A fault isolation method for offshore wind power DC collection lines based on half-bridge MMC current limiting control

Abstract

The present invention discloses a fault isolation method for an offshore wind power DC collection line based on half-bridge MMC current limiting control, comprising the following steps: S1. collecting the current of each collection line; S2. calculating the absolute value of the current gradient of each collection line and performing a starting element discrimination. If the discrimination criteria are met, the collection line is deemed to have a fault and execution continues; otherwise, the method returns to S1; S3. locking the DC transformer at the head end of the collection line; S4. switching the MMC converter on the low-voltage side of DCT0 to a current limiting control mode; S5. starting the collection line relay protection device to select the fault line; S6. performing a disconnector disconnection condition discrimination. If the discrimination criteria are met, execution continues; otherwise, the current of each collection line is continuously collected; S7. disconnecting the disconnectors at both ends of the fault line according to the line selection result of the protection device. The method of the present invention can ensure that the current limiting control is quickly activated and the fault current is attenuated to zero when a collection line fault occurs, thereby enabling the use of disconnectors to isolate the fault, reducing the construction cost of offshore wind power and meeting the requirements for lightweight offshore wind power construction.

Description

Technical Field

[0001] The present invention belongs to the field of power systems and relates to the technical field of fault current limiting control, and specifically to a fault isolation method for an offshore wind power DC collection line based on half-bridge MMC current limiting control. Background Art

[0002] Currently, offshore wind power in my country is primarily located in coastal areas, less than 50 km from shore. However, offshore wind farm sites are becoming increasingly difficult to locate. Far-flung offshore wind power can be consumed locally, offers high-quality wind energy, large unit capacity, and does not occupy land resources. This facilitates the development of new power systems and is expected to become a major focus for future offshore wind power development.

[0003] To accommodate long-distance, high-capacity transmission, offshore wind power aggregation is trending from AC to DC transmission and from centralized to distributed commutation. DC aggregation is more suitable for long-distance, high-capacity transmission. This has led to the emergence of offshore wind power all-DC aggregation and transmission systems based on a DC transformer step-up structure, which are expected to become the primary form of offshore wind power transmission.

[0004] However, DC systems have a significant low inertia characteristic, leading to rapid fault development and propagation. Furthermore, power electronics have weak current tolerances. DC faults can easily damage power electronics and have a significant impact on the power grid. Therefore, effectively suppressing DC system fault currents and enabling fault identification and rapid isolation are crucial for protecting equipment and ensuring safe and stable system operation. Summary of the Invention

[0005] In order to solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a fault isolation method for offshore wind power DC collection line based on half-bridge MMC current limiting control. The method of the present invention can quickly start and accurately switch to the current limiting control mode within 1ms, and can clear the fault current of the line and isolate the fault within 10ms through the half-bridge MMC combined with the isolating switch. While ensuring the safe and stable operation of power equipment and systems, it greatly reduces the construction cost of offshore wind power and meets the lightweight requirements of offshore platform construction.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for isolating faults in an offshore wind power DC collection line based on half-bridge MMC current limiting control comprises the following steps:

[0008] Step 1: Build a fully DC collection and transmission system for offshore wind power;

[0009] The constructed offshore wind power all-DC collection and transmission system includes multiple wind turbine rectifiers. Each wind turbine rectifier is boosted by a distributed DC transformer and then connected to the collection line. Multiple wind turbines are connected to the busbar through the collection line. Then, the centralized DC transformer DCT0 is used to boost the voltage and connect to the transmission line. Finally, it is connected to the onshore AC system through the onshore inverter station MMC.

[0010] Step 2: Sample the current signal i at each collection line protection installation n (t);

[0011] Step 3: Calculate the absolute value of the current gradient of each collection line Perform startup component identification. If the criteria are met, it is a fault and the process continues. Otherwise, return to step 2:

[0012] The absolute value of the current gradient of each collecting line The discriminant of the calculation and startup components is shown in formula (1).

[0013]

[0014] Where n is the number of the collection line, k and j are the sampling numbers, i n (kj) is the current of the nth collection line corresponding to the kjth sampling moment, is the current gradient of the nth sink line calculated at the kth sampling moment, ε1 is the setting threshold of the starting element, which should be greater than the maximum current gradient under normal operating conditions;

[0015] The setting principle of ε1 is shown in formula (2):

[0016]

[0017] Where k rel1 is the reliability coefficient of the threshold setting of the starting element, It is the maximum current gradient under normal operating conditions;

[0018] Step 4: Block the distributed DC transformer at the head end of the collection line;

[0019] After the distributed DC transformer is locked, the power flowing into the wind turbine lateral collection line is interrupted; the secondary capacitor at the outlet is discharged, and the discharge circuit is a second-order RLC circuit, with the current decaying naturally.

[0020] Step 5: The MMC converter on the low-voltage side of the centralized DC transformer DCT0 switches to current limiting control mode;

[0021] Calculate the reduction coefficient k and multiply it by the pole control output signal of the MMC converter, i.e., the bridge arm voltage reference value. The reduction coefficient k is output by the difference signal between the DC current and the reference value through the PI current limiting control, which can achieve continuous adaptive adjustment. The specific calculation formula of the reduction coefficient k is shown in formula (3).

[0022]

[0023] Among them, K p and K I are the proportional coefficient and the integral coefficient respectively, and k is limited to be output in [0,1];

[0024] The bridge arm voltage reference value after the MMC converter switches to current limiting control is shown in formula (4):

[0025] u′ p =ku p (4)

[0026] u′ n =ku n

[0027] Among them, u p 、u n They are the upper and lower bridge arm voltage reference values ​​before current limiting control is put into use;

[0028] At this time, the number of submodules put into use in the upper and lower bridge arms of the MMC converter is shown in formula (5):

[0029] n′ p =kn p (5)

[0030] n′ n =kn n

[0031] Among them, n p 、n n are the number of submodules put into operation in the upper and lower arms of the MMC converter before the current limiting control is put into operation;

[0032] By reducing the number of submodules, capacitor discharge is reduced, thereby suppressing fault current;

[0033] Step 6: Start the relay protection device of the collection line to select the fault line;

[0034] Step 7: Determine whether the fault current meets the disconnection condition of the disconnector. If so, continue the process; otherwise, continue collecting the current of each collection line.

[0035] The breaking condition is shown in formula (6):

[0036] I f <Ilimit (6)

[0037] Among them, I f is the collected fault current of the collection line, I limit The maximum current that the isolating switch can interrupt;

[0038] Step 8: Open the disconnectors at both ends of the fault line according to the line selection results of the relay protection device in step 6.

[0039] Preferably, the wind turbine rectifier adopts a two-level VSC structure; the MMC converter on the low-voltage side of the centralized DC transformer DCT0 adopts a half-bridge MMC structure, and the high-voltage side adopts a diode uncontrolled rectifier structure; the distributed DC transformer adopts a full-bridge three-level LLC resonant converter structure.

[0040] Preferably, there are four wind turbine rectifiers, namely, the first wind turbine rectifier VSC1, the second wind turbine rectifier VSC2, the third wind turbine rectifier VSC3 and the fourth wind turbine rectifier VSC4; correspondingly, there are four distributed DC transformers, namely, the first distributed DC transformer (DCT1), the second distributed DC transformer (DCT2), the third distributed DC transformer (DCT3) and the fourth distributed DC transformer (DCT4).

[0041] Preferably, the sampling frequency of the current at the collecting line protection installation described in step 1 is 2 kHz.

[0042] Preferably, the setting threshold ε1 of the starting element in step 2 is set to 0.078. Compared with the prior art, the present invention has the following advantages:

[0043] 1. By adopting a full-bridge three-level LLC resonant DC transformer, the systems on both sides of the transformer can be electrically isolated after a DC fault, replacing the AC circuit breaker to solve the problem of AC inrush after a fault.

[0044] 2. By improving the control of the half-bridge MMC and coordinating the disconnector to replace the full-bridge MMC and other topologies with fault self-clearing capabilities or DC circuit breakers to isolate faults, the construction cost of offshore wind power is reduced and the requirements for lightweight offshore platform construction are met.

[0045] 3. It can realize fast and accurate control within an ultra-short data window (1ms) and clear faults within 10ms, effectively ensuring the safe and stable operation of power equipment and systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a power transmission system topology diagram suitable for the method of the present invention.

[0047] Figure 2 It is the distributed DC transformer model in the system of the present invention.

[0048] Figure 3 This is the centralized DC transformer model in the system of the present invention.

[0049] Figure 4 It is a flow chart of the method of the present invention.

[0050] Figure 5 It is the line current waveform at the protection installation under bipolar short-circuit fault at the end of collecting line 1.

[0051] Figure 6 It is the line voltage waveform at the protection installation under bipolar short circuit fault at the end of collecting line 1. DETAILED DESCRIPTION

[0052] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0053] like Figure 1 Figure 1 shows a simulation model of an offshore wind power all-DC transmission system. The rated power of a single wind farm is 100 MV·A, and the rated DC output voltage of the rectifier is ±5 kV. The transformation ratios of the distributed DC transformers DCT1 to DCT4 are ±5 kV / ±60 kV, and the transformation ratio of the centralized offshore DC booster platform DCT0 is ±60 kV / ±400 kV. Each DC transmission line is 30 km long, and the transmission line is 100 km long.

[0054] like Figure 2 As shown in FIG, the structural topology of the distributed DC transformer (DCT1 to DCT4) is given.

[0055] like Figure 3 As shown in FIG, the structural topology of the centralized DC transformer DCT0 is given.

[0056] When a bipolar short circuit fault occurs at the end of the collection line 1 within 10ms, the method provided by the present invention can be used to accurately input and quickly limit the fault current. Figure 4 As shown, the following steps are included:

[0057] Step 1: Build an offshore wind power all-DC collection and transmission system; the constructed offshore wind power all-DC collection and transmission system includes multiple wind turbine rectifiers. Each wind turbine rectifier is connected to the collection line after being stepped up by a distributed DC transformer. Multiple wind turbines are connected to the bus through the collection line, and then stepped up by a centralized DC transformer DCT0 and connected to the transmission line. Finally, they are connected to the onshore AC system through the onshore inverter station MMC.

[0058] The rated power of a single wind turbine is 100MW, and the rated DC output voltage of the wind turbine rectifier is ±5kV. The distributed DC transformer has a transformation ratio of ±5kV / ±60kV, while the centralized DC transformer has a transformation ratio of ±60kV / ±400kV. Each collection line is 30km long, and the transmission line is 100km long.

[0059] The wind turbine rectifier controls the active power and reactive power of the wind turbine, the distributed DC transformers DCT1 to DCT4 control the DC voltage and reactive power of the system, the centralized DC transformer DCT0 controls the active power and reactive power of the system, and the onshore inverter station controls the DC voltage and AC voltage of the system.

[0060] Step 2: sampling the current signal i1(t) of the collection line 1;

[0061] To ensure that the start signal can be issued within 1ms after the fault occurs, at least one current signal needs to be collected within 1ms. Considering the development process of the fault and the economy of the sampling equipment, the sampling frequency is taken as 2kHz.

[0062] The positive current value at the end of line 1 from 6.5ms to 17ms is shown in Table 1:

[0063] Table 1

[0064]

[0065] Step 2: Calculate the absolute value of the current gradient of the collection line 1 according to formula (1) as shown in Table 2.

[0066] Table 2

[0067]

[0068] The data in the table shows that the absolute values ​​of the calculated current gradients are relatively small during normal operation. ε1 is the threshold for the start-up element, which should avoid the maximum absolute value of the current gradient under normal operation. The maximum absolute value of the current gradient during steady-state operation before the fault is 0.039 at 7.5ms.

[0069] In this example, the reliability coefficient k of formula (2) is rel1 Take 2, so the starting threshold ε1 = 0.039*2 = 0.078 is calculated by formula (2); at the moment of fault occurrence, the absolute value of the calculated current gradient is 0.4349> 0.078, the starting criterion is met, it is judged that a fault has occurred, and execution continues;

[0070] It can be seen that the current gradient obtained by sampling at the moment the fault occurs can meet the startup criterion. If the fault occurs between two samplings, and in the worst case the fault occurs just after the previous sampling is completed, the criterion can also ensure that the output is within 0.5ms.

[0071] Step 4: Block the distributed DC transformers (DCT1 to DCT4) at the head end of the collection line;

[0072] DCT1-DCT4 are full-bridge, three-level LLC resonant DC transformers. Upon fault detection, blocking DCT1-DCT4 isolates both systems, effectively shutting off the continuous inrush current from the lateral fault point in the wind farm. However, the secondary capacitors discharge, using a second-order RLC circuit, causing the current to decay naturally.

[0073] Step 5: The MMC converter on the low-voltage side of the centralized DC transformer DCT0 switches to current limiting control mode;

[0074] In formula (3), K p Take 1.2, K I Take 0.2. At the same time, considering that this method uses an isolating switch to interrupt the fault current, and the current breaking capacity of the isolating switch is very weak, the DC current reference value I dcref Take 0.

[0075] The value k calculated by equation (3) decreases from its initial value of 1 to 0 within 1ms. Equation (5) shows that when k = 0, all submodules are bypassed, blocking the capacitor discharge circuit and thus suppressing the fault current. Unlike the method of directly bypassing all submodules, this method gradually reduces the number of submodules in operation to 0, mitigating the risk of overvoltage. The current limiting control method essentially reduces the number of submodules in operation, thereby reducing capacitor discharge and thus suppressing the fault current.

[0076] Furthermore, because the high-voltage side of the centralized DC transformer DCT0 utilizes a diode-uncontrolled rectifier structure, which allows only unidirectional power transmission, the onshore AC system does not experience a surge in current at the fault point. Consequently, the fault area at this point becomes a passive system, and any generated fault current is dissipated and decays to zero within the circuit.

[0077] Step 6: Start the relay protection device of the collection line to select the fault line;

[0078] The relay protection device determines that the collection line 1 is the fault line.

[0079] Step 7: Determine whether the fault current meets the disconnection condition of the disconnector according to formula (6);

[0080] In this example, the maximum permissible breaking capacity of the isolating switch is 5A. From the data in Table 1, we can see that at 16ms, I f =0.0024<0.005, the disconnector meets the opening conditions and continues to execute.

[0081] Step 8: Combined with the protection device selection results in step 6, the isolation switches at both ends of the collection line 1 are opened at 16ms to isolate the fault line. The waveforms of the positive and negative currents of the collection line during the entire isolation process are as follows: Figure 5 As shown, the positive and negative voltage waveforms of the collection line are as follows Figure 6 shown.

Claims

1. A method for isolating faults in an offshore wind power DC collection line based on half-bridge MMC current limiting control, characterized in that: The following steps are involved: Step 1: Build a fully DC collection and transmission system for offshore wind power; The constructed offshore wind power all-DC collection and transmission system includes multiple wind turbine rectifiers. Each wind turbine rectifier is boosted by a distributed DC transformer and then connected to the collection line. Multiple wind turbines are connected to the busbar through the collection line, and then boosted by a centralized DC transformer before being connected to the transmission line. Finally, it is connected to the onshore AC system through an onshore inverter station. Step 2: Sample the current signal i at each collection line protection installation n (t); Step 3: Calculate the absolute value of the current gradient of each collection line Perform startup component identification. If the criteria are met, it is a fault and the process continues. Otherwise, return to step 2: The absolute value of the current gradient of each collecting line The discriminant of the calculation and startup components is shown in formula (1). Where n is the number of the collection line, k and j are the sampling numbers, i n (kj) is the current of the nth collection line corresponding to the kjth sampling moment, is the current gradient of the nth sink line calculated at the kth sampling moment, ε1 is the setting threshold of the starting element, which is greater than the maximum current gradient under normal operating conditions; The setting principle of ε1 is shown in formula (2): Where k rel1 is the reliability coefficient of the threshold setting of the starting element, It is the maximum current gradient under normal operating conditions; Step 4: Block the distributed DC transformer at the head end of the collection line; After the distributed DC transformer is locked, the power flowing into the wind turbine lateral collection line is interrupted; the secondary capacitor at the outlet is discharged, and the discharge circuit is a second-order RLC circuit, with the current decaying naturally. Step 5: The MMC converter on the low-voltage side of the centralized DC transformer switches to current limiting control mode; Calculate the reduction coefficient k and multiply it by the pole control output signal of the MMC converter, i.e., the bridge arm voltage reference value. The reduction coefficient k is output by the difference signal between the DC current and the reference value through the PI current limiting control, which can achieve continuous adaptive adjustment. The specific calculation formula of the reduction coefficient k is shown in formula (3). Among them, K p and K I are the proportional coefficient and the integral coefficient respectively, and k is limited to be output in [0,1]; The bridge arm voltage reference value after the MMC converter switches to current limiting control is shown in formula (4): u' p =stand p (4) u' n =stand n Among them, u p 、u n They are the upper and lower bridge arm voltage reference values ​​before current limiting control is put into use; At this time, the number of submodules put into use in the upper and lower bridge arms of the MMC converter is shown in formula (5): n′ p =kn p (5) n′ n =kn n Among them, n p 、n n are the number of submodules put into operation in the upper and lower arms of the MMC converter before the current limiting control is put into operation; By reducing the number of submodules, capacitor discharge is reduced, thereby suppressing fault current; Step 6: Start the relay protection device of the collection line to select the fault line; Step 7: Determine whether the fault current meets the disconnection condition of the disconnector. If so, continue the process; otherwise, continue collecting the current of each collection line. The breaking condition is shown in formula (6): I f <I limit (6) Among them, I f is the collected fault current of the collection line, I limit The maximum current that the isolating switch can interrupt; Step 8: Open the disconnectors at both ends of the fault line according to the line selection results of the relay protection device in step 6.

2. A method for isolating faults in an offshore wind power DC collection line based on half-bridge MMC current limiting control according to claim 1, characterized in that: The wind turbine rectifier adopts a two-level VSC structure; the MMC converter on the low-voltage side of the centralized DC transformer adopts a half-bridge MMC structure, and the high-voltage side adopts a diode uncontrolled rectifier structure; the distributed DC transformer adopts a full-bridge three-level LLC resonant converter structure.

3. The method for isolating faults in an offshore wind power DC collection line based on half-bridge MMC current limiting control according to claim 1, characterized in that: There are four wind turbine rectifiers, namely the first wind turbine rectifier, the second wind turbine rectifier, the third wind turbine rectifier and the fourth wind turbine rectifier; correspondingly, there are four distributed DC transformers, namely the first distributed DC transformer, the second distributed DC transformer, the third distributed DC transformer and the fourth distributed DC transformer.

4. The method for isolating faults in an offshore wind power DC collection line based on half-bridge MMC current limiting control according to claim 1, characterized in that: The sampling frequency of the current at the collecting line protection installation described in step 1 is 2 kHz.

5. The method for isolating faults in an offshore wind power DC collection line based on half-bridge MMC current limiting control according to claim 1, characterized in that: The setting threshold ε1 of the starting element described in step 2 is set to 0.078.

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