Abstract:
An uninterruptible power supply and method for controlling same are disclosed. The controlling method includes the steps of bypassing the first AC power to the output terminal via the bypass loop and the switch and converting a second AC power having a voltage, phase and frequency substantially equal to that of the first AC power by an inverter when the first AC power is normal; and switching the second AC power to the output terminal via the switch when the phase or frequency of the first AC power is changed so as to generate a difference value between the first AC power and the second AC power and the difference value is larger than a predetermined difference value.

Description:
FIELD OF THE INVENTION 
   The present invention is related to a power supply system and the control method thereof, and more particularly to an uninterruptible power supply and the control method thereof. 
   BACKGROUND OF THE INVENTION 
   With the rapid progress of information technology and the rapid development of the high-tech industry, most of the sophisticated electronic instruments and equipment rely on high-quality power supply to maintain a normal operation. Among a variety of power-supplying solutions, uninterruptible power supply can ensure a nonstop and a high-quality power supply. Therefore, uninterruptible power supply has become the best solution for providing a high-quality power supply. Because different uninterruptible power supplies have different power conversion efficiency, the use of uninterruptible power supply will pose a 10%-30% surcharge on the utilities every year. 
   Referring to  FIG. 1 , the circuitry of a conventional uninterruptible power supply is shown. As shown in  FIG. 1 , the uninterruptible power supply  1  includes an AC/DC converter  11 , a charger circuit  12 , a battery module  13 , a DC/DC converter  14 , an inverter  15 , a controller  16 , a static transfer switch  17  and a bypass route  18 . The function and the association of each circuit elements of the uninterruptible power supply  1  are described as follows. 
   When the input power source Vin is supplying power normally, the controller  15  will manipulate the AC/DC converter  11  to convert the input AC voltage Vin into a DC voltage having a predetermined voltage level and provide this DC voltage for the charger circuit  12  and the inverter  15 . In the meantime, the controller  16  will manipulate the inverter  15  to convert this DC voltage into a standard and reliable AC voltage. The output AC voltage V 1  of the inverter  15  is provided for the load  19  through the static transfer switch  17  (where the load voltage Vout is the output AC voltage V 1  of the inverter  15 ). In the meantime, the charger circuit  12  will convert the DC voltage outputted from the AC/DC converter  11  into a DC voltage tailored to charge the battery module  13 . 
   When the input power source Vin is unavailable for supplying power due to blackout or brownout and thereby causing the degradation of power, the controller  16  will manipulate the DC/DC converter  14  to convert the DC voltage outputted from the battery module  13  into a DC voltage required by the inverter  15 . Next, the inverter  15  will convert the DC voltage outputted from the DC/DC converter  14  into an AC voltage, which is provided for the load  19  through the static transfer switch  17 . In this case, the power used by the load  19  is supplied by the battery module  13  that is formed by a plurality of batteries. More specifically, the duration of the battery module  13  for sustaining supplying power is dependent on the number of batteries of the battery module  13 . 
   When the input power source Vin is supplying power normally, the AC/DC converter  11  of the uninterruptible power supply  1  will convert the input AC voltage Vin into a DC power, and then the inverter  15  will convert the DC power into a standard and reliable AC power. The output AC power V 1  of the inverter  15  is provided for the load  15  through the static transfer switch  17 . Because the AC/DC converter  11  and the inverter  15  will output energy when the input power source Vin in supplying power normally, the power conversion efficiency of the AC/DC converter  11  and the inverter  15  will lower the power utilization. 
   In order to improve the power utilization, another operating method for the uninterruptible power supply is proposed. This operating method is carried out in a manner that when the input power source Vin is supplying power normally, the controller  16  will manipulate the static transfer switch  17  to provide the input AC power Vin for the load  19 . That is, when the whole system is normal, the input AC power Vin is provided for the load  19  through the bypass route  18 . In the meantime, the inverter  15  will perform power conversion process to output AC power through the static transfer switch  17 , and the peak voltage of the output voltage V 1  of the inverter  15  is a predetermined rated peak voltage Vp 1 . In the meantime, the charger circuit  12  will convert the DC voltage outputted from the AC/DC converter  11  into a DC voltage tailored to charge the battery module  13 . 
   When the peak voltage or the frequency of the input voltage Vin is abnormal, for example, when the peak voltage is increased or decreased by 10% of the rated peak voltage or when the frequency is increased or decreased by 5% of the rated frequency, the controller  16  will manipulate the static transfer switch  17  to provide the output voltage V 1  of the inverter  15  for the load  19  through the static transfer switch  17  (where the load voltage Vout is the output AC voltage V 1  of the inverter  15 ), thereby enhancing power utilization. The uninterruptible power supply employing such control method is called economic-mode uninterruptible power supply, or ECO-mode uninterruptible power supply. Nevertheless, such control method and control configuration is applicable to non-inductive load and non-motorized load. In the case of inductive load and motorized load, when the peak voltage or frequency of the input voltage Vin is abnormal and the static transfer switch  17  switches the power delivery route to provide the output voltage V 1  of the inverter  15  for the load  19 , the phase difference between the input voltage Vin and the output voltage V 1  of the inverter  15  would be large. When the phase difference between the input voltage Vin and the output voltage V 1  of the inverter  15  is sufficiently large, for example, above 20 degrees, the uninterruptible power supply would perform asynchronous power conversion and cause inrush current. This would even burn down the uninterruptible power supply and inhibit the uninterruptible power supply from supplying power to the load  19 . 
   Referring to  FIG. 2 , the waveform diagram showing the voltage waveforms and current waveforms associated with the uninterruptible power supply. As indicated in  FIG. 2 , the peak voltage, frequency or phase of the input voltage Vin is abnormal at time t 1 . In the meantime, the controller  16  will manipulate the static transfer switch  17  to switch the power delivery route to provide the output voltage V 1  of the inverter  15  for the load  19 . However, the phase difference between the input voltage Vin and the output voltage V 1  of the inverter  15  is quite large, for example, above 20 degrees, so that a large voltage difference is generated between the load voltage Vout and the output voltage V 1  of the inverter  15 . Under this condition, the static transfer switch will switch the power delivery route so as to cause a large inrush current on the load current lout. This would even burn down the uninterruptible power supply and inhibit the uninterruptible power supply from supplying power to the load  19 . 
   Hence, it is urgent for those skilled in the art to develop a control method for use by an uninterruptible power supply to remove the above-mentioned drawbacks encountered by the prior art. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an uninterruptible power supply system and the control method thereof. When the input power of the uninterruptible power supply system is available, the uninterruptible power supply can supply the input power to a load through a bypass route, and the peak voltage, phase and frequency of the inverter of the uninterruptible power supply can be varied synchronously with the power source. On the other hand, when the input power is unstable, the output voltage of the inverter is provided for the load before the voltage difference between the input power and the output voltage of the inverter is enlarged. Therefore, there will be a small inrush current if the load is an inductive load or a motorized load, so that the uninterruptible power supply can be protected from being burned down. 
   To this end, a preferred embodiment of the present invention is to provide a control method for an uninterruptible power supply, wherein the uninterruptible power supply includes a power input terminal for receiving a first AC voltage, a battery module, an AC/DC converter for receiving the first AC voltage from the power input terminal and converting the first AC voltage into a DC voltage, a charger circuit, an inverter for converting the DC voltage into a second AC voltage, a bypass route, a switch, a controller, and a power output terminal. The control method includes the following steps of: (a) determining if a peak voltage of the first AC voltage, a frequency of the first AC voltage, a phase difference between the first AC voltage and the second AC voltage is abnormal; and (b) when it is determined that the peak voltage of the first AC voltage, the frequency of the first AC voltage, the phase difference between the first AC voltage and the second AC voltage is normal, supplying the first AC voltage to the power output terminal through the bypass route and the switch; and when it is determined that the peak voltage of the first AC voltage, the frequency of the first AC voltage, the phase difference between the first AC voltage and the second AC voltage is abnormal, switching the switch to supply the second AC voltage to the power output terminal. 
   To this end, another aspect of the present invention is related to a uninterruptible power supply, including a battery module for storing electric power; an AC/DC converter for receiving a first AC voltage from a power input terminal and converting the first AC voltage into a DC voltage; a charger circuit connected to the AC/DC converter and the battery module for charging the battery module; an inverter connected to the AC/DC converter and the charger circuit for converting the DC voltage into a second AC voltage; a bypass route connected to the power input terminal; a switch connected to the bypass route, the inverter and a power output terminal; and a controller connected to the power input terminal, the AC/DC converter, the charger circuit and the inverter for controlling the uninterruptible power supply. The uninterruptible power supply is configured to carry out the following control method: (a) determining if a peak voltage of the first AC voltage, a frequency of the first AC voltage, a phase difference between the first AC voltage and the second AC voltage is abnormal; and (b) when it is determined that the peak voltage of the first AC voltage, the frequency of the first AC voltage, the phase difference between the first AC voltage and the second AC voltage is normal, supplying the first AC voltage to the power output terminal through the bypass route and the switch; and when it is determined that the peak voltage of the first AC voltage, the frequency of the first AC voltage, the phase difference between the first AC voltage and the second AC voltage is abnormal, switching the switch to supply the second AC voltage to the power output terminal. 
   Now the foregoing and other features and advantages of the present invention will be best understood through the following descriptions with reference to the accompanying drawings, wherein: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a conventional uninterruptible power supply; 
       FIG. 2  is a timing diagram showing the waveforms associated with the voltage conversion process carried out by the conventional uninterruptible power supply; 
       FIG. 3  is a circuit diagram showing an uninterruptible power supply according to a preferred embodiment of the present invention; 
       FIG. 4  is a flowchart illustrating the control steps applied to the uninterruptible power supply according to a preferred embodiment of the present invention; and 
       FIG. 5  is a timing diagram showing the waveforms associated with the voltage conversion process carried out by the uninterruptible power supply of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment embodying the features and advantages of the present invention will be expounded in following paragraphs of descriptions. It is to be realized that the present invention is allowed to have various modification in different respects, all of which are without departing from the scope of the present invention, and the description herein and the drawings are to be taken as illustrative in nature, but not to be taken as limitative. 
     FIG. 3  shows the structure of an uninterruptible power supply according to a preferred embodiment of the present invention. As shown in  FIG. 3 , the inventive uninterruptible power supply  3  includes an AC/DC converter  31 , a charger circuit  32 , a battery module  33 , a DC/DC converter  34 , an inverter  35 , a controller  36 , a switch  37 , a power input terminal  31   a,  a DC bus  31   b,  a power output terminal  37   a,  and a bypass route  38 . The function and control method of the uninterruptible power supply  3  are described as follows. 
   In the present embodiment, the power input terminal  31   a  is configured to receive an input voltage Vin, which is set as a first AC voltage. The AC/DC converter  31  is connected between the power input terminal  31   a  and the DC bus  31   b  for converting the first AC voltage Vin into a DC voltage having a predetermined voltage level. The charger circuit  32  is connected between the DC bus  31   b  and the battery module  33  for converting the DC voltage outputted from the AC/DC converter  31  into a DC voltage tailored to charge the battery module  33 . The DC/DC converter  34  is connected between the battery module  33  and the DC bus  31   b  for converting the DC voltage of the battery module  33  into a DC voltage required by the inverter  35 . The inverter  35  is connected between the DC bus  31   b  and the switch  37  for converting the DC voltage of the DC bus  31   b  into a standard and reliable output AC voltage V 1 , which is set as a second AC voltage. The switch  37  is connected to the bypass route  38 , the inverter  35  and the power output terminal  37   a,  and can be implemented by a silicon controlled rectifier (SCR), a triode AC switch (TRIAC), an insulated gate bipolar transistor (IGBT), a MOSFET, or a relay. The bypass route  38  is connected between the switch  37  and the power input terminal  31   a,  and the controller  36  is connected to the power input terminal  31   a,  the AC/DC converter  31 , the charger circuit  32 , the DC/DC converter  34  and the inverter  35  for controlling the operation of the uninterruptible power supply  3 . 
   Referring to  FIG. 4 , the flowchart illustrating the control method for use by the uninterruptible power supply according to the present invention is shown. As shown in  FIG. 4 , the steps of the control method are described as follows: 
   Step S 10 : Start the control procedure for use by the uninterruptible power supply. 
   Step S 11 : Determine if the peak voltage of the input voltage Vin is abnormal. In the present embodiment, if the peak voltage of the input voltage Vin is increased or decreased by a predetermined percentage of the rated peak voltage, for example, ±10%, it is determined that the peak voltage of the input voltage Vin is abnormal. If it is determined that the peak voltage of the input voltage Vin is abnormal, the method continues with step S 16 ; 
   Step S 12 : Determine if the frequency of the input voltage Vin is abnormal. In the present embodiment, if the frequency of the input voltage Vin is increased or decreased by a predetermined percentage of the rated frequency, for example, ±5%, it is determined that the frequency of the input voltage Vin is abnormal. If it is determined that the frequency of the input voltage Vin is abnormal, the method continues with step S 16 ; 
   Step S 13 : Determine if the phase difference between the input voltage Vin and the output voltage VI of the inverter  35  is abnormal. In the present embodiment, if the phase different exceeds a predetermined phase difference, for example, 10 degrees, it is determined that the phase difference between the input voltage Vin and the output voltage V 1  of the inverter  35  is abnormal. If the phase difference between the input voltage Vin and the output voltage V 1  of the inverter  35  is abnormal, the method continues with step S 16 ; 
   Step S 14 : Provide the input voltage Vin for the load  39  through the switch  37 . That is, the input voltage Vin is provided for the load  39  through the bypass route  38 . Under this condition, the load voltage Vout is the same as the input voltage Vin. 
   Step S 15 : Adjust peak voltage and the phase of the output voltage V 1  of the inverter  35  to be identical with those of the input voltage Vin. That is, the peak voltage and the phase of the output voltage V 1  of the inverter  35  will vary synchronously with the peak voltage of the input voltage Vin. In some alternative embodiments, the frequency of the output voltage V 1  of the inverter  35  is the same with the frequency of the input voltage Vin, so that the frequency of the output voltage V 1  of the inverter  35  will vary synchronously with the frequency of the input voltage Vin. Next, the method continues with step S 11 . 
   Step S 16 : Provide the output voltage V 1  of the inverter  35  for the load  39  through the switch  37 . Under this condition, the load voltage Vout is the same as the output voltage V 1  of the inverter  35 . 
   Step S 17 : Adjust the peak voltage of the output voltage V 1  of the inverter  35  to be identical to the rated peak voltage Vp 1 . In some alternative embodiments, the frequency of the output voltage V 1  of the inverter  35  is adjusted to be identical to the rated frequency. Next, the method continues with step S 11 . 
   The above-mentioned control method will be executed repetitively, so that the uninterruptible power supply can supply electric power to the load  39  stably. Certainly, in other alternative embodiments the sequence of the steps S 11  to S 13  can be altered. 
     FIG. 5  shows the waveforms associated with the power conversion process carried out by the uninterruptible power supply according to the present invention. As shown in  FIG. 3  and  FIG. 5 , the input voltage Vin, that is, the first AC voltage, will be available before time t 2 , and the controller  36  will manipulate the switch  37  to supply the input voltage Vin to the load  39 . That is, the input voltage Vin will be provided for the load  39  through the bypass route  38 . Meanwhile, the AC/DC converter  31  will convert the input voltage Vin into a DC voltage, and the inverter  35  will convert this DC voltage into a stable output voltage V 1 , which is the second AC voltage. In some alternative embodiments, the frequency of the output voltage V 1  of the inverter  35  is the same with that of the input voltage Vin, so that the frequency of the output voltage V 1  of the inverter  35  will vary synchronously with the frequency of the input voltage Vin. In addition, the controller  36  will manipulate the inverter  35  so that the peak voltage and phase of the output voltage V 1  of the inverter  35  will be the same with those of the input voltage Vin. That is, the peak voltage and phase of the output voltage V 1  of the inverter  35  will vary synchronously with the peak voltage and phase of the input voltage Vin. In the meantime, if the capacity of the battery module  33  is insufficient, the charger circuit  32  can convert the DC voltage outputted from the AC/DC converter  31  into a DC voltage tailored to charge the battery module  33 , thereby charging the battery module  33 . 
   In the present embodiment, although the inverter  35  is still in operation, the load  39  is powered by the input voltage Vin. Under this condition, the output current It of the inverter  35  is zero, and the whole uninterruptible power supply  3  will consume energy only when the battery module  33  is charging, and thus the power efficiency of the uninterruptible power supply  3  is quite high. 
   Referring to  FIG. 3  and  FIG. 5  again, the peak voltage of the input voltage (that is, the first AC voltage) is maintained at the rated peak voltage Vp 1  before time t 2 . However, the peak voltage of the input voltage Vin will be lowered from the rated peak voltage Vp 1  to a second peak voltage Vp 2  at time t 2 . Also, the phase and frequency of the output voltage V 1  of the inverter  35  will be changed rapidly so that the output voltage V 1  (that is, the second AC voltage) of the inverter  35  can not be synchronized with the input voltage Vin. However, the rapid change of the phase and frequency of the input voltage Vin causes the phase difference between the input voltage Vin and the output voltage V 1  of the inverter  35 . Therefore, a first phase difference d 1  is generated between the input voltage Vin and the output voltage V 1  of the inverter  35  at time t 2 . With the continuing instability of the input voltage Vin, the phase difference will be enlarged with time. At time t 3 , a second phase difference d 2  between the input voltage Vin and the output voltage V 1  of the inverter  35  is generated and exceeds a predetermined phase difference, for example, 10 degrees, and thus the controller  36  will manipulate the switch  37  to switch the power delivery route to supply the output voltage V 1  of the inverter  35  to the load  39 . 
   The peak voltage of the output voltage VI of the inverter  35  is subject to change with the input voltage Vin. In some alternative embodiments, the frequency of the output voltage V 1  of the inverter  35  is subject to change with the frequency of the input voltage Vin. At time t 3 , the voltage difference between the input voltage Vin and the output voltage V 1  of the inverter  35  is caused by the second phase difference d 2 . The controller  36  can use the voltage difference between the input voltage Vin and the output voltage V 1  of the inverter  35  to detect if the input voltage Vin is stable. Before the phase difference between the input voltage Vin and the output voltage V 1  of the inverter  35  is enlarged, the switch  37  operates to switch the power delivery route so that the output voltage V 1  of the inverter  35  is supplied to the load  39 . In the meantime, the second phase difference d 2  will limit the voltage difference between the input voltage Vin and the output voltage V 1  of the inverter  35 . Although the load  39  is an inductive load or a motorized load, the load current lout and the output current I 1  of the inverter  35  will not cause a large inrush current when the switch  37  switches the power delivery route to supply the output voltage V 1  of the inverter  35  to the load  39  at time t 3 . Therefore, the uninterruptible power supply will be protected from being burned down and the load will be continuously powered. 
   After the power delivery route is switched, that is, after time t 3  is elapsed, the controller  36  will manipulate the inverter  35  to adjust the peak voltage of the output voltage V 1  to the rated peak voltage Vp 1 . In some alternative embodiments, the frequency of the output voltage V 1  of the inverter  35  will be adjusted to the rated frequency. In the meantime, the peak voltage and frequency of the load voltage Vout are identical to the rated peak voltage Vp 1  and rated frequency, respectively. Even if the input voltage Vin is unavailable, the DC/DC converter  34  can convert the voltage of the battery module  33  into the required voltage for the inverter  35 , so that the uninterruptible power supply  3  can supply electric power to the load  39  stably. In some alternative embodiments, the battery module  33  can be made up of a plurality of batteries, and its power supplying time is dependent on the number of batteries. Certainly, the DC/DC converter can be an optional element in some alternative embodiments. In such embodiments, the battery module will be directly connected to the DC bus (not shown), and a switch circuit is arranged to control if the battery module is supplying power to the DC bus. 
   Another situation that may occur during operation is that the peak voltage of the input voltage Vin decreases or increases to a level being unbearable for the load  39 . In the present embodiment, the peak voltage of the input voltage Vin is set to decrease or increase by a predetermined percentage of the rated peak voltage, for example, 10%. In this case, the controller  36  will manipulate the switch  37  to switch the power delivery route so that the output voltage V 1  of the inverter  35  is provided for the load  39 . After the power delivery route is switched, the controller  36  will manipulate the inverter  35  to adjust the peak voltage of the output voltage V 1  to be identical to the rated peak voltage Vp 1 . In some alternative embodiments, the frequency of the output voltage V 1  of the inverter  35  will be adjusted to be identical to the rated frequency. It should be noted that the peak voltage of the output voltage V 1  of the inverter  35  varies synchronously with the peak voltage of the input voltage Vin, and the frequency of the output voltage V 1  of the inverter  35  varies synchronously with the frequency of the input voltage Vin in some alternative embodiments, the voltage difference between the output voltage V 1  of the inverter  35  and the input voltage Vin is limited. Although the load  39  is an inductive load or a motorized load, the load current Iout and the output current I 1  of the inverter  35  will not cause a large inrush current when the switch  37  switches the power delivery route to supply the output voltage V 1  of the inverter  35  to the load  39 . Therefore, the uninterruptible power supply  3  will be protected from being burned down and the load  39  will be continuously powered. 
   Likewise, after a period of time, the peak voltage of the input voltage Vin returns to normal. The output voltage V 1  of the inverter  35  will be synchronous with the input voltage Vin, that is, the phase difference between the output voltage V 1  of the inverter  35  and the input voltage Vin will be zero, and their peak voltage and frequency are the same, as shown in the waveforms after time t 4  in  FIG. 5 . In the meantime, the controller  36  will manipulate the switch  37  to supply the input voltage Vin to the load  39  through the bypass route  38 . 
   In conclusion, the uninterruptible power supply and the control method thereof according to the present invention is able to allow the uninterruptible power supply to supply power to the load  39  through a bypass route when the input voltage Vin is normal, and the peak voltage and phase and frequency of the output voltage V 1  of the inverter  35  is varied synchronously with the peak voltage and phase and frequency of the input voltage Vin. When the input voltage Vin is unstable, the input voltage Vin is not synchronous with the output voltage V 1  of the inverter  35  and a phase difference is generated thereby. Before the phase difference is enlarged, the controller  36  manipulates the switch  37  to switch the power delivery route to supply the output voltage V 1  of the inverter  35  to the load  39 . Therefore, even if the load  39  is an inductive load or a motorized load, the uninterruptible power supply can be protected from being burned down due to the large inrush current. Also, the load can be continuously powered and the power efficiency can be improved. 
   While the present invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention need not be restricted to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the present invention which is defined by the appended claims.