CN114285054A - Distributed energy storage state of charge (SOC) balancing strategy based on virtual impedance self-adjustment - Google Patents
Distributed energy storage state of charge (SOC) balancing strategy based on virtual impedance self-adjustment Download PDFInfo
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Abstract
The invention relates to a distributed energy storage state of charge balancing strategy based on virtual impedance self-adjustment, which comprises the following steps: providing a new V-I droop control expression based on traditional V-I droop control adopted by load distribution of a microgrid, wherein the new V-I droop control expression introduces self-adaptive associated virtual impedance and self-adjusting virtual impedance; designing a self-adaptive associated virtual impedance; designing a self-adjusting associated virtual impedance; the microgrid system works in a new V-I droop control mode, and the sampling holder adjusts virtual impedance. The invention can overcome the problem of mismatched parameters, realize the charge state balance and accurate load distribution of the distributed energy storage units, optimize the charging and discharging efficiency of the energy storage units and maintain good electric energy quality.
Description
Technical Field
The invention belongs to the technical field of coordination and cooperation among distributed energy storage in a system, and particularly relates to a distributed energy storage state of charge balancing strategy based on virtual impedance self-adjustment.
Background
Renewable energy power generation and distributed energy technology are concerned under the situation of environmental pollution and energy shortage, a micro grid is widely used as a novel power system capable of effectively receiving renewable energy, demonstration engineering is widely applied, compared with an alternating current micro grid, a direct current micro grid has no harmonic wave and reactive power balance, and meanwhile, the problem of excessive loss of direct current/alternating current conversion is solved, due to the inherent intermittency and uncertainty of distributed energy, the micro grid system generally needs to introduce an energy storage system consisting of distributed energy storage units to realize peak clipping and valley filling and maintain the power balance in the system, so that the output of each energy storage unit is coordinated, the balance of the charge states of the distributed energy storage units is realized, and a research hotspot is formed, and under the condition of unbalanced charge states, part of the energy storage units are overcharged or overdischarged, the service life of the energy storage units is directly shortened, droop control is generally used for load distribution of DC/DC converters connected in parallel on a common bus, because the traditional droop control adopts fixed virtual impedance, although the output current of each group of converters can be accurately distributed, the state of charge of distributed energy storage units cannot be balanced, in order to solve the problem of unbalanced state of charge, the current method comprises the steps of dynamically balancing the load power and the state of charge by adding the state of charge into an armature resistor, or utilizing the n-order correlation of a droop coefficient and the state of charge, adjusting the balancing rate of the state of charge according to the magnitude of n, realizing the balance of the state of charge in the charging and discharging process by a control strategy, enhancing the adaptive droop control by adopting a cooperative physical network, and mutually cooperating battery energy storage systems to balance the state of charge among the battery energy storage systems, and when the situation occurs, a constant voltage charging mode is carried out and the self-adaptive droop concept is used for processing, but the research does not really consider the problem of mismatching parameters in the direct-current micro-grid system, and the method cannot be widely applied.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, and provides a distributed energy storage state-of-charge balancing strategy based on virtual impedance self-adjustment, which can overcome the problem of mismatched parameters, realize the state-of-charge balancing and accurate load distribution of distributed energy storage units, optimize the charging and discharging efficiency of the energy storage units and maintain good electric energy quality.
The technical scheme adopted by the invention is as follows: a distributed energy storage state of charge balancing strategy based on virtual impedance self-adjustment comprises the following steps:
step S1: based on the traditional V-I droop control adopted by the load distribution of the micro-grid, a new V-I droop control expression is provided, wherein the new V-I droop control expression introduces self-adaptive associated virtual impedance and self-adjusting virtual impedance,
the conventional V-I droop control expression is:
Vi＝V*-RiIi
in the formula, ViRepresenting the output voltage, V, of the DC/DC converter connected with the ith group of distributed energy storage units*Represents the system standard reference voltage value, RiRepresents the virtual impedance of the ith group of distributed energy storage units, IiRepresenting the output current value of the ith group of distributed energy storage units;
the new V-I droop control expression is:
Vi＝V*-RSiIi-RaiIi
in the formula, RSiRepresenting an adaptively associated virtual impedance, RaiRepresents a self-adjusting virtual impedance, at which time the virtual impedance RiRepresents RSiAnd RaiSumming;
step S2: designing the self-adaptive associated virtual impedance, wherein the expression is as follows:
wherein the content of the first and second substances,
in the formula, SoCiRepresenting the current state of charge, SoC, of the ith group of distributed energy storage unitsrefReference value, C, representing the state of chargeDiRepresents the capacity of the ith group of distributed energy storage units, R0Representing the initial value of the virtual impedance, ksDenotes the adjustment coefficient, A denotes the acceleration factor, ksThe value of the sign represents a sign function, when the output value of the sign function is 1, the distributed energy storage unit is in a charging mode, and when the output value of the sign function is-1, the distributed energy storage unit is in a discharging mode;
step S3: the self-adjusting associated virtual impedance is designed,
adding the self-adjusting virtual impedance on the basis of the virtual impedance to obtain the following expression:
wherein R isi、RjRespectively representing the virtual impedance of the distributed energy storage units of the ith group and the jth group, Rai、RajSelf-adjusting virtual impedance, R, of distributed energy storage units respectively representing ith and jth groupsSi、RSjRespectively representing the self-adaptive associated virtual impedance R of the distributed energy storage units of the ith group and the jth grouplinei、RlinejRespectively representing the line impedance of the distributed energy storage units of the ith group and the jth group to the common coupling point;
step S4: the micro-grid system works in a new V-I droop control mode, the sampling holder adjusts virtual impedance, and the expression is as follows:
Rai_(n+1)＝Rai_n+Hi_n
Hi_n＝ki(Ii_n-Iref_n)
in the formula, Rai_(n+1)Self-adjusting virtual impedance, R, representing the n +1 th sampling periodai_nSelf-adjusting virtual impedance, H, representing the nth sampling periodi_nA load distribution adjustment item representing the ith group of distributed energy storage units in the nth sampling period, Ii_nRepresenting the output current, I, of the nth sampling periodref_nReference value, k, representing the output current for the nth sampling periodiThe current adjustment factor is obtained.
Specifically, in step 2, the current state of charge SoC of the ith group of distributed energy storage unitsiThe expression of (a) is:
in the formula, SoCi0And m represents the ratio of the output end voltage of the DC/DC converter to the end voltage of the storage battery.
Specifically, in step 4, when the microgrid system operates in the new V-I droop control mode, the central processing unit sends a synchronization signal to the sample holder to trigger the sample holder to adjust the virtual impedance.
The invention has the beneficial effects that: the invention introduces self-adjusting virtual impedance on the basis of the traditional virtual impedance, the sampling holder is used for self-adaptively adjusting the virtual impedance of droop control in each sampling period, the problem of unmatched parameters is solved, further the charge state balance and the accurate load distribution of the distributed energy storage unit are realized, the charge and discharge efficiency of the energy storage unit can be effectively optimized on the premise of ensuring the accurate load distribution, and the good electric energy quality is maintained.
Drawings
FIG. 1 is a flow chart of the steps of the present invention;
FIG. 2 is a topological structure diagram of the microgrid system of the present invention;
FIG. 3 is a schematic diagram of a distributed energy storage unit according to the present invention;
FIG. 4 is an equivalent circuit diagram of the local load of the present invention;
FIG. 5 is a control schematic block diagram of the present invention;
FIG. 6 is a graph of load distribution after a first sampling period in a discharge state;
FIG. 7 is a graph of load distribution after a first sampling period in a charged state;
fig. 8 is a schematic diagram illustrating a state of charge simulation of the energy storage unit in case 1 during stable charging;
fig. 9 is a schematic diagram illustrating simulation of output current when the energy storage unit is stably charged in case 1;
fig. 10 is a schematic diagram illustrating simulation of dc bus voltage when the energy storage unit is stably charged in case 1;
fig. 11 is a schematic diagram illustrating the state of charge simulation of the energy storage unit in case 1 during stable discharge;
fig. 12 is a simulation diagram of the output current when the energy storage unit in case 1 is stably discharged;
fig. 13 is a schematic diagram illustrating simulation of dc bus voltage during stable discharge of the energy storage unit in case 1;
fig. 14 is a schematic diagram of power distribution in the microgrid system in case 2;
fig. 15 is a schematic diagram of the state of charge simulation of the energy storage unit in case 2;
fig. 16 is a simulation diagram of the output current of the energy storage unit in case 2;
fig. 17 is a schematic diagram of dc bus voltage simulation in case 2;
fig. 18 is a schematic diagram of power distribution in the microgrid system in case 3;
fig. 19 is a schematic diagram of the state of charge simulation of the energy storage unit in case 3;
fig. 20 is a simulation diagram of the output current of the energy storage unit in case 3;
fig. 21 is a simulation diagram of dc bus voltage in case 3.
Detailed Description
The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments of the present invention, belong to the protection scope of the present invention, and are specifically described below with reference to the embodiments.
As shown in fig. 1, the present invention comprises the steps of:
step S1: providing a new V-I droop control expression based on traditional V-I droop control adopted by load distribution of a microgrid, wherein the new V-I droop control expression introduces self-adaptive associated virtual impedance and self-adjusting virtual impedance;
step S2: designing a self-adaptive associated virtual impedance;
step S3: designing a self-adjusting associated virtual impedance;
step S4: the microgrid system works in a new V-I droop control mode, and the sampling holder adjusts virtual impedance.
The principle analysis before the implementation of the invention is as follows:
a typical topological structure of a microgrid system is shown in fig. 2, a distributed power supply, distributed energy storage units and various loads are connected to a public direct current bus through corresponding power electronic converters, the line impedance and the local load are respectively positioned at an outlet of each converter and a local direct current bus, and the problem of mismatching of line parameters is objectively existed, when an interconnection converter connected with a microgrid and an alternating current grid is opened, the direct current microgrid operates in an independent operation mode, a photovoltaic and a fan usually operate in a maximum power tracking mode to obtain the maximum output, at the moment, the distributed energy storage units in the system need to be matched with renewable energy sources for power generation, the defects caused by the intermittent nature of the renewable energy sources are overcome, for example, when the energy emitted by the renewable energy sources is greater than the load requirement at the moment in an actual scene, the energy storage units are charged to absorb the redundant energy, and on the contrary, the energy storage units provide the energy required by heavy load, the system is always maintained in an energy dynamic equilibrium state.
The load distribution of the microgrid generally adopts traditional V-I or V-P droop control, the traditional V-I droop can more intuitively reflect the relation between the voltage and the charge state of the distributed energy storage units, and the expression is as follows:
Vi＝V*-RiIi (1)
in the formula, ViRepresenting the output voltage, V, of the DC/DC converter connected with the ith group of distributed energy storage units*Represents the system standard reference voltage value, RiRepresents the virtual impedance of the ith group of distributed energy storage units, IiRepresenting the output current value of the ith group of distributed energy storage units;
in order to ensure the power supply reliability of the independently operated micro-grid, N groups of distributed energy storage units with the same or similar capacity are connected in parallel to a direct current bus through a converter, as shown in figure 3, wherein DESUiAnd DESUjRepresenting any two groups of distributed energy storage units, Rlinei、RlinejRespectively representing the line impedance, V, of the distributed energy storage units of the ith group and the jth group to the common coupling pointpccIs a DC bus voltage, RpccIs an equivalent load resistance, RlocalIs the local load.
The state of charge of the distributed energy storage unit representsThe energy currently stored by the energy storage units and the current state of charge (SoC) of the ith group of distributed energy storage unitsiDefined according to coulomb counting as:
in the formula, SoCi0Representing the initial value of the state of charge of the ith group of distributed energy storage units, m representing the ratio of the voltage of the output end of the DC/DC converter to the voltage of the storage battery, CDiThe capacity of the ith group of distributed energy storage units is represented, and the distributed energy storage units are closely related to the initial values, the capacities of the energy storage units and the output currents according to the formula.
Derivation of equation (2) yields the rate of change of the state of charge, i.e., the trend of change of the state of charge, and the obtained equation is as follows:
as can be seen from equation (3), the state of charge balance is affected by the capacity of the distributed energy storage units and the output current thereof, and the change rates of the state of charge tend to be consistent with the state of charge balance, which requires that each distributed energy storage unit must realize accurate load current sharing.
The expression for each converter output current from fig. 3 can be found as:
bringing formula (1) into formula (4) to obtain
The relationship of the charge state change rate between any two groups of distributed energy storage units is obtained according to the formulas (3), (4) and (5), wherein i, j is 1,2, N and i is not equal to j,
from the formula (6), it is known that the state of charge balance of the distributed energy storage units is influenced by the original virtual impedance, meanwhile, the unbalanced degree of the state of charge is deepened due to the existence of the equivalent line impedance, and the influence of unmatched line impedance cannot be overcome by the conventional V-I droop control, so that the load currents cannot be accurately distinguished, and therefore, the state of charge of each distributed energy storage unit cannot be balanced.
In a local load connected with a local direct current bus and a distributed energy storage unit, the problem that parameters are not matched can directly influence the output current average of the energy storage unit, the balance control on the state of charge is influenced, research and analysis are facilitated, the local load is equivalently converted into an equivalent public load to participate in load distribution of a system, an equivalent circuit is shown in fig. 4, and fig. 4 shows that DESU is equivalent to DESUiThe connected branch, according to the equivalent circuit, obtains the following two equations:
wherein, IDiThe current value of the line impedance branch, ILiFor the current values of the local load branches, equations (7) and (8) are combined, and the equivalent line impedance is obtained as follows:
the problem of the unmatched parameters after conversion is converted into the problem of the unmatched line impedance parameters, and the local load is equivalent to the load distribution of the common bus side participating in the system.
The distributed energy storage state of charge equalization strategy based on virtual impedance self-adjustment provided by the invention is introduced:
according to the analysis, the energy balance inside the independent operation of the direct-current microgrid is maintained by the distributed energy storage units, in order to overcome the problem of mismatched parameters and realize the optimal SoC control of the state of charge of the distributed energy storage units, the invention provides a new V-I droop control expression which is expressed as follows:
Vi＝V*-RSiIi-RaiIi (10)
in the formula, RSiRepresenting an adaptively associated virtual impedance, RaiRepresents a self-adjusting virtual impedance, at which time the virtual impedance RiRepresents RSiAnd RaiAnd (4) summing.
Firstly, designing a self-adaptive associated virtual impedance, which is specifically designed as follows:
in the formula, SoCiRepresenting the current state of charge, SoC, of the ith group of distributed energy storage unitsrefReference value, C, representing the state of chargeDiRepresents the capacity of the ith group of distributed energy storage units, R0Representing the initial value of the virtual impedance, ksDenotes the adjustment coefficient, A denotes the acceleration factor, ksThe value of the A and the value of the A influence the convergence degree and the convergence speed of the state of charge, an exponential function is introduced to enable the traditional virtual impedance to be directly related to the SoC, sign represents a sign function, and when the output value of the sign function is 1, the distributed energy storage unit is in a charging mode and signsWhen the output value of the function is-1, the distributed energy storage unit is in a discharge mode;
taking the charging of the energy storage system as an example, the output value of the sign function is 1, and the simultaneous (1), (2) and (11) can be obtained
In an energy storage system, DESUiAnd DESUjThe SoC deviation of (a) is:
the derivation of equation (15) can be:
according to the Taylor series expansion, the first order expansion of the e-exponential function is approximately expressed as:
ex＝1+x (17)
the coupling type (16) and (17) approximately obtain:
as can be seen from equation (18), when the SoC deviation between the two sets of energy storage units approaches zero, the change rate of the corresponding SoC also becomes zero, so that SoC equalization is achieved, overcharge of the distributed energy storage units is successfully avoided, and similarly, the phenomenon of overdischarge is also avoided.
As can be seen from the formulas (5) and (6), the problem of mismatching parameters inevitably exists in the micro-grid system, and the self-adjusting virtual impedance is added on the basis of the virtual impedance to obtain the following expression:
wherein R isi、RjRespectively representing the virtual impedance of the distributed energy storage units of the ith group and the jth group, Rai、RajSelf-adjusting virtual impedance, R, of distributed energy storage units respectively representing ith and jth groupsSi、RSjRespectively representing the self-adaptive associated virtual impedance R of the distributed energy storage units of the ith group and the jth grouplinei、RlinejAnd respectively representing the line impedance of the distributed energy storage units of the ith group and the jth group to the common coupling point.
A control schematic block diagram of a distributed energy storage state-of-charge equalization strategy based on virtual impedance self-adjustment is shown in fig. 5, where S/H is a sample holder, a trigger signal S is a switching function, when a conventional V-I droop control mode is used, a worker does not trigger a signal, S is 0, the sample holder does not operate, when a new V-I droop control mode proposed by the present invention is used, a central controller sends a trigger signal to trigger the sample holder, and the sample holder adjusts virtual impedance, and the expression is:
Rai_(n+1)＝Rai_n+Hi_n (20)
Hi_n＝ki(Ii_n-Iref_n) (21)
in the formula, Rai_(n+1)Self-adjusting virtual impedance, R, representing the n +1 th sampling periodai_nSelf-adjusting virtual impedance, H, representing the nth sampling periodi_nA load distribution adjustment item representing the ith group of distributed energy storage units in the nth sampling period, Ii_nRepresenting the output current, I, of the nth sampling periodref_nReference value, k, representing the output current for the nth sampling periodiThe current adjustment factor is obtained.
When the central controller sends out a trigger signal to trigger the sampling retainer, communication is carried out between local adjacent distributed energy storage units, related electrical quantities are interacted, corresponding electrical quantities are calculated according to an equation (12) and an equation (22), the corresponding electrical quantities are compared with local sampling results, and then the sampling retainer adjusts virtual impedance.
To further illustrate the strategy proposed by the present invention, two sets of distributed energy storage units DESU with the same capacity are usediAnd DESUjFor example, specific timing analysis is performed by taking equations (20) to (22) into equation (10), and the following expressions are obtained:
Vi_(n+1)＝V*-RSiIi_n-Rai_(n+1)Ii_n＝V*-RSiIi_n-(Rai_n+Hi_n)Ii_n (23)
Vi_(n+1)＝V*-RSiIi_n-[Rai_n+ki(Ii_n-Iref_n)]Ii_n (24)
the microgrid system starts to work in a traditional V-I droop mode, when a trigger signal S is equal to 1, the working mode is switched, namely self-adjusting virtual impedance is introduced, after a first sampling period,
Vi_1＝V*-RSiIi_0-(Rai_0+Hi_1)Ii_0 (25)
at this time, the SoC deviation between the distributed energy storage units can be expressed as:
in the formula, TsampleRepresenting the sampling period of S/H,
when the distributed energy storage unit is in a discharge working mode and the mismatch parameter meets Rlinei＞RlinejAt the moment, the output currents of the two groups of energy storage units meet the relation that I is more than 0 in the first sampling periodi(0)＜Ij(0)According to the formula (2), the SoC of the two groups of energy storage units satisfies the SoCi(0)＞SoCj(0)And k isi(Ii(0)-Iave(0))＜0＜ki(Ij(0)-Iave(0)) The load distribution curve after the first sampling period is shown in FIG. 6, and S/H adaptively adjusts the virtual impedance according to the current load distribution deviation to compensate the error caused by the mismatching parameter, i.e. DESUiIncreasing the virtual impedance after the first sampling period, while the DESUjThe virtual impedance is reduced after the first sampling period, as can be seen from fig. 6, I is satisfied after the first sampling periodi(0)＜Ii(1)，Ij(1)＜Ij(0)Along with the continuous adjustment of the sampling holder, the current distribution error between the energy storage units is continuously reduced until the error is completely eliminated, and as can be seen from equation (26), the error of the SoC between the energy storage units is also eliminated, SoC equalization control is realized, and if R is equal to R, SoC equalization control is realizedlinei＜RlinejThe same is true.
The adjustment of the charging mode of the energy storage unit is analyzed, and fig. 7 shows thatj(0)＜Ij(1)＜Ii(1)＜Ii(0)Less than 0, SoC satisfies SoCi(1)＜SoCj(1)And k isi(Ii(0)-Iave(0))＜0＜ki(Ij(0)-Iave(0)) After the first sampling period, the DESU, similar to the discharge modeiThe virtual impedance of the cell is reduced and the DESU is usedjAnd the virtual impedance is increased, and the corresponding SoC has no deviation as the load distribution error between the energy storage units is gradually reduced to zero along with the sampling period.
The simulation analysis is as follows:
in order to verify the effectiveness and feasibility of the control strategy provided by the invention, three groups of energy storage units (DESU) are built by using MATLAB1、DESU2、DESU3) The microgrid simulation model is analyzed, the simulation parameters of the microgrid system are set as shown in table 1, the capacity of the energy storage units adopted for simulation is 100Ah, case 1 verifies whether the balance of the charge state can be realized during the charging/discharging of the distributed energy storage units, case 2 verifies the effectiveness of the control strategy under the condition of unmatched local load switching, and case 3 verifies the effectiveness of the distributed energy storage units under the condition of unmatched local load switchingThe invention proposes the effectiveness of the strategy under the condition of energy fluctuation.
TABLE 1 microgrid System parameters
When the micro-grid system normally and independently operates, the energy storage system composed of the distributed energy storage units maintains the power balance in the system, namely the distributed energy storage units are mutually coordinated to balance the power difference between the distributed power supply and the load, when the power sent by the PV system is greater than the load demand, the energy storage system absorbs the redundant power and works in a charging mode, otherwise, when the power required by the load exceeds the power sent by the photovoltaic system, the energy storage system sends the power to assist the photovoltaic unit to supply energy to the load, the distributed energy storage units work in a discharging mode, and the waveforms of the energy storage system under the conditions of stable charging and discharging are respectively shown in figures 8-10 and figures 11-13.
The simulation results of the distributed energy storage stable charging are shown in fig. 8-10, and the systems 0-5s work in the conventional droop mode, as shown in fig. 8, due to the objective influence of the impedance of the unmatched line, the SoC of the distributed energy storage unit cannot be balanced in the period, the distribution of the output load current of the distributed energy storage unit is shown in fig. 9, it can be seen that the impedance of the unmatched line also influences the load distribution of the energy storage system, the load current cannot be uniformly distributed, the state of the micro-grid system is switched in 5s, the virtual impedance self-adjustment control strategy provided by the invention is introduced, as shown in fig. 8 and 9, the controller adaptively adjusts the virtual impedance of each group of energy storage units of the microgrid system, after that, the output current of the distributed energy storage unit is gradually equalized under the regulation of the sampling holder, and the equalization of the SoC is realized at the same time.
The simulation results of the distributed energy storage stable discharge are shown in fig. 11-13, the microgrid system operates in a conventional droop mode, due to the existence of unmatched line impedance, accurate load distribution of the energy storage system cannot be realized, in addition, the deviation exists in the SoC among the distributed energy storage units, the system is switched to a virtual impedance self-adjusting control strategy in 5s, as shown in figures 11 and 12, after that, the current deviation of the distributed energy storage units is gradually reduced and finally equal, the deviation of the SoC also gradually approaches zero, the distributed energy storage units can not generate the condition of over charge/discharge, the life of the energy storage unit is optimized to maintain good power quality, as shown in figure 13, under the virtual impedance self-adjustment control strategy provided by the invention, the bus voltage of the direct current micro-grid system is always maintained in a normal range (plus or minus 5%), and the safety of the system is guaranteed.
Case 2
The embodiment is used for carrying out verification on the influence of the unmatched local loads of the micro-grid system, wherein the characteristics and the changes of the unmatched local loads are shown in table 2, the local loads are also observed except for the unmatched line impedances in the micro-grid system, the output and the charge state of the distributed energy storage units are directly influenced, and simulation results under the influence of the local loads are shown in fig. 14-16.
The power distribution in the microgrid system is shown in fig. 14, when a 0-5s system operates in a traditional droop mode, SoC has deviation, accurate load distribution cannot be realized, 5s starts to switch to an optimized droop control strategy, a virtual impedance self-adjustment control strategy is introduced, as can be seen from fig. 15 and 16, under the condition of virtual impedance self-adaptive adjustment, the influence of unmatched parameters is successfully eliminated, the load current of the distributed energy storage units is accurately distributed, the SoC error of each unit is gradually reduced, the local load is switched out of the system at the moment of 7s, as shown in fig. 15 and 16, the switching of unmatched local loads does not affect the stability of the controller, and finally SoC balancing control is realized, meanwhile, as shown in fig. 17, the direct-current bus voltage of the microgrid system is always maintained in an ideal range in the whole process, almost no deviation exists, and the microgrid system stably operates.
TABLE 2 mismatch local load parameter
In this case, the effectiveness of a control strategy is checked when the output of a photovoltaic system changes when a microgrid independently operates, the output of a PV system changes along with the change of illumination intensity at 7S, the states of distributed energy storage units which need to be coordinated and changed mutually are switched from a charging mode to a discharging mode to maintain the power balance of the system, the power distribution waveform inside the system is shown in fig. 18, the output current and SoC of the distributed energy storage units cannot realize good distribution and balance under the influence of mismatched parameters, the service life of the distributed energy storage units is directly influenced, a trigger signal S is switched from 0 to 1 at 5S, the system is switched to a droop mode of virtual impedance self-adjustment provided by the invention, the energy storage system is switched from the charging mode to the discharging mode along with the change of the photovoltaic power at 7S, and as can be seen from fig. 18-21, along with the adjustment of a sampling holder, the accurate distribution of the load current is realized, the charge state is gradually balanced, and the fluctuation of PV does not influence the robust performance of the control strategy provided by the invention.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Claims (3)
1. A distributed energy storage state of charge equalization strategy based on virtual impedance self-adjustment is characterized by comprising the following steps:
step S1: based on the traditional V-I droop control adopted by the load distribution of the micro-grid, a new V-I droop control expression is provided, wherein the new V-I droop control expression introduces self-adaptive associated virtual impedance and self-adjusting virtual impedance,
the conventional V-I droop control expression is:
Vi＝V*-RiIi
in the formula, ViRepresenting the output voltage, V, of the DC/DC converter connected with the ith group of distributed energy storage units*Represents the system standard reference voltage value, RiRepresents the virtual impedance of the ith group of distributed energy storage units, IiRepresenting the output current value of the ith group of distributed energy storage units;
the new V-I droop control expression is:
Vi＝V*-RSiIi-RaiIi
in the formula, RSiRepresenting an adaptively associated virtual impedance, RaiRepresents a self-adjusting virtual impedance, at which time the virtual impedance RiRepresents RSiAnd RaiSumming;
step S2: designing the self-adaptive associated virtual impedance, wherein the expression is as follows:
wherein the content of the first and second substances,
in the formula, SoCiRepresenting the current state of charge, SoC, of the ith group of distributed energy storage unitsrefReference value, C, representing the state of chargeDiRepresents the capacity of the ith group of distributed energy storage units, R0Representing the initial value of the virtual impedance, ksDenotes the adjustment coefficient, A denotes the acceleration factor, ksThe value of the sum A influences the convergence degree and the convergence speed of the state of charge, sign represents a sign function, when the output value of the sign function is 1, the distributed energy storage unit is in a charging mode,when the output value of the sign function is-1, the distributed energy storage unit is in a discharge mode;
step S3: the self-adjusting associated virtual impedance is designed,
adding the self-adjusting virtual impedance on the basis of the virtual impedance to obtain the following expression:
wherein R isi、RjRespectively representing the virtual impedance of the distributed energy storage units of the ith group and the jth group, Rai、RajSelf-adjusting virtual impedance, R, of distributed energy storage units respectively representing ith and jth groupsSi、RSjRespectively representing the self-adaptive associated virtual impedance R of the distributed energy storage units of the ith group and the jth grouplinei、RlinejRespectively representing the line impedance of the distributed energy storage units of the ith group and the jth group to the common coupling point;
step S4: the micro-grid system works in a new V-I droop control mode, the sampling holder adjusts virtual impedance, and the expression is as follows:
Rai_(n+1)＝Rai_n+Hi_n
Hi_n＝ki(Ii_n-Iref_n)
in the formula, Rai_(n+1)Self-adjusting virtual impedance, R, representing the n +1 th sampling periodai_nSelf-adjusting virtual impedance, H, representing the nth sampling periodi_nA load distribution adjustment item representing the ith group of distributed energy storage units in the nth sampling period, Ii_nRepresenting the output current, I, of the nth sampling periodref_nReference value, k, representing the output current for the nth sampling periodiThe current adjustment factor is obtained.
2. The virtual impedance self-adjustment based distributed energy storage state of charge balancing strategy according to claim 1, wherein in the step 2, the current state of charge SoC of the ith group of distributed energy storage unitsiThe expression of (a) is:
in the formula, SoCi0And m represents the ratio of the output end voltage of the DC/DC converter to the end voltage of the storage battery.
3. The distributed energy storage state of charge balancing strategy based on virtual impedance self-adjustment according to claim 1, characterized in that: in the step 4, when the microgrid system works in a new V-I droop control mode, the central processing unit sends a synchronization signal to the sampling holder to trigger the sampling holder to adjust the virtual impedance.
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