Coordinated control system for ac fault ride-through of new energy through flexible direct current grid based on stability boundary switching

Abstract

The application discloses a new energy through flexible direct current grid-connected alternating current fault ride-through cooperative control system based on stability boundary switching, comprising: 1, based on the new energy during the symmetrical fault of the alternating current line, the active power zero is sent to the flexible direct current system, and the phase-locked loop synchronization stability criterion is obtained; 2, the criterion is used as the conservative lower boundary of the existence of the active power positive sent to the flexible direct current system; 3, when the fault resistance exceeds the conservative lower boundary, the phase-locked loop synchronization control is carried out on the new energy, and when the fault resistance does not reach the conservative lower boundary, the power synchronization control is carried out on the new energy. The application can enhance the robustness and reliability of the new energy grid-connected system under different fault depths, thereby effectively improving the synchronization stability of the system during the fault ride-through period.

Description

Technical Field

[0001] This invention relates to the field of flexible DC power transmission and new energy grid connection technology, specifically to a synchronous control method for a fault-crossing cooperative system of new energy through a flexible DC grid connection system. It is particularly applicable to a control method that maintains the synchronous operation of new energy power generation units without disconnecting from the grid and prevents active power backfeeding from the receiving end grid when a short-circuit fault occurs on a three-phase AC line. Background Technology

[0002] With the advancement of the "dual carbon" target, my country's large-scale new energy bases are currently mainly located in the western desert and Gobi regions, while load centers are located thousands of kilometers away in the eastern regions. Adopting flexible DC transmission technology to achieve long-distance, large-capacity transmission of wind and solar power has become the mainstream solution. However, when a ground fault occurs on the AC line at the sending end of the new energy transmission system via the flexible DC system, the system faces multiple severe challenges: on the one hand, the fault causes a voltage drop at the grid connection point, which may trigger the phase-locked loop (PLL) of the new energy power plant to lose synchronization; on the other hand, to support the voltage, the sending-end converter station will inject current into the fault point, easily resulting in a "backward" flow of active power to the fault point. Due to the isolation effect of the flexible DC system, this power disturbance cannot be detected by the receiving-end grid, which will seriously disrupt the power balance of the receiving-end system. Therefore, achieving coordinated control between the new energy power plant and the converter station at the flexible DC transmission end during a fault, and ensuring the synchronous, grid-connected operation of the new energy power plant, is crucial to ensuring the safe and stable operation of the system.

[0003] Existing control measures for new energy transmission via flexible direct current lines during AC line faults mainly focus on current limiting control at the new energy power plant or the sending-end converter station itself. Control strategies often involve the new energy power plant executing a "low voltage ride-through" curve based on voltage drops, or the sending-end converter station using voltage reduction and current limiting. These two methods often operate independently. While these methods can prevent equipment overcurrent blocking to some extent, they fail to fundamentally solve the problem of active power backfeeding caused by the sending-end converter station maintaining voltage, and they do not fully consider the instability risk of the new energy phase-locked loop (PLL) when the fault depth is deep. Contingency plans for PLL instability often rely on a single electrical quantity threshold as a switching criterion, or directly communicate with the sending-end converter station to obtain a synchronization angle. If these strategies fail to accurately reflect the dynamic boundaries of the PLL, they may lead to erroneous operation or loss of the optimal intervention opportunity. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention proposes a collaborative control system for AC fault ride-through of new energy sources via flexible DC grid connection based on stability boundary switching. The aim is to conservatively assess and maintain the phase-locked loop synchronization stability of the new energy converter during AC line faults, enabling it to coordinate with the converter station at the flexible DC transmission end and preventing the receiving-end grid from feeding back active power to the fault point. This optimizes the active support capability during fault ride-through while ensuring stable operation without disconnecting from the grid.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: The present invention discloses a collaborative control system for AC fault ride-through of new energy sources connected to the grid via flexible DC, based on stability boundary switching. The system includes: a grounding resistor. R f The system comprises a three-phase AC transmission line, a new energy power generation unit, a flexible DC transmission unit, and a receiving-end power grid; wherein the new energy power generation unit is connected to the sending-end converter station of the flexible DC transmission unit via the three-phase AC transmission line, and the grounding resistance... R f The flexible DC transmission unit is grounded through a three-phase AC transmission line and then connected to the receiving-end power grid. The new energy power generation unit includes: a new energy converter; The flexible DC transmission unit includes: a sending-end converter station, a receiving-end converter station, and a DC transmission line located between the two converter stations; The grounding resistance R f A symmetrical AC fault is formed by a short circuit to ground in the three-phase AC transmission line. The new energy converter adjusts the phase of its output current during an AC symmetrical fault in the current single-loop control mode. The synchronization stability boundary of the phase-locked loop (PLL) during an AC symmetrical fault is calculated to obtain the phase of the PLL output during the AC symmetrical fault. or power loop output phase ; The given active current command Reactive current command Phase of the phase-locked loop output or power loop output phase After being modulated by a single-loop controller, a pulse signal is obtained to drive the new energy converter to transmit power to the fault point and the flexible DC transmission unit through the three-phase AC transmission line. This enables constant voltage and frequency control of the sending-end converter station, resulting in a stable AC voltage used to track the given AC voltage amplitude. ; During a symmetrical fault, the sending-end converter station, in coordination with the new energy converter, actively reduces the AC voltage amplitude given by the outer loop controller. This allows the new energy converter to simultaneously reduce power compensation at the fault point and ensures that the remaining power after compensation by the new energy converter flows into the sending-end converter station. The DC transmission line receives the power output from the sending-end converter station and transmits it to the receiving-end converter station. The receiving-end converter station transmits the incoming power to the receiving-end power grid under constant DC voltage and reactive power control mode.

[0006] The AC fault ride-through cooperative control system for new energy grid connection via flexible DC based on stability boundary switching, as described in this invention, is characterized in that the new energy converter obtains the grid connection point voltage phase during AC symmetrical faults according to the following steps. or power loop output phase ; Step 1: Sample the active power flowing through the sending-end converter station. P r and current and the amplitude of the output current of the new energy converter. and phase Line resistance between the new energy converter and the fault point R 1 and line reactance X 1. And the line resistance between the fault point and the sending-end converter station. R 0 and line reactance X 0; Step 2: Using the voltage phase of the sending-end converter station as a reference, define the phase of the phase-locked loop output. The difference between the voltage phase angle of the sending-end converter station and the voltage phase angle of the sending-end converter station is the power angle. ,and Therefore, the q-axis component of the grid connection point voltage can be established using equation (1). With the angle relational equations : (1) In equation (1), Indicates the amplitude coefficient. Indicates additional items, Indicates the argument of X. j Represents the imaginary unit. X This indicates terms that are only related to impedance. X | indicates X The modulus of , and we have: (2) (3) Step 3: Use equation (4) to obtain the current flowing through the sending-end converter station. And use equation (3) to Perform coordinate rotation transformation to obtain d-axis components Thus, using equation (6), the voltage reference value of the sending-end converter station when zero active current is fed into the flexible DC transmission unit can be obtained. : (4) (5) (6) In equation (5), This indicates the amplitude of the output current of the new energy converter, and , This indicates the phase of the output current of the new energy converter, and , This represents the current vector output by the new energy converter, and ; Step 4: Construct the first q-axis component of the grid connection point voltage using equation (7). u q1 With the angle curves between and the second q-axis component of the grid connection point voltage u q2 With the angle curves between and obtain and All intersections between them: (7) Step 5: Select the intersection point that satisfies equation (8) from all intersection points as the stable equilibrium point: (8) Step 6: Traverse At the same time, adjust the grounding resistance R f The resistance value allows for the attainment of a stable equilibrium point during the process of a symmetrical AC fault transitioning from a high-resistance fault to a metallic fault. lower bound ; Step 7, let =0, and according to equation (5), for Adjustments were made to obtain the active voltage reduction value of the sending-end converter station. Thus, using equation (9), we can obtain the expected positive active power of the new energy converter being fed into the flexible DC transmission unit during an AC symmetrical fault. P r_new : (9) Step 8, if > Then, using equation (10), phase-locked loop synchronous control is performed on the new energy converter, and the grid connection point voltage phase during AC symmetrical faults is obtained. Conversely, equation (11) is used to perform power synchronization control on the new energy converter, and the power loop output phase during AC symmetrical faults is obtained. ; (10) (11) In equations (10)-(11), It is the rated angular frequency of the power grid. Indicates deviation, This represents the proportional gain of the phase-locked loop controller. This represents the integral coefficient of the phase-locked loop controller. This represents the proportional gain of the power loop controller.

[0007] Compared with the prior art, the beneficial effects achieved by the present invention are: 1. This invention proposes a new energy grid-connected synchronous control method based on stability boundary switching. During AC line faults, compared with relying on a single electrical quantity threshold for judgment, it can conservatively predict the risk of phase-locked loop instability, thereby preventing the risk of phase-locked loop instability caused by deep faults in new energy converters and significantly improving the synchronization stability and robustness of the system during fault ride-through.

[0008] 2. This invention constructs a collaborative control architecture between the renewable energy converter and the flexible direct-transmission converter station during faults. By combining the renewable energy side's autonomous adjustment of active and reactive current commands based on the fault depth with the sending-end converter station's proactive voltage reduction based on fault severity, dynamic coordination between the two is achieved. This collaborative strategy not only effectively limits fault current and the backflow of active power to the fault point, ensuring power balance in the receiving-end grid, but also solves the problem that existing independent control strategies cannot coordinate and balance synchronization stability and system power balance. Attached Figure Description

[0009] Figure 1 This is a control structure diagram of the fault ride-through collaborative control system of the new energy source through the flexible DC grid-connected system in this invention; Figure 2 This is the equivalent circuit diagram of a single unit of new energy connected to the flexible DC grid in this invention; Figure 3 This is a diagram illustrating the conditions for the existence of a stable equilibrium point during fault-crossing cooperative control in this invention. Figure 4 This is a flowchart of the new energy grid-connected synchronous control method based on stability boundary switching according to the present invention. Detailed Implementation

[0010] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0011] In this embodiment, to address the issues of phase-locked loop instability and active power backfeeding from the receiving-end grid during AC fault ride-through, a collaborative control system for new energy and flexible DC transmission is proposed, combining synchronous stability and active power support capabilities during fault ride-through. This system calculates the conservative boundary of phase-locked loop synchronous stability at the converter station at the flexible DC transmission end when active power transmission is zero. Based on this boundary, it actively switches the synchronous control strategy of the new energy converter when active power transmission is positive at the converter station at the flexible DC transmission end. Simultaneously, it collaboratively adjusts the sending-end voltage reference value of the flexible DC transmission system to complete AC fault ride-through. This collaborative control system can effectively improve the synchronous stability of new energy power generation units and the active grid support capability under deep symmetrical faults. Specifically, the collaborative control system includes: a grounding resistor... R f The system consists of a three-phase AC transmission line, a new energy power generation unit, a flexible DC transmission unit, and a receiving-end power grid; wherein, the new energy power generation unit is connected to the sending-end converter station of the flexible DC transmission unit via the three-phase AC transmission line, and the grounding resistance is... R f The flexible DC transmission unit is connected to the receiving-end power grid via a three-phase AC transmission line grounding system.

[0012] The new energy power generation unit includes: one new energy converter; A flexible DC transmission unit includes: a sending-end converter station, a receiving-end converter station, and a DC transmission line located between the two converter stations; Grounding resistance R f A symmetrical AC fault is formed by a short circuit to ground in a three-phase AC transmission line.

[0013] During an AC symmetrical fault in the single-loop current control mode, the new energy converter adjusts the phase of its output current. To calculate the synchronization stability boundary of the phase-locked loop during an AC symmetrical fault, the following steps are performed to obtain the phase of the phase-locked loop output during the AC symmetrical fault. or power loop output phase .

[0014] Step 1, as follows Figure 1 As shown, the active power of the sampled stream when it passes through the sending-end converter station P r and current and the amplitude of the output current of the new energy converter. and phase Line resistance between the new energy converter and the fault point R 1 and line reactance X 1. And the line resistance between the fault point and the sending-end converter station. R 0 and line reactance X0 is used to calculate the grid connection point voltage and use it for the d-axis orientation of the phase-locked loop to obtain the phase synchronization between the new energy converter and the sending-end converter station.

[0015] Step 2, establish as follows Figure 2 The single-unit equivalent circuit shown uses the voltage phase of the sending-end converter station as a reference to define the phase of the phase-locked loop output. The difference between the voltage phase angle of the sending-end converter station and the voltage phase angle of the sending-end converter station is the power angle. ,and Therefore, the q-axis component of the grid connection point voltage can be established using equation (1). With the angle relational equations : (1) In equation (1), Indicates the amplitude coefficient. Indicates additional items, Indicates the argument of X. j Represents the imaginary unit. X This indicates terms that are only related to impedance. X | indicates X The modulus of , and we have: (2) (3) Step 3, based on Figure 2 The equivalent circuit shown uses equation (4) to obtain the current flowing through the sending-end converter station. And use equation (3) to Perform coordinate rotation transformation to obtain d-axis components Setting equation (5) to zero yields equation (6), and thus equation (6) is used to obtain the voltage reference value of the sending-end converter station when the active current is zero and fed into the flexible DC transmission unit. Regarding the amplitude of the output current of the new energy converter Phase , angle and fault resistor The expression.

[0016] (4) (5) (6) In equation (5), This indicates the amplitude of the output current of the new energy converter, and , This indicates the phase of the output current of the new energy converter, and , This represents the current vector output by the new energy converter, and .

[0017] Step 4: When the phase-locked loop has synchronous stability and the grid connection point voltage is d-axis oriented, u q= 0 always holds true, and the equation (1) will always hold true. u q Decomposed into the first q-axis component u q1 Second q-axis component u q2 In order to facilitate u q= The solvable problem of 0 is transformed into an analytical curve. and curve The intersection problem. The first q-axis component of the grid connection point voltage is constructed using equation (7). u q1 With the angle curves between and the second q-axis component of the grid connection point voltage u q2 With the angle curves between and obtain and All intersections between them: (7) Step 5, as follows Figure 3 As shown, when hour, u q2 yes u q1 The tangent lines therefore have only one intersection point, which is the boundary of the untakeable values; when hour, u q2 and u q1 There are two intersection points, which form the boundary of possible values. Among these two intersection points, the following condition must be met: One point is a stable equilibrium point, and the other is an unstable equilibrium point; therefore When the equilibrium point lies between the two, a stable equilibrium point must exist. We select the intersection point that satisfies equation (8) from all intersection points as the stable equilibrium point. Furthermore, according to Lyapunov's method, the stable equilibrium point is locally asymptotically stable. (8) Step 6, as follows Figure 4 As shown, when a fault is detected, both the new energy converter and the sending-end converter station simultaneously receive the fault ride-through signal LVRT_Flag. At this time, circuit parameters are collected and iterated. At the same time, adjust the grounding resistanceR f The resistance value allows for the attainment of a stable equilibrium point during the process of a symmetrical AC fault transitioning from a high-resistance fault to a metallic fault. lower bound .

[0018] Step 7, let =0 means that the new energy converter generates active power at full capacity during AC faults, and according to equation (5), for Adjustments were made to obtain the active voltage reduction value of the sending-end converter station. Thus, using equation (9), we can obtain the expected positive active power of the new energy converter being fed into the flexible DC transmission unit during an AC symmetrical fault. P r_new And use it as the active power reference value for the power synchronization control of new energy converters: (9) Step 8, as follows Figure 1 As shown, if > It is assumed that the phase-locked loop has a stable equilibrium point and synchronous stability during AC faults. Therefore, equation (10) is used to perform phase-locked loop synchronous control on the new energy converter, and the phase of the grid connection point voltage during AC symmetrical faults is obtained. Conversely, if it is assumed that the phase-locked loop has no stable equilibrium point and is unstable during AC faults, then equation (11) is used to perform power synchronization control on the new energy converter, and the power loop output phase during AC symmetrical faults is obtained. ; (10) (11) In equations (10)-(11), It is the rated angular frequency of the power grid. Indicates deviation, This represents the proportional gain of the phase-locked loop. This represents the integral coefficient of the phase-locked loop. This represents the power loop proportional coefficient.

[0019] The given active current command Reactive current command Phase of the phase-locked loop output or power loop output phase After being modulated by a single-loop controller, a pulse signal is obtained to drive the new energy converter to transmit power to the fault point and the flexible DC transmission unit through a three-phase AC transmission line. This enables constant voltage and frequency control of the sending-end converter station, resulting in a stable AC voltage used to track the given AC voltage amplitude. .

[0020] During symmetrical faults, the sending-end converter station, in coordination with the new energy converter, actively reduces the AC voltage amplitude given by the outer loop controller. This allows the new energy converter to simultaneously reduce power compensation at fault points, and the remaining power after compensation by the new energy converter flows into the sending-end converter station.

[0021] The DC transmission line receives the power output from the sending-end converter station and transmits it to the receiving-end converter station; The receiving-end converter station transmits the incoming power to the receiving-end power grid under constant DC voltage and reactive power control mode.

Claims

1. A collaborative control system for AC fault ride-through of new energy sources via flexible DC grid connection based on stability boundary switching, characterized in that, include: Grounding resistance R f The system comprises a three-phase AC transmission line, a new energy power generation unit, a flexible DC transmission unit, and a receiving-end power grid; wherein the new energy power generation unit is connected to the sending-end converter station of the flexible DC transmission unit via the three-phase AC transmission line, and the grounding resistance... R f The flexible DC transmission unit is grounded through a three-phase AC transmission line and then connected to the receiving-end power grid. The new energy power generation unit includes: a new energy converter; The flexible DC transmission unit includes: a sending-end converter station, a receiving-end converter station, and a DC transmission line located between the two converter stations; The grounding resistance R f A symmetrical AC fault is formed by a short circuit to ground in the three-phase AC transmission line. The new energy converter adjusts the phase of its output current during an AC symmetrical fault in the current single-loop control mode. The synchronization stability boundary of the phase-locked loop (PLL) during an AC symmetrical fault is calculated to obtain the phase of the PLL output during the AC symmetrical fault. or power loop output phase ; The given active current command Reactive current command Phase of the phase-locked loop output or power loop output phase After being modulated by a single-loop controller, a pulse signal is obtained to drive the new energy converter to transmit power to the fault point and the flexible DC transmission unit through the three-phase AC transmission line. This enables constant voltage and frequency control of the sending-end converter station, resulting in a stable AC voltage used to track the given AC voltage amplitude. ; During a symmetrical fault, the sending-end converter station, in coordination with the new energy converter, actively reduces the AC voltage amplitude given by the outer loop controller. This allows the new energy converter to simultaneously reduce power compensation at the fault point, and the remaining power after compensation by the new energy converter flows into the sending-end converter station. The DC transmission line receives the power output from the sending-end converter station and transmits it to the receiving-end converter station. The receiving-end converter station transmits the incoming power to the receiving-end power grid under constant DC voltage and reactive power control mode.

2. The AC fault ride-through cooperative control system for new energy sources connected to the grid via flexible DC transmission based on stability boundary switching, as described in claim 1, is characterized in that... The new energy converter obtains the grid-connected point voltage phase during AC symmetrical faults by following these steps. or power loop output phase ; Step 1: Sample the active power flowing through the sending-end converter station. P r and current and the amplitude of the output current of the new energy converter. and phase Line resistance between the new energy converter and the fault point R 1 and line reactance X 1. And the line resistance between the fault point and the sending-end converter station. R 0 and line reactance X 0; Step 2: Using the voltage phase of the sending-end converter station as a reference, define the phase of the phase-locked loop output. The difference between the voltage phase angle of the sending-end converter station and the voltage phase angle of the sending-end converter station is the power angle. ,and Therefore, the q-axis component of the grid connection point voltage can be established using equation (1). With the angle relational equations : (1) In equation (1), Indicates the amplitude coefficient. Indicates additional items, Indicates the argument of X. j Represents the imaginary unit. X This indicates terms that are only related to impedance. X | indicates X The modulus of , and we have: (2) (3) Step 3: Use equation (4) to obtain the current flowing through the sending-end converter station. And use equation (3) to Perform coordinate rotation transformation to obtain d-axis components Thus, using equation (6), the voltage reference value of the sending-end converter station when zero active current is fed into the flexible DC transmission unit can be obtained. : (4) (5) (6) In equation (5), This indicates the amplitude of the output current of the new energy converter, and , This indicates the phase of the output current of the new energy converter, and , This represents the current vector output by the new energy converter, and ; Step 4: Construct the first q-axis component of the grid connection point voltage using equation (7). u q1 With the angle curves between and the second q-axis component of the grid connection point voltage u q2 With the angle curves between and obtain and All intersections between them: (7) Step 5: Select the intersection point that satisfies equation (8) from all intersection points as the stable equilibrium point: (8) Step 6: Traverse At the same time, adjust the grounding resistance R f The resistance value allows for the attainment of a stable equilibrium point during the process of a symmetrical AC fault transitioning from a high-resistance fault to a metallic fault. lower bound ; Step 7, let =0, and according to equation (5), for Adjustments were made to obtain the active voltage reduction value of the sending-end converter station. Thus, using equation (9), we can obtain the expected positive active power of the new energy converter being fed into the flexible DC transmission unit during an AC symmetrical fault. P r_new : (9) Step 8, if > Then, using equation (10), phase-locked loop synchronous control is performed on the new energy converter, and the grid connection point voltage phase during AC symmetrical faults is obtained. Conversely, equation (11) is used to perform power synchronization control on the new energy converter, and the power loop output phase during AC symmetrical faults is obtained. ; (10) (11) In equations (10)-(11), It is the rated angular frequency of the power grid. Indicates deviation, This represents the proportional gain of the phase-locked loop controller. This represents the integral coefficient of the phase-locked loop controller. This represents the proportional gain of the power loop controller.

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