Fault ride-through method based on fan crowbar and energy dissipation device of flexible direct current system

Abstract

The application discloses a kind of based on fan crowbar and the failure crossing method of HVDC system energy consumption device, in view of the higher cost of existing HVDC system energy consumption device design scheme, and only rely on fan crowbar circuit cannot guarantee offshore wind power-HVDC system reliable failure crossing problem, research the cooperative control strategy of HVDC system and wind turbine converter, utilize wind turbine unit's own crowbar circuit and the energy consumption device of HVDC system is completed together failure crossing operation, under the condition of guaranteeing system reliable failure crossing, reduce the capacity of HVDC system energy consumption device, effectively reduce engineering construction cost.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation technology, specifically relating to a fault ride-through method based on a wind turbine crowbar and a flexible vertical system energy dissipation device. Background Technology

[0002] Currently, the construction of offshore wind farms is gradually shifting from short-distance, small-capacity projects to large-scale, deep-sea operations. Traditional AC transmission technology, due to its mature technology and low equipment cost, is well-suited for near-shore wind farms with lower capacity and shorter distances. However, when facing deep-sea areas, it suffers from high line costs, significant reactive power losses, and weak support for wind farms. Therefore, when the line length is long, flexible high-voltage direct current (HVDC) transmission technology is generally a more economical and efficient transmission solution.

[0003] To address the power surplus problem in flexible DC transmission systems during AC grid failures, the current mainstream solution is to connect energy-consuming devices in parallel with the DC bus. However, for high-power DC transmission systems, the capacity requirements of these energy-consuming devices are substantial, construction costs are high, and heat dissipation is difficult, all of which negatively impact the system's safe and stable operation as well as its economic benefits.

[0004] Currently, all full-power wind turbines in actual operation are equipped with skid circuits of unit-level capacity to cope with grid faults. However, when a fault occurs in the onshore AC grid, the control objective of offshore converter stations is generally to maintain the frequency and voltage stability of the offshore AC grid, thus isolating the impact of the fault on the DC side. The wind turbines are less directly impacted and often do not activate their skid circuits during the fault. In fact, if all wind turbines could activate their skid circuits immediately after an AC grid fault, the overall output power of the wind farm could be rapidly reduced, preventing a large amount of surplus power from being generated on the DC bus of the flexible DC transmission system. However, the effectiveness of this fault ride-through strategy relies on the wind turbines' ability to quickly detect onshore AC grid faults. The action time of the flexible DC transmission system's fault ride-through strategy needs to be on a millisecond timescale; relying solely on wind turbine skid circuits cannot guarantee reliable fault ride-through for the system. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of high cost of existing flexible DC system energy consumption device designs and the inability to guarantee reliable fault ride-through of offshore wind power-flexible DC systems by relying solely on wind turbine crowbar circuits. This invention provides a collaborative control strategy for flexible DC systems and wind turbine converters, which utilizes the wind turbine's own crowbar circuits and the energy consumption devices of the flexible DC system to jointly complete fault ride-through operation, thereby reducing the capacity of the flexible DC system's energy consumption devices and effectively reducing engineering construction costs.

[0006] To achieve the above-mentioned objectives, this method adopts the following technical solution:

[0007] A fault ride-through method based on a wind turbine crowbar and a flexible DC system energy dissipation device is characterized in that: the fault ride-through method utilizes the wind turbine crowbar circuit and the flexible DC system energy dissipation device, and achieves system fault ride-through operation through coordinated control of the receiving-end MMC converter, the sending-end MMC converter, and the wind turbine. The specific fault ride-through steps are as follows:

[0008] When a fault occurs in the AC grid at the receiving end, the voltage of the AC grid at the receiving end decreases, the output current of the MMC converter at the receiving end reaches the limit value, the output power of the MMC converter at the receiving end is limited, and the DC bus voltage of the flexible DC system increases.

[0009] When the DC bus voltage of the flexible DC system exceeds the first voltage reference value, the AC voltage reference value U at the sending end is reduced through the sending-end MMC converter. gdqref ;

[0010] When the AC grid voltage at the sending end decreases, the AC side voltage of the wind turbine grid-side converter decreases, the output current of the wind turbine grid-side converter reaches the limit value, and surplus power is generated inside the wind turbine. The surplus power inside the wind turbine causes the DC bus voltage of the wind turbine to rise, the wind turbine crowbar circuit is put into operation, consumes the surplus power, and maintains the stability of the DC bus voltage of the wind turbine.

[0011] When a single-phase ground fault or a two-phase ground fault occurs in the receiving-end power grid, the DC bus voltage of the flexible DC system rises slowly and will not rise to the second voltage reference value during the fault ride-through process. In this case, in the wind turbine crowbar circuit and the flexible DC system energy consumption device, the system fault ride-through can be achieved by consuming the surplus power through the wind turbine crowbar circuit alone.

[0012] When a three-phase ground fault occurs in the receiving-end power grid, the DC bus voltage of the flexible DC system rises rapidly and will rise to the second voltage reference value during the fault ride-through process. When the DC bus voltage of the flexible DC system exceeds the second voltage reference value, the energy dissipation device of the flexible DC system is also put into operation to ensure that the DC bus voltage of the flexible DC system does not rise to the system protection threshold during the fault ride-through process.

[0013] After the AC grid fault at the receiving end is cleared, the output power capacity of the receiving-end MMC converter is restored, and the energy-consuming devices of the flexible DC system are taken out of operation; the sending-end AC voltage reference value U is transmitted through the sending-end MMC converter. gdqref Adjust to the rated value;

[0014] After the AC voltage at the sending end is adjusted to the rated value, the output power capacity of the wind turbine grid-side converter is restored. The DC bus voltage is adjusted to the rated value through the wind turbine grid-side converter, the wind turbine skid circuit is taken out of operation, and the fault ride-through process ends.

[0015] Furthermore, the sending-end MMC control system for implementing the control method includes: a sending-end Park transformation module, a sending-end AC voltage reference value calculation module, a sending-end AC voltage control module, a sending-end positive sequence current control module, a sending-end negative sequence current control module, a sending-end differential mode voltage calculation module, a sending-end Park inverse transformation module, a sending-end internal circulating current control module, a sending-end reference voltage calculation module, and a modulation module;

[0016] The AC voltage reference value calculation module at the sending end is based on the DC bus voltage U of the flexible DC system. dc Adjust the AC voltage reference value U gdqref The specific implementation method is as follows:

[0017]

[0018] u gqref =0

[0019] Among them, u gdref and u gqref The reference value of the sending end voltage U gdqref The d-axis and q-axis components.

[0020] Furthermore, the wind turbine grid-side converter control system that implements the aforementioned control method adopts a dual-loop grid-following control strategy, consisting of an outer loop for DC bus voltage and reactive power control and an inner loop for current control.

[0021] The beneficial effects of this invention are:

[0022] By adopting the technical solution of this invention, the fault ride-through operation can be completed by using the wind turbine's own crowbar circuit and the energy dissipation device of the flexible DC system. Under the condition of ensuring reliable fault ride-through of the system, the capacity of the energy dissipation device of the flexible DC system is reduced, effectively reducing the engineering construction cost. Attached Figure Description

[0023] Figure 1 The present invention provides a flowchart of a fault ride-through method based on a wind turbine skid bar and a flexible vertical system energy dissipation device.

[0024] Figure 2 This is a typical topology diagram of the offshore wind power flexible DC transmission system of the present invention. It includes: 1-Wind turbine generator, 2-Wind turbine-side converter, 3-Wind turbine skid circuit, 4-Wind turbine grid-side converter, 5-Wind turbine step-up transformer, 6-Sending-end connection transformer, 7-Sending-end MMC converter, 8-Flexible DC system energy dissipation device, 9-Receiving-end MMC converter, 10-Receiving-end connection transformer, 11-Receiving-end AC grid.

[0025] Figure 3This is a schematic diagram of a specific example of the sending-end converter valve control system in this invention, including: 101-Park conversion module, 102-AC voltage reference value calculation module, 103-AC voltage control module, 104-positive sequence current control module, 105-negative sequence current control module, 106-differential mode voltage reference value calculation module, 107-Park inverse conversion module, 108-internal circulating current control module, 109-bridge arm voltage calculation module, and 110-modulation module. Detailed Implementation

[0026] 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.

[0027] like Figure 2 As shown, in this embodiment of the invention, the offshore wind power flexible DC transmission system includes: a wind turbine generator 1, a wind turbine-side converter 2, a wind turbine skid circuit 3, a wind turbine grid-side converter 4, a wind turbine step-up transformer 5, a sending-end connection transformer 6, a sending-end MMC converter 7, a flexible DC system energy dissipation device 8, a receiving-end MMC converter 9, a receiving-end connection transformer 10, and a receiving-end AC grid 11.

[0028] like Figure 1 As shown, in this embodiment of the invention, the specific fault-crossing steps are as follows:

[0029] When a fault occurs in the AC grid at the receiving end, the voltage of the AC grid at the receiving end decreases, the output current of the MMC converter 9 at the receiving end reaches the limit value, the output power of the MMC converter 9 at the receiving end is limited, and the DC bus voltage of the flexible DC system increases.

[0030] When the DC bus voltage of the flexible DC system exceeds the first voltage reference value, the sending-end AC voltage reference value U is reduced using the sending-end MMC converter 7. gdqref ;

[0031] When the AC grid voltage at the sending end decreases, the AC side voltage of the wind turbine grid-side converter 4 decreases, the output current of the wind turbine grid-side converter 4 reaches the limit value, and surplus power is generated inside the wind turbine.

[0032] The excess power inside the fan causes the DC bus voltage of the fan to rise. The fan crowbar circuit 3 is put into operation to consume the excess power and maintain the DC bus voltage of the fan stable.

[0033] When a single-phase ground fault or a two-phase ground fault occurs in the receiving-end power grid, the DC bus voltage of the flexible DC system rises slowly and will not rise to the second voltage reference value during the fault ride-through process. At this time, in the wind turbine crowbar circuit 3 and the flexible DC system energy consumption device 8, the surplus power can be consumed by the wind turbine crowbar circuit 3 to achieve system fault ride-through.

[0034] When a three-phase ground fault occurs in the receiving-end power grid, the DC bus voltage of the flexible DC system rises rapidly and will rise to the second voltage reference value during the fault ride-through process. When the DC bus voltage of the flexible DC system exceeds the second voltage reference value, the energy dissipation device 8 of the flexible DC system is also put into operation to ensure that the DC bus voltage of the flexible DC system does not rise to the system protection threshold during the fault ride-through process.

[0035] After the AC grid fault at the receiving end is cleared, the output power capacity of the receiving-end MMC converter 9 is restored, and the power dissipation device 8 of the flexible DC system is taken out of operation; the sending-end AC voltage reference value U is transmitted through the sending-end MMC converter 7. gdqref Adjust to the rated value;

[0036] After the AC voltage at the sending end is adjusted to the rated value, the output power capacity of the wind turbine grid-side converter 4 is restored. The DC bus voltage is adjusted to the rated value through the wind turbine grid-side converter 4, the wind turbine crowbar circuit 3 is taken out of operation, the offshore wind power flexible DC transmission system returns to normal operation, and the fault ride-through process ends.

[0037] The second voltage reference value is higher than the first voltage reference value but lower than the DC bus voltage protection threshold of the flexible DC system. In this embodiment of the invention, the first voltage reference value is designed to be 1.1 times the rated DC voltage, and the second voltage reference value is designed to be 1.15 times the rated DC voltage.

[0038] like Figure 3 As shown in the embodiment of the present invention, the sending-end MMC converter control system for implementing the fault ride-through method based on the wind turbine crowbar and the energy dissipation device of the flexible DC system includes: a Park conversion module 101, an AC voltage reference value calculation module 102, an AC voltage control module 103, a positive sequence current control module 104, a negative sequence current control module 105, a differential mode voltage reference value calculation module 106, a Park inverse conversion module 107, an internal circulating current control module 108, a bridge arm voltage calculation module 109, and a modulation module 110.

[0039] In this embodiment of the invention, the Park conversion module 101 respectively converts the three-phase voltage U gabc and three-phase current I gabc Performing the Park transformation yields the voltage vector U in the synchronously rotating coordinate system. gdq and current vector I gdq .

[0040] AC voltage reference value calculation module 102 calculates the DC bus voltage U of the flexible DC system. dc Adjust the AC voltage reference value U gdqref The specific implementation method is as follows:

[0041]

[0042] u gqref =0

[0043] Among them, u gdref and u gqref The reference value of the sending end voltage U gdqref The d-axis and q-axis components.

[0044] AC voltage control module 103 controls the d-axis and q-axis voltages U gdq Control is achieved through a PI controller, making it follow the given reference value u. gdref and u gqref The output of the AC voltage PI controller, after passing through a limiting circuit, is used as the reference value i for the positive sequence currents of the d and q axes, respectively. gdref+ and i gqref+ The specific implementation method of AC voltage control module 103 is as follows:

[0045]

[0046]

[0047] Wherein: F PI1 (s) is the transfer function of the AC voltage PI controller, k p1 k is the proportionality coefficient. i1 Let i be the integral coefficient. gdref+ i gqref+ Corresponding to the current vector I gdqref+ The d-axis and q-axis components.

[0048] Positive sequence current control module 104 controls the d-axis and q-axis currents I. gdq A PI controller is used for control, making it follow the reference value I. gdqref+ The output of the positive-sequence current PI controller is used as the positive-sequence differential output voltage U. difdq+ The specific implementation method of the positive sequence current control module 104 is as follows:

[0049]

[0050]

[0051] Wherein: F PI2 (s) is the transfer function of the positive-sequence current PI controller, k p2 k is the proportionality coefficient. i2 U is the integral coefficient, L1 is the equivalent inductance including the sending-end connection transformer and the bridge arm reactor, and u is the integral coefficient. difd+ ,u difq+ Corresponding to voltage vector U difdq+ d-axis and q-axis components, ω g This is the angular frequency of the grid voltage.

[0052] Negative sequence current control module 105 controls the d-axis and q-axis currents I. gdq A resonant controller is used for control, making it follow the reference value I. gdqref- The output of the negative sequence current resonant controller is used as the negative sequence differential mode output voltage U. difdq- The specific implementation method of the negative sequence current control module 105 is as follows:

[0053]

[0054]

[0055] Wherein: F R1 (s) is the transfer function of the negative-sequence current resonant controller, k g1 ω is the gain coefficient of the negative-sequence current resonant controller. c The cutoff frequency is set to ±100Hz. In this module, the resonant frequency is set to 12Hz.

[0056] Differential mode voltage reference value calculation module 106 calculates the differential mode output voltage U based on the positive sequence differential mode output voltage. difdq+ With negative sequence differential mode output voltage U difdq- The differential mode voltage reference value U is calculated. difdq .

[0057] Park inverse converter module 107 references differential voltage U. difdq Performing the inverse Park transform, we obtain the differential-mode voltage reference value U in the stationary three-phase coordinate system. difabc .

[0058] The internal circulating current control module 108 uses a resonant controller to control I. cabc To suppress the internal circulating current, the reference value is set to zero. The output of the internal circulating current resonant controller serves as the common-mode voltage reference value U. comabc The specific implementation method of the internal circulation control module 108 is as follows:

[0059]

[0060]

[0061] Wherein: F R2 (s) is the transfer function of the internal circulating resonant controller, k g2 ω is the gain coefficient of the internal circulating resonant controller. c The cutoff frequency is set to ±100Hz. In this module, the resonant frequency is set to 12Hz.

[0062] Bridge arm voltage calculation module 109 uses differential mode voltage reference value U difabcand common-mode voltage reference value U comabc The reference voltage U of the upper and lower bridge arms is calculated. prefabc with U nrefabc The specific calculation method is as follows:

[0063]

[0064] Among them, u prefa ,u prefb and u prefc Corresponding to voltage vector U prefabc The a-axis, b-axis, and c-axis components, u nrefa ,u nrefb and u nrefc Corresponding to voltage vector U nrefabc The a-axis, b-axis and c-axis components.

[0065] The modulation module 110 generates modulation commands based on the reference voltages of the upper and lower bridge arms to control the MMC converter at the sending end.

[0066] In this embodiment of the invention, the wind turbine grid-side converter control system adopts a dual-loop grid-following control strategy, which uses an outer loop for DC bus voltage and reactive power control and an inner loop for current control.

[0067] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive 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. A fault ride-through method based on a wind turbine crowbar and a flexible vertical system energy dissipation device, characterized in that: The fault ride-through method utilizes the wind turbine crowbar circuit and the power dissipation device of the flexible DC system. Through the coordinated control of the receiving-end MMC converter, the sending-end MMC converter, and the wind turbine, the system achieves fault ride-through operation. The specific fault ride-through steps are as follows: When a fault occurs in the AC grid at the receiving end, the voltage of the AC grid at the receiving end decreases, the output current of the MMC converter at the receiving end reaches the limit value, the output power of the MMC converter at the receiving end is limited, and the DC bus voltage of the flexible DC system increases. When the DC bus voltage of the flexible DC system exceeds the first voltage reference value, the AC voltage reference value U at the sending end is reduced through the sending-end MMC converter. gdqref ; When the AC grid voltage at the sending end decreases, the AC side voltage of the wind turbine grid-side converter decreases, the output current of the wind turbine grid-side converter reaches the limit value, and surplus power is generated inside the wind turbine. The surplus power inside the wind turbine causes the DC bus voltage of the wind turbine to rise, the wind turbine crowbar circuit is put into operation, consumes the surplus power, and maintains the stability of the DC bus voltage of the wind turbine. When a single-phase ground fault or a two-phase ground fault occurs in the receiving-end power grid, the DC bus voltage of the flexible DC system rises slowly and will not rise to the second voltage reference value during the fault ride-through process. In this case, in the wind turbine crowbar circuit and the flexible DC system energy consumption device, the system fault ride-through can be achieved by consuming the surplus power through the wind turbine crowbar circuit alone. When a three-phase ground fault occurs in the receiving-end power grid, the DC bus voltage of the flexible DC system rises rapidly and will rise to the second voltage reference value during the fault ride-through process. When the DC bus voltage of the flexible DC system exceeds the second voltage reference value, the energy dissipation device of the flexible DC system is also put into operation to ensure that the DC bus voltage of the flexible DC system does not rise to the system protection threshold during the fault ride-through process. After the AC grid fault at the receiving end is cleared, the output power capacity of the receiving-end MMC converter is restored, and the energy-consuming devices of the flexible DC system are taken out of operation; the sending-end AC voltage reference value U is transmitted through the sending-end MMC converter. gdqref Adjust to the rated value; After the AC voltage at the sending end is adjusted to the rated value, the output power capacity of the wind turbine grid-side converter is restored. The DC bus voltage is adjusted to the rated value through the wind turbine grid-side converter, the wind turbine skid circuit is taken out of operation, and the fault ride-through process ends.

2. The fault ride-through method based on the energy dissipation device of the wind turbine crowbar and flexible vertical system according to claim 1, characterized in that: The sending-end MMC control system for implementing the control method includes: a sending-end Park transformation module, a sending-end AC voltage reference value calculation module, a sending-end AC voltage control module, a sending-end positive sequence current control module, a sending-end negative sequence current control module, a sending-end differential mode voltage calculation module, a sending-end Park inverse transformation module, a sending-end internal circulating current control module, a sending-end reference voltage calculation module, and a modulation module. The AC voltage reference value calculation module at the sending end is based on the DC bus voltage U of the flexible DC system. dc Adjust the AC voltage reference value U gdqref The specific implementation method is as follows: u gqref ＝0 Among them, u gdref and u gqref The reference value of the sending end voltage U gdqref The d-axis and q-axis components.

3. The fault ride-through method based on the energy dissipation device of the wind turbine crowbar and flexible vertical system according to claim 1, characterized in that: The wind turbine grid-side converter control system that implements the aforementioned control method adopts a dual-loop grid-following control strategy, consisting of an outer loop for DC bus voltage and reactive power control and an inner loop for current control.

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