Method for determining fault clearing angle of new energy grid-connected system based on improved equal-area rule

Abstract

The application provides a method for determining a fault removal angle of a new energy grid-connected system based on an improved equal-area rule, comprising the following steps: determining a topological structure of the new energy grid-connected system, and determining a transient model of the new energy grid-connected system based on the topological structure of the new energy grid-connected system; constructing an equal-area model based on the transient model of the new energy grid-connected system; determining the fault removal angle of the new energy grid-connected system based on the equal-area model; and considering the negative damping effect of the new energy grid-connected system based on the improved equal-area rule, so that the fault removal angle can be accurately determined, and accurate data support can be provided for the fault removal of the new energy grid-connected system and the stability analysis of the system.

Description

Technical Field

[0001] This invention relates to a method for determining power grid parameters, and more particularly to a method for determining the fault clearing angle of a renewable energy grid-connected system based on an improved equal area rule. Background Technology

[0002] The installed capacity of renewable energy sources such as photovoltaics and wind power has been increasing year by year, leading to changes in the structure and dynamic characteristics of the power system.

[0003] The equal-area rule and Lyapunov's direct method are used to analyze the transient stability of grid-connected renewable energy converter systems. The higher-order nonlinear characteristics of renewable energy converters increase the difficulty of constructing Lyapunov functions, especially in multi-machine interconnected systems. Therefore, the equal-area rule is frequently used to analyze the transient stability of grid-connected renewable energy converter systems.

[0004] When a transient instability fault occurs in a new energy grid-connected system, fault clearing is achieved through the fault clearing angle. The fault clearing angle is a key indicator of whether the system can return to a steady state after fault clearing and a key indicator of system stability margin. Currently, the critical value of the fault clearing angle is determined based on the equal area rule. However, the fault clearing angle determined by the equal area rule in the existing technology only considers the positive damping interval and the scaling and simplification of the damping term, and weakens the negative damping effect through parameter optimization. This is because the existing technology cannot solve how to determine the impact of the negative damping effect on the acceleration and deceleration area. However, due to the influence of the phase-locked loop and the method of only considering the positive damping region, the fault clearing angle cannot meet the needs of actual new energy grid-connected systems.

[0005] Therefore, in order to solve the above-mentioned technical problems, it is urgent to propose a new technical approach. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a method for determining the fault clearing angle of a new energy grid-connected system based on an improved equal area rule. Based on the improved equal area rule, the negative damping effect of the new energy grid-connected system is also taken into account. A work model is constructed for the negative damping effect before and after the critical fault clearing angle. This method makes up for the fact that the fault clearing angle calculation results are too conservative or too optimistic due to the unclear boundary of the influence of the negative damping effect on the acceleration and deceleration area. Thus, a more accurate fault clearing angle is calculated within the actual transient stability boundary, providing accurate data support for the fault clearing and stability analysis of the new energy grid-connected system.

[0007] This invention provides a method for determining the fault clearing angle of a renewable energy grid-connected system based on an improved equal-area rule, comprising:

[0008] Determine the topology of the new energy grid-connected system, and determine the transient model of the new energy grid-connected system based on the topology;

[0009] Constructing an equal-area model based on the transient model of a new energy grid-connected system;

[0010] The fault clearing angle of the new energy grid-connected system is determined based on the equal area model.

[0011] Furthermore, the transient model of the new energy grid-connected system is specifically as follows:

[0012] Determine the q-axis component of the grid connection point voltage of the new energy grid-connected system. :

[0013] (1);

[0014] Determine the output phase angle of the PI controller in the new energy grid-connected system. :

[0015] (2);

[0016] Determine the phase difference between the generator terminal voltage and the grid voltage of the new energy unit. :

[0017] (3);

[0018] Substituting equations (1) and (3) into equation (2) and differentiating with respect to t, we obtain the transient model:

[0019] (4);

[0020] in:

[0021] (5);

[0022] Where: J represents the equivalent inertial time constant of the new energy grid-connected system, T m T represents the equivalent mechanical torque of a new energy grid-connected system. e—dur D represents the equivalent electromagnetic torque of a new energy grid-connected system. dur U represents the equivalent damping of the new energy grid-connected system. g Indicates the grid voltage, i dref and i qref These represent the d-axis current reference value and the q-axis current reference value in the coordinate system, respectively; ω PLL The output voltage angular frequency of the new energy unit is represented by m, and the transformer turns ratio of the new energy grid-connected system is represented by L. g R represents the mains inductance. g Represents the grid resistance, ω gk represents the angular frequency of the grid voltage. pPLL and k iPLL These are the proportional and integral coefficients of the PI controller for the phase-locked loop, i. td i represents the d-axis component of the output current of the new energy grid-connected system. tq Ugn represents the q-axis component of the output current of the new energy grid-connected system, and Ugn represents the grid voltage amplitude during non-fault periods.

[0023] Furthermore, the equal area model is as follows:

[0024] The equal area includes the acceleration area and the deceleration area, wherein:

[0025] The accelerated area model is as follows:

[0026] (6)

[0027] The deceleration area model is as follows:

[0028] (7);

[0029] Acceleration area and maximum angular frequency deviation The relational model is as follows: (8);

[0030] The energy conservation equation is: (9)

[0031] (10);

[0032] Substituting equations (6), (7), (8), and (10) into equation (9), and using the Newton-Raphson method for iterative calculation, the critical fault clearing angle δ is obtained. c and δ d ;

[0033] Where: WD_ represents the work done by the equivalent negative damping, δ max This indicates the maximum phase difference between the output voltage of the new energy unit and the grid voltage. This represents the reference value of the d-axis component of the output current of the new energy unit during the fault period. Indicates the voltage amplitude of the power grid during the fault. This indicates the initial phase difference between the output voltage of the new energy unit and the grid voltage at the moment the fault occurs. δ represents the reference value of the d-axis component of the output current of the new energy unit during non-fault periods. d This represents the phase difference between the voltage of the new energy generating unit and the grid voltage when the equivalent damping is 0.

[0034] Furthermore, the equivalent negative damping work WD_ is determined by the following method:

[0035] (11);

[0036] in: This indicates the angular frequency deviation between the output voltage of the new energy grid-connected system and the grid voltage.

[0037] Furthermore, the phase difference δ between the new energy unit and the grid voltage when the equivalent damping is 0 is determined using the following method. d :

[0038] When δ0 < δ d <δ c When the equivalent damping is zero, the phase difference δ d for:

[0039] (12);

[0040] When δ c <δ<δ max When the equivalent damping is zero, the phase difference δ d for:

[0041] (13).

[0042] The beneficial effects of this invention are as follows: Based on the improved equal area rule, this invention also considers the negative damping effect of the new energy grid-connected system, and constructs a work model for the negative damping effect before and after the critical fault clearing angle. This makes up for the overly conservative or overly optimistic fault clearing angle calculation results caused by the unclear boundary of the negative damping effect's influence on the acceleration and deceleration area. Thus, a more accurate fault clearing angle is calculated within the actual transient stability boundary, providing accurate data support for fault clearing and system stability analysis of the new energy grid-connected system. Attached Figure Description

[0043] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0044] Figure 1 This is a schematic diagram of a new energy grid-connected system.

[0045] Figure 2 This is a control topology diagram for a new energy generating unit.

[0046] Figure 3 A schematic diagram considering the influence of damping effect and fault clearing angle distribution characteristics on acceleration and deceleration area.

[0047] Figure 4 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0048] The present invention will be further described in detail below:

[0049] This invention provides a method for determining the fault clearing angle of a renewable energy grid-connected system based on an improved equal-area rule, comprising:

[0050] Determine the topology of the new energy grid-connected system, and determine the transient model of the new energy grid-connected system based on the topology;

[0051] Constructing an equal-area model based on the transient model of a new energy grid-connected system;

[0052] The fault clearing angle of the new energy grid-connected system is determined based on the equal area model.

[0053] Specifically: among them, Figure 1 and Figure 2 The diagram showcases the grid connection system structure of the new energy power plant and the control topology of the new energy generating units. The new energy generating units are connected to the grid via a transformer substation and a combiner substation. Among them, i dc and u dc These represent the DC-side current and voltage, respectively. tabc and u gabc These are the grid connection point voltage and the power supply voltage, respectively. tabc This is the grid-connected current. abc and i abc These represent the terminal voltage and output current of the grid-connected new energy power generation unit, respectively. C1 is the DC capacitor, L1 is the filter inductor, and L... g For the mains inductance, R g This represents the grid resistance.

[0054] The control strategies for new energy generating units include PLL control, outer loop control, and current loop control. The input to PLL control is the q-axis component u of the generator terminal voltage of the new energy generating unit. q After PI control, the output angular frequency deviation ∆ω is further integrated to output the phase angle θ. PLL This provides phase angle information for the Park transformation of voltage and current. The output current i of the new energy unit... abc The dq-axis current components i are obtained after Park transformation. dq0 Combined with the command value i of the dq-axis current component of the outer loop control input dqref0 The command value u of the dq-axis current component of the output terminal voltage. cdqref u cdqref The switching devices are controlled by a PWM modulation stage.

[0055] The transient model of the new energy grid-connected system is as follows:

[0056] Determine the q-axis component of the grid connection point voltage of the new energy grid-connected system. :

[0057] (1);

[0058] Determine the output phase angle of the PI controller in the new energy grid-connected system. :

[0059] (2);

[0060] Determine the phase difference between the generator terminal voltage and the grid voltage of the new energy unit. :

[0061] (3);

[0062] Substituting equations (1) and (3) into equation (2) and differentiating with respect to t, we obtain the transient model:

[0063] (4);

[0064] in:

[0065] (5);

[0066] Where: J represents the equivalent inertial time constant of the new energy grid-connected system, T m T represents the equivalent mechanical torque of a new energy grid-connected system. e—dur This represents the equivalent electromagnetic torque of a new energy grid-connected system, that is: T e Including the equivalent electromagnetic torque T during the fault e—dur The equivalent electromagnetic torque during fault periods and in non-fault periods is calculated using the same formula, derived from U. g The value of U determines whether the fault occurs or not. g The values ​​are different, D dur This represents the equivalent damping during a fault in the renewable energy grid-connected system, i.e., the equivalent damping during the fault period, and its situation is similar to T. e Similarly, the calculation formulas for both fault periods and non-fault periods use D. dur The calculation formula only applies to U during fault periods and non-fault periods. g and i dref Different values ​​for U g Indicates the grid voltage, i dref and i qref These represent the d-axis current reference value and the q-axis current reference value in the coordinate system, respectively; ω PLL The output voltage angular frequency of the new energy unit is represented by m, and the transformer turns ratio of the new energy grid-connected system is represented by L. g R represents the mains inductance. g Represents the grid resistance, ω g k represents the angular frequency of the grid voltage. pPLL and kiPLL These are the proportional and integral coefficients of the PI controller for the phase-locked loop, i. td i represents the d-axis component of the output current of the new energy grid-connected system. tq The q-axis component of the output current of the renewable energy grid-connected system is represented by Ugn, which represents the grid voltage amplitude during non-fault periods, and i dref and i qref These represent the d-axis current reference value and the q-axis current reference value in the coordinate system, respectively. Figure 2 As shown, the two current reference values ​​are determined by the outer loop control during fault periods and non-fault periods, which is the prior art.

[0067] From formulas (4) and (5), it can be seen that when the equivalent damping D < 0, it can be considered that the equivalent mechanical torque T increases, which in turn leads to an increase in the equivalent mechanical power work and deteriorates the transient stability of the grid-connected renewable energy power station system. Therefore, when using the equal area rule to analyze the transient stability of the grid-connected renewable energy power station system, the influence of equivalent negative damping must be considered.

[0068] Therefore, the equal area model is specifically as follows:

[0069] The equal area includes the acceleration area and the deceleration area, wherein:

[0070] The accelerated area model is as follows:

[0071] (6)

[0072] The deceleration area model is as follows:

[0073] ;

[0074] Depend on Figure 3 It can be seen that, First increase then decrease, and at phase angle δ c Maximum angular frequency deviation occurs In [δ0, δ c Within this region, the work done by the system's equivalent mechanical torque is converted into equivalent kinetic energy. Therefore, the deviation between the acceleration area and the maximum angular frequency can be determined. The relational model is as follows: (8);

[0075] The energy conservation equation is: (9)

[0076] (10);

[0077] Substituting equations (6), (7), (8), and (10) into equation (9), and using the Newton-Raphson method for iterative calculation, the critical fault clearing angle δ is obtained. cand δ d ;

[0078] Where: WD_ represents the work done by the equivalent negative damping, δ max This indicates the maximum phase difference between the output voltage of the new energy unit and the grid voltage. This represents the reference value of the d-axis component of the output current of the new energy unit during the fault period. Indicates the voltage amplitude of the power grid during the fault. This indicates the initial phase difference between the output voltage of the new energy unit and the grid voltage at the moment the fault occurs. δ represents the reference value of the d-axis component of the output current of the new energy unit during non-fault periods. d This represents the phase difference between the voltage of the renewable energy unit and the grid voltage when the equivalent damping is 0. Figure 3 It can be seen that when δ c >δ d At this time, the equivalent negative damping region appears during both the fault persistence and fault recovery phases; that is, the work done by the equivalent negative damping affects not only the equivalent acceleration area but also the equivalent deceleration area. When δ c <δ d At this time, the equivalent negative damping region appears in the fault recovery stage, that is, the work done by the equivalent negative damping only affects the equivalent deceleration area.

[0079] The equivalent negative damping work WD_ is determined by the following method:

[0080] (11);

[0081] in: This indicates the angular frequency deviation between the output voltage of the new energy grid-connected system and the grid voltage.

[0082] The phase difference δ between the new energy generating unit and the grid voltage when the equivalent damping is 0 is determined by the following method. d :

[0083] When δ0 < δ d <δ c When the equivalent damping is zero, the phase difference δ d for:

[0084] (12);

[0085] When δ c <δ d <δ max When the equivalent damping is zero, the phase difference δ d for:

[0086] (13); in [δ0, δ d ] is the equivalent positive damping, in [δ d, δ max [This represents the equivalent negative damping.] δ cannot be directly determined during calculation. d and δ c Therefore, we can first assume δ d <δ c Therefore, at this time, δ d Formula (12) is selected, the acceleration / deceleration area is selected from the lower part of formulas (6) and (7), and the calculation formula for WD_ is selected from the upper part of formula (11). After substituting these formulas into formula (9), the Newton-Raphson method is used for iterative calculation to determine δ. d and δ c At this time, δ d and δ c Compare, if δ is not satisfied d <δ c， This indicates that the currently selected calculation formula is inappropriate, so we choose δ. d >δ c The formula for the corresponding case repeats the above operations to determine the final δ. d and δ c Therefore, based on the improved equal area rule, the negative damping effect of the new energy grid-connected system is also taken into account. A work model is constructed for the negative damping effect before and after the critical fault clearing angle. This makes up for the overly conservative or overly optimistic fault clearing angle calculation results caused by the unclear boundary of the negative damping effect on the acceleration and deceleration area. Thus, a more accurate fault clearing angle is calculated within the actual transient stability boundary, providing accurate data support for fault clearing and stability analysis of the new energy grid-connected system.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for determining the fault clearing angle of a renewable energy grid-connected system based on an improved equal-area rule, characterized in that: include: Determine the topology of the new energy grid-connected system, and determine the transient model of the new energy grid-connected system based on the topology; Constructing an equal-area model based on the transient model of a new energy grid-connected system; The fault clearing angle of the new energy grid-connected system is determined based on the equal area model.

2. The method for determining the fault clearing angle of a new energy grid-connected system based on the improved equal area rule according to claim 1, characterized in that: The transient model of the new energy grid-connected system is as follows: Determine the q-axis component of the grid connection point voltage of the new energy grid-connected system. : (1)； Determine the output phase angle of the PI controller in the new energy grid-connected system. : (2)； Determine the phase difference between the generator terminal voltage and the grid voltage of the new energy unit. : (3)； Substituting equations (1) and (3) into equation (2) and differentiating with respect to t, we obtain the transient model: (4)； in: (5)； Where: J represents the equivalent inertial time constant of the new energy grid-connected system, T m T represents the equivalent mechanical torque of a new energy grid-connected system. e—dur D represents the equivalent electromagnetic torque of a new energy grid-connected system. dur U represents the equivalent damping of the new energy grid-connected system. g Indicates the grid voltage, i dref and i qref These represent the d-axis current reference value and the q-axis current reference value in the coordinate system, respectively; ω PLL The output voltage angular frequency of the new energy unit is represented by m, and the transformer turns ratio of the new energy grid-connected system is represented by L. g R represents the mains inductance. g Represents the grid resistance, ω g k represents the angular frequency of the grid voltage. pPLL and k iPLL These are the proportional and integral coefficients of the PI controller for the phase-locked loop, i. td i represents the d-axis component of the output current of the new energy grid-connected system. tq Ugn represents the q-axis component of the output current of the new energy grid-connected system, and Ugn represents the grid voltage amplitude during non-fault periods.

3. The method for determining the fault clearing angle of a new energy grid-connected system based on the improved equal area rule according to claim 2, characterized in that: The equal area model is as follows: The equal area includes the acceleration area and the deceleration area, wherein: The accelerated area model is as follows: (6) The deceleration area model is as follows: (7)； Acceleration area and maximum angular frequency deviation The relational model is as follows: (8); The energy conservation equation is: (9) （10）； Substituting equations (6), (7), (8), and (10) into equation (9), and using the Newton-Raphson method for iterative calculation, the critical fault clearing angle δ is obtained. c and δ d ; Where: WD_ represents the work done by the equivalent negative damping, δ max This indicates the maximum phase difference between the output voltage of the new energy unit and the grid voltage. This represents the reference value of the d-axis component of the output current of the new energy unit during the fault period. Indicates the voltage amplitude of the power grid during the fault. This indicates the initial phase difference between the output voltage of the new energy unit and the grid voltage at the moment the fault occurs. δ represents the reference value of the d-axis component of the output current of the new energy unit during non-fault periods. d This represents the phase difference between the voltage of the new energy generating unit and the grid voltage when the equivalent damping is 0.

4. The method for determining the fault clearing angle of a new energy grid-connected system based on the improved equal area rule according to claim 3, characterized in that: The equivalent negative damping work WD_ is determined by the following method: (11)； in: This indicates the angular frequency deviation between the output voltage of the new energy grid-connected system and the grid voltage.

5. The method for determining the fault clearing angle of a new energy grid-connected system based on the improved equal area rule according to claim 3, characterized in that: The phase difference δ between the new energy generating unit and the grid voltage when the equivalent damping is 0 is determined by the following method. d : When δ0 < δ d < δ c When the equivalent damping is zero, the phase difference δ d for: (12)； When δ c <δ< δ max When the equivalent damping is zero, the phase difference δ d for: (13)。

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