Inverter grid-connected system overall impedance frequency sweeping method under unbalanced working condition

Abstract

The invention is suitable for the technical field of inverter grid-connected system impedance measurement, and provides an inverter grid-connected system overall impedance frequency sweeping method under an unbalanced working condition. Comprising the following steps: respectively injecting a plurality of positive-sequence disturbance voltage signals with different frequencies and a plurality of negative-sequence disturbance voltage signals with different frequencies into a disturbance injection point on a power grid side of an inverter grid-connected system, and collecting voltage signals and current signals of the disturbance injection point when the plurality of positive-sequence disturbance voltage signals with different frequencies are injected; acquiring a voltage signal and a current signal of a disturbance injection point when a plurality of negative sequence disturbance voltage signals with different frequencies are injected; and based on the collected voltage signals and current signals, obtaining positive sequence impedance measurement values and negative sequence impedance measurement values of the whole inverter grid-connected system under a plurality of different frequencies. According to the invention, the overall impedance frequency sweeping of the inverter grid-connected system under the unbalanced working condition can be realized.

Description

Technical Field

[0001] The present application belongs to the technical field of impedance measurement of inverter grid-connected systems, and in particular, relates to an overall impedance frequency sweeping method of an inverter grid-connected system under unbalanced working conditions. Background Art

[0002] With the massive amount of new energy being connected to the distribution network through power electronic inverters, the dynamic characteristics of the traditional distribution network have undergone fundamental changes. The interaction between the inverter and the distribution network has caused a series of oscillation problems, which has brought considerable challenges to the reliable operation of the distribution network. In addition, in the actual distribution network, due to the random fluctuations of user loads and the large-scale use of single-phase loads, the distribution network often has three-phase imbalance. Therefore, the inverter is often in an unbalanced working state. The impedance analysis method has the advantages of simple measurement and clear physical meaning, and has been widely used in analyzing the stability of the inverter grid-connected system. However, in existing studies, the frequency sweep of the inverter impedance under unbalanced conditions usually includes positive sequence impedance, negative sequence impedance and coupling impedance. The frequency sweep process is relatively complicated and the accuracy is not high. In addition, there is little research on the overall impedance sweep of the inverter grid-connected system under unbalanced conditions. In order to more conveniently analyze the impedance characteristics of the overall impedance of the inverter grid-connected system under unbalanced conditions and lay a theoretical foundation for the stability analysis of the grid-connected system, it is urgent to propose a feasible overall impedance sweep method for the inverter grid-connected system under unbalanced conditions. Summary of the invention

[0003] The embodiment of the present application provides a method for sweeping the overall impedance of an inverter grid-connected system under unbalanced conditions, which can solve the problem that the overall impedance sweeping of the inverter grid-connected system under unbalanced conditions cannot be achieved.

[0004] The embodiment of the present application provides a method for sweeping the overall impedance of an inverter grid-connected system under unbalanced working conditions, including:

[0005] A plurality of positive-sequence disturbance voltage signals of different frequencies and a plurality of negative-sequence disturbance voltage signals of different frequencies are injected at a disturbance injection point; the disturbance injection point is located at the grid side of the inverter grid-connected system, a plurality of positive-sequence disturbance voltage signals are injected simultaneously, a plurality of negative-sequence disturbance voltage signals are injected simultaneously, and the positive-sequence disturbance voltage signal and the negative-sequence disturbance voltage signal are injected separately;

[0006] When a plurality of positive-sequence disturbance voltage signals of different frequencies are injected, a voltage signal and a current signal at a disturbance injection point are collected; and when a plurality of negative-sequence disturbance voltage signals of different frequencies are injected, a voltage signal and a current signal at a disturbance injection point are collected;

[0007] Based on the collected voltage signal and current signal, the positive sequence impedance measurement value and negative sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies are obtained.

[0008] Optionally, based on the collected voltage signal and current signal, the positive sequence impedance measurement value and the negative sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies are obtained, including:

[0009] Perform Fourier decomposition on the collected voltage and current signals to obtain the amplitudes and phases of the three-phase voltage and three-phase current at multiple different frequencies;

[0010] The amplitudes and phases of the three-phase voltages and three-phase currents at multiple different frequencies are calculated by a symmetrical component method to obtain positive-sequence disturbance voltage components and negative-sequence disturbance voltage components of the three-phase voltages at multiple different frequencies, and positive-sequence disturbance current components and negative-sequence disturbance current components of the three-phase currents at multiple different frequencies;

[0011] Based on the positive-sequence disturbance voltage component and negative-sequence disturbance voltage component of the three-phase voltage at multiple different frequencies, and the positive-sequence disturbance current component and negative-sequence disturbance current component of the three-phase current at multiple different frequencies, the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies are calculated.

[0012] Optionally, a plurality of positive-sequence disturbance voltage signals of different frequencies include a positive-sequence disturbance voltage signal V per_p1 , positive sequence disturbance voltage signal V per_p2 , positive sequence disturbance voltage signal V per_p3 , positive sequence disturbance voltage signal V per_p4 , positive sequence disturbance voltage signal V per_p5 and positive sequence disturbance voltage signal V per_p6 .

[0013] Optionally, a plurality of negative-sequence disturbance voltage signals of different frequencies include a negative-sequence disturbance voltage signal V per_n1 , negative sequence disturbance voltage signal V per_n2 , negative sequence disturbance voltage signal V per_n3 , negative sequence disturbance voltage signal V per_n4 , negative sequence disturbance voltage signal V per_n5 And negative sequence disturbance voltage signal V per_n6 .

[0014] Optional, positive sequence disturbance voltage signal V per_p1 And negative sequence disturbance voltage signal V per_n1 for:

[0015]

[0016] Positive sequence disturbance voltage signal V per_p2 And negative sequence disturbance voltage signal V per_n2 for:

[0017]

[0018] Positive sequence disturbance voltage signal V per_p3 And negative sequence disturbance voltage signal V per_n3 for:

[0019]

[0020] Positive sequence disturbance voltage signal V per_p4 And negative sequence disturbance voltage signal V per_n4 for:

[0021]

[0022] Positive sequence disturbance voltage signal V per_p5 And negative sequence disturbance voltage signal V per_n5 for:

[0023]

[0024] Positive sequence disturbance voltage signal V per_p6 And negative sequence disturbance voltage signal V per_n6 for:

[0025]

[0026] Among them, V p1 V per_p1 The amplitude, f p1 V per_p1 frequency, t is time, V n1 V per_n1 The amplitude, f n1 V per_n1 The frequency, f p1 =f n1 , V p2 V per_p2 The amplitude, f p2 V per_p2 Frequency, V n2 V per_n2 The amplitude, f n2 V per_n2 The frequency, f p2 =f n2 , V p3 V per_p3 The amplitude, f p3 V per_p3 Frequency, V n3 V per_n3 The amplitude, f n3 V per_n3 The frequency, f p3 =f n3 , V p4 V per_p4 The amplitude, f p4V per_p4 Frequency, V n4 V per_n4 The amplitude, f n4 V per_n4 The frequency, f p4 =f n4 , V p5 V per_p5 The amplitude, f p5 V per_p5 Frequency, V n5 V per_n5 The amplitude, f n5 V per_n5 The frequency, f p5 =f n5 , V p6 V per_p6 The amplitude, f p6 V per_p6 Frequency, V n6 V per_n6 The amplitude, f n6 V per_n6 The frequency, f p6 =f n6 .

[0027] Optionally, the impedance sweep method further includes:

[0028] For each frequency, the measured positive-sequence impedance value of the inverter grid-connected system as a whole at the frequency is compared with the theoretical value of the positive-sequence impedance to obtain the verification result of the positive-sequence impedance measurement value at the frequency, and the measured negative-sequence impedance value of the inverter grid-connected system as a whole at the frequency is compared with the theoretical value of the negative-sequence impedance to obtain the verification result of the negative-sequence impedance measurement value at the frequency.

[0029] Optionally, the impedance sweep method further includes:

[0030] Adjust multiple different frequencies, and return to the step of injecting multiple positive-sequence disturbance voltage signals of different frequencies and multiple negative-sequence disturbance voltage signals of different frequencies at the disturbance injection point based on the adjusted multiple different frequencies, until the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole are measured at all frequencies in the frequency band to be measured.

[0031] The above solution of the present application has the following beneficial effects:

[0032] In an embodiment of the present application, the inverter and the grid impedance are taken as a whole, the positive-sequence disturbance voltage signal and the negative-sequence disturbance voltage signal are injected twice at the disturbance injection point on the grid side, and the voltage signal and the current signal of the disturbance injection point are collected when multiple positive-sequence disturbance voltage signals of different frequencies are injected at the same time, and the voltage signal and the current signal of the disturbance injection point are collected when multiple negative-sequence disturbance voltage signals of different frequencies are injected at the same time. Finally, based on the collected voltage signal and current signal, the positive-sequence impedance measurement value and the negative-sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies can be obtained, and the overall impedance frequency scanning of the inverter grid-connected system under unbalanced conditions can be realized, laying a theoretical foundation for the stability analysis of the grid-connected system.

[0033] Other beneficial effects of the present application will be described in detail in the subsequent specific implementation section. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0035] Figure 1 The topological structure diagram of the grid-connected inverter system with grid-connected droop control is shown in FIG.

[0036] Figure 2 A flow chart of an overall impedance sweep method for an inverter grid-connected system under unbalanced conditions provided by an embodiment of the present application;

[0037] Figure 3 A schematic diagram of injecting a disturbance voltage provided in an embodiment of the present application;

[0038] Figure 4a It is the positive sequence impedance sweep diagram of the overall impedance sweep result of the inverter grid-connected system;

[0039] Figure 4b The negative sequence impedance sweep diagram is the overall impedance sweep result of the inverter grid-connected system. DETAILED DESCRIPTION

[0040] In the following description, specific details such as specific system structures, technologies, etc. are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present application. However, it should be clear to those skilled in the art that the present application may also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present application.

[0041] It should be understood that when used in the present specification and the appended claims, the term "comprising" indicates the presence of described features, wholes, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or combinations thereof.

[0042] It should also be understood that the term “and / or” used in the specification and appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0043] As used in the specification and appended claims of this application, the term "if" can be interpreted as "when" or "uponce" or "in response to determining" or "in response to detecting", depending on the context. Similarly, the phrase "if it is determined" or "if [described condition or event] is detected" can be interpreted as meaning "uponce it is determined" or "in response to determining" or "uponce [described condition or event] is detected" or "in response to detecting [described condition or event]", depending on the context.

[0044] In addition, in the description of the present application specification and the appended claims, the terms "first", "second", "third", etc. are only used to distinguish the descriptions and cannot be understood as indicating or implying relative importance.

[0045] References to "one embodiment" or "some embodiments" etc. described in the specification of this application mean that one or more embodiments of the present application include specific features, structures or characteristics described in conjunction with the embodiment. Therefore, the statements "in one embodiment", "in some embodiments", "in some other embodiments", "in some other embodiments", etc. that appear in different places in this specification do not necessarily refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized in other ways. The terms "including", "comprising", "having" and their variations all mean "including but not limited to", unless otherwise specifically emphasized in other ways.

[0046] In view of the problem that the overall impedance sweep of the inverter grid-connected system under unbalanced conditions cannot be achieved at present, an embodiment of the present application provides a method for the overall impedance sweep of the inverter grid-connected system under unbalanced conditions. By treating the inverter and the grid impedance as a whole, a positive-sequence disturbance voltage signal and a negative-sequence disturbance voltage signal are injected twice at the disturbance injection point on the grid side, and the voltage signal and current signal of the disturbance injection point are collected when multiple positive-sequence disturbance voltage signals of different frequencies are injected at the same time, and the voltage signal and current signal of the disturbance injection point are collected when multiple negative-sequence disturbance voltage signals of different frequencies are injected at the same time. Finally, based on the collected voltage signal and current signal, the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies can be obtained, thereby achieving the overall impedance sweep of the inverter grid-connected system under unbalanced conditions, and laying a theoretical foundation for the stability analysis of the grid-connected system.

[0047] The overall impedance frequency sweeping method of the inverter grid-connected system under unbalanced conditions provided by the present application is exemplarily described below in conjunction with specific embodiments.

[0048] It is understandable that the embodiment of the present application does not limit the structure of the inverter grid-connected system, and the above impedance sweep method provided by the present application can be adapted to all inverter grid-connected systems. For ease of understanding, the embodiment of the present application takes the grid-connected droop control inverter grid-connected system (DFDCS) as an example to illustrate the impedance sweep method. The derivation process of the positive and negative sequence impedance theoretical value of DFDCS includes the following steps:

[0049] Step 1: Construct the impedance model of DFDCS unbalanced working condition taking into account the frequency coupling effect:

[0050] The topological structure of the above-mentioned grid-connected droop control inverter system is as follows: Figure 1 As shown in the figure, V dc is the DC side voltage; e a 、e b and e c is the bridge arm midpoint voltage; i La 、i Lb and i Lc is the filter inductor current; i Ld 、i Lq is the d-axis and q-axis inductor current; u a 、u b and u c is the voltage at the point of common coupling (PCC); u d and u q is the d-axis and q-axis grid-connected voltage; i a 、i b and i c is the grid-connected point current; i d and iq is the grid-connected current of d-axis and q-axis; c dq is the modulation signal in the synchronous rotating coordinate system; c abc is the three-phase modulation signal in the stationary coordinate system; L f is the filter inductor; C f is the filter capacitor; Z Labc is an unbalanced load; L g is the grid inductance; R g is the grid resistance; U gabc is the grid voltage; θ is the inverter output phase angle; P is the output active power; Q is the output reactive power; P ref is the active power reference value; Q ref is the reactive power reference value; u dref and u qref is the reference voltage of d-axis and q-axis; G del (s) = e -sTs is the delay transfer function, s is the frequency domain operator, Ts is the sampling period, abc / dq is the coordinate transformation module from the abc coordinate system to the dq coordinate system, and PWM is pulse width modulation, which is used to output gate signals to control the inverter.

[0051] The two-dimensional impedance matrix of DFDCS under unbalanced working condition taking into account the frequency coupling effect is established by harmonic linearization method. Let c a [±f p ]=A1V p +A2V p1 +A3I p +A4I p1 , c a [±f p1 ]=B1V p +B2V p1 +B3I p +B4I p1 , c a [±f p ] is the disturbance frequency f p The a phase modulation signal component, c a [±f p1 ] is the disturbance frequency f p1 The a-phase modulation signal component, f p1 =f p -2f1, f1 is the fundamental frequency, then the impedance matrix expression is as follows:

[0052]

[0053] In the formula, s=σ+jω, ω=2*π*f p , f p is the disturbance frequency, K m= 0.5, ω0 = 2 * π * 50; and are the positive-sequence self-impedance, positive-negative sequence coupling impedance, and negative-sequence self-impedance of the DFDCS respectively, where j is the imaginary unit; A1 is the polynomial coefficient of V in c a [±f p , A2 is the polynomial coefficient of V in c p [±f a [±f p , A3 is the polynomial coefficient of I in c p1 [±f a [±f p , A4 is the polynomial coefficient of I in c p [±f a [±f p , B1 is the polynomial coefficient of V in c p1 [±f a [±f p1 , B2 is the polynomial coefficient of V in c p [±f a [±f p1 , B3 is the polynomial coefficient of I in c p1 [±f a [±f p1 , B4 is the polynomial coefficient of I in c p [±f a [±f p1 , V p1 is the perturbation voltage at frequency f p , V p is the coupled-frequency voltage at frequency f p1 , I p1 is the response current at frequency f p , I p is the coupled-frequency current at frequency f p1 ; Y p1 (s) = 1 + s con L 2 C f ; Since the focus of this application is on the description of the frequency sweep method rather than on constructing the inverter impedance model, and this frequency sweep method is applicable to the frequency sweep of other inverter impedance models under various working conditions, the specific expressions of A1~A4 and B1~B4 are not given here. f ; Since the focus of this application is on the description of the frequency sweep method rather than on constructing the inverter impedance model, and this frequency sweep method is applicable to the frequency sweep of other inverter impedance models under various working conditions, the specific expressions of A1~A4 and B1~B4 are not given here.

[0054] Under unbalanced working conditions, the expressions of the grid impedance matrix Z g (s) and the load impedance matrix Z L (s) are:

[0055]

[0056] The total impedance matrix Z on the grid side grid (s) is expressed as:

[0057]

[0058] Where a = e j120° ; Z k When the subscript k of (s) is g, it means the grid side; when k is L, it means the load side; Z g (s) is the two-dimensional matrix of grid impedance; Z L (s) is the two-dimensional matrix of load impedance; Z kpp , Z kpn , Z knp , Z knn is the corresponding element of the two-dimensional impedance matrix of the power grid / load; Z ka , Z kb , Z kc is the three-phase impedance of the power grid / load abc; is the total impedance matrix element on the grid side.

[0059] Step 2: Construct a single-input single-output impedance model of DFDCS:

[0060] In order to simplify the complexity of subsequent inverter grid-connected system overall impedance sweep, a single-input single-output impedance model of the inverter grid-connected system overall impedance is constructed.

[0061] The single input and single output impedance of the inverter and the total impedance of the grid side can be expressed as:

[0062]

[0063]

[0064]

[0065]

[0066] In the formula, and They are the equivalent positive and negative sequence impedances of the inverter and the equivalent positive and negative sequence impedances of the grid side respectively.

[0067] From the above formula, the overall impedance single-input single-output positive and negative sequence impedance model of the inverter grid-connected system can be obtained as follows:

[0068]

[0069] In the formula, and They are the theoretical values ​​of the overall positive and negative sequence impedances of the inverter grid-connected system, respectively.

[0070] like Figure 2As shown, the overall impedance sweep method of the inverter grid-connected system under unbalanced conditions provided in the embodiment of the present application includes the following steps:

[0071] Step 21 : injecting a plurality of positive-sequence disturbance voltage signals of different frequencies and a plurality of negative-sequence disturbance voltage signals of different frequencies at a disturbance injection point.

[0072] The above disturbance injection point is located on the grid side of the inverter grid-connected system (such as Figure 1 Middle U gabc At the mark), multiple positive-sequence disturbance voltage signals are injected simultaneously, multiple negative-sequence disturbance voltage signals are injected simultaneously, and the positive-sequence disturbance voltage signals and negative-sequence disturbance voltage signals are injected separately. That is, Figure 3 As shown, when injecting disturbance voltage, the positive sequence disturbance voltage signal and the negative sequence disturbance voltage signal can be input at the disturbance injection point twice in succession. It should be pointed out that the grid impedance is classified as the inverter side impedance at this time. Exemplarily, multiple positive sequence disturbance voltage signals of different frequencies can be injected first, and then multiple negative sequence disturbance voltage signals of different frequencies can be injected.

[0073] It is worth mentioning that the measurement efficiency can be improved by injecting multiple positive / negative sequence disturbance voltage signals of different frequencies simultaneously.

[0074] In some embodiments of the present application, the above-mentioned multiple different frequencies can be 6 different frequencies. Correspondingly, the positive-sequence disturbance voltage signals of the multiple different frequencies include the positive-sequence disturbance voltage signal V per_p1 , positive sequence disturbance voltage signal V per_p2 , positive sequence disturbance voltage signal V per_p3 , positive sequence disturbance voltage signal V per_p4 , positive sequence disturbance voltage signal V per_p5 and positive sequence disturbance voltage signal V per_p6 ; A plurality of negative-sequence disturbance voltage signals of different frequencies include a negative-sequence disturbance voltage signal V per_n1 , negative sequence disturbance voltage signal V per_n2 , negative sequence disturbance voltage signal V per_n3 , negative sequence disturbance voltage signal V per_n4 , negative sequence disturbance voltage signal V per_n5 And negative sequence disturbance voltage signal V per_n6 .

[0075] Specifically, the positive sequence disturbance voltage signal V per_p1 And negative sequence disturbance voltage signal V per_n1 for:

[0076]

[0077] Positive sequence disturbance voltage signal V per_p2 And negative sequence disturbance voltage signal Vper_n2 for:

[0078]

[0079] Positive sequence disturbance voltage signal V per_p3 And negative sequence disturbance voltage signal V per_n3 for:

[0080]

[0081] Positive sequence disturbance voltage signal V per_p4 And negative sequence disturbance voltage signal V per_n4 for:

[0082]

[0083] Positive sequence disturbance voltage signal V per_p5 And negative sequence disturbance voltage signal V per_n5 for:

[0084]

[0085] Positive sequence disturbance voltage signal V per_p6 And negative sequence disturbance voltage signal V per_n6 for:

[0086]

[0087] Among them, V p1 V per_p1 The amplitude, f p1 V per_p1 frequency, t is time, V n1 V per_n1 The amplitude, f n1 V per_n1 The frequency, f p1 =f n1 , V p2 V per_p2 The amplitude, f p2 V per_p2 Frequency, V n2 V per_n2 The amplitude, f n2 V per_n2 The frequency, f p2 =f n2 , V p3 V per_p3 The amplitude, f p3 V per_p3 Frequency, V n3 V per_n3 The amplitude, f n3 V per_n3 The frequency, fp3 =f n3 , V p4 V per_p4 The amplitude, f p4 V per_p4 Frequency, V n4 V per_n4 The amplitude, f n4 V per_n4 The frequency, f p4 =f n4 , V p5 V per_p5 The amplitude, f p5 V per_p5 Frequency, V n5 V per_n5 The amplitude, f n5 V per_n5 The frequency, f p5 =f n5 , V p6 V per_p6 The amplitude, f p6 V per_p6 Frequency, V n6 V per_n6 The amplitude, f n6 V per_n6 The frequency, f p6 =f n6 .

[0088] Step 22, collecting the voltage signal and current signal of the disturbance injection point when injecting multiple positive sequence disturbance voltage signals of different frequencies, and collecting the voltage signal and current signal of the disturbance injection point when injecting multiple negative sequence disturbance voltage signals of different frequencies.

[0089] Step 23, based on the collected voltage signal and current signal, obtain the positive sequence impedance measurement value and negative sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies.

[0090] In some embodiments of the present application, the above step 23, based on the collected voltage signal and current signal, obtains the positive sequence impedance measurement value and the negative sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies in a specific implementation manner as follows:

[0091] Step 23.1, perform Fourier decomposition on the collected voltage signal and current signal to obtain the amplitude and phase of the three-phase voltage and three-phase current at multiple different frequencies.

[0092] It should be noted that it is necessary to perform Fourier decomposition on the voltage signal and current signal collected when multiple positive-sequence disturbance voltage signals of different frequencies are injected to obtain the amplitudes and phases of the three-phase voltage and three-phase current at multiple different frequencies; at the same time, it is also necessary to perform Fourier decomposition on the voltage signal and current signal collected when multiple negative-sequence disturbance voltage signals of different frequencies are injected to obtain the amplitudes and phases of the three-phase voltage and three-phase current at multiple different frequencies.

[0093] Step 23.2, calculate the amplitude and phase of the three-phase voltage and three-phase current at multiple different frequencies by the symmetrical component method, and obtain the positive-sequence disturbance voltage component and negative-sequence disturbance voltage component of the three-phase voltage at multiple different frequencies, and the positive-sequence disturbance current component and negative-sequence disturbance current component of the three-phase current at multiple different frequencies.

[0094] It should be noted that by calculating the amplitudes and phases of the three-phase voltages and three-phase currents at multiple different frequencies obtained when the positive-sequence disturbance voltage signal is injected through the symmetrical component method, the positive-sequence disturbance voltage components of the three-phase voltages and the positive-sequence disturbance current components of the three-phase currents at multiple different frequencies can be obtained; by calculating the amplitudes and phases of the three-phase voltages and three-phase currents at multiple different frequencies obtained when the negative-sequence disturbance voltage signal is injected through the symmetrical component method, the negative-sequence disturbance voltage components of the three-phase voltages and the negative-sequence disturbance current components of the three-phase currents at multiple different frequencies can be obtained.

[0095] Step 23.3, based on the positive-sequence disturbance voltage component and negative-sequence disturbance voltage component of the three-phase voltage at multiple different frequencies, and the positive-sequence disturbance current component and negative-sequence disturbance current component of the three-phase current at multiple different frequencies, calculate the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies.

[0096] Specifically, the positive sequence impedance measurement value and negative sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies can be calculated by the following formula:

[0097]

[0098]

[0099] In the formula, Z tot_p_mea_x , Z tot_n_mea_x They are the measured values ​​of positive sequence impedance and negative sequence impedance respectively. Here, taking the number of frequencies as 6 as an example, x = 1 to 6, representing the xth frequency; V px is the positive sequence disturbance voltage component of the three-phase voltage at the xth frequency obtained by Fourier decomposition, including amplitude and phase angle information; I px is the positive sequence disturbance voltage component of the three-phase current at the xth frequency obtained by Fourier decomposition, including amplitude and phase angle information; V nxis the negative-sequence disturbance voltage component of the three-phase voltage at the xth frequency obtained by Fourier decomposition, including amplitude and phase angle information; I nx It is the negative sequence disturbance voltage component of the three-phase current at the xth frequency obtained by Fourier decomposition, which contains amplitude and phase angle information. When the positive direction of the ammeter is from the inverter to the grid, and Take a negative sign in front, otherwise take a positive sign.

[0100] It should be noted that in order to measure the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole at all frequency points in the frequency band to be measured (such as 1Hz~10000Hz), multiple different frequencies in step 21 can be adjusted, and based on the adjusted multiple different frequencies, return to execute step 21 (injecting multiple positive-sequence disturbance voltage signals of different frequencies and multiple negative-sequence disturbance voltage signals of different frequencies at the disturbance injection point) until the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole at all frequencies in the frequency band to be measured are obtained.

[0101] That is, the above steps 21 to 23 may be repeated and the frequency of the disturbance injected in step 21 may be changed to perform frequency sweeping until the positive sequence impedance measurement values ​​and the negative sequence impedance measurement values ​​of all frequency points in the frequency band to be measured are measured.

[0102] In some embodiments of the present application, after calculating the positive-sequence impedance measurement value and the negative-sequence impedance measurement value of the inverter grid-connected system as a whole at each frequency, it is necessary to verify the accuracy of the positive-sequence impedance measurement value and the negative-sequence impedance measurement value. Specifically, for each frequency, the positive-sequence impedance measurement value of the inverter grid-connected system as a whole at the frequency and the positive-sequence impedance theoretical value (through the formula The calculated value) is compared to obtain the verification result of the positive sequence impedance measurement value at the frequency, and the negative sequence impedance measurement value of the inverter grid-connected system at the frequency is compared with the negative sequence impedance theoretical value (through the formula The calculated value is compared to obtain the verification result of the negative sequence impedance measurement value at the frequency.

[0103] It should be noted that, regardless of whether it is a positive-sequence impedance measurement value or a negative-sequence impedance measurement value, as long as its deviation from the corresponding theoretical value is within a preset range, the verification result of the impedance measurement value is considered to be: the accuracy meets the requirements; otherwise, the verification result of the impedance measurement value is considered to be: the accuracy does not meet the requirements.

[0104] In practical applications, after obtaining the overall impedance measurement value of the inverter grid-connected system, the measured value and the theoretical value can be plotted on a Bode diagram by the point plotting method to verify the effectiveness of the overall impedance scanning method of the inverter grid-connected system proposed in this application.

[0105] To achieve the above purpose, build the following in MATLAB / Simulink Figure 1 The DFDCS simulation shown in the figure has the following simulation parameters: see Table 1. Add the positive and negative sequence disturbance voltage module on the grid side, and the measurement frequency range is 1Hz to 10000Hz. The simulation results of the overall positive and negative sequence impedance of the inverter grid-connected system are shown in Figure 4a and Figure 4b shown. Figure 4a and Figure 4b The black solid line in the middle is the theoretical value of the positive and negative sequence impedance, and the black hollow circles are the measured values ​​of the positive and negative sequence impedance. It can be seen from the figure that the theoretical value and the measured value can perfectly correspond to each other, which verifies the effectiveness of the overall impedance sweep result proposed in this application.

[0106]

[0107]

[0108] Table 1

[0109] It is worth mentioning that the impedance scanning method provided in the application embodiment treats the inverter and the grid impedance as a whole, injects the positive-sequence disturbance voltage signal and the negative-sequence disturbance voltage signal at the disturbance injection point on the grid side twice respectively, and collects the voltage signal and current signal of the disturbance injection point when injecting multiple positive-sequence disturbance voltage signals of different frequencies at the same time, and collects the voltage signal and current signal of the disturbance injection point when injecting multiple negative-sequence disturbance voltage signals of different frequencies at the same time. Finally, based on the collected voltage signal and current signal, the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies can be obtained, thereby realizing the overall impedance scanning of the inverter grid-connected system under unbalanced conditions, laying a theoretical foundation for the stability analysis of the grid-connected system.

[0110] In addition, the impedance sweep method is not limited to a single converter or a single working condition, has high universality, and has the advantages of fast sweep speed and high precision.

[0111] The above is a preferred embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles described in the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A method for sweeping the overall impedance of an inverter grid-connected system under unbalanced working conditions, characterized in that: include: Injecting a plurality of positive-sequence disturbance voltage signals of different frequencies and a plurality of negative-sequence disturbance voltage signals of different frequencies at a disturbance injection point; The disturbance injection point is located at the grid side of the inverter grid-connected system, a plurality of positive-sequence disturbance voltage signals are injected simultaneously, a plurality of negative-sequence disturbance voltage signals are injected simultaneously, and the positive-sequence disturbance voltage signal and the negative-sequence disturbance voltage signal are injected separately; When a plurality of positive-sequence disturbance voltage signals of different frequencies are injected, a voltage signal and a current signal of the disturbance injection point are collected; and when a plurality of negative-sequence disturbance voltage signals of different frequencies are injected, a voltage signal and a current signal of the disturbance injection point are collected; Based on the collected voltage signal and current signal, the positive sequence impedance measurement value and negative sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies are obtained.

2. The impedance sweep method according to claim 1, characterized in that: The method of obtaining the positive sequence impedance measurement value and the negative sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies based on the collected voltage signal and current signal includes: Perform Fourier decomposition on the collected voltage and current signals to obtain the amplitudes and phases of the three-phase voltage and three-phase current at multiple different frequencies; The amplitudes and phases of the three-phase voltages and three-phase currents at multiple different frequencies are calculated by a symmetrical component method to obtain positive-sequence disturbance voltage components and negative-sequence disturbance voltage components of the three-phase voltages at multiple different frequencies, and positive-sequence disturbance current components and negative-sequence disturbance current components of the three-phase currents at multiple different frequencies; Based on the positive-sequence disturbance voltage component and negative-sequence disturbance voltage component of the three-phase voltage at multiple different frequencies, and the positive-sequence disturbance current component and negative-sequence disturbance current component of the three-phase current at multiple different frequencies, the positive-sequence impedance measurement value and negative-sequence impedance measurement value of the inverter grid-connected system as a whole at multiple different frequencies are calculated.

3. The impedance sweep method according to claim 2, characterized in that: The plurality of positive-sequence disturbance voltage signals with different frequencies include a positive-sequence disturbance voltage signal V per_p1 , positive sequence disturbance voltage signal V per_p2 , positive sequence disturbance voltage signal V per_p3 , positive sequence disturbance voltage signal V per_p4 , positive sequence disturbance voltage signal V per_p5 and positive sequence disturbance voltage signal V per_p6 .

4. The impedance sweep method according to claim 3, characterized in that: The plurality of negative-sequence disturbance voltage signals of different frequencies include a negative-sequence disturbance voltage signal V per_n1 , negative sequence disturbance voltage signal V per_n2 , negative sequence disturbance voltage signal V per_n3 , negative sequence disturbance voltage signal V per_n4 , negative sequence disturbance voltage signal V per_n5 And negative sequence disturbance voltage signal V per_n6 .

5. The impedance sweep method according to claim 4, characterized in that: The positive sequence disturbance voltage signal V per_p1 and the negative sequence disturbance voltage signal V per_n1 for: The positive sequence disturbance voltage signal V per_p2 and the negative sequence disturbance voltage signal V per_n2 for: The positive sequence disturbance voltage signal V per_p3 and the negative sequence disturbance voltage signal V per_n3 for: The positive sequence disturbance voltage signal V per_p4 and the negative sequence disturbance voltage signal V per_n4 for: The positive sequence disturbance voltage signal V per_p5 and the negative sequence disturbance voltage signal V per_n5 for: The positive sequence disturbance voltage signal V per_p6 and the negative sequence disturbance voltage signal V per_n6 for: Among them, V p1 V per_p1 The amplitude, f p1 V per_p1 frequency, t is time, V n1 V per_n1 The amplitude, f n1 V per_n1 The frequency, f p1 =f n1 , V p2 V per_p2 The amplitude, f p2 V per_p2 Frequency, V n2 V per_n2 The amplitude, f n2 V per_n2 The frequency, f p2 =f n2 , V p3 V per_p3 The amplitude, f p3 V per_p3 Frequency, V n3 V per_n3 The amplitude, f n3 V per_n3 The frequency, f p3 =f n3 , V p4 V per_p4 The amplitude, f p4 V per_p4 Frequency, V n4 V per_n4 The amplitude, f n4 V per_n4 The frequency, f p4 =f n4 , V p5 V per_p5 The amplitude, f p5 V per_p5 Frequency, V n5 V per_n5 The amplitude, f n5 V per_n5 The frequency, f p5 =f n5 , V p6 V per_p6 The amplitude, f p6 V per_p6 Frequency, V n6 V per_n6 The amplitude, f n6 V per_n6 The frequency, f p6 =f n6 .

6. The impedance sweep method according to claim 1, characterized in that: The impedance sweep method further comprises: For each frequency, the measured positive-sequence impedance value of the inverter grid-connected system as a whole at the frequency is compared with the theoretical value of the positive-sequence impedance to obtain a verification result of the positive-sequence impedance measured value at the frequency, and the measured negative-sequence impedance value of the inverter grid-connected system as a whole at the frequency is compared with the theoretical value of the negative-sequence impedance to obtain a verification result of the negative-sequence impedance measured value at the frequency.

7. The impedance sweep method according to claim 1, characterized in that: The impedance sweep method further comprises: The multiple different frequencies are adjusted, and based on the adjusted multiple different frequencies, the step of injecting the positive-sequence disturbance voltage signals of multiple different frequencies and the negative-sequence disturbance voltage signals of the multiple different frequencies at the disturbance injection point is returned to be executed until the positive-sequence impedance measurement value and the negative-sequence impedance measurement value of the inverter grid-connected system as a whole are measured at all frequencies in the frequency band to be measured.

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