WO2019075879A1

WO2019075879A1 - Running mode conversion method for alternating-current/direct-current hybrid microgrid

Info

Publication number: WO2019075879A1
Application number: PCT/CN2017/115203
Authority: WO
Inventors: 史林军; 吴峰; 史江峰; 张万阔; 林克曼
Original assignee: 河海大学
Priority date: 2017-10-16
Filing date: 2017-12-08
Publication date: 2019-04-25
Also published as: CN107681700A; CN107681700B

Abstract

A running mode conversion method for an alternating-current/direct-current hybrid microgrid. The used alternating-current/direct-current hybrid microgrid has high flexibility and has high renewable energy absorption ability, running modes, conversion conditions and conversion modes of a microgrid alternating-current side and a microgrid direct-current side in different states are provided with regard to a switching mode of the alternating-current/direct-current hybrid microgrid in different running states, seamless switching among running modes can be achieved, and a technical support is provided for safe and stable running of the alternating-current/direct-current hybrid microgrid.

Description

Method for converting AC/DC hybrid microgrid operation mode

Technical field

The invention belongs to the field of power system micro-grid, and relates to an AC-DC hybrid micro-grid operation mode conversion method.

Background technique

With the increasing energy crisis, the Micro Grid (MG) has developed rapidly. Since the microgrid contains multiple types of distributed power generators (DGs), energy storage devices, and coordination controllers in different states, the power source, energy management, and power quality requirements of the MG are different from those of the large power grid. grid energy management system has functions different from the power grid, it needs to develop according to MG and MG constituent unit of the operating characteristics of a reasonable run strategy to ensure economic and reliable operation of ^microgrids]. Therefore, it is of great significance to study the operation mode and conversion mode of the microgrid.

This year, the microgrid energy management system has become one of the research hotspots, and there are many research results. LIANG Chengshan,WU Zhen,LI Peng.Study on key technologies of microgrid[J].Transactions of China Electrotechnical Society, 2014,29(2):1-12. To study the economic optimization problem of off-grid MG system on island, combined with energy storage The constraints of the device establish an optimization model that takes into account various operational and environmental costs. Literature: Tian Po. Research on AC/DC hybrid microgrid modeling and converter control technology [D]. Jinan: Shandong University, 2014. EMS maintains the stability of the internal voltage and frequency of the microgrid and optimizes the output of each DG in the microgrid. Two features. Literature Wang Chengshan, Hong Bowen, Guo Li, et al.General Modeling Method for Micro-grid Optimization Scheduling of Cooling, Heating and Power Supply[J].The Chinese Journal of Electrical Engineering establishes a general model for the microgrid for cogeneration of cold and heat, considering the electricity A series of constraints and flue gas and steam constraints optimize system operation. Literature Wang Chengshan, Hong Bowen, Guo Li. Scheduling strategy of optical storage microgrid in different scenarios[J].Power grid technology research Microgrid model and various constraints in different states, design two microgrid optimization operation strategies One is to calculate the depreciation expense of the energy storage device during operation to optimize the calculation, and the other is to optimize the calculation without considering the depreciation expense of the energy storage device during operation, and obtain two different optimized operation methods. LI Hongwen, GUO Li, WANG Chengshan, et al. Multi-objective dynamic optimization scheduling model and method for microgrid[J]. Power automation equipment has established an optimal scheduling model that takes into account both environmental and economic objectives, taking into account environmental costs. Compared with the operating cost of the microgrid, the genetic algorithm is used to calculate and obtain the optimal scheduling scheme. Literature Zhang Ye, Guo Li, Jia Hongjie, et al. Energy management method for hybrid energy storage system based on smoothing control[J]. Power System Automation proposes an energy management strategy for hybrid energy storage devices, respectively for supercapacitors with high power density and The battery with high energy density is controlled, and the pre-control strategy of the terminal voltage of SC is introduced. The simulation platform is used to simulate and verify the above control method strategy. Literature Hao Yuchen, Dou Xiaobo, Wu Zaijun, et al. Microgrid hierarchical distributed energy optimization Management [J]. The power automation equipment has established an optimization model that takes into account the diesel generator set and the demand side load, and accurately estimates the operation and maintenance cost of the energy storage device in the microgrid by the rain flow counting method, according to the short-term load forecasting and Ultra-short-term load forecasting technology to develop and adjust the energy management strategies of the day and day, so that the units in the micro-grid are in optimal operation.

For the existing research objects are AC microgrid or DC microgrid, there are few studies on the operation mode of the new microgrid such as AC/DC hybrid microgrid.

Therefore, there is a need for an AC/DC hybrid microgrid operating mode conversion method.

Summary of the invention

The object of the present invention is to provide an AC/DC hybrid microgrid operation mode conversion method in order to solve the above problems.

The present invention achieves the above objects by the following technical solutions:

An AC/DC hybrid microgrid operation module conversion method, the AC/DC hybrid microgrid comprises an AC microgrid and a DC microgrid, wherein the AC microgrid and the DC microgrid are connected by an AC/DC interconnect converter PCS,

The AC microgrid includes an AC bus, a PV array PV, an energy storage device, and an AC side load, wherein the PV array, the energy storage device, and the load are connected to the AC bus; the AC microgrid is switched to the grid through the PCC. Mode or off-grid mode of operation;

When the PCC is closed, the AC-DC interconnect converter PCS operates in a constant voltage control mode, and the AC-side grid-connected operation mode includes a first AC operation mode.

When the PCC is disconnected, the AC/DC interconnect converter PCS is switched to the PQ control mode, and the AC side off-network operation mode includes a second AC operation mode, a second two AC operation mode, and a second three AC operation mode;

The first alternating current operation mode is that all the photovoltaic arrays PV are operated in the MPPT mode, the energy storage device is standby or charged and discharged, and the AC side load is all put into use;

The second alternating current operation mode is that the energy storage device operates in the V/f mode, and all the photovoltaic array PVs are operated in the MPPT mode, and the AC side loads are all put into use;

The second two alternating current operation mode is that the energy storage device operates in the V/f mode, and part of the photovoltaic array PV is cut off, and the remaining photovoltaic array PVs are operated in the MPPT mode, maintaining the frequency and voltage of the alternating microgrid, and the AC side loads are all put into use;

The second three AC operation mode is that the energy storage device first outputs the maximum power, and the AC/DC interconnection converter PCS increases the power input to the AC microgrid or cuts off part of the AC side load until the energy storage device recovers V. /f control mode, all PV array PVs are operated in MPPT mode;

When the PCC is closed, the AC microgrid operates in a first AC mode of operation;

When the PCC is disconnected, the AC microgrid is switched from the first alternating current operating mode to the second alternating current operating mode;

When P _PV +P _PCS -P _bch-max >=P _load , the AC microgrid switches from the second AC operating mode to the second AC operating mode;

When the P _PV +P _PCS -P _bch-max <P _load , the AC microgrid switches from the second alternating current operating mode to the second alternating current operating mode;

When the P _PV +P _PCS +P _bdi-max <P _load , the AC microgrid switches from the second AC mode to the second AC mode;

When P _PV +P _PCS +P _bdi-max >P _load , the AC microgrid switches from the second three AC operation mode to the second AC operation mode;

Where, P _PV is the power output of the PV array PV, P _PCS is the power of the AC/DC interconnect converter PCS flowing into the AC side of the microgrid, P _bch-max is the maximum charging power of the energy storage device, and P _bdi-max is the storage The maximum discharge power can be installed, and P _load is the power consumed by the AC side load;

The DC microgrid includes a DC bus, a second PV array PV, a second energy storage device, and a DC side load, and the second PV array PV, the second energy storage device, and the DC side load are connected to the DC bus;

When the PCC is closed and the AC/DC interconnect converter PCS adopts a constant voltage control mode, the operating mode of the DC microgrid includes a first DC operating mode;

When the AC/DC interconnect converter PCS adopts the PQ control mode or standby, the operating mode of the DC microgrid includes a second direct current operation mode, a second two-DC operation mode, and a second three-DC operation mode;

The first DC operating mode is to maintain the DC bus voltage at the first voltage by using the AC/DC interconnect converter PCS, all the second PV array PVs are operated in the MPPT mode, and the second energy storage device performs charging and discharging control, the DC side The load is fully put into use;

The second direct current operation mode is to maintain the DC bus voltage at the second voltage by using the second energy storage device, all the second photovoltaic array PVs are operated in the MPPT mode, and the AC/DC interconnection converter PCS performs charging and discharging control, and the DC The side loads are all put into use;

The second two-DC operation mode is that the second energy storage device operates in a maximum power charging or full charge standby state, and a part of the second photovoltaic array PV is cut off, and the remaining second photovoltaic array PVs are operated in the MPPT mode, so that the second energy storage device Operate in constant voltage control mode to maintain DC bus voltage, DC side load is fully put into use;

The second three-DC operation mode is to cut off part of the DC side load so that the discharge power of the second energy storage device is less than The maximum discharge power until the second energy storage device operates in the constant voltage mode, and all the second photovoltaic array PVs operate in the MPPT mode;

_Switching from the second all-current operation mode to the second two-DC operation mode when P _PV2 -P _PCS2- P _bch-max2 >=P _load2 ;

_Switching from the second two-DC operation mode to the second constant-current operation mode when P _PV2 -P _PCS2- P _bch-max2 <P _load2 ;

_Switching from the second all-current operation mode to the second three-DC operation mode when P _PV2 -P _PCS2+ P _bdi-max2 <P _load2 ;

_Switching from the second three-DC operation mode to the second constant-current operation mode when P _PV2 -P _PCS2+ P _bdi-max2 >P _load2 ;

Where, P _PV2 is the output power of the DC side photovoltaic power of the micro grid, P _PCS2 is the power of the DC bus input to the AC/DC interconnect converter PCS, and P _bch-max2 is the maximum charging power allowed by the second energy storage device, P _load2 For the power of the DC side load, P _bdi-max2 is the maximum discharge power of the second energy storage device.

Further, the energy storage device comprises a battery and a super capacitor. The energy storage device on the AC side also includes a super capacitor, which can fully utilize the characteristics of fast charging and discharging of the super capacitor.

Further, the first voltage is 400V.

Further, the second voltage is 400V.

Further, in the first DC operating mode, the second energy storage device performs constant power charging and discharging control.

Advantageous Effects: The AC/DC hybrid microgrid operating module conversion method of the present invention adopts an AC/DC hybrid microgrid with high flexibility and strong regenerative energy absorption capability, and proposes that the microgrid AC side and the microgrid DC side are different. The operation mode and the conversion conditions and the conversion mode in the state can realize seamless switching between the operation modes, and provide technical support for the safe and stable operation of the AC-DC hybrid microgrid.

DRAWINGS

1 is a schematic structural view of an AC-DC microgrid;

2 is a control mode of the first alternating current operation mode;

Figure 3 is a control mode of the second alternating current operation mode;

Figure 4 is a control mode of the second two alternating current operation modes;

Figure 5 is a control mode of the second three alternating current operation modes;

6 is a schematic diagram of conversion of an AC side AC side operation mode;

Figure 7 is a control mode of the first DC operating mode;

Figure 8 is a control mode of the second continuous flow operation mode;

Figure 9 is a control mode of the second two-DC operation mode;

Figure 10 is a control mode of the second three-DC operation mode;

11 is a schematic diagram of conversion of a DC side operating mode of a microgrid;

Figure 12 is the relationship between the PCC and the operating state of the interconnected PCS;

Figure 13 is a first DC operating mode simulation voltage curve;

Figure 14 is a second constant current operation mode simulation voltage curve;

Figure 15 is a voltage curve of the DC bus of the grid-connected off-grid;

Figure 16 is an unplanned off-grid AC bus voltage on the AC side of the microgrid;

Figure 17 shows the frequency of the unplanned off-grid AC bus on the AC side of the microgrid;

Figure 18 is a hierarchical control structure of a microgrid;

Figure 19 shows the basic control timing of the seamless switching operation mode of the microgrid;

Figure 20 is a schematic diagram of the main power mode switching.

Detailed ways

The technical solutions in the embodiments of the present invention are clearly and completely described in the following with reference to the accompanying drawings in the embodiments of the present invention. It is obvious that the described embodiments are only a part of the embodiments of the present invention, but not all embodiments. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without creative efforts are within the scope of the present invention.

Referring to FIG. 1 , the AC side and the DC side of the AC/DC hybrid microgrid of the present invention both include a PV array PV, an energy storage device, and a load, wherein the AC side energy storage device includes a battery and a super capacitor, and the AC bus passes through The common connection point is connected to the distribution network, and the DC side and the AC side are connected by an AC/DC converter that enables bidirectional flow of energy. Priority is given to some DC-type DGs and loads connected to the DC bus. At the same time, in order to ensure that the DC side and the AC side of the MG can operate independently under special conditions, a part of the PV array is also incorporated on the AC bus, and part of the AC load is directly Connect to the AC bus. The microgrid constructed by the invention has high flexibility and strong regenerative energy absorption capacity.

AC/DC hybrid microgrid system model:

(1) Photovoltaic power generation system model

The power output of the PV array is:

Where: f _PV is the power derating factor of the photovoltaic system, which represents the ratio of the actual output power of the photovoltaic system to the output power under the rated conditions, which is used to account for the surface contamination of the photovoltaic panel, the covering of rain and snow, and the aging of the photovoltaic panel itself. The loss is generally 0.9; Y _PV is the PV array capacity, kW; I _T is the actual illuminance, kW / m ² ; I _S is the illuminance under the standard test conditions, generally 1kW / m ² ; α _p is the temperature coefficient of power , % / ° C; T _cell is the surface temperature of the current photovoltaic cell, ° C, can be estimated according to the current ambient temperature; T _{cell, STC} is the temperature of the photovoltaic cell under standard test conditions, generally taken 25 ° C.

(2) Battery model

The battery model adopts the Kinetic Battery Model (KiBaM) model, and assumes that the battery port voltage is constant, and the charge and discharge current of the battery in each time step is constant, regardless of the influence of environmental conditions.

The total energy stored by the battery pack at any time is equal to the sum of available energy and binding energy, ie:

W=W ₁ +W ₂ (2)

Where W ₁ is available energy; W ₂ is bound energy.

According to the actual charge and discharge power of the battery pack, the available energy of the battery pack after charging and discharging can be calculated:

Where W _1,0 is the available energy of the battery pack at the initial time (kW·h); W _1,end is the available energy of the battery pack at the end time (kW·h); W ₀ is the total energy of the battery pack at the initial time (kW· h); P is the battery pack discharge (positive) or charge (negative) power (kW), does not include charge and discharge loss; Δt is the time interval (in the algorithm, ie time step, h); c is the battery capacity The ratio represents the ratio of available energy to total energy in the fully charged state of the battery; k is the battery rate constant (h ^-1 ), indicating the conversion rate of available energy and bound energy.

Combining equations (2) and (3), taking into account that the total energy reduction is equal to the discharge amount PΔt, the binding energy of the battery pack after charging and discharging can be obtained:

W _2,end =W ₀ -PΔt-W _1,end (4)

Where W _2,end is the binding energy (kW·h) of the battery pack at the end time.

The available energy W _{1 at} any time satisfies the relationship: ₀ ≤ W ₁ ≤ cW _max , where W _max represents the maximum storable energy (kW·h) of the battery. Combining the energy step W _{at the end of the} time step _and calculating the equation (3), the single-step maximum allowable charging power and the maximum charging and discharging power of the battery KiBaM model can be obtained ^[9] , respectively:

In order to prevent overcharging and overdischarging of the battery, the maximum charging current and speed constraint of the battery should also be taken into account in the maximum charging power constraint, and the corresponding maximum charging power (kW) is obtained as follows:

Where N _bat is the total number of battery strings connected in parallel; I _max is the maximum charging current of the battery (A); U _N is the rated voltage (V) of the battery; α _c is the maximum charging rate of the battery (A / (Ah)).

Combined with the limitation of the charging and discharging power of the battery in the KiBaM model, and taking into account the charge and discharge loss, the final battery charging and discharging power is limited to:

-P _bat,cmax ≤P _bat ≤P _bat,dmax (9)

P _bat,dmax =η _bat,d P _bat,cmax,kbm (11)

Where η _bat,c is the battery charging efficiency; η _bat,d is the battery discharge efficiency.

(3) Converter model

Since the optical storage grid-connected power generation system includes both the DC bus and the AC bus, the converter needs to be rectified or inverted. The model can use the following expression:

Where: P _{con, AC} represents the AC side power (kW) of the converter, positive in the inverter, negative in the rectification, the same below; P _{con, DC} represents the sum of the DC side power of the converter (kW). η _inv and η _rec represent the efficiency of converter inversion and rectification; R _inv and R _rec represent the maximum active power (kW) of the converter inverter and rectification, which is numerically equal to its rated capacity.

The above formula can represent either a bidirectional converter model or a unidirectional rectifier or inverter. For example, a unidirectional inverter model can be expressed as follows:

(4) Load modeling

The load in the microgrid is roughly divided into two categories: important load and participation in the demand side management load. The important load needs to ensure uninterrupted power supply; the participating demand side management load is divided into three categories: interruptible load, translatable load and elastic load (also known as controllable load). The invention regards the participation demand side management load as all interruptible load, the interruptible load as the non-important load, and the micro grid dispatcher has the interruption right of such load.

AC side operation mode

The AC side of the microgrid switches its grid-connected or off-grid operation mode through the PCC. The present invention divides the grid connection of the micro-grid on the AC side into two main modes.

(1) Connected to the AC side

The first AC operation mode: the PV array operates in the MPPT mode, and the energy storage standby or charging and discharging with a certain strategy, as shown in FIG. 2 .

(2) AC side off the network

While the PCC is disconnected, the interconnected PCS is switched from the constant voltage mode to the PQ control mode.

The second AC operation mode: the difference between the output power of the photovoltaic power P _PV and the power P _PCS of the interconnection PCS flowing into the micro-grid AC side and the power consumption of the _load P _load is the maximum charging power P _bch-max and maximum of the energy storage. Between the discharge power P _bdi-max , the energy storage is operated in the V/f mode, and the photovoltaic array operates in the MPPT mode, as shown in FIG. 3 .

The second two-AC operation mode: the sum of the output power P _{PV of the} photovoltaic and the power P _PCS of the interconnection PCS flowing into the AC side of the micro-grid is greater than the power of the _load P _load and the maximum charging power of the energy storage at this time (P _bch-max or 0) In the sum, some PV arrays are cut off, other PVs are still running in MPPT mode, and the energy storage is operated in V/f control mode to maintain the frequency and voltage of the AC side of the microgrid. The specific control method is shown in Figure 4.

Run the second three AC operation mode: the power of PV output P _PV , the power P _{PCS of the} interconnected PCS flowing into the AC side of the microgrid and the maximum discharge power P _bdi-max of the energy storage are less than the power of the load, and the energy storage is maximum at this time. Power output, the system still has power shortage, the energy management system will increase the energy input from the interconnected PCS to the AC side (when the interconnected PCS is running in constant power control) or the AC side load is cut off (the interconnected PCS is in the standby state) ), so that the energy storage restores the V/f control mode. The specific control method is shown in Figure 5.

The relationship between the above modes is shown in Fig. 6.

The transition conditions between modes are shown in Table 1:

Table 1 Micro-grid AC side operating mode transition conditions

DC side operation mode:

Similar to the AC side of the microgrid, the DC side is divided into two main modes according to the control mode of the interconnected PCS.

(1) When interconnecting PCS constant voltage control mode (connected to the AC side)

The first DC operating mode: using the interconnected PCS to maintain the DC bus voltage at 400V, the PV array operates in MPPT mode, and the stored energy is charged and discharged according to a given command, as shown in Figure 7.

(2) When the interconnected PCS adopts the PQ control mode or standby mode (when the AC side is connected to the network and disconnected from the network, if the interconnected PCS is in the voltage regulation mode, it should be switched to the PQ mode at the moment of conversion)

The second all-current operation mode: using the energy storage to maintain the DC bus voltage at 400V, the PV array operates in the MPPT mode, and the interconnected PCS performs charging and discharging control according to a given command, and the control mode is as shown in FIG.

The second two-DC operation mode: if the output power P _{PV2 of the} DC side of the micro-grid and the power-P _PCS2 of the interconnected PCS input DC bus are greater than the maximum charging power allowed by the energy storage at this time (P _bch-max2 or 0) ) and the load power P _load2 , the voltage will rise, the mode is changed from the second direct current operation mode to the second two-DC operation mode, and the energy storage operation is in the maximum power charging or full charge standby state, and the cutting is performed. Part of the photovoltaic, so that the energy storage is still running in the constant voltage control mode to maintain the DC bus voltage, the control mode is shown in Figure 9; as the load increases or the PV output decreases, when the PV output is less than the load, the gradual input is removed. The photovoltaics are restored to the second continuous flow mode of operation.

The second three-DC operation mode: if the output power P _{PV2 of the} photovoltaic side on the DC side of the micro-grid, the maximum discharge power P _bdi-max2 at the time of energy storage and the power-P _PCS2 of the interconnected PCS flowing into the DC bus are less than the power of the load P _Load2 , at this time the voltage will drop, from the second direct current operation mode to the second three-current operation mode, in order to make the energy storage still operate in the constant pressure mode, the load is cut off according to the load level so that the discharge power of the stored energy is less than its maximum Discharge power. When the stored energy is switched from the discharge to the charged state, the load is gradually input until the second continuous flow operation mode is restored.

The relationship between the operating modes of the DC side of the microgrid is shown in Figure 11, and the switching conditions between the modes on the DC side of the microgrid are shown in Table 2.

Table 2 DC grid DC operating mode transition conditions

Since the operation modes of the AC side and the DC side of the microgrid are separately proposed, there is a certain interaction relationship between the two, mainly the relationship between the operating state of the PCC and the interconnected PCS, as shown in FIG.

Among them, the PCC has two operating modes, closed and disconnected, which can be switched to each other; the interconnected PCS has three operating modes of constant voltage control, standby and PQ control, and the three switches between each other. There is a certain relationship between the PCC and the operating mode of the interconnected PCS. When the PCC is closed, the interconnected PCS can run in any mode, but mainly operates in the constant voltage mode; when the PCC is disconnected, the interconnected PCS can only run in the PQ control and standby. Mode, not in constant voltage control mode, and mainly in PQ control mode.

Operation mode switching simulation verification

The invention performs simulation verification by taking the micro grid as the example of the grid connection state.

Since the microgrid operation mode is divided according to the grid connection or not, it is only necessary to simulate the parallel/off-grid handover of the microgrid. The simulation first needs to start the microgrid.

(1) Microgrid DC side start

The first DC operating mode: the DC bus is charged by the interconnected PCS, the voltage is maintained at 400V, and then the PV array and the load operating in the MPPT mode are respectively connected, and the DC side energy storage is charged and discharged according to the given command.

The process is simulated by the model built in Simulink. The curve of the DC bus during the simulation is shown in Fig. 13.

When the interconnected PCS is started at 0.05s, the DC bus is charged. During the charging process, the maximum voltage reaches 475V, and the voltage tends to be stable after 0.2s. In the 0.45s, 0.7s and 0.8s respectively, the 19.4kW, 6.2kW and 5.58kW PV arrays are connected, and the voltage fluctuations are all within 5%; in the 0.9s, 1.1s and 1.2s, respectively, 20kW, 5.1kW and 4.9kW are incorporated. Load, voltage fluctuations are also within 5%.

In fact, interconnecting the PCS to charge the DC bus is a soft-start process that does not create a large overvoltage on the DC side. The photovoltaic inverter also has a starting process, and the power gradually becomes larger and there is no impact. This simulation is to simulate the overvoltage level of the microgrid under the most extreme conditions.

The second direct current operation mode (self-starting): the DC side is mainly energy storage, the energy storage charges the DC bus, and then the constant voltage control is performed, and the PV array and the load operating in the MPPT mode are respectively accessed in a certain order. The interconnected PCS performs charge and discharge control according to the given command. The simulated voltage waveform is shown in Figure 14.

In Figure 14, the DC bus is charged by the energy storage device, and then merged into 5kW load and 5kW PV at 0.4s and 0.8s respectively. Although the DC side has a small energy storage capacity, it can still be maintained under such large power fluctuations. The DC bus voltage fluctuates within 10%.

The AC side of the microgrid can also be self-started and activated by an external circuit, which is not studied by the present invention.

(2) DC side DC/DC switching

When the DC side of the microgrid is switched from the grid-connected state to the off-grid state, it can be divided into two cases: planned off-network and unplanned off-net.

Planned off-network means that the system obtains state switching instructions in advance, and systematically adjusts the relationship between distributed generation and output of each energy storage unit and controllable load, while ensuring power supply to important loads in the system.

Unplanned off-network is the control process when the grid fails or is disconnected. When the AC side of the microgrid is connected to the large grid, the interconnected PCS operates in the constant voltage control mode. At this time, the process of connecting the DC side of the microgrid to the grid is complicated, and the voltage stability is The influence is relatively large, which is the main research object of the DC side of the micro-grid connected to the network. The main switching process of the grid-connected network is as follows: the protection device detects the voltage amplitude and frequency anomaly of the AC side of the micro-grid, sends a disconnection command to the grid-connected switch, and sends a mode switching command to the interconnected PCS by the switch position feedback signal, interconnecting the PCS. The DC side energy storage interface of the microgrid communicates with the dry contact signal. After the interconnected PCS receives the switch position signal, the operation mode is switched from the constant voltage control mode to the standby or constant power control mode, and the energy storage DC converter is switched from the charging and discharging modes to The constant voltage mode controls the DC bus voltage, and the grid switch controls the disconnection. The three cooperate to complete the seamless switching of the system from the grid to the off-grid. Figure 15 shows the variation of the DC bus voltage when simulating the process of grid-connected off-grid in Simulink.

In Figure 15, at 1.5 s, the interconnected PCS is disconnected due to AC side fault or other reasons. The coordinating controller detects that the PCS is disconnected and sends a switching operation mode command to the energy storage converter. The energy storage converter is charged by constant power. The discharge control mode is switched to the constant voltage mode to maintain the DC bus voltage. In this process, due to the power imbalance, the voltage rises briefly to 404V, and the regulation effect of the energy storage rapidly decreases.

When the AC side is connected to the large grid and the interconnected PCS is operating in the constant power control mode, the DC-connected DC-connected network process has the same effect on the DC side of the micro-grid as the DC-side input load, and will not be discussed here.

When the grid is restored and the AC side is connected to the grid, the coordination controller sends instructions to the interconnected PCS. After the interconnected PCS receives the signal, it communicates with the DC side energy storage converter using dry contacts, and the grid converter operates. Switching from the standby or constant power control mode to the constant voltage control mode, after the DC side energy storage converter receives the interlock signal, it switches from the voltage regulation mode to the constant power charge/discharge or standby mode, and the three cooperate to complete the system off-grid. Seamless switching of the web.

(3) Micro-grid AC side/off-network switching

1) Grid-connected network process

The switching of the AC side of the micro-grid from the grid-connected to the off-grid state can also be divided into two types: planned off-network and unplanned off-network.

Unplanned off-network process, when the large power grid fails or the PCC is disconnected, the PCC communicates with the AC side energy storage and the interconnected PCS signals, the energy storage is converted from the constant power control mode to the V/F control mode, and the interconnected PCS is converted by the constant voltage mode. It is a constant power control or standby mode, and its power is locked to the power of the interconnected PCS at the moment of switching. If the stored energy is discharged at the maximum discharge power or the maximum charging power is still unable to maintain the voltage and frequency of the AC side of the microgrid, the power of the interconnected PCS can be adjusted to an appropriate value to achieve the purpose of maintaining the stability of the AC side of the microgrid.

The voltage of the AC busbar during unplanned off-grid is shown in Figure 16, and the frequency is shown in Figure 17.

At 0.5 seconds, the PCC is disconnected and the energy storage is switched from the PQ control mode to the V/f control mode. It is known from Figures 16 and 17, that the voltage appears to have a certain jitter, but it quickly returns to normal, and the bus frequency is basically constant.

2) Off-net to grid connection process

When the large power grid resumes stability, the coordination controller issues a synchronous closing command to the PCC, and the PCC performs the synchronous closing. The energy storage is converted from the V/f control to the PQ control, and the micro-grid AC side completes the off-grid-to-network connection process.

The invention also discloses a microgrid control structure, and the three-layer control structure adopted by the microgrid is as shown in FIG. 18.

(1) First layer control

The DG or energy storage device can often operate in a variety of control modes depending on the condition of the microgrid, including maximum power point tracking (MPPT) control, constant voltage control, constant voltage/constant frequency (V/f) control, and the like.

When the microgrid is connected to the grid, the distribution network ensures that the voltage and frequency of the AC side of the microgrid are stable, and the DG is often output with maximum power; the DC side of the microgrid adopts the master-slave control mode, and the AC side or energy storage of the microgrid is used as the main power source to maintain DC bus voltage.

When the microgrid is operating off-grid, a certain DG in the AC side of the microgrid ensures voltage and frequency stability. Usually, the energy storage device adopts V/f control to ensure that the voltage and frequency are at specified values, and other DGs use constant power control (PQ control) and output at maximum power; the energy storage device on the DC side of the microgrid adopts constant voltage control as the main power source. Other distributed DGs are output at maximum power. The energy storage device needs to be able to compensate for the power difference caused by the load change. Therefore, the energy storage must be able to accurately control the output within its allowable charge and discharge power range, and quickly compensate the power difference.

(2) Layer 2 control

The second layer is the coordinated control layer, which functions to reduce the negative impact of the distribution network due to DG output and load changes when the microgrid is in the grid-connected state. The microgrid can be integrated into the distribution as a stable and controllable load. Network, through the micro-grid coordination controller to coordinate control of each unit to achieve tie line power control; when the micro-grid is in the off-grid state, through the master-slave control to ensure that the voltage and frequency are within the specified range, the main power supply compensates for the DG output or The difference in power generated by load changes.

This layer control can also implement the switching function between the MG modes. In order to reduce the impact of this function on the operating parameters of the microgrid, it is necessary to have functions such as MG fault detection and synchronous detection, and coordinate switching between control modes. In this paper, the master-slave control mode is adopted. The control timing is shown in Figure 19. The main control mode of each DG in the MG is switched. When the control mode of the main power source is switched, the output power fluctuation should be as small as possible. Figure 20 shows the control structure used in this paper. The PQ control and V/f control of the main power supply use the same current inner loop. When the switching control method is used, only the voltage outer loop is switched. In this process, in order to minimize the impact generated when the control mode is switched, the control logic and algorithm need to be applied reasonably.

(3) Layer 3 control

This layer mainly realizes the energy management function of the microgrid, and optimizes the operation of the microgrid through corresponding optimization algorithms:

1) When the microgrid is in the grid-connected state, calculate the optimal power value of the tie line between the microgrid and the distribution network (as the reference value of the second-layer control target of the micro-grid);

2) When the microgrid is in the off-grid state, adjusting the information of each distributed DG output power reference value, the microgrid can be in the most economical operation state.

Claims

An AC/DC hybrid microgrid operation module conversion method, characterized in that the AC/DC hybrid microgrid comprises an AC microgrid and a DC microgrid, and the AC microgrid and the DC microgrid are connected by an AC/DC interconnect converter PCS ,

The AC microgrid includes an AC bus, a PV array PV, an energy storage device, and an AC side load, wherein the PV array, the energy storage device, and the load are connected to the AC bus; the AC microgrid is switched to the grid through the PCC. Mode or off-grid mode of operation;

When the PCC is closed, the AC-DC interconnect converter PCS operates in a constant voltage control mode, and the AC-side grid-connected operation mode includes a first AC operation mode.

When the PCC is disconnected, the AC/DC interconnect converter PCS is switched to the PQ control mode, and the AC side off-network operation mode includes a second AC operation mode, a second two AC operation mode, and a second three AC operation mode;

The first alternating current operation mode is that all the photovoltaic arrays PV are operated in the MPPT mode, the energy storage device is standby or charged and discharged, and the AC side load is all put into use;

The second alternating current operation mode is that the energy storage device operates in the V/f mode, and all the photovoltaic array PVs are operated in the MPPT mode, and the AC side loads are all put into use;

The second two alternating current operation mode is that the energy storage device operates in the V/f mode, and part of the photovoltaic array PV is cut off, and the remaining photovoltaic array PVs are operated in the MPPT mode, maintaining the frequency and voltage of the alternating microgrid, and the AC side loads are all put into use;

The second three AC operation mode is that the energy storage device first outputs the maximum power, and the AC/DC interconnection converter PCS increases the power input to the AC microgrid or cuts off part of the AC side load until the energy storage device recovers V. /f control mode, all PV array PVs are operated in MPPT mode;

When the PCC is closed, the AC microgrid operates in a first AC mode of operation;

When the PCC is disconnected, the AC microgrid is switched from the first alternating current operating mode to the second alternating current operating mode;

When P PV +P PCS -P bch-max >=P load , the AC microgrid switches from the second AC operating mode to the second AC operating mode;

When the P PV +P PCS -P bch-max <P load , the AC microgrid switches from the second alternating current operating mode to the second alternating current operating mode;

When the P PV +P PCS +P bdi-max <P load , the AC microgrid switches from the second AC mode to the second AC mode;

When P PV +P PCS +P bdi-max >P load , the AC microgrid switches from the second three AC operation mode to the second AC operation mode;

Where, P PV is the power output of the PV array PV, P PCS is the power of the AC/DC interconnect converter PCS flowing into the AC side of the microgrid, P bch-max is the maximum charging power of the energy storage device, and P bdi-max is the storage The maximum discharge power can be installed, and P load is the power consumed by the AC side load;

The DC microgrid includes a DC bus, a second PV array PV, a second energy storage device, and a DC side load, and the second PV array PV, the second energy storage device, and the DC side load are connected to the DC bus;

When the PCC is closed and the AC/DC interconnect converter PCS adopts a constant voltage control mode, the operating mode of the DC microgrid includes a first DC operating mode;

When the AC/DC interconnect converter PCS adopts the PQ control mode or standby, the operating mode of the DC microgrid includes a second direct current operation mode, a second two-DC operation mode, and a second three-DC operation mode;

The first DC operating mode is to maintain the DC bus voltage at the first voltage by using the AC/DC interconnect converter PCS, all the second PV array PVs are operated in the MPPT mode, and the second energy storage device performs charging and discharging control, the DC side The load is fully put into use;

The second direct current operation mode is to maintain the DC bus voltage at the second voltage by using the second energy storage device, all the second photovoltaic array PVs are operated in the MPPT mode, and the AC/DC interconnection converter PCS performs charging and discharging control, and the DC The side loads are all put into use;

The second two-DC operation mode is that the second energy storage device operates in a maximum power charging or full charge standby state, and a part of the second photovoltaic array PV is cut off, and the remaining second photovoltaic array PVs are operated in the MPPT mode, so that the second energy storage device Operate in constant voltage control mode to maintain DC bus voltage, DC side load is fully put into use;

The second three-DC operation mode is to cut off part of the DC side load so that the discharge power of the second energy storage device is less than its maximum discharge power until the second energy storage device operates in the constant voltage mode, and all the second PV array PVs are in the MPPT mode. run;

Switching from the second all-current operation mode to the second two-DC operation mode when P PV2 -P PCS2- P bch-max2 >=P load2 ;

Switching from the second two-DC operation mode to the second constant-current operation mode when P PV2 -P PCS2- P bch-max2 <P load2 ;

Switching from the second all-current operation mode to the second three-DC operation mode when P PV2 -P PCS2+ P bdi-max2 <P load2 ;

Switching from the second three-DC operation mode to the second constant-current operation mode when P PV2 -P PCS2+ P bdi-max2 >P load2 ;

Where, P PV2 is the output power of the DC side photovoltaic power of the micro grid, P PCS2 is the power of the DC bus input to the AC/DC interconnect converter PCS, and P bch-max2 is the maximum charging power allowed by the second energy storage device, P load2 For the power of the DC side load, P bdi-max2 is the maximum discharge power of the second energy storage device.
The AC/DC hybrid microgrid operation module conversion method according to claim 1, wherein the energy storage device comprises a battery and a super capacitor.
The AC/DC hybrid microgrid operation module conversion method according to claim 1, wherein the first voltage is 400V.
The AC/DC hybrid microgrid operation module conversion method according to claim 1, wherein the second voltage is 400V.
The AC/DC hybrid microgrid operation module conversion method according to claim 1, wherein the second energy storage device performs constant power charge and discharge control in the first DC operation mode.
The AC/DC hybrid microgrid operation module conversion method according to claim 1, wherein

Both the photovoltaic array PV and the second photovoltaic array PV are represented by the following formula:

Where, P PV is the output power of the photovoltaic array, f PV is the power derating factor of the photovoltaic system, indicating the ratio of the actual output power of the photovoltaic system to the output power under the rated condition; Y PV is the PV array capacity; I T is the actual illumination ; I S is the illuminance under standard test conditions; α p is the temperature coefficient of power; T cell is the surface temperature of the current photovoltaic cell; T cell, STC is the temperature of the photovoltaic cell under standard test conditions.
The AC/DC hybrid microgrid operation module conversion method according to claim 1, wherein the energy storage device comprises a battery, and the charge and discharge power of the battery is limited to:

-P bat,cmax ≤P bat ≤P bat,dmax

P bat,dmax =η bat,d P bat,cmax,kbm

Where η bat,c is the charging efficiency of the battery; η bat,d is the discharge efficiency of the battery, P bat,cmax,kbm is the maximum allowable charging power of the battery single step P bat, dmax,kbm is the maximum charging and discharging power of the battery single step, P Bat,cmax,mcc are the maximum charging powers that take into account the maximum charging current of the battery. P bat,cmax,mcr are the maximum charging powers that are limited by the maximum charging rate of the battery.

Where N bat is the total number of battery strings connected in parallel; I max is the maximum charging current of the battery; U N is the rated voltage of the battery; α c is the maximum charging rate of the battery, and W is the total energy stored by the battery at any time, which is equal to the available The sum of energy and binding energy, namely: W = W 1 + W 2 , where W 1 is the available energy; W 2 is the binding energy; the available energy of the battery after charging and discharging is represented by the following formula:

Where W 1,0 is the available energy of the battery at the initial time; W 1,end is the energy available to the battery at the end time; W 0 is the total energy of the battery at the initial time; P is the discharge or charging power of the battery; Δt is the time Interval; c is the ratio of battery capacity, indicating the ratio of available energy to total energy in the fully charged state of the battery; k is the battery rate constant;

The binding energy of the battery pack after charging and discharging:

W 2,end =W 0 -PΔt-W 1,end

Where W 2,end is the binding energy of the battery pack at the end time;

The available energy W 1 at any time satisfies the following formula: 0 ≤ W 1 ≤ cW max , where W max represents the maximum storable energy of the battery.
The AC/DC hybrid microgrid operation module conversion method according to claim 1, wherein

The AC-DC interconnect converter PCS is represented by the following formula:

Where: P con, AC represents the AC side power of the converter, positive in the inverter, negative in the rectification; P con, DC respectively represent the sum of the DC side power of the converter, η inv and η rec respectively represent the current Inverter and rectification efficiency; R inv and R rec represent the maximum active power of the converter inverter and rectification, respectively.