Inverter and control method thereof

Abstract

The embodiment of the invention discloses an inverter and a control method thereof, and the method comprises the steps: obtaining the voltage of a grid-connected point, determining the actual current effective value of a capacitor according to the actual current instantaneous value of the capacitor obtained through the differential operation of the voltage of the grid-connected point, determining whether a resonance protection condition is met or not according to the actual current effective value, and controlling the operation state of the inverter. And when the actual current effective value meets the resonance protection condition, the inverter is timely controlled to stop outputting, so that the inverter needs to timely stop transmitting resonant frequency sub-harmonics to a power grid. According to the technical scheme, no extra resonance detection device needs to be added, hardware cost is saved, complex parameter adjustment can be avoided, resonance protection of the inverter can be achieved, damage to internal components of the inverter is avoided, the service life of the inverter is prolonged, the reliability of a power grid is improved, and high practicability is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected power generation, and in particular to an inverter and a control method thereof. Background Art

[0002] When the new energy grid-connected inverter is working, the electric energy it outputs needs to pass through an LC or LCL filter before it can be connected to the grid, or multiple inverters are operated in parallel, resulting in changes in the impedance of the public grid connection point. The output impedance of the inverter interacts with the grid impedance and forms resonance at a specific frequency. For example, resonance is more likely to occur in a weak grid environment.

[0003] Existing resonance protection schemes are mostly suppression or prevention measures, which usually require external equipment or adjustment of control parameters.

[0004] Improper selection of external devices, control parameters, etc. or changes in the operating conditions of the grid-connected point cause the inverter grid-connected system to resonate. The resonant voltage or current has not yet reached the inverter voltage and current over-high protection threshold, and most of the resonant current will pass through the capacitor. The inverter generally only samples the inverter side current. At this time, the inverter cannot detect the abnormality and cannot cut off the inverter operation in time. The harmonic loss of the power grid will further increase, interfering with detection and communication equipment, reducing the reliability of the power grid, and even causing damage to the electronic components, capacitors, inductors, etc. inside the inverter, reducing the service life of the inverter. Summary of the invention

[0005] The present invention provides an inverter and a control method thereof, so as to timely detect the grid-connected resonance situation of the inverter and realize resonance protection of the inverter.

[0006] According to one aspect of the present invention, a control method for an inverter is provided, wherein the inverter is connected to a power grid at a grid connection point through a filter circuit, the filter circuit includes a capacitor, and the control method includes:

[0007] Obtaining the grid connection point voltage, and determining the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage;

[0008] Whether the resonance protection condition is met is determined according to the actual effective current value of the capacitor, and the operating state of the inverter is controlled to perform grid-connected resonance protection on the inverter.

[0009] Optionally, the grid connection point voltage is obtained, and the actual current effective value of the capacitor is determined according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage, including:

[0010] Obtaining grid connection point voltages at multiple sampling points in at least one power frequency cycle;

[0011] Differentiate the grid connection point voltage to determine the instantaneous value of the actual current of the capacitor corresponding to the sampling point;

[0012] Determine the effective value of the actual current of the capacitor according to the instantaneous values of the current corresponding to each sampling point in at least one power frequency cycle.

[0013] Optionally, the resonance protection condition includes: the effective value of the actual current of the capacitor is greater than the set protection threshold.

[0014] Optionally, before obtaining the grid connection point voltage and determining the effective value of the actual current of the capacitor according to the differentiation of the grid connection point voltage, it further includes:

[0015] Determine the target voltage effective value of the harmonic superimposed on the grid connection point voltage according to the voltage distortion rate threshold and the fundamental wave effective value of the grid connection point voltage during the test experiment;

[0016] Carry out the test experiment in the way of superimposing harmonics on the grid connection point voltage, and determine the set protection threshold according to the test current effective value of the capacitor when the grid connection point voltage does not superimpose harmonics, and the test current effective value of the capacitor when the grid connection point voltage superimposes the target voltage effective value and harmonics of the target harmonic number; where the target harmonic number corresponds to the resonance frequency of the grid connection resonance of the inverter.

[0017] Optionally, before carrying out the test experiment in the way of superimposing harmonics on the grid connection point voltage, it further includes:

[0018] Obtain the resonance frequency range of the grid connection resonance of the inverter;

[0019] Determine the target harmonic number of the harmonics superimposed on the grid connection point voltage according to the resonance frequency range.

[0020] Optionally, determining the target harmonic number of the harmonics superimposed on the grid connection point voltage according to the resonance frequency range includes:

[0021] Determine the target harmonic number of the harmonics superimposed on the grid connection point voltage according to the minimum resonance frequency in the resonance frequency range.

[0022] Optionally, after the test experiment and before controlling the operating state of the inverter according to the magnitude relationship between the effective value of the actual current of the capacitor and the set protection threshold to perform grid connection resonance protection on the inverter, it further includes:

[0023] Update the set protection threshold according to the product of the ratio of the per-unit value of the grid connection point voltage to the per-unit value of the grid connection point voltage during the test experiment and the set protection threshold.

[0024] Optionally, the resonance protection condition includes that the change amount of the effective value of the actual current of the capacitor within a preset time interval exceeds the set change threshold.

[0025] Optionally, determining whether the resonance protection condition is met according to the actual effective current value of the capacitor, and controlling the operating state of the inverter to perform grid-connected resonance protection on the inverter include:

[0026] If the resonance protection condition is met according to the actual current effective value, the inverter is controlled to stop output after a first set delay time, and the capacitor is determined to be in an overcurrent fault state and the overcurrent fault state is stored and / or reported;

[0027] If it is determined that the resonance protection condition is not met according to the actual current effective value, determine whether the capacitor is in an overcurrent fault state;

[0028] If so, after a delay of a second set time, the overcurrent fault state of the capacitor is cleared and the inverter is controlled to restart;

[0029] If not, the inverter is controlled to keep running.

[0030] According to another aspect of the present invention, an inverter is provided, which is controlled by the inverter control method according to any embodiment of the present invention.

[0031] The inverter and control method of the embodiment of the present invention obtain the grid connection point voltage, and determine the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage, determine whether the resonance protection condition is met according to the actual current effective value, and control the operation state of the inverter to perform grid connection resonance protection on the inverter. When the actual current effective value meets the resonance protection condition, the inverter is promptly controlled to stop output, so that the inverter should promptly stop transmitting the resonance frequency subharmonic to the power grid. The technical solution of the embodiment of the present invention can realize the resonance protection of the inverter without adding an additional resonance detection device, save hardware costs and avoid complex parameter adjustment, avoid damage to the internal components of the inverter, extend the service life of the inverter, improve the reliability of the power grid, and has strong practicality.

[0032] It should be understood that the contents described in this section are not intended to identify the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0034] Figure 1 It is a structural diagram of the connection between the inverter and the power grid;

[0035] Figure 2 is a flow chart of a control method for an inverter provided by an embodiment of the present invention;

[0036] Figure 3 is a flow chart of another inverter control method provided by an embodiment of the present invention;

[0037] Figure 4 is a flow chart of another inverter control method provided by an embodiment of the present invention;

[0038] Figure 5 It is a flow chart of another inverter control method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0040] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0041] An embodiment of the present invention provides a control method for an inverter. Figure 1 This is a schematic diagram of the structure of the inverter connected to the grid, refer to Figure 1 , the inverter 10 is connected to the power grid 30 at the grid connection point PCC through the filter circuit 20, the filter circuit includes a capacitor C1, and in some embodiments, the filter circuit 20 also includes an inductor ( Figure 1 (not shown), wherein the structure of the filter circuit can be an LC filter circuit or an LCL filter circuit.

[0042] Figure 2 is a flow chart of a control method for an inverter provided by an embodiment of the present invention, with reference to Figure 2, the control method of the inverter includes:

[0043] S110, obtaining the grid connection point voltage, and determining the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage.

[0044] Specifically, according to the definition of capacitance C = Q / V, where C represents the capacitance, Q represents the amount of charge stored in the capacitor, and V represents the voltage across the capacitor. The calculation formula for the instantaneous current of the capacitor is Icap = dQ / dt, that is, the instantaneous current Icap of the capacitor is equal to the rate of change of the charge Q with time t. Based on this, the calculation formula for the instantaneous current of the capacitor can be expressed as Accordingly, the instantaneous current value of the capacitor can be obtained by differential operation of the grid-connected point voltage. In this step, the actual instantaneous current value of the capacitor refers to the instantaneous current value of the capacitor during the actual grid-connected operation of the inverter, so as to distinguish it from the instantaneous test current value of the capacitor during the test experiment in other embodiments of the present invention. After the actual instantaneous current value of the capacitor is determined, the actual effective current value of the capacitor can be calculated according to the relationship between the actual instantaneous current value of the capacitor and the actual effective current value.

[0045] S120, determining whether a resonance protection condition is met according to the effective value of the actual capacitor current, and controlling the operating state of the inverter to perform grid-connected resonance protection on the inverter.

[0046] When the inverter is connected to the grid and resonates, such as when the LCL resonates, the network composed of the grid impedance, the grid-side inductance, and the capacitor has a very low impedance to the voltage of the resonant frequency, and most of the resonant current will pass through the capacitor. From the perspective of energy exchange, first of all, the capacitor is a component of the circuit. In the inverter circuit containing inductors and capacitors, the inductor and the capacitor together constitute a resonant circuit. When the resonance condition is met (that is, the inductive reactance is equal to the capacitive reactance), a current path will be formed. Secondly, the characteristics of the capacitor determine that it can allow AC current to pass. Although the capacitor is equivalent to an open circuit in a DC circuit, the situation is different in an AC circuit and in a resonant state. For AC signals, the capacitor will continuously charge and discharge. During resonance, due to the periodic changes in the voltage and current at the grid connection point, the voltage across the capacitor also changes periodically, which will generate continuous charging and discharging currents, and the peak value of the current will increase due to resonance, which will increase the capacitor current; from the perspective of the change in impedance characteristics, in the resonant state, the reactance values ​​of the inductor and capacitor in the circuit are equal and offset each other, making the total impedance of the circuit purely resistive and reaching the minimum value. According to Ohm's law I=U / R, when the voltage remains unchanged, the current will inevitably increase as the resistance decreases, which in turn leads to an increase in the capacitor current. Therefore, it is possible to determine whether the inverter has grid-connected resonance by the capacitor current, for example, by determining whether the inverter has grid-connected resonance by the size of the capacitor current and / or the change in current capacitance.

[0047] In this embodiment, the resonance protection condition can be used as a basis for judging whether to perform resonance protection on the inverter. Optionally, the resonance protection condition can be determined based on the working condition that the grid-connected resonance needs to be detected when the inverter is grid-connected, for example, it can be determined by detecting the voltage distortion rate of the grid-connected resonance of the inverter as needed. In this step, whether the resonance protection condition is met is determined according to the actual current effective value of the capacitor. In some embodiments, the resonance protection condition can include that the actual current effective value of the capacitor is greater than the set protection threshold. In other embodiments, the resonance protection condition can include that the change in the actual current effective value of the capacitor within a preset time interval exceeds the set change threshold; in other embodiments, the resonance protection condition can also be that the actual current effective value of the capacitor is greater than the set protection threshold and the change in the actual current effective value of the capacitor within a preset time interval exceeds the set change threshold. Among them, the preset time interval can be set according to actual needs. Optionally, the preset time interval is greater than or equal to the duration of at least two power frequency cycles. In some embodiments, the preset time interval is less than or equal to the duration of 10 power frequency cycles. In other optional embodiments of the present invention, other resonance protection conditions regarding the actual current effective value can also be set, and the embodiments of the present invention are not specifically limited here. By judging whether the actual current effective value of the capacitor meets the resonance protection condition, when the actual current effective value meets the resonance protection condition, the inverter is promptly controlled to stop output, so that the inverter should promptly stop transmitting the resonant frequency subharmonics to the power grid.

[0048] The control method of the inverter of this embodiment obtains the grid connection point voltage, and determines the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage, determines whether the resonance protection condition is met according to the actual current effective value, and controls the operation state of the inverter to perform grid connection resonance protection on the inverter. When the actual current effective value of the capacitor meets the resonance protection condition, the inverter is promptly controlled to stop output, so that the inverter should promptly stop transmitting the resonant frequency subharmonic to the power grid. The technical solution of this embodiment does not need to add an additional resonance detection device, saves hardware costs and avoids complex parameter adjustments, so as to realize the resonance protection of the inverter, avoid damage to the internal components of the inverter, extend the service life of the inverter, improve the reliability of the power grid, and has strong practicality.

[0049] In some embodiments, S110 includes: obtaining the grid-connected point voltage of multiple sampling points in at least one power frequency cycle; performing a differential operation on the grid-connected point voltage to determine the actual current instantaneous value of the capacitor corresponding to the sampling point; and determining the actual current effective value of the capacitor based on the instantaneous current value corresponding to each sampling point in at least one power frequency cycle.

[0050] Specifically, in order to accurately calculate the effective value of the current, it is necessary to consider the change of the current in a complete cycle. If the sampling time is less than one power frequency cycle, it is impossible to fully obtain the waveform characteristics of the current, including the complete information of the fundamental and harmonics. For example, if only half a cycle is sampled, for some asymmetric current waveforms (including even harmonics, etc.), the calculated effective value will be too small. For a periodic function i(t), the calculation formula for its actual current effective value Icaprms is as follows:

[0051]

[0052] Where i(t) is the actual instantaneous current value of the capacitor in the above embodiment, T represents the power frequency period, and the calculation formula of Icaprms can be rewritten as:

[0053]

[0054] According to the above formula, the actual current effective value of the capacitor can be determined by the actual current instantaneous value of the capacitor corresponding to multiple sampling points in one power frequency cycle, and the actual current effective value of the capacitor can be determined according to the actual current instantaneous value of the capacitor corresponding to multiple sampling points in one power frequency cycle. The waveform characteristics of the current can be fully obtained to ensure the accuracy of the calculated current waveform effective value. In other optional embodiments of the present invention, when calculating the actual current effective value, the actual current instantaneous value of multiple sampling points can also be used to calculate the actual current effective value during a period exceeding one power frequency cycle, and the embodiments of the present invention are not specifically limited here.

[0055] In some embodiments, the resonance protection condition includes that the actual effective value of the current of the capacitor is greater than a set protection threshold.

[0056] For an inverter that has experienced grid-connected resonance, a set protection threshold can be determined through historical data of the inverter's grid-connected resonance, and the set protection threshold can be stored. For example, the set protection threshold is determined based on the actual effective current value of the capacitor corresponding to the inverter's grid-connected resonance. The set protection threshold can be less than or equal to the actual effective current value of the capacitor when the inverter experienced grid-connected resonance in historical situations, and greater than the actual effective current value of the capacitor when the inverter did not experience grid-connected resonance. In this way, when the inverter is running, it can be determined that the resonance protection condition is met when the actual current effective value is greater than the set protection threshold, and the inverter can be protected in time by controlling the inverter's working state, such as controlling the inverter to stop running.

[0057] For an inverter that has never experienced grid-connected resonance, the protection threshold can be set by testing the inverter. Figure 3 is a flowchart of another inverter control method provided by an embodiment of the present invention, referring to Figure 3, the control method of the inverter includes:

[0058] S210. Determine a target voltage effective value of the grid connection point voltage superimposed with harmonics according to a voltage distortion rate threshold and a fundamental wave effective value of the grid connection point voltage during a test experiment.

[0059] Among them, the voltage distortion rate threshold may correspond to the critical voltage distortion rate corresponding to the inverter grid-connected resonance that needs to be detected. Exemplarily, when the voltage distortion rate reaches 10%, the inverter grid-connected resonance needs to be detected, and in this case the voltage distortion rate threshold is equal to 10%; when the voltage distortion rate reaches 15%, the inverter grid-connected resonance needs to be detected, and in this case the voltage distortion rate threshold is equal to 15%. The voltage distortion rate threshold may be the distortion rate threshold of the inverter grid-connected point voltage.

[0060] Specifically, for an inverter that has not experienced grid-connected resonance, it is necessary to simulate the situation of grid-connected resonance by superimposing harmonic voltage on the grid-connected point voltage. In this case, it is necessary to determine the target harmonic order and target voltage amplitude of the superimposed harmonic. In some cases, for a specific inverter, the harmonic order when grid-connected resonance occurs is known. In this case, it is sufficient to obtain the target voltage amplitude of the harmonic. According to the definition of voltage distortion rate, voltage distortion rate refers to the percentage of the root sum of the squares of the effective values ​​of each harmonic to the effective value of the fundamental wave. When the voltage distortion rate threshold is determined and the fundamental effective value of the grid-connected point voltage is known, the effective value of the harmonic can be obtained. The target voltage amplitude of the harmonic can be determined based on the effective value of the harmonic. For example, when the voltage distortion rate threshold is 9.09%, it is known that the amplitude corresponding to the fundamental effective value of the grid-connected point voltage is 220V, then it can be determined that the target voltage amplitude corresponding to the target voltage effective value of the harmonic of the grid-connected point voltage is 20V.

[0061] S220, conduct a test experiment by superimposing harmonics on the grid connection point voltage, and determine the set protection threshold value based on the effective value of the capacitor test current when the grid connection point voltage is not superimposed with harmonics, and the effective value of the capacitor test current when the grid connection point voltage is superimposed with the effective value of the target voltage and the harmonics of the target harmonic order.

[0062] The target harmonic order corresponds to the resonant frequency of the grid-connected resonance of the inverter.

[0063] Specifically, the total harmonic current effective value can be calculated according to the following formula:

[0064]

[0065] Among them, Iz represents the effective value of the total harmonic current, I0 represents the effective value of the test current of the capacitor when the grid-connected voltage is not superimposed with harmonics, and In represents the effective value of the test current of the capacitor when the grid-connected voltage is superimposed with the effective value of the target voltage and the nth harmonic, where n is the target harmonic order. In some embodiments, a protection threshold value Iup=Iz can also be set; in other embodiments, the protection threshold value can be set to be greater than Iz to avoid misjudgment; in other embodiments, the protection threshold value can also be set to be less than Iz to ensure that grid-connected resonance can be discovered early.

[0066] When the harmonic order of the superimposed harmonics is unknown, it is necessary to determine the target harmonic order based on the resonant frequency of the inverter grid-connected resonance. Exemplarily, when the resonant frequency is known, the target resonant order is determined based on the ratio of the resonant frequency to the fundamental frequency. Exemplarily, the resonant frequency is 1kHz and the fundamental frequency is 60Hz. The target harmonic order corresponding to the resonant frequency can be n=1kHz / 60Hz, that is, the resonant frequency of 1kHz corresponds to the 16th harmonic in a 60Hz power grid.

[0067] When the resonant frequency is unknown, before the test experiment is performed by superimposing harmonics on the grid-connected voltage, the method also includes: obtaining the resonant frequency range of the inverter grid-connected resonance; and determining the target harmonic order of the grid-connected voltage superimposed harmonics according to the resonant frequency range.

[0068] Among them, the frequency range of the inverter grid-connected resonance can be obtained by collecting the resonant frequency when multiple inverters are grid-connected and resonant, and / or determined according to engineering design experience. Exemplarily, the LCL resonant frequency of the grid-connected inverter is generally between 1kHz-5kHz. When determining the target harmonic number of superimposed harmonics according to the resonant frequency range, in some embodiments, the target harmonic number of the superimposed harmonics of the grid-connected point voltage is determined according to the minimum resonant frequency in the resonant frequency range. Exemplarily, when the resonant frequency range is in the range of 1kHz-5kHz, the target harmonic number of the superimposed harmonics of the grid-connected point voltage is determined according to the resonant frequency of 1kHz. Exemplarily, when the fundamental frequency is 60Hz, the target harmonic number is 16. By determining the target harmonic number of the superimposed harmonics of the grid-connected point voltage according to the minimum resonant frequency in the resonant frequency range, it can be made that under the set protection threshold determined according to the target harmonic number, when the harmonics of each resonant frequency within the resonant frequency range occur, the resonance protection condition can be met, thereby ensuring that when resonance within the resonant frequency range occurs, the inverter is promptly resonantly protected. In other embodiments, the target harmonic order may also be determined based on other resonant frequencies in the resonant frequency range, for example, the median value or at least one resonant frequency between the minimum resonant frequency and the median value.

[0069] S230: Setting a protection threshold.

[0070] S240, obtaining the grid connection point voltage, and determining the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage.

[0071] S250, updating the set protection threshold according to the ratio of the per-unit value of the grid connection point voltage to the per-unit value of the grid connection point voltage during the test experiment and the product of the set protection threshold.

[0072] Specifically, in actual operation, the grid connection point voltage of the inverter and the grid connection point voltage may be unstable, for example, relative to a rated value greater than the grid connection point voltage or less than the rated value of the grid connection point voltage. In this step, the set protection threshold is updated according to the ratio of the per-unit value of the grid connection point voltage to the per-unit value of the grid connection point voltage during the test experiment during the actual operation of the inverter, and the product of the set protection threshold set in S230, so that the set protection threshold can be matched with the actual operating condition of the inverter, ensuring that when the grid connection point voltage changes, the grid connection resonance of the inverter is timely judged, and resonance protection is performed in time. Exemplarily, when the grid connection point voltage increases, if the maximum is 1.3 times the rated grid connection point voltage, the distortion rate remains at 10%, then the fundamental current and the harmonic current are both 1.3 times the original, and the ratio of the total current to 1 times the grid connection point voltage is also 1.3. If the per-unit value of the grid connection point voltage during the test experiment is 1, the updated set protection threshold is 1.3 times the set protection threshold determined according to the test experiment.

[0073] S250: Determine whether the resonance protection condition is met according to the actual current effective value, and control the operating state of the inverter to perform grid-connected resonance protection on the inverter.

[0074] Among them, when the actual current effective value is greater than the updated set protection threshold, it is determined that the resonance protection condition is met. In this case, resonance protection can be performed by controlling the inverter to stop running; when the actual current effective value is less than or equal to the updated set protection threshold, it is determined that the resonance protection condition is not met, and the inverter can continue to run.

[0075] The following verifies the accuracy of the effective value of the capacitor current determined by integrating the grid-connected point voltage.

[0076] 1) When the grid-connected voltage is not superimposed with harmonics, verify that the effective values ​​of the capacitor current when the inverter is not running and when it is running at full load are basically equal, indicating that the switching current has no effect on the calculation of the capacitor current. The calculation formula for the theoretical capacitor current is Ical = Uphase*Xc, where Ical represents the theoretical capacitor current, Uphase represents the rated value of the grid-connected phase voltage, and Xc represents the capacitive reactance of the capacitor. According to the calculation formula for the capacitive reactance of the capacitor, Xc = 1 / 2πfC, f represents the grid frequency, and C represents the capacitance value. The theoretical calculation formula can be obtained: Ical = Uphase*2*π*f*C. Assuming that the grid-connected phase voltage of the inverter is 346V, the grid frequency is 60Hz, and the capacitance value C is 35E-6F, the final theoretical calculation value is Ical = Uphase*2*π*f*C = 4.56A. Table 1 is the instantaneous value of the capacitor current calculated by sampling the grid-connected voltage differential, and then according to the formula The calculated current effective value Icaprms of the instantaneous value in one power frequency cycle shows that the current effective value Icaprms is very close to the theoretical capacitor current Ical, which also proves that the method of using voltage differential to calculate the capacitor current is effective. Table 1 shows the corresponding current effective values ​​Icaprm obtained by the calculation method according to the embodiment of the present invention when harmonics are not superimposed.

[0077] Table 1

[0078] Mutually Current effective value Icaprm a 4.89760685 b 4.59852362 c 4.63262987

[0079] 1) When the grid-connected voltage is superimposed with the 20th harmonic with an amplitude of 20V, the distortion rate is 9.09%. The theoretical calculated value is as follows: The 20th harmonic current I 20 =Uphase*2*π*f 20 *C=20*2*π*(20*60)*35E-6=5.28A, where f 20 Indicates the frequency of the 20th harmonic current, according to the calculation formula of the total harmonic current:

[0080]

[0081] The total harmonic current can be calculated Table 2 shows the corresponding current effective values ​​Icaprm obtained by calculation according to the embodiment of the present invention when the 20th harmonic is superimposed.

[0082] Table 2

[0083] Mutually Current effective value Icaprm a 7.22663261 b 6.70746851 c 7.02990198

[0084] 3) When the grid-connected voltage is superimposed with a 40th harmonic with an amplitude of 20V, the theoretical calculated value is as follows: The theoretical calculated value of the 40th harmonic current is as follows: The 40th harmonic current I 40=Uphase*2*π*f 40 *C=20*2*π*(40*60)*35E-6=10.56A, where f 40 Indicates the frequency of the 40th harmonic current, according to the calculation formula of the total harmonic current:

[0085]

[0086] The total harmonic current can be calculated Table 3 shows the corresponding current effective values ​​Icaprm obtained by calculation according to the embodiment of the present invention when the 40th harmonic is superimposed.

[0087] Table 3

[0088] Mutually Current effective value Icaprm a 11.4461823 b 11.2753668 c 10.9768753

[0089] According to the above verification, when the amplitude of superimposed harmonics is equal, the lower the resonant frequency, the smaller the current increase, and the range of the resonant frequency. For example, according to engineering design experience, the LCL resonant frequency of the grid-connected inverter is generally between 1kHz and 5kHz. Assuming that the resonant frequency is 1kHz and the fundamental frequency is 60Hz, the harmonic number corresponding to the resonant frequency at this time can be calculated as n=1kHz / 60Hz≈16, that is, the resonant frequency of 1kHz corresponds to the 16th harmonic in a 60Hz power grid.

[0090] The total harmonic current of the 16th harmonic with an amplitude of 20V superimposed on the grid connection point voltage is calculated similarly to the above method. The 16th harmonic current I 16 =Uphase*2*π*f 40 *C=20*2*π*(16*60)*35E-6≈4.22A, and the total harmonic current can be obtained Table 4 shows the corresponding current effective values ​​Icaprm obtained by calculation according to the embodiment of the present invention when the 40th harmonic is superimposed.

[0091] Table 4

[0092]

[0093]

[0094] In summary, considering the current bearing capacity of capacitors, the purpose of resonance protection is to prevent the control system from diverging and causing the capacitor current to continue to increase. Therefore, the protection value can be appropriately increased to 6.5A (at 1 times the voltage per unit value). That is, when I capRms When the current is >6.5A, the inverter triggers the resonance protection and stops output to avoid damage to electrical equipment.

[0095] Figure 4is a flowchart of another inverter control method provided by an embodiment of the present invention, referring to Figure 4 , the control method of the inverter includes:

[0096] S301, initialization, setting a protection threshold.

[0097] S310, obtaining the grid connection point voltage, and determining the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage.

[0098] S320: Determine whether a resonance protection condition is met according to the actual current effective value.

[0099] If yes, execute S330.

[0100] If not, execute S360.

[0101] S330, delaying a first set time.

[0102] Optionally, the first set time is greater than at least 3 power frequency cycles. Within the first set time, S320 is continuously executed, that is, when the actual current effective value always meets the holding condition within the first set time, S340 is executed, so that misjudgment caused by short-term current mutation can be avoided, ensuring that the resonance protection will not be triggered by mistake.

[0103] S340, control the inverter to stop output.

[0104] That is, after delaying the first set time, the inverter is controlled to stop outputting. In this way, when resonance occurs in the inverter grid-connected system, the inverter should promptly stop transmitting the resonant frequency subharmonics to the grid, thereby performing resonance protection on the inverter and helping to improve grid reliability.

[0105] S350: Determine that the capacitor is in an overcurrent fault state and store and / or report it.

[0106] S360: Determine whether the capacitor is in an overcurrent fault state.

[0107] If yes, execute S370 to delay for a second set time.

[0108] S380, controlling the inverter to restart to clear the overcurrent fault state of the capacitor.

[0109] That is, after delaying the second set time, the inverter is controlled to restart.

[0110] Optionally, the second set time is greater than the first set time. During the second set time, S360 is continuously performed. In this way, the inverter can be controlled to restart after the capacitor remains in the overcurrent fault state for the second set time, thereby avoiding the adverse effects on the inverter control caused by a short-term misjudgment of whether S360 is in the overcurrent fault state.

[0111] If not, that is, it is determined that the capacitor is not in an overcurrent fault state, S390 is executed to control the inverter to keep running.

[0112] Figure 5 is a flowchart of another inverter control method provided by an embodiment of the present invention, referring to Figure 5 , the control method of the inverter includes:

[0113] S401, initialization, setting a protection threshold.

[0114] S410, obtaining the grid connection point voltage, and determining the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage.

[0115] S420: Determine whether the actual current effective value is greater than the set protection threshold.

[0116] If so, execute S430.

[0117] If not, execute S460.

[0118] S430, delaying a first set time.

[0119] S440, control the inverter to stop output.

[0120] S450: Determine that the capacitor is in an overcurrent fault state and store and / or report it.

[0121] S460: Determine whether the capacitor is in an overcurrent fault state.

[0122] If yes, execute S470 to delay for a second set time.

[0123] S480, clear the overcurrent fault state of the capacitor and control the inverter to restart.

[0124] That is, after delaying the second set time, the inverter is controlled to restart.

[0125] If not, execute S490 and control the inverter to keep running.

[0126] An embodiment of the present invention further provides an inverter, which is controlled by the control method of the inverter of any embodiment of the present invention and has the beneficial effects of the control method of the inverter of any embodiment of the present invention.

[0127] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the present invention can be executed in parallel, sequentially or in different orders, as long as the desired results of the technical solution of the present invention can be achieved, and this document does not limit this.

[0128] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A control method for an inverter, characterized in that: The inverter is connected to the grid at the grid connection point through a filter circuit, the filter circuit includes a capacitor, and the control method includes: Obtaining a grid connection point voltage, and determining an actual current effective value of the capacitor according to an actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage; Whether a resonance protection condition is met is determined according to the actual current effective value, and the operating state of the inverter is controlled to perform grid-connected resonance protection on the inverter.

2. The inverter control method according to claim 1, characterized in that: The step of obtaining the grid connection point voltage and determining the actual current effective value of the capacitor according to the actual current instantaneous value of the capacitor obtained by differential operation of the grid connection point voltage includes: Acquire the grid connection point voltage at a plurality of sampling points in at least one power frequency cycle; Performing a differential operation on the grid connection point voltage to determine an actual instantaneous current value of the capacitor corresponding to the sampling point; The actual effective current value of the capacitor is determined according to the instantaneous current value corresponding to each sampling point in the at least one power frequency cycle.

3. The inverter control method according to claim 1 or 2, characterized in that: The resonance protection condition includes: the actual effective current value of the capacitor is greater than a set protection threshold.

4. The control method of the inverter according to claim 3, characterized in that: Before obtaining the grid connection point voltage and determining the actual current effective value of the capacitor according to the differential of the grid connection point voltage, the method further includes: Determine a target voltage effective value of the grid connection point voltage superimposed with harmonics according to a voltage distortion rate threshold and a fundamental wave effective value of the grid connection point voltage during a test experiment; A test experiment is performed by superimposing harmonics on the grid-connected point voltage, and the set protection threshold is determined according to the effective value of the test current of the capacitor when the grid-connected point voltage is not superimposed with harmonics, and the effective value of the test current of the capacitor when the grid-connected point voltage is superimposed with the effective value of the target voltage and harmonics of the target harmonic order; wherein the target harmonic order corresponds to the resonant frequency of the grid-connected resonance of the inverter.

5. The control method of the inverter according to claim 4, characterized in that: Before the test experiment is performed by superimposing harmonics on the grid connection point voltage, the method further includes: Obtaining a resonant frequency range of the inverter grid-connected resonance; The target harmonic order of the harmonics superimposed on the grid connection point voltage is determined according to the resonant frequency range.

6. The control method of the inverter according to claim 5, characterized in that: The step of determining the target harmonic order of the harmonics superimposed on the grid connection point voltage according to the resonant frequency range includes: The target harmonic order for superimposing harmonics on the grid connection point voltage is determined according to the minimum resonant frequency in the resonant frequency range.

7. The inverter control method according to claim 4, characterized in that: After the test experiment and before controlling the operating state of the inverter according to the magnitude relationship between the actual current effective value and the set protection threshold value to perform grid-connected resonance protection on the inverter, the method further includes: The set protection threshold is updated according to the product of the ratio of the per-unit value of the grid connection point voltage to the per-unit value of the grid connection point voltage during the test experiment and the set protection threshold.

8. The inverter control method according to claim 1 or 2, characterized in that: The resonance protection condition includes that a change in the actual current effective value of the capacitor within a preset time interval exceeds a set change threshold.

9. The inverter control method according to claim 1, characterized in that: The determining whether the resonance protection condition is met according to the actual current effective value, and controlling the operating state of the inverter to perform grid-connected resonance protection on the inverter, comprises: If it is determined according to the effective value of the actual current of the capacitor that the resonance protection condition is met, after a delay of a first set time, the inverter is controlled to stop outputting, and the capacitor is determined to be in an overcurrent fault state and the overcurrent fault is stored and / or reported; If it is determined according to the actual current effective value that the resonance protection condition is not satisfied, determining whether the capacitor is in the overcurrent fault state; If so, after a delay of a second set time, the overcurrent fault state of the capacitor is cleared and the inverter is controlled to restart; if not, the inverter is controlled to keep running.

10. An inverter, characterized in that: The inverter control method according to any one of claims 1 to 9 is used for control.

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