DC bus capacitor protection method and grid-connected inverter

Abstract

The invention discloses a direct-current bus capacitor protection method and a grid-connected inverter, the direct-current end of the grid-connected inverter is connected with a direct-current bus capacitor, and the alternating-current end of the grid-connected inverter is connected with a power grid; the method comprises the following steps: acquiring a grid-connected current of the grid-connected inverter; obtaining a discrete degree characteristic value corresponding to the grid-connected current based on the grid-connected current; and determining whether the DC bus capacitor needs to be protected or not according to the discrete degree characteristic value. The method comprises the following steps: obtaining a discrete degree characteristic value corresponding to a grid-connected current through the grid-connected current of a grid-connected inverter; determining whether the direct current bus capacitor needs to be protected or not according to the discrete degree characteristic value; a hardware circuit does not need to be additionally arranged, and protection of the direct current bus capacitor can be realized only by occupying software resources.

Description

Technical Field

[0001] The present application relates to the field of grid-connected technology, and in particular to a DC bus capacitor protection method and a grid-connected inverter. Background Art

[0002] The development of new energy has become a global consensus to cope with the increasingly severe energy crisis. However, with the increase in the penetration rate and capacity level of new energy power generation, it is easy to cause changes in the impedance of the public grid connection point, causing resonance in the grid-connected system, resulting in overcurrent damage to the bus capacitor, and causing the grid-connected inverter to fail and unable to generate electricity normally, which brings great challenges to the safe operation of the power grid and user electrical equipment.

[0003] The existing technology, for example, in high-power inverters such as centralized inverters and distributed inverters, mainly adds a bus capacitor current sampling circuit, and then judges and implements bus capacitor protection based on the bus capacitor current. This method requires additional capacitor current sampling circuits and requires the control chip to have spare sampling channels, which directly leads to an increase in hardware costs. Summary of the invention

[0004] The present application aims to provide a DC bus capacitor protection method and a grid-connected inverter to solve the problem of overcurrent damage to the DC bus capacitor in a grid-connected system.

[0005] In one aspect, the present application provides a DC bus capacitor protection method, which is applied to a grid-connected inverter, wherein the DC end of the grid-connected inverter is connected to the DC bus capacitor, and the AC end of the grid-connected inverter is connected to the power grid; the method comprises:

[0006] Obtaining a grid-connected current of the grid-connected inverter;

[0007] Based on the grid-connected current, obtaining a discrete degree characteristic value corresponding to the grid-connected current;

[0008] Whether the DC bus capacitor needs to be protected is determined according to the discrete degree characteristic value.

[0009] On the other hand, the present application provides a grid-connected inverter, characterized in that the DC end of the grid-connected inverter is connected to the DC bus capacitor, and the AC end of the grid-connected inverter is connected to the power grid; the grid-connected inverter also includes a control unit, and the control unit is configured to execute the DC bus capacitor protection method.

[0010] The DC bus capacitor protection method and grid-connected inverter provided in the embodiments of the present application obtain a discrete degree characteristic value corresponding to the grid-connected current through the grid-connected current of the grid-connected inverter; thereby determining whether the DC bus capacitor needs to be protected based on the discrete degree characteristic value; there is no need to add additional hardware circuits, and only software resources need to be occupied to achieve the protection of the DC bus capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 A schematic diagram of a grid-connected system provided in an embodiment of the present application;

[0012] Figure 2 A schematic diagram of a DC bus capacitor protection method provided in an embodiment of the present application;

[0013] Figure 3 Another schematic diagram of the DC bus capacitor protection method provided in an embodiment of the present application.

[0014] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0015] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer and more understandable, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0016] It should be understood that, although the terms "first", "second", etc. used herein can describe different elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or part from another region, layer or part. Therefore, the first element, component, region, layer or part discussed below can be referred to as the second element, component, region, layer or part without departing from the teachings of the exemplary embodiments.

[0017] In the description of this application, the terms "low-order harmonic components" and "high-order harmonic components" refer to harmonic signals whose frequencies are integer multiples of the original signal frequency. In the power system, low-order harmonics mainly refer to harmonics whose frequencies are integer multiples of 100Hz, 150Hz, 200Hz, etc. in a 50Hz power grid, while high-order harmonics generally refer to harmonic signals with frequencies above 2kHz.

[0018] In the description of the present application, the terms “exceed” and “reach” may include a situation of being equal to or may not include a situation of being equal to.

[0019] Figure 1 A schematic diagram of a grid-connected system provided in an embodiment of the present application.

[0020] like Figure 1 As shown, the grid-connected system includes a grid-connected inverter Q1, the DC end of the grid-connected inverter Q1 is connected to the DC bus capacitor C1, and the AC end of the grid-connected inverter Q1 is connected to the grid through the grid switch K1. Further, the grid-connected inverter Q1 also includes a control unit, and the control unit may include at least one processor. The processor may be implemented as an array of multiple logic gates, or may be implemented as a combination of a general-purpose microcontroller and a memory, in which a program executable in the microcontroller is stored, such as the program of the DC bus capacitor protection method described below. It will be understood by those skilled in the art that the processor may be implemented in other forms of hardware.

[0021] Based on the above-mentioned grid-connected system, in one example, the control unit is configured to obtain the grid-connected current of the grid-connected inverter; based on the grid-connected current, obtain a discrete degree characteristic value corresponding to the grid-connected current; and determine whether the DC bus capacitor needs to be protected according to the discrete degree characteristic value.

[0022] In this example, the control unit can obtain the grid-connected current of the grid-connected inverter through the detection module in the grid-connected inverter Q1, without adding additional hardware circuits, and only needs to occupy the software resources of the control unit to realize the protection of the DC bus capacitor.

[0023] In one example, the control unit is configured to obtain a current component of the grid-connected current in a preset coordinate system based on the grid-connected current; filter the current component to obtain a harmonic component corresponding to the current component; and obtain a discrete degree characteristic value corresponding to the grid-connected current based on the harmonic component corresponding to the current component.

[0024] The preset coordinate system includes but is not limited to a two-phase coordinate system, such as a stationary α-β coordinate system, a rotating dq coordinate system, and the like.

[0025] In one example, the control unit is configured to filter out the fundamental wave component and the low-order harmonic component in the current component through a high-pass filter to obtain the high-order harmonic component corresponding to the current component.

[0026] In one example, the control unit is configured to refresh the cumulative sum and cumulative square values ​​corresponding to the harmonic components corresponding to the current components; when the refresh times reach the data length of the preset discrete degree characteristic value, the discrete degree characteristic value corresponding to the grid-connected current is obtained based on the cumulative sum and cumulative square values ​​corresponding to the harmonic components.

[0027] It should be noted that the cumulative sum value corresponding to the harmonic components can be expressed by the following formula: gd_sum (k) = Igd_sum (k-1)+I gd_filter , where I gd_sum (k) represents the current cumulative sum value, I gd_sum (k-1) represents the previous cumulative sum, I gd_filter It is understood that the first cumulative sum is I gd_filter .

[0028] The accumulated square value corresponding to the harmonic component can be expressed by the following formula: gd_squ_sum (k) = I gd_squ_sum (k-1)+(I gd_filter ) 2 , where I gd_squ_sum (k) represents the current accumulated square value, I gd_squ_sum (k-1) represents the previous accumulated square value, I gd_filter It is understood that the first cumulative square value is (I gd_filter ) 2 .

[0029] In one example, the control unit is configured to calculate the discrete degree characteristic value corresponding to the grid-connected current by the following formula:

[0030] Among them I gd_std represents the discrete degree characteristic value, n represents the data length of the preset discrete degree characteristic value, n is an integer greater than 2, I gd_squ_sum Indicates the accumulated square value corresponding to the harmonic component, I gd_sum Indicates the cumulative sum value corresponding to the harmonic components.

[0031] In one example, the control unit is configured to compare the discrete degree characteristic value with a preset characteristic threshold; if the discrete degree characteristic value exceeds the preset characteristic threshold for a number of times reaching a preset number threshold, the DC bus capacitor is protected; otherwise, the DC bus capacitor is not protected.

[0032] Specifically, when the control unit determines to protect the DC bus capacitor, it controls the grid-connected inverter to exit the grid-connected state. Further, the control unit can also control the grid-connected switch K1 to be disconnected.

[0033] In one example, the control unit is configured to compare the discrete degree characteristic value with a first preset characteristic threshold, and compare the discrete degree characteristic value with a second preset characteristic threshold; if the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold reaches a first preset number threshold or the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold reaches a second preset number threshold, the DC bus capacitor is protected; otherwise, the DC bus capacitor is not protected.

[0034] Specifically, if the discrete degree characteristic value exceeds the first preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold increases by one; if the discrete degree characteristic value does not exceed the first preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold decreases by one; wherein the lower limit value of the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold is zero.

[0035] If the discrete degree characteristic value exceeds the second preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold is incremented once; wherein the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold is reset after a preset interval time, for example, reset once every 30 seconds, specifically, it can be reset once.

[0036] Figure 2 A schematic diagram of a DC bus capacitor protection method provided in an embodiment of the present application.

[0037] like Figure 2 As shown, the method comprises the steps of:

[0038] S11, obtaining the grid-connected current of the grid-connected inverter;

[0039] S12. Based on the grid-connected current, obtaining a discrete degree characteristic value corresponding to the grid-connected current;

[0040] S13: Determine whether the DC bus capacitor needs to be protected according to the discrete degree characteristic value.

[0041] In one example, obtaining a discrete degree characteristic value corresponding to the grid-connected current based on the grid-connected current includes:

[0042] According to the grid-connected current, obtaining a current component of the grid-connected current in a preset coordinate system;

[0043] Performing filtering on the current component to obtain a harmonic component corresponding to the current component;

[0044] Based on the harmonic components corresponding to the current components, a discrete degree characteristic value corresponding to the grid-connected current is obtained.

[0045] In one example, filtering the current component to obtain a harmonic component corresponding to the current component includes:

[0046] The fundamental wave component and the low-order harmonic component in the current component are filtered out to obtain the high-order harmonic component corresponding to the current component.

[0047] In one example, obtaining the discrete degree characteristic value corresponding to the grid-connected current based on the harmonic component corresponding to the current component includes:

[0048] Based on the harmonic component corresponding to the current component, refreshing the accumulated sum value and the accumulated square value corresponding to the harmonic component;

[0049] When the number of refreshes reaches a preset data length of the discrete degree characteristic value, the discrete degree characteristic value corresponding to the grid-connected current is obtained based on the cumulative sum value and the cumulative square value corresponding to the harmonic component.

[0050] In one example, the discrete degree characteristic value corresponding to the grid-connected current is calculated by the following formula:

[0051] Among them I gd_std represents the discrete degree characteristic value, n represents the data length of the preset discrete degree characteristic value, n is an integer greater than 2, I gd_squ_sum Indicates the accumulated square value corresponding to the harmonic component, I gd_sum Indicates the cumulative sum value corresponding to the harmonic components.

[0052] In one example, determining whether the DC bus capacitor needs to be protected according to the discrete degree characteristic value includes:

[0053] Comparing the discrete degree characteristic value with a preset characteristic threshold;

[0054] If the number of times that the discrete degree characteristic value exceeds the preset characteristic threshold reaches the preset number threshold, the DC bus capacitor is protected; otherwise, the DC bus capacitor is not protected.

[0055] In one example, the method includes:

[0056] Comparing the discrete degree characteristic value with a first preset characteristic threshold, and comparing the discrete degree characteristic value with a second preset characteristic threshold;

[0057] If the number of times that the discrete degree characteristic value exceeds the first preset characteristic threshold reaches a first preset number threshold or the number of times that the discrete degree characteristic value exceeds the second preset characteristic threshold reaches a second preset number threshold, the DC bus capacitor is protected; otherwise, the DC bus capacitor is not protected.

[0058] In one example, the method includes:

[0059] If the discrete degree characteristic value exceeds the first preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold is incremented once; if the discrete degree characteristic value does not exceed the first preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold is decremented once;

[0060] If the discrete degree characteristic value exceeds the second preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold is incremented once; wherein the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold is reset after a preset interval time.

[0061] In one example, the method further includes:

[0062] When it is determined to protect the DC bus capacitor, the grid-connected inverter is controlled to exit the grid-connected state.

[0063] Figure 3 Another schematic diagram of the DC bus capacitor protection method provided in an embodiment of the present application.

[0064] like Figure 3 As shown, the method comprises the steps of:

[0065] S21, collect the instantaneous value of three-phase grid-connected current I ga ,I gb and I gc .

[0066] S22, the instantaneous value of the three-phase grid-connected current I ga ,I gb ,I gc After the coordinate change, the current component I in the rotating dq coordinate system is obtained gd ,I gq .

[0067] S23, using a high-pass filter to filter the current component I gd The low-order harmonic components and fundamental components are filtered out, and the high-order harmonic components are retained to obtain I gd_filter .

[0068] S24, for high-order harmonic components I gd_filterThe corresponding accumulated sum and accumulated square values ​​are refreshed.

[0069] Specifically, I gd_sum (k) = I gd_sum (k-1)+I gd_filter , where I gd_sum (k) represents the current cumulative sum value, I gd_sum (k-1) represents the previous cumulative sum, I gd_filter It is understood that the first cumulative sum is I gd_filter .

[0070] I gd_squ_sum (k) = I gd_squ_sum (k-1)+(I gd_filter ) 2 , where I gd_squ_sum (k) represents the current accumulated square value, I gd_squ_sum (k-1) represents the previous accumulated square value, I gd_filter It is understood that the first cumulative square value is (I gd_filter ) 2 .

[0071] S25. Determine whether the number of refresh times reaches the data length of the preset discrete degree characteristic value.

[0072] In this example, the data length of the preset discrete degree characteristic value is 64, and the data refresh frequency is 18 kHz.

[0073] S26, if the refresh times reach the preset data length of the discrete degree characteristic value, then based on the cumulative sum and cumulative square values ​​corresponding to the harmonic components, the discrete degree characteristic value corresponding to the grid-connected current is obtained. If the refresh times do not reach the preset data length of the discrete degree characteristic value, then continue refreshing.

[0074] Specifically, the discrete degree characteristic value corresponding to the grid-connected current is calculated by the following formula:

[0075] Among them I gd_std represents the discrete degree characteristic value, n represents the data length of the preset discrete degree characteristic value, n is an integer greater than 2, I gd_squ_sum Indicates the accumulated square value corresponding to the harmonic component, I gd_sum Indicates the cumulative sum value corresponding to the harmonic components.

[0076] S27, determine the discrete degree characteristic value I gd_std Is it greater than or equal to the first preset characteristic threshold I gd_std_refThe value of the first preset characteristic threshold is determined according to the parameters of the inverter and the bus capacitor. In a specific example, the first preset characteristic threshold I gd_std_ref It can be set between 100 and 200, and specifically 150.

[0077] S28, if the discrete degree eigenvalue I gd_std Greater than or equal to the first preset characteristic threshold I gd_std_ref , then Cnt increases once, that is, Cnt=Cnt+1.

[0078] S29, if the discrete degree eigenvalue I gd_std Less than the first preset characteristic threshold I gd_std_ref , then Cnt decreases once, that is, Cnt=Cnt-1. The lower limit of Cnt is 0.

[0079] S30, determine the discrete degree characteristic value I gd_std Is it greater than or equal to the second preset characteristic threshold I gd_std_ref2 The value of the second preset characteristic threshold is determined according to the parameters of the inverter and the bus capacitor. In a specific example, the second preset characteristic threshold I gd_std_ref2 It can be set between 200 and 300, specifically 250.

[0080] S31, if the discrete degree characteristic value I gd_std Greater than or equal to the second preset characteristic threshold I gd_std_ref2 , then Cnt2 increases once, that is, Cnt2 = Cnt2 + 1. Further, Cnt2 is cleared to 0 every 30 seconds.

[0081] It should be noted that the above step S30 can be executed simultaneously with step S27.

[0082] S32: Determine whether the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold reaches the first preset number threshold or whether the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold reaches the second preset number threshold. That is, determine whether Cnt is greater than or equal to CntSet, or determine whether Cnt2 is greater than or equal to Cnt2Set, where CntSet is the first preset number threshold and Cnt2Set is the second preset number threshold.

[0083] S33: If Cnt≥CntSet or Cnt2≥Cnt2Set, trigger the fault protection, for example, control the grid-connected inverter to exit the grid-connected state, and control the grid-connected switch K1 to disconnect.

[0084] The preferred embodiments of the present application are described above with reference to the accompanying drawings, but the scope of the present application is not limited thereby. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present application shall be within the scope of the present application.

Claims

1. A DC bus capacitor protection method, applied to a grid-connected inverter, wherein the DC end of the grid-connected inverter is connected to the DC bus capacitor, and the AC end of the grid-connected inverter is connected to the power grid; characterized in that: The method comprises: Obtaining a grid-connected current of the grid-connected inverter; Based on the grid-connected current, obtaining a discrete degree characteristic value corresponding to the grid-connected current; Whether the DC bus capacitor needs to be protected is determined according to the discrete degree characteristic value.

2. The method according to claim 1, characterized in that The obtaining, based on the grid-connected current, a discrete degree characteristic value corresponding to the grid-connected current includes: According to the grid-connected current, obtaining a current component of the grid-connected current in a preset coordinate system; Performing filtering on the current component to obtain a harmonic component corresponding to the current component; Based on the harmonic components corresponding to the current components, a discrete degree characteristic value corresponding to the grid-connected current is obtained.

3. The method according to claim 2, characterized in that The filtering the current component to obtain the harmonic component corresponding to the current component includes: The fundamental wave component and the low-order harmonic component in the current component are filtered out to obtain the high-order harmonic component corresponding to the current component.

4. The method according to claim 2, characterized in that: The obtaining, based on the harmonic component corresponding to the current component, a discrete degree characteristic value corresponding to the grid-connected current includes: Based on the harmonic component corresponding to the current component, refreshing the accumulated sum value and the accumulated square value corresponding to the harmonic component; When the number of refreshes reaches a preset data length of the discrete degree characteristic value, the discrete degree characteristic value corresponding to the grid-connected current is obtained based on the cumulative sum value and the cumulative square value corresponding to the harmonic component.

5. The method according to claim 4, characterized in that The discrete degree characteristic value corresponding to the grid-connected current is calculated by the following formula: Among them I gd_std represents the discrete degree characteristic value, n represents the data length of the preset discrete degree characteristic value, n is an integer greater than 2, I gd_squ_sum Indicates the accumulated square value corresponding to the harmonic component, I gd_sum Indicates the cumulative sum value corresponding to the harmonic components.

6. The method according to claim 1, characterized in that The determining whether the DC bus capacitor needs to be protected according to the discrete degree characteristic value includes: Comparing the discrete degree characteristic value with a preset characteristic threshold; If the number of times that the discrete degree characteristic value exceeds the preset characteristic threshold reaches the preset number threshold, the DC bus capacitor is protected; otherwise, the DC bus capacitor is not protected.

7. The method according to claim 6, characterized in that The method comprises: Comparing the discrete degree characteristic value with a first preset characteristic threshold, and comparing the discrete degree characteristic value with a second preset characteristic threshold; If the number of times that the discrete degree characteristic value exceeds the first preset characteristic threshold reaches a first preset number threshold or the number of times that the discrete degree characteristic value exceeds the second preset characteristic threshold reaches a second preset number threshold, the DC bus capacitor is protected; otherwise, the DC bus capacitor is not protected.

8. The method according to claim 7, characterized in that The method comprises: If the discrete degree characteristic value exceeds the first preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold is incremented once; if the discrete degree characteristic value does not exceed the first preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the first preset characteristic threshold is decremented once; If the discrete degree characteristic value exceeds the second preset characteristic threshold, the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold is incremented once; wherein the number of times the discrete degree characteristic value exceeds the second preset characteristic threshold is reset after a preset interval time.

9. The method according to claim 1, characterized in that: The method further comprises: When it is determined to protect the DC bus capacitor, the grid-connected inverter is controlled to exit the grid-connected state.

10. A grid-connected inverter, characterized in that: The DC end of the grid-connected inverter is connected to the DC bus capacitor, and the AC end of the grid-connected inverter is connected to the power grid; the grid-connected inverter also includes a control unit, which is configured to execute the DC bus capacitor protection method described in any one of claims 1-9.

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