Cooperative control method between offshore wind power low-frequency power stations

Abstract

The invention discloses an inter-station cooperative control method for offshore wind power low-frequency power grids, and the method can achieve the power flow control of the offshore wind power low-frequency power grids while guaranteeing the voltage stability of the offshore low-frequency power grids through the reasonable design of a control strategy of an onshore M3C converter and switching the control modes of different onshore M3C converters under different operation conditions. The problem of unbalanced power flow distribution of the offshore wind power low-frequency power grid is avoided, and the method has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of renewable energy power generation, and in particular relates to a method for coordinated control between offshore wind power low-frequency power stations. Background Art

[0002] At present, the construction of offshore wind farms is gradually developing from short distance and small capacity to deep sea and large scale. When the offshore wind farm is more than a certain distance from the shore, the traditional industrial frequency AC transmission method has the problem of excessive reactive power consumption of the equivalent capacitance of the submarine cable. Since the transmission frequency of the flexible low-frequency transmission method is significantly lower than the industrial frequency transmission method, the charging power of the low-frequency AC submarine cable is small, which can improve the transmission capacity of the AC submarine cable. At the same time, compared with the flexible DC transmission method, the flexible low-frequency transmission has the advantages of current zero-crossing breaking and easy networking. It can use wind turbines to directly output low-frequency electricity without the need for an offshore converter platform. Therefore, the offshore wind power flexible low-frequency AC transmission method has the advantages of both industrial frequency AC transmission and flexible DC transmission, and has advantages in the medium and long-distance offshore wind power networking and transmission scenarios.

[0003] In the point-to-point low-frequency transmission system, the low-frequency AC voltage is established by the onshore M3C converter, which generally adopts a fixed low-frequency voltage control strategy. In the offshore wind power low-frequency power grid, since the system contains multiple onshore M3C converters, if the fixed low-frequency voltage control strategy is adopted, the power flow of the system cannot be effectively controlled. It is necessary to consider the coordination of different M3C converters, to ensure the stability of the offshore low-frequency power grid, and to allocate the power flowing through different M3C converters. Therefore, it is urgent to conduct research on the coordinated control method between offshore wind power low-frequency power grids. Summary of the invention

[0004] The purpose of the present invention is to solve the problem that the power flow of offshore wind power low-frequency power grid is difficult to be effectively controlled, and to provide a method for coordinated control between offshore wind power low-frequency power grids. By reasonably designing the control strategies of different onshore M3C converters, the power flow control of offshore wind power low-frequency power grid can be achieved while ensuring the voltage stability of the offshore low-frequency power grid.

[0005] In order to achieve the above-mentioned invention object, the present method adopts the following technical scheme:

[0006] A method for coordinated control of low-frequency offshore wind power grids, characterized in that the offshore wind power grid comprises: an offshore low-frequency wind turbine, a low-frequency collection submarine cable, M offshore low-frequency booster stations, a low-frequency circuit breaker, a low-frequency transmission submarine cable, N onshore low-frequency connection transformers, N onshore M3C converters, and N onshore power frequency connection transformers;

[0007] The offshore low-frequency wind turbines are connected to low-frequency collection submarine cables. The generated electricity is collected at sea through low-frequency collection submarine cables and then connected to M offshore low-frequency booster stations. After boosting at the offshore low-frequency booster stations, the electricity is transmitted to land through low-frequency transmission submarine cables. The low-frequency transmission submarine cables connect all offshore low-frequency booster stations and onshore low-frequency connection transformers to form an offshore low-frequency power grid. Each offshore low-frequency booster station and onshore low-frequency connection transformer are connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables. Low-frequency circuit breakers are provided at both ends of each section of low-frequency transmission submarine cable to cut off the faulty line when a fault occurs in the submarine cable. The N onshore low-frequency connection transformers are connected to N onshore M3C converters, and the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converters, and then connected to the onshore industrial frequency AC power grid through N onshore industrial frequency connection transformers.

[0008] Among N onshore M3C converters, one onshore M3C converter adopts a fixed low-frequency voltage control strategy, responsible for maintaining the voltage stability of the offshore low-frequency AC power grid; N-1 onshore M3C converters adopt a fixed low-frequency side power control strategy, responsible for controlling the active power and reactive power flowing into the corresponding onshore M3C converter; when the onshore M3C converter adopting the fixed low-frequency voltage control strategy fails, the offshore wind power low-frequency power grid centralized control system selects one from the other N-1 onshore M3C converters, switches its control strategy to the fixed low-frequency voltage control strategy, and is responsible for maintaining the voltage stability of the offshore low-frequency AC power grid during the fault; the active power reference value of the N-1 onshore M3C converters adopting the fixed low-frequency side power control strategy is given by the offshore wind power low-frequency power grid centralized control system according to the power grid dispatching instruction, and the reactive power reference value is given by the offshore wind power low-frequency power grid centralized control system according to the actual demand for reactive power of the offshore power grid.

[0009] Further, the onshore M3C converter control system for implementing the control method includes: a low-frequency voltage control module, a low-frequency power control module, a low-frequency current control module, a low-frequency Park inverse transformation module, a low-frequency internal circulation control module, a power frequency phase-locked loop module, a capacitor voltage and reactive power control module, a power frequency current control module, a power frequency Park inverse transformation module, a power frequency internal circulation control module, a bridge arm voltage calculation module, and a modulation module;

[0010] The low-frequency voltage control module controls the d-axis and q-axis low-frequency voltages through a first PI controller so that they follow a given reference value, and the output of the low-frequency AC voltage control module is a first low-frequency current reference value; the low-frequency power control module controls the active power and reactive power on the low-frequency side through a second PI controller so that they follow a given reference value, and the output of the low-frequency power control module is a second low-frequency current reference value; the low-frequency current control module controls the low-frequency d-axis and q-axis currents using a third PI controller so that they follow the low-frequency current reference value, and the output of the low-frequency current control module is a low-frequency output voltage; when the onshore M3C converter adopts a fixed low-frequency voltage control strategy, the low-frequency current reference value selects a first low-frequency current reference value; when the onshore M3C converter adopts a fixed low-frequency side power control strategy, the low-frequency current reference value selects a second low-frequency current reference value; the low-frequency Park inverse transformation module performs a Park inverse transformation on the low-frequency output voltage to obtain a low-frequency output voltage in a stationary three-phase coordinate system, and the angle used by the low-frequency Park inverse transformation is a low-frequency grid reference phase; the low-frequency internal circulation control module realizes the suppression of the low-frequency internal circulation, and its output is a low-frequency internal circulation voltage;

[0011] The power frequency phase-locked loop module calculates the power frequency grid voltage phase according to the power frequency grid voltage; the capacitor voltage and reactive power control module controls the capacitor voltage average value and the power frequency reactive power through the fourth PI controller, so that they follow the given reference values ​​respectively, and the output of the capacitor voltage and reactive power control module is the power frequency current reference value; the power frequency current control module controls the d and q axis power frequency currents using the fifth PI controller, so that they follow the power frequency current reference value, and the output of the power frequency current control module is the power frequency output voltage; the power frequency Park inverse transformation module performs Park inverse transformation on the power frequency output voltage to obtain the power frequency output voltage in the stationary three-phase coordinate system, and the angle used by the power frequency Park inverse transformation is the power frequency grid voltage phase; the power frequency internal circulation control module realizes the suppression of the power frequency internal circulation, and its output is the power frequency internal circulation voltage;

[0012] The bridge arm voltage calculation module calculates the reference voltage of the nine bridge arms of the onshore M3C converter using the low-frequency output voltage, the low-frequency internal circulating voltage, the power frequency output voltage, and the power frequency internal circulating voltage; the modulation module generates a modulation instruction according to the reference voltage of the nine bridge arms of the onshore M3C converter to realize the control of the onshore M3C converter.

[0013] The beneficial effects of the present invention are:

[0014] By adopting the technical solution of the present invention, through reasonably designing the control strategy of the onshore M3C converter, the control modes of different onshore M3C converters are switched under different operating conditions, so as to ensure the voltage stability of the offshore low-frequency power grid while realizing the power flow control of the offshore wind power low-frequency power grid, and avoid the problem of unbalanced power flow distribution in the offshore wind power low-frequency power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a typical topological diagram of the offshore wind power low-frequency power grid of the present invention.

[0016] Figure 2 This is a typical topological diagram of the onshore M3C converter in the present invention.

[0017] Figure 3 It is a specific example system schematic diagram of the onshore M3C converter control system in the present invention. DETAILED DESCRIPTION

[0018] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0019] like Figure 1 As shown, in an embodiment of the present invention, the offshore wind power low-frequency power grid includes: an offshore low-frequency wind turbine 1, a low-frequency collection submarine cable 2, M offshore low-frequency booster stations 3, a low-frequency circuit breaker 4, a low-frequency transmission submarine cable 5, N onshore low-frequency connection transformers 6, N onshore M3C converters 7, and N onshore industrial frequency connection transformers 8.

[0020] The electricity generated by the offshore low-frequency wind turbines 1 is collected at sea through the low-frequency collection submarine cable 2, connected to M offshore low-frequency booster stations 3, and after boosting at the offshore low-frequency booster stations 3, is transmitted to land through the low-frequency transmission submarine cable 5. The low-frequency transmission submarine cable 5 connects all the offshore low-frequency booster stations 3 and the onshore low-frequency connection transformer 6 to form an offshore low-frequency power grid. Each offshore low-frequency booster station and the onshore low-frequency connection transformer are connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables. Figure 1 The connection method given in the figure is a connection method adopted in this embodiment, and other connection methods that can meet the above requirements can also achieve the technical effect of the present invention; at both ends of each section of low-frequency transmission submarine cable 5, a low-frequency circuit breaker 4 is set to cut off the faulty line when a fault occurs in the submarine cable, so as to ensure the normal operation of other parts of the offshore low-frequency power grid; the N onshore low-frequency connection transformers 6 are connected to the N onshore M3C converters 7 in a one-to-one correspondence, and the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converter 7. The N onshore M3C converters 7 are then connected to the onshore industrial frequency AC power grid through N one-to-one corresponding onshore industrial frequency connection transformers 8, and are connected to the onshore industrial frequency AC power grid together.

[0021] like Figure 2 As shown, in the embodiment of the present invention, the onshore M3C converter 7 is composed of three frequency conversion modules, each frequency conversion module includes three bridge arm branches, and the entire M3C converter is composed of nine bridge arm branches in total. Each bridge arm branch is composed of multiple cascaded full-bridge sub-modules and bridge arm inductors connected in series. The three-phase AC systems on both sides of the M3C converter are connected through the bridge arm branches, and each phase of the system on one side is connected to the three phases of the system on the other side through three bridge arm branches.

[0022] like Figure 3 As shown, in an embodiment of the present invention, an onshore M3C converter control system for implementing a method for coordinated control of low-frequency power stations of offshore wind power includes: a low-frequency voltage control module 101, a low-frequency power control module 102, a low-frequency current control module 103, a low-frequency Park inverse transformation module 104, a low-frequency internal circulation control module 105, a power frequency phase-locked loop module 106, a capacitor voltage and reactive power control module 107, a power frequency current control module 108, a power frequency Park inverse transformation module 109, a power frequency internal circulation control module 110, a bridge arm voltage calculation module 111, and a modulation module 112.

[0023] In the embodiment of the present invention, the low-frequency voltage control module 101 controls the low-frequency d-axis and q-axis voltages U gdq1 The first PI controller is used to control the following reference values ​​u gd1ref and u gq1ref , the output of the first PI controller passes through the limiting link and is used as the d-axis and q-axis components i of the first low-frequency current reference value. vd1ref1 and i vq1ref1 The specific implementation of the low-frequency voltage control module 101 is as follows:

[0024]

[0025] Among them: F PI1 (s) is the transfer function of the low-frequency voltage PI controller, k p1 is the proportionality coefficient, k i1 is the integral coefficient, i vd1ref1 ,i vq1ref1 Corresponding to the current vector I vdq1ref1 The d-axis and q-axis components of .

[0026] In the embodiment of the present invention, the low-frequency power control module 102 controls the low-frequency side active power P g1 and reactive power Q g1 The second PI controller is used to control the following reference values ​​P g1 and Q g1The output of the second PI controller is limited by the amplitude limiting link and is used as the d-axis and q-axis components i of the second low-frequency current reference value. vd1ref2 and i vq1ref2 The specific implementation of the low-frequency power control module 102 is as follows:

[0027]

[0028] Among them: F PI2 (s) is the transfer function of the low-frequency power PI controller, k p2 is the proportionality coefficient, k i2 is the integral coefficient, i vd1ref2 ,i vq1ref2 Corresponding to the current vector I vdq1ref2 The d-axis and q-axis components of .

[0029] The low-frequency current control module 103 controls the low-frequency d-axis and q-axis currents I vdq1 The third PI controller is used to control it to follow the d and q axis low-frequency current reference value i vd1ref and i vq1ref The output of the third PI controller passes through the limiting link and is used as the low-frequency output voltage U vdq1 When the onshore M3C converter adopts a fixed low-frequency voltage control strategy, the low-frequency current reference value selects the first low-frequency current reference value i vd1ref1 and i vq1ref1 When the onshore M3C converter adopts the fixed low-frequency side power control strategy, the low-frequency current reference value selects the second low-frequency current reference value i vd1ref2 and i vq1ref2 The specific implementation of the low-frequency current control module 103 is as follows:

[0030]

[0031] Among them: F PI3 (s) is the transfer function of the low-frequency current PI controller, k p3 is the proportionality coefficient, k i3 is the integral coefficient, L1 is the equivalent inductance including the low-frequency side converter transformer and bridge arm reactor, u vd1 ,u vq1 The corresponding voltage vector is U vdq1 The d-axis and q-axis components, ω g1 is the angular frequency of the low-frequency grid voltage.

[0032] The low-frequency Park inverse transformation module 104 is configured to convert the low-frequency output voltage U vdq1 The low-frequency output voltage U in the stationary three-phase coordinate system is calculated vαβ1 The specific implementation of the low-frequency Park inverse transform module 104 is as follows:

[0033]

[0034] Among them, u vd1 ,u vq1 The corresponding voltage vector is U vdq1 The d-axis and q-axis components, u vα1 ,u vβ1 The corresponding voltage vector is U vαβ1 The α-axis, β-axis components, θ g1 It is the reference phase of the low-frequency AC power grid.

[0035] The low-frequency internal circulation control module 105 converts the low-frequency internal circulation I cαβ1 As the feedback value of the controller, the output of the low-frequency internal circulation controller is used as the low-frequency internal circulation voltage U cαβ1 The specific implementation of the low-frequency internal circulation control module 105 is as follows:

[0036]

[0037] Among them, u cα1 ,u cβ1 The corresponding voltage vector is U cαβ1 The α-axis and β-axis components, i cα1 ,i cβ1 Corresponding to the current vector I cαβ1 The α-axis and β-axis components.

[0038] The power frequency phase-locked loop module 106 generates a power frequency grid voltage U gabc2 , calculate the power frequency grid voltage phase θ g2 .

[0039] The capacitor voltage and reactive power control module 107 controls the capacitor voltage average value U c , and the power frequency reactive power Q g2 The fourth PI controller is used to control the following reference values ​​U cref and Q g2ref The output of the fourth PI controller is used as the reference value i of the d-axis and q-axis currents after the limit link. vd2ref and i vq2ref The specific implementation of the capacitor voltage and reactive power control module 107 is as follows:

[0040]

[0041] Among them: F PI4 (s) is the transfer function of capacitor voltage and reactive power PI controller, k p4 is the proportionality coefficient, k i4 is the integral coefficient, i vd2ref ,ivq2ref Corresponding to the current vector I vdq2ref The d-axis and q-axis components of .

[0042] The power frequency current control module 108 controls the power frequency d and q axis currents I vdq2 The fifth PI controller is used to control it to follow the reference value I vdq2ref , the output of the fifth PI controller is used as the power frequency output voltage U vdq2 The specific implementation of the power frequency current control module 108 is as follows:

[0043]

[0044] Among them: F PI5 (s) is the transfer function of the power frequency current PI controller, k p5 is the proportionality coefficient, k i5 is the integral coefficient, L2 is the equivalent inductance of the power frequency side converter transformer and bridge arm reactor, u vd2 ,u vq2 The corresponding voltage vector is U vdq2 The d-axis and q-axis components, ω g2 is the angular frequency of the industrial frequency grid voltage.

[0045] The power frequency Park inverse transformation module 109 outputs the power frequency voltage U vdq2 Perform Park inverse transformation to obtain the power frequency output voltage U in the stationary three-phase coordinate system vαβ2 The angle used by the power frequency Park inverse transformation is the power frequency grid voltage phase θ g2 The specific implementation of the power frequency Park inverse transformation module 109 is as follows:

[0046]

[0047] Among them, u vd2 ,u vq2 The corresponding voltage vector is U vdq2 The d-axis and q-axis components, u vα2 ,u vβ2 The corresponding voltage vector is U vαβ2 The α-axis and β-axis components.

[0048] The power frequency internal circulation control module 110 converts the power frequency internal circulation I cαβ2 As the feedback value of the controller, it can be controlled in a stationary coordinate system without rotating the coordinates. The output of the power frequency internal circulating current controller is used as the power frequency internal circulating current voltage U cαβ2 The specific implementation of the power frequency internal circulation control module 110 is as follows:

[0049]

[0050] Among them, u cα2 ,u cβ2 The corresponding voltage vector is U cαβ2 The α-axis and β-axis components, i cα2 ,i cβ2 Corresponding to the current vector I cαβ2 The α-axis and β-axis components.

[0051] The bridge arm voltage calculation module 111 uses the low-frequency output voltage U vdq1 , low frequency internal circulating current voltage U cαβ1 , power frequency output voltage U vdq2 , power frequency internal circulating current voltage U cαβ2 , the reference voltage of the nine bridge arms of the M3C frequency-changing valve is calculated.

[0052] The modulation module 112 generates a modulation instruction according to the reference voltage of the nine bridge arms of the M3C frequency-changing valve to realize the control of the M3C frequency-changing valve.

[0053] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that 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 creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made to the present invention by those skilled in the art based on the disclosure of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for coordinated control of low-frequency offshore wind power stations, characterized in that: The offshore wind power low-frequency power grid includes: offshore low-frequency wind turbines, low-frequency collection submarine cables, M offshore low-frequency booster stations, low-frequency circuit breakers, low-frequency transmission submarine cables, N onshore low-frequency connection transformers, N onshore M3C converters, and N onshore power frequency connection transformers; The offshore low-frequency wind turbines are connected to low-frequency collection submarine cables. The generated electricity is collected at sea through low-frequency collection submarine cables and then connected to M offshore low-frequency booster stations. After boosting at the offshore low-frequency booster stations, the electricity is transmitted to land through low-frequency transmission submarine cables. The low-frequency transmission submarine cables connect all offshore low-frequency booster stations and onshore low-frequency connection transformers to form an offshore low-frequency power grid. Each offshore low-frequency booster station and onshore low-frequency connection transformer are connected to the offshore low-frequency power grid through at least two low-frequency transmission submarine cables. Low-frequency circuit breakers are provided at both ends of each section of low-frequency transmission submarine cable to cut off the faulty line when a fault occurs in the submarine cable. The N onshore low-frequency connection transformers are connected to N onshore M3C converters, and the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converters, and then connected to the onshore industrial frequency AC power grid through N onshore industrial frequency connection transformers. Among N onshore M3C converters, one onshore M3C converter adopts a fixed low-frequency voltage control strategy, responsible for maintaining the voltage stability of the offshore low-frequency AC power grid; N-1 onshore M3C converters adopt a fixed low-frequency side power control strategy, responsible for controlling the active power and reactive power flowing into the corresponding onshore M3C converter; when the onshore M3C converter adopting the fixed low-frequency voltage control strategy fails, the offshore wind power low-frequency power grid centralized control system selects one from the other N-1 onshore M3C converters, switches its control strategy to the fixed low-frequency voltage control strategy, and is responsible for maintaining the voltage stability of the offshore low-frequency AC power grid during the fault; the active power reference value of the N-1 onshore M3C converters adopting the fixed low-frequency side power control strategy is given by the offshore wind power low-frequency power grid centralized control system according to the power grid dispatching instruction, and the reactive power reference value is given by the offshore wind power low-frequency power grid centralized control system according to the actual demand for reactive power of the offshore power grid.

2. The method for coordinated control of low-frequency offshore wind power stations according to claim 1, characterized in that: The onshore M3C converter control system for implementing the control method includes: a low-frequency voltage control module, a low-frequency power control module, a low-frequency current control module, a low-frequency Park inverse transformation module, a low-frequency internal circulation control module, a power frequency phase-locked loop module, a capacitor voltage and reactive power control module, a power frequency current control module, a power frequency Park inverse transformation module, a power frequency internal circulation control module, a bridge arm voltage calculation module, and a modulation module; The low-frequency voltage control module controls the d-axis and q-axis low-frequency voltages through a first PI controller so that they follow a given reference value, and the output of the low-frequency AC voltage control module is a first low-frequency current reference value; the low-frequency power control module controls the active power and reactive power on the low-frequency side through a second PI controller so that they follow a given reference value, and the output of the low-frequency power control module is a second low-frequency current reference value; the low-frequency current control module controls the low-frequency d-axis and q-axis currents using a third PI controller so that they follow the low-frequency current reference value, and the output of the low-frequency current control module is a low-frequency output voltage; when the onshore M3C converter adopts a fixed low-frequency voltage control strategy, the low-frequency current reference value selects a first low-frequency current reference value; when the onshore M3C converter adopts a fixed low-frequency side power control strategy, the low-frequency current reference value selects a second low-frequency current reference value; the low-frequency Park inverse transformation module performs a Park inverse transformation on the low-frequency output voltage to obtain a low-frequency output voltage in a stationary three-phase coordinate system, and the angle used by the low-frequency Park inverse transformation is a low-frequency grid reference phase; the low-frequency internal circulation control module realizes the suppression of the low-frequency internal circulation, and its output is a low-frequency internal circulation voltage; The power frequency phase-locked loop module calculates the power frequency grid voltage phase according to the power frequency grid voltage; the capacitor voltage and reactive power control module controls the capacitor voltage average value and the power frequency reactive power through the fourth PI controller, so that they follow the given reference values ​​respectively, and the output of the capacitor voltage and reactive power control module is the power frequency current reference value; the power frequency current control module controls the d and q axis power frequency currents using the fifth PI controller, so that they follow the power frequency current reference value, and the output of the power frequency current control module is the power frequency output voltage; the power frequency Park inverse transformation module performs Park inverse transformation on the power frequency output voltage to obtain the power frequency output voltage in the stationary three-phase coordinate system, and the angle used by the power frequency Park inverse transformation is the power frequency grid voltage phase; the power frequency internal circulation control module realizes the suppression of the power frequency internal circulation, and its output is the power frequency internal circulation voltage; The bridge arm voltage calculation module calculates the reference voltage of the nine bridge arms of the onshore M3C converter using the low-frequency output voltage, the low-frequency internal circulating voltage, the power frequency output voltage, and the power frequency internal circulating voltage; the modulation module generates a modulation instruction according to the reference voltage of the nine bridge arms of the onshore M3C converter to realize the control of the onshore M3C converter.

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