Following-networking combined low-voltage ride-through control method based on transient support

Abstract

The invention discloses a tracking-network construction combined low-voltage ride-through control method based on transient support. The method comprises the following steps: S1, analyzing transient voltage characteristics and requirements of a network construction-tracking network converter; s2, analyzing a transient voltage support target of the network construction-network following type hybrid multi-machine system; s3, constructing a network construction-network following type hybrid multi-machine system control strategy; and S4, on the basis of the network construction-network following type hybrid multi-machine system control strategy constructed in the S3, optimizing the low voltage ride-through control of the network following type converter and the transient voltage control of the network construction type converter. According to the control method disclosed by the invention, the advantages of the network-building / network-following converter are complemented, the control between the network-building / network-following converter and the network-building / network-following converter is coordinated, and the transient voltage of the power grid is actively supported by using the multi-machine converter system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of grid-connected control of new energy power systems, and particularly relates to a follow-grid combined low-voltage ride-through control method based on transient support. BACKGROUND

[0002] With the increasing proportion of new energy power generation equipment represented by wind power and photovoltaic power in modern alternating current power systems, large-scale grid-connected converters have put forward higher control requirements for transient voltage of the power system. The fluctuation and recovery process of transient voltage caused by faults, control, etc. deeply affect the control process of the converter system. Under the background of multi-loop large-capacity and large-scale regional grid interconnection, transient voltage stability is also related to the voltage / frequency stability of interconnected grids and the cascading reaction of power grid faults. Therefore, the transient voltage support capability of the power grid has become an important constraint for reducing fault impact and ensuring stable operation of the power grid. The research on the interaction between new energy stations and the transient voltage of the power grid is mostly concentrated in the stable control of single-machine converter equipment and high-low voltage fault ride-through. Therefore, there is still a large research space in the transient voltage support of multi-machine converter.

[0003] In summary, under the background of a large number of new energy access to the power grid, the main difficulties of transient voltage support for multi-machine converter systems with follow-grid and grid-forming control modes include:

[0004] 1) Reliably improving the short-circuit capacity of the station and the stability of phase-locked synchronization under transient conditions: Grid-forming converters have strong adaptability to weak grids due to their voltage source characteristics. However, the speed and accuracy of its reactive power output are greatly affected by the power synchronization control strategy and its parameters. The reactive power output capability under transient conditions is also limited by current limiting and active power disturbance. Under transient conditions, the target of grid-forming control is greatly affected by transient voltage disturbance, and the output current of the converter is easily beyond the limit, which will cause the protection action of the grid-forming converter to exit or enter the controlled current source control mode, thereby losing the original controllable voltage support capability. In addition, due to the large time constant of active power control, the active power is easily oscillated in a large range under transient conditions. Considering the current output capability of the converter, the active power oscillation will limit the voltage stability capability of the converter.

[0005] 2) Having sufficient reactive power output to change the voltage: The follow-grid converter is equivalent to a controlled current source externally, and can output accurate reactive power, but has poor adaptability to weak grids. The reactive power output capability under transient conditions is mainly limited by the capacity and stability of the converter itself. Under steady state, active power output is usually the main output, which is limited by the capacity of the converter device, and the active and reactive power needs to be redistributed under transient conditions. The reliability of synchronization with the grid decreases with the decrease of the grid strength, and the output fluctuation of the follow-grid converter under weak grid transient conditions is also not conducive to system stability. SUMMARY

[0006] To solve the above technical problems, the application provides a follow-network-constructed network joint low-voltage ride-through control method based on transient support, which takes the follow-network-constructed network converter system as a transient voltage active support scheme, and the constructed network converter provides a reliable and stable voltage source in a weak power grid, improves the system short-circuit capacity and provides stable phase-locked synchronization conditions, and the follow-network converter provides sufficient compensation capacity. The constructed network converter ensures the voltage source characteristics and limits overcurrent through active-reactive adaptive fast regulation control, thereby improving the voltage stability of the constructed network converter; the follow-network converter outputs sufficient reactive current to raise the output voltage of the point of common coupling under transient conditions through adaptive compensation control. The application complements the advantages of the constructed network / follow-network converter and coordinates the control between the two, and actively supports the transient voltage of the power grid by using a multi-machine converter system.

[0007] The technical scheme adopted by the application is as follows:

[0008] The follow-network-constructed network joint low-voltage ride-through control method based on transient support comprises the following steps:

[0009] S1: analyzing the transient voltage characteristics and requirements of the constructed network-follow-network converter;

[0010] S2: analyzing the transient voltage support target of the constructed network-follow-network hybrid multi-machine system;

[0011] S3: constructing a control strategy for the constructed network-follow-network hybrid multi-machine system;

[0012] S4: based on the control strategy for the constructed network-follow-network hybrid multi-machine system constructed in S3, optimizing the low-voltage ride-through control of the follow-network converter and the transient voltage control of the constructed network converter, respectively.

[0013] In S1, the voltage at the point of common coupling of the constructed network-follow-network converter parallel equivalent circuit; and the equivalent voltage source single branch current of the constructed network converter are expressed as:

[0014]

[0015] wherein, I s is the system current; Y s is the system admittance; U t is the equivalent voltage source voltage of the system; I eqt is the equivalent total injection current of all equivalent controlled sources; Y t is the sum of all converter equivalent admittances; I eq,k is the equivalent controlled current source current of the kth follow-network converter; Y veq,j is the equivalent series admittance of the equivalent controlled voltage source of the jth constructed network converter; Yieq,k Yk is the equivalent parallel admittance of the kth grid- following converter; U eq,j Uj is the equivalent controlled voltage source voltage of the jth grid- forming converter. eq,1 ,…,U eq n1 is the equivalent controlled voltage source of the grid- forming converter;

[0016] I v,1 ,…,I v,n1 is the branch current of the equivalent controlled voltage source of the grid- forming converter;

[0017] Y veq,1 ,…,Y veq,n1 is the equivalent series admittance of the equivalent controlled voltage source of the grid- forming converter;

[0018] I eq,1 ,…,I eq,n2 is the equivalent controlled current source of the grid- following converter;

[0019] I i,1 ,…,I i,n2 is the branch current of the grid- following converter;

[0020] Y ieq,1 ,…,Y ieq,n2 is the equivalent parallel admittance of the grid- following converter;

[0021] U L is the voltage of the point of common coupling, which is also the phase- locked voltage of the grid- following converter;

[0022] n1 is the number of grid- forming converters; n2 is the number of grid- following converters.

[0023] By analyzing equation (1), it can be seen that the voltage U L of the point of common coupling contains the equivalent voltage source U t of the power system and the equivalent controlled voltage source component U eq,i of each grid- forming converter.

[0024] Considering the starting moment of the transient voltage condition of the power grid, the electrical quantities in the field station remain unchanged, and the branch current change of the controlled source caused by the change of the system voltage is further written as equation (2) from equation (1):

[0025]

[0026] In equation (2): ΔI v,1 is the branch current change of the grid- forming converter, ΔI i,1 is the branch current change of the grid- following converter; and ΔU t is the change of the system voltage.

[0027] As can be seen from formula (2), the large system voltage change at the initial moment of the transient state will cause the rapid change of the branch current of the grid-following converter; at this time, the current change is not limited by the control of the grid-following converter itself, and the total current change will be shared by multiple grid-following converter branches.

[0028] In S2, the transient voltage support target includes the following:

[0029] 1) In the weak grid fault condition, rely on the grid-forming converter to provide stable voltage for the grid-following converter, improve the short-circuit capacity under low voltage, and enable the grid-following converter to reliably synchronize with the grid through the phase-locked loop to accurately and stably inject target current into the grid.

[0030] 2) The grid-following converter needs to output accurate reactive power to support the system voltage during the transient state, while ensuring the smooth recovery of the grid voltage after the fault is cleared.

[0031] 3) The grid-forming converter cannot be forced to change its voltage source external characteristic to current source external characteristic due to current limiting during the transient state, which cannot provide reliable support voltage.

[0032] 4) The dynamic characteristic of the active power output of the grid-forming converter should have strong damping to ensure the stability of the controlled voltage source grid-forming converter during the transient process.

[0033] In S3, the control mode of the grid-forming-grid-following hybrid multi-machine system is aimed at power coordination control and voltage coordination control during the transient state.

[0034] During the transient state, the output voltage regulation speed of the grid-forming converter should be greater than the power regulation speed of the grid-following converter, and to ensure the stability of the grid-forming converter, the active and reactive reference values are changed to ensure the voltage source characteristic limitation and overcurrent limitation of the grid-forming converter; Reference: Li, J., Zou, F., You, H., et al. Adaptive low-voltage ride-through control strategy for grid-forming energy storage converters[J]. Transactions of China Electrotechnical Society, 2025, 40(09): 2724-2737. DOI:10.19595 / j.cnki.1000-6753.tces.240850.

[0035] According to the formula (13) and (21) in the literature, the active and reactive reference values are adjusted to ensure the voltage source characteristic of the grid-forming converter and limit the overcurrent, and the formula is as follows:

[0036]

[0037] In the formula: P * is the adjusted active power reference value, P refP if V gf is the grid voltage, δ is the power angle, L g is the inductance, Q * is the adjusted reactive power reference value, k q is the reactive power integral coefficient, V ifref is the converter output voltage reference value, Q is the reactive power compensation amount;

[0038] By adjusting the virtual damping, the impact of power angle change on voltage support is reduced, thereby improving the voltage stability of grid-connected converters. Reference: Wu Z, Li Z, Liu J, et al. JDL coordinated control strategy of energy storage converter based on VSG [J]. Power System Protection and Control, 2025, 53(07): 112-122. DOI: 10.19783 / j.cnki.pspc.240824.

[0039] The reference adjusts the virtual damping adaptively to achieve voltage stability according to formula (19) in the literature, as follows:

[0040]

[0041] In the formula: D p0 is the initial damping coefficient of grid-connected converter, D p1 is the adaptive damping coefficient, P e is the actual output value of active power, T d is the action threshold of virtual damping.

[0042] The grid-connected converter outputs sufficient reactive current in transient state through adaptive compensation control to raise the output voltage of the point of common coupling. Reference: Chen ZY, Yang LH, Shi JZ, et al. Analysis of transient stability of grid-connected converter and grid-following converter system considering transient mode switching [J]. Proceedings of the Chinese Society of Electrical Engineering, 2025, 45(15): 6025-6038. DOI: 10.13334 / j.0258-8013.pcsee.241359.

[0043] The reference controls the impedance Zg and Z GFM according to formula (4) in the literature to increase the output of sufficient reactive current, as follows:

[0044]

[0045] In the formula: I GFM is the grid-connected output current; U GFM is the grid-connected converter output voltage; Z GFM is the grid-connected converter impedance; Z gis the grid impedance; U g is the voltage voltage; I GFL is the grid-connected converter output voltage.

[0046] In the S4, optimization strategies are proposed for low voltage ride through control of grid-connected converter and transient voltage control of grid-forming converter, including:

[0047] 4.1: Active-reactive adaptive fast regulation strategy for low voltage ride through control of grid-connected converter:

[0048] 4.1.1: An active-reactive adaptive updating method during voltage sag is proposed, including the following:

[0049]

[0050] In formula (7), S n , Q ref , represent the rated power, active power instruction during voltage sag, reactive power instruction under normal conditions, reactive power instruction during voltage sag, respectively; represent the per-unit value of output voltage amplitude; α and β are the reactive regulation coefficient and active regulation coefficient, respectively;

[0051] 4.1.2: Determine the value of α and β parameters:

[0052] α can control the size of reactive power output, so α can be used to change the size of grid-connected converter output reactive current, thereby enhancing the transient support capability of the system.

[0053] β can adjust the minimum value of output active power, and smaller β can reduce the active power output by grid-connected converter during fault, thereby improving the power angle stability of the system.

[0054] 4.2: Low voltage ride through control of grid-connected converter:

[0055] When the grid-connected converter is in steady state operation, it basically operates in unity power factor state. When the control system detects the fault state, the system will quickly switch to low voltage ride through state. At this time, the system will lock the PQ loop, and output active and reactive power through direct control of dq axis current, while ensuring that the current does not exceed the limit. An adaptive generation method of d, q axis current instruction signal during fault is designed, and its formula is:

[0056]

[0057] In formula (8), I d is the d-axis current, I q is the q-axis current, S NU is rated voltage, k1, k2, k3 are fault adjustable parameters; u N U is rated voltage, k1, k2, k3 are fault adjustable parameters; u c U is rated voltage, k1, k2, k3 are fault adjustable parameters; u

[0058] In the event of a fault, first, the output current of the q-axis is calculated according to the fault drop degree, and then the output current of the d-axis is calculated. Thus, the transient reactive output of the controlled voltage source grid-connected converter can be improved, and the overall output current of the grid-connected converter can be ensured to be within a reliable range.

[0059] In order to ensure that the transient reactive control does not affect the normal steady-state regulation, the transient reactive control is put into and out according to the control point voltage condition. Specifically, under the transient operating condition, the voltage drop is set to 0.9, 0.5 or below to switch according to the range of the adaptive strategy.

[0060] The technical effects of the grid-connected converter based on transient support are as follows:

[0061] 1) The grid-connected converter is controlled by active-reactive adaptive fast regulation, which ensures the voltage source characteristics of the converter and limits the overcurrent, thereby improving the voltage stability of the grid-connected converter.

[0062] 2) The grid-connected converter is controlled by adaptive compensation, which makes it output sufficient reactive current to lift the output voltage of the point of common coupling in the transient state, complements the advantages of the grid-connected converter and the grid-connected converter, coordinates the control between the two, and actively supports the transient voltage of the grid-connected grid-connected hybrid multi-machine system.

[0063] 3) The strategy proposed in the present application has small oscillation, stable fault, good voltage support, and short-circuit current not exceeding the standard; it provides a fault ride-through solution for the grid with fast voltage support and strong stability, and can still ensure safe operation of the system in the grid with SCR≥2. BRIEF DESCRIPTION OF DRAWINGS

[0064] The present application will be further described below in conjunction with the drawings and examples:

[0065] Figure 1 is the equivalent circuit diagram of the converter parallel architecture of the present application.

[0066] Figure 2 is the overall architecture schematic diagram of the hybrid multi-machine control strategy of the present application.

[0067] Figure 3 is the active and reactive power instruction curve of the present application.

[0068] Figure 4(a) is a low voltage ride-through test when SCR=2 Figure 1 ;

[0069] Figure 4(b) is a low voltage ride through test with SCR = 2 Figure 2 ;

[0070] Figure 4(c) is a low voltage ride through test with SCR = 2 Figure 3 .

[0071] Figure 5(a) is a low voltage ride through test with SCR = 3 Figure 1 ;

[0072] Figure 5(b) is a low voltage ride through test with SCR = 3 Figure 2 ;

[0073] Figure 5(c) is a low voltage ride through test with SCR = 3 Figure 3 .

[0074] Figure 6(a) is a simulation case with different strategies with SCR = 2 Figure 1 ;

[0075] Figure 6(b) is a simulation case with different strategies with SCR = 2 Figure 2 ;

[0076] Figure 6(c) is a simulation case with different strategies with SCR = 2 Figure 3 ;

[0077] Figure 6(d) is a simulation case with different strategies with SCR = 2 Figure Four

[0078] Figure 6(e) is a simulation case with different strategies with SCR = 2 Figure Five

[0079] Figure 7(a) is a simulation case with different strategies with SCR = 3 Figure 1 ;

[0080] Figure 7(b) is a simulation case with different strategies with SCR = 3 Figure 2 ;

[0081] Figure 7(c) is a simulation case with different strategies with SCR = 3 Figure 3 ;

[0082] Figure 7(d) is a simulation case with different strategies with SCR = 3 Figure Four

[0083] Figure 7(e) is a simulation case with different strategies with SCR = 3 Figure Five DETAILED DESCRIPTION

[0084] The follow-follow network joint low voltage ride through control method based on transient support includes the following steps:

[0085] Step 1: Build the overall control architecture of the hybrid multi-machine system

[0086] Step 2: For the low voltage ride through control of the grid-connected converter, an adaptive generation method of the d, q axis current command signal during fault is proposed. For the problems existing in the transient voltage control of the grid-connected converter, a control method combining active-reactive adaptive fast regulation with dynamic adaptive virtual damping control is proposed;

[0087] Step 3: Based on MATLAB&Simulink, an electromagnetic transient simulation verification platform of the grid-connected-following hybrid multi-machine system connected to an infinite grid is established;

[0088] Step 4: The low voltage ride through performance of the grid-connected-following hybrid multi-machine system under short-circuit current ratios of 2 and 3 is tested;

[0089] Step 5: After the test, the effectiveness of the strategy is analyzed.

[0090] (I): The overall control architecture of the grid-connected-following hybrid multi-machine system is established:

[0091] In the step 1, the problem studied by the present application is the transient support problem of the new energy station under the condition of a large short-time voltage drop in the transient voltage problem. The problems to be solved are as follows:

[0092] 1. What is the relationship between the transient support demand of the new energy station and the voltage drop amplitude of the PCC point and the size of the line impedance.

[0093] 2. For the grid-connected converter, how to design the strategy to ensure that it can provide transient voltage for the following grid-connected converter when the voltage is short-time and large amplitude, so that the phase-locked loop can remain stable.

[0094] 3. For the following grid-connected converter, how to design the strategy to ensure that it can accurately and quickly provide reactive current to support the voltage of the PCC point when the voltage is short-time and large amplitude.

[0095] The problems are solved through the following analysis:

[0096] 1.1. Analysis of the key points of the transient voltage support of the grid-connected-following hybrid multi-machine system:

[0097] The units in the new energy station are often connected to the unit transformer through the line via the converter, and then connected to the grid via a section of transmission line. Considering that the grid voltage sag time generally does not exceed 1 second, the dynamic change process of the converter DC bus capacitor voltage can be ignored. At this time, the dynamic processes of the machine-side converter and the grid-side converter are considered to be relatively decoupled, so an independent voltage source is used to replace the machine-side converter part when modeling. At this time, the new energy station is regarded as a grid-connected-following hybrid multi-machine system, and the transient voltage support of the new energy station is the transient voltage support of the grid-connected-following hybrid multi-machine system.

[0098] New energy converters in grid-formation and grid-following hybrid multi-machine systems can be divided into grid-formation converters and grid-following converters according to different control modes. According to the different effects of different converter control modes, the grid-formation converter can be regarded as a controlled voltage source; the grid-following converter can be regarded as a controlled current source. For such a grid-formation and grid-following hybrid multi-machine system with grid-following and grid-formation control modes, the difficulties of transient voltage support mainly include:

[0099] ①Reliably improve the short-circuit capacity of the substation and the stability of phase-locked synchronization under transient operating conditions.

[0100] ②Have enough reactive power output to change the voltage.

[0101] 1.2. Analysis of the transient voltage characteristics and requirements of grid-formation and grid-following converters:

[0102] According to the control characteristics of the grid-formation and grid-following converter, the equivalent circuit of its parallel connection is as shown in Figure 1 The expression of the common coupling point voltage of the grid-formation and grid-following converter parallel equivalent circuit and the single branch current of the grid-formation converter equivalent voltage source is:

[0103]

[0104] Where, I s is the system current; Y s is the system admittance; U eq,1 ,…,U eq ,n1 is the equivalent controlled voltage source of the grid-formation converter; I v,1 ,…,I v,n1 is the branch current of the equivalent controlled voltage source of the grid-formation converter; Y veq,1 ,…,Y veq,n1 is the equivalent series admittance of the grid-formation converter equivalent controlled voltage source; I eq,1 ,…,I eq,n2 is the grid-following converter equivalent controlled current source; I i,1 ,…,I i,n2 is the grid-following converter branch current; Y ieq,1 ,…,Y ieq,n2 is the equivalent parallel admittance of the grid-following converter; U L is the common coupling point voltage, which is also the phase-locked voltage of the grid-following converter; n1 is the number of grid-formation converters; n2 is the number of grid-following converters.

[0105] Analysis of equation (1) shows that the common coupling point voltage U L contains the equivalent voltage source U t of the power system and the equivalent controlled voltage source component U eq,iWhen the voltage sag occurs in the power system, the equivalent voltage source U t When the voltage amplitude drops, especially when the voltage amplitude drops to 20% or less, on the one hand, due to the control bandwidth of the phase-locked loop and the system impedance, the grid-following converter is prone to instability, while the grid-forming converter can provide a reliable synchronization source for the grid-following converter which is equivalent to a controlled current source to ensure the stable output of its current; on the other hand, when the system voltage drops significantly, the presence of the grid-forming converter can maintain the voltage amplitude of U L to a certain extent, thereby ensuring the short-circuit capacity of the new energy station, that is, ensuring the voltage support capability of the station to the system.

[0106] From the above analysis, it can be seen that the grid-forming converter provides a stable and reliable output voltage, which is the key to ensuring the transient reactive power support capability of the station.

[0107] Considering the starting time of the transient voltage condition of the power grid, the electrical quantities in the station remain unchanged, and the change of the controlled source branch current caused by the change of the system voltage can be further written as formula (1):

[0108]

[0109] In the formula: ΔI v,1 is the change of the grid-forming converter branch current, ΔI i,1 is the change of the grid-following converter branch current; ΔU t is the change of the system voltage.

[0110] As can be seen from formula (2), a large change in the system voltage at the initial time of the transient condition will cause a rapid change in the grid-following converter branch current. At this time, the current change is not limited by the control of the converter itself, and multiple converter branches will share the total current change. Considering the current-carrying and heating limitations of power devices, and in order to prevent the current change of a converter branch from being greater than that of other branches due to poor transient current sharing, it is necessary to limit the amplitude of the transient overcurrent of the grid-forming-grid-following converter at the subsequent time through the control of the converter to reduce its impact on power electronic devices and improve the safety margin.

[0111] Under the transient condition, there is often a large active power and voltage fluctuation, and the phase control of the grid-forming converter comes from the active power control. Appropriately increasing the active power damping under the transient condition can improve the stability of the synchronization phase angle, which is conducive to the stable operation of the controlled voltage source converter. Under the premise of reliable phase-locked synchronization and certain short-circuit capacity, the grid-following converter needs to output a large amount of accurate reactive power to support the voltage under the transient voltage condition, while the grid-following converter mainly outputs active power under normal conditions. Therefore, the grid-following converter needs to perform adaptive compensation control according to the transient condition.

[0112] 1.3. Analysis of the transient voltage support target of grid-forming-following hybrid multi-machine system:

[0113] Based on the above analysis of the transient voltage control requirements and the control characteristics of grid-forming-following converters, the advantages of grid-forming-following converters can be complementary, and the control between them can be coordinated to actively support the transient voltage of the grid-forming-following hybrid multi-machine system. The key points of transient support capability include the following:

[0114] ① In the weak grid fault operating condition, rely on the grid-forming converter to provide stable voltage for the following converter, improve the short-circuit capacity under low voltage conditions, so that the following converter can reliably synchronize with the grid through the phase-locked loop and accurately and stably inject target current into the grid.

[0115] ② The following converter needs to output accurate reactive power to support the system voltage in transient conditions, while ensuring the smooth recovery of the grid voltage after fault clearance.

[0116] ③ In the transient condition, the grid-forming converter cannot be forced to change its voltage source external characteristic to current source external characteristic due to current limiting, and cannot provide reliable support voltage.

[0117] ④ The dynamic characteristics of the active power output of the grid-forming converter should have strong damping to ensure the stability of the controlled voltage source converter in the transient process.

[0118] 1.4. Overall architecture of the control strategy of the grid-forming-following hybrid multi-machine system:

[0119] The overall control architecture of the grid-forming-following hybrid multi-machine system is shown in Figure 2 , where U Cd , U Cq are the dq components of the voltage on the filter capacitor, I g is the output current of the converter, i gdref , i gqref are the given values of the dq-axis currents, i gd , i gq are the actual values of the dq-axis currents, P ref is the given value of active power, Q ref is the given value of reactive power, and U Cdref is the given value of control voltage. The control mode of the grid-forming-following converter in the hybrid multi-machine system mainly includes power coordination control and voltage coordination control in steady state and transient conditions. Table 1 is a summary of the hybrid multi-machine control mode architecture.

[0120] Table 1 Hybrid multi-machine control mode architecture table

[0121]

[0122]

[0123] Under transient conditions, the output voltage regulation speed of a grid-type converter is greater than that of a ground-type converter. To ensure the stability of the grid-type converter, adaptive active-reactive power control is used to limit the voltage source characteristics and overcurrent of the converter; adaptive virtual damping is used to reduce the impact of power angle changes on voltage support, thereby improving the voltage stability of the grid-type converter; and adaptive compensation control is used to ensure that the ground-type converter outputs sufficient reactive current under transient conditions to raise the output voltage of the common coupling point.

[0124] (II): Optimization of strategies for low-voltage ride-through control and transient voltage control in grid-connected converters:

[0125] In the hybrid support strategy of grid-connected converter, optimization strategies are proposed for low-voltage ride-through control of grid-connected converter and transient voltage control of grid-connected converter, respectively:

[0126] 2.1. Active-Reactive Adaptive Fast Adjustment Strategy for Low Voltage Ride-Through Control of Grid-Type Converters:

[0127] By adaptively adjusting reactive power through output voltage amplitude, and considering the limited short-circuit capacity of the system, active power needs to be adjusted in conjunction with the system's short-circuit capacity and reactive power. When the voltage drop is not severe, it is assumed that a certain amount of active power output is still required to ensure power output; when the voltage drop is severe, active power output should be suppressed as much as possible to ensure a stable power angle during fault periods. This is because the two active power commands are mathematically related to... The function, in order to prevent due to Changes in the active power command can lead to abrupt changes and deteriorate the stability of the power angle, necessitating that the function remain numerically continuous. To ensure the characteristics of the controlled voltage source under voltage sag conditions of arbitrary magnitudes, a feasible adaptive update method for active and reactive power during voltage sags is proposed.

[0128]

[0129] In equation (3), S represents the per-unit value of the output voltage amplitude. n , Q ref , These represent the active power command at rated power, active power command during voltage sag, reactive power command under normal conditions, and reactive power command during voltage sag, respectively. α and β are the reactive power regulation coefficient and active power regulation coefficient, respectively.

[0130] 2.2. Determine the values ​​of parameters α and β:

[0131] Alpha can control the size of the reactive power output, so alpha can be used to change the size of the grid-connected type converter output reactive current, and then enhance the transient support capability of the system. After debugging, the recommended value range of alpha is 0.9-1.7. The simulation value built in the present application is 1.5.

[0132] Beta can adjust the minimum value of the output active power, and smaller beta can reduce the active power output by the grid-connected type converter during fault, thereby improving the power angle stability of the system; but too small system output active power may lead to the output power of the power generation device unable to be output from the converter, which will lead to the DC bus voltage of the converter rising, thereby affecting the low voltage ride through capability of the system. Therefore, the value of the parameter beta should not be too small. After debugging, the recommended value range of beta is 0.05-0.4. The simulation value built in the present application is 0.2. The active power instruction and reactive power instruction generation under voltage sag condition is as shown in Figure 3 .

[0133] 2.3. Low voltage ride through control of grid-following type converter:

[0134] The transient voltage control block diagram of the grid-following type converter is as shown in Figure 2 , wherein i d_fault_ref , i q_fault_ref are respectively the d-axis and q-axis currents output in the fault state. The improvement of the grid-following type converter in the hybrid support strategy envisaged in the present application mainly focuses on how to improve the transient reactive power output capability. The specific method is as follows:

[0135] When the grid-following type converter is in the steady state operation condition, it basically operates in the unit power factor state. When the control system detects the fault state, the system will quickly switch to the low voltage ride through state. At this time, the system will lock the PQ ring, and output active and reactive power through direct control of the dq-axis current, while ensuring that the current does not exceed the limit. The present application designs a self-adaptive generation method of the d-axis and q-axis current instruction signals during fault, and the formula is as follows:

[0136]

[0137] In the formula, I d is the d-axis current, I q is the q-axis current, S N is the rated power, and U NFor rated voltage, k1, k2, k3 are fault adjustable parameters. At the time of fault, first calculate the output current of q-axis according to the fault drop degree, and then calculate the output current of d-axis. Thus the transient reactive output of the controlled current source type converter can be improved, and the overall output current of the converter can be ensured within a reliable range. To ensure that the transient reactive control does not affect the normal steady-state regulation, the transient reactive control is put into and out according to the voltage condition of the control point.

[0138] (Three): Based on MATLAB&Simulink, a grid-following hybrid system simulation verification platform is built:

[0139] An electromagnetic transient simulation verification platform of grid-constructing and grid-following hybrid multi-machine system connected to an infinite grid is built on MATLAB&Simulink.

[0140] (Four): Test low voltage ride through performance under different short circuit ratios:

[0141] The low voltage ride through performance of the grid-constructing and grid-following hybrid multi-machine system under short circuit current ratios of 2 and 3 is tested to ensure that the proposed low voltage ride through strategy can play a stable role in the grid-constructing and grid-following hybrid multi-machine system and be applicable under different grid intensities. The specific steps are as follows:

[0142] 4.1 Define initial conditions:

[0143] First, determine the initial values of all control parameters. For example:

[0144] a, grid equivalent inductance: L g ;

[0145] b, DC side bus voltage: U dc ;

[0146] c, virtual inertia: J;

[0147] d, grid rated voltage: U N ;

[0148] 4.2 Test working conditions:

[0149] In order to ensure that the proposed low voltage ride through strategy can play a stable role in the grid-constructing and grid-following hybrid multi-machine system and be applicable under different grid intensities, the low voltage ride through performance under short circuit current ratios of 2 and 3 is tested. For each kind of low voltage ride through under each scenario, four working conditions of voltage sag are set: voltage sag to 0.8pu, 0.6pu, 0.4pu, 0.2pu; the fault time is set to 1 second (greater than 0.625 seconds of national standard). The simulation results are shown in Figures 4(a) to 4(c) , Figures 5(a) to 5(c) .

[0150] (V): Strategy reliability analysis:

[0151] 5.1. Analysis of simulation results:

[0152] Fig. 4(a), Fig. 5(a) show that in the power grid with short-circuit ratio of 2-3, the active power and voltage curves are obtained by using the adaptive strategy proposed in the application. As can be seen from the figure, under the conditions that the grid voltage drops by 0.2, 0.4, 0.6, and 0.8 respectively, the active power can be well adapted to switch the active reference value during the fault according to the fault drop degree, and the short-circuit current is limited by reducing the active power and kept stable during the fault.

[0153] Fig. 4(b), Fig. 5(b) show that in the power grid with short-circuit ratio of 2-3, the reactive power and current curves are obtained by using the adaptive strategy proposed in the application. As can be seen from the figure, under the conditions that the grid voltage drops by 0.2, 0.4, 0.6, and 0.8 respectively, the reactive power can be well adapted to switch the reactive reference value during the fault according to the fault drop degree, and the voltage is supported by increasing the reactive power and kept stable during the fault.

[0154] Fig. 4(c), Fig. 5(c) show that in the power grid with short-circuit ratio of 2-3, the grid-type converter power angle and the grid-type converter power angle curves are obtained by using the adaptive strategy proposed in the application. As can be seen from the figure, under the conditions that the grid voltage drops by 0.2, 0.4, 0.6, and 0.8 respectively, the power angle remains stable, indicating that the stability of the hybrid system has good robustness.

[0155] At the same time, according to the simulation analysis, some other conclusions are obtained:

[0156] a. When the strategy works, the increased reactive power and the reduced active power are mainly related to the drop degree of the voltage of the infinite grid, and the correlation degree with the port voltage of the converter itself and the grid strength is small.

[0157] b. The weaker the grid is, the smaller the short-circuit current ratio is, and the better the voltage support ability of the hybrid system is. This is mainly reflected in the port voltage, which is lifted more in the weak grid condition. However, the transient stability of the system is also worse (the system is more difficult to stabilize), which is mainly reflected in that when SCR=2, the system can only guarantee that the fault clearing time is greater than 1 second; and the remaining conditions can guarantee that the fault clearing time is greater than 2 seconds.

[0158] 5.2. Comparative analysis:

[0159] To verify the superiority of the proposed strategy, typical related papers are selected for comparative analysis. The related papers are Wang Panbao, Wang Peng, Li Senguang, et al. Grid fault dynamic current limiting control strategy of grid-connected inverter [J]. High Voltage Technology, 2022, 48(10): 3829-3837. DOI: 10.13336 / j.1003-6520.hve.20220627.

[0160] The strategy proposed in the above papers is as follows:

[0161]

[0162] In the formula: S F is the allowed apparent power after the grid fault, U n is the rated voltage amplitude; S n is the rated apparent power, is the active power reference value, is the reactive power reference value injected into the grid, U pu is the grid voltage amplitude.

[0163] In order to ensure the superiority of the proposed low voltage ride through strategy in the system, the strategy without strategy, i.e. the traditional low penetration strategy (strategy 3, i.e. the given active power and reactive power reference value), the strategy proposed in the reference (strategy 2), and the strategy proposed in the invention (strategy 1) are set for comparison and verification. The working condition is set as the grid voltage drop to 0.2, SCR = 2, 3, and the fault time is 1s. The simulation result graph is shown in Figures 6(a) to 6(e) , Figures 7(a) to 7(e) .

[0164] Fig. 6(a) and Fig. 7(a) are active power curves. As shown in Fig. 6(a) and Fig. 7(a), among the three strategies, the strategy proposed in the invention has the best effect, can enter the low penetration state well, and can maintain stability in the low penetration state. On the other hand, strategy 2 can enter the low penetration state, but in the transient process, it appears to be under-damped and oscillates. Strategy 3 loses stability.

[0165] Fig. 6(b) and Fig. 7(b) are reactive power curves. As shown in Fig. 6(b) and Fig. 7(b), among the three strategies, the strategy proposed in the invention can well increase the reactive power in the low penetration state to support the voltage of the hybrid system, and can maintain system stability during the fault period. Strategy 2 can enter the low penetration state, but in the transient process, it appears to be under-damped and oscillates. Strategy 3 loses stability.

[0166] Fig. 6(c), Fig. 7(c) are voltage curves, from which it can be seen that, in the three strategies, the strategy proposed in the application can keep the voltage constant during low penetration, and the oscillation problem does not occur; strategy 2, during low penetration, the voltage is unstable and voltage oscillation occurs after disconnection, and strategy 3 is always unstable;

[0167] Fig. 6(d), Fig. 7(d) are current curves, from which it can be seen that, in the three strategies, the strategy proposed in the application can well limit the short-circuit current during low penetration, so as to keep it within a reasonable range, however, strategy 2 can limit the short-circuit current, but the current waveform has been severely distorted, and there is a long oscillation problem during fault recovery, and strategy 3 is unstable;

[0168] Fig. 6(e), Fig. 7(e) are power angle curves, from which it can be seen that, in the three strategies, the strategy proposed in the application can well keep the power angle stable during low penetration and steady state, further illustrating that the strategy proposed in the application can well improve the system stability, however, strategy 2 is unstable during fault, from the above analysis, the active power, reactive power, voltage and current curves can also know that there is a serious instability problem during fault, and strategy 3 is unstable during low penetration and steady state;

[0169] The strategies are summarized as follows:

[0170] 1) the strategy proposed in the application (strategy 1):

[0171] The network-construction type converter improves the active power and reactive power loop by introducing key control parameters, and the voltage at the PCC point is lifted from 0.2pu to about 0.7pu after the fault occurs, and the reactive power support capability is better than that of strategy 2 and strategy 3. At the same time, the network-following type converter accurately outputs 1 reactive current based on the stable phase reference of the improved phase-locked loop, and the reactive power response speed is significantly improved compared with the traditional strategy. After the fault is cleared, the voltage is restored to 0.95pu within 1 second, and the current peak value is strictly limited to below 2.2pu, the short-circuit current limitation effect is good under the condition of SCR=3, and the voltage support is strong, which also verifies the strong robustness of the strategy.

[0172] 2) conventional strategy (strategy 2):

[0173] The traditional fixed current limiting grid-forming control strategy performs poorly under the same working conditions. Simulation shows that its reactive power response has a significant delay (about 100 ms), resulting in a minimum PCC point voltage recovery of only 0.5 pu and a recovery time of up to 2 seconds. Due to the use of static virtual impedance design, the short-circuit current peak reaches 2 pu, triggering the current limiting protection of the grid-forming converter, which is forced to switch to current source mode, causing the loss of system equivalent inertia. Although this strategy can maintain basic stability, it has obvious shortcomings in voltage support accuracy and transient oscillation suppression.

[0174] 3) No strategy working condition (strategy 3):

[0175] Without any coordinated control strategy, the system is completely unstable under severe faults. The grid-forming converter loses stability due to strong voltage support, causing the current to directly lose stability at 0.2 seconds, triggering the overcurrent protection to exit operation; the grid-following converter loses synchronization due to the loss of PCC point voltage, and its reactive power continues to oscillate, further exacerbating voltage collapse. This working condition verifies the necessity of grid-forming-grid-following collaborative control, and it is difficult for a single control mode to meet the dual needs of short-circuit current suppression and active voltage support.

[0176] In summary:

[0177] ① The strategy proposed by the invention: small oscillation, stable fault, good voltage support, and short-circuit current not exceeding the standard;

[0178] ② The strategy proposed by the literature: large oscillation, stable fault, average voltage support, and short-circuit current not exceeding the standard;

[0179] ③ Without strategy: large oscillation, unstable fault, average voltage support, and short-circuit current seriously exceeding the standard;

[0180] ④ The strategy proposed by the invention provides a fault ride-through solution that combines fast voltage support and strong stability, and can still ensure safe operation of the system in an extremely weak grid with SCR≥2.

Claims

1. A transient support based coordinated low voltage ride through control method for a follow-the-sun grid, characterized in that The method comprises the following steps: S1: analyzing transient voltage characteristics and requirements of grid-forming-grid-following converter; S2: analyzing transient voltage support target of grid-forming-grid-following hybrid multi-machine system; S3: constructing control strategy of grid-forming-grid-following hybrid multi-machine system; S4: based on the control strategy of grid-forming-grid-following hybrid multi-machine system constructed in S3, optimization is proposed for low voltage ride through control of grid-following converter and transient voltage control of grid-forming converter.

2. The method of claim 1, wherein the method is implemented in a follow-the- sun grid support system. In S1, the voltage at the common coupling point of the parallel equivalent circuit of the grid-forming-grid-following converter; and the equivalent voltage source single branch current of the grid-forming converter are expressed as: （1）； wherein I s is the system current; Y s is the system admittance; is the system equivalent voltage source voltage; is the equivalent total injected current of all equivalent controlled sources; is the sum of all converter equivalent admittances; is the equivalent controlled current source current of the kth grid- following converter; is the equivalent series admittance of the equivalent controlled voltage source of the jth grid- forming converter; is the equivalent shunt admittance of the kth grid- following converter; is the equivalent controlled voltage source voltage of the jth grid- forming converter; U eq,1 , …, U eq , n1 is the equivalent controlled voltage source of the grid- forming converter; I v,1 ,… I v,n1 a branch current of an equivalent controlled voltage source of the grid-forming converter; Y veq,1 ,… Y veq,n1 equivalent series admittance of the equivalent controlled voltage source for the grid-forming converter I eq,1 ,…, I eq,n2 For the grid-connected converter, the equivalent controlled current source is I i,1 ,… I i,n2 for a grid-connected converter branch current; Y ieq,1 ,… Y ieq,n2 Y for the equivalent parallel admittance of the network-connected converter; U L For the public coupling point voltage, it is also the voltage for phase-locked with the grid-connected converter; n 1 is the number of grid-forming converters; n 2 is the number of grid-following converters; Analyzing equation (1), the common coupling point voltage U L Equivalent voltage source of power system U t Component and equivalent controlled voltage source component of each network type converter .

3. The follow-the-sun, support-based, combined low-voltage ride-through control method according to claim 2, characterized in that: Considering the starting moment of the transient voltage condition of the power grid, the electrical quantities in the station remain unchanged, and the change of the controlled source branch current caused by the change of the system voltage is further written as formula (1): （2）； In formula (2), ΔI v,1 is a change in the branch current of the grid-forming converter, ΔI i,1 is a change in the branch current of the grid-following converter; is a change in the system voltage; As can be seen from formula (2), the rapid change of the grid-following converter branch current will be caused by the larger system voltage change at the initial moment of the transient condition; at this time, the current change is not limited by the self-control of the grid-following converter, and multiple grid-following converter branches will share the overall current change.

4. The follow-the-sun, support-based, combined low-voltage ride-through control method according to claim 3, characterized in that: In S2, the transient voltage support target includes the following: 1) In the weak grid fault condition, the grid-forming converter provides stable voltage for the grid-following converter, improves the short-circuit capacity under low voltage, and enables the grid-following converter to reliably synchronize with the grid through the phase-locked loop to accurately and stably inject target current into the grid; 2) The grid-following converter needs to output accurate reactive power to support the system voltage in the transient condition, while ensuring the smooth recovery of the grid voltage after the fault is cleared; 3) In the transient condition of the grid-forming converter, the voltage source external characteristic cannot be forcibly converted into the current source external characteristic due to current limiting, so as to provide reliable support voltage; 4) The dynamic characteristic of the active power output of the grid-forming converter should have strong damping to ensure the stability of the controlled voltage source type grid-forming converter.

5. The follow-the-sun, support-based, combined low-voltage ride-through control method according to claim 4, characterized in that: In S3, the control mode of the grid-forming-grid-following converter in the grid-forming-grid-following hybrid multi-machine system is for power coordination control and voltage coordination control in the transient condition; In the transient condition, the output voltage regulation speed of the grid-forming converter is greater than the power regulation speed of the grid-following converter, in order to ensure the stability of the grid-forming converter, the active and reactive reference values are changed to ensure the voltage source characteristic limitation and overcurrent limitation of the grid-forming converter; The reference values of active and reactive power are adjusted to ensure the voltage source characteristic and overcurrent limitation of the grid-forming converter, and the formula is as follows: （3）； （4）； wherein: P * is the adjusted active power reference value, P ref is the unadjusted active power reference value, V if is the converter output voltage, V gf is the grid voltage, δ is the power angle, L g is the inductance, Q * is the adjusted reactive power reference value, k q is the reactive power integral coefficient, V ifref is the converter output voltage reference value, Q is the reactive power compensation amount; The influence of power angle change on voltage support is reduced by adjusting the virtual damping, thereby improving the voltage stability of the grid-forming converter; the formula is as follows: （5）； In the formula, D p0 is the initial damping coefficient of the grid-forming converter, D p1 is the adaptive damping coefficient, P e is the actual output value of the active power, T d is the action threshold of the virtual damping.

6. The follow-the-sun, support-based, combined low-voltage ride-through control method according to claim 5, wherein: The grid-following converter outputs sufficient reactive current in the transient condition through adaptive compensation control to raise the output voltage of the common coupling point, which is as follows: The control impedance Zg and Z GFM The different to increase the issuance of Zg and Z The different to increase the issuance of Zg and Z （6）； where: I GFM is the grid-forming output current; U GFM is the grid-forming inverter output voltage; Z GFM is the grid-forming inverter impedance; Z g is the grid impedance; U g is the voltage voltage; I GFL is the grid-following inverter output voltage.

7. The follow-the-sun, support-based, combined low-voltage ride-through control method according to claim 6, characterized in that: In S4, the active-reactive adaptive fast regulation strategy for low voltage ride through control of the grid-forming converter is as follows: 4.

1. 1: an active-reactive adaptive updating method during voltage sag is proposed, including the following: （7）； In formula (7), respectively represent the rated power, the active power instruction at the time of voltage sag, the reactive power instruction at the normal time, and the reactive power instruction at the time of voltage sag; represents the unit of the output voltage amplitude; α、β respectively are the reactive power regulation coefficient and the active power regulation coefficient.

8. The follow-the-sun grid-forming low-voltage ride-through control method based on transient support according to claim 7, characterized in that: 4.

1. 2: Determining α , β Parameter values: The size of the reactive power output can be controlled, thus The size of the reactive current output of the network-forming converter can be changed, thus enhancing the transient support capability of the system. The minimum value of the output active power can be adjusted, and the smaller The active power output by the network-forming converter during a fault can be reduced, thereby improving the power angle stability of the system.

9. The transient support based follow-the-sun network coordinated low voltage ride through control method of claim 8, wherein: The follow-the-sun grid-forming low-voltage ride-through control method based on transient support according to claim 7, characterized in that: When the grid-connected converter is in steady-state operation, it basically operates in unity power factor state; when the control system detects a fault state, the system will quickly switch to low voltage ride-through state; at this time the system will lock the PQ loop, and through direct control dq The method rapidly outputs active and reactive power in the form of shaft current, while ensuring that the current does not exceed the limit value, and a fault d 、 q The adaptive generation method of the shaft current command signal is as follows: （8）； In formula (8), is d shaft current, is q shaft current, is rated power, is rated voltage, k 1 、k 2 、k 3 is a fault adjustable parameter; denotes the unit of the voltage amplitude of the phase-locked point of the grid-connected type converter.

10. The method of claim 9, wherein the method further comprises: At the time of fault, firstly, the output current of the shaft is calculated according to the fault drop degree q , and then the output current of the shaft is calculated d ; thus the transient reactive output of the controlled voltage source type grid-connected type converter can be improved, and the overall output current of the grid-connected type converter can be ensured within a reliable range; in order to ensure that the transient reactive control does not affect the normal steady-state regulation, the transient reactive control is put into and withdrawn according to the control point voltage.

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