JP6955978B2 - Hydropower system and control method - Google Patents

Description

The present invention relates to a hydroelectric power generation system and a control method having a function of determining a stall state of a hydroelectric power generation device and returning it to a normal state.

A hydroelectric power generation device is a system that uses the kinetic energy of running water for power generation. The main components of the hydroelectric power generator include a turbine that rotates in response to the flow of water, a generator that is connected to the turbine and converts rotational energy into electrical energy, and a control device that controls the output of the generator and the turbine.
Since the optimum power to be extracted from the generator changes depending on the flow velocity, the control device measures the flow velocity, the rotation speed of the water turbine, or the power generation voltage of the generator to determine the optimum power to be extracted from the generator to generate power. Control so that the power generated by the machine matches the optimum value.

In order to determine the optimum power to be extracted from the generator and control it so that the amount of power of the generator matches the optimum value, install a hydraulic generator in the waterway in advance and measure the flow velocity, generated power, and power generation characteristics. It is necessary to set the optimum value and create the table characteristics by the control map or the like. Therefore, there are factors such as cost increase such as measurement work before the operation of the hydroelectric power generation system.

As a method of eliminating the work of pre-measurement and setting of the optimum value, a method of controlling hydroelectric power generation by maximum power point tracking control called MPPT control by mountain climbing or the like has been proposed (for example, Patent Document 1). MPPT control is also used in wind power generation (for example, Patent Document 2).

Japanese Unexamined Patent Publication No. 2016-185006 Japanese Unexamined Patent Publication No. 2010-200533

When MPPT control is applied to a hydroelectric power generation control device, the rotation speed of the turbine is lowered and the generated power is also lowered, which may lead to a stall state, which makes it difficult to apply MPPT control to hydroelectric power generation.
In Patent Document 2, although it is MPPT control in wind power generation, a control for coping with stall is proposed. That is, every time the MPPT control is used to search for the optimum operating point of the generator, the time derivative of the output voltage or the time derivative of the output current is calculated, and the output power at the operating point and the calculated time derivative of the output power are combined with each other. The stall is determined by whether or not the relationship of is satisfied with the stall boundary condition. When it is determined to be stall, the load on the generator is released or reduced.

However, by releasing or reducing the load on the generator, when the stall boundary condition is no longer satisfied, if the load is returned to the original value, the stall boundary condition will be satisfied immediately, and hunting for power limitation will occur. May occur.

The present invention solves the above-mentioned problems, and an object of the present invention is to appropriately control the output power without performing prior measurement, and to reduce or release the load on the generator even in a stall state. It is an object of the present invention to provide a hydroelectric power generation system and a control method capable of returning from a stall state to a normal power generation state, obtaining a large amount of power generation, and preventing hunting of power control.

In the hydraulic power generation system of the present invention, the number of rotations of the water wheel 1 is adjusted by adjusting the water wheel 1 that rotates by hydraulic power, the generator 3 that converts the rotational energy of the water wheel 1 into electric energy, and the load power of the generator 3. It is a hydraulic generator system provided with a control device 4 for controlling.
The control device 4
The electric power past-current value detecting means 16 for detecting the output power past value and the output power present value of the generator 3 and
The electric power past-current value storage means 17 for storing the detected output power past value and output power current value of the generator 3 and
The rotation speed past-current value detecting means 18 for detecting the rotation speed past value and the rotation speed current value of the generator 3 and
The rotation speed past-current value storage means 19 for storing the detected rotation speed past value and rotation speed current value of the generator 3, and
The differential power calculation means 20 for calculating the difference power between the past output power value and the current output power value of the generator 3 and
The difference rotation speed calculation means 21 for calculating the difference rotation speed between the past value of the output rotation speed and the current value of the output rotation speed of the generator 3 and
The stall determination means 22 for determining whether or not the generator is in a stall state based on the difference power and the difference rotation speed, and the load power of the generator 3 are reduced or released when the stall determination means 22 determines the stall. Equipped with load reduction / opening means 23
In the stall determination means 22, the stall determination area A for determining stall and the non-stall determination area B for determining non-stall are classified by the determination curves a and b showing the relationship between the difference power and the difference rotation speed, and The determination curves a and b include a stall determination curve a for determining stall when the differential power increases and a return determination curve b for determining non-stall when the differential power decreases, and both curves a and b. The interval becomes the stall boundary region C in the hysteresis region.

According to this configuration, the control device 4 basically monitors the generated power and the like, and controls the output power according to a predetermined control rule to control the rotation speed of the water turbine 1.
During this time, the stall determination means 22 determines whether or not the generator is in a stall state based on the difference power and the difference rotation speed, and when the stall determination means is determined, the load reduction / opening means 23 is used to determine the generator 3 Reduce or release the load power of. When the water turbine 1 stalls in this way, the load power is reduced or released, so that the stall state can be restored to the normal power generation state, and a large amount of power generation can be obtained.
Since the stall is determined by the difference power and the difference rotation speed between the present and the past, an appropriate determination can be made. Further, since the stall boundary region C of the hysteresis region is provided, hunting of power control due to the stall determination is prevented.
The number of revolutions is the number of revolutions per unit time, in other words, the number of revolutions.

In the hydroelectric power generation system of the present invention, the control device 4 includes MPPT control means 6 for tracking and controlling the maximum power operating point with respect to the output fluctuation of the generator 3, and every time the search for the maximum power operating point is performed, the control device 4 is provided. The stall determination by the stall determination means 22 and the processing by the load reduction / release means 23 may be performed.
According to the MPPT control that follows and controls the maximum power operating point, efficient power generation can be performed even if the pre-measurement work of the flow velocity and the rotation speed of the turbine at the site where the turbine 1 is installed is omitted. However, when MPPT control is applied to the hydroelectric power generation system, the rotation speed of the turbine 1 is lowered and the generated power is also lowered, resulting in a stall state, and it may not be possible to return to the normal state only by the control. With respect to this, according to the hydroelectric power generation system of the present invention, since the stall determination is performed and the load power is reduced or stopped at the time of stall, the stall state can be restored without causing a large reduction in generated power due to stall. Efficient control can be performed by MPPT control.

The control method of the hydraulic power generation system of the present invention is the rotation of the water wheel 1 by adjusting the load power of the water wheel 1 that rotates by hydraulic power, the generator 3 that converts the rotational energy of the water wheel 1 into electric energy, and the load power of the generator 3. It is a control method of a hydraulic generator system applied to a hydraulic generator system including a control device 4 for controlling a number.
For each control cycle of the control device 4,
The difference power calculation process (S6) for obtaining the difference power between the past value and the current value of the output power, and
The difference rotation speed calculation process (S5) for obtaining the difference rotation speed between the past value and the current value of the rotation value of the generator 1 and
A stall determination process (S6) for determining whether or not the relationship between the difference power and the difference rotation speed is a stall depending on whether or not a predetermined stall boundary condition is satisfied.
Including the load reduction / opening process (S7) of reducing or releasing the load of the generator (3) when the stall is determined in the stall determination process (S6).
As the stall boundary condition, the stall determination area A for determining stall and the non-stall determination area B for determining non-stall are classified by the determination curves a and b showing the relationship between the difference power and the difference rotation speed, and the determination is made. The curve defines a stall determination curve a for determining stall when the differential power increases and a return determination curve b for determining non-stall when the differential power decreases, and hysteresis is performed between the curves a and b. It becomes the stall boundary area C of the area,
It is characterized by that.

According to this control method, the hydroelectric power generation system of the present invention can be returned to the normal power generation state even if a stall state occurs, and a large amount of power generation can be obtained. Further, since the stall boundary region C of the hysteresis region is provided, hunting of power control due to the stall determination is prevented.

In the method of the present invention, the control device 4 includes MPPT control means 6 for tracking and controlling the maximum power operating point with respect to the output fluctuation of the generator 3, and each time the tracking control searches for the operating point, the control device 4 is provided. The difference power calculation process (S6), the difference rotation number calculation process (S5), the stall determination process (S7), and the load reduction / release process (S8) may be performed.
According to MPPT control, efficient power generation can be performed even if the pre-measurement work of the flow velocity and the number of revolutions of the turbine is omitted, but the control alone cannot restore the normal state when the stall state occurs. In some cases. With respect to this, according to the method of the present invention, the stall determination is performed and the load power is reduced or stopped in the stall state. Efficient control is possible.

The hydraulic power generation system of the present invention includes a water wheel that rotates by hydraulic power, a generator that converts the rotational energy of the water wheel into electrical energy, and a control device that adjusts the load power of the generator to control the rotation speed of the water wheel. A hydraulic generator system provided, wherein the control device includes a power past-current value detecting means for detecting an output power past value and an output power current value of the generator, and a detected output power past value of the generator. And the power past-current value storage means for storing the output power current value, the rotation number past-current value detecting means for detecting the rotation number past value and the rotation number current value of the generator, and
Difference power calculation for calculating the difference power between the detected current value of the generator and the past value of the output power of the generator, which stores the past value and the current value of the generator. It is determined from the means, the difference rotation number calculation means for calculating the difference rotation speed between the past value of the output rotation speed of the generator and the current value of the output rotation speed, and whether or not the generator is in a stall state based on the difference power and the difference rotation speed. The stall determination means is provided with a load reduction / release means for reducing or releasing the load of the generator when the stall determination means determines the stall, and the stall determination means determines the stall. The region and the non-stray determination region to be determined as non-stray are classified by a determination curve showing the relationship between the difference power and the difference rotation speed, and the determination curve shows a stall determination to determine stall when the difference power increases. There is a curve and a return judgment curve that determines non-stray when the difference power drops, and since the stall boundary region of the hysteresis region is between the two curves, appropriate output power can be controlled without prior measurement. In addition, even if the generator is in a stalled state, by reducing or releasing the load on the generator, it is possible to return from the stalled state to the normal power generation state, obtain a large amount of generated power, and prevent hunting of power control. Can be done.

The control method of the hydraulic power generation system of the present invention is a water wheel that rotates by hydraulic power, a generator that converts the rotational energy of the water wheel into electric energy, and a control that controls the rotation speed of the water wheel by adjusting the load power of this generator. It is a control method of a hydraulic generator system applied to a hydraulic generator system equipped with a device, and is a differential power calculation process for obtaining the difference power between the past value and the current value of the output power for each control cycle of the control device. And whether the relationship between the difference rotation number calculation process for obtaining the difference rotation speed between the past value and the current value of the rotation value of the generator and the difference power and the difference rotation speed satisfies a predetermined stall boundary condition. It includes a stall determination process for determining whether or not the generator is stall depending on whether or not it is present, and a load reduction / opening process for reducing or releasing the load of the generator when it is determined to be stall in the stall determination process. As the stall boundary condition, the stall determination region determined to be stall and the non-stray determination region determined to be non-stray are classified by a determination curve showing the relationship between the difference power and the difference rotation speed, and the determination curve includes the difference power. A stall judgment curve that determines stall when increases and a return determination curve that determines non-stray when the difference power decreases are defined, and the interval between the two curves is the stall boundary region of the hysteresis region. Appropriate output power can be controlled without measurement, and even if the generator is in a stalled state, the load on the generator can be reduced or released to restore the stalled state to the normal power generation state and obtain a large amount of generated power. It is possible to prevent hunting of power control.

It is explanatory drawing which shows the outline of the hydroelectric power generation system which concerns on one Embodiment of this invention. It is a block diagram which shows the conceptual structure of the hydroelectric power generation system. It is explanatory drawing of the reference example of a stall determination area and a non-stall determination area. It is explanatory drawing of the stall determination area, the non-stall determination area, and the stall boundary area set in the same embodiment. It is a flow chart which shows the procedure of the control method performed by the hydroelectric power generation system. It is the schematic explanatory drawing of another example of the hydroelectric generator used in the hydroelectric power generation system.

An embodiment of the present invention will be described with reference to the drawings. This hydroelectric power generation system is an example of a horizontal axis type (propeller type) hydroelectric power generator. The kinetic energy of water flowing through the water channel (not shown) causes the water wheel 1 to rotate, and the main shaft 2 of the water wheel 1 rotates the generator 3. The generator 3 is, for example, a three-phase synchronous generator using a permanent magnet, and is connected to the spindle 2 by a coupling (not shown) or the like. A speed increaser 25 may be provided between the spindle 2 and the generator 3 as shown in the example of FIG.
When a load is connected to the generator 2 and an output is taken, torque is applied to the turbine 1 from the generator 3, and the rotation of the turbine 1 is braked. When the load power is increased, the rotation speed of the turbine 3 is slowed down, and when the load power is lightened, the rotation speed of the turbine 3 is increased.
As a load of the generator 3, a load circuit is connected via a control device 4, and the control device 4 increases or decreases the torque of the generator 3 according to the flow velocity, and controls the water turbine to rotate at an optimum rotation speed. ing. A DC / DC converter, an inverter, or the like is used for the control device 4. The load circuit 5 is an electric device or a load system.

FIG. 2 shows a specific example of the control device 4. The control device 4 includes a main circuit unit 6 that supplies the generated power of the generator 3 to the load circuit 5, and a control circuit unit 7 that controls the main circuit unit 6, and in addition, a battery 8 that stores the generated power. It has.
The main circuit unit 6 includes a rectifier 9, a converter 10, an ammeter 11, a voltmeter 24, and a switching means 12 that are interposed between the battery 8 and the power generation 3 in this order.

The rectifier 9 is a device that rectifies the three-phase alternating current power generated by the generator 3 to direct current, and is composed of a half-bridge circuit of a semiconductor switching element.
The converter 10 comprises, for example, a boost chopper. It may be used as a step-down chopper.
The switching means 12 is a means for switching whether or not to turn on / off the rectified DC power and supply it to the battery 8. The switching means 12 may be a semiconductor switching element or a contact switch. The switching means 12 can be switched on and off by a control signal output from the gate circuit 15.

The battery 8 and the load circuit 5 are in parallel, and the power generated by the generator 3 can be supplied to the load circuit 5 while being charged to the battery 8.
Since the battery 8 and the load circuit 5 are connected in parallel to the output side of the generator 3 in the main circuit unit 6, the output voltage becomes substantially constant. Therefore, the output power of the generator 3 can be adjusted by adjusting the duty of the converter 10 by opening and closing the switching means 12.

In this example, the control circuit unit 7 performs basic control by the MPPT control means 13, and controls the stall of the turbine 1 by the stall response control means 14. Both the MPPT control means 13 and the stall response control means 14 adjust the output power by controlling the opening and closing of the switching means 12 under the control of the gate circuit 15.
The control circuit unit 7 may perform basic control by a basic control means that employs a control method different from that of the MPPT control means 13.

The MPPT control means 13 changes the operating point of the generator 3 so as to always follow the maximum output operating point in control with respect to the fluctuation of the output power of the generator 3, so that the maximum output from the generator 3 is reached. It is a means for controlling the extraction of power. The maximum output operating point is an operating point that maximizes the output to the load circuit 5 detected by the ammeter 11 and the voltmeter 24 among the operating points obtained for each sampling by the MPPT control means 13.

The stall response control means 14 is a power past / present value detecting means 16, a power past / present value storing means 17, a rotation number past / present value detecting means 18, a rotation number past / present value storing means 19, a difference power calculation means 20, and a difference rotation number. It has a calculation means 21, a stall determination means 22, and a load reduction / release means 23.
The stall response control means 14 repeatedly controls a series of stall determinations. In this embodiment, every time the MPPT control is used to search for the operating point, the stall determination means 22 determines the stall, and the load reduction / release means 23 performs processing.

The electric power past-current value detecting means 16 detects the output electric power past value and the output electric power current value of the generator 3. The current output power value is the output power at the time of sampling this time when sampling is performed for each control of the series of stall determinations, and in this example, sampling is performed every time the MPPT control is used to search for an operating point. The past output power value is, for example, the output power in the sampling one time before this time. The past output power value may be the average value of the output power of a plurality of recent samplings. The sampled output power is, for example, a value obtained by multiplying the current obtained by the ammeter 11 by the voltage obtained by the voltmeter 24. It may be a value multiplied by a constant voltage which is the voltage of the battery 8. A power meter (not shown) may be provided for detection.
The electric power past-current value storage means 16 is a means for storing the detected output power past value and output power current value of the generator 3.

The rotation speed past / present value detecting means 18 is a means for detecting the rotation speed past value and the rotation speed current value of the generator 3 from a rotation detector (not shown) provided in the generator 3. The rotation speed past value and the rotation speed current value are the rotation speeds of the previous time and the present time of the sampling as well as the output voltage past value and the output voltage current value.
The rotation speed past / present value storage means 19 is a means for storing the detected rotation speed past value and rotation speed current value of the generator 3.

The difference power calculation means 20 is a means for calculating the difference power ΔP between the output power past value and the output power current value of the generator 3 stored in the power past-current value storage means 19.
The difference rotation speed calculation means 13 is a means for calculating the difference rotation speed ΔN between the output rotation speed past value and the output rotation speed current value of the generator 3 stored in the rotation speed past / current value storage means 19.

The stall determining means 22 is a means for determining whether or not the vehicle is in a stall state depending on whether or not the stall boundary condition satisfied by the difference power ΔP and the difference rotation speed ΔN is satisfied.
Specifically, as shown in FIG. 4, the determination curve a, showing the relationship between the difference power ΔP and the difference rotation speed ΔN between the stall determination area A for determining stall and the non-stall determination area B for determining non-stall. It is classified by b. The determination curves a and b are not a single determination curve d as in the example of FIG. 3, but a stall determination curve a that determines stall when the differential power ΔP increases and a non-stall when the differential power decreases. The return determination curve b is defined, and the area between the curves a and b is the stall boundary region C in the hysteresis region.

The load reducing / releasing means 23 is a means for reducing or releasing the load power of the generator 3 when the stall determining means 22 determines that the stall has occurred. Specifically, the load reducing / releasing means 23 reduces the duty of the converter 10 by opening / closing the switching means 12 via the gate circuit 15, or reduces or releases the load power by continuing to open the switching means 12. conduct.

Along with FIG. 5, the flow of stall response control in the hydroelectric power generation system having the above configuration will be described. FIG. 6 shows a flow of control performed by the stall response control means 14 of FIG. 2, and the control of FIG. 2 is repeated every time the MPPT control is used to search for an operating point.
First, the rotation speed detection process (S1) detects the rotation speed past value and the rotation speed current value of the generator 3, and the power detection process (S2) detects the output power past value and the output power current value of the generator 3. I do.
These detected past rotation speed values (previous value N0), current rotation speed value (N1), past output power value (previous value P0), and current output power value (P1) are the rotation speed storage process (S3) and In the power storage process (S4), the number of revolutions is stored in the past / present value storage means 19 and the past / present value storage means 17. The stored contents are updated every control cycle.

The difference rotation number ΔN between the stored rotation number past value (previous value N0) and the rotation number current value (N1) is calculated in the difference rotation number calculation process (S5), and the stored output power past value (previous value P0) is calculated. ) And the current output power value (P1), the difference power ΔP is calculated in the difference power calculation process (S6).

In the stall determination process (S7), the stall determination means 22 determines whether or not the calculated difference rotation speed ΔN and the difference power Δ are stalls depending on whether or not the predetermined stall boundary conditions are satisfied. If it has not stalled, the control ends as it is.
If it is determined to be stall, in the load reduction / opening process (S8), the load reduction / opening means 23 prolongs the opening time of the switching means 12 via the gate circuit 15 or keeps the switching means 12 open to generate electric power. Reduce or release the load power of the machine 3.

If the control device 4 follows the maximum power point by MPPT control in the hydroelectric power generation system, the turbine 1 may fall into a stall state, resulting in a significant decrease in the generated power.
However, as described above, the stall state is determined by the control device 4, and the load power is reduced or released, so that the normal power generation state can be obtained. This makes it possible to obtain a large amount of generated power.

When determining the stall state as described above, if the stall determination area A and the non-stall determination area B determined to be non-stall are separated by one determination curve d as shown in FIG. 3, it is determined to be stall. After the load power is reduced or released to a non-stall state, the flow velocity of the water flow does not change significantly, so the stall may occur again and hunting for stall determination may occur.

However, in this embodiment, as the stall boundary condition, as shown in FIG. 4, the stall determination area A for determining stall and the non-stall determination area B for determining non-stall have a relationship between the difference power and the difference rotation speed. It is divided by the two judgment curves a and b shown. That is, the determination curve defines a stall determination curve a that determines stall when the differential power increases and a return determination curve b that determines non-stall when the differential power decreases. It is defined as the stall boundary region C in the hysteresis region.
Whether to judge by the stall judgment curve a or the return judgment curve b is determined by storing a flag or the like as to whether the stall state or the stall state was in the previous control cycle, and the previous time is the non-stall state. If there is, the stall determination curve a is used for determination, and if the previous stall state was present, the return determination curve b is used for determination.
When the stall determination curve a is exceeded and the stall is once determined, even if the stall determination curve a or less is reached due to the reduction or opening of the load power, the stall determination state is maintained and the stall determination curve is lower than the return determination curve b. Determined to be in a non-stall state. Therefore, hunting for stall determination is prevented and control is stable.

It should be considered that the embodiments disclosed this time are exemplary in all respects and not restrictive. The scope of the present invention is shown by the scope of claims rather than the above description, and it is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1 ... Water wheel 3 ... Generator 4 ... Control device 5 ... Load circuit 6 ... Main circuit unit 7 ... Control circuit unit 8 ... Battery 13 ... MPPT control means 14 ... Stall response control means 16 ... Electric power Past and present value detecting means 17 ... Electric power Past / current value storage means 18 ... Rotation number Past / present value detection means 19 ... Rotation number Past / present value storage means 20 ... Difference power calculation means 21 ... Difference rotation number calculation means 22 ... Stall determination means 23 ... Load reduction / release means

Claims (4)

It is a hydraulic generator system equipped with a hydraulic turbine, a generator that converts the rotational energy of the turbine into electrical energy, and a control device that adjusts the load power of this generator to control the rotation speed of the turbine. hand,
The control device is
A power past-current value detecting means for detecting the output power past value and the output power current value of the generator, and
A power past-current value storage means for storing the detected output power past value and output power current value of the generator, and
The rotation speed past-current value detecting means for detecting the rotation speed past value and the rotation speed current value of the generator, and
A rotation speed past-current value storage means for storing the detected rotation speed past value and rotation speed current value of the generator, and
Difference power calculation means for calculating the difference power between the output power past value and the output power current value of the generator, and the difference rotation speed for calculating the difference rotation speed between the output rotation speed past value and the output rotation speed current value of the generator. Computational means and
A stall determining means for determining whether or not the vehicle is in a stall state based on the difference power and the difference rotation speed,
It is provided with a load reducing / releasing means for reducing or releasing the load of the generator when the stall determining means determines that the stall has occurred.
In the stall determination means, the stall determination region for determining stall and the non-stall determination region for determining non-stall are classified by a determination curve showing the relationship between the difference power and the difference rotation speed, and the determination curve is different. There is a stall judgment curve that determines stall when the power increases and a return determination curve that determines non-stall when the difference power decreases, and the area between the two curves is the stall boundary region of the hysteresis region.
Hydropower system. In the hydroelectric power generation system according to claim 1, the control device includes MPPT control means for tracking and controlling the maximum power operating point with respect to the output fluctuation of the generator, and stalls each time the search for the maximum power operating point is performed. A hydroelectric power generation system that determines stall by a determination means and performs processing by the load reduction / release means. It is applied to a hydraulic generator system equipped with a hydraulic wheel, a generator that converts the rotational energy of the water wheel into electrical energy, and a control device that adjusts the load power of this generator to control the rotation speed of the water wheel. It is a control method of the hydraulic generator system.
For each control cycle of the control device
The difference power calculation process for finding the difference power between the past value and the current value of the output power,
The difference rotation speed calculation process for obtaining the difference rotation speed between the past value and the current value of the rotation value of the generator, and
A stall determination process for determining whether or not the relationship between the difference power and the difference rotation speed is a stall depending on whether or not a predetermined stall boundary condition is satisfied.
Including the load reduction / opening process of reducing or releasing the load of the generator when it is determined to be stall in the stall determination process.
As the stall boundary condition, the stall determination region for determining stall and the non-stall determination region for determining non-stall are classified by a determination curve showing the relationship between the difference power and the difference rotation speed, and the determination curve includes the difference power. A stall judgment curve that determines stall when the value rises and a return judgment curve that determines non-stall when the difference power decreases are defined, and the stall boundary region of the hysteresis region is defined between the two curves.
A method of controlling a water power generation system, which is characterized by the fact that. In the control method of the hydraulic power generation system according to claim 3, the control device includes MPPT control means for tracking and controlling the maximum power operating point with respect to the output fluctuation of the generator, and the operating point is controlled by the tracking control. A control method for a water power generation system that undergoes the difference power calculation process, the difference rotation number calculation process, the stall determination process, and the load reduction / opening process each time a search is performed.

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