A method for designing a grid-connected converter considering anti-saturation current limiting strategy

Abstract

The application discloses a grid-forming converter design method considering an anti-saturation current limiting strategy, which is applied to a grid-forming converter grid-connected system controlled by a VSG and comprises the following steps: (1) deriving a necessary and sufficient condition for switching a grid-forming converter GFMC from a conventional control CC mode to a current saturation control CSC mode; (2) deriving a necessary condition and a sufficient condition for the GFMC to exit from the CSC mode and return to the CC mode; and (3) obtaining a power angle curve of the system in the CC mode and the CSC mode, constructing a new criterion based on a desaturation region, and designing parameters of the grid-forming converter. By using the application, a control conflict problem dominated by the converter can be avoided, and the stability of a new power system is improved.

Description

Technical Field

[0001] This invention belongs to the field of power grid control, and in particular relates to a design method for grid-type converters that considers anti-saturation current limiting strategies. Background Technology

[0002] With the rapid development of renewable energy (RES) and the advancement of high-voltage direct current (HVDC) transmission technology, modern power systems are gradually evolving into power electronics-dominated systems. The diversification of power generation methods and control strategies poses significant challenges to the security and stability of large power grids. To improve system stability, grid-connected management (GFM) and grid-following technology (GFL) have attracted widespread attention. Compared to GFL devices based on phase-locked loops (PLLs), GFMCs possess the ability to independently establish voltage amplitude and phase angle. Typical GFM control strategies include droop control and virtual synchronous generator (VSG) control. The former can provide frequency and voltage regulation for the system; while the latter can dynamically provide inertia support by simulating the oscillation equation of a synchronous generator. Domestic and international research shows that adding a low-pass filter (LPF) to droop control can make it equivalent to VSG control. Despite having similar dynamic characteristics, GFMCs using VSG control differ fundamentally from traditional synchronous generators (SGs) due to their weaker overcurrent capability. During fault and recovery phases, the current in GFMCs is more likely to reach the threshold, leading to irreversible damage to power electronic components.

[0003] Various methods have been proposed both domestically and internationally to protect converters from damage. One of the most effective methods is limiting the controller's current reference value, also known as the direct method. By adding a current limiter between the voltage inner loop and the inner current loop, the current reference value is locked when it exceeds the upper limit. Besides the direct method, indirect control of system state variables can also achieve a similar current limiting effect. Virtual impedance (VI) control, as a pre-loop of the voltage inner loop control, virtually changes the system's impedance characteristics, reducing the converter's voltage reference value and thus reducing the device's current output. Compared to VI control, adjusting the power synchronization loop (PSL) can also suppress the growth of the virtual power angle (VPA), avoiding overcurrent and power angle instability.

[0004] All of the above methods can effectively alleviate the overcurrent problem during the fault ride-through (FRT) phase. However, due to the switching of control logic, various new stability problems dominated by the GFMC also arise. Taking the traditional current reference value limiting strategy as an example, the deceleration zone of the GFMC will be significantly reduced during the transient process, making it more prone to power angle instability. Even if a stable operating point (SEP) exists, the GFMC may not be able to recover to its original operating state because it stabilizes at an abnormal SEP or cannot exit the current saturation mode. To solve the above problems, various anti-saturation strategies have been proposed to limit abnormal integration of the voltage inner loop, such as: 1) preventing the integrator from continuously saturating by offsetting the integration deviation; 2) freezing the integrator of the voltage inner loop during current saturation to directly avoid abnormal integration. Although some studies have explored the desaturation conditions of different current saturation controls from the perspective of state variables, the derived desaturation conditions are based on simplification assumptions, and the evolution process of the conditions is still unclear.

[0005] The current saturation strategy of grid-connected converters (GFMCs) is designed to protect equipment from overcurrent damage. However, the specific operating logic is usually not known to the public.

[0006] In summary, few studies have fundamentally explored the desaturation conditions and their evolution patterns during rapid transient processes. Therefore, it is urgent to conduct research on the evolution of desaturation conditions in grid-type converters, thereby characterizing the system's desaturation boundary and providing further suggestions for controller design of grid-type converters from the perspective of control strategy switching. Summary of the Invention

[0007] This invention provides a grid-type converter design method that considers anti-saturation current limiting strategy, which can avoid converter-dominated control conflict problems and improve the stability of new power systems.

[0008] A grid-connected converter design method considering anti-saturation current limiting strategy, applied to a VSG-controlled grid-connected converter system, includes:

[0009] (1) Derive the necessary and sufficient conditions for the grid-type converter GFMC to switch from conventional control CC mode to current saturation control CSC mode.

[0010] (2) Derive the necessary and sufficient conditions for the grid-type converter GFMC to exit CSC mode and return to CC mode;

[0011] (3) Obtain the power angle curves of the system in CC mode and CSC mode, construct a new criterion based on the desaturation region, and design the parameters of the grid-type converter.

[0012] In step (1), the necessary and sufficient condition for GFMC to switch from conventional control CC mode to current saturation control CSC mode is expressed as:

[0013]

[0014] In the formula, Indicates the current reference value, I max δ represents the upper limit of the current amplitude, and θ represents the phase angle difference. PSL -θ SYS θ PSL θ represents the voltage phase of the converter. SYS Represents the voltage phase of an infinite system; Definition Let represent the i-th root of the following expression when equality is taken, with the roots sorted in ascending order. The solution set of the following expression is defined as the saturation domain δ. sa :

[0015]

[0016] In the formula, They represent and phase angle, and Y represents the elements in the first row and first column of the system admittance matrix, and the elements in the first row and second column, respectively. ra and Y rb They represent and amplitude; φ d express The phase difference with the d-axis is equal to 0 in the steady state; U GFM and U SYS These represent the voltage amplitudes of the converter and the system, respectively.

[0017] Considering the effect of the reactive-voltage loop, U GFM Satisfy the following equation:

[0018]

[0019] U GFM The relationship with δ can be solved using the following formula:

[0020]

[0021] In the formula, Q GFM and These represent the reactive power of the converter and its reference value, respectively; U GFM and U SYS K represents the voltage amplitude of the converter and the system, respectively; Q This represents the reactive power-voltage droop coefficient; The values ​​represent the reference values ​​of the converter's d-axis and q-axis voltages in the dq coordinate system; u0 represents the steady-state voltage at the grid connection point; and a, b, and c represent U... GFM Intermediate variables in the calculation process.

[0022] In step (2), the necessary condition for GFMC to exit CSC mode and return to CC mode is expressed as follows:

[0023]

[0024] In the formula, Indicates the current reference value, I max δ represents the upper limit of the current amplitude, and θ represents the phase angle difference. PSL -θ SYS θ PSL θ represents the voltage phase of the converter. SYS Represents the voltage phase of an infinite system; Definition Let δ represent the i-th root when the following expression holds true, with the roots sorted in ascending order. Define the δ interval that satisfies the following expression as the desaturation region δ. dsa :

[0025]

[0026] In the formula, K δ0 K δ1 K δ2 These represent the coefficients of the second harmonic, first harmonic, and constant components, respectively, α δ1 α δ2 These represent the phase of the second harmonic and the first harmonic component, respectively.

[0027] The equations for determining current saturation and desaturation are different. For a given system power angle operating domain Θ, the necessary and sufficient condition for desaturation is:

[0028]

[0029] In the formula, C Θ δ sa Describe the set δ sa The complement of the entire set Θ;

[0030] Define δ cft =δ sa ∩δ dsa To control the conflict domain, when At this time, the system may run into the conflict domain. At this time, multiple control logics of GFMC are triggered simultaneously, which manifests as an abnormal current source and causes significant oscillations in the system.

[0031] In step (3), by analyzing P GFMThe -δ curve is used to determine the stable point of the system, thereby obtaining the power angle curves of the system in CC mode and CSC mode; where P GFM δ represents the active power of the converter; θ represents the phase angle difference. PSL -θ SYS θ PSL θ represents the voltage phase of the converter. SYS This represents the voltage phase of an infinite system.

[0032] In step (3), when designing the parameters of the grid-type converter, the following two problems should be avoided:

[0033] When the work point moves to the conflict zone δ cft At times, due to the rapid and repeated switching of control strategies, the system faces the risk of high-frequency oscillations;

[0034] When the saturated SEP is located in the necessary and sufficient desaturation region Otherwise, the system faces the potential danger of permanently running in CSC mode and will never be able to independently return to the original SEP.

[0035] In step (3), the new criterion based on the desaturation region is as follows:

[0036] for Must meet:

[0037]

[0038] R(Θ,δ sa ∪δ dsa )≤σ

[0039] In the formula, δ cft Indicates the conflict area of ​​the control strategy. C represents the virtual power angle corresponding to the stable equilibrium point of the system in CSC mode. Θ δ dsa Denotes the necessary desaturation region δ dsa The complement of set B in the execution domain Θ; R(A,B) represents a user-defined function that calculates the length of the relative complement of set B with respect to set A; σ represents a small tolerance value.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] This invention proposes a complete set of desaturation domain analysis methods for grid-connected converters that consider anti-saturation current limiting strategies. By deriving the necessary and sufficient conditions for the current saturation of grid-connected converters during inrush / outrush, a new type of stability problem dominated by the converter—control conflict—is identified. At the same time, a controller parameter design criterion considering the desaturation domain is proposed to avoid the generation of such control conflict problems and improve the stability of the new power system. Attached Figure Description

[0042] Figure 1 A schematic diagram of a grid-connected system for a VSG-controlled grid-connected converter;

[0043] Figure 2 A schematic diagram of the voltage inner loop control logic for a grid-type converter;

[0044] Figure 3 A schematic diagram of the anti-saturation current limiting strategy;

[0045] Figure 4 This is a schematic diagram of the necessary desaturation region in an embodiment of the present invention;

[0046] Figure 5 This is a schematic diagram of the power angle curves of the GFMC under different states in an embodiment of the present invention;

[0047] Figure 6 A schematic diagram showing how the GFMC can stabilize at the initial unsaturated SEP when the system encounters a three-phase short-circuit fault;

[0048] Figure 7 This is a schematic diagram showing how the GFMC eventually stabilizes at the saturated SEP when the system encounters a three-phase short-circuit fault. Detailed Implementation

[0049] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.

[0050] In this embodiment of the invention, the grid-connected system of the grid-connected converter is as follows: Figure 1 As shown, the converter uses VSG control. The specific control logic of the voltage inner loop is as follows: Figure 2 As shown, the anti-saturation current control strategy is as follows: Figure 3 As shown.

[0051] Where J and D represent virtual inertia and damping coefficient, respectively. GFM and These represent the active power of the converter and its reference value, respectively. Q GFM and These represent the reactive power of the converter and its reference value, respectively. ω PSL ,θ PSL , ω0 represents the angular velocity of the power synchronization loop, the voltage phase, the derivative of the angular velocity, and the rated angular velocity of the system, respectively. θ SYS U represents the voltage phase of an infinitely large system. GFM and U SYS K represents the voltage amplitude of the converter and the system, respectively. Q This represents the reactive power-voltage droop coefficient. These represent the reference values ​​of the converter's d-axis and q-axis voltages in the dq coordinate system, respectively. These represent the reference values ​​of the dq current output from the voltage inner loop and the current limiter, respectively. CS This indicates a current saturation trigger signal. These represent the d-axis and q-axis integral gains of the voltage inner loop integrator, respectively. These represent the d-axis and q-axis proportional gains of the voltage inner loop integrator, respectively. These represent the outputs of the d-axis and q-axis integrators in the inner voltage loop, respectively. max This indicates the upper limit of the current amplitude. This represents the reference current dq used during current saturation.

[0052] The following is a detailed description of a grid-type converter design method considering anti-saturation current limiting strategy according to the present invention, which mainly includes the following steps:

[0053] 1. System External Characteristic Model

[0054] Since the GFMC can be modeled as a controlled voltage source under normal operating conditions, the algebraic admittance model of the system can be described as follows:

[0055]

[0056] In the formula, Y a ,Y b ,Y c ,Y d These represent the four submatrices of the system admittance matrix. s and U s These represent the current and voltage column vectors at the power supply nodes in the system, respectively. U ns This represents the voltage column vector of the remaining passive nodes.

[0057] The Kron-reduction method is used to eliminate passive nodes, so the above formula can be transformed into:

[0058]

[0059] Extending the above equation, we have:

[0060]

[0061] In the formula, the subscripts GFM and SYS represent the state variables of the GFMC and the infinite system, respectively. Current saturation control (CSC) switches the GFMC from a voltage source to a current source, thereby changing the spatial distribution of node types. In this case, to solve for the unknowns, I is transformed through algebraic operations. GFM Move to the right side of the equation, that is

[0062]

[0063] 2. Derivation of current saturation triggering condition

[0064] When the amplitude of the current reference value is greater than I max The GFMC will switch from CC to CSC. Before reaching the current threshold, the GFMC exhibits voltage source characteristics, and its internal control loop can be considered as a circuit with a transfer function G≈1. In this operating mode, we can obtain:

[0065]

[0066] In the formula, They represent and The phase angle. φ d express The phase difference with the d-axis is zero in the steady state. δ represents the phase angle difference θ. PSL -θ SYS θ PSL θ represents the voltage phase of the converter. SYS This represents the voltage phase of an infinite system.

[0067] Considering the effect of the reactive-voltage loop, U GFM Satisfy the following equation:

[0068]

[0069] U GFM The relationship with δ can be solved using the following formula:

[0070]

[0071] Will I max Substitute By expressing the equations and rearranging them, we can obtain the conditions for transforming from CC to CSC:

[0072]

[0073] definition Let represent the i-th root when equality holds in the above expression, with the roots sorted in ascending order. Define the solution set of the above expression as the saturation domain δ. saTherefore, the necessary and sufficient condition for GFMC to switch to current saturation mode can be expressed as:

[0074]

[0075] 3. Derivation of the current saturation exit condition

[0076] Under current saturation conditions, the integrator in the inner voltage loop is frozen, forcing the voltage control of the grid-type converter to become open-loop control. At this point, it is necessary to solve... u should be determined first d ,u q Q GFM , The expression is given. As analyzed above, in CSC mode, the external characteristics of the system satisfy the following formula:

[0077]

[0078] Multiply both sides by the rotation factor e -jδ You can get u d ,u q :

[0079]

[0080] In the formula, They represent and The phase angle. express The phase angle.

[0081] Therefore, reactive power can be calculated using the formula Q. GFM =u q i d -u d i q We arrive at this conclusion. After simplification, Q... GFM and The expression is:

[0082]

[0083] In current saturation mode, The formula for calculation is:

[0084]

[0085] will u d ,u q Q GFM , Substituting into the above equation, we can obtain The expression for δ:

[0086]

[0087] In the formula, the coefficients of each term are as follows:

[0088]

[0089] when Less than I max Only then can the GFMC exit CSC mode and return to CC mode. (The last part, "I," appears to be a typo and can be left as is.) max Substituting current saturation From the expression, the necessary condition for GFMC desaturation can be obtained as follows:

[0090]

[0091] definition Let represent the i-th root when equality holds in the above expression, with the roots sorted in ascending order. Define the δ-interval satisfying the above expression as the desaturation region δ. dsa Therefore, the necessary condition for current desaturation can be expressed as:

[0092]

[0093] Because the judgment equations for current saturation and desaturation are different, GFMC may trigger multiple control strategies simultaneously, leading to conflicts between different control logics. When the following VPA conflict region δ cft When the space is not empty, control strategy conflicts will occur.

[0094]

[0095] For a given system power angle operating domain Θ, the necessary and sufficient condition for desaturation is:

[0096]

[0097] In the formula, C Θ δ sa Describe the set δ sa The complement of the whole set Θ.

[0098] Based on the set of parameters, the desaturation region under that parameter set can be plotted. Taking the necessary desaturation region as an example, such as... Figure 4 As shown.

[0099] By performing intersection or union operations on the saturation and desaturation regions, it can be determined whether the configured control parameters meet the requirements.

[0100] 4. GFMC Controller Design Method Based on Desaturation Region

[0101] By analyzing P GFM The -δ curve can be used to determine the stable point of the system. Wherein, P... GFM=u d i d +u q i q .

[0102]

[0103] From this, the power angle curves of the system in CC mode and CSC mode can be obtained, such as Figure 5 As shown.

[0104] Based on the preceding analysis, two problems need to be avoided when designing a GFMC controller: 1) When the operating point moves to the conflict region δ cft At this time, due to the rapid and repeated switching of control strategies, the system will face the risk of high-frequency oscillation. Fundamentally, control logic conflicts indicate a flaw in the controller design, which is unacceptable. 2) When the saturated SEP is located in the necessary and sufficient desaturation region. Otherwise, the system will face the potential danger of permanently running in CSC mode and will never be able to independently return to the original SEP.

[0105] To prevent inappropriate desaturation conditions from adversely affecting the system, this invention proposes a new criterion based on the desaturation region, providing a reference for the design of GFMC controllers. The following criteria must be met:

[0106]

[0107] R(Θ,δ sa ∪δ dsa )≤σ (21d)

[0108] Equations (21a)-(21d) outline the four requirements of the proposed criterion. Equation (21a) ensures that there are no conflict domains in the operating domain. Equations (21b) and (21c) ensure that the system has an unsaturated SEP and prevent the attraction domain of the saturated SEP from capturing the operating point during transient processes, thus avoiding the possibility of the system stabilizing at the saturated SEP. Furthermore, satisfying equation (21d) will mitigate the oscillations generated when the system exits CSC mode.

[0109] 5. Method Validation

[0110] The correctness of the method proposed in this invention patent is verified by the MATLAB-ODE solver.

[0111] A fundamental requirement of the GFMC is that the operating point can return to the initial SEP (System Operating Point) after a power system fault is cleared. During the fault recovery phase, the GFMC may operate in a conflict zone. Figure 6 It demonstrates the observed conflicts between control strategies.

[0112] In this verification case study, a three-phase ground fault occurred on the AC transmission line, causing the operating point of the GFMC to change from... Figure 6 In (a), point A moves to point B. During the fault, this point moves along the saturated P. GFM The -δ curve moves from B to C until the fault is cleared. As δ increases, the GFMC enters CSC mode a second time, accompanied by a control conflict (i.e., from D to C). Clearly, control conflicts are unacceptable in actual operation. Figure 6 As shown in (a) and (b), GFMC should operate in CSC mode, rather than frequently switching between CC and CSC modes. For better understanding, Figure 6 Figure (c) illustrates the specific relationships between the saturation domain, desaturation domain, and conflict domain. Because... Less than when At that time, the system will enter the conflict domain, that is... Figure 6 The gray area in (c). Finally, when the necessary and sufficient desaturation condition is met, GFMC exits CSC.

[0113] When the operating point enters the attraction region of the saturated SEP in CSC mode, but its trajectory does not pass through the necessary and sufficient desaturation region, the GFMC will stabilize at the saturated SEP and will be unable to exit CSC mode. Figure 7 As shown.

[0114] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for designing a grid-connected converter considering an anti-saturation current limiting strategy, applied to a grid-connected system of a grid-connected converter controlled by a VSG, characterized in that, Comprise: (1) the necessary and sufficient conditions for the grid-forming converter GFMC to switch from the conventional control CC mode to the current saturation control CSC mode; denoted as: wherein represents a current reference value, I max represents a current amplitude upper limit, δ represents a phase angle difference θ PSL - θ SYS , θ PSL represents a voltage phase of the converter, θ SYS represents a voltage phase of an infinite system; defined represents the i-th root when the following equation is equal, the roots are sorted in ascending order, and the solution set of the following equation is defined as the saturation region δ sa : wherein denote the phase angles of and and denote the elements of the first row, first column and the first row, second column of the system admittance matrix, Y ra and Y rb denote the amplitudes of and ; φ d denotes the phase difference of and the d-axis, which equals 0 in the steady state phase.​ U GFM and U SYS denote the voltage amplitudes of the converter and the system, respectively; (2) the necessary and sufficient conditions for the grid-forming converter GFMC to exit from the CSC mode and return to the CC mode; Wherein, the necessary conditions for the GFMC to exit from the CSC mode and return to the CC mode are denoted as: In the formula, definitions denotes the i-th root of the equation of the following formula, the size of the root is sorted in ascending order, and the interval δ satisfying the following formula is defined as the desaturation domain δ dsa : wherein K δ0 , K δ1 , K δ2 represent the coefficients of the constant, the one- frequency, the two- frequency components, respectively, and α δ1 , α δ2 represent the phases of the one- frequency, the two- frequency components, respectively. (3) obtaining the power angle curve of the system in the CC mode and the CSC mode, constructing a new criterion based on the desaturation region, and designing the parameters of the grid-forming converter; as follows: By analyzing P GFM -δ curve to determine the stable point of the system, thereby obtaining the power angle curve of the system in the CC mode and the CSC mode; wherein, P GFM represents the active power of the converter; The new criterion based on the desaturation region is as follows: For Must satisfy: R(Θ,δ sa ∪δ dsa )≤σ where δ cft denotes the conflict region of the control strategy, denotes the virtual power angle corresponding to the stable equilibrium point of the system in the CSC mode, C Θ δ dsa denotes the necessary desaturation region δ dsa the complement of the operating region Θ; R(A, B) denotes a custom function that calculates the length of the relative complement of set B with respect to set A; σ denotes a tolerance value, 2. The method of designing a network-forming converter according to claim 1, wherein, Considering the role of the reactive-voltage loop, U GFM satisfies the following equation: U GFM The relationship with delta is solved by the following equation: where Q GFM and represent the reactive power of the converter and its reference value, respectively; K Q represents the reactive-voltage droop coefficient; represent the converter d-axis and q-axis voltage reference values in the dq coordinate system; u0represents the steady-state voltage at the point of common coupling, a, b, c represent U GFM intermediate variables in the calculation process.

3. The method of designing a network-forming converter according to claim 1, wherein, The judgment equations of current saturation and desaturation are different, and for a specified system power angle operating region Θ, the necessary and sufficient conditions for desaturation are: where C Θ δ sa denotes the set δ sa complement in the universal set Θ; Definition of δ cft = δ sa ∩ δ dsa For control conflict domain, when When the system runs into the conflict domain, the multiple control logics of GFMC are triggered simultaneously, which appears as abnormal current source and causes significant oscillation of the system.

4. The method of designing a network-forming converter according to claim 1, wherein, In step (3), when designing the parameters of the grid-forming converter, the following two problems need to be avoided: When the operating point moves to the conflict region δ cft Due to the rapid repeated switching of the control strategy, the system is at risk of high-frequency oscillation; When the saturated SEP is located outside the sufficient and necessary desaturation domain The system faces the potential danger of permanently operating in the CSC mode and can never return to the original SEP independently.

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