Method for improving stability of network-forming converter by considering voltage and phase transients

Abstract

The invention discloses a method for improving stability of a network-forming converter by considering voltage and phase transients, which solves the problem of over-current control blind areas existing at the initial stage of a fault and at the moment of clearing in a traditional single current limiting method and overcomes the defect that the stability is sharply deteriorated when a traditional strategy is used for coping with a composite fault. The method comprises the following steps: S1, obtaining a virtual synchronous machine swing equation under a synchronous rotating coordinate system and an equivalent circuit model under the action of a circular current limiter; and S2, obtaining a nonlinear relation between the output power and the power angle of the converter, and analyzing a transient stability domain of the converter during a fault period. And S3, obtaining a mixed current limiting strategy, deducing an equivalent circuit model corresponding to the strategy, and further obtaining an output power expression corresponding to the strategy. And S4, obtaining a power damping term acting on an active power control loop, further obtaining a synchronization stability improvement strategy, and deducing a closed-loop model describing the system at the moment.

Description

Technical Field

[0001] This invention belongs to the field of power transmission and distribution technology, and specifically relates to a method for improving the stability of grid-type converters that takes into account voltage and phase transients. Background Technology

[0002] With the widespread application of energy storage inverters in power grids, the synchronization mechanism provided by the inherent inertia of synchronous generators is undergoing fundamental changes. While this shift enhances grid flexibility, it also reduces system inertia, weakens grid strength, and consequently increases the grid's sensitivity to transient instability factors. To address these challenges, grid-mode (GFM) control strategies have been proposed, simulating the operating behavior of synchronous generators to achieve key functions such as frequency regulation and virtual inertia. However, due to hardware limitations, the overcurrent capability of GFM converters is typically only 1.2 pu, far lower than the 6 pu level of synchronous generators. Therefore, developing effective overcurrent protection measures is crucial to ensure the safe and stable operation of GFM converters.

[0003] Several mature technical solutions exist for overcurrent protection of GFM converters, among which the most widely used fall into three categories: DC current limiting, indirect methods utilizing virtual impedance, and hybrid methods combining both principles. A common direct method—a priority-based current limiter—forces the converter into grid-following (GFL) mode upon fault occurrence. This mode switching is undesirable during grid-connected operation because it delays voltage source control recovery, potentially impairing transient stability and grid support capabilities. Another direct method—a circular current limiter (CCL)—models the converter as a voltage source with a series variable virtual resistor, enabling smooth recovery after a fault. Xiong X et al. analyzed an indirect current limiting method based on adaptive virtual impedance (AVI) in their literature, which primarily reduces the voltage command during overcurrent by adjusting the voltage reference value; however, the adjustment process is susceptible to measurement noise, thus reducing robustness. Hybrid current limiting methods, including combining d-axis limiting with AVI and combining CCL with virtual impedance, have shown potential to enhance transient stability. However, for certain combinations, such as the combination of CCL with a fixed virtual impedance, the stability margin may decrease.

[0004] It is worth noting that the above studies analyzed single, simple faults, neglecting complex concurrent disturbances. Among all severe grid fault disturbances, combined fault events (such as simultaneous voltage drops and power factor angle changes) are the most challenging, as stated in standards such as IEEE Standard 1564-2014. Taul MG et al. pointed out in their literature that the difference between the X / R ratio of the fault impedance and the grid impedance distorts the amplitude and phase angle of the voltage at the point of common connection, which can lead to severe power oscillations and transient instability. This highlights the urgent need for grids with concurrent faults to consider a CCL converter stability analysis architecture to ensure the converter's resilience under real-world grid conditions.

[0005] In summary, the transient stability of GFM converters faces a dual threat: internally, the triggering of overcurrent protection mechanisms alters the converter's dynamic characteristics; externally, severe impacts from concurrent grid faults. These situations can lead to inaccurate stability assessments and trigger unique fault mechanisms, such as destructive overcurrents during fault recovery. Research on hybrid current-limiting methods remains significantly lacking, with little consideration given to the complexity of actual concurrent faults, and no framework for concurrent fault analysis applicable to current-limited GFM converters has yet been established. Therefore, a method for improving the stability of grid-connected converters against concurrent voltage and phase transient (PAJ) faults is urgently needed. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the technical problem to be solved by this invention is to provide a method for improving the stability of grid-type converters that considers voltage and phase transients, thereby solving the problem of overcurrent control blind spots in the initial stage of a fault and the instant of clearing the fault in traditional single current limiting methods, and overcoming the defect of traditional strategies in the rapid deterioration of stability when dealing with complex faults.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for improving the stability of a grid-type converter considering voltage and phase transients includes the following steps:

[0009] S1: Based on the topology and control block diagram of the grid converter, the virtual synchronous machine swing equation of the grid converter in the synchronous rotating coordinate system and the equivalent circuit model under the action of the circular current limiter are obtained.

[0010] S2: Based on the virtual synchronous machine swing equation and the combined fault of voltage sag and phase jump in the power grid, the nonlinear relationship between the converter output power and the power angle is obtained, and the transient stability domain during the fault is analyzed.

[0011] S3: Based on the overcurrent capacity constraint of the grid-type converter, a hybrid current limiting strategy based on a circular current limiter and adaptive virtual impedance is obtained. The equivalent circuit model corresponding to the hybrid current limiting strategy is derived, and the output power expression corresponding to the strategy is further obtained. The influence of key parameters on transient stability is analyzed. S4: Based on the real-time resistance value of the adaptive virtual impedance and the q-axis component of the common coupling point voltage in the output power expression corresponding to the hybrid current limiting strategy, a power damping term acting on the active power control loop is obtained. Then, a synchronous stability improvement strategy is obtained, and the closed-loop model describing the system at this time is derived.

[0012] S5: Based on the closed-loop model of the system after introducing the power damping term, the phase plane response of the stability improvement method under voltage and phase transients is obtained, and its effect on expanding the system stability boundary is analyzed and verified.

[0013] Preferably, in S1, the mathematical representation of the virtual synchronizer swing equation is as follows:

[0014]

[0015] Where δ is the virtual power angle, representing the relationship between E and V. g The angle difference between them, E represents the terminal voltage of the converter, V g Indicates the voltage amplitude of the power grid. The first derivative of the virtual work angle. The second derivative of the virtual work angle.

[0016] D p J represents the virtual damping and inertia of the synchronization element, and P represents the virtual damping and inertia of the synchronization element. ref P and P represent the power command and output power of the converter, respectively.

[0017] Preferably, in S1, the mathematical representation of the circular current limiter acting on the converter control is as follows:

[0018]

[0019] Where Iref sdq represents the converter output current command value generated by the voltage outer loop. I represents the current command value output by the CCL limiter. max This indicates the limit value set by CCL.

[0020] Preferably, in step S1, the equivalent circuit model of the circular current limiter is represented as follows:

[0021]

[0022] Among them, R eV represents the equivalent series resistance in the external circuit when the CCL limiter is introduced. When it is 0, it means the CCL is not triggered; when it is positive, it means the CCL is triggered. E represents the terminal voltage of the converter. g Represents the grid voltage amplitude, θ represents the grid phase drop angle, and I max R represents the limit value set by CCL. g X g Let denot represent the equivalent resistance and reactance of the power grid, respectively. Let δ be defined as the virtual power angle, expressed as: , where θ E θ represents the phase of the converter terminal voltage. g Indicates the phase of the grid voltage.

[0023] Preferably, in S2, the combined fault of voltage sag and phase jump in the power grid is characterized in the mathematical model by time-varying voltage parameters and phase angle jump values, respectively.

[0024] Preferably, in step S2, the nonlinear relationship between the converter output power and the power angle is expressed as follows:

[0025]

[0026] Where δ represents the virtual power angle, R e V represents the equivalent series resistance of the CCL limiter in the external circuit, E represents the terminal voltage of the converter, and V represents the series resistance of the CCL limiter in the external circuit. g R represents the voltage amplitude of the power grid, θ represents the phase angle drop angle of the power grid, and R represents the voltage amplitude of the power grid. g X g These represent the equivalent resistance and reactance of the power grid, respectively.

[0027] Preferably, in step S3, the mathematical model of the hybrid current limiting strategy with adaptive virtual impedance is expressed as follows:

[0028]

[0029] Among them, R v X v K represents virtual resistance and virtual reactance, respectively. pr I represents the impedance current limiting factor. mag I represents the amplitude of the inverter output current. th This represents the threshold value for triggering the virtual impedance circuit, and σ represents the ratio of virtual impedances X. v / R v ;

[0030] The effect of virtual impedance on the external circuit is achieved through virtual voltage drop, which is expressed as:

[0031]

[0032] Among them, V zd V zq R represents the voltage drop caused by the virtual impedance. v X v Representing virtual resistance and virtual reactance respectively, I sd I sq These represent the d-axis and q-axis components of the inverter output current, respectively.

[0033] Preferably, considering a combined fault involving a voltage dip and a phase jump in the grid, the active power output of the converter is expressed as:

[0034]

[0035] Where δ represents the virtual power angle, R e V represents the equivalent series resistance of the CCL limiter in the external circuit, E represents the terminal voltage of the converter, and V represents the series resistance of the CCL limiter in the external circuit. g R represents the voltage amplitude of the power grid, θ represents the phase angle drop angle of the power grid, and R represents the voltage amplitude of the power grid. v X v R represents virtual resistance and virtual reactance, respectively. g X g These represent the equivalent resistance and reactance of the power grid, respectively.

[0036] Using Kirchhoff's electrical relationship that the converter output current equals the limiting value when CCL is triggered, the expression for the equivalent series resistance at this time is derived as follows:

[0037]

[0038] Among them, R e This represents the equivalent series resistance in the external circuit when the CCL limiter is introduced. When it is 0, it indicates that the CCL is not triggered; when it is positive, it indicates that the CCL is triggered. E represents the terminal voltage of the converter, V. g Represents the grid voltage amplitude, θ represents the grid phase drop angle, and I max R represents the limit value set by CCL. g X g R represents the equivalent resistance and reactance of the power grid, respectively. v X v They represent virtual resistance and virtual reactance, respectively, and δ is defined as the virtual power angle.

[0039] Preferably, in step S4, the mathematical model of the power damping term is expressed as follows:

[0040]

[0041] Among them, P damp R represents the damping power term. v V represents virtual resistance.pccq V represents the q-axis component of the grid connection point voltage. pcc δ represents the amplitude of the voltage at the grid connection point, and δ represents the virtual power angle.

[0042] Preferably, in step S4, the synchronization stability improvement strategy is implemented by modifying the swing equation of the virtual synchronizer.

[0043] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0044] This invention combines a circular current limiter with an adaptive virtual impedance to form a hybrid current limiter, solving the overcurrent control blind zone problem that exists in traditional single current limiting methods during the initial stage of a fault and the instant of clearing. This hybrid structure not only ensures strict current limitation throughout the fault period, but also enhances the damping characteristics of the system by introducing an adaptively adjusted virtual impedance, thus providing crucial support for transient stability while limiting current.

[0045] This invention addresses the severe fault scenario of concurrent voltage sags and phase jumps that are prone to occur in power grids. It proposes introducing a power damping term based on q-axis voltage and an adaptive virtual resistance into the active power control loop. This method can accurately sense the system's instability trend and generate a suppressive torque that directly acts on angular acceleration, effectively overcoming the shortcomings of traditional strategies that suffer from drastic stability deterioration when dealing with such combined faults, and significantly expanding the system's stability boundary.

[0046] These features and advantages of the present invention will be disclosed in detail in the following specific embodiments and accompanying drawings. Attached Figure Description

[0047] Figure 1 This is a schematic flowchart of the steps of a method for improving the stability of a grid-type converter that takes into account voltage and phase transients according to the present invention. Figure 2 The system structure and control block diagram of a grid-type converter; Figure 3 The equivalent circuit diagram of the grid-type converter considering CCL; Figure 4 A diagram showing the power angle curve of a grid-type converter considering CCL under concurrent fault conditions; Figure 5 The control block diagram is for a hybrid current limiting converter that combines CCL and adaptive virtual impedance. Figure 6 The equivalent circuit diagram of a grid-type converter under the action of a hybrid current limiter; Figure 7a The power angle curve and simulation verification diagram of the grid-type converter under the action of the hybrid current limiter; Figure 7bThe power angle curve and simulation verification diagram of the grid-type converter under the action of the hybrid current limiter; Figure 8 This is a control block diagram for a power synchronization loop with a power damping term. Figure 9a The response diagram of a grid-type converter with enhanced hybrid current limiting under concurrent fault conditions; Figure 9b The response diagram of a grid-type converter with enhanced hybrid current limiting under concurrent fault conditions. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be explained and described below with reference to the accompanying drawings. However, the following embodiments are only preferred embodiments of the present invention and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments in the implementation methods without creative effort are all within the protection scope of the present invention.

[0049] Those skilled in the art will understand that, without conflict, the features in the following embodiments and implementations can be combined with each other.

[0050] like Figure 1 As shown, this embodiment provides a method for improving the stability of a grid-type converter considering voltage and phase transients, which is applied to a stability improvement control system for a grid-type converter, and includes the following steps:

[0051] Step 1: Based on the topology and control block diagram of the grid converter, derive the mathematical representation of the circular current limiter during triggering, establish a large-signal mathematical model considering the circular current limiter, and further obtain its virtual synchronous machine swing equation and equivalent circuit model in the synchronous rotating coordinate system. The system topology and control block diagram are as follows: Figure 2 As shown.

[0052] The voltage at the point of common coupling (PCC) is denoted as V. pcc The converter's terminal voltage is denoted as E, and the converter's output current is denoted as I. s The grid-connected current of the converter is denoted as I. g R g X g L represents the equivalent resistance and reactance of the power grid, respectively. f C f These represent the filter inductance and capacitance values ​​of the converter's terminal filter, respectively. P and Q represent the active and reactive power fed into the grid by the converter via the PCC, respectively. Its control system includes an active power synchronization loop, a voltage control loop, and a current control loop. The output value of the voltage loop becomes the final limiting current command value through the CCL. The synchronization mechanism generates the reference angle θ of the rotating coordinate system.ref , serving as the reference for each control loop in the synchronous rotating coordinate system. Wherein, D p J represents the virtual damping and inertia of the synchronization element, and K... pv K iv K represents the proportional coefficient and integral coefficient of the voltage PI controller, respectively. pc K ic These represent the proportional coefficient and integral coefficient of the current PI controller, respectively.

[0053] according to Figure 2 The control block diagram of the circular current limiter described, and its mathematical representation for its effect on converter control, can be expressed as follows:

[0054]

[0055] Where Iref sdq represents the converter output current command value generated by the voltage outer loop. I represents the current command value output by the CCL limiter. max This indicates the limit value set by CCL. This represents the magnitude of the converter's output current. When the CCL is triggered, only the magnitude of the current decreases, while its angle remains unchanged. Therefore, a GFM converter with CCL can be modeled as one with a variable virtual resistor R. e Series voltage sources, such as Figure 3 As shown in the diagram. Based on this equivalent circuit diagram, the amplitude of the converter output current can be derived, expressed as:

[0056]

[0057] When CCL is triggered, assuming the current loop bandwidth is large enough and has sufficient dynamic performance, the output current amplitude is equal to the CCL's limiting value I. max Using Kirchhoff's voltage law at this point, the expression for the variable virtual resistance can be derived:

[0058]

[0059] Among them, R e This represents the equivalent series resistance in the external circuit when the CCL limiter is introduced. A value of 0 indicates that the CCL is not triggered, while a positive value indicates that the CCL is triggered. E represents the converter's terminal voltage, V. g Represents the grid voltage amplitude, θ represents the grid phase drop angle, and I max R represents the limit value set by CCL. g X g Let represent the equivalent resistance and reactance of the power grid, respectively, and δ be defined as the virtual power angle.

[0060] according to Figure 2 The control block diagram of the described power synchronization link can be used to derive the swing equation of the virtual synchronizer, which is mathematically represented as:

[0061]

[0062] Where δ is the virtual power angle, representing the relationship between E and V. g The angular difference between them, E is fixed on the d-axis, D p J represents the virtual damping and inertia of the synchronization element, and P represents the virtual damping and inertia of the synchronization element. ref P and P represent the power command and output power of the converter, respectively.

[0063] Step 2: Based on the swing equation and the combined fault of voltage sag and phase jump in the power grid, the fault under concurrent fault conditions is derived, and the nonlinear relationship between the converter output power and the power angle is obtained. Furthermore, the transient response of the fault process is described by the power angle curve, and the transient stability domain is analyzed by the equivalent area method.

[0064] The nonlinear relationship between converter output power and power angle can be derived from... Figure 3 The equivalent circuit described is derived mathematically as follows:

[0065]

[0066] Where δ represents the virtual power angle, R e V represents the equivalent series resistance of the CCL limiter in the external circuit, E represents the terminal voltage of the converter, and V represents the series resistance of the CCL limiter in the external circuit. g R represents the voltage amplitude of the power grid, θ represents the phase angle drop angle of the power grid, and R represents the voltage amplitude of the power grid. g X g Let represent the equivalent resistance and reactance of the power grid, respectively. Using the above mathematical relationships and the swing equation of the virtual synchronous machine, a second-order transient analysis model of a grid-type converter with a CCL limiter under concurrent fault conditions is obtained.

[0067] Based on the mathematical model described above, theoretical analysis and simulation verification were performed on the system described in Table 1. The power angle curve of the analyzed system is shown below. Figure 4 As shown in the figure, the results reveal the dynamic trajectory of the work angle during the transient process and clarify that the equilibrium point of the system is determined by the P-δ curve and P... ref The intersection point of the curves is determined, P ref This represents the mechanical input of SG. At the rated voltage (V) g In both the 1p.u. and current limiting modes (light blue curve), the system exhibits two equilibrium points: the stable equilibrium point (SEP), represented by a solid red dot, which indicates the steady-state operating point of the system; and the unstable equilibrium point (UEP), represented by a hollow red circle.

[0068] Figure 4 The transient process being addressed refers to two fault conditions with the same fault duration: V g From the rated value to 0.5 times the rated value, and V g A composite fault event occurs when the power output drops to 0.5 times the rated value, accompanied by a 60° phase angle jump. In the first fault scenario, the system trajectory moves from the pre-fault steady-state operating point (red dot) to point a. To maintain transient stability, the fault must be cleared before the power angle reaches the critical cutoff point at point b. In this case, because the acceleration region (light green area) is smaller than the available deceleration region (light red area), the system is able to return to its original steady-state operating point, and stability is maintained. Conversely, in the second fault scenario, the composite fault significantly expands the acceleration region (light green area). This increase reduces the transient stability margin of the GFM inverter, thereby exacerbating the risk of synchronous instability.

[0069] Table 1 Example Parameter Table parameter Value (pu) <![CDATA[Rated power (P ref )]]> 2.75MW (1 p.u.) <![CDATA[Rated grid voltage (V g )]]> 690V (1p.u.) <![CDATA[DC voltage (V dc )]]> 1200V (1p.u.) <![CDATA[Grid resistance (R g )]]> 3.58mΩ (0.021pu) <![CDATA[Grid reactance (L g )]]> 131µH (0.2pu) <![CDATA[Filter inductor (L f )]]> 17.9µH (0.032pu) <![CDATA[Filter capacitor (C f )]]> 1.26mF (0.068pu) Virtual inertia (J) 60p.u. <![CDATA[Virtual damping (D p )]]> 60p.u. <![CDATA[Voltage loop proportional and integral coefficients (K pv / K iv )]]> 1p.u. / 5p.u. <![CDATA[Current loop proportional and integral coefficients (K pc / K ic )]]> 1p.u. / 10p.u. AVI impedance ratio (σ) 5 <![CDATA[AVI trigger threshold (I th )]]> 1.1 pu <![CDATA[Current limit value (I max )]]> 1.2pu

[0070] Step 3: Based on the overcurrent capacity constraint of the converter, a hybrid current limiting strategy based on a circular current limiter and adaptive virtual impedance is obtained. The equivalent circuit model corresponding to this strategy is obtained by the influence of virtual impedance on the external circuit characteristics. The output power expression corresponding to this strategy is further derived, and the influence of key parameters on transient stability is analyzed.

[0071] Based on the overcurrent capacity constraint of the converter, a hybrid current limiting strategy based on a circular current limiter (CCL) and adaptive virtual impedance (AVI) is obtained, such as... Figure 5 As shown. Specifically, the AVI dynamically adjusts its virtual resistance and virtual reactance based on the difference between the amplitude of the converter output current and a preset threshold. The mathematical description of its control law is as follows:

[0072]

[0073] Among them, R v X v K represents virtual resistance and virtual reactance, respectively. pr I represents the current limiting coefficient. mag I represents the amplitude of the inverter output current. th This represents the threshold value for triggering the virtual impedance circuit, and σ represents the ratio of virtual impedances X. v / R v The effect of virtual impedance on the external circuit is achieved through virtual voltage drop, such as... Figure 6 As shown, the grid-type converter employing the hybrid strategy can be equivalent to a voltage source E connected in series with a variable resistor R introduced by the CCL. e And the variable virtual impedance Z introduced by AVIv =R v +jX v The resulting virtual voltage drop can be expressed as:

[0074]

[0075] Among them, V zd V zq R represents the voltage drop caused by the virtual impedance. v X v Representing virtual resistance and virtual reactance respectively, I sd I sq These represent the d-axis and q-axis components of the inverter output current, respectively.

[0076] according to Figure 6 The equivalent circuit model described above is used to further derive the output power model corresponding to this strategy. Considering the combined fault of voltage amplitude sag and phase jump in the grid, the active power output by the converter is:

[0077]

[0078] Where δ represents the virtual power angle, R e V represents the equivalent series resistance of the CCL limiter in the external circuit, E represents the terminal voltage of the converter, and V represents the series resistance of the CCL limiter in the external circuit. g R represents the voltage amplitude of the power grid, θ represents the phase angle drop angle of the power grid, and R represents the voltage amplitude of the power grid. v X v R represents virtual resistance and virtual reactance, respectively. g X g These represent the equivalent resistance and reactance of the power grid, respectively.

[0079] according to Figure 6 In the aforementioned equivalent circuit model, the equivalent resistance of the CCL to the external circuit will also change. Using Kirchhoff's electrical relationship that the converter output current equals the limiting value when the CCL is triggered, the expression for the equivalent series resistance at this time can be derived as follows:

[0080]

[0081] Among them, R e This represents the equivalent series resistance in the external circuit when the CCL limiter is introduced. A value of 0 indicates that the CCL is not triggered, while a positive value indicates that the CCL is triggered. E represents the converter's terminal voltage, V. g Represents the grid voltage amplitude, θ represents the grid phase drop angle, and I max R represents the limit value set by CCL. g X g R represents the equivalent resistance and reactance of the power grid, respectively. vX v They represent virtual resistance and virtual reactance, respectively, and δ is defined as the virtual power angle.

[0082] The P-δ characteristic curve of the proposed hybrid current limiting control strategy is as follows: Figure 7a As shown. Compared to the CCL scheme, the hybrid current-limiting control strategy exhibits stronger power transfer capability. In this control architecture, K pr Reflecting the main degree of freedom of control, this parameter is adjusted to achieve current limiting capability, but it also has a crucial impact on system stability. K pr An excessive increase in K may lead to the disappearance of SEP, resulting in the loss of synchronous stability. Therefore, K pr The choice constitutes a critical design trade-off, requiring careful consideration of both current limiting performance and transient stability margin.

[0083] Figure 7b The proposed hybrid current limiting strategy was verified through simulation under two severe fault conditions: V g The amplitude decreased to 0.5 pu, accompanied by a 60° power angle transient, and V g A drop to a deeper 0.1 pu is accompanied by a 60° power angle transient. Both fault conditions last for 0.5 seconds. Proportional gain K pr It is set to the maximum value of 1.4. Current limiting factor K pr It was set to the maximum value of 1.4. Simulation results confirm that the output current for both fault conditions is strictly limited to within 1.2 per unit during fault occurrence and clearing. However, at V... g In a deep fault scenario with a power angle transient of 60° and a drop to 0.1 pu (as shown by the red line), transient stability is significantly compromised. Despite the significant effect of current limiting, δ continues to accelerate, indicating a deterioration in stability, which cannot be achieved by simply increasing K. pr This does not alleviate the decline in stability.

[0084] Step 4: Based on the real-time resistance value of the adaptive virtual impedance and the q-axis component of the common coupling point voltage, a power damping term acting on the active power control loop is obtained, thereby obtaining a synchronization stability improvement strategy, and deriving the closed-loop model describing the system at this time.

[0085] Specifically, the power damping term P damp It is composed of the adaptive virtual resistor and the q-axis component of the common coupling point voltage, and its mathematical expression is:

[0086]

[0087] Among them, R vV represents the virtual resistance, characterizing the severity of the current overcurrent in the system and the extent of the effect of the virtual impedance element; pccq The q-axis component representing the grid connection point voltage directly reflects the power angle deviation between the converter's internal potential and the grid voltage, i.e., the degree of deviation of the synchronization link. Under normal operation, both are close to zero, and this damping term is not activated; only when a serious fault such as a voltage sag or phase jump occurs, causing both to increase significantly at the same time, will this link generate a strong negative feedback damping signal.

[0088] By injecting the power damping term into the active power control loop, a synchronization stability improvement strategy is obtained. This strategy is implemented by modifying the swing equation of the virtual synchronizer, such as... Figure 8 As shown, inject P damp The subsequent system dynamic equations are expressed as follows:

[0089]

[0090] This equation of motion shows that P damp The introduction of the term is equivalent to the inherent damping D of the system. p Based on this, an adaptive damping force related to angular velocity deviation and fault depth is added. The torque generated by this force is always opposite to the acceleration direction of the power angle, thus effectively counteracting the acceleration energy accumulated during the fault and suppressing power angle loss of synchronization.

[0091] Step 5: Based on the closed-loop model of the system after introducing the power damping term, obtain the phase plane response of the stability improvement method under voltage and phase transients, and analyze and verify its effect on extending the stability boundary of the system.

[0092] Based on the synchronous control strategy that introduces a power damping term, the converter system shown in Table 1 is analyzed. An extreme combined fault scenario is set: grid voltage V... g The power decreased to an amplitude of 0.1 pu, accompanied by a 60° power angle transient, and increased the fault duration to 1 second. For example... Figure 9a As shown in the phase trajectory comparison, the unenhanced hybrid synchronization control strategy (blue trajectory) accelerated the power angle δ to 2.98 rad when the fault was cleared, far exceeding the unstable equilibrium point (UEP), causing the system to lose synchronization; while after adopting the enhanced synchronization strategy (yellow trajectory), its maximum power angle was successfully suppressed to within 1.81 rad, and it eventually converged stably back to the original equilibrium point. Figure 9b The actual waveforms shown further demonstrate that this strategy, while strictly limiting current, completely eliminates the continuous oscillations of power and power angle, achieves smooth and rapid stable recovery, and significantly extends the system's low-voltage ride-through time and transient stability margin.

[0093] Specifically, the verification results show that the proposed enhanced hybrid current-limiting control strategy substantially expands the system's stability domain by injecting adaptive damping. Under the same extreme combined fault conditions, the system's transient stability boundary is improved from being unable to maintain synchronization to successfully traversing long-term faults and restoring stable operation. This demonstrates the significant effect of the proposed method in expanding the stability boundary and provides an effective approach to solving the synchronization instability problem of grid-connected converters under complex power grid faults.

[0094] The above description is merely a specific embodiment of the invention, but the scope of protection of the invention is not limited thereto. Those skilled in the art should understand that the invention includes, but is not limited to, the contents described in the accompanying drawings and the specific embodiments above. Any modifications that do not depart from the functional and structural principles of the invention will be included within the scope of the claims.

Claims

1. A method for improving the stability of a grid-type converter considering voltage and phase transients, characterized in that, Includes the following steps: S1: Based on the topology and control block diagram of the grid converter, the virtual synchronous machine swing equation of the grid converter in the synchronous rotating coordinate system and the equivalent circuit model under the action of the circular current limiter are obtained. S2: Based on the virtual synchronous machine swing equation and the combined fault of voltage sag and phase jump in the power grid, the nonlinear relationship between the converter output power and the power angle is obtained, and the transient stability domain during the fault is analyzed. S3: Based on the overcurrent capacity constraint of the grid-type converter, a hybrid current limiting strategy based on a circular current limiter and an adaptive virtual impedance is obtained. The equivalent circuit model corresponding to the hybrid current limiting strategy is derived, and the output power expression corresponding to the strategy is further obtained. The influence of key parameters on transient stability is analyzed. S4: Based on the real-time resistance value of the adaptive virtual impedance and the q-axis component of the common coupling point voltage in the output power expression corresponding to the hybrid current limiting strategy, a power damping term acting on the active power control loop is obtained, thereby obtaining a synchronous stability improvement strategy, and deriving the closed-loop model describing the system at this time. S5: Based on the closed-loop model of the system after introducing the power damping term, the phase plane response of the stability improvement method under voltage and phase transients is obtained, and its effect on expanding the system stability boundary is analyzed and verified.

2. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 1, characterized in that, In S1, the mathematical representation of the virtual synchronizing machine swing equation is as follows: Where δ is the virtual power angle, representing the relationship between E and V. g The angle difference between them, E represents the terminal voltage of the converter, V g Indicates the voltage amplitude of the power grid. The first derivative of the virtual work angle. The second derivative of the virtual work angle. D p J represents the virtual damping and inertia of the synchronization element, and P represents the virtual damping and inertia of the synchronization element. ref P and P represent the power command and output power of the converter, respectively.

3. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 1, characterized in that, In S1, the mathematical representation of the circular current limiter's effect on converter control is as follows: Where Iref sdq represents the converter output current command value generated by the voltage outer loop. I represents the current command value output by the CCL limiter. max This indicates the limit value set by CCL.

4. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 3, characterized in that, In S1, the equivalent circuit model of the circular current limiter is represented as follows: Among them, R e V represents the equivalent series resistance in the external circuit when the CCL limiter is introduced. When it is 0, it means the CCL is not triggered; when it is positive, it means the CCL is triggered. E represents the terminal voltage of the converter. g Represents the grid voltage amplitude, θ represents the grid phase drop angle, and I max R represents the limit value set by CCL. g X g Let denot represent the equivalent resistance and reactance of the power grid, respectively. Let δ be defined as the virtual power angle, expressed as: , where θ E θ represents the phase of the converter terminal voltage. g Indicates the phase of the grid voltage.

5. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 1, characterized in that, In S2, the combined fault of voltage sag and phase jump in the power grid is characterized in the mathematical model by time-varying voltage parameters and phase angle jump values, respectively.

6. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 5, characterized in that, In S2, the nonlinear relationship between the converter output power and the power angle is expressed as follows: Where δ represents the virtual power angle, R e V represents the equivalent series resistance of the CCL limiter in the external circuit, E represents the terminal voltage of the converter, and V represents the series resistance of the CCL limiter in the external circuit. g R represents the voltage amplitude of the power grid, θ represents the phase angle drop angle of the power grid, and R represents the voltage amplitude of the power grid. g X g These represent the equivalent resistance and reactance of the power grid, respectively.

7. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 1, characterized in that, In S3, the mathematical model of the hybrid current limiting strategy with adaptive virtual impedance is expressed as follows: Among them, R v X v K represents virtual resistance and virtual reactance, respectively. pr I represents the impedance current limiting factor. mag I represents the amplitude of the inverter output current. th This represents the threshold value for triggering the virtual impedance circuit, and σ represents the ratio of virtual impedances X. v / R v ; The effect of virtual impedance on the external circuit is achieved through virtual voltage drop, which is expressed as: Among them, V zd V zq R represents the voltage drop caused by the virtual impedance. v X v Representing virtual resistance and virtual reactance respectively, I sd I sq These represent the d-axis and q-axis components of the inverter output current, respectively.

8. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 7, characterized in that, Considering the combined fault of voltage amplitude drop and phase jump in the grid, the active power output of the converter is expressed as: Where δ represents the virtual power angle, R e V represents the equivalent series resistance of the CCL limiter in the external circuit, E represents the terminal voltage of the converter, and V represents the series resistance of the CCL limiter in the external circuit. g R represents the voltage amplitude of the power grid, θ represents the phase angle drop angle of the power grid, and R represents the voltage amplitude of the power grid. v X v R represents virtual resistance and virtual reactance, respectively. g X g These represent the equivalent resistance and reactance of the power grid, respectively. Using Kirchhoff's electrical relationship that the converter output current equals the limiting value when CCL is triggered, the expression for the equivalent series resistance at this time is derived as follows: Among them, R e This represents the equivalent series resistance in the external circuit when the CCL limiter is introduced. When it is 0, it indicates that the CCL is not triggered; when it is positive, it indicates that the CCL is triggered. E represents the terminal voltage of the converter, V. g Represents the grid voltage amplitude, θ represents the grid phase drop angle, and I max R represents the limit value set by CCL. g X g R represents the equivalent resistance and reactance of the power grid, respectively. v X v They represent virtual resistance and virtual reactance, respectively, and δ is defined as the virtual power angle.

9. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 1, characterized in that, In S4, the mathematical model of the power damping term is expressed as follows: Among them, P damp R represents the damping power term. v V represents virtual resistance. pccq V represents the q-axis component of the grid connection point voltage. pcc δ represents the amplitude of the voltage at the grid connection point, and δ represents the virtual power angle.

10. The method for improving the stability of a grid-type converter considering voltage and phase transients according to claim 1, characterized in that, In S4, the synchronization stability improvement strategy is implemented by modifying the swing equation of the virtual synchronizer.

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