Phase-locked oscillation suppression method based on virtual equivalent transmission reactance

Abstract

The invention provides a phase-locked oscillation suppression method based on virtual equivalent transmission reactance, and relates to the technical field of new energy grid connection, and the method comprises the steps: obtaining the power transfer amount between a tracking network type new energy PLL and an energy storage MCC according to Y-delta conversion, substituting a linearized model into a control equation of the energy storage MCC, obtaining a two-degree-of-freedom coupling model, and carrying out the calculation of the two-degree-of-freedom coupling model. Based on the principle of a dynamic vibration absorber, an optimal equivalent transmission reactance and an optimal additional parallel resistance value are obtained according to a model, a phase-locked oscillation suppression strategy based on a virtual equivalent transmission reactance is designed according to the optimal equivalent transmission reactance and the optimal additional parallel resistance value, and an optimal reference value of energy storage output active power is obtained. Under the scene of interaction of a following network type system and a novel construction network type system, the virtual equivalent transmission reactance oscillation suppressor is designed, so that the equivalent electrical distance between the new energy and the stored energy is changed, the power grid strength of the new energy grid-connected system is equivalently improved, and the reliability of the new energy grid-connected system is improved. And a new suppression measure is provided for subsynchronous oscillation dominated by the phase-locked loop in a specific scene.

Description

Technical Field

[0001] This invention relates to the field of new energy grid connection technology, and in particular to a method for suppressing phase-locked loop oscillations based on virtual equivalent transmission reactance. Background Art

[0002] With the development of new power systems dominated by new energy sources, AC systems are gradually exhibiting the "dual high" characteristics of high proportion of new energy and high proportion of power electronic equipment. Especially in the long-distance transmission mode of wind power transmission systems, insufficient short-circuit ratios and weak system support for grid-connected new energy units such as wind power are problematic. Under the "dual high" characteristics of the grid, the low damping and weak inertia of new energy sources significantly increase the risk of inducing subsynchronous oscillations in the system. Therefore, it is urgent to improve the grid-connected stability of wind turbine units. Currently, there are application scenarios where grid-connected energy storage is configured with new energy power plants. The combined grid connection of new energy and energy storage has become a necessary measure to improve system stability in the future. Among various grid-connected control methods, matching control only needs to measure the DC bus voltage of the full-power converter to achieve autonomous synchronization with the grid, exhibiting a fast response speed. Furthermore, the DC capacitor integral stage directly provides the equivalent inertia of the system, eliminating the need for additional inertia simulation. Based on these advantages, matching control converters have been tested and verified in multiple locations and have broad application prospects in the future. When energy storage employs matched control, the dynamic changes in active power flow caused by the DC-side capacitor voltage of the converter are matched to the rotational dynamics of the rotor motion equation of a synchronous motor, thereby achieving a natural power synchronization grid connection effect similar to that of a synchronous motor. This provides an inertial regulation effect on the grid. Furthermore, the addition of DC-side resistance damping in the control structure provides an equivalent damping effect to the system, further avoiding the problem of undamped oscillation instability. However, after energy storage is connected to the grid, the lack of energy transfer analysis between the energy storage and the system means that the oscillation suppression effect and the parameter design in the control structure still need to be optimized. Introducing virtual equivalent transmission reactance to establish a controllable vibration coupling relationship between new energy and energy storage can provide a new solution for further improving the grid connection safety of new energy units.

[0003] Therefore, it is essential to design a phase-locked loop suppression method based on virtual equivalent transmission reactance. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a phase-locked loop suppression method based on virtual equivalent transmission reactance.

[0005] To achieve the above objectives, the present invention provides the following solution: This invention provides a method for suppressing phase-locked loop oscillations based on virtual equivalent transmission reactance, comprising: Step 1: Based on the Y-Δ transformation, obtain the power transfer between the grid-connected new energy PLL and the energy storage MCC; Step 2: Based on the power transfer between the grid-connected new energy PLL and the energy storage MCC, substitute the linearized model into the control equation of the energy storage MCC to obtain a two-degree-of-freedom coupled model of the new energy and energy storage joint grid-connected system. Step 3: Based on the principle of the dynamic vibration absorber, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC under the two degrees of freedom are obtained according to the two-degree-of-freedom coupling model. Step 4: Based on the two-degree-of-freedom coupling model, obtain the optimal equivalent transmission reactance of the system under two degrees of freedom and the optimal additional parallel resistance value of the energy storage MCC. Design a phase-locked loop oscillation suppression strategy based on the virtual equivalent transmission reactance to obtain the optimal reference value of the active power output of the energy storage.

[0006] Preferably, in step 1, the power transfer between the grid-connected new energy PLL and the energy storage MCC is obtained according to the Y-Δ transformation, specifically as follows: After grid-connected energy storage is constructed in conjunction with grid-connected renewable energy sources, there is energy interaction between the grid-connected inverter and the energy storage. Let the voltage at the renewable energy inverter terminal be... U 1∠ i 1. The energy storage terminal voltage is U 2∠ i 2. The grid voltage is U 3∠ i 3. The grid connection point voltage is U t ∠ i t After Y-Δ transformation, the equivalent reactance between new energy and energy storage is X 12 The equivalent reactance between energy storage and the grid is X 23 The linearized active power output of the energy storage is expressed as: ; In the formula, i 10 , i 20 as well as i 30 These represent the initial terminal voltage phases of the grid-connected converter, energy storage, and AC grid, respectively.

[0007] Preferably, in step 2, based on the power transfer between the grid-connected renewable energy PLL and the energy storage MCC, the linearized model is substituted into the control equation of the energy storage MCC to obtain a two-degree-of-freedom coupled model of the renewable energy and energy storage joint grid-connected system, specifically: Based on the power transfer between the grid-connected new energy PLL and the energy storage MCC, the linearized second-order differential equation of the PLL is obtained as follows: ; In the formula, s For differential operators, U t The voltage amplitude at the grid connection point. i t This refers to the voltage phase at the wind turbine's grid connection point. i 1 represents the PLL output phase. K pll_p , K pll_i These represent the proportional gain and integral gain of the PI stage, respectively. Among them, energy storage adopts an additional DC side parallel DC resistor. R The matching control mechanism is based on the electromechanical characteristics of synchronous generators in power system transient analysis. If the system is under small disturbance, it is approximately considered that... u dc * =1, after linearization u dc Regarding active power P S2 - P Dynamic equations and energy storage output phase of 2 i The relationship between 2 is represented as: ; In the formula, C For DC filter capacitors, u dc This is the DC capacitor voltage. N c To match the proportional coefficient, P s2 This refers to the active power on the DC side of the converter. Under small disturbance conditions, the phase change of the grid-type energy storage is approximately equal to that of the PLL tracking the grid connection point. Δ P Substituting the expression for 2 into the above equation, we obtain the two-degree-of-freedom coupled model under the interaction of the PLL and the energy storage MCC, which is: ; In the formula, Δ P e This refers to power disturbances encountered during normal system operation.

[0008] Preferably, in step 3, based on the principle of the dynamic vibration absorber, the optimal equivalent transmission reactance and the optimal additional parallel resistance values ​​of the energy storage MCC of the system under two degrees of freedom are obtained according to the two-degree-of-freedom coupling model, specifically as follows: By substituting variables into the obtained two-degree-of-freedom coupled model of the interaction between the PLL and the energy storage MCC, it is transformed into a standard second-order two-degree-of-freedom differential equation model, as follows: ; In the formula, Δ i The oscillation curve of 1 will have two common intersection points M and N. By optimizing the oscillation frequency ratio, the optimal equivalent transmission reactance can be obtained. By adjusting the damping ratio parameter, the optimal additional parallel resistance value under matched control can be obtained. The optimal equivalent transmission reactance and the optimal additional parallel resistance under matching control are calculated as follows: .

[0009] Preferably, in step 4, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC are obtained according to the two-degree-of-freedom coupling model. A phase-locked loop suppression strategy based on the virtual equivalent transmission reactance is designed to obtain the optimal reference value of the energy storage output active power, specifically: Based on the calculated optimal equivalent transmission reactance Optimal Additional Parallel Resistance Value under Matching Control R The correction amount Δ of the energy storage output active power is obtained. P ES for: ; In the formula, Δ X 21 Defined as virtual equivalent transmission reactance; Add a correction factor Δ of the active power output from the energy storage to the virtual mechanical power under energy storage matching control. P ES Subtract electromagnetic power P s2 The reference value of the active power of energy storage is calculated. P ref for: ; The active power reference value of this energy storage is input into the inner loop control to implement a phase-locked oscillation suppression strategy based on virtual equivalent transmission reactance.

[0010] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: This invention provides a phase-locked loop (PLL) oscillation suppression method based on virtual equivalent transmission reactance. The method includes obtaining the power transfer between a grid-connected renewable energy PLL and a matching-controlled energy storage converter (MCC) based on Y-Δ transformation; substituting the linearized model of the power transfer between the PLL and MCC into the control equations of the MCC to obtain a two-degree-of-freedom coupled model of the renewable energy and energy storage combined grid-connected system; obtaining the optimal equivalent transmission reactance and optimal additional parallel resistance of the MCC under the two-degree-of-freedom coupling model based on the principle of dynamic vibration absorbers; designing a PLL oscillation suppression strategy based on virtual equivalent transmission reactance; and obtaining the optimal reference value for the active power output of the energy storage. This invention defines a new virtual equivalent transmission reactance by establishing the power transfer relationship between the renewable energy unit PLL and the matched-controlled energy storage converter (MCC). X 21 Furthermore, under the interaction of the two, based on the conditions of optimal frequency ratio and optimal damping ratio, and with the optimization objective of reducing the overall phase angle amplitude of the grid-connected system in a specific subsynchronous frequency band, the coupled model after solving the equations of motion of the two is mathematically derived, and the optimized virtual equivalent transmission reactance parameters are obtained. X 21 And matching control of optimal additional parallel resistance parameters R Compared to traditional matched control, the optimal parameters X 21 This provides a new additional active power correction amount Δ for the control loop. P ES Optimal additional parallel resistance parameters R This provides optimized virtual mechanical power for the control loop, thereby providing a new reference value for the power output of energy storage, further improving the energy transfer effect of the grid-connected system. A phase-locked loop oscillation suppressor based on virtual equivalent transmission reactance was designed. Finally, simulation verified the superiority of this oscillation suppression strategy over the existing subsynchronous oscillation suppression strategy under grid control, providing a new solution for further improving the grid connection safety of new energy units. Attached Figure Description

[0011] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0012] Figure 1 This is a schematic diagram of the phase-locked loop suppression method based on virtual equivalent transmission reactance according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the grid connection structure of the new energy storage system according to an embodiment of the present invention; Figure 3 This is a block diagram of the phase-locked loop control according to an embodiment of the present invention; Figure 4 The actual simulated waveform of the PLL phase angle and the amplitude-frequency response characteristic curve of the embodiment of the present invention are shown. Figure 5 This is a structural diagram of a phase-locked loop suppressor based on virtual equivalent transmission reactance according to an embodiment of the present invention; Figure 6 This is a diagram of the simulation system structure. Figure 7 The simulation parameter diagram; Figure 8 A comparison of power oscillation responses with only inertia and additional damping; Figure 9 The diagram shows the power response of the wind turbine under different control conditions. Detailed Implementation

[0013] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0014] The purpose of this invention is to provide a method for suppressing phase-locked loop oscillations based on virtual equivalent transmission reactance. In scenarios involving interaction with grid-connected and novel grid-connected systems, by designing a virtual equivalent transmission reactance oscillation suppressor, the equivalent electrical distance between new energy sources and energy storage is changed, effectively increasing the grid strength of the new energy grid-connected system. This provides a new suppression measure for subsynchronous oscillations dominated by phase-locked loops in specific scenarios.

[0015] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0016] This invention takes a grid-connected new energy unit as an example to analyze the oscillation suppression problem of subsynchronous oscillation in the system under the dominance of the phase-locked loop (PPL) and the interaction between the grid and the system. The system structure is as follows: Figure 2 As shown. The phase-locked loop control block diagram is as follows. Figure 3 As shown. In weak grid connection mode, the new energy unit uses a phase-locked loop to track the phase of the grid connection point in real time, and in order to ensure energy utilization, it is usually in maximum power point tracking mode. The energy storage side adopts matching control, which is achieved by establishing the DC capacitor voltage. u dcWith output voltage angular frequency oh The matching relationship between them is established, and the inertia of the DC capacitor is used to achieve autonomous synchronization with the power grid, simulating the power support capability of the synchronous machine and improving grid friendliness.

[0017] like Figure 1 As shown, this invention provides a method for suppressing phase-locked loop oscillations based on virtual equivalent transmission reactance, comprising: Step 1: Based on the Y-Δ transformation, obtain the power transfer between the grid-connected new energy PLL and the energy storage MCC; Step 2: Based on the power transfer between the grid-connected new energy PLL and the energy storage MCC, substitute the linearized model into the control equation of the energy storage MCC to obtain a two-degree-of-freedom coupled model of the new energy and energy storage joint grid-connected system. Step 3: Based on the principle of the dynamic vibration absorber, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC under the two degrees of freedom are obtained according to the two-degree-of-freedom coupling model. Step 4: Based on the two-degree-of-freedom coupling model, obtain the optimal equivalent transmission reactance of the system under two degrees of freedom and the optimal additional parallel resistance value of the energy storage MCC. Design a phase-locked loop oscillation suppression strategy based on the virtual equivalent transmission reactance to obtain the optimal reference value of the active power output of the energy storage.

[0018] In step 1, the power transfer between the grid-connected renewable energy PLL and the energy storage MCC is obtained based on the Y-Δ transformation, specifically: like Figure 2 As shown, after grid-connected energy storage is constructed in conjunction with grid-connected renewable energy, there is energy interaction between the grid-connected inverter (MCC) and the energy storage. Let the voltage at the renewable energy inverter terminal be... U 1∠ i 1. The energy storage terminal voltage is U 2∠ i 2. The grid voltage is U 3∠ i 3. The grid connection point voltage is U t ∠ i t After Y-Δ transformation, the equivalent reactance between new energy and energy storage is X 12 The equivalent reactance between energy storage and the grid is X 23 The linearized active power output of the energy storage is expressed as: ; In the formula, i 10 , i 20 as well as i 30These represent the initial terminal voltage phases of the grid-connected converter, energy storage, and AC grid, respectively.

[0019] In step 2, based on the power transfer between the grid-connected renewable energy PLL and the energy storage MCC, the linearized model is substituted into the control equation of the energy storage MCC to obtain a two-degree-of-freedom coupled model of the renewable energy and energy storage joint grid-connected system, specifically: like Figure 3 The diagram shows the control structure of a phase-locked loop (PLL). Based on the power transfer between the grid-connected new energy PLL and the energy storage MCC, the linearized second-order differential equation of the PLL is obtained as follows: ; In the formula, s For differential operators, U t The voltage amplitude at the grid connection point. i t This refers to the voltage phase at the wind turbine's grid connection point. i 1 represents the PLL output phase. K pll_p , K pll_i These represent the proportional gain and integral gain of the PI stage, respectively. The core idea of ​​energy storage MCC (Multi-Channel Control) lies in using the dynamic changes in the DC-side capacitor voltage of the converter to match the rotational dynamics of the synchronous machine rotor, thereby achieving natural power synchronization and grid connection performance similar to that of a synchronous machine. u dc The dynamic changes in active power on both the AC and DC sides are characterized. To avoid undamped oscillations, the energy storage in this invention employs an additional DC-side parallel DC resistor. R The matching control mechanism, and based on the electromechanical characteristics of synchronous generators in power system transient analysis, when the system is under small disturbance, u dc The range of variation is not large, so in the above formula, it is approximately considered that... u dc * =1, therefore after linearization u dc Regarding active power P S2 - P Dynamic equations and energy storage output phase of 2 i The relationship between 2 can be represented as: ; In the formula, C For DC filter capacitors, u dc This is the DC capacitor voltage. N c To match the proportional coefficient,P s2 This refers to the active power on the DC side of the converter. Under small disturbance conditions, the phase change of the grid-type energy storage is approximately equal to that of the PLL tracking the grid connection point. Δ P Substituting the expression for 2 into the above equation, we obtain the two-degree-of-freedom coupled model under the interaction of the PLL and the energy storage MCC, which is: ; In the formula, Δ P e This refers to power disturbances encountered during normal system operation.

[0020] In step 3, based on the principle of the dynamic vibration absorber, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC under the two degrees of freedom are obtained according to the two-degree-of-freedom coupling model, specifically: By substituting variables into the obtained two-degree-of-freedom coupled model of the interaction between the PLL and the energy storage MCC, it is transformed into a standard second-order two-degree-of-freedom differential equation model, as follows: ; In the formula, K 23 = U 2 U 3cos( i 20 - i 30 ) / X 23 , K 21 = U 2 U 1cos( i 20 -θ 10 ) / X 21 , a 2 = K pll_i U t , b 2 = K 23 / M , c 2 = K 21 / M, M = C / N 2 c, D =1 / ( N 2 c R ), ξ=D / (2 Mb ); Let the disturbance quantity Then we can solve it as follows: ; To simplify calculations, variable substitution is introduced here. ,definition or 1, or 2, or 3 represents the corresponding oscillation frequency ratios. or 1= b / a , or 2= c / a , or 3= oh e / a ; In a two-degree-of-freedom system, Δ i The oscillation curve of 1 will have two common intersection points. In order to more intuitively represent the oscillation response of the phase angle at different frequencies, a reference phase amplitude parameter is introduced. i 0= P e / a 2 Analyzing the structure of the above equation, we can see that the expression for the phase angle amplitude of the PLL can be divided into two parts, namely, the expression containing the damping term. E 2 x 2, F 2 x 2 and without damping terms E 1 x 1, F 1 x 1. Therefore, when the damping ratio approaches zero, the response value of the PLL phase angle can be expressed as: i 0( E 1 / F 1), and when the damping ratio approaches infinity, the response value of the PLL phase angle can be expressed as i 0( E 2 / F 2) To make the ordinates at the two common intersection points equal, it is only necessary to make the response values ​​equal when the damping ratio approaches zero and infinity, that is: ; Simplifying, we get:

[0021] The above expression has two roots. or 31 = l M and or 32 = l N According to Vieta's formulas, we have: ; Since M and N are the common intersection points of all curves, therefore l M and l N The oscillatory responses at the two points should be equal for any value of the damping ratio. Therefore, as the damping ratio approaches infinity, from: ; Therefore, we can obtain: ; By combining the equations, we can obtain that when the frequency is greater than... or 2. Optimal frequency ratio of the system when fixed or The expression is: ; Substituting the optimal frequency ratio into the equation, we can obtain the x-coordinates of the two common intersection points: ; The optimal frequency ratio obtained through the above solution ensures that the two common points M and N are adjusted to equal heights, i.e., their ordinates are equal. Furthermore, regardless of damping... x Regardless of the value, the oscillation response curve will always pass through points M and N, and the maximum amplitude will not be lower than the ordinates of points M and N. Therefore, to obtain the optimal vibration reduction effect, according to the extreme value condition, only when the phase angle response value Δ... i 1 relative to parameters or For the sensitivity of 2 and 3 to satisfy the following formula, M and N can potentially reach their highest points, namely: ; For ease of calculation, variable substitution is introduced here, that is: ; It can be equivalent to: ; Under the optimal frequency ratio, the above formula can be expressed as: ; Therefore, by substituting the coordinates of the two common intersection points, the optimal damping ratio can be obtained. x 2 Although a reasonable choice of damping ratio can achieve an extremum at the common intersection point, it is usually impossible to obtain the ideal situation of achieving extrema at both intersection points simultaneously. Therefore, this invention takes... x M and x N The average value is taken as the optimal damping ratio, that is: ; Substituting the x-coordinates of the two common intersection points into the equation, we get: ; The above formula illustrates the importance of appropriate selection. x The value of can only make the curve reach an extreme value at one of the points M or N. Therefore, in order to achieve a better optimization effect, we should choose . x M and x N The average value is taken as the optimal damping ratio, that is: ; The final calculated optimal equivalent transmission reactance and optimal additional parallel resistance for matching control are as follows: .

[0022] Set the initial frequency ratio or 1 = 0.79 or 2=0.93, and to ensure the versatility and performance requirements of the phase-locked loop, after setting the system parameters, the system can be obtained under different damping ratios. x Below, the oscillating response of the system phase angle |Δ i 1 / i 0|About oh e The oscillation response curves show two common intersection points, M and N. When the frequency is within these intersection points, the damping is negatively correlated with the overall system amplitude; when it is outside these intersection points, it is positively correlated. Therefore, for disturbances at specific frequencies, the system can achieve anti-resonance, i.e., transfer the system's oscillations from the higher-amplitude degree of freedom to other degrees of freedom, and by setting appropriate damping, avoid power amplification caused by resonance, thus effectively suppressing the system's phase and power oscillations. Figure 4 (a)(b) and (c)(d) represent the equivalent transmitted reactance parameters of the PLL after phase fluctuations caused by disturbances. X 21 and additional parallel resistor R The simulated waveforms and amplitude-frequency response curves of the phase angle output for different values ​​show that, with continuous optimization of this parameter, when... X 21 Take 0.21, R When the value is 1.44, the oscillation amplitude of the system is significantly reduced and the system's vibration suppression effect reaches its optimal level.

[0023] In step 4, based on the two-degree-of-freedom coupling model, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC are obtained. A phase-locked loop suppression strategy based on the virtual equivalent transmission reactance is designed to obtain the optimal reference value of the energy storage output active power, specifically: like Figure 5 As shown, based on the calculated optimal equivalent transmission reactance... Optimal Additional Parallel Resistance Value under Matching Control R The correction amount Δ of the energy storage output active power is obtained. P ES for: ; In the formula, Δ X 21 Defined as virtual equivalent transmission reactance; Add a correction factor Δ of the active power output from the energy storage to the virtual mechanical power under energy storage matching control. P ES Subtract electromagnetic power P s2 The reference value of the active power of energy storage is calculated. P ref for: ; The active power reference value of this energy storage is input into the inner loop control to implement a phase-locked oscillation suppression strategy based on virtual equivalent transmission reactance.

[0024] To verify the effectiveness of the virtual equivalent transmission reactance control strategy in suppressing subsynchronous oscillations induced by the phase-locked loop of new energy units, this invention utilizes DIgSILENT / PowerFactory to build a system such as... Figure 6 The simulation system shown is a 3-unit, 9-node wind-storage grid-connected system, comprising a wind farm (50 2MW doubly-fed induction generator (DFIG) wind turbines) and a 10MW / 39kAh (10kV) energy storage battery. The three synchronous generators have capacities of 80MW, 60MW, and 24MW respectively. The wind-storage combined power generation system transmits power through a double-circuit line, Line 11-7. The simulation system assumes a DC energy storage capacitor of 170μF and a constant wind speed of 8m / s. For detailed DFIG wind turbine parameters, please refer to [link to relevant documentation]. Figure 7 ; The wind turbine's output power is transmitted via a dual-circuit line. At 10 seconds, the simulated transmission line... 11-7 When the circuit is disconnected once, the system's short-circuit ratio decreases, inducing subsynchronous oscillations. As mentioned earlier, in matching control, to equivalently simulate converter switching losses, a DC resistor is often connected in parallel with the DC capacitor to simulate the damping effect of the additional DC-side resistance, such as... Figure 8 The figures show the DC-side inertia response alone and the power oscillation response of the system with DC-side damping control, respectively. It can be seen that the oscillation recovery time of the system becomes shorter after the addition of the parallel resistor, thus verifying the effectiveness of the additional DC-side damping control. like Figure 9As shown, four operating conditions were set. In condition 1, without any additional control of energy storage, the system experienced continuous amplified oscillations due to the lack of damping. In condition 2, under the control of only the DC capacitor inertial response, the power oscillations of the system eventually tended to converge, but the suppression effect was still not good. In condition 3, after adding a DC parallel resistor, the oscillations were suppressed relatively quickly under the damping design for subsynchronous oscillations, but there was still room for optimization. In condition 4, after optimizing the DC-side parallel resistor parameters and applying a virtual equivalent transmission reactance control strategy, the system achieved the best oscillation suppression effect, and the stability of the wind power grid-connected system was effectively guaranteed.

[0025] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0026] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for suppressing phase-locked loop oscillations based on virtual equivalent transmission reactance, characterized in that, include: Step 1: Based on the Y-Δ transformation, obtain the power transfer between the grid-connected new energy PLL and the energy storage MCC; Step 2: Based on the power transfer between the grid-connected new energy PLL and the energy storage MCC, substitute the linearized model into the control equation of the energy storage MCC to obtain a two-degree-of-freedom coupled model of the new energy and energy storage joint grid-connected system. Step 3: Based on the principle of the dynamic vibration absorber, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC under the two degrees of freedom are obtained according to the two-degree-of-freedom coupling model. Step 4: Based on the two-degree-of-freedom coupling model, obtain the optimal equivalent transmission reactance of the system under two degrees of freedom and the optimal additional parallel resistance value of the energy storage MCC. Design a phase-locked loop oscillation suppression strategy based on the virtual equivalent transmission reactance to obtain the optimal reference value of the active power output of the energy storage.

2. The method according to claim 1, characterized in that, In step 1, the power transfer between the grid-connected renewable energy PLL and the energy storage MCC is obtained based on the Y-Δ transformation, specifically: After grid-connected energy storage is constructed in conjunction with grid-connected renewable energy sources, there is energy interaction between the grid-connected inverter and the energy storage. Let the voltage at the renewable energy inverter terminal be... U 1∠ θ 1. The energy storage terminal voltage is U 2∠ θ 2. The grid voltage is U 3∠ θ 3. The grid connection point voltage is U t ∠ θ t After Y-Δ transformation, the equivalent reactance between new energy and energy storage is X 12 The equivalent reactance between energy storage and the grid is X 23 The linearized active power output of the energy storage is expressed as: ； In the formula, θ 10 , θ 20 as well as θ 30 These represent the initial terminal voltage phases of the grid-connected converter, energy storage, and AC grid, respectively.

3. The method according to claim 2, characterized in that, In step 2, based on the power transfer between the grid-connected renewable energy PLL and the energy storage MCC, the linearized model is substituted into the control equation of the energy storage MCC to obtain a two-degree-of-freedom coupled model of the renewable energy and energy storage joint grid-connected system, specifically: Based on the power transfer between the grid-connected new energy PLL and the energy storage MCC, the linearized second-order differential equation of the PLL is obtained as follows: ； In the formula, s For differential operators, U t The voltage amplitude at the grid connection point. θ t This refers to the voltage phase at the wind turbine's grid connection point. θ 1 represents the PLL output phase. K pll_p , K pll_i These represent the proportional gain and integral gain of the PI stage, respectively. Among them, energy storage adopts an additional DC side parallel DC resistor. R The matching control mechanism is based on the electromechanical characteristics of synchronous generators in power system transient analysis. If the system is under small disturbance, it is approximately considered that... u dc * =1, after linearization u dc Regarding active power P S2 - P Dynamic equations and energy storage output phase of 2 θ The relationship between 2 is represented as: ； In the formula, C For DC filter capacitors, u dc This is the DC capacitor voltage. N c To match the proportional coefficient, P s2 This refers to the active power on the DC side of the converter. Under small disturbance conditions, the phase change of the grid-type energy storage is approximately equal to that of the PLL tracking the grid connection point. Δ P Substituting the expression for 2 into the above equation, we obtain the two-degree-of-freedom coupled model under the interaction of the PLL and the energy storage MCC, which is: ； In the formula, Δ P e This refers to power disturbances encountered during normal system operation.

4. The method according to claim 3, characterized in that, In step 3, based on the principle of the dynamic vibration absorber, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC under the two degrees of freedom are obtained according to the two-degree-of-freedom coupling model, specifically: By substituting variables into the obtained two-degree-of-freedom coupled model of the interaction between the PLL and the energy storage MCC, it is transformed into a standard second-order two-degree-of-freedom differential equation model, as follows: ； In the formula, Δ θ The oscillation curve of 1 will have two common intersection points M and N. By optimizing the oscillation frequency ratio, the optimal equivalent transmission reactance can be obtained. By adjusting the damping ratio parameter, the optimal additional parallel resistance value under matched control can be obtained. The optimal equivalent transmission reactance and the optimal additional parallel resistance under matching control are calculated as follows: 。 5. The method according to claim 4, characterized in that, In step 4, based on the two-degree-of-freedom coupling model, the optimal equivalent transmission reactance and the optimal additional parallel resistance of the energy storage MCC are obtained. A phase-locked loop suppression strategy based on the virtual equivalent transmission reactance is designed to obtain the optimal reference value of the energy storage output active power, specifically: Based on the calculated optimal equivalent transmission reactance Optimal Additional Parallel Resistance Value under Matching Control R The correction amount Δ of the energy storage output active power is obtained. P ES for: ； In the formula, Δ X 21 Defined as virtual equivalent transmission reactance; Add a correction factor Δ of the active power output from the energy storage to the virtual mechanical power under energy storage matching control. P ES Subtract electromagnetic power P s2 The reference value of the active power of energy storage is calculated. P ref for: ； The active power reference value of this energy storage is input into the inner loop control to implement a phase-locked oscillation suppression strategy based on virtual equivalent transmission reactance.

Images