Photovoltaic string medium-voltage convergence system

Abstract

The invention discloses a photovoltaic string medium-voltage convergence system, and belongs to the technical field of photovoltaic power generation, the system realizes voltage level jump through a + / -1500V bipolar symmetric architecture, and a plurality of 1500V photovoltaic strings are configured to have positive electrodes and negative electrodes symmetrically arranged to form a medium-voltage convergence system with the total output voltage of 3000V. And meanwhile, the stability of the system is maintained through the midpoint high-resistance grounding device, and the voltage limitation of a traditional 1500V system is broken through on the premise of not changing the insulation and voltage resistance level (1500V) of the existing equipment. According to the scheme, the direct current side transmission loss and the cable cost are remarkably reduced, the confluence radius is expanded, existing photovoltaic modules, cables and matched equipment are compatible, and an efficient and economical confluence solution is provided for a large photovoltaic power station.

Description

Technical Field

[0001] This application relates to the field of photovoltaic power generation technology, and more specifically, to a medium-voltage combiner system for photovoltaic strings. Background Technology

[0002] In recent years, the global photovoltaic (PV) power generation industry has experienced explosive growth, with installed capacity continuously expanding. Driven by technological iteration and economies of scale, the cost of PV power generation has significantly decreased, and the cost per kilowatt-hour has become competitive with traditional coal-fired power generation, marking the industry's full entry into the era of grid parity. To continuously improve power generation efficiency and system economics, the voltage level of PV systems has gradually increased from 600V to 1000V, and further evolved to 1500V. Currently, the 1500V system, with its advantages in reducing line losses and saving initial investment, has become the mainstream technical solution for large-scale PV power plants.

[0003] However, as photovoltaic power plants expand to megawatts (MW) and even gigawatts (GW), especially in high-irradiance regions like Northwest and North my country, the technical bottlenecks of existing 1500V systems are becoming increasingly apparent. Limited by the 1500V withstand voltage rating of photovoltaic modules, the DC-side bus voltage typically cannot exceed 1500V, leading to significant problems in power transmission for low-voltage bus systems, such as a sharp increase in line losses and limited bus capacity. According to GB50797-2012, "Design Code for Photovoltaic Power Plants," under standard test conditions, the voltage drop of the cable from the photovoltaic string to the DC side of the inverter should not exceed 2%.

[0004] Existing 1500V systems suffer from the following technical shortcomings: First, limited combiner radius – the low voltage level results in a larger current under the same power transmission conditions, significantly increasing cable losses and restricting the expansion of the combiner range; second, decreased transmission efficiency – the ohmic loss of long-distance DC cables is proportional to the square of the current, severely affecting the overall system efficiency; third, deteriorated economic efficiency – large-section cables are required to meet voltage drop limitations, leading to a significant increase in cable costs. Therefore, given that current photovoltaic system equipment is still generally compatible with the 1500V withstand voltage level, there is an urgent need to propose an innovative combiner system solution to overcome the DC-side voltage limitations, effectively solve the technical problems of high transmission loss and difficulty in capacity expansion, and further improve the overall performance and economic efficiency of photovoltaic power plants. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of existing technologies and provide a medium-voltage combiner system for photovoltaic strings. While maintaining the ground insulation withstand voltage level of all equipment such as photovoltaic strings and cables at 1500V, the total voltage of the system is increased to 3000V through a ±1500V bipolar symmetrical combiner architecture, which effectively reduces DC transmission loss and cable costs, while being fully compatible with the equipment system of existing photovoltaic power plants.

[0006] The objective of this application is achieved through the following technical solution: In a first aspect, this application proposes a photovoltaic string medium-voltage combiner system, comprising: Multiple first photovoltaic strings and second photovoltaic strings, with an output voltage level of 1500V for both first and second photovoltaic strings; A bipolar busbar is used to connect the positive output terminal of the first photovoltaic string in series with the negative output terminal of the second photovoltaic string to form a ±1500V bipolar symmetrical structure with a total output voltage of 3000V. The ±1500V bipolar symmetrical structure includes a positive busbar, a negative busbar, and a neutral point. The neutral point is grounded through the grounding resistance, and the insulation withstand voltage rating to ground of the first photovoltaic string, the second photovoltaic string, and the connecting cable is 1500V.

[0007] In one possible implementation, the bipolar combiner is a centralized MPPT combiner box, the input end of which is connected to the first photovoltaic string and the second photovoltaic string, and the output end forms a ±1500V bipolar symmetrical structure. The system also includes a positive voltage equalization device and a negative voltage equalization device installed at the input end of the centralized MPPT combiner box, which are used to maintain the voltage balance between the positive bus and the negative bus to ground.

[0008] In one possible implementation, the positive and negative voltage equalization devices are resistive-capacitive voltage equalization circuits.

[0009] In one possible implementation, the bipolar combiner includes: a first DC / DC converter, with its input terminal connected to the output terminal of a first photovoltaic string, the positive terminal of its output terminal connected to the positive bus, and the negative terminal of its output terminal connected to the neutral point; The second DC / DC converter has its input terminal connected to the output terminal of the second photovoltaic string, its positive terminal connected to the neutral point, and its negative terminal connected to the negative bus. Both the first and second DC / DC converters implement MPPT control and automatically balance the output power, ensuring that their output voltages are both 1500V.

[0010] In one possible implementation, the first DC / DC converter and the second DC / DC converter are integrated into a distributed MPPT combiner box.

[0011] In one possible implementation, the system further includes: The voltage detection unit is configured to monitor the voltage of the positive bus and the negative bus to ground in real time; The protection control unit, which is communicatively connected to the voltage detection unit, is configured to disconnect the series connection of the first photovoltaic string and / or the second photovoltaic string, or cut off the output of the combiner device, when an abnormal voltage is detected at either pole to ground.

[0012] In one possible implementation, the protection control unit is configured to determine that a single-pole grounding fault has occurred and perform protection actions when the voltage to ground of the positive or negative busbar drops below a first preset threshold or rises above a second preset threshold. The main solution and its various further alternatives described above can be freely combined to form multiple solutions, all of which are solutions that can be adopted and are claimed in this application; furthermore, the (non-conflicting alternatives) can also be freely combined with each other and with other alternatives. Those skilled in the art, after understanding the solution of this application, will realize from the prior art and common general knowledge that there are many combinations, all of which are technical solutions to be protected in this application, and will not be exhaustively listed here.

[0013] This application discloses a medium-voltage combiner system for photovoltaic strings, belonging to the field of photovoltaic power generation technology. This system achieves a voltage level leap through a ±1500V bipolar symmetrical architecture, configuring multiple 1500V photovoltaic strings in a symmetrical arrangement of positive and negative terminals to form a medium-voltage combiner system with a total output voltage of 3000V. Simultaneously, a high-resistance grounding device at the midpoint maintains system stability. This overcomes the voltage limitations of traditional 1500V systems without changing the insulation withstand voltage rating (1500V) of existing equipment. This solution significantly reduces DC-side transmission loss and cable costs, expands the combiner radius, and is compatible with existing photovoltaic modules, cables, and supporting equipment, providing an efficient and economical combiner solution for large-scale photovoltaic power plants. Attached Figure Description

[0014] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 A schematic diagram of a photovoltaic string medium-voltage combiner system according to an embodiment of this application is shown.

[0016] Figure 2 A schematic diagram of a centralized single-pole MPPT combiner system proposed in an embodiment of this application is shown. Detailed Implementation

[0017] The following specific examples illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification. This application can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this application. It should be noted that, unless otherwise specified, the following embodiments and features in the embodiments can be combined with each other.

[0018] Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] In existing technologies, 1500V systems mainly suffer from the following technical defects: 1) Limited bus radius: The low voltage level results in a larger current for the same power transmission, significantly increasing cable losses and restricting the expansion of the bus radius; 2) Reduced transmission efficiency: The ohmic loss of long-distance DC cables increases with the square of the current, severely affecting the overall system efficiency; 3) Decreased economic efficiency: Large cross-sectional area cables are required to control voltage drop, leading to a significant increase in cable costs. Therefore, it is urgent to develop an innovative bus solution that, while being compatible with existing 1500V withstand voltage equipment systems, overcomes the DC-side voltage limitations to solve the problems of transmission loss and capacity expansion.

[0020] Therefore, to address the economic and technical challenges of high DC-side transmission loss and limited combiner radius in large-scale photovoltaic power plants, this application proposes a medium-voltage combiner system for photovoltaic strings. This system achieves a voltage level jump through a ±1500V bipolar symmetrical architecture (total voltage 3000V) while maintaining the insulation withstand voltage of all equipment at 1500V. This solution significantly improves combiner efficiency and is fully compatible with existing photovoltaic modules, cables, and supporting equipment, which will be described in detail below.

[0021] Please refer to Figure 1 , Figure 1 This paper illustrates a structural schematic diagram of a photovoltaic string medium-voltage combiner system according to an embodiment of this application. The photovoltaic string medium-voltage combiner system includes: Multiple first photovoltaic strings and second photovoltaic strings, with an output voltage level of 1500V for both first and second photovoltaic strings; A bipolar busbar is used to connect the positive output terminal of the first photovoltaic string in series with the negative output terminal of the second photovoltaic string to form a ±1500V bipolar symmetrical structure with a total output voltage of 3000V. The ±1500V bipolar symmetrical structure includes a positive busbar, a negative busbar, and a neutral point. The neutral point is grounded through the grounding resistance, and the insulation withstand voltage rating to ground of the first photovoltaic string, the second photovoltaic string, and the connecting cable is 1500V.

[0022] Figure 1 In this system, the positive output of the first DC / DC converter is connected to the positive bus (+) of the system, and its negative output is connected to the positive output of the second DC / DC converter, forming the system midpoint. The negative output of the second DC / DC converter is connected to the negative bus (-) of the system. Through this series architecture, each of the two DC / DC converters outputs Udc / 2 (i.e., 1500V), together constructing a total output voltage of 3000V. Maintaining the 1500V insulation level of all photovoltaic modules, combiner boxes, and cables, the two DC / DC converters independently execute MPPT control to automatically achieve power balance. The system midpoint is grounded via a high-resistance grounding device to ensure stable system operation. In the event of a single-pole grounding fault, the system can quickly disconnect the corresponding DC / DC combiner box output by monitoring the pole-to-ground voltage in real time, achieving reliable protection.

[0023] The system mainly includes multiple photovoltaic power generation units, a bipolar combiner device, and a grounding protection system. The multiple photovoltaic power generation units consist of several first photovoltaic strings (PV1) and several second photovoltaic strings (PV2). Both the first photovoltaic strings (PV1) and the second photovoltaic strings (PV2) are DC power generation units composed of photovoltaic modules connected in series, with a rated output voltage of 1500V, which is completely consistent with the current mainstream photovoltaic power station equipment withstand voltage standards.

[0024] A bipolar combiner is used to electrically connect the positive output terminal of a first photovoltaic string (PV1) to the negative output terminal of a second photovoltaic string (PV2) in series. Through this specific series connection, two DC power supplies with a voltage of 1500V to ground are combined into a new DC power supply with a potential difference of 3000V, forming a ±1500V bipolar symmetrical structure with a total output voltage of 3000V. This structure specifically includes a positive bus with a potential of +1500V to ground, a negative bus with a potential of -1500V to ground, and a neutral point with a potential of approximately 0V to ground.

[0025] The grounding protection system includes a high-resistance grounding resistor, through which the neutral point of the bipolar DC bus is reliably grounded. This high-resistance grounding method can effectively limit the fault current when a single-pole ground fault occurs in the system.

[0026] It is particularly important to emphasize that, in the entire system constructed in this scheme, although the inter-electrode operating voltage is as high as 3000V, the highest voltage that any photovoltaic string (PV1, PV2), combiner box, DC cable, and all related supporting equipment can withstand is still only its DC voltage to ground of 1500V. Therefore, this invention successfully achieves a leap from 1500V to 3000V DC side voltage without breaking the insulation withstand voltage level of existing 1500V equipment in the photovoltaic industry chain, thereby significantly reducing transmission current and losses, and significantly improving system efficiency and economy.

[0027] The bipolar combiner is a centralized MPPT combiner box. The input end of the centralized MPPT combiner box is connected to the first photovoltaic string and the second photovoltaic string, and the output end forms a ±1500V bipolar symmetrical structure. The system also includes a positive voltage equalization device and a negative voltage equalization device installed at the input end of the centralized MPPT combiner box to maintain the voltage balance between the positive bus and the negative bus and ground.

[0028] The bipolar combiner unit specifically employs a centralized MPPT (Maximum Power Point Tracking) combiner box. The DC input terminal of this centralized MPPT combiner box is connected to the output terminals of the first photovoltaic string (PV1) and the second photovoltaic string (PV2) to receive the 1500V DC power generated by them. The combiner box integrates an MPPT control unit and a Boost converter circuit, among other power conversion modules. Its DC output terminal directly forms a ±1500V bipolar symmetrical structure, providing positive and negative buses with ground potentials of +1500V and -1500V respectively.

[0029] To ensure the stability and reliability of this bipolar symmetrical structure during operation, the system further incorporates a positive voltage equalization device and a negative voltage equalization device at the input of the centralized MPPT combiner box. These two equalization devices are connected in parallel between the positive and negative terminals, respectively. Their main function is to dynamically adjust and maintain the voltage balance between the positive and negative buses and ground, preventing neutral point potential drift caused by slight differences in the output characteristics of the two photovoltaic strings or asymmetry in their ground insulation impedance. This ensures that the voltage between the positive and negative buses and ground is stably maintained at approximately ±1500V.

[0030] The positive and negative voltage equalization devices are RC voltage equalization circuits. These circuits consist of resistors and capacitors connected in parallel, with their parallel terminals connected between the positive and negative busbars and the neutral point, respectively. Through the dissipation of the resistive components and the dynamic energy storage and release of the capacitive components, the circuit can automatically adjust the current flowing through it, effectively suppressing neutral point potential drift caused by differences in component characteristics or insulation fluctuations. This dynamically maintains the stability and balance of the positive and negative busbar voltages to ground, ensuring the reliable operation of the ±1500V bipolar symmetrical architecture.

[0031] The bipolar combiner includes: a first DC / DC converter, whose input terminal is connected to the output terminal of the first photovoltaic string, whose positive terminal is connected to the positive bus, and whose negative terminal is connected to the neutral point; The second DC / DC converter has its input terminal connected to the output terminal of the second photovoltaic string, its positive terminal connected to the neutral point, and its negative terminal connected to the negative bus. Both the first and second DC / DC converters implement MPPT control and automatically balance the output power, ensuring that their output voltages are both 1500V.

[0032] Both the first and second DC / DC converters operate independently and perform maximum power point tracking (MPPT) control, ensuring that their respective connected photovoltaic strings always operate at their optimal power generation state. Simultaneously, through the synergistic effect of the series-connected circuit characteristics and their internal control logic, these two DC / DC converters can automatically balance their respective output power and precisely control and stabilize their output voltage at 1500V, thus naturally maintaining voltage balance between the positive bus, negative bus, and neutral point, guaranteeing the stability and efficiency of the system architecture.

[0033] The first and second DC / DC converters are integrated into a distributed MPPT combiner box.

[0034] The first and second DC / DC converters are physically integrated and packaged within a distributed MPPT combiner box. This highly integrated design combines two independent MPPT power conversion units, corresponding control circuits, and necessary electrical connections into a unified physical device. As an independent outdoor electrical device, the distributed MPPT combiner box connects to DC cables from the first and second photovoltaic strings at its inputs, while its outputs directly provide positive bus, neutral point, and negative bus interfaces.

[0035] The system also includes: The voltage detection unit is configured to monitor the voltage of the positive bus and the negative bus to ground in real time; The protection control unit, which is communicatively connected to the voltage detection unit, is configured to disconnect the series connection of the first photovoltaic string and / or the second photovoltaic string, or cut off the output of the combiner device, when an abnormal voltage is detected at either pole to ground.

[0036] The voltage detection unit is configured to monitor the voltage between the positive bus and ground, as well as the voltage between the negative bus and ground, in real time. It typically employs a high-precision voltage sensor or a voltage divider sampling circuit, continuously transmitting the acquired voltage signals to the processing center. The protection control unit and the voltage detection unit are connected via electrical signals or a data bus to form a closed-loop control system. This unit is pre-configured with a logic judgment program for real-time analysis of the received voltage data. When an abnormality is detected in the voltage between either pole (positive or negative) and ground, such as a voltage drop significantly below or a rise significantly above the rated 1500V, the unit determines that a single-pole grounding fault or other insulation fault has occurred. Once the fault is confirmed, the protection control unit immediately generates a protection command, driving the corresponding circuit breaker, contactor, or fast switch to operate. Its protection strategies include, but are not limited to: controlling the disconnection of the series connection between the first photovoltaic string (PV1) and / or the second photovoltaic string (PV2) to resolve the fault structure, or directly cutting off the total output circuit of the bipolar combiner.

[0037] The protection control unit is configured to determine a single-pole ground fault and perform protection actions when the voltage to ground of the positive or negative bus drops below a first preset threshold or rises above a second preset threshold.

[0038] The protection control unit incorporates precise fault determination logic. This unit is configured to continuously receive positive and negative voltage signals to ground from the voltage detection unit and compare them with internally preset voltage thresholds. The first preset threshold is set to a lower value close to zero potential (e.g., 200V) to detect abnormal voltage drops; the second preset threshold is set to a higher value close to the rated inter-pole voltage (e.g., 2800V) to detect abnormal voltage increases. When the positive bus voltage to ground or the negative bus voltage to ground is detected to drop below the first preset threshold or rise above the second preset threshold, the logic determines that a single-pole ground fault (e.g., positive grounding or negative grounding) has occurred. Once the fault determination condition is met, the protection control unit immediately triggers and executes corresponding protection actions, such as disconnecting the fault circuit, thereby achieving rapid and accurate fault isolation and ensuring system safety.

[0039] Figure 2A schematic diagram of a centralized unipolar MPPT combiner system according to an embodiment of this application is shown. As shown, the system includes a first photovoltaic string (PV1) and a second photovoltaic string (PV2), both of which are conventional photovoltaic power generation units with an output voltage of 1500V. PV1 and PV2 are connected in series after being combined, specifically, the positive output of PV1 is connected to the negative output of PV2, together forming a DC power supply architecture with a total voltage of 3000V. This series connection point forms the neutral point of the system.

[0040] The system neutral point is reliably grounded through a high-resistance grounding device. At the same time, a positive and negative resistance-capacitance equalization device (not directly shown in the figure, but a necessary component of this scheme) is installed at the incoming end of the combiner box to dynamically maintain the balance of the positive and negative bus voltages to ground, ensuring that the system operates stably under a symmetrical voltage of ±1500V.

[0041] The key to this topology lies in boosting the DC-side voltage to 3000V through a series architecture without altering the original 1500V insulation level of the photovoltaic modules (PV1, PV2) and DC cables (such as L1, L2), significantly reducing transmission losses and cable costs. The boosted DC power is ultimately delivered to the subsequent DC / DC or DC / AC converter, whose MPPT control strategy is compatible with conventional systems. The system also features fault protection capabilities; in the event of a single-pole ground fault, it can quickly disconnect the series connection between PV1 and PV2 by monitoring the pole-to-ground voltage in real time, ensuring equipment and system safety.

[0042] Compared with the prior art, the embodiments of this application have the following beneficial effects: First, by increasing the system voltage to 3000V, the transmission current is reduced, thereby significantly reducing cable loss and investment costs.

[0043] Secondly, independent MPPT control of the string overcomes mismatch losses, while high-voltage transmission reduces DC-side energy loss.

[0044] Third, it has the ability to quickly detect and isolate faults, and can automatically achieve power balance to ensure the safe and stable operation of the system.

[0045] Fourth, it is fully compatible with existing 1500V equipment systems and is particularly suitable for large-scale photovoltaic power generation bases of 100MW and above.

[0046] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A photovoltaic string voltage bus system, comprising: The system comprises: a plurality of first and second photovoltaic strings, the output voltage of the first and second photovoltaic strings being 1500V; a bipolar busbar device for connecting the positive output of the first photovoltaic string and the negative output of the second photovoltaic string in series to form a ±1500V bipolar symmetric structure with a total output voltage of 3000V, the ±1500V bipolar symmetric structure comprising a positive busbar, a negative busbar and a neutral point; a grounding resistor, the neutral point being grounded through the grounding resistor, wherein the ground insulation withstand voltage of the first photovoltaic string, the second photovoltaic string and the connecting cable is 1500V.

2. The photovoltaic string medium bus system of claim 1, wherein, The bipolar busbar device is a centralized MPPT busbar box, the input end of the centralized MPPT busbar box being connected with the first and second photovoltaic strings, and the output end forming the ±1500V bipolar symmetric structure.

3. The photovoltaic string medium bus system of claim 2, wherein, The system further comprises positive and negative voltage equalization devices arranged at the input end of the centralized MPPT busbar box, for maintaining the voltage balance of the positive busbar and the negative busbar to ground.

4. The photovoltaic string medium bus system of claim 1, wherein, The positive and negative voltage equalization devices are resistance-capacitance voltage equalization circuits.

5. The photovoltaic string medium bus system of claim 1, wherein, The bipolar busbar device comprises: a first DC / DC converter, the input end of which being connected with the output end of the first photovoltaic string, the positive output end being connected with the positive busbar, and the negative output end being connected with the neutral point; a second DC / DC converter, the input end of which being connected with the output end of the second photovoltaic string, the positive output end being connected with the neutral point, and the negative output end being connected with the negative busbar; 6. The photovoltaic string medium bus system of claim 1, wherein, The first and second DC / DC converters both perform MPPT control and automatically balance the output power, so that the output voltage of each of them is 1500V.

7. The photovoltaic string medium bus system of claim 1, wherein, The first and second DC / DC converters are integrated in the centralized MPPT busbar box. The system further comprises: a voltage detection unit configured to monitor the voltage of the positive busbar and the negative busbar to ground in real time; 8. The photovoltaic string medium bus system of claim 1, wherein, a protection control unit in communication connection with the voltage detection unit, configured to control the disconnection of the series connection of the first and / or second photovoltaic strings, or the cutting off of the output of the busbar device, when detecting that any of the voltages to ground is abnormal. The protection control unit is configured to determine that a single-pole grounding fault occurs and perform a protection action when detecting that the voltage of the positive busbar or the negative busbar to ground drops below a first preset threshold or rises above a second preset threshold.

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