A DC / DC converter and control method suitable for offshore wind farms
By employing a combination of boost converters and buck converters in offshore wind farms, along with specific control methods, the problems of DC/DC converters failing to establish grid voltage and short-circuit fault ride-through in offshore wind farms have been solved. This enables black start and fault isolation in offshore wind farms, reducing system cost and size.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- STATE GRID JIANGSU ELECTRIC POWER CO LTD RESEARCH INSTITUTE
- Filing Date
- 2022-03-18
- Publication Date
- 2026-05-26
AI Technical Summary
Existing DC/DC converters cannot actively establish the grid voltage in the reverse direction in offshore wind farms, and they do not have short-circuit fault ride-through capability.
A combination of boost converter and buck converter is adopted, along with black start control, phase shift multiplexing control and short-circuit fault ride-through control methods, to achieve voltage establishment and fault isolation during wind farm startup, steady-state power generation and fault stages, respectively.
It enables black start and fault ride-through of offshore wind farms, ensuring the reliable operation of the power transmission system, and reduces cost and size by reducing the use of submodule capacitors and switching devices.
Smart Images
Figure CN114583969B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a DC / DC converter and control method suitable for offshore wind farms, belonging to the field of offshore wind power and power electronic conversion technology in power systems. Background Technology
[0002] In recent years, the capacity and distance from shore of offshore wind farms have been increasing. The reactive voltage and loss problems caused by AC cable transmission have led to the transformation of offshore wind farms into all-DC wind farms with DC collection and DC transmission.
[0003] Offshore wind farms often require the rectified output of wind turbines to be boosted once to form a medium-voltage DC bus for wind power, and then boosted again through an offshore substation to connect to a high-voltage DC bus. This technology uses a DC / DC converter based on a medium-frequency isolation transformer to replace the power frequency transformer in the original AC convergence process, making the structure more compact.
[0004] However, the current DC / DC converters still have the following problems that need to be solved: 1) Offshore wind farms are isolated grids. When black start is required, the current DC / DC converters often cannot actively establish the voltage of the wind farm's internal grid in reverse; 3) The current DC / DC converters do not have the ability to ride through short-circuit faults. Summary of the Invention
[0005] This invention provides a DC / DC converter and control method suitable for offshore wind farms, solving the problems disclosed in the background art.
[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:
[0007] A DC / DC converter suitable for offshore wind farms includes several boost converters and several buck converters;
[0008] The step-up converter includes a step-up transformer, an MMC connected to the primary side of the step-up transformer, and a diode uncontrolled rectifier connected to the secondary side of the step-up transformer. The MMC is connected in parallel to the medium-voltage DC bus of the wind farm, and the diode uncontrolled rectifier is connected in series to the high-voltage DC bus.
[0009] The step-down converter includes a step-down transformer, an MMC connected to the primary side of the step-down transformer, and a diode uncontrolled rectifier connected to the secondary side of the step-down transformer. The MMC is connected in parallel to the high-voltage DC bus, and the diode uncontrolled rectifier is connected in series to the medium-voltage DC bus of the wind farm.
[0010] The MMC is a three-phase half-bridge type MMC. The upper arm and the corresponding lower arm of the three-phase half-bridge type MMC have a symmetrical structure. Each arm includes multiple MMC sub-modules and arm inductors connected in series. The arm inductors of the upper arm and the corresponding lower arm are connected.
[0011] The MMC submodule includes a power switch. T 1. Power switch T 2 and DC capacitor C HB Power switch T 1 and power switch T Both are connected in reverse parallel with diodes, power switch T 1's emitter and power switch T 2 collector connection, power switch T The collector of 2 serves as the input terminal of the MMC submodule, and the power switch... T The emitter of 2 serves as the output terminal of the MMC submodule, and the DC capacitor... C HB The negative terminal is connected to the power switch. T 2's emitter, DC capacitor C HB The positive terminal is connected to the power switch. T The collector of 1.
[0012] The diode uncontrolled rectifier is a 6-pulse diode rectifier, a 12-pulse diode rectifier, or a phase-shifted multiplexed diode rectifier.
[0013] A control method for a DC / DC converter suitable for offshore wind farms, comprising:
[0014] During the wind farm startup phase, the step-up converter is not operated, and the step-down converter is controlled by a black-start control method.
[0015] During the steady-state power generation phase of the wind farm, the buck converter is controlled to not work, and the boost converter is controlled by the phase-shifting multiplexing control method.
[0016] During the short-circuit fault stage of the medium-voltage DC bus or high-voltage DC bus in the wind farm, the short-circuit fault ride-through control method is used to control the boost converter and buck converter.
[0017] During the wind farm startup phase, the step-up converter is kept inactive, and a black-start control method is used to control the buck converter, including:
[0018] During the wind farm startup phase, the trigger pulse of the MMC in the boost converter is blocked, and the output voltage of the MMC in the buck converter is controlled to raise the voltage of the medium-voltage DC bus in the wind farm.
[0019] If the voltage of the medium-voltage DC bus in the wind farm rises to the rated threshold, the trigger pulse of the MMC in the boost converter is unlocked, and the trigger pulse of the MMC in the buck converter is locked.
[0020] During the steady-state power generation phase of a wind farm, the buck converter is controlled to remain inactive, and the boost converter is controlled using a phase-shifting multiplexing control method, including:
[0021] During the steady-state power generation phase of the wind farm, the trigger pulse of the MMC in the buck converter is blocked, and the modulation ratio of the AC modulation voltage of the MMC in the boost converter is kept consistent, while the phase angle difference between the AC modulation voltages of the MMC is kept at π / (3). i ),in, i This represents the number of MMCs in the boost converter.
[0022] During the short-circuit fault phase of the medium-voltage DC bus or high-voltage DC bus in a wind farm, a short-circuit fault ride-through control method is used to control the step-up converter and the buck converter, including:
[0023] If a short-circuit fault occurs on the medium-voltage DC bus of the wind farm, the trigger pulses of the MMC in the boost converter and buck converter are blocked, and the diodes in the boost converter are controlled to prevent the rectifier from isolating the short-circuit fault current.
[0024] If a short-circuit fault occurs on the high-voltage DC bus, the trigger pulses of the MMC in the boost converter and buck converter are blocked, and the diodes in the buck converter are controlled to isolate the short-circuit fault current from the rectifier.
[0025] The beneficial effects achieved by this invention are as follows: This invention realizes the voltage boost and power transmission of the full DC wind farm bus through a boost converter, establishes the AC side grid voltage during the wind farm startup phase through a buck converter, provides the startup power of the wind farm, realizes the black start of the offshore wind farm, and through the control of the MMC and diode uncontrolled rectifier, it can realize fault ride-through under short-circuit faults of both DC buses, ensuring the reliable operation of the power transmission system. Attached Figure Description
[0026] Figure 1 This is a circuit diagram of the DC / DC converter of the present invention;
[0027] Figure 2 This is the circuit topology diagram of MMC;
[0028] Figure 3 This is the circuit topology diagram of the MMC submodule. Detailed Implementation
[0029] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.
[0030] like Figure 1 As shown, a DC / DC converter suitable for offshore wind farms includes several boost converters and several buck converters. The specific number of boost converters and buck converters depends on the actual situation, and in general, the number of boost converters and buck converters is similar.
[0031] The step-up converter includes a step-up transformer, an MMC (Modular Multilevel Converter) connected to the primary side of the step-up transformer, and a diode uncontrolled rectifier connected to the secondary side of the step-up transformer. The MMC is connected in parallel to the medium-voltage DC bus of the wind farm, and the diode uncontrolled rectifier is connected in series to the high-voltage DC bus. The MMC realizes DC / AC conversion, and the diode uncontrolled rectifier realizes AC / DC conversion.
[0032] Figure 1 In China, boost converters include i All MMC inputs are connected in parallel and connected to the medium-voltage DC bus of the wind farm. The outputs of the MMCs are connected to the primary side of a step-up transformer, which is a medium-frequency transformer with a turns ratio of 1:N. H N H >1. The input terminal of the diode uncontrolled rectifier is connected to the secondary side of the step-up transformer. The output terminals of all diode uncontrolled rectifiers are connected in series and connected to the high-voltage DC bus.
[0033] The step-down converter includes a step-down transformer, an MMC connected to the primary side of the step-down transformer, and a diode uncontrolled rectifier connected to the secondary side of the step-down transformer. The MMC is connected in parallel to the high-voltage DC bus, and the diode uncontrolled rectifier is connected in series to the medium-voltage DC bus of the wind farm.
[0034] Figure 1 Only one buck converter is shown in the diagram; the rest are omitted. It is an inverting circuit compared to a boost converter. All MMC inputs are connected in parallel and then connected to the high-voltage DC bus. The outputs of the MMCs are connected to the primary side of the buck transformer, which is also a medium-frequency transformer with a turns ratio of N. V :1,N V >1. The secondary side of the step-down transformer is connected to the input terminal of the diode uncontrolled rectifier. The output terminals of all diode uncontrolled rectifiers are connected in series and connected in series to the medium-voltage DC bus of the wind farm.
[0035] The aforementioned DC / DC converters are mainly used in DC booster stations for offshore wind farms. The boost converter is full power and is used to boost the voltage of the entire DC wind farm bus and transmit power. The buck converter has a smaller power and is used to establish the AC side grid voltage during the wind farm startup phase, providing startup power for the wind farm and enabling black start of the offshore wind farm. Furthermore, by controlling the MMC and diode uncontrolled rectifier, fault ride-through under short-circuit faults on both DC buses can be achieved, ensuring the reliable operation of the power transmission system.
[0036] All MMCs in a DC / DC converter have the same structure; here, a three-phase half-bridge MMC is used, such as... Figure 2 As shown, the MMC is a three-phase half-bridge type MMC. The upper bridge arm and the corresponding lower bridge arm of the three-phase half-bridge type MMC have a symmetrical structure. Each bridge arm includes multiple MMC sub-modules and bridge arm inductors connected in series. As shown in the figure, there are n MMC sub-modules and 1 bridge arm inductor connected in series. The bridge arm inductor of the upper bridge arm is connected to the bridge arm inductor of the corresponding lower bridge arm. This connection point is the AC output terminal of the bridge arm.
[0037] like Figure 3 As shown, the MMC submodule includes a power switch. T 1. Power switch T 2 and DC capacitor C HB Power switch T 1 and power switch T Both 2 are connected in reverse parallel with diodes, as shown in the diagram. D 1 and D 2. Power switch T 1's emitter and power switch T 2 collector connection, power switch T The collector of 2 serves as the input terminal of the MMC submodule, and the power switch... T The emitter of 2 serves as the output terminal of the MMC submodule, and the DC capacitor... C HB The negative terminal is connected to the power switch. T 2's emitter, DC capacitor C HB The positive terminal is connected to the power switch. T The collector of 1.
[0038] The diode uncontrolled rectifier is a 6-pulse diode rectifier, a 12-pulse diode rectifier, or a phase-shifted multiplexed diode rectifier.
[0039] To achieve the different functions of the aforementioned DC / DC converter at different stages, this invention also discloses corresponding control methods for the converter, including: during the wind farm startup stage, controlling the boost converter to not operate and using a black-start control method to control the buck converter; during the wind farm steady-state power generation stage, controlling the buck converter to not operate and using a phase-shifting multiplexing control method to control the boost converter; and during the short-circuit fault stage of the medium-voltage DC bus or high-voltage DC bus in the wind farm, using a short-circuit fault ride-through control method to control both the boost converter and the buck converter.
[0040] During the startup phase of a wind farm, there is no startup voltage or startup power. This necessitates the use of a step-down converter to provide power from the high-voltage DC bus side. The specific process is as follows:
[0041] 11) During the wind farm startup phase, the trigger pulse of the MMC in the boost converter is blocked, and the output voltage of the MMC in the buck converter is controlled to raise the voltage of the medium-voltage DC bus in the wind farm.
[0042] During the wind farm startup phase, the trigger pulse of the MMC in the boost converter is blocked, and the boost converter does not operate. Energy is transferred from the high-voltage DC bus to the medium-voltage DC bus of the wind farm by the buck converter. The MMC of the buck converter is in inverter mode, generating AC side voltage according to the modulation command. u ma , u mb , u mc After rectification, the output is a medium-voltage DC voltage. U s Used to establish the voltage of the wind farm's internal grid;
[0043] The wind farm's starting voltage and starting power are both provided by a step-down converter. The wind turbines start up and connect to the grid according to conventional control strategies. The wind turbines begin generating electricity and output power, gradually transitioning from a load state to a power supply state, through control... u ma , u mb , u mc To control the rise of the medium-voltage DC bus voltage in the wind farm.
[0044] 22) If the voltage of the medium-voltage DC bus in the wind farm rises to the rated threshold, unlock the trigger pulse of the MMC in the boost converter and lock the trigger pulse of the MMC in the buck converter.
[0045] After the wind farm is started up, the voltage of the medium-voltage DC bus in the wind farm is raised to the rated threshold of the DC side of the boost converter. At this time, the high-voltage unidirectional DC boost converter is turned on, that is, the trigger pulse of MMC in the boost converter is unlocked, so that the power generation of the wind farm begins to transfer energy from the boost converter to the high-voltage DC bus.
[0046] The trigger pulse of the MMC in the locked buck converter reduces the power flowing through the buck converter to zero until all active power flows through the boost converter, and the wind farm starts up and is connected to the grid.
[0047] During the steady-state power generation phase of the wind farm, the buck converter is controlled to be inactive, and the boost converter is controlled using a phase-shifting multiplexing control method. This helps reduce the harmonic content of the high-voltage DC bus and improves the output performance of the uncontrolled rectifier. The specific process is as follows: During the steady-state power generation phase of the wind farm, the trigger pulse of the MMC in the buck converter is blocked, and the modulation ratio of the AC modulation voltage of the MMC in the boost converter is kept consistent, and the phase angle difference between the AC modulation voltages of the MMC is kept at π / (3). i ),in, i This represents the number of MMCs in the boost converter.
[0048] That is, in the steady-state power generation stage of a wind farm, in the step-up converter, i The modulation ratio of each MMC AC modulation voltage remains consistent, i.e., the MMC output voltage... u a1 ~ u ai , u b1 ~ u bi , u c1 ~ u ci The amplitude remains consistent; and the modulation angle is controlled so that in the boost converter, i The phase angle difference between the MMC AC modulation voltages remains at π / (3). i To minimize the harmonic content of the high-voltage DC bus.
[0049] During the short-circuit fault phase of the medium-voltage DC bus or high-voltage DC bus in a wind farm, short-circuit fault isolation is required to greatly improve the reliability of system operation. Therefore, the short-circuit fault ride-through control process adopted is as follows:
[0050] 21) If a short-circuit fault occurs on the medium-voltage DC bus in the wind farm, the trigger pulses of the MMC in the boost converter and buck converter are blocked, and the diodes in the boost converter are controlled to isolate the short-circuit fault current from the rectifier.
[0051] The sampling current of the medium-voltage DC bus in the wind farm can be detected. If a short-circuit fault is detected, all IGBT trigger pulses of the MMC are immediately removed. The uncontrolled rectifier diodes on the high-voltage side of the boost converter isolate the short-circuit fault current, and the MMC output in the buck converter is simultaneously deactivated. u ma , u mb , u mc If the value is 0, there is no fault current flowing through the buck converter.
[0052] 22) If a short-circuit fault occurs on the high-voltage DC bus, the trigger pulses of the MMC in the boost converter and buck converter are blocked, and the diodes in the buck converter are controlled to isolate the short-circuit fault current from the rectifier.
[0053] The sampling current of the high-voltage DC bus can be detected. If a short-circuit fault is detected, all IGBT trigger pulses of the MMC are immediately removed, and the MMC output in the boost converter... u a1 ~ u ai , u b1 ~ u bi , u c1 ~ u ci When the value is 0, no fault current flows in the boost converter, and the uncontrolled rectifier diodes on the voltage side of the buck converter will isolate the short-circuit fault current.
[0054] The aforementioned converter not only meets the voltage boosting and power transmission requirements of the full DC wind farm bus, but also enables autonomous startup and grid connection during the wind farm startup phase. Furthermore, it allows for fault ride-through under short-circuit faults on both DC buses, ensuring reliable operation of the transmission system. In addition, compared to existing full MMC transmission systems, the main power transmission channel of this converter is the boost converter, while the buck converter only carries a small portion (1%~2%) of the system's rated power during the wind farm startup phase. Therefore, the submodule capacitor values and rated current of the switching devices are significantly reduced. Given that submodule capacitors and switching devices account for a large portion of the MMC size and cost, this converter will have a more compact size and lower cost, resulting in better economic benefits.
[0055] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. A control method for a DC / DC converter suitable for an offshore wind farm, characterized in that, The DC / DC converter includes several boost converters and several buck converters; the boost converter includes a boost transformer, an MMC connected to the primary side of the boost transformer, and a diode uncontrolled rectifier connected to the secondary side of the boost transformer. The MMC is connected in parallel to the medium-voltage DC bus of the wind farm, and the diode uncontrolled rectifier is connected in series to the high-voltage DC bus; the buck converter includes a buck transformer, an MMC connected to the primary side of the buck transformer, and a diode uncontrolled rectifier connected to the secondary side of the buck transformer. The MMC is connected in parallel to the high-voltage DC bus, and the diode uncontrolled rectifier is connected in series to the medium-voltage DC bus of the wind farm. The method includes: During the wind farm startup phase, the step-up converter is not operated, and the step-down converter is controlled by a black-start control method. During the steady-state power generation phase of the wind farm, the buck converter is controlled to not work, and the boost converter is controlled by the phase-shifting multiplexing control method. During the short-circuit fault stage of the medium-voltage DC bus or high-voltage DC bus in the wind farm, the short-circuit fault ride-through control method is used to control the boost converter and buck converter.
2. The control method of a DC / DC converter suitable for an offshore wind farm according to claim 1, characterized in that, During the wind farm startup phase, the step-up converter is kept inactive, and a black-start control method is used to control the buck converter, including: During the wind farm startup phase, the trigger pulse of the MMC in the boost converter is blocked, and the output voltage of the MMC in the buck converter is controlled to raise the voltage of the medium-voltage DC bus in the wind farm. If the voltage of the medium-voltage DC bus in the wind farm rises to the rated threshold, the trigger pulse of the MMC in the boost converter is unlocked, and the trigger pulse of the MMC in the buck converter is locked.
3. The control method for a DC / DC converter suitable for offshore wind farms according to claim 1, characterized in that, During the steady-state power generation phase of a wind farm, the buck converter is controlled to remain inactive, and the boost converter is controlled using a phase-shifting multiplexing control method, including: During the steady-state power generation phase of the wind farm, the trigger pulse of the MMC in the buck converter is blocked, and the modulation ratio of the AC modulation voltage of the MMC in the boost converter is kept consistent, while the phase angle difference between the AC modulation voltages of the MMC is kept at π / (3). i ),in, i This represents the number of MMCs in the boost converter.
4. The control method for a DC / DC converter suitable for offshore wind farms according to claim 1, characterized in that, During the short-circuit fault phase of the medium-voltage DC bus or high-voltage DC bus in a wind farm, a short-circuit fault ride-through control method is used to control the step-up converter and the buck converter, including: If a short-circuit fault occurs on the medium-voltage DC bus of the wind farm, the trigger pulses of the MMC in the boost converter and buck converter are blocked, and the diodes in the boost converter are controlled to prevent the rectifier from isolating the short-circuit fault current. If a short-circuit fault occurs on the high-voltage DC bus, the trigger pulses of the MMC in the boost converter and buck converter are blocked, and the diodes in the buck converter are controlled to isolate the short-circuit fault current from the rectifier.