Battery energy storage system based on series-parallel compensation

Abstract

The invention discloses a battery energy storage system based on series-parallel connection compensation, the battery energy storage system comprises m battery energy storage units (m is greater than or equal to 2), each battery energy storage unit is formed by connecting single battery packs connected in series and the input side of an LLC resonant converter in parallel, and the SOC of a battery module is adjusted by controlling the input current of the LLC resonant converter; the input sides of the m battery energy storage units are connected in series to form an input port, and the output sides are connected in parallel to form an output port; the output port and the input port are connected in series to form a common DC bus, and the output voltage of the LLC resonant converter is controlled to compensate the terminal voltage change of the battery module caused by charging and discharging; the system has the beneficial effects that the rated power of the converter used in battery grouping can be greatly reduced, the cost of the battery energy storage system is reduced, the service efficiency and the service life of the battery are improved, and the system is high in power density, high in dynamic response speed, stable and reliable.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of battery energy storage, and in particular to a battery energy storage system based on series-parallel compensation. BACKGROUND

[0002] The volatility, intermittence and unpredictability in the renewable energy generation process have caused impact and challenge to the power system. In order to solve the power fluctuation problem caused by renewable energy generation, a method of configuring an energy storage system near a new energy source or a distributed new energy source can be used to achieve the purpose of providing stable power output to the power grid. Among various energy storage technologies, battery energy storage, which belongs to the chemical energy storage category, is the earliest to be applied to the power system, is relatively mature in technology, can realize large-capacity storage, has relatively low unit energy cost and system cost, has good modularity, fast response speed, and high commercialization degree, and has been highly concerned. It is the most practical and most widely used energy storage technology. Its characteristics are that the power and energy can be flexibly configured according to different application requirements, the response speed is fast, it is not limited by external conditions such as geography, and it is suitable for large-scale application and mass production.

[0003] In a traditional battery energy storage system, single batteries can be combined in different series and parallel combinations to meet the needs of high voltage and large capacity, mainly divided into two connection modes of "first series and then parallel" and "first parallel and then series" to form an energy storage battery pack, and then connected with a full-power high-voltage converter. The battery series-parallel connection and the subsequent full-power high-voltage converter scheme has certain differences in the capacity, internal resistance and other parameters of each single battery in the battery pack, and the performance of the entire battery pack is far inferior to that of the single battery.

[0004] The battery energy storage system based on H-bridge cascading and MMC is only suitable for AC access occasions and has limited application range. The use of DC-DC converters to form battery energy storage units for series connection can only meet the demand for high voltage, and in parallel mode, the batteries still need to be connected in series to form high voltage. Therefore, the battery consistency has not been solved, and the converter used is full-power controlled, and the converter power capacity is large. The energy storage system based on series-parallel compensation battery grouping uses partial power control of the converter, but the output voltage range is large. When the battery voltage is low, the output voltage is high, and when the battery voltage is high, the output voltage is low. The design power of the converter is greater than the actual operating power, which reduces the efficiency of the converter.

[0005] In summary, the existing battery energy storage system is faced with low voltage and small capacity of single battery. In actual application, multiple single batteries are usually connected in series and parallel to meet the demand of high voltage and large capacity. However, the grouping mode of directly connecting multiple single batteries in series and parallel has many defects. Most notably, the inconsistency of single batteries leads to serious performance degradation of the whole battery group, which not only leads to insufficient capacity utilization of the battery group, but also greatly shortens the service life of the battery group. When the battery grouping is applied to the energy storage system, a dedicated charging and discharging equipment is needed. The existing solution is to equip a converter with the same power as the system, which will cause the power of the converter to be large and the cost of the system to increase. Secondly, under the traditional voltage type control, the dynamic response of the converter is poor, the capacity of the converter is designed to be large, the size of the battery energy storage system is too large, and the cost of the system is high, and so on. Therefore, it is necessary to study and improve the battery energy storage system. SUMMARY

[0006] The present application is designed to solve the above technical problems.

[0007] The technical scheme adopted by the present application to solve the technical problems is:

[0008] A battery energy storage system based on series and parallel compensation, the battery energy storage system comprises m battery energy storage units (m≥2), each battery energy storage unit is composed of a single battery group connected in series and an LLC resonant converter input side connected in parallel, and the SOC of the battery module is adjusted by controlling the input current of the LLC resonant converter;

[0009] The input side of the m battery energy storage units is connected in series to form an input port, and the output side is connected in parallel to form an output port;

[0010] The output port and the input port are connected in series to form a common DC bus, and the output voltage of the LLC resonant converter is controlled to compensate for the change in the terminal voltage of the battery module caused by charging and discharging;

[0011] Wherein, the total output power P dc The power P c satisfies the relationship:

[0012]

[0013] The target of reducing the rated power of the LLC resonant converter can be achieved.

[0014] The battery energy storage system based on series and parallel compensation, the LLC resonant converter adopts a digital control architecture, which includes a main control chip DSP, a sampling circuit, a driving circuit and a protection circuit;

[0015] The main control chip DSP is used for executing voltage / current sampling, communication interaction and closed-loop control algorithm;

[0016] The sampling circuit is responsible for collecting voltage and current signals in the main circuit and sending them to the main control chip for control.

[0017] The drive circuit amplifies the PWM signal output by the main control chip DSP to a driving pulse square wave and sends it to the switching tube.

[0018] The protection circuit is responsible for monitoring overvoltage / overcurrent and triggering protection actions to take measures when exceeding the safety range to achieve the goal of the protection circuit.

[0019] The battery energy storage system based on series-parallel compensation, the main control chip DSP acquires external voltage and current signals through an analog-to-digital converter ADC and exchanges real-time data with a touch screen through a serial port communication SCI to obtain instruction values and start-stop signals; these data will be used for closed-loop control, and the output of the closed-loop control is sent to an ePWM module to generate a PWM wave, and finally the DSP sends the PWM wave to the drive circuit; the main control chip DSP generates three kinds of control mode pulses through the ePWM module:

[0020] Mode one: forward variable frequency control: adjust the switching frequency by changing the TBPRD value to control the ePWM wave frequency;

[0021] Mode two: reverse phase shift control: adjust the pulse phase difference by changing the PHASE value to control the ePWM wave phase;

[0022] Mode three: reverse buck-boost control: adjust the duty cycle by changing the CMP value to control the ePWM duty cycle.

[0023] The battery energy storage system based on series-parallel compensation, the LLC resonant converter energy conversion adopts a resonant capacitor voltage control strategy, including:

[0024] Mode one, forward variable frequency control: double closed-loop control is performed with the converter output voltage as the outer loop and the resonant capacitor voltage as the inner loop; the control is as follows: the resonant capacitor voltage is sampled at the moment when the power switching tube Q2 is turned off, the difference between the output voltage sampling value and the instruction value is sent to the first PI regulator, and the output thereof is used as the instruction value of the resonant capacitor voltage inner loop; then the instruction value of the resonant capacitor voltage inner loop is compared with the resonant capacitor voltage sampling value, the difference is sent to the second PI regulator, and the final output TBPRD value is sent to the ePWM module of the DSP to generate driving pulses of different frequencies, thereby adjusting the output power;

[0025] Mode two, reverse phase-shift control: single closed-loop control is adopted, and the resonant capacitor voltage is taken as the controlled quantity; the control is specifically as follows: the sampling value of the resonant capacitor voltage is compared with the instruction value, and the difference is sent to the PI regulator, and the output of the regulator directly changes the PHASE value of the ePWM module, and the reverse transmission power is controlled by adjusting the phase shift angle, so as to improve the dynamic response speed of the converter;

[0026] Mode three, reverse buck-boost control: single closed-loop or double closed-loop control is adopted, and the duty cycle is taken as the control freedom; the control is specifically as follows: the voltage or current input to the converter is taken as the control target, and the sampling value is compared with the instruction value, and the difference is output after the PI regulator, and the CMP value of the ePWM module is changed, and the reverse transmission power is controlled by adjusting the duty cycle of the driving pulse.

[0027] The battery energy storage system based on series-parallel compensation, the energy conversion of the LLC resonant converter adopts a control strategy in the control total voltage mode, the strategy is: total voltage outer loop, inductance current inner loop control, and a droop control link of regulating battery SOC is superimposed in the inductance current inner loop; in order to realize the SOC balance of each battery pack, the inductance current compensation value ΔI L * is taken after being negated and then superimposed on the instruction value I o_ref of the inductance current; after obtaining the instruction value regulated by the voltage outer loop, the inductance current inner loop of each LLC resonant converter adjusts the inductance current according to the instruction value and the inductance current sampling value of itself, and the discharge current of each battery pack can be adjusted under the condition of ensuring the stability of the total output voltage;

[0028] The SOC droop control adopts a characteristic curve of the inductance current compensation value and the SOC, and the droop coefficient R is positive resistance, and the relationship expression is formula (10); in the formula, ΔI L is the inductance current compensation value; ΔI Lmax , ΔI Lmin are respectively the maximum compensation value and the minimum compensation value of the inductance current; R is the SOC droop coefficient; SOC ref , SOC min , SOC max are respectively the reference value, the minimum value and the maximum value of the SOC;

[0029]

[0030] The corresponding inductance current compensation value of formula (10) corresponds to the SOC droop characteristic curve. Within the allowed SOC range, the compensation value of the inductance current and the difference between the SOC reference value and the actual SOC value are in proportional relationship. To avoid overcharge and overdischarge of the battery pack, the actual SOC value range must be between 0 and 1. When the allowed SOC range is exceeded, the compensation value of the inductance current must be within a reasonable and effective range.

[0031] The battery energy storage system based on series-parallel compensation, when discharging, the control strategy under the control of the total voltage mode: the battery module with a larger SOC value will increase the discharge current, and the battery module with a smaller SOC value will decrease the discharge current. After a period of adjustment, the SOC values of each battery module gradually tend to be consistent.

[0032] The battery energy storage system based on series-parallel compensation, when charging, the control strategy under the control of the total voltage mode: the battery module with a larger SOC value will decrease the charging current, and the battery module with a smaller SOC value will increase the charging current. After a period of adjustment, the SOC values of each battery module gradually tend to be consistent.

[0033] The battery energy storage system based on series-parallel compensation, the energy conversion of the LLC resonant converter includes a control strategy under the control of the total current mode. When multiple batteries are connected in series and parallel on the same DC bus, the total output voltage of the system can not be controlled, but the total charging and discharging current of the system needs to be controlled. To control the charging and discharging current of the entire battery string, the control strategy under the control of the total current mode adds a control link for adjusting the total current on the outer loop of the total voltage based on the control strategy under the control of the total voltage. The PI output of the total current adjustment is the given value of the system total voltage, and the PI output of the total voltage adjustment is the given value of the system total inductance current. Then, the inductance current is distributed among the three LLC resonant converters according to the SOC droop control.

[0034] The battery energy storage system based on series-parallel compensation, the LLC resonant converter is a full-bridge converter or a DAB converter.

[0035] The battery energy storage system based on series-parallel compensation, each battery energy storage unit is configured with a bypass switch. In case of failure, the unit is cut off and the system is maintained to run, supporting hot plug and redundancy control.

[0036] The above at least one technical solution adopted by the embodiments of the present application can achieve the following beneficial effects: by utilizing the feature that the voltage of the battery module after being connected in series is close to the voltage of the DC bus, a control strategy based on the resonant capacitor voltage with high dynamic response is adopted, so that the LLC resonant converter in the battery energy storage system only flows through a part of the battery emission or absorption power, the rated power of the converter used in the battery group can be greatly reduced, the cost of the battery energy storage system is reduced, the use efficiency and service life of the battery are improved, the system not only has high power density and fast dynamic response speed, but also is stable and reliable. BRIEF DESCRIPTION OF DRAWINGS

[0037] The accompanying drawings, which are included to provide a further understanding of the present application and are incorporated in and constitute a part of this application, illustrate embodiments of the present application and serve to explain the present application. In the drawings:

[0038] Figure 1 It is a topology of the battery energy storage system based on series-parallel compensation;

[0039] Figure 2 It is a digital control circuit block diagram of the converter;

[0040] Figure 3 It is a main control logic block diagram;

[0041] Figure 4(a) is a main program flow chart of the software control flow;

[0042] Figure 4(b) is an interrupt program flow chart of the software control flow;

[0043] Figure 5 It is a waveform diagram under forward operation;

[0044] Figure 6 It is a resonant capacitor voltage control block diagram;

[0045] Figure 7 It is a typical waveform diagram of reverse phase shift control;

[0046] Figure 8 It is a topology diagram of total voltage mode control;

[0047] Figure 9 It is a control strategy diagram of total voltage mode control;

[0048] Figure 10 It is an inductance current compensation value and SOC droop curve diagram;

[0049] Figure 11 It is a topology diagram of total current mode control;

[0050] Figure 12 It is a control strategy diagram of total current mode control. DETAILED DESCRIPTION

[0051] In order to make the purposes, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely below in conjunction with the specific embodiments of the present application and corresponding drawings. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0052] The technical solutions provided by the embodiments of the present application will be described in detail below in conjunction with the drawings.

[0053] In order to facilitate the understanding of the embodiments of the present application, a battery energy storage system based on series-parallel compensation is shown in Figure 1 The battery energy storage system includes m battery energy storage units (m≥2), each of which is composed of a series-connected single battery group and an LLC resonant converter input side in parallel, and the SOC of the battery module is adjusted by controlling the input current of the LLC resonant converter;

[0054] The input side of the m battery energy storage units is connected in series to form an input port, and the output side is connected in parallel to form an output port;

[0055] The output port and the input port are connected in series to form a common DC bus, and the output voltage of the LLC resonant converter is controlled to compensate for the change in the terminal voltage of the battery module caused by charging and discharging;

[0056] It is assumed that the power flowing through the m LLC resonant converters is the same, and the input voltage, input current and output current satisfy the following relationships:

[0057]

[0058] It can be known from Figure 1 The total output voltage of the battery energy storage system is

[0059]

[0060] The total output current of the system is

[0061]

[0062] The total output power of the system is

[0063] P dc =V dc I dc =m(m+1)V o I o (4)

[0064] In the battery energy storage system, the power flowing through the LLC resonant converter used is

[0065] P c =V B1 I i1 +V B2 I i2 +L+V Bm I im =mV o I o (5)

[0066] Based on the above derivation, and comparing equations (4) and (5), it can be seen that under this circuit topology, the actual output power of the system has the following relationship with the power flowing through the LLC resonant converter.

[0067]

[0068] As can be seen from the equation, since m>1, the output power on the DC side of the system is greater than the power flowing through the LLC resonant converter. Therefore, in practical applications, the rated power of the converter can be reduced, thereby reducing investment costs.

[0069] The battery energy storage system based on series-parallel compensation is provided in which each battery energy storage unit is equipped with a bypass switch, which disconnects the unit and maintains system operation in case of failure, and supports hot-swapping and redundancy control.

[0070] If the number of battery energy storage units in the system is sufficient to ensure the stability of the DC bus voltage, then if a battery module or LLC resonant converter in the system fails, the faulty part can be disconnected via a bypass switch, while the remaining parts continue to operate normally, thus achieving hot-swapping of the battery energy storage units. This not only facilitates the maintenance and open-circuit measurement of the battery energy storage units but also enables the system to achieve redundant control. Therefore, battery energy storage systems based on series-parallel compensation not only have high power density but are also stable and reliable.

[0071] (1) Digital control implementation scheme

[0072] LLC resonant converters employ digital control, such as... Figure 2 As shown in the block diagram of the digital control circuit, the battery energy storage system based on series-parallel compensation, the LLC resonant converter adopts a digital control architecture, including: a main control chip DSP, a sampling circuit, a drive circuit and a protection circuit;

[0073] The main control chip, DSP, is used to execute voltage / current sampling, communication interaction, and closed-loop control algorithms.

[0074] The sampling circuit is responsible for collecting voltage and current signals from the main circuit and sending them to the main control chip for control purposes.

[0075] The driving circuit amplifies the PWM signal output by the master control chip DSP to a pulse square wave with driving capability, and sends the pulse square wave to the switch tube.

[0076] The protection circuit is responsible for monitoring overvoltage / overcurrent and triggering protection actions, and taking measures when exceeding the safety range to achieve the goal of the protection circuit.

[0077] The digital signal processing technology (Digital Signal Processing, DSP) is the core of digital control, and the internal control logic is as shown in Figure 3 .

[0078] The battery energy storage system based on series-parallel compensation, the master control chip DSP acquires external voltage and current signals through an analog-to-digital converter ADC, and exchanges real-time data with a touch screen through a serial port communication SCI to obtain instruction values and start-stop signals; these data will be used for closed-loop control, and the output of the closed-loop control is sent to an ePWM module to generate a PWM wave, and finally the DSP sends the PWM wave to a driving circuit; the master control chip DSP generates three kinds of control mode pulses through the ePWM module:

[0079] Mode one: forward variable frequency control: adjusting the switching frequency, controlling the ePWM wave frequency by changing the TBPRD value;

[0080] Mode two: reverse phase shift control: adjusting the pulse phase difference, controlling the ePWM wave phase by changing the PHASE value;

[0081] Mode three: reverse buck-boost control: adjusting the duty cycle, controlling the ePWM duty cycle by changing the CMP value.

[0082] Figure 4(a) is a system control software program flowchart, the main program is first initialized and configured for system clock, port and interrupt when running, after completing the initialization configuration, enters the main loop, and waits for an interrupt. The main loop mainly executes start-stop control, software fault protection and other programs. Under normal operation of the converter, in each cycle T s enter the ePWM interrupt service program to execute the control program, change the frequency, phase or duty cycle of ePWM, etc., to achieve the purpose of changing the switch tube driving pulse.

[0083] The interrupt control program design flowchart of digital control is shown in Figure 4(b). The commutation program determines the running direction of the converter, and different programs are executed. When running forward, forward variable frequency control is executed to generate and change the TBPRD of ePWM, thereby changing the switching frequency of the driving pulse. When running in reverse, first determine the voltage instruction value V ref1The ratio of the secondary side voltage V2 determines whether the reverse boost or buck operation. The reverse boost control generates and changes the CMP of ePWM, thereby changing the duty cycle of the driving pulse; the reverse phase shift control generates and changes the PHASE of ePWM, thereby changing the phase difference of the driving pulse.

[0084] The battery energy storage system based on series-parallel compensation, the LLC resonant converter energy conversion adopts a resonant capacitor voltage control strategy, comprising:

[0085] Mode one, forward frequency conversion control: double closed loop control is performed with the converter output voltage as the outer ring and the resonant capacitor voltage as the inner ring; the control is specifically as follows: the resonant capacitor voltage is sampled at the turn-off moment of the power switch tube Q2, the difference between the output voltage sampling value and the instruction value is sent to the first PI regulator, and the output thereof is taken as the instruction value of the resonant capacitor voltage inner ring; the instruction value of the resonant capacitor voltage inner ring is compared with the sampling value of the resonant capacitor voltage, and the difference is sent to the second PI regulator, and finally the output TBPRD value is sent to the ePWM module of the DSP to generate driving pulses of different frequencies, thereby adjusting the output power;

[0086] Mode two, reverse phase shift control: single closed loop control is adopted, and the resonant capacitor voltage is taken as the controlled quantity; the control is specifically as follows: the sampling value of the resonant capacitor voltage is compared with the instruction value, and the difference is sent to the PI regulator, and the output of the regulator directly changes the PHASE value of the ePWM module, and the reverse transmission power is controlled by adjusting the phase shift angle, thereby improving the dynamic response speed of the converter;

[0087] Mode three, reverse boost-buck control: single closed loop or double closed loop control is adopted, and the duty cycle is taken as the control freedom degree; the control is specifically as follows: the voltage or current input to the converter is taken as the control target, and the sampling value thereof is compared with the instruction value, and the difference is sent to the PI regulator after adjustment, and the output changes the CMP value of the ePWM module, and the reverse transmission power is controlled by adjusting the duty cycle of the driving pulse.

[0088] (2) Control strategy based on resonant capacitor voltage

[0089] The resonant capacitor voltage is one of the key parameters of the energy conversion of the LLC resonant converter, and directly reflects the energy transmission process. When load disturbance or input voltage fluctuation occurs, the resonant capacitor voltage can be accurately controlled, thereby realizing stable adjustment of the power of the LLC resonant converter.

[0090] Due to the presence of a reactive resonant circuit, the input current from the voltage source to the resonant cavity typically consists of two parts: forward current and reverse current. The net input current can be represented by the net input charge in each switching cycle. Since the net input charge directly reflects the input power, and since the input power equals the output power when losses are ignored, the net input charge can accurately reflect the output power. Furthermore, the net input charge can be calculated by the voltage change of the resonant capacitor during the time period of energy exchange between the input voltage source and the resonant cavity. Therefore, the resonant capacitor voltage reflects the change in output power, and can be used as a controlled variable in closed-loop control.

[0091] First, we analyze the forward operation, assuming Q1 and Q2 are bridge arms A, and Q3 and Q4 are bridge arms B. The high-side switch of one bridge arm and the low-side switch of the other bridge arm always turn on and off simultaneously. Energy is transferred from the voltage source to the resonant cavity during the two half-switching cycles. Therefore, the net charge of the two half-cycles should be calculated to determine the net input charge for the entire cycle. The energy exchange cycle occurs when switches Q2 and Q3 are turned off (t). ALoff At this point, the resonant current begins to flow back to the input voltage source, and the energy exchange cycle begins when the Q1 and Q4 switches are turned off. AHoff The time ends. Afterwards, the parasitic capacitance C... Q1 C Q4 When charged to the input voltage, the waveform of the LLC resonant converter in forward operation is as follows: Figure 5 As shown.

[0092] The energy exchange cycle can be identified from the resonant current: there is a negative input charge from the turn-off point of switches Q2 and Q3 to the zero-crossing point of the resonant current, and a positive input charge from the zero-crossing point to the turn-off point of switches Q1 and Q4. The net input charge for the entire switching cycle can be determined from t... ALoff Time to t AHoff Resonant capacitor voltage V at time 1 cr The calculation is derived from equation (7).

[0093] Q net =2·[Q neg +Q pos ] = 2C r [v Cr (t ALoff )-v Cr (t AHoff (7)

[0094] Further analysis of the energy exchange process reveals that the charge on the parasitic capacitance should also be included in the net charge. After switches Q2 and Q3 are turned off, the parasitic capacitance C... Q1 C Q4The resonant current charges from -V1 to V1. In the above equation, this part of charge is considered to return to the voltage source, but in fact this part of energy remains in the resonant cavity, so it should be deducted from the negative input charge. Similarly, after the switch Q1, Q4 is off, the input current continues to flow until the parasitic capacitor C Q2 , C Q3 is charged from -V1 to V1. This part of charge should be added to the positive input charge. Therefore, the net input charge amount, the net input current, and the net input power formula after considering the parasitic capacitor are as follows:

[0095]

[0096] Therefore, in the forward operation, only sampling the resonant capacitor voltage when Q2 is off can detect the average input current and power. The control principle of the LLC resonant converter controlled by the resonant capacitor voltage is shown in Figure 6 , the output voltage is the outer ring, and the resonant capacitor voltage is the inner ring. The difference between the command value and the sampling value of the output voltage outer ring is sent to the PI regulator, and the output value of the outer ring is used as the command value of the resonant capacitor voltage inner ring, and compared with the sampling value of the resonant capacitor voltage, and the difference is sent to the PI regulator, and the final TBPRD is sent to the ePWM module of the DSP to generate driving pulses of different frequencies. It should be noted that although the resonant capacitor voltage is a high-frequency alternating current signal, only the resonant capacitor voltage value at the moment when the Q2 switch is off is needed for control.

[0097] The waveforms under reverse phase-shift control are shown in Figure 7 , assuming that Q5 and Q6 are C bridge arms, the expressions for the resonant capacitor voltage under reverse phase-shift control are respectively formula (9). Therefore, the resonant capacitor voltage can also be used as the controlled quantity to participate in the closed loop during reverse operation, thereby improving the dynamic response speed of the converter.

[0098]

[0099] The control strategy under the total voltage mode is: total voltage outer ring, inductance current inner ring control, and superimposing the droop control link of adjusting the battery SOC in the inductance current inner ring; in order to realize the SOC balance of each battery pack, the inductance current compensation value ΔI L * is taken and then superimposed on the command value I o_ref of the inductance current; after obtaining the command value adjusted by the voltage outer ring, the inductance current inner ring of each LLC resonant converter adjusts the inductance current according to the command value and the inductance current sampling value of itself, and adjusts the discharge current of each battery pack under the condition of ensuring the stability of the total output voltage;

[0100] (3) Control total voltage mode

[0101] When the energy storage system is running with load, a common working condition is to control the output voltage to be a given value. In the control total voltage mode working condition, the experimental circuit topology for controlling the total voltage is shown in Figure 8 . In the process of system operation, in order to keep the common DC bus voltage stable, the control strategy shown in Figure 9 is adopted. The basis of the control strategy is the total voltage outer loop and the inductor current inner loop control, and the droop control link for adjusting the battery SOC is superimposed on the inductor current inner loop.

[0102] In order to realize the SOC (State of Charge) balance of each battery pack, the inductor current compensation value ΔI L * is taken reversely and then superimposed on the command value I o_ref of the inductor current. After obtaining the command value adjusted by the voltage outer loop, the inductor current inner loop of each LLC resonant converter adjusts the inductor current according to the command value and the inductor current sampling value of itself. In the case of ensuring the stability of the total output voltage, the discharge current of each battery pack can be adjusted.

[0103] The SOC droop control adopts the characteristic curve of the inductor current compensation value and the SOC. The droop coefficient R is positive resistance, and its relationship expression is formula (10). In formula (10), ΔI L is the inductor current compensation value; ΔI Lmax , ΔI Lmin are respectively the maximum compensation value and the minimum compensation value of the inductor current; R is the SOC droop coefficient; SOC ref , SOC min , SOC max are respectively the reference value, the minimum value and the maximum value of the SOC.

[0104]

[0105] The inductor current compensation value and the SOC droop characteristic curve corresponding to formula (10) are shown in Figure 10 . As can be seen from the figure, within the allowed SOC range, the compensation value of the inductor current and the difference between the SOC reference value and the actual value of the SOC are in proportional relationship. According to the definition of the SOC, in order to avoid the overcharge and overdischarge of the battery pack, the actual SOC value range must be between 0 and 1. When the allowed SOC range is exceeded, the compensation value of the inductor current must be ensured to be within a reasonable and effective range.

[0106] The battery energy storage system based on series-parallel compensation, in discharging, the control strategy under the control of total voltage mode: the battery module with larger SOC value will increase the discharging current, the battery module with smaller SOC value will decrease the discharging current, after a period of adjustment, the SOC values of each battery module gradually tend to be consistent.

[0107] In discharging, set the reference value SOC of SOC ref All are smaller than the actual SOC values of the three battery groups, the SOC values of the three battery groups at the beginning of discharging are SOC1, SOC2 and SOC3, and satisfy the relationship SOC1>SOC2>SOC3. Figure 9 It can be known that the inductance current compensation value relationship obtained by SOC droop control in this case is ΔI L1 <ΔI L2 <ΔI L3 <0, after inversion, -ΔI L1 >-ΔI L2 >-ΔI L3 >0. When the three inverted inductance current compensation values are superimposed on the reference value of the inductance current, the inductance current reference values of the three LLC resonant converters will satisfy I Lref1 >I Lref2 >I Lref3 >0, here the discharging current is defined as positive. Therefore, the battery module with larger SOC value will increase the discharging current, the battery module with smaller SOC value will decrease the discharging current, after a period of adjustment, the SOC values of each battery module can gradually tend to be consistent.

[0108] The battery energy storage system based on series-parallel compensation, in charging, the control strategy under the control of total voltage mode: the battery module with larger SOC value will decrease the charging current, the battery module with smaller SOC value will increase the charging current, after a period of adjustment, the SOC values of each battery module gradually tend to be consistent.

[0109] In charging, set the reference value SOC of SOC, all are larger than the actual SOC values of the three battery groups, the SOC values of the three battery groups at the beginning of charging are SOC1, SOC2 and SOC3, and satisfy the relationship SOC1<SOC2<SOC3. Figure 9 It can be known that the inductance current compensation value relationship obtained by SOC droop control in this case is ΔI L1 >ΔI L2 >ΔI L3 , after inversion, -ΔI L1 <-ΔI L2 <-ΔI L3 When the three inverted inductance current compensation values are superimposed on the reference value of the inductance current, the inductance current reference values of the three LLC resonant converters will satisfy ILref1 <I Lref2 <I Lref3 <0, the charging current is defined as negative. Therefore, the battery module with a larger SOC value will decrease the charging current, and the battery module with a smaller SOC value will increase the charging current, and after a period of adjustment, the SOC values of the battery modules can gradually tend to be consistent. As can be seen, this control strategy can also achieve the SOC equalization adjustment of each battery module in the battery group system during charging.

[0110] The battery energy storage system based on series-parallel compensation, the energy conversion of the LLC resonant converter includes a control strategy in a total current mode, and the strategy is that when a plurality of batteries are connected in series and parallel on the same DC bus, the total output voltage of the system can not be controlled, but the total charging and discharging current of the system needs to be controlled; in order to control the charging and discharging current of the whole string of batteries, on the basis of the control strategy of controlling the total voltage, the control strategy in the total current mode adds a control link of adjusting the total current on the outer loop of the total voltage; the PI output of adjusting the total current is a given value of the total voltage of the system, the PI output of adjusting the total voltage is a given value of the total inductance current of the system, and then the inductance current is distributed among the three LLC resonant converters according to the SOC droop control.

[0111] (4) Control total current mode

[0112] When a plurality of batteries are connected in series and parallel on the same DC bus, the total output voltage of the system can not be controlled, but the total charging and discharging current of the system needs to be controlled. In the control mode of the total current, the experimental circuit topology adopted is as shown in Figure 11 , and the control strategy adopted is as shown in Figure 12 . It can be known from Figure 12 that in order to control the charging and discharging current of the whole string of batteries, on the basis of the control strategy of controlling the total voltage, the control strategy adds a control link of adjusting the total current on the outer loop of the total voltage. The PI output of adjusting the total current is a given value of the total voltage of the system, the PI output of adjusting the total voltage is a given value of the total inductance current of the system, and then the inductance current is distributed among the three LLC resonant converters according to the SOC droop control.

[0113] The battery energy storage system based on series-parallel compensation, the LLC resonant converter is a full-bridge converter or a DAB converter.

[0114] The above only describes the embodiments of the present application and is not used to limit the present application. The present application can have various changes and modifications for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of claims of the present application.

Claims

1. A battery energy storage system based on series-parallel compensation, characterized by: The battery energy storage system comprises m battery energy storage units (m≥2), each of which is formed by a single battery module in series and an LLC resonant converter input side in parallel, and the SOC of the battery module is adjusted by controlling the input current of the LLC resonant converter; The input side of the m battery energy storage units is connected in series to form an input port, and the output side is connected in parallel to form an output port; The output port and the input port are connected in series to form a common DC bus, and the output voltage of the LLC resonant converter is controlled to compensate for the change in the terminal voltage of the battery module caused by charging and discharging; Wherein, the total output power P dc The single LLC resonant converter handles power P c Satisfies the relationship: The target of reducing the rated power of the LLC resonant converter is achieved.

2. The series-parallel compensation based battery energy storage system of claim 1, wherein: The LLC resonant converter adopts a digital control architecture, which comprises a master control chip DSP, a sampling circuit, a driving circuit and a protection circuit; The master control chip DSP is used for voltage / current sampling, communication interaction and closed-loop control algorithm; The sampling circuit is responsible for collecting voltage and current signals in the main circuit and sending them to the master control chip for control; The driving circuit amplifies the PWM signal output by the master control chip DSP to a pulse square wave with driving capability and sends it to the switch tube; The protection circuit is responsible for monitoring overvoltage / overcurrent and triggering protection actions, and measures are taken when the safety range is exceeded to achieve the target of the protection circuit.

3. The series-parallel compensation based battery energy storage system of claim 2, wherein: The master control chip DSP acquires external voltage and current signals through an analog-to-digital converter ADC and exchanges real-time data with a touch screen through a serial port communication SCI to obtain instruction values and start / stop signals; the data will be used for closed-loop control, and the output of the closed-loop control is sent to an ePWM module to generate a PWM wave, which is finally sent to the driving circuit by the DSP; the master control chip DSP generates three control mode pulses through the ePWM module: Mode one: forward frequency conversion control: adjust the switching frequency by changing the TBPRD value to control the wave frequency of the ePWM; Mode two: reverse phase shift control: adjust the pulse phase difference by changing the PHASE value to control the wave phase of the ePWM; Mode three: reverse buck-boost control: adjust the duty cycle by changing the CMP value to control the duty cycle of the ePWM.

4. The series-parallel compensation based battery energy storage system of claim 3, wherein: The LLC resonant converter energy conversion adopts a resonant capacitor voltage control strategy, which comprises: Mode one, forward frequency conversion control: double closed-loop control is performed on the converter output voltage as the outer loop and the resonant capacitor voltage as the inner loop; the control is as follows: the resonant capacitor voltage is sampled at the turn-off time of the power switch tube Q2, the difference between the output voltage sampling value and the instruction value is sent to the first PI regulator, and the output of the first PI regulator is used as the instruction value of the resonant capacitor voltage inner loop; the instruction value of the resonant capacitor voltage inner loop is compared with the sampling value of the resonant capacitor voltage, the difference is sent to the second PI regulator, and the final output TBPRD value is sent to the ePWM module of the DSP to generate a driving pulse with different frequencies, thereby adjusting the output power; Mode two, reverse phase-shift control: single closed-loop control is adopted, and the resonant capacitor voltage is taken as the controlled quantity; the control is specifically as follows: the sampling value of the resonant capacitor voltage is compared with the instruction value, and the difference value is sent to the PI regulator, and the output of the regulator directly changes the PHASE value of the ePWM module, the reverse transmission power is controlled by adjusting the phase shift angle, and then the dynamic response speed of the converter is improved; Mode three, reverse buck-boost control: single closed-loop or double closed-loop control is adopted, and the duty cycle is taken as the control freedom; the control is specifically as follows: the voltage or current input to the converter is taken as the control target, the sampling value is compared with the instruction value, and the difference value is output after the PI regulator, and the CMP value of the ePWM module is changed, and the reverse transmission power is controlled by adjusting the duty cycle of the driving pulse.

5. The series-parallel compensation based battery energy storage system of claim 3, wherein: The LLC resonant converter energy conversion includes a control strategy in a total voltage mode, which is: total voltage outer loop, inductance current inner loop control, and a droop control link for adjusting battery SOC superimposed on the inductance current inner loop; to realize the SOC balance of each battery pack, the inductance current compensation value ΔI L * The inductance current compensation value ΔI is obtained by taking the complement of the SOC droop control and then superimposing it on the instruction value I o_ref After obtaining the instruction value adjusted by the voltage outer loop, the inductance current inner loop of each LLC resonant converter adjusts the inductance current according to the instruction value and the inductance current sampling value of itself, and can adjust the discharge current of each battery pack under the condition of ensuring the stability of the total output voltage. The SOC droop control adopts a characteristic curve of inductance current compensation value and SOC, and the droop coefficient R is positive resistance, and the relationship expression is formula (10); wherein, ΔI L is the inductance current compensation value; ΔI Lmax , ΔI Lmin are respectively the maximum compensation value and the minimum compensation value of the inductance current; R is the SOC droop coefficient; SOC ref , SOC min , SOC max are respectively the reference value, the minimum value and the maximum value of the SOC. The corresponding inductor current compensation value of formula (10) and the SOC droop characteristic curve are in proportional relationship within the allowed SOC range; In order to avoid the overcharge and overdischarge of the battery pack, the actual SOC value range must be between 0 and 1, and when the allowed SOC range is exceeded, the compensation value of the inductor current must be in a reasonable and effective range.

6. The series-parallel compensation based battery energy storage system of claim 5, wherein: During discharging, the control strategy under the total voltage mode is that the battery module with a larger SOC value will increase the discharging current, the battery module with a smaller SOC value will decrease the discharging current, and after a period of adjustment, the SOC values of the battery modules gradually tend to be consistent.

7. The series-parallel compensation based battery energy storage system of claim 5, wherein: During charging, the control strategy under the total voltage mode is that the battery module with a larger SOC value will decrease the charging current, the battery module with a smaller SOC value will increase the charging current, and after a period of adjustment, the SOC values of the battery modules gradually tend to be consistent.

8. The series-parallel compensation based battery energy storage system of claim 5, wherein: The energy conversion of the LLC resonant converter includes the control strategy under the total current mode, and the strategy is as follows: in order to control the charging and discharging current of the whole battery, the control strategy under the total current mode adds a control link of adjusting the total current on the total voltage outer loop on the basis of the control strategy under the total voltage mode; the PI output of adjusting the total current is the given value of the system total voltage, the PI output of adjusting the total voltage is the given value of the system total inductor current, and then the inductor current is distributed among the three LLC resonant converters according to the SOC droop control.

9. The series-parallel compensation based battery energy storage system of claim 1, wherein: The LLC resonant converter is a full-bridge converter or a DAB converter.

10. The series-parallel compensation based battery energy storage system of claim 1, wherein: Each battery energy storage unit is configured with a bypass switch, which is cut off when a fault occurs and maintains system operation, supports hot plug and redundancy control.

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