A photovoltaic power generation optimization layout system

Abstract

The present invention discloses a photovoltaic power generation optimization layout system, comprising: photovoltaic modules, a DC busbar trunking unit, a photovoltaic grid-connected DC / DC device, a DC metering cabinet, and a DC power distribution cabinet; the DC metering cabinet and the DC power distribution cabinet are both arranged inside a DC power distribution room, and the photovoltaic modules, the DC busbar trunking unit, and the photovoltaic grid-connected DC / DC device are all arranged on the roof of the DC power distribution room; the photovoltaic modules are connected to the DC busbar trunking unit, the DC metering cabinet is connected to the DC power distribution cabinet through a first cable, the photovoltaic grid-connected DC / DC device is connected to the DC metering cabinet through a second cable, and the DC busbar trunking unit is connected to the photovoltaic grid-connected DC / DC device through a third cable; the present invention can reduce the amplitude and the rising rate of the short-circuit current provided by the photovoltaic grid-connected DC / DC device during inter-pole short circuit, and reduce the probability of the photovoltaic grid-connected DC / DC device providing an extremely large short-time overcurrent without adding additional electrical equipment.

Description

Technical Field

[0001] The present invention relates to the field of power distribution structures, and particularly to a photovoltaic power generation optimization layout system. Background Art

[0002] With the increasingly prominent problem of fossil energy shortage, photovoltaic power generation technology, as a renewable power source technology, has developed rapidly. A photovoltaic power generation system utilizes the photovoltaic effect of solar cells to directly convert solar radiation energy into electrical energy and is a power generation system electrically connected to the public power grid. Its connection to the public power grid can be in AC form or DC form. Since DC access of the photovoltaic system has higher energy efficiency, with the popularization and application of DC distribution systems, it can be foreseen that distributed photovoltaic systems will more often adopt DC form to access DC distribution grids in the future.

[0003] Since the DC grid has not been fully popularized at the present stage, the types of mature products of photovoltaic DC grid-connected DC / DC devices are not complete, and most of them need to be customized and are expensive. Therefore, at present, distributed photovoltaic with DC grid connection mostly adopts indoor centralized grid connection mode to save the cost of grid connection devices.

[0004] Taking roof photovoltaic as an example, in the prior art one, usually the photovoltaic modules are placed on the roof. After being converged on the roof by a DC busbar box, they are introduced into the photovoltaic grid-connected DC / DC grid connection device placed in the DC distribution room through cables, then through DC metering, and finally connected to the DC distribution cabinet, as follows Figure 1 shown. This layout scheme of the prior art adopts a centralized DC / DC device installed indoors, which is convenient to install and easy to implement, and is widely used at the present stage. From the Figure 1 shown layout diagram, the main circuit diagram of DC grid connection of the distributed photovoltaic power generation system can be drawn as Figure 2 shown.

[0005] Figure 3 Shown is a commonly used topological structure of a DC / DC converter for DC grid connection of a photovoltaic power generation system. The electrical energy output by the photovoltaic array is connected through the low-voltage side (or high-voltage side), and the high-voltage side (or low-voltage side) is connected in parallel with the DC grid. When a feeder pole-to-pole short circuit occurs, for example, Figure 2 the short circuit of f2 or f3, f4 shown, in the Figure 1 shown layout mode, Figure 2 the cables L1 and L2 in it are both short (generally not exceeding 5m). Therefore, the voltage across the port capacitor of the photovoltaic grid-connected DC / DC converter will drop to an extremely low value instantaneously during the short circuit fault, and the capacitor will discharge rapidly to generate a large discharge current, thus having a serious impact on the protection, setting, etc. of the entire system.

[0006] Figure 4The figure shows the output current waveform of the photovoltaic power generation system when a fault occurs at f3 at 0.5 s with the total length of cable L1 and cable L2 being 10 m. It can be seen that at the moment of fault occurrence, the capacitor at the DC / DC port discharges rapidly, and the output current of the photovoltaic power generation system rises rapidly. It reaches a peak value of 3.08 kA after about 0.5 ms, which is 9.6 times the rated operating current. The fault current drops to a very low value after about 4.9 ms, and then starts to oscillate. The oscillation ends after about 50 ms, and the short-circuit current is all provided by the photovoltaic array, stabilizing at 0.45 kA. The parameters used in the simulation are as follows: the rated voltage of the system is UPV = 750 V, assuming constant light intensity and constant stability, the output current under normal operation of the photovoltaic is 0.32 kA, the short-circuit current is 0.45 kA, and the capacitance size CPV at the outlet of the DC / DC converter is 3000 uF.

[0007] From the simulation calculation results, the main disadvantages of the prior art one are as follows: 1. Since the photovoltaic grid-connected DC / DC device is too close to the DC distribution cabinet, the electrical distance is short and the impedance is small. When a pole-to-pole short circuit occurs in the feeder, the port capacitors of the grid-connected DC / DC device will all discharge rapidly and provide a large short-circuit current, which is extremely likely to cause misoperation of the protection and even damage the equipment. 2. Any pole-to-pole feeder short circuit will cause the grid-connected DC / DC device to discharge rapidly and provide a large short-circuit current. Therefore, for occasions with more feeders, the probability of the grid-connected DC / DC device providing a large short-term overcurrent will increase significantly.

[0008] Regarding the overcurrent problem of the prior art one, in the prior art two, the method of Figure 5 as shown in the figure is adopted to suppress the overcurrent by connecting a series current-limiting reactor, that is, connecting a series current-limiting reactor at the outlet of the photovoltaic grid-connected DC / DC converter to slow down the discharge speed of the capacitor and reduce the rising rate and peak value of the fault current. Figure 6 The figure shows the output current waveform of the photovoltaic power generation system at 0.5 s when a fault occurs at f3 with a 1 mH current-limiting reactor connected in series at both the positive and negative poles. The peak value of the short-circuit current appears after a delay of 4.8 ms, and the peak current is reduced to 1.25 kA, a reduction of about 59.4% in the peak current; the fault current drops to a very low value after about 29 ms, and then the port capacitor of the DC / DC converter, the current-limiting reactor, the line reactance, etc. form an oscillation circuit, and the short-circuit current oscillates continuously around 0.45 kA. Figure 6 Compared with Figure 4 it can be seen that after connecting the series current-limiting reactor, the peak value of the fault current, the rising rate of the short-circuit current, and the discharge speed of the port capacitor are all suppressed.

[0009] The series current-limiting reactance principle used in the second prior art is simple, easy to implement, and has obvious effects, which is a commonly used method in current engineering. However, this method also has the following disadvantages: 1. It is difficult to select the current-limiting reactor. The iron-core reactor is prone to saturation under large current and loses its current-limiting effect; the air-core reactor has large leakage magnetic flux, which is likely to affect surrounding equipment, and has a large volume, requiring a large floor area. 2. It adds primary equipment, resulting in corresponding increases in project costs, floor area, etc. Especially when the rated current is large, the reactor has a large volume for convenient heat dissipation. 3. It is not easy to determine the inductance value of the current-limiting reactor. If the inductance value is too small, the current-limiting effect is not obvious; if the inductance value is too large, the reactor has a large volume and is likely to cause system instability. In addition, the reactor may also resonate with capacitors in the system.

[0010] Therefore, there is an urgent need for an arrangement scheme on the market that can reduce the amplitude and rising rate of the short-circuit current provided by the PV grid-connected DC / DC device during inter-pole short circuit and reduce the probability of the PV grid-connected DC / DC device providing extremely large short-term overcurrents without adding additional electrical equipment. Summary of the Invention

[0011] The present invention provides a PV power generation optimization arrangement system that can reduce the amplitude and rising rate of the short-circuit current provided by the PV grid-connected DC / DC device during inter-pole short circuit and reduce the probability of the PV grid-connected DC / DC device providing extremely large short-term overcurrents without adding additional electrical equipment.

[0012] To solve the above technical problems, an embodiment of the present invention provides a PV power generation optimization arrangement system, including: PV modules, a DC busbar box, a PV grid-connected DC / DC device, a DC metering cabinet, and a DC power distribution cabinet;

[0013] Both the DC metering cabinet and the DC power distribution cabinet are arranged inside the DC power distribution room, and the PV modules, the DC busbar box, and the PV grid-connected DC / DC device are all arranged on the roof of the DC power distribution room; the PV modules are connected to the DC busbar box, the DC metering cabinet is connected to the DC power distribution cabinet through a first cable, the PV grid-connected DC / DC device is connected to the DC metering cabinet through a second cable, and the DC busbar box is connected to the PV grid-connected DC / DC device through a third cable.

[0014] As a preferred solution, the distance between the DC busbar box and the PV grid-connected DC / DC device is less than 10 m; both the DC busbar box and the PV grid-connected DC / DC device are arranged on the roof of the DC power distribution room.

[0015] As a preferred solution, the distance between the DC metering cabinet and the PV grid-connected DC / DC device is greater than 100 m. The DC metering cabinet is arranged inside the DC distribution room, and the PV grid-connected DC / DC device is arranged on the roof of the DC distribution room.

[0016] As a preferred solution, the length of the first cable is less than that of the second cable.

[0017] As a preferred solution, the length of the third cable is less than that of the second cable.

[0018] Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

[0019] 1. In the technical solution of the present invention, by arranging the PV grid-connected DC / DC device on the roof and arranging both the DC metering cabinet and the DC power distribution cabinet inside the DC distribution room, the connection distance between the PV grid-connected DC / DC device and the DC metering cabinet becomes longer, that is, the second cable becomes longer, which can effectively reduce the short-circuit current amplitude and rising rate and is only invalid for the inter-pole fault on the outlet side of the PV grid-connected DC / DC device. Therefore, the probability of the PV grid-connected DC / DC device providing a large short-time overcurrent can be greatly reduced.

[0020] 2. By increasing the length of the second cable, the short-circuit current amplitude and rising rate can be effectively reduced, achieving an effect similar to that of a series current-limiting reactor.

[0021] 3. By arranging the PV grid-connected DC / DC device in the area close to the PV modules, the length of the second cable can be effectively increased, the length of the third cable can be reduced, and the total length of the first cable, the second cable, and the third cable remains unchanged, without additional cable costs and cable losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 : Schematic diagram of the layout structure of the photovoltaic power generation system of the prior art 1;

[0023] Figure 2 : DC grid-connected main circuit diagram of the photovoltaic power generation system of the prior art 1;

[0024] Figure 3 : Schematic diagram of the topology structure of the DC / DC converter of the prior art 1;

[0025] Figure 4 : Output current waveform diagram of the photovoltaic power generation system during the inter-pole short circuit of the feeder when the total length of the cables L1 and L2 in the prior art 1 is 10 m;

[0026] Figure 5 : Schematic diagram of the principle of the series current-limiting reactor method of the prior art 2;

[0027] Figure 6: Output current waveform diagram of the photovoltaic power generation system during the inter-pole short circuit of the feeder after adding a 2mH fault current limiter in the prior art II;

[0028] Figure 7 : Output current waveform diagram of the photovoltaic power generation system during the inter-pole short circuit of the feeder when the total length of cables L1 and L2 is 100m in the embodiment of the present invention;

[0029] Figure 8 : Output current waveform diagram of the photovoltaic power generation system during the inter-pole short circuit of the feeder when the total length of cables L1 and L2 is 200m in the embodiment of the present invention;

[0030] Figure 9 : Structural schematic diagram of the photovoltaic power generation optimization layout system in the embodiment of the present invention. Detailed implementation manners

[0031] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention.

[0032] Please refer to Figure 1 , a preferred embodiment of the present invention provides a photovoltaic power generation optimization layout system, including: photovoltaic modules, a DC combiner box, a photovoltaic grid-connected DC / DC device, a DC metering cabinet, and a DC power distribution cabinet;

[0033] The DC metering cabinet and the DC power distribution cabinet are both arranged inside the DC power distribution room, and the photovoltaic modules, the DC combiner box, and the photovoltaic grid-connected DC / DC device are all arranged on the roof of the DC power distribution room; the photovoltaic modules are connected to the DC combiner box, the DC metering cabinet is connected to the DC power distribution cabinet through a first cable, the photovoltaic grid-connected DC / DC device is connected to the DC metering cabinet through a second cable, and the DC combiner box is connected to the photovoltaic grid-connected DC / DC device through a third cable.

[0034] The technical solution of the present invention sets the photovoltaic grid-connected DC / DC device on the roof and arranges the DC metering cabinet and the DC power distribution cabinet inside the DC power distribution room, so that the connection distance between the photovoltaic grid-connected DC / DC device and the DC metering cabinet becomes longer, that is, the second cable becomes longer, which can effectively reduce the short-circuit current amplitude and rise rate, and is only ineffective for the inter-pole fault on the outlet side of the photovoltaic grid-connected DC / DC device. Therefore, the probability of the photovoltaic grid-connected DC / DC device providing a very large short-time overcurrent can be greatly reduced.

[0035] In a preferred embodiment, the distance between the DC busbar box and the PV grid-connected DC / DC device is less than 10 m; both the DC busbar box and the PV grid-connected DC / DC device are arranged on the roof of the DC distribution room. In a preferred embodiment, the distance between the DC metering cabinet and the PV grid-connected DC / DC device is greater than 100 m, the DC metering cabinet is arranged inside the DC distribution room, and the PV grid-connected DC / DC device is arranged on the roof of the DC distribution room. Specifically, both the DC busbar box and the PV grid-connected DC / DC device are arranged on the roof and are adjacent to each other. This solution arranges the PV grid-connected DC / DC device in the area close to the PV modules, which can effectively increase the length of the second cable and reduce the length of the third cable. The total length of the first cable, the second cable, and the third cable remains unchanged, without additional cable costs and cable losses. That is, the length of the first cable is less than the length of the second cable, and the length of the third cable is less than the length of the second cable.

[0036] By increasing the length of cable L2 and reducing the length of cable L3, the present invention can effectively reduce the amplitude and rising rate of the short-circuit current, achieving an effect similar to that of a series-connected current-limiting reactor.

[0037] The following combines this technical solution to elaborate on the technical principle of the present invention in detail.

[0038] From the analysis of the prior art 1, it can be seen that the too short lengths of cables L1 and L2 and the too small system impedance are one of the main reasons for the excessive short-circuit current (about 10 times the rated current) provided by the PV power generation system during pole-to-pole short circuit. Figure 7 and Figure 8 respectively show the output current waveforms of the PV power generation system when a feeder pole-to-pole short circuit occurs at the 0.5 s moment when the total lengths of cables L1 and L2 are 100 m and 200 m. When the cable length reaches 100 m, the short-circuit current provided by the PV system reaches a peak value of 1.13 kA after about 3.5 ms, a 63.4% decrease compared with the cable length of 10 m. The fault current drops to a very low value after about 23 ms, then starts to oscillate, and the oscillation ends after about 120 ms. The short-circuit current is all provided by the PV array and stabilizes at 0.45 kA. When the cable length reaches 200 m, the short-circuit current provided by the PV system reaches a peak value of 0.91 kA after about 5.6 ms, a 70.5% decrease compared with the cable length of 10 m. The fault current drops to a very low value after about 32 ms, and then starts to oscillate. It can be seen that increasing the total length of cables L1 and L2 can effectively reduce the amplitude and rising rate of the short-circuit current, achieving an effect similar to that of a series-connected current-limiting reactor.

[0039] Considering that cable L1 is the connecting cable between the metering cabinet and the DC power distribution cabinet, to ensure the accuracy of metering, the metering cabinet and the DC power distribution cabinet are generally not allowed to be arranged separately, and the length of cable L1 cannot be too long. Therefore, only by finding a way to increase the length of cable L2. Since photovoltaic modules are generally arranged in outdoor open areas or on rooftops, etc., there is still a certain distance from the DC power distribution room. If the grid-connected DC / DC device is arranged in the area close to the photovoltaic modules, the length of cable L2 can be effectively increased, and at the same time, the length of L3 can be reduced. Therefore, there will be no additional cable cost and cable loss. If the grid-connected DC / DC device adopts a box structure that can be arranged outdoors, it can be arranged near the photovoltaic combiner box; if the grid-connected DC / DC device adopts a cabinet structure that cannot be arranged outdoors, it can be arranged at a certain indoor position on the side close to the photovoltaic modules. For the rooftop photovoltaic power generation system, the following Figure 9 layout scheme can be adopted. The photovoltaic modules and the DC combiner box are installed on the rooftop; the grid-connected photovoltaic DC / DC device is arranged close to the DC combiner box and is also installed on the rooftop; the DC power distribution cabinet and the DC metering cabinet are installed in the DC power distribution room. The electric energy generated by the photovoltaic modules is collected by the DC combiner box, then connected to the grid-connected photovoltaic DC / DC device through a cable, and then connected to the DC metering cabinet in the DC power distribution room through a cable, and finally connected to the DC power distribution cabinet to achieve DC grid connection.

[0040] Combined with Figure 2 It can be seen that this scheme can effectively reduce the amplitude and rise rate of the short-circuit current provided by the photovoltaic system for any feeder pole line fault (such as f2, f3, f4), and it is only invalid for the pole-to-pole fault (f1) on the outlet side of the grid-connected DC / DC device. Therefore, the probability of the grid-connected DC / DC device providing a very large short-time overcurrent can be greatly reduced.

[0041] The advantages of the technical solution of the present invention are as follows:

[0042] 1. It is simple to implement, does not add additional electrical equipment, has a low project cost, and occupies less land.

[0043] 2. Compared with the use of a current-limiting reactor, the design of the present invention is simple and the effect is remarkable.

[0044] 3. It can effectively reduce the amplitude and rise rate of the instantaneous overcurrent provided by the photovoltaic power generation system during the pole-to-pole short circuit of the feeder.

[0045] 4. Only when a pole-to-pole short circuit occurs at the outlet of the grid-connected DC / DC device, the photovoltaic power generation system will provide a large instantaneous overcurrent, which greatly reduces the probability of the photovoltaic power generation system providing a very large short-time overcurrent.

[0046] The specific embodiments described above further elaborate on the objectives, technical solutions, and beneficial effects of the present invention. It should be understood that the above descriptions are only specific embodiments of the present invention and are not used to limit the protection scope of the present invention. In particular, it is pointed out that for those skilled in the art, any modifications, equivalent replacements, improvements, etc., made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A photovoltaic power generation optimization layout system, characterized in that, Including: Photovoltaic modules, DC busbar boxes, photovoltaic grid-connected DC / DC devices, DC metering cabinets, and DC power distribution cabinets; Both the DC metering cabinet and the DC power distribution cabinet are arranged inside the DC power distribution room, and the photovoltaic modules, the DC busbar box, and the photovoltaic grid-connected DC / DC device are all arranged on the roof of the DC power distribution room; the photovoltaic modules are connected to the DC busbar box, the DC metering cabinet is connected to the DC power distribution cabinet through a first cable, the photovoltaic grid-connected DC / DC device is connected to the DC metering cabinet through a second cable, and the DC busbar box is connected to the photovoltaic grid-connected DC / DC device through a third cable; The length of the first cable is less than the length of the second cable; The length of the third cable is less than the length of the second cable; The distance between the DC busbar box and the photovoltaic grid-connected DC / DC device is less than 10 m. When the total length of the first cable and the second cable reaches 100 m, the short-circuit current provided by the photovoltaic system decreases by 63.4% compared with when the total length of the first cable and the second cable is 10 m.

2. The photovoltaic power generation optimization layout system according to claim 1, wherein The distance between the DC metering cabinet and the photovoltaic grid-connected DC / DC device is greater than 100 m.

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