A black start method suitable for a CSC-MMC offshore wind power DC system

Abstract

This invention provides a black-start method suitable for CSC-MMC offshore wind power DC systems. Offshore wind turbines are collected by a converter to an AC bus, rectified by the CSC, and transmitted via submarine DC cable to an onshore half-bridge / full-bridge hybrid MMC. After inversion, the signal is connected to the onshore AC grid. During the black-start phase, the receiving-end hybrid MMC uses a full-bridge submodule to output a negative voltage to start the wind turbine, achieving stable control without switching the black-start resistor. The advantages of this invention are: using a CSC converter reduces the size and weight of the offshore platform, thus lowering construction costs; since the receiving end of the DC transmission system is connected to the grid, it has high requirements for voltage stability and is prone to DC faults; the voltage source characteristics of the receiving-end hybrid MMC ensure the stability of the onshore voltage; and the AC / DC voltage decoupling capability of the hybrid MMC enables unblocked DC fault ride-through while providing AC voltage support.

Description

Technical Field

[0001] This invention relates to the fields of DC power transmission and power electronics technology, specifically to a black start method applicable to the CSC-MMC offshore wind power DC system. Background Technology

[0002] my country possesses abundant offshore wind energy resources. As offshore wind power development gradually extends to deeper waters, DC transmission technology has become a superior approach for large-scale transmission of offshore wind power. Currently, the widely used converter type in engineering projects is the Modular Multilevel Converter (MMC). With the increase in transmission capacity and voltage levels, the demand for power modules in MMCs has increased, especially the demand for higher energy storage capacitors. This has led to a significant increase in the size, weight, and scale of the converter, thereby increasing the construction and operation and maintenance costs of offshore converter platforms.

[0003] Active commutated current source converters (CSCs) do not have DC-side energy storage capacitors or require large-area AC filter fields. They are smaller in size and lighter in weight, and can also supply power to passive systems, making them particularly suitable for offshore wind power applications. However, as current source converters, CSCs have weak voltage and frequency support capabilities at the receiving end, resulting in large voltage fluctuations. Therefore, a new topology is urgently needed at the receiving end.

[0004] Some scholars have proposed that using an MMC (Multi-Content Capacitor) structure at the receiving end can leverage the voltage source characteristics of the MMC to ensure the stability of onshore voltage. However, the MMC structure has a high probability of DC faults, and it still relies on DC circuit breakers to clear DC faults. However, DC circuit breakers cannot fully utilize the technical advantages of selective fault clearing, and power transmission needs to be interrupted briefly after a fault. In addition, there is currently no black-start method for hybrid systems. Traditional black-start methods still require switching black-start resistors on the AC side of the offshore platform during system startup, but three-phase AC side black-start resistors increase the size and weight of the offshore platform, increasing the construction cost of the offshore platform. Summary of the Invention

[0005] To achieve lightweight offshore platforms, stable DC fault ride-through, and overcome the problems in black start of offshore wind power DC systems, this invention proposes a black start method suitable for CSC-MMC offshore wind power DC systems. The key feature is that, compared to using MMC or CSC, the receiving-end converter structure employs a half-bridge / full-bridge hybrid MMC, which can achieve unblocked DC fault ride-through while maintaining AC voltage support capability. Based on the negative voltage output capability of the full-bridge submodule in the hybrid MMC, the AC voltage of the wind farm can be established without switching the black start resistor, achieving stable control of the AC voltage of the offshore wind farm.

[0006] To reduce AC and DC harmonics, the offshore converter employs a 12-pulse CSC, consisting of two cascaded 6-pulse converters with transformer connections of YY and Y-Δ, respectively, with a 30° phase difference. Each 6-pulse CSC uses fully controlled switching devices capable of withstanding reverse voltage connected in series, employing an EFFM modulation strategy with two modulation degrees of freedom, θ and α, ensuring that each arm conducts for 120° in each cycle. The onshore converter uses a semi-hybrid MMC, employing nearest-level modulation to achieve received-end voltage support, DC fault clearing, and black start.

[0007] The overall structure of the offshore wind power DC system proposed in this invention is as follows: large-scale offshore wind turbines are collected by their respective converters and short-distance AC cables to the offshore AC bus, then rectified and sent out by the offshore CSC converter, and sent to the onshore converter via submarine DC cable. Finally, the onshore hybrid MMC converter inverts the DC power and connects it to the onshore AC grid.

[0008] The formulas for calculating the active power P and reactive power Q flowing into the AC side of the offshore CSC are as follows: (1)

[0009] (1)

[0010] Where A = B= ω is the angular frequency, X L For AC series inductance, X C For AC filter capacitors, U pm I represents the phase voltage amplitude of the AC bus in an offshore wind farm. dc For direct current, k T For the transformer turns ratio, α r For the firing angle of the offshore CSC converter station, θ r The compensation angle is used. Therefore, there are two control degrees of freedom, enabling independent control of active and reactive power.

[0011] The mathematical model of the receiver-side MMC is shown in equation (2):

[0012] (2)

[0013] Where P and Q represent the active and reactive power outputs of the AC system; U s U is the AC bus voltage; c X is the voltage at the AC output of the MMC. L K T δ represents the leakage reactance and turns ratio of the converter transformer; δ is the voltage phase difference between the MMC AC outlet and the bus position.

[0014] Assuming the voltage modulation ratio of the MMC is m, then U C The expression is:

[0015] (3)

[0016] As can be seen from equation (3), the active power transmission of MMC mainly relies on δ. When δ < 0, MMC absorbs active power from the AC side, and otherwise sends reactive power to the AC side. Attached Figure Description

[0017] Figure 1 This is a topology diagram of an offshore wind power DC system based on CSC-MMC provided by the present invention;

[0018] Figure 2(a) is a control strategy diagram of the offshore converter station provided by the present invention;

[0019] Figure 2(b) is a control strategy diagram of the onshore converter station provided by the present invention;

[0020] Figure 3 This is a diagram illustrating the black start steps of an offshore wind power DC system provided by the present invention;

[0021] Figure 4 This is a simulation result diagram of the black start stage of the offshore wind power DC system provided by the present invention. Detailed Implementation

[0022] The preferred embodiments will now be described in detail with reference to the accompanying drawings. It should be emphasized that the following description is merely exemplary and not intended to limit the scope or application of the invention.

[0023] Figure 1 This is a topology diagram of an offshore wind power DC transmission system. (Example:) Figure 1 As shown, after the offshore wind turbines are connected to their respective turbine-side converters and grid-side converters, they are connected to the offshore CSC converter station via short-distance AC cables. The power is then transmitted to the onshore MMC converter station via submarine DC cables, and finally inverted at the onshore converter station and connected to the onshore AC grid. The offshore sending-end converter adopts a 12-pulse CSC, which is composed of two 6-pulse CSCs cascaded together. The onshore receiving-end converter station adopts a hybrid MMC, which is composed of a combination of half-bridge submodules (HBSM) and full-bridge submodules (FBSM).

[0024] In the CSC topology at the sending end, each CSC converter valve has a filter capacitor C connected in parallel at its AC side outlet, which is then connected to the AC bus via a series filter inductor L0 and a converter transformer T; a smoothing reactor L is connected in series on the DC side. dc .

[0025] In the hybrid MMC topology at the receiving end, each bridge arm consists of a reactor L and M full-bridge submodules and NM half-bridge submodules connected in series, and then connected to the AC bus via a converter transformer T.

[0026] During the stable wind power transmission phase, the detailed control strategies for each converter are as follows:

[0027] (1) Maritime CSC Control Strategy:

[0028] The offshore CSC employs constant active and reactive power control. The control strategy, as shown in Figure 2(a), mainly consists of a power control loop, an inner-loop current control loop, and a trigger angle inverse calculation loop. Regarding the power controller, the active and reactive power reference values ​​P... dcref and Q dcref With the measured active power P r and reactive power Q r After comparison, and based on the reference value V of the offshore AC bus voltage... pm The reference value i of the dq-axis component of the grid-side current can be obtained through equation (2). dref and i qref By passing the grid-side current reference value through the inner loop current control circuit, the dq component i of the equivalent valve outlet current reference value can be obtained. dref and i qref Then the trigger angle α can be calculated. r and compensation angle θ r It is used to generate the trigger signal for the converter valve.

[0029] (2) Land-based hybrid MMC control strategy:

[0030] The onshore hybrid MMC master station adopts a constant DC voltage and constant reactive power control strategy, as shown in Figure 2(b). dcref with U dc These are the commanded and measured values ​​of the voltage across the receiving-end MMC, respectively. These are used as input signals to the receiving-end MMC master station control loop, and after passing through a PI controller and a limiting controller, i is obtained. dref Q dcref Setting it to 0 and obtaining Q from actual measurement dc Compared to obtaining i through the PI stage and the limiting stage qref After decoupling via dq, it then passes through a PI stage and connects with V. d V q Comparison yields U d and U q U is then obtained through inverse dq transformation. abcref It is used to generate the trigger signal for the converter valve.

[0031] Figure 3 This is a diagram of the black start procedure for an offshore wind farm. Step 1: BRK and BRK1 are disconnected. The onshore hybrid MMC starts up and charges itself. The sending-end CSC and the receiving-end hybrid MMC work together to establish the offshore AC voltage.

[0032] Step 2: After establishing a stable offshore AC voltage that meets grid connection requirements, BRK closes, the onshore hybrid MMC switches to constant DC voltage control, outputs negative voltage on the DC side, and the onshore hybrid MMC sends starting power back to the wind farm to supply power to the auxiliary equipment of the wind turbine, thus starting the auxiliary equipment of the wind turbine.

[0033] Step 3: BRK1 is closed, the wind turbine starts and connects to the grid, and the offshore CSC and wind turbine work together to control the offshore AC voltage. The control mode switching indicator for the offshore CSC is obtained by measuring the change in local DC power ΔP or the rate of change dP / dt, without communication. Control mode switching occurs when ΔP is greater than zero or dP / dt is greater than zero.

[0034] Step 4: The DC voltage and DC current of the onshore hybrid MMC rise to the rated value, the black start process ends, and the system enters steady-state operation.

[0035] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Those skilled in the art can still make modifications or equivalent substitutions to the specific implementation of the present invention by referring 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 protection scope of the claims of the present invention pending approval.

Claims

1. A black start method suitable for a CSC-hybrid MMC offshore wind power DC system, characterized in that: The offshore sending-end converter is an active commutation current source converter (CSC) based on fully controlled devices, and the onshore receiving-end converter is a half-bridge-full-bridge hybrid modular multilevel converter (MMC) based on fully controlled devices. A black start method for offshore wind power DC systems is proposed using the half-bridge-full-bridge hybrid MMC.

2. The system topology according to claim 1, characterized in that, The marine sending-end converter CSC is composed of m six 6-pulse CSCs in cascade, m ≥1, each bridge arm of the CSC has a plurality of series-connected fully-controlled switching devices. The land receiving-end converter uses a hybrid MMC, each bridge arm of which has a plurality of series-connected fully-controlled switching devices.

3. The system topology of claim 1, wherein, The marine CSC employs a dual-degree-of-freedom modulation strategy, while the land-based MMC employs a nearest-level modulation strategy.

4. The system topology of claim 1, wherein, The fully controllable device can be a semiconductor switching device that can withstand reverse voltage.

5. The black start method of claim 1, wherein, By utilizing the negative voltage output capability of the full-bridge submodule in the semi-full hybrid MMC, it is possible to establish AC voltage for the wind farm without switching the black start resistor.

6. The sink topology of claim 1, wherein, The land-based MMC adopts a half-bridge-full-bridge hybrid structure, which has the ability to decouple AC and DC voltages, and can achieve unblocked DC fault ride-through while maintaining AC voltage support capability.

Images