Improved droop control method for sodium ion battery parallel system in micro-grid

Abstract

The invention discloses an improved droop control method for a sodium-ion battery parallel system in a micro-grid, and the method comprises the following steps: 1, calculating the actual droop control coefficient of a sodium-ion battery based on the charge state, health state and initial droop control coefficient of the sodium-ion battery, and further calculating the voltage reference of the sodium-ion battery; step 2, obtaining a direct current bus side voltage, and constructing a voltage compensator as a secondary compensation link of a sodium ion battery voltage reference by using a PI regulator, thereby ensuring that the direct current bus side voltage is stabilized at a voltage compensation reference value, and obtaining an actual droop control coefficient meeting the stability and the voltage reference of the sodium ion battery; and step 3, obtaining the actual output voltage and the actual inductive current of the sodium ion battery, and calculating the duty ratio of the bidirectional DC / DC converter by combining the adjusted voltage reference of the sodium ion battery obtained in the step 2. According to the method disclosed by the invention, each sodium-ion battery can be in an optimal working state.

Description

Technical Field

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

[0002] In DC microgrids, energy storage systems play a vital role, but facing large-scale energy storage needs, a single energy storage system is difficult to cope with. Therefore, modular technology is generally adopted, such as Figure 1 As shown, multiple sodium-ion batteries are used as the energy storage system, and each sodium-ion battery is 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 needs.

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

[0004] The parallel operation control strategy of the prior art droop control method is to realize the dynamic distribution of the power load of the parallel system in combination with the state of charge (SOC) of the energy storage battery, and adopts a fixed droop coefficient. It does not consider the problem of the overall performance degradation of the parallel system caused by the excessive difference in the health state (SOH) between the energy storage batteries, and cannot ensure the balance of SOH, and may even increase the difference in SOH within the energy storage system. In addition, the fixed droop coefficient cannot flexibly respond to 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 sodium-ion battery parallel system. If no effective control strategy is adopted, some sodium-ion batteries in poor health will reach the cut-off voltage prematurely and exit the operating state, which not only affects the stability of the DC bus voltage, but may also have an adverse effect on the entire DC microgrid. Summary of the invention

[0006] The present invention provides an improved droop control method for a sodium ion battery parallel system in a microgrid, so as to solve the problems that the droop control method adopted by the sodium ion battery parallel system in the prior art does not take the health state SOH into consideration and a fixed droop coefficient is difficult to cope with the change of the health state SOH.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is: An improved droop control method for a sodium ion battery parallel system in a microgrid, wherein each sodium ion battery is connected to a DC bus through a bidirectional DC / DC converter, and a plurality of sodium ion batteries are connected in parallel to form a sodium ion parallel system, characterized in that the method comprises the following steps: Get the i State of charge of a sodium-ion battery SOC i and health status SOH i , calculate the state of charge SOC i The difference between the state of charge of all sodium-ion batteries and the average value Δ SOC i , calculate the first i The actual droop control coefficient of a sodium-ion battery K droop , as shown in formula (1): , in, n is the power index; Step 2: Get the DC bus voltage U load The voltage compensator is constructed with a PI regulator as the first i Sodium-ion battery voltage reference The secondary compensation link uses the voltage compensation reference value U ref , DC bus voltage U load Difference U ref - U load As the input of the voltage compensator, the proportional integral of the voltage compensator is used to obtain the i Sodium-ion battery voltage reference The compensation signal Δ U , as shown in formula (4): , in, G v is the transfer function of the secondary compensation link; R linei For the i The line resistance of the bidirectional DC / DC converter output line corresponding to each sodium-ion battery; In order to ensure the DC bus voltage U load Stable at voltage compensation reference value U ref ,make U load =U ref , substituting into formula (4) to obtain U load = U ref Initial droop control coefficient at k 0 ; Then, it will satisfy U load = U ref Initial droop control coefficient at k 0 Substituting into formula (1), the calculation is satisfied U load = U ref The actual droop control coefficient when K droop , and then the calculation is satisfied U load = U ref The i Voltage reference for sodium-ion batteries , complete the i Voltage reference for sodium-ion batteries of adjustment.

[0008] Step 3: Get the i The actual output voltage of a sodium-ion battery U o , actual inductor current , combined with the adjusted i Voltage reference for sodium-ion batteries , calculate the i The duty cycle of the bidirectional DC / DC converter corresponding to the sodium-ion battery is output to the i A bidirectional DC / DC converter corresponding to a sodium-ion battery is provided to achieve improved droop control.

[0009] Furthermore, in step 2, the voltage reference The calculation formula (2) is: .

[0010] Furthermore, in step 2, the proportional integral formula (3) of the voltage compensator is: , in, K p3 is the proportional coefficient of the voltage compensator; K i3 is the integral coefficient of the voltage compensator.

[0011] Furthermore, in step 3, a voltage controller constructed by a PI regulator and a current controller constructed by a PI regulator are constructed, and the regulated first i Voltage reference for sodium-ion batteries And the actual output voltage U o Input to the voltage controller for proportional integration to obtain the inductor current reference That is, the output value obtained by the voltage controller, the inductor current reference obtained by the voltage controller and the actual inductor current The input is sent to the current controller for proportional integration, and the duty cycle is finally obtained by the current controller.

[0012] Furthermore, the proportional-integral formula (6) of the voltage controller in step 3 is: , in, 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. U load = U ref Time Δ U =0.

[0013] Furthermore, the proportional-integral formula (7) of the current controller in step 3 is: , in, 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.

[0014] Compared with the prior art, the present invention has the following advantages: The method of the present invention is effective in improving bus voltage stability and enhancing power distribution accuracy. By accurately distributing power load, the SOH difference between energy storage units is effectively reduced. By effectively combining the SOC and SOH of the sodium ion battery, the power load can be effectively distributed, so that the sodium ion battery with a larger SOH in the parallel system can output more power and have a greater discharge depth. Not only can the bus voltage be stabilized at an ideal value, but the SOH difference between the sodium ion batteries is also reduced, which is beneficial to extending the service life of the sodium ion battery and improving the energy utilization rate of the energy storage system.

[0015] The method of the present invention can make each sodium ion battery in the best working state, realize the dynamic distribution of power, reduce the SOH difference between sodium ion batteries, extend the service life of the parallel energy storage system, and ensure the long-term stable operation of the energy storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the structure of a sodium ion battery parallel system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0018] like Figure 1 As 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 shown in FIG. Figure 1 As shown, each sodium ion battery is 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 to the DC bus in parallel. The method of this embodiment includes the following steps: Step 1: Use the state estimation algorithm to estimate the current i State of charge of a sodium-ion battery SOC i and health status SOH i , calculate the average state of charge of all sodium-ion batteries SOC av , and calculate the i State of charge of a sodium-ion battery SOC i and all sodium-ion battery state of charge SOC i The difference between the mean values ​​Δ SOC i , that is, Δ SOC i =SOC i -SOC av .

[0019] Next, based on the i The difference Δ corresponding to the sodium ion battery SOC i , health status SOH i And the initial droop control coefficient is set k 0 , calculate the i The actual droop control coefficient of a sodium-ion battery K droop, as shown in formula (1): , in, n is the power index, the power index n Determines the difference Δ SOC i Actual droop control coefficient K droop The intensity of the impact directly affects the efficiency and speed of the system's energy distribution.

[0020] Then, get the i The output current of the bidirectional DC / DC converter corresponding to each sodium ion battery to the DC bus I o , set the i Fixed voltage compensation reference value for sodium-ion batteries U ref , combined with the above actual droop control coefficient K droop , calculate the i Voltage reference for sodium-ion batteries , as shown in formula (2): .

[0021] Step 2: Get the DC bus voltage U load The voltage compensator is constructed with a PI regulator as the first i Voltage reference for sodium-ion batteries The secondary compensation link is the voltage compensator link.

[0022] In the secondary compensation link, the fixed voltage compensation reference value is set U ref , DC bus voltage U load Difference U ref - U load As the input of the voltage compensator, the proportional integral of the voltage compensator is used to obtain the i Sodium-ion battery voltage reference The compensation signal Δ U , the proportional integral formula of the voltage compensator is shown in formula (3): .

[0023] The formula without voltage compensator is U load = U ref - Kdroop × I 0- R linei × I 0, after adding the voltage compensator, it increases more ( U ref - U load )× G v , G v is the proportional-integral transfer function of the voltage compensator, from which the following formula (4) can be obtained: , in, G v is the transfer function of the secondary compensation link, i.e., the voltage compensator link; R linei For the i The line resistance of the output line of the bidirectional DC / DC converter corresponding to each sodium-ion battery.

[0024] In order to ensure the DC bus voltage U load Stable at voltage compensation reference value U ref , that is, U load = U ref ,but U ref - U load is 0, the input of the voltage compensator is 0, and substituting it into formula (4) can be calculated to satisfy U load = U ref Initial droop control coefficient at k 0 , the calculation formula is shown in formula (5): , According to formulas (1) and (2), when the initial droop control coefficient k 0 After the change, the actual droop control coefficient K droop also changes, so the corresponding i Voltage reference for sodium-ion batteries Therefore, in this embodiment, the satisfaction calculated according to formula (5) is U load = U ref Initial droop control coefficient at k0 Substituting into formula (1), the calculation is satisfied U load = U ref The actual droop control coefficient when K droop , and then satisfy U load = U ref The actual droop control coefficient when K droop Substituting into formula (2), we can calculate U load = U ref The i Voltage reference for sodium-ion batteries Thus, the i Voltage reference for sodium-ion batteries Secondary compensation adjustment.

[0025] Step 3: Get the i The actual output voltage of a sodium-ion battery U o , and the actual inductor current That is, the inductor current in the DC / DC. The inductor in the DC / DC plays the role of energy storage and filtering, helping to stabilize the output voltage. In addition, a voltage controller constructed by a PI regulator and a current controller constructed by a PI regulator are constructed.

[0026] The adjusted i Voltage reference for sodium-ion batteries And the complementary signal Δ output by the voltage compensator U The actual output voltage is subtracted from the sum U o , that is, ,by As the input signal, it is input to the voltage controller for proportional integration to obtain the inductor current reference, which is the output value obtained by the voltage controller. , as shown in formula (6): , in, 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. U load = U ref Time ΔU =0, so the actual input of the voltage controller is the adjusted i Voltage reference for sodium-ion batteries , actual output voltage U o .

[0027] Then, the inductor current reference obtained by the voltage controller and the actual inductor current The difference As the input signal, it is input to the current controller for proportional integration, and finally the duty cycle is obtained by the current controller. d , as shown in formula (7): , in, 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.

[0028] Finally, the i The duty cycle calculated for a sodium-ion battery d Output to i A bidirectional DC / DC converter corresponding to each sodium-ion battery is provided, thereby achieving improved droop control.

[0029] This embodiment introduces a secondary compensation link to improve the droop method, which can overcome the DC bus voltage drop caused by the droop coefficient, stabilize the DC bus voltage near the expected value, and effectively maintain the stability of the system voltage.

[0030] The preferred embodiments of the present invention are 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. The various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction, and such combinations should also be regarded as the contents disclosed in the present disclosure as long as they do not violate the concept of the present invention. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0031] The present invention is not limited to the specific details of the above-mentioned embodiments. Within the technical concept of the present invention and without departing from the design concept of the present invention, various modifications and improvements made to the technical solution of the present invention by technical personnel in this field should fall within the protection scope of the present invention. The technical contents for which protection is sought in the present invention have been fully recorded in the claims.

Claims

1. An improved droop control method for a sodium ion battery parallel system in a microgrid, wherein each sodium ion battery is connected to a DC bus through a bidirectional DC / DC converter, and a plurality of sodium ion batteries are connected in parallel to form a sodium ion parallel system, characterized in that: The following steps are involved: Step 1: Get the i State of charge of a sodium-ion battery SOC i and health status SOH i , calculate the state of charge SOC i The difference between the state of charge of all sodium-ion batteries and the average value Δ SOC i , calculate the first i The actual droop control coefficient of a sodium-ion battery K droop : ， in, n is the power index; k 0 is the initial droop control coefficient; Step 2: Get the DC bus voltage U load The voltage compensator is constructed with a PI regulator as the first i Sodium-ion battery voltage reference The secondary compensation link uses the voltage compensation reference value U ref , DC bus voltage U load Difference U ref - U load As the input of the voltage compensator, the proportional integral of the voltage compensator is used to obtain the i Sodium-ion battery voltage reference The compensation signal Δ U : ， in, G v is the transfer function of the secondary compensation link; R linei For the i The line resistance of the bidirectional DC / DC converter output line corresponding to each sodium-ion battery; I o is the output current of the DC bus; Step 3: Get the i The actual output voltage of a sodium-ion battery U o , actual inductor current , combined with the adjusted i Voltage reference for sodium-ion batteries , calculate the i The duty cycle of the bidirectional DC / DC converter corresponding to the sodium-ion battery is output to the i A bidirectional DC / DC converter corresponding to a sodium-ion battery is provided to achieve improved droop control.

2. The improved droop control method for the sodium ion battery parallel system in a microgrid according to claim 1, characterized in that: In step 2, in order to ensure that the DC bus voltage U load Stable at voltage compensation reference value U ref ,make U load = U ref , substituting into formula (4) to obtain U load = U ref Initial droop control coefficient at k 0 ; Then, it will satisfy U load = U ref Initial droop control coefficient at k 0 Substituting into formula (1), the calculation is satisfied U load = U ref The actual droop control coefficient when K droop , and then the calculation is satisfied U load = U ref The i Voltage reference for sodium-ion batteries , complete the i Voltage reference for sodium-ion batteries of adjustment.

3. The improved droop control method for the sodium ion battery parallel system in a microgrid according to claim 1, characterized in that: In step 2, the voltage reference The calculation formula (2) is: 。 4. The improved droop control method for a sodium ion battery parallel system in a microgrid according to claim 1, characterized in that: In step 2, the proportional-integral formula (3) of the voltage compensator is: ， in, K p3 is the proportional coefficient of the voltage compensator; K i3 is the integral coefficient of the voltage compensator.

5. The improved droop control method for a sodium ion battery parallel system 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 constructed, and the regulated first i Voltage reference for sodium-ion batteries And the actual output voltage U o Input to the voltage controller for proportional integration to obtain the inductor current reference That is, the output value obtained by the voltage controller, the inductor current reference obtained by the voltage controller and the actual inductor current The input is sent to the current controller for proportional integration, and the duty cycle is finally obtained by the current controller.

6. The improved droop control method for a sodium ion battery parallel system in a microgrid according to claim 5, characterized in that: The proportional-integral formula (6) of the voltage controller in step 3 is: ， in, 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. U load = U ref Time Δ U =0.

7. The improved droop control method for a sodium ion battery parallel system in a microgrid according to claim 5, characterized in that: The proportional-integral formula (7) of the current controller in step 3 is: ， in, 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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