Stability improvement control method for offshore wind power low-frequency networking system

Abstract

The invention discloses a stability improvement control method for an offshore wind power low-frequency networking system, and the method comprises the steps: designing a low-frequency power grid stability control module in a land M3C converter control strategy, achieving the reasonable distribution of the power of different M3C converters in an offshore wind power low-frequency AC power grid, and avoiding the overcurrent problem; the inertia of an offshore wind power low-frequency networking system can be improved, the stability of a low-frequency power grid during offshore wind power output fluctuation is improved, and the method has a good application prospect.

Description

Technical Field

[0001] The present invention belongs to the field of renewable energy power generation, and specifically relates to a stability improvement control method for an offshore wind power low-frequency networking system. 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 offshore wind power low-frequency networking system, the offshore power grid is established by M3C converters. When there are multiple receiving-end M3C converters, it is necessary to consider the coordination of different M3C converters, not only to ensure the stability of the offshore low-frequency power grid, but also to distribute the power flowing through different M3C converters to avoid overcurrent problems; in addition, the offshore wind power low-frequency networking system also has the problem of insufficient inertia, and the sudden fluctuation of new energy output can easily cause the power grid to become unstable. At present, there is no relevant research on the solution to the above problems. Therefore, it is urgent to study the technical solution of the stability improvement control method of the offshore wind power low-frequency networking system. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of coordination between different M3C converters in an offshore wind power low-frequency networking system and the problem of insufficient inertia of the low-frequency networking system, and to provide a control method for improving the stability of an offshore wind power low-frequency networking system. By optimizing the control strategy of the onshore M3C converter, the power of different M3C converters can be reasonably distributed while ensuring the inertia stability of the offshore low-frequency power grid.

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

[0006] A method for improving the stability of an offshore wind power low-frequency networking system, characterized in that the offshore wind power low-frequency networking system comprises: an offshore low-frequency wind turbine, a low-frequency collection submarine cable, an offshore low-frequency booster station, a low-frequency circuit breaker, a low-frequency transmission submarine cable, an onshore low-frequency connection transformer, an onshore M3C converter, and an onshore power frequency connection transformer;

[0007] The electricity generated by the offshore low-frequency wind turbines is collected at sea through low-frequency collection submarine cables, connected to the offshore low-frequency booster station, boosted at the offshore low-frequency booster station, and then transmitted to the land through low-frequency transmission submarine cables. The low-frequency transmission submarine cables connect multiple offshore low-frequency booster stations and onshore low-frequency connection transformers to form an offshore low-frequency power grid. Low-frequency circuit breakers are provided at both ends of each section of the low-frequency transmission submarine cable to cut off the faulty line when a fault occurs in the low-frequency transmission submarine cable, so as to ensure the normal operation of other parts of the offshore low-frequency power grid; the onshore low-frequency connection transformer is connected to the onshore M3C converter, converts the low-frequency electric energy into industrial frequency electric energy through the onshore M3C converter, and then connects to the onshore industrial frequency AC power grid through the onshore industrial frequency connection transformer;

[0008] The M3C control system for realizing the stability improvement control method of the offshore wind power low-frequency networking system includes: a low-frequency power grid stability control module, a low-frequency AC voltage 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;

[0009] The low-frequency grid stability control module adopts a virtual synchronous machine control strategy to calculate the d- and q-axis low-frequency voltage reference values ​​and the low-frequency grid reference phase, and is responsible for balancing the power distribution between different onshore M3C converters and providing inertia support for the offshore low-frequency grid; the low-frequency AC voltage control module controls the d- and q-axis low-frequency voltages through a first PI controller to make them follow a given reference value, and the output of the low-frequency AC voltage control module is a low-frequency current reference value; the low-frequency current control module uses a second PI controller to control the low-frequency d- and q-axis currents to make them follow the low-frequency current reference value, and the output of the low-frequency current control module is a low-frequency output voltage; 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 the low-frequency grid reference phase; the low-frequency internal circulating current control module realizes the suppression of the low-frequency internal circulating current, and its output is a low-frequency internal circulating current voltage;

[0010] 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 third 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 fourth 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;

[0011] 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.

[0012] Furthermore, the low-frequency grid stability control module refers to the q-axis voltage reference value u gq1ref Given as 0, the reference d-axis voltage reference value u gd1ref and the reference phase θ g1 Calculated according to the following method:

[0013]

[0014] Among them, u gd1ref (k+1) is the d-axis voltage reference value of the next sampling period, |U g1 (k)| is the voltage amplitude of this sampling period, U ref is the voltage amplitude reference value, Q g1ref is the reactive power reference value, Q g1 (k) is the reactive power of this sampling period, K is the virtual excitation coefficient, D q is the reactive damping coefficient;

[0015] θ g1 (k+1)=∫ω g1 (k+1)dt

[0016]

[0017] Among them, θ g1 (k+1) is the reference phase of the next sampling period, ω g1 (k+1) is the angular frequency of the next sampling period, ωg1 (k) is the angular frequency of this sampling period, ω n1 is the rated angular frequency, P g1ref is the active power reference value, P g1 (k) is the active power of this sampling period, J is the virtual rotor moment of inertia, D p is the active damping coefficient.

[0018] The beneficial effects of the present invention are as follows: by optimizing the control strategy of the M3C converter, it is possible to achieve a reasonable distribution of the power of different M3C converters in the offshore wind power low-frequency AC power grid, avoid overcurrent problems, and improve the inertia of the offshore wind power low-frequency networking system, thereby improving the stability of the low-frequency power grid when the offshore wind power output fluctuates. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a typical topology diagram of the offshore wind power low-frequency networking system of the present invention.

[0020] Figure 2 This is a typical topological diagram of the onshore M3C converter in the present invention.

[0021] Figure 3 It is a specific example system schematic diagram of the onshore M3C converter control system in the present invention. DETAILED DESCRIPTION

[0022] 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.

[0023] like Figure 1 As shown, in an embodiment of the present invention, the offshore wind power low-frequency networking system includes: an offshore low-frequency wind turbine 1, a low-frequency collection submarine cable 2, an offshore low-frequency booster station 3, a low-frequency circuit breaker 4, a low-frequency transmission submarine cable 5, an onshore low-frequency connection transformer 6, an onshore M3C converter 7, and an onshore industrial frequency connection transformer 8.

[0024] The electricity generated by the offshore low-frequency wind turbine 1 is collected at sea through the low-frequency collection submarine cable 2, connected to the offshore low-frequency boosting station 3, and after boosting at the offshore low-frequency boosting station 3, it is transmitted to the land through the low-frequency transmission submarine cable 5. The low-frequency transmission submarine cable 5 connects multiple offshore low-frequency boosting stations 3 and multiple onshore low-frequency connection transformers 6 to form an offshore low-frequency power grid. At both ends of each section of the low-frequency transmission submarine cable, a low-frequency circuit breaker 4 is provided to cut off the faulty line when a fault occurs in the low-frequency transmission submarine cable, so as to ensure the normal operation of other parts of the offshore low-frequency power grid; the onshore low-frequency connection transformer 6 is connected to the onshore M3C converter 7, and the low-frequency electric energy is converted into industrial frequency electric energy through the onshore M3C converter 7, and then connected to the onshore industrial frequency AC power grid through the onshore industrial frequency connection transformer 8;

[0025] 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.

[0026] like Figure 3 As shown, in an embodiment of the present invention, an onshore M3C converter control system that implements a stability improvement control method for an offshore wind power low-frequency networking system includes: a low-frequency grid stability control module 101, a low-frequency AC voltage 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.

[0027] In the embodiment of the present invention, the low-frequency grid stability control module 101 adopts a virtual synchronous machine control strategy to calculate the d-axis and q-axis low-frequency voltage reference values ​​and the low-frequency grid reference phase, which is responsible for balancing the power distribution between different onshore M3C converters and providing inertia support for the low-frequency grid; wherein, the reference q-axis voltage reference value u gq1ref Given as 0, the reference d-axis voltage reference value u gd1ref and the reference phase θ g1 Calculated according to the following method:

[0028]

[0029] Among them, u gd1ref (k+1) is the d-axis voltage reference value of the next sampling period, |U g1 (k)| is the voltage amplitude of this sampling period, U ref is the voltage amplitude reference value, Q g1ref is the reactive power reference value, Q g1 (k) is the reactive power of this sampling period, K is the virtual excitation coefficient, D q is the reactive damping coefficient;

[0030] θ g1 (k+1)=∫ω g1 (k+1)dt

[0031]

[0032] Among them, θ g1(k+1) is the reference phase of the next sampling period, ω g1 (k+1) is the angular frequency of the next sampling period, ω g1 (k) is the angular frequency of this sampling period, ω n1 is the rated angular frequency, P g1ref is the active power reference value, P g1 (k) is the active power of this sampling period, J is the virtual rotor moment of inertia, D p is the active damping coefficient.

[0033] The low-frequency AC voltage control module 102 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 is used as the reference value i of the d-axis and q-axis currents after the limit link. vd1ref and i vq1ref The specific implementation of the low-frequency AC voltage control module 102 is as follows:

[0034]

[0035] Among them: F PI1 (s) is the transfer function of the low-frequency AC voltage PI controller, k p1 is the proportionality coefficient, k i1 is the integral coefficient, i vd1ref ,i vq1ref Corresponding to the current vector I vdq1ref The d-axis and q-axis components of .

[0036] The low-frequency current control module 103 controls the low-frequency d-axis and q-axis currents I vdq1 The second PI controller is used to control it to follow the reference value i of the d and q axis currents. vd1ref and i vq1ref The output of the second PI controller passes through the limiting link and is used as the low-frequency output voltage U vdq1 ; The specific implementation of the low-frequency current control module is as follows:

[0037]

[0038] Among them: F PI2 (s) is the transfer function of the low-frequency current PI controller, k p2 is the proportionality coefficient, k i2 is the integral coefficient, L 1 is the equivalent inductance of the low-frequency side converter transformer and bridge arm reactor, u vd1 ,u vq1 The corresponding voltage vector is U vdq1The d-axis and q-axis components, ω g1 is the angular frequency of the low-frequency grid voltage.

[0039] 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:

[0040]

[0041] 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.

[0042] 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:

[0043]

[0044] 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.

[0045] 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 .

[0046] 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 third PI controller is used to control the following reference values ​​U cref and Q g2ref The output of the third 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:

[0047]

[0048] Among them: F PI3 (s) is the transfer function of capacitor voltage and reactive power PI controller, k p3 is the proportionality coefficient, k i3 is the integral coefficient, i vd2ref ,i vq2ref Corresponding to the current vector I vdq2ref The d-axis and q-axis components of .

[0049] The power frequency current control module 108 controls the power frequency d and q axis currents I vdq2 The fourth PI controller is used to control it to follow the reference value I vdq2ref , the output of the fourth 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:

[0050]

[0051] Among them: F PI4 (s) is the transfer function of the power frequency current PI controller, k p4 is the proportionality coefficient, k i4 is the integral coefficient, L 2 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.

[0052] 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:

[0053]

[0054] 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.

[0055] 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:

[0056]

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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 improving the stability of an offshore wind power low-frequency networking system, characterized in that: The offshore wind power low-frequency networking system includes: offshore low-frequency wind turbines, low-frequency collection submarine cables, offshore low-frequency booster stations, low-frequency circuit breakers, low-frequency transmission submarine cables, onshore low-frequency connection transformers, onshore M3C converters, and onshore power frequency connection transformers; The electricity generated by the offshore low-frequency wind turbines is collected at sea through low-frequency collection submarine cables, connected to the offshore low-frequency booster station, boosted at the offshore low-frequency booster station, and then transmitted to the land through low-frequency transmission submarine cables. The low-frequency transmission submarine cables connect multiple offshore low-frequency booster stations and onshore low-frequency connection transformers to form an offshore low-frequency power grid. Low-frequency circuit breakers are provided at both ends of each section of the low-frequency transmission submarine cable to cut off the faulty line when a fault occurs in the low-frequency transmission submarine cable, so as to ensure the normal operation of other parts of the offshore low-frequency power grid; the onshore low-frequency connection transformer is connected to the onshore M3C converter, converts the low-frequency electric energy into industrial frequency electric energy through the onshore M3C converter, and then connects to the onshore industrial frequency AC power grid through the onshore industrial frequency connection transformer; The M3C control system for realizing the stability improvement control method of the offshore wind power low-frequency networking system includes: a low-frequency power grid stability control module, a low-frequency AC voltage 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 grid stability control module adopts a virtual synchronous machine control strategy to calculate the d- and q-axis low-frequency voltage reference values ​​and the low-frequency grid reference phase, and is responsible for balancing the power distribution between different onshore M3C converters and providing inertia support for the offshore low-frequency grid; the low-frequency AC voltage control module controls the d- and q-axis low-frequency voltages through a first PI controller to make them follow a given reference value, and the output of the low-frequency AC voltage control module is a low-frequency current reference value; the low-frequency current control module uses a second PI controller to control the low-frequency d- and q-axis currents to make them follow the low-frequency current reference value, and the output of the low-frequency current control module is a low-frequency output voltage; 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 the low-frequency grid reference phase; the low-frequency internal circulating current control module realizes the suppression of the low-frequency internal circulating current, and its output is a low-frequency internal circulating current 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 third 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 fourth 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.

2. The offshore wind power low-frequency networking system stability improvement control method according to claim 1 is characterized by: The low-frequency grid stability control module refers to the q-axis voltage reference value u gqlref Given as 0, the reference d-axis voltage reference value u gdlref and the reference phase θ g1 Calculated according to the following method: Among them, u gd1ref (k+1) is the d-axis voltage reference value of the next sampling period, |U g1 (k)| is the voltage amplitude of this sampling period, U ref is the voltage amplitude reference value, Q g1ref is the reactive power reference value, Q g1 (k) is the reactive power of this sampling period, K is the virtual excitation coefficient, D q is the reactive damping coefficient; θ g1 (k+1)=∫ω g1 (k+1)dt Among them, θ g1 (k+1) is the reference phase of the next sampling period, ω g1 (k+1) is the angular frequency of the next sampling period, ω g1 (k) is the angular frequency of this sampling period, ω n1 is the rated angular frequency, P g1ref is the active power reference value, P g1 (k) is the active power of this sampling period, J is the virtual rotor moment of inertia, D p is the active damping coefficient.

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