Method for enhancing power of network construction type converter based on variable switching frequency under dynamic state of power grid

Abstract

The invention discloses a variable switching frequency-based power enhancement method for a network-constructing type converter under a dynamic state of a power grid, and relates to the field of network-constructing type converter control. According to the method, the power module loss and thermal network model is constructed, the junction temperature of the power device of the network-forming type converter is estimated to keep the junction temperature unchanged, the relation model of the power reference value and the switching frequency of the converter is established, and the power-switching frequency relation model is looked up after the power reference value is obtained through the network-forming algorithm during the dynamic period of the power grid. And obtaining the switching frequency corresponding to the current power reference value for underclocking operation, thereby reducing the switching loss of the power module, and ensuring that the maximum power output capability of the converter is maintained under the dynamic state of the power grid until the switching frequency is reduced to the lowest amplitude limit. The method for enhancing the power of the constructed network type current converter effectively improves the voltage and frequency supporting capability under the dynamic state of the power grid.

Description

Technical Field

[0001] The present invention belongs to the field of grid-type converter control, and in particular relates to a grid-type converter power enhancement method based on variable switching frequency under dynamic power grid conditions. Background Art

[0002] Grid-following control is externally equivalent to a current source. During dynamic grid voltage events, such as low-voltage fault ride-through, overcurrent is prevented due to the current limiting effect of the inner current loop. However, for grid-following converters, when a grid voltage sag occurs, the converter's virtual internal potential e remains constant for a moment, resulting in a significant voltage drop between the converter and the grid, and thus, a significant overcurrent. Traditional synchronous generators have strong overcurrent capabilities, capable of withstanding overcurrents 5 to 7 times greater. However, power electronic converters based on virtual synchronous machine control have limited overcurrent tolerance, and excessive currents can cause permanent damage to power electronic components.

[0003] To address the above issues, there are currently two common ways for grid-type converters to cope with grid dynamics such as low voltage ride-through and frequency fluctuations. One is to switch grid-type control to grid-following control when the grid voltage drops. Since grid-following control is externally equivalent to a current source, it directly limits the current by changing the current reference value, thereby suppressing overcurrent. However, this will cause the converter to lose its ability to actively support the grid, and if the switching method is improper during the switching process, overcurrent can easily occur. Another method is to not switch modes, retain the grid-type characteristics of the converter, and reduce overcurrent by reducing the voltage drop difference between the converter and the grid. For example, a virtual impedance method can be used. Virtual impedance simulates the output resistance and reactance of the converter and achieves the purpose of current limiting by reducing the terminal voltage of the converter. Alternatively, the voltage reference can be changed to control the converter output voltage and thus reduce overcurrent. This essentially retains the voltage source characteristics of the converter, reduces the voltage difference between the converter and the grid, and thus suppresses overcurrent.

[0004] Power electronic devices have a limited ability to withstand overcurrent. When the grid voltage drops, converters must not only meet the reactive power requirements specified in national standards but also output active power to support the grid. While existing current-limiting measures can suppress overcurrent, simply reducing output current will affect the converter's output power, significantly weakening the grid-connected converter's ability to actively support the grid. This can easily lead to an imbalance between active load supply and demand, and pose the risk of frequency instability in the power system. Therefore, a control strategy is needed to suppress overcurrent while minimizing the active power output capacity of the grid-connected converter and ensuring stable power system operation. Summary of the Invention

[0005] In response to the problems existing in the prior art, the present invention provides a method for enhancing the power of a grid-type converter based on variable switching frequency under dynamic grid conditions, which can effectively solve the problem that existing current limiting measures reduce the output capacity of the grid-type converter and are not conducive to supporting the grid.

[0006] To solve the above technical problems, the present invention provides the following technical solution: a method for enhancing the power of a grid-connected converter based on variable switching frequency under dynamic grid conditions, comprising the following steps:

[0007] S1. Constructing a grid-type converter model, a power module loss model, and a power module thermal network model; then, based on the grid-type converter model, the power module loss model, and the power module thermal network model, obtaining a corresponding relationship between an active power reference value and a switching frequency of the grid-type converter that meets preset conditions; and further establishing a power-switching frequency relationship model under different voltage drop levels in an offline manner;

[0008] S2. Detect the dynamic process of the power grid and obtain the corresponding power reference value P under the dynamic process based on the network control algorithm. ref , Q ref ; Input the power reference value into the offline power and switching frequency relationship model to obtain the corresponding switching frequency f sw , determine the switching frequency f sw Is it greater than or equal to the minimum limit value? If so, no current limiting operation is performed; otherwise, current limiting operation is performed to reduce the operating current of the grid-type converter.

[0009] Furthermore, in the aforementioned step S1, the power reference value P ref and Q ref As input, the time domain waveforms of current and voltage are output, and a grid-type converter model is constructed;

[0010] The power module loss model is constructed with current, voltage and given switching frequency data as input and the total loss of the power module as output;

[0011] Taking the total loss of the power module as input and the actual junction temperature of the power module T j For output, a power module thermal network model is constructed.

[0012] Furthermore, in the aforementioned step S1, the corresponding relationship between the active power reference value and the switching frequency of the grid-type converter that meets the preset conditions is obtained, and a power-switching frequency relationship model under different voltage drop levels is established in an offline manner. The specific method is as follows: the total power module loss calculated by the power module loss model is input into the power module thermal network model to calculate the actual junction temperature T of the power module. j ; The junction temperature calculation result T j With a given reference junction temperature T jNComparison is performed. If the junction temperature exceeds the reference junction temperature, the given switching frequency is reduced, and the re-given switching frequency is input into the power module loss model to calculate the module loss. The junction temperature is then input into the thermal network model to calculate the junction temperature, which is then compared with the reference junction temperature again until the calculated junction temperature does not exceed the reference junction temperature. The corresponding active power reference value that does not exceed the reference junction temperature is obtained.

[0013] The corresponding active power reference value and switching frequency relationship of the grid-type converter that does not exceed the reference junction temperature are saved; different active power reference values ​​are selected and the above steps are repeated to establish the corresponding relationship between power and switching frequency. By changing different grid drop levels, the power reference value Q is selected according to the voltage drop level. ref , and obtain the relationship model between power and switching frequency under different voltage drop levels.

[0014] Furthermore, the aforementioned step S2 is specifically as follows:

[0015] The control system detects the voltage drop and frequency fluctuation of the power grid in real time, and obtains the corresponding power reference value P under the dynamic process based on the specific network control algorithm. ref , Q ref ; Input the power reference value as input to the offline established power and switching frequency relationship model to obtain the corresponding switching frequency f under the dynamic process sw ; Determine the switching frequency f sw Whether it meets the minimum limit requirement, if it is lower than the minimum limit, the power reference value is limited to reduce the operating current of the grid-type converter, and the updated power reference value P is ref , Q ref Input the power and switching frequency relationship model established offline to obtain the corresponding switching frequency f under current limiting conditions sw , until the switching frequency f sw To meet the minimum limit, if the switching frequency f sw If the minimum limit requirement is met, that is, it is greater than or equal to the minimum limit, current limiting operation will not be performed; finally, a new power reference value and switching frequency control parameters are configured for the converter to achieve high-power frequency reduction operation.

[0016] Furthermore, the aforementioned power module includes IGBT, MOSFET, and wide bandgap devices.

[0017] Furthermore, in one cycle, the power module and diode conduction loss P Icon 、P Fcon It is expressed as follows:

[0018]

[0019] Where V ce0 、V F0is the saturation voltage drop of the power module and the threshold voltage of the diode, r ce 、r F are the on-resistance of the power module and diode respectively, i a is the current flowing through the power device, d I and d F are the duty cycles corresponding to when the power module and diode are turned on.

[0020] Furthermore, in the aforementioned method for enhancing the power of a grid-type converter based on variable switching frequency under a dynamic grid, within a switching cycle, the turn-on loss and turn-off loss of the device are expressed as a quadratic function of the current, and the turn-on loss and turn-off loss of the power module E on and E off It is expressed as follows:

[0021]

[0022] Where a0, a1, a2, b0, b1, b2 are E on 、E off with i a Coefficients of the quadratic function; V, V rated are the actual voltage and rated voltage of the device respectively;

[0023] The switching loss of the power module in one switching cycle is expressed as follows:

[0024]

[0025] Where, f sw is the switching frequency;

[0026] The reverse recovery loss of the diode E rec Expressed as:

[0027]

[0028] Where c0, c1, and c2 are E rec with i a Coefficients of quadratic functions;

[0029] The diode switching loss in one switching cycle is expressed as:

[0030]

[0031] The power loss of the power module during one switching cycle is expressed as:

[0032] P IT =P Icon +P Isw (6)

[0033] The diode power loss in one switching cycle is expressed as:

[0034] P FT =P Fcon +P Fsw (7)

[0035] By obtaining the grid-type converter operating parameters required for power module loss calculation, the device loss value can be obtained. The device junction temperature value can be obtained through the thermal impedance network, which facilitates real-time reading of the device junction temperature.

[0036] Furthermore, the junction temperature of the aforementioned chip is expressed as:

[0037] T j =T a +P loss Z thj→a (t) (8)

[0038] Where, T j is the chip temperature, T a is the ambient temperature, P loss is the chip loss value, Z thj→a (t) represents the total impedance from the chip to the environment;

[0039] The thermal impedance network includes the thermal impedance from the chip to the housing, from the housing to the heat sink, and from the heat sink to the environment. The total impedance Z thj→a (t) is expressed as:

[0040]

[0041] Where, thermal resistance R i (i=1,2...6) represents the steady-state junction temperature of the power device, thermal capacitance C i (i=1, 2...6) represents the rate of change of the junction temperature of the power device.

[0042] Furthermore, the aforementioned method for enhancing the power of a grid-type converter based on variable switching frequency under dynamic grid conditions uses a grid-type converter to provide reactive power support to the grid during a voltage drop in the grid.

[0043] During the grid voltage sag, the reactive current output by the converter should satisfy the following formula:

[0044]

[0045] Where U N is the rated voltage of the grid, I N is the rated current of the converter;

[0046] According to the reactive current I qref and reactive power given value Q refThe relationship between the reactive power output and the reactive power output can be obtained as follows:

[0047]

[0048] Furthermore, the aforementioned grid dynamics include low voltage fault ride-through, frequency fluctuation, load mutation, and synchronous frequency resonance.

[0049] Compared with the prior art, the beneficial technical effects of the present invention using the above technical solution are as follows:

[0050] 1. The present invention considers that the junction temperature of the device is an important factor in measuring whether the device can work normally for a long time, and uses the junction temperature of the device to evaluate the service life of the device;

[0051] 2. The present invention considers the impact of variable switching frequency on the output capacity of the grid-type converter, effectively improving the active output capacity of the grid-type converter under grid voltage drops and enhancing the converter's support capacity for the grid;

[0052] 3. The present invention uses Matlab / Simulink to build an electrothermal simulation model of a grid-type converter. Combined with various grid voltage drop scenarios, the active power of the grid-type converter is increased quickly and efficiently by adjusting the switching frequency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a flow chart of the method of the present invention.

[0054] Figure 2 This is a flow chart of establishing a power and switching frequency relationship model in the present invention.

[0055] Figure 3 This is the Foster thermal network model diagram.

[0056] Figure 4 This is the electrothermal simulation model diagram of the grid-type converter.

[0057] Figure 5 This is a schematic diagram of the output power of a grid-type two-level converter when the grid voltage drops by 50% Ug. In the figure, (a) is a schematic diagram of the output reactive power, (b) is a schematic diagram of the output active power at a switching frequency of 20 kHz, (c) is a schematic diagram of the output active power at a switching frequency of 18 kHz, and (d) is a schematic diagram of the output active power at a switching frequency of 16 kHz.

[0058] Figure 6 This is a schematic diagram of the relationship between the switching frequency and output power of a grid-type two-level converter. DETAILED DESCRIPTION

[0059] In order to better understand the technical content of the present invention, specific embodiments are given and described below with reference to the accompanying drawings.

[0060] Various aspects of the present invention are described herein with reference to the accompanying drawings, which show a number of illustrative embodiments. The embodiments of the present invention are not limited to those described in the accompanying drawings. It should be understood that the present invention can be implemented by any of the various concepts and embodiments described above, as well as the concepts and implementations described in detail below, because the concepts and embodiments disclosed herein are not limited to any particular implementation. In addition, some aspects disclosed herein may be used alone or in any appropriate combination with other aspects disclosed herein.

[0061] like Figure 1 As shown, the present invention provides a method for enhancing the power of a grid-type converter based on variable switching frequency under dynamic grid conditions, comprising the following steps:

[0062] S1. Constructing a grid-type converter model, a power module loss model, and a power module thermal network model; then, based on the grid-type converter model, the power module loss model, and the power module thermal network model, obtaining a corresponding relationship between an active power reference value and a switching frequency of the grid-type converter that meets preset conditions; and further establishing a power-switching frequency relationship model under different voltage drop levels in an offline manner;

[0063] Specifically, if Figure 2 As shown, the power reference value P ref and Q ref As input, the time domain waveforms of current and voltage are output to build a grid-type converter model; with current, voltage and given switching frequency data as input, the total loss of the power module is output to build a power module loss model; with the total loss of the power module not input, the actual junction temperature T j For output, a power module thermal network model is constructed;

[0064] Obtain the corresponding relationship between the active power reference value and switching frequency of the grid-type converter that meets the preset conditions, and establish the power and switching frequency relationship model under different voltage drop degrees in an offline manner. The specific method is as follows: the total power module loss calculated by the power module loss model is input into the constructed power module thermal network model to calculate the actual junction temperature T of the power module. j ; The junction temperature calculation result T j With a given reference junction temperature T jN Comparison is performed. If the junction temperature exceeds the reference junction temperature, the given switching frequency is reduced, and the re-given switching frequency is input into the power module loss model to calculate the module loss. The junction temperature is then input into the thermal network model to calculate the junction temperature, which is then compared with the reference junction temperature again until the calculated junction temperature does not exceed the reference junction temperature. The corresponding active power reference value that does not exceed the reference junction temperature is obtained.

[0065] The corresponding active power reference value and switching frequency relationship of the grid-type converter that does not exceed the reference junction temperature are saved; different active power reference values ​​are selected and the above steps are repeated to establish the corresponding relationship between power and switching frequency. By changing different grid drop levels, the power reference value Q is selected according to the voltage drop level. ref , and obtain the relationship model between power and switching frequency under different voltage drop levels.

[0066] S2. Detect the dynamic process of the power grid and obtain the corresponding power reference value P under the dynamic process based on the network control algorithm. ref , Q ref ; where Q ref Select according to national standards; input the power reference value into the power and switching frequency relationship model established offline to obtain the corresponding switching frequency f sw , determine the switching frequency f sw Is it greater than or equal to the minimum limit value? If so, no current limiting operation is performed; otherwise, current limiting operation is performed to reduce the operating current of the grid-type converter.

[0067] The specific implementation of step S2 is as follows: the control system detects the voltage drop and frequency fluctuation of the power grid in real time, and obtains the power reference value P corresponding to the dynamic process based on the specific network control algorithm, such as virtual synchronous machine, droop control, etc. ref , Q ref ; Input the power reference value as input to the offline established power and switching frequency relationship model to obtain the corresponding switching frequency f under the dynamic process sw ; Determine the switching frequency f sw Whether it meets the minimum limit requirement, if it is lower than the minimum limit, the power reference value is limited to reduce the operating current of the grid-type converter, and the updated power reference value P is ref , Q ref Input the power and switching frequency relationship model established offline to obtain the corresponding switching frequency f under current limiting conditions sw , until the switching frequency f sw To meet the minimum limit, if the switching frequency f sw If the minimum amplitude requirement is met, that is, it is greater than or equal to the minimum amplitude limit, current limiting operation is not performed; finally, a new power reference value and switching frequency control parameters are configured for the converter to achieve high-power frequency reduction operation. In this embodiment, the power module includes IGBTs, MOSFETs, and wide-bandgap devices.

[0068] Furthermore, during the process of turning on and off the device, the current flowing through the device and the voltage waveform across it overlap, which will produce turn-on and turn-off losses. In a switching cycle, the turn-on and turn-off losses of the device are expressed as a quadratic function of the current. The turn-on and turn-off losses of the power module are E on and Eoff It is expressed as follows:

[0069]

[0070] Where a0, a1, a2, b0, b1, b2 are E on 、E off with i a Coefficients of the quadratic function; V, V rated are the actual voltage and rated voltage of the device respectively;

[0071] The switching loss of the power module in one switching cycle is expressed as follows:

[0072]

[0073] Where, f sw is the switching frequency;

[0074] The reverse recovery loss of the diode E rec Expressed as:

[0075]

[0076] Where c0, c1, and c2 are E rec with i a Coefficients of quadratic functions;

[0077] The diode switching loss in one switching cycle is expressed as:

[0078]

[0079] The power loss of the power module during one switching cycle is expressed as:

[0080]

[0081] The diode power loss in one switching cycle is expressed as:

[0082]

[0083] By obtaining the grid-type converter operating parameters required for power module loss calculation, the device loss value can be obtained. The device junction temperature value can be obtained through the thermal impedance network, which facilitates real-time reading of the device junction temperature.

[0084] Figure 3 For the Foster thermal network model, the junction temperature of the chip is expressed as:

[0085] T j =T a +P loss Z thj→a (t) (8)

[0086] Where, T j is the chip temperature, T a is the ambient temperature, P loss is the chip loss value, Z thj→a (t) represents the total impedance from the chip to the environment;

[0087] The data sheet of the power module usually provides the thermal impedance network model of the module in the form of thermal resistance and time constant for user convenience. The thermal impedance network includes the thermal impedance from the chip to the housing, from the housing to the heat sink, and the heat sink to the environment. The total impedance Z thj→a (t) is expressed as:

[0088]

[0089] Where, thermal resistance R i (i=1,2...6) represents the steady-state junction temperature of the power device, R i The larger the value, the greater the steady-state junction temperature; the thermal capacity C i (i=1,2...6) represents the change rate of the junction temperature of the power device. i The smaller the time constant, the faster the junction temperature changes, and the R i and C i The greater the number, the more accurate the junction temperature of the device.

[0090] Reactive power set value Q ref The selection principle is as follows: During a grid voltage sag, a grid-type converter is required to provide reactive power support to the grid. According to GB / T 19964-2012, during a grid voltage sag, the reactive current output by the converter should meet the following requirements:

[0091]

[0092] Where U N is the rated voltage of the grid, I N is the rated current of the converter;

[0093] According to the reactive current I qref and reactive power given value Q ref The relationship between the reactive power output and the reactive power output can be obtained as follows:

[0094]

[0095] In order to verify the power enhancement method of grid-type converter based on variable switching frequency under low voltage ride-through, this application example is built on the Matlab / Simulink platform as follows Figure 4The grid-type electrothermal simulation model shown in the figure is used. At the same time, a grid voltage drop fault is set at 0.5s to verify the power enhancement characteristics of the grid-type converter. The specific parameters are shown in Table 1. The initial switching frequency of the converter is 20kHz, and the junction temperature of the power module IGBT device at rated voltage and rated power is 71.3℃, which is used as the reference junction temperature.

[0096] Table 1 Simulation parameters

[0097]

[0098] Figure 5 This diagram shows the output power of a two-level grid-connected converter when the grid voltage drops by 50% Ug. (a) shows the output reactive power, (b) shows the output active power at a switching frequency of 20 kHz, (c) shows the output active power at a switching frequency of 18 kHz, and (d) shows the output active power at a switching frequency of 16 kHz. The converter is required to output 15 kVar reactive power. As the switching frequency decreases from 20 kHz to 18 kHz and then to 16 kHz, the converter's output power gradually increases from 27.8 kW to 30.1 kW and then to 32.6 kW.

[0099] Figure 6 The following are the simulation results of the converter output active power under three different grid voltage drop levels and different switching frequencies. It can be seen that when the grid voltage drops by 5% U g 50% U g , 90% U g When the switching frequency is reduced from 20kHz to 16kHz, the output active power of the grid-connected two-level converter increases, and the supporting capacity of the power grid is greatly improved, which shows that the method proposed in the present invention is feasible and effective.

[0100] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0101] While the present invention has been described above with reference to preferred embodiments, this is not intended to limit the present invention. Persons skilled in the art will readily appreciate that various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions, characterized in that: The following steps are involved: S1. Constructing a grid-type converter model, a power module loss model, and a power module thermal network model; then, based on the grid-type converter model, the power module loss model, and the power module thermal network model, obtaining a corresponding relationship between an active power reference value and a switching frequency of the grid-type converter that meets preset conditions; and further establishing a power-switching frequency relationship model under different voltage drop levels in an offline manner; S2. Detect the dynamic process of the power grid and obtain the corresponding power reference value P under the dynamic process based on the network control algorithm. ref , Q ref ; Input the power reference value into the offline power and switching frequency relationship model to obtain the corresponding switching frequency f sw , determine the switching frequency f sw Is it greater than or equal to the minimum limit value? If so, no current limiting operation is performed; otherwise, current limiting operation is performed to reduce the operating current of the grid-type converter.

2. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: In step S1, the power reference value P ref and Q ref As input, the time domain waveforms of current and voltage are output, and a grid-type converter model is constructed; The power module loss model is constructed with current, voltage and given switching frequency data as input and the total loss of the power module as output; Taking the total loss of the power module as input and the actual junction temperature of the power module T j For output, a power module thermal network model is constructed.

3. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: In step S1, the corresponding relationship between the active power reference value and the switching frequency of the grid-type converter that meets the preset conditions is obtained, and a power-switching frequency relationship model under different voltage drop levels is established in an offline manner. The specific method is as follows: the total power module loss calculated by the power module loss model is input into the power module thermal network model to calculate the actual junction temperature T of the power module. j ; The junction temperature calculation result T j With a given reference junction temperature T jN Comparison is performed. If the junction temperature exceeds the reference junction temperature, the given switching frequency is reduced, and the re-given switching frequency is input into the power module loss model to calculate the module loss. The junction temperature is then input into the thermal network model to calculate the junction temperature, which is then compared with the reference junction temperature again until the calculated junction temperature does not exceed the reference junction temperature. The corresponding active power reference value that does not exceed the reference junction temperature is obtained. The corresponding active power reference value and switching frequency relationship of the grid-type converter that does not exceed the reference junction temperature are saved; different active power reference values ​​are selected and the above steps are repeated to establish the corresponding relationship between power and switching frequency. By changing different grid drop levels, the power reference value Q is selected according to the voltage drop level. ref , and obtain the relationship model between power and switching frequency under different voltage drop levels.

4. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: Step S2 is specifically as follows: The control system detects the voltage drop and frequency fluctuation of the power grid in real time, and obtains the corresponding power reference value P under the dynamic process based on the specific network control algorithm. ref , Q ref ; Input the power reference value as input to the offline established power and switching frequency relationship model to obtain the corresponding switching frequency f under the dynamic process sw ; Determine the switching frequency f sw Whether it meets the minimum limit requirement, if it is lower than the minimum limit, the power reference value is limited to reduce the operating current of the grid-type converter, and the updated power reference value P is ref , Q ref Input the power and switching frequency relationship model established offline to obtain the corresponding switching frequency f under current limiting conditions sw , until the switching frequency f sw To meet the minimum limit, if the switching frequency f sw If the minimum limit requirement is met, that is, it is greater than or equal to the minimum limit, current limiting operation will not be performed; finally, a new power reference value and switching frequency control parameters are configured for the converter to achieve high-power frequency reduction operation.

5. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: Power modules include IGBTs, MOSFETs, and wide bandgap devices.

6. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: In one cycle, the power module and diode conduction loss P Icon 、P Fcon It is expressed as follows: Where V ce0 、V F0 is the saturation voltage drop of the power module and the threshold voltage of the diode, r ce 、r F are the on-resistance of the power module and diode respectively, i a is the current flowing through the power device, d I and d F are the duty cycles corresponding to when the power module and diode are turned on.

7. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: In a switching cycle, the turn-on loss and turn-off loss of the device are expressed as a quadratic function of the current. The turn-on loss and turn-off loss of the power module are E on and E off It is expressed as follows: Where a0, a1, a2, b0, b1, b2 are E on 、E off with i a Coefficients of the quadratic function; V, V rated are the actual voltage and rated voltage of the device respectively; The switching loss of the power module in one switching cycle is expressed as follows: Where, f sw is the switching frequency; The reverse recovery loss of the diode E rec Expressed as: Where c0, c1, and c2 are E rec with i a Coefficients of quadratic functions; The diode switching loss in one switching cycle is expressed as: The power loss of the power module during one switching cycle is expressed as: P IT =P Icon +P Isw (6) The diode power loss in one switching cycle is expressed as: P FT =P Fcon +P Fsw (7) By obtaining the grid-type converter operating parameters required for power module loss calculation, the device loss value can be obtained. The device junction temperature value can be obtained through the thermal impedance network, which facilitates real-time reading of the device junction temperature.

8. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: The junction temperature of the chip is expressed as: T j =T a +P loss Z thj→a (t) (8) Where, T j is the chip temperature, T a is the ambient temperature, P loss is the chip loss value, Z thj→a (t) represents the total impedance from the chip to the environment; The thermal impedance network includes the thermal impedance from the chip to the housing, from the housing to the heat sink, and from the heat sink to the environment. The total impedance Z thj→a (t) is expressed as: Where, thermal resistance R i (i=1,2...6) represents the steady-state junction temperature of the power device, thermal capacitance C i (i=1, 2...6) represents the rate of change of the junction temperature of the power device.

9. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: During the period when the grid voltage drops, the grid-type converter is used to provide reactive support to the grid. During the grid voltage sag, the reactive current output by the converter should satisfy the following formula: Where U N is the rated voltage of the grid, I N is the rated current of the converter; According to the reactive current I qref and reactive power given value Q ref The relationship between the reactive power output and the reactive power output can be obtained as follows:

10. The method for enhancing power of a grid-connected converter based on variable switching frequency under dynamic grid conditions according to claim 1, characterized in that: The grid dynamics include low voltage fault ride-through, frequency fluctuation, load mutation, and synchronous frequency resonance.

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