CN114865745A - A hybrid energy storage system based on supercapacitor state of charge - Google Patents

Abstract

The invention discloses a hybrid energy storage system based on the state of charge of a super capacitor, comprising: a super capacitor, a storage battery, a power electronic converter, and a low-pass filter. The invention realizes the function of balancing the fluctuations of power generation and consumption power of the hybrid energy storage system, and changes the filtering time constant of the low-pass filter in real time through the fuzzy control strategy and the state of charge of the super capacitor, so that the state of charge and discharge of the super capacitor is kept at Within a reasonable range, the overcharge and overdischarge of the supercapacitor are avoided, the fast response characteristics of the supercapacitor are fully utilized, the charge and discharge power of the hybrid energy storage module is optimized, and the performance of the hybrid energy storage system is improved under the premise of stabilizing the DC bus voltage. The dynamic characteristics prolong the service life of the energy storage device.

Description

A hybrid energy storage system based on supercapacitor state of charge

technical field

The invention belongs to the field of energy storage, in particular to a hybrid energy storage system based on the state of charge of a super capacitor.

Background technique

In recent years, new energy sources such as wind power and photovoltaics have been widely used in distributed DC microgrid systems. However, the connection of a large number of new energy sources to the microgrid has brought about problems such as large fluctuations, low reliability and unpredictability. The DC microgrid composed of load and load cannot operate safely and stably. Energy storage devices can stabilize voltage fluctuations caused by new energy sources, and are flexible in control and easy to use, so they are widely used in microgrid systems. Energy storage devices can be divided into high energy density energy storage and high power density energy storage. High energy density energy storage has higher energy density, but lower power density and cannot be frequently charged and discharged; The energy density is large, the response rate is fast, and it can be charged and discharged quickly, but the energy density is small. When the bus voltage fluctuates due to load changes, a single energy storage device cannot meet the requirements of high power density and high energy density at the same time. The hybrid energy storage system composed of high-energy-density batteries and high-power-density supercapacitors (SCs) can give full play to the complementary characteristics of batteries and supercapacitors, smooth the power fluctuations of the microgrid, and achieve stable operation of the microgrid system.

The power control strategy of the hybrid energy storage system includes the smoothing filter control method and the correction method based on the battery power limit. The smoothing filter control method uses low-pass filtering to obtain the compensation power frequency band corresponding to the battery and super capacitor, so as to calculate the low-pass filter. The time constant T of the device is obtained, respectively, and the respective target smoothing power is obtained. The filter time constant T of this method is fixed. If the input of new energy fluctuates greatly, problems such as overcharging and overdischarging of batteries and supercapacitors are likely to occur due to the charging and discharging state of the energy storage unit and the limitation of charging and discharging power. The correction method based on the battery power limit is to smooth the fluctuation of new energy and effectively manage the battery charge and discharge according to the charge and discharge state of the battery and the charge and discharge power limit. When the battery state of charge (SOC) reaches the upper and lower limits, the battery charge and discharge is prohibited; When the SOC of the battery is between the upper and lower limits, according to the reference power and rated capacity of the battery, the charging and discharging power of the battery can meet the requirements. However, this method does not consider the state of charge of the supercapacitor. The supercapacitor has a small storage energy. During the operation, the state of charge is prone to change greatly, so it is easy to reach a saturated or depleted state, causing the supercapacitor to be overcharged or overcharged. put.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a hybrid energy storage system based on the supercapacitor state of charge, so as to solve the above-mentioned problems in the prior art.

In order to achieve the above object, the present invention provides a hybrid energy storage system based on the supercapacitor state of charge, comprising:

Super capacitors, batteries, power electronic converters, low-pass filters;

The low-pass filter is used to filter the target power of the hybrid energy storage system to obtain low-frequency power and high-frequency power to be suppressed;

The super capacitor is used for charging or discharging the high-frequency power together with the power electronic converter, so as to smooth the fluctuation of the high-frequency power;

The storage battery is used for charging or discharging the low-frequency power together with the power electronic converter, so as to smooth the fluctuation of the low-frequency power.

Optionally, the low-pass filter filters the target power of the hybrid energy storage system, and after obtaining the low-frequency power and high-frequency power to be suppressed, the low-frequency power is used as the target power of the battery. , and the high frequency power is taken as the target power of the super capacitor.

Optionally, the acquisition method of the low frequency power is:

Among them, P _BATREF is the low-frequency power, P _HESS is the target power of the hybrid energy storage system, T is the filtering time constant of the low-pass filter, and s is the complex frequency.

Optionally, the method for obtaining the high-frequency power is:

Among them, P _SCREF is the high frequency power.

Optionally, a fuzzy controller is provided in the low-pass filter, and the fuzzy controller is used to adjust the power distribution of the battery and the super capacitor in the hybrid energy storage system.

Optionally, the fuzzy controller adjusts the filter time constant of the low-pass filter in real time according to the state of charge of the super capacitor, and adjusts the filter time constant of the battery and the super capacitor in the hybrid energy storage system based on the adjusted filter time constant. power distribution.

Optionally, the calculation method of the state of charge of the super capacitor is:

Among them, SOC(t) and SOC(0) are the SOC values of the supercapacitor at the current moment and the initial moment, respectively; C _e is the rated capacity of the supercapacitor; _isc is the charge and discharge current of the supercapacitor.

Optionally, the calculation method of the filtering time constant is:

Among them, μ _1j (t) is the input membership function value of the supercapacitor SOC at time t, μ _2k (t) is the jth input membership function value corresponding to the output current of the renewable energy at time t, and T _jk is the corresponding The output membership function value of .

Optionally, the method for adjusting the power distribution of the hybrid energy storage system based on the adjusted filter time constant includes:

When the hybrid energy storage system is in the charging stage, if the state of charge of the supercapacitor is greater than 0.8, the filtering time constant is decreased; if the state of charge of the supercapacitor is less than 0.3, the filtering time constant is increased;

When the hybrid energy storage system is in the discharge stage, if the state of charge of the supercapacitor is greater than 0.8, the filtering time constant is increased; if the state of charge of the supercapacitor is less than 0.3, the filtering time constant is decreased; if the supercapacitor state of charge is less than 0.3 When the state of charge is between 0.3 and 0.8, the filter time constant is kept unchanged.

The technical effect of the present invention is:

The invention can change the filter time constant in real time according to the state of charge (SOC) of the supercapacitor, so that the charge and discharge state of the supercapacitor can be kept within a reasonable range, give full play to the fast response characteristics of the supercapacitor, and optimize the charge and discharge of the hybrid energy storage module. On the premise of stabilizing the DC bus voltage, the dynamic characteristics of the hybrid energy storage module are improved, and the service life of the energy storage device is prolonged.

Description of drawings

The accompanying drawings constituting a part of the present application are used to provide further understanding of the present application, and the schematic embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation of the present application. In the attached image:

1 is a topology diagram of a DC microgrid system in an embodiment of the present invention;

2 is a topology diagram of a main circuit of a hybrid energy storage system in an embodiment of the present invention;

3 is a block diagram of a low-pass filtering control strategy in an embodiment of the present invention;

4 is a block diagram of a power distribution low-pass filter based on fuzzy control in an embodiment of the present invention;

Fig. 5 is the SOC input membership function diagram of the super capacitor in the embodiment of the present invention;

6 is a current input membership function diagram of a super capacitor in an embodiment of the present invention;

Fig. 7 is the output membership function diagram of the super capacitor in the embodiment of the present invention;

8 is a reference power diagram of the super capacitor before and after applying the fuzzy control allocation power strategy when the SOC of the super capacitor is (0, 0.2) in an embodiment of the present invention;

9 is a reference power diagram of the super capacitor before and after applying the fuzzy control allocation power strategy when the SOC of the super capacitor is (0.2, 0.3) in an embodiment of the present invention;

10 is a reference power diagram of the super capacitor before and after applying the fuzzy control allocation power strategy when the SOC of the super capacitor is (0.8, 0.9) in the embodiment of the present invention;

11 is a reference power diagram of the super capacitor before and after applying the fuzzy control allocation power strategy when the SOC of the super capacitor is (0.9, 1) in the embodiment of the present invention;

FIG. 12 is a comparison diagram of the bus voltage before and after applying the fuzzy control power distribution strategy when the SOC of the super capacitor is (0.9, 1) in an embodiment of the present invention;

13 is a comparison diagram of battery output power before and after applying a fuzzy control power distribution strategy when the SOC of the super capacitor is at (0.9, 1) in an embodiment of the present invention;

14 is a comparison diagram of the bus voltage before and after applying the fuzzy control power distribution strategy when the SOC of the super capacitor is (0.2, 0.3) in an embodiment of the present invention;

15 is a comparison diagram of battery output power before and after applying a fuzzy control allocation power strategy when the SOC of the super capacitor is at (0.2, 0.3) according to an embodiment of the present invention;

16 is an SOC diagram of a super capacitor before and after applying a fuzzy control allocation power strategy in an embodiment of the present invention;

FIG. 17 is an SOC diagram of a super capacitor before and after applying a fuzzy control power distribution strategy in an embodiment of the present invention.

Detailed ways

It should be noted that the embodiments in the present application and the features of the embodiments may be combined with each other in the case of no conflict. The present application will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

It should be noted that the steps shown in the flowcharts of the accompanying drawings may be executed in a computer system, such as a set of computer-executable instructions, and, although a logical sequence is shown in the flowcharts, in some cases, Steps shown or described may be performed in an order different from that herein.

Example 1

As shown in Figures 1-7, this embodiment provides a power distribution method for a hybrid energy storage system based on the state of charge of a super capacitor, including:

1. DC microgrid with hybrid energy storage system

The DC microgrid structure using the hybrid energy storage system is shown in Figure 1, which is mainly composed of three parts: renewable energy, constant power loads (CPLs) and hybrid energy storage systems (HESS). The CPLs are realized by the closed-loop control resistance of the power electronic converter, the HESS balances the DC bus voltage fluctuation, and the renewable energy is represented by a current source.

The topology of the HESS parallel bidirectional DC-DC converter is shown in Figure 2. Among them, V _DC is the DC bus voltage, _IS is the output current of the renewable energy, I _CPL is the current of the constant power load, C _BUS is the DC bus capacitor, I _CBUS is the DC bus capacitor current, and I _REF is the output of the hybrid energy storage system Current, I _BAT and I _SC are the output currents of the battery and the super capacitor respectively, V _BAT and V _SC are the terminal voltages of the battery and the super capacitor respectively, L ₁ and L ₂ are the main circuit inductances of the battery and the super capacitor respectively, SW ₁ and SW _3. SW ₂ and SW ₄ respectively constitute the upper and lower switch tubes of the main circuit of the battery and the super capacitor DC-DC converter.

According to the state of the switch tube of the main circuit of the battery and super capacitor, it can be obtained from Figure 2

2. Control link of hybrid energy storage system

The hybrid energy storage system consists of a super capacitor (SC) and a battery. The super capacitor has the characteristics of high power density, fast response, and long cycle life, which can smooth the high frequency power part with high fluctuation frequency and small amplitude; the battery has high energy The density characteristic can smooth the low frequency power part of low fluctuation frequency and large value. Therefore, it is necessary to add a filter link, and divide the power shortage by the low-pass filtering method. When the new energy or local load power fluctuates greatly, based on the good energy complementary characteristics of the battery and the super capacitor, through the low-pass filter, the battery can absorb or compensate the low frequency power, and the super capacitor can absorb or compensate the high frequency power, so as to maintain the DC The bus voltage is stable.

In the hybrid energy storage system, the real-time power distribution between the supercapacitor and the battery is the primary problem. The low-pass filter is used to filter the target smoothing power P _HESS of the hybrid energy storage system, and the obtained low-frequency component is used as the target smoothing power of the battery. The residual power part after low-pass filtering is used as the target smoothing power of the supercapacitor, as shown in Figure 3. Among them, P _BATREF is the target flattening power of the battery; P _SCREF is the target flattening power of the super capacitor, T is the filtering time constant, and the transfer function of the low-pass filter is

The target flattening power P _HESS is distributed to the battery and the super capacitor respectively after the low-pass filtering link. The target flattening power P _BATREF of the battery and the target flattening power P _SCREF of the super capacitor are respectively

It can be known from equations (4) and (5) that P _SCREF changes rapidly due to the change of P _HESS , showing high-frequency fluctuations, while P _BATREF changes slowly with P _HESS .

3. Consider fuzzy control of supercapacitor SOC

In the hybrid energy storage system, the capacity of the supercapacitor is limited, and the state of charge (SOC) will change greatly during operation, resulting in overcharge and overdischarge of the supercapacitor. Traditional power distribution control, such as smoothing filter control, uses a fixed filter time constant T, the target power of super capacitors and batteries is fixed, and the power between energy storage units cannot be changed with the change of load power. On the other hand, the SOC value of the supercapacitor is associated with the time constant T, so that the time constant T changes with the change of the SOC value of the supercapacitor, thereby dynamically adjusting the output power of the supercapacitor and maintaining the SOC of the supercapacitor at a certain value range to prevent overcharge and overdischarge of supercapacitors.

The ampere-hour measurement method is commonly used to estimate the SOC of a supercapacitor, which is

Among them, SOC(t) and SOC(0) are the SOC values of the supercapacitor at the current moment and the initial moment, respectively; C _e is the rated capacity of the supercapacitor; _isc is the charging and discharging current of the supercapacitor, which is defined as positive during discharge and as burden.

Taking the derivation of formula (6), we can get:

Based on equation (7), it can be seen that since the rated capacity remains unchanged, the rate of change of the supercapacitor SOC increases with the increase of the charging and discharging current. Therefore, changing the time constant T to control the charging and discharging power of the supercapacitor can keep the SOC of the supercapacitor stable. Within a certain range.

4. Apply the low-pass filter time constant of fuzzy control

The reference currents for batteries and supercapacitors are:

Among them, I _BATREF is the battery reference current, and I _SCREF is the super capacitor reference current.

According to the SOC of the supercapacitor, fuzzy control is used to dynamically adjust the filtering time constant T of the low-pass filter, optimize the power distribution of the hybrid energy storage system, and realize the protection of the supercapacitor. Based on the above characteristics, as shown in Figure 4, the input of the fuzzy controller is the supercapacitor reference current I _SCREF and the state of charge SOC of the supercapacitor, and the filtering time constant T is the output.

When the SOC of the supercapacitor is between 0.3-0.8, T is constant.

When the SOC of the supercapacitor belongs to (0, 0.2), the filter time constant T is adjusted so that the supercapacitor is charged preferentially to avoid the discharge of the supercapacitor.

When the SOC of the supercapacitor belongs to (0.2, 0.3), when the supercapacitor is charged, T increases, and the supercapacitor is charged preferentially. On the contrary, when the supercapacitor discharges, reducing T, the discharge power of the supercapacitor decreases.

When the SOC of the supercapacitor is (0.8, 0.9), when the supercapacitor is charged, T decreases, and the supercapacitor power decreases; when the supercapacitor is discharged, T increases, and the supercapacitor discharge power increases.

When the SOC of the supercapacitor is (0.9,1), the supercapacitor is discharged first and charging is prohibited.

The fuzzy set of the supercapacitor SOC is {SS, S, M, L, LL}, representing {very small, small, medium, large, very large}, respectively. At the same time, the fuzzy sets of the supercapacitor current I _SCREF are {N, P}, representing {charging, discharging} respectively; the fuzzy sets of T are {NB, NM, NS, M, PS, PM, PB}, representing {negative Large, Negative Medium, Negative Small, Medium, Positive Small, Positive Medium, Positive Large}. The input and output membership functions of the fuzzy controller are shown in Figures 5, 6 and 7. The specific fuzzy control rules are shown in Table 2. The fuzzy control output adopts the center of gravity method to achieve de-blurring, as shown in Equation 10, and obtains the filtering time constant T.

Among them, μ _1j (t) corresponds to the input membership function value of the supercapacitor SOC at time t, μ _2k (t) corresponds to the jth input membership function value corresponding to I _s at time t, and T _jk is the corresponding output membership function degree function value.

Table 1

When the energy storage system is in the charging stage, if the SOC of the supercapacitor is too high, reduce the filtering time constant T, thereby reducing the power absorbed by the supercapacitor; if the SOC of the supercapacitor is low, increase the time constant T, thereby increasing the supercapacitor absorbed work. When the energy storage system is in the discharge stage, if the SOC of the supercapacitor is high, increase the time constant T, thereby increasing the power released by the supercapacitor; if the SOC of the supercapacitor is low, decrease the time constant T, thereby reducing the output of the supercapacitor If the SOC of the supercapacitor is moderate, keep the time constant T unchanged.

Embodiment 2

As shown in FIGS. 8-17 , this embodiment provides an application example of a method for power distribution of a hybrid energy storage system based on the state of charge of a super capacitor.

This embodiment uses MATLAB/Simulink software to build a DC microgrid system model including a hybrid energy storage system. The simulation parameters of the microgrid system are shown in Table 1. The simulation applies the fuzzy control distribution power strategy (FUZZY) to the hybrid energy storage system of the predictive current control (MPCC), and compares the output power changes of the supercapacitors before and after the use of the fuzzy control distribution power strategy. .

1) Fuzzy control

After applying the fuzzy control allocation power strategy, the filtering time constant of the low-pass filter can be dynamically adjusted according to the SOC of the supercapacitor. When the SOCs of the supercapacitors are (0,0.2), (0.2,0.3), (0.8,0.9) and (0.9,1), respectively, the reference powers of the supercapacitors before and after applying the fuzzy control allocation strategy are shown in Figures 8 and 9, respectively. , 10 and 11.

Table 2

It can be seen from Figure 8-11 that after applying the fuzzy control power distribution strategy, different filter time constants are selected according to the SOC of the supercapacitor, which significantly adjusts the reference power of the supercapacitor and optimizes the power distribution of the hybrid energy storage system. Absorption/release of power is smoother. The fuzzy control distribution power strategy can distribute the power of the hybrid energy storage system more effectively, effectively change the SOC of the supercapacitor, and ensure that the supercapacitor always works in a safe and stable area.

When the SOC of the supercapacitor is (0.9, 1), the bus voltage and battery output power before and after applying the fuzzy control distribution strategy are shown in Figures 12 and 13, respectively. The results in Figures 11, 12 and 13 show that when the SOC of the supercapacitor is large and the fuzzy control power distribution strategy is adopted, the supercapacitor discharge current increases, the regulation time is prolonged, and the corresponding battery regulation time is shortened, and the fuzzy control allocation power strategy is applied. The front and rear bus voltages remain stable and the adjustment time remains unchanged; similarly, when the supercapacitor needs to be charged, the charging current is significantly reduced, the adjustment time is shortened, and the DC bus voltage remains stable.

When the SOC of the supercapacitor is (0.2, 0.3), the bus voltage and battery output power before and after applying the fuzzy control distribution strategy are shown in Figures 14 and 15, respectively. The results in Figures 9, 14 and 15 show that when the SOC of the supercapacitor is (0.2, 0.3) and the fuzzy control power distribution strategy is adopted, the supercapacitor charging current increases, the regulation time is prolonged, and the corresponding battery regulation time is shortened, and the fuzzy control is applied. Before and after the power distribution strategy, the bus voltage remains stable and the adjustment time remains unchanged; when the supercapacitor discharges, the discharge current decreases significantly, the adjustment time is shortened, and the DC bus voltage remains stable.

Figure 16 shows the SOC change curve of the supercapacitor before and after applying the fuzzy control power distribution strategy when the SOC of the supercapacitor is small. Figure 17 shows the SOC change curve of the supercapacitor before and after applying the fuzzy control power distribution strategy when the SOC of the supercapacitor is large. The results in Figures 16 and 17 show that when the fuzzy control power distribution strategy based on the supercapacitor SOC is adopted, the filtering time constant T will change adaptively, and the reference discharge or charging power of the supercapacitor will also change with the change of T. If the SOC of the supercapacitor is large, the reference discharge power of the supercapacitor will increase, thereby outputting a larger discharge current, so that the SOC of the supercapacitor quickly drops to a safe range to avoid overcharging. At the same time, the reference charging power of the super capacitor is reduced, the charging current is reduced, and the adjustment time is shortened. On the contrary, if the SOC of the supercapacitor is small, due to the change of the filter time constant T, the reference charging power of the supercapacitor will increase, and the charging current will increase; The SOC of the capacitor rises rapidly to a safe range to avoid over-discharge.

The simulation results show that the fuzzy control power distribution strategy considering the supercapacitor SOC proposed in this patent can effectively change the state of charge of the supercapacitor in a short time, optimize the power of the supercapacitor, protect the supercapacitor, and ensure that the hybrid energy storage system is always Work in a safe and stable area.

The above are only the preferred specific embodiments of the present application, but the protection scope of the present application is not limited to this. Substitutions should be covered within the protection scope of this application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims (9)

1. a hybrid energy storage system based on supercapacitor state of charge, is characterized in that, comprises supercapacitor, accumulator, power electronic converter, low-pass filter; The low-pass filter is used to filter the target power of the hybrid energy storage system to obtain low-frequency power and high-frequency power to be suppressed; The super capacitor is used for charging or discharging the high-frequency power together with the power electronic converter, so as to smooth the fluctuation of the high-frequency power; The storage battery is used for charging or discharging the low-frequency power together with the power electronic converter, so as to smooth the fluctuation of the low-frequency power. 2. The hybrid energy storage system based on the supercapacitor state of charge according to claim 1, wherein the low-pass filter filters the target power of the hybrid energy storage system to obtain the low frequency that needs to be suppressed After the process of power and high-frequency power, the low-frequency power is used as the target power of the battery, and the high-frequency power is used as the target power of the super capacitor. 3. The hybrid energy storage system based on the supercapacitor state of charge according to claim 2, wherein the acquisition method of the low-frequency power is:

Among them, P _BATREF is the low-frequency power, P _HESS is the target power of the hybrid energy storage system, T is the filtering time constant of the low-pass filter, and s is the complex frequency. 4. The hybrid energy storage system based on the supercapacitor state of charge according to claim 2, wherein the acquisition method of the high-frequency power is:

Among them, P _SCREF is the high frequency power. 5. The hybrid energy storage system based on the supercapacitor state of charge according to claim 2, wherein the low-pass filter is provided with a fuzzy controller, and the fuzzy controller is used to adjust the hybrid energy storage Power distribution of batteries and supercapacitors in the system. 6. The hybrid energy storage system based on the supercapacitor state of charge according to claim 5, wherein the fuzzy controller adjusts the filter time constant of the low-pass filter in real time according to the state of charge of the supercapacitor , the power distribution of the battery and the super capacitor in the hybrid energy storage system is adjusted based on the adjusted filter time constant. 7. The hybrid energy storage system based on the state of charge of the supercapacitor according to claim 6, wherein the calculation method of the state of charge of the supercapacitor is:

Among them, SOC(t) and SOC(0) are the SOC values of the supercapacitor at the current moment and the initial moment, respectively; C _e is the rated capacity of the supercapacitor; _isc is the charge and discharge current of the supercapacitor. 8. The hybrid energy storage system based on the supercapacitor state of charge according to claim 6, wherein the calculation method of the filtering time constant is:

Among them, μ _1j (t) is the input membership function value of the supercapacitor SOC at time t, μ _2k (t) is the jth input membership function value corresponding to the output current of the renewable energy at time t, and T _jk is the corresponding The output membership function value of . 9. The hybrid energy storage system based on the supercapacitor state of charge according to claim 6, wherein the method for adjusting the power distribution of the hybrid energy storage system based on the adjusted filter time constant comprises: When the hybrid energy storage system is in the charging stage, if the state of charge of the supercapacitor is greater than 0.8, the filtering time constant is decreased; if the state of charge of the supercapacitor is less than 0.3, the filtering time constant is increased; When the hybrid energy storage system is in the discharge stage, if the state of charge of the supercapacitor is greater than 0.8, the filtering time constant is increased; if the state of charge of the supercapacitor is less than 0.3, the filtering time constant is decreased; if the supercapacitor state of charge is less than 0.3 When the state of charge is between 0.3 and 0.8, the filter time constant is kept unchanged.

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