A method for evaluating equivalent inertia of asynchronous motors during inertial response phase

Abstract

This invention discloses a method for estimating the equivalent inertia of an asynchronous motor during the inertial response phase, relating to the technical field of power system inertia assessment. In recent years, my country's power grid has gradually evolved into a power system with a high proportion of power electronics. Studying the inertial response of asynchronous motors in power systems, evaluating their effective inertia at the inertial time scale, and their support for system frequency regulation are of great significance for inertia assessment in power systems with a high proportion of power electronics.

Description

Technical Field

[0001] The present invention relates to the field of power system frequency stabilization, and in particular to a method for evaluating the equivalent inertia of an asynchronous motor in an inertial response phase. Background Art

[0002] In recent years, with the massive investment in large-scale new energy, energy storage and direct current transmission projects, the number and capacity of power electronic equipment in many power grids in my country have increased rapidly, and the power grid is gradually evolving into a high-proportion power electronic power system. Compared with traditional power systems, the system inertia is insufficient, and the proportion of inertia on the load side is gradually increasing. As the main load of the power system, the inertia of the asynchronous motor cannot be ignored. Existing research has not clearly analyzed the frequency dynamic characteristics of the asynchronous motor from a mechanism point of view and has not continued to evaluate the frequency support capability of the asynchronous motor at the inertia response time scale. The present invention studies the inertial response of the asynchronous motor in the power system, evaluates the effective inertia of the asynchronous motor at the inertia time scale of the asynchronous motor, and its support role in the system frequency regulation, which is of great significance to the evaluation of the inertia of the power system and the field of power system frequency stability.

[0003] Therefore, this patent proposes to establish an asynchronous motor model under electromechanical transient conditions, derive its transfer function, and analyze the time-varying characteristics and influencing factors of the asynchronous motor's inertia presented to the power grid. Based on the idea of ​​frequency support capability during the inertia response phase, the asynchronous motor's ability to support system frequency is evaluated. This paper proposes an evaluation model for the equivalent inertia of the asynchronous motor, which quantifies the inertial support capacity of dynamic loads. The model's frequency support capability and the asynchronous motor's dynamic characteristics after frequency disturbances are analyzed. Summary of the Invention

[0004] In order to study the inertia response of asynchronous motors in power systems, evaluate the effective inertia of asynchronous motors under the inertia time scale of asynchronous motor evaluation and their supporting role in system frequency regulation, the present invention proposes a method for evaluating the equivalent inertia of asynchronous motors in the inertia response stage. First, a small signal model of the asynchronous motor under electromechanical transient conditions is established, and its transfer function related to the power consumption of the asynchronous motor and the system frequency deviation is derived. At the same time, this patent derives the effective inertia of the asynchronous motor to the power system and analyzes the time-varying characteristics of the inertia presented by the asynchronous motor to the power grid. Based on the idea of ​​frequency support capability in the inertia response stage, the factors affecting the inertia frequency response of the asynchronous motor are analyzed. An equivalent inertia evaluation model for asynchronous motors is proposed to quantitatively reflect the inertia frequency support capability of dynamic loads.

[0005] The present invention adopts the following technical solutions to solve the above technical problems:

[0006] A method for evaluating the equivalent inertia of an asynchronous motor during the inertial response phase, comprising the following steps:

[0007] Step 1: Establish a small signal model of the asynchronous motor under electromechanical transient conditions;

[0008] Step 2: Construct the transfer function between the frequency and electromagnetic power of the asynchronous motor;

[0009] Step 3: Propose an equivalent inertia evaluation model for asynchronous motors to quantify the frequency support capability;

[0010] Furthermore, the effective inertia analysis method and equivalent inertia estimation model for asynchronous motors are characterized by: in step 1, the relationship between active power and mechanical power is first solved for the asynchronous motor's electromechanical transient equivalent circuit. Using a small-signal model, the frequency support capability of the asynchronous motor during the inertial response phase is determined, and inertia is estimated, resulting in the corresponding frequency response transfer function and effective inertia.

[0011] Furthermore, the relationship between active power and mechanical power of the asynchronous motor under electromechanical transient state is characterized by the fact that the electromagnetic power P can be described by the rotor motion equation of the asynchronous motor. e and mechanical power P m The relationship between the two when power imbalance occurs is as follows:

[0012] Rotor equation:

[0013] Active power:

[0014] Mechanical power: P m =ω r k[α+(1-α)(1-s slip ) ρ ]

[0015] Where H am is the inertia constant of the asynchronous motor; ω r is the angular velocity of the asynchronous motor rotor; ΔP e is the offset electromagnetic power of the asynchronous motor; ΔP m is the offset mechanical power of the asynchronous motor; r s and x s is the equivalent resistance and leakage reactance of the stator winding; r r and x r is the equivalent resistance and leakage reactance of the rotor winding; x m is the mutual inductive reactance of the stator and rotor; s slip is the slip ratio; k is the load factor; α is the constant torque component; and ρ is an index related to the motor's load mechanical characteristics. In practical applications, k and ρ are time-varying variables. To study the frequency support capability of an asynchronous motor during inertial response, a quadratic function was used, taking ρ = 2 and k = 1.85 as an example.

[0016] Slip:

[0017] Where: ω is the angular velocity of the system; ω r is the rotor speed of the asynchronous motor.

[0018] Furthermore, the effective inertia analysis method and equivalent inertia evaluation model of the asynchronous motor are characterized by: in the above step 1, when the system frequency is subject to load disturbance and the change range is small, the frequency support capability of the asynchronous motor in the inertia response phase is solved by using a small signal model and the inertia evaluation is performed. Linearization is performed at the initial operating point, and the slip rate s is converted to slip The change in system speed ω and asynchronous motor speed ω r The change in .

[0019] Slip rate small signal model:

[0020] Where: ω r0 is the initial value of the asynchronous motor rotor speed; ω0 is the initial value of the system speed.

[0021] Similarly, at the initial working point, the active power P e The change in slip rate s slip and asynchronous motor speed ω r The change in .

[0022] Electromagnetic power small signal model: ΔP e =f es Δs slip

[0023] Mechanical power small signal model: ΔP m =f ms Δs slip +f mw Δω r

[0024]

[0025] f ms =kω r0 ρ(1-α)(1-s slip0 ) ρ-1

[0026] f mw =k[α+(1-α)(1-s slip0 ) ρ ]

[0027] Where: f es is the disturbance Δs at the initial operating point slip Corresponding electromagnetic power ΔP e The slope value; sslip0 is the initial value of the slip rate; f ms and f mw is the disturbance Δs at the initial operating point slip and Δω r Corresponding mechanical power ΔP m The slope of .

[0028] At the initial working point, linearization is performed and the following relationship can be obtained:

[0029] 2H am sΔω r =ΔP e -ΔP m

[0030] Furthermore, the effective inertia analysis method and equivalent inertia evaluation model of the asynchronous motor are characterized in that: in the step 2, the input Δω and the output ΔP can be directly solved by the Mason gain formula. e The transfer function G(s).

[0031] Transfer function:

[0032]

[0033]

[0034]

[0035] Where: K1, K2 and K3 are the parameters in the transfer function without s. According to the above formula, the effective inertia of the asynchronous motor to the power system can be obtained in the following form:

[0036] Effective inertia:

[0037] Furthermore, the effective inertia analysis method and equivalent inertia estimation model for an asynchronous motor are characterized by: in step 3, a frequency response model of the asynchronous motor is first constructed to derive a load response model in the complex frequency domain. Using the derived results, an inverse Lagrange transform is performed to solve a time-domain relationship. Considering the same frequency support capability, the equivalent inertia estimation model for the asynchronous motor is then calculated.

[0038] Furthermore, the asynchronous motor frequency response model is characterized by aggregating the rotor motion equations of all generators in the entire network into a single-machine model. Equivalent fitting is performed on the dynamic link of the prime mover-speed regulation system, and the single-machine model is used to represent the entire system, resulting in a system frequency response model (SFR). Based on the SFR model, the asynchronous motor frequency response transfer function is added, and the dynamic load of a certain node is analyzed as a single-machine model. An equivalent synchronous motor with the same frequency support capability is set to replace the original asynchronous motor.

[0039] Equivalent synchronous motor: G2(s)=T m s

[0040] System inertia:

[0041] Where: H g,i and S g,i is the inertia time constant and capacity of the i-th generator; n is the number of generators in the system; T m is the equivalent inertia of the synchronous motor, and its value should be equal to the final equivalent inertia of the asynchronous motor.

[0042] When the system load fluctuates, the transfer function through the power system becomes a frequency change, Δf. The electromagnetic power consumed by the synchronous and asynchronous motors, respectively, is shown below.

[0043]

[0044]

[0045]

[0046] Where: Δf is the frequency change of the step disturbance signal after passing through the power system transfer function; P ge is the electromagnetic power consumed by the synchronous motor; P ae It is the electromagnetic power consumed by the asynchronous motor.

[0047] Furthermore, the Laplace inverse transform is performed to solve the relationship in the time domain, which is characterized in that after the Laplace inverse transform, the electromagnetic power of the motor after the load disturbance is specifically as follows.

[0048] Synchronous motor power:

[0049] Asynchronous motor power:

[0050]

[0051] To evaluate the frequency support capability of an asynchronous motor's inertia, it's necessary to determine its equivalent inertia. Within the inertia response time, when a synchronous motor and an asynchronous motor are subjected to the same system frequency disturbance, the inertia of an asynchronous motor that exhibits the same frequency support capability can be equivalent to the inertia of a synchronous motor with the same characteristics.

[0052] Perform time domain integration at the same inertia response time.

[0053] Equivalent inertia:

[0054]

[0055] Where: t g is the inertia response time.

[0056] When two motors with the same equivalent inertia are subjected to the same load disturbance of the system, under the inertia time scale, before the intervention of a frequency modulation, the energy fed back to the system by the two motors is the same, and the change in the system frequency is the same.

[0057] Compared with the existing technology, the above technical solution adopted in this paper has the following beneficial effects:

[0058] 1. This invention proposes a small-signal model for asynchronous motors when the system frequency is subject to load disturbances and the frequency variation range is relatively small. This model can reflect the mechanism of asynchronous motors under small frequency disturbances and provides a new approach to asynchronous motor modeling.

[0059] 2. The present invention proposes an effective inertia evaluation method for asynchronous motors. This model can effectively reflect the frequency support capability of asynchronous motors and effectively express the time-varying inertia characteristics of asynchronous motors during the inertial response stage, which is of great significance to the frequency security of power systems.

[0060] 3. The present invention proposes an equivalent inertia evaluation model for asynchronous motors, which can effectively reflect the effective inertia size of asynchronous motors and can reasonably quantify and evaluate the support capability of asynchronous motors for system frequency, which is of great significance for identifying the frequency recovery and stabilization stage of power systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 A specific flow chart of the method of the present invention

[0062] Figure 2 The equivalent circuit of the asynchronous motor under mechanical transient state

[0063] Figure 3 The equivalent circuit of an asynchronous motor

[0064] Figure 4 This is the block diagram of the asynchronous motor small signal model

[0065] Figure 5 The system frequency response model considering asynchronous motor load

[0066] Figure 6 is the time curve of the system frequency change

[0067] Figure 7 The time curve of the electromagnetic power change of the asynchronous motor

[0068] Figure 8 The effective inertia curve of the asynchronous motor in the inertia response stage

[0069] Figure 9 System frequency change curve

[0070] Figure 10 PSASP three-machine nine-node simulation model

[0071] Figure 11 Comparison of PSASP simulation and asynchronous motor frequency response model curves

[0072] Figure 12 Frequency curve for asynchronous motor and static load DETAILED DESCRIPTION

[0073] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings:

[0074] The present invention may be implemented in many different forms and should not be considered limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0075] The present invention discloses a method for evaluating the equivalent inertia of an asynchronous motor during the inertial response phase, which is characterized by comprising the following steps:

[0076] Step 1: Establish a small signal model of the asynchronous motor under electromechanical transient conditions;

[0077] Step 2: Construct the transfer function between the frequency and electromagnetic power of the asynchronous motor;

[0078] Step 3: Propose an equivalent inertia evaluation model for asynchronous motors to quantify the frequency support capability;

[0079] Furthermore, the effective inertia analysis method and equivalent inertia estimation model for asynchronous motors are characterized by: in step 1, the relationship between active power and mechanical power is first solved for the asynchronous motor's electromechanical transient equivalent circuit. Using a small-signal model, the frequency support capability of the asynchronous motor during the inertial response phase is determined, and inertia is estimated, resulting in the corresponding frequency response transfer function and effective inertia.

[0080] Furthermore, the relationship between active power and mechanical power of the asynchronous motor under electromechanical transient state is characterized by the fact that the electromagnetic power P can be described by the rotor motion equation of the asynchronous motor. e and mechanical power P m The relationship between the two when power imbalance occurs is as follows:

[0081] Rotor equation:

[0082] Active power:

[0083] Mechanical power: P m =ω r k[α+(1-α)(1-s slip ) ρ ]

[0084] Where H am is the inertia constant of the asynchronous motor; ω r is the angular velocity of the asynchronous motor rotor; ΔP e is the offset electromagnetic power of the asynchronous motor; ΔP m is the offset mechanical power of the asynchronous motor; r s and x s is the equivalent resistance and leakage reactance of the stator winding; r r and x r is the equivalent resistance and leakage reactance of the rotor winding; x m is the mutual inductive reactance of the stator and rotor; s slip is the slip ratio; k is the load factor; α is the constant torque component; and ρ is an index related to the motor's load mechanical characteristics. In practical applications, k and ρ are time-varying variables. To study the frequency support capability of an asynchronous motor during its inertial response, a quadratic function is used, taking ρ = 2 and k = 1.85 as an example.

[0085] Slip:

[0086] Where: ω is the angular velocity of the system; ω r is the rotor speed of the asynchronous motor.

[0087] Furthermore, the effective inertia analysis method and equivalent inertia evaluation model of the asynchronous motor are characterized by: in the above step 1, when the system frequency is subject to load disturbance and the change range is small, the frequency support capability of the asynchronous motor in the inertia response phase is solved by using a small signal model and the inertia evaluation is performed. Linearization is performed at the initial operating point, and the slip rate s is converted to slip The change in system speed ω and asynchronous motor speed ω r The change in .

[0088] Slip rate small signal model:

[0089] Where: ω r0 is the initial value of the asynchronous motor rotor speed; ω0 is the initial value of the system speed.

[0090] Similarly, at the initial working point, the active power P e The change in slip rate s slip and asynchronous motor speed ω r The change in .

[0091] Electromagnetic power small signal model: ΔP e =f es Δs slip

[0092] Mechanical power small signal model: ΔP m =f ms Δs slip +f mw Δω r

[0093]

[0094] f ms =kω r0 ρ(1-α)(1-s slip0 ) ρ-1

[0095] f mw =k[α+(1-α)(1-s slip0 ) ρ ]

[0096] Where: f es is the disturbance Δs at the initial operating point slip Corresponding electromagnetic power ΔP e The slope value; s slip0 is the initial value of the slip rate; f ms and f mw is the disturbance Δs at the initial operating point slip and Δω r Corresponding mechanical power ΔPm The slope of .

[0097] At the initial working point, linearization is performed and the following relationship can be obtained:

[0098] 2H am sΔω r =ΔP e -ΔP m

[0099] Furthermore, the effective inertia analysis method and equivalent inertia evaluation model of the asynchronous motor are characterized in that: in the step 2, the input Δω and the output ΔP can be directly solved by the Mason gain formula. e The transfer function G(s).

[0100] Transfer function:

[0101]

[0102]

[0103]

[0104] Where: K1, K2 and K3 are the parameters in the transfer function without s. According to the above formula, the effective inertia of the asynchronous motor to the power system can be obtained in the following form:

[0105] Effective inertia:

[0106] Furthermore, the effective inertia analysis method and equivalent inertia estimation model for an asynchronous motor are characterized by: in step 3, a frequency response model of the asynchronous motor is first constructed to derive a load response model in the complex frequency domain. Using the derived results, an inverse Lagrange transform is performed to solve a time-domain relationship. Considering the same frequency support capability, the equivalent inertia estimation model for the asynchronous motor is then calculated.

[0107] Furthermore, the asynchronous motor frequency response model is characterized by aggregating the rotor motion equations of all generators in the entire network into a single-machine model. Equivalent fitting is performed on the dynamic link of the prime mover-speed regulation system, and the single-machine model is used to represent the entire system, resulting in a system frequency response model (SFR). Based on the SFR model, the asynchronous motor frequency response transfer function is added, and the dynamic load of a certain node is analyzed as a single-machine model. An equivalent synchronous motor with the same frequency support capability is set to replace the original asynchronous motor.

[0108] Equivalent synchronous motor: G2(s)=T ms

[0109] System inertia:

[0110] Where: H g,i and S g,i is the inertia time constant and capacity of the i-th generator; n is the number of generators in the system; T m is the equivalent inertia of the synchronous motor, and its value should be equal to the final equivalent inertia of the asynchronous motor.

[0111] When the system load fluctuates, the transfer function through the power system becomes a frequency change, Δf. The electromagnetic power consumed by the synchronous and asynchronous motors, respectively, is shown below.

[0112]

[0113]

[0114]

[0115] Where: Δf is the frequency change of the step disturbance signal after passing through the power system transfer function; P ge is the electromagnetic power consumed by the synchronous motor; P ae It is the electromagnetic power consumed by the asynchronous motor.

[0116] Furthermore, the Laplace inverse transform is performed to solve the relationship in the time domain, which is characterized in that after the Laplace inverse transform, the electromagnetic power of the motor after the load disturbance is specifically as follows.

[0117] Synchronous motor power:

[0118] Asynchronous motor power:

[0119]

[0120] To evaluate the frequency support capability of an asynchronous motor's inertia, it's necessary to determine its equivalent inertia. Within the inertia response time, when a synchronous motor and an asynchronous motor are subjected to the same system frequency disturbance, the inertia of an asynchronous motor that exhibits the same frequency support capability can be equivalent to the inertia of a synchronous motor with the same characteristics.

[0121] Perform time domain integration at the same inertia response time.

[0122] Equivalent inertia:

[0123]

[0124] Where: t g is the inertia response time.

[0125] When two motors with the same equivalent inertia are subjected to the same load disturbance of the system, under the inertia time scale, before the intervention of a frequency modulation, the energy fed back to the system by the two motors is the same, and the change in the system frequency is the same.

[0126] Implementation Cases

[0127] On the MATLAB / Simulink simulation platform, we built Figure 3 To verify the proposed effective inertia of the asynchronous motor and its changes after system frequency disturbances, the asynchronous motor must be connected to an equivalent power system. The asynchronous motor's parameters are detailed in Table 1 below. For ease of calculation, all parameters in this model are expressed in per-unit values. Furthermore, to verify the actual power system frequency response curve, the frequency support capability of the proposed asynchronous motor was verified using the WSCC9V7 model on the PSASP simulation platform.

[0128] Table 1 Asynchronous motor parameters

[0129]

[0130] Working condition 1: Inertia verification and analysis

[0131] According to the above, in the system model, M = 6.02s, D = 0.8. To verify the frequency response process of the asynchronous motor, the system does not apply a frequency modulation measure. The output is the change in frequency Δf and the change in asynchronous motor power ΔP e , in steady state, Δf=0 and ΔP e =0.

[0132] At 10s in the simulation experiment, a step signal ΔP is applied to the system. fh =0.005pu. By monitoring the change in system frequency Δf and the electromagnetic power deviation ΔP of the asynchronous motor e , to detect the effect of asynchronous motor on system frequency support. The results are as follows Figure 6 and Figure 7 shown.

[0133] Based on the above experiments, Figure 7 It shows that when the system load increases and there is no new external power supply or power output increase, the system frequency decreases and finally stabilizes at 49.896Hz. Figure 7The curve shows that as the system frequency decreases, the electromagnetic power of the asynchronous motor decreases. This further proves that as the system frequency decreases, the electromagnetic power of the asynchronous motor decreases, reducing the generator load, providing an inertial response to the system, and supporting the regulation of the system frequency.

[0134] When the system experiences active power imbalance disturbance, each generator set shares the disturbance power instantaneously according to the size of inertia. Generally, the primary frequency regulation is involved after 3s. In this paper, the inertia response time t g In the inertia response stage, the effective inertia of the asynchronous motor is as follows: Figure 8 As shown in the figure, as the system frequency decreases, the system slip rate responds first, and the effective inertia is small. After that, the kinetic energy stored in the asynchronous motor rotor is gradually released, and the effective inertia of the asynchronous motor gradually increases.

[0135] Case 2: Equivalent inertia evaluation

[0136] Based on the above system parameters, the equivalent inertia T of the asynchronous motor can be derived: m =3.6204. After derivation and calculation, it is proved that the inertia of the asynchronous motor to the system is different from its own inertia. At the moment of 0s, a ΔP is added to the system. fh = 0.003 pu disturbance, observe the inertia response time t g =3s system frequency change.

[0137] from Figure 9 The curve shows that the actual frequency variation curve of the asynchronous motor has the same trend as the frequency variation curve of the synchronous motor with the equivalent inertia of the asynchronous motor proposed above. And under the same inertia time, the frequency variation is consistent. g =3s, the frequency change Δf is 0.038Hz. The simulation results verify the effectiveness of the equivalent inertia evaluation model.

[0138] Case 3: Complex case verification

[0139] To verify the accuracy of the inertia response curve obtained in the simulation model above, a simulation system based on PSASP was built in this paper, and the generator used a fifth-order model. The asynchronous motor parameters were based on the data in Table 1, and the total inertia of the three-machine system was calculated to be M = 6.02. In the steady-state case, the system Δf = 0 and ΔP e = 0. At t = 0, a ΔP is applied to the system load bus M1. e =0.02pu disturbance, and observe the frequency changes of the busbars at the system load nodes M2 and M3. To verify the frequency response of the asynchronous motor, the system does not apply primary frequency modulation measures.

[0140] Figure 11The PSASP system frequency curve is compared with the frequency curve obtained based on the asynchronous motor frequency response model constructed above. It can be seen that the waveforms and steady-state frequency deviations of the two models are basically consistent, verifying the accuracy of the proposed model.

[0141] Table 2 Nadir points of each load frequency curve

[0142]

[0143] Figure 12 The loads carried by the power system are static loads and asynchronous motor loads with the same power. Compared with the above experiment, in order to be more in line with the actual design, the system adds a speed regulator, a voltage regulator and a PSS device, and applies ΔP e = 0.05pu disturbance. From Table 2 and Figure 12 The curve shows that asynchronous motors with inertia are more effective in preventing frequency fluctuations. The nadir point of the system frequency curve is later, and the power system frequency fluctuation is significantly smaller than that of static loads. This effectively prevents system frequency fluctuations and provides support for the power system frequency. Furthermore, due to the inertia's ability to prevent frequency fluctuations, system frequency recovery is slower than that of static loads without inertia.

[0144] The above embodiments are only for illustrating the technical ideas of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made on the basis of the technical solutions of the present invention in accordance with the technical ideas proposed by the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for evaluating the equivalent inertia of an asynchronous motor during the inertial response phase, characterized in that: The steps include: Step 1: Establish a small signal model of the asynchronous motor under electromechanical transient conditions; Step 2: Construct the transfer function between the frequency and active power of the asynchronous motor; Step 3: Propose an equivalent inertia evaluation model for asynchronous motors to quantitatively evaluate the asynchronous motor's ability to support system frequency. In step 1, the relationship between active power and mechanical power is first solved for the equivalent circuit of the asynchronous motor under electromechanical transient state; the frequency support capability of the asynchronous motor in the inertial response phase is solved using a small signal model, and the inertia is evaluated, and the frequency response transfer function and effective inertia of the response are obtained; the active power P can be described by the rotor motion equation of the asynchronous motor e and mechanical power P m The relationship between the two when power imbalance occurs is as follows: Rotor equation: Active power: Mechanical power: P m =ω r k[α+(1-α)(1-s slip ) ρ ] Where H am is the inertia constant of the asynchronous motor; ω r is the angular velocity of the asynchronous motor rotor; ΔP e is the offset active power of the asynchronous motor; ΔP m is the offset mechanical power of the asynchronous motor; r s and x s is the equivalent resistance and leakage reactance of the stator winding; r r and x r is the equivalent resistance and leakage reactance of the rotor winding; x m is the mutual inductive reactance of the stator and rotor; s slip is the slip ratio; k is the load factor; α is the constant torque part; ρ is the load mechanical characteristic index of the motor; In practical applications, k and ρ are time variables; U t Indicates terminal voltage; Slip ratio: Where: ω is the angular velocity of the system; ω r is the rotor speed of the asynchronous motor; In step 1, when the system frequency is subject to load disturbance and the change range is small, the frequency support capability of the asynchronous motor in the inertial response phase is solved by using the small signal model and the inertia evaluation is performed; linearization is performed at the initial operating point, and the slip rate s is converted to slip The change in system speed ω and asynchronous motor speed ω r The change in Slip rate small signal model: Where: ω r0 is the initial value of the asynchronous motor rotor speed; ω0 is the initial value of the system speed; Δs slip Indicates the change in slip rate; Δω r Indicates the change in the angular velocity of the motor rotor; Δω indicates the change in the angular velocity of the system; similarly, at the initial working point, the active power P e The change in slip rate s slip and asynchronous motor speed ω r The change in Active power small signal model: ΔP e =f es Δs slip Mechanical power small signal model: ΔP m =f ms Δs slip +f mw Δω r f ms =kω r0 p(1-a)(1-s) slip0 ) ρ-1 f mw =k[α+(1-α)(1-s slip0 ) ρ ] Where: f es is the disturbance Δs at the initial operating point slip Corresponding active power ΔP e The slope value; s slip0 is the initial value of the slip rate; f ms and f mw is the disturbance Δs at the initial operating point slip and Δω r Corresponding mechanical power ΔP m The slope of At the initial working point, linearization is performed and the following relationship can be obtained: 2H am sΔω r =ΔP e -ΔP m ; In step 2, the input Δω and output ΔP can be directly solved by the Mersenne gain formula. e The transfer function G(s); Transfer function: Where s is the Laplace transform operator, representing a complex frequency domain variable; K1, K2, and K3 are parameters in the transfer function excluding s. Based on the above formula, the effective inertia of the asynchronous motor to the power system can be obtained as follows: Effective inertia:

2. The method for estimating equivalent inertia of an asynchronous motor in the inertial response phase according to claim 1, characterized in that: In step 3, the frequency response model of the asynchronous motor is first constructed to derive the response model of the load in the complex frequency domain. Using the known derived results, the relationship in the time domain is solved through the inverse Lagrange transform. Considering the same frequency support capability, the equivalent inertia evaluation model of the asynchronous motor is solved.

3. The method for estimating equivalent inertia of an asynchronous motor in the inertial response phase according to claim 1, characterized in that: The rotor motion equations of all generators in the entire network are aggregated into a single-machine model. Equivalent fitting is performed on the dynamic link of the prime mover-speed regulation system. The single-machine model is used to represent the entire system, resulting in a system frequency response model (SFR). Based on the SFR model, the frequency response transfer function of the asynchronous motor is added, and the dynamic load of a certain node is analyzed as a single-machine model. An equivalent synchronous motor with the same frequency support capability is set to replace the original asynchronous motor. Equivalent synchronous motor: G2(s)=T m s System inertia: Where: H g,i and S g,i is the inertia time constant and capacity of the i-th generator; n is the number of generators in the system; T m is the equivalent inertia of the synchronous motor, and its value should be equal to the equivalent inertia of the asynchronous motor ultimately required; When the system load fluctuates, the transfer function of the power system is converted into a frequency change Δf. Δf passes through the synchronous motor and the asynchronous motor, and the active power consumed by the system is as follows: Where: Δf is the frequency change of the step disturbance signal after passing through the power system transfer function; P ge is the active power consumed by the synchronous motor; P ae is the active power consumed by the asynchronous motor; D is the system damping coefficient.

4. The method for estimating the equivalent inertia of an asynchronous motor in the inertial response phase according to claim 2, wherein the relationship in the time domain is solved after the Laszlo inverse transform, and is characterized in that: After the inverse Laplace transform, the active power of the motor after the load disturbance is as follows: Synchronous motor power: Asynchronous motor power: Where: P ge P is the electromagnetic power and active power consumed by the synchronous motor; ae is the electromagnetic power and active power consumed by the asynchronous motor; D is the system damping coefficient; K1, K2 and K3 are the parameters in the transfer function without s; M is the system inertia; T m is the equivalent inertia of the synchronous motor; H am is the effective inertia; in order to evaluate the frequency support capability of the asynchronous motor inertia, the equivalent inertia of the asynchronous motor needs to be calculated; in order to evaluate the frequency support capability of the asynchronous motor inertia, the equivalent inertia of the asynchronous motor needs to be calculated; within the inertia response time, when the synchronous motor and the asynchronous motor are subjected to the same system frequency disturbance, the inertia of the asynchronous motor that exhibits the same frequency support capability to the system can be equivalent to the inertia of the synchronous motor with the same characteristics; Under the same inertia response time, perform time domain integration, Equivalent inertia: Where: t g is the inertia response time; When two motors with the same equivalent inertia are subjected to the same load disturbance of the system, under the inertia time scale, before the intervention of a frequency modulation, the energy fed back to the system by the two motors is the same, and the change in the system frequency is the same.

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