Method and system for calculating voltage and frequency of small hydropower island power grid based on excitation control

Abstract

The invention discloses a small hydropower island power grid voltage and frequency calculation method and system based on excitation control. The method comprises the following steps: determining system parameters of an island power grid; on the basis of the system parameters, an excitation voltage calculation value is calculated through an excitation voltage calculation model, and an excitation voltage interval to which the excitation voltage calculation value belongs is judged; and calculating the steady-state voltage and frequency of the island power grid according to the excitation voltage calculation value and the interval to which the excitation voltage calculation value belongs. According to the method, the adjusting capacity of the excitation system of the small hydroelectric generating set and the dynamic relation between the mechanical power and the frequency are fully considered, the steady-state operating point of the system after the island is formed is quantitatively analyzed, a scientific basis is provided for judging whether an anti-island protection device needs to be configured or not, the economical efficiency is improved, and the cost is reduced. And potential safety hazards caused by non-planned islands to power grid dispatching and equipment maintenance are effectively avoided.

Description

Technical Field

[0001] This invention relates to the field of power system protection and control, specifically to a method and system for calculating voltage and frequency in a small hydropower islanded power grid based on excitation control. Background Technology

[0002] With the rapid advancement of my country's new power system construction, the penetration rate of distributed clean energy has significantly increased, and the distribution network structure has become increasingly complex and diversified. Particularly in provinces rich in small hydropower resources, such as Sichuan, Yunnan, and Hunan, a large number of small hydropower units have been connected to the distribution network. While promoting the consumption of renewable energy, this has also brought new safety operation challenges. Because small hydropower units typically possess a certain degree of autonomous voltage regulation capability, they may cause surrounding loads to continue operating independently during local grid faults or planned power outages, forming unplanned islands. Such islands not only threaten the safety of maintenance personnel but may also damage connected equipment due to voltage and frequency runaway, and even trigger wider grid faults.

[0003] Currently, there is considerable research on anti-islanding protection technologies for distributed photovoltaic (PV) power generation systems both domestically and internationally, and related standards are relatively mature. However, the operating characteristics of small hydropower units differ significantly from those of intermittent power sources such as PV. PV power generation relies on power electronic interfaces for grid connection, and its output is affected by weather conditions. In contrast, small hydropower units are mostly synchronous generators directly connected to the grid, possessing rotational inertia and excitation systems, enabling them to autonomously adjust voltage and frequency within a certain range. This characteristic means that small hydropower units may maintain transient stability after island formation and enter a new steady-state operating point, thereby increasing the risk of unplanned islanding persisting. Existing assessment methods are mostly based on the power balance principle, neglecting the dynamic adjustment capability of the small hydropower excitation system and the coupling relationship between the prime mover's power and frequency, thus making it difficult to accurately determine whether islanding can stably persist.

[0004] Furthermore, islanding is closely related to the location and capacity of the generating units, as well as the impedance characteristics (resistance, inductance, and capacitance components) of the local load. Without a refined model that considers excitation regulation and mechanical characteristics, existing methods cannot effectively assess the steady-state voltage and frequency after islanding, making it difficult to determine whether anti-islanding protection devices are needed. Currently, some regions adopt a "one-size-fits-all" configuration strategy, which not only increases investment costs but may also affect power supply reliability due to protection malfunctions. Therefore, there is an urgent need to establish a steady-state analysis method that reflects the excitation regulation characteristics and frequency response capabilities of small hydropower units to assess the sustainable operating conditions of islanded power grids.

[0005] In summary, as the proportion of distributed generation in power distribution systems continues to increase, the risk of localized islanding becomes increasingly prominent. For distributed generation systems like small hydropower, which possess autonomous regulation capabilities, developing accurate islanding steady-state analysis methods is of significant practical importance. Summary of the Invention

[0006] To address the aforementioned problems in the prior art, this invention provides a method and system for calculating the voltage and frequency of a small hydropower islanded power grid based on excitation control. By fully considering the excitation regulation characteristics of small hydropower and the relationship between the mechanical power and frequency of small hydropower, it accurately assesses whether an anti-islanding protection device needs to be configured in the area, thereby improving economic efficiency and reducing the burden of scheduling, operation and maintenance.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following steps: S1, determine the system parameters of the isolated power grid, including: the capacity of the small hydropower unit, synchronous reactance, etc. Maximum allowable operating excitation voltage and minimum value And the equivalent resistance R, equivalent inductance L, and equivalent capacitance C of the load; S2, based on the system parameters, calculate the excitation voltage value Ef using the excitation voltage calculation model, and determine the excitation voltage range to which Ef belongs. The range includes: the normal range [ , Overvoltage range ( (, +∞) and undervoltage range (-∞, ); S3, based on the calculated excitation voltage value Ef and its corresponding interval, calculate the steady-state voltage Vg and frequency f of the islanded power grid: When Ef falls within the normal range, the terminal voltage Vg is set to the rated voltage Vg0, and the frequency f is calculated based on Vg0. When Ef falls within the overvoltage range, the excitation voltage is set to... By combining the synchronous generator equations and the active power balance equations, the steady-state voltage can be obtained. And then according to Calculate the frequency f; When Ef belongs to the undervoltage range, the excitation voltage is set to By combining the synchronous generator equations and the active power balance equations, the steady-state voltage can be obtained. And then according to Calculate the frequency f. Furthermore, the establishment of the excitation voltage calculation model in step S2 includes the following steps: S201, based on the basic characteristics of synchronous generators, establishes a correlation calculation model between excitation voltage and terminal voltage Vg, active power P and reactive power Q; S202, Based on the mechanical power model of the water turbine and the active power consumed by the equivalent resistance of the load, a correlation calculation model is established between frequency and small hydropower terminal voltage and equivalent resistance of the load. S203, Based on the load RLC parallel model, establish a calculation model for reactive power and frequency; S204, combining the above three calculation models, based on the synchronous generator reactance... It is related to frequency and can be expressed as:

[0008] in, The reference frequency for the power grid; This refers to the equivalent reactance of the synchronous generator at the reference frequency. This is the current actual frequency.

[0009] The final calculation formula for the excitation voltage calculation model is as follows:

[0010] in, This is a reference value for the terminal voltage of a small hydropower unit; Rated active power for small hydropower; The equivalent resistance of the load within the islanded area; The equivalent capacitance within the island's boundaries; The equivalent inductance within the isolated area; .

[0011] Furthermore, the correlation calculation model between the excitation voltage and the terminal voltage Vg, active power P, and reactive power Q in step S201 is as follows: Based on the no-load electromotive force equation of a synchronous generator:

[0012] in, This is the excitation voltage; Let I be the terminal voltage of the small hydropower generator; I is the stator current, and the formula for calculating I is:

[0013] Where P is active power and Q is reactive power; Combining the above two equations, we get:

[0014] Furthermore, the correlation calculation model between frequency f, small hydropower generator terminal voltage, and equivalent load resistance in step S202 is as follows: Based on the condition of no speed regulator, the output power of small hydropower With frequency Relationship:

[0015] in, For the mechanical power of small hydroelectric turbines, The mechanical power of a small hydroelectric turbine under fundamental frequency conditions. As the reference frequency, This is the actual frequency; Active power based on load consumption Some calculation formulas are as follows:

[0016] Where R is the equivalent resistance of the load; Meanwhile, since the active power provided by the turbine will be consumed by the load, we have:

[0017] Combining the above three equations, we get: .

[0018] Furthermore, the calculation model for reactive power and frequency in step S203 is as follows:

[0019] in, This indicates the reactive power of the load.

[0020] Furthermore, in step S3, when Ef belongs to the normal range, the formula for calculating frequency f is:

[0021] in, The reference frequency for the power grid; This represents the mechanical power of a small hydroelectric turbine under fundamental frequency conditions.

[0022] Furthermore, in step S3, when Ef belongs to the overvoltage range, the formula for calculating the frequency f is:

[0023] in, This represents the maximum allowable operating voltage at the terminal of the small hydropower generator.

[0024] Furthermore, in step S3, when Ef belongs to the undervoltage range, the formula for calculating the frequency f is:

[0025] in, This is the minimum allowable operating voltage at the terminal of the small hydropower generator.

[0026] The present invention also provides a voltage and frequency calculation system for a small hydropower islanded grid based on excitation control, comprising a processor and a computer-readable storage medium interconnected thereto, wherein the computer-readable storage medium stores a computer program, which is executed by the processor to implement the steps of the above-described method for calculating voltage and frequency of a small hydropower islanded grid based on excitation control.

[0027] The present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the above-described method for calculating the voltage and frequency of a small hydropower island grid based on excitation control.

[0028] Compared with the prior art, the advantages of the present invention are as follows: This invention addresses the current lack of risk assessment methods for small hydropower units operating in islanded conditions by proposing a method for calculating the steady-state voltage and frequency of an islanded power grid that considers excitation regulation characteristics. This method fully takes into account the excitation system regulation capability of small hydropower units and the dynamic relationship between their mechanical power and frequency. By quantitatively analyzing the steady-state operating point of the system after island formation, it provides a scientific basis for determining whether anti-islanding protection devices are needed. Based on the assessment results obtained from this method, differentiated configurations can be implemented without affecting system safety: for scenarios where stable island operation cannot be maintained, anti-islanding protection devices can be omitted, thereby saving equipment investment and improving economic efficiency; while for small hydropower units that may form sustainable islands, corresponding protections can be installed in a timely manner to effectively avoid the safety hazards brought by unplanned islands to power grid dispatching and equipment maintenance. Attached Figure Description

[0029] Figure 1 This is a flowchart illustrating an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of a small hydropower station and the downstream load of a circuit breaker according to an embodiment of the present invention. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0031] The technical solution adopted in this embodiment includes the following steps: S1, determine the system parameters of the isolated power grid, including: the capacity of the small hydropower unit, synchronous reactance, etc. Maximum allowable operating excitation voltage and minimum value And the equivalent resistance R, equivalent inductance L, and equivalent capacitance C of the load; S2, based on the system parameters, calculate the excitation voltage value Ef using the excitation voltage calculation model, and determine the excitation voltage range to which Ef belongs. The range includes: the normal range [ , Overvoltage range ( (, +∞) and undervoltage range (-∞, ); S3, based on the calculated excitation voltage value Ef and its corresponding interval, calculate the steady-state voltage Vg and frequency f of the islanded power grid: When Ef falls within the normal range, the terminal voltage Vg is set to the rated voltage Vg0, and the frequency f is calculated based on Vg0. When Ef falls within the overvoltage range, the excitation voltage is set to... By combining the synchronous generator equations and the active power balance equations, the steady-state voltage can be obtained. And then according to Calculate the frequency f; When Ef belongs to the undervoltage range, the excitation voltage is set to By combining the synchronous generator equations and the active power balance equations, the steady-state voltage can be obtained. And then according to Calculate the frequency f. Preferably, the establishment of the excitation voltage calculation model in step S2 includes the following steps: S201, based on the basic characteristics of synchronous generators, establishes a correlation calculation model between excitation voltage and terminal voltage Vg, active power P and reactive power Q; S202, Based on the mechanical power model of the water turbine and the active power consumed by the equivalent resistance of the load, a correlation calculation model is established between frequency and small hydropower terminal voltage and equivalent resistance of the load. S203, Based on the load RLC parallel model, establish a calculation model for reactive power and frequency; S204, combining the above three calculation models, based on the synchronous generator reactance... It is related to frequency and can be expressed as:

[0032] in, The reference frequency for the power grid; This refers to the equivalent reactance of the synchronous generator at the reference frequency. This is the current actual frequency.

[0033] The final calculation formula for the excitation voltage calculation model is as follows:

[0034] in, This is a reference value for the terminal voltage of a small hydropower unit; Rated active power for small hydropower; The equivalent resistance of the load within the islanded area; The equivalent capacitance within the island's boundaries; The equivalent inductance within the isolated area; .

[0035] Preferably, the correlation calculation model between the excitation voltage and the terminal voltage Vg, active power P, and reactive power Q in step S201 is as follows: Based on the no-load electromotive force equation of a synchronous generator:

[0036] in, This is the excitation voltage; Let I be the terminal voltage of the small hydropower generator; I is the stator current, and the formula for calculating I is:

[0037] Where P is active power and Q is reactive power; Combining the above two equations, we get:

[0038] Preferably, the correlation calculation model between frequency f and small hydropower generator terminal voltage and equivalent load resistance in step S202 is as follows: Based on the condition of no speed regulator, the output power of small hydropower With frequency Relationship:

[0039] in, For the mechanical power of small hydroelectric turbines, The mechanical power of a small hydroelectric turbine under fundamental frequency conditions. As the reference frequency, This is the actual frequency; Active power based on load consumption Some calculation formulas are as follows:

[0040] Where R is the equivalent resistance of the load; Meanwhile, since the active power provided by the turbine will be consumed by the load, we have:

[0041] Combining the above three equations, we get: .

[0042] like Figure 2 The structure of the small hydropower station and the downstream load of the circuit breaker shown is preferably represented by the following calculation model for reactive power and frequency in step S203:

[0043] in, This indicates the reactive power of the load.

[0044] Preferably, in step S3, when Ef belongs to the normal range, the formula for calculating frequency f is:

[0045] in, The reference frequency for the power grid; This represents the mechanical power of a small hydroelectric turbine under fundamental frequency conditions.

[0046] Preferably, in step S3, when Ef belongs to the overvoltage range, the formula for calculating the frequency f is:

[0047] in, This represents the maximum allowable operating voltage at the terminal of the small hydropower generator.

[0048] The derivation of this calculation formula is as follows: When Ef belongs to the overvoltage range, let Substituting into the excitation voltage calculation model, we can obtain:

[0049] make The above formula can be simplified to

[0050] After rearranging into a polynomial of u, it can be simplified to:

[0051] in, , , , .

[0052] Solving this expression yields an expression for u greater than 0, which provides the result. ,in This represents the terminal voltage of the small hydroelectric generator when the excitation voltage reaches its maximum value. Substitute this result into...

[0053] have to .

[0054] Preferably, in step S3, when Ef belongs to the undervoltage range, the formula for calculating the frequency f is:

[0055] in, This is the minimum allowable operating voltage at the terminal of the small hydropower generator.

[0056] The derivation of this calculation formula is as follows: When Ef belongs to the overvoltage range, let , Substituting into the excitation voltage calculation model, we can obtain:

[0057] Organized into The polynomial above can be simplified to:

[0058] in, , , , .

[0059] Solving the cubic equation, we can obtain An expression greater than 0 yields... , This represents the terminal voltage of the small hydroelectric generator when the excitation voltage reaches its minimum value. Substitution

[0060] The expression for frequency can then be obtained as: .

[0061] The overall flowchart of this embodiment is as follows: Figure 1 As shown.

[0062] Compared with the prior art, the advantages of this embodiment are: This embodiment addresses the current lack of risk assessment methods for small hydropower units operating in islanded mode by proposing a method for calculating the steady-state voltage and frequency of an islanded power grid that considers excitation regulation characteristics. This method fully takes into account the excitation system regulation capability of small hydropower units and the dynamic relationship between their mechanical power and frequency. By quantitatively analyzing the steady-state operating point of the system after island formation, it provides a scientific basis for determining whether anti-islanding protection devices are needed. Based on the assessment results obtained by this method, differentiated configurations can be achieved without affecting system safety: for scenarios where stable island operation cannot be maintained, anti-islanding protection devices can be omitted, thereby saving equipment investment and improving economic efficiency; while for small hydropower units that may form sustainable islands, corresponding protections can be installed in a timely manner to effectively avoid the safety hazards brought by unplanned islands to power grid dispatching and equipment maintenance.

[0063] This embodiment also provides a voltage and frequency calculation system for a small hydropower islanded grid based on excitation control, including a processor and a computer-readable storage medium connected to each other. The computer-readable storage medium stores a computer program, which is executed by the processor to implement the steps of the above-described method for calculating the voltage and frequency of a small hydropower islanded grid based on excitation control.

[0064] This embodiment also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the above-described method for calculating the voltage and frequency of a small hydropower island grid based on excitation control.

[0065] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-readable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create a machine for implementing the process. Figure 1 One or more processes and / or boxes Figure 1The computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to operate in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The functions specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable apparatus for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0066] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for calculating voltage and frequency in a small hydropower islanded power grid based on excitation control, characterized in that, Includes the following steps: S1, determine the system parameters of the isolated power grid, including: the capacity of the small hydropower unit, synchronous reactance, etc. Maximum allowable operating excitation voltage and minimum value And the equivalent resistance R, equivalent inductance L, and equivalent capacitance C of the load; S2, based on the system parameters, calculate the excitation voltage value Ef using the excitation voltage calculation model, and determine the excitation voltage range to which Ef belongs. The range includes: the normal range [ , Overvoltage range ( (, +∞) and undervoltage range (-∞, ); S3, based on the calculated excitation voltage value Ef and its corresponding interval, calculate the steady-state voltage Vg and frequency f of the islanded power grid: When Ef falls within the normal range, the terminal voltage Vg is set to the rated voltage Vg0, and the frequency f is calculated based on Vg0. When Ef falls within the overvoltage range, the excitation voltage is set to... By combining the synchronous generator equations and the active power balance equations, the steady-state voltage can be obtained. And then according to Calculate the frequency f; When Ef belongs to the undervoltage range, the excitation voltage is set to By combining the synchronous generator equations and the active power balance equations, the steady-state voltage can be obtained. And then according to Calculate the frequency f.

2. The method for calculating voltage and frequency of a small hydropower islanded power grid based on excitation control according to claim 1, characterized in that, The establishment of the excitation voltage calculation model in step S2 includes the following steps: S201, based on the basic characteristics of synchronous generators, establishes a correlation calculation model between excitation voltage and terminal voltage Vg, active power P and reactive power Q; S202, Based on the mechanical power model of the water turbine and the active power consumed by the equivalent resistance of the load, a correlation calculation model is established between frequency and small hydropower terminal voltage and equivalent resistance of the load. S203, Based on the load RLC parallel model, establish a calculation model for reactive power and frequency; S204, combining the above three calculation models, based on the synchronous generator reactance... It is related to frequency and can be expressed as: in, The reference frequency for the power grid; This refers to the equivalent reactance of the synchronous generator at the reference frequency. This refers to the current actual frequency; The final calculation formula for the excitation voltage calculation model is as follows: in, This is a reference value for the terminal voltage of a small hydropower unit; Rated active power for small hydropower; The equivalent resistance of the load within the islanded area; The equivalent capacitance within the island's boundaries; The equivalent inductance within the isolated area; .

3. The method for calculating voltage and frequency of a small hydropower islanded power grid based on excitation control according to claim 2, characterized in that, The correlation calculation model between the excitation voltage and the terminal voltage Vg, active power P, and reactive power Q in step S201 is as follows: Based on the no-load electromotive force equation of a synchronous generator: in, This is the excitation voltage; Let I be the terminal voltage of the small hydropower generator; I is the stator current, and the formula for calculating I is: Where P is active power and Q is reactive power; Combining the above two equations, we get: 。 4. The method for calculating voltage and frequency of a small hydropower islanded power grid based on excitation control according to claim 2, characterized in that, The correlation calculation model between frequency f, small hydropower generator terminal voltage, and equivalent load resistance in step S202 is as follows: Based on the condition of no speed regulator, the output power of small hydropower With frequency Relationship: in, For the mechanical power of small hydroelectric turbines, The mechanical power of a small hydroelectric turbine under fundamental frequency conditions. As the reference frequency, This is the actual frequency; Active power based on load consumption Some calculation formulas are as follows: Where R is the equivalent resistance of the load; Meanwhile, since the active power provided by the turbine will be consumed by the load, we have: Combining the above three equations, we get: 。 5. The method for calculating voltage and frequency of a small hydropower islanded power grid based on excitation control according to claim 2, characterized in that, The calculation model for reactive power and frequency in step S203 is as follows: in, This indicates the reactive power of the load.

6. The method for calculating voltage and frequency of a small hydropower islanded power grid based on excitation control according to claim 1, characterized in that, In step S3, when Ef belongs to the normal range, the formula for calculating frequency f is: in, The reference frequency for the power grid; This represents the mechanical power of a small hydroelectric turbine under fundamental frequency conditions.

7. The method for calculating voltage and frequency of a small hydropower islanded power grid based on excitation control according to claim 1, characterized in that, In step S3, when Ef belongs to the overvoltage range, the formula for calculating the frequency f is: in, This represents the maximum allowable operating voltage at the terminal of the small hydropower generator.

8. The method for calculating voltage and frequency of a small hydropower islanded power grid based on excitation control according to claim 1, characterized in that, In step S3, when Ef belongs to the undervoltage range, the formula for calculating the frequency f is: in, This is the minimum allowable operating voltage at the terminal of the small hydropower generator.

9. A voltage and frequency calculation system for a small hydropower islanded power grid based on excitation control, characterized in that, The device includes an interconnected processor and a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program that is executed by the processor to implement the steps of the method for calculating the voltage and frequency of a small hydropower island grid based on excitation control as described in any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, which is executed by a processor to implement the steps of the method for calculating the voltage and frequency of a small hydropower island grid based on excitation control as described in any one of claims 1 to 8.

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