CN105514966A - Energy storage optimization and coordination control method for direct-current micro grid group - Google Patents

Abstract

The invention relates to an energy storage optimization and coordination control method for a direct-current micro grid group. When multiple sub micro grids are operated in parallel, an energy storage system in each sub micro grid automatically adjusts power distribution according to the maximum charging and discharging power and the state of charge (SOC) of the corresponding energy storage system, and therefore the safety of the energy storage systems is improved while distributed generated power fluctuation can be smoothed and the bus voltage quality can be improved; when the energy storage systems of all the sub micro grids cannot effectively buffer the system power supply and demand, an energy storage power station needs to be connected to control stabilization of the bus voltage. According to the energy storage optimization and coordination control method for the direct-current micro grid group, the virtual impedance of droop controllers of all energy storage units in the energy storage power station is adjusted by adopting fuzzy control to achieve automatic distribution of the power among the different energy storage units and SOC balance, fuzzy input is segmented, the SOC balancing speed of the fuzzy input in different ranges is increased by adopting fuzzy control, bus voltage drop caused by droop control is compensated by adopting a bus voltage feedback control method, and the bus voltage quality is improved.

Description

Energy storage optimization and coordination control method for direct-current micro-grid group

Technical Field

The invention relates to a design technology of a microgrid coordinated controller, in particular to a direct-current microgrid group energy storage optimization and coordinated control method.

Background

The microgrid is used as an autonomous system with self control and self energy management, and can work in a grid-connected mode and an off-grid mode (Wang Chengshan, Wushu, Lepeng. research on the micro grid key technology [ J ]. report on electrotechnical technology, 2014,29(2): 1-12). For example, fig. 1 is a structure diagram of a microgrid system, compared with an ac microgrid, a dc microgrid formed by establishing dc transmission lines in the microgrid and connecting each distributed power source with an energy storage system can improve the utilization rate of a converter and reduce loss, and is favorable for complementary advantages and coordinated control among various micro sources (royal, zhilongrong, li and ming, etc.. voltage hierarchical coordinated control of a wind power dc microgrid [ J ]. china electrical engineering report, 2013,33(4):16-24), and the dc system does not have problems in terms of phase synchronization, harmonic and reactive power loss (WuTF, SunKH, KuoCL, actual. predictivecurntrolled 5-kwsite-phase transformation and conversion efficiency and conversion for dc-micro applications [ J ]. power electronics, ieee transmission, 25 (2010, 3012): microgrid 76, pacifying grid oriented technologies [ dcoverhead, wo 72, wo 80, wo 12, wo 12, wo, gu learns phoenix, three-phase load flow calculation of a weak ring power distribution network containing various distributed power supplies [ J ], China Motor engineering Proc, 2009,29(13): 35-40).

The dc micro-grid is generally relatively small in scale, and the load fluctuation and external interference are frequent (special reference in wang shang). In order to realize various distributed power generation accesses and efficient and reliable power supply at a user side, a direct current microgrid can be divided into a plurality of sub-microgrids (IEEEStands. IEEEStasta1547.4-2011. Guidadoresign, operation, and coordinated distributed power systems with the same power system [ S ].2011) in a planned way, so that the direct current microgrids with similar geographic positions are connected through corresponding control strategies through direct current buses to form a direct current microgrid group, and the stable and efficient and reliable power supply of the microgrid group can be remarkably improved (HeM, GiesslemanM. reliability-communication-oriented electrical parameter-organizing and networking information relating to technical microgrid [ C ]// intelligent power supply) (IEEE & 5: IEEE & S1). Therefore, the coordination control among each distributed power source, the energy storage system, the load, the circuit breaker and the sub-microgrid in the dc microgrid needs to be researched according to a specific dc microgrid networking structure so as to ensure that the microgrid can supply power stably and reliably under different operating states (haru mei, toming, zhangcheng, etc.. multi-energy power generation experimental platform and energy management information integration [ J ]. power system automation, 2010(1): 106-. In addition, the energy storage system can improve the capability of the microgrid group for dealing with emergencies and the operation stability (Tianpegen, Xiaoxi, Dingluxing, and the like.) the capacity configuration method of the multi-element composite energy storage system of the autonomous microgrid group [ J ] the power system automation, 2013,37(1): 168-.

The technical scheme is that a layered coordination control model aiming at realizing the maximization of the power generation efficiency is established in the literature (chapter key, aiqian, wangxincha. application of a multi-agent system in a microgrid [ J ]. power system automation, 2008,32(24):80-82), but the control design of a single microgrid system is only researched, and a microgrid group formed by a plurality of planned island microgrids is not specifically analyzed. The document (LopesJ, MoreiraCL, madureiraag. definingcontroll systems for micro requirements systems and devices [ J ]. power systems, ieee transactions, 2006,21(2):916 and 924) adopts a hierarchical coordination control strategy, and the upper-level central controller adjusts the working state and load switching of the bottom-level distributed power controller according to the system power supply and demand conditions. The document (thoughts, gold, wang strong steel, and the like, a multi-microgrid system hierarchical coordination control strategy [ J ] of a series and parallel structure, power system automation, 2013,37(12):13-18) provides a tie line power and grid-connected synchronization coordination control strategy of the multi-microgrid system, and the amplitude difference, the frequency difference and the phase difference of voltages on two sides of a closing switch can be reduced to be within a synchronization range. However, the existing research still involves little coordination control among a plurality of sub-microgrid systems under different operating states, which is important for normal operation and efficient and reliable load power supply of the multi-sub-microgrid system (LiangChe, mohammadsharehdehpour. dcmicrogrids: economic operation and enhancement of resource by hierarchy higher control. ieee trans. on smart grid, 2014, 5 (5): 2517-2526).

Disclosure of Invention

The invention provides a direct current micro-grid group energy storage optimization and coordination control method aiming at the problem of coordination control among a plurality of sub-micro-grid systems in different operation states.

The technical scheme of the invention is as follows: a plurality of sub-micro-grids are connected with a high-capacity energy storage power station through a direct current bus to form a direct current micro-grid group, each sub-micro-grid comprises a distributed power generation system, a low-capacity energy storage system and a load, the direct current micro-grid group is stably controlled to operate through two layers of coordination, and the bottom layer adopts decentralized control: the distributed power generation system in the sub-micro grid adopts MPPT to control photovoltaic to output electric energy with maximum power; the small-capacity energy storage system in the sub-microgrid adopts a power control method to automatically distribute load power requirements and is disconnected with the microgrid to protect energy storage when the safe capacity limit is reached; the load is connected to the direct current bus directly or after power conversion is carried out through a DC/DC converter according to specific requirements; each energy storage unit in the energy storage power station is segmented according to the size by adopting the residual capacity difference value of different energy storage units, each segment is subjected to fuzzy droop control to dynamically distribute load power, and the voltage drop of a bus caused by the droop control is compensated through voltage feedback control;

the upper control adopts centralized control: controlling the parallel operation among the sub-micro-grids and between the sub-micro-grids and the energy storage power station according to the bus voltage fluctuation range of each sub-micro-grid, and dividing the system operation into 4 working modes: 1) the direct-current micro-grid group operates off-grid and each sub-micro-grid operates independently; 2) the direct-current micro-grid group operates in an off-grid mode, and the multiple sub-micro-grids operate in parallel; 3) the direct-current micro-grid group operates in an off-grid mode, and the sub-micro-grids operate in parallel with the energy storage power station; 4) and when the direct-current micro-grid group is connected to the grid, the alternating-current main grid stabilizes the voltage.

The 4 working modes in the upper layer control are as follows:

operating mode 1): each sub-microgrid independently and stably operates, each distributed power generation system controls maximum power generation through MPPT, a small-capacity energy storage system in the sub-microgrid system is used for controlling the stability of the voltage of each microgrid bus and balancing the power supply and demand of the microgrid bus, and the power required to be provided by the small-capacity energy storage system is as follows:

P_{battery_x}＝P_{load_x}-P_{DG_x}，P_{battery_x}represents the charge and discharge power P of the small-capacity energy storage system in the xth sub-microgrid_{load_x}Representing the load power in the xth sub-microgrid, P_{DG_x}Representing the generated power of a distributed power generation system in the xth sub-microgrid;

operating mode 2): when the distributed power generation system and the small-capacity energy storage system in each sub-microgrid still cannot meet the requirement of load power utilization, the fluctuation of the bus voltage will exceed the allowable range, and at the moment, the sub-microgrids with the bus voltage increased and the voltage dropped can run in parallel through the circuit breakers, so that each sub-microgrid meets the following power balance,

$\underset{x=1}{\overset{u}{\Σ}}{P}_{load\_x}=\underset{x=1}{\overset{u}{\Σ}}{P}_{battery\_x}+\underset{x=1}{\overset{u}{\Σ}}{P}_{DG\_x}$

wherein u represents the number of the sub-micro-grids operated in parallel, and u is more than or equal to 1 and less than or equal to 3;

operating mode 3): the power supply and demand of the system still cannot be balanced after the sub-micro-grids are operated in parallel, the bus voltage has large fluctuation, the micro-grids and the energy storage power station are operated in parallel, at the moment, the system meets the following power balance,

$P_{ESS}=\underset{x=1}{\overset{u}{\Σ}}{P}_{DG\_x}-\underset{x=1}{\overset{u}{\Σ}}{P}_{battery\_x}-\underset{x=1}{\overset{u}{\Σ}}{P}_{load\_x}$

wherein, P_ESSThe charging and discharging power of the energy storage power station is represented, u represents the number of sub-micro-grids which are operated in parallel, u is more than or equal to 1 and less than or equal to 3, in the mode, the energy storage power station absorbs or discharges power to control the stability of bus voltage, the energy storage power station controls the charging and discharging balance of the residual capacity SOC of each energy storage unit in the energy storage power station by adopting fuzzy droop, and voltage drop caused by droop control and disturbance is compensated by introducing voltage feedback control;

operating mode 4): the direct-current micro-grid group is connected with an alternating-current main grid through a grid-connected inverter, the alternating-current main grid maintains stable bus voltage and charges a small-capacity energy storage system in the energy storage power station and the sub-micro-grids, and the alternating-current main grid needs to provide power requirements outside each distributed power generation system,

P_grid＝P_load-P_DG-P_battery-P_ESS，

P_DGthe total power generation power P of the distributed power generation system in the direct current micro-grid group_loadTotal load power, P, for a DC microgrid group_batteryThe total charging and discharging power P of all sub-micro-grid small-capacity energy storage systems_ESSIndicating charge and discharge power, P, of the energy storage station_gridAnd the exchange power values of the alternating main network and the micro-grid group are represented, each distributed power generation system generates power according to the maximum power under the control of MPPT, and each small-capacity energy storage system is in a charging state if not full.

The charge and discharge power of the small-capacity energy storage systems in the sub-micro-grids operated in parallel is controlled by the following formula:

ζ＝(b_x)′P_{battery_x}，

$\mathrm{\ζ}={\left(V_{dc}\right)}^{\′}=1-\frac{1}{0.5\underset{x=1}{\overset{u}{\Σ}}{P}_{DG\_x}^{\max }}\underset{x=1}{\overset{u}{\Σ}}{P}_{DG\_x}$

where zeta is a constant and-1. ltoreq. zeta.ltoreq.1, V_dcIs a DC bus voltage, P_{DG_x}Andthe actual generating power and the maximum generating power of each distributed generating system in the xth sub-microgrid are respectively, u represents the number of sub-microgrids which are operated in parallel, u is more than or equal to 1 and less than or equal to 3, u is 1 when the three sub-microgrids are operated independently,

${\left(b_x\right)}^{\&prime;}=\left\{\begin{array}{ccc}{\left(\frac{{\mathrm{SOC}}_x}{{{\mathrm{SOC}}_x}^{*}}\right)}^{\mathrm{\&lambda;}}{b}_x,& if& P_{battery\_x}<0\\ {\left(\frac{{{\mathrm{SOC}}_x}^{*}}{{\mathrm{SOC}}_x}\right)}^{\mathrm{\&lambda;}}{b}_x,& if& P_{battery\_x}>0\end{array}\right.$

therein, SOC_x ^*And SOC_xRespectively are reference values and instantaneous values of the state of charge of the small-capacity energy storage system in the xth sub-microgrid, wherein lambda is a constant,

${{\mathrm{SOC}}_x}^{*}=\frac{({\mathrm{SOC}}_x^{\min }+{\mathrm{SOC}}_x^{max})}{2}$

in the formula,andand respectively limiting the minimum and maximum safe capacity of the SOC of the small-capacity energy storage system in the xth sub-microgrid.

The invention has the beneficial effects that: according to the direct-current microgrid group energy storage optimization and coordination control method, when a plurality of microgrids run in parallel, the control method adopted by the energy storage systems in each microgrid can automatically adjust power distribution according to the maximum charging and discharging power of each energy storage system and the residual electric quantity SOC thereof, so that the distributed generation power fluctuation can be smoothed, the bus voltage quality is improved, and meanwhile, the safety of the energy storage system is improved. When each sub-microgrid energy storage system cannot effectively buffer the power supply requirement of the system, the sub-microgrid energy storage system needs to be connected to an energy storage power station to control the stability of bus voltage. For the charge and discharge control of the energy storage power station, fuzzy control is adopted to adjust the virtual impedance of droop controllers of each energy storage unit in the energy storage power station so as to realize the automatic distribution of power and the balance of SOC among different energy storage units, fuzzy input is segmented, namely SOC difference values among different energy storage units, and fuzzy input in different ranges is respectively subjected to fuzzy control so as to accelerate the balance speed of the SOC. In addition, the bus voltage drop caused by droop control is compensated by adopting a bus voltage feedback control method, so that the bus voltage quality is improved.

Drawings

FIG. 1 is a diagram of a microgrid system architecture;

FIG. 2 is a block diagram of the system control of the present invention;

FIG. 3 is a diagram of the energy storage system control architecture of the present invention;

FIG. 4 is a block diagram of bus voltage feedback control according to the present invention;

fig. 5 is a schematic diagram of a dc microgrid group hierarchical coordination control strategy according to the present invention;

fig. 6 is a diagram of a hysteresis control method for a switching point of a working mode of a direct-current microgrid group according to the present invention;

FIG. 7a is a SOC balance diagram using blur according to the present invention;

FIG. 7b is a SOC balance diagram without blur according to the present invention;

FIG. 7c is a graph of bus voltage curves using bus voltage feedback and ambiguity in accordance with the present invention;

FIG. 7d is a graph of bus voltage without bus voltage feedback but with ambiguity;

fig. 8a is a bus voltage diagram of the sub-microgrid 1 according to the present invention;

fig. 8b is a voltage diagram of the sub-microgrid 2 bus according to the present invention;

fig. 8c is a bus voltage diagram of the sub-microgrid 3 according to the present invention;

fig. 8d is a graph of energy storage power in each sub-microgrid according to the present invention;

FIG. 8e is a power diagram of the energy storage plant of the present invention;

FIG. 8f is a SOC diagram of the energy storage unit 1 in the energy storage power station of the present invention;

FIG. 8g is a SOC diagram of the energy storage unit 2 in the energy storage power station of the present invention;

FIG. 9a is a SOC diagram of the energy storage unit 1 in the energy storage power station of the present invention;

fig. 9b is a SOC diagram of the energy storage unit 2 in the energy storage power station of the invention.

Detailed Description

The microgrid group is controlled to stably and reliably operate by adopting a two-layer coordination control strategy. The bottom layer adopts decentralized control, MPPT is adopted to control a photovoltaic distributed power generation system and the like to output electric energy with the maximum power under the control, a small-capacity energy storage system in the sub-microgrid adopts an improved power control method to automatically distribute load power requirements and is disconnected with the microgrid to protect energy storage when the safety capacity limit is reached, and the load can be directly connected to a direct current bus or connected to the direct current bus after power conversion is carried out through a DC/DC converter according to specific requirements. Each energy storage unit in the energy storage power station dynamically distributes load power by adopting a segmented fuzzy droop control strategy, and compensates the problem of bus voltage drop caused by droop control through voltage feedback control. The upper-layer control adopts centralized control, and the control strategy controls the parallel operation among the sub-micro-grids and the energy storage power grid according to the bus voltage fluctuation range of each sub-micro-grid. In addition, the system is divided into four working modes according to the parallel-connection and off-network operation and the operation conditions of each sub-microgrid, and the bottom-layer control strategy and the upper-layer control strategy are reasonably selected, so that the system can automatically and smoothly switch under different operation conditions and provide high-quality electric energy.

The coordination control method designed by the scheme is suitable for a microgrid group comprising a plurality of sub-microgrids, and for convenience of description and simulation analysis, three sub-microgrids are taken to explore interactive operation among the sub-microgrids under different disturbances. Each sub-microgrid is composed of a distributed power generation system, a small-capacity energy storage system, a load and the like. The specific control structure is shown in fig. 2, each distributed power supply in the sub-microgrid 1 shown in the figure utilizes new energy to the maximum extent by adopting an MPPT algorithm, a direct-current load adopts constant-voltage control to improve power supply quality, a small-capacity energy storage system adopts a droop control method to smooth power fluctuation, and when different sub-microgrids are operated in parallel, the respective small-capacity energy storage systems of the sub-microgrids are operated in parallel, so that under the adopted droop control method, the small-capacity energy storage systems of the sub-microgrids can provide load power according to the maximum allowed power in the same proportion. The structures and control methods in the piconets 2 and 3 are the same as those of the piconet 1. In addition, the three sub-microgrids and the high-capacity energy storage power station are connected through a direct-current bus, and therefore a direct-current microgrid group is formed.

In various operating states, P_{DG_x}Where x is 1,2,3 denotes the generated power of the distributed power generation system in the xth sub-microgrid, P_DGIs the total power generation power of the distributed power generation system in the micro grid group, andrepresenting the load power in the xth sub-microgrid, P_loadIs the total power of the load of the microgrid group, andrepresents the charge and discharge power P of the small-capacity energy storage system in the xth sub-microgrid_batteryThe total power of charging and discharging of the three sub-micro-grid small-capacity energy storage systems is metP_ESSIndicating the charging and discharging power of the energy storage station, U_{dc_x}Where x is 1,2,3 denotes a bus voltage of each subgrid, and U is_dcFor storing terminal voltage of power station, P_gridAnd expressing the exchange power value of the alternating main network and the micro-grid group.

1. The operation mode of the direct-current microgrid group is as follows:

the direct-current microgrid usually works in a grid-connected state and an off-grid state, and system operation is divided into 4 working modes according to series-parallel operation conditions among multiple-sub microgrid systems and different external interferences (such as distributed generation power, load fluctuation and the like): 1) the off-grid operation is carried out, and each sub-microgrid operates independently; 2) the off-grid operation and the parallel operation of the multiple sub-microgrids are realized; 3) the sub-microgrid and the energy storage power station are operated in parallel; 4) and when the grid is connected, the alternating current main network stabilizes voltage.

1) Working mode 1:

under the working mode, each sub-microgrid independently and stably operates, each distributed power generation system still controls the maximum power generation through MPPT, a small-capacity energy storage system in each sub-microgrid system is used for controlling the voltage stability of each microgrid bus and balancing the power supply and demand of each microgrid, and the power required to be provided by the small-capacity energy storage system is

P_{battery_x}＝P_{load_x}-P_{DG_x}(4)

2) The working mode 2 is as follows:

when the distributed power generation system and the small-capacity energy storage system in each sub-microgrid still cannot meet the requirement of load power utilization, the fluctuation of bus voltage can exceed the allowable range, and the sub-microgrid with the increased bus voltage and the dropped voltage can run in parallel through a circuit breaker. The mode is favorable for balancing the power supply and demand of each sub-microgrid. Each sub-microgrid satisfies the following power balance,

$\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{load\_x}=\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{battery\_x}+\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}---\left(5\right)$

and u represents the number of the sub-micro-grids in parallel operation, and u is more than or equal to 1 and less than or equal to 3, so that when a plurality of sub-micro-grids are in parallel operation, the bus voltage can be effectively stabilized, and the loss caused by frequent power exchange between the sub-micro-grids and the energy storage power station is reduced.

3) Working mode 3:

the power supply and demand of the system still cannot be balanced after the sub-micro-grids are operated in parallel, the bus voltage has large fluctuation, and the micro-grids and the energy storage power station are considered to be operated in parallel at the moment, so that the mode is entered. At this time, the system satisfies the following power balance

$P_{ESS}=\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}-\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{battery\_x}-\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{load\_x}---\left(6\right)$

And u represents the number of the sub-micro-grids running in parallel, and u is more than or equal to 1 and less than or equal to 3, in the mode, the energy storage power station absorbs or releases power to control the bus voltage to be stable, the energy storage power station controls the charge-discharge balance of each energy storage unit SOC by adopting fuzzy droop, and the voltage drop caused by droop control and disturbance is compensated by introducing voltage feedback control.

4) The working mode 4 is as follows:

at the moment, the micro-grid group is in grid-connected operation, the micro-grid group is connected with the alternating current main grid through a grid-connected inverter (G-VSC), and the alternating current main grid maintains the stable bus voltage and charges the energy storage power station and the small-capacity energy storage system in the sub-micro-grids. At this point, the ac main grid needs to provide power requirements outside of each distributed power generation system,

P_grid＝P_load-P_DG-P_battery-P_ESS(7)

in the mode, each distributed power generation system generates power according to the maximum power under the control of MPPT, and each small-capacity energy storage system is in a charging state if not full.

2. The bottom layer control method of the direct-current microgrid group comprises the following steps:

each distributed power generation system in the direct-current micro-grid adopts MPPT to control power generation, and a direct-current load can be directly connected with a direct-current bus through a DC/DC converter or adopt constant-voltage control to supply power. The small-capacity energy storage system plays a key role in stabilizing bus voltage, balancing system power supply and demand and the like, and particularly when the system operates in an island, the small-capacity energy storage system directly influences system stable operation and high-quality power supply.

1) The control strategy adopted by the small-capacity energy storage system in the sub-microgrid is as follows:

the fluctuation value of the bus voltage determines the charging and discharging power of the small-capacity energy storage system. Defining:

ζ＝(V_dc)′(8)

wherein, zeta is constant and-1 ≦ zeta ≦ 1, and DC bus voltage V_dcζ can be obtained by the following formula:

$\mathrm{\&zeta;}={\left(V_{dc}\right)}^{\&prime;}=1-\frac{1}{0.5\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}^{\max }}\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}---\left(9\right)$

wherein, P_{DG_x}Andthe actual power generation power and the maximum power generation power of each distributed power generation system in the xth sub-microgrid are respectively, u represents the number of sub-microgrids which are operated in parallel, u is more than or equal to 1 and less than or equal to 3, and u is 1 when the three sub-microgrids are operated independently, so that (V)_dc) ' between-1 and 1, in island, if 0<(V_dc)′<1, charging the small-capacity energy storage system in the sub-microgrid operating in parallel to absorb surplus power, and if the small-capacity energy storage system is-1<(V_dc)′<And 0, discharging the small-capacity energy storage system in the sub-microgrid in parallel operation to stabilize the bus voltage.

At this time, the small-capacity energy storage systems in the sub-microgrids operating in parallel can be charged and discharged according to the following formula:

ζ＝b_xP_{battery_x}(10)

wherein, P_{battery_x}The bipolar variable represents the actual charging and discharging power of the small-capacity energy storage system in the xth sub-microgrid, a positive value represents discharging, and a negative value represents charging. b_xThe droop coefficient of the small-capacity energy storage system in the xth sub-microgrid is as follows:

$b_x=\frac{1}{P_{battery\_x}^{\max }}---\left(11\right)$

wherein,the maximum allowable charge-discharge power, ζ, of the small-capacity energy storage systems in the xth sub-microgrid is a constant, so that the small-capacity energy storage systems in each sub-microgrid can bear load power in the same proportion to the respective maximum allowable power according to equation (12).

However, since the amount of electricity and the charging power of the energy storage battery vary with the change in SOC during actual operation, b can be expressed by the following equation (13)_xAnd (7) correcting.

${\left(b_x\right)}^{\&prime;}=\left\{\begin{array}{ccc}{\left(\frac{{\mathrm{SOC}}_x}{{{\mathrm{SOC}}_x}^{*}}\right)}^{\mathrm{\&lambda;}}{b}_x,& if& P_{battery\_x}<0\\ {\left(\frac{{{\mathrm{SOC}}_x}^{*}}{{\mathrm{SOC}}_x}\right)}^{\mathrm{\&lambda;}}{b}_x,& if& P_{battery\_x}>0\end{array}\right.---\left(13\right)$

Therein, SOC_x ^*And SOC_xThe reference value and the instantaneous value of the state of charge of the small-capacity energy storage system in the xth sub-microgrid are respectively. Lambda is a constant, and it can be seen that the larger the lambda value is, the larger the influence of SOC on the charge and discharge power of the energy storage system is, where lambda is 1 and SOC is taken as_x ^*Can be defined as:

${{\mathrm{SOC}}_x}^{*}=\frac{({\mathrm{SOC}}_x^{\min }+{\mathrm{SOC}}_x^{max})}{2}---\left(14\right)$

in the formula,andand respectively limiting the minimum and maximum safe capacity of the SOC of the small-capacity energy storage system in the xth sub-microgrid. Up to this point, the charge and discharge power of each energy storage system can be controlled by the following formula:

ζ＝(b_x)′P_{battery_x}(15)

through the analysis, when the sub-microgrids in the microgrid group run in parallel, the respective energy storage systems can automatically adjust the charging and discharging power according to the control method. Specifically, the energy storage system with a large SOC has a relatively small charging power and a relatively large discharging power, and the energy storage system with a small SOC has the opposite. Therefore, the SOC balance of the energy storage systems of the sub-micro-grids can be balanced, and the overcharge or the overdischarge of the energy storage systems in a certain sub-micro-grid can be effectively avoided, so that the safety of energy storage and the stability of the system are improved.

2) The control strategy adopted by the energy storage power station is as follows:

the virtual impedance value of droop controllers of each energy storage unit in the energy storage power station is automatically adjusted by adopting fuzzy control, so that the self-operation between the charging and discharging power of the energy storage units is realizedAnd (4) dynamic distribution. The control method provided by the invention is suitable for an energy storage power station system formed by a plurality of groups of energy storage units, and for simplifying analysis, two groups of energy storage units are taken. For example, fig. 3 is a control structure diagram of an energy storage power station, the input of a fuzzy controller, that is, SOC difference values of different energy storage units in the energy storage power station are segmented according to their magnitudes, the fuzzy control is respectively applied to the SOC difference values in different ranges to automatically adjust the virtual impedance Rd of the droop controller of each energy storage unit, and the droop controller applied to the energy storage units detects the bus voltage signal V as shown in fig. 2_DCAnd the power distribution of different energy storage units is automatically adjusted by utilizing the P-V characteristic of droop control. In addition, the bus voltage feedback control shown in fig. 3 obtains the reference value V of the droop controller through PI control by detecting the bus voltage and comparing the bus voltage with the reference voltage_refOf the dynamic correction value DeltaV_refTherefore, the bus voltage drop caused by droop control can be compensated, and the voltage quality is improved.

A, droop control:

the charge-discharge power balance of the energy storage unit can be realized through droop control. The output voltage of the energy storage unit is as follows:

V_DC＝V_ref-I_Li×Rd_i(16)

wherein, Rd_ii is 1,2 is the virtual impedance of the energy storage unit droop controller in the energy storage power station, V_DCIs the DC bus voltage, V_refIs a bus reference voltage, I_LiIs the output current of the converter of the energy storage unit in the energy storage power station. Virtual impedance Rd of droop controller of each energy storage unit_iThe energy storage units with smaller values have larger charge and discharge power, so that the SOC difference values of different energy storage units are taken as fuzzy input, the virtual impedance of the droop controllers of the energy storage units is adjusted through fuzzy control, and the automatic distribution of the charge and discharge power of the energy storage units in the energy storage power stations can be realized. Specifically, for energy storage units with relatively low SOC, Rd is reduced_iThe value is increased to increase the charging power, and the value is adjusted to be larger to decrease the discharging power during discharging. And vice versa.

The control strategy of the energy storage power station is suitable for multiple groups of energy storage units, and the actual balance effect of the SOC of the energy storage units is explored by adopting two groups of energy storage units. Under the MPPT control, the distributed power generation system may be regarded as a Constant Power Supply (CPS), and the value of the virtual impedance Rd of the energy storage unit droop controller may be limited by the following equation to prevent the energy storage charging and discharging power from exceeding the maximum allowable value.

$V_{DC}=\frac{\frac{V_{ref}}{R_{deq}}+\sqrt{{\left(\frac{V_{ref}}{R_{deq}}\right)}^2+4P_{DG}(\frac{1}{R_{deq}}+\frac{1}{R_{load}})}}{2(\frac{1}{R_{deq}}+\frac{1}{R_{load}})}---\left(17\right)$

In the formula, R_deqIs the equivalent virtual resistance, R, of the droop controller of the energy storage unit when the distributed power generation system is regarded as a constant power source_loadIs a load resistor. Because the generated power and the bus voltage of each distributed power generation system are both positive, the value range of Rd can be solved through the equation (17).

B: and (3) segmented fuzzy control:

and segmenting the SOC difference values among different energy storage units, and respectively adopting fuzzy control to adjust the virtual impedance value of the droop controller of each energy storage unit according to the SOC difference values with different sizes. Specifically, when the SOC difference is large, the virtual impedance difference of each energy storage unit is increased to accelerate the SOC balancing speed, and when the SOC difference is small, the virtual impedance difference of each energy storage unit is decreased to reduce or even eliminate the SOC steady-state deviation. Therefore, the SOC difference values are segmented according to the size, so that the SOC difference values among different energy storage units can be quickly and effectively balanced, and the safe and reliable power supply of the energy storage system is ensured. Specific simulation analysis will be given in the next section.

And C, bus voltage feedback control:

the method is based on droop control, so that the problem of bus voltage sag is caused, for the bus voltage sag, as shown in fig. 4, feedback compensation control is introduced, compensation quantity output by a PI controller is dynamically superposed on a reference voltage value of a fuzzy droop controller of each energy storage unit converter, automatic compensation of the bus voltage is realized, and voltage sag caused by the droop control can be controlled.

3. The upper-layer control method of the direct-current micro-grid group comprises the following steps:

the adopted direct-current microgrid group comprises a plurality of groups of sub-microgrids and an energy storage power station, and the parallel operation among the sub-microgrids can effectively balance the shortage of power among systems, so that the stability of the system is improved. Particularly, when the voltage of the parallel operation bus of the multi-sub micro-grid still does not meet the power supply requirement, the parallel operation bus can be connected with the energy storage power station to further balance the power supply and demand of the system through the charging and discharging of the energy storage power station. However, frequent power conversion between sub-micro grids and energy storage power stations increases power loss and generates harmonic pollution and the like, so that measures must be taken to control switching of different operation modes of the sub-micro grids and the energy storage power stations, and power loss is reduced as much as possible on the premise of high-quality reliable power supply.

The voltage stratification method used herein is shown in fig. 5, and varies according to the bus voltage (Δ V) of each sub-microgrid_dc) To control the switching of the operating modes of the system in order to avoidFrequent switching between the operating modes during the run-free process, as shown in fig. 6, hysteresis control is employed at the switching point. Switching threshold voltage U in this context_t1And U_t25V and 10V are respectively taken. Where, S ═ 1,2, and 3 indicate that the system is in the 1 st, 2 nd, and 3 rd operating modes, respectively.

4. Analysis by calculation example:

in order to verify the effectiveness of the multi-sub microgrid coordinated control strategy based on energy storage optimization, a simulation model shown in fig. 1 is built in Matlab/Simulink. In the simulation, 3 groups of direct current microgrid systems are adopted, the bus voltage of each sub-microgrid is 380V, and the disturbances such as illumination or wind speed change are set for the sub-microgrid systems respectively. The energy storage power station adopts two groups of energy storage units with the capacity of 4.5Kwh and the charge states of 69.2 percent and 70 percent respectively. In simulation, the working voltage of a direct current load is 380V, or power is supplied after the direct current load is converted by a DC/DC converter.

The effectiveness of the control strategy on the four working modes is verified through simulation analysis, and the parallel operation characteristics between the sub-micro-networks and the energy storage power station are mainly analyzed. Particularly, when the multiple sub-micro-grids run in parallel, the respective energy storage systems can be charged and discharged in the same proportion according to the respective charging and discharging power limits and SOC according to the adopted control method, so that the safety and the reliability of the energy storage systems in the sub-micro-grids are effectively improved. In addition, the fuzzy self-adaptive droop control adopted by the energy storage power station can automatically adjust the power distribution among the energy storage units within the maximum charge-discharge power allowable range, and can quickly balance the SOC. Aiming at the problem of bus voltage drop caused by droop control, the high-quality power supply can be realized and the system stability is improved by introducing the reference voltage feedback compensation quantity for control. And (3) adopting named values for all main parameters in simulation.

1) Bus voltage compensation control simulation analysis:

in order to verify the effectiveness of dynamic load power distribution and bus voltage compensation of the energy storage power station control strategy, an optical storage direct current microgrid simulation model is built, and a simulation structure adopts a microgrid 3 in fig. 1. Specifically, the energy storage control strategy in the sub-microgrid 3 adopts segmented fuzzy self-adaptive droop control and adopts voltage closed-loop feedback to compensate the voltage drop of the bus. The method comprises the steps of respectively simulating and analyzing the balance effect of SOC (system on chip) of two groups of energy storage units and the actual control effect of bus voltage under three conditions of taking a constant value for a droop coefficient of an energy storage unit droop controller, adopting subsection fuzzy self-adaptive droop control and adopting fuzzy control and adopting bus voltage compensation control. In addition, other simulation parameters and control methods are as described in the above section, and are not described again.

As shown in fig. 7, in the simulation process, the generated power of the photovoltaic system is respectively reduced and increased at 2s and 4s, and then the energy storage system stabilizes the bus voltage through charge and discharge control. Fig. 7a and 7b are graphs of SOC balancing effects of two energy storage units when fuzzy adaptive droop control and droop coefficients are used to take fixed values, respectively. As can be seen from fig. 7a, the charging and discharging power of the two energy storage units can be automatically adjusted in the charging and discharging processes, so that the energy storage unit with a smaller SOC has a larger charging power and a smaller discharging power, and vice versa, thereby realizing the rapid balance of the SOCs of the different energy storage units. As can be seen from fig. 7b, when the droop coefficient takes a fixed value, the two energy storage units bear load power on average, and SOC balance cannot be achieved. Fig. 7c and 7d are the bus voltage when the bus voltage feedback compensation is adopted and the bus voltage compensation is not adopted, and both adopt fuzzy control to automatically adjust the droop coefficient of the energy storage droop controller, so that the SOC balance effect is still as shown in fig. 7 a. Comparing fig. 7c and 7d, it can be seen that when the bus voltage compensation is adopted, when the external interference occurs, the fluctuation of the bus voltage is small, and the voltage quality is high after the bus voltage enters the steady state.

The analysis shows that the SOC among different energy storage units can be rapidly balanced by adopting the segmented fuzzy self-adaptive droop control, and the bus voltage drop caused by the droop control can be compensated by adopting the bus voltage compensation control, so that the bus voltage quality is improved. Therefore, the energy storage control method provided by the invention is proved to effectively improve the quality of electric energy while improving the safety and reliability of the energy storage system.

2) Analyzing the operating characteristics of the microgrid group:

in the simulation, each distributed power supply controls maximum power generation by adopting MPPT (maximum power point tracking), an energy storage system in each sub-microgrid is limited by maximum charge-discharge power, and the SOC of two energy storage power stations in the energy storage power stations is 69.2% and 70% respectively. The independent operation of each sub-microgrid, the parallel operation of the sub-microgrid and the parallel operation of the energy storage power station are mainly explored. The interaction of the multi-sub microgrid and the energy storage power station is analyzed through simulation, and the microgrid group can reliably and stably operate and effectively improve the voltage quality under the control strategy.

When the simulation starts, the energy storage in the sub-microgrid is limited by the maximum charging and discharging power, and large bus voltage drop can be caused when large power fluctuation occurs. As shown in fig. 8a, 8b and 8c, the sub-microgrid bus voltages are respectively about 380.1V, 355V and 386V, wherein the bus voltage fluctuations of both the sub-network 2 and the sub-network 3 have exceeded the threshold value U_t1And U_t2Thus at 0.5s subnetworks 2 and 3 are operating in parallel and subnet 1 remains operating in island. It can be seen from FIGS. 8b and 8c that the bus voltages were improved to about 371V, but still exceeded the threshold voltage U_t1In the process, the charging and discharging power of the energy storage system in each sub-microgrid is shown in fig. 8d, and it can be seen that after the sub-network 2 and the sub-network 3 are operated in parallel, the energy storage system can automatically adjust the power distribution according to the respective maximum charging and discharging power and considering the SOC, so that the safety of the energy storage system is effectively improved. At 1s, the subnetworks 2 and 3 are connected with the energy storage power station in parallel to operate to stabilize the bus voltage, and the parallel bus voltage is recovered to about 378.8V to meet the design requirement. Meanwhile, the power generation power of the sub-microgrid 2 is increased, and the energy storage system cannot effectively balance the power supply and demand of the system due to the fact that the energy storage system exceeds the designed charge-discharge power range, so that the voltage is greatly increased and is about 400V. At 1.5s, the three subnets and the energy storage power station run in parallel, redundant electric energy is used for charging the energy storage power station, the voltage of each subnet bus is about 379.2V, the fluctuation range is small, and the requirement of high-quality power supply is basically met. The generated power of each sub-microgrid is recovered to be normal in 2.0s, so that the energy storage power station and each sub-microgrid are disconnected to operate independently, and the voltage of each sub-grid bus can be better stabilized under the regulation and control of the corresponding energy storage systemAnd at about 380V, the system supplies power safely.

It is noted that, in the above process, the energy storage power station operates in parallel with the microgrid group at 1.0s, and exits at 2.0 s. In this process, the two energy storage units of the energy storage power station rapidly balance the SOC of the two energy storage units according to the adopted control method, as shown in fig. 8f and 8g, specifically, during discharging, adjust the energy storage unit with larger/smaller SOC to larger/smaller discharging power, and vice versa. In addition, the adopted bus voltage feedback compensation control method can effectively compensate bus voltage drop caused by droop control. Therefore, the SOC among the energy storage units can be quickly balanced and the bus voltage quality can be effectively improved while the safe and reliable power supply is ensured.

3) Analyzing the charging characteristic of the energy storage power station during grid-connected operation:

the main simulation analysis has been carried out the energy storage power station and has been had and had been had little net parallel operation and had exchanged the parallel operation of main network characteristic charging characteristic in the process. The energy storage power station adopts two groups of energy storage units with SOC of 69.2% and 70% respectively.

Fig. 9a and 9b are SOC diagrams of two energy storage units of the energy storage power station respectively, the micro-grid group, the energy storage power station and the alternating main network are operated in parallel when 0-1.0s is taken, the micro-grid group and the alternating main network charge the energy storage power station together, and the charging power of the two energy storage units is automatically distributed according to the SOC of the energy storage power station. And when the time is 1.0-2.0s, the energy storage power station is connected with the alternating current main network and disconnected with the micro-grid group, and at the moment, because only the alternating current main network charges the power station, the charging power is slowed down. And when 2.0s, the alternating current main network is disconnected with the energy storage power station.

As can be seen from fig. 9a and 9b, in the switching process of each operation mode, the system can smoothly and quickly respond under the control method adopted, so that the safe and reliable power supply of the energy storage system is ensured.

Claims (3)

1. A direct-current microgrid group energy storage optimization and coordination control method is characterized in that a plurality of sub-microgrids and a high-capacity energy storage power station are connected through a direct-current bus to form a direct-current microgrid group, each sub-microgrid comprises a distributed power generation system, a low-capacity energy storage system and a load, the direct-current microgrid group is controlled to operate stably by adopting two layers of coordination,

the bottom layer adopts dispersion control: the distributed power generation system in the sub-micro grid adopts MPPT to control photovoltaic to output electric energy with maximum power; the small-capacity energy storage system in the sub-microgrid adopts a power control method to automatically distribute load power requirements and is disconnected with the microgrid to protect energy storage when the safe capacity limit is reached; the load is connected to the direct current bus directly or after power conversion is carried out through a DC/DC converter according to specific requirements; each energy storage unit in the energy storage power station is segmented according to the size by adopting the residual capacity difference value of different energy storage units, each segment is subjected to fuzzy droop control to dynamically distribute load power, and the voltage drop of a bus caused by the droop control is compensated through voltage feedback control;

the upper control adopts centralized control: controlling the parallel operation among the sub-micro-grids and between the sub-micro-grids and the energy storage power station according to the bus voltage fluctuation range of each sub-micro-grid, and dividing the system operation into 4 working modes: 1) the direct-current micro-grid group operates off-grid and each sub-micro-grid operates independently; 2) the direct-current micro-grid group operates in an off-grid mode, and the multiple sub-micro-grids operate in parallel; 3) the direct-current micro-grid group operates in an off-grid mode, and the sub-micro-grids operate in parallel with the energy storage power station; 4) and when the direct-current micro-grid group is connected to the grid, the alternating-current main grid stabilizes the voltage.

2. The direct-current microgrid group energy storage optimization and coordination control method according to claim 1, characterized in that 4 working modes in the upper-layer control are as follows:

operating mode 1): each sub-microgrid independently and stably operates, each distributed power generation system controls maximum power generation through MPPT, a small-capacity energy storage system in the sub-microgrid system is used for controlling the stability of the voltage of each microgrid bus and balancing the power supply and demand of the microgrid bus, and the power required to be provided by the small-capacity energy storage system is as follows:

P_{battery_x}＝P_{load_x}-P_{DG_x}，P_{battery_x}represents the charge and discharge power P of the small-capacity energy storage system in the xth sub-microgrid_{load_x}Representing the load power in the xth sub-microgrid, P_{DG_x}Representing the generated power of a distributed power generation system in the xth sub-microgrid;

operating mode 2): when the distributed power generation system and the small-capacity energy storage system in each sub-microgrid still cannot meet the requirement of load power utilization, the fluctuation of the bus voltage will exceed the allowable range, and at the moment, the sub-microgrids with the bus voltage increased and the voltage dropped can run in parallel through the circuit breakers, so that each sub-microgrid meets the following power balance,

$\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{load\_x}=\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{battery\_x}+\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}$

wherein u represents the number of the sub-micro-grids operated in parallel, and u is more than or equal to 1 and less than or equal to 3;

operating mode 3): the power supply and demand of the system still cannot be balanced after the sub-micro-grids are operated in parallel, the bus voltage has large fluctuation, the micro-grids and the energy storage power station are operated in parallel, at the moment, the system meets the following power balance,

$P_{ESS}=\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}-\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{battery\_x}-\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{load\_x}$

wherein, P_ESSThe charging and discharging power of the energy storage power station is represented, u represents the number of sub-micro-grids which are operated in parallel, u is more than or equal to 1 and less than or equal to 3, in the mode, the energy storage power station absorbs or discharges power to control the stability of bus voltage, the energy storage power station controls the charging and discharging balance of the residual capacity SOC of each energy storage unit in the energy storage power station by adopting fuzzy droop, and voltage drop caused by droop control and disturbance is compensated by introducing voltage feedback control;

operating mode 4): the direct-current micro-grid group is connected with an alternating-current main grid through a grid-connected inverter, the alternating-current main grid maintains stable bus voltage and charges a small-capacity energy storage system in the energy storage power station and the sub-micro-grids, and the alternating-current main grid needs to provide power requirements outside each distributed power generation system,

P_grid＝P_load-P_DG-P_battery-P_ESS，

P_DGthe total power generation power P of the distributed power generation system in the direct current micro-grid group_loadTotal load power, P, for a DC microgrid group_batteryThe total charging and discharging power P of all sub-micro-grid small-capacity energy storage systems_ESSIndicating charge and discharge power, P, of the energy storage station_gridExpressing the exchange power value of the alternating main network and the micro-grid group, and each distributed power generation system performs power generation according to the maximum power under the control of MPPT (maximum power point tracking)And if the small-capacity energy storage systems are not fully charged, the small-capacity energy storage systems are in a charging state.

3. The direct-current microgrid group energy storage optimization and coordination control method of claim 2, wherein charging and discharging power of the small-capacity energy storage systems in the sub-microgrids operated in parallel is controlled by the following formula:

ζ＝(b_x)′P_{battery_x}，

$\mathrm{\&zeta;}={\left(V_{dc}\right)}^{\&prime;}=1-\frac{1}{0.5\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}^{\max }}\underset{x=1}{\overset{u}{\&Sigma;}}{P}_{DG\_x}$

where zeta is a constant and-1. ltoreq. zeta.ltoreq.1, V_dcIs a DC bus voltage, P_{DG_x}Andthe actual generated power and the maximum generated power of each distributed generation system in the xth sub-microgrid are respectively,u represents the number of the sub-microgrids which are operated in parallel, u is more than or equal to 1 and less than or equal to 3, u is more than or equal to 1 when the three sub-microgrids are operated independently,

${\left(b_x\right)}^{\&prime;}=\left\{\begin{array}{cc}{\left(\frac{{\mathrm{SOC}}_x}{{{\mathrm{SOC}}_x}^{*}}\right)}^{\mathrm{\&lambda;}}{b}_x,& \begin{array}{cc}if& P_{battery\_x}<0\end{array}\\ {\left(\frac{{{\mathrm{SOC}}_x}^{*}}{{\mathrm{SOC}}_x}\right)}^{\mathrm{\&lambda;}}{b}_x,& \begin{array}{cc}if& P_{battery\_x}>0\end{array}\end{array}\right.$

therein, SOC_x ^*And SOC_xRespectively are reference values and instantaneous values of the state of charge of the small-capacity energy storage system in the xth sub-microgrid, wherein lambda is a constant,

${{\mathrm{SOC}}_x}^{*}=\frac{({\mathrm{SOC}}_x^{\min }+{\mathrm{SOC}}_x^{max})}{2}$

in the formula,andand respectively limiting the minimum and maximum safe capacity of the SOC of the small-capacity energy storage system in the xth sub-microgrid.