Online de-icing method for distribution network considering coordinated optimization of active capacity and disposal speed

Abstract

This application provides an online de-icing method for a distribution network that takes into account the coordinated optimization of active capacity and disposal speed. The method includes: obtaining current thermal environment parameters; determining the critical de-icing current value required for de-icing the current line in the current environment based on the current thermal environment parameters; obtaining the load current in the current distribution line load; determining a first value corresponding to the first amplitude of the de-icing current injected into the fourth bridge arm and a second value corresponding to the first phase of the injected de-icing current based on the load current and the critical de-icing current value; determining a reference value for the injected de-icing current based on the first and second values; collecting the DC side voltage of the midpoint clamping unit and the capacitor voltage of the cascaded H-bridge unit submodule; and determining a first switching signal for the midpoint clamping unit and a second switching signal for the cascaded H-bridge unit based on the DC side voltage, the capacitor voltage, and the injected de-icing current reference value. This method effectively reduces online de-icing time consumption and rationally optimizes the overall capacity of the HIS.

Description

Technical Field

[0001] The present application relates to the field of online ice melting in distribution networks, and specifically to an online ice melting method for distribution networks that takes into account the coordinated optimization of active capacity and disposal speed. Background Art

[0002] With its unique advantages of simultaneously melting ice on power lines and providing power to the grid, online ice-melting technology has garnered widespread attention from scholars both domestically and internationally. Taking into account the zero-sequence isolation of the distribution transformers at both ends of the distribution network, online ice-melting of distribution lines can be achieved without impacting grid power supply by flexibly regulating the zero-sequence current of the distribution network. Based on this, Huazhong University of Science and Technology achieved online ice-melting of distribution lines by dynamically injecting zero-sequence current using an adjustable transformer. However, while the aforementioned solution has a relatively small device capacity, the single-phase ice-melting method results in a relatively low ice-melting rate.

[0003] In addition, artificially creating a single-phase grounding fault to construct a zero-sequence current circulation loop may pose a challenge to the safe operation of the distribution network. To this end, Fuzhou University configured a cascaded H-bridge (CHB) at both ends of the ice-covered line to construct a zero-sequence reactive current circulation loop, thereby effectively eliminating the safety hazards of the single-phase grounding fault branch during the ice melting period, and the three-phase simultaneous ice melting method makes the ice melting disposal rate higher. However, the CHB branch can only output reactive current to ensure DC energy balance, and does not consider the coordinated optimization problem of load current and ice melting current, resulting in a large active device capacity and high cost. In addition, the existing online ice melting solution only works during the ice-covered period of the distribution network, and the equipment utilization rate is low. Therefore, online ice melting technology that takes into account both equipment cost and utilization efficiency is in urgent need of a breakthrough. Summary of the Invention

[0004] In order to overcome the above technical defects, the present application provides an online ice melting method for distribution network that takes into account the coordinated optimization of active capacity and disposal speed.

[0005] To achieve the above objectives, this application is implemented according to the following technical solutions:

[0006] This application provides a distribution network online ice melting method that takes into account the coordinated optimization of active capacity and disposal speed, including:

[0007] Get the current thermal environment parameters;

[0008] Determining a critical ice-melting current value required for ice melting in the current environment circuit based on the current thermal environment parameters;

[0009] Obtain the load current in the current distribution line load;

[0010] Determining, based on the load current and the critical ice-melting current value, a first value corresponding to a first amplitude of the ice-melting current injected into the fourth bridge arm and a second value corresponding to a first phase of the ice-melting current injected;

[0011] determining a reference value of injected ice-melting current based on the first value and the second value;

[0012] Collect the DC side voltage of the midpoint clamp unit and the capacitor voltage of the cascaded H-bridge unit submodule;

[0013] Based on the DC link voltage, the capacitor voltage and the injected ice-melting current reference value, a first switching signal of the midpoint clamp unit and a second switching signal of the cascaded H-bridge unit are determined.

[0014] Optionally, the thermal environment parameters include the following:

[0015] Temperature, wind speed, convection, and auxiliary heat transfer coefficient.

[0016] Optionally, determining a first value corresponding to a first amplitude of the injected ice-melting current and a second value corresponding to a first phase of the injected ice-melting current based on the load current and the critical ice-melting current value includes:

[0017] Based on the load current, determining a second amplitude and a second phase value corresponding to the load current;

[0018] Based on the second amplitude, the second phase and the critical ice-melting current value, a first value corresponding to a first amplitude of the injected ice-melting current and a second value corresponding to a first phase of the injected ice-melting current are determined.

[0019] Optionally, determining a first value corresponding to a first amplitude of the injected ice-melting current and a second value corresponding to a first phase of the injected ice-melting current based on the second amplitude and the second phase value and the critical ice-melting current value includes:

[0020] Based on the second amplitude and the critical ice-melting current value, a first ratio of the critical ice-melting current value to the load current is determined, specifically:

[0021] ;

[0022] Where, is the critical ice melting current value, is the second amplitude, is the first ratio;

[0023] Based on the second amplitude, the second phase value, and the injected ice-melting current, a first expression corresponding to the third amplitude and the third phase of the superimposed line current is determined, specifically:

[0024] ;

[0025] Where, is the third amplitude, For the third phase, is the second phase value, is the first amplitude, For the first phase, in the A, B, and C phases, 0, -1 and 1 respectively;

[0026] The critical ice melting current value is used as a constraint condition of the first expression. The constraint condition is as follows:

[0027] ;

[0028] Where, is the amplitude of the phase A line current, is the amplitude of the C-phase line current; is the point that satisfies the first expression;

[0029] Determine the second value based on the constraint condition and the first expression, wherein the difference between the second value and the second amplitude is 60 degrees;

[0030] Obtain two-phase ice melting phasor diagram;

[0031] Based on the second value and the two-phase ice melting phasor diagram, a second expression is determined, where the second expression is:

[0032] ;

[0033] Based on the second expression and the first ratio, a first value is determined as:

[0034] ;

[0035] Optionally, the determining the first switching signal of the midpoint clamp unit and the second switching signal of the cascaded H-bridge unit based on the DC link voltage, the capacitor voltage, and the injected ice-melting current reference value includes:

[0036] Determining a voltage-stabilized active current reference value based on the DC side voltage and the capacitor voltage;

[0037] Get reactive power compensation instructions;

[0038] Determining a reactive compensation current reference value based on the reactive compensation instruction;

[0039] Based on the voltage-stabilized active current reference value, the reactive compensation current reference value, and the injected ice-melting current reference value, a first switching signal of the midpoint clamp unit and a second switching signal of the cascaded H-bridge unit are determined.

[0040] Optionally, determining the first switching signal of the midpoint clamp unit and the second switching signal of the cascaded H-bridge unit based on the voltage-stabilized active current reference value, the reactive compensation current reference value, and the injected ice-melting current reference value includes:

[0041] Determining a three-phase bridge arm modulation wave based on the voltage-stabilized active current reference value and the reactive compensation current reference value;

[0042] Obtaining a first output voltage of the fourth bridge arm;

[0043] determining a fourth bridge arm modulation wave based on the injected ice-melting current reference value and the first output voltage;

[0044] Determining a first switching signal of the midpoint clamping unit based on the three-phase bridge arm modulation wave and the fourth bridge arm modulation wave;

[0045] A second switching signal of the cascaded H-bridge unit is determined based on the first switching signal of the midpoint clamp unit and the DC side voltage.

[0046] Optionally, determining the second switching signal of the cascaded H-bridge unit based on the first switching signal of the midpoint clamp unit and the DC side voltage includes:

[0047] Determining a second output voltage of the three-phase bridge arm of the midpoint clamping unit and a third output voltage of the fourth bridge arm of the midpoint clamping unit based on the first switching signal of the midpoint clamping unit and the DC side voltage;

[0048] A second switching signal of the cascaded H-bridge unit is determined based on the three-phase bridge arm modulation wave, the fourth bridge arm modulation wave, the second output voltage, and the third output voltage.

[0049] Optionally, after the step of determining the first switching signal of the midpoint clamp unit and the second switching signal of the cascaded H-bridge unit based on the DC link voltage, the capacitor voltage, and the injected ice-melting current reference value, the step further includes:

[0050] The first switching signal and the second switching signal are input into the driving control signal of the switching device to control the switching action.

[0051] This application has the following beneficial effects:

[0052] The method proposed in this application analyzes the coordinated optimization mechanism of the actively injected ice-melting current and the load current on the basis of considering the contribution of the load current to line ice melting. Based on the above analysis, an optimized two-phase rotation ice-melting method suitable for HIS is proposed, which achieves effective compression of online ice-melting time consumption and reasonable optimization of the overall capacity of HIS.

[0053] In addition to the above-described purposes, features and advantages, the present application has other purposes, features and advantages. The present application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0055] Figure 1 It is a flow chart of an online ice melting method for a distribution network taking into account the coordinated optimization of active capacity and disposal speed provided in an embodiment of the present application.

[0056] Figure 2 This is a schematic diagram of the architecture of the HIS solution provided in the embodiment of the present application;

[0057] Figure 3 This is a schematic diagram of the two-phase ice melting phase provided in the embodiment of the present application. Figure 3 (a) is the phasor diagram of load current, Figure 3 (b) Schematic diagram of the phasor of load current and line current in the two-phase rotation ice melting mode;

[0058] Figure 4 This is a comparison diagram of the prior art provided in the embodiment of the present application that does not consider the load current and the present application that considers the capacity of the load current device. DETAILED DESCRIPTION

[0059] The embodiments of the present application are described in detail below with reference to the accompanying drawings, but the present application can be implemented in many different ways as defined and covered by the claims.

[0060] It should be noted that the terms "first", "second", "third", etc. in the claims, description and drawings of the present application are used to distinguish similar objects and are not used to describe a specific order or sequence. The data used in this way are interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in a sequence other than that illustrated or described herein. In addition, the terms "including", "having" and their variations are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or modules is not necessarily limited to those steps or modules clearly listed, but may include other steps or modules that are not clearly listed or inherent to these processes, methods, products or devices.

[0061] In order to provide a more vivid explanation of the issues raised in the background technology, Figure 1 As shown, this application proposes an online de-icing method for distribution networks that takes into account the coordinated optimization of active capacity and disposal speed, including:

[0062] Step S101: obtaining current thermal environment parameters;

[0063] Since this application is based on the HIS solution, the architecture of the HIS solution needs to be described in detail, such as Figure 2 As shown in FIG, the HIS consists of a multi-functional active ice-melting device (MAID) at the head end of the line and a grounding transformer (GT) at the end end. is the neutral point voltage of the grid, is the voltage at the device grid connection point, is the line impedance, is the line-to-ground capacitance, is the line-to-ground resistance, The GT ground current. When the grid is operating normally, the MAID provides reactive power support. When the distribution line is iced, the MAID and the GT operate in tandem, utilizing zero-sequence ice-melting current to achieve online ice melting, effectively improving the ice-melting device's utilization and power supply reliability.

[0064] During ice-melting on transmission lines, thermal environment parameters refer to physical quantities directly related to heat generation, transfer, and the ice melting process. They describe the thermal state and thermal interaction patterns of the ice-melting system (e.g., conductors, ice layer, and surrounding environment). Their core function is to quantify heat balance relationships and guide the design of key parameters such as ice-melting current and time, ensuring ice-melting efficiency and safety. Thermal environment parameters generally include temperature, wind speed, convection, and auxiliary heat transfer coefficient.

[0065] Step S102: determining a critical ice-melting current required for ice melting of the circuit in the current environment based on the current thermal environment parameters;

[0066] In the transmission line ice-covered scenario, the critical ice melting current This refers to the minimum current value required to melt ice-covered conductors. It is a key parameter in ice-melting technology, used to measure the critical state when, under specific environmental conditions, the heat generated by the current flowing through the conductor just offsets the system's heat dissipation (including heat absorption by the ice).

[0067] After obtaining the above thermal environment parameters, combined with the line type and icing type, the critical ice melting current can be calculated according to the following formulas (1)-(3): Specific values:

[0068] (1)

[0069] (2)

[0070] (3)

[0071] Where: is the critical ice melting current (A); is the ice melting time (h); is the thickness of the ice layer (cm); is the resistance per unit length of wire at 0°C ; is the difference between the conductor temperature and the outside air temperature (°C); is the equivalent ice layer conduction thermal resistance (℃·cm / W); is the equivalent thermal resistance of convection and radiation (℃·cm / W); is the ice density, generally taken as 0.9g / cm 3 ; is the outer diameter of the conductor after ice coating (cm); is the wire diameter (cm); is the wind speed (m / s); is the thermal conductivity (W / (℃·cm)), in the wet growth icing scenario, =2.27×10 -2 .

[0072] Step S103: Obtain the load current in the current distribution line load;

[0073] At this point, by detecting the three-phase sinusoidal AC current in the distribution line, the load current in the current distribution line can be determined. It should be noted that this application takes into account the existing load current in the line when injecting the ice-melting current, while the prior art does not. As a result, the injected ice-melting current value is too large, resulting in a dramatic increase in the device capacity.

[0074] Step S104: Based on the load current and the critical ice-melting current, determine a first value corresponding to a first amplitude of the ice-melting current injected into the fourth bridge arm and a second value corresponding to a first phase of the ice-melting current injected.

[0075] This application does not ignore the contribution of load current to ice melting operation, but considers the superposition of load current and injected ice melting current, that is, the superposition of three-phase sinusoidal load current and injected zero-sequence ice melting current together constitutes the line current.

[0076] After the load current is obtained, the second amplitude corresponding to the load current can be determined and the second phase value , then according to the second magnitude and the second phase value , and critical ice melting current value , the first amplitude of the injected ice melting current can be determined The corresponding first value and the first phase of the injected ice melting current The corresponding second value is:

[0077] First, a first ratio of the critical ice melting current to the load current is determined based on the second amplitude and the critical ice melting current. for:

[0078] (4)

[0079] Here we assume that the ice melting current injected into the fourth bridge arm is The expression is the following formula (5):

[0080] (5)

[0081] Where, is the grid angular frequency.

[0082] At this time, due to the second amplitude The value of has been determined, and we need to find the value that matches the first amplitude. The corresponding specific first value and the first phase The corresponding second value.

[0083] Due to the calculated critical ice melting current

[0084] According to the HIS scheme, the following formula (6) can be obtained. Its core principle is the superposition of the original line current (load current) and the injected ice-melting current after diversion:

[0085] (6)

[0086] Where, is the line current;

[0087] Further analysis of formula (6) yields the first expression (7) corresponding to the third amplitude and third phase of the superimposed line current:

[0088] (7)

[0089] Where, is the third amplitude, For the third phase, in the A, B, and C phases, 0, -1 and 1 respectively;

[0090] The critical ice melting current value is used as the constraint condition of formula (7), that is, formula (8):

[0091] (8)

[0092] Where, is the amplitude of the phase A line current, is the amplitude of the phase C line current, is the point that satisfies the first expression.

[0093] Then, based on formulas (7) and (8), the second value can be determined, that is, the first phase of the ice-melting current injected into the fourth bridge arm can be expressed as formula (9):

[0094] (9)

[0095] According to the relationship in formula (9), we can determine Figure 3 The two-phase ice melting phasor diagram is shown, and then the second expression (10) is determined using the cosine theorem based on the second value and the two-phase ice melting phasor diagram:

[0096] (10)

[0097] Then, according to the second expression (10) and formula (4), the first value can be obtained as:

[0098] (11)

[0099] Step S105: determining an injected ice-melting current reference value based on the first value and the second value;

[0100] Determine the first value and the second value. Since the application follows the two-phase melting sequence of AB, BC, and CA, the injected melting current phases differ by 120 degrees. After determining the first value and the second value, the injected melting current reference value can be determined. , which is as follows (12):

[0101] (12)

[0102] Where, Indicates the time of two-phase ice melting. This application adopts two-phase ice melting, so the total time is 1.5 .

[0103] Step S106: collecting the DC side voltage of the midpoint clamp unit and the capacitor voltage of the cascaded H-bridge unit submodule;

[0104] In order to realize the online ice melting of HIS solution, it is necessary to collect the DC side voltage of the neutral point clamped (NPC) unit. and the sum of the capacitor voltages of the cascaded H-bridge (CHB) unit submodules .

[0105] Step S107: determining a first switching signal of a midpoint clamp unit and a second switching signal of a cascaded H-bridge unit based on the DC link voltage, the capacitor voltage, and the injected ice-melting current reference value.

[0106] According to the DC side voltage and the capacitor voltage, the DC side voltage reference value and the capacitor voltage reference value are obtained. Then, according to the sum of the DC side voltage reference value and the capacitor voltage reference value, the difference between the DC side voltage and the capacitor voltage is subjected to PI feedback closed-loop control (voltage stabilization outer loop) to calculate the voltage stabilization active current, see formula (13):

[0107] (13)

[0108] Where, To stabilize the active current, is a complex frequency domain variable, are the proportional coefficient and integral coefficient of the PI controller respectively, is the DC side voltage reference value, is the capacitor voltage reference value.

[0109] At this time, the reactive power compensation instruction issued by the superior dispatcher is obtained , the reactive compensation current reference value can be calculated , the calculation process is shown in formula (14):

[0110] (14)

[0111] Where: is the grid voltage active vector.

[0112] Then, based on the voltage-stabilized active current reference value, the reactive compensation current reference value, and the injected ice-melting current reference value, the first switching signal of the midpoint clamping unit and the second switching signal of the cascaded H-bridge unit are determined. The specific process is as follows:

[0113] Based on the synchronous rotating coordinate system, the three-phase bridge arm modulation wave is determined according to the reference value of the voltage-stabilized active current and the reference value of the reactive compensation current. The specific process is: Get the sampling value of the voltage-stabilized active current and reactive compensation current sampling value , the difference between the reference value of the voltage-stabilized active current and the sampling value of the voltage-stabilized active current , and the difference between the reactive compensation current reference value and the reactive compensation circuit sampling value , respectively perform decoupling PI feedback closed-loop control (current inner loop) to obtain the three-phase bridge arm modulation wave 、 、 , as shown in Equations (15) to (16):

[0114] (15)

[0115] Where, is the active component of the voltage at the device grid connection point, is the reactive component of the voltage at the device grid connection point, is the proportional coefficient of the inner loop PI controller, is the integral coefficient of the inner loop PI controller, is the grid angular frequency, is the filter inductor, is the reactive component of the grid voltage:

[0116] (16)

[0117] Where, is the angle between the d-axis and the phase reference axis.

[0118] At this time, the first output voltage of the fourth bridge arm is obtained And the injected ice melting current sampling value , by calculating the difference between the fourth bridge arm output current reference value and the sampling value Perform PR feedback closed-loop control and add the fourth bridge arm output voltage to the control variable Feedforward control to obtain the modulation wave of the fourth bridge arm , as shown in formula (17):

[0119] (17)

[0120] Where: is the control coefficient of the PR controller.

[0121] Then, according to the modulation wave of the three-phase bridge arm and the modulation wave of the fourth bridge arm, the first switching signal of the midpoint clamping unit can be determined. Taking phase A as an example, as shown in formulas (18) to (19):

[0122] (18)

[0123] (19)

[0124] Where, They are the switching functions of the upper and lower bridge arms of phase A of the NPC unit respectively.

[0125] It should be noted that the relationship between the arm switching functions of the other phase bridge arms and the arm switching function of the A phase bridge arm is the same. In order to avoid repetition, they will not be described one by one here.

[0126] Then, the second switching signal of the cascaded H-bridge unit is determined according to the first switching signal of the midpoint clamp unit and the DC side voltage. The specific process is as follows:

[0127] According to the first switching signal and the DC side voltage, the second output voltage of the three-phase bridge arm of the midpoint clamping unit is determined 、 、 and the third output voltage of the fourth bridge arm of the midpoint clamp unit ;

[0128] Take phase A as an example to illustrate, the reference voltage The NPC circuit bears the main fundamental component. The output voltage of the NPC unit A phase bridge arm for:

[0129] (20)

[0130] It should be noted that the second output voltage 、 and the third output voltage of the fourth bridge arm of the midpoint clamp unit The calculation is also determined using a similar process to formula (20).

[0131] Then, according to the three-phase bridge arm modulation wave 、 、 , the fourth bridge arm modulation wave , the second output voltage With the third output voltage , determining a second switching signal of the cascaded H-bridge unit;

[0132] CHB unit modulation strategy (carrier phase shift modulation)

[0133] The CHB unit of each bridge arm adopts the carrier phase shifting-sinusoidal pulse width modulation (CPS-SPWM) strategy. Taking phase A as an example, the CHB unit modulates the voltage The expression is:

[0134] (twenty one)

[0135] CHB unit submodule switch function Expressed as:

[0136] (twenty two)

[0137] Where: For the N The CHB unit submodule modulates the carrier, and its corresponding phase is .

[0138] The first and second switching signals are input into the driving control signals of the switching devices to control the switching action, achieving voltage regulation control for the three-phase bridge arm and ice-melting current control for the fourth bridge arm. Different control signals are input at different time periods to complete the regulation of the entire ice-melting process.

[0139] Capacity comparison

[0140] The ice melting current injected by the active ice melting device corresponding to the traditional ice melting current injection method and the ice melting current method proposed in this application and load current The relationships are:

[0141] (twenty three)

[0142] The traditional solution does not consider the influence of load current. Under certain circumstances, load current will reduce the injected ice melting current. However, the method proposed in this application fully considers the coordination between injection current and load current, thus achieving the optimization of equipment capacity. Figure 4 As shown in Equation (24), the capacity reduction rate of the proposed scheme compared with the traditional method is:

[0143] (twenty four)

[0144] In summary, the method proposed in this application, on the basis of considering the contribution of load current to line ice melting, analyzes the coordinated optimization mechanism of actively injected ice melting current and load current. Based on the above analysis, an optimized two-phase rotation ice melting method suitable for HIS is proposed, which achieves effective compression of online ice melting time consumption and reasonable optimization of the overall capacity of HIS.

[0145] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A distribution network online ice melting method taking into account the coordinated optimization of active capacity and disposal speed, characterized in that: include: Get the current thermal environment parameters; Determining a critical ice-melting current value required for ice melting in the current environment circuit based on the current thermal environment parameters; Obtain the load current in the current distribution line load; Determining, based on the load current and the critical ice-melting current value, a first value corresponding to a first amplitude of the ice-melting current injected into the fourth bridge arm and a second value corresponding to a first phase of the ice-melting current injected; determining a reference value of injected ice-melting current based on the first value and the second value; Collect the DC side voltage of the midpoint clamp unit and the capacitor voltage of the cascaded H-bridge unit submodule; Determining a first switching signal of a midpoint clamp unit and a second switching signal of a cascaded H-bridge unit based on the DC link voltage, the capacitor voltage, and the injected ice-melting current reference value; The determining, based on the load current and the critical ice-melting current value, of a first value corresponding to a first amplitude of the ice-melting current injected into the fourth bridge arm and a second value corresponding to a first phase of the ice-melting current injected into the fourth bridge arm includes: Based on the load current, determining a second amplitude and a second phase value corresponding to the load current; Based on the second amplitude and the critical ice-melting current value, a first ratio of the critical ice-melting current value to the load current is determined, specifically: ； Where, is the critical ice melting current value, is the second amplitude, is the first ratio; Based on the second amplitude, the second phase value, and the injected ice-melting current, a first expression corresponding to the third amplitude and the third phase of the superimposed line current is determined, specifically: ； Where, is the third amplitude, For the third phase, is the second phase value, is the first amplitude, For the first phase, in the A, B, and C phases, 0, -1 and 1 respectively; The critical ice melting current value is used as a constraint condition of the first expression. The constraint condition is as follows: ； Where, is the amplitude of the phase A line current, is the amplitude of the C-phase line current; is the point that satisfies the first expression; Determine the second value based on the constraint condition and the first expression, wherein the difference between the second value and the second amplitude is 60 degrees; Obtain two-phase ice melting phasor diagram; Based on the second value and the two-phase ice melting phasor diagram, a second expression is determined, where the second expression is: ； Based on the second expression and the first ratio, a first value is determined as: 。 2. The method according to claim 1, characterized in that The thermal environment parameters include the following: Temperature, wind speed, convection, and auxiliary heat transfer coefficient.

3. The method according to claim 1, characterized in that The determining, based on the DC link voltage, the capacitor voltage, and the injected ice-melting current reference value, of the first switching signal of the midpoint clamp unit and the second switching signal of the cascaded H-bridge unit includes: Determining a voltage-stabilized active current reference value based on the DC side voltage and the capacitor voltage; Get reactive power compensation instructions; Determining a reactive compensation current reference value based on the reactive compensation instruction; Based on the voltage-stabilized active current reference value, the reactive compensation current reference value, and the injected ice-melting current reference value, a first switching signal of the midpoint clamp unit and a second switching signal of the cascaded H-bridge unit are determined.

4. The method according to claim 3, characterized in that The determining of the first switching signal of the midpoint clamp unit and the second switching signal of the cascaded H-bridge unit based on the voltage-stabilized active current reference value, the reactive compensation current reference value, and the injected ice-melting current reference value includes: Determining a three-phase bridge arm modulation wave based on the voltage-stabilized active current reference value and the reactive compensation current reference value; Obtaining a first output voltage of the fourth bridge arm; determining a fourth bridge arm modulation wave based on the injected ice-melting current reference value and the first output voltage; Determining a first switching signal of the midpoint clamping unit based on the three-phase bridge arm modulation wave and the fourth bridge arm modulation wave; A second switching signal of the cascaded H-bridge unit is determined based on the first switching signal of the midpoint clamp unit and the DC side voltage.

5. The method according to claim 4, characterized in that The determining the second switching signal of the cascaded H-bridge unit based on the first switching signal of the midpoint clamp unit and the DC side voltage includes: Determining a second output voltage of the three-phase bridge arm of the midpoint clamping unit and a third output voltage of the fourth bridge arm of the midpoint clamping unit based on the first switching signal of the midpoint clamping unit and the DC side voltage; A second switching signal of the cascaded H-bridge unit is determined based on the three-phase bridge arm modulation wave, the fourth bridge arm modulation wave, the second output voltage, and the third output voltage.

6. The method according to claim 1, characterized in that After the step of determining the first switching signal of the midpoint clamp unit and the second switching signal of the cascaded H-bridge unit based on the DC link voltage, the capacitor voltage, and the injected ice-melting current reference value, the method further includes: The first switching signal and the second switching signal are input into the driving control signal of the switching device to control the switching action.

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