Calculation method for constant AC voltage controller parameters of flexible DC converter station

Abstract

This invention discloses a method for calculating the parameters of a constant AC voltage controller in a flexible DC converter station, including obtaining the short-circuit ratio S of the flexible DC converter station connected to a weak AC system. scr Obtain the equivalent capacitance C at the AC system connection point of the flexible DC converter station. pcc Obtain the simplified AC voltage closed-loop transfer function F. uu (s); Obtain the initial value K of the simplified AC voltage closed-loop transfer function unit step response. stepu and equivalent time constant τ u ; Calculate the constant AC voltage controller parameter k puac and k iuac Obtain the active power controller parameters and verify the calculated constant AC voltage controller parameters k. puac and k iuac Does the parameter constraint condition meet? This invention, through the proposed method for calculating the parameters of the constant AC voltage controller in flexible DC converter stations, reduces the time spent by engineering researchers in selecting outer loop controller parameters and improves research efficiency.

Description

[0001] Technical Field

[0002] The present invention relates to a method for analytically calculating the parameters of an outer-loop controller for a flexible HVDC system based on a voltage source converter (VSC) connected to a weak AC system, and specifically to a method for calculating the parameters of a fixed AC voltage controller for a flexible DC converter station. Background Art

[0003] Flexible HVDC technology based on a modular multilevel converter (MMC) uses power electronic devices with self-turn-off capabilities, providing a solid foundation for the decoupled control of active and reactive power. Flexible HVDC overcomes the fundamental defect of conventional HVDC that requires an AC power source to provide commutation voltage. Its superior control and operation performance have caused a major transformation in the field of power transmission, providing a better solution for scenarios such as multiple DC drop points at load centers, asynchronous grid interconnection across regions, support for weak AC systems in remote areas, and new energy transmission.

[0004] The control mode of a flexible DC converter station connected to an AC system is closely related to whether there is a synchronous power source and the strength of the AC system. The short circuit ratio (SCR), as an index to measure the strength of the AC system, is defined as the ratio of the short-circuit capacity at the point of common coupling (PCC) to the rated DC power. When SCR≥3, it is considered a strong power grid; when 2≤SCR<3, it is a weak power grid; and when 1<SCR<2, it is an extremely weak power grid. Weak AC systems mainly exhibit the following characteristics: 1) Long transmission lines result in large equivalent line impedances and weak anti-disturbance capabilities; 2) A small number of synchronous generators leads to low moment of inertia, weak AC voltage and frequency support capabilities, and weak power angle stability; 3) There is a strong non-linear coupling relationship between active power and reactive power.

[0005] When a flexible DC converter station is connected to a weak AC system, the control system generally adopts a double closed-loop control mode, where the outer loop uses a fixed AC voltage control to enhance the support for the weak AC system. Although there have been many studies on the stability of a flexible DC converter station connected to a weak AC system and the influence of outer-loop controller parameters on stability, the research on the analytical calculation of outer-loop controller parameters is still very lacking.

[0006] To reduce the time spent by engineering researchers in selecting outer loop controller parameters and improve research efficiency, this invention patent proposes a method for calculating the parameters of a constant AC voltage controller for a flexible DC converter station. For example... Figure 1 The diagram shows a simplified main circuit for connecting a flexible DC converter station to a weak AC system: In the diagram, U g and U s R represents the equivalent power source and the phase voltage amplitude at point PCC, where δ is the phase difference; g and L g For the equivalent resistance of the AC system, C pcc R is the equivalent capacitance at point PCC; t and L t For the equivalent resistance and leakage inductance of the converter transformer; I g I c and I s For AC system, equivalent capacitance and converter station current; P g and Q g It refers to the active and reactive power injected into the PCC point of the AC system; Q c It is the equivalent capacitance C pcc Reactive power injected into the PCC point; P s and Q s It refers to the active and reactive power injected into the converter transformer by the AC system; u dc and i dc These are DC voltage and DC current. For example... Figure 2 The diagram shows the structure of the control equations for the outer loop constant AC voltage of a flexible DC converter station: In the diagram, u sd and u sq The voltage at point PCC is represented by its d-axis and q-axis components in the synchronous dq rotating coordinate system; the low-pass filter includes, but is not limited to, first-order forms; U * s This is a reference value for the phase voltage amplitude at the PCC point; G uac (s)=k upac +k ipac / s is the AC voltage controller, where k upac and k ipac Here, i represents the proportionality coefficient and the integral coefficient, respectively, and s is the Laplace operator; * sd This is the reference value for the d-axis current. Figure 3 The diagram shows the steps for analytical calculation of parameters for a constant AC voltage controller. Summary of the Invention

[0007] The method for calculating the parameters of the constant AC voltage controller in a flexible DC converter station provided by this invention includes the following steps:

[0008] S1. Obtain the short-circuit ratio S of the flexible DC converter station connected to the weak AC system. scr ;

[0009] S2. Determine the equivalent capacitance C at the AC system connection point of the flexible DC converter station based on the actual AC system lines and the compensated capacitive reactive power. pcc ;

[0010] S3. Obtain the simplified AC voltage closed-loop transfer function F uu (s);

[0011] S4. Obtain the initial value K of the simplified AC voltage closed-loop transfer function unit step response. stepu and equivalent time constant τ u ;

[0012] S5. Calculate the constant AC voltage controller parameter k puac and k iuac ;

[0013] S6. Obtain the active power controller parameters and verify the constant AC voltage controller parameters k obtained in step S5. puac and k iuac Does it meet the parameter constraints?

[0014] Step S1 involves obtaining the short-circuit ratio S of the flexible DC converter station connected to the weak AC system. scr The short-circuit ratio S of the flexible DC converter station connected to the weak AC system is calculated using the following formula. scr :

[0015]

[0016] In the formula, ω1 is the fundamental angular frequency, U sN P is the rated amplitude of the phase voltage at point PCC. dcN The rated active power of the PCC point, L g This is the equivalent inductance of the AC system.

[0017] When the flexible DC converter station is connected to a weak AC system as described in step S2, the equivalent capacitance C at the AC system connection point of the flexible DC converter station is determined based on the actual AC system lines and the compensated capacitive reactive power. pcc ;

[0018] When the flexible DC converter station is connected to a weak AC system as described in step S3, the simplified AC voltage closed-loop transfer function F is obtained. uu The simplified AC voltage closed-loop transfer function F is calculated using the following formula (s). uu (s):

[0019]

[0020] In the formula, k puac and kiuac These are the proportional and integral coefficients of the AC voltage controller, τ. u The equivalent time constant of the simplified AC voltage closed-loop transfer function, under the zero steady-state power condition, is expressed as:

[0021]

[0022] Step S4 describes obtaining the initial value K of the simplified AC voltage closed-loop transfer function unit step response. stepu and equivalent time constant τ u K stepu It is recommended to use a value in the range of 0.25 to 0.35, τ u Choose a value within the range of 60ms to 120ms; K is recommended. stepu It is recommended to take 0.3, τ u Take 80ms.

[0023] The initial value K of the unit step response of the simplified AC voltage closed-loop transfer function obtained in step S4, as described in step S5. stepu and equivalent time constant τ u Then, combined with the simplified AC voltage closed-loop transfer function F obtained in step S3 uu (s) can calculate the constant AC voltage controller parameter k puac and k iuac The specific formula for calculating the AC voltage controller parameter k is as follows. puac and k iuac :

[0024]

[0025] The short-circuit ratio S obtained in step S1 for connecting the flexible DC converter station to the weak AC system scr The equivalent capacitance C at the AC system connection point of the flexible DC converter station obtained in step S2. pcc The rated active power P of the flexible DC converter station dcN PCC point phase voltage rated amplitude U sN 1. Fundamental angular frequency ω1; 2. Initial step value K for a given unit step response. stepu and time response constant τ u Substituting these values ​​into the above formula, the proportional coefficient and integral coefficient of the AC voltage controller can be calculated.

[0026] Step S6 involves obtaining the active power controller parameters and verifying the constant AC voltage controller parameters k obtained in step S5. puac and k iuac Whether the parameter constraints are met is verified using the following formula to determine the AC voltage controller parameter k. puac and kiuac Does the parameter constraint condition meet?

[0027]

[0028]

[0029]

[0030]

[0031] In the formula, max(x1, x2) is the function that takes the maximum value of x1 and x2, and k pp and k ip These are the proportional and integral coefficients of the active power controller. and These represent the d-axis and q-axis currents of a flexible DC converter station operating at full power inverter conditions. Generally, as long as k... pp and k ip The values ​​are reasonable, and the parameters of the AC voltage controller can meet the parameter constraints. Attached Figure Description

[0032] Figure 1 A simplified main circuit diagram for connecting a flexible DC converter station to a weak AC system.

[0033] Figure 2 This is a schematic diagram of the control equations for the outer ring constant AC voltage of a flexible DC converter station.

[0034] Figure 3 Flowchart for the analytical calculation of parameters for a constant AC voltage controller. Detailed Implementation

[0035] like Figure 3 The flowchart shown is a process for implementing the method of the present invention: The method for calculating the parameters of the constant AC voltage controller in a flexible DC converter station provided by the present invention includes the following steps:

[0036] S1. Obtain the short-circuit ratio S of the flexible DC converter station connected to the weak AC system. scr It is defined as the short-circuit capacity of the AC system at the PCC point of a flexible DC converter station relative to the rated active power P. dcN The ratio. In specific implementation, the short-circuit ratio S of the flexible DC converter station connected to the weak AC system is calculated using the following formula. scr :

[0037]

[0038] In the formula, ω1 is the fundamental angular frequency, U sN P is the rated amplitude of the phase voltage at point PCC. dcNThe rated active power of the PCC point, L g Let L be the equivalent inductance of the AC system. If the equivalent inductance L of the AC system is unknown... g Then the AC system short-circuit capacity at PCC can be calculated by dividing it by the rated active power P of the converter station. dcN You can get it immediately.

[0039] S2. When a flexible DC converter station is connected to a weak AC system, due to the significant inductive reactive power consumption of the lines and the converter transformer, a capacitor is typically connected in parallel at the PCC point of the flexible DC converter station to compensate for the inductive reactive power consumed by the AC system in order to reduce the rated capacity of the converter station. Considering the existence of distributed capacitance of the AC system lines, the equivalent capacitance of the flexible DC converter station at the PCC can be calculated as C. pcc This value needs to be determined based on the actual AC system lines and the capacitive reactive power compensation.

[0040] S3. When a flexible DC converter station is connected to a weak AC system, obtain the simplified AC voltage closed-loop transfer function F. uu (s). In specific implementation, the simplified AC voltage closed-loop transfer function F is calculated using the following formula. uu (s):

[0041]

[0042] In the formula, k puac and k iuac These are the proportional and integral coefficients of the AC voltage controller, τ. u The equivalent time constant of the simplified AC voltage closed-loop transfer function, under the zero steady-state power condition, is expressed as:

[0043]

[0044] S4. In step S3, the simplified form F of the AC voltage closed-loop transfer function was obtained. uu (s), i.e., the first-order transfer function with zeros in the left half-plane, and then obtaining the initial value K of its unit step response. stepu and equivalent time constant τ u K stepu It is recommended to use a value in the range of 0.25 to 0.35, τ u Choose a value within the range of 60ms to 120ms; K is recommended. stepu It is recommended to take 0.3, τ u Take 80ms.

[0045] S5. The initial value K of the simplified AC voltage closed-loop transfer function unit step response obtained in step S4. stepu and equivalent time constant τu Then, combined with the simplified AC voltage closed-loop transfer function F obtained in step S3 uu (s) can be used to calculate the constant AC voltage controller parameter k. puac and k iuac In practical implementation, the following formula is used to calculate the constant AC voltage controller parameter k. puac and k iuac :

[0046]

[0047] The short-circuit ratio S obtained in step S1 for connecting the flexible DC converter station to the weak AC system scr The equivalent capacitance C at the AC system connection point of the flexible DC converter station obtained in step S2. pcc The rated active power P of the flexible DC converter station dcN PCC point phase voltage rated amplitude U sN , fundamental angular frequency ω1, K stepu and τ u Substituting these values ​​into the above formula, the proportional coefficient and integral coefficient of the AC voltage controller can be calculated.

[0048] S6. Taking the constant active power and constant AC voltage control of a flexible DC converter station as an example, the AC voltage controller parameter k is calculated in step S5. puac and k iuac Next, substitute the following expression to verify whether the parameter constraints are met:

[0049]

[0050]

[0051] In the formula, max(x1, x2) is the function that takes the maximum value of x1 and x2, and k pp and k ip These are the proportional and integral coefficients of the active power controller. and These represent the d-axis and q-axis currents of a flexible DC converter station operating at full power inverter conditions. Generally, as long as k... pp and k ip The values ​​are reasonable, and the parameters of the AC voltage controller can meet the parameter constraints.

Claims

1. The calculation method for the constant AC voltage controller parameters of a flexible DC converter station includes the following steps: S1. Obtain the short-circuit ratio of the flexible DC converter station connected to the weak AC system. ; S2. Obtain the equivalent capacitance at the AC system connection point of the flexible DC converter station. ; S3. Obtain the simplified AC voltage closed-loop transfer function ; S4. Obtain the initial values ​​of the simplified AC voltage closed-loop transfer function and the unit step response. and equivalent time constant ; S5. Calculate the parameters of the constant AC voltage controller. and ; and These are the proportional and integral coefficients of the AC voltage controller, respectively. S6. Obtain the active power controller parameters and verify the constant AC voltage controller parameters obtained in step S5. and Does it meet the parameter constraints? in, The calculation of constant AC voltage controller parameters and ,include: The initial value of the simplified AC voltage closed-loop transfer function unit step response obtained in step S4 and equivalent time constant Then, combined with the simplified AC voltage closed-loop transfer function obtained in step S3 Calculate the parameters of the constant AC voltage controller. and The parameters of the constant AC voltage controller are calculated using the following formula. and : ； In the formula, Rated active power of flexible DC converter station The rated amplitude of the phase voltage at the PCC point, ω is the fundamental angular frequency.

2. The method for calculating the parameters of the constant AC voltage controller in a flexible DC converter station according to claim 1, characterized in that... Step S1 involves obtaining the short-circuit ratio of the flexible DC converter station connected to the weak AC system. The short-circuit ratio of the flexible DC converter station connected to the weak AC system is calculated using the following formula. : ； In the formula, This is the equivalent inductance of the AC system.

3. The method for calculating the parameters of the constant AC voltage controller in a flexible DC converter station according to claim 1, characterized in that... Step S2: Determine the equivalent capacitance at the AC system connection point of the flexible DC converter station based on the actual AC system lines and the compensated capacitive reactive power. .

4. The method for calculating the parameters of the constant AC voltage controller in a flexible DC converter station according to claim 2, characterized in that... Step S3 describes obtaining the simplified AC voltage closed-loop transfer function. The simplified AC voltage closed-loop transfer function is calculated using the following formula. : ； In the formula, The equivalent time constant of the simplified AC voltage closed-loop transfer function, under the zero steady-state power condition, is expressed as: 。 5. The method for calculating the parameters of the constant AC voltage controller in a flexible DC converter station according to claim 4, characterized in that, Step S4 describes obtaining the initial value of the simplified AC voltage closed-loop transfer function unit step response. and equivalent time constant ,in The value range is 0.25~0.

35. The value range is 60ms to 120ms.

6. The method for calculating the parameters of the constant AC voltage controller in a flexible DC converter station according to claim 5, characterized in that... Step S6 involves obtaining the active power controller parameters and verifying the constant AC voltage controller parameters obtained in step S5. and Whether the parameter constraints are met is verified using the following formula to determine the AC voltage controller parameters. and Does the parameter constraint condition meet? ； ； ； In the formula, To obtain and The maximum value function in and These are the proportional and integral coefficients of the active power controller. and These are the d-axis and q-axis currents of the flexible DC converter station operating at full power inverter conditions.

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