter works by following the maximum power point of the source. For this, the following condition must be fulfilled: Here, R g is the equivalent resistance from the terminal of the controlled current source.
A PV system in isolated operation can be represented by the circuit shown in Figure 4. At the input loop, the current is given by That can be expressed in the way of Therefore, according Equation (6), it must fulfill Equation (9) is useful when the system is operating in isolated mode, since it allows to determine the duty cycle under different load conditions when the system is transferring the maximum power. In many applications, the PV system is connected to a bipolar grid in parallel with other sources. When the same model of controlled sources is considered, the equivalent circuit from the bipolar network is useful because it allows the analysis of the power flows and establishing the range of powers when the converter works in CCM mode. In this case, from the terminals of the bipolar grid, the equivalent circuit of the converter can be obtained. Figure 5 shows the equivalent voltage, which is kV S , and the value equivalent resistance, 2k 2 R S . Under the conditions of maximum power where Vg is the output voltage of the PV system when the maximum power P is transferred. For a balanced bipolar grid of voltage ± Vdc, at the positive output, transferred power can be expressed by So, from the equivalent circuit in Figure 5, the next equation can be established as When Io + is substituted in Equation (12) from Equation (11) Obtaining k from Equation (13) and taking into account that the system must supply power, it is deduced Therefore, k only depends on the voltage at the connection point of the converter and the output voltage of the photovoltaic system at the maximum power point (MPP).
Equation (14) allows to obtain the duty cycle of the converter for different conditions of grid voltage and input voltage at the MPP.
From the point of view of the converter component design, Table 1 shows the average values of voltages and currents for each of the converter elements when it works in steady state. It is assumed that the converter operates in CCM. This mode of operation is the preferred in power systems due to its higher efficiency, so this operation mode is the most adequate for the proposed application. It should be noted that the voltage at the C2 capacitor (Ćuk side) is higher than the voltage at the C1 capacitor (SEPIC side). Choosing the appropriate values for the inductors and capacitors is of special importance for the proper operation of the converter. Under the conditions of maximum power where V g is the output voltage of the PV system when the maximum power P is transferred.
For a balanced bipolar grid of voltage ± V dc , at the positive output, transferred power can be expressed by So, from the equivalent circuit in Figure 5, the next equation can be established as When I o + is substituted in Equation (12) from Equation (11) 2kV g − V dc Obtaining k from Equation (13) and taking into account that the system must supply power, it is deduced Therefore, k only depends on the voltage at the connection point of the converter and the output voltage of the photovoltaic system at the maximum power point (MPP).
Equation (14) allows to obtain the duty cycle of the converter for different conditions of grid voltage and input voltage at the MPP.
From the point of view of the converter component design, Table 1 shows the average values of voltages and currents for each of the converter elements when it works in steady state. It is assumed that the converter operates in CCM. This mode of operation is the preferred in power systems due to its higher efficiency, so this operation mode is the most adequate for the proposed application. It should be noted that the voltage at the C 2 capacitor (Ćuk side) is higher than the voltage at the C 1 capacitor (SEPIC side). Choosing the appropriate values for the inductors and capacitors is of special importance for the proper operation of the converter. Table 1. Performance of the combination of SEPIC (single-ended primary-inductor converter) and Cuk converters.
The selection of the L 1 inductor depends on the desired ripple value of the input current. Its peak-to-peak value is given by ∆i L1 is a percentage of the input current average. For the choice of the L 2 and L 3 inductors, it is assumed that the converter works in CCM in a power range.
On the other hand, the combination of SEPIC andĆuk converters works in CCM when the next expressions are fulfilled as 2L eq where L eq and R o are defined according Equation (5). The selection of the link capacitors C 1 and C 2 , for the SEPIC andĆuk sides, is based on assuming that the voltage in those capacitors must be constant. The resonant frequencies (ω r ) must be lower than the switching frequency (ω S ), in order to ensure constant voltage.
To ensure that the converter operation mode is neither discontinuous quasi-resonant mode (DQRM) nor discontinuous capacitor voltage mode (DCVM), the resonant frequency must be ω S > ω r ; with ω 2 In the same way, the voltage ripple in C 1 and C 2 is given by Electronics 2020, 9, 1171 8 of 14 The selection of the output capacitors C o1 and C o2 is also made to obtain the desired voltage ripple value. Its peak-to-peak value is given by 8·L 2 ·C· f S 2 , forĆuk side (20)

Experimental Prototype
An experimental prototype of the proposed DC-DC converter has been designed, based on the circuit shown in Figure 3. The prototype is a scaled model that allows the verification of its operation in the laboratory. Figure 6 shows the experimental prototype. The selection of the output capacitors Co1 and Co2 is also made to obtain the desired voltage ripple value. Its peak-to-peak value is given by

Experimental Prototype
An experimental prototype of the proposed DC-DC converter has been designed, based on the circuit shown in Figure 3. The prototype is a scaled model that allows the verification of its operation in the laboratory. Figure 6 shows the experimental prototype.
The designed converter allows the connection of a PV system to a bipolar DC microgrid of ± 100 V. In this design, an array consisting of three monocrystalline photovoltaic modules from Isofoton, model ISF-250, have been considered. Table 2 shows its performance at an irradiance of 1000 W/m 2 and cell temperature of 25 °C.   Figure 7 shows the variation in the output voltage for different irradiances and different temperatures.
With respect to the power, it varies from 72 (100 W/m 2 , 25 °C) to 796 W (1000 W/m 2 , 10 °C), which will be taken as design values to the converter. The designed converter allows the connection of a PV system to a bipolar DC microgrid of ±100 V. In this design, an array consisting of three monocrystalline photovoltaic modules from Isofoton, model ISF-250, have been considered. Table 2 shows its performance at an irradiance of 1000 W/m 2 and cell temperature of 25 • C.  Figure 7 shows the variation in the output voltage for different irradiances and different temperatures.
With respect to the power, it varies from 72 (100 W/m 2 , 25 • C) to 796 W (1000 W/m 2 , 10 • C), which will be taken as design values to the converter.

Converter Design
The selection of the components of the SEPIC-Ćuk combined converter shown in Figure 3 is stablished according the peak power of the string where the converter will be connected. Therefore, the prototype will be designed for a power of 1 kW (0.25 kW above maximal power). The voltage at maximum power for different irradiance can vary from 75 to 98 V. Thus, according to Equation (14), the duty cycle of the converter will be between 0.51 and 0.57. The switching period is established at 40 µs. Table 3 shows the main design features of the converter. The maximum input current is given for the maximum power, it is 8.17 A. On the other hand, the maximum ripple of the input current is set to 4%. The application of Equation (15) results in an inductance, L1, of 5 mH.
For the lower power, 72 W, the input voltage is 88 V (100 W/m 2 , 25°C), and the maximum load Ro can be calculated, which results in Ro = 103 Ω.
For a switching period (Ts) of 40 µs and fS = 100fr, the Equation (18) are satisfied for a minimum capacity value of 101 µF (for C1 and C2). Therefore, a standard value of 470 µF is chosen. With respect to the voltage ripple, the application of Equation (19) with C1 = C2 = 470 µF allows to obtain a ripple of 0.4%, when the load is maximum, which is when D = 0.52 and Ro = 11.1 Ω.

Converter Design
The selection of the components of the SEPIC-Ćuk combined converter shown in Figure 3 is stablished according the peak power of the string where the converter will be connected. Therefore, the prototype will be designed for a power of 1 kW (0.25 kW above maximal power). The voltage at maximum power for different irradiance can vary from 75 to 98 V. Thus, according to Equation (14), the duty cycle of the converter will be between 0.51 and 0.57. The switching period is established at 40 µs. Table 3 shows the main design features of the converter. The maximum input current is given for the maximum power, it is 8.17 A. On the other hand, the maximum ripple of the input current is set to 4%. The application of Equation (15) results in an inductance, L 1 , of 5 mH.
For the lower power, 72 W, the input voltage is 88 V (100 W/m 2 , 25 • C), and the maximum load R o can be calculated, which results in R o = 103 Ω.
103 Ω, according to Equation (17), the inductance L eq must be greater than 0.47 mH. Furthermore, considering L 2 = L 3 , its values will be of 1.03 mH. In this design, L 2 = L 3 = 1 mH was chosen. For a switching period (T s ) of 40 µs and f S = 100f r , the Equation (18) are satisfied for a minimum capacity value of 101 µF (for C 1 and C 2 ). Therefore, a standard value of 470 µF is chosen. With respect to the voltage ripple, the application of Equation (19) with C 1 = C 2 = 470 µF allows to obtain a ripple of 0.4%, when the load is maximum, which is when D = 0.52 and R o = 11.1 Ω.
Finally, for the selection of the output capacitors C o1 and C o2 , Equation (20) is applied. For a low-voltage ripple, C = 470 µF has been considered, which provides an estimated voltage ripple of 0.4% for V o + and 0.1% for V o − . Table 4 summarizes the characteristics of the SEPIC-Ćuk combined converter components implemented.

Control Strategy
For the control of the converter, several strategies can be applied to the proposals in the technical literature for converters of one input and one output [18][19][20][21][22][23][24]. In this work, the technique chosen allows maximum power point tracking. The perturb and observe method (P and O) is one of the most used in practice [24]. It is an iterative method where the reference voltage varies according to the increase in power, obtained from the input voltage and current to the converter. Figure 8 shows the flowchart of the P and O strategy.

Experimental Results
The experimental system set-up is shown in Figure 9. The DC-DC converter will connect to a bipolar DC microgrid of ± 100 V. It has been designed for a maximum power of 5 kW. In the test, three resistors have been connected. They have the following values: RL + = RL − = 47/2 Ω and RL = 47 Ω.
When the switch S1 is closed and S2 is open, the Vdc+ and Vdc− voltages have an average value of 100 V. The current for each conductor is IL + = 8.5 A and IL − = − 8.5 A, so that the power consumed by The control strategy has been implemented in the TMS320F28335 Delfino Microcontroller by Texas Instruments Dallas, Texas, USA. The measuring sensors used are the LA 25-NP for the currents and the LV 25-1000 for the voltage, both by LEM International SA, Geneva, Switzerland.

Experimental Results
The experimental system set-up is shown in Figure 9. The DC-DC converter will connect to a bipolar DC microgrid of ± 100 V. It has been designed for a maximum power of 5 kW. In the test, three resistors have been connected. They have the following values: R L + = R L − = 47/2 Ω and R L = 47 Ω.  The DC source is adjusted to an output voltage of 182.4 V, with an internal resistance (Rs) of 11.1 Ω. Therefore, the DC source will transfer the maximum power (750 W) when it supplies a current of 8.17 A and with an output voltage of 91.8 V.
In the following test, the switch S1 is open and the switch S2 is closed. Thus, the PV system is operating in island mode. According with Equation (4), Ro = 5.9 Ω for the resistance values RL + , RL − and RL. Therefore, the application of Equation (9) Figure 11 shows the gate source signal, Vg, drain source voltage, Vds, and the input current waveform. The measured duty cycle in these conditions is 41%, similar to the theoretical value. When the switch S1 is closed and S2 is open, the V dc+ and V dc− voltages have an average value of 100 V. The current for each conductor is I L + = 8.5 A and I L − = − 8.5 A, so that the power consumed by the set is 1.7 kW.
The PV system is emulated by a DC source, so the point of maximum power can be adjusted to a desired value, modifying its internal resistance and voltage. Figure 10 shows the curves I-V and P-V of a photovoltaic string of three series of modules of Isofoton ISF-250 for a given irradiance. In this situation, the maximum power is given for 750 W with a voltage of 91.8 V and a current of 8.2 A.  Figure 10 shows the curves I-V and P-V of a photovoltaic string of three series of modules of Isofoton ISF-250 for a given irradiance. In this situation, the maximum power is given for 750 W with a voltage of 91.8 V and a current of 8.2 A.
The DC source is adjusted to an output voltage of 182.4 V, with an internal resistance (Rs) of 11.1 Ω. Therefore, the DC source will transfer the maximum power (750 W) when it supplies a current of 8.17 A and with an output voltage of 91.8 V.
In the following test, the switch S1 is open and the switch S2 is closed. Thus, the PV system is operating in island mode. According with Equation (4), Ro = 5.9 Ω for the resistance values RL + , RL − and RL. Therefore, the application of Equation (9) Figure 11 shows the gate source signal, Vg, drain source voltage, Vds, and the input current waveform. The measured duty cycle in these conditions is 41%, similar to the theoretical value. The DC source is adjusted to an output voltage of 182.4 V, with an internal resistance (R s ) of 11.1 Ω. Therefore, the DC source will transfer the maximum power (750 W) when it supplies a current of 8.17 A and with an output voltage of 91.8 V.
In the following test, the switch S1 is open and the switch S2 is closed. Thus, the PV system is operating in island mode. According with Equation (4), R o = 5.9 Ω for the resistance values R L + , R L − and R L . Therefore, the application of Equation (9) Figure 11 shows the gate source signal, V g , drain source voltage, V ds , and the input current waveform. The measured duty cycle in these conditions is 41%, similar to the theoretical value.
Electronics 2020, 9, x FOR PEER REVIEW 13 of 15 (a) (b) Figure 11. Gate source signal, Vg, and drain source voltage, Vds (a), and input current waveform (b) when the photovoltaic system operates in island mode. When the PV system and source are connected (Figure 9, S1 and S2 are closed), the voltages at the point of connection are Vdc+ = −Vdc− = 99.7 V. From the theoretical point of view, Equation (14) must be fulfilled, therefore at the maximum power point, k must be 1.09 and the theoretical duty cycle will be 52%. The converter supplies a current IGD+ = −IGD− = 3.2 A, so the power transferred by the converter is 638 W. The input voltage (VPV) is 92 V and the input current is (IPV) 8.1 A. Figure 12 shows the gate source signal, Vg, drain source voltage, Vds, and the input current waveform. In this case, the measured duty cycle is about 51%. In this test, the measured efficiency of the converter is 86.5%.
(a) (b) Figure 12. Gate source signal, Vg, and drain source voltage, Vds (a), and input current waveform (b) when the photovoltaic system operates in parallel with a bipolar DC source.

Conclusions
Bipolar DC grids have become an adequate solution for high-power DC microgrids. When compared with monopolar DC grids, their main advantage lies in a greater power transmission capacity.
A distributed generation system can be connected to the bipolar DC microgrid through DC-DC Figure 11. Gate source signal, V g , and drain source voltage, V ds (a), and input current waveform (b) when the photovoltaic system operates in island mode. When the PV system and source are connected (Figure 9, S1 and S2 are closed), the voltages at the point of connection are V dc+ = −V dc− = 99.7 V. From the theoretical point of view, Equation (14) must be fulfilled, therefore at the maximum power point, k must be 1.09 and the theoretical duty cycle will be 52%. The converter supplies a current I GD+ = −I GD− = 3.2 A, so the power transferred by the converter is 638 W. The input voltage (V PV ) is 92 V and the input current is (I PV ) 8.1 A. Figure 12 shows the gate source signal, V g , drain source voltage, V ds , and the input current waveform. In this case, the measured duty cycle is about 51%. In this test, the measured efficiency of the converter is 86.5%.
Electronics 2020, 9, x FOR PEER REVIEW 13 of 15 (a) (b) Figure 11. Gate source signal, Vg, and drain source voltage, Vds (a), and input current waveform (b) when the photovoltaic system operates in island mode. When the PV system and source are connected (Figure 9, S1 and S2 are closed), the voltages at the point of connection are Vdc+ = −Vdc− = 99.7 V. From the theoretical point of view, Equation (14) must be fulfilled, therefore at the maximum power point, k must be 1.09 and the theoretical duty cycle will be 52%. The converter supplies a current IGD+ = −IGD− = 3.2 A, so the power transferred by the converter is 638 W. The input voltage (VPV) is 92 V and the input current is (IPV) 8.1 A. Figure 12 shows the gate source signal, Vg, drain source voltage, Vds, and the input current waveform. In this case, the measured duty cycle is about 51%. In this test, the measured efficiency of the converter is 86.5%.
(a) (b) Figure 12. Gate source signal, Vg, and drain source voltage, Vds (a), and input current waveform (b) when the photovoltaic system operates in parallel with a bipolar DC source.

Conclusions
Bipolar DC grids have become an adequate solution for high-power DC microgrids. When compared with monopolar DC grids, their main advantage lies in a greater power transmission capacity.
A distributed generation system can be connected to the bipolar DC microgrid through DC-DC converters. There are different converter configurations with bipolar output, however, all of them Figure 12. Gate source signal, V g , and drain source voltage, V ds (a), and input current waveform (b) when the photovoltaic system operates in parallel with a bipolar DC source.

Conclusions
Bipolar DC grids have become an adequate solution for high-power DC microgrids. When compared with monopolar DC grids, their main advantage lies in a greater power transmission capacity. A distributed generation system can be connected to the bipolar DC microgrid through DC-DC converters. There are different converter configurations with bipolar output, however, all of them have several power switches. In this paper, a converter based on the combination of a SEPIC and aĆuk converter is proposed. This topology has, as main advantages, a reduced number of components and the use of a single switch.
In the proposed converter control strategy, any of the techniques described in the literature applied to converters of a single output can be used. Among the different control strategies applied to PV systems, the most commonly used are those that allow maximum power point tracking.
In this paper, an experimental prototype DC-DC converter with bipolar output based on the combination of SEPIC andĆuk converters was developed. This converter allows the connection of a PV system to a bipolar DC grid. In the control, the perturb and observe method has been applied which has allowed maximum power point tracking. The results obtained allowed to verify the behavior of the converter with the applied strategy.
Author Contributions: All of the Authors conceived and designed the experiments; performed the experiments; analyzed the data; contributed reagents/materials/analysis tools; wrote the paper. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.