CSC-MMC cascade topology suitable for offshore wind power output DC system

Abstract

The invention provides a CSC-MMC hybrid cascade direct current power transmission topology suitable for an offshore wind power transmission system. The CSC-MMC hybrid cascade type converter has the advantages that the converter structure adopted on land is a CSC-MMC hybrid cascade type structure; the receiving end of the offshore wind power direct-current power transmission system is connected with a power grid, the direct-current system is expected to perform frequency active support on the power grid, so that the receiving end is expected to adopt an MMC structure, and the method is characterized in that active support on the frequency of the power grid can be realized by utilizing the external characteristic that the MMC presents a voltage source on the alternating-current side power grid; the CSC-MMC hybrid structure has the characteristics of a current source, in order to facilitate black start and power flow reversal, the receiving end cascades the CSC and the MMC to form the CSC-MMC hybrid structure, active frequency support for a power grid is realized, black start and power flow reversal can be facilitated, and the receiving end is on the land, so that the occupied area of the cascading structure does not need to be considered, and the CSC-MMC hybrid structure has practical engineering application.

Description

Technical Field

[0001] The present invention relates to the field of direct current transmission and power electronics technology, and in particular to a receiving-end CSC-MMC hybrid direct current transmission topology suitable for an offshore wind power direct current system. Background Art

[0002] my country is rich in offshore wind energy resources. As offshore wind power development gradually moves towards deep sea areas, DC transmission technology has become a better way to deliver large-scale offshore wind power. The type of converter currently widely used in engineering is the Modular Multilevel Converter (MMC), which has the characteristics of multiple output levels, no commutation failure, and the ability to achieve active and reactive decoupling control. However, with the increase in transmission capacity and voltage level, the demand for power modules of MMC has increased, especially the demand for higher energy storage capacitors, which has led to a significant increase in the size, weight and scale of the converter, thereby increasing the construction cost and operation and maintenance cost of the offshore converter platform.

[0003] Active commutation current source converter (CSC) does not have DC side energy storage capacitors, nor does it require a large AC filter field. It is small in size, light in weight, and can also power passive systems, making it particularly suitable for offshore wind power applications. However, as a current source converter, CSC has a weak voltage support capability for the receiving end, and the voltage fluctuates greatly. It can only rely on current changes to reflect changes in voltage frequency, so a new topology is urgently needed at the receiving end.

[0004] Most of the current hybrid DC transmission topologies use a hybrid structure of LCC-MMC, combining the advantages of LCC and MMC to achieve good operating characteristics. However, the thyristors used in LCC in the hybrid structure of LCC-MMC are semi-controlled devices, which cannot be controlled to open and close, so they are difficult to apply in offshore wind power DC transmission systems. Summary of the invention

[0005] In order to realize the active support of the offshore wind power transmission system to the power grid and overcome the problems existing in the traditional hybrid cascade DC transmission system used in offshore wind power, the present invention proposes a CSC-MMC hybrid DC transmission topology suitable for offshore wind power DC system. It is characterized in that the receiving-end converter structure can realize the voltage and frequency support of the receiving end of the offshore wind power system by using CSC-MMC compared with LCC-MMC or CSC; the CSC structure in CSC-MMC can match the CSC at the sending end to power the passive system, which is convenient for realizing the black start of the offshore wind farm; and the MMC structure in CSC-MMC can realize the stable control of the AC voltage of the offshore wind farm and can track the frequency changes well.

[0006] In order to reduce the harmonics on the AC and DC sides, the offshore converter uses a 12-pulse CSC, which is composed of two 6-pulse converters in cascade. The transformer connection forms are YY and Y-Δ, with a phase difference of 30°. Each 6-pulse CSC three-phase six-bridge arm is connected in series with fully controlled switching devices that can withstand reverse pressure. The EFFM modulation strategy is adopted, with two modulation degrees of freedom θ and α, and it is ensured that each bridge arm is turned on for 120° in each cycle to achieve independent decoupling control of active and reactive power at the sending end. The onshore converter is composed of a mixed cascade of 6-pulse CSC and MMC. Each 6-pulse CSC three-phase six-bridge arm is connected in series with fully controlled switching devices that can withstand reverse pressure. Since MMC has two control degrees of freedom, the receiving-end CSC can simply adopt the FFM modulation strategy. Each bridge arm is turned on for 120° in each cycle with one modulation degree of freedom α. MMC adopts the nearest level approximation modulation to achieve support for the receiving-end power grid.

[0007] The overall structure of the offshore wind power transmission system proposed in the present invention is as follows: large-scale offshore wind turbines are collected to the offshore AC bus through their respective converters and short-distance AC cables, and then rectified and transmitted by the offshore CSC converter, through the submarine DC cable to the onshore converter, and finally inverted by the onshore CSC-MMC converter and integrated into the onshore AC power grid.

[0008] The mathematical model of the offshore wind power transmission system proposed in the present invention is as follows.

[0009] The calculation formula of active power P and reactive power Q flowing into the AC side of offshore CSC from the sending end is as follows:

[0010]

[0011] in, ω is the angular frequency, X L is the AC series inductance, X C is the AC filter capacitor, U pm is the AC bus phase voltage amplitude of the offshore wind farm, I dc is the DC current, k T is the transformer ratio, α r is the trigger angle of the offshore CSC converter station, θ r is the compensation angle. Therefore, there are two control degrees of freedom, which can realize independent control of active and reactive power.

[0012] The mathematical model of the mixed cascade of CSC and MMC at the receiving end is as shown in equations (2) and (3):

[0013]

[0014] Formula (2) is the calculation formula for the active power P and reactive power Q on the AC side of the CSC using fundamental frequency modulation, where: ω is the angular frequency, L is the AC series inductance, C is the AC filter capacitor, U pm is the AC bus phase voltage amplitude of the offshore wind farm.

[0015]

[0016] Formula (3) is the active and reactive power output of the AC system of MMC. The specific mathematical model is as follows: Figure 1 ; Us is the AC bus voltage; Uc is the voltage at the MMC AC outlet; X L , K T is the leakage reactance and transformation ratio of the converter transformer; δ is the voltage phase difference between the MMC AC outlet and the bus position;

[0017] Assuming that the voltage modulation ratio of MMC is m, then U C The expression is:

[0018]

[0019] It can be seen from formula (4) that the active power transmission of MMC mainly depends on δ. When δ < 0, MMC absorbs active power from the AC side, otherwise it sends reactive power to the AC side. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a mathematical model diagram of the receiving-end MMC of the offshore wind power DC transmission system provided by the present invention.

[0021] Figure 2 It is a receiving-end CSC-MMC cascade topology diagram of the offshore wind power transmission DC system provided by the present invention.

[0022] Figure 3 It is a control strategy diagram of the sending-end converter station and the receiving-end converter station provided by the present invention.

[0023] Figure 4 It is the result of simulating the present invention in PSCAD. DETAILED DESCRIPTION

[0024] 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.

[0025] Figure 2 This is the topological structure diagram of the offshore wind power DC transmission system. Figure 2As shown in the figure, after the offshore wind turbines are connected to their own machine-side converters and grid-side converters, they are connected to the offshore CSC converter station through short-distance AC cables, transmitted to the onshore CSC converter station through submarine DC cables, and then inverted by the onshore converter station and connected to the onshore AC power grid. The offshore sending-end converter uses a 12-pulse CSC, which is cascaded by two 6-pulse CSCs, and the onshore receiving-end converter station uses a hybrid transmission structure composed of a cascade of six-pulse CSCs and MMCs.

[0026] In the CSC topology of the sending and receiving ends, each CSC converter valve has a filter capacitor C in parallel at the AC side outlet, which is then connected to the 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 .

[0027] In the MMC topology at the receiving end, each bridge arm is composed of a reactor L and N sub-modules (SM) connected in series, and then connected to the AC bus through a converter transformer T.

[0028] In the stage of stable wind power transmission, the detailed control strategies of each converter are as follows:

[0029] (1) Offshore CSC control strategy:

[0030] Offshore CSC adopts constant voltage and frequency control. The control strategy is as follows Figure 3 As shown in (a), it mainly consists of power control link, inner loop current control link and trigger angle back calculation link. Regarding the power controller part, the voltage and frequency reference value V ref and F ref After comparing with the measured voltage V and frequency F, and according to the offshore AC bus voltage reference value U pm The reference value i of the grid-side current dq axis component can be obtained by formula (2): dref and i qref The grid-side current reference value is passed through the inner loop current control link to obtain the dq component i of the equivalent valve outlet current reference value. rvdref and i rvqref , then the trigger angle α can be calculated inversely r and compensation angle θ r , used to generate a trigger signal for the converter valve.

[0031] (2) Onshore CSC control strategy:

[0032] Since the onshore CSC adopts the CSC-MMC hybrid cascade structure, the CSC end adopts a constant DC voltage control strategy such as Figure 3 As shown in (b), according to the measured DC voltage U csc With DC voltage reference value U cscref After comparison, through the PI link and the limiting link, the leading trigger angle β of CSC can be obtained.i , thus obtaining the delayed trigger angle α i , used to generate the corresponding trigger pulse. The MMC master station adopts the control strategy of fixed DC voltage and fixed reactive power, such as Figure 3 As shown in (c), U mmcref with U mmc They are the command value and the measured value of the voltage at both ends of the receiving MMC, which are used as the input signal of the control link of the receiving MMC master station. After the PI link and the limit link, i mmcdref , Q mmcref Set to 0 and the actual measured Q mmc Compared with the i obtained through the PI link and the limiting link mmcqref After dq decoupling, it passes through the PI link and V mmcd , V mmcq Compare to get U mmcd and U mmcq After the dq inverse transformation, we get U abcref , used to generate the trigger signal of the converter valve. Figure 3 (a) and Figure 3 (b) It can be seen that the voltage of the DC line is maintained by the receiving-end CSC and the MMC master station. The transmission system can obtain the reference value of the DC current based on the ratio of the total power demand P at the receiving end to the receiving-end voltage.

[0033] A CSC-MMC hybrid cascade DC transmission model was built in PSCAD, and the simulation results are shown in the figure below. Figure 4 As shown, the effectiveness of the topology proposed by the present invention is verified. The active power is set to 1000MW, the DC voltage is 400kV, and the reference values ​​of the active power and reactive power are set to 1000MVA.

[0034] 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 CSC-MMC hybrid cascaded DC transmission topology suitable for an offshore wind power DC system, characterized in that: The offshore sending-end converter is an actively commutated current source converter CSC based on fully controlled devices, and the onshore receiving-end converter is a hybrid structure of CSC-MMC.

2. The system topology according to claim 1, characterized in that: The onshore CSC-MMC is composed of m 6-pulse CSCs and MMCs in cascade, where m≥1. Each bridge arm of the CSC and MMC is composed of several fully controlled switching devices and sub-modules connected in series.

3. The system topology according to claim 1, characterized in that: The fully controlled device may adopt a semiconductor switch device that can withstand reverse voltage.

4. The system topology according to claim 1, characterized in that: Since the CSC-MMC cascade structure is adopted onshore, the active support of the grid frequency can be achieved by utilizing the external characteristics of the MMC presenting a voltage source to the AC side grid.

5. The system topology according to claim 1, characterized in that: Since both onshore and offshore have CSC structures and are current source converters, black start and power flow reversal are more convenient.

6. The system topology according to claim 1, characterized in that: The offshore CSC adopts a dual-degree-of-freedom modulation strategy, while the CSC in the onshore CSC-MMC adopts a baseband modulation strategy and the MMC adopts a nearest level modulation strategy.

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