Power System

Abstract

To prevent switching hunting of a changeover switch at an open power failure, in a power supply system that achieves both an uninterruptible power supply function and a load leveling function by using a common distributed power supply while satisfying FRT requirements.SOLUTION: A power supply system 100 provided between a commercial power system 10 and an important load 30 to supply a power to the important load, comprises: a distributed power supply 2; a changeover switch 3; an impedance element 4 connected in parallel to the changeover switch; a system side voltage detector 61; non-fundamental wave current injection means; and a non-fundamental wave voltage detector 82. In a case where a system voltage becomes lower than a first setting value, the changeover switch is opened to continuously operate the distributed power supply in a state where the distributed power supply and the commercial power system are connected with each other via the impedance element, and injection of a non-fundamental wave current by the non-fundamental wave current injection means is started. After the changeover switch is opened, in a case where the detected system voltage becomes equal to or more than the first setting value and a non-fundamental wave voltage becomes equal to or less than a second setting value, the changeover switch is closed again.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power supply system. [Background technology]

[0002] Conventionally, a power supply system that satisfies FRT requirements while simultaneously achieving both an uninterruptible power supply function and a load leveling function by using a common distributed power source has been proposed, as shown in Patent Document 1.

[0003] The power supply system of Patent Document 1 is configured by providing a changeover switch such as a semiconductor switch between a commercial power system and an important load, an impedance element connected in parallel to the changeover switch, and a distributed power supply on the important load side of the changeover switch. Also, a parallel-off switch is provided on the commercial power system side of the changeover switch. When an instantaneous voltage drop (hereinafter also referred to as a sag) occurs in the commercial power system, this power supply system opens the changeover switch to connect the commercial power system and the distributed power supply via the impedance element, and the distributed power supply continues operation including reverse power flow (FRT operation). On the other hand, when the commercial power system returns to a healthy state, the changeover switch is turned on to resume normal operation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6338131 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above-mentioned power supply system, during an open power outage of the commercial power system, depending on the system conditions, the increase in system voltage caused by the opening of the changeover switch may be mistaken for a recovery from a momentary sag (power restoration), and the changeover switch may be immediately turned on again. Then, immediately after the changeover switch is turned on again, the system voltage may drop, which may be mistaken for a momentary sag, and the changeover switch may be turned on again. Thereafter, switching hunting may occur, in which the changeover switch is repeatedly turned on and off, and this may have an adverse effect on important loads.

[0006] As examples of system conditions that cause this type of opening and closing hunting, there are two cases shown in FIG.

[0007] For example, if the impedance element connected in parallel to the changeover switch is a capacitor (C) or coil (L), the receiving point voltage V r This may cause hunting when the changeover switch is opened and closed (Case 1).

[0008] In addition, the voltage at the receiving point V due to an open power outage r After the changeover switch is opened due to the drop in V, the receiving point voltage V is increased by LC series resonance caused by the high reactance L of the receiving transformer reactance of other consumers with distributed power sources and the internal reactance of the generator, and the large-capacity SC on the line side. r will rise, which may cause hunting when the changeover switch is opened and closed (Case 2).

[0009] Therefore, the present invention has been made to solve the above problems, and its main objective is to prevent opening and closing hunting of the change-over switch during an open power outage in a power supply system that satisfies the FRT requirements and achieves both an uninterruptible power supply function and a load leveling function using a common distributed power source. [Means for solving the problem]

[0010] That is, the power supply system according to the present invention is provided between a commercial power system and an important load, and supplies power to the important load, and includes a distributed power source connected to a power line for supplying power from the commercial power system to the important load, a changeover switch provided on the power line closer to the commercial power system than the distributed power source and for opening and closing the power line, an impedance element connected in parallel to the changeover switch on the power line, a system side voltage detection unit that detects a system voltage on the commercial power system side of the changeover switch, non-fundamental current injection means that injects a non-fundamental current, which is a current having a frequency different from the fundamental frequency of the commercial power system, into the power line, and and a non-fundamental wave voltage detection unit that detects a non-fundamental wave voltage, which is the voltage of the non-fundamental wave current on the commercial power system side, and is configured to open the change-over switch when the detected system voltage falls below a predetermined first set value, continue operation of the distributed power source in a state in which the distributed power source and the commercial power system are connected via the impedance element, and start injection of the non-fundamental wave current by the non-fundamental wave current injection means, and, after opening the change-over switch, restart the change-over switch when the detected system voltage becomes equal to or greater than the first set value and the detected non-fundamental wave voltage becomes equal to or less than a predetermined second set value.

[0011] In such a power supply system, a changeover switch is provided on the commercial power system side of the distributed power source in the power line, and an impedance element is connected in parallel to the changeover switch. When the voltage on the commercial power system side falls below the first set value, the changeover switch is opened, so that the demand side equipment is connected to the commercial power system via the impedance element even during a momentary sag. This makes it possible to prevent a voltage drop to important loads during a momentary sag while satisfying the FRT requirements of the distributed power source. After the changeover switch is opened, the changeover switch is turned on again when the detected system voltage is equal to or higher than the first set value and the non-fundamental wave voltage is equal to or lower than the second set value. Therefore, for example, when the system voltage rises due to LC series resonance during an open power outage, hunting in which the changeover switch is repeatedly turned on and off again can be prevented, and adverse effects on important loads can be prevented.

[0012] Moreover, it is preferable that the power supply system is configured to stop injection of the non-fundamental current by the non-fundamental current injection means when the changeover switch is turned on again.

[0013] In a specific embodiment of the power supply system, the non-fundamental current is an integer harmonic current. The integer harmonic refers to a harmonic whose frequency is an integer multiple (2 or more) of the fundamental wave. Effect of the Invention

[0014] According to the present invention configured in this manner, in a power supply system that satisfies the FRT requirements while achieving both an uninterruptible power supply function and a load leveling function using a common distributed power source, it is possible to prevent opening and closing hunting of the change-over switch during an open power outage. [Brief description of the drawings]

[0015] [Figure 1] 1 is a diagram illustrating a schematic configuration of a power supply system according to an embodiment of the present invention; [Diagram 2] 11A and 11B are diagrams illustrating simulation results of the operation of the power supply system of the present embodiment when an open circuit power outage occurs. [Diagram 3] 11A and 11B are diagrams illustrating simulation results of the operation of the power supply system of the present embodiment when an open circuit power outage occurs. [Figure 4] 1A and 1B are diagrams illustrating an example in which opening and closing hunting of a changeover switch occurs in a conventional power supply system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A power supply system 100 according to an embodiment of the present invention will be described below with reference to the drawings.

[0017] As shown in FIG. 1 , the power supply system 100 of this embodiment is provided between a commercial power system 10 and important loads 30, and performs a function as an uninterruptible power supply system (uninterruptible power supply function) that supplies power to the important loads 30 in the event of an abnormality in the commercial power system 10, and a function as a distributed power supply system (load leveling function) that levels the load by providing forward and reverse power flows to the commercial power system 10.

[0018] Here, the commercial power system 10 is a power supply network of a power company (electric utility company) and includes a power plant, a power transmission system, and a power distribution system. The important loads 30 are loads that should stably supply power even during system abnormalities such as power outages and voltage drops, and although there is only one important load in FIG. 1, there may be multiple important loads.

[0019] Specifically, the power supply system 100 includes a distributed power source 2, a changeover switch 3 that connects the commercial power system 10 to the distributed power source 2 and important load 30, an impedance element 4 connected in parallel to the changeover switch 3, a parallel-off switch 5 provided on the commercial power system 10 side of the changeover switch 3, a system side voltage detection unit 61 that detects the voltage on the commercial power system 10 side of the changeover switch 3, a system abnormality detection unit 62 that detects at least an instantaneous voltage drop and a power outage from the detected voltage of the system side voltage detection unit 61, and a control unit 7 that controls the open / closed states of the changeover switch 3 and the parallel-off switch 5.

[0020] The distributed power source 2 is connected to a power line L1 for supplying power from a commercial power system 10 to an important load 30. The distributed power source 2 is connected to the commercial power system 10, and may be, for example, a DC power generation facility 21 such as a solar power generation facility or a fuel cell, a power storage device (power storage device) 22 such as a secondary battery (storage battery), a power generation facility (not shown) that rectifies the electric energy output in AC such as a wind power generation facility or a micro gas turbine into DC and is connected to the system using a power conversion device, or an AC power generation facility 23 such as a synchronous generator or an induction generator. The DC power generation facility 21 and the power storage device (power storage device) 22 are equipped with a power conversion device (not shown). The power supply system 100 is equipped with at least the power storage device 22, and may also have any of the above-mentioned distributed power sources 2.

[0021] The changeover switch 3 is provided on the power line L1 closer to the commercial power system 10 than the connection point of the distributed power source 2 and opens and closes the power line L1. For example, a changeover switch capable of high speed switching, such as a semiconductor switch or a hybrid switch combining a semiconductor switch and a mechanical switch, can be used. For example, when a semiconductor switch is used, the switching time can be reduced to 2 ms or less, and the power can be cut off regardless of the zero point. Also, when a hybrid switch is used, the switching time can be reduced to 2 ms or less, and not only can the power be cut off regardless of the zero point, but the current loss can be reduced to zero. The changeover switch 3 is controlled to be opened and closed by the control unit 7.

[0022] The impedance element 4 is connected in parallel to the changeover switch 3 in the power line L1, and is a current-limiting reactor in this embodiment.

[0023] The parallel-off switch 5 is provided on the power line L1 closer to the commercial power system 10 than the distributed power sources 2 (here, closer to the changeover switch 3), and is, for example, a mechanical switch.

[0024] The system-side voltage detection unit 61 detects the voltage on the power line L1 closer to the commercial power system 10 than the changeover switch 3 via a voltage transformer. Specifically, the system-side voltage detection unit 61 is connected to the commercial power system 10 side closer to the parallel circuit made up of the changeover switch 3 and the impedance element 4 via a voltage transformer.

[0025] The system abnormality detection unit 62 compares the detected voltage detected by the system side voltage detection unit 61 with a first preset value to detect a system abnormality such as an instantaneous voltage drop or a power outage. Specifically, the system abnormality detection unit 62 detects an instantaneous voltage drop when the detected voltage falls below the first preset value. The system abnormality detection unit 62 also detects a power outage when the duration of the state in which the detected voltage is below the first preset value is equal to or greater than a predetermined value (longer than the duration of the instantaneous drop). After detecting an instantaneous voltage drop or a power outage, the system abnormality detection unit 62 detects the return of the commercial power system 10 to a healthy state when the duration of the state in which the detected voltage is equal to or greater than the first preset value is equal to or greater than a predetermined value.

[0026] The control unit 7 switches the open / closed states of the changeover switch 3 and the parallel-off switch 5 in response to a signal from the system abnormality detection unit 62, thereby switching the operating state of the power supply system 100.

[0027] Specifically, when an instantaneous voltage drop is detected by the system abnormality detection unit 62, the control unit 7 opens the change-over switch 3. Then, with the commercial power system 10, the distributed power sources 2, and the important loads 30 connected via the impedance elements 4, operation by the distributed power sources 2 continues, and charging and discharging is performed so that the voltage and frequency of the important loads 30 are stabilized, thereby adjusting the power flow balance.

[0028] Furthermore, when a power outage is detected by the system abnormality detection unit 62, the control unit 7 opens the parallel-off switch 5 in addition to opening the changeover switch 3, and causes the distributed power source 2 to operate in an independent mode.

[0029] Therefore, in order to prevent opening and closing hunting of the change-over switch 3 during an open power outage, the power supply system 100 of this embodiment is equipped with non-fundamental current injection means 81 that injects a non-fundamental current, which is a current having a frequency different from the fundamental frequency of the commercial power system 10, into the power line L1, a non-fundamental voltage detection unit 82 that detects a non-fundamental voltage, which is the voltage value of the injected non-fundamental current, and a non-fundamental voltage monitoring unit 83 that compares the detected non-fundamental voltage with a predetermined second set value.

[0030] The non-fundamental current injection means 81 injects a non-fundamental current into the power line L1 on the commercial power system 10 side of the changeover switch 3. Specifically, the non-fundamental current injection means 81 injects a non-fundamental current into the commercial power system 10 side of the parallel circuit consisting of the changeover switch 3 and the impedance element 4. The non-fundamental current injection means 81 is a current injection device configured to be able to inject a current of any frequency into the power line L1, and in this embodiment, it injects an integer-order harmonic current into the power line L1 as a non-fundamental current. The integer-order harmonic is a harmonic whose frequency is an integer multiple of the fundamental wave or more, and examples of such harmonic include even-order harmonics such as second order and fourth order. The non-fundamental current injection means 81 in this embodiment is connected to the power line L1 via an open / close switch 81s, and is configured to start and stop injection of the non-fundamental current by opening and closing the open / close switch 81s based on a control signal from the control unit 7.

[0031] Non-fundamental voltage detection unit 82 detects a non-fundamental voltage on the power line L1 closer to the commercial power system 10 than the changeover switch 3, via a potential transformer. Specifically, non-fundamental voltage detection unit 82 is connected via a potential transformer to the commercial power system 10 side closer to the parallel circuit made up of the changeover switch 3 and the impedance element 4. Note that non-fundamental voltage detection unit 82 is configured to detect a harmonic voltage of the same order as the injected non-fundamental current.

[0032] The non-fundamental wave voltage monitoring unit 83 compares the non-fundamental wave voltage detected by the non-fundamental wave voltage detection unit 82 with a predetermined second set value to determine whether the non-fundamental wave voltage is less than or equal to the second set value, and outputs the determination result to the control unit 7.

[0033] In this embodiment, the control unit 7 is configured to, after detecting an instantaneous voltage drop or a power outage, re-close the changeover switch 3 based on the detection result of the system abnormality detection unit 62 and the determination result of the non-fundamental wave voltage monitoring unit 83. Specifically, when the system abnormality detection unit 62 detects a system abnormality (i.e., the detected voltage falls below the first set value), the control unit 7 opens the changeover switch 3 and closes the open / close switch 81s to start injection of a non-fundamental wave current by the non-fundamental wave current injection means 81. When the system abnormality detection unit 62 detects recovery from the system abnormality (i.e., the duration of the state in which the detected voltage is equal to or greater than the first set value is equal to or greater than a predetermined value) and the non-fundamental wave voltage monitoring unit 83 determines that the non-fundamental wave voltage is equal to or less than the second set value, the control unit 7 re-closes the changeover switch 3 and opens the open / close switch 81s to stop injection of a non-fundamental wave current by the non-fundamental wave current injection means 81. In other words, even if the voltage detected by the system abnormality detection unit 62 is equal to or greater than the first set value, if the non-fundamental wave voltage detected by the non-fundamental wave voltage monitoring unit 83 is greater than the second set value, the control unit 7 will not turn the change-over switch 3 back on.

[0034] Next, the operation of the power supply system 100 of this embodiment (during normal operation, during a momentary voltage drop, and during a power outage) will be described.

[0035] (1) Continuous operation When no system abnormality is detected by the system abnormality detection unit 62, the control unit 7 closes the change-over switch 3 and the parallel-off switch 5. In this case, the distributed power source 2 and the important load 30 are connected to the commercial power system 10 via the change-over switch 3. The impedance element 4 is connected in parallel to the change-over switch 3, but since a current flows through the change-over switch 3 side, which has a lower impedance value, power is exchanged between the commercial power system 10 and the distributed power source 2 and the important load 30 on the change-over switch 3 side. Peak cut and peak off can be achieved by the reverse power flow by the distributed power source 2. Note that during this normal operation, the open / close switch 81s is open, and no non-fundamental current is injected into the power line L1.

[0036] (2) When a voltage drop occurs and when it is restored When a short circuit accident (e.g., a three-phase short circuit) occurs on the commercial power system 10 side, the voltage on the commercial power system 10 side drops. This voltage drop is detected by the system-side voltage detection unit 61. When the voltage detected by the system-side voltage detection unit 61 is less than the first set value, the control unit 7 opens the change-over switch 3. As a result, the impedance element 4 connected in parallel to the change-over switch 3 is inserted, and power is supplied while maintaining a reverse power flow in a manner that limits the overcurrent flowing from the distributed power source 2 to the commercial power system 10 side, and a voltage drop in the supply voltage of the important load 30 is prevented (UPS operation is performed for the important load 30). At this time, the control unit 7 also turns on the open / close switch 81s, and starts injection of a non-fundamental current into the power line L1 by the non-fundamental current injection means 81. After the changeover switch 3 is opened, if the detected voltage becomes equal to or greater than the first set value and the detected non-fundamental wave voltage becomes equal to or less than a predetermined second set value, the changeover switch 3 is turned on again and the opening / closing switch 81s is opened to stop the injection of the non-fundamental wave current.

[0037] (3) Operation during and after parallel-off due to a drop in grid voltage (caused by a grid power outage, etc.) After detecting the above-mentioned momentary sag, if the detected voltage of the grid-side voltage detection unit 61 satisfies a predetermined disconnection condition (i.e., the duration of the state in which the detected voltage is below the first set value is equal to or longer than a predetermined value), the control unit 7 opens the disconnection switch 5 and also opens the open / close switch 81s to stop the injection of non-fundamental current. As a result, the distributed power source 2 is disconnected from the commercial power grid 10 and enters the independent operation mode. Note that, since the changeover switch 3 has already been opened, the disconnection switch 5 is opened without any overcurrent.

[0038] (4) Operation when the grid recovers after being disconnected (when power is restored) When the duration of the state in which the voltage detected by the grid-side voltage detection unit 61 is equal to or higher than the first set value reaches a predetermined value or more, and the non-fundamental wave voltage detected by the non-fundamental wave voltage detection unit 82 falls to or lower than the second set value, the control unit 7 turns on the changeover switch 3 again. After that, when the synchronization test condition is met based on the detected voltage of the grid-side voltage detection unit 61 and the detected voltage of the power source-side voltage detection unit (the magnitude, frequency, and phase of the voltage of the distributed power source 2 match the magnitude, frequency, and phase of the voltage of the commercial power grid 10), the control unit 7 turns on the parallel-off switch 5. This resumes the operation during normal operation as described above (1).

[0039] Next, the results of a simulation of the operation of the power supply system 100 of this embodiment during an open circuit power outage are shown in FIGS.

[0040] FIG. 2 shows the time variations of the open / closed state of the changeover switch 3 during an open power outage, the injection current [A] of the second harmonic, the instantaneous effective value [V] of the detected second harmonic, the open power outage determination signal, and the power restoration detection signal based on the system voltage detected by the system side voltage detection unit 61. FIG. 3 shows the time variations of the system voltage V detected by the system side voltage detection unit 61 at the same time as FIG. 2. r [V], system voltage V r The graph shows the time changes of the instantaneous effective value [V] of the load current [V], the open / close state of the changeover switch 3, and the critical load voltage [V].

[0041] As shown in Fig. 2 and Fig. 3, when the instantaneous effective value of the system voltage falls below the first set value, the changeover switch 3 is opened and the injection of the second harmonic current is started. Soon after, the instantaneous effective value of the second harmonic rises and exceeds the second set value (open power failure judgment level). When the duration during which the second harmonic exceeds the second set value exceeds a predetermined set time, the open power failure judgment signal is turned on. This set time is set to be (Condition 1) "longer than the time during which an open power failure is not detected due to the transient phenomenon at the time of the occurrence of the momentary sag and the time at the time of recovery" and (Condition 2) "longer than the time during which the transient phenomenon of the instantaneous effective value of the inter-harmonic at the time of the occurrence of the open power failure converges". This open power failure judgment signal masks the subsequent power recovery detection signal, and the changeover switch 3 is not turned on again even if the time during which the instantaneous effective value of the system voltage exceeds the first set value exceeds the set time. As a result, when an open power failure occurs, opening and closing hunting of the changeover switch 3 does not occur, and it is understood that no transient waveform occurs in the important load voltage.

[0042] According to the power supply system 100 of this embodiment configured as described above, the changeover switch 3 is provided on the power line L1 closer to the commercial power system 10 than the distributed power source 2, and an impedance element 4 is connected in parallel to the changeover switch 3, and when the voltage on the commercial power system 10 side falls below a first set value, the changeover switch 3 is opened, so that even during a momentary sag, the distributed power source 2 and important load 30 are connected to the commercial power system 10 via the impedance element 4. In this way, the power supply system 100 does not disconnect the distributed power source 2 and important load 30 from the commercial power system 10 either during normal operation or during a momentary sag, and therefore it is possible to prevent a voltage drop at the important load 30 during a momentary sag while satisfying the FRT requirements for the distributed power source 2. Then, after the changeover switch 3 is opened, the changeover switch is turned on again when the detected system voltage becomes equal to or higher than the first set value and the voltage of the injected non-fundamental wave component becomes equal to or lower than the second set value, so that, for example, when the system voltage rises due to LC series resonance during an open power outage, hunting in which the changeover switch 3 is repeatedly turned on and off again can be prevented, and adverse effects on the important loads 30 can be prevented. As a result, in the power supply system 100 that satisfies the FRT requirements and achieves both an uninterruptible power supply function and a load leveling function by using a common distributed power source 2, opening and closing hunting of the changeover switch 3 during an open power outage can be prevented.

[0043] <Other Modified Embodiments> The present invention is not limited to the above-described embodiment.

[0044] For example, the impedance element 4 may be a capacitor, or may be a combination of a reactor, a resistor, or a capacitor.

[0045] Furthermore, the grid-side voltage detection unit 61 of the above embodiment may be provided in the grid-connection protection device. Examples of grid-connection protection devices defined in the grid-connection regulations include an overvoltage relay (OVR), an undervoltage relay (UVR), a directional short circuit relay (DSR), an earth fault overvoltage relay (OVGR), an overfrequency relay (OFR), an underfrequency relay (UFR), a transfer cutoff device, and the like. In this case, the control unit 7 may open the disconnection switch 5 when any one of the interconnection protection devices is activated. The control unit 7 may also turn on the disconnection switch 5 when all the grid-connection protection devices are in an inoperative state and the detection voltage of the grid-side voltage detection unit 61 and the detection voltage of the power supply-side voltage detection unit satisfy the synchronization test condition. With this configuration, since the voltage detection unit provided in the interconnection protection device is used, there is no need to provide a separate grid-side voltage detection unit, and the device configuration can be simplified.

[0046] Furthermore, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications are possible without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0047] 100 Power Supply System 10...Commercial power system 30...Important load L1...power line 2...Distributed power supply 3. Changeover switch 4. Impedance Element 5. Parallel-off switch 61 Grid side voltage detection section 62 System abnormality detection unit 7 Control section 81...Non-fundamental current injection means 82 Non-fundamental wave voltage detection section 83 Non-fundamental wave voltage monitoring unit

Claims

1. A power supply system provided between a commercial power system and a critical load, for supplying power to the critical load, comprising: A distributed power source connected to a power line for supplying power from the commercial power system to the critical load; a changeover switch that is provided on the power line closer to the commercial power system than the distributed power source and that opens and closes the power line; an impedance element connected in parallel to the changeover switch in the power line; a system side voltage detection unit that detects a system voltage on the commercial power system side of the changeover switch; a non-fundamental current injection means for injecting a non-fundamental current, which is a current having a frequency different from a fundamental frequency of the commercial power system, into the power line; a non-fundamental voltage detection unit that detects a non-fundamental voltage that is a voltage of the non-fundamental current on the commercial power system side of the changeover switch, when the detected system voltage falls below a predetermined first setting value, the change-over switch is opened, and the distributed power source continues to operate in a state in which the distributed power source and the commercial power system are connected via the impedance element, and injection of the non-fundamental wave current by the non-fundamental wave current injection means is started; a power supply system configured to, after the changeover switch is opened, re-open the changeover switch when the detected system voltage becomes equal to or greater than the first set value and the detected non-fundamental wave voltage becomes equal to or less than a predetermined second set value.

2. 2. The power supply system according to claim 1, wherein the changeover switch is turned on again and the injection of the non-fundamental current by the non-fundamental current injection means is stopped.

3. 3. The power supply system according to claim 1, wherein the non-fundamental current is an integer-order harmonic current.

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