Patent Publication Number: US-2023155379-A1

Title: Power system stabilizing system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-142998, filed Aug. 26, 2020 and PCT/JP2021/029336, filed Aug. 6, 2021; the entire contents all of which are incorporated herein by reference. 
    
    
     FIELD 
     An embodiment of the present invention relates to a power system stabilizing system. 
     BACKGROUND 
     Recently, introduction of regeneration energy has progressed. Since most regeneration energy is generated by variable power supplies of which an output varies depending on weather conditions such as solar power generation or wind power generation, there is concern about an influence on the stability of a power system. Accordingly, demand for a power system stabilizing system including a regeneration energy power supply (hereinafter referred to as a “regenerative power supply”) as a control target is expected to increase. 
     In general, a power system stabilizing system prevents occurrence of an abnormal phenomenon such as a step-out phenomenon (a transient stability abnormality), a frequency abnormality, a voltage abnormality, or an overload of a power system or prevents an increasing extension of such an abnormal phenomenon to the whole system. 
       FIG.  17    is a diagram showing a configuration of a power system stabilizing system SSC including a regenerative power supply as a control target according to the related art. The power system stabilizing system SSC includes, for example, an accident detector  1 , an arithmetic processor  2 , and a controller  3 . 
     The accident detector  1  detects occurrence of a system accident in a power system. 
     When a system accident is detected by the accident detector  1 , the arithmetic processor  2  determines a regenerative power supply which is to be disconnected from the power system on the basis of a type of the system accident. “Disconnection” is disconnection from a power system and may be referred to below as “cutoff,” “power restriction,” or “dropping.” Control for forcibly cutting off some power supplies from a power system to prevent extension of an influence of an accident to the whole power system is referred to as power supply restriction or power restriction. In the following description, a control target such as a regenerative power supply which is selected as a target of power supply restriction is referred to as a power restriction target. 
     The controller  3  cuts off a power restriction target on the basis of a power restriction command from the arithmetic processor  2 . In the power system stabilizing system SSC for a system including regenerative power supplies, when the number of regenerative power supplies which are control targets is too large, the controller  3  is generally divisionally configured as a distribution device  3   a  and a reception device  3   b . For example, a configuration in which one distribution device  3   a  and multiple reception devices  3   b  are provided is employed, and a transmission hardware volume of the arithmetic processor  2  is curbed by causing the distribution device  3   a  to receive a control command output from the arithmetic processor  2  and causing the distribution device  3   a  to transmit the control command to the reception devices  3   b.    
     A regenerative power supply which has been cut off by the power system stabilizing system SSC at the time of occurrence of a system accident may be reconnected when reclosing of a place in which the system accident has occurred succeeds and the system is restored to a system state before the system accident occurred. 
     However, the power system stabilizing system SSC performs control for cutting off a power restriction target at the time of occurrence of a system accident, but does not perform a check of restoration from the system accident and reconnection of a regenerative power supply after the power system has been restored from the system accident. Accordingly, a check of restoration from a system accident and reconnection of a regenerative power supply are generally carried out by a human system such as an operator. For example, an operator goes to the site and carries out an operation of checking that the power system has been restored from the system accident and then reconnecting a regenerative power supply which has been cut off. Accordingly, time is required for from a check of restoration from an accident to reconnection, and thus a time in which a regenerative power supply is disconnected from the power system and an output thereof is stopped (hereinafter referred to as a “stop time”) may be elongated. This problem with the stop time is not limited to a case in which a regenerative power supply cut off by the power system stabilizing system SSC is reconnected, but is common even when a regenerative power supply is disconnected due to an influence of a system accident, an isolated operation preventing function of a power conditioner, or the like. As a result, there is concern about deterioration in system stability, and there is a likelihood of warranty costs of a power company for power generation business operators increasing or a likelihood of restriction of emergency control. 
     An objective of the present invention is to provide a power system stabilizing system that can shorten a stop time of a power supply. 
     A power system stabilizing system according to an embodiment includes an accident detector, a power restriction target selector, a cutoff controller, and a reconnection controller. The accident detector is configured to detect a system accident of a power system. The power restriction target selector is configured to select power restriction targets which are required for stability maintenance of the power system out of a plurality of power supplies included in the power system or connected to the power system according to a type of the system accident detected by the accident detector. The cutoff controller is configured to cut off the power restriction targets selected by the power restriction target selector. A system restoration checker is configured to check that the power system has been restored from the system accident on the basis of system information of the power system. The reconnection controller is configured to reconnect some or all of the power restriction targets cut off by the cutoff controller when the system restoration checker checks that the power system has been restored from the system accident. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram showing a configuration of a power system E to which a power system stabilizing system SSC 1  according to a first embodiment is applied. 
         FIG.  2    is a diagram showing a configuration of the power system stabilizing system SSC 1  according to the first embodiment. 
         FIG.  3    is a diagram showing an example of details included in a control table. 
         FIG.  4    is a flowchart showing a routine of processes which are performed by the power system stabilizing system SSC 1  according to the first embodiment. 
         FIG.  5    is a diagram showing a configuration of a power system stabilizing system SSC 2  according to a second embodiment. 
         FIG.  6    is a flowchart showing a routine of processes which are performed by the power system stabilizing system SSC 2  according to the second embodiment. 
         FIG.  7    is a diagram showing a configuration of a power system stabilizing system SSC 3  according to a third embodiment. 
         FIG.  8    is a flowchart showing a routine of processes which are performed by the power system stabilizing system SSC 3  according to the third embodiment. 
         FIG.  9    is a diagram showing a configuration of a power system stabilizing system SSC 4  according to a fourth embodiment. 
         FIG.  10    is a flowchart showing a routine of processes which are performed by the power system stabilizing system SSC 4  according to the fourth embodiment. 
         FIG.  11    is a diagram showing a configuration of a power system stabilizing system SSC 5  according to a fifth embodiment. 
         FIG.  12    is a flowchart showing a routine of processes which are performed by the power system stabilizing system SSC 5  according to the fifth embodiment. 
         FIG.  13    is a diagram showing a configuration of a power system stabilizing system SSC 6  according to a sixth embodiment. 
         FIG.  14    is a flowchart showing a routine of processes which are performed by the power system stabilizing system SSC 4  according to the sixth embodiment. 
         FIG.  15    is a diagram showing a configuration of a power system stabilizing system SSC 7  according to a seventh embodiment. 
         FIG.  16    is a flowchart showing a routine of processes which are performed by the power system stabilizing system SSC 7  according to the seventh embodiment. 
         FIG.  17    is a diagram showing a configuration of a power system stabilizing system according to the related art. 
     
    
    
     DETAILED DESCRIPTION 
     A power system stabilizing system according to an embodiment includes an accident detector, a power restriction target selector, a cutoff controller, and a reconnection controller. The accident detector is configured to detect a system accident of a power system. The power restriction target selector is configured to select power restriction targets which are required for stability maintenance of the power system out of a plurality of power supplies included in the power system or connected to the power system according to a type of the system accident detected by the accident detector. The cutoff controller is configured to cut off the power restriction targets selected by the power restriction target selector. A system restoration checker is configured to check that the power system has been restored from the system accident on the basis of system information of the power system. The reconnection controller is configured to reconnect some or all of the power restriction targets cut off by the cutoff controller when the system restoration checker checks that the power system has been restored from the system accident. According to the embodiment, it is possible to provide a power system stabilizing system that can shorten a stop time of a power supply. 
     Hereinafter, a power system stabilizing system according to an embodiment will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG.  1    is a diagram showing a configuration of a power system E to which a power system stabilizing system SSC 1  according to a first embodiment is applied. The power system E includes various types of equipment such as power supplies including regenerative power supplies, power lines, and transformers. The power system E may include a synchronous power generator in addition to regenerative power supplies as long as it includes regenerative power supplies. For example, the power system E includes a main system  10 , a power distributing substation  11 , and a plurality of external systems  12 - k  (k=1, 2, . . . , n, where n is an integer equal to or greater than 2). In the following description, signs added after hyphens, which indicate external systems to which elements belong, may be omitted. 
     The power distributing substation  11  supplies electric power from the main system  10  which is a system more significant than the power distributing substation  11  to the external systems  12  which is a system less significant than the power distributing substation  11 . 
     Each external system  12 - k  includes, for example, a power supply line  20 - k , a bus line  30 - k , one or more transformers  40 - k , and one or more regenerative power supplies  50 - k.    
     The power supply line  20 - k  is a connection line for connecting the power distributing substation  11  and the bus line  30 - k.    
     One or more regenerative power supplies  50 - k  are connected to the bus line  30 - k  via one or more transformers  40 - k . Loads such as electrical appliances of a consumer may be connected to the bus line  30 - k  via the transformer  40 - k . In the example shown in  FIG.  1   , a plurality of regenerative power supplies  50 - k  are electrically connected to the bus line  30 - k , but the invention is not limited thereto and the number of regenerative power supplies  50 - k  connected to the bus line  30 - k  may be one. 
     Each regenerative power supply  50 - k  is an asynchronous power generator using regeneration energy. The regenerative power supplies  50 - k  are, for example, distributed power supplies which are small-scale power generation equipment distributed in the vicinity of power consumers or the like. Each regenerative power supply  50 - k  includes, for example, a power generation device  51 - k  such as a solar cell and a power conditioner system (PCS)  52 - k.    
     The PCS  52 - k  includes an inverter that converts DC electric power output from the power generation device  51 - k  to AC electric power. The PCS  52 - k  has, for example, an isolation operation preventing function of detecting isolated operation on the basis of a change in frequency or voltage of the main system  10  side and disconnecting the regenerative power supply  50 - k  from the main system  10 . 
     A breaker  60 - k  is provided in the power supply line  20 - k . When the breaker  60 - k  is switched from an electrified state (an on state) to an unelectrified state (an off state), the regenerative power supplies  50 - k  are cut off (disconnected) from the main system  10 . When the breaker  60 - k  is switched from the unelectrified state to the electrified state, the regenerative power supplies  50 - k  are connected to the main system  10 . When a regenerative power supply  50 - k  is disconnected and then is connected again to the main system  10 , this is called reconnection. 
     The power system stabilizing system SSC 1  is, for example, an online pre-calculation type power system stabilizing system. The power system stabilizing system SSC 1  performs transient stability calculation for each preset postulated accident using system information acquired online from the power system E, determines control details for each postulated accident, and sets a control table on the basis of the result of determination. When a system accident actually occurs, the power system stabilizing system SSC 1  performs control by comparing an accident type of the system accident with the control table. The system information includes, for example, information on a connection state, a power demand-supply state, and a power flow state of the power system E. The connection state of the power system E includes, for example, connection information of breakers or disconnectors included in the main system  10  and the external systems  12 - k . The power demand-supply state includes, for example, information on outputs or loads of the regenerative power supplies  50 - k  of the main system  10  and the external system  12 - k , active power and reactive power of power lines or transformers included in the main system  10  and the external systems  12 - k , a voltage level or a current level in the power system E, and phases of the power system E (for example, a phase of a voltage). 
     The control details are power supplies which are disconnected when a system accident occurs. The control details may include turbine early valve actuation (EVA) for curbing acceleration of a power supply in addition to power supply restriction. The configuration of the power system stabilizing system SSC 1  will be described below. 
       FIG.  2    is a diagram showing the configuration of the power system stabilizing system SSC 1 . The power system stabilizing system SSC 1  includes, for example, an accident detector  100 , an arithmetic processor  200 , and a controller  300 . These constituents are implemented, for example, by causing a hardware processor such as a central processing unit (CPU) to execute a program (software). Some or all of these constituents may be implemented by hardware (a circuit unit including circuitry) such as a large scale integration (LSI) chip, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a graphics processing unit (GPU), or may be implemented by a combination of software and hardware. The program may be stored in a storage device (a storage device including a non-transitory storage medium) such as a hard disk drive (HDD) or a flash memory in advance or may be stored in a detachable storage medium (a non-transitory storage medium) such as a DVD or a CD-ROM and may be installed in the storage device by setting the storage medium to a drive device. The storage device is implemented, for example, by an HDD, a flash memory, an electrically erasable programmable read only memory (EEPROM), a read only memory (ROM), or a random access memory (RAM). 
     The accident detector  100 , the arithmetic processor  200 , and the controller  300  which are constituents of the power system stabilizing system SSC 1  are connected by communication equipment  400  (for example, a signal line or a communication device). 
     The accident detector  100  is installed, for example, in a substation. The accident detector  100  acquires system information from the power system E, detects occurrence of a system accident in the power system E, and additionally determines a type of a system accident that is detected. When occurrence of a system accident is detected, the accident detector  100  determines an accident type and transmits information of the accident type to the arithmetic processor  200 . The installation position of the accident detector  100  is not particularly limited, and the accident detector  100  may be mounted, for example, in the same device in which the arithmetic processor  200  or the controller  300  is installed. 
     The arithmetic processor  200  is installed in a place in which communication with another device is possible and may be installed in the same place in which the accident detector  100  or the controller  300  is installed. The arithmetic processor  200  may have a function of the accident detector  100  or the controller  300 . The arithmetic processor  200  is installed in a place in which connection to a load dispatching information network N such as a central load dispatching and liaison office is possible such that system information can be acquired online. 
     The arithmetic processor  200  includes, for example, a pre-calculator  210  and a post-calculator  220 . 
     The pre-calculator  210  and the post-calculator  220  may be installed in the same device or may be installed in different devices. The pre-calculator  210  may set a control table on the basis of system information (load dispatching online data) collected from the power system E via the load dispatching information network N, for example, using a method described in Japanese Unexamined Patent Application, First Publication No. 2011-61911, or Japanese Unexamined Patent Application, First Publication No. 2019-126155. For example, the pre-calculator  210  repeatedly performs preparing collecting current system information, preparing an analysis system model, performing transient stability calculation on a plurality of postulated accidents, selecting power restriction targets required for stability maintenance of the power system, setting the power restriction targets in the control table, and updating the control table in a first storage device  500  of the post-calculator  220 .  FIG.  3    is a diagram showing an example of details included in the control table. In the control table, information of power restriction targets is correlated with each postulated accident type. The postulated accident type includes, for example, a monitoring point (for example, a power line) at which an accident is monitored and information indicating a phase of the accident. The control table includes, for example, a postulated accident type number which is information for identifying an accident type and information of power restriction targets. In the control table shown in  FIG.  3   , the regenerative power supplies  50 - 1 ,  50 - 2 , and  50 - 3  shown in  FIG.  1    are set to be restricted when an accident with a postulated accident type number “1” occurs, and the regenerative power supplies  50 - 2  and  50 - 3  are set to be restricted when an accident with a postulated accident type number “2” occurs. The pre-calculator  210  transmits the control table to the post-calculator  220 . 
     The post-calculator  220  includes a first storage device  500 , a second storage device  510 , a power restriction target selector  520 , and a system restoration checker  530 . 
     The control table transmitted from the pre-calculator  210  is stored in the first storage device  500 . System information from the power system E and reconnection target setting information for setting whether to reconnect each power restriction target are stored in the second storage device  510 . In the reconnection target setting information, information indicating whether a power restriction target is to be selected as a reconnection target is set for each power restriction target. In the following description, a power supply which can be selected as a reconnection target (a power restriction target) is referred to as a “reconnectable power supply.” It is preferable that a reconnectable power supply be a regenerative power supply. That is, the power restriction target may include a power supply other than a regenerative power supply, but it is preferable that the reconnection target not include a power supply other than a regenerative power supply. 
     The power restriction target selector  520  selects power restriction targets required for stability maintenance of the power system E out of a plurality of power supplies connected to the power system E on the basis of the type of the system accident detected by the accident detector  100 . The plurality of power supplies connected to the power system E may be only regenerative power supplies or may include regenerative power supplies and power generators other than the regenerative power supplies. The power restriction targets may include only regenerative power supplies or may include regenerative power supplies and power generators other than the regenerative power supplies. The power restriction target selector  520  acquires information on the accident type of the power system E from the accident detector  100 , acquires a combination of power restriction targets in the accident type correlated with the accident type through comparison with the control table stored in the first storage device  500 , selects the acquired combination of power restriction targets as actual power restriction targets, and transmits a power restriction command to the controller  300  controlling the selected power restriction targets. The power restriction targets are not limited to regenerative power supplies, and may include power generators other than regenerative power supplies. 
     The system restoration checker  530  performs a restoration checking process of checking whether the power system has been restored from the system accident on the basis of the system information of the power system E. The restoration checking process is continuously performed until it is checked that the power system has been restored from the system accident. When it is checked that the power system has been restored from the system accident, the system restoration checker  530  transmits a power restriction release command to the controller  300 . 
     For example, when the power system E has been restored to a state before the system accident occurred on the basis of the system information of the power system E after the system accident occurred, the system restoration checker  530  may determine that the power system has been restored from the system accident. For example, the system restoration checker  530  may determine whether reclosing has succeeded by a breaker or a disconnector in a place in which the system accident occurred (an accident place) in the main system  10  on the basis of the system information and determine that the power system has been restored from the system accident. Reclosing means, for example, switching a breaker or a disconnector corresponding to the accident place to reconnect the accident place to the power system E after the accident place has been cut off from the power system E by switching the breaker or the disconnector to the unelectrified state. When reclosing has failed, the breaker or the disconnector corresponding to the accident place is controlled in the unelectrified state. Accordingly, the system restoration checker  530  may determine that reclosing has succeeded when the breaker or the disconnector corresponding to the accident place is maintained in the electrified state after reclosing has been performed. The system restoration checker  530  may determine that reclosing has succeeded when the power demand-supply state (for example, a power demand-supply state such as a voltage level, a voltage phase, or a power flow rate of the accident place of the main system  10 ) after reclosing has been performed has been restored to the power demand-supply state before the system accident occurred. The power demand-supply state before the system accident occurred is stored, for example, in the second storage device  510 . The system restoration checker  530  may not determine whether reclosing has succeeded, and may compare the power demand-supply state of the power system E before the system accident occurred with the power demand-supply state after the system accident occurred and determine that the power system has been restored from the system accident when the power demand-supply state after the system accident occurred is restored to the power demand-supply state before the system accident occurred. 
     The controller  300  includes, for example, a cutoff controller  600  and a reconnection controller  610 . 
     The cutoff controller  600  cuts off the power restriction targets selected by the power restriction target selector  520 . For example, the cutoff controller  600  cuts off the power restriction targets by transmitting a cutoff command for the power restriction targets on the basis of the power restriction command received from the post-calculator  220 . 
     When the system restoration checker  530  checks that the power system has been restored from the system accident, the reconnection controller  610  reconnects some or all of reconnectable power supplies to the power system E on the basis of the reconnection target setting information stored in the second storage device  510  out of the power restriction targets cut off by the cutoff controller  600 . For example, when the power restriction release command is received from the post-calculator  220 , the reconnection controller  610  reconnects the power restriction targets by transmitting a reconnection command for the power restriction targets for which the cutoff command has been output from the cutoff controller  600 . 
       FIG.  4    is a flowchart showing an example of a routine of processes of power restriction and reconnection which is performed by the power system stabilizing system SSC 1 . 
     When a system accident occurs in the main system  10 , the accident detector  100  detects the system accident and outputs information of a type of the detected system accident to the post-calculator  220  (Step S 101 ). When the type of the system accident is received from the accident detector  100 , the power restriction target selector  520  acquires a combination of power restriction targets in the postulated accident type correlated with the received accident type from the control table and selects the acquired combination of power restriction targets as actual power restriction targets (Step S 102 ). The power restriction target selector  520  transmits a power restriction command for controlling the selected power restriction targets to the controller  300 . The cutoff controller  600  cuts off the power restriction targets by transmitting a cutoff command for the power restriction targets on the basis of the power restriction command from the power restriction target selector  520  (Step S 103 ). For example, when a regenerative power supply  50 - 1  and a regenerative power supply  50 - 2  are selected as the power restriction targets, the cutoff controller  600  controls a breaker  60 - 1  and a breaker  60 - 2  such that they are in the unelectrified state to cut off the regenerative power supply  50 - 1  and the regenerative power supply  50 - 2  from the main system  10  by outputting the cutoff command to the breaker  60 - 1  and the breaker  60 - 2 . 
     After the regenerative power supply  50 - 1  and the regenerative power supply  50 - 2  have been disconnected from the main system  10 , the system restoration checker  530  performs a restoration checking process of collecting system information from the power system E and checking whether the power system has been restored from the system accident on the basis of the collected system information (Step S 104 ). When restoration from the system accident is checked as the result of the restoration checking process, the system restoration checker  530  transmits a power restriction release command to the controller  300 . When the power restriction release command is received from the system restoration checker  530 , the reconnection controller  610  reconnects some or all of reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets cut off in the process of Step S 103  as reconnection targets to the power system E by outputting a reconnection command (Step S 105 ). For example, when the regenerative power supply  50 - 1  and the regenerative power supply  50 - 2  are cut off from the main system  10  in the process of Step S 103 , the reconnection controller  610  performs control such that the breaker  60 - 1  and the breaker  60 - 2  are switched from the unelectrified state to the electrified state by outputting the reconnection command to the breaker  60 - 1  and the breaker  60 - 2 , and thus reconnects the regenerative power supply  50 - 1  and the regenerative power supply  50 - 2  to the main system  10 . 
     Since the power system stabilizing system SSC 1  according to the first embodiment checks restoration from a system accident on the basis of system information of the power system E, it is not necessary to check restoration from the system accident using a human system and it is possible to shorten the stop time of the regenerative power supplies  50 . Since the power system stabilizing system SSC 1  reconnects cutoff regenerative power supplies  50  after checking restoration from the system accident, it is not necessary to perform a reconnection operation using a human system and it is possible to further shorten the stop time. 
     Second Embodiment 
     A power system stabilizing system SSC 2  according to a second embodiment will be described below. In the following description, elements having the same functions as described above in the first embodiment will be referred to by the same names and reference signs, and specific description of the functions will be omitted. 
       FIG.  5    is a diagram showing a configuration of the power system stabilizing system SSC 2  according to the second embodiment. The power system stabilizing system SSC 2  is different from the power system stabilizing system SSC 1  according to the first embodiment shown in  FIG.  2   , in that power supplies disconnected to the power system E are also reconnected in addition to the cutoff power restriction targets. 
     As shown in  FIG.  5   , the power system stabilizing system SSC 2  includes, for example, an accident detector  100 , an arithmetic processor  200 B, and a controller  300 B. The accident detector  100 , the arithmetic processor  200 B, and the controller  300 B which are constituents of the power system stabilizing system SSC 2  are connected by communication equipment  400  (for example, a signal line or a communication device). 
     The arithmetic processor  200 B includes, for example, a pre-calculator  210  and a post-calculator  220 B. 
     The post-calculator  220 B includes a first storage device  500 , a second storage device  510 , a power restriction target selector  520 , a system restoration checker  530 , and a disconnected power supply detector  540 . 
     The disconnected power supply detector  540  detects regenerative power supplies  50 - k  which are regenerative power supplies  50  other than the power restriction targets selected by the power restriction target selector  520  and which are disconnected from the power system E out of a plurality of regenerative power supplies  50 - k . In the following description, a regenerative power supply which is a regenerative power supply  50  other than the power restriction targets and which is disconnected from the power system E may be referred to as a “disconnected power supply.” 
     A disconnected power supply is, for example, a regenerative power supply  50  which is disconnected from the power system E by an isolated operation preventing function of a PCS  52 - k . The disconnected power supply detector  540  collects system information including disconnection information (for example, information indicating the on/off state of the breaker  60 - k ) indicating whether each regenerative power supply  50 - k  is disconnected by the isolated operation preventing function from a plurality of PCSs  52 - k  and detects disconnected power supplies on the basis of the collected system information and information of the power restriction targets. The disconnected power supply detector  540  transmits information of the detected disconnected power supplies to the controller  300 B. 
     The controller  300 B includes, for example, a cutoff controller  600  and a reconnection controller  610 B. 
     When the system restoration checker  530  checks that the power system has been restored from the system accident, the reconnection controller  610 B reconnects some or all of reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets cut off by the cutoff controller  600  and some or all of the disconnected power supplies detected by the disconnected power supply detector  540  to the power system E. 
       FIG.  6    is a flowchart showing an example of a routine of processes of power restriction and reconnection which is performed by the power system stabilizing system SSC 2 . Steps S 201  to S 203  in the flowchart shown in  FIG.  6    are the same processes as Steps S 101  to S 103  in the flowchart shown in  FIG.  4   , and thus description thereof will be omitted. 
     The disconnected power supply detector  540  detects disconnected power supplies out of a plurality of regenerative power supplies  50 - k  (Step S 204 ). For example, the disconnected power supply detector  540  detects the disconnected power supplies by comparing information of the power restriction targets selected in Step S 202  with the disconnection information received from the power system E. The disconnected power supply detector  540  outputs the information of the disconnected power supplies to the controller  300 B. It is also conceivable that there is no disconnected power supply. In this case, the disconnected power supply detector  540  may transmit information indicating that there is no disconnected power supply to the controller  300 B. For example, in Step S 204 , the disconnected power supply detector  540  may detect whether there is a disconnected power supply on the basis of the information of the power restriction targets and the disconnection information, output the information indicating that there is no disconnected power supply to the controller  300 B when there is no disconnected power supply, and output information of disconnected power supplies to the controller  300 B when the disconnected power supplies are detected. 
     When a power restriction target is cut off from the main system  10 , the system restoration checker  530  performs a restoration checking process on the basis of the system information from the power system E (Step S 205 ). When it is checked that the power system has been restored from the system accident, the system restoration checker  530  transmits a power restriction release command to the controller  300 B. When the information of the disconnected power supplies is acquired by the disconnected power supply detector  540  and the power restriction release command is received from the system restoration checker  530 , the reconnection controller  610  reconnects some or all of reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets cut off in the process of Step S 203  and some or all of the disconnected power supplies detected in the process of Step S 204  as reconnection targets to the power system E by outputting a reconnection command for the power restriction targets and the disconnected power supplies (Step S 206 ). For example, when the regenerative power supply  50 - 1  and the regenerative power supply  50 - 2  which are power restriction targets are cut off from the main system  10  in Step S 203  and the regenerative power supply  50 - 3  which is not a power restriction target is disconnected, the regenerative power supply  50 - 3  is detected as a disconnected power supply in the process of Step S 204 , and thus the reconnection controller  610  performs control such that the breakers  60 - 1  to  60 - 3  are switched from the unelectrified state to the electrified state by outputting the reconnection command to the breakers  60 - 1  to  60 - 3  and reconnects the regenerative power supplies  50 - 1  to  50 - 3  to the main system  10 . 
     With the power system stabilizing system SSC 2  according to the second embodiment, it is possible to achieve the same advantages as in the power system stabilizing system SSC 1  according to the first embodiment and it is also possible to shorten the stop time of the regenerative power supplies  50  which have been disconnected due to a reason such as an isolated operation preventing function or an influence of a main system accident other than cutoff control by reconnecting the regenerative power supplies  50  which are disconnected in addition to the power restriction targets as the reconnection targets. 
     Third Embodiment 
     A power system stabilizing system SSC 3  according to a third embodiment will be described below. In the following description, elements having the same functions as described above in the first embodiment and the second embodiment will be referred to by the same names and reference signs, and specific description of the functions will be omitted. 
       FIG.  7    is a diagram showing a configuration of the power system stabilizing system SSC 3  according to the third embodiment. The power system stabilizing system SSC 3  shown in  FIG.  7    is different from the power system stabilizing system SSC 2  shown in  FIG.  5   , in that detection of disconnected power supplies is performed after power restriction targets have been reconnected. 
     As shown in  FIG.  7   , the power system stabilizing system SSC 3  includes, for example, an accident detector  100 , an arithmetic processor  200 C, and a controller  300 C. The accident detector  100 , the arithmetic processor  200 C, and the controller  300 C which are constituents of the power system stabilizing system SSC 3  are connected by communication equipment  400  (for example, a signal line or a communication device). 
     The arithmetic processor  200 C includes, for example, a pre-calculator  210  and a post-calculator  220 C. 
     The post-calculator  220 C includes a first storage device  500 , a second storage device  510 , a power restriction target selector  520 , a system restoration checker  530 , and an additional disconnected power supply detector  550 . 
     The additional disconnected power supply detector  550  detects disconnected power supplies in the same way as the disconnected power supply detector  540 . However, the additional disconnected power supply detector  550  performs the process of detecting disconnected power supplies after having reconnected the power restriction targets selected by the power restriction target selector  520 . When disconnected power supplies are detected, the additional disconnected power supply detector  550  transmits information of the disconnected power supplies to the controller  300 C. 
     The controller  300 C includes, for example, a cutoff controller  600  and a reconnection controller  610 C. 
     When the system restoration checker  530  checks that the power system has been restored from the system accident, the reconnection controller  610 C reconnects some or all of reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets cut off by the cutoff controller  600  to the power system E. The reconnection controller  610 C reconnects some or all of the disconnected power supplies detected by the additional disconnected power supply detector  550  to the power system E after having reconnected some or all of the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets to the power system E. 
       FIG.  8    is a flowchart showing an example of a routine of processes of power restriction and reconnection which is performed by the power system stabilizing system SSC 3 . Steps S 301  to S 304  in the flowchart shown in  FIG.  8    are the same processes as Steps S 101  to S 104  in the flowchart shown in  FIG.  4   , and thus description thereof will be omitted. 
     When a power restriction release command is received from the system restoration checker  530 , the reconnection controller  610  reconnects the power restriction targets cut off by the cutoff controller  600  by outputting a reconnection command for the power restriction targets (Step S 305 ). 
     The additional disconnected power supply detector  550  performs the process of detecting disconnected power supplies after the process of Step S 305  has been performed and outputs information of disconnected power supplies to the controller  300 C when the disconnected power supplies are detected (Step S 306 ). 
     When the main system  10  returns to the system state before the system accident occurred and the power restriction targets cut off by the cutoff controller  600  are reconnected to the main system  10 , the disconnected power supplies may be reconnected regardless of control by the power system stabilizing system SSC 3 . In the following description, reconnection of disconnected power supplies regardless of control by the power system stabilizing system SSC 3  is referred to as “automatic reconnection.” For example, when the power restriction targets cut off by the cutoff controller  600  are reconnected to the main system  10 , the main system  10  may be stabilized, the isolated operation preventing function for the disconnected power supplies may be released, and the disconnected power supplies may be automatically reconnected. Accordingly, when reconnection is performed by the power system stabilizing system without considering the automatic reconnection, an unnecessary process of reconnecting the disconnected power supplies which have been automatically reconnected is performed by the power system stabilizing system. Therefore, the power system stabilizing system SSC 3  reconnects only disconnected power supplies which have not been automatically reconnected out of a plurality of disconnected power supplies by detecting whether there is a disconnected power supply after automatic reconnection has been performed and performing reconnection in connection of automatic reconnection. For example, the additional disconnected power supply detector  550  performs a process of detecting a disconnected power supply when a predetermined time elapses after the power restriction targets cut off by the cutoff controller  600  have been reconnected. The predetermined time is set in advance to be longer than a time until automatic reconnection is performed after the power restriction targets have been reconnected. 
     When information of disconnected power supplies is received from the additional disconnected power supply detector  550 , the reconnection controller  610 C reconnects some or all of the disconnected power supplies detected in Step S 306  to the power system E by outputting a reconnection command for the disconnected power supplies (Step S 307 ). For example, it is assumed that the regenerative power supply  50 - 1  is a power restriction target and the regenerative power supply  50 - 2  and the regenerative power supply  50 - 3  are disconnected power supplies. In this case, the regenerative power supply  50 - 1  is reconnected in Step S 305 , and the process of detecting a disconnected power supply is performed by the additional disconnected power supply detector  550  when the predetermined time has elapsed thereafter. Here, when the regenerative power supply  50 - 2  is automatically reconnected with reconnection of the regenerative power supply  50 - 1 , the additional disconnected power supply detector  550  detects only the regenerative power supply  50 - 3  as the disconnected power supply in Step S 306 . Accordingly, the reconnection controller  610 C reconnects the regenerative power supply  50 - 3  after having automatically reconnected the regenerative power supply  50 - 2 . 
     With the power system stabilizing system SSC 3  according to the third embodiment, it is possible to achieve the same advantages as in the power system stabilizing system SSC 1  according to the first embodiment, and multi-phase reconnection of detecting disconnected power supplies when the predetermined time elapsed after the power restriction targets have been reconnected and then reconnecting the detected disconnected power supplies is performed in consideration of automatic reconnection of a disconnected power supply. 
     Fourth Embodiment 
     A power system stabilizing system SSC 4  according to a fourth embodiment will be described below. In the following description, elements having the same functions as described above in the first to third embodiments will be referred to by the same names and reference signs, and specific description of the functions will be omitted. 
       FIG.  9    is a diagram showing a configuration of the power system stabilizing system SSC 4  according to the fourth embodiment. The power system stabilizing system SSC 4  shown in  FIG.  9    is different from the power system stabilizing system SSC 1  shown in  FIG.  2   , in that whether restoration from a system accident is restoration of a partial system from a permanent accident or whether restoration from a system accident is restoration from a transient accident to a system state before the accident occurred on the basis of system information of the power system in the process of checking restoration from the system accident, an amount of reconnectable electric power is calculated when only a partial system is restored from the permanent accident, and regenerative power supplies to be reconnected are selected according to the calculated amount of reconnectable electric power. 
     As shown in  FIG.  9   , the power system stabilizing system SSC 4  includes, for example, an accident detector  100 , an arithmetic processor  200 D, and a controller  300 D. 
     The arithmetic processor  200 D includes, for example, a pre-calculator  210  and a post-calculator  220 D. 
     The post-calculator  220 D includes a first storage device  500 , a second storage device  510 , a power restriction target selector  520 , a system restoration checker  530 D, a reconnectable power calculator  560 , and a reconnectable target selector  570 . 
     The system restoration checker  530 D performs a restoration checking process of checking whether the power system has been restored from a system accident on the basis of the system information of the power system E. As the restoration from a system accident, two restoration states including partial restoration in which only a partial system is restored from a permanent accident and complete restoration in which the power system is restored from a transient accident to a system state before the accident occurred are considered. The system restoration checker  530 D determines which of partial restoration and complete restoration the restoration from the system accident is in the restoration checking process. For example, when it is determined that the state (for example, the power demand-supply state) of the power system E checked after the predetermined time has elapsed on the basis of the system information is restored to the state before the system accident occurred, the system restoration checker  530 D determines that the restoration from the system accident is complete restoration. For example, when it is determined that the system state (for example, the power demand-supply state) of the power system E checked after the predetermined time has elapsed on the basis of the system information is not restored to the system state before the system accident occurred, the system restoration checker  530 D may determine that the restoration from the system accident is partial restoration. 
     The system restoration checker  530 D may determine that the restoration from the system accident is complete restoration, for example, through the same process as the restoration checking process in the first embodiment. For example, when a demand-supply state (for example, a voltage level, a voltage phase, or a power flow rate in an accident place of the main system  10 ) of the power system E when the predetermined time has elapsed after the system accident occurred is not restored to the demand-supply state before the system accident occurred but is restored to a predetermined level, the system restoration checker  530 D may determine that the restoration from the system accident is partial restoration. 
     When the system restoration checker  530 D determines that the restoration from the system accident is partial restoration, the reconnectable power calculator  560  calculates a reconnectable power which is an amount of electric power which can be supplied to the main system  10  through reconnection. The reconnectable power is, for example, an upper limit of the reconnectable power in consideration of whether an overload occurred after the reconnection. For example, the reconnectable power calculator  560  calculates the reconnectable power by performing power flow calculation or the like using current system information. 
     The reconnectable target selector  570  selects reconnectable targets out of power supplies which are disconnected such that the total value of amounts of electric power (reconnection powers) supplied from all the reconnected regenerative power supplies  50 - k  to the main system  10  when the regenerative power supplies  50 - k  are reconnected to the power system E is less than the reconnectable power calculated by the reconnectable power calculator  560 . The power supplies which are disconnected may be reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the disconnected power restriction targets, may be disconnected power supplies, or may be both thereof. The reconnectable target selector  570  selects the reconnection targets such that the total value of reconnectable powers is less than the reconnectable power calculated by the reconnectable power checker  14  through calculation such as optimal combination or quasi-optimal combination on the basis of a preset priority order. For example, the reconnectable target selector  570  acquires reconnectable powers of the regenerative power supplies  50  which are disconnected out of information of outputs of the regenerative power supplies  50 - k  or information of the loads included in the system information. Then, the reconnectable target selector  570  selects the reconnection targets out of the power restriction targets such that the total value of the reconnection powers is less than the reconnectable power on the basis of the priority order in which a power restriction target has a higher priority than a disconnected power supply. It is preferable that the total value of the reconnection powers be as close to the reconnectable power calculated by the reconnectable power checker  14  as possible. When a load is connected to an external system  12 - k , a value obtained by subtracting an amount of electric power consumed in the load from the outputs of the regenerative power supplies  50 - k  may be defined as the reconnection power. The reconnectable target selector  570  transmits information of the selected reconnection targets to the controller  300 D. 
     The controller  300 D includes, for example, a cutoff controller  600  and a reconnection controller  610 D. 
     The reconnection controller  610 D reconnects some or all of the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets cut off by the cutoff controller  600  to the power system E when the system restoration checker  530 D determines that the power system is restored from a transient accident to the system state before the system accident occurred, and reconnects only the reconnection targets selected by the reconnectable target selector  570  to the power system E when the system restoration checker  530  determines that only a partial system is restored from a permanent accident which is the main system accident. 
       FIG.  10    is a flowchart showing an example of a routine of processes of power restriction and reconnection which is performed by the power system stabilizing system SSC 4 . Steps S 401  to S 403  in the flowchart shown in  FIG.  10    are the same processes as Steps S 101  to S 103  in the flowchart shown in  FIG.  4   , and thus description thereof will be omitted. 
     The system restoration checker  530  determines which of partial restoration and complete restoration the restoration from the system accident is when a power restriction target is cut off (Step S 404 ) and transmits a power restriction command to the controller  300 D when it is determined that the restoration from the system accident is complete restoration. The reconnection controller  610 D reconnects all the power restriction targets by outputting a cutoff command for some or all of the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets cut off in Step S 403  when the power restriction command is received (Step S 405 ). When it is determined in Step S 403  that the restoration from the system accident is partial restoration, the process of Step S 405  is not performed, and calculation of a reconnectable power (Step S 406 ) and selection of reconnection targets based on the reconnectable power (Step S 407 ) are performed. Then, the reconnection controller  610 D reconnects the reconnection targets to the power system E by outputting a reconnection command to all the breakers  60 - k  of the reconnection targets selected in Step S 407  (Step S 408 ). 
     With the power system stabilizing system SSC 4  according to the fourth embodiment, it is possible to achieve the same advantages as in the power system stabilizing system SSC 1  according to the first embodiment. In addition, when only a partial system is restored from a permanent accident, it is possible to minimize the number of regenerative power supplies  50  to be stopped by calculating an optimal reconnection power based on the system information and reconnecting the regenerative power supplies  50  according to the reconnection power. 
     Fifth Embodiment 
     A power system stabilizing system SSC 5  according to a fifth embodiment will be described below. In the following description, elements having the same functions as described above in the first to fourth embodiments will be referred to by the same names and reference signs, and specific description of the functions will be omitted. 
       FIG.  11    is a diagram showing a configuration of the power system stabilizing system SSC 5  according to the fifth embodiment. The power system stabilizing system SSC 5  shown in  FIG.  11    is different from the power system stabilizing system SSC 1  shown in  FIG.  2   , in that a reconnection target is selected in consideration of a power demand-supply balance of the main system  10 . 
     As shown in  FIG.  11   , the power system stabilizing system SSC 5  includes, for example, an accident detector  100 , an arithmetic processor  200 E, and a controller  300 E. 
     The arithmetic processor  200 E includes, for example, a pre-calculator  210  and a post-calculator  220 E. 
     The post-calculator  220 E includes a first storage device  500 , a second storage device  510 , a power restriction target selector  520 , a system restoration checker  530 , a power demand-supply balance checker  700 , a necessary reconnection power calculator  710 , and a reconnection control target selector  720 . 
     The power demand-supply balance checker  700  checks whether an abnormality of a power demand-supply balance has occurred in the main system  10  in a state in which power restriction targets are cut off. Whether an abnormality of the power demand-supply balance has occurred in the main system  10  may be checked by the power demand-supply balance checker  700 , for example, on the basis of an abnormality in level of a frequency or a rate of change of the frequency of the main system  10 . An example of an abnormality in level of the frequency is, for example, that the level of the frequency is outside of a predetermined range. An abnormality of the rate of change of the frequency is, for example, that the rate of change of the frequency is greater than a predetermined value for determining whether the rate of change of the frequency is abnormal. 
     When an abnormality of the power demand-supply balance has been checked by the power demand-supply balance checker  700 , the necessary reconnection power calculator  710  calculates a reconnection power necessary for releasing an abnormality of the power demand-supply balance (hereinafter referred to as a “necessary reconnection power”). When it is considered that the abnormality of the power demand-supply balance of the main system  10  is released by reconnecting the regenerative power supplies  50 - k , it may not be effective even if the total value of amounts of electric power supplied from the reconnection targets to the main system  10  is much greater or less than the necessary reconnection power. On the other hand, it may be difficult to select the reconnection targets such that the total value is equal to the necessary reconnection power. Accordingly, the necessary reconnection power calculator  710  may calculate only one necessary reconnection power, but preferably calculates two necessary reconnection powers including an upper limit and a lower limit. Power flow calculating using system information or the like can be used to calculate the necessary reconnection power. 
     The reconnection control target selector  720  selects reconnection targets required for releasing an abnormality of the power demand-supply balance on the basis of the necessary reconnection power. For example, the reconnection control target selector  720  selects regenerative power supplies  50  which are reconnection targets such that the total value of amounts of electric power supplied to the main system  10  by reconnecting the regenerative power supplies  50 - k  reaches the necessary reconnection power calculated by the necessary reconnection power calculator  710 . For example, when one necessary reconnection power is calculated by the necessary reconnection power calculator  710 , the reconnection control target selector  720  selects the reconnection targets out of regenerative power supplies  50 - k  which are disconnected such as the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets or the disconnected power supplies such that the total value is as close to the necessary reconnection power as possible. For example, when two necessary reconnection powers including the upper limit and the lower limit are calculated by the necessary reconnection power calculator  710 , the reconnection control target selector  720  selects the reconnection targets out of regenerative power supplies  50 - k  which are disconnected such as the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets or the disconnected power supplies such that the total value is in a range defined by the upper limit and the lower limit. Similarly to the method used by the reconnectable target selector  570 , a method of selecting the reconnection targets such that the total value of reconnection powers calculated through calculation such as optimal combination or quasi-optimal combination on the basis of the preset priority order is equal to a value calculated as the necessary reconnection power or the like can be considered as the method of selecting the reconnection targets. The reconnection control target selector  720  transmits information of the selected reconnection targets to the controller  300 E. 
     The controller  300 E includes, for example, a cutoff controller  600  and a reconnection controller  610 E. 
     The reconnection controller  610 E reconnects only the reconnection targets selected by the reconnection control target selector  720  to the power system E. 
       FIG.  12    is a flowchart showing an example of a routine of processes of power restriction and reconnection which is performed by the power system stabilizing system SSC 5 . Steps S 501  to S 504  in the flowchart shown in  FIG.  10    are the same processes as Steps S 101  to S 104  in the flowchart shown in  FIG.  4   , and thus description thereof will be omitted. 
     When a power supply which is not assumed beforehand drops due to a main system accident, an abnormality of the power demand-supply balance may not be released even if onset control for cutting off power restriction targets is performed in Step S 503 . Accordingly, the power system stabilizing system SSC 5  checks the power demand-supply balance after the onset control and reconnects regenerative power supplies  50  for supplying an amount of electric power for releasing an abnormality of the power demand-supply balance to the power system E when the abnormality of the power demand-supply balance is not released. 
     For example, when the system restoration checker  530  checks that restoration from the system accident has been completed (Step S 504 ), the power demand-supply balance checker  700  determines whether an abnormality of the power demand-supply balance of the main system  10  has occurred in a state in which the power restriction targets are cut off (Step S 505 ). When it is determined in Step S 505  that an abnormality of the power demand-supply balance has not occurred (the power demand-supply balance is normal), the controller  300 E does not perform reconnection of the regenerative power supplies  50 - k . When it is determined in Step S 505  that an abnormality of the power demand-supply balance has occurred, calculation of a necessary reconnection power which is an amount of electric power for releasing the abnormality of the power demand-supply balance (Step S 506 ) and selection of reconnection targets based on the necessary reconnection power (Step S 507 ) are performed. Then, the reconnection controller  610 D reconnects the reconnection targets to the power system E by outputting a reconnection command to all the breakers  60 - k  of the reconnection targets selected in Step S 507  (Step S 508 ). 
     With the power system stabilizing system SSC 5  according to the fifth embodiment, it is possible to achieve the same advantages as in the power system stabilizing system SSC 1  according to the first embodiment. In addition, it is possible to curb deterioration in system stability by reconnecting appropriate regenerative power supplies  50  according to an amount of electric power necessary for releasing an abnormality of the power demand-supply balance in consideration of the power demand-supply balance. 
     Sixth Embodiment 
     A power system stabilizing system SSC 6  according to a sixth embodiment will be described below. In the following description, elements having the same functions as described above in the first to fifth embodiments will be referred to by the same names and reference signs, and specific description of the functions will be omitted. 
       FIG.  13    is a diagram showing a configuration of the power system stabilizing system SSC 6  according to the sixth embodiment. The power system stabilizing system SSC 6  shown in  FIG.  13    is different from the power system stabilizing system SSC 1  shown in  FIG.  2   , in that it is checked that reconnection of regenerative power supplies  50  to the power system E does not cause any problem before reconnection is actually performed. 
     As shown in  FIG.  13   , the power system stabilizing system SSC 6  includes, for example, an accident detector  100 , an arithmetic processor  200 F, and a controller  300 F. 
     The arithmetic processor  200 F includes, for example, a pre-calculator  210  and a post-calculator  220 F. 
     The post-calculator  220 F includes a first storage device  500 , a second storage device  510 , a power restriction target selector  520 , a system restoration checker  530 , a post-reconnection system state checker  800 , and a reconnection target reselector  810 . 
     When the power system is restored from a system accident, the post-reconnection system state checker  800  selects all the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets as reconnection targets and checks whether a system abnormality will occur in the main system  10  when the selected power supplies are reconnected to the power system E. The system abnormality is, for example, an abnormal phenomenon such as an abnormality in frequency (such as an increase in frequency) or an overload. For example, power flow calculation using the system information can be used to check the system abnormality. The post-reconnection system state checker  800  may perform a check by predicting outputs of reconnected power restriction targets or disconnected power supplies from maximum values of the outputs, outputs of near regenerative power supplies  50 - k , or the like. 
     When the post-reconnection system state checker  800  checks that a system abnormality occurs, the reconnection target reselector  810  reselects reconnection targets. Reduction of selection targets based on a preset priority order or the like can be used to reselect reconnection targets. For example, when the post-reconnection system state checker  800  checks that a system abnormality occurs, the reconnection target reselector  810  may perform the reselection by more preferentially reducing the disconnected power supplies than the power restriction targets out of the selected reconnection targets or may perform the reselection by preferentially reducing the regenerative power supplies  50  of which an output is low when they are reconnected. When the reconnection targets are reselected, the post-reconnection system state checker  800  checks whether a system abnormality occurs when the reconnection targets selected through the reselection are reconnected. The reselection of reconnection targets is repeatedly performed until the post-reconnection system state checker  800  checks that a system abnormality does not occur. 
     The controller  300 F includes, for example, a cutoff controller  600  and a reconnection controller  610 F. 
     When the post-reconnection system state checker  800  checks that a system abnormality does not occur, the reconnection controller  610 F reconnects the reconnection targets selected at that time to the power system E. 
       FIG.  14    is a flowchart showing an example of a routine of processes of power restriction and reconnection which is performed by the power system stabilizing system SSC 6 . Steps S 601  to S 604  in the flowchart shown in  FIG.  14    are the same processes as Steps S 101  to S 104  in the flowchart shown in  FIG.  4   , and thus description thereof will be omitted. 
     When the system restoration checker  530  checks restoration from the system accident (Step S 604 ), the post-reconnection system state checker  800  selects all the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets as reconnection targets (Step S 605 ) and checks whether a system abnormality occurs in the main system  10  when the selected power restriction targets are reconnected to the power system E (Step S 606 ). When it is checked in Step S 606  that a system abnormality does not occur, the system restoration checker  530  transmits information of the reconnection targets selected at that time to the controller  300 F. When it is checked in Step S 606  that a system abnormality occurs, the reconnection target reselector  810  reselects reconnection targets (Step S 607 ), and it is checked again in Step S 606  whether a system abnormality occurs in the main system  10  when the reselected reconnection targets are reconnected to the power system E. The reconnection controller  610 D reconnects the reconnection targets to the power system E by outputting a reconnection command to all the breakers  60 - k  of the selected reconnection targets (Step S 608 ). 
     With the power system stabilizing system SSC 6  according to the sixth embodiment, it is possible to achieve the same advantages as in the power system stabilizing system SSC 1  according to the first embodiment. In addition, by checking whether a system abnormality occurs due to reconnection beforehand when the regenerative power supplies  50 - k  are reconnected in consideration of that the regenerative power supplies  50 - k  are variable power supplies of which an output varies depending on weather conditions or the like and reconnecting only the reconnection targets checked not to cause a system abnormality, it is possible to curb occurrence of an unnecessary system abnormality such as deterioration in system stability due to the reconnection. 
     The post-reconnection system state checker  800  and the reconnection target reselector  810  according to the sixth embodiment can be applied to the second to fifth embodiments. For example, when the post-reconnection system state checker  800  is applied to the second embodiment, the post-reconnection system state checker  800  may select the reconnectable power supplies set in the reconnection target setting information stored in the second storage device  510  out of the power restriction targets and the disconnected power supplies as reconnection targets and check whether a system abnormality occurs before Step S 206 . For example, when the post-reconnection system state checker  800  is applied to the third embodiment, the post-reconnection system state checker  800  may select the power restriction targets as reconnection targets and check whether a system abnormality occurs before Step S 305  and may select the disconnected power supplies as reconnection targets and check whether a system abnormality occurs before Step S 307 . For example, when the post-reconnection system state checker  800  is applied to the fourth embodiment, the post-reconnection system state checker  800  may check whether a system abnormality occurs in the reconnection targets selected in Step S 407  before Step S 408 . For example, when the post-reconnection system state checker  800  is applied to the fifth embodiment, the post-reconnection system state checker  800  may check whether a system abnormality occurs in the reconnection targets selected in Step S 507  before Step S 508 . 
     Seventh Embodiment 
     A power system stabilizing system SSC 7  according to a seventh embodiment will be described below. In the following description, elements having the same functions as described above in the first to sixth embodiments will be referred to by the same names and reference signs, and specific description of the functions will be omitted. 
       FIG.  15    is a diagram showing a configuration of the power system stabilizing system SSC 7  according to the seventh embodiment. The power system stabilizing system SSC 7  shown in  FIG.  15    is different from the power system stabilizing system SSC 1  shown in  FIG.  2   , in that a reconnection checker  900 , a failure frequency checker  910 , and a warning output  920  are provided. 
     As shown in  FIG.  15   , the power system stabilizing system SSC 7  includes, for example, an accident detector  100 , an arithmetic processor  200 , and a controller  300 G. 
     The controller  300 G includes, for example, a cutoff controller  600 , a reconnection controller  610 G, a reconnection checker  900 , a failure frequency checker  910 , and a warning output  920 . The warning output  920  may be provided in the arithmetic processor  200 . 
     The reconnection controller  610 G has the function of the reconnection controller  610 . 
     After the reconnection controller  610 G has output a reconnection command for reconnection targets, the reconnection checker  900  checks whether reconnection of the reconnection targets has failed for each reconnection target. For example, the reconnection checker  900  checks connection states of the breakers  60 - k  corresponding to the reconnection targets on the basis of connection information of the breakers  60 - k  included in the system information after the reconnection command has been output, determines that the reconnection has succeeded when the connection states are the electrified state, and determines that the reconnection has failed when the connection states are the unelectrified state. 
     The failure frequency checker  910  counts the number of failures (hereinafter referred to as a “failure frequency”) S determined by the reconnection checker  900  for each reconnection target. The failure frequency checker  910  compares the counted failure frequency S with a preset upper limit Sth and notifies the warning output  920  when the failure frequency S is greater than the upper limit Sth. For example, the failure frequency checker  910  may notify the warning output  920  of information of the reconnection targets in which the failure frequency S is greater than the upper limit Sth. 
     The warning output  920  transmits warning information including information indicating that the reconnection has failed to the outside (for example, an external communication terminal) in a predetermined format. The communication terminal is, for example, a computer or a mobile terminal (for example, a smartphone or a tablet terminal) which is owned by an operator or a manager of the power system stabilizing system SSC 7 . The predetermined format may be electronic mail notification to the communication terminal, display of text information on a screen of the communication terminal, or output of vocal information from a speaker of the communication terminal. The warning information may include at least one of information of a reconnection target of which the failure frequency is greater than the upper limit, information of a position of the reconnection target, and information of the breaker  60 - k  corresponding to the reconnection target. 
       FIG.  16    is a flowchart showing an example of a routine of processes of power restriction and reconnection which is performed by the power system stabilizing system SSC 7 . Steps S 701  to S 704  in the flowchart shown in  FIG.  16    are the same processes as Steps S 101  to S 104  in the flowchart shown in  FIG.  4   , and thus description thereof will be omitted. 
     When the system restoration checker  530  checks that the power system has been restored from the system accident and a power restriction release command is received from the system restoration checker  530 , the reconnection controller  610 G selects all the power restriction targets which are cut off as reconnection targets and outputs a reconnection command to the reconnection targets (Step S 705 ). After the reconnection command for the reconnection targets has been output, the reconnection checker  900  performs a reconnection checking process of determining whether the reconnection has succeeded for each reconnection target (Step S 706 ). Regarding a reconnection target of which the reconnection has succeeded out of the plurality of reconnection targets, the failure frequency S thereof is initialized (Step S 707 ), and the reconnection process is completed. Regarding a reconnection target of which the reconnection has failed out of the plurality of reconnection targets, the failure frequency checker  910  counts the failure frequency S of the reconnection target (Step S 708 ). Then, the failure frequency checker  910  determines whether the failure frequency S is greater than the upper limit Sth (Step S 709 ), and the reconnection controller  610 G outputs a reconnection command for the power restriction target of which the reconnection has failed when the failure frequency S is not greater than the upper limit Sth (Step S 710 ). When the process of Step S 710  is performed, the reconnection checker  900  performs the reconnection checking process on the reconnection target of which the reconnection has failed again in Step S 706 . When there is a reconnection target of which the failure frequency S is greater than the upper limit Sth in Step S 709 , the warning output  920  transmits, for example, warning information including information of the reconnection target to an external communication terminal (Step S 711 ). Initialization of the failure frequency S may be performed by the failure frequency checker  910  with output of the warning information from the warning output  920  as a trigger. 
     With the power system stabilizing system SSC 7  according to the seventh embodiment, it is possible to achieve the same advantages as in the power system stabilizing system SSC 1  according to the first embodiment. In addition, it is possible to rapidly notify an operator, a manager, or the like of the power system stabilizing system SSC 7  that the reconnection has failed by outputting a warning when the reconnection has failed and to prompt the operator, the manager, or the like to cope with the failure in reconnection. 
     The reconnection checker  900 , the failure frequency checker  910 , and the warning output  920  according to the seventh embodiment can be applied to the second to sixth embodiments. For example, the processes of Steps S 709  to S 714  may be performed after the regenerative power supplies  50 - k  have been reconnected in the second to sixth embodiments. 
     According to at least one of the aforementioned embodiments, since the system restoration checker  530  or the system restoration checker  530 D configured to check that the power system has been restored from a system accident on the basis of system information of the power system after a regenerative power supply  50 - k  which is a power restriction target has been cut off due to occurrence of the system accident is provided, it is possible to shorten the stop time of the regenerative power supply  50 . 
     The power system stabilizing systems according to the aforementioned embodiments may be power system stabilizing systems using only regenerative power supplies as targets or may be power system stabilizing systems using both of regenerative power supplies and power supplies (power generators) other than regenerative power supplies. 
     The term “˜ unit” described in this specification refers to a unit for processing at least one function or operation, and this may be implemented by hardware or software or may be implemented by a combination of hardware and software. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.