Electricity-hydrogen hybrid energy storage DC microgrid coordination control method based on bus voltage layering

Abstract

The invention discloses an electricity-hydrogen hybrid energy storage direct-current micro-grid coordination control method based on bus voltage layering. The method comprises the following steps that 1, a mathematical model of an electricity-hydrogen hybrid energy storage direct-current micro-grid system is built, and a wind power generation system model, an electrochemical energy storage system model and a hydrogen energy storage system model are built; 2, dividing operation conditions for the system based on the bus voltage fluctuation range; 3, droop control and constant pressure control based on a fuzzy algorithm are adopted by the hydrogen energy storage system; 4, the electrochemical energy storage system adopts similar virtual synchronous machine control and constant voltage value control; according to the electricity-hydrogen hybrid energy storage direct-current micro-grid coordination control method based on bus voltage layering, the bus voltage of the direct-current micro-grid is kept stable under the source load fluctuation condition, good dynamic performance is achieved, and the problem of over-charging and over-discharging of a hydrogen energy storage system is effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of hybrid energy storage energy management, and in particular to a coordinated control method of an electric-hydrogen hybrid energy storage DC microgrid based on bus voltage stratification. Background Art

[0002] Since wind speed is random and intermittent, the voltage and frequency will fluctuate when wind power is connected to the grid. Energy storage technology has the ability to transmit power bidirectionally and flexibly dispatch, which can effectively alleviate the volatility of wind power. The limitations of the scale and reliability of traditional energy storage devices make them unsuitable for long-term operation. Therefore, it is particularly important to build a diversified energy storage system. Hydrogen energy, as a clean energy, has the characteristics of high energy density, high conversion efficiency, long-term storage and long-distance transmission. Using wind power to produce hydrogen can not only solve the problem of wind power consumption, but also provide a green and pollution-free energy storage solution. The research on coordinated control of wind-hydrogen coupling system mainly focuses on smoothing power fluctuations, while less attention is paid to the stability of bus voltage when the source and load fluctuate. Although most strategies can make the DC microgrid operate stably, they do not fully consider the problem of voltage mutation when the power fluctuates. In addition, when considering the control strategy of the underlying hydrogen energy storage system, the research is not comprehensive enough. Summary of the invention

[0003] The purpose of the present invention is to overcome the above-mentioned shortcomings and provide a coordinated control method for a DC microgrid with electric-hydrogen hybrid energy storage based on bus voltage stratification, so as to solve the problem of bus voltage volatility in a DC microgrid system under the influence of wind power and fluctuating loads, and improve the overcharging and over-discharging problems of the hydrogen energy storage system.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: a coordinated control method of an electric-hydrogen hybrid energy storage DC microgrid based on bus voltage stratification, comprising the following steps:

[0005] Step 1: Build a mathematical model of the electric-hydrogen hybrid energy storage DC microgrid system, and establish wind power generation system models, electrochemical energy storage system models, and hydrogen energy storage system models;

[0006] Step 2: Based on the bus voltage fluctuation range, divide the system into operating conditions;

[0007] Step 3: The hydrogen energy storage system adopts droop control and constant voltage control based on fuzzy algorithm; in normal mode, droop control based on fuzzy algorithm is adopted to dynamically optimize the output power of hydrogen storage according to the hydrogen storage transition state and voltage deviation, so that it can accelerate back to the normal range according to the hydrogen storage state when the hydrogen storage transition state is in a critical state; in extreme mode, constant voltage control is adopted to maintain bus voltage stability;

[0008] Step 4: The electrochemical energy storage system adopts quasi-virtual synchronous machine control and constant voltage control value; in normal mode, AVSG control is used to simulate the inertial support effect of capacitor on voltage, and the change rate of bus voltage during fluctuation is slowed down by flexibly adjusting the inertia coefficient; in extreme mode, constant power control is adopted;

[0009] Step 5: Simulate the constructed electric-hydrogen hybrid energy storage DC microgrid system to verify the effectiveness of the proposed electric-hydrogen hybrid energy storage coordinated control strategy under fluctuating conditions.

[0010] Preferably, the step 1 is specifically as follows:

[0011] 1.1. Establish a mathematical model of wind power generation system;

[0012] The wind power generation system consists of a direct-drive wind turbine system and a direct-drive wind turbine generator system;

[0013] The PMSG direct-drive fan system is responsible for capturing wind energy and converting it into mechanical energy. The output mechanical power P W It is expressed as:

[0014]

[0015] Where ρ is the air density; A is the area swept by the impeller; v w is the wind speed; C p is the wind energy utilization coefficient, which is determined by the tip speed ratio λ and the pitch angle β;

[0016] The stator voltage equation of the PMSG direct-drive wind turbine generator in the dq coordinate system is:

[0017]

[0018] Among them, u sd 、u sq Represents the d-axis and q-axis components of the generator stator winding voltage respectively; R s is the stator winding resistance; p is the differential operator; ω r is the rotor electrical angular velocity; L d , L q They are the d-axis and q-axis inductances of the stator winding respectively; i sd 、i sq The d-axis and q-axis components of the stator current respectively; ψ f is the permanent magnetic flux;

[0019] Electromagnetic torque equation:

[0020] T e =n p [(L d -L q )i sd isq +ψ f i sq ];

[0021] Among them, n p is the number of motor pole pairs;

[0022] 1.2. Establish the mathematical model of electrochemical energy storage system, the expression is:

[0023]

[0024]

[0025] C p =C t (T b -25);

[0026] Where E is the internal electromotive force of the battery; E0 is the initial internal electromotive force; K is the polarization voltage constant; A and B are the voltage variation coefficient and capacity variation coefficient respectively; Q n is the rated capacity of the battery; i(t) is the charge and discharge current; S SoC is the percentage of remaining battery power; C t is the polarization effect coefficient; T b is the battery temperature; N b-s With N b-p are the number of batteries connected in series and in parallel in the battery pack respectively;

[0027] 1.3. Establish a mathematical model of hydrogen energy storage unit;

[0028] The hydrogen energy storage unit consists of a proton exchange membrane electrolyzer, a hydrogen storage tank, and a proton exchange membrane fuel cell;

[0029] The voltage model of the PEM electrolyzer is composed of the open circuit reversible voltage U ocv , electromotive force loss U generated by water electrolysis act 、Ohm polarization electromotive force U ohm constitute:

[0030] U el =U ocv +U act +U ohm ;

[0031] The open circuit reversible voltage is expressed as:

[0032]

[0033] U rev =1.229-0.0009(T el -298);

[0034] The electromotive force loss generated by water electrolysis is expressed as:

[0035]

[0036] The Ohm polarization electromotive force is expressed as:

[0037] U ohm =R m i el ;

[0038]

[0039] Among them, U el is the output voltage of the PEM electrolyzer; R is the gas constant; T el is the electrolytic cell temperature; F is the Faraday constant; P H2 , P O2 and b are hydrogen pressure, oxygen pressure and water activity; α cat , α an is the charge transfer coefficient between the positive and negative electrodes; i el is the electrolytic cell current density; i cat 、i an is the exchange current density between the anode and cathode; R mem is the equivalent resistance; δ m is the thickness of the proton exchange membrane; σ m is the ionic conductivity;

[0040] The efficiency model of hydrogen production by PEM electrolyzer is:

[0041]

[0042] Among them, η el is the electrolytic cell efficiency; a is the stoichiometric coefficient; γ A is the anode activation area ratio; δ m0 ,δ an and δ cat are the initial proton exchange membrane thickness, anode plate thickness and cathode plate thickness respectively; k0, σ a and σ c are the initial conductivity of the electrolyte, the conductivity of the anode plate and the conductivity of the cathode plate respectively; C H is the hydrogen diffusion coefficient; S H is the hydrogen solubility coefficient; ΔP is the hydrogen pressure difference on both sides of the membrane;

[0043] Proton exchange membrane fuel cells are used to burn hydrogen to generate electricity. The voltage of the fuel cell is determined by the open circuit voltage Activation overvoltage Fuel cell ohmic overvoltage and fuel cell concentration difference overvoltage U conc constitute:

[0044]

[0045] The open circuit voltage of the fuel cell is expressed as:

[0046]

[0047] The activation overvoltage is expressed as:

[0048]

[0049] The ohmic overvoltage of a fuel cell is expressed as:

[0050]

[0051] Where N is the number of fuel cells; E n is the Nernst instantaneous voltage; A is the Tafel slope; i0 is the exchange current; τ is the dynamic response time; i fc is the fuel cell output current; T fc is the ambient temperature of the fuel cell; z is the number of transferred electrons; R int is the equivalent internal resistance; P H2 and P O2 are the gas pressures of hydrogen and oxygen respectively; ε is the electron transfer coefficient; k is the Boltzmann constant; h is the Planck constant, and ΔG is the change in Gibbs free energy;

[0052] The hydrogen storage tank is used to store the hydrogen produced by the electrolyzer, and its pressure mathematical model is expressed as:

[0053]

[0054] Among them, P st is the pressure in the hydrogen tank; T is the temperature of the hydrogen storage tank; V b is the volume of the hydrogen storage tank; Z is the compression coefficient; N H2 is the internal hydrogen storage capacity; N0 is the hydrogen storage capacity at the initial moment; η fc is the fuel cell conversion efficiency; I el is the electrolytic cell current; I fc is the fuel cell current;

[0055] The hydrogen storage state of the hydrogen storage tank is defined as:

[0056]

[0057] Among them, P N It is the upper limit of the pressure of the hydrogen storage tank.

[0058] Preferably, in step 2, based on the bus voltage fluctuation range, the system is divided into operating conditions as follows;

[0059] 2.1 Mode 1: u dc ≤u dc.min When the voltage fluctuates over a large range, the load power shortage is large; the battery pack is at the maximum power - P bat.max To discharge, the fuel cell switches to constant voltage control and stabilizes the DC bus voltage at u dc.min ; If the load lacks too much power, the load can be reduced if necessary to ensure voltage stability;

[0060] 2.2 Mode 2: u dc.min dc dc.L When the voltage is abnormal within a small range, the wind power supply of the system is insufficient. The fuel cell adopts droop control and the battery controlled by AVSG to stabilize the bus voltage and realize reasonable power distribution.

[0061] 2.3 Mode 3: u dc.L ≤u dc ≤u dc.H When the voltage is higher than the reference value, the system works in the charging state, and the electrolyzer and the battery absorb the excess power; when the voltage is lower than the reference value, the system works in the discharging state, and the electrolyzer and the fuel cell make up for the power shortage; in order to prevent the frequent start-up of the electrolyzer and the fuel cell and improve the efficiency, within a certain voltage range, the battery alone bears the task of energy support;

[0062] 2.4, Mode 4: u dc.H dc dc.max When the voltage is abnormal within a small range, the system wind power supply is in excess, and the electrolyzer adopts droop control and the battery controlled by AVSG to stabilize the bus voltage and realize reasonable power distribution.

[0063] 2.5. Mode 5: u dc ≥u dc.max This situation belongs to a large range of voltage fluctuations; the wind turbine energy supply is much greater than the load consumption, and the battery pack is at maximum power P bat.max To charge, the electrolyzer switches to constant voltage control and stabilizes the DC bus voltage at u dc.max , if there is too much excess power, switch the fan operation mode to constant pressure control;

[0064] Preferably, the step 3 is specifically as follows:

[0065] ​​​​The hydrogen energy storage unit adopts droop control mode. In order to prevent the system hydrogen energy storage system SOH from exceeding the safe operation range during the charging and discharging process, and at the same time make the system pursue a smaller DC bus voltage deviation, the droop control is improved, and the SOH and DC bus voltage deviation are taken into consideration. In the design of the droop control coefficient, the fuzzy algorithm is used to obtain the droop control coefficient; the changes in SOH and DC bus voltage will cause the adjustment of the droop coefficient of each hydrogen energy storage unit, thereby adaptively adjusting the output power of each unit;

[0066] The new droop control strategy formula for hydrogen energy storage units is as follows:

[0067]

[0068] in, is the reference output power of the electrolyzer, k el is the electrolytic cell droop control coefficient, is the reference output power of the fuel cell, k fc is the fuel cell droop control coefficient, P fcmin is the minimum output power of the fuel cell; P elmin is the minimum output power of the electrolyzer;

[0069] The droop coefficient calculation formula is as follows:

[0070]

[0071] Among them, P elmax and P fcmax is the maximum output power of the electrolyzer and fuel cell; k0 and k1 are the balance adjustment factors, which are determined by the fuzzy algorithm;

[0072] According to the charging and discharging state of the electric-hydrogen hybrid energy storage, the fuzzy algorithm rules are divided into PEM-FLC and PEMFC-FLC modules, and a dual-input-single-output form is adopted. The input variables are the hydrogen storage tank SOH and the DC bus voltage deviation Δu, and the output variables are the balancing adjustment factors k0 and k1; the input variable Δu is defined as 5 subsets {VS, S, M, B, VB}, representing {very small, small, medium, large, very large}, respectively, and the domain is [0, 0.1]; the input variable SOH is defined as 5 subsets {VS, S, M, B, VB}, representing {very small, small, medium, large, very large}, respectively, and the domain is [0, 1]; the output variable defines 5 subsets {Z, S, M, B, M}, representing the balancing factors {very small, small, medium, large, very large}, respectively, and the domain is [0, 1];

[0073] When the voltage fluctuation of the microgrid is small, the battery is responsible for maintaining the safety of the bus voltage; at this time, the hydrogen energy storage unit enters the standby state, and the balancing adjustment factors k0 and k1 are Z; when the microgrid has a lot of excess power, the PEM is started to produce hydrogen. When the SOH is low and the voltage deviation is large, the balancing adjustment factor k0 is increased to increase the hydrogen production power of the PEM and consume the unbalanced power in the microgrid; with the gradual increase of SOH and the decrease of voltage deviation, the balancing adjustment factor k0 should be reduced to avoid the rapid increase of the hydrogen storage tank pressure exceeding the upper limit; when the SOH is VB, the balancing adjustment factor k0 is 0, and the hydrogen storage tank pressure is close to When the power shortage of the microgrid is small, the battery will discharge the power alone, just like the charging situation. When the power shortage is large, PEMFC will be started to make up for the shortage. When SOH is high and the voltage deviation is large, the balance adjustment factor k1 should be increased to improve the output power of PEMFC. As the output power of the fuel cell increases, SOH gradually decreases and the voltage deviation gradually decreases. At this time, the balance adjustment factor k0 should be reduced, and the output power of PEMFC should be reduced to avoid the rapid decrease of the hydrogen storage tank pressure beyond the lower limit. When the SOH of the hydrogen storage tank reaches the lower limit, PEMFC stops working.

[0074] Preferably, the step 4 specifically includes:

[0075] The droop control strategy of the hydrogen energy storage unit will cause the DC bus voltage of the electric-hydrogen hybrid energy storage system to deviate from the set reference value, and when the source-load fluctuates, the bus voltage will also fluctuate; in order to slow down the rate of change of the bus voltage during fluctuations, the battery adopts a virtual synchronous generator control; specifically, it is analogous to the VSG control in the AC microgrid, and the droop and virtual inertia functions can be realized through the one-to-one correspondence between AC and DC variables; the AVSG controls the port characteristics of the converter to simulate the capacitor charging and discharging characteristics, providing inertial support for the system during power fluctuations and improving the voltage stability of the microgrid;

[0076] The expression of AVSG control is:

[0077]

[0078] Among them, u N is the bus voltage rating; is the voltage reference value; k droop is the droop coefficient; |du dc / dt| is the rate of change of bus voltage; k a , k b To flexibly adjust the relevant control parameters of the virtual inertia coefficient; C vir0 To deal with the value of the virtual inertia coefficient when the power fluctuates slightly; x is the threshold;

[0079] C virrepresents the simulated capacitance, which symbolizes the inertia strength of the system. When the system has power disturbance, the larger the simulated capacitance, the smaller the voltage change rate of the busbar will be, which is more conducive to voltage stability. When the system is running stably, |du dc / dt| is 0, at this time, only the droop link plays a role in the control, allocating power to the battery; to avoid the system from taking too long to recover from the steady state voltage when the power is frequently fluctuating at a low level; therefore, when the voltage fluctuation is low, a fixed value is used to achieve voltage fluctuation suppression and rapid recovery. When the voltage change rate is high, C vir The value is random. dc / dt| increases, and the voltage change rate decreases accordingly, avoiding the problem of voltage mutation caused by frequent load switching and fluctuations in new energy output;

[0080]

[0081] Where, Ssoc is the value of SOC; S N is the rated capacity of the battery; μ ref is the SOC relative power coefficient; Δu dc is the voltage fluctuation range of the DC microgrid;

[0082] In addition, in order to limit the overcharge and overdischarge of the battery, it is necessary to limit the power in the droop coefficient;

[0083]

[0084] Among them, P dis is the discharge power limit; P ch is the charging power limit.

[0085] Beneficial effects of the invention: The invention aims at the problem of bus voltage fluctuation in a DC microgrid system under the influence of wind power and fluctuating loads, and proposes a coordinated control method for a DC microgrid with electric-hydrogen hybrid energy storage based on bus voltage stratification; first, the strategy divides the system operation state into normal operating conditions and extreme operating conditions according to the bus voltage fluctuation range; then, under normal operating conditions, a droop control strategy based on a fuzzy logic algorithm is adopted, and the output power of the hydrogen energy storage system is optimized by combining the bus voltage fluctuation and the proposed strategy of the hydrogen storage state; at the same time, the battery adopts a control method similar to a virtual synchronous generator to provide necessary inertial support for the bus voltage; under extreme conditions, the voltage stability is ensured by switching the control mode of the DC microgrid system; finally, the effectiveness of the strategy is verified in the Matlab / Simulink environment, and the results show that the stability of the DC microgrid bus voltage under source-load fluctuation conditions can be improved, and the overcharge and over-discharge problems of the hydrogen energy storage system can be effectively improved; the DC microgrid bus voltage is kept stable under source-load fluctuation conditions through the coordinated control method for a DC microgrid with electric-hydrogen hybrid energy storage based on bus voltage stratification, and has good dynamic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0086] Figure 1 It is the system control frame of the electric-hydrogen hybrid energy storage DC power grid in the present invention;

[0087] Figure 2 It is the droop control strategy based on fuzzy algorithm in the present invention;

[0088] Figure 3 This is the AVSG control block diagram of the present invention;

[0089] Figure 4 is the fuzzy logic reasoning result of PEM-FLC and PEMFC-FLC in the present invention;

[0090] Figure 5 The result diagram of the simulation of the hybrid energy storage in the charging state is selected as the normal operation scenario for the present invention;

[0091] Figure 6 This is a result diagram of the simulation of a critical hydrogen storage state operation scenario selected for the present invention. DETAILED DESCRIPTION

[0092] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0093] Example 1: Figure 1-3 As shown, a control method for a photovoltaic storage coupled hydrogen production system based on INC mainly includes the following main steps:

[0094] Step 1: Build a mathematical model of the electric-hydrogen hybrid energy storage DC microgrid system, and establish wind power generation system models, electrochemical energy storage system models, and hydrogen energy storage system models;

[0095] Step 2: Based on the bus voltage fluctuation range, divide the system into operating conditions;

[0096] Step 3: The hydrogen energy storage system adopts droop control and constant voltage control based on fuzzy algorithm. In normal mode, droop control based on fuzzy algorithm is adopted to dynamically optimize the output power of hydrogen storage according to the hydrogen storage transition state and voltage deviation, so that it can accelerate back to the normal range according to the hydrogen storage state when the hydrogen storage transition state is in a critical state; in extreme mode, constant voltage control is adopted to maintain bus voltage stability;

[0097] Step 4: The electrochemical energy storage system adopts virtual synchronous machine control and constant voltage control. In normal mode, AVSG control is used to simulate the inertial support of the capacitor on the voltage, and the change rate of the bus voltage during fluctuations is slowed down by flexibly adjusting the inertia coefficient; in extreme mode, constant power control is used;

[0098] Step 5: Simulate the constructed electric-hydrogen hybrid energy storage DC microgrid system to verify the effectiveness of the proposed electric-hydrogen hybrid energy storage coordinated control strategy under fluctuating conditions.

[0099] Specifically, in step 1, a mathematical model of the electric-hydrogen hybrid energy storage DC microgrid system is constructed, and a wind power generation system model, an electrochemical energy storage system model and a hydrogen energy storage system model are established. The system structure diagram is shown in FIG. Figure 1 As shown,

[0100] (1) Establish a mathematical model of wind power generation system.

[0101] The wind power generation system consists of a direct-drive wind turbine system and a direct-drive wind turbine generator system.

[0102] The PMSG direct-drive fan system is mainly responsible for capturing wind energy and converting it into mechanical energy. The output mechanical power P W It can be expressed as:

[0103]

[0104] Where ρ is the air density; A is the area swept by the impeller; v w is the wind speed; C p is the wind energy utilization coefficient, and its size is determined by the tip speed ratio λ and the pitch angle β.

[0105] The stator voltage equation of the PMSG direct-drive wind turbine generator in the dq coordinate system is:

[0106]

[0107] Among them, u sd 、u sq Represents the d-axis and q-axis components of the generator stator winding voltage respectively; R s is the stator winding resistance; p is the differential operator; ω r is the rotor electrical angular velocity; L d , L q They are the d-axis and q-axis inductances of the stator winding respectively; i sd 、i sq The d-axis and q-axis components of the stator current respectively; ψ f is the permanent magnet flux.

[0108] Electromagnetic torque equation:

[0109] T e =n p [(L d -L q )i sd i sq +ψ f i sq ]

[0110] Among them, n p is the number of motor pole pairs.

[0111] (2) Establish a mathematical model of the electrochemical energy storage system, expressed as:

[0112]

[0113] C p =C t (T b -25)

[0114] Where E is the internal electromotive force of the battery; E0 is the initial internal electromotive force; K is the polarization voltage constant; A and B are the voltage variation coefficient and capacity variation coefficient respectively; Q n is the rated capacity of the battery; i(t) is the charge and discharge current; S SoC C is the remaining battery power percentage (state of charge); t is the polarization effect coefficient; T b is the battery temperature; N b-s With N b-p are the number of batteries connected in series and in parallel in the battery pack respectively.

[0115] (3) Establishing a mathematical model of hydrogen energy storage unit

[0116] The hydrogen energy storage unit consists of a proton exchange membrane electrolyzer, a hydrogen storage tank and a proton exchange membrane fuel cell.

[0117] The voltage model of the PEM electrolyzer is composed of the open circuit reversible voltage U ocv , electromotive force loss U generated by water electrolysis act 、Ohm polarization electromotive force U ohm constitute:

[0118] U el =U ocv +U act +U ohm

[0119] The open circuit reversible voltage can be expressed as:

[0120]

[0121] U rev =1.229-0.0009(T el -298)

[0122] The electromotive force loss generated by water electrolysis can be expressed as:

[0123]

[0124] Ohmic polarization electromotive force can be expressed as:

[0125] U ohm =R m i el

[0126]

[0127] Among them, U el is the output voltage of the PEM electrolyzer; R is the gas constant; T el is the electrolytic cell temperature; F is the Faraday constant; P H2 , P O2 and b are hydrogen pressure, oxygen pressure and water activity; α cat , α an is the charge transfer coefficient between the positive and negative electrodes; i el is the electrolytic cell current density; i cat 、i an is the exchange current density between the anode and cathode; R mem is the equivalent resistance; δ m is the thickness of the proton exchange membrane, which is 0.254 mm; σ m is the ionic conductivity.

[0128] The efficiency model of hydrogen production by PEM electrolyzer is:

[0129]

[0130] Among them, η el is the electrolytic cell efficiency; a is the stoichiometric coefficient; γ A is the anode activation area ratio; δ m0 ,δ an and δ cat are the initial proton exchange membrane thickness, anode plate thickness and cathode plate thickness respectively; k0, σ a and σ c are the initial conductivity of the electrolyte, the conductivity of the anode plate and the conductivity of the cathode plate respectively; C H is the hydrogen diffusion coefficient; S H is the hydrogen solubility coefficient; ΔP is the hydrogen pressure difference on both sides of the membrane.

[0131] Proton exchange membrane fuel cells are used to burn hydrogen to generate electricity. The voltage of the fuel cell is determined by the open circuit voltage Activation overvoltage Fuel cell ohmic overvoltage and fuel cell concentration difference overvoltage U conc constitute:

[0132]

[0133] The open circuit voltage of a fuel cell can be expressed as:

[0134]

[0135] The activation overvoltage can be expressed as:

[0136]

[0137]

[0138] The ohmic overvoltage of a fuel cell is expressed as:

[0139]

[0140] Where N is the number of fuel cells; E n is the Nernst instantaneous voltage; A is the Tafel slope; i0 is the exchange current; τ is the dynamic response time; i fc is the fuel cell output current; T fc is the ambient temperature of the fuel cell; z is the number of transferred electrons; R int is the equivalent internal resistance; P H2 and P O2 are the gas pressures of hydrogen and oxygen respectively; ε is the electron transfer coefficient; k is the Boltzmann constant; h is the Planck constant, and ΔG is the change in Gibbs free energy.

[0141] The hydrogen storage tank is used to store the hydrogen produced by the electrolyzer. Its pressure mathematical model can be expressed as:

[0142]

[0143] Among them, P st is the pressure in the hydrogen tank; T is the temperature of the hydrogen storage tank; V b is the volume of the hydrogen storage tank; Z is the compression coefficient; N H2 is the internal hydrogen storage capacity; N0 is the hydrogen storage capacity at the initial moment; η fc is the fuel cell conversion efficiency; I el is the electrolytic cell current; I fc is the fuel cell current.

[0144] The hydrogen storage state of the hydrogen storage tank is defined as:

[0145]

[0146] Among them, P N It is the upper limit of the pressure of the hydrogen storage tank.

[0147] Furthermore, in step 2, based on the bus voltage fluctuation range, the system is divided into operating conditions; specifically as follows:

[0148] Based on the bus voltage fluctuation range, the system is divided into operating conditions;

[0149] 1) Mode 1: u dc ≤u dc.min When the voltage fluctuates in a large range, the load power shortage is large. The battery pack is operated at the maximum power (-P bat.max ) to discharge, the fuel cell switches to constant voltage control, stabilizing the DC bus voltage at u dc.min If the load is short of too much power, the load can be reduced if necessary to ensure voltage stability.

[0150] 2) Mode 2: u dc.min dc dc.L At this time, the voltage is abnormal in a small range, and the wind power supply of the system is insufficient. The fuel cell adopts droop control and the battery controlled by AVSG to stabilize the bus voltage and realize reasonable power distribution.

[0151] 3) Mode 3: u dc.L ≤u dc ≤u dc.H When the voltage is higher than the reference value, the system works in the charging state, and the electrolyzer and the battery consume the excess power; when the voltage is lower than the reference value, the system works in the discharging state, and the electrolyzer and the fuel cell make up for the power shortage; in order to prevent the frequent start-up of the electrolyzer and the fuel cell and improve the efficiency, within a certain voltage range, the battery alone bears the task of energy support.

[0152] 4) Mode 4: u dc.H dc dc.max When the voltage is abnormal within a small range, the system wind power supply is in excess, and the electrolyzer adopts droop control and the battery controlled by AVSG to stabilize the bus voltage and realize reasonable power distribution.

[0153] 5) Mode 5: u dc ≥u dc.max This is a large-scale voltage fluctuation. The wind turbine supply is much greater than the load consumption, and the battery pack is running at maximum power (P bat.max ) to charge, the electrolyzer switches to constant voltage control, and the DC bus voltage is stabilized at u dc.max ,If the excess power is too large, the fan operation mode can be switched to constant pressure control if necessary.

[0154] Among the above five modes, modes 1 and 5 reach the boundary of the safe operation constraint of the DC bus voltage, which are extreme cases. In this mode, the electric-hydrogen hybrid energy storage will switch the control strategy to coordinately smooth the source-load power fluctuation and stabilize the DC bus voltage.

[0155] ​​​​Furthermore, in step 3, the hydrogen energy storage system adopts droop control and constant voltage control based on fuzzy algorithm. In normal mode, droop control based on fuzzy algorithm is adopted to dynamically optimize the output power of hydrogen storage according to the hydrogen storage transition state and voltage deviation, so that it can accelerate back to the normal range according to the hydrogen storage state when the hydrogen storage transition state is in a critical state; in extreme mode, constant voltage control is adopted to maintain bus voltage stability; the details are as follows:

[0156] Hydrogen energy storage units mainly use droop control. Traditional droop control can achieve power distribution of hydrogen energy storage units to a certain extent. However, under different hydrogen storage conditions, it is impossible to adaptively change the output power ratio of hydrogen energy storage in hybrid energy storage according to SOH. When SOH is too high or too low, it is impossible to adjust the output power of hydrogen energy storage to return it to the ideal state, resulting in hydrogen energy storage being in a critical state for a long time and frequent start and stop, which not only affects the service life, but also loses the supporting role of hydrogen energy storage for the microgrid system, making its stability worse.

[0157] In order to prevent the SOH of the hydrogen energy storage system from exceeding the safe operation range during the charging and discharging process, and at the same time make the system pursue a smaller DC bus voltage deviation, this paper improves the traditional droop control, takes SOH and DC bus voltage deviation into consideration in the design of the droop control coefficient, and uses fuzzy algorithm to obtain the droop control coefficient. The changes in SOH and DC bus voltage will cause the adjustment of the droop coefficient of each hydrogen energy storage unit, thereby adaptively adjusting the output power of each unit.

[0158] The new droop control strategy formula a of the hydrogen energy storage unit is as follows:

[0159]

[0160] in, is the reference output power of the electrolyzer, k el is the electrolytic cell droop control coefficient, is the reference output power of the fuel cell, k fc is the fuel cell droop control coefficient, P fcmin is the minimum output power of the fuel cell; P elmin is the minimum output power of the electrolyzer.

[0161] The droop coefficient calculation formula is as follows:

[0162]

[0163] Among them, P elmax and P fcmax is the maximum output power of the electrolyzer and fuel cell; k0 and k1 are balance adjustment factors, which are determined by the fuzzy algorithm.

[0164] The droop control strategy based on fuzzy algorithm is as follows Figure 2 As shown in the figure, according to the charging and discharging state of the electric-hydrogen hybrid energy storage, the fuzzy algorithm rules are divided into PEM-FLC and PEMFC-FLC modules, using a dual-input-single-output form, the input variables are the hydrogen storage tank SOH and the DC bus voltage deviation Δu, and the output variables are the balancing adjustment factors k0 and k1. The input variable Δu is defined as 5 subsets {VS, S, M, B, VB}, representing {very small, small, medium, large, very large}, respectively, and the domain is [0, 0.1]; the input variable SOH is defined as 5 subsets {VS, S, M, B, VB}, representing {very small, small, medium, large, very large}, respectively, and the domain is [0, 1]; the output variable defines 5 subsets {Z, S, M, B, M}, representing the balancing factors {very small, small, medium, large, very large}, respectively, and the domain is [0, 1].

[0165] The basic idea of ​​the fuzzy rule design of this module is: when the voltage fluctuation of the microgrid is small, the battery is mainly responsible for maintaining the safety of the bus voltage. At this time, the hydrogen energy storage unit enters the standby state, and the balancing adjustment factors k0 and k1 are Z. When the microgrid has a lot of excess power, the PEM is started to produce hydrogen. When the SOH is low and the voltage deviation is large, the balancing adjustment factor k0 is increased to increase the hydrogen production power of the PEM and consume the unbalanced power in the microgrid; with the gradual increase of SOH and the decrease of voltage deviation, the balancing adjustment factor k0 should be reduced at this time to avoid the rapid increase of the hydrogen storage tank pressure exceeding the upper limit; when the SOH is VB, the balancing adjustment factor k0 is 0, and the hydrogen storage tank pressure is close to the upper limit, and the PEM stops producing hydrogen. When the microgrid has a power shortage, the battery is discharged alone when the microgrid has a small power shortage, just like the charging situation; when the power shortage is large, the PEMFC is started to make up for the shortage. When SOH is high and the voltage deviation is large, the balance adjustment factor k1 should be increased to improve the output power of PEMFC; as the output power of the fuel cell increases, SOH gradually decreases and the voltage deviation gradually decreases. At this time, the balance adjustment factor k0 should be reduced, and the output power of PEMFC should be reduced to avoid the hydrogen storage tank pressure from decreasing rapidly and crossing the lower limit; when the hydrogen storage tank SOH reaches the lower limit, PEMFC stops working. The fuzzy logic reasoning results of PEM-FLC and PEMFC-FLC are shown in Figure 3 shown.

[0166] Furthermore, the step 4 is specifically as follows:

[0167] The droop control strategy of the hydrogen energy storage unit will cause the DC bus voltage of the electric-hydrogen hybrid energy storage system to deviate from the set reference value, and when the source-load fluctuates, the bus voltage will also fluctuate. In order to slow down the rate of change of the bus voltage during fluctuations, the battery adopts a virtual synchronous generator control. The principle is analogous to the VSG control in the AC microgrid. Through the one-to-one correspondence between AC and DC variables, the droop and virtual inertia functions can be realized. The AVSG controls the port characteristics of the converter to simulate the capacitor charging and discharging characteristics, providing inertial support for the system during power fluctuations and improving the voltage stability of the microgrid. The AVSG control block diagram is shown below. Figure 4 shown.

[0168] The expression of AVSG control is:

[0169]

[0170] Among them, u N is the bus voltage rating; is the voltage reference value; k droop is the droop coefficient; |du dc / dt| is the rate of change of bus voltage; k a , k b To flexibly adjust the relevant control parameters of the virtual inertia coefficient; C vir0 To deal with the value of the virtual inertia coefficient when small power fluctuations occur; x is the threshold.

[0171] From the above formula, we can see that C vir Represents the simulated capacitance, which symbolizes the inertia strength of the system. When the system experiences power disturbance, the larger the simulated capacitance, the smaller the voltage change rate of the busbar will be, which is more conducive to voltage stability. When the system is running stably, |du dc / dt| is 0, at this time, only the droop link plays a role in the control, allocating power to the battery. In order to avoid the system from taking too long to recover from the steady state voltage when the power is fluctuating frequently. Therefore, when the voltage fluctuation is low, a fixed value is used to achieve voltage fluctuation suppression and rapid recovery. When the voltage change rate is high, C vir The value is random. dc / dt| becomes larger and larger, at this time the voltage change rate decreases accordingly, avoiding the problem of voltage mutation caused by frequent load switching and fluctuations in new energy output.

[0172]

[0173] Where, Ssoc is the value of SOC; S N is the rated capacity of the battery; μ ref is the SOC relative power coefficient; Δu dc is the voltage fluctuation range of the DC microgrid.

[0174] In addition, in order to limit overcharge and over-discharge of the battery, it is necessary to limit the power in the droop factor.

[0175]

[0176] Among them, P dis is the discharge power limit; P ch Charging power limit

[0177] Example 2: Figure 5 As shown, the present invention selects the normal operation scenario and the hybrid energy storage is in the charging state for simulation. In the charging mode, it is assumed that the load remains unchanged, the electrolyzer temperature is always maintained at 25°C, the initial SOH is set to 50%, the SOH is set to 45%, and the wind speed changes every 5 seconds to simulate the sudden change of wind power. The simulation results are shown in Figure 5 shown.

[0178] As the power imbalance increases, the discharge power of the hybrid energy storage system is increased accordingly through the adjustment of the control strategy. The fuzzy algorithm can automatically adjust the droop coefficient of the hydrogen energy storage system according to the voltage deviation, thereby increasing the hydrogen production power of the electrolyzer. The battery controlled by AVSG provides the necessary inertial support for the DC bus voltage, allowing the battery to provide greater power to suppress voltage mutations. Through the mutual coordination of electrical energy and hydrogen energy, the power balance is maintained, the voltage fluctuation is effectively suppressed, and the safe and stable operation of the system is ensured. It meets the requirements of the present invention.

[0179] Example 3: Figure 6 As shown, the present invention selects the critical hydrogen storage state operation scenario for simulation, sets the SOH to 79%, the SOC to 45%, and the power changes of the fan and the load are as follows Figure 6 The simulation results are shown in Figure 6. The hydrogen energy storage control strategy based on fuzzy droop can automatically optimize the charging and discharging power of hydrogen energy storage according to the SOH state of hydrogen energy storage, so that it is maintained within the normal range, reducing the number of switching times of the energy storage device operation mode. When the hydrogen energy storage is in a deep charging state and the DC microgrid power is insufficient, it is allocated to discharge with a higher power. When the DC microgrid power is in excess, it is allocated to charge with a lower power. It meets the requirements of the present invention.

[0180] The above embodiments are only preferred technical solutions of the present invention and should not be regarded as limiting the present invention. The protection scope of the present invention shall be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A coordinated control method for an electric-hydrogen hybrid energy storage DC microgrid based on bus voltage stratification, characterized in that: The following steps are involved: Step 1: Build a mathematical model of the electric-hydrogen hybrid energy storage DC microgrid system, and establish wind power generation system models, electrochemical energy storage system models, and hydrogen energy storage system models; Step 2: Based on the bus voltage fluctuation range, divide the system into operating conditions; Step 3: The hydrogen energy storage system adopts droop control and constant voltage control based on fuzzy algorithm; in normal mode, droop control based on fuzzy algorithm is adopted to dynamically optimize the output power of hydrogen storage according to the hydrogen storage transition state and voltage deviation, so that it can accelerate back to the normal range according to the hydrogen storage state when the hydrogen storage transition state is in a critical state; in extreme mode, constant voltage control is adopted to maintain bus voltage stability; Step 4: The electrochemical energy storage system adopts quasi-virtual synchronous machine control and constant voltage control value; in normal mode, AVSG control is used to simulate the inertial support effect of capacitor on voltage, and the change rate of bus voltage during fluctuation is slowed down by flexibly adjusting the inertia coefficient; in extreme mode, constant power control is adopted; Step 5: Simulate the constructed electric-hydrogen hybrid energy storage DC microgrid system to verify the effectiveness of the proposed electric-hydrogen hybrid energy storage coordinated control strategy under fluctuating conditions.

2. The coordinated control method of a DC microgrid with electric-hydrogen hybrid energy storage based on bus voltage stratification according to claim 1 is characterized by: The step 1 is specifically as follows: 1.

1. Establish a mathematical model of wind power generation system; The wind power generation system consists of a direct-drive wind turbine system and a direct-drive wind turbine generator system; The PMSG direct-drive fan system is responsible for capturing wind energy and converting it into mechanical energy. The output mechanical power P W It is expressed as: Where ρ is the air density; A is the area swept by the impeller; v w is the wind speed; C P is the wind energy utilization coefficient, which is determined by the tip speed ratio λ and the pitch angle β; i is an intermediate variable; The stator voltage equation of the PMSG direct-drive wind turbine generator in the dq coordinate system is: Among them, u sd 、u sq Represents the d-axis and q-axis components of the generator stator winding voltage respectively; R s is the stator winding resistance; p is the differential operator; ω r is the rotor electrical angular velocity; L d , L q They are the d-axis and q-axis inductances of the stator winding respectively; i sd 、i sq The d-axis and q-axis components of the stator current respectively; ψ f is the permanent magnetic flux; Electromagnetic torque equation: T e =n p [(L d -L q )i sd i sq +ψ f i sq ]; Among them, n p is the number of motor pole pairs; 1.

2. Establish the mathematical model of electrochemical energy storage system, the expression is: C p =C t (T b -25); Where E is the internal electromotive force of the battery; E0 is the initial internal electromotive force; K is the polarization voltage constant; A and B are the voltage variation coefficient and capacity variation coefficient respectively; Q n is the rated capacity of the battery; i(t) is the charge and discharge current; S SoC is the percentage of remaining battery power; C t is the polarization effect coefficient; T b is the battery temperature; N b-s With N b-p are the number of batteries connected in series and in parallel in the battery pack respectively; 1.

3. Establish a mathematical model of hydrogen energy storage unit; The hydrogen energy storage unit consists of a proton exchange membrane electrolyzer, a hydrogen storage tank, and a proton exchange membrane fuel cell; The voltage model of the PEM electrolyzer is composed of the open circuit reversible voltage U ocv , electromotive force loss U generated by water electrolysis act 、Ohm polarization electromotive force U ohm constitute: IN el =U ocv +U act +U ohm ; The open circuit reversible voltage is expressed as: U rev =1.229-0.0009(T el -298); The electromotive force loss generated by water electrolysis is expressed as: The Ohm polarization electromotive force is expressed as: U ohm =R m i el ; Among them, U el is the output voltage of the PEM electrolyzer; R is the gas constant; T el is the electrolytic cell temperature; F is the Faraday constant; P H2 , P O2 and b are hydrogen pressure, oxygen pressure and water activity; α cat , α an is the charge transfer coefficient between the positive and negative electrodes; i el is the electrolytic cell current density; i cat 、i an is the exchange current density between the anode and cathode; R mem is the equivalent resistance; δ m is the thickness of the proton exchange membrane; σ m is the ionic conductivity; The efficiency model of hydrogen production by PEM electrolyzer is: Among them, η el is the electrolytic cell efficiency; a is the stoichiometric coefficient; γ A is the anode activation area ratio; δ m0 , δ an and δ cat are the initial proton exchange membrane thickness, anode plate thickness and cathode plate thickness respectively; k0, σ a and σ c are the initial conductivity of the electrolyte, the conductivity of the anode plate and the conductivity of the cathode plate respectively; C H is the hydrogen diffusion coefficient; S H is the hydrogen solubility coefficient; ΔP is the hydrogen pressure difference on both sides of the membrane; Proton exchange membrane fuel cells are used to burn hydrogen to generate electricity. The voltage of the fuel cell is determined by the open circuit voltage Activation overvoltage Fuel cell ohmic overvoltage and fuel cell concentration difference overvoltage U conc constitute: The open circuit voltage of the fuel cell is expressed as: The activation overvoltage is expressed as: The ohmic overvoltage of a fuel cell is expressed as: Where N is the number of fuel cells; E n is the Nernst instantaneous voltage; A is the Tafel slope; i0 is the exchange current; τ is the dynamic response time; i fc is the fuel cell output current; T fc is the ambient temperature of the fuel cell; z is the number of transferred electrons; R int is the equivalent internal resistance; P H2 and P O2 are the gas pressures of hydrogen and oxygen respectively; ε is the electron transfer coefficient; k is the Boltzmann constant; h is the Planck constant, and ΔG is the change in Gibbs free energy; The hydrogen storage tank is used to store the hydrogen produced by the electrolyzer, and its pressure mathematical model is expressed as: Among them, P st is the pressure in the hydrogen tank; T is the temperature of the hydrogen storage tank; V b is the volume of the hydrogen storage tank; Z is the compression coefficient; N H2 is the internal hydrogen storage capacity; N0 is the hydrogen storage capacity at the initial moment; η fc is the fuel cell conversion efficiency; I el is the electrolytic cell current; I fc is the fuel cell current; The hydrogen storage state of the hydrogen storage tank is defined as: Among them, P N It is the upper limit of the pressure of the hydrogen storage tank.

3. The coordinated control method of a DC microgrid with electric-hydrogen hybrid energy storage based on bus voltage stratification according to claim 1 is characterized by: In step 2, based on the bus voltage fluctuation range, the system is divided into operating conditions as follows; 2.1 Mode 1: u dc ≤u dc.min When the voltage fluctuates over a large range, the load power shortage is large; the battery pack is at the maximum power - P bat.max To discharge, the fuel cell switches to constant voltage control and stabilizes the DC bus voltage at u dc.min ; If the load lacks too much power, the load can be reduced if necessary to ensure voltage stability; 2.2 Mode 2: u dc.min dc dc.L When the voltage is abnormal within a small range, the wind power supply of the system is insufficient. The fuel cell adopts droop control and the battery controlled by AVSG to stabilize the bus voltage and realize reasonable power distribution.​​ 2.3 Mode 3: u dc.L ≤u dc ≤u dc.H When the voltage is higher than the reference value, the system works in the charging state, and the electrolyzer and the battery absorb the excess power; when the voltage is lower than the reference value, the system works in the discharging state, and the electrolyzer and the fuel cell make up for the power shortage; in order to prevent the frequent start-up of the electrolyzer and the fuel cell and improve the efficiency, within a certain voltage range, the battery alone bears the task of energy support; 2.4, Mode 4: u dc.H dc dc.max When the voltage is abnormal within a small range, the system wind power supply is in excess, and the electrolyzer adopts droop control and the battery controlled by AVSG to stabilize the bus voltage and realize reasonable power distribution.​​ 2.

5. Mode 5: u dc ≥u dc.max This situation belongs to a large range of voltage fluctuations; the wind turbine energy supply is much greater than the load consumption, and the battery pack is at maximum power P bat.max To charge, the electrolyzer switches to constant voltage control and stabilizes the DC bus voltage at u dc.max , if the excess power is too much, switch the fan operation mode to constant pressure control.

4. The coordinated control method of a DC microgrid with electric-hydrogen hybrid energy storage based on bus voltage stratification according to claim 1 is characterized by: The step 3 is as follows: The hydrogen energy storage unit adopts droop control mode. In order to prevent the system hydrogen energy storage system SOH from exceeding the safe operation range during the charging and discharging process, and at the same time make the system pursue a smaller DC bus voltage deviation, the droop control is improved, and the SOH and DC bus voltage deviation are taken into consideration. In the design of the droop control coefficient, the fuzzy algorithm is used to obtain the droop control coefficient; the changes in SOH and DC bus voltage will cause the adjustment of the droop coefficient of each hydrogen energy storage unit, thereby adaptively adjusting the output power of each unit; The new droop control strategy formula for hydrogen energy storage units is as follows: in, is the reference output power of the electrolyzer, k el is the electrolytic cell droop control coefficient, is the reference output power of the fuel cell, k fc is the fuel cell droop control coefficient, P fcmin is the minimum output power of the fuel cell; P elmin is the minimum output power of the electrolyzer; The droop coefficient calculation formula is as follows: Among them, P elmax and P fcmax is the maximum output power of the electrolyzer and fuel cell; k0 and k1 are the balance adjustment factors, which are determined by the fuzzy algorithm; According to the charging and discharging state of the electric-hydrogen hybrid energy storage, the fuzzy algorithm rules are divided into PEM-FLC and PEMFC-FLC modules, and a dual-input-single-output form is adopted. The input variables are the hydrogen storage tank SOH and the DC bus voltage deviation Δu, and the output variables are the balancing adjustment factors k0 and k1; the input variable Δu is defined as 5 subsets {VS, S, M, B, VB}, representing {very small, small, medium, large, very large}, respectively, and the domain is [0, 0.1]; the input variable SOH is defined as 5 subsets {VS, S, M, B, VB}, representing {very small, small, medium, large, very large}, respectively, and the domain is [0, 1]; the output variable defines 5 subsets {Z, S, M, B, M}, representing the balancing factors {very small, small, medium, large, very large}, respectively, and the domain is [0, 1]; When the voltage fluctuation of the microgrid is small, the battery is responsible for maintaining the safety of the bus voltage; at this time, the hydrogen energy storage unit enters the standby state, and the balancing adjustment factors k0 and k1 are Z; when the microgrid has a lot of excess power, the PEM is started to produce hydrogen. When the SOH is low and the voltage deviation is large, the balancing adjustment factor k0 is increased to increase the hydrogen production power of the PEM and consume the unbalanced power in the microgrid; with the gradual increase of SOH and the decrease of voltage deviation, the balancing adjustment factor k0 should be reduced to avoid the rapid increase of the hydrogen storage tank pressure exceeding the upper limit; when the SOH is VB, the balancing adjustment factor k0 is 0, and the hydrogen storage tank pressure is close to When the power shortage of the microgrid is small, the battery will discharge the power alone, just like the charging situation. When the power shortage is large, PEMFC will be started to make up for the shortage. When SOH is high and the voltage deviation is large, the balance adjustment factor k1 should be increased to improve the output power of PEMFC. As the output power of the fuel cell increases, SOH gradually decreases and the voltage deviation gradually decreases. At this time, the balance adjustment factor k0 should be reduced, and the output power of PEMFC should be reduced to avoid the rapid decrease of the hydrogen storage tank pressure beyond the lower limit. When the SOH of the hydrogen storage tank reaches the lower limit, PEMFC stops working.

5. The coordinated control method of a DC microgrid with electric-hydrogen hybrid energy storage based on bus voltage stratification according to claim 1 is characterized by: The step 4 specifically includes: The droop control strategy of the hydrogen energy storage unit will cause the DC bus voltage of the electric-hydrogen hybrid energy storage system to deviate from the set reference value, and when the source-load fluctuates, the bus voltage will also fluctuate; in order to slow down the rate of change of the bus voltage during fluctuations, the battery adopts a virtual synchronous generator control; specifically, it is analogous to the VSG control in the AC microgrid, and the droop and virtual inertia functions can be realized through the one-to-one correspondence between AC and DC variables; the AVSG controls the port characteristics of the converter to simulate the capacitor charging and discharging characteristics, providing inertial support for the system during power fluctuations and improving the voltage stability of the microgrid; The expression of AVSG control is: Among them, u N is the bus voltage rating; is the voltage reference value; k droop is the droop coefficient; |du dc / dt| is the rate of change of bus voltage; k a , k b To flexibly adjust the relevant control parameters of the virtual inertia coefficient; C vir0 To deal with the value of the virtual inertia coefficient when the power fluctuates slightly; x is the threshold; C vir represents the simulated capacitance, which symbolizes the inertia strength of the system. When the system has power disturbance, the larger the simulated capacitance, the smaller the voltage change rate of the busbar will be, which is more conducive to voltage stability. When the system is running stably, |du dc / dt| is 0, at this time, only the droop link plays a role in the control, allocating power to the battery; to avoid the system from taking too long to recover from the steady state voltage when the power is frequently fluctuating at a low level; therefore, when the voltage fluctuation is low, a fixed value is used to achieve voltage fluctuation suppression and rapid recovery. When the voltage change rate is high, C vir The value is random. dc / dt| increases, and the voltage change rate decreases accordingly, avoiding the problem of voltage mutation caused by frequent load switching and fluctuations in new energy output; Where, Ssoc is the value of SOC; S N is the rated capacity of the battery; μ ref is the SOC relative power coefficient; Δu dc is the voltage fluctuation range of the DC microgrid; In addition, in order to limit the overcharge and overdischarge of the battery, it is necessary to limit the power in the droop coefficient; Among them, P dis is the discharge power limit; P ch is the charging power limit.

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