A method for quantitatively analyzing transient damping energy of a grid-connected converter hybrid system

Abstract

The application discloses a kind of follow-construction converter hybrid system transient damping energy quantitative analysis method.For the challenge that the transient damping effect of hybrid system relative phase angle movement is difficult to accurately characterize and analytically quantitatively analyze, by introducing relative phase angle δ pg -Relative speed ω pg Phase plane diagram will be the damping effect energy generated by relative motion be described in corresponding trajectory integral in phase plane, by introducing relative phase angle δ pg -D pi Construction network converter speed ω g Phase plane diagram, the damping energy generated by construction network type converter speed is described in corresponding phase plane area;And respectively through δ pg -ω pg Approximation in phase plane trajectory and δ pg -D pi ω g Approximation in phase plane rectangular area respectively estimates the energy consumed by two kinds of damping effects, quantitatively describes the influence of two kinds of damping energy in transient process, realizes the critical damping dissipation energy quantification of follow-network type converter-construction network type converter hybrid system and fault critical clearing time calculation considering damping effect.

Description

Technical Field

[0001] The present 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 converter synchronization strategies: grid-following control and grid-forming control. Grid-following control achieves grid synchronization through a phase-locked loop (PLL) structure, enabling precise power injection and exhibiting the characteristics of a controlled current source. Grid-forming converters achieve grid connection through power synchronization, enabling autonomous voltage and frequency support and exhibiting the characteristics of a controlled voltage source. In future power systems with a higher proportion of renewable energy, both grid-following and grid-forming converters will coexist in large numbers for a long time, jointly influencing the behavioral characteristics and stability boundaries of future power systems.

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

[0004] The traditional quantitative analysis method is mainly the energy function method. This method can provide an intuitive and quantitative analysis of the conversion process between the kinetic energy of each 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 the transient damping energy of a parallel-parallel converter system, which includes the following steps:

[0006] 1) Based on the circuit structure and impedance value of the parallel-parallel system, the output current of the grid-following converter, and the terminal voltage of the grid-forming converter, the expressions for the synchronous voltage of the grid-following converter and the output active power of the grid-forming converter are obtained;

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

[0008] 3) According to the speed related term of the relative angle transient motion equation of the hybrid system, expressions of the network type damping power and the network configuration type damping power in the hybrid system are obtained;

[0009] 4) The network type damping power and the network configuration type damping power in the relative angle transient motion are integrated with respect to the relative angle, and an energy expression of the damping power generated in the transient process of the hybrid system is obtained;

[0010] 5) The network configuration type transient damping energy is visually displayed in the phase plane diagram of the relative angle-network configuration speed, and then the network configuration type transient damping energy is mapped to the area surrounded by the phase trajectory and the horizontal axis in the phase plane diagram;

[0011] 6) The critical value of the network configuration type damping energy is obtained according to the rectangular area determined by the state point at the fault clearing time and the unstable equilibrium point of the relative angle transient motion system, which is used to approximate the area surrounded in step 5) corresponding to the critical stable trajectory;

[0012] 7) The projection of the network type transient critical stable trajectory in the phase plane diagram of the relative angle-relative speed is approximated;

[0013] For the transient trajectory projected in the negative network damping interval of the phase plane diagram, an equivalent second-order network system is introduced, the critical stable boundary of the equivalent second-order network system in the phase plane diagram is used for approximation, the network type damping power is integrated along the approximate trajectory, the network type damping energy corresponding to the critical stable trajectory is obtained, and according to 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 negative network damping interval.

[0014] For the transient trajectory projected in the positive network damping interval of the phase plane diagram, a line segment connecting the state point at the fault clearing time, the intersection point of the approximate boundary of the negative damping region and the zero damping plane is used for trajectory approximation; the network type damping power is integrated along the approximate trajectory, the network type damping energy corresponding to the critical stable trajectory is obtained, and according to 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 network damping interval.

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

[0016] (1) The present application adopts a customized phase plane, and directly displays the network configuration type damping effect in the relative motion of the network type converter-network configuration type converter hybrid system in the customized phase plane, overcoming the defect that the existing transient analysis tool is difficult to directly analyze the damping dissipation energy.

[0017] (2) The present invention adopts a damping energy approximation scheme based on rectangular approximation for grid-type transient critical damping and a damping energy approximation scheme based on phase plane critical stability trajectory approximation for grid-type transient critical damping, thereby realizing the analytical solution of the transient stability critical fault clearing point of the grid-type converter-grid-type converter hybrid system taking into account the complex damping effect. It has the advantages of fast solution speed and high accuracy, and can realize the online calculation of the critical fault clearing time of the hybrid system. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0019] Figure 2 (a) is a schematic diagram of the visualization of the network-type transient damping energy;

[0020] Figure 2 (b) is the diagram of the approximate scheme of the network-type damping energy;

[0021] Figure 3 This is an approximate scheme diagram for grid-type transient damping energy. DETAILED DESCRIPTION

[0022] To explain the present invention in more detail, the present invention is further described in detail below by taking a 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 short-circuit fault of the load as an example, in combination with the accompanying drawings and embodiments.

[0023] Figure 1 The following figure is a schematic diagram of a hybrid system with two converters. The grid-following converter achieves synchronization with the grid through a 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 output voltage phase. The output current of the grid-following converter is Among them I dref , I qref is the active current reference and reactive current reference output by the grid-following converter, θ p is the phase-locked loop output angle of the grid-following converter, and its transient motion equation is as follows:

[0024]

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

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

[0027]

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

[0029] also, Figure 1 The structure of the hybrid circuit is also given, where Z1, Z2, Z L They 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.

[0030] In combination with the above-mentioned parallel-parallel system, this embodiment proposes a transient damping energy quantitative analysis method, which is described in detail below.

[0031] The present invention's method for quantitatively analyzing transient damping energy in a hybrid system of a grid-following converter and a grid-forming converter addresses two types of damping effects (defined as grid-following damping and grid-forming damping) in the transient relative power angle motion equation of the hybrid system. It introduces two types of phase plane diagrams to analyze the energy consumed by the two types of damping torques separately. It also proposes an approximate transient damping energy method for these two types of damping effects, quantitatively characterizing the impact of transient damping energy. Furthermore, this method is combined with traditional energy function methods to quantitatively characterize the transient stability boundary of the hybrid system, taking into account complex damping effects, and to quantitatively calculate the critical fault clearing time.

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

[0033] Step 1: Based on the circuit structure and impedance value of the parallel-parallel system, the output current of the grid-following converter, and the terminal voltage of the grid-forming converter, the expression for the synchronous voltage of the grid-following converter and the expression for the output active power of the grid-forming converter are obtained;

[0034] This embodiment characterizes the synchronous voltage q-axis quadrature component U of the grid-type converter based on the output current I1 of the grid-type converter, the terminal voltage E2 of the grid-type converter, and the impedance in the circuit. 1q The output active power P of the grid-connected 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. Indicates the operation of taking the imaginary part of a complex number; U1 is the synchronous voltage of the grid-following converter, that is, 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] Wherein, 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, which is expressed 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 is the equivalent reference power and equivalent output power of the grid-type converter, and its expression is:

[0047]

[0048] D0 and D1 are the two damping coefficients of the damping term related to relative velocity, respectively. peq is the damping coefficient of the damping term related to the speed of the grid-type converter, and its specific expression is:

[0049]

[0050] Step 3: Based on the velocity-related terms of the transient motion equation of the hybrid system relative to the power angle, the expressions of the grid-following damping power and the grid-forming damping power in the hybrid system are obtained; where P is defined as dp and P dg are the grid-following damping power and grid-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 grid-forming converter is The relative power angle at the moment of fault clearing is The relative speed is against Figure 1 In the 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 When the transient total energy V of the parallel system is defined pf (δ pg ,ω pg ), V pf (δ pg ,ω pg ) can be written as:

[0054]

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

[0056]

[0057] Substitute the corresponding relative power angle δ pgand 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

[0058] Step 4: Integrate the grid-type damping power and the network-type damping power in the relative power angle transient motion 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;

[0059] The two damping torques P in the relative motion dp and P dg Relative power angle δ pg By integrating, we can get the damping torque work expression of the hybrid system transient process as follows, and define W dp and W dg It is the following network damping energy and the building network damping energy in the hybrid system.

[0060]

[0061] Step 5: Visualize the network-type transient damping energy in the phase plane diagram of relative power angle-network velocity; 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;

[0062] According to W in the above expression dg Item, customized vertical axis is ω g D peq times, the horizontal axis is δ pg Phase plane diagram, and then the power angle from Change to The energy consumed by the network-type damping torque in the process is mapped to the area enclosed by the phase trajectory and the horizontal axis in the angle range. The consumed network-type damping energy is the inverse of the enclosed area, such as Figure 2 As shown in (a).

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

[0064] The critical value of the network-type positive damping energy consumed after the fault is cleared is use and The area of ​​the enclosed rectangle S dga approximate like Figure 2 (b)dga The expression is:

[0065]

[0066] in For fault clearing critical relative power angle and critical grid-type converter speed, is the steady-state relative power angle at the unstable equilibrium point of the hybrid system.

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

[0068] a) For the transient trajectory projection that falls within 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. The grid damping power is integrated along the approximate trajectory to obtain the grid damping energy corresponding to the critical stability trajectory. Based on the energy conservation relationship of the system after fault clearance, the fault clearance point corresponding to the transient critical stability of the hybrid system is obtained when the fault clearance point falls within the negative grid damping interval. The details are as follows:

[0069] According to W in this expression dp The equivalent second-order network system is introduced as follows:

[0070]

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

[0072]

[0073] 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 Damping energy consumed in the process, E p 、E k are the potential energy and kinetic energy of the equivalent second-order following network system respectively.

[0074] According to the nonlinear motion equation of the second-order system, the corresponding energy term E p 、E k and The expression is:

[0075]

[0076] For the power angle interval where the damping coefficient of the grid-following damping power is negative (grid-following negative damping coefficient interval), the damping energy of the equivalent second-order grid-following system is used. Critical value of grid-type damping energy in approximate series-parallel systems like Figure 3 As shown in (a). The energy relationship of the second-order grid system is obtained as follows:

[0077]

[0078] When the critical fault clearance angle is in the power angle interval where the damping coefficient of the grid-following damping power is negative (grid-following negative damping coefficient interval), the system transient critical stability fault clearance point 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] Based on this, the critical stability boundary of the hybrid system in the negative damping power angle range can be obtained, and it can be expressed in the relative power angle phase plane δ pg -ω pg The projection in is defined as

[0083] b) For the transient trajectory projection that falls within the positive damping interval of the grid in the phase plane diagram, the trajectory is approximated by a line segment connecting the state point at the fault clearing moment, the approximate boundary of the negative damping region, and the intersection of the zero damping plane. The grid damping power is integrated along the approximate trajectory to obtain the grid damping energy corresponding to the critical stability trajectory. Based on the energy conservation relationship of the system after fault clearing, the fault clearing point corresponding to the transient critical stability of the hybrid system is obtained when the fault clearing point falls within the positive grid damping interval. The details are as follows:

[0084] For the power angle interval where the damping coefficient of the grid-following damping power is positive (grid-following positive damping coefficient interval), the following formula is used: arrive The line segment approximates the hybrid system fault cleared at the power angle from Move to δ zd The critical stability trajectory of Figure 3(b) is shown, and its expression is:

[0085]

[0086] where δ zd is the power angle value corresponding to the damping coefficient of the grid-type 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), the grid-following damping energy in the positive damping power angle interval is obtained based on the approximate trajectory integral, and its critical value is:

[0089]

[0090] in:

[0091]

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

[0093]

[0094] 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:

[0095]

[0096] The present invention is for Figure 1 The grid-following converter-grid-forming converter hybrid system shown in the figure is verified using the following per-unit parameter settings: The reference current I dref +jI qref =1.50+j0.00p.u., the reference voltage amplitude of the grid-type converter is U ref The rated value is 1.0pu, the line impedance is Z1=0.01+j0.05p.u., Z2=0.02+j0.20p.u., the equivalent load impedance Z L The proportional coefficient K of the grid-type converter phase-locked loop is 0.30 pu. p and the integral coefficient K iThe equivalent damping coefficient and inertia time constant of the grid-forming converter are 20 p.u. and 5 s respectively, and the active power reference value of the grid-forming converter is 0.70 p.u.

[0097] The fault critical clearing time is used as an index to quantitatively measure the accuracy of different critical stability criteria. The actual critical fault clearing time t R is 298 ms. 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 that the approximate result of the traditional method is no longer conservative and cannot be used as a reference for fault clearing time. According to the critical stability criterion obtained by the proposed method by approximately considering the transient damping energy after fault clearing, the critical fault clearing time t D is 288 ms, which is close to the real fault clearing time and conservatively estimates the influence of the two types of damping on transient stability. Therefore, the 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 transient damping energy quantitative analysis method.

[0099] The above description of the embodiments is for the purpose of facilitating the understanding and application of the present application by those skilled in the art. 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 inventive labor. Therefore, the present application is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art to the present application according to the disclosure of the present application should be within the scope of protection of the present application, such as considering reactive power control in the control structure shown in Figure 1 , analyzing with different fault types, etc. Therefore, the claims are intended to cover all variations within the true concept and scope of the present application.

Claims

1. A method for quantitative analysis of transient damping energy of a parallel-parallel converter system, characterized in that: The following steps are involved: 1) Based on the circuit structure and impedance value of the parallel-parallel system, the output current of the grid-following converter, and the terminal voltage of the grid-forming converter, the expressions for the synchronous voltage of the grid-following converter and the output active power of the grid-forming converter are obtained; 2) Based on the expression in 1), the transient power angle motion equations of the grid-following converter and the grid-forming converter are simultaneously derived to obtain the relative power angle transient motion equation of the hybrid system; 3) Obtain the expressions for the grid-following damping power and the grid-forming damping power in the parallel-parallel system; 4) Integrate the network damping power and the network-structured damping power in the relative power angle transient motion with respect to the relative power angle to obtain the energy expression for 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; 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) using the rectangular area determined by the state point at the moment of fault clearing and the unstable equilibrium point of the relative power angle transient motion system to approximate the area enclosed in step 5) corresponding to the critical stable trajectory, and obtaining the critical value of the network-type damping energy based on the rectangular area; 7) Approximate the projection of the critical stability trajectory on the phase plane diagram of relative power angle-relative velocity, 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 within 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. The grid damping power is integrated along the approximate trajectory to obtain the grid 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 within the negative grid damping interval. For the transient trajectory projection falling within the positive damping interval of the grid in the phase plane diagram, the trajectory is approximated by a line segment connecting the state point at the fault clearing moment, the approximate boundary of the negative damping region, and the intersection of the zero damping plane; the grid damping power is integrated along the approximate trajectory to obtain the grid damping energy corresponding to the critical stability trajectory; based on the energy conservation relationship of the system after fault clearing, the fault clearance point corresponding to the transient critical stability of the hybrid system is obtained when the fault clearance point falls within the positive grid damping interval.

2. The method for quantitative analysis of transient damping energy of a parallel-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. Indicates the operation of taking the imaginary part of a complex number; U1 is the synchronous voltage of the grid-following converter, that is, its port voltage, U 1q 、 are the q-axis quadrature 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-following 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-type converter is: Among them, I2 is the current vector output by the grid-type converter, * is the conjugate symbol, and U ref 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 parallel-parallel converter system according to claim 2, 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-forming 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 parallel-parallel converter system according to claim 3 is characterized in that: In step 3), in the parallel-parallel system, the grid-type damping power expression is: The expression of network damping power is: P dg =-D peq (oh g -oh gu ) P dp and P dg are the grid-following damping power and grid-forming damping power in the hybrid system.

5. The method for quantitative analysis of transient damping energy of a parallel-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 are 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 transient process d , the expression is: W dp and W dg It is the following network damping energy and the building network damping energy in the hybrid system.

6. The method for quantitative analysis of transient damping energy of a parallel-parallel converter system according to claim 5, 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, customized vertical axis is ω g D peq times, the horizontal axis is δ pg Phase plane diagram, and then the relative power angle from 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 the phase plane diagram, 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 parallel-parallel converter system according to claim 6, 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 S dga approximate S dga The expression is: in is the fault clearance critical relative power angle and critical grid-type 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 parallel-parallel converter system according to claim 7, characterized in that: Step 7) include: 7.1) According to the W in the expression of damping torque work in the transient process of the hybrid system, 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 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 approximate series-parallel systems The energy relationship of the equivalent second-order grid system is obtained as follows: 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 within the negative damping coefficient interval of the grid, the system transient critical stability fault clearance point 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 range of the grid, use arrive The line segment approximates the hybrid system fault cleared at the power angle from Move to δ 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 range of the grid, the grid-type damping energy in the positive damping power angle range 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 clearing point of the hybrid system is directly obtained according to the following algebraic equation when the critical fault clearing point is in the positive grid damping region:

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