Off-grid micro-grid system and secondary frequency modulation control method thereof

Abstract

The invention discloses an off-grid micro-grid system and a secondary frequency modulation control method thereof. The secondary frequency modulation control method comprises the steps that the grid frequency of an off-grid micro-grid is acquired, and whether the grid frequency is in a frequency dead zone is judged; if the power grid frequency is in the frequency dead zone, adjusting active power output of all units in the equipment by adopting a secondary frequency modulation strategy in the frequency dead zone, so that the power grid frequency is recovered to the rated power grid frequency; and if the power grid frequency is outside the frequency dead zone, adjusting active power output of at least one unit in the equipment by adopting a secondary frequency modulation strategy outside the frequency dead zone, so that the power grid frequency is recovered to the rated power grid frequency. According to the invention, by adding the network construction type energy storage unit equipment in the off-grid micro-grid and respectively adopting different secondary frequency modulation strategies based on the working conditions inside and outside the frequency dead zone, the method can be used for constructing a network, facilitates the access of network-following type new energy power generation equipment and controllable load equipment, can also be used for smoothing new energy fluctuation and energy time shift, and improves the energy efficiency. And stable operation of the off-grid micro-grid is realized.

Description

Technical Field

[0001] The present application relates to the field of power grid technology, and in particular to an off-grid microgrid system and a secondary frequency modulation control method thereof. Background Art

[0002] An off-grid microgrid is a small power system that does not rely on the main grid and operates independently. It is widely used in remote areas, emergency power supply and special scenarios. In recent years, with the decline in the cost of solar energy and energy storage technology, off-grid microgrids have become more economical and feasible; and the advancement of intelligent control technology, such as the application of microgrid energy management systems, has made its operation more efficient; in addition, the functional modular design has also enhanced the flexibility and scalability of the system.

[0003] Off-grid microgrids are developing rapidly, but they still face challenges, such as the energy balance problem caused by the volatility of renewable energy and the complexity of system integration, that is, wind and photovoltaic power generation may fluctuate greatly in time and space scales. Summary of the invention

[0004] In view of this, the purpose of this application is to provide an off-grid microgrid system and a secondary frequency regulation control method thereof, so as to smooth out the fluctuation of new energy and energy time shift and realize the stable operation of the off-grid microgrid.

[0005] The technical solutions adopted by this application to solve the above technical problems are as follows:

[0006] On one hand, the present application provides a secondary frequency modulation control method for an off-grid microgrid system, the secondary frequency modulation control method comprising:

[0007] Obtain the grid frequency of the off-grid microgrid and determine whether the grid frequency is within the frequency dead zone;

[0008] If the grid frequency is within the frequency dead zone, the secondary frequency regulation strategy within the frequency dead zone is used to adjust the active power output of all units in the device so that the grid frequency is restored to the rated grid frequency;

[0009] If the grid frequency is outside the frequency dead zone, the secondary frequency regulation strategy outside the frequency dead zone is adopted to adjust the active power output of at least one unit in the device so that the grid frequency is restored to the rated grid frequency;

[0010] Among them, the equipment includes new energy power generation units, grid-type energy storage units and controllable load units.

[0011] On the other hand, the present application provides an off-grid microgrid system, characterized in that the off-grid microgrid system includes a new energy power generation unit, a grid-type energy storage unit, a controllable load unit and an energy management system, and the energy management system is configured to execute a secondary frequency modulation control method.

[0012] The off-grid microgrid system and secondary frequency regulation control method provided in the embodiments of the present application can be used for grid construction, facilitating the access of grid-following new energy power generation equipment and controllable load equipment, and can also be used to smooth out new energy fluctuations and energy time shifts, thereby achieving stable operation of the off-grid microgrid, by adding a grid-building energy storage unit device to the off-grid microgrid and adopting different secondary frequency regulation strategies based on the working conditions inside and outside the frequency dead zone. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A schematic diagram of an off-grid microgrid system provided in an embodiment of the present application;

[0014] Figure 2 A schematic diagram of a secondary frequency modulation control method for an off-grid microgrid system provided in an embodiment of the present application;

[0015] Figure 3 This is a schematic diagram of PI control in the secondary frequency modulation strategy outside the frequency dead zone provided in an embodiment of the present application.

[0016] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0017] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer and more understandable, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0018] Figure 1 Schematic diagram of an off-grid microgrid system provided in an embodiment of the present application.

[0019] like Figure 1 As shown, the off-grid microgrid system provided in the embodiment of the present application includes a new energy power generation unit, a grid-connected energy storage unit, a controllable load unit, an uncontrollable load unit, a diesel generator and an energy management system (EMS).

[0020] New energy power generation units include, for example, photovoltaic power generation units, wind power generation units, etc.; the configuration capacity of grid-type energy storage units should not be less than 15% of the load rated capacity; controllable load units include, for example, hydrogen production units, electrolyzers, etc.; diesel generators can be configured as small-capacity diesel generators, generally used to assist black start.

[0021] The data symbols of the above units are defined as follows: the power data symbols of the controllable load unit and the uncontrollable load unit are both positive; the power data symbols of the new energy power generation unit are all positive; the power data symbol of the grid-type energy storage unit is defined as negative for charging and positive for discharging.

[0022] At present, wind power and photovoltaic power generation equipment are mainly connected to the grid and cannot operate independently. Therefore, adding a grid-building energy storage unit to an off-grid microgrid can be used to build a grid, facilitate the access of grid-building new energy power generation equipment and controllable load equipment, and can also be used to smooth new energy fluctuations and energy time shifts to achieve stable operation of the off-grid microgrid.

[0023] In an off-grid microgrid, the grid-type energy storage unit is responsible for balancing the active power deviation between wind power, photovoltaic and other new energy power generation units and the load. When the load demand exceeds the real-time upper limit of wind power, photovoltaic and other new energy power generation units, the grid-type energy storage unit outputs active power to meet the load demand; when the load demand is less than the real-time upper limit of wind power, photovoltaic and other new energy power generation units, the grid-type energy storage unit absorbs excess active power. In this process, the grid-type energy storage unit quickly responds to the active power deviation in the off-grid microgrid on the one hand; on the other hand, according to the active power command and actual active power received by the grid-type energy storage unit, the frequency of the microgrid is adjusted according to the mechanical characteristics of the traditional synchronous machine.

[0024] The energy management system needs to perform real-time data interaction with the above devices. Specifically, the energy management system is configured to execute the secondary frequency modulation control method.

[0025] like Figure 2 As shown, the secondary frequency modulation control method of the off-grid microgrid system provided in the embodiment of the present application includes:

[0026] Step S11, obtaining the grid frequency of the off-grid microgrid and determining whether the grid frequency is within the frequency dead zone.

[0027] In one example, the grid frequency can be detected by a measurement and control instrument configured in the off-grid microgrid system to determine whether the grid frequency is within a frequency dead zone.

[0028] Step S12: If the grid frequency is within the frequency dead zone, a secondary frequency regulation strategy within the frequency dead zone is used to adjust the active power output of all units in the device so that the grid frequency is restored to the rated grid frequency.

[0029] The equipment includes new energy power generation units, grid-type energy storage units and controllable load units.

[0030] In one example, a secondary frequency regulation strategy within a frequency dead zone is used to regulate the active power output of all units in the device, including:

[0031] According to the frequency modulation coefficient and frequency dead zone of the grid-forming energy storage unit, the allowable active power adjustment range of the grid-forming energy storage unit is determined;

[0032] Based on the active power adjustment range, determine the active power setting values ​​of all units in the device;

[0033] According to the active power setting values ​​of all units in the device, the active power output of all units in the device is adjusted.

[0034] Among them, when determining the active power setting value of all units in the equipment, the following conditions need to be met:

[0035] P pv / wtg +P diesel +P ess =P ld_unctrl +P ld_ctrlled +P loss , where P pv / wtg is the actual active power output of the new energy power generation unit, P diesel is the actual active power output of the diesel generator, P ess is the actual output active power of the grid-type energy storage unit, P ld_unctrl is the actual active power output of the uncontrollable load unit, P ld_ctrlled is the actual output active power of the controllable load unit, P loss is the line loss.

[0036] The active power setting value of all units in the device is limited to [P Act -P Ess_Dec , P Act +P Ess_Dec ]; among them, P Act is the current active power value of all units in the device, P Ess_Dec It is the lower limit of the active power adjustment range.

[0037] For example, EMS calculates the allowable active power regulation range [P Ess_Dec , P Ess_Inc ].

[0038] Allowable active power adjustment range [P Ess_Dec , P Ess_Inc ] indicates that during the microgrid regulation process, when the active power increment of the grid-connected energy storage unit is within this range, it can maintain the grid frequency within the dead zone through autonomous frequency regulation.

[0039] Therefore, during the energy dispatching process of the microgrid, the EMS should strictly distribute power according to the basic control conditions of the microgrid, as shown below:

[0040] P wtg +P pv +P diesel +P ess =P ld_unctrl +P ld_ctrlled +P loss, where P wtg is the actual active power output of the wind power generation unit, P pv It is the actual active power output of the photovoltaic power generation unit.

[0041] Considering the differences in communication cycles and regulation rates of various devices, it should be ensured that during the entire regulation process, the maximum active power shortage of the microgrid caused by the communication cycle and regulation rate is within [P Ess_Dec , P Ess_Inc ] range.

[0042] In order to meet the above requirements, when EMS allocates power to each device, the active power setting value of the power source (wind, solar, storage and discharge) and load (hydrogen production, storage and charging) should be limited to [P Act -P Ess_Dec , P Act +P Ess_Dec ], where P Act It is the current active power value of the power supply or load.

[0043] At this time, when each device is under normal control, the energy in the microgrid is dynamically balanced, and after the EMS distributes power to each device, the grid frequency is restored to the rated grid frequency.

[0044] In one example, the secondary frequency modulation control method further includes:

[0045] When the grid frequency is within the frequency dead zone, the active power value fed back by the grid-forming energy storage unit is obtained, and the obtained active power value is sent to the grid-forming energy storage unit as the active power setting value.

[0046] EMS sends the active power value fed back by the energy storage as the active power setting value to the grid-forming energy storage unit. Firstly, it can offset the response and calculation errors caused by long-term small adjustments, and secondly, it can maintain the consistency between the active power setting value and the active power output value of the grid-forming energy storage unit.

[0047] Step S13: If the grid frequency is outside the frequency dead zone, a secondary frequency regulation strategy outside the frequency dead zone is used to adjust the active power output of at least one unit in the device so that the grid frequency is restored to the rated grid frequency.

[0048] In one example, the active power output of at least one unit in the device is adjusted by using a secondary frequency modulation strategy outside the frequency dead zone, including:

[0049] When the grid-forming energy storage unit is not in the maximum discharge state or the maximum charge state and the grid frequency has remained stable, the active power output of the grid-forming energy storage unit is adjusted based on the active power setting value of the grid-forming energy storage unit;

[0050] When the grid-type energy storage unit is in the maximum discharge state or the maximum charge state and the grid frequency is still fluctuating, the active power setting value of all units in the device is determined based on the grid frequency deviation, and the active power output of all units in the device is adjusted according to the active power setting value of all units in the device.

[0051] It should be noted that when the grid frequency exceeds the limit, EMS requires equipment (new energy generation units, grid-connected energy storage units, and controllable load units) to participate in secondary frequency regulation, which must ultimately meet two conditions:

[0052] "Energy balance in microgrid" and "active power setting value of grid-type energy storage unit is consistent with active power output value". Frequency overlimit is often caused by equipment failure and load switching, and it is impossible to monitor all nodes on the line, so it is impossible to calculate the shortage according to the aforementioned basic control conditions of microgrid. Therefore, it is mainly adjusted through PI control algorithm.

[0053] When the frequency exceeds the limit, there are two situations:

[0054] Case 1: The grid-connected energy storage unit has stabilized the frequency and the microgrid has energy balance. In theory, the frequency can be restored as long as the "active setting value of the grid-connected energy storage unit is consistent with the output value".

[0055] Case 2: The grid-connected energy storage unit is in the maximum discharge state or the maximum charge state, the microgrid is still energy unbalanced, and the grid frequency continues to be disturbed or increased. At this time, it is necessary to achieve "energy balance within the microgrid" and "the active setting value of the grid-connected energy storage unit is consistent with the output value".

[0056] In one example, the active power setting value of the grid-type energy storage unit is calculated based on the active power setting value of the grid-type energy storage unit before the secondary frequency modulation is triggered and the current active power value of the grid-type energy storage unit.

[0057] For example, for a grid-forming energy storage unit, the active power setting value of the grid-forming energy storage unit can be calculated according to the following formula:

[0058] P Ess_Set =P Ess_LockSet +(P Ess_Act -P EssLockSet )*T cycle / T SecDelay

[0059] Among them, P Ess_LockSet P is the active power setting value of the grid-type energy storage unit before the secondary frequency modulation is triggered; Ess_Act is the current active power value of the grid-type energy storage unit; T cycle is the period of the secondary frequency modulation algorithm; T SecDelay is the secondary frequency modulation delay time; PEss_Set is the calculated active power setting value of the grid-connected energy storage unit.

[0060] In one example, determining active power setting values ​​of all units in a device based on a grid frequency deviation includes:

[0061] Subtract the grid frequency from the rated grid frequency to obtain the grid frequency deviation;

[0062] After the grid frequency deviation passes through the PI regulator, the active power setting value of all units in the equipment is obtained.

[0063] like Figure 3 As shown in the figure, PI control is performed by monitoring the frequency in the microgrid and calculating the grid frequency deviation, and the active power demand of the site is allocated to each unit in the equipment. Among them, the multi-source active power distribution is based on the real-time power that can be generated by photovoltaic and wind power feedback, the current state of charge (State of Charge, SOC) and charge and discharge limits of the current grid-type energy storage system, and the active power adjustment range of the controllable load. The active power instructions of wind power, photovoltaic, grid-type energy storage, and controllable load are comprehensively evaluated and calculated to reduce the active power shortage supported by the grid-type energy storage unit.

[0064] Specifically, the detected grid frequency ω gird Compared with the rated grid frequency, the grid frequency deviation is obtained, and the microgrid active power shortage instruction is generated through PI control. By adjusting the active output of the power supply or controlled load, the grid frequency is restored to the rated grid frequency. is the target active value of the secondary frequency modulation output; The target active value of the wind power system after the target active value of the secondary frequency regulation output is processed by multi-source distribution; The target active value of the photovoltaic system after the target active value of the secondary frequency regulation output is processed by multi-source distribution; The target active value of the secondary frequency modulation output is the target active value of the grid-following energy storage system after multi-source distribution processing; The target active value of the controllable load system after the target active value of the secondary frequency modulation output is processed by multi-source distribution; To construct the target active value of grid energy storage.

[0065] In one example, the secondary frequency modulation control method further includes:

[0066] When the grid frequency exceeds the permitted frequency, the system enters emergency control mode to increase the rate at which all units in the device reduce their active power.

[0067] Specifically, when the grid frequency exceeds the allowable frequency, it means that the capacity of the grid-connected energy storage unit is insufficient, and the frequency continues to change and cannot be restored to the normal range. At this time, it is necessary to enter the emergency control mode, and then increase the rate at which the power supply and load reduce active power until it shuts down, that is, emergency cut-off of the load / power supply.

[0068] The above-mentioned use of energy storage units for networking maintains the balance of power supply and demand in the off-grid microgrid system, improves the system stability margin, and enables the off-grid microgrid to operate stably. By distinguishing the working conditions inside and outside the frequency dead zone, different secondary frequency regulation strategies are adopted respectively. In the frequency dead zone, the active power is autonomously adjusted by the networking type energy storage unit equipment to maintain the stability of the microgrid frequency; when the frequency exceeds the limit, the system coordinates wind, light, energy storage and controllable loads through the PI control algorithm to achieve rapid frequency recovery and energy balance. At the same time, the present application can effectively reduce the power shortage caused by the communication cycle and the difference in regulation rate during the regulation process, and significantly improve the frequency stability and frequency regulation accuracy of the microgrid.

[0069] The preferred embodiments of the present application are described above with reference to the accompanying drawings, but the scope of the present application is not limited thereby. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present application shall be within the scope of the present application.

Claims

1. A secondary frequency modulation control method for an off-grid microgrid system, characterized in that: The secondary frequency modulation control method comprises: Obtaining the grid frequency of the off-grid microgrid, and determining whether the grid frequency is within a frequency dead zone; If the grid frequency is within the frequency dead zone, a secondary frequency regulation strategy within the frequency dead zone is adopted to adjust the active power output of all units in the device so that the grid frequency is restored to the rated grid frequency; If the grid frequency is outside the frequency dead zone, a secondary frequency regulation strategy outside the frequency dead zone is adopted to adjust the active power output of at least one unit in the device so that the grid frequency is restored to the rated grid frequency; Among them, the equipment includes a new energy power generation unit, a grid-type energy storage unit and a controllable load unit.

2. The secondary frequency modulation control method according to claim 1, characterized in that: The method of using the secondary frequency modulation strategy within the frequency dead zone to adjust the active power output of all units in the device includes: Determining the active power adjustment range allowed by the grid-type energy storage unit according to the frequency modulation coefficient and frequency dead zone of the grid-type energy storage unit; Based on the active power adjustment interval, determining active power setting values ​​of all units in the device; According to the active power setting values ​​of all the units in the device, the active power output of all the units in the device is adjusted.

3. The secondary frequency modulation control method according to claim 2, characterized in that: When determining the active power setpoints of all units in the device, the following conditions need to be met: P pv / stg +P diesel +P ess =P ld_unctrl +P ld_ctrlled +P loss , where P pv / wtg is the actual output active power of the new energy power generation unit, P diesel is the actual active power output of the diesel generator, P ess is the actual output active power of the grid-type energy storage unit, P ld_unctrl is the actual output active power of the uncontrollable load unit, P ld_ctrlled is the actual output active power of the controllable load unit, P loss is the line loss.

4. The secondary frequency modulation control method according to claim 2, characterized in that: The active power setting value of all units in the device is limited to [P Act -P Ess_Dec , P Act +P Ess_Dec ]; among them, P Act is the current active power value of all units in the device, P Ess_Dec is the lower limit of the active power adjustment range.

5. The secondary frequency modulation control method according to claim 2, characterized in that: The secondary frequency modulation control method further comprises: When the grid frequency is within the frequency dead zone, the active power value fed back by the grid-forming energy storage unit is obtained, and the obtained active power value is sent to the grid-forming energy storage unit as an active power setting value.

6. The secondary frequency modulation control method according to claim 1, characterized in that: The method of using a secondary frequency modulation strategy outside the frequency dead zone to adjust the active power output of at least one unit in the device includes: When the grid-forming energy storage unit is not in a maximum discharge state or a maximum charge state and the grid frequency has remained stable, the active power output of the grid-forming energy storage unit is adjusted based on the active power setting value of the grid-forming energy storage unit; When the grid-type energy storage unit is in the maximum discharge state or the maximum charge state and the grid frequency is still fluctuating, the active power setting value of all units in the device is determined based on the grid frequency deviation, and the active power output of all units in the device is adjusted according to the active power setting value of all units in the device.

7. The secondary frequency modulation control method according to claim 6, characterized in that: The method further comprises: Based on the active power setting value of the grid-forming energy storage unit before the secondary frequency modulation is triggered and the current active power value of the grid-forming energy storage unit, the active power setting value of the grid-forming energy storage unit is calculated.

8. The secondary frequency modulation control method according to claim 6, characterized in that: Determining the active power setting values ​​of all units in the device based on the grid frequency deviation includes: Subtracting the grid frequency from the rated grid frequency to obtain the grid frequency deviation; After the grid frequency deviation passes through a PI regulator, the active power setting values ​​of all units in the device are obtained.

9. The secondary frequency modulation control method according to claim 6, characterized in that: The secondary frequency modulation control method further comprises: When the grid frequency exceeds the permitted frequency, an emergency control mode is entered to increase the rate at which all units in the device reduce their active power.

10. An off-grid microgrid system, characterized in that: The off-grid microgrid system includes a new energy power generation unit, a grid-connected energy storage unit, a controllable load unit and an energy management system, and the energy management system is configured to execute the secondary frequency regulation control method described in any one of claims 1-9.

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