Low voltage ride through control method for assembling virtual synchronous generator in virtual power plant

Abstract

The invention provides a low voltage ride through control method for configuring a virtual synchronous generator for a virtual power plant. Firstly, a transient model of a traditional low-voltage ride-through control method for assembling a virtual synchronous generator in a virtual power plant is constructed; through analysis of an output current dynamic curve, a transient synchronous instability mechanism caused by insufficient reactive current support in a traditional low-voltage ride-through control method is disclosed. Secondly, determining a feasible region of a reactive current injection coefficient according to an existence criterion of a transient stability balance point, and taking the feasible region as a quantitative index of a transient stability margin; on the basis, a proportional-integral adaptive stability control method based on the fault voltage is provided, and a calculation method of an adaptive integral gain parameter Ki is designed. According to the method, the operation stability and the low-voltage ride-through capability of the virtual power plant assembled with the virtual synchronous generator in a power grid fault scene can be remarkably improved, and a key technical support is provided for safe grid connection and efficient operation of the virtual power plant in a novel power system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power systems, in particular to a low voltage ride through control method of a virtual power plant equipped with a virtual synchronous generator. BACKGROUND

[0002] With the rapid development of new energy power generation technology, the penetration rate of distributed energy continues to rise. As the core carrier of integrating scattered distributed energy and realizing efficient use of energy and collaborative scheduling of power grid, virtual power plant (VPP) has become a key component of new power system construction. To solve the problem of "low inertia and weak damping" caused by the grid-connected of traditional inverter interface distributed energy, the virtual synchronous generator (VSG) technology is widely used in the grid-connected control of virtual power plant because it can simulate the inertia support, damping adjustment and frequency and voltage regulation characteristics of synchronous generators, effectively improving the dynamic response consistency of virtual power plant and power grid.

[0003] However, low voltage drop events caused by power grid faults (such as short circuit, lightning strike, etc.) pose a serious challenge to virtual power plants equipped with virtual synchronous generators - the low voltage ride through (LVRT) capability has become a mandatory technical requirement for the grid-connected operation of virtual power plants. The existing virtual synchronous generator-low voltage ride through control method has the following core defects: first, there is little research on the reactive current injection coefficient (K coefficient) in low voltage ride through. Some scholars have found through simulation research that the size of K coefficient has a great influence on the transient synchronous stability of grid-connected inverters, and the feasible region of K coefficient can be quantified in the presence of transient stability point; in addition, some scholars point out that if the K coefficient does not match the grid parameters during the fault, it will lead to transient synchronous instability; second, the existing method pays little attention to the effect of K coefficient under dynamic current, which has a key influence on the low voltage ride through control method. According to the low voltage ride through guide, the port current of grid-connected inverter changes dynamically with the PCC voltage, and the voltage is accurately controlled by the phase-locked loop. Due to the nonlinear coupling between the low voltage ride through control loop and the phase-locked loop, the output current will show time-varying characteristics in the transient process. The existing technology has the following shortcomings:

[0004] 1) The research depth of K coefficient of reactive current injection for LVRT control is insufficient. In the existing LVRT control method, K coefficient is usually set as a fixed value, which cannot be dynamically adjusted according to fault voltage, system state or operating condition, resulting in the inability to fully utilize the voltage support capability of the system and the difficulty in coping with the complex demands of different fault scenarios; 2) The research on time-varying characteristics of dynamic current is insufficient. Most researches only focus on the influence mechanism of LVRT method on the static characteristics of the system, or conduct stability evaluation under constant current conditions, without fully considering the interaction of LVRT method with the time-varying characteristics of output current. SUMMARY

[0005] The technical problem solved by the present application is to provide a low-voltage ride-through control method for a virtual power plant equipped with a virtual synchronous generator, which has the advantages of significantly improving the operation stability and low-voltage ride-through capability of the virtual power plant equipped with the virtual synchronous generator in a power grid fault scenario, so as to overcome the shortcomings of the prior art.

[0006] To solve the above technical problems, the present application adopts the following technical scheme: a low-voltage ride-through control method for a virtual power plant equipped with a virtual synchronous generator, characterized in that it comprises the following steps: S1) construction of a low-voltage ride-through control model for a virtual power plant equipped with a virtual synchronous generator; S2) dynamic current characteristic analysis of a traditional low-voltage ride-through control method; S3) derivation of a feasible region of a reactive current injection coefficient K; S4) establishment of a P I coefficient adaptive stability control method based on fault voltage; S5) adaptive integral gain K i parameter design; the virtual power plant equipped with a virtual synchronous generator system comprises a phase-locked synchronization module, a voltage outer loop controller, a low-voltage ride-through controller, and a current closed loop; through a phase-locked synchronization mechanism, the three-phase voltage and current signals of a point of common coupling (PCC) are converted to a synchronous rotating dq coordinate axis through Park transformation; in a steady state condition, the voltage outer loop controller generates a current reference value of the dq axis according to a grid voltage deviation, and the inner loop realizes a preset power output of the system through current vector control, assuming that the DC side voltage is a constant value, i q_ref and i d_ref are determined by the voltage outer loop during the steady state, when a power grid fault occurs, the voltage outer loop control is converted into a low-voltage ride-through control to preferentially provide a reactive current, i q_ref1 and i d_ref1 are determined by the low-voltage ride-through control loop during the fault, the reactive current reference value i q_ref1 is calculated by formula (1), and the active current reference value i d_ref1 is determined by the remaining current, which is formula (2), and the phase angle output by the phase-locked loop is formula (3).

[0007] (1)

[0008] (2)

[0009] (3)

[0010] In the formula, K is a reactive current injection coefficient, U pcc is a per-unit value, I N is a rated current; is an angular frequency output by a phase-locked loop, represents the rotational speed of the d-axis relative to U g , and U q is a q-axis component of a PCC voltage. ​

[0011] The stable operation of the virtual power plant equipped with the virtual synchronous generator system relies on the phase-locked loop control, and the output voltage is consistent with the grid voltage phase through phase tracking. g The angle between the d-axis and U

[0012] (4)

[0013] In the formula, is the grid phase, and the frequency is usually a power frequency constant ω v , and δ is the difference between the phase-locked loop output phase and the grid phase.

[0014] The vector relationship between the point of common coupling voltage and the grid voltage is:

[0015] (5)

[0016] In the formula, U pcc is the point of common coupling voltage, I pcc is the point of common coupling output current, R is the equivalent grid resistance, X is the equivalent grid reactance, U z is the equivalent grid impedance voltage drop, and U g is the grid voltage.

[0017] The phase-locked loop realizes frequency synchronization through d-axis tracking of the Upcc phase, and when the port voltage Upcc phase is affected by voltage and current disturbances, the phase deviation thereof from the d-axis phase causes angular frequency adjustment, when the Upcc phase lags behind the d-axis phase, the d-axis rotation angular frequency will be reduced to the phase-locked state, and when the Upcc phase leads the d-axis, i.e., Uq>0, the d-axis rotation angular frequency will be increased to the phase alignment.

[0018] The above technical scheme has the following beneficial effects: on the basis of the traditional low-voltage ride-through method, the proportional-integral coefficient adaptive controller is introduced to compensate for the reactive current, the transient stability margin is widened, and the transient instability problem caused by insufficient reactive current in the traditional method is effectively improved. The present application can significantly improve the operation stability and low-voltage ride-through capability of the virtual power plant equipped with the virtual synchronous generator in the grid fault scene, and provides key technical support for the safe grid connection and efficient operation of the virtual power plant in the new power system.

[0019] The application is a low voltage ride through (LVRT) control method for a virtual synchronous generator (VSG) configured in a virtual power plant. First, a transient model of a conventional LVRT control method for a VSG configured in a virtual power plant is constructed. Through analysis of the output current dynamic curve, a transient synchronous instability mechanism caused by insufficient reactive current support of the conventional LVRT control method is revealed. Second, according to the existence criterion of the transient stability equilibrium point, the feasible region of the reactive current injection coefficient is determined, and is used as a quantitative index of the transient stability margin. On this basis, a proportional-integral adaptive stability control method based on fault voltage is proposed, and a calculation method of the adaptive integral gain parameter Ki is designed. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed in the following embodiment or prior art description will be briefly introduced. Obviously, the drawings in the following description are only exemplary, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of the provided drawings.

[0021] The structures, proportions, sizes, etc. shown in the specification are only used to cooperate with the content disclosed in the specification, to be understood and read by those skilled in the art, and do not define the limiting conditions for the implementation of the present application, so they do not have technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effect and purpose that can be achieved by the present application, should still fall within the scope of the technical content disclosed by the present application.

[0022] Figure 1 System control block diagram for a virtual power plant equipped with a virtual synchronous generator;

[0023] Figure 2 Conventional low voltage ride through control block diagram for a virtual power plant equipped with a virtual synchronous generator;

[0024] Figure 3 (a) iq variation curve with K and δ, (b) id variation curve with K and δ, and dq axis output current variation curve with K and δ;

[0025] Figure 4 Output current limit region under the low voltage ride through method;

[0026] Figure 5 Reactive current matching curve;

[0027] Figure 6 PI coefficient adaptive stability low voltage ride through control method;

[0028] Figure 7The system voltage response waveform under different control methods when the grid voltage is shallowly dropped;

[0029] Figure 8 The system voltage response waveform under different control methods when the grid voltage is deeply dropped;

[0030] Figure 9 The workflow diagram of the present application. DETAILED DESCRIPTION

[0031] The specific embodiments of the present application will be further described below in conjunction with the accompanying drawings. It should be noted that the description of these embodiments is used to help understand the present application, but does not constitute a limitation on the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0032] Referring to Figures 1-9 The present application proposes a low voltage ride through control method for a virtual power plant equipped with a virtual synchronous generator. First, a transient model of the conventional low voltage ride through control method for a virtual power plant equipped with a virtual synchronous generator is established. By analyzing the dynamic curve of the output current, the transient synchronous instability mechanism caused by insufficient reactive current support of the conventional low voltage ride through control method is revealed. Second, according to the existence criterion of the transient stability equilibrium point, the feasible region of the reactive current injection coefficient is determined, and it is used as a quantitative indicator of the transient stability margin. Based on the above analysis, a proportional-integral adaptive stability control method based on fault voltage is proposed, and the adaptive integral gain K i The parameter design method is as follows:

[0033] Step 1: Construction of the low voltage ride through control model of the virtual power plant equipped with the virtual synchronous generator.

[0034] The virtual power plant equipped with the virtual synchronous generator system is composed of a phase-locked synchronization module, a voltage outer loop controller, a low voltage ride through controller, and a current closed loop. The system control block diagram is shown in Figure 1 The three-phase voltage and current signals at the point of common coupling are converted to the synchronous rotating dq coordinate axis through Park transformation by the phase-locked synchronization mechanism. Under steady state conditions, the voltage outer loop controller generates the current reference value of the dq axis according to the grid voltage deviation, and the inner loop realizes the system preset power output through current vector control. Assuming that the DC side voltage is a constant value, i q_ref and i d_ref are determined by the voltage outer loop. When the grid fails, the voltage outer loop control is converted to the low voltage ride through control to preferentially provide reactive current. During the fault period, i q_ref1 and i d_ref1 are determined by the low voltage ride through control loop, and the reactive current reference value i q_ref1The active current reference value i can be calculated using equation (1). d_ref1 It is determined by the residual current, as shown in equation (2). Figure 1 The output phase angle of the intermediate phase-locked loop The expression for is shown in equation (3).

[0035] (1)

[0036] (2)

[0037] (3)

[0038] In the formula, K is the reactive current injection coefficient, and U pcc I is the per-unit value. N Rated current; This is the output angular frequency of the phase-locked loop. Indicates the d-axis relative to U g rotational speed, U q This represents the q-axis component of the PCC voltage.

[0039] The stable operation of a virtual power plant equipped with a virtual synchronous generator system relies on phase-locked loop control, the core of which lies in accurately tracking the grid U. g Phase, achieved by phase tracking to ensure the output voltage rotates in phase with the grid voltage. Define the d-axis and U... g The included angle is the virtual work angle δ, as shown in equation (4).

[0040] (4)

[0041] In the formula, For an infinitely large power grid, its frequency is typically the power frequency constant ω. v δ is the difference between the phase-locked loop output phase and the grid phase.

[0042] Depend on Figure 1 As shown, according to Kirchhoff's laws, the vector relationship between the voltage at the point of common coupling and the grid voltage is given by equation (5).

[0043] (5)

[0044] In the formula, U pcc I is the voltage at the point of common coupling. pcc The output current at the point of common coupling, R is the equivalent grid resistance, X is the equivalent grid reactance, and U... z For the equivalent grid impedance voltage drop, U g This is the grid voltage.

[0045] Phase-locked loop tracks U via d-axis pcc Phase synchronization achieves frequency synchronization. When U gWhen a voltage dip occurs, the response speed of the inner current loop is significantly faster than that of the phase-locked loop (PLL) during the dynamic process. At this point, i can be considered... q i d Reference value i can be tracked in real time d_ref i q_ref Port voltage U pcc When the phase is affected by voltage and current disturbances, its phase deviation from the d-axis causes angular frequency adjustment. When U pcc When the phase lags behind the d-axis phase, U q <0, as can be seen from equation (3), the rotational angular frequency of the d-axis will decrease until the d-axis and U are perpendicular. pcc Overlap, to achieve U q =0, completing the phase-locked loop process. Similarly, when U pcc Phase leads the d-axis, i.e., U q When the value is greater than 0, the angular frequency of the d-axis rotation increases until the phase is aligned.

[0046] The core of the phase-locked loop synchronization mechanism lies in maintaining U q =0. During the transient process, the virtual synchronous generator system exhibits controlled current source characteristics. According to equation (3), U at this time... q Influenced by multiple factors, including output current, grid voltage drop, and grid equivalent impedance, the phase-locked loop (PLL) synchronization control problem can be transformed into U... q Dynamic adjustment is required. This necessitates achieving U under the influence of output current, grid voltage drop, and grid impedance coupling. q =0 stable control.

[0047] Step 2: Dynamic current characteristics analysis of traditional low voltage ride-through control methods.

[0048] From equation (5), it can be seen that under the condition of a power grid fault, when the voltage fault voltage and the equivalent impedance of the power grid are constant, I pcc Determines U q =0, therefore, we need to focus on the impact of dynamic current on stability.

[0049] According to equation (3), the output frequency and phase of the phase-locked loop are determined by the q-axis component U of the voltage at the point of common connection in the dq coordinate system. q Drive adjustment, in the dq coordinate system, U q and U d They can be represented as shown in equation (6).

[0050] (6)

[0051] In the formula, U gf This refers to the grid fault voltage.

[0052] Figure 2The traditional low-voltage ride-through control block diagram for a virtual power plant equipped with a virtual synchronous generator is given. From equation (6), it can be seen that the voltage at the point of common coupling U under grid fault conditions is... pcc The voltage expression is shown in equation (7).

[0053] (7)

[0054] In the formula, It is the impedance angle. .

[0055] During power grid faults, the dynamic process of the phase-locked loop is affected by i q Impact, i q The reference value is determined by both the low-voltage ride-through method and the phase-locked loop response. Equations (1), (2), and (7) are combined to establish the dynamic current equation for the transient process i. q i d Quantitative analysis was performed. The dynamic output current exhibits implicit functional characteristics, related to the K coefficient and the virtual power angle δ, i q and i d The mathematical expression for i is shown in equation (8). During the dynamic process of the phase-locked loop... q Let δ and K be time-varying functions.

[0056] (8)

[0057] Figure 3 shows the output current i q i d The dynamic curves show the variation of K coefficient and virtual power angle δ. During a fault, the system preferentially injects reactive current to maintain voltage support capability. Figure 3(a) shows i q The curves showing the variation of K and δ are shown in the figure. q The minimum value is -1.2pu. When the virtual power angle δ changes, the larger K coefficient accelerates i. q The amplitude limit is reached. As the K coefficient increases, i... q The absolute value increases accordingly; when the K coefficient is small, i q The absolute value shows a trend of first increasing and then decreasing. Figure 3(b) shows i d The dynamic curves of K and δ, i d It is determined by the residual current of the virtual synchronous generator system. During the virtual power angle change, i d Numerical adjustments are synchronized. The larger the K coefficient, the faster i approaches zero. It is worth noting that when the K coefficient is small, i... d It exhibits a dynamic characteristic of first decreasing and then increasing.

[0058] Step 3: Derivation of the feasible region of reactive current injection coefficient K.

[0059] When the q-axis voltage component at PCC is zero, the phase-locked loop achieves phase synchronization tracking of the power grid. At this time, the system reaches a transient stable state and satisfies the condition of equation (11).

[0060] (11)

[0061] In the formula, U gf This is the voltage of the power grid fault. This refers to the virtual power angle during the transient equilibrium of the grid-connected system.

[0062] Voltage amplitude at PCC and It can be represented as shown in equations (12) and (13).

[0063] (12)

[0064] (13)

[0065] According to the phase-locked loop synchronization principle, when the phase-locked loop is synchronized stably, U q =0. When -1 ≤ sinδ e When ω ≤ 1 and transient synchronization is stable pll =ω v At this point, the system satisfies the conditions for synchronous stability, as shown in equation (14).

[0066] (14)

[0067] From equation (14), it can be seen that when U gf When the voltage drops too much, the grid voltage is less than U1. At this time, U in equation (11)... q The constant non-zero value causes a type of synchronization instability in the phase-locked loop, where no transient equilibrium point exists. When U... gf When U ≥ U1, U q It could be zero, in which case there exists a transient stable equilibrium point δ. e Satisfying U q =0 condition. Therefore, it can be seen that the fault voltage depth of the power grid directly affects the synchronization capability of the phase-locked loop.

[0068] A virtual synchronous generator system is equivalent to a controlled current source, and the output current flowing through the line impedance directly determines the magnitude of U1. In transmission networks, the grid reactance is typically much larger than the resistance, and under high reactance, U1 is prone to occur. gf In the case of <U1, a type I synchronous instability of the phase-locked loop occurs. Furthermore, the system output current characteristics during the fault are determined by the low-voltage ride-through control equations (1) and (2). The fault output current is affected by both the PCC voltage drop and the K coefficient of the low-voltage ride-through control loop. When the system reaches transient equilibrium, each control element transitions from transient to steady-state operation, and the dynamic current i d i qTracking the low-voltage ride-through reference current to reach the transient steady-state current command value i d_ref1 with i q_ref1 Further quantitative analysis of i d and i q Based on the transient stability condition of the phase-locked loop and the restricted domain, the following assumptions are made: Assume U gf The falling speed is faster than U pcc Change, see U gf The values ​​are constant, and the grid impedances R and X are fixed. Combining equations (1), (2), and (14), we finally obtain i. d i q Restricted domain, specific results as follows Figure 4 As shown. Figure 4 The output current limiting range under low voltage ride-through control and the limiting control setting i are demonstrated. d with|i q |The range is from 0 to i max i max Take 1.2. Under the low voltage ride-through control limit range, the arc represents the output current limit trajectory, where the red dashed line represents the fault voltage and impedance constraints. The deeper the voltage drop, the smaller the dashed area. The greater the grid impedance, the smaller the area also becomes.

[0069] To quantify the impact of fault voltage and grid impedance on the reactive current limiting range under transient equilibrium conditions, three cases are discussed based on different voltage dips and impedance magnitudes:

[0070] Case 1, when In a power transmission network, X is much larger than R. At this time, i q Any value of i satisfies equation (11), q The range of values ​​is

[0071] (15)

[0072] Scenario 2, when i q Take -i max The expression (11) still holds true. When the maximum value of iq is set to 0, i needs to be retuned q The boundary value. By solving the quadratic equation using the boundary conditions of equation (11), we can obtain i. q The range is

[0073] (16)

[0074] Scenario 3, when The effective region satisfying equation (14) is further reduced. The extreme values ​​of the q-axis projection must satisfy the boundary conditions of equation (11), at which point i q The range is

[0075] (17)

[0076] The i in the transient stable case of the system is obtained through equations (15)-(17). q Complete constraint domain. Under low voltage ride-through control, the voltage across the equivalent power supply output terminal and the equivalent grid load characteristic is matched through the output current. When the power supply output current characteristic curve i... q If the current limiting characteristic curve at the load end does not match, the two characteristic curves will not intersect, the system will not have a steady-state operating point, and the low voltage ride-through will fail. Figure 5 This shows the matching between the reactive current output characteristic curve at the power supply end (red curve) and the reactive current limiting characteristic curve at the load end (current limiting domain in three cases). Based on the reactive current matching curve, the actual feasible domain of the K coefficient can be quantified.

[0077] According to the reactive current matching principle, i q_ref1 The required value of i is equal to the grid connection stability requirement. q Therefore, the feasible region of the K coefficient is derived. Taking case 3 as an example, point A represents the maximum reactive current demand i. qmax This point corresponds to the PCC voltage U. pcc_iqmax The slope between points AC is defined as K. max This represents the upper limit of the feasible region. Similarly, the minimum reactive current requirement i is analyzed. qmin The voltage U corresponding to point B pcc_iqmin The slope of point BC corresponds to K. min , representing the lower bound of the feasible region. The feasible regions of the K coefficients for the above three cases are respectively expressed as:

[0078] (18)

[0079] (19)

[0080] Step 4: An adaptive stability control method based on fault voltage PI coefficient.

[0081] Based on the traditional low-voltage ride-through method with a fixed K-coefficient, reactive current compensation is applied according to the PCC voltage drop. An integral term is added to the controller, and the integral coefficient is adaptively adjusted to control the dynamic compensation amount of reactive current. When a grid fault causes U... pcc <U ref When this occurs, a dynamic current compensation adjustment mechanism is activated, and the adaptive integral controller outputs a dynamic compensation current in real time. This method effectively improves transient synchronization stability by enhancing reactive current support capability.

[0082] Let U ref =0.9, when the voltage drop is less than Uref At the threshold, steady-state voltage outer loop control is enabled to provide a current reference value for the system. When the PCC voltage drops beyond U... ref When the range is within a certain range, a PI coefficient adaptive stability control method needs to be adopted. At this time, the mathematical expressions for the reference values ​​of reactive and active current are shown in equation (20), and the detailed structure is as follows: Figure 6 As shown. During the fault period K p Coefficient is lower than K min At this time, insufficient reactive current injection may occur. In this case, the integral controller generates a compensation component based on the fault voltage. The compensation increases the reactive current while suppressing the active current, ultimately achieving accurate matching between the current output and the fault condition.

[0083] (20)

[0084] In the formula, K i This represents the gain parameter of the integral controller. K p As a proportional parameter, it is consistent with the definition of the aforementioned K coefficient.

[0085] Step 5: Adaptive Integral Gain K i Parameter design method.

[0086] Integral gain K i The design goal is to ensure U q The condition = 0 is met. When the traditional low-voltage ride-through method is insufficient in reactive current support under severe faults, the control method can activate the compensation mechanism if the integral gain meets the set conditions. The compensation mechanism expands the feasible region of Kp by dynamically adjusting the reactive current, ultimately making Kp equal to the reactive current. p Returning to the effective operating range enables stable system control.

[0087] In actual control systems, the rate of change of the PCC voltage is faster than the dynamic process of the controller in the improved method. To simplify the analysis, assume U... pcc Falling to the minimum value U pccmin The ultimate clearing time parameter was obtained by simulating the instability state. During the fault, when there was no dynamic reactive current injection, the measured ultimate clearing time was 0.05 seconds. Based on this operating condition, K... i Value range analysis, K i The parameter separation expression is shown below:

[0088] (twenty one)

[0089] Considering the dynamic process of the PLL, U at this time pcc The expression is

[0090] (twenty two)

[0091] Considering the most severe voltage drop conditions in the power grid, let And the current reaches the critical condition of limiting, at which point U pccmin The mathematical expression is as follows

[0092] (twenty three)

[0093] Combining equations (21) and (23), K... i The conservative maximum estimate of the parameters is as follows:

[0094] (twenty four)

[0095] In the formula, K imax This represents the maximum integral coefficient under the most severe three-phase short-circuit condition of the power grid. Under this condition, U... gf The voltage drops sharply to zero, at which point according to K imax The parameters allow the integral controller to provide sufficient reactive current to support the stability of the virtual synchronous generator system.

[0096] The output of the integral controller serves as the compensation value for dynamic reactive current, and K needs to be discussed in detail. imax The influence of parameters on transient stability. According to the dynamic current analysis in Figure 3, when K... p ≤K min At the initial stage of the fault i q The absolute value shows an increasing trend, but as the work angle shifts, i q The absolute value begins to decrease, and the integral controller outputs a reactive current compensation value based on the fault voltage drop, continuously accumulating the compensation component. Therefore, during the fault period, K... imax The value of needs to meet the requirements under the most severe voltage drop condition, so that the system can provide sufficient compensating reactive current to suppress the increasing trend of iq. However, under the steady-state condition of the system, U pcc with U ref There is a stable error between them, so K is used. imax This will accumulate an excessive reverse reactive current compensation component during the steady state period, and there will be an overcompensation problem during the fault period.

[0097] To address the above issues, the integral gain is dynamically corrected as follows: Figure 6 As shown, K is corrected in real time based on the drop in fault voltage during the fault period. i The integral gain correction expression is as follows:

[0098] (25)

[0099] In the formula, U pcc0 This represents the steady-state per-unit voltage value at PCC.

[0100] From the above equation, it can be seen that during steady state, U pcc =U pcc0 At this time, K i =0, the integral controller is locked, and only the proportional controller operates. During a fault, U pcc <U pcc0 At this time, K i >0, the integral gain adjusts the output in real time according to the voltage drop. As the voltage drop deepens, the integral gain increases to provide more reactive current compensation and enhance synchronization stability.

[0101] It should be noted that this invention was designed considering the worst-case scenario of the grid voltage dropping to 0; therefore, the control parameters are set conservatively. The core of the stable control method is the introduction of an integral controller to inject a dynamic compensation component into the q-axis current. Wherein, K... p With K i All parameters are set to positive values, and their signs are the same as |i q |To maintain consistency, the output value of the integral controller serves as the compensation component of the reactive current reference value. This control loop has completed its adjustment action before the phase-locked loop starts. During a fault, the integral controller is activated and runs, causing dynamic adjustment of the feasible region of the K coefficient. The specific change patterns are shown in equations (26) and (27).

[0102] (26)

[0103] (27)

[0104] As can be seen from the above formula, the reactive power compensation effect of the integral controller effectively reduces K. min With K max K p The coefficient remains at a constant set value, originally at K. min The following K p The coefficients are adjusted by the integral controller and then enter the new feasible region.

[0105] To verify the effectiveness of this invention, a virtual synchronous generator system simulation model was established using the MATLAB / Simulink simulation platform. The controller parameters are shown in Table 1. A three-phase short-circuit fault was simulated on the grid side. At t=2s, the degree of voltage drop was varied to simulate two typical fault conditions: shallow voltage drop and deep voltage drop, thus verifying the effectiveness of the invention.

[0106] Table 1 Simulation parameters of the main circuit and controller of the virtual synchronous generator system

[0107] Parameter Value Parameter Value Rated capacity / S B ]] 2000 kVA Line inductance / L g ]]> 0.11 mH Direct current bus voltage / U dc ]]> 1000V Line resistance / R g ]]> 0.0036 Ω Grid voltage / U g ]] 380V LVRT reactive current injection coefficient / K p ]] 2.4 Rated frequency / f 50 Hz Phase-locked loop PI controller / (k pPLL + k iPLL / s)]]> 60+1400 / s Filtering capacitor / C f ]]> 154.8 μF voltage outer loop PI controller / (k pU + k iU / s)]]> 7.06+811.43 / s Filter inductance / L f ]]> 0.022 mH Current outer loop PI controller / (k pI + k iI / s) 0.2977+20.423 / s Grid-connected resistance / R f ]]> 0.0036 Ω Current limiting / i max ]]> 1.2 p.u.

[0108] The system voltage response waveforms under different control methods during a shallow voltage dip in the power grid are as follows: Figure 7As shown, when the grid voltage drops to 0.7 pu, by Figure 7 (b) It can be seen that the method of this patent has a smaller oscillation amplitude and better voltage stability than the traditional method. Within [2.376s-3s], the voltage support effect of this patent method is significant, with the final voltage reaching 0.862pu, while the traditional method only reaches 0.758pu. When the voltage drops to 0.3pu, the system voltage response waveforms under different control methods are as follows: Figure 8 As shown, the method of this patent can support continuous voltage recovery, while the voltage of traditional methods eventually fluctuates around 0.5 pu. In summary, the method of this invention, compared with traditional methods, adds reactive current compensation, improves system stability, and provides effective support for grid voltage.

[0109] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A low-voltage ride-through control method for a virtual power plant equipped with a virtual synchronous generator, characterized in that, Includes the following steps: S1) Construction of a low-voltage ride-through control model for a virtual power plant equipped with a virtual synchronous generator; S2) Dynamic current characteristics analysis of traditional low-voltage ride-through control methods; S3) Derivation of the feasible region of the reactive current injection coefficient K; S4) Establish P based on fault voltage I Coefficient adaptive stability control method; S5) Adaptive Integral Gain K i Parameter design; The virtual power plant system, equipped with a virtual synchronous generator, includes a phase-locked synchronization module, an outer voltage loop controller, a low-voltage ride-through controller, and a current closed loop. Through the phase-locked synchronization mechanism, the three-phase voltage and current signals at the point of common coupling are converted to a synchronous rotating dq coordinate axis via Parker transformation. Under steady-state conditions, the outer voltage loop controller generates a current reference value for the dq axis based on the grid voltage deviation, while the inner loop achieves the system's preset power output through current vector control. Assuming the DC-side voltage is constant, during steady-state operation, i... q_ref with i d_ref Determined by the voltage outer loop, when a grid fault occurs, the voltage outer loop control switches to low-voltage ride-through control to prioritize providing reactive current. During the fault period, i q_ref1 with i d_ref1 The reactive current reference value i is determined by the low-voltage ride-through control loop. q_ref1 The active current reference value i is calculated from equation (1). d_ref1 The phase angle of the phase-locked loop output is determined by the residual current, as shown in equation (2). The expression is given by equation (3); （1） （2） （3） In the formula, K is the reactive current injection coefficient, and U pcc I is the per-unit value. N Rated current; This is the output angular frequency of the phase-locked loop. Indicates the d-axis relative to U g rotational speed, U q This represents the q-axis component of the PCC voltage. The stable operation of the virtual power plant equipped with a virtual synchronous generator system relies on phase-locked loop control, which achieves phase tracking to ensure that the output voltage rotates in phase with the grid voltage; the d-axis and U-axis are defined. g The included angle is the virtual power angle δ, which is given by equation (4). （4） In the formula, For an infinitely large power grid, its frequency is typically the power frequency constant ω. v δ is the difference between the phase-locked loop output phase and the grid phase; The vector relationship between the point of common coupling voltage and the grid voltage is as follows: （5） In the formula, U pcc I is the voltage at the point of common coupling. pcc The output current at the point of common coupling, R is the equivalent grid resistance, X is the equivalent grid reactance, and U... z For the equivalent grid impedance voltage drop, U g This refers to the grid voltage. The phase-locked loop achieves frequency synchronization by tracking the Upcc phase along the d-axis. When the phase of the port voltage Upcc is affected by voltage and current disturbances, its phase deviation from the d-axis causes angular frequency adjustment. When the Upcc phase lags behind the d-axis phase, the rotational angular frequency of the d-axis will decrease to phase-locked. When the Upcc phase leads the d-axis, i.e., Uq>0, the rotational angular frequency of the d-axis will increase to phase alignment.

2. The low voltage ride-through control method according to claim 1, characterized in that: The output frequency and phase of the phase-locked loop are determined by the q-axis component U of the voltage at the point of common coupling in the dq coordinate system. q Drive adjustment, in the dq coordinate system, U q and U d They are represented as follows: （6） In the formula, U gf This refers to the voltage at which the power grid fails. Voltage at the point of common coupling U under power grid fault conditions pcc The voltage expression is given by equation (7); （7） In the formula, It is the impedance angle. ; During power grid faults, the dynamic process of the phase-locked loop is affected by i q Impact, i q The reference value is jointly determined by the low-voltage ride-through method and the phase-locked loop response; the dynamic current equation is established by combining equations (1), (2) and (7) for the transient process i q i d Quantitative analysis was performed; the dynamic output current exhibits implicit functional characteristics, and is related to the K coefficient and the virtual power angle δ, i q and i d The mathematical expression is given by equation (8); during the dynamic process of the phase-locked loop, i q Let be a time-varying function of δ and K; （8） During a fault, the system preferentially injects reactive current to maintain voltage support capability.

3. The low voltage ride-through control method according to claim 2, characterized in that: In step S3, when the q-axis voltage component at PCC is zero, the phase-locked loop achieves grid phase synchronization tracking. At this time, the system reaches a transient stable state and satisfies the condition of equation (11). （11） In the formula, U gf This is the voltage of the power grid fault. This refers to the virtual power angle during the transient equilibrium of the grid-connected system. Voltage amplitude at PCC and It can be expressed as equations (12) and (13); （12） （13） When the phase-locked loop is synchronously stable, U q =0; when -1 ≤ sinδ e When ω ≤ 1 and transient synchronization is stable pll =ω v At this time, the system satisfies the conditions for synchronous stability as expressed in equation (14). （14） From equation (14), it can be seen that when U gf When <U1, the grid voltage drops too much; at this time, U in equation (11) q The constant non-zero value causes the phase-locked loop to experience a type of synchronization instability, where no transient equilibrium point exists; when U... gf When U ≥ U1, U q It could be zero, in which case there exists a transient stable equilibrium point δ. e Satisfying U q If the condition is 0, then the fault voltage depth of the power grid directly affects the synchronization capability of the phase-locked loop. The virtual synchronous generator system is equivalent to a controlled current source, and the output current flowing through the line impedance directly determines the size of U1. In power transmission networks, the grid-connected reactance is usually much larger than the resistance, and under large reactance, U is prone to occur. gf <U1 causes phase-locked loop type I synchronous instability; during the fault, the system output current characteristics are determined by the low voltage ride-through control formulas (1) and (2); the fault output current is affected by both the PCC voltage drop and the K coefficient of the low voltage ride-through control loop. When the system reaches transient equilibrium, each control element transitions from transient to steady-state operation, and the dynamic current i d i q Tracking the low-voltage ride-through reference current to reach the transient steady-state current command value i d_ref1 with i q_ref1 Further quantitative analysis d and i q Based on the transient stability condition of the phase-locked loop and the restricted domain, the following assumptions are made: Assume U gf The falling speed is faster than U pcc Change, see U gf Given constant values ​​and grid impedances R and X being fixed values; combining equations (1), (2), and (14), we finally obtain i d i q Restricted domain.

4. The low voltage ride-through control method according to claim 3, characterized in that: Output current limiting range under low voltage ride-through control, limiting control setting i d with|i q |The range is from 0 to i max i max The value is 1.

2.

5. The low voltage ride-through control method according to claim 3 or 4, characterized in that: To quantify the impact of fault voltage and grid impedance on the reactive current limiting range under transient equilibrium conditions, three cases are categorized based on different voltage dips and impedance magnitudes. Case 1, when In a power transmission network, X is much larger than R. At this time, i q Any value of i satisfies equation (11), q The range of values ​​is （15） Scenario 2, when i q Even when taking -imax, equation (11) is still satisfied; i q When the maximum value is set to 0, i needs to be retuned q The boundary value; by solving the quadratic equation using the boundary conditions of equation (11), we can obtain i. q The range is （16） Scenario 3, when The effective region satisfying equation (14) is further reduced; the extreme values ​​of the q-axis projection must satisfy the boundary conditions of equation (11), at which point i q The range is （17） From equations (15)-(17), we obtain i under the transient stable condition of the system. q Complete restricted domain.

6. The low voltage ride-through control method according to claim 5, characterized in that: The practical feasible region of the K coefficient is obtained based on reactive current matching curve quantization. （18） （19）。 7. The low voltage ride-through control method according to claim 6, characterized in that: In step S4, based on the fixed K-coefficient method of the traditional low-voltage ride-through method, reactive current is compensated according to the degree of PCC voltage drop. An integral element is added to the controller, and the integral coefficient is adaptively adjusted to control the dynamic compensation amount of reactive current; when a grid fault causes U pcc <U ref When the dynamic current compensation adjustment mechanism is activated, the adaptive integral controller outputs the dynamic compensation current in real time. Let U ref =0.9, when the voltage drop is less than U ref When the threshold is reached, steady-state voltage outer loop control is enabled to provide a current reference value for the system; when the PCC voltage drops beyond U... ref When the range is wide, a PI coefficient adaptive stability control method needs to be adopted. At this time, the mathematical expressions for the reactive and active current reference values ​​are as shown in equation (20). When K is in fault period p Coefficient is lower than K min At this time, insufficient reactive current injection may occur. In this case, the integral controller generates a compensation component based on the fault voltage. The compensation increases the reactive current while suppressing the active current, ultimately achieving accurate matching between the current output and the fault condition. （20） In the formula, K i K represents the gain parameter of the integral controller. p This is a proportional parameter.

8. The low voltage ride-through control method according to claim 7, characterized in that: Integral gain K i The design goal is to ensure U q =0 condition is established; Assume U pcc Falling to the minimum value U pccmin The ultimate cut-off time parameter is obtained by simulating the unstable state; Based on the absence of dynamic reactive current injection during the fault, for K i Value range analysis, K i The parameter separation expression is given by equation (21). （21） Considering the dynamic process of the PLL, U at this time pcc The expression for is equation (22). （22） Considering the most severe voltage drop conditions in the power grid, let And the current reaches the critical condition of limiting, at which point U pccmin The mathematical expression is equation (23). （23） Combining equations (21) and (23), K... i The conservative maximum estimate of the parameter is given by equation (24). （24） According to K imax The parameters, the integral controller provides reactive current to support the stability of the virtual synchronous generator system, and corrects Ki in real time based on the drop in fault voltage during the fault period. The integral gain correction expression is Equation (25). （25） Introducing an integral controller to inject a dynamic compensation component into the q-axis current; K p With K i All parameters are set to positive values, and their signs are the same as |i q |To keep consistent, the output value of the integral controller is used as the compensation component of the reactive current reference value; this control loop has completed the adjustment action before the phase-locked loop starts, and the integral controller is activated during the fault, causing dynamic adjustment of the feasible domain of the K coefficient; the change law is as shown in equations (26) and (27). （26） （27） The reactive power compensation effect of the integral controller reduces K min With K max ;K p The coefficient remains at a constant set value, originally at K. min The following K p The coefficients are adjusted by the integral controller and then enter the new feasible region.

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