DC microgrid voltage stabilization control method based on hybrid energy storage state of charge balancing

Abstract

The application belongs to the field of direct current micro-grid energy management, and particularly relates to a direct current micro-grid voltage stabilization control method based on mixed energy storage state of charge balance. Whether the mixed energy storage can be processed is judged according to the source-load power difference fluctuation, then the state of charge of the lithium battery and the super capacitor is judged to determine the region where the mixed energy storage state of charge is located, which is two limited working zones and a normal working zone; the frequency division control filter time constant is adjusted based on the region, the power distribution of the mixed energy storage is reasonably distributed on the basis of the super capacitor processing high-frequency power and the lithium battery processing low-frequency power, and the state of charge of the two energy storages is translated to the normal working zone, that is, the balance target is achieved. In the case of stabilizing the system voltage, the number of times that the mixed energy storage state of charge reaches the limit value can be reduced in the mixed energy storage operation process, and the purpose of balancing the mixed energy storage state of charge and prolonging the service life is achieved.

Description

Technical Field

[0001] This invention belongs to the field of DC microgrid energy management, specifically relating to a DC microgrid voltage regulation control method based on hybrid energy storage state of charge balancing. Background Technology

[0002] Against the backdrop of "dual carbon" (carbon dioxide, carbon emissions, and carbon sequestration), clean energy sources, represented by wind power and photovoltaics, are beginning to be widely used in power systems. However, due to the intermittent and random nature of clean energy, its integration into the main power grid can affect its stable operation. In contrast, DC distribution networks have the advantage of being friendly to clean energy. They not only do not have voltage phase and frequency issues, but also save a lot of power electronic devices, reduce losses, and have greater flexibility and controllability.

[0003] The versatility of DC microgrid modes contributes to their flexibility. Microgrids can operate in islanded mode or interconnected with the main grid. However, in islanded mode, the core principle remains the same: maintaining stable bus voltage and improving power quality. Nevertheless, since the power sources and loads in a microgrid are influenced by external factors, a major challenge lies in how to rationally allocate power output and achieve coordinated control of each unit under the existing load conditions.

[0004] Traditional power allocation strategies that consider the state of charge (SOC) of supercapacitors adjust filter constants based on the known SOC of the supercapacitor, determining whether it falls within the charge / discharge restriction zone, upper / lower limit zone, or operating zone. This aims to minimize supercapacitor discharge and maximize charging within the discharge restriction zone, and minimize charging and maximize discharge within the charging restriction zone. However, this strategy only considers the supercapacitor's SOC, neglecting the lithium battery's SOC, and overlooking the possibility that the lithium battery's SOC might not be suitable for handling the power not processed by the supercapacitor. Furthermore, the traditional strategy's secondary power allocation aims to ensure supercapacitor health, essentially transferring power that the supercapacitor should be charging / discharging to the battery. Over time, this can lead to an imbalance in the SOC between the two energy storage systems, making control difficult and impacting system stability.

[0005] A design method for converter control parameters in a hybrid energy storage system (CN114050590A) is described in the prior art. This patent mainly concerns the PI tuning design and stability analysis of the hybrid energy storage converter. The selection of the filter constant involves choosing filter time constants T that meet stability requirements in the high-frequency, mid-frequency, and low-frequency regions. In the process of power distribution between the supercapacitor and the battery, the influence of constant power load on system stability is fully considered, thereby compensating for the negative impedance characteristics of the constant power load. However, it does not consider the influence of the hybrid energy storage state of charge (i.e., capacity) in energy distribution, nor the specific issue of how to allocate the filter constant. Summary of the Invention

[0006] To address the shortcomings and deficiencies of existing technologies, this invention aims to improve upon the traditional time-dependent filter constant control method. By establishing a normal operating range and a restricted range within a limit zone, power distribution is achieved using first-order frequency division control with a normal filter time constant when the supercapacitor or battery is in a restricted operating range approaching overcharge or over-discharge. This adjusts the filter time constant within a certain range to tilt power distribution, reducing charging for energy storage nearing overcharge and increasing charging for energy storage nearing over-discharge. This maintains a balanced state of charge for hybrid energy storage and keeps the bus voltage stable while ensuring the smoothness of lithium battery power output.

[0007] To achieve the above objectives, the present invention provides a DC microgrid voltage regulation control method based on hybrid energy storage state-of-charge balancing, which includes the following steps:

[0008] S1, the DC bus voltage U is sampled in each sampling period. dc Photovoltaic unit output current I pv Load unit input current I L And the port voltage U of lithium batteries and supercapacitors. bat U sc Current I bat I sc State of charge (SOC) bat SOC sc Perform sampling;

[0009] S2. The output power P of the photovoltaic unit is calculated through the sampled signal. G Load unit input power P L And obtain the difference between the two |P G -P L | Compare the rated power of the lithium battery with this difference. If the lithium battery meets the output conditions, proceed to S3.

[0010] S3, Input the two energy storage states of charge (SOC) bat SOC sc and P G -P L The positive and negative states are judged, the working area of ​​the hybrid energy storage is determined according to the charge state of the two energy storage systems, and the filter constant is adjusted in combination with the requirements of the DC microgrid to obtain the final first-order filter time constant.

[0011] S4. The difference between the DC bus reference voltage and the sampled value is adjusted by PI control to obtain the reference current value that the hybrid energy storage unit needs to meet. The reference current value is separated by the first-order filter time constant obtained by S3 to obtain the reference current values ​​of the lithium battery and the supercapacitor respectively. Finally, the duty cycle of the two energy storage PWM signals is obtained by adjusting the current loop to drive the two DC-DC circuits to achieve the goal.

[0012] Preferably, in step S2, when the power demand of the microgrid exceeds the output range of the lithium battery, considering that the supercapacitor is a power-type load, although it can temporarily meet the demand by generating power as a power source, its state of charge will enter a limit under long-term operation and it will not be able to meet the power demand; therefore, the lithium battery power is used as the judgment condition, and the supercapacitor is used as an auxiliary energy storage to smooth the output smoothness of the battery and extend its life.

[0013] Preferably, there are two situations where the microgrid power exceeds the load demand: when the photovoltaic power output is much greater than the load demand and exceeds the maximum charging power range of the lithium battery, the photovoltaic unit control is adjusted and power limiting control is adopted to ensure that the microgrid power demand can be met by hybrid energy storage; when the photovoltaic power output is much less than the load demand and exceeds the maximum discharging power range of the lithium battery, the secondary load needs to be disconnected to maintain the stability of the bus voltage.

[0014] Preferably, the specific control method for step S4 is as follows:

[0015] The DC microgrid bus voltage is used as the sampled value. The error between the expected value and the sampled value is converted into the expected current value after being adjusted by a PI controller. Then, the expected current value is divided into high-frequency power and low-frequency power by a first-order low-frequency filter. The high-frequency power is handled by the supercapacitor, and the low-frequency power is handled by the battery.

[0016] Preferably, the range of the filtering time constant in step S3 is [0, T]. d State of charge (SOC) of batteries and supercapacitors bat SOC sc Divided into over-discharge regions 0≤SOC≤SOC min Workspace SOC min ≤SOC≤SOC max Overcharge zone SOC max ≤SOC≤1; where SOC exists within the working area. high SOC low Two warning thresholds;

[0017] Preferably, the work area is further divided into two restricted work areas and one normal work area;

[0018] a. Within the normal operating range, the output filter time constant is the rated filter time constant;

[0019] b. Within the restricted operating area, adjust the filter time constant according to the different states of charge of the hybrid energy storage;

[0020] When operating in the restricted range below the state of charge of hybrid energy storage:

[0021]

[0022] When hybrid energy storage charging is required, the filter time constant output is:

[0023]

[0024] When hybrid energy storage and discharge are required, the filter time constant output is:

[0025]

[0026] Hybrid energy storage state of charge is in the following restricted operating range:

[0027]

[0028] When hybrid energy storage charging is required, the filter time constant output is:

[0029]

[0030] When hybrid energy storage and discharge are required, the filter time constant output is:

[0031]

[0032] When the hybrid energy storage state of charge is in the following normal operating range:

[0033]

[0034] When hybrid energy storage charging and discharging is required, the filter time constant output is:

[0035]

[0036] If one energy storage is located in the working area and the other in the non-working area, the energy storage in the working area will maintain the bus voltage stability by constant voltage control. If both energy storages are overcharged and need to be recharged, the photovoltaic system will implement power limiting control. If both energy storages are over-discharged and need to be discharged, the secondary load will need to be disconnected.

[0037] Compared with existing hybrid energy storage power allocation methods, the present invention has the following advantages:

[0038] (1) The state of charge of hybrid energy storage is taken into account, and power is allocated reasonably according to this factor. Warning and restriction are carried out before the state of charge exceeds the limit, so that the energy storage near the overcharge area is discharged more and charged less, and the energy storage near the over-discharge area is charged more and discharged less. While ensuring that the demand is met, the balance of the state of charge is maintained, thus ensuring the operational reliability and endurance of hybrid energy storage.

[0039] (2) Compared with the traditional method of adjusting the state of charge of one energy storage device after it exceeds the limit, which increases the number of charge and discharge cycles and losses, this method adjusts and controls the device before it exceeds the limit, and there is no power interaction between energy storage devices, which reduces losses and improves lifespan.

[0040] (3) This invention is an improvement on the basis of hybrid frequency division. Compared with the separation and regulation of general hybrid energy storage, this invention has the effect of smoothing the power output of the battery. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention will be described in detail below with reference to specific implementations and accompanying drawings.

[0042] Figure 1 This is a diagram of the DC microgrid architecture of the present invention;

[0043] Figure 2 This is the power allocation diagram of the hybrid energy storage SOC in this invention;

[0044] Figure 3 This is the SOC region division diagram of the present invention;

[0045] Figure 4 This is a diagram of the DC microgrid energy management of the present invention;

[0046] Figure 5 This is the source-load power diagram of the present invention;

[0047] Figure 6 This is the DC bus diagram of the present invention;

[0048] Figure 7 This is a diagram showing the lithium battery output power of the present invention;

[0049] Figure 8 This is a diagram of the supercapacitor output power of the present invention;

[0050] Figure 9 This is a state-of-charge diagram of the lithium battery of the present invention;

[0051] Figure 10 This is the state-of-charge diagram of the supercapacitor of the present invention. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0053] It should also be noted that, in order to avoid obscuring the invention with unnecessary details, only the structures and / or processing steps closely related to the solution according to the invention are shown in the accompanying drawings, while other details that are not closely related to the invention are omitted.

[0054] See as Figures 1-10 As shown, the specific implementation adopts the following technical solution:

[0055] A voltage regulation control method for DC microgrids based on hybrid energy storage state-of-charge balancing is proposed, applicable to DC microgrids based on hybrid energy storage. The DC microgrid architecture consists of typical photovoltaic (PV) power generation units, hybrid energy storage units, and load units. The PV power generation units control their output power by adjusting the duty cycle of the Boost circuit; the energy flow of the PV power generation units is unidirectional. Similarly, the load units are on the demand side, unidirectionally consuming electrical energy. The hybrid energy storage, acting as an intermediate regulating link, interacts with the DC bus power through a bidirectional DC-DC circuit, maintaining stable bus voltage.

[0056] The DC microgrid voltage regulation control method based on hybrid energy storage state-of-charge balancing includes the following steps:

[0057] S1, the DC bus voltage U is sampled in each sampling period. dc Photovoltaic unit output current I pv Load unit input current I L And the port voltage U of lithium batteries and supercapacitors. bat U sc Current I bat I sc State of charge (SOC) bat SOC sc Perform sampling.

[0058] Maintaining a stable DC bus voltage under fluctuating source load conditions requires efficient power exchange of the bus capacitors, i.e., rapid response of hybrid energy storage power to source load power fluctuations.

[0059]

[0060] Among them, C dc This is the value of the DC bus supercapacitor, P. HESS The power generated by hybrid energy storage.

[0061] S2. The output power P of the photovoltaic unit is calculated through the sampled signal. G Load unit input power P L And obtain the difference between the two |P G -P L The lithium battery's rated power is compared with this difference. If the lithium battery meets the output conditions, it enters S3.

[0062] When the power demand of a microgrid exceeds the output range of a lithium battery, considering that a supercapacitor, as a power-type load, can temporarily meet the demand by providing power, its state of charge will reach a limit under long-term operation, thus failing to meet the power demand. Therefore, the lithium battery power is used as the criterion, and the supercapacitor serves as an auxiliary energy storage device to smooth the output smoothness of the battery and extend its lifespan.

[0063] There are two situations where the power demand of a microgrid exceeds the output of a lithium battery: When the photovoltaic power output is much greater than the load demand and exceeds the maximum charging power range of the lithium battery, the photovoltaic unit control is adjusted and power limiting control is adopted to ensure that the power demand of the microgrid can be met by hybrid energy storage; when the photovoltaic power output is much less than the load demand and exceeds the maximum discharging power range of the lithium battery, secondary loads need to be disconnected to maintain the stability of the bus voltage.

[0064] S3, Input the two energy storage states of charge (SOC) bat SOC sc and P G -P L The positive and negative states are judged, the working area of ​​the hybrid energy storage is determined according to the charge state of the two energy storage systems, and the filter constant is adjusted in combination with the requirements of the DC microgrid to obtain the final first-order filter time constant.

[0065] The filtering time constant of this invention is in the range of [0, T]. d State of charge (SOC) of batteries and supercapacitors bat SOC sc Divided into over-discharge regions 0≤SOC≤SOC min Workspace SOC min ≤SOC≤SOC max Overcharge zone SOC max ≤SOC≤1. Within the working area, there exists a SOC... high SOC low Two warning thresholds.

[0066] like Figure 3 As shown, the work area is further divided into two restricted work areas and one normal work area:

[0067] When operating in the restricted range below the state of charge of hybrid energy storage:

[0068]

[0069] When hybrid energy storage charging is required, the filter time constant output is:

[0070]

[0071] When hybrid energy storage and discharge are required, the filter time constant output is:

[0072]

[0073] Hybrid energy storage state of charge is in the following restricted operating range:

[0074]

[0075] When hybrid energy storage charging is required, the filtered time constant output is:

[0076]

[0077] When hybrid energy storage and discharge are required, the filter time constant output is:

[0078]

[0079] When the hybrid energy storage state of charge is in the following normal operating range:

[0080]

[0081] When hybrid energy storage charging and discharging is required, the filter time constant output is:

[0082]

[0083] When hybrid energy storage is in the operating range and frequency division control is applied, there are two scenarios: When in the normal operating range, the supercapacitor and battery distribute power according to the normal low-pass filter time constant, without the need for secondary power distribution. However, when in the restricted operating range, based on the obtained state of charge (SOC) of the hybrid energy storage, the filter constant is adjusted to allow energy storage with a high SOC to discharge more and charge less, while energy storage with a low SOC to charge more and discharge less. Under long-term operation, this will eventually return the system to the normal operating range.

[0084] S4. The difference between the DC bus reference voltage and the sampled value is adjusted by PI control to obtain the reference current value that the hybrid energy storage unit needs to meet. The reference current value is separated by the first-order filter time constant obtained by S3 to obtain the reference current values ​​of the lithium battery and the supercapacitor respectively. Finally, the duty cycle of the two energy storage PWM signals is obtained by adjusting the current loop to drive the two DC-DC circuits to achieve the goal.

[0085] like Figure 4 As shown, the hybrid frequency division control strategy of this invention is implemented when the hybrid energy storage is feasible to maintain the microgrid power and the state of charge is normal. However, if one energy storage is located in the working area and the other in the non-working area, the energy storage in the working area adopts constant voltage control to maintain the bus voltage stability; if both energy storages are overcharged, and charging is required, the photovoltaic system adopts power-limiting constant voltage control; if both energy storages are over-discharged, and discharging is required, the secondary load needs to be disconnected.

[0086] The feasibility of this invention is verified by maintaining the bus voltage through a DC microgrid voltage regulation control strategy based on hybrid energy storage state-of-charge equilibrium, as detailed below:

[0087] DC bus reference voltage U dcref =400V, battery rated voltage U bat =168, maximum voltage U of the supercapacitor sc =192V, dual energy storage SOC min＝ 20. SOC low =40, SOC high =60, SOC max =80, the time filter constant range is [0,0.2].

[0088] like Figure 5 , 6 As shown, the bus voltage remains stable at 400V despite the following changes in external conditions. The initial photovoltaic irradiance and temperature are 800W / m². 2 At 25℃, the power is 3900W, and the initial energy load is 3100W. Illuminance decreases by 200W / m in 0.5 seconds. 2 It incorporated a 1000W energy load; the light intensity returned to 800W / m² in 2 seconds. 2 Simultaneously, a 1000W energy-type load was disconnected; after 3 seconds, an 800W energy-type load was added, along with a 200Hz, 200W power-type load; finally, after 4 seconds, the illumination became 900W / m. 2 The load increased by 1000W, and the photovoltaic load returned to 800W / m in 5 seconds. 2 3900W.

[0089] like Figure 7 , 8As shown in Figures 9 and 10, the power and state of charge (SOC) of the hybrid energy storage system can be observed. Taking the initial SOC of the battery and supercapacitor as examples (75% and 39.78%) while maintaining a stable bus voltage, the traditional strategy allows the supercapacitor to charge and discharge normally. However, the battery is in a near-overcharged state (80%), requiring measures to slow down its charging. The original strategy did not restrict the near-overcharged battery. This strategy, however, controls the supercapacitor to charge more and discharge less based on the SOC of the two energy storage systems. As can be seen from the figures, compared to the traditional strategy, the SOC of this strategy shifts towards a healthier state. Over time, this strategy will become increasingly effective.

[0090] Based on frequency division control, this invention incorporates the state of charge of both lithium batteries and supercapacitors and divides them into restricted and normal operating areas. By rationally allocating power, it improves the endurance of the hybrid energy storage system. This control method reduces the number of times the hybrid energy storage exceeds its limits, thereby reducing losses, extending the service life of the hybrid energy storage, and also has certain speed and stability.

[0091] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0092] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A DC microgrid voltage regulation control method based on hybrid energy storage state-of-charge balancing, characterized in that: It includes the following steps: (S1) sampling the DC bus voltage U dc , the photovoltaic cell output current I pv , the load cell input current I L , and the port voltage U of the lithium battery and super capacitor bat , U sc , the current I bat , I sc , the state of charge SOC bat , SOC sc ; (S2) The output power P of the photovoltaic unit is calculated by sampling the signal. G , Load unit input power P L And obtain the difference between the two |P G -P L | Compare the lithium battery's rated power with this difference. If the lithium battery meets the output requirements, proceed to the next step. (S3) Input the two energy storage states of charge (SOC) bat SOC sc and P G -P L The positive and negative states are judged, the working area of ​​the hybrid energy storage is determined according to the charge state of the two energy storage systems, and the filter constant is adjusted in combination with the requirements of the DC microgrid to obtain the final first-order filter time constant. (S4) The difference between the DC bus reference voltage and the sampled value is adjusted by PI control to obtain the reference current value that the hybrid energy storage unit needs to meet. The reference current value is separated by the first-order filter time constant obtained by S3 to obtain the reference current values ​​of the lithium battery and the supercapacitor respectively. Finally, the duty cycle of the two energy storage PWM signals is obtained by adjusting the current loop to drive the two DC-DC circuits to achieve the goal. The range of the filtering time constant in step (S3) is [0, T]. d State of charge (SOC) of batteries and supercapacitors bat SOC sc Divided into over-discharge regions 0≤SOC≤SOC min Workspace SOC min ≤SOC≤SOC max Overcharge zone SOC max ≤SOC≤1; where SOC exists within the working area. high SOC low Two warning thresholds; The work area is further divided into two restricted work areas and one normal work area; (a) The filter time constant is output as the rated filter time constant within the normal working range; (b) Within the restricted working area, adjust the filter time constant according to the different states of charge of the hybrid energy storage; When operating in the restricted range below the state of charge of hybrid energy storage: ; When hybrid energy storage charging is required, the filter time constant output is: ; When hybrid energy storage and discharge are required, the filter time constant output is: ; Hybrid energy storage state of charge is in the following restricted operating range: ; When hybrid energy storage charging is required, the filter time constant output is: ; When hybrid energy storage and discharge are required, the filter time constant output is: ; When the hybrid energy storage state of charge is in the following normal operating range: ; When hybrid energy storage charging and discharging is required, the filter time constant output is: ; If one energy storage is located in the working area and the other in the non-working area, the energy storage in the working area will maintain the bus voltage stability by constant voltage control. If both energy storages are overcharged and need to be recharged, the photovoltaic system will implement power limiting control. If both energy storages are over-discharged and need to be discharged, the secondary load will need to be disconnected.

2. The DC microgrid voltage regulation control method based on hybrid energy storage state-of-charge balancing according to claim 1, characterized in that: In step (S2), when the power demand of the microgrid exceeds the output range of the lithium battery, the lithium battery power is used as the judgment condition, and the supercapacitor is used as an auxiliary energy storage to smooth the output smoothness of the battery and extend its life.

3. The DC microgrid voltage regulation control method based on hybrid energy storage state-of-charge balancing according to claim 2, characterized in that: There are two situations where the microgrid's power demand exceeds the limit: When the power output of photovoltaics far exceeds the power demand of the load and exceeds the maximum charging power range of lithium batteries, the photovoltaic unit control is adjusted and power limiting control is adopted to enable the power demand of the microgrid to be met by hybrid energy storage. When the power output of the photovoltaic system is far less than the power demand of the load and exceeds the maximum discharge power range of the lithium battery, it is necessary to disconnect the secondary load to maintain the stability of the bus voltage.

4. The DC microgrid voltage regulation control method based on hybrid energy storage state-of-charge balancing according to claim 1, characterized in that: The specific control method for step (S4) is as follows: The DC microgrid bus voltage is used as the sampled value. The error between the expected value and the sampled value is converted into the expected current value after being adjusted by a PI controller. Then, the expected current value is divided into high-frequency power and low-frequency power by a first-order low-frequency filter. The high-frequency power is handled by the supercapacitor, and the low-frequency power is handled by the battery.

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