Active thermal control method and system for photovoltaic converter during low voltage ride-through

Abstract

The present invention discloses an active thermal control method and system for a photovoltaic converter during low voltage ride-through, which determines whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid; and when the low voltage fault occurs in the power grid, performs low voltage temperature control on the power grid based on an active thermal control strategy. At this time, a virtual impedance current limiting control strategy and an adaptive power adjustment strategy are used to reduce the maximum junction temperature of the insulated gate bipolar transistor inside the converter to a large extent, and to suppress its maximum temperature cycle to a certain extent; at the same time, by combining an adaptive switching frequency adjustment strategy, the temperature cycle of the insulated gate bipolar transistor inside the converter is further suppressed, thereby reducing the maximum temperature cycle and the maximum junction temperature of the insulated gate bipolar transistor to a large extent, thereby greatly reducing the short-term overstress of the photovoltaic converter during low voltage ride-through, and improving the reliability and life of the insulated gate bipolar transistor inside the converter.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic converters, and in particular to an active thermal control method and system for photovoltaic converters during low voltage ride-through. Background Art

[0002] With the widespread application of renewable energy grid-connected power generation technology, the traditional power system dominated by synchronous machines is gradually transforming into a power electronics system dominated by converters. Since the photovoltaic converter based on the virtual synchronous control strategy has an effective synchronization mechanism with the power grid and can achieve self-synchronization and grid connection through its own internal adjustment, it is widely involved in the grid-connected power generation and energy consumption of renewable energy.

[0003] In photovoltaic converters, core power devices such as insulated gate bipolar transistors are often the most vulnerable components, and their reliability and life have received widespread attention in the industry. In order to improve the reliability and life of power semiconductor devices such as insulated gate bipolar transistors, a variety of active thermal control methods are used to adjust the electrothermal stress of internal insulated gate bipolar transistors. Generally speaking, the electrothermal stress of insulated gate bipolar transistors inside power electronic equipment can be divided into short-term overstress and long-term wear stress. Most active thermal control methods focus on wear stress on a long-term time scale. However, for photovoltaic converters during low voltage ride-through, the impact of overstress on a short-term time scale is also crucial. At this time, the short-term overstress of photovoltaic converters during low voltage ride-through is too large, which affects the reliability and life of the insulated gate bipolar transistors inside the converter. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art. The present invention provides a method and system for active thermal control of a photovoltaic converter during low voltage ride-through, which determines whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid; and when the low voltage fault occurs in the power grid, the low voltage temperature control of the power grid is performed based on an active thermal control strategy. At this time, the maximum junction temperature of the insulated gate bipolar transistor inside the converter is reduced to a large extent through a virtual impedance current limiting control strategy and an adaptive power adjustment strategy, and its maximum temperature cycle is suppressed to a certain extent; at the same time, by combining an adaptive switching frequency adjustment strategy, the temperature cycle of the insulated gate bipolar transistor inside the converter is further suppressed, thereby greatly reducing the maximum temperature cycle and the maximum junction temperature of the insulated gate bipolar transistor, thereby greatly reducing the short-term overstress of the photovoltaic converter during low voltage ride-through, and improving the reliability and life of the insulated gate bipolar transistor inside the converter.

[0005] In order to solve the above technical problems, an embodiment of the present invention provides a method for active thermal control of a photovoltaic converter during low voltage ride-through, comprising:

[0006] When the grid-side voltage is normal, a virtual synchronization control strategy is implemented based on the photovoltaic converter to ensure continuous and stable grid connection;

[0007] Phase-locking the power grid based on a phase-locked loop to obtain the voltage amplitude of the power grid in real time;

[0008] Determining whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid;

[0009] And when the low-voltage fault occurs in the power grid, the low-voltage temperature control of the power grid is performed based on the active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy. At the same time, the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low-voltage ride-through is reduced based on the adaptive reference power adjustment strategy.

[0010] Optionally, when the grid-side voltage is normal, executing a virtual synchronization control strategy based on the photovoltaic converter to ensure continuous and stable grid connection includes:

[0011] When the grid-side voltage is normal, collecting the photoelectric converter;

[0012] and executing a virtual synchronization control strategy based on the photovoltaic converter to ensure continuous and stable grid connection;

[0013] The expression of the virtual synchronization control strategy is as follows:

[0014] ;

[0015] ;

[0016] in, J is the moment of inertia of the converter, D p and D q are the damping coefficients of the converter, ω out and V out is the angular frequency and internal potential output by the converter in real time, ω 0 and V 0 is the angular frequency and internal potential of the given reference of the converter, P ref and Q ref The active and reactive power reference values ​​are given to the converter inputs. P out and Q outOutput active and reactive power to the converter in real time.

[0017] Optionally, the phase-locking the power grid based on a phase-locked loop to obtain the voltage amplitude of the power grid in real time includes:

[0018] collecting the power grid;

[0019] Associating a phase locked loop with the power grid;

[0020] And the power grid is phase-locked according to the phase-locked loop, wherein the power grid voltage is abc-dq transformed to realize the output of dq-axis power grid voltage, and the phase-locked loop phase-locks the q-axis power grid voltage obtained in real time through PI control to ensure that the q-axis power grid voltage is 0, so as to obtain the voltage amplitude of the power grid in real time.

[0021] Optionally, judging whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid includes:

[0022] Detecting changes in the voltage amplitude of the power grid in real time;

[0023] And the voltage amplitude of the power grid is compared with a preset voltage amplitude. If the voltage amplitude of the power grid is lower than the preset voltage amplitude, a low voltage fault occurs in the power grid; or, if the voltage amplitude of the power grid is higher than the preset voltage amplitude, the power grid is in a normal state.

[0024] Optionally, judging whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid further includes:

[0025] ;

[0026] in, v th It is the lower limit of the voltage threshold that the grid voltage is allowed to fluctuate under normal circumstances and serves as the preset voltage amplitude; v g is the grid voltage amplitude; v gd and v gq are the grid voltages in the dq coordinate system respectively.

[0027] Optionally, when the low voltage fault occurs in the power grid, low voltage temperature control is performed on the power grid based on an active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride-through is reduced based on the adaptive reference power adjustment strategy, including:

[0028] Monitoring the low voltage fault of the power grid and determining the low voltage ride through period of the photovoltaic converter;

[0029] During the low voltage ride-through of the photovoltaic converter, the grid voltage is lower than a normal voltage level;

[0030] When a low-voltage fault occurs in the power grid, low-voltage temperature control is performed on the power grid based on an active thermal control strategy;

[0031] Collecting the real-time three-phase current output by the photoelectric converter, and constraining the temperature cycle caused by the current change during the low voltage ride-through period based on the three-phase current and adaptively changing the switching frequency control strategy;

[0032] And performing initial current limiting processing based on a virtual impedance current limiting control strategy, and reducing the current level during the low voltage ride-through period based on an adaptive reference power adjustment strategy.

[0033] Optionally, when the low voltage fault occurs in the power grid, low voltage temperature control is performed on the power grid based on an active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride through is constrained based on the three-phase current and the adaptive change switching frequency control strategy, and at the same time, initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride through is reduced based on the adaptive reference power adjustment strategy, and also includes:

[0034] During low voltage ride-through, the photovoltaic converter further limits the maximum temperature cycle of the internal insulated gate bipolar transistor based on the adaptive switching frequency adjustment strategy; when the output current of the photovoltaic converter is greater than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be reduced to limit the excessive increase of loss, thereby reducing the further increase of junction temperature; and when the output current of the photovoltaic converter is less than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be increased to increase the loss and increase the heat generation;

[0035] The control expression of the adaptive switching frequency adjustment strategy is as follows:

[0036] ;

[0037] in, f sw1 and f sw2 is the switching frequency at different current levels; k 1 and k 2 is the proportional gain coefficient of the switching frequency; b 1 and b 2 is the adjustment coefficient of the switching frequency; i amp0 is the steady-state output current amplitude of the converter before adding the active thermal control scheme; i amp It is the real-time output current amplitude of the converter.

[0038] Optionally, when the low voltage fault occurs in the power grid, low voltage temperature control is performed on the power grid based on an active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride through is constrained based on the three-phase current and the adaptive change switching frequency control strategy, and at the same time, initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride through is reduced based on the adaptive reference power adjustment strategy, and also includes:

[0039] During the low voltage ride-through period, the photovoltaic converter will perform initial current limiting processing through the virtual impedance control strategy. The expression of the virtual impedance current limiting control strategy is as follows:

[0040] ;

[0041] ;

[0042] in, I th To set the current limiting threshold, k R and k X are the resistance and reactance coefficients of the virtual impedance, R v and X v are virtual resistance and virtual reactance respectively, dV d and dV q are the d-axis and q-axis voltage drops caused by virtual impedance, P 0 and k 0 are the reference active power initial value and power variation coefficient, k 1 and k 2 is the proportional gain coefficient of the switching frequency,b 1 and b 2 is the adjustment coefficient of the switching frequency, f sw1 and f sw2 is the switching frequency at different current levels, i amp0 is the steady-state output current amplitude of the converter before adding the active thermal control scheme.

[0043] Optionally, when the low voltage fault occurs in the power grid, low voltage temperature control is performed on the power grid based on an active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride through is constrained based on the three-phase current and the adaptive change switching frequency control strategy, and at the same time, initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride through is reduced based on the adaptive reference power adjustment strategy, and also includes:

[0044] During the low voltage ride-through period, the photovoltaic converter reduces the set active reference value based on the adaptive reference power adjustment strategy, thereby reducing the current level during the low voltage ride-through period, so as to achieve the purpose of reducing the maximum junction temperature level during the low voltage ride-through period;

[0045] The specific expression of the adaptive reference power adjustment strategy is as follows:

[0046] ;

[0047] in, P 0 is the initial value of reference active power; V 0 is the sinusoidal peak-to-peak value of the rated grid voltage; k 0 is the power variation coefficient.

[0048] In addition, the present invention further provides an active thermal control system for a photovoltaic converter during low voltage ride-through, the active thermal control system for a photovoltaic converter during low voltage ride-through comprising:

[0049] The grid-connected module is used to execute a virtual synchronization control strategy based on the photovoltaic converter when the grid-side voltage is normal to ensure continuous and stable grid connection;

[0050] A phase-locked module is used to phase-lock the power grid based on a phase-locked loop to obtain the voltage amplitude of the power grid in real time;

[0051] A fault detection module is used to determine whether a low voltage fault occurs in the power grid based on the change of the voltage amplitude of the power grid;

[0052] And a control module, which is used to perform low-voltage temperature control on the power grid based on an active thermal control strategy when a low-voltage fault occurs in the power grid. The real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy. At the same time, the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low-voltage ride-through is reduced based on the adaptive reference power adjustment strategy.

[0053] In an embodiment of the present invention, through the method in the embodiment of the present invention, when the grid-side voltage is normal, a virtual synchronization control strategy is executed based on the photovoltaic converter to ensure continuous and stable grid connection; the grid is phase-locked based on a phase-locked loop to obtain the voltage amplitude of the grid in real time; based on the change of the voltage amplitude of the grid, it is determined whether a low-voltage fault occurs in the grid; and when the low-voltage fault occurs in the grid, the grid is subjected to low-voltage temperature control based on an active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, based on the virtual impedance current limiting control The control strategy is used to perform initial current limiting processing, and the current level during the low voltage ride-through period is reduced based on the adaptive reference power adjustment strategy. At this time, the maximum junction temperature of the insulated gate bipolar transistor inside the converter is reduced to a large extent through the virtual impedance current limiting control strategy and the adaptive power adjustment strategy, and its maximum temperature cycle is suppressed to a certain extent; at the same time, by combining the adaptive switching frequency adjustment strategy, the temperature cycle of the insulated gate bipolar transistor inside the converter is further suppressed, thereby greatly reducing the maximum temperature cycle and the maximum junction temperature of the insulated gate bipolar transistor, thereby greatly reducing the short-term overstress of the photovoltaic converter during the low voltage ride-through period, and improving the reliability and life of the insulated gate bipolar transistor inside the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0055] Figure 1 is a schematic flow chart of an active thermal control method for a photovoltaic converter during low voltage ride-through in an embodiment of the present invention;

[0056] Figure 2 is a flow chart of S11 in the method for active thermal control of a photovoltaic converter during low voltage ride-through in an embodiment of the present invention;

[0057] Figure 3 is a flow chart of S12 in the method for active thermal control of a photovoltaic converter during low voltage ride-through in an embodiment of the present invention;

[0058] Figure 4 is a flow chart of S13 in the method for active thermal control of a photovoltaic converter during low voltage ride-through in an embodiment of the present invention;

[0059] Figure 5 is a flow chart of S14 in the method for active thermal control of a photovoltaic converter during low voltage ride-through in an embodiment of the present invention;

[0060] Figure 6 is a structural block diagram of a photovoltaic converter based on virtual synchronous control in a photovoltaic converter active thermal control method during low voltage ride-through in an embodiment of the present invention;

[0061] Figure 7 is a schematic diagram of a thermal network model of a power module in an active thermal control method for a photovoltaic converter during a low voltage ride-through period in an embodiment of the present invention;

[0062] Figure 8 1 is a schematic diagram of the principle structure of a phase-locked loop in the active thermal control method for a photovoltaic converter during a low voltage ride-through period in an embodiment of the present invention;

[0063] Fig. 9 is a structural block diagram of an active thermal control scheme in an active thermal control method for a photovoltaic converter during a low voltage ride-through period in an embodiment of the present invention;

[0064] Fig.10 It is a schematic diagram of the junction temperature output waveform of the insulated gate bipolar transistor inside the converter when the active thermal control method is not used in the active thermal control method of the photovoltaic converter during the low voltage ride-through in the embodiment of the present invention;

[0065] Fig.11 It is a schematic diagram of the junction temperature output waveform of the insulated gate bipolar transistor inside the converter using the active thermal control method in the active thermal control method of the photovoltaic converter during the low voltage ride-through period in the embodiment of the present invention;

[0066] Fig.12 Schematic diagram of the structure of the active thermal control system of the photovoltaic converter during the low voltage ride-through period in the embodiment of the present invention;

[0067] Fig.13 The figure is a hardware diagram of an electronic device according to an exemplary embodiment. DETAILED DESCRIPTION

[0068] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0069] Example

[0070] See also Figures 1 to 13 , an active thermal control method for a photovoltaic converter during low voltage ride-through, applied to an active thermal control scenario of a photovoltaic converter during low voltage ride-through; the active thermal control method for a photovoltaic converter during low voltage ride-through includes:

[0071] Step S11: When the grid-side voltage is normal, a virtual synchronization control strategy is executed based on the photovoltaic converter to ensure continuous and stable grid connection;

[0072] Step S12: Phase-locking the power grid based on a phase-locked loop to obtain the voltage amplitude of the power grid in real time;

[0073] Step S13: determining whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid;

[0074] Step S14: When the low-voltage fault occurs in the power grid, the power grid is subjected to low-voltage temperature control based on an active thermal control strategy, wherein the real-time three-phase current output by the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy. At the same time, initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low-voltage ride-through is reduced based on the adaptive reference power adjustment strategy.

[0075] In an embodiment of the present invention, through the method in the embodiment of the present invention, it is determined whether a low-voltage fault occurs in the power grid based on the change of the voltage amplitude of the power grid; and when the low-voltage fault occurs in the power grid, the low-voltage temperature control of the power grid is performed based on the active thermal control strategy. At this time, through the virtual impedance current limiting control strategy and the adaptive power adjustment strategy, the maximum junction temperature of the insulated gate bipolar transistor inside the converter is greatly reduced, and its maximum temperature cycle is suppressed to a certain extent; at the same time, by combining the adaptive switching frequency adjustment strategy, the temperature cycle of the insulated gate bipolar transistor inside the converter is further suppressed, thereby greatly reducing the maximum temperature cycle and the maximum junction temperature of the insulated gate bipolar transistor, thereby greatly reducing the short-term overstress of the photovoltaic converter during low-voltage ride-through, and improving the reliability and life of the insulated gate bipolar transistor inside the converter.

[0076] In step S11, when the grid-side voltage is normal, a virtual synchronization control strategy is executed based on the photovoltaic converter to ensure continuous and stable grid connection;

[0077] In the specific implementation process of the present invention, the specific steps may be:

[0078] S111: When the grid-side voltage is normal, collecting the photoelectric converter;

[0079] S112: Execute a virtual synchronization control strategy based on the photovoltaic converter to ensure continuous and stable grid connection.

[0080] In an embodiment of the present application, when the grid-side voltage is normal, the photovoltaic converter is collected and managed, so that a virtual synchronization control strategy is executed based on the photovoltaic converter to ensure continuous and stable grid connection.

[0081] The expression of the virtual synchronization control strategy is as follows:

[0082] ;

[0083] ;

[0084] in, J is the moment of inertia of the converter, D p and D q are the damping coefficients of the converter, ω out and V out is the angular frequency and internal potential output by the converter in real time, ω 0 and V 0 is the angular frequency and internal potential of the given reference of the converter, P ref and Q ref The active and reactive power reference values ​​are given to the converter inputs. P out and Q out Output active and reactive power to the converter in real time.

[0085] Furthermore, in order to better compare the output junction temperature of the IGBT before and after the grid-side fault, it is necessary to establish an electrothermal model for the IGBT in the photovoltaic converter. The power loss expression of the IGBT and its anti-parallel diode inside the converter is as follows:

[0086] ;

[0087] ;

[0088] in, P c,I and P sw,I are the conduction loss and switching loss of the insulated gate bipolar transistor, P c,D and P sw,D are the conduction loss and switching loss of the anti-parallel diode, i c ′ and v ce are the collector current and collector-emitter voltage of the insulated gate bipolar transistor, i D and v D are the forward current and voltage of the anti-parallel diode, E sw,I and E sw,D are the switching energies of the insulated gate bipolar transistor and the anti-parallel diode, respectively.

[0089] According to the thermoelectric analogy theory, the heat propagation process of the chip inside the power module can be equivalent, and the heat transfer process can be simulated by a thermal equivalent circuit. The obtained chip loss is sent to the established thermal network model, and the junction temperature output value of the insulated gate bipolar transistor and diode chip can be obtained. The specific expression is as follows:

[0090] ;

[0091] ;

[0092] in, T j,I and T j,D and are the junction temperatures of the IGBT and diode respectively, T c and is the case temperature of the power module, R jc and C jc are the thermal resistance from junction to case and the thermal capacitance from junction to case of the insulated gate bipolar transistor, R jc ′ is the thermal resistance from junction to case of the insulated gate bipolar transistor.

[0093] In step S12, the power grid is phase-locked based on a phase-locked loop to obtain the voltage amplitude of the power grid in real time;

[0094] In the specific implementation process of the present invention, the specific steps may be:

[0095] S121: collecting the power grid;

[0096] S122: Associating a phase-locked loop with the power grid;

[0097] S123: The power grid is phase-locked according to the phase-locked loop, wherein the power grid voltage is transformed from abc to dq to realize the output of the dq-axis power grid voltage. The phase-locked loop phase-locks the q-axis power grid voltage obtained in real time through PI control to ensure that the q-axis power grid voltage is 0, and the power grid voltage is transformed from abc to dq to realize the output of the dq-axis power grid voltage, so as to obtain the voltage amplitude of the power grid in real time.

[0098] In an embodiment of the present application, the power grid is collected, and an association is performed between a phase-locked loop and the power grid to facilitate associating the phase-locked loop and the power grid, thereby phase-locking the power grid according to the phase-locked loop, wherein the power grid voltage is abc-dq transformed to achieve the output of the dq-axis power grid voltage, and the phase-locked loop phase-locks the q-axis power grid voltage obtained in real time through PI control to ensure that the q-axis power grid voltage is 0, so as to obtain the voltage amplitude of the power grid in real time.

[0099] In order to better obtain the grid-side voltage amplitude of the photovoltaic converter, a phase-locked loop is used to phase-lock the grid to obtain the grid voltage amplitude in real time.

[0100] The principle structure diagram of the phase-locked loop is as follows: Figure 8 As shown. Among them, v gabc and v gdq They are the obtained abc three-phase real-time grid voltage and the corresponding value in the dq coordinate system, ω g is the actual output angular frequency of the power grid. Generally speaking, the phase-locked loop phase-locks the real-time q-axis power grid voltage through PI control to ensure that the q-axis power grid voltage is 0. At the same time, in order to better perform the phase-locked operation, the three-phase power grid voltage must first be transformed from abc to dq to achieve the output of the dq-axis power grid voltage. The expression is as follows:

[0101] ;

[0102] in, v g0 is the zero-sequence voltage of the power grid.

[0103] In step S13, judging whether a low voltage fault occurs in the power grid based on the change of the voltage amplitude of the power grid;

[0104] In the specific implementation process of the present invention, the specific steps may be:

[0105] S131: Detecting changes in the voltage amplitude of the power grid in real time;

[0106] S132: Compare the voltage amplitude of the power grid with a preset voltage amplitude. If the voltage amplitude of the power grid is lower than the preset voltage amplitude, a low voltage fault occurs in the power grid; or, if the voltage amplitude of the power grid is higher than the preset voltage amplitude, the power grid is in a normal state.

[0107] In an embodiment of the present application, changes in the voltage amplitude of the power grid are detected in real time, and changes in the voltage amplitude of the power grid are monitored so as to compare the voltage amplitude of the power grid with a preset voltage amplitude. If the voltage amplitude of the power grid is lower than the preset voltage amplitude, a low-voltage fault occurs in the power grid; or, if the voltage amplitude of the power grid is higher than the preset voltage amplitude, the power grid is in a normal state.

[0108] After the grid voltage is acquired in real time through the phase-locked loop, it is necessary to detect the grid voltage change in real time to determine whether a low voltage fault occurs in the grid. The determination of whether a low voltage fault occurs in the grid based on the change of the voltage amplitude of the grid also includes:

[0109] ;

[0110] in, v th It is the lower limit of the voltage threshold that the grid voltage is allowed to fluctuate under normal circumstances and serves as the preset voltage amplitude; v g is the grid voltage amplitude; v gd and v gq are the grid voltages in the dq coordinate system respectively.

[0111] Collect the real-time output three-phase current of the photoelectric converter and perform abc-dq transformation. The expression is as follows:

[0112] ;

[0113] in, i 0 is the zero-sequence current of the converter. According to the relationship between the three-phase power system and the dq rotating coordinate system, the amplitude of the real-time output current of the converter can be obtained: i amp , its transformation expression is as follows:

[0114] ;

[0115] The three-phase current amplitudes obtained here in real time will be used for subsequent adaptive changes in the switching frequency control strategy, thereby further constraining the temperature cycle caused by the large current changes during the low voltage ride-through period.

[0116] S14: When the low voltage fault occurs in the power grid, low voltage temperature control is performed on the power grid based on an active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride-through is reduced based on the adaptive reference power adjustment strategy;

[0117] In the specific implementation process of the present invention, the specific steps may be:

[0118] S141: monitoring the low voltage fault of the power grid and determining a low voltage ride-through period of the photovoltaic converter;

[0119] S142: During the low voltage ride-through of the photovoltaic converter, the grid voltage is lower than a normal voltage level;

[0120] S143: When a low voltage fault occurs in the power grid, performing low voltage temperature control on the power grid based on an active thermal control strategy;

[0121] S144: collecting the real-time output three-phase current of the photovoltaic converter, and constraining the temperature cycle caused by the current change during the low voltage ride-through period based on the three-phase current and adaptively changing the switching frequency control strategy;

[0122] S145: Performing initial current limiting processing based on a virtual impedance current limiting control strategy, and reducing the current level during the low voltage ride-through period based on an adaptive reference power adjustment strategy.

[0123] In the embodiment of the present application, during the low voltage ride-through period of the photovoltaic converter, the grid voltage will be lower than the normal voltage level. Since the internal potential of the converter output does not change instantly, a certain potential difference will be formed between it and the grid voltage, and a certain fault impact current will be generated on the line.

[0124] At this time, the low voltage fault of the power grid is monitored, and the low voltage ride-through period of the photovoltaic converter is determined; during the low voltage ride-through period of the photovoltaic converter, the power grid voltage is lower than the conventional voltage level; when a low voltage fault occurs in the power grid, the low voltage temperature control of the power grid is performed based on an active thermal control strategy.

[0125] At the same time, the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy; the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride-through is reduced based on the adaptive reference power adjustment strategy.

[0126] Therefore, by limiting the output current of the converter during low voltage ride-through, the output junction temperature of the insulated gate bipolar transistor inside the converter can be controlled, thereby reducing the maximum junction temperature and maximum temperature cycle of the insulated gate bipolar transistor. At the same time, the power loss of the insulated gate bipolar transistor inside the converter is proportional to the switching frequency. f sw There is a positive correlation between the switching frequency and the temperature cycle of the insulated gate bipolar transistor, so the temperature cycle of the insulated gate bipolar transistor can be further reduced by adaptively changing the switching frequency.

[0127] For the photovoltaic converter, there will be an output current greater than iamp0 at the moment of low voltage ride-through, and the junction temperature of the insulated gate bipolar transistor will increase; at the moment of grid fault removal, the grid voltage returns to normal level, and the converter will switch back to the normal virtual synchronous control mode. At this moment, the output current of the converter will reach the minimum value, and accordingly, the output junction temperature of the insulated gate bipolar transistor will also drop to a certain extent.

[0128] In order to further reduce the temperature cycle caused by large current changes, an adaptive switching frequency adjustment strategy is added to the active thermal control scheme to further limit the maximum temperature cycle of the internal insulated gate bipolar transistor. When the converter output current is greater than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be reduced to limit the excessive increase in loss, thereby reducing the further increase in junction temperature; and when the converter output current is less than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be increased to increase the loss and increase the heat generation.

[0129] During low voltage ride-through, the photovoltaic converter further limits the maximum temperature cycle of the internal insulated gate bipolar transistor based on the adaptive switching frequency adjustment strategy; when the output current of the photovoltaic converter is greater than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be reduced to limit the excessive increase of loss, thereby reducing the further increase of junction temperature; and when the output current of the photovoltaic converter is less than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be increased to increase the loss and increase the heat generation;

[0130] The control expression of the adaptive switching frequency adjustment strategy is as follows:

[0131] ;

[0132] in, f sw1 and f sw2 is the switching frequency at different current levels; k 1 and k 2 is the proportional gain coefficient of the switching frequency; b 1 and b 2 is the adjustment coefficient of the switching frequency; i amp0 is the steady-state output current amplitude of the converter before adding the active thermal control scheme; i amp It is the real-time output current amplitude of the converter.

[0133] The active thermal control scheme proposed in the present invention reduces the maximum junction temperature of the insulated gate bipolar transistor inside the converter to a large extent through the virtual impedance current limiting control strategy and the adaptive power adjustment strategy, and suppresses its maximum temperature cycle to a certain extent; at the same time, by combining the adaptive switching frequency adjustment strategy, the temperature cycle of the insulated gate bipolar transistor inside the converter is further suppressed. Therefore, by adding the active thermal control scheme, the maximum temperature cycle and the maximum junction temperature of the insulated gate bipolar transistor can be reduced to a large extent, thereby greatly reducing the short-term overstress of the photovoltaic converter during the low voltage ride-through period, and improving the reliability and life of the insulated gate bipolar transistor inside the converter.

[0134] During the low voltage ride-through period, the photovoltaic converter will perform initial current limiting processing through the virtual impedance control strategy. The expression of the virtual impedance current limiting control strategy is as follows:

[0135] ;

[0136] ;

[0137] in, I th To set the current limiting threshold, k R and k X are the resistance and reactance coefficients of the virtual impedance, R v and X v are virtual resistance and virtual reactance respectively, dV d and dV q are the d-axis and q-axis voltage drops caused by virtual impedance, P 0 and k 0 are the reference active power initial value and power variation coefficient, k 1 andk 2 is the proportional gain coefficient of the switching frequency, b 1 and b 2 is the adjustment coefficient of the switching frequency, f sw1 and f sw2 is the switching frequency at different current levels, i amp0 is the steady-state output current amplitude of the converter before adding the active thermal control scheme.

[0138] At this time, the virtual impedance control strategy is mainly applied to the modulated voltage signal in the form of voltage drop. Therefore, the dq axis voltage signal obtained by virtual synchronous control will be based on the original voltage drop generated by the virtual impedance, and finally form the modulated voltage signal required for PWM modulation.

[0139] At the same time, in order to further reduce the overall current level of the photovoltaic converter during low voltage ride-through, an adaptive reference power adjustment strategy is added. This strategy can ensure that the converter reduces the set active reference value during low voltage ride-through, thereby reducing the current level during low voltage faults, so as to achieve the purpose of reducing the maximum junction temperature level during low voltage ride-through.

[0140] Therefore, during the low voltage ride-through period, the photovoltaic converter reduces the set active reference value based on the adaptive reference power adjustment strategy, thereby reducing the current level during the low voltage ride-through period, so as to achieve the purpose of reducing the maximum junction temperature level during the low voltage ride-through period;

[0141] The specific expression of the adaptive reference power adjustment strategy is as follows:

[0142] ;

[0143] in, P 0 is the initial value of reference active power; V 0 is the sinusoidal peak-to-peak value of the rated grid voltage; k 0 is the power variation coefficient.

[0144] In an embodiment of the present invention, through the method in the embodiment of the present invention, when the grid-side voltage is normal, a virtual synchronization control strategy is executed based on the photovoltaic converter to ensure continuous and stable grid connection; the grid is phase-locked based on a phase-locked loop to obtain the voltage amplitude of the grid in real time; based on the change of the voltage amplitude of the grid, it is determined whether a low-voltage fault occurs in the grid; and when the low-voltage fault occurs in the grid, the grid is subjected to low-voltage temperature control based on an active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, based on the virtual impedance current limiting control The control strategy is used to perform initial current limiting processing, and the current level during the low voltage ride-through period is reduced based on the adaptive reference power adjustment strategy. At this time, the maximum junction temperature of the insulated gate bipolar transistor inside the converter is reduced to a large extent through the virtual impedance current limiting control strategy and the adaptive power adjustment strategy, and its maximum temperature cycle is suppressed to a certain extent; at the same time, by combining the adaptive switching frequency adjustment strategy, the temperature cycle of the insulated gate bipolar transistor inside the converter is further suppressed, thereby greatly reducing the maximum temperature cycle and the maximum junction temperature of the insulated gate bipolar transistor, thereby greatly reducing the short-term overstress of the photovoltaic converter during the low voltage ride-through period, and improving the reliability and life of the insulated gate bipolar transistor inside the converter.

[0145] In addition, actual tests and verifications were carried out on a grid-connected photovoltaic converter based on a virtual synchronous control strategy. The internal power device used was Infineon's FS100R12KT4G ( I c,max =100A, V CE,max =1200V), the real-time output junction temperature change of the insulated gate bipolar transistor inside the converter was obtained. The converter was directly connected to the grid at 0s, and the output junction temperature of the internal insulated gate bipolar transistor reached a dynamic balance in steady state at 0.8s. The grid voltage dropped at 1.2s, dropping to 70% of the original voltage; the grid voltage fault will last for 0.5s, and the grid-side fault will be cleared at 1.7s. When the active thermal control method is not used, the junction temperature output waveform of the insulated gate bipolar transistor inside the converter is as follows Fig.10 As shown. Fig.10 It can be seen that when the converter does not take any active thermal control method during the low voltage ride-through period, the internal insulated gate bipolar transistor will have an instantaneous junction temperature spike of about 198.624°C after the fault is removed; then the junction temperature will gradually recover and an instantaneous junction temperature trough will appear, about 121.334°C, at which time the maximum thermal cycle amplitude is about 77.29°C. Therefore, during the grid fault period, the converter will have a higher junction temperature peak and temperature cycle, and will be subjected to greater electrothermal stress, which can easily cause the failure of the insulated gate bipolar transistor.

[0146] When the active thermal control method proposed in the present invention is used during low voltage ride-through, the junction temperature output waveform of the insulated gate bipolar transistor inside the converter is as follows: Fig.11 As shown. Fig.11 It can be seen that after adding the active thermal control method during low voltage ride-through proposed by the present invention, the maximum junction temperature of the insulated gate bipolar transistor inside the converter will be greatly reduced to about 145.835°C, a decrease of about 26.6%. At the same time, the arrival time of the instantaneous junction temperature trough of the insulated gate bipolar transistor will be advanced, and its amplitude is about 120.644°C. At this time, the temperature cycle of the insulated gate bipolar transistor inside the converter will be greatly reduced to 25.191°C, about one-third of the original.

[0147] Obviously, the active thermal control method proposed in the present invention can significantly reduce the maximum junction temperature and temperature cycle of the insulated gate bipolar transistor, thereby reducing the short-term electrothermal stress it is subjected to, and further improving its reliability and lifespan.

[0148] Firstly, a photovoltaic converter model based on virtual synchronous control strategy is established to collect the output voltage and current of the photovoltaic converter in real time, and perform corresponding abc-dq transformation for subsequent processing.

[0149] Then, a corresponding electro-thermal model is established for the insulated gate bipolar transistor inside the photovoltaic converter, and the calculated device loss is input into the thermal network model to obtain the real-time output junction temperature waveform of the insulated gate bipolar transistor.

[0150] The grid-side voltage of the converter is then detected to determine whether the converter is in a low voltage ride-through period. At the same time, the output current amplitude of the converter is obtained in real time through calculation to facilitate subsequent active thermal control.

[0151] Finally, the active thermal control scheme during LVRT is executed. When the grid-side voltage of the converter is lower than vth, the active thermal control scheme is activated.

[0152] This solution uses the synergistic effects of virtual impedance current limiting control strategy, adaptive reference power adjustment strategy and adaptive switching frequency adjustment strategy to jointly reduce the maximum junction temperature and temperature cycle of the insulated gate bipolar transistor, thereby improving the reliability and life of the insulated gate bipolar transistor.

[0153] The key variable parameters in each embodiment can be as follows:

[0154]

[0155] Example

[0156] Fig.12Schematic diagram of the structure of the active thermal control system of the photovoltaic converter during the low voltage ride-through period in the embodiment of the present invention. Fig.12 As shown, an active thermal control system for a photovoltaic converter during low voltage ride-through, the active thermal control system for a photovoltaic converter during low voltage ride-through includes:

[0157] The grid-connected module 21 is used to execute a virtual synchronization control strategy based on the photovoltaic converter when the grid-side voltage is normal, so as to ensure continuous and stable grid connection;

[0158] A phase locking module 22, configured to phase lock the power grid based on a phase locked loop, so as to obtain the voltage amplitude of the power grid in real time;

[0159] a fault detection module 23, configured to determine whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid; and

[0160] The control module 24 is used to perform low-voltage temperature control on the power grid based on an active thermal control strategy when the low-voltage fault occurs in the power grid, wherein the real-time three-phase current output by the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low-voltage ride-through is reduced based on the adaptive reference power adjustment strategy.

[0161] Example

[0162] See also Fig.13 , refer to the following Fig.13 The electronic device 40 according to this embodiment of the present invention will be described. Fig.13 The electronic device 40 shown is only an example and should not bring any limitation to the functions and scope of use of the embodiments of the present invention.

[0163] like Fig.13 As shown, the electronic device 40 is in the form of a general computing device. The components of the electronic device 40 may include but are not limited to: at least one processing unit 41, at least one storage unit 42, and a bus 43 connecting different system components (including the storage unit 42 and the processing unit 41).

[0164] The storage unit stores program codes, which can be executed by the processing unit 41, so that the processing unit 41 executes the steps according to various exemplary embodiments of the present invention described in the above “Embodiment Method” section of this specification.

[0165] The storage unit 42 may include a readable medium in the form of a volatile storage unit, such as a random access memory unit (RAM) 421 and / or a cache memory unit 422 , and may further include a read-only memory unit (ROM) 423 .

[0166] The storage unit 42 may also include a program / utility 424 having a set (at least one) of program modules 425, such program modules 425 including but not limited to: an operating system, one or more application programs, other program modules, and program data, each of which or some combination may include an implementation of a network environment.

[0167] Bus 43 may represent one or more of several types of bus structures, including a memory unit bus or memory unit controller, a peripheral bus, an accelerated graphics port, a processing unit, or a local bus using any of a variety of bus architectures.

[0168] The electronic device 40 may also communicate with one or more external devices (e.g., keyboards, pointing devices, Bluetooth devices, etc.), one or more devices that enable a trainee to interact with the electronic device 40, and / or any device that enables the electronic device 40 to communicate with one or more other computing devices (e.g., routers, modems, etc.). Such communication may be performed via an input / output (I / O) interface 44. Furthermore, the electronic device 40 may also communicate with one or more networks (e.g., a local area network (LAN), a wide area network (WAN), and / or a public network, such as the Internet) via a network adapter 45. Fig.13 As shown, the network adapter 45 communicates with other modules of the electronic device 40 via the bus 43. It should be understood that although Fig.13 Not shown, other hardware and / or software modules may be used in conjunction with the electronic device 40, including but not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data backup planning systems.

[0169] Through the description of the above implementation, it is easy for those skilled in the art to understand that the example implementation described here can be implemented by software, or by software combined with necessary hardware. Therefore, the technical solution according to the implementation of the present disclosure can be embodied in the form of a software product, which can be stored in a non-volatile storage medium (which can be a CD-ROM, a USB flash drive, a mobile hard disk, etc.) or on a network, and includes a number of instructions to enable a computing device (which can be a personal computer, a server, a terminal device, or a network device, etc.) to execute the method according to the implementation of the present disclosure.

[0170] Those skilled in the art can understand that all or part of the steps in the various methods of the above embodiments can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium, and the storage medium can include: a read-only memory (ROM), a random access memory (RAM), a disk or an optical disk, etc. In addition, it stores computer program instructions, and when the computer program instructions are executed by a computer, the computer executes the above method.

[0171] In addition, the above is a detailed introduction to the active thermal control method and system of the photovoltaic converter during low voltage ride-through provided by the embodiments of the present invention. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as a limitation on the present invention.

Claims

1. An active thermal control method for photovoltaic converters during low voltage ride-through. It is characterized in that include: When the grid-side voltage is normal, a virtual synchronization control strategy is implemented based on the photovoltaic converter to ensure continuous and stable grid connection; Phase-locking the power grid based on a phase-locked loop to obtain the voltage amplitude of the power grid in real time; Determining whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid; And when the low-voltage fault occurs in the power grid, the low-voltage temperature control of the power grid is performed based on the active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low-voltage ride-through is reduced based on the adaptive reference power adjustment strategy; When the low voltage fault occurs in the power grid, the low voltage temperature control of the power grid is performed based on the active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride-through is reduced based on the adaptive reference power adjustment strategy, including: Monitoring the low voltage fault of the power grid and determining the low voltage ride through period of the photovoltaic converter; During the low voltage ride-through of the photovoltaic converter, the grid voltage is lower than a normal voltage level; When a low-voltage fault occurs in the power grid, low-voltage temperature control is performed on the power grid based on an active thermal control strategy; Collecting the real-time three-phase current output by the photoelectric converter, and constraining the temperature cycle caused by the current change during the low voltage ride-through period based on the three-phase current and adaptively changing the switching frequency control strategy; and performing initial current limiting processing based on a virtual impedance current limiting control strategy, and reducing the current level during the low voltage ride-through period based on an adaptive reference power adjustment strategy; During low voltage ride-through, the photovoltaic converter further limits the maximum temperature cycle of the internal insulated gate bipolar transistor based on the adaptive switching frequency adjustment strategy; when the output current of the photovoltaic converter is greater than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be reduced to limit the excessive increase of loss, thereby reducing the further increase of junction temperature; and when the output current of the photovoltaic converter is less than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be increased to increase the loss and increase the heat generation; The control expression of the adaptive switching frequency adjustment strategy is as follows: ； in, f sw1 and f sw2 is the switching frequency at different current levels; k 1 and k 2 is the proportional gain coefficient of the switching frequency; b 1 and b 2 is the adjustment factor of the switching frequency; i amp0 is the steady-state output current amplitude of the converter before adding the active thermal control scheme; i amp It is the amplitude of the real-time output current of the converter; During the low voltage ride-through period, the photovoltaic converter will perform initial current limiting processing through the virtual impedance control strategy. The expression of the virtual impedance current limiting control strategy is as follows: ； ； in, I th To set the current limiting threshold, k R and k X are the resistance and reactance coefficients of the virtual impedance, R v and X v are virtual resistance and virtual reactance respectively, dV d and dV q are the d-axis and q-axis voltage drops caused by virtual impedance respectively; and Output the d-axis and q-axis currents of the three-phase current to the optoelectronic converter in real time; During the low voltage ride-through period, the photovoltaic converter reduces the set active reference value based on the adaptive reference power adjustment strategy, thereby reducing the current level during the low voltage ride-through period, so as to achieve the purpose of reducing the maximum junction temperature level during the low voltage ride-through period; The specific expression of the adaptive reference power adjustment strategy is as follows: ； in, P 0 is the initial value of reference active power; V 0 is the sinusoidal peak-to-peak value of the rated voltage of the power grid; k 0 is the power variation coefficient; v th It is the lower limit of the voltage threshold that the grid voltage is allowed to fluctuate under normal circumstances and serves as the preset voltage amplitude; v g is the grid voltage amplitude.

2. The active thermal control method for photovoltaic converter during low voltage ride-through according to claim 1, It is characterized in that When the grid-side voltage is normal, a virtual synchronization control strategy is executed based on the photovoltaic converter to ensure continuous and stable grid connection, including: When the grid-side voltage is normal, collecting the photoelectric converter; and executing a virtual synchronization control strategy based on the photovoltaic converter to ensure continuous and stable grid connection; The expression of the virtual synchronization control strategy is as follows: ； ； in, J is the moment of inertia of the converter, D p and D q are the damping coefficients of the converter, ω out and V out is the angular frequency and internal potential output by the converter in real time, ω 0 and V 0 Given the reference angular frequency and internal potential of the converter, P ref and Q ref The active and reactive power reference values ​​are given to the converter inputs. P out and Q out Output active and reactive power to the converter in real time.

3. The active thermal control method for photovoltaic converter during low voltage ride-through according to claim 1, It is characterized in that The phase-locking of the power grid based on a phase-locked loop to obtain the voltage amplitude of the power grid in real time includes: collecting the power grid; Associating a phase locked loop with the power grid; And the power grid is phase-locked according to the phase-locked loop, wherein the power grid voltage is abc-dq transformed to realize the output of dq-axis power grid voltage, and the phase-locked loop phase-locks the q-axis power grid voltage obtained in real time through PI control to ensure that the q-axis power grid voltage is 0, so as to obtain the voltage amplitude of the power grid in real time.

4. The active thermal control method for photovoltaic converter during low voltage ride-through according to claim 1, It is characterized in that The determining whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid comprises: Detecting changes in the voltage amplitude of the power grid in real time; And the voltage amplitude of the power grid is compared with a preset voltage amplitude. If the voltage amplitude of the power grid is lower than the preset voltage amplitude, a low voltage fault occurs in the power grid; or, if the voltage amplitude of the power grid is higher than the preset voltage amplitude, the power grid is in a normal state.

5. The active thermal control method for photovoltaic converter during low voltage ride-through according to claim 4, It is characterized in that The method of judging whether a low voltage fault occurs in the power grid based on the change in the voltage amplitude of the power grid further includes: ； in, v th It is the lower limit of the voltage threshold that the grid voltage is allowed to fluctuate under normal circumstances and serves as the preset voltage amplitude; v g is the grid voltage amplitude; v gd and v gq are the grid voltages in the dq coordinate system respectively.

6. An active thermal control system for photovoltaic converters during low voltage ride-through. It is characterized in that The photovoltaic converter active thermal control system during low voltage ride-through adopts the photovoltaic converter active thermal control method during low voltage ride-through as described in any one of claims 1 to 5, and the photovoltaic converter active thermal control system during low voltage ride-through includes: The grid-connected module is used to execute a virtual synchronization control strategy based on the photovoltaic converter when the grid-side voltage is normal to ensure continuous and stable grid connection; A phase locking module, used to phase lock the power grid based on a phase locked loop to obtain the voltage amplitude of the power grid in real time; A fault detection module, used for determining whether a low voltage fault occurs in the power grid based on a change in the voltage amplitude of the power grid; and a control module, which is used to perform low-voltage temperature control on the power grid based on an active thermal control strategy when a low-voltage fault occurs in the power grid, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low-voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low-voltage ride-through is reduced based on the adaptive reference power adjustment strategy; When the low voltage fault occurs in the power grid, the low voltage temperature control of the power grid is performed based on the active thermal control strategy, wherein the real-time output three-phase current of the photovoltaic converter is collected, and the temperature cycle caused by the current change during the low voltage ride-through is constrained based on the three-phase current and the adaptive change of the switching frequency control strategy, and at the same time, the initial current limiting processing is performed based on the virtual impedance current limiting control strategy, and the current level during the low voltage ride-through is reduced based on the adaptive reference power adjustment strategy, including: Monitoring the low voltage fault of the power grid and determining the low voltage ride through period of the photovoltaic converter; During the low voltage ride-through of the photovoltaic converter, the grid voltage is lower than a normal voltage level; When a low-voltage fault occurs in the power grid, low-voltage temperature control is performed on the power grid based on an active thermal control strategy; Collecting the real-time three-phase current output by the photoelectric converter, and constraining the temperature cycle caused by the current change during the low voltage ride-through period based on the three-phase current and adaptively changing the switching frequency control strategy; and performing initial current limiting processing based on a virtual impedance current limiting control strategy, and reducing the current level during the low voltage ride-through period based on an adaptive reference power adjustment strategy; During low voltage ride-through, the photovoltaic converter further limits the maximum temperature cycle of the internal insulated gate bipolar transistor based on the adaptive switching frequency adjustment strategy; when the output current of the photovoltaic converter is greater than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be reduced to limit the excessive increase of loss, thereby reducing the further increase of junction temperature; and when the output current of the photovoltaic converter is less than the normal current level, the switching frequency of the insulated gate bipolar transistor will be forced to be increased to increase the loss and increase the heat generation; The control expression of the adaptive switching frequency adjustment strategy is as follows: ； in, f sw1 and f sw2 is the switching frequency at different current levels; k 1 and k 2 is the proportional gain coefficient of the switching frequency; b 1 and b 2 is the adjustment factor of the switching frequency; i amp0 is the steady-state output current amplitude of the converter before adding the active thermal control scheme; i amp It is the amplitude of the real-time output current of the converter; During the low voltage ride-through period, the photovoltaic converter will perform initial current limiting processing through the virtual impedance control strategy. The expression of the virtual impedance current limiting control strategy is as follows: ； ； in, I th To set the current limiting threshold, k R and k X are the resistance and reactance coefficients of the virtual impedance, R v and X v are virtual resistance and virtual reactance respectively, dV d and dV q are the d-axis and q-axis voltage drops caused by virtual impedance respectively; and Output the d-axis and q-axis currents of the three-phase current to the optoelectronic converter in real time; During the low voltage ride-through period, the photovoltaic converter reduces the set active reference value based on the adaptive reference power adjustment strategy, thereby reducing the current level during the low voltage ride-through period, so as to achieve the purpose of reducing the maximum junction temperature level during the low voltage ride-through period; The specific expression of the adaptive reference power adjustment strategy is as follows: ； in, P 0 is the initial value of reference active power; V 0 is the sinusoidal peak-to-peak value of the rated voltage of the power grid; k 0 is the power variation coefficient; v th It is the lower limit of the voltage threshold that the grid voltage is allowed to fluctuate under normal circumstances and serves as the preset voltage amplitude; v g is the grid voltage amplitude.

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