Directly-hung energy storage direct current collection system alternating current fault coordination control method and device

Abstract

The application discloses a direct-hanging energy storage direct-current collection system AC fault coordination control method and device, and relates to the technical field of energy storage. The direct-current collection system comprises a direct-hanging energy storage, a photovoltaic generator set, a flexible converter and an energy storage coordination controller. When an AC fault occurs, the energy storage coordination controller judges whether the photovoltaic generator set enters an AC fault ride-through mode according to the AC side voltage of the flexible converter or the direct-current bus voltage. When the direct-hanging energy storage does not have the capability of controlling the direct-current bus voltage, the energy storage coordination controller sends a limited output power instruction to the photovoltaic generator set. When the direct-hanging energy storage has the capability of controlling the direct-current bus voltage, the direct-hanging energy storage is switched from fixed active power control to fixed direct-current voltage control according to the over-limit quantity of the direct-current bus voltage by using a ladder control mode. When the direct-current bus voltage is restored, the direct-hanging energy storage is switched from fixed direct-current voltage control to fixed active power control by using a ladder control mode according to the direct-current bus voltage, and the photovoltaic generator set is controlled to operate in a maximum power tracking mode.

Description

Technical Field

[0001] This invention belongs to the field of DC power transmission grid and equipment technology, specifically relating to a method and system for coordinated control of AC faults in a direct-connected energy storage DC collection system. Background Technology

[0002] Compared to traditional thermal power, hydropower, and new energy bases, the "Shagohuang" region boasts abundant wind and solar resources, making it suitable for the centralized construction of large-scale new energy bases. However, these bases are located at the end of the AC power grid, and traditional AC collection and transmission methods face challenges such as insufficient system strength, homogeneity of power sources at both ends, and inadequate system flexibility and stability. Furthermore, they require a large number of SVG (Static Var Generator), energy storage, and synchronous condenser auxiliary support devices. Medium-voltage direct current (DC) generation technology, on the other hand, avoids these problems associated with traditional AC collection and transmission. It offers stable power transmission, high transmission capacity, and is suitable for new energy base collection and transmission scenarios with wide collection areas and long transmission distances. Therefore, medium-voltage DC generation technology is gradually becoming an option for new energy base transmission scenarios.

[0003] Due to the randomness and volatility of photovoltaics, the channel capacity utilization rate of the converter threshold is low when only photovoltaics are used. At the same time, considering that the converter threshold will reduce the active power output when AC faults occur, it is necessary to rely on DC energy dissipation devices or direct-connected power-type energy storage and energy-type energy storage to limit the rise of DC voltage and avoid DC overvoltage system blockage. Energy dissipation devices can only be used to consume active power and cannot effectively suppress the randomness and volatility of photovoltaics. Therefore, the application of direct-connected energy storage photovoltaic all-DC systems in the medium voltage field has broad application prospects.

[0004] The increased complexity of the grid connection due to the use of DC boost collection necessitates ensuring the reliable operation of photovoltaic power plants and preventing transient impacts on the main grid. This requires coordinated control of the photovoltaic DC power generation system. However, current research on DC transmission projects focuses on the control of flexible converters and DC transformers themselves, as well as the AC fault ride-through technology at the receiving end of flexible converters using DC energy-consuming devices. Research has not been conducted on the use of direct-connected power storage, energy storage, and coordinated control of photovoltaic, wind power, and other new energy sources with flexible converters to achieve AC fault ride-through technology at the receiving end of flexible converters. Therefore, there is an urgent need to study the use of DC-side direct-connected power storage, energy storage, and the coordinated operation of flexible converters and new energy power generation equipment to ensure the safe and reliable operation of photovoltaic power plants when connected to the main grid via DC boost collection. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method and apparatus for coordinated control of AC faults in a direct-connected energy storage DC collection system. It proposes a coordinated strategy applicable to direct-connected energy storage, converter threshold, and photovoltaic power generation units in AC faults within a full DC power generation system, thereby providing a method for coordinated control of AC faults in a direct-connected energy storage DC collection system.

[0006] The present invention adopts the following technical solution.

[0007] This invention proposes a collaborative control method for AC faults in a direct-connected energy storage DC collection system. The DC collection system includes: direct-connected energy storage, photovoltaic generator set, flexible converter, and energy storage coordination controller. The direct-connected energy storage includes power-type energy storage and energy-type energy storage. The photovoltaic generator set includes multiple photovoltaic power generation units. The DC sides of the direct-connected energy storage, photovoltaic generator set, and flexible converter are all connected to the DC bus, and the AC side of the flexible converter is connected to the AC power grid.

[0008] The methods include:

[0009] During steady-state operation, under the constraints of SOC balance and charge / discharge cycles, the energy storage coordination controller allocates active power discharge commands to power-type energy storage and active power charging commands to energy-type energy storage. Both power-type and energy-type energy storage perform constant active power control.

[0010] In the event of an AC fault, if the DC controller of the flexible converter determines that the DC collection system has entered the AC fault ride-through mode based on the grid connection point voltage, the energy storage coordination controller determines whether the photovoltaic generator has entered the AC fault ride-through mode based on the AC side voltage or DC bus voltage of the flexible converter.

[0011] When the photovoltaic generator enters the AC fault ride-through mode, if the direct-connected energy storage does not have the ability to control the DC bus voltage, the energy storage coordination controller sends an output power limit command to the photovoltaic generator. Upon receiving the output power limit command, the photovoltaic generator determines whether the inverter has triggered a lockout based on the output DC voltage, and determines the number of photovoltaic inverters that do not trigger a lockout based on the DC transformer loss. When the direct-connected energy storage has the ability to control the DC bus voltage, the energy storage coordination controller switches the direct-connected energy storage from constant active power control to constant DC voltage control using a stepped control method based on the DC bus voltage exceeding the limit.

[0012] When the DC bus voltage recovers, the energy storage coordination controller adopts a stepped control method based on the DC bus voltage to switch the direct-connected energy storage from constant DC voltage control to constant active power control, and controls the photovoltaic generator to operate in maximum power point tracking mode.

[0013] Preferably, if the grid connection point voltage of the DC controller of the flexible converter is less than a set first voltage threshold, the DC collection system is determined to enter the AC fault ride-through mode, wherein the set first voltage threshold is 0.9 pu;

[0014] When the energy storage coordination controller detects that the AC side voltage of the flexible converter is less than the set first voltage threshold or the DC bus voltage is greater than the set second voltage threshold, it determines that each photovoltaic power generation unit enters the AC fault ride-through mode; otherwise, each photovoltaic power generation unit operates in the maximum power point tracking mode. The set second voltage threshold is 1.1 pu.

[0015] Preferably, when determining whether the photovoltaic generator set has entered the AC fault ride-through mode, if the rate of change of the DC bus voltage is greater than the second voltage threshold, all power-type energy storage units are switched to constant DC voltage control.

[0016] Preferably, when all the directly connected energy storage devices are in a charging state, if the charging power is greater than a set first power threshold or the SOC is greater than a set first SOC threshold, then it is determined that the directly connected energy storage devices do not have the ability to control the DC bus voltage; otherwise, when all the directly connected energy storage devices are in a charging state, if the charging power is not greater than a set first power threshold and the SOC is not greater than a set first SOC threshold, then it is determined that the directly connected energy storage devices have the ability to control the DC bus voltage; wherein, the set first power threshold is 0.9 pu and the set first SOC threshold is 0.9.

[0017] Preferably, the energy storage coordinating controller issues a power limiting command to the photovoltaic generator set based on the fault type and the fault level determined by the AC voltage drop depth, setting the upper limit of the output active power. As shown in the following formula:

[0018]

[0019] In the formula, To output the real-time value of active power, The limiting coefficient is based on the fault type. The limiting factor is based on the fault level. This is the power limiting value based on the AC voltage drop depth.

[0020] Preferably, when a photovoltaic inverter that receives a power limit command detects that its output DC voltage is greater than a set second voltage threshold, the number of photovoltaic inverters in each DC collection group that do not trigger the blocking is calculated using the following formula. :

[0021]

[0022] In the formula, This refers to the power loss of the DC transformer in the DC collection group. This refers to the active power output of the photovoltaic inverter in maximum power point tracking mode.

[0023] The number of photovoltaic inverters that trigger the blocking is determined based on the actual number of photovoltaic inverters operating in the DC aggregation group and the calculated number of photovoltaic inverters that do not trigger the blocking.

[0024] Preferably, energy storage systems for control switching are selected based on their operating status and operating parameters; including:

[0025] Power-type energy storage devices that are in the charging state and whose charging power is greater than the set second power threshold and whose SOC is greater than the set second SOC threshold will not perform control switching; all other power-type energy storage devices will perform control switching.

[0026] Energy storage devices that are in the charging state and whose charging power is greater than the set second power threshold and whose SOC is greater than the set second SOC threshold will not be subject to control switching; all other energy storage devices will be subject to control switching.

[0027] The second power threshold is 0.8 pu, and the second SOC threshold is 0.8.

[0028] Preferably, if the number of power-type energy storage devices performing control switching is not zero, then when the DC bus voltage is greater than the set second voltage threshold and less than the set third voltage threshold, the power-type energy storage in the discharge state is switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set third voltage threshold and less than the set fourth voltage threshold, the power-type energy storage in the charging state is switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set fourth voltage threshold and less than the set fifth voltage threshold, the energy-type energy storage in the discharge state is switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set fifth voltage threshold, the energy-type energy storage in the charging state is switched from constant power control to constant DC voltage control.

[0029] The third voltage threshold is set to 1.12 pu, the fourth voltage threshold is set to 1.13 pu, and the fifth voltage threshold is set to 1.14 pu.

[0030] Preferably, if the number of power-type energy storage devices performing control switching is zero, then when the DC bus voltage is greater than the set second voltage threshold and less than the set sixth voltage threshold, the energy storage device in the discharge state is switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set sixth voltage threshold, the energy storage device in the charging state is switched from constant power control to constant DC voltage control; wherein, the set sixth voltage threshold is 1.11 pu.

[0031] Preferably, when all direct-connected energy storage is switched to constant DC voltage control, the inverter detects that the output DC voltage is greater than the set seventh voltage threshold and triggers a temporary lockout of the inverter, wherein the seventh voltage threshold is 1.15 pu.

[0032] Preferably, when the DC bus voltage is less than the set eighth voltage threshold, the power-type energy storage preferentially switches from constant DC voltage control to constant active power control, and the eighth voltage threshold is 1.03 pu;

[0033] When the DC bus voltage is less than the set ninth voltage threshold, the energy storage system switches from constant DC voltage control to constant active power control. The ninth voltage threshold is 1.02 pu.

[0034] When the inverter detects that the output DC voltage is less than the set tenth voltage threshold, each photovoltaic power generation unit operates in maximum power point tracking mode, where the tenth voltage threshold is 1.01 pu.

[0035] This invention also proposes an AC fault collaborative control device for a direct-connected energy storage DC collection system, the collaborative control device comprising:

[0036] The steady-state control module is used to allocate active power discharge commands to power-type energy storage and active power charging commands to energy-type energy storage under SOC equalization constraints and charge / discharge cycle constraints during steady-state operation. Both power-type and energy-type energy storage perform constant active power control.

[0037] The fault control module is used to handle AC faults. If the DC controller of the flexible converter determines that the DC collection system has entered AC fault ride-through mode based on the grid connection point voltage, the energy storage coordination controller determines whether the photovoltaic generator set has entered AC fault ride-through mode based on the AC side voltage or DC bus voltage of the flexible converter. When the photovoltaic generator set enters AC fault ride-through mode, if the direct-connected energy storage does not have the ability to control the DC bus voltage, the energy storage coordination controller sends an output power limit command to the photovoltaic generator set. Upon receiving the output power limit command, the photovoltaic generator set determines whether the inverter triggers a lockout based on the output DC voltage and determines the number of photovoltaic inverters that do not trigger a lockout based on the DC transformer loss. When the direct-connected energy storage has the ability to control the DC bus voltage, the energy storage coordination controller switches the direct-connected energy storage from constant active power control to constant DC voltage control using a stepped control method based on the DC bus voltage exceeding the limit. When the DC bus voltage recovers, the energy storage coordination controller switches the direct-connected energy storage from constant DC voltage control to constant active power control using a stepped control method based on the DC bus voltage, and controls the photovoltaic generator set to operate in maximum power point tracking mode.

[0038] The present invention is also a terminal, including a processor and a storage medium; the storage medium is used to store instructions; the processor is used to perform operations according to the instructions to execute the steps of the method.

[0039] The present invention is also a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method.

[0040] The beneficial effects of this invention, compared with the prior art, include at least the following: For the future adoption of DC transmission technology for renewable energy in desert areas, this invention proposes a DC-side direct-connected energy storage system in conjunction with existing energy sources to achieve AC fault ride-through. Compared to using DC energy-consuming devices, direct-connected energy storage can absorb and store active power. When the fault ride-through ends, the active power can be output to the AC system. Furthermore, to improve the utilization rate of flexible converters, since the photovoltaic capacity is generally greater than the output capacity of the flexible converter, the charging and discharging of the energy storage system can suppress renewable energy fluctuations while effectively preventing overload of the flexible converter. This invention avoids the failure of relying solely on a single device (such as energy storage) for fault ride-through (where equipment failure prevents the absorption of active power), which can cause DC voltage rise and equipment overvoltage shutdown. Moreover, compared to parallel energy storage solutions on the AC side, which can only suppress renewable energy fluctuations and provide reactive power support to the AC grid during AC fault ride-through, this invention does not benefit the DC grid. Attached Figure Description

[0041] Figure 1 This is a flowchart of the AC fault collaborative control method for a direct-connected energy storage DC collection system proposed in this invention. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0043] The direct-connected energy storage DC collection system includes: direct-connected energy storage, photovoltaic generator set, flexible converter, and energy storage coordination controller. The direct-connected energy storage, photovoltaic generator set, and flexible converter are all connected to the DC bus. The flexible converter controls the DC bus voltage and provides reactive power to the AC grid.

[0044] The system includes n sets of direct-connected energy storage directly connected to the high-voltage DC bus. The direct-connected energy storage includes power-type energy storage and energy-type energy storage. Each set of direct-connected energy storage has two operating modes: constant active power control and constant DC voltage control. During steady-state operation, the direct-connected energy storage charges and discharges in constant active power control mode based on the dispatch power command, and maintains the remaining state of charge (SOC) in an ideal state, where n≥2.

[0045] The photovoltaic generator set includes n DC collection groups. Each DC collection group includes: 1 DC transformer and multiple photovoltaic power generation units. The MPPT controllers of the multiple photovoltaic power generation units are connected in parallel and then stepped up through a DC transformer. Each DC collection group is connected to a high-voltage DC bus, and each photovoltaic power generation unit operates in maximum power point tracking mode.

[0046] The flexible converter controls the DC voltage of the DC collection system and provides reactive power to the AC grid. The DC controller of the flexible converter detects the voltage at the grid connection point in real time.

[0047] This invention provides a method for coordinated control of AC faults in a direct-connected energy storage DC collection system, such as... Figure 1 As shown, the method includes:

[0048] Step 1: During steady-state operation, under the constraints of SOC balance and charge / discharge cycles, the energy storage coordination controller allocates active power discharge commands to power-type energy storage and active power charging commands to energy-type energy storage. Both power-type and energy-type energy storage perform constant active power control.

[0049] Specifically, the energy storage coordinating controller receives active power commands from the dispatch center. Under the premise of maintaining SOC balance and limiting the number of daily charge and discharge cycles, it prioritizes the allocation of active power discharge commands to power-type energy storage and the allocation of active power charging commands to energy-type energy storage. This facilitates a rapid switch to ride-through control mode during AC faults and a rapid reduction in DC voltage.

[0050] Step 2: In the event of an AC fault, if the DC controller of the flexible converter determines that the DC collection system has entered the AC fault ride-through mode based on the grid connection point voltage, the energy storage coordination controller determines whether the photovoltaic generator has entered the AC fault ride-through mode based on the AC side voltage or DC bus voltage of the flexible converter.

[0051] Specifically, when the DC controller of the flexible converter detects that the grid connection point voltage is less than a set first voltage threshold, it determines that the DC collection system has entered the AC fault ride-through mode. The set first voltage threshold is 0.9 pu. When entering the AC fault ride-through mode, the flexible converter limits the active current, which reduces the active power injected into the AC grid by the DC collection system. Since each photovoltaic power generation unit operates in maximum power point tracking mode, the active power injected into the DC bus by the photovoltaic power generation unit remains unchanged, resulting in the DC bus voltage being raised. Therefore, this invention proposes a control strategy whereby the energy storage coordination controller determines whether the DC power generation system has entered the AC fault ride-through mode based on the AC side voltage of the flexible converter or the DC bus voltage when the DC controller of the flexible converter determines that the DC collection system has entered the AC fault ride-through mode based on the grid connection point voltage. This achieves AC fault ride-through at the receiving end of the flexible converter by using direct-connected power storage, energy-type energy storage, and new energy sources such as photovoltaic and wind power in coordinated control with the flexible converter.

[0052] Specifically, when the energy storage coordinating controller detects that the AC side voltage of the flexible converter is less than the set first voltage threshold or the DC bus voltage is greater than the set second voltage threshold, it determines that each photovoltaic power generation unit has entered the AC fault ride-through mode; otherwise, each photovoltaic power generation unit still operates in the maximum power point tracking mode; wherein, the set second voltage threshold is 1.1 pu.

[0053] However, once all photovoltaic units enter fault ride-through mode, their active power exchange with the grid becomes unstable and uncontrollable. At this point, the DC bus essentially loses its stable power balance point with the AC grid; without control, the DC bus voltage will fluctuate drastically, causing the photovoltaic inverters to disconnect from the grid due to DC overvoltage, leading to the collapse of the entire system. Furthermore, based on the rate of change of the DC bus voltage to assist in judging the degree of AC fault limiting and whether the photovoltaic generator has entered AC fault ride-through mode, when the rate of change of the DC bus voltage exceeds the second voltage threshold, all power-type energy storage switches to constant DC voltage control, and then further adjustments are made based on whether the direct-connected energy storage has the ability to control the DC bus voltage.

[0054] Step 3: The photovoltaic generator set enters the AC fault ride-through mode. When the direct-connected energy storage does not have the ability to control the DC bus voltage, the energy storage coordination controller sends an output power limit command to the photovoltaic generator set. Upon receiving the output power limit command, the photovoltaic generator set determines whether the inverter is triggered to lock out based on the output DC voltage, and determines the number of photovoltaic inverters that are not triggered to lock out based on the DC transformer loss.

[0055] Specifically, when all connected energy storage devices are in a charging state, if the charging power is greater than a set first power threshold or the SOC is greater than a set first SOC threshold, then the connected energy storage devices are determined to lack the ability to control the DC bus voltage; otherwise, when all connected energy storage devices are in a charging state, if the charging power is not greater than a set first power threshold and the SOC is not greater than a set first SOC threshold, then the connected energy storage devices are determined to have the ability to control the DC bus voltage; wherein, the set first power threshold is 0.9 pu and the set first SOC threshold is 0.9.

[0056] When the direct-connected energy storage lacks the ability to control the DC bus voltage, the energy storage coordination controller issues a power limiting command to the photovoltaic generator based on the fault type and fault level. The photovoltaic inverter reduces its active power output according to the power limiting command. The power limiting command limits power in stages according to the fault type and fault level, as shown in the following formula:

[0057]

[0058] In the formula, This is the upper limit of the output active power. To output the real-time value of active power, The limiting coefficient is based on the fault type. The limiting factor is based on the fault level. This is a power limiting value based on the AC voltage sag depth;

[0059] For three-phase faults The preferred value is 0.3 for two-phase faults. The preferred value is 0.6 for single-phase faults. The preferred value is 0.8; an AC voltage drop depth of 0.2 pu corresponds to an emergency fault. The preferred value is 0.3 and A value of 0pu corresponds to a severe fault; an AC voltage drop depth of 0.7pu indicates a critical fault. The preferred value is 0.6 and The value is 0.5 pu; an AC voltage drop depth of 0.9 pu corresponds to a minor fault. The preferred value is 0.8 and The value is 0.8 pu; when there is no fault. The value is 1.0pu.

[0060] In the method proposed in this invention, when the photovoltaic generator set enters the AC fault ride-through mode, if the direct-connected energy storage cannot limit the DC bus voltage, the energy storage coordination controller sends an output power limit command to the inverter of each photovoltaic power generation unit to avoid the DC bus voltage exceeding the limit.

[0061] When a photovoltaic inverter that receives a power limit command detects that its output DC voltage exceeds a set second voltage threshold, the number of photovoltaic inverters in each DC collection group that do not trigger the blocking is calculated using the following formula. :

[0062]

[0063] In the formula, This refers to the power loss of the DC transformer in the DC collection group. This refers to the active power output of the photovoltaic inverter in maximum power point tracking mode.

[0064] If the number of photovoltaic inverters actually operating within the DC aggregation group The number of photovoltaic inverters that do not trigger the blocking If the numbers are equal, none of the photovoltaic inverters in the DC aggregation group will trigger the blocking; if the actual number of photovoltaic inverters operating in the DC aggregation group is equal... Greater than the number of photovoltaic inverters that do not trigger the blocking Then the DC collection group - The invention triggers a temporary lockout for photovoltaic inverters. It determines the target number of photovoltaic inverters operating in the DC collection group based on the loss of the DC transformer, and determines the number of photovoltaic inverters that will be temporarily locked out based on the difference between the actual number of operating inverters and the target number of operating inverters, so as to quickly reduce the active power output and maintain DC voltage stability.

[0065] When the DC voltage drops to the set threshold, the photovoltaic inverter exits the temporary lockout and resumes active power output according to the slope.

[0066] Step 4: The photovoltaic generator enters the AC fault ride-through mode. When the direct-connected energy storage has the ability to control the DC bus voltage, the energy storage coordination controller adopts a stepped control method to switch the direct-connected energy storage from constant active power control to constant DC voltage control based on the DC bus voltage exceeding the limit.

[0067] Specifically, when the direct-connected energy storage is capable of regulating the DC bus voltage, the direct-connected energy storage control device monitors the DC bus voltage in real time. When the direct-connected energy storage detects that the DC bus voltage is greater than the set second voltage threshold, the direct-connected energy storage switches the power control to DC voltage control. To avoid all direct-connected energy storage devices switching to constant DC voltage mode at the same time, which would cause large fluctuations in the DC voltage of the DC system, the direct-connected energy storage adopts a stepped control method to switch from constant active power control to constant DC voltage control.

[0068] Specifically, step 4 includes:

[0069] Step 4.1: Based on the operating status and parameters of the energy storage, select the energy storage to perform control switching; including:

[0070] Power-type energy storage devices that are in the charging state and whose charging power is greater than the set second power threshold and whose SOC is greater than the set second SOC threshold will not perform control switching; all other power-type energy storage devices will perform control switching.

[0071] Energy storage devices that are in the charging state and whose charging power is greater than the set second power threshold and whose SOC is greater than the set second SOC threshold will not be subject to control switching; all other energy storage devices will be subject to control switching.

[0072] The second power threshold is 0.8 pu, and the second SOC threshold is 0.8.

[0073] Step 4.2: If the number of power-type energy storage units undergoing control switching is not zero, then when the DC bus voltage is greater than the set second voltage threshold and less than the set third voltage threshold, the power-type energy storage in the discharge state will be switched from constant active power control to constant DC voltage control; when the DC bus voltage is greater than the set third voltage threshold and less than the set fourth voltage threshold, the power-type energy storage in the charging state will be switched from constant active power control to constant DC voltage control; when the DC bus voltage is greater than the set fourth voltage threshold and less than the set fifth voltage threshold, the energy-type energy storage in the discharge state will be switched from constant active power control to constant DC voltage control; when the DC bus voltage is greater than the set fifth voltage threshold, the energy-type energy storage in the charging state will be switched from constant active power control to constant DC voltage control.

[0074] The third voltage threshold is set to 1.12 pu, the fourth voltage threshold is set to 1.13 pu, and the fifth voltage threshold is set to 1.14 pu.

[0075] Step 4.3: If the number of power-type energy storage devices performing control switching is zero, then when the DC bus voltage is greater than the set second voltage threshold and less than the set sixth voltage threshold, the energy storage devices in the discharge state will be switched from constant active power control to constant DC voltage control; when the DC bus voltage is greater than the set sixth voltage threshold, the energy storage devices in the charging state will be switched from constant active power control to constant DC voltage control; wherein, the set sixth voltage threshold is 1.11 pu.

[0076] Step 4.4: When all direct-connected energy storage devices are switched to constant DC voltage control, if the inverter detects that the output DC voltage is greater than the set seventh threshold, the inverter will be temporarily locked.

[0077] When all direct-connected energy storage is switched to constant DC voltage control, the DC voltage continues to rise. The photovoltaic inverter control device monitors the low-voltage side bus voltage in real time. When the DC bus voltage is greater than the set seventh voltage threshold, the photovoltaic inverter triggers temporary blocking processing to quickly reduce the active power output. The seventh voltage threshold is 1.15 pu.

[0078] Step 5: When the DC bus voltage recovers, the energy storage coordination controller uses a stepped control method based on the DC bus voltage to switch the direct-connected energy storage from constant DC voltage control to constant active power control, and controls the photovoltaic generator to operate in maximum power point tracking mode, including:

[0079] When the DC bus voltage is less than the set eighth voltage threshold, the power-type energy storage will switch from constant DC voltage control to constant active power control, and reduce the charging power to 0 according to the set slope. The eighth voltage threshold is 1.03 pu.

[0080] When the DC bus voltage is less than the set ninth voltage threshold, the energy storage switches from constant DC voltage control to constant active power control, and reduces the charging power to 0 according to the set slope. The ninth voltage threshold is 1.02 pu.

[0081] When the inverter detects that the output DC voltage is less than the set tenth voltage threshold, the inverter exits the temporary lockout, and each photovoltaic power generation unit operates in maximum power point tracking mode, restoring active power output according to the set slope. The tenth voltage threshold is 1.01 pu.

[0082] When the flexible converter unit detects that the AC voltage at the grid connection point has recovered, the flexible converter resumes its active power transmission capability.

[0083] This control method utilizes a collaborative control approach for AC faults in a direct-connected energy storage DC collection system to achieve fault ride-through capability of the energy-free system during AC faults in the entire DC power generation system, thereby reducing human intervention and ensuring the safe and stable operation of the power grid.

[0084] This invention also proposes an AC fault collaborative control device for a direct-connected energy storage DC collection system, the collaborative control device comprising:

[0085] The steady-state control module is used to allocate active power discharge commands to power-type energy storage and active power charging commands to energy-type energy storage under SOC equalization constraints and charge / discharge cycle constraints during steady-state operation. Both power-type and energy-type energy storage perform constant active power control.

[0086] The fault control module is used to handle AC faults. If the DC controller of the flexible converter determines that the DC collection system has entered AC fault ride-through mode based on the grid connection point voltage, the energy storage coordination controller determines whether the photovoltaic generator set has entered AC fault ride-through mode based on the AC side voltage or DC bus voltage of the flexible converter. When the photovoltaic generator set enters AC fault ride-through mode, if the direct-connected energy storage does not have the ability to control the DC bus voltage, the energy storage coordination controller sends an output power limit command to the photovoltaic generator set. Upon receiving the output power limit command, the photovoltaic generator set determines whether the inverter triggers a lockout based on the output DC voltage and determines the number of photovoltaic inverters that do not trigger a lockout based on the DC transformer loss. When the direct-connected energy storage has the ability to control the DC bus voltage, the energy storage coordination controller switches the direct-connected energy storage from constant active power control to constant DC voltage control using a stepped control method based on the DC bus voltage exceeding the limit. When the DC bus voltage recovers, the energy storage coordination controller switches the direct-connected energy storage from constant DC voltage control to constant active power control using a stepped control method based on the DC bus voltage, and controls the photovoltaic generator set to operate in maximum power point tracking mode.

[0087] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0088] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0089] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0090] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A method for coordinated control of AC faults in a direct-connected energy storage DC collection system, the DC collection system comprising: The system comprises a direct-connected energy storage unit, a photovoltaic (PV) generator set, a flexible converter, and an energy storage coordination controller. The direct-connected energy storage unit includes power-type energy storage and energy-type energy storage. The PV generator set includes multiple PV power generation units. The DC side of the direct-connected energy storage unit, the PV generator set, and the flexible converter are all connected to a DC bus, and the AC side of the flexible converter is connected to an AC power grid. The method is characterized by comprising: During steady-state operation, under the constraints of SOC balance and charge / discharge cycles, the energy storage coordination controller allocates active power discharge commands to power-type energy storage and active power charging commands to energy-type energy storage. Both power-type and energy-type energy storage perform constant active power control. In the event of an AC fault, if the DC controller of the flexible converter determines that the DC collection system has entered the AC fault ride-through mode based on the grid connection point voltage, the energy storage coordination controller determines whether the photovoltaic generator has entered the AC fault ride-through mode based on the AC side voltage or DC bus voltage of the flexible converter. When the photovoltaic generator enters the AC fault ride-through mode, if the direct-connected energy storage does not have the ability to control the DC bus voltage, the energy storage coordination controller sends an output power limit command to the photovoltaic generator. Upon receiving the output power limit command, the photovoltaic generator determines whether the inverter has triggered a lockout based on the output DC voltage, and determines the number of photovoltaic inverters that do not trigger a lockout based on the DC transformer loss. When the direct-connected energy storage has the ability to control the DC bus voltage, the energy storage coordination controller switches the direct-connected energy storage from constant active power control to constant DC voltage control using a stepped control method based on the DC bus voltage exceeding the limit. When the DC bus voltage recovers, the energy storage coordination controller adopts a stepped control method based on the DC bus voltage to switch the direct-connected energy storage from constant DC voltage control to constant active power control, and controls the photovoltaic generator to operate in maximum power point tracking mode.

2. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 1, characterized in that, If the grid connection point voltage of the DC controller of the flexible converter is less than the set first voltage threshold, the DC collection system is determined to enter the AC fault ride-through mode, wherein the set first voltage threshold is 0.9 pu; When the energy storage coordination controller detects that the AC side voltage of the flexible converter is less than the set first voltage threshold or the DC bus voltage is greater than the set second voltage threshold, it determines that each photovoltaic power generation unit enters the AC fault ride-through mode; otherwise, each photovoltaic power generation unit operates in the maximum power point tracking mode. The set second voltage threshold is 1.1 pu.

3. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 2, characterized in that, While determining whether the photovoltaic generator set has entered AC fault ride-through mode, when the rate of change of the DC bus voltage is greater than the second voltage threshold, all power-type energy storage units switch to constant DC voltage control.

4. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 2, characterized in that, When all connected energy storage devices are in a charging state, if the charging power is greater than the set first power threshold or the SOC is greater than the set first SOC threshold, it is determined that the connected energy storage devices do not have the ability to control the DC bus voltage; otherwise, when all connected energy storage devices are in a charging state, if the charging power is not greater than the set first power threshold and the SOC is not greater than the set first SOC threshold, it is determined that the connected energy storage devices have the ability to control the DC bus voltage; wherein, the set first power threshold is 0.9 pu and the set first SOC threshold is 0.

9.

5. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 4, characterized in that, The energy storage coordinating controller issues a power limiting command to the photovoltaic generator set based on the fault type and the fault level determined by the AC voltage drop depth, setting the upper limit of the output active power. As shown in the following formula: In the formula, To output the real-time value of active power, The limiting coefficient is based on the fault type. The limiting factor is based on the fault level. This is the power limiting value based on the AC voltage drop depth.

6. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 4, characterized in that, When a photovoltaic inverter that receives a power limit command detects that its output DC voltage exceeds a set second voltage threshold, the number of photovoltaic inverters in each DC collection group that do not trigger the blocking is calculated using the following formula. : In the formula, This refers to the power loss of the DC transformer in the DC collection group. This refers to the active power output of the photovoltaic inverter in maximum power point tracking mode. The number of photovoltaic inverters that trigger the blocking is determined based on the actual number of photovoltaic inverters operating in the DC aggregation group and the calculated number of photovoltaic inverters that do not trigger the blocking.

7. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 4, characterized in that, Based on the energy storage's operating status and parameters, select energy storage systems for control switching; including: Power-type energy storage devices that are in the charging state and whose charging power is greater than the set second power threshold and whose SOC is greater than the set second SOC threshold will not perform control switching; all other power-type energy storage devices will perform control switching. Energy storage devices that are in the charging state and whose charging power is greater than the set second power threshold and whose SOC is greater than the set second SOC threshold will not be subject to control switching; all other energy storage devices will be subject to control switching. The second power threshold is 0.8 pu, and the second SOC threshold is 0.

8.

8. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 7, characterized in that, If the number of power-type energy storage devices performing control switching is not zero, then when the DC bus voltage is greater than the set second voltage threshold and less than the set third voltage threshold, the power-type energy storage in the discharge state will be switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set third voltage threshold and less than the set fourth voltage threshold, the power-type energy storage in the charging state will be switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set fourth voltage threshold and less than the set fifth voltage threshold, the energy-type energy storage in the discharge state will be switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set fifth voltage threshold, the energy-type energy storage in the charging state will be switched from constant power control to constant DC voltage control. The third voltage threshold is set to 1.12 pu, the fourth voltage threshold is set to 1.13 pu, and the fifth voltage threshold is set to 1.14 pu.

9. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 8, characterized in that, If the number of power-type energy storage devices performing control switching is zero, then when the DC bus voltage is greater than the set second voltage threshold and less than the set sixth voltage threshold, the energy storage devices in the discharge state will be switched from constant power control to constant DC voltage control; when the DC bus voltage is greater than the set sixth voltage threshold, the energy storage devices in the charging state will be switched from constant power control to constant DC voltage control; wherein, the set sixth voltage threshold is 1.11 pu.

10. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 9, characterized in that, When all direct-connected energy storage is switched to constant DC voltage control, the inverter will trigger a temporary lockout when it detects that the output DC voltage is greater than the set seventh voltage threshold, where the seventh voltage threshold is 1.15 pu.

11. The AC fault collaborative control method for a direct-connected energy storage DC collection system according to claim 10, characterized in that, When the DC bus voltage is less than the set eighth voltage threshold, the power-type energy storage will preferentially switch from constant DC voltage control to constant active power control. The eighth voltage threshold is 1.03 pu. When the DC bus voltage is less than the set ninth voltage threshold, the energy storage system switches from constant DC voltage control to constant active power control. The ninth voltage threshold is 1.02 pu. When the inverter detects that the output DC voltage is less than the set tenth voltage threshold, each photovoltaic power generation unit operates in maximum power point tracking mode, where the tenth voltage threshold is 1.01 pu.

12. A collaborative control device for AC faults in a direct-connected energy storage DC collection system, used to implement the method described in any one of claims 1 to 11; wherein, The DC collection system includes: direct-connected energy storage, photovoltaic generator sets, flexible converters, and an energy storage coordination controller. The direct-connected energy storage includes power-type energy storage and energy-type energy storage. The photovoltaic generator sets include multiple photovoltaic power generation units. The DC sides of the direct-connected energy storage, photovoltaic generator sets, and flexible converters are all connected to the DC bus, and the AC side of the flexible converters is connected to the AC power grid. The system is characterized by a coordination control device including: The steady-state control module is used to allocate active power discharge commands to power-type energy storage and active power charging commands to energy-type energy storage under SOC equalization constraints and charge / discharge cycle constraints during steady-state operation. Both power-type and energy-type energy storage perform constant active power control. The fault control module is used to handle AC faults. If the DC controller of the flexible converter determines that the DC collection system has entered AC fault ride-through mode based on the grid connection point voltage, the energy storage coordination controller determines whether the photovoltaic generator set has entered AC fault ride-through mode based on the AC side voltage or DC bus voltage of the flexible converter. When the photovoltaic generator set enters AC fault ride-through mode, if the direct-connected energy storage does not have the ability to control the DC bus voltage, the energy storage coordination controller sends an output power limit command to the photovoltaic generator set. Upon receiving the output power limit command, the photovoltaic generator set determines whether the inverter triggers a lockout based on the output DC voltage and determines the number of photovoltaic inverters that do not trigger a lockout based on the DC transformer loss. When the direct-connected energy storage has the ability to control the DC bus voltage, the energy storage coordination controller switches the direct-connected energy storage from constant active power control to constant DC voltage control using a stepped control method based on the DC bus voltage exceeding the limit. When the DC bus voltage recovers, the energy storage coordination controller switches the direct-connected energy storage from constant DC voltage control to constant active power control using a stepped control method based on the DC bus voltage, and controls the photovoltaic generator set to operate in maximum power point tracking mode.

13. A terminal, comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to perform the steps of the method according to any one of claims 1-11.

14. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the program implements the steps of the method according to any one of claims 1-11.

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