PHOTOVOLTAIC ENERGY CONVERSION SYSTEM FOR INTERCONNECTION TO UNBALANCED ELECTRICAL DISTRIBUTION NETWORKS.

Abstract

The present invention relates to a photovoltaic power conversion system (PVECS). This system comprises a photovoltaic (PV) array connected to a DC / DC boost converter, which is implemented with a maximum power point tracker (MPPT) for the PV modules. This converter is then interconnected to the electrical distribution network via a voltage source converter (VSC) in inverter mode, which is implemented with a DC voltage control structure. One of the most concerning disturbances in electrical distribution networks is voltage imbalance.Considering the above, the objective of this invention is to provide a PVECS system capable of simultaneously achieving the following characteristics under unbalanced voltage conditions in the interconnection network: continuous operation, maximum power extraction from the photovoltaic array, continuous generation of active and reactive power, balanced generation currents, negligible harmonic content, and maintenance of a unity power factor. The validation of this proposal has been analytically modeled, implemented in simulation, and tested through real-time simulation, thus evaluating different case studies both numerically and experimentally. Finally, the operational advantages of the PVECS system are demonstrated when operating under unbalanced voltage conditions, compared to other proposals analyzed in the scientific literature.

Description

The technical field of the present invention lies within electrical engineering, specifically relating to a photovoltaic energy conversion system (PVECS) interconnected to an electrical distribution network with unbalanced voltages. The proposed conversion system, regardless of whether the distribution network to which it is interconnected has balanced voltages or not, can guarantee: continuous photovoltaic energy generation, mitigation of total harmonic distortion (THD), unity power factor (PF), and balanced generation currents. BACKGROUND OF THE INVENTION Electrical energy is one of the fundamental pillars of human progress; therefore, electrical systems represent a priority on the political, economic, scientific, and social agendas of most countries in the world. Electricity generation from renewable or non-conventional energy sources has increased considerably in the last decade, due to improvements in efficiency and costs. qj cznn / zznz / e / YiAi Because of this, electricity generation through photovoltaic (PV) solar technology is considered one of the most prolific in many countries. For example, in Mexico, the installed capacity of distributed generation through renewable energy sources (DREA) increased from 117.53 MW in 2015 to 1,196.52 MW in 2020, with 99.28% corresponding to photovoltaic technologies. This technology transforms the solar irradiance that falls on the surface of a device called a solar panel and converts it into direct current (DC) electrical power. This phenomenon is known as the PV effect. That said, to create a PVECS (Photovoltaic Electrical and Solar Combined Cycle) system, power electronics converters are necessary. First, the solar panels are connected to a DC / DC boost converter, which performs two functions: increasing the DC voltage and tracking the maximum power point (MPPT) of the photovoltaic modules. MPPT is a control algorithm implemented in the boost converter that is essential in photovoltaic systems to ensure maximum power extraction under varying irradiance and temperature conditions. Subsequently, the output of the step-up converter is interconnected to the electrical distribution network through a voltage source converter (VSC) qj cznn / zznz / e / YiAi in inverter mode, maintaining a constant DC voltage link and regulating the generated active and reactive power. One of the most concerning disturbances in electrical distribution networks is voltage imbalance. This phenomenon occurs when there is a reduction or increase in voltage magnitude in one or more phases of the network, or when the relative angular difference between the network phases is not equal. The main causes of this phenomenon in the electrical network include: variations in line impedance, energization of large industrial loads, variations in the demand of single-phase residential loads, variations in the demand of two-phase commercial loads, line faults far from the generation node, and the integration of renewable energy sources. Currently, this type of adverse power quality phenomenon is mitigated by the power converters themselves, which are an integral element in the generation, transmission, distribution, and storage of energy from renewable sources. However, to guarantee the efficiency, quality, reliability, continuity, and sustainability of an electrical grid, photovoltaic power plants with a capacity of less than 500 kW are typically disconnected when a voltage imbalance occurs. Recent literature has proposed multiple control strategies for interconnecting a PVECS (Photovoltaic Energy Storage and Storage System) to an electrical grid. However, most of these schemes are based on the dqO (Quality of Voltage) reference framework, demonstrating optimal performance only under balanced grid voltages. That said, in an operation with unbalanced voltages, a PVECS with a classic control structure has poor performance, since it tends to cause: harmonic distortion in the AC currents of the network, unwanted reactive power generated and a large number of 2ω oscillations in the DC voltage, thus generating adverse effects on the quality of the power and causing the temporary disconnection of the system itself. Thus, the present invention demonstrates a PVECS that, through an MPPT algorithm and a novel control structure, maintains photovoltaic energy generation regardless of adverse conditions such as power grid imbalances. Furthermore, it can also mitigate total harmonic distortion (THD), ensure a unity power factor, and maintain balanced generation currents. Within the prior art of the present invention, it is worth highlighting patents that present grid-connected photovoltaic energy conversion systems. US patent 7,479,774 B2, granted in 2006 to YUAN ZE UNIVERSITY, proposes a solar energy conversion system consisting of a wide-range DC / DC boost converter followed by a single-phase full-bridge inverter. The invention allows for the interconnection of solar panels to a single phase; therefore, three systems are required for three-phase integration. It also includes an adaptive observation and perturbation method to obtain the MPPT (Maximum Power Point Tracking) of the generation source. Furthermore, to achieve greater system efficiency, the invention implements Adaptive Total Sliding Mode Control (ATSMC) to ensure a unity power factor between the system and the grid. Similarly, the patent CN101917017B of KBEIJING JIATONG qj cznn / zznz / e / YiAi UNIVERSITY patent, awarded in 2010, presents a three-phase PVECS (Powered Energy Storage System) consisting of two conversion stages, a storage system, and an LCL (Liquid Content) filter. The solar panels and the storage system are interconnected via a power electronics stage. Subsequently, both elements are connected to an electrical grid through a three-phase VSC (Voltage-Stepped Converter) inverter and an LCL filter. The invention includes two control structures in the system that, together, allow for maximum power extraction from the generation source, manage the charging and discharging of the storage system, control the DC link voltage, and control the active and reactive power generation by the PVECS. For its part, patent CN1033715717B, granted in 2013 by North China Electric Power University, presents a three-phase PVECS with a three-stage topology. First, a boost-DC / DC converter is used, followed by a three-phase DC / AC inverter converter (VSC). This inverter is then connected to the electrical grid via an LCL filter. It is important to note that this invention does not implement an MPPT algorithm to obtain the maximum possible power from the solar panels. Furthermore, to provide the system with operational stability, a high power factor, harmonic blocking, and high efficiency under abnormal grid conditions, a proportional resonant (PR) current control with an additional PR harmonic compensator is implemented in the inverter converter. Zhejiang University Zju patent CN103944186A, granted in 2014, describes a three-phase PVECS consisting of a single conversion stage: a three-phase VSC inverter. Due to the absence of a boost power electronics stage, the system must be connected to the electrical grid via a 60 Hz transformer. The invention focuses primarily on ensuring system operation, controlling generated power, and maintaining a stable DC voltage. The control structure implemented in the inverter stage is a conventional dqO control system combined with a zero-sequence voltage suppression module. Also, KEHUA DATA CORPORATION's patent CN1054712338A, granted in 2015, presents a single-phase PVECS consisting of three stages: a DC / DC boost converter, a bidirectional DC / DC converter connected to a storage system, and a full-bridge DC / AC inverter. To regulate oscillations at the DC voltage node, the invention implements a control structure in the system that measures the low-frequency voltage ripple in real time. If the voltage ripple is in a positive half-cycle, the storage system is charged. If the voltage ripple is in a negative half-cycle, the storage system is discharged. A three-phase PVECS is presented in patent CN106451544A, granted by the Guangzhou Institute of Energy Conversion in 2016. In this system, the solar panels share their negative terminal with a battery, which, via a bidirectional converter, powers a capacitor C2. This capacitor, in turn, is connected in series with a capacitor C1, whose positive terminal is connected to the phase of the panels. Subsequently, the capacitor array qj cznn / zznz / e / YiAi is connected to a three-stage, three-phase inverter. This invention does not include a step-up converter; therefore, a conventional AC transformer is required to connect the system to an electrical grid. To obtain maximum power extraction from the generation source, the system controls the voltage across the capacitor by charging and discharging the storage system through C2. Additionally, a power compensator control is included in the inverter, which is based on a conventional dqO structure and guarantees balanced generation currents at all times. qj cznn / zznz / e / YiAi Other documents that, in combination with those mentioned above, define as a whole the state of the art closest to the proposed invention, are shown in Table 1 below. Table 1. Other Technique Status Papers. Patent number. Year Current assignment. US20100133904A1 2010 General Electric Co CN101951011B 2010 Nanjing University of Aeronautics US9502904B2 2011 Eaton Intelligent Power Ltd CN103390900 2013 Shanghai Power University of Electric CN1010104A 2014 State Grid Corp of China SGCC CN104218595A 2014 Jinling Institute of Technology CN105703652A 2016 Beijing Jiaotong University W02020057635A1 2018 HUAWEI TECHNOLOGIES CO.,LTD. CN111641228A 2020 Tianjin University CN111756046A 2020 Sungrow Power Supply Co Ltd Based on the foregoing, it is necessary to mention that the patents described above are the most closely related to the present invention. Specifically, with the aim of interconnecting photovoltaic energy conversion systems to the electrical grid, several proposals have been developed to date; however, their attention to a common problem in distribution networks, such as unbalanced voltages, is practically nonexistent, implementing only MPPT algorithms and very general control structures. For this reason, under the title of “Photovoltaic Energy Conversion System for Interconnection to Unbalanced Electrical Distribution Networks”, an invention is proposed that not only addresses the problem of unbalanced network voltages in PVECS, but also proposes a simple and easy-to-implement control structure that does not require changes in the frame of reference or complex algorithms. OBJECTS OF THE INVESTIGATION. The main objective of this invention is to provide a photovoltaic energy conversion system that, when interconnected to an electrical distribution network, guarantees its continuity of operation, regardless of whether the network presents adverse conditions in the quality of the energy such as unbalanced voltages. It is an additional objective to provide a PVECS that maintains balanced generation currents and a unity power factor, even though it is interconnected to an unbalanced electrical distribution network. It is a further object of the present invention to provide a PVECS that, interconnected to a distribution network with unbalanced voltages, maintains a stable DC voltage in the link between the boost converter and the inverting VSC converter. qj cznn / zznz / e / YiAi Another objective is to ensure that the total harmonic distortion, present in the current waveforms of the PVECS generation of the present invention, is mitigated for the most part. It is yet another proportional object a PVECS that, under any operating situation, the system guarantees the maximum power extraction from the photovoltaic energy source by means of an MPPT algorithm implemented in the boost converter. These objects, features and advantages of the PVECS of this application will be evident to a technician in the field from the detailed description of certain sections and the accompanying Figures, as well as the attached claims. BRIEF DESCRIPTION OF THE INVENTION. One of the most common phenomena in electrical distribution networks is voltage imbalance. For this reason, it is important that photovoltaic power generation plants interconnected to these networks implement suitable conversion systems that address the problems caused by this phenomenon and guarantee continuous generation at all times. Thus, the present invention describes a PVECS (Power Electronic Converter System) which, through power electronic converters and a suitable control structure, is capable of addressing the various problems caused by the aforementioned phenomenon. Therefore, under such operating conditions, the main attributes that the PVECS of this invention, qj cznn / zznz / e / YiAi, can offer are: • Balanced generation currents and a unity power factor; this is important for photovoltaic generation plants because it guarantees their continuous generation. This is because, normally, when these problems occur, the system needs to be disconnected from the electrical grid. • A robust control structure that also requires simple implementation in the system, since it only needs a single PI control loop and does not require changing variables to different reference frames. • Mitigation of total harmonic distortion; which allows for a reduction of adverse effects on the electrical grid itself. • Maximum power extraction; which ensures that the system delivers the maximum amount of power possible to the electrical grid at all times and under any operation. Once all the above points have been analyzed, through several case studies, it is possible to establish that the PVECS proposed in the invention has the potential for easy and rapid implementation in PV generation plants, thus generating various technical and economic advantages in the electrical distribution networks. The PVECS of the present invention (Fig. 1) is defined primarily by two power electronics stages. The first stage consists of a boost converter (Fig. 3) and an MPPT algorithm (Fig. 5), whose objective is to increase the magnitude of the DC voltage generated by the solar panels (Fig. 2) and ensure the maximum extraction of power from them. Subsequently, the second stage of the PVECS consists of a three-phase inverter VSC (Fig. 6) accompanied by a control structure (Fig. 8). The main task of this control is to guarantee generation currents and a unity power factor, regardless of whether the grid voltages to which the system is interconnected are balanced or not. To achieve this, it is assumed that the voltages across the coupling inductors between the VSC and the grid are balanced and in quadrature with respect to the grid at all times. Under this condition, the control technique produces independent modulating signals for the converter depending on the presence or absence of imbalances. Finally, the modulation stages for the boost converter (Fig. 4) and for the three-phase inverting VSC converter (Fig. 7) are also included in order to intellectually protect the set of power electronics elements and their proper operation of the present invention. The detailed description of each of the elements necessary to form the present invention will be described in more detail later. BRIEF DESCRIPTION OF THE FIGURES. The appended claims detail the novel aspects considered characteristic of the present invention. However, due to its method of operation and organization, the invention itself, along with its advantages, will be better understood from the following detailed description of its embodiments, read in conjunction with the drawings described below: qj cznn / zznz / e / YiAi Figure 1 corresponds to a diagram showing the topology of the PVECS, consisting of a photovoltaic energy source, a boost converter with an implemented MPPT algorithm, and a VSC converter with a control for imbalances. Figure 2 corresponds to a diagram representing the equivalent model of a 5 solar panel, which consists of a current source connected in parallel to a diode and a resistor Rsh which are subsequently connected to a series resistor Rs. Figure 3 shows the internal components of a boost converter. It consists of an inductor, a diode, a power transistor, and two capacitors. The control signal input to the power converter and a load, which is later replaced by the converter in Figure 6, are also shown. Figure 4 presents the elements necessary to perform PWM modulation, which generates a trigger signal that is introduced into the power transistor of the boost converter in Figure 3, allowing it to function correctly. Figure 5 shows the flowchart corresponding to the MPPT algorithm implemented in the boost converter of Figure 3. This algorithm's main objective is to obtain the maximum power extraction from the solar panels, such as the one in Figure 2, at all times. Figure 6 shows a diagram of the internal components of an inverting VSC converter. It consists of three parallel structures of two power transistors each, qj cznn / zznz / e / YiAi, connected in series. The six modulating signals that are input to the power transistors to achieve the desired operation are also identified. Figure 7 shows the elements necessary to perform SPWM modulation that generates the trigger signals required for the operation of the PVECS three-phase inverter VSC converter. The sinusoidal mabcson signals are variable and come from the control structure shown in Figure 8. Figure 8 shows the control structure diagram implemented in the three-phase inverter VSC of the PVECS of the present invention. This diagram illustrates the process for producing independent modulating signals necessary for SPWM modulation, which in turn is required for the converter's operation. Figure 9 shows the behavior of the electrical variables in the PVECS, such as DC link voltage, active power, and reactive power generated. Specifically, Figure 9 shows the dynamic behavior of these variables as a function of the solar irradiance present on the solar panels, since this exhibits different changes at various times of the day. Figure 10 shows a graph illustrating the dynamic range of voltages in the electrical grid, AC generation currents, and the modulating signals of the PVECS of the present invention. This graph considers a case study where the voltage of phases a and b of the grid experiences a drop of 70% and 80%, respectively. qj cznn / zznz / e / YiAi DETAILED DESCRIPTION OF THE INVENTION. Referring to Figure 1, all the elements that make up the PVECS of the present invention are shown, which is interconnected to an unbalanced AC electrical network. This photovoltaic energy conversion system comprises the following steps: 1. Consider nodes 101 and 102 as the input of the temperature and solar irradiance variables to the PV array 103, respectively. 2. Consider connection nodes 301 and 302 as the positive and negative output poles of the PV array 103, respectively. 3. Connect the positive terminal of capacitor CB1104 to node 301 and the negative terminal to node 302. 4. Connect the boost converter in parallel to capacitor CB1104 and the PV 103 array 105. Positive CD input to node 301 and negative input to node 302. 5. Input into the MPPT algorithm block 108 the DC voltage signal VAPV106 measured on capacitor CB1104 and a reference current IAPV107. 6. Input the duty cycle signal D 503 from MPPT algorithm block 108 to the boost converter 105. 7. Consider connection nodes 308 and 309 as the positive and negative output poles of the boost converter 105, respectively. 8. Connect the positive terminal of capacitor CB2109 to node 308 and the negative terminal to node qj cznn / zznz / e / YiAi 309. 9. Connect the VSC 112 converter in parallel to capacitor CB2109 and boost converter 105. Connect the positive DC input to node 308 and the negative input to node 309. 10. Introduce to the proposed control block 113 the DC voltage VCD110 measured in capacitor CB2109 and a reference voltage 111. 11. Input the mains voltages (116-118) to the proposed control block 113 of the present invention 12. Input the mains voltages (116-118) to a phase-tracking loop (PLL) 114 to obtain a sawtooth wave ωA. 13. Input the output of PLL 114 to the PLL 115 signal block to obtain three sinusoidal signals offset from each other by 120° (804-806). 14. Introduce the sinusoidal signals (804-806) from block 114 to the proposed control block 113 of the present invention. 15. Introduce the mabc(703-705) modulating signals from block 113 to the three-phase VSC converter 112. 16. Consider the connection nodes (601-603) as the AC output nodes of the three-phase VSC 112, which contain the voltages Vtabc. 17. Using coupling inductances Labc(119-121) and their corresponding parasitic resistances rabc(122-124), connect nodes (601-603) with nodes (116-118), which correspond to the unbalanced AC network and contain the voltages Vgabc. qj cznn / zznz / e / YiAi Referring to Figure 2, an equivalent circuit for a solar panel is shown, which describes its actual behavior. This photovoltaic power generation equipment comprises the following steps: a) Consider connection nodes 202 and 203 as the positive and negative output poles of a current-dependent source 201 whose supplied current is equal to the photogenerated current. b) Connect the anode and cathode of diode 204 to connection nodes 202 and 203, respectively. c) Consider the parallel resistance Rsh207 as the representation of the nonlinearities in the equipment. d) Connect the ends of the series resistor Rsh207 to connection nodes 205 and 206, so that the resistance is in parallel with diode 204 and current source 201. e) Consider the series resistance Rs208 as the representation of the charge of the semiconductor material and the metallic contacts. f) Connect the ends of resistor Rs208 to connection nodes 205 and 301, so that the resistor is in series with resistor 207. g) Finally, consider connection nodes 301 and 302 as the DC voltage output VPV209 of the solar panel. Referring to Figure 3, the topology of a boost converter is shown, whose main function is to increase the DC input voltage. Also, when used in a solar panel array, they allow the implementation of MPPT algorithms to achieve maximum power extraction. This topology is structured according to the following steps: a) Consider nodes 301 and 302 as the positive and negative input terminals of the boost converter 105, establishing a parallel connection with capacitor CB1104. b) Connect the ends of the inductor LB303 to connection nodes 301 and 304. c) Connect the collector and emitter of power transistor 306 to connection nodes 304 and 305, respectively. d) Consider S± as a trigger signal that is introduced to the base of the power transistor 306 and that comes from the PWM modulation. e) Connect the anode and cathode of the DB307 diode to connection nodes 304 and 308, respectively. f) Connect the positive and negative terminals of capacitor CB2109 to connection nodes 308 and 309, respectively. It is important to note that, in practical terms, node 309 is the same as node 305. g) Consider connection nodes 310 and 311 as the output nodes of the boost converter 105, where a load 312 can be connected. Specifically, in this invention, the VSC converter 112 is connected to nodes 310 and 311. Finally, it is important to mention that, in practical terms, nodes 310 and 311 are the same as nodes 308 and 309. qj cznn / zznz / e / YiAi Referring to Figure 4, the schematic shows the elements necessary to perform PWM modulation and generate the trigger signal for the power transistor of the boost converter. This modulation is structured according to the following steps: a) Consider 401 as a module that generates a triangular signal necessary for pulse width modulation. b) Consider 108 as a block representing the MPPT algorithm, whose inputs are the DC voltage VAPV106 measured in the capacitor CB1104 and a reference current IAPV107. c) Consider the output of the MPPT 108 algorithm block as a variable signal D 402 that will decide the duty cycle of the PWM modulation output trigger signal. d) Connect the duty cycle signal D 402 to the PWM block 403. e) Consider the PWM block 403 as a comparison module between the triangular signal To coming from module 401 and the duty cycle signal D 402. It is important to mention that the duty cycle signal D could not come from an MPPT algorithm but could just be a constant signal. f) Finally, the SPWM modulation block 403 generates the Sr trigger signal which is fed into the power transistor 306 of the boost converter 105. Referring to Figure 5, the flowchart for the MPPT algorithm is shown, whose inputs are the DC voltage VAPV and the reference current IAPV. Its output is the duty cycle D, which is useful in PWM modulation to make the boost converter functional. This flowchart consists of the following steps: a) Read the DC voltage VAPV(Jc) and the reference current IAPV(k). b) Ask if a condition is met. The condition is ΔR = P( / c) — P( / c — 1) = 0. If the condition is met, return to the beginning of the algorithm. If the condition is not met, proceed to step c. c) Ask if a condition is met. The condition is ΔR > 0. If the condition is met, proceed to step d. If the condition is not met, proceed to step e. d) Check if a condition is met. The condition is V^py( / c) — VapvW < 0. If the condition is met, the work cycle D is equal to D = D + ΔP and the algorithm returns to the beginning. If the condition is not met, the work cycle D is equal to D — D — ΔZ) and the algorithm returns to the beginning. e) Check if a condition is met. The condition is—< θ·. If the condition is met, the work cycle D is equal to D — D — ΔO and the algorithm returns to the beginning. If the condition is not met, the work cycle D is equal to D = D + Δ£ and the algorithm returns to the beginning. Referring to Figure 6, the topology of a VSC is shown. A VSC is a three-phase bidirectional converter that performs DC / AC power conversion. It consists of three parallel structures of two power transistors in series. This topology is structured according to the following steps: a) Consider connection nodes 601, 602 and 603 as the three-phase output ports of the VSC. b) Connect the emitter 604 and the collector 605 of the power transistor to connection node 601, which forms the first arm of the VSC. c) Connect the emitter 606 and the collector 607 of the power transistor to connection node 602, which forms the second arm of the VSC. d) Connect the emitter 608 and the collector 609 of the power transistor to connection node 603, which forms the third arm of the VSC. e) Connect the collectors of power transistors 604, 606, 608 to form connection node 308. f) Connect the emitters of power transistors 605, 607, 609 to form connection node 309. g) The trigger signals (S1-S6) come from the SPWM modulation of Figure 7. qj cznn / zznz / e / YiAi Referring to Figure 7, the schematic shows the elements necessary to perform SPWM modulation and generate the trigger signals for the power transistors of a VSC. This modulation is structured according to the following steps: a) Consider 701 as a block that produces a triangular signal Useful for sinusoidal pulse width modulation. b) Consider 702 as a module that generates three sinusoidal signals mabc(703-705) of unit magnitude and with a phase shift of 120° electrical between them. c) Consider that, in the present invention, the variable sinusoidal signals mabc(703-705) are delivered by the unbalanced control 113. However, these signals can be only constant signals without the action of any control. d) Finally, consider the SPWM block 706 as a comparison stage between the modulating signals (703-705) and the triangular signal To coming from module 701, in order to generate the trigger signals (S1-S6). Referring to Figure 8, the control structure proposed in this invention is shown, which is applicable to the PVECS inverter VSC. The scheme of this control structure comprises the following steps: a) Consider connection nodes 110 and 111 as the points where the voltages VCD and VCDref are located, respectively. b) Perform a subtraction 801 of the voltage VCD110 minus the voltage VCDref 111. c) Pass the result of the subtraction 801 through a PI controller 802, the result of which will be the virtual approximate magnitude of the voltage 803. d) Consider (807-809) as the multiplications between the virtual voltage V / j 803 and the sinusoidal signals sin (ωϋ + 0“bc} (804-806). qj cznn / zznz / e / YiAi e) Perform the subtractions (810-812) of the network voltages Vgabc(116-118) minus the results of the multiplications (807-809). f) Enter the results of the sums (810-812) into the profits (813-815) to double their value. g) Consider the blocks (819-821) as divisions between the VCD voltage present at the nodes (816-818) and the result of the gains (813-815). h) Finally, consider the result of the divisions (819-821) as the modulating signals mfbc(703-705), respectively. ANALYTICAL DESCRIPTION OF THE PRESENT INVENTION. Figure 1 shows an extended and detailed view of the PVECS implemented in the present invention. It illustrates all the electrical and electronic components of the conversion stages and control structures used. Modeling of the photovoltaic panel. An ideal model of a solar panel consists solely of a dependent current source 201 connected in parallel to a diode 204. However, to complete a more realistic behavior, a parallel resistor Rsh207 is added to the model, representing the nonlinearities in the equipment, as well as a series resistor Rs208, representing the charge of the semiconductor material and the metal contacts. The current delivered IPV by the PV panel 103 is described by the following equation: qj cznn / zznz / e / YiAi PV=4—D ~ sh (1) Q / cznn / zznz / e / Y where ILes is the current delivered by the dependent source, IDes is the current flowing through diode 204 and Istles is the current present in resistor Rsh207. Using the model proposed by William Bradford Shockley, the current IDen the diode 204 is proportional to the saturation current Ioy and is described by equation (2). d — o ÍQVd' exp ——p\nkT. (2) where n is the ideal diode factor; k = 1.381x1023J / K is the constant of Boltzmann; q = 1.602 x 10¹⁹ C is the charge of the electron and T is the absolute temperature of the panel. Furthermore, the current lshen through the resistance Rsh207 is described as: _ VPV+ IPVRS¡sh - ñ ' 'Ksh where Vpv is the output voltage of the PV 103 panel. Subsequently, assuming that there are multiple PV 103 panels connected in series ( / Vs) and parallel (Np), by replacing (2) and (3) in (1), the current generated by the PV 103 panel can also be described as: ¡PV ¡*¡p¡L Np¡o í QÍVpv + ¡PV¡s)\Λexp ---Γ7——1 \ NsnkT J _ .. Vpv + ¡pyRsPNsRsh(4) Modeling of the DC / DC boost converter. The DC / DC boost converter 3 has two functions: first, to boost the DC voltage of the PV panel 103; second, to obtain the maximum power extraction from the PV panel 103, using the MPPT algorithm 108. Where, the basic operating principle consists of the continuous exchange between an active switching state (ton) and an inactive state (t0^). qj cznn / zznz / e / YiAi Thus, during a state t0 / / , switch 306 is closed, causing inductor LB303 to increase its current and store the energy supplied by the solar panel. Conversely, during a state ton, switch 306 is closed, causing inductor LB303 to release the previously stored energy. At the end of an on / off cycle of the switch, the current in inductor LB303 should be the same as at the beginning, resulting in: Vjníty on , (V¿n(t) Vout(ty)toff__zcv - + -- U Furthermore, depending on the state (tone tOff) and switching period (To) of the 10 switch, the duty cycle in this can be expressed by equations 8 or 9, respectively. 0=^2(6) To 1-0 = ^(7) * O Based on the above, by substituting (6) and (7) into (5), we can obtain: ^n(t) DT0(MC - VoutW (1 - D)T0 ----L-----+------------L--------------=°(8) lb^b Finally, solving for Vout we can obtain the output voltage of the boost converter 3 using the equation: V = Vin(9) It can be noted that, as the duty cycle D increases, the output voltage of the converter increases; however, when large values ​​for D are present, considerable losses are obtained due to the parasitic resistance associated with the inductor LB303. Pulse Width Modulation Technique. Pulse-width modulation (PWM) 4 compares a triangular carrier signal Tocon at a pre-set frequency with the duty cycle signal D402, which can be generated by the MPPT algorithm 108 or be a constant signal. This comparison generates a trigger signal Sxcon with a variable pulse width 10, which activates the switching state of switch 306. To Igoritmo MPPT. The MPPT algorithm 108 is a process where the impedance observed by the PV panel 103 is continuously adjusted, operating near the maximum power point corresponding to the temperature 101, irradiance 102, and load. In the present invention, the perturb-and-observe (PO) method is used, which has characteristics such as compatibility with any PV array module and ease of implementation. The operating principle of the PO method consists of continuously changing the operating point of the PV panel 103 by perturbing the associated duty cycle. Q / cznn / zznz / e / Y to the PWM modulation 4 of the boost converter 3. After each disturbance, the voltage and current of the PV panel 103 are measured, calculating the instantaneous power P(k). If the measured power exceeds the previous power value P( / c — 1), the disturbance continues in the same direction, i.e., increasing or decreasing the duty cycle D 402. This flow diagram of this MPPT method can be observed in Figure 5. VSC converter modeling. In the present invention, a VSC 112 converter is used to interconnect the PVECS to a three-phase AC electrical network with nominal voltage V^bc. The voltages at the AC output terminals (601-603) of this device are V“bc and it is interconnected to the network by means of the 10 inductors Labc(119-121) and the resistors rabc(122-124). On the other hand, the DC input terminals of the VSC 112 are connected to the output of the boost converter 105 by means of a capacitor CB2109. For the correct operation of a VSC, it must be true that VCD> Vg_LL. Assuming that the switching frequency is ten times greater than the electrical network frequency, it is valid to represent the VSC 112 as that of Figure 6 with the averaged model of equation (10). Vtabc(t) = - mabc(t) VCD(t) (10) Applying Kirchhoff's voltage laws to the VSC 112 of Figure 1, we obtain expressions (11) and (12) in the time domain, which describe the relationship between the AC and DC parameters of the converter, respectively. qj cznn / zznz / e / YiAi dl?bc1 / hh\ -j- = Tabc (~rJi ~Vt+VCW) Cí-L Li ' ' (11) Q / cznn / zznz / e / Y - ^“GcdCO—Icde(O) (12) Ul Cy Sinusoidal Pulse Width Modulation Technique. Sinusoidal pulse-width modulation (SPWM) 7 compares a three-phase sinusoidal signal (modulator) mabc(703-705) with a triangular signal (carrier) To. The latter sets the frequency at which the VSC 112 switches are activated, generating a variable-width signal resembling a sine wave. The ^35 switches of VSC 112 are triggered when the modulating signals exceed the magnitude of the carrier signal. The switching states of switches S246 are complementary. Proposed control law for the interconnection of a PVECS to an unbalanced network. In order for control law 8 of the present invention (shown in Figure 8) to guarantee balanced generation currents and a unity power factor in each phase, regardless of the total power delivered or whether the grid voltages (116-118) to VSC 112 are balanced or not, the following condition must be met: 1. The voltages across the coupling inductances (119-121) V^bc, must be perfectly balanced between them in magnitude, \V“1 = kí| = ikíi. Furthermore, these voltages must be in quadrature (90°) with respect to Vgabc(116-118), 3^bc= dgbc+ 90°, regardless of whether the latter are balanced or not. To meet this condition, the voltages (601-603) V^bc vary in magnitude and phase, while the voltages across the coupling inductors (119-121) VBbc remain in quadrature with respect to V£bc (116-118). In this way, the power factor remains unity, while the generating currents I^bcCA are balanced. To be able to vary this voltage, assuming that in equation (10) the voltage VCD110 is constant, the control scheme must be able to independently generate modulating variables (703-705) mabc with different magnitudes and angles. From Figure 8, the following mathematical expression for V“bc can be inferred: \VL\z.0Bbc= \Vg\ / -9gbc-\Vt\^bc(13) Applying Ohm's law to the left-hand side, equation (13) can be rewritten as follows: [| / Δ|Ζ0^] [r +jXL] = \Vg\¿e“bc-\Vt\¿0“bc(14) By substituting equation (10) into equation (14) and expressing the result in phasor form, the following expression is obtained: pL V(*l)2+ O)2|z + tan-1(y)) = \vg\¿egbc- 2 ' lmWcVcd(15)Solving for \m\ze^>c(703-705) and substituting equation (13) into equation (15), the above expression can be described as: qj cznn / zznz / e / YiAi \Vg\¿0“bc- |PL|z(e?bc+ tan'1|m|z <c= 2———^-----vcd (16)Using equation (16), a control scheme can be used to obtain the magnitude and phase of the modulating signals |m|z.0^c(703.705), which allow the regulation of VCD110 and guarantee a unity power factor. To develop this control scheme, the VSC 112 of the PVECS must be analyzed in the frequency domain. Applying the transform of Laplace, the input AC currents and DC voltage are expressed below: , 1 =1—sf.L+r^---- * * (18)sCB2 To consider the PI control loop (802) it is necessary to make the following considerations: 1) The reference DC voltage 111 can be approximated as VCDref ~ VCD; 2) The voltages across the coupling inductances (119-121) are approximated by 7 / « | VL|. Therefore, V[ can be defined as the controller output in equation (19). (19) Where kpy are the proportional and integral gains, respectively. The complete model of control 113 is given by equation (20), which results from substituting equation (19) into equation (16). ~VL 'SÍn(04C+tan-1(v)) VCDref Best method of implementing the invention. First of all, it is important to mention that the present description should not be considered as limiting, but rather as an illustrative description. This section presents the numerical validation of the mathematical models described above. Specifically, it shows the results of a case study to analytically corroborate the performance of the PVECES of the present invention. Figures 9 and 10 illustrate the dynamic responses of irradiance, active and reactive power, AC and DC voltages, and currents generated in the system during a condition of changing irradiance and unbalanced grid voltages. The PVECS data used for this simulation are shown in Table 2. qj cznn / zznz / e / YiAi Table 2. General Operating Data. Parameter Value. Solar Irradiance 1000 [W / m2] Voltage VAPV 309 [V] Active Power 15 [kW] Reactive Power 0 [VAR] Voltage VDC 450 [V] Inductance LB 23.7 [mH] Capacitor CB1 115.92 [pF] Boost Converter Frequency 8000 [Hz] Capacitor CB2 1772.53 [pF] Inductances Labc 1.29 [mH] Resistances rabc 0.1 [Ω] Mains Voltages Vgabc 127 RMS [V] Mains Frequency fred 60 [Hz] Starting with Figure 9, section a) shows the simulated solar irradiance for different times of day. During the simulation period between 0 and 1.5s (sections A and B), the irradiance reaches a maximum value of 1000 VK / m², after which it drops to 500 W / m² by 2.5s. It is important to note that the generated active power can only be transmitted if the DC voltage remains constant during the change in irradiance. Having said that, Figure 9 b) shows the behavior of the VCD voltage. It is noteworthy that two disturbances occur during the simulation period. The first (section B) happens at 0.8s and is caused by an imbalance in the grid voltages (Vga = 0.7 pu, Vg = 0.8 pu, Vgc = 1 pu), which is shown later. The second (section C) corresponds to the change in solar irradiance between 1.5s and 2.5s. In both cases, thanks to the implemented control structure, the PVECS's VCD voltage is maintained at its reference of 450 V. Similarly, the active and reactive power generated are shown in Figures 9 c) and d). It can be noted that during section B, the power values ​​undergo a slight change, as does the DC voltage. However, both power values ​​return to their established values. On the other hand, during the change in section C, the active power generated decreases from 15 kW to 7.5 kW in 2.5 s (section D). At the same time, the reactive power generated undergoes another slight change; however, it remains at its reference value of 0 VAR. Subsequently, Figure 10a) shows the grid voltages and the imbalance they experience at 0.8s. Applying an increase, section b) shows that two of the phases suffer an unequal voltage drop. Meanwhile, the behavior of the modulating signals mabcde of the inverter section of the PVECS during the simulation period is shown in Figures 10d) and f). It is worth noting that, in order to produce balanced generation currents, these currents behave entirely according to the voltage imbalance applied to the grid. Finally, the behavior of the generation currents during the simulation period is shown in Figures 10 c) and e). When a voltage imbalance occurs, the currents must increase in magnitude to compensate for the power demand. Subsequently, when the irradiance changes and active power generation decreases, the currents also decrease. Simultaneously, it can be observed that the currents remain balanced, sinusoidal, and free of harmonic content at all times, maintaining a unity power factor. NEW FEATURES OF THE INVENTION. The present invention comprises a PVECS that, through the combination of its conversion stages and control structures, has the capacity to interconnect to an unbalanced electrical grid without losing any of its operational advantages. Although PVEC systems are a known technology within the field of photovoltaics, there is no record of any intellectual property that reports this characteristic in a PVEC system with the topological structure and control structure presented here. Specifically, the novelty of the present invention is to allow the proposed system, under a scenario of unbalanced voltages in the interconnection network, to jointly meet at all times the following characteristics: continuity of operation, maximum power extraction from the photovoltaic array, continuous generation of active and reactive power, balanced generation currents, having a negligible harmonic content and maintaining a unity power factor.

Claims

1. A photovoltaic energy conversion system with the ability to interconnect to an unbalanced electrical grid without losing any of its operational advantages, characterized in that it comprises a PV array 103 that delivers electrical energy by means of the variables of temperature and solar irradiance; a capacitor CB1 104 electrically connected to a PV array 103; a boost converter 105 electrically connected to a capacitor CB1104; a maximum power point tracking (MPPT) control module 108 that produces a duty cycle signal D; a PWM modulation submodule that delivers a trigger signal Sx; a capacitor CB2 109 electrically connected to the boost converter 306; a voltage source converter (VSC) 112 electrically connected to the capacitor CB2 109; a control module 113 electrically connected to the VSC converter 112; and an SPWM modulation submodule that generates the trigger signals (S1-S6).a phase-tracking loop (PLL) signal module 115 electrically connected to the control module 113; a PLL module 114 electrically connected to the signal module 115; an arrangement of inductors (119-121) and resistors (122-124) interconnected between the VSC converter 112 and an unbalanced electrical distribution network.

2. The photovoltaic energy conversion system according to claim 1, characterized in that its PV arrangement 103 delivers electrical energy to the capacitor CB1 104 through the connection nodes (301 and 302) using the temperature and solar irradiance variables.

3. The photovoltaic energy conversion system according to claim 1, characterized in that its boost converter 105 comprises nodes (301 and 302) as the positive and negative input ports, nodes (308 and 309) as the positive and negative output ports, as well as the electrically connected inductor LB 303, power transistor 306 and diode 307.

4. The photovoltaic energy conversion system according to claim 1, characterized in that its step-up converter 105 raises the magnitude of the DC voltage from the PV array 103 to a value suitable for use by the VSC converter 112 5. The photovoltaic energy conversion system according to claim 1, characterized in that its MPPT module 105, by means of reading the DC voltage 106 and the current 107, generates a duty cycle D 503 which is delivered to the boost converter 105, ensuring maximum power extraction from the PV array 103 as a function of solar irradiance and temperature.

6. The photovoltaic energy conversion system according to claim 1, characterized in that its PWM modulation submodule 4, by means of the comparison between a triangular signal 401 and the duty cycle D 503, generates a trigger signal S± used by the power transistor 306 of the boost converter 105.

7. The photovoltaic energy conversion system according to claim 1, characterized in that its VSC 112 converter comprises nodes (308 and 309) as the positive and negative input ports, nodes (601-603) as the AC output ports, and electrically connected power transistors (604-609).

8. The photovoltaic energy conversion system according to claim 1, characterized in that its VSC 112 converter performs DC / AC power conversion and interconnects the system with an electrical network.

9. The photovoltaic energy conversion system according to claim 1, characterized in that its SPWM modulation submodule 7, by means of the comparison between a triangular signal 701 and the mabc modulating signals (703-705), generates the trigger signals used by the power transistors (604-609) of the VSC converter 112.

10. The photovoltaic energy conversion system according to claim 1, characterized in that its voltage control module 113, by means of the voltage VCD 110, reference voltage VCDref, the magnitude of the network voltages V^bc (116-118) and the phase of the voltages in the inductors (119-121) and resistors (122-124), generates under dynamic conditions, independent phase mabc modulating signals (703-705).

11. The control structure in claim 10, which improves a photovoltaic energy conversion system, characterized in that it comprises the following steps: i. Performing a subtraction 801 of the voltage VCD 110 minus the voltage VCDref 111. ii. Passing the result of the subtraction 801 through a PI controller 802, the result of which will be the approximate virtual magnitude of the voltage V'Li 803. iii. Consider (807-809) as the multiplications between the virtual voltage 803 and the sinusoidal signals sin(wt + (804-806). iv. Perform the subtractions (810-812) of the network voltages Vgabc (116-118) minus the results of the multiplications (807-809). v. Input the results of the additions (810-812) to the gains (813-815) to double their value. vi. Consider the blocks (819-821) as divisions between the voltage VCD present at the nodes (816-818) and the result of the gains (813-815). vii.Consider the result of the divisions (819-821) as the mfbc modulating signals (703-705), respectively.

12. The photovoltaic energy conversion system according to claim 1, wherein, in the presence of unbalanced voltages in the interconnection network, the following advantages are obtained: i. Continuous operation. ii. Maximum power extraction from the photovoltaic array. iii. Continuous generation of active and reactive power. iv. Balanced generation currents. v. Negligible ammonia content. vi. Unity power factor.

Images