CSC-MMC hybrid DC power transmission topology suitable for offshore wind power DC system

Abstract

The invention provides a CSC-MMC hybrid DC power transmission topology suitable for an offshore wind power DC system. The offshore wind turbine generators are collected to an offshore alternating current bus through respective converters, then are rectified and sent out by an offshore active commutation type current source converter (CSC), are transmitted to a land modular multilevel converter (MMC) through a seabed direct current cable, and are merged into a land alternating current power grid after being inverted. The technical scheme provided by the invention has the advantages that the receiving end of the offshore wind power direct-current power transmission system is connected with the power grid, and the requirement on voltage stability is high, so that the receiving end of the offshore wind power transmission system is designed into an MMC structure, the stability of onshore voltage can be ensured by utilizing the voltage source special type of the MMC, meanwhile, the frequency support for an onshore power grid can be realized, and the reliability of the system is improved. In addition, the size and the weight of the converter do not need to be considered on the land, the converter has a wider application prospect, and the function of a seaborne direct-current power transmission receiving end can be better achieved.

Description

Technical Field

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

[0002] my country has abundant offshore wind energy resources. As offshore wind power development gradually moves to deep sea areas, DC transmission technology has become a better way to deliver large-scale offshore wind power. The type of converter widely used in current projects is Modular Multilevel Converter (MMC). 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 area 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. The modulation strategy currently used by CSC can use fundamental frequency modulation CSC (Fundamental Frequency modulation-based CSC, FFM-CSC), which reduces DC voltage fluctuations and switching losses, and also reduces the difficulty of voltage balancing of series switching devices. It solves a series of defects of the previous mainstream application of pulse width modulation (PWM), such as high switching frequency leading to high switching losses and high requirements for voltage balancing characteristics of series devices. However, FFM-CSC has only a single control degree of freedom. At this time, active power and reactive power are fully coupled, and it is impossible to simultaneously control the AC voltage and amplitude of the offshore wind farm, making it difficult to apply to offshore wind power DC transmission systems. Summary of the invention

[0004] In order to realize the lightweight of offshore platforms, stabilize the voltage at the receiving end on land and overcome the problems existing in the use of traditional hybrid DC transmission systems in offshore wind power, the present invention proposes a CSC-MMC hybrid DC transmission topology suitable for offshore wind power DC systems. It is characterized in that the sending end adopts a control strategy based on enhanced fundamental frequency modulation CSC (Enhanced FundamentalFrequency Modulation, EFFM-CSC), which has two control degrees of freedom; at the same time, since CSC is smaller in size than MMC, it can realize the lightweight of offshore platforms when applied to offshore sending-end converters; and for the receiving end, since the receiving end of the offshore wind power DC transmission system is connected to the power grid, the voltage stability requirements are high. Since the receiving end of the offshore wind power transmission system uses the MMC structure, the voltage source characteristics of MMC can be used to ensure the stability of the onshore voltage. In addition, the converter does not need to consider the volume and weight on land, and has more complete technology, which can better realize the function of the receiving end of offshore DC transmission. The CSC with fully controlled devices can power the passive system and realize the black start of the offshore wind farm. The coordinated control strategy based on the offshore EFFM-CSC and the onshore converter can realize the stable control of the AC voltage of the offshore wind farm.

[0005] 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, to ensure that each bridge arm is turned on for 120° in each cycle. The onshore converter uses MMC, which uses the nearest level modulation to achieve terminal voltage support.

[0006] 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 busbar through their respective converters and short-distance AC cables, and then rectified and transmitted by the offshore CSC converter, transmitted to the onshore converter via the submarine DC cable, and finally inverted by the onshore MMC converter and connected to the onshore AC power grid.

[0007] 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:

[0008]

[0009] 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, kT 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.

[0010] The mathematical model of the receiving end MMC is as follows:

[0011]

[0012] Where P and Q are the active and reactive power output of the AC system; Us is the AC bus voltage; Uc is the voltage at the MMC AC outlet; XL and KT are 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;

[0013] Assuming that the voltage modulation ratio of MMC is m, then U C The expression is:

[0014]

[0015] It can be seen from formula (3) 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

[0016] Figure 1 The present invention provides a CSC-MMC hybrid DC transmission topology suitable for an offshore wind power DC system;

[0017] Fig. 2 (a) Offshore CSC control strategy;

[0018] Fig. 2(b) Onshore MMC control strategy;

[0019] Figure 3 It is a result diagram of verifying the topology proposed by the present invention in PSCAD; DETAILED DESCRIPTION

[0020] 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.

[0021] Figure 1 This is the topological structure diagram of the offshore wind power DC transmission system. Figure 1 As 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 MMC converter station via submarine DC cables, and then connected to the onshore AC power grid after being inverted at the onshore converter station. The offshore sending-end converter uses a 12-pulse CSC, which is composed of two 6-pulse CSCs cascaded, and the onshore receiving-end converter station uses MMC.

[0022] In the CSC topology at the sending end, 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 .

[0023] In the MMC topology structure 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.

[0024] In the stage of stable wind power transmission, the detailed control strategies of each converter are as follows:

[0025] (1) Offshore CSC control strategy:

[0026] The offshore CSC adopts fixed active power and reactive power control. The control strategy is shown in Figure 2(a), which mainly consists of power control link, inner loop current control link and trigger angle back calculation link. Regarding the power controller part, the active and reactive reference values ​​V dcref and f dcref The measured active power V r and reactive power f r After comparison, and according to the offshore AC bus voltage reference value V 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. dref and i qref Then the trigger angle α can be calculated back r and compensation angle θ r , used to generate a trigger signal for the converter valve.

[0027] (2) Onshore MMC control strategy:

[0028] The onshore MMC master station adopts a control strategy of constant DC voltage and constant reactive power, as shown in Figure 2(b). dcref with U dc 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 dref , Q dcref Set to 0 and the actual measured Q dc Compared with the i obtained through the PI link and the limiting link qref After dq decoupling, it passes through the PI link and V d , V q Compare to get U d and Uq After the dq inverse transformation, we get U abcref , used to generate a trigger signal for the converter valve.

[0029] Finally, according to the topology and control strategy proposed in this invention, a model was built in PSCAD and simulation verification was performed to verify the rationality of the topology proposed in this invention. The simulation parameters are: DC voltage is 400kV, active power is 1000MW. The power reference value is 1000MVA. The simulation results are as follows: Figure 3 shown.

[0030] 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 topological structure suitable for offshore wind power DC transmission system, characterized in that In offshore AC systems, the sending-end converter uses an active-commutated current source converter CSC based on fully-controlled devices, and in onshore AC systems, the receiving-end converter uses a modular multilevel converter MMC based on fully-controlled devices.

2. The system topology according to claim 1, characterized in that: The offshore sending-end converter CSC is composed of m 6-pulse CSCs in cascade, m ≥ 1, and each bridge arm of the CSC is composed of several fully controlled switch devices connected in series. The onshore receiving-end converter uses MMC, and each bridge arm is composed of several submodules connected in series.

3. The system topology according to claim 1, characterized in that: The offshore CSC adopts improved fundamental frequency modulation (EFFM) with a switching frequency of 150Hz and has dual control degrees of freedom to achieve independent decoupling control of active and reactive power.

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

5. The system topology according to claim 1, characterized in that: The characteristics of the MMC voltage source can be used to actively support the grid frequency.

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