CN117578621B - Control method of grid-built inverter - Google Patents

Abstract

The invention relates to the technical field of inverter control, in particular to a grid-built inverter control method, which comprises the steps of obtaining a floating interval model and an exceeding model according to collected data, then carrying out change analysis on the floating interval model, obtaining the limit slope and the average slope of all the floating interval models according to slope change in the floating interval model, and then respectively determining a limit floating interval and an average floating interval according to the numerical distribution condition of the limit slope and the average slope; and then processing the standard exceeding model according to the same original model, determining a dangerous floating interval and a dangerous average interval, and finally determining the current value monitored in real time according to an analysis object, thereby determining the control mode adopted at present, being capable of flexibly making proper decisions according to different conditions and past data performances and being capable of more effectively carrying out flexible control of the grid-built inverter.

Description

A control method for grid-connected inverter

Technical Field

The present invention relates to the technical field of inverter control, and in particular to a grid-type inverter control method.

Background technique

The grid-forming (GFM) control strategy is a control strategy that enables the converter to support the power grid without a synchronous generator. The grid-forming control strategy mainly simulates the generation characteristics and frequency synchronization mechanism of the synchronous generator to achieve the self-synchronization function and output a given voltage amplitude and phase. Unlike the traditional grid-based inverter that adjusts the active and reactive power injection by controlling the active current and reactive current, the grid-forming inverter controls the injected power by adjusting the amplitude and phase of the common coupling point (PCC) voltage. The response of the grid-forming inverter to grid faults is very different from that of the grid-based inverter. The grid-forming inverter can respond quickly to any grid event in time, while the grid-based inverter needs to detect the current actual operation status in real time. Although this fast response is better than the grid-based inverter and can maintain the grid-connected operation state when the grid fails, in actual applications, since the grid-forming control exhibits voltage source characteristics, the rapid increase of the inverter current caused by the grid fault will cause overcurrent problems. Traditional synchronous generators can support currents up to 7 times their rated current, but grid-connected inverters can only handle 20% to 50% overcurrent, so fault current limiting measures for grid-connected inverters need to be considered.

The current current limiting measures of grid-connected inverters usually adopt the method of switching control types. For example, when the current does not exceed the allowable value, the grid-connected operation of the grid-connected control is maintained. When a fault is detected, it switches to the grid following mode or directly runs off the grid. This operation mode cannot better play the role of the grid-connected inverter, and grid failures may sometimes be only temporary problems, with short-term overcurrents that will not reach the upper limit, but the corresponding control methods are adapted, which affects their actual use.

Summary of the invention

In view of this, an object of the present invention is to propose a control method for a grid-type inverter to solve the problem that the existing control type switching method cannot better play the role of the grid-type inverter.

Based on the above purpose, the present invention provides a grid-connected inverter control method, comprising the following steps:

S1. Take the grid-type inverter as the analysis object, take the circuit where the grid-type inverter is located as the target object, obtain all fault conditions that occur in the target object during the target period, and obtain the fault current rise model of each fault;

S2. According to the maximum current value that the analysis object can withstand, that is, the upper limit, the fault current climbing model is screened, and the fault current climbing model whose maximum current value is not lower than the set quantization line and less than the upper limit is marked as a floating interval model, and the fault current climbing model whose maximum current value is not lower than the upper limit is marked as an over-standard model;

S3, analyzing the floating interval model to obtain the limit slope and average slope of all floating interval models;

S4. According to the numerical distribution of the limit slope and the average slope of the floating interval model, determine the limit floating interval and the average floating interval respectively;

S5. Analyze the exceeding-standard model to obtain the limit slope and average slope of all exceeding-standard models, wherein the limit slope of the exceeding-standard model is the maximum slope before the upper limit current of the set analysis object appears;

S6, according to the numerical distribution of the limit slope and the average slope of the exceeding standard model, respectively determine the dangerous floating interval and the dangerous average interval in the same manner as in step S4;

S7. Monitor the real-time situation of the analysis object, obtain the real-time current of the analysis object and the instantaneous growth rate of the current, and generate a switching signal according to the interval in which the highest instantaneous growth rate is reached, when the real-time current of the analysis object reaches the preset maximum current corresponding to the interval, and switch the analysis object to the grid following mode or off-grid operation.

Preferably, the target period is a period from the current time to six months ago.

Preferably, in step S1, the method for acquiring the fault current rise model includes:

When a fault occurs in the target object, two preset unit time periods are reversed, and then the real-time current of the target object is obtained once every unit time period, and X ₁ time periods are obtained continuously, where X ₁ is a preset value;

Plot the graph of time and real-time current value to obtain the corresponding target object fault current climbing model.

Preferably, the preset value _X1 in step S1 satisfies monitoring the target object until the real-time current reaches the maximum value and returns to normal, or after the real-time current reaches the maximum value, it drops to the middle stage between the normal current and the current corresponding to the maximum value.

Preferably, in step S2, the quantization line is set to 0.8 times the tolerance upper limit.

Preferably, in step S3, the method for obtaining the limit slope and the average slope of all floating interval models includes:

S31, select any floating interval model, obtain its maximum current value, and mark it as the upper limit current value;

S32, obtaining the point where the maximum slope in the floating interval model is located, and marking it as the limit slope;

S33, obtaining the slope of the current value when it changes from the initial current value to the upper limit current value in the floating interval model, and marking it as the average slope;

Steps S31-S33 are performed on all other floating interval models to obtain the limit slopes and average slopes of all floating interval models, which are _Gi , i=1, ..., n, _Pi , i=1, ..., n, _Gi and _Pi are in a one-to-one correspondence, and n means that there are n limit slopes and average slopes.

Preferably, the method for determining the limit floating interval and the average floating interval in step S4 is:

S41. Get the mean value of _Gi , mark it as P, and calculate the deviation value W of Gi using the following formula:

;

S42, if W is less than _X2 , mark the interval from the minimum value to the maximum value of _Gi as the limit floating interval, otherwise execute step S43;

S43, selecting the _Gi corresponding to the maximum value of |Gi _{- P} | in a descending order, deleting the _Gi value, and recalculating the W values of the remaining _Gi according to step S41;

S44, determine whether W is less than X ₂ , if not, repeat step S43 until W is less than X ₂ , obtain the number of _Gi values deleted at this time, divide it by n to get the deletion ratio, if the deletion ratio is lower than X ₃ , mark the range from the minimum to the maximum value of the remaining _Gi values as the limit floating interval, otherwise determine the limit floating interval according to the minimum and maximum values of the original _Gi ;

_Pi is processed in the same manner as steps S41-S44, and the result obtained is marked as an average floating interval.

Preferably, step S7 specifically includes:

When the instantaneous growth rate of the real-time current reaches only the average floating interval at most, when the current value reaches 95% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation;

When the instantaneous growth rate of the real-time current reaches the maximum limit floating range, when the current value reaches 90% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation;

When the instantaneous growth rate of the real-time current reaches the highest dangerous average interval, when the current value reaches 85% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation;

When the instantaneous growth rate of the real-time current reaches the highest dangerous floating interval, when the current value reaches 80% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation.

Preferably, in step S6, the highest instantaneous growth rate is specifically the maximum value of the instantaneous growth rate before the current rises to 80% of the maximum current that the analysis object can withstand.

The beneficial effects of the present invention are as follows: the present invention can divide the extreme floating interval, the average floating interval, the dangerous floating interval and the dangerous average interval, so as to determine the current control method to be adopted according to the real-time monitored current value of the analysis object, and can flexibly make appropriate decisions according to different situations and past data performances, and can more effectively perform flexible control of the grid-type inverter. The present invention is simple, effective, and easy to use.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only for the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

FIG. 1 is a schematic flow chart of a method for controlling a grid-connected inverter according to an embodiment of the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

As shown in FIG1 , an embodiment of the present specification provides a grid-connected inverter control method, comprising the following steps:

S1. Take the grid-type inverter as the analysis object, take the circuit where the grid-type inverter is located as the target object, obtain all fault conditions that occur in the target object in the target time period, and obtain the fault current climbing model of each fault. In this embodiment, the target time period is set to be pushed forward six months from the current time, that is, the time period from six months ago to the current time. Of course, other times can also be used here, which can be set by the administrator. First, when a fault occurs in the target object, two unit time periods will be pushed forward, and then the real-time current of the target object will be obtained once every unit time period, and _X1 time periods will be obtained continuously, where _X1 is a preset value, and _X1 satisfies the monitoring of the target object until the real-time current reaches the highest value until it returns to normal, or after the real-time current reaches the highest value, it drops to the median stage of the normal current and the current corresponding to the highest value; the specific length of a time period here is set by the administrator, and then a graph between the time point and the real-time current value is drawn to obtain the fault current climbing model of the corresponding target object, and then the fault current climbing model of each time in the target time period is obtained.

S2. According to the maximum current value that the analysis object can withstand, that is, the upper limit, the fault current climbing model is screened, and the fault current climbing model whose maximum current value is not lower than the set quantization line and less than the upper limit is marked as a floating interval model, and the fault current climbing model whose maximum current value is not lower than the upper limit is marked as an over-standard model. For example, the specific screening method is:

First, the highest current value that the corresponding analysis object can withstand is obtained, and it is marked as the upper limit of the withstand value. The value obtained by multiplying the upper limit of the withstand value by 0.8 is marked as the quantization line. All the current values in the fault current climbing model that are lower than the quantization line are removed;

The current maximum value in the fault current climbing model that is not lower than the quantization line and lower than the upper limit is marked as a floating interval model;

The remaining fault current rise models are marked as exceeding models.

S3. Analyze the floating interval model to obtain the limit slope and average slope of all floating interval models. For example, the specific analysis method is:

S31, select any floating interval model, obtain its maximum current value, and mark it as the upper limit current value;

S32, obtaining the point where the maximum slope in the floating interval model is located, and marking it as the limit slope;

S33, obtaining the slope of the current value when it changes from the initial current value to the upper limit current value in the floating interval model, and marking it as the average slope;

S34. Perform steps S31-S33 on all other floating interval models to obtain the limit slopes and average slopes of all floating interval models, which are _Gi , i=1, ..., n, _Pi , i=1, ..., n, _Gi and _Pi are in a one-to-one correspondence, and n means there are n limit slopes and average slopes.

S4. According to the numerical distribution of the limit slope and the average slope of the floating interval model, the limit floating interval and the average floating interval are determined respectively. Specifically, the limit floating interval and the average floating interval are determined including:

S41: Get the mean value of Gi, mark it as P, and then use the formula to calculate the deviation value W of Gi. The specific calculation formula is:

;

S42: When W is less than or equal to X2, the interval from the minimum value to the maximum value of Gi is automatically calibrated as the limit floating interval;

Otherwise, the corresponding Gi values will be automatically selected in order from large to small according to the order |Gi-P|. Each time a Gi value is selected, it will be deleted, and the W value of the remaining Gi will be automatically calculated. If the W value is still greater than X2, the next Gi value will be selected in order, deleted, and the W value will be recalculated until the W value is less than or equal to X2. The number of Gi values deleted at this time is obtained, and it is divided by n to obtain the deletion ratio. If the deletion ratio is lower than X3, the range from the minimum to the maximum value of the remaining Gi values is marked as the extreme floating interval, otherwise the extreme floating interval is determined according to the minimum and maximum values of the original Gi.

Here X2 and X3 are both preset values;

S43: obtaining the limit floating range;

S44: Process Pi according to the same principle as steps S41-S43, and mark the final result as the average floating interval.

S5. Analyze the over-standard model to obtain the limit slope and average slope of all over-standard models, where the limit slope of the over-standard model is the maximum slope before the upper limit current of the set analysis object appears. The specific analysis method includes:

S51, select any over-standard model, obtain its maximum current value, and mark it as the upper limit current value;

S52, obtaining the point where the maximum slope in the exceeding standard model is located, and marking it as the limit slope;

S53, obtaining the slope of the current value when the current value changes from the initial value to the upper limit value in the exceeding standard model, and marking it as the average slope;

S54. Perform steps S51-S53 on all other exceeding-standard models to obtain the limiting slopes and average slopes of all exceeding-standard models, which are _Gi ', i=1, ..., n', _Pi ', i=1, ..., n', respectively. _Gi ' and _Pi ' are in one-to-one correspondence, and n' means that there are n' limiting slopes and average slopes.

For example, the upper limit current of the analysis object set above is 95% of the maximum current that the analysis object can withstand.

S6, according to the numerical distribution of the limit slope and the average slope of the exceeding standard model, respectively determine the dangerous floating interval and the dangerous average interval in the same manner as in step S4;

S7, monitor the real-time situation of the analysis object, obtain the real-time current of the analysis object and the instantaneous growth rate of the current, and generate a switching signal when the real-time current of the analysis object reaches the preset maximum current corresponding to the interval according to the interval of the highest instantaneous growth rate, and switch the analysis object to the grid following mode or off-grid operation. As an implementation method, the specific analysis process is as follows:

When the instantaneous growth rate of the real-time current reaches only the average floating interval at most, when the current value reaches 95% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation;

When the instantaneous growth rate of the real-time current reaches the maximum limit floating range, when the current value reaches 90% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation;

When the instantaneous growth rate of the real-time current reaches the highest dangerous average interval, when the current value reaches 85% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation;

When the instantaneous growth rate of the real-time current reaches the highest dangerous floating interval, when the current value reaches 80% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation.

Some of the data in the above formula are calculated by removing the dimensions and taking their numerical values. The formula is a formula that is closest to the actual situation obtained by simulating a large amount of collected data through software. The preset parameters and preset thresholds in the formula are set according to the actual situation or obtained through simulation of a large amount of data.

It should be understood by those skilled in the art that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present invention (including the claims) is limited to these examples. Under the concept of the present invention, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity.

The present invention is intended to cover all such substitutions, modifications and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims (9)

1. A grid-connected inverter control method, characterized in that the method comprises the following steps: S1. Take the grid-type inverter as the analysis object, take the circuit where the grid-type inverter is located as the target object, obtain all fault conditions that occur in the target object during the target period, and obtain the fault current rise model of each fault; S2. According to the maximum current value that the analysis object can withstand, that is, the upper limit, the fault current climbing model is screened, and the fault current climbing model whose maximum current value is not lower than the set quantization line and less than the upper limit is marked as a floating interval model, and the fault current climbing model whose maximum current value is not lower than the upper limit is marked as an over-standard model; S3, analyzing the floating interval model to obtain the limit slope and average slope of all floating interval models; S4. According to the numerical distribution of the limit slope and the average slope of the floating interval model, determine the limit floating interval and the average floating interval respectively; S5. Analyze the exceeding-standard model to obtain the limit slope and average slope of all exceeding-standard models, wherein the limit slope of the exceeding-standard model is the maximum slope before the upper limit current of the set analysis object appears; S6, according to the numerical distribution of the limit slope and the average slope of the exceeding standard model, respectively determine the dangerous floating interval and the dangerous average interval in the same manner as in step S4; S7. Monitor the real-time situation of the analysis object, obtain the real-time current of the analysis object and the instantaneous growth rate of the current, and generate a switching signal according to the interval in which the highest instantaneous growth rate is reached, when the real-time current of the analysis object reaches the preset maximum current corresponding to the interval, and switch the analysis object to the grid following mode or off-grid operation. 2. The grid-type inverter control method according to claim 1, characterized in that the target time period is a period from the current time to six months ago. 3. The grid-connected inverter control method according to claim 1, characterized in that, in step S1, the fault current rise model is obtained in a manner comprising: When a fault occurs in the target object, two preset unit time periods are reversed, and then the real-time current of the target object is obtained once every unit time period, and X ₁ time periods are obtained continuously, where X ₁ is a preset value; Plot the graph of time and real-time current value to obtain the corresponding target object fault current climbing model. 4. The grid-connected inverter control method according to claim 3 is characterized in that the preset value _X1 in step S1 satisfies the monitoring of the target object until the real-time current reaches the maximum value until it returns to normal, or after the real-time current reaches the maximum value, it drops to the middle stage between the normal current and the current corresponding to the maximum value. 5. The grid-type inverter control method according to claim 1, characterized in that in step S2, the quantization line is set to 0.8 times the upper limit. 6. The grid-connected inverter control method according to claim 1, characterized in that, in step S3, the method of obtaining the limit slope and the average slope of all floating interval models comprises: S31, select any floating interval model, obtain its maximum current value, and mark it as the upper limit current value; S32, obtaining the point where the maximum slope in the floating interval model is located, and marking it as the limit slope; S33, obtaining the slope of the current value when it changes from the initial current value to the upper limit current value in the floating interval model, and marking it as the average slope; Steps S31-S33 are performed on all other floating interval models to obtain the limit slopes and average slopes of all floating interval models, which are _Gi , i=1, ..., n, _Pi , i=1, ..., n, _Gi and _Pi are in a one-to-one correspondence, and n means that there are n limit slopes and average slopes. 7. The grid-connected inverter control method according to claim 6, characterized in that the method for determining the limit floating interval and the average floating interval in step S4 is: S41. Get the mean value of _Gi , mark it as P, and calculate the deviation value W of Gi using the following formula: , S42, if W is less than _X2 , mark the interval from the minimum value to the maximum value of _Gi as the limit floating interval, otherwise execute step S43; S43, selecting the _Gi corresponding to the maximum value of |Gi _{- P} | in a descending order, deleting the _Gi value, and recalculating the W values of the remaining _Gi according to step S41; S44, determine whether W is less than X ₂ , if not, repeat step S43 until W is less than X ₂ , obtain the number of _Gi values deleted at this time, divide it by n to get the deletion ratio, if the deletion ratio is lower than X ₃ , mark the range from the minimum to the maximum value of the remaining _Gi values as the limit floating interval, otherwise determine the limit floating interval according to the minimum and maximum values of the original _Gi ; _Pi is processed in the same manner as steps S41-S44, and the result obtained is marked as an average floating interval. 8. The grid-connected inverter control method according to claim 1, wherein step S7 specifically comprises: When the instantaneous growth rate of the real-time current reaches only the average floating interval at most, when the current value reaches 95% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation; When the instantaneous growth rate of the real-time current reaches the maximum limit floating range, when the current value reaches 90% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation; When the instantaneous growth rate of the real-time current reaches the highest dangerous average interval, when the current value reaches 85% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation; When the instantaneous growth rate of the real-time current reaches the highest dangerous floating interval, when the current value reaches 80% of the maximum current that the analysis object can withstand, a switching signal is generated to switch the analysis object to the grid following mode or directly off-grid operation. 9. The grid-type inverter control method according to claim 1, characterized in that, in step S6, the highest instantaneous growth rate is specifically the maximum value of the instantaneous growth rate before the current rises to 80% of the maximum current that the analysis object can withstand.