CN116345524A - Fault-tolerant direct-current transformer substation for offshore wind power boost collection - Google Patents

Abstract

The invention provides a fault-tolerant direct-current transformer substation for offshore wind power boost collection, which comprises a plurality of power transmission groups, wherein each power transmission group comprises a wind power plant, an inverter and a transformer; the positive power end of the wind power plant is connected with the first connecting end of the inverter, and the negative power end of the wind power plant is connected with the second connecting end of the inverter; two ends of the main side of the transformer are respectively connected with a third connecting end and a fourth connecting end of the inverter; secondary sides of the transformers of the plurality of power transmission groups are connected in series and then connected with the modularized multi-level converter to form an intermediate frequency loop; the modularized multi-level converter is connected with a direct current network. The invention realizes the functions of topology fault reconstruction and fault isolation based on the power electronic switch and the modulation mode in the converter, has the fault-tolerant operation capability of the short-circuit fault of the wind field side port, and ensures the high-reliability operation capability of the multi-port direct-current transformer substation when a certain port fails.

Description

Fault-tolerant DC substation for offshore wind power boost collection

technical field

The invention relates to the technical field of renewable energy power generation, in particular to a fault-tolerant direct current substation for boosting and collecting offshore wind power.

Background technique

The proportion of renewable energy in the power system will continue to increase. Among them, offshore wind power will become an important part of the future power system due to its small land occupation, low pollution caused by development, continuous stability, and huge reserves.

Offshore wind power grid-connected methods mainly include high voltage alternating current (HVAC), high voltage direct current (HVDC), and frequency division transmission technology. Among them, HVDC technology is more suitable for meeting the long-distance power transmission needs of offshore wind farms due to its low line loss. In recent years, theoretical research on MTDC circuit topology, power characteristics and control strategies is being continuously improved.

Existing research on HVDC technology is mostly limited to single-channel wind farm input and single-channel output scenarios. Based on inverter, rectifier technology and single-phase intermediate frequency transformers, the output power of a single wind farm can be integrated into the DC transmission system to realize wind power. Grid-connected, while reducing space requirements for offshore platforms. However, when multiple offshore wind farms need to be connected to the grid, this technical solution will bring additional equipment and transmission line costs.

Document 1, the patent document with the publication number CN112290527A discloses a collector-based offshore wind power DC collection network structure, including: n wind turbine groups, n collectors, offshore converter stations, onshore converter stations, n is an integer not less than 1, wherein each of the wind turbine groups includes k wind turbines, k is an integer not less than 1; each of the wind turbines outputs a medium and low voltage DC voltage VDCij, i is an integer and 1 ≤i≤n, j is an integer and 1≤j≤k, VDCi1～VDCik are all delivered to collector i; collector i outputs medium-voltage DC voltage to the offshore converter station, and the offshore converter station transfers the medium-voltage DC bus The voltage is raised to HVDC voltage and sent to the onshore converter station. But this patent document still has the defect of high transmission line cost.

Document 2, H.Liu, M.S.A.Dahidah, J.Yu, R.T.Naayagi and M.Armstrong, Design and Control of Unidirectional DC–DC Modular Multilevel Converter for Offshore DCCollection Point: Theoretical Analysis and Experimental Validation[J].IEEETransactions on Power Electronics, 2019,34(6):5191-5208. This document proposes an improved, unidirectional dc-dc modular multilevel converter for the application scenario of offshore wind farms being integrated into high-voltage direct current transmission systems. The proposed converter consists of a single-phase MMC inverter coupled with a series rectifier module through an intermediate frequency transformer with multiple secondary windings. The modulation characteristics of the converter enable extension at different voltage levels, except current In addition to the characteristics of isolation, the transformer also provides a step gain for the output voltage. In terms of efficiency, loss and equipment applicability, the converter is comparable to the most competitive unidirectional cascaded DC-DC converter, such as input series output series and Compared with the parallel input and output in series, it has advantages. In addition, unlike the traditional d-q control method that includes many transformation methods, this document adopts a simple proportional resonance control strategy in the static coordinate system to directly act on the AC output of the MMC. The excellent performance of the converter proposed in this document is verified by analyzing the design, simulation and experimental results. However, this document uses a transformer with multiple secondary windings, connects MMC and multiple diode rectifiers in series, and realizes the integration of offshore wind farms into the HVDC power transmission system. However, in a multi-winding transformer, bidirectional power transmission between any two ports is possible. , due to the characteristics of multi-winding transformers, the high coupling between the transmission power of each port makes the power characteristics of the system complex, and the manufacturing method of multi-winding high-voltage intermediate frequency transformers is also more complicated and the cost is higher. .

Document 3, S.Zhao, Y.Chen, S.Cui, B.J.Mortimer and R.W.De Doncker, Three-PortBidirectional Operation Scheme of Modular-Multilevel DC–DC Converters Interconnecting MVDC and LVDC Grids[J].IEEE Transactions on PowerElectronics,2021, 36(7):7342-7348. This document is based on the modular multilevel DC converter (MMDC), which is an attractive solution for connecting the medium-voltage grid to the low-voltage grid. Modular multilevel converters (MMC) and full-bridge converters (FBs) on the low-voltage side, MMDC can be flexibly adjusted to different rated power and voltage levels, however, the existing MMDC is limited to two-port operation, and the DC voltage The output power of FBs must be consistent, and this paper proposes a control strategy that enables three-port MMDCs to operate bidirectionally independently and connect the medium-voltage grid with two low-voltage grids with different rated voltages, and proposes an integrated operation strategy, including two modes of operation with different modulation schemes, which optimizes the current of the MMC transformer over the full load range, compared to the most common three-port isolated DC-DC converter: the three-port active bridge, where the three-port The power transmission relationship between them is simple, the amount of calculation required for bidirectional control is small, and the power circulation is essentially eliminated. The experimental results verify the reliability and effectiveness of the operation strategy proposed in this document. However, this document proposes a three-port MMDC converter topology and control strategy, including two operating modes, but does not propose a fault-tolerant operation method after a fault. At the same time, it does not involve the research on the expansion of the number of circuit ports, and only has three ports. The MMDC converter is not suitable for the application scenario of multi-port DC collection.

Contents of the invention

Aiming at the defects in the prior art, the object of the present invention is to provide a fault-tolerant DC substation for boosting and collecting offshore wind power.

According to the present invention, a fault-tolerant DC substation for offshore wind power boosting collection includes a plurality of power transmission groups, and each power transmission group includes a wind farm, an inverter and a transformer;

The positive power supply terminal of the wind farm is connected to the first connection terminal of the inverter, and the negative power supply terminal of the wind farm is connected to the second connection terminal of the inverter;

Both ends of the main side of the transformer are respectively connected to the third connection end and the fourth connection end of the inverter;

The secondary sides of the transformers of multiple power transmission groups are connected in series and then connected with a modular multilevel converter to form an intermediate frequency circuit; the modular multilevel converter is connected with a DC power grid.

Preferably, an inductor is connected in series in the intermediate frequency circuit.

Preferably, each power transmission group also includes a current-limiting reactance;

The positive power supply end of the wind farm is connected to one end of the current-limiting reactance, and the other end of the current-limiting reactance is connected to the first connection end of the inverter.

Preferably, the DC port of the modular multilevel converter on the high voltage side is connected to the DC grid through a high voltage cable.

Preferably, each power transmission group also includes a voltage stabilizing capacitor;

One end of the voltage stabilizing capacitor is connected to the first connection end of the inverter, and the other end of the voltage stabilizing capacitor is connected to the second connection end of the inverter.

Preferably, the inverter on the wind farm side is a low-voltage inverter.

Preferably, the converter on the high voltage side is a modular multilevel converter.

Preferably, the output power of each wind farm port is controlled by adjusting the duty ratio of the output voltage of each inverter and the phase shift angle relative to the output voltage of the modular multilevel converter.

风场向电网进行功率传输。Preferably, the series voltage V _P on the secondary side of the transformer and the output voltage V _MMC of the modular multilevel converter are both 7-level waveforms with the same waveform, and the phase of V _P is ahead of the phase of V _MMC by a phase shift angle

The wind farm transmits power to the grid.

隔离故障端口，容错运行非故障端口。Preferably, it is possible to perform fault-tolerant operation when the port of the wind farm fails: when a DC fault occurs at the port of the wind farm, the pulse blocking protection method is used to isolate the faulty port, V _P changes from a 7-level waveform to a 5-level waveform, and V _MMC Then it changes into a 5-level waveform, and the V _P phase leads the phase shift angle relative to the V _MMC phase

Isolate faulty ports and run non-faulty ports with fault tolerance.

Compared with the prior art, the present invention has the following beneficial effects:

1. Based on inverter technology and MMC technology, the present invention establishes connections between multiple offshore wind farms and onshore DC power grids by using intermediate frequency transformers, and realizes electrical isolation between offshore wind farms and DC power grids;

2. Based on the reconfiguration control mode of the internal switch of the substation, the present invention can realize the flexible removal of the fault port, reduce the fault range, and not affect the normal operation of the remaining wind power DC access ports;

3. The topology scheme proposed by the present invention can flexibly change the number of ports according to actual application requirements, while still maintaining a relatively simple power transmission characteristic between ports.

Description of drawings

Other characteristics, objects and advantages of the present invention will become more apparent by reading the detailed description of non-limiting embodiments made with reference to the following drawings:

FIG. 1 is a topology diagram of a fault-tolerant DC substation for offshore wind power boost collection in an embodiment;

Figure 2 is an equivalent circuit diagram of a multi-port DC substation in normal working mode;

Figure 3 is the voltage and current phasor diagram of the intermediate frequency circuit;

Fig. 4 is a schematic structural diagram of a single wind farm and an inverter unit;

Fig. 5 is a control block diagram of the normal operation of the DC substation;

Figure 6 is a schematic diagram of the fault-tolerant operation of the DC substation;

Figure 7 is the equivalent circuit of the substation in the fault-tolerant operation mode;

Fig. 8 is a block diagram of the fault-tolerant operation control of the DC substation;

Fig. 9 is a schematic diagram of a four-port DC substation model;

Fig. 10 is a schematic diagram of voltage waveforms at each port of a four-port DC substation;

Figure 11 is a schematic diagram of the simulation and calculation results of the transmission power at each port of the DC substation under open-loop control;

Fig. 12 is a schematic diagram of driving pulses of FB1 switching tubes S ₁ -S ₄ before and after a fault occurs;

Fig. 13 is a schematic diagram of the sum Vp waveform of the secondary side voltage of the fault transformer;

Figure 14 is a schematic diagram of the transmission power of each port before and after the fault occurs;

FIG. 15 is a schematic diagram of simulated values and calculated values of transmission power of non-faulty ports before and after a fault occurs.

Detailed ways

The present invention will be described in detail below in conjunction with specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

Example 1:

As shown in Figure 1, this embodiment provides a fault-tolerant DC substation for offshore wind power boosting collection, including a plurality of power transmission groups, each power transmission group includes a wind farm, an inverter, and a transformer. The positive power supply end is connected to the first connection end of the inverter, the negative power supply end of the wind farm is connected to the second connection end of the inverter, and the two ends of the main side of the transformer are respectively connected to the third connection end and the fourth connection end of the inverter. At the end, the secondary sides of the transformers of multiple power transmission groups are connected in series and then connected to the modular multilevel converter to form an intermediate frequency circuit, and the modular multilevel converter is connected to the DC power grid. Series inductance in the intermediate frequency circuit.

Each power transmission group also includes a current-limiting reactance, the positive power supply end of the wind farm is connected to one end of the current-limiting reactance, and the other end of the current-limiting reactance is connected to the first connection end of the inverter. Each power transmission group also includes a voltage stabilizing capacitor, one end of the voltage stabilizing capacitor is connected to the first connection end of the inverter, and the other end of the voltage stabilizing capacitor is connected to the second connection end of the inverter.

By adjusting the duty ratio of the output voltage of each inverter and the phase shift angle relative to the output voltage of the modular multilevel converter, the output power of each wind farm port is controlled, and the ratio of the output power of each wind farm port It all depends on the ratio of the duty cycle of each inverter output voltage.

The converter on the high voltage side is a modular multilevel converter. The inverter on the side of the wind farm adopts a low-voltage inverter. The DC port of the modular multilevel converter on the high-voltage side is connected to the DC grid through a high-voltage cable to realize power transmission.

风场向电网进行功率传输。能够进行风电场端口故障时的容错运行：当风电场端口发生直流故障时，采用脉冲闭锁保护的方式，隔离故障端口，V_P由7电平波形变化为5电平波形，V_MMC随之变化为5电平波形，V_P相位相对于V_MMC相位领先移相角/>

隔离故障端口，容错运行非故障端口。The series voltage V _P on the secondary side of the transformer and the output voltage V _MMC of the modular multilevel converter are both 7-level waveforms with the same waveform, and the phase of V _P is ahead of the phase of V _MMC by the phase shift angle

The wind farm transmits power to the grid. Fault-tolerant operation when the wind farm port is faulty: when a DC fault occurs at the wind farm port, the pulse blocking protection method is used to isolate the faulty port, V _P changes from a 7-level waveform to a 5-level waveform, and V _MMC changes accordingly is a 5-level waveform, V _P phase is leading phase shift angle relative to V _MMC phase />

Isolate faulty ports and run non-faulty ports with fault tolerance.

This embodiment proposes a multi-port DC substation topology solution with port fault isolation capability for the offshore wind farm to connect to the land DC power grid. Compared with the existing technical solutions, this embodiment has the following characteristics:

It has the functions of multi-channel offshore wind power DC feed-in/single-channel high-voltage DC output power conversion and boost conversion. The low-voltage side is based on a power electronic converter, which inverts the output DC current of the offshore wind farm into a two-level or multi-level AC current, and performs power conversion and series boost through the power transformer. The high-voltage side is based on a Modular Multilevel Converter (MMC). The number of sub-modules is determined according to the number of wind farm ports and the type of power electronic converter. sent to the DC grid.

In the substation, the secondary side of the transformer is connected in series to realize DC collection with a high step-up ratio. Establish a medium-high frequency connection between the transformer port of the wind farm and the MMC port, reduce the size of the AC transformer, and reduce the space and weight requirements of the offshore platform.

Based on the internal power electronic switch and modulation method of the converter, it realizes the topology fault reconstruction and fault isolation functions, and has the fault-tolerant operation capability of the wind farm side port short-circuit fault, ensuring the high reliability operation capability of the multi-port DC substation when a certain port fails. .

The DC substation system based on multi-input and single-channel boost output proposed in this embodiment can collect the output power of multiple offshore wind farms and transmit it to the DC power grid through a single-channel DC cable, which saves the cost of transmission lines and has Fault isolation, fault-tolerant operation capability, to ensure that when a single port fails, the non-faulty port can still transmit power normally.

Example 2:

The difference between this embodiment and Embodiment 1 is that the inverter is a midpoint clamped inverter.

Example 3:

The difference between this embodiment and Embodiment 1 is that the inverter is a Vienna inverter.

Example 4:

Those skilled in the art can understand this embodiment as a more specific description of Embodiment 1.

As shown in Figure 1, this embodiment provides a multi-port DC substation topology. This embodiment involves renewable energy power generation, offshore wind power boosting and collection operation, especially the multi-port boosting substation topology design, fault-tolerant operation and high reliability. Reconfiguration of permanent faults, the specific technical implementation scheme is as follows, DC substation topology scheme design:

In the multi-port DC substation topology of this embodiment, the power supply side includes n wind farms (WF ₁ , WF ₂ ...WF _n ), voltage stabilizing capacitors and current limiting reactances (C ₁ , C ₂ ...C _n ; L ₁ , L ₂ ...L _n ), inverters (Inv ₁ , Inv ₂ ...Inv _n ) and transformers (Tr ₁ , Tr ₂ ...Tr _n ) connected thereto. The secondary side of each transformer is connected in series, and forms an intermediate frequency circuit with a modular multilevel converter (MMC) to realize power transmission. The inductance L _t is connected in series in the intermediate frequency loop to limit the loop current, and the MMC is connected to the DC grid to transmit the output power of the wind farm to the grid, and finally realize DC integration. The low-voltage side inverter unit can be implemented based on two-level or three-level inverters such as full-bridge inverters, mid-point clamped inverters, and Vienna inverters.

Operation control and power characteristics of multi-port DC substation:

A. Normal operation mode:

For the DC substation topology shown in Figure 1, according to Kirchhoff's law, the voltage and current equations can be listed as shown in formula (1). In the formula, the directions of each current are shown in Figure 9, and V _MMC represents the MMC AC port Voltage, L _t represents the inductance value of the auxiliary inductance connected in series in the AC circuit, I _MMC represents the output current of the MMC, V _a1u and V _a2u represent the total voltage of the upper half of the _a1 bridge arm and _a2 bridge arm of the MMC respectively, V _a1L , V _a2L represents the total voltage of the bridge arm _a1 and the lower half of the bridge arm _a2 in the MMC, L ₀ represents the inductance value of the bridge arm of the MMC, V _MV represents the voltage of the medium-voltage DC grid, and i _a1u and i _a2u represent the voltage in the MMC respectively. a ₁ bridge arm, a ₂ bridge arm current in the upper half of the bridge arm, i _a1l and i _a2l represent the current in the a ₁ bridge arm and a ₂ bridge arm in the MMC respectively.

Simplify and eliminate V _MMC , the formula (2) can be obtained:

L_eq＝L_t+L₀，式(2)可化简为式(3)：make:

L _eq ＝L _t +L ₀ , formula (2) can be simplified to formula (3):

In the formula, V _P is the series voltage of each secondary side of each transformer. According to the formula (3), the equivalent circuit of the DC substation is obtained, as shown in Fig. 2 .

According to the equivalent circuit, using the Fourier series, the output voltages V ₁ , V ₂ ... V _n of each inverter, and the output voltage V _s of the MMC can be expressed as the sum of the fundamental wave and each harmonic, as shown in Formula (4) shows:

为MMC输出电压V_S相对于V_P的移相角。In the formula, V _P1 , V _P2 ...V _Pn are the fundamental wave voltage output by each transformer and the amplitude of each harmonic voltage, which is a function of the harmonic order n, and its value is determined by the wind field voltage and the modulation mode of each inverter. Decide.

It is the phase shift angle of MMC output voltage V _S relative to _VP .

For the fundamental wave and each harmonic voltage, calculate the current expression corresponding to the frequency according to the vector diagram shown in Figure 3, and calculate its power. Superposition is performed to obtain the expression of the intermediate frequency loop current as shown in formula (5):

The power transmitted by each harmonic voltage is superimposed to obtain the expression of the intermediate frequency loop current I _L as shown in formula (5), where ω represents the angular frequency of each voltage of the AC loop.

According to formula (4) and formula (5), the instantaneous transmission power of each port is calculated as:

Among them, P ₁ , P ₂ ... P _n is the instantaneous transmission power of each port, and P _S is the instantaneous receiving power of the MMC port.

Because the integral value of the odd-order harmonic current in a period of the other order harmonic voltage is zero. Therefore, for each harmonic voltage, we only need to pay attention to the corresponding harmonic current to calculate the average power transmitted by the harmonic voltage in the intermediate frequency circuit. The average transmission power of each port is shown in formula (7):

In Equation (7), V _P1 , V _P2 ...V _Pn are all determined by the modulation mode of the inverter and the voltage of each wind field. If the effect of higher harmonics is ignored, the power distribution between ports is as shown in Equation (8): Show:

P ₁ :P ₂ :...:P _n =V _p1 :V _p2 :...:V _pn (8)

Therefore, the output power distribution of the wind farm can be controlled only by changing the modulation mode of the corresponding inverter.

As shown in Figure 4, for a single wind farm and inverter unit, when the voltage of the stabilizing capacitor is V _C , the output power of the inverter is P _n , and the output power of the wind farm is P _WFn , the voltage variation of the stabilizing capacitor is as follows: As shown in formula (9), where C is the capacitance of the voltage stabilizing capacitor.

Therefore, in the normal operation mode, when the output power P _WFn of the wind farm changes, the inverter control strategy is used to adjust the port transmission power P _n to stabilize the voltage of the stabilizing capacitor V _C .

从而决定各逆变器内部电力电子开关与调制方式，以及MMC各子模块控制方式，通过脉冲宽度调制器(Pulse width modulation,PWM)，向逆变器与MMC发出控制脉冲，控制各风电场端口输出功率，从而保持各风电场电压稳定。In normal operation mode, the substation control block diagram is shown in Figure 5. Sampling the voltage of the voltage stabilizing capacitors of each wind farm, and calculating the fundamental wave of the output voltage of each inverter and each harmonic component V _p1 , V _p2 ...V _pn , and the output voltage of the MMC relative to the secondary transformer The phase shift angle of side series voltage

In order to determine the internal power electronic switch and modulation mode of each inverter, as well as the control mode of each sub-module of the MMC, through the pulse width modulator (Pulse width modulation, PWM), send control pulses to the inverter and MMC to control the ports of each wind farm Output power, so as to keep the voltage of each wind farm stable.

B. Fault-tolerant operation mode for short-circuit faults on the wind farm side:

Taking the DC-to-ground short-circuit fault in wind farm WF1 as an example, the fault-tolerant operation mode of the substation is illustrated.

As shown in Figure 6, when WF ₁ has a ground short-circuit fault, the pulse blocking protection method is adopted to control the inverter Inv ₁ to output zero voltage. Equivalently, the fault port transformer T _r1 is short-circuited, and the other ports are still normal at this time. Work.

After a fault occurs, the equivalent circuit of the substation in the fault-tolerant operation mode is shown in Fig. 7.

For the average transmission power of each port under normal operation described in formula (7), it is only necessary to set the corresponding port voltage _Vp1 to 0V, and the expression of the average transmission power of the port under fault-tolerant operation can be obtained as shown in formula (10):

Therefore, in the fault-tolerant operation mode, the transmission power of the faulty port is zero, and the remaining ports can still transmit power normally. At this time, the power allocation between ports is shown in Equation (11). Compared with Equation (8), the power allocation characteristics of non-faulty ports before and after the fault will not change.

P ₂ :P ₃ :...:P _n =V _p2 :V _p3 :...:V _pn (11)

After a fault occurs, the fault-tolerant operation control block diagram of the substation is shown in Figure 8. By means of pulse blocking and cutting, the transformer corresponding to the faulty wind field is controlled to output 0V voltage, and equivalently, the corresponding transformer is short-circuited to realize fault removal. At this time, the power distribution characteristics of the remaining non-faulty ports do not change, and can still be controlled by the voltage balance control method during normal operation.

Working principle: Under normal working conditions, the voltage waveforms of each AC port are shown in Figure 10. The three groups of full-bridge inverters output three-level voltages with different duty ratios, and the output of the grid-side modular multilevel converter and The voltage waveform of the series voltage on the secondary side of the transformer group is the same as the 7-level voltage. At this time, the current expression in the AC loop is shown in formula (5).

If higher harmonics are ignored, the average transmission power of each port can be calculated as shown in Equation (7), and the power distribution between ports is shown in Equation (8). The power distribution is completely determined by the voltage duty cycle of each port. Therefore, by increasing or decreasing the duty cycle of the output voltage of each full-bridge inverter, the increase or decrease of the output power of the corresponding wind farm port can be controlled.

Under fault conditions, the average transmission power of each port is shown in equation (10), and the power distribution among ports is shown in equation (11). obvious change. It is still possible to change the output power of the corresponding wind farm port by controlling the increase or decrease of the duty cycle of the output voltage of each full-bridge inverter.

从而实现风场向电网的功率传输。As shown in Figure 1 and Figure 10, the series voltage V _P on the secondary side of the transformer and the output voltage V _MMC of the modular multilevel converter are both 7-level waveforms with the same waveform, and the phase of V _P is shifted ahead of the phase of V _MMC horn

In this way, the power transmission from the wind farm to the grid is realized.

从而实现故障端口的隔离，以及非故障端口的容错运行。As shown in Figure 1 and Figure 13, when a DC fault occurs at the port of the wind farm, the pulse blocking protection method is used to isolate the faulty port. V _P changes from a 7-level waveform to a 5-level waveform, and V _MMC changes accordingly It is a 5-level waveform, V _P phase leads the phase shift angle relative to V _MMC phase

In this way, the isolation of faulty ports and the fault-tolerant operation of non-faulty ports are realized.

Example 5:

Those skilled in the art can understand this embodiment as a more specific description of Embodiment 1.

In order to verify the technical solution proposed in this application, this embodiment establishes a system model as shown in Figure 7 based on the Matlab/Simulink simulation environment, and the application scenario is: a four-port DC substation.

According to the topology of the DC substation proposed in this application, a four-port DC substation model is designed as shown in Figure 9, which is used to connect three wind farms with the onshore DC power grid. Full-bridge inverters FB ₁ , FB ₂ , and FB ₃ are used to connect the offshore Wind field and intermediate frequency transformer.

The voltage control target of the voltage stabilizing capacitor at the outlet of the wind farm is 10kV, the rated voltage of the DC grid is 30kV, and the rated power of a single wind farm is 10MW. The system parameters are shown in Table 1.

Table 1 Basic system parameters of DC substation

Wind farm voltage stabilizing capacitors C ₁ , C ₂ , C ₃ 1mF Rated capacity of intermediate frequency transformer 5MW Rated frequency of intermediate frequency transformer 5kHz IF loop inductance L _t 500uH MMC arm inductance L ₀ 5uH MMC sub-module capacitance C _SM 0.01F

A. Normal operation power characteristic verification

For the four-port DC substation shown in Figure 9, the output voltage waveform of each port is shown in Figure 10. V ₁ , V ₂ , and V ₃ are two-level waveforms, and V _p is the superposition of V ₁ , V ₂ , and V ₃ , which is a six-level waveform. Vs is the same as the six-level waveform, and the phase shift angle is lagging behind Vp

For the voltage waveform shown in Figure 10, using the Fourier sequence, V _P1 , V _P2 ... V _Pn in formula (4) is shown in formula (12):

Substituting Equation (12) into Equation (6), the average transmission power of each port of the substation can be obtained as shown in Equation (13):

In order to verify the power characteristics, the full-bridge inverter duty ratio d1, d2, d3 is different, and the output voltage of the MMC is verified at different phase shift angles relative to the output voltage of the full bridge, and the simulation value of the average transmission power of each port is recorded. According to formula (12), consider the calculated value obtained by the fundamental wave and the 3rd, 5th, and 7th harmonics, and compare them. The results are shown in Table 2.

Table 2 Simulation and calculation results of transmission power at each port of DC substation under open-loop control

Draw the simulation results and calculation results in Table 2 into a line graph, as shown in Figure 11, using formula (14). Calculate the calculation accuracy of the average transmission power of each port, where Value_cal is the power calculation value, where Value_Cal is the power calculation value, and Value_Sim is the power simulation value.

Calculation results: acc _P1 =96.18%; acc _P2 =96.94%; acc _P3 =96.87%, verifying the correctness of formula (12) for the calculation of transmission power.

B. Fault-tolerant operation verification

For the four-port DC substation model, the fault-tolerant operation mode of the wind farm WF1 after a short circuit occurs is simulated. The three wind farms are represented by current sources, WF1, WF2, WF3 respectively output power: 5MW, 2.5MW, 3.5MW. Each port uses PI control independently, the control amount is the voltage of the wind farm voltage stabilizing capacitor, and the control target is 10kV. The simulation time is 0.2s, and a short-circuit fault of the wind farm occurs at 0.1s.

After the short-circuit fault of WF ₁ occurs, the driving pulses of the switching tubes of FB ₁ are shown in Figure 12. The two IGBTs of the upper half bridge arm are kept off, and the two IGBTs of the lower half bridge arm are turned on, so that the output voltage of FB1 is 0V, which is equivalent to , Short circuit the transformer Inv1. In this way, the fault point is isolated from the normal operating system, and the normal operation of the non-faulty port is avoided.

Figure 13 shows the waveforms of the sum of secondary side voltages of the three transformers V _p before and after the fault. Compared with before and after the fault, the voltage waveform changes from six levels to four levels. Indicates that the output voltage of the inverter at the faulty port is 0V, the transformer is successfully short-circuited, and the non-faulty port can still work normally.

Before and after the fault, the transmission power of each port is shown in Figure 14. After the fault occurs, the transmission power of the non-faulty port drops briefly, and then the power transmission capability before the fault can be restored through PI control. Figure 15 compares the simulated and calculated values of the port transmission power before and after the fault, and it is found that the calculation error before and after the fault is within 5%. The experimental results show that the power transmission system has the capability of fault-tolerant operation, and the non-faulty ports can still transmit power normally after the fault occurs, and the power characteristics of the system do not change.

Based on inverter technology and modular multilevel converter technology, the invention uses intermediate frequency transformers to establish connections between multiple offshore wind farms and onshore DC power grids. And the electrical isolation between the offshore wind farm and the DC grid is realized.

Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. In the case of no conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims (10)

1. The fault-tolerant direct-current transformer substation for offshore wind power boost collection is characterized by comprising a plurality of power transmission groups, wherein each power transmission group comprises a wind farm, an inverter and a transformer;

the positive power end of the wind power plant is connected with the first connecting end of the inverter, and the negative power end of the wind power plant is connected with the second connecting end of the inverter;

two ends of the main side of the transformer are respectively connected with a third connecting end and a fourth connecting end of the inverter;

secondary sides of the transformers of the plurality of power transmission groups are connected in series and then connected with the modularized multi-level converter to form an intermediate frequency loop; the modularized multi-level converter is connected with a direct current network.

2. The fault tolerant direct current substation for offshore wind power boost integration of claim 1, wherein an inductor is connected in series in the intermediate frequency loop.

3. The fault tolerant dc substation for offshore wind boost pooling of claim 1, wherein each group of power transmission groups further comprises a current limiting reactance;

the positive power end of the wind power plant is connected with one end of the current limiting reactance, and the other end of the current limiting reactance is connected with the first connecting end of the inverter.

4. The fault tolerant dc substation for offshore wind boost pooling of claim 1, wherein the dc port of the modular multilevel converter at the high voltage side is connected to a dc grid through a high voltage cable.

5. A fault tolerant dc substation for offshore wind power boost pooling as defined in claim 3, wherein each group of power transmission groups further comprises a voltage stabilizing capacitor;

one end of the voltage stabilizing capacitor is connected with the first connecting end of the inverter, and the other end of the voltage stabilizing capacitor is connected with the second connecting end of the inverter.

6. The fault tolerant direct current substation for offshore wind power boost pooling of claim 1, wherein the inverter on the wind farm side employs a low voltage inverter.

7. The fault tolerant direct current transformer substation for offshore wind power boost integration of claim 1, wherein the converter at high voltage side is a modular multilevel converter.

8. The fault tolerant dc substation for offshore wind boost integration of claim 1, wherein the output power of each wind farm port is controlled by adjusting the duty cycle of each inverter output voltage and the phase shift angle with respect to the modular multilevel converter output voltage.

9. The fault tolerant direct current transformer substation for offshore wind power boost pooling of claim 1, wherein the transformer secondary side series voltage V _P Output voltage V of modularized multi-level converter _MMC All have the same 7-level waveform, V _P Phase relative to V _MMC Phase leading phase shifting angle

The wind farm transmits power to the grid.

10. The fault tolerant direct current substation for offshore wind farm boost integration of claim 1, wherein fault tolerant operation at wind farm port failure is enabled: when the wind farm port has direct current fault, a pulse locking protection mode is adopted to isolate the fault port, V _P Change from 7 level wave to 5 level wave, V _MMC Then changes into 5 level waveform, V _P Phase relative to V _MMC Phase leading phase shifting angle

Isolating the fault port and fault-tolerant running the non-fault port.

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