Positive and negative trigger angle coordination control strategy suitable for current source converter

Abstract

The invention provides a positive and negative trigger angle coordination control strategy of a current source converter (CSC) for an offshore wind power delivery system. The positive and negative trigger angle coordination control strategy of the offshore wind power delivery system comprises a positive trigger angle and a negative trigger angle, the CSC is composed of a full-control device reverse blocking type IGCT and is actively turned off after each device is turned on by a 120-degree electrical angle, the range of alpha can be expanded to-90 degrees to 90 degrees during rectification operation, a positive trigger angle operation mode is in a range of 0-90 degrees, and a negative trigger angle operation mode is in a range of-90 degrees to 0 degrees. During inversion operation, the range of alpha can be expanded to 90 degrees to-90 degrees, the operation mode is a positive trigger angle operation mode when the angle ranges from 90 degrees to 180 degrees, and the operation mode is a negative trigger angle operation mode when the angle ranges from-180 degrees to-90 degrees. According to the control strategy provided by the invention, two CSCs in a converter station are utilized to operate in a positive firing angle mode and a negative firing angle mode respectively, so that the CSCs have two control degrees of freedom. According to the technical scheme, direct current fluctuation can be effectively reduced, loss can be reduced, the CSC has two degrees of freedom, independent control over the amplitude and frequency of offshore alternating current voltage is achieved, and stable output of offshore wind energy is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power and direct current transmission, and in particular to a positive and negative trigger angle coordinated control strategy applicable to a current source converter. Background Art

[0002] At present, the overall trend of offshore wind power development is to develop in deep sea and large capacity, and DC transmission technology has become the mainstream transmission technology for offshore wind power transmission. The core of DC transmission is the converter. Which converter topology should be adopted to make the offshore wind power transmission scheme have reliability and economic advantages has become a challenging problem. The line commutated converter (LCC) has the advantages of low engineering cost and low operating loss, but there is a problem of commutation failure. At the same time, LCC needs to be equipped with reactive power compensation and filtering equipment, which greatly increases the volume and weight of the offshore platform. The modular multilevel converter (MMC) uses a fully controlled switching device IGBT, which greatly improves the operating performance and does not have the problem of commutation failure. However, there are problems of high cost and limited transmission capacity. Therefore, the offshore wind power transmission scheme using MMC has the fundamental problems of large volume and weight and high cost. The offshore wind power transmission scheme is transmitted through a diode rectifier (DR), which effectively reduces the construction cost and switching loss. However, DR does not have control capabilities and needs to adopt a grid-type wind turbine. And the DR cannot work in reverse mode, and the system does not have the ability to start.

[0003] The current source converter (CSC) based on the reverse resistance IGCT combines the advantages of both LCC and MMC converters. Compared with LCC, the CSC commutation process does not rely on the grid voltage, which can completely solve the commutation failure problem of LCC and realize system startup; compared with MMC, CSC uses fewer devices, has no DC side energy storage capacitor, is small in size and light in weight. Therefore, CSC has broad application prospects in the field of offshore wind power transmission.

[0004] At present, the widely used control strategies of CSC are pulse width modulation (PWM) and fundamental frequency modulation (FFM). However, PWM-CSC has the problems of high switching loss, severe DC fluctuation and difficulty in voltage balancing of devices, and cannot be directly applied to offshore wind power DC transmission systems; while FFM-CSC reduces DC voltage fluctuation, but has only one control degree of freedom, the trigger angle, and cannot control the AC voltage and frequency at sea at the same time. Summary of the invention

[0005] In order to overcome the above-mentioned problems of large fluctuation of PWM-CSC DC voltage and only one control degree of freedom of FFM-CSC, the present invention proposes that two CSCs in the converter station are respectively in positive trigger angle mode and negative trigger angle mode, and through mutual coordination, the CSC converter station has two control degrees of freedom. It is characterized in that the CSC is composed of a fully controlled device reverse resistance type IGCT, which can be actively turned off after opening 120° electrical angle. When rectifying, the range of α can be expanded to -90°~90°, of which 0~90° is the positive trigger angle mode, and -90°~0 is the negative trigger angle mode; when inverting, the range of α can be expanded to 90°~-90°, of which 90°~180° is the positive trigger angle mode, and -180°~-90° is the negative trigger angle mode.

[0006] The converter station topology applicable to the control strategy of the present invention is a CSC with n 12-pulse CSCs cascaded, n≥1. Here, n=1 is taken as an example for explanation, that is, a 12-pulse CSC is taken as an example for explanation. The 12-pulse CSC is composed of two 6-pulse converters connected in series on the DC side and in parallel on the AC side, namely the high valve group CSC1 and the low valve group CSC2; each bridge arm of the high and low valve groups is composed of a plurality of reverse resistance IGCT devices connected in series; the three-phase capacitor C1 connected in star is connected in parallel on the AC outlet side of the CSC1 bridge arm, and then connected to the star-delta transformer T1 through the three-phase reactor L1 in series, and connected to the AC power grid; the three-phase capacitor C2 connected in star is connected in parallel on the AC outlet side of the CSC2 bridge arm, and then connected to the star-delta transformer T2 through the three-phase reactor L2, and connected to the AC power grid.

[0007] Since the positive and negative trigger angle coordinated control strategy increases the control freedom, the offshore AC voltage and frequency can be coordinated and controlled by adjusting the positive trigger angle and the negative trigger angle.

[0008] According to Kirchhoff's voltage and current laws, we can get:

[0009]

[0010] in, X L =j2πfL; j is the phase unit (j=a, b, c); I clj is the j-phase current phasor flowing through the filter capacitor; I lj is the j-phase current phasor flowing through the valve side of the converter transformer; U clj is the j-phase capacitor voltage phasor; U rj is the j-phase voltage on the offshore AC bus; C is the filter capacitor; L is the filter inductance L0 and the converter transformer leakage inductance L T The sum of T is the transformer ratio.

[0011] Assume U rjWhen the phase is 0 and the trigger angle of CSC is α, the current phasor I at the outlet of CSC can be obtained. rvlj for:

[0012]

[0013] Based on the relationship between the phase-shifting transformer, the single 12-pulse CSC output current I can be obtained. rj for:

[0014]

[0015] in U rm is the phase voltage amplitude of the offshore AC bus.

[0016] Set the firing angle of CSC1 to α r1 , the trigger angle of CSC2 is set to α r2 , so the current I at the AC busbar on the sea side srj for:

[0017]

[0018] According to the instantaneous power theory, the active power and reactive power absorbed by the offshore dual 12-pulse CSC converter station can be obtained as follows:

[0019]

[0020] Design of offshore AC voltage amplitude U rm The control strategy of frequency f requires discussion of the two intermediate variables α of the control trigger angle of the offshore CSC. F and α V with U rm The relationship between and f is used to correspond the control quantity and the controlled quantity.

[0021]

[0022]

[0023] Through U rm 2 and f 2 For α d and α q Taking partial derivatives, we can analyze that in α F and α V Within the operating range, f and α F It shows a monotonically increasing relationship, U rm and α V It shows a monotonically increasing relationship, so a coordinated control strategy for the positive and negative trigger angles of the offshore CSC can be designed to coordinate the frequency f of the offshore AC system with its reference value f refBy comparison, after the PI controller, the trigger angle α can be obtained F , the sea AC voltage amplitude U rm With its reference value U rm_ref By comparison, after the PI controller, the trigger angle α can be obtained F Finally, the trigger angle α can be obtained by back calculation. r1 and α r2 , used for CSC r1 and CSC r2 Carry out regulation to maintain stable operation of offshore AC voltage amplitude and frequency.

[0024] The beneficial effect of the present invention is that the offshore wind power DC transmission system based on the active commutation current source converter adopts a positive and negative trigger angle coordinated control strategy, which enables the offshore converter station to have two control degrees of freedom without relying on long-distance communication, and can stably control the amplitude and frequency of the offshore low-frequency AC voltage, and reduce the DC voltage fluctuation and switching loss of the DC link, thereby realizing large-scale offshore wind power grid connection. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The invention provides an offshore wind power direct current transmission system based on a current source converter;

[0026] Figure 2 It is the CSC trigger angle operation circle diagram provided by the present invention;

[0027] Figure 3 This is the offshore CSC positive and negative trigger angle coordinated control strategy provided by the present invention. DETAILED DESCRIPTION

[0028] The preferred embodiments are described in detail below in conjunction with the accompanying drawings. It should be emphasized that the following description is only exemplary and is not intended to limit the scope and application of the present invention.

[0029] Figure 1 This is a topological diagram of an offshore wind power DC transmission system based on a current source converter. Figure 1 As shown, the AC power output by the large-scale wind farm passes through the offshore booster station and is transmitted to the offshore CSC converter station via the collecting AC cable, and is converted into DC power and transmitted to the onshore CSC converter station. Both the offshore CSC converter station and the offshore CSC converter station use double twelve-pulse CSC, which is composed of two 12-pulse CSCs cascaded. A filter capacitor C is connected in parallel to the AC side outlet of each CSC converter valve, and then connected to the low-frequency system AC bus through a series filter inductor L0 and a converter transformer T; a smoothing reactor L is connected in series on the DC side. dc .

[0030] Figure 2It is the CSC trigger angle operation circle diagram. CSC is composed of fully controlled devices, reverse resistance type IGCT, which can actively shut down after each device is turned on for 120° electrical angle. Thus, the CSC trigger angle operation circle diagram is obtained, such as Figure 3 As shown in the figure, when the rectifier is running, the range of α can be expanded to -90°~90°, where 0~90° is the positive trigger angle operation mode, and -90°~0 is the negative trigger angle operation mode; when the inverter is running, the range of α can be expanded to 90°~-90°, where 90°~180° is the positive trigger angle operation mode, and -180°~-90° is the negative trigger angle operation mode. When the LC circuit is not considered, the reactive direction of the converter is closely related to the operating range of α. It can be analyzed that different intervals of the trigger angle will affect the reactive absorption and emission of the converter.

[0031] Offshore CSC positive and negative trigger angle coordinated control strategy Figure 3 As shown, the frequency f of the marine AC system is compared with its reference value f ref By comparison, the trigger angle intermediate variable α can be obtained through the PI controller F , and then the sea AC voltage amplitude U rm With its reference value U rm_ref By comparison, the trigger angle intermediate variable α can be obtained through the PI controller V Finally, the trigger angle α can be obtained by back calculation. r1 and α r2 , used for CSC r1 and CSC r2 Carry out regulation to maintain stable operation of offshore AC voltage amplitude and frequency.

[0032] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Ordinary technicians in the relevant field can still modify or make equivalent substitutions to the specific implementation methods of the present invention with reference to the above embodiments. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention to be approved.

Claims

1. A positive and negative trigger angle coordinated control strategy for a current source converter, characterized in that: There are two twelve-pulse CSC converters in the offshore converter station, CSC1 operates in the positive trigger angle operation mode, and CSC2 operates in the negative trigger angle operation mode. Through the mutual coordination of the two twelve-pulse CSCs, the offshore converter station has two degrees of freedom, realizing independent control of the offshore AC voltage amplitude and frequency.

2. The positive and negative trigger angle coordinated control strategy according to claim 1 is characterized in that: The offshore converter station has two control degrees of freedom: the firing angle α r1 and α r2 When the rectifier is running, the trigger angle range is -90°~90°, among which 0~90° is the positive trigger angle operation mode, and -90°~0 is the negative trigger angle operation mode; when the inverter is running, the trigger angle range can be expanded to 90°~-90°, among which 90°~180° is the positive trigger angle operation mode, and -180°~-90° is the negative trigger angle operation mode.

3. The positive and negative trigger angle coordinated control strategy according to claim 1 is characterized in that: The offshore wind power DC transmission system based on current source converter is a pseudo-bipolar connection method. The offshore converter station consists of two twelve-pulse CSC converters. Each bridge arm of the CSC is composed of several fully controlled switching devices connected in series.

4. According to the positive and negative trigger angle coordinated control strategy described in claim 1, a method for controlling voltage amplitude and frequency at sea is proposed, characterized in that: For offshore wind power DC transmission systems based on current source converters, the positive trigger angle α r1 and negative firing angle α r2 The two control degrees of freedom enable the offshore converter station to control the voltage and frequency of the offshore AC system, and the inverter station to control the DC voltage and reactive power, ultimately achieving stable transmission of offshore wind energy.

5. The offshore voltage amplitude and frequency control method according to claim 4, characterized in that: The offshore voltage amplitude and frequency control method includes a control link and a trigger angle back calculation link. According to the voltage and frequency measurement values ​​and reference values ​​of the offshore AC bus, the trigger angle intermediate variable of the offshore CSC is obtained through the PI link and the limit link. After the trigger angle back calculation link, the positive trigger angle α can be obtained. r2 and negative firing angle α r2 Two control quantities are used to generate trigger signals for each bridge arm.

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