Overvoltage suppression method based on intermediate-frequency DRU-MMC direct-current power transmission system

Abstract

The invention discloses an overvoltage suppression method based on an intermediate-frequency DRU-MMC direct-current power transmission system, and the method comprises the steps: improving the operation frequency of an offshore alternating-current system to an intermediate-frequency range (such as 100-200Hz), employing a diode uncontrolled rectification unit in a rectification station, and reducing the weight and size of equipment and a platform; aiming at the situation that direct-current voltage is raised due to the fact that power output of an intermediate-frequency DRU-MMC direct-current power transmission system is blocked due to faults of an onshore alternating-current system, an offshore alternating-current system voltage reduction strategy, an offshore alternating-current system frequency increasing strategy and a direct-current energy consumption device switching strategy are cooperatively called in a layered mode, a progressive protection mechanism is formed, and the recovery sequence of the strategies is emphasized. The method is easy to implement, high in reliability, capable of simulating the voltage change characteristic when the DRU-MMC system has the direct current side fault, high in universality and high in application value in actual engineering.

Description

Technical Field

[0001] This invention belongs to the field of power system transmission and distribution technology, specifically relating to an overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system. Background Technology

[0002] To date, most operational offshore wind farms use purely flexible DC transmission systems. However, under current technological conditions, flexible DC transmission systems based on MMC (Modular Multilevel Converter) have some unavoidable problems: offshore converter stations have numerous MMC sub-modules, large capacitor volumes, heavy offshore converter platforms, and high investment costs; in addition, the high structural and operational complexity of MMCs increases the probability of failure to some extent, resulting in relatively high operation and maintenance costs for offshore converter stations; these problems have become the main factors restricting offshore wind power DC transmission and grid connection schemes. Therefore, a promising low-cost offshore wind power transmission technology has been proposed at this stage, namely, the medium-frequency uncontrolled rectifier technology (see reference [Zhang Zheren, Tang Yingjie, Xu Zheng. Offshore wind power transmission scheme using medium-frequency uncontrolled rectifier DC system [J]. China Electric Power, 2020, 53(07):80-91]). By increasing the operating frequency of the offshore AC system to the medium-frequency range (e.g., 100-200Hz) and using DRU (diode uncontrolled rectifier unit) on the rectifier side, the weight and size of the equipment and platform are reduced. It has the advantages of simple sending-end system, small platform size, light weight and low cost.

[0003] However, there are still many challenges to overcome in the application of medium-frequency uncontrolled rectifier technology in actual offshore wind power transmission projects. One important issue is its overvoltage suppression strategy. Overvoltage not only affects the insulation strength of power equipment, but also relates to the safety and reliability of the transmission system, and its impact is particularly significant on medium-frequency DRU-MMC DC systems. The overvoltage suppression strategy of offshore wind power DC transmission systems based on medium-frequency uncontrolled rectifier technology is one of the key issues that it must face and urgently needs to solve.

[0004] For DC faults in medium-frequency DRU-MMC DC systems, since the DC lines use submarine cables, DC faults are generally considered permanent. In DC engineering, the strategy of first blocking the converter and then opening the AC circuit breaker is usually used to handle DC faults. For AC faults in medium-frequency DRU-MMC DC systems, since the probability of faults in the onshore AC grid is higher than that on the offshore side, research on AC fault crossing in medium-frequency DRU-MMC DC systems focuses on onshore grid faults. When a fault occurs in the onshore AC system, the system output power is blocked but the output power remains unchanged, resulting in a power surplus and a voltage rise. The main solutions for handling faults in the onshore AC system (i.e., the receiving end) include improving the grid-side topology, configuring DC energy dissipation devices, reducing the load on wind farms, and the coordinated use of these methods (see reference [Gong Yipin. Analysis and Coordinated Control of AC Faults at the Sending End of Offshore Wind Power Transmission System via Flexible DC [D]. Shandong University, 2023]). Currently, there is no established and mature operational experience to draw upon worldwide for research on overvoltage suppression strategies for medium-frequency DRU-MMC DC systems. However, overvoltage suppression strategies are crucial for the stable operation of medium-frequency DRU-MMC systems, thus requiring urgent research. Summary of the Invention

[0005] In view of the above, the present invention provides an overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system. It is simple to implement, highly reliable, can simulate the voltage change characteristics of the DRU-MMC system under DC-side faults, has strong versatility, and has application value in practical engineering.

[0006] An overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system is proposed. Specifically, the operating frequency of the offshore AC system is increased to the medium-frequency range (e.g., 100-200Hz). In response to the situation where the DC voltage rises due to the obstruction of power output of the DC transmission system caused by the failure of the onshore AC system, the offshore AC system voltage reduction strategy, the offshore AC system frequency increase strategy, and the DC energy consumption device switching strategy are called in a layered and coordinated manner to form a progressive protection mechanism. The overvoltage suppression of the DC transmission system is achieved through the logical interlocking and recovery sequence between the strategies.

[0007] Furthermore, the DC transmission system includes a grid-type offshore wind farm, a DRU-based offshore rectifier station, and an MMC-based onshore inverter station. The offshore wind farm contains multiple offshore wind turbines, which are connected to an AC bus via a medium-frequency AC collector submarine cable. The AC side of the offshore rectifier station is connected to the AC bus via a converter transformer. The DC side of the offshore rectifier station is connected to the DC side of the onshore inverter station via a high-voltage DC submarine cable. The AC side of the onshore inverter station is connected to the onshore power grid via a converter transformer. The offshore wind farm, the medium-frequency AC collector submarine cable, and the converter transformer of the offshore rectifier station constitute the offshore AC system. The AC side of the onshore inverter station, the converter transformer of the onshore inverter station, and the onshore power grid constitute the onshore AC system.

[0008] Furthermore, the aforementioned offshore AC system voltage reduction strategy reduces the DC voltage U of the DC transmission system by decreasing the d-axis voltage command value of the offshore wind turbine grid-side converter. dc Specifically: when U dc >1.1U dcN At that time, the d-axis voltage command value of the grid-side converter of the offshore wind turbine is set to U. f * d ', to quickly reduce the active power injected into DC lines by offshore wind turbines;

[0009]

[0010] in: and These are the d-axis voltage command values ​​of the offshore wind turbine grid-side converter before and after the fault, U dcN k is the rated DC voltage. fd This is the voltage regulation coefficient;

[0011] When U dc Restored to below 1.05U dcN At that time, the d-axis voltage command value of the offshore wind turbine grid-side converter will be readjusted to...

[0012] Furthermore, the voltage regulation coefficient k fd The expression:

[0013]

[0014] Among them: U max This represents the maximum DC voltage reached by the DC transmission system without any overvoltage suppression measures.

[0015] Furthermore, the DC power consumption device switching strategy involves installing DC power consumption devices in parallel on the DC side of the onshore inverter station. These devices consist of switching devices and power-consuming resistors connected in series. When the DC voltage U of the DC transmission system... dc Rise by more than 1.1U dcN When U is activated, the DC power dissipation device is put into operation, causing the surplus power to be consumed in the power dissipation resistor; when U dc Restored to below 1.05U dcN When necessary, disconnect the DC power-consuming device.

[0016] Furthermore, the expression for the resistance value R of the energy-consuming resistor is as follows:

[0017]

[0018] Where: P wind To reduce the output power of offshore wind farms after lowering the d-axis voltage command value of the grid-side converter of offshore wind turbines, U av This represents the average DC voltage when the DC power-consuming device is in operation during the fault period.

[0019] Furthermore, the aforementioned offshore AC system frequency upscaling strategy reduces the DC voltage U of the DC transmission system by increasing the AC system frequency command value of the offshore wind turbine grid-side converter. dc Specifically: when the voltage reduction strategy of the offshore AC system and the switching strategy of the DC power consumption device are activated, U dc The upward slope exceeds the preset threshold or U within a short period of time dc Still greater than 1.1U dcN The AC system frequency command value for the grid-side converter of the offshore wind turbine is set to f. * ':

[0020] f * '=f * +k f (U dc -U dcN )

[0021] Where: f * and f * 'These represent the AC system frequency command values ​​of the offshore wind turbine grid-side converter before and after the fault, U dcN k is the rated DC voltage. f This is the frequency adjustment coefficient;

[0022] When U dc Stable at U dcN Furthermore, the voltage reduction strategy for the offshore AC system and the switching strategy for DC power dissipation devices have both been discontinued, and the AC system frequency command value of the offshore wind turbine grid-side converter has been readjusted to f. * .

[0023] Furthermore, the frequency adjustment coefficient k f The expression:

[0024]

[0025] Among them: U max Δf represents the maximum DC voltage reached by the DC transmission system without any overvoltage suppression strategy. max This represents the maximum frequency variation of the maritime communication system.

[0026] Furthermore, the offshore AC system voltage reduction strategy and the DC energy dissipation device switching strategy are the fastest and most direct power regulation methods. Both should be activated simultaneously to form a combined force to quickly curb the rising trend of DC voltage, reduce the injection of offshore wind turbines, and actively release voltage from the energy dissipation resistors. If the DC voltage continues to rise or fails to fall effectively after the above two strategies are activated, the offshore AC system frequency increase strategy should be activated.

[0027] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the above-described overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system.

[0028] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system.

[0029] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0030] 1. For medium-frequency DRU-MMC DC transmission systems, this invention hierarchically and collaboratively invokes the voltage reduction strategy of the offshore AC system, the frequency increase strategy of the offshore AC system, and the switching strategy of DC energy-consuming devices to form a progressive protection mechanism. It also emphasizes the recovery sequence between strategies, filling the current research gap in overvoltage suppression strategies caused by power output obstruction, and can provide a certain reference for future engineering design.

[0031] 2. This invention has strong versatility. Theoretically, this overvoltage suppression strategy is not only applicable to DC systems based on medium-frequency uncontrolled rectifier technology, but also to many other types of power transmission systems. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the topology of the DRU-MMC DC transmission system in an embodiment of the present invention.

[0033] Figure 2 This is a schematic diagram of the pressure reduction control strategy for the marine AC system in an embodiment of the present invention.

[0034] Figure 3 This is a schematic diagram of the switching control strategy for DC power consumption devices in an embodiment of the present invention.

[0035] Figure 4 This is a schematic diagram of the frequency upsampling control strategy for a marine AC system in an embodiment of the present invention.

[0036] Figure 5 This is a flowchart illustrating the overvoltage suppression method for a DC transmission system in an embodiment of the present invention.

[0037] Figure 6 This is a schematic diagram of the DC voltage waveforms before and after the overvoltage suppression strategy is applied when the AC system at the receiving end fails in the verification example of this invention.

[0038] Figure 7 This is a schematic diagram of the system output power waveform after adopting an overvoltage suppression strategy when the AC system at the receiving end fails in the verification example of this invention.

[0039] Figure 8 This is a schematic diagram of the voltage waveform of the offshore AC system after adopting an overvoltage suppression strategy when the receiving-end AC system fails in a verification example of the present invention. Detailed Implementation

[0040] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] The DRU-MMC DC transmission system topology in this embodiment is as follows: Figure 1 As shown, the offshore wind farm transmits power to the onshore AC grid via a DRU rectifier station, a long-distance high-voltage DC submarine cable, and an MMC inverter station. The DRU-MMC high-voltage DC transmission system is used to realize power transmission. The offshore AC system operates in the medium frequency range (e.g., 100-200Hz). The rectifier station uses diode uncontrolled rectifier units to reduce the weight and size of equipment and platforms.

[0042] When a fault in the onshore AC system causes a power output interruption in the DRU-MMC DC transmission system, resulting in a rise in DC voltage, a layered and coordinated protection mechanism is implemented, incorporating the offshore AC system's voltage reduction strategy, frequency increase strategy, and DC power consumption device switching strategy. This forms a progressive protection mechanism, emphasizing logical interlocking and recovery sequence between strategies. The specific implementation process is as follows: Figure 5 As shown:

[0043] (1) The fan measures the DC voltage and compares it with a threshold value. When the DC voltage U dc >1.1U dcN (U dcN When the DC voltage is at its rated value, enable as follows: Figure 2The voltage reduction control strategy for the offshore AC system shown reduces the d-axis voltage command value of the input voltage controller in the wind turbine grid-side converter to:

[0044]

[0045] Where, k fd It can be designed as:

[0046]

[0047] In the formula: These are the d-axis voltage command values ​​of the input voltage controller in the grid-side converter of the wind turbine before and after the fault, U max This represents the maximum overvoltage that can be achieved without overvoltage suppression strategies.

[0048] When U dc Restored to below 1.05U dcN At that time, the d-axis voltage command value U of the input voltage controller in the grid-side converter of the wind turbine f * d Back to normal.

[0049] (2) Enable simultaneously, such as Figure 3 The DC power dissipation device switching control strategy shown first involves installing DC power dissipation devices on the DC side of the onshore inverter station. When the DC line voltage U... dc The voltage rises above the upper limit of the voltage threshold U H =1.1U dcN The DC power dissipation device is triggered to work, so that the surplus power is consumed in the power dissipation resistor.

[0050] The expression for calculating the resistance R of the energy-consuming resistor is as follows:

[0051]

[0052] Where: P wind To reduce the output power of offshore wind farms after the AC voltage command value, U av This represents the average DC voltage when the DC power-consuming device is in operation during the fault period.

[0053] When U dc Restored to below 1.05U dcN At this time, the DC power-consuming device is disconnected. The voltage reduction strategy of the offshore AC system and the switching strategy of the DC power-consuming device, as the fastest and most direct power regulation methods, should operate almost simultaneously to form a combined force to quickly curb the voltage rise trend, reduce injection on the wind turbine side, and actively release pressure through the power-consuming resistor.

[0054] (3) After the voltage reduction strategy of the offshore AC system and the switching strategy of DC energy-consuming devices are put into application, if U dcStill rising (U dc The rising slope dU dc / dt exceeds the preset threshold of 0.2U dcN ) or failed to fall back effectively (after a set short time of 100ms, U dc >1.1U dcN Enable as follows Figure 4 The frequency upscaling control strategy for the offshore AC system shown here increases the frequency of the wind farm's AC grid to:

[0055] f *' =f * +Δf=f * +k f (U dc -U dcN )

[0056] Where, k f It can be designed as:

[0057]

[0058] In the formula: f * f *' These represent the command values ​​for controlling the power of the offshore AC system by the wind turbine grid-side converter before and after the fault, Δf. max This represents the maximum frequency change.

[0059] The system is confirmed to be completely stable (U dc Stable at U dcN After the nearby (and both the offshore AC system voltage reduction method and the DC energy dissipation device switching method have been withdrawn) the frequency of the wind farm's AC grid returned to normal, the frequency recovery should also be a ramp process to avoid impacting the wind turbines and the grid.

[0060] The frequency upscaling strategy of the offshore AC system is relatively slow to act and has a significant impact on the power grid. Therefore, as a backup measure, it is only activated when the response of the offshore AC system's voltage reduction strategy and the DC power consumption device switching strategy is insufficient to control overvoltage.

[0061] Verification Example

[0062] To verify the feasibility of the overvoltage suppression strategy for the medium-frequency DRU-MMC DC transmission system of this invention, we built an electromagnetic transient simulation model in PSCAD / EMTDC. The parameters in the example are shown in Table 1:

[0063] Table 1

[0064]

[0065] Assuming the system was in stable operation before the fault, a three-phase metallic short-circuit fault occurred on the receiving end AC bus at t = 1.0s, and the fault was cleared 100ms later. Simulations were performed with and without the overvoltage suppression strategy of this invention. Figures 6-8 As shown, Figure 6 Due to the implementation of the overvoltage suppression strategy after the fault, the maximum fault voltage of the DC system is much smaller than the maximum DC fault voltage of the system when the overvoltage suppression strategy is not implemented, and it can gradually transition to the normal operating level after the fault is cleared in 1.1s. Figure 7 After a fault, the system output power was interrupted. After 1.1 seconds, as the DC power-consuming device was disconnected and the voltage amplitude and frequency of the marine AC system returned to normal levels, the system output power gradually returned to normal operation. This is due to the voltage reduction and frequency increase methods used in the marine AC system overvoltage suppression strategy. Figure 8 The voltage amplitude of the offshore AC system tends to decrease after a fault, but recovers to normal operating levels after the fault is cleared. The simulation results show that the fault ride-through characteristics of the receiving-end AC system are better after adopting the voltage suppression strategy of this invention. The overvoltage suppression strategy can effectively suppress DC overvoltage caused by power output obstruction, and the AC system voltage, system power, and DC voltage all return to normal operating levels after the fault ends.

[0066] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. An overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system, characterized in that: The operating frequency of the marine AC system is increased to the medium frequency range. In response to the situation where the DC transmission system's power output is blocked due to a fault in the onshore AC system, causing the DC voltage to rise, the marine AC system's voltage reduction strategy, frequency increase strategy, and DC energy consumption device switching strategy are invoked. Through logical interlocking and recovery sequence between the strategies, the overvoltage suppression of the DC transmission system is achieved.

2. The overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system according to claim 1, characterized in that: The aforementioned offshore AC system voltage reduction strategy involves lowering the DC voltage U of the DC transmission system by reducing the d-axis voltage command value of the offshore wind turbine grid-side converter. dc Specifically: when U dc >1.1U dcN At that time, the d-axis voltage command value of the offshore wind turbine grid-side converter is set to... To reduce the active power injected into DC lines by offshore wind turbines; in: and These are the d-axis voltage command values ​​of the offshore wind turbine grid-side converter before and after the fault, U dcN k is the rated DC voltage. fd This is the voltage regulation coefficient; When U dc Restored to below 1.05U dcN At that time, the d-axis voltage command value of the offshore wind turbine grid-side converter will be readjusted to...

3. The overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system according to claim 2, characterized in that: The voltage regulation coefficient k fd The expression: Among them: U max This represents the maximum DC voltage reached by the DC transmission system without any overvoltage suppression measures.

4. The overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system according to claim 1, characterized in that: The DC power dissipation device switching strategy involves installing DC power dissipation devices in parallel on the DC side of the onshore inverter station. These devices consist of switching devices and a power dissipation resistor connected in series. When the DC voltage U of the DC transmission system... dc Rise by more than 1.1U dcN When U is activated, the DC power dissipation device is put into operation, causing the surplus power to be consumed in the power dissipation resistor; when U dc Restored to below 1.05U dcN When necessary, disconnect the DC power-consuming device.

5. The overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system according to claim 4, characterized in that: The expression for the resistance R of the energy-consuming resistor is as follows: Where: P wind To reduce the output power of offshore wind farms after lowering the d-axis voltage command value of the grid-side converter of offshore wind turbines, U av This represents the average DC voltage when the DC power-consuming device is in operation during the fault period.

6. The overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system according to claim 1, characterized in that: The aforementioned offshore AC system frequency upscaling strategy involves increasing the AC system frequency command value of the offshore wind turbine grid-side converter to reduce the DC voltage U of the DC transmission system. dc Specifically: when the voltage reduction strategy of the offshore AC system and the switching strategy of the DC power consumption device are activated, U dc The upward slope exceeds the preset threshold or U within a short period of time dc Still greater than 1.1U dcN The AC system frequency command value for the grid-side converter of the offshore wind turbine is set to f. * ': f * '=f * +k f (U dc -U dcN ) Where: f * and f * 'These represent the AC system frequency command values ​​of the offshore wind turbine grid-side converter before and after the fault, U dcN k is the rated DC voltage. f This is the frequency adjustment coefficient; When U dc Falling back to U dcN Furthermore, the voltage reduction strategy for the offshore AC system and the switching strategy for DC power dissipation devices have both been discontinued, and the AC system frequency command value of the offshore wind turbine grid-side converter has been readjusted to f. * .

7. The overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system according to claim 6, characterized in that: The frequency adjustment coefficient k f The expression: Among them: U max Δf represents the maximum DC voltage reached by the DC transmission system without any overvoltage suppression strategy. max This represents the maximum frequency variation of the maritime communication system.

8. The overvoltage suppression method based on a medium-frequency DRU-MMC DC transmission system according to claim 1, characterized in that: The aforementioned offshore AC system voltage reduction strategy and DC energy dissipation device switching strategy are the fastest and most direct power regulation methods. When both are activated simultaneously, they quickly curb the rising trend of DC voltage, reduce the injection of offshore wind turbines, and actively release voltage from the energy dissipation resistors. If the DC voltage continues to rise or fails to fall effectively after the above two strategies are activated, the offshore AC system frequency increase strategy will be activated.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the overvoltage suppression method based on the medium-frequency DRU-MMC DC transmission system as described in any one of claims 1 to 8.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the overvoltage suppression method based on the medium-frequency DRU-MMC DC transmission system as described in any one of claims 1 to 8.

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