Junction temperature control method and device of inverter switching device

Abstract

The embodiment of the invention discloses a junction temperature control method and device for an inverter switching device, and the method comprises the steps: determining a target reactive current for supporting the operation of a power grid according to a measured power grid impedance parameter under the condition of low voltage ride through, and determining the minimum bus voltage of an inverter under the target reactive current, and then the output reactive current of the inverter is controlled to be the target reactive current, so that stable operation of the power grid can be supported under the condition of low voltage ride through of the power grid. The bus voltage of the inverter is adjusted between the preset maximum bus voltage and the preset minimum bus voltage, the target bus voltage meeting the condition that the junction temperature of the switching device does not exceed the preset junction temperature upper limit is determined, and the junction temperature of the switching device in the inverter does not exceed the safety range by controlling the inverter to operate at the target bus voltage subsequently. And the operation safety of the system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of junction temperature control of power electronic devices, and in particular to a junction temperature control method and device of an inverter switch device. Background Art

[0002] In photovoltaic power generation systems, since grid-connected inverters are often used in remote areas, their transmission lines are long, the resistance-inductance ratio is low, and the power grid generally exhibits weak grid characteristics. When the grid voltage drops, the inverter needs to provide support to the grid by outputting reactive current.

[0003] However, due to the impedance difference of the weak grid, the reactive current output by the inverter varies. If there is a lack of effective control measures, the junction temperature of the inverter switching device is likely to exceed the safety range, threatening the operational safety of the system. Summary of the invention

[0004] The present invention provides a method and device for controlling the junction temperature of an inverter switch device, so as to optimize the inverter bus voltage so that the junction temperature of the switch device does not exceed a safe range, thereby improving the safety of system operation.

[0005] According to one aspect of the present invention, a method for controlling junction temperature of an inverter switching device is provided, comprising:

[0006] In case of low voltage ride-through, measure the grid impedance parameters;

[0007] Determine the target reactive current supporting the operation of the grid according to the grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current;

[0008] Controlling the output reactive current of the inverter to be the target reactive current;

[0009] The bus voltage of the inverter is adjusted between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, and the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

[0010] Optionally, the inverter is connected in a two-stage system; before adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine the target bus voltage, the method further includes:

[0011] The bus voltage of the inverter is controlled to remain unchanged, the active output power of the inverter is adjusted, and the active output power when the junction temperature of the switching device reaches a preset junction temperature upper limit is determined to be the maximum active output power;

[0012] The bus voltage of the inverter is adjusted between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, including:

[0013] The reactive current of the inverter is controlled to maintain the target reactive current. When the active output power of the inverter is the maximum active output power, the bus voltage of the inverter is adjusted between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage.

[0014] Optionally, controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches a preset junction temperature upper limit is the maximum active output power, includes:

[0015] The bus voltage of the inverter is controlled to remain unchanged, and the active output power of the inverter is gradually increased from 0;

[0016] In the process of gradually increasing the active output power of the inverter, obtaining the shell temperature of each switching device in the inverter;

[0017] When the junction temperature of the switching device corresponding to the shell temperature of at least one switching device reaches a preset junction temperature upper limit, the corresponding active output power is determined as the maximum active output power.

[0018] Optionally, controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine the target bus voltage includes:

[0019] The reactive current of the inverter is controlled to maintain the target reactive current. When the active output power of the inverter is the maximum active output power, the bus voltage is controlled to gradually decrease from the maximum bus voltage to the minimum bus voltage.

[0020] In the process of the maximum bus voltage gradually decreasing to the minimum bus voltage, the shell temperature of each switching device is obtained, and the temperature margin between the maximum junction temperature of each switching device and the preset junction temperature upper limit is determined according to the shell temperature of each switching device;

[0021] The bus voltage when the temperature margin is maximum is determined as the target bus voltage.

[0022] Optionally, when the reactive current of the inverter is controlled to maintain the target reactive current and the active output power of the inverter is the maximum active output power, the bus voltage of the inverter is adjusted between a preset maximum bus voltage and a minimum bus voltage to determine the target bus voltage, the method further includes:

[0023] Control the inverter to operate at a target reactive current, a maximum active output power and a target bus voltage;

[0024] Recording the grid impedance parameters and the target reactive current, maximum active output power and target bus voltage corresponding to the grid impedance parameters as historical data;

[0025] After determining the grid impedance parameters, it also includes:

[0026] Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, maximum active output power and target bus voltage corresponding to the grid impedance parameter in the historical data;

[0027] If not, the steps of determining a target reactive current for supporting the operation of the power grid according to the power grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

[0028] Optionally, the inverter is connected to a single-stage system, the single-stage system includes at least two photovoltaic strings; adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage includes:

[0029] Adjust the number of PV strings connected to the inverter;

[0030] At each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is adjusted between a preset maximum bus voltage and a minimum bus voltage, and the junction temperature of each switching device is obtained, and the temperature margin is determined according to the maximum junction temperature of the switching device and a preset junction temperature upper limit;

[0031] The target number of PV strings connected to the inverter and the target bus voltage are determined based on the temperature margin.

[0032] Optionally, adjust the number of PV strings connected to the inverter, including:

[0033] Adjust the number of PV strings connected to the inverter in ascending order;

[0034] At each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is adjusted between the preset maximum bus voltage and the minimum bus voltage, and the junction temperature of each switching device is obtained. The temperature margin is determined according to the maximum junction temperature of the switching device and the preset junction temperature upper limit, including:

[0035] Under each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is controlled to gradually decrease from the maximum bus voltage to the minimum bus voltage, and the junction temperature of each switching device is obtained, and the temperature margin is determined according to the maximum junction temperature of the switching device and the preset junction temperature upper limit;

[0036] The target number of PV strings connected to the inverter and the target bus voltage are determined according to the temperature margin, including: when i+1 PV strings are connected to the inverter, if the temperature margin is equal to 0, i is determined as the target number, and the bus voltage with the maximum temperature margin corresponding to the target number is determined as the target bus voltage, wherein i is greater than or equal to 1 and less than the total number of PV strings.

[0037] Optionally, after determining the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin, the method further includes:

[0038] Control the inverter to operate at a target reactive current, connect a target number of PV strings, and a target bus voltage;

[0039] Recording the grid impedance parameters and the target reactive current, target quantity and target bus voltage corresponding to the grid impedance parameters as historical data;

[0040] After determining the grid impedance parameters, it also includes:

[0041] Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, target quantity, and target bus voltage corresponding to the grid impedance parameter in the historical data;

[0042] If not, the steps of determining a target reactive current for supporting the operation of the power grid according to the power grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

[0043] Optionally, the grid impedance parameters include grid impedance and grid inductive reactance; when low voltage ride-through occurs, the grid impedance parameters are measured, including:

[0044] In the event of low voltage ride-through, the grid impedance is calculated by disturbing the active output power of the inverter, and the grid inductive reactance is calculated by the reactive output power of the inverter;

[0045] Determine the target reactive current supporting the grid operation according to the grid impedance parameters, control the inverter output to reach the target reactive current, and determine the minimum bus voltage of the inverter under the target reactive current, including:

[0046] Determine the target reactive current according to the grid inductive reactance, the target voltage of the common connection point between the inverter and the grid, the grid voltage and the grid angular frequency;

[0047] The minimum bus voltage is determined according to the target reactive current, grid voltage, grid inductance, and parameters of the filter inductance in the inverter.

[0048] According to another aspect of the present invention, there is provided a junction temperature control device for an inverter switching device, comprising:

[0049] A measurement module is used to measure grid impedance parameters in the event of low voltage ride-through;

[0050] A first determination module is used to determine a target reactive current supporting the operation of the power grid according to the power grid impedance parameter, and to determine a minimum bus voltage of the inverter under the target reactive current;

[0051] A control module, used for controlling the output reactive current of the inverter to be a target reactive current;

[0052] The second determination module adjusts the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, and the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

[0053] The junction temperature control method and device of the inverter switch device of the embodiment of the present invention can support the stable operation of the power grid when low voltage crossing occurs by determining the target reactive current to support the operation of the power grid according to the measured power grid impedance parameters and determining the minimum bus voltage of the inverter under the target reactive current, and then controlling the output reactive current of the inverter to be the target reactive current. The bus voltage of the inverter is adjusted between the preset maximum bus voltage and the minimum bus voltage, and the target bus voltage that satisfies the junction temperature of the switch device does not exceed the preset junction temperature upper limit is determined. Subsequently, the inverter is controlled to operate under the target bus voltage to ensure that the junction temperature of the switch device in the inverter does not exceed the safety range, thereby improving the safety of system operation.

[0054] It should be understood that the contents described in this section are not intended to identify the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. 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 creative work.

[0056] Figure 1 A flow chart of a method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention;

[0057] Figure 2 It is a structural diagram of a two-stage system;

[0058] Figure 3is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention;

[0059] Figure 4 It is a schematic diagram of a single-phase topology structure of a neutral point clamped (NPC) three-level inverter;

[0060] Figure 5 They are the equivalent thermal impedance models of the switching device IGBT and the diode;

[0061] Figure 6 It is a schematic diagram of the junction temperature partition of the switching device in the inverter;

[0062] Figure 7 It is a specific determination flow chart of the maximum active output power;

[0063] Figure 8 It is a schematic diagram of the junction temperature curves of different switching devices of the NPC inverter under different active currents;

[0064] Fig. 9 It is a specific determination flow chart of the target bus voltage;

[0065] Fig.10 It is a schematic diagram of the change of junction temperature of the switching device under different bus voltages;

[0066] Fig.11 is another schematic diagram of the change in junction temperature of the switching device under different bus voltages;

[0067] Fig.12 is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention;

[0068] Fig.13 It is a structural diagram of a single-stage system;

[0069] Fig.14 is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention;

[0070] Fig.15 A schematic diagram showing the change in junction temperature of a switching device during bus voltage adjustment when only one photovoltaic string is connected to an inverter;

[0071] Fig.16 Another schematic diagram showing the change of junction temperature of the switching device during the process of adjusting the bus voltage when only one photovoltaic string is connected to the inverter;

[0072] Fig.17 is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention;

[0073] Fig.18 It is the waveform diagram of power disturbance measurement grid impedance parameters;

[0074] Fig.19 It is to disturb the junction temperature change of the active current power switch device under the target reactive current of the inverter;

[0075] Fig. 20 A schematic diagram showing the change of the overall system temperature margin when the bus voltage is disturbed, when both the active current and the target reactive current corresponding to the maximum active output power are determined;

[0076] Fig.21 Another schematic diagram showing the change of the overall system temperature margin when the bus voltage is disturbed, when both the active current and the target reactive current corresponding to the maximum active output power are determined;

[0077] Fig. 22 A schematic diagram showing the change of the overall system temperature margin when the bus voltage is disturbed when the DC switch is closed to three paths;

[0078] Fig.23 Another schematic diagram showing the change of the overall system temperature margin when the bus voltage is disturbed when the DC switch is closed to three paths;

[0079] Fig.24 It is a structural schematic diagram of a junction temperature control device of an inverter switch device provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0080] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings 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 should fall within the scope of protection of the present invention.

[0081] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0082] Figure 1 A flow chart of a method for controlling the junction temperature of an inverter switch device provided in an embodiment of the present invention. This embodiment can be applied to controlling the inverter to output reactive current to support the power grid in the event of low voltage ride-through, and controlling the junction temperature of the switch device in the inverter by optimizing the bus voltage of the inverter, so that the junction temperature of the switch device does not exceed the preset junction temperature upper limit. The method can be executed by an inverter switch device junction temperature control device, and the inverter switch device junction temperature control device can be implemented in the form of hardware and / or software. The inverter can be connected to a DC power supply and a power grid respectively, wherein the DC power supply includes but is not limited to photovoltaic rectifiers and batteries, and the batteries include but are not limited to storage batteries and lithium batteries. Figure 1 As shown, the method includes:

[0083] S110. When low voltage ride-through occurs, measure grid impedance parameters.

[0084] In the event of low voltage crossing, the power grid exhibits weak grid characteristics and the system enters a low voltage crossing state. A weak grid means that under the combined effect of nonlinear loads and line impedance, the power grid exhibits certain inductive characteristics, and when the location where the photovoltaic equipment is connected to the grid changes, the grid inductance relative to the common coupling point will also fluctuate. This non-ideal power grid is called a weak grid. A weak grid means that under certain conditions, the power grid exhibits lower stability and dynamic response capabilities, which is particularly evident in distributed power generation and remote areas. In this step, in the event of low voltage crossing, the grid impedance parameters are first measured. Optionally, the grid impedance parameters are determined by disturbing the active output power and reactive output power of the inverter.

[0085] S120. Determine a target reactive current for supporting grid operation according to grid impedance parameters, and determine a minimum bus voltage of the inverter under the target reactive current.

[0086] Specifically, in the event of low voltage ride-through, the inverter is required to provide reactive current to ensure the operation of the power grid. In this step, after the grid impedance parameters are determined, the target reactive current supporting the operation of the power grid is determined according to the grid impedance parameters. Under the target reactive current, the common connection point between the inverter and the power grid can be restored to the voltage before the low voltage ride-through occurs in the power grid.

[0087] After determining the target reactive current, the minimum bus voltage of the inverter is determined according to the target reactive current. The minimum bus voltage must satisfy the requirement that no overmodulation will occur under the Space Vector Pulse Width Modulation (SVPWM) modulation strategy, thereby ensuring the stability and safety of the system.

[0088] S130. Control the output reactive current of the inverter to be a target reactive current.

[0089] In this step, the output reactive current of the inverter is controlled to be equal to the target reactive current, which can support the stable operation of the power grid when low voltage ride-through occurs in the power grid.

[0090] S140, adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, where the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

[0091] Optionally, the preset maximum bus voltage is equal to the rated value of the bus voltage.

[0092] In this step, the bus voltage is adjusted between the maximum bus voltage and the minimum bus voltage. During this process, the junction temperature of the switching device in the inverter can be obtained in real time. Since the junction temperature of the switching device is not easy to obtain directly, in some embodiments, the junction temperature of the switching device can be obtained by obtaining the shell temperature of the switching device and obtaining the junction temperature of the switching device according to the relationship between the junction temperature and the shell temperature of the switching device. Optionally, in a voltage interval with the maximum bus voltage and the minimum bus voltage as the interval endpoints, one of the bus voltages that satisfies that the junction temperature of each switching device in the inverter does not exceed the preset junction temperature upper limit is determined as the target bus voltage. Subsequently, the inverter can be controlled to operate at the target bus voltage to ensure that the junction temperature of the switching device in the inverter does not exceed the safety range, thereby improving the safety of system operation.

[0093] Among them, adjusting the bus voltage between the maximum bus voltage and the minimum bus voltage may be to gradually reduce the bus voltage from the maximum bus voltage, or to gradually increase the bus voltage from the minimum bus voltage, or to gradually increase from an intermediate bus voltage between the minimum bus voltage and the maximum bus voltage to the maximum bus voltage, and then gradually reduce from the intermediate bus voltage to the minimum bus voltage. Adjusting the bus voltage between the maximum bus voltage and the minimum bus voltage may also be other adjustment methods, which are not specifically limited in the embodiments of the present invention.

[0094] It should be noted that the inverter switch device junction temperature control method of this embodiment can be applied to a two-stage system and a single-stage system. Optionally, when the method is applied to a two-stage system, in the process of adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage, the active output power of the inverter is controlled to be a fixed value, and the fixed value can be obtained in advance to ensure that the junction temperature of the switch device in the inverter does not exceed the preset junction temperature upper limit under the active output power, target bus voltage and target reactive current corresponding to the fixed value of the inverter. When the method is applied to a single-stage system, since the bus voltage output characteristics of the photovoltaic cell in the single-stage system will change with the change of the photovoltaic active output power, the target active power of the inverter corresponding to the target bus voltage can also be obtained in the process of S140.

[0095] The junction temperature control method of the inverter switch device of this embodiment can support the stable operation of the power grid when low voltage ride-through occurs by determining the target reactive current supporting the operation of the power grid according to the measured power grid impedance parameters and determining the minimum bus voltage of the inverter under the target reactive current, and then controlling the output reactive current of the inverter to be the target reactive current. The bus voltage of the inverter is adjusted between the preset maximum bus voltage and the minimum bus voltage, and the target bus voltage that satisfies the junction temperature of the switch device does not exceed the preset junction temperature upper limit is determined. Subsequently, the inverter is controlled to operate under the target bus voltage to ensure that the junction temperature of the switch device in the inverter does not exceed the safety range, thereby improving the safety of system operation.

[0096] Optionally, the inverter is connected in a two-stage system. Figure 2 This is a schematic diagram of the structure of a two-stage system. Figure 2The two-stage system is a distributed system structure, which is usually composed of a front stage and a back stage. In the front stage, a DC / DC converter 10 (such as a boost circuit) is used to increase the DC voltage; in the back stage, an inverter 20 (such as a three-phase half-bridge inverter) is used to convert the DC voltage into an AC voltage and connect it to the power grid 40. That is, in the two-stage system, the inverter 20 and the DC power supply 30 are connected by a DC / DC converter 10, and the inverter 20 is connected to the power grid 40 at a common connection point PCC. The DC power supply 30 of the two-stage system can be composed of a photovoltaic array, a battery or a lithium battery, so as to achieve stable power supply under different working conditions. Figure 3 is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention, with reference to Figure 3 , the junction temperature control method of the inverter switch device includes:

[0097] S210. When low voltage ride-through occurs, measure grid impedance parameters.

[0098] S220. Determine a target reactive current for supporting grid operation according to grid impedance parameters, and determine a minimum bus voltage of the inverter under the target reactive current.

[0099] S230. Control the output reactive current of the inverter to be a target reactive current.

[0100] S240, controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches a preset junction temperature upper limit is the maximum active output power.

[0101] Specifically, in this step, the output reactive current of the inverter is controlled to be maintained at the target reactive current, and the bus voltage of the inverter is controlled to remain unchanged. Optionally, the bus voltage of the inverter is controlled to be the rated value of the bus voltage, that is, when the output reactive current of the inverter is the target reactive current and the bus voltage of the inverter remains unchanged at the rated value, the active output power of the inverter is adjusted, and at each active output power, the junction temperature of each switch device in the inverter is obtained respectively, for example, the junction temperature of the switch device is determined by obtaining the shell temperature of the switch device, and then the active output power when the junction temperature of the switch device reaches the preset junction temperature upper limit is determined, and the active output power is determined as the maximum active output power. In this step, by determining the maximum active output power when the junction temperature of the switch device is the preset junction temperature upper limit, the inverter can be controlled to output active power according to the maximum active output power when the inverter is controlled to run, thereby providing stronger power support for the power grid.

[0102] Optionally, S240 includes: controlling the bus voltage of the inverter to remain unchanged, and gradually increasing the active output power of the inverter from 0; in the process of gradually increasing the active output power of the inverter, obtaining the shell temperature of each switching device in the inverter; when the junction temperature of the switching device corresponding to the shell temperature of at least one switching device reaches a preset junction temperature upper limit, the corresponding active output power is determined as the maximum active output power.

[0103] Figure 4 This is a schematic diagram of a single-phase topology structure of a neutral point clamped (NPC) three-level inverter. Figure 4 , the switching area of ​​the inverter includes the insulated-gate bipolar transistor (IGBT) and the diode. Since the junction temperature of the switching device is difficult to measure directly, the case temperature of the IGBT and the diode can be measured separately, and then the junction temperature of the IGBT can be determined based on the case temperature, junction-case thermal impedance and loss of the IGBT, and the junction temperature of the diode can be determined based on the junction temperature, junction-case thermal impedance and loss of the diode. Figure 5 They are the equivalent thermal impedance models of the switching device IGBT and diode respectively. According to the equivalent thermal impedance model, the junction temperature of the IGBT and diode can be calculated by equations (1) and (2).

[0104] (1)

[0105] (2)

[0106] Where T IJ 、T DJ are the junction temperatures of the IGBT and diode, T Icase 、T Dcase are the case temperatures of the IGBT and diode, respectively, I , P D are the power loss on IGBT and diode, R IJC , R DJC are the junction-to-case thermal impedances of the IGBT and diode, respectively.

[0107] The power loss of IGBT and diode includes switching loss and conduction loss. The conduction loss of IGBT and diode can be calculated by equation (3) and (4) respectively. I , R D are the resistance values ​​of IGBT and diode when they are turned on, and the saturation voltage drop V ce 、V D With R I , R DThe relationship between the junction temperature and the device can be found in the device manual.

[0108] P I1 =V ce I c +I C 2 R I (3)

[0109] P D1 =V D I D +I D 2 R D (4)

[0110] Where P I1 , P D1 are the conduction losses of IGBT and diode respectively, V ce 、V D are the saturation voltage drops of IGBT and diode, respectively. c ,I D are the currents flowing through the IGBT and the diode respectively.

[0111] The switching loss of IGBT and diode can be calculated by equation (5) and (6). The energy of IGBT and diode switching loss can be obtained by referring to the device manual.

[0112] P I2 =f sw E I (I C ) (5)

[0113] P D2 =f sw E D (I D ) (6)

[0114] Where P I2 , P D2 is the switching loss of IGBT and diode, f sw is the switching frequency, E I and E I are the switching loss energies of IGBT and diode respectively.

[0115] Figure 6 This is a schematic diagram of the junction temperature partition of the switching device in the inverter, refer to Figure 6, the junction temperature of the switching device when the inverter is running is set to be divided into three areas, namely the safe operation area, the warning operation area and the fault operation area. When the junction temperature of the switching device is within the safe operation area and the warning operation area, the inverter can operate normally. When it is in the fault operation area, the inverter needs to stop working in a short time, otherwise it will damage the power switching device and cause irreversible impact on the overall system. Optionally, in a two-stage system, the preset junction temperature upper limit can be equal to the temperature upper limit T1 of the warning operation area. In other optional embodiments, in order to ensure the safe operation of the system, the preset junction temperature upper limit can also be less than the temperature upper limit T1 of the warning operation area, but greater than the temperature upper limit T2 of the safe operation area.

[0116] As the active output power of the inverter gradually increases, the junction temperature of each switch device shows an increasing trend. Optionally, the active output power of the switch device that first reaches the preset junction temperature upper limit during the process of the active output power of the inverter gradually increasing is determined as the maximum active output power. In this way, it can be ensured that when the inverter is subsequently controlled to operate at the maximum active output power, the junction temperature of each switch device will not exceed the preset junction temperature upper limit, thereby ensuring the safe operation of the system.

[0117] Figure 7 This is a specific flow chart for determining the maximum active output power. Figure 7 The process of determining the maximum active output power includes: controlling the reactive output current of the inverter according to the target reactive current value, controlling the bus voltage of the inverter to be fixed, and setting the active output power output of the inverter to 0; increasing the active output power of the inverter according to a first preset step size; judging whether the maximum junction temperature of each switching device in the inverter reaches a preset temperature upper limit; if so, determining that the current active output power is the maximum active output power; if not, returning to execute the step of increasing the active output power of the inverter according to the first preset step size.

[0118] Figure 8 It is a schematic diagram of the junction temperature curves of different switching devices of NPC inverter under different active currents, where Figure 8 Only one junction temperature curve is schematically shown. Under different working conditions, the junction temperature variation of the same switch device in the inverter may be different, and the junction temperature curves of the switch devices in different inverters may also be different. Figure 4 As shown, the NPC inverter includes IGBTs and diodes. According to the different functions of IGBTs, they can be divided into two types, namely, type IGBT and type II IGBT, and the two types of IGBTs are respectively denoted as DCSW and ACSW; according to the different functions of diodes, they can be divided into three types, namely, type I diodes, type II diodes and type III diodes, and the three types of diodes are respectively denoted as DCD, ACD and NPD. Figure 4 and Figure 8, taking the preset junction temperature upper limit as 110℃ as an example, in the process of the active output power gradually increasing ( Figure 8 In the example of increasing the active output power by increasing the active current output by the inverter), in each switching device, the third type diode NPD reaches 110°C first, and the active output power corresponding to the active current when the third type diode NPD reaches 110°C first is determined as the maximum active output power.

[0119] S250, controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine the target bus voltage.

[0120] In this step, while controlling the inverter output to maintain the target reactive current and the maximum active output power, the target bus voltage is determined between the maximum bus voltage and the minimum bus voltage. The target bus voltage satisfies that the junction temperature of each switching device in the inverter does not exceed the preset junction temperature upper limit. In this way, the target bus voltage of the inverter under the conditions of the target reactive current and the maximum active output power to support the operation of the power grid can be determined, ensuring that in the event of a grid voltage drop, the inverter can support the operation of the power grid and maintain the maximum output active power. By optimizing the bus voltage, the inverter can be prevented from overloading, the thermal stress and loss of the switching devices can be reduced, and the adaptability of the inverter under complex power grid conditions can be enhanced.

[0121] Optionally, S250 includes: controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, controlling the bus voltage to gradually decrease from the maximum bus voltage to the minimum bus voltage; in the process of gradually decreasing the maximum bus voltage to the minimum bus voltage, obtaining the shell temperature of each switching device, and determining the temperature margin between the maximum junction temperature in each switching device and the preset junction temperature upper limit according to the shell temperature in each switching device; determining the bus voltage when the temperature margin is the maximum as the target bus voltage. In some embodiments, the bus voltage corresponding to other temperature margins greater than 0 can also be set as the target bus voltage.

[0122] The maximum junction temperature of each switching device is the maximum value of the junction temperatures corresponding to each switching device included in the inverter. Optionally, the temperature margin is equal to the difference between the preset junction temperature upper limit and the maximum junction temperature of each switching device. By determining the bus voltage corresponding to the maximum temperature margin as the target bus voltage, it can be ensured that when the inverter is operating at the target bus voltage, the margin between the junction temperature of each switching device and the preset junction temperature upper limit is large, further reducing the thermal stress and loss of the switching device.

[0123] In some embodiments, the bus voltage can also be controlled to gradually increase from the minimum bus voltage to the maximum bus voltage. In the process of gradually increasing from the minimum bus voltage to the maximum bus voltage, the shell temperature of each switching device is obtained, and the temperature margin is calculated. Then, the bus voltage when the temperature margin is maximum is determined as the target bus voltage.

[0124] Combination Figure 6 In some embodiments, the difference between the maximum junction temperature in each switching device and the upper limit of the junction temperature in the safe operation area can also be determined as the temperature margin, and the bus voltage when the temperature margin is the smallest can be determined as the target bus voltage. Fig. 9 This is the specific flow chart for determining the target bus voltage. Fig. 9 The process of determining the target bus voltage includes: controlling the inverter output to be the target reactive current and the maximum active output power, and the bus voltage to be the preset maximum bus voltage; determining the junction temperature of the switching device according to the obtained shell temperature of the switching device; selecting the maximum junction temperature and the upper limit of the junction temperature in the safe operation area as the difference to obtain the temperature margin; reducing the bus voltage, for example, the bus voltage can be reduced according to the second preset step size; determining whether the temperature margin is reduced; if so, returning to the step of reducing the bus voltage; if not, increasing the bus voltage, and determining the bus voltage with the smallest temperature margin as the target bus voltage.

[0125] Fig.10 It is a schematic diagram of the change of junction temperature of switching devices under different bus voltages. Fig.11 This is another schematic diagram of the junction temperature changes of the switching devices under different bus voltages. In the process of the bus voltage decreasing from the maximum bus voltage to the minimum bus voltage, the junction temperature changes of the switching devices in the inverter mainly include Fig.10 and Fig.11 There are two cases. When the temperature margin is taken as the difference between the maximum junction temperature and the upper limit of the junction temperature in the safe operation area of ​​the switching device, Fig.10 The junction temperature change is shown in Figure 2. As the bus voltage decreases from the maximum bus voltage to the minimum bus voltage, the temperature margin first decreases and then increases. Fig.11 The junction temperature change is shown in Figure 1. As the bus voltage decreases from the maximum bus voltage to the minimum bus voltage, the temperature margin gradually decreases. For both change trends, the bus voltage with the smallest temperature margin can be determined as the target bus voltage by judging whether the temperature margin change decreases during the decrease of the bus voltage, thereby ensuring that the determined target bus voltage will not cause the inverter to overload and reduce thermal stress and loss.

[0126] When the temperature margin is equal to the difference between the preset upper junction temperature limit and the maximum junction temperature of each switching device, Fig.10The junction temperature change is shown in Figure 2. As the bus voltage decreases from the maximum bus voltage to the minimum bus voltage, the temperature margin increases first and then decreases. Fig.11 The junction temperature change is shown in Figure 1. As the bus voltage decreases from the maximum bus voltage to the minimum bus voltage, the temperature margin gradually increases. For both change trends, by judging whether the temperature margin is the largest during the decrease of the bus voltage, the bus voltage with the largest temperature margin can be determined as the target bus voltage, thereby ensuring that the determined target bus voltage will not cause the inverter to overload and reduce thermal stress and loss.

[0127] Fig.12 is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention, with reference to Fig.12 , the junction temperature control method includes:

[0128] S310. When low voltage ride-through occurs, measure grid impedance parameters.

[0129] S320: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data.

[0130] Optionally, when the absolute value of the difference between the current grid impedance parameter and the grid impedance parameter in the historical data is less than or equal to a set threshold, it is determined that the current grid impedance parameter matches the grid impedance parameter in the historical data, otherwise it is determined that the current grid impedance parameter does not match the grid impedance parameter in the historical data. In some embodiments, when the current grid impedance parameter is equal to the grid impedance parameter in the historical data, it is determined that the current grid impedance parameter matches the grid impedance parameter in the historical data, otherwise it is determined that the current grid impedance parameter does not match the grid impedance parameter in the historical data.

[0131] If yes, then S330 is executed to control the inverter operation according to the target reactive current, maximum active output power and target bus voltage corresponding to the grid impedance parameter in the historical data. That is, the active output power connected to the inverter is controlled to be equal to the maximum active output power, the reactive current output by the inverter is the target reactive current, and the bus voltage of the inverter is the target bus voltage.

[0132] If not, execute S340.

[0133] S340. Determine a target reactive current for supporting grid operation according to grid impedance parameters, and determine a minimum bus voltage of the inverter under the target reactive current.

[0134] S350, controlling the output reactive current of the inverter to be a target reactive current.

[0135] S360, controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches a preset junction temperature upper limit is the maximum active output power.

[0136] S370, controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage.

[0137] S380, control the inverter to operate at the target reactive current, maximum active output power and target bus voltage. In this way, it can be ensured that in the event of low voltage ride-through, the inverter outputs the target reactive current to support the grid operation, outputs the maximum active output power under the condition of meeting the reactive power demand, provides stronger power support for the grid, and ensures that the junction temperature of the switch device will not exceed the limit under the target bus voltage.

[0138] S390. Record the grid impedance parameters and the target reactive current, maximum active output power and target bus voltage corresponding to the grid impedance parameters as historical data.

[0139] For example, the historical data may be stored in a storage medium or in the cloud, so as to be used as a reference for controlling the inverter when low voltage ride-through occurs in the power grid.

[0140] In some embodiments, the inverter is connected in a single-stage system. Fig.13 This is a schematic diagram of the structure of a single-stage system. Fig.13 In a single-stage system, there is no DC / DC converter between the inverter 20 and the DC power supply 30, and the inverter 20 is connected to the power grid 40 at a common connection point PCC. In a single-stage system, the DC power supply 30 is mainly a photovoltaic array, and the photovoltaic array includes at least two photovoltaic strings. Since the bus voltage output characteristics of the photovoltaic cell in the single-stage system will change with the change of the photovoltaic active output power, the junction temperature control method of the inverter switch device in the single-stage system will be different from that in the two-stage system. Fig.14 is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention, with reference to Fig.14 , the junction temperature control method of the inverter switch device includes:

[0141] S410: When low voltage ride-through occurs, measure grid impedance parameters.

[0142] S420: Determine a target reactive current for supporting grid operation according to grid impedance parameters, and determine a minimum bus voltage of the inverter under the target reactive current.

[0143] S430, controlling the output reactive current of the inverter to be a target reactive current.

[0144] S440. Adjust the number of photovoltaic strings connected to the inverter.

[0145] Optionally, the single-stage system includes at least two photovoltaic strings, which are connected via a DC switch ( Fig.13 In the embodiment of the present invention, the number of photovoltaic strings connected to the inverter is controlled by controlling the number of closed DC switches (K1...Kn, where n is the total number of DC switches). By controlling the number of closed DC switches, the number of photovoltaic strings connected to the inverter can be controlled. Optionally, the number of photovoltaic strings connected to the inverter is adjusted in order from small to large. Exemplarily, the number of photovoltaic strings connected to the inverter is first controlled to be 1, and then the number of photovoltaic strings connected to the inverter is gradually increased. For example, one photovoltaic string is added to be connected to the inverter each time, that is, the number of photovoltaic strings connected to the inverter is controlled in order of 1, 2, 3...n, where n is the total number of photovoltaic strings in the single-stage system. In this case, each DC switch controls the connection of one photovoltaic string to the inverter.

[0146] S450. At each number of photovoltaic strings connected to the inverter, adjust the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage, obtain the junction temperature of each switching device, and determine the temperature margin according to the maximum junction temperature of the switching device and a preset junction temperature upper limit.

[0147] In some embodiments, the temperature margin is equal to the difference between the upper junction temperature limit and the maximum junction temperature. In the process of adjusting the number of photovoltaic strings connected to the inverter, the inverter is controlled to output a target reactive current to support grid operation.

[0148] Optionally, S450 includes: at each number of photovoltaic strings connected to the inverter, controlling the bus voltage of the inverter to gradually decrease from the maximum bus voltage to the minimum bus voltage, and obtaining the junction temperature of each switching device, and determining the temperature margin according to the maximum junction temperature of the switching device and a preset junction temperature upper limit.

[0149] Fig.15 A schematic diagram showing the change in junction temperature of the switching device during bus voltage adjustment when only one PV string is connected to the inverter. Fig.16 Another schematic diagram shows the change in junction temperature of a switch device during bus voltage adjustment when only one PV string is connected to the inverter. The change in junction temperature of the same switch device may be different under different working conditions. The change in junction temperature of the switch device may also be different in different inverters. Figure 4 and Fig.15, when the preset junction temperature upper limit is 110℃, during the entire adjustment process of the bus voltage, the maximum junction temperature of each switch device does not reach the preset junction temperature upper limit, and the temperature margin is greater than 0. In this case, the number of PV strings connected to the inverter needs to be increased to determine the target number of PV strings connected to the inverter and the target bus voltage. Figure 4 and Fig.16 , when the preset junction temperature upper limit is 110℃, during the entire adjustment process of the bus voltage, the maximum junction temperature of each switching device does not reach the preset junction temperature upper limit, and the temperature margin is greater than 0. In this case, it is also necessary to increase the number of PV strings connected to the inverter to determine the target number of PV strings connected to the inverter and the target bus voltage.

[0150] S460: Determine a target number of photovoltaic strings connected to the inverter and a target bus voltage according to the temperature margin.

[0151] In the case where the temperature margin is equal to the difference between the junction temperature upper limit and the maximum junction temperature, S460 may include: when i+1 photovoltaic strings are connected to the inverter, if the temperature margin is equal to 0, i is determined as the target number, and the bus voltage corresponding to the maximum temperature margin under the target number is determined as the target bus voltage, wherein i is greater than or equal to 1 and less than the total number of photovoltaic strings. In this way, it can be ensured that under the determined target number, the active output power of the inverter is large to provide stronger power support for the power grid. And under the target bus voltage of the target number, there is a large temperature margin between the junction temperature of the switching device and the preset junction temperature upper limit, reducing thermal stress and loss.

[0152] Exemplarily, S460 specifically includes: controlling the number of photovoltaic strings connected in parallel to the inverter to be j, controlling the bus voltage of the inverter to decrease from a preset maximum bus voltage to a minimum bus voltage, and obtaining the junction temperature of each switching device; j is a positive integer greater than or equal to 1, and j is less than or equal to the total number of photovoltaic strings; determining the temperature margin between the maximum junction temperature in each switching device and the preset junction temperature upper limit according to the shell temperature in each switching device; when the temperature margin is equal to 0, determining the target number of photovoltaic strings connected in parallel to the inverter to be j-1, and determining the bus voltage with the maximum temperature margin corresponding to the target number as the target bus voltage; when the temperature margin is greater than 0, setting j=j+1, and returning to the step of controlling the number of photovoltaic strings connected in parallel to the inverter to be j.

[0153] Fig.17 is a flow chart of another method for controlling junction temperature of an inverter switch device provided by an embodiment of the present invention, with reference to Fig.17 Optionally, the junction temperature control method of the inverter switching device includes:

[0154] S510: When low voltage ride-through occurs, measure grid impedance parameters.

[0155] S520: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data.

[0156] If yes, then S530 is executed to control the inverter operation according to the target reactive current, target quantity and target bus voltage corresponding to the grid impedance parameter in the historical data. That is, the number of photovoltaic strings connected to the inverter is controlled to be equal to the target quantity, the reactive current output by the inverter is the target reactive current, and the bus voltage of the inverter is the target bus voltage.

[0157] If not, execute S540.

[0158] S540: Determine a target reactive current for supporting grid operation according to the grid impedance parameter, and determine a minimum bus voltage of the inverter under the target reactive current.

[0159] S550. Control the output reactive current of the inverter to be a target reactive current.

[0160] S560: Adjust the number of photovoltaic strings connected to the inverter.

[0161] S570. At each number of photovoltaic strings connected to the inverter, adjust the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage, obtain the junction temperature of each switching device, and determine the temperature margin based on the maximum junction temperature of the switching device and a preset junction temperature upper limit.

[0162] S580: Determine a target number of photovoltaic strings connected to the inverter and a target bus voltage according to the temperature margin.

[0163] S590, control the inverter to operate with the target reactive current, target number of photovoltaic strings connected and target bus voltage. In this way, it can be ensured that in the event of low voltage ride-through, the inverter outputs the target reactive current to support the grid operation, and the inverter is controlled to have a larger active output power under the condition of meeting the reactive power demand, providing stronger power support for the grid, and ensuring that the junction temperature of the switch device does not exceed the limit under the target bus voltage.

[0164] S600. Record the grid impedance parameter and the target reactive current, target quantity, and target bus voltage corresponding to the grid impedance parameter as historical data.

[0165] Based on the above embodiments, Figure 2 and Fig.13 , optionally, the grid impedance parameters include the grid impedance R g and grid inductive reactance L g; In S110, S210, S310, S410, and S510, in the event of low voltage ride-through, the grid impedance parameters are measured, including: in the event of low voltage ride-through, the grid impedance is calculated by disturbing the active output power of the inverter, and the grid inductive reactance is calculated by the reactive output power of the inverter.

[0166] Fig.18 It is the waveform diagram of power disturbance measurement grid impedance parameters. Fig.18 When low voltage ride-through occurs in the power grid, the impedance parameters of the power grid are measured first. The measurement diagram is as follows: Fig.18 As shown, the measurement is mainly divided into two parts: active disturbance measurement grid impedance and reactive disturbance measurement grid inductive reactance. The calculation formulas are shown in formula (7) and formula (8) respectively.

[0167] (7)

[0168] (8)

[0169] Where R g is the grid impedance, L g is the grid inductive reactance, is the d-axis variation of the grid voltage, is the q-axis variation of the grid voltage, is the d-axis variation of the grid current, is the q-axis variation of the grid current, ω is the grid angular frequency, and its value is 2πf, where f is the grid frequency.

[0170] Optionally, S120, S220, S340, S420, and S540 determine the target reactive current to support the operation of the grid according to the grid impedance parameters, control the inverter output to achieve the target reactive current, and determine the minimum bus voltage of the inverter under the target reactive current, including: determining the target reactive current according to the grid inductance, the target voltage of the common connection point between the inverter and the grid, the grid voltage, and the grid angular frequency; determining the minimum bus voltage according to the target reactive current, the grid voltage, the grid inductance, and the parameters of the filter inductance in the inverter.

[0171] Specifically, since the resistance in the weak grid is relatively small, R can be ignored in the calculation. g After obtaining the grid impedance parameters, the reactive current required for the inverter to restore the common connection point, that is, the target reactive current, can be obtained through the formula.

[0172] I q =(V pcc -V g ) / ωL g (9)

[0173] Where V pccis the target voltage that the inverter and the grid common connection point need to restore to, V g is the grid voltage.

[0174] The parameters of the grid-connected inverter filter inductor are expressed in L x According to formula (10), when the SVPWM modulation strategy is adopted, the inverter bus voltage V bus The minimum value of , that is, the minimum bus voltage.

[0175] V bus =(ω(L g +L x )I q +V g )*2 / 1.15 (10)

[0176] The junction temperature control method of the inverter switch device according to the above embodiment of the present invention is described below in combination with specific parameters.

[0177] For the two-stage system embodiment parameters: the rated bus voltage is 1500V, the rated grid-connected AC line voltage is 800V, the grid-side phase voltage is 461.89V, and the peak value is 461.89*1.414=653.11V. The low voltage ride-through depth is 0.5pu, the grid inductance is 4mH, and the inverter filter inductance is 500uH.

[0178] According to formula (9), we can get:

[0179] I q =(V pcc -V g ) / ωL g =(653.11-653.11*0.5) / 2π*50*0.004=260 (A)

[0180] According to formula (10), we can get:

[0181] V bus =(ω(L g +L x )I q +V g )*2 / 1.15=(2π*50*(0.004+0.0005)*260+(653.11*0.5))*2 / 1.15=1206.84(V)

[0182] The calculation shows that the reactive current to be compensated (that is, the target reactive current) is 260A, and the minimum bus voltage is 1206V. In order to increase the margin, the sum of the calculated minimum bus voltage and the set margin voltage can be selected as the final minimum bus voltage. For example, the final minimum bus voltage can be determined to be 1210V. Fig.19 It is the change in junction temperature of power switch device under the target reactive current of the inverter, which disturbs the active current. Fig.19 and Figure 8 Correspondingly, it can be seen that as the active current changes, the maximum junction temperature of the switching device also changes, and it is finally determined that the active current corresponding to the maximum active output power is 21A.

[0183] Fig. 20 A schematic diagram shows that the active current and target reactive current corresponding to the maximum active output power are determined, and the change of the temperature margin of the overall system when the bus voltage is disturbed is shown. Taking the temperature margin equal to the difference between the upper limit of the junction temperature and the maximum junction temperature as an example, it can be seen from the figure that the temperature margin of the overall system is the largest when the bus voltage is 1287V. Therefore, 1287V is selected as the bus voltage under the low voltage ride-through depth and grid impedance parameters. Fig.21 Another schematic diagram shows the change of the overall system temperature margin when the bus voltage is disturbed, when the active current and target reactive current corresponding to the maximum active output power are determined. It can be seen from the figure that as the bus voltage decreases, the system temperature margin gradually increases. Therefore, the minimum bus voltage value of 1210V is selected as the bus voltage under the low voltage ride-through depth and grid impedance parameters.

[0184] Parameters of the single-stage system embodiment: the rated bus voltage is 1500V, the rated grid-connected AC line voltage is 800V, the low voltage ride-through depth is 0.5pu, the grid inductance is 4mH, the inverter filter inductance is 500uH, when only one DC switch is closed, the photovoltaic cell terminal voltage is 1530V, the maximum power point voltage is 1250V, and the maximum power point current is 7.2A. When multiple DC switches are closed, they form a parallel relationship, the voltage does not change, and the current increases exponentially. When the DC switch is closed to four circuits, the junction temperature of the power switch device exceeds the upper limit of the warning operation value (that is, the preset junction temperature upper limit) when the bus voltage is disturbed, so the previous state is selected, and the DC switch is closed in three circuits. At this time, the bus voltage system temperature margin is disturbed. Fig. 22 A schematic diagram showing the change of the overall system temperature margin when the bus voltage is disturbed when the DC switch is closed to three paths. Fig. 22 As shown, the state with the largest temperature margin is selected as the inverter system control parameter. At this time, the bus voltage is 1282V and the output active current is 21A. Fig.23 Another schematic diagram shows the change of the overall system temperature margin when the bus voltage is disturbed when the DC switch is closed to three paths. Fig.23 As shown, as the bus voltage decreases, the system temperature margin gradually increases, and the lowest bus voltage 1210V is selected as the inverter system control parameter, at which time its active current is 22.2A.

[0185] Through the above embodiments, it can be found that the junction temperature control method of the inverter switching device during low voltage ride-through provided by the present invention can not only output the corresponding reactive current to support the power grid as required, but also determine the maximum active current output capacity under this condition, and on this basis, disturb the bus voltage to determine the bus voltage value with the minimum inverter junction temperature, establish the bus voltage and system temperature margin curve, reduce the overall loss of the inverter, and facilitate thermal management.

[0186] The embodiment of the present invention further provides a junction temperature control device for an inverter switch device. Fig.24 is a schematic diagram of a junction temperature control device of an inverter switch device provided by an embodiment of the present invention, with reference to Fig.24 , the junction temperature control device comprises:

[0187] The measuring module 610 is used to measure the impedance parameters of the power grid when low voltage ride-through occurs;

[0188] A first determination module 620 is used to determine a target reactive current supporting the operation of the power grid according to the power grid impedance parameter, and to determine a minimum bus voltage of the inverter under the target reactive current;

[0189] A control module 630, configured to control the output reactive current of the inverter to be the target reactive current;

[0190] The second determination module 640 adjusts the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, wherein the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

[0191] The junction temperature control device of the inverter switch device provided in the embodiment of the present invention can execute the junction temperature control method of the inverter switch device provided in any embodiment of the present invention, and has the corresponding functional modules and beneficial effects of the execution method.

[0192] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the present invention can be executed in parallel, sequentially or in different orders, as long as the desired results of the technical solution of the present invention can be achieved, and this document does not limit this.

[0193] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for controlling junction temperature of an inverter switching device, characterized in that: include: In case of low voltage ride-through, measure the grid impedance parameters; Determining a target reactive current for supporting grid operation according to the grid impedance parameter, and determining a minimum bus voltage of the inverter under the target reactive current; Controlling the output reactive current of the inverter to be the target reactive current; The bus voltage of the inverter is adjusted between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, wherein the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

2. The method for controlling junction temperature of an inverter switch device according to claim 1, characterized in that: The inverter is connected in a two-stage system; before adjusting the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, the method further includes: Controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches the preset junction temperature upper limit is the maximum active output power; The step of adjusting the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage to determine a target bus voltage includes: The reactive current of the inverter is controlled to maintain the target reactive current. When the active output power of the inverter is the maximum active output power, the bus voltage of the inverter is adjusted between a preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage.

3. The method for controlling junction temperature of an inverter switch device according to claim 2, characterized in that: The controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches the preset junction temperature upper limit is the maximum active output power, includes: Controlling the bus voltage of the inverter to remain unchanged, and gradually increasing the active output power of the inverter from 0; In the process of gradually increasing the active output power of the inverter, obtaining the shell temperature of each of the switching devices in the inverter; When the junction temperature of the switching device corresponding to the shell temperature of at least one of the switching devices reaches the preset junction temperature upper limit, the corresponding active output power is determined as the maximum active output power.

4. The method for controlling junction temperature of an inverter switch device according to claim 2, characterized in that: The controlling the reactive current of the inverter to maintain the target reactive current and the active output power of the inverter being the maximum active output power, adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, comprises: controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, controlling the bus voltage to gradually decrease from the maximum bus voltage to the minimum bus voltage; In the process of the maximum bus voltage gradually decreasing to the minimum bus voltage, the shell temperature of each of the switching devices is acquired, and the temperature margin between the maximum junction temperature of each of the switching devices and the preset junction temperature upper limit is determined according to the shell temperature of each of the switching devices; The bus voltage when the temperature margin is maximum is determined as the target bus voltage.

5. The method for controlling junction temperature of an inverter switch device according to claim 2, characterized in that: Under the condition that the reactive current of the inverter is controlled to maintain the target reactive current and the active output power of the inverter is the maximum active output power, after adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage, the method further includes: Controlling the inverter to operate with the target reactive current, the maximum active output power and the target bus voltage; Recording the grid impedance parameter and the target reactive current, the maximum active output power and the target bus voltage corresponding to the grid impedance parameter as historical data; After determining the grid impedance parameter, the method further includes: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, the maximum active output power and the target bus voltage corresponding to the grid impedance parameter in the historical data; If not, the steps of determining a target reactive current for supporting grid operation according to the grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

6. The method for controlling junction temperature of an inverter switch device according to claim 1, characterized in that: The inverter is connected to a single-stage system, and the single-stage system includes at least two photovoltaic strings; the bus voltage of the inverter is adjusted between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, including: adjusting the number of photovoltaic strings connected to the inverter; At each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is adjusted between a preset maximum bus voltage and the minimum bus voltage, and the junction temperature of each of the switching devices is obtained, and the temperature margin is determined according to the maximum junction temperature of the switching device and the preset junction temperature upper limit; A target number of photovoltaic strings connected to the inverter and the target bus voltage are determined according to the temperature margin.

7. The method for controlling junction temperature of an inverter switch device according to claim 6, characterized in that: The adjusting the number of photovoltaic strings connected to the inverter comprises: Adjusting the number of photovoltaic strings connected to the inverter in ascending order; The step of adjusting the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage at each number of photovoltaic strings connected to the inverter, acquiring the junction temperature of each switching device, and determining the temperature margin according to the maximum junction temperature of the switching device and the preset junction temperature upper limit comprises: At each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is controlled to gradually decrease from the maximum bus voltage to the minimum bus voltage, and the junction temperature of each switching device is obtained, and the temperature margin is determined according to the maximum junction temperature of the switching device and the preset junction temperature upper limit; The method of determining the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin includes: when i+1 photovoltaic strings are connected to the inverter, if the temperature margin is equal to 0, i is determined as the target number, and the bus voltage with the maximum temperature margin corresponding to the target number is determined as the target bus voltage, wherein i is greater than or equal to 1 and less than the total number of the photovoltaic strings.

8. The method for controlling junction temperature of an inverter switch device according to claim 6, characterized in that: After determining the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin, the method further includes: Controlling the inverter to operate at the target reactive current, connecting the target number of photovoltaic strings and the target bus voltage; Recording the grid impedance parameter and the target reactive current, the target quantity and the target bus voltage corresponding to the grid impedance parameter as historical data; After determining the grid impedance parameter, the method further includes: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, the target quantity, and the target bus voltage corresponding to the grid impedance parameter in the historical data; If not, the steps of determining a target reactive current for supporting grid operation according to the grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

9. The method for controlling junction temperature of an inverter switch device according to claim 1, characterized in that: The grid impedance parameters include grid impedance and grid inductance; when low voltage ride through occurs, measuring the grid impedance parameters includes: In the event of low voltage ride-through, the grid impedance is calculated by disturbing the active output power of the inverter, and the grid inductive reactance is calculated by the reactive output power of the inverter; The step of determining a target reactive current for supporting grid operation according to the grid impedance parameter, controlling the inverter output to reach the target reactive current, and determining a minimum bus voltage of the inverter under the target reactive current includes: Determining the target reactive current according to the grid inductive reactance, the target voltage of the common connection point between the inverter and the grid, the grid voltage and the grid angular frequency; The minimum bus voltage is determined according to the target reactive current, the grid voltage, the grid inductance, and parameters of the filter inductance in the inverter.

10. A junction temperature control device for an inverter switch device, characterized in that: include: A measurement module is used to measure grid impedance parameters in the event of low voltage ride-through; A first determination module is used to determine a target reactive current supporting the operation of the power grid according to the power grid impedance parameter, and to determine a minimum bus voltage of the inverter under the target reactive current; A control module, used for controlling the output reactive current of the inverter to be the target reactive current; The second determination module adjusts the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, wherein the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

Images