Design method of network-constructing type current converter considering anti-saturation current amplitude limiting strategy

Abstract

The invention discloses a network construction type converter design method considering an anti-saturation current amplitude limiting strategy, which is applied to a VSG-controlled network construction type converter grid-connected system and comprises the following steps: (1) deducing necessary and sufficient conditions for switching a network construction type converter GFMC from a conventional control CC mode to a current saturation control CSC mode; (2) deducing necessary conditions and necessary and sufficient conditions for the GFMC to exit from the CSC mode and return to the CC mode; and (3) power angle curves of the system in the CC mode and the CSC mode are acquired, a new criterion based on a desaturation region is constructed, and parameters of the network-forming type converter are designed. According to the invention, the problem of control conflict dominated by the converter can be avoided, and the stability of the novel power system is improved.

Description

Technical Field

[0001] The invention belongs to the field of power grid control, and in particular relates to a design method for a grid-type converter taking into account an anti-saturation current limiting strategy. Background Art

[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 production methods and control strategies has brought great challenges to the security and stability of large power grids. In order to improve the stability of the system, grid-building (GFM) and grid-following (GFL) technologies have attracted widespread attention. Compared with GFL devices based on phase-locked loops (PLLs), GFMCs have the ability to independently establish voltage amplitudes and phase angles. 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 swing equation of synchronous generators. Domestic and foreign studies have shown that by adding a low-pass filter (LPF) to droop control, it can be made equivalent to VSG control. Despite having similar dynamic characteristics, GFMCs controlled by VSGs still have fundamental differences from traditional synchronous generators (SGs) due to their weaker overcurrent capabilities. During the fault and recovery stages, the current of GFMCs is more likely to reach the threshold, resulting in irreversible damage to power electronic components.

[0003] Various methods have been proposed at home and abroad to protect the converter from damage. One of the most effective methods is to limit the current reference value of the controller, also known as the direct method. By adding a current limiter between the voltage inner loop and the inner current loop, when the current reference value exceeds the upper limit, its value will be locked. In addition to the above direct methods, indirect control of the 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, reduces the voltage reference value of the converter by virtually changing the impedance characteristics of the system, thereby reducing the current output of the device. Compared with VI control, the adjustment of the power synchronization loop (PSL) can also suppress the growth of the virtual power angle (VPA) and avoid overcurrent and power angle instability.

[0004] All of the above methods can effectively alleviate the overcurrent problem in the fault ride-through (FRT) stage. However, due to the switching of control logic, various new stability problems dominated by GFMC also arise. Taking the traditional current reference value limiting strategy as an example, during the transient process, the deceleration zone of GFMC will be significantly reduced, which makes it more prone to power angle instability. Even if there is a stable operating point (SEP), GFMC may not be able to recover to its original operating state due to stabilization in an abnormal SEP or inability to exit the current saturation mode. In order to solve the above problems, a variety of anti-saturation strategies have been proposed to limit the abnormal integration of the voltage inner loop, such as: 1) Preventing continuous saturation of the integrator by offsetting the integral deviation 2) Freezing the integrator of the voltage inner loop during the current saturation period 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 simplified assumptions, and the evolution process of the conditions is still unclear.

[0005] The current saturation strategy of grid-forming converters (GFMCs) aims to protect the equipment from overcurrent damage. However, its detailed operation logic is usually not known to the public.

[0006] In summary, since few studies have fundamentally explored the desaturation conditions and their evolution patterns in fast transient processes, it is urgent to study the evolution law of the desaturation conditions of grid-type converters, so as to characterize the desaturation boundary of the system and provide further suggestions for the controller design of grid-type converters from the perspective of control strategy switching. Summary of the invention

[0007] The present invention provides a design method for a grid-type converter taking into account an anti-saturation current limiting strategy, which can avoid the control conflict problem dominated by the converter and improve the stability of the new power system.

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

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

[0010] (2) Derive the necessary and sufficient conditions for the grid-connected converter GFMC to exit from 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-connected converter.

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

[0013]

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

[0015]

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

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

[0018]

[0019] U GFM The relationship with δ is solved by the following formula:

[0020]

[0021] In the formula, Q GFM and Respectively represent the reactive power of the converter and its reference value; U GFM and U SYS Respectively represent the voltage amplitude of the converter and the system; K Q Indicates reactive power-voltage droop coefficient; represents the reference value of the converter d-axis and q-axis voltage in the dq coordinate system; u 0 represents the steady-state voltage of the grid-connected point, a, b, and c represent U GFM Intermediate variables in the calculation process.

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

[0023]

[0024] In the formula, Indicates the current reference value, I max represents the upper limit of the current amplitude, δ represents the phase angle difference θ PSL -θ SYS ,θ PSL represents the voltage phase of the converter, θ SYS Represents the voltage phase of an infinite system; definition It represents the i-th root of the following equation when the equal sign is taken. The roots are sorted in ascending order. The δ interval that satisfies the following equation is defined as the desaturation domain δ dsa :

[0025]

[0026] In the formula, K δ0 , K δ1 , K δ2 Respectively represent the coefficients of the 2nd frequency, 1st frequency, and constant components, α δ1 , α δ2 They represent the phase of the 2x frequency and 1x frequency components respectively.

[0027] The judgment equations for current saturation and desaturation are different. For a specified system power angle operation domain Θ, the necessary and sufficient conditions for desaturation are:

[0028]

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

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

[0031] In step (3), by analyzing P GFM-δ curve to determine the stability point of the system, and thus obtain the power angle curve of the system in CC mode and CSC mode; where P GFM represents the active power of the converter; δ represents the phase angle difference θ PSL -θ SYS ,θ PSL represents the voltage phase of the converter, θ SYS 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 need to be avoided:

[0033] When the working point moves to the conflict area δ cft When the control strategy is switched repeatedly, the system faces the risk of high-frequency oscillation.

[0034] When the saturated SEP is in the necessary and sufficient desaturation region When this happens, the system is potentially in danger of operating in CSC mode permanently and will never be able to independently return to the original SEP.

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

[0036] for Need to meet:

[0037]

[0038] R(θ,δ sa ∪δ dsa )≤σ

[0039] In the formula, δ cft represents the conflicting area of ​​control strategies, represents the virtual power angle corresponding to the stable equilibrium point of the system in CSC mode, C Θ δ dsa Denotes the necessary desaturation region δ dsa The complement in the running domain Θ; R(A,B) represents a custom function, whose function is to calculate the length of the relative complement of set B relative to set A; σ represents a smaller allowable value,

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

[0041] The present invention proposes a set of desaturation domain analysis methods for grid-type converters taking into account anti-saturation current limiting strategies. By deriving the necessary and sufficient conditions for the grid-type converter to enter / exit current saturation, a new type of stability problem dominated by the converter - control conflict - is determined. At the same time, a controller parameter design criterion taking into account the desaturation domain is proposed, which avoids the occurrence of such control conflict problems and improves the stability of the new power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Schematic diagram of grid-connected system of grid-connected converter controlled by VSG;

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

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

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

[0046] Figure 5 Schematic diagram of power angle curve of GFMC in different states in an embodiment of the present invention;

[0047] Figure 6 It is a schematic diagram of GFMC being able to stabilize at the initial unsaturated SEP when the system encounters a three-phase short circuit fault;

[0048] Figure 7 It is a schematic diagram of the GFMC finally stabilizing at the saturated SEP when the system encounters a three-phase short circuit fault. DETAILED DESCRIPTION

[0049] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0050] In the embodiment of the present invention, the grid-connected converter system is as follows: Figure 1 As shown in Figure 1, the converter is controlled by VSG. 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 shown.

[0051] Where J and D represent the virtual inertia and damping coefficient respectively. GFM and Respectively represent the active power of the converter and its reference value. GFM and Respectively represent the reactive power of the converter and its reference value. PSL ,θ PSL , ω 0 Respectively represent the angular velocity of the power synchronization loop, the voltage phase, the differential of the angular velocity, and the rated angular velocity of the system. SYS Represents the voltage phase of an infinite system. U GFM and U SYS Represent the voltage amplitude of the converter and the system respectively. Q Represents the reactive power-voltage droop coefficient. They respectively represent the converter d-axis and q-axis voltage reference values ​​in the dq coordinate system. Respectively represent the dq current reference values ​​output by the voltage inner loop and the current limiter. CS Indicates the current saturation trigger signal. They represent the d-axis and q-axis integral gains of the voltage inner loop integrator respectively. They represent the d-axis and q-axis proportional gains of the voltage inner loop integrator respectively. They represent the outputs of the d-axis and q-axis integrators of the voltage inner loop, respectively. max Indicates the upper limit of the current amplitude. Represents the dq reference current used during current saturation.

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

[0053] 1. External characteristic model of the system

[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:

[0055]

[0056] Where Y a ,Y b ,Y c ,Y d Respectively represent the four sub-matrices of the system admittance matrix. s and U s Respectively represent the current and voltage column vectors of the system and the power node. ns Represents the column vector of voltages at 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] Expanding the above equation, we have:

[0060]

[0061] Where, the subscripts GFM and SYS denote the state variables of the GFMC and infinite system, respectively. Current saturation control (CSC) causes the GFMC to switch 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 GFM Moving to the right side of the equation, we get

[0062]

[0063] 2. Derivation of current saturation trigger conditions

[0064] When the current reference value is greater than I max , GFMC will switch from CC to CSC. Before reaching the current threshold, GFMC has voltage source characteristics, and the internal control loop can be regarded as a link with transfer function G≈1. In this operating mode, it can be obtained:

[0065]

[0066] In the formula, Respectively and The phase angle of φ d express The phase difference with the d-axis is equal to 0 in the steady state. δ represents the phase angle difference θ PSL -θ SYS ,θ PSL represents the voltage phase of the converter, θ SYS Represents the voltage phase of an infinite system.

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

[0068]

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

[0070]

[0071] Will I max Substituted By re-arranging the equations, we can find the conditions for transitioning from CC to CSC:

[0072]

[0073] definition It represents the i-th root when the above equation is equal, and the roots are sorted in ascending order. The solution set of the above equation is defined as the saturation domain δ sa, so the necessary and sufficient conditions for GFMC to switch to current saturation mode can be expressed as:

[0074]

[0075] 3. Derivation of current saturation exit conditions

[0076] In the current saturation state, the integrator of the voltage inner loop is frozen, and the voltage control of the grid-type converter is forced to become open-loop control. First, we should determine u d ,u q ,Q GFM , 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 , we can get u d ,u q :

[0079]

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

[0081] Therefore, the reactive power can be calculated by the formula Q GFM =u q i d -u d i q After simplification, Q GFM and The expression is:

[0082]

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

[0084]

[0085] will u d ,u q ,Q GFM , Substituting into the above formula, we can get The expression of δ is:

[0086]

[0087] In the formula, the coefficients are:

[0088]

[0089] when Less than I max GFMC can exit CSC mode and return to CC mode only when I max Substituting the current saturation The necessary condition for GFMC desaturation is:

[0090]

[0091] definition It represents the i-th root when the above equation is equal, and the roots are sorted in ascending order. The δ interval that satisfies the above equation is defined as the desaturation domain δ dsa Therefore, the necessary condition for current desaturation can be expressed as:

[0092]

[0093] Due to the different judgment equations for current saturation and desaturation, GFMC may trigger multiple control strategies at the same time, resulting in conflicts between different control logics. cft When it is not empty, a control strategy conflict will occur

[0094]

[0095] For a specified system power angle operating domain Θ, the necessary and sufficient conditions for desaturation are:

[0096]

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

[0098] According to the set parameter set, the desaturation domain under the parameter set can be drawn. Taking the necessary desaturation domain as an example, Figure 4 shown.

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

[0100] 4. GFMC controller design method based on desaturation domain

[0101] By analyzing P GFM -δ curve can determine the stability point of the system. GFM=u d i d +u q i q .

[0102]

[0103] Thus, the power angle curve of the system in CC mode and CSC mode can be obtained, such as Figure 5 shown.

[0104] According to the previous analysis, two problems need to be avoided when designing the GFMC controller: 1) When the working point moves to the conflict area δ cft When the control strategy is switched repeatedly, the system will face the risk of high-frequency oscillation. Fundamentally, the control logic conflict indicates that the controller design is flawed, which is not allowed. 2) When the saturated SEP is in the sufficient and necessary desaturation region If this is not the case, the system will potentially be in permanent CSC mode and will no longer be able to independently return to the original SEP.

[0105] In order to prevent inappropriate desaturation condition configuration from causing adverse effects on the system, this invention patent proposes a new criterion based on the desaturation region to provide a reference for the design of GFMC controller. The criteria to be met are as follows:

[0106]

[0107] R(θ,δ sa ∪δ dsa )≤σ (21d)

[0108] Among them, equations (21a)-(21d) give the four requirements of the proposed criterion. Equation (21a) ensures that there is no conflict domain in the operating domain. Equations (21b) and (21c) ensure that the system has an unsaturated SEP and prevent the attraction domain where the saturated SEP is located from capturing the operating point during the transient process, avoiding the possibility of the system stabilizing at the saturated SEP. In addition, satisfying equation (21d) will reduce the oscillation generated when the system exits the CSC mode.

[0109] 5. Method Validation

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

[0111] After the fault in the power system is cleared, it is a basic requirement of GFMC that the operating point can return to the initial SEP. During the fault recovery phase, GFMC may operate to the conflict area. Figure 6 The observed conflicts between control strategies are demonstrated.

[0112] In this verification case study, a three-phase ground short circuit fault occurs in the AC transmission line, causing the GFMC to operate from Figure 6 In (a), A moves to B. During the fault, this point moves along the saturated P GFM The -δ curve moves from B to C until the fault is eliminated. As δ increases, GFMC enters CSC mode for the second time, accompanied by control conflict (i.e., from D to ). Obviously, control conflict is not allowed in actual operation. Figure 6 As shown in (a) and (b), GFMC should operate in CSC mode instead of switching between CC and CSC modes at high frequency. For better understanding, Figure 6 (c) shows the specific relationship between the saturation domain, desaturation domain, and conflict domain. Less than when When the system enters the conflict domain, Figure 6 Finally, when the necessary and sufficient desaturation conditions are met, GFMC exits CSC.

[0113] When the working point enters the attraction region of the saturated SEP in the CSC mode, and its trajectory does not pass through the necessary and sufficient desaturation region, the GFMC will stabilize at the saturated SEP and will never be able to exit the CSC mode. Figure 7 shown.

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

Claims

1. A design method for a grid-connected converter considering an anti-saturation current limiting strategy, applied to a grid-connected converter grid-connected system controlled by a VSG, characterized in that: include: (1) Derive the necessary and sufficient conditions for the grid-connected converter GFMC to switch from the conventional control CC mode to the current saturation control CSC mode; (2) Derive the necessary and sufficient conditions for the grid-connected converter GFMC to exit from CSC mode and return to CC mode; (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-connected converter.

2. The grid-type converter design method considering the anti-saturation current limiting strategy according to claim 1 is characterized in that: In step (1), the necessary and sufficient conditions for GFMC to switch from the conventional control CC mode to the current saturation control CSC mode are expressed as: In the formula, Indicates the current reference value, I max represents the upper limit of the current amplitude, δ represents the phase angle difference θ PSL -θ SYS ,θ PSL represents the voltage phase of the converter, θ SYS Represents the voltage phase of an infinite system; definition It represents the i-th root of the following equation when the equal sign is taken. The roots are sorted in ascending order. The solution set of the following equation is defined as the saturation domain δ sa : In the formula, Respectively and The phase angle, and Respectively represent the elements of the first row and first column and the first row and second column of the system admittance matrix, Y ra and Y rb Respectively and The amplitude of φ d express The phase difference with the d-axis is equal to 0 in the steady state; U GFM and U SYS Represent the voltage amplitudes of the converter and the system respectively.

3. The design method of a grid-type converter considering an anti-saturation current limiting strategy according to claim 2, characterized in that: Considering the effect of reactive power-voltage loop, U GFM Satisfies the following equation: U GFM The relationship with δ is solved by the following formula: In the formula, Q GFM and Respectively represent the reactive power of the converter and its reference value; U GFM and U SYS Respectively represent the voltage amplitude of the converter and the system; K Q Indicates reactive power-voltage droop coefficient; represents the reference value of the converter d-axis and q-axis voltage in the dq coordinate system; u0 represents the steady-state voltage at the grid-connected point, a, b, c represent U GFM Intermediate variables in the calculation process.

4. The design method of a grid-type converter considering an anti-saturation current limiting strategy according to claim 2, characterized in that: In step (2), the necessary condition for GFMC to exit from CSC mode and return to CC mode is expressed as: In the formula, Indicates the current reference value, I max represents the upper limit of the current amplitude, δ represents the phase angle difference θ PSL -θ SYS ,θ PSL represents the voltage phase of the converter, θ SYS Represents the voltage phase of an infinite system; definition It represents the i-th root of the following equation when the equal sign is taken. The roots are sorted in ascending order. The δ interval that satisfies the following equation is defined as the desaturation domain δ dsa : In the formula, K δ0 , K δ1 , K δ2 Respectively represent the coefficients of the 2nd frequency, 1st frequency, and constant components, α δ1 , α δ2 They represent the phase of the 2-fold and 1-fold components respectively.

5. The design method of a grid-type converter considering an anti-saturation current limiting strategy according to claim 4, characterized in that: The judgment equations for current saturation and desaturation are different. The system power angle operation domain Θ, the necessary and sufficient conditions for desaturation are: In the formula, C Θ δ sa Represents the set δ sa the complement of the set Θ; Definition of δ cft =δ sa ∩δ dse To control the conflict domain, when When the system is in a conflict domain, the system may run into the conflict domain. At this time, various control logics of GFMC are triggered simultaneously, which appears as an abnormal current source and causes significant oscillation of the system.

6. The design method of a grid-type converter considering an anti-saturation current limiting strategy according to claim 5, characterized in that: In step (3), by analyzing P GFM -δ curve to determine the stability point of the system, and thus obtain the power angle curve of the system in CC mode and CSC mode; where P GFM represents the active power of the converter; δ represents the phase angle difference θ PSL -θ SYS ,θ PSL represents the voltage phase of the converter, θ SYS Represents the voltage phase of an infinite system.

7. The design method of a grid-type converter considering an anti-saturation current limiting strategy according to claim 5, characterized in that: In step (3), when designing the parameters of the grid-type converter, the following two problems need to be avoided: When the working point moves to the conflict area δ cft When the control strategy is switched repeatedly, the system faces the risk of high-frequency oscillation. When the saturated SEP is in the necessary and sufficient desaturation region When this happens, the system is potentially in danger of operating in CSC mode permanently and will never be able to independently return to the original SEP.

8. According to the design method of grid-type converter considering anti-saturation current limiting strategy as claimed in claim 7, in step (3), the new criterion based on the desaturation region is as follows: for Need to meet: R(Θ,δ sa ∪δ dsa )≤σ In the formula, δ cft represents the conflicting area of ​​control strategies, represents the virtual power angle corresponding to the stable equilibrium point of the system in CSC mode, C Θ δ dse Denotes the necessary desaturation region δ dsa The complement in the running domain Θ; R(A, B) represents a custom function, whose function is to calculate the length of the relative complement of set B relative to set A; σ represents a smaller allowable value,

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