Patent Publication Number: US-2023163629-A1

Title: Microgrid control system, control method of microgrid, and intelligent electronic device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a microgrid control system, a control method of a microgrid, and an intelligent electronic device. 
     Priority is claimed on Japanese Patent Application No. 2021-189106, filed Nov. 22, 2021, the content of which is incorporated herein by reference. 
     Description of Related Art 
     Microgrids, which are small-scale power systems that supply electric power from distributed power supplies to power loads, are attracting attention from the viewpoints of using renewable energy, stable electric power supply, and the like. States in which a microgrid is operated include a grid connection mode in which the microgrid is operated in a state of being connected to a higher-ordered system and an island mode in which the microgrid is operated separately from the higher-ordered system. Published Japanese Translation No. 2020-526174 of the PCT International Publication discloses a synchronization method when a microgrid is mutually connected to a neighboring power transmission system. 
     SUMMARY OF THE INVENTION 
     However, in a microgrid, a fault current in the island mode is less than that in the gird connection mode, so although a fault current can be detected in each of the island mode alone and the gird connection mode alone, if a switching operation is performed therebetween, there is a problem that the detection of a fault current may not be possible due to a difference in individual fault currents. 
     The present disclosure was made in view of such circumstances, and an object thereof is to provide a microgrid control system, a control method of a microgrid, and an intelligent electronic device that can detect a fault current by a protection relay even when a switching operation is performed between an island mode and a grid connection mode. 
     The present disclosure was made to obtain the above-described object, and an aspect of the present disclosure is a microgrid control system including: a first intelligent electronic device to detect an open/closed state of a switch coupling a microgrid and a higher-ordered system; a protection relay to disconnect a consumer from the microgrid when detecting a fault current; and a second intelligent electronic device including protection relay-controlling circuitry to apply, to the protection relay, a set value according to at least the open/closed state of the switch, wherein the protection relay detects the fault current using the set value applied by the second intelligent electronic device. 
     Another aspect of the present disclosure is that in the microgrid control system described above, the second intelligent electronic device includes a communicator to receive a notification indicating the open/closed state from the first intelligent electronic device. 
     Another aspect of the present disclosure is that in the microgrid control system described above, the set value applied to the protection relay when the open/closed state is in a closed state is greater than that when the open/closed state is in an open state. 
     Another aspect of the present disclosure is that in the microgrid control system described above, the set value applied to the protection relay is a value according to a magnitude of a power supply source in the microgrid when the open/closed state is in an open state. 
     Another aspect of the present disclosure is a control method of a microgrid including: detecting an open/closed state of a switch coupling the microgrid and a higher-ordered system; applying a set value according to at least the open/closed state to a protection relay in the microgrid; and disconnecting, by the protection relay, a consumer from the microgrid when the protection relay detects a fault current using the applied set value. 
     Another aspect of the present disclosure is an intelligent electronic device including: protection relay-controlling circuitry to control a protection relay in a microgrid and to apply, to the protection relay, a set value according to at least an open/closed state of a switch coupling the microgrid and a higher-ordered system. 
     According to the present disclosure, it is possible to detect a fault current by a protection relay even when a switching operation is performed between an island mode and a grid connection mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic block diagram showing a configuration of a power transmission/distribution system  10  according to an embodiment of the present disclosure. 
         FIG.  2    is a schematic block diagram showing a configuration of an intelligent electronic device (hereinafter, referred to as “IED”)  20  according to the embodiment. 
         FIG.  3    is a schematic block diagram showing a configuration of a protection relay  30  according to the embodiment. 
         FIG.  4    is a schematic block diagram showing a configuration of a management device  250  according to the embodiment. 
         FIG.  5    is a table showing Content Example 1 of a set value determination table according to the embodiment. 
         FIG.  6    is a table showing Content Example 2 of the set value determination table according to the embodiment. 
         FIG.  7    is a flowchart showing operations of IEDs  235  and  243  according to the embodiment. 
         FIG.  8    is a sequence diagram showing an operation of a microgrid  200  according to the embodiment. 
         FIG.  9    is a schematic diagram showing an example of a display screen of the management device  250  according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.  FIG.  1    is a schematic block diagram showing a configuration of a power transmission/distribution system  10  according to an embodiment. The power transmission/distribution system  10  includes a higher-ordered system  100  and a microgrid  200 . The higher-ordered system  100  includes a power transmission system  110  and a large-scale power plant  120 . The power transmission system  110  transmits, to the microgrid  200 , electric power that the large-scale power plant  120  supplies. The large-scale power plant  120  is a power supply source that supplies electric power to the higher-ordered system  100  by using thermal power generation, hydroelectric power generation, nuclear power generation, or other power generation. Although  FIG.  1    shows only one large-scale power plant  120  as a power supply source included in the higher-ordered system  100 , the higher-ordered system  100  may include a plurality of power supply sources including small-scale sources. 
     The microgrid  200  includes a switch  210 , an intelligent electronic device (hereinafter, referred to as “IED”)  211  (first intelligent electronic device), a high-voltage system  220 , a small-scale power plant  221 , an IED  222 , a low-voltage system  230 , a storage battery  231 , an IED  232 , a consumer  233 , a protection relay  234 , an IED  235  (second intelligent electronic device), a distribution line  240 , a consumer  241 , a protection relay  242 , an IED  243  (second intelligent electronic device), and a management device  250 . The IEDs  211 ,  222 ,  232 ,  235  and  243  and the management device  250  are connected through a wired or wireless network (for example, an Ethernet (registered trademark)) (not shown) to be communicable and perform communication using, for example, a generic object oriented substation events (GOOSE) defined by an international electrotechnical commission (IEC) 61850. Each of the IEDs  211 ,  222 ,  232 ,  235  and  243  and the management device  250  is a structure that includes at least controlling circuitry or processing circuitry. In the embodiment, a microgrid control system  1 A is configured of at least the IED  211 , the IED  235 , and the protection relay  234  among these, and another microgrid control system  1 B is configured of at least the IED  211 , the IED  243 , and the protection relay  242 . 
     The switch  210  is installed at a position (for example, a substation) at which the higher-ordered system  100  and the microgrid  200  are coupled. When the switch  210  is in a closed state, the higher-ordered system  100  and the microgrid  200  are connected together, and when the switch  210  is in an open state, the higher-ordered system  100  and the microgrid  200  are disconnected from each other. The IED  211  is installed in the vicinity of the switch  210  and is connected to the switch  210  in a wired manner. When the open/closed state of the switch  210  changes, the IED  211  notifies other IEDs, which include the IED  235  and the IED  243 , and the management device  250  of the changed open/closed state. The IED  211  detects the open/closed state of the switch  210 . At the time the switch  210  is in a closed state, the microgrid  200  is in the grid connection mode in which the microgrid  200  can receive electric power from the higher-ordered system  100 . At the time the switch  210  is in an open state, the microgrid  200  is in the island mode in which the microgrid  200  does not receive electric power from the higher-ordered system  100 . Therefore, the open/closed state of the switch  210  notified by the IED  211  also indicates whether the microgrid  200  is in the grid connection mode or the island mode. 
     The high-voltage system  220  is a power transmission system having the highest voltage in the microgrid  200  and is connected to the higher-ordered system  100  through the switch  210 . The small-scale power plant  221  is a power supply source that supplies electric power to the microgrid  200  by using thermal power generation, hydroelectric power generation, solar power generation, wind power generation, or other power generation. The small-scale power plant  221  is connected to the high-voltage system  220 . The IED  222  is installed in the vicinity of the small-scale power plant  221  and is connected to the small-scale power plant  221  in a wired manner. The IED  222  notifies other IEDs, which include the IED  235  and the IED  243 , and the management device  250  whether or not the small-scale power plant  221  is supplying electric power. 
     The low-voltage system  230  is a power transmission/distribution system having a voltage lower than that of the high-voltage system  220  and is connected to the high-voltage system  220  through a substation facility (not shown). The storage battery  231  stores electric power from the microgrid  200  and supplies electric power to the microgrid  200 . The storage battery  231  may be a secondary battery such as a lithium-ion battery or may be a facility that stores electric power by storing hydrogen generated by electrolyzing water. The IED  232  is installed in the vicinity of the storage battery  231  and is connected to the storage battery  231  in a wired manner. The IED  232  notifies other IEDs, which include the IED  235  and the IED  243 , and the management device  250  whether or not the storage battery  231  is supplying electric power. 
     The consumer  233  is a consumer connected to the microgrid  200 . The consumer is a person who needs electricity to be supplied and receives and uses the supplied electricity. The consumer  233  is a relatively large consumer who receives electric power from the microgrid  200  at the voltage of the low-voltage system  230 . Therefore, the consumer  233  receives electric power at a higher voltage than the general consumer  241  that will be described later. The protection relay  234  is installed between the low-voltage system  230  and the consumer  233 . When the protection relay  234  detects a current exceeding an applied set value, the detected current is regarded as a fault current, and the protection relay  234  disconnects the consumer  233  from the microgrid  200 . The IED  235  is installed in the vicinity of the protection relay  234  and is connected to the protection relay  234  in a wired manner. The IED  235  controls the protection relay  234  of the consumer  233 . For example, the IED  235  applies a set value to the protection relay  234  on the basis of a notification from another IED and notifies the management device  250  of the applied set value. 
     The distribution line  240  is a distribution line from the low-voltage system  230  to the general consumer  241 . The distribution line  240  may include a pole transformer that steps down a voltage to the supply voltage for the consumer  241 . The consumer  241  is a consumer connected to the microgrid  200 . The protection relay  242  is installed between the distribution line  240  and the consumer  241 . When the protection relay  242  detects a current exceeding an applied set value, the protection relay  242  disconnects the consumer  241  from the microgrid  200 . The IED  243  is installed in the vicinity of the protection relay  242  and is connected to the protection relay  242  in a wired manner. The IED  243  controls the protection relay  242  of the consumer  241 . For example, the IED  243  applies a set value to the protection relay  242  on the basis of a notification from another IED and notifies the management device  250  of the applied set value. 
     The management device  250  receives notifications from the IEDs  211 ,  222 ,  232 ,  235  and  243  and notifies an operator of the state of the microgrid  200  through display of a screen or the like. Examples of the state of the microgrid  200  of which the operator is notified include a determination concerning whether the microgrid  200  is in the island mode or the grid connection mode, a power supply status of each of the power supply sources (small-scale power plant  221  and storage battery  231 ), and set values applied to the protection relays  234  and  242 . The management device  250  may be configured to inform the IEDs  235  and  243  of set values to be applied to the protection relays  234  and  242 , respectively, in accordance with operator operations. The management device  250  may be configured to apply set value determination tables that will be described later to the IEDs  235  and  243  in accordance with operator operations. 
     Although the microgrid  200  includes the high-voltage system  220  and the low-voltage system  230  in  FIG.  1   , the present disclosure is not limited thereto. The microgrid  200  may include more than two systems or may include only one system. Although the microgrid  200  includes the small-scale power plant  221  and the storage battery  231  as power supply sources, the present disclosure is not limited thereto. The microgrid  200  may include more than two power supply sources or may include only one power supply source. 
     Notifications to other IEDs by the IEDs  211 ,  222  and  232  may be performed using any one of broadcast, multicast, and unicast protocols. 
       FIG.  2    is a schematic block diagram showing a configuration of an IED  20  according to the embodiment. The IED  235  and the IED  243  have the same configuration as the IED  20 . The IED  20  includes a communicator  21 , protection relay-controlling circuitry  22 , a protection relay interface (hereinafter, referred to as “IF”)  23 , and a storage  24 . The communicator  21  communicates with other devices. For example, the communicator  21  receives a notification indicating the open/closed state of the switch  210  from the IED  211  that detects the open/closed state thereof. The communicator  21  notifies the management device  250  of the set value applied to the protection relay connected with the IED  20 . The protection relay-controlling circuitry  22  controls the protection relay connected with the IED  20 . The protection relay-controlling circuitry  22  determines a set value according to at least the open/closed state indicated by the above-described notification with reference to the set value determination table stored in the storage  24 . The protection relay-controlling circuitry  22  applies the determined set value to the protection relay connected with the IED  20  through the protection relay IF  23 . For example, when the switch  210  is in an open state, the protection relay-controlling circuitry  22  applies, to the protection relay, a set value less than that when the switch  210  is in a closed state. The storage  24  stores the set value determination table that will be described later. The protection relay IF  23  is a connection interface with the protection relay. The protection relay-controlling circuitry  22  of the IED  235  controls the protection relay  234 , and the protection relay-controlling circuitry  22  of the IED  243  controls the protection relay  242 . The protection relay IF  23  of the IED  235  is connected to the protection relay  234 , and the protection relay IF  23  of the IED  243  is connected to the protection relay  242 . 
     The protection relay-controlling circuitry  22  may determine a set value on the basis of the electric power supplied by the power supply source of the microgrid  200  when the switch  210  is in an open state. For example, the protection relay-controlling circuitry  22  may determine a set value to be applied to the protection relay using the set value determination table that stores a correspondence relationship between a set value and a combination of the open/closed state of the switch  210  and the electric power supplied by the power supply source of the microgrid  200 . The set value determined by the protection relay-controlling circuitry  22  when the switch  210  is in an open state may be decreased value as the electric power supplied by the power supply source of the microgrid  200  decreases. 
     For example, the communicator  21  may receive, from the IED  222 , a notification whether or not the small-scale power plant  221  is supplying electric power and receive, from the IED  232 , a notification whether or not the storage battery  231  is supplying electric power, and the protection relay-controlling circuitry  22  may determine the electric power supplied by the power supply source of the microgrid  200  on the basis of these notifications. For example, the storage  24  may store in advance the electric power supplied by each of the power supply sources of the microgrid  200 , and the protection relay-controlling circuitry  22  may add together the electric power supplied by power supply sources in which notifications indicating that these power supply sources are supplying electric power have been issued, among these, and may regard the added electric power as the total electric power supplied by the power supply sources of the microgrid  200 . 
     For example, the communicator  21  receives, from the IED  222 , a notification indicating the magnitude of the electric power supplied by the small-scale power plant  221  and receives, from the IED  232 , a notification indicating the magnitude of the electric power supplied by the storage battery  231 . If the storage battery  231  is in a state of storing electric power, the magnitude of the electric power supplied thereby may be shown as a negative value. The protection relay-controlling circuitry  22  may add together the magnitudes of the electric power indicated by these notifications and regard the obtained magnitude as the total magnitude of the electric power supplied by the power supply sources of the microgrid  200 . 
       FIG.  3    is a schematic block diagram showing a configuration of a protection relay  30  according to the embodiment. The protection relay  234  and the protection relay  242  have the same configuration as the protection relay  30 . The protection relay  30  includes an IED-IF  31 , controlling circuitry  32 , a storage  33 , a current measurer  34 , and a relay  35 . The IED-IF  31  is a connection interface with the IED  235 , the IED  243  and the like. For example, the applying of a set value to the protection relay  30  is performed through the IED-IF  31 . The controlling circuitry  32  controls the operation of the protection relay  30 . For example, the controlling circuitry  32  makes the storage  33  store the applied set value. When the current measured by the current measurer  34  exceeds the set value, the controlling circuitry  32  determines that a fault current is detected and turns off the relay  35 . The current measurer  34  measures the current that is being supplied to the consumer. When the relay  35  is turned on, electric power is supplied from the microgrid  200  to the consumer therethrough, and when the relay  35  is turned off, the relay  35  disconnects the consumer from the microgrid  200 . 
       FIG.  4    is a schematic block diagram showing a configuration of the management device  250  according to the embodiment. The management device  250  includes a communicator  251 , controlling circuitry  252 , a display  253 , and a storage  254 . The communicator  251  communicates with the IEDs  211 ,  222 ,  232 ,  235  and  243 . The controlling circuitry  252  receives notifications from the IEDs  211 ,  222 ,  232 ,  235  and  243  through the communicator  251  and generates a display image showing the state of the microgrid  200  on the basis of these notifications. The controlling circuitry  252  makes the storage  254  store the contents of the notifications from the IEDs  211 ,  222 ,  232 ,  235  and  243 . The contents of the notifications are stored in the storage  254  in this way, whereby the controlling circuitry  252  can ascertain how the contents of the notifications change. The display  253  includes an image display device such as a liquid crystal display and an organic electro luminescence (EL) display and displays a display image generated by the controlling circuitry  252  on the screen. 
       FIG.  5    is a table showing Content Example 1 of the set value determination table according to the embodiment. Content Example 1 is, for example, a content example of the set value determination table stored in the storage  24  of the IED  243 . In Content Example 1, a set value “A1 [A]” is associated with a combination of the open/closed state “closed state” of the switch  210  and electric power “-” of the power supply source of the microgrid  200 . The electric power “-” indicates that the set value does not depend on the magnitude of the electric power of the power supply source of the microgrid  200 . This is because a fault current in the closed state, that is, in the grid connection mode is large so that an influence rate of the power supply source of the microgrid  200  on the fault current is small. The symbol [A] indicates that the unit is ampere. Similarly, in Content Example 1, a set value “A2 [A]” is associated with a combination of the open/closed state “open state” of the switch  210  and electric power “X1 [kW]” of the power supply source of the microgrid  200 . The symbol [kW] indicates that the unit is kilowatts. 
     In Content Example 1, a set value “A3 [A]” is associated with a combination of the open/closed state “open state” of the switch  210  and electric power “X2 [kW]” of the power supply source of the microgrid  200 , and a set value “A4 [A]” is associated with a combination of the open/closed state “open state” of the switch  210  and electric power “X3 [kW]” of the power supply source of the microgrid  200 . X1&gt;X2&gt;X3 is satisfied and A1&gt;A2&gt;A3&gt;A4 is satisfied. That is, the set value in the open state (island mode) is less than that in the closed state (grid connection mode). In the open state, the set value decreases as the electric power of the power supply source decreases. Thereby, even if the magnitude of the fault current differs depending on the operation state of the power supply source, it is possible to detect a fault current by the protection relays  234  and  242 . 
     X1 may be set to the sum of the electric power supplied by the small-scale power plant  221  and the electric power supplied by the storage battery  231 . If the electric power supplied by the small-scale power plant  221  is greater than the electric power supplied by the storage battery  231 , X2 may be set to the electric power supplied by the small-scale power plant  221 , and X3 may be set to the electric power supplied by the storage battery  231 . On the other hand, the electric power supplied by the small-scale power plant  221  is less than the electric power supplied by the storage battery  231 , X2 may be set to the electric power supplied by the storage battery  231 , and X3 may be set to the electric power supplied by the small-scale power plant  221 . 
     Although the set value determination table has specific numerical values as the magnitudes of the power supply source associated with the set values in  FIG.  5   , the table may have ranges of numerical values associated with the set values. For example, in the set value determination table, the set value “A2 [A]” may be associated with a combination of the open/closed state “open state” and the power supply source “X1 [kW] or more,” the set value “A3 [A]” may be associated with a combination of the open/closed state “open state” and the power supply source “X2 [kW] or more and less than X1 [kW],” and the set value “A4 [A]” may be associated with a combination of the open/closed state “open state” and the power supply source “less than X2 [kW].” The same applies to  FIG.  6    that will be described later. 
     Although the set value determination table has specific numerical values as the magnitudes of the power supply source associated with the set values in  FIG.  5   , the table may have combinations of power supply sources, which are supplying electric power, associated with the set values. For example, in the set value determination table, the set value “A2 [A]” may be associated with a combination of the open/closed state “open state” and the power supply source “small-scale power plant  221  and storage battery  231 ,” the set value “A3 [A]” may be associated with a combination of the open/closed state “open state” and the power supply source “small-scale power plant  221 ,” and the set value “A4 [A]” may be associated with a combination of the open/closed state “open state” and the power supply source “storage battery  231 .” The same applies to  FIG.  6    that will be described later. 
       FIG.  6    is a table showing Content Example 2 of the set value determination table according to the embodiment. Content Example 2 is, for example, a content example of the set value determination table stored in the IED  235 . That is, Content Example 2 is a content example of the set value determination table in a case in which the consumer  233  receives electric power at a voltage higher than that of Content Example 1. In Content Example 2, a set value “B1 [A]” is associated with a combination of the open/closed state “closed state” of the switch  210  and the electric power “-” of the power supply source of the microgrid  200 . Similarly, a set value “B2 [A]” is associated with a combination of the open/closed state “open state” of the switch  210  and the electric power “X1 [kW]” of the power supply source of the microgrid  200 , a set value “B3 [A]” is associated with a combination of the open/closed state “open state” of the switch  210  and the electric power “X2 [kW]” of the power supply source of the microgrid  200 , and a set value “B4 [A]” is associated with a combination of the open/closed state “open state” of the switch  210  and the electric power “X3 [kW] of the power supply source of the microgrid  200 . 
     In Content Example 2, B1&gt;B2&gt;B3&gt;B4 is satisfied. That is, the set value in the open state (island mode) is less than that in the closed state (grid connection mode). In the open state, the set value decreases as the electric power of the power supply source decreases. A1&lt;B1, A2&lt;B2, A3&lt;B3, and A4&lt;B4 are satisfied. That is, the set value further increases when the electric power is received at a higher voltage. Thereby, even if the magnitude of the fault current differs depending on the consumer, it is possible to detect a fault current by the protection relays  234  and  242 . 
       FIG.  7    is a flowchart showing the operation of the protection relay-controlling circuitry  22  according to the embodiment. Hereinafter, the protection relay-controlling circuitry  22  will be described as the protection relay-controlling circuitry  22  of the IED  243 , which controls the protection relay  242 . In a case of the protection relay-controlling circuitry  22  of the IED  235 , which controls the protection relay  234 , the protection relay  242  in the following description is read as the protection relay  234 . 
     First, the protection relay-controlling circuitry  22  obtains the open/closed state of the switch  210  (Step ST 1 ). The open/closed state is an open/closed state indicated by a notification, which has been transmitted by the IED  211  and has been received by the communicator  21 . Subsequently, the protection relay-controlling circuitry  22  determines whether or not the obtained open/closed state indicates a closed state (Step ST 2 ). When it is determined that the open/closed state indicates the closed state (Step ST 2 -Yes), the protection relay-controlling circuitry  22  determines whether or not the open/closed state has changed (Step ST 3 ). For example, the protection relay-controlling circuitry  22  may make the storage  24  store the previously obtained open/closed state and may compare the stored previously obtained open/closed state with the open/closed state obtained at this time, thereby determining whether or not the open/closed state has changed. 
     When it is determined that the open/closed state has changed (Step ST 3 -Yes), the protection relay-controlling circuitry  22  applies the set value in the closed state to the protection relay  242  through the protection relay IF  23  with reference to the set value determination table (Step ST 4 ), and the process of the protection relay-controlling circuitry  22  returns to Step ST 1 . When it is determined in Step ST 3  that the open/closed state has not changed (Step ST 3 -No), the process of the protection relay-controlling circuitry  22  returns to Step ST 1  without doing anything. 
     When it is determined in Step ST 2  that the open/closed state does not indicate the closed state (indicates the open state) (Step ST 2 -No), the protection relay-controlling circuitry  22  obtains the magnitude of the power supply source of the microgrid  200  (Step ST 5 ). The magnitude of the power supply source is a value calculated by the protection relay-controlling circuitry  22  on the basis of notifications, which has been transmitted by the IED  222  and the IED  232  and has been received by the communicator  21 . 
     For example, when a notification transmitted by the IED  222  indicates that the small-scale power plant  221  is supplying electric power and a notification transmitted by the IED  232  also indicates that the storage battery  231  is supplying electric power, the protection relay-controlling circuitry  22  regards the sum of the electric power of the small-scale power plant  221  and the electric power of the storage battery  231  as the magnitude of the power supply source. When a notification transmitted by the IED  222  indicates that the small-scale power plant  221  is supplying electric power and a notification transmitted by the IED  232  indicates that the storage battery  231  is not supplying electric power, the protection relay-controlling circuitry  22  regards the electric power of the small-scale power plant  221  as the magnitude of the power supply source. When notifications transmitted by the IED  222  and the IED  232  indicate the electric power supplied by the small-scale power plant  221  and the electric power supplied by the storage battery  231 , respectively, the protection relay-controlling circuitry  22  may regard the total electric power indicated by these notifications as the magnitude of the power supply source. 
     Subsequently, the protection relay-controlling circuitry  22  determines whether or not the open/closed state or the magnitude of the power supply source has changed (Step ST 6 ). With regard to the open/closed state, for example, the protection relay-controlling circuitry  22  may store the previously obtained open/closed state in the storage  24  and may compare the stored previously obtained open/closed state with the open/closed state obtained at this time, thereby determining whether or not the open/closed state has changed. With regard to the magnitude of the power supply source, for example, the protection relay-controlling circuitry  22  may store the previously obtained magnitude of the power supply source in the storage  24  and may compare the stored previously obtained magnitude of the power supply source with the magnitude of the power supply source obtained at this time, thereby determining whether or not the magnitude of the power supply source has changed. 
     When it is determined that either the open/closed state or the magnitude of the power supply source has changed (Step ST 6 -Yes), the protection relay-controlling circuitry  22  applies the set value according to the magnitude of the power supply source with reference to the set value determination table to the protection relay  242  through the protection relay IF  23  (Step ST 7 ), and the process of the protection relay-controlling circuitry  22  returns to Step ST 1 . When it is determined in Step ST 6  that neither the open/closed state nor the magnitude of the power supply source has changed (Step ST 6 -No), the process of the protection relay-controlling circuitry  22  also returns to Step ST 1 . 
       FIG.  8    is a sequence diagram showing the operation of the microgrid  200  according to the embodiment. When the sequence shown in  FIG.  8    starts, it is assumed that the switch  210  is in a closed state and the small-scale power plant  221  and the storage battery  231  are not suppling electric power. When the small-scale power plant  221  is in operation and starts supplying electric power, the IED  222  issues a notification that the small-scale power plant  221  is supplying electric power (Sequence S 1 ). This notification is received by the IED  235  that controls the protection relay  234  and the IED  243  that controls the protection relay  242 . Since the switch  210  is in the closed state and the mode is in the grid connection mode, the IED  235  and the IED  243  do not change the set values for the protection relay  234  and the protection relay  242 , respectively. 
     Subsequently, when the switch  210  changes to the open state for some reason, the IED  211  issues a notification that the switch  210  is in the open state (Sequence S 2 ). This notification is received by the IED  232  of the storage battery  231 , the IED  235  that controls the protection relay  234 , and the IED  243  that controls the protection relay  242 . Since the switch  210  changes to the open state and the mode is in the island mode, the IED  235  and the IED  243  apply the set values according to the power supply source of the microgrid  200  to the protection relay  234  and the protection relay  242 , respectively. At this time, only the small-scale power plant  221  is supplying electric power, so the IED  235  and the IED  243  apply the set values according to the electric power of the small-scale power plant  221  to the protection relay  234  and the protection relay  242 , respectively. 
     The IED  232  receives the notification of Sequence S 2  and controls the storage battery  231  to start supplying electric power. The IED  232  issues a notification that the storage battery  231  is supplying electric power (Sequence S 3 ). This notification is received by the IED  235  that controls the protection relay  234  and the IED  243  that controls the protection relay  242 . Thereby, although the mode has not changed from the island mode, both of the small-scale power plant  221  and the storage battery  231  are set to supply electric power, so the IED  235  and the IED  243  apply the set values according to the total electric power of the small-scale power plant  221  and the storage battery  231  to the protection relay  234  and the protection relay  242 , respectively. 
     Subsequently, when the switch  210  returns to the closed state, the IED  211  issues a notification that the switch  210  is in the closed state (Sequence S 4 ). This notification is received by the IED  232  of the storage battery  231 , the IED  235  that controls the protection relay  234 , and the IED  243  that controls the protection relay  242 . Since the switch  210  changes to the closed state and the mode is in the grid connection mode, the IED  235  and the IED  243  apply the set values according to the closed state to the protection relay  234  and the protection relay  242 , respectively. 
     The IED  232  receives the notification of Sequence S 4  and controls the storage battery  231  to stop supplying electric power. The IED  232  issues a notification that the storage battery  231  stops supplying electric power (Sequence S 5 ). This notification is received by the IED  235  that controls the protection relay  234  and the IED  243  that controls the protection relay  242 . Thereby, although the magnitude of the power supply source changes, the open/closed state is maintained to the closed state, so the IED  235  and the IED  243  do not change the set values. 
       FIG.  9    is a schematic diagram showing an example of a display screen of the management device  250  according to the embodiment. The example of the display screen of  FIG.  9    is an example of the display screen in which the open/closed state of the switch  210  changes from the closed state to the open state at 10:02 on October 21. The management device  250  receives, from the IED  211 , a notification indicating that the switch  210  is in the open state and displays “[switch  210 ] closed→open (10/21 10:02)” near the icon of the IED  211 . The date and time in parentheses are the date and time when this notification has been received or the date and time when the notification has been issued. 
     The management device  250  displays information regarding the switch  210  whose open/closed state has been changed in the notification, the microgrid  200  coupled to the switch  210 , and the higher-ordered system  100 . The management device  250  displays an area in which the microgrid  200  is installed on a map. The management device  250  receives, from the IED  235 , a notification indicating that the set value is set to B2 and displays “[protection relay  234 ] set value: decrease (B1→B2) (10/21 10:02)” near the icon of the IED  235 . Similarly, the management device  250  receives, from the IED  243 , a notification indicating that the set value is set to A2 and displays “[protection relay  242 ] set value: decrease (A1→A2) (10/21 10:02)” near the icon of the IED  243 . 
     The management device  250  receives a notification indicating that the small-scale power plant  221  is supplying electric power from the IED  222  at 11:23 on October 19 and displays “[small-scale power plant  221 ] it is supplying electric power (10/19 11:23)” near the icon of the IED  222 . Similarly, the management device  250  receives a notification indicating that the storage battery  231  is supplying electric power from the IED  232  at 10:02 on October 21 and displays “[storage battery  231 ] it is supplying electric power (10/21 10:02)” near the icon of the IED  232 . When the displaying is performed in this way, an operator can easily ascertain the situations of the microgrid  200 , the protection relays  234  and  242 . 
     Although the set value is associated with the combination of the open/closed state and the power supply source in the set value determination table of the above-described embodiment, the set value may be associated with the open/closed state. For example, the association of the set value with the open/closed state is suitable when the magnitude of the power supply source does not change or is small in the island mode such as when the microgrid  200  has only one power supply source. 
     The microgrid control system of the above-described embodiment includes the IED  211  that detects the open/closed state of the switch coupling the microgrid  200  and the higher-ordered system  100 , and the IED  235  or the IED  243  that controls the protection relay  234  or the protection relay  242  in the microgrid  200 , and the IED  235  or the IED  243  includes the communicator  21  that receives a notification indicating the open/closed state from the IED  211 , and the protection relay-controlling circuitry  22  that applies a set value according to at least the open/closed state indicated by the notification to the protection relay  234  or the protection relay  242 . Thereby, the set value according to whether or not the microgrid is connected to the higher-ordered system is applied to the protection relay  234  or the protection relay  242 . Therefore, even when the mode is in the island mode, the protection relay  234  or the protection relay  242  can detect a fault current. Since the IEDs  235  and  243  autonomously set the set values, it is possible to easily add similar IEDs to the microgrid control system. 
     A program for realizing the function of the IED  20  shown in  FIG.  2    may be recorded on a non-transitory computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and be executed to realize the function of the IED  20 . The “computer system” described herein includes an OS and hardware such as peripheral devices. 
     The “non-transitory computer-readable recording medium” refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, or a DVD, or a storage device such as a hard disk or an SSD built in a computer system. The “non-transitory computer-readable recording medium” includes a medium that holds a program for a certain period of time such as a volatile memory inside a computer system. The above program may be for realizing part of the above-described functions and may be for realizing the above-described functions in combination with a program recorded in the computer system in advance. 
     Each functional block of the IED  20  shown in  FIG.  2    described above may be individually realized as a chip or part or all of them may be integrated into a chip. A method for making an integrated circuit is not limited to an LSI and may be realized by a dedicated circuit or a general-purpose processor. Either hybrid or monolithic may be used. Some of the functions may be realized by hardware and some of the functions may be realized by software. 
     When technologies such as integrated circuits that replace LSIs emerge due to advances in semiconductor technology, it is possible to use integrated circuits based on these technologies for the IEDs of the present disclosure. 
     Each of the controlling circuitry and the processing circuitry described in the above embodiment may include a memory storing instructions for realizing the above-described functions of respective circuitry, and a processor that executes the instructions. The storage described in the above embodiment includes a HDD, an SSD, a RAM, and a non-volatile memory such as a flash memory. 
     Hereinbefore, the embodiment of the present disclosure has been described in detail with reference to the drawings, but the specific configuration is not limited to this embodiment and includes design changes and the like within the scope of the present disclosure.