Direct-current boost collection converter of offshore wind power full-direct-current sending-out system

Abstract

The invention provides a direct-current boost collection converter of an offshore wind power full-direct-current sending-out system, and the converter can quickly suppress residual direct-current fault current, so that quick direct-current fault isolation is realized by utilizing a direct-current switch. And the fault clearing time is obviously shortened from about 500ms to below 14ms, so that rapid system recovery is realized. The working principle of the topology under the conditions of normal operation and direct-current fault is elaborated, and enhanced power equalization control is provided, so that direct-current voltage and current equalization among sub-modules of the topology is realized. In addition, after interleaving control is adopted, the number of direct-current reactors of the topology is remarkably reduced. A simulation result verifies the effectiveness of the hybrid DC / DC converter and the control scheme thereof in the aspects of blocking fault residual current and promoting direct current fault ride-through by using the DCS.

Description

Technical Field

[0001] The present application belongs to the field of converters, and specifically relates to a DC boost converter for an offshore wind power all-DC transmission system. Background Art

[0002] Offshore wind power (OWP) is key to achieving global decarbonization goals. Typical OWP uses medium voltage AC (MVAC) for aggregation and high voltage DC (HVDC) for long-distance transmission. However, with the rapid increase in offshore wind power aggregation distances and installed capacity, a full DC approach—combining medium voltage DC (MVDC) aggregation with HVDC transmission—is considered a viable solution for high-capacity, large-scale OWP aggregation. Its advantages include reduced offshore platform size, lower power losses in MVDC aggregation converters, higher power transmission capacity, and greater system stability.

[0003] A major difficulty of the full DC system is DC fault protection, which must be considered from the perspective of the entire system, including key equipment such as the DC-link converter. Existing multifunctional DC-link converters with DC fault isolation capabilities rely on DC circuit breakers to isolate faults on the high-voltage DC side. DC circuit breakers are bulky and have high investment costs. Existing LCL-based thyristor DC / DC converters have bidirectional power flow and DC fault clearing capabilities, but the voltage boost ratio of the converter is limited. Existing DC autotransformers with fault isolation capabilities have large fault currents in the diodes of the half-bridge submodules. In addition, due to the lack of electrical isolation in the DC autotransformer, the system recovers slowly after the DC fault is cleared. Existing DC / DC converters use resonant circuits to achieve zero voltage turn-on and zero current turn-off. However, since the resonant resistance is close to zero, a large fault current will be generated during a DC short-circuit fault. Existing DC / DC converters consist of two traditional Modular multilevel converters (MMCs) are connected face-to-face. This topology enables continuous power transmission during DC faults through DC circuit breakers. However, the use of DC circuit breakers results in high investment costs for protective devices. Existing fault ride-through strategies based on boost-type submodules (MMCs) achieve fault ride-through under various fault conditions, but also significantly increase hardware complexity and the difficulty of parameter design. Existing MMCs with integrated energy dissipation devices combine the complementary advantages of MMCs and boost converters (DC boost converters) to provide a reliable solution for fault ride-through. However, the integrated energy dissipation devices may face the risk of system aging or failure due to frequent switching. Existing hybrid modular multilevel DC / DC converters (MMDCs) combine half-bridge and full-bridge submodules as well as thyristors and diodes. Hybrid MMDCs can use primary-side SMs to mitigate the impact of DC faults and quickly isolate DC faults. Although MMDCs meet the requirements of medium-voltage, large-capacity DC power acquisition, they suffer from large bridge arm power fluctuations, resulting in larger submodule capacitances.

[0004] Another promising candidate for high-capacity aggregate converters is the input-parallel output-series (IPOS) DC / DC converter. It not only leverages existing low-voltage equipment and circuit topologies but also improves system reliability and power capacity. A dual-active bridge converter is installed in each wind turbine's nacelle. The turbines are then connected in an IPOS configuration, directly boosting the voltage to a high-voltage level for transmission, thus avoiding the need for offshore platforms for boosting. However, this configuration requires the wind turbine nacelles to be insulated from the high-voltage level. Furthermore, the discharge of DC capacitors after a fault can generate large fault currents. To avoid capacitor discharge during a fault and reduce equipment costs, a single active bridge (SAB) converter was developed. Given the unidirectional nature of offshore wind power transmission, SABs use diode rectification instead of a full-bridge circuit on the high-voltage side.

[0005] After detecting a fault, isolated DC / DC converters can lock all IGBTs to block DC faults and prevent the fault from propagating from the high / medium voltage side to the medium / high voltage side. Therefore, DC switches (DCS) can be used instead of DCCBs (DC circuit breakers) to isolate faults, reducing the cost of protective equipment. However, due to the residual energy stored in the DC cables and DC reactors, the fault current still flows through the diodes, DC cables, reactors, and the fault point, such as Figure 1 As shown in Figure 2. The residual energy circulates on the HVDC side and slowly dissipates through the resistance of the DC cables and reactors. The DCS cannot disconnect quickly, which prolongs the fault clearing time. Figure 2 As shown in Figure 1, the DC fault current of a typical DC / DC converter decays to 100A in about 500ms. When the current is lower than 100A, the DCS can be turned on for fault isolation. However, system recovery is slow, affecting power supply reliability. Summary of the Invention

[0006] In order to solve the problems of poor fault current suppression and slow system recovery in the existing offshore wind power technology, a DC boost converter for an offshore wind power full DC transmission system is proposed.

[0007] One end of the DC reactor is connected to the positive electrode of the high-voltage DC output terminal, and the other end of the DC reactor is connected to the positive output terminal of the first submodule; the negative output terminal of the i-th submodule is connected to the positive output terminal of the i+1-th module, i∈[1,2,3,···,N-1], and the negative output terminal of the N-th submodule is connected to the negative electrode of the high-voltage DC output terminal;

[0008] The positive input terminals of the 1st submodule to the Nth submodule are simultaneously connected to the positive pole of the medium voltage DC input terminal, and the negative input terminals of the 1st submodule to the Nth submodule are simultaneously connected to the negative pole of the medium voltage DC input terminal;

[0009] The first submodule includes a residual fault current blocking circuit, a transformer T1, a leakage inductor L S1 and a first full-bridge circuit;

[0010] The residual fault current blocking circuit includes a first bridge arm and a second bridge arm, the first bridge arm and the second bridge arm are connected in parallel, one end of the first bridge arm is connected to one end of the second bridge arm as the positive output end of the first submodule, and the other end of the first bridge arm is connected to the other end of the second bridge arm as the negative output end of the first submodule, the first bridge arm includes a diode D1, an IGBT Q1, a diode D2 and an IGBT Q2 connected in series in sequence, and the second bridge arm includes a diode D3, an IGBT Q3, a diode D4 and an IGBT Q4 connected in series in sequence, the middle point of the first bridge arm is connected to one end of the secondary side of the transformer T1, and the middle point of the second bridge arm is connected to the other end of the secondary side of the transformer T1; the first full-bridge circuit is simultaneously connected to the leakage inductance L S1 One end is connected to the other end of the primary side of transformer T1, and the leakage inductance L S1 The other end is connected to the primary end of transformer T1;

[0011] The 2nd to Nth submodules are single active bridge converters;

[0012] When the offshore wind power full DC transmission system is operating normally, the 1st to Nth submodules are used for power transmission from medium voltage DC to high voltage DC; when a DC fault occurs in the offshore wind power full DC transmission system, the residual fault current blocking circuit is used to block the residual fault current.

[0013] A power balancing control method for a DC boost converter of an offshore wind power all-DC transmission system, comprising:

[0014] Step 1: Set the medium voltage DC voltage reference value V MVDC * , get the actual value of medium voltage DC voltage V MVDC , average voltage of submodule and the DC output voltage v of the jth submodule oj , j∈[2,3,4,···,N];

[0015] Step 2: Set the medium voltage DC voltage reference value V MVDC * And the actual value of medium voltage DC voltage V MVDC The difference is input to the first PI controller to obtain the off time t off , the off time t off After doubling the amount, we get twice the off time 2t off ;

[0016] Step 3: Average the submodule voltage The DC output voltage v of the jth submoduleoj The difference is input to the j-th PI controller to obtain the compensation off time t of the j-th submodule. comj ;

[0017] Step 4: Double the off time 2t off Input to the first PWM module, obtain the first gating signal and control the shutdown of the first submodule according to the first gating signal; double the shutdown time 2t off Compensation off time t of the jth submodule comj The difference is input to the j-th PWM module to obtain the j-th gating signal, and the j-th sub-module is turned off according to the j-th gating signal.

[0018] A staggered control method for a DC boost converter of an offshore wind power all-DC transmission system, comprising:

[0019] Step 1: Obtain the number N and switching period t of the DC boost converter submodules of the offshore wind power full DC transmission system s ;

[0020] Step 2: Take the first submodule switch on time as the reference time t z , the opening time of the jth submodule is t zj , t zj =t j +(t s / 2) / N.

[0021] Beneficial effects: The present application proposes a DC boost-collecting converter for an offshore wind power all-DC transmission system, which is a new hybrid DC / DC converter (HSAB) based on a single active bridge with parallel input and series output. By adopting a fault current blocking circuit, the residual DC fault current can be quickly suppressed to zero, thereby using DCS to achieve rapid fault isolation, and the fault clearing time is significantly shortened from about 500ms to less than 14ms, thereby achieving rapid recovery of the system. The key parameters of the HSAB are designed, and an enhanced control strategy is proposed. By adjusting the duty cycle of each sub-module, DC voltage and current balancing between the high-voltage side and medium-voltage side sub-modules is achieved. Interleaved control is adopted to reduce the size of the DC reactor. The DC boost-collecting converter and power balancing control method for the offshore wind power all-DC transmission system proposed in the present application improve the power supply reliability and provide a very promising option for the collection and transmission of large-scale offshore wind power. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a residual fault current path of an isolated DC / DC converter in the prior art;

[0023] Figure 2This is a comparison diagram of the fault current blocking waveform of the existing DC converter and the fault current blocking waveform of the DC boost-sink converter of the offshore wind power full DC transmission system according to the specific embodiment of the present application;

[0024] Figure 3 This is the overall structural diagram of existing offshore wind power;

[0025] Figure 4 This is a topological diagram of a DC boost converter for an offshore wind power all-DC transmission system according to a specific embodiment of the present application;

[0026] Figure 5 This is a working waveform diagram of the DC boost converter of the offshore wind power all-DC transmission system according to a specific embodiment of the present application;

[0027] Figure 6 The current paths of the DC boost converter in different working periods of the offshore wind power full DC transmission system according to the specific embodiment of the present application;

[0028] Figure 7 This is a working waveform diagram of the DC boost converter of the offshore wind power full DC transmission system during a fault period according to a specific embodiment of the present application;

[0029] Figure 8 The fault current flow path of the DC boost converter in fault state I of the offshore wind power full DC transmission system according to the specific embodiment of the present application;

[0030] Figure 9 This is the fault equivalent circuit of the DC boost converter of the offshore wind power full DC transmission system in fault state I according to the specific embodiment of the present application;

[0031] Figure 10 The fault current flow path of the DC boost converter in fault state II of the offshore wind power full DC transmission system according to the specific embodiment of the present application;

[0032] Figure 11 This is the fault equivalent circuit of the DC boost converter of the offshore wind power full DC transmission system in fault state II according to the specific embodiment of the present application;

[0033] Figure 12 This is a comparison diagram of the peak fault current of the DC boost converter of the offshore wind power full DC transmission system according to the specific embodiment of the present application;

[0034] Figure 13 This is a comparison diagram of the capacitor voltage peaks of the DC boost converter of the offshore wind power full DC transmission system according to a specific embodiment of the present application;

[0035] Figure 14This is a power balancing control block diagram of a DC boost converter in an offshore wind power all-DC transmission system according to a specific embodiment of the present application;

[0036] Figure 15 This is a diagram showing the simulation results of power balancing control of a DC boost converter in an offshore wind power all-DC transmission system according to a specific embodiment of the present application;

[0037] Figure 16 The voltage and current waveforms of the DC boost converter of the offshore wind power all-DC transmission system according to the specific embodiment of the present application when interleaved control is adopted;

[0038] Figure 17 The voltage and current waveforms of the DC boost converter of the offshore wind power full DC transmission system according to the specific embodiment of this application when interleaving control is not adopted

[0039] Figure 18 A four-terminal DC grid structure with a DC boost converter for an offshore wind power all-DC transmission system according to a specific embodiment of the present application;

[0040] Figure 19 This is a fault current suppression simulation waveform of the DC boost converter of the offshore wind power full DC transmission system according to a specific embodiment of the present application;

[0041] Figure 20 This is a fault ride-through simulation waveform of the DC boost converter of the offshore wind power full DC transmission system according to the specific implementation of the present application. DETAILED DESCRIPTION

[0042] Specific implementation method 1: The following is combined with the attached embodiment of the present invention Figure 1 To the attached Figure 20 , illustrate this embodiment, and clearly and completely describe the technical solutions in the embodiments of the present invention:

[0043] A DC boost converter for an offshore wind power all-DC transmission system, comprising: a DC reactor and N submodules;

[0044] One end of the DC reactor is connected to the positive electrode of the high-voltage DC output terminal, and the other end of the DC reactor is connected to the positive output terminal of the first submodule; the negative output terminal of the i-th submodule is connected to the positive output terminal of the i+1-th module, i∈[1,2,3,···,N-1], and the negative output terminal of the N-th submodule is connected to the negative electrode of the high-voltage DC output terminal;

[0045] The positive input terminals of the 1st submodule to the Nth submodule are simultaneously connected to the positive pole of the medium voltage DC input terminal, and the negative input terminals of the 1st submodule to the Nth submodule are simultaneously connected to the negative pole of the medium voltage DC input terminal;

[0046] The first submodule includes a residual fault current blocking circuit, a transformer T1, a leakage inductor L S1 and a first full-bridge circuit;

[0047] The residual fault current blocking circuit includes a first bridge arm and a second bridge arm, the first bridge arm and the second bridge arm are connected in parallel, one end of the first bridge arm is connected to one end of the second bridge arm as the positive output end of the first submodule, and the other end of the first bridge arm is connected to the other end of the second bridge arm as the negative output end of the first submodule, the first bridge arm includes a diode D1, an IGBT Q1, a diode D2 and an IGBT Q2 connected in series in sequence, and the second bridge arm includes a diode D3, an IGBT Q3, a diode D4 and an IGBT Q4 connected in series in sequence, the middle point of the first bridge arm is connected to one end of the secondary side of the transformer T1, and the middle point of the second bridge arm is connected to the other end of the secondary side of the transformer T1; the first full-bridge circuit is simultaneously connected to the leakage inductance L S1 One end is connected to the other end of the primary side of transformer T1, and the leakage inductance L S1 The other end is connected to the primary end of transformer T1;

[0048] The 2nd to Nth submodules are single active bridge converters;

[0049] When the offshore wind power full DC transmission system is operating normally, the 1st to Nth submodules are used for power transmission from medium voltage DC to high voltage DC; when a DC fault occurs in the offshore wind power full DC transmission system, the residual fault current blocking circuit is used to block the residual fault current.

[0050] Specifically, the overall structure of full DC offshore wind power is as follows: Figure 3 As shown, the DC boost converter topology of the offshore wind power full DC transmission system of this application is shown in FIG. Figure 4 As shown in the figure, the onshore MMC (Modular Multilevel Converter) controls the HVDC (High Voltage Direct Current) voltage, the DC boost converter of the offshore wind power all-DC transmission system controls the MVDC (Medium Voltage Direct Current) voltage, and the DC WTs (DC Wind Turbines) control the output power according to the Maximum Power Point Tracking (MPPT) algorithm.

[0051] Further, the residual fault current blocking circuit includes a diode D1, a diode D2, a diode D3, a diode D4, an IGBT Q1, an IGBT Q2, an IGBT Q3, an IGBT Q4, a capacitor C1, a capacitor C2, a capacitor C3 and a capacitor C4;

[0052] The cathode of diode D1 and the cathode of diode D3 are simultaneously connected to the other end of DC reactor L, the anode of diode D1 is simultaneously connected to the emitter of IGBT Q1 and one end of capacitor C1, the collector of IGBT Q1 and the other end of capacitor C1 are simultaneously connected to the cathode of diode D2, the anode of diode D2 is simultaneously connected to the emitter of IGBT Q2 and one end of capacitor C2, and the collector of IGBT Q2 and the other end of capacitor C2 are simultaneously connected to the positive output end of the second submodule; the anode of diode D3 is simultaneously connected to the emitter of IGBT Q3 and one end of capacitor C3, the collector of IGBT Q3 and the other end of capacitor C3 are simultaneously connected to the cathode of diode D4, the anode of diode D4 is simultaneously connected to the emitter of IGBT Q4 and one end of capacitor C4, and the collector of IGBT Q4 and the other end of capacitor C4 are simultaneously connected to the positive output end of the second submodule; the collector of IGBT Q1 and the other end of capacitor C1 are simultaneously connected to one end of the secondary side of transformer T1, and the collector of IGBT Q3 and the other end of capacitor C3 are simultaneously connected to the other end of the secondary side of transformer T1;

[0053] When the offshore wind power full DC transmission system operates normally, IGBT Q1, IGBT Q2, IGBT Q3 and IGBT Q4 remain turned on. When a DC fault occurs in the offshore wind power full DC transmission system, IGBT Q1, IGBT Q2, IGBT Q3 and IGBT Q4 are locked.

[0054] Furthermore, the first full-bridge circuit includes IGBT S1, IGBT S2, IGBT S3, IGBT S4, diode D S1 , diode D S2 , diode D S3 , diode D S4 and capacitor C in ;

[0055] Capacitor C in One end, IGBT S1 collector, diode D S1 Cathode, IGBT S3 collector and diode D S3 The cathode is connected to the positive terminal of the medium voltage DC input terminal, and the capacitor C in The other end is connected to the negative terminal of the medium voltage DC input terminal; the emitter of IGBT S1 and the diode D S1 The anode is connected to the collector of IGBT S2 and diode D S2 Cathode connection, IGBT S2 emitter and diode D S2 The anode is also connected to the negative terminal of the medium voltage DC input terminal; the emitter of IGBT S3 and the diode D S3 The anode is connected to the collector of IGBT S4 and diode D S4 Cathode connection, IGBT S4 emitter and diode D S4The anode is also connected to the negative pole of the medium voltage DC input terminal;

[0056] IGBT S1 emitter and diode D S1 The anode and the leakage inductance L S1 One end is connected, leakage inductance L S1 The other end is connected to the primary side of transformer T1, the emitter of IGBT S3 and diode D S3 The anode is connected to the other end of the primary side of transformer T1.

[0057] Furthermore, the jth submodule includes a capacitor C inj 、IGBT S j1 、IGBT S j2 、IGBT S j3 、IGBT S j4 , diode D Sj1 , diode D Sj2 , diode D Sj3 , diode D Sj4 , diode D j1 , diode D j2 , diode D j3 , diode D j4 , transformer T j and leakage inductance L Sj ;j∈[2,3,4,···,N];

[0058] Capacitor C inj One end, IGBT S j1 Collector, diode D Sj1 Anode, IGBT S j3 Collector and diode D Sj3 The anode is also connected to the positive terminal of the medium voltage DC input terminal, and the capacitor C inj The other end is connected to the negative pole of the medium voltage DC input terminal; the emitter of IGBT Sj1 and diode D Sj1 Anode and IGBT S j2 Collector and diode D Sj2 Cathode connection, IGBT S j2 Emitter and diode D Sj2 The anode is also connected to the negative terminal of the medium voltage DC input terminal; IGBTS j3 Emitter and diode D Sj3 Anode and IGBTS j4 Collector and diode D Sj4 Cathode connection, IGBT S j4 Emitter and diode D Sj4 The anode is also connected to the negative terminal of the medium voltage DC input terminal; IGBT S j1 Emitter and diode D S1The anode and the leakage inductance L Sj One end is connected, leakage inductance L Sj The other end is connected to the transformer T j Primary side connection, IGBT S j3 Emitter and diode D Sj3 Anode transformer T j The other end of the original side is connected;

[0059] Diode D j1 Cathode and diode D j2 The cathode is connected to the negative output terminal of the j-1th submodule, and the diode D j1 Anode and diode D j3 Cathode connection, diode D j2 Anode and diode D j4 Cathode connection, diode D j1 Anode and transformer T j One end of the secondary side is connected to the diode D j2 Anode and transformer T j The other end of the secondary side is connected;

[0060] When j∈[2,3,4,···,N-1], the diode D j3 Anode and diode D j4 The anode is connected to the positive output terminal of the j+1th submodule at the same time; when j=N, the diode D j3 Anode and diode D j4 The anode is also connected to the negative pole of the high voltage DC output terminal.

[0061] Specifically, if Figure 4 As shown in FIG, the topology of the DC boost-collecting converter of the offshore wind power full DC transmission system is a hybrid single active bridge converter (HSAB). The high-voltage side of the first submodule of the DC boost-collecting converter of the offshore wind power full DC transmission system is configured with a residual fault current blocking circuit (FCBC). The remaining submodules are composed of typical SABs (unidirectional active bridges). All submodules are connected in parallel on the medium-voltage side and in series on the high-voltage side to boost the DC voltage to a high-voltage level. Each bridge arm of the FCBC includes an IGBT (insulated gate bipolar IGBT) Q m (m=1, 2, 3, 4) and capacitance C m The parallel connection of capacitor C m With diode D mIn the DC boost-and-converter of an offshore wind power all-DC transmission system, the majority of the power is transmitted through the second to the Nth submodule, while the submodule with the residual fault current blocking circuit (FCBC) only needs to transmit a small portion of the power and block the residual DC fault current, thereby minimizing the capacity (for example, only 10%). A centralized DC reactor L is connected to the HVDC (high-voltage direct current) side of the DC boost-and-converter of the offshore wind power all-DC transmission system to filter harmonics. This topology eliminates the need for capacitors on the HVDC side, avoiding the surge discharge current of the DC capacitors during a DC fault and reducing the size and weight of the DC / DC converter.

[0062] During normal operation, IGBT Q1, IGBT Q2, IGBT Q3, and IGBT Q4 in the FCBC remain on, capacitors C1, C2, C3, and C4 are bypassed, and the capacitor voltage is zero. Therefore, during normal operation, the submodule with the FCBC operates as a SAB (Single Active Bridge) in the same manner as the other submodules. In the event of a DC fault, IGBT Q1, IGBT Q2, IGBT Q3, and IGBT Q4 are blocked, and capacitors C1, C2, C3, and C4 are included in the fault current flow path, such as Figure 4 The DC fault current charges the capacitor. As the capacitor voltage increases, the residual fault current is quickly suppressed to zero, thereby achieving rapid fault clearing.

[0063] Figure 2 The fault current waveforms of the DC boost converter of the offshore wind power all-DC transmission system of this application and the typical IPOS-SAB are compared. A two-pole DC short circuit fault occurs at t = 0.5s, and the converter is blocked at t = 0.501s. For a typical DC / DC converter, the residual fault current continues to circulate on the HVDC side, such as Figure 1 As shown in the figure, in the DC boost converter of the offshore wind power all-DC transmission system, capacitors C1, C2, C3, and C4 absorb the energy of the fault current while also providing a blocking voltage. As a result, the fault current is suppressed to zero in less than 14ms (compared to approximately 500ms for a typical DC / DC converter), enabling rapid fault clearing.

[0064] In order to quickly resume normal operation after the fault is cleared, an energy-dissipating resistor is connected in parallel with capacitors C1, C2, C3, and C4 through a mechanical switch to achieve rapid discharge of the capacitors, thereby turning on IGBT Q1, IGBT Q2, IGBT Q3, and IGBT Q4 to achieve rapid recovery.

[0065] When the offshore wind power full DC transmission system of the present application operates normally, IGBT Q1, IGBT Q2, IGBT Q3 and IGBT Q4 of the FCBC remain in the on state, and the 1st to Nth submodules all operate in the same manner. Taking this as an example, the working principle of the DC boost converter of the offshore wind power full DC transmission system is analyzed.

[0066] The DC boost converter of the offshore wind power full DC transmission system works in buck mode (step-down mode). The typical working waveform is as follows: Figure 5 As shown, where t s is the switching period of the DC boost converter of the offshore wind power full DC transmission system, t on is the conduction time of IGBT S1 and IGBT S3. The signals of the two IGBTs in the same-phase bridge arm are complementary, that is, the signals of IGBT S1 and IGBT S2 are complementary, and the signals of IGBT S3 and IGBT S4 are complementary. The voltage ratio between the left and right ends of the DC reactor L is defined as the voltage matching ratio k:

[0067]

[0068] Among them, V HVDC is the HVDC (high voltage direct current) voltage, v o is the output voltage of the DC boost converter, V MVDC is the common MVDC (medium voltage direct current) voltage of N submodules, and n is the transformer transformation ratio.

[0069] like Figure 5 As shown, each switching cycle is divided into six working intervals. Figure 6 The current paths in different working periods are given in Figure 1. The switching cycle starts at time t0, when IGBT S1 is turned on, IGBT S4 is continuously turned on, and IGBT S2 and IGBT S3 are turned off.

[0070] State I[t0, t1]: In this working range, the medium voltage DC side voltage v M Equal to the medium voltage DC voltage V MVDC , the high voltage DC side voltage of the transformer is v H =nV MVDC ,like Figure 5 Middle (g) and Figure 6 As shown in (a). The output voltage of the DC boost converter is v o =|v H |=nV MVDC , and is greater than the HVDC voltage V in formula (1) HVDC The DC boost converter operates at a reduced voltage, so the output current i oIt increases linearly, as shown in formula (2), and flows through IGBT Q1, diode D1, IGBT Q4 and diode D4;

[0071]

[0072] Among them, Δi o1 is the change in output current of the DC boost converter in state I;

[0073] State II[t1, t2]: At t1, IGBT S1 is turned off, IGBT S2 is turned on, IGBT S3 is turned off, and IGBT S4 remains on. H =v M =0 and v o =0, such as Figure 5 (b). Since v H =0,i o = 0 flows through both bridge arms of FCBC at the same time, the change in the output current of the DC boost converter in state II is Δi o2 The expression is as follows:

[0074]

[0075] Due to the transformer leakage inductance L s ,i M The current flows through the transformer MV winding, IGBT S4 and diode D S2 , while i H The current flows through the transformer's HV winding and the two bridge arms of the FCBC. Therefore, the current flowing through diodes D1 and D4 is greater than the current flowing through diodes D2 and D3. According to Kirchoff's current (node current) law, the following formula is obtained:

[0076]

[0077] Among them, i D1 、i D2 、i D3 and i D4 are the currents of diode D1, diode D2, diode D3 and diode D4 respectively.

[0078] State III[t2, t3]: at time t2, i M and i H Drop to zero, i o Flows through the two bridge arms of the high-voltage side FCBC, such as Figure 6 (c) shows the output voltage v o =|v H |=0, the change in the output current of the DC boost converter in state III Δi o3 for:

[0079]

[0080] State IV[t3, t4]: At t3, IGBT S1 is turned off, IGBT S4 is turned off, IGBT S3 is turned on, and IGBT S2 remains on. Similar to state I, the MVAC and HVAC voltages of the transformer in this working state are v M =-V MVDC and v H =-nV MVDC ,i o increases linearly, flowing through IGBT Q2 and diode D2, as well as IGBT Q3 and diode D3. Figure 6 (d) The change in the output current of the DC boost converter in state IV is Δi o4 for:

[0081]

[0082] State V[t4, t5]: At t4, IGBT S3 is turned off, IGBT S4 is turned on, and IGBT S2 remains on, then v H =v L =v o =0,i o Flows through both bridge arms simultaneously, such as Figure 6 (e) The change in the output current Δi of the DC boost converter in state V o5 for:

[0083]

[0084] State VI[t5, t6]: At t5, i M Drop to 0, i o Continue to flow through the two bridge arms of FCBC, such as Figure 6 (f) is shown. Output voltage v o =0, the change in output current Δi of the DC boost converter in state VI o6 for:

[0085]

[0086] When a short circuit fault is applied between two poles on the HVDC side of the DC boost converter topology of the offshore wind power full DC transmission system, such as Figure 4 As shown in the figure, the typical operating waveform during the fault period is as follows Figure 7 Before t'0, the circuit operates normally, IGBTQ1 to IGBTQ4 are turned on, capacitors C1, C2, C3 and C4 are short-circuited, and the capacitor voltage v C1 、vC2 、v C3 and v C4 All are zero.

[0087] Fault state I[t0'~t1']: After a DC short circuit fault occurs at time t0', the DC voltage V HVDC quickly dropped to zero, as Figure 7 (a) shows the output voltage v of the DC boost converter. o It remains basically unchanged before the fault is detected and is completely applied to the DC side inductor L, resulting in the output current i o Rapidly increasing, such as Figure 7 (b) and (c). The fault current path in fault state I is as follows Figure 8 As shown, and can be represented by Figure 9 The RL charging circuit shown is equivalent, where R e is the equivalent resistance of the DC reactor and cable, L e is the equivalent inductance of the DC reactor and cable. Figure 9 The equivalent voltage V e Expressed as:

[0088] V e =NnV MVDC (9)

[0089] The DC boost converter is locked at time t1'. Under fault state I, the DC boost converter conduction time t on '(like Figure 5 (a) shows the inductor current increment i during the period o_charge for:

[0090]

[0091] Since the equivalent resistance R e Small and off time t off Relatively short (eg Figure 5 (c)), so the turn-off time t can be ignored off Through the equivalent resistance R e Reactance L e The total increase of inductor current from t0' to t1' is:

[0092]

[0093] DC boost converter peak current I faultmax Approximately:

[0094] I faultmax =I o +i o_inc (12)

[0095] Among them I o is the output current of the DC boost sink converter during normal operation.

[0096] Fault state II[t1'~t2']: When the DC output current i o At t1' the peak fault current I faultmax When IGBT Q1, IGBTQ2, IGBTQ3 and IGBTQ4 are locked, the fault current will be commutated from IGBTQ1, IGBTQ2, IGBTQ3 and IGBTQ4 to capacitors C1, C2, C3 and C4. Figure 10 As shown; the equivalent RLC circuit is as Figure 11 As shown; Figure 7 As shown in (c), (h) and (i), the capacitor voltage increases from zero and the fault current begins to decrease.

[0097] Assume C1=C2=C3=C4=C, Figure 11 The equivalent capacitance C in e Expressed as:

[0098] C e =C(13)

[0099] Figure 11 In the equation, the equivalent capacitance C e Voltage on for:

[0100]

[0101] in:

[0102]

[0103] Wherein, A1, A2, Z1 and Z2 are the first intermediate parameter, the second intermediate parameter, the third intermediate parameter and the fourth intermediate parameter respectively; is the capacitance C e The initial voltage is 0; i o (0) is the reactance L e The initial current is I faultmax ; Therefore, formula (16) can be rewritten as

[0104]

[0105] From formula (14), we can get that the current flowing through the reactor L e Fault current for:

[0106]

[0107] like Figure 10 and Figure 11 As shown, the output voltage v of the DC boost converter is o for:

[0108]

[0109] in:

[0110] like Figure 7 As shown in (c), (h) and (i), when the fault current i o When the capacitor is reduced to zero, the reactance L e Continuously charge and reach the peak at t2' Then we have:

[0111] v C1max =v C2max =v C3max =v C4max (twenty one)

[0112] Fault state III [t2'~t3']: At t2', the fault current i o is consumed to zero, and due to the voltage difference between the midpoints of the bridge arms, a small current still flows through the transformer (i.e., i H >0). Therefore, diodes D2 and D3 first withstand the voltage, such as Figure 7 (f) and (g), the expression is:

[0113] v D1 =v C1 +v C2 (twenty two)

[0114] v D3 =v C3 +v C4 (twenty three)

[0115] where v D3 、v D3 、v D3 and v D3 are the voltages of diode D1, diode D2, diode D3 and diode D4 respectively;

[0116] After t3', the output current and voltage drop to zero (i o =0 and v o =0). The two diodes on the same bridge arm share the voltage on the capacitor:

[0117] v D1 +v D2 =v C1 +v C2 (twenty four)

[0118] v D3 +v D4 =v C3 +v C4 (25)

[0119] Furthermore, assuming that a DC short circuit fault occurs in the DC boost-sink converter at time t0', and IGBT S1, IGBT S2, IGBT S3, IGBT S4, IGBT Q1, IGBT Q2, IGBT Q3, and IGBT Q4 are all turned off at time t1', the peak fault current of the DC boost-sink converter is calculated as follows:

[0120] I faultmax =I o +i o_inc , where I faultmax is the peak fault current, I o is the output current of the DC boost converter during normal operation of the offshore wind power full DC transmission system, i o_inc is the total increment of DC reactor current from t0' to t1', where t s is the duty cycle of IGBT S1, IGBT S2, IGBT S3 and IGBT S4; i o_charge is the current increment of the DC reactor during the on-time of a single IGBT working cycle, where t on_ab is the conduction time of IGBT S1, IGBT S2, IGBT S3, IGBT S4, IGBT Q1, IGBT Q2, IGBT Q3 or IGBT Q4 after a DC short circuit fault occurs in the DC boost-sink converter, V e is the equivalent voltage of the RL charging circuit of the DC boost converter from time t0' to time t1', R e is the equivalent resistance of the DC reactor and cable in the equivalent RL charging circuit of the DC boost converter from time t0' to time t1', L e t a The equivalent inductance of the DC reactor and cable in the equivalent RL charging circuit of the DC boost converter from t0' to t1'; V e =NnV MVDC , where n is the transformation ratio of transformer T, V MVDC is the medium voltage DC output voltage.

[0121] Furthermore, the maximum voltage of capacitors C1, C2, C3, and C4 is calculated as follows:

[0122] Among them, V C1maxis the maximum voltage of capacitor C1, V C2max is the maximum voltage of capacitor C2, V C3max is the maximum voltage of capacitor C3, V C4max is the maximum voltage of capacitor C4, and C is the capacitance value of capacitors C1, C2, C3 and C4.

[0123] Specifically, the key parameters of the DC boost converter of the offshore wind power full DC transmission system of the present application include energy absorption capacitors C1-C4 and DC reactor L, and the above parameters are designed with DC faults in mind.

[0124] DC reactor L for peak fault current I faultmax Table 1 Peak fault current I faultmax With the change of inductance L and Figure 12 The peak fault current theoretical value obtained by formula (12) is compared with the simulation results obtained by the main parameters of the simulation system in Table 2. As the DC inductance L increases from 22.5mH to 62.5mH, the peak fault current simulation results I faultmax Reducing the current from 8.9kA to 6.1kA reduces the current stress on the converter and cables. However, a larger DC reactor will increase the cost, weight, volume, etc. Therefore, when designing the DC inductor L, a trade-off between factors such as fault performance and cost must be considered. From Table 1 and Figure 12 It can be seen that the theoretical value is consistent with the simulation result.

[0125] Table 1 Peak fault current I faultmax With the change of inductance L

[0126]

[0127] Table 2 Main parameters of the simulation system

[0128]

[0129] Similarly, as the capacitance changes, the maximum voltage v across the capacitors C1-C4 derived from equation (21) is Cmax As shown in Table 3, the peak voltage of the capacitor v Cmax With the change of capacitance C and Figure 13 As shown in the figure, the results are compared with the simulation results. When the energy absorption capacitor C increases from 180μF to 300μF, the maximum voltage v on capacitors C1, C2, C3 and C4 is Cmax When the voltage decreases from 91.7kV to 69.5kV, the difference between the simulation value and the theoretical value is between 6.8% and 7.5%.

[0130] Table 3 Capacitor peak voltage v Cmax With the change of capacitance C

[0131]

[0132]

[0133] In order to improve the economy of this topology, the rated voltage of FCBC is designed to be the maximum capacitor voltage v Cmax Choosing larger capacitors reduces the maximum capacitance voltage and rated voltage of the FCBC, thus reducing costs. However, it takes longer to suppress the fault current to zero, which slows down fault isolation. Smaller capacitors speed up fault isolation but result in higher voltage stress on the FCBC (including its capacitors).

[0134] When the output current reaches the peak value I faultmax When IGBT Q1, IGBT Q2, IGBT Q3 and IGBT Q4 in FCBC are turned off, the current stored in the equivalent inductance L e The energy in is then transferred to capacitors C1, C2, C3, and C4, as shown in equation (26):

[0135]

[0136] When I faultmax When set to 2pu, the energy stored in the equivalent inductor E fault is fixed, so the maximum voltage across the capacitor is:

[0137]

[0138] The maximum voltage on the capacitor can be obtained from formulas (21) and (27), and its value is consistent with the simulation results, as shown in Table 2I Capacitor peak voltage V Cmax With the change of capacitance C and Figure 13 shown.

[0139] Specific embodiment 2: A power balancing control method for a DC boost converter of an offshore wind power all-DC transmission system, comprising:

[0140] Step 1: Set the medium voltage DC voltage reference value V MVDC * , get the actual value of medium voltage DC voltage V MVDC , average voltage of submodule and the DC output voltage v of the jth submodule oj , j∈[2,3,4,···,N];

[0141] Step 2: Set the medium voltage DC voltage reference value V MVDC * And the actual value of medium voltage DC voltage V MVDCThe difference is input to the first PI controller to obtain the off time t off , the off time t off After doubling the amount, we get twice the off time 2t off ;

[0142] Step 3: Average the submodule voltage The DC output voltage v of the jth submodule oj The difference is input to the j-th PI controller to obtain the compensation off time t of the j-th submodule. comj ;

[0143] Step 4: Double the off time 2t off Input to the first PWM module, obtain the first gating signal and control the shutdown of the first submodule according to the first gating signal; double the shutdown time 2t off Compensation off time t of the jth submodule comj The difference is input to the j-th PWM module to obtain the j-th gating signal, and the j-th sub-module is turned off according to the j-th gating signal.

[0144] Specifically, for Figure 3 The DC OWF shown in the figure, the DC boost converter of the offshore wind power full DC transmission system of this application adopts the following Figure 14 The control structure and control method shown in the figure are used to control the MVDC (medium voltage direct current) side voltage: the measured MVDC voltage V CMDC The voltage is compared with its reference voltage and the difference is fed to the PI controller to adjust the off time t off ,according to Figure 5 , t off The expression is:

[0145] t off =t s / 2-t on (28)

[0146] In actual engineering, the submodules of the DC boost converter of the offshore wind power full DC transmission system of this application may be subjected to unbalanced voltage and current on the high voltage side and the medium voltage side respectively, which will reduce the life of the equipment. Therefore, based on the MVDC voltage controller, a method such as Figure 14 The power balancing control method shown in the figure is used to achieve reasonable distribution of DC voltage and current. The output of the MVDC voltage controller is 2t off The power balancing controller equipped for each submodule is used to adjust the average voltage of the jth submodule. And DC voltage output v oj The difference between the two is fed to the PI controller. The output compensation time t comj Added to MVDC voltage controller 2toff The output is then fed to the PWM module to control the jth submodule to shut down. For the DC boost converter of the offshore wind power full DC transmission system with N submodules, only N-1 power balancing controllers are required.

[0147] When the output voltage v of the jth submodule oj When it is greater than the average DC voltage, that is, v oj >V HVDC / N, the output of the power balancing controller t comj Negative, turn-off time 2t off -t comj increases, resulting in the output voltage v oj Decrease. On the contrary, 2t off -t comj Reduced, thereby achieving power balance between sub-modules.

[0148] Figure 15 The simulation results before and after the power balancing control is put into use are given. For the sake of simplicity, the DC boost converter of the offshore wind power full DC transmission system composed of three SABs (i.e. N=3) is considered. Figure 15 As shown, the closing time 2t of the three submodules before the enhanced control is enabled at 0.25s off Due to non-ideal operating conditions, such as device parasitics, voltage and current differences were observed between the submodules before 0.25s. At 0.25s, power balancing control was activated to adjust the 2toff of each submodule, achieving equal distribution of DC voltage and current.

[0149] Specific embodiment 3: A staggered control method for a DC boost converter of an offshore wind power all-DC transmission system, characterized by comprising:

[0150] Step 1: Obtain the number N and switching period t of the DC boost converter submodules of the offshore wind power full DC transmission system s ;

[0151] Step 2: Take the first submodule switch on time as the reference time t z , the opening time of the jth submodule is t zj , t zj =t j +(t s / 2) / N.

[0152] Specifically, the DC boost converter of the offshore wind power all-DC transmission system in this application requires a reactor on the DC side to control the output current and filter harmonics. To reduce the DC reactor requirement, a control strategy based on interleaved control is proposed, in which the triangular carriers of N submodules are phase-shifted by π / N radians. The following analyzes the DC current ripple characteristics with and without interleaved control.

[0153] Considering the proposed DC boost converter of the offshore wind power full DC transmission system with three submodules (i.e., N=3), the low level and high level of the output voltage under interleaved control are respectively and like Figure 16 Therefore, the voltage across the reactor is low, which is Current ripple I o_rip for:

[0154]

[0155] Without interleaving control, the output voltage v o The low and high levels are 0 and 3nV respectively. MVDC ,like Figure 17 (e) is shown. When v o =0, the voltage on the DC reactor is -V HVDC , generating current ripple I o_rip2 for:

[0156]

[0157] Considering the matching ratio k is 0.8, Figure 16 and Figure 17 The voltage and current waveforms are shown with and without interleaved control. By adopting interleaved control, the peak-to-peak output voltage of the DC boost converter of the offshore wind power full DC transmission system is reduced from 3nV to MVDC Reduced to nV MVDC , respectively as Figure 20 (e) and Figure 19 (e) As shown. The low level of the voltage across the DC reactor changes from -V HVDC Reduce to -1 / 6V HVDC , so the output current i o The ripple is reduced from 337A to 59A, such as Figure 17 (a) and Figure 16 (a). Furthermore, the current ripples of 337A and 59A are consistent with the results of equations and respectively. The above analysis shows that, when the output current ripple requirements are consistent, interleaved control can significantly reduce the number of DC reactors, thereby reducing the size and cost of the converter.

[0158] Formula (11) is derived based on the absence of interleaved control. When interleaved control is used, the increment of the inductor current at high level is:

[0159]

[0160] At the same time, at low level, the increment of inductor current is

[0161]

[0162] Therefore, when staggered control is used, the total increase in fault current is:

[0163]

[0164] In order to verify the effectiveness of the proposed topology and control method, a simulation model was established in the PSCAD / EMTDC (electromagnetic transient simulation software) environment. Figure 18 The offshore four-terminal DC grid shown in the figure, in which the DC boost-collecting converter of the offshore wind power full DC transmission system of the present application serves as the offshore converter station. The onshore converter station includes a first modular multilevel converter (MMC1) and a second modular multilevel converter (MMC2). MMC1 and MMC2 regulate the DC voltage of the DC grid through droop control, while the offshore first DC boost-collecting converter (HSAB1) and the second DC boost-collecting converter (HSAB2) respectively control the MVDC collection side voltage of the first offshore wind power (OWF1) and the second offshore wind power (OWF2). The DC boost-collecting converter of the offshore wind power full DC transmission system of the present application consists of ten submodules (i.e., N=10), of which the first submodule adopts the FCBC module and the remaining submodules adopt the typical SAB. This means that the rated power of the FCBC is only 1 / 10 of the rated power of the HSAB. The DC OWF is represented by a centralized control model. A DCS (DC switch) and a DCCB (DC circuit breaker) are respectively equipped at the offshore and onshore terminals of each DC cable for fault isolation. The detailed parameters of the simulation model are shown in Table 2. Figure 18 As shown, a permanent two-pole short circuit fault occurs in the first DC cable (Cable1) at 0.5s.

[0165] The DC fault occurs at t = 0.5s, and the DC voltage v bus1 to zero, such as Figure 19 As shown in (a), the first DC circuit breaker (DCCB1) and the fourth DC circuit breaker (DCCB4) are opened after a fault is detected to partially isolate the fault. Figure 19As shown in (b), the DC voltage of the non-fault part of the DC grid fluctuates briefly, but remains near the rated value. The second offshore wind turbine (OWF2), the second DC boost converter (HSAB2), the first modular multilevel converter (MMC1) and the second modular multilevel converter (MMC2) remain in operation to continuously transmit power to the onshore AC grid. After the fault occurs, the DC terminal current i o1 Rapidly rising, at t = 0.501s, i o1 Reaching 2pu, i.e. 4kA, HSAB1 is locked, e.g. Figure 19 (c) The second DC cable (Cable2) discharges to the fault point, and the current i flowing through the second DC switch (DCS2) is DCS2 Reverse, such as Figure 19 As shown in (e), the current flowing through the first DC switch (DCS1) and i o1 Feed power to the fault point together. o1 The rapid rise and DCS2 In the reverse direction, the DC fault can be located on the first DC cable (Cable1).

[0166] When HSAB1 is locked at 0.501s, the internal AC voltage and current of HSAB drop to zero, as shown in Figure 19 (g) and (h). After the converter is locked, the capacitor in FCBC will be introduced into the fault current path, and the fault current will charge the capacitor. As the voltage of FCBC capacitor increases, the fault current i flowing through DCS1 DCS1 At 0.525s, it is quickly suppressed to within the range of 100A, and the capacitor voltage v C Reaching a peak value of 0.1pu, such as Figure 19 As shown in (d) and (f), the submodule with FCBC only needs to be designed with 1 / 10 of the rated power, which reduces the cost.

[0167] The difference is that when the fault current is circulated and consumed in a typical SABs system, it takes about 500ms to decay to 100A, see Figure 19 (c) and (d). The proposed HSAB can quickly suppress the fault current, thereby accelerating the fault isolation of DCS, and the cost of capacitors and IGBTs is moderate.

[0168] The simulation results of DC fault ride-through of four-terminal DC grid system are shown in Figure 2. Figure 20 As shown. Although the DC voltage v bus1 After the fault occurs, it drops to zero, but the DC voltage of the non-fault part is basically kept near the rated value, which benefits from the fast fault current breaking capability of DCCB1 and DCCB4. Figure 20 As shown in (a) and (b).

[0169] When the fault current i DCS1 When the current is suppressed below 100A in 0.525s, DCS1 is disconnected to isolate the fault from the offshore HSAB1, and at this time the current i DCS1 1 drops to zero, see Figure 17 (e) To achieve fault recovery after fault isolation, the mechanical switch is closed and the capacitor in the FCBC is rapidly discharged through the resistor connected in parallel with it, as shown in Figure 20 (g) At t = 0.55s, the capacitors are fully discharged, and then IGBTs Q1-Q4 are turned on to bypass the capacitors in FCBC. HSAB1 is unlocked at 0.552s to establish the HVDC voltage v bus1 ,like Figure 20 (a). When v bus1 When it recovers to near the rated value, DCCB4 closes, and then OWF1 resumes power generation, see Figure 20 (c) Since Cable 1 is disconnected, the DC fault is isolated and the power of OWF 1 is retransmitted to the DC grid. All the power generated by OWF 1 is transmitted to the shore through Cable 2, and the current through Cable 2 increases as follows: Figure 20 (f) shown.

[0170] During the initial phase of system restart, the current briefly overshoots and then reaches Figure 20 In the steady state shown in (c), (d) and (f), the DC bus voltage v bus1 and v bus2 Similar, see Figure 20 (a) and (b). The onshore MMC will also be disturbed during the fault period. The MMC bridge arm current is well controlled within the range of 2pu to avoid converter damage. Figure 20 (h). Since the onshore MMC adopts DC voltage droop control, the transmission power of the offshore wind farm is distributed between MMC1 and MMC2. The transmission power of the onshore converter station before and after the fault is slightly different, see Figure 20 (i).

[0171] Although the offshore DC grid was affected by the most severe type of DC fault, the system recovered rapidly within 100ms and continued to transmit power to offshore wind power. Figure 20 The results shown indicate that the fault current blocking capability of the proposed HSAB enables the offshore DC grid to quickly ride through DC faults and ensure reliable power supply.

[0172] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the invention. It should be understood that many modifications may be made to the illustrative embodiments, and that other arrangements may be devised, without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in ways other than those described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be employed in conjunction with other described embodiments.

Claims

1. A DC boost converter for an offshore wind power all-DC transmission system, characterized in that: include: DC reactor and N submodules; One end of the DC reactor is connected to the positive electrode of the high-voltage DC output terminal, and the other end of the DC reactor is connected to the positive output terminal of the first submodule; the negative output terminal of the i-th submodule is connected to the positive output terminal of the i+1-th module, i∈[1,2,3,···,N-1], and the negative output terminal of the N-th submodule is connected to the negative electrode of the high-voltage DC output terminal; The positive input terminals of the 1st submodule to the Nth submodule are simultaneously connected to the positive pole of the medium voltage DC input terminal, and the negative input terminals of the 1st submodule to the Nth submodule are simultaneously connected to the negative pole of the medium voltage DC input terminal; The first submodule includes a residual fault current blocking circuit, a transformer T1, a leakage inductor L S1 and a first full-bridge circuit; The residual fault current blocking circuit includes a first bridge arm and a second bridge arm, the first bridge arm and the second bridge arm are connected in parallel, one end of the first bridge arm is connected to one end of the second bridge arm as the positive output end of the first submodule, and the other end of the first bridge arm is connected to the other end of the second bridge arm as the negative output end of the first submodule, the first bridge arm includes a diode D1, an IGBT Q1, a diode D2 and an IGBT Q2 connected in series in sequence, and the second bridge arm includes a diode D3, an IGBT Q3, a diode D4 and an IGBT Q4 connected in series in sequence, the middle point of the first bridge arm is connected to one end of the secondary side of the transformer T1, and the middle point of the second bridge arm is connected to the other end of the secondary side of the transformer T1; the first full-bridge circuit is simultaneously connected to the leakage inductance L S1 One end is connected to the other end of the primary side of transformer T1, and the leakage inductance L S1 The other end is connected to the primary end of transformer T1; The 2nd to Nth submodules are single active bridge converters; When the offshore wind power full DC transmission system is operating normally, the 1st to Nth submodules are used for power transmission from medium voltage DC to high voltage DC; when a DC fault occurs in the offshore wind power full DC transmission system, the residual fault current blocking circuit is used to block the residual fault current.

2. The DC boost converter for an offshore wind power all-DC transmission system according to claim 1, characterized in that: The residual fault current blocking circuit includes a diode D1, a diode D2, a diode D3, a diode D4, an IGBT Q1, an IGBT Q2, an IGBT Q3, an IGBT Q4, a capacitor C1, a capacitor C2, a capacitor C3, and a capacitor C4; The cathode of diode D1 and the cathode of diode D3 are simultaneously connected to the other end of DC reactor L, the anode of diode D1 is simultaneously connected to the emitter of IGBT Q1 and one end of capacitor C1, the collector of IGBT Q1 and the other end of capacitor C1 are simultaneously connected to the cathode of diode D2, the anode of diode D2 is simultaneously connected to the emitter of IGBT Q2 and one end of capacitor C2, and the collector of IGBT Q2 and the other end of capacitor C2 are simultaneously connected to the positive output end of the second submodule; the anode of diode D3 is simultaneously connected to the emitter of IGBT Q3 and one end of capacitor C3, the collector of IGBT Q3 and the other end of capacitor C3 are simultaneously connected to the cathode of diode D4, the anode of diode D4 is simultaneously connected to the emitter of IGBT Q4 and one end of capacitor C4, and the collector of IGBT Q4 and the other end of capacitor C4 are simultaneously connected to the positive output end of the second submodule; the collector of IGBT Q1 and the other end of capacitor C1 are simultaneously connected to one end of the secondary side of transformer T1, and the collector of IGBT Q3 and the other end of capacitor C3 are simultaneously connected to the other end of the secondary side of transformer T1; When the offshore wind power full DC transmission system operates normally, IGBT Q1, IGBT Q2, IGBT Q3 and IGBT Q4 remain turned on. When a DC fault occurs in the offshore wind power full DC transmission system, IGBT Q1, IGBT Q2, IGBT Q3 and IGBT Q4 are locked.

3. The DC boost converter for an offshore wind power all-DC transmission system according to claim 2, characterized in that: The first full-bridge circuit includes IGBT S1, IGBT S2, IGBT S3, IGBT S4, diode D S1 , diode D S2 , diode D S3 , diode D S4 and capacitor C in ; Capacitor C in One end, IGBT S1 collector, diode D S1 Cathode, IGBT S3 collector and diode D S3 The cathode is connected to the positive terminal of the medium voltage DC input terminal, and the capacitor C in The other end is connected to the negative terminal of the medium voltage DC input terminal; the emitter of IGBT S1 and the diode D S1 The anode is connected to the collector of IGBT S2 and diode D S2 Cathode connection, IGBT S2 emitter and diode D S2 The anode is also connected to the negative terminal of the medium voltage DC input terminal; the emitter of IGBT S3 and the diode D S3 The anode is connected to the collector of IGBT S4 and diode D S4 Cathode connection, IGBT S4 emitter and diode D S4 The anode is also connected to the negative pole of the medium voltage DC input terminal; IGBT S1 emitter and diode D S1 The anode and the leakage inductance L S1 One end is connected, leakage inductance L S1 The other end is connected to the primary side of transformer T1, the emitter of IGBT S3 and diode D S3 The anode is connected to the other end of the primary side of transformer T1.

4. The DC boost converter for an offshore wind power all-DC transmission system according to claim 3, characterized in that: The jth submodule includes capacitor C inj 、IGBT S j1 、IGBT S j2 、IGBT S j3 、IGBT S j4 , diode D Sj1 , diode D Sj2 , diode D Sj3 , diode D Sj4 , diode D j1 , diode D j2 , diode D j3 , diode D j4 , transformer T j and leakage inductance L Sj ;j∈[2,3,4,···,N]; Capacitor C inj One end, IGBT S j1 Collector, diode D Sj1 Anode, IGBT S j3 Collector and diode D Sj3 The anode is also connected to the positive terminal of the medium voltage DC input terminal, and the capacitor C inj The other end is connected to the negative pole of the medium voltage DC input terminal; the emitter of IGBT Sj1 and diode D Sj1 Anode and IGBT S j2 Collector and diode D Sj2 Cathode connection, IGBT S j2 Emitter and diode D Sj2 The anode is also connected to the negative terminal of the medium voltage DC input terminal; IGBTS j3 Emitter and diode D Sj3 Anode and IGBT S j4 Collector and diode D Sj4 Cathode connection, IGBT S j4 Emitter and diode D Sj4 The anode is also connected to the negative terminal of the medium voltage DC input terminal; IGBT S j1 Emitter and diode D S1 The anode and the leakage inductance L Sj One end is connected, leakage inductance L Sj The other end is connected to the transformer T j Primary side connected, IGBT S j3 Emitter and diode D Sj3 Anode transformer T j The other end of the original side is connected; Diode D j1 Cathode and diode D j2 The cathode is connected to the negative output terminal of the j-1th submodule, and the diode D j1 Anode and diode D j3 Cathode connection, diode D j2 Anode and diode D j4 Cathode connection, diode D j1 Anode and transformer T j One end of the secondary side is connected to the diode D j2 Anode and transformer T j The other end of the secondary side is connected; When j∈[2,3,4,···,N-1], the diode D j3 Anode and diode D j4 The anode is connected to the positive output terminal of the j+1th submodule at the same time; when j=N, the diode D j3 Anode and diode D j4 The anode is also connected to the negative pole of the high voltage DC output terminal.

5. The DC boost converter for an offshore wind power all-DC transmission system according to claim 1, characterized in that: Assume that a DC short circuit fault occurs in the DC boost-sink converter at time t0', and IGBT S1, IGBT S2, IGBT S3, IGBT S4, IGBT Q1, IGBT Q2, IGBT Q3, and IGBT Q4 are all turned off at time t1'. The peak fault current of the DC boost-sink converter is calculated as follows: I faultmax =I o +i o_inc , where I faultmax is the peak fault current, I o is the output current of the DC boost converter when the offshore wind power full DC transmission system is operating normally, i o_inc is the total increment of DC reactor current from t0' to t1', where t s is the duty cycle of IGBT S1, IGBT S2, IGBT S3 and IGBT S4; i o_charge is the current increment of the DC reactor during the on-time of a single IGBT working cycle, where t on_ab is the conduction time of IGBT S1, IGBT S2, IGBT S3, IGBT S4, IGBT Q1, IGBT Q2, IGBT Q3 or IGBT Q4 after a DC short circuit fault occurs in the DC boost converter, V e is the equivalent voltage of the RL charging circuit of the DC boost converter from time t0' to time t1', R e is the equivalent resistance of the DC reactor and cable in the equivalent RL charging circuit of the DC boost converter from time t0' to time t1', L e t a The equivalent inductance of the DC reactor and cable in the equivalent RL charging circuit of the DC boost converter from t0' to t1'; V e =NnV MVDC , where n is the transformation ratio of transformer T, V MVDC is the medium voltage DC output voltage.

6. The DC boost converter for an offshore wind power all-DC transmission system according to claim 5, characterized in that: The maximum voltage calculation method of capacitors C1, C2, C3 and C4 is: Among them, V C1max is the maximum voltage of capacitor C1, V C2max is the maximum voltage of capacitor C2, V C3max is the maximum voltage of capacitor C3, V C4max is the maximum voltage of capacitor C4, and C is the capacitance value of capacitors C1, C2, C3 and C4.

7. The power balancing control method for a DC boost converter of an offshore wind power all-DC transmission system according to claim 1, characterized in that: include: Step 1: Set the medium voltage DC voltage reference value V MVDC * , get the actual value of medium voltage DC voltage V MVDC , average voltage of submodule and the DC output voltage v of the jth submodule oj , j∈[2,3,4,···,N]; Step 2: Set the medium voltage DC voltage reference value V MVDC * And the actual value of medium voltage DC voltage V MVDC The difference is input to the first PI controller to obtain the off time t off , the off time t off After doubling the amount, we get twice the off time 2t off ; Step 3: Submodule average voltage V HVDC =V HVDC / N and the DC output voltage v of the jth submodule oj The difference is input to the j-th PI controller to obtain the compensation off time t of the j-th submodule. comj ; Step 4: Double the off time 2t off Input to the first PWM module, obtain the first gating signal and control the shutdown of the first submodule according to the first gating signal; double the shutdown time 2t off Compensation off time t of the jth submodule comj The difference is input to the j-th PWM module to obtain the j-th gating signal, and the j-th sub-module is turned off according to the j-th gating signal.

8. The interleaved control method for DC boost converters of an offshore wind power all-DC transmission system according to claim 1, characterized in that: include: Step 1: Obtain the number N and switching period t of the DC boost converter submodules of the offshore wind power full DC transmission system s ; Step 2: Take the first submodule switch on time as the reference time t z , the opening time of the jth submodule is t zj , t zj =t j +(t s / 2) / N.

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