Hybrid mode control method for new energy station through flexible direct export and grid integration

Abstract

The application discloses a new energy station flexible direct current external sending follow-structure network fusion type hybrid mode control method and belongs to the electrical engineering field.The application comprises sampling and power control, fusion power ring control, follow-structure network control, obtaining the equivalent short-circuit ratio of a flexible direct current transmission system, and the follow-structure network fusion type hybrid mode control combined with the strength representation function of the flexible direct current transmission system.The application introduces the representation function to carry out adaptive adjustment of the fusion degree of the follow-structure network two control modes to realize the fusion type hybrid mode control, combines the advantages of the follow-network type and structure network type control, greatly improves the stability of the grid-connected converter, and especially solves the stability problem of the grid-connected converter in a weak networking scene through multi-stage voltage step-up and external sending through the flexible direct current system.The application can make the grid-connected converter not only have a fast active power response but also have a certain reactive power support capacity, so that the grid-connected converter can stably operate in the weak networking scene with a large impedance fluctuation.

Description

Technical Field

[0001] This invention relates to the field of electrical engineering, and in particular to a hybrid control method for new energy power plants that transmit power via flexible DC transmission and integrates with the grid. Background Technology

[0002] With the commissioning of large-scale new energy power plants, the penetration rate of new energy power generation continues to increase. Most of these large-scale new energy power plants are connected to the power grid through long-distance transmission lines and multi-stage transformers to collect and boost voltage, and then connected to the power grid through flexible DC transmission systems. This makes the power system exhibit significant "weak interconnection" characteristics, specifically manifested in low system inertia, a drop in the short-circuit ratio (SCR) of the point of common coupling (PCC) to a critical value and the time-varying fluctuation characteristics.

[0003] Currently, most renewable energy power plants still adopt a current-source grid-following (GFL) operation mode to achieve maximum power point tracking, thereby ensuring high renewable energy utilization and rapid power response capabilities. However, under conditions of high grid impedance and "weak grid connection" characteristics, traditional grid-following converters suffer from stability issues such as wideband oscillations. From the perspective of stability analysis under small disturbances, relevant literature has proposed a series of improvement measures. For example...

[0004] 1) The paper “Adaptive Control of Grid-Connected Inverters Based on Online Grid Impedance Measurements”, published in the IEEE Transactions on Sustainable Energy in 2014, proposes a control method based on the adaptive adjustment of phase-locked loop parameters based on online impedance identification. By adjusting the bandwidth of the phase-locked loop, the stability of the grid-connected inverter under short-circuit ratio changes is improved, giving it a certain oscillation suppression capability.

[0005] 2) M. Li, X. Zhang., The Control Strategy for the Grid-Connected Inverter Through Impedance Reshaping in q-Axis, and its Stability, published in the 2020 IEEE Journal of Emerging and Selected Topics in Power Electronics [J]. EEE, Journal of Emerging, Selected, Topics, in Power, Electronics, 2020, 9(3): 3229-3242. (“Control Strategy and Stability Analysis of Grid-Connected Converter for Q-Axis Impedance Reshaping under Weak Grid”) This paper addresses the issue that phase-locked loops and grid voltage feedforward can cause negative resistance in the q-axis output impedance of the inverter in the low-frequency band, leading to instability under weak grid conditions. It proposes an impedance controller that reshapes the q-axis impedance to positive resistance in the low-frequency band, improving the anti-interference capability of the grid-connected converter without affecting the d-axis output characteristics.

[0006] However, under extreme grid conditions with high grid impedance, even with improved grid-connected converter control strategies, power transmission imbalances may still occur, leading to system instability. To address the fundamental problems of grid-connected converters, related research has proposed a control strategy for grid-forming (GFM) converters. In this strategy, the frequency and phase are set by the grid-connected converter itself, and it functions as a controlled voltage source, possessing a certain degree of active support capability, enabling stable operation under grid conditions with extremely low short-circuit ratios. For example:

[0007] 1) X.,Gao,,D.,Zhou.,Stability Analysis of Grid-Following and Grid-Forming Converters Based on State-Space Modelling[J].,IEEE,Transactions,on,IndustryApplications,2024,60(3):,4910-4920. (“Stability Analysis of Grid-Following and Grid-Forming Converters Based on State-Space Model”) This paper analyzes the stability boundaries of grid-following and grid-forming inverters under different short-circuit ratios through state-space modeling and eigenvalue trajectory analysis, and concludes that grid-following converters are suitable for strong grids, while grid-forming converters are more suitable for weak grids.

[0008] As concluded in the aforementioned literature, grid-based control still suffers from insufficient stability under strong power grid conditions. Furthermore, related studies have shown that grid-based control exhibits problems such as slow power regulation speed, strong multi-machine coupling, and poor economic efficiency. Therefore, relying solely on either grid-following or grid-based control modes is insufficient to ensure the stability of renewable energy power plants in weakly gridded systems with significant impedance fluctuations. Considering the complementary nature of grid-following and grid-based converters in terms of applicable scenarios and advantages / disadvantages, related research has explored the relationship between the two in depth. For example:

[0009] 1) Y.,Li,Y.,Gu,T.,C.,Green.,Revisiting,Grid-Forming,and,Grid-Following,Inverters:A,Duality,Theory[J].,IEEE,Transactions,on,Power,Systems,,2022,,37(6):,4541-4554. (“Revisiting Grid-Forming and Grid-Following Inverters: Duality Theory”) This paper proposes a dual theory, analyzes in detail the symmetrical relationship between grid-forming and grid-following inverters in terms of control structure, interaction with the grid, and applicable scenarios, and clearly reveals the essential connection and difference between the two types of inverters.

[0010] Combining the inherent connections and complementary characteristics of the two types of grid-connected converters, some scholars have proposed a dual-mode switching control strategy for grid-connected converters to address the stability issues under scenarios of significant grid impedance fluctuations. For example:

[0011] 1) M.,Li,X.,Zhang.,Impedance,Adaptive,Dual-Mode,Control,of,Grid-Connected,Inverters,With,Large,Fluctuation,of,SCR,and,Its,Stability,Analysis,Based,on,D-Partition,Method[J].,IEEE,Transactions,on,Power,Electronics,,2021,,36(12):,14420-14435. (“D-Partition Method-based Adaptive Dual-Mode Control of Grid-Connected Inverters under Large Fluctuations in Short-Circuit Ratio and Its Stability Analysis”) This paper proposes a dual-mode switching control strategy of current source mode and voltage source mode. It quantitatively analyzes the dual-mode switching boundary by combining the D-partition method. Different control modes are switched under different short-circuit ratios, realizing the complementary advantages of the two converter control strategies and effectively improving the stability of the converter under a wide range of short-circuit ratios.

[0012] However, the dual-mode switching control strategy is still a time-sharing operation of a single mode. Direct switching will cause voltage and current surges, and the nonlinearity of the switching process poses a serious threat to the stable operation of the converter. Although the surge can be reduced by improving the soft switching mechanism, it can only be reduced, not eliminated. Furthermore, fluctuations in grid strength may lead to frequent switching and misjudgments between the two modes.

[0013] To address the shortcomings of dual-mode switching control, some scholars have proposed a grid-connected hybrid control strategy that integrates grid construction and operation, aiming to achieve complementary advantages of two grid-connected converters and effectively improve the stability of grid-connected converters in weak grid scenarios. For example:

[0014] 1) Published in 2024 IEEE Transactions on Industrial Electronics (IEEE,Transactions,on,Industrial,Electronics) F.,Han,,X.,Zhang,,M.,Li.,Stability,Control,for,Grid-Connected,Inverters,Based,on,Hybrid-Mode,of,Grid-Following,and,Grid-Forming[J].,IEEE,Transactions,on,Industrial,Electronics,,2024,,71(9):,10750-10760. (“Stability Control of Grid-Connected Inverters Based on Hybrid Mode”) This paper proposes a grid-connected inverter control strategy based on a hybrid mode of grid-connected grid. It adopts a phase-locked loop-free structure based on power synchronization, which significantly reduces the capacitive negative resistance region of the grid-connected inverter output impedance and significantly improves its stability over a wide SCR range.

[0015] However, the aforementioned hybrid grid-connected control strategy, with its fixed weighting ratio in the modulation stage, while ensuring stability of the short-circuit ratio over a wide range, lacks flexibility. Furthermore, the analysis focuses on a single-unit infinite bus system with relatively simple impedance characteristics. In contrast, due to geographical characteristics, most renewable energy power plants require high-voltage collection and transmission via flexible DC transmission systems. The introduction of flexible DC transmission systems increases the impedance complexity of the connected system, transforming it from a simple resistive-inductive impedance to a complex impedance with multi-timescale characteristics and greater flexibility. This increased impedance complexity places more stringent demands on the stability of renewable energy power plants, and the fixed weighting ratio in the hybrid grid-connected control strategy is insufficient to meet the stability requirements of grid-connected converters in complex scenarios.

[0016] In summary, the existing technology has the following problems:

[0017] 1) Existing grid-connected converters based on grid-following and grid-connecting control modes exhibit significant stability issues under scenarios with large fluctuations in grid impedance. Grid-following converters have a faster active power response, but lack voltage support capability, and the phase-locked loop (PLL) and grid voltage feedforward affect their stability under weak grid conditions. Grid-connecting converters have stronger reactive power support capability, but their power response is slower, and insufficient damping affects their stability under strong grid conditions.

[0018] 2) Existing research on the two control modes mainly focuses on the switching between the two modes. It is still a time-sharing operation of a single mode. The nonlinearity of the switching process poses a serious threat to the stable operation of the converter. Furthermore, grid impedance fluctuations may lead to frequent switching and misjudgment between the two modes.

[0019] 3) Existing integrated hybrid mode control strategies for grid-connected systems adopt a fixed weighting ratio in the modulation stage and analyze a simple single-machine infinite bus system. However, the system complexity increases when the power is transmitted through a flexible DC transmission system, and the impedance with multi-time-scale characteristics makes it impossible to guarantee the stability of the grid-connected converter under a fixed weighting ratio. Summary of the Invention

[0020] The technical problem this invention aims to solve is to overcome the shortcomings of the switching process in the original dual-mode switching control strategy while combining the advantages of grid-connected and grid-connected converters, and to overcome the limitations of the grid-connected hybrid mode control strategy in complex scenarios. This allows renewable energy power plants to achieve rapid active power response and strong reactive power support even when transmitting power via a flexible DC transmission system. This invention proposes a grid-connected hybrid mode control method for renewable energy power plants transmitting power via flexible DC.

[0021] The technical solution of the present invention is as follows:

[0022] A hybrid control method for renewable energy power plants transmitting power via flexible DC transmission to the grid is disclosed. The renewable energy power plant comprises n grid-connected converters with identical structures. Each grid-connected converter includes a DC source, a main converter, and an LCL filter. The LCL filter includes a bridge arm-side inductor, a filter capacitor, and a grid-side inductor. The outputs of the n grid-connected converters are connected in parallel at a common coupling point and then connected to the AC grid via a flexible DC transmission system. The control method for the n grid-connected converters is identical and includes the following steps.

[0023] Step 1: Sample the bridge arm current i of the main converter. a i b i c Sampling filter capacitor voltage u Ca ,u Cb ,u Cc Then, the αβ axis components of the bridge arm side current are obtained through Clark transformation. α i β and the αβ axis component of the filter capacitor voltage u Cα ,u Cβ The output active power P and output reactive power Q of the grid-connected converter were calculated.

[0024] Step 2, set the rated output voltage V of the grid-connected converter. n Rated angular frequency ω n Active power reference value P ref and reactive power reference value Q ref Inputting the fused power loop yields the synchronization phase angle θ and the reference value u for the dq-axis voltage of the mesh-type control. Cdref u CqrefAnd the reference value of dq axis current i for mesh control dref i qref ;

[0025] Step 3, based on the synchronization phase angle θ, adjust the bridge arm side current i a i b i c and filter capacitor voltage u Ca ,u Cb ,u Cc Perform Park transforms to obtain the dq-axis components i of the bridge arm side current. d i q and the dq-axis component of the filter capacitor voltage u Cd ,u Cq ;

[0026] Step 4, based on the reference value u of the dq axis voltage for mesh-type control. Cdref ,u Cqref And the reference value of dq axis current i for mesh control dref i qref The grid-connected converter is subjected to both grid-connected control and grid-following control to obtain the grid-connected control dq-axis modulation signal e. d1 ,e q1 and mesh control dq axis modulation signal e d2 ,e q2 ;

[0027] Step 5: Obtain the equivalent output impedance Z of the flexible DC transmission system through small-signal modeling and impedance frequency domain injection method. eq And it is characterized as the equivalent short-circuit ratio (SCR) eq ;

[0028] Step 6: Introduce the intensity characterization function f of the flexible DC transmission system, and combine it with the dq-axis modulation signal e of the grid-type control. d1 ,e q1 and mesh control dq axis modulation signal e d2 ,e q2 The calculated dq-axis modulation signal e of the hybrid-mode control converter is obtained. d ,e q .

[0029] Preferably, the formulas for calculating the output active power P and output reactive power Q in step 1 are as follows:

[0030]

[0031] Preferably, the synchronization phase angle θ and the reference value u of the dq-axis voltage in step 2 are... Cdref ,u Cqref The reference value i of the dq-axis current is obtained from the droop control circuit and the mesh control circuit.dref i qref From the current calculation stage, the expressions for the droop control stage and the current calculation stage are as follows:

[0032]

[0033] In the formula, m q The reactive power droop factor is m. p ω is the active power droop factor, ω is the reference angular frequency, and t is the operating time of the grid-connected converter.

[0034] Preferably, the mesh-type control dq-axis modulation signal e in step 4 d1 ,e q1 The solution is as follows:

[0035] The inductor current command signal i for mesh-type control is obtained through the filter capacitor voltage control equation. dref1 i qref1 Its expression is:

[0036]

[0037] In the formula, K pv K is the proportional control coefficient for capacitor voltage. iv is the integral control coefficient of the capacitor voltage, and s is the Laplace operator;

[0038] The dq-axis modulation signal e of the network-controlled system is obtained through the inner loop control equation of the inductor current. d1 ,e q1 Its expression is:

[0039]

[0040] In the formula, K pi1 K is the proportional control coefficient for the inductor current in a network-type control system. ii1 The integral control coefficient for the inductor current in a network-type control;

[0041] The following mesh control dq axis modulation signal e d2 ,e q2 The expression is obtained by using the inner loop control equation of the inductor current in grid-type control.

[0042]

[0043] In the formula, K pi2 K is the proportional control coefficient for the inductor current in mesh-controlled systems. ii2 This is the integral control coefficient for the inductor current in mesh-controlled systems.

[0044] Preferably, the equivalent short-circuit ratio (SCR) described in step 5...eq The expression is:

[0045]

[0046] In the formula, I base This is the reference current.

[0047] Preferably, the expression for the intensity characterization function f of the flexible DC transmission system in step 6 is:

[0048]

[0049] In the formula, α is the adjustment parameter;

[0050] The hybrid mode control converter modulation signal e d ,,e q The modulation signal of the grid-connected converter after the integrated hybrid mode control is calculated as follows:

[0051]

[0052] Preferably, the flexible DC transmission system includes a sending-end converter station, a transformer, a receiving-end converter station, and a DC cable. One end of the sending-end converter station is connected to the output end of the grid-connected converter through a common coupling point, and the other end is connected to one end of the receiving-end converter station through a DC cable. The other end of the receiving-end converter station is connected to the AC power grid through a transformer.

[0053] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0054] 1. This invention overcomes the stability problem of grid-connected converters with single current source characteristics and grid-connected converters with voltage source characteristics under large impedance fluctuations. The proposed integrated hybrid mode control method can operate stably under a wide range of short-circuit ratios, effectively improving the robustness of the converter in weak grid scenarios.

[0055] 2. The integrated hybrid mode control method proposed in this invention overcomes the shortcomings of the dual-mode switching control strategy, which is still a time-sharing operation of a single-mode converter, and effectively solves the unavoidable nonlinear impact during the switching process, thus ensuring the stability of the converter in complex scenarios.

[0056] 3. The integrated hybrid mode control method proposed in this invention combines the advantages of both grid-following and grid-building control modes. It not only has a fast active power response but also a strong reactive power support capability. It can effectively respond to power flow changes in flexible DC systems and provide support capabilities when voltage drops or frequency instability occur at the common coupling point.

[0057] 4. The integrated hybrid mode control method proposed in this invention is an integrated hybrid mode control method with adjustable integration degree. It can flexibly adjust the integration degree through the system characterization function under different flexible DC transmission system strength conditions. It can both ensure that the grid-type control ratio is adjusted under strong grid conditions to improve power response speed, and adjust the grid-type control ratio under weak grid conditions to improve active support capability. Attached Figure Description

[0058] Figure 1 This is a schematic diagram of the overall structure of the system involved in this invention.

[0059] Figure 2 This is a schematic diagram of the control structure of the network-integrated hybrid mode control method of the present invention.

[0060] Figure 3 This is a schematic diagram of the control structure of the fused power loop in this invention.

[0061] Figure 4 This is a schematic diagram of the root network control structure in this invention.

[0062] Figure 5 This is a flowchart of the network-integrated hybrid mode control method of the present invention.

[0063] Figure 6 The voltage waveform diagram is shown when using the traditional grid-type control method.

[0064] Figure 7 The voltage waveform diagram is shown using the traditional grid-type control method.

[0065] Figure 8 The voltage waveform diagram is shown for the hybrid mode control method of the present invention. Detailed Implementation

[0066] The technical solution of the present invention will now be clearly and completely described in conjunction with the accompanying drawings.

[0067] Figure 1 This is a schematic diagram of the overall structure of the circuit involved in this invention. Figure 1 As can be seen, the new energy power station includes n grid-connected converters with the same structure. Each grid-connected converter includes a DC source, a main converter, and an LCL filter. The LCL filter includes a bridge arm-side inductor, a filter capacitor, and a grid-side inductor. The output terminals of the n grid-connected converters are connected in parallel at a common coupling point and then connected to the power grid through a flexible DC transmission system.

[0068] In this embodiment, the flexible DC transmission system includes a sending-end converter station, a transformer, a receiving-end converter station, and DC cables. One end of the sending-end converter station is connected to the output terminal of the grid-connected converter through a common coupling point, and the other end is connected to one end of the receiving-end converter station through a DC cable. The other end of the receiving-end converter station is connected to the AC power grid through a transformer.

[0069] Figure 1 In the middle, V dc L1 is the DC voltage, L2 is the bridge arm inductor, L2 is the grid inductor, and PCC is the common coupling point.

[0070] Figure 2 This is a control structure diagram of the hybrid mode control method for new energy power plants with integrated grid and DC transmission proposed in this invention. Figure 2 As can be seen, the control method described in this invention obtains a modulation signal independent of the grid control by designing a fusion power loop controller, and then realizes the synthesis of the modulation signal and the fusion hybrid mode control of the grid-connected converter by introducing a flexible DC transmission system characterization function.

[0071] Figure 5 This is a flowchart of the grid-connected hybrid mode control method of the present invention. In this invention, the control method for n grid-connected converters is the same, including the following steps;

[0072] Step 1: Sample the bridge arm current i of the main converter. a i b i c Sampling filter capacitor voltage u Ca ,u Cb ,u Cc Then, the αβ axis components of the bridge arm side current are obtained through Clark transformation. α i β and the αβ axis component of the filter capacitor voltage u Cα ,u Cβ The output active power P and output reactive power Q of the grid-connected converter are calculated.

[0073] In this embodiment, the formulas for calculating the output active power and output reactive power are as follows:

[0074]

[0075] Step 2, set the rated output voltage V of the grid-connected converter. n Rated angular frequency ω n Reference value of active power P of converter ref and reactive power reference value Q ref Inputting the fused power loop yields the synchronization phase angle θ and the reference value u for the dq-axis voltage of the mesh-type control. Cdref ,uCqref And the reference value of dq axis current i for mesh control dref i qref .

[0076] In this embodiment, the synchronization phase angle θ and the reference value u of the dq-axis voltage of the mesh-type control are... Cdref ,u Cqref The reference value i of the dq-axis current is obtained from the droop control circuit and the mesh control circuit. dref i qref From the current calculation stage, the expressions for the droop control stage and the current calculation stage are as follows:

[0077]

[0078] In the formula, m q The reactive power droop factor is m. p ω is the active power droop factor, ω is the reference angular frequency, and t is the operating time of the grid-connected converter.

[0079] Figure 3 This is a schematic diagram of the control structure of the fused power loop in this invention.

[0080] Step 3, based on the synchronization phase angle θ, adjust the bridge arm side current i a i b i c and filter capacitor voltage u Ca ,u Cb ,u Cc Perform Park transforms to obtain the dq-axis components i of the bridge arm side current. d i q and the dq-axis component of the filter capacitor voltage u Cd ,u Cq .

[0081] Step 4, based on the reference value u of the dq axis voltage for mesh-type control. Cdref ,u Cqref And the reference value of dq axis current i for mesh control dref i qref The grid-connected converter is subjected to both grid-connected control and grid-following control to obtain the grid-connected control dq-axis modulation signal e. d1 ,e q1 and mesh control dq axis modulation signal e d2 ,e q2 .

[0082] In this embodiment, the mesh-type control dq-axis modulation signal e d1 ,e q1 The solution is as follows:

[0083] The inductor current command signal i for mesh-type control is obtained through the filter capacitor voltage control equation. dref1 i qref1 Its expression is:

[0084]

[0085] In the formula, K pv K is the proportional control coefficient for capacitor voltage. iv is the integral control coefficient of the capacitor voltage, and s is the Laplace operator;

[0086] The dq-axis modulation signal e of the network-controlled system is obtained through the inner loop control equation of the inductor current. d1 ,e q1 Its expression is:

[0087]

[0088] In the formula, K pi1 K is the proportional control coefficient for the inductor current in a network-type control system. ii1 This is the integral control coefficient of the inductor current in a network-type control.

[0089] In this embodiment, the following mesh control dq-axis modulation signal e d2 ,e q2 The expression is obtained by using the inner loop control equation of the inductor current in grid-type control.

[0090]

[0091] In the formula, K pi2 K is the proportional control coefficient for the inductor current in mesh-controlled systems. ii2 This is the integral control coefficient for the inductor current in mesh-controlled systems.

[0092] In this embodiment, K pv =0.05, K iv =120, K pi1 =4,K ii1 =200, K pi2 =4,K ii2 =200.

[0093] Figure 4 This is a diagram of the root network control structure in this invention.

[0094] Step 5: Obtain the equivalent output impedance Z of the flexible DC transmission system through small-signal modeling and impedance frequency domain injection method. eq And it is characterized as the equivalent short-circuit ratio (SCR) eq .

[0095] In this embodiment, the equivalent short-circuit ratio (SCR) eq The expression is:

[0096]

[0097] In the formula, I base This is the reference current.

[0098] Step 6: Introduce the intensity characterization function f of the flexible DC transmission system, and combine it with the dq-axis modulation signal e of the grid-type control. d1 ,e q1 and mesh control dq axis modulation signal e d2 ,e q2 The calculated dq-axis modulation signal e of the hybrid-mode control converter is obtained. d ,e q .

[0099] In this embodiment, the expression for the intensity characterization function f of the flexible DC transmission system is:

[0100]

[0101] In the formula, α is the adjustment parameter.

[0102] The hybrid mode control converter modulation signal e d ,,e q The modulation signal of the grid-connected converter after the integrated hybrid mode control is calculated as follows:

[0103]

[0104] The modulation signal e of the converter is controlled by a hybrid mode. d ,e q To achieve integrated hybrid mode control of grid-connected converters.

[0105] To verify the hybrid control method of new energy power plants with integrated grid and DC transmission provided by this invention, simulations were performed. The simulation structure diagram is shown below. Figure 1 As shown.

[0106] In the simulation, Figure 1 The power generation system of the new energy power station shown was simulated and compared using different control methods for power transmission via flexible DC transmission.

[0107] Figure 6 The figure shows the voltage waveform using a traditional grid-type control method, where U... a U b U c The three-phase voltage at the point of common coupling. As shown in the diagram, changing the equivalent impedance of the flexible DC transmission system at 0.5 seconds affects the SCR.eq When the voltage is reduced from 6 to 2, the strength of the flexible DC transmission system weakens, the stability of traditional grid-connected control deteriorates under weak grid conditions, and voltage distortion occurs significantly.

[0108] Figure 7 The voltage waveform diagram using the traditional grid-type control method is shown. The equivalent impedance of the flexible DC transmission system is changed at 0.5 seconds. (SCR) eq When the voltage is increased from 2 to 6, the strength of the flexible DC transmission system increases. The stability of the traditional grid-type control becomes worse under the strong grid, and the voltage becomes significantly distorted, eventually diverging.

[0109] Figure 8 To illustrate the voltage waveform diagram of the hybrid control method integrated with the grid structure of this invention, the equivalent impedance of the flexible DC transmission system is changed at 0.8 seconds. (SCR) eq The ratio of 6 to 2 is changed. After the switch, the strength characterization function f of the flexible DC transmission system is adaptively adjusted, reducing the proportion of grid-based control and increasing the proportion of grid-connected control, so as to ensure that the voltage waveform is stable before and after the switch and has stronger support capability under weak grid conditions.

[0110] In summary, Figure 6 , Figure 7 and Figure 8 The simulation waveform analysis and comparison of different control methods shown in the figure verify the effectiveness of the hybrid mode control proposed in this invention.

Claims

1. A hybrid control method for a new energy power station transmitting power via flexible DC transmission to the grid, wherein the new energy power station comprises n grid-connected converters with identical structures, each grid-connected converter comprising a DC source, a main converter, and an LCL filter, the LCL filter comprising a bridge arm-side inductor, a filter capacitor, and a grid-side inductor; the output terminals of the n grid-connected converters are connected in parallel at a common coupling point and then connected to the AC grid via a flexible DC transmission system; characterized in that... The control method for n grid-connected converters is the same, including the following steps; Step 1: Sample the bridge arm current of the grid-connected converter. , , Sampling filter capacitor voltage , , Then, the bridge arm side current is obtained through Clark transformation. Axial components , and filter capacitor voltage Axial components , The output active power of the grid-connected converter was calculated. and output reactive power ; Step 2, set the rated output voltage of the grid-connected converter. Rated angular frequency Active power reference value and reactive power reference value Input the fused power loop to obtain the synchronization phase angle. Reference values ​​for dq-axis voltage in mesh-type control , Reference values ​​for dq-axis current in mesh control , ; Step 3, based on the synchronization phase angle Current on the bridge arm side , , and filter capacitor voltage , , Perform Park transforms to obtain the dq-axis components of the bridge arm currents. , and the dq-axis components of the filter capacitor voltage , ; Step 4, based on the reference value of the dq-axis voltage of the network control , Reference values ​​for dq-axis current in mesh control , The grid-connected converter is subjected to both grid-connected control and grid-following control to obtain the dq-axis modulation signal for grid-connected control. , and mesh control dq axis modulation signal , ; Step 5: Obtain the equivalent output impedance of the flexible DC transmission system through small-signal modeling and impedance frequency domain injection method. And it is characterized as the equivalent short-circuit ratio ; The equivalent short-circuit ratio The expression is: In the formula, The reference current; Step 6: Introduce the intensity characterization function of the flexible DC transmission system. And combined with mesh-type control of dq-axis modulation signals , and mesh control dq axis modulation signal , The dq-axis modulation signal of the hybrid-mode control converter was calculated. , ; The strength characterization function of the flexible DC transmission system The expression is: In the formula, To adjust the parameters; The integrated hybrid mode control converter modulation signal , The formula for calculation is: 。 2. The method for controlling a hybrid mode of new energy power plants transmitting power via flexible direct current and grid integration according to claim 1, characterized in that, Step 1 describes the output active power and output reactive power The calculation formulas are as follows: 。 3. The method for controlling a hybrid mode of new energy power plants transmitting power via flexible direct current and grid integration according to claim 1, characterized in that, The synchronization phase angle described in step 2 Reference values ​​for dq-axis voltage in mesh-type control , The reference value of the dq-axis current is obtained from the droop control circuit and the mesh control circuit. , From the current calculation stage, the expressions for the droop control stage and the current calculation stage are as follows: In the formula, This is the reactive power droop factor. This is the active power droop coefficient. As the reference angular frequency, t This refers to the operating time of the grid-connected converter.

4. The method for controlling a hybrid mode of new energy power plants transmitting power via flexible direct current and grid integration according to claim 1, characterized in that, Step 4 describes the mesh-type control of the dq-axis modulation signal. , The solution is as follows: The inductor current command signal for mesh-type control is obtained through the filter capacitor voltage control equation. , Its expression is: In the formula, This is the proportional control coefficient for the capacitor voltage. This represents the integral control coefficient for the capacitor voltage. For the Laplace operator; The dq-axis modulation signal of the network-controlled system is obtained through the inner loop control equation of the inductor current. , Its expression is: In the formula, This is the proportional control coefficient for the inductor current in a network-type control system. The integral control coefficient for the inductor current in a network-type control; The following mesh control dq axis modulation signal , The expression is obtained by using the inner loop control equation of the inductor current in grid-type control. In the formula, This is the proportional control coefficient for the inductor current in grid-type control. This is the integral control coefficient for the inductor current in mesh-controlled systems.

5. The method for controlling a hybrid mode of new energy power plants transmitting power via flexible direct current and grid integration according to claim 1, characterized in that, The flexible DC transmission system includes a sending-end converter station, a transformer, a receiving-end converter station, and DC cables. One end of the sending-end converter station is connected to the output end of the grid-connected converter through a common coupling point, and the other end is connected to one end of the receiving-end converter station through a DC cable. The other end of the receiving-end converter station is connected to the AC power grid through a transformer.

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