Method for calculating power stable operation area of network-constructing converter station based on network-constructing capability

Abstract

The application discloses a network-constructing type converter station power stable operation area calculation method based on network-constructing capacity, and relates to the technical field of power electronics. The method comprises the following steps: based on the control characteristics of the network-constructing type converter station, a network frequency stability constraint equation is established; based on the relationship between the valve outlet voltage on the AC side of the network-constructing type converter station and the AC bus voltage, a power constraint range considering the network-constructing capacity of the network-constructing type converter station is established; an operation circuit model of the network-constructing type converter station under complex frequency domain is established, and factors influencing transient components in the network-constructing type converter station are analyzed; the current limiting effect of the network-constructing type converter station is considered, and the power stable operation area of the network-constructing type converter station based on the network-constructing capacity is calculated. The application adds the "network-constructing support capacity constraint" and the "current limiting network-constructing support capacity constraint" under the influence of the current limiting and the transient components, is suitable for the power stable operation evaluation of the AC system provided with voltage support by the network-constructing type converter station, is simple in logic, and is targeted.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method for calculating the power stability operating area of ​​a grid-type converter station based on grid-building capability. Background Technology

[0002] my country's large-scale exploitable new energy sources are mainly concentrated in the Gobi and desert regions of the Northwest. These new energy bases are large in scale and widely distributed. At the same time, they face problems such as the lack of conventional power supply support for the AC collection grid and the lack of AC main grid connection. As a result, the AC collection grids of new energy bases in the Gobi and desert regions are huge and have frequent power fluctuations, making it difficult to transmit new energy power to other regions.

[0003] Grid-forming (GFM) converter stations can independently control the output voltage amplitude and phase within their operating power range, providing voltage support for such grid structures. Therefore, the transmission of renewable energy from renewable energy bases via AC grids through grid-forming converter stations offers a reference solution to the aforementioned problems. For this type of grid, grid-forming converter stations must both generate reactive current to support the AC grid voltage and ensure the stable transmission of active power from renewable energy sources. Considering the vulnerability of power electronic devices, the converter needs to incorporate current limiting circuitry. Increased reactive current generation will consume active current margin, but it also helps support AC voltage, thus improving the active power output capacity of renewable energy bases to some extent. Therefore, it is necessary to consider the power operating range within the grid support capacity of the grid-forming converter station to match the actual power of the renewable energy base and ensure the stable transmission of renewable energy from desert areas.

[0004] For systems providing voltage support to grid-connected converter stations, grid performance constraints must be considered, namely, the converter station's ability to maintain frequency stability. This includes frequency deviation Δf and the rate of change of frequency (RoCoF) to ensure the grid-connected converter station can stably support the AC system. However, conventional converter station power range constraints include modulation ratio constraints, voltage stability constraints, capacitor voltage fluctuation constraints, converter transformer capacity constraints, and converter capacity constraints. There is currently no stable power operation range for grid-connected converter stations considering the impact of grid capability constraints.

[0005] To address the aforementioned issues, this invention proposes a method for calculating the stable power operation area of ​​a grid-type converter station based on grid-building capabilities. Summary of the Invention

[0006] The purpose of this invention is to provide a method for calculating the stable power operating region of a grid-type converter station based on its grid-building capability, thereby solving the problems mentioned in the background section. This invention addresses the drawing of the stable power operating region of a system in a renewable energy transmission system that provides voltage support to a grid-type converter station, ensuring the grid-building capability is adequate. Conventional methods for drawing the stable power operating region of converter stations lack constraints on the grid-building capability of grid-type converter stations, potentially resulting in a large stable power operating region and the risk that the AC grid power of the renewable energy base may exceed the power supported by the grid-building capability of the converter station.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for calculating the power stability operating region of a grid-type converter station based on grid construction capability includes the following steps:

[0009] S1. Based on the power control characteristics of grid-type converter stations and the principle of constructing AC side voltage in grid-type converter stations, establish grid frequency stability constraint equations.

[0010] S2. Based on the voltage control characteristics of grid-type converter stations and the relationship between the AC side valve outlet voltage and the AC bus voltage of grid-type converter stations, substitute the S1 grid-type frequency stability constraint equation to establish a power constraint range that considers the grid-type converter station's grid-type capability.

[0011] S3. Establish a computational circuit model of a grid-type converter station in the complex frequency domain and analyze the influencing factors of transient components in the grid-type converter station;

[0012] S4. Considering the current limiting effect of the grid-type converter station, and leaving a margin for the transient component attenuation of the grid-type converter station in S3, the power stability operation constraint of the grid-type converter station described in S2 is adopted to calculate the power stability operation area of ​​the grid-type converter station based on the grid-building capability.

[0013] Preferably, S1 is specifically as follows:

[0014] The AC collection grid lacks conventional power source support, such as supporting coal-fired power plants, and is not connected to the existing AC grid via transmission lines. It relies solely on grid-type converter stations to provide voltage support for the renewable energy AC collection grid and is responsible for transmitting power from the renewable energy base. Grid-type converter stations achieve power synchronization and construct their own AC-side voltage by controlling their output voltage. The corresponding active power control employs virtual synchronization control, and the power control characteristics are as follows:

[0015]

[0016] Where: J is the moment of inertia, D is the damping coefficient, and ω is the converter angular frequency. n k is the rated angular frequency. w P is the active power droop coefficient. vThe active power of the grid-type converter station.

[0017] The corresponding network frequency stability constraint equation is:

[0018]

[0019] Among them: U g U is the AC power grid bus voltage. GFM For the output voltage of a grid-connected converter, X = x T +x L / 2,x T For the reactance of the converter transformer, x L For the bridge arm reactance, Δf max For the maximum frequency deviation, RoCoF max This represents the maximum rate of change of frequency.

[0020] Preferably, step S2 is as follows:

[0021] The voltage of a grid-type converter station adopts reactive power droop control, and the corresponding voltage control characteristics are as follows:

[0022] U GFM =U ref +k q (Q ref -Q v )

[0023] Among them: U GFM For the output voltage of the grid-type converter, U ref For the reference voltage, k q Q is the reactive power droop factor. ref Q is the reactive power reference value. v This represents the actual reactive power of the converter station.

[0024] In a grid-type converter station, controlling reactive power to maintain the output voltage amplitude affects the AC bus voltage amplitude. The relationship between the bus voltage and the AC side valve outlet voltage of the converter station is as follows:

[0025]

[0026] Where: P g Q represents the active power of the AC bus. g This refers to the reactive power of the AC bus.

[0027] Combining the above formulas yields the power constraint range that takes into account the grid-building capability of the converter stations.

[0028] Preferably, step S3 is as follows:

[0029] The transient current component of the system affects the current supply capacity of the grid-type converter station, and the degree of influence weakens as the transient component decays. Since the system is three-phase symmetrical and the AC component of the bridge arm current is half of the line current, the transient current expression corresponding to the single-phase bridge arm current is as follows:

[0030]

[0031] Where: i pa (t) represents the transient current of the upper arm of phase a, i a (0) represents the transient current component, τ is the transient current decay time constant, and R ac L ac R0 and L0 are the AC side resistance and inductance, respectively, and the bridge arm resistance and inductance are the bridge arm resistance and inductance, respectively.

[0032] Preferably, step S4 is as follows:

[0033] When the system voltage drops, grid-type converter stations will generate additional reactive power to maintain the AC voltage. However, due to current limiting constraints, this increased reactive current generation will consume the active current margin. Furthermore, the system transient current component affects the current-providing capacity of grid-type DC converter stations, and this impact weakens as the transient component decays. Therefore, it is necessary to consider the reactive power support capacity of grid-type converter stations under the influence of current limiting and to allow for a margin for transient component decay. The corresponding reactive power droop coefficient expression is:

[0034] σ min ≤σ≤σ max

[0035] Where: k q U is the actual value of the reactive power droop factor, k is the per-unit value of the reactive power droop factor, and U is the actual value of the reactive power droop factor. T,GFM S is the valve-side voltage of the converter transformer. GFM The rated capacity of the grid-type converter station is given by σ, where σ is the current limiting value. max The maximum allowable current amplitude within 100ms for a grid-type converter station corresponds to the "current amplitude limiting grid support capability constraint", σ min This refers to the current limit value that allows grid-type converter stations to operate for a relatively long period of time, corresponding to the "grid support capability constraint".

[0036] Compared with existing technologies, this invention provides a method for calculating the equivalent impedance of a new energy voltage source converter station based on grid-connected control, which has the following advantages:

[0037] This invention proposes a method for calculating the stable power operation region of a grid-type converter station based on grid-building capability. Considering the existing constraints on the stable power operation region of converter stations, and taking into account the influence of current limiting and transient components, the method defines the power operation region of a grid-type converter station that meets the grid-building capability requirements. This is manifested as the addition of "grid-building support capability constraints" and "current-limited grid-building support capability constraints" under the influence of current limiting and transient components. This method is applicable to the power stability assessment of AC systems supported by voltage from grid-type converter stations and has significant implications for practical engineering applications. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings involved in the embodiments are now briefly described. Obviously, the drawings in the following description are merely illustrative of some embodiments of the present invention. For those skilled in the art, other forms of drawings can be constructed based on these drawings without creative effort.

[0039] Figure 1 This is a schematic diagram of the control loop and current limiting of the grid-type converter station corresponding to the calculation method of the power stable operation area of ​​the grid-type converter station based on grid-type capability proposed in Embodiment 1 of the present invention.

[0040] Figure 2 The method for calculating the stable operating area of ​​a grid-type converter station based on grid-building capability proposed in Embodiment 1 of this invention includes virtual synchronous control for primary frequency regulation and voltage droop control for the grid-type converter station.

[0041] Figure 3 The operational circuit model of the grid-type converter station in the complex frequency domain in Embodiment 1 of the present invention is to transform the transient current decay problem in the time domain into a steady-state current solution problem in the complex frequency domain, where sL is the operational impedance and Li(0) is the excitation source;

[0042] Figure 4 This refers to the stable power operation region of the grid-type converter station in Embodiment 1 of the present invention, considering constraints such as modulation ratio, voltage stability, capacitor voltage fluctuation, converter transformer capacity, converter capacity, and grid construction capability. Detailed Implementation

[0043] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0044] This invention proposes a method for calculating the power stable operation region of a grid-type converter station based on grid-building capability. This method is applicable to the power operation region of a grid-type converter station that meets the grid-building capability requirements, taking into account current limiting and transient component influences. The grid-type converter station employs virtual synchronous control with an additional primary frequency regulation stage, and the voltage uses reactive power droop control. The method for calculating the power stable operation region adds "grid-building support capability constraints" and "current-limited grid-building support capability constraints" to the existing power stable operation region constraints of converter stations, taking into account current limiting and transient component influences. This method is suitable for evaluating the power stable operation of AC systems supported by voltage from grid-type converter stations.

[0045] The following describes the method for calculating the stable power operation area of ​​a grid-type converter station based on grid-building capability, as proposed in this invention, with reference to relevant accompanying drawings and specific examples. The details are as follows.

[0046] Example 1:

[0047] This invention proposes a method for calculating the power stability operating region of a grid-type converter station based on grid-building capability. This method is implemented based on the calculation of grid-building capability constraints of the grid-type converter station with additional primary frequency regulation virtual synchronous control and reactive power droop control. The specific calculation method is as follows:

[0048] First, based on the power control characteristics of grid-type converter stations and the principle of AC side voltage construction, a grid-type converter station frequency stability constraint equation is established. Then, based on the voltage control characteristics of grid-type converter stations and the relationship between the AC side valve outlet voltage and the AC bus voltage, the grid-type converter station frequency stability constraint equation is substituted to establish a power constraint range considering the grid-type converter station's grid-building capability. Next, a complex frequency domain operational circuit model of the grid-type converter station is established to analyze the transient component influencing factors. Finally, the current limiting effect of the grid-type converter station is reflected in the reactive power droop coefficient calculation, enabling the mapping of the power stability operating region of the grid-type converter station based on its grid-building capability.

[0049] Based on the above, the calculation method for the power stability operation area of ​​grid-type converter stations based on grid construction capability specifically includes the following:

[0050] The active power control of the grid-type converter station adopts virtual synchronous control, and the power control characteristics are as follows:

[0051]

[0052] Where: J is the moment of inertia, D is the damping coefficient, and ω is the converter angular frequency. n k is the rated angular frequency. w P is the active power droop coefficient. v The active power of the grid-type converter station.

[0053] The corresponding network frequency stability constraint equation is:

[0054]

[0055] Among them: U g U is the AC power grid bus voltage. GFM For the output voltage of a grid-connected converter, X = x T +x L / 2,x T For the reactance of the converter transformer, x L For the bridge arm reactance, Δf max For the maximum frequency deviation, RoCoF max This represents the maximum rate of change of frequency.

[0056] The voltage of a grid-type converter station adopts reactive power droop control, and the corresponding voltage control characteristics are as follows:

[0057] U GFM =U ref +k q (Q ref -Q v (3)

[0058] Among them: U GFM For the output voltage of the grid-type converter, U ref For the reference voltage, k q Q is the reactive power droop factor. ref Q is the reactive power reference value. v This represents the actual reactive power of the converter station.

[0059] In a grid-type converter station, controlling reactive power to maintain the output voltage amplitude affects the AC bus voltage amplitude. The relationship between the bus voltage and the AC side valve outlet voltage of the converter station is as follows:

[0060]

[0061] Where: P g Q represents the active power of the AC bus. g This refers to the reactive power of the AC bus.

[0062] Substituting equations (3) and (4) into equation (2) yields the power constraint range considering the grid-building capability of the converter station.

[0063] The transient current component of the system affects the current supply capacity of the grid-type converter station, and the degree of influence weakens as the transient component decays. Since the system is three-phase symmetrical and the AC component of the bridge arm current is half of the line current, the transient current expression corresponding to the single-phase bridge arm current is as follows:

[0064]

[0065] Where: ipa (t) represents the transient current of the upper arm of phase a, i a (0) represents the transient current component, τ is the transient current decay time constant, and R ac L ac R0 and L0 are the AC side resistance and inductance, respectively, and the bridge arm resistance and inductance are the bridge arm resistance and inductance, respectively.

[0066] When the system voltage drops, grid-type converter stations will generate additional reactive power to maintain the AC voltage. However, due to current limiting constraints, this increased reactive current generation will consume the active current margin. Furthermore, the system transient current component affects the current-providing capacity of grid-type DC converter stations, and this impact weakens as the transient component decays. Therefore, it is necessary to consider the reactive power support capacity of grid-type converter stations under the influence of current limiting and to allow for a margin for transient component decay. The corresponding reactive power droop coefficient expression is:

[0067]

[0068] Where: k q U is the actual value of the reactive power droop factor, k is the per-unit value of the reactive power droop factor, and U is the actual value of the reactive power droop factor. T,GFM S is the valve-side voltage of the converter transformer. GFM The rated capacity of the grid-type converter station is given by σ, where σ is the current limiting value. max The maximum allowable current amplitude within 100ms for a grid-type converter station corresponds to the "current amplitude limiting grid support capability constraint", σ min This refers to the current limit value that allows grid-type converter stations to operate for a relatively long period of time, corresponding to the "grid support capability constraint".

[0069] The present invention will now be described in further detail with reference to the accompanying drawings:

[0070] Figure 1 This is a schematic diagram of the control loop and current limiting of a grid-type converter station corresponding to the calculation method for the stable operation region of the grid-type converter station based on grid-building capability proposed in Embodiment 1 of the present invention. Figure 1 As shown, the grid-type converter station uses phase information generated by virtual synchronous control to perform dq transformation on the acquired voltage and current, and then uses outer loop voltage control and inner loop current control to achieve trigger control of the converter. The current setting value generated by the outer loop voltage control is input to the inner loop current control after passing through a current limiting circuit. Therefore, when analyzing the grid-connection capability of the grid-type converter station, it is necessary to consider current limiting while also allowing for a margin for transient current decay.

[0071] Figure 2This diagram illustrates the virtual synchronization control and voltage droop control of a grid-type converter station with added primary frequency regulation, corresponding to the calculation method for the stable operating region of the grid-type converter station based on grid-building capability proposed in Embodiment 1 of this invention. The virtual synchronization control adds a primary frequency regulation stage, rapidly adjusting the active power of the grid-type converter station based on the difference between the actual angular frequency and the rated angular frequency, thus achieving primary frequency regulation of the system. The corresponding virtual synchronization control introduces virtual inertia to smooth frequency fluctuations caused by new energy power fluctuations, and uses a damping coefficient to suppress system oscillations. Voltage control employs reactive power droop control, which monitors the difference between the system reactive power and the rated value, and adjusts the outer loop voltage setting in a timely manner to control the system voltage amplitude.

[0072] Figure 3 In Embodiment 1 of this invention, the operational circuit model of a grid-type converter station in the complex frequency domain is used. The transient current decay problem in the time domain is transformed into a steady-state current solution problem in the complex frequency domain. For the operational circuit corresponding to the transient current dissipation on the valve side of the converter station, the voltage of the sub-module capacitor can be considered to be zero or very low during the transient current decay time period, and the bridge arm is approximately short-circuited. The operational circuit of the converter station is simplified, where sL is the operational impedance and Li(0) is the excitation source. Since the system is inductive, the power fluctuation of new energy sources causes the grid-type converter station to generate transient current components, which encroach on the current margin of the grid-type converter station.

[0073] Figure 4 This refers to the stable power operation region of a grid-type converter station in Embodiment 1 of the present invention, considering constraints such as modulation ratio, voltage stability, capacitor voltage fluctuation, converter transformer capacity, converter capacity, and grid construction capability. Compared to conventional converter stations, the stable power operation region of the converter station is reduced when considering grid construction capability. The "grid construction support capability constraint" corresponds to the power stability range boundary, representing the power operation region where the grid-type converter station can operate long-term. The "current limiting grid construction support capability constraint" corresponds to the power support limit boundary, representing the upper limit of the grid-type converter station's support capability, including transient component attenuation.

[0074] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for calculating the power stability operating region of a grid-type converter station based on grid construction capability, characterized in that, Includes the following steps: S1. Based on the power control characteristics of grid-connected converter stations and the principle of constructing AC side voltage in grid-connected converter stations, establish grid-connected frequency stability constraint equations; specifically including the following: Grid-type converter stations employ virtual synchronous control, with the following corresponding power control characteristics: (1) in, J It is the moment of inertia; D The damping coefficient; ω The converter angular frequency; ω n The rated angular frequency; k w This is the active power droop coefficient; P v The active power of the grid-type converter station; The corresponding network frequency stability constraint equation is: (2) in, U g This refers to the AC power grid bus voltage. U GFM This refers to the output voltage of a grid-type converter. X = x T + x L / 2, x T For the reactance of the converter transformer, x L For bridge arm reactance; Δf max This represents the maximum frequency deviation. RoCoF max The maximum rate of change of frequency; S2. Based on the voltage control characteristics of the grid-type converter station and the relationship between the AC side valve outlet voltage and the AC bus voltage of the grid-type converter station, substitute the grid frequency stability constraint equation obtained in S1 to establish a power constraint range that considers the grid-type converter station's grid-building capability. S3. Establish a computational circuit model of a grid-type converter station in the complex frequency domain and analyze the influencing factors of transient components in the grid-type converter station; S4. Considering the current limiting effect of grid-type converter stations, and leaving a margin for the attenuation of transient components of grid-type converter stations in S3, the power stability operation constraint of grid-type converter stations described in S2 is adopted to calculate the power stability operation area of ​​grid-type converter stations based on grid-building capability.

2. The method for calculating the power stability operating area of ​​a grid-type converter station based on grid construction capability according to claim 1, characterized in that, S2 specifically includes the following: The voltage of a grid-type converter station adopts reactive power droop control, and the corresponding voltage control characteristics are as follows: (3) in, U GFM This refers to the output voltage of a grid-type converter. U ref Reference voltage; k q This is the reactive power droop coefficient; Q ref This is a reference value for reactive power. Q v This represents the actual reactive power of the converter station. In a grid-type converter station, controlling reactive power to maintain the output voltage amplitude affects the AC bus voltage amplitude. The relationship between the bus voltage and the AC side valve outlet voltage of the converter station is as follows: (4) in, P g For AC bus active power, Q g This refers to the reactive power of the AC bus. Substituting equations (3) and (4) into equation (2) yields the power constraint range considering the grid-building capability of the converter station.

3. The method for calculating the power stability operating area of ​​a grid-type converter station based on grid construction capability according to claim 1, characterized in that, S3 specifically includes the following: The transient current component of the system affects the current supply capacity of the grid-type converter station, and the degree of influence weakens as the transient component decays. Since the system is three-phase symmetrical and the AC component of the bridge arm current is half of the line current, the transient current expression corresponding to the single-phase bridge arm current is as follows: (5) in, i pa ( t () represents the transient current of the upper arm of phase a; i a (0) represents the transient current component; τ The transient current decay time constant; R ac , L ac These are the AC side resistance and inductance, respectively; R 0、 L 0 represents the bridge arm resistance and inductance, respectively.

4. The method for calculating the power stability operating region of a grid-type converter station based on grid construction capability according to claim 1, characterized in that, S4 specifically includes the following: When the system voltage drops, grid-type converter stations generate additional reactive power to maintain AC voltage. Considering the reactive power support capacity of grid-type converter stations under the influence of current limiting, and leaving a margin for transient component attenuation of grid-type converter stations, the corresponding reactive power droop coefficient expression is: (6) in, k q This represents the actual value of the reactive power droop coefficient. k This is the per-unit value of the reactive power droop coefficient; U T,GFM This refers to the valve-side voltage of the converter transformer. S GFM Rated capacity for grid-type converter stations; σ This is the current limiting value; σ max The maximum current limit allowed within 100ms for grid-type converter stations corresponds to the "current limit grid support capability constraint"; σ min This refers to the current limit value for long-term operation in grid-type converter stations, corresponding to the "constraint when the support capacity of the grid-type converter station is weak".

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