CN112072697B - Modular direct-current microgrid structure reconstruction method and operation control method - Google Patents
Modular direct-current microgrid structure reconstruction method and operation control method Download PDFInfo
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Abstract
A reconstruction method and an operation control method of a modular direct-current microgrid structure are based on a traditional energy router structure applied to an alternating-current and direct-current microgrid, a converter of each module is connected to the same direct-current bus in the modular direct-current microgrid to form a generalized energy router structure, and the operation of a system is divided into three modes: the system comprises a grid-connected PQ mode, a superior power grid participation regulation mode and an island mode, a modularized direct current micro-grid structure is realized based on a generalized energy router structure, information interaction and internal energy management are carried out by means of the structure, and a fault part is cut off in time and structure reconstruction is carried out. The invention can reduce the number of the circuit breakers in the direct current micro-grid, can ensure the flexible and reliable operation of the direct current micro-grid under any condition to the maximum extent, ensures the minimum output of the stored energy and the large grid, prolongs the service life of the battery and reduces the cost of purchasing electricity to the large grid.
Description
Technical Field
The invention belongs to the field of direct current micro-grid modular structures and operation control, and relates to a modular direct current micro-grid structure reconstruction method and an operation control method thereof.
Background
In recent years, the proportion of distributed motors capable of efficiently utilizing clean energy in a power grid gradually increases, and as a micro-grid is a system for coordinating contradictions between a large power grid and a distributed power supply, is an important component of a future intelligent power distribution network and represents the development trend of a future distributed energy power supply system, the national energy agency provides a great advance for research and construction of the micro-grid through documents such as guidance suggestions about promoting construction of a new energy micro-grid demonstration project and notices of a power distribution network construction transformation action plan (2015-2020). The micro-grid generally has the advantages of high energy utilization efficiency, high energy supply reliability, less pollutant discharge, good running economy and the like.
Among them, the DC microgrid does not need an energy conversion device formed by a DC/AC or AC/DC converter when each power generation unit and load are connected, so that the cost can be reduced, the loss can be reduced, mutual coordination control between each micro source is facilitated, and the problems of reactive loss and phase deviation do not exist, so that the DC microgrid is deeply researched by students. Due to the rapid development of power electronics, the application of the power electronics in micro-grids is also becoming more and more widespread. Energy routers represented by application in alternating current and direct current micro-grids become hot contents of research through flexible power flow control and wide-area information interaction. Energy routers can provide diverse functions, such as: the flexible, adjustable and bidirectional flow of energy between the micro-grid and a superior grid is realized through control; the energy management in the micro-grid is completed, and the efficiency, reliability and economy of the system are improved; and wide-area data acquisition is carried out from various devices in real time, and the wide-area data acquisition is provided for a control center of the micro-grid and a main grid control center to assist in completing the functions of operation state detection and fault diagnosis. The literature, "research on energy router-based microgrid control strategy" explains in detail the advantages of energy routers in an alternating-current microgrid, and analyzes the specific operation mode and hierarchical control of the energy routers. The direct-current micro-grid has the characteristics of small scale, single system stability standard measurement, stable direct-current bus voltage and the like, and is favorable for matching of each subsequent power generation unit and selection of a control method. Therefore, the direct current micro-grid and the direct current micro-grid are combined to form a new structure, the advantages of simple structure, convenience in control, flexibility in operation and the like of the direct current micro-grid are achieved, and the structure reconstruction is facilitated.
However, the dc micro grid is still not very widely constructed and applied under the conditions of outstanding advantages, easy control and vigorous development of power electronics. After the national institute of government issued the text of electricity change No. 9 in 2015, the construction of the direct-current micro-grid in China is strongly supported in policy, but the construction is not yet fully laid out. The reason for this is that the direct current microgrid lacks large moment of inertia, resulting in too small output damping, too low inertia, and is susceptible to frequent external interference and load fluctuation. The method mainly comprises the steps of large one-time investment, slow output and relatively complex technology of a micro-grid project, so that the inherent requirements of the construction and the driving force of the economic level are insufficient. Therefore, modules are divided into modules according to the operation stability of the microgrid, so that the modules have different stabilizing effects on the microgrid, the microgrid is not required to be planned from the beginning during design, the modules are simplified into combinations of different modules, later construction is facilitated, and the 'constructed microgrid' is changed into a 'combined microgrid'.
The document "design and research of a novel modular microgrid" shows a design scheme of a novel modular new energy microgrid by taking a microgrid type charging station designed for a certain logistics park in actual engineering as an example, the whole photovoltaic is divided into one module, and the stored energy is divided into another module as compensation. This design is through surveying on the spot, and abundant illumination resource of make full use of, and the several large-scale warehouse top in garden possesses the structure of installation roof photovoltaic. The modular scheme has the advantages that the modular scheme is combined with the existing power equipment in the current market, and the existing power products are utilized as much as possible or the available products are simply transformed and combined. The document "Hybrid AC _ DC micro controlled architecture with comprehensive control for energy management of smart building" refers to a cell as a micro grid, where each building is a separate module, so as to ensure the internal stability of each module, i.e. the output of the power generation unit and the output of the load unit are substantially the same, and maintain a self-balancing mode. The modularization researched at the present stage is basically that the geographic position characteristics of the micro-grid are combined to divide modules, so that the design during the construction of the power grid is simplified, and the subsequent extension is facilitated.
Disclosure of Invention
The invention aims to provide a modular direct-current microgrid structure reconstruction method and an operation control method thereof. A generalized energy router structure is provided, a modularized direct current micro-grid structure is realized, information interaction and internal energy management are carried out by means of the structure, and a fault part is cut off in time and structure reconstruction is carried out. The structure ensures stable operation of the system, simplifies the structure of the direct current structure control target and the modular unit, is favorable for flexible and stable reconstruction of the structure and is favorable for later expansion. Based on the structure and the operation control method thereof, the flexibility, stability and reliability of the direct-current micro-grid are improved, and the control of the direct-current micro-grid is facilitated.
In the direct-current micro-grid, according to the difference between the load and the geographical position and the function of the distributed motor, the part in the area is called as a 'module', and the modular structure facilitates the selection of a control method and the unification of subsequent construction. The controllable distributed motor, the energy storage, the diesel generator and the like are divided into controllable units; dividing uncontrollable distributed motors, loads and the like into uncontrollable units; the upper-level grid and the corresponding bidirectional converter are divided into 'upper-level grid units'. Each module may contain one or more controllable units and/or uncontrollable units or one of the superordinate grid unit, controllable unit or uncontrollable unit, and the different modules thus produced have different effects on the dc microgrid. Based on a traditional energy router structure applied to an alternating current-direct current micro-grid, a converter of each module is connected to the same direct current bus in a modularized direct current micro-grid, a switch for cutting off faults and disconnecting the modules from the grid is installed on the bus to form a generalized energy router structure, and the energy, the operation mode and the faults of the micro-grid are managed and detected so as to facilitate structural reconstruction during faults. The operation and control method of the structure is designed, and the operation of the system is divided into three modes: the method comprises the steps of designing a grid-connected PQ mode, a superior power grid participation regulation mode and an island mode, and designing a distributed motor control method of a controllable unit when a generalized energy router on-off structure is reconstructed in each mode.
In order to achieve the purpose, the invention adopts the technical scheme that:
a modular direct current micro-grid structure reconstruction method and an operation control method are provided, and the modular direct current micro-grid structure is characterized in that each module is connected to the same direct current bus through a generalized energy router. The generalized energy router comprises converters of all the modules and a circuit breaker for controlling the on-off of the modules. The different modules divided according to different functions on the direct current micro-grid are mainly divided into three types: the module I only comprises an uncontrollable unit and a superior power grid unit; a second module comprising an energy storage controllable unit and an uncontrollable unit; and a third module comprising a controllable unit and an uncontrollable unit of the micro gas turbine.
(1) A first module: the module is connected with a superior power grid, and additional controllable distributed motors or energy storage and other controllable units are not needed for regulation and control. The upper grid unit in this module therefore comprises: the bidirectional converter is connected with a superior power grid, and the superior power grid is equivalent to an infinite power supply; the uncontrollable unit comprises: and nearby uncontrollable distributed generators such as loads, photovoltaics, wind power and the like. Only one such module is included in the dc microgrid.
(2) And a second module: in the direct current micro-grid, because a large number of uncontrollable units which fluctuate randomly exist, the uncontrollable units are balanced locally. For economic reasons, it is impossible to include all micro gas turbines in the controllable modules balanced in place, so the energy storage is selected to balance fluctuation in real time, peak clipping and valley filling. The controllable unit in this module only contains the stored energy; the uncontrollable unit comprises a nearby load and an uncontrollable distributed generator such as photovoltaic, wind power and the like. One or more modules can exist in the direct-current microgrid, and the modules are the modules with the highest occurrence frequency.
(3) And a third module: the controllable module containing the energy storage in the direct-current micro-grid rapidly realizes real-time balance of fluctuation, but the charging and discharging times of the energy storage have great influence on the service life of the battery, so that secondary regulation of the controllable module containing the micro gas turbine for balancing voltage fluctuation is applied for economic consideration. The controllable units in this module contain energy storage and micro gas turbines or only micro gas turbines; the uncontrollable unit comprises a nearby load and an uncontrollable distributed generator such as photovoltaic, wind power and the like. Since the micro gas turbine is to be included on the longest divided path to the maximum extent when the generalized energy router control module is turned off, the micro gas turbine is generally in the middle portion of the dc microgrid, and there is generally only one module in the dc microgrid.
The content of the specific units contained in each module is shown in the following table.
TABLE 1 Each Module contains the specific Unit content
According to the module structure, the operation modes are divided into three types according to the action of a superior power grid on a direct-current micro-power grid, wherein the three types are respectively as follows: a grid-connected PQ mode, a superior grid participation control mode and an island mode. The upper-level power grid sends information to the generalized energy router in the direct current micro-grid, and mode conversion is carried out by the generalized energy router.
(1) Grid-connected PQ mode: in this mode, the upper grid is in demand for the dc microgrid and cannot participate in the bus voltage regulation of the dc microgrid, so the bidirectional converter adopts PQ control. Transmission of PQ-controlled reference value P from a superordinate network to a direct-current microgridrefAnd QrefAnd the bidirectional converter of the direct current micro-grid is used for receiving, and the upper-level grid at the moment is similar to the uncontrollable unit part. The interior of the system needs to satisfy the formula (1),
Pcm＝Pload+pgrid(1) in the formula, PcmIs the output power of the controllable unit, PloadAs an uncontrolled unitOutput power, PgridThe output power of the power grid to the micro-power grid can be positive or negative.
Therefore, when the direct-current bus voltage of the direct-current micro-grid system is too high and the output power of the micro gas turbine reaches the lowest limit value, energy is absorbed by stored energy, and if the stored energy reaches the SOC reaching the upper limit, wind and light are abandoned to the uncontrollable unit; and when the voltage of a direct current bus of the direct current micro-grid system is reduced, the energy storage SOC reaches the lower limit and the output of the micro gas turbine reaches the upper limit, correspondingly cutting off the load of the uncontrollable unit.
(2) The upper-level power grid participates in a regulation mode: in this mode, the higher-level grid participates in the regulation of the direct-current microgrid, and the controllable units of the higher-level grid and the direct-current microgrid share the regulation of the direct-current bus. Based on the economic principle, the interior of the direct-current micro-grid needs to meet the basic self-sufficiency, and when the higher-level grid is required to be regulated, the power requirement on the higher-level grid is also reduced as much as possible. Therefore, the bidirectional converter adopts droop control, the specific control process is as the formula (2),
in the formula of UdcIs the DC bus voltage value, U* dcIs the rated value of DC bus voltage, k is droop coefficient, P is output power value0Is the output power rating.
In the mode, the bidirectional converter adopts droop control, so that quick response to voltage fluctuation of a direct current bus is met, electricity can not be purchased to a superior power grid at the voltage recovery rate, electricity purchasing cost is reduced, and electricity is guaranteed to be sold to the superior power grid when the output power of the micro gas turbine reaches the minimum limit value and the stored energy reaches the SOC (state of charge) to reach the upper limit. The electricity purchasing cost is saved to the maximum extent, and wind abandonment, light abandonment and load abandonment are not carried out.
(3) Island mode: in this mode, the bidirectional converter is disconnected, so that the upper-level power grid is not connected with the direct-current micro-grid and does not participate in the regulation of the direct-current bus voltage. The direct current microgrid needs to automatically balance the fluctuation generated by the uncontrollable unit, the running working condition of the direct current microgrid is similar to the working condition in the grid-connected PQ mode, and the bidirectional converter plays a role in isolation.
In the above three operation modes, when the generalized energy router detects that the output power of a certain module has a problem, it sends information to the control centers of the microgrid and the superior power grid, and the control centers send out the on-off signal of the module to reconfigure the structure of the dc microgrid, so that the control method inside the unit will change to some extent, and the specific content is as follows.
(1) In a grid-connected PQ mode, when a generalized energy router disconnects a certain module, the direct current microgrid structure is reconstructed into a small direct current microgrid structure consisting of a plurality of modules or modules. At this time, the direct current microgrid module structure comprising the module III has a micro gas turbine with constant voltage control, so the module structure has stable voltage, and the control method of the units in the module does not need any change. When the module I is independently arranged in a direct current module structure, the direct current micro-grid is required to send signals to a superior grid because the structure does not contain a controllable unit at the moment, and P is changedrefAnd QrefSo as to match the module with the uncontrollable unit in the module, thereby ensuring the stable bus voltage in the module structure. When the second module exists alone or together with the first module in a direct current microgrid module structure, in order to meet the requirements of the uncontrollable unit and the first module, the energy storage control mode of the controllable unit in the second module needs to be changed into secondary control of droop control to meet the voltage stability of a direct current bus in the module, the secondary regulation of droop is as shown in a formula (3),
in the formula of UdcIs the DC bus voltage value, U*’ dcIs the rated value of the voltage of the secondary DC bus, k is the droop coefficient, P is the output power value, P0To output a rated power value, where U*’ dcThe calculation formula (2) is as shown in (4),
(2) under the condition that a superior power grid participates in a regulation mode, when a generalized energy router switches on or off a certain module, the direct-current micro-grid structure is reconstructed into a small direct-current micro-grid structure formed by a plurality of modules or modules. At this time, the module structure including any one of the first module and the second module is subjected to droop control, so that the energy storage of the controllable unit in the module or the control mode of the bidirectional converter is directly changed into secondary droop control. When the first module and the second module exist in the same direct current micro-grid module structure, only the energy storage droop control is converted into secondary droop control, so that the economic principle is met. The internal control method of the direct current micro-grid module structure containing the module III is consistent with the control method.
(3) In island mode, when a generalized energy router disconnects a module, there is no module one in the structure. The second module is independently arranged in the modular structure, and the droop control of the stored energy is changed into secondary droop so as to ensure the voltage stability of the direct current bus. The internal control method of the direct current micro-grid module structure containing the module III is consistent with the two methods.
The invention has the beneficial effects that:
(1) the generalized energy router is integrated into the direct-current micro-grid, faults in the module are effectively isolated, the faults are guaranteed not to be transmitted to any part of the system, the number of circuit breakers in the direct-current micro-grid is reduced, stability is improved, and cost is reduced. At the moment, the direct current microgrid carries out structural reconstruction, the control mode of the internal units is changed, and the voltage of each part of the bus of the direct current microgrid is ensured to be in a stable state.
(2) The control target of the structure is simple-the direct current bus voltage is stabilized near the rated value. Each module divided according to the geographic position and the function in the direct current micro-grid has a specific structure and function, and selection of a control mode of a unit inside the module is facilitated.
(3) The components of the direct current micro-grid are divided into different units according to different geographical positions and balanced output of the direct current micro-grid, and different modules have different balancing and stabilizing effects on the direct current micro-grid due to different units formed in the modules. The modularized structure avoids the need of specific design when the direct-current micro-grid is established every time, and corresponding modules can be selected to form the direct-current micro-grid according to requirements, so that the designed micro-grid is converted into an assembled micro-grid, and the micro-grid and subsequent expansion are conveniently established.
(4) Under the normal working condition, the control mode of the energy storage part is droop control; when a generalized energy router switches off a certain module, the droop control needs to be changed into secondary droop control according to requirements so as to ensure the voltage stability of a direct current bus in the module structure. And the bidirectional converter of the superior power grid selects PQ control, droop control or isolation disconnection according to different operation modes. When a certain module is disconnected under the condition of grid connection, the bidirectional converter needs to ensure enough response to an uncontrollable unit of a module structure, and changes a corresponding numerical value of PQ control or converts the corresponding numerical value into secondary droop control to ensure that a bus is stable. The micro gas turbine needs to be controlled in a constant voltage mode all the time, and does not need to be changed due to mode change or breaker break-off, because enough power support is provided for the direct current micro grid to stabilize the bus voltage. The control mode can ensure that the energy storage operates efficiently, flexibly and quickly to the maximum extent, the output of the energy storage and the large power grid is minimum, the service life of the battery is ensured, and the cost of purchasing electricity to the large power grid is reduced.
Drawings
FIG. 1 is a block diagram of a modular DC microgrid with a generalized energy router;
FIG. 2 is a diagram of a DC microgrid architecture deployed in cells;
FIG. 3 is a diagram of module information interaction in different operating modes;
fig. 4 is a generalized energy router operational control block diagram.
Detailed Description
The module division of the direct current micro-grid, the composition of the module internal units and the operation control method of the system are described in detail with reference to the attached drawings, as follows:
the modularized direct-current micro-grid is characterized in that each component of the direct-current micro-grid is divided into a controllable unit, an uncontrollable unit and a superior grid unit according to functions of the direct-current micro-grid, each unit is divided into a first module, a second module and a third module according to geographical positions and contained components, and the modules are connected to the same direct-current bus through a generalized energy router. The generalized energy router is formed by a current transformer common bus of each module and a switch which is arranged on the bus and used for isolating faults and modules from a network, manages and detects the energy, the operation mode and the faults of the microgrid, has an information interaction function, and can transmit the information of the operation mode and the faults to each current transformer so as to be convenient for the structural reconstruction of the faults, and the specific module direct current microgrid structure containing the circuit breaker is shown in figure 1.
In the figure, the modular structure is a first module consisting of a superior power grid unit and an uncontrollable unit, a second module consisting of a controllable unit A and an uncontrollable unit, a third module consisting of a controllable unit B and an uncontrollable unit, and a generalized energy router consisting of a first line, a second line, a breaker a, a breaker B, a breaker c and a breaker d. The types, the number and the connection positions of the modules are determined according to the actual requirements of the required direct current micro-grid.
The cell contents included in each block are developed specifically to obtain a structure diagram shown in fig. 2. As will be described in more detail below,
the module is composed of an upper power grid module 9 and an uncontrollable unit. The upper power grid module 9 is formed by connecting the upper power grid 5 to a transformer 7 through an alternating current transmission line 6 to convert high voltage into low voltage, connecting the converted high voltage into low voltage to a bidirectional converter 8 through the alternating current transmission line 6, and connecting the bidirectional converter 8 to a direct current bus. The bidirectional converter is a medium for connecting the direct-current micro-grid with a superior grid, and receives superior grid signals to determine the operation mode of the direct-current micro-grid: and the PQ mode and the droop mode realize interaction between a superior power grid and a direct-current micro-power grid and play a role in isolation to enable the direct-current micro-power grid to run in an isolated island mode. The uncontrollable unit is composed of a load 1 and an uncontrollable distributed motor 2. The load 1 is all loads nearby, the uncontrollable distributed motor 2 comprises photovoltaic power, wind power and the like, and the control mode is the MPPT mode. The direct-current micro-grid only comprises one module I.
The second module consists of a controllable unit A and an uncontrollable unit. The controllable unit a is composed of controllable distributed motors 4 and may contain a plurality of collections. In the direct current micro-grid, because a large number of uncontrollable units which fluctuate randomly exist, the uncontrollable units are balanced locally. In consideration of economy, all the micro gas turbines cannot be contained in the controllable modules balanced on site, energy storage real-time balance fluctuation is selected, peak clipping and valley filling are performed, so that the controllable distributed motors 4 in the controllable units A are energy storage or multiple energy storage, droop control is selected in a control mode, and the charging and discharging frequency of the energy storage is reduced while the energy storage is utilized to the maximum extent. In order to ensure the stability of the bus voltage when the bus voltage is independently operated, drooping secondary control needs to be set. One or more modules can exist in the direct-current microgrid, and the modules are the modules with the highest occurrence frequency.
And the third module consists of a controllable unit B and an uncontrollable unit. The controllable unit B is composed of a controllable distributed motor 3. The controllable module containing the energy storage in the direct-current micro-grid rapidly realizes real-time balance of fluctuation, but the charging and discharging times of the energy storage have great influence on the service life of the battery, and secondary regulation of the voltage fluctuation is balanced by the controllable module containing the micro gas turbine in consideration of economy. Therefore, the controllable distributed motor 3 is a micro gas turbine, and the control mode is constant voltage control in order to ensure the stability of the direct current bus voltage. Since the generalized energy router control module is turned off and the micro gas turbine is included to the maximum extent on the longest path divided by the circuit breaker, the micro gas turbine is generally in the middle of the dc microgrid, and there is generally only one module in the dc microgrid.
The generalized energy router is composed of a current transformer, a circuit breaker a, a circuit breaker b and a circuit breaker c in a module I, a module II and a module III, and information interaction between the generalized energy router and a central controller is shown in fig. 3. The control of the system realizes flexible, adjustable and bidirectional energy flow between the direct-current micro-grid and the upper-level grid, completes internal energy management of the direct-current micro-grid, collects data in real time and sends the data to control centers of the direct-current micro-grid and the upper-level grid, and assists in completing switching of operation modes and reconstruction of a direct-current micro-grid structure when a module fails.
The operation modes of the modularized direct-current micro-grid can be divided into three types: a grid-connected PQ mode, a superior grid participation control mode and an island mode. Under different operation modes, when the generalized energy router detects a module fault and then switches on and off a circuit breaker, and a direct-current micro-grid structure is reconstructed, the control mode of elements in each unit inside the module may be changed accordingly. A control block diagram of the relationship between the generalized energy router and the operation mode switching is shown in fig. 4, and its specific contents are as follows.
(1) Grid-connected PQ mode: when the upper-level power grid controls the output power of the direct-current microgrid, the central controller sends an instruction signal to the direct-current microgrid central controller, and the microgrid central controller controls the bidirectional converter in the generalized energy router to send a signal, so that the control mode is PQ control. At this time, the inverter in the module I is subjected to PQ control, the inverter in the module II is subjected to constant voltage control, and the inverter in the module III is subjected to droop control. When the generalized energy router detects that a fault module exists in the direct-current micro-grid and the output power of the module is lower than the lower limit, the generalized energy router transmits information to the grid control center and sends an instruction to remove the fault module. Because each module is connected on same direct current bus, direct current microgrid structure carries out the reconsitution at this moment. If module one fails, its internal PQ control needs to be on PrefAnd QrefThe value is modified to meet the requirement that the bus power grid is in a stable range, the second module is connected with the third module, and the bus voltage can be stable without changing a control method. If the second module fails, the internal control mode needs to be changed into secondary droop control, namely U is changed* dcThe voltage of the direct current bus is stable, the first module is connected with the third module, and the bus voltage can be stable without changing a control method. If module three fails, PQ control in module one needs to be on PrefAnd QrefThe value is modified to meet the requirement that the bus power grid is in a stable range, and the control mode in the third module needs to be changed into secondary droop control, so that the bus voltage stability can be met.
(2) Higher-level power grid participation regulation and controlMode (2): when the upper-level power grid participates in the voltage control of the direct-current microgrid bus, the central controller sends an instruction signal to the direct-current microgrid central controller, and the microgrid central controller sends a signal to the bidirectional converter in the generalized energy router to control the bidirectional converter in a droop control mode. At this time, the converter in the module I is droop control, the converter in the module II is constant voltage control, and the converter in the module III is droop control. If the module I fails, the internal control mode needs to be changed into secondary droop control, namely U is changed* dcThe value meets the requirement of stable voltage of the direct current bus, the second module is connected with the third module, and the bus voltage can be stable without changing a control method. If the second module fails, the internal control mode needs to be changed into secondary droop control, namely U is changed* dcThe voltage of the direct current bus is stable, the first module is connected with the third module, and the bus voltage can be stable without changing a control method. If the module three fails, the control modes in the module one and the module three need to be changed into secondary droop control, so that the bus voltage stability can be met.
(3) Island mode: when the upper-level power grid does not participate in any regulation and control of the direct-current microgrid, the central controller of the upper-level power grid sends an instruction signal to the central controller of the direct-current microgrid, and the central controller of the microgrid sends a locking signal to the bidirectional converter in the generalized energy router. At this time, the load in the first module is balanced and absorbed by other modules, the current transformer in the second module is under constant voltage control, and the current transformer in the third module is under droop control. If the second module fails, the internal control mode needs to be changed into secondary droop control, namely U is changed* dcThe value meets the requirement of stable voltage of the direct current bus, and the module III can also meet the requirement of stable voltage of the bus without changing a control method. If the third module fails, the internal control mode of the second module needs to be changed into secondary droop control, namely, U is changed* dcThe value satisfies the voltage stability of the direct current bus.
The above-mentioned embodiments only express the embodiments of the present invention, but not should be understood as the limitation of the scope of the invention patent, it should be noted that, for those skilled in the art, many variations and modifications can be made without departing from the concept of the present invention, and these all fall into the protection scope of the present invention.
Claims (1)
1. A reconstruction method and an operation control method of a modular direct current micro-grid structure are characterized in that each module of the modular direct current micro-grid structure is connected to the same direct current bus through a generalized energy router; the generalized energy router comprises a current transformer of each module and a circuit breaker for controlling the on-off of the modules; the different modules divided according to different functions on the direct current micro-grid are mainly divided into three types: the module I only comprises an uncontrollable unit and a superior power grid unit; a second module comprising an energy storage controllable unit and an uncontrollable unit; a third module containing a controllable unit and an uncontrollable unit of the micro gas turbine;
the direct-current micro-grid only comprises a module I; one or more modules II can exist in the direct-current micro-grid; only one module III generally exists in the direct-current micro-grid; the content of the specific units contained in each module is shown in table 1:
TABLE 1 Each Module contains the specific Unit content
According to the module structure, the operation modes are divided into three types according to the action of a superior power grid on a direct-current micro-power grid, wherein the three types are respectively as follows: a grid connection PQ mode, a superior grid participation regulation mode and an island mode; the upper-level power grid sends information to a generalized energy router in the direct current micro-grid, and mode conversion is carried out by the generalized energy router;
(1) grid-connected PQ mode: in this mode, the upper-level grid has a demand on the direct-current microgrid and cannot participate in the bus voltage regulation of the direct-current microgrid, so that the bidirectional converter adopts PQ control; transmission of PQ-controlled reference value P from a superordinate network to a direct-current microgridrefAnd QrefAnd the bidirectional converter of the direct current micro-grid receives the data, and the superior grid at the moment is similar to the uncontrollable unit part; the system needs to satisfy the formula (1):
Pcm＝Pload+Pgrid (1)
in the formula, PcmIs the output power of the controllable unit, PloadIs the output power of the uncontrollable cell, PgridThe output power of the power grid to the micro-power grid can be positive or negative;
therefore, when the direct-current bus voltage of the direct-current micro-grid system is too high and the output power of the micro gas turbine reaches the lowest limit value, energy is absorbed by stored energy, and if the stored energy reaches the SOC reaching the upper limit, wind and light are abandoned to the uncontrollable unit; when the voltage of a direct current bus of the direct current micro-grid system is reduced, the energy storage SOC reaches the lower limit and the output of the micro gas turbine reaches the upper limit, correspondingly cutting off the load of the uncontrollable unit;
(2) the upper-level power grid participates in a regulation mode: in the mode, the upper-level power grid participates in the regulation of the direct-current micro-power grid, and the regulation of a direct-current bus is shared by the controllable units of the upper-level power grid and the direct-current micro-power grid; based on the economic principle, the bidirectional converter adopts droop control, and the specific control process is as the following formula (2):
in the formula of UdcIs the DC bus voltage value, U* dcIs the rated value of DC bus voltage, k is droop coefficient, P is output power value0Is the output power rating;
in the mode, the bidirectional converter adopts droop control, so that the electricity purchasing cost is saved to the maximum extent, and wind, light and load abandoning is not performed;
(3) island mode: in the mode, the bidirectional converter is disconnected, so that a superior power grid is not connected with the direct-current micro-grid and does not participate in the regulation of the direct-current bus voltage; the direct current microgrid needs to automatically balance the fluctuation generated by the uncontrollable unit, the running working condition of the direct current microgrid is similar to the working condition in the grid-connected PQ mode, and the bidirectional converter plays a role in isolation;
in the three operation modes, when the generalized energy router detects that the output power of a certain module has a problem, the generalized energy router sends information to the control centers of the micro-grid and the superior grid, and sends a switching-off signal of the module to enable the direct-current micro-grid to carry out structural reconstruction, at the moment, the control method in the unit changes, and the specific content is as follows;
a, under a grid-connected PQ mode, when a generalized energy router switches on or off a certain module, reconstructing a direct current micro-grid structure into a small direct current micro-grid structure consisting of a plurality of modules or modules; at the moment, the direct current microgrid module structure comprising the module III has a micro gas turbine with constant-pressure control, so that the module structure has stable voltage, and the control method of units in the module does not need any change; when the module I is independently arranged in a direct current module structure, the direct current micro-grid is required to send signals to a superior grid because the structure does not contain a controllable unit at the moment, and P is changedrefAnd QrefLet P standrefAnd QrefThe voltage of the bus in the module structure is ensured to be stable by matching with the uncontrollable unit in the module; when the second module exists alone or together with the module in a dc microgrid module structure, in order to meet the requirements of the uncontrollable unit and the first module, the energy storage control mode of the controllable unit in the second module needs to be changed into secondary control of droop control to meet the voltage stability of the dc bus in the module, and the secondary droop regulation is as shown in formula (3):
in the formula of UdcIs the DC bus voltage value, U*’ dcIs the rated value of the voltage of the secondary DC bus, k is the droop coefficient, P is the output power value, P0To output a rated power value, where U*’ dcThe calculation formula (4) is as follows:
b, under the condition that the upper-level power grid participates in a regulation mode, when a generalized energy router switches on or off a certain module, reconstructing the direct-current micro-grid structure into a small-sized direct-current micro-grid structure consisting of a plurality of modules or modules; at the moment, the module structure comprising any one of the first module and the second module is subjected to droop control, so that the energy storage of a controllable unit in the module or the control mode of the bidirectional converter is directly changed into secondary droop control; when the first module and the second module exist in the same direct-current microgrid module structure, only energy storage droop control is converted into secondary droop control so as to meet the economic principle; the internal control method of the direct current micro-grid module structure containing the module III is consistent with the method in the step a under the grid-connected PQ mode;
c, in an island mode, when a generalized energy router switches on or off a certain module, a module I does not exist in the structure; the second module independently exists in the modular structure, and the droop control of the stored energy is changed into secondary droop so as to ensure the voltage stability of the direct-current bus; and c, the internal control method of the direct-current microgrid module structure containing the module III is consistent with the method in the step a grid-connected PQ mode.
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