A coordinated control method for a DC microgrid with wind power generation and hybrid energy storage

Abstract

A coordinated control method for a DC microgrid containing wind power generation and hybrid energy storage includes the following steps: building a wind power generation system model so that it is connected to a DC microgrid through a PWM rectifier; controlling the switching state of the grid-side input voltage of the PWM rectifier, and dividing the three bridge arm input voltage vectors of the PWM rectifier into multiple switching states; controlling the instantaneous active power and reactive power of the PWM rectifier through a direct power control strategy; using a low-pass filter to distribute the power borne by the supercapacitor and the battery, calculating the SOC and terminal voltage of the battery and the supercapacitor, and designing a partition limit management strategy for the energy storage device; by comparing the power generated by the wind turbine in the system with the power required by the load, determining the control mode of the bidirectional DC / DC converter. The method of the present invention can avoid complex parameter settings and a large number of calculations, and while taking into account the service life of the energy storage device, the control is flexible, thereby improving the economic efficiency of operation.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrid control, and in particular to a coordinated control method of a direct current microgrid including wind power generation and hybrid energy storage. Background Art

[0002] At present, the proportion of clean energy in modern power systems is increasing, and countries around the world are paying more attention to the development of new energy. However, new energy sources such as wind energy and solar energy have the characteristics of intermittency and randomness, which leads to defects such as discontinuity and instability in their output power during power generation. If they are directly connected to the power grid without adopting appropriate control strategies, it will have a great impact on the safe and reliable operation of the power grid.

[0003] The urgent demand for low-carbon energy has promoted the development of microgrids, which are a new type of grid that combines distributed power sources, energy storage devices, controllable loads and other units. It has flexible AC and DC power supply modes and can operate in both isolated and grid-connected states. Compared with AC microgrids, DC microgrids have the advantages of efficiently accepting distributed power generation units such as wind power and photovoltaics, loads and energy storage units, simple control methods, no reactive power flow in AC microgrids, and flexible networking, and have been further developed and widely concerned by experts at home and abroad.

[0004] Since there is no flow of reactive power in the DC microgrid and there is no need to consider complex controls such as frequency and phase in the AC system, the DC bus voltage becomes an important indicator for measuring the stable operation of the DC microgrid. Due to the intermittent nature of renewable energy generation and the unpredictability of load fluctuations, energy storage devices need to be integrated into the DC microgrid to maintain the power balance of the system under normal voltage levels. As an important component of the DC microgrid, the energy storage system plays a very important role in smoothing system power fluctuations and stabilizing the DC bus voltage: the energy storage device can provide energy support and power compensation for the power system by storing and releasing energy. In addition, the development of energy storage technology will effectively solve the supply and demand contradiction between distributed power generation units and load demand, and improve the power supply stability of the entire microgrid.

[0005] However, it is difficult for a single energy storage device to simultaneously meet the characteristics of high energy density, high power density and long service life. This requires combining two or more different energy storage elements into a hybrid energy storage system to give full play to the advantages of each energy storage device under a suitable control strategy, achieve complementary advantages, and suppress the power fluctuations of the microgrid.

[0006] Among various energy storage media, battery energy storage system (BESS) has the advantages of high energy density, high capacity and mature technology; while supercapacitors (SC) have the characteristics of high power density and fast response speed. They can respond quickly to power mutations and can perform charge and discharge frequently. They have good development prospects in stabilizing voltage and power balance and are suitable for microgrids with renewable energy generation and frequent load power fluctuations. It can be seen that batteries and supercapacitors complement each other. The hybrid energy storage system composed of batteries and supercapacitors is used in microgrids, which can give full play to the advantages of both and fully meet the growing power and energy needs of microgrids. Summary of the invention

[0007] The present invention proposes a coordinated control method for a DC microgrid containing wind power generation and hybrid energy storage, which uses a low-pass filter to distribute the power borne by supercapacitors and batteries to fully utilize the advantages of two different types of energy storage devices; in addition, the bidirectional flow of energy between the energy storage device and the DC bus side is controlled by a bidirectional DC / DC converter, which is conducive to maintaining the stable operation of the power system. Compared with traditional methods, the method of the present invention can avoid complex parameter settings and a large number of calculations, and the control is flexible while taking into account the service life of the energy storage device, thereby improving the economic efficiency of operation.

[0008] The technical solution adopted by the present invention is:

[0009] A coordinated control method for a DC microgrid including wind power generation and hybrid energy storage comprises the following steps:

[0010] Step 1: Build a wind power generation system model and connect it to the DC microgrid through a PWM rectifier;

[0011] Step 2: Based on the characteristics of the PWM rectifier, the switching state of the grid-side input voltage of the PWM rectifier is controlled, and the three bridge arm input voltage vectors of the PWM rectifier are divided into multiple switching states;

[0012] Step 3: Control the instantaneous active power and reactive power of the PWM rectifier through a direct power control strategy, thereby further controlling the instantaneous current, reducing the current harmonic distortion rate and the phase difference between current and voltage;

[0013] Step 4: Comprehensively consider the characteristics of power-type energy storage and energy-type energy storage, and use a low-pass filter to distribute the power borne by the supercapacitor and the battery. In addition, in order to take into account the service life of the energy storage device and prevent the energy storage device from being overcharged or over-discharged, the SOC and terminal voltage of the battery and supercapacitor are calculated to design a partition limit management strategy for the energy storage device;

[0014] Step 5: When the output power of the wind power generation system in the DC microgrid is less than the power required by the load, the bidirectional DC / DC converter operates in the Boost working mode, and the energy storage device discharges to make up for the power shortage in the wind power generation system;

[0015] When the output power of the wind power generation system in the DC microgrid is greater than the power required by the load, the bidirectional DC / DC converter operates in the Buck working mode, and the surplus power in the wind power generation system is provided to the energy storage device for charging.

[0016] In step 1, considering that the direct-drive permanent magnet synchronous generator (PMSG) has the advantages of high power density, small size, and light weight, the wind turbine generator used in the present invention is PMSG. Figure 1 As shown. Its main control task is to accurately control the output torque and power of PMSG, and the realization of this precise control goal is inseparable from the machine-side PWM converter. Since the structure of the machine-side PWM converter has three groups of bridge arms, a sine wave with equal frequency and amplitude and a phase difference of 120° is added to the three groups of bridge arms as a modulation signal. Each bridge arm has two switching states: on and off. That is, if a bridge arm is in the on state, it means that the upper bridge arm of the bridge arm is on and the lower bridge arm is off. If a bridge arm is in the off state, it means the opposite. The switching function S is used. j (j=a,b,c) to describe the on-off state of the bridge arm, that is:

[0017]

[0018] The stator voltage and electromagnetic torque equations of the wind turbine equivalent model are shown in equations (2) and (3) respectively:

[0019]

[0020]

[0021] Where: u d 、i d , L d Represent the direct-axis component of stator voltage, the direct-axis component of stator current and the direct-axis inductance of stator winding respectively;

[0022] u q 、i q and L q They represent the quadrature-axis component of the stator voltage, the quadrature-axis component of the stator current and the quadrature-axis inductance of the stator winding respectively;

[0023] R s and ψ fare the stator resistance and rotor permanent magnet flux respectively;

[0024] ω e is the generator rotor angular frequency; T e is the electromagnetic torque; p n is the pole pair number.

[0025] In step 2, the input voltage u of the PWM rectifier grid side r It can be controlled by the on and off state of the power switch device. Considering that the power supply voltage u is usually constant, the PWM rectifier input voltage u is controlled r The amplitude and phase of the AC side current i can be controlled by adjusting the amplitude and phase of the AC side current i, thereby minimizing the harmonic distortion rate of the current and reducing the phase difference between the current and voltage to improve the power factor.

[0026] When the PWM rectifier is in normal working state, one of the upper and lower bridge arms of each bridge arm is in the on state and the other is in the off state; for example, S a S b S c =001 means that the switch tubes S4, S6, and S5 are turned on, and the other three switch tubes are turned off. Then the three bridge arms have a total of 8 switch combination working states, among which 000 and 111 respectively represent that all the lower bridge arms of the three bridge arms are turned on and all the upper bridge arms are turned on. In these two switching states, the amplitude of the voltage vector on the AC side of the rectifier is zero, which is called the "zero vector".

[0027] The three bridge arm input voltage vectors u of the PWM rectifier r There are 8 switch states, namely: V0 (000), V1 (100), V2 (110), V3 (010), V4 (011), V5 (001), V6 (101), V7 (111), and the input voltage is as follows:

[0028]

[0029] Where U dc is the DC bus voltage, j represents the selected bridge arm, S u , S v , S w They respectively represent the switching states of the bridge arms of the three-phase rectifier circuit.

[0030] It can be seen that the input voltage of the PWM rectifier is 2 / 3U dc The voltage vector can be decomposed into the vector u in the two-phase stationary coordinate system through the three-phase / two-phase coordinate transformation. α and u β , so as to determine the sector to which the voltage vector belongs, and draw the sector according to the voltage vector diagram, such as Figure 2 shown.

[0031] In step 3, the control strategies for model prediction mainly include vector control, direct torque control and direct power control, etc. Considering the defects of single vector control such as large amount of calculation and complex control system, and direct torque control has the disadvantages of unstable switching frequency, large flux and current fluctuations, etc., the present invention adopts direct power control.

[0032] Direct power control strategy refers to the control of instantaneous current by controlling the instantaneous active and reactive power of PWM rectifier under the premise of stable AC voltage; reducing the current harmonic distortion rate and the phase difference between current and voltage, and has the advantages of simple control algorithm, faster system response speed and lower harmonic distortion rate. The present invention adopts MPDPC strategy based on space voltage vector, and the model predictive control block diagram is as follows Figure 3 The control structure of voltage outer loop and power inner loop can reduce the current harmonic distortion rate and improve the system operation performance.

[0033] The mathematical model of PWM rectifier is:

[0034]

[0035] Where u is the voltage vector on the generator side, i is the current vector on the generator side, and u r is the grid-side voltage vector of the rectifier; R and L are the resistance and reactance of the filter circuit respectively.

[0036] The model prediction algorithm is as follows:

[0037] Since u=|u|e jωt , so the rate of change of the grid voltage is expressed as:

[0038]

[0039] In the formula, ω represents the angular frequency of the system, e jωt It is the phasor form of the power supply voltage u expressed by Euler formula, and the meaning of j is the same as above.

[0040] According to the instantaneous power theory, the complex power S is calculated as shown in formula (7):

[0041] S=p+jq=1.5(i * u) (7)

[0042] Among them, i * represents the conjugate of the phase i, p and q represent the real and imaginary parts of the complex power S, namely the active power and reactive power, respectively.

[0043] Combining equations (5), (6) and (7), the change of the complex power S is decomposed and the change of the complex power S can be obtained as:

[0044]

[0045] Among them, u r * is the input voltage u on the rectifier side r conjugation of;

[0046] Decomposing the real and imaginary parts of the complex power S yields:

[0047]

[0048] In the formula, Re(u r * u),Im(u r * u) respectively represent u r * The real and imaginary parts of the phase product with the power supply voltage u.

[0049] Among them, formula (9) provides a basis for calculating the active and reactive power deviations, so that the predicted values ​​of active and reactive power in the next control cycle can be calculated, as shown in formulas (10) and (11):

[0050]

[0051]

[0052] Where, t sp Indicates the control period.

[0053] p k+1 With q k+1 They represent the active power and reactive power at time k+1 respectively; p k With q k They represent the active power and reactive power at time k respectively; u k , They represent the power supply voltage and the rectifier side input voltage at time k respectively.

[0054] represents the conjugate complex number of the input voltage on the rectifier side at time k; and They represent the real part and imaginary part of the product of the conjugate complex number of the input voltage on the rectifier side and the power supply voltage at time k respectively;

[0055] MPDPC directly controls the power and selects the most suitable voltage vector so that the calculated power is as close as possible to the given power value, thereby achieving the goal of minimizing the cost function g.

[0056]

[0057] Where P ref , Q ref Represent the given active and reactive power respectively.

[0058] By controlling the reactive power setpoint Q ref The control objective is to make the active and reactive power as close to their given power values ​​as possible, so as to minimize the complex power fluctuation.

[0059] In step 4, due to the volatility and intermittency of wind power generation, the power balance and bus voltage stability in the microgrid will be seriously affected. At the same time, considering the respective characteristics of power-type energy storage and energy-type energy storage, hybrid energy storage can be used to store or release electric energy to smooth out power fluctuations and stabilize the DC bus voltage in the microgrid. The energy storage system is mainly composed of a hybrid energy storage device and a bidirectional DC / DC converter. The bidirectional DC / DC converter can control the bidirectional flow of energy between the energy storage side and the DC bus side. Effectively solve the contradiction between the supply and demand of distributed energy generation and load demand, and further improve the stability of the microgrid.

[0060] The mathematical models of batteries and supercapacitors are as follows: Figure 4 and Figure 5 shown.

[0061] Battery terminal voltage U bat The expression of the state of charge (SOC) is:

[0062] U bat =ER b i bat (13)

[0063] Where E is the voltage source, R b is a constant internal resistance, U bat is the battery terminal voltage, i bat Indicates the battery current;

[0064]

[0065] In the formula, E rest 、E rated They respectively represent the rated capacity and remaining capacity of the battery.

[0066] Supercapacitors have the characteristics of high power density, long cycle life and easy maintenance, so they play an important role in power fluctuations. In actual engineering, supercapacitors are mainly used in Figure 5 The classic RC series model shown.

[0067] The circuit equation of the supercapacitor classic RC model equivalent circuit is:

[0068]

[0069] In the formula, C and R s They represent the ideal capacitance and equivalent series internal resistance in the classic supercapacitor model, respectively. sc is the working current of the supercapacitor; u sc is the terminal voltage of the supercapacitor.

[0070] The mathematical expression of SOC of supercapacitor is:

[0071]

[0072] In the formula, Q N is the total charge storage capacity of the supercapacitor; Q t is the amount of charge stored in the supercapacitor at time t;

[0073] u max 、u min are the maximum and minimum operating voltages of the supercapacitor respectively; u0 is the initial voltage value of the supercapacitor.

[0074] In step 4, the power distribution borne by the battery and the supercapacitor is achieved through a low-pass filter. According to the respective characteristics of the battery and the supercapacitor, the battery bears the low-frequency part of the power change, and the supercapacitor bears the high-frequency power fluctuation. The transfer function of the low-pass filter is:

[0075]

[0076] Where, T is the filter time constant; s is the differential operator;

[0077] Substituting s=jω into equation (17), we can obtain the transfer function and amplitude-frequency characteristic function of the low-pass filter as shown in equation (18) and equation (19), respectively:

[0078]

[0079]

[0080] The amplitude-frequency characteristics of the low-pass filter are as follows: Figure 6 As shown, the low-pass filter has a strong inhibitory effect on the passage of high-frequency signals, and low-frequency signals are more likely to pass through. c=1 / T is the cutoff frequency of the low-pass filter. When the filter time constant T is larger, the cutoff frequency is lower, that is, the lower the frequency of the signal allowed to pass through the filter, more signals can pass through the low-pass filter, and the obtained signal is smoother; according to the power relationship of each unit in the DC microgrid and the principle of the low-pass filter, the power borne by the hybrid energy storage device in the system operation state can be obtained as:

[0081] P hess =P bat +P sc =P load -P wt (20)

[0082] Among them, P hess Indicates the power borne by the hybrid energy storage device, P bat Indicates the power borne by the battery, P sc Indicates the power borne by the supercapacitor, P load Indicates the power required by the load, P wt Indicates the power generated by the fan;

[0083] The power P borne by the hybrid energy storage device hess The smoothed part after low-pass filtering is used as the reference power borne by the battery:

[0084]

[0085] In formula (21), the filtering time constant T can be determined according to the frequency band that the battery needs to smooth the power fluctuation. The hybrid energy storage device bears the power P hess The remaining fluctuation in is borne by the supercapacitor:

[0086]

[0087] The partition limit management strategy of energy storage device is:

[0088] When the battery SOC or the supercapacitor terminal voltage U sc Below its minimum limit SOC min or Usc_ min When the battery or supercapacitor is not allowed to discharge, only charging is allowed; when the battery SOC or the supercapacitor terminal voltage U sc Above its maximum limit SOC max or U sc_max When the energy storage device is in a state of being charged, it is forbidden to charge the energy storage device, and only the energy storage device is allowed to discharge. This is to prevent excessive charging and discharging from damaging the energy storage device and affecting its cycle life.

[0089] Considering that deep charging and discharging have a great impact on the cycle life of the energy storage device, the control of the supercapacitor and the battery must follow the aforementioned energy storage device zoning limit management strategy, and the power fluctuations in the system are smoothed by the charging and discharging of the energy storage device. When the terminal voltage of the supercapacitor reaches its charge limit or discharge limit state, the battery assumes all power tasks of the energy storage device. When the SOC of the battery reaches the charge limit or discharge limit state, the supercapacitor assumes all power tasks of the energy storage device.

[0090] In step 5, when the bidirectional DC / DC converter operates in the buck mode, the energy on the DC bus side flows to the energy storage side, and the opposite is true when the bidirectional DC / DC converter operates in the boost mode. The topological structure diagram and control block diagram are shown in FIG. Figure 7 and Figure 8 shown.

[0091] In order to realize the bidirectional flow of power between the energy storage side and the DC bus side, the actual working current i of the energy storage is firstly ref The error is sent to the PI controller, and then the output of the PI controller is limited to protect the switch tube. When there is a power surplus on the DC bus side, that is, the wind power generation system output power P wt Greater than the load demand power P load In this case, the power fluctuation in the system is smoothed by storing electric energy in energy storage. The bidirectional DC / DC converter works in Buck mode. At this time, the T1 tube is turned on and the T2 tube is in the off state. By controlling the duty cycle of the T1 tube, the energy on the DC bus side flows to the energy storage side, thereby achieving the purpose of charging the energy storage by the system.

[0092] The terminal voltage of the energy storage device in Buck mode is:

[0093]

[0094] Where U0 and U dc are the terminal voltage of energy storage and the voltage on the DC bus side respectively; t on and t off They are the time when tube T1 is in the on and off states respectively.

[0095] When there is a power shortage in the microgrid, that is, the wind power system output power P wt Less than the load power requirement P load At this time, it is necessary to provide power compensation for the system by releasing electric energy through energy storage, and the bidirectional DC / DC converter works in Boost mode; at this time, the T2 tube is turned on and the T1 tube is in the off state. By controlling the duty cycle of the T2 tube, the energy on the energy storage side flows to the DC bus side, achieving the purpose of providing power compensation for the system by energy storage discharge.

[0096] The DC bus voltage in Boost mode is:

[0097]

[0098] The present invention provides a coordinated control method for a DC microgrid including wind power generation and hybrid energy storage, and the technical effects are as follows:

[0099] 1) In step 1 of the control method of the present invention, the advantages of the direct-drive permanent magnet synchronous generator are: high power density, small size, light weight, etc. Wind power generation has attracted much attention in the development of distributed energy due to its advantages such as mature technology and large-scale development.

[0100] 2) In step 2 of the control method of the present invention, vector control and model prediction direct power control are used to effectively reduce the power error in the system, control the grid-side current to be a sine wave and in phase with the voltage, enable the system to operate in a unit power factor state, effectively reduce the harmonic pollution caused by irregular opening and closing of power electronic devices and improve the power quality of the DC microgrid.

[0101] 3) In step 3 of the control method of the present invention, direct power control refers to controlling the instantaneous active and reactive power of the PWM rectifier to achieve the purpose of controlling the instantaneous current under the premise of stable AC voltage, thereby reducing the current harmonic distortion rate and the phase difference between current and voltage. It has the advantages of simple control algorithm, faster system response speed and lower harmonic distortion rate.

[0102] 4) In step 4 of the control method of the present invention, the respective characteristics of power-type energy storage and energy-type energy storage are comprehensively considered, and the power borne by the supercapacitor and the battery is distributed by a low-pass filter. In addition, in order to take into account the service life of the energy storage device and prevent the energy storage device from being in an overcharge or over-discharge state, the power and terminal voltage borne by the supercapacitor and the battery are calculated, and a partition limit management strategy for the energy storage device is designed;

[0103] 5) In step 5 of the control method of the present invention, the Boost and Buck working modes of the bidirectional DC / DC converter are used to realize the bidirectional flow of energy between the energy storage side and the DC bus side. When there is a power surplus on the DC bus side, the bidirectional DC / DC converter operates in the Buck mode to charge the energy storage device. When there is a power shortage on the DC bus side, the bidirectional DC / DC converter operates in the Boost mode, and the energy storage device discharges to supplement the power shortage in the system.

[0104] 6) The present invention combines the advantages of both supercapacitors and batteries, introduces source-load power difference information, and passes the low-frequency part of power change to the battery through a low-pass filter, which can reduce its cycle times and optimize its charging and discharging process. Since supercapacitors have the advantages of high power density and fast response speed, the high-frequency part of power change can be passed to supercapacitors. In addition, the working mode of the energy storage device is determined by a bidirectional DC / DC converter to prevent the energy storage device from being in a frequent charging and discharging state, thereby extending the service life of the energy storage equipment and improving the economic efficiency of system operation.

[0105] 7) The method of the present invention can avoid complex parameter settings and a large amount of calculations, and while providing flexible control, it also takes into account the service life of the energy storage device, thereby improving the economic efficiency of operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0106] Figure 1 This is a structural diagram of a DC microgrid containing wind power and energy storage units.

[0107] Figure 2 This is the voltage vector sector division diagram.

[0108] Figure 3 Block diagram for the model predictive control.

[0109] Figure 4 Schematic diagram of the mathematical model of the battery.

[0110] Figure 5 Schematic diagram of the mathematical model of supercapacitor.

[0111] Figure 6 This is the amplitude-frequency characteristic diagram of the low-pass filter.

[0112] Figure 7 Bidirectional DC / DC converter topology diagram

[0113] Figure 8 This is the control block diagram of the bidirectional DC / DC converter. DETAILED DESCRIPTION

[0114] In view of the problems of large power pulsation, violent fluctuation of DC bus voltage, large input current harmonics and low power factor in DC microgrids with wind power generation, the present invention proposes a coordinated control method for DC microgrids with wind power access. Taking the direct-drive permanent magnet synchronous generator as the research object, combined with model prediction and direct power control, a dual-vector model prediction direct power control strategy suitable for wind power generation system is proposed. Based on the vector control strategy and the introduction of zero vector, the power pulsation and current harmonics in the wind power generation system are effectively reduced to ensure that the wind generator operates in a high power factor state. In addition, by additionally configuring an energy storage device, the DC bus voltage instability and power fluctuation problems caused by the uneven distribution of wind power generation time can be smoothed. The simulation results show that the proposed control strategy is conducive to achieving unity power factor, sinusoidal grid-side current, effectively suppressing power fluctuations in the system and improving the operating stability of the DC microgrid.

[0115] In view of the situation that there is power fluctuation in a DC microgrid containing wind power generation, considering that different energy storage devices have their own advantages and disadvantages in suppressing power fluctuations. The present invention proposes a coordinated control method for a DC microgrid containing hybrid energy storage and wind power generation, which adopts a hybrid energy storage system composed of batteries and supercapacitors to smooth out power fluctuations in the DC microgrid, uses a low-pass filter to distribute the power borne by the battery and the supercapacitor, and then controls the working mode of the bidirectional DC / DC converter by comparing the power generated by the wind turbine with the power required by the load, thereby achieving the effect of maintaining power balance in the system.

[0116] A coordinated control method for a DC microgrid including wind power generation and hybrid energy storage specifically comprises the following steps:

[0117] Step 1: Build a DC microgrid system including a wind power generation system and a hybrid energy storage system as a test system for the stability of the power system. Use MATLAB / Simulink simulation software to build a DC microgrid system containing a hybrid energy storage system. The rated value of the DC bus voltage studied is set to 600V, the maximum output power of the wind power generation system is 10kW, and the load power is 5kW. In order to speed up the change of the supercapacitor terminal voltage and the battery SOC, the following settings are made: the upper and lower limits of the supercapacitor voltage operation are 170V and 130V respectively, and the rated capacity is 10F; the battery capacity is 30A·h, the terminal voltage is 150V, and the SOC works normally between 20% and 90%. Other parameters can be shown in Table 1:

[0118] Table 1 DC microgrid related parameters

[0119]

[0120] In this DC microgrid system, the output power of the wind generator can be accurately controlled by controlling the three-phase PWM rectifier, and the input voltage of the three bridge arms can be accurately adjusted by controlling the on-off state of the three groups of bridge arms of the PWM rectifier, thereby accurately controlling the output power of the wind generator.

[0121] When the rectifier is in normal working state, one of the upper and lower bridge arms of each bridge arm must be in the on state and the other in the off state. For example, S a S b S c =001 means that the switch tubes S4, S6, and S5 are turned on, and the other three switch tubes are turned off. The three bridge arms have a total of 8 switch combination working states, of which 000 and 111 respectively represent that all the lower bridge arms of the three bridge arms are turned on and all the upper bridge arms are turned on. In these two switching states, the amplitude of the voltage vector on the AC side of the rectifier is zero, which is called the "zero vector". Then the three bridge arm input voltage vectors u r There are 8 switch states, namely V0 (000), V1 (100), V2 (110), V3 (010), V4 (011), V5 (001), V6 (101), V7 (111), and the input voltage is as follows:

[0122]

[0123] Step 2: In order to stabilize the DC bus voltage, define ΔP as the difference between the output power of the wind power generation system and the power required by the load, that is, ΔP = P wt -P load , P wt With P load Respectively represent the output power of the wind power generation system and the power required by the load. The present invention sets a source-load power difference signal ΔP lay As the switching point between the battery and supercapacitor working modes, when |ΔP|≥|ΔP lay |, the supercapacitor bears the power fluctuation in the system. When |ΔP|<|ΔP lay |, the battery bears the power fluctuation in the system. In the energy storage system, each unit generally distributes the load power in proportion to its capacity, ΔP lay Can be set to

[0124] △P lay =γ△P max (25)

[0125] In formula (25), γ is the ratio of the supercapacitor action area to the maximum source-load power difference of the system, ΔP max is the maximum source-load power difference that may occur in the system, and β is the ratio of the supercapacitor capacity to the entire energy storage device capacity in the system. ΔP layThe value of cannot be too small, otherwise the advantages of the two energy storage devices cannot be effectively utilized. In summary, it is appropriate for γ to be (0.5~1)β.

[0126] In addition, considering that the capacity of supercapacitors is generally low, it is necessary to set its terminal voltage U sc In the normal upper and lower limits U sc_max , U sc_min Considering the fact that the battery cannot be charged and discharged frequently, its SOC must be set within the normal upper and lower limits to increase its service life. max , SOC min The terminal voltage of the supercapacitor is 160V and the SOC of the battery is 80%, both of which meet the requirements.

[0127] Step 3: Allocate the power required by the supercapacitor and the battery through a low-pass filter, as shown in equations (8) and (9), respectively.

[0128]

[0129] In the formula, the filtering time constant T can be determined according to the frequency band that the battery needs to smooth the power fluctuation. hess The remaining fluctuation in is borne by the supercapacitor:

[0130]

[0131] Step 4: The present invention adopts the MPDPC strategy based on the space voltage vector, and adopts the control structure of the voltage outer loop and the power inner loop. Through direct power control, the instantaneous active and reactive power of the PWM rectifier can be effectively controlled to achieve the purpose of controlling the instantaneous current, reducing the phase difference between the current and the voltage, and controlling the current harmonic distortion rate to below 5% specified by the national standard. It has the advantages of simple control algorithm, faster system response speed and lower harmonic distortion rate.

[0132]

[0133]

[0134] In the formula, t sp Indicates the control period.

[0135] MPDPC selects the most suitable voltage vector by directly controlling the power so that the calculated power is as close as possible to the given power value, thereby achieving the goal of minimizing the cost function g.

[0136]

[0137] By controlling the reactive power setpoint Q refThe control objective is to make the active and reactive powers as close to their given power values ​​as possible, so as to minimize the complex power fluctuation.

[0138] Step 5: The bidirectional DC / DC converter can realize the bidirectional flow of energy between the energy storage side and the DC bus side. When the power generated by the wind power generation is greater than the power required by the load, the surplus power in the system is provided to the energy storage for charging; when the power generated by the wind power generation is less than the power required by the load, the energy storage device is used to discharge to compensate for the power shortage in the system. The power components borne by the supercapacitor and the battery are distributed through the low-pass filter.

[0139] Figure 1 It is a DC microgrid structure containing wind power and energy storage units. The DC microgrid as a whole includes a hybrid energy storage system, a bidirectional DC / DC converter control system, a wind power generation system, and a PWM rectifier. Among them, the wind turbine is connected to the DC bus through a three-phase PWM rectifier, which can accurately control the output power of the wind turbine and the input voltage of the rectifier grid side. The hybrid energy storage system realizes the bidirectional flow of energy between the battery and supercapacitor and the DC bus side through a bidirectional DC / DC converter.

[0140] Figure 2 The rectifier input voltage vector division diagram is shown in Figure 2. Through the three-phase / two-phase coordinate transformation, the voltage vector can be decomposed into the vector u in the two-phase stationary coordinate system. α and u β , thereby determining the sector to which the voltage vector belongs, and thus accurately controlling the input voltage of the rectifier.

[0141] Figure 3 It is a model predictive control block diagram, which adopts a control structure of voltage outer loop and power inner loop. It controls the instantaneous active and reactive power of the PWM rectifier to achieve the purpose of controlling the instantaneous current, reduce the current harmonic distortion rate and the phase difference between current and voltage, and has the advantages of simple control algorithm, faster system response speed and lower harmonic distortion rate.

[0142] Figure 6 The figure is the amplitude-frequency characteristic diagram of the low-pass filter. The low-pass filter can be used to divide the power component borne by the hybrid energy storage device into high-frequency and low-frequency parts, which are respectively borne by the supercapacitor and the battery.

[0143] Figure 7 and Figure 8 They are the topological structure diagram and control block diagram of the bidirectional DC / DC converter respectively. The two working modes of Boost and Buck of the bidirectional DC / DC converter can control the bidirectional flow of energy between the energy storage side and the DC bus side, thereby smoothing the power fluctuations in the system and improving the stability of the DC microgrid operation.

Claims

1. A coordinated control method for a DC microgrid containing wind power generation and hybrid energy storage, characterized in that The following steps are involved: Step 1: Build a wind power generation system model and connect it to the DC microgrid through a PWM rectifier; Step 2: Based on the characteristics of the PWM rectifier, the switching state of the grid-side input voltage of the PWM rectifier is controlled, and the three bridge arm input voltage vectors of the PWM rectifier are divided into multiple switching states; Step 3: Control the instantaneous active power and reactive power of the PWM rectifier through a direct power control strategy; Step 4: Use a low-pass filter to distribute the power borne by the supercapacitor and the battery, calculate the SOC and terminal voltage of the battery and the supercapacitor respectively, and design a partition limit management strategy for the energy storage device; Step 5: When the output power of the wind power generation system in the DC microgrid is less than the power required by the load, the bidirectional DC / DC converter operates in the Boost working mode, and the energy storage device discharges; When the output power of the wind power generation system in the DC microgrid is greater than the power required by the load, the bidirectional DC / DC converter operates in the Buck working mode, and the surplus power in the wind power generation system is provided to the energy storage device for charging; In step 1, the PWM rectifier has three groups of bridge arms, and a sine wave with equal frequency and amplitude and a phase difference of 120° is added to the three groups of bridge arms as a modulation signal. Each bridge arm has two switching states: on and off. That is, when a bridge arm is in the on state, it means that the upper bridge arm of the bridge arm is on and the lower bridge arm is off. When a bridge arm is in the off state, it means the opposite. The switching function S is used. j (j=a,b,c) to describe the on-off state of the bridge arm, that is: The stator voltage and electromagnetic torque equations of the wind turbine equivalent model are shown in equations (2) and (3) respectively: Where: u d 、i d , L d Respectively represent the direct-axis component of stator voltage, the direct-axis component of stator current and the direct-axis inductance of stator winding; u q 、i q and L q Respectively represent the quadrature-axis component of the stator voltage, the quadrature-axis component of the stator current and the quadrature-axis inductance of the stator winding; R s and ψ f are the stator resistance and rotor permanent magnet flux respectively; ω e is the generator rotor angular frequency; T e is the electromagnetic torque; p n is the pole pair number; In step 3, the direct power control strategy refers to controlling the instantaneous active and reactive power of the PWM rectifier under the premise of stable AC voltage to achieve the purpose of controlling the instantaneous current; The mathematical model of PWM rectifier is: Where u is the voltage vector on the generator side, i is the current vector on the generator side, and u r is the grid-side voltage vector of the rectifier; R and L are the resistance and reactance of the filter circuit respectively; The model prediction algorithm is as follows: Since u=|u|e jωt , so the rate of change of the grid voltage is expressed as: In the formula, ω represents the angular frequency of the system, e jωt It is the phasor form of the power supply voltage u expressed by Euler formula, and the meaning of j is the same as above. The complex power S is calculated as shown in formula (7): S=p+jq=1.5(i * u) (7) Among them, i * represents the conjugate of the phasor i, p and q represent the real and imaginary parts of the complex power S, i.e., the active power and reactive power, respectively; Combining equations (5), (6) and (7), the change of the complex power S is decomposed to obtain the change of the complex power S: Among them, u r * is the input voltage u on the rectifier side r conjugation of; Decomposing the real and imaginary parts of the complex power S yields: In the formula, Re(u r * u),Im(u r * u) represents the real part and imaginary part of the phase product of ur* and the power supply voltage u respectively; Among them, formula (9) provides a basis for calculating the active and reactive power deviations, thereby calculating the predicted values ​​of active and reactive power in the next control cycle, as shown in formulas (10) and (11): Where, t sp Indicates the control cycle; p k+1 With q k+1 Respectively represent the active power and reactive power at time k+1; p k With q k They represent the active power and reactive power at time k respectively; u k , They represent the power supply voltage and the input voltage on the rectifier side at time k respectively; represents the conjugate complex number of the input voltage on the rectifier side at time k; and They represent the real part and imaginary part of the product of the conjugate complex number of the input voltage on the rectifier side and the power supply voltage at time k respectively; By directly controlling the power, the most suitable voltage vector is selected so that the calculated power is as close as possible to the given power value, thereby achieving the goal of minimizing the cost function g. Where P ref , Q ref Represent the given active and reactive power respectively; In step 4, the power distribution borne by the battery and the supercapacitor is achieved through a low-pass filter. According to the respective characteristics of the battery and the supercapacitor, the battery bears the low-frequency part of the power change, and the supercapacitor bears the high-frequency power fluctuation. The transfer function of the low-pass filter is: Where, T is the filter time constant; s is the differential operator; Substituting s=jω into equation (17), we can obtain the transfer function and amplitude-frequency characteristic function of the low-pass filter as shown in equation (18) and equation (19), respectively: The power borne by the hybrid energy storage device when the system is in operation is: P hess =P bat +P sc =P load -P wt (20) Among them, P hess Indicates the power borne by the hybrid energy storage device, P bat Indicates the power borne by the battery, P sc Indicates the power borne by the supercapacitor, P load Indicates the power required by the load, P wt Indicates the power generated by the fan; The power P borne by the hybrid energy storage device hess The smoothed part after low-pass filtering is used as the reference power borne by the battery: In formula (21), the filtering time constant T can be determined according to the frequency band that the battery needs to smooth the power fluctuation. The hybrid energy storage device bears the power P hess The remaining fluctuation in is borne by the supercapacitor: In step 4, the partition limit management strategy of the energy storage device is: When the battery SOC or the supercapacitor terminal voltage U sc Below its minimum limit SOC min or Usc_ min When the battery or supercapacitor is not allowed to discharge, only the battery or supercapacitor is allowed to charge; when the battery SOC or the supercapacitor terminal voltage U sc Above its maximum limit SOC max or U sc_max When the energy storage device is in a state of being charged, the energy storage device is prohibited from charging and only discharging is allowed.

2. A coordinated control method for a DC microgrid including wind power generation and hybrid energy storage according to claim 1, characterized in that: In step 2, when the PWM rectifier is in a normal working state, one of the upper and lower bridge arms of each bridge arm is in an on state and the other is in an off state; the three bridge arms have a total of 8 switch combination working states, wherein 000 and 111 respectively represent that all lower bridge arms of the three bridge arms are on and all upper bridge arms are on, and the amplitude of the voltage vector on the AC side of the rectifier in these two switching states is zero; The three bridge arm input voltage vectors u of the PWM rectifier r There are 8 switch states, namely: V0 (000), V1 (100), V2 (110), V3 (010), V4 (011), V5 (001), V6 (101), V7 (111), and the input voltage is as follows: Where U dc is the DC bus voltage, j represents the selected bridge arm, S u , S v , S w They respectively represent the switching states of the bridge arms of the three-phase rectifier circuit.

3. A coordinated control method for a DC microgrid including wind power generation and hybrid energy storage according to claim 2, characterized in that: In step 2, the input voltage of the PWM rectifier is 2 / 3U dc The voltage vector can be decomposed into the vector u in the two-phase stationary coordinate system through the three-phase / two-phase coordinate transformation. α and u β , so as to determine the sector to which the voltage vector belongs, and draw the sector according to the voltage vector diagram.

4. A coordinated control method for a DC microgrid including wind power generation and hybrid energy storage according to claim 2, characterized in that: In step 4, the battery terminal voltage U bat The expression of state of charge SOC is: U bat =E-R b i bat (13) Where E is the voltage source, R b is a constant internal resistance, U bat is the battery terminal voltage, i bat Indicates the battery current; In the formula, E rest 、E rated Respectively represent the rated capacity and remaining capacity of the battery; The circuit equation of the supercapacitor classic RC model equivalent circuit is: In the formula, C and R s They represent the ideal capacitance and equivalent series internal resistance in the classic supercapacitor model, respectively. sc is the working current of the supercapacitor; u sc is the terminal voltage of the supercapacitor; The mathematical expression of SOC of supercapacitor is: In the formula, Q N is the total charge storage capacity of the supercapacitor; Q t is the charge stored in the supercapacitor at time t; u max 、u min are the maximum and minimum operating voltages of the supercapacitor respectively; u0 is the initial voltage value of the supercapacitor.

5. A coordinated control method for a DC microgrid including wind power generation and hybrid energy storage according to claim 1, characterized in that: In step 5, in order to realize the bidirectional flow of power between the energy storage side and the DC bus side, the actual working current i of the energy storage is firstly ref The error is sent to the PI controller, and then the output of the PI controller is limited to protect the switch tube. When there is a power surplus on the DC bus side, that is, the wind power generation system output power P wt Greater than the load demand power P load When the power fluctuation in the system is smoothed by storing electric energy in energy storage, the bidirectional DC / DC converter works in Buck mode. At this time, the T1 tube is turned on and the T2 tube is turned off. By controlling the duty cycle of the T1 tube, the energy on the DC bus side flows to the energy storage side, so that the system can charge the energy storage. The terminal voltage of the energy storage device in Buck mode is: Where U0 and U dc are the terminal voltage of energy storage and the voltage on the DC bus side respectively; t on and t off are the time when tube T1 is in the on and off state respectively; When there is a power shortage in the microgrid, that is, the wind power system output power P wt Less than the load power requirement P load At this time, it is necessary to provide power compensation for the system by releasing electric energy through energy storage, and the bidirectional DC / DC converter works in Boost mode; at this time, the T2 tube is turned on, and the T1 tube is in the off state. By controlling the duty cycle of the T2 tube, the energy on the energy storage side flows to the DC bus side, achieving the purpose of providing power compensation for the system by energy storage discharge; The DC bus voltage in Boost mode is:

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