Patent Publication Number: US-2022224220-A1

Title: Power supply apparatus and control method of power supply apparatus

Description:
FIELD OF THE INVENTION 
     The invention relates to a power supply apparatus and a control method of a power supply apparatus. 
     DESCRIPTION OF RELATED ART 
     A power supply apparatus including an energy storage device, such as a secondary battery, to back up or to smooth power supply to a power system, etc., by using a power generation system is known. Such power supply apparatus includes a converter converting the DC power output by the energy storage device into AC power at a desired voltage and frequency. When the power supply from the power generation system drops or stops, such power supply apparatus operates so as to supply necessary AC power to the power system or specific power usage equipment. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent “Patent No. 6058147”. 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     For an isolated power system in a remote island or in the mountains, the application of a power supply apparatus including an energy storage device with a large capacity and the stabilization of power supply are desired. In such power system, such power supply apparatus is used to smooth the output of a power generation system using natural energy and as the backup of a power generation system requiring fuel, such as a diesel power generator. In such application, specifically when the power supply apparatus including an energy storage device supplies power to the power system, even when a short circuit failure, etc., occurs in the power system, it is considered that the operation of the power supply apparatus does not stop and supplying power as much as possible is necessary. 
     An aspect of the invention is made in view of the above issue, and the objective thereof is to realize a power supply apparatus including an energy storage device and capable of keeping supplying power as much as possible even in a case where a short circuit failure, etc., occurs in the power system. 
     Means for Solving Problems 
     To solve the above issue, a power supply apparatus according to an aspect of the invention includes: an energy storage device; and a converter for converting a DC output of the energy storage device into an AC output; a current measurer, measuring a current of the AC output; a voltage measurer, measuring a voltage of the AC output; and a control unit, controlling the converter. The power supply apparatus includes a configuration in which the control unit performs control on the converter, such that, at a time when a value of the current exceeds a first limit value, the value of the current becomes a predetermined value greater than the first limit value by lowering the voltage to be less than a normal value. 
     To solve the above issue, a power supply apparatus according to another aspect of the invention includes: an energy storage device; and a converter for converting a DC output of the energy storage device into an AC output; a current measurer, measuring a current of the AC output; a voltage measurer, measuring a voltage of the AC output; and a control unit, controlling the converter. The power supply apparatus includes a configuration in which: the control unit is provided with a current upper limit setting unit, a target voltage setting unit, and an output instruction unit. The current upper limit setting unit operates, so as to: at a time when a value of the current exceeds a first limit value, change a current upper limit value from the first limit value to a second limit value greater than the first limit value, and at a time when the value of the current drops below the first limit value, change the current upper limit value from the second limit value to the first limit value. The output instruction unit controls the converter, so that the voltage becomes a target voltage calculated by the target voltage setting unit. The target voltage setting unit, in a case where the value of the current exceeds the first limit value, lowers the target voltage to be less than a normal value, so that the value of the current becomes the current upper limit value. 
     To solve the above issue, a control method of a power supply apparatus according to an aspect of the invention is a control method of a power supply apparatus including an energy storage device and a converter for converting a DC output of the energy storage device into an AC output. The control method includes a configuration for performing control on the converter, such that, at a time when a value of a current of the AC output exceeds a first limit value, the value of the current becomes a predetermined value greater than the first limit value by lowering a voltage of the AC output to be less than a normal value. 
     Inventive Effects 
     According to the power supply apparatus according to an aspect of the invention, a power supply apparatus including an energy storage device and capable of keeping supplying power as much as possible even in a case where a short circuit failure, etc., occurs in the power system can be provided. 
     According to the control method of the power supply apparatus according to an aspect of the invention, a power supply apparatus including an energy storage device and capable of keeping supplying power as much as possible even in a case where a short circuit failure, etc., occurs in the power system can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a configuration of a power supply apparatus and a power system to which the power supply apparatus is applied according to an embodiment of the invention. 
         FIG. 2  is a block diagram schematically illustrating a configuration of a control unit of the power supply apparatus according to an embodiment of the invention. 
         FIG. 3  is a diagram illustrating a control logic of a control unit of a power supply apparatus according to Embodiment 1 of the invention. 
         FIG. 4  is a time chart illustrating signal waveforms of respective units of the power supply apparatus according to Embodiment 1 of the invention. 
         FIG. 5  is a time chart illustrating signal waveforms of respective units of the power supply apparatus according to Embodiment 2 of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 
     In the following, an embodiment of the invention will be described in detail. 
     &lt;Configuration of Power System to which Power Supply Apparatus is Applied&gt; 
       FIG. 1  is a diagram illustrating a power supply apparatus  1  according to an embodiment. In  FIG. 1 , the entirety of a power system  100  to which the power supply apparatus  1  is applied is shown. The power supply apparatus  1  is an apparatus including an energy storage device  10  and capable of storing power. The alternated current power (AC output) output by the power supply apparatus  1  is supplied to multiple feeders  90 . 
     Each of the feeders  90  is formed by a breaker  91  and a load  92 . In each of the feeders  90 , when a short circuit failure occurs in the feeder  90 , the breaker  91  detects the continuation of an overcurrent for a predetermined time and trips to disconnect the feeder  90  from the power system  100 . 
     While not shown in  FIG. 1 , in the power system  100 , a power generation system using natural energy, such as a solar power generation system or a wind power generation system, may also be arranged in parallel with the power supply apparatus  1 . Alternatively, a power generation system using fuel, such as a diesel power generator or a cogeneration system, may also be arranged in parallel with the power supply apparatus  1 . The power supply apparatus  1  may at least be applied as the backup of the output of an arbitrary one of the power generation systems. In this sense, the power supply apparatus  1  is also an uninterruptible power supply (UPS). 
     An isolated power system in a remote island or mountains can serve as a specific example of the power system  100 . If a power generation system using natural energy is used in such power system, by applying the power supply apparatus  1  including the energy storage device  10 , the power supply using natural energy is smoothed. Alternatively, if a power generation system using fuel is used in such power system  100 , the power supply apparatus  1  may be adopted as a backup power supply in case of a failure of the power generation system. 
     However, the specific example of the power system  100  is not limited to an isolated power system in a remote island or mountains, and may also be a power system in a factory using a power generation system using natural energy or other power generation systems. Even in a power system in a factory, the effects and functions of the example disclosed in the invention are exhibited in a similar manner. 
     At the time of performing power supply to the power system  100 , the power supply apparatus  1  according to the embodiment does not stop operation as much as possible even when a short circuit failure occurs in the feeders  90 , and still operates to keep supplying power to the power system  100 . In the following descriptions, for the ease of understanding, it is described that only the power supply apparatus  1  supplies power to the power system  100 . However, if the power supply apparatus  1  supplies power to the power system  100 , the operation of the power supply apparatus  1  remains the same even if the power supply apparatus  1  is arranged in parallel with another power generation system. 
     &lt;Configuration of Power Supply Apparatus&gt; 
     As shown in  FIG. 1 , the power supply apparatus  1  includes the energy storage device  10 , a DC-AC converter  20  (converter), a filter  30 , a current measurer  40 , a voltage measurer  50 , and a control unit  60 . 
     The energy storage device  10  is a device storing input power therein as energy, and outputting the stored energy as direct current power (DC output) in accordance with needs. The energy storage device  10  may be a device including a secondary battery, such as a lithium-ion battery, a sodium-sulfur (NaS) battery, a redox flow battery, a lead-acid battery. 
     However, the energy storage device  10  is not limited to a device including a secondary battery. As the energy storage device  10 , any arbitrary unit having a function of storing electric energy, such as a capacitor, a superconducting power storage unit, a flywheel power storage unit, a compressed air type power storage unit, can be used. Also, the energy storage device  10  being a device outputting DC power is a concept which also includes the case where the power temporarily output internally as AC power is converted into DC by a rectifying circuit or a converter, etc., and output. 
     The DC-AC converter  20  is a device converting DC power (DC output) output by the energy storage device  10  into AC power (AC output). The DC-AC converter  20  converts DC power into AC power with a desired voltage, frequency used by the power system  100  via the filter  30  by using pulse width modulation (PWM) in accordance with an output command from the control unit  60 . The filter  30  is a filter for removing harmnoics included in the output of the DC-AC converter  20 . The current measurer  40  and the voltage measurer  50  respectively measure the current and the voltage of the AC power (AC output) output by the power supply apparatus  1  and transmit such information to the control unit  60 . 
     &lt;Configuration of Control Unit&gt; 
       FIG. 2  is a block diagram schematically illustrating a configuration of the control unit  60  of the power supply apparatus  1  according to the embodiment. As shown in  FIG. 2 , the control unit  60  is provided with respective functional blocks, which are a current upper limit setting unit  61 , a target voltage setting unit  62 , and an output instruction unit  63 . 
     Firstly, referring to  FIG. 2 , the operation executed by the power supply apparatus  1  under the control of the control unit  60  is briefly described. The output instruction unit  63  which is a region shown as encircled by a broken line in  FIG. 2  is a functional block which a control unit controlling the output of the DC-AC converter  20  usually includes. That is, the output instruction unit  63  usually refers to the voltage measurement value detected by the voltage measurer  50 , and performs feedback control of the DC-AC converter  20  to output a target voltage which is normally a rated voltage with reference to the voltage measurement value detected by the voltage measurer  50 . 
     However, in the control unit  60  of the embodiment, while the target voltage input to the output instruction unit  63  is usually a voltage command (normal value) that is normally the rated voltage, the target voltage is changed as follows in a case such as when a short circuit failure occurs in the feeder  90 . 
     The current upper limit setting unit  61  operates as follows based on the current measurement value detected by the current measurer  40 . At a usual time when the current measurement value is smaller than a first limit value, the current upper limit setting unit  61  calculates the first limit value as a current upper limit value. Here, the first limit value is set at a current value at which the DC-AC converter  20  is not damaged even if such current keeps being output. 
     When a short circuit failure occurs in any of the feeders  90 , since an failure current flows, the current output by the DC-AC converter  20  increases drastically. Thus, the current measurement value exceeds the first limit value. In the case where the current measurement value exceeds the first limit value, the current upper limit setting unit  61  increases the calculated current upper limit value to a second limit value. 
     The second limit value is a value greater than the first limit value, and is selected from values equal to or less than a short-time overload level of the DC-AC converter  20 . The second limit value is set according to a current value so that the breaker  91  trips with an overcurrent continuing for a predetermined time in the breaker  91  of the feeder  90  in which a short circuit failure occurs. As a specific value of the second limit value, a value that is 1.5 times to 2 times the first limit value is appropriate. 
     It is preferable that the current upper limit setting unit  61  changes the current upper limit value from the first limit value to the second limit value so that the value gradually changes. This is because the control of the output voltage of the power supply apparatus  1  changing with the current upper limit value can be stably executed. 
     In the case where the current measurement value exceeds the first limit value, the target voltage setting unit  62  calculates a voltage value dropping from the voltage command (normally the rated voltage) as the target voltage. By doing so, a feedback is made to the output command calculated by the output instruction unit  63 , so that the current measurement value becomes the current upper limit value (a predetermined value also greater than the first limit value). 
     As a result, when the short circuit failure continues, the output voltage (voltage measurement value) of the power supply apparatus  1  becomes a voltage value dropping from the voltage command, and a balance is reached when the output current (current measurement value) becomes the second limit value which is the current upper limit value. By doing so, these values are controlled to be substantially constant, and the AC power (AC output) is output from the power supply apparatus  1 . 
     Thus, the breaker  91  of the feeder  90  in which the short circuit failure occurs trips, and the filter  90  is disconnected. This is because the second limit value is set to a value that tripping occurs with an overcurrent (failure current) continuing for a predetermined time in the breaker  91  of the feeder  90  in which the short circuit failure occurs. 
     When the place where the short circuit failure occurs is disconnected from the power system  100 , the failure current no longer occurs, the current output by the DC-AC converter  200  drastically decreases and returns to a value smaller than the first limit value. When the current measurement value drops below the first limit value, the current upper limit setting unit  61  calculates the first limit value as the voltage upper limit value. That is, the setting of the voltage upper limit value returns to normal. 
     Since the current measurement value does not exceed the current upper limit value, the target voltage setting unit  62  calculates the voltage command (normally the rated voltage) as the target voltage. The output instruction unit  63  feedback-controls the DC-AC converter  20 , so as to output a voltage which is the voltage command. 
     At the time of restoring the output voltage of the power supply apparatus  1  to the normal value, it is preferable to exert control so that the output voltage changes gradually. This is because that, when the output voltage increases drastically, there is a concern that a transient overcurrent (excitation inrush current) occurs in a transformer in the power system  100 , and the DC-AC converter  20 , etc., is damaged. 
     Embodiment 1 
     In the following, Embodiment 1 as a specific example using the power supply apparatus  1  and the operation thereof will be described.  FIG. 3  is a diagram illustrating a control logic of a control unit  60 A of the power supply apparatus  1  in Embodiment 1.  FIG. 4  is a time chart illustrating the voltage measurement value (output voltage of the power supply apparatus  1 ), the current measurement value (output current of the power supply apparatus  1 ), and the control signal at each point of the control part  60 A in Embodiment 1. 
     The output command in Embodiment 1 is an instantaneous value of the output voltage instructed to the DC-AC converter  20 . An instantaneous value Vsin (ωt) is a product of a phase component sin (ωt) and an amplitude component V. A phase command representing the phase component sin (ωt) is calculated by a functional block of phase calculation  607  based on a frequency command (equivalent to ω). 
     The amplitude component V is calculated as an amplitude command S 1 . Since the amplitude is non-negative, in the calculation of the amplitude command S 1 , a limiter  606  limiting a signal less than 0 is provided. The output command is generated by multiplying the amplitude command S 1  by the phase command by using a multiplier  608 . 
     The portion shown as being encircled by a broken line in  FIG. 3  is equivalent to the output instruction unit  63  in  FIG. 2 . Usually, the portion excluding the portion shown as being encircled by the broken line has no influence on the output command from the control unit  60 . Usually, a difference (deviation S 3 ) between the voltage command (normally the rated voltage) and the voltage measurement value is input to a voltage controller  605 . 
     The voltage controller  605  is a functional block performing normal PI control. The voltage controller  605  calculates a voltage controller output S 4  based on the deviation S 3  and the integration of the deviation S 3  from the past. If the deviation S 3  and the integration of the deviation S 3  are 0, the voltage controller  605  continues to calculate a constant value as the voltage controller output S 4 . If the deviation S 3  and the integration of the deviation S 3  are other than 0, the voltage controller  605  calculates the voltage controller output S 4  so that the deviation S 3  is toward 0. By doing so, the control unit  60 A, at usual times (before a time T 1  of  FIG. 4 ), feedback-controls the DC-AC converter  20 , so that the voltage measurement value becomes the voltage command (normally the rated voltage). 
     In order to feed the current measurement value from the current measurer  40   a  back to the output command in the case where a short circuit failure, for example, occurs in the feeder  90 , functional blocks equivalent to the current upper limit setting unit  61  and the target voltage setting unit  62  of  FIG. 2  are provided in the control unit  60 A of Embodiment 1. 
     Assuming a normal feedback control system (only the portion shown as being encircled by a broken line in  FIG. 3 ) without these functions, in the case where a short circuit failure, for example, occurs in the feeder  90 , the operation is as follows. 
     When the short circuit failure occurs in one of the feeders  90 , the output current (current measurement value) of the power supply device  1  increases drastically, and the output voltage (voltage measurement value) drops. Then, a state where there is a difference between the voltage command and the voltage measurement value (the deviation S 3  being positive) occurs, and the voltage controller  605  attempts to increase the amplitude command. 
     However, there is a limit on the current which the DC-AC converter  20  is able to output, and the output voltage (voltage measurement value) cannot return to the voltage command (normal value). The power supply apparatus  1  comes to a state in which the voltage as in the command of the voltage controller  605  cannot be output. When the short circuit failure continues, since the deviation S 3  stays positive, the integration of the deviation S 3  in the voltage controller  605  accumulates, and the voltage controller  605  further increase the amplitude command. 
     In such state, when the breaker  91  trips to disconnect the feeder in which the failure occurs, the supply of the failure current disappears, and it is possible for the DC-AC converter  20  to output a voltage in accordance with the amplitude command S 1  from the voltage controller  605 . At this time, the amplitude command S 1  exceeds the voltage command (normal value), and the output voltage drastically increases. 
     Then, a transient overcurrent (excitation inrush current) flows in the transformer in the power system  100 , and there is a possibility that the DC-AC converter  20 , etc., be damaged. At this time, even if the amplitude command is the voltage command (normal value), since the output voltage also increases drastically, there is also a possibility that the DC-AC converter  20 , etc., be damaged in such case. 
     Actually, in the control unit  60 A of Embodiment 1, in the case where a short circuit failure occurs in the feeder  90 , the operation is as follows. 
     A current upper limit L 1  of  FIG. 3  is the same as the one calculated by the current upper limit setting unit  61  of  FIG. 2 . At the normal time when the current upper limit value L 1  is smaller than the first limit value (before the time T 1  in  FIG. 4 ), the current upper limit value is the first limit value. When the current measurement value exceeds the first limit value (at the time T 1 ) from the normal state, the current upper limit value L 1  is increased to the second limit value. The current upper limit value L 1  increases gradually from the first limit value to the second limit value (time T 1  to time T 2 ). 
     When the state in which the current measurement value exceeds the first limit value is resolved (time T 3 ), the current upper limit value L 1  returns to the first limit value after (time T 4 ) a predetermined period (time T 3  to time T 4 ). The signal waveform of the current upper limit value L 1  is shown together with the graph of the current measurement value of  FIG. 4 . 
     The difference (deviation) between the current upper limit value L 1  and the current measurement value (a value proportional to the effective value of the current waveform calculated by a functional block of effective value calculation  601 , in the case where the current measurement value is the instantaneous value) is input to the current controller  602 . The current controller  602  is a functional block performing normal PI control. If the deviation and the integration of the deviation are 0, the current controller  602  continues to calculate 0 as the output. If the deviation and the integration of the deviation are other than 0, the voltage controller  605  calculates the output so that the deviation S 3  is toward 0. 
     In the case of being negative through a limiter  603 , the output of the current controller  602  is calculated directly as a current suppression command S 2 . In a case elsewhere, with the functioning of the limiter, the current suppression command S 2  is 0, and the current measurement value is not reflected in the voltage command. 
     The current suppression command S 2  is multiplied by a properly set proportional coefficient K by using a gain  604  and added to the voltage command. The current suppression command S 2  is further properly added to the voltage controller output S 4 . A sum (not negative) of the current suppression command S 2  and the voltage controller output S 4  becomes the amplitude command S 1 . 
     At the time T 1  of  FIG. 4 , a short circuit failure occurs in three phase lines in one of the feeders  90 . Here, a three-phase balanced failure is assumed. Then, the current measurement value exceeds the current upper limit value (first limit value) and increases drastically. The current controller  602  calculates a negative value to reduce the current. The current suppression command S 2  is multiplied by K and added to the voltage command, and the voltage controller input S 3  (deviation) becomes a greater negative value. As a result, the voltage controller output S 4  drops, and the amplitude command S 1  drops below the voltage command (normal value). 
     As a result of the feedback control, the current measurement value becomes the current upper limit value L 1 , the amplitude command S 1  is balanced and becomes constant when becoming a voltage value smaller than the voltage command, and the voltage controller input S 3  (deviation) becomes substantially 0 (after the time T 1  to the time T 3 ). Accordingly, even when the short circuit failure continues, the deviation does not accumulate in the voltage controller  605 . Even when the short circuit failure continues, as described above, the output voltage of the power supply apparatus  1  is controlled to a value (a value lower than the normal value) instructed by the control unit  60 A. 
     At this time, the deviation, which is the input of the current controller  602 , also becomes 0. Specifically, at the time when the current upper limit value L 1  becomes the second limit value from the time T 2  to the time T 3 , the DC-AC converter  20  is feedback-controlled so that the current measurement value becomes the second limit value. The power supply apparatus  1  keeps outputting the current of the second limit value, and as a result, the breaker  91  of the feeder in which the failure occurs trips at the time T 3 , and the feeder in which the failure occurs is disconnected. The abnormal current is removed, and the output current (current measurement value) of the power supply device  1  returns to the normal value below the first limit value. Then, the current suppression command S 2  becomes 0, and is not added to the voltage command. 
     As a result of PI control, the voltage controller  605  gradually restores the voltage controller output S 4  to the voltage command (normal value). By doing so, the voltage measurement value gradually returns (time T 3  to time T 5 ) to the voltage command (normal value) from the small value at the time when the short circuit failure continues. 
     Embodiment 2 
     Embodiment 2 shows that the short circuit failure which occurs in one of the feeders  90  is the result of a short circuit failure of two phase lines in a three-phase unbalanced failure. The apparatus configuration of the power supply apparatus  1  is the same as that of Embodiment 1.  FIG. 5  illustrates the waveforms of the voltage measurement value and the current measurement value in the case of Embodiment 2. The respective times T 1  to T 5  shown in  FIG. 5  are the same as the case of Embodiment 1. 
     As shown in the drawings, even if the failure is a short circuit failure of two phase lines, the result is the same as the case of Embodiment 1. Immediately after the short circuit failure occurs at the time T 1 , the output voltage of the power supply apparatus  1  is narrowed down. From the time T 1  to the time T 2 , the output current (current measurement value) of the phase with the maximum current is controlled to gradually increase from the first limit value to the second limit value. 
     When the state in which the output current (current measurement value) of the power supply apparatus  1  is at the second limit value continues, the feeder in which the failure occurs is disconnected at the time T 3 . Then, from the time T 3  to the time T 5 , the output voltage (voltage measurement value) of the power supply apparatus  1  gradually returns to the voltage command (normal value). 
     &lt;Effects&gt; 
     The power supply apparatus  1  according to the embodiment includes the energy storage device  10  and the DC-AC converter  20  converting the DC output of the energy storage device  10  into an AC output. Therefore, the power supply apparatus  1  functions as a power supply for smoothing or as a backup of another power generation system supplying power to the power system  100 . Accordingly, according to the power supply apparatus  1 , it is possible to supply power during the night in the case where the another power generation system is a solar power generation system, for example, or supply power when it is windless in the case where the another power generation system is a wind power generation system. In addition, the power supply apparatus  1  may function as a backup in case that the another power generation system fails. 
     In the power supply apparatus  1  according to the embodiment, when the power supply apparatus  1  supplies power to the power system  100 , stoppage still suppressed at the time when a short circuit failure occurs in one of the feeders  90 , the failure current flows out, and the voltage of the power system  100  drops. 
     In a power generation system using a synchronous generator, there is sufficient capacity for supplying an emergency current greater than a rated current. Therefore, even in the case where a short circuit failure occurs, it is common to not disconnect the synchronous generator from the system to stop operation. However, in the power generation system using a DC-AC converter, the capacity for supplying an emergency current to the DC-AC converter is not as sufficient, and damages may be caused if the overcurrent continues. Therefore, it is common to disconnect from the system to stop the operation in the case where a short circuit failure occurs. 
     However, in the power supply apparatus  1  according to the embodiment, even in the case where a short circuit failure occurs, the power supply apparatus  1  controls the output voltage in a narrowed state to continue the power supply to the system, so as to supply a current with a current value (second limit value) temporarily greater than the rated current to the system. Then, the power supply apparatus  1  functions to be able to cause the breaker  91  of the feeder in which the failure occurs to trip to disconnect the feeder in which the failure occurs from the system. Therefore, while the power supply apparatus  1  is a power supply using the DC-AC converter  20 , the power supply to the power system  100  can continue even at the time of abnormality such as the time when a short circuit failure occurs. 
     At the time of the return due to the disconnection of the feeder where the failure occurs, the power supply apparatus  1  exerts control, so that the output voltage (voltage measurement value) is restored gradually. Therefore, the situation where a transient overcurrent (polarized inrush current) occurs due to the transformer in the power system  100  in the case where the output voltage (voltage measurement value) increases drastically to damage the DC-AC converter  20  of the power supply apparatus  1  is effectively suppressed. 
     Accordingly, if the power supply apparatus  1  according to the embodiment is used in the power system  100 , the power supply can continue as much as possible regardless of the occurrence of the short circuit failure. 
     In the embodiment, the return of the current upper limit value by using the current upper limit setting unit  61  from the second limit value to the first limit value is performed by detecting that the current measurement value returns to the first limit value or less. However, the return to the first limit value may also be performed through the current upper limit setting unit  61  receiving a signal indicating the disconnection of the feeder from the breaker  91  which trips. 
     In the embodiment, it is described that the control of the respective phases is not particularly distinguished based on phases, but is executed through general control. At this time, the control unit may exert control to adopt, as the voltage measurement value and the current measurement value, the minimum value among the phases for the voltage and the maximum value among the phases for the current. Alternatively, a three-phase instantaneous effective value (the root mean square of the instantaneous voltage values of the respective phases) may also be adopted as the current measurement value. However, regarding the control of each phase, the control unit may also perform control individually for each phase. In this case, the current upper limit value may be generally determined and may also be determined for each phase. 
     In the embodiment, the voltage command and the frequency command are considered as constant values, particularly considered as rated values, and not considered as values that change. This is equivalent to the case where the power system is used under the so-called constant voltage constant frequency. However, in the case where the power supply apparatus  1  cooperates with a power generation system such as a diesel power generator, in which voltage or frequency changes in accordance with the load state, the voltage command and the frequency command may also be adjusted in correspondence with such change. 
     [Example of Realizing by Software] 
     The respective functional blocks (particularly the control units  60 ,  60 A) of the power supply apparatus  1  may be realized by logic circuits (hardware) formed by integrated circuits (IC chips), and may also be realized by software. 
     In the case of the latter, the power supply apparatus  1  includes a computer executing the command of a program which is the software for realizing the respective functions. The computer, for example, includes at least one processor (control apparatus), and includes at least one recording medium storing the program and readable by the computer. In addition, in the computer, the processor reads the program from the recording medium and executes the program, thereby achieving the objective of the invention. As the processor, for example, a central processing unit (CPU) can be used. 
     As the recording medium, in addition to a non-transient tangible medium, such as a read only memory (ROM) and others, a tape, a disc, a card, a semiconductor memory, a programmable logic circuit, etc., can also be used. In addition, the computer may further include a random access memory (RAM) for expanding the program. In addition, the program may also be supplied to the computer via an arbitrary transmission medium (communication network, broadcast wave, etc.) capable of transmitting the program. In addition, an aspect of the invention may also be realized in the form of data signals embedded in carrier waves, in which the program is embodied, through electronic transmission. 
     SUMMARY 
     A power supply apparatus according to Aspect  1  of the invention includes: an energy storage device; and a converter for converting a DC output of the energy storage device into an AC output; a current measurer, measuring a current of the AC output; a voltage measurer, measuring a voltage of the AC output; and a control unit, controlling the converter. The power supply apparatus includes a configuration in which the control unit performs control on the converter, such that, at a time when a value of the current exceeds a first limit value, the value of the current becomes a predetermined value greater than the first limit value by lowering the voltage to be less than a normal value. 
     According to the above configuration, a power supply apparatus including an energy storage device and capable of keeping supplying power as much as possible even in a case where a short circuit failure, etc., occurs in the power system can be realized. 
     The power supply apparatus according to Aspect  2  of the invention may be configured as follows: according to Aspect  1  above, the control unit performs the control, at the time when the value of the current exceeds the first limit value, so that the value of the current gradually transitions from the first limit value to the second limit value. 
     According to the above configuration, in the case where a short circuit failure, etc., occurs, the state of current control which controls and outputs the current value at a desired value can be maintained, and, at the time of a short circuit failure, etc., a state in which the AC output is controlled by the power supply apparatus can be maintained. 
     The power supply apparatus according to Aspect  3  of the invention may be configured as follows: according to Aspect  1  or  2  above, during the control of lowering the voltage to be less than the normal value, in a case where the value of the current drops below the first limit value, the control unit controls the converter so as to restore the voltage to the normal value. 
     According to the above configuration, a configuration which restores the state of control of the power supply apparatus to a normal state in a case where a feeder in which an failure occurs is disconnected can be specifically realized. 
     The power supply apparatus according to Aspect  4  of the invention may be configured as follows: according to Aspect  3  above, the control unit restores the voltage to the normal value so that the voltage gradually transitions to the normal value. 
     According to the above configuration, the occurrence of a transient overcurrent (polarized inrush current) which damages a DC-AC converter of the power supply apparatus and which is caused by a transformer in a power system as the voltage increases drastically can be suppressed. 
     A power supply apparatus according to Aspect  5  of the invention includes: an energy storage device; and a converter for converting a DC output of the energy storage device into an AC output; a current measurer, measuring a current of the AC output; a voltage measurer, measuring a voltage of the AC output; and a control unit, controlling the converter. The power supply apparatus includes a configuration in which: the control unit is provided with a current upper limit setting unit, a target voltage setting unit, and an output instruction unit. The current upper limit setting unit operates, so as to: at a time when a value of the current exceeds a first limit value, change a current upper limit value from the first limit value to a second limit value greater than the first limit value, and at a time when the value of the current drops below the first limit value, change the current upper limit value from the second limit value to the first limit value. The output instruction unit controls the converter, so that the voltage becomes a target voltage calculated by the target voltage setting unit. The target voltage setting unit, in a case where the value of the current exceeds the first limit value, lowers the target voltage to be less than a normal value, so that the value of the current becomes the current upper limit value. 
     According to the above configuration, a power supply apparatus including an energy storage device and capable of keeping supplying power as much as possible even in a case where a short circuit failure, etc., occurs in the power system can be realized. 
     A control method of a power supply apparatus according to Aspect  6  of the invention is a control method of a power supply apparatus including an energy storage device and a converter for converting a DC output of the energy storage device into an AC output. The control method includes a configuration for performing control on the converter, such that, at a time when a value of a current of the AC output exceeds a first limit value, the value of the current becomes a predetermined value greater than the first limit value by lowering a voltage of the AC output to be less than a normal value. 
     According to the above configuration, a power supply apparatus including an energy storage device and capable of keeping supplying power as much as possible even in a case where a short circuit failure, etc., occurs in the power system can be realized. 
     The invention is not limited to the above embodiments, examples, and the like, and various modifications can be made within the scope of the claims. The technical scope of the invention also includes embodiments obtained by appropriately combining the technical means disclosed in the embodiments and the like. Furthermore, new technical features can be formed by combining the respective disclosed technical means. 
     REFERENCE SIGNS LIST 
     
         
           100 : Power system; 
           1 : Power supply apparatus; 
           10 : Energy storage device; 
           20 : DC-AC converter; 
           30 : Filter; 
           40 : Current measurer; 
           50 : Voltage measurer; 
           60 ,  60 A: Control unit; 
           61 : Current upper limit setting unit; 
           62 : Target voltage setting unit; 
           63 : Output instruction unit; 
           90 : Feeder; 
           91 : Breaker; 
           92 : Load