Inverter power supply three-phase short-circuit fault current analytic method considering phase-locked loop and current inner loop transient process

Abstract

The application discloses an inverter power three-phase short-circuit fault current analytic method considering a phase-locked loop and a current inner loop transient process, and belongs to the technical field of photovoltaic grid-connected system fault analysis. The application studies the transient process of SRF-PLL under different damping ratios, analyzes the influence of the transient process of SRF-PLL on the coordinate transformation of PCC point voltage and current, finally considers the influence of the transient process of SRF-PLL and the current inner loop on the fault current, introduces a correction coefficient, and decomposes the phase current component expressions of the inverter port fault current into the phase current components of the inverter port current under the first influence condition and the phase current components of the inverter port current under the second influence condition. Through the application, the transient characteristics of the IIDG can be more accurately analyzed, the short-circuit characteristic analysis of the IIDG is perfected, and a reference basis is provided for the fault characteristic analysis of the IIDG.

Description

TECHNICAL FIELD

[0001] The application relates to an inverter power three-phase short-circuit fault current analysis method considering the transient process of a phase-locked loop and a current inner loop, and belongs to the technical field of fault analysis of a photovoltaic grid-connected system. BACKGROUND

[0002] The "double carbon" target is proposed, and energy transformation is urgently needed, among which an inverter type distributed generator (IIDG) has begun to attract widespread attention. The IIDG is mainly composed of photovoltaic power and direct-drive wind turbines. They are usually connected to the grid in the form of an inverter power supply. Affected by a large number of power electronic switching devices, the system presents very different fault characteristics from traditional distribution networks when a short-circuit fault occurs. Therefore, it is necessary to analyze the short-circuit fault current of the IIDG to provide a reference for the transient mechanism analysis and protection design of the new power system. The phase-locked loop (PLL) is a key control link for the IIDG to be connected to the grid. When a short-circuit fault occurs in the grid, the transient characteristics will directly affect the fault characteristics of the inverter power supply. Therefore, it is necessary to discuss the transient process of the phase-locked loop when analyzing the fault current. At present, the existing research on the analysis of the short-circuit current characteristics of the inverter power supply pays more attention to the influence of the transient process of the double closed-loop control of the inverter on the fault characteristics of the inverter power supply, and ignores the influence of the transient process of the phase-locked loop after the fault.

[0003] Therefore, it is necessary to consider the influence of the transient process of the phase-locked loop and the current inner loop when analyzing the fault current, and to provide a reference basis for the fault characteristic analysis of the IIDG. SUMMARY

[0004] The application provides an inverter power three-phase short-circuit fault current analysis method considering the transient process of a phase-locked loop and a current inner loop, studies the transient process of an SRF-PLL under different damping ratios, analyzes the influence of the transient process of the SRF-PLL on the coordinate transformation of the PCC point voltage and current, and finally considers the influence of the transient process of the SRF-PLL and the current inner loop on the fault current and analyzes the same. Through the application, the transient characteristics of the IIDG can be more accurately analyzed, the short-circuit characteristic analysis of the IIDG is improved, and a reference basis is provided for the fault characteristic analysis of the IIDG.

[0005] The technical scheme of the application is as follows:

[0006] The application discloses a kind of inverter power supply three-phase short-circuit fault current analytical method considering phase-locked loop and current inner loop transient process, comprising the following steps: step one: respectively calculate the output phase angle expression of phase-locked loop in underdamped and overdamped state;Step two: analyze the current inner loop transient process, obtain the current inner loop transient response expression;Step three: considering the influence of phase-locked loop output phase angle after fault occurs, solve the voltage and current d,q components of grid-connected point PCC;Considering the influence of current inner loop transient process after fault occurs, the port control equation of grid-connected inverter based on grid voltage directional vector control is introduced and substituted into the voltage and current d,q components of grid-connected point PCC, to obtain the expression of inverter port voltage d,q axis component;Step four: the expression of inverter port voltage d,q axis component is converted into the expression of each phase voltage component of inverter port voltage;Step five: according to the relationship between each phase voltage component of inverter port voltage and each phase current component of inverter port fault current, the expression of each phase current component of inverter port fault current is obtained;According to different influence conditions, the correction coefficient is introduced, and the expression of each phase current component of inverter port fault current is decomposed into each phase current component of inverter port current under the first influence condition and each phase current component of inverter port current under the second influence condition;Among them, the influence condition includes the first influence condition and the second influence condition, and the first influence condition is affected by the phase-locked loop transient process;The second influence condition is affected by the phase-locked loop and the current inner loop transient process.

[0007] Further, the step four is specifically: according to the expression of inverter port voltage d,q axis component, the three-phase expression of inverter port voltage is obtained by inverse park transformation;According to the expression of each phase of inverter port voltage after inverse park transformation, the expression of each phase component of inverter port voltage affected by the phase-locked loop transient process and the expression of each phase voltage component of inverter port voltage affected by the phase-locked loop and the current inner loop transient process are established.

[0008] Further, taking phase a as an example, according to the expression of phase a of inverter port voltage after inverse park transformation, the expression of phase a component of inverter port voltage after inverse park transformation is established. a And the expression of phase a voltage component of inverter port voltage affected by the phase-locked loop transient process and the expression of phase a voltage component of inverter port voltage affected by the phase-locked loop and the current inner loop transient process are as follows:

[0009] u a = u a.pll + u a.c

[0010] In the formula, u a.pll It is the phase a voltage component of inverter port voltage affected by the phase-locked loop transient process, and u a.cThe a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop and the current inner loop.

[0011] Further, when the phase-locked loop is in the under-damped state, the a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop is the a-phase voltage component u a.pll1 of the inverter port voltage affected by the output phase angle of the phase-locked loop in the under-damped state, and the a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop and the current inner loop is the a-phase voltage component u a.c1 of the inverter port voltage affected by the output phase angle of the phase-locked loop in the under-damped state and the transient process of the current inner loop, which is expressed as

[0012]

[0013]

[0014] wherein k u1 , k u2 , k u3 , and k u4 are amplitude coefficients of u a.pll1 ; γ u1 , γ u2 are phase coefficients of u a.pll1 ; ω0 is the angular velocity of the power frequency; ω s is the angular frequency of the harmonic; γ pll is an intermediate variable; ω n is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop; k cu1 , k cu2 , k cu3 are amplitude coefficients of u a.c1 ; γ cu1 , γ cu2 are phase coefficients of u a.c1 ; and τ i is the decay time constant of the current inner loop.

[0015] Further, when the phase-locked loop is in the over-damped state, the a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop is the a-phase voltage component u a.pll2 of the inverter port voltage affected by the output phase angle of the phase-locked loop in the over-damped state, and the a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop and the current inner loop is the a-phase voltage component u a.c2 of the inverter port voltage affected by the output phase angle of the phase-locked loop in the over-damped state and the transient process of the current inner loop, which is expressed as

[0016]

[0017] where k′ u1 , k′ u2 , k′ u3 , k′ u4 , k′ u5 , k′ u6 are the respective amplitude coefficients of u a.pll2 ; γ′ u1 , γ′ u2 , γ′ u3 , γ′ u4 , γ′ u5 , γ′ u6 are the phase coefficients of u a.pll2 ; λ1, λ2 are intermediate variables; k′ cu1 , k′ cu2 , k′ cu3_1 , k′ cu3_2 are the respective amplitude coefficients of u a.c2 ; γ′ cu1 , γ′ u2 , γ′ u3_1 , γ′ u3_2 are the phase coefficients of u a.c2 ; τ i is the current inner loop damping time constant; ω0 is the power frequency angular velocity; ω n is the natural oscillation frequency of the phase-locked loop; and ξ is the damping ratio of the phase-locked loop.

[0018] Further, in the under-damped state of the phase-locked loop, according to different influence conditions, a correction coefficient is introduced, and the phase current component expressions of the inverter port fault current are decomposed into an a-phase current component i a.pll1 of the inverter port current affected by the output phase angle of the phase-locked loop in the under-damped state, an a-phase current component i a.c1 of the inverter port current affected by the output phase angle of the phase-locked loop in the under-damped state and the transient process of the current inner loop, and the specific expressions are as follows:

[0019]

[0020] where k i0 is an amplitude component of i a.pll1 ; k i1 , k i2 , k i3 , k i4 , k i3_1 , k i4_1 are amplitude components of i a.pll1 related to amplitude correction coefficients; γ i0 is a phase component of i a.pll1 ; γ i1 , γ i2 , γ i3 , γi4 γ i3_1 γ i4_1 For i a.pll1 The phase component with respect to the phase correction coefficient; ω0 is the power frequency angular velocity; ω n ω is the natural oscillation frequency of the phase-locked loop; s ξ is the harmonic angular frequency; ξ is the damping ratio of the phase-locked loop; k ci1 ,k ci2 ,k ci3_1 For i a.c1 amplitude components; γ ci1 ,γ ci2 ,γ ci3_1 For i a.c1 Phase component; τ i is the time constant of the inner current loop decay.

[0021] Furthermore, under the overdamped state of the phase-locked loop (PLL), a correction coefficient is introduced based on different influencing conditions. This decomposes the expression for each phase current component of the inverter port fault current into the a-phase current component i of the inverter port current affected by the PLL output phase angle under overdamped conditions. a.pll2 The a-phase current component i of the inverter port current is affected by the combined influence of the phase angle of the phase-locked loop output and the transient process of the inner current loop under overdamped conditions. a.c2 Specifically:

[0022]

[0023]

[0024] In the formula, k′ i0 For i a.pll2 amplitude components; k′ i1 、k′ i2 、k′ i3 、k′ i4 、k′ i5 、k′ i6 For i a.pll2 The amplitude component with respect to the amplitude correction factor; γ′ i0 For i a.pll2 Phase component; γ′ i1 ,γ′ i2 ,γ′ i3 ,γ′ i4 ,γ′ i5 ,γ′ i6 For i a.pll2 The phase component with respect to the phase correction coefficient; ω0 is the power frequency angular velocity; ω n k′ is the natural oscillation frequency of the phase-locked loop. ci1 、k′ ci2 、k′ ci3_1 、k′ci3_2 For i a.c2 amplitude components; γ′ ci1 ,γ′ ci2 ,γ′ ci3_1 ,γ′ ci3_2 For i a.c2 Phase component; τ i λ1 and λ2 are intermediate variables; λ1 and λ2 are the time constant of the inner current loop decay.

[0025] The beneficial effects of this invention are as follows: Based on the classic synchronous rotating coordinate system phase-locked loop (PLL), this invention reveals the influence mechanism of phase-locked loop deviation on fault current under three-phase short-circuit faults through analysis, and analyzes the harmonic characteristics of fault current, providing a reference basis for the analysis of the influence of SRF-PLL on fault current characteristics; by deriving the expression of each phase current component of the inverter port fault current considering the phase deviation of the PLL at the beginning of the fault, there will be non-power frequency oscillation attenuation components, thus it can more clearly characterize the transient characteristics of fault current than the traditional fault current analytical expression that ignores phase-locked loop deviation. Attached Figure Description

[0026] Figure 1 This is a flowchart of the present invention;

[0027] Figure 2 The equivalent topology and control block diagram of the grid-connected inverter power supply;

[0028] Figure 3 This is a schematic diagram of the inverter port output fault current when the SRF-PLL is underdamped.

[0029] Figure 4 This is a schematic diagram of the inverter port output fault current when the SRF-PLL is in an overdamped state. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be arbitrarily combined with each other.

[0031] When a three-phase short-circuit fault occurs in the inverter power supply, the amplitude and phase angle of the grid voltage will change abruptly, affecting the dynamic performance of the phase-locked loop (PLL) and further impacting the fault characteristics of the inverter power supply under the transient time scale. Therefore, the transient process of the PLL must be discussed when performing fault current analysis.

[0032] The embodiments of the present invention will now be described with reference to the accompanying drawings.

[0033] Example 1: As Figures 1-4 As shown, according to a first aspect of the present invention, a method for analyzing the three-phase short-circuit fault current of an inverter power supply, taking into account the transient processes of the phase-locked loop and the inner current loop, is provided, comprising the following steps:

[0034] Step 1: When the SRF-PLL is in underdamped state, the expressions for each part of the inverter port fault current are as follows:

[0035] After a fault occurs, the phase angle calculation formula for the phase-locked loop output in the underdamped state of the SRF-PLL is as follows:

[0036]

[0037] Where: Δθ pll1 (t) represents the phase angle output by the phase-locked loop in the underdamped state after the fault occurs; ω n ω is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop; s The harmonic angular frequencies are A, B, and γ. pll All are intermediate variables; Δθ is the voltage phase jump variable at the grid connection point; e is an exponential constant.

[0038] After a fault occurs, the phase angle calculation formula for the SRF-PLL output phase loop in the overdamped state is as follows:

[0039]

[0040] Where: Δθ pll2 (t) represents the phase angle output by the phase-locked loop in the overdamped state after the fault occurs; ω n ξ is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop; λ1, λ2, C1, and C2 are all intermediate variables.

[0041] Step 2: Analyze the transient process of the inner current loop to obtain the expression for the transient response of the inner current loop;

[0042] The transient responses of the inner current loop along the d and q axes are considered as independent first-order elements. In this case, the transient response i of the inner current loop... i The expression for (t) is:

[0043]

[0044] in: The current command value of the inner current loop before the fault (i.e., including the d-axis and q-axis current command values ​​of the inner current loop before the fault). ), This refers to the current command value of the inner current loop after the fault (i.e., including the d-axis and q-axis current command values ​​of the inner current loop after the fault occurs). );Δi * The difference between the current command value before and after entering low-voltage charging (i.e., including) );τ i is the time constant of the inner current loop decay.

[0045] After a fault occurs, the system will enter a low-voltage ride-through phase. During this low-voltage ride-through, the command values ​​for the d-axis and q-axis currents in the inner current loop are as follows:

[0046]

[0047] in, These are the d-axis and q-axis current command values ​​for the inner current loop after the fault occurs; U T The per-unit value of the grid connection point voltage; i max This is the per-unit value of the maximum flowable current of the inverter; The current command value for the d-axis before the fault occurred is given by the voltage outer loop during normal system operation. Due to the first-order inertial element settling time τ... i.s(Δ＝0.05s) =3τ i Therefore, the transient process of the inner current loop will last from 1.5ms to 15ms, and this process cannot be simply ignored.

[0048] Step 3: Considering the phase angle effect of the phase-locked loop output after the fault occurs (i.e., considering the transient process effect of the phase-locked loop), the voltage and current d,q component expressions of the grid connection point PCC are as follows:

[0049]

[0050] Where: e d e q These are the d-axis and q-axis components of the voltage at the grid connection point PCC, respectively; e a e b e c These are the three-phase components of the PCC voltage at the grid connection point; E m The per-unit value of the PCC voltage at the grid connection point; i d i q These are the d-axis and q-axis components of the PCC current at the grid connection point, respectively; I a I b I c These are the three-phase components of the PCC current at the grid connection point; The angle between the d and q-axis components of the current command value; i m T represents the amplitude of the inverter's output current after the fault occurs. 3s / 2r Here is the Park transformation matrix; Δθ pllThe phase angle of the phase-locked loop output (if it is the phase angle output in the underdamped state of the phase-locked loop after a fault, it is represented by Δθ). pll1 (t); if it is the phase angle output by the phase-locked loop under overdamped state after a fault occurs, then it represents Δθ. pll2 (t)); Δθ p ′ ll This represents the dynamic deviation of the phase-locked loop.

[0051] Furthermore, considering the impact of the transient process of the inner current loop after a fault occurs, the port control equation of the grid-connected inverter based on grid voltage-oriented vector control is introduced, i.e., the port voltage expression is as follows:

[0052]

[0053] In the formula, The port voltage is represented by its d- and q-axis components; R and L represent the filter's equivalent resistance and inductance, respectively; K ip K ii These represent the proportional and integral control parameters of the inner current loop, respectively; ω0 represents the power frequency angular velocity. These are the d and q axis command values ​​for the inner current loop (after a fault occurs, the d and q axis command values ​​for the inner current loop are adopted). ).

[0054] will i d i q With e d e q Substituting these expressions into the port voltage expression above, we obtain the expression for the inverter port voltage in the d,q coordinate system as follows:

[0055]

[0056] Where: u d u q Inverter port voltage d, q-axis component; M 11 ~M 13 M 21 ~M 23 N1 and N2 are constant terms; These are the differences between the d-axis current command value and the q-axis current command value before and after entering the low-voltage circuit.

[0057] Step 4: Based on the d and q-axis components of the inverter port voltage, obtain the three-phase expression u of the inverter port voltage through inverse Parker transformation. abcBased on the phase expressions of the inverter port voltage after the inverse Parker transformation, we establish the phase components of the inverter port voltage after the inverse Parker transformation, the phase components of the inverter port voltage affected by the phase-locked loop transient process, and the phase components of the inverter port voltage affected by the combined transient processes of the phase-locked loop and the current inner loop, as the first equation.

[0058] Taking phase a as an example, the expression for phase a of the inverter port voltage in the inverse Parker transformation is u. a for:

[0059]

[0060] Based on the expression for the a-phase voltage of the inverter port voltage after the inverse Parker transformation, the expressions for the a-phase component of the inverter port voltage after the inverse Parker transformation, the a-phase voltage component of the inverter port voltage affected by the phase-locked loop transient process, and the a-phase voltage component of the inverter port voltage affected by the combined transient processes of the phase-locked loop and the inner current loop are established as the first equation for the a-phase, as follows: u a =u a.pll +u a.c Wherein: M 11 M 12 M 13 M 21 M 22 M 23 N1 and N2 are constant terms; θ0 represents the phase angle before the fault occurs; u a.pll The phase a voltage component of the inverter port voltage affected by the transient process of the phase-locked loop (i.e., the phase a voltage component of the inverter port voltage affected by the output phase angle of the phase-locked loop under underdamped conditions) is represented as u. a.pll1 The phase a component of the inverter port voltage affected by the phase angle of the phase-locked loop output under overdamped conditions is expressed as u. a.pll2 ), u a.c The a-phase voltage component of the inverter port voltage, which is affected by the transient processes of the phase-locked loop and the inner current loop (i.e., the a-phase voltage component of the inverter port voltage affected by the phase-locked loop output phase angle and the transient processes of the inner current loop under underdamped conditions, is expressed as u). a.c1 The phase a voltage component of the inverter port voltage, which is affected by the combined effects of the phase-locked loop output phase angle and the transient process of the inner current loop under overdamped conditions, is expressed as u. a.c2 .

[0061] The a-phase voltage component u of the inverter port voltage affected by the phase angle of the phase-locked loop output under underdamped conditions. a.pll1 The expression is:

[0062]

[0063] Where: ku1 ,k u2 ,k u3 ,k u4 For u a.pll1 Amplitude coefficient; γ u1 γ u2 ω is the phase coefficient; ω0 is the power frequency angular velocity, its value is 2πf0, and f0 is the system rated frequency; A, B, γ pll All are intermediate variables; ω n ξ is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop; M 11 M 12 M 13 M 21 M 22 M 23 N1 and N2 are constant terms.

[0064] The a-phase voltage component of the inverter port voltage, which is affected by both the output phase angle of the phase-locked loop and the transient process of the inner current loop under underdamped conditions, is expressed as u. a.c1 The expression is:

[0065]

[0066]

[0067] Where: k cu1 ,k cu2 ,k cu3 For u a.c1 Amplitude coefficient; γ cu1 γ cu2 τ is the phase coefficient; ω0 is the power grid frequency angular velocity, its value is 2πf0, f0 is the system rated frequency; i Let ω be the time constant of the inner current loop decay. n ξ is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop; N1 and N2 are constant terms; A and B are intermediate variables.

[0068] The phase a component of the inverter port voltage affected by the phase angle of the phase-locked loop output under overdamped conditions is represented as u. a.pll2 The expression is:

[0069]

[0070] Where: k u ′1、k u ′2、k u ′3、k u ′4、k u ′5、k u ′6 is u a.pll2 The various amplitude coefficients, γ u′1、γ u ′2、γ u ′3、γ u ′4、γ u ′5、γ u ′6 is the phase coefficient; ω0 is the power grid frequency angular velocity, its value is 2πf0, f0 is the system rated frequency; λ1, λ2, A, B are intermediate variables; M 11 M 12 M 13 M 21 M 22 M 23 N1 and N2 are constant terms; E m This is the per-unit value of the PCC voltage at the grid connection point.

[0071] The phase a voltage component u of the inverter port voltage, which is affected by the combined effects of the phase angle of the phase-locked loop output and the transient process of the inner current loop under overdamped conditions. a.c2 The expression is:

[0072]

[0073] Where: k c ′ u1 k c ′ u2 k c ′ u3_1 k c ′ u3_2 For u a.c2 The various amplitude coefficients; γ c ′ u1 γ u ′2、γ u ′ 3_1 γ u ′ 3_2 λ1 and λ2 are phase coefficients; ω0 is the power grid frequency angular velocity, with a value of 2πf0, where f0 is the system rated frequency; λ1 and λ2 are intermediate variables; A and B are intermediate variables; N1 and N2 are constant terms.

[0074] Step 5: Based on the relationship between the phase voltage components of the inverter port voltage and the phase current components of the inverter port fault current, obtain the expression for the phase current components of the inverter port fault current; substitute the first equation for each phase into the expression for the phase current components of the inverter port fault current; according to different influencing conditions, decompose it into the phase current components of the inverter port current under the first influencing condition and the phase current components of the inverter port current under the second influencing condition; wherein, the influencing conditions include the first influencing condition and the second influencing condition. The first influencing condition is affected by the transient process of the phase-locked loop; the second influencing condition is affected by the transient processes of the phase-locked loop and the current inner loop.

[0075] For example, taking phase a as an example, the relationship between the phase a voltage component of the inverter port voltage and the phase a current component of the port current is as follows:

[0076]

[0077] In the formula, e a This is the voltage of phase a at the grid connection point PCC.

[0078] Based on the above formula, the expression for the phase a current component of the inverter port fault current is obtained as follows:

[0079]

[0080] In the formula, C represents an arbitrary constant;

[0081] When the SRF-PLL is in an underdamped state, the expression for the a-phase voltage component of the inverter port voltage in the underdamped state is u. a =u a1 =u a.pll1 +u a.c1 Substitute R and L Depending on the different influencing conditions, it can be decomposed into the a-phase current component of the inverter port current affected by the output phase angle of the phase-locked loop under underdamped conditions, and the a-phase current component of the inverter port current affected by the output phase angle of the phase-locked loop and the transient process of the inner current loop under underdamped conditions.

[0082] When the SRF-PLL is in underdamped mode, the expressions for each part of the inverter port fault current are as follows:

[0083] The expression for the a-phase current component of the inverter port current affected by the phase angle of the phase-locked loop output under underdamped conditions is as follows:

[0084]

[0085]

[0086] In the formula, i a.pll1 Let k be the a-phase current component of the inverter port current affected by the phase angle of the phase-locked loop output under underdamped conditions. u.fix k is the amplitude correction factor. i0 k i1 k i2 k i3 k i4 k i3_1 k i4_1 All are amplitude components; γ u.fix γ is the phase correction coefficient. i0 γ i1 γ i2 γ i3γ i4 γ i3_1 γ i4_1 R is the phase component; L is the equivalent resistance of the filter; L is the equivalent inductance of the filter; ω0 is the power grid frequency angular velocity, which is 2πf0, where f0 is the system rated frequency; ω n ω is the natural oscillation frequency of the phase-locked loop; s ξ is the harmonic angular frequency; ξ is the damping ratio of the phase-locked loop.

[0087] The expression for the a-phase current component of the inverter port current, which is affected by both the output phase angle of the phase-locked loop under underdamped conditions and the transient process of the inner current loop, is as follows:

[0088]

[0089]

[0090] In the formula, i a.c1 The a-phase current component of the inverter port current is affected by the combined influence of the phase-locked loop output phase angle and the transient process of the inner current loop under underdamped conditions; k ci1 ,k ci2 ,k ci3_1 All are amplitude components; γ ci1 ,γ ci2 ,γ ci3_1 R is the phase component; L is the equivalent resistance of the filter; L is the equivalent inductance of the filter; ω0 is the power grid frequency angular velocity, which is 2πf0, where f0 is the system rated frequency; τ i Let ω be the time constant of the inner current loop decay. n ξ is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop.

[0091] When the SRF-PLL is in an overdamped state, the expressions for each part of the inverter port fault current are as follows:

[0092] The a-phase current component i of the inverter port current affected by the phase angle of the phase-locked loop output under overdamped conditions. a.pll2 The expression is:

[0093]

[0094]

[0095] In the formula, k i ′0、k i ′1、k i ′2、k i ′3、k i ′4、k i ′5、k i ′6 are all amplitude components, γ i ′0~γi ′6 are all phase components; k u.fix γ is the amplitude correction factor; u.fix R is the phase correction coefficient; L is the equivalent resistance of the filter; L is the equivalent inductance of the filter; ω0 is the power grid frequency angular velocity, which is 2πf0, where f0 is the system rated frequency; τ i Let ω be the time constant of the inner current loop decay. n ξ is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop.

[0096] The a-phase current component i of the inverter port current is affected by the combined influence of the phase angle of the phase-locked loop output and the transient process of the inner current loop under overdamped conditions. a.c2 The expression is:

[0097]

[0098]

[0099] In the formula, k′ ci1 、k′ ci2 、k′ ci3_1 、k′ ci3_2 All are amplitude components; γ′ ci1 ,γ′ ci2 ,γ′ ci3_1 ,γ′ ci3_2 All are phase components; R is the equivalent resistance of the filter; L is the equivalent inductance of the filter; ω0 is the power grid frequency angular velocity, its value is 2πf0, f0 is the system rated frequency; τ i λ1, λ2, C1, and C2 are the time constants of the inner current loop decay; N1 and N2 are constant terms; λ1, λ2, C1, and C2 are all intermediate variables.

[0100] According to a second aspect of the present invention, a three-phase short-circuit fault current analysis system for an inverter power supply that takes into account the transient processes of the phase-locked loop and the inner current loop is provided, comprising modules of any of the methods described above.

[0101] According to a third aspect of the present invention, a processor is provided, the processor being configured to perform operations including performing the inverter power supply three-phase short-circuit fault current analysis method that takes into account the transient processes of the phase-locked loop and the inner current loop as described in any one of the preceding embodiments.

[0102] Example 2: This invention relates to a method for analyzing the three-phase short-circuit fault current of an inverter power supply, taking into account the transient processes of the phase-locked loop and the inner current loop. The method is performed according to the following steps: Taking the equivalent topology of a grid-connected inverter power supply as an example, its equivalent control block diagram is as follows... Figure 2 As shown in Table 1, assuming a three-phase short-circuit fault occurs in the power grid system, the parameters of the photovoltaic grid-connected system are as follows:

[0103] Table 1 Specific parameters of photovoltaic grid-connected system

[0104] Parameter Value Parameter Value Rated capacity / (MVA) 0.345 Transformer T1 transformation ratio 0.26 / 25 Current inner loop proportional control parameter k ip ]]> 0.83 Filter equivalent inductance / mH 1 Current inner loop integral control parameter k ii ]] 0.083 Filter equivalent resistance / mΩ 0.1 Phase-locked loop proportional control parameter k ppll ]]> 1098 SRF-PLL natural oscillation angular frequency ω n / rad / s 120 Phase-locked loop integral control parameter k ipll ]]> 43902

[0105] In such Figure 2 A three-phase short-circuit fault occurs at point f. After the fault, the voltage amplitude of the PCC at the grid connection point drops from 0.328kV to 0.103kV, and the phase angle jumps backward by approximately 10.4°. At this time, according to the grid connection technical regulations, the command values ​​of the d-axis and q-axis currents in the inner current loop after the low-voltage ride-through are...

[0106] 1. Analysis of the transient process of a phase-locked loop

[0107] When a three-phase short circuit occurs in a photovoltaic grid-connected system, analyze the transient process of the SRF-PLL, assuming the input phase-locked loop grid connection point PCC voltage e abc (i.e. e) a e b e c The phase angle before the fault was θ0, and the per-unit value of the PCC voltage at the grid connection point was E. m Its expression for the d and q axis components can be obtained as e. dq =T 2r / 3s e abc =[E m cos(θ0-θ pll (t))E m sin(θ0-θ pll (t))] T ; where Δθ pll (t)=θ0-θ pll (t), Δθ pll (t) represents Δθ pll1 (t) / Δθ pll2 (t), θ pll (t) represents θ pll1 (t) / θ pll2 (t).

[0108] When the deviation of the phase-locked loop output is small, the following approximation can be made according to the Taylor expansion formula: sin(θ0-θ) pll )≈(θ0-θ pll ), at this time e q Substitute into the open-loop transfer function of the phase-locked loop In this process, the transfer function θ is obtained. pll The expression for (s) is:

[0109]

[0110] Where the definition These are the natural oscillation frequency and damping ratio of the SRF-PLL, respectively; k ipllWith k ppll These are the integral and proportional control parameters of the phase-locked loop, respectively; when the SRF-PLL is in an underdamped state, i.e., ξ < 1, the phase transition signal Δθ is controlled according to the specific parameters in Table 1. ε The response of (t) is:

[0111] Δθ pll1 (t) = -0.1815 + 0.3345e -56.7t cos(36.6246t+0.9973) (2)

[0112] When the SRF-PLL is in an overdamped state, i.e., ξ > 1, for the phase-jumping signal Δθ ε The response of (t) is:

[0113] Δθ pll2 (t) = -0.1815 + 0.2549e -125.7744t -0.0734e -36.2256t (3)

[0114] 2. Find the transient analytical expression for the inner current loop.

[0115] The inner current loop is designed according to the classic Type I system, and its PI element proportional and integral parameters are usually set as follows:

[0116]

[0117] Where, τ i Let τ be the decay time constant of the inner current loop. To ensure that the current control speed is fast enough and the bandwidth of the closed-loop system is not too large, τ i The timeframe is typically chosen within the range of 0.5 to 5 ms. τ can be calculated based on the data in Table 1. i =1.2ms.

[0118] When a three-phase fault occurs at the grid connection point PCC, the voltage drops to 0.314 pu. The transient responses of the inner current loop on the d and q axes are considered as independent first-order elements. The expression for the transient response of the inner current loop at this time is:

[0119]

[0120] According to the regulations for photovoltaic grid connection technology, the command values ​​for the d-axis and q-axis currents in the inner current loop during low voltage ride-through are as follows:

[0121]

[0122] 3. Considering the phase angle effect of the phase-locked loop output after the fault occurs (i.e., considering the effect of the phase-locked loop transient process), solve for the expression of the inverter port voltage in the d and q axes.

[0123] (1) Solve for the expressions of the grid-side voltage d and q axes.

[0124] When a three-phase fault occurs on the AC side of the photovoltaic system, the per-unit value of the PCC voltage at the grid connection point E m A momentary voltage drop occurs. Let the actual phase angle of the voltage before the fault be θ0, and the actual phase angle of the voltage after the phase jump at the fault be θ1. Δθ = θ1 - θ0, therefore, the expression for the PCC voltage at the grid connection point after the fault is:

[0125]

[0126] After the fault, the voltage at point PCC is e abc After the Park transformation, we can obtain e. d e q The expression is:

[0127]

[0128] (2) Solve for the expressions of the grid-side currents d and q on the axes.

[0129] After the fault, the current command value according to the low voltage ride-through requirement is:

[0130]

[0131] in: This indicates the amplitude of the inverter's output current after a fault. According to photovoltaic grid-connected technology regulations, during the low-voltage ride-through period after a fault, the output fault current of the grid-connected inverter cannot exceed 1.2 times the rated current, i.e., i m The value is 1.2. The grid-connected point PCC current i considering the influence of phase-locked loop dynamic deviation abc After the Parker transformation, we can obtain i d i q The expression is:

[0132]

[0133] Substituting the expressions for the grid-connected point PCC voltage and current in the d and q axes, equations (9) and (11), along with the transient process equation (5) of the current inner loop, into the inverter port control equations, we obtain the expression u for the inverter port voltage in the d and q axes. dq :

[0134]

[0135] Δθ pll The phase angle output by the phase-locked loop is Δθ. p ′ ll This represents the dynamic deviation of the phase-locked loop (PLL). The expressions for both depend on the damping state of the PLL.

[0136] 4. Based on the voltage expressions of the inverter ports along the d and q axes, solve for the three-phase voltage expressions of the inverter ports. The following formula takes phase a as an example.

[0137]

[0138] According to the data in Table 1, when the SRF-PLL is in an underdamped state, ξ = 0.84 is taken, and the specific expression is as follows:

[0139]

[0140] Where: ω n ξ = 57.6; ω0 = 2πf0 is the power frequency angular frequency, and f0 is the rated frequency.

[0141] According to the data in Table 1, when the SRF-PLL is in an overdamped state, ξ = 1.2 is taken, and the specific expression is as follows:

[0142]

[0143] Among them: λ1=-125.7744; λ2=-36.2256; (λ1+λ2)=-162; ω n ξ = 81; ω0 = 2πf0 is the power frequency angular frequency, and f0 is the rated frequency.

[0144] The SRF-PLL requires 20-100ms to track voltage phase changes, while the transient process time of the inner current loop, as analyzed above, is known to be 1.5-15ms. This transient process is shorter than that of the PLL, and can be considered to have largely decayed in the latter half of the SRF-PLL transient process, thus having a relatively small impact on the fault steady-state current. For considering the SRF-PLL transient process, u... a.pll steady-state value u a 'for:

[0145]

[0146] In the formula, u d1 with u q1 u d with u q The steady-state value is clearly shown in equation (18), and it is evident that the voltage calculation result deviates from the actual voltage value. The above analysis shows that even if the transient process of the inner current loop has a relatively small impact on the fault steady-state current, there is still a deviation because this transient process is not considered. Therefore, it is necessary to correct this deviation using the following amplitude correction coefficient and phase correction coefficient:

[0147]

[0148] The corrected u a.pll The magnitudes of all cosine terms are changed to the original k. u.fix It is twice as much as the original γ in phase angle. u.fix For ease of description, we will continue to use "u" below. a.pll Represents the corrected u a.pll .

[0149] 5. Solve the analytical expression for the three-phase fault current at the inverter port.

[0150] Based on the physical equation of the inverter port voltage, the analytical expression for the fault current can be obtained as follows:

[0151]

[0152] When the SRF-PLL is in an underdamped state, substituting equations (9), (14), and (15) into equation (20) yields the following expression, the overall formula of which is i a1 =i a.pll1 +i a.c1 As attached Figure 3 As shown.

[0153]

[0154] When the SRF-PLL is in an overdamped state, substituting equations (9), (16), and (17) into equation (20) yields the following expression, the overall formula of which is i a2 =i a.pll2 +i a.c2 As attached Figure 4 As shown.

[0155]

[0156] The principle of this invention is as follows:

[0157] Current research analyzing the short-circuit current characteristics of inverter power supplies focuses primarily on the impact of the inverter's dual-loop control transient process on the inverter's fault characteristics, neglecting the influence of the phase-locked loop (PLL) transient process after a fault. After a fault occurs, the amplitude and phase angle of the grid voltage undergo abrupt changes, affecting the dynamic performance of the PLL and further impacting the inverter's fault characteristics on the transient timescale. Therefore, the PLL transient process must be discussed in fault current analysis. This invention considers the influence of both the PLL and the inner current loop transient processes when analyzing fault current. Based on an equivalent inverter model, using the SRF-PLL as an example, it studies the SRF-PLL transient process under different damping ratios, analyzes the impact of the SRF-PLL transient process on the coordinate transformation of the grid-connected point PCC voltage and current, and finally considers and analyzes the influence of the SRF-PLL and the inner current loop transient processes on the fault current. This invention can more accurately analyze the transient characteristics of inverter-type distributed generation (IIDG), improve the short-circuit characteristic analysis of IIDG, and provide a basis for IIDG fault characteristic analysis.

[0158] 1. The phase angles of the SRF-PLL output under underdamped and overdamped states are specifically as follows:

[0159] Inverter-type power supplies are connected to the AC grid via inverters and filters, with a unified equivalent model as follows: Figure 2 As shown, assuming the total output of the inverter power supply remains unchanged after a fault, the DC side of the inverter can be considered a constant power source. When a fault occurs, the outer voltage loop is disconnected, while the command value of the inner current loop is directly calculated based on the voltage drop depth according to grid connection specifications; that is, the inner current loop directly tracks the command value. Therefore, the fault characteristics of the inverter power supply are significantly affected by the inner current loop. Figure 2 The influence of the outer voltage loop is ignored, and the outer voltage loop part is omitted. Figure 2 middleu abc The three-phase voltage at the inverter port is e, and the subsequent calculation of the three-phase current is also the output current at the port. abc i abc These are the three-phase voltage and current on the grid side, θ pll This is the phase angle output by the phase-locked loop.

[0160] When a three-phase short circuit occurs in a photovoltaic grid-connected system, analyze the transient process of the SRF-PLL, assuming the input phase-locked loop grid connection point PCC voltage e abc (i.e. e) a e b e c The phase angle before the fault was θ0, and the per-unit value of the PCC voltage at the grid connection point was E. m Its expression for the d and q axis components can be obtained as e. dq =T 2r / 3s eabc =[E m cos(θ0-θ pll (t)) E m sin(θ0-θ pll (t))] T ; where Δθ pll (t)=θ0-θ pll (t), Δθ pll (t) represents Δθ pll1 (t) / Δθ pll2 (t), θ pll (t) represents θ pll1 (t) / θ pll2 (t).

[0161] When the deviation of the phase-locked loop output is small, the following approximation can be made according to the Taylor expansion formula: sin(θ0-θ) pll )≈(θ0-θ pll ), at this time e q Substitute into the open-loop transfer function of the phase-locked loop In this process, the transfer function θ is obtained. pll The expression for (s) is:

[0162]

[0163] Where the definition These are the natural oscillation frequency and damping ratio of the SRF-PLL, respectively; k ipll With k ppll These are the integral and proportional control parameters of the phase-locked loop, respectively.

[0164] When the SRF-PLL is in an underdamped state, i.e., ξ < 1, for the phase-jumping signal Δθ ε The response of (t) is:

[0165]

[0166] Among them: A, B, γ pll All are intermediate variables.

[0167] When the SRF-PLL is in an overdamped state, i.e., ξ > 1, for the phase-jumping signal Δθ ε The response of (t) is:

[0168]

[0169] Among them, λ1, λ2, C1, and C2 are all intermediate variables.

[0170] 2. The aforementioned transient process of the inner current loop is specifically as follows:

[0171] The inner current loop is often designed according to a typical Type I system, and its P-element proportional and integral parameters are usually set as follows: Where, τ i Let τ be the inner loop decay time constant. To ensure sufficiently fast current control speed and prevent excessive bandwidth in the closed-loop system, the design must consider specific application scenarios and inverter switching frequencies. i The range of 0.5–5 ms is typically chosen. When the influence of the SRF-PLL transient process on the inner current loop is not considered, the d-axis and q-axis transient responses of the inner current loop can be treated as independent first-order inertial elements. In this case, the expression for the transient response of the inner current loop is:

[0172]

[0173] in, and These are the current command values ​​for the inner current loop before and after the fault, respectively. These represent the differences between the d-axis current command value and the q-axis current command value before and after low-voltage ride-through, respectively. e is an exponential constant. The d-axis and q-axis current command values ​​during low-voltage ride-through are respectively:

[0174] Due to the settling time τ of the first-order inertial element i.s(Δ＝0.05s) =3τ i Therefore, the transient process of the inner current loop will last from 1.5ms to 15ms, and this process cannot be simply ignored.

[0175] 3. Considering the phase angle effect of the phase-locked loop output after the fault occurs (i.e., considering the effect of the phase-locked loop transient process), solve for the voltage and current components of the grid-connected point PCC in the d and q axes. Finally, solve for the inverter port voltage expressions in the d and q axes, specifically:

[0176] When a symmetrical fault occurs on the photovoltaic AC side, the per-unit value Em of the PCC voltage at the grid connection point experiences a momentary drop. Let the actual phase angle of the voltage before the fault be θ0, and the actual phase angle of the voltage after the phase jump at the fault be θ1. Then Δθ = θ1 - θ0, meaning the expression for the PCC voltage at the grid connection point after the fault is:

[0177]

[0178] After the fault, the voltage at point PCC is e abc After the Park transformation, we can obtain e. d e q The expression is:

[0179]

[0180] The current command value for low-voltage ride-through after a fault can be expressed as follows:

[0181]

[0182] in, This indicates the amplitude of the inverter's output current after a fault. According to photovoltaic grid-connected technology regulations, during the low-voltage ride-through period after a fault, the output fault current of the grid-connected inverter cannot exceed 1.2 times the rated current, i.e., i m The value is 1.2. The grid-connected point PCC current i considering the influence of phase-locked loop dynamic deviation abc After the Parker transformation, we can obtain i d i q The expression is:

[0183]

[0184] In the underdamped state It is a time-varying phase-locked error, Δθ at the moment of fault occurrence. p ′ ll =0, when the phase-locked loop finally tracks the fault transition angle, then Δθ p ′ ll It will equal 0 again. At this point, i dq With e dq Substituting this into the port control equations of the grid-connected inverter based on grid voltage-oriented vector control:

[0185]

[0186] The expression for the inverter port voltage on the d and q axes can be obtained as follows:

[0187]

[0188] When the SRF-PLL is in overdamped state, the inverter port voltage expression u dq Solution process and voltage expression u under underdamped state dq The solution process is the same; the difference lies in the phase angle Δθ of the phase-locked loop output. pll Dynamic deviation of phase-locked loop Δθ at the time of failure p ′ ll =0, when the phase-locked loop finally tracks the fault transition angle, then Δθ p ′ ll It will decay to 0.

[0189] 4. Convert the d- and q-axis components of the inverter ports into three-phase voltage expressions, specifically:

[0190] If the SRF-PLL is in an underdamped state, taking the voltage of phase a as an example, u a The specific expression is:

[0191] via u dq The inverse Parker transformation yields the phase voltages at the inverter ports. Taking phase a as an example, the phase a voltage after considering the inverse Parker transformation error is:

[0192] u a =u d cos(θ0+Δθ pll )+u q sin(θ0+Δθ pll )

[0193] ≈u d (cosθ0-sinθ0Δθ pll )+u q (sinθ0+cosθ0Δθ pll )

[0194] Expanding the above formula, we can further obtain:

[0195]

[0196] As shown in the above equation, the voltage of phase a at the port of the photovoltaic grid-connected inverter contains two components, one of which contains only Δθ. pll , Δθ p ′ ll The time constant τ is not included in the decay time constant. i The attenuation term indicates that this part of the voltage component is only affected by the transient process of the SRF-PLL, while another part of the component also contains an attenuation time constant τ. i The attenuation term and Δθ pll This indicates that this voltage component is affected by the transient effects of the SRF-PLL and the inner current loop, denoted as u, respectively. a.pll with u a.c If the SRF-PLL is in an underdamped state, the specific expression is:

[0197]

[0198] If the SRF-PLL is in an overdamped state, taking the phase a voltage as an example, the specific expression is:

[0199]

[0200] The SRF-PLL requires 20-100ms to track voltage phase changes, while the transient process time of the inner current loop, as analyzed above, is known to be 1.5-15ms. This transient process is shorter than that of the PLL, and can be considered to have largely decayed in the latter half of the SRF-PLL transient process, thus having a relatively small impact on the fault steady-state current. For considering the SRF-PLL transient process, u... a.pll steady-state value u′ afor:

[0201]

[0202] In the formula, u d1 with u q1 u d with u q The steady-state value clearly shows that the result of the voltage calculation formula deviates from the actual voltage value. Through the above analysis, it can be seen that even if the transient process of the inner current loop has a relatively small impact on the fault steady-state current, a deviation still exists because this transient process is not considered. Therefore, corrections are needed using the following amplitude and phase correction coefficients:

[0203]

[0204] The corrected u a.pll The magnitudes of all cosine terms are changed to the original k. u.fix It is twice as much as the original γ in phase angle. u.fix For ease of description, we will continue to use "u" below. a.pll Represents the corrected u a.pll .

[0205] 5. Solve the analytical expression for the three-phase fault current at the inverter port.

[0206] When the SRF-PLL is in an underdamped state, the inverter port output fault current is analyzed as follows:

[0207] The physical equations for the port voltages of a three-phase inverter power supply, taking phase a as an example:

[0208]

[0209] From the above formula, we can further derive the expression for the fault current of the inverter power supply as follows:

[0210]

[0211] Because the DC attenuation term has a small amplitude and a long attenuation time, this part is ignored in the fault current analysis of this transient process. The u under the underdamped state is then considered... a, e a Substituting the fault current expression, we can obtain the expression for the A-phase short-circuit current i output by the SRF-PLL in underdamped state. a1 for:

[0212] i a1 =i a.pll1 +i a.c1

[0213]

[0214] When the SRF-PLL is in an overdamped state, the fault current calculation method is basically the same as that in the underdamped state:

[0215] i a2 =i a.pll2 +i a.c2

[0216]

[0217] Through the fault current expression i a1 =i a.pll1 +i a.c1 i a2 =i a.pll2 +i a.c2 Analysis reveals that the fault current consists of four parts. The first part is the steady-state fault current after the transient process of the SRF-PLL and the inner current loop ends, i.e., the power frequency quantity. The existence of the power frequency quantity is not affected by the SRF-PLL and the inner current loop. The second part is the oscillating decay current affected by the SRF-PLL, whose time decay constant is consistent with that of the SRF-PLL. Its influence on the fault current continues until the transient process of the SRF-PLL ends. The third part is also the oscillating decay current affected by the SRF-PLL, whose time decay constant is half that of the SRF-PLL. The fourth part is the decay component containing the decay time constant of the inner current loop. Its decay time is much smaller than that of the SRF-PLL, and it decays faster. It only has an effect in the early stage of the fault.

[0218] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for analyzing three-phase short-circuit fault current of an inverter power supply considering transient process of phase-locked loop and current inner loop, characterized in that, The method comprises the following steps: Step one: calculate the output phase angle expression of the phase-locked loop in under-damped and over-damped states respectively; Step two: analyze the transient process of the current inner loop to obtain the transient response expression of the current inner loop; Step three: consider the influence of the phase-locked loop output phase angle after the fault occurs, and solve the voltage and current d, q components of the grid-connected point PCC; Considering the influence of the transient process of the current inner loop after the fault occurs, the port control equation of the grid-connected inverter based on the grid voltage directional vector control is introduced and substituted into the voltage and current d, q components of the grid-connected point PCC to obtain the expression of the d, q axis components of the inverter port voltage; Step four: convert the expression of the d, q axis components of the inverter port voltage into the expression of the phase voltage components of the inverter port voltage; Step five: according to the relationship between the phase voltage components of the inverter port voltage and the phase current components of the inverter port fault current, the expression of the phase current components of the inverter port fault current is obtained; according to different influence conditions, a correction coefficient is introduced to decompose the expression of the phase current components of the inverter port fault current into the phase current components of the inverter port current under the first influence condition and the phase current components of the inverter port current under the second influence condition; wherein the influence conditions include the first influence condition and the second influence condition, the first influence condition is affected by the transient process of the phase-locked loop, and the second influence condition is affected by the transient processes of the phase-locked loop and the current inner loop.

2. The method of claim 1, wherein, The step four is specifically: According to the d, q axis component expression of the inverter port voltage, the three-phase expression of the inverter port voltage is obtained through inverse Park transformation; according to the phase expression of the inverter port voltage after inverse Park transformation, the expression of the phase components of the inverter port voltage after inverse Park transformation, the phase voltage components of the inverter port voltage affected by the transient process of the phase-locked loop, and the phase voltage components of the inverter port voltage affected by the transient processes of the phase-locked loop and the current inner loop is established.

3. The method of claim 2, wherein, Taking the a phase as an example, according to the a phase expression of the inverter port voltage of the inverse Park transformation, the a phase component u of the inverter port voltage after the inverse Park transformation is established a The a phase voltage component of the inverter port voltage affected by the phase-locked loop transient process and the a phase voltage component of the inverter port voltage affected by the phase-locked loop and the current inner loop transient process are expressed as follows: u a = u a.pll + u a.c wherein u a.pll is the a-phase voltage component of the inverter port voltage affected by the phase-locked loop transient process, u a.c is the a-phase voltage component of the inverter port voltage affected by the phase-locked loop transient process, u The a-phase voltage component of the inverter port voltage affected by the transient processes of the phase-locked loop and the current inner loop.

4. The method of claim 3, wherein, In the under-damped state of the phase-locked loop, the a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop is the a-phase voltage component of the inverter port voltage affected by the output phase angle of the phase-locked loop in the under-damped state, denoted as u a.pll1 The a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop and the current inner loop is the a-phase voltage component of the inverter port voltage affected by the output phase angle of the phase-locked loop in the under-damped state and the transient process of the current inner loop, denoted as u a.c1 The specific expression is as follows: wherein k u1 ,k u2 ,k u3 ,k u4 is the amplitude coefficient of u a.pll1 ; γ u1 , γ u2 is the phase coefficient of u a.pll1 ; ω0is the angular velocity of the power frequency; ω s is the angular frequency of the harmonic; γ pll is an intermediate variable; ω n is the natural oscillation frequency of the phase-locked loop; ξ is the damping ratio of the phase-locked loop; k cu1 ,k cu2 ,k cu3 is the amplitude coefficient of u a.c1 ; γ cu1 , γ cu2 is the phase coefficient of u a.c1 ; τ i is the current inner loop decay time constant.

5. The method of claim 3, wherein, In the over-damped state of the phase-locked loop, the a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop is the a-phase voltage component of the inverter port voltage affected by the output phase angle of the phase-locked loop in the over-damped state, denoted as u a.pll2 The a-phase voltage component of the inverter port voltage affected by the transient process of the phase-locked loop and the current inner loop is the a-phase voltage component of the inverter port voltage affected by the output phase angle of the phase-locked loop in the over-damped state and the transient process of the current inner loop, denoted as u a.c2 The specific expression is as follows: where k u ′1, k u ′2, k u ′3, k u ′4, k u ′5, k u ′6 are the amplitude coefficients of u a.pll2 ; γ u ′1, γ u ′2, γ u ′3, γ u ′4, γ u ′5, γ u ′6 are the phase coefficients of u a.pll2 ; λ1, λ2 are intermediate variables; k c ′ u1 , k c ′ u2 , k c ′ u3_1 , k c ′ u3_2 are the amplitude coefficients of u a.c2 ; γ c ′ u1 , γ cu ′2, γ cu ′ 3_1 , γ cu ′ 3_2 are the phase coefficients of u a.c2 ; τ i is the current inner loop time constant; ω0is the power frequency angular velocity; ω n is the phase-locked loop natural oscillation frequency; and ξ is the phase-locked loop damping ratio.

6. The method of claim 1, wherein, In the under-damped state of the phase-locked loop, according to the different influence conditions, a correction coefficient is introduced, and the phase current component expression of the inverter port fault current is decomposed into the a-phase current component i a.pll1 of the inverter port current affected by the output phase angle of the phase-locked loop in the under-damped state a.c1 , and the a-phase current component i a.c1 of the inverter port current affected by the output phase angle of the phase-locked loop in the under-damped state and the transient process of the current inner loop, specifically: wherein k i0 is the amplitude component of i a.pll1 ; k i1 , k i2 , k i3 , k i4 , k i3_1 , k i4_1 is the amplitude component of i a.pll1 with respect to the amplitude correction coefficient; γ i0 is the phase component of i a.pll1 ; γ i1 , γ i2 , γ i3 , γ i4 , γ i3_1 , γ i4_1 is the phase component of i a.pll1 with respect to the phase correction coefficient; ω0is the power frequency angular velocity; ω n is the natural oscillation frequency of the phase-locked loop; ω s is the harmonic angular frequency; ξ is the damping ratio of the phase-locked loop; k ci1 , k ci2 , k ci3_1 is the amplitude component of i a.c1 ; γ ci1 , γ ci2 , γ ci3_1 is the phase component of i a.c1 ; τ i is the current inner loop decay time constant.

7. The method of claim 1, wherein, When the phase-locked loop (PLL) is in an overdamped state, a correction coefficient is introduced based on different influencing conditions. This decomposes the expression for each phase current component of the inverter port fault current into the a-phase current component i of the inverter port current affected by the PLL output phase angle under overdamped conditions. a.pll2 The a-phase current component i of the inverter port current is affected by the combined influence of the phase angle of the phase-locked loop output and the transient process of the inner current loop under overdamped conditions. a.c2 Specifically: In the formula, k i ′0 is i a.pll2 amplitude components; k i ′1、k i ′2、k i ′3、k i ′4、k i ′5、k i ′6 for i a.pll2 The amplitude component with respect to the amplitude correction factor; γ i ′0 is i a.pll2 Phase component; γ i ′1、γ i ′2、γ i ′3、γ i ′4、γ i ′5、γ i ′6 for i a.pll2 The phase component with respect to the phase correction coefficient; ω0 is the power frequency angular velocity; ω n k is the natural oscillation frequency of the phase-locked loop. c ′ i1 k c ′ i2 k c ′ i3_1 k c ′ i3_2 For i a.c2 amplitude components; γ c ′ i1 γ c ′ i2 γ c ′ i3_1 γ c ′ i3_2 For i a.c2 Phase component; τ i λ1 and λ2 are intermediate variables; λ1 and λ2 are the time constant of the inner current loop decay.

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