Method for analyzing primary frequency modulation characteristics of system with high proportion of non-synchronous electromechanical power sources

Abstract

The application discloses a kind of high proportion non-synchronous electromechanical power access system primary frequency modulation characteristic analysis method.The application calculates steady-state frequency deviation factor evaluation index according to the proportion of synchronous machine power supply and the proportion of network-type converter in non-synchronous machine power supply in power system, and measures the primary frequency modulation characteristic of high proportion non-synchronous electromechanical power access system according to the index.The greater the steady-state frequency deviation factor, the smaller the frequency deviation of high proportion non-synchronous electromechanical power access system under the same power disturbance, and the better the primary frequency modulation characteristic of the system.The primary frequency modulation characteristic analysis method provided by the application can be used to study the influence of factors such as the proportion of non-synchronous machine power supply and the proportion of network-type converter in non-synchronous machine power supply on the steady-state frequency support capability of high proportion non-synchronous electromechanical power access system, thereby guiding new energy development and grid connection, and the implementation method is simple and efficient.

Description

Technical Field

[0001] This invention belongs to the field of power system transmission and distribution technology, and relates to a method for analyzing the primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power sources connected to it. Background Technology

[0002] Currently, the power system is at a critical stage of transitioning towards a high proportion of renewable energy and a high proportion of power electronics (i.e., "dual high"). The integration of asynchronous power sources will break the dominance of traditional synchronous generators and profoundly affect the dynamic characteristics of the power system. Grid-following converters maintain synchronization with the grid power source through phase-locked loops (PLLs), offering the advantage of fast response speed and showing broad application prospects in large-scale renewable energy grid integration. Modular multilevel converters (MMCs) are an important implementation method for grid-following converters.

[0003] However, grid-connected MMCs lack frequency response characteristics and cannot participate in system frequency regulation or provide frequency support. Their large-capacity grid connection and gradual replacement of traditional synchronous generators will weaken the power system's steady-state frequency support capability. The steady-state frequency support capability of a traditional power system mainly refers to the system's ability to resist active power disturbances and maintain frequency stability through primary frequency regulation. Taking a sudden increase in load as an example, after a brief lag, the synchronous generator governor acts on the steam inlet valve (or water inlet valve), increasing power generation and causing the frequency to rise. Because primary frequency regulation is differential control, the system frequency is lower than the initial value at this time, resulting in a steady-state frequency deviation.

[0004] To address this issue, scholars both domestically and internationally have proposed control strategies for grid-forming converters. Grid-forming converters are typically implemented using "virtual synchronous machines" (ZHONG QC, WEISSG. Synchronverters: inverters that mimic synchronous generators[J]. IEEE Transactions on Industrial Electronics, 2011, 58(4): 1259-1267.). The aim is to utilize the flexible and controllable characteristics of the converter to make it have external characteristics similar to synchronous generators through control, thereby participating in system frequency regulation.

[0005] However, to date, the vast majority of published literature primarily studies the impact of various converter control strategies on frequency stability, with very little research on frequency stability evaluation indicators for systems with a high proportion of asynchronous power sources connected to the grid. For systems with a high proportion of asynchronous power sources connected to the grid, where both grid-connected and grid-attached MMCs are potentially integrated, it is essential to study primary frequency regulation characteristic evaluation indicators. By proposing scientific and accurate evaluation indicators, we can effectively measure the primary frequency regulation characteristics of systems with a high proportion of renewable energy, providing theoretical guidance on frequency stability for the large-capacity development and grid connection of renewable energy. Summary of the Invention

[0006] In view of the deficiencies of the prior art, the present invention provides a method for analyzing the primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power supply. This method effectively evaluates the system's steady-state frequency support capability and primary frequency regulation characteristics by calculating the system's steady-state frequency deviation factor index.

[0007] Therefore, the present invention adopts the following technical solution: a method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system, which includes the following steps:

[0008] 1) Based on the power output of synchronous generators and asynchronous machines, establish a primary frequency regulation model for a high proportion of asynchronous machine power supply connected to the system;

[0009] 2) Based on the capacity and control strategy of each asynchronous generator in the power system, calculate the proportion of synchronous generators and the proportion of grid-connected converters in the asynchronous generators; based on the capacity and droop coefficient of each synchronous generator in the power system, calculate the equivalent droop coefficient of all synchronous generators.

[0010] 3) Revise the definition of the steady-state frequency deviation factor in traditional power systems and derive an evaluation index suitable for measuring the primary frequency regulation characteristics of systems with a high proportion of asynchronous machine power sources connected to the system;

[0011] 4) Based on the evaluation indicators, analyze the impact of the proportion of asynchronous power supplies and the proportion of grid-connected converters in asynchronous power supplies on the primary frequency regulation characteristics of a high proportion of asynchronous power supplies connected to the system.

[0012] Furthermore, the primary frequency regulation model for a high-proportion asynchronous machine power supply system mainly consists of the total generator output P of the system. SYS With the total system load P L The relative relationship determines that when P SYS >P L The system frequency increases; when P SYS <P L The system frequency decreases.

[0013] Furthermore, the total output P of the system generator SYS The total output P of the equivalent synchronous generator setCON Total output P of asynchronous generator units NEW Composition, total installed capacity of generators in the system S SYS The total capacity S of the equivalent synchronous generator sets CON Total capacity of asynchronous generator units S NEW constitute:

[0014] P SYS =P CON +P NEW ,

[0015] S SYS =S CON +S NEW ,

[0016] Assuming the power grid contains N traditional generators, the total output and total capacity of the equivalent synchronous generators are respectively the actual active power P of each synchronous generator set. CON,i Sum and capacity S CON,i The sum is:

[0017]

[0018]

[0019] Furthermore, the proportion of asynchronous power supplies α in a high-proportion asynchronous power supply system is calculated using the following expression:

[0020]

[0021] Among them, K PCON The ratio of total output of all synchronous generators to total installed capacity:

[0022]

[0023] Furthermore, due to the different requirements for operating technology and conditions of different types of generating units, the actual operating K... PCON The value fluctuates within a certain range, and the K mentioned above PCON The preferred value is 0.85.

[0024] Furthermore, the asynchronous power supply adopts grid-connected converter control or grid-linked converter control, with the grid-connected converter providing frequency support to the power grid; the total output P of the asynchronous power supply... NEW Power output P from grid-type converter FORM and grid-type converter output P FOLL Composition, total power supply capacity S of asynchronous machine NEW The capacity S of the grid-type converter FORM And grid converter capacity S FOLL constitute:

[0025] P NEW =P FORM +P FOLL ,

[0026] S NEW =S FORM +S FOLL ,

[0027] The proportion γ of grid-type converters in the asynchronous power supply system is calculated using the following expression:

[0028]

[0029] Furthermore, for N synchronous generators in a power system, the equivalent droop factor R of all synchronous generators... eq Calculated using the following expression:

[0030]

[0031] Among them, P CON,i With P N,i These are the actual active power and rated active power of each generator, R. CON,i This represents the equivalent droop coefficient for each generator.

[0032] Furthermore, for systems with a high proportion of asynchronous machine power supply, based on the traditional definition of frequency deviation factor, the corrected frequency deviation factor β... X Represented as:

[0033]

[0034] Among them, the proportion of asynchronous machine power sources in the α power system, R eq γ is the equivalent droop factor for all synchronous generators, γ is the proportion of grid-connected converters in asynchronous power supplies, and D is the equivalent droop factor for all synchronous generators. PFORM D is the equivalent damping coefficient of the grid-type converter. L This is the load frequency adjustment coefficient.

[0035] Furthermore, β X As an evaluation metric, it measures the primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power supplies; based on β X The system steady-state frequency deviation Δf and load disturbance ΔP L The relationship is calculated using the following expression:

[0036]

[0037] Among them, f N P is the steady-state frequency of the power system. L This represents the total system load.

[0038] According to the evaluation index βX The primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power supplies are analyzed as follows:

[0039] 1) When the proportion γ of the grid-type converter in the asynchronous power supply remains constant, as the proportion α of the asynchronous power supply gradually increases, the steady-state frequency deviation factor β... X The continuous decrease indicates that an increase in the proportion of asynchronous power sources will lead to a weakening of the power system's steady-state frequency support capability.

[0040] 2) When the proportion of asynchronous power supply α remains constant, as the proportion of grid-type converters in the asynchronous power supply γ gradually increases, the steady-state frequency deviation factor continuously increases. This indicates that increasing the proportion of grid-type converters in the asynchronous power supply can effectively enhance the steady-state frequency support capability of high-proportion asynchronous power systems and improve the frequency stability of the system.

[0041] Compared with the prior art, the present invention has the following beneficial technical effects:

[0042] 1. This invention proposes a method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system. By calculating the steady-state frequency deviation factor index of the system, the steady-state frequency support capability and primary frequency regulation characteristics of the system can be effectively evaluated.

[0043] 2. The system steady-state frequency deviation factor index derived in this invention can be used to analyze the main influencing factors of the primary frequency regulation characteristics of the system, providing theoretical guidance on frequency stability for the large-capacity development and grid connection of new energy sources. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the topology of the high-proportion asynchronous machine power supply access system of the present invention;

[0045] Figure 2 This is a schematic diagram of the control structure of the mesh-type MMC of the present invention;

[0046] Figure 3 This is a schematic diagram of the virtual speed controller structure of the network-type MMC of the present invention;

[0047] Figure 4 This is a schematic diagram illustrating the steady-state frequency deviation factor variation characteristics of the system of the present invention;

[0048] Figure 5 This is a simulation diagram of the frequency response of the mesh-type MMC system of the present invention (the upper figure is the frequency change diagram, and the lower figure is the output change diagram).

[0049] Figure 6 This is a simulation diagram of the frequency response of the network-type MMC system of the present invention (the upper figure is the frequency variation diagram, and the lower figure is the output variation diagram). Detailed Implementation

[0050] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0051] High-proportion asynchronous power supply systems can adopt Figure 1 The system model shown is equivalent to an MMC grid-connected system, where MMC is an asynchronous power source that can be controlled using either grid-connected or grid-following control; G1 and G2 are both traditional synchronous generators. L1, L2, and L3 are all system loads. When the power generated by MMC, G1, and G2 is balanced with the power absorbed by L1, L2, and L3, the system frequency stabilizes at 50Hz.

[0052] Asynchronous power supplies can employ either grid-based MMC control or follow-grid MMC control. The follow-grid MMC control structure is as follows: Figure 2 As shown. A grid-type converter can be implemented by adding a virtual speed governor to the power outer loop of the MMC. The structure of the virtual speed governor is as follows: Figure 3 As shown, s is the Laplace operator, D is the MMC equivalent damping coefficient, Y is the turbine blade position, and T is the torque vector. G T is the time constant of the speed controller. RH T CH F is the typical time constant of a steam turbine. HP Indicates the virtual speed governor control parameters; △ω represents the deviation between the MMC angular velocity and the rated angular velocity; △P ref P represents the power increment output by the virtual speed controller. ref0 Indicates the MMC rated power command value; P ref This represents the power command value after correction by the virtual speed controller.

[0053] For systems with a high proportion of asynchronous power supply, based on the traditional definition of frequency deviation factor, the corrected frequency deviation factor β... X It can be represented as:

[0054]

[0055] Among them, the proportion of asynchronous machine power sources in the α power system, R eq γ is the equivalent droop factor for all synchronous generators, γ is the proportion of grid-connected converters in asynchronous power supplies, and D is the equivalent droop factor for all synchronous generators. PRORM D is the equivalent damping coefficient of the grid-type converter. L This is the load frequency adjustment coefficient.

[0056] β X It can be used as an evaluation metric to measure the primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power supplies. Based on β... X The system steady-state frequency deviation Δf and load disturbance ΔP L The relationship can be calculated using the following expression:

[0057]

[0058] Among them, f N This is the steady-state frequency of the power system, typically the power frequency of 50Hz. It can be seen that β... X The larger the value of β, the smaller the steady-state frequency deviation of the power grid under the same power disturbance. X It is inversely proportional to the steady-state frequency deviation of the power grid. Therefore, β X The larger the value, the better the frequency stability of the power grid.

[0059] Based on the above analysis, it can be seen that the corrected steady-state frequency deviation factor β X Characterization. In actual power grids, the equivalent droop coefficient R of synchronous generators. eq It varies within the range of 0.04 to 0.06. Load frequency regulation coefficient D L It varies within the range of 1.2 to 1.8. In this embodiment, R is taken as... eq =0.05, D L =1.5, D PFORM =20. Calculate the ratio of asynchronous power supply α and the ratio of grid-connected converters in the asynchronous power supply γ to the frequency deviation factor β of the MMC grid-connected system. X The effects are as follows Figure 4 As shown. By Figure 4 It can be seen that:

[0060] (1) When the proportion γ of the grid-type converter in the asynchronous power supply remains constant, as the proportion α of the asynchronous power supply gradually increases, the steady-state frequency deviation factor β... X The continuous decrease indicates that an increase in the proportion of asynchronous power sources will lead to a weakening of the power system's steady-state frequency support capability.

[0061] (2) When the proportion of asynchronous power supply α remains unchanged, as the proportion of grid-type converters in asynchronous power supply γ gradually increases, the steady-state frequency deviation factor continuously increases. This indicates that increasing the proportion of grid-type converters in asynchronous power supply can effectively enhance the steady-state frequency support capability of high-proportion asynchronous power systems and improve the frequency stability of the system.

[0062] In PSCAD / EMTDC, build a topology for a system consisting of a synchronous generator and an MMC, such as... Figure 1 As shown in Table 1, the system parameters are shown in Table 2, and the synchronous generator parameters are shown in Table 3. The MMC can adopt a grid-following control strategy or a grid-based control strategy based on a virtual governor. The structure of the virtual governor is shown in Table 4. Figure 3 As shown in Table 3, the parameters are as follows.

[0063] Table 1 System Parameters

[0064]

[0065]

[0066] Table 2 Synchronous Generator Parameters

[0067] physical quantity numerical values Rated capacity 240MW inertia constant 4s Mechanical damping 0 Sag coefficient 0.05 Rotor lead time constant 0.01s Rotor hysteresis time constant 0.01s

[0068] Table 3 Virtual Speed ​​Regulator Parameters

[0069] physical quantity numerical values <![CDATA[D PFORM ]]> 20 <![CDATA[T G ]]> 0.2s <![CDATA[F HP ]]> 0.3pu <![CDATA[T RH ]]> 7s <![CDATA[T CH ]]> 0.3s

[0070] The asynchronous power supply adopts a grid-connected MMC control strategy. Simulation studies are conducted on the frequency response characteristics of the grid-connected MMC system under active load disturbances. In the grid-connected MMC system, the asynchronous power supply accounts for α = 45.45%, the grid-connected converter accounts for γ = 0, and the frequency deviation factor is β. X =10.909. When t=20s, the load experiences disturbances of 40, 60, and 80MW respectively. The system frequency change and MMC output change are as follows: Figure 5 As shown.

[0071] A virtual speed controller is added to the outer loop of the MMC power supply. A grid-connected MMC control strategy is adopted for the asynchronous machine power supply. The frequency response characteristics of the grid-connected MMC system under active load disturbances are studied through simulation. In the grid-connected MMC system, the asynchronous machine power supply accounts for α = 45.45%, the grid-connected converter accounts for γ = 1, and the frequency deviation factor is β. X =20. When t=5s, the load experiences disturbances of 40, 60, and 80MW respectively. The system frequency change and MMC output change are as follows: Figure 6 As shown.

[0072] Depend on Figure 5 It can be seen that before the load power disturbance, the active power output of each power source in the system was balanced with the load, and the steady-state frequency of the system was 50.024Hz. With the occurrence of the load disturbance, the system frequency first dropped, and then the synchronous generator governor activated, increasing the output of the synchronous generator and supporting a gradual recovery of the system frequency. After a transient process of approximately 30 seconds, the system frequency returned to stability. There was a steady-state frequency deviation before and after the active power disturbance. Throughout the entire process, the active power output of the grid-connected MMC remained unchanged, proving that the grid-connected MMC does not possess frequency support characteristics. Therefore, large-capacity grid-connected MMCs connected to the grid will worsen the frequency stability of the system. Furthermore, when the load disturbances were 40, 60, and 80MW, the steady-state frequencies of the system were 49.82, 49.72, and 49.62Hz, respectively, with corresponding steady-state frequency deviations of 0.20, 0.30, and 0.40Hz.

[0073] Depend on Figure 6It can be seen that the frequency response characteristics of the grid-connected MMC system are basically the same as those of the grid-connected MMC system. Before the load disturbance, the steady-state frequency of the system is 50.00Hz. After the load disturbance, the frequency drops due to power imbalance. Subsequently, the power supply increases its output, and the frequency gradually recovers and eventually stabilizes. There is a steady-state frequency deviation before and after the system experiences an active power disturbance. In addition, when the frequency is below 50Hz, the grid-connected MMC can change the active power output through the virtual speed controller. The more the system frequency decreases, the more the MMC increases its output. This proves that the virtual speed controller control strategy can adjust the active power output of the MMC according to the system frequency change, providing frequency support for the system. Furthermore, when the load disturbances were 40, 60, and 80 MW, the frequencies after the system recovered to steady state were 49.88, 49.82, and 49.76 Hz, respectively, with corresponding steady-state frequency deviations of 0.12, 0.18, and 0.24 Hz, which were significantly smaller than the steady-state frequency deviation of the grid-connected MMC system. This demonstrates that using a virtual speed governor in the outer loop of the MMC power system can significantly optimize the primary frequency regulation performance of the MMC.

[0074] To further demonstrate the accuracy of the frequency deviation factor and the effectiveness of the virtual governor control strategy, the theoretical and simulated values ​​of the steady-state frequency deviation of the grid-connected MMC system and the grid-connected MMC system under different load disturbances were recorded, as shown in Table 4. It can be seen that under the same load disturbance, the steady-state frequency deviation of the grid-connected MMC system is significantly smaller than that of the grid-connected MMC system. Furthermore, based on β... X The calculated theoretical value is basically consistent with the simulation value obtained based on the PSCAD model, proving that β X The effectiveness. For MMC high-capacity access systems, β can be used. X Evaluate the system's steady-state frequency support capability.

[0075] Table 4. Theoretical and Simulated Values ​​of Steady-State Frequency Deviation

[0076]

[0077] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for analyzing the primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power supply, characterized in that, Includes the following steps: 1) Based on the power output of synchronous generators and asynchronous machines, establish a primary frequency regulation model for a high proportion of asynchronous machine power supply connected to the system; 2) Based on the capacity and control strategy of each asynchronous generator in the power system, calculate the proportion of synchronous generators and the proportion of grid-connected converters in the asynchronous generators; based on the capacity and droop coefficient of each synchronous generator in the power system, calculate the equivalent droop coefficient of all synchronous generators. 3) Revise the definition of the steady-state frequency deviation factor in traditional power systems and derive an evaluation index suitable for measuring the primary frequency regulation characteristics of systems with a high proportion of asynchronous machine power sources connected to the system; 4) Based on evaluation indicators, analyze the impact of the proportion of asynchronous power supplies and the proportion of grid-connected converters in asynchronous power supplies on the primary frequency regulation characteristics of a high proportion of asynchronous power supplies connected to the system. For systems with a high proportion of asynchronous power supply, based on the traditional definition of frequency deviation factor, the corrected frequency deviation factor is... β X Represented as: ， in, α The proportion of asynchronous power sources in the power system. R eq Let be the equivalent droop factor for all synchronous generators. γ The proportion of grid-type converters in asynchronous power supplies. D PRORM The equivalent damping coefficient of the grid-type converter. D L This is the load frequency adjustment coefficient; β X As an evaluation metric, it measures the primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power supplies; according to β X The steady-state frequency deviation of the system is Δ f With load disturbance Δ P L The relationship is calculated using the following expression: ， in, f N For the steady-state frequency of the power system, P L This represents the total system load.

2. The method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system according to claim 1, characterized in that, The primary frequency regulation model of a high-proportion asynchronous power supply system mainly consists of the total output of the system's generators. P SYS With total system load P L Relative relationships determine that when P SYS > P L The system frequency increases; when P SYS < P L The system frequency decreases.

3. The method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system according to claim 1, characterized in that, Total output of system generators P SYS Total output of equivalent synchronous generator units P CON Total output of asynchronous generator units P NEW Composition, total installed capacity of the system generator S SYS Total capacity of equivalent synchronous generator sets S CON Total capacity of asynchronous generator units S NEW constitute: ， ， Assuming the power grid includes conventional generators N If the total output and total capacity of the equivalent synchronous generators are respectively the actual active power of each synchronous generator unit. P CON,i Sum and capacity S CON,i The sum is: ， 。 4. The method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system according to claim 3, characterized in that, High proportion of asynchronous power supplies in the system; asynchronous power supply ratio α Calculated using the following expression: ， in, K PCON The ratio of total output of all synchronous generators to total installed capacity: 。 5. The method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system according to claim 4, characterized in that, The aforementioned K PCON The value is 0.

85.

6. The method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system according to claim 1, characterized in that, The asynchronous power supply uses either grid-connected or grid-following converter control; the total output of the asynchronous power supply... P NEW Power output from grid-type converter P FORM and grid converter output P FOLL Composition, total capacity of asynchronous power supply S NEW Capacity of grid-type converter S FORM and grid converter capacity S FOLL constitute: ， ， The proportion of grid-type converters in asynchronous power supplies of the system γ Calculated using the following expression: 。 7. The method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system according to claim 3, characterized in that, In power systems N A synchronous generator, and the equivalent droop factor of all synchronous generators. R eq Calculated using the following expression: ， in, P N,i This refers to the rated active power of each generator. R CON,i This represents the equivalent droop coefficient for each generator.

8. The method for analyzing the primary frequency regulation characteristics of a high-proportion asynchronous machine power supply system according to claim 1, characterized in that, According to the evaluation indicators β X The primary frequency regulation characteristics of a system with a high proportion of asynchronous machine power supplies are analyzed as follows: 1) When the proportion of grid-type converters in asynchronous power supplies γ When it remains unchanged, with the proportion of asynchronous machine power supply α Gradually increase, steady-state frequency deviation factor β X The continuous decrease indicates that an increase in the proportion of asynchronous power sources will lead to a weakening of the power system's steady-state frequency support capability. 2) When the asynchronous power supply accounts for a certain percentage α When the ratio remains constant, as the proportion of grid-type converters in asynchronous power supplies increases... γ The gradual increase in the steady-state frequency deviation factor indicates that increasing the proportion of grid-type converters in asynchronous power supplies can effectively enhance the steady-state frequency support capability of high-proportion asynchronous power systems and improve the frequency stability of the system.

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