Method for calculating power stable operation area of network construction type converter station based on network construction capability

Abstract

The invention discloses a method for calculating a stable operation area of power of a network construction type converter station based on network construction capability, and relates to the technical field of power electronics. The method comprises the following steps: establishing a network construction frequency stability constraint equation based on control characteristics of a network construction type converter station; establishing a power constraint range considering the network construction capability of the network construction type converter station based on the relationship between the outlet voltage of the AC side valve of the network construction type converter station and the voltage of the AC bus; establishing a network construction type converter station operational circuit model under the complex frequency domain, and analyzing transient component influence factors in the network construction type converter station; and considering the current amplitude limiting influence of the network construction type converter station, and calculating the stable power operation region of the network construction type converter station based on the network construction capability. According to the method, the network construction supporting capability constraint and the current amplitude limiting network construction supporting capability constraint under the influence of the current amplitude limiting and the transient component are newly added, the method is suitable for power stable operation evaluation of the alternating current system with voltage support provided by the network construction type converter station, the logic is simple, and the method has pertinence.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and particularly to a method for calculating the power stable operation region of a grid-forming converter station based on grid-forming ability. Background Art

[0002] Large-scale exploitable new energy in China is mainly concentrated in the Gobi and desert areas in the northwest. New energy bases in such areas are large in scale and wide in distribution area. At the same time, they face problems such as the lack of conventional power source support and no connection to the AC main grid in the AC collection grid, resulting in a huge AC collection grid and frequent power fluctuations in the new energy bases in the desert and Gobi areas, and it is difficult to transmit the new energy power out.

[0003] A grid-forming (GFM) converter station can independently control the output voltage amplitude and phase within the operating power range, and can provide voltage support for such a grid. Therefore, after the new energy bases are collected through the AC grid and transmitted through the grid-forming converter station, it can provide a reference solution to solve the above problems. For such a grid, the grid-forming converter station not only needs to generate reactive current to support the voltage of the AC grid, but also needs to ensure the stable transmission of the active power of the new energy. Considering the vulnerability of power electronic devices, a current limiting link needs to be introduced into the converter. The increase in reactive current will occupy the margin of active current, and at the same time, the increase in reactive current helps to support the AC voltage, and to a certain extent, improves the active output ability of the new energy power station. Therefore, it is necessary to consider the power operation region within the grid-forming support ability of the grid-forming converter station to match the actual power of the new energy base and ensure the stable transmission of the new energy in the desert and Gobi areas.

[0004] For the system that provides voltage support by the above-mentioned grid-forming converter station, it is necessary to consider the grid-forming performance constraints, that is, the ability of the converter station to maintain frequency stability, specifically including the frequency deviation Δf and the rate of change of frequency (RoCoF), to ensure that the grid-forming converter station can stably support the AC system. However, the power range constraint conditions of the conventional converter station include modulation ratio constraint, voltage stability constraint, capacitor voltage fluctuation constraint, converter transformer capacity constraint, and converter capacity constraint. There is no power stable operation region of the grid-forming converter station considering the influence of grid-forming ability constraints.

[0005] To solve the above problems, the present invention proposes a method for calculating the power stable operation region of a grid-forming converter station based on grid-forming ability. Summary of the Invention

[0006] The object of the present invention is to provide a method for calculating the power stable operation area of a network-forming converter station based on the network-forming ability to solve the problems raised in the above background technology; the present invention aims to draw the power stable operation area of the system that ensures the network-forming ability in the new energy transmission system that provides voltage support for the network-forming converter station. The drawing of the power stable operation area of the conventional converter station lacks the constraint on the network-forming ability of the network-forming converter station, which may result in a relatively large power stable operation area, leading to the risk that the AC grid power of the new energy base exceeds the power supported by the network-forming ability of the network-forming converter station.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for calculating the power stable operation area of a network-forming converter station based on the network-forming ability, comprising the following steps:

[0009] S1. Based on the power control characteristics of the network-forming converter station and the principle of constructing the AC side voltage by the network-forming converter station, establish a network-forming frequency stability constraint equation;

[0010] S2. Based on the voltage control characteristics of the network-forming converter station and the relationship between the AC side valve outlet voltage and the AC bus voltage of the network-forming converter station, substitute into the network-forming frequency stability constraint equation in S1 to establish a power constraint range considering the network-forming ability of the network-forming converter station;

[0011] S3. Establish an operational circuit model of the network-forming converter station in the complex frequency domain and analyze the influencing factors of the transient components in the network-forming converter station;

[0012] S4. Considering the influence of the current limit of the network-forming converter station and leaving a margin for the attenuation of the transient components of the network-forming converter station in S3, adopt the power stable operation constraint of the network-forming converter station described in S2 to calculate the power stable operation area of the network-forming converter station based on the network-forming ability.

[0013] Preferably, the S1 is specifically as follows:

[0014] The AC collection grid has no support from conventional power sources, such as supporting coal-fired power plants, and is not connected to the existing AC grid through transmission lines. Only the network-forming converter station provides voltage support for the new energy AC collection grid and is responsible for the external transmission of the power of the new energy base. The network-forming converter station realizes its own power synchronization and constructs the AC side voltage by controlling the output voltage. The corresponding active power control adopts virtual synchronous control, and the power control characteristics are:

[0015]

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

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

[0018]

[0019] Where: U g Is the AC grid bus voltage, U GFM Is the output voltage of the network-forming converter, X = x T + x L / 2, x T Is the reactance of the converter transformer, x L Is the arm reactance, Δf max Is the maximum frequency deviation, RoCoF max Is the maximum rate of change of frequency.

[0020] Preferably, the S2 is specifically as follows:

[0021] The network-forming converter station voltage adopts reactive power droop control, and the corresponding voltage control characteristic is:

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

[0023] Where: U GFM Is the output voltage of the network-forming converter, U ref Is the reference voltage, k q Is the reactive power droop coefficient, Q ref Is the reactive power reference value, Q v Is the actual reactive power of the converter station.

[0024] The network-forming converter station controls reactive power to maintain the output voltage amplitude, which affects the AC bus voltage amplitude. The relationship between the bus voltage and the converter station AC side valve outlet voltage is:

[0025]

[0026] Where: P g Is the active power of the AC bus, Q g Is the reactive power of the AC bus.

[0027] Combining the above formulas is the power constraint range considering the network-forming ability of the network-forming converter station.

[0028] Preferably, the S3 is specifically as follows:

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

[0030]

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

[0032] Preferably, the S4 is specifically as follows:

[0033] When the system voltage drops, in order to maintain the AC voltage, the network-forming converter station will increase the reactive power output. However, restricted by the current limiting link, the increase in reactive current will occupy the margin of active current. In addition, the transient current component of the system affects the current supply capacity of the network-forming DC converter station, and the degree of influence weakens with the attenuation of the transient component. Therefore, it is necessary to consider the reactive power support ability of the network-forming converter station under the influence of current limiting and leave a margin for the attenuation of the transient component of the network-forming converter station. The corresponding expression of the reactive power droop coefficient is:

[0034] σ min ≤σ≤σ max

[0035] Where: k q is the actual value of the reactive power droop coefficient, k is the per-unit value of the reactive power droop coefficient, U T,GFM is the valve side voltage of the converter transformer, S GFM is the rated capacity of the network-forming converter station, σ is the current limit value, σ max is the maximum current limit value allowed for the network-forming converter station within 100 ms, corresponding to the "current limit network support ability constraint", σ min is the current limit value allowed for the network-forming converter station to operate for a long time, corresponding to the "network support ability constraint".

[0036] Compared with the prior art, the present invention provides a method for calculating the equivalent impedance of a new energy voltage source converter station based on grid-following control, having the following beneficial effects:

[0037] The present invention proposes a method for calculating the power stable operation area of a network-forming converter station based on network-forming ability. On the basis of considering the constraint conditions of the power stable operation area of the existing converter station, the power operation area of the network-forming converter station that meets the network-forming ability under the influence of the current limit and transient components of the network-forming converter station is considered. It is manifested as the newly added "network-forming support ability constraint" and "current limit network-forming support ability constraint" under the influence of current limit and transient components, and is applicable to the power stable operation assessment of an AC system supported by a network-forming converter station, which has important significance for practical engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings involved in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only schematic illustrations of some embodiments of the present invention, and those skilled in the art can also construct other forms of drawings based on these drawings without creative efforts.

[0039] Figure 1 It is a schematic diagram of the control link and current limit of the network-forming converter station corresponding to the method for calculating the power stable operation area of the network-forming converter station based on network-forming ability proposed in Embodiment 1 of the present invention;

[0040] Figure 2 It is the virtual synchronous control and voltage-reactive power droop control of the additional primary frequency modulation of the network-forming converter station corresponding to the method for calculating the power stable operation area of the network-forming converter station based on network-forming ability proposed in Embodiment 1 of the present invention;

[0041] Figure 3 It is the operational circuit model of the network-forming converter station in the complex frequency domain of Embodiment 1 of the present invention, which converts the transient current decay problem in the time domain into the solution of the steady-state current in the complex frequency domain, where sL is the operational impedance and Li(0) is the excitation source;

[0042] Figure 4 It is the power stable operation area of the network-forming converter station considering the modulation ratio constraint, voltage stability constraint, capacitor voltage fluctuation constraint, converter transformer capacity constraint, converter capacity constraint, and network-forming ability constraint in Embodiment 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments.

[0044] The present invention proposes a method for calculating the power stable operation area of a network-forming converter station based on network-forming ability, which is applicable to the power operation area of a network-forming converter station that meets the network-forming ability under the influence of current limiting and transient components. The network-forming converter station adopts virtual synchronous control and an additional primary frequency modulation link, and the voltage adopts reactive power droop control; the power stable operation area calculation method is based on the existing constraints of the power stable operation area of the converter station, and newly adds the "network-forming support ability constraint" and "current-limited network-forming support ability constraint" under the influence of current limiting and transient components, which is applicable to the power stable operation evaluation of an AC system supported by a network-forming converter station.

[0045] The following combines relevant drawings and specific examples to illustrate the method for calculating the power stable operation area of a network-forming converter station based on network-forming ability proposed by the present invention, and the specific content is as follows.

[0046] Example 1:

[0047] The present invention proposes a method for calculating the power stable operation area of a network-forming converter station based on network-forming ability. This method is realized by calculating the network-forming ability constraints of a network-forming converter station based on virtual synchronous control with additional primary frequency modulation and reactive power droop control. The specific calculation method is as follows:

[0048] First, based on the power control characteristics of the network-forming converter station and the principle of constructing the AC side voltage of the network-forming converter station, establish a network-forming frequency stability constraint equation; then, based on the voltage control characteristics of the network-forming converter station and the relationship between the AC side valve outlet voltage and the AC bus voltage of the network-forming converter station, substitute it into the network-forming frequency stability constraint equation to establish a power constraint range considering the network-forming ability of the network-forming converter station; then, establish an operational circuit model of the network-forming converter station in the complex frequency domain to analyze the influencing factors of transient components in the network-forming converter station; finally, reflect the influence of the current limit of the network-forming converter station in the reduction of the reactive power droop coefficient to realize the drawing of the power stable operation area of the network-forming converter station based on network-forming ability.

[0049] Based on the above content, the method for calculating the power stable operation area of a network-forming converter station based on network-forming ability specifically includes the following content:

[0050] The active power control of the network-forming converter station adopts virtual synchronous control, and the power control characteristics are:

[0051]

[0052] where: J is the moment of inertia, D is the damping coefficient, ω is the converter angular frequency, ω n is the rated angular frequency, k w is the active power droop coefficient, P v is the active power of the network-forming converter station.

[0053] The corresponding power grid frequency stability constraint equation is as follows:

[0054]

[0055] Where: U g is the AC power grid bus voltage, U GFM is the output voltage of the power grid-forming converter, X = x T + x L / 2, x T is the reactance of the converter transformer, x L is the arm reactance, Δf max is the maximum frequency deviation, RoCoF max is the maximum rate of frequency change.

[0056] The voltage of the power grid-forming converter station adopts reactive power droop control, and the corresponding voltage control characteristic is:

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

[0058] Where: U GFM is the output voltage of the power grid-forming converter, U ref is the reference voltage, k q is the reactive power droop coefficient, Q ref is the reactive power reference value, Q v is the actual reactive power of the converter station.

[0059] The power grid-forming converter station controls the reactive power to maintain the output voltage amplitude, which affects the amplitude of the AC bus voltage. The relationship between the corresponding bus voltage and the AC side valve outlet voltage of the converter station is:

[0060]

[0061] Where: P g is the active power of the AC bus, Q g is the reactive power of the AC bus.

[0062] Substitute Equation (3) and Equation (4) into Equation (2) to obtain the power constraint range considering the power grid-forming ability of the power grid-forming converter station.

[0063] The transient current component of the system affects the current-providing ability of the power grid-forming converter station, and the influence degree weakens with the attenuation of the transient component. Since the system is three-phase symmetrical and the AC component in the arm current is half of the line current, the expression of the transient current corresponding to the single-phase arm current is:

[0064]

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

[0066] When the system voltage drops, in order to maintain the AC voltage, the network-forming converter station will increase the reactive power output. However, restricted by the current limiting link, the increase in reactive current will occupy the margin of active current. In addition, the transient current component of the system affects the current supply ability of the network-forming HVDC converter station, and the degree of influence weakens with the decay of the transient component. Therefore, it is necessary to consider the reactive power support ability of the network-forming converter station under the influence of current limiting and leave a margin for the decay of the transient component of the network-forming converter station. The corresponding expression of the reactive power droop coefficient is:

[0067]

[0068] Among them: k q is the actual value of the reactive power droop coefficient, k is the per-unit value of the reactive power droop coefficient, U T,GFM is the valve-side voltage of the converter transformer, S GFM is the rated capacity of the network-forming converter station, σ is the current limit value, σ max is the maximum current limit value allowed for the network-forming converter station within 100 ms, corresponding to the "current limit network-forming support ability constraint", σ min is the current limit value allowed for the network-forming converter station to operate for a long time, corresponding to the "network-forming support ability constraint".

[0069] The following further elaborates on the present invention in conjunction with the accompanying drawings:

[0070] Figure 1 It is a schematic diagram of the control link and current limit of the network-forming converter station corresponding to the calculation method of the power stable operation area of the network-forming converter station based on the network-forming ability proposed in Embodiment 1 of the present invention. As Figure 1 shown, after the network-forming converter station performs dq transformation on the collected voltage and current using the phase information generated by virtual synchronous control, the trigger control of the converter is realized through outer-loop voltage control and inner-loop current control. Among them, the current set value generated by the outer-loop voltage control is input to the inner-loop current link after passing through the current limiting link. Therefore, when analyzing the network-forming ability of the network-forming converter station, it is necessary to consider the current limit and leave a margin for the decay of the transient current.

[0071] Figure 2This is a schematic diagram of the virtual synchronous control with additional primary frequency regulation and voltage-reactive droop control of a network-forming converter station corresponding to the calculation method of the power stable operation area of the network-forming converter station based on the network-forming ability in Embodiment 1 of the present invention. The virtual synchronous control adds a primary frequency regulation link, and quickly adjusts the active power of the network-forming converter station through the difference between the actual angular frequency and the rated angular frequency to achieve the primary regulation of the system frequency. The corresponding virtual synchronous control suppresses the frequency fluctuations caused by the new energy power fluctuations by introducing virtual inertia and cooperates with the damping coefficient to suppress the system oscillation. The voltage control adopts reactive droop control, and adjusts the set value of the voltage outer loop in a timely manner by monitoring the difference between the system reactive power and the rated value to achieve the control of the system voltage amplitude.

[0072] Figure 3 This is the operational circuit model of the network-forming converter station in the complex frequency domain in Embodiment 1 of the present invention. It converts the transient current decay problem in the time domain into the solution of the steady-state current in the complex frequency domain. For the operational circuit corresponding to the transient current dissipation on the valve side of the converter station, it can be considered that the capacitor voltage of the sub-module is zero or very low during the transient current decay period, and the bridge arm is approximately short-circuited, so as to simplify the operational circuit of the converter station. Among them, sL is the operational impedance, and Li(0) is the excitation source. Since the system is inductive, the new energy power fluctuations cause the network-forming converter station to generate transient current components, occupying the current margin of the network-forming converter station.

[0073] Figure 4 This is the power stable operation area of the network-forming converter station considering the modulation ratio constraint, voltage stability constraint, capacitor voltage fluctuation constraint, converter transformer capacity constraint, converter capacity constraint, and network-forming ability constraint in Embodiment 1 of the present invention. Compared with the conventional converter station, the power stable operation area of the converter station decreases when considering the network-forming ability. Among them, the "network-forming support ability constraint" corresponds to the boundary of the power stable range and is the power operation area where the network-forming converter station can operate for a long time; the "current limit network-forming support ability constraint" corresponds to the power support limit boundary and is the upper limit of the support ability of the network-forming converter station including the attenuation of transient components.

[0074] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. A method for calculating the power stable operation area of a network-forming converter station based on the network-forming ability, characterized in that It includes the following steps: S1. Based on the power control characteristics of the network-forming converter station and the principle of the network-forming converter station to construct the AC-side voltage, establish a network-forming frequency stability constraint equation; S2. Based on the voltage control characteristics of the network-forming converter station and the relationship between the AC-side valve outlet voltage and the AC bus voltage of the network-forming converter station, substitute the network-forming frequency stability constraint equation obtained in S1 to establish a power constraint range considering the network-forming ability of the network-forming converter station; S3. Establish an operational circuit model of the network-forming converter station in the complex frequency domain and analyze the influencing factors of transient components in the network-forming converter station; S4. Considering the influence of the current limit of the network-forming converter station, leave a margin for the attenuation of the transient components of the network-forming converter station in S3, and adopt the power stable operation constraint of the network-forming converter station described in S2 to calculate the power stable operation area of the network-forming converter station based on the network-forming ability.

2. The method for calculating the power stable operation area of a network-forming type converter station based on network-forming ability according to claim 1, characterized in that, The specific content of S1 includes the following: The network-forming converter station adopts virtual synchronous control, and the corresponding power control characteristics are: Among them, J is the moment of inertia; D is the damping coefficient; ω is the converter angular frequency; ω n is the rated angular frequency; k w is the active power droop coefficient; P v is the active power of the network-forming converter station; The corresponding network-forming frequency stability constraint equation is: Among them, U g is the AC grid bus voltage; U GFM is the output voltage of the network-forming converter; X = x T + x L / 2, x T is the reactance of the converter transformer, and x L is the arm reactance; Δf max is the maximum frequency deviation; RoCoF max is the maximum rate of change of frequency.

3. The method for calculating the power stable operation region of a network-forming converter station based on the network-forming ability according to claim 2, characterized in that, The specific content of S2 includes the following: The voltage of the network-forming converter station adopts reactive power droop control, and the corresponding voltage control characteristics are: U GFM = U ref + k q (Q ref - Q v ) (3) Among them, U GFM is the output voltage of the network-forming converter; U ref is the reference voltage; k q is the reactive power droop coefficient; Q ref is the reactive power reference value; Q v is the actual reactive power of the converter station; The network-forming converter station controls the reactive power to maintain the output voltage amplitude, which affects the AC bus voltage amplitude. The corresponding relationship between the bus voltage and the AC-side valve outlet voltage of the converter station is: Among them, P g is the active power of the AC bus, and Q g is the reactive power of the AC bus; Substitute Equation (3) and Equation (4) into Equation (2) to obtain the power constraint range considering the network-forming ability of the network-forming converter station.

4. The method for calculating the power stable operation area of a network-forming converter station based on network-forming ability according to claim 1, wherein The specific content of S3 includes the following: The system transient current component affects the current supply ability of the network-forming converter station, and the influence degree weakens with the attenuation of the transient component. Since the system is three-phase symmetric and the AC component in the arm current is half of the line current, the expression of the transient current corresponding to the single-phase arm current is: where, i pa (t) is the transient current of the upper arm of phase a; i a (0) is the transient current component; τ is the transient current decay time constant; R ac , L ac are the resistance and inductance of the AC side respectively; R0 and L0 are the resistance and inductance of the bridge arm respectively.

5. The method for calculating the power stable operation area of a network-forming converter station based on network-forming ability according to claim 1, characterized in that The specific content of S4 includes the following: When the system voltage drops, the network-forming converter station increases the reactive power to maintain the AC voltage; considering the reactive power support ability of the network-forming converter station under the influence of current limit and leaving a margin for the attenuation of the transient components of the network-forming converter station, the corresponding reactive power droop coefficient expression is: Among them, k q is the actual value of the reactive power droop coefficient; k is the per-unit value of the reactive power droop coefficient; U T,GFM is the valve-side voltage of the converter transformer; S GFM is the rated capacity of the network-forming converter station; σ is the current limit value; σ max is the maximum current limit value allowed within 100ms for the network-forming converter station, corresponding to the "current limit network-forming support ability constraint"; σ min is the current limit value for long-term operation in the network-forming converter station, corresponding to the "network-forming support ability constraint".

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