Frequency conversion system and offshore wind power transmission system

Abstract

The utility model discloses a frequency conversion system and an offshore wind power transmission system, and relates to the technical field of electric power, the frequency conversion system comprises a rectification end current source converter, an inversion end current source converter, a current limiting device and a controller, and the current limiting device is connected in series to a direct current line between the rectification end current source converter and the inversion end current source converter; the current limiting device comprises a first circuit and a second circuit which are connected in parallel between the power input end of the current limiting device and the power output end of the current limiting device, the first circuit is provided with at least one full-control device, the second circuit is provided with at least one resistor, and the resistance value of the first circuit is smaller than that of the second circuit; when the full-control device is in the first working state, the first circuit is in an on state; and when the full-control device is in the second working state, the first circuit is in an open-circuit state. According to the invention, the DC overcurrent of the DC line between the rectifier end current source converter and the inverter end current source converter can be suppressed.

Description

Technical Field

[0001] The present application relates to the technical field of electric power, and in particular, to a frequency conversion system and an offshore wind power transmission system. Background Art

[0002] Compared with onshore wind power, offshore wind power has advantages such as rich resources and high stability. The flexible low-frequency power transmission technology can be applied in the offshore wind power output system to send offshore wind power into the onshore power grid. Among them, due to the excellent performance of the Fundamental Frequency Modulation-based Current Source Converter (FFM-CSC), it can be applied to the onshore frequency conversion station of the offshore wind power transmission system.

[0003] For the stable operation of the power system, it is usually required that the offshore wind power output system has the fault ride-through ability. Specifically, when the AC voltage of the onshore power grid drops, it will cause a DC overcurrent in the DC line between the rectifier end CSC and the inverter end CSC of the onshore frequency conversion station, and then it may cause damage to the DC line between the rectifier end CSC and the inverter end CSC. Summary of the Utility Model

[0004] Embodiments of the present application provide a frequency conversion system and an offshore wind power transmission system to solve the problem that the DC line between the rectifier end CSC and the inverter end CSC may be damaged due to the AC voltage drop of the onshore power grid in the related art.

[0005] To solve the above technical problems, the present application is implemented as follows;

[0006] In a first aspect, embodiments of the present application provide a frequency conversion system, including a rectifier end current source converter, an inverter end current source converter, a current limiting device, and a controller. The power output end of the rectifier end current source converter is electrically connected to the power input end of the current limiting device through a first DC line, and the power output end of the current limiting device is electrically connected to the power input end of the inverter end current source converter through a second DC line;

[0007] The current limiting device includes a first circuit and a second circuit connected in parallel between the power input end and the power output end of the current limiting device. The first circuit is provided with at least one fully controlled device, the second circuit is provided with at least one resistor, and the resistance value of the first circuit is less than the resistance value of the second circuit;

[0008] The controller is electrically connected to the fully controlled device, and the fully controlled device includes a first working state and a second working state;

[0009] When the fully controlled device is in the first operating state, the first circuit is in a conducting state;

[0010] When the fully controlled device is in the second operating state, the first circuit is in an open state.

[0011] In a second aspect, an embodiment of the present application provides an offshore wind power transmission system, including an offshore wind power generation system, a power supply grid, and the frequency conversion system described in the first aspect. The power output end of the offshore wind power generation system is electrically connected to the power input end of the rectifier end current source converter, and the power output end of the inverter end current source converter is electrically connected to the power input end of the power supply grid.

[0012] In the embodiment of the present application, when the fully controlled device is in the first operating state, the first circuit is in a conducting state. Since the resistance value of the first circuit is smaller than that of the second circuit, the current output by the rectifier end current source converter flows through the first circuit to the inverter end current source converter; when the fully controlled device is in the second operating state, the first circuit is in an open state, and the current output by the rectifier end current source converter flows through the second circuit to the inverter end current source converter. Since at least one resistor is provided in the second circuit, the current flowing through the second circuit is equivalent to increasing the voltage magnitude at the power input end of the inverter end current source converter. When the AC voltage of the onshore power grid drops in the embodiment of the present application, by controlling the fully controlled device to be in the second operating state, it is equivalent to increasing the voltage magnitude at the power input end of the inverter end current source converter. In this way, the voltage difference between the power output end of the rectifier end current source converter and the power input end of the inverter end current source converter can be reduced, thereby suppressing the DC overcurrent in the first DC line and the second DC line between the rectifier end current source converter and the inverter end current source converter, and further reducing the risk of damage to the first DC line and the second DC line. Description of the Drawings

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for describing the embodiments of the present application will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0014] Figure 1 It is a circuit schematic diagram of a frequency conversion system provided by an embodiment of the present application;

[0015] Figure 2 It is a circuit schematic diagram of an offshore wind power transmission system provided by an embodiment of the present application;

[0016] Figure 3Flowchart of a fault ride-through method provided by an embodiment of the present application;

[0017] Figure 4 Waveform diagram of the per-unit value of the voltage of an offshore AC bus provided by an embodiment of the present application;

[0018] Figure 5 Waveform diagram of the per-unit value of the voltage of an onshore AC bus provided by an embodiment of the present application;

[0019] Figure 6 Waveform diagram of a current frequency provided by an embodiment of the present application;

[0020] Figure 7 Waveform diagram of a DC current provided by an embodiment of the present application;

[0021] Figure 8 Waveform diagram of the active power of a wind turbine provided by an embodiment of the present application;

[0022] Figure 9 Waveform diagram of the reactive power of a wind turbine provided by an embodiment of the present application;

[0023] Figure 10 Waveform diagram of the active power of a rectifier-side CSC provided by an embodiment of the present application;

[0024] Figure 11 Waveform diagram of the reactive power of a rectifier-side CSC provided by an embodiment of the present application. Detailed implementation manners

[0025] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0026] To understand the technical solutions in the embodiments of the present application more clearly, the relevant content of the present application is described below.

[0027] Compared with onshore wind power, offshore wind power has the advantages of rich resources, high stability, and less demand for land resources, and has great development potential. Promoting the development of offshore wind power plays a significant role in continuously expanding the installed capacity of wind power and increasing the new energy penetration rate of the power system. When the siting of offshore wind farms extends to deep and far sea areas, traditional high-voltage AC transmission technology is difficult to meet the requirements of the offshore wind power transmission system for transmission distance and transmission capacity. In recent years, flexible low-frequency transmission technology, which combines the advantages of high-voltage DC transmission and high-voltage AC transmission, has good engineering application prospects in the scenario of transmitting and connecting medium and far sea wind power to the grid. Among them, the basic principle of low-frequency transmission technology is: by reducing the operating frequency of the AC system to reduce the impedance of the transmission line, thereby bringing significant improvements in the power transmission capacity of the AC system, obvious improvement in the voltage distribution along the line, and improvement in voltage stability. In the field of flexible low-frequency transmission, due to the excellent performance of the Fundamental Frequency Modulation-based Current Source Converter (FFM-CSC) (no DC side energy storage capacitor, no large-area AC filter field, small volume, light weight, and capable of powering passive systems), it can be applied to the onshore frequency conversion system of the offshore wind power output system.

[0028] With the continuous expansion of the scale of offshore wind farms, the influence of the operating characteristics of the offshore wind power output system on the onshore main grid becomes more and more significant. For the stable operation of the power system, it is usually required that the offshore wind power output system has the ability to ride through faults. Among them, the ability to ride through faults is a general requirement for the dynamic performance of grid-connected wind farms, stipulating that the wind farm can remain connected to the grid during an AC grid fault and can quickly and smoothly return to the normal state after the fault is cleared. Specifically, when the AC voltage of the onshore power grid drops, it will cause a DC overcurrent in the DC line between the rectifier CSC and the inverter CSC of the onshore frequency conversion station. The following will explain it in combination with relevant formulas.

[0029] 1. Explanation of the generation of the above DC overcurrent from the energy perspective

[0030] When the AC voltage of the onshore power grid drops, based on the requirement of fault ride-through, the offshore wind power system needs to continue to operate stably, that is, the offshore wind power system still transmits power to the onshore power grid. The change formula of the DC current in the DC line between the rectifier CSC and the inverter CSC is shown in Formula 1 below. Based on Formula 1, it can be obtained that since the inverter CSC cannot output electric power, the electric power output by the offshore wind power system accumulates in the smoothing reactor between the rectifier CSC and the inverter CSC, causing the DC current to rise.

[0031]

[0032] Among them, idc is the direct current of the direct current line between the rectifier terminal CSC and the inverter terminal CSC, P w is the active power output by the offshore wind power system; P grid is the active power output by the inverter terminal CSC; L dc is the DC smoothing reactor. Due to the AC voltage dip of the onshore power grid, the active power P grid output by the inverter terminal CSC decreases, resulting in an excessive increase in i dc that is, a DC overcurrent is generated.

[0033] 2. Explanation of the generation of the above DC overcurrent based on the DC loop

[0034] The DC voltage U dcr at the power output end of the rectifier terminal CSC and the DC voltage U dci at the power input end of the inverter terminal CSC are expressed as:

[0035]

[0036]

[0037] where X C and X L are the impedances of the capacitor and inductor in the CSC respectively, U rm is the amplitude of the offshore AC phase voltage, U im is the amplitude of the onshore AC phase voltage, α r is the lagging trigger angle of the rectifier terminal CSC, and β i is the lagging trigger angle of the inverter terminal CSC.

[0038] The direct current I dc of the direct current line between the rectifier terminal CSC and the inverter terminal CSC is expressed as:

[0039]

[0040] where R dc is the resistance value of the direct current line, L dc is the inductance of the direct current line. For the description of other parameters, refer to the descriptions in Formula 2 and Formula 3 above.

[0041] When a severe three-phase short circuit fault occurs in the onshore power grid, the AC voltage suddenly drops, resulting in a significant drop in the above U dci . The active power that can be transmitted by the onshore variable frequency system drops significantly. At the same time, based on the requirement of fault ride-through, when the onshore power grid is short-circuited, the offshore wind power system still transmits power to the onshore power grid, and U dcr basically remains unchanged. Therefore, in the DC loop, U dcr and Udci The voltage difference is too large, resulting in a rapid increase in the DC current I dc and thus generating a transient overcurrent.

[0042] In summary, when the AC voltage of the onshore power grid drops, a DC overcurrent will be generated in the DC line between the rectifier - side CSC and the inverter - side CSC, which may further cause damage to the DC line between the rectifier - side CSC and the inverter - side CSC. Based on this, the embodiments of the present application provide a frequency - conversion system and an offshore wind power transmission system, which can suppress the DC overcurrent in the DC line between the rectifier - side CSC and the inverter - side CSC when the AC voltage of the onshore power grid drops, thereby reducing the risk of damage to the DC line between the rectifier - side CSC and the inverter - side CSC.

[0043] The frequency - conversion system provided by the embodiments of the present application will be described first below.

[0044] Referring to Figure 1 and Figure 2 , the frequency - conversion system of the embodiments of the present application includes a rectifier - side current - source converter, an inverter - side current - source converter, a current - limiting device 3, and a controller. The power output end of the rectifier - side current - source converter is electrically connected to the power input end of the current - limiting device 3 through a first DC line, and the power output end of the current - limiting device 3 is electrically connected to the power input end of the inverter - side current - source converter through a second DC line;

[0045] The current - limiting device 3 includes a first circuit and a second circuit connected in parallel between the power input end and the power output end of the current - limiting device 3. The first circuit is provided with at least one fully - controlled device 31, and the second circuit is provided with at least one resistor 32. The resistance value of the first circuit is less than the resistance value of the second circuit;

[0046] The controller is electrically connected to the fully - controlled device 31. The fully - controlled device 31 includes a first working state and a second working state;

[0047] When the fully - controlled device 31 is in the first working state, the first circuit is in a conducting state;

[0048] When the fully - controlled device 31 is in the second working state, the first circuit is in an open - circuit state.

[0049] The above rectifier terminal current source converter is the rectifier terminal CSC 1. The power input terminal of the rectifier terminal CSC 1 is used to be electrically connected to the power output terminal of the offshore wind power system, and is used to rectify the alternating current output by the offshore wind power system into direct current. The above inverter terminal current source converter is the inverter terminal CSC 2. The power input terminal of the inverter terminal CSC 2 is electrically connected to the power input terminal of the rectifier terminal CSC 1 through the second DC line, the current limiting device 3 and the first DC line, and is used to invert the direct current output by the rectifier terminal CSC 1 into alternating current with a frequency meeting the requirements, that is, to invert the direct current into industrial frequency alternating current meeting the grid connection requirements.

[0050] The above current limiting device 3 includes a first circuit and a second circuit connected in parallel between the power input terminal and the power output terminal of the current limiting device 3. The first circuit is provided with at least one fully controlled device 31. The fully controlled device 31 is also called a self-turn-off device, which refers to a power electronic device that can be controlled to conduct and turn off through a control signal. As an example, the fully controlled device 31 in the embodiments of the present application may include an integrated gate-commutated thyristor (IGCT) and / or an insulated gate bipolar transistor (IGBT). The above fully controlled device 31 has two working states. The corresponding control signal can be sent to the full controller through the above controller to control the fully controlled device 31 to be in the corresponding working state. When the fully controlled device 31 is in the first working state (i.e., conducting), the first circuit is in a conducting state. Since the resistance 32 value of the first circuit is less than the resistance 32 value of the second circuit, the current output by the rectifier terminal CSC 1 flows through the first circuit to the inverter terminal CSC 2. When the fully controlled device 31 is in the second working state (i.e., off), the first circuit is in an open state, and the current output by the rectifier terminal CSC 1 flows through the second circuit to the inverter terminal CSC 2. Since at least one electrical resistor 32 is provided in the second circuit, the current output by the rectifier terminal CSC 1 flows through the second circuit to the inverter terminal CSC 2, which is equivalent to increasing the DC voltage of the inverter terminal CSC 2, that is, equivalent to U in Formula 4. dci Increase.

[0051] In the embodiments of the present application, the fault ride-through of the variable frequency system can be realized by controlling the state of the fully controlled device 31, and the following is a specific description thereof.

[0052] When the AC voltage of the onshore power supply grid does not suddenly drop below the third preset value, the fully controlled device 31 can be controlled to be in the first working state, the first circuit is in a conducting state, and the current output by the rectifier terminal CSC 1 flows through the first DC line, the first circuit and the second DC line to the inverter terminal CSC 2.

[0053] When the AC voltage of the onshore power supply grid suddenly drops below a third preset value, U in the above formula 4 dci will also correspondingly drop, and the fully controlled device 31 can be controlled to be in the second working state, the second circuit is in a conducting state, and the current output by the rectifier end CSC 1 flows through the first DC line, the second circuit, and the second DC line to the inverter end CSC 2. Since at least one resistor 32 is provided in the second circuit, it is equivalent to increasing the DC voltage U of the inverter end CSC 2 dci . U dci increases, and it can reduce the voltage difference between U dcr and U dci when the AC voltage of the onshore power grid suddenly drops. Based on formula 4, the voltage difference between U dcr and U dci decreases, and the DC overcurrent I of the first DC line and the second DC line between the rectifier end current source converter and the inverter end current source converter dc can be effectively suppressed.

[0054] When the AC voltage of the onshore power supply grid returns from the state of dropping to less than the third preset value to the state where the AC voltage is greater than the fourth preset value, the fully controlled device 31 can be controlled to be in the first working state, the first circuit is in a conducting state, and the current output by the rectifier end CSC 1 flows through the first DC line, the first circuit, and the second DC line to the inverter end CSC 2. Among them, for the stability of the frequency conversion system, when the AC voltage of the onshore power grid returns to the state where the AC voltage is greater than the fourth preset value, after a delay period (for example, 20 milliseconds), the fully controlled device 31 can be controlled to be in the first working state.

[0055] The above third preset value and fourth preset value can be set based on specific requirements. As an example, the above third preset value can be 0.2 times the rated power frequency voltage value, that is, 0.2 p.u.; the above fourth preset value can be 0.8 times the rated power frequency voltage value, that is, 0.8 p.u. In the embodiments of the present application, based on the magnitude of the AC voltage of the onshore power grid, the working state of the fully controlled device 31 is correspondingly controlled, and the DC overcurrent can be greatly reduced when the onshore alternating current drops, so that the offshore wind power system 5 can maintain non-disconnection operation during the AC grid fault and can quickly and smoothly return to the normal state after the fault is cleared.

[0056] It is worth noting that the specific quantities of the fully controlled devices 31 in the first circuit and the resistors 32 in the second circuit can be set according to actual requirements. Among them, the quantity of the fully controlled devices 31 in the first circuit is related to the withstand voltage parameter of the fully controlled devices 31 and the voltage across the resistors 32 in the second circuit. For example, when the voltage across the resistors 32 in the second circuit is 10 kV, if the withstand voltage parameter of the fully controlled devices 31 in the first circuit is 5 kV, at least two 5-kV fully controlled devices 31 need to be set in the first circuit. The quantity of the resistors 32 in the second circuit is related to the voltage dip situation of the power supply grid, and the quantity of the resistors 32 can be determined according to the historical voltage dip parameters of the power supply grid.

[0057] In the embodiment of the present application, through the above settings, in the case of an onshore AC voltage dip, the DC overcurrents of the first DC line and the second DC line between the rectifier-side current source converter and the inverter-side current source converter can be suppressed, so that the offshore wind power system 5 can maintain grid-connected operation during an AC grid fault and can quickly and smoothly return to the normal state after the fault is cleared.

[0058] When the power supply grid is in a steady state (the steady state means that the AC voltage is greater than the above fourth preset value), the current output by the voltage rectifier-side CSC 1 flows through the first DC line, the first circuit, and the second circuit to the inverter-side CSC 2. Due to the on-state voltage drop of the fully controlled devices 31 in the first circuit, the current damage is relatively large. Based on this, the embodiment of the present application sets a third circuit composed of only a metal wire and a mechanical switch 33. When the power supply grid is in a steady state, the third circuit is controlled to conduct, and the current output by the voltage rectifier-side CSC 1 flows through the first DC line, the first circuit, and the third circuit to the inverter-side CSC 2, thereby reducing losses. The following is a specific description thereof.

[0059] Optionally, the current limiting device 3 further includes a third circuit connected in parallel between the power input end and the power output end of the current limiting device 3. The third circuit includes a mechanical switch 33 for controlling the on-off state of the third circuit, and the resistance value of the third circuit is smaller than the resistance value of the first circuit.

[0060] The following combines Figure 3 to illustrate how to control the working state of the fully controlled devices 31 based on the AC voltage of the power supply grid when the third circuit is provided in the current limiting device 3.

[0061] When the AC voltage of the onshore power supply grid does not suddenly drop to less than the third preset value (0.2 p.u.), the fully controlled devices 31 are controlled to be in the second working state and the mechanical switch 33 is controlled to close, and the current output by the rectifier-side CSC 1 flows through the first DC line, the third circuit, and the second DC line to the inverter-side CSC 2.

[0062] When the AC voltage of the onshore power supply grid suddenly drops below a third preset value, the natural commutation technology is adopted to quickly disconnect the mechanical switch 33 and control the fully controlled device 31 to be in the first working state through the controller, and the current transfers from the third circuit to the first circuit; subsequently, the controller controls the fully controlled device 31 to be in the second working state, the current transfers from the first circuit to the second circuit, and the resistor 32 of the second circuit realizes current limiting and DC voltage support.

[0063] When the AC voltage of the onshore power supply grid recovers from the state of dropping below the third preset value to the state where the AC voltage is greater than the fourth preset value (0.8 p.u.), the closing of the mechanical switch 33 can be controlled with a delay of 20 ms, and the current output by the rectifier terminal CSC 1 flows through the first DC line, the third circuit, and the second DC line to the inverter terminal CSC 2.

[0064] To verify the technical effects of the embodiments of the present application, the embodiments of the present application perform simulations on the offshore wind power transmission system, and perform corresponding control on the above current limiting device 3 based on the state of the power supply grid, thereby obtaining Figures 4 to 11 the waveform diagram shown. Refer to Figures 4 to 11 , before the moment t = 7 s, the system is in a stable operation state, and the wind turbine generator outputs an active power of 1000 MW; at t = 7 s, a solid grounding fault occurs in the receiving-end AC system, and the duration is 100 ms. According to Figures 4 to 9 it can be obtained that before and after the fault (i.e., around 7 s), the AC voltage amplitude and frequency fluctuate slightly near the rated value, and the overall fluctuation of the offshore wind power transmission system is small, and stable operation can be achieved.

[0065] In the embodiments of the present application, when the power supply grid is in a steady state, the third circuit is controlled to conduct, and the current output by the voltage rectifier terminal CSC 1 flows through the first DC line, the first circuit, and the third circuit to the inverter terminal CSC 2, which is beneficial to reducing losses.

[0066] Optionally, the fully controlled device 31 includes a first integrated gate-commutated thyristor IGCT.

[0067] IGCT has strong current-carrying and voltage-withstanding capabilities. By setting the fully controlled device 31 to include IGCT, it is beneficial to improve the safety and stability of the first circuit.

[0068] Optionally, the second DC line includes a smoothing reactor 4.

[0069] In this embodiment, by setting the second DC line to include a smoothing reactor 4, it is beneficial to suppress the DC voltage fluctuation of the DC line.

[0070] Optionally, the rectifier - side current - source converter includes a first three - winding transformer 11 and a second IGCT 12. The power output terminal of the first three - winding transformer 11 is electrically connected to the power input terminal of the second IGCT 12, and the power output terminal of the second IGCT 12 is the power output terminal of the rectifier - side current - source converter. The inverter - side current - source converter includes a second three - winding transformer 21 and a third IGCT 22. The power output terminal of the third IGCT 22 is electrically connected to the power input terminal of the second three - winding transformer 21, and the power input terminal of the third IGCT 22 is the power input terminal of the inverter - side current - source converter.

[0071] In this embodiment, the power input terminal of the first three - winding transformer 11 is electrically connected to the power output terminal of the offshore wind power generation system 5, and the power output terminal of the third IGCT 22 is electrically connected to the power supply grid. The power output terminal of the second IGCT 12 is the power output terminal of the rectifier - side current - source converter. The second IGCT 12 includes two output terminals, namely the positive pole and the negative pole of the second IGCT 12. The power input terminal of the third IGCT 22 is the power input terminal of the inverter - side current - source converter. The third IGCT 22 also includes two power input terminals, namely the positive pole and the negative pole of the third IGCT 22. It is worth noting that the positive pole of the second IGCT 12 is electrically connected to the negative pole of the third IGCT 22 through a third circuit, and the negative pole of the second IGCT 12 is electrically connected to the negative pole of the third IGCT 22 through a fourth circuit. The current - limiting device 3 in the embodiment of the present application can be arranged in the third circuit or the fourth circuit. When the current - limiting device 3 is arranged in the third circuit, the above - mentioned first DC line is the DC line between the positive pole of the second IGCT 12 and the power input terminal of the current - limiting device 3, and the above - mentioned second DC line is the DC line between the negative pole of the third IGCT 22 and the power output terminal of the current - limiting device 3. When the current - limiting device 3 is arranged in the fourth circuit, the above - mentioned first DC line is the DC line between the negative pole of the second IGCT 12 and the power input terminal of the current - limiting device 3, and the above - mentioned second DC line is the DC line between the positive pole of the third IGCT 22 and the power output terminal of the current - limiting device 3. In some embodiments, the fourth circuit is grounded, so the current - limiting device 3 can be arranged in the third circuit.

[0072] The embodiment of the present application also provides an offshore wind power transmission system, which includes an offshore wind power generation system 5, a power supply grid, and the frequency - conversion system in any of the above embodiments. The power output terminal of the offshore wind power generation system 5 is electrically connected to the power input terminal of the rectifier - side current - source converter, and the power output terminal of the inverter - side current - source converter is electrically connected to the power input terminal of the power supply grid.

[0073] It should be noted that any of the above embodiments of the variable frequency system can be applied to the offshore wind power transmission system in this embodiment, and the same technical effects can be achieved. To avoid repetition, it will not be elaborated here.

[0074] Optionally, the offshore wind power transmission system further includes an offshore power aggregation system 6. The offshore wind power generation system 5 includes a plurality of wind turbines. The power output end of the offshore wind power generation system 5 is the power output ends of the plurality of wind turbines. The power output ends of the plurality of wind turbines are electrically connected to the power input end of the rectifier end current source converter through the offshore power aggregation system 6.

[0075] In this embodiment, by arranging the power output ends of the plurality of wind turbines to be electrically connected to the power input end of the rectifier end CSC 1 through offshore power aggregation, compared with arranging the plurality of wind turbines to be respectively electrically connected to the power input end of the rectifier end CSC 1, it is beneficial to save the transmission cables between the plurality of wind turbines and the rectifier end CSC 1.

[0076] Optionally, each of the plurality of wind turbines includes an inverter. The offshore power aggregation system 6 includes a step-up transformer. The power output end of the inverter is electrically connected to the power input end of the step-up transformer. The power output end of the step-up transformer is electrically connected to the power input end of the rectifier end current source converter;

[0077] Wherein, the inverter of the wind turbine is used to lower the frequency of the alternating current generated by the wind turbine to a first preset value, and the step-up transformer is used to raise the voltage of the alternating current output by the offshore power aggregation system 6 to a second preset value.

[0078] In this embodiment, by lowering the frequency of the alternating current generated by the wind turbine to the first preset value and raising the voltage of the alternating current to the second preset value through the step-up transformer of the offshore power aggregation system 6, it is beneficial to reduce the loss of the offshore transmission cables.

[0079] Optionally, the power output end of the inverter end current source converter is connected to the power input end of the power supply grid through a transmission cable. A detection device for detecting the transmission voltage of the transmission cable is provided in the transmission cable.

[0080] In this embodiment, by providing the above detection device, the voltage of the power supply grid can be detected in a timely manner, so as to facilitate controlling the on-off of the first circuit and the third circuit of the current limiting device 3 based on the voltage of the power supply grid.

[0081] The above specific embodiments do not constitute a limitation on the protection scope of the present disclosure. Those skilled in the art should understand that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present disclosure shall be included within the protection scope of the present disclosure.

Claims

1. A frequency conversion system, characterized in that: It includes a rectifier-end current source converter, an inverter-end current source converter, a current limiting device and a controller, wherein the power output end of the rectifier-end current source converter is electrically connected to the power input end of the current limiting device through a first DC line, and the power output end of the current limiting device is electrically connected to the power input end of the inverter-end current source converter through a second DC line; The current limiting device comprises a first circuit and a second circuit connected in parallel between a power input end of the current limiting device and a power output end of the current limiting device, the first circuit is provided with at least one fully controlled device, the second circuit is provided with at least one resistor, and the resistance value of the first circuit is smaller than the resistance value of the second circuit; The controller is electrically connected to the fully-controlled device, and the fully-controlled device includes a first working state and a second working state; When the fully-controlled device is in the first working state, the first circuit is in a conducting state; When the fully-controlled device is in the second working state, the first circuit is in an open circuit state.

2. The frequency conversion system according to claim 1, characterized in that: The current limiting device also includes a third circuit connected in parallel between the power input end of the current limiting device and the power output end of the current limiting device, the third circuit includes a mechanical switch for controlling the on and off state of the third circuit, and the resistance value of the third circuit is smaller than the resistance value of the first circuit.

3. The frequency conversion system according to claim 1 or 2, characterized in that: The fully controlled device comprises a first integrated gate-commutated thyristor IGCT.

4. The frequency conversion system according to claim 1 or 2, characterized in that: The second DC line includes a smoothing reactor.

5. The frequency conversion system according to claim 1 or 2, characterized in that: The rectifier-end current source converter includes a first three-winding transformer and a second IGCT, wherein the power output end of the first three-winding transformer is electrically connected to the power input end of the second IGCT, and the power output end of the second IGCT is the power output end of the rectifier-end current source converter.

6. The frequency conversion system according to claim 5, characterized in that: The inverter-end current source converter includes a second three-winding transformer and a third IGCT, the power output end of the third IGCT is electrically connected to the power input end of the second three-winding transformer, and the power input end of the third IGCT is the power input end of the inverter-end current source converter.

7. An offshore wind power transmission system, characterized in that: It comprises an offshore wind power generation system, a power supply grid, and the frequency conversion system as described in any one of claims 1 to 5, wherein the power output end of the offshore wind power generation system is electrically connected to the power input end of the rectifier-end current source converter, and the power output end of the inverter-end current source converter is electrically connected to the power input end of the power supply grid.

8. The offshore wind power transmission system according to claim 7, characterized in that: The offshore wind power transmission system also includes an offshore power collection system. The offshore wind power generation system includes multiple wind turbines. The power output end of the offshore wind power generation system is the power output end of the multiple wind turbines. The power output ends of the multiple wind turbines are electrically connected to the power input end of the rectifier-end current source converter through the offshore power collection system.

9. The offshore wind power transmission system according to claim 8, characterized in that: Each of the plurality of wind turbines comprises a converter, the offshore power collection system comprises a step-up transformer, the power output end of the converter is electrically connected to the power input end of the step-up transformer, and the power output end of the step-up transformer is electrically connected to the power input end of the rectifier-end current source converter; The converter of the wind turbine set is used to reduce the frequency of the alternating current generated by the wind turbine set to a first preset value, and the step-up transformer is used to increase the voltage of the alternating current output by the offshore power collection system to a second preset value.

10. The offshore wind power transmission system according to claim 7 or 8, characterized in that: The power output end of the inverter-end current source converter is connected to the power input end of the power supply grid through a transmission cable, and a detection device for detecting the transmission voltage of the transmission cable is provided in the transmission cable.

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