CN106786729A - A kind of microgrid unsteady flow energy storage device and its energy management method - Google Patents

Abstract

The invention relates to the field of new energy power generation and its energy management, in particular to a micro-grid variable current energy storage device and its energy management method, including: a six-switch seven-level single-phase inverter unit, and a bidirectional DC/AC conversion for a storage battery Inverter unit, bidirectional DC/AC converter unit for supercapacitor, microgrid converter energy storage device control and energy management unit, photovoltaic MPPT unit, battery charge and discharge management unit, supercapacitor charge and discharge management unit and load management unit. The invention can integrate photovoltaically converted electric energy into the power grid, and use the energy storage device to balance system power fluctuations and peak-shaving and valley-filling according to the energy flow of the system. The six-switch seven-level single-phase inverter is used, which requires less power switching devices, low switching loss, and high output power quality. The energy storage device adopts a bidirectional DC/AC converter. Compared with the traditional converter, it uses fewer switching devices, has a wide voltage output range, and has the advantages of simple SPWM modulation. It has broad application prospects in the field of microgrid applications.

Description

A micro-grid variable current energy storage device and its energy management method

technical field

The invention relates to the field of new energy power generation and its energy management, in particular to a micro-grid variable current energy storage device and its energy management method.

Background technique

With the strong support of national policies in recent years, more and more distributed power generation such as household photovoltaic power generation, community photovoltaic power generation, and enterprise office building photovoltaic power generation has been incorporated into the public grid. Although the use of these new energy sources is conducive to low-carbon emission reduction and increased economic benefits, they are obviously intermittent and unstable due to environmental conditions. To impact and interfere. Microgrid technology is an ideal means to solve the above contradictions. Microgrid is a small power generation and distribution system that can realize self-control and management. It usually includes distributed power sources, energy storage devices, converter devices, related loads and Energy management device. Among them, the structure of the converter that plays the role of energy conversion and the energy management method are important directions of microgrid research. Most of the current low-power photovoltaic systems use non-isolated inverter structures, such as single-phase full-bridge inverters. This type of non-isolated inverter has disadvantages such as poor output common-mode voltage characteristics and high leakage current to ground. The average number is low, harmonics are injected to the grid side, and the filter device is bulky. In addition, the access of large-scale distributed photovoltaic power generation systems also brings challenges to grid energy management. Intermittent distributed power sources affect the voltage stability of local micro-grids. The role of power supply peak shaving and valley filling requires the arrangement of energy storage devices in the microgrid. Most of the energy storage devices of existing small and medium power grid-connected systems are installed on the side of the DC bus, with small capacity, multi-level energy conversion, and low efficiency. At the same time, the decentralized arrangement is not conducive to unified energy management. The invention provides a micro-grid variable current energy storage device and its energy management method. The energy storage part of the device adopts the form of direct connection to the AC side of the micro-grid, which reduces the energy conversion loss and is not limited by the geographical location of the micro-power supply. It can be arranged in the appropriate position of the microgrid to facilitate unified energy management. The energy storage part adopts a DC/AC converter composed of two bidirectional current-mode BOOST converters. Compared with traditional converters, it has the advantages of less switching devices, wide voltage output range, and simple SPWM modulation. The grid-connected inverter part of the device adopts a six-switch seven-level single-phase inverter, which is composed of six power switching devices connected in anti-parallel and divided into three groups in series, and a capacitor is connected in parallel between each group, and controlled by PWM modulation. It can realize single-phase seven-level AC output. The DC bus voltage segmental droop control is adopted to determine the relationship between the DC bus voltage and the charging and discharging of the energy storage device. The fuzzy control strategy determined by the system power difference and the SOC status of the energy storage device itself determines the charging and discharging actions of the battery and super capacitor. It has broad application prospects in the field of distributed power supply and micro grid application.

Contents of the invention

(1) Solved technical problems

Aiming at the deficiencies of the prior art, the present invention provides a micro-grid variable current energy storage device and its energy management method, which uses a six-switch seven-level single-phase inverter to track the voltage output by the photovoltaic array and its maximum power tracking circuit Invert to the AC grid. In addition, according to the energy operation and management of the microgrid, through the energy storage structure directly connected to the AC side, the energy conversion between the AC side of the microgrid and the DC of the energy storage device is realized. Discharge fuzzy control strategy to achieve optimal scheduling and use of energy such as balancing system power fluctuations, peak shaving and valley filling. The structure of the device is simple and the function is rich. Compared with the prior art, the six-switch seven-level single-phase inverter has better output power quality, requires less power switching devices, low switching loss, and simple control. The energy storage part adopts a DC/AC converter composed of two bidirectional current-mode BOOST converters. Compared with traditional converters, it has the advantages of less switching devices, wide voltage output range, and simple modulation.

(2) Technical solutions

To achieve the above object, the present invention is achieved through the following technical solutions:

A micro-grid variable current energy storage device and an energy management method thereof, comprising: a six-switch seven-level single-phase inverter unit, a bidirectional DC/AC converter unit for a storage battery, a bidirectional DC/AC converter unit for a supercapacitor, Microgrid converter and energy storage device control and energy management unit, photovoltaic MPPT unit, battery charge and discharge management unit, supercapacitor charge and discharge management unit, load management unit, characterized in that: the micro power supply in the microgrid is provided by a photovoltaic array, After passing through the BOOST type photovoltaic MPPT unit, it is input to the six-switch seven-level single-phase inverter unit.

Preferably, the inverter inverts the DC power into power-frequency AC and inputs it into the AC microgrid, and the energy storage device includes a bidirectional DC/AC converter unit for the storage battery and a bidirectional DC/AC converter unit for the supercapacitor.

Preferably, both the storage battery and the supercapacitor are directly connected to the AC side through a bidirectional DC/AC converter to balance system power fluctuations.

Preferably, the battery charging and discharging management unit and the supercapacitor charging and discharging management unit adopt a fuzzy control strategy determined by the system power difference and its own SOC status.

Preferably, the control and energy management unit of the microgrid converter and the energy storage device adopts segmental droop control of the DC bus voltage to determine the relationship between the DC bus voltage and the charging and discharging of the energy storage device, control the action of the energy storage device and balance the power fluctuation of the system, the load The management unit switches and switches microgrid loads in stages to ensure reliable power supply for important loads.

Preferably, the six-switch seven-level single-phase inverter unit used is composed of six power switching devices connected in anti-parallel in pairs and divided into three groups in series, and a capacitor is connected in parallel between each group. Through PWM modulation control, single-phase inverters can be realized. Phase seven level AC output.

(3) Beneficial effects

The invention provides a micro-grid variable current energy storage device and its energy management method. The device can not only realize the integration of photovoltaic converted electric energy into the power grid, but also use the energy storage device to balance system power fluctuations and reduce energy consumption according to the energy flow of the system. Peak filling valley and so on. The energy storage part is directly connected to the AC side, eliminating the need for intermediate energy conversion links, which improves the overall efficiency of the system. The six-switch seven-level single-phase inverter used requires less power switching devices, low switching loss, and high output power quality. The energy storage device adopts a bidirectional DC/AC converter composed of two bidirectional current-mode BOOST converters. Compared with traditional converters, it uses fewer switching devices, has a wide voltage output range, and has the advantages of simple SPWM modulation. It has great potential in microgrid applications. Broad application prospects.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. Those skilled in the art can also obtain other drawings based on these drawings without creative work.

Fig. 1 is a control flowchart of the present invention;

Fig. 2 is the circuit structural diagram of six switch seven level single-phase inverters in the device of the present invention;

Fig. 3 is a structural schematic diagram of a hybrid energy storage circuit composed of a supercapacitor and a storage battery;

Fig. 4 is a block diagram of the voltage outer loop control system of the energy storage device;

Fig. 5 is a block diagram of the inductive current inner loop control system of the BOOST converter on the left side of the battery pack;

Figure 6 is a block diagram of the energy management fuzzy control strategy and the overall control structure of the energy storage device.

detailed description

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments It is a part of embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

Implementation Mode 1: In this implementation mode, taking the working process of a six-switch seven-level single-phase inverter as an example, the schematic diagram of the circuit structure is shown in Figure 1, explaining the working process of the device converting light into electric energy and inverting and connecting to the grid. Assuming that the output voltage of the photovoltaic array is Vpv, it is boosted to Vdc after passing through the maximum power tracking circuit. According to the on and off states of the power switching devices in the inverter, the inverter can operate in the following modes:

Define the switch function as

Then the voltage of each node in Figure 1 can be expressed by the switching function as

Then, the inverter output AC side voltage can be expressed as

v _ad =v _ab +v _bc v _cd =(S ₁ -S ₂ )V ₁ +(S ₂ -S ₃ )V ₂ (3)

In the same way, the switching function of each node current can be expressed as

i _c ＝i ₃ -i ₂ ＝(S ₃ -S ₂ )i _o (5)

According to the relationship between the capacitor voltage and current, it can be known that

It can be seen from the above formula that the voltage V2 at both ends of the capacitor C2 is related to the switching function. In order to realize the seven-level output of the inverter, V2=V1/3 can be controlled through the closed-loop PI. At this time, the formula (3) can be rewritten as

In steady state operation, the voltage across capacitor C1 is Vdc boosted by the photovoltaic array after passing through the maximum power tracking circuit, and the output voltage can be in the form of ±Vdc, ±2/3Vdc, ±1/3Vdc and 0, a total of seven levels.

Embodiment 2: In this embodiment, the control method of the energy storage device and its boost DC/AC converter is analyzed. The energy storage device adopts the hybrid energy storage form of battery pack and super capacitor. The charging and discharging time constant of the battery pack is large, and the service life is seriously reduced as the number of charging and discharging increases. The power balance of the low-frequency part of the load changes. Compared with batteries, supercapacitors charge and discharge faster, have higher efficiency and longer life, and are responsible for the power balance of high-frequency parts of load changes. The circuit structure diagram is shown in Figure 2. The structure of the boost DC/AC converter connected to the battery pack and the supercapacitor is exactly the same. Taking the battery pack and its DC/AC converter as an example for analysis, the power switch devices batt_S1, batt_S3 and RLbatt1 constitute the left BOOST type converter unit, and the power switch devices batt_S2, batt_S4 and RLbatt2 constitute the right side BOOST type converter unit. unit. These two identical Boost DC converter units are connected in the form of a differential circuit, and they output a bidirectional power flow of an anti-phase low-frequency sinusoidal pulsating DC voltage, and the AC power grid is connected across the output terminals of the two DC converter units. When the storage battery transmits power to the grid, the inverter switches batt_S1 and batt_S2 invert the input DC voltage Vbatt into low-frequency AC currents iLbatt1 and iLbatt2 with high-frequency components superimposed, which are converted into High-quality low-frequency sinusoidal pulsating DC voltages vC1 and vC2, when the grid transfers energy to the battery, the rectifier switches batt_S3 and batt_S4 work in the inverter state, while the inverter switches batt_S1 and batt_S2 work in the rectifier state. Assuming that the positive direction of each physical quantity is shown in Figure 2, the energy storage part of the device can operate in the following modes.

Mode 1: When Vg>0, Ig<0, iLbatt1>0, iLbatt2<0, the DC conversion unit on the right side flows through the reverse current, and the AC power supply side of the microgrid feeds energy to the DC power supply, working in the BUCK charging mode. On the contrary, the DC conversion unit on the left side provides energy from the battery to the AC power supply side, and works in BOOST discharge mode. Since the current on the side of the AC power supply is negative, the energy transmitted by the battery in the reverse direction is greater than the energy transmitted in the forward direction. The difference between the two is the energy fed back to the DC power supply from the AC power supply side.

Mode 2: When Vg>0, Ig<0, iLbatt1<0, iLbatt2>0, the forward current flowing through the right DC conversion unit increases, and the battery supplies energy to the AC power side, working in BOOST discharge mode. The DC conversion unit on the left side flows through the reverse current to reduce, and the energy is fed from the side of the AC power supply to the DC power supply, and it works in the BUCK charging mode. At this time, the current on the side of the AC power supply gradually increases but is still negative, so the energy transmitted by the battery in the reverse direction is still greater than the energy transmitted in the forward direction. The AC source still feeds energy back to the battery.

Mode 3: When Vg>0, Ig>0, iLbatt1<0, iLbatt2>0, the DC conversion unit on the right side flows a forward current, and the battery supplies energy to the AC power side, working in BOOST discharge mode. The DC conversion unit on the left flows through the reverse current, feeds energy from the side of the AC power supply to the battery, and works in the BUCK charging mode. At this time, the current on the side of the AC power supply is positive, and the battery starts to provide energy to the side of the AC power supply.

Mode 4: When Vg<0, Ig>0, iLbatt1<0, iLbatt2>0, the output voltage becomes negative at this time, but since the output current is still positive, this working mode is the same as Mode 2.

Mode 5: When Vg<0, Ig>0, iLbatt1>0, iLbatt2<0, the output voltage becomes negative at this time, but since the output current is still positive, this working mode is the same as Mode 3.

Mode 6: When Vg<0, Ig<0, iLbatt1>0, iLbatt2<0, the right DC conversion unit flows through the reverse current, and the energy is fed from the side of the AC power supply to the battery, and it works in the BUCK charging mode. The DC conversion unit on the left flows a positive current to provide energy to the AC power supply side. At this time, the voltage and current of the AC power supply side are both negative, and the battery starts to provide energy to the AC power supply side of the microgrid.

From the above analysis, it can be seen that when it is necessary to transmit energy from the battery to the AC side, the energy storage converter can work in modes 3, 5, and 6. For the two groups of energy storage converters, they are individually controlled, and they can respectively generate a sine wave unipolar voltage output with a DC bias. The drive unit of the energy storage converter in the left group and the drive unit of the right energy storage converter use PWM modulation to make the output voltages of the two converters differ from each other by 180°. Since the AC power supply is connected between the two converters, the converters respectively The output DC bias voltages will cancel each other at the AC power supply end, resulting in a bipolar AC voltage output. When it is necessary to transmit energy from the AC side to the battery, the energy storage converter can work in modes 1, 2, and 4.

Each BOOST DC conversion unit is independently controlled. Taking the left converter as an example, based on the continuous time average model of the BOOST converter, the following equation can be obtained

V _batt -V _RLbatt1 = (1-d _batt1 )V _c1 (8)

V _scap -V _RLscap1 =(1-d _scap1 )V _c1 (9)

i _C1 +i _O1 ＝(1-d _batt1 )i _Lbatt1 +(1-d _scap1 )i _Lscap1 (10)

In the formula, represents the conduction-to-duty ratio of the two power switching devices batt_S1 and scap_S1.

Suppose the grid-side voltage reference value can be expressed as

The two DC converters on the left and right can output two different sine wave voltages with DC bias, which can be realized by controlling the voltages at both ends of C1 and C2 according to the following reference values.

where is the DC bias value, which can be given according to the following formula

V _bias ≥V _batt +V _o /2 (13)

The block diagram of the control system with the capacitor voltage as the control target of the outer ring is shown in Figure 3. The inner loop adopts the direct current control of the inductor current, and each Boost converter has the same structure. Taking the boost converter on the left side of the battery as an example, the structure diagram of the inner loop control system is shown in Figure 4.

Embodiment 3: In this embodiment, the energy control strategy and management method in the present invention are analyzed. The segmental droop control of the DC bus voltage is adopted to determine the relationship between the DC bus voltage and the charging and discharging of the energy storage device, and the action of the energy storage device is controlled to balance the power fluctuation of the system. Combined with the actual system load power, photovoltaic array power, grid-connected inverter rated power, and energy storage device capacity, set 5 segmentation points for the DC bus voltage value (corresponding to Vth1>Vth2>Vth3>Vth4>Vth5), and the corresponding There will be five working modes, which will be analyzed in detail below.

System working mode 1: When Vth1<V dc<V dcN, V dcN represents the rated value of the DC bus voltage when the photovoltaic power source alone provides energy in the microgrid, grid-connected power Pinv=PI+kind(V dc-Vth1), In the formula, PI represents the first-level important load power in the microgrid, kind is the grid-connected power coefficient, which can be adjusted according to the actual situation, the maximum value is (PinvN-PI)/(Vdc-Vth1), and PinvN is the rated value of the inverter Grid-connected power; at this time, the energy provided by the photovoltaic power supply not only meets the first-level important load, but also has a surplus, and the load control unit can control other loads to enter the system. If the SOC of the energy storage device is ≤0.9, the energy storage battery will be charged with the rated charging current and constant current, and the supercapacitor bank will balance the high-frequency charging current to cooperate with the battery pack; Allow terminal voltage float charging or enter standby state directly.

System working mode 2: When Vth2<V dc<Vth1, the inverter grid-connected power Pinv=PI+kind(Vdc-Vth2), at this time, the energy provided by the photovoltaic power supply not only meets the first-level important load, but also has a margin. The load control unit can control other load input systems. If SOC≤0.9, the battery pack will be charged with the variable current command value determined by the fuzzy power distribution unit, and the supercapacitor bank will balance the high-frequency charging current to work with the battery pack; if SOC>0.9, the battery pack and the supercapacitor pack will Both charge according to the maximum allowable terminal voltage floating voltage or directly enter the standby state.

System working mode three: when Vth3≤V dc≤Vth2, inverter grid-connected power Pinv=PI+kind(V dc-Vth3), other loads in the system can be input through the load control unit, energy balance and power fluctuations are controlled by energy storage device bears. The bidirectional Boost DC/AC converter of the battery pack balances low-frequency power fluctuations, and the bidirectional Boost DC/AC converter of the supercapacitor pack uses the DC bus voltage Vth2 as the control target to balance high-frequency power fluctuations.

System working mode 4: When Vth4≤V dc<Vth3, inverter grid-connected power Pinv=PI+kind(V dc-Vth4), if SOC≥0.2, the battery pack will follow the discharge current command given by the fuzzy power distribution unit The value is released, and the supercapacitor bank balances the high-frequency discharge current to cooperate with the battery pack; if the SOC<0.2, the converters in the energy storage device are all in a standby state. At this time, in order to ensure the power supply of important loads in the system, other loads are disconnected by the load control unit.

System working mode 5: When Vth5≤V dc<Vth4, the inverter grid-connected power Pinv=PI+kind(V dc-Vth5), the energy provided by the photovoltaic power supply is insufficient, and the system power shortage is serious; if SOC≥0.2, the battery pack Discharge with the maximum discharge current, and the supercapacitor bank balances the high-frequency discharge current to work with the battery pack; when V dc ≥ Vth4, it can be considered to put the load according to the load level. If SOC<0.2, in order to prevent power loss, the converters of the energy storage subsystem are all in a standby state.

The core function of the microgrid is to balance power fluctuations, that is, energy balance, DC bus voltage stability, and a schematic diagram of the fuzzy control strategy for system power distribution and energy management, as shown in Figure 5. The DC bus capacitor is the hub of energy exchange, and the state change of the bus capacitor voltage determines the stability of the system. The reference value Pdc-ref of the Cdc exchange power can be obtained through a proportional-integral link from the current-voltage relationship at both ends of the capacitor. Comparing this value with the power PPV provided by the photovoltaic array, the power difference between the photovoltaic power supply and the microgrid can be obtained, which is the power reference value Psto-ref of the energy storage system. Psto-ref is a rapidly changing quantity due to the inherent volatility of photovoltaic power generation and variability of load. Due to the large inertia of the battery pack in the energy storage system, the supercapacitor pack can follow rapidly changing quantities. Therefore, a low-pass filter is used to distribute the power frequency decoupling between the battery pack and the super capacitor pack, combined with the SOC status of the energy storage device, fuzzy logic control is adopted, and the control decision is based on the value of Pbatt-ref and SOC value, fuzzy control The output Pbatt* is used as a given power signal controlled by the PQ of the energy storage device, and then through the voltage and current double closed-loop adjustment of each conversion unit, the charging and discharging of the energy storage device is realized, and the system power balance is realized.

It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is a relationship between these entities or operations. There is no such actual relationship or order between them. Furthermore, the term "comprises", "comprises" or any other variation thereof is intended to cover a non-exclusive inclusion such that a process, method, article, or apparatus comprising a set of elements includes not only those elements, but also includes elements not expressly listed. other elements of or also include elements inherent in such a process, method, article, or device. Without further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article or apparatus comprising said element.

The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it can still be described in the foregoing embodiments Modifications are made to the recorded technical solutions, or equivalent replacements are made to some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (6)

1. A micro-grid variable current energy storage device and an energy management method thereof, comprising: a six-switch seven-level single-phase inverter unit (1), a bidirectional DC/AC converter unit (2) for a storage battery, and a supercapacitor Bidirectional DC/AC converter unit (3), microgrid converter and energy storage device control and energy management unit (4), photovoltaic MPPT unit (5), battery charge and discharge management unit (6), supercapacitor charge and discharge management unit (7), load management unit (8), characterized in that: the micro power supply in the microgrid is provided by the photovoltaic array, and after passing through the BOOST type photovoltaic MPPT unit (5), it is input to the six-switch seven-level single-phase inverter unit ( 1). 2. The micro-grid variable current energy storage device and its energy management method according to claim 1, characterized in that: the inverter converts DC power into power-frequency AC power and inputs it into the AC micro-grid, and the energy storage device includes a battery A bidirectional DC/AC converter unit (2) and a supercapacitor are used for a bidirectional DC/AC converter unit (3). 3. The micro-grid variable current energy storage device and its energy management method according to claim 1, characterized in that: both the storage battery and the supercapacitor are directly connected to the AC side through a bidirectional DC/AC converter to balance system power fluctuations . 4. The micro-grid variable current energy storage device and its energy management method according to claim 1, characterized in that: the storage battery charge and discharge management unit (6) and the supercapacitor charge and discharge management unit (7) adopt the system power difference and The fuzzy control strategy determined by its own SOC status. 5. The micro-grid converter energy storage device and its energy management method according to claim 1, characterized in that: the control and energy management unit (4) of the micro-grid converter and energy storage device adopts DC bus voltage segmental drooping Control, determine the relationship between the DC bus voltage and the charge and discharge of the energy storage device, control the action of the energy storage device and balance the power fluctuation of the system, and the load management unit (8) switches the microgrid load in stages to ensure reliable power supply of important loads. 6. The micro-grid variable current energy storage device and its energy management method according to claim 1, characterized in that: the adopted six-switch seven-level single-phase inverter unit (1) is composed of six power switching devices and two Two anti-parallel connections are divided into three groups in series, and a capacitor is connected in parallel between each group. Through PWM modulation control, single-phase seven-level AC output can be realized.