Photovoltaic grid-connected system and ac fault ride-through method and device thereof

Abstract

The application discloses a photovoltaic grid-connected system and an alternating current fault ride-through method and device thereof, and comprises a plurality of photovoltaic arrays, a plurality of photovoltaic grid-connected inverters, a direct current collection line and a step-up transformer. Each photovoltaic grid-connected inverter is composed of six bridge arms, wherein each bridge arm comprises a plurality of photovoltaic sub-modules; each photovoltaic sub-module comprises a front-stage MMC converter sub-module and a rear-stage DC-DC converter. When an alternating current fault occurs in the photovoltaic grid-connected system, a hierarchical three-stage control of a photovoltaic converter stage, a bridge arm stage and a sub-module stage is combined to independently regulate the output of each photovoltaic sub-module, ensure that photovoltaic MPPT control is not completely disabled during the fault ride-through, inhibit overvoltage of the photovoltaic sub-module when the grid voltage drops, and realize the alternating current fault ride-through of the photovoltaic grid-connected system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of photovoltaic power generation, in particular to a photovoltaic grid-connected system and an alternating current fault ride-through method and device thereof. BACKGROUND

[0002] A photovoltaic power generation system mainly comprises a photovoltaic module for absorbing solar radiation and converting it into direct current power, a photovoltaic inverter for converting the direct current power into alternating current power, and related cables and supporting facilities. The photovoltaic inverter is the core part of the entire photovoltaic power generation system. Currently, two-level and three-level photovoltaic inverters are relatively mature. With the continuous development of technology and the increasing requirements for the performance of inverters, photovoltaic inverters are developing towards multi-level technology. Voltage source converters (VSC) are the core equipment of a flexible direct current transmission system (VSC-HVDC). Among its many topologies, the modular multilevel converter (MMC) has the characteristics of modularization, easy expansion, and fewer harmonics, and can meet the technical application requirements of multi-level photovoltaic grid-connected inverters.

[0003] In a large photovoltaic power generation grid-connected system, the number of photovoltaic arrays is in the tens of thousands. Combining photovoltaic power generation with modular multilevel converter technology can improve the independent control of each sub-module unit, maximize the utilization of solar energy of the photovoltaic array, and make it work in the maximum power state as much as possible, thereby greatly improving the power generation efficiency of the photovoltaic system. In addition, the multi-level inverter can realize multi-level voltage output to improve the grid connection quality. Compared with the same scale of traditional photovoltaic alternating current grid-connected systems, the photovoltaic direct current grid-connected system based on the modular multilevel converter greatly reduces the number of grid-connected inverters and improves the system operation stability. The fault ride-through technology of the photovoltaic grid-connected system is the key to ensuring the safe and stable operation of the entire power grid after large-scale photovoltaic access. How to coordinate the MMC and the modular photovoltaic power generation unit to achieve fault ride-through is a problem to be solved. SUMMARY

[0004] Therefore, the technical problem to be solved by the present application is how to coordinate the MMC and the modular photovoltaic power generation unit to achieve fault ride-through, so as to provide a photovoltaic grid-connected system and an alternating current fault ride-through method and device thereof.

[0005] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0006] In a first aspect, an embodiment of the present application provides a photovoltaic grid-connected system, comprising: a plurality of photovoltaic arrays, a plurality of photovoltaic grid-connected inverters, a direct current collection line, and a step-up transformer, wherein the direct current side of each photovoltaic grid-connected inverter is connected to the plurality of photovoltaic arrays through the direct current collection line, and the alternating current side of each photovoltaic grid-connected inverter is connected to an alternating current system through the step-up transformer; each photovoltaic grid-connected inverter is composed of six bridge arms, wherein each bridge arm comprises: a plurality of photovoltaic sub-modules; the direct current side of each photovoltaic sub-module is connected to a photovoltaic array, and the alternating current side of each photovoltaic sub-module is connected to the alternating current system through the step-up transformer; when the photovoltaic grid-connected system has an alternating current fault, a three-level control of a photovoltaic converter level, a bridge arm level, and a sub-module level is combined to independently regulate the output of each photovoltaic sub-module.

[0007] In an embodiment, each photovoltaic sub-module comprises: a front-stage MMC converter sub-module and a rear-stage DC-DC converter; the low-voltage side of the rear-stage DC-DC converter is connected to a photovoltaic array, and the high-voltage side of the rear-stage DC-DC converter is connected to the direct current side of the front-stage MMC converter sub-module; the alternating current side of the front-stage MMC converter sub-module is connected to the alternating current system through the step-up transformer.

[0008] In a second aspect, an embodiment of the present application provides an alternating current fault ride-through method of a photovoltaic grid-connected system, characterized in that the method is applied to the photovoltaic grid-connected system of the first aspect, and the method comprises: judging whether the photovoltaic grid-connected system has an alternating current fault; when the photovoltaic grid-connected system has an alternating current fault, combining a three-level control of a photovoltaic converter level, a bridge arm level, and a sub-module level to independently regulate the output of each photovoltaic sub-module.

[0009] In an embodiment, the process of combining a three-level control of a photovoltaic converter level, a bridge arm level, and a sub-module level to independently regulate the output of each photovoltaic sub-module comprises: after generating a reactive current reference instruction according to a photovoltaic grid-connected low-voltage ride-through standard, performing photovoltaic converter level control on the front-stage MMC converter sub-module by using power outer loop control and current inner loop control; obtaining a photovoltaic array output power reference value based on an MPPT control link; after correcting the photovoltaic array output power reference value by using bridge arm level control and sub-module level control, obtaining the duty cycle of the rear-stage DC-DC converter by using power control.

[0010] In an embodiment, the process of the photovoltaic converter stage control of the front-stage MMC converter sub-module includes: calculating the deviation value of the reference value of the sum of the capacitor voltages of all photovoltaic sub-modules of the six bridge arms minus the actual value, and obtaining the active current reference instruction after PI conversion of the difference value; limiting the active current reference instruction and the reactive current reference instruction to obtain the active current control instruction and the reactive current control instruction; inputting the active current control instruction and the reactive current control instruction into the current inner loop to obtain the bridge arm modulation wave of the front-stage MMC converter sub-module; and obtaining the switching sequence of the front-stage MMC converter sub-module based on the bridge arm modulation wave and using a preset modulation method.

[0011] In an embodiment, the process of obtaining the duty cycle of the rear-stage DC-DC converter includes: obtaining the photovoltaic array output voltage reference value by using the MPPT controller based on the photovoltaic array output voltage measurement value and the output current measurement value; obtaining the photovoltaic array output power reference value after PI conversion of the difference value between the photovoltaic array output voltage reference value and the photovoltaic array output voltage measurement value; correcting the photovoltaic array output power reference value by using the single photovoltaic sub-module output power correction value output by the bridge arm stage control and the sub-module stage control; obtaining the duty cycle of the rear-stage DC-DC converter after PI conversion of the difference value between the corrected photovoltaic array output power reference value and the actual value of the photovoltaic array output power, so as to adjust the output of the rear-stage DC-DC converter.

[0012] In an embodiment, the process of obtaining the single photovoltaic sub-module output power correction value by the bridge arm stage control includes: judging whether the actual value of the sum of the capacitor voltages of all photovoltaic sub-modules of a single bridge arm is greater than a set threshold value; and obtaining the output power correction value of each photovoltaic sub-module of the bridge arm after PI conversion and multiplication of the deviation value of the actual value of the sum of the capacitor voltages of all photovoltaic sub-modules of the single bridge arm minus the reference value by the power distribution proportionality coefficient.

[0013] In an embodiment, the process of obtaining the single photovoltaic sub-module output power correction value by the sub-module stage control includes: obtaining the average value of the capacitor voltages of the photovoltaic sub-modules of the bridge arm by dividing the actual value of the sum of the capacitor voltages of all photovoltaic sub-modules of the single bridge arm by the number of the photovoltaic sub-modules of the bridge arm; judging whether the capacitor voltage measurement value of each photovoltaic sub-module of the bridge arm is greater than the average value; and obtaining the output power correction value of the single photovoltaic sub-module of the bridge arm after PI conversion and multiplication of the deviation value of the capacitor voltage measurement value of the photovoltaic sub-module minus the average value by the power correction proportionality coefficient when the capacitor voltage measurement value of the photovoltaic sub-module is greater than the average value.

[0014] In a third aspect, an AC fault ride-through device for a photovoltaic grid-connected system is provided, comprising: a judging module configured to judge whether an AC fault occurs in the photovoltaic grid-connected system; and a three-level control module configured to, when the AC fault occurs in the photovoltaic grid-connected system, combine three-level control of a photovoltaic converter level, a bridge arm level and a submodule level to independently regulate output of each photovoltaic submodule.

[0015] In a fourth aspect, a computer device is provided, comprising: at least one processor, and a memory communicatively connected to the at least one processor, wherein the memory stores instructions executable by the at least one processor, and the instructions are executed by the at least one processor to cause the at least one processor to perform the AC fault ride-through method for a photovoltaic grid-connected system according to the second aspect of the embodiments of the present application.

[0016] In a fifth aspect, a computer readable storage medium is provided, which stores computer instructions for causing a computer to perform the AC fault ride-through method for a photovoltaic grid-connected system according to the second aspect of the embodiments of the present application.

[0017] The technical scheme of the present application has the following advantages:

[0018] 1. The photovoltaic grid-connected system provided by the present application connects a plurality of photovoltaic arrays through a direct current collection line at the direct current side of each photovoltaic grid-connected inverter, and connects the alternating current system through a step-up transformer at the alternating current side of each photovoltaic grid-connected inverter. Each photovoltaic grid-connected inverter is composed of six bridge arms, each of which includes a plurality of photovoltaic submodules, the direct current side of each photovoltaic submodule is connected to a photovoltaic array, and the alternating current side of each photovoltaic submodule is connected to the alternating current system through a step-up transformer. Each photovoltaic submodule includes a front-stage MMC converter submodule and a rear-stage DC-DC converter, the low-voltage side of the rear-stage DC-DC converter is connected to a photovoltaic array, the high-voltage side of the rear-stage DC-DC converter is connected to the direct current side of the front-stage MMC converter submodule, and the alternating current side of the front-stage MMC converter submodule is connected to the alternating current system through a step-up transformer. When an AC fault occurs in the photovoltaic grid-connected system, three-level control of the photovoltaic converter level, the bridge arm level and the submodule level is combined to independently regulate the output of each photovoltaic submodule, to ensure that the photovoltaic MPPT control does not completely fail during the fault ride-through, while suppressing overvoltage of the photovoltaic submodule when the grid voltage drops, and realizing AC fault ride-through of the photovoltaic grid-connected system.

[0019] 2.The AC fault ride-through method of the photovoltaic grid-connected system provided by the application, which combines three-level control of the photovoltaic converter level, the bridge arm level and the submodule level to independently regulate the output of each photovoltaic submodule without locking the DC transformer and adding any additional hardware facilities, ensures that the photovoltaic MPPT control does not completely fail during the fault ride-through, suppresses the overvoltage of the photovoltaic submodule when the grid voltage drops, and realizes the AC fault ride-through of the photovoltaic grid-connected system. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the drawings needed in the specific embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0021] Figure 1 The composition diagram of a specific example of the photovoltaic grid-connected system in the embodiment of the application;

[0022] Figure 2 The specific topology diagram of the photovoltaic submodule in the embodiment of the application;

[0023] Figure 3 The flowchart of a specific example of the AC fault ride-through method of the photovoltaic grid-connected system in the embodiment of the application;

[0024] Figure 4 The flowchart of another specific example of the AC fault ride-through method of the photovoltaic grid-connected system in the embodiment of the application;

[0025] Figure 5 The flowchart of a specific example of the photovoltaic converter level control in the embodiment of the application;

[0026] Figure 6 The control block diagram of the photovoltaic converter level control in the embodiment of the application;

[0027] Figure 7 The simulation effect diagram of the grid-side AC system reactive current support during the fault ride-through in the embodiment of the application;

[0028] Figure 8 The flowchart of another specific example of the AC fault ride-through method of the photovoltaic grid-connected system in the embodiment of the application;

[0029] Figure 9 The control block diagram of the photovoltaic submodule output step-by-step hierarchical control in the embodiment of the application;

[0030] Figure 10It is an effect diagram of AC fault ride-through simulation of the photovoltaic grid-connected system in the embodiment of the present application.

[0031] Figure 11 It is a component diagram of a specific example of the AC fault ride-through method of the photovoltaic grid-connected system in the embodiment of the present application.

[0032] Figure 12 It is a component diagram of a specific example of the computer device provided in the embodiment of the present application. DETAILED DESCRIPTION

[0033] The technical solutions of the present application will be described clearly and completely below in conjunction with the drawings. 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 skilled in the art without creative labor fall within the protection scope of the present application.

[0034] In the description of the present application, it should be noted that the orientations or positional relationships indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", "third" are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0035] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the internal communication of two elements, it can be wireless connection, or wired connection. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0036] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as there is no conflict between them.

[0037] Embodiment 1

[0038] The embodiment of the present application provides a photovoltaic grid-connected system, such as Figure 1As shown, the photovoltaic grid-connected system comprises a plurality of photovoltaic arrays, a plurality of photovoltaic grid-connected inverters, a direct current collection line, and a step-up transformer, wherein the direct current side of each photovoltaic grid-connected inverter is connected to the plurality of photovoltaic arrays through the direct current collection line, and the alternating current side of each photovoltaic grid-connected inverter is connected to the alternating current system through the step-up transformer.

[0039] Optionally, as shown in Figure 1 each photovoltaic grid-connected inverter is composed of six bridge arms, wherein each bridge arm comprises a plurality of photovoltaic sub-modules (PVSMs) in each photovoltaic sub-module, the direct current side of each photovoltaic sub-module is connected to a photovoltaic array, and the alternating current side of each photovoltaic sub-module is connected to the alternating current system through the step-up transformer. Figure 1

[0040] Optionally, as shown in Figure 2 each photovoltaic sub-module comprises a front-stage MMC converter sub-module and a rear-stage DC / DC converter, the low-voltage side of the rear-stage DC / DC converter is connected to a photovoltaic array, the high-voltage side of the rear-stage DC / DC converter is connected to the direct current side of the front-stage MMC converter sub-module, and the alternating current side of the front-stage MMC converter sub-module is connected to the alternating current system through the step-up transformer.

[0041] Specifically, a plurality of front-stage MMC converter sub-modules are connected in series to form a bridge arm and are connected to the alternating current system, for converting the direct current voltage converted by the DC / DC converter into alternating current voltage. The MMC converter sub-module can adopt a half-bridge sub-module (HBSM), a full-bridge sub-module (FBSM), or a hybrid sub-module (HSM) with a mixed connection of multiple sub-modules, Figure 2 and the like, and a full-bridge sub-module topology is used for illustration inThe rear-stage DC / DC converter is connected to the photovoltaic array, for converting the direct current voltage output by the photovoltaic panel into a step-up converted direct current voltage. The DC / DC converter can adopt a single active bridge (SAB), a dual active bridge (DAB), or a Boost topology, Figure 2 and a DC / DC converter with a SAB topology is used for illustration in

[0042] Embodiment 2

[0043] The embodiment of the present application provides an alternating current fault ride-through method of a photovoltaic grid-connected system, applied to the photovoltaic grid-connected system of embodiment 1, as shown in Figure 3 the method comprises the following steps.

[0044] Step S11: determining whether the photovoltaic grid-connected system has an alternating current fault.

[0045] Specifically, when the power grid fails, the grid-side AC system voltage measurement value is obtained. The deviation value of the grid-side AC system voltage reference value minus the measurement value is calculated, and the deviation value is used as the photovoltaic grid-connected system AC fault ride-through criterion. When the deviation value is in the voltage drop range specified in the photovoltaic system access power system guide, a fault ride-through signal is generated, which is used to implement the photovoltaic converter level, bridge arm level, and submodule level hierarchical three-level control. The grid-side AC system voltage reference value is set according to the specific working condition of the photovoltaic DC grid-connected system.

[0046] Step S12: When the photovoltaic grid-connected system fails, the photovoltaic converter level, bridge arm level, and submodule level hierarchical three-level control are combined to independently adjust the output of each photovoltaic submodule.

[0047] Specifically, the embodiment of the application utilizes the cooperative control of the MMC converter and the modular photovoltaic submodule to realize a method for AC fault ride-through of a new photovoltaic DC grid-connected system based on a modular multilevel converter. Without blocking the DC transformer and without adding any additional hardware facilities, the method realizes independent adjustment of the photovoltaic submodule output through photovoltaic converter level, bridge arm level, and submodule level hierarchical three-level control, ensures that the photovoltaic MPPT control does not completely fail during fault ride-through, and suppresses the overvoltage of the photovoltaic submodule when the grid voltage drops, thereby realizing AC fault ride-through of the photovoltaic DC grid-connected system.

[0048] Optionally, as shown in Figure 4 The process of combining the photovoltaic converter level, bridge arm level, and submodule level hierarchical three-level control to independently adjust the output of each photovoltaic submodule includes steps S21-S22, and specifically as follows:

[0049] Step S21: After generating the reactive current reference instruction according to the photovoltaic grid-connected low-voltage ride-through standard, the power outer loop control and the current inner loop control are used to perform photovoltaic converter level control on the front-stage MMC converter submodule.

[0050] Optionally, as shown in Figure 5 The process of performing photovoltaic converter level control on the front-stage MMC converter submodule includes steps S31-S34, and specifically as follows:

[0051] Step S31: The deviation value of the reference value of the sum of the capacitor voltages of all photovoltaic submodules of the six bridge arms minus the actual value is calculated, and the difference value is converted by PI to obtain the active current reference instruction.

[0052] Step S32: The active current reference instruction and the reactive current reference instruction are limited to obtain the active current control instruction and the reactive current control instruction.

[0053] Specifically, the control of the front-end MMC converter submodule includes a power outer loop control module and a current inner loop control module. The power outer loop control module includes an active power outer loop and a reactive power outer loop. The active power outer loop uses DC voltage control to maintain the stability of the converter submodule capacitor voltage and to transfer the active power generated by the photovoltaic system to the AC grid. The reactive power outer loop can use constant reactive power control or AC voltage control to exchange reactive power with the grid side and support the stability of the AC system voltage at the grid connection point.

[0054] Step S33: Input the active current control command and reactive current control command into the inner current loop to obtain the bridge arm modulation wave of the front-end MMC converter submodule.

[0055] Step S34: Based on the bridge arm modulation wave, the switching sequence of the front-end MMC converter submodule is obtained using a preset modulation method.

[0056] Specifically, the inner current loop control is used to generate the bridge arm modulation wave for controlling the switching of the upstream MMC converter submodule. During fault ride-through, the upstream MMC converter submodule will remain connected to the grid and will provide reactive current support to the grid-side AC system.

[0057] For example, such as Figure 6 As shown, the active current command i is calculated by the DC voltage control loop in the power outer loop control module of the MMC converter (actually a photovoltaic grid-connected inverter). d_ref The reactive power control loop calculates the reactive current command i. q_ref After being limited, it serves as the control command for the inner current loop control module. and In this stage, due to the limited active power transmission capacity of the MMC converter during grid voltage dips during faults, it is necessary to maximize its advantages as a voltage source converter, prioritizing reactive power generation to provide reactive current support to the grid-side AC system. Simultaneously, while ensuring the output current does not exceed the current withstand capability of the photovoltaic converter, the active current component is then output to effectively limit the power transmitted from the grid-connected inverter system to the AC grid. For example... Figure 7 The figure shown is a simulation waveform diagram of reactive current support of the grid-side AC system during fault ride-through, obtained by using the fault ride-through method of this embodiment of the invention.

[0058] The characteristic of photovoltaic converter-level control lies in calculating the reference value u, which is the sum of the capacitor voltages of all photovoltaic submodules in the six arms of the MMC converter. sum_ref Subtract the actual value u sum The deviation value is converted into the active current command i after PI conversion. d_ref, to realize the inverter stage photovoltaic output control. Wherein, the reference value u sum_ref Generally set to 1.0 p.u (unit value).

[0059] Step S22: based on the MPPT control link, the photovoltaic array output power reference value is obtained; after the photovoltaic array output power reference value is corrected by the bridge arm stage control and the sub-module stage control, the duty cycle of the post-stage DC-DC converter is obtained by power control.

[0060] Optionally, as Figure 8 The process of obtaining the duty cycle of the post-stage DC-DC converter includes steps S41-S44, and specifically as follows:

[0061] Step S41: based on the photovoltaic array output voltage measurement value and the output current measurement value, the photovoltaic array output voltage reference value is obtained by using the MPPT controller.

[0062] Exemplarily, the embodiment of the application obtains the duty cycle of the post-stage DC-DC converter based on the control block diagram shown in Figure 9 The control of the post-stage DC-DC converter includes a photovoltaic module voltage controller and a power controller. The photovoltaic module voltage controller includes an MPPT controller and a voltage controller, the MPPT controller is used to obtain a photovoltaic array output voltage instruction meeting the maximum power point tracking control, and the voltage controller is used to obtain a photovoltaic array output power instruction according to the photovoltaic array output voltage instruction. The photovoltaic module power controller is used to generate a duty cycle instruction of an IGBT device switching signal for controlling the post-stage DC-DC converter. During the fault ride-through, the post-stage DC-DC converter will limit the active power input by the photovoltaic array to the grid-connected inverter system, maintain the photovoltaic sub-module capacitor voltage stable, and avoid the protection of the sub-module due to overvoltage.

[0063] Specifically, Figure 9 The photovoltaic array output voltage measurement value U pv and the output current measurement value I pv are obtained as the input signals of the MPPT controller, and then the photovoltaic array output voltage reference value U pvref is calculated by the MPPT controller.

[0064] Step S42: the photovoltaic array output voltage reference value is subtracted from the photovoltaic array output voltage measurement value, and the difference value is converted by PI to obtain the photovoltaic array output power reference value.

[0065] Specifically, Figure 9 The photovoltaic array output voltage reference value U pvref is calculated by subtracting the measurement value U pvThe deviation value. After PI conversion, the reference value P of the photovoltaic array output power is obtained. pvref0 .

[0066] Step S43: Correct the photovoltaic array output power reference value using the output power correction value of a single photovoltaic submodule output from the bridge arm level control and submodule level control.

[0067] Optionally, the process of obtaining the output power correction value of a single photovoltaic submodule by the bridge arm level control includes: (1) determining whether the actual value of the sum of the capacitor voltages of all photovoltaic submodules in a single bridge arm is greater than the set threshold value; (2) when the actual value of the sum of the capacitor voltages of all photovoltaic submodules in a single bridge arm is greater than the set threshold value, subtracting the deviation value of the reference value from the actual value of the sum of the capacitor voltages of all photovoltaic submodules in a single bridge arm, and after PI conversion and multiplying by the power distribution ratio coefficient, obtaining the output power correction value of each photovoltaic submodule in that bridge arm.

[0068] For example, with Figure 9 Taking the control block diagram shown as an example, the bridge arm level control process is as follows: Determine the actual value U of the sum of the capacitor voltages of all photovoltaic submodules in a single bridge arm of the MMC converter. FBsum Is it greater than the set threshold value U? FBsum_lim If so, calculate the actual value U of the sum of the capacitor voltages of all photovoltaic submodules in a single arm of the MMC converter. FBsum Subtract the reference value U FBsum_ref The deviation value, after PI conversion and multiplication by the power allocation ratio coefficient K, yields the output power correction value ΔP for each photovoltaic submodule of that bridge arm. pv1 If not, then the output power correction value ΔP of the photovoltaic submodule in that bridge arm is... pv1 It is zero. Here, the threshold value U is the sum of the capacitor voltages of all photovoltaic submodules in a single arm of the MMC converter. FBsum_lim The setting is based on the specific operating conditions of the photovoltaic DC grid-connected system. The reference value U is the sum of the capacitor voltages of all photovoltaic submodules in a single arm of the MMC converter. FBsum_ref It is generally set to 1.0 pu (per unit).

[0069] The characteristic of arm-level control is that it calculates the actual value U of the sum of the capacitor voltages of all photovoltaic submodules contained in a single arm of the MMC converter. FBsum Subtract the reference value U FBsum_ref The deviation value is converted by PI and multiplied by the power allocation ratio coefficient K to obtain the output power correction value ΔP for each photovoltaic module in a single bridge arm. pv2 To achieve control of photovoltaic output at the bridge arm level.

[0070] Optionally, the process of obtaining the output power correction value of a single photovoltaic submodule at the submodule level includes: (1) dividing the actual value of the sum of the capacitor voltages of all photovoltaic submodules in a single bridge arm by the number of photovoltaic submodules in that bridge arm to obtain the average value of the capacitor voltages of the photovoltaic submodules in that bridge arm; (2) determining whether the measured value of the capacitor voltage of each photovoltaic submodule in that bridge arm is greater than the average value; (3) when the measured value of the capacitor voltage of the photovoltaic submodule is greater than the average value, subtracting the deviation value of the average value from the measured value of the capacitor voltage of the photovoltaic submodule, and then multiplying it by the power correction ratio coefficient after PI conversion to obtain the output power correction value of a single photovoltaic submodule in that bridge arm.

[0071] For example, with Figure 9 Taking the control block diagram shown as an example, the sub-module level control process is as follows: Divide the actual value of the sum of the capacitor voltages of all photovoltaic sub-modules in a single arm of the MMC converter by the number of photovoltaic sub-modules in that arm to obtain the average capacitor voltage E of the photovoltaic modules in that arm. sm_avg Determine the measured capacitor voltage E of each photovoltaic submodule in this bridge arm. sm Is it greater than the average value E? sm_avg If so, then calculate the measured value E of the photovoltaic submodule capacitor voltage. sm Subtract the average value E sm_avg The deviation value, after PI conversion and multiplication by the power correction ratio K, yields the output power correction value ΔP for a single photovoltaic submodule of that bridge arm. pv2 If not, then the photovoltaic array output power correction value ΔP pv2 It is zero.

[0072] The characteristic of submodule-level control is that it calculates the measured value E of the capacitor voltage of a single photovoltaic submodule in the MMC converter. sm Subtract the average value E sm_avg The deviation value, after PI conversion and multiplication by the power correction ratio K, yields the photovoltaic submodule output power correction value ΔP. pv1 This enables submodule-level photovoltaic output control.

[0073] Step S44: The difference between the corrected photovoltaic array output power reference value and the actual photovoltaic array output power value is calculated. The difference is then converted by PI to obtain the duty cycle of the subsequent DC-DC converter, so as to adjust the output power of the subsequent DC-DC converter.

[0074] In summary, when a power grid fault occurs, based on Figure 9 The control block diagram shown illustrates that during fault ride-through, the DC / DC converter limits the active power input from the photovoltaic array to the grid-connected inverter system, maintaining stable capacitor voltage in the photovoltaic submodules and preventing them from being protected against overvoltage. Specifically, this includes the following steps:

[0075] (1): Obtain the measured value U of the photovoltaic array output voltage. pvand output current measurement value I pv The input signal to the MPPT controller is used to calculate the reference value U of the photovoltaic array output voltage. pvref .

[0076] (2): such as Figure 4 As shown, calculate the reference value U of the photovoltaic array output voltage. pvref Subtract the measured value U pv The deviation value. After PI conversion, the reference value P of the photovoltaic array output power is obtained. pvref0 .

[0077] (3): The photovoltaic array output power correction value ΔP obtained from the converter bridge arm stage control in S3. pv1 The photovoltaic array output power correction value ΔP obtained from the photovoltaic module-level control in S4 pv2 The photovoltaic array output power reference value P obtained by superimposing the values ​​in (2) together pvref0 Generate the corrected photovoltaic array output power reference value P pvref .

[0078] (4): Obtain the measured value U of the DC voltage on the DC bus at the output side of the DC / DC converter. pv_out and the DC current measurement value I of the DC bus on the output side of the DC / DC converter pv_out Multiplying the two values ​​yields the actual output power P of the photovoltaic array. dc .

[0079] (5): Calculate the reference value P of the photovoltaic array output power obtained from T3. pvref Subtracting (4) from the actual output power P of the photovoltaic array dc The deviation value. After PI conversion, the duty cycle command D of the DC / DC converter is obtained. pv The resulting duty cycle instruction D pv By modulating the switching signals of the IGBT devices in the DC / DC converter, the on / off state of the IGBT devices is controlled, thereby stabilizing the capacitor voltage of the photovoltaic submodule and effectively limiting the power input from the photovoltaic array to the grid-connected inverter system.

[0080] like Figure 10 The figure shown is a typical fault ride-through simulation waveform obtained by using the novel AC fault ride-through method for a photovoltaic DC grid-connected system based on a modular multilevel photovoltaic converter proposed in this embodiment of the invention.

[0081] Example 3

[0082] This invention provides an AC fault ride-through device for a photovoltaic grid-connected system, such as... Figure 11 As shown, it includes:

[0083] The judgment module is used to determine whether an AC fault has occurred in the photovoltaic grid-connected system; this module executes the method described in step S11 of embodiment 1, which will not be repeated here.

[0084] The three-level control module is used to combine the hierarchical three-level control of the photovoltaic converter level, bridge arm level and sub-module level when an AC fault occurs in the photovoltaic grid-connected system, so as to independently adjust the output of each photovoltaic sub-module; this module executes the method described in step S12 of embodiment 2, which will not be repeated here.

[0085] Example 4

[0086] This invention provides a computer device, such as... Figure 12 As shown, the system includes: at least one processor 401, such as a CPU (Central Processing Unit), at least one communication interface 403, a memory 404, and at least one communication bus 402. The communication bus 402 is used to enable communication between these components. The communication interface 403 may include a display screen or a keyboard; optionally, the communication interface 403 may also include a standard wired interface or a wireless interface. The memory 404 may be high-speed RAM (Random Access Memory) or non-volatile memory, such as at least one disk storage device. Optionally, the memory 404 may also be at least one storage device located remotely from the aforementioned processor 401. The processor 401 can execute the AC fault ride-through method of the photovoltaic grid-connected system of Embodiment 2. The memory 404 stores a set of program code, and the processor 401 calls the program code stored in the memory 404 to execute the AC fault ride-through method of the photovoltaic grid-connected system of Embodiment 2.

[0087] The communication bus 402 can be a peripheral component interconnect (PCI) bus or an extended industry standard architecture (EISA) bus, etc. The communication bus 402 can be divided into an address bus, a data bus, and a control bus, etc. For ease of representation, Figure 12 The symbol is represented by only one line, but this does not mean that there is only one bus or one type of bus.

[0088] The memory 404 can include a volatile memory, such as a random-access memory (RAM), and / or can include a non-volatile memory, such as a flash memory, a hard disk drive (HDD) or a solid-state drive (SSD). The memory 404 can also include a combination of the above-mentioned types of memories.

[0089] The processor 401 can be a central processing unit (CPU), a network processor (NP), or a combination of CPU and NP.

[0090] The processor 401 can further include a hardware chip. The hardware chip can be an application-specific integrated circuit (ASIC), a programmable logic device (PLD) or a combination thereof. The PLD can be a complex programmable logic device (CPLD), a field-programmable gate array (FPGA), a generic array logic (GAL) or any combination thereof.

[0091] Optionally, the memory 404 is further configured to store program instructions. The processor 401 can invoke the program instructions to implement the AC fault ride-through method of the grid-connected photovoltaic system as described in Embodiment 2.

[0092] The embodiment of the present application further provides a computer readable storage medium, and the computer readable storage medium stores computer executable instructions. The computer executable instructions can execute the AC fault ride-through method of the photovoltaic grid-connected system in the embodiment 2. The storage medium can be a disk, an optical disk, a read-only memory (ROM), a random access memory (RAM), a flash memory, a hard disk drive (HDD) or a solid-state drive (SSD), etc. The storage medium can also include a combination of the above-mentioned storage devices.

[0093] Obviously, the above-mentioned embodiments are only examples for clearly illustrating the present application, and are not intended to limit the embodiments. Based on the above-mentioned description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments cannot be enumerated, and the obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A photovoltaic grid-connected system, characterized by, The application relates to a photovoltaic grid-connected system, which comprises a plurality of photovoltaic arrays, a plurality of photovoltaic grid-connected inverters, a direct-current collection line and a step-up transformer. The direct-current side of each photovoltaic grid-connected inverter is connected with the plurality of photovoltaic arrays through the direct-current collection line, and the alternating-current side of each photovoltaic grid-connected inverter is connected with an alternating-current system through the step-up transformer. Each photovoltaic grid-connected inverter is composed of six bridge arms, wherein each bridge arm comprises a plurality of photovoltaic sub-modules; the direct-current side of each photovoltaic sub-module is connected with a photovoltaic array, and the alternating-current side of each photovoltaic sub-module is connected with an alternating-current system through the step-up transformer. Each photovoltaic sub-module comprises a front-stage MMC converter sub-module and a rear-stage DC-DC converter; the low-voltage side of the rear-stage DC-DC converter is connected with a photovoltaic array, the high-voltage side of the rear-stage DC-DC converter is connected with the direct-current side of the front-stage MMC converter sub-module, and the alternating-current side of the front-stage MMC converter sub-module is connected with an alternating-current system through the step-up transformer. When the photovoltaic grid-connected system occurs alternating-current fault, the photovoltaic converter stage, the bridge arm stage and the sub-module stage are combined to independently adjust the output of each photovoltaic sub-module. The process of combining the photovoltaic converter stage, the bridge arm stage and the sub-module stage to independently adjust the output of each photovoltaic sub-module comprises the following steps: after reactive current reference instructions are generated according to photovoltaic grid-connected low-voltage ride-through standards, power outer loop control and current inner loop control are utilized to perform photovoltaic converter stage control on the front-stage MMC converter sub-module; photovoltaic array output power reference values are obtained based on an MPPT control link; after the photovoltaic array output power reference values are corrected by utilizing bridge arm stage control and sub-module stage control, duty cycles of the rear-stage DC-DC converter are obtained by utilizing power control. The process of obtaining the duty cycles of the rear-stage DC-DC converter comprises the following steps: photovoltaic array output voltage reference values are obtained by utilizing an MPPT controller based on photovoltaic array output voltage measurement values and output current measurement values; the photovoltaic array output voltage reference values are subtracted from the photovoltaic array output voltage measurement values, and the difference is converted by PI to obtain photovoltaic array output power reference values; the photovoltaic array output power reference values are corrected by utilizing single photovoltaic sub-module output power correction values output by the bridge arm stage control and the sub-module stage control; the corrected photovoltaic array output power reference values are subtracted from actual photovoltaic array output power values, and the difference is converted by PI to obtain the duty cycles of the rear-stage DC-DC converter, so as to adjust the output of the rear-stage DC-DC converter. The process of obtaining single photovoltaic sub-module output power correction values by the bridge arm stage control comprises the following steps: whether the actual value of the sum of the capacitor voltages of all photovoltaic sub-modules of a single bridge arm is greater than a set threshold value is judged; when the actual value of the sum of the capacitor voltages of all photovoltaic sub-modules of a single bridge arm is greater than a set threshold value, the actual value of the sum of the capacitor voltages of all photovoltaic sub-modules of the single bridge arm is subtracted from the deviation value of the reference value, and the result is converted by PI and multiplied by a power distribution proportionality coefficient to obtain the output power correction value of each photovoltaic sub-module of the bridge arm. ​ The process of obtaining the single photovoltaic submodule output power correction value by the submodule level control comprises: dividing the actual value of the sum of the capacitor voltages of all photovoltaic submodules of a single bridge arm by the number of photovoltaic submodules of the bridge arm to obtain the average value of the capacitor voltages of the photovoltaic submodules of the bridge arm; judging whether the measured value of the capacitor voltage of each photovoltaic submodule of the bridge arm is greater than the average value; when the measured value of the capacitor voltage of the photovoltaic submodule is greater than the average value, subtracting the deviation value of the measured value of the capacitor voltage of the photovoltaic submodule from the average value, and then performing PI conversion and multiplying by a power correction proportionality coefficient to obtain the single photovoltaic submodule output power correction value of the bridge arm.

2. An AC fault ride-through method for a photovoltaic grid-connected system, characterized in that, The photovoltaic grid-connected system of claim 1, the method comprising: judging whether the photovoltaic grid-connected system has an AC fault; when the photovoltaic grid-connected system has an AC fault, combining the photovoltaic converter level, the bridge arm level and the submodule level three-layer control to independently adjust the output of each photovoltaic submodule; the process of combining the photovoltaic converter level, the bridge arm level and the submodule level three-layer control to independently adjust the output of each photovoltaic submodule, comprising: after generating a reactive current reference instruction according to the photovoltaic grid-connected low-voltage ride-through standard, using power outer loop control and current inner loop control to perform photovoltaic converter level control on the front-stage MMC converter submodule; obtaining a photovoltaic array output power reference value based on an MPPT control link; after correcting the photovoltaic array output power reference value using the bridge arm level control and the submodule level control, using power control to obtain the duty cycle of the rear-stage DC-DC converter; the process of obtaining the duty cycle of the rear-stage DC-DC converter, comprising: obtaining a photovoltaic array output voltage reference value using an MPPT controller based on the photovoltaic array output voltage measurement value and the output current measurement value; subtracting the photovoltaic array output voltage reference value from the photovoltaic array output voltage measurement value, and then performing PI conversion on the difference to obtain the photovoltaic array output power reference value; correcting the photovoltaic array output power reference value using the single photovoltaic submodule output power correction value output by the bridge arm level control and the submodule level control; subtracting the corrected photovoltaic array output power reference value from the actual value of the photovoltaic array output power, and then performing PI conversion on the difference to obtain the duty cycle of the rear-stage DC-DC converter to adjust the output of the rear-stage DC-DC converter; the process of obtaining the single photovoltaic submodule output power correction value by the bridge arm level control, comprising: judging whether the actual value of the sum of the capacitor voltages of all photovoltaic submodules of a single bridge arm is greater than a set threshold value; when the actual value of the sum of the capacitor voltages of all photovoltaic submodules of a single bridge arm is greater than a set threshold value, subtracting the deviation value of the reference value from the actual value of the sum of the capacitor voltages of all photovoltaic submodules of a single bridge arm, and then performing PI conversion and multiplying by a power distribution proportionality coefficient to obtain the output power correction value of each photovoltaic submodule of the bridge arm; The process of obtaining the single photovoltaic submodule output power correction value by the submodule level control comprises: dividing the actual value of the sum of the capacitance voltages of all photovoltaic submodules of a single bridge arm by the number of photovoltaic submodules of the bridge arm to obtain the average value of the photovoltaic submodule capacitance voltages of the bridge arm; judging whether the measured value of the photovoltaic submodule capacitance voltage of each photovoltaic submodule of the bridge arm is greater than the average value; when the measured value of the photovoltaic submodule capacitance voltage is greater than the average value, subtracting the deviation value of the measured value of the photovoltaic submodule capacitance voltage from the average value, and then performing PI conversion and multiplying by a power correction proportionality coefficient to obtain the single photovoltaic submodule output power correction value of the bridge arm.

3. The AC fault ride-through method of a photovoltaic grid-connected system according to claim 2, wherein, The process of performing photovoltaic converter level control on the front-stage MMC converter submodule comprises: calculating the deviation value of the reference value of the sum of the capacitance voltages of all photovoltaic submodules of the six bridge arms from the actual value, and obtaining the active current reference instruction by performing PI conversion on the difference value; limiting the active current reference instruction and the reactive current reference instruction to obtain the active current control instruction and the reactive current control instruction; inputting the active current control instruction and the reactive current control instruction into the current inner loop to obtain the bridge arm modulation wave of the front-stage MMC converter submodule; obtaining the switching sequence of the front-stage MMC converter submodule based on the bridge arm modulation wave and by using a preset modulation method.

4. An AC fault ride-through device for a photovoltaic grid-connected system, characterized by, comprises: a judging module configured to judge whether an AC fault occurs in the photovoltaic grid-connected system; a three-level control module configured to, when the AC fault occurs in the photovoltaic grid-connected system, combine photovoltaic converter level control, bridge arm level control and submodule level control to independently adjust the output of each photovoltaic submodule; The process of combining photovoltaic converter level control, bridge arm level control and submodule level control to independently adjust the output of each photovoltaic submodule comprises: after generating the reactive current reference instruction according to the photovoltaic grid-connected low-voltage ride-through standard, performing photovoltaic converter level control on the front-stage MMC converter submodule by using power outer loop control and current inner loop control; obtaining the photovoltaic array output power reference value based on the MPPT control link; after correcting the photovoltaic array output power reference value by using bridge arm level control and submodule level control, obtaining the duty cycle of the rear-stage DC-DC converter by using power control; The process of obtaining the duty cycle of the rear-stage DC-DC converter comprises: obtaining the photovoltaic array output voltage reference value by using the MPPT controller based on the photovoltaic array output voltage measurement value and the output current measurement value; obtaining the photovoltaic array output power reference value by performing PI conversion on the difference between the photovoltaic array output voltage reference value and the photovoltaic array output voltage measurement value; correcting the photovoltaic array output power reference value by using the single photovoltaic submodule output power correction value output by the bridge arm level control and the submodule level control; obtaining the duty cycle of the rear-stage DC-DC converter by performing PI conversion on the difference between the corrected photovoltaic array output power reference value and the actual value of the photovoltaic array output power, so as to adjust the output of the rear-stage DC-DC converter; The process of obtaining the output power correction value of the single photovoltaic submodule by the bridge arm level control includes: judging whether the actual value of the sum of the capacitor voltages of all photovoltaic submodules of a single bridge arm is greater than a set threshold value; when the actual value of the sum of the capacitor voltages of all photovoltaic submodules of the single bridge arm is greater than the set threshold value, subtracting the deviation value of the actual value of the sum of the capacitor voltages of all photovoltaic submodules of the single bridge arm from a reference value, and then obtaining the output power correction value of each photovoltaic submodule of the bridge arm through PI conversion and multiplication by a power distribution proportionality coefficient; The process of obtaining the output power correction value of the single photovoltaic submodule by the submodule level control includes: dividing the actual value of the sum of the capacitor voltages of all photovoltaic submodules of a single bridge arm by the number of photovoltaic submodules of the bridge arm to obtain the average value of the capacitor voltages of the photovoltaic submodules of the bridge arm; judging whether the measured value of the capacitor voltage of each photovoltaic submodule of the bridge arm is greater than the average value; when the measured value of the capacitor voltage of the photovoltaic submodule is greater than the average value, subtracting the deviation value of the measured value of the capacitor voltage of the photovoltaic submodule from the average value, and then obtaining the output power correction value of the single photovoltaic submodule of the bridge arm through PI conversion and multiplication by a power correction proportionality coefficient.

5. A computer device, comprising: Comprise: At least one processor and a memory connected in communication with the at least one processor, wherein the memory stores instructions executable by the at least one processor, and the instructions are executed by the at least one processor to enable the at least one processor to perform the AC fault ride-through method of the photovoltaic grid-connected system according to any one of claims 2-3.

6. A computer-readable storage medium, characterized in that, The computer readable storage medium stores computer instructions for enabling the computer to perform the AC fault ride-through method of the photovoltaic grid-connected system according to any one of claims 2-3.

Images