A fault ride-through control method and system for a modular multi-level matrix converter

Abstract

The present invention discloses a fault ride-through control method and system for a modular multilevel matrix converter (M3C). The control method forces the wind farm to reduce its output power according to its own low voltage ride-through method by changing the AC voltage instruction on the low-frequency side of the M3C, thereby adjusting the power balance on both sides of the M3C. The M3C power frequency side adopts a control method that prioritizes active power balance and outputs residual reactive power to support the grid-side voltage drop, and is divided into three control schemes based on the system's demand for active and reactive power: active power and reactive power, active power priority, and reactive power priority. The control method proposed in the present invention does not require any additional hardware, fully utilizes the control capability of the M3C itself, and can effectively improve the fault ride-through capability of the modular multilevel matrix converter in the offshore wind power frequency division transmission system.

Description

Technical Field

[0001] The present invention relates to the technical field of frequency-divided power transmission, and in particular to a fault ride-through control method and system for a modular multi-level matrix converter. Background Art

[0002] With the development of offshore wind power frequency division transmission technology, modular multilevel matrix converter (M3C) has attracted widespread attention as one of the important devices in offshore wind power transmission systems.

[0003] Offshore wind power frequency-splitting transmission systems typically connect low-frequency offshore wind farms and onshore power grids. When a sudden fault in the onshore power grid causes a voltage drop, without fault ride-through control measures in place, the wind farm continues to output power normally. The M3C's input active power remains unchanged, while its output capacity is limited by the maximum grid-connected current. As the grid voltage drops, the grid-connected current increases until it reaches the current limit, threatening the M3C's normal operation. If the M3C is forced to shut down due to overcurrent protection, the impact is further expanded. As a key frequency conversion device in the system, a failure of the M3C can severely impact the safe and stable operation of the grid and other equipment.

[0004] When the power grid fails, a load shedding circuit can be added to dissipate excess active power. Alternatively, the wind farm and M3C can be managed as a single entity, with the dispatch center regulating the wind farm output to ensure power balance within the M3C's regulation capacity under grid failure conditions. The former requires large-capacity load shedding circuits, which adds additional system costs. The latter places high demands on real-time and reliable communication. Communication delays or failures hinder rapid adjustment of wind farm output and maintain power balance between the two sides of the M3C. Summary of the Invention

[0005] The object of the present invention is to provide a fault ride-through control method and system for a modular multi-level matrix converter in an offshore wind power frequency-divided transmission system, which does not require additional unloading circuits and communications, and the wind turbine generator can maintain a conventional fault ride-through method, so as to solve the problems raised in the above background technology.

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

[0007] A fault ride-through control method for a modular multi-level matrix converter, the control method comprising the following steps:

[0008] Step 1: Detect the current grid voltage amplitude and determine the grid state based on the voltage amplitude;

[0009] Step 2: Determine the system's requirements for active power and reactive power;

[0010] Step 3: Switch the M3C low-frequency side voltage instruction and the power frequency side reactive current according to the grid state and the demand.

[0011] Optionally, step 3 includes: determining a low-frequency side voltage instruction according to the demand.

[0012] Optionally, step 3 includes:

[0013] Step 3-1: When the grid voltage U gd When the voltage drops, the low-frequency side voltage command U sd_ref Switch to

[0014]

[0015] Among them, U gd is the per-unit value of the grid voltage after the drop, k is the margin coefficient, is the actual average voltage per unit value of the M3C submodule capacitor, The maximum per-unit average voltage that the M3C submodule capacitor can withstand;

[0016] Step 3-2: When the grid voltage U gd When the power frequency side drops, the reactive current reference value switches to

[0017]

[0018] Among them, i gmax is the maximum current value allowed by M3C, i gd_ref It is the reference value of the active current on the power frequency side of M3C.

[0019] Optionally, the system's demand for active power and reactive power is divided into three scenarios: active and reactive power consideration, active power priority, and reactive power priority.

[0020] Optionally, in the active and reactive power consideration scenario, the low-frequency side voltage drops with the power-frequency side voltage in equal amplitude. At this time, the margin coefficient k in the low-frequency side voltage command is 0.

[0021] Optionally, in the active power priority scenario, the voltage drop on the low-frequency side is made smaller than that on the power frequency side. The voltage on the low-frequency side is maintained at a steady-state value of 1pu according to the instantaneous voltage drop. The margin coefficient at this time can be calculated.

[0022] Optionally, in the reactive power priority scenario, the voltage drop on the low-frequency side is made greater than that on the power frequency side. According to the voltage drop moment, the voltage on the low-frequency side is made to drop directly to 0.2pu, and the margin coefficient at this time can be calculated.

[0023] Optionally, if the voltage amplitude drops below 90%, it is determined that the power grid is in a fault state.

[0024] Optionally, the control method changes the AC voltage instruction on the low-frequency side of M3C to force the wind farm to reduce its output power according to its own low voltage ride-through method, thereby adjusting the power balance on both sides of M3C. The M3C power frequency side adopts a control method that prioritizes active power balance and outputs residual reactive power to support the grid-side voltage drop.

[0025] A fault ride-through control system for a modular multi-level matrix converter, the control system comprising:

[0026] A voltage detection module is used to detect the current grid voltage amplitude and determine the grid status based on the voltage amplitude;

[0027] A demand determination module, used to determine the system's demand for active power and reactive power;

[0028] The control module is used to switch the M3C low-frequency side voltage instruction and the power frequency side reactive current according to the grid state and the demand.

[0029] The beneficial effect of adopting the above technical solution is that the fault ride-through control method does not require any additional hardware, fully utilizes the control capability of M3C itself, and can effectively improve the fault ride-through capability of the modular multi-level matrix converter in the offshore wind power frequency division transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] Figure 1 This is a structural diagram of the wind farm frequency division transmission system in the present invention;

[0032] Figure 2 This is a block diagram of the M3C low-frequency side fault ride-through control method of the present invention;

[0033] Figure 3 This is a block diagram of the M3C power frequency side fault ride-through control method of the present invention;

[0034] Figure 4 This is a control flow chart of a fault ride-through control method for a modular multi-level matrix converter in an offshore wind power frequency-fractionated transmission system according to the present invention;

[0035] Figure 5 is a voltage waveform diagram of the system when a power grid fails in an embodiment of the present invention;

[0036] Figure 6 is a waveform diagram of active and reactive power of the system when a power grid fails in an embodiment of the present invention;

[0037] Figure 7 is a current waveform diagram of the system when a power grid fails in an embodiment of the present invention;

[0038] Figure 8 1 is a waveform diagram of the average capacitor voltage of the M3C submodule when a power grid fault occurs in an embodiment of the present invention; DETAILED DESCRIPTION

[0039] The following is a clear and complete description of the technical solutions in the embodiments of the present invention, in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts are within the scope of protection of the present invention.

[0040] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0041] In general, the present invention discloses a fault ride-through control method for a modular multi-level matrix converter in an offshore wind power frequency division transmission system. The wind farm frequency division transmission system structure is as follows: Figure 1 As shown in Figure 1, the output power of the offshore wind farm is transmitted to the low-frequency side of M3C through the low-frequency submarine cable, and the industrial frequency side of M3C is connected to the 50Hz power grid.

[0042] Figure 2 This is the block diagram of the M3C low-frequency side fault ride-through control method. gd When the voltage drops, the low-frequency side voltage command U sd_ref Switch to

[0043]

[0044] Among them, U gd is the per-unit value of the grid voltage after the drop, k is the margin coefficient, is the actual average voltage per unit value of the M3C submodule capacitor, is the average voltage per unit value that the M3C submodule capacitor can withstand.

[0045] As shown in formula (1), the voltage on the low-frequency side follows the voltage drop on the power-frequency side, forcing the wind farm output to decrease. At the same time, the average capacitor voltage of the M3C submodule is used The margin can be increased and the margin coefficient k can be changed to adjust the drop amplitude to adapt to three different power regulation demand scenarios.

[0046] Figure 3This is the block diagram of the M3C power frequency side fault ride-through control method. gd When the power frequency side drops, the reactive current reference value switches to

[0047]

[0048] Among them, i gmax is the maximum allowable current value of M3C, i gd_ref It is the reference value of the active current on the power frequency side of M3C.

[0049] As shown in formula (2), the power frequency side switches to a control method that prioritizes active power balance and generates reactive power in the remainder. Under the premise of ensuring power balance and DC voltage stability on both sides, reactive power is output as much as possible to provide voltage support.

[0050] Further, Figure 4 This is a flow chart of the fault ride-through control method. The control method changes the AC voltage command on the low-frequency side of the M3C to force the wind farm to reduce its output power according to its own low-voltage ride-through method, thereby adjusting the power balance on both sides of the M3C. The M3C power frequency side adopts a control method that prioritizes active power balance and outputs residual reactive power to support the grid-side voltage drop. Based on the system's requirements for regulating active and reactive power, the control scheme is divided into three scenarios: active and reactive power balance, active power priority, and reactive power priority. The process includes:

[0051] Step 1: Detect the current grid voltage amplitude. If the voltage amplitude drops below 90%, it is determined that the grid is in a fault state.

[0052] Step 2: Switch the M3C low-frequency side voltage command and power frequency side control strategy according to the grid status determined in step 1;

[0053] Step 2-1: When the grid voltage U gd When the voltage drops, the low-frequency side voltage command U sd_ref Switch to

[0054]

[0055] Among them, U gd is the per-unit value of the grid voltage after the drop, k is the margin coefficient, is the actual average voltage per unit value of the M3C submodule capacitor, is the average voltage per unit value that the M3C submodule capacitor can withstand.

[0056] As shown in formula (1), the voltage on the low-frequency side follows the voltage drop on the power-frequency side, forcing the wind farm output to decrease. At the same time, the average capacitor voltage of the M3C submodule is used The margin can be increased and the margin coefficient k can be changed to adjust the drop amplitude to adapt to three different power regulation demand scenarios.

[0057] Step 2-2: When the grid voltage U gd When the power frequency side drops, the reactive current reference value switches to

[0058]

[0059] Among them, i gmax is the maximum allowable current value of M3C, i gd_ref It is the reference value of the active current on the power frequency side of M3C.

[0060] As shown in formula (2), the power frequency side switches to a control method that prioritizes active power balance and generates reactive power in the remainder. Under the premise of ensuring power balance and DC voltage stability on both sides, reactive power is output as much as possible to provide voltage support.

[0061] Step 3: Determine the low-frequency side voltage command in step 2 according to the system's demand for active and reactive power, which is divided into three scenarios: active and reactive power consideration, active power priority, and reactive power priority.

[0062] In the scenario where both active and reactive power are taken into consideration, the low-frequency side voltage drops with the power-frequency side voltage by the same amplitude. At this time, the margin coefficient k in the low-frequency side voltage command is 0.

[0063] In the active power priority scenario, in order to maximize the active power transmission of M3C and reduce wind curtailment, the voltage drop on the low-frequency side is made smaller than that on the power frequency side. According to the voltage drop moment, the low-frequency side voltage is maintained at a steady-state value of 1pu. The margin coefficient at this time can be calculated.

[0064] In the reactive power priority scenario, in order to maximize the reactive power output of the M3C power frequency side and reduce the active power transmitted to the low frequency side, the voltage drop on the low frequency side is made greater than that on the power frequency side. According to the voltage drop moment, the voltage on the low frequency side is directly dropped to 0.2pu (the voltage when the wind turbine is not disconnected from the grid under the most serious fault condition). The margin coefficient at this time can be calculated.

[0065] In order to further demonstrate the feasibility and correctness of the implementation plan, this note simulates the three-phase symmetrical drop calculation example of the grid voltage based on Matlab / Simulink. The system simulation parameters are as follows: the rated power of the wind farm is 12.5MW, the submarine cable transmission distance is 30km; the low-frequency side frequency is 20Hz, and the effective value of the line voltage is 10kV; the rated capacity of the M3C converter is 12.5MW, the submodule capacitance is 10mF, the submodule average capacitance voltage is 4500V, the number of bridge arm submodules is 6, and the bridge arm inductance is 10mH; the power frequency side frequency is 50Hz, and the effective value of the line voltage is 35kV. Before the grid fault occurred, the wind farm output rated power and the system operated at unity power factor; at 1.2s, a three-phase symmetrical fault occurred in the power frequency side grid voltage, and the voltage dropped to 0.4pu, lasting for 625ms, and the grid voltage recovered at 1.825s. The simulation waveforms of the system voltage, power, current, and submodule average capacitance voltage are shown as follows. Figure 5 、 Figure 6 、 Figure 7 and Figure 8 As shown, where V wd / V wq , I wd / I wq 、P wabc / Q wabc are the dq axis voltage, current component and active / reactive power component at the wind farm side respectively; V sd / V sq , I sd / I sq 、P sabc / Q sabc are the dq axis voltage, current components and active / reactive power components of the low-frequency side of M3C respectively; V gd / V gq , I gd / I gq 、P guvw / Q guvw are the dq axis voltage, current component and active / reactive power component of the M3C power frequency side, V dc_avg is the average capacitor voltage of each submodule.

[0066] like Figure 5 、 6 , 7, and 8, the voltage V gd During the dip period, the power frequency side current increases to the limit value of 1.1pu. (1) When active and reactive power are controlled, the power P transmitted by the wind farm to the low frequency side of M3C is sabc As the grid voltage drops, the active power P on both sides drops. sabc With P guvw Close to balance, the average capacitor voltage of the submodule is V dc_avg Stable at 1pu, while the power frequency side outputs reactive power Qguvw Grid voltage V gd It has a certain supporting effect (from 0.4pu to about 0.44pu). (2) When active power priority control is adopted, the voltage V gd Drops to 0.4pu, low-frequency side voltage V sd The initial drop is maintained at 1pu, and the power imbalance on both sides of M3C charges the submodule capacitor, V dc_avg It increases slowly but remains within the threshold of 1.2pu. dc_avg Increase, V sd Slowly decrease, P sabc With P wabc The constant amplitude drop in active and reactive power control is changed to a slow drop, which can provide more active power to the system. After the grid voltage recovers, the power stored in the M3C bridge arm capacitor can continue to be transmitted to the grid. (3) When reactive power priority control is adopted, the voltage V sd Directly drop to 0.2pu, and control the V sd =0.44pu, the voltage and active power drop more. After the active power input of M3C is reduced, its power frequency side has more margin to output reactive power, thereby providing more voltage support to the faulty grid (from 0.4pu to 0.48pu). dc_avg Stable at 1pu.

[0067] The present invention also provides a fault ride-through control system for a modular multi-level matrix converter, the control system comprising:

[0068] A voltage detection module is used to detect the current grid voltage amplitude and determine the grid status based on the voltage amplitude;

[0069] A demand determination module, used to determine the system's demand for active power and reactive power;

[0070] The control module is used to switch the M3C low-frequency side voltage instruction and the power frequency side reactive current according to the grid state and the demand.

[0071] The various embodiments in this specification are described in a progressive manner. Similar parts between the various embodiments can be referred to in conjunction with each other. Each embodiment focuses on the differences between the other embodiments. In particular, the system embodiments are generally similar to the method embodiments, so the description is relatively simple. For relevant parts, refer to the description of the method embodiments.

[0072] The foregoing is merely an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application should all be included within the scope of the claims of the present application.

Claims

1. A fault ride-through control method for a modular multi-level matrix converter, characterized in that: The control method comprises the following steps: Step 1: Detect the current grid voltage amplitude and determine the grid state based on the voltage amplitude; Step 2: Determine the system's requirements for active power and reactive power; Step 3: Switch the M3C low-frequency side voltage instruction and the power frequency side reactive current according to the grid status and the demand; Step 3-1: When the grid voltage U gd When the voltage drops, the low-frequency side voltage command U sd_ref Switch to Among them, U gd is the per-unit value of the grid voltage after the drop, k is the margin coefficient, is the actual average voltage per unit value of the M3C submodule capacitor, The maximum per-unit average voltage that the M3C submodule capacitor can withstand; Step 3-2: When the grid voltage U gd When the power frequency side drops, the reactive current reference value switches to Among them, i gmax is the maximum current value allowed by M3C, i gd_ref It is the reference value of the active current on the power frequency side of M3C.

2. The fault ride-through control method of a modular multi-level matrix converter according to claim 1, wherein: The step 3 includes: determining a low-frequency side voltage instruction according to the demand.

3. The fault ride-through control method of a modular multi-level matrix converter according to claim 1, wherein: The system's demand for active power and reactive power is divided into three scenarios: active and reactive power balance, active power priority, and reactive power priority.

4. The fault ride-through control method of a modular multi-level matrix converter according to claim 3, wherein: In the active and reactive power consideration scenario, the low-frequency side voltage drops with the power-frequency side voltage in equal amplitude. At this time, the margin coefficient k in the low-frequency side voltage command is 0.

5. The fault ride-through control method of a modular multi-level matrix converter according to claim 3, wherein: In the active power priority scenario, the voltage drop on the low-frequency side is made smaller than that on the power frequency side. The voltage on the low-frequency side is maintained at a steady-state value of 1pu according to the instantaneous voltage drop. The margin coefficient at this time can be calculated.

6. The fault ride-through control method of a modular multi-level matrix converter according to claim 3, wherein: In the reactive power priority scenario, the voltage drop on the low-frequency side is set to be greater than that on the power frequency side. The voltage on the low-frequency side is set to drop directly to 0.2pu according to the instant of voltage drop. The margin coefficient at this time can be calculated.

7. The fault ride-through control method of a modular multi-level matrix converter according to claim 1, wherein: If the voltage amplitude drops below 90%, the grid is determined to be in a fault state.

8. The fault ride-through control method for a modular multi-level matrix converter according to claim 1, wherein: The control method changes the AC voltage command on the low-frequency side of the M3C to force the wind farm to reduce its output power according to its own low voltage ride-through method, thereby adjusting the power balance on both sides of the M3C. The M3C power frequency side adopts a control method that prioritizes active power balance and outputs residual reactive power to support the grid-side voltage drop.

9. A fault ride-through control system for a modular multi-level matrix converter, characterized in that: According to any one of claims 1 to 8, the control system comprises: A voltage detection module is used to detect the current grid voltage amplitude and determine the grid status based on the voltage amplitude; A demand determination module, used to determine the system's demand for active power and reactive power; The control module is used to switch the M3C low-frequency side voltage instruction and the power frequency side reactive current according to the grid state and the demand.

Images