Fault ride-through method of grid-forming wind turbine through diode rectification direct current sending-out system

Abstract

The application discloses a kind of network type fan through diode rectification DC sending system fault ride-through method, for the power surplus problem of network type fan through diode rectification sending system under the fault of receiving end ac power grid, a kind of fault ride-through method is proposed, receiving end MMC converter and the collaborative control strategy of wind turbine generator set are used, when the fault of receiving end ac power grid occurs, surplus power of DC power transmission system is transferred to fan DC bus, and surplus power is realized by fan crowbar circuit The accommodation, without additional measures such as energy consumption device of DC power transmission system, the fault ride-through of network type fan through diode rectification DC sending system can be realized, with significant economic benefits and good application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy power generation, and particularly relates to a fault ride-through method of a grid-connected wind turbine through a diode rectification DC sending-out system. BACKGROUND

[0002] At present, offshore wind power is developing towards the deep sea, and the flexible DC power transmission technology based on the modular multilevel converter (MMC) topology has the advantages of low manufacturing difficulty, low system loss, high waveform quality, etc., and has a very good application prospect in long-distance power collection and transmission. However, in a large-capacity offshore wind farm HVDC system, the offshore converter station using the MMC topology structure has a large volume and weight, and its construction and transportation cost is very high, which is not conducive to the on-grid parity of offshore wind power. Compared with the MMC topology structure, the use of diode topology can significantly reduce the volume and weight of the equipment, thereby reducing the construction cost of the offshore converter station. Since the diode topology cannot control the offshore AC voltage, the wind turbine sent out using this topology structure needs to use a grid-connected control strategy.

[0003] The sending-end AC grid voltage and the DC voltage of the DC power transmission system jointly determine the transmission power of the diode converter. When the receiving-end AC grid fails, the surplus power will cause the DC bus voltage to rise, at which time the active power output by the diode will decrease, causing the surplus power to accumulate in the offshore AC grid. This fault characteristic is significantly different from the offshore wind power DC transmission system based on the MMC converter, and the fault ride-through method needs to be further studied in combination with the control strategy of the grid-connected wind turbine. SUMMARY

[0004] The purpose of the present application is to solve the problem of power surplus of a grid-connected wind turbine through a diode rectification DC sending-out system under a receiving-end AC grid fault, and to provide a fault ride-through method of a grid-connected wind turbine through a diode rectification DC sending-out system. Through the coordinated control of the receiving-end MMC converter and the wind turbine, the surplus power of the system is transferred to the wind turbine DC bus, and the surplus power is consumed through the wind turbine crowbar circuit, so that the fault ride-through of the system is realized without the need for additional energy consumption devices of the DC power transmission system.

[0005] In order to achieve the above-mentioned purpose of the application, the method adopts the following technical scheme:

[0006] A fault ride-through method of a grid-connected wind turbine through a diode rectification DC sending-out system, characterized in that the grid-connected wind turbine through a diode rectification DC sending-out system comprises: a wind turbine, a wind turbine machine-side converter, a wind turbine crowbar circuit, a wind turbine grid-side converter, a wind turbine booster transformer, a sending-end coupling transformer, a sending-end diode converter, a receiving-end MMC converter, and a receiving-end coupling transformer.

[0007] The wind turbine is connected to the offshore AC power grid via a turbine-side converter, a turbine DC bus, and a turbine grid-side converter. The voltage is then stepped up to the offshore AC bus voltage level by a turbine step-up transformer. The turbine crowbar circuit is connected in parallel to the turbine DC bus. Multiple turbine step-up transformers converge on the offshore AC bus and step up the voltage to a level matching the DC transmission system via a sending-end connection transformer. The voltage is then rectified to DC by a sending-end diode converter, transmitted to the receiving end, and inverted to AC by a receiving-end MMC converter. Finally, the voltage is connected to the receiving-end AC power grid via a receiving-end connection transformer.

[0008] The control system used to achieve the fault ride-through method includes: a wind turbine grid-side converter control system, a wind turbine machine-side converter control system, a wind turbine skid circuit control system, and a receiving-end MMC converter control system.

[0009] The wind turbine grid-side converter control system adopts a grid-type control strategy based on active-frequency coupling and reactive-voltage coupling, which is responsible for maintaining the stability of the sending-end AC grid. During the system fault ride-through phase, when the angular frequency of the sending-end AC grid increases, it can reduce the active power output of the wind turbine grid-side converter.

[0010] The wind turbine-side converter control system adopts a constant DC bus voltage and reactive power control mode during normal system operation to maintain the stability of the wind turbine DC bus voltage; during the system fault ride-through phase, it switches to a constant active power and reactive power control mode, with the active power reference value given by the maximum power point tracking module.

[0011] The wind turbine crowbar circuit control system adjusts its own activation status according to the DC bus voltage of the wind turbine. It intermittently activates during the fault ride-through phase, consumes the surplus power in the system, and maintains the DC bus voltage of the wind turbine within a certain range.

[0012] The receiving-end MMC converter control system adopts a constant DC bus voltage and reactive power control strategy. Based on the angular frequency of the sending-end AC grid, it adjusts the DC bus voltage of the DC transmission system to maintain the active power balance within the system.

[0013] Furthermore, the specific implementation method of the wind turbine-side converter control system is as follows: During the normal operation phase of the system, a constant DC bus voltage and reactive power control mode is adopted, and the specific implementation method is as follows:

[0014] i sdref =F PI7 (s)(Q sref -Q s )

[0015]

[0016] i sqref= F PI5 (s) (U dcref - dc )

[0017]

[0018] wherein: F PI5 (s) is the transfer function of the DC bus voltage PI controller, k p5 is the proportional coefficient, k i5 is the integral coefficient, F PI7 (s) is the transfer function of the reactive power PI controller, k p7 is the proportional coefficient, k i7 is the integral coefficient, i sdref , i sqref correspond to the d-axis, q-axis components of the current vector I sdqref , U dcref is the fan DC bus voltage reference value, U dc is the fan DC bus voltage, Q sref is the reactive power reference value, Q s is the reactive power;

[0019] In the system fault ride-through phase, switch to the fixed active power and reactive power control mode, and the active power reference value is given by the maximum power tracking module, and the specific implementation manner is as follows:

[0020] i sdref = F PI7 (s) (Q sref -Q s )

[0021]

[0022] i sqref = F PI6 (s) (P sref -P s )

[0023]

[0024] wherein: F PI6 (s) is the transfer function of the active power PI controller, k p6 is the proportional coefficient, k i6 is the integral coefficient, F PI7 (s) is the transfer function of the reactive power PI controller, k p7 is the proportional coefficient, k i7 is the integral coefficient, i sdref , i sqref correspond to the d-axis, q-axis components of the current vector I sdqref , Psref P is an active power reference value s Q is an active power sref Q is a reactive power reference value s Q is a reactive power

[0025] The beneficial effects of the present application are:

[0026] By adopting the technical scheme of the present application, the coordinated control strategy of the receiving-end MMC converter and the wind turbine is adopted, when the receiving-end AC power grid fails, the surplus power of the DC power transmission system can be transferred to the wind turbine DC bus, and the surplus power is consumed through the wind turbine crowbar circuit, without the need for additional measures such as energy consumption devices of the DC power transmission system, so that the fault ride-through of the grid-connected wind turbine through the diode rectification DC transmission system can be realized, which has significant economic benefits and good application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The fault ride-through flow chart of the grid-connected wind turbine through the diode rectification DC transmission system fault ride-through method of the present application.

[0028] Figure 2 The typical topology diagram of the grid-connected wind turbine through the diode rectification DC transmission system of the present application.

[0029] Figure 3 The specific example system principle diagram of the wind turbine machine side converter control method of the present application.

[0030] Figure 4 The specific example system principle diagram of the wind turbine grid side converter control method of the present application.

[0031] Figure 5 The specific example system principle diagram of the receiving-end MMC converter control method of the present application. DETAILED DESCRIPTION

[0032] In order to more specifically describe the present application, the technical scheme of the present application will be described in detail below in combination with the drawings and specific embodiments.

[0033] In the embodiment of the present application, the grid-connected wind turbine through the diode rectification DC transmission system is as shown in Figure 2 The grid-connected wind turbine through the diode rectification DC transmission system includes a wind turbine 1, a wind turbine machine side converter 2, a wind turbine crowbar circuit 3, a wind turbine grid side converter 4, a wind turbine booster transformer 5, a sending-end coupling transformer 6, a sending-end diode converter 7, a receiving-end MMC converter 8, and a receiving-end coupling transformer 9.

[0034] The wind power generator 1 is connected to the offshore AC power grid through a fan machine side converter 2, a fan DC bus and a fan grid side converter 4, is boosted to the voltage level of the offshore AC bus through a fan booster transformer 5, and a fan pry bar circuit 3 is connected in parallel to the fan DC bus; after a plurality of fan booster transformers 5 are collected on the offshore AC bus, the voltage level matched with the DC power transmission system is boosted through a sending end coupling transformer 6, is rectified to DC through a sending end diode converter 7 after being sent to the receiving end, is inverted to AC through a receiving end MMC converter 8, and is connected to the receiving end AC power grid through a receiving end coupling transformer 9.

[0035] In the embodiment of the application, the schematic diagram of the fan machine side converter control system is as shown in Figure 3 The schematic diagram of the fan machine side converter control system is as shown in

[0036] The implementation mode of the DC bus voltage control module 104 is as follows:

[0037] i sqref =F PI5 (s)(U dcref -U dc )

[0038]

[0039] Wherein: F PI5 (s) is the transfer function of the PI controller, k p5 is the proportional coefficient, k i5 is the integral coefficient, i sqref is the q-axis component of the current vector I sdqref , U dcref is the fan DC bus voltage reference value, and U dc is the fan DC bus voltage.

[0040] The implementation mode of the active power control module 106 is as follows:

[0041] i sqref =F PI6 (s)(P sref -P s )

[0042]

[0043] wherein: F PI6 (s) is the transfer function of the PI controller, k p6 is the proportional coefficient, k i6 is the integral coefficient, i sqref is the q-axis component of the current vector I sdqref , P sref is the active power reference value, P s is the active power.

[0044] The implementation of the reactive power control module 103 is as follows:

[0045] i sdref = F PI7 (s)(Q sref - Q s )

[0046]

[0047] wherein: F PI7 (s) is the transfer function of the PI controller, k p7 is the proportional coefficient, k i7 is the integral coefficient, i sdref is the d-axis component of the current vector I sdqref , Q sref is the reactive power reference value, Q s is the reactive power.

[0048] The implementation of the current inner loop control module 107 is as follows:

[0049]

[0050] wherein: F PI8 (s) is the transfer function of the PI controller, k p8 is the proportional coefficient, k i8 is the integral coefficient, u sdref , u sqref correspond to the d-axis, q-axis components of the voltage vector U sdqref , i sd , i sq correspond to the d-axis, q-axis components of the current vector I sdq , ω r is the rotor angular frequency, L s is the stator inductance of the fan, and Ψ is the rotor permanent magnet flux linkage.

[0051] In the embodiment of the application, the fan grid-side converter 4 adopts a network configuration type control strategy based on active-power-frequency coupling and reactive-power-voltage coupling, and is responsible for maintaining the stability of the sending end AC power grid; during the system fault ride-through stage, when the angle frequency of the sending end AC power grid rises, the active power output by the fan grid-side converter can be reduced; the control system of the fan grid-side converter is shown in FIG. 2, and includes a maximum power tracking module 201, an active-power-frequency coupling control module 202, a reactive-power-voltage coupling control module 203, a Park transformation module 204, a voltage outer loop control module 205, a current inner loop control module 206, a Park inverse transformation module 207 and a modulation module 208. Figure 4

[0052] In the active-power-frequency coupling control module 202, the reference phase θ g (k+1) is calculated according to the following method:

[0053] θ g (k+1) = ∫ω g (k+1)dt

[0054]

[0055] wherein θ g (k+1) is the reference phase of the next sampling period, ω g (k+1) is the angle frequency of the next sampling period, ω g (k) is the angle frequency of the current sampling period, ω n is the rated angle frequency, P gref is the active power reference value, P g (k) is the active power of the current sampling period, J is the virtual rotor moment of inertia, and D p is the active damping coefficient.

[0056] In the reactive-power-voltage coupling control module 203, the d-axis voltage reference value u gdref (k+1) is calculated according to the following method:

[0057]

[0058] wherein u gdref (k+1) is the d-axis voltage reference value of the next sampling period, |U g (k)| is the voltage amplitude of the current sampling period, U ref is the voltage amplitude reference value, Q gref is the reactive power reference value, Q g (k) is the reactive power of the current sampling period, K is the virtual excitation coefficient, and D q is the reactive damping coefficient.

[0059] The implementation mode of the voltage outer loop control module 205 is as follows: ​

[0060]

[0061] wherein: F PI3 (s) is the transfer function of the PI controller, k p3 is the proportional coefficient, k i3 is the integral coefficient, i gdref , i gqref correspond to the d-axis, q-axis components of the current vector I gdqref , u gdref , u gqref correspond to the d-axis, q-axis components of the voltage vector reference value U gdqref , u gd , u gq correspond to the d-axis, q-axis components of the voltage vector reference value U gdq .

[0062] The implementation of the current inner loop control module 206 is as follows:

[0063]

[0064] wherein: F PI4 (s) is the transfer function of the PI controller, k p4 is the proportional coefficient, k i4 is the integral coefficient, u vdref , u vqref correspond to the d-axis, q-axis components of the voltage vector U vdqref , u gd , u gq correspond to the d-axis, q-axis components of the voltage vector U gdq , i gd , i gq correspond to the d-axis, q-axis components of the current vector I gdq , ω g is the grid voltage angular frequency, L g is the filter inductance.

[0065] In the embodiment of the application, the control system of the receiving-end MMC converter 8 is as shown in Figure 5 , and comprises: a DC current reference value calculation module 301, a DC voltage reference value calculation module 302, a DC bus voltage and reactive power control module 303, a differential mode current control module 304, a Park inverse transformation module 305, a common mode current control module 306, a bridge arm voltage calculation module 307, and a modulation module 308.

[0066] The specific implementation of the DC current reference value calculation module 301 is as follows:

[0067] I dcref = F PI1 (s)(ωg -ω gref )

[0068]

[0069] wherein: I dcref is the DC current reference value, ω gref is the sending end power grid angular frequency reference value, ω g is the sending end power grid angular frequency, F PI1 (s) is the transfer function of the angular frequency PI controller, k p1 is the proportional coefficient, k i1 is the integral coefficient.

[0070] The specific implementation of the DC voltage reference value calculation module 302 is as follows:

[0071] U dc2ref = U dc2N -F PI2 (s)(I dcref -I dc )

[0072]

[0073] wherein: I dcref is the DC current reference value, I dc is the DC current, U dc2ref is the DC bus voltage reference value, U dc2N is the DC bus voltage rated value, F PI2 (s) is the transfer function of the DC current PI controller, k p2 is the proportional coefficient, k i2 is the integral coefficient.

[0074] In the embodiment of the application, the control system of the fan crowbar circuit 3 adjusts its input state according to the fan DC bus voltage, is intermittently put into operation in the fault ride-through stage, consumes the surplus power in the system, and maintains the fan DC bus voltage within a certain range.

[0075] In the embodiment of the application, the specific process of the wind power diode sending-out system starting method based on active-frequency coupling network construction is as follows: Figure 1When the receiving end AC power grid fails, the output power capability of the receiving end MMC converter decreases, and the surplus power will cause the DC bus voltage of the DC power transmission system to rise, while the sending end AC grid voltage remains unchanged, so the sending end diode converter output active power decreases, at this time the sending end grid angular frequency has a rising trend. Since the wind turbine grid-side converter adopts the active-frequency coupling grid-forming control strategy, the wind turbine grid-side converter will maintain the sending end grid angular frequency stable by reducing the active power, at this time the wind turbine machine-side converter switches from the constant DC bus voltage control mode to the constant active power control mode, and the wind turbine machine-side converter outputs active power according to the actual output power of the wind turbine, so that the power starts to accumulate on the wind turbine DC bus, causing the wind turbine DC bus voltage to rise, and the wind turbine crowbar circuit is put into operation to consume the surplus power. After the receiving end AC power grid recovers to normal, the DC bus voltage of the DC power transmission system recovers to the rated value, the sending end diode converter output active power capability recovers, and the sending end AC grid angular frequency has a decreasing trend. Since the active-frequency coupling grid-forming control strategy is adopted, the wind turbine grid-side converter output active power increases, at this time the wind turbine machine-side converter switches from the constant active power control mode to the constant DC bus voltage control mode, the wind turbine DC bus voltage recovers to the rated value, the wind turbine crowbar circuit exits operation, and the system recovers to the normal operating state.

[0076] The above description of the embodiments is to facilitate the understanding and application of the present application by those skilled in the art. Those skilled in the art can easily make various modifications to the above embodiments, and apply the general principles described herein to other embodiments without inventive labor. Therefore, the present application is not limited to the above embodiments, and the improvements and modifications of the present application made by those skilled in the art according to the disclosure of the present application should be within the scope of protection of the present application.

Claims

1. A method for grid forming wind turbine fault ride through of a diode rectified DC transmission system, characterized in that, The network type fan rectified by a diode direct current sending-out system comprises a wind turbine, a fan machine side converter, a fan pry bar circuit, a fan network side converter, a fan booster transformer, a sending end coupling transformer, a sending end diode converter, a receiving end MMC converter and a receiving end coupling transformer. The wind turbine is connected to the offshore AC power grid through the fan machine side converter, the fan DC bus and the fan network side converter, and is boosted to the voltage level of the offshore AC bus through the fan booster transformer; the fan pry bar circuit is connected in parallel to the fan DC bus; after being collected on the offshore AC bus, a plurality of fan booster transformers are boosted to the voltage level matched with the DC power transmission system through the sending end coupling transformer, are rectified to DC through the sending end diode converter, are inverted to AC through the receiving end MMC converter after being transmitted to the receiving end, and are connected to the receiving end AC power grid through the receiving end coupling transformer; The control system for realizing the fault ride-through method comprises a fan network side converter control system, a fan machine side converter control system, a fan pry bar circuit control system and a receiving end MMC converter control system; The fan network side converter control system adopts a network type control strategy based on active power-frequency coupling and reactive power-voltage coupling, and is responsible for maintaining the stability of the sending end AC power grid; during the system fault ride-through stage, the active power output by the fan network side converter can be reduced when the angle frequency of the sending end AC power grid is increased; The fan machine side converter control system adopts a constant DC bus voltage and reactive power control mode during the normal operation stage of the system, is responsible for maintaining the stability of the fan DC bus voltage, and switches to a constant active power and reactive power control mode during the system fault ride-through stage, with the active power reference value being given by a maximum power tracking module; The fan pry bar circuit control system adjusts its input state according to the fan DC bus voltage, is intermittently put into operation during the fault ride-through stage, consumes the surplus power in the system, and maintains the fan DC bus voltage within a certain range; The receiving end MMC converter control system adopts a constant DC bus voltage and reactive power control strategy, adjusts the DC bus voltage of the DC power transmission system according to the angle frequency of the sending end AC power grid, and maintains the active power balance in the system; During the system fault ride-through stage, the receiving end MMC converter control system switches to a constant active power and reactive power control mode, with the active power reference value being given by a maximum power tracking module, and the specific implementation manner is as follows: i sdref = F PI7 (s) (Q sref - Q s ) i sqref = F PI6 (s) (P sref - P s ) Wherein: F PI6 (s) is the transfer function of the active power PI controller, k p6 k is the proportionality coefficient. i6 F is the integral coefficient. PI7 (s) is the transfer function of the reactive power PI controller, k p7 k is the proportionality coefficient. i7 Let i be the integral coefficient. sdref i sqref Corresponding to the current vector I sdqref d-axis and q-axis components, P sref P is the active power reference value. s For active power, Q sref Q is the reactive power reference value. s This refers to reactive power.

2. The network type fan rectified by a diode direct current sending-out system fault ride-through method according to claim 1, characterized in that: The specific implementation manner of the fan machine side converter control system is as follows: during the normal operation stage of the system, a constant DC bus voltage and reactive power control mode is adopted, and the specific implementation manner is as follows: i sdref = F PI7 (s) (Q sref - Q s ) i sqref = F PI5 (s) (U dcref - U dc ) Wherein: F PI5 (s) is the transfer function of the DC bus voltage PI controller, k p5 is the proportional coefficient, k i5 is the integral coefficient, F PI7 (s) is the transfer function of the reactive power PI controller, k p7 is the proportional coefficient, k i7 is the integral coefficient, i sdref , sqref corresponding to the d-axis, q-axis components of the current vector I sdqref , dcref is the fan DC bus voltage reference value, U dc is the fan DC bus voltage, Q sref is the reactive power reference value, Q s is the reactive power.

Images