CN112600188A  Multienergystorage SOC (system on chip) balanced segmentation selfadaptive droop control method for directcurrent microgrid  Google Patents
Multienergystorage SOC (system on chip) balanced segmentation selfadaptive droop control method for directcurrent microgrid Download PDFInfo
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 CN112600188A CN112600188A CN202011417558.1A CN202011417558A CN112600188A CN 112600188 A CN112600188 A CN 112600188A CN 202011417558 A CN202011417558 A CN 202011417558A CN 112600188 A CN112600188 A CN 112600188A
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 238000004146 energy storage Methods 0.000 title claims abstract description 87
 230000011218 segmentation Effects 0.000 title claims description 5
 WHXSMMKQMYFTQSUHFFFAOYSAN lithium Chemical compound [Li] WHXSMMKQMYFTQSUHFFFAOYSAN 0.000 claims abstract description 98
 229910052744 lithium Inorganic materials 0.000 claims abstract description 98
 238000007599 discharging Methods 0.000 claims abstract description 18
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 230000001133 acceleration Effects 0.000 claims description 3
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 230000000295 complement Effects 0.000 abstract description 2
 230000003044 adaptive Effects 0.000 description 16
 238000004088 simulation Methods 0.000 description 11
 238000010586 diagram Methods 0.000 description 9
 238000011084 recovery Methods 0.000 description 6
 238000010248 power generation Methods 0.000 description 3
 230000005540 biological transmission Effects 0.000 description 2
 230000000694 effects Effects 0.000 description 2
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 230000000087 stabilizing Effects 0.000 description 2
 238000010521 absorption reaction Methods 0.000 description 1
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 238000004140 cleaning Methods 0.000 description 1
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Classifications

 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
 H02J1/00—Circuit arrangements for dc mains or dc distribution networks
 H02J1/14—Balancing the load in a network

 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
 H02J1/00—Circuit arrangements for dc mains or dc distribution networks
 H02J1/10—Parallel operation of dc sources

 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
 H02J1/00—Circuit arrangements for dc mains or dc distribution networks
 H02J1/10—Parallel operation of dc sources
 H02J1/106—Parallel operation of dc sources for load balancing, symmetrisation, or sharing

 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
 H02J7/0013—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries acting upon several batteries simultaneously or sequentially
 H02J7/0014—Circuits for equalisation of charge between batteries

 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
 H02J7/34—Parallel operation in networks using both storage and other dc sources, e.g. providing buffering
 H02J7/35—Parallel operation in networks using both storage and other dc sources, e.g. providing buffering with light sensitive cells
Abstract
The invention discloses a segmented selfadaptive droop control method for multienergystorage SOC balance of a directcurrent microgrid, which comprises the steps that a controller collects SOC and actual capacity information of each energy storage unit, droop coefficients are reset through a segmented selfadaptive droop coefficient algorithm, the charge and discharge power of the energy storage units is controlled, and the rapid balance of the SOC is realized; in order to compensate for the inherent voltage drop of the traditional droop control, a bus voltage deviation is utilized to output a common voltage deviation amount delta U through a PI regulator, so that a droop curve is compensated, and the bus voltage is automatically recovered; the sectional selfadaptive droop coefficient algorithm dynamically adjusts the droop coefficient according to the SOC deviation of the lithium battery; when the SOC deviation is large, adjusting the droop coefficient to discharge (charge) the maximum power of the lithium battery with high (low) SOC during discharging (charging), and simultaneously controlling the other group of lithium batteries to complement the residual power to accelerate the balancing speed; when the SOC deviation is small, the droop coefficient is optimized on the basis of considering different line impedances and actual capacities, and SOC balance control is achieved.
Description
Technical Field
The invention relates to the field of a segmented adaptive droop coefficient algorithm and application thereof in multienergystorage SOC (system on chip) balance control of a directcurrent microgrid in independent operation, in particular to a segmented adaptive droop control method for multienergystorage SOC balance of a directcurrent microgrid.
Background
In recent years, international society has paid more and more attention to the development of clean renewable energy, and more scholars are invested in the research of renewable energy distributed power generation. In order to overcome the problem of power randomness of renewable energy power, a scheme of local consumption by using a microgrid is provided and widely accepted by researchers. The direct current microgrid has the advantages of low manufacturing cost, less loss, high efficiency, simple structure, convenient control and the like, and is distinguished from a plurality of microgrid schemes. Because the distributed energy power has the unpredictability of intermittence and load fluctuation, in order to ensure the stable operation of the microgrid, a distributed energy storage system is often required to be arranged in the direct current microgrid to play a role in stabilizing the power difference in the microgrid.
In a directcurrent microgrid, when a plurality of groups of Energy Storage units (DESUs) run in parallel, traditional droop control is adopted, and due to the fact that line impedance and the actual capacity of the Energy Storage units are different, the State of Charge (SOC) of the Energy Storage units deviates, so that part of the Energy Storage units are overcharged or overdischarged, and the service life of a lithium battery is seriously influenced. The droop control of the fuzzy control and the adaptive control can realize the SOC balance control, but the balance speed is slow.
The conventional droop control and SOC imbalance generation mechanism:
1. conventional droop control for energy storage systems
In the independently operated dc microgrid, in order to control the bus voltage and the output current of the lithium battery at the same time, a PI regulator double closed loop structure of a bus voltage outer loop and a battery current inner loop is often adopted. Droop control is generally adopted to obtain the DC/DC output reference voltage of the energy storage system, so as to realize power distribution among the energy storage units. The droop control expression is:
U_{out}＝U_{ref}kI_{out} (1)
in the formula of U_{ref}A reference voltage for the converter port; u shape_{out}For a converterPort voltage (i.e., dc bus voltage); k is a droop coefficient; i is_{out}Outputting current for the converter.
2. Mechanism analysis for generating SOC imbalance of lithium battery
The lithium battery charge state calculation formula is as follows:
in the formula, SOC is the state of charge of the lithium battery; SOC_{0}The initial state of charge of the lithium battery; i (t) is the output current of the lithium battery; c_{e}The maximum capacity of the lithium battery.
Considering the line impedance and the actual capacity of the lithium battery, the rate of change of the SOC of the two groups of lithium batteries obtained from equations (1) and (2) is:
in the formula, R_{line1}、k_{1}、U_{ref1}The circuit impedance, the droop coefficient and the port voltage reference value of the energy storage unit 1 are respectively; r_{line2}、k_{2}、U_{ref2}Line impedance, droop coefficient, port voltage reference value of the energy storage unit 2.
Under normal conditions U_{ref1}＝U_{ref2}From formula (3):
from the formula (4), it can be known that theoretically, the energy storage units with the same capacity have differences in actual capacity due to various production and operation reasons, and the SOC of multiple groups of lithium batteries has deviation due to line impedance differences.
In summary, the mobile phone of the present invention provides a segment adaptive droop control method for multienergy storage SOC balancing of a dc microgrid, where a segment adaptive droop coefficient algorithm dynamically adjusts a droop coefficient according to the magnitude of SOC deviation, so as to accelerate the SOC balancing speed of an energy storage unit. .
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a segmented adaptive droop control method for multienergystorage SOC (system on chip) balance of a directcurrent microgrid, wherein a segmented adaptive droop coefficient algorithm dynamically adjusts a droop coefficient according to the SOC deviation of a lithium battery; when the SOC deviation is large, adjusting the droop coefficient to discharge (charge) the maximum power of the lithium battery with high (low) SOC during discharging (charging), and simultaneously controlling the other group of lithium batteries to complement the residual power to accelerate the balancing speed; when the SOC deviation is small, the droop coefficient is optimized on the basis of considering different line impedances and actual capacities, and SOC balance control is achieved. The automatic recovery control of the bus voltage is adopted to realize the nodifference control of the bus voltage, control the bus voltage to be stable and improve the power supply quality.
In order to achieve the purpose, the invention is realized by the following technical scheme: a multienergy storage SOC balanced segmentation selfadaptive droop control method for a direct current microgrid comprises the following steps: the controller collects the SOC and actual capacity information of each energy storage unit, the droop coefficient is reset through a segmented selfadaptive droop coefficient algorithm, the charging and discharging power of the energy storage units is controlled, and the quick equalization of the SOC is realized; in order to compensate for the inherent voltage drop of the traditional droop control, a bus voltage deviation is utilized to output a common voltage deviation amount delta U through a PI regulator, so that a droop curve is compensated, and the bus voltage is automatically recovered; the energy storage unit comprises a first energy storage unit 1 and a second energy storage unit 2, U_{out1}、U_{out2}The DC/DC output voltages, I, of the first energy storage unit 1 and the second energy storage unit 2, respectively_{bat1}、I_{bat2}Output currents of the first energy storage unit 1 and the second energy storage unit 2, U_{bus}Is the bus voltage.
The segmented adaptive droop coefficient algorithm is as follows:
the average of the two groups of lithium batteries is:
the state of charge deviation of the ith group of cells is
ΔSOC_{i}＝SOC_{i}SOC_{avg} (10)
(1) When the deviation  Δ SOC  is > 5%
1) Power P of energy storage system needing discharging_{dmax}≤P＜2P_{dmax}(P_{dmax}Rated discharge power for lithium battery), or power P to be charged_{cmax}≤P＜2P_{cmax}(P_{cmax}Rated charging power for lithium battery):
charging state: and controlling the group of batteries with the smaller SOC to charge at the rated charging power, and absorbing the residual power by the group of batteries with the larger SOC. The droop coefficient is reset as follows:
in the formula, SOC_{max}Is the maximum value of SOC in all lithium batteries.
Discharge state: and controlling the group of batteries with the larger SOC to discharge at the rated discharge power, and releasing the residual power from the group of batteries with the smaller SOC. The droop coefficient is reset as follows:
in the formula, SOC_{min}Is the minimum value of SOC in all lithium batteries.
2) When the discharge power P of the energy storage system is less than P_{dmax}Or charging power P < P_{cmax}And during discharging, the droop coefficient of the group of lithium batteries with the highest SOC or the droop coefficient of the group of lithium batteries with the lowest SOC is forced to take a small value, so that the maximum power is released or absorbed, and the droop coefficients of the rest lithium batteries are adjusted according to the difference value of the respective SOC and the average SOC, so that the smooth transition of the power is realized as far as possible. The droop coefficient is reset as follows:
(2) when the deviation  delta SOC  is less than or equal to 5 percent
As can be seen from equation (4), it is ensured that there is no deviation between the two sets of SOCs, and that the following equation holds:
and the droop coefficients are dynamically adjusted according to the SOC changes of the two groups of lithium batteries, so that the power can be distributed according to the proportion of the actual capacity. The lithium battery with large actual capacity absorbs less power in a steady state and releases more power, and the lithium battery with small actual capacity is opposite to the lithium battery with large actual capacity. Therefore, the SOC indifferent balance of the lithium batteries with different capacities can be realized. The sag factor is set as follows:
in the formula, α is an acceleration factor, and the sign differs depending on the charge/discharge state.
In summary, when  Δ SOC  is greater than 5%, the droop coefficient is dynamically adjusted according to equations (11) to (14), so that the discharge (charge) power of the lithium battery with the highest (low) SOC is maximum, and the corresponding power of the other group of lithium batteries is minimum, thereby accelerating the SOC balancing speed; when the deviation  delta SOC  is less than or equal to 5%, the droop coefficient is adaptively adjusted according to the formula (16), so that the charging and discharging power of the energy storage unit is automatically distributed according to the actual capacity and the SOC, and the SOC deviation is thoroughly eliminated.
The bus voltage automatic recovery control method comprises the following steps: conventional droop control always produces a size of kI_{out}If the transmission power is too large, the voltage deviation of the bus can cause serious drop of the bus voltage. In order to overcome the bus voltage drop caused by the traditional droop control and ensure the operation at rated loadAnd the stability of the down bus voltage increases the recovery control of the bus voltage. And increasing delta U offset on the basis of droop control, translating a droop curve, compensating bus voltage drop caused by a droop coefficient, and realizing nondeviation control of the bus voltage. The bus voltage can be represented by:
in the formula, K_{p}、K_{i}The proportional coefficient and the integral coefficient of the bus voltage PI regulator are respectively.
The invention has the beneficial effects that: the invention discloses a method for adaptively calculating a droop coefficient according to SOC deviation of each energy storage unit. When the deviation is large, the SOC is balanced as fast as possible under the condition that the charging and discharging power of the energy storage unit is controlled not to exceed the rated power by adjusting the droop coefficient; when the deviation is small, zero deviation control of multiple groups of energy storage SOC can be realized through selfadaptive optimization of the vertical coefficient. And the bus voltage deviation caused by droop control can be well compensated by utilizing the bus voltage automatic recovery control. The power of the energy storage units can be reasonably distributed under different microgrid operation conditions, so that the SOC is rapidly balanced, and meanwhile, the bus voltage is controlled to be free of deviation.
Drawings
The invention is described in detail below with reference to the drawings and the detailed description;
FIG. 1 is a topological structure diagram of a DC microgrid of the present invention;
FIG. 2 is an equivalent circuit diagram of the DC microgrid of the present invention;
FIG. 3 is a schematic diagram of the control method of the present invention;
fig. 4(a) is a simulation waveform diagram illustrating the difference between the line impedance and the actual capacity of the lithium battery in the conventional droop control according to the embodiment of the present invention;
fig. 4(b) is a simulation waveform diagram of the line impedance and the actual capacity of the lithium battery of the multienergystorage SOC equalization control based on the piecewise adaptive droop coefficient algorithm according to the embodiment of the present invention;
FIG. 5(a) is a diagram of an adaptive droop control method according to an embodiment of the present inventionP_{dmax}≤P<2P_{dmax}Simulating a waveform under the condition of discharging;
FIG. 5(b) is a diagram illustrating P of multienergystorage SOC equalization control based on the piecewise adaptive droop coefficient algorithm according to an embodiment of the present invention_{dmax}≤P<2P_{dmax}Simulating a waveform under the condition of discharging;
FIG. 6 shows P in an embodiment of the present invention_{cmax}≤P<2P_{cmax}Simulating a waveform under the charging condition;
FIG. 7 shows P in an embodiment of the present invention<P_{dmax}Simulating a waveform under the condition of discharging;
FIG. 8 shows P in an embodiment of the present invention<P_{cmax}And simulating a waveform diagram under the charging condition.
Detailed Description
In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further described with the specific embodiments.
Referring to fig. 1 to 8, the following technical solutions are adopted in the present embodiment: a multienergy storage SOC balanced segmentation selfadaptive droop control method for a direct current microgrid comprises the following steps: the controller collects the SOC and actual capacity information of each energy storage unit, the droop coefficient is reset through a segmented selfadaptive droop coefficient algorithm, the charging and discharging power of the energy storage units is controlled, and the quick equalization of the SOC is realized; in order to compensate for the inherent voltage drop of the traditional droop control, a bus voltage deviation is utilized to output a common voltage deviation amount delta U through a PI regulator, so that a droop curve is compensated, and the bus voltage is automatically recovered; the energy storage unit comprises a first energy storage unit 1 and a second energy storage unit 2, U_{out1}、U_{out2}The DC/DC output voltages, I, of the first energy storage unit 1 and the second energy storage unit 2, respectively_{bat1}、I_{bat2}Output currents of the first energy storage unit 1 and the second energy storage unit 2, U_{bus}Is the bus voltage.
The topological structure of the independent operation direct current microgrid is shown in figure 1 and comprises a photovoltaic power generation system, an energy storage system and a load. The photovoltaic power generation system serves as a micro source to provide energy for the bus bar. The two groups of energy storage units absorb redundant energy on the bus or supplement the power shortage of the bus together, the voltage of the bus is maintained to be stable, the normal operation of a load is guaranteed, and the power in the energy storage system is automatically distributed by utilizing droop control. The parameters of the microsource of the direct current microgrid are shown in table 1.
TABLE 1 DC microgrid system parameters
Determination of the droop coefficient range of the present embodiment:
the equivalent circuit of the dc microgrid is shown in fig. 2. The microsource is generally controlled by Maximum Power Point Tracking (MPPT), so that the microsource can be equivalent to a constant Power source or a constant current source; the energy storage unit is used for stabilizing power fluctuation in the network, maintaining the voltage stability of the direct current bus and being equivalent by a model of a constant voltage source and impedance in series connection; the load may be equated with an equivalent impedance.
From the equivalent circuit we can get:
in the formula, P_{source}Total power emitted for the microsources; r_{load}Is the equivalent impedance of the load; k ═ k + R_{line}Is the equivalent impedance, R, of the energy storage system_{line}Is the transformer line impedance.
The droop coefficient can be calculated from equation (5) as:
and (4) carrying the maximum variation range allowed by the bus voltage into the formula (6) to obtain the maximum value of the droop coefficient.
According to the actual capacity and the discharge rate of the lithium battery, the minimum value of the droop coefficient can be determined, and the method comprises the following steps:
in the formula of U_{bat}Is the port voltage of the lithium battery; i is_{bat}Is the output current of the lithium battery.
From (7), it is possible:
the maximum chargedischarge current of the lithium battery can be determined according to the actual capacity, and the maximum chargedischarge current is substituted into the formula (8) to obtain the minimum value of the droop coefficient.
In order to eliminate SOC deviation caused by different actual capacities and line impedances of all energy storage units, avoid overcharge and overdischarge of a single energy storage unit and solve the problem of deviation of droop control bus voltage, the invention designs a multienergystorage SOC balance droop control strategy of a piecewise selfadaptive droop coefficient algorithm, and a control block diagram of the multienergystorage SOC balance droop control strategy is shown in FIG. 3. Wherein, U_{out1}、U_{out2}DC/DC output voltages, I, of energy storage units 1 and 2, respectively_{bat1}、I_{bat2}Output currents, U, of energy storage units 1 and 2, respectively_{bus}Is the bus voltage. The controller collects the SOC and actual capacity information of each energy storage unit, the droop coefficient is reset through a segmented selfadaptive droop coefficient algorithm, the charge and discharge power of the energy storage units is controlled, and the quick balance of the SOC is achieved. In order to compensate for the inherent voltage drop of the traditional droop control, a bus voltage deviation is utilized to output a common voltage deviation amount delta U through a PI regulator, so that a droop curve is compensated, and the bus voltage is automatically recovered.
The piecewise adaptive droop coefficient algorithm of the present embodiment is as follows:
the average of the two groups of lithium batteries is:
the state of charge deviation of the ith group of cells is
ΔSOC_{i}＝SOC_{i}SOC_{avg} (10)
(1) When the deviation  Δ SOC  is > 5%
1) Power P of energy storage system needing discharging_{dmax}≤P＜2P_{dmax}(P_{dmax}Rated discharge power for lithium battery), or power P to be charged_{cmax}≤P＜2P_{cmax}(P_{cmax}Rated charging power for lithium battery):
charging state: and controlling the group of batteries with the smaller SOC to charge at the rated charging power, and absorbing the residual power by the group of batteries with the larger SOC. The droop coefficient is reset as follows:
in the formula, SOC_{max}Is the maximum value of SOC in all lithium batteries.
Discharge state: and controlling the group of batteries with the larger SOC to discharge at the rated discharge power, and releasing the residual power from the group of batteries with the smaller SOC. The droop coefficient is reset as follows:
in the formula, SOC_{min}Is the minimum value of SOC in all lithium batteries.
2) When the discharge power P of the energy storage system is less than P_{dmax}Or charging power P < P_{cmax}And during discharging, the droop coefficient of the group of lithium batteries with the highest SOC or the droop coefficient of the group of lithium batteries with the lowest SOC is forced to take a small value, so that the maximum power is released or absorbed, and the droop coefficients of the rest lithium batteries are adjusted according to the difference value of the respective SOC and the average SOC, so that the smooth transition of the power is realized as far as possible. The droop coefficient is reset as follows:
(2) when the deviation  delta SOC  is less than or equal to 5 percent
As can be seen from equation (4), it is ensured that there is no deviation between the two sets of SOCs, and that the following equation holds:
and the droop coefficients are dynamically adjusted according to the SOC changes of the two groups of lithium batteries, so that the power can be distributed according to the proportion of the actual capacity. The lithium battery with large actual capacity absorbs less power in a steady state and releases more power, and the lithium battery with small actual capacity is opposite to the lithium battery with large actual capacity. Therefore, the SOC indifferent balance of the lithium batteries with different capacities can be realized. The sag factor is set as follows:
in the formula, α is an acceleration factor, and the sign differs depending on the charge/discharge state.
In summary, when  Δ SOC  is greater than 5%, the droop coefficient is dynamically adjusted according to equations (11) to (14), so that the discharge (charge) power of the lithium battery with the highest (low) SOC is maximum, and the corresponding power of the other group of lithium batteries is minimum, thereby accelerating the SOC balancing speed; when the deviation  delta SOC  is less than or equal to 5%, the droop coefficient is adaptively adjusted according to the formula (16), so that the charging and discharging power of the energy storage unit is automatically distributed according to the actual capacity and the SOC, and the SOC deviation is thoroughly eliminated.
Conventional droop control always produces a size of kI_{out}If the transmission power is too large, the voltage deviation of the bus can cause serious drop of the bus voltage. In order to overcome the bus voltage drop caused by the traditional droop control, ensure the stability of the bus voltage under the rated load operation and increase the recovery control of the bus voltage. Increasing delta U offset on the basis of droop control, translating a droop curve, and compensating a parent brought by a droop coefficientAnd the line voltage is dropped, and the nondeviation control of the bus voltage is realized. The bus voltage can be represented by:
in the formula, K_{p}、K_{i}The proportional coefficient and the integral coefficient of the bus voltage PI regulator are respectively.
Example 1: in order to verify the effectiveness of the control method, a simulation model is set up in MATLAB/Simulink simulation software according to the topology of FIG. 1 and the parameters of Table 1, and simulation verification is carried out. The maximum discharge rate of the lithium batteries is 0.5C and the maximum charge rate of the lithium batteries is 0.2C, so that the maximum charge/discharge power of the two groups of lithium batteries is 10kW/25kW respectively.
1. Simulation research on different line impedance and actual capacity of lithium battery
The initial SOC of the two groups of lithium batteries is the same, the initial SOC is set to be 70%, the photovoltaic power is set to be 25kW, and the load power is 40 kW. The results of the simulation of the conventional droop control and the control strategy presented herein are shown in fig. 4. As can be seen from fig. 4(a), the SOC of the two batteries is deviated due to the difference between the line impedance and the actual capacity of the lithium battery, and the bus voltage is lower than 750V. As can be seen from fig. 4(b), a multienergystorage SOC equalization droop control strategy based on a segmented adaptive droop coefficient algorithm is adopted, the difference between the discharge power of the lithium battery 1 and the discharge power of the lithium battery 2 is controlled by dynamically adjusting the droop coefficient, the influence of the line impedance and the difference between the actual capacities of the lithium batteries is eliminated, and the SOC deviation of the two groups of lithium batteries is always 0; bus voltage is maintained at 750V, and bus voltage automatic recovery control realizes voltage deviationfree control and stabilizes bus voltage.
2. Adaptive sectional type droop control strategy simulation research
(1) Discharge power P of energy storage system_{dmax}≤P＜2P_{dmax}
The initial SOC of the two groups of lithium batteries are respectively SOC_{1}＝70％、SOC_{2}50%, photovoltaic power is 10kW, and the load demand is 40 kW. The power shortage in the network is 30kW, and the power shortage is larger than a group of lithium batteries and can be borne by the lithium batteries25kW maximum discharge power. The results of the adaptive droop control and the control strategy simulation presented herein are shown in fig. 5.
As can be seen from fig. 5(a), the adaptive droop control can realize SOC equalization control, but the equalization speed is slow, and the bus voltage 740V has a deviation. As can be seen from fig. 5(b), when the deviation of the SOC of the two groups of lithium batteries is greater than 5% (before 0.5 s), the lithium battery 1 with the larger SOC outputs the maximum allowable discharge power of 25kW, and the remaining 5kW power shortage is compensated by the lithium battery 2, so that the SOC is equalized rapidly as possible; after 0.5s, the SOC deviation of the two groups of lithium batteries is reduced to be within 5%, the power in the energy storage system is distributed according to a droop coefficient of a formula (16), and the SOC deviation is finally eliminated; bus voltage is 750V, and bus voltage is stable, has improved the power supply quality.
(2) Charging power P of energy storage system_{cmax}≤P＜2P_{cmax}
The initial SOC of the two groups of lithium batteries is SOC_{1}＝50％、SOC_{2}30%, photovoltaic power is 55kW, and the load demand is 40kW, and the power surplus is 15kW in the net, is greater than a set of lithium cell rated charging power 10 kW. The simulation waveform is shown in fig. 6, and it can be seen that the lithium battery 2 with the lower SOC level is charged with 10kW of rated power, and the remaining 5kW of power is absorbed by the lithium battery 1. And 2.5s, the SOC of the two groups of lithium batteries are balanced, and the rapid balance of the SOC is realized.
(3) Discharge power P of energy storage system is less than P_{dmax}
The initial SOC of the two groups of lithium batteries is SOC_{1}＝70％、SOC_{2}50%, photovoltaic power is 25kW, and the load demand is 40kW, and the power shortage is 15kW in the net. The simulation waveform is shown in fig. 7, and it can be seen that the lithium battery 1 releases 15kW of total power shortage in the network, and the SOC of the two groups of lithium batteries is balanced when 1.5 s.
(4) Charging power P of energy storage system is less than P_{cmax}
The initial SOC of the two groups of lithium batteries is SOC_{1}＝50％、SOC_{2}30%, photovoltaic power is 49kW, and the load demand is 40kW, and the power surplus is 9kW in the net. The simulation waveform is shown in fig. 8, the absorption power of the lithium battery 1 with high SOC is almost 0, and the lithium battery 2 absorbs the rest of the whole lithium battery in the gridAnd 9kW power. And 2s, the two groups of lithium batteries realize SOC balance.
The foregoing shows and describes the general principles and broad features of the present invention and advantages thereof. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (3)
1. A multienergy storage SOC balanced segmentation selfadaptive droop control method for a direct current microgrid is characterized by comprising the following steps: the controller collects the SOC and actual capacity information of each energy storage unit, the droop coefficient is reset through a segmented selfadaptive droop coefficient algorithm, the charging and discharging power of the energy storage units is controlled, and the quick equalization of the SOC is realized; in order to compensate for the inherent voltage drop of the traditional droop control, a bus voltage deviation is utilized to output a common voltage deviation amount delta U through a PI regulator, so that a droop curve is compensated, and the bus voltage is automatically recovered; the energy storage unit comprises a first energy storage unit 1 and a second energy storage unit 2, U_{out1}、U_{out2}The DC/DC output voltages, I, of the first energy storage unit 1 and the second energy storage unit 2, respectively_{bat1}、I_{bat2}Output currents of the first energy storage unit 1 and the second energy storage unit 2, U_{bus}Is the bus voltage.
2. The method for the piecewise selfadaption droop control on the multienergystorage SOC balance of the directcurrent microgrid according to claim 1, wherein the piecewise selfadaption droop coefficient algorithm is as follows: the average of the two groups of lithium batteries is:
the state of charge deviation of the ith group of cells is
ΔSOC_{i}＝SOC_{i}SOC_{avg} (10)
(1) When the deviation  Δ SOC  is > 5%
1) Power P of energy storage system needing discharging_{dmax}≤P＜2P_{dmax}(P_{dmax}Rated discharge power for lithium battery), or power P to be charged_{cmax}≤P＜2P_{cmax}(P_{cmax}Rated charging power for lithium battery):
charging state: controlling a group of batteries with smaller SOC to charge at rated charging power, and absorbing the residual power by a group of batteries with larger SOC; the droop coefficient is reset as follows:
in the formula, SOC_{max}The maximum value of SOC in all lithium batteries;
discharge state: controlling a group of batteries with larger SOC to discharge at rated discharge power, and releasing the residual power from a group of batteries with smaller SOC; the droop coefficient is reset as follows:
in the formula, SOC_{min}The minimum value of SOC in all lithium batteries;
2) when the discharge power P of the energy storage system is less than P_{dmax}Or charging power P < P_{cmax}During discharging, the droop coefficient of the group of lithium batteries with the highest SOC or the droop coefficient of the group of lithium batteries with the lowest SOC is forced to take a small value, so that the maximum power is released or absorbed, and the droop coefficients of the other lithium batteries are adjusted according to the difference value of the respective SOC and the average SOC, so that smooth transition of the power is realized as far as possible; the droop coefficient is reset as follows:
(2) when the deviation  delta SOC  is less than or equal to 5 percent
As can be seen from equation (4), it is ensured that there is no deviation between the two sets of SOCs, and that the following equation holds:
the droop coefficients are dynamically adjusted according to the change of the SOC of the two groups of lithium batteries, so that power can be distributed according to the proportion of actual capacity; the lithium battery with large actual capacity absorbs less power and releases more power in a steady state, and the lithium battery with small actual capacity is opposite to the lithium battery with large actual capacity; therefore, the SOC (system on chip) uniform balance of lithium batteries with different capacities can be realized; the sag factor is set as follows:
in the formula, alpha is an acceleration factor, and the sign is different according to the chargedischarge state;
in summary, when  Δ SOC  is greater than 5%, the droop coefficient is dynamically adjusted according to equations (11) to (14), so that the discharge (charge) power of the lithium battery with the highest (low) SOC is maximum, and the corresponding power of the other group of lithium batteries is minimum, thereby accelerating the SOC balancing speed; when the deviation  delta SOC  is less than or equal to 5%, the droop coefficient is adaptively adjusted according to the formula (16), so that the charging and discharging power of the energy storage unit is automatically distributed according to the actual capacity and the SOC, and the SOC deviation is thoroughly eliminated.
3. The method for the piecewise selfadaption droop control of the multienergystorage SOC balance of the directcurrent microgrid according to claim 1, characterized by comprising the following steps of recovering and controlling bus voltage; increasing delta U offset on the basis of droop control, translating a droop curve, compensating bus voltage drop caused by a droop coefficient, and realizing bus voltage deviationfree control; the bus voltage can be represented by:
in the formula, K_{p}、K_{i}The proportional coefficient and the integral coefficient of the bus voltage PI regulator are respectively.
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