Voltage control method for flexible HVDC system based on controllable area of receiving converter station

Abstract

The present application relates to the field of power system protection and control, in particular to a voltage control method for a flexible HVDC system based on a controllable area of a receiving end converter station, comprising collecting active and reactive power and AC bus voltage amplitude of the receiving end converter station at the moment of a fault, establishing an equivalent model of the receiving end power grid and calculating an equivalent fault transition resistance; calculating active and reactive power control reference values of the receiving end converter station and updating the current active and reactive power control reference values of the receiving end converter station; calculating an unloading action time according to the controllable area of the receiving end converter station; when the current time is greater than the unloading action time, an unloading circuit is put into operation until the DC voltage of the flexible HVDC system drops to 1.03 times the rated voltage; the present application can maximize the extension of the DC voltage out-of-limit time while meeting the safety and stability requirements of the receiving end power grid, and can avoid and reduce the number of unloading as much as possible.

Description

Technical Field

[0001] This invention relates to the field of power system protection and control, and specifically to a voltage control method for flexible DC transmission systems based on the controllable domain of the receiving-end converter station. Background Technology

[0002] Flexible DC transmission systems can provide active power support to AC grids and dynamically compensate for reactive power. Adjusting the power output of flexible DC transmission systems to improve the stability of the sending and receiving grids has become an important means of operating and controlling hybrid AC / DC grids. However, under grid fault conditions at the receiving end, the active power of the sending and receiving converter stations is no longer equal. As the power imbalance time increases, the unbalanced power is continuously stored in the DC capacitor in the form of electric field energy, causing the DC voltage to gradually rise. DC overvoltage may induce damage to power semiconductor devices or even block the flexible DC transmission system, threatening the stability of the sending and receiving grids.

[0003] Dissipating unbalanced power through unloading is the most common method to avoid DC overvoltage. However, centralized DC unloading has drawbacks such as large voltage change rate and difficulty in voltage equalization of switching devices. Technicians have made improvements in the installation location, topology, and control algorithm of unloading. They analyzed the transfer mechanism of unbalanced power under AC faults and proposed an AC-side energy dissipation method based on bidirectional thyristor valve groups and an onshore AC-side energy dissipation method suitable for offshore wind power transmission systems. Based on modular multilevel converters, they proposed a flexible energy dissipation topology based on unidirectional current H-bridge submodules and a centralized DC chopper topology based on coupling transformers. They also proposed an unloading control algorithm based on nearest-level approximation modulation, improving DC voltage control accuracy. However, unloading triggers large pulse currents, and the dissipation of unbalanced power as heat increases heat dissipation costs and is detrimental to active power absorption. Especially in multi-terminal flexible DC transmission systems or DC grids, the current rise rate and peak value flowing through unloading are significantly increased, making the risks of unloading considerable.

[0004] In summary, unloading helps suppress DC voltage rise. However, unloading causes temporary short circuits at both the sending-end and receiving-end converter stations, potentially leading to a loss of control of the flexible DC transmission system. The receiving-end converter station's DC voltage control capability should be fully utilized to minimize unloading while ensuring DC voltage safety. The receiving-end converter station's DC voltage control capability depends on its reactive power and maximum permissible AC current. If relying solely on the coordinated active and reactive power of the receiving-end converter station is insufficient to prevent DC overvoltage, unloading should be further utilized in conjunction with the receiving-end converter station.

[0005] How to characterize the controllable domain of active and reactive power of the receiving-end converter station under the requirements of DC voltage safety and the safety and stability of the receiving-end power grid, so as to determine the control reference values ​​of active and reactive power of the receiving-end converter station and coordinate them with unloading switching, so as to minimize or reduce the number of unloading times, has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0006] To address the shortcomings of existing technologies and effectively solve the problem of characterizing the controllable domain of the receiving-end converter station under receiving-end grid faults, this invention proposes a voltage control method for flexible DC transmission systems based on the controllable domain of the receiving-end converter station. This method aims to maximize the DC voltage over-limit time while meeting the safety and stability requirements of the receiving-end grid and minimizing the number of load shedding operations. The method includes establishing an equivalent model of the receiving-end grid to obtain the controllable domain of the active and reactive power of the receiving-end converter station. By controlling the active and reactive power of the receiving-end converter station within this controllable domain, the method maximizes the DC voltage over-limit time while meeting the safety and stability requirements of the receiving-end grid. Figure 1 Specifically, it includes the following steps:

[0007] S101. Monitor the AC bus voltage of the receiving-end converter station in real time. When the AC bus voltage drops below 0.95 times the rated voltage, determine that a fault has occurred in the receiving-end power grid and execute S102. Otherwise, continue to monitor the AC bus voltage of the receiving-end converter station.

[0008] S102: Collect the active and reactive power of the receiving-end converter station and the amplitude of the AC bus voltage at the moment of the fault, establish an equivalent model of the receiving-end power grid, and calculate the equivalent fault transition resistance.

[0009] S103: Calculate and change the active and reactive power control reference values ​​of the receiving-end converter station;

[0010] S104: Calculate the unloading action time;

[0011] S105: Determine if the current time is greater than the unloading action time. If the current time is greater than the unloading action time, execute S106.

[0012] S106: Engage the unloading circuit until the DC voltage of the flexible DC transmission system drops to 1.03 times the rated voltage.

[0013] Furthermore, the equivalent model of the receiving-end power grid is a two-port network consisting of a constant voltage source, a series reactive branch, and an equivalent fault transition resistance connected in parallel.

[0014] Furthermore, the calculation of the equivalent fault transition resistance includes:

[0015]

[0016] in, This is the equivalent fault transition resistance. , and These are parameters determined based on the equivalent model of the receiving-end power grid.

[0017] Furthermore, parameters are determined based on the equivalent model of the receiving-end power grid. , and The process includes:

[0018]

[0019]

[0020]

[0021] in, This represents the amplitude of the AC bus voltage at the receiving end of the converter station at the moment of the fault. This represents the active power of the receiving-end converter station at the instant of the fault. This represents the reactive power of the receiving-end converter station at the instant of the fault. The equivalent reactance of the receiving-end power grid, To connect the equivalent reactance of the transformer, This represents the equivalent potential amplitude of the receiving-end power grid.

[0022] Furthermore, the process of calculating the active and reactive power control reference values ​​for the receiving-end converter station includes:

[0023]

[0024]

[0025] in, This is the active power control reference value for the receiving-end converter station. This is the reactive power control reference value for the receiving-end converter station. A, B, and C are the minimum reactive power required by the receiving-end power grid to be provided by the receiving-end converter station, and are parameters determined based on the equivalent model of the receiving-end power grid.

[0026] Furthermore, the unloading action time is defined as the power imbalance time when the intersection of the controllable domain of the receiving-end converter station that ensures DC voltage safety and the controllable domain of the receiving-end converter station that meets the safety and stability requirements of the receiving-end power grid contains only one active and reactive power operating point of the receiving-end converter station, and the reactive power at this operating point is exactly equal to the minimum reactive power required to be provided by the receiving-end converter station. The unloading action time is expressed as:

[0027]

[0028]

[0029]

[0030]

[0031] in, For the unloading action time, This represents the initial active power of the power-sending converter station before the fault. The equivalent capacitance of a flexible DC transmission system; Rated DC voltage; The maximum allowable voltage coefficient is represented by A, B, and C, which are parameters determined based on the equivalent model of the receiving-end power grid.

[0032] Furthermore, the controllable domain of the receiving-end converter station that ensures DC voltage safety is:

[0033]

[0034] in, The active power of the receiving-end converter station. This represents the power imbalance time.

[0035] Furthermore, the controllable domain of the receiving-end converter station that meets the security and stability requirements of the receiving-end power grid is:

[0036]

[0037] in, The active power of the receiving-end converter station. Let A be the reactive power of the receiving-end converter station, and let B, C be the parameters determined based on the equivalent model of the receiving-end power grid.

[0038] Furthermore, the calculation of parameters A, B, and C, determined based on the equivalent model of the receiving-end power grid, includes:

[0039]

[0040]

[0041]

[0042] in, This represents the maximum permissible AC current coefficient for the receiving-end converter station. This is the rated AC current of the receiving-end converter station.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] 1. Existing technologies are limited by the difficulty in obtaining fault parameters of the receiving-end power grid, making it impossible to characterize and fully utilize the DC voltage control capability of the receiving-end converter station, which may lead to an increase in the number of unloading operations during the duration of the receiving-end power grid fault. This invention establishes an equivalent model of the receiving-end power grid and calculates the equivalent fault transition resistance, clearly defining the controllable domain of the receiving-end converter station that ensures DC voltage safety and the safety and stability requirements of the receiving-end power grid. By enabling the receiving-end converter station to provide the maximum active power under the maximum allowable AC current limit while meeting the minimum reactive power requirements of the receiving-end power grid, the amount of unloading during the duration of the receiving-end power grid fault can be reduced.

[0045] 2. Existing technologies avoid DC overvoltage under power grid faults at the receiving end by unloading, but unloading triggers large pulse currents, and the unbalanced power is dissipated as heat, which increases heat dissipation costs and is not conducive to active power absorption, thus its application limitations are becoming increasingly apparent. This invention considers the interaction between the DC voltage safety margin and the active and reactive power control strategies of the receiving-end converter station, and characterizes the controllable domain of the receiving-end converter station to ensure DC voltage safety and the safety and stability of the receiving-end power grid. Based on the characteristic that the controllable domain of the receiving-end converter station decreases with the power imbalance time, the unloading action time is set to avoid unnecessary unloading inputs. Attached Figure Description

[0046] Figure 1 A flowchart of a voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station;

[0047] Figure 2 This is a schematic diagram of a flexible DC transmission system under a receiving-end grid fault.

[0048] Figure 3 The figures are effect diagrams under the embodiments of the present invention, wherein (a) is a graph showing the relationship between time and DC voltage, (b) is a graph showing the relationship between time and AC bus voltage, (c) is a graph showing the relationship between time and active power of the receiving-end converter station, and (d) is a graph showing the relationship between time and reactive power of the receiving-end converter station. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] This invention proposes a voltage control method for flexible DC transmission systems based on the controllable domain of the receiving-end converter station. This method includes establishing an equivalent model of the receiving-end power grid to obtain the controllable domain of the active and reactive power at the receiving-end converter station. By controlling the active and reactive power of the receiving-end converter station within this controllable domain, the method aims to maximize the extension of the DC voltage over-limit time while meeting the safety and stability requirements of the receiving-end power grid. Figure 1 Specifically, it includes the following steps:

[0051] S101. Monitor the AC bus voltage of the receiving-end converter station in real time. When the AC bus voltage drops below 0.95 times the rated voltage, determine that a fault has occurred in the receiving-end power grid and execute S102. Otherwise, continue to monitor the AC bus voltage of the receiving-end converter station.

[0052] S102: Collect the active and reactive power of the receiving-end converter station and the amplitude of the AC bus voltage at the moment of the fault, establish an equivalent model of the receiving-end power grid, and calculate the equivalent fault transition resistance.

[0053] S103: Calculate and change the active and reactive power control reference values ​​of the receiving-end converter station;

[0054] S104: Calculate the unloading action time;

[0055] S105: Determine if the current time is greater than the unloading action time. If the current time is greater than the unloading action time, execute S106.

[0056] S106: Engage the unloading circuit until the DC voltage of the flexible DC transmission system drops to 1.03 times the rated voltage.

[0057] In the specific implementation of this invention, the equivalent model of the receiving-end power grid is a two-port network consisting of a constant voltage source, a series reactive branch, and an equivalent fault transition resistance connected in parallel.

[0058] In the specific implementation of this invention, the voltage of the voltage source is the rated voltage of the receiving-end power grid, the series reactance is the equivalent reactance of the receiving-end power grid under normal operation, and the equivalent fault transition resistance is determined as follows:

[0059]

[0060] in, This is the equivalent fault transition resistance. , and These are parameters determined based on the equivalent model of the receiving-end power grid.

[0061] parameter , and Calculate using the following formula:

[0062]

[0063]

[0064]

[0065] in, This represents the amplitude of the AC bus voltage at the receiving end of the converter station at the moment of the fault. This represents the active power of the receiving-end converter station at the instant of the fault. This represents the reactive power of the receiving-end converter station at the instant of the fault. The equivalent reactance of the receiving-end power grid, To connect the equivalent reactance of the transformer, This represents the equivalent potential amplitude of the receiving-end power grid.

[0066] In the specific implementation of this invention, the active power and reactive power control reference values ​​of the receiving-end converter station are determined according to the following method:

[0067]

[0068]

[0069] in, This is the active power control reference value for the receiving-end converter station. This is the reactive power control reference value for the receiving-end converter station. A, B, and C are the minimum reactive power required by the receiving-end power grid to be provided by the receiving-end converter station, and are parameters determined based on the equivalent model of the receiving-end power grid.

[0070] In the specific implementation of this invention, the process of determining parameters A, B, and C based on the equivalent model of the receiving-end power grid includes:

[0071]

[0072]

[0073]

[0074] in, This represents the maximum permissible AC current coefficient for the receiving-end converter station. This is the rated AC current of the receiving-end converter station.

[0075] In the specific implementation of this invention, the unloading action time is determined so that the intersection of the controllable domain of the receiving-end converter station that ensures DC voltage safety and the controllable domain of the receiving-end converter station that meets the safety and stability requirements of the receiving-end power grid contains only one active and reactive power operating point of the receiving-end converter station, and the reactive power of this operating point is exactly equal to the power imbalance time when the receiving-end converter station is required to provide the minimum reactive power. It is determined as follows:

[0076]

[0077] in, For the unloading action time, parameters Calculate using the following formula:

[0078]

[0079] in, The initial active power of the converter station at the sending end before the fault occurred;

[0080] parameter Calculate using the following formula:

[0081]

[0082] in, The equivalent capacitance of a flexible DC transmission system; Rated DC voltage; This is the maximum allowable voltage coefficient;

[0083] parameter Calculate using the following formula:

[0084] .

[0085] In the specific implementation of this invention, the controllable region of the receiving-end converter station that ensures DC voltage safety is determined by the following inequality:

[0086]

[0087] in, The active power of the receiving-end converter station. This represents the power imbalance time.

[0088] In the specific implementation of this invention, the controllable domain of the receiving-end converter station that meets the safety and stability requirements of the receiving-end power grid is determined by the following inequality:

[0089]

[0090] in, The active power of the receiving-end converter station. Let A be the reactive power of the receiving-end converter station, and let B, C be the parameters determined based on the equivalent model of the receiving-end power grid.

[0091] To verify the effectiveness of the present invention, as follows Figure 2 The following analysis uses a schematic diagram of a flexible DC transmission system under a receiving-end grid fault as an example. Figure 2In this embodiment, the flexible DC transmission system adopts a pseudo-bipolar structure with a maximum allowable voltage coefficient of 1.1 and a maximum allowable current coefficient of 1.2. During normal operation, the sending-end converter station employs constant active and reactive power control, while the receiving-end converter station employs constant DC voltage and reactive power control.

[0092] In this embodiment, point f is taken at 2.15s (i.e. Figure 2 Middle U f The scenario is a three-phase short-circuit fault occurring at a location (where the fault is cleared after 150ms). To verify the effectiveness of the voltage control method for flexible DC transmission systems based on the controllable domain of the receiving-end converter station disclosed in this invention, the controllable group was configured to implement load shedding to achieve DC voltage control when the DC voltage rose to 1.06 pu. The DC voltage, the AC bus voltage of the receiving-end converter station, and the active and reactive power of the receiving-end converter station were recorded and analyzed.

[0093] In this embodiment, the equivalent fault transition resistance is calculated according to the formula in step S102 above. The calculated equivalent fault transition resistance is: ;

[0094] The active and reactive power reference values ​​of the receiving-end converter station are calculated according to the formula in step S103 above. The calculated active power control reference value of the receiving-end converter station is 0.88pu and the reactive power control reference value of the receiving-end converter station is 0.12pu.

[0095] The unloading action time is calculated according to the formula in step S104 above, and the calculated unloading action time is 2.367s.

[0096] Figure 3 Figures (a) to (d) show the waveforms of DC voltage, AC bus voltage of the receiving-end converter station, active power, and reactive power of the flexible DC transmission system under a three-phase short-circuit fault, respectively. The horizontal axis represents time, and the vertical axis represents DC voltage, AC bus voltage of the receiving-end converter station, active power, and reactive power of the receiving-end converter station, respectively. The solid lines represent the curves of the flexible DC transmission system voltage control method based on the controllable domain of the receiving-end converter station disclosed in this invention, and the dashed lines represent the curves of the comparison group.

[0097] like Figure 3 As shown in (a), the DC voltage continuously rises during the receiving-end grid fault. The DC voltage change trend of the method of this invention and the method of the control group are basically the same before 2.249s. When the DC voltage reaches 1.06pu, the control group is started. The receiving-end grid fault is cleared at 2.3s, and the AC bus voltage gradually recovers. The method of this invention does not engage unloading during the fault duration. At the time of fault clearing, the DC voltage rises to the maximum value of 1.09pu.

[0098] Depend on Figure 3 As shown in (b) to (d), the reactive power and AC bus voltage at the receiving-end converter station are very similar under both methods. However, in the comparison group, the DC voltage oscillates due to repeated switching during unloading. Simultaneously, unloading conduction causes polar-to-ground faults in the flexible DC transmission system, resulting in fluctuations in active power at both the sending-end and receiving-end converter stations. This invention, by optimizing the unloading action time, can maximize the extension of the DC voltage over-limit time while meeting the safety and stability requirements of the receiving-end power grid, thereby minimizing and avoiding the number of unloading operations.

[0099] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A voltage control method for flexible DC transmission systems based on the controllable domain of the receiving-end converter station, characterized in that, Specifically, the following steps are included: S101: Monitor the AC bus voltage of the receiving-end converter station in real time. When the AC bus voltage drops below 0.95 times the rated voltage, it is determined that a fault has occurred in the receiving-end power grid, and S102 is executed. Otherwise, continue to monitor the AC bus voltage of the receiving-end converter station. S102: Collect the active and reactive power of the receiving-end converter station and the amplitude of the AC bus voltage at the moment of the fault, establish an equivalent model of the receiving-end power grid, and calculate the equivalent fault transition resistance. S103: Calculate the active power and reactive power control reference values ​​of the receiving-end converter station, and update the current active power and reactive power control reference values ​​of the receiving-end converter station. S104: Calculate the unloading action time based on the controllable domain of the receiving-end converter station; the unloading action time is the power imbalance time when the intersection of the controllable domain of the receiving-end converter station that ensures DC voltage safety and the controllable domain of the receiving-end converter station that meets the safety and stability requirements of the receiving-end power grid contains only one active and reactive power operating point of the receiving-end converter station, and the reactive power of this operating point is exactly equal to the minimum reactive power required to be provided by the receiving-end converter station. The unloading action time is expressed as: ； ； ； ； in, For the unloading action time, This represents the initial active power of the power-sending converter station before the fault. The equivalent capacitance of a flexible DC transmission system; Rated DC voltage; The maximum allowable voltage coefficient is represented by A, B, and C, which are parameters determined based on the equivalent model of the receiving-end power grid. S105: Determine if the current time is greater than the unloading action time. If the current time is greater than the unloading action time, execute S106. S106: Engage the unloading circuit until the DC voltage of the flexible DC transmission system drops to 1.03 times the rated voltage.

2. The voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station according to claim 1, characterized in that, The equivalent model of the receiving-end power grid is a two-port network consisting of a constant voltage source, a series reactive branch, and an equivalent fault transition resistance connected in parallel.

3. The voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station according to claim 2, characterized in that, The calculation of the equivalent fault transition resistance includes: in, This is the equivalent fault transition resistance. , and These are parameters determined based on the equivalent model of the receiving-end power grid.

4. The voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station according to claim 3, characterized in that, Parameters are determined based on the equivalent model of the receiving-end power grid. , and The process includes: in, This represents the amplitude of the AC bus voltage at the receiving end of the converter station at the moment of the fault. This represents the active power of the receiving-end converter station at the instant of the fault. This represents the reactive power of the receiving-end converter station at the instant of the fault. The equivalent reactance of the receiving-end power grid, To connect the equivalent reactance of the transformer, This represents the equivalent potential amplitude of the receiving-end power grid.

5. The voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station according to claim 1, characterized in that, The process of calculating the active and reactive power control reference values ​​for the receiving-end converter station includes: in, This is the active power control reference value for the receiving-end converter station. This is the reactive power control reference value for the receiving-end converter station. A, B, and C are the minimum reactive power required by the receiving-end power grid to be provided by the receiving-end converter station, and are parameters determined based on the equivalent model of the receiving-end power grid.

6. The voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station according to claim 1, characterized in that, The controllable domain of the receiving-end converter station that ensures DC voltage safety is: in, The active power of the receiving-end converter station. This represents the power imbalance time.

7. The voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station according to claim 1, characterized in that, The controllable domain of the receiving-end converter station that meets the safety and stability requirements of the receiving-end power grid is: in, The active power of the receiving-end converter station. Let A be the reactive power of the receiving-end converter station, and let B, C be the parameters determined based on the equivalent model of the receiving-end power grid.

8. The voltage control method for a flexible DC transmission system based on the controllable domain of the receiving-end converter station according to claim 1, 5, or 7, characterized in that, The calculation of parameters A, B, and C, determined based on the equivalent model of the receiving-end power grid, includes: in, This represents the maximum permissible AC current coefficient for the receiving-end converter station. The rated AC current of the receiving-end converter station, The equivalent reactance of the receiving-end power grid, This is the equivalent fault transition resistance. To connect the equivalent reactance of the transformer, This represents the equivalent potential amplitude of the receiving-end power grid.

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