CN115313453B - Multi-energy-storage-system coordination control method based on SOC improved droop control algorithm - Google Patents
Multi-energy-storage-system coordination control method based on SOC improved droop control algorithm Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/28—Arrangements for balancing of the load in a network by storage of energy
- H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
- H02J7/0029—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries with safety or protection devices or circuits
- H02J7/00302—Overcharge protection
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
- H02J7/0029—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries with safety or protection devices or circuits
- H02J7/00306—Overdischarge protection
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
- H02J7/0047—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries with monitoring or indicating devices or circuits
- H02J7/0048—Detection of remaining charge capacity or state of charge [SOC]
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
- H02J7/007—Regulation of charging or discharging current or voltage
- H02J7/00712—Regulation of charging or discharging current or voltage the cycle being controlled or terminated in response to electric parameters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/20—The dispersed energy generation being of renewable origin
- H02J2300/22—The renewable source being solar energy
- H02J2300/24—The renewable source being solar energy of photovoltaic origin
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E70/00—Other energy conversion or management systems reducing GHG emissions
- Y02E70/30—Systems combining energy storage with energy generation of non-fossil origin
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Abstract
The invention relates to the technical field of power electronics, and discloses a multi-energy storage system coordination control method based on an SOC improved droop control algorithm, which comprises the following steps: step 1: a plurality of energy storage batteries are adopted to run in parallel on an independently-running light storage direct current micro-grid; step 2: calculating the state of charge (SOC) of the energy storage battery in real time, and determining the SOC variation of the energy storage battery; step 3: according to the SOC variation of the energy storage battery in the step 2, an improved sagging control coefficient calculation method is provided; step 4: and carrying out energy storage balance rate coordination control according to the improved sagging control coefficient. Compared with the prior art, the invention adopts the multi-energy storage system to adjust unbalanced system power, and provides an improved sagging control method, and the SOC is introduced to calculate and control the charge and discharge power of the multi-energy storage battery, so that the overcharge or the overdischarge of the battery is effectively avoided, and the voltage fluctuation of the direct current bus is ensured to be within a control range.
Description
Technical Field
The invention relates to the technical field of parallel operation control of converters, in particular to a multi-energy storage system coordination control method based on an SOC improved droop control algorithm.
Background
With the continuous increase of the development proportion of the distributed power supply, when the optical storage direct current micro-grid running in isolation runs stably, the situation of excessive power or insufficient power of the whole system can be caused due to the problem of the indirect power generation of the distributed power supply and random load switching, and the stable running of the system is influenced. Generally, droop control is adopted in energy storage system control, but conventional droop control can make the charge and discharge power of the energy storage units identical, and may cause the problem of overcharge or overdischarge of the energy storage units.
At present, the micro-grid energy storage mode mainly adopts a hybrid energy storage system and a multi-energy storage system, the power density of a storage battery in the hybrid energy storage system is low, the power density of a super energy storage capacitor opposite to the power density of the storage battery is higher, and a plurality of scholars propose a hybrid energy storage control strategy according to the characteristics. However, the mixed compensation control strategy is adopted, so that the requirements of electric energy quality and fluctuation load are met, but the increase of charge and discharge times is unfavorable for the use of the storage battery. Therefore, the multi-energy storage battery direct-current micro-grid adopting local information communication gradually becomes a research hot spot at home and abroad at present, and the multi-energy storage battery is used in parallel with the direct-current micro-grid through a DC-DC circuit, so that the problem of overlarge charge and discharge current of a single storage battery can be solved. Most of equalization researches on the charge states of the storage batteries (SOC) used in parallel adopt an improved sagging control technology, the power of the energy storage converter is adjusted in real time by calculating the real-time SOC participation control algorithm design of the energy storage batteries, and finally the equalization effect of the SOC is achieved, so that the storage batteries of the multi-energy storage system can adjust the charging power according to the State quantity of the self battery, the problem of overcharging or overdischarging of the storage batteries is avoided, and meanwhile, the voltage stability of the direct current bus is maintained.
In future development of micro-grids, an energy storage battery device is an indispensable part, but the service life of an energy storage unit is greatly shortened by a traditional droop control mode in the same power charge-discharge mode, so that the problem of real-time dynamic adjustment of charge-discharge power and direct current bus voltage fluctuation based on an SOC improved droop control algorithm needs to be fully considered.
Disclosure of Invention
The invention aims to: aiming at the problems in the prior art, the invention provides a multi-energy-storage system coordination control method based on an SOC improved sagging control algorithm, which adopts a multi-energy-storage system to adjust unbalanced system power, and provides an improved sagging control method, wherein the SOC is introduced to calculate and control the charging and discharging power of a multi-energy-storage battery, so that the overcharge or overdischarge of the battery is effectively avoided, and the voltage fluctuation of a direct-current bus is ensured to be within a control range.
The technical scheme is as follows: the invention provides a multi-energy storage system coordination control method based on an SOC improved droop control algorithm, which comprises the following steps:
Step 1: a plurality of energy storage batteries are adopted to run in parallel on an independently-running light storage direct current micro-grid;
step 2: calculating the state of charge (SOC) of the energy storage battery in real time, and determining the SOC variation of the energy storage battery;
step 3: according to the SOC variation of the energy storage battery in the step 2, an improved droop control coefficient calculation method is provided:
S c.max、Sc.min represents the maximum value and the minimum value of the initial electric quantity in the energy storage system respectively; A reference value representing the calculated current distribution coefficient of the storage battery in operation, the value of which is equal to 0 in the case of charge operation and 1 in the case of discharge; alpha i is the i-th storage battery current distribution coefficient, and the current distribution coefficients of a plurality of storage batteries are 1; when the capacities of the plurality of storage batteries are set to be the same, the charge state change quantity of the storage batteries only depends on the current during working;
step 4: and carrying out energy storage balance rate coordination control according to the improved sagging control coefficient.
Further, the step 2 specifically includes: according to the state of charge (SOC) of the energy storage battery, power distribution is realized, a calculation formula of the energy storage battery is added, and a current expression calculation formula is adopted:
In the formula, soc i (t) and Soc i (0) are state quantities of energy storage batteries connected in parallel at the t moment and the starting moment respectively; i i (τ) represents the current level when the ith energy storage battery is operated; c i is the capacity of the ith energy storage battery; according to the above, the SOC variation of the energy storage battery can be obtained by arrangement:
further, when the secondary battery is charged, the battery, The value of (2) is equal to 0, so that the smaller the current distribution coefficient of the battery is, the smaller the absorption power is, and the larger the current distribution coefficient of the battery is, the larger the charging power is, when the SOC is smaller; when the battery is discharged/>The value of (2) is equal to 1, so that the larger the current distribution coefficient is, the larger the output power is, and the smaller the current distribution coefficient is, the smaller the discharge power is, when the SOC is smaller.
Further, the balance formula of the optical storage direct current micro-grid independently operated in the step1 is as follows:
PLoad＝PDGi+Pstorage
Pstorage＝Pstorage1+Pstorage2+…+Pstoragen
Where P Load is the total power of the load; p DGi is the output power of the distributed power supply; p storage is the output power of the energy storage unit.
Further, the operation modes of the independently operated optical storage direct current micro-grid are divided into 4 types:
mode 1: all normal operation modes of the energy storage system are not failed;
Mode 2: the energy storage unit cannot work and retract when the energy storage system fails;
mode 3: the energy storage system is wholly withdrawn due to overcharging;
mode 4: when the energy storage system exits from working, part of DG units exit from running;
According to the mode 1 and the mode 2, the power distribution is carried out according to the state of charge SOC when the energy storage unit does not exit operation.
Further, the specific process of the step 4 is as follows:
By adopting a plurality of energy storage batteries to be connected in parallel, the outlet voltage U dc1、Udc2...Udcn of the battery converter should satisfy the following conditions:
Udc1＝Udc2＝...＝Udcn
The droop coefficient is selected as:
Wherein, Setting a reference voltage for a direct current bus; i bati is the energy storage converter output current; alpha i is the i-th battery current distribution coefficient; u dc_ref is the voltage reference value after the droop coefficient calculation; the power relation of each energy storage unit is as follows:
α1Pbat1＝α2Pbat2＝...＝αnPbatn。
The beneficial effects are that:
According to the method, in the traditional sagging control method, the SOC state quantity of each energy storage unit is introduced and applied to a control algorithm, and the micro-grid system can dynamically adjust the charge and discharge power of the multiple energy storage units in real time according to the SOC state quantity, so that the problem that the traditional multiple energy storage charge and discharge power is the same is solved, the overcharge or overdischarge of the storage battery is avoided, and the service life of the multiple energy storage battery is prolonged. Based on the SOC improved droop control algorithm, the charging and discharging power of the multiple energy storage units is dynamically adjusted in real time, and the voltage of the direct current bus of the micro-grid system can be kept constant.
Drawings
FIG. 1 is a conventional droop control algorithm;
FIG. 2 is a battery sag control curve;
FIG. 3 is an isolated light storage DC micro-grid topology of the present invention;
FIG. 4 is a block diagram of an energy storage control system of the present invention;
FIG. 5 is a graph showing the power curve of the energy storage system according to embodiment 1 of the present invention, wherein (a) is the power curve of the energy storage system according to the present invention based on the SOC improvement control; (b) an energy storage system power curve for conventional droop control;
FIG. 6 shows the voltage of the bus when DG fluctuates in case 1 of the embodiment of the present invention;
FIG. 7 is a graph showing the power curve of the energy storage system according to embodiment 2 of the present invention, wherein (a) is the power curve of the energy storage system according to the present invention based on the SOC improvement control; (b) an energy storage system power curve for conventional droop control;
FIG. 8 is a graph showing the bus voltage at load fluctuation in case 2 of the present invention;
FIG. 9 is a graph showing the power curve of the energy storage system according to embodiment 3 of the present invention, wherein (a) is the power curve of the energy storage system according to the present invention based on the SOC improvement control; (b) an energy storage system power curve for conventional droop control;
FIG. 10 shows the bus voltage at the time of partial energy storage withdrawal in case 3 of the present invention.
Detailed Description
The invention is further described below with reference to the accompanying drawings. The following examples are only for more clearly illustrating the technical aspects of the present invention, and are not intended to limit the scope of the present invention.
The invention discloses a multi-energy storage system coordination control method based on an SOC improved droop control algorithm, which realizes charge and discharge power adjustment of a multi-energy storage battery unit according to an SOC real-time state and maintains the voltage stability of a direct current bus of a micro-grid. Thereby improving the service life of the energy storage unit equipment and enhancing the stability of the micro-grid. The invention is realized by the following technical scheme:
Currently, droop control is mainly divided into two types of control of "current-voltage" and "power-voltage", the present invention uses I-U droop control as an example analysis, and a control block diagram thereof is shown in fig. 1.
In the context of figure 1 of the drawings,Setting a reference voltage for a direct current bus; i bati is the energy storage converter output current; k is the resistive droop coefficient in droop control; u dc_ref is the voltage reference value after the droop coefficient calculation; g u(s) and G i(s) are a voltage outer loop PI controller and a current inner loop PI controller of voltage-current double closed loop control, respectively; l and R L are the energy storage converter inductance and inductance equivalent resistance; k 1 is the amplified signal gain; p d is the system disturbance power; and C is the voltage stabilizing capacitance value of the direct current bus. The droop coefficient is selected as:
the sagging curve thereof is shown in FIG. 2 according to the formula (4-1).
In fig. 2, U H、UL is the maximum value and the minimum value of the voltage stabilizing operation of the dc bus of the micro-grid, respectively; i c_limit and i f_limit represent maximum current allowed when the energy storage system is charged and discharged; A reference value representing the dc bus voltage. When the voltage and the current run in the first quadrant, the storage battery is in a discharging mode; when the voltage and current are operating in the second quadrant, the battery is in a charging mode.
The invention adopts three energy storage batteries to be used in parallel, so that the outlet voltages U dc1、Udc2 and U dc3 of the battery converter should satisfy the following conditions:
Udc1＝Udc2＝Udc3 (4-2)
the combination of the formula (4-1) and the formula (4-2) can further obtain the following power relations of each energy storage unit under the traditional control:
k1Pbat1＝k2Pbat2＝k3Pbat3 (4-3)
As can be seen from the formula (4-3), the conventional droop control generally adopts the same constant droop coefficient, so that the energy storage unit charges and discharges with the same power, which may cause the problems of overcharging of the energy storage battery with too high SOC and overdischarging of the energy storage battery with too low SOC, and greatly reduce the service life and utilization rate of the energy storage battery. Therefore, the invention further provides an energy storage balance rate control strategy based on the SOC, and the balance speed of the energy storage system is improved.
The invention mainly aims at an independently operated light storage direct current micro-grid structure as shown in figure 3, and a balance formula of the whole grid can be obtained from the flow direction of power in figure 3, wherein the balance formula is as follows:
PLoad＝PDGi+Pstorage (4-4)
Wherein:
Pstorage＝Pstorage1+Pstorage2+…+Pstoragen (4-5)
In the above formula, P Load is the total power P DGi of the load, and is the output power of the distributed power supply; p storage is the output power of the energy storage unit.
As known from the above equation, any fluctuation in unit power will cause the busbar voltage to fluctuate, thereby causing the micro-grid to operate abnormally. The invention divides the running mode of the direct current micro-grid into 4 types, and the energy storage system has no fault and is in all normal running modes (mode 1); the energy storage unit cannot work to exit (mode 2) when the energy storage system fails; the energy storage system is wholly withdrawn due to overcharging (mode 3); when the energy storage system is out of operation, part of DG units are out of operation (mode 4), and each DG unit can operate in proportion to the capacity in this mode. The mode 1 and the mode 2 of the invention mainly aim at considering that the energy storage unit does not exit from running and carrying out power distribution according to the state of charge (SOC); while modes 3 and 4 are mainly used for controlling the photovoltaic output, so that the simulation analysis is not specifically performed in relation to the present disclosure.
Mode 1: the method mainly researches busbar voltage fluctuation caused by load switching and distributed power supply fluctuation in the optical storage direct-current micro-grid, and stabilizes direct-current busbar voltage through charging and discharging of an energy storage unit. In this mode, the photovoltaic DG unit adopts an MPPT operating mode to operate at a maximum output power position, and the energy storage system adaptively provides full-network fluctuating power according to a state of charge (SOC). In this mode, the grid power relationship is:
Pstorage＝PLoad-PDG (4-6)
Mode 2: in the mode, certain storage batteries do not participate in the system operation due to maintenance or other uncontrollable factors, and the power shortage of the system is regulated by charging and discharging other normally-operating energy storage units. At this time, the distributed photovoltaic power supply adopts a disturbance observation method to enable the distributed photovoltaic power supply to work in an MPPT maximum power mode. At this time, the system is quickly changed from the mode 1 to the mode 2, and the energy storage unit which works normally adjusts the output power in real time according to the SOC to maintain the power supply of the system. At this time, the power relationship during normal operation of the system is:
Pstorage_m＝PLoad-PDG (4-7)
In the formula (4-8), m represents the number of energy storage units which can normally operate.
Mode 3: the mode is mainly used for researching that the energy storage cell is fully charged and is withdrawn from the regulation operation due to the fact that the energy storage cell is continuously charged due to the fact that the continuous operation voltage of the photovoltaic cell is too high. Under the condition that the direct current micro-grid still continuously outputs redundant power, the voltage of the direct current bus is increased, and at the moment, the energy storage-free unit regulates that each photovoltaic power supply should exit from the MPPT control mode to supply power to the system load in proportion to the self capacity. The adaptive droop control algorithm of the invention is adopted to adaptively adjust the output power of the DG unit to realize the power balance of the whole network. At this time, the total network power should satisfy:
PLoad＝PDG (4-9)
Mode 4: when the independent direct current micro-grid operates in the working mode 3, part of photovoltaic DG units cannot provide electric energy due to faults, and the rest photovoltaic DG units are self-adaptively increased in generation power to be in a system load power supply mode. At this time, DG unit output power is:
wherein n is the number of photovoltaic DG units capable of operating normally.
The invention mainly researches an energy storage unit working mode 1 and a mode 2 of the SOC, so that when the photovoltaic cell is set to run, the photovoltaic power generation system adopts a disturbance observation method to realize MPPT maximum power control. The storage battery has the advantage of high energy density, but the service life of the storage battery is reduced due to the fact that the single storage battery is charged and discharged with high power for a long time, and therefore the problem that the charging and discharging currents of the single storage battery are overlarge due to the fact that 3 energy storage batteries are operated in parallel is solved. The invention mainly considers the influence of state of charge (SOC), and performs charging and discharging by comparing the difference value between the real-time voltage and the reference voltage of the direct current bus, and can define the difference value as delta U:
Where U dc is the actual measurement of the DC bus voltage, Is a dc bus voltage reference. When the voltage deviation delta U is more than 0, the power supply is judged to send out power which is larger than the load demand, and the energy storage system absorbs the redundant power of the power supply and the load demand to charge; when the delta U is smaller than 0, the system power is judged to be insufficient, and the energy storage system releases energy to supply power to the load together with the power supply.
The invention considers that the power distribution is realized according to the state of charge (SOC) of the energy storage battery, so that a calculation formula of the energy storage battery is added, and a current expression calculation formula is adopted:
In the formulas (4-12), soc i (t) and Soc i (0) represent the state quantity of the energy storage battery at the time t and the starting time respectively of the i-th energy storage battery connected in parallel; i i (τ) represents the current level when the ith energy storage battery is operated; c i is the capacity of the ith energy storage cell. According to the above formula, the arrangement formula (4-12) can obtain the SOC variation of the energy storage battery as follows:
As can be seen from the formulas (4-13), the SOC variation is related to the current of the battery during operation and the capacity of the battery, and when the capacities of 3 groups of batteries are set to be the same, the state of charge variation of the battery only depends on the current during operation, so that the invention proposes an improved formula as follows:
In the formulas (4-14), S c.max、Sc.min represents the maximum value and the minimum value of the electric quantity in the energy storage system respectively; A reference value representing the calculated current distribution coefficient of the storage battery in operation, the value of which is equal to 0 in the case of charge operation and 1 in the case of discharge; α i is the i-th battery current distribution coefficient, and the invention takes three batteries as research objects, so that the distribution coefficient of three energy storage units meets α 1+α2+α3 =1.
As can be seen from (4-14), when the battery is chargedThe value of (2) is equal to 0, so that the smaller the current distribution coefficient of the battery is, the smaller the absorption power is, and the larger the current distribution coefficient of the battery is, the larger the charging power is, when the SOC is smaller. When the battery is discharged/>The value of (2) is equal to 1, so that the larger the current distribution coefficient is, the larger the output power is, and the smaller the current distribution coefficient is, the smaller the discharge power is, when the SOC is smaller. Thus, the overcharge and overdischarge of certain energy storage units are avoided, and the service life of the energy storage battery is prolonged.
Therefore, by adopting three energy storage batteries in parallel, the outlet voltage U dc1、Udc2...Udcn of the battery converter should satisfy:
Udc1＝Udc2＝...＝Udcn
The droop coefficient is selected as:
Wherein, Setting a reference voltage for a direct current bus; i bati is the energy storage converter output current; alpha i is the i-th battery current distribution coefficient; u dc_ref is the voltage reference value after the droop coefficient calculation; the power relation of each energy storage unit is as follows:
α1Pbat1＝α2Pbat2＝α3Pbatn。
in summary, a block diagram of the control current coefficient distribution of the energy storage system of the present invention is shown in fig. 4.
In order to verify the effectiveness of a sagging control method introducing SOC improvement, a simulation model is built in Matlab/Simulink. The initial three storage batteries SOC are 80%, 70% and 60% respectively; the capacity of the storage battery is 50Ah; the set value of the DC bus voltage is 400V; the maximum power generation power is respectively 50kW, 50kW and 25kW when the 3 photovoltaic cells are used for stable operation. The invention verifies the effectiveness of the invention for the following 3 working conditions:
Case 1: the photovoltaic DG unit adopts an MPPT stable working mode, and the energy storage system adopts the SOC-based current distribution method. At the beginning of simulation, the energy storage system releases energy to maintain the normal operation of the system, and the photovoltaic power generation is increased from 60kW to about 120kW at 1s, and the simulation results are shown in fig. 5 and 6.
As shown in fig. 5 (b), when the energy storage system uses the conventional droop control method, the output power of the energy storage unit is consistent all the time, which will cause overcharge and overdischarge of some storage batteries, and reduce the service life of the storage batteries.
As can be seen from fig. 5 (a), with the control method of the present invention, when the output of the photovoltaic cell is increased, the energy storage unit rapidly absorbs the surplus energy stabilizing system, and when the SOC is large, the energy storage unit absorbs less power, and when the SOC is small, the energy storage unit absorbs more power. As is apparent from fig. 5 (a), when the SOC is large, the output power of the energy storage unit is large at the time of large current, and when the SOC is small, the output power of the energy storage unit is small at the time of small current, and the energy storage system adjusts the charge and discharge power in real time according to the SOC to avoid overcharge and overdischarge of the energy storage unit, so that the service life of the storage battery is prolonged. As can be further seen from fig. 6, when the generated power fluctuates, the energy storage system can rapidly adjust the output to maintain the stable bus voltage, which illustrates the feasibility of the control method provided by the invention.
Case 2: the influence on the system stability when the system load fluctuates is mainly studied. At the beginning of the simulation, the energy storage system output power maintains the system to stably run, and about 90kW of load power is cut off at 1s, and the result is shown in fig. 7 and 8.
As can be seen from fig. 7 and 8, when the load fluctuates, the energy storage system rapidly adjusts the charging and discharging state, and rapidly changes from the original discharging operation state to the charging operation state, so that the bus voltage is stabilized at the set value of 400V, and the voltage fluctuation is small.
As shown in fig. 7, when the load is removed, the energy storage system can quickly absorb the redundant energy to adjust the charge and discharge power. As further seen in fig. 7 (a), the energy storage system operates in two operating states, a discharge mode at 0-1 s and a charge mode at 1-2 s. When the energy storage unit works for 0-1 s, the energy storage unit outputs power in proportion to the SOC, the load power provided by the energy storage unit with larger SOC is larger, and the load power provided by the energy storage unit with smaller SOC is smaller. When the energy storage system works for 1-2 s, the energy storage unit absorbs power in inverse proportion to the SOC, the smaller the SOC, the larger the energy storage unit absorbs power from the system, and the larger the SOC, the smaller the energy storage unit absorbs power from the system. In summary, the energy storage unit distributes power according to different SOCs, which is beneficial to avoiding overcharge or overdischarge of the energy storage system. As shown in fig. 7 (b), under the conventional droop control mode, the energy storage system is always charged and discharged with the same power, and the problem of overcharge or overdischarge may occur, which affects the service life of the energy storage battery.
Through the analysis of the two conditions, under the condition that energy storage can work normally, the energy storage unit can adjust output power according to the SOC size by adopting the control method provided by the invention, so that the overcharge and overdischarge of the energy storage system are avoided, and meanwhile, the voltage of the bus can be maintained to be stable at a set value, so that the effectiveness of the control method provided by the invention is verified.
Case 3: the isolated light storage direct current micro-grid storage battery can be out of operation due to faults or other reasons, and the influence on the system caused by the fact that part of energy storage units are out of operation is studied. At the initial stage of the simulation, the energy storage system works in a discharging state to maintain the normal operation of the system, and when the operation reaches 1s, the storage battery 3 exits the system operation due to the fault, and the simulation result diagrams are shown in fig. 9 and 10.
As can be seen from fig. 9 and 10, when the energy storage unit 3 is out of operation, the other two energy storage units are adjusted to increase the output force, the bus voltage fluctuation is small and the bus voltage is quickly recovered to the set value, so as to maintain the system stable.
As can be seen from fig. 9 (a), the energy storage system can distribute power output according to SOC when operating at 0-1 s; when the energy storage unit 3 is out of operation for 1s, the other two storage batteries can still distribute power output according to the SOC, so that the control objective is realized to avoid overcharge and overdischarge of the energy storage system. However, as shown in fig. 9 (b), under the conventional droop control, each energy storage unit averagely bears the system power, which can cause the energy storage system to be overcharged and overdischarged, and the service life of the storage battery is affected.
The foregoing embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the present invention and to implement the same, not to limit the scope of the present invention. All equivalent changes or modifications made according to the spirit of the present invention should be included in the scope of the present invention.
Claims (1)
1. The multi-energy storage system coordination control method based on the SOC improved droop control algorithm is characterized by comprising the following steps of:
Step 1: a plurality of energy storage batteries are adopted to run in parallel on an independently-running light storage direct current micro-grid;
the balance formula of the independently operated light storage direct current micro-grid is as follows:
PLoad＝PDGi+Pstorage
Pstorage＝Pstorage1+Pstorage2+···+Pstoragen
Where P Load is the total power of the load; p DGi is the output power of the distributed power supply; p storage is the output power of the energy storage unit;
The operation modes of the independently operated optical storage direct current micro-grid are divided into 4 types:
mode 1: all normal operation modes of the energy storage system are not failed;
Mode 2: the energy storage unit cannot work and retract when the energy storage system fails;
mode 3: the energy storage system is wholly withdrawn due to overcharging;
mode 4: when the energy storage system exits from working, part of DG units exit from running;
according to the mode 1 and the mode 2, power distribution is carried out according to the state of charge (SOC) when the energy storage unit is not withdrawn from operation;
step 2: calculating the state of charge (SOC) of the energy storage battery in real time, and determining the SOC variation of the energy storage battery;
According to the state of charge (SOC) of the energy storage battery, power distribution is realized, a calculation formula of the energy storage battery is added, and a current expression calculation formula is adopted:
In the formula, soc i (t) and Soc i (0) are state quantities of energy storage batteries connected in parallel at the t moment and the starting moment respectively; i i (τ) represents the current level when the ith energy storage battery is operated; c i is the capacity of the ith energy storage battery; according to the above, the SOC variation of the energy storage battery can be obtained by arrangement:
when the secondary battery is being charged up, The value of (2) is equal to 0, so that the smaller the current distribution coefficient of the battery is, the smaller the absorption power is, and the larger the current distribution coefficient of the battery is, the larger the charging power is, when the SOC is smaller; when the battery is discharged/>The value of (2) is equal to 1, so that the larger the current distribution coefficient of the battery is, the larger the output power is, and the smaller the current distribution coefficient of the battery is, the smaller the discharge power is, when the SOC is smaller;
step 3: according to the SOC variation of the energy storage battery in the step 2, an improved droop control coefficient calculation method is provided:
S c.max、Sc.min represents the maximum value and the minimum value of the initial electric quantity in the energy storage system respectively; A reference value representing the calculated current distribution coefficient of the storage battery in operation, the value of which is equal to 0 in the case of charge operation and 1 in the case of discharge; alpha i is the i-th storage battery current distribution coefficient, and the current distribution coefficients of a plurality of storage batteries are 1; when the capacities of the plurality of storage batteries are set to be the same, the charge state change quantity of the storage batteries only depends on the current during working;
Step 4: and performing energy storage balance rate coordination control according to the improved droop control coefficient:
By adopting a plurality of energy storage batteries to be connected in parallel, the outlet voltage U dc1、Udc2…Udcn of the battery converter should satisfy the following conditions:
Udc1＝Udc2＝...＝Udcn
The droop coefficient is selected as:
Wherein, Setting a reference voltage for a direct current bus; i bati is the energy storage converter output current; alpha i is the i-th battery current distribution coefficient; u dc_ref is the voltage reference value after the droop coefficient calculation; the power relation of each energy storage unit is as follows:
α1Pbat1＝α2Pbat2＝...＝αnPbatn。
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