Method for dividing network-constructed energy storage limiting saturation mode and quantifying transient stability boundary

Abstract

The application discloses a network-constructing energy storage limiting saturation mode division and transient stability boundary quantification method. The method corrects the analysis and evaluation errors caused by the default instantaneous current limiting in the traditional transient analysis by establishing a transient evolution model considering the saturation process. Based on the relative size relationship between the fault instantaneous apparent current and the limiting amplitude threshold, the instantaneous saturation and the delay saturation double mode are determined and divided. In combination with the standard sinusoidal reduction representation of the multi-state power-angle curve and the equal-area rule, an active power instruction transient stability boundary quantification evaluation strategy for generalized short-circuit fault scenes is proposed, which provides a core quantitative basis for active transient adaptive control under large disturbances. The application can accurately capture the transient synchronous safety interval under various fault processes and current limiting conditions, and lays a solid foundation for the safe and stable operation of grid-connected new energy power generation equipment and transient synchronous fault ride-through.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation control technology, specifically involving a method for dividing grid-type energy storage into limited saturation modes and quantifying transient stability boundaries. Background Technology

[0002] With the rapid evolution of high-proportion renewable energy and new power systems, the power grid exhibits significant characteristics of "low inertia and weak damping," resulting in a substantial decrease in the system's transient stability capability in response to external disturbances and faults. Grid-connected (GFM) converters, due to their ability to simulate the external characteristics of synchronous generators, can proactively provide mechanical inertia and voltage support similar to synchronous machines to weak power grids, and have become key equipment for maintaining the stable operation of high-proportion renewable energy systems. On the other hand, in the construction of renewable energy power systems supplying power to heavy-haul railways in desert and Gobi areas, the safety margin and disturbance resistance capability of the all-green power system are becoming increasingly critical; as the core equipment supporting this system, the precise quantification of the limiting saturation mechanism and transient stability boundary of grid-connected energy storage directly determines its grid-connection performance and operational reliability.

[0003] However, unlike synchronous machines which possess extremely high instantaneous overload thermal capacity (able to withstand 6-8 times the overcurrent), GFM converters based on power electronic devices have extremely poor overload capacity, typically only possessing 20%-40% overcurrent capability. To ensure the safety of hardware equipment, current limiters are widely used in grid-type equipment. However, the introduction of the limiter is a highly nonlinear element. When the current limiter activates, the GFM converter will switch from being controlled by a voltage source to being controlled by a controlled current source. This will cause severe distortion of its electromagnetic power-power angle characteristic curve, lowering the system's equilibrium point and even causing power angle divergence and instability.

[0004] Currently, most existing technologies for overcurrent limiting and transient stability analysis of grid-type energy storage are based on voltage dip fault analysis. For example, the literature ["A method for characterizing the current-constrained power angle characteristics of grid-type voltage source converters in transient stability analysis." IEEE Transactions on Energy Conversion, 2023, Vol. 38, No. 2, pp. 1338-1351] proposes a transient analysis method that characterizes the effect of the limiter by equivalent output impedance. This method, when analyzing the triggering process of the limiter, assumes that the converter immediately enters saturation at the moment of fault, ignoring the gradual current ramp-up process caused by the coupling between fault depth and control parameters. In actual generalized short-circuit faults, the converter often exhibits a "hysteretic saturation" characteristic, meaning that it initially accelerates on the unsaturated fault power curve in the early stages of the fault, and only enters the saturation curve after a period of ramp-up. Existing technologies neglect this hysteresis saturation dynamic transition process and the huge transient kinetic energy accumulated during the hysteresis window, resulting in limited accuracy of the derived transient stability boundary. This makes it impossible to adaptively generalize fault scenarios, severely restricting the active power support capability and safe operation of grid-based energy storage.

[0005] Furthermore, existing technologies have also focused on enhancing transient stability under current-limited conditions. For example, the literature [Analysis and Solutions to the Impact of Current Limiting Measures on the Transient Stability of Grid-Type Converters. International Journal of Power and Energy Systems, 2024, Vol. 158, No. 109919] quantitatively analyzes the impact of current limiting on the transient synchronous stability of grid-type converters and improves the stability margin during current limiting by introducing virtual active power in the control loop. However, these methods mainly focus on stability analysis and control compensation after the converter enters the current-limited state. They have not yet distinguished between instantaneous saturation and hysteretic saturation based on the relative relationship between the fault current and the limiting threshold, nor have they incorporated the current ramp-up process and kinetic energy accumulation before entering saturation into the unified quantification of the active power command boundary. Therefore, they are difficult to directly apply to stability boundary assessment under different fault depths and current-limiting thresholds.

[0006] Therefore, there is an urgent need to establish an analytical method that can clearly delineate the saturation evolution path and accurately quantify and derive the corresponding safe active power range, so as to provide a core quantitative theoretical basis for improving the ride-through performance and adaptive stabilization control of grid-type energy storage under generalized faults. Summary of the Invention

[0007] To address the challenge of uncertain transient stability boundaries caused by the complex saturation evolution process of grid-type energy storage in existing technologies, this invention provides a method for dividing the saturation mode of grid-type energy storage and quantifying the transient stability boundary. This method can accurately calculate and quantify the safe range of active power commands under different saturation modes.

[0008] A method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage includes the following steps: (1) For grid-connected energy storage systems, a transient large-signal mathematical model of the system is constructed, which includes a virtual synchronous control swing equation and a hybrid current limiting control equation containing a virtual impedance and a current limiter; when a short-circuit fault occurs at the grid connection point, the system power angle and grid voltage are collected in real time as initial state parameters. (2) Based on the transient large signal mathematical model and initial state parameters, calculate the apparent current output of the grid-type energy storage converter at the moment of fault, and according to the relative magnitude relationship between the apparent current and the current limiting threshold, divide the saturation dynamic evolution behavior of the current limiter into instantaneous saturation mode and hysteresis saturation mode. (3) Construct the power-power angle characteristic curves of the grid-type energy storage converter under different states of the current limiter, and unify them into a standard sine function form by reducing their order; using the power angle motion trajectory as a reference, use the equal area rule to quantify the active power command safety range that the grid-type energy storage converter can maintain the transient synchronous stability of the system under the instantaneous saturation mode and the hysteresis saturation mode respectively.

[0009] Furthermore, the expression for the virtual synchronous control swing equation in step (1) is as follows:

[0010]

[0011] in: J For virtual inertia, Δ ω The deviation between the internal potential angular frequency of the grid-type energy storage converter and the grid angular frequency. P ref This is an active power command. P This refers to the actual active power output of the grid-type energy storage converter. D p This is the virtual damping coefficient. δ The phase difference between the internal potential of the grid-connected energy storage converter and the grid voltage is the system power angle. ω b As the reference angular velocity, t Indicates time.

[0012] Furthermore, the expression for the hybrid current-limiting control equation in step (1) is as follows:

[0013]

[0014] in: U v This represents the equivalent transient voltage drop of the virtual impedance. R v and X v These are the set virtual resistance and virtual reactance, respectively. I This refers to the actual output current of the grid-type energy storage converter. I sat This is the current command after current limiting. I max The rate limiting threshold, The preset constant saturation current phase angle, j The imaginary unit, e It is a natural constant.

[0015] Furthermore, the calculation expression for the apparent current in step (2) is as follows:

[0016] in: I ( δ 0) is the apparent current output of the grid-connected energy storage converter at the moment of fault. δ 0 represents the system power angle at the moment of the fault.E The internal potential amplitude of the grid-type energy storage converter. V g This represents the voltage amplitude of the power grid immediately following the fault. R eq The equivalent total resistance of the main circuit of the system and R eq = R v + R g , X eq The equivalent total reactance of the main circuit of the system and X eq = X v + X g , R g and X g These are the equivalent resistance and equivalent reactance of the power grid under fault conditions, respectively.

[0017] Further, in step (2), the calculated apparent current is compared with the current limiting threshold: if the apparent current is greater than or equal to the current limiting threshold, it is determined that the grid-type energy storage converter directly enters the limiting state at the moment the fault occurs, that is, the current limiter triggers the instantaneous saturation mode; if the apparent current is less than the current limiting threshold, it is determined that the grid-type energy storage converter will first move along the unsaturated power characteristic curve under the fault state, and then enter the limiting state after the current climbs to the limiting boundary, that is, the current limiter triggers the hysteresis saturation mode.

[0018] Furthermore, in step (3), under the condition that the current limiter is not saturated, the standard sinusoidal function expression of the power-power angle characteristic curve of the grid-type energy storage converter is as follows:

[0019]

[0020] Under the saturation state of the current limiter, the standard sinusoidal function expression of the power-power angle characteristic curve of the grid-type energy storage converter is as follows:

[0021]

[0022] in: P f ( δ The power angle of the grid-type energy storage converter under unsaturated current limiter conditions corresponds to the system power angle on its power-power angle characteristic curve. δ The actual output active power, Psat ( δ The power angle of the grid-type energy storage converter under saturation conditions is the corresponding system power angle on its power-power angle characteristic curve. δ The actual output active power, P f0 , P f,m , These represent the output power offset, output power amplitude, and curve phase offset under the unsaturated state of the current limiter, respectively. P sat0 , P sat,m , These represent the output power offset, output power amplitude, and curve phase offset under saturation conditions of the current limiter, respectively. Z eq The equivalent impedance magnitude of the main circuit of the system and , θ z The equivalent impedance angle of the main circuit of the system and θ z =arctan( X eq / R eq ).

[0023] Furthermore, the equal area rule in step (3) is based on the extreme points and intersection points of the standard sine function of the power-power angle characteristic curve to calculate the acceleration area and deceleration area in the transient evolution process. The critical condition is that the maximum deceleration area that the system can provide can completely offset the accumulated acceleration kinetic energy, and the analytical solution of the stable interval is performed.

[0024] Furthermore, in step (3), in the instantaneous saturation mode, to ensure that the deceleration area of ​​the system within the same cycle is greater than the acceleration area, and to avoid positive or negative divergence of the power angle, the active power command safety range that the grid-type energy storage converter can maintain for transient synchronous stability of the system is as follows:

[0025]

[0026] in: P sat ( δ 0) represents the power angle corresponding to the power-power angle characteristic curve of the grid-type energy storage converter under the saturation state of the current limiter. δ 0 is the actual output active power.

[0027] Furthermore, in step (3), when the current limiter is in hysteresis-type saturation mode, a critical power command exists when the acceleration kinetic energy and deceleration potential energy accumulated by the system before entering the saturation state are exactly completely canceled out. P crt At this time, the safe range of active power command that the grid-type energy storage converter can maintain transient synchronization and stability of the system is as follows:

[0028]

[0029]

[0030] in: δ sat This is the critical power angle at which the system transitions from the fault curve to the saturation power curve. δ UEP The system's unstable equilibrium operating point under the saturated power characteristic curve and δ UEP =π- δ 0.

[0031] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the above-described method for classifying grid-type energy storage limiting saturation modes and quantifying transient stability boundaries.

[0032] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described method for dividing grid-type energy storage into limited saturation modes and quantizing transient stability boundaries.

[0033] Compared with the prior art, the present invention has the following beneficial technical effects: 1. This invention establishes a large-signal equivalent analysis model that considers the dynamics of the current limiter. Based on the rigorous calculation of the apparent current at the moment of the fault, the complex nonlinear saturation dynamics are reduced to two typical modes: "instantaneous" and "hysteresis". This corrects the theoretical defect in traditional analysis that only regards saturation as an instantaneous action, and provides an accurate boundary classification criterion for transient analysis under complex fault scenarios.

[0034] 2. This invention unifies the multimodal power curves under different limiting states into a sine function, and combines the power angle trajectory and the equal area rule to quantify and derive for the first time the closed algebraic formulas for the stable lower and upper limits of active power command considering the hysteresis saturation process. This reduces the risk of instability and divergence caused by traditional analysis that ignores saturation dynamics, and greatly ensures the transient safety margin of grid-type energy storage. Attached Figure Description

[0035] Picture 1This is a block diagram of the grid-connected topology of a grid-type energy storage system and its control system.

[0036] Picture 2 To generalize the transient response waveforms of different amplitude-limiting saturation modes under fault scenarios, (a) in the figure corresponds to V g =0.2pu and I max When =1.3pu, (b) corresponds to V g =0.8 pu and I max When =1.3 pu, (c) corresponds to V g =0.2pu and I max =2.4 PU.

[0037] Picture 3 A block diagram of a grid-type energy storage mathematical model that takes into account limited amplitude saturation nonlinear dynamics.

[0038] Picture 4 The diagram shows the triggering mechanism and transient evolution path of different amplitude-limited saturation modes. In the diagram, (a) corresponds to the instantaneous saturation mode and (b) corresponds to the hysteresis saturation mode.

[0039] Picture 5 This diagram illustrates the quantitative analysis of the equal-area rule for transient stability boundary under instantaneous saturation mode and the derivation of the active power command boundary. Figure (a) corresponds to... P ref > P sat ( δ In case 0), (b) corresponds to P sat0 < P ref ≤ P sat ( δ In case 0).

[0040] Picture 6 This diagram illustrates the quantitative analysis of the equal-area rule for transient stability boundary under hysteresis-type saturation mode and the derivation of the active power command boundary. Figure (a) corresponds to... P ref = P crt In the case of (b), P ref > P crt In this case, (c) corresponds to P f ( δ 0) <P ref < P crt In this case, (d) corresponds to P f0 < P ref ≤ P f ( δ In case 0). Detailed Implementation

[0041] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] This embodiment takes a GFM converter with a rated power of 1.5MW, a rated voltage of 690V, and a rated frequency of 50Hz as the research object. The main circuit is connected to the grid through an LC filter. Its specific operation and control loop parameters are shown in Table 1. Table 1

[0043] like Picture 1 As shown, the grid-connected energy storage system in this embodiment consists of a DC-side energy storage unit, a three-phase voltage source converter, an LC filter, a grid connection line, and a control system. Picture 1 middle V dc This is the DC bus voltage. L f and C f These are the filter inductor and the filter capacitor, respectively. i abc For the three-phase current on the AC side of the converter, u pabc The voltage at the grid connection point is the three-phase voltage; the grid uses Thevenin equivalents, where... R g and X g These are the equivalent resistance and reactance of the power grid, respectively. V g ∠ θ g This is the equivalent voltage vector of the power grid. The power calculation process is based on... u pabc and i abc Calculate active power P and reactive power Q The VSG (Virtual Synchronous Generator) synchronization control system operates based on active power commands. P ref Actual active power P Virtual dampingD p and virtual inertia J The frequency deviation Δ is obtained ω angular frequency ω and internal phase θ VSG and define the system power angle δ = θ VSG - θ g The reactive power-voltage control circuit operates according to the reactive power command. Q ref Actual reactive power Q Sag coefficient K q and rated potential E n Generate voltage amplitude command E The voltage outer loop generates a current command. I ref The current limiter limits it to I sat The inner current loop PI (proportional-integral) controller is combined with virtual impedance. R v + jX v Generate converter voltage command E Subsequently, the power devices are driven through dq / abc coordinate transformation and PWM (pulse width modulation) stages; U p For the grid connection point voltage vector, U v This is the equivalent voltage drop generated by the virtual impedance.

[0044] For the aforementioned grid-connected energy storage system, this embodiment provides a method for classifying the current-limited saturation mode and quantizing the transient stability boundary of the grid-connected energy storage system. This method aims to solve the problems of nonlinear transient instability and boundary quantization caused by the operation of the current-limiting mechanism. The specific implementation process is as follows: (1) Equivalent modeling of transient large signals for grid-connected energy storage.

[0045] like Picture 1 As shown, the grid-connected main circuit of the grid-connected energy storage system uses an LC filter circuit connected to the grid, which is equivalent to a Thevenin voltage source in series with the grid impedance. To achieve grid-connected characteristics while suppressing short-circuit overcurrent, the VSG control and hybrid current-limiting strategy of the converter are shown in the following equation; the complete system transient mathematical model is as follows: Picture 3 As shown, where a sat A flag indicating the amplitude limiting saturation state. V f Indicates the voltage amplitude during a fault.s This represents the Laplace operator.

[0046]

[0047]

[0048] in: U v This represents the equivalent transient voltage drop of the virtual impedance. I sat Output the current command after current limiting for the current loop. I max The rate limiting threshold, This is the preset saturation current phase angle.

[0049] In the specific implementation process, the system physical parameters shown in Table 1 are substituted into the network model: the equivalent impedance of the main power grid is R g =0.025pu, X g =0.25pu; the configured feedforward virtual impedance is set to R v =0.01pu, X v =0.18pu. Therefore, under fault conditions, the total equivalent resistance and equivalent reactance of the main circuit are respectively:

[0050]

[0051] The equivalent impedance magnitude of the main circuit is ≈0.43pu, equivalent impedance angle is θ z =arctan( X eq / R eq =arctan(0.43 / 0.035)≈1.49rad (85.4°). Assume the initial rated internal potential amplitude of the converter is maintained at... E =1.0 pu, the power angle latched at the moment of the fault is δ 0 = 0.46 rad.

[0052] (2) Division of instantaneous and hysteresis saturation modes of limiter and waveform analysis in multiple scenarios.

[0053] After a symmetrical short-circuit fault occurs in the system, the output current of the GFM converter will undergo drastic transient changes due to the voltage drop at the grid connection point. For example... Picture 2 As shown, the voltage amplitude of the system after the grid voltage drops.V g and current limiting threshold I max The differences in waveforms result in three distinct actual transient response waveforms. Picture 2 The solid blue line represents the experimental / simulation waveform, and the dashed green line represents the numerical analytical trajectory of the mathematical model of this invention. The two lines highly overlap, which strongly verifies the accuracy of the model. Picture 4 (a) and (b) describe the triggering mechanisms of instantaneous saturation and hysteretic saturation, respectively, which are closely related to the current and power characteristics during a fault. This embodiment is based on this triggering mechanism. Picture 2 Rigorous mode classification and quantitative analysis were performed on the typical waveforms: The first waveform is as follows: Picture 2 As shown in (a) above, in this scenario, the voltage depth at the grid connection point drops to V g =0.2pu, the current limiter safety threshold is set to a relatively small value. I max =1.3 pu. Based on network algebra relationships, calculate the instantaneous apparent current amplitude of the converter output at the moment the fault occurs. I ( δ 0)|:

[0054] Substitute the initial work angle δ 0 = 0.46 rad, and cos( δ The result is obtained by calculating 0) = 0.90:

[0055] Due to the calculated instantaneous apparent current | I ( δ 0)|=1.914pu> I max =1.3pu indicates that the apparent current at the moment of the fault step directly exceeded the limiter's allowable upper limit, and the converter directly switched into the limit cutoff state without any delay, which means that the instantaneous saturation mode was triggered.

[0056] exist Picture 2 In the experimental waveform (a) above, the output current... I At the moment of fault triggering t Within 2 seconds, the current surged sharply, breaking through the 1.3 pu current-limiting threshold. Subsequently, the current was completely leveled off and locked at the 1.3 pu saturation line. At this point, since the converter is equivalent to a constant current source externally, the output active power... P Under strong compression and reshaping, it exhibits periodic oscillations of active power with extremely low amplitude.

[0057] The second waveform is as follows: Picture 2 As shown in (b) of the diagram, in this scenario, a shallow external fault occurs, resulting in a shallow voltage drop and a voltage amplitude after the drop. V g =0.8pu, the current limiting threshold is set to a relatively small value. I max =1.3 pu. Based on network algebraic relationships, calculate the instantaneous current output of the network at the moment of the fault:

[0058] Calculate the current at the instant of the fault. I ( δ 0)|=1.053pu< I max =1.3pu, indicating that no overcurrent occurred in the system at the moment of the fault step, and the converter operating point first jumps to the normal power curve under the fault.

[0059] exist Picture 2 In the experimental / simulation waveform of (b) in the figure, the output current I At t=2s, only a small jump occurs, followed by an increase in the power angle due to power imbalance, and the current gradually climbs along the fault characteristic curve. After a brief climb period of approximately 100ms, the current reaches the 1.3pu threshold at t≈2.1s and enters current-limiting mode, i.e., the system triggers a hysteresis-type saturation mode. After entering saturation, the output active power exhibits wide-amplitude, long-period fluctuations, and the average active power decreases significantly.

[0060] The third waveform is as follows: Picture 2 As shown in (c) above, in this scenario, the voltage depth drops to V g =0.2 pu, but the safe operating threshold of the hardware current limiter is set to a relatively wide range. I max =2.4 pu. The instantaneous current calculated at the moment of the fault is | I ( δ 0)|=1.914pu, because | I ( δ 0)|=1.914pu< I max =2.4pu, indicating that the instantaneous current did not reach the protection limit; the system also first switched to the fault curve operation, and then the current slowly climbed upward as the power angle increased.

[0061] like Picture 2 As shown in the red shaded area in (c), the system experienced a significant and long-term hysteresis ramp-up period (from...). t =2.0s continued to climb to t≈2.5s, with a hysteresis window exceeding 500ms). As the power angle diverges sharply, the current... t At approximately 2.5 seconds, the current finally climbed to the 2.4 pu current-limiting line and was forced to switch into a saturated constant current state. Due to the large amount of acceleration kinetic energy accumulated within the half-second hysteresis window, the system surpassed the transient unstable equilibrium point and output active power. P It exhibits extremely strong deep-amplitude periodic unstable oscillations.

[0062] (3) Sinusoidal unified characterization of multi-state power-power angle characteristic curves.

[0063] To facilitate a unified algebraic solution for transient stability boundary conditions, this invention unifies the fault power curve when the current limiter is unsaturated and the saturation power curve when the current limiter is saturated into a standard sine function form, and substitutes the system physical parameters to obtain the following specific numerical characteristic relationships: For unsaturated fault power curves P f ( δ ), and its corresponding analytical bias. P f0 Amplitude P f,m and phase shift The specific calculation is as follows:

[0064]

[0065]

[0066] Therefore, the power characteristic curve equation under the unsaturated fault state is:

[0067] For the saturation power curve P sat ( δ The preset saturation current phase angle is set to... ϕ =1.18rad, substituting the system parameters into the calculation yields: At the threshold I max =1.3 pu, such as Picture 2 Under the working condition corresponding to (a) in the text:

[0068]

[0069]

[0070] The equation for the saturated power characteristic curve at this point is:

[0071] At the threshold I max =2.4 pu, such as Picture 2 Under the operating condition corresponding to (c) in the figure: based on the electrical derivation of saturated active power in the impedance network, the bias and power amplitude physically satisfy... P sat0 =0.20pu and P sat,m =0.48pu; combined with impedance angle θ z =1.49 rad and saturation phase angle ϕ =1.18rad, the resulting saturated power characteristic curve equation is specifically simplified and substituted as follows:

[0072] (4) Quantization of active power command boundaries in instantaneous saturation mode.

[0073] When the system triggers a transient saturation mode, such as Picture 2 As shown in (a), since the converter directly switches to limiting mode at the moment of the fault, the transient synchronous stability trajectory of the system will be completely determined by the power characteristic curve under saturation. P sat ( δ ) = 0.06 + 0.26sin( δ +0.39) and the input active power reference command P ref Joint decision, such as Picture 5 As shown.

[0074] To ensure the converter's power angle remains stable at the new equilibrium operating point and to prevent divergence caused by the acceleration area constantly being larger than the deceleration area, the active power command... P ref It must be strictly limited to a range P sat0 < P ref ≤ P sat ( δ 0) inside.

[0075] By substituting specific values, the safe range of the active power command under this operating condition can be calculated as follows:

[0076] from Picture 2 As can be seen from the characteristic waveform in (a), in I max =1.3pu andV g Under the 0.2 PU condition, although the amplitude of the power curve is extremely compressed, if we take the active power command... P ref Set within a safe range (e.g., take...) P ref =0.15pu, corresponding to an active power output of approximately 225kW), the system then possesses a new equilibrium point that satisfies transient energy balance, such as Picture 5 As shown in (b), the power angle will rapidly converge again after a fault, allowing the output active power to achieve stable operation after experiencing small fluctuations with rapid decay, ensuring the system can achieve stable fault ride-through. Conversely, if the power command... P ref If this safety limit is exceeded, the accelerating kinetic energy will never be balanced by the decelerating potential energy, and the system will inevitably experience severe monotonically positive divergence of the power angle and transient instability, such as... Picture 5 As shown in (a) in the figure.

[0077] (5) Quantization of active power command boundary in hysteresis saturation mode.

[0078] When the system determines that a hysteresis-type saturation mode has been triggered, such as Picture 2 As shown in the operating conditions corresponding to (b) and (c), the operating point is along the fault curve. P f ( δ It moves and accumulates kinetic energy, and then cuts into the saturation characteristic curve after the current rises to the limiting boundary. P sat ( δ The transient process in this mode is divided into three stages, requiring a more detailed analysis of the power command range, such as... Picture 6 As shown.

[0079] To ensure that the power angle does not exceed the unstable equilibrium point after undergoing a complex hysteresis-saturated multimodal transition. δ UEP It is essential to ensure that the transient acceleration area accumulated during the initial stage of the fault and the delayed climb is less than the maximum deceleration area that can be provided after saturation. Substituting into the equal area rule, we can analyze the integral formula as follows:

[0080] Substituting the specific numerical curves and the integral equation of the equal area rule, under the critical impedance configuration parameters, the limiting active power critical value is obtained simultaneously as 0.55 pu; combined with the fault offset lower limit of 0.188 pu, the safe transient stability boundary interval of the active power command under this hysteresis mode is quantified as follows:

[0081] exist Picture 2 In the operating condition shown in (c), the power grid experienced an event related to... Picture 2 (a) shows the same depth symmetrical drop, but because the current protection threshold is broadened to I max =2.4 PU, the system failed to limit the output instantly, instead entering a hysteresis window lasting over 500 ms. During this period, if the converter maintains a high active power command setting (e.g., P ref =0.60pu, exceeding the calculated critical boundary. P crt =0.55pu), the power angle will diverge and rise sharply with extremely high acceleration, such as Picture 6 As shown in (b), this results in the accumulation of extremely large and irreversible transient acceleration kinetic energy during the unsaturated hysteresis period. When the system is in t =2.5s, δ sat When the torque reaches approximately 1.65 rad and enters saturation, the reduced electromagnetic torque after amplitude limiting and compression is completely insufficient to pull back the power angle, which has already experienced severe overshoot, causing the power angle to completely exceed the saturation unstable equilibrium point. δ UEP ≈2.06 rad, which directly explains Picture 2 Active electromagnetic power in (c) P The reason why deep-amplitude periodic unstable oscillations eventually occurred.

[0082] Conversely, in Picture 2 In the shallow fault condition shown in (b), due to the shallow voltage drop at the grid connection point, the apparent current at the moment of the fault is small, and the system also enters a hysteresis ramp-up period. Because of the shallow voltage drop, the accelerating torque is small, and the current limiting threshold is... I max When the active power is 1.3 PU, the hysteresis time is extremely short, and the transient kinetic energy accumulated by the system during the hysteresis window is very weak. Therefore, as long as the active power command is... P ref Controlled within the calculated safety boundaries, such as Picture 6 As shown in (c), after the system enters saturation, it can provide sufficient deceleration area. After experiencing a very short period of fluctuation, the power angle can achieve foldback convergence in the saturated constant current state, thereby achieving stable fault ride-through.

[0083] also, Picture 6 (a) in the text describes the power angle trajectory corresponding to the critical power command in the hysteresis saturation mode. At this time, the kinetic energy at the operating point can be consumed just before entering saturation, thus ensuring stability. Picture 6(d) describes the trajectory when the power command is extremely small. In this case, the initial direction of motion is reversed, but sufficient braking energy is available, thus maintaining stability. It should be noted that the above two trajectory characteristics are not the same as... Picture 2 The waveforms do not correspond to those in different scenarios, but rather represent transient characteristics of different power command ranges obtained under the method of this invention.

[0084] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage, characterized in that, Includes the following steps: (1) For grid-connected energy storage systems, a transient large-signal mathematical model of the system is constructed, which includes a virtual synchronous control swing equation and a hybrid current limiting control equation containing a virtual impedance and a current limiter; when a short-circuit fault occurs at the grid connection point, the system power angle and grid voltage are collected in real time as initial state parameters. (2) Based on the transient large signal mathematical model and initial state parameters, calculate the apparent current output of the grid-type energy storage converter at the moment of fault, and according to the relative magnitude relationship between the apparent current and the current limiting threshold, divide the saturation dynamic evolution behavior of the current limiter into instantaneous saturation mode and hysteresis saturation mode. (3) Construct the power-power angle characteristic curves of the grid-type energy storage converter under different states of the current limiter, and unify them into a standard sine function form by reducing their order; using the power angle motion trajectory as a reference, use the equal area rule to quantify the active power command safety range that the grid-type energy storage converter can maintain the transient synchronous stability of the system under the instantaneous saturation mode and the hysteresis saturation mode respectively.

2. The method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 1, characterized in that, The expression for the virtual synchronous control swing equation in step (1) is as follows: ; ; in: J For virtual inertia, Δ ω The deviation between the internal potential angular frequency of the grid-type energy storage converter and the grid angular frequency. P ref This is an active power command. P This refers to the actual active power output of the grid-type energy storage converter. D p This is the virtual damping coefficient. δ The phase difference between the internal potential of the grid-connected energy storage converter and the grid voltage is the system power angle. ω b As the reference angular velocity, t Indicates time.

3. The method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 1, characterized in that, The expression for the hybrid current-limiting control equation in step (1) is as follows: ; ; in: U v This represents the equivalent transient voltage drop of the virtual impedance. R v and X v These are the set virtual resistance and virtual reactance, respectively. I This refers to the actual output current of the grid-type energy storage converter. I sat This is the current command after current limiting. I max The rate limiting threshold, The preset constant saturation current phase angle, j The imaginary unit, e It is a natural constant.

4. The method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 3, characterized in that, The expression for calculating the apparent current in step (2) is as follows: ; in: I ( δ 0) is the apparent current output of the grid-connected energy storage converter at the moment of fault. δ 0 represents the system power angle at the moment of the fault. E The internal potential amplitude of the grid-type energy storage converter. V g This represents the voltage amplitude of the power grid immediately following the fault. R eq The equivalent total resistance of the main circuit of the system and R eq = R v + R g , X eq The equivalent total reactance of the main circuit of the system and X eq = X v + X g , R g and X g These are the equivalent resistance and equivalent reactance of the power grid under fault conditions, respectively.

5. The method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 1, characterized in that, In step (2), the calculated apparent current is compared with the current limiting threshold: if the apparent current is greater than or equal to the current limiting threshold, it is determined that the grid-type energy storage converter directly enters the limiting state at the moment the fault occurs, that is, the current limiter triggers the instantaneous saturation mode; if the apparent current is less than the current limiting threshold, it is determined that the grid-type energy storage converter will first move along the unsaturated power characteristic curve under the fault state, and then enter the limiting state after the current climbs to the limiting boundary, that is, the current limiter triggers the hysteresis saturation mode.

6. The method for dividing the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 4, characterized in that, In step (3), when the current limiter is not saturated, the standard sine function expression of the power-power angle characteristic curve of the grid-type energy storage converter is as follows: ; ; Under the saturation state of the current limiter, the standard sinusoidal function expression of the power-power angle characteristic curve of the grid-type energy storage converter is as follows: ; ; in: P f ( δ The power angle of the grid-type energy storage converter under unsaturated current limiter conditions corresponds to the system power angle on its power-power angle characteristic curve. δ The actual output active power, P sat ( δ The power angle of the grid-type energy storage converter under saturation conditions is the corresponding system power angle on its power-power angle characteristic curve. δ The actual output active power, P f0 , P f,m , These represent the output power offset, output power amplitude, and curve phase offset under the unsaturated state of the current limiter, respectively. P sat0 , P sat,m , These represent the output power offset, output power amplitude, and curve phase offset under saturation conditions of the current limiter, respectively. Z eq The equivalent impedance magnitude of the main circuit of the system and , θ z The equivalent impedance angle of the main circuit of the system and θ z =arctan( X eq / R eq ).

7. The method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 1, characterized in that: The equal area rule in step (3) is based on the extreme points and intersections of the standard sine function of the power-power angle characteristic curve to calculate the acceleration area and deceleration area in the transient evolution process. The critical condition is that the maximum deceleration area that the system can provide can completely offset the accumulated acceleration kinetic energy, and the analytical solution of the stable interval is performed.

8. The method for classifying the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 6, characterized in that, In step (3), in the instantaneous saturation mode, to ensure that the deceleration area of ​​the system within the same cycle is greater than the acceleration area and to avoid positive or negative divergence of the power angle, the active power command safety range that the grid-type energy storage converter can maintain the transient synchronous stability of the system is as follows: ; ; in: P ref This is an active power command. P sat ( δ 0) represents the power angle corresponding to the power-power angle characteristic curve of the grid-type energy storage converter under the saturation state of the current limiter. δ 0 is the actual output active power.

9. The method for dividing the limited saturation mode and quantifying the transient stability boundary of grid-type energy storage according to claim 6, characterized in that, In step (3), when the current limiter is in hysteresis saturation mode, a critical power command exists when the acceleration kinetic energy and deceleration potential energy accumulated by the system before entering saturation state are exactly completely canceled out. P crt At this time, the safe range of active power command that the grid-type energy storage converter can maintain transient synchronization and stability of the system is as follows: ; ; ; in: P ref This is an active power command. δ sat This is the critical power angle at which the system transitions from the fault curve to the saturation power curve. δ UEP The system's unstable equilibrium operating point under the saturated power characteristic curve and δ UEP =π- δ 0.

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