Stability control method for new energy power generation system based on power spring under weak power grid

Abstract

The application discloses a new energy power generation system stability control method based on a power spring under a weak power grid, and belongs to the technical field of new energy grid connection and smart grid control. The control method comprises the following steps: constructing an equivalent impedance model of a new energy power generation system based on a power spring and establishing a point of common coupling node equation; determining a phase control strategy of the power spring based on the node equation, and establishing a load side output impedance model containing the power spring; and analyzing overall impedance characteristics of the system, and revealing a remodeling relationship of the system impedance by the power spring through equivalent virtual impedance. The application does not require complex communication, has low hardware cost, effectively expands the stable operation boundary of the new energy power generation system under the weak power grid through phase control and virtual impedance remodeling of the power spring, and cooperatively realizes double promotion of the stability of the new energy power generation system and local power supply quality, thereby providing a practical path for engineering application of the power spring in a large-scale new energy access scene.

Description

Technical Field

[0001] This invention relates to the field of new energy grid connection and smart grid control technology, specifically to a stability control method for a new energy power generation system based on electric springs under weak grid conditions. Background Technology

[0002] In recent years, the penetration rate of new energy power generation has risen sharply. The core contradiction of high-proportion new energy grid connection lies in the conflict between its volatility and the grid's stability requirements. To resolve this contradiction, current solutions mainly rely on two technical approaches: one is to optimize its operating characteristics through flexible control strategies, and the other is to provide active support to the grid by adding compensation devices or energy storage systems, for example: 1) Gao Yuan's 2021 paper, "Research on Improved Control Methods for Grid-Connected Inverters Considering Power Grid Stability," introduces various grid-connected control strategies for new energy sources, such as virtual synchronization control, droop control, and schedulable virtual oscillator control, to provide inertia and stability adjustment mechanisms and improve the stability of power grids with high penetration rates of new energy sources.

[0003] 2) Chen Yahao, in his 2019 paper "Research on Subsynchronous Oscillation Problem of Wind Power Grid-Connected System Considering Static Var Compensator", proposed that the subsynchronous oscillation problem can be avoided and the system stability improved by optimizing the parameters of the static var compensator control loop.

[0004] 3) Wu Jiajie, Chen Xin, Zhang Donghui, et al., published an article entitled "Analysis and Improvement Strategies of Grid-based Energy Storage Converter in New Energy Access Scenarios" in Volume 44, Issue 23 of the Proceedings of the Chinese Society for Electrical Engineering in 2024. The article proposes that grid-based energy storage converters can provide transient frequency / voltage support for new energy units, increase system rotational inertia, and thus improve the stability of new energy systems.

[0005] Existing technologies for improving the stability of grid-connected systems for new energy power plants have the following problems: 1) "Overcompensation" or "extensive" governance: Traditional devices attempt to stabilize the entire power grid or grid connection point, requiring huge capacity to cope with the worst operating conditions, resulting in poor economic efficiency.

[0006] 2) Lack of "precision" and "coordination" concepts: failure to distinguish between critical loads (which must be supplied with stable power) and non-critical loads (which can withstand certain fluctuations) in the power grid, resulting in a waste of control resources.

[0007] Electric springs actively absorb or compensate for fluctuations in active and reactive power in power lines by injecting a controllable compensation voltage into non-critical loads. This stabilizes the voltage across critical loads at their rated values, effectively improving power quality on the user side and achieving "load-side" intelligence. Currently, electric springs are used for power factor correction, voltage and frequency stabilization, and reducing energy storage costs, for example: 1) Yin Fagen's 2024 paper, "Research on Voltage and Frequency Control of Islanded Microgrids with the Participation of Electric Springs," studies the voltage and frequency control of islanded microgrids with the participation of electric springs. Taking node voltage and system angular frequency as the control objects, it achieves node voltage stability and frequency regulation performance improvement based on consistency control.

[0008] 2) Zhang Jing, Wu Zezhi, Lu Xiuchang, et al., published a paper entitled "Research on Coordinated Control Strategy of 'Photovoltaic-Storage-DC-Flexible' System Considering DC Power Spring" in Volume 40, Issue 07 of "Power Supply and Utilization" in 2023. This paper proposes an energy management mode and coordinated control strategy for a "photovoltaic-storage-DC-flexible" system considering DC power spring, which can provide effective support to the power grid in emergency situations.

[0009] Electric spring technology possesses millisecond-level response capabilities, effectively tracking fluctuations in renewable energy (rapid response); it utilizes existing non-critical loads as regulation resources, avoiding the cost of expensive energy storage batteries (economic efficiency); and it can simultaneously or independently regulate active and reactive power, possessing the potential to simultaneously support voltage and frequency (bidirectional regulation capability), demonstrating promising application prospects. However, current applications of electric spring technology are mostly limited to distribution networks, microgrids, or individual user sides to ensure the power quality of specific critical loads; there is a lack of overall architecture and control methods for its application in large-scale renewable energy power plants (such as megawatt-level wind farms / photovoltaic power plants), failing to fully leverage its role in improving the stability of renewable energy power generation systems at the grid level.

[0010] In summary, existing technologies lack a solution that can take a holistic view of the power plant and utilize electric springs to economically, efficiently, and synergistically improve the overall stability (including voltage, frequency, and inertia) of new energy grid connection. Summary of the Invention

[0011] The technical problem this invention aims to solve is the lack of an economical, efficient, and synergistic system solution for improving the overall stability of renewable energy power generation systems connected to the grid. This invention provides a method for studying the stability of renewable energy power generation systems based on electric springs under weak grid conditions. It aims to improve the stability of renewable energy power generation systems from a global perspective by reshaping the grid-side impedance using electric springs, thereby fundamentally and actively enhancing the stability of the renewable energy power generation system while also considering the power supply quality of critical loads.

[0012] The technical solution of the present invention is as follows.

[0013] A stability control method for a new energy power generation system based on an electric spring under weak power grid conditions is disclosed. The system includes a new energy power plant, an electric spring, non-critical loads, critical loads, and the power grid. The output of the new energy power plant is connected to the power grid via a common coupling point. The electric spring and the non-critical load are connected in series to form a smart load, which is then connected in parallel with the critical load and connected to the common coupling point. The new energy power plant is composed of… m The system consists of several new energy power generation units with identical parameters, each including a grid-connected converter and a unit transformer connected in series. L t , m The output terminal of the unit transformer is connected in parallel to the output terminal of the new energy power station; the grid-connected converter includes a first DC power supply. V dc First three-phase two-level inverter bridge, grid-connected converter side filter inductor L f Grid-connected converter filter capacitor C f and damping resistor R d and the filter capacitor of the grid-connected converter C f and damping resistor R d The resulting branch is denoted as the filter capacitor-damping resistor branch; the topology of the power spring includes a second DC power supply. V dc2 Second and third phase two-level inverter bridge, power spring filter inductor L and power spring filter capacitor C The power grid is equivalent to a series of line resistors connected in series. R g Line reactance X g and ideal voltage source v g The steps include: Step 1: Denote the common coupling point as PCC, and sample the following parameters: PCC voltage. and current Critical load impedance Z CL and voltage Non-critical load impedance Z NCL and voltage Electric spring voltage Electric spring filter inductor current i L ; Step 2: Construct the equivalent impedance model of the new energy power generation system based on electric springs, and establish the active power balance equation and reactive power balance equation of the PCC node. Step 3: Based on the active power balance equation and reactive power balance equation of the PCC node, solve for the power spring phase control angle. The phase control strategy for the electric spring is determined, and the output impedance of each phase on the load side containing the electric spring, viewed from the left side of the PCC, is obtained. Z Esout ; Step 4: Based on the equivalent impedance model of the new energy power generation system and the overall impedance characteristics of the new energy power generation system using the electric spring, the relationship between the electric spring and the system impedance reshaping through the equivalent virtual impedance is obtained, and the phase control angle of the electric spring is adjusted. By changing the amplitude and polarity of the positive / negative sequence output impedance on the load side containing the electric spring, the overall impedance of the system can be reshaped.

[0014] Preferably, the topology of the equivalent impedance model of the new energy power generation system based on electric springs described in step 2 is as follows: m Equivalent current source of grid-connected converter Outputs in parallel m The equivalent output impedance R of the grid-connected converter D1 Then, in series m The equivalent impedance R of the unit transformer Dm The power grid impedance is connected in series via PCC. Z g and ideal voltage source v g Equivalent voltage source of electric spring v ES The output impedance of each phase on the load side, including the electric spring, as seen from the PCC. Z Esout Both are connected to the PCC; the equivalent output impedance R D1 = Z CCM / m, Z CCM For each The equivalent output impedance of the grid-connected converter; The m The equivalent impedance RD of the unit transformer m = Z Lt / m , The equivalent impedance in the complex frequency domain of a unit transformer 。

[0015] The active power balance equation and reactive power balance equation of the PCC node are as follows:

[0016] In the formula, and These are the active power and reactive power of new energy sources, respectively. and These are the active power and reactive power of the power grid, respectively. and These are the active and reactive power of the critical load, respectively. and These represent the active and reactive power of non-critical loads, respectively. and Let represent the active and reactive power of the electric spring, respectively, expressed as follows:

[0017] In the formula, Z g,pu This is the per-unit value of the line impedance. R g,pu This is the per-unit value of the line resistance. X g,pu This is the per-unit value of the line reactance. Z CL,pu This refers to the per-unit value of the critical load impedance. Z NCL,pu These are per-unit values ​​for non-critical load impedance. V pcc,pu This represents the per-unit value of the amplitude of the PCC voltage at the point of common coupling. V CL,pu This is the per-unit value of the critical load voltage. V NCL,pu These are per-unit values ​​for non-critical load voltages. For the critical load power angle, For non-critical load power angles, The phase of the PCC voltage at the point of common coupling lags behind the grid voltage. and , The phase of the non-critical load voltage leading the PCC voltage at the point of common coupling is [not specified]. and .

[0018] Preferably, step 3 is implemented as follows: Step 3.1: In a new energy power generation system containing an electric spring, the point of common coupling (PCC) voltage is taken as the critical load voltage, and the control target is set to make the PCC voltage amplitude per unit value... V pcc,pu =1.0; Step 3.2, given the system impedance parameters, determine the control target. V pcc,pu Substituting 1.0 into the active power balance equation and reactive power balance equation of the PCC node, and solving them simultaneously, we obtain the unique electric spring phase control angle that satisfies both equations. ; Step 3.3, Real-time detection of the phase of the grid voltage Combined with the phase control angle of the electric spring The phase of the PCC voltage is calculated. ; Step 3.4, with V pcc,pu =1.0 and the phase of the PCC voltage To achieve the control objective, a voltage controller is used. H vc Its function is to regulate the output PCC point voltage to ensure the steady-state accuracy of the system, while also providing a reference signal for the inner current loop; the inner current loop uses the inductor current as the control quantity, and is controlled by the current controller. H ic This is used to improve the dynamic performance of the system, ultimately forming the phase control strategy for the electric spring. Step 3.5: Based on the phase control strategy and the Mason gain formula, the output impedance of each phase on the load side, including the electric spring, as viewed from the left side of the PCC, is derived. Z Esout .

[0019] Preferably, step 4 is implemented as follows: Step 4.1: Establish the positive sequence output impedance of the grid-connected inverter. and negative sequence output impedance ; Step 4.2 introduces the positive sequence impedance ratio for determining the stability of the new energy power generation system. and negative sequence impedance ratio Their expressions are as follows:

[0020] In the formula, This is the equivalent impedance in the complex frequency domain of the line. The equivalent impedance in the complex frequency domain of a unit transformer; Step 4.3: Analyze the overall impedance characteristics of the new energy power generation system based on electric springs, and establish the new positive sequence impedance ratio after introducing electric springs. and the new negative sequence impedance ratio Their expressions are as follows:

[0021] In the formula, and These are the positive-sequence output impedance and negative-sequence output impedance on the load side containing the electric spring, respectively. Step 4.4: Determine the impedance reshaping relationship of the electric spring. Specifically, consider that each phase of the three-phase electric spring is independently controlled using a controller with the same structure, and let... That is, the equivalent virtual impedance provided by the electric spring is symmetrical in the positive and negative sequences; By adjusting the phase control angle of the electric spring ,Change and The amplitude and polarity of the system are determined to reshape the overall impedance of the system, so that the system impedance ratio satisfies the Nyquist stability criterion.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention applies electric springs to the overall architecture of large-scale new energy power plants (such as megawatt-level wind farms / photovoltaic power plants) at the system level, establishing a phase control strategy based on the common coupling point (PCC) node equation. Closed-loop tracking can be achieved through voltage and current controllers, eliminating the need for complex communication or global synchronization systems. This results in low hardware costs and ease of retrofitting and upgrading existing new energy power generation systems.

[0023] 2. This invention reveals the mechanism by which an electric spring, as an equivalent virtual impedance, reshapes the system impedance: by adjusting the phase control angle of the electric spring, the amplitude and polarity of the output impedance are changed, thereby achieving overall impedance reshaping of the system and transforming the originally unstable system with "weak damping and low inertia" into a stable system, effectively improving the stability of the new energy power generation system under weak power grid conditions.

[0024] 3. This invention clarifies for the first time the coupling law between the power ratio of non-critical loads, the allowable voltage fluctuation range of non-critical loads and the minimum short-circuit ratio, quantifies the relationship between non-critical loads and system stability, and provides a clear engineering basis for system load configuration.

[0025] 4. This invention quantifies the relationship between the power spring capacity, phase-locked loop configuration parameters, and system stability. Engineers can flexibly select the power spring capacity according to cost and performance requirements, achieving a predictable and adjustable design with enhanced stability.

[0026] 5. This invention can not only stabilize the voltage and current at the common coupling point, but also ensure the stability of the critical load voltage, thus achieving a dual improvement in the stability performance of the new energy power generation system and the power supply quality of the critical load.

[0027] 6. This invention makes full use of existing non-critical loads in the station as the "medium" for energy regulation, eliminating the need for or significantly reducing the need for expensive energy storage battery configurations, resulting in significant economic benefits; at the same time, non-critical loads still perform their original functions during the regulation process, so energy is not wasted, and only their power consumption is intelligently shifted, resulting in high overall energy efficiency. Attached Figure Description

[0028] Figure 1 This is a topology diagram of the new energy power generation system based on electric springs involved in this invention.

[0029] Figure 2 This is an equivalent impedance model for a new energy power generation system based on electric springs.

[0030] Figure 3 This is a block diagram of the phase control of the electric spring proposed in this invention.

[0031] Figure 4 The positive sequence output impedance of the grid-connected converter when the power spring is not engaged. / m The Bode plot.

[0032] Figure 5 The output impedance of the grid-connected converter under varying grid strength conditions when the power spring is not engaged. Intersection curve with the equivalent impedance on the grid side.

[0033] Figure 6 This is a positive sequence impedance ratio diagram used to determine the stability of a new energy power generation system under varying grid strength conditions when the power spring is not engaged.

[0034] Figure 7 In order to be in SCR Under a weak power grid with a voltage rating of 1.4, the output impedance of the grid-connected converter before and after the power spring is applied is... A comparison of the intersection curves with the equivalent impedance on the grid side.

[0035] Figure 8 In order to be in SCR =2 Under a weak power grid, the output voltage waveform of PCC before and after the power spring is turned on.

[0036] Figure 9 In order to be in SCR =2 Under a weak power grid, the output current waveform of PCC before and after the power spring is engaged.

[0037] Figure 10 In order to be in SCR =2 Under a weak power grid, the waveform and amplitude of the output key load voltage before and after the power spring is turned on.

[0038] Figure 11 In order to be in SCR=1.4 Under a weak power grid, the output voltage waveform of PCC when the power spring is not engaged.

[0039] Figure 12 In order to be in SCR =1.4 Under a weak power grid, the PCC output current waveform when the electric spring is not engaged.

[0040] Figure 13 In order to be in SCR =1.4 Under a weak power grid, the output voltage waveform of PCC after the power spring is engaged.

[0041] Figure 14 In order to be in SCR =1.4 Under a weak power grid, the output current waveform of the PCC after the electric spring is engaged.

[0042] Figure 15 In order to be in SCR =1.4 Under a weak power grid, after the power spring is engaged, the waveforms of the output power spring voltage and the non-critical load voltage are shown.

[0043] Figure 16 In order to be in SCR =1.4 Under a weak power grid, after the power spring is put into operation, the amplitude of the output power spring voltage and the non-critical load voltage is equal to 1.4. Detailed Implementation

[0044] The technical solution of the present invention will now be clearly and completely described in conjunction with the accompanying drawings.

[0045] Figure 1 This is a topology diagram of the new energy power generation system based on an electric spring, as involved in this invention. Figure 1 As can be seen, the system involved in this method includes a new energy power station, an electric spring, non-critical loads, critical loads, and a power grid. The output end of the new energy power station is connected to the power grid through a common coupling point. The electric spring and the non-critical load are connected in series to form a smart load, which is then connected in parallel with the critical load and connected to the common coupling point.

[0046] The new energy power station is composed of m The system consists of several new energy power generation units with identical parameters, each including a grid-connected converter and a unit transformer connected in series. L t , m The output terminal of the unit transformer is connected in parallel to the output terminal of the new energy power station. The grid-connected converter includes a first DC power supply. V dc1 First three-phase two-level inverter bridge, grid-connected converter side filter inductor L f Grid-connected converter filter capacitor C f and damping resistorR d and the filter capacitor of the grid-connected converter C f and damping resistor R d The resulting branch is denoted as the filter capacitor-damping resistor branch.

[0047] The topology of the electric spring includes a second DC power supply. V dc2 Second and third phase two-level inverter bridge, power spring filter inductor L and power spring filter capacitor C .

[0048] The power grid is equivalent to a series of line resistors connected in series. R g Line reactance X g and ideal voltage source v g .

[0049] In this embodiment, the non-critical load refers to a dissipative load, and the critical load refers to a load that requires precise voltage at the opposite end.

[0050] Specifically, in this embodiment, the non-critical load refers to loads with less stringent requirements for the opposite-end voltage, including electric kettles, electric stoves, lighting systems, etc.; the critical load refers to loads with precise requirements for the opposite-end voltage, including vital sign monitoring medical equipment, data centers, monitoring and ventilation equipment in coal mines, etc.

[0051] This invention provides a method for studying the stability of a new energy power generation system based on an electric spring under weak power grid conditions. The specific steps are as follows: Step 1: Denote the common coupling point as PCC, and sample the following parameters: PCC voltage. and current Critical load impedance Z CL and voltage Non-critical load impedance Z NCL and voltage Electric spring voltage Electric spring filter inductor current i L .

[0052] Step 2: Construct the equivalent impedance model of the new energy power generation system based on electric springs, and establish the active power balance equation and reactive power balance equation of the PCC node.

[0053] In this embodiment, Figure 2This is an equivalent impedance model for a new energy power generation system based on an electric spring. (The model is derived from...) Figure 2 As can be seen, the topology of the equivalent impedance model of the new energy power generation system based on electric springs described in step 2 is as follows: m Equivalent current source of grid-connected converter Outputs in parallel m The equivalent output impedance R of the grid-connected converter D1 Then, in series m The equivalent impedance R of the unit transformer Dm The power grid impedance is connected in series via PCC. Z g and ideal voltage source v g Equivalent voltage source of electric spring v ES The output impedance of each phase on the load side, including the electric spring, as seen from the PCC. Z Esout Both are connected to the PCC; the equivalent output impedance R D1 = Z CCM / m, Z CCM For each The equivalent output impedance of the grid-connected converter; The m The equivalent impedance RD of the unit transformer m = Z Lt / m , The equivalent impedance in the complex frequency domain of a unit transformer 。

[0054] The active power balance equation and reactive power balance equation of the PCC node are as follows:

[0055] In the formula, and These are the active power and reactive power of new energy sources, respectively. and These are the active power and reactive power of the power grid, respectively. and These are the active and reactive power of the critical load, respectively. and These represent the active and reactive power of non-critical loads, respectively. and Let represent the active and reactive power of the electric spring, respectively, expressed as follows:

[0056] In the formula, Z g,pu This is the per-unit value of the line impedance. R g,pu This is the per-unit value of the line resistance. X g,pu This is the per-unit value of the line reactance. Z CL,pu This refers to the per-unit value of the critical load impedance. Z NCL,pu These are per-unit values ​​for non-critical load impedance. V pcc,pu This represents the per-unit value of the amplitude of the PCC voltage at the point of common coupling. V CL,pu This is the per-unit value of the critical load voltage. V NCL,pu These are per-unit values ​​for non-critical load voltages. For the critical load power angle, For non-critical load power angles, The phase of the PCC voltage at the point of common coupling lags behind the grid voltage. and , The phase of the non-critical load voltage leading the PCC voltage at the point of common coupling is [not specified]. and .

[0057] In this embodiment, the renewable energy power station operates at full capacity; both critical and non-critical loads are resistive loads. If the electric spring only provides reactive power compensation, then the voltage of the electric spring is perpendicular to the voltage of the non-critical load, i.e. Using the grid voltage as a reference, the expressions involved in the active and reactive power balance equations at the PCC node are updated as follows:

[0058] Step 3: Based on the active power balance equation and reactive power balance equation of the PCC node, solve for the power spring phase control angle. The phase control strategy for the electric spring is determined, and the output impedance of each phase on the load side containing the electric spring, viewed from the left side of the PCC, is obtained. Z Esout .

[0059] In this embodiment, step 3 is implemented as follows: Step 3.1: In a new energy power generation system containing an electric spring, the point of common coupling (PCC) voltage is taken as the critical load voltage, and the control target is set to make the PCC voltage amplitude per unit value... Vpcc,pu =1.0.

[0060] Step 3.2, given the system impedance parameters, determine the control target. V pcc,pu Substituting 1.0 into the active power balance equation and reactive power balance equation of the PCC node, and solving them simultaneously, we obtain the unique electric spring phase control angle that satisfies both equations. .

[0061] Step 3.3, Real-time detection of the phase of the grid voltage Combined with the phase control angle of the electric spring The phase of the PCC voltage is calculated. .

[0062] Step 3.4, with V pcc,pu =1.0 and the phase of the PCC voltage To achieve the control objective, a voltage controller is used. H vc Its function is to regulate the output PCC point voltage to ensure the steady-state accuracy of the system, while also providing a reference signal for the inner current loop; the inner current loop uses the inductor current as the control quantity, and is controlled by the current controller. H ic This is used to improve the dynamic performance of the system, ultimately forming a phase control strategy for the electric spring.

[0063] Figure 3 This is a block diagram of the phase control of the electric spring proposed in this invention.

[0064] Step 3.5: Based on the phase control strategy and the Mason gain formula, the output impedance of each phase on the load side, including the electric spring, as viewed from the left side of the PCC, is derived. Z Esout .

[0065] Step 4: Based on the equivalent impedance model of the new energy power generation system and the overall impedance characteristics of the new energy power generation system using the electric spring, the relationship between the electric spring and the system impedance reshaping through the equivalent virtual impedance is obtained, and the phase control angle of the electric spring is adjusted. By changing the amplitude and polarity of the positive / negative sequence output impedance on the load side containing the electric spring, the overall impedance of the system can be reshaped.

[0066] In this embodiment, step 4 is implemented as follows: Step 4.1: Establish the positive sequence output impedance of the grid-connected inverter. and negative sequence output impedance .

[0067]

[0068] In the formula, The voltage of the DC power supply. For modulator gain, Indicates control delay, The sampling period is This is the decoupling coefficient of the current loop. For voltage feedforward coefficients, This represents the steady-state value of the fundamental amplitude of the output voltage of the grid-connected converter. This is the reactive power reference value for grid-connected converters. The droop coefficient is the inverted sag coefficient. This is a reference value for the active power of the grid-connected converter. Voltage of the filter capacitor in the grid-connected converter v Cfabc The fundamental amplitude, Voltage of the filter capacitor in the grid-connected converter v Cfabc The initial phase, For the filter inductor current on the grid-connected converter side i Lfabc The fundamental amplitude, For the filter inductor current on the grid-connected converter side i Lfabc The initial phase, This indicates a positive sequence current loop regulator. This represents a negative-sequence current loop regulator, where 's' represents the Laplace operator. Indicates the rated angular frequency of the power grid. This represents a positive-sequence low-pass filter. This represents a negative-order low-pass filter. This represents the closed-loop gain of the positive-sequence phase-locked loop. This represents the closed-loop gain of the negative-sequence phase-locked loop.

[0069] In this embodiment, the following diagrams are drawn. Figure 4 The positive sequence output impedance of the grid-connected converter when the power spring is not engaged is shown. / m From the Bode plot, it can be seen that the simulated frequency sweep results are consistent with the established... / m The theoretical calculation results of the expression are consistent across a wide frequency range, verifying the correctness of the impedance model expression.

[0070] Step 4.2 introduces the positive sequence impedance ratio for determining the stability of the new energy power generation system. and negative sequence impedance ratio Their expressions are as follows:

[0071] In the formula, This is the equivalent impedance in the complex frequency domain of the line. This is the equivalent impedance in the complex frequency domain of a unit transformer.

[0072] The strength of a power grid is determined by the equivalent short-circuit ratio of the power generation system of new energy power plants, and the magnitude of the short-circuit ratio is closely related to the equivalent impedance of the lines: the larger the equivalent impedance of the lines, the lower the short-circuit ratio, and the weaker the power grid. In this embodiment, a diagram is drawn. Figure 5 The output impedance of the grid-connected converter is shown when the power spring is not engaged, under varying grid strength conditions. The intersection curve with the equivalent impedance on the grid side is shown in the figure. Figure 5 It is evident that the output impedance of the grid-connected converter exhibits a capacitive negative damping region with a phase less than -90°. As the short-circuit ratio decreases from 20 to 1, the grid strength gradually weakens, the impedance curve intersection frequency gradually decreases, the phase margin gradually decreases, and the stability of the new energy power generation system declines. When the short-circuit ratio is 2, the phase margin at the impedance curve intersection frequency is 0, and the power generation system is in a critical stable state.

[0073] In this embodiment, further by drawing Figure 6 The diagram shown is a positive sequence impedance ratio used to determine the stability of a new energy power generation system under varying grid strength conditions when the power spring is not engaged. Figure 6 It can be seen that when the phase frequency characteristic of the converter output impedance crosses 180°, at SCR =20 and SCR The amplitude-frequency response of the impedance ratio curves corresponding to 2 is less than 0dB, therefore the system is stable, 1≤ SCR When the value is less than 2, the impedance ratio amplitude-frequency characteristic is greater than 0dB, therefore the system is unstable.

[0074] In summary, based on the impedance ratio criterion, stability analysis was conducted to determine the performance of the new energy power plant's power generation system. SCR It is unstable under weak power grid conditions with a strength of ≤2.

[0075] Step 4.3: Analyze the overall impedance characteristics of the new energy power generation system based on electric springs, and establish the new positive sequence impedance ratio after introducing electric springs. and the new negative sequence impedance ratio Their expressions are as follows:

[0076] In the formula, and These are the positive-sequence output impedance and negative-sequence output impedance on the load side containing the electric spring, respectively.

[0077] Step 4.4: Determine the impedance reshaping relationship of the electric spring. Specifically, consider that each phase of the three-phase electric spring is independently controlled using a controller with the same structure, and let... This means that the equivalent virtual impedance provided by the electric spring is symmetrical in both the positive and negative sequences.

[0078] By adjusting the phase control angle of the electric spring ,Change and The amplitude and polarity of the system are determined to reshape the overall impedance of the system, so that the system impedance ratio satisfies the Nyquist stability criterion.

[0079] In this embodiment, the non-critical load power ratio is configured as a = 40% (a is the percentage of the total system load power), and the short-circuit ratio is... SCR Under a weak power grid with a value of 1.4, by drawing... Figure 7 The output impedance of the grid-connected converter before and after the power spring is applied is shown. By comparing the intersection curves with the equivalent impedance on the grid side, it can be seen that the virtual impedance provided by the electric spring reshapes the overall impedance of the system, turning the originally unstable system into a stable one. This proves that the introduction of the electric spring effectively expands the stable operating boundary of the system.

[0080] In this embodiment, to demonstrate the beneficial effects of the present invention, when a=40%, SCR Under the condition of 2, the working conditions of the new energy power generation system before and after the power spring is put into operation were simulated. Figure 8 , Figure 9 , Figure 10 The simulation results show the voltage waveform at the common coupling point, the current waveform at the common coupling point, and the voltage and amplitude waveforms of the critical load before and after the electric spring is engaged. Simulation results indicate that before engaging the electric spring, the voltage at the common coupling point drops to 0.89 pu; after engaging the electric spring, the voltage at the common coupling point stabilizes at 1.0 pu, and the voltage of the critical load also stabilizes at its rated value. The electric spring effectively supports the system, achieving a dual improvement in the stability of the new energy power generation system and the power supply quality of the critical load.

[0081] When a=40%, SCR =1.4、 V NCL,min The operating conditions of the new energy power generation system before and after the power spring was put into operation were simulated under the condition of 0.85pu (i.e., the allowable fluctuation range of non-critical load voltage is ≥0.85pu). Figure 11 , Figure 12 Before the electric spring is engaged, the output voltage and current waveforms of the system's common coupling point PCC are shown. Figure 13 , Figure 14The waveforms of the PCC output voltage and output current are shown after the electric spring is engaged. Figure 15 , Figure 16 The waveforms and amplitudes of the electric spring voltage and the non-critical load voltage after the electric spring is engaged. Figure 11-14 The results show that before the electric spring is engaged, the electrical quantity at the common coupling point is unstable; after the electric spring is engaged, the output of the common coupling point returns to stability. Figure 15 This indicates that the electric spring voltage leads the non-critical load voltage by 90°, and the electric spring only provides capacitive reactive power compensation. Figure 16 This indicates that the output non-critical load voltage is within the allowable fluctuation range.

Claims

1. A stability control method for a new energy power generation system based on an electric spring under a weak power grid. The system includes a new energy power station, an electric spring, non-critical loads, critical loads, and a power grid. The output of the new energy power station is connected to the power grid via a common coupling point. The electric spring and the non-critical load are connected in series to form a smart load, which is then connected in parallel with the critical load and connected to the common coupling point. The new energy power station consists of... m The system consists of several new energy power generation units with identical parameters, each including a grid-connected converter and a unit transformer connected in series. L t , m The output terminal of the unit transformer is connected in parallel to the output terminal of the new energy power station; the grid-connected converter includes a first DC power supply. V dc First three-phase two-level inverter bridge, grid-connected converter side filter inductor L f Grid-connected converter filter capacitor C f and damping resistor R d and the grid-connected converter filter capacitor C f and damping resistor R d The resulting branch is denoted as the filter capacitor-damping resistor branch; the topology of the power spring includes a second DC power supply. V dc2 Second and third phase two-level inverter bridge, power spring filter inductor L and power spring filter capacitor C The power grid is equivalent to a series of line resistors connected in series. R g Line reactance X g and ideal voltage source v g Its characteristics are, Includes the following steps: Step 1: Denote the common coupling point as PCC, and sample the following parameters: PCC voltage. and current Critical load impedance Z CL and voltage Non-critical load impedance Z NCL and voltage Electric spring voltage Electric spring filter inductor current i L ; Step 2: Construct the equivalent impedance model of the new energy power generation system based on electric springs, and establish the active power balance equation and reactive power balance equation of the PCC node. Step 3: Based on the active power balance equation and reactive power balance equation of the PCC node, solve for the power spring phase control angle. The phase control strategy for the electric spring is determined, and the output impedance of each phase on the load side containing the electric spring, viewed from the left side of the PCC, is obtained. Z Esout ; Step 4: Based on the equivalent impedance model of the new energy power generation system and the overall impedance characteristics of the new energy power generation system using the electric spring, the relationship between the electric spring and the system impedance reshaping through the equivalent virtual impedance is obtained, and the phase control angle of the electric spring is adjusted. By changing the amplitude and polarity of the positive / negative sequence output impedance on the load side containing the electric spring, the overall impedance of the system can be reshaped.

2. The stability control method for a new energy power generation system based on an electric spring under a weak power grid according to claim 1, characterized in that, The topology of the equivalent impedance model of the new energy power generation system based on electric springs described in step 2 is as follows: m Equivalent current source of grid-connected converter Outputs in parallel m The equivalent output impedance R of the grid-connected converter D1 Then, in series m The equivalent impedance R of the unit transformer Dm The power grid impedance is connected in series via PCC. Z g and ideal voltage source v g Equivalent voltage source of electric spring v ES The output impedance of each phase on the load side, including the electric spring, as seen from the PCC. Z Esout Both are connected to the PCC; the equivalent output impedance R D1 = Z CCM / m, Z CCM For each The equivalent output impedance of the grid-connected converter; The m The equivalent impedance RD of the unit transformer m = Z Lt / m , This is the equivalent impedance in the complex frequency domain of a unit transformer. The active power balance equation and reactive power balance equation of the PCC node are as follows: In the formula, and These are the active power and reactive power of new energy sources, respectively. and These are the active power and reactive power of the power grid, respectively. and These are the active and reactive power of the critical load, respectively. and These represent the active and reactive power of non-critical loads, respectively. and Let represent the active and reactive power of the electric spring, respectively, expressed as follows: In the formula, Z g,pu This is the per-unit value of the line impedance. R g,pu This is the per-unit value of the line resistance. X g,pu This is the per-unit value of the line reactance. Z CL,pu This refers to the per-unit value of the critical load impedance. Z NCL,pu These are per-unit values ​​for non-critical load impedance. V pcc,pu This represents the per-unit value of the amplitude of the PCC voltage at the point of common coupling. V CL,pu This is the per-unit value of the critical load voltage. V NCL,pu These are per-unit values ​​for non-critical load voltages. For the critical load power angle, For non-critical load power angles, The phase of the PCC voltage at the point of common coupling lags behind the grid voltage. and , The phase of the non-critical load voltage leading the PCC voltage at the point of common coupling is [not specified]. and .

3. The stability control method for a new energy power generation system based on an electric spring under a weak power grid according to claim 1, characterized in that, The implementation process of step 3 is as follows: Step 3.1: In a new energy power generation system containing an electric spring, the point of common coupling (PCC) voltage is taken as the critical load voltage, and the control target is set to make the PCC voltage amplitude per unit value... V pcc,pu =1.0; Step 3.2, given the system impedance parameters, determine the control target. V pcc,pu Substituting 1.0 into the active power balance equation and reactive power balance equation of the PCC node, and solving them simultaneously, we obtain the unique electric spring phase control angle that satisfies both equations. ; Step 3.3, Real-time detection of the phase of the grid voltage Combined with the phase control angle of the electric spring The phase of the PCC voltage is calculated. ; Step 3.4, with V pcc,pu =1.0 and the phase of the PCC voltage To achieve the control objective, a voltage controller is used. H vc Its function is to regulate the output PCC point voltage to ensure the steady-state accuracy of the system, while also providing a reference signal for the inner current loop; the inner current loop uses the inductor current as the control quantity, and is controlled by the current controller. H ic This is used to improve the dynamic performance of the system, ultimately forming the phase control strategy for the electric spring. Step 3.5: Based on the phase control strategy and the Mason gain formula, the output impedance of each phase on the load side, including the electric spring, as viewed from the left side of the PCC, is derived. Z Esout .

4. The stability control method for a new energy power generation system based on an electric spring under a weak power grid according to claim 1, characterized in that, The implementation process of step 4 is as follows: Step 4.1: Establish the positive sequence output impedance of the grid-connected inverter. and negative sequence output impedance ; Step 4.2 introduces the positive sequence impedance ratio for determining the stability of the new energy power generation system. and negative sequence impedance ratio Their expressions are as follows: In the formula, This is the equivalent impedance in the complex frequency domain of the line. The equivalent impedance in the complex frequency domain of a unit transformer; Step 4.3: Analyze the overall impedance characteristics of the new energy power generation system based on electric springs, and establish the new positive sequence impedance ratio after introducing electric springs. and the new negative sequence impedance ratio Their expressions are as follows: In the formula, and These are the positive-sequence output impedance and negative-sequence output impedance on the load side containing the electric spring, respectively. Step 4.4: Determine the impedance reshaping relationship of the electric spring. Specifically, consider that each phase of the three-phase electric spring is independently controlled using a controller with the same structure, and let... That is, the equivalent virtual impedance provided by the electric spring is symmetrical in the positive and negative sequences; By adjusting the phase control angle of the electric spring ,Change and The amplitude and polarity of the system are determined to reshape the overall impedance of the system, so that the system impedance ratio satisfies the Nyquist stability criterion.

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