Method for evaluating voltage instability risk caused by new energy fault ride-through

Abstract

The invention discloses a risk assessment method for voltage instability caused by new energy fault ride-through, belongs to the technical field of voltage stability control, is suitable for a grid-connected power generation system based on an inverter, and comprises the following steps: S1, constructing a quasi-steady-state equation for analyzing a balance point of the grid-connected power generation system; s2, substituting the fault response characteristics of the inverter into the quasi-steady-state equation to obtain a piecewise equation about the grid-connected point voltage; s3, drawing a grid voltage-grid connection point voltage curve according to the piecewise equation; and S4, performing risk assessment on voltage instability by taking the length of the non-equilibrium point interval on the grid voltage-grid connection point voltage curve as a quantitative index. According to the method, the magnitude of the balance-point-free interval is used as a risk assessment index for quantifying the voltage instability caused by fault ride-through switching control of the new energy unit, reference is provided for guiding fault ride-through parameter setting of the new energy unit, and the stability of a new energy power system is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of voltage stability control in new energy systems, and specifically relates to a method for assessing the risk of voltage instability caused by new energy fault ride-through. Background Art

[0002] Renewable energy gathering areas are often located in remote areas, connected to the main grid through long-distance transmission lines. The number of synchronous units in the vicinity is small and the grid structure is weak, resulting in prominent voltage stability problems in renewable energy gathering areas.

[0003] There are a lot of studies on transient overvoltage and voltage stability in areas where new energy converges. Existing studies have pointed out that grid failures and surplus reactive power generated by new energy can lead to overvoltage problems. Regarding voltage stability, existing studies are mainly divided into two categories. One is based on methods such as static PV curves or short-circuit ratio indicators, which demonstrates that the access of new energy will deteriorate the static voltage stability of the system; the other is based on time domain simulation, pointing out that by modifying the control, new energy units can improve the voltage stability of the system. However, in recent years, a new type of voltage instability phenomenon has appeared in an actual wind farm, which is manifested as wind turbines repeatedly entering the low voltage ride-through mode, and control switching causes the unit output power and grid connection point voltage to continue to oscillate. This voltage instability phenomenon is different from the formation mechanism of the conventional voltage instability mentioned above. The existing technology has made a preliminary mechanism analysis of this phenomenon, pointing out that this phenomenon is caused by the control switching of new energy units, but how to quantitatively evaluate the risks of this new type of voltage instability phenomenon and the influencing factors still lack relevant technology. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, a method for assessing the risk of voltage instability caused by fault crossing of new energy sources is provided in the present invention, which solves the problem that the prior art lacks a method for quantitatively assessing the impact of different factors on the risk of instability, thereby affecting the design of control parameters of new energy units.

[0005] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is: a method for assessing the risk of voltage instability caused by fault ride-through of new energy, which is applicable to a grid-connected power generation system based on an inverter, comprising the following steps: S1. Construct a quasi-steady-state equation to analyze the equilibrium point of the grid-connected power generation system; S2. Substitute the fault response characteristics of the inverter into the quasi-steady-state equation to obtain a piecewise equation for the grid connection point voltage; S3. Draw a grid voltage-grid connection point voltage curve according to the piecewise equation; S4. The length of the non-balanced point interval on the grid voltage-grid connection point voltage curve is used as a quantitative indicator to conduct risk assessment on voltage instability.

[0006] Furthermore, in step S1, when constructing the quasi-steady-state equation, the following assumptions exist: The grid-connected power generation system has reached a balanced state, ignoring the current control dynamics, and the active and reactive currents accurately track the reference instructions, and the grid-connection point voltage orientation is adopted.

[0007] Furthermore, in step S1, the quasi-steady-state equation constructed is:

[0008] In the formula, Indicates the grid connection point voltage, Represents the equivalent resistance of the power grid, , Respectively represent the d and q components of the output current, represents the equivalent impedance of the power grid, Indicates the grid voltage.

[0009] Further, in step S2, the fault response characteristic of the inverter is expressed as:

[0010] In the formula, , Respectively represent the reference instructions for active power and reactive power, Indicates the grid connection point voltage, Indicates the maximum current that the inverter can withstand. Indicates the startup threshold for entering low voltage ride-through mode. Indicates the dynamic reactive current coefficient.

[0011] Furthermore, for the piecewise equation, , and As a demarcation point, it satisfies ; in, Indicates the voltage of the dividing point 1, Indicates the voltage of the dividing point 2, Indicates the voltage of the dividing point 3.

[0012] Furthermore, in step S3, the drawn grid voltage-grid point voltage curve includes a non-balance point interval caused by the fault ride-through switching control of the new energy unit. In this non-balance point interval, there is no stable balance point in the grid-connected power generation system, and the new energy unit repeatedly enters fault ride-through, resulting in voltage instability.

[0013] Furthermore, in step S4, the length of the non-balance point interval Δ U g It is expressed as:

[0014]

[0015]

[0016] In the formula, Indicates the startup threshold for entering low voltage ride-through mode. Represents the dynamic reactive current coefficient, Indicates the short-circuit ratio of the AC system, represents the AC system impedance ratio, Indicates the grid voltage corresponding to the low voltage ride-through start threshold before entering the low voltage ride-through mode. It means that after entering the low voltage ride-through mode, the grid voltage corresponding to the low voltage ride-through start threshold is reached when the grid connection point voltage reaches the low voltage ride-through start threshold.

[0017] The beneficial effects of the present invention are: The method of the present invention is based on the quasi-steady-state model of the new energy grid-connected system, considers the low voltage ride-through control strategy of the inverter, and derives the no-balance point interval from the perspective of the existence of the system balance point. The size of the interval is used as a quantitative risk assessment indicator for voltage instability caused by fault ride-through switching control of the new energy unit, which provides a reference for guiding the setting of fault ride-through parameters of the new energy unit and improves the stability of the new energy power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A flow chart of a method for assessing the risk of voltage instability caused by fault ride-through of new energy provided by the present invention.

[0019] Figure 2 This is a topological diagram of a typical inverter grid-connected power generation system provided by the present invention.

[0020] Figure 3 The present invention provides a grid voltage-grid connection point voltage curve.

[0021] Figure 4 Different short circuit ratios provided by the present invention K SCR The theoretical calculation results are compared with the time domain simulation results.

[0022] Figure 5 Different low voltage ride-through start thresholds provided by the present invention U th The theoretical calculation results are compared with the time domain simulation results.

[0023] Figure 6 Different impedance ratios provided by the present invention K XR The theoretical calculation results are compared with the time domain simulation results. DETAILED DESCRIPTION

[0024] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0025] The embodiment of the present invention provides a method for assessing the risk of voltage instability caused by fault ride-through of new energy sources, which is applicable to a grid-connected power generation system based on an inverter, such as Figure 1 As shown, the following steps are included: S1. Construct a quasi-steady-state equation to analyze the equilibrium point of the grid-connected power generation system; S2. Substitute the fault response characteristics of the inverter into the quasi-steady-state equation to obtain a piecewise equation for the grid connection point voltage; S3. Draw a grid voltage-grid connection point voltage curve according to the piecewise equation; S4. The length of the non-balanced point interval on the grid voltage-grid connection point voltage curve is used as a quantitative indicator to conduct risk assessment on voltage instability.

[0026] The voltage instability risk assessment method provided by the present invention is applicable to inverter-based grid-connected power generation systems, including wind power, photovoltaic power generation and energy storage, etc. The typical inverter-based grid-connected power generation system topology is as follows: Figure 2 As shown, the control strategy adopts a typical voltage outer loop-current inner loop double closed-loop vector control based on a phase-locked loop, and the present invention makes improvements on this basis.

[0027] exist Figure 2 As shown in the figure, for the sake of generality, the "other parts" of the system are not specifically given. For wind turbines, this part is the machine-side converter and prime mover; for photovoltaic power generation systems, this part is the photovoltaic array and DC / DC converter; for energy storage systems, this part is the energy storage system and DC / DC converter. The DC side is equipped with load shedding protection to maintain DC voltage stability. L f and C f They are filter inductor and capacitor respectively; u a , u b , u c is the three-phase voltage at the grid connection point, i a , i b , ic To output three-phase current; u ga , u gb , u gc is the three-phase grid voltage, R g and L g are the equivalent resistance and equivalent inductance of the power grid respectively.

[0028] The method of the present invention analyzes the voltage instability phenomenon caused by fault ride-through switching control from the perspective of system balance point. Figure 2 The system voltage dynamic equation is shown in formula (1); (1) In the formula, u d , u q Represents the d-axis and q-axis components of the grid-connected point voltage, represented by u a , u b , u c through abc-dq Transformation is obtained; i d , i q Represents the d and q axis components of the output current, i a , i b , i c through abc-dq Transformation is obtained; u gd , u gq Represents the d and q axis components of the grid voltage.

[0029] In order to obtain a quasi-steady-state equation to analyze the equilibrium point, in step S1, when constructing the quasi-steady-state equation, the following assumptions exist: The grid-connected power generation system has reached a balanced state, the current control dynamics are ignored (i.e., the differential term in equation (1) is set to 0), and the active and reactive currents accurately track the reference instructions, and the grid-connected point voltage orientation is adopted.

[0030] The quasi-steady-state equation constructed is: (2) In the formula, Indicates the grid connection point voltage, Represents the equivalent resistance of the power grid, , Respectively represent the d and q components of the output current, represents the equivalent impedance of the power grid, Indicates the grid voltage.

[0031] According to equations (1) and (2), and combined with the output power equation, the PV curve between active power and voltage can be drawn to analyze voltage stability. This method is a conventional method for voltage stability analysis. However, this method cannot reflect the influence of the inverter control strategy and cannot analyze the voltage instability caused by fault-crossing switching control. To this end, the present invention further considers the fault response characteristics of the inverter, which is expressed as: (3) In the formula, , Respectively represent the reference instructions for active power and reactive power, Indicates the grid connection point voltage, Indicates the maximum current that the inverter can withstand. Indicates the startup threshold for entering low voltage ride-through mode. Represents the dynamic reactive current coefficient. Assuming that the active current and reactive current can accurately track the reference instruction, there is i d = i d, ref and i q = i q,ref Established.

[0032] Substituting equation (3) into equation (2), we can get a piecewise equation for the grid connection point voltage. For the piecewise equation, , and As a demarcation point, it satisfies ; in, Indicates the voltage of the dividing point 1, Indicates the voltage of the dividing point 2, Indicates the voltage of the dividing point 3.

[0033] According to the obtained piecewise equation, the special feature of the present invention is that, unlike the conventional active power-voltage curve, the embodiment of the present invention chooses to draw the grid voltage-grid connection point voltage curve, such as Figure 3 shown.

[0034] Similar to the traditional PV curve, the grid voltage-grid connection point voltage curve used in the present invention also has a saddle node bifurcation point indicating system instability. However, unlike the PV curve, the grid voltage-grid connection point voltage curve drawn in the present invention includes a non-balance point interval caused by the fault ride-through switching control of the new energy unit. In this non-balance point interval ( Figure 3 The shaded area Δ U g In the interval), there is no stable equilibrium point in the grid-connected power generation system, and the new energy units repeatedly enter fault ride-through, resulting in voltage instability.

[0035] In step S4 of the embodiment of the present invention, by combining equations (2) and (3), the length of the non-equilibrium point interval Δ U g It is expressed as: (4) (5) (6) In the formula, Indicates the startup threshold for entering low voltage ride-through mode. Represents the dynamic reactive current coefficient, Indicates the short-circuit ratio of the AC system, represents the AC system impedance ratio, Indicates the grid voltage corresponding to the low voltage ride-through start threshold before entering the low voltage ride-through mode. It means that after entering the low voltage ride-through mode, the grid voltage corresponding to the low voltage ride-through start threshold is reached when the grid connection point voltage reaches the low voltage ride-through start threshold.

[0036] in, The relationship with the grid parameters is , The relationship with the grid parameters is .

[0037] The quantitative index Δ in the embodiment of the present invention U g The influence of grid parameters, including short-circuit ratio and impedance ratio, and control parameters of new energy units, including low voltage ride-through starting threshold and dynamic reactive current coefficient, on this new type of voltage instability phenomenon is characterized.

[0038] In order to further illustrate the effectiveness and accuracy of the index proposed in this invention, the theoretical calculation results obtained according to formula (6) are compared with the electromagnetic transient time domain simulation results. Figures 4 to 6 The time domain simulation results are obtained by setting the grid voltage to drop from the rated value at a rate of 0.025 pu / s.

[0039] Depend on Figures 4 to 6 It can be seen that the theoretical calculation results are very close to the actual simulation results. The comparison results show that the proposed index can accurately quantify the voltage instability risk caused by fault ride-through switching control.

[0040] The present invention uses specific embodiments to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present invention.

[0041] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A method for assessing the risk of voltage instability caused by fault ride-through of new energy sources, characterized in that: Applicable to the inverter-based grid-connected power generation system, including the following steps: S1. Construct a quasi-steady-state equation to analyze the equilibrium point of the grid-connected power generation system; S2. Substitute the fault response characteristics of the inverter into the quasi-steady-state equation to obtain a piecewise equation for the grid connection point voltage; S3. Draw a grid voltage-grid connection point voltage curve according to the piecewise equation; S4. The length of the non-balanced point interval on the grid voltage-grid connection point voltage curve is used as a quantitative indicator to conduct risk assessment on voltage instability.

2. The method for assessing voltage instability risk caused by fault ride-through of new energy sources according to claim 1 is characterized in that: In step S1, when constructing the quasi-steady-state equation, the following assumptions exist: The grid-connected power generation system has reached a balanced state, ignoring the current control dynamics, and the active and reactive currents accurately track the reference instructions, and the grid-connection point voltage orientation is adopted.

3. The method for assessing voltage instability risk caused by fault ride-through of new energy sources according to claim 2 is characterized in that: In step S1, the quasi-steady-state equation constructed is: In the formula, Indicates the grid voltage, Represents the equivalent resistance of the power grid, , Respectively represent the d and q components of the output current, represents the equivalent impedance of the power grid, Indicates the grid voltage.

4. The method for assessing voltage instability risk caused by fault ride-through of new energy sources according to claim 1 is characterized in that: In step S2, the fault response characteristic of the inverter is expressed as: In the formula, , Respectively represent the reference instructions for active power and reactive power, Indicates the grid voltage, Indicates the maximum current that the inverter can withstand. Indicates the startup threshold for entering low voltage ride-through mode. Indicates the dynamic reactive current coefficient.

5. The method for assessing voltage instability risk caused by fault ride-through of new energy sources according to claim 4 is characterized in that: For the piecewise equation, , and As a demarcation point, it satisfies ; in, Indicates the voltage of the dividing point 1, Indicates the voltage of the dividing point 2, Indicates the voltage of the dividing point 3.

6. The method for assessing voltage instability risk caused by fault ride-through of new energy sources according to claim 1 is characterized in that: In step S3, the drawn grid voltage-grid connection point voltage curve includes a non-balance point interval caused by the fault ride-through switching control of the new energy unit. In this non-balance point interval, there is no stable balance point in the grid-connected power generation system, and the new energy unit repeatedly enters fault ride-through, resulting in voltage instability.

7. The method for assessing voltage instability risk caused by fault ride-through of new energy sources according to claim 6 is characterized in that: In step S4, the length of the non-balance point interval Δ U g It is expressed as: In the formula, Indicates the startup threshold for entering low voltage ride-through mode. Represents the dynamic reactive current coefficient, Indicates the short-circuit ratio of the AC system, represents the AC system impedance ratio, Indicates the grid voltage corresponding to the low voltage ride-through start threshold before entering the low voltage ride-through mode. It means that after entering the low voltage ride-through mode, the grid voltage corresponding to the low voltage ride-through start threshold is reached when the grid connection point voltage reaches the low voltage ride-through start threshold.

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