A reconfigurable battery energy storage and off-grid system and method

Abstract

The present invention proposes a reconfigurable battery energy storage grid-connected and off-grid system, which includes: a reconfigurable battery energy storage system, a DC-DC conversion circuit, and an inverter; the reconfigurable battery energy storage system is connected to the DC side of the inverter through a DC-DC circuit; a reconfigurable battery energy storage grid-connected and off-grid method, which includes: 1) performing PI constant current discharge control on the DC-DC conversion circuit designed between the reconfigurable battery energy storage system and the DC side of the inverter; 2) controlling the inverter using a VSG control strategy; 3) performing seamless switching control of the reconfigurable battery energy storage system from off-grid to grid-connected using a grid-connected seamless switching control strategy; 4) performing seamless switching control of the reconfigurable battery energy storage system from grid-connected to off-grid using an off-grid seamless switching control strategy; the present invention realizes smooth switching between off-grid operation mode and grid-connected operation mode in a reconfigurable battery energy storage system.

Description

Technical Field

[0001] The invention relates to a reconfigurable battery energy storage and off-grid system and method, belonging to the technical field of power energy storage. Background Art

[0002] Distributed energy storage has two typical operating modes: off-grid and grid-connected. Smooth switching between modes is an important guarantee for the stable operation of distributed energy storage. Since the control objects change before and after off-grid to grid-connected, the frequency, amplitude and phase of the inverter output voltage are inconsistent, which also leads to a large impact current in the process of switching from off-grid to grid-connected, affecting the stable operation of the energy storage system. Therefore, how to reduce the impact current in the on-grid / off-grid switching process is an urgent problem to be solved.

[0003] With the rapid development of electrochemical energy storage, the inherent safety of energy storage systems has become increasingly prominent. The "short board effect" of the battery system brought about by the fixed series-parallel mode of traditional battery energy storage systems is the essential reason for the safety and economic problems of current electrochemical energy storage systems. In recent years, reconfigurable battery energy storage systems have received widespread attention. Each battery pack in the system is deeply coupled with a control switch, which can quickly diagnose and remove faulty batteries while maintaining the normal operation of electrochemical energy storage, greatly improving the availability and reliability of the system, and providing a new path for the safety and economy of electrochemical energy storage systems. During the on-grid / off-grid switching process of the reconfigurable battery energy storage system, the on-off time nodes of the battery control switch have a certain impact on the stability of the on-grid / off-grid switching process. It is urgent to study the timing coordination of the control switch and the physical state switching of the on-grid / off-grid switch of the reconfigurable battery energy storage system during the on-grid / off-grid switching process. Summary of the invention

[0004] The present invention proposes a reconfigurable battery energy storage and off-grid system and method, the purpose of which is to achieve smooth switching between an off-grid operation mode and a grid-connected operation mode in a reconfigurable battery energy storage system.

[0005] The technical solution of the present invention is a reconfigurable battery energy storage and off-grid system, which includes: a reconfigurable battery energy storage system, a DC-DC conversion circuit, and an inverter; the reconfigurable battery energy storage system is connected to the DC side of the inverter through a DC-DC circuit.

[0006] Furthermore, the reconfigurable battery energy storage system includes a reconfigurable battery network; the reconfigurable battery network includes a plurality of battery packs, the plurality of battery packs in the reconfigurable battery network are distributed in N parallel and M series, and each battery pack is connected to a control switch S ij Connect in series to form a series unit, i≤M, j≤N, S ij Represents the number of each battery pack and the control switch connected in series with the battery pack. Every N series units connected in parallel are connected with a bypass switch Si The parallel connection forms a parallel unit, and M such parallel units are connected in series to form a reconfigurable battery network.

[0007] Furthermore, the DC-DC conversion circuit includes an inductor L bat , resistor R bat , capacitor V dc , the first switch tube K1, the first switch tube K2; due to the discharge process, the terminal voltage V of the battery energy storage system can be reconstructed bat will change, so the reconfigurable battery energy storage system adopts constant current discharge and real-time detection of the output current I of the reconfigurable battery energy storage system bat , the output current I of the reconfigurable battery energy storage system bat and the expected discharge current I* bat The current difference is obtained by subtracting the current difference, and the obtained current difference generates a PWM modulation signal through a PI controller and acts on the first switch tube K1 and the first switch tube K2 to perform PI constant current discharge control to ensure that the reconfigurable battery energy storage system performs constant current discharge on the DC side of the inverter after passing through the DC-DC conversion circuit.

[0008] A reconfigurable battery energy storage and off-grid method, the method comprising:

[0009] 1) Perform PI constant current discharge control on the DC-DC conversion circuit designed between the reconfigurable battery energy storage system and the DC side of the inverter to ensure that the reconfigurable battery energy storage system discharges the inverter at a constant current;

[0010] 2) The inverter is controlled using the VSG control strategy;

[0011] 3) Use the grid-connected seamless switching control strategy to perform seamless switching control of the reconfigurable battery energy storage system from off-grid to grid-connected;

[0012] 4) Use the off-grid seamless switching control strategy to perform seamless switching control of the reconfigurable battery energy storage system from grid-connected to off-grid.

[0013] Furthermore, the inverter is controlled by adopting a VSG control strategy, specifically including:

[0014] 2-1) Real-time detection of the output three-phase voltage e before the inverter filter abc , the output three-phase voltage u after inverter filtering abc 、Current before filtering i Labc And the filtered current i abc ; The output three-phase voltage e before the inverter is filtered abc Perform dq coordinate transformation to obtain e d 、e q , the output three-phase voltage u after inverter filtering abcPerform dq coordinate transformation to obtain u d 、u q , the current before filtering i Labc Perform dq coordinate transformation to obtain i Ld 、i Lq , the filtered current i abc Perform dq coordinate transformation to obtain i d 、i q ;

[0015] 2-2) For u d 、u q 、i d 、i q The power calculation is performed to obtain the instantaneous active power P and instantaneous reactive power Q output by the inverter. The power calculation formula is as follows:

[0016]

[0017] 2-3): In order to simulate the frequency modulation characteristics of the synchronous generator, a speed regulator is applied in VSG control. The droop characteristics between the active power and frequency of the speed regulator are expressed by formula (2):

[0018] P m =P ref +k p (ω0-ω) (2);

[0019] Where P m Represents the mechanical power of the synchronous generator; P ref is the reference active power; k p is the active power-frequency droop coefficient; ω0 is the grid synchronization angular velocity; ω is the mechanical angular velocity of the virtual synchronous generator when off-grid;

[0020] The rotor motion equation of the synchronous generator is simulated as shown in formula (3):

[0021]

[0022] Where, J is the moment of inertia; D is the damping torque; P e represents electromagnetic power;

[0023] Substituting formula (3) into formula (2), we can obtain:

[0024]

[0025] Transform formula (4) into Laplace:

[0026]

[0027] 2-4) Use formula (6) to perform QE excitation control, and use QE excitation control to adjust the virtual potential E of the virtual synchronous generator to adjust the terminal voltage and reactive power of the inverter:

[0028]

[0029] Where Q is the instantaneous reactive power value output by the inverter after filtering, Q ref is the reactive power instruction of the inverter, U R is the command value of the inverter terminal voltage effective value, U abc is the real value of the inverter terminal voltage, k q is the reactive power-voltage droop coefficient, k i is the integral coefficient, E0 is the no-load potential of the virtual synchronous generator, which represents the terminal voltage when the inverter is running off-grid without load, E is the virtual potential of the virtual synchronous generator model when off-grid, and s is the Laplace operator;

[0030] 2-5) The PWM modulation signal is obtained through voltage and current double closed-loop control, in which both the voltage outer loop and the current inner loop adopt PI control. The current inner loop and the voltage outer loop equations are shown in equations (7) and (8) respectively:

[0031]

[0032]

[0033] In the formula, R f is the line resistance; C f is the filter capacitor; L is the filter inductor, and L=L f +L g , where L f is the inverter side filter inductor, L g It is the grid-side filter inductor.

[0034] Furthermore, the grid-connected seamless switching control strategy specifically includes:

[0035] 3-1) Constantly detect the frequency f and amplitude |U of the inverter output voltage abc |, phase θ;

[0036] 3-2) When receiving the grid-connected signal, first determine whether the inverter output voltage and the grid voltage meet the conditions shown in formula (9):

[0037]

[0038] In the formula, f g is the frequency of the grid voltage, |U g | is the amplitude of the grid voltage, θ gis the phase of the grid voltage, f is the frequency of the inverter output voltage, |U abc | is the amplitude of the inverter output voltage, θ is the phase of the inverter output voltage, f limit is the set grid-connected voltage frequency threshold, U limit is the set grid-connected voltage amplitude threshold, θ limit is the set grid-connected voltage phase threshold;

[0039] If the conditions shown in formula (9) are met, the grid-connected / off-grid switch is directly closed for grid-connected operation; if the conditions shown in formula (9) are not met, pre-synchronization control is required first, and then the grid-connected / off-grid switch is closed for grid-connected operation after the conditions shown in formula (9) are met;

[0040] 3-3) Set the load required active power to P R , when connected to the grid, the active power transmitted to the grid is P g ; During the process of the reconfigurable battery energy storage system from off-grid to grid-connected, the reconfigurable battery energy storage system is in a constant current discharge state. To meet the output power demand, more battery packs need to be connected in series by closing the control switch to discharge; when connected to the grid, the output power of the reconfigurable battery energy storage system needs to meet the conditions shown in formula (10):

[0041] P R +P g ≤P bat ≤(1+σ)(P R +P g ) (10);

[0042] Where P R is the active power required by the load, P g P is the active power transmitted to the grid when connected to the grid. bat is the output power of the reconfigurable battery energy storage system, σ is the corresponding safety threshold parameter;

[0043] 3-4) Set the expected output active power of the inverter in the VSG control strategy to the output power of the reconfigurable battery energy storage system; the energy storage is a constant current discharge. When the load increases, more battery packs need to be connected in series according to the load demand power requirements. The series connection increases the voltage, and the current remains unchanged, so the output power of the reconfigurable battery energy storage system increases. Here, the expected output power of the inverter is set to the output power of the reconfigurable battery energy storage system;

[0044] 3-5) Set the following three timing coordination conditions between grid-connected switches: the first is that the closing time of the control switch is ahead of the grid-connected / off-grid switch; the second is that the closing time of the control switch is synchronized with the grid-connected / off-grid switch; the third is that the closing time of the control switch lags behind the grid-connected / off-grid switch;

[0045] 3-6) Compare the timing coordination conditions between the above three grid-connected switches. According to the waveform of the three-phase current output by the inverter, compare the current impact size of the three working conditions from off-grid to grid-connected process. According to the waveform of the three-phase voltage and power output by the inverter, compare the smoothness and oscillation amplitude of the three-phase voltage and power output by the inverter from off-grid to grid-connected process in the three working conditions, determine the optimal switch timing coordination condition, and ensure the stable operation of the reconfigurable battery energy storage system from off-grid to grid-connected.

[0046] Furthermore, the pre-synchronization control includes a phase compensation link Δθ and an amplitude compensation link ΔU, specifically including:

[0047] 3-2-1) The difference between the inverter output three-phase voltage and the grid voltage is calculated. The deviation is output as a regulation value ΔU by the PI regulator. ΔU is related to the virtual potential E (E = ΔE) in the off-grid mode. u +ΔE Q +E0) are superimposed to obtain the reference voltage amplitude U ref ;

[0048] 3-2-2) The phase difference between the inverter output three-phase voltage and the grid voltage is made. The deviation is output through the PI regulator to adjust the value Δθ. Δθ is added to the off-grid mode reference voltage phase ωt to form the reference phase θ of the pre-synchronization link. m ;

[0049] 3-2-3) Reference voltage amplitude U ref With reference phase θ m The three-phase reference voltage is obtained by superposition This enables pre-synchronization control of voltage amplitude and phase.

[0050] Furthermore, the closing time of the control switch is ahead of the on-grid / off-grid switch, as shown in formula (11): con ≤tt s , t sw =t p =t+t d (11);

[0051] The control switch closing time is synchronized with the grid-connected / off-grid switch, as shown in formula (12): t con =t sw , t sw =t p =t+t d (12);

[0052] The closing time of the control switch lags behind the on-grid / off-grid switch, as shown in formula (13): t con >t sw , t sw =t p =t+td (13);

[0053] Where, t con is the time point when the control switch is closed; t is the time point when the grid-connected signal is received; t sw is the time point when the on / off-grid switch is closed; t p is the time point when pre-synchronization is completed; t s It is the time for the inverter output power to reach stability after the control switch is closed; t d Adjust the time for pre-synchronization; tt s Indicates that the time point of the grid-connected signal is taken as the reference and the time point of the grid-connected signal is taken as the reference. s Time length, t+t d Indicates the delay t based on the time point of the grid-connected signal d Length of time.

[0054] Furthermore, the off-grid seamless switching control strategy specifically includes:

[0055] 4-1) Always monitor the output power of the inverter and the reconfigurable battery energy storage system;

[0056] 4-2) Set the load required active power to P R , when connected to the grid, the active power transmitted to the grid is P g , at this time, the output power of the reconfigurable battery energy storage system meets the conditions shown in formula (10):

[0057] P R +P g ≤P bat ≤(1+σ)(P R +P g ) (10);

[0058] Where P R is the active power required by the load, P g P is the active power transmitted to the grid when connected to the grid. bat is the output power of the reconfigurable battery energy storage system, σ is the corresponding safety threshold parameter;

[0059] During the process from grid connection to off-grid operation, the reconfigurable battery energy storage system is in a constant current discharge state. To meet the output power demand, the control switch needs to be disconnected to isolate the redundant battery packs. When off-grid, the output power of the reconfigurable battery energy storage system needs to meet the conditions shown in formula (14):

[0060] P R ≤P bat ≤(1+λ)P R (14);

[0061] In the formula, λ is the corresponding safety threshold parameter;

[0062] 4-3) Setting the reference active power of the inverter in the VSG control strategy to the output power of the reconfigurable battery energy storage system;

[0063] 4-4) When receiving the off-grid signal, the on-grid / off-grid switch is disconnected synchronously, and the following three timing coordination conditions between the off-grid switches are set: the first one is that the control switch disconnection time is ahead of the on-grid / off-grid switch; the second one is that the control switch disconnection time is synchronized with the on-grid / off-grid switch; the third one is that the control switch disconnection time lags behind the on-grid / off-grid switch;

[0064] 4-5) Compare the timing coordination conditions between the above three off-grid switches, and compare the current impact size of the three working conditions from off-grid to grid-connected according to the waveform of the three-phase current output by the inverter; compare the smoothness and oscillation amplitude of the three-phase voltage and power output by the inverter from off-grid to grid-connected according to the waveform of the three-phase voltage and power output by the inverter; determine the optimal switch timing coordination condition based on the above method, analyze the oscillation amplitude of the inverter output power, and determine the optimal switch timing coordination condition to ensure the stable operation of the reconfigurable battery energy storage system from grid-connected to off-grid.

[0065] Furthermore, the control switch disconnection time is ahead of the on-grid / off-grid switch, as shown in formula (15): t c <t a =t w (15);

[0066] The control switch disconnection time is synchronized with the on / off grid switch, as shown in formula (16): t c =t a =t w (16);

[0067] The disconnection time of the control switch lags behind that of the on-grid / off-grid switch, as shown in formula (17): t c >t a =t w (17);

[0068] Where, t c is the time point when the control switch is disconnected; t a is the time point when the off-grid signal is received; t w It is the time point when the on / off-grid switch is disconnected; if one time point is smaller than another time point, it means it is before the other time point; if two time points are equal, it means they are at the same moment.

[0069] Beneficial effects of the present invention:

[0070] The present invention realizes smooth switching between an off-grid operation mode and a grid-connected operation mode in a reconfigurable battery energy storage system, is beneficial to the safe and reliable operation of the reconfigurable battery energy storage system, and has good use and promotion effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Attached Figure 1 It is a schematic diagram of the process of the present invention.

[0072] Attached Figure 2 Schematic diagram of the structure of the reconfigurable battery energy storage system.

[0073] Attached Figure 3 It is a schematic diagram of a bidirectional DC-DC conversion circuit.

[0074] Attached Figure 4 This is the constant current control block diagram.

[0075] Attached Figure 5 It is a reconfigurable new energy storage overall architecture.

[0076] Attached Figure 6 This is the pre-synchronization control flow chart.

[0077] Attached Figure 7 This is the overall control block diagram of the reconfigurable new energy storage grid-connected system.

[0078] Attached Figure 8 A schematic diagram of the off-grid to grid-connected switching process in which the closing time of the control switch is ahead of the on-grid / off-grid switch.

[0079] Attached Fig. 9 Schematic diagram of the off-grid to grid switching process for controlling the switch closing time to be synchronized with the grid-connected / off-grid switch.

[0080] Attached Fig.10 A schematic diagram of the off-grid to grid-connected switching process in which the closing time of the control switch lags behind the on-grid / off-grid switch.

[0081] Attached Fig.11 A schematic diagram of the grid-to-off-grid switching process in which the disconnection time of the control switch is ahead of the grid-connected / off-grid switch.

[0082] Attached Fig.12 A schematic diagram of the grid-to-off-grid switching process in which the disconnection time of the control switch is synchronized with the grid-connected / off-grid switch.

[0083] Attached Fig.13 A schematic diagram of the grid-to-off-grid switching process in which the disconnection time of the control switch lags behind that of the grid-connected / off-grid switch. DETAILED DESCRIPTION

[0084] A reconfigurable battery energy storage and off-grid system comprises a reconfigurable battery energy storage system, a DC-DC conversion circuit, and an inverter; the reconfigurable battery energy storage system is connected to the DC side of the inverter via a DC-DC circuit.

[0085] The reconfigurable battery energy storage system includes a reconfigurable battery network; the reconfigurable battery network includes a plurality of battery packs, the plurality of battery packs in the reconfigurable battery network are distributed in N parallel and M series, and each battery pack is connected to a control switch S ij Connect in series to form a series unit, i≤M, j≤N, S ij Represents the number of each battery pack and the control switch connected in series with the battery pack. Every N series units connected in parallel are connected with a bypass switch S i The parallel connection forms a parallel unit, and M such parallel units are connected in series to form a reconfigurable battery network.

[0086] Each battery pack in the reconfigurable battery network is preferably composed of a plurality of single cells connected in parallel, and a single battery pack is used as a minimum control unit to construct a reconfigurable battery network; the control switch S ij A high frequency power electronic switch is preferred.

[0087] The DC-DC conversion circuit includes an inductor L bat , resistor R bat , capacitor V dc , the first switch tube K1, the first switch tube K2; due to the discharge process, the terminal voltage V of the battery energy storage system can be reconstructed bat will change, so the reconfigurable battery energy storage system adopts constant current discharge and real-time detection of the output current I of the reconfigurable battery energy storage system bat , the output current I of the reconfigurable battery energy storage system bat and the expected discharge current I* bat The current difference is obtained by subtracting the current difference, and the obtained current difference generates a PWM modulation signal through a PI controller and acts on the first switch tube K1 and the first switch tube K2 to perform PI constant current discharge control to ensure that the reconfigurable battery energy storage system performs constant current discharge on the DC side of the inverter after passing through the DC-DC conversion circuit.

[0088] A reconfigurable battery energy storage and off-grid method, the method comprising the following steps:

[0089] 1) Perform PI constant current discharge control on the DC-DC conversion circuit designed between the reconfigurable battery energy storage system and the DC side of the inverter to ensure that the reconfigurable battery energy storage system discharges the inverter at a constant current;

[0090] 2) The inverter is controlled using the VSG (virtual synchronous generator) control strategy;

[0091] 3) Use the grid-connected seamless switching control strategy to perform seamless switching control of the reconfigurable battery energy storage system from off-grid to grid-connected;

[0092] 4) Use the off-grid seamless switching control strategy to perform seamless switching control of the reconfigurable battery energy storage system from grid-connected to off-grid.

[0093] The inverter is controlled by adopting the VSG control strategy, which specifically includes the following steps:

[0094] 2-1) Real-time detection of the output three-phase voltage e before the inverter filter abc , the output three-phase voltage u after inverter filtering abc 、Current before filtering i Labc And the filtered current i abc ; The output three-phase voltage e before the inverter is filtered abc Perform dq coordinate transformation to obtain e d 、e q , the output three-phase voltage u after inverter filtering abc Perform dq coordinate transformation to obtain u d 、u q , the current before filtering i Labc Perform dq coordinate transformation to obtain i Ld 、i Lq , the filtered current i abc Perform dq coordinate transformation to obtain i d 、i q The value obtained by the above dq coordinate transformation is used for the voltage and current double closed-loop control in step 2-5; dq coordinate transformation is an existing decoupling control method, which can transform the three-phase winding of the asynchronous motor into an equivalent two-phase winding, and transform the rotating coordinate system into an orthogonal stationary coordinate system, so as to obtain the relationship between voltage and current expressed by DC quantity;

[0095] 2-2) For u obtained in step 2-1 d 、u q 、i d 、i q The power calculation is performed to obtain the instantaneous active power P and instantaneous reactive power Q output by the inverter. The power calculation formula is as follows:

[0096]

[0097] 2-3) In order to simulate the frequency modulation characteristics of synchronous generators, a speed regulator is applied in VSG control. The droop characteristics between the active power and frequency of the speed regulator are expressed by formula (2):

[0098] P m =P ref +k p(ω0-ω) (2);

[0099] Where P m Represents the mechanical power of the synchronous generator; P ref is the reference active power; k p is the active power-frequency droop coefficient; ω0 is the grid synchronization angular velocity; ω is the mechanical angular velocity of the virtual synchronous generator when off-grid;

[0100] In order to change the rigid external characteristics of the inverter, virtual inertia and damping are further added to simulate the rotor motion equation of the synchronous generator, as shown in formula (3):

[0101]

[0102] Where, J is the moment of inertia; D is the damping torque; P e represents electromagnetic power;

[0103] Substituting formula (3) into formula (2), we can obtain:

[0104]

[0105] Transform formula (4) into Laplace:

[0106]

[0107] Where s is the Laplace operator;

[0108] 2-4) Adjust the effective value of the inverter terminal voltage U by adjusting the virtual potential E of the virtual synchronous generator abc and reactive power Q, where the QE excitation control equation controlled by the VSG (virtual synchronous generator) is shown in formula (6). Formula (6) is used to perform QE excitation control, and the virtual potential E of the virtual synchronous generator is adjusted by QE excitation control to adjust the terminal voltage and reactive power of the inverter:

[0109]

[0110] Where Q is the instantaneous reactive power value output by the inverter after filtering, Q ref is the reactive power instruction of the inverter, U R is the command value of the inverter terminal voltage effective value, U abc is the real value of the inverter terminal voltage, k q is the reactive power-voltage droop coefficient, k i is the integral coefficient, E0 is the no-load potential of the virtual synchronous generator, which represents the terminal voltage when the inverter is running off-grid without load, E is the virtual potential of the virtual synchronous generator model when off-grid, and s is the Laplace operator;

[0111] 2-5) The PWM modulation signal is obtained through voltage and current double closed-loop control, in which both the voltage outer loop and the current inner loop adopt PI control. The current inner loop and the voltage outer loop equations are shown in equations (7) and (8) respectively:

[0112]

[0113]

[0114] In the formula, R f is the line resistance; C f is the filter capacitor; L is the filter inductor, and L=L f +L g , where L f is the inverter side filter inductor, L g It is the grid-side filter inductor.

[0115] The grid-connected seamless switching control strategy specifically includes the following steps:

[0116] 3-1) Constantly detect the frequency f and amplitude |U of the inverter output voltage abc |, phase θ;

[0117] 3-2) When receiving the grid-connected signal, first determine whether the inverter output voltage and the grid voltage meet the conditions shown in formula (9):

[0118]

[0119] In the formula, f g is the frequency of the grid voltage, |U g | is the amplitude of the grid voltage, θ g is the phase of the grid voltage, f is the frequency of the inverter output voltage, |U abc | is the amplitude of the inverter output voltage, θ is the phase of the inverter output voltage, f limit is the set grid-connected voltage frequency threshold, U limit is the set grid-connected voltage amplitude threshold, θ limit is the set grid-connected voltage phase threshold;

[0120] If the conditions shown in formula (9) are met, the grid-connected / off-grid switch is directly closed for grid-connected operation; if the conditions shown in formula (9) are not met, pre-synchronization control is required first, and then the grid-connected / off-grid switch is closed for grid-connected operation after the conditions shown in formula (9) are met;

[0121] 3-3) Set the load required active power to P R , when connected to the grid, the active power transmitted to the grid is P g; During the process of the reconfigurable battery energy storage system from off-grid to grid-connected, the reconfigurable battery energy storage system is in a constant current discharge state. To meet the output power demand, more battery packs need to be connected in series by closing the control switch to discharge; when connected to the grid, the output power of the reconfigurable battery energy storage system needs to meet the conditions shown in formula (10):

[0122] P R +P g ≤P bat ≤(1+σ)(P R +P g ) (10);

[0123] Where P R is the active power required by the load, P g P is the active power transmitted to the grid when connected to the grid. bat is the output power of the reconfigurable battery energy storage system, σ is the corresponding safety threshold parameter;

[0124] 3-4) Set the expected output active power of the inverter in the VSG control strategy to the output power of the reconfigurable battery energy storage system; the energy storage is a constant current discharge. When the load increases, more battery packs need to be connected in series according to the load demand power requirements. The series connection increases the voltage, and the current remains unchanged, so the output power of the reconfigurable battery energy storage system increases. Here, the expected output power of the inverter is set to the output power of the reconfigurable battery energy storage system;

[0125] 3-5) Set the following three timing coordination conditions between grid-connected switches: the first is that the closing time of the control switch is ahead of the grid-connected / off-grid switch; the second is that the closing time of the control switch is synchronized with the grid-connected / off-grid switch; the third is that the closing time of the control switch lags behind the grid-connected / off-grid switch;

[0126] 3-6) Compare the timing coordination working conditions between the above three grid-connected switches, compare the current impact sizes of the three working conditions from off-grid to grid-connected according to the waveform of the three-phase current output by the inverter, and compare the smoothness and oscillation amplitude of the three-phase voltage and power output by the inverter in the three working conditions from off-grid to grid-connected according to the waveform of the three-phase voltage and power output by the inverter, and judge the optimal switch timing coordination working condition; the present invention makes judgments based on two aspects: one is the size of the impact current during the switching process from off-grid to grid-connected. The smaller the impact current, the higher the grid-connected safety of the system; the second is the smoothness of the inverter output power and voltage curves during the entire off-grid and grid-connected process. The better the smoothness of the working condition, that is, the smaller the oscillation, the more stable the working condition; to ensure the stable operation of the reconfigurable battery energy storage system from off-grid to grid-connected.

[0127] The pre-synchronization control includes a phase compensation link Δθ and an amplitude compensation link ΔU, specifically including:

[0128] 3-2-1) The difference between the inverter output three-phase voltage and the grid voltage is calculated. The deviation is output as a regulation value ΔU by the PI regulator. ΔU is related to the virtual potential E (E = ΔE) in the off-grid mode. u +ΔE Q +E0) are superimposed to obtain the reference voltage amplitude U ref ;

[0129] 3-2-2) The phase difference between the inverter output three-phase voltage and the grid voltage is made. The deviation is output through the PI regulator to adjust the value Δθ. Δθ is added to the off-grid mode reference voltage phase ωt to form the reference phase θ of the pre-synchronization link. m ;

[0130] 3-2-3) Reference voltage amplitude U ref With reference phase θ m The three-phase reference voltage is obtained by superposition This enables pre-synchronization control of voltage amplitude and phase.

[0131] The control switch closing time is ahead of the grid-connected / off-grid switch, as shown in formula (11): t con ≤tt s , t sw =t p =t+t d (11);

[0132] The control switch closing time is synchronized with the grid-connected / off-grid switch, as shown in formula (12): t con =t sw , t sw =t p =t+t d (12);

[0133] The closing time of the control switch lags behind the on-grid / off-grid switch, as shown in formula (13): t con >t sw , t sw =t p =t+t d (13);

[0134] Where, t con is the time point when the control switch is closed; t is the time point when the grid-connected signal is received; t sw is the time point when the on / off-grid switch is closed; t p is the time point when pre-synchronization is completed; t s It is the time taken for the inverter output power to reach stability after the control switch is closed; t d The time used for pre-synchronization adjustment; tt s Indicates that the time point of the grid-connected signal is taken as the reference and the time point of the grid-connected signal is taken as the reference.s Time length, t+t d Indicates the delay t based on the time point of the grid-connected signal d Length of time.

[0135] The off-grid seamless switching control strategy specifically includes the following steps:

[0136] 4-1) Always monitor the output power of the inverter and the reconfigurable battery energy storage system;

[0137] 4-2) Set the load required active power to P R , when connected to the grid, the active power transmitted to the grid is P g , at this time, the output power of the reconfigurable battery energy storage system meets the conditions shown in formula (10):

[0138] P R +P g ≤P bat ≤(1+σ)(P R +P g ) (10);

[0139] Where P R is the active power required by the load, P g P is the active power transmitted to the grid when connected to the grid. bat is the output power of the reconfigurable battery energy storage system, σ is the corresponding safety threshold parameter;

[0140] During the process from grid connection to off-grid operation, the reconfigurable battery energy storage system is in a constant current discharge state. To meet the output power demand, the control switch needs to be disconnected to isolate the redundant battery packs. When off-grid, the output power of the reconfigurable battery energy storage system needs to meet the conditions shown in formula (14):

[0141] P R ≤P bat ≤(1+λ)P R (14);

[0142] In the formula, λ is the corresponding safety threshold parameter;

[0143] 4-3) Setting the reference active power of the inverter in the VSG control strategy to the output power of the reconfigurable battery energy storage system;

[0144] 4-4) When receiving the off-grid signal, the on-grid / off-grid switch is disconnected synchronously, and the following three timing coordination conditions between the off-grid switches are set: the first one is that the control switch disconnection time is ahead of the on-grid / off-grid switch; the second one is that the control switch disconnection time is synchronized with the on-grid / off-grid switch; the third one is that the control switch disconnection time lags behind the on-grid / off-grid switch;

[0145] 4-5) Compare the timing coordination conditions between the above three off-grid switches, and compare the current impact sizes of the three working conditions from off-grid to grid-connected according to the waveform of the three-phase current output by the inverter; compare the smoothness and oscillation amplitude of the three-phase voltage and power output by the inverter from off-grid to grid-connected according to the waveform of the three-phase voltage and power output by the inverter; judge the optimal switching timing coordination condition according to the above method, analyze the oscillation amplitude of the inverter output power, and judge the optimal switching timing coordination condition. The judgment is made based on two aspects: one is the size of the impact current during the switching process from off-grid to grid-connected. The smaller the impact current, the safer the system; the second is the smoothness of the inverter output power and voltage curves during the entire off-grid and grid-connected process. The condition with better smoothness, that is, the smaller the oscillation, the more stable the condition is, ensuring the stable operation of the reconfigurable battery energy storage system from grid-connected to off-grid.

[0146] The control switch disconnection time is ahead of the on / off grid switch, as shown in formula (15): t c <t a =t w (15);

[0147] The control switch disconnection time is synchronized with the on / off grid switch, as shown in formula (16): t c =t a =t w (16);

[0148] The disconnection time of the control switch lags behind that of the on-grid / off-grid switch, as shown in formula (17): t c >t a =t w (17);

[0149] Where, t c is the time point when the control switch is disconnected; t a is the time point when the off-grid signal is received; t w It is the time point when the on / off-grid switch is disconnected; if one time point is smaller than another time point, it means it is before the other time point; if two time points are equal, it means they are at the same moment.

[0150] The present invention discloses a reconfigurable battery energy storage and off-grid method, which can solve the fluctuation problem of the reconfigurable battery energy storage system during on-grid / off-grid switching. A large impact current may be generated during the process of the inverter switching from off-grid to on-grid operation mode. The voltage phase and amplitude compensation pre-synchronization control strategy effectively ensures the synchronization of the inverter with the power grid during the pre-synchronization process, and can effectively suppress the grid impact current. At the same time, the timing coordination problem of the control switch of the reconfigurable battery energy storage system and the physical state switching of the on-grid / off-grid switch during the off-grid to on-grid switching process is considered, and the magnitude of the impact current during the off-grid to on-grid switching process under different timing conditions is set and analyzed. As well as the stability and smoothness of the inverter output power, the optimal switch timing coordination condition is selected to ensure the seamless switching of the reconfigurable battery energy storage system from off-grid to grid-connected; no impact current will be generated during the inverter switching from grid-connected to off-grid operation mode, but the timing coordination problem of the physical state switching of the control switch and the grid-connected / off-grid switch in the reconfigurable battery energy storage system has a certain impact on the stability of the grid-connected to off-grid switching process. Therefore, the stability and smoothness of the inverter output power under different timing conditions during the off-grid to grid-connected switching process are analyzed, and the optimal switch timing coordination condition is selected to ensure the stable operation of the reconfigurable battery energy storage system during the grid-connected to off-grid switching process.

[0151] The present invention solves the timing coordination problem of the control switch and the physical state switching of the on-grid / off-grid switch in the on-grid / off-grid switching process of the reconfigurable battery energy storage system, and analyzes the impact current size of different timing conditions in the off-grid to on-grid switching process, as well as the stability and smoothness of the inverter output power. The optimal switch timing coordination condition can be selected to ensure the smooth transition of the reconfigurable battery energy storage system from off-grid to on-grid; by analyzing the stability and smoothness of the inverter output power under different timing conditions, the optimal switch timing coordination condition can be selected to ensure the stable operation of the reconfigurable battery energy storage system in the on-grid to off-grid switching process.

[0152] Example

[0153] A specific application of the reconfigurable battery energy storage and off-grid method of the present invention to implement a reconfigurable battery energy storage and off-grid operation model is as follows:

[0154] (1) The load active power requirement is set to 20 kW, and the expected discharge current is set to 20 A. When connected to the grid, in addition to supplying power to the load normally, 10 kW of power needs to be transmitted to the grid. The voltage variation range of each battery pack is set to 434.7826 V to 581.9936 V, and the voltage variation range corresponds to the state of charge (SOC) variation range of 5% to 95%.

[0155] (2) To meet the power required by the load, two battery groups in good condition are selected to be connected in series for discharge when off-grid; to meet the requirements of grid-connected operation, more battery groups are connected in series for discharge by closing the control switch when connected to the grid; here, two battery groups are connected for discharge when off-grid, and three battery groups are connected for discharge when connected to the grid.

[0156] (3) When connected to the grid, the following three timing coordination conditions are set between the grid-connected switches: the closing time of the control switch is ahead of the grid-connected / off-grid switch; the closing time of the control switch is synchronized with the grid-connected / off-grid switch; the closing time of the control switch lags behind the grid-connected / off-grid switch; when disconnected from the grid, the following three timing coordination conditions are set between the off-grid switches: the disconnection time of the control switch is ahead of the grid-connected / off-grid switch; the disconnection time of the control switch is synchronized with the grid-connected / off-grid switch; the disconnection time of the control switch lags behind the grid-connected / off-grid switch.

[0157] (4) Set the time point of 1s to receive the grid-connected signal and the time point of 3s to receive the off-grid signal; compare the timing coordination conditions of the three grid-connected switches, analyze the current impact size of the off-grid to grid-connected process in the time period of 0.9s to 1.2s, and the stability and smoothness of the inverter output power, and determine the optimal switch timing coordination condition; compare the timing coordination conditions of the three off-grid switches, analyze the stability and smoothness of the inverter output power in the time period of 2.9s to 3.2s, and determine the optimal switch timing coordination condition.

Claims

1. A reconfigurable battery energy storage and off-grid system, characterized by It includes a reconfigurable battery energy storage system, a DC-DC conversion circuit, and an inverter; the reconfigurable battery energy storage system is connected to the DC side of the inverter via a DC-DC circuit; A method for performing reconfigurable battery energy storage and off-grid operation using the reconfigurable battery energy storage and off-grid operation system comprises: 1) Perform PI constant current discharge control on the DC-DC conversion circuit designed between the reconfigurable battery energy storage system and the DC side of the inverter to ensure that the reconfigurable battery energy storage system discharges the inverter at a constant current; 2) The inverter is controlled using the VSG control strategy; 3) Use the grid-connected seamless switching control strategy to perform seamless switching control of the reconfigurable battery energy storage system from off-grid to grid-connected; 4) Use the off-grid seamless switching control strategy to perform seamless switching control of the reconfigurable battery energy storage system from grid-connected to off-grid; The grid-connected seamless switching control strategy specifically includes: 3-1) Constantly detect the frequency f and amplitude |U of the inverter output voltage abc |, phase θ; 3-2) When receiving the grid-connected signal, first determine whether the inverter output voltage and the grid voltage meet the conditions shown in formula (9): In the formula, f g is the frequency of the grid voltage, |U g | is the amplitude of the grid voltage, θ g is the phase of the grid voltage, f is the frequency of the inverter output voltage, |U abc | is the amplitude of the inverter output voltage, θ is the phase of the inverter output voltage, f limit is the set grid-connected voltage frequency threshold, U limit is the set grid-connected voltage amplitude threshold, θ limit is the set grid-connected voltage phase threshold; If the conditions shown in formula (9) are met, the grid-connected / off-grid switch is directly closed for grid-connected operation; if the conditions shown in formula (9) are not met, pre-synchronization control is required first, and then the grid-connected / off-grid switch is closed for grid-connected operation after the conditions shown in formula (9) are met; 3-3) Set the load required active power to P R , when connected to the grid, the active power transmitted to the grid is P g ; During the process of the reconfigurable battery energy storage system from off-grid to grid-connected, the reconfigurable battery energy storage system is in a constant current discharge state. To meet the output power demand, more battery packs need to be connected in series by closing the control switch to discharge; when connected to the grid, the output power of the reconfigurable battery energy storage system needs to meet the conditions shown in formula (10): P R +P g ≤P bat ≤(1+σ)(P R +P g ) (10); Where P R is the active power required by the load, P g P is the active power transmitted to the grid when connected to the grid. bat is the output power of the reconfigurable battery energy storage system, σ is the corresponding safety threshold parameter; 3-4) Set the expected output active power of the inverter in the VSG control strategy to the output power of the reconfigurable battery energy storage system; the energy storage is a constant current discharge. When the load increases, more battery packs need to be connected in series according to the load demand power requirements. The series connection increases the voltage, and the current remains unchanged, so the output power of the reconfigurable battery energy storage system increases. Here, the expected output power of the inverter is set to the output power of the reconfigurable battery energy storage system; 3-5) Set the following three timing coordination conditions between grid-connected switches: the first is that the closing time of the control switch is ahead of the grid-connected / off-grid switch; the second is that the closing time of the control switch is synchronized with the grid-connected / off-grid switch; the third is that the closing time of the control switch lags behind the grid-connected / off-grid switch; 3-6) Compare the timing coordination conditions between the above three grid-connected switches. According to the waveform of the three-phase current output by the inverter, compare the current impact size of the three working conditions from off-grid to grid-connected. According to the waveform of the three-phase voltage and power output by the inverter, compare the smoothness and oscillation amplitude of the three-phase voltage and power output by the inverter from off-grid to grid-connected in the three working conditions, and determine the optimal switch timing coordination condition to ensure the stable operation of the reconfigurable battery energy storage system from off-grid to grid-connected. The off-grid seamless switching control strategy specifically includes: 4-1) Always monitor the output power of the inverter and the reconfigurable battery energy storage system; 4-2) Set the load required active power to P R , when connected to the grid, the active power transmitted to the grid is P g , at this time, the output power of the reconfigurable battery energy storage system meets the conditions shown in formula (10): P R +P g ≤P bat ≤(1+σ)(P R +P g ) (10); Where P R is the active power required by the load, P g P is the active power transmitted to the grid when connected to the grid. bat is the output power of the reconfigurable battery energy storage system, σ is the corresponding safety threshold parameter; During the process from grid connection to off-grid operation, the reconfigurable battery energy storage system is in a constant current discharge state. To meet the output power demand, the control switch needs to be disconnected to isolate the redundant battery packs. When off-grid, the output power of the reconfigurable battery energy storage system needs to meet the conditions shown in formula (14): P R ≤P bat ≤(1+λ)P R (14); In the formula, λ is the corresponding safety threshold parameter; 4-3) Setting the reference active power of the inverter in the VSG control strategy to the output power of the reconfigurable battery energy storage system; 4-4) When receiving the off-grid signal, the on-grid / off-grid switch is disconnected synchronously, and the following three timing coordination conditions between the off-grid switches are set: the first one is that the control switch disconnection time is ahead of the on-grid / off-grid switch; the second one is that the control switch disconnection time is synchronized with the on-grid / off-grid switch; the third one is that the control switch disconnection time lags behind the on-grid / off-grid switch; 4-5) Compare the timing coordination conditions between the above three off-grid switches, and compare the current impact size of the three working conditions from off-grid to grid-connected according to the waveform of the three-phase current output by the inverter; compare the smoothness and oscillation amplitude of the three-phase voltage and power output by the inverter from off-grid to grid-connected according to the waveform of the three-phase voltage and power output by the inverter; determine the optimal switch timing coordination condition based on the above method, analyze the oscillation amplitude of the inverter output power, and determine the optimal switch timing coordination condition to ensure the stable operation of the reconfigurable battery energy storage system from grid-connected to off-grid.

2. A reconfigurable battery energy storage and off-grid system according to claim 1, characterized in that The reconfigurable battery energy storage system includes a reconfigurable battery network; the reconfigurable battery network includes a plurality of battery packs, the plurality of battery packs in the reconfigurable battery network are distributed in N parallel and M series, and each battery pack is connected to a control switch S ij Connect in series to form a series unit, i≤M, j≤N, S ij Represents the number of each battery pack and the control switch connected in series with the battery pack. Every N series units connected in parallel are connected with a bypass switch S i The parallel connection forms a parallel unit, and M such parallel units are connected in series to form a reconfigurable battery network.

3. A reconfigurable battery energy storage and off-grid system according to claim 1, characterized in that The DC-DC conversion circuit includes an inductor L bat , resistor R bat , capacitor V dc , the first switch tube K1, the first switch tube K2; due to the discharge process, the terminal voltage V of the battery energy storage system can be reconstructed bat will change, so the reconfigurable battery energy storage system adopts constant current discharge and real-time detection of the output current I of the reconfigurable battery energy storage system bat , the output current I of the reconfigurable battery energy storage system bat and the expected discharge current I* bat The current difference is obtained by subtracting the current difference, and the obtained current difference generates a PWM modulation signal through a PI controller and acts on the first switch tube K1 and the first switch tube K2 to perform PI constant current discharge control to ensure that the reconfigurable battery energy storage system performs constant current discharge on the DC side of the inverter after passing through the DC-DC conversion circuit.

4. The reconfigurable battery energy storage and off-grid system according to claim 1 is characterized in that The inverter is controlled by adopting the VSG control strategy, which specifically includes: 2-1) Real-time detection of the output three-phase voltage e before the inverter filter abc , the output three-phase voltage u after inverter filtering abc 、Current before filtering i Labc And the filtered current i abc ; The output three-phase voltage e before the inverter is filtered abc Perform dq coordinate transformation to obtain e d 、e q , the output three-phase voltage u after inverter filtering abc Perform dq coordinate transformation to obtain u d 、u q , the current before filtering i Labc Perform dq coordinate transformation to obtain i Ld 、i Lq , the filtered current i abc Perform dq coordinate transformation to obtain i d 、i q ; 2-2) For u d 、u q 、i d 、i q The power calculation is performed to obtain the instantaneous active power P and instantaneous reactive power Q output by the inverter. The power calculation formula is as follows: 2-3): In order to simulate the frequency modulation characteristics of the synchronous generator, a speed regulator is applied in VSG control. The droop characteristics between the active power and frequency of the speed regulator are expressed by formula (2): P m =P ref +k p (ω0-ω) (2); Where P m Represents the mechanical power of the synchronous generator; P ref is the reference active power; k p is the active power-frequency droop coefficient; ω0 is the grid synchronization angular velocity; ω is the mechanical angular velocity of the virtual synchronous generator when off-grid; The rotor motion equation of the synchronous generator is simulated as shown in formula (3): Where, J is the moment of inertia; D is the damping torque; P e represents electromagnetic power; Substituting formula (3) into formula (2), we can obtain: Transform formula (4) into Laplace: 2-4) Use formula (6) to perform QE excitation control, and use QE excitation control to adjust the virtual potential E of the virtual synchronous generator to adjust the terminal voltage and reactive power of the inverter: Where Q is the instantaneous reactive power value output by the inverter after filtering, Q ref is the reactive power instruction of the inverter, U R is the command value of the inverter terminal voltage effective value, U abc is the real value of the inverter terminal voltage, k q is the reactive power-voltage droop coefficient, k i is the integral coefficient, E0 is the no-load potential of the virtual synchronous generator, which represents the terminal voltage when the inverter is running off-grid without load, E is the virtual potential of the virtual synchronous generator model when off-grid, and s is the Laplace operator; 2-5) The PWM modulation signal is obtained through voltage and current double closed-loop control, in which both the voltage outer loop and the current inner loop adopt PI control. The current inner loop and the voltage outer loop equations are shown in equations (7) and (8) respectively: In the formula, R f is the line resistance; C f is the filter capacitor; L is the filter inductor, and L=L f +L g , where L f is the inverter side filter inductor, L g It is the grid-side filter inductor.

5. The reconfigurable battery energy storage and off-grid system according to claim 1 is characterized in that The pre-synchronization control includes a phase compensation link Δθ and an amplitude compensation link ΔU, specifically including: 3-2-1) The difference between the inverter output three-phase voltage and the grid voltage is calculated. The deviation is output as the adjustment value ΔU by the PI regulator. ΔU is superimposed on the virtual potential E in the off-grid mode to obtain the reference voltage amplitude U ref ; 3-2-2) The phase difference between the inverter output three-phase voltage and the grid voltage is made. The deviation is output through the PI regulator to adjust the value Δθ. Δθ is added to the off-grid mode reference voltage phase ωt to form the reference phase θ of the pre-synchronization link. m ; 3-2-3) Reference voltage amplitude U ref With reference phase θ m The three-phase reference voltage is obtained by superposition This enables pre-synchronization control of voltage amplitude and phase.

6. The reconfigurable battery energy storage and off-grid system according to claim 1 is characterized in that The control switch closing time is ahead of the grid-connected / off-grid switch, as shown in formula (11): t con ≤tt s , t sw =t p =t+t d (11); The control switch closing time is synchronized with the grid-connected / off-grid switch, as shown in formula (12): t con =t sw , t sw =t p =t+t d (12); The closing time of the control switch lags behind the on-grid / off-grid switch, as shown in formula (13): t con >t sw , t sw =t p =t+t d (13); Where, t con is the time point when the control switch is closed; t is the time point when the grid-connected signal is received; t sw is the time point when the on / off-grid switch is closed; t p is the time point when pre-synchronization is completed; t s To control the time it takes for the inverter output power to stabilize after the switch is closed; t d Adjust the time for pre-synchronization; tt s Indicates that the time point of the grid-connected signal is taken as the reference and the time point of the grid-connected signal is taken as the reference. s Time length, t+t d Indicates the delay t based on the time point of the grid-connected signal d Length of time.

7. The reconfigurable battery energy storage and off-grid system according to claim 1 is characterized by: The control switch disconnection time is ahead of the on / off grid switch, as shown in formula (15): t c <t a =t w (15); The control switch disconnection time is synchronized with the on / off grid switch, as shown in formula (16): t c =t a =t w (16); The disconnection time of the control switch lags behind that of the on-grid / off-grid switch, as shown in formula (17): t c >t a =t w (17); Where, t c is the time point when the control switch is disconnected; t a is the time point when the off-grid signal is received; t w It is the time point when the on / off-grid switch is disconnected; if one time point is smaller than another time point, it means it is before the other time point; if two time points are equal, it means they are at the same moment.

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