Patent Publication Number: US-2016233560-A1

Title: Power Storage System and Maintenance Method of the Power Storage System

Description:
TECHNICAL FIELD 
     The present invention relates to a power storage system using storage batteries and a maintenance technology including preventive maintenance thereof. 
     BACKGROUND ART 
     Large scale power storage systems capable of charging/discharging a large capacity of power by connecting a plurality of storage batteries serially and in parallel have been developed as power storage systems. A power storage system charges storage batteries by feeding from a power system, a photovoltaic power generating system, and the like and supplies stable and necessary power to a load in a load facility by discharging from the storage batteries. Application examples of a large scale power storage system are to perform peak cut and peak shift of load power consumed in load facilities such as buildings and factories and leveling of natural energy such as sunlight. 
     As storage batteries to be used for a power storage system, lithium-ion storage batteries are used instead of conventional lead-acid storage batteries. Depending on conditions such as capacity density and mass, lithium-ion storage batteries are often more advantageous than lead-acid storage batteries. Unlike lead-acid storage batteries, in case of lithium-ion storage batteries, it is necessary to continuously monitor states of the storage batteries such as voltage in order to ensure soundness. That is, a power storage system is provided with a function of monitoring states of the storage batteries. 
     The above function determines and detects states of the storage batteries such as a normal state, an abnormal state, or a warning state by monitoring to transmit signals in accordance with the detections. For example, when it is determined that the voltage of the storage batteries is outside the range of use, a signal indicating an abnormal state is output, and when it is determined that it is close to a threshold value within the range of use, a signal indicating warning is output. The power storage system outputs an alarm when states of the storage batteries such as an abnormal state or a warning state are detected and partially stops operating the system with respect to a portion including the corresponding storage batteries in the detected states, that is, the charging/discharging function. 
     In the power storage system, a maintenance person performs maintenance works such as checking malfunctions of the portion including the corresponding storage batteries in accordance with the alarm based on the above signal. The maintenance person then eliminates the malfunction, ensures safety, and resumes operations of this portion. Works of the maintenance person include maintenance actions such as periodic maintenance checkups, diagnosis, and examination of storage batteries in the above states such as an abnormal state or a warning state and replacement of parts. 
     A prior-art example related to power storage systems is Japanese Patent Application Laid-Open Publication No. 2012-244843 (Patent Document 1). Patent Document 1 discloses a configuration of providing terminals for replacing storage batteries in a storage battery system in which a plurality of storage batteries are connected serially and in parallel. 
     RELATED ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2012-244843 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the prior-art power storage system, when arranged to monitor storage batteries such as lithium-ion storage batteries as described above, operations of the system, that is, charging/discharging functions are stopped in units of storage battery groups including the corresponding storage battery/storage batteries in accordance with signals of detection of states such as an abnormal state or a warning state due to failures or degradation of the storage batteries. Maintenance works including preventive maintenance by a maintenance person are performed in a state in which operations in units of storage battery groups are stopped. Therefore, the stopped states in units of storage battery groups continue until malfunctions of the storage batteries are eliminated by the maintenance works. The storage battery groups cannot be utilized for charging/discharging during the stop time. Such a stop time in the storage battery groups occurs also for storage batteries in a normal state other than the storage batteries in an abnormal state. 
     When the stop time of operation in units of storage battery groups of the power storage system becomes long as described above, a problem in that the convenience for users is lowered is caused. Prior-art power storage systems including Patent Document 1 are not provided with means for coping with stop times of storage battery groups based on monitoring of the storage batteries described above, and there is room for improvement with respect to convenience. 
     It is an object of the present invention to provide a technology capable of reducing a stop time of operation in units of storage battery groups during maintenance works including preventive maintenance of storage battery facilities and enhancing an operation rate of the entire system with respect to the above-described power storage system. 
     Means for Solving the Problems 
     Representative embodiments of the present invention are a power storage system and a maintenance method including preventive maintenance of the power storage system, and they are characterized in that they include configurations as indicated below. 
     A power storage system according to one embodiment includes one or more relay panels connected to one or more power control devices; a plurality of storage battery panels connected to the relay panels and equipped with one or more storage batteries; and one or more maintenance devices connected to the relay panels when performing maintenance works including preventive maintenance by a maintenance person, in which the storage battery panel transmits a first signal in which based on monitoring of a state of the storage battery, the state thereof is detected or determined to the superordinate relay panel, the relay panel transmits a second signal based on the first signal to the superordinate power control device, the maintenance device transmits, based on an operation of designating a storage battery panel to be maintained by the maintenance person, a third signal to the superordinate relay panel of the storage battery panel to be maintained, and the relay panel masks and invalidates the first signal from the storage battery panel to be maintained or the second signal based on the first signal in accordance with the third signal. 
     A maintenance method of the power storage system according to one embodiment is a maintenance method including preventive maintenance of a power storage system which has one or more relay panels connected to one or more power control devices, a plurality of storage battery panels connected to the relay panel and equipped with one or more storage batteries, and one or more maintenance devices connected to the relay panel when performing maintenance works including preventative maintenance by a maintenance person, the maintenance method of the power storage system including: a first step of transmitting, based on an operation by the maintenance person, a masking start instructing signal from the maintenance device to the superordinate relay panel of a storage battery panel to be maintained and masking and invalidating, in accordance with the masking start instructing signal, a first signal transmitted from the storage battery panel to be maintained to the superordinate relay panel or a second signal transmitted from the relay panel to the superordinate power control device based on the first signal by the relay panel; a second step of performing, based on an operation by the maintenance person, maintenance works for the storage battery panel to be maintained in a state in which the storage battery panel to be maintained is electrically disconnected and electrically connecting the storage battery panel after the maintenance; and a third step of transmitting, based on an operation by the maintenance person, a masking cancel instructing signal from the maintenance device to the superordinate relay panel of the storage battery panel to be maintained and canceling the state of masking the first signal or the second signal and validating the first signal or the second signal in accordance with the masking cancel instructing signal by the relay panel. 
     A maintenance method of the power storage system according to one embodiment is a maintenance method including preventive maintenance of a power storage system which has one or more relay panels connected to one or more power control devices, a plurality of storage battery panels connected to the relay panel and equipped with one or more storage batteries, and one or more maintenance devices connected to the relay panel when performing maintenance works including preventive maintenance by a maintenance person, the maintenance method of the power storage system including: a first step of transmitting a first signal in which based on monitoring of a state of the storage battery, the state thereof is detected or determined to the superordinate relay panel by the storage battery panel; a second step of transmitting a second signal based on the first signal to the superordinate power control device by the relay panel; a third step of stopping an operation in units of a subordinate relay panel and the plurality of storage battery panels connected to the relay panel by the power control device based on the second signal; a fourth step of transmitting, based on an operation by the maintenance person, a masking start instructing signal from the maintenance device to the superordinate relay panel of the storage battery panel to be maintained and masking and invalidating, in accordance with the masking start instructing signal, a first signal transmitted from the storage battery panel to be maintained to the superordinate relay panel or a second signal transmitted from the relay panel to a superordinate power control device based on the first signal by the relay panel; a fifth step of resuming, based on an operation by the maintenance person, operation in units of the subordinate relay panel and the plurality of storage battery panels by the power control device in a state in which the storage battery panel to be maintained is electrically disconnected; a sixth step of performing, based on an operation by the maintenance person, maintenance works for the storage battery panel to be maintained and electrically connecting the storage battery panel after the maintenance; and a seventh step of transmitting, based on an operation by the maintenance person, a masking cancel instructing signal from the maintenance device to the superordinate relay panel of the storage battery panel to be maintained and canceling the state of masking the first signal or the second signal and validating the first signal or the second signal in accordance with the masking cancel instructing signal by the relay panel. 
     EFFECTS OF THE INVENTION 
     According to representative embodiments of the present invention, it is possible to reduce a stop time of operation in units of storage battery groups in performing maintenance works including preventive maintenance of storage battery facilities and enhance an operation rate of the entire system. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a view showing a configuration of a power storage system according to one embodiment of the present invention; 
         FIG. 2  is a view showing a functional block configuration of the power storage system according to the present embodiment; 
         FIG. 3  is a view showing a configuration in units of relay panels according to the present embodiment; 
         FIG. 4  is a view showing a configuration of a supervising BCU, a BCU and a storage battery monitoring unit; 
         FIG. 5  is a view showing a first process flow in performing maintenance works using the power storage system and the maintenance method according to the present embodiment; 
         FIG. 6  is a view showing a second process flow in performing maintenance works using the power storage system and the maintenance method according to the present embodiment; 
         FIG. 7  is a view showing a display example of a maintenance device of the power storage system according to the present embodiment; and 
         FIG. 8  is a view showing an example of signals of the power storage system according to the present embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment, and the repetitive description thereof will be omitted. 
     In the description, maintenance includes preventive maintenance in a broad sense. That is, the maintenance methods according to the embodiments include preventive maintenance methods. 
     Premise 
     Problems and details of a prior-art power storage system and maintenance method as premises will be additionally described hereinafter. A prior-art power storage method transmits signals indicating the states such as an abnormal state or a warning state in performing maintenance works in a storage battery facility by a maintenance person, for example, when storage batteries are replaced due to a failure of a single storage battery, as a mechanism of monitoring storage batteries. Accordingly, the prior-art power storage system once stops operating the system, that is, charging/discharging functions in units of storage battery groups including the corresponding storage batteries. This is due to a mechanism of controlling power in units of storage battery groups. For ensuring soundness, it is necessary to maintain states of voltages or SOCs of the plurality of storage batteries to be identical as much as possible. 
     The maintenance person performs maintenance works in a state in which the storage battery groups are stopped. The maintenance person confirms a storage battery in a state with some malfunction such as a failure or degradation and performs diagnosis, examination or replacement of storage batteries or peripheral equipment as necessary. The cause of malfunction is eliminated by the maintenance works and after the normal state is confirmed, the power storage system resumes operation in units of storage battery groups. The stopped state of the storage battery groups continues until the cause of malfunction is eliminated. During the stop time, even though most storage batteries among the plurality of storage batteries are in a normal state, they cannot be utilized for charging/discharging. 
     When the stop time in units of storage battery groups becomes long, the user might feel inconvenient. For example, when it might take a long time to examine malfunctions of the storage batteries, the stop time becomes long. Moreover, particularly when the power storage system is applied to infrastructure directly linked to individual lives, it is preferable to shorten the stop time as much as possible and to utilize storage batteries in a normal state as much as possible. It is desirable to perform maintenance works in a state in which storage battery groups are operated as much as possible while ensuring safety in performing maintenance works by the maintenance person. 
     Note that the storage battery system of Patent Document 1 cannot cope with the problem of stop time when considering an application to a configuration of monitoring storage batteries. This system detects states such as an abnormal state or a warning state due to interruption of communication during monitoring of storage batteries when storage batteries using terminals for replacing are replaced. Accordingly, this system will stop operating the portion including the corresponding storage batteries to be replaced. That is, the stop time of the storage battery groups will become long in performing maintenance works. 
     EMBODIMENTS 
     The power storage system of the present embodiment shown in  FIG. 1  and the like enables maintenance works while storage battery groups are in an operating state in performing maintenance works including preventive maintenance of the storage battery facility and it realizes reductions in the stop time of the storage battery groups. Therefore, the power storage system of the present embodiment provides a masking function of masking a signal of monitoring of the storage batteries based on operations in the maintenance device by the maintenance person as shown in  FIG. 2  and the like. The maintenance method of the power storage system of the present embodiment includes a step of performing maintenance works based on operations in the maintenance device by the maintenance person by the masking function which regards the storage battery groups of the power storage system as in an operating state, as shown in  FIGS. 5 and 6 . 
     In the present embodiment, objects of maintenance works are storage battery panels. In performing masking, a signal is transmitted from the maintenance device to a relay panel for instructing masking. Upon reception of the signal, the relay panel masks and invalidates the signal based on monitoring of storage batteries from a storage battery panel to be maintained. With this arrangement, a superordinate power control device can be operated in units of storage battery groups including the storage battery panel to be maintained. Then, the maintenance person performs electrical disconnection in units of storage battery panel to be maintained, so that the maintenance person can perform maintenance works in a state in which the storage battery group except for the storage battery panel to be maintained is operating. The power storage system of the present embodiment is characterized in that it not only enables the above operations but also enables disconnection of a stopped portion in the power storage system, irrespective of forms of superordinate power control devices. 
     Power Storage System ( 1 ) 
       FIG. 1  shows an overall configuration of the power storage system of the present embodiment. The power storage system of the present embodiment includes a plurality of power control devices  1 , a plurality of relay panels  2 , a plurality of storage battery panels  3  and a maintenance device  5 . The power storage system of  FIG. 1  includes PCS 1  to PCSN 1  which are N 1  number of power control devices  1 , R 1  to RN 2  which are N 2  number of relay panels  2 , and L 1  to LN 3  which are N 3  number of storage battery panels  3 . One power control device  1  connects N 4  number of relay panels  2 . For example, the case in which N 4  is 3 is shown. Further, N 2  is N 1 ×N 4 . One relay panel  2  connects N 5  number of storage battery panels  3 . N 3  is N 2 ×N 5 . For example, the case in which N 5  is 10 is shown. In the present embodiment, N 2  number of relay panels  2  and N 3  number of storage battery panels  3  are all defined as objects and ranges to which the masking function in performing maintenance works are applicable. 
     Reference numeral  101  denotes a PCS unit in which one power control device  1 , a plurality of relay panels  2  and a plurality of storage battery panels  3  connected thereto form a group, wherein there are N 1  number thereof ranging from C 1  to CN 1 . Reference numeral  102  denotes a relay panel unit in which one relay panel  2  and a plurality of storage battery panels  3  connected thereto form a group, wherein there are N 2  number thereof corresponding to R 1  to RN 2 . Reference numeral  103  denotes a storage battery panel unit in which a plurality of storage battery packs  4  ( FIG. 2 ) constituted by one storage battery panel  3  form a group, wherein there are N 3  number thereof corresponding to L 1  to LN 3 . 
     The overall power storage system of  FIG. 1  is constituted by connecting from C 1  which is the first PCS unit  101  connected to PCS 1  to CN 1  which is the N 1 th PCS unit  101  connected to PCSN 1 . Reference characters C 1  to CN 1  which are PCS units  101  are of identical configuration. Reference characters PCS 1  to PCSN 1  are of identical configuration, and it is possible to consider that a single power control device  1  is constituted by connecting or uniting them. Outputs of a single power control device  1  are connected in parallel depending on a necessary amount of power. Reference characters PCS 1  to PCSN 1  respectively supply power corresponding to a sum of power of each PCS unit  101 . The power is designed in accordance with power required in a load facility to which they are connected, for example. 
     In a single PCS unit  101 , one power control device  1  is connected to (N 4 =3) number of relay panels  2  through a main circuit wire  6 . For example, reference characters R 1  to R 3  which are three relay panels  2  are connected to PCS 1 . In a single relay panel  2 , (N 5 =10) number of storage battery panels  3  are connected in parallel. For example, reference characters L 1  to L 10  which are 10 storage battery panels  3  are connected to R 1  in parallel. A single storage battery panel  3  is equipped with a plurality of storage battery packs  4  ( FIGS. 2 and 3 ). For example, reference characters b 1  to bN 6  which are N 6  numbers of storage battery packs  4  ( FIG. 3 ) are serially connected to L 1 . A total of N 3  (=N 2 ×N 5 ) number of storage battery panels  3  are connected in parallel in a single PCS unit  101 . 
     The power control device  1  is a PCS (power conditioning system) and performs control of the power storage system and has a function of controlling charge/discharge of the storage battery groups connected thereto through the relay panels  2 . The PCS is connected to a power system, a load facility, a photovoltaic power generating system, and the like. The PCS charges the storage batteries with power from the power system, the photovoltaic power generating system, or the like and supplies power obtained by discharging from the storage batteries to the load of the load facility. When states of the storage batteries such as an abnormal state or a warning state are detected by a signal from a subordinate relay panel  2 , the power control device  1  generates an alarm and stops operating the PCS unit  101  which is the subordinate system portion. 
     Specifically, the PCS includes a DC/DC converter, a DC/AC inverter, and the like as a function of converting direct current (DC) or alternating current (AC) power for stably adjusting and utilizing power of the storage batteries or photovoltaic generation. 
     Note that, in the power storage system of the present embodiment, the power control devices  1  do not need to be equipped with specific functions, and it is also possible to use hardware and software of conventional PCS. Stopping operations of storage batteries and the like in the present embodiment means stopping charging/discharging functions, and in the stopped state, charge of the storage batteries and discharge from the storage batteries do not work. 
     The maintenance device  5  is connected to the relay panels  2  by the maintenance person U when maintenance works are required. The maintenance device  5  is constituted in the form of a personal computer and the like. The maintenance device  5  may also be constituted by an exclusive device or circuit. Reference numeral  113  denotes L 2  which is an example of storage battery panel  3  to be maintained. Reference numeral  112  denotes R 1  which is an example of a relay panel  2  to be connected as a superordinate of the storage battery panel  3  to be maintained. For example, the maintenance device  5  is connected to R 1  in performing maintenance works of, for example, L 2 , as an object to be maintained. 
     The connection of the maintenance device  5  and the relay panel  2  (R 1 ) of  FIG. 1  is one example, and it is possible to connect any one of the relay panels  2 . Also, by providing a plurality of maintenance devices  5 , different maintenance devices  5  may be connected to some of the relay panels  2 , respectively. Moreover, a single maintenance device  5  may be connected to some of the relay panels  2 . Further, a plurality of maintenance persons may use the respective maintenance devices  5 . 
     Also, in the power storage system of  FIG. 1  and the like, connections between the components may be direct connections or indirect connections in which other elements are interposed therebetween. For example, the connection between the maintenance device  5  and the relay panel  2  may be an indirect connection in which a communication network, a communication device, or the like are interposed therebetween, so that remote operations, that is, remote maintenance works can be performed. 
     In the power storage system of  FIG. 1  and the like, elements such as the power control device  1  and the relay panel  2  are provided in plural number; however, the present invention is not limited to this, and a configuration employing a single number of each device is also possible. For example, it might be a system using a single relay panel  2 . It is also possible that the storage battery panel  3  is equipped with a single storage battery. It is further possible to provide a power storage system without the relay panel  2 ; in this case, a plurality of storage battery panels  3  are connected to the power control device  1 . 
     Power Storage System ( 2 ) 
       FIG. 2  shows connection of the devices of the respective components of the power storage system of  FIG. 1  and a configuration of functional blocks. The power control device  1  includes a control unit  11 , a communication unit  12 , and a power connecting unit  13 . The control unit  11  includes a CPU, a memory, and the like and performs control processes of the power control device  1 . The communication unit  12  performs communication processes with a communication unit of a supervising BCU  21  of the relay panel  2 . The power connecting unit  13  is a portion connecting a plurality of terminals of a parallel connecting unit  22  of the relay panel  2  in a group and including terminals for inputting/outputting storage battery voltages. 
     The relay panel  2  includes the supervising BCU  21 , the parallel connecting unit  22 , and a connecting unit  23 . The supervising BCU  21  includes a CPU, a memory, and the like and is a component of a storage battery control unit in a broad sense to be described later. The supervising BCU  21  is communicated and connected to the control unit  11  of the superordinate power control device  1  and BCUs  31  of the respective subordinate storage battery panels  3 . The parallel connecting unit  22  is an interface unit for connecting a plurality of storage battery panels  3  (for example, L 1  to L 10 ) in parallel the relay panel  2 . The parallel connecting unit  22  includes terminals for connecting main circuit wires of the respective storage battery panels  3 . The connecting unit  23  is an interface unit for connecting the maintenance device  5  to the relay panel  2  and includes a terminal for connecting, for example, a cable. The relay panel  2  is provided with functions of connecting, monitoring, calculating SOCs, and the like of the plurality of storage battery panels  3 . 
     The storage battery panel  3  is equipped with a plurality of BCUs  31 , a plurality of storage battery monitoring units  32  and a storage battery block  40 . The BCU  31  includes a CPU, a memory, and the like and is a component of the storage battery control unit in a broad sense to be described later. The storage battery block  40  includes a plurality of serially connected storage battery packs  4 , disconnecting switches  41  on both ends, and a fuse  42  on one end. The storage battery panel  3  is provided with functions of connecting, monitoring, calculating SOCs, and the like of the storage battery packs  4 . 
     The relay panel  2  and storage battery panel  3  include a storage battery control unit (BCU: battery control unit) in a broad sense. In the present embodiment, the storage battery control unit is comprised of the supervising BCU  21  provided in the relay panel  2  and one or more BCUs  31  provided in the storage battery panel  3 . In other words, the BCUs  31  are subordinate BCUs, and the supervising BCU  21  is a superordinate BCU. The supervising BCU  21  communicates and connects a plurality of BCUs  31  to supervise and control the plurality of BCUs  31 . The supervising BCU  21  comprehensively determines the state of the storage battery pack  4  groups using signals from the plurality of BCUs  31 . The supervising BCU  21  and the BCUs  31  are provided with implementations corresponding to the masking function. The BCUs  31  and the BCU  21  are implemented by, for example, circuit boards. 
     The BCUs  31  each are of identical configuration, and representative one of the BCUs  31  is connected to the supervising BCU  21  of the superordinate relay panel  2 , for example. The plurality of storage battery monitoring units  32  are connected to the BCU  31 . The plurality of storage battery monitoring units  32  function as monitoring units of the storage battery block  40 . One storage battery monitoring unit  32  monitors and detects states such as voltage, current, and temperature of one or more storage batteries pack  4  in the storage battery block  40 . The storage battery monitoring units  32  each are of identical configuration. Note that it is also possible to employ a form in which the storage battery monitoring units  32  are integrated into the BCU  31 . The BCU  31  use the plurality of storage battery monitoring units  32  to continuously monitor states of equipment and circuits such as the plurality of storage battery packs  4  in the storage battery block  40  in the storage battery panel  3  or the peripheral power sources thereof. 
     The maintenance device  5  includes a control unit  51 , a communication unit  52 , a connecting unit  53 , and a screen unit  54 . The control unit  51  performs control processes of the maintenance device  5 . The communication unit  52  performs communicating processes with the relay panel  2  and the power control device  1  connected through the connecting unit  53 . The connecting unit  53  is an interface unit for connection in the case in which the maintenance device  5  is connected to the relay panel  2  and the like. For example, the connecting unit  23  of the relay panel  2  and the connecting unit  53  of the maintenance device  5  are connected via a cable for performing communication. The screen unit  54  configures and provides a screen serving as a user interface when the maintenance person U performs maintenance works based on the control processes of the control unit  51 . 
     The maintenance device  5  performs processes corresponding to support functions including the masking function in performing maintenance works and provides a screen corresponding to these functions and shown in  FIG. 7  to be described later by the screen unit  54 . The maintenance person can input instructions of starting or cancelling masking using the screen. Accordingly, a masking signal M is transmitted from the maintenance device  5  to the relay panel  2 . 
     Reference character F 1  denotes a flag signal which is transmitted from, for example, L 1  or L 2  which are storage battery panels  3  to, for example, reference character R 1  which is a superordinate relay panel  2  based on monitoring. Reference character F 2  denotes a flag signal which is transmitted from R 1  which is the relay panel  2  to, for example, reference character PCS 1  which is a superordinate power control device  1 . When a mask signal M is received from the maintenance device  5 , the relay panel  2  performs a process of masking the corresponding flag signal to exclude a storage battery panel  3  to be maintained and designated by the mask signal M from an object to be monitored. In the present embodiment, R 1  which is a relay panel  2  transmits a flag signal F 2  to the PCS 1  which is a superordinate power control device  1  in a state in which it has been determined that the flag signal F 1  from storage battery panel as an object to be masked is invalid. Accordingly, the superordinate power control device  1  is set to be a state in which operation of the PCS unit  101  including the object storage battery panel  3  is allowed to be resumed from a state in which operation thereof is not allowed to be stopped or is allowed to be stopped once. 
     Power Storage System ( 3 ) 
       FIG. 3  shows a configuration of connecting a plurality of storage battery panels  3  and main circuit wires in a relay panel unit  102 . A plurality of storage battery panels  3  are connected in parallel to, for example, R 1 , which is a relay panel  2 , through switches, main circuit wires  301  of positive electrode and main circuit wires  302  of negative electrode. Within a single storage battery block  40 , there are serially connected a plurality of storage battery cells ( FIG. 4 ) of N 6  number of storage battery packs  4  indicated as b 1  to bN 6 . One storage battery pack  4  includes one or more storage battery cells  7 . All of the storage battery cells  7  are lithium-ion storage batteries in the present embodiment. Note that, in  FIG. 3 , the configuration within the storage battery panel  3  is simplified with only the storage battery block  40  illustrated. In the configuration example of  FIG. 3 , the plurality of storage battery cells  7  connected to the relay panel  2  are connected such that N 6  number of storage battery packs  4  are connected in series and (N 5 =10) number of storage battery panels  3  are connected in parallel, wherein there are (N 5 ×N 6 ) number of storage battery cells in total. 
     Disconnecting switches  41  are provided on both ends of the storage battery blocks  40 . One disconnecting switch  41  on one end of the storage battery blocks  40  is provided with the fuse  42 . Based on control of the BCU  31 , ON/OFF states of the disconnecting switches  41  are switched in accordance with functions of charge/discharge and storage battery protection. Also, the disconnecting switches  41  are switched ON/OFF based on control of the BCU  31  in attaching and detaching storage battery blocks  40 . The fuse  42  is a protection circuit and is automatically cut off when the voltage difference between the plurality of storage battery packs  4  is large and there is an overload state, for example. Note that it is possible to omit the fuse  42  in some cases and the disconnecting switch  41  may have a function corresponding to that of the fuse  42 . 
     In the storage battery blocks  40  in the plurality of storage battery panels  3  connected in parallel to the relay panel  2 , when a failure occurs in one storage battery block  40 , it is possible to perform operation in a degraded state using the remaining ones of the plurality of storage battery blocks  40  connected in parallel. When the storage battery pack  4  indicated by, for example,  310 , which is one storage battery pack  4  in a storage battery block  40  fails and a replacement work in units of the storage battery block  40  is necessary, a flag signal F 1  indicating an abnormal state or the like is transmitted to the superordinate relay panel  2  based on monitoring of this storage battery panel  3 . 
     It is necessary that values of the voltage and SOC (state of charge: referred to as charged state or charging rate) of the storage battery packs  4  connected in parallel to the relay panel  2  and in the plurality of storage battery blocks  40  are uniform at predetermined values. That is, it is necessary that differences in potential and SOC fall within predetermined ranges. When they are not uniform, currents exceeding prescribed values might flow in replacing storage battery blocks  40  and the like, thereby not being safe. 
     Failure Case Example 
     In  FIG. 1  and the like, examples in which malfunctions such as a failure of storage battery facilities and abnormal and warning states corresponding thereto occur according to the prior-art power storage system are as follows. Note that it is assumed that the prior-art power storage system includes storage battery facilities similar to those of  FIG. 1 . Further, it is assumed that in the example in which a failure occurs, one storage battery pack  4 , for example,  310  in  FIG. 3 , of L 2  which is, for example, a storage battery panel in  FIG. 1  fails. It is assumed that it is necessary to maintain one storage battery block  40  including the storage battery pack  4  of  310  in L 2 . 
     In this case, in the prior-art, power storage system, a flag signal F 1  indicating an abnormal state or the like due to the above failure is transmitted from the L 2  which is the storage battery panel  3  to R 1  which is a superordinate relay panel  2  by the function of monitoring the storage batteries, and a similar flag signal F 2  is transmitted from R 1  to PCS 1  which is a superordinate power control device  1 . Based on this flag signal F 2 , the PCS 1  outputs an alarm and further stops operation in units of the PCS  101  of C 1  including R 1  which includes L 2 . In the stopped state of the PCS unit  101  of C 1 , all of the 30 storage battery panels  3 , L 1  to L 30 , become a state in which charge/discharge functions are prohibited. Until the malfunction of the storage battery panel  3  which is L 2  including the faulty storage battery pack  4  is eliminated by maintenance works performed by a maintenance person, the stopped state of the PCS unit  101  of C 1  continues. 
     According to the prior-art power storage system and the maintenance method thereof, the power control device can control operation and stop in units of PCSs but it cannot control charge/discharge in units of individual relay panels or in units of individual storage battery panels of storage battery groups in units of PCSs since they are connected in parallel. Supposing that in a storage battery group of a PCS unit, one storage battery panel is in a state in which charge/discharge is stopped while another storage battery panel is in a state in which charge/discharge is in operation, a difference in SOC occurs in the storage battery group due to the differences in charge/discharge. Accordingly, overcharge, overdischarge, or the like may occur, so that it is impossible to ensure soundness. Therefore, the prior-art power storage system controls operation and stop in units of PCSs for ensuring soundness. The prior-art power storage system stops operation in units of relay panels including these storage battery panels and in units of PCSs including these relay panels, even in a case in which a single storage battery panel fails. 
     Masking Function 
     In contrast, the power storage system of the present embodiment utilizes the masking function based on an operation from the maintenance device by a maintenance person in order to enable maintenance works in a state in which PCS units  101  of the storage battery pack  4  groups are operating. Accordingly, it is possible to instruct masking to temporally exclude a storage battery panel  3  to be maintained from an object to be monitored in the storage battery packs  4 . In other words, it is possible to temporally mask and invalidate signals indicating states such as an abnormal state, a warning state, or the like which are transmitted from the storage battery panels  3  to the power control device  1  via the relay panels  2  by monitoring the storage battery packs  4 . 
     Since the superordinate power control device  1  does not receive flag signals F 2  indicating states such as an abnormal state, a warning state, or the like by executing the masking, storage battery pack  4  groups of PCS units  101  including storage battery panels  3  to be maintained will be in states in which they are not stopped, that is, in which they are operating. Masking allows the power control device  1  to see the subordinate relay panels  2  and the like being in a normal state. It is possible to perform maintenance works such as replacement of the storage battery blocks  40  by the maintenance person in a condition in which PCS units  101  are operating. When signals indicating states such as a warning state are being issued, it is possible to arbitrarily execute masking in accordance with judgments and confirmations of the states by the maintenance person. 
     Flag Signals and Mask Signals 
       FIG. 4  shows a configuration of the supervising BCU  21  of the relay panels  2 , the BCU  31  and the storage battery monitoring unit  32  of the storage battery panel  3  and a configuration example of flag signals and masking signals. 
     The storage battery monitoring unit  32  includes a control unit  321 , a calculating unit  322 , a measuring unit  323  and a communication unit  324 . The measuring unit  323  measures the voltage, current, temperature, and the like of the storage battery cells  7 . The calculating unit  322  calculates SOC values and the like of the storage battery cells  7  using the measured values measured by the measuring unit  323 . The communication unit  324  performs communication processes with the BCU  31 . The control unit  321  refers to the measured values, makes determinations related to the states of the storage battery cells  7 , and transmits a detection signal  401  indicating the states detected through the determination to the BCU 1  through the communication unit  324 . It is also possible to use detailed code signals to be described later as the detection signals  401 . 
     The BCU  31  includes a control unit  311 , a calculating unit  312 , and a communication unit  313 . The control unit  311  makes determinations related to the states of the storage battery cells  7  using the detection signals  401  from the storage battery monitoring unit  32  and transmits flag signals F 1  indicating states such as a normal state, an abnormal state, a warning state obtained through the determination to the supervising BCU  21  of the superordinate relay panel  2  via the communication unit  313 . The calculating unit  312  calculates SOC values and the like of the storage battery blocks  40  using the measured values transmitted from the storage battery monitoring unit  32 . The communication unit  313  performs communication processes with the supervising BCU  21  and the storage battery monitoring unit  32 . 
     States of the flag signals F 1  and F 2  are three types such as a normal state, a warning state, or an abnormal state in the present embodiment. Note that it is possible to use detailed code signals to be described later as the flag signals. 
     The supervising BCU  21  of the relay panel  2  receives a flag signal F 1  from the subordinate storage battery panel  3 . The control unit  211  determines the state of the subordinate storage battery panel  3  using the flag signal F 1  and transmits a flag signal F 2  based on the determination to the superordinate power control device  1  via the communication unit  213 . The calculating unit  212  calculates SOC values and the like in units of storage battery pack  4  groups of the plurality of storage battery panels  3  using information from the storage battery panel  3  and the flag signal F 1 . Note that the supervising BCU  21  may also transmit the flag signal F 1  as it is from the subordinate storage battery panel  3  to the superordinate power control device  1  as the flag signal F 2 . 
     In accordance with operations by the maintenance person, a masking signal M is transmitted from the maintenance device  5  to the supervising BCU  21  of the superordinate relay panel  2  to which the storage battery panel  3  to be maintained is connected. The supervising BCU  21  of the relay panel  2  receives the masking signal from the maintenance device  5 . There are a masking start instructing signal M 1  and a masking cancel instructing signal M 2  as the masking signals M. The masking start instructing signal M 1  is an instructing signal for starting masking and the masking cancel instructing signal M 2  is an instructing signal for canceling masking. When the masking start instructing signal M 1  is received, the supervising BCU  21  starts invalidation by the masking of the flag signal F 2  based on the flag signal F 1  so as to exclude the storage battery panel  3  to be designated by the signal M 1  from an object to be monitored. When the masking cancel instructing signal M 2  is received, the supervising BCU  21  cancels the masking state of the flag signal F 2  and validates the flag signal. 
     In the present embodiment, invalidation by the masking is assumed that the flag signal F 2  itself is not transmitted to the power control device  1  in a case in which the content of the original flag signal F 2  indicates the state such as an abnormal state or a warning state. In another embodiment, invalidation by the masking may be assumed that the state such as an abnormal state or a warning state which is the content of the flag signal F 2  is changed to a normal state to be transmitted to the power control device  1 . In still another embodiment, invalidation by the masking may be assumed that the state such as an abnormal state or a warning state, which is the content of the flag signal F 2 , is changed to a state indicating that specific masking is being performed to be transmitted to the power control device  1 . Note that, in such a case, the power control device  1  needs to be implemented with a different function than usual, so that it can recognize states of masking indicated by the flag signals F 2 . 
     Upon reception of the flag signal F 2  from the supervising BCU  21  of the subordinate relay panel  2 , the control unit  11  of the power control device  1  stops operations of charge/discharge functions of the PCS unit  101  including the subordinate relay panel  2  when the state indicated by the flag signal F 2  is an abnormal state or a warning state. When flag signal F 2  indicating an abnormal state or the like has not been received or when a flag signal F 2  indicating a normal state has been received, the control unit  11  of the power control device  1  operates charge/discharge functions in the PCS unit  101  including the subordinate relay panel  2 . 
     Process Flow ( 1 ) When Performing Maintenance 
       FIG. 5  shows a process flow when maintenance checkup of a storage battery panel unit  103  is performed as an example of maintenance works including preventive maintenance by a maintenance person, as a first process flow in performing maintenance works using the power storage system and the maintenance method of the first embodiment. Reference character S 1  and the like indicate steps of processes and works. Preventive maintenance as used herein indicates maintenance checkup when stopping of the system and the like has been planned in advance.
         (S 1 ) In step S 1 , the maintenance device  5  is connected to a supervising BCU  21  of a superordinate relay panel  2  (For example, R 1 ) of a storage battery panel  3  (For example, L 2 ) to be maintained through preventive maintenance by a maintenance person U in a state in which the power storage system is in operation. The above-described connecting unit  53  and the connecting unit  23  are connected. The maintenance device  5  displays a screen for maintenance works by the screen unit  54 . The screen includes items for utilizing the masking function as in  FIG. 7 .   (S 2 ) In step S 2 , the maintenance person U uses the screen of the maintenance device  5  to designate the storage battery panel  3  (L 2 ) to be maintained through preventive maintenance and to input a masking start instruction for the storage battery panel (L 2 ). Based on this input operation, the control unit  51  of the maintenance device  5  transmits a masking start instructing signal M 1  for the storage battery panel  3  (L 2 ) to be maintained through preventive maintenance to the supervising BCU  21  of the relay panel  2  (R 1 ).       

     Upon reception of the masking start instructing signal M 1 , the supervising BCU  21  of the relay panel  2  (R 1 ) masks and invalidates the flag signal F 2  for the storage battery panel  3  (L 2 ) to be maintained through preventive maintenance and designated by the signal. By starting the masking, the storage battery panel (L 2 ) to be maintained through preventive maintenance becomes a state in which it is excluded from the object to be monitored when seen from the power control device  1 , and an operating state of the relay panel unit including the storage battery panel  3  (L 2 ) and the PCS unit are maintained.
         (S 3 ) In step S 3 , prior to maintenance works in step S 4 , the storage battery panel  3  (L 2 ) to be maintained through preventive maintenance is disconnected from the main circuit wires  301 ,  302  through manual operation by the maintenance person U by, for example, switching off (OFF) the disconnecting switch  41  corresponding to a breaker. Note that the maintenance person may also detach the BCU  31 , depending on contents of maintenance. Accordingly, the storage battery panel  3  (L 2 ) to be maintained through preventive maintenance is in a state in which it is electrically disconnected from the power storage system, so that operations can be safely performed. In a prior-art power storage system, since a signal indicating states such as an abnormal state or a warning state is transmitted or communication completely interrupted by disconnecting the storage battery panel, operation of a portion including this storage battery panel is stopped based on determination of a superordinate power control device. In the present embodiment, due to the masking function, operation of the portion is not stopped, so that it is possible to perform maintenance works while storage battery panels  3  other than the storage battery panel  3  to be maintained are in an operating state.   (S 4 ) In step S 4 , the maintenance person U performs works of maintenance checkup related to the storage battery panel  3  (L 2 ) to be maintained through preventive maintenance as the maintenance work. As a result of maintenance checkup, for example, the storage battery block  40  including the storage battery pack  4  is replaced. The maintenance person detaches the object storage battery block  40  and attaches a storage battery block  40  after replacement in a state in which the breaker of the storage battery panel  3  is OFF.   (S 5 ) In step S 5 , the maintenance person U confirms the total voltage of the superordinate relay panel  2  (R 1 ) of the storage battery panel  3  (L 2 ) to be maintained through preventive maintenance after the maintenance work. This total voltage is a total voltage of the relay panel unit and is a voltage obtained by summing voltages of all of the plurality of storage battery panels  3  connected to the relay panel  2 . This total voltage is also referred to as parallel restoration voltage. The total voltage can be confirmed by connecting an exclusive measurement device to the relay panel  2  for confirming measured values such as potential differences or SOC differences. If the relay panel  2  includes a function of measuring total voltages, it is also possible to use such means. Alternatively, if the maintenance device  5  includes a function of measuring total voltages, it is also possible to use it. Confirmation of the total voltage corresponds to confirming that the potential differences and SOC differences in the storage battery pack  4  groups are small, thereby ensuring soundness.       

     The maintenance person confirms whether the total voltage or the relay panel unit is within a prescribed range, for example, within ±3.0 V. After confirming that the value is within this range, the maintenance person performs operations of connecting and restoring the storage battery panel  3  (L 2 ) to be maintained to the main circuit wires  301 ,  302  of the relay panel  2  (R 1 ) by switching ON the disconnecting switch  41  corresponding to the breaker. If the value is not within this range, the maintenance person performs charge/discharge using a known electronic load device and the like to adjust the voltage to fall within this range.
         (S 6 ) In step S 6 , the maintenance person U inputs an instruction of canceling the masking of the maintained storage battery panel  3  (L 2 ) as an object of the instruction on the screen of the maintenance device  5 . Based on the input operation, the maintenance device  5  transmits a masking cancel instructing signal M 2  which regards the maintained storage battery panel  3  (L 2 ) as an object thereof to the supervising BCU  21  of the relay panel  2 . Upon reception of the masking cancel instructing signal M 2 , the supervising BCU  21  of the relay panel  2  cancels the masking state of the object storage battery panel  3  (L 2 ) which has been started in step S 2  to validate the flag signal F 2  to the superordinate power control device  1 .   (S 7 ) In step S 7 , the maintenance person U detaches the maintenance device  5  from the relay panel  2 . By performing the above steps, it is possible to perform works of maintenance checkup as maintenance works performed in an operating state with a portion thereof being disconnected and to perform preventive maintenance in a state in which storage battery groups are not stopped.       

     Process Flow ( 2 ) When Performing Maintenance 
       FIG. 6  shows a process flow when coping with a failure in a case in which a failure occurs in a storage battery panel unit  103  as an example of maintenance works by a maintenance person, as a second process flow in performing maintenance works using the power storage system and the maintenance method of the first embodiment. Maintenance as used herein indicates that a state such as an abnormal state of the system and the like is returned to a normal state through recovery. In  FIG. 6 , steps indicated by solid line frames are substantially same to the steps of  FIG. 5 , and steps indicated broken line frames are portions different from the steps of  FIG. 5 .
         (S 11 ) In step S 11 , the BCU 3  of a storage battery panel  3  (For example, L 2 ) detects and determines states such as an abnormal state or a warning state of storage battery cells  7  based on a detection signal  401  of monitoring from the storage battery monitoring unit  32 . For example, when a voltage of a storage battery cell  7  exceeds a prescribed value, the BCU  31  determines that there is an abnormal state due to a failure of the storage battery cell  7 .   (S 12 ) In step S 12 , the BCU  31  of the storage battery panel  3  (L 2 ) transmits a flag signal F 1  indicating an abnormal state in accordance with the detection in step S 11  to a supervising BCU  21  of a superordinate relay panel  2  (R 1 ).   (S 13 ) In step S 13 , upon reception of the flag signal F 1  from the storage battery panel  3  (L 2 ), the supervising BCU  21  of the relay panel  2  (R 1 ) determines a state of the relay panel unit including the storage battery panel  3  and transmits a flag signal F 2  indicating the determined state to a superordinate power control device  1  (PCS 1 ). For example, the supervising BCU  21  transmits a flag signal F 2  indicating an abnormal state similarly to the content of the flag signal F 1 .   (S 14 ) Is step S 14 , upon reception of the flag signal  32  from the relay panel  2  (R 1 ), the power control device  1  (PCS 1 ) generates an alarm in accordance with the abnormal state indicated by the flag signal F 2 . Along with that, the power control device  1  (PCS 1 ) once stops operation of the charge/discharge functions of the storage battery pack  4  group of a PCS unit  101  (C 1 ) including the storage battery panel  3  (L 2 ) in the abnormal state indicated by the flag signal F 2 . The maintenance person U confirms the necessity of maintenance by receiving the alarm.   (S 15 ) In step S 15 , the maintenance person U connects the maintenance device  5  to the supervising BCU  21  of the superordinate relay panel  2  (R 1 ) of the storage battery panel  3  (L 2 ) to be maintained in a state in which the power storage system is in operation, similarly to the above-described step S 1 .   (S 16 ) In step S 16 , the maintenance person U confirms information of the storage battery panel  3  (L 2 ) of which an abnormal state has been detected on the screen of the maintenance device  5 . The maintenance person inputs a masking start instruction for the storage battery panel  3  (L 2 ) as an object to be maintained on the screen. Accordingly, the maintenance device  5  transmits a corresponding masking start instructing signal M 1  to the supervising BCU  21  of the relay panel  2  (R 1 ). Upon reception of the masking start instructing signal M 1 , the supervising BCU  21  of the relay panel  2  (R 1 ) masks and invalidates the flag signal F 2  for the storage battery panel  3  (L 2 ) to be maintained.   (S 17 ) In step S 17 , the storage battery panel  3  (L 2 ) to be maintained is disconnected from the main circuit wires  301 ,  302  through manual operation by the maintenance person U.   (S 18 ) In step S 18 , since no flag signal F 2  indicating an abnormal state is received due to the masking in step S 16 , the superordinate power control device  1  (PCS 1 ) resumes operation of the PCS unit  101  (C 1 ) including the storage battery panel  3  (L 2 ) to be maintained. That is, the PCS unit  101  (C 1 ) changes from the stopped state in step S 14  to the operating state. Only the storage battery panel  3  (L 2 ) to be maintained from among the PCS unit  101  (C 1 ) is in the disconnected state in step S 17 .   (S 19 ) In step S 19 , the maintenance person U confirms the abnormal state due to a failure and performs maintenance works such as replacing storage battery packs  4  with respect to the storage battery panel  3  (L 2 ) to be maintained which is in the disconnected state in step S 17 .   (S 20 ) In step S 20 , the maintenance person U confirms the total voltage of the superordinate relay panel  2  (R 1 ) of the storage battery panel  3  (L 2 ) to be maintained. The maintenance person confirms whether the total voltage of the relay panel unit is within a prescribed range, for example, within ±3.0 V. After confirming, the maintenance person performs operations of connecting and restoring the storage battery panel  3  (L 2 ) to be maintained to the main circuit wires  301 ,  302  of the relay panel  2  (R 1 ).   (S 21 ) In step S 21 , the maintenance person U inputs an instruction of canceling the masking of the maintained storage battery panel  3  (L 2 ) as an object of the instruction on the screen of the maintenance device  5 . Accordingly, the maintenance device  5  transmits a corresponding masking cancel instructing signal M 2  to the supervising BCU  21  of the relay panel  2 . Upon reception of the masking cancel instructing signal M 2 , the supervising BCU  21  of the relay panel  2  validates the flag signal F 2  to the superordinate power control device  1  by canceling the masking state of the object storage battery panel  3  (L 2 ) which has been started in step S 16 .   (S 22 ) In step S 22 , the maintenance person U detaches the maintenance device  5  from the relay panel  2 . It is possible to perform maintenance works in a short stop time by performing the above steps.       

     As another example of maintenance works, maintenance works for peripheral equipment and circuits such as the power source other than the storage battery packs  4  provided in the storage battery panels  3  as objects to be maintained can be similarly performed. Further, while the above is an example in which detection and determination is performed as the “abnormal” state due to a failure, the masking function is similarly applicable also to cases of other states. For example, when detecting and determining interruption of communication through monitoring of the storage battery packs  4  as a “warning” state, it is possible to perform masking of the flag signal F 2  indicating warning thereby. 
     Note that, as a matter of course, in operation, top priority is given to ensuring safety when utilizing the masking function in performing maintenance works, and masking is executed upon confirmation and judgment of states by the maintenance person. For example, when it has been confirmed that effects of a storage battery panel  3  in an “abnormal” state are limited to be within the storage battery panel  3 , the maintenance person can perform masking of this storage battery panel  3  to be maintained. 
     Note that, depending on power storage systems, it is also possible to stop the entire function of monitoring the storage batteries. However, in a case, since the entire power storage system is in a stopped state, it is not desirable. By recording detection signals  401  by monitoring and related information thereof, the power storage system can perform reference or examination at a later time. 
     Screen Example 
       FIG. 7  shows a screen example of a masking function in performing maintenance works in the maintenance device  5 . The maintenance device  5  provides an interface for supporting maintenance works including the masking function. The maintenance person can easily utilize the masking function while confirming states of the power storage system on the screen of the maintenance device  5 . The maintenance person sets an instruction of masking a storage battery panel  3  to be maintained on the screen for the masking function. 
     On the screen of  FIG. 7 , the table  700  includes items of relay panel  701 , storage battery panel  702 , state  703  and masking  704 . The relay panel  701  displays a piece of identification information of the relay panels  2 . The storage battery panel  702  displays pieces of identification information of the storage battery panels  3 . The state  703  displays states of the storage battery panels  3  based on the above-described flag signals. The states include a normal state, a warning state, an abnormal state, and the like. It is also possible to use code information which is described later and is more detailed than the flag signals in the states  703 . The masking  704  displays check buttons for designating presence/absence of masking for each of the storage battery panels  3 . 
     By referring to the table  700 , the maintenance person can confirm states of the plurality of storage battery panels  3  for each of the relay panels  2 , and by turning on the check buttons on the item of the masking  704 , it is possible to designate masking of storage battery panels  3  to be maintained. 
     The maintenance person can instruct start of masking corresponding to the state of the masking  704  in the table  700  by pressing a masking start button indicated by  711 . Corresponding thereto, the above-described masking start instructing signal M 1  is transmitted. Further, by pressing the masking cancel button indicated by  712 , it is possible to instruct cancel of masking corresponding to the state of the masking in the table  700 . Corresponding thereto, the above-described masking start instructing signal M 2  is transmitted. 
     The screen of the masking function is not limited to the style of the table shown in  FIG. 7 , and it is also possible to employ style in which a connection configuration as that of  FIG. 1  is shown graphically wherein the maintenance person can designate an object storage battery panel therefrom. 
     Code Signals 
       FIG. 8  shows examples of code signals which is a configuration example related to signals issued based on monitoring of the storage batteries. A prior-art power storage system issues code signals illustrated in  FIG. 8  as signals by monitoring. Also in the power storage system of the present embodiment, not only the above-described flag signals, but also such code signals can be used. 
     In the table of  FIG. 8 , there are indicated, in the order from the left column, code  801 , item  802 , detected value  803 , restore  804  and type  805 . The code  801  is indicated here as a numeric value such as “01.” The item  802  indicates, for example, explanations of contents or states of the code  801 . The detected value  803  indicates that the code  801  is generated under the condition when the value of the detected value  803  is detected. The restore  804  indicates that restorations are made under the condition when values of  804  are satisfied. The type  805  indicates values in which a plurality of codes are roughly divided into several types wherein there are three types of abnormal, warning, and normal values corresponding to the above-described flag signals. The above-described flag signals F 1 , F 2  indicate an abnormal state, a warning state, or the like corresponding to the values of the types of codes. 
     Referring to some code examples, code “ 00 ” indicates a normal state, and no flags such as abnormal or warning are generated. Code “ 01 ” indicates “operational upper limit voltage” which is output when the voltage of storage batteries becomes 500 V or more, and the type is “warning.” Code “ 02 ” indicates “overcharge” which is output when the total voltage of a relay panel becomes 520 V or more, and the type is “abnormal.” Code “ 03 ” indicates “operational lower limit voltage” which is output when the voltage of storage batteries becomes 360 V or less, and the type is “warning.” Code “ 04 ” indicates “overdischarge” which is output when the total voltage of a relay panel becomes 300 V or less, and the type is “abnormal.” 
     Code “ 05 ” indicates “overcurrent” which is output when the voltage of storage batteries becomes 180 A or more, and the type is “warning.” Code “ 08 ” indicates “abnormal storage battery temperature” which is output when the temperature of a storage battery pack  4  becomes 60 degrees C. or more, and the type is “abnormal.” Code “ 09 ” indicates “communication interruption” which is output when communication between a BCU  31  and a supervising BCU  21  has been interrupted for two minutes or more, and the type is “abnormal.” Code “ 10 ” indicates “BCU failure” which is output when a BCU  31  fails, and the type is “abnormal.” 
     Codes indicating “abnormal” as the type, particularly “ 02 ,” “ 08 ,” and “ 10 ,” for example, are codes which require attention in that operation of these portions needs to be stopped immediately when these states are detected. In such a case, the maintenance person gives top priority to coping with these abnormalities, and utilization of the masking function should be judged carefully. 
     In cases of codes indicating “warning” as the type, for example, code “ 01 ,” when the warning state is detected, operation of the corresponding portion in accordance with judgments and confirmations of the state by the maintenance person may cause no problems in most cases, since the warning state is not so serious as the abnormal state. In this case, utilizing the above-described masking function to set the system in an operating state will reduce the stop time. 
     Effects and the Like 
     As explained so far, according to the power storage system and the maintenance method of the present embodiments, it is possible to reduce the time of stopping operation in units of storage battery groups in performing maintenance works of the storage battery facilities to thereby improve the operation rate of the entire system. Accordingly, convenience for users is improved. The present embodiments make it possible to perform maintenance works in a state in which storage battery groups other than storage batteries to be maintained are in operation by utilizing the masking function with respect to the time of stopping storage battery groups based on monitoring of the storage batteries. Specifically, it is possible to perform maintenance works in a state in which storage battery groups are in operation, in a large-sized power storage system which includes a large number of lithium ion rechargeable storage batteries and the function of monitoring thereof and is assumed to be used regularly. When a malfunction of the facility including storage batteries is generated, a maintenance person can utilize the masking function in accordance with a judgment of the state and degree of the malfunction. The maintenance person can easily utilize the masking function while confirming the state on a screen of the maintenance device. 
     In the foregoing, the invention made by the inventors of the present invention has been concretely described based on the embodiments. However, it is needless to say that the present invention is not limited to the foregoing embodiments and various modifications and alterations can be made within the scope of the present invention. Some modification examples of the present embodiment are as follows.
         (1) An embodiment may also be configured such that the relay panel  2  is implemented with the same function as the masking function of the maintenance device  5 . For example, a button for utilizing masking function is provided on an operation panel of the relay panels  2 . The maintenance person inputs masking instructions through the function of the relay panel  2 .   (2) As for the masking function of the above embodiment, flag signal F 2  transmitted from the supervising BCU  21  of the relay panel  2  to the power control device  1  is an object to be masked. In another embodiment of the masking function, flag signal F 1  from the BCU  31  of the storage battery panel  3  may also be an object to be masked. For example, a masking signal from the maintenance device  5  is transmitted to the BCU  31  of the storage battery panel  3  via the relay panel  2 . The BCU  31  of the storage battery panel  3  performs masking of the flag signal F 1  from the storage battery panel  3  to the relay panel  2  as an object in accordance with the masking signal, for example, a masking start instructing signal M 1 . Accordingly, the flag signal F 2  which is based on the flag signals F 1  is also masked.   (3) An embodiment may also be configured such that whether the masking function is applicable or not is set and controlled in accordance with detailed states of the storage battery panel  3 . The relay panel  2  detects or determines detailed states of the storage battery panel  3  using the above-described detection signals, flag signals or code signals. The relay panel  2  determines whether masking by masking signals is applicable or not in accordance with states of the storage battery panel  3  and executes masking in accordance with the determination. For example, in a case of a flag indicating a “warning” state or in a case of a specific code, the relay panel  2  determines that masking is applicable, and in a case of a flag indicating an “abnormal” state or in a case of a specific code, the relay panel  2  determines that masking is not applicable. A screen for setting to utilize this function is provided by the maintenance device  5 . The maintenance person sets in what kind of states and signals masking is applicable by using the screen. The above-described function makes it possible to perform conventional control and operation in specific states.   (4) The maintenance device  5  may display not only a normal state or an abnormal state as a state of the storage battery panel but also states of electric disconnection and connection corresponding to the above-described steps S 3  and S 5 , for example.       

     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a power storage system and preventive maintenance and a maintenance work thereof. 
     EXPLANATION OF REFERENCE CHARACTERS 
       1  . . . Power control device,  2  . . . Relay panel,  3  . . . Storage battery panel,  4  . . . Storage battery pack,  5  . . . Maintenance device,  6  . . . Main circuit wire,  7  . . . Storage battery cell,  11  . . . Control unit,  12  . . . Communication unit,  13  . . . Power connecting unit,  21  . . . Supervising BCU,  22  . . . Parallel connecting unit,  23  . . . Connecting unit,  32  . . . Storage battery monitoring unit,  40  . . . Storage battery block,  41  . . . Disconnecting switch,  42  . . . Fuse,  51  . . . Control unit,  52  . . . Communication unit,  53  . . . Connecting unit,  54  . . . Screen unit,  101  . . . PCS unit,  102  . . . Relay panel unit,  103  . . . Storage battery panel unit, F 1 , F 2  . . . Flag signal, M . . . Mask signal, M 1  . . . Masking start instructing signal, M 2  . . . Masking cancel instructing signal.