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

Abstract

The invention discloses a multi-direct-current coordination control method and system based on VSC-HVDC and LCC-HVDC, and the method comprises the steps: determining the emergency power support amount Pshort needed by a receiving end system when a non-blocking direct-current line meets the large signal stability after the blocking of the LCC-HVDC is detected; if the required emergency power support amount Pshort is smaller than the maximum power support amount PVSCth of the VSC-HVDC, the VSC-HVDC preferentially carries out active power support; if the required emergency power support amount Pshort is greater than the maximum power support amount PVSCth of the VSC-HVDC, but less than the sum PVSCth + PLCCth of the maximum power support amount of the VSC-HVDC and the maximum power support amount of the LCC-HVDC, the VSC-HVDC and the LCC-HVDC of the non-blocking DC line need to coordinate active power support, and the VSC-HVDC carries out reactive power support at the same time; and if the maximum power support quantities of the VSC-HVDC and the LCC-HVDC still cannot ensure the stability of the system, additional generator tripping or load shedding measures need to be taken. According to the method, the frequency stability of the receiving end power grid can be improved after the LCC-HVDC is subjected to direct current blocking.

Description

Technical Field

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

[0002] With the formation of multi-infeed DC transmission system in my country, the grid structure has become more complex and diverse, bringing greater challenges to the safe and stable operation of the grid. Due to the close electrical distance between multiple DC converter stations in the multi-infeed DC transmission system, there are interactive coupling effects between AC and DC systems, between different DC lines, and between the multi-infeed DC transmission system and nearby power electronic devices. When the AC system fails, it may cause commutation failure in the conventional DC transmission system. Further single-circuit DC commutation failure may cause commutation failure in multiple DC circuits, resulting in large fluctuations in transmission power; if the AC fault is not removed in time (such as switch failure or bus differential protection refusal after the fault), the impact of multiple DC commutation failures is serious, which may lead to DC locking. The interruption of DC power transmission will seriously affect the safe and stable operation of the power system. In particular, the multi-infeed DC transmission system may also experience simultaneous commutation failure or successive commutation failure (first commutation failure). In the recovery stage after the first commutation failure, there is a transient interactive effect between the DC system and the AC power grid. If the 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 disturbance conditions, DC systems generally have a short-term overload capacity of 1.5 times 3s and a long-term overload capacity of 1.1 times. Using the characteristics of fast controllable power 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 drops and power surges will have a non-negligible impact on DC lines. Therefore, it is necessary to conduct large-signal stability analysis on non-blocking DC lines to ensure that non-blocking DC lines meet the large-signal stability criteria. In recent years, domestic and foreign scholars have studied large-signal stability analysis methods for power electronic converters. Common methods include Lyapunov direct method, Takagi-Sugeno fuzzy model method, reverse trajectory tracking method, mixed potential function theory, etc., and further quantify the degree of disturbance deviation that the system can withstand through the asymptotic stability domain. In recent years, mixed potential function theory has been widely used in large-signal stability analysis of power electronic systems, such as PWM rectifier cascade systems, DC microgrids and DC distribution systems, and multi-electric aircraft power systems, but few studies have conducted large-signal stability analysis of DC transmission systems based on mixed potential functions. As for the relevant 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 needs to consume a large amount of reactive power when increasing the 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, reactive power coordinated control should be considered. The system voltage stability can be improved by urgently investing in the spare reactive power of the converter station to support the increase of DC power. Based on the advantage of flexible DC that can 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 power grid emergency control system can reduce the cost of conventional machine shedding and load shedding control on the basis of improving the transient stability of the power grid.

[0004] In general, the influencing factors considered in the existing research on the safety and stability control technology of multi-infeed DC 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 DC transmission system and the mechanism analysis of different types of safety and stability problems during the fault evolution of the multi-infeed DC 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 view of the shortcomings of the existing technology, so as to make full use of the overload capacity of the non-locking DC system and improve the safe and stable operation capability of the power system.

[0006] In order 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 primary frequency modulation, P L Active power change caused by load regulation effect;

[0008] S2. Determine the emergency active power support amount 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-blocked 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-blocked DC line VSC-HVDC VSC :

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

[0013] Before step S1, the method 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] Wherein, 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 grid voltage of the rectifier station; is the DC voltage per unit value.

[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 analyzes the mechanism of different types of safety and stability problems in the process of fault evolution of the multi-infeed DC transmission system. On the basis that the non-locked DC line meets 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 the measures such as machine shedding, load shedding, reactive voltage control, etc., a control strategy with targeted improvement of the multi-infeed DC transmission system is proposed to form a more complete safety and stability control technology system, which provides 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 the DC lockout of LCC-HVDC occurs, 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, 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 the transient stability of the power grid;

[0020] Figure 2 It is a flow chart of the emergency power support strategy based on the coordination of VSC-HVDC and LCC-HVDC;

[0021] Figure 3 The simulated waveform is supported only by LCC-HVDC emergency power;

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

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

[0024] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are 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 creative work are 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 1 As shown. MMC has become the preferred converter topology for flexible DC transmission systems. The VSC-HVDC described below all adopt the MMC structure. DC emergency power support is classified as the second line of defense for grid stability control. Its rapid increase or decrease of DC power can achieve the stable control effect of cutting the machine or load. When conventional DC LCC-HVDC performs active power regulation, the converter station consumes 40% to 60% of the active capacity of reactive power. This part of reactive power is usually provided by reactive compensation devices and filters, and frequent switching of filters will affect their life. Emergency power support only by LCC-HVDC may cause reactive imbalance in the converter station, causing the problem of bus voltage fluctuation in the converter station, further affecting the operating state of the DC system during emergency power support. VSC-HVDC can realize four-quadrant power operation and complete independent control of active power and reactive power, so its dynamic reactive support capability can be used to improve the AC bus voltage stability problem. When a blocking fault occurs in the 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 the coordination of VSC-HVDC and LCC-HVDC. The specific content of the invention is as follows:

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

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

[0029] (2) Non-blocked 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. The power support amount ΔP VSC P short ; If the system requires emergency power support P short Greater than the maximum power support of VSC-HVDC P VSC_th , then the VSC-HVDC power support is its maximum power support P VSC_th .

[0030]

[0031] (3) Non-blocked DC line LCC-HVDC emergency active power support ΔP 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 maximum power support of VSC-HVDC 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 machine or load ΔP Load:If 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 machine or load, and the cutting amount ΔP Load P short -P VSC_th -P LCC_th .

[0034]

[0035] (5) Non-blocked DC line VSC-HVDC emergency reactive power support ΔQ VSC :When the system requires emergency power support P short Greater than the maximum power support of VSC-HVDC P VSC_th When the VSC-HVDC is in the state of being overloaded, 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-HVDC reactive power support ΔQ VSC The change 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 It is the reactive power absorbed by the AC bus inverter station side when no active power support is provided. The reactive power absorbed by the AC bus inverter station side is composed 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 between VSC-HVDC and LCC-HVDC is shown in the attached figure. Figure 2 As shown. The sufficient condition for VSC-HVDC to maintain stable operation under large signal disturbance 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 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 Respectively represent the equivalent capacitance voltage of each upper and lower bridge arm of MMC; v sd is the d-axis component of the grid voltage of the rectifier station; is the DC voltage per unit value.

[0041] In order to verify the correctness of the proposed emergency power support strategy, a multi-input DC transmission system consisting of LCC-HVDC1, LCC-HVDC2 and VSC-HVDC is built in the embodiment of the present invention. The DC blocking of LCC-HVDC1 causes a power shortage in the receiving power grid, and emergency power support is performed using the non-blocking DC lines VSC-HVDC and LCC-HVDC2 with a long-term overload capacity of 1.1 times. The DC rated power of the non-blocking DC lines VSC-HVDC and LCC-HVDC2 is 1000MW, and the power support rate is set to 3000MW / s.

[0042] Working condition I: The initial operating power of the faulty DC line LCC-HVDC1 is set to 500MW, and the initial operating power of the non-blocked DC lines VSC-HVDC and LCC-HVDC2 is set to 700MW. At 2s, DC blocking occurs in LCC-HVDC1. Figure 3 This is the simulation waveform of emergency power support provided only by the non-blocked DC line LCC-HVDC2. It can be seen that the grid frequency remains at around 49.5Hz after the fault transient. However, due to the additional reactive power loss of the converter station caused by LCC-HVDC2 in active power support, the bus voltage of the converter station fluctuates.

[0043] Condition II: Maintain 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. After 2s, LCC-HVDC1 is DC blocked, 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 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 DC blocked at 2s. The emergency power support required by the system at this time is P shortGreater than the maximum power support of VSC-HVDC P VSC_th , VSC-HVDC and LCC-HVDC2 jointly provide emergency power support. Figure 5 The waveform of the emergency power support strategy based on the coordination of VSC-HVDC and LCC-HVDC is simulated. It can be seen that the system frequency can be kept stable after the fault transient state under the emergency power support strategy. And compared with the voltage waveform of VSC-HVDC only providing active power support, VSC-HVDC providing both active and reactive power support can suppress the bus voltage fluctuation of the converter station caused by LCC-HVDC power support.

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

[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 memory.

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

[0050] Example 3

[0051] Embodiment 3 of the present invention provides a computer-readable storage medium corresponding to the above 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 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 present application can adopt the form of complete hardware embodiments, complete software embodiments, or embodiments in combination with software and hardware. Moreover, the present 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 embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal 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 generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. 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 operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0056] Although the preferred embodiments of the present application have been described, those skilled in the art may make other 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 falling within the scope of the present application.

[0057] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also 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 primary frequency modulation, P L Active power change caused by load regulation effect; S2. Determine the emergency active power support amount 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-blocked 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-blocked DC line VSC-HVDC VSC : Q' LCC To support active power, the AC bus inverter station absorbs reactive power, Q LCC It absorbs reactive power on 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 in claim 2, the large signal stability criterion is: in, L, 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 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 grid voltage of the rectifier station; is the DC voltage per unit value.

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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