Medium-voltage direct-current energy storage system and control method

Abstract

The invention provides a medium-voltage direct-current energy storage system and a control method. The medium-voltage direct-current energy storage system is composed of a plurality of modules. Each module is provided with a plurality of energy storage ports and a grid-connected port. The energy storage port of the module is connected with a plurality of energy storage units, and the grid-connected port is connected in series with the medium-voltage DC bus. Each module comprises a plurality of branches, and each branch is formed by connecting four power switch devices in series. The upper end points of the branches are mutually connected to serve as module output anodes. The lower end points of the branches are mutually connected to serve as output cathodes. The middle connection point of the branch is connected with the anode of the energy storage battery unit through an inductor and is connected with the cathode of the energy storage unit. The middle connection points are mutually connected to the middle point of the output capacitor. The energy storage system adopts a double-layer staggered modulation method. The first layer is interlaced control among modules, and the second layer is interlaced control among multiple branches in the modules. The control strategy of balanced charging and discharging of the energy storage battery is considered, and the problem that the residual electric quantity of the energy storage battery is unbalanced in the operation process is avoided.

Description

Technical Field

[0001] The present invention belongs to the field of electric power generation, and in particular relates to a medium voltage direct current energy storage system and a control method. Background Art

[0002] With the development of power electronics technology, medium-voltage DC systems have been gradually applied in power generation, transmission, and power consumption due to their high efficiency, good stability, and high power quality. The main application areas of medium-voltage DC systems include large-scale renewable energy power generation bases such as Shagohuang Base, offshore wind power and photovoltaic power generation systems, and regional medium-voltage DC distribution network systems such as industrial parks. At present, the development of medium-voltage DC equipment such as ±30kV photovoltaic medium-voltage DC converters and 10kV medium-voltage DC transformers has been completed and demonstrated. As an important part of the medium-voltage DC system, large-capacity energy storage has the function of smoothing power fluctuations in renewable energy systems such as photovoltaics and stabilizing the medium-voltage DC bus. In the medium-voltage DC system, energy storage is incorporated into the medium-voltage DC bus, and the voltage level needs to reach tens to hundreds of kilovolts, and the capacity needs to reach tens to hundreds of megavolts. The DC transformer solution requires multiple conversion links such as high-frequency inversion, high-frequency transformer, and high-frequency rectification. The equipment cost is high, the efficiency is low, the volume is large, and the reliability is low. It is necessary to develop a high-power and low-cost medium-voltage DC energy storage system.

[0003] Chinese invention application CN115663864 discloses a DC cascade energy storage system, which is composed of n full-bridge DC / DC power modules. However, the DC / DC modules of this technology can be connected to low input and output voltage levels. The system is connected to the medium-voltage DC bus through a first-stage step-down and a first-stage step-up. Specifically, the energy storage unit is first stepped down by DC / DC, and then the output of multiple groups of modules is connected in series to achieve step-up and connected to the medium-voltage DC bus, resulting in low output voltage levels of the energy storage unit and the module. The system requires a large number of modules to be connected in series to meet the medium-voltage DC grid-connected requirements, which increases the system cost and failure rate. At present, the voltage of large-capacity energy storage will reach 1.5kV or even higher, so the use of this technology requires high-voltage switching tubes, and the equipment cost is high. At the same time, due to the inconsistent parameters of the energy storage units connected to each port of the topology, it is easy to cause uneven power of each module, affecting the stable operation of the DC converter. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a medium-voltage DC energy storage system and a control method. The system has a modular structure and realizes energy storage and incorporation into the medium-voltage DC bus through two-stage boosting. The module topology in the system has a high boost ratio, which can realize high boost ratio conversion of energy storage to the grid-connected side. The module is a multi-port structure, which provides multiple interfaces for energy storage series modules. Through the series connection of high-power modules, the system capacity can reach more than 100 megawatts. At the same time, the energy storage system adopts a double-layer interleaved modulation method to reduce the grid-connected current ripple and reduce the high-voltage grid-connected filter inductance. The first layer is interleaved control between modules, and the second layer is interleaved control between multiple branches within the module. The present invention proposes a module balancing control technology for a medium-voltage DC energy storage system that takes into account the remaining capacity of the energy storage battery to solve the problem of inconsistent remaining capacity of energy storage units caused by inconsistent output characteristics of each energy storage unit.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] A medium-voltage direct current energy storage system adopts a modular solution and is composed of a first module to an nth module; each module has multiple energy storage ports and a grid-connected port; the energy storage port of the module is connected to multiple energy storage units, and the grid-connected port is connected in series to a medium-voltage direct current bus; the voltage of the energy storage port is a low-voltage direct current; when the energy storage unit is discharged, the energy storage unit realizes a boost conversion through a module circuit, and realizes a secondary boost through the series connection of the grid-connected ports of multiple modules, and finally is boosted to a medium-voltage direct current and connected to a medium-voltage direct current bus; when the energy storage unit is charged, the voltage is stepped down from the medium-voltage direct current bus to charge the energy storage unit.

[0007] Furthermore, the module comprises m branches, wherein the jth branch is formed by connecting power switch devices Qij1 to Qij4 in series, wherein i is the module number and j is the branch number; the upper end points of branches Ai1, Ai2, ...Aim are connected to each other as the module output positive electrode Mi; the lower end points of branches Ei1, Ei2, ...Eim are connected to each other as the output negative electrode Ni; the middle connection point Bij of the branch is connected to the positive electrode of the energy storage battery unit BATij through the inductor Lij, and the middle connection point Dij is connected to the negative electrode of the energy storage battery unit BATij; the middle connection points Cij are connected to each other and connected to the midpoint Oi of the output capacitor; capacitor CAPi1 and capacitor CAPi2 are connected in series, with their positive poles connected to the module positive pole Mi, and their negative poles connected to the module negative pole Ni.

[0008] Furthermore, the medium voltage DC energy storage system adopts a double-layer interleaved modulation method, the first layer is interleaved control between modules, and the second layer is interleaved control between multiple branches within the module.

[0009] Furthermore, when the medium voltage DC energy storage system includes n modules, the reference phases of the first module to the nth module differ by T / n respectively, where T is the switching period of each switching device in the module, thereby realizing the first layer of inter-module staggered control.

[0010] Furthermore, the multiple branches in the second layer module are interleaved and controlled, and the phase of each branch of the module is shifted back by T / m based on the reference phase of the module, where m is the total number of branches in the module, thereby achieving interleaved modulation between modules and branches.

[0011] Furthermore, by clamping the midpoints of the output capacitors CAPi1 and CAPi2, i is the module number, so that the maximum voltage borne by each switch tube of the module is 50% of the grid-connected side voltage of the module.

[0012] The present invention also provides a control method for a medium voltage DC energy storage system, which adopts a dual closed loop method, wherein the outer loop is an output voltage loop or an output current loop according to actual needs, and the inner loop is a battery current loop, and comprises the following steps:

[0013] Step 1: In constant voltage grid-connected mode, the grid-connected voltage command V o_ref Subtract the instantaneous value of the grid voltage v o , the energy storage system current command I is obtained through the first regulator ref In the constant current grid-connected mode, the grid-connected current command I o_ref Subtract the instantaneous value of the grid voltage i o , the energy storage system current command I is obtained through the second regulator ref ; According to the actual required operation mode, select the operation mode and use the corresponding current command as the output command of the mode selector;

[0014] Step 2: Calculate the remaining power of each energy storage battery unit and sum them up to get the total remaining power of the medium voltage DC energy storage system. , where SOC_modij represents the remaining capacity of each energy storage battery, i is the module number, j is the branch number, the total remaining capacity is multiplied by the inverse of the number of modules 1 / n, n is the number of modules, and the average value of the remaining capacity of the module SOC_ ave ; Set SOC_ ave Subtract the remaining power of module i , through the 3_i regulator, the module remaining power current command I is obtained ref_socmodi ;

[0015] Step 3: Set the energy storage system current command I ref Subtract the module remaining power current command I of module i ref_modsoci Get the final current instruction I of module i ref_modi , where i is the module number;

[0016] Step 4: Set the module final current instruction I ref_modi And the remaining power SOC_ of each branch energy storage battery of module i modi1 、SOC_ modi2 ...SOC_modim , the current command I of each branch of module i is obtained through the 4_i regulator ref_bati1 ,I ref_bati2 ……I ref_batim , where i is the module number and m is the number of branches in the module;

[0017] Step 5: Set the current command I of each branch of module i ref_bati1 ,I ref_bati2 ……I ref_batim Subtract the energy storage port sampling current I of the corresponding branch ref_i1 ,I ref_i2 ……I ref_im , through the 5th i1 Regulator, No. 5_ i2 Regulator... No. 5_ im Regulator, get the duty cycle D_ of each branch of module i i1 ,D_ i2 ...D_ im ;

[0018] Step 6: The reference phase from the 1st module to the nth module is shifted back by T / n. The phase of each branch of the module is shifted back by T / m based on the reference phase of the module, where m is the total number of branches in the module, thereby achieving interleaved modulation between modules and branches to form a triangular wave of each branch;

[0019] Step 7: Duty cycle D_ of each branch of module i i1 ,D_ i2 , ...D_ im The triangular waves of each branch are compared through the i1th comparator, the i2th comparator, ... the imth comparator to obtain the PWM signals PWM_i1, PWM_i2, ... PWM_im of each branch, and the switch tube of the control module is turned on and off.

[0020] Beneficial effects:

[0021] (1) The system of the present invention has a modular structure, and realizes energy storage and incorporation into the medium-voltage DC bus through two-stage boosting. The DC conversion module has a high boosting ratio. The first stage realizes DC boosting of the energy storage unit through the module; the second stage realizes secondary DC boosting through the output series connection of the DC conversion module. Therefore, the system has a high boosting ratio. Under the same DC bus voltage, the number of modules in series can be effectively reduced, thereby improving system reliability and reducing costs.

[0022] (2) The DC boost module of the present invention realizes high boost ratio conversion from energy storage to the grid-connected side. The module topology can withstand high voltage levels on the input and output sides through inductive boost and capacitor clamping, thereby increasing the access voltage of the energy storage battery.

[0023] (3) The module of the present invention has a multi-port structure, which provides multiple interfaces for the energy storage series module, avoiding the energy storage circulation problem caused by the energy storage unit being connected in parallel before being connected to the DC conversion module; by connecting high-power modules in series, the system capacity can reach more than 100 megawatts.

[0024] (4) The inter-module and intra-module branches of the present invention adopt an interleaved modulation method to effectively reduce the grid-connected current ripple and the high-voltage filter inductance on the grid-connected side.

[0025] (5) The present invention proposes a module balancing control technology for a medium voltage DC energy storage system that takes into account the remaining capacity of the energy storage battery, thereby solving the problem of inconsistent remaining capacity of energy storage units caused by inconsistent output characteristics of each energy storage unit, and effectively improving the reliability and availability of the energy storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a structural block diagram of the medium voltage DC energy storage system of the present invention;

[0027] Figure 2 This is a modulation waveform diagram of the medium voltage DC energy storage system module of the present invention;

[0028] Figure 3 The figure is a flow chart of a control method of a medium voltage DC energy storage system of the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0030] like Figure 1 As shown, the medium-voltage DC energy storage system of the embodiment of the present invention adopts a modular solution, which is composed of the 1st module to the nth module. Each module has multiple energy storage ports and a grid-connected port. The energy storage port of the module is connected to multiple energy storage units, and the grid-connected port is connected in series to the medium-voltage DC bus. The voltage of the energy storage port is low-voltage DC, and the access voltage of the low-voltage DC port can be from hundreds of volts to thousands of volts. When the energy storage unit is discharged, the energy storage unit realizes a boost conversion through the module circuit, and realizes a secondary boost through the series connection of the grid-connected ports of multiple modules, and finally it is upgraded to medium-voltage DC and connected to the medium-voltage DC bus. The medium-voltage DC bus voltage is generally tens of kilovolts. Conversely, when the energy storage unit is charged, the voltage is stepped down from the medium-voltage DC bus to the low voltage to charge the energy storage unit.

[0031] Below Figure 1The first module in the example is used to illustrate the composition of the modules in the medium-voltage DC energy storage system. The first module includes m branches, of which the i-th branch is composed of power switch devices Q1i1~power switch devices Q1i4 in series, where i is an integer from 1 to m. The upper and lower ends of the branches A11, A12, ...A1m are connected to each other as the module output positive electrode M1. The lower ends of the branches E11, E12, ...E1m are connected to each other as the output negative electrode N1. The middle connection point B1i of the branch is connected to the positive electrode of the energy storage battery unit BAT1i through the inductor L1i, and the middle connection point D1i is connected to the negative electrode of the energy storage battery unit BAT1i. The middle connection points C1i are connected to each other and connected to the midpoint O1 of the output capacitor. Capacitors CAP11 and CAP12 are connected in series, with their positive poles connected to the module positive pole M1 and their negative poles connected to the module negative pole N1.

[0032] The medium-voltage DC energy storage system adopts a two-layer interleaved modulation method to reduce the grid-connected current ripple and the high-voltage grid-connected filter inductance. The first layer is interleaved control between modules, and the second layer is interleaved control between multiple branches within the module. When the medium-voltage DC energy storage system contains n modules, the reference phases of the first module to the nth module differ by T / n, where n is the switching cycle, thereby realizing the first layer of interleaved control between modules, that is, the duty cycle is generated through the control loop, and the modules are turned on by interleaved control. If the total number of modules is n, the reference phase difference of each module is set to T / n, thereby realizing interleaved control between modules.

[0033] The interleaving control between multiple branches in the second layer module is as follows: Figure 2 shown.

[0034] Figure 2 It is the modulation waveform in the boost discharge mode taking the first module including 3 branches as an example. In the discharge stage of the energy storage unit, the module operates in the boost mode, and the power switch device Q112, the power switch device Q122 and the power switch device Q132 are turned on alternately for T / 3 time. The duty cycle of the two middle switches of each branch is the same, that is, the power switch device Q112 and the power switch device Q113, the power switch device Q122 and the power switch device Q123, the power switch device Q132 and the power switch device Q133 have the same duty cycle. The power switch device Q113, the power switch device Q123 and the power switch device Q133 are turned on alternately with the power switch device Q112, the power switch device Q122 and the power switch device Q132 with a difference of T / 2, and the other switch tubes are always in the off state during the discharge stage. In the charging stage of the energy storage unit, the module operates in the buck mode.

[0035] The charging mode is opposite to the discharging mode. The power switch devices Q112, Q122, Q132, Q113, Q123 and Q133 are always off, and the power switch devices Q111, Q121 and Q131 are turned on alternately for T / 3 time. The upper and lower switch tubes of each branch have the same duty cycle, that is, the power switch devices Q111 and Q114, the power switch devices Q121 and Q124, and the power switch devices Q131 and Q134 have the same duty cycle. The power switch devices Q111, Q121 and Q131 are turned on alternately with the power switch devices Q114, Q124 and Q134 with a difference of T / 2.

[0036] By clamping the midpoints of the output capacitors CAP11 and CAP12, the maximum voltage borne by each switch tube of the first module is 50% of the voltage on the grid-connected side of the module, thereby reducing the voltage stress of the switch tube. In Chinese patent application CN115663864, the voltage stress of the module switch tube is the voltage on the grid-connected side of the module. Therefore, although the same switching devices are used, the input voltage and output voltage of the DC conversion module in the present invention are greatly improved compared with existing solutions such as CN115663864, thereby reducing the number of modules used in the energy storage system, increasing the voltage levels on the energy storage side and the high-voltage side, and improving the reliability and adaptability of the system.

[0037] The medium voltage DC energy storage system in the present invention is composed of multiple energy storage units and modules connected in series. Due to the inconsistency between the module hardware parameters and the energy storage battery parameters, the residual charge of the energy storage battery units will be inconsistent during long-term operation, which directly affects the system's operating performance and the life of the energy storage unit. In response to this problem, the present invention also proposes a control strategy that takes into account the balanced charging and discharging of the energy storage battery to avoid the problem of uneven residual charge of the energy storage battery during operation.

[0038] like Figure 3 As shown, the control method of the medium voltage DC energy storage system of the present invention adopts a double closed loop mode, the outer loop can be an output voltage loop or an output current loop according to actual needs, and the inner loop is a battery current loop, which specifically includes the following steps:

[0039] Step 1: In constant voltage grid-connected mode, the grid-connected voltage command V o_ref Subtract the instantaneous value of the grid voltage v o , the energy storage system current command I is obtained through the first regulator ref In the constant current grid-connected mode, the grid-connected current command I o_ref Subtract the instantaneous value of the grid voltage i o, the energy storage system current command I is obtained through the second regulator ref ; According to the actual required operation mode, select the operation mode and use the corresponding current command as the output command of the mode selector;

[0040] Step 2: Calculate the remaining power of each energy storage battery unit and sum them up to get the total remaining power of the medium voltage DC energy storage system. , where SOC_modij represents the remaining capacity of each energy storage battery, i is the module number, and j is the branch number. The total remaining capacity is multiplied by the inverse of the number of modules 1 / n, where n is the number of modules, to obtain the average value of the module remaining capacity SOC_ ave . Set SOC_ ave Subtract the remaining power of module i , through the 3_i regulator, the module remaining power current command I is obtained ref_socmodi ;

[0041] Step 3: Set the energy storage system current command I ref Subtract the module remaining power current command I of module i ref_modsoci Get the final current instruction I of module i ref_modi , where i is the module number;

[0042] Step 4: Set the module final current instruction I ref_modi And the remaining power SOC_ of each branch energy storage battery of module i modi1 、SOC_ modi2 ...SOC_ modim , the current command I of each branch of module i is obtained through the 4_i regulator ref_bati1 ,I ref_bati2 ……I ref_batim , where i is the module number and m is the number of branches in the module.

[0043] Step 5: Set the current command I of each branch of module i ref_bati1 ,I ref_bati2 ……I ref_batim Subtract the energy storage port sampling current I of the corresponding branch ref_i1 ,I ref_i2 ……I ref_im , through the 5th i1 Regulator, No. 5_ i2 Regulator... No. 5_ im Regulator, get the duty cycle D_ of each branch of module i i1 ,D_ i2 ...D_ im ;

[0044] Step 6: The reference phase from the 1st module to the nth module is shifted back by T / n. The phase of each branch of the module is shifted back by T / m based on the reference phase of the module, where m is the total number of branches in the module, thereby achieving interleaved modulation between modules and branches to form a triangular wave of each branch;

[0045] Step 7: Duty cycle D_ of each branch of module i i1 ,D_ i2 , ...D_ im The triangular waves of each branch are compared through the i1th comparator, the i2th comparator, ... the imth comparator to obtain the PWM signals PWM_i1, PWM_i2, ... PWM_im of each branch, and the switch tube of the control module is turned on and off.

[0046] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A medium voltage DC energy storage system, characterized in that: A modular solution is adopted, consisting of the 1st module to the nth module; each module has multiple energy storage ports and a grid-connected port; the energy storage port of the module is connected to multiple energy storage units, and the grid-connected port is connected in series to the medium-voltage DC bus; the voltage of the energy storage port is low-voltage DC; when the energy storage unit is discharged, the energy storage unit realizes a boost conversion through the module circuit, and realizes a secondary boost through the series connection of the grid-connected ports of multiple modules, and finally boosts it to medium-voltage DC and connects it to the medium-voltage DC bus; when the energy storage unit is charged, the voltage is stepped down from the medium-voltage DC bus to low voltage to charge the energy storage unit.

2. A medium voltage DC energy storage system according to claim 1, characterized in that: The module comprises m branches, wherein the jth branch is formed by connecting power switch devices Qij1 to Qij4 in series, wherein i is the module number and j is the branch number; the upper end points of branches Ai1, Ai2, ...Aim are connected to each other as the output positive electrode Mi of module i; the lower end points of branches Ei1, Ei2, ...Eim are connected to each other as the output negative electrode Ni; the middle connection point Bij of the branch is connected to the positive electrode of the energy storage battery unit BATij through the inductor Lij, and the middle connection point Dij is connected to the negative electrode of the energy storage battery unit BATij; the middle connection points Cij are connected to each other and connected to the midpoint Oi of the output capacitor; the capacitor CAPi1 and the capacitor CAPi2 are connected in series, with their positive electrodes connected to the module positive electrode Mi and their negative electrodes connected to the module negative electrode Ni.

3. A medium voltage DC energy storage system according to claim 2, characterized in that: The medium voltage DC energy storage system adopts a two-layer interleaved modulation method. The first layer is the interleaved control between modules, and the second layer is the interleaved control between multiple branches within the module.

4. A medium voltage DC energy storage system according to claim 3, characterized in that: When the medium voltage DC energy storage system includes n modules, the reference phases of the first module to the nth module differ by T / n, where T is the switching period of each switching device in the module, thereby realizing the first layer of inter-module staggered control.

5. A medium voltage DC energy storage system according to claim 3, characterized in that: The second layer implements interleaving control between multiple branches within a module. The phase of each branch of the module is shifted back by T / m based on the reference phase of the module, where m is the total number of branches in the module, thereby achieving interleaving modulation between modules and branches.

6. A medium voltage DC energy storage system according to claim 2, characterized in that: By clamping the midpoint of the output capacitor CAPi1 and the capacitor CAPi2, i is the module number, so that the maximum voltage that each switch tube of the module can withstand is 50% of the grid-connected side voltage of the module.

7. A control method for a medium voltage DC energy storage system, characterized in that: A dual closed loop method is adopted, the outer loop is the output voltage loop or the output current loop according to actual needs, and the inner loop is the battery current loop, including the following steps: Step 1: In constant voltage grid-connected mode, the grid-connected voltage command V o_ref Subtract the instantaneous value of the grid voltage v o , the energy storage system current command I is obtained through the first regulator ref , in constant current grid-connected mode, the grid-connected current command I o_ref Subtract the instantaneous value of the grid voltage i o , the energy storage system current command I is obtained through the second regulator ref ; According to the actual required operation mode, select the operation mode and use the corresponding current command as the output command of the mode selector; Step 2: Calculate the remaining power of each energy storage battery unit and sum them up to get the total remaining power of the medium voltage DC energy storage system. , where SOC_modij represents the remaining capacity of the energy storage battery, i is the module number, j is the branch number, the total remaining capacity is multiplied by the inverse of the number of modules 1 / n, n is the number of modules, and the average value of the remaining capacity of the module SOC_ ave ; Set SOC_ ave Subtract the remaining power of module i , through the 3_i regulator, the module i remaining power current command I is obtained ref_socmodi ; Step 3: Set the energy storage system current command I ref Subtract the module remaining power current command I of module i ref_modsoci Get the final current instruction I of module i ref_modi , where i is the module number; Step 4: Set the module final current instruction I ref_modi And the remaining power SOC_ of each branch energy storage battery of module i modi1 、SOC_ modi2 ...SOC_ modim , the current command I of each branch of module i is obtained through the 4_i regulator ref_bati1 ,I ref_bati2 ……I ref_batim , where i is the module number and m is the number of branches in the module; Step 5: Set the current command I of each branch of module i ref_bati1 ,I ref_bati2 ……I ref_batim Subtract the energy storage port sampling current I of the corresponding branch ref_i1 ,I ref_i2 ……I ref_im , through the 5th i1 Regulator, No. 5_ i2 Regulator... No. 5_ im Regulator, get the duty cycle D_ of each branch of module i i1 ,D_ i2 ...D_ im ; Step 6: The reference phase from the first module to the nth module is shifted back by T / n, and the phase of each branch of the module is shifted back by T / m based on the reference phase of the module, where m is the total number of branches of the module, thereby realizing inter-module and inter-branch intermodulation and forming a triangular wave of each branch; Step 7: Duty cycle D_ of each branch of module i i1 ,D_ i2 , ...D_ im The triangular waves of each branch are compared through the i1th comparator, the i2th comparator, ... the imth comparator to obtain the PWM signals PWM_i1, PWM_i2, ... PWM_im of each branch, and the switch tube of the control module is turned on and off.

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