Multi-DC coordinated control method and system based on VSC-HVDC and LCC-HVDC

Abstract

The present invention discloses a multi-DC coordinated control method and system based on VSC-HVDC and LCC-HVDC. Under the condition that the non-blocked DC line meets the large signal stability, when the LCC-HVDC blocking is detected, the emergency power support amount P required by the receiving system is determined. short If the required emergency power support amount P short Less than the maximum power support of VSC-HVDC P VSC_th , then VSC-HVDC will give priority to active power support; if the required emergency power support amount P short Greater than the VSC-HVDC maximum power support P VSC_th , but less than the sum of the maximum power support of VSC-HVDC and LCC-HVDC P VSC_th +P LCC_th , the non-blocking DC line VSC-HVDC and LCC-HVDC need to coordinate active power support, and the VSC-HVDC also provides reactive power support. If the maximum power support of the VSC-HVDC and LCC-HVDC still cannot ensure system stability, additional generator or load shedding measures are necessary. This invention can improve the frequency stability of the receiving power grid after DC blocking occurs in the LCC-HVDC.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current (HVDC) transmission, and in particular to a multi-DC coordinated control method and system based on VSC-HVDC and LCC-HVDC. Background Art

[0002] With the development of multi-infeed HVDC transmission systems in my country, the grid structure has become more complex and diverse, posing greater challenges to the safe and stable operation of the grid. Due to the close electrical distances between multiple DC converter stations within a multi-infeed HVDC system, cross-coupling effects exist between the AC and DC systems, between different DC lines, and between the multi-infeed HVDC system and nearby power electronic devices. When an AC system fault occurs, it can cause commutation failure in the conventional HVDC transmission system. Further commutation failure in a single DC circuit can trigger commutation failure in multiple DC circuits, leading to significant fluctuations in transmission power. If the AC fault is not promptly cleared (e.g., due to switch failure or bus differential protection failure), the impact of multiple DC circuit commutation failures can be severe, potentially leading to DC blocking. DC power transmission interruption will seriously impact the safe and stable operation of the power system. In particular, multi-infeed HVDC systems can experience simultaneous or sequential commutation failures (initial commutation failure). During the recovery phase after the initial commutation failure, transient interactions occur between the DC system and the AC grid. If commutation conditions are still not met, subsequent commutation failures will occur, resulting in multiple DC power drops. If the subsequent commutation failure is not suppressed in time, it will also lead to DC lockout. At this time, the large-capacity power impact may cause the unit power angle to become unstable.

[0003] Under large disturbances, DC systems typically have a short-term overload capacity of 1.5 times 3s and a long-term overload capacity of 1.1 times. Leveraging the rapid power controllability and large regulation capacity of non-blocking DC lines for emergency power support is an effective measure to improve the transient stability of power systems after faults. However, large disturbances such as grid voltage sags and power surges can have significant impacts on DC lines. Therefore, large-signal stability analysis of non-blocking DC lines is necessary to ensure that they meet large-signal stability criteria. In recent years, researchers both domestically and internationally have studied large-signal stability analysis methods for power electronic converters. Common methods include the Lyapunov direct method, the Takagi-Sugeno fuzzy model method, the reverse trajectory tracking method, and the mixed potential function theory. Furthermore, the asymptotic stability region is used to quantify the degree of disturbance deviation that the system can withstand. In recent years, the mixed potential function theory has been widely used for large-signal stability analysis of power electronic systems, such as cascaded PWM rectifier systems, DC microgrids and DC distribution systems, and the power systems of multi-electric aircraft. However, limited research has been conducted on large-signal stability analysis of DC transmission systems based on the mixed potential function. As for the research on emergency power support technology, some researchers have analyzed the mechanism of DC emergency power support to improve the transient stability of the system. In addition, the DC transmission system consumes a large amount of reactive power when increasing active power. Insufficient reactive power leads to a decrease in the voltage of the converter bus, which will also have a certain impact on the power transmission of the DC system. When designing the DC emergency power support strategy, it is necessary to consider the coordinated control of reactive power. The emergency use of the converter station's spare reactive power can be used to improve the system voltage stability and support the increase in DC power. Based on the advantage of flexible DC in being able to independently control active power and reactive power, researchers analyzed the power support priority under the coexistence of conventional DC and flexible DC. The results show that incorporating flexible DC into the grid emergency control system can reduce the cost of conventional generator and load shedding control while improving the transient stability of the grid.

[0004] In summary, the influencing factors considered in existing research on the safety and stability control technology of multi-infeed HVDC transmission systems are still relatively simple, and there is a lack of comprehensive consideration of the multi-level interactive influencing factors of the multi-infeed HVDC transmission system and the mechanism analysis of different types of safety and stability problems during the fault evolution process of the multi-infeed HVDC transmission system. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a multi-DC coordinated control method and system based on VSC-HVDC and LCC-HVDC in response to the shortcomings of the existing technology, so as to make full use of the overload capacity of the non-blocking DC system and improve the safe and stable operation capability of the power system.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a multi-DC coordinated control method based on VSC-HVDC and LCC-HVDC, comprising the following steps:

[0007] S1. Determine the emergency power support amount P required by the system short :P short =P F -P Y -P L Among them, P F is the active power shortage caused by the faulty DC line blocking, P Y is the power increase of the system’s primary frequency modulation, P L Active power change caused by load regulation effect;

[0008] S2. Determine the emergency active power support ΔP of the non-blocked DC line VSC-HVDC VSC : P VSC_th is the maximum power support of VSC-HVDC;

[0009] Determine the emergency active power support ΔP of non-blocking DC line LCC-HVDC LCC : P LCC_th is the maximum power support of LCC-HVDC;

[0010] Determine the amount of machine or load to be cut ΔP Load :

[0011] Determine the emergency reactive power support ΔQ of non-blocking DC lines VSC-HVDC VSC :

[0012] Q' LCC To support active power, the AC bus inverter station absorbs reactive power, Q LCC It refers to the reactive power absorbed by the AC bus inverter station side when no active power support is provided.

[0013] Before step S1 , the process further includes: performing the following judgment: when a blocking fault occurs in the faulty DC line and the non-blocking DC line meets the large signal stability criterion, entering step S1 .

[0014] The large signal stability criterion is:

[0015]

[0016] Where, L and R represent the MMC bridge arm filter inductance and parasitic resistance; L line 、R line Indicates the equivalent inductance and resistance of the DC line; Pline is the active power of the DC line; R0 is the equivalent resistance on the AC side; C0 is the equivalent capacitance on the DC side; k idp is the total energy inner loop controller parameter; k ip is the current loop controller parameter in the rectifier station; k vp is the DC voltage loop controller parameter of the rectifier station; It is a Δ-type alternating current; v cpz 、v cnz Represents the equivalent capacitance voltage of each upper and lower bridge arm of MMC; v sd is the d-axis component of the rectifier station grid voltage; is the per-unit value of DC voltage.

[0017] Based on the same inventive concept, the present invention also provides a multi-DC coordinated control system based on VSC-HVDC and LCC-HVDC, including a memory, a processor, and a computer program stored in the memory; the processor executes the computer program to implement the steps of the above method.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention comprehensively considers the multi-level interactive influencing factors of the multi-infeed DC transmission system, and conducts a mechanism analysis of different types of safety and stability problems in the fault evolution process of the multi-infeed DC transmission system. On the basis of the non-locked DC line meeting the large signal stability criterion, the overload capacity of the non-locked DC system is fully utilized to take emergency power support measures, and coordinated with measures such as generator shedding, load shedding, reactive voltage control, etc., to propose a control strategy with targeted improvements to the multi-infeed DC transmission system, forming a more complete safety and stability control technology system, providing a theoretical basis and technical support for ensuring the safe and stable operation of the power grid, thereby improving the safe and stable operation capability of the power system. The method of the present invention can improve the frequency stability of the receiving-end power grid after DC locking occurs in LCC-HVDC, and utilizes VSC-HVDC reactive power support to compensate for the reactive power consumption of the converter station caused by the increase in the active power of LCC-HVDC, thereby reducing the voltage fluctuation of the converter bus of the receiving-end power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the impact of multi-DC power coordination on grid transient stability;

[0020] Figure 2 Flowchart of emergency power support strategy based on coordination between VSC-HVDC and LCC-HVDC;

[0021] Figure 3 This is the simulation waveform supported only by LCC-HVDC emergency power;

[0022] Figure 4 This is the simulation waveform supported only by VSC-HVDC emergency power;

[0023] Figure 5 Simulated waveforms for the emergency power support strategy based on the coordination of VSC-HVDC and LCC-HVDC. DETAILED DESCRIPTION

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0025] Example 1

[0026] The embodiment of the present invention is directed to a multi-infeed DC transmission system consisting of conventional DC transmission and flexible DC transmission, as shown in the attached figure. Figure 1 As shown, MMC has become the preferred converter topology for flexible DC transmission systems. The VSC-HVDC systems described below all employ the MMC structure. DC emergency power support is categorized as the second line of defense for grid stability control. Its rapid ramping up or down of DC power can achieve stable control by shedding generators or loads. Conventional DC LCC-HVDC systems consume 40% to 60% of their active capacity in reactive power during active power regulation. This reactive power is typically provided by reactive power compensation devices and filters, and frequent switching of filters can affect their lifespan. Using only LCC-HVDC for emergency power support may cause reactive power imbalance at the converter station, leading to bus voltage fluctuations at the converter station and further impacting the DC system's operational status during emergency power support. VSC-HVDC can achieve four-quadrant power operation and independent control of active and reactive power, thus leveraging its dynamic reactive power support capabilities to improve AC bus voltage stability. When a blocking fault occurs on a faulty DC line and the non-blocking DC line meets the large signal stability criterion, an embodiment of the present invention proposes an emergency power support control strategy based on coordination between VSC-HVDC and LCC-HVDC. The specific invention content is as follows:

[0027] (1) Determine the emergency power support required by the system P short :Assume that the active power shortage caused by the faulty DC line is P F , the system's primary frequency modulation power increase is P Y , the active power change caused by the load regulation effect is P L , then the emergency power support required by the system is P short for:

[0028] P short =P F -P Y -P L

[0029] (2) Non-blocking DC line VSC-HVDC emergency active power support ΔP VSC :If the system requires emergency power support P short Less than the maximum power support of VSC-HVDC P VSC_th , VSC-HVDC will give priority to power support, thus avoiding the additional reactive power consumption caused by LCC-HVDC power support. Its power support amount ΔP VSC P short If the system requires emergency power support P short Greater than the VSC-HVDC maximum power support P VSC_th , then the VSC-HVDC power support is its maximum power support P VSC_th .

[0030]

[0031] (3) Emergency active power support ΔP for non-blocking DC line LCC-HVDC LCC :If the system requires emergency power support P short Less than the maximum power support of VSC-HVDC P VSC_th , then LCC-HVDC does not provide emergency power support; if the system requires emergency power support P short Greater than the VSC-HVDC maximum power support P VSC_th , but less than the sum of the maximum power support of VSC-HVDC and LCC-HVDC P VSC_th +P LCC_th , then the LCC-HVDC power support ΔP LCC P short -P VSC_th If the system requires emergency power support P short Greater than the sum of the maximum power support of VSC-HVDC and LCC-HVDC P VSC_th +P LCC_th , then the LCC-HVDC power support is its maximum power support P LCC_th .

[0032]

[0033] (4) Cutting the machine or load ΔP LoadIf the maximum power support of VSC-HVDC and LCC-HVDC still cannot ensure system stability, it is necessary to take measures such as cutting the generator or shedding the load. The cutting amount ΔP Load P short -P VSC_th -P LCC_th .

[0034]

[0035] (5) Non-blocking DC line VSC-HVDC emergency reactive power support ΔQ VSC :When the system requires emergency power support P short Greater than the VSC-HVDC maximum power support P VSC_th When the VSC-HVDC is in the state of emergency, it needs to provide reactive power support without exceeding the maximum overload capacity to compensate for the additional reactive power loss caused by the emergency power support of the non-blocked DC line LCC-HVDC. VSC Changes in reactive power absorbed by the AC bus inverter station before and after active power support for LCC-HVDC.

[0036]

[0037] Among them, Q' LCC To support active power, the AC bus inverter station absorbs reactive power, Q LCC The reactive power absorbed by the AC bus inverter station when no active power support is provided. The reactive power absorbed by the AC bus inverter station consists of the reactive power consumption of the inverter station and the reactive power compensation of the filter device.

[0038] The emergency power support flow chart based on the coordination of VSC-HVDC and LCC-HVDC is shown in the attached figure. Figure 2 As shown in Figure 2, the sufficient condition for VSC-HVDC to maintain stable operation under large signal disturbances is:

[0039]

[0040] Where L and R represent the MMC bridge arm filter inductance and parasitic resistance; L line 、R line Indicates the equivalent inductance and resistance of the DC line; P line is the active power of the DC line; R0 represents the equivalent resistance on the AC side; C0 represents the equivalent capacitance on the DC side; k idp is the total energy inner loop controller parameter; k ip is the current loop controller parameter in the rectifier station; k vp is the DC voltage loop controller parameter of the rectifier station; It is a Δ-type alternating current; v cpz 、v cnz Represents the equivalent capacitance voltage of each upper and lower bridge arm of MMC; v sd is the d-axis component of the rectifier station grid voltage; is the per-unit value of DC voltage.

[0041] To verify the correctness of the proposed emergency power support strategy, a multi-infeed DC transmission system consisting of LCC-HVDC1, LCC-HVDC2, and VSC-HVDC was constructed in this embodiment of the present invention. When DC blocking of LCC-HVDC1 resulted in a power shortage in the receiving grid, emergency power support was provided by utilizing the 1.1-fold long-term overload capacity of the non-blocking DC lines VSC-HVDC and LCC-HVDC2. The rated DC power of both the non-blocking DC lines VSC-HVDC and LCC-HVDC2 was 1000 MW, and the power support rate was set to 3000 MW / s.

[0042] Working condition I: Set the initial operating power of the faulty DC line LCC-HVDC1 to 500MW, and the initial operating power of the non-blocked DC lines VSC-HVDC and LCC-HVDC2 to 700MW. At 2s, DC blocking occurs on LCC-HVDC1. Figure 3 This is a simulation waveform showing emergency power support provided only by the non-blocked DC link LCC-HVDC2. It shows that the grid frequency remains around 49.5 Hz after the fault transient. However, the additional reactive power losses at the converter station caused by the LCC-HVDC2's active power support result in fluctuations in the converter station bus voltage.

[0043] Condition II: Maintaining the same fault conditions as Condition I, set the initial operating power of the faulty DC line LCC-HVDC1 to 500MW, and the initial operating power of the non-blocked DC lines VSC-HVDC and LCC-HVDC2 to 700MW. At 2s, DC blocking occurs on LCC-HVDC1, and VSC-HVDC provides emergency power support. Figure 4 The following is the simulation waveform of the emergency power support provided by the non-blocked DC line VSC-HVDC. It can be seen that the system frequency can remain stable after the fault transient. Figure 4 With attached Figure 3 By comparison, it can be seen that giving priority to emergency power support by VSC-HVDC can ensure the voltage stability of the AC bus of the inverter station.

[0044] Condition III: Set the initial operating power of LCC-HVDC1 to 1000MW, the initial operating power of the non-blocked DC lines VSC-HVDC and LCC-HVDC2 to 700MW, and LCC-HVDC1 is blocked at 2s. The emergency power support required by the system at this time is P shortGreater than the VSC-HVDC maximum power support P VSC_th , VSC-HVDC and LCC-HVDC2 jointly provide emergency power support. Figure 5 The simulated waveforms for an emergency power support strategy based on coordination between VSC-HVDC and LCC-HVDC show that, with the emergency power support strategy, system frequency can remain stable after a fault transient. Furthermore, compared to the voltage waveform when VSC-HVDC only provides active power support, the simultaneous active and reactive power support provided by VSC-HVDC can suppress converter station bus voltage fluctuations caused by LCC-HVDC power support.

[0045] Therefore, after LCC-HVDC DC blocking, on the basis of ensuring the stability of large signals of non-blocked DC lines such as VSC-HVDC, the emergency power support strategy based on the coordination of non-blocked DC lines VSC-HVDC and LCC-HVDC proposed in the embodiment of the present invention can effectively improve the frequency stability of the receiving power grid, and use the VSC-HVDC reactive power support to compensate for the reactive power consumption of the converter station caused by the active power increase of LCC-HVDC, which can effectively reduce the voltage fluctuation of the converter bus of the receiving power grid.

[0046] Example 2

[0047] Embodiment 2 of the present invention provides a control method corresponding to the above-mentioned embodiment 1, including a memory, a processor, and a computer program stored in the memory; the processor executes the computer program in the memory to implement the steps of the method in the above-mentioned embodiment 1.

[0048] In some implementations, the memory may be a high-speed random access memory (RAM), and may also include a non-volatile memory, such as at least one disk storage.

[0049] In other implementations, the processor may be a central processing unit (CPU), a digital signal processor (DSP), or other general-purpose processors, which are not limited here.

[0050] Example 3

[0051] Embodiment 3 of the present invention provides a computer-readable storage medium corresponding to the above-mentioned embodiment 1, on which a computer program / instruction is stored. When the computer program / instruction is executed by a processor, the steps of the method of the above-mentioned embodiment 1 are implemented.

[0052] Computer readable storage media can be tangible devices that hold and store instructions used by instruction execution devices. Computer readable storage media can be, for example, but not limited to, electronic storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any combination thereof.

[0053] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiment of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal translation scripting language JavaScript, etc.

[0054] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the steps in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0055] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0056] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present application.

[0057] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A multi-DC coordinated control method based on VSC-HVDC and LCC-HVDC, characterized in that: The following steps are involved: S1. Determine the emergency power support amount P required by the system short :P short =P F -P Y -P L ; Among them, P F is the active power shortage caused by the faulty DC line blocking, P Y is the power increase of the system’s primary frequency modulation, P L Active power change caused by load regulation effect; S2. Determine the emergency active power support ΔP of the non-blocked DC line VSC-HVDC VSC : P VSC_th is the maximum power support of VSC-HVDC; Determine the emergency active power support ΔP of non-blocking DC line LCC-HVDC LCC : P LCC_th is the maximum power support of LCC-HVDC; Determine the amount of machine or load to be cut ΔP Load : Determine the emergency reactive power support ΔQ of non-blocking DC lines VSC-HVDC VSC : Q' LCC To support active power, the AC bus inverter station absorbs reactive power, Q LCC It refers to the reactive power absorbed by the AC bus inverter station side when no active power support is provided.

2. The multi-DC coordinated control method based on VSC-HVDC and LCC-HVDC according to claim 1 is characterized in that: Before step S1, the method further includes: The following judgment is made: when a blocking fault occurs in the faulty DC line and the non-blocking DC line meets the large signal stability criterion, step S1 is entered.

3. According to the multi-DC coordinated control method based on VSC-HVDC and LCC-HVDC as claimed in claim 2, the large signal stability criterion is: in, L and R represent the MMC bridge arm filter inductance and parasitic resistance; L line 、R line Indicates the equivalent inductance and resistance of the DC line; P line is the active power of the DC line; R0 represents the equivalent resistance on the AC side; C0 represents the equivalent capacitance on the DC side; k idp is the total energy inner loop controller parameter; k ip is the current loop controller parameter in the rectifier station; k vp is the DC voltage loop controller parameter of the rectifier station; It is a Δ-type alternating current; v cpz 、v cnz Represents the equivalent capacitance voltage of each upper and lower bridge arm of MMC; v sd is the d-axis component of the rectifier station grid voltage; is the per-unit value of DC voltage.

4. A multi-DC coordinated control system based on VSC-HVDC and LCC-HVDC, comprising a memory, a processor, and a computer program stored in the memory; characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 3.

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