CN110649590B - Energy cooperative control method for networking type direct-current micro-grid - Google Patents

Abstract

The application provides a networking type direct current micro-grid energy cooperative control method, which comprises the steps of dividing different voltage level layers according to direct current bus voltage values of a direct current micro-grid, setting the voltage level layers into working modes, wherein at least one local unit in each working mode controls the direct current bus voltage, and keeping the power balance of a system; a control method is designed for each local unit in the direct current micro-grid, and different working modes are switched when the voltage of the direct current bus is changed. The networking type direct current micro-grid energy cooperative control method provided by the application can implement and adjust the working mode according to the working voltage value of the direct current micro-grid, maintain the power balance of the system and meet the requirement of 'plug and play' of each unit under the condition of no communication.

Description

A networked DC microgrid energy collaborative control method

Technical field

The present invention relates to the technical field of DC microgrid energy coordination, and in particular, to a networked DC microgrid energy collaborative control method.

Background technique

In the research on microgrids, DC microgrids connect distributed power sources and energy storage through DC buses to provide energy to the corresponding loads of the system. Since most of the electric energy generated by new energy power generation units such as photovoltaics is DC, the use of DC microgrids not only reduces The AC-DC conversion device is eliminated, reducing costs and unnecessary losses, and there are no problems such as frequency fluctuations and reactive power losses in the DC grid. Therefore, research on DC microgrids is becoming more and more popular.

Since distributed energy resources are usually intermittent and volatile, the microgrid system needs to be connected to a reliable public power grid. At the same time, with the development of communication technology and power electronics technology, traditional power grids are gradually transforming and developing into smart grids, which will Form an intelligent energy network based on energy router (ER). As a key device connecting the public grid and microgrid, the energy router can improve the consumption of distributed energy and the flexible use of electric energy.

The control methods of smart microgrids are mainly divided into master-slave control, peer-to-peer control and hierarchical control. Among them, hierarchical control is widely used in smart microgrid systems. At present, research on smart microgrids mainly focuses on energy router topology and microgrid system architecture, and there is less research on the interaction between microgrids and public power grids. Therefore, it is very practical to propose a method for distributed control of networked DC microgrids including energy routers, taking into account the SOC conditions and rated power limits of energy storage units, and achieving the "plug and play" requirements of each unit without communication conditions. Is necessary.

Contents of the invention

The purpose of this section is to outline some aspects of embodiments of the invention and to briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section, the abstract and the title of the invention to avoid obscuring the purpose of this section, the abstract and the title of the invention, and such simplifications or omissions cannot be used to limit the scope of the invention.

In order to solve the above technical problems, the present invention provides the following technical solution: a networked DC microgrid energy collaborative control method, including:

Divide different voltage levels according to the DC bus voltage value of the DC microgrid, set the voltage levels to each working mode, and have at least one local unit in each working mode to control the DC bus voltage to maintain system power balance;

The control method is designed for each local unit in the DC microgrid, and different working modes are switched when the DC bus voltage changes.

As a preferred solution of the networked DC microgrid energy collaborative control method of the present invention, the working mode is divided according to the DC bus voltage value, and the DC bus voltage value refers to the upper and lower limits of the DC bus voltage fluctuation range U _H2 ~ U _L2 and the upper and lower limits of the layer voltage value range U _H1 ~ U _L1 .

As a preferred solution of the networked DC microgrid energy collaborative control method of the present invention, the local unit includes an energy router, and the energy router connects the DC microgrid and the public power grid.

As a preferred solution of the energy collaborative control method of the networked DC microgrid of the present invention, the DC microgrid includes a power generation unit, an energy storage unit and a load unit.

As a preferred solution of the networked DC microgrid energy collaborative control method of the present invention, wherein: when the DC bus voltage value is within U _L1 ~ U _H1 , the DC microgrid is set to operating mode 1; In the working mode 1, the power generation unit adopts MPPT control according to the principle of maximum power generation consumption, with the energy storage unit as the main control unit to maintain system power and energy balance, and the energy router is in a standby state.

As a preferred solution of the networked DC microgrid energy collaborative control method of the present invention, when the DC bus voltage value is within U _L1 ~ U _H1 , the energy storage unit may enter the power limited state, relying on the energy router To maintain the stability of the DC bus voltage, the distributed power supply works in MPPT mode, and the DC microgrid is in working mode 2 at this time.

As a preferred solution of the networked DC microgrid energy collaborative control method of the present invention, when the DC bus voltage value is within the range of (U _H1 , U _H2 ), there is excess power in the DC microgrid system. At this time, energy can be output to the AC grid through the energy router and the DC bus voltage can be controlled. At this time, the DC microgrid is in working mode 2-1;

When the DC bus voltage value is within the range of (U _L1 , U _L2 ), the DC microgrid system lacks energy. At this time, the AC grid can provide energy to the DC microgrid through the energy router. At this time, the DC microgrid is in working mode. 2-2.

As a preferred solution of the networked DC microgrid energy collaborative control method of the present invention, when the DC bus voltage range is within the DC bus voltage upper limit U _H2 and the maximum limit U _H3 , the energy router works in a power limited state. , the distributed power generation unit will switch from MPPT mode to step-down constant power mode to maintain system power balance.

As a preferred solution of the networked DC microgrid energy collaborative control method of the present invention, wherein: for the energy storage unit, SOC-based adaptive droop control is adopted; for the rectification stage of the energy router, dual-loop control is adopted, For the isolation stage of the energy router, double closed-loop control of voltage loop and current loop is adopted; for the power generation unit, power reduction constant voltage control is adopted.

Beneficial effects of the present invention: The networked DC microgrid energy collaborative control method provided by the present invention can adjust the working mode according to the working voltage value of the DC microgrid, maintain the system power balance, and meet the requirements of each unit under no communication conditions. Plug and Play” requirement.

Description of drawings

In order to explain the technical solutions of the embodiments of the present invention more clearly, the drawings needed to be used in the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. Those of ordinary skill in the art can also obtain other drawings based on these drawings without exerting any creative effort. in:

Figure 1 shows the DC microgrid structure used in the present invention;

Figure 2 shows the energy router topology used in the present invention;

Figure 3 is a two-quadrant droop curve controlled by the energy storage unit of the present invention;

Figure 4 shows the rectification stage control of the energy router used in the present invention;

Figure 5 shows the isolation level control of the energy router used in the present invention;

Figure 6 shows the photovoltaic power generation unit control used in the present invention;

Figure 7 is a working condition diagram of the DC microgrid under photovoltaic fluctuation conditions according to the present invention;

Figure 8 is a working condition diagram of the DC microgrid under load fluctuation and power grid failure conditions according to the present invention;

Figure 9 is a step diagram of the energy collaborative control method of the networked DC microgrid of the present invention.

Detailed ways

In order to make the above objects, features and advantages of the present invention more obvious and understandable, the specific implementation modes of the present invention will be described in detail below with reference to the accompanying drawings.

Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described here. Those skilled in the art can do so without departing from the connotation of the present invention. Similar generalizations are made, and therefore the present invention is not limited to the specific embodiments disclosed below.

Example 1

Figure 1 shows the DC microgrid structure used in the present invention. It can be seen from Figure 1 that the DC microgrid studied by the present invention includes distributed power generation units represented by photovoltaics, which can compensate for the energy deficit of the microgrid and ensure the microgrid Balanced energy storage units, multiple types of load units, and energy routers (ER) connecting the DC microgrid and the AC grid support the bidirectional flow of energy between the DC microgrid and the external grid. Each unit is connected to the DC bus through a bidirectional DC-DC converter.

The invention relates to a networked DC microgrid energy collaborative control method, which includes the following steps:

Step 1: Divide different voltage levels according to the bus voltage value of the DC microgrid, define different voltage levels as different working modes, and ensure that at least one unit in each working mode controls the DC bus voltage to maintain system power. balance.

In a DC microgrid, whether the bus voltage is stable is a direct indication of whether the active power in the system is balanced. The specific working modes divided according to the DC bus voltage value are as follows: set the DC bus rated voltage U to 380V, and the change amount of the DC bus voltage ΔU. Among them, the upper and lower limits of the DC bus fluctuation range are U _H2 and U _L2 , and the upper and lower limits of the layered voltage are The lower limits are U _H1 and U _L1 , and the DC microgrid operating mode is divided into the following operating modes:

Working mode 1: The fluctuation range of the DC bus rated voltage |ΔU|<10V, that is, the voltage is between U _L1 and U _H1 . The distributed power generation unit adopts MPPT control in accordance with the principle of maximum consumption of renewable energy power generation in China, and the energy storage unit is the main one. The control unit maintains the system power and energy balance, and the energy router is in standby state.

Working mode 2: When the bus voltage fluctuation range is 10V<|ΔU|<20V, the supply and demand difference between the power generation unit and the load unit in the microgrid continues to increase or decrease, and the energy storage unit may enter a power limiting state. At this time, you can The energy router is relied upon to maintain DC bus voltage stability. Distributed power supplies still work in MPPT mode.

This working mode can be subdivided into two modes: When the bus voltage is in the (U _H1 , U _H2 ) range, there is excess power in the DC microgrid system. At this time, energy can be output to the AC grid through the energy router and the DC bus can be controlled. voltage, defined as working mode 2-1 at this time; when the bus voltage is in the (U _L1 , U _L2 ) range, there is a lack of energy in the DC microgrid system. At this time, the AC grid can provide energy to the DC microgrid through the energy router, defined This is working mode 2-2.

If the local load demand cannot be met and the bus voltage drops too low, in order to maintain the stability of the bus voltage, the energy router adopts load shedding work, and the DC microgrid receives energy through the energy router.

Working mode 3: The excess power generation of the power generation unit causes the bus voltage to be high. When the bus voltage range is (U _H2 ~ U _H3 ), where U _H3 is the maximum limit of the DC bus voltage fluctuation, the energy router works in a power limited state, and distributed generation The unit will switch from MPPT mode to buck constant power mode to maintain system power balance.

Step 2: Design a control method for each local unit in the DC microgrid to cope with the switching of different working modes of the system when the DC bus voltage changes.

For the energy storage unit, this paper adopts SOC-based adaptive droop control.

Droop control cannot be improved based on the power distribution characteristics of different capacities of energy storage units. The traditional droop control equation is expressed as:

U _refi =U ₀ -k _i P _refi (2)

In the formula, U _refi and P _refi are the output voltage and output active power of the i-th energy storage unit respectively, U ₀ is the rated bus reference voltage value, and k _i is the droop coefficient.

The improved sag coefficient is expressed as:

In the formula, k _i ′ is the improved droop coefficient; SOC _i , SOC _max , and SOC _min are the SOC value and the upper and lower SOC limits of the energy storage unit respectively; when P _refi <0, the energy storage unit works in the charging mode, P _refi When >0, the energy storage unit works in discharge mode. The improved droop control formula is expressed as:

U _refi =U ₀ -k′ _i P _refi (4)

Taking the charging mode as an example: By default, the output voltages of each energy storage unit are equal, and the combined formulas (2) to (4) can be obtained:

From equation (5) and Figure 4, we can know that in charging mode, the energy storage unit with a larger SOC has smaller charging power, and in the discharge mode, the energy storage unit with a smaller SOC discharges slower, and vice versa. This strategy can effectively realize the reasonable distribution of the output power of the energy storage unit and meet the plug-and-play requirements.

Considering that frequent charging and discharging of energy storage will affect battery life, it is set that when the SOC of the energy storage unit reaches the limit, the energy storage stops charging or discharging.

The basic structure of the energy router is shown in Figure 2, which is mainly divided into rectifier stage, isolation stage (DAB), and inverter stage. The research part of this article is mainly a DC microgrid, so the inverter stage is not included. Energy routers can realize power exchange between power grids, while isolation levels can ensure power quality and system stability. When there is excess power in the DC microgrid, the energy can be transferred to the AC grid through the energy router; when there is a power shortage in the DC microgrid, the energy router will transmit power to the DC microgrid through the AC grid.

For the rectifier stage of the energy router, a double-closed-loop feedback control strategy is adopted to ensure the stability of the output voltage. The double-loop control of the voltage loop and current loop also ensures response speed. It can be seen from the mathematical model of the rectifier system that the voltage and current of the rectifier stage are coupled, so it is necessary to design a voltage and current decoupling module to achieve static error-free control. The system control is shown in Figure 5, where, U ₀ is the reference value and actual sampling value of the DC side voltage, i _abc is the three-phase input current on the AC side, i _d and i _q are the current d-axis and q-axis current values after dq transformation,/> are the reference values of the d-axis and q-axis of the current inner loop, u _d and u _q are the d-axis and q-axis voltage values of the AC side three-phase input voltage after dq transformation,/> is the inner loop output voltage value.

For isolation stage control, since the isolation stage is directly connected to the busbar of the DC microgrid, the main goal of the isolation stage is to output a stable DC bus voltage. Therefore, double closed-loop control is adopted, as shown in Figure 5, where U _{dc_ref} is the DC bus voltage reference value, U _dc is the actual DC bus voltage value, I _{dc_ref} is the DC side current reference value obtained through the voltage outer loop, I _dc is the actual current value.

For the photovoltaic power generation unit, when the DC microgrid works in operating mode 3, the photovoltaic power generation unit has residual power, which affects the stable operation of the DC microgrid. Therefore, similar to ER control, the photovoltaic power generation unit control is switched to power reduction and constant voltage control. . As shown in Figure 6, where V _pv is the terminal voltage of the photovoltaic power generation unit, I _pv is the output current of the photovoltaic power generation unit, I _{pv_ref} is the reference current obtained through PI control, and V _{pv_ref} is the voltage reference value. In order to prove the effectiveness of the coordinated control strategy method of the present invention, this embodiment builds a DC microgrid model as shown in Figure 1, which includes 1 group of distributed photovoltaic power generation units with a rated power of 15kW; 2 groups of energy storage energy sources, with energy storage The rated power of unit 1 and energy storage unit 2 are 3kW and 6kW respectively, and the safety limit range of charging and discharging is 20% to 90%; all distributed units are connected to the AC power grid through energy routers, and the rated power of the energy router is 6kW. The rated bus voltage of the DC microgrid is set to 380V, and the voltage range divided in this article is shown in Table 1.

Table 1 Voltage level interval division

Figure 7 shows the operating status of the DC microgrid under the condition of photovoltaic fluctuations of the model of the present invention. In the initial state, the power generated by the distributed photovoltaic unit is 8kW, the initial DC load in the system is 4.5kW, the energy storage unit works in the discharge state, the power of energy storage unit 1 and energy storage unit 2 are 1.5kW and 2kW respectively, and the SOC value They are 70% and 60% respectively. The energy router is in standby state. At this time, the system is running in working mode 1, and the DC bus voltage corresponds to 384.4V.

When t=1s, the photovoltaic unit power generation increases to 12kW, the total power of the DC microgrid system increases, and the DC bus voltage rises accordingly. The energy storage unit loses control over the DC bus voltage and becomes power limited control. At this time, the system works in the voltage level of mode 2-1, and the DC bus voltage rises to about 396.9V. The remaining power in the system is transmitted to the AC grid through the energy router, and the DC bus voltage is controlled to be stable. The photovoltaic system works in MPPT mode.

At t=2s, the photovoltaic unit power generation continues to increase to 14.7kW, the DC bus voltage continues to increase to 407V, and the energy router continues to output power to the AC grid. At this time, the energy router is running in the maximum power state. At this time, in order to maintain the power balance in the system, the photovoltaic unit switches from MPPT mode to constant power mode and controls the DC bus voltage. At this time, the system is running in operating mode 3.

At the same time, when the improved adaptive droop control is used, the energy storage unit with a larger SOC bears more output power, while the energy storage unit with a smaller SOC outputs less power. Compared with the traditional droop control strategy, this The control strategy can reasonably allocate output power according to the SOC of the energy storage unit.

Figure 8 shows the operating status of the DC microgrid under load fluctuation conditions and power grid fault conditions of the model of the present invention. In the initial state, the photovoltaic unit is not working, the initial load in the DC microgrid is 12kW, the energy storage unit provides energy to the load at maximum power output, and the SOC of energy storage unit 1 and unit 2 is 80% and 70%. At the same time, the energy router also transmits power to the microgrid and controls the DC bus voltage. At this time, the system is running in working mode 2-2, and the DC bus voltage is approximately 366.7V.

When t=1s, the photovoltaic unit starts to output power, the output power is 13.4kW, the DC bus voltage rises to about 387V, and the system is working in mode 1 at this time. The energy router does not need to draw power delivery from the AC grid, so it is in a standby state, during which bus voltage control is performed by the energy storage unit.

At t=2s, the load power in the DC microgrid system is reduced by 4kW. The energy router transmits the excess power in the system to the AC grid and controls the DC bus voltage to be stable. At this time, the system works in working mode 2-1.

When t=3s, the AC grid fails, the energy router cuts off the connection with the AC grid, and the DC microgrid can run in island mode. When t=3.6s, the AC grid returns to normal operation, and the DC microgrid can still return to the operating mode before the fault. At the same time, the improved adaptive droop control can also achieve reasonable power output of the energy storage unit and cope with fault conditions that occur in the system.

It should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solution of the present invention can be carried out. Modifications or equivalent substitutions without departing from the spirit and scope of the technical solution of the present invention shall be included in the scope of the claims of the present invention.

Claims (2)

1. A networking type direct current micro-grid energy cooperative control method is characterized in that: comprising the steps of (a) a step of,

dividing different voltage level layers according to the voltage value of a direct current bus of the direct current micro-grid, setting the voltage level layers into working modes, wherein at least one local unit in each working mode controls the voltage of the direct current bus, and keeping the power balance of the system;

designing a control method aiming at each local unit in the direct current micro-grid, and switching different working modes aiming at the change of the direct current bus voltage;

the working mode is divided according to the voltage value of the direct current bus, and the voltage value of the direct current bus refers to the upper limit and the lower limit U of the voltage fluctuation range of the direct current bus _H2 ～U _L2 And upper and lower limits U of the layered voltage value range _H1 ～U _L1 ；

The local unit comprises an energy router, and the energy router is connected with the direct-current micro-grid and the public grid; the voltage range of the direct current bus is that the voltage of the direct current bus is upper limit U _H2 And maximum limit U _H3 When the energy router works in a power limiting state, the distributed power generation unit is switched from an MPPT mode to a step-down constant power modeTo maintain system power balance;

for the energy storage unit, adopting self-adaptive droop control based on SOC; the rectification stage of the energy router is controlled by double rings, and the isolation stage of the energy router is controlled by double closed loops of a voltage ring and a current ring; adopting power reduction constant voltage control for the power generation unit;

when the voltage value of the direct current bus is U _L1 ～U _H1 When the direct current micro-grid is in the internal state, the direct current micro-grid is set to be in a working mode 1; in the working mode 1, the power generation unit adopts MPPT control according to the maximum power generation absorption principle, an energy storage unit is used as a main control unit to maintain the balance of system power and energy, and an energy router is in a standby state;

when the voltage value of the direct current bus is U _H1 ～U _H2 When the power is in the range, the power in the direct-current micro-grid system is excessive, at the moment, energy can be output to the alternating-current power grid through the energy router, the direct-current bus voltage is controlled, and at the moment, the direct-current micro-grid is in a working mode 2-1;

when the voltage value of the direct current bus is U _L1 ～U _L2 When the energy is within the range, the energy is lacking in the direct-current micro-grid system, the alternating-current power grid can provide energy for the direct-current micro-grid through the energy router, and the direct-current micro-grid is in a working mode 2-2;

if the local load demand cannot be met and the bus voltage is reduced too low, the energy router adopts load switching work to maintain the bus voltage stable, and the direct current micro-grid receives energy through the energy router;

the bus voltage is higher due to the excessive power generation of the power generation unit, and the bus voltage range is U _H2 ～U _H3 When U is _H3 For the maximum limit of the voltage fluctuation of the direct current bus, the energy router works in a power limiting state, and the distributed power generation unit is switched from an MPPT mode to a step-down constant power mode so as to maintain the power balance of the system to be a working mode 3;

when the redundant power exists in the direct-current micro-grid, the energy can be transmitted to the alternating-current grid through the energy router; when power shortage occurs in the direct-current micro-grid, the energy router can transmit power to the direct-current micro-grid through the alternating-current grid;

aiming at the rectifying stage of the energy router, a double-closed loop feedback control strategy is adopted to ensure the stability of the output voltage, wherein the double-loop control of the voltage loop and the current loop also ensures the response speed; the mathematical model of the rectification system shows that the voltage and the current of the rectification stage are coupled, so that a voltage and current decoupling module is required to be designed, and the static-difference-free control is realized;

for the control of the isolation stage, the isolation stage is directly connected with the bus of the direct current micro-grid, so that the main aim of the isolation stage is to output stable direct current bus voltage, and double closed loop control is adopted;

for the photovoltaic power generation unit, when the direct-current micro-grid works in the working mode 3, residual power exists in the photovoltaic power generation unit, so that stable operation of the direct-current micro-grid is affected, and the photovoltaic power generation unit control is switched to the power-down constant-voltage control similar to ER control.

2. The networking type direct current micro grid energy cooperative control method according to claim 1, wherein the method comprises the following steps: the direct-current micro-grid comprises a power generation unit, an energy storage unit and a load unit.