Power generation system and power conversion device

Abstract

The invention provides a power generation system and a power conversion device. The power generation system comprises one or more power conversion devices, a transformer and a control unit. And under the condition that the first fault ride-through is finished, the control unit controls the reactive output current of the power conversion device to be a current value obtained by superposing the reactive compensation current on the basis of the reactive reference current. The reactive reference current is a reactive output current of the power conversion device recovered to a first current value in a time period from the end of the first fault ride-through to the normal recovery of the voltage of the power grid; the direction of the reactive compensation current is opposite to that of the first reactive output current, and the first reactive output current is the reactive output current of the transformer after the second fault ride-through is finished under the condition that the transformer is in power-off connection with the power conversion device and is in conductive connection with the power grid. The second fault ride-through is the same as the first fault ride-through. And furthermore, the influence of the reactive current in the magnetizing inrush current of the output end of the transformer on the zero clearing stage of the reactive output current of the transformer can be reduced.

Description

Technical Field

[0001] The present application relates to the field of power supply technology, and in particular to a power generation system and a power conversion device. Background Art

[0002] For a photovoltaic power station, before it is connected to the grid, it must have the ability to ride through a fault before it is allowed to be connected to the grid. For example, Figure 1 As shown, multiple inverters in the photovoltaic power station are connected in parallel and then boosted to 35KV by a transformer before being connected to the high-voltage grid. Before the photovoltaic power station is connected to the grid, a voltage mutation device is used on the high-voltage side of the transformer to simulate a grid fault, and the high-voltage side voltage and high-voltage side current information of the transformer are collected to determine whether the photovoltaic power station has the fault ride-through capability. If so, the photovoltaic power station meets the grid connection conditions.

[0003] The schematic diagram of the standard voltage and current of the inverter during the fault ride-through of the photovoltaic power station is as follows: Figure 2 As shown, during the fault occurrence and steady state period (i.e., the time period from t0 to t2), the grid voltage u drops or rises rapidly, and the inverter current responds to the change of the grid voltage u, injecting a reactive current i proportional to the change of the grid voltage into the grid. q1 , active current i d1 After the fault ride-through is completed (i.e. after time t2), the reactive current i q1 As the grid voltage u recovers and quickly clears to zero, the active current i d1 It will slowly recover according to the preset gradient. The high-voltage side current of the transformer is the product of the sum of the output currents of all inverters and the turns ratio of the transformer. Based on this, the high-voltage side current of the transformer during and after the fault ride-through of the photovoltaic power station can be obtained. The national standard document "GB_T 19964-2024" stipulates that after the fault is restored, the exit time of the reactive current of the photovoltaic power station (that is, the reactive current on the high-voltage side of the transformer) should be less than 30ms; the standard document of the Southern Power Grid "Technical Specifications for Photovoltaic Power Station Access to the Grid" stipulates that during the exit of the reactive current of the photovoltaic power station, the reactive current fluctuation is less than 5%I n, where I n is the rated current of the photovoltaic power station.

[0004] Inverter according to Figure 2 The standard shown requires that when responding to changes in reactive current, the high-voltage side current of the transformer, which is the assessment object of the photovoltaic power station, will be affected by the transformer characteristics. When the current of the inverter meets the standard requirements, the current deviation caused by the transformer's own excitation and demagnetization process will also cause the high-voltage side current data to fail to meet the above-mentioned standards.

[0005] Taking low voltage ride-through as an example, when the primary side of the transformer is unloaded (i.e., the transformer is electrically disconnected from the inverter) and the secondary side of the transformer is connected to the grid, Figure 2The fault recovery phase shown in the figure starts from time t2 (i.e. after the fault ride-through is completed). ab and u bc ) rises rapidly from zero to the rated voltage of 35KV. The change of the transformer secondary voltage is accompanied by the transformer's own excitation process, which causes a huge excitation surge current on the transformer secondary side. The reactive current in the excitation surge current on the transformer secondary side will cause the reactive current i q2 Large fluctuations occurred during the zeroing process (i.e. Figure 3 The rectangular frame part in the figure) and the reactive current i q2 The speed of the zeroing stage is slow, so that the photovoltaic power station cannot pass the fault ride-through grid-connected indicators. Therefore, how to reduce the reactive current in the excitation inrush current of the secondary side of the transformer after the fault ride-through is completed is particularly important for the impact of the reactive current zeroing stage on the high-voltage side. Summary of the invention

[0006] The present application provides a power generation system and a power conversion device, which can reduce or even offset the influence of reactive current in the excitation surge current at the output end of the transformer after the fault ride-through is completed on the reactive output current zeroing stage of the transformer.

[0007] In the first aspect, the present application provides a power generation system, which includes one or more power conversion devices, a transformer and a control unit. The input end of the power conversion device is used to connect to a photovoltaic string or an energy storage battery, the output end of the power conversion device is connected to the input end of the transformer, and the output end of the transformer is used to connect to the power grid. The control unit is used to control the reactive output current of the power conversion device to be the current value after the reactive compensation current is superimposed on the reactive reference current when the first fault crossing ends. The reactive reference current is the reactive output current of the power conversion device that recovers to the first current value during the period from the end of the first fault crossing to the voltage of the power grid returning to normal (such as within 30ms after the end of the first fault crossing), and the first current value is the reactive output current value of the power conversion device before the first fault crossing, such as 0; the direction of the reactive compensation current is opposite to the direction of the first reactive output current, and the first reactive output current is the reactive output current of the transformer after the second fault crossing ends when the power conversion device is electrically disconnected from the transformer and the transformer is conductively connected to the power grid. The second fault crossing is the same as the first fault crossing. Here, the end time of the second fault ride-through is earlier than the occurrence time of the first fault ride-through. In other words, the first fault ride-through can be understood as the current fault ride-through, and the second fault ride-through can be understood as the same historical fault ride-through as the current fault ride-through.

[0008] It should be noted that after the power generation system ends the fault ride-through, the reactive output current of the transformer cannot be quickly cleared mainly due to the influence of the reactive current in the excitation inrush current caused by the voltage change at the output end of the transformer during the grid voltage recovery process. Based on this, in the power generation system provided by the present application, the reactive current output by the power conversion device is the superposition of the reactive reference current and the reactive compensation current, wherein the reactive compensation current is used to reduce or even offset the influence of the reactive current in the excitation inrush current at the output end of the transformer on the reactive output current of the transformer. Specifically, the direction of the reactive compensation current is opposite to the direction of the first reactive output current, wherein the first reactive output current can be understood as the reactive current in the excitation inrush current caused by the voltage change at the output end of the transformer during the grid voltage recovery process under the same historical fault ride-through as the current fault ride-through, i.e., the first fault ride-through, so that the sum of the reactive compensation currents output by at least one power conversion device can offset at least part of the reactive current in the excitation inrush current at the output end of the transformer. It is worth mentioning that the second fault ride-through is a grid state automatically simulated by manual operation or a device in the system (such as a voltage mutation generation device simulation) before the power generation system provided by the present application is connected to the grid. The power conversion device or other device in the power generation system records the electrical parameters of one or more devices in the power generation system under different simulated grid states, and then after the power generation system is connected to the grid, the power conversion device outputs the corresponding reactive current for the actual fault scenario of the grid, thereby shortening the clearing time of the reactive output current of the transformer. In addition, in addition to being opposite in direction to the first reactive output current, the reactive compensation current has a numerical relationship with the first reactive output current in that the absolute value of the reactive compensation current is less than or equal to the absolute value of the first reactive output current, so as to avoid excessive reactive current compensation.

[0009] In a first possible implementation, when the first fault ride-through is a low voltage ride-through, the absolute value of the difference between the minimum value of the grid voltage drop depth during the second fault ride-through and the minimum value of the grid voltage drop depth during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and the grid voltage drop depth is the ratio of the grid voltage to the rated voltage when the grid voltage drops.

[0010] In this embodiment, the solution provided by the present application for reducing or even offsetting the effect of the reactive current in the excitation surge current at the output end of the transformer on the reactive output current zeroing stage of the transformer after the fault crossing is completed is applicable to the application scenario of low voltage crossing of the power generation system. In addition, in the low voltage crossing scenario of the power generation system, the second fault crossing can be a low voltage crossing that is exactly the same as the first fault crossing, that is, a low voltage crossing in which the minimum value of the grid voltage drop depth during the fault crossing process is the same as the minimum value of the grid voltage drop depth during the first fault crossing process; the second fault crossing can also be a low voltage crossing that is approximately the same as the first fault crossing, that is, any low voltage crossing in which there is a certain deviation between the minimum value of the grid voltage drop depth during the fault crossing process and the minimum value of the grid voltage drop depth during the first fault crossing process, so that the solution is highly flexible and applicable.

[0011] In a second possible implementation, when the first fault ride-through is a high voltage ride-through, the absolute value of the difference between the maximum value of the grid voltage increase amplitude during the second fault ride-through and the maximum value of the grid voltage increase amplitude during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and when the grid voltage increase amplitude is the grid voltage increase, the ratio of the grid voltage to the rated voltage.

[0012] In this embodiment, the solution provided by the present application for reducing or even offsetting the effect of the reactive current in the excitation surge current at the output end of the transformer on the reactive output current zeroing stage of the transformer after the fault crossing is completed is applicable to the application scenario of high voltage crossing of the power generation system. In addition, in the high voltage crossing scenario of the power generation system, the second fault crossing can be a high voltage crossing that is exactly the same as the first fault crossing, that is, a high voltage crossing in which the maximum value of the grid voltage increase amplitude during the fault crossing process is the same as the maximum value of the grid voltage increase amplitude during the first fault crossing process; the second fault crossing can also be a high voltage crossing that is approximately the same as the first fault crossing, that is, any high voltage crossing in which there is a certain deviation between the maximum value of the grid voltage increase amplitude during the fault crossing process and the maximum value of the grid voltage increase amplitude during the first fault crossing process, so that the solution is highly flexible and has strong applicability.

[0013] In a third possible implementation, the ratio of the reactive compensation current to the first reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of one or more power conversion devices, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the number of turns of the secondary winding in the transformer.

[0014] In this embodiment, the sum of the reactive compensation currents actively compensated by at least one power conversion device can offset the reactive current in the excitation surge current at the output end of the transformer, thereby offsetting the influence of the reactive current in the excitation surge current at the output end of the transformer on the reactive output current clearing stage of the transformer after the fault ride-through is completed.

[0015] In a fourth possible implementation, the direction of the reactive compensation current is opposite to the direction of the second reactive output current, wherein the second reactive output current is the reactive output current of the transformer when the power conversion device is conductively connected to the transformer, the transformer is conductively connected to the grid, and the reactive output current of the power conversion device is the reactive reference current after the second fault crossing is completed.

[0016] It should be noted that after the power generation system ends the fault ride-through, the reactive output current of the transformer cannot be quickly cleared to zero. In addition to being affected by the reactive current (referred to as reactive current 1) in the excitation surge current caused by the voltage change at the output end of the transformer during the grid voltage recovery process, it is also affected by the reactive current (referred to as reactive current 2) induced at the output end of the transformer by part of the active current output by the power conversion device. Based on this, the reactive compensation current needs to reduce or even offset the joint influence of reactive current 1 and reactive current 2 on the reactive output current zeroing stage of the transformer. Specifically, the direction of the reactive compensation current is opposite to that of the second reactive output current, wherein the second reactive output current can be understood as the sum of reactive current 1 and reactive current 2 under the same historical fault ride-through as the current fault ride-through, i.e., the first fault ride-through, so that the sum of reactive compensation currents output by at least one power conversion device can offset at least part of the reactive current in the sum of reactive current 1 and reactive current 2, thereby shortening the clearing time of the reactive current. In addition, in addition to being opposite in direction to the second reactive output current, the reactive compensation current is numerically related to the second reactive output current in that the absolute value of the reactive compensation current is less than or equal to the absolute value of the second reactive output current to avoid excessive reactive current compensation.

[0017] In a fifth possible implementation, the ratio of the reactive compensation current to the second reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of one or more power conversion devices, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the secondary winding in the transformer.

[0018] In this embodiment, the sum of the reactive compensation currents actively compensated by at least one power conversion device can offset the reactive current in the excitation surge current at the output end of the transformer and the active output current of the power conversion device, which is induced as a reactive component current at the output end of the transformer, thereby offsetting the influence of the reactive current in the excitation surge current at the output end of the transformer and the active output current of the power conversion device on the reactive output current zeroing stage of the transformer after the fault ride-through is completed, thereby making the reactive output current of the transformer quickly clear within 30ms after the fault ride-through is completed and the current fluctuation is less than 5%I n.

[0019] In the second aspect, the present application provides a power conversion device, the input end of which is connected to a photovoltaic string or an energy storage battery, the output end of which is connected to the input end of a transformer, and the output end of the transformer is connected to a power grid. The power conversion device is used to control the reactive output current of the power conversion device to be a current value after the reactive compensation current is superimposed on the reactive reference current when the first fault crossing ends. Among them, the reactive reference current is the reactive output current of the power conversion device that recovers to the first current value during the time period from the end of the first fault crossing to the voltage of the power grid returning to normal, and the first current value is the reactive output current value of the power conversion device before the first fault crossing, such as 0; the direction of the reactive compensation current is opposite to the direction of the first reactive output current, and the first reactive output current is the reactive output current of the transformer after the second fault crossing ends when the power conversion device is electrically disconnected from the transformer and the transformer is conductively connected to the power grid. The second fault crossing is the same as the first fault crossing.

[0020] In a first possible implementation, when the first fault ride-through is a low voltage ride-through, the absolute value of the difference between the minimum value of the grid voltage drop depth during the second fault ride-through and the minimum value of the grid voltage drop depth during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and the grid voltage drop depth is the ratio of the grid voltage to the rated voltage when the grid voltage drops.

[0021] In a second possible implementation, when the first fault ride-through is a high voltage ride-through, the absolute value of the difference between the maximum value of the grid voltage increase during the second fault ride-through and the maximum value of the grid voltage increase during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and the grid voltage increase is the ratio of the grid voltage to the rated voltage when the grid voltage increase is the grid voltage increase.

[0022] In a third possible implementation, the ratio of the reactive compensation current to the first reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of power conversion devices connected to the transformer, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the number of turns of the secondary winding in the transformer.

[0023] In a fourth possible implementation, the direction of the reactive compensation current is opposite to the direction of the second reactive output current, wherein the second reactive output current is the reactive output current of the transformer when the power conversion device is conductively connected to the transformer, the transformer is conductively connected to the grid, and the reactive output current of the power conversion device is the reactive reference current after the second fault crossing is completed.

[0024] In a fifth possible implementation, the ratio of the reactive compensation current to the second reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of power conversion devices connected to the transformer, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the secondary winding in the transformer.

[0025] It should be understood that the implementation and beneficial effects of the above-mentioned aspects of the present application can be referenced to each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a fault ride-through schematic diagram of a photovoltaic power station provided by the prior art;

[0027] Figure 2 It is a voltage and current schematic diagram of inverter fault ride-through provided by the prior art;

[0028] Figure 3 It is a schematic diagram of voltage and current on the high-voltage side of the transformer when the low voltage ride-through is provided in the present application and the transformer is disconnected from the inverter;

[0029] Figure 4 It is a schematic diagram of an application scenario of the power generation system provided by this application;

[0030] Figure 5 It is a structural schematic diagram of the power generation system provided by the present application;

[0031] Figure 6 is a circuit diagram of a transformer under non-ideal conditions provided by the present application;

[0032] Figure 7 This is a schematic diagram of the primary and secondary voltages and currents of a transformer under fault ride-through under the influence of the primary and secondary current phase difference caused by transformer excitation provided by the present application;

[0033] Figure 8 It is a schematic diagram of voltage and current at the output end of the power conversion device and the output end of the transformer under low voltage ride-through provided by the present application. DETAILED DESCRIPTION

[0034] The power generation system and power conversion device provided in this application can be applied to various application fields such as photovoltaic power generation, energy storage power generation, new energy smart microgrid, power transmission and distribution, etc. The power conversion device provided in this application can be an inverter, an energy storage converter (Power Convers ion System, PCS), which is applicable to different application scenarios, such as photovoltaic power supply scenarios, energy storage power supply scenarios, and photovoltaic-storage hybrid power supply scenarios. The photovoltaic power supply scenario is used as an example for explanation.

[0035] See also Figure 4 , Figure 4is a schematic diagram of an application scenario of the power generation system provided in this application. In the photovoltaic power supply scenario, the power generation system and power conversion device provided in this application are respectively Figure 4 The photovoltaic power generation system and inverter shown in the figure include n inverters (inverter 11, ..., inverter 1n), a transformer and a control unit, where n is an integer greater than 1. The input end of each inverter in the n inverters is used to connect to the photovoltaic string, the output ends of the n inverters are connected in parallel to the input end of the transformer, and the output end of the transformer is used to connect to the AC power grid.

[0036] In the energy storage power supply scenario, the power generation system and power conversion device provided in this application are respectively an energy storage power generation system and a PCS, and the energy storage power generation system includes n PCSs, a transformer and a control unit. Among them, the input end of each of the n PCSs is used to connect to the energy storage battery, the output ends of the n PCSs are connected in parallel to the input end of the transformer, and the output end of the transformer is used to connect to the AC power grid. Each PCS is used to convert the DC power of the energy storage battery connected to it into AC power and output it to the power grid. In addition, the PCS can also rectify the AC power of the power grid into DC power and charge the energy storage battery.

[0037] After the power generation system is connected to the grid, the control unit detects the grid voltage in real time, and after determining that the grid has failed based on the grid voltage, controls the reactive output current and active output current of each inverter or PCS to achieve fault ride-through. Fault ride-through is defined as follows: When a power system fault or disturbance causes a change in the voltage or frequency at the grid connection point of the power generation system, the power generation system can ensure continuous operation without disconnecting from the grid within a certain voltage and frequency change range and time interval.

[0038] The above is only an example of the application scenarios of the power generation system provided by the present application, and is not an exhaustive list. The present application does not limit the application scenarios.

[0039] Combine the following Figures 5 to 8 The working principles of the power generation system and the power conversion device provided in the present application are illustrated.

[0040] See also Figure 5 , Figure 5 Schematic diagram of a power generation system provided by this application. Figure 5As shown, the power generation system includes power conversion devices 11, ..., power conversion devices 1n, transformer 2 and control unit 3, where n is a positive integer. Among them, the input end of the power conversion device 11 is used to connect to the DC power supply 41, ..., the input end of the power conversion device 1n is used to connect to the DC power supply 4n, the output end of the power conversion device 11, ..., and the output end of the power conversion device 1n are all connected to the input end of the transformer 2, and the output end of the transformer 2 is used to connect to the power grid. The DC power supply includes a photovoltaic string or an energy storage battery. The power grid here can be a real power grid or a simulated power grid. Specifically, before the power generation system is incorporated into the real power grid, the power grid connected to the output end of the transformer 2 is a simulated power grid (such as a voltage mutation generating device); after the power generation system is incorporated into the real power grid, the power grid connected to the output end of the transformer 2 is a real power grid. The power generation system provided in the present application can reduce or even offset the control method of the reactive current in the excitation inrush current at the output end of the transformer after the fault crossing ends and the influence of the reactive output current zeroing stage of the transformer, which is suitable for the fault crossing application scenario after the power generation system is connected to the grid.

[0041] Considering that the reactive current in the excitation surge current caused by the voltage change at the output end of the transformer during the grid voltage recovery process will have a significant impact on the reactive output current zeroing stage of the transformer, the present application reduces or even offsets the impact of the reactive current in the excitation surge current at the output end of the transformer on the reactive output current zeroing stage of the transformer after the fault ride-through is completed by making the power conversion device output reactive compensation current. The specific implementation method is as follows:

[0042] In one embodiment, when the first fault ride-through ends, each power conversion device controls its own reactive output current to be the current value after the reactive compensation current is superimposed on the reactive reference current. The reactive reference current is the reactive output current of each power conversion device that recovers to the first current value during the period from the end of the first fault ride-through to the time when the voltage of the power grid returns to normal. The first current value is the reactive output current value of the power conversion device before the first fault ride-through, that is, when the power grid is normal, such as 0. Exemplarily, the reactive reference current is Figure 2 i in the time period from t2 to t3 q1 The direction of the reactive compensation current is opposite to the direction of the first reactive output current, which is the reactive output current of transformer 2 after the second fault ride-through is completed when each power conversion device is electrically disconnected from the transformer and the transformer is conductively connected to the grid. The second fault ride-through is the same as the first fault ride-through.

[0043] Here, the first fault ride-through and the second fault ride-through can be the same in a strict sense or in a sense of certain deviation, specifically including the following situations: when the first fault ride-through is a low voltage ride-through, the absolute value of the difference between the minimum value of the grid voltage drop depth during the second fault ride-through and the minimum value of the grid voltage drop depth during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and when the grid voltage drop depth is the voltage drop of the grid, the ratio of the grid voltage to the rated voltage; when the first fault ride-through is a high voltage ride-through, the absolute value of the difference between the maximum value of the grid voltage increase amplitude during the second fault ride-through and the maximum value of the grid voltage increase amplitude during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and when the grid voltage increase amplitude is the voltage increase of the grid, the ratio of the grid voltage to the rated voltage. In addition, the end time of the second fault ride-through is earlier than the occurrence time of the first fault ride-through. In other words, the first fault ride-through can be understood as the actual fault ride-through currently occurring during the power generation system being connected to the grid, and the second fault ride-through can be understood as the fault ride-through that is the same as the current actual fault ride-through and is simulated before the power generation system is connected to the grid.

[0044] Specifically, after the power generation system is connected to the grid, the control unit 3 detects the grid voltage in real time, and in the event of a grid fault, for example, when the grid voltage is greater than the first voltage threshold (the grid voltage is too high, the power generation system enters high voltage ride-through) or less than the second voltage threshold (the grid voltage is too low, the power generation system enters low voltage ride-through), controls each power conversion device to output reactive output current and active output current to ensure that the power generation system does not go off the grid. Among them, the first voltage threshold is greater than the rated voltage Un of the grid, and the second voltage threshold is less than the rated voltage Un of the grid. Exemplarily, the first voltage threshold is 1.1 times Un, and the second voltage threshold is 0.9 times Un. Here, for the specific change trend of the reactive output current and the active output current during the fault ride-through of the power conversion device, please refer to Figure 2 The part from t0 to t2 will not be expanded here.

[0045] During the current fault ride-through, the control unit 3 determines the historical fault ride-through (i.e., the second fault ride-through) that is the same as the current fault ride-through from the first fault ride-through history list, and determines the reactive output current of the transformer 2 after the historical fault ride-through is completed as the first reactive output current i when each power conversion device is electrically disconnected from the transformer and the transformer is conductively connected to the grid. qh1 Here the first reactive output current i qh1It can be understood as the reactive current in the excitation surge current caused by the voltage change at the output end of the transformer during the grid voltage recovery process under the same historical fault ride-through as the current fault ride-through. Afterwards, the control unit 3 calculates the total reactive compensation current i required for the n power conversion devices to compensate for the reactive current in the excitation surge current at the output end of the transformer 2 based on the turns ratio N of the transformer 2. ql1 =-N*i qh1 , so that the reactive compensation current i required by each power conversion device to compensate for the reactive current in the excitation surge current at the output end of transformer 2 can be calculated q1* =i ql1 / n=-N*i qh1 / n, and the reactive compensation current i q1* The first fault ride-through history list may be fault ride-through history data acquired in a fault ride-through simulation scenario when each power conversion device is electrically disconnected from the transformer and the transformer is conductively connected to the power grid.

[0046] Afterwards, when the fault ride-through ends, specifically, when the fault ride-through ends (i.e., when the grid voltage begins to recover), each power conversion device controls its own reactive output current to be the reactive compensation current i superimposed on the reactive reference current of the power conversion device. q1* The current value after the fault ride-through is adjusted, so that the sum of the reactive compensation currents output by the n power conversion devices and the reactive current in the excitation surge current at the output end of the transformer cancel each other out, thereby canceling the influence of the reactive current in the excitation surge current at the output end of the transformer on the reactive output current clearing stage of the transformer after the fault ride-through is completed.

[0047] It should be noted that, in the above embodiment, the direction of the reactive compensation current is opposite to the direction of the first reactive output current, and the ratio of the reactive compensation current to the first reactive output current is k1=-N / n. In fact, in order to avoid excessive reactive current compensation, the reactive compensation current is opposite to the direction of the first reactive output current in terms of value, and the relationship between the reactive compensation current and the first reactive output current must satisfy that the absolute value of the reactive compensation current is less than or equal to the absolute value of the first reactive output current. Specifically, when k1 takes any value in the interval [-N / n, 0), the sum of the reactive compensation currents actively compensated by the n power conversion devices can offset at least part of the reactive current in the excitation surge current at the output end of the transformer, thereby achieving the effect of reducing the influence of the reactive current in the excitation surge current at the output end of the transformer after the fault ride-through on the reactive output current zeroing stage of the transformer.

[0048] It can be understood that, since the reactive output current of the transformer cannot be quickly cleared to zero within 30ms after the end of the fault ride-through and the fluctuation is large, it is mainly caused by the reactive current in the excitation inrush current caused by the voltage change in the grid voltage recovery process at the output end of the transformer. Therefore, the power conversion device provided by the present application controls the reactive output current of the power conversion device by starting to superimpose the reactive compensation current on the basis of the reactive reference current at the end of the current fault ride-through, wherein the reactive compensation current is used to reduce or even offset the influence of the reactive current in the excitation inrush current at the output end of the transformer on the zeroing of the reactive output current of the transformer. Specifically, the direction of the reactive compensation current is opposite to the direction of the first reactive output current, wherein the first reactive output current can be understood as the reactive current in the excitation inrush current caused by the voltage change in the grid voltage recovery process at the output end of the transformer under the same historical fault ride-through as the current fault ride-through, so that the sum of the reactive compensation currents compensated by the n power conversion devices can offset at least part of the reactive current in the excitation inrush current at the output end of the transformer, thereby reducing or even offsetting the influence of the reactive current in the excitation inrush current at the output end of the transformer on the zeroing stage of the reactive output current of the transformer after the fault ride-through ends.

[0049] Furthermore, the presence of the excitation inductance in the transformer will cause part of the active output current of the power conversion device to affect the reactive output current clearing stage of the transformer 2 during the active output current recovery process of the power conversion device, thereby causing the reactive output current of the transformer 2 to fail to clear quickly within 30ms after the fault ride-through and the current fluctuation to be greater than 5%I n. The specific reasons are as follows:

[0050] See also Figure 6 , Figure 6 is a circuit diagram of a transformer under non-ideal conditions provided by this application. Figure 6 As shown, the primary side of the transformer is equivalent to the excitation inductance L f In parallel with the coil, due to the excitation inductance L f The existence of the primary current i in and the secondary current i 2 are not equal, the primary current i in and the secondary current i 2 There are deviations in the phase and amplitude of the primary current i in and the secondary current i 2 There is a phase deviation, and the primary voltage U in and the secondary voltage U out The phases of the primary and secondary sides are the same, therefore, the active currents of the primary and secondary sides are inconsistent, and the reactive currents of the primary and secondary sides are also inconsistent. After the active current component of the primary side is induced to the secondary side, there is a certain phase difference with the secondary side voltage, that is, part of the active current component will be induced as reactive current, and similarly, the reactive current of the primary side will also include active current after being induced to the secondary side.

[0051] See also Figure 7 , Figure 7 This is a schematic diagram of the primary and secondary voltages and currents of a transformer under fault ride-through under the influence of the primary and secondary current phase difference caused by transformer excitation provided by the present application. Figure 7 As shown in the figure, in the fault exit phase after time t2, the primary reactive current i q1 (i.e. the reactive output current of the power conversion device) is quickly cleared, and the primary active current i d1 (i.e., the active output current of the power conversion device) is slowly restored according to the gradient setting. During this process, the primary reactive current i q1 After clearing, due to the primary active current i d1 The slow recovery will be affected by the phase difference between the primary and secondary currents caused by the transformer excitation, making the primary active current i d1 A part of it will be induced as the secondary side reactive current i q2 (i.e. the reactive output current of transformer 2), so the secondary reactive current i q2 During the zeroing process, a period of time will appear along with the primary active current i d1 The slow change process of gradient recovery corresponds to Figure 7 The rectangular frame part in .

[0052] In summary, the reactive current in the excitation surge current caused by the voltage change at the output end of the transformer during the grid voltage recovery process (referred to as reactive current 1), and the reactive current induced at the output end of the transformer by part of the active output current of the power conversion device during the active output current recovery process of the power conversion device (referred to as reactive current 2), will both affect the reactive output current zeroing stage of the transformer 2. Based on this, the present application replaces the first reactive output current in the above embodiment with a second reactive output current including the first reactive output current, and the second reactive output current is the reactive output current of the transformer 2 after the second fault crossing when each power conversion device is conductively connected to the transformer 2, the transformer 2 is conductively connected to the grid, and the reactive output current of each power conversion device after the second fault crossing is the reactive reference current, to reduce or even offset the reactive current in the excitation surge current at the output end of the transformer and part of the active output current of the power conversion device is induced as the reactive current at the output end of the transformer, and the influence on the reactive output current zeroing stage of the transformer is jointly achieved. The specific implementation method is as follows:

[0053] Specifically, after the power generation system is connected to the grid, the control unit 3 detects the grid voltage in real time, and in the event of a grid fault, controls each power conversion device to output reactive output current and active output current to ensure that the power generation system does not disconnect from the grid. During the current fault ride-through, the control unit 3 determines the same historical fault ride-through as the current fault ride-through from the second fault ride-through history list, and determines the reactive output current of the transformer 2 after the historical fault ride-through as the second reactive output current i when each power conversion device is conductively connected to the transformer, the transformer is conductively connected to the grid, and the reactive output current of the power conversion device after the historical fault ride-through is the reactive reference current. qh2 Here the second reactive output current i qh2 It can be understood as the sum of reactive current 1 and reactive current 2 under the same historical fault ride-through as the current fault ride-through. Then, the control unit 3 calculates the total reactive compensation current i required for the n power conversion devices to actively compensate for reactive current 1 and reactive current 2 based on the turns ratio N of the transformer 2. ql2 =-N*i qh2 , so that the reactive compensation current i required by each power conversion device to compensate reactive current 1 and reactive current 2 can be calculated q2* =i ql2 / n=-N*i qh2 / n, and the reactive compensation current i q2* The second fault ride-through history list may be fault ride-through history data acquired in a fault ride-through simulation scenario when the transformer is conductively connected to each power conversion device and the power grid and the reactive output current of the power conversion device is the reactive reference current.

[0054] Afterwards, when the fault ride-through ends, specifically, when the fault ride-through ends, each power conversion device controls its own reactive output current to be the reactive compensation current i superimposed on the reactive reference current of the power conversion device. q2* The current value after the fault ride-through is obtained, so that the sum of the reactive compensation currents output by the n power conversion devices can offset the sum of reactive current 1 and reactive current 2, thereby making the reactive output current of transformer 2 quickly cleared within 30ms after the fault ride-through is completed and the current fluctuation is less than 5%I n.

[0055] It should be noted that, in the above embodiment, the direction of the reactive compensation current is opposite to the direction of the second reactive output current, and the ratio of the reactive compensation current to the second reactive output current k2=-N / n is used as an example for introduction. In fact, in order to avoid excessive reactive current compensation, in addition to being opposite to the direction of the second reactive output current in direction, the relationship between the reactive compensation current and the second reactive output current in value must satisfy that the absolute value of the reactive compensation current is less than or equal to the absolute value of the second reactive output current. Specifically, when k2 takes any value in the interval [-N / n, 0), the sum of the reactive compensation currents actively compensated by the n power conversion devices can offset at least part of the reactive current in the sum of the reactive current 1 and the reactive current 2, thereby achieving the effect of reducing the reactive current in the excitation inrush current at the output end of the transformer after the fault ride-through is completed and part of the active output current of the power conversion device is induced as the reactive current at the output end of the transformer, and jointly affecting the reactive output current zeroing stage of the transformer.

[0056] For a better understanding, the following Figure 8 , taking low voltage ride-through as an example, this article will introduce it. Figure 8 As shown, during the time period from t0 to t2, the power conversion device determines the grid fault based on the grid voltage u, and controls its own reactive output current i based on the grid voltage drop depth. q1 and active output current i d1 , in order to perform low voltage ride through. And in the time period from t1 to t2, the power conversion device obtains reactive compensation current by exchanging data with the control unit 3. Then, at the end of the low voltage ride through (i.e., at time t2), the power conversion device controls its own reactive output current i q1 The reactive output current i of the power conversion device is obtained by adding the reactive compensation current to the reactive reference current. q1 Current waveform in the time period from t2 to t4. Obviously, at time t3, the reactive output current i of transformer 2 is q2 The reset is completed, the time interval between time t2 and time t3 is less than or equal to 30ms, and the reactive output current i q1 It is still outputting. Therefore, it can be known that the power conversion device can make the reactive output current of the transformer 2 quickly clear to zero within 30ms after the fault ride-through is completed, and the current fluctuation is less than 5%I n by actively compensating the reactive compensation current.

[0057] It can be understood that since the reactive output current of the transformer cannot be quickly cleared to zero and fluctuates greatly within 30ms after the end of fault crossing, it is caused by the reactive current 1 and the reactive current 2. Therefore, the power conversion device provided in the present application controls the reactive output current of the power conversion device by starting to superimpose the reactive compensation current on the reactive reference current at the end of the current fault crossing, wherein the reactive compensation current is used to reduce or even offset the joint influence of the reactive current 1 and the reactive current 2 on the reactive output current clearing stage of the transformer 2. Specifically, the direction of the reactive compensation current is opposite to that of the second reactive output current. The second reactive output current can be understood as the sum of reactive current 1 and reactive current 2 under the same historical fault crossing as the current fault crossing, so that the sum of reactive compensation currents actively compensated by n power conversion devices can offset at least part of the reactive current in the sum of reactive current 1 and reactive current 2, thereby reducing or even offsetting the reactive current in the excitation surge current at the output end of the transformer after the fault crossing and part of the active output current of the power conversion device being induced as the reactive current at the output end of the transformer, and jointly affecting the reactive output current clearing stage of the transformer.

[0058] In the present application, at the end of the current fault ride-through, the power conversion device starts to superimpose a reactive compensation current on the basis of a reactive reference current, and the direction of the reactive compensation current is opposite to the direction of the first reactive output current or the second reactive output current under the same historical fault ride-through as the current fault ride-through, so as to control the reactive output current of the power conversion device, so that the sum of the reactive compensation currents actively compensated by n power conversion devices can offset part of the reactive current in reactive current 1, or offset at least part of the reactive current in the sum of reactive current 1 and reactive current 2, thereby reducing or even offsetting the influence of the reactive current in the excitation surge current at the output end of the transformer after the fault ride-through on the reactive output current zeroing stage of the transformer, or reducing or even offsetting the reactive current in the excitation surge current at the output end of the transformer after the fault ride-through and part of the active output current of the power conversion device being induced as the reactive current at the output end of the transformer, and jointly on the reactive output current zeroing stage of the transformer.

[0059] The above are only specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A power generation system, characterized in that: The power generation system comprises one or more power conversion devices, a transformer and a control unit, wherein: The input end of the power conversion device is used to connect to a photovoltaic string or an energy storage battery, the output end of the power conversion device is connected to the input end of the transformer, and the output end of the transformer is used to connect to a power grid; The control unit is used to control the reactive output current of the power conversion device to be a current value obtained by superimposing a reactive compensation current on a reactive reference current when the first fault ride-through is completed, wherein the reactive reference current is the reactive output current of the power conversion device that recovers to a first current value during the time period from the completion of the first fault ride-through to the restoration of the voltage of the power grid to normal, and the first current value is the reactive output current value of the power conversion device before the first fault ride-through; the direction of the reactive compensation current is opposite to that of the first reactive output current, and the first reactive output current is the reactive output current of the transformer after the second fault ride-through is completed when the power conversion device is electrically disconnected from the transformer and the transformer is conductively connected to the power grid; the second fault ride-through is the same as the first fault ride-through.

2. The power generation system according to claim 1, characterized in that: When the first fault ride-through is a low voltage ride-through, the absolute value of the difference between the minimum value of the grid voltage drop depth during the second fault ride-through and the minimum value of the grid voltage drop depth during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and the grid voltage drop depth is the ratio of the voltage of the grid to the rated voltage when the grid voltage drops.

3. The power generation system according to claim 1, characterized in that: When the first fault ride-through is a high voltage ride-through, the absolute value of the difference between the maximum value of the grid voltage increase during the second fault ride-through and the maximum value of the grid voltage increase during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and the grid voltage increase is the ratio of the grid voltage to the rated voltage when the grid voltage is increased.

4. The power generation system according to any one of claims 1 to 3, characterized in that: The ratio of the reactive compensation current to the first reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of the one or more power conversion devices, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the number of turns of the secondary winding in the transformer.

5. The power generation system according to any one of claims 1 to 4, characterized in that: The direction of the reactive compensation current is opposite to the direction of the second reactive output current, wherein the second reactive output current is the reactive output current of the transformer when the power conversion device is conductively connected to the transformer, the transformer is conductively connected to the power grid, and the reactive output current of the power conversion device is the reactive reference current after the second fault ride-through is completed.

6. The power generation system according to claim 5, characterized in that: The ratio of the reactive compensation current to the second reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of the one or more power conversion devices, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the number of turns of the secondary winding in the transformer.

7. A power conversion device, characterized in that: The input end of the power conversion device is connected to a photovoltaic string or an energy storage battery, the output end of the power conversion device is connected to the input end of a transformer, and the output end of the transformer is connected to a power grid; The power conversion device is used to control the reactive output current of the power conversion device to be a current value obtained by superimposing a reactive compensation current on a reactive reference current when a first fault ride-through is completed, wherein the reactive reference current is the reactive output current of the power conversion device that is restored to a first current value during a period from the completion of the first fault ride-through to the restoration of the voltage of the power grid to normal, and the first current value is the reactive output current value of the power conversion device before the first fault ride-through; the direction of the reactive compensation current is opposite to that of the first reactive output current, and the first reactive output current is the reactive output current of the transformer after the second fault ride-through is completed when the power conversion device is electrically disconnected from the transformer and the transformer is conductively connected to the power grid; the second fault ride-through is the same as the first fault ride-through.

8. The power conversion device according to claim 7, characterized in that: When the first fault ride-through is a low voltage ride-through, the absolute value of the difference between the minimum value of the grid voltage drop depth during the second fault ride-through and the minimum value of the grid voltage drop depth during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and the grid voltage drop depth is the ratio of the voltage of the grid to the rated voltage when the grid voltage drops.

9. The power conversion device according to claim 7, characterized in that: When the first fault ride-through is a high voltage ride-through, the absolute value of the difference between the maximum value of the grid voltage increase during the second fault ride-through and the maximum value of the grid voltage increase during the first fault ride-through is less than or equal to 0.2 times the rated voltage of the grid, and the grid voltage increase is the ratio of the grid voltage to the rated voltage when the grid voltage is increased.

10. The power conversion device according to any one of claims 7 to 9, characterized in that: The ratio of the reactive compensation current to the first reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of power conversion devices connected to the transformer, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the number of turns of the secondary winding in the transformer.

11. The power conversion device according to any one of claims 7 to 10, characterized in that: The direction of the reactive compensation current is opposite to the direction of the second reactive output current, wherein the second reactive output current is the reactive output current of the transformer when the power conversion device is conductively connected to the transformer, the transformer is conductively connected to the power grid, and the reactive output current of the power conversion device is the reactive reference current after the second fault ride-through is completed.

12. The power conversion device according to claim 11, characterized in that: The ratio of the reactive compensation current to the second reactive output current is the inverse of the ratio of the turns ratio of the transformer to the number of power conversion devices connected to the transformer, and the turns ratio of the transformer is the ratio of the number of turns of the primary winding to the number of turns of the secondary winding in the transformer.

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