Hybrid direct current transformer suitable for offshore wind power direct current collection and active fault current absorption control method

Abstract

The invention discloses a hybrid direct-current transformer suitable for offshore wind power direct-current collection and an active fault current absorption control method, belongs to the field of offshore wind power, and aims to solve the problem of fault circulating current in a medium-voltage direct-current MVDC collection network after a direct-current transformer DCT is locked. The modular multi-level converter comprises a medium-voltage side modular multi-level converter MMC1, a high-voltage side modular multi-level converter MMC2 and a medium-frequency transformer MFT, the direct current side of the MMC1 serves as a medium-voltage direct current MVDC side of the hybrid direct current transformer, currents of the n cables are collected to the MVDC side, and each cable is provided with a direct current high-speed switch HSS; the alternating current side of the MMC1 and the alternating current side of the MMC2 are connected with the primary side and the secondary side of the MFT respectively, and the direct current side of the MMC2 serves as the high-voltage direct current HVDC side of the hybrid direct current transformer and is transmitted to a land receiving end power grid through the sea; each bridge arm of the MMC2 is composed of NHV half-bridge sub-modules HBSM (Half Bridge Submodules); each bridge arm of the MMC1 is formed by mixing NH half-bridge sub-modules HBSM and NF full-bridge sub-modules FBSM, and the number of the full-bridge sub-modules of each bridge arm of the MMC1 is NF = 0.5 NMV (mAC-mDC).

Description

Technical Field

[0001] The present invention relates to a hybrid DC transformer applicable to offshore wind power DC collection and an active fault current absorption control method, belonging to the field of offshore wind power. Background Art

[0002] Traditional offshore wind power grid connection technologies adopt medium-voltage AC collection and high-voltage DC power transmission. However, with the continuous increase in the single-machine capacity of wind turbines (such as 20 MW) and the scale of offshore wind farms, the all-DC technology is regarded as a feasible solution for large-capacity and long-distance offshore wind power grid connection due to advantages such as reduced volume of offshore platforms, lower collection losses of medium-voltage DC MVDC (abbreviation of Medium Voltage Direct Current), stronger power transmission capacity, and enhanced overall system stability. The all-DC technology refers to medium-voltage DC MVDC (abbreviation of Medium Voltage Direct Current) collection and high-voltage DC HVDC (abbreviation of High Voltage Direct Current) transmission.

[0003] Since the DC fault current has no natural zero-crossing point and the DC grid impedance is low, considering the severity of DC faults and the difficulty of installing large and costly DC circuit breakers offshore, DC faults pose a major challenge to the application of medium-voltage DC MVDC technology in large-scale offshore wind power collection. By connecting DC wind turbines in series or in a series-parallel scheme, the DC voltage can be directly boosted to the medium-voltage DC MVDC or high-voltage DC HVDC level. This scheme has advantages in terms of land occupation and cost. However, during a fault, the faulty wind turbine needs to be bypassed, and the remaining normal units need to withstand the full DC voltage. Therefore, a higher voltage margin needs to be designed for the DCWT converter of the DC wind turbine.

[0004] The isolated front-to-front DC transformer FTF-DCT (FTF is the abbreviation of front-to-front, face-to-face; DCT is the abbreviation of Direct Current Transformer, DC transformer) based on the modular multilevel converter (MMC) is considered an ideal choice for all-DC offshore wind farms due to its modular design, electrical isolation ability, and high step-up ratio characteristics. After detecting a DC fault, the FFT-DCT can block the fault by locking all IGBTs of the switching tubes, avoiding the propagation of the fault from the high-voltage side to the medium-voltage side or from the medium-voltage side to the high-voltage side, so as to isolate the fault using a DC high-speed switch HSS instead of a DC circuit breaker, reducing the cost of protection equipment. However, due to the residual energy stored in DC cables and inductor components (such as arm reactors), the fault current will still form a path through the anti-parallel diodes, DC cables, and the fault point, as Figure 1As shown, the DC transformer consists of two MMCs (MMC1 and MMC2), and the MMC adopts half-bridge sub-modules HBSM. The residual energy continuously circulates on the medium-voltage DC MVDC side and can only be slowly dissipated through the parasitic resistance of the DC cable and the arm reactor, resulting in a long fault clearing time. As Figure 2 shown, it takes about 200 ms for the DC side current of the traditional DC transformer DCT to decay below 100 A (the opening threshold of the HSS), resulting in slow system recovery and affecting power supply reliability. Summary of the Invention

[0005] To solve the problem of fault circulating current in the medium-voltage DC MVDC collection network after the DC transformer DCT is blocked, the present invention provides a hybrid DC transformer applicable to offshore wind power DC collection and an active fault current absorption control method.

[0006] On the one hand, the present invention provides a hybrid DC transformer applicable to offshore wind power DC collection, including a medium-voltage side modular multilevel converter MMC1, a high-voltage side modular multilevel converter MMC2, and an intermediate-frequency transformer MFT; the DC side of the medium-voltage side modular multilevel converter MMC1 serves as the medium-voltage DC MVDC side of the hybrid DC transformer, and the currents of n cables are collected to the medium-voltage DC MVDC side, and a DC high-speed switch HSS is set for each cable; the AC sides of the medium-voltage side modular multilevel converter MMC1 and the high-voltage side modular multilevel converter MMC2 are respectively connected to the primary and secondary sides of the intermediate-frequency transformer MFT, and the DC side of the high-voltage side modular multilevel converter MMC2 serves as the high-voltage DC HVDC side of the hybrid DC transformer and is transmitted to the onshore load through the sea;

[0007] Each arm of the high-voltage side modular multilevel converter MMC2 is composed of N HV half-bridge sub-modules HBSM; each arm of the medium-voltage side modular multilevel converter MMC1 is composed of N H half-bridge sub-modules HBSM and N F full-bridge sub-modules FBSM in a mixed manner, and the number of sub-modules N MV of each arm of MMC1 H = N F + N F . The number N

[0008]

[0009] In the formula, m AC is the AC modulation ratio, and m DC is the DC modulation ratio.

[0010] On the other hand, the present invention provides an active fault current absorption control method for a hybrid DC transformer applicable to offshore wind power DC collection. When a fault occurs, the medium-voltage side modular multilevel converter MMC1 controls the DC voltage bias V i by inputting the difference between the measured current I iR of the i-th fault cable and the reference value 0 into a PI controller, so as to control the voltage V DCo on the medium-voltage DC MVDC side, thereby actively absorbing the fault current in the medium-voltage DC MVDC collection network; suppressing the fault current flowing through the DC switch to zero, and realizing zero-current breaking of the HSS on the fault cable, where i = 1, 2,..., n; DC

[0011] The medium-voltage side modular multilevel converter MMC1 uses the average capacitor voltage control to construct the direct-axis current reference value of the current loop, so as to stabilize the capacitor voltage of the sub-module at the rated value;

[0012] When a fault occurs, the high-voltage side modular multilevel converter MMC2 uses the reduced AC voltage control to construct the direct-axis voltage of the voltage loop.

[0013] Preferably, the control process of the medium-voltage side modular multilevel converter MMC1 is as follows:

[0014] The current loop outputs a control signal according to the direct-axis current reference value I dref , the quadrature-axis current reference value I qref , and then generates the abc-phase AC voltage reference value v abcref through the dq / abc transformation. According to the DC voltage bias V DCo , the abc-phase AC voltage reference value v abcref and the command output by the circulating current suppression control CCSC, the upper-bridge-arm drive voltage reference value v pref and the lower-bridge-arm drive voltage reference value v nref are obtained to drive the MMC1 arm;

[0015] Among them, the direct-axis current reference value I dref is switched in two modes: normal and fault. In the normal state, the direct-axis current reference value I dref is: the difference between the voltage V DC on the medium-voltage DC MVDC side and the DC-side voltage reference value V DCref is input into a PI controller and then output after amplitude limiting; in the fault state, the direct-axis current reference value I dref is: the difference between the average value V CFarg of the sub-module capacitor voltage and the capacitor voltage reference value is input into a PI controller and then output after amplitude limiting;

[0016] Among them, the DC voltage bias V DCo ​It is switched between normal and fault modes. Under normal conditions, the DC voltage bias V DCo is: half of the rated value V DCn of the medium-voltage DC MVDC side voltage; under fault conditions, the DC voltage bias V DCo is: the difference between the measured current I i of the switch HSS iR on the i-th faulty cable and the reference value 0 is input to the PI controller, and then it is half of the value after amplitude limiting.

[0017] Preferably, the control process of the high-voltage side modular multilevel converter MMC2 is as follows:

[0018] The voltage loop outputs a voltage loop control command according to the direct-axis voltage reference value V dref and the quadrature-axis voltage reference value V qref , outputs a current loop control command through the current loop, and then generates an abc-phase AC voltage reference value v abcref through dq / abc transformation. According to the DC voltage bias V DCo , the abc-phase AC voltage reference value v abcref and the command output by the circulating current suppression control CCSC, the upper-arm drive voltage reference value v pref and the lower-arm drive voltage reference value v nref are obtained to drive the MMC2 arm;

[0019] Among them, the direct-axis voltage reference value V dref is switched between normal and fault modes. Under normal conditions, the direct-axis voltage reference value V dref is: 1 pu; under fault conditions, the direct-axis voltage reference value V dref is input after voltage reduction, and the amplitude V mf of the reduced AC voltage is limited to:

[0020]

[0021] In the formula, k is a constant coefficient between (0, 1), and m ACn is the rated AC modulation ratio.

[0022] Advantages of the present invention: The present invention proposes a hybrid DC transformer for handling DC faults in a medium-voltage DC (MVDC) collection network. By operating at a reduced AC voltage, the proportion of full-bridge sub-modules (FBSMs) required for the hybrid DC transformer (HDCT) is significantly reduced to 10%, significantly reducing costs and losses. A new DC fault current control strategy is proposed, using the HDCT to absorb the fault circulating current in the MVDC collection network, so that the residual energy in the MVDC collection network is dissipated through the HDCT, and a high-speed DC switch is used to quickly isolate the fault. The fault clearing time is significantly shortened from approximately 300 ms to within 16.5 ms, enabling rapid system recovery. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the fault circulating current path after the conventional DC transformer (DCT) on the medium-voltage side is blocked during a fault;

[0024] Figure 2 It is a comparison chart of the fault currents of the conventional DC transformer (DCT) and the hybrid DC transformer (HDCT) of the present invention after a DC fault occurs at 1.5 s;

[0025] Figure 3 It is a layout diagram of a fully DC offshore wind farm;

[0026] Figure 4 It is a schematic structural diagram of the hybrid DC transformer applicable to DC collection of offshore wind power according to the present invention;

[0027] Figure 5 It is the DC fault characteristics after the hybrid DC transformer (HDCT) of the present invention is blocked at 1.5 s: where Figure 5 (a) DC fault circulating current path; Figure 5 (b) DC current of the hybrid DC transformer (HDCT) and DC cable current diagram;

[0028] Figure 6 It is an equivalent circuit diagram of the positive and negative cables between the fault point on the faulty cable and the medium-voltage port of the hybrid DC transformer (HDCT);

[0029] Figure 7 It is the control block diagram proposed by the hybrid DC transformer of the present invention;

[0030] Figure 8 It is the arm voltage diagram of the modular multilevel converter (MMC1) of the hybrid DC transformer (HDCT) of the present invention in two stages of active fault current control, where Figure 8 (a) is the arm voltage that may be generated in stage 1, Figure 8 (b) is the arm voltage that may be generated in stage 2;

[0031] Figure 9are the DC voltage, current, and power waveforms of a fully DC offshore wind farm: among them Figure 9 (a), Figure 9 (b), Figure 9 (c) are the curve graphs of the medium - voltage side voltage, current, and power of the hybrid DC transformer HDCT respectively, Figure 9 (d) is the cable current I 1R , I2, and I3 (i.e., I 2R and I 3R ) curve, Figure 9 (e) is the DC voltage curve of the wind turbine group 1, Figure 9 (f) is the curve of the fault circulating current I 1L of the wind turbine group 1;

[0032] Figure 10 is the detailed waveform diagram of the active fault current absorption control of the present invention, where Figure 10 (a) is the DC voltage diagram of - MMC1 of the hybrid DC transformer HDCT, Figure 10 (b) is the DC fault current I 1R curve flowing through the DC switch HSS1 on the fault cable;

[0033] Figure 11 is the DC fault detection and location diagram based on local current measurement, combined with current magnitude and direction;

[0034] Figure 12 is the internal AC waveform diagram of the hybrid DC transformer HDCT, where Figure 12 (a) is the AC voltage waveform diagram, Figure 12 (b) is the AC current waveform diagram;

[0035] Figure 13 is the bridge - arm current waveform diagram of the hybrid DC transformer HDCT, where Figure 13 (a), Figure 13 (b) are the upper and lower bridge - arm current waveform diagrams of the hybrid DC transformer HDCT - MMC1 respectively, Figure 13 (c), Figure 13 (d) are the upper and lower bridge - arm current waveform diagrams of the hybrid DC transformer HDCT - MMC2 respectively;

[0036] Figure 14 is the on - shore MMC waveform diagram, where Figure 14 (a) is the DC voltage waveform diagram, Figure 14 (b) is the DC current waveform diagram, Figure 14 (c) is the DC power waveform diagram. Detailed implementation manner

[0037] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0038] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments may be combined with each other.

[0039] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but it is not limited to the present invention.

[0040] Specific Embodiment 1: The following will be described in conjunction with Figures 2 to 5 、 Figure 7 to illustrate this embodiment. The hybrid DC transformer applicable to offshore wind power DC collection described in this embodiment is shown in Figure 7 and includes a medium-voltage side modular multilevel converter MMC1, a high-voltage side modular multilevel converter MMC2, and an intermediate-frequency transformer MFT; the DC side of the medium-voltage side modular multilevel converter MMC1 serves as the medium-voltage DC MVDC side of the hybrid DC transformer, and the currents of n cables are collected to the medium-voltage DC MVDC side, and a DC high-speed switch HSS is provided for each cable; the AC side of the medium-voltage side modular multilevel converter MMC1 and the AC side of the high-voltage side modular multilevel converter MMC2 are respectively connected to the primary and secondary sides of the intermediate-frequency transformer MFT, and the DC side of the high-voltage side modular multilevel converter MMC2 serves as the high-voltage DC HVDC side of the hybrid DC transformer and is transmitted to the onshore load via the sea;

[0041] Each arm of the high-voltage side modular multilevel converter MMC2 is composed of N HV half-bridge sub-modules HBSM; each arm of the medium-voltage side modular multilevel converter MMC1 is composed of N H half-bridge sub-modules HBSM and N F full-bridge sub-modules FBSM in a mixed manner. The number of sub-modules N MV of each arm of MMC1 is N H = N F + N F , and the number N F of the full-bridge sub-modules FBSM is:

[0042]

[0043] In the formula, m AC is the AC modulation ratio, and m DC is the DC modulation ratio.

[0044] When MMC1 and MMC2 are transformed, the required control angle θ is uniformly generated by the angle generation module in Figure 7 .

[0045] The DC transformer described in this embodiment is a hybrid setting of full - bridge and half - bridge, and contains two converters. By operating at a reduced AC voltage, the proportion of full - bridge sub - modules FBSM required for the hybrid DC transformer HDCT is significantly reduced to 10%, significantly reducing costs and losses.

[0046] The hybrid DC transformer described in this embodiment is applied to the DC collection of offshore wind power. An example is given below. The fully - DC OWF (abbreviation for Offshore Wind Farm, offshore wind farm) under study is as Figure 3 shown. Its rated capacity is 1200 MW and it consists of 3 wind turbine groups, each with a rated capacity of 400 MW, including 40 10 - MW DC wind turbines (DC windturbine, DCWT). Each DC wind turbine DCWT includes a wind turbine generator, an AC / DC converter and a DC / DC converter, which boosts the low voltage to a medium voltage of ±50 kV. The main DC transformer DCT is located on the offshore platform and is used to boost the medium voltage of ±50 kV to a high voltage of ±500 kV.

[0047] Under normal operating mode, Figure 3 in MMC1 controls the medium - voltage DC MVDC voltage, and MMC2 regulates the internal AC voltage. The on - shore MMC controls the high - voltage DC HVDC voltage. In the DC wind turbine DCWT, the AC / DC converter controls the internal DC voltage, and the DC / DC converter controls the wind turbine power.

[0048] To suppress the fault circulating current during DC faults in the medium - voltage DC MVDC collection network, the present invention proposes a hybrid DC transformer HDCT, whose topological structure is as Figure 4 shown. The number of sub - modules in each arm of the proposed hybrid DC transformer HDCT on the medium - voltage DC MVDC side and the high - voltage DC HVDC side are N MV and N HV respectively. On the medium - voltage side, MMC1 adopts a hybrid arm, and each arm contains N H half - bridge sub - modules FBSM and N F full - bridge sub - modules FBSM. In order to reduce losses and costs, only a small number of half - bridge sub - modules FBSM are replaced with full - bridge sub - modules FBSM. In addition, the internal AC link of the hybrid DC transformer HDCT uses a medium - frequency transformer (medium frequency transformer, MFT) to reduce the volume of passive devices and losses.

[0049] Figure 2 compares the hybrid DC transformer HDCT of the present invention withFigure 1 The DC terminal current of the traditional DC transformer DCT after a pole-to-pole (P2P) metallic short-circuit fault occurs at t = 1.5 s. The DC transformer DCT locks out 1 ms after the fault occurs. For the traditional DC transformer DCT, as Figure 1 shown, the residual fault current circulates on the medium-voltage DC (MVDC) side, while the capacitors of the full-bridge sub-modules (FBSMs) in the proposed topology are charged by the fault current, providing a reverse blocking voltage for the residual fault current circulation, causing the fault current to rapidly decay to near 0 within 3 ms, significantly shortening the clearing time compared to the approximately 200 ms clearing time of the traditional DC transformer DCT, achieving fast fault isolation.

[0050] Figure 5 Shows the fault characteristics of the hybrid DC transformer (HDCT) of the present invention when a P2P DC fault occurs. The fault occurs at the midpoint of cable 1 at t = 1.5 s, and all DC wind turbine (DCWT) converters and the proposed hybrid DC transformer (HDCT) of the present invention lock out at t = 1.501 s.

[0051] As Figure 5 (b) shows, the current I T at the hybrid DC transformer (HDCT) terminal is quickly suppressed to 0, but the fault current still circulates in the MVDC collection network. The measured current I i of the switch HSS iR on the i-th fault cable (i = 2, 3) is the current flowing through the DC switch on cable i. After cable 1 fails, as shown by the red line in Figure 5 (a), the current I iR flows through the wind turbine group i, cable i, HSS1, and cable 1 to the fault location. The discharge currents of cables 2 and 3 and part of the current of cable 1 flow through HSS1, causing HSS1 to wait until the residual energy stored in the MVDC collection network dissipates through the parasitic resistance of the fault current path before it can be opened. Due to the low resistance and high current amplitude in the MVDC collection network, the fault current circulation decays slowly, and the current I 1R flowing through HSS1 takes more than 300 ms to drop to 0, prolonging the fault isolation time. As shown by the green line in Figure 5 (a), the current I 1L circulates between the fault point and the converter of the wind turbine group 1, without affecting the opening of HSS1.

[0052] Specific implementation method two: The following is combined with Figures 3 to 14To describe this embodiment, the active fault current absorption control method of the hybrid DC transformer applicable to offshore wind power DC collection is implemented based on the hybrid DC transformer applicable to offshore wind power DC collection described in Embodiment 1. When a fault occurs, the medium-voltage side modular multilevel converter MMC1 controls the switch HSS on the i-th fault cable by i inputting the difference between the measured current I iR of the switch and the reference value 0 into a PI controller to adjust the DC voltage bias V DCo , thereby controlling the voltage V DC on the medium-voltage DC MVDC side to actively absorb the fault current in the medium-voltage DC MVDC collection network; suppressing the fault current flowing through the DC switch to zero and achieving zero-current breaking of the HSS on the fault cable, where i = 1, 2,..., n;

[0053] The medium-voltage side modular multilevel converter MMC1 uses the average capacitor voltage control to construct the direct-axis current reference value of the current loop to keep the capacitor voltage of the sub-module stable at the rated value;

[0054] When a fault occurs, the high-voltage side modular multilevel converter MMC2 uses the reduced AC voltage control to construct the direct-axis voltage of the voltage loop.

[0055] The control process of the medium-voltage side modular multilevel converter MMC1 is as follows:

[0056] The current loop outputs a control signal according to the direct-axis current reference value I dref , the quadrature-axis current reference value I qref , and then generates the abc-phase AC voltage reference value v abcref through the dq / abc transformation. According to the DC voltage bias V DCo , the abc-phase AC voltage reference value v abcref and the command output by the circulating current suppression control CCSC, the upper-bridge-arm drive voltage reference value v pref and the lower-bridge-arm drive voltage reference value v nref are obtained to drive the MMC1 arm;

[0057] Among them, the direct-axis current reference value I dref is switched between normal and fault modes. In the normal state, the direct-axis current reference value I dref is: the difference between the voltage V DC on the medium-voltage DC MVDC side and the DC side voltage reference value V DCref is input into a PI controller and then output after amplitude limiting; in the fault state, the direct-axis current reference value I dref is: the difference between the average value V CFarg of the sub-module capacitor voltage and the capacitor voltage reference value is input into a PI controller and then output after amplitude limiting;

[0058] Among them, the DC voltage bias V DCo Switches between normal and fault modes. In the normal state, the DC voltage bias V DCo is: half of the rated voltage V DCn of the medium-voltage DC MVDC side; in the fault state, the DC voltage bias V DCo is: half of the value obtained by inputting the difference between the measured current I i of the switch HSS iR on the i-th faulty cable and the reference value 0 into a PI controller and then limiting the amplitude.

[0059] The control process of the high-voltage side modular multilevel converter MMC2 is as follows:

[0060] The voltage loop outputs a voltage loop control command based on the direct-axis voltage reference value V dref , the quadrature-axis voltage reference value V qref , outputs a current loop control command through the current loop, and then generates an abc-phase AC voltage reference value v abcref through dq / abc transformation. According to the DC voltage bias V DCo , the abc-phase AC voltage reference value v abcref and the command output by the circulating current suppression control CCSC, the upper-arm drive voltage reference value v pref and the lower-arm drive voltage reference value v nref are obtained to drive the MMC2 arm;

[0061] Among them, the direct-axis voltage reference value V dref switches between normal and fault modes. In the normal state, the direct-axis voltage reference value V dref is: 1 pu; in the fault state, the direct-axis voltage reference value V dref is input with a reduced voltage, and the amplitude V mf of the reduced AC voltage is limited to:

[0062]

[0063] In the formula, k is a constant coefficient between (0, 1), and m ACn is the rated AC modulation ratio.

[0064] Figure 3 In the layout of the all-DC offshore wind farm shown, the DC transformer DCT adopts the hybrid DC transformer HDCT of the present invention. Although as Figure 2 and Figure 5 (b) shows, the proposed hybrid DC transformer HDCT can block the DC-side current I T, but the fault circulating current still flows through the HSS. Therefore, the present invention further proposes an active fault current absorption strategy for the hybrid DC transformer HDCT, which realizes the rapid disconnection of the HSS to isolate the fault by actively absorbing the fault current from the medium-voltage DC MVDC collection network.

[0065] Key factors affecting the fault circulating current. Figure 5 (a) The cable 1 is divided into two sections by the fault. For simplicity of analysis, each section is represented by a π equivalent circuit. At this time, the part between the fault point and the medium-voltage DC MVDC side of the hybrid DC transformer HDCT can be modeled as Figure 6 the equivalent circuit shown, where V f is the fault point voltage, V DC is the medium-voltage DC MVDC side voltage of the hybrid DC transformer HDCT, and Z and Y are the cable impedance and admittance respectively.

[0066] According to Kirchhoff's law, Figure 6 the current and voltage in the circuit can be expressed as:

[0067] I 1Z = I 1R + V Y Y / 2 (1)

[0068] V Y = V f / 2 - I 1Z Z (2)

[0069] -V f + 2I 1Z Z + V DC = 0 (3)

[0070] Combining equations (1) and (2) gives

[0071]

[0072] Substituting equation (4) into equation (3) gives the current I 1R as

[0073]

[0074] According to equation (5), the current I 1R flowing through the HSS1 depends on the difference between the fault point voltage V f (close to 0) and the medium-voltage DC MVDC side voltage V DC of the hybrid DC transformer HDCT, and it can be controlled by adjusting the medium-voltage DC MVDC side voltage V DC of the hybrid DC transformer HDCT. Based on this, the present paper proposes an active fault current absorption control for the hybrid DC transformer HDCT, which uses the HSS to achieve rapid fault isolation.

[0075] Blocking the hybrid DC transformer HDCT can only suppress its DC-side current to zero, but cannot eliminate the fault circulating current fed by the fault-free cable. The present invention proposes an active control for the residual DC fault current of the proposed hybrid DC transformer HDCT.

[0076] The proposed control scheme is as Figure 7 shown. By inputting the difference between the measured current I i of the switch HSS iR on the fault cable and the reference value 0 into the PI controller, the DC voltage bias V DCo is adjusted, thereby controlling the voltage V DC on the medium-voltage DC MVDC side of the hybrid DC transformer HDCT to actively absorb the fault current in the medium-voltage DC MVDC collection network. The fault current flowing through the DC switch is thus suppressed to zero, realizing zero-current breaking of the HSS on the fault cable.

[0077] To stabilize the capacitor voltage of the sub-module at the rated value V Cref during the fault, an average capacitor voltage control is designed to give the d-axis current reference value I dref , as Figure 7 shown, where V CFavg is the average capacitor voltage of the full-bridge sub-module FBSM. During normal operation, I dref is output by the DC voltage loop.

[0078] The fault cable is judged comprehensively by the direction and magnitude of the cable current. After the DC fault occurs, only the current of the fault cable reverses, and the direction of the current of the fault-free cable remains unchanged, as Figure 5 shown. Before the converter connected to the medium-voltage DC MVDC network is blocked, each DC current rises. The protection threshold of the current I iR is set to -1 pu. Combining the current direction (reverse) and magnitude (exceeding the threshold) can locate the fault cable, and the current is locally measured without communication.

[0079] When a metallic P2P fault occurs, the voltage V f at the fault point drops to near zero, and all half-bridge sub-modules HBSM of the MMC1 can be bypassed, and only the full-bridge sub-module FBSM is connected to the current path to generate a negative voltage to actively absorb the fault current in the medium-voltage DC MVDC collection network. At this time, the MMC1 is equivalent to each arm containing N F full-bridge sub-modules FBSM, and the lowest DC voltage that can be generated is:

[0080]

[0081] where ∑V CF is N FThe sum of the capacitor voltages of all full-bridge sub-modules FBSM. In the fault current absorption control of the present invention, the lower limit of the PI controller limit is set to V in Equation (6) DCFmin .

[0082] The hybrid DC transformer HDCT of the present invention only requires a small number of full-bridge sub-modules FBSM to actively absorb the fault circulating current in the medium-voltage DC MVDC collection network. The method for determining the number of required full-bridge sub-modules FBSM will be elaborated in detail below.

[0083] Ignoring the output of the circulating current suppression controller (CCSC), the upper and lower arm voltages v pj and v nj of the hybrid DC transformer HDCT can be expressed as:

[0084]

[0085] where v ACj (j = A, B, C) is the AC voltage of phase j.

[0086] The AC modulation ratio m AC The DC modulation ratio m DC is defined as:

[0087]

[0088] where V m is the amplitude of the AC voltage, and V DCn is the rated value of the medium-voltage DC MVDC voltage. The range of the arm voltage v arm can be expressed by the AC and DC modulation ratios and the rated DC voltage:

[0089]

[0090] In the proposed DC fault current absorption control, the hybrid DC transformer HDCT generates a negative voltage on the medium-voltage side through the negative voltage generation ability of the full-bridge sub-module FBSM, that is, the DC modulation ratio m DC < 0, so the DC bias of the arm voltage is negative. Assuming that the sub-module capacitor voltage is balanced near the rated value, the reduced DC voltage V DC on the medium-voltage DC MVDC side can be expressed by the lower limit of the arm voltage in Equation (9) and the rated sub-module capacitor voltage V DCn / N MV as:

[0091]

[0092] For the required medium-voltage DC MVDC voltage V in Equation (10) DC, the number of full-bridge sub-modules FBSM required by MMC1 is:

[0093]

[0094] It can be seen from Equation (11) that the number of required full-bridge sub-modules FBSM depends on the AC modulation ratio m AC and the DC modulation ratio m DC . As m DC decreases (m DC <0), the negative voltage generation ability of MMC1 is enhanced, and the fault circulating current can be suppressed to zero faster, but the number of required full-bridge sub-modules FBSM will increase. Therefore, when setting the minimum DC modulation ratio m DC , it is necessary to carefully balance performance indicators such as fault clearing speed, cost, and loss.

[0095] It can be seen from Equation (11) that another variable affecting the number of required full-bridge sub-modules FBSM is the AC modulation ratio m AC inside the hybrid DC transformer HDCT, and this value can be adjusted by the hybrid DC transformer HDCT. For this reason, the following AC voltage reduction control strategy is proposed. During normal operation, the d-axis voltage reference value V Figure 7 of MMC2 is set to the rated value and switches to V dref after the proposed fault current absorption control is started. V mf , V mf is the amplitude of the AC voltage after step-down. The design of V mf in the AC voltage reduction control is as follows.

[0096] Define half of the AC voltage amplitude V mf and the minimum negative DC voltage V DCFmin generated by MMC1 of the hybrid DC transformer HDCT as the required modulation ratio in the AC voltage reduction control:

[0097]

[0098] According to the magnitude of the DC fault current, the process of the proposed control can be divided into two stages, which have different requirements for the amplitude V mf of the AC voltage after step-down:

[0099] Stage 1: Stage 1 is the time period from the start of the fault current absorption control to the moment when the fault current flowing through the HSS is controlled to zero. During this stage, the DC bias of the arm voltage decreases from V DCn / 2 to a negative value, and the AC modulation ratio m ACf needs to satisfy m ACf ≤1 to maintain the arm voltage within the range of [-∑V CF , 0], as shown in Figure 8(as shown in (a)). In practice, the time of Stage 1 is usually short, and the duration depends on the lowest negative DC voltage V that the hybrid DC transformer HDCT-MMC1 can generate DCFmin .

[0100] Stage 2: The period from when the fault current is suppressed to near zero to the start of the recovery process is defined as Stage 2. Since the fault current is 0 in this stage, the DC voltage of MMC1 of the hybrid DC transformer HDCT and the DC bias of the arm voltage are both near 0 to match the zero voltage at the fault point. Therefore, the feasible range of the arm voltage generated by MMC1 of the hybrid DC transformer HDCT is [-∑V CF , ∑V CF , as shown in Figure 8 (b).

[0101] From the above analysis, it can be seen that Stage 1 has more stringent requirements for the arm voltage range. The arm voltage range of MMC1 of the hybrid DC transformer HDCT is [-∑V CF , 0], and its DC bias is -∑V CF / 2. Therefore, the upper limit value of V m regulated by MMC2 of the hybrid DC transformer HDCT is:

[0102]

[0103] When the AC voltage amplitude V m = 1 pu during normal operation, substituting V DCn in Equation (13) with V m rated AC modulation ratio m ACn , we get:

[0104]

[0105] Each arm of MMC1 of the hybrid DC transformer HDCT can be equivalently regarded as a lumped capacitor charged by the current i C , and this current is the product of the arm modulation wave m Cref and the arm current i arm :

[0106] i C = m Cref i arm (15)

[0107] The arm modulation wave m Cref is composed of an AC component and a DC bias, and the latter is close to zero in Stage 2. Therefore, to ensure that the sub-module capacitor can be charged / discharged in the capacitor voltage balance, the AC component of m Cref cannot be zero. This indicates that the reduced AC voltage amplitude V m must be greater than 0. Therefore Figure 7 V inmf The constraints of the range are as follows:

[0108]

[0109] where k is a constant coefficient, and considering the fault transient and the dynamics of the AC voltage controller, its value needs to be greater than 0 and less than 1.

[0110] Adopt Figure 3 The performance of the hybrid DC transformer HDCT adopting the proposed active fault current control strategy is verified by using the model shown in the PSCAD / EMTDC simulation environment. The detailed parameters of the system are shown in Table I. According to Equation (11), the ratio of the full-bridge sub-module FBSM of each arm of the hybrid DC transformer HDCT-MMC1 is selected as N F / N MV = 10%. Each wind turbine group is represented by an aggregated model consisting of 1 wind turbine generator, 1 AC / DC converter, and 1 DC / DC converter.

[0111] Table I Parameters of the test system

[0112]

[0113] A permanent P2P DC fault is applied to the midpoint of the collection cable 1 at t0 = 1.5 s, and the simulation results of the system adopting the proposed topology and control are as follows.

[0114] After the DC fault occurs, as Figure 9 (a) and Figure 10 (a) show, the DC voltage of the medium-voltage DC MVDC collection network drops suddenly. The DC wind turbine DCWT locks when the DC output current is higher than the protection threshold of 2 pu. The DC fault is comprehensively judged by the abnormal amplitude and direction of the local current measurement. As Figure 10 (b) and Figure 11 show, the current I 1R flowing through HSS1 reaches the threshold of -1 pu at t1 = 1.5007 s. The fault currents I2 and I3 from the normal cables (i.e., I 2R and I 3R ) increase in magnitude and feed into the fault location, but the direction remains unchanged. The fault is located at cable 1, and the hybrid DC transformer HDCT switches to the proposed fault current control at t1 = 1.5007 s.

[0115] After the fault occurs, the fault currents I2 and I3 fed by cables 2 and 3 flow through HSS1, causing I 1R to increase in the reverse direction, as Figure 9(d) As shown in Fig. (11). At t1 = 1.5007 s, the proposed DC fault current control of the hybrid DC transformer HDCT is initiated, actively absorbing the fault current from the medium-voltage DC MVDC collection network, and reducing the internal AC voltage to 0.05 pu, as Figure 12 (a) shows.

[0116] Figure 9 The waveforms of DC voltage, current, and power in the all-DC offshore wind farm are shown as follows: (a), (b), (c) represent the voltage, current, and power on the medium-voltage side of the hybrid DC transformer HDCT. (d) shows the cable currents I 1R , I2, and I3 (i.e., I 2R and I 3R ). (e) shows the DC voltage of the first wind turbine group. (f) shows the fault circulating current I 1L of the first wind turbine group.

[0117] During the fault current suppression stage from t1 to t2 = 1.5165 s, the hybrid DC transformer HDCT outputs a negative DC voltage, as Figure 10 (a) shows. At this time, the voltage differences between the medium-voltage DC MVDC side of the hybrid DC transformer HDCT and the DC buses of the second and third wind turbine groups cause the cable currents I2 and I3 to increase, as Figure 9 (d) shows. The voltage difference between the medium-voltage side of the hybrid DC transformer HDCT and the fault point also reduces the rate of change of the current I 1R , as shown in the Figure 11 amplification part. Therefore, the hybrid DC transformer HDCT actively absorbs the fault current on the three DC cables into its medium-voltage side, resulting in the increase of the DC side current I T to quickly release the energy of the medium-voltage DC MVDC collection network, as shown in Figure 9 (b) and (d). Approximately 16.5 ms after the fault occurs (i.e., at t2 = 1.5165 s), the current I 1R is quickly suppressed to within ±100 A. At this time, the switch HSS1 of cable 1 is disconnected to isolate the fault.

[0118] Refer to Figure 10 for the detailed waveforms of the active fault current absorption control shown. After the fault isolation, the hybrid DC transformer HDCT-MMC1 switches back to the DC voltage control mode at t3 = 1.5215 s, and the DC voltage of the medium-voltage DC MVDC network gradually recovers to the rated value within 20 ms without significant transient oscillations, as Figure 9 (a) shows. The DC voltage V WT1 of the first wind turbine group connected to the fault point remains close to zero due to the DC fault, as shown in Figure 9 (e). The fault circulating current I 1LGradually dissipates and has no effect on the opening of HSS1.

[0119] The AC voltage reference value of the hybrid DC transformer HDCT-MMC2 ramps up to the rated value within 20 ms starting from t4 = 1.5265 s, as shown in Figure 12 (a). When the system completes the restoration of DC voltage and AC voltage at t5 = 1.5465 s, the wind turbine groups 2 and 3 ramp up the active power reference to the rated value within 100 ms, as shown in Figure 9 (c) and Figure 14 (c). I2 and I3 return to the rated value, while due to the isolation of the faulty wind turbine group, I T returns to 0.67 pu, as shown in Figure 9 (b) and Figure 9 (d). The system relies on the remaining fault-free wind turbine groups to resume normal operation.

[0120] As shown in Figure 13 , during the MVDC fault, the arm currents of the hybrid DC transformer HDCT are well controlled. The peak arm currents of MMC1 and MMC2 are 1.6488 pu and 1.7271 pu respectively, both lower than the protection threshold of 2 pu. The fault in the MVDC collection network interrupts the power transmission and causes disturbances to the onshore MMC, as shown in Figure 14 . After the fault isolation, the onshore MMC resumes normal operation.

[0121] Figure 11 DC fault detection and location based on local current measurement, combining current magnitude and direction are given.

[0122] Although the present invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed, as long as they do not depart from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the different dependent claims and the features described herein can be combined in a manner different from that described in the original claims. It should also be understood that the features described in connection with a single embodiment can be used in other described embodiments.

Claims

1. A hybrid DC transformer applicable to offshore wind power DC collection, characterized in that, It includes a medium-voltage side modular multilevel converter MMC1, a high-voltage side modular multilevel converter MMC2 and an intermediate-frequency transformer MFT; the DC side of the medium-voltage side modular multilevel converter MMC1 serves as the medium-voltage DC MVDC side of the hybrid DC transformer, the currents of n cables are gathered to the medium-voltage DC MVDC side, and a DC high-speed switch HSS is set for each cable; the AC side of the medium-voltage side modular multilevel converter MMC1 and the AC side of the high-voltage side modular multilevel converter MMC2 are respectively connected to the primary and secondary sides of the intermediate-frequency transformer MFT, and the DC side of the high-voltage side modular multilevel converter MMC2 serves as the high-voltage DC HVDC side of the hybrid DC transformer and is transmitted to the onshore receiving grid via sea. Each bridge arm of the high-voltage side modular multilevel converter MMC2 consists of N HV Each bridge arm of the medium voltage side modular multilevel converter MMC1 consists of N H A half-bridge submodule HBSM and N F The number of full-bridge sub-modules FBSM is mixed, and the number of sub-modules in each bridge arm of MMC1 is N MV =N H +N F , the number of full-bridge submodules FBSM N F for: where m AC is the AC modulation ratio, and m DC is the DC modulation ratio.

2. The active fault current absorption control method for a hybrid DC transformer applicable to offshore wind power DC collection is implemented based on the hybrid DC transformer applicable to offshore wind power DC collection described in claim 1, and is characterized in that When a fault occurs, the medium voltage side modular multilevel converter MMC1 switches the i-th fault cable on the HSS i The measured current I iR The difference between the reference value 0 is input into the PI controller to adjust the DC voltage bias V DCo , thereby controlling the medium voltage DC MVDC side voltage V DC , to actively absorb the fault current in the medium voltage DC MVDC collection network; so that the fault current flowing through the DC switch is suppressed to zero, and the zero current disconnection of the HSS on the fault cable is achieved, i = 1, 2, ..., n; The medium-voltage side modular multilevel converter MMC1 uses the average capacitor voltage control to construct the direct-axis current reference value of the current loop so as to stabilize the sub-module capacitor voltage at the rated value. When a fault occurs, the high-voltage side modular multilevel converter MMC2 uses the reduced AC voltage control to construct the direct-axis voltage of the voltage loop.

3. The active fault current absorption control method of the hybrid DC transformer applicable to the DC collection of offshore wind power according to claim 2, wherein The control process of the medium-voltage side modular multilevel converter MMC1 is as follows: The current loop outputs a control signal according to the direct-axis current reference value I dref and the quadrature-axis current reference value I qref , and then generates the abc-phase AC voltage reference value v abcref through dq / abc transformation. According to the DC voltage offset V DCo , the abc-phase AC voltage reference value v abcref and the command output by the circulating current suppression control CCSC, the upper-bridge-arm drive voltage reference value v pref and the lower-bridge-arm drive voltage reference value v nref are obtained to drive the MMC1 arm; Among them, the direct-axis current reference value I dref Switches between normal and fault modes. Under normal conditions, the direct-axis current reference value I dref Is: the difference between the medium-voltage DC MVDC side voltage V DC And the DC side voltage reference value V DCref Is input into the PI controller and then output after amplitude limiting; in case of a fault, the direct-axis current reference value I dref Is: the difference between the average value of the sub-module capacitor voltage V CFarg And the capacitor voltage reference value is input into the PI controller and then output after amplitude limiting; Among them, the DC voltage bias V DCo Switches between normal and fault modes. In the normal state, the DC voltage bias V DCo Is: half of the rated voltage of the medium-voltage DC (MVDC) side V DCn In the fault state, the DC voltage bias V DCo Is: half of the value obtained by inputting the difference between the measured current I i Of the switch HSS iR On the i-th faulty cable and the reference value 0 into a PI controller and then through amplitude limiting.

4. The active fault current absorption control method of the hybrid DC transformer applicable to offshore wind power DC collection according to claim 2, wherein, The control process of the high-voltage side modular multilevel converter MMC2 is as follows: The voltage loop outputs a voltage loop control command based on the direct-axis voltage reference value V dref and the quadrature-axis voltage reference value V qref . The current loop control command is output through the current loop, and then the abc-phase AC voltage reference value v abcref is generated through the dq / abc transformation. Based on the DC voltage offset V DCo , the abc-phase AC voltage reference value v abcref and the command output by the circulating current suppression control CCSC, the upper-bridge-arm drive voltage reference value v pref and the lower-bridge-arm drive voltage reference value v nref are obtained to drive the MMC2 arm; Among them, the direct-axis voltage reference value V dref Switches between normal and fault modes. In normal operation, the direct-axis voltage reference value V dref is: 1 pu; in the event of a fault, the direct-axis voltage reference value V dref has a reduced voltage input, and the amplitude V of the reduced AC voltage mf is limited to: where k is a constant coefficient between (0, 1), and m ACn is the rated AC modulation ratio.

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