Medium-voltage chain type energy storage system topology and control method thereof

Abstract

The invention provides a medium-voltage chain type energy storage system topology and a control method thereof, and relates to the technical field of power system electric energy quality control and new energy grid connection. The method has a sag supporting function and can be used for a medium-voltage power distribution network. The medium-voltage chain type energy storage system topologically integrates a three-phase parallel side chain type STATCOM (static synchronous compensator), a three-phase series side chain type STATCOM and an energy storage device, and comprises a transformer secondary winding in triangular connection. The three-phase parallel side chained STATCOM adopts positive and negative sequence voltage and current double-loop control, the three-phase series side chained STATCOM adopts an interphase direct-current voltage equalizing strategy of zero-sequence component injection through an angle connection winding, and the chained STATCOMs on the two sides both adopt a network construction type control method. The parallel side chain type STATCOM, the series side chain type STATCOM, the energy storage device and the triangular connection secondary winding are integrated, and a network construction type control strategy is combined, so that the operation stability and the electric energy quality of the system in a medium-voltage scene are improved, and a grid-connected point sag support function and direct-current voltage balance control of the chain type device are realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power system power quality control and new energy grid connection technology, and particularly relates to a medium-voltage chain energy storage system topology and a control method thereof. BACKGROUND

[0002] With the increasing proportion of new energy such as photovoltaic and wind power in the medium-voltage distribution network, the fluctuation, intermittence and low inertia characteristics of the new energy lead to serious challenges of power quality and safety and stability of the power grid. Voltage sag, as a common fault of the distribution network, can cause sensitive loads to shut down or performance to decline, resulting in significant economic losses. At the same time, new energy units are prone to be disconnected from the grid under voltage sag, further exacerbating the instability of the power grid. Existing medium-voltage energy storage systems mainly focus on power smoothing, and the response speed of voltage sag support is slow and the energy scheduling is not accurate. The control strategy is mainly grid-connected, and lacks the voltage / frequency support capability of a synchronous generator, which is easy to lose stability during power grid disturbance. Therefore, there is an urgent need for a medium-voltage chain energy storage system topology and control method that integrates fast voltage sag support, grid-forming control and efficient energy management. SUMMARY

[0003] The present application aims to provide a medium-voltage chain energy storage system topology and a control method thereof, which integrates a three-phase parallel side chain STATCOM (static synchronous compensator), a three-phase series side chain STATCOM and an energy storage device in the medium-voltage chain energy storage system topology, creates a zero sequence loop by adding a secondary winding connected in a delta configuration, realizes inter-phase active power exchange, and realizes fast voltage sag support, system inertia support and power quality optimization in combination with a grid-forming control strategy, thereby improving the safe and stable operation level of the medium-voltage distribution network in the high penetration scenario of new energy.

[0004] To achieve the above-mentioned purpose, in a first aspect, the present application provides a medium-voltage chain energy storage system topology, comprising a multi-winding transformer, a three-phase parallel side chain STATCOM, a three-phase series side chain STATCOM, N DC capacitors and N energy storage devices, N being a positive integer; the multi-winding transformer comprises a secondary winding L ac2 , L bc2 , L cc2 , L a0 , L b0 , L c0 and N primary windings; the secondary windings L ac2 , L bc2 , L cc2 are connected in series in the three-phase line of the power grid, and the secondary windings L a0 , L b0 , L c0 are connected in a delta configuration. The three-phase parallel side-chain STATCOM consists of N cascaded H-bridge modules, each H-bridge module including four switching transistors; the input of the first H-bridge module is connected to the grid input, and the output of the Nth H-bridge module is connected to the three-phase common coupling point; the input of the m-th H-bridge module is connected to the output of the (m-1)-th H-bridge module, and the output of the m-th H-bridge module is connected to the input of the (m+1)-th H-bridge module, where m is a positive integer and 1 < m < N; A three-phase series-connected side-chain STATCOM includes N H-bridge modules, each H-bridge module includes four switching transistors, and each H-bridge module is connected to the corresponding primary winding. The H-bridge module of a three-phase parallel side-chain STATCOM has the same structure as the H-bridge module of a three-phase series side-chain STATCOM. Each H-bridge module shares the corresponding DC capacitor and energy storage device on the DC side.

[0005] According to the medium-voltage chain-type energy storage system topology provided by the present invention, it further includes a three-phase reactor; the input terminal of the first H-bridge module of the three-phase parallel side-chain STATCOM is connected to the grid input terminal through the connection of the three-phase reactor.

[0006] In a second aspect, the present invention provides a control method for a medium-voltage chain energy storage system topology as described in the first aspect, comprising: The three-phase parallel side-chain STATCOM and the three-phase series side-chain STATCOM are controlled independently in a grid-like manner. The grid-like control uses the energy of the DC capacitor to simulate the rotor energy of the synchronous generator, realizes the synchronization of DC voltage control with the H-bridge module, establishes the matching relationship between DC voltage and system angular frequency, and thus realizes the grid-like operation of the three-phase parallel side-chain STATCOM and the three-phase series side-chain STATCOM.

[0007] According to the control method of a medium-voltage chain energy storage system topology provided by the present invention, the three-phase parallel side-chain STATCOM adopts positive and negative sequence voltage and current dual-loop control, which includes: DC voltage control, positive sequence decoupling control, negative sequence decoupling control and output control.

[0008] According to the control method of a medium-voltage chain energy storage system topology provided by the present invention, the DC voltage control includes: controlling the reference value U dc_ref Subtract the three-phase DC voltage U dc_a U dc_b U dc_c The difference is obtained by averaging the values, and this difference is used by the PI controller to adjust the output reference d-axis positive sequence current command i. d +* .

[0009] According to the control method of a medium-voltage chain energy storage system topology provided by the present invention, the positive sequence decoupling control includes: referencing the d-axis positive sequence current command id +* the difference between the reference d-axis positive-sequence current command i d + the difference between the reference q-axis positive-sequence current command i q +* the difference between the reference q-axis positive-sequence current command i q + the difference between the reference q-axis positive-sequence current command i d + and the difference between the reference q-axis positive-sequence current command i ωLi q + the difference between the reference q-axis positive-sequence current command i q + and the difference between the reference q-axis positive-sequence current command i ωLi d + and the difference between the reference q-axis positive-sequence current command i

[0010] According to the control method of the medium-voltage chain energy storage system topology provided by the application, the negative-sequence decoupling control comprises: taking the difference between the reference d-axis negative-sequence current command i d -* the difference between the reference q-axis negative-sequence current command i d - the difference between the reference q-axis negative-sequence current command i q -* the difference between the reference q-axis negative-sequence current command i q - the difference between the reference q-axis negative-sequence current command i ωLi q + the difference between the reference q-axis negative-sequence current command i ωLi d + the difference between the reference q-axis negative-sequence current command i

[0011] According to the control method of the medium-voltage chain energy storage system topology provided by the application, the output control comprises: superimposing the positive-sequence modulation voltage and the negative-sequence modulation voltage to obtain a modulation voltage , and performing phase-shift carrier sine pulse modulation to obtain the switching signal of the switching tube of the H-bridge module of the three-phase parallel side chain STATCOM.

[0012] According to the control method of the medium-voltage chain energy storage system topology provided by the application, the three-phase series side chain STATCOM adopts zero sequence voltage injection inter-phase DC voltage equalization control, active power between three phases is redistributed through injection of zero sequence voltage, so that the balance of three-phase DC voltage is realized.

[0013] According to the control method of the medium-voltage chain energy storage system topology provided by the application, the inter-phase DC voltage equalization control comprises: Supposing that the negative sequence voltage and the negative sequence current are both 0, only the injection of zero sequence voltage is considered, the sub-winding L a0 , L b0 , L c0 As a zero sequence component path, zero sequence current is generated, and the three-phase active power output by the three-phase series side chain STATCOM is: (6) In the formula, U p , U0 respectively represent the effective values of the positive sequence and zero sequence components of the output voltage; I p , I0 respectively represent the effective values of the positive sequence and zero sequence components of the output current; the reference direction is the angle of the positive sequence voltage, that is, the initial phase of the positive sequence voltage U p is 0, and θ0 represents the initial phase of the zero sequence voltage output by the three-phase series side chain STATCOM. respectively represent the initial phases of the positive sequence current and the zero sequence current output by the three-phase series side chain STATCOM. ω is the system angular frequency; P is the active power, P A0 , P B0 , P C0 is called zero sequence cluster active power, and the zero sequence cluster active power satisfies: (7) The amplitude and phase of the injected zero sequence voltage are: (8) (9).

[0014] Compared with the prior art, the application has at least the following technical effects: The application provides a medium-voltage chain energy storage system topology and a control method thereof. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the drawings.

[0016] In the drawings: Figure 1 Fig. 1 is a structural schematic diagram of the medium-voltage chain energy storage system topology in the application; Figure 2 Fig. 2 is a network configuration control strategy block diagram of the application; Figure 3 Fig. 3 is a positive and negative sequence voltage and current double-loop control block diagram of the three-phase parallel side chain STATCOM in the application; Figure 4 Fig. 4 is a phase-to-phase voltage balancing control strategy block diagram of the three-phase series side chain STATCOM in the application; Figure 5 Fig. 5 is a network configuration effect simulation diagram of the chain STATCOM in the application; Figure 6 Fig. 6 is a common connection point voltage sag support result simulation diagram in the application; Figure 7 Fig. 7 is a phase-to-phase DC voltage balancing effect simulation diagram in the application. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical solutions and advantages of the application more clear, the technical solutions in the application will be clearly and completely described in the following with reference to the drawings in the application. Obviously, the described embodiments are some embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor belong to the protection scope of the application.

[0018] Some embodiments of the present application will be described in detail with reference to the drawings. The following embodiments and features can be combined with each other in the case of no conflict.

[0019] Please refer to Figure 1 The embodiment of the present application provides a medium-voltage chain energy storage system topology with temporary support function, which comprises a multi-winding transformer, a three-phase parallel side chain STATCOM (static synchronous compensator), a three-phase series side chain STATCOM, N DC capacitors and N energy storage devices, N being a positive integer; the multi-winding transformer comprises a secondary winding L ac2 , L bc2 , L cc2 , L a0 , L b0 , L c0 and N primary windings; the secondary winding L ac2 , L bc2 , L cc2 is connected in series in a three-phase line of a power grid, and the secondary winding L a0 , L b0 , L c0 is connected in a delta connection. The three-phase parallel side chain STATCOM comprises N H-bridge modules connected in cascade, each H-bridge module comprising four switching tubes; an input end of a first H-bridge module is connected to a power grid input end, and an output end of an Nth H-bridge module is connected to a three-phase common connection point; an input end of an mth H-bridge module is connected to an output end of an (m-1) th H-bridge module, and an output end of the mth H-bridge module is connected to an input end of an (m+1) th H-bridge module, m being a positive integer and 1 The three-phase series side chain STATCOM comprises N H-bridge modules, each H-bridge module comprising four switching tubes, and each H-bridge module being connected to a corresponding primary winding. The H-bridge modules of the three-phase parallel side chain STATCOM and the H-bridge modules of the three-phase series side chain STATCOM are identical in structure, and the DC side of each H-bridge module shares a corresponding DC capacitor and energy storage device.

[0020] Specifically, the medium-voltage chain energy storage system topology further comprises three-phase reactors L ac1 , L bc1 , L cc1 ; and an input end of a first H-bridge module of the three-phase parallel side chain STATCOM is connected to the power grid input end through the three-phase reactors.

[0021] In some embodiments, the power grid input end A, B and C is connected in parallel to the three-phase parallel side chain STATCOM. Taking phase a as an example, an mth H-bridge module comprises four switching tubes Q am11 , Q am12 , Q am13 , Qam14 The first H-bridge module includes four switching tubes Q a111 , Q a112 , Q a113 , Q a114 The emitter of the switching tube Q a111 is connected to the collector of the switching tube Q a113 , and is connected to the grid through the connection a-phase reactance L ac1 The emitter of the switching tube Q a112 is connected to the collector of the switching tube Q a114 , and is connected to the common connection point of the emitter of the switching tube Q a211 and the collector of the switching tube Q a213 , and so on; the emitter of the switching tube Q aN11 of the Nth H-bridge module is connected to the collector of the switching tube Q aN13 , and is connected to the common connection point of the emitter of the switching tube Q a(N-1)12 and the collector of the switching tube Q a(N-1)14 of the N-1th H-bridge module; the emitter of the switching tube Q aN12 is connected to the collector of the switching tube Q aN14 , and is connected to the three-phase common connection point.

[0022] The grid load end (i.e. the grid output end) A', B', C' is connected in series with the three-phase series side chain STATCOM through a multi-winding transformer. Taking the a-phase as an example, the mth H-bridge module includes four switching tubes Q am21 , Q am22 , Q am23 , Q am24 In the mth H-bridge module, the emitter of the switching tube Q am21 is connected to the collector of the switching tube Q am23 , the emitter of the switching tube Q am22 is connected to the collector of the switching tube Q am24 , and the output end of the H-bridge module is connected to the primary winding L am of the multi-winding transformer, and the secondary windings of the multi-winding transformer are L ac2 and L a0 The DC capacitors and energy storage devices of the first, mth, and Nth H-bridge modules are C a1 , C am , C aN and E a1 , E am , E aN .

[0023] The secondary windings L ac2 , L bc2 , L cc2Connected in series in a three-phase line from grid input terminals A, B, C to grid output terminals A', B', C'. a0 b0 c0 The three-phase connection provides a zero sequence loop for the inter-phase and intra-phase voltage balancing control and double frequency ripple suppression control of the three-phase series side chain STATCOM, thereby constructing a three-phase power channel.

[0024] In the medium voltage chain energy storage system topology of the application, each stage controls the output of the corresponding primary winding through two H-bridge modules, which can smooth the current and voltage of the DC capacitor and energy storage device for output, and finally transmits energy to the grid output terminals A', B', C' through the secondary winding.

[0025] Please refer to Figure 2 Another embodiment of the application provides a control method for the medium voltage chain energy storage system topology in the foregoing embodiments, comprising: the three-phase parallel side chain STATCOM at the generator end and the three-phase series side chain STATCOM at the load end are independently controlled in a network type. The DC voltage equation of the converter is: (1) In the formula, P L is the grid side converter output power; P R is the load side converter output power; U dc is the DC voltage; U dc0 is the rated DC voltage; H C is the DC capacitor inertia time constant.

[0026] The rotor motion equation of the synchronous generator can be expressed as: (2) In the formula, P m is the input mechanical power of the synchronous machine; P e is the output electromagnetic power of the synchronous machine; ω m is the rotor speed, ω m0 is the rated rotor speed; H J is the rotor inertia time constant.

[0027] It can be seen that due to the certain duality between the DC capacitor of the converter and the rotor motion frequency of the synchronous phase modifier in structure, the DC capacitor energy can be used to simulate the rotor energy of the synchronous generator, the DC voltage control and the converter synchronization can be realized, and the DC voltage U dc ​​The converter is matched with the alternating current voltage angle frequency (i.e. system angle frequency) ω, so as to realize the network operation of the two-end STATCOM.

[0028] (3) In the formula, ω n is a rated frequency; U dc is a DC voltage; U dc0 is a rated DC voltage.

[0029] In the prior art, the STATCOM at the generator end generally adopts a reactive power-voltage control strategy. By collecting the port voltage and the connecting inductance current of the converter, the reactive power is calculated through a power link. Then, the reference voltage amplitude V ref is obtained through a voltage control link containing a PI controller, and the dq axis components of the converter output voltage are generated through the voltage outer loop and the current inner loop.

[0030] Referring to Figure 3 , the three-phase parallel side chain STATCOM adopts positive and negative sequence voltage and current double-loop control. The core of the positive and negative sequence voltage and current double-loop control is to realize the accurate control of the STATCOM through positive and negative sequence decoupling control, and the control process can be divided into several main parts: DC voltage control, positive sequence decoupling control, negative sequence decoupling control and output control.

[0031] The DC voltage control, i.e. the voltage outer loop control, compares the average value of the three-phase DC voltages U dc_a , U dc_b and U dc_c with the reference value U dc_ref , subtracts the average value of the three-phase DC voltages U dc_ref , U dc_a and U dc_b from the reference value U dc_c to obtain a difference value, and then the difference value is adjusted through a PI controller to output the reference d-axis positive sequence current command i d +* .

[0032] The positive sequence decoupling control. The input signals are the reference d-axis positive sequence current command i d +* and the reference q-axis positive sequence current command i q + , the sampling d-axis positive sequence current i d + and the sampling q-axis positive sequence current i q + . The reference d-axis positive sequence current command i d +* is subtracted from the sampling d-axis positive sequence current i d +the difference between the reference q-axis positive sequence current command i q +* and the sampling q-axis positive sequence current i q + The difference between the reference q-axis positive sequence current command i sq cd and the sampling q-axis positive sequence current i cq d is respectively adjusted by a PI controller, the signal is converted from the dq coordinate system to the abc coordinate system, and then the inverse Park transformation is performed to obtain the positive sequence modulation voltage. Moreover, after the signal is converted from the dq coordinate system to the abc coordinate system, it needs to be decoupled before the inverse Park transformation. In the case of a symmetrical and non-distorted grid voltage and load, the mathematical model of the control system can be simplified as formula (4): (4) In the formula, R is the connection resistance, L is the connection inductance, is the connection part of the three-phase parallel side chain STATCOM and the system, Figure 1 The connection inductance L in the formula is L a1 , L b1 , L c1 ; ω is the system angular frequency, e sd , e sq are the d-axis component and the q-axis component of the grid output voltage respectively; u cd , u cq are the d-axis component and the q-axis component of the three-phase parallel side chain STATCOM output voltage respectively.

[0033] It can be seen from formula (4) that the dq components of the system are coupled with each other and jointly affect the inverter output voltage. In order to eliminate the coupling of the current components and realize the separate control of the positive sequence dq components, the decoupling design is performed on them in the control process.

[0034] Define the intermediate variable: , and formula (4) can be transformed into: (5) At this time, the d-axis and q-axis components of the system act independently and are no longer coupled with each other. Therefore, before the inverse Park transformation, the d-axis signal is reversed and the corresponding system sampling d-axis positive sequence voltage e d + is added to it. ωLi q + After the q-axis signal is reversed, the corresponding system sampling q-axis positive sequence voltage e q + is subtracted from it. ωLi d + .

[0035] Negative sequence decoupling control. The input signals are the reference d-axis negative sequence current command i d -* and the reference q-axis negative sequence current command iq -* , the sampled d-axis negative sequence current i d - and the sampled q-axis negative sequence current i q - The difference between the reference d-axis negative sequence current instruction i d -* and the sampled d-axis negative sequence current i d - The difference between the reference q-axis negative sequence current instruction i q -* and the sampled q-axis negative sequence current i q - The signals output by the PI controller after adjustment are converted from the dq coordinate system to the abc coordinate system, and then subjected to inverse Park transformation to obtain the negative sequence modulation voltage. In addition, the signals obtained in the negative sequence decoupling control process also need to be subjected to decoupling before inverse Park transformation, similar to positive sequence decoupling control, but the phase sequence is opposite to that of positive sequence decoupling control, and in the case of symmetrical grid three-phase voltage, the grid-side voltage does not contain negative sequence components, so the intermediate variable is Therefore, before inverse Park transformation, the d-axis signal is reversed and subtracted by ωLi q + , and the q-axis signal is reversed and added by ωLi d + .

[0036] Output control. The positive sequence modulation voltage and the negative sequence modulation voltage are superimposed to obtain the modulation voltage which is subjected to phase-shifted carrier sine pulse modulation (CPS-SPWM) to obtain the switching signals of the switching tubes of the H-bridge module of the three-phase parallel side chain STATCOM.

[0037] Please refer to Figure 4 , the three-phase series side chain STATCOM of the application adopts zero sequence voltage injection for inter-phase DC voltage equalization control. The target of inter-phase DC voltage equalization control is to keep the three-phase DC voltages U dc_a , U dc_b , U dc_c balanced. If each phase changes the active power of single-phase input according to the deviation of inter-phase DC voltage by introducing a regulator, it will cause the coupling of the second layer three-phase active power control and the first layer total active power control, which is not conducive to the stability of the system. Therefore, by injecting zero sequence voltage to redistribute the active power among the three phases, the balance of the three-phase DC voltage is achieved. The specific process is as follows: Assuming that the negative sequence voltage and the negative sequence current are both 0, only the injection of zero sequence voltage is considered, the secondary windings L a0 , Lb0 , L c0 As a zero sequence component path will produce zero sequence current, the three-phase active power output by the three-phase series side chain STATCOM is: (6) In the formula, U p , U0 respectively represent the effective value of the positive sequence and zero sequence components of the output voltage; I p , I0 respectively represent the effective value of the positive sequence and zero sequence components of the output current; the reference direction is the angle of the positive sequence voltage, i.e. the initial phase of the positive sequence voltage U p is 0, and θ0 represents the initial phase of the zero sequence voltage output by the three-phase series side chain STATCOM. , respectively, represent the initial phase of the positive sequence current and the zero sequence current output by the three-phase series side chain STATCOM. ω is the system angular frequency. The three-phase active power P A , P B , P C is composed of two parts, where P is the active power, and P A0 , P B0 , P C0 is called zero sequence cluster active power, and the zero sequence cluster active power satisfies: (7) According to formula (6) and (7), it can be concluded that by adjusting the amplitude and phase of the zero sequence voltage output by the three-phase series side chain STATCOM, the total active power will not change, but the distribution of active power among the three-phase converter chains can be changed. Compared with the traditional zero sequence voltage injection method, the adjustable active power range of the present application is larger.

[0038] The amplitude and phase of the injected zero sequence voltage are: (8) (9).

[0039] In the formula, sign represents a sign function, sign(P B0 )=1 when P B0 >0; sign(P B0 )=-1 when P B0 <0; and sign(P B0 )=0 when P B0 =0.

[0040] Therefore, the application provides a medium-voltage chain energy storage system topology and a control method thereof, which has a voltage sag support function and can be used in a medium-voltage power distribution network. The medium-voltage chain energy storage system topology integrates a three-phase parallel side chain STATCOM, a three-phase series side chain STATCOM and an energy storage device, and contains a triangularly connected transformer secondary winding. The three-phase parallel side chain STATCOM adopts positive and negative sequence voltage and current double-loop control, the three-phase series side chain STATCOM adopts a zero sequence component injection inter-phase DC voltage equalization strategy through an angularly connected winding, and both side chain STATCOMs adopt a grid-forming control method. By integrating the parallel side chain STATCOM, the series side chain STATCOM, the energy storage device and the triangularly connected secondary winding and combining the grid-forming control strategy, the application improves the operation stability and power quality of the system in a medium-voltage scenario, realizes voltage sag support function at a grid-connected point and DC voltage equalization control of the chain device.

[0041] Please refer to Figure 5 In order to verify the effect of the grid-forming control method of the medium-voltage chain energy storage system topology of the application, a simulation model is built using MATLAB / Simulink, the C-phase voltage of the public access point is allowed to drop by 30% for 0.2s, and the phase angle and frequency change of the AC end of the converter are detected. When the C-phase voltage drops, the phase angle of the AC end of the converter is not affected, and periodically changes between 0 and 2 , and the frequency quickly stabilizes around the rated value 50Hz of the grid frequency, which reflects the voltage source property of the grid-forming STATCOM and verifies the effectiveness of the grid-forming control strategy.

[0042] Please refer to Figure 6 In order to verify the voltage sag support effect of the application, the C-phase voltage of the public access point is allowed to drop by 30% for 0.2s, and the voltage change of the public access point is detected. When the voltage drops, the voltage of the public access point quickly decreases, but under the action of the grid-forming STATCOM, the voltage quickly stabilizes, which verifies the voltage sag support effect of the application.

[0043] Please refer to Figure 7 In order to verify the inter-phase DC side equalization control strategy based on zero sequence injection, the three-phase DC voltage is detected, and at 0.5s, a negative sequence current is injected to simulate an unbalanced grid condition. It can be seen from Figure 7 that after the STATCOM is put into use, the three-phase DC voltage quickly stabilizes, and after the negative sequence current is injected, the three-phase DC voltage of the STATCOM is unbalanced, but under the action of the inter-phase equalization strategy, the three-phase inter-phase voltage gradually reaches equilibrium, which verifies the feasibility of the control strategy.

[0044] In summary, compared with the prior art, the application has the following advantages: This invention proposes a medium-voltage chain-type energy storage system topology and its control method. This medium-voltage chain-type energy storage system topology integrates a three-phase parallel side-chain STATCOM, a three-phase series side-chain STATCOM, and an energy storage device, replacing the APF (Active Power Filter) used in general UPQC (Unified Power Quality Conditioner) type devices. This reduces hardware costs and integrates multiple functions such as voltage sag support, improved power quality, and enhanced system oscillation damping effect, thereby improving the overall utilization efficiency of the equipment. It has good adaptability under medium and high voltage conditions and good economic efficiency.

[0045] The secondary winding L of the transformer connected to the three-phase series-connected side-chain STATCOM of this invention a0 L b0 L c0 The delta connection provides a zero-sequence loop for the phase-to-phase and phase-to-phase voltage equalization control of the three-phase series-connected STATCOM, thereby constructing a three-phase power path. This allows the three-phase series-connected STATCOM to achieve phase-to-phase DC voltage balance by injecting zero-sequence current, improving its ability to regulate system power fluctuations.

[0046] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A medium-voltage chain energy storage system topology, characterized in that, It includes a multi-winding transformer, a three-phase parallel-connected STATCOM, a three-phase series-connected STATCOM, N DC capacitors, and N energy storage devices, where N is a positive integer; the multi-winding transformer includes a secondary winding L ac2 L bc2 L cc2 L a0 L b0 L c0 And N primary windings; L secondary windings ac2 L bc2 L cc2 Connected in series in a three-phase power grid line, the secondary winding L a0 L b0 L c0 Connect the triangles; The three-phase parallel side-chain STATCOM consists of N cascaded H-bridge modules, each H-bridge module including four switching transistors; the input of the first H-bridge module is connected to the grid input, and the output of the Nth H-bridge module is connected to the three-phase common coupling point; the input of the m-th H-bridge module is connected to the output of the (m-1)-th H-bridge module, and the output of the m-th H-bridge module is connected to the input of the (m+1)-th H-bridge module, where m is a positive integer and 1 < m < N; A three-phase series-connected side-chain STATCOM includes N H-bridge modules, each H-bridge module includes four switching transistors, and each H-bridge module is connected to the corresponding primary winding. The H-bridge module of a three-phase parallel side-chain STATCOM has the same structure as the H-bridge module of a three-phase series side-chain STATCOM. Each H-bridge module shares the corresponding DC capacitor and energy storage device on the DC side.

2. The medium-voltage chain energy storage system topology according to claim 1, characterized in that, It also includes three-phase reactors; the input of the first H-bridge module of the three-phase parallel side-chain STATCOM is connected to the grid input via the three-phase reactors.

3. A control method for a medium-voltage chained energy storage system topology as described in claim 1 or 2, characterized in that, include: The three-phase parallel side-chain STATCOM and the three-phase series side-chain STATCOM are independently controlled in a grid configuration. The grid configuration control uses the energy of a DC capacitor to simulate the rotor energy of a synchronous generator, realizes DC voltage control and synchronization with the H-bridge module, establishes a matching relationship between the DC voltage and the system angular frequency, and thus realizes the grid configuration operation of the three-phase parallel side-chain STATCOM and the three-phase series side-chain STATCOM.

4. The control method for the topology of the medium-voltage chain energy storage system according to claim 3, characterized in that, The three-phase parallel side-chain STATCOM adopts a dual-loop control of positive and negative sequence voltage and current, which includes: DC voltage control, positive sequence decoupling control, negative sequence decoupling control and output control.

5. The control method for the topology of the medium-voltage chain energy storage system according to claim 4, characterized in that, The DC voltage control includes: setting the reference value U... dc_ref Subtract the three-phase DC voltage U dc_a U dc_b U dc_c The difference is obtained by averaging the values, and this difference is used by the PI controller to adjust the output reference d-axis positive sequence current command i. d +* .

6. The control method for the topology of a medium-voltage chain energy storage system according to claim 5, characterized in that, The positive-sequence decoupling control includes: referencing the d-axis positive-sequence current command i d +* With the sampling d-axis positive sequence current i d + The difference, reference q-axis positive sequence current command i q +* With the sampling q-axis positive sequence current i q + The difference is processed by a PI controller to transform the output signal from the dq coordinate system to the abc coordinate system. Furthermore, the d-axis component of the three-phase parallel-chain STATCOM output voltage is inverted and then sampled at the positive-sequence d-axis voltage e is added. d + And add ωLi q + The q-axis component of the output voltage of the three-phase parallel-connected side-chain STATCOM is inverted and then sampled with the positive-sequence q-axis voltage e. q + And subtract ωLi d + Then, after the inverse Park transform, the positive sequence modulation voltage is obtained; where L is the connecting inductor and ω is the system angular frequency.

7. The control method for the topology of a medium-voltage chain energy storage system according to claim 6, characterized in that, The negative sequence decoupling control includes: referencing the d-axis negative sequence current command i d -* With the sampled d-axis negative sequence current i d - The difference, reference q-axis negative sequence current command i q -* With the sampling q-axis negative sequence current i q - The difference is transformed from the dq coordinate system to the abc coordinate system by the PI controller, and the d-axis component of the three-phase parallel-chain STATCOM output voltage is inverted and subtracted. ωLi q + The q-axis component of the output voltage of the three-phase parallel-connected side-chain STATCOM is reversed and then applied. ωLi d + Then, after the inverse Park transformation, the negative sequence modulation voltage is obtained.

8. The control method for the topology of the medium-voltage chain energy storage system according to claim 7, characterized in that, The output control includes: superimposing the positive-sequence modulation voltage and the negative-sequence modulation voltage to obtain the modulation voltage. After phase-shift carrier sinusoidal pulse modulation, the switching signals of the switching transistors of the H-bridge module of the three-phase parallel side-chain STATCOM are obtained.

9. The control method for the topology of the medium-voltage chain energy storage system according to claim 3, characterized in that, The three-phase series-connected side-chain STATCOM uses phase-to-phase DC voltage equalization control by injecting zero-sequence voltage. By injecting zero-sequence voltage, the active power between the three phases is redistributed, thereby achieving the balance of the three-phase DC voltage.

10. The control method for the topology of a medium-voltage chain energy storage system according to claim 9, characterized in that, The phase-to-phase DC voltage equalization control includes: Assuming both negative sequence voltage and negative sequence current are zero, and only considering the injection of zero sequence voltage, the secondary winding L of the delta connection... a0 L b0 L c0 As a zero-sequence component path, it will generate zero-sequence current, resulting in the following three-phase active power output from the three-phase series-connected STATCOM: (6) In the formula, U p U0 and U0 represent the effective values ​​of the positive-sequence and zero-sequence components of the output voltage, respectively; I p I0 and I0 represent the effective values ​​of the positive-sequence and zero-sequence components of the output current, respectively; with the angle of the positive-sequence voltage as the reference direction, i.e., the positive-sequence voltage U0 p The initial phase is 0, and θ0 represents the initial phase of the zero-sequence voltage output by the three-phase series-connected STATCOM. These represent the initial phases of the positive-sequence current and zero-sequence current output by a three-phase series-connected STATCOM, respectively. ω P is the system angular frequency; P is the active power. A0 P B0 P C0 This is called zero-order cluster active power, and the zero-order cluster active power satisfies: (7) The magnitude and phase of the injected zero-sequence voltage are: (8) (9)。

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