Improved Droop Control Method for Parallel Sodium-Ion Battery Systems in Microgrids

Abstract

The present invention discloses an improved droop control method for a parallel system of sodium-ion batteries in a microgrid, comprising the following steps: Step 1, based on the state of charge, state of health, and initial droop control coefficient of the sodium-ion battery, calculate the actual droop control coefficient of the sodium-ion battery, and further calculate the voltage reference of the sodium-ion battery; Step 2, obtain the voltage on the DC bus side, and construct a voltage compensator with a PI regulator as a secondary compensation link for the voltage reference of the sodium-ion battery, so as to ensure that the voltage on the DC bus side is stabilized at the voltage compensation reference value ， Obtain the actual droop control coefficient and the voltage reference of the sodium-ion battery that meet the stability; Step 3, obtain the actual output voltage and the actual inductor current of the sodium-ion battery, and combine with the adjusted voltage reference of the sodium-ion battery obtained in Step 2 to calculate the duty ratio of the bidirectional DC / DC converter. The method of the present invention can enable each sodium-ion battery to be in the best working state.

Description

Technical Field

[0001] The present invention relates to the field of control methods for parallel systems of sodium-ion batteries in a microgrid, and specifically to an improved droop control method for a parallel system of sodium-ion batteries in a microgrid. Background Art

[0002] In a DC microgrid, the energy storage system plays a crucial role. However, in the face of large-scale energy storage requirements, a single energy storage system is difficult to cope with. Therefore, generally, modular technology is adopted, such as Figure 1 As shown, multiple sodium-ion batteries are used as the energy storage system. Each sodium-ion battery is respectively connected to the DC bus through a bidirectional converter DC / DC, thereby forming a parallel system of multiple sodium-ion batteries to meet large-scale requirements.

[0003] In the parallel system of sodium-ion batteries, its parallel operation control strategy is crucial for the stable and efficient operation of the system. The existing parallel operation control strategy for the parallel system of sodium-ion batteries is to adopt a droop control method. Droop control generally refers to U-I droop control, and the current-voltage distribution is realized by adjusting the droop coefficient of the corresponding bidirectional DC / DC converter of each sodium-ion battery.

[0004] The existing parallel operation control strategy of the droop control method is to combine the state of charge (SOC) of the energy storage battery to realize the dynamic distribution of the power load of the parallel system, and a fixed droop coefficient is adopted. It does not consider the problem of the overall performance degradation of the parallel system caused by too large a difference in the state of health (SOH) between energy storage batteries, and cannot ensure the balance of SOH. It may even increase the difference in SOH within the energy storage system. Moreover, the fixed droop coefficient cannot flexibly cope with the change of SOH, which may lead to unreasonable power distribution in the parallel energy storage system.

[0005] There are differences in the internal states of each sodium-ion battery in the parallel system of sodium-ion batteries. If no effective control strategy is adopted, it will cause some sodium-ion batteries with poor state of health to reach the cut-off voltage in advance and exit the operating state, which not only affects the stability of the DC bus voltage, but may also have an adverse impact on the entire DC microgrid. Summary of the Invention

[0006] The present invention provides an improved droop control method for a parallel system of sodium-ion batteries in a microgrid to solve the problems existing in the droop control method adopted by the parallel system of sodium-ion batteries in the prior art, namely, not considering the state of health SOH and being difficult to cope with the change of the state of health SOH with a fixed droop coefficient.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] Improved droop control method for a parallel system of sodium-ion batteries in a microgrid. Each sodium-ion battery is connected to the DC bus through a bidirectional DC / DC converter, and multiple sodium-ion batteries are connected in parallel to form a sodium-ion parallel system. The method is characterized by the following steps:

[0009] Obtain the i state of charge SOC i and health state SOH i of the SOC i th sodium-ion battery, and calculate the difference Δ SOC i between the state of charge i of the K droop th sodium-ion battery and the average state of charge of all sodium-ion batteries, and calculate the actual droop control coefficient

[0010] of the

[0011] th sodium-ion battery, as shown in formula (1): n where

[0012] is the power exponent; U load Step 2: Obtain the voltage i on the DC bus side, and construct a voltage compensator with a PI regulator as the secondary compensation link for the voltage reference of the U ref th sodium-ion battery. In the secondary compensation link, use the difference U load between the voltage compensation reference value U ref - U load as the input of the voltage compensator, and obtain the compensation signal Δ i for the voltage reference of the U th sodium-ion battery through the proportional integration of the voltage compensator, as shown in formula (4):

[0013] where

[0014] is the transfer function of the secondary compensation link; G v is the line resistance of the output line of the bidirectional DC / DC converter corresponding to the R linei th sodium-ion battery; i To ensure the voltage

[0015] on the DC bus sideU load Stabilize at the voltage compensation reference value U ref , let U load = U ref , substitute into formula (4) and calculate to obtain the initial droop control coefficient that satisfies U load = U ref when k 0 ; then, substitute the initial droop control coefficient that satisfies U load = U ref when k 0 into formula (1), calculate to obtain the actual droop control coefficient that satisfies U load = U ref when K droop , and then calculate to obtain the voltage reference of the U load = U ref th sodium-ion battery when i , and complete the adjustment of the voltage reference of the th sodium-ion battery i . .

[0016] Step 3: Obtain the actual output voltage i of the U o th sodium-ion battery, the actual inductor current , combine with the adjusted voltage reference of the i th sodium-ion battery obtained in step 2 , calculate the duty cycle of the bidirectional DC / DC converter corresponding to the i th sodium-ion battery, and output it to the bidirectional DC / DC converter corresponding to the i th sodium-ion battery to achieve improved droop control.

[0017] Furthermore, in step 2, the calculation formula (2) of the voltage reference is:

[0018] .

[0019] Furthermore, in step 2, the proportional-integral formula (3) of the voltage compensator is:

[0020] ,

[0021] Among them, K p3 is the proportional coefficient of the voltage compensator; K i3 is the integral coefficient of the voltage compensator.

[0022] Furthermore, in step 3, a voltage controller constructed by a PI regulator and a current controller constructed by a PI regulator are built. The regulated voltage reference i of the th sodium-ion battery obtained in step 2 U o and the actual output voltage are input into the voltage controller for proportional integration to obtain the inductor current reference i.e., the output value obtained by the voltage controller. The inductor current reference obtained by the voltage controller and the actual inductor current are input into the current controller for proportional integration, and finally the duty cycle is obtained by the current controller.

[0023] Furthermore, the proportional integration formula (6) of the voltage controller in step 3 is:

[0024] ,

[0025] Among them, K p1 is the proportional coefficient of the voltage controller; K i1 is the integral coefficient of the voltage controller; Δ U is the compensation signal output by the voltage compensator. When it satisfies U load = U ref , Δ U = 0.

[0026] Furthermore, the proportional integration formula (7) of the current controller in step 3 is:

[0027] ,

[0028] Among them, d is the duty cycle; K p2 is the proportional coefficient of the current controller; K i2 is the integral coefficient of the current controller.

[0029] Compared with the prior art, the advantages of the present invention are:

[0030] The effectiveness of the method of the present invention in improving the bus voltage stability and enhancing the power distribution accuracy is demonstrated by the fact that through precise power load distribution, the SOH difference between energy storage units is effectively reduced; by effectively combining the SOC and SOH of sodium-ion batteries, the power load can be effectively distributed, enabling the sodium-ion batteries with a larger SOH in the parallel system to output more power and have a greater depth of discharge. This not only stabilizes the bus voltage at the ideal value but also reduces the SOH difference between sodium-ion batteries, which is beneficial to extending the service life of sodium-ion batteries and improving the energy utilization rate of the energy storage system.

[0031] The method of the present invention enables each sodium-ion battery to be in the optimal operating state, realizes dynamic power distribution, reduces the SOH difference between sodium-ion batteries, extends the service life of the parallel energy storage system, and ensures the long-term stable operation of the energy storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the structural schematic diagram of the sodium-ion battery parallel system in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The present invention will be further described below in conjunction with the drawings and embodiments.

[0034] As Figure 1 shown, this embodiment discloses an improved droop control method for a sodium-ion battery parallel system in a microgrid. The sodium-ion battery parallel system of this embodiment is as Figure 1 shown. Each sodium-ion battery is respectively connected to the DC bus through a bidirectional DC / DC converter. Each sodium-ion battery and its corresponding bidirectional DC / DC converter serve as an energy storage module, and the sodium-ion batteries in multiple energy storage modules are connected in parallel to the DC bus. The method of this embodiment includes the following steps:

[0035] Step 1: Adopt a state estimation algorithm to estimate the state of charge i and health state SOC i of the current SOH i th sodium-ion battery, calculate the average value SOC av of the state of charge of all sodium-ion batteries, and calculate the difference Δ i between the state of charge SOC i of the SOC i th sodium-ion battery and the average value of the state of charge of all sodium-ion batteries, that is, Δ SOC i Δ SOC i =SOC i -SOCav 。

[0036] Next, based on the difference Δ i corresponding to the SOC i th sodium-ion battery, the state of health SOH i and the set initial droop control coefficient k 0 , calculate the actual droop control coefficient i of the K droop th sodium-ion battery, as shown in formula (1):

[0037] ,

[0038] where n is the power exponent, and the power exponent n determines the intensity of the influence of the difference Δ SOC i on the actual droop control coefficient K droop , thus directly affecting the efficiency and speed of system energy distribution.

[0039] Then, obtain the output current i of the bidirectional DC / DC converter corresponding to the I o th sodium-ion battery to the DC bus, the set voltage compensation reference value i fixed for the U ref th sodium-ion battery, and combine the above actual droop control coefficient K droop to calculate the voltage reference i of the th sodium-ion battery, as shown in formula (2):

[0040] 。

[0041] Step 2. Obtain the voltage U load on the DC bus side, and construct a voltage compensator with a PI regulator as the secondary compensation link, i.e., the voltage compensator link, for the voltage reference i of the th sodium-ion battery.

[0042] In the secondary compensation link, with the set fixed voltage compensation reference value U ref , the difference U load between the voltage U ref -U load As the input of the voltage compensator, the proportional integral of the voltage compensator is used to obtain the i voltage reference of the th sodium-ion battery U compensation signal Δ

[0043] .

[0044] The formula without the voltage compensator is U load = U ref - K droop × I 0 - R linei × I 0. After adding the voltage compensator, an additional ( U ref - U load ) × G v is added. G v is the transfer function of the proportional integral of the voltage compensator. From this, the following formula (4) can be obtained:

[0045] .

[0046] Among them, G v is the transfer function of the secondary compensation link, i.e., the voltage compensator link; R linei is the line resistance of the output line of the i th sodium-ion battery corresponding bidirectional DC / DC converter.

[0047] To ensure that the DC bus voltage U load is stabilized at the voltage compensation reference value U ref , that is, let U load = U ref , then U ref - U load is 0, and the input of the voltage compensator is 0. Substituting it into formula (4) can calculate and obtain the initial droop control coefficient U load = U ref when k0 , the calculation formula is as shown in formula (5):

[0048] ,

[0049] According to formulas (1) and (2), it can be known that when the initial droop control coefficient k 0 changes, the actual droop control coefficient K droop also changes. Therefore, the corresponding i th sodium-ion battery voltage reference also changes accordingly. Thus, in this embodiment, the initial droop control coefficient calculated according to formula (5) that satisfies U load = U ref is substituted into formula (1) to calculate the actual droop control coefficient that satisfies k 0 = U load = U ref The actual droop control coefficient K droop is then substituted into formula (2) to calculate the voltage reference of the U load = U ref th sodium-ion battery that satisfies K droop = U load = U ref The voltage reference of the i th sodium-ion battery . Thus, the secondary compensation adjustment of the voltage reference of the i th sodium-ion battery is completed.

[0050] Step 3: Obtain the actual output voltage i of the U o th sodium-ion battery, and the actual inductor current That is, the inductor current in the DC / DC. The inductor in the DC / DC plays a role in energy storage and filtering to help stabilize the output voltage. And, a voltage controller constructed by a PI regulator and a current controller constructed by a PI regulator are built.

[0051] Subtract the actual output voltage from the sum of the adjusted voltage reference i of the th sodium-ion battery obtained in step 2 and the supplementary signal Δ U output by the voltage compensatorU o , that is, obtain , taking as the input signal and input it into the voltage controller for proportional integration to obtain the reference inductor current, that is, the output value obtained by the voltage controller , as shown in formula (6):

[0052] ,

[0053] Among them, K p1 is the proportional coefficient of the voltage controller; K i1 is the integral coefficient of the voltage controller; Δ U is the compensation signal output by the voltage compensator. When U load = U ref , Δ U = 0. Therefore, the actual input of the voltage controller is the adjusted voltage reference of the i rd sodium-ion battery obtained in step 2 , the actual output voltage U o .

[0054] Then, the difference between the reference inductor current obtained by the voltage controller and the actual inductor current is used as the input signal and input into the current controller for proportional integration. Finally, the duty cycle d is obtained by the current controller, as shown in formula (7):

[0055] ,

[0056] Among them, d is the duty cycle, K p2 is the proportional coefficient of the current controller, K i2 is the integral coefficient of the current controller.

[0057] Finally, the duty cycle i calculated for the d th sodium-ion battery is output to the bidirectional DC / DC converter corresponding to the i th sodium-ion battery, thereby realizing the improved droop control.

[0058] In this embodiment, a secondary compensation link is introduced to improve the droop method, which can overcome the DC bus voltage drop caused by the droop coefficient, can stabilize the DC bus voltage near the expected value, and effectively maintains the stability of the system voltage.

[0059] The preferred embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. The embodiments described in the present invention are merely descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Among the various specific technical features described in the above specific embodiments, they can be combined in any suitable manner without contradiction. As long as such a combination does not violate the idea of the present invention, it should also be regarded as the content disclosed in this disclosure. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0060] The present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention and without departing from the design idea of the present invention, various variations and improvements made by those skilled in the art to the technical solution of the present invention should all fall within the protection scope of the present invention. The technical content claimed by the present invention has been fully recorded in the claims.

Claims

1. Improved droop control method for a parallel sodium-ion battery system in a microgrid. Each sodium-ion battery is connected to the DC bus through a bidirectional DC / DC converter, and multiple sodium-ion batteries are connected in parallel to form a sodium-ion parallel system, characterized in that, Including the following steps: Step 1: Obtain the i state of charge of the SOC i nth sodium-ion battery SOH i and its health state, SOC i calculate the difference Δ between the state of charge of the SOC i nth sodium-ion battery and the average state of charge of all sodium-ion batteries, i and calculate the actual droop control coefficient of the K droop nth sodium-ion battery: ， Among them, n is the power exponent; k 0 is the initial droop control coefficient; Step 2: Obtain the DC bus voltage U load , and construct a voltage compensator with a PI regulator as the secondary compensation link for the i th sodium-ion battery voltage reference . In the secondary compensation link, use the voltage compensation reference value U ref , the DC bus voltage U load The difference U ref - U load as the input of the voltage compensator, and obtain the compensation signal Δ i for the th sodium-ion battery voltage reference U : ， Among them, G v is the transfer function of the secondary compensation link; R linei is the i line resistance of the output line of the bidirectional DC / DC converter corresponding to the I o th sodium-ion battery; is the output current of the DC bus; Calculated to satisfy U load = U ref At the time of the i voltage reference of the sodium-ion battery , complete the adjustment of the voltage reference of the i sodium-ion battery ; The calculation formula (2) of the voltage reference is as follows: ； Step 3: Obtain the actual output voltage of the i th sodium-ion battery U o , the actual inductor current . Combine with the regulated voltage reference of the i th sodium-ion battery obtained in Step 2 to calculate the duty cycle of the bidirectional DC / DC converter corresponding to the i th sodium-ion battery, and output it to the bidirectional DC / DC converter corresponding to the i th sodium-ion battery to implement improved droop control.

2. The improved droop control method for the parallel system of sodium-ion batteries in a microgrid according to claim 1, characterized in that, In step 2 described above, to ensure that the voltage on the DC bus side U load is stabilized at the voltage compensation reference value U ref , let U load = U ref , substitute it into formula (4) to calculate and obtain the initial droop control coefficient U load = U ref when k 0 ; then, substitute the initial droop control coefficient U load = U ref when k 0 into formula (1), calculate and obtain the actual droop control coefficient U load = U ref when K droop , and further calculate and obtain the voltage reference U load = U ref of the i th sodium-ion battery when , and complete the adjustment of the voltage reference i of the th sodium-ion battery.

3. The improved droop control method for the parallel system of sodium-ion batteries in the microgrid according to claim 1, wherein In step 2, the proportional-integral formula (3) of the voltage compensator is: ， Among them, K p3 is the proportionality coefficient of the voltage compensator; K i3 is the integral coefficient of the voltage compensator.

4. The improved droop control method for the parallel system of sodium-ion batteries in a microgrid according to claim 1, characterized in that, In step 3, a voltage controller constructed by a PI regulator and a current controller constructed by a PI regulator are built. The regulated voltage reference of the i nth sodium-ion battery obtained in step 2 and the actual output voltage U o are input into the voltage controller for proportional integration to obtain the inductor current reference i.e., the output value obtained by the voltage controller. The inductor current reference obtained by the voltage controller and the actual inductor current are input into the current controller for proportional integration, and finally the duty cycle is obtained by the current controller.

5. The improved droop control method for the parallel system of sodium-ion batteries in a microgrid according to claim 4, characterized in that, In step 3, the proportional-integral formula (6) of the voltage controller is: ， Among them, K p1 is the proportional coefficient of the voltage controller; K i1 is the integral coefficient of the voltage controller; Δ U is the compensation signal output by the voltage compensator. When satisfying U load = U ref at this time, Δ U = 0.

6. The improved droop control method for the parallel system of sodium-ion batteries in a microgrid according to claim 4, characterized in that In step 3, the proportional-integral formula (7) of the current controller is: ， Among them, d is the duty cycle; K p2 is the proportional coefficient of the current controller; K i2 is the integral coefficient of the current controller.

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