Hybrid energy storage system and grid-connected-to-off-grid control method

Abstract

The invention provides a hybrid energy storage system and a grid-connected-to-off-grid control method, the system comprises an electrochemical energy storage system, a flywheel energy storage system and a control device, when a power grid is normal, the control device controls the battery energy storage system and the flywheel energy storage system to perform charging and discharging control, an excitation system in the flywheel energy storage system operates independently, and the flywheel energy storage system operates independently. The control by a control device is avoided; when a power grid breaks down, the control device disconnects the power grid and controls the hybrid energy storage system to enter an off-grid operation mode at the same time, in the process of converting a grid-connected order into an off-grid order, the flywheel energy storage inner generator is directly excited by the control device, and along with rapid increase of excitation current, the energy storage generator rapidly converts mechanical energy into electric energy to be released to a load. Electrochemical energy storage and flywheel energy storage are combined, in the off-grid mode, an excitation system of the flywheel energy storage generator is directly excited by the control device, the flywheel energy storage generator has the advantages of rapid strong excitation and the like, and the problem of impact load in PCS off-grid operation is solved.

Description

Technical Field

[0001] The present invention relates to the field of energy storage systems, and more specifically, to a hybrid energy storage system and a grid-connected to off-grid control method. Background Art

[0002] The growth of a high proportion of renewable energy has brought great development opportunities to the energy storage industry. In some grids with poor stability or microgrid systems, energy storage systems generally have the need to operate off-grid. Due to the diversity and uncertainty of loads, energy storage systems may have startup failure problems when operating off-grid.

[0003] The energy storage system is mainly composed of batteries and process control system PCS, which realizes grid-connected and off-grid switching. When the grid fails, PCS detects the grid failure and automatically enters the off-grid state according to the set program. At this time, if off-grid load operation is required, the impact current generated by some high-power inductive impact loads will cause PCS overcurrent protection and cannot operate normally off-grid.

[0004] When operating off-grid, the generator in the flywheel energy storage releases energy to the high-power impact load, and the flywheel speed drops rapidly, causing the exciter's ability to provide excitation current to drop rapidly. At the same time, the impact load power increases approximately linearly, causing the demagnetization reaction of the motor armature in the flywheel energy storage to gradually increase, further causing the generator output voltage to drop, and ultimately leading to system de-energization. Summary of the invention

[0005] The present invention aims at the technical problems existing in the prior art and provides a hybrid energy storage system and a grid-connected to off-grid control method, which can solve the problem that the existing energy storage system cannot carry impact loads when operating off-grid.

[0006] According to a first aspect of the present invention, there is provided a hybrid energy storage system, comprising an electrochemical energy storage system, a flywheel energy storage system and a control device; When the power grid is operating normally, the control device collects the voltage signal and the current signal of the power grid, and when it is determined that the power grid fails according to the voltage signal and the current signal of the power grid, the power grid is disconnected, and the hybrid energy storage system enters the off-grid operation mode; The current actual charging and discharging power of the electrochemical energy storage system, the current actual charging and discharging power of the flywheel energy storage system and the current real-time power of the load are respectively obtained by the control device; Determining whether it is an impact load according to the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system, and the current real-time power of the load; If it is an impact load, the control device is used to control the excitation of the generator in the flywheel energy storage system, so that the generator converts mechanical energy into electrical energy and releases it to the load; During the release process, when it is determined that the hybrid energy storage system enters a stable state, control the electrochemical energy storage system to take over all loads, and control the flywheel energy storage system to perform small-power charging to cope with the next impact load.

[0007] Based on the above technical solutions, the present invention can also be improved as follows.

[0008] Optionally, the electrochemical energy storage system includes a battery energy storage unit and a process control system PCS. The battery energy storage unit is connected to the PCS through a DC cable. The PCS is connected to the bus through a circuit breaker QF1. The flywheel energy storage system includes a flywheel energy storage unit, an AC / DC power module, and a DC / AC power module. The flywheel energy storage unit is connected to the DC / AC power module through an AC cable. The DC / AC power module is connected to the AC / DC power module through a DC cable. The AC / DC power module is connected to the bus through a circuit breaker QF2. The mains power is connected to the bus through a circuit breaker QF1. The load is connected to the bus through a circuit breaker QF3. The control device is respectively connected to the electrochemical energy storage system, the flywheel energy storage system, the power grid, and the load.

[0009] According to a second aspect of the present invention, there is provided a grid-connected to off-grid control method for a hybrid energy storage system, including: When the power grid is operating normally, the control device collects the voltage signal and current signal of the power grid. When it is determined according to the voltage signal and current signal of the power grid that a fault occurs in the power grid, disconnect the power grid, and the hybrid energy storage system enters the off-grid operation mode; The control device respectively obtains the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system, and the current real-time power of the load; According to the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system, and the current real-time power of the load, determine whether it is an impact load; If it is an impact load, the control device performs excitation control on the generator in the flywheel energy storage system, so that the generator converts mechanical energy into electrical energy and releases it to the load; During the release process, when it is determined that the hybrid energy storage system enters a stable state, control the electrochemical energy storage system to take over all loads, and control the flywheel energy storage system to perform small-power charging to cope with the next impact load.

[0010] Optionally, when the power grid is operating normally, the control device collects the voltage signal and current signal of the power grid. When it is determined according to the voltage signal and current signal of the power grid that a fault occurs in the power grid, disconnect the power grid, and the hybrid energy storage system enters the off-grid operation mode, including: When the power grid is operating normally, the control device collects the voltage signal and current signal of the power grid. According to the collected voltage signal and current signal of the power grid, the mutation rates of the power grid voltage, frequency, and phase angle are obtained as , , and the voltage sag time ; When any of the following conditions is met, it is determined that a fault has occurred in the power grid: Condition 1: ≥Un and ≥0.15 s; Condition 2: ≥0.8Un and ≥0.625 s; Condition 3: <0.4273 and 0.625< ≤2 s; Condition 4: ≥5 and ≥0.2 s; Condition 5: ≥5 and ≥1 s; When it is determined that a fault has occurred in the power grid, the control device outputs a trip signal to control the circuit breaker on the power grid to disconnect the power grid, and the hybrid energy storage system enters the off-grid operation mode.

[0011] Optionally, the electrochemical energy storage system includes a battery energy storage unit and a process control system PCS. The battery energy storage unit is connected to the PCS through a DC cable, and the PCS is connected to the bus through a circuit breaker QF1; Obtaining the current actual charge and discharge power of the electrochemical energy storage system through the control device includes: Obtaining the charge and discharge power P1 sent by the control device to the PCS for the electrochemical energy storage system; Obtaining the charge and discharge power P2 of the battery energy storage unit read by the PCS according to the CAN cable, and obtaining the maximum charge and discharge power P3 that the PCS itself can output; Then the current actual charge and discharge power Ppcs of the PCS = min(P1, P2, P3), and Ppcs is the current actual charge and discharge power of the electrochemical energy storage system.

[0012] Optionally, the flywheel energy storage system includes a flywheel energy storage unit, an AC / DC power module, and a DC / AC power module. The flywheel energy storage unit is connected to the DC / AC power module through an AC cable. The DC / AC power module is connected to the AC / DC power module through a DC cable. The AC / DC power module is connected to the bus through a circuit breaker QF2. The current actual charge-discharge power of the flywheel energy storage system includes: Obtain the charge-discharge power P4 issued by the control device to the flywheel energy storage unit; read the charge-discharge power P5 of the flywheel energy storage unit and the maximum charge-discharge power P6 of the AC / DC power module; and obtain the maximum charge-discharge power P7 uploaded by the DC / AC power module to the control device. Then, when the control device does not turn on the excitation, the current actual charge-discharge power Pfy of the flywheel energy storage system is Pfy = min(P4, P5, P6, P7).

[0013] Optionally, the load is connected to the bus through a circuit breaker QF3. Obtaining the current real-time power of the load includes: Collect the voltage u at the upper end of the circuit breaker QF3 through the control device 2 and the current signal i 2 , then the current real-time power of the load is .

[0014] Optionally, determining whether it is an impact load according to the current actual charge-discharge power of the electrochemical energy storage system, the current actual charge-discharge power of the flywheel energy storage system, and the current real-time power of the load includes: When Pload > Pfy × K1 and Pload ≥ Ppcs × K2, it is determined that the load is unstable and is an impact load, where Pload is the current real-time power of the load, Pfy is the current actual charge-discharge power of the flywheel energy storage system, Ppcs is the current actual charge-discharge power imported by the electrochemical energy storage system, K1 is the overload coefficient of the flywheel energy storage system, and K2 is the overload coefficient of the PCS.

[0015] Optionally, during the release process, when it is determined that the hybrid energy storage system enters a stable state, control the electrochemical energy storage system to take over all loads, and control the flywheel energy storage system to perform small-power charging to cope with the next impact load, including: When Pload < Pfy × K3 or Pload < Ppcs × K4, it is determined that the load is stable, and the hybrid energy storage system enters a steady operation state, where Pload is the current real-time power of the load, Pfy is the current actual charge-discharge power of the flywheel energy storage system, Ppcs is the current actual charge-discharge power imported by the electrochemical energy storage system, K3 is the stability coefficient of the flywheel energy storage system, and K4 is the stability coefficient of the PCS; The control device issues control commands to the PCS and the flywheel energy storage system to control the gradual reduction of the output power of the flywheel energy storage system until it drops to zero power, and the electrochemical energy storage system takes over all the loads.

[0016] A hybrid energy storage system and a grid-connected to off-grid control method provided by the present invention. When the power grid is normal, the control device controls the charge and discharge of the battery energy storage and the flywheel energy storage according to the set program. The excitation system inside the flywheel energy storage system operates independently and is not controlled by the control device. When the power grid fails, the control device controls QF0 to trip, and at the same time controls the hybrid energy storage system to enter the off-grid operation mode. During the grid-connected to off-grid stage, the control device controls the flywheel energy storage system to take over most of the loads, and the electrochemical energy storage system takes over a small part of the loads. When entering the steady state stage, the electrochemical energy storage system takes over all the loads, and the flywheel energy storage system performs small-power charging to prepare for the next impact load. During this grid-connected to off-grid process, the generator in the flywheel energy storage is directly excited by the control device. As the excitation current rapidly increases, the energy storage generator rapidly converts mechanical energy into electrical energy and releases it to the load. By combining the electrochemical energy storage and the flywheel energy storage, in the off-grid mode, the excitation system of the flywheel energy storage generator is directly excited by the control device, which has advantages such as fast and strong excitation, and solves the problem of the PCS operating with impact loads in the off-grid mode. Brief Description of the Drawings

[0017] Figure 1 It is the structural schematic diagram of a hybrid energy storage system provided by the present invention; Figure 2 It is the flow schematic diagram of a grid-connected to off-grid control method of a hybrid energy storage system provided by the present invention; Figure 3 It is the overall schematic diagram of the grid-connected to off-grid control method of the hybrid energy storage system. Detailed Embodiments

[0018] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. In addition, the technical features in the various embodiments or individual embodiments provided by the present invention can be combined with each other arbitrarily to form a feasible technical solution. Such combination is not restricted by the order of steps and / or the pattern of structural composition, but must be based on the fact that those of ordinary skill in the art can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the protection scope required by the present invention.

[0019] Figure 1 FIG. is a structural diagram of a hybrid energy storage system provided by the present invention. As Figure 1 shown, the hybrid energy storage system mainly includes an electrochemical energy storage system, a flywheel energy storage system and a control device connected to the AC side. The electrochemical energy storage system consists of a battery energy storage unit and a process control system PCS. The battery energy storage unit is connected to the PCS through a DC cable, and the PCS is connected to the outside through an AC cable. The flywheel energy storage system consists of a flywheel energy storage unit, an AC / DC power module and a DC / AC power module. The flywheel energy storage unit is connected to the DC / AC module through an AC cable, the DC / AC power module is connected to the AC / DC power module through a DC cable, and the AC / DC power module is connected to the outside through an AC cable. The control device integrates a control interface, a collection interface, a communication interface, a power supply interface and an excitation interface.

[0020] See Figure 1 , the PCS is connected to the bus through the circuit breaker QF1, the AC / DC power module is connected to the bus through the circuit breaker QF2, the commercial power is connected to the bus through the circuit breaker QF1, and the load is connected to the bus through the circuit breaker QF3. The control device is respectively connected to the electrochemical energy storage system, the flywheel energy storage system, the power grid and the load. Among them, the control interface in the control device is used to control the closing and opening of external circuit breakers, etc.; the collection interface is used to collect the voltage and current signals at the terminals of the circuit breakers QF1, QF2, QF3 and QF4; the communication interface is used to realize the communication between the control device and other units; the excitation interface is connected to the excitation system inside the flywheel energy storage unit and is used to perform strong excitation on the generator inside the flywheel when the off-grid load has impact.

[0021] Among them, in the prior art, the excitation system of the flywheel energy storage system is controlled by itself. When operating off-grid, the generator in the flywheel energy storage system releases energy to a large-power impact load, and the flywheel speed drops rapidly, resulting in a rapid decline in the ability of the exciter to provide excitation current. At the same time, the impact load power increases approximately linearly, making the armature demagnetization reaction of the motor in the flywheel energy storage gradually strengthen. In the off-grid mode, the excitation system of the generator inside the flywheel energy storage system of the present invention is directly excited by the control device, with fast and strong excitation, enabling the normal start of the off-grid operation of the hybrid energy storage system.

[0022] Figure 1 The working principle of the hybrid energy storage system is as follows: When the power grid is operating normally, the control device collects the voltage signal and current signal of the power grid. When it is determined that the power grid fails according to the voltage signal and current signal of the power grid, the power grid is disconnected, and the hybrid energy storage system enters the off-grid operation mode. The control device respectively obtains the current actual charge-discharge power of the electrochemical energy storage system, the current actual charge-discharge power of the flywheel energy storage system, and the current real-time power of the load. According to the current actual charge-discharge power of the electrochemical energy storage system, the current actual charge-discharge power of the flywheel energy storage system, and the current real-time power of the load, it is determined whether it is an impact load. If it is an impact load, the control device performs excitation control on the generator in the flywheel energy storage system, so that the generator converts mechanical energy into electrical energy and releases it to the load. During the release process, when it is determined that the hybrid energy storage system enters a stable state, the control device controls the electrochemical energy storage system to take over all loads, and controls the flywheel energy storage system to perform small-power charging to cope with the next impact load.

[0023] See Figure 2 and Figure 3 , which provides a grid-connected to off-grid control method for a hybrid energy storage system of the present invention, mainly including the following steps: Step 1, when the power grid is operating normally, the control device collects the voltage signal and current signal of the power grid. When it is determined that the power grid fails according to the voltage signal and current signal of the power grid, the power grid is disconnected, and the hybrid energy storage system enters the off-grid operation mode.

[0024] It can be understood that when the power grid is operating normally, the control device controls the charge-discharge control of the battery energy storage system and the flywheel energy storage system according to the set program, and the excitation system inside the flywheel energy storage system operates independently and is not controlled by the control device.

[0025] When the power grid operates normally, the control device collects the voltage signal and current signal of the power grid, and based on the collected voltage signal and current signal of the power grid, the mutation rates of the power grid voltage, frequency and phase angle are respectively , , and the voltage sag time ; When any of the following conditions is met, it is determined that a fault has occurred in the power grid: Condition 1: ≥Un and ≥0.15 s; Condition 2: ≥0.8Un and ≥0.625 s; Condition 3: <0.4273 and 0.625< ≤2 s; Condition 4: ≥5 and ≥0.2 s; Condition 5: ≥5 and ≥1 s; When it is determined that there is a power grid fault, the control device immediately outputs a tripping signal through the control interface, which acts on QF0 to trip and disconnect the power grid from the hybrid energy storage system. At this time, the hybrid energy storage system enters the off-grid operation mode.

[0026] Step 2: The control device respectively obtains the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system, and the current real-time power of the load.

[0027] It can be understood that in the off-grid operation mode of the hybrid energy storage system, the control device collects the parameters of the battery energy storage unit, AC / DC power module, DC / AC power and flywheel energy storage unit through the communication interface, such as power signal, voltage, current, frequency, fault parameters, etc. At the same time, the charge and discharge power is sent to the battery energy storage unit, AC / DC power module, DC / AC power and flywheel energy storage unit through the communication interface.

[0028] The control device respectively obtains the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system, and the current real-time power of the load.

[0029] Among them, obtaining the current actual charge-discharge power of the electrochemical energy storage system through the control device includes: obtaining the charge-discharge power P1 sent by the control device to the PCS for the electrochemical energy storage system; obtaining the charge-discharge power P2 of the battery energy storage unit read by the PCS according to the CAN cable, and obtaining the maximum charge-discharge power P3 that the PCS itself can output. Then the current actual charge-discharge power Ppcs of the PCS = min(P1, P2, P3), and Ppcs is the current actual charge-discharge power of the electrochemical energy storage system.

[0030] Obtaining the current actual charge-discharge power of the flywheel energy storage system includes: obtaining the charge-discharge power P4 sent by the control device to the flywheel energy storage unit; reading the charge-discharge power P5 of the flywheel energy storage unit and the maximum charge-discharge power P6 of the AC / DC power module; and obtaining the maximum charge-discharge power P7 uploaded by the DC / AC power module to the control device. Then, when the control device does not turn on the excitation, the current actual charge-discharge power Pfy of the flywheel energy storage system = min(P4, P5, P6, P7).

[0031] Obtaining the current real-time power of the load includes: collecting the voltage u at the upper end of the circuit breaker QF3 through the control device 2 and the current signal i 2 , then the current real-time power of the load is .

[0032] Step 3, determine whether it is an impact load according to the current actual charge-discharge power of the electrochemical energy storage system, the current actual charge-discharge power of the flywheel energy storage system, and the current real-time power of the load.

[0033] It can be understood that in Step 2, the control device respectively obtains the current actual charge-discharge power of the electrochemical energy storage system, the current actual charge-discharge power of the flywheel energy storage system, and the current real-time power of the load.

[0034] When Pload > Pfy × K1 and Pload ≥ Ppcs × K2, it indicates that the power of the actual load at this time exceeds the overload capacity that the hybrid energy storage system can bear, and it is determined that the load is unstable and is an impact load. Among them, Pload is the current real-time power of the load, Pfy is the current actual charge-discharge power of the flywheel energy storage system, Ppcs is the current actual charge-discharge power introduced by the electrochemical energy storage system, K1 is the overload coefficient of the flywheel energy storage system, and K2 is the overload coefficient of the PCS.

[0035] Step 4, if it is an impact load, perform excitation control on the generator in the flywheel energy storage system through the control device, so that the generator converts mechanical energy into electrical energy and releases it to the load.

[0036] It is understandable that when the load is an impact load, the generator in the flywheel energy storage system is directly excited by the control device. As the excitation current rapidly increases, the energy storage generator quickly converts mechanical energy into electrical energy and releases it to the load, and the load of the load will gradually decrease.

[0037] Step 5, during the release process, when it is determined that the hybrid energy storage system enters a steady state, control the electrochemical energy storage system to take over all loads, and control the flywheel energy storage system to perform low-power charging to cope with the next impact load.

[0038] During the load release process, when it is detected that the load of the load Pload < Pfy × K3 or Pload < Ppcs × K4, it is considered that the load is stable at this time, and the hybrid energy storage system can enter stable operation. Wherein, Pload is the current real-time power of the load, Pfy is the current actual charge and discharge power of the flywheel energy storage system, Ppcs is the current actual charge and discharge power imported by the electrochemical energy storage system, K3 is the stability coefficient of the flywheel energy storage system, and K4 is the stability coefficient of the PCS.

[0039] After the hybrid energy storage system enters a steady state, control commands are sent to the PCS and the flywheel energy storage system through the control device to control the output power of the flywheel energy storage system to be gradually reduced until it reaches zero power, and the electrochemical energy storage system takes over all loads.

[0040] If the high-power load is random, the control device can set a program and send it to the flywheel energy storage system to charge to full capacity and standby to prepare for subsequent impact loads.

[0041] If the power of the electrochemical energy storage is exhausted and the mains power has not been restored, the control device controls the hybrid energy storage system to shut down.

[0042] A hybrid energy storage system and a grid-connected to off-grid control method provided by an embodiment of the present invention. When the grid is normal, the control device controls the charge and discharge control of the battery energy storage system and the flywheel energy storage system. The excitation system inside the flywheel energy storage system operates independently and is not controlled by the control device; when the grid fails, the control device disconnects the grid, and at the same time controls the hybrid energy storage system to enter the off-grid operation mode. During the process of grid-connected to off-grid, the generator in the flywheel energy storage is directly excited by the control device. As the excitation current rapidly increases, the energy storage generator quickly converts mechanical energy into electrical energy and releases it to the load. The present invention combines electrochemical energy storage and flywheel energy storage. In the off-grid mode, the excitation system of the flywheel energy storage generator is directly excited by the control device, which has advantages such as fast and strong excitation, and solves the problem of the PCS operating with impact loads in the off-grid mode.

[0043] It should be noted that in the above embodiments, the descriptions of the respective embodiments have their own emphases. For parts not described in detail in a certain embodiment, reference may be made to the relevant descriptions of other embodiments.

[0044] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic inventive concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments as well as all changes and modifications falling within the scope of the present invention.

[0045] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A hybrid energy storage system, characterized in that: The hybrid energy storage system includes an electrochemical energy storage system, a flywheel energy storage system and a control device; When the power grid is operating normally, the control device collects the voltage signal and the current signal of the power grid, and when it is determined that the power grid fails according to the voltage signal and the current signal of the power grid, the power grid is disconnected, and the hybrid energy storage system enters the off-grid operation mode; The current actual charging and discharging power of the electrochemical energy storage system, the current actual charging and discharging power of the flywheel energy storage system and the current real-time power of the load are respectively obtained by the control device; Determining whether it is an impact load according to the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system, and the current real-time power of the load; If it is an impact load, the control device is used to control the excitation of the generator in the flywheel energy storage system, so that the generator converts mechanical energy into electrical energy and releases it to the load; During the release process, when it is determined that the hybrid energy storage system has entered a stable state, the electrochemical energy storage system is controlled to take over all loads, and the flywheel energy storage system is controlled to perform low-power charging to cope with the next impact load.

2. The hybrid energy storage system according to claim 1, characterized in that: The electrochemical energy storage system includes a battery energy storage unit and a process control system PCS, the battery energy storage unit is connected to the PCS via a DC cable, the PCS is connected to the bus via a circuit breaker QF1, the flywheel energy storage system includes a flywheel energy storage unit, an AC / DC power module and a DC / AC power module, the flywheel energy storage unit is connected to the DC / AC power module via an AC cable, the DC / AC power module is connected to the AC / DC power module via a DC cable, the AC / DC power module is connected to the bus via a circuit breaker QF2, the mains is connected to the bus via a circuit breaker QF1, the load is connected to the bus via a circuit breaker QF3, and the control device is respectively connected to the electrochemical energy storage system, the flywheel energy storage system, the power grid and the load.

3. A grid-connected to off-grid control method for a hybrid energy storage system according to claim 1, characterized in that: include: When the power grid is operating normally, the voltage signal and current signal of the power grid are collected through the control device. When the power grid is determined to have a fault according to the voltage signal and current signal of the power grid, the power grid is disconnected and the hybrid energy storage system enters the off-grid operation mode; The current actual charging and discharging power of the electrochemical energy storage system, the current actual charging and discharging power of the flywheel energy storage system and the current real-time power of the load are respectively obtained through the control device; Determining whether it is an impact load according to the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system, and the current real-time power of the load; If it is an impact load, the control device is used to control the excitation of the generator in the flywheel energy storage system, so that the generator converts mechanical energy into electrical energy and releases it to the load; During the release process, when it is determined that the hybrid energy storage system has entered a stable state, the electrochemical energy storage system is controlled to take over all loads, and the flywheel energy storage system is controlled to perform low-power charging to cope with the next impact load.

4. The grid-connected to off-grid control method according to claim 3, characterized in that: When the power grid is operating normally, the control device collects the voltage signal and the current signal of the power grid, and when it is determined that the power grid fails according to the voltage signal and the current signal of the power grid, the power grid is disconnected, and the hybrid energy storage system enters the off-grid operation mode, including: When the power grid is operating normally, the control device collects the voltage signal and current signal of the power grid, and obtains the mutation rates of the power grid voltage, frequency and phase angle according to the collected voltage signal and current signal of the power grid. , , And get the voltage drop time ; When any of the following conditions is met, it is determined that the power grid fails: Condition 1: ≥Un and ≥0.15s; Condition 2: ≥0.8Un and ≥0.625s; Condition 3: < 0.4273 and 0.625 < ≤ 2s; Condition 4: ≥5 and ≥0.2s; Condition 5: ≥5 and ≥1s; When it is determined that a fault occurs in the power grid, a trip signal is outputted through the control device to control the circuit breaker on the power grid to disconnect the power grid, and the hybrid energy storage system enters an off-grid operation mode.

5. The grid-connected to off-grid control method according to claim 3, characterized in that: The electrochemical energy storage system includes a battery energy storage unit and a process control system PCS, the battery energy storage unit is connected to the PCS via a DC cable, and the PCS is connected to the busbar via a circuit breaker QF1; Obtaining the current actual charging and discharging power of the electrochemical energy storage system through the control device includes: Obtain the power P1 sent by the control device to the PCS for charging and discharging the electrochemical energy storage system; Obtain the chargeable and dischargeable power P2 of the battery energy storage unit read by the PCS according to the CAN cable, and obtain the maximum chargeable and dischargeable power P3 that the PCS itself can output; Then the current actual charging and discharging power Ppcs of the PCS=min(P1, P2, P3), where Ppcs is the current actual charging and discharging power of the electrochemical energy storage system.

6. The grid-connected to off-grid control method according to claim 3, characterized in that: The flywheel energy storage system includes a flywheel energy storage unit, an AC / DC power module and a DC / AC power module, wherein the flywheel energy storage unit is connected to the DC / AC power module via an AC cable, the DC / AC power module is connected to the AC / DC power module via a DC cable, and the AC / DC power module is connected to the busbar via a circuit breaker QF2; The current actual charging and discharging power of the flywheel energy storage system includes: Obtaining the charging and discharging power P4 sent by the control device to the flywheel energy storage unit; Read the chargeable and dischargeable power P5 of the flywheel energy storage unit and the maximum chargeable and dischargeable power P6 of the AC / DC power module; and obtain the maximum chargeable and dischargeable power P7 that the DC / AC power module can upload to the control device; When the control device does not start excitation, the current actual charging and discharging power Pfy of the flywheel energy storage system is min (P4, P5, P6, P7).

7. The grid-connected to off-grid control method according to claim 3, characterized in that: The load is connected to the busbar through the circuit breaker QF3 to obtain the current real-time power of the load, including: The control device collects the voltage u2 and current signal i2 at the upper end of the circuit breaker QF3, and the current real-time power of the load is .

8. The grid-connected to off-grid control method according to claim 3, characterized in that: The determining whether it is an impact load according to the current actual charge and discharge power of the electrochemical energy storage system, the current actual charge and discharge power of the flywheel energy storage system and the current real-time power of the load includes: When Pload>Pfy×K1 and Pload≥Ppcs×K2, the load is determined to be unstable and is an impact load, where Pload is the current real-time power of the load, Pfy is the current actual charging and discharging power of the flywheel energy storage system, Ppcs is the current actual charging and discharging power imported by the electrochemical energy storage system, K1 is the overload coefficient of the flywheel energy storage system, and K2 is the overload coefficient of the PCS.

9. The grid-connected to off-grid control method according to claim 3, characterized in that: During the release process, when it is determined that the hybrid energy storage system enters a stable state, the electrochemical energy storage system is controlled to take over all loads, and the flywheel energy storage system is controlled to perform low-power charging to cope with the next impact load, including: When Pload<Pfy×K3 or Pload<Ppcs×K4, it is determined that the load is stable and the hybrid energy storage system enters a stable operation state, wherein Pload is the current real-time power of the load, Pfy is the current actual charge and discharge power of the flywheel energy storage system, Ppcs is the current actual charge and discharge power imported by the electrochemical energy storage system, K3 is the stability coefficient of the flywheel energy storage system, and K4 is the stability coefficient of the PCS; The control device sends control instructions to the PCS and the flywheel energy storage system to control the gradual reduction of the output power of the flywheel energy storage system until it drops to zero power, and the electrochemical energy storage system takes over all the loads.

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