Patent Publication Number: US-9906074-B2

Title: Power supply system

Description:
TECHNICAL FIELD 
     The present invention relates to a power supply system, and particularly to a power supply switching circuit used for an uninterruptable power-supply system (UPS). 
     BACKGROUND ART 
     An uninterruptable power-supply system (UPS) is configured to supply power continuously to a load. The UPS is stopped in some cases due to failure or maintenance of the UPS. To prepare for such cases, a bypass power supply is provided in parallel with the UPS. 
     A UPS generally includes an inverter. When the inverter fails, the bypass power supply provides power to a load. When the voltage of the bypass power supply is not in synchronization with the output voltage of the UPS, a large magnetizing inrush current disadvantageously flows into a load transformer. 
     When the bypass power supply provides power to the load, an output switch of the UPS is turned off. However, a certain amount of time is required to completely turn the output switch off. Therefore, there is a possibility that the magnetizing inrush current flows not only into the load transformer but also into an isolation transformer provided in the UPS. 
     Japanese Patent Laying-Open No. 8-256431 (PTD 1) discloses a static inrush current suppressing device for suppressing a transformer magnetizing inrush current. This device includes a thyrister connected to a primary side of a transformer, an instrument transformer connected to a primary side of the thyrister, a peak voltage detecting circuit connected to a secondary side of the instrument transformer, and a thyrister gate output circuit turning the thyrister on at a timing when the peak voltage detecting circuit detects a peak voltage. The transformer is magnetized by the peak voltage. Accordingly, the transformer magnetizing inrush current can be suppressed. 
     CITATION LIST 
     Patent Document 
     PTD 1: Japanese Patent Laying-Open No. 8-256431 
     SUMMARY OF INVENTION 
     Technical Problem 
     To provide power to a load without interruption even when a UPS is stopped, shortening a time for switching a power supply is required. Further, synchronizing a voltage of a bypass power supply with an output voltage of the UPS is also required. 
     An object of the present invention is to provide a power supply system capable of shortening a switching time of a power supply as well as suppressing a magnetizing inrush current. 
     Solution to Problem 
     A power supply system according to one aspect of the present invention includes an uninterruptable power-supply apparatus supplying power to a load, a switch connecting a bypass power supply to the load, and a switching circuit turning the switch on when the uninterruptable power-supply apparatus is stopped. In a case where the uninterruptable power-supply apparatus is stopped, the switching circuit turns the switch on when a phase of an output voltage of the bypass power supply is equal to a phase of an output voltage of the uninterruptable power-supply apparatus at a stopped timing. 
     Advantageous Effects of Invention 
     According to the present invention, a time for switching power supplies between an uninterruptable power-supply system and a bypass power supply can be shortened. Further, according to the present invention, a magnetizing inrush current can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  represents a configuration of a power supply system according to one embodiment of the present invention. 
         FIG. 2  is a functional block diagram representing a switching circuit shown in  FIG. 1 . 
         FIG. 3  represents waveforms for describing operation of the power supply system according to one embodiment of the present invention. 
         FIG. 4  represents waveforms for describing another operation of the power supply system according to one embodiment of the present invention. 
         FIG. 5  represents a configuration of a comparative example of the power supply system according to the embodiment of the present invention. 
         FIG. 6  represents waveforms for describing operation of the power supply system shown in  FIG. 5 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. The same or corresponding parts in the drawings have the same reference numerals allotted, and description thereof will not be repeated. 
       FIG. 1  represents a configuration of a power supply system according to one embodiment of the present invention. Referring to  FIG. 1 , a power supply system  100  supplies power, which is provided by a bypass power supply  1  or an input power supply  2 , to a load  3 . Bypass power supply  1  and input power supply  2  are alternate-current power supplies. In  FIG. 1 , bypass power supply  1  and input power supply  2  are illustrated as three-phase alternate-current power supplies. However, these power supplies may be single-phase power supplies. Types of bypass power supply  1  and input power supply  2  are not particularly limited. 
     Power supply system  100  includes an uninterruptable power-supply apparatus (UPS)  5 , a thyrister switch  6 , and a switching circuit  7 . UPS  5  includes a converter  11 , a battery  12 , an inverter  13 , an isolation transformer  14 , an output switch  15 , a capacitor  16 , and a synchronizing circuit  17 . Load transformer  4  is provided in parallel with load  3 . 
     Converter  11  converts alternate-current power from input power supply  2  into direct-current power. The direct-current power from converter  11  is supplied to battery  12  and inverter  13 . During operation of converter  11 , battery  12  stores the direct-current power from converter  11 . When converter  11  is stopped, battery  12  supplies direct-current power to inverter  13 . Capacitor  16  smoothes the direct-current power inputted to inverter  13 . 
     Inverter  13  converts direct-current power into alternate-current power (for example, three-phase alternate-current). Output switch  15  switches whether or not to output the alternate-current power from inverter  13  to load  3 . For example, output switch  15  is achieved by a mechanical switch (for example, contactor). In a case where output switch  15  is in the on-state, the alternate-current power from inverter  13  is supplied to load  3  and load transformer  4  via isolation transformer  14 . In this case, thyrister switch  6  is in the off-state. 
     Synchronizing circuit  17  synchronizes a phase of a voltage Vinv with a phase of a voltage Vbyp. Voltage Vinv is a voltage outputted from inverter  13  via isolation transformer  14 . Voltage Vbyp is an output voltage of bypass power supply  1 . For example, synchronizing circuit  17  is achieved by a PLL (Phase Lock Loop) circuit. In the case where output switch  15  is in the on-state, a voltage Vo inputted to load  3  and load transformer  4  is equal to voltage Vinv. 
     Switching circuit  7  monitors voltage Vinv. For example, in the case where the supply of voltage Vinv is stopped due to failure of inverter  13 , switching circuit  7  turns thyrister switch  6  on. Further, output switch  15  is turned off. For example, switching circuit  7  turns output switch  15  off. Another circuit may turn output switch  15  off. 
     When thyrister switch  6  is turned on, bypass power supply  1  is connected to load  3  and load transformer  4 . Therefore, power from bypass power supply  1  is supplied to load  3  and load transformer  4 . In this case, voltage Vo is equal to voltage Vbyp. 
     According to one embodiment of the present invention, switching circuit  7  stores a phase of voltage Vinv which is exhibited at a timing when UPS  5  stops the supply of power to load  3 . Further, switching circuit  7  monitors voltage Vbyp. Switching circuit  7  turns thyrister switch  6  on when the phase of voltage Vbyp matches with the stored phase of voltage Vinv. 
     According to this embodiment, a length of a period during which the supply of power to load  3  is stopped can be set within one cycle of an alternate current (for example, 1/50 seconds or 1/60 seconds). In other words, according to this embodiment, a period of during which the supply of power to load  3  is stopped can be shortened. 
     Further, according to this embodiment, the phase of voltage Vo can be maintained when voltage Vo is switched from voltage Vinv to voltage Vbyp. Therefore, before and after voltage Vo is switched from voltage Vinv to voltage Vbyp, the phase of voltage Vo is changed continuously. Accordingly, the magnetizing inrush current flowing into load  3  and load transformer  4  can be suppressed. 
       FIG. 2  is a functional block diagram representing the switching circuit shown in  FIG. 1 . Referring to  FIG. 2 , switching circuit  7  includes phase detectors  21 ,  23 , a storage  22 , and a switch controller  24 . 
     Phase detector  21  detects the phase of voltage Vinv of UPS  5 . For example, phase detector  21  measures a value of voltage Vinv to detect the phase of voltage Vinv. Specifically, an amplitude and a cycle of voltage Vinv are acquired, so that the phase of voltage Vinv can be determined based on a voltage value of voltage Vinv. The amplitude and cycle of voltage Vinv may be given to phase detector  21  in advance, or may be acquired by phase detector  21  during operation of UPS  5 . 
     Storage  22  stores the phase of voltage Vinv at the stopped timing of UPS  5 . For example, in the case where the phase of voltage Vinv does not change, phase detector  21  stores a value of the phase in storage  22 . Alternatively, phase detector  21  may update the value stored in storage  22  with a detection value of the phase of voltage Vinv. In the case where the phase of voltage Vinv does not change, the value stored in storage  22  does not change. Therefore, storage  22  can store the phase of voltage Vinv at the stopped timing of UPS  5 . 
     Phase detector  23  detects a phase of voltage Vbyp of bypass power supply  1 . For example, phase detector  23  uses the same method as the detection method of phase detector  21  to detect the phase of voltage Vbyp. 
     Switch controller  24  compares the phase of Vinv stored in storage  22  with the phase of voltage Vbyp. In the case where the phase of voltage Vbyp matches with the phase of voltage Vinv, switch controller  24  turns thyrister switch  6  on. 
     In the case where UPS  5  is recovered, thyrister switch  6  is turned off. A control method for turning thyrister switch  6  off is not particularly limited. According to one embodiment, in the case where the phase of Vinv detected by phase detector  21  changes temporally, and the phase matches with the phase of voltage Vbyp, switch controller  24  turns thyrister switch  6  off. Switch controller  24  may further turn output switch  15  on. With such a control, the phase of voltage Vo supplied to load  3  and load transformer  4  can be made continuous. 
     Switch controller  24  can acquire the value stored in storage  22  periodically to determine if UPS  5  is operated or stopped. As described above, phase detector  21  uses the detection value of the phase of voltage Vinv to update the value stored in storage  22 . Switch controller  24  can determine that UPS  5  is stopped in accordance with no change in the value stored in storage  22 . 
     On the other hand, switch controller  24  can determine that UPS  5  is operated in accordance with a change in the value stored in storage  22 . In this case, switch controller  24  does not turn thyrister switch  6  on. A control for turning thyrister switch  6  off can be achieved in accordance with the method described above. According to one embodiment, switch controller  24  turns thyrister switch  6  off in the case where the phase of voltage Vinv matches with the phase of voltage Vbyp. 
       FIG. 3  represents waveforms for describing operation of the power supply system according to one embodiment of the present invention. Referring to  FIGS. 1 and 3 , inverter  13  is stopped at a time t1. The phase of voltage Vinv and the phase of voltage Vo are “a.” On the other hand, the phase of voltage Vbyp at time t1 is “c.” The phase “c” and the phase “a” are different. 
     The phase of voltage Vbyp is equal to “a” at a time t2. Therefore, switching circuit  7  turns thyrister switch  6  on. The supply of power to load  3  and load transformer  4  is restarted from time t2. The period from time t1 to time t2 is a period within one cycle T. After a lapse of a certain time (for example, 50 ms) from time t1, output switch  15  is turned off completely (time t3). However, according to the present embodiment, the supply of power to the load is recovered before time t3. 
       FIG. 4  represents waveforms for describing another operation of the power supply system according to one embodiment of the present invention. Referring to  FIG. 4 , times t1, t3 correspond respectively to times t1, t3 shown in  FIG. 3 . In the embodiment shown in  FIG. 4 , voltage Vbyp and voltage Vinv are synchronized. In other words, the phase of voltage Vbyp and the phase of voltage Vinv are equal to each other. 
     According to  FIG. 4 , thyrister switch  6  can be turned on substantially at the same timing with the timing at which inverter  13  is stopped. Therefore, the period during which the supply of power to load  3  (and load transformer  4 ) is stopped can be further shortened. 
       FIG. 5  represents a configuration of a comparative example of the power supply system according to the present embodiment. Referring to  FIGS. 1 and 5 , a power supply system  101  is different from power supply system  100  in including a switching circuit  7 A in place of switching circuit  7 . Since the configuration of other parts of power supply system  101  are similar to the configuration of corresponding parts of power supply system  100 , subsequent description will not be repeated. 
     In the case where UPS  5  is stopped, switching circuit  7 A turns thyrister switch  6  on. Specifically, switching circuit  7 A turns thyrister switch  6  on after a lapse of a certain time from turning output switch  15  off completely. 
     In the case where a frequency of bypass power supply  1  is changed, voltage Vinv cannot be synchronized with voltage Vbyp immediately. In the case where voltage Vinv cannot be synchronized with voltage Vbyp, inverter  113  is operated with a fixed free-running frequency. 
       FIG. 6  represents waveforms for describing operation of the power supply system shown in  FIG. 5 . Referring to  FIGS. 5 and 6 , the phase of voltage Vinv and the phase of voltage Vbyp are shifted each other. In the case where UPS  5  is stopped in this state, not only switching the power supply from input power supply  2  to bypass power supply  1  but also suppressing the magnetizing inrush current with respect to isolation transformer  114  is required. Therefore, according to the configuration shown in  FIG. 5 , thyrister switch  6  is turned on after output switch  15  is turned off. 
     Inverter  13  is stopped at time tn. As described above, a certain time is required to completely turn output switch  15  off after inverter  13  is stopped. Output switch  15  is completely turned off at time t12. 
     Thyrister switch  6  turns on after a lapse of a fixed delay time from turning output switch  15  off completely (time t13). From time t11 to time t13, the supply of power to load  3  is halted (Vo=0). Therefore, there is a possibility that operation of load  3  is stopped. 
     The phase of voltage Vo at time t11 is equal to phase “a” of voltage Vinv. On the other hand, the phase of voltage Vo at time t13 is equal to phase “b” of voltage Vbyp. Phase “b” is different from phase “a.” Since the phase is changed discontinuously, a large magnetizing inrush current disadvantageously flows into load transformer  4 . 
     On the other hand, as shown in  FIGS. 3 and 4 , according to the embodiment of the present invention, the period during which the supply of power to the load is stopped is a period within one cycle of an alternate current. Therefore, a possibility that the load is stopped becomes small. 
     Further, according to the present embodiment, the supply of power to load  3  can be restarted while maintaining the state of phase exhibited at the stopped timing of the supply of power to load  3 . Accordingly, the magnetizing inrush current to load  3  and load transformer  4  can be suppressed. 
     The present invention can be applied to the configuration that the bypass power supply provides power to the load when the UPS is stopped. Therefore, the cause of the stoppage of the UPS is not limited to failure of the UPS, and it may include maintenance of the UPS. 
     It is to be understood that the embodiments disclosed herein are only way of example, and not to be taken by way of limitation. The scope of the present invention is not limited by the description above, but rather by the terms of the appended claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       1  bypass power supply;  2  input power supply;  3  load;  4  load transformer;  5  uninterruptable power-supply apparatus;  6  thyrister switch;  7 ,  7 A switching circuit;  11  converter;  12  battery;  13  inverter;  14  isolation transformer;  15  output switch;  16  capacitor;  17  synchronizing circuit;  21 ,  23  phase detector;  22  storage;  24  switch controller;  100 ,  101  power supply system.