Fault ride-through method and device for embedded flexible direct current system network construction type current converter

Abstract

The embodiment of the invention relates to the technical field of flexible direct-current power transmission, in particular to a fault ride-through method and device for an embedded flexible direct-current system grid-forming type converter, and the method comprises the steps: adaptively adjusting a reactive power instruction value of grid-forming type control when a fault of an alternating-current system is detected, and calculating the reactive power adjustment amount during the fault period, thereby achieving the fault ride-through of the grid-forming type converter. And adding the reactive power adjusting quantity and the reactive power instruction value before the fault occurs to obtain the reactive power instruction value during the fault period, and adaptively adjusting the reactive power instruction value after the fault is detected instead of fixing an original instruction or being driven by droop control, so that the control system always works in a linear controllable area, and the control efficiency is improved. And overcurrent and controller saturation are avoided, so that the stability and waveform quality of the output voltage of the converter during the fault period are ensured, and the transient stability of the system is maintained.

Description

Technical Field

[0001] This application relates to the field of flexible DC transmission technology, and in particular to a fault ride-through method and apparatus for a grid-type converter in an embedded flexible DC system. Background Technology

[0002] With the continuous development of new energy technologies, large-scale external hydropower, wind power, and solar power will continue to be connected to the power grid, which will have a significant impact on the dynamic and static characteristics and power flow distribution of the power grid. At the same time, the risk of faults increases in areas with a high proportion of new energy connections, necessitating research into fault ride-through problems under scenarios of large-scale new energy integration. With the acceleration of urbanization, the power grid load is showing a rapid and intensive growth trend, while the electrical distance between power generation centers and load centers is relatively long, leading to a continuous increase in long-distance centralized large-scale transmission capacity. The overall economic development level of the power grid is high, the demand for electricity supply is large, and the problem of multiple DC feeds is significant. Load growth places higher demands on the power grid layout, structure, and capacity.

[0003] In applications such as isolated power supply and renewable energy grid integration, embedded flexible DC transmission systems often experience frequent AC faults in the AC system connected to the flexible DC converter due to the randomness and volatility of renewable energy access. This can lead to renewable energy disconnection from the grid and even large-scale power outages, seriously jeopardizing the stable operation of the power system. Therefore, there is an urgent need to develop fault ride-through strategies adapted to embedded flexible DC transmission systems. Currently, flexible DC converters operate primarily using two control methods: one is grid-following control, which tracks the frequency and phase information of the grid connection point voltage to achieve power transfer between renewable energy and the grid, exhibiting passive following characteristics. However, renewable energy generation is random, intermittent, and volatile, making the grid highly susceptible to voltage and frequency fluctuations. The other is grid-building control, which simulates the voltage and frequency regulation characteristics of synchronous generators to provide the system with virtual flexible DC transmission. Using voltage source converters, it can independently adjust active and reactive power output, improving the transmission capacity of the AC system and easily forming a DC grid. It has significant competitiveness in applications such as renewable energy generation and grid integration, isolated city power supply, and AC system interconnection.

[0004] However, when dealing with AC faults, the grid-type control uses reactive power-voltage droop control in the reactive power control loop. When a fault occurs, it leads to AC voltage support, controller saturation and loss of control, current overcurrent, resulting in unstable output AC voltage, high harmonic content, and easy divergence and instability, which affects the fault ride-through performance of new energy sources. Summary of the Invention

[0005] To address this issue, the present invention provides a fault ride-through method and apparatus for a grid-type converter in an embedded flexible DC system. This method overcomes the problems in the prior art where, when dealing with AC faults, the grid-type control uses reactive power-voltage droop control in the reactive power control loop. This leads to AC voltage support, controller saturation and loss of control, current overcurrent, unstable output AC voltage, high harmonic content, and easy divergence and instability, thus affecting the fault ride-through performance of new energy sources.

[0006] In a first aspect, the present invention provides a fault ride-through method for an embedded flexible DC system grid-type converter, comprising:

[0007] Step S1: Detect the real-time operating status of the AC system. When a fault is detected in the AC system, adaptively adjust the reactive power command value of the grid-type control.

[0008] Step S2: Calculate the reactive power adjustment during the fault period, and add the reactive power adjustment to the reactive power command value before the fault occurred to obtain the reactive power command value during the fault period.

[0009] Step S3: Use the calculated reactive power command value during the fault period as the target value of the converter output reactive power.

[0010] Step S4: After the AC system fault is detected to have disappeared, the reactive power command value is restored to the original reactive power command value before the fault, either by the set recovery rate or by a direct step.

[0011] Further, in step S1, the adaptive adjustment of the reactive power command value of the grid-type control includes:

[0012] The reactive power command value is adaptively adjusted according to the magnitude of the AC voltage.

[0013] Furthermore, the adaptive adjustment of the reactive power command value based on the magnitude of the AC voltage includes:

[0014] Set the AC voltage threshold;

[0015] When the AC voltage is detected to be greater than or equal to the AC voltage threshold, it is determined that no AC fault has occurred, the reactive power command value is not adjusted, and the real-time operating status of the AC system is continuously monitored.

[0016] When the AC voltage is detected to be lower than the AC voltage threshold, an AC fault is determined to have occurred, and reactive power command value adjustment is initiated.

[0017] Further, in step S2, calculating the reactive power adjustment during the fault includes:

[0018] The AC voltage difference is obtained by subtracting the AC voltage before the fault from the AC voltage after the fault.

[0019] Multiply the AC voltage difference by the reactive power adjustment coefficient to obtain the reactive power adjustment during the fault period.

[0020] Furthermore, the method for calculating the reactive power regulation coefficient is as follows:

[0021] ;

[0022] In the formula, k is the reactive power regulation coefficient, and Q rated U0 is the rated reactive power value, and U0 is the rated AC voltage value.

[0023] Furthermore, in step S2, the method for calculating the reactive power command value during the fault period is as follows:

[0024] ;

[0025] In the formula, This represents the reactive power command value during the fault period. This is the reactive power command value before the fault occurred. This refers to the reactive power adjustment during a fault.

[0026] Furthermore, the calculation steps for the reactive power regulation amount also include:

[0027] The AC voltage difference is filtered, and the reactive power adjustment is calculated using the AC voltage difference after low-pass filtering.

[0028] Furthermore, in step S4, the method for determining whether the AC system fault has disappeared includes:

[0029] When the detected AC voltage is greater than the AC voltage threshold, it is determined that the AC system fault has disappeared.

[0030] Further, in step S1, the adaptive adjustment of the reactive power command value of the grid-type control includes:

[0031] The voltage reference value is calculated using a virtual synchronous machine control algorithm based on the instantaneous active power, reactive power, active power reference value, and reactive power reference value.

[0032] Based on the voltage reference value, the modulation voltage is obtained using voltage and current dual closed-loop control;

[0033] The control signal for the converter is generated based on the modulation voltage, and the converter is driven to perform corresponding control actions.

[0034] Secondly, embodiments of this application provide an embedded flexible DC system grid-type converter fault ride-through device, comprising:

[0035] The monitoring module is used to detect the real-time operating status of the AC system;

[0036] An analysis module, which is connected to the monitoring module, is used to adaptively adjust the reactive power command value of the grid-type control when the monitoring module detects a fault in the AC system, and to restore the reactive power command value to the original reactive power command value before the fault by a set recovery rate or by a direct step after the AC system fault is detected to have disappeared.

[0037] The calculation module, which is connected to both the monitoring module and the analysis module, is used to calculate the reactive power adjustment during the fault period based on the analysis results of the analysis module. The reactive power adjustment is added to the reactive power command value before the fault occurs to obtain the reactive power command value during the fault period. The calculated reactive power command value during the fault period is used as the target value of the reactive power output of the converter.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows: when a fault is detected in the AC system, the reactive power command value of the grid-type control is adaptively adjusted, and the reactive power adjustment amount during the fault period is calculated. The reactive power adjustment amount is added to the reactive power command value before the fault occurs to obtain the reactive power command value during the fault period. The reactive power command value is adaptively adjusted after the fault is detected, instead of sticking to the original command or letting the droop control drive it. This makes the control system always work in the linear controllable region, avoids current overcurrent and controller saturation, thereby ensuring the stability of the converter output voltage and waveform quality during the fault period and maintaining the transient stability of the system.

[0039] Furthermore, in this invention, faults can be determined simply by comparing the real-time AC voltage with the AC voltage threshold. Voltage is one of the most basic and easily measured state variables in a power system, which improves the reliability of the fault detection process. Voltage drop is one of the most direct and fastest indicators of AC faults. Voltage-based detection does not require complex calculations or delays and can identify the occurrence and disappearance of faults almost instantaneously, thus gaining valuable time for subsequent control and regulation.

[0040] Furthermore, this invention takes into account that instantaneous noise, harmonics, or small disturbances are inevitable in grid voltage measurement. Directly using the original voltage difference to calculate ΔQ will cause frequent fluctuations in reactive power commands, leading to oscillations in the control system. Filtering can remove high-frequency noise and invalid disturbances, ensuring that the voltage difference used for core calculation is a stable and reliable representative signal, avoiding erroneous adjustments, ensuring the stability and accuracy of reactive power commands, and making the entire fault ride-through control strategy more reliable and robust, capable of coping with complex actual engineering environments. Attached Figure Description

[0041] Figure 1This is a schematic diagram of a multi-terminal embedded flexible DC transmission system according to an embodiment of this application;

[0042] Figure 2 This is a schematic diagram of the fault ride-through method for a grid-type converter according to an embodiment of this application;

[0043] Figure 3 This is a flowchart illustrating the adjustment of reactive power command values ​​in an embodiment of this application. Detailed Implementation

[0044] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0045] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0046] Please see Figures 1-3 As shown, Figure 1 This is a schematic diagram of a multi-terminal embedded flexible DC transmission system according to an embodiment of this application. Figure 2 This is a schematic diagram of the fault ride-through method for a grid-type converter according to an embodiment of this application. Figure 3 This is a flowchart illustrating the adjustment of reactive power command values ​​in an embodiment of this application.

[0047] The technical solution provided in this application includes the following steps:

[0048] Step S1: Detect the real-time operating status of the AC system. When a fault is detected in the AC system, adaptively adjust the reactive power command value of the grid-type control.

[0049] Step S2: Calculate the reactive power adjustment during the fault period, and add the reactive power adjustment to the reactive power command value before the fault occurred to obtain the reactive power command value during the fault period.

[0050] Step S3: Use the calculated reactive power command value during the fault period as the target value of the converter output reactive power.

[0051] Step S4: After the AC system fault is detected to have disappeared, restore the reactive power command value to the original reactive power command value before the fault.

[0052] Specifically, in this embodiment, a converter controller is used to detect the AC voltage to determine whether a fault has occurred in the AC system.

[0053] Specifically, in this embodiment, the recovery rate is adjustable. Different recovery rates are configured according to the grid connection requirements of new energy sources to achieve gradual or step recovery of AC voltage.

[0054] In this invention, when a fault is detected in the AC system, the reactive power command value of the grid-type control is adaptively adjusted, and the reactive power adjustment amount during the fault period is calculated. The reactive power adjustment amount is added to the reactive power command value before the fault occurs to obtain the reactive power command value during the fault period. The reactive power command value is adaptively adjusted after the fault is detected, instead of sticking to the original command or letting the droop control drive it. This ensures that the control system always works in the linear controllable region, avoids current overcurrent and controller saturation, thereby ensuring the stability of the converter output voltage and waveform quality during the fault period and maintaining the transient stability of the system.

[0055] Specifically, in step S1, the adaptive adjustment of the reactive power command value of the grid-type control includes:

[0056] The reactive power command value is adaptively adjusted according to the magnitude of the AC voltage.

[0057] Specifically, the adaptive adjustment of the reactive power command value based on the magnitude of the AC voltage includes:

[0058] Set the AC voltage threshold;

[0059] When the AC voltage is detected to be greater than or equal to the AC voltage threshold, it is determined that no AC fault has occurred, the reactive power command value is not adjusted, and the real-time operating status of the AC system is continuously monitored.

[0060] When the AC voltage is detected to be lower than the AC voltage threshold, an AC fault is determined to have occurred, and reactive power command value adjustment is initiated.

[0061] In this invention, faults can be determined simply by comparing the real-time AC voltage with the AC voltage threshold. Voltage is one of the most basic and easily measured state variables in a power system, which improves the reliability of the fault detection process. Voltage drop is one of the most direct and fastest indicators of AC faults. Voltage-based detection does not require complex calculations or delays and can identify the occurrence and disappearance of faults almost instantaneously, thus gaining valuable time for subsequent control and regulation.

[0062] Specifically, in step S2, calculating the reactive power adjustment during the fault includes:

[0063] The AC voltage difference is obtained by subtracting the AC voltage before the fault from the AC voltage after the fault.

[0064] Multiplying the AC voltage difference by the reactive power regulation coefficient yields the reactive power regulation during the fault period, i.e.: .

[0065] Specifically, in this embodiment, the reactive power adjustment amount ;

[0066] In the formula, The AC voltage threshold for determining AC system faults can be set according to actual engineering requirements. This is the real-time measured value of AC voltage. This refers to the rated AC voltage. When the AC voltage is less than... At that time, it was determined that an AC system fault had occurred. .

[0067] Specifically, in this embodiment, when an AC voltage is detected... If this is the case, then the AC system fault is considered to have disappeared. The reactive power command value is normal.

[0068] Specifically, the calculation method for the reactive power regulation coefficient is as follows:

[0069] ;

[0070] In the formula, k is the reactive power regulation coefficient, used to ensure that the current does not flow during AC faults, and Q rated U0 is the rated reactive power value, and U0 is the rated AC voltage value.

[0071] Specifically, in step S2, the method for calculating the reactive power command value during the fault period is as follows:

[0072] ;

[0073] In the formula, This represents the reactive power command value during the fault period. This is the reactive power command value before the fault occurred. This refers to the reactive power adjustment during a fault.

[0074] Specifically, in this embodiment, the converter uses the calculated reactive power command value during the fault period as the converter output reactive power target.

[0075] Specifically, the calculation steps for the reactive power regulation amount also include:

[0076] The AC voltage difference is filtered, and the reactive power adjustment is calculated using the AC voltage difference after low-pass filtering.

[0077] In this invention, it is considered that there are inevitably instantaneous noises, harmonics, or small disturbances in the grid voltage measurement. Directly using the original voltage difference to calculate ΔQ will cause frequent fluctuations in reactive power commands, which will cause oscillations in the control system. The filtering process can filter out high-frequency noise and invalid disturbances, ensuring that the voltage difference used for core calculation is a stable, reliable, and representative signal. This avoids erroneous adjustments, ensures the stability and accuracy of reactive power commands, and makes the entire fault ride-through control strategy more reliable and robust, capable of coping with complex actual engineering environments.

[0078] Specifically, in step S4, the method for determining whether the AC system fault has disappeared includes:

[0079] When the detected AC voltage is greater than the AC voltage threshold, a 200ms delay is made to determine that the AC system fault has disappeared.

[0080] Specifically, in step S1, the adaptive adjustment of the reactive power command value of the grid-type control includes:

[0081] The voltage reference value is calculated using a virtual synchronous machine control algorithm based on the instantaneous active power, reactive power, active power reference value, and reactive power reference value.

[0082] Based on the voltage reference value, the modulation voltage is obtained using voltage and current dual closed-loop control;

[0083] The control signal for the converter is generated based on the modulation voltage, and the converter is driven to perform corresponding control actions.

[0084] The embedded flexible DC system grid-type converter fault ride-through device provided in this application embodiment includes:

[0085] The monitoring module is used to detect the real-time operating status of the AC system;

[0086] An analysis module, which is connected to the monitoring module, is used to adaptively adjust the reactive power command value of the grid-type control when the monitoring module detects a fault in the AC system, and to restore the reactive power command value to the original reactive power command value before the fault by a set recovery rate or by a direct step after the AC system fault is detected to have disappeared.

[0087] The calculation module, which is connected to both the monitoring module and the analysis module, is used to calculate the reactive power adjustment during the fault period based on the analysis results of the analysis module. The reactive power adjustment is added to the reactive power command value before the fault occurs to obtain the reactive power command value during the fault period. The calculated reactive power command value during the fault period is used as the target value of the reactive power output of the converter.

[0088] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A fault ride-through method for a grid-type converter in an embedded flexible DC system, characterized in that, include: Step S1: Detect the real-time operating status of the AC system. When a fault is detected in the AC system, adaptively adjust the reactive power command value of the grid-type control. Step S2: Calculate the reactive power adjustment during the fault period, and add the reactive power adjustment to the reactive power command value before the fault occurred to obtain the reactive power command value during the fault period. Step S3: Use the calculated reactive power command value during the fault period as the target value of the converter output reactive power. Step S4: After the AC system fault is detected to have disappeared, restore the reactive power command value to the original reactive power command value before the fault.

2. The method according to claim 1, characterized in that, In step S1, the reactive power command value of the adaptive grid-type control includes: The reactive power command value is adaptively adjusted according to the magnitude of the AC voltage.

3. The method according to claim 2, characterized in that, The adaptive adjustment of the reactive power command value based on the magnitude of the AC voltage includes: Set the AC voltage threshold; When the AC voltage is detected to be greater than or equal to the AC voltage threshold, it is determined that no AC fault has occurred, the reactive power command value is not adjusted, and the real-time operating status of the AC system is continuously monitored. When the AC voltage is detected to be lower than the AC voltage threshold, an AC fault is determined to have occurred, and reactive power command value adjustment is initiated.

4. The method according to claim 1, characterized in that, In step S2, calculating the reactive power adjustment during the fault includes: The AC voltage difference is obtained by subtracting the AC voltage before the fault from the AC voltage after the fault. Multiply the AC voltage difference by the reactive power adjustment coefficient to obtain the reactive power adjustment during the fault period.

5. The method according to claim 4, characterized in that, The method for calculating the reactive power regulation coefficient is as follows: ； In the formula, k is the reactive power regulation coefficient, and Q rated U0 is the rated reactive power value, and U0 is the rated AC voltage value.

6. The method according to claim 4, characterized in that, In step S2, the method for calculating the reactive power command value during the fault period is as follows: ; In the formula, This represents the reactive power command value during the fault period. This is the reactive power command value before the fault occurred. This refers to the reactive power adjustment during a fault.

7. The method according to claim 4, characterized in that, The calculation steps for the reactive power regulation amount also include: The AC voltage difference is filtered, and the reactive power adjustment is calculated using the AC voltage difference after low-pass filtering.

8. The method according to claim 1, characterized in that, In step S4, the method for determining whether the AC system fault has disappeared includes: When the detected AC voltage is greater than the AC voltage threshold, it is determined that the AC system fault has disappeared.

9. The method according to claim 1, characterized in that, In step S1, the reactive power command value of the adaptive grid-type control includes: The voltage reference value is calculated using a virtual synchronous machine control algorithm based on the instantaneous active power, reactive power, active power reference value, and reactive power reference value. Based on the voltage reference value, the modulation voltage is obtained using voltage and current dual closed-loop control; The control signal for the converter is generated based on the modulation voltage, and the converter is driven to perform corresponding control actions.

10. A fault ride-through device for an embedded flexible DC system grid-type converter, characterized in that, include: The monitoring module is used to detect the real-time operating status of the AC system; An analysis module, which is connected to the monitoring module, is used to adaptively adjust the reactive power command value of the grid-type control when the monitoring module detects a fault in the AC system, and to restore the reactive power command value to the original reactive power command value before the fault by a set recovery rate or by a direct step after the AC system fault is detected to have disappeared. The calculation module, which is connected to both the monitoring module and the analysis module, is used to calculate the reactive power adjustment during the fault period based on the analysis results of the analysis module. The reactive power adjustment is added to the reactive power command value before the fault occurs to obtain the reactive power command value during the fault period. The calculated reactive power command value during the fault period is used as the target value of the reactive power output of the converter.

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