Quantitative analysis method for transient damping energy of heel-structure converter series-parallel system

Abstract

The invention discloses a quantitative analysis method for transient damping energy of a heel-structure converter series-parallel system. Aiming at the challenge that the transient damping effect of the relative power angle motion of the hybrid system is difficult to accurately characterize and analyze and quantitatively analyze, damping effect energy generated by the relative motion is described by a corresponding track integral in a phase plane by introducing a relative power angle delta pg-relative speed omega pg phase plane graph; the method comprises the following steps: by introducing a relative power angle deltapg-Dpi to multiply a phase plane graph of the speed omegag of a network-constructing converter, describing damping energy generated by the speed of the network-constructing converter by the area of a corresponding phase plane; energy consumed by the two types of damping effects is estimated through trajectory approximation in a delta pg-omega pg phase plane and rectangular area approximation in a delta pg-Dpi omega g phase plane, and influences of the two types of damping energy in the transient process are quantitatively described; the critical damping dissipated energy quantification and the fault critical clearing time calculation of the grid following type converter-grid constructing type converter series-parallel system considering the damping effect are realized.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic control, and in particular relates to a method for quantitatively analyzing transient damping energy of a parallel-parallel converter system. Background Art

[0002] There are two main types of grid-connected synchronization strategies for grid-connected converters: grid-following control and grid-building control. The grid-following control strategy achieves synchronization with the grid through a phase-locked loop structure, which can achieve precise power injection and exhibits a controlled current source characteristic. The grid-building converter achieves grid-connected function through power synchronization, which can achieve autonomous voltage and frequency support characteristics and exhibits a controlled voltage source characteristic. In future power systems with a higher proportion of new energy, grid-following converters and grid-building converters will coexist in large numbers for a long time, jointly affecting the behavioral characteristics and stability boundaries of future power systems.

[0003] However, unlike synchronous generators, the transient motion characteristics of grid-following converters and grid-forming converters are determined by their control strategies, which show complex damping effects. The phase-locked loop structure in the grid-following converter will introduce a cosine-varying damping power, and the equivalent large damping effect in the grid-forming converter will introduce a large dissipated damping power, which in turn leads to energy changes related to the transient motion trajectory. In addition, in the hybrid system of grid-following converter and grid-forming converter, due to the different grid-connection algorithms of the two converters, the relative motion description between the converters under transient conditions is affected by the two types of grid-connection algorithms, making the influence mechanism of the damping effect on the relative motion of the parallel system unclear, which brings huge challenges to the transient synchronous stability analysis and system relay protection setting of the hybrid system.

[0004] The traditional quantitative analysis method is mainly the energy function method, which can conduct an intuitive and quantitative analysis of the conversion process between the kinetic energy of each machine rotor and the potential energy of the parallel system. However, it is difficult to consider the energy change effect of the damping torque on the relative motion, resulting in unclear transient stability boundaries of the hybrid system and the inability to quickly calculate the critical fault clearing time of the hybrid system. Summary of the invention

[0005] In order to quantitatively analyze the transient damping effect of a parallel-parallel system, the present invention proposes a method for quantitatively analyzing transient damping energy of a parallel-parallel converter system, which includes the following steps:

[0006] 1) According to the circuit structure and impedance value of the hybrid system, the output current of the grid-following converter and the terminal voltage of the grid-forming converter, the synchronous voltage expression of the grid-following converter and the output active power expression of the grid-forming converter are obtained;

[0007] 2) According to the expression in 1), the transient power angle motion equations of the grid-following converter and the grid-forming converter are combined to obtain the relative power angle transient motion equation of the hybrid system;

[0008] 3) Based on the velocity-related terms of the relative power angle transient motion equation of the hybrid series-parallel system, obtain the expressions of the grid-connected damping power and the grid-forming damping power in the hybrid series-parallel system;

[0009] 4) Integrate the grid-connected damping power and the grid-forming damping power in the relative power angle transient motion with respect to the relative power angle to obtain the energy expression generated by the damping power during the transient process of the hybrid series-parallel system;

[0010] 5) Visualize the grid-forming transient damping energy in the phase plane of the relative power angle - grid-forming speed; furthermore, map the grid-forming transient damping energy to the area enclosed by the phase trajectory and the horizontal axis in the phase plane;

[0011] 6) Use the rectangular area determined by the state point at the fault clearing moment and the unstable equilibrium point of the relative power angle transient motion system to approximately represent the enclosed area in step 5) corresponding to the critical stable trajectory, and obtain the critical value of the grid-forming damping energy according to the rectangular area;

[0012] 7) Approximate the projection of the grid-connected transient critical stable trajectory in the phase plane of the relative power angle - relative speed;

[0013] Among them, for the transient trajectory whose projection in the phase plane falls in the grid-connected negative damping interval, introduce an equivalent second-order grid-connected system, use the critical stable boundary of the equivalent second-order grid-connected system in the phase plane for approximation, integrate the grid-connected damping power along the approximate trajectory, obtain the grid-connected damping energy corresponding to the critical stable trajectory, and according to the energy conservation relationship of the system after the fault clearing, obtain the fault clearing point corresponding to the transient critical stability of the hybrid series-parallel system when the fault clearing point falls in the negative grid-connected damping interval;

[0014] For the projection of the transient trajectory falling in the grid-connected positive damping interval in the phase plane, use the line segment connecting the state point at the fault clearing moment, the intersection point of the approximate boundary of the negative damping area and the zero damping plane for trajectory approximation; integrate the grid-connected damping power along the approximate trajectory, obtain the grid-connected damping energy corresponding to the critical stable trajectory; according to the energy conservation relationship of the system after the fault clearing, obtain the fault clearing point corresponding to the transient critical stability of the hybrid series-parallel system when the fault clearing point falls in the positive grid-connected damping interval.

[0015] Based on the above technical solutions, the present invention has the following beneficial technical effects:

[0016] (1) The present invention adopts a customized phase plane and visually displays the grid-forming damping effect existing in the relative motion of the grid-connected converter - grid-forming converter hybrid system in the customized phase plane, overcoming the defect that it is difficult for existing transient analysis tools to visually analyze the damping dissipation energy.

[0017] (2) The present invention adopts a damping energy approximation scheme based on rectangular approximation for the grid-forming transient critical damping and a damping energy approximation scheme based on the approximate critical stable trajectory of the phase plane for the grid-following transient critical damping, realizing the analytical solution of the transient stability critical fault clearing point of the grid-following converter-grid-forming converter hybrid system considering complex damping effects. It has the advantages of fast solving speed and high accuracy, and can realize the online calculation of the critical fault clearing time of the hybrid system. Description of the Drawings

[0018] Figure 1 It is a schematic diagram of the grid-following control structure, the grid-forming control structure and the circuit structure of the grid-following and grid-forming converter hybrid system;

[0019] (a)(b)

[0020] Figure 2 It is a schematic diagram of the visualization of the grid-forming transient damping energy and the grid-forming damping energy approximation scheme diagram;

[0021] Figure 3 It is a grid-following transient damping energy approximation scheme diagram. Detailed Embodiment

[0022] To explain the present invention in more detail, the following takes the hybrid system of a grid-following converter with constant current control and a grid-forming converter with constant terminal voltage control under a three-phase ground short-circuit fault of the load as an example, and further elaborates on the present invention in combination with the drawings and embodiments.

[0023] Figure 1 It is a schematic diagram of the hybrid system with two converters. The grid-following converter realizes synchronization with the power grid through the phase-locked loop structure and adjusts the active power output by adjusting the output current; the grid-forming converter adjusts the active power output by adjusting the phase of the output voltage. The output current of the grid-following converter is where I dref , I qref are the reference active current and the reference reactive current output by the grid-following converter, and θ p is the output angle of the phase-locked loop of the grid-following converter, and its transient motion equation is as follows:

[0024]

[0025] where ω p is the speed of the grid-following converter, and K i , K p are the integral coefficient and the proportional coefficient of the phase-locked loop respectively, and U 1q , They are respectively the q-axis quadrature component of the grid-following converter PLL-detected grid-connected point voltage of the converter (hereinafter referred to as the synchronous voltage of the grid-following converter) and its derivative with respect to time.

[0026] The amplitude of the terminal voltage E2 of the grid-forming converter is its reference value U ref , and the phase is θ g . The power angle swing equation of the grid-forming converter is as follows:

[0027]

[0028] Among them, ω g is the speed of the grid-forming converter, and P0, P e2 , D p and H respectively represent the reference value of the active power, the active power output, the damping coefficient, and the virtual inertia time constant of the grid-forming converter.

[0029] In addition, Figure 1 the structure of the hybrid circuit is also given, where Z1, Z2, Z L are respectively the line impedance of the grid-following converter connecting the load, the line impedance of the grid-forming converter connecting the load, and the load impedance.

[0030] Combined with the above hybrid system, this embodiment proposes a transient damping energy quantitative analysis method, which will be described in detail below.

[0031] The transient damping energy quantitative analysis method for the grid-following converter-grid-forming converter hybrid system of the present invention aims at two types of damping effects (respectively defined as the grid-following damping effect and the grid-forming damping effect) in the transient relative power angle motion equation of the hybrid system, introduces two types of phase plane diagrams, realizes the separate analysis of the energy consumed by the two types of damping torques, and proposes an approximate method for the transient damping energy of the two types of damping effects, quantitatively characterizing the influence of the transient damping energy. Further, this method is linked with the traditional energy function method to realize the quantitative characterization of the transient stability boundary of the hybrid system considering complex damping effects and the quantitative calculation of the critical clearing time of the fault.

[0032] The method of this embodiment is implemented according to the following steps:

[0033] Step 1: Obtain the synchronous voltage expression of the grid-following converter and the output active power expression of the grid-forming converter according to the circuit structure and impedance values of the hybrid system, the output current of the grid-following converter, and the terminal voltage of the grid-forming converter;

[0034] This embodiment represents the q-axis quadrature component U of the synchronous voltage of the grid-following converter and the output active power P of the grid-forming converter based on the output current I1 of the grid-following converter, the terminal voltage E2 of the grid-forming converter, and the impedance in the circuit 1q of the grid-following convertere2 For the q-axis quadrature component U of the synchronous voltage of the grid-following converter 1q and its time derivative It can be written as:

[0035]

[0036] in, It represents the operation of taking the real part of a complex number. represents the operation of taking the imaginary part of a complex number; U1 is the synchronous voltage of the grid-following converter, i.e., its port voltage, δ pg =θ p -θ g is the relative power angle between the grid-following converter and the grid-forming converter, R 1eq , X 1eq 、U2、 The intermediate physical quantity used to simplify the expression definition is as follows:

[0037]

[0038] According to the circuit relationship of the hybrid system, the output active power of the grid-connected converter can also be expressed by the output current of the grid-connected converter, the port voltage of the grid-connected converter and the circuit impedance. The expression is:

[0039]

[0040] Among them, I2 is the current vector output by the grid-type converter, and * is the conjugate symbol.

[0041] Step 2: Combine the transient power angle motion equations of the grid-following converter and the grid-forming converter to obtain the relative power angle transient motion equation of the hybrid system; Step 2 is as follows:

[0042] By combining the transient power angle motion equations of the grid-following converter and the grid-forming converter, the relative power angle transient motion equation of the grid-following converter-grid-forming converter hybrid system is obtained, and its expression is as follows:

[0043]

[0044] Among them, ω pg =ω p -ω g is the relative speed between the grid-following converter and the grid-forming converter; ω gu is the steady-state velocity of the system at the unstable equilibrium point, P m1eq , P e1eq is the equivalent reference power and equivalent output power of the grid-following converter, and its expression is:

[0045]

[0046] P m2eq 、P e2eq are the equivalent reference power and equivalent output power of the network-forming converter, and their expressions are:

[0047]

[0048] D0 and D1 are the two damping coefficients of the damping term related to the relative speed, and D peq is the damping coefficient of the damping term related to the speed of the network-forming converter, and its specific expression is:

[0049]

[0050] Step 3: Obtain the expressions of the grid-following damping power and network-forming damping power in the hybrid system according to the speed-related terms of the relative power angle transient motion equation of the hybrid system; where P dp and P dg are the grid-following damping power and network-forming damping power in the hybrid system.

[0051]

[0052] P dg =-D peq (ω g -ω gu )

[0053] When a fault occurs in the power system, the initial relative power angle of the grid-following converter and the network-forming converter is The relative power angle at the fault clearing moment is The relative speed is For the Figure 1 system, after the fault is cleared, when the relative power angle of the two converters is δ pg and the relative speed is ω pg , the transient total energy V pf (δ pg ,ω pg ) of the parallel system is defined, and V pf (δ pg ,ω pg ) can be written as:

[0054]

[0055] Among them, E0 is a constant representing the potential energy reference value. Taking the unstable equilibrium position as the zero potential energy surface, then E0 is:

[0056]

[0057] Substitute the corresponding relative power angle δ pgRelative speed ω with the rotor pg , at the fault clearing moment, the transient total energy is In the critically stable case, the relative power angle reaches the unstable equilibrium position At this point, the relative speed of the two converters is 0, so there is a critical transient total energy

[0058] Step 4: Integrate the network-type damping power and the grid-forming damping power in the relative power angle transient motion with respect to the relative power angle to obtain the energy expression generated by the damping power in the transient process of the hybrid system;

[0059] Integrate the two damping torques P dp and P dg with respect to the relative power angle δ pg to obtain the following expression for the work done by the damping torque in the transient process of the hybrid system. Define W dp and W dg as the network-following damping energy and the grid-forming damping energy in the hybrid system.

[0060]

[0061] Step 5: Visualize the grid-forming transient damping energy in the relative power angle - grid-forming speed phase plane diagram; and then map the grid-forming transient damping energy to the area enclosed by the phase trajectory and the horizontal axis in the phase plane diagram;

[0062] According to the W dg term in the above expression, customize the vertical axis to be D g times ω peq and the horizontal axis to be δ pg of the phase plane diagram. Then, map the energy consumed by the work done by the grid-forming damping torque during the change of the power angle from to to the area enclosed by the phase trajectory and the horizontal axis in this angular range. The consumed grid-forming damping energy is the opposite of the enclosed area, as shown in Figure 2 (a).

[0063] Step 6: Use the rectangular area determined by the state point at the fault clearing moment and the unstable equilibrium point of the relative power angle transient motion system to approximate the enclosed area corresponding to the critical stable trajectory in Step 5, and obtain the critical value of the grid-forming damping energy according to the rectangular area;

[0064] The critical value of the positive grid-forming damping energy consumed after the fault clearing is Use and to enclose the rectangular area S dga to approximate as shown in Figure 2 (b). Sdga The expression is as follows:

[0065]

[0066] where is the critical relative power angle for fault clearing and the critical speed of the network-forming converter, is the steady-state relative power angle at the unstable equilibrium point of the hybrid system.

[0067] Step 7: Approximate the projection of the network-following transient critical stability trajectory on the relative power angle - relative speed phase plane, and integrate the relative power angle along the approximate trajectory to obtain an approximate value of the network-following damping critical energy;

[0068] a) For the projection of the transient trajectory in the network-following negative damping interval on the phase plane, approximate it using the critical stability boundary of the equivalent second-order network-following system in the phase plane. Integrate the network-following damping power along the approximate trajectory to obtain the network-following damping energy corresponding to the critical stability trajectory. According to the energy conservation relationship of the system after fault clearing, obtain the fault clearing point corresponding to the transient critical stability of the hybrid system when the fault clearing point falls in the negative network-following damping interval, as follows:

[0069] According to the W dp term in this expression, introduce the equivalent second-order network-following system as follows:

[0070]

[0071] For the introduced equivalent second-order network-following system, the points on its stability boundary satisfy the following energy relationship:

[0072]

[0073] where is the critical stability boundary of the equivalent second-order network-following system in the δ pg -ω pg phase plane, is the intersection point of the stability boundary of the equivalent second-order network-following system and δ pg = δ0, is the damping energy consumed during the process of moving from δ0 to the unstable equilibrium point along the stability boundary of the equivalent second-order network-following system, and E p , E k are the potential energy and kinetic energy of the equivalent second-order network-following system respectively.

[0074] According to the nonlinear motion equation of the second-order system, the expressions of its corresponding energy terms E p , E k and are as follows:

[0075]

[0076] For the power angle interval with a negative damping coefficient of the grid-following damping power (grid-following negative damping coefficient interval), the damping energy of the equivalent second-order grid-following system is adopted. The critical value of the grid-following damping energy in the approximate series-parallel system As Figure 3 (a) shows. Obtained from the energy relationship of the second-order grid-following system, its expression is:

[0077]

[0078] For the case where the critical clearing angle of the fault is in the power angle interval with a negative damping coefficient of the grid-following damping power (grid-following negative damping coefficient interval), the transient critical stable fault clearing point of the system can be directly obtained according to the following algebraic equation:

[0079]

[0080] Among them, when the relative power angle of the two converters is δ pg and the relative speed is ω pg When, the transient total energy V of the parallel system is defined pf (δ pg , ω pg ):

[0081]

[0082] Accordingly, the critical stable boundary of the series-parallel system in the negative damping power angle interval can be obtained, and its projection in the relative power angle phase plane δ pg -ω pg in is defined as

[0083] b) For the projection of the transient trajectory in the positive grid-following damping interval in the phase plane diagram, a line segment connecting the state point at the fault clearing moment, the intersection point of the approximate boundary of the negative damping region and the zero damping plane is used for trajectory approximation; the grid-following damping power is integrated along the approximate trajectory to obtain the grid-following damping energy corresponding to the critical stable trajectory; according to the energy conservation relationship of the system after the fault clearing, the fault clearing point corresponding to the transient critical stability of the series-parallel system when the fault clearing point falls in the positive grid-following damping interval is obtained, specifically as follows:

[0084] For the power angle interval with a positive damping coefficient of the grid-following damping power (grid-following positive damping coefficient interval), the line segment from to is used to approximately represent the critical stable trajectory of the series-parallel system after the fault clearing when the power angle moves from to δ zd As Figure 3(as shown in (b), its expression is:

[0085]

[0086] where δ zd is the power angle value corresponding to the damping coefficient of the grid-following damping power being zero, that is:

[0087]

[0088] For the power angle interval where the damping coefficient of the grid-following damping power is positive (grid-following positive damping coefficient interval), based on the approximate trajectory integration, the grid-following damping energy in the positive damping power angle interval is obtained, and its critical value is:

[0089]

[0090] where:

[0091]

[0092] For the case where the critical fault clearing corner is in the power angle interval where the damping coefficient of the grid-following damping power is positive (grid-following positive damping coefficient interval), the critical value of the grid-following damping energy can be obtained by summing the approximate values in the negative damping power angle interval and the positive damping power angle interval, and its expression is:

[0093]

[0094] Therefore, based on the following algebraic equation, the transient critical stable fault clearing point of the hybrid system can be directly obtained in the case of the positive grid-following damping region:

[0095]

[0096] The present invention Figure 1 For the grid-following converter - grid-forming converter hybrid system shown in dref +jI qref of the grid-following converter is 1.50 + j0.00 p.u., the rated value of the reference voltage amplitude U ref of the grid-forming converter is 1.0 p.u., the line impedances are Z1 = 0.01 + j0.05 p.u. and Z2 = 0.02 + j0.20 p.u. respectively, and the equivalent load impedance Z L is 0.30 p.u. The proportional coefficient K p and the integral coefficient K iThey are 0.07 p.u. and 0.01 p.u. respectively. The equivalent damping coefficient and inertia time constant of the network-forming converter are 20 p.u. and 5 s respectively, and the active power reference value of the network-forming converter is 0.70 p.u.

[0097] Using the critical clearing time of the fault as an index can quantitatively measure the accuracy of different critical stability criteria. The actual critical fault clearing time t of the system R is 298 ms. Calculating according to the critical stability condition obtained by the traditional energy function method without considering the damping effect, the critical fault clearing time t L is 308 ms, which is greater than the actual critical fault clearing time t R , resulting in the approximation result of the traditional method not being conservative anymore and unable to be used as a reference for the fault clearing moment. While calculating according to the critical stability criterion obtained by approximately considering the transient damping energy after the fault clearing with the proposed method, the critical fault clearing time t D is 288 ms, which is close to the true fault clearing time and conservatively estimates the influence of the two types of damping effects on the transient stability. Therefore, its result can be used to guide the fault clearing of the system.

[0098] The above analysis results verify the effectiveness and advantages of the proposed quantitative analysis method for transient damping energy.

[0099] The above description of the embodiments is to enable those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments. Those skilled in the art should make improvements and modifications to the present invention within the protection scope of the present invention according to the disclosure of the present invention, such as considering reactive power control in the control structure shown in Figure 1 and analyzing with different fault types. Therefore, the claims are intended to cover all variants within the true concept and scope of the present invention.

Claims

1. A method for quantitative analysis of transient damping energy of a series-parallel converter system, characterized in that: The following steps are involved: 1) According to the circuit structure and impedance value of the hybrid system, the output current of the grid-following converter and the terminal voltage of the grid-forming converter, the synchronous voltage expression of the grid-following converter and the output active power expression of the grid-forming converter are obtained; 2) According to the expression in 1), the transient power angle motion equations of the grid-following converter and the grid-forming converter are combined to obtain the relative power angle transient motion equation of the hybrid system; 3) Obtain the expressions of the grid-following damping power and grid-forming damping power in the hybrid system; 4) Integrate the grid-type damping power and the grid-type damping power in the transient motion of the relative power angle with respect to the relative power angle to obtain the energy expression of the damping power generated in the transient process of the hybrid system; 5) Visualize the network-type transient damping energy in the phase plane diagram of relative power angle-network velocity; and then map the network-type transient damping energy to the area enclosed by the phase trajectory and the horizontal axis in the phase plane diagram; 6) The rectangular area determined by the state point at the time of fault clearing and the unstable equilibrium point of the relative power angle transient motion system is used to approximate the area enclosed in step 5) corresponding to the critical stable trajectory, and the critical value of the grid-type damping energy is obtained according to the rectangular area; 7) Approximate the projection of the critical stability trajectory on the phase plane diagram of relative power angle-relative speed, and integrate the relative power angle along the approximate trajectory to obtain the approximate value of the critical energy of the grid-type damping; Among them, for the transient trajectory projection that falls in the negative damping interval of the grid in the phase plane diagram, the critical stability boundary of the equivalent second-order grid system in the phase plane diagram is used for approximation, and the grid-type damping power is integrated along the approximate trajectory to obtain the grid-type damping energy corresponding to the critical stability trajectory. According to the energy conservation relationship of the system after the fault is cleared, the fault clearance point corresponding to the transient critical stability of the hybrid system is obtained when the fault clearance point falls in the negative grid damping interval; For the transient trajectory projection that falls in the positive damping interval of the grid in the phase plane diagram, the trajectory is approximated by using a line segment connecting the state point at the moment of fault clearing, the approximate boundary of the negative damping region and the intersection of the zero damping plane; the grid-type damping power is integrated along the approximate trajectory to obtain the grid-type damping energy corresponding to the critical stability trajectory; based on the energy conservation relationship of the system after the fault is cleared, the fault clearing point corresponding to the transient critical stability of the hybrid system is obtained when the fault clearing point falls in the positive grid damping interval.

2. The method for quantitative analysis of transient damping energy of a series-parallel converter system according to claim 1 is characterized in that: In step 1), the synchronous voltage expression of the grid-following converter is: in, It represents the operation of taking the real part of a complex number. represents the operation of taking the imaginary part of a complex number; U1 is the synchronous voltage of the grid-following converter, i.e., its port voltage, and U 1q , are the q-axis orthogonal component of the synchronous voltage of the grid-following converter and its differential with respect to time, δ pg =θ p -θ g is the relative power angle between the grid-following converter and the grid-forming converter, θ p ,ω p They are the power angle and speed of the grid-following converter, θ g is the power angle of the grid-type converter, Z1, Z2, Z L are the line impedance of the load connected to the grid-type converter, the line impedance of the load connected to the grid-type converter, and the load impedance, respectively. dref ,I qref is the active current reference and reactive current reference output by the grid-following converter, E2 is the terminal voltage vector of the grid-forming converter, R 1eq , X 1eq 、U2、 The intermediate physical quantity used to simplify the expression definition is as follows: The output active power expression of the grid-connected converter is: Where I2 is the current vector output by the grid-type converter, * is the conjugate symbol, and U ref It is the reference value of the voltage amplitude at the grid-type converter terminal.

3. The method for quantitative analysis of transient damping energy of a series-parallel converter system according to claim 2 is characterized in that: In step 2), the relative power angle transient motion equation of the hybrid system is: Among them, ω pg =ω p -ω g is the relative speed between the grid-following converter and the grid-building converter, ω g is the speed of the grid-type converter, ω gu is the steady-state velocity of the system at the unstable equilibrium point, P m1eq , P e1eq is the equivalent reference power and equivalent output power of the grid-following converter; P m2eq , P e2eq is the equivalent reference power and equivalent output power of the grid-type converter; D0 and D1 are the two damping coefficients of the relative speed related damping term, respectively. peq is the damping coefficient of the speed-related damping term of the grid-type converter.

4. The method for quantitative analysis of transient damping energy of a series-parallel converter system according to claim 3 is characterized in that: In step 3), in the hybrid system, the grid-type damping power expression is: The expression of the grid-type damping power is: P dg =-D peq (oh g -oh gu ) P dp and P dg It is the grid-following damping power and grid-building damping power in the hybrid system.

5. The method for quantitative analysis of transient damping energy of a series-parallel converter system according to claim 4 is characterized in that: The step 4) is specifically as follows: The two damping powers P in the relative power angle transient motion dp and P dg Respectively for the relative power angle δ pg Integrate to obtain the grid-following damping energy and grid-forming damping energy in the hybrid system, and thus obtain the damping torque work W of the hybrid system in the transient process: d , the expression is: W dp and W dg It is the following network damping energy and the constituting network damping energy in the hybrid system.

6. The method for quantitative analysis of transient damping energy of a series-parallel converter system according to claim 5 is characterized in that: The step 5) is: according to the W in the damping torque work expression of the transient process of the hybrid system dg Item, the custom vertical axis is ω g D peq times, the horizontal axis is δ pg The phase plane diagram of Change to The energy consumed by the work done by the network-type damping torque in the process is mapped to the area enclosed by the phase trajectory and the horizontal axis in this angle range, and the consumed network-type damping energy is the inverse of the enclosed area.

7. The method for quantitative analysis of transient damping energy of a series-parallel converter system according to claim 6 is characterized in that: The step 6) is: after the fault is cleared, the critical value of the network-type positive damping energy consumed is use and The area of ​​the enclosed rectangle is S dga approximate S dga The expression is: in is the fault clearance critical relative power angle and critical grid-connected converter speed corresponding to the critical stability of the system after the fault, is the steady-state relative power angle at the unstable equilibrium point of the hybrid system.

8. The method for quantitative analysis of transient damping energy of a series-parallel converter system according to claim 7 is characterized in that: Step 7) include: 7.1) According to the expression of damping torque work in the transient process of the hybrid system, W dp The equivalent second-order network system is introduced as follows: For the introduced equivalent second-order grid-following system, the points on its stable boundary satisfy the following energy relationship: in For the equivalent second-order grid-following system in δ pg -ω pg The critical stability boundary of the phase plane, is the stability boundary of the equivalent second-order grid-following system and δ pg = the intersection point of δ0, To move from δ0 to the unstable equilibrium point along the stable boundary of the equivalent second-order grid system The damping energy consumed in the process, E p 、E k are the potential energy and kinetic energy of the equivalent second-order grid-following system respectively; 7.2) For the negative damping coefficient range of the grid, the damping energy of the equivalent second-order grid system is used Critical value of grid-type damping energy in an approximate series-parallel system It is obtained from the energy relationship of the equivalent second-order grid-following system, and its expression is: E p 、E k are the potential energy and kinetic energy of the equivalent second-order grid-following system, respectively. They are defined in the same way as the kinetic energy and potential energy in the relative power angle transient motion, and their expressions are: 7.3) For the case where the critical fault clearance angle is in the negative damping coefficient interval of the grid, the transient critical stability fault clearance point of the system is directly obtained according to the following algebraic equation: in, When the relative power angle of the two converters is δ pg And the relative speed is ω pg When the transient total energy V of the parallel system is defined pf (δ pg ,ω pg ): E0 is a constant representing the reference value of potential energy. Taking the unstable equilibrium position as the zero potential energy surface, E0 is: Substitute the corresponding power angle δ pg and the rotor relative speed ω pg , when the fault is cleared, the total transient energy is In the critical stability case, the relative power angle reaches the unstable equilibrium position At the position where the relative speed of the two converters is 0, the critical transient total energy is Based on this, the approximate critical stability boundary of the hybrid system in the negative damping power angle range can be obtained analytically, and its relative power angle phase plane δ pg -ω pg The projection in is defined as 7.4) For the positive damping coefficient interval of the grid, use arrive The line segment approximates the hybrid system fault after the power angle changes from Move to Delta zd The critical stability trajectory of is expressed as: Among them, δ zd is the power angle value corresponding to the damping coefficient of the grid-type damping power being zero; 7.5) For the positive damping coefficient interval of the grid, the grid-type damping energy in the positive damping power angle interval is obtained based on the approximate trajectory integral, and its critical value is: Among them, the simplified expressions of parameters A1 and A2 are: 7.6) For the case where the critical fault clearance angle is in the positive damping coefficient interval of the grid, the critical value of the grid-type damping energy is obtained by summing the approximate value of the negative damping power angle interval and the approximate value of the positive damping power angle interval, and its expression is: Therefore, the transient critical stability fault clearance point of the hybrid system can be directly obtained according to the following algebraic equation when the critical fault clearance point is in the positive grid damping region:

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