Short-circuit accident detection method for a microgrid system, a microgrid system, and a voltage source inverter

Abstract

To provide a method for detecting a short circuit fault in a microgrid system, a microgrid system, and a voltage source inverter that can identify the location of a short circuit fault.SOLUTION: In a method for detecting a short circuit fault in a microgrid system connected to an upper system and having a voltage source inverter and a supply system connected to the output side of the voltage source inverter, the supply system includes a circuit breaker provided in a plurality of electrical circuits in one or a plurality of distribution boards, and when the upper system is in a power outage and the microgrid is operating, at least one circuit breaker in the supply system is tripped by the output current at the time of soft start of the voltage source inverter to detect a short circuit fault.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a short - circuit accident detection method for a micro - grid system, a micro - grid system, and a voltage - source inverter.

Background Art

[0002] In recent years, micro - grids have attracted attention because of their advantages such as ensuring energy supply during disasters, improving energy utilization efficiency, and activating regional industries through regional energy utilization. A micro - grid grasps the power flow of the lower - level system during normal times and can be disconnected from the upper - level system during a large - scale power outage of the upper - level system due to disasters or the like, and can supply power independently using distributed energy resources.

[0003] Patent Document 1 discloses a micro - grid system including an inverter power source as a distributed energy resource, and the AC terminal of the inverter power source is connected to a power distribution system via a step - up transformer.

[0004] A plurality of switchboards are installed in the micro - grid system. Inside each switchboard, a plurality of electric circuits connected to a plurality of electrical loads are provided, and each electric circuit is provided with a circuit breaker that automatically shuts off the circuit when an accident (such as a short - circuit or overload) occurs in the electrical load or the like, so as to disconnect the accident location from the micro - grid system.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] When a short - circuit accident occurs in a micro - grid system powered by an inverter power supply, if the inverter power supply responds faster to over - current than the circuit breaker, the inverter power supply stops operating and stops the over - current before the circuit breaker shuts off the circuit. Therefore, there is a possibility that the short - circuit location cannot be identified because the short - circuit location is not disconnected by the circuit breaker.

[0007] The present invention has been made in view of such circumstances, and an object thereof is to provide a short - circuit accident detection method for a micro - grid system, a micro - grid system, and a voltage - source inverter that can identify a short - circuit accident location.

Means for Solving the Problem

[0008] This application includes a plurality of means for solving the above problems. For example, the short - circuit accident detection method of the micro - grid system according to the present embodiment is a short - circuit accident detection method for a micro - grid system including a voltage - source inverter connected to an upper - level system and a supply system connected to the output side of the voltage - source inverter. The supply system includes circuit breakers provided in a plurality of circuits of each of one or more switchboards. When the upper - level system experiences a power outage during micro - grid operation, at least one circuit breaker in the supply system is tripped by the output current during the soft - start of the voltage - source inverter to detect a short - circuit accident.

Effect of the Invention

[0009] According to the present invention, a short - circuit accident location can be identified.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Embodiments for Carrying Out the Invention

[0011] Hereinafter, the present invention will be described based on the drawings showing its embodiments. FIG. 1 is a diagram showing an example of the configuration of the microgrid system of this embodiment. The microgrid system is connected to the upper system 1 via the switch 2. The upper system is a transmission line, a distribution line, etc. owned by a general power transmission and distribution company or the like. In normal times, the switch 2 is in a closed state, and the microgrid system receives power supply through the upper system 1.

[0012] The microgrid system includes a voltage source inverter 10, a step-up transformer 51 that boosts the output of the voltage source inverter 10, a supply system 5 connected to the step-up transformer 51, an in-plant load 40, a step-down transformer 54 provided between the supply system 5 and the in-plant load 40, etc. The step-up transformer 51 can boost, for example, 210V to 6600V.

[0013] The supply system 5 is, for example, a 6.6 kV (high voltage) distribution system and includes a plurality of step-down transformers 4 and a plurality of switchboards (not shown) connected to the secondary side (low voltage side) of each step-down transformer 4. A plurality of electric circuits connected to a plurality of electrical loads are provided in each of the plurality of switchboards, and circuit breakers 6, 6,... that automatically cut off the electric circuit when an accident (such as a short circuit or overload) occurs in the electric circuit, such as an electrical load, are installed in each electric circuit. Also, a current measuring device 7 may be provided on the primary side (high voltage side) of the step-down transformer 4. The current measuring device 7 outputs, for example, the measured current value to an EMS (energy management system) (not shown). Further, the supply system 5 includes a distribution line 3 that connects between the step-down transformer 4 and the upper system 1, the step-up transformer 51, and the step-down transformer 54. An isolator 2a is connected to the downstream side of the distribution line 3, and it can be separated from the adjacent system on the downstream side of the isolator 2a. A circuit breaker 6 is connected to the step-down transformer 4.

[0014] Disconnect switches 65 and 66 are provided on the high voltage side and the low voltage side of the step-down transformer 54.

[0015] In the event of a power outage of the upper-level system 1 due to a disaster or the like, as a means of accident recovery, the switch 2 is switched from the closed state to the open state, and the microgrid system is disconnected from the upper-level system 1. Also, by switching the switch 2a from the closed state to the open state, the microgrid system can be disconnected from the adjacent system. Further, the opening switches 65 and 66 are always in the closed state. Then, in order to supply power to the supply system 5, the operation of the voltage source inverter 10 is started (so-called black start: operation start when no voltage is applied to the supply system 5). When the output voltage of the voltage source inverter 10 reaches the rated voltage (for example, about 200 V), a voltage of 6600 V is supplied to the distribution line 3 by the step-up transformer 51.

[0016] When the voltage source inverter is started during black start, an exciting current flows to excite the step-up transformer on the output side of the voltage source inverter. Depending on the situation, if the exciting current transiently becomes an excessive current and exceeds the upper limit value of the output current of the voltage source inverter, the current protection circuit of the voltage source inverter trips (overcurrent trip) and the voltage startup fails. Although the voltage startup of the voltage source inverter is repeated multiple times, there may be cases where an overcurrent trip occurs each time and the voltage source inverter cannot operate. In the present embodiment, soft start is executed when starting up the output voltage of the voltage source inverter during black start. Hereinafter, the soft start will be described.

[0017] FIG. 2 is a diagram showing a first example of the configuration of the voltage source inverter 10 of the present embodiment. The voltage source inverter 10 includes a switch 11 provided on the input terminal side, a coil 12 (inductor) connected to the switch 11, a DC / DC converter 13 connected to the coil 12, a DC / AC converter 15 connected to the DC / DC converter 13, a coil 16 connected to the DC / AC converter 15, a voltage regulating transformer 18 connected to the coil 16, a switch 19 provided between the voltage regulating transformer 18 and the output terminal, a controller 100 that detects the voltage on the secondary side of the voltage regulating transformer 18 and outputs a control signal to the DC / AC converter 15, a capacitor 14 connected in parallel to the circuit path between the DC / DC converter 13 and the DC / AC converter 15, and a capacitor 17 connected in parallel to the circuit path between the coil 16 and the voltage regulating transformer 18, and the like.

[0018] The LP gas stored in the gas bulk is supplied to the LP gas generator, and the DC power generated by the LP gas generator is supplied to the voltage source inverter 10. The voltage source inverter 10 converts the DC voltage supplied from the input terminal into a required DC voltage by the DC / DC converter 13. The converted DC voltage is input to the DC / AC converter 15. The DC / AC converter 15 converts the input DC voltage into an AC voltage, and inputs the converted AC voltage to the voltage regulating transformer 18 via an LC filter (coil 16 and capacitor 17). The voltage regulating transformer 18 adjusts the voltage so that the converted AC voltage becomes the rated voltage (for example, about 200 V) of the voltage source inverter 10. Note that the power source that supplies DC power to the voltage source inverter 10 is not limited to the LP gas generator, and other DC power sources may also be used.

[0019] The controller 100 executes soft start so that the output current does not exceed the upper limit value when the output voltage of the voltage source inverter 10 rises. Specifically, the controller 100 (voltage source inverter 10) executes soft start to maintain the output voltage at a predetermined voltage while raising the output voltage of the voltage source inverter 10 to the rated voltage during a power failure of the upper system 1.

[0020] FIG. 3 is a diagram showing an example of a method for controlling the output voltage of the voltage source inverter 10 in the first example during soft start. In FIG. 3, the vertical axis represents voltage (the effective value of the AC voltage), and the horizontal axis represents time. The starting point of operation is represented by 0. In FIG. 3, for the sake of convenience, the change in voltage is shown by a broken line, but the actual change in voltage is not limited to the example in FIG. 3. Also, the actual voltage waveform is a set of sine waves whose peaks of the effective value (crest value) change as shown in FIG. 3. The controller 100 controls the DC / AC converter 15 so that the output voltage increases from 0V to a predetermined voltage during the acceleration time Δt1 from the start of operation to the first time point t1. The controller 100 controls the DC / AC converter 15 so that the output voltage is maintained at the predetermined voltage during the standby period Δt2 from the first time point t1 to the second time point t2. The controller 100 controls the DC / AC converter 15 so that the output voltage increases from the predetermined voltage to the rated voltage during the acceleration time Δt3 from the second time point t2 to the third time point t3.

[0021] The predetermined voltage can be appropriately set within the range from 0V to the rated voltage. The acceleration time Δt1 can be appropriately set, for example, to a time between 0 seconds and 5 seconds, the standby time Δt2 can be appropriately set, for example, to a time between 0.1 seconds and 5 seconds, and the acceleration time Δt3 can be appropriately set, for example, to a time between 0.1 seconds and 5 seconds. The acceleration times Δt1, Δt3, the standby time Δt2, and the predetermined voltage can be set by the user in advance. As shown in FIG. 3, by maintaining the output voltage at the predetermined voltage while the output voltage is rising to the rated voltage, the sudden inrush current to the excitation of the step-up transformer 51 and the load equipment connected to the supply system 5 can be suppressed, and the voltage source inverter 10 (inverter power supply) can be started up without stopping during black start.

[0022] Also, since the rising speed of the output voltage during the acceleration times Δt1, Δt3 can be adjusted, the sudden inrush current to the excitation of the step-up transformer 51 and the load equipment connected to the supply system 5 can be suppressed, and the voltage source inverter 10 can be started up without stopping during black start.

[0023] In the first example shown in FIGS. 2 and 3, the configuration was to control the transition of the output voltage of the voltage source inverter 10. However, the soft start is not limited to the configuration of the first example. For example, not only the output voltage of the voltage source inverter 10 but also the output current may be detected to execute the soft start. Hereinafter, the soft start considering the output current will be described.

[0024] FIG. 4 is a diagram showing a second example of the configuration of the voltage source inverter 10 of the present embodiment. The difference from the first example shown in FIG. 2 is that the controller 100 detects not only the voltage on the secondary side of the voltage regulating transformer 18 but also the output current on the secondary side of the voltage regulating transformer 18 and outputs a control signal to the DC / AC converter 15. Since other configurations are the same as those of the first example, the description is omitted.

[0025] When a power failure occurs in the upper system 1, the controller 100 (voltage source inverter 10) increases the output voltage of the voltage source inverter 10 when the output current of the voltage source inverter 10 is below the current threshold, maintains the output voltage when the output current exceeds the current threshold, and after maintaining the output voltage, adjusts the output voltage to increase it to the rated voltage so that the output current becomes below the current threshold.

[0026] FIG. 5 is a diagram showing an example of a method for controlling the output voltage and output current of the voltage source inverter 10 in the second example during soft start. In FIG. 5, the vertical axis represents voltage (effective value of AC voltage) and current (effective value of AC current), and the horizontal axis represents time. The actual voltage waveform and current waveform are a set of sine waves in which the peak of the effective value (crest value) changes as shown in FIG. 5. The controller 100 raises the output voltage of the voltage source inverter 10 to the initial set value. The initial set value is the acceleration time t from when the output voltage reaches the rated voltage from 0V. The acceleration time t can be appropriately set in the range of, for example, 0 seconds to 5 seconds. The controller 100 detects the output voltage and output current together with the start of operation (start-up) of the voltage source inverter 10.

[0027] When the output current is less than or equal to the current threshold, the controller 100 increases the output voltage to the initial set value. When the output current exceeds the current threshold, the controller 100 maintains the voltage value of the output voltage at that time. Maintaining the output voltage at that voltage value corresponds to setting the slope of the output voltage (the increment of voltage with respect to time) to 0. After the controller 100 maintains the output voltage at that voltage value, it increases the output voltage to the rated voltage while adjusting the slope of the output voltage so that the output current is less than or equal to the current threshold.

[0028] FIG. 6 shows an example of a processing procedure for implementing the control method shown in FIG. 5 by the controller 100. The controller 100 starts soft start with the set acceleration time t (S11) and detects the output current of the voltage source inverter 10 (S12). Thereafter, the controller 100 detects the output voltage and output current at a predetermined sampling period.

[0029] The controller 100 determines whether the output current > the threshold value (current threshold value) (S13). If the output current is not > the threshold value (NO in S13), the controller 100 increases the output voltage to the rated voltage with the set acceleration time t (S14). The controller 100 determines whether the output voltage has reached the rated voltage (S15). If it has not reached the rated voltage (NO in S15), the controller 100 continues the processing after step S13. When the rated voltage is reached (YES in S15), the controller 100 ends the soft start assuming that the soft start is completed (S16) and ends the processing.

[0030] If the output current > the threshold value (YES in S13), the controller 100 sets the slope of the output voltage (the increment of voltage with respect to time) to 0 (S17) and determines whether the output current > the threshold value (current threshold value) (S18). If the output current is not > the threshold value (NO in S18), the controller 100 increases the slope of the output voltage (S19) and determines whether the output current > the threshold value (current threshold value) (S20).

[0031] When the output current is not greater than the threshold value (NO in S20), the controller 100 increases the output voltage to the rated voltage (S21), determines whether the output voltage has reached the rated voltage (S22), and if the rated voltage has not been reached (NO in S22), continues the processing after step S18. When the rated voltage has been reached (YES in S22), the controller 100 performs the processing of step S16.

[0032] In step S18 or step S20, when the output current is greater than the threshold value (YES in S18 or YES in S20), the controller 100 decreases the slope of the output voltage (S23) and determines whether the output current is greater than the threshold value (S24). When the output current is not greater than the threshold value (NO in S24), the controller 100 continues the processing after step S19. When the output current is greater than the threshold value (YES in S24), the controller 100 ends the soft start as unable to complete the soft start (S25) and ends the processing.

[0033] FIG. 7 is a diagram showing an example of the transition of the output voltage and output current of the voltage source inverter 10 of the present embodiment and the output voltage and output current of the voltage source inverter of the comparative example during black start. FIG. 7A shows the output voltage and output current of the voltage source inverter 10 of the present embodiment, and FIG. 7B shows the output voltage and output current of the voltage source inverter of the comparative example. The voltage source inverter of the comparative example does not have a soft start function as in the present embodiment. The actual voltage waveform and current waveform are a set of sine waves whose effective value (crest value) peaks transition as shown in FIG. 7.

[0034] As shown in FIG. 7A, in the voltage source inverter 10 of the present embodiment, since it has a soft start function, the output voltage reaches the rated voltage after passing through a soft start period as exemplified in FIG. 3, for example. Also, the output current is suppressed from transiently increasing sharply by the soft start, gradually increases with a relatively gentle rise time, and increases to the current required by the load without exceeding the upper limit value. Note that the soft start function is not limited to the example of FIG. 7A, and for example, the method shown in FIG. 5 may also be used.

[0035] On the other hand, as shown in FIG. 7B, in the voltage source inverter of the comparative example, since it does not have a soft start function, the output voltage increases constantly (compared with the case of having a soft start), and the output current also becomes a sudden inrush current due to the magnetization of the transformer, the capacitive component of the electrical load, etc., and an overcurrent trip occurs by exceeding the upper limit value, resulting in a failure to start up the voltage source inverter. If the voltage source inverter has a restart function, the startup is repeated, but there is a possibility that the startup fails due to the same phenomenon, resulting in a situation where the voltage source inverter cannot operate.

[0036] As described above, according to the present embodiment, it is possible to suppress the sudden inrush current to the magnetization of the boost transformer and the load equipment connected to the supply system, and start up the voltage source inverter 10 without stopping it during black start.

[0037] Next, the case where a short circuit accident occurs in the supply system 5 will be described.

[0038] FIG. 8 is a diagram showing a first example of a short circuit accident location in the supply system 5 of the microgrid system. In FIG. 8, the short circuit location is indicated by an X mark. In the example of FIG. 8, it is a short circuit accident on the secondary side (low voltage side) of the step-down transformer 4, for example, a short circuit between two lines of a single-phase three-wire system. The example of FIG. 8 illustrates a state where the voltage source inverter 10 is supplying power to the supply system 5.

[0039] FIG. 9 is a diagram showing an example of the operating characteristic curve of the circuit breaker 6. In FIG. 9, the vertical axis represents the operating time, and the horizontal axis represents the current. The operating characteristic curve of the circuit breaker 6 has the characteristics of both time-delay tripping and instantaneous tripping. The time-delay tripping is a characteristic that matches the allowable current-time characteristic of the electric wire and further prevents operation at the starting current of the load device. When the overcurrent is large, the operating time is short, and when it is small, the operating time is long. The instantaneous tripping is a characteristic that causes instantaneous operation when a large current such as a short-circuit current due to a short-circuit accident flows. In FIG. 9, it is the tripping in the current region indicated by "instantaneous tripping". Also, the time from the occurrence of the short circuit to the start of the tripping operation is called the relay time. The relay time is, for example, several ms (such as 2 ms to 5 ms). When a short-circuit current flows exceeding the relay time, the circuit breaker 6 instantaneously operates to cut off the circuit. That is, the response (instantaneous tripping operation time) of the circuit breaker 6 to the short-circuit current is on the order of several ms. For the sake of convenience, FIG. 9 shows the operating characteristic curve as a single curve, but actually there is a width, and the operating characteristic is represented in the region between two curves showing the minimum value and the maximum value.

[0040] FIG. 10 is a diagram showing an example of the responses of the circuit breaker 6 and the voltage source inverter 10 to a short-circuit accident. As shown in FIG. 10, assume that a short-circuit accident occurs on the downstream side of the circuit breaker 6 shown in FIG. 8 at time point t1. In this case, an excessive short-circuit current flows through the output of the voltage source inverter 10. When the output current of the voltage source inverter 10 exceeds the upper limit value, the voltage source inverter 10 stops operating for the purpose of protecting the voltage source inverter 10. The overcurrent operating time (response speed) for stopping the operation of the voltage source inverter 10 is on the order of approximately μs. In the example of FIG. 10, at time point t2 when a time on the order of μs has elapsed from the occurrence of the short circuit at time point t1, the overcurrent operating time occurs and the voltage source inverter 10 stops operating. Since the supply source that supplies the short-circuit current stops, the short-circuit current becomes 0 at time point t2.

[0041] On the one hand, as described above, the relay time (instantaneous tripping operation time) of the circuit breaker 6 is on the order of several milliseconds, which is longer than the overcurrent operation time. Therefore, the circuit breaker 6 does not respond to the short-circuit current, the circuit breaker 6 does not cut off the circuit, and the circuit remains closed. That is, when a short-circuit accident occurs in the supply system 5, the voltage source inverter 10 stops operating before being cut off by the circuit breaker 6, and the short-circuit current stops flowing.

[0042] The voltage source inverter 10 that has stopped operating executes soft start and resumes operation when the reclosing standby time (for example, 5 seconds, etc.) has elapsed from the stop of operation by the automatic reclosing system. In the present embodiment, the output characteristics of the voltage source inverter 10 at the time of soft start are utilized to cut off the circuit breaker 6 at the location where the short-circuit accident has occurred, and the short-circuit accident location is detected. Hereinafter, the short-circuit accident detection method will be described.

[0043] FIG. 11 shows an example of the output current characteristics of the voltage source inverter 10 during soft start in reclosing. In FIG. 11, the vertical axis represents current, and the horizontal axis represents time. As illustrated in FIG. 7, the output current of the voltage source inverter 10 during soft start gradually increases with a relatively gentle rise time and increases to the current required by the load that is connected without exceeding the upper limit value. However, in this case, since the circuit is not cut off by the circuit breaker 6, theoretically, the output current of the voltage source inverter 10 gradually increases with a relatively gentle rise time toward the short-circuit current value. The short-circuit current value can be estimated by voltage ÷ short-circuit resistance value (for example, 0.5 Ω, etc.). The upper limit value is the upper limit value of the output current at which the voltage source inverter 10 stops operating, and is set, for example, by rated current × coefficient. The coefficient can be, for example, 1.5, etc., but is not limited thereto.

[0044] As shown in Fig. 11, the output current of the voltage source inverter 10 gradually increases by soft start. However, when the instantaneous tripping current value corresponding to the instantaneous tripping characteristic of the circuit breaker 6 is reached before reaching the upper limit value, the circuit breaker 6 shuts off the circuit. That is, the circuit breaker 6 shuts off the circuit before the voltage source inverter 10. As a result, the short - circuit location is separated from the supply system 5, and the short - circuit current becomes zero. That is, the short - circuit accident location can be detected by the location of the circuit breaker 6 that has been tripped. After that, when the output voltage of the voltage source inverter 10 reaches the rated voltage, the soft start ends.

[0045] As described above, at least one circuit breaker 6 in the supply system 5 can be tripped by the output current during the soft start of the voltage source inverter 10 to detect a short - circuit accident. Thus, when a short - circuit accident occurs in the supply system 5 supplied with power by the inverter power supply, even if the inverter power supply stops operating and stops the short - circuit current before the circuit breaker shuts off the circuit, and the short - circuit location cannot be separated by the circuit breaker 6 and the short - circuit accident location cannot be identified, the short - circuit accident location can still be identified.

[0046] More specifically, in the short - circuit accident detection method of the micro - grid system of the present embodiment, the over - current operation time for stopping the operation of the voltage source inverter 10 is shorter than the instantaneous tripping operation time of the circuit breaker 6. After a short - circuit accident occurs in the supply system 5 and the operation of the voltage source inverter 10 stops, the operation of the voltage source inverter 10 is restarted by a soft start that suppresses the rise of the output voltage and output current of the voltage source inverter 10. Before the output current of the voltage source inverter 10 reaches the upper limit value, the output current characteristic by the soft start of the voltage source inverter 10 is brought within the instantaneous tripping characteristic range of the circuit breaker 6, and at least one circuit breaker in the supply system 5 can be tripped. Thereby, the short - circuit accident location can be identified.

[0047] Further, the voltage source inverter 10 includes a controller 100 (execution unit) that executes a soft start for maintaining the output voltage at a predetermined voltage while increasing the output voltage to the rated voltage. A short - circuit accident can be detected by tripping at least one circuit breaker 6 in the supply system 5 due to the output current during the soft start by the controller 100.

[0048] FIG. 12 is a diagram showing a second example of a short - circuit accident location in the supply system 5 of the microgrid system. In FIG. 12, the short - circuit location is indicated by an 'X' mark. In the example of FIG. 12, it is a short - circuit accident on the secondary side (low - voltage side) of the step - down transformer 4, for example, a short - circuit between two lines of a single - phase three - wire system. Also, the state where the voltage source inverter 10 is supplying power to the supply system 5 is illustrated. An ammeter 7 is provided on the primary side (high - voltage side) of the step - down transformer 4 upstream of the short - circuit location. The current value measured by the ammeter 7 is output to an EMS (Energy Management System). As shown in FIG. 11, the output current of the voltage source inverter 10 during the soft start in the re - connection system gradually increases with a relatively gentle rise time and increases to the current required by the connected load without exceeding the upper limit value. In this case, since the circuit breaker 6 has not tripped, theoretically, the output current of the voltage source inverter 10 gradually increases with a relatively gentle rise time towards the short - circuit current value. If the output current of the voltage source inverter 10 can be measured by the ammeter 7, it can be known that there is a short - circuit location downstream of the location where the ammeter 7 is provided. Therefore, even if the circuit breaker 6 does not trip, the approximate short - circuit location can be identified. Even if it takes time to measure with the ammeter 7, since the output current during the soft start increases gently, current measurement by the ammeter 7 is possible.

[0049] As described above, in the short - circuit accident detection method of the microgrid system, an ammeter 7 for measuring the current flowing through the distribution line in the supply system 5 may be provided, and a short - circuit accident may be detected based on the result of measuring the output current of the voltage source inverter 10 during the soft start with the ammeter 7.

[0050] As described above, according to the present embodiment, by using the soft start during the reclosing of the voltage source inverter 10, the output current during soft start gradually increases to trip the circuit breaker 6, so that the short - circuit accident location can be identified.

[0051] (Appendix 1) A method for detecting a short - circuit accident in a micro - grid system is a method for detecting a short - circuit accident in a micro - grid system including a voltage source inverter connected to an upper - level system and a supply system connected to the output side of the voltage source inverter. The supply system includes circuit breakers provided in a plurality of electric circuits of each of one or more switchboards. When the upper - level system experiences a power outage and the micro - grid is operating, at least one circuit breaker in the supply system is tripped by the output current during the soft start of the voltage source inverter to detect a short - circuit accident.

[0052] (Appendix 2) The method for detecting a short - circuit accident in a micro - grid system is, in Appendix 1, the over - current operation time for stopping the operation of the voltage source inverter is shorter than the instantaneous tripping operation time of the circuit breaker. After a short - circuit accident occurs in the supply system and the operation of the voltage source inverter stops, the operation of the voltage source inverter is restarted by a soft start that suppresses the rise of the output voltage and output current of the voltage source inverter. Before the output current of the voltage source inverter reaches the upper limit value, the output current characteristics during the soft start of the voltage source inverter are brought within the instantaneous tripping characteristic range of the circuit breaker to trip at least one circuit breaker in the supply system.

[0053] (Appendix 3) The method for detecting a short - circuit accident in a micro - grid system is, in Appendix 1 or Appendix 2, provided with a current measuring device for measuring the current flowing through the distribution line in the supply system, and a short - circuit accident is detected based on the result of measuring the output current during the soft start of the voltage source inverter by the current measuring device.

[0054] (Appendix 4) In the short - circuit accident detection method of the micro - grid system, in any one of Appendices 1 to 3, the voltage - source inverter executes a soft start to increase the output voltage of the voltage - source inverter to a predetermined voltage from the start of operation to the first time point.

[0055] (Appendix 5) A micro - grid system is a micro - grid system connected to a higher - level system and comprising a voltage - source inverter and a supply system connected to the output side of the voltage - source inverter. The supply system includes circuit breakers provided in a plurality of electric circuits of each of one or more switchboards. When the micro - grid is operating during a power outage of the higher - level system, at least one circuit breaker in the supply system is tripped by the output current during the soft start of the voltage - source inverter to detect a short - circuit accident.

[0056] (Appendix 6) A voltage - source inverter is a voltage - source inverter that is connected to a higher - level system and supplies power to a micro - grid system having a supply system. The voltage - source inverter includes an execution unit that executes a soft start to maintain the output voltage at a predetermined voltage while increasing the output voltage to the rated voltage. At least one circuit breaker in the supply system is tripped by the output current during the soft start by the execution unit to detect a short - circuit accident.

[0057] The matters described in each embodiment can be combined with each other. Also, the independent claims and dependent claims described in the claims can be combined with each other in all possible combinations regardless of the citation form. Further, the claims use a form (multiple - claim form) of describing a claim that cites two or more other claims, but it is not limited to this. A form of describing a multiple - claim (multi - multiple - claim) that cites at least one multiple - claim may be used.

Description of Reference Numerals

[0058] 1 Higher - level system 4 Step - down transformer 5 Supply system 6 Circuit breaker 7 Galvanometer 10 Voltage source inverter 51 Step-up transformer 54 Step-down transformer

Claims

1. A method for detecting a short - circuit accident in a micro - grid system including a voltage - source inverter connected to a higher - level system and a supply system connected to an output side of the voltage - source inverter, comprising: The supply system includes circuit breakers provided in a plurality of electric circuits of each of one or more switchboards; During micro - grid operation when the higher - level system has a power outage, at least one circuit breaker in the supply system is tripped by an output current during soft - start of the voltage - source inverter to detect a short - circuit accident. A method for detecting a short - circuit accident in a micro - grid system.

2. The over - current operation time for stopping the operation of the voltage - source inverter is shorter than the instantaneous tripping operation time of the circuit breaker; After a short - circuit accident occurs in the supply system and the operation of the voltage - source inverter stops, the operation of the voltage - source inverter is restarted by a soft - start that suppresses the rise of the output voltage and output current of the voltage - source inverter; Before the output current of the voltage - source inverter reaches the upper limit value, the output - current characteristic during soft - start of the voltage - source inverter is brought within the instantaneous - tripping characteristic range of the circuit breaker to trip at least one circuit breaker in the supply system. The method for detecting a short - circuit accident in a micro - grid system according to Claim 1.

3. Comprising an ammeter for measuring a current flowing through a distribution line in the supply system; A short - circuit accident is detected based on a result of measuring the output current during soft - start of the voltage - source inverter with the ammeter. The method for detecting a short - circuit accident in a micro - grid system according to Claim 1 or Claim 2.

4. The voltage - source inverter: Performs a soft - start that increases the output voltage of the voltage - source inverter to a predetermined voltage from the start of operation to a first time point. The method for detecting a short - circuit accident in a micro - grid system according to Claim 1 or Claim 2.

5. A micro - grid system including a voltage - source inverter connected to a higher - level system and a supply system connected to an output side of the voltage - source inverter, comprising: The supply system includes circuit breakers provided in a plurality of electric circuits of each of one or more switchboards; During micro - grid operation when the higher - level system has a power outage, at least one circuit breaker in the supply system is tripped by an output current during soft - start of the voltage - source inverter to detect a short - circuit accident. A micro - grid system.

6. A voltage source inverter that is connected to a higher-level system and supplies power to a microgrid system having a supply system, comprising an execution unit that executes a soft start for maintaining the output voltage at a predetermined voltage while raising the output voltage to the rated voltage, detecting a short-circuit accident by tripping at least one circuit breaker in the supply system by the output current during the soft start by the execution unit, the voltage source inverter.

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