A method for risk assessment of voltage instability caused by new energy fault ride-through

Abstract

The present invention discloses a method for risk assessment of voltage instability caused by new energy fault ride-through, belonging to the technical field of voltage stability control, and applicable to grid-connected power generation systems based on inverters, including: S1, constructing a quasi-steady-state equation for analyzing the equilibrium 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 for the grid connection point voltage; S3, plotting the grid voltage-grid connection point voltage curve according to the piecewise equation; S4, using the length of the interval without equilibrium points on the grid voltage-grid connection point voltage curve as a quantization index to conduct risk assessment of voltage instability. The present invention uses the size of the interval without equilibrium points as a quantization index for risk assessment of voltage instability caused by the fault ride-through switching control of new energy units, provides a reference for guiding the setting of fault ride-through parameters of new energy units, and improves the stability of the new energy power system.

Description

Technical Field

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

[0002] New energy collection 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 framework is weak, resulting in prominent voltage stability problems in new energy collection areas.

[0003] There are relatively rich studies on the transient overvoltage and voltage stability problems in new energy collection areas. Existing studies have pointed out that grid faults and surplus reactive power generated by new energy will both cause overvoltage problems. Regarding voltage stability, existing studies are mainly divided into two categories. One category is based on methods such as static P-V curves or short-circuit ratio indicators to demonstrate that the access of new energy will deteriorate the static voltage stability of the system; the other category is based on time-domain simulation, indicating that by modifying the control, new energy units can improve the voltage stability of the system. However, in a certain actual wind farm in recent years, a new type of voltage instability phenomenon has occurred, manifested as wind turbines repeatedly entering the low voltage ride-through mode, and the control switching causes the output power of the units and the grid-connected point voltage to continuously oscillate. The formation mechanism of this voltage instability phenomenon is different from the aforementioned conventional voltage instability. The existing technology has made a preliminary mechanism analysis on this phenomenon, pointing out that this phenomenon is caused by the control switching of new energy units. However, there is still a lack of relevant technology on how to quantitatively evaluate the risk of this new type of voltage instability phenomenon and its influencing factors. Summary of the Invention

[0004] Aiming at the above deficiencies in the prior art, a method for risk assessment of voltage instability caused by new energy fault ride-through provided by the present invention solves the problem in the prior art that there is a lack of a method for quantitatively evaluating the influence of different factors on the instability risk, thereby affecting the design of control parameters of new energy units.

[0005] In order to achieve the above invention purpose, the technical solution adopted by the present invention is: A method for risk assessment of voltage instability caused by new energy fault ride-through, applicable to grid-connected power generation systems based on inverters, includes the following steps:

[0006] S1. Construct a quasi-steady state equation for analyzing the equilibrium point of the grid-connected power generation system;

[0007] S2. Substitute the fault response characteristics of the inverter into the quasi-steady state equation to obtain a piecewise equation for the grid-connected point voltage;

[0008] S3. Draw the grid voltage-grid-connected point voltage curve according to the piecewise equation;

[0009] S4. Use the length of the interval without equilibrium points on the grid voltage - point - of - connection voltage curve as a quantization index to conduct a risk assessment of voltage instability.

[0010] Further, in the step S1, when constructing the quasi - steady - state equation, the following assumptions are made:

[0011] The grid - connected power generation system has reached an equilibrium state, ignoring the current control dynamics, and the active current and reactive current accurately track the reference commands, and the point - of - connection voltage orientation is adopted.

[0012] Further, in the step S1, the constructed quasi - steady - state equation is:

[0013]

[0014] In the formula, represents the point - of - connection voltage, represents the equivalent resistance of the grid, and respectively represent the d - component and q - component of the output current, represents the equivalent impedance of the grid, represents the grid voltage.

[0015] Further, in the step S2, the fault response characteristic of the inverter is expressed as:

[0016]

[0017] In the formula, and respectively represent the reference commands of the active power and reactive power, represents the point - of - connection voltage, represents the maximum current that the inverter can withstand, represents the starting threshold for entering the low - voltage ride - through mode, represents the dynamic reactive current coefficient.

[0018] Further, for the piece - wise equation, taking , and as the demarcation points, it satisfies ;

[0019] Among them, represents the voltage of demarcation point 1, represents the voltage of demarcation point 2, represents the voltage of demarcation point 3.

[0020] Furthermore, in the step S3, the grid voltage - connection point voltage curve drawn includes a non - equilibrium point interval caused by the fault - ride - through switching control of the new - energy unit. Within this non - equilibrium point interval, there is no stable equilibrium point in the grid - connected power generation system, and the new - energy unit repeatedly enters the fault - ride - through, resulting in voltage instability.

[0021] Furthermore, in the step S4, the length Δ of the non - equilibrium point interval U g is expressed as:

[0022]

[0023]

[0024]

[0025] In the formula, represents the starting threshold for entering the low - voltage ride - through mode, represents the dynamic reactive current coefficient, represents the short - circuit ratio of the AC system, represents the impedance ratio of the AC system, represents the grid voltage corresponding to the low - voltage ride - through starting threshold of the connection point voltage before entering the low - voltage ride - through mode, represents the grid voltage corresponding to the low - voltage ride - through starting threshold of the connection point voltage after entering the low - voltage ride - through mode.

[0026] The beneficial effects of the present invention are as follows:

[0027] Based on the quasi - steady - state model of the new - energy grid - connected system and considering the low - voltage ride - through control strategy of the inverter, the present invention method derives the non - equilibrium point interval from the perspective of the existence of the system equilibrium point, uses the size of this interval as a risk assessment index for quantifying the voltage instability caused by the fault - ride - through switching control of the new - energy unit, provides a reference for guiding the setting of the fault - ride - through parameters of the new - energy unit, and improves the stability of the new - energy power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flowchart of the method for assessing the voltage instability risk caused by new - energy fault - ride - through provided by the present invention.

[0029] Figure 2 is a typical inverter grid - connected power generation system topology diagram provided by the present invention.

[0030] Figure 3 is the grid voltage - connection point voltage curve provided by the present invention.

[0031] Figure 4 is for different short - circuit ratios provided by the present invention K SCRComparison between the theoretical calculation results and the time-domain simulation results under

[0032] Figure 5 For different low-voltage ride-through starting thresholds provided by the present invention U th Comparison between the theoretical calculation results and the time-domain simulation results under

[0033] Figure 6 For different impedance ratios provided by the present invention K XR Comparison between the theoretical calculation results and the time-domain simulation results under Specific embodiments

[0034] The specific embodiments of the present invention will be described below to facilitate those skilled in the art to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are within the scope of protection.

[0035] An embodiment of the present invention provides a method for assessing the voltage instability risk caused by new energy fault ride-through, which is applicable to grid-connected power generation systems based on inverters, such as Figure 1 shown, and includes the following steps:

[0036] S1. Construct a quasi-steady-state equation for analyzing the equilibrium point of the grid-connected power generation system;

[0037] 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;

[0038] S3. Draw the grid voltage-grid connection point voltage curve according to the piecewise equation;

[0039] S4. Use the length of the non-equilibrium point interval on the grid voltage-grid connection point voltage curve as a quantization index to assess the voltage instability risk.

[0040] The above voltage instability risk assessment method provided by the present invention is applicable to grid-connected power generation systems based on inverters, including wind power, photovoltaic, energy storage, etc. A typical topological structure of a grid-connected power generation system based on an inverter is as Figure 2 shown, and its control strategy adopts a typical voltage outer loop-current inner loop double closed-loop vector control based on a phase-locked loop. The present invention is improved on this basis.

[0041] At Figure 2As shown, without loss of generality, the "other parts" of the system are not specifically given. For a wind turbine generator set, this part is the machine-side converter and the prime mover; for a photovoltaic power generation system, this part is the photovoltaic array and the DC / DC converter; for an energy storage system, this part is the energy storage system and the DC / DC converter. The DC side is equipped with a dump protection to maintain the stability of the DC voltage. In the figure L f and C f are the filter inductor and capacitor respectively; u a , u b , u c are the three-phase voltages at the grid connection point, i a , i b , i c are the output three-phase currents; u ga , u gb , u gc are the three-phase grid voltages, R g and L g are the grid equivalent resistance and equivalent inductance respectively.

[0042] The method of the present invention analyzes the voltage instability phenomenon caused by the fault ride-through switching control from the perspective of the system equilibrium point. According to Figure 2 the dynamic equation of the system voltage can be obtained as shown in Equation (1);

[0043] (1)

[0044] In the formula, u d , u q represent the d-axis and q-axis components of the grid connection point voltage, which are obtained by u a , u b , u c through abc-dq transformation; i d , i q represent the d-axis and q-axis components of the output current, which are obtained by i a , i b , i c throughabc-dq Obtained by transformation; u gd , u gq represent the d-axis and q-axis components of the grid voltage.

[0045] To obtain the quasi-steady-state equation for analyzing the equilibrium point, in step S1, when constructing the quasi-steady-state equation, the following assumptions are made:

[0046] The grid-connected power generation system has reached an equilibrium state, the current control dynamics are ignored (i.e., the differential terms in Equation (1) are set to 0), the active current and reactive current accurately track the reference commands, and the grid-connected point voltage orientation is adopted.

[0047] The constructed quasi-steady-state equation is:

[0048] (2)

[0049] In the formula, represents the grid-connected point voltage, represents the grid equivalent resistance, , respectively represent the d-axis and q-axis components of the output current, represents the grid equivalent impedance, represents the grid voltage.

[0050] According to Equations (1) and (2), and combined with the output power equation, the P-V curve between the active power and the voltage can be plotted to analyze the 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 phenomenon caused by the fault ride-through switching control. Therefore, in the present invention, the fault response characteristics of the inverter are further considered, which are expressed as:

[0051] (3)

[0052] In the formula, , respectively represent the reference commands of the active power and the reactive power, represents the grid-connected point voltage, represents the maximum current that the inverter can withstand, represents the starting threshold for entering the low voltage ride-through mode, represents the dynamic reactive current coefficient. Since it is assumed that the active current and the reactive current can accurately track the reference commands, there are i d = i d, ref and i q = i q,ref holds.

[0053] 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 ;

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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:

[0058] (4)

[0059] (5)

[0060] (6)

[0061] 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.

[0062] Among them, The relationship with the grid parameters is , The relationship with the grid parameters is .

[0063] The quantization index Δ in the embodiments of the present invention U g Characterizes the influence of grid parameters, including short - circuit ratio and impedance ratio, and new - energy unit control parameters, including low - voltage ride - through start - up threshold and dynamic reactive current coefficient, on this new type of voltage instability phenomenon.

[0064] To further illustrate the effectiveness and accuracy of the index proposed by the present invention, the theoretical calculation results obtained according to Equation (6) are compared with the electromagnetic transient time - domain simulation results, as Figures 4 to 6 shown. Among them, the time - domain simulation results are obtained by setting the grid voltage to start dropping from the rated value at a speed of 0.025 p.u. / s.

[0065] It can be seen from Figures 4 to 6 that the theoretical calculation results are very close to the actual simulation results, and the comparison results show that the proposed index can accurately quantify the voltage instability risk caused by the fault - ride - through switching control.

[0066] Specific embodiments are applied in the present invention to elaborate on the principle and implementation manner 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 of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

[0067] Those of ordinary skill in the art will realize that the embodiments described here are for helping readers understand the principle of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations without departing from the essence of the present invention according to the technical revelations disclosed in the present invention, and these deformations 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. Use the length of the non-balanced point interval on the grid voltage-grid connection point voltage curve as a quantitative indicator to conduct risk assessment on voltage instability; The length of the interval without equilibrium point 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.

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 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.

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 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.

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.

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