Inverter single-phase / two-phase island detection method and device based on negative sequence component

Abstract

The application discloses an inverter single-phase / two-phase island detection method and device based on negative sequence components and belongs to the technical field of inverter island detection, which comprises the following steps: injecting periodic reactive disturbance current into a power grid through each inverter, including four disturbance modes in one disturbance period, each disturbance mode lasts N rated grid periods, and the amplitudes of positive and negative reactive disturbance currents are not equal; for each inverter, the following steps are performed: (S1) obtaining the voltage negative sequence component vector of a common coupling point in the current rated grid period, and judging whether disturbance reset is needed; (S2) updating the number of rated grid periods AI_CNT of the current disturbance mode, if AI_CNT is not equal to 0, turning to (S1), otherwise, turning to (S3); (S3) point multiplying the voltage negative sequence component vector change amount of the current and the previous disturbance mode; (S4) judging whether the island detection criterion is met according to the point multiplication result. The application can improve the accuracy of single-phase or two-phase island detection and can be applied to multi-machine grid-connected working conditions.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of inverter island detection, and more particularly, to an inverter single-phase / two-phase island detection method and device based on negative sequence components. BACKGROUND

[0002] When the grid fails, the grid-connected inverter fails to detect the outage state in time and stops, and continues to supply power to the local load, which is called islanding effect. The islanding effect poses a certain threat to sensitive loads and the safety of maintenance personnel. Therefore, the grid-connected inverter needs to continuously detect whether there is an islanding effect in the grid-connected power generation system during normal operation. The IEC62116Ed.2 (2014) standard on islanding detection stipulates that the islanding detection time must be less than 2s.

[0003] Common islanding detection methods include passive and active detection methods. Compared with passive methods, active methods have smaller detection blind spots and higher detection accuracy, and are more widely used in actual industrial products. Active methods include active frequency shift, active phase shift, negative sequence component injection, harmonic component injection, active disturbance, and reactive disturbance. Among them, the reactive disturbance is widely used because it has the advantages of small impact on grid power quality, no impact on grid stability, and simple implementation. When the grid-connected power generation system is in an islanding state, this method injects a reactive power disturbance to cause the system frequency to deviate. The larger the disturbance amplitude, the greater the deviation, and eventually triggers the under-frequency, over-frequency, and other related protections, thereby achieving islanding state detection. However, these methods have the problem of detection failure when the grid-connected system is in a single-phase or two-phase islanding state, and additional single-phase or two-phase islanding detection methods need to be added.

[0004] Some methods inject a reactive power disturbance to detect whether the phase angle difference between three-phase voltages changes. However, in actual situations, the grid has some fluctuations, which can easily cause inaccurate phase angle detection at zero-crossing points, and each phase needs to obtain the corresponding phase angle, which requires a large amount of calculation. Some methods improve on this basis by detecting whether the three-phase voltage negative sequence component changes to make a judgment. Specifically, if the three-phase voltage negative sequence component is detected to exceed a certain size, it is determined that single-phase or two-phase islanding detection occurs. However, when the grid is unbalanced or severely distorted, three-phase voltage negative sequence components can also be generated, so this method is prone to misjudgment. In addition, for photovoltaic, wind power, and other distributed power generation systems, as the power generation capacity increases, the capacity of a single grid-connected inverter cannot meet the demand, and multiple grid-connected inverters need to be connected in parallel to expand the capacity, but the above methods do not involve multiple grid-connected working conditions. SUMMARY

[0005] In view of the defects of the prior art and the demand for improvement, the present application provides a single-phase / two-phase island detection method and device for inverters based on negative sequence components, which aims to improve the accuracy of single-phase or two-phase island detection and can be applied to multi-machine grid-connected working conditions.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a single-phase / two-phase island detection method for inverters based on negative sequence components is provided, comprising:

[0007] Periodic reactive disturbance currents are injected into the grid by each inverter, and an island detection step is performed for each inverter; the reactive disturbance current includes four disturbance modes in one disturbance period, which are positive reactive disturbance, zero disturbance, negative reactive disturbance and zero disturbance in turn, each disturbance mode lasts for N rated grid periods, and the current amplitudes of positive and negative reactive disturbances are not equal; 3≤N≤5;

[0008] For any one inverter, the island detection step comprises:

[0009] (S1) Obtain the negative sequence component of the point of common coupling voltage in the current rated grid period and transform it into the two-phase rotating coordinate system to obtain a voltage negative sequence component vector;

[0010] (S2) Update the number of rated grid periods AI_CNT that the current disturbance mode has lasted according to AI_CNT=(AI_CNT+1)mod N, if AI_CNT≠0 after updating, then go to step (S1) after the next rated grid period arrives; otherwise, go to step (S3);

[0011] (S3) Calculate the change of the voltage negative sequence component vector in the current rated grid period, and multiply it with the change of the voltage negative sequence component vector calculated in the last rated grid period of the previous disturbance mode to obtain the multiplication result in the current disturbance mode;

[0012] (S4) According to the multiplication results in the current disturbance mode and the three previous disturbance modes, judge whether T(0)≥N th , T(1)≤-N th , T(2)≥N th and T(3)≤-N th are met at the same time; if yes, it is determined that a single-phase / two-phase island error occurs, and the work of the current inverter is stopped; otherwise, switch the disturbance mode of the current inverter to the next disturbance mode of the current disturbance mode, and go to step (S1) after the next rated grid period arrives;

[0013] Wherein, T(0), T(1), T(2) and T(3) represent the dot product results calculated under positive reactive disturbance, zero disturbance after positive reactive disturbance, negative reactive disturbance and zero disturbance after negative reactive disturbance in the four disturbance modes in sequence; N th is a preset island criterion threshold.

[0014] Further, the amplitude of the positive reactive disturbance in the periodic reactive disturbance current is and the amplitude of the negative reactive disturbance is respectively as follows:

[0015]

[0016] Wherein, i dref represents the d-axis current reference value of the inverter output; K1 and K2 are reactive disturbance coefficients, K1>0, K2>0, and K1≠K2; T(n) represents the dot product result under the latest complete disturbance mode; η≥0.

[0017] Further, η=0.02.

[0018] Further, K1=0.015, K2=0.0075; or, K1=0.0075, K2=0.015.

[0019] Further,

[0020] Wherein, k is the smaller value of the reactive disturbance coefficients K1 and K2, U pccd is the size of the d-axis of the point of common coupling rated voltage.

[0021] Further, N=4.

[0022] Further, the step (S1) further comprises: after obtaining the voltage negative sequence component vector in the current rated grid cycle, judging whether a preset disturbance reset criterion is met, if yes, setting the current injected reactive disturbance current in each inverter to the same state for disturbance reset;

[0023] The disturbance reset criterion is:

[0024] |SUM1 now -SUM2 now |≥M th

[0025] Wherein, SUM1 now and SUM2 nowrespectively represent the first and second accumulated negative sequence components calculated in the current rated grid period; the first accumulated negative sequence component is the accumulated sum of the average value of the common coupling point negative sequence voltage q-axis component or d-axis component calculated in each rated grid period in a complete disturbance sequence composed of i-8N+1-i-4N rated grid periods; the second accumulated negative sequence component is the accumulated sum of the average value of the common coupling point negative sequence voltage q-axis component or d-axis component calculated in each rated grid period in a complete disturbance sequence composed of i-4N+1-i rated grid periods; M th is a preset disturbance reset threshold, M th >0; i is the serial number of the current rated grid period.

[0026] Further,

[0027]

[0028] wherein, SUM1 next and SUM2 next respectively represent the first and second accumulated negative sequence components in the next rated grid period of the current rated grid period; a[] represents the average value of the common coupling point negative sequence voltage q-axis component or d-axis component calculated in the corresponding rated grid period.

[0029] Further, the disturbance reset criterion further comprises: the time interval between the current time and the last time of disturbance reset is greater than 2s.

[0030] According to another aspect of the present application, there is provided an inverter single-phase / two-phase island detection device based on negative sequence components, comprising:

[0031] a disturbance injection module for injecting periodic reactive disturbance current into the grid through each inverter; the reactive disturbance current comprises four disturbance modes in one disturbance period, in sequence, positive reactive disturbance, zero disturbance, negative reactive disturbance and zero disturbance, each disturbance mode lasts for N rated grid periods, and the current amplitudes of the positive and negative reactive disturbances are not equal; 3≤N≤5;

[0032] and an island detection module for performing an island detection step on each inverter respectively; for any one inverter, the island detection step comprises:

[0033] (S1) obtaining the negative sequence component of the common coupling point voltage in the current rated grid period and transforming it into the two-phase rotating coordinate system to obtain a voltage negative sequence component vector;

[0034] (S2) updating the rated grid cycle number AI_CNT in which the current disturbance mode has lasted according to AI_CNT=(AI_CNT+1)mod N, if AI_CNT≠0 after updating, then turning to step (S1) after the next rated grid cycle arrives; otherwise, turning to step (S3);

[0035] (S3) calculating the variation of the voltage negative sequence component vector in the current rated grid cycle, and multiplying the variation with the variation of the voltage negative sequence component vector calculated in the last rated grid cycle of the last disturbance mode to obtain a multiplication result under the current disturbance mode;

[0036] (S4) judging whether T(0)≥N th , T(1)≤-N th , T(2)≥N th and T(3)≤-N th are simultaneously satisfied according to the multiplication results under the current disturbance mode and the three disturbance modes before the current disturbance mode; if yes, determining that a single-phase / two-phase island error occurs, and stopping the operation of the current inverter; otherwise, switching the disturbance mode of the current inverter to the next disturbance mode of the current disturbance mode, and turning to step (S1) after the next rated grid cycle arrives;

[0037] wherein, T(0), T(1), T(2) and T(3) represent the multiplication results calculated under the positive reactive disturbance, the zero disturbance after the positive reactive disturbance, the negative reactive disturbance and the zero disturbance after the negative reactive disturbance respectively in the four disturbance modes; N th is a preset island criterion threshold.

[0038] Overall, the above technical scheme conceived by the present application can achieve the following beneficial effects:

[0039] (1) The present application performs single-phase / two-phase island detection based on the variation of three-phase voltage negative sequence components, specifically, taking the multiplication value of the variation of two adjacent voltage negative sequence vectors as a detection criterion quantity, and establishing a connection with the reactive disturbance mode to form an island detection criterion. Compared with the existing method of directly performing island detection based on three-phase voltage negative sequence components, the present application can exclude the interference of three-phase voltage negative sequence components caused by grid imbalance or serious distortion, ensure that the detection quantity is caused only by the injected reactive disturbance, and effectively improve the accuracy of single-phase / two-phase island detection.

[0040] (2) The present application uses the periodic reactive disturbance current injected by the inverter into the grid, and the current amplitude of the positive reactive disturbance is not equal to the amplitude of the negative reactive disturbance current, which ensures that when multiple inverters are connected in parallel, the injected reactive disturbance will not be completely diluted, thereby ensuring that island detection can be accurately realized in the case of multiple inverters connected in parallel.

[0041] (3) In the preferred scheme of the present application, in the periodic reactive disturbance current injected into the power grid by the inverter, on the basis that the positive reactive disturbance current amplitude is not equal to the negative reactive disturbance current amplitude, a point multiplication operation result positive feedback ηT(n) is introduced, thereby being able to reduce the reactive disturbance amplitude while ensuring that the detection blind area remains unchanged, and reducing the impact on the output power quality.

[0042] (4) In the preferred scheme of the present application, in the periodic reactive disturbance current, the positive reactive disturbance amplitude and the related parameters in the negative reactive disturbance are designed. Specifically, the coefficient η=0.02 in the point multiplication operation result positive feedback is designed. This parameter design can better balance the detection blind area and the impact of the reactive disturbance on the output power quality in the case of introducing the point multiplication operation result positive feedback, and obtain better comprehensive benefits. On this basis, the current amplitudes of the positive and negative reactive disturbances are designed to be 0.015 and 0.0075. Based on this parameter design, it can be ensured that while multiple inverters are working normally and there is a certain reactive disturbance, the impact of the disturbance on the power quality is minimized.

[0043] (5) In the preferred scheme of the present application, based on the size of the PCC point rated voltage on the d-axis U pccd and the injected reactive disturbance coefficient determines the reasonable range of the island detection threshold in the island detection criterion, further ensuring the accuracy of single-phase / two-phase island detection.

[0044] (6) In the preferred scheme of the present application, a corresponding disturbance reset criterion is set. In each rated grid period, it is judged whether the disturbance reset criterion is met according to the three-phase voltage negative sequence component of the PCC, and the disturbance is reset when it is met, so that the disturbance states of the inverters are the same, the reactive disturbance of each inverter is synchronized, and the accuracy of subsequent island detection can be further improved. Further preferably, the disturbance reset criterion is |SUM1 now -SUM2 now |≥M th The difference between the average value accumulations of the d-axis or q-axis components of the three-phase negative sequence voltage of the PCC in the adjacent two complete disturbance sequences is comprehensively considered, which can effectively avoid the error caused by judging based on the calculation result of a single rated grid period, thereby accurately capturing the situation where an island may occur, triggering disturbance reset, realizing the synchronization of the reactive disturbance of each inverter, and effectively improving the accuracy of subsequent island detection. Further preferably, the disturbance reset is only performed when the time interval from the last disturbance reset exceeds 2s, which can avoid prolonging the island detection time due to repeated reset while ensuring the accuracy of island detection. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 This is a schematic diagram of an existing three-phase grid-connected power generation system experiencing a single-phase islanding condition.

[0046] Figure 2 This is a schematic diagram of an existing three-phase grid-connected power generation system experiencing a two-phase islanding condition.

[0047] Figure 3 The present invention provides an embodiment of the three-phase voltage vector change at the PCC point caused by the injected reactive power disturbance under single-phase / two-phase islanding conditions; wherein, (a) is the three-phase voltage vector change at the PCC point caused by the injected reactive power disturbance under single-phase islanding conditions, and (b) is the three-phase voltage vector change at the PCC point caused by the injected reactive power disturbance under two-phase islanding conditions.

[0048] Figure 4 The vector diagram of the negative sequence component of the three-phase voltage at the PCC point caused by the injected reactive power disturbance in the dq coordinate system is provided in an embodiment of the present invention.

[0049] Figure 5 The negative order component vector coordinates, vector differences between adjacent modes, and dot product results of adjacent vector differences are provided for different reactive disturbance modes in the embodiments of the present invention.

[0050] Figure 6 A schematic diagram of a periodic reactive power disturbance sequence provided in an embodiment of the present invention;

[0051] Figure 7 A schematic diagram of the disturbance reset criterion and its calculation method provided in an embodiment of the present invention;

[0052] Figure 8 This is a flowchart of the island detection steps provided in an embodiment of the present invention;

[0053] Figure 9 This is a structural block diagram of the experimental verification platform provided in an embodiment of the present invention;

[0054] Figure 10 for Figure 9 The experimental verification platform is shown with waveforms of two-phase and single-phase islanding detection during single-unit grid-connected operation; where (a) is the waveform of two-phase islanding detection during single-unit grid-connected operation, and (b) is the waveform of single-phase islanding detection during single-unit grid-connected operation.

[0055] Figure 11 for Figure 9 The experimental verification platform is shown with two-phase and single-phase islanding detection waveforms when the two machines are connected to the grid. Among them, (a) is the two-phase islanding detection waveform when the single machine is connected to the grid, and (b) is the single-phase islanding detection waveform when the single machine is connected to the grid.

[0056] Figure 12 for Figure 9The experimental verification platform is shown with waveforms of two-phase and single-phase islanding detection during the operation of three units connected to the grid. Among them, (a) is the waveform of two-phase islanding detection during the operation of a single unit connected to the grid, and (b) is the waveform of single-phase islanding detection during the operation of a single unit connected to the grid.

[0057] Figure 13 for Figure 9 The experimental results shown are the results of the islanding protection time repeatability test of the platform when operating with two machines in parallel and three machines in parallel; where (a) is the result of the islanding protection time repeatability test when operating with two machines in parallel and (b) is the result of the islanding protection time repeatability test when operating with three machines in parallel.

[0058] Figure 14 for Figure 9 The experimental verification platform is shown in waveforms under asymmetrical and severely distorted power grid conditions; where (a) is the waveform under asymmetrical power grid conditions and (b) is the waveform under severely distorted power grid conditions. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0060] In this invention, the terms "first," "second," etc. (if present) in the invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0061] Before explaining the technical solution of the present invention in detail, the single-phase islanding and two-phase islanding conditions of the three-phase grid-connected power generation system are briefly described below.

[0062] like Figure 1 As shown, when a line fault occurs in any phase of a three-phase grid-connected power generation system, if the active and reactive power output of the three-phase grid-connected inverter is close to the active and reactive power consumed by the three-phase RLC load, the system will continue to maintain single-phase islanded operation. Similarly, as... Figure 2 As shown, when any two phases of a three-phase grid-connected power generation system experience a line break fault on the grid side, the system may continue to maintain two-phase islanded operation.

[0063] When a single-phase or two-phase islanding occurs in the system, the reactive power disturbance injected by the grid-connected inverter will cause changes in the voltage phasor of the disconnected phase, while the voltage of the non-disconnected phase remains under grid control. In this case, the three-phase voltage at the PCC (point of common coupling) will exhibit negative-sequence and zero-sequence components. Single-phase islanding operation is as follows: Figure 3As shown in (a), taking phase A circuit breaker as an example, when a positive reactive power disturbance N1 is injected, the voltage vector changes from A to A1; when a negative reactive power disturbance -N2 is injected, the voltage vector changes from A to A2; the phase B and phase C vectors remain unchanged. The two-phase islanding condition is as follows: Figure 3 As shown in (b), taking the A and B phases being disconnected as an example, when a positive reactive power disturbance N1 is injected, voltage vector A changes to A1, and voltage vector B changes to B1. When a negative reactive power disturbance -N2 is injected, voltage vector A changes to A2, and voltage vector B changes to B2, while the voltage vector of phase C remains unchanged. Therefore, by detecting the changes in the negative sequence components of the three-phase voltages, it is possible to determine whether single-phase or two-phase islanding has occurred. However, when the power grid is unbalanced or severely distorted, three-phase voltage negative sequence components may also be generated. Relying solely on the three-phase voltage negative sequence components for detection can easily lead to misjudgments.

[0064] To improve the accuracy of single-phase or two-phase islanding detection, this invention provides a method and apparatus for single-phase / two-phase islanding detection of inverters based on negative sequence components. The overall approach is to detect islanding based on the change in the negative sequence voltage vector, rather than directly on the negative sequence voltage vector itself. This eliminates interference caused by grid asymmetry or severe distortion, ensuring that the detected quantity is solely due to injected reactive power disturbances. Furthermore, based on the pattern of the change in the negative sequence voltage vector under single-phase / two-phase islanding conditions and its relationship with reactive power disturbance modes, a reasonable islanding detection criterion is designed to reduce the possibility of misjudgment and effectively improve the accuracy of single-phase / two-phase islanding detection.

[0065] To achieve accurate detection in single-phase / two-phase islanding scenarios, and also in scenarios with multiple inverters connected to the grid, this invention improves the reactive power disturbance injected into the grid by each inverter. This ensures that the current amplitude of the positive reactive power disturbance is not equal to the current amplitude of the negative reactive power disturbance, guaranteeing that the reactive power disturbance injected when any inverter is connected in parallel is not completely diluted. In practical applications, the current amplitude of the positive reactive power disturbance can be greater than or less than that of the negative reactive power disturbance. Without loss of generality, the following embodiments will use the example of the positive reactive power disturbance having a greater current amplitude than the negative reactive power disturbance. For ease of description, the following embodiments will use symbols... and These represent the current amplitudes of positive and negative reactive disturbances, respectively. In the present application, the reactive disturbance current injected by each inverter into the power grid includes four disturbance modes in a disturbance period, in turn, positive reactive disturbance, zero disturbance, negative reactive disturbance and zero disturbance, each disturbance mode lasts for N rated grid periods, 3≤N≤5; in the present application, the frequency of the point of common coupling (PCC) is detected and island detection is performed in the last rated grid period of each disturbance mode, if the duration of each disturbance mode is too long, it will lead to too long island detection time, if the duration of each disturbance mode is too short, it may lead to inaccurate detection results because the system has not yet reached a stable state, based on this, the present application sets the duration of each disturbance mode to 3-5 rated grid periods, which can effectively avoid related problems. In order to balance the detection time and the detection accuracy, as a preferred embodiment, in the following embodiments, N=4 is set, that is, each disturbance mode lasts for 4 rated grid periods, accordingly, a complete disturbance period lasts for 16 rated grid periods. In the last rated grid period of each disturbance mode, the steady-state value of the negative sequence component of the PCC point voltage under the current disturbance mode is obtained, and an island detection criterion is constructed based on the obtained results to perform single-phase / two-phase island detection.

[0066] As shown in Figure 4 , the three-phase voltage negative sequence component measured at the PCC is transformed into the dq coordinate system to obtain a negative sequence component vector, the size of the projection of the vector on the d-axis and the q-axis can form coordinates to describe the vector. Figure 4 In the present application, respectively, are the corresponding negative sequence voltage vectors when no reactive disturbance is injected, positive reactive disturbance is injected, and negative reactive disturbance is injected. When the three-phase voltage of the power grid remains balanced and the output power of the inverter is exactly equal to the consumed power of the load, is a zero vector. respectively, are the change amounts of the negative sequence component vectors corresponding to the injected positive reactive disturbance and negative reactive disturbance.

[0067] The present application researches and finds that the change amounts of the negative sequence component vectors of two adjacent disturbance modes are in the same direction or opposite directions, therefore, the dot product of the two vector change amounts is equal to or opposite to the product of the lengths of the two vector change amounts, as shown in Figure 5 , that is, the dot product of the two vector change amounts contains the size and direction information of the two vector change amounts. Based on this, the present embodiment converts the vector operation into numerical operation by performing dot product operation on the two vector change amounts, and forms the following island detection criterion by establishing a relationship between the dot product operation result and the reactive disturbance mode:

[0068] T(0)≥N th &&T(1)≤-N th &&T(2)≥N th&&T(3)≤-N th

[0069] Wherein, T(0), T(1), T(2) and T(3) represent the dot product operation results of positive reactive disturbance, zero disturbance after positive reactive disturbance, negative reactive disturbance and zero disturbance after negative reactive disturbance in current disturbance mode and its previous three disturbance modes in sequence; N th The threshold value of the preset island criterion, which can be set reasonably according to actual working conditions, is usually small, and the detection is relatively easy, but the detection error is large, on the contrary, the threshold value is large, and the detection error is small, but the detection difficulty is relatively large, in order to ensure that the threshold value is within a reasonable range, in the application, the size of the criterion threshold N th is determined by the size of the rated voltage of the PCC point on the d-axis U pccd and the injected reactive disturbance coefficient k, and the specific expression is:

[0070]

[0071] After the inverter injects the reactive disturbance current into the power grid, the total amount of reactive power will not be 0, which will affect the power factor of the system and reduce the quality of the output power; in order to reduce the influence, as an optional embodiment, in the following embodiment, the injected reactive disturbance current further introduces the positive feedback of the dot product operation result ηT(n) on the basis that the current amplitudes of positive and negative reactive disturbances are not equal; based on this, in the periodic reactive disturbance current, the amplitude of positive reactive disturbance and the amplitude of negative reactive disturbance are as follows:

[0072]

[0073]

[0074] i dref represents the d-axis current reference value of the inverter output; K1 and K2 are reactive disturbance coefficients, K1>0, K2>0, and K1≠K2; T(n) represents the dot product operation result in the latest complete disturbance mode; η≥0, and its specific value can be set according to the range of possible dot product operation results and the upper and lower limits of the injected reactive disturbance; the values of K1, K2 and η are different, and the detection blind area and the influence on the quality of the output power will be different, in order to achieve a good balance, in the following embodiment, the specific values of the related parameters are: K1=0.015, K2=0.0075, and η=0.02. It should be noted that the parameter values here are only optimal values, and should not be understood as the only limitation of the application, in some other embodiments of the application, the parameters can also be adjusted flexibly according to actual needs.

[0075] In the above island detection threshold relationship, k represents the smaller value of reactive disturbance coefficients K1 and K2, which is 0.0075 in the following embodiment; for a system with an AC grid rated phase voltage effective value of 220V, in the following embodiment, N th The value is 0.5.

[0076] For ease of description, in the following embodiments, AI_MODE represents the current mode of reactive disturbance, which takes values from 0 to 3 in a cycle, and the values from 0 to 3 represent positive reactive disturbance, zero disturbance, negative reactive disturbance and zero disturbance, respectively; AI_CNT represents the number of rated grid cycles that each disturbance mode has lasted, which also takes values from 0 to 3 in a cycle; then the reactive disturbance current waveform injected by a single inverter into the grid is as shown in the following figure: Figure 6 Based on the above parameter setting, when AI_MODE takes different values, the corresponding reactive disturbance can be represented as follows:

[0077] When AI_MODE = 0,

[0078] When AI_MODE = 1, Δi q = 0;

[0079] When AI_MODE = 2,

[0080] When AI_MODE = 3, Δi q = 0;

[0081] Where, Δi q represents the current amplitude in the zero disturbance mode.

[0082] The present application ensures that the injected reactive disturbance is not completely diluted when multiple inverters are connected in parallel, thereby ensuring that island detection can be accurately realized in the case of multiple inverters connected in parallel.

[0083] In the case of multiple inverters connected in parallel, the injected reactive power disturbance of each inverter is synchronized, and the result of island detection is more accurate and fast, therefore, as a preferred embodiment, the present application sets a suitable disturbance reset criterion, which can accurately determine the possibility of island occurrence and reset the reactive disturbance of each inverter, so that the injected reactive disturbance current of each inverter is in the same state, thereby effectively improving the accuracy of island detection while shortening the island detection time.

[0084] In each rated grid cycle, it is judged whether disturbance reset is needed according to a disturbance reset criterion. In order to avoid misjudgment caused by only using single-cycle frequency deviation, in the following embodiments, the disturbance reset criterion is designed by integrating the accumulative sum of the average values of the q-axis components of the PCC negative sequence voltage calculated in two adjacent complete disturbance sequences (each complete disturbance sequence includes 16 rated grid cycles).

[0085] Specifically, in any ith rated grid cycle, the average value of the q-axis component of the PCC negative sequence voltage vector in the rated grid cycle is defined as

[0086] a[i] = |U PCCq_neg_avg

[0087] wherein U PCCq_neg_avg represents the average value of the q-axis component of the PCC negative sequence voltage vector in the rated grid cycle; it should be noted that in some other embodiments of the present application, the average value of the d-axis component of the PCC negative sequence voltage vector can also be used to define a[] in the corresponding rated grid cycle;

[0088] Based on a[], the first accumulative negative sequence component SUM1 in any ith rated grid cycle is defined as now the accumulative sum of the average values of the q-axis or d-axis components of the PCC negative sequence voltage calculated in the ith-8N+1-i-4N rated grid cycle; the second accumulative negative sequence component SUM2 in any ith rated grid cycle is defined as now the accumulative sum of the average values of the q-axis or d-axis components of the PCC negative sequence voltage calculated in the ith-4N+1-i rated grid cycle; then for the current rated grid cycle t=32t0 (t0 represents the rated grid cycle), the first accumulative negative sequence component SUM1 now and the second accumulative negative sequence component SUM2 now in the current rated grid cycle are respectively as follows:

[0089]

[0090]

[0091] Based on the above calculation, the disturbance reset criterion is specifically as follows:

[0092] D SUM = |SUM1 now -SUM2 now | ≥ M th

[0093] wherein M th is a preset disturbance reset threshold, and Mth 0; when the multi-machine grid-connected system is in a normal grid-connected state, D SUM 0; when the system has a single-phase or two-phase island, since there is always a certain amount of reactive disturbance in the system, the negative sequence voltage component of the PCC point will change; D SUM When the disturbance reset criterion is exceeded, it means that the negative sequence component deviation is large, and an island may occur, and disturbance reset needs to be performed; alternatively, in the following embodiments, by clearing AI_MODE and AI_CNT, the reactive disturbance currents injected into the grid by each inverter are set to the same state, and disturbance reset is completed; it should be noted that in some other embodiments of the present application, AI_MODE of each inverter can also be set to other same values on the basis of clearing AI_CNT, and disturbance reset is completed. Since all inverters sample the same PCC point voltage, the reactive disturbance sequence will be reset at the same time, thereby realizing synchronization of the reactive disturbance sequence among the inverters.

[0094] Since the first cumulative frequency deviation and the second cumulative frequency deviation need to be calculated in each rated grid period, and the calculation of the two cumulative frequency deviations involves a large amount of repeated calculation in adjacent two rated grid periods, in order to reduce the amount of calculation, a sliding window method is used for calculation, that is, on the basis of the calculation result of the current rated grid period, the difference value of the next rated grid period relative to the calculation result of the current rated grid period is added as the calculation result of the next rated grid period, then as shown in Figure 7 the first cumulative negative sequence component SUM1 next and the second cumulative negative sequence component SUM2 next in the next rated grid period t=33t0, the calculation formulas are as follows:

[0095]

[0096] Since multiple repeated resets can greatly prolong the island detection time, in order to avoid this problem, in the following embodiments, the disturbance reset criterion further includes: the time interval between the current time and the last disturbance reset is greater than 2s, and 2s is the required island detection time. This constraint ensures that the reset action cannot be performed again within 2s after the last reset.

[0097] The following is an embodiment.

[0098] Embodiment 1:

[0099] A single-phase / two-phase island detection method for inverters based on a negative sequence component, comprising:

[0100] injecting the above periodic reactive disturbance current into the grid by each inverter, and performing an island detection step for each inverter respectively;

[0101] For any inverter, the island detection procedure as shown in Figure 8 includes:

[0102] (S1) obtaining the negative sequence component of the point of common coupling voltage in the current rated grid period and transforming it into the two-phase rotating coordinate system to obtain the voltage negative sequence component vector;

[0103] As a preferred embodiment, step (S1) of the present embodiment further includes:

[0104] After obtaining the voltage negative sequence component vector in the current rated grid period, it is determined whether the above disturbance reset criterion is met. If yes, the AI_MODE and AI_CNT corresponding to the current injected reactive disturbance current in each inverter are both cleared, so that the reactive disturbance current states are the same, to perform disturbance reset. After the disturbance reset is completed, step (S2) is entered. If the disturbance reset criterion is not met, step (S2) is directly entered;

[0105] (S2) updating the number of rated grid periods AI_CNT in which the current disturbance mode has lasted according to AI_CNT=(AI_CNT+1)mod4. If AI_CNT≠0 after updating, step (S1) is entered after the next rated grid period arrives. Otherwise, step (S3) is entered;

[0106] (S3) calculating the change of the voltage negative sequence component vector in the current rated grid period, and performing point multiplication between the change and the change of the voltage negative sequence component vector calculated in the last rated grid period of the last disturbance mode to obtain the point multiplication result under the current disturbance mode;

[0107] The reactive disturbance current of the current inverter is updated accordingly based on the point multiplication result;

[0108] (S4) determining whether the above island detection criterion is met according to the point multiplication results under the current disturbance mode and the three disturbance modes before the current disturbance mode, i.e., whether T(0)≥0.5, T(1)≤-0.5, T(2)≥0.5 and T(3)≤-0.5 are met at the same time, and the time interval between the current time and the last disturbance reset is greater than 2s. If yes, it is determined that a single-phase / two-phase island error occurs, and the work of the current inverter is stopped. If the island detection criterion is not met, the disturbance mode of the current inverter is switched to the next disturbance mode of the current disturbance mode, and accordingly, AI_MODE will be updated to (AI_MODE+1)mod4. After the next rated grid period arrives, step (S1) is entered;

[0109] Wherein, T(0), T(1), T(2) and T(3) represent the dot product calculation results in the positive reactive disturbance, zero disturbance after the positive reactive disturbance, negative reactive disturbance and zero disturbance after the negative reactive disturbance of the four disturbance modes in turn.

[0110] Generally, the embodiment takes the dot product value of the adjacent two voltage negative sequence vector changes as the detection criterion quantity, and establishes a connection with the reactive disturbance mode to form an island detection criterion. This criterion is not prone to misjudgment when the power grid is unbalanced or severely distorted; by setting the reactive disturbance sequence reset criterion, the reactive disturbance sequence synchronization between the inverters is realized, ensuring that this method can be applied to multi-machine grid-connected conditions; this synchronization method does not require wired or wireless communication between inverters, effectively reducing the cost of the system. Compared with detecting the phase angle difference between the three-phase voltages, detecting the three-phase voltage negative sequence component is more convenient and consumes less computing resources of the digital controller.

[0111] Embodiment 2:

[0112] A kind of inverter single-phase / two-phase island detection device based on negative sequence component, comprising:

[0113] Disturbance injection module, for injecting the periodic reactive disturbance current described above to power grid by each inverter;

[0114] And island detection module, for performing island detection step to each inverter respectively;For any one inverter, the island detection step includes:

[0115] (S1) obtains the negative sequence component of the voltage at the point of common coupling in the current rated grid cycle and transforms to two-phase rotating coordinate system, obtains voltage negative sequence component vector;

[0116] As a preferred embodiment, step (S1) of the embodiment further includes:

[0117] After obtaining the voltage negative sequence component vector in the current rated grid cycle, it is judged whether the disturbance reset criterion is met, if yes, the AI_MODE and AI_CNT corresponding to the current injected reactive disturbance current in each inverter are all cleared, so that the reactive disturbance current state is the same, to reset the disturbance, after the disturbance reset is finished, step (S2) is entered;If the disturbance reset criterion is not met, step (S2) is directly entered;

[0118] (S2) updates the number of rated grid cycles AI_CNT that the current disturbance mode has lasted according to AI_CNT=(AI_CNT+1)mod 4, if AI_CNT≠0 after updating, step (S1) is entered after the next rated grid cycle arrives;Otherwise, step (S3) is entered;

[0119] (S3) calculate the change of the voltage negative sequence component vector in the current rated grid cycle, and multiply it with the change of the voltage negative sequence component vector calculated in the last rated grid cycle of the previous disturbance mode to obtain a point multiplication result under the current disturbance mode;

[0120] The reactive disturbance current of the current inverter is updated accordingly based on the point multiplication result;

[0121] (S4) according to the point multiplication results under the current disturbance mode and the three disturbance modes before it, judge whether the above island detection criterion is met, that is, whether T(0)≥0.5, T(1)≤-0.5, T(2)≥0.5 and T(3)≤-0.5 are met at the same time, and the time interval from the current time to the last disturbance reset is greater than 2s, if it is satisfied, it is determined that single-phase / two-phase island error occurs, and the work of the current inverter is stopped; otherwise, the disturbance mode of the current inverter is switched to the next disturbance mode of the current disturbance mode, accordingly, AI_MODE will be updated to (AI_MODE+1)mod 4, after the next rated grid cycle is reached, step (S1) is entered;

[0122] Wherein, T(0), T(1), T(2) and T(3) represent the point multiplication results calculated under the positive reactive disturbance, zero disturbance after positive reactive disturbance, negative reactive disturbance and zero disturbance after negative reactive disturbance in the four disturbance modes respectively.

[0123] In this embodiment, the specific implementation of each module can refer to the description in the above method embodiment, which will not be repeated here.

[0124] The feasibility and effectiveness of the single-phase / two-phase island detection method are verified by experiments. As shown in Figure 9 The experimental verification platform used in the experiment is composed of three three-phase grid-connected inverters, an adjustable three-phase RLC load and a three-phase circuit breaker. The experimental steps are as follows:

[0125] (1) connect the RLC load and the inverter to the grid in turn, and ensure that the output power of the inverter and the consumed power of the RLC load are equal;

[0126] (2) form single-phase or two-phase island working condition by short-circuiting two phases or one phase of the three-phase circuit breaker;

[0127] (3) disconnect the common connection point circuit breaker switch, at this time the grid is disconnected, simulating island occurrence condition;

[0128] (4) measure the time between the disconnection of the circuit breaker and the disconnection of the inverter relay with an oscilloscope, which is the island protection time;

[0129] (5) When multiple machines are connected to the grid, repeatable tests are conducted.

[0130] Figure 9 The experimental verification platform shown is running in single-unit grid-connected mode. The waveforms for two-phase and single-phase islanding detection are as follows: Figure 10 As shown in (a) and (b); Figure 10 The waveform diagram shown verifies that the method provided by this invention can realize single-phase and two-phase islanding detection in a stand-alone grid-connected system. Figure 9 The experimental verification platform shown in the figure displays the waveforms for two-phase and single-phase islanding detection during grid-connected operation of the two machines, respectively. Figure 11 As shown in (a) and (b) above, the waveforms for two-phase and single-phase islanding detection during three-unit grid-connected operation are respectively as follows: Figure 12 As shown in (a) and (b) in the figure, the results of the repeatability test of islanding protection time during two-machine grid-connected operation and three-machine grid-connected operation are respectively as follows: Figure 13 As shown in (a) and (b), Figure 11 , Figure 12 as well as Figure 13 The proposed method has been verified to achieve single-phase and two-phase islanding detection in multi-machine grid-connected systems, with the islanding protection time consistently less than 800ms, meeting the requirement of less than 2s for islanding protection time in IEC 62116Ed.2(2014). Waveform diagrams under grid asymmetry and severe grid distortion conditions are shown below. Figure 14 As shown in (a) and (b), Figure 14 The experimental results shown verify that the method proposed in this invention will not misjudge situations under conditions of power grid asymmetry and severe power grid distortion. The above experiments fully demonstrate the feasibility and effectiveness of this invention.

[0131] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A single-phase / two-phase island detection method for an inverter based on a negative sequence component, characterized by, Comprising: Injecting periodic reactive disturbance current to the grid through each inverter, and performing island detection step for each inverter respectively; the reactive disturbance current comprises four disturbance modes in a disturbance period, in turn, positive reactive disturbance, zero disturbance, negative reactive disturbance and zero disturbance, each disturbance mode lasts N rated grid periods, and the current amplitudes of positive and negative reactive disturbance are not equal; 3≤N≤5； For any one inverter, the island detection step comprises: (S1) obtaining the negative sequence component of the common coupling point voltage in the current rated grid period and transforming to the two-phase rotating coordinate system to obtain the voltage negative sequence component vector; (S2) updating the number of rated grid periods AI_CNT that the current disturbance mode has lasted according to AI_CNT=(AI_CNT+1)mod N, if AI_CNT≠0 after updating, then after the next rated grid period arrives, step (S1) is entered; otherwise, step (S3) is entered; (S3) calculating the change of the voltage negative sequence component vector in the current rated grid period, and multiplying it with the change of the voltage negative sequence component vector calculated in the last rated grid period of the last disturbance mode to obtain the multiplication result under the current disturbance mode; (S4) According to the point multiplication operation results under the current disturbance mode and the previous three disturbance modes, it is judged whether T(0)≥N th , T(1)≤-N th , T(2)≥N th and T(3)≤-N th are satisfied at the same time; if satisfied, it is determined that a single-phase / two-phase island error occurs, and the operation of the current inverter is stopped; otherwise, the disturbance mode of the current inverter is switched to the next disturbance mode of the current disturbance mode, and after the next rated grid period is reached, step (S1) is entered. Wherein, T(0), T(1), T(2) and T(3) represent the dot product calculation results under the positive reactive disturbance, the zero disturbance after the positive reactive disturbance, the negative reactive disturbance and the zero disturbance after the negative reactive disturbance in the four disturbance modes in turn; N th is a preset island criterion threshold.

2. The negative sequence component based single-phase / two-phase island detection method for inverters as claimed in claim 1, wherein, In the periodic reactive disturbance current, the amplitude of the positive reactive disturbance is equal to the amplitude of the negative reactive disturbance respectively as follows: wherein i dref represents the d-axis current reference value of the inverter output; K1 and K2 are reactive disturbance coefficients, K1>0, K2>0, and K1≠K2; T(n) represents the point multiplication result under the latest complete disturbance mode; and η≥0.

3. The negative sequence component based single-phase / two-phase island detection method for inverters as claimed in claim 2, wherein, η=0.

02.

4. The negative sequence component based single-phase / two-phase island detection method for inverters as claimed in claim 3, wherein, K1=0.015, K2=0.0075; or, K1=0.0075, K2=0.

015.

5. The negative sequence component based single-phase / two-phase island detection method for inverters as claimed in claim 2, wherein, where k is the smaller of the reactive disturbance coefficients K1 and K2, U pccd is the size of the rated voltage at the point of common coupling on the d-axis.

6. The negative sequence component based single-phase / two-phase island detection method for inverters as claimed in claim 1, wherein, N＝4。 7. The negative sequence component based single-phase / two-phase island detection method for inverters according to any one of claims 1 to 6, characterized in that, The step (S1) further comprises: after obtaining the voltage negative sequence component vector in the current rated grid period, judging whether the preset disturbance reset criterion is met, if yes, setting the current injected reactive disturbance current of each inverter to the same state for disturbance reset; The disturbance reset criterion is: |SUM1 now -SUM2 now |≥M th SUM1 now and SUM2 now respectively represent a first accumulated negative sequence component and a second accumulated negative sequence component calculated in a current rated grid period; the first accumulated negative sequence component is an accumulated sum of average values of the point of common coupling negative sequence voltage q-axis component or d-axis component calculated in each rated grid period in a complete disturbance sequence composed of i-8N+1-i-4N rated grid periods; the second accumulated negative sequence component is an accumulated sum of average values of the point of common coupling negative sequence voltage q-axis component or d-axis component calculated in each rated grid period in a complete disturbance sequence composed of i-4N+1-i rated grid periods; M th is a preset disturbance reset threshold, M th >0; i is a serial number of the current rated grid period.

8. The inverter single-phase / two-phase island detection method based on negative sequence component according to claim 7, characterized in that, SUM1 next SUM2 next respectively the first and second cumulative negative sequence components in the next rated grid cycle following the current rated grid cycle; a[] represents the average of the calculated point of common coupling negative sequence voltage q-axis component or d-axis component in the corresponding rated grid cycle.

9. The negative sequence component based single-phase / two-phase island detection method for inverters as claimed in claim 8, wherein, The disturbance reset criterion further comprises: the time interval from the current time to the last disturbance reset is greater than 2s.

10. An inverter single-phase / two-phase island detection device based on negative sequence components, characterized by, Comprising: A disturbance injection module, configured to inject periodic reactive disturbance current to the grid through each inverter; the reactive disturbance current comprises four disturbance modes in a disturbance period, in turn, positive reactive disturbance, zero disturbance, negative reactive disturbance and zero disturbance, each disturbance mode lasts N rated grid periods, and the current amplitudes of positive and negative reactive disturbance are not equal; 3≤N≤5; And an island detection module, configured to perform island detection step for each inverter respectively; For any one inverter, the island detection step comprises: (S1) obtaining the negative sequence component of the common coupling point voltage in the current rated grid period and transforming to the two-phase rotating coordinate system to obtain the voltage negative sequence component vector; (S2) updating the number of rated grid periods AI_CNT that the current disturbance mode has lasted according to AI_CNT=(AI_CNT+1)mod N, if AI_CNT≠0 after updating, then after the next rated grid period arrives, step (S1) is entered; otherwise, step (S3) is entered; (S3) calculating a variation of the voltage negative sequence component vector in the current rated power grid cycle, and point-multiplying the variation of the voltage negative sequence component vector calculated in the last rated power grid cycle of the previous disturbance mode to obtain a point-multiplication result under the current disturbance mode; (S3) calculating a variation of the voltage negative sequence component vector in the current rated power grid cycle, and point-multiplying the variation of the voltage negative sequence component vector calculated in the last rated power grid cycle of the previous disturbance mode to obtain a point-multiplication result under the current disturbance mode; (S3) calculating a variation of the voltage negative sequence component vector in the current rated power grid cycle, and point-m (S4) Based on the dot product results of the current perturbation mode and the three preceding perturbation modes, determine whether T(0)≥N is satisfied simultaneously. th T(1)≤-N th T(2)≥N th And T(3)≤-N th If the condition is met, a single-phase / two-phase islanding error is determined to have occurred, and the current inverter is stopped from operating; otherwise, the disturbance mode of the current inverter is switched to the next disturbance mode of the current disturbance mode, and after the next rated grid cycle is reached, the process proceeds to step (S1). Wherein, T(0), T(1), T(2) and T(3) represent the dot product calculation results under the positive reactive disturbance, the zero disturbance after the positive reactive disturbance, the negative reactive disturbance and the zero disturbance after the negative reactive disturbance in the four disturbance modes in turn; N th is a preset island criterion threshold.

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