Power control of energy storage system for wind power smoothing and short circuit fault protection method

Abstract

The application provides a power control and short-circuit fault protection method for a wind power smoothing energy storage system, a DC direct-hanging battery energy storage system suitable for wind power fluctuation power smoothing, a power control method and a DC short-circuit fault protection method, and the energy storage system is used as a current source to provide or absorb current to a DC bus to realize power control, and a filter inductor and a grid-connected inductor are designed; energy transmission of the DC direct-hanging energy storage is limited between DC and DC, an AC inductor is not needed, the number of bridge arms is reduced, the number of required grid-connected inductors is reduced, the value of the designed inductance is reduced, and the cost can be effectively reduced; under the same voltage, the number of required batteries and battery management systems is only 1 / 6 of that of an MMC-BESS, and the cost of the batteries and the battery management systems is greatly reduced. The energy storage system is directly hung on the DC bus, and when a DC line fault occurs, the fault can be quickly removed by directly locking the half-bridge sub-modules.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery energy storage, in particular to a power control and short-circuit fault protection method for a wind power smoothing energy storage system. BACKGROUND

[0002] From a large perspective, the DC transmission and grid connection mode of offshore wind power can be divided into offshore wind power AC collection and DC transmission and grid connection, offshore wind power base multi-terminal DC transmission and grid connection to multi-voltage level DC grid connection technology. With the maturity and cost reduction of key power electronic equipment technology, commercial application has begun to promote from demonstration projects.

[0003] Due to changes in wind speed, wind turbine generators are prone to cause grid voltage and power fluctuation problems, as well as reactive voltage control and power quality problems caused thereby. It is necessary to balance through the adjustment of conventional power sources and energy storage systems. Battery energy storage is not limited by geographical and geological conditions, is flexible to install, and is convenient to implement, and is the main form of energy storage application in the field of new energy power generation. Combining large-capacity battery energy storage systems with wind turbine generators can effectively suppress or alleviate the volatility of wind power and reduce the impact of wind power on the grid.

[0004] In the application of battery energy storage, the energy storage converter (PCS) realizes the bidirectional flow of energy between the energy storage system and the grid, and is the key equipment for connecting the energy storage battery to the power system. Its performance directly affects the effectiveness and indicators of the energy storage system. The power conversion system topology directly determines the electrical structure and integration method of the battery energy storage system (BESS). Under the constraint conditions of the performance of current power devices and the characteristics of batteries, different topologies can realize different levels of battery energy storage system capacity. In the application of battery energy storage, the modularity of circuit topology is used to reduce the scale of battery series and parallel connection, thereby refining the granularity of battery management and power control. Reducing the difficulty of battery selection and assembly also increases the means of battery management and control.

[0005] From the power level, the PCS can be divided into single-stage and double-stage; from the topology, the current mainstream application and research focuses on two-level, three-level and multi-level structures. Among them, multi-level is divided into cascaded H-bridge and modular multi-level (MMC).

[0006] The research and application of conventional two-level energy storage PCS has been mature, and most of the energy storage engineering PCS adopts two-level structure. Considering the efficiency factor, the application of single-stage two-level is mainly used. Due to the restriction of battery series scale, two-level energy storage PCS adopts low-voltage grid connection, so the single machine capacity is small, from several kW to MW, and most of them are less than 500 kW. Two-level energy storage PCS mostly uses LCL filter to improve the quality of grid-connected current while reducing cost and volume, with high loop order, complex control, and stability problems when multiple machines are connected in parallel.

[0007] Three-level energy storage converter needs to control the average DC current injected into the DC neutral point to achieve the balance of the upper and lower battery packs. Under the same device conditions, three-level energy storage converter can get higher output voltage and higher capacity than two-level energy storage converter, and the direct output voltage can reach several kV, and the single machine capacity can reach about 3 MW, which is suitable for occasions with large demand for single machine capacity.

[0008] Cascaded H Bridge (CHB) and Modular Multilevel Converter (MMC) structures have the characteristics of high modularity, excellent output characteristics and unique voltage expansion method. The control granularity of PCS based on the above two structures is one energy storage unit, which can promote the cascade utilization of batteries from the perspective of circuit control. Bypassing a certain energy storage unit in failure can realize redundant fault-tolerant operation. Through differential control of different energy storage units, this structure can also realize intra-phase and inter-phase balancing of energy storage batteries.

[0009] MMC topology was first applied in the field of flexible DC power transmission. Modular Multilevel Converter based Battery Energy Storage System (MMC-BESS) combines MMC and energy storage, which has attracted attention at home and abroad, and is a very promising topology structure in large-scale battery energy storage. The research on MMC-BESS is still gradually developing, and the existing research mainly focuses on modulation methods, power control, battery balancing, two-stage capacitor voltage balancing, circulating current control, elimination of battery current fluctuation, DC fault protection and processing, etc. At present, MMC-BESS is mostly in the research and demonstration engineering stage, and the research and application of MMC-BESS need to be further developed.

[0010] Another way of DC side energy storage is DC direct hanging energy storage, which directly installs energy storage devices on the DC side. There is little research on DC direct hanging energy storage, and there is no engineering application case of DC direct hanging device at present.

[0011] At present, the parallel boosting scheme of low-voltage energy storage converter in AC energy storage is the most mature, and is applied most in engineering; the AC direct hanging scheme of cascaded H bridge (CHB) directly connects the energy storage system to the medium-voltage power grid without transformer, reduces the loss and cost, and is relatively mature in research, and has breakthroughs in engineering application.

[0012] The closest prior art to the direct current direct hanging energy storage technology is MMC-BESS (modular multilevel battery energy storage system).

[0013] The MMC (modular multilevel) topology is first applied to flexible DC transmission. Although MMC-BESS, as the combination of MMC and battery energy storage, is also modular multilevel in structure as flexible DC transmission, and has many same technical aspects as MMC, the sub-module circuit structure is different, the number of modules is greatly different, and it has special features in modulation method, sub-module balancing, main circuit voltage level and parameter design, module control, etc. MMC-BESS has two forms of battery arrangement: centralized arrangement on the common DC bus and dispersed arrangement in the sub-module.

[0014] Compared with the direct current direct hanging energy storage in which the energy storage device is directly installed on the DC side, the energy storage device of the MMC-BESS with dispersed battery arrangement is integrated in the MMC sub-module, that is, the MMC sub-module is composed of a half bridge and an energy storage device, and the energy storage device can be directly connected in parallel with the sub-module capacitor or connected in parallel with the sub-module capacitor through a bidirectional DC / DC converter. In this topology, the MMC is a three-port network, and energy can be transmitted between the DC side, the AC side and the battery, which can provide AC and DC integrated energy storage function, and also can play the roles of AC / DC interconnection, energy buffering and maintaining grid stability. Since the connection between the AC side and the battery in this topology, there is a large frequency-doubled current with pulsation on the battery side, which has an adverse effect on the battery life, and a series inductance needs to be connected in series on the battery side for filtering.

[0015] Since the half-bridge MMC cannot effectively handle the fault on the common DC bus side, in the case that the high-voltage DC circuit breaker is not mature, the use of AC side circuit breaker to cut off the DC fault current is almost the only economic and feasible means for MMC-HVDC to handle DC side fault. For this reason, researchers have proposed various solutions such as full-bridge MMC, clamped double sub-module MMC and hybrid MMC. Through evaluation and comparison of various hybrid topologies in terms of DC fault handling capacity, power devices and efficiency, the hybrid of half-bridge topology and other topologies can achieve better performance. At present, researchers have carried out principle verification on the operation control of the hybrid MMC-BESS based on full-bridge and half-bridge when the common DC bus is faulty.

[0016] There are still some problems in the application of large-scale battery energy storage: the dynamic response characteristics of multi-PCS parallel systems are still difficult to describe, and their interaction with the grid is unclear. Battery energy storage technologies suitable for large-scale (hundreds of MW) applications are not yet mature.

[0017] As the number of PCS units connected in parallel increases, various stability issues arise, including: due to transformer leakage reactance and line impedance, multiple PCS units become associated with and coupled to the power grid, forming a complex high-order circuit structure. At this time, the resonant characteristics of the PCS will change, and additional resonant peaks will be generated at high and low frequencies on the basis of the original resonant peaks. Traditional control strategies are difficult to suppress them, resulting in an increase in the harmonic content of the output current of each PCS. In severe cases, it may even cause the voltage at the PCC point to resonate, leading to the shutdown of the entire energy storage system.

[0018] In a single-stage MMC-BESS, the battery side is connected to the AC side, and frequency pulsation at the power frequency and twice the power frequency is unavoidable, which has an adverse effect on battery life. A large inductor needs to be added between the battery and the sub-module to eliminate low-frequency oscillation current, which increases costs.

[0019] In the MMC-BESS topology, battery energy storage is highly coupled within the MMC submodule. A battery failure could lead to converter shutdown. Furthermore, the MMC converter itself has three phase units, each consisting of an upper arm and a lower arm. Each arm requires an energy storage device, significantly increasing the number of energy storage batteries and grid-connected inductors, thus increasing the cost of batteries and the battery management system and resulting in poor economic efficiency.

[0020] In handling DC faults, the half-bridge MMC topology cannot effectively handle faults on the common DC bus side due to the presence of AC current; the hybrid topology of MMC-BESS full-bridge and half-bridge increases design costs and control difficulty. Summary of the Invention

[0021] This invention proposes a power control and short-circuit fault protection method for energy storage systems to smooth out wind power fluctuations. Specifically, it relates to a DC direct-connected battery energy storage system suitable for smoothing fluctuating offshore wind power and its power control and DC short-circuit fault protection method. This greatly reduces the cost of batteries and battery management systems. Furthermore, this invention uses an energy storage system directly connected to a DC bus, which can quickly isolate the fault by directly blocking the half-bridge sub-module when a DC line fault occurs.

[0022] The present invention adopts the following technical solution.

[0023] The application relates to a power control and short-circuit fault protection method for a wind power smoothing energy storage system, which can be used for the control of offshore wind power, and the method takes a direct-parallel DC direct-hanging energy storage device between the positive and negative poles of a wind power DC transmission line as a current source, and comprises the following steps.

[0024] In step S1, the energy storage device samples a wind power current on the DC line to obtain an original sending-end output DC current i tr ;

[0025] In step S2, the energy storage device filters the sending-end output DC current sampled in step S1 to obtain a smooth and stable current, that is, an ideal state transmission current i ref ;

[0026] In step S3, the energy storage device subtracts the filtered current i ref from the sampled sending-end output DC current i tr to obtain a current i bess that needs to be provided or absorbed by the energy storage system.

[0027] In step S4, the current of the energy storage system is controlled according to the required current of the energy storage system to realize the smoothing of the transmission current, that is, the power smoothing.

[0028] The DC direct-hanging energy storage device is composed of a plurality of series-connected sub-modules, and the number of the series-connected sub-modules is N; the sub-module comprises a battery energy storage unit and a half-bridge power unit connected to the battery energy storage unit, and further comprises a filter inductance connected to the battery energy storage unit; the series-connected sub-modules are connected to a DC bus through a grid-connected inductance; the battery energy storage unit is used for storing and releasing electric energy; the upper bridge arm of the half-bridge structure of the half-bridge power unit is an upper tube, and the lower bridge arm is a lower tube.

[0029] When the wind power DC transmission line is in normal operation, the voltage between the positive and negative poles of the line is basically unchanged, at this time, the power P=UI, and the power fluctuation is smoothed, that is, the current fluctuation at the input end is smoothed.

[0030] When the DC bus current fluctuates, the DC direct-hanging energy storage device is put into or cut off the integrated sub-modules, the sub-module port voltage is controlled, the sub-module current flow direction is controlled, the power is smoothed by smoothing the DC bus current, and the power control or the DC short-circuit fault is handled.

[0031] The filtering in step S2 adopts a first-order low-pass filtering; the transfer function of the first-order low-pass filter is expressed in a formula as

[0032]

[0033] After the above formula is discretized by using a z transform, the formula is obtained as

[0034]

[0035] where τ = RC is the time constant of the system;

[0036] The output-input relationship of the first-order low-pass filter is expressed by the formula

[0037]

[0038] where i in (k) is the input state quantity at time k, i out (k) is the output state quantity at time k, i out (k-1) is the output state quantity at time k-1; the greater the time constant τ, the smaller the difference between the output at time k and the output at time k-1, and the better the smoothing effect of the filter.

[0039] In step S3, the current required to be provided or absorbed by the energy storage system is expressed by the formula

[0040] i bess = i tr -i ref Formula four

[0041] i bess When positive, the current flows from the DC power grid to the DC directly-hung energy storage device, and the energy storage device absorbs power;

[0042] i bess When negative, the current flows from the energy storage device to the DC power grid, and the energy storage system provides power.

[0043] The filter inductance in the sub-module, the filter inductance reactance at the switching frequency is m times the capacitance reactance of the parallel capacitor, m is 10-100, so as to smooth the charge and discharge current at the battery end, and reduce the adverse effects of high-frequency current on the service life of the battery in the battery energy storage unit.

[0044] The design method of the grid-connected inductance adopts the following method:

[0045] Step A1, carrier phase-shift modulation is adopted for each sub-module, so that the equivalent switching frequency after modulation is N times the switching frequency, N is the total number of sub-modules; the ripple current is 1 / k of the rated current I N , and k is in the range of 10-50, and the grid-connected inductance is valued accordingly;

[0046] Step A2, after derivation and arrangement, the absolute value calculation formula of the DC current ripple during carrier phase-shift modulation is:

[0047] where |Δi dc | is the absolute value of the DC current ripple, N is the number of sub-modules, L S is the grid-connected inductance value, and Tc is the carrier frequency, U DC is the DC bus voltage, U B is the energy storage battery voltage, INT is the integer function; and the expression of the grid-connected inductance under the carrier phase-shift modulation is obtained as follows:

[0048]

[0049] wherein L S is the grid-connected inductance value under the carrier phase-shift modulation;

[0050] combined with the value of k, there are thereby realizing the calculation of the grid-connected inductance.

[0051] The upper tube of the half-bridge structure includes a switch tube T1, and the lower tube includes a switch tube T2.

[0052] The operating states of the sub-modules in the DC direct-hanging energy storage device include:

[0053] Normal operating state: two switch tubes, one of which is open and the other is closed, are in two working conditions, which are normal operating states;

[0054] Locking state: when both tubes are closed, the sub-module works in a locking state;

[0055] Short circuit state: when both switch tubes are open, the energy storage system runs to avoid this state.

[0056] The application adopts a DC direct-hanging energy storage device to propose a method for suppressing offshore wind power fluctuations and power control, and realizes DC short-circuit fault handling. Specifically includes:

[0057] 1. The energy storage system is directly connected in parallel between the DC bus, and provides or absorbs current to the DC bus as a current source to realize power control.

[0058] 2. Design filter inductance and grid-connected inductance. Compared with another existing DC energy storage method-MMC-BESS (Modular Multilevel Battery Energy Storage System), the energy transfer of the DC direct-hanging technology is limited between DC and DC, and does not require AC inductance, while the number of bridge arms is reduced, the required grid-connected inductance is also reduced, and the design of inductance value is also reduced, which can effectively reduce the cost.

[0059] 3. Compared with MMC-BESS, the number of batteries of the DC direct-hanging topology is reduced by 1 / 6 under the same voltage, which greatly reduces the cost of the battery and the battery management system.

[0060] 4. The energy storage system is directly connected to the DC bus, and when the DC line fails, the fault can be quickly removed by directly locking the half-bridge sub-module.

[0061] The application has the advantages that:

[0062] 1. It can control the current of the DC bus through the DC direct connection topology, thereby achieving DC bus power smoothing.

[0063] 2. The DC-side direct-connected topology avoids the impact of frequency doubling current caused by the AC side of MMC-BESS on the energy storage battery. The design of the filter inductor on the energy storage battery side can effectively avoid the influence of high-frequency square wave signals.

[0064] 3. Compared with the MMC method, this invention does not have AC inductors, which reduces the number of bridge arm inductors and the inductive reactance of the filter inductor, effectively saving costs.

[0065] 4. The present invention adopts a DC direct-connection topology. Under the same voltage, the number of batteries is only 1 / 6 of that of the MMC method. In the present invention, the reduction of the number of BMS (Battery Management System) can effectively reduce costs. Moreover, compared with the MMC method, the present invention is simpler in terms of control decoupling.

[0066] 5. In the event of a DC fault, this invention can effectively cut off the short-circuit current by simply locking all sub-modules; thus solving the problem that MMC-BESS cannot cut off DC faults. Attached Figure Description

[0067] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0068] Appendix Figure 1 This is a schematic diagram of the topology of the flexible DC transmission system containing DC direct-connected energy storage according to the present invention;

[0069] Appendix Figure 2 This is a schematic diagram of the technical solution for power suppression according to the present invention;

[0070] Appendix Figure 3 This is a schematic diagram of the DC direct-connected energy storage system of the present invention;

[0071] Appendix Figure 4 This is a schematic diagram of the DC-connected energy storage system submodule in this invention. Detailed Implementation

[0072] As shown in the figure, the power control and short-circuit fault protection method of the energy storage system for wind power power smoothing can be used for the control of offshore wind power. The method uses a DC direct-connected energy storage device directly connected in parallel between the positive and negative poles of the wind power DC transmission line as a current source. The method includes the following steps.

[0073] Step S1: The energy storage device samples the wind power current on the DC line to obtain the original output DC current i at the sending end. tr ;

[0074] Step S2, the energy storage device filters the sending end output direct current obtained by sampling in step S1 to obtain a smooth and stable current, that is, an ideal state of the delivered current i ref

[0075] Step S3, the energy storage device filters the sending end output direct current i tr obtained by sampling ref , and the filtered current i bess ;

[0076] Step S4, according to the required current of the energy storage system, the current of the energy storage system is controlled to realize the suppression of the delivered current, that is, the power suppression.

[0077] The DC direct-hanging energy storage device is composed of a plurality of sub-modules in series, and the number of the series sub-modules is N; the sub-module includes a battery energy storage unit and a half-bridge power unit connected thereto, and further includes a filter inductance connected to the battery energy storage unit; the series sub-modules are connected to a DC bus through a grid-connected inductance; the battery energy storage unit is used for storing and releasing electric energy; the upper bridge arm of the half-bridge structure of the half-bridge power unit is an upper tube, and the lower bridge arm is a lower tube;

[0078] When the wind power DC transmission line is in normal operation, the voltage between the positive and negative poles of the line is basically unchanged, at this time the power P=UI, and the power fluctuation is the current fluctuation at the input end;

[0079] When the DC bus current fluctuates, the DC direct-hanging energy storage device puts into or cuts off the integrated sub-modules, controls the sub-module port voltage, and then controls the sub-module current flow direction, thereby suppressing the DC bus current to suppress the power and control or handle the DC short circuit fault.

[0080] The filtering in step S2 adopts a first-order low-pass filter; the transfer function of the first-order low-pass filter is expressed by the formula

[0081]

[0082] After the above formula is discretized by z transform, the formula is obtained

[0083]

[0084] In the formula, τ=RC is the time constant of the system;

[0085] The output and input relationship of the first-order low-pass filter is expressed by the formula

[0086]

[0087] In the formula, i in (k) is the input state quantity at time k​out (k) is the output state quantity at time k out (k-1) is the output state quantity at time k-1; the greater the time constant τ, the smaller the difference between the output at time k and k-1, and the better the smoothing effect of the filter.

[0088] In step S3, the current required to be provided or absorbed by the energy storage system is expressed by the formula

[0089] i bess = i tr - i ref Formula four

[0090] i bess When positive, the current flows from the DC power grid to the DC directly-connected energy storage device, and the energy storage device absorbs power;

[0091] i bess When negative, the current flows from the energy storage device to the DC power grid, and the energy storage system provides power.

[0092] The filter inductance in the sub-module, the filter inductance reactance at the switching frequency is m times the parallel capacitance reactance, m is 10-100, so as to smooth the charge and discharge current at the battery end and reduce the adverse effects of high-frequency current on the service life of the battery in the battery energy storage unit.

[0093] The design method of the grid-connected inductance adopts the following method:

[0094] Step A1, carrier phase-shift modulation is adopted for each sub-module, so that the equivalent switching frequency after modulation is N times the switching frequency, N is the total number of sub-modules; the ripple current is 1 / k of the rated current I N , k is in the range of 10-50, and the grid-connected inductance is valued accordingly;

[0095] Step A2, after derivation and arrangement, the absolute value calculation formula of the DC current ripple during carrier phase-shift modulation is:

[0096]

[0097] In the formula, |Δi dc | is the absolute value of the DC current ripple, N is the number of sub-modules, L S is the grid-connected inductance value,

[0098] T c is the carrier frequency, U DC is the DC bus voltage, U B is the energy storage battery voltage, and INT is the integer function.

[0099] Accordingly, the expression of the grid-connected inductance under carrier phase-shift modulation is:

[0100]

[0101] In the formula, L S is the grid-connected inductance value under carrier phase shift modulation;

[0102] Combining the value of k, we have Thus, the grid-connected inductance is calculated.

[0103] The upper tube of the half-bridge structure includes a switch tube T1, and the lower tube includes a switch tube T2;

[0104] The operating states of the sub-modules in the DC direct hanging energy storage device include:

[0105] Normal operating state: two switch tubes, one of which is open and the other is closed, are in two operating conditions, which are normal operating states;

[0106] Locking state: when both tubes are closed, the sub-module works in a locking state;

[0107] Short circuit state: when both switch tubes are open, it is a short circuit, and the energy storage system needs to avoid this state.

[0108] Embodiment 1:

[0109] In this example, a method for suppressing offshore wind power fluctuations and power control is proposed using a DC direct hanging energy storage device, and DC short circuit fault handling is realized. Specifically, it includes:

[0110] 1. The energy storage system is directly connected in parallel between the DC bus, acting as a current source to provide or absorb current to the DC bus to achieve power control.

[0111] 2. Design filter inductance and grid-connected inductance. Compared with another existing DC energy storage method-MMC-BESS (Modular Multilevel Battery Energy Storage System), the energy transfer of the DC direct hanging technology is limited between DC and DC, and does not require AC inductance. At the same time, the number of bridge arms is reduced, the required grid-connected inductance is also reduced, and the design of inductance value is also reduced, which can effectively reduce the cost.

[0112] 3. Compared with MMC-BESS, the number of batteries in the DC direct hanging topology is reduced by 1 / 6, greatly reducing the cost of batteries and battery management systems.

[0113] 4. The energy storage system is directly connected to the DC bus, and in the event of a DC line fault, the fault can be quickly removed by directly locking the half-bridge sub-module.

[0114] Embodiment 2:

[0115] In this example, the DC direct hanging energy storage device is composed of a plurality of identical sub-modules connected in series; the sub-modules include a battery energy storage unit and a half-bridge power unit;

[0116] The battery energy storage unit is used for storing input electric energy and outputting electric energy when the DC bus power and voltage decrease.

[0117] The half-bridge power unit includes inductance, capacitance, insulated gate bipolar transistor (IGBT), large capacity diode and thyristor, which are used for controlling the input and removal of the battery energy storage unit; the half-bridge structure of the circuit of the half-bridge power unit is provided with a large capacity diode in reverse parallel connection on the upper tube to increase the current carrying capacity, and a thyristor in forward parallel connection on the lower tube to facilitate the removal of the sub-module.

[0118] The upper tube in the half-bridge structure is provided with a plurality of diodes in reverse parallel connection, so that the current carrying capacity of the first parallel connection structure formed thereby is more than five times that of the upper tube to enhance the current path when the DC bus power is surplus.

[0119] The lower tube in the half-bridge structure is provided with a thyristor in forward parallel connection with high current carrying capacity, so that the current carrying capacity of the second parallel connection structure formed thereby is more than five times that of the lower tube, and the battery energy storage unit in parallel connection with the half-bridge is removed from the bypass.

[0120] The series connection method between the sub-modules is as follows:

[0121] The upper tube and the lower tube of the half-bridge structure of the sub-module are provided with a first connection point connected with the upper tube and the lower tube, and the first connection point is a series input end of the sub-module; the series input end is connected with a grid-connected inductor or a higher-level sub-module;

[0122] The lower tube of the half-bridge structure of the sub-module is provided with a second connection point, and the second connection point is a series output end of the sub-module; the connection device of the series output end includes a lower-level sub-module.

Claims

1. A power control and short-circuit fault protection method for wind power smoothing energy storage systems, applicable to offshore wind power control, characterized by: The method uses a DC-connected energy storage device directly connected in parallel between the positive and negative poles of a wind power DC transmission line as a current source, and the method includes the following steps; Step S1: The energy storage device samples the wind power current on the DC line to obtain the original output DC current i at the sending end. tr ; Step S2: The energy storage device filters the DC current sampled in step S1 to obtain a smooth and stable current, which is the ideal current i being transmitted. ref ; Step S3: The energy storage device outputs a DC current i to the sampled sending end. tr and the filtered current i ref By subtracting the current, we obtain the current i that the energy storage system needs to provide or absorb. bess ; Step S4: Control the current of the energy storage system according to the current required by the energy storage system to achieve current smoothing, that is, power smoothing. The DC-connected energy storage device consists of multiple sub-modules connected in series, with N sub-modules in total. Each sub-module includes a battery energy storage unit and a connected half-bridge power unit, as well as a filter inductor connected to the battery energy storage unit. The series-connected sub-modules are connected to the DC bus via a grid-connected inductor. The battery energy storage unit is used for storing and releasing electrical energy. The upper arm of the half-bridge structure of the half-bridge power unit is the upper tube, and the lower arm is the lower tube. When the wind power DC transmission line is operating normally, the voltage between the positive and negative poles of the line remains basically constant. At this time, the power P = UI, and smoothing out power fluctuations is equivalent to smoothing out current fluctuations at the input end. When the DC bus current fluctuates, the DC direct-connected energy storage device can put into operation or disconnect its integrated sub-modules, control the sub-module port voltage, and then control the sub-module current flow. By smoothing the DC bus current, the power can be smoothed, and power control or DC short-circuit faults can be handled. The filtering in step S2 uses a first-order low-pass filter; the transfer function of the first-order low-pass filter is expressed by the formula: After discretizing the above equation using the z-transform, we obtain the formula. In the formula, τ is the system's time constant; The relationship between the output and input of a first-order low-pass filter is expressed by the formula: In the formula, i in (k) represents the input state variable at time k, i out (k) represents the output state variable at time k, i out (k-1) represents the output state quantity at time k-1; the larger the time constant τ is, the smaller the difference between the output at time k and time k-1, and the better the smoothing effect of the filter. The design method for the grid-connected inductor adopts the following approach: Step A1: Apply carrier phase-shift modulation to each submodule so that the equivalent switching frequency after modulation is N times the switching frequency, where N is the total number of submodules. Ripple current is the rated current I N 1 / k1, where k1 ranges from 10 to 50, and the value of the grid-connected inductor is selected accordingly; Step A2: After derivation and simplification, the formula for calculating the absolute value of DC current ripple during carrier phase-shift modulation is as follows: In the formula |Δi dc | represents the absolute value of the DC current ripple, N is the total number of submodules, and L S T is the grid-connected inductance value under carrier phase-shift modulation. c For the carrier period, U DC U is the DC bus voltage. B Here, INT represents the energy storage battery voltage, and INT is the floor function. Therefore, the expression for the grid-connected inductance value under carrier phase-shift modulation is: In the formula L S This is the grid-connected inductance value under carrier phase-shift modulation; Considering the possible values ​​of k, we have This enables the calculation of grid-connected inductance.

2. The method for power control and short-circuit fault protection of a wind power smoothing energy storage system according to claim 1, characterized in that: In step S3, the current required to be provided or absorbed by the energy storage system is expressed by the formula: i bess =i tr -i ref Formula 4; i bess When positive, the current flows from the DC grid to the DC-connected energy storage device, and the energy storage device absorbs power; i bess When the current is negative, the current flows from the energy storage device to the DC grid, and the energy storage system provides power.

3. The method for power control and short-circuit fault protection of a wind power smoothing energy storage system according to claim 1, characterized in that: The filter inductor in the submodule has an inductive reactance that is m times the capacitive reactance of the capacitor connected in parallel at the switching frequency, where m is between 10 and 100. This is to smooth the charging and discharging current at the battery terminal and reduce the adverse effects of high-frequency current on the battery life of the battery energy storage unit.

4. The method for power control and short-circuit fault protection of an energy storage system for wind power smoothing according to claim 1, characterized in that: The upper tube of the half-bridge structure includes switch T1, and the lower tube includes switch T2; The operating status of the submodules in a DC direct-connected energy storage device includes: Normal operating conditions: The two operating conditions in which one of the two switching transistors is turned on and the other is turned off are considered normal operating conditions. Locked state: When both tubes are turned off, the submodule operates in the locked state; Short circuit condition: A short circuit occurs when both switching transistors are turned on. This condition must be avoided when operating an energy storage system.

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