Coordination control method of offshore wind power multi-terminal flexible direct current system for absorbing surplus power

Abstract

The invention belongs to the field of offshore wind power multi-terminal flexible direct current systems, and particularly relates to an offshore wind power multi-terminal flexible direct current system coordination control method for absorbing surplus power, which comprises the following steps that: when a certain shore alternating current power grid of an offshore wind power multi-terminal flexible direct current system breaks down, an offshore converter station starts energy control after detecting that direct current voltage exceeds a threshold value; increasing the surplus power in the energy reference value absorption system according to the change of the direct current voltage; and meanwhile, the shore converter station predicts the change of the direct current voltage, and when the predicted value of the direct current voltage exceeds the limit value of the direct current voltage, the shore converter station also starts energy control and continues to absorb surplus power in the system. According to the method, the surplus power in the system can be effectively consumed under the conditions that the normal operation of the offshore wind turbine generator is not influenced and the project investment is not increased, so that the safety and the economical efficiency of the system are improved.

Description

Technical Field

[0001] The present invention belongs to the field of offshore wind power multi-terminal flexible direct current systems, and in particular to a coordinated control method for an offshore wind power multi-terminal flexible direct current system for absorbing surplus power. Background Art

[0002] The offshore wind power multi-terminal flexible direct current system has the advantages of diverse operation modes and flexible control, and can provide an economical and reliable operation mode in the interconnection of multiple power grids. However, compared with the traditional two-terminal flexible direct current system, the multi-terminal flexible direct current system is more complex in structure and control, and the probability of failure during power transmission also increases. When the onshore AC power grid of the offshore wind power multi-terminal flexible direct current system fails, the output power of the onshore converter station drops significantly. Due to the decoupling effect of the multi-terminal flexible direct current system, the offshore wind farm cannot feel the failure of the onshore AC system and continues to inject active power into the multi-terminal flexible direct current system. At this time, the input power of the multi-terminal flexible direct current system is greater than the output power, which will generate a large amount of surplus power in the system, thereby charging a large number of sub-module capacitors in multiple MMC converter stations, causing the DC voltage of the multi-terminal flexible direct current system to rise significantly. In severe cases, it will cause the converter station to lock, thereby causing the offshore wind farm to be disconnected from the grid, endangering the stable operation of the system.

[0003] However, current research is mainly focused on two-terminal flexible DC systems. In multi-terminal flexible DC systems, surplus power is mainly absorbed through energy-consuming resistors. However, energy-consuming resistors are expensive and dissipate surplus power in the form of heat, which results in a large amount of energy waste and is therefore less economical.

[0004] Chinese patent CN118825986A discloses a coordinated control method for an offshore wind power multi-terminal flexible DC system containing a DC energy consumption device. When a fault occurs in a certain onshore power grid, the DC voltage of the DC system rises; when the system DC voltage exceeds the set threshold, two offshore converter stations are controlled in turn to absorb surplus power and reduce the output power of the wind turbine set at the same time. When the energy of the offshore converter station approaches the limit, the DC energy consumption device is put into use to continue to dissipate the surplus power in the system.

[0005] This patent reduces the power input into the multi-terminal flexible direct current system of the offshore wind farm, and when the energy absorbed by the two offshore converter stations approaches the limit, the DC energy consumption device is put into use to continue to dissipate the surplus power in the system. On the one hand, it will affect the normal operation of the offshore wind turbines, and on the other hand, the increase in the investment in DC energy consumption devices will increase the burden on the system. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a coordinated control method for an offshore wind power multi-terminal flexible DC system that absorbs surplus power. By studying the active energy control method of the offshore converter station proposed in the literature and the active energy control method of the onshore converter station of the multi-terminal flexible DC system proposed in the literature, the decoupling of the energy and bridge arm voltage of all converter stations is achieved in the offshore wind power multi-terminal flexible DC system, so that the energy of all converter stations can be actively controlled.

[0007] The technical solution adopted by the present invention to solve the technical problem is:

[0008] The present invention provides a method for coordinated control of an offshore wind power multi-terminal flexible direct current system for absorbing surplus power, comprising: when a fault occurs in an onshore AC power grid of the offshore wind power multi-terminal flexible direct current system, the offshore converter station starts energy control after detecting that the DC voltage exceeds a threshold value, and raises an energy reference value according to changes in the DC voltage to absorb surplus power in the system; at the same time, the onshore converter station predicts changes in the DC voltage, and when the predicted DC voltage exceeds the limit value of the DC voltage, the onshore converter station also starts energy control to continue absorbing surplus power in the system.

[0009] Furthermore, the method specifically includes the following steps:

[0010] Step 1: When a fault occurs in an onshore AC power grid of the offshore wind power multi-terminal flexible DC system, the output power of the offshore wind power multi-terminal flexible DC system is reduced, and the surplus power generated in the system charges the sub-module capacitor in the MMC converter station, causing the DC voltage of the multi-terminal flexible DC system to increase;

[0011] Step 2: When the DC voltage is greater than the threshold That is, at 1.02pu, the two offshore converter stations first start energy control, raise the energy reference value as the DC voltage changes, and use their own energy storage margin to absorb the surplus power in the system, and when the DC voltage reaches the limit of 1.1pu, the energy of the two offshore converter stations also rises to the maximum value;

[0012] Step 3: At the same time, the two onshore converter stations make real-time predictions on the changes in DC voltage. When it is predicted that the DC voltage will exceed 1.1pu, the two onshore converter stations also start energy control to raise the energy reference value and continue to absorb the surplus power in the system.

[0013] Further, in step 2, when the DC voltage exceeds the threshold That is, when the DC voltage is 1.02 pu, the two offshore converter stations first start energy control, raise the energy reference value according to the change of DC voltage, absorb the surplus power in the system, and meet the requirement that when the DC voltage rises to the limit value, the energy of the offshore converter station also reaches the maximum value. The energy reference values ​​of the two offshore converter stations are expressed as:

[0014]

[0015] in, and are the energy reference values ​​of the two offshore converter stations respectively; and are the maximum energy values ​​of the two offshore converter stations respectively; and When the DC voltage reaches the threshold The energy of two offshore converter stations at that time; is the maximum allowable value of DC voltage; V dc is the DC voltage of the multi-terminal flexible DC system.

[0016] Furthermore, in step 3, while the two offshore converter stations absorb the surplus power, the two onshore converter stations make real-time predictions of the DC voltage. When an asymmetric fault occurs in the flexible DC system, the DC voltage changes periodically with a period ΔT of 10ms. The DC voltage is predicted once every period ΔT, and the DC voltage prediction value is The expression is:

[0017]

[0018] in, As the DC voltage rises to the threshold The DC voltage value at the next n-1 cycles; As the DC voltage rises to the threshold The DC voltage value at the last n cycles.

[0019] Furthermore, in step 3, if the DC voltage prediction value does not exceed the limit value when the fault ends, the two offshore converter stations adjust the energy reference value as the DC voltage decreases, so that the submodule capacitor releases energy; if the DC voltage prediction value reaches the limit value before the fault ends, the two onshore converter stations start energy control and continue to absorb the surplus power in the system. Similarly, it should be satisfied that when the DC voltage reaches the limit value, the energy of the onshore converter station also reaches the maximum value. The energy reference values ​​of the two onshore converter stations are respectively expressed as:

[0020]

[0021] in, and are the energy reference values ​​of the two onshore converter stations respectively; and are the maximum energies of the two onshore converter stations respectively; and The energy of the two onshore converter stations when the DC voltage prediction value reaches 1.1pu; It is the DC voltage when the predicted DC voltage reaches 1.1pu.

[0022] Furthermore, in step three, after the fault ends, the offshore converter station and the onshore converter station adjust the energy reference value to release energy to the rated value as the DC voltage decreases, and the offshore wind power multi-terminal flexible DC system returns to steady-state operation.

[0023] The advantages and positive effects of the present invention are:

[0024] 1. The present invention proposes a method for coordinated control of an offshore wind power multi-terminal flexible direct current system that absorbs surplus power. When a fault occurs in a shore AC power grid of an offshore wind power multi-terminal flexible direct current system, the offshore converter station first starts energy control, raises the energy reference value as the DC voltage rises, and uses the energy storage margin of its own converter station to absorb the surplus power in the system. At the same time, the onshore converter station predicts the change of the DC voltage. When the DC voltage prediction value exceeds the limit, the onshore converter station starts energy control and continues to absorb the surplus power in the system.

[0025] 2. The present invention makes full use of the energy storage margin of multiple MMCs in the offshore wind power multi-terminal flexible direct current system, and proposes a control method for absorbing surplus power during an onshore AC power grid failure. This method can effectively absorb the surplus power in the system without affecting the normal operation of the offshore wind turbines and without increasing the project investment, thereby improving the safety and economy of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a topological structure diagram of the offshore wind power multi-terminal flexible direct current system of the present invention;

[0027] Figure 2 This is a topological diagram of a converter station in an offshore wind power multi-terminal flexible direct current system of the present invention;

[0028] Figure 3 The energy control diagram of the MMC offshore converter station of the present invention;

[0029] Figure 4 This is the energy control diagram of the MMC onshore converter station of the present invention;

[0030] Figure 5 It is a flow chart of the coordinated control method of the offshore wind power multi-terminal flexible direct current system for absorbing surplus power according to the present invention;

[0031] FIG6( a ) is a simulated waveform diagram of the onshore AC voltage on the grid side of the onshore converter station 1 in the case of a single-phase grounding fault according to the present invention;

[0032] FIG6( b ) is a simulation waveform diagram of the output power of the onshore converter station in the case of a single-phase grounding fault according to the present invention;

[0033] FIG6( c ) is a DC voltage simulation waveform diagram of a multi-terminal flexible DC system with a single-phase grounding fault according to the present invention;

[0034] FIG6( d ) is a simulation waveform diagram of the average voltage of the submodule capacitor of the offshore converter station in the case of a single-phase grounding fault according to the present invention;

[0035] FIG6( e ) is a simulation waveform diagram of the average voltage of the submodule capacitor of the onshore converter station in the case of a single-phase grounding fault according to the present invention;

[0036] FIG6( f ) is an energy simulation waveform diagram of an offshore converter station with a single-phase grounding fault according to the present invention;

[0037] FIG6( g) is a waveform diagram of energy simulation of an onshore converter station with a single-phase grounding fault according to the present invention;

[0038] FIG. 7( a ) is a simulated waveform diagram of the onshore AC voltage on the grid side of the onshore converter station 1 in the case of a two-phase grounding fault according to the present invention;

[0039] FIG7( b ) is a simulation waveform diagram of the output power of the onshore converter station with a two-phase grounding fault according to the present invention;

[0040] FIG7( c ) is a DC voltage simulation waveform diagram of a multi-terminal flexible DC system with a two-phase grounding fault according to the present invention;

[0041] FIG7( d ) is a simulation waveform diagram of the average voltage of the submodule capacitor of the offshore converter station with a two-phase grounding fault according to the present invention;

[0042] FIG7( e ) is a simulation waveform diagram of the average voltage of the submodule capacitor of the onshore converter station in the case of a two-phase grounding fault according to the present invention;

[0043] FIG7( f ) is an energy simulation waveform diagram of an offshore converter station with a two-phase grounding fault according to the present invention;

[0044] FIG7( g) is a waveform diagram of energy simulation of an onshore converter station with a two-phase grounding fault according to the present invention;

[0045] FIG8( a ) is a simulated waveform diagram of the onshore AC voltage on the grid side of the onshore converter station 1 in the case of a three-phase grounding fault according to the present invention;

[0046] FIG8( b ) is a simulation waveform diagram of the output power of the onshore converter station with a three-phase grounding fault according to the present invention;

[0047] FIG8( c ) is a DC voltage simulation waveform diagram of a multi-terminal flexible DC system with a three-phase grounding fault according to the present invention;

[0048] FIG8( d ) is a simulation waveform diagram of the average voltage of the submodule capacitor of the offshore converter station with a three-phase grounding fault according to the present invention;

[0049] FIG8( e ) is a simulation waveform diagram of the average voltage of the submodule capacitor of the onshore converter station in the case of a three-phase grounding fault according to the present invention;

[0050] FIG8( f ) is an energy simulation waveform diagram of an offshore converter station with a three-phase grounding fault according to the present invention;

[0051] FIG8( g ) is a waveform diagram of energy simulation of an onshore converter station with a three-phase grounding fault according to the present invention. DETAILED DESCRIPTION

[0052] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0053] The present invention establishes Figure 1 The offshore wind power multi-terminal flexible direct current system shown in the figure includes two offshore wind farms, a four-terminal flexible direct current system and two onshore AC power grids. Some parameters of the system are shown in Table 1:

[0054] Table 1 Some parameters of offshore wind power multi-terminal flexible direct current system

[0055]

[0056] Figure 2 The topological structure diagram of the converter station in the offshore wind power multi-terminal flexible direct current system. C0 is the rated voltage of the submodule capacitor of the MMC converter station, and C0 is the capacitance value of the submodule capacitor. The energy is expressed as:

[0057]

[0058] Where N is the number of submodule capacitors in a single bridge arm of the MMC converter station. In actual projects, a certain voltage margin is usually left when selecting submodule capacitors, which is generally 1.5 times the rated voltage. Therefore, the energy of the MMC converter station can reach up to 2.25 times the rated operating condition. It is expressed as:

[0059]

[0060] Figure 3 and Figure 4 They are the energy control diagrams of the MMC offshore converter station and the onshore converter station, respectively. By introducing the DC modulation ratio m of the DC current loop dc , increasing the control dimension to three dimensions, realizing the decoupling of the converter station bridge arm voltage and the DC voltage, thereby realizing the control of the energy of the MMC converter station.

[0061] During stable operation, the two offshore converter stations control the AC voltage and frequency on the wind turbine side, and the two onshore converter stations control the DC voltage and reactive power of the multi-terminal flexible DC system.

[0062] The present invention provides a method for coordinated control of an offshore wind power multi-terminal flexible direct current system for absorbing surplus power, comprising: when a fault occurs in an onshore AC power grid of the offshore wind power multi-terminal flexible direct current system, the offshore converter station starts energy control after detecting that the DC voltage exceeds a threshold value, and raises an energy reference value according to changes in the DC voltage to absorb surplus power in the system; at the same time, the onshore converter station predicts changes in the DC voltage, and when the predicted DC voltage exceeds the limit value of the DC voltage, the onshore converter station also starts energy control to continue absorbing surplus power in the system.

[0063] like Figure 5 As shown, it is a flow chart of the coordinated control method of the offshore wind power multi-terminal flexible direct current system for absorbing surplus power of the present invention, which specifically includes the following steps:

[0064] Step 1: When a fault occurs in an onshore AC power grid of the offshore wind power multi-terminal flexible DC system, the output power of the offshore wind power multi-terminal flexible DC system decreases, and the surplus power generated in the system charges the sub-module capacitors in the MMC converter station, causing the DC voltage of the multi-terminal flexible DC system to increase.

[0065] Specifically, when a fault occurs in one of the onshore AC power grids, the output power of the multi-terminal flexible DC system drops significantly. However, due to the decoupling effect of the multi-terminal flexible DC system, the offshore wind farm cannot feel the fault onshore, and the power input to the multi-terminal flexible DC system remains unchanged. The input power of the multi-terminal flexible DC system is greater than the output power, thereby generating surplus power in the system to charge a large number of sub-module capacitors in the MMC converter station, causing the DC voltage to rise rapidly.

[0066] Step 2: When the DC voltage is greater than the threshold That is, at 1.02pu, the two offshore converter stations first start energy control, raise the energy reference value as the DC voltage changes, use their own energy storage margin to absorb the surplus power in the system, and meet the requirement that when the DC voltage reaches the limit of 1.1pu, the energy of the two offshore converter stations also rises to the maximum value.

[0067] When the DC voltage exceeds the threshold That is, when the DC voltage is 1.02 pu, the two offshore converter stations first start energy control, raise the energy reference value according to the change of DC voltage, absorb the surplus power in the system, and meet the requirement that when the DC voltage rises to the limit, the energy of the offshore converter station also reaches the maximum value. Therefore, the energy reference values ​​of the two offshore converter stations are expressed as:

[0068]

[0069] in, and are the energy reference values ​​of the two offshore converter stations respectively; and are the maximum energy values ​​of the two offshore converter stations respectively; and When the DC voltage reaches the threshold The energy of two offshore converter stations at that time; is the maximum allowable value of DC voltage, usually 1.1pu; V dc is the DC voltage of the multi-terminal flexible DC system.

[0070] Step 3: At the same time, the two onshore converter stations make real-time predictions on the changes in DC voltage. When it is predicted that the DC voltage will exceed 1.1pu, the two onshore converter stations also start energy control to raise the energy reference value and continue to absorb the surplus power in the system.

[0071] While the two offshore converter stations absorb surplus power, the two onshore converter stations make real-time predictions of the DC voltage. When an asymmetric fault occurs in the flexible DC system, the DC voltage changes periodically with a period ΔT of 10ms. Therefore, the DC voltage can be predicted every 10ms. The expression is:

[0072]

[0073] in, As the DC voltage rises to the threshold The DC voltage value at the next n-1 cycles; As the DC voltage rises to the threshold The DC voltage value at the last n cycles; for a 220kV transmission line, the circuit breaker action time is usually 120ms, so a maximum of 11 predictions can be made.

[0074] If the DC voltage prediction value does not exceed the limit value when the fault ends, the two offshore converter stations adjust the energy reference value as the DC voltage decreases, so that the submodule capacitor releases energy. If the DC voltage prediction value reaches the limit value before the fault ends, the two onshore converter stations start energy control and continue to absorb the surplus power in the system. Similarly, it should be satisfied that when the DC voltage reaches the limit value, the energy of the onshore converter station also reaches the maximum value. Therefore, the energy reference values ​​of the two onshore converter stations are expressed as:

[0075]

[0076] in, and are the energy reference values ​​of the two onshore converter stations respectively; and are the maximum energies of the two onshore converter stations respectively; and The energy of the two onshore converter stations when the DC voltage prediction value reaches 1.1pu; It is the DC voltage when the predicted DC voltage reaches 1.1pu.

[0077] When the fault ends, the offshore converter station and the onshore converter station adjust the energy reference value to release energy to the rated value as the DC voltage drops, and the offshore wind power multi-terminal flexible DC system returns to steady-state operation.

[0078] The present invention makes full use of the energy storage margin of multiple MMC converter stations in an offshore wind power multi-terminal flexible DC system, and proposes a coordinated control method for an offshore wind power multi-terminal flexible DC system that absorbs surplus power. According to this method, when an onshore AC power grid fails, multiple MMC converter stations can be used to absorb the surplus power in the DC system, thereby suppressing DC overvoltage.

[0079] In order to verify the effectiveness of the present invention, the following simulation model was built in PSCAD / EMTDC electromagnetic transient simulation software: Figure 1 The simulation model of the offshore wind power multi-terminal flexible DC system is shown. Single-phase grounding fault, two-phase grounding fault and three-phase grounding fault are set in the AC side grid of the onshore converter station 1. The fault time is 4.0s and the fault duration is 90ms. Due to the short fault time, the circuit breaker on the AC side will not operate.

[0080] The simulation results of single-phase grounding fault are shown in Figure 6.

[0081] As can be seen from Figures 6(a), 6(b) and 6(c), after the fault occurs, the output power of the onshore converter station 1 begins to decrease, so that the output power of the onshore converter station 2 reaches the maximum, and the DC voltage begins to rise. Under the control method proposed in the present invention, the increase in the DC voltage is very small.

[0082] As shown in Figures 6(d), 6(e), 6(f) and 6(g), under the control method proposed in the present invention, the two offshore converter stations quickly began to absorb surplus power after the fault occurred, and the energy exceeded 1.7pu. At the same time, the two onshore converter stations have been predicting the DC voltage, but the DC voltage has never exceeded 1.1pu, so the two onshore converter stations failed to start energy control. At this time, the energy of the onshore converter station is coupled with the DC voltage.

[0083] The simulation results of two-phase grounding fault are shown in Figure 7.

[0084] It can be seen from Figure 7(a), Figure 7(b) and Figure 7(c) that the output power of the onshore converter station 1 drops rapidly after the fault occurs. Under the control method proposed in the present invention, the DC voltage only reaches 1.07pu.

[0085] As can be seen from Figure 7(d), Figure 7(e), Figure 7(f) and Figure 7(g), under the control method proposed by the present invention, the two offshore converter stations quickly start energy control to absorb the surplus power in the system, and the energy exceeds 1.75pu. At the same time, the two onshore converter stations continue to predict the DC voltage. When the DC voltage prediction value exceeds the limit in the fourth prediction, the two onshore converter stations immediately start energy control to continue to absorb the surplus power in the system, and the energy eventually exceeds 1.7pu.

[0086] The simulation results of the three-phase grounding fault are shown in Figure 8.

[0087] As can be seen from Figures 8(a), 8(b) and 8(c), after the fault occurs, the output power of the onshore converter station 1 quickly drops to 0MW, and the DC voltage begins to rise rapidly. Under the control method proposed in the present invention, the DC voltage only exceeds 1.09pu at most, and does not reach the limit of 1.1pu.

[0088] As can be seen from Figures 8(d), 8(e), 8(f) and 8(g), under the control method proposed by the present invention, after the fault occurs, the two offshore converter stations quickly start energy control and use their own energy storage margin to absorb surplus power, and the energy exceeds 2.0pu. When the two onshore converter stations perform the second DC voltage prediction, the DC voltage prediction value exceeds 1.1pu, and then they immediately start energy control to absorb surplus power, and the maximum energy also exceeds 2.0pu.

[0089] It can be seen that the present invention proposes a coordinated control method for an offshore wind power multi-terminal flexible direct current system that absorbs surplus power by making full use of the energy storage margin of multiple MMC converter stations in the offshore wind power multi-terminal flexible direct current system. This method can effectively absorb the surplus power in the system without affecting the normal operation of the offshore wind turbines and without increasing the project investment, thereby improving the safety and economy of the system.

[0090] The above description is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept, which all belong to the protection scope of the present invention.

Claims

1. A method for coordinated control of an offshore wind power multi-terminal flexible direct current system for absorbing surplus power, characterized in that: include: When a fault occurs in an onshore AC power grid of an offshore wind power multi-terminal flexible DC system, the offshore converter station detects that the DC voltage exceeds the threshold and starts energy control, raising the energy reference value according to the change of DC voltage to absorb the surplus power in the system; at the same time, the onshore converter station predicts the change of DC voltage, and when the predicted DC voltage exceeds the DC voltage limit, the onshore converter station also starts energy control to continue absorbing the surplus power in the system.

2. The method according to claim 1, characterized in that The specific steps include: Step 1: When a fault occurs in an onshore AC power grid of the offshore wind power multi-terminal flexible DC system, the output power of the offshore wind power multi-terminal flexible DC system is reduced, and the surplus power generated in the system charges the sub-module capacitor in the MMC converter station, causing the DC voltage of the multi-terminal flexible DC system to increase; Step 2: When the DC voltage is greater than the threshold That is, at 1.02pu, the two offshore converter stations first start energy control, raise the energy reference value as the DC voltage changes, and use their own energy storage margin to absorb the surplus power in the system, and when the DC voltage reaches the limit of 1.1pu, the energy of the two offshore converter stations also rises to the maximum value; Step 3: At the same time, the two onshore converter stations make real-time predictions on the changes in DC voltage. When it is predicted that the DC voltage will exceed 1.1pu, the two onshore converter stations also start energy control to raise the energy reference value and continue to absorb the surplus power in the system.

3. The method according to claim 2, characterized in that In step 2, when the DC voltage exceeds the threshold That is, when the DC voltage is 1.02 pu, the two offshore converter stations first start energy control, raise the energy reference value according to the change of DC voltage, absorb the surplus power in the system, and meet the requirement that when the DC voltage rises to the limit value, the energy of the offshore converter station also reaches the maximum value. The energy reference values ​​of the two offshore converter stations are expressed as: in, and are the energy reference values ​​of the two offshore converter stations respectively; and are the maximum energy values ​​of the two offshore converter stations respectively; and When the DC voltage reaches the threshold The energy of two offshore converter stations at that time; is the maximum allowable value of DC voltage; V dc is the DC voltage of the multi-terminal flexible DC system.

4. The method according to claim 3, characterized in that In step 3, while the two offshore converter stations absorb the surplus power, the two onshore converter stations make real-time predictions of the DC voltage. When an asymmetric fault occurs in the flexible DC system, the DC voltage changes periodically with a period ΔT of 10ms. The DC voltage is predicted once every period ΔT, and the DC voltage prediction value is The expression is: in, As the DC voltage rises to the threshold The DC voltage value at the next n-1 cycles; As the DC voltage rises to the threshold The DC voltage value at the last n cycles.

5. The method according to claim 4, characterized in that In step 3, if the DC voltage prediction value does not exceed the limit value when the fault ends, the two offshore converter stations adjust the energy reference value as the DC voltage decreases, so that the submodule capacitor releases energy; if the DC voltage prediction value reaches the limit value before the fault ends, the two onshore converter stations start energy control and continue to absorb the surplus power in the system. Similarly, it should be satisfied that when the DC voltage reaches the limit value, the energy of the onshore converter station also reaches the maximum value. The energy reference values ​​of the two onshore converter stations are respectively expressed as: in, and are the energy reference values ​​of the two onshore converter stations respectively; and are the maximum energies of the two onshore converter stations respectively; and The energy of the two onshore converter stations when the DC voltage prediction value reaches 1.1pu; It is the DC voltage when the predicted DC voltage reaches 1.1pu.

6. The method according to claim 5, characterized in that In step three, when the fault ends, the offshore converter station and the onshore converter station adjust the energy reference value to release energy to the rated value as the DC voltage decreases, and the offshore wind power multi-terminal flexible DC system returns to steady-state operation.

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