Operating procedure for a sub-network in case of decoupling from the power grid

Abstract

The invention relates to an operating method for a system comprising a battery converter connected between a battery and a sub-grid, and at least one further power converter connected between energy generation units and / or loads and the sub-grid. A power supply grid can be connected to the sub-grid via a grid disconnector. The method comprises the steps of feeding in generation power through the at least one further power converter, feeding in the generation power in line with the grid when the grid disconnector is closed, and disconnecting the power supply grid from the sub-grid by opening the grid disconnector.The process then proceeds with a grid-forming feed-in of battery power by the battery converter into the sub-grid, wherein the fed-in battery power comprises active power and / or reactive power, and a grid-forming feed-in of the generated power by the at least one further power converter, wherein the fed-in generated power comprises active power and / or reactive power. Once the voltage and frequency have been regulated within predefined value ranges by the grid-forming feed-in after the disconnection, the battery power is fed into the grid and the generated power is fed into the grid following the grid, so that the voltage and frequency within the sub-grid are maintained within the predefined value ranges.

Description

[0001] The invention relates to an operating method for an energy system. Problem

[0002] In light of climate change, efforts are being made to replace fossil fuel energy production with energy production from renewable energy sources. The most common renewable energy sources are wind and solar. Unlike wind energy, solar energy can also be converted into electricity using private photovoltaic systems. This electricity is used in the generating household or fed into a public energy grid. To ensure the stability of the energy supply grid and also of a sub-grid, such as a single local grid or a larger property, efforts are being made to decouple these sub-grids from the overall grid and make them self-sufficient in the event of a fault.

[0003] German patent application DE 10 2012 023 424 A1 discloses a power distribution system with at least one energy source and at least one electrical storage device. This system can be separated from the utility grid and operated as an isolated grid. In isolated operation, the system's sole inverter operates to regulate voltage and form the grid. The inverter must provide all the system's power on its own, requiring high apparent power, which leads to high costs.

[0004] German patent application DE 10 2019 116 254 A1 describes a power generation system with at least two inverters that can be operated either in voltage-impressing or current-impressing mode. In voltage-impressing mode, one, several, or all inverters can form an isolated grid or act as a grid-shaping element for a higher-level grid. A division of tasks is unknown, so all inverters have the same complexity.

[0005] The German published patent application DE 10 2017 112 491 A1 shows a similar operating procedure for a wind turbine.

[0006] The problem here is that in the event of decoupling from the energy supply network, the loads in the sub-grid must continue to be supplied by stable operation of the sub-grid. Technical solution

[0007] This problem is solved by the operating method according to the main claim. The subject matter of the subclaims constitutes further developments according to the invention.

[0008] The operating method according to the invention for a system with a battery converter connected between a battery and a sub-grid, and at least one further power converter connected between energy generation units and / or loads and the sub-grid,

[0009] wherein a power supply grid is connectable to the subgrid via a grid disconnector, discloses the steps of feeding in generating power by the additional power converters, feeding in the generating power in a grid-following manner when the grid disconnector is closed, and disconnecting the power supply grid from the subgrid by opening the grid disconnector. After the disconnection, a grid-forming feed-in of battery power by the battery converter into the subgrid follows, wherein the fed-in battery power comprises active power and / or reactive power, and a grid-forming feed-in of the generating power by at least one additional power converter, wherein the fed-in generating power comprises active power and / or reactive power.The process is characterized by, as soon as voltage and frequency are regulated after separation by the grid-forming feed-in in predefined value ranges, grid-forming feed-in of the battery power and grid-following feed-in of the generation power, so that voltage and frequency within the sub-grid are kept in predefined value ranges.

[0010] The system represents a sub-grid, such as a local distribution grid. A battery converter is equipped with a battery, also known as a district storage system. The system also includes additional power converters, via which energy supply units, such as photovoltaic systems, and / or loads are connected to the sub-grid. Furthermore, the system can be connected to a power grid via a grid disconnector. With an existing connection to the power grid, the system or sub-grid can draw electrical power from the power grid and feed it into it.

[0011] The battery converter and battery serve to stabilize the sub-grid. It is possible for any surplus energy generated in the sub-grid, for example by photovoltaics, to be stored in the battery and then withdrawn from the storage system at a different time, for example in the evening hours when PV power is low, without placing a strain on the higher-level energy supply grid. The system according to the invention is initially operated in normal mode, with the other power converters feeding generated power into the sub-grid. The feeding of generated power is grid-following. This means that the generated power is fed into the sub-grid in the form of active power and / or reactive power. Grid-following feed-in means that the generated power is divided into active and reactive power in such a way that the grid parameters, i.e. voltage and frequency, of the sub-grid are kept within the permissible range.During this time, the power grid and the subgrid are continuously monitored for faults. If a fault is detected, this is considered a fault, and the power grid is disconnected from the subgrid by opening the mains disconnect switch.

[0012] This fault can be a failure of the power supply network, but can also be an overload in the sub-network.

[0013] A power grid failure can result, for example, from a short circuit within the nearby power grid or a transformer failure. In this case, electrical power can no longer be drawn from the power grid. Rather, there is a rapid voltage drop in the power grid, which, without appropriate countermeasures, leads to a voltage drop in the subgrid, meaning that loads can no longer be supplied.

[0014] An overload condition occurs when the power supply units feed too much active power into the sub-grid via the additional power converters. In a system with synchronous machines, this excess active power would increase the frequency in the system. Generating units in the sub-grid according to the invention are primarily not synchronous machines. Furthermore, the sub-grid is connected to the higher-level power supply network, so that the frequency in the sub-grid is not increased even in the event of an overload in the sub-grid. Consequently, no active power throttling occurs in the additional power converters. Active power can be supplied from the sub-grid via the transformer to the power supply network up to the design limit of the transformer or other network equipment. However, if this capacity limit of the transformer or other network equipment is reached, an overload condition occurs.

[0015] In the event of any fault, the subgrid is disconnected from the power generation grid by opening the disconnector. In particular, in the event of an overload, the subgrid is disconnected from the power supply grid to protect the grid equipment. The battery converter then feeds power stored in the battery into the subgrid, shaping the grid, or extracts energy from the subgrid, shaping the grid. "Grid-shaping" here means that the battery converter applies the battery power to the voltage in such a way that the frequency and voltage are supported, and generation and consumption are consistent. Furthermore, the fed-in battery power includes active power and / or reactive power, depending on the grid parameters of voltage and frequency in the subgrid.

[0016] After the power generation grid is disconnected, the additional converters feed in generating power to the grid. In particular, grid-shaping feed-in occurs because the loads continue to run throughout the entire period, and the battery converter and the additional converters stabilize the subgrid as an island grid. By following the procedure described above when disconnecting the subgrid from the power grid, it is possible to operate the subgrid autonomously thanks to the battery power fed in by the battery converter and the generating power fed in by the additional converters. In particular, at high feed-in power, the frequency can increase to such an extent that the additional converters reduce their feed-in power or increase their consumption.

[0017] As soon as the voltage and frequency of the sub-grid are regulated within the respective specified value ranges after disconnection from the power supply grid, the battery converter is in a grid-forming feed-in mode so that the voltage and frequency within the sub-grid are kept within the specified value ranges.

[0018] Grid-following feed-in means that a power converter, also called a transformer, is synchronized to an existing grid and follows this grid in terms of frequency and voltage. The power converter follows the grid as quickly as its control allows. In addition, a power converter with grid-following feed-in is current-impressing. Grid-shaping feed-in means that a power converter with grid-shaping feed-in can differ from an existing grid in terms of phase (where the frequency is indirectly influenced via a phase shift) and / or voltage. Thus, an existing grid can be influenced by grid-shaping feed-in. With suitable control, several voltage-impressing power converters with grid-shaping feed-in can be capable of maintaining or forming a grid. A grid-forming power converter is capable of forming and maintaining a grid on its own.When forming a network, the power converter can determine the frequency and voltage of the network to be formed.

[0019] According to one embodiment, the operating method further comprises specifying the frequency of the sub-grid by the battery converter in the event of an overload in the sub-grid, wherein the overload is determined by the disconnection in step C.

[0020] In this embodiment, operation in the event of an overload is further specified. In particular, this embodiment specifies that the battery converter adjusts the frequency for the subgrid so that the other power converters reduce or increase their power according to the specified frequency. They follow the frequency / power specifications of the relevant standards (e.g., VDE-AR-N 4105 or VDE-AR-N 4110). In this case, the mains disconnect switch is connected to sensors that can detect an overload. The sensors are not limited to a specific technology.

[0021] The battery converter and the other power converters can either detect the disconnection from the power grid independently via sensors (e.g., the frequency changes after the disconnection, thus indicating an overload), or the switch can notify the battery converter and the other power converters of the disconnection via a suitable communication mechanism such as PLC (Power Line Communication) via a power cable or an explicit communication connection. Alternatively, or in addition, the battery converters and power converters can also detect a disconnection independently by measuring the electrical parameters frequency and voltage at their respective grid connection points.

[0022] According to one embodiment, the operating method further comprises that the frequency set by the battery converter is increased compared to the predetermined value range of the frequency in order to reduce the active power fed into the further power converters.

[0023] This embodiment specifies how the reduction in active power in the subgrid is achieved. The additional power converters are equipped with a control system that adjusts the active power fed in depending on the frequency of the grid, in this case the subgrid. By setting a frequency in the subgrid that is higher than the standard frequency of the respective power grid in this area (e.g., 50 Hz in Europe) using the battery converter, the active power fed in by the additional power converters is reduced. In addition, the power drawn from the subgrid by controllable loads can be increased at such an increased frequency by such loads increasing their power or switching on above a certain frequency. This eliminates the overload situation and establishes a power balance in the subgrid.

[0024] According to one embodiment, the operating method further comprises adjusting the frequency set by the battery converter such that there is no longer any overload in the sub-grid, wherein voltage and frequency are subsequently regulated within a predetermined value range.

[0025] This configuration specifies how long the regulation is maintained in the event of an overload. Specifically, the frequency set by the battery converter is increased relative to the normal frequency until the overload no longer exists and a power balance is achieved in the subgrid. The battery converter automatically regulates the frequency of the subgrid back to the specified value range, specifically to a nominal frequency, as soon as the battery converter needs to supply power to the subgrid because the power from the generators in the subgrid is no longer sufficient.

[0026] According to one embodiment, the system is further connected to the power grid via a decoupling reactor, and the operating method further comprises providing or receiving active power by the battery converter in the event of a grid failure so that the frequency in the sub-grid is kept within the specified value range, and providing so much reactive power by the additional converters before opening, regardless of the position of the grid disconnector, that the voltage is kept within the specified value range (e.g., the additional converters can follow a static reactive power setpoint or adjust their reactive power according to the normative specifications according to, for example, VDE-AR-N 4105 in accordance with the voltage change).

[0027] In the event of a grid failure, the voltage in the power grid drops to zero. A sub-grid must remain connected to the power grid for a standardized period of time to support it, so that the power grid does not collapse permanently due to a lack of generators in the event of a short-term outage. To prevent the voltage in the sub-grid from also dropping to zero, the additional converters provide enough reactive power to keep the voltage in the sub-grid within the specified range. This works by feeding reactive power into the sub-grid, resulting in a voltage difference across the decoupling reactor. This allows the sub-grid voltage to differ from the grid voltage.

[0028] During this time between the grid failure and the disconnection from the power grid, the battery converter feeds mainly active power, and reactive power is also possible, into the sub-grid so that the loads continue to be supplied with active power and the voltage in the sub-grid is supported.

[0029] Overall, an advantage of this design is that even in the event of a grid failure, before the power grid is disconnected (yet), the sub-grid can maintain the voltage and supply the loads with active power. This means that the sub-grid continues to operate as normal and does not itself fail. The frequency in the sub-grid is supported by the grid-shaping feed-in of battery power, and the voltage in the sub-grid is supported by the reactive power feed-in of generated power from the other power converters. Reactive power for voltage support comes from the other power converters and the battery power converters. The battery power converter contributes the additional portion of dynamic reactive power required to maintain the voltage.

[0030] According to one embodiment, the operating method further comprises that the further power converters provide the reactive power in such a way that the voltage in the sub-grid at the decoupling reactor is higher or lower than on the grid side.

[0031] As already described above, the other converters also provide reactive power in the event of a grid failure, resulting in a voltage drop across the decoupling reactor. It is possible that the voltage in the sub-grid is higher than in the grid. In special cases, the voltage in the sub-grid on the load side may also be lower than in the grid, for example, if an overvoltage occurs in the grid.

[0032] According to one embodiment, the operating method further comprises that the mains failure is detected by a measuring device and / or is detected by opening the mains disconnect switch.

[0033] In this embodiment, it is specified that the grid failure is detected by a measuring device and / or the opening of the grid disconnector. The detection via a measuring device is designed such that the grid failure is first detected via sensors. Subsequently, a dedicated signal is sent to the grid disconnector, the battery converter, and the other power converters so that these elements are also notified. After a predetermined time, the grid disconnector will then disconnect the sub-grid from the power grid. The battery converter and the other power converters will activate the control system described in the previous embodiments. Alternatively or additionally, the grid disconnector can also be equipped with appropriate sensors for detecting a grid failure.The battery converter and the other power converters then either detect the grid failure themselves via the opening or are notified via communication channels. A combination of these is also possible. For example, the measuring device can activate the grid failure control, while opening the grid disconnector terminates this control and initiates island-grid operation.

[0034] According to one embodiment, the operating method further comprises that the battery converter and the further power converters comprise controls which have different inertias or different settings with respect to the amplitude inertia (associated with the reactive power) or with respect to the phase inertia (associated with the active power).

[0035] The operation of the battery converter and the other power converters differs in the event of a grid failure and in island grid operation. Using the example of the other power converters, initially only the reactive power is changed during a grid failure and fed into the sub-grid to maintain the voltage in the sub-grid. Once the grid disconnector is opened, the other power converters begin feeding in reactive power to shape the grid (according to their characteristic curve (also known as reactive power droop)). To avoid side effects during a sudden switchover from a pure reactive power feed-in to another feed-in, the controls of the other power converters are equipped with higher inertias so that the change in feed-in occurs gradually. This means that when there is a voltage drop, the other power converters begin feeding reactive power into the sub-grid to maintain the voltage across the decoupling reactor.The reactive power control of the other converters is designed with such a slowness that the feed-in of reactive power is not stopped immediately when the voltage returns to the normal range, but rather is gradually reduced. In the event of a grid failure, the battery converter is designed to initially supply active power primarily to the loads in the subgrid and only subsequently feed in the necessary differential reactive power required to maintain the voltage.

[0036] According to one embodiment of the above operating method, the battery converter and the further power converter comprise controls which have different inertias from one another.

[0037] In this embodiment, the inertias of the battery converter and the additional power converters are different from each other. This allows the battery converter and the additional power converters to perform different tasks in the event of a grid failure or overload. For example, the battery converter can provide active power due to a high phase inertia, while the additional power converters can provide reactive power due to a high amplitude inertia.

[0038] According to one embodiment of the operating method according to the invention, the inertias of the battery converter and the further power converters differ in the settings for the amplitude inertia with respect to reactive power and the phase inertia with respect to active power.

[0039] In this embodiment, the inertias of the battery converter and the additional power converters differ from one another. This allows the battery converter and the additional power converters to perform different tasks in the event of a grid failure or overload. For example, the battery converter can provide active power due to a high phase inertia, and the additional power converters can provide reactive power due to a high amplitude inertia. According to one embodiment, the operating method further comprises the control of the battery converter having different inertias depending on the operating mode.

[0040] In the operating mode following a grid failure, it is important that the battery converter can quickly supply a new load in the subgrid with active power. To do this, it exhibits increased inertia in the event of a grid failure, i.e., it maintains voltage and frequency. This absorbs fluctuations in the feed-in from the other power converters. For example, if the frequency provided by the battery converter in the subgrid is higher than the specified frequency range in the subgrid, the active power feed-in from the other power converters can quickly decrease. With its high inertia, the battery converter can then compensate for any insufficient active power in the subgrid and reduce the frequency accordingly until equilibrium is reached.

[0041] According to the above embodiment of the operating method, the control of the battery converter, in the event of a mains failure, has high inertias in the amplitude and phase inertia.

[0042] In this embodiment, the battery converter provides both active and reactive power in the event of a fault, regardless of the frequency and voltage parameters that arise in the subgrid due to the fault. The battery converter can therefore maintain the subgrid's operation. It should be noted that the power grid can be, for example, a medium-voltage or high-voltage grid. Furthermore, the power grid can be another grid into which electrical power can be fed and from which electrical power can be drawn.

[0043] Furthermore, the mains disconnector and the bypass switch can be designed as different or identical switch types. Both switches can be relays or other electromechanical switches. Generally, the mains disconnector and the bypass switch must be switches that can be equipped with their own measuring electronics or controlled by a control system and that switch or can be switched safely and in accordance with standards. In particular, these switches are medium-voltage switches and part of a medium-voltage switchgear system.

[0044] In the following, the invention is illustrated by means of the figures, where Fig. 1 shows a system according to the invention, Fig. 2 shows an operating method according to the invention, Fig. 3 shows an operating method according to the invention according to one embodiment, and Fig. 4 shows an operating method according to the invention according to a further embodiment.

[0045] The figures are explained in detail below.

[0046] Fig. Figure 1 shows a system 1 according to the invention. The system 1 comprises a sub-grid connected to a power supply network 10 via a mains disconnect switch 12 and a decoupling choke 16. Optionally, the decoupling choke 16 can be bypassed via a bypass switch 18 connected in parallel.

[0047] A battery converter 20 is connected to the sub-grid. A battery 22 is connected to the battery converter 20. The battery converter 20 is capable of transferring electrical power from the battery 22 to the sub-grid. Likewise, the battery converter 20 is capable of transferring electrical power from the sub-grid to the battery 22.

[0048] Further power converters 24, 26 are also connected to the sub-grid. The further power converters 24, 26 can, for example, be power converters for supplying a load, such as in an electrolysis system, or power converters for operating a generator, such as in a domestic photovoltaic system. The further power converters 24, 26 can also be designed as battery power converters. In general, the power converters 24, 26 are capable of feeding power into the sub-grid and / or extracting power from the sub-grid. Furthermore, loads 28 can be connected to the sub-grid. These loads 28 are preferably designed as AC loads.

[0049] In Fig. Figure 2 shows an operating method according to the invention for an upcoming system according to the invention. The method begins with step 100, in which generating power is fed into the sub-grid by the additional power converters 24, 26. As specified in step 102, generating power is fed into the sub-grid in a grid-following manner. This means that the voltage and frequency of the fed-in generating power correspond to the specified voltage and frequency of the sub-grid. Step 102 is maintained until step 104 is continued. In step 104, the sub-grid is disconnected from the power grid. The disconnection occurs by opening the grid disconnect switch 12.

[0050] Once the subgrid has been disconnected from the power grid in step 104, the method continues with step 106. In step 106, battery power from battery 22, which is connected to battery converter 20, is fed into the subgrid by the battery converter 20, forming the grid. The battery power fed into the subgrid from battery 22, which is connected to battery converter 20, includes active power and / or reactive power.

[0051] After or simultaneously with step 106, step 108 is executed. In step 108, generated power is fed into the subgrid by at least one of the additional power converters 24, 26, wherein the fed-in generated power comprises active power and / or reactive power. Steps 106 and 108 are executed until the voltage and frequency, after the subgrid is disconnected from the energy supply grid 10, are regulated within predetermined value ranges by the grid-shaping feed-in of the battery converter 20 and the at least one additional power converter 24, 26. Once this has occurred, the method continues with step 200.

[0052] In step 200, battery power from the battery 22, which is connected to the battery converter 20, is fed into the sub-grid by the battery converter 20 and generation power by the at least one further power converter 24, 26 in a grid-forming manner, so that voltage and frequency within the sub-grid are maintained within the specified value range.

[0053] In Fig. 3 shows an embodiment of the operating method for the system according to the invention. Differences from the operating method shown in Fig. 2 described operating method according to the invention. The Fig. The operating method according to the invention shown in Figure 3 relates to a situation in which an overload is detected in the subnetwork. Steps 100 and 102 are associated with the Fig. The operating method according to the invention described in section 2 is identical. In step 104a, it is determined that an overload exists in the sub-network via a suitable measuring device connected to the mains disconnector 12, or by the switch itself, which has such a measuring device. After disconnecting the sub-network from the power supply network 10 in step 104a, the method continues with step 106a.

[0054] In step 106a, the battery converter 20 sets the frequency of the sub-grid by feeding battery power into the sub-grid. After or simultaneously with step 106a, step 108 is executed, as in Fig. 2. The procedure ends as described in the description of Fig. 2, with step 200.

[0055] In Fig. 4 shows a further embodiment of the operating method for the system according to the invention. In the following, only differences from the Fig. 2 described operating method according to the invention. The Fig. The operating method according to the invention shown in Figure 4 relates to a situation in which a power failure of the energy supply network 10 is detected.

[0056] Steps 100 and 102 are to be repeated with the Fig. The operating method described in Figure 2 is identical. In step 104b, it is determined that a power failure in the power grid 10 is present via a suitable measuring device connected to the power disconnector 12, or by the power disconnector 12 itself, which has such a measuring device. After disconnecting the sub-grid from the power grid 10 in step 104b, the method continues with step 106b.

[0057] In step 106b, battery power from battery 22 is provided by battery converter 20. The battery power is provided in a grid-shaping manner such that the frequency in the sub-grid is maintained within the specified value range. Simultaneously with step 106b, step 108b is executed. In step 108b, the additional converters 24, 26 feed sufficient reactive power into the sub-grid to maintain the voltage within the specified value range according to suitable characteristics or through their grid-shaping behavior (amplitude inertia). The method ends, as in the description of Fig. 2, with step 200. List of reference symbols 10 Energy supply network 12 mains disconnect switches 14 Transformer 16 Decoupling choke 18 bypass switches 20 battery converters 22 Battery 24 power converters 26 power converters 100-200 steps

Claims

[1] Operating method for a system with a battery converter (20) connected between a battery (22) and a sub-grid, and at least one further power converter (24, 26) connected between power generation units and / or loads and the sub-grid, wherein a power supply network can be connected to the sub-network via a mains disconnector (12), with the steps: Feeding (100) of generating power by the at least one further power converter (24, 26), grid-following feed-in (102) of the generating power when the grid disconnector (12) is closed, Separating (104) the power supply network (10) from the sub-network by opening the mains disconnector (12), grid-forming feeding (106) of battery power by the battery converter (20) into the sub-grid, wherein the fed-in battery power comprises active power and / or reactive power, grid-forming feed-in (108) of the generating power by the at least one further power converter (24, 26), wherein the fed-in generating power comprises active power and / or reactive power, and characterized by , as soon as the voltage and frequency are regulated within predetermined value ranges after the separation (104) by the grid-forming feed-in (106, 108), grid-forming feed-in (200) of the battery power and grid-following feed-in of the generation power, so that the voltage and frequency within the sub-grid are kept within the predetermined value ranges. [2] Operating method according to claim 1, comprising adjusting (106a) the frequency of the sub-grid by the battery converter (20) in the event of an overload in the sub-grid, wherein the overload is detected by disconnecting (104) the power supply network from the sub-grid. [3] Operating method according to claim 2, wherein the set frequency is increased compared to the predetermined value range for the frequency in order to reduce the active power fed into the further power converters (24, 26). [4] Operating method according to claim 3, wherein, the frequency set by the battery converter is increased until there is no longer any overload in the sub-grid, whereby voltage and frequency are then regulated within the specified value range. [5] Operating method according to claim 1, wherein the system is further connected to the power grid via a decoupling choke (16) comprising a Providing (106b) or absorbing active power by the battery converter (20) in the event of a grid failure, so that the frequency in the sub-grid is kept within the specified value range, and a Providing (108b) so much reactive power by the further converters (24, 26) regardless of the position of the mains disconnect switch (12) that the voltage is kept within the specified value range. [6] Operating method according to claim 5, wherein the further power converters (24, 26) and the battery converter (20) provide the reactive power such that the voltage in the sub-network at the decoupling reactor (16) is higher or lower than on the grid side. [7] Operating method according to one of claims 5 or 6, wherein the mains failure is detected by a measuring device and / or by opening the mains disconnect switch (12). [8] Operating method according to one of claims 5 to 7, wherein the battery converter (20) and the further power converter (24, 26) comprise controls which have different inertias from one another. [9] Operating method according to claim 8, wherein the inertias of the battery converter (20) and the further power converters (24, 26) differ in the settings for the amplitude inertia with respect to reactive power and the phase inertia with respect to active power. [10] Operating method according to one of the preceding claims, wherein the control of the battery converter (20) has different inertias depending on the operating mode. [11] Operating method according to claim 10, wherein the control of the battery converter (20) in the event of a mains failure comprises an increase in inertias in the amplitude and phase inertia.

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