Voltage compensation system and uninterruptible power supply
Patent Information
- Authority / Receiving Office
- TW · TW
- Patent Type
- Applications
- Current Assignee / Owner
- DELTA ELECTRONICS INC(CN)
- Filing Date
- 2025-01-15
- Publication Date
- 2026-08-01
AI Technical Summary
Existing voltage compensation systems face issues with power failures due to mismatched AC power source phases and long relay flight times, requiring larger capacitors and increased costs, as they rely on a single AC power source during switching, leading to voltage fluctuations and inefficiencies.
A voltage compensation system with a bidirectional voltage conversion device and energy buffer that uses an energy tank to release electrical energy to the DC bus during switching periods, maintaining voltage stability through a controller, allowing for a smaller capacitor size and reduced costs.
The system achieves rapid dynamic discharge of the DC bus voltage, reducing the need for large capacitors and overall system size while maintaining stable power supply, thereby lowering costs and ensuring efficient operation.
Smart Images
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Abstract
Description
Technical Field
[0001] This case relates to a power supply system, particularly a voltage compensation system and an uninterruptible power supply system. Prior Technology
[0002] The voltage compensation system includes a DC bus and a converter. The DC bus can selectively receive two AC power sources and convert them into DC power to supply the load via the converter. Each AC power source is connected to the DC bus via a corresponding relay. When the voltage of one of the two AC power sources is abnormal, the relay connected to the abnormal AC power source will disconnect, causing the DC bus to wait to receive the other AC power source. Since the input phases of the two AC power sources are not exactly the same, and the relay flight time is relatively long, the voltage compensation system is prone to power failure. In addition, when the DC bus is waiting to receive another AC power source, it means that neither AC power source has been supplied yet, which requires a larger capacitor in the DC bus to provide the output voltage, resulting in a larger overall size and increased cost of the voltage compensation system.
[0003] Therefore, developing a voltage compensation system that overcomes the above-mentioned shortcomings is an urgent need at present. Summary of the Invention
[0004] The purpose of this invention is to provide a voltage compensation system in which, during the pause period when the first switching device switches between the first AC input source and the second AC input source, the controller controls the operation of the bidirectional voltage conversion device of the energy buffer device. The energy tank releases electrical energy to the DC bus via the bidirectional voltage conversion device to compensate for the DC voltage on the DC bus. Therefore, the voltage compensation system of this invention can transmit all the charging voltage of the energy tank to the DC bus via the bidirectional voltage conversion device of the energy buffer device to achieve a rapid dynamic discharge effect. Thus, the capacitor volume in the DC bus of the voltage compensation system of this invention is small enough to meet the discharge effect, resulting in a smaller overall size and lower cost for the voltage compensation system.
[0005] To achieve the above objectives, one embodiment of this invention is a voltage compensation system, comprising a first switching device, a voltage conversion device, a DC bus, an energy buffer device, and a controller. The first switching device is used to switch between a first AC input source and a second AC input source. The voltage conversion device, via the first switching device, selectively receives either the first or second AC input source and converts either the first or second AC input source into a DC voltage. The DC bus is used to transmit the DC voltage. The energy buffer device is connected to the DC bus and includes a bidirectional voltage conversion device and an energy tank. One end of the bidirectional voltage conversion device is connected to the DC bus. The energy tank is connected to the other end of the bidirectional voltage conversion device. During the pause when the first switching device is switching between the first and second AC input sources, the bidirectional voltage conversion device is operated to allow the energy tank to compensate for the DC voltage of the DC bus.
[0006] To achieve the above objectives, another embodiment of this invention is an uninterruptible power supply (UPS) system, comprising a first switching element, a second switching element, a power factor corrector, an energy buffer, and a controller. The first switching element is electrically connected to a first AC input source, and the second switching element is electrically connected to a second AC input source. The power factor corrector is electrically connected to both the first and second switching elements, and selectively switches between them to allow the power factor corrector to receive either the first or second AC input source, thereby generating a DC voltage with a first operating voltage at its output. The output of the energy buffer is electrically connected to the output of the power factor corrector. During periods of inactivity when the first and second switching elements are switching between the first and second AC input sources, the controller enables the energy buffer to compensate for the DC voltage and maintain it at a second operating voltage. Simple Explanation of the Diagram
[0007] Figure 1A is a schematic diagram of the circuit structure of the voltage compensation system according to the first embodiment of this case; Figure 1B is a schematic diagram of the circuit structure of the voltage compensation system of the second embodiment of this case; Figure 2 is a waveform timing diagram of the internal components of the voltage compensation system shown in Figure 1A; Figure 3 is a detailed circuit diagram of the bidirectional voltage conversion device of the energy buffer device in the voltage compensation system shown in Figure 1A; and Figure 4 is a schematic diagram of the detailed circuit structure of the controller of the voltage compensation system shown in Figure 1A. Implementation
[0008] Some typical embodiments that embody the features and advantages of this case will be described in detail in the following description. It should be understood that this case can have various variations in different forms, all of which do not depart from the scope of this case, and the descriptions and drawings therein are essentially for illustrative purposes and not for limiting this case.
[0009] Please refer to Figure 1A, which is a schematic diagram of the circuit structure of the voltage compensation system of the first embodiment of this case. As shown in Figure 1A, the voltage compensation system 1 of this embodiment is used to receive and convert a first AC input source AC1 and a second AC input source AC2 to supply power to the load L, wherein the first AC input source AC1 and the second AC input source AC2 are two different AC input sources. The voltage compensation system 1 includes a first switching device 2, a voltage conversion device 3, a DC bus 4, a DC / DC conversion circuit 5, a backup device 6, an energy buffer device 8, and a controller 9.
[0010] The first switching device 2 is used to switch between a first AC input source AC1 and a second AC input source AC2, selectively switching between the first AC input source AC1 and the second AC input source AC2, thereby selectively receiving either the first AC input source AC1 or the second AC input source AC2. When either the first AC input source AC1 or the second AC input source AC2 malfunctions, the first switching device 2 will experience a pause period when switching between the first AC input source AC1 and the second AC input source AC2, during which the first switching device 2 cannot transmit power normally. In this embodiment, the first switching device 2 includes a first switching element 21 and a second switching element 22. The first switching element 21 is connected to the first AC input source AC1 to transmit power when the first AC input source AC1 is active, and the second switching element 22 is connected to the second AC input source AC2 to transmit power when the second AC input source AC2 is active.
[0011] A voltage conversion device 3 is connected to the first switching element 21 and the second switching element 22 of the first switching device 2 to selectively receive either a first AC input source AC1 or a second AC input source AC2 via the switching states of the first switching element 21 and the second switching element 22, and converts the received first AC input source AC1 or second AC input source AC2 into a DC voltage V1. A DC bus 4 receives and transmits the DC voltage V1 provided by the voltage conversion device 3. During the stagnation period when the first switching device 2 switches between the first AC input source AC1 and the second AC input source AC2, the DC bus 4 cannot receive electrical energy, causing the DC voltage V1 on the DC bus 4 to attenuate. A DC / DC conversion circuit 5 is connected between the DC bus 4 and the load L to convert the DC voltage V1 on the DC bus 4 to supply power to the load L.
[0012] The backup device 6 includes a DC / DC converter 61, a battery module 62, and a second switching device 63. The DC / DC converter 61 includes an input terminal 611 and an output terminal 612. The battery module 62 provides battery voltage to the input terminal 621 of the DC / DC converter 61. The second switching device 63 is connected between the input terminal 621 of the DC / DC converter 61 and the battery module 62. When the first AC input source AC1 and / or the second AC input source AC2 malfunctions, the second switching device 63 is activated, causing the DC / DC converter 61 to switch the battery voltage provided by the battery module 62 to a third operating voltage.
[0013] An energy buffer device 8 is connected between the output terminal 612 of the DC / DC converter 61 and the DC bus 4, and includes an energy tank 81 and a bidirectional voltage conversion device 82. The energy tank 81 is connected to the output terminal 612 of the DC / DC converter 61 to store the third operating voltage provided by the DC / DC converter 61 as a charging voltage V2. During the pause period when the first switching device 2 switches between the first AC input source AC1 and the second AC input source AC2, the charging voltage V2 stored inside the energy tank 81 is provided. The bidirectional voltage conversion device 82 is connected between the energy tank 81 and the DC bus 4, and selectively releases the charging voltage V2 stored in the energy tank 81 to the DC bus 4, or charges the energy tank 81 with the DC voltage V1 on the DC bus 4. In other words, the third operating voltage in this embodiment is delivered to the DC bus 4 via the energy tank 81 and the bidirectional voltage conversion device 82.
[0014] Compared to the backup device 6 of the voltage compensation system 1 shown in Figure 1A, which is connected to the DC bus 4 via the energy buffer device 8, in some embodiments, the backup device of the voltage compensation system can be directly connected to the DC bus. As shown in Figure 1B, the backup device 6 of the voltage compensation system 1a is directly connected to the DC bus 4. In other words, the output terminal 612 of the DC / DC converter 61 and the bidirectional voltage conversion device 82 of the energy buffer device 8 are electrically connected to the DC bus 4, so that the charging voltage V2 provided by the DC / DC converter 61 and the DC voltage V1 on the energy buffer device 8 can be directly provided to the DC bus 4 at the same time.
[0015] Please refer to Figure 1A again. The controller 9 controls the operation of the bidirectional voltage conversion device 82 based on the DC voltage V1 on the DC bus 4. During the stagnation period when the first AC input source AC1 or the second AC input source AC2 is abnormal and the first switching device 2 switches between the first AC input source AC1 and the second AC input source AC2, the controller 9 detects that the DC voltage V1 on the DC bus 4 has dropped from the first operating voltage to below the low voltage threshold. The controller 9 determines that the DC voltage V1 on the DC bus 4 is undervoltage. The controller 9 operates the bidirectional voltage conversion device 82 so that the energy tank 81 provides the internally stored charging voltage V2 as the first compensation voltage, and the bidirectional voltage conversion device 82 converts the first compensation voltage into a second compensation voltage on the DC bus 4. That is, the energy tank 81 releases electrical energy to the DC bus 4 through the bidirectional voltage conversion device 82 so that the energy tank 81 compensates for the DC voltage V1 on the DC bus 4, so that the DC voltage V1 on the DC bus 4 is restored to the second operating voltage. In one embodiment, the user can adjust the settings of the controller 9 according to actual needs to change the magnitude of the second compensation voltage, thereby adjusting the second operating voltage. In one embodiment, the second operating voltage is lower than the first operating voltage, wherein the energy tank 81 has a capacitor or other energy storage device. Because the second operating voltage is lower than the first operating voltage, while maintaining the normal operation of the load L and maintaining a steady state, power saving can be achieved, and the charging time of the energy tank 81 can be reduced. In addition, a capacitor with a lower capacitance value can also be selected in the energy tank 81 to reduce the size and cost of the circuit. In another embodiment, the second operating voltage is higher than or equal to the first operating voltage to ensure that the load L operates normally or efficiently.
[0016] In one embodiment, when the controller 9 detects that the first AC input source AC1 or the second AC input source AC2 has recovered, i.e., the standby period has ended, the bidirectional voltage conversion device 82 is operated to allow the voltage conversion device 3 to switch the first AC input source AC1 or the second AC input source AC2 to charge the energy tank 81. In another embodiment, when the controller 9 detects that the energy tank 81 has discharged for a predetermined time, the bidirectional voltage conversion device 82 is operated to allow the voltage conversion device 3 to switch the first AC input source AC1 or the second AC input source AC2 to charge the energy tank 81. The predetermined time is longer than the standby period. Since the standby period (i.e., dead time) of a typical power switching is fixed or calculable, the discharge time can also be calculated to determine the end time of the standby period.
[0017] Please refer to Figure 2 in conjunction with Figure 1A, where Figure 2 is a waveform timing diagram of the internal components of the voltage compensation system shown in Figure 1A. In Figure 2, the four waveforms from top to bottom respectively show the voltage timing diagrams of the AC input sources (AC1, AC2), the DC voltage V1 of the DC bus 4, the charging voltage V2 of the energy tank 81, and the load L. Before time t0, the first AC input source AC1 is in a steady state, so the DC voltage V1 of the DC bus 4 and the charging voltage V2 of the energy tank 81 remain fixed. Between time t0 and t1, during the stagnation period between the first AC input source AC1 being de-energized and the second AC input source AC2 being switched, the DC voltage V1 of the DC bus 4 decreases, and the controller 9 detects that the operation of the first AC input source AC1 and the second AC input source AC2 is in a stagnation period. Between times t1 and t2, controller 9 controls the operation of bidirectional voltage converter 82, causing it to release electrical energy to DC bus 4 based on the charging voltage V2 stored in energy tank 81. This compensates for the DC voltage V1 on DC bus 4, causing the charging voltage V2 stored in energy tank 81 to decrease, and the DC voltage V1 on DC bus 4 to rise to its maximum capacity and remain stable. Between times t2 and t3, the standby period ends, i.e., when the first AC input source AC1 or the second AC input source AC2 resumes operation, the second AC input source AC2 begins supplying power, and the DC voltage V1 on DC bus 4 continues to remain stable, while the charging voltage V2 stored in energy tank 81 remains fixed. Between times t3 and t4, the charging voltage V2 stored in energy tank 81 and / or the DC voltage V1 on DC bus 4 recharge energy tank 81, causing the charging voltage V2 stored in energy tank 81 to rise. At all the times mentioned above, the voltage across the load L remains in a steady state.
[0018] As can be seen from the above, during the pause period when the first switching device 2 switches between the first AC input source AC1 and the second AC input source AC2, the controller 9 of the voltage compensation system 1 controls the operation of the bidirectional voltage conversion device 82 of the energy buffer device 8. The energy tank 81 releases electrical energy to the DC bus 4 through the bidirectional voltage conversion device 82 to compensate for the DC voltage V1 on the DC bus 4. Therefore, compared with the traditional voltage compensation system, the voltage compensation system 1 of this invention can transmit all the charging voltage V2 of the energy tank 81 to the DC bus 4 through the bidirectional voltage conversion device 82 of the energy buffer device 8 to achieve the effect of rapid dynamic discharge. Therefore, the capacitor volume in the DC bus 4 of the voltage compensation system 1 of this invention is small enough to meet the discharge effect. Thus, the overall size of the voltage compensation system 1 of this invention is smaller and the cost is reduced.
[0019] Please refer to Figures 3 and 4, and in conjunction with Figures 1A and 2. Figure 3 is a detailed circuit diagram of the bidirectional voltage conversion device of the energy buffer device in the voltage compensation system shown in Figure 1A, and Figure 4 is a detailed circuit diagram of the controller of the voltage compensation system shown in Figure 1A. As shown in Figure 3, the bidirectional voltage conversion device 82 includes a first switch 821, a second switch 822, an inductor 823, and a current detection unit 824. The first end of the first switch 821 is electrically connected to the DC bus 4. The first end of the second switch 822 is electrically connected to the second end of the first switch 821, and the second end of the second switch 822 is also electrically connected to the DC bus 4. One end of the inductor 823 is electrically connected to the first end of the second switch 822 and the second end of the first switch 821. The current detection unit 824 is connected between the other end of the inductor 823 and the energy tank 81 to detect the detection current i2 flowing into the bidirectional voltage conversion device 82 from the energy tank 81, and then calculates the first compensation voltage. As shown in Figure 4, the controller 9 is connected to the bidirectional voltage conversion device 82 of the DC bus 4 and the energy buffer device 8, and includes a first subtractor 91, a first proportional-integral control unit 92, a first limiter 93, a second subtractor 94, a second proportional-integral control unit 95, a second limiter 96, and a comparator 97. The first subtractor 91 subtracts the preset voltage reference value Vref in the controller 9 from the DC voltage V1 on the DC bus 4 to obtain the voltage difference. The first proportional-integral control unit 92 is used to adjust the voltage difference. The first limiter 93 adjusts the voltage difference accordingly. The first saturation upper limit value is obtained. The second subtractor 94 subtracts the first saturation upper limit value provided by the first limiter 93 from the detected current i2 detected by the current detection unit 824 of the bidirectional voltage conversion device 82 to obtain the current difference value. The second proportional-integral control unit 95 is used to adjust the current difference. The second limiter 96 adjusts the current difference accordingly. The second saturation upper limit is obtained. Comparator 97 obtains PWM control signals P1 and P2 based on the comparison result between the second saturation upper limit and the preset original control signal P0 within controller 9, causing the duty cycles of PWM control signals P1 and P2 to rise to their maximum values to control the operation of the bidirectional voltage converter 82. For example, it controls the first switch 821 and the second switch 822 within the bidirectional voltage converter 82 respectively, releasing the charging voltage V2 stored in energy tank 81 to DC bus 4 via the bidirectional voltage converter 82, thereby adjusting the second compensation voltage to compensate for the DC voltage V1 on DC bus 4, where the DC voltage V1 on DC bus 4 is greater than the charging voltage V2 stored in energy tank 81. In one embodiment, the voltage reference value Vref within controller 9 is adjustable; for example, the voltage reference value Vref is the voltage reference value plus the voltage adjustment value, allowing controller 9 to adjust the voltage adjustment value according to the required electrical energy for the DC voltage V1 on DC bus 4.
[0020] In one embodiment, the voltage conversion device in the voltage compensation system can be replaced by a power factor corrector, so that the first switching element and the second switching element of the first switching device, the power factor corrector, the DC bus, the DC / DC conversion circuit, the backup device, the energy buffer device and the controller together constitute an uninterruptible power supply system. Similarly, during the standby period when the first switching element and the second switching element switch the first AC input source and the second AC input source, the controller of the uninterruptible power supply system enables the energy buffer device to compensate for the DC voltage and maintain it at the second operating voltage.
[0021] In summary, during the pause period when the first switching device switches between the first AC input source and the second AC input source, the controller of the voltage compensation system in this case controls the operation of the bidirectional voltage conversion device of the energy buffer device. The energy tank releases electrical energy to the DC bus via the bidirectional voltage conversion device to compensate for the DC voltage on the DC bus. Therefore, compared with traditional voltage compensation systems, the voltage compensation system in this case can transmit all the charging voltage of the energy tank to the DC bus via the bidirectional voltage conversion device of the energy buffer device to achieve a rapid dynamic discharge effect. Therefore, the capacitor volume in the DC bus of the voltage compensation system in this case is smaller to meet the discharge effect, resulting in a smaller overall size and lower cost for the voltage compensation system in this case.
[0022] 1.1a: Voltage compensation system AC1: First AC input source AC2: Second AC input source 2: First switching device 21: First switching element 22: Second switching element 3: Voltage conversion device 4: DC bus V1: DC voltage 5: DC / DC conversion circuit 6: Backup device 61: DC / DC converter 62: Battery Module 63: Second switching device 8: Energy buffer device 81: Energy Bar V2: Charging voltage 82: Bidirectional voltage conversion device 821: First Switch 822: Second Switch 823: Inductor 824: Current Detection Unit 9: Controller 91: First Subtractor 92: First proportional-integral control unit 93: First Limiter 94: Second Subtractor 95: Second proportional-integral control unit 96: Second Limiter 97: Comparator Vref: Voltage reference value Voltage difference Current difference P0: Raw control signal P1: PWM control signal t0, t1, t2, t3, t4: Time points
Claims
1. A voltage compensation system, comprising: A first switching device for switching between a first AC input source and a second AC input source; A voltage conversion device selectively receives the first AC input source or the second AC input source via the first switching device, and converts the first AC input source or the second AC input source into a DC voltage; A DC bus for transmitting the DC voltage; an energy buffer connected to the DC bus, and including: a bidirectional voltage converter, wherein one end of the bidirectional voltage converter is connected to the DC bus; The device includes an energy tank connected to the other end of the bidirectional voltage converter; and a controller that operates the bidirectional voltage converter during a pause in the switching of the first AC input source and the second AC input source by the first switching device, allowing the energy tank to compensate for the DC voltage of the DC bus.
2. The voltage compensation system as claimed in claim 1, wherein the controller detects during the pause period that the DC voltage on the DC bus drops from a first operating voltage to below a low voltage threshold, and the controller determines that the DC voltage on the DC bus is undervoltage.
3. The voltage compensation system as claimed in claim 2, wherein after the controller determines that the DC voltage is undervoltage, the energy tank provides a first compensation voltage, and the bidirectional voltage conversion device converts the first compensation voltage into a second compensation voltage and applies it to the DC bus, so that the DC voltage is restored to a second operating voltage.
4. The voltage compensation system as claimed in claim 3, wherein the second operating voltage is lower than the first operating voltage.
5. The voltage compensation system as claimed in claim 3, wherein the second operating voltage is higher than or equal to the first operating voltage.
6. The voltage compensation system as claimed in claim 1, wherein the voltage compensation system further includes a backup device, and the backup device includes: A DC / DC converter, including an output terminal; And a battery module that provides a battery voltage to an input terminal of the DC / DC converter; A second switching device is disposed between the DC / DC converter and the battery module; wherein when the first AC input source and / or the second AC input source malfunctions, the second switching device is activated, and the DC / DC converter converts the battery voltage to a third operating voltage and outputs the third operating voltage to the DC bus.
7. The voltage compensation system as claimed in claim 6, wherein the output of the DC / DC converter is electrically connected to the energy tank, allowing the third operating voltage to pass through the energy tank and the bidirectional voltage conversion device to the DC bus.
8. The voltage compensation system as claimed in claim 6, wherein the output of the DC / DC converter and the output of the bidirectional voltage conversion device are electrically connected to the DC bus.
9. The voltage compensation system as claimed in claim 1, wherein the bidirectional voltage conversion device comprises: a first switch, wherein a first terminal of the first switch is electrically connected to the DC bus; a second switch, wherein a first terminal of the second switch is electrically connected to a second terminal of the first switch, and the second terminal of the second switch is electrically connected to the DC bus; an inductor, wherein one end of the inductor is electrically connected to the first terminal of the second switch and the second terminal of the first switch; and a current detection unit connected between the other end of the inductor and the energy tank to detect a current flowing into the bidirectional voltage conversion device from the energy tank, and thereby calculate a first compensation voltage.
10. The voltage compensation system as claimed in claim 9, wherein the controller includes a first subtractor, a first proportional-integral control unit, a first limiter, a second subtractor, a second proportional-integral control unit, a second limiter, and a comparator. The first subtractor subtracts a voltage reference value from the DC voltage on the DC bus to obtain a voltage difference. The first proportional-integral control unit adjusts the voltage difference. The first limiter obtains a first saturation upper limit value based on the adjusted voltage difference. The second subtractor subtracts the first saturation upper limit value from the detected current detected by the current detection unit to obtain a current difference. The second proportional-integral control unit adjusts the current difference. The second limiter obtains a second saturation upper limit value based on the adjusted current difference. The comparator obtains a PWM control signal based on a comparison result of the second saturation upper limit value and an original control signal to control the first switch and the second switch, and thereby adjust a second compensation voltage.
11. An uninterruptible power supply (UPS) system, comprising: a first switching element and a second switching element, wherein the first switching element is electrically connected to a first AC input source and the second switching element is electrically connected to a second AC input source; a power factor corrector electrically connected to the first switching element and the second switching element, wherein the first switching element and the second switching element selectively switch the power factor corrector to receive either the first AC input source or the second AC input source, thereby generating a DC voltage having a first operating voltage at an output terminal of the power factor corrector; an energy buffer device, wherein an output terminal of the energy buffer device is electrically connected to the output terminal of the power factor corrector; and a controller, wherein during a pause in the switching of the first AC input source and the second switching element between the first switching element and the second switching element, the controller enables the energy buffer device to compensate for the DC voltage and maintain it at a second operating voltage.
12. The uninterruptible power supply system as claimed in claim 11, wherein during the pause period the controller detects that the DC voltage drops from the first operating voltage to below a low-voltage threshold, the controller determines that the DC voltage is undervoltage, and enables the energy buffer device.
13. The uninterruptible power supply system as claimed in claim 11, wherein the second operating voltage is lower than the first operating voltage; or the second operating voltage is higher than or equal to the first operating voltage.
14. The uninterruptible power supply system as claimed in claim 11, wherein the energy buffer device comprises: A bidirectional voltage converter, wherein one output terminal of the bidirectional voltage converter serves as the output terminal of the energy buffer device and is electrically connected to the output terminal of the power factor corrector; And an energy tank connected to an input terminal of the bidirectional voltage converter; wherein when the controller enables the energy buffer device, the bidirectional voltage converter is operated to discharge the energy tank to compensate for the DC voltage.
15. The uninterruptible power supply system as claimed in claim 14, wherein when the controller detects that the first AC input source or the second AC input source has been restored, the bidirectional voltage conversion device is operated to allow the power factor corrector to switch the first AC input source or the second AC input source to charge the energy tank.
16. The uninterruptible power supply system as claimed in claim 14, wherein when the controller detects that the energy tank is discharging for a predetermined time, the bidirectional voltage conversion device is operated to allow the power factor corrector to switch the first AC input source or the second AC input source to charge the energy tank; wherein the predetermined time is longer than the stagnation period.