JP7617510B2 - Power System - Google Patents

Description

The present invention relates to a power supply system.

Conventionally, the system shown in Patent Document 1 has been considered as a power supply system that uses a common distributed power source to achieve both uninterruptible power supply functionality and load leveling functionality while satisfying FRT requirements.

The power supply system of Patent Document 1 is configured by providing a changeover switch, such as a semiconductor switch, between the commercial power system and the 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.

Patent No. 6338131

However, in the above-mentioned power supply system, during an open power outage of the commercial power grid, 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.

As examples of system conditions that may cause this type of opening and closing hunting, there are two cases as shown in Figure 4.

For example, when the impedance element connected in parallel to the change-over switch is a capacitor (C) or a coil (L), LC series resonance between the impedance element and the inductive reactance of the internal and external loads or the phase-leading capacitor (SC) may cause the receiving point voltage _Vr to rise, resulting in hunting when the change-over switch is opened or closed (Case 1).

In addition, after the change-over switch is opened due to a drop in the receiving point voltage _Vr caused by an open power outage, the receiving point voltage _Vr rises due to 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, etc., and the large-capacity SC on the line side, which may cause hunting when the change-over switch is opened or closed (Case 2).

The present invention was made to solve the above problems, and its main objective is to prevent hunting between opening and closing of the changeover switch during an open power outage in a power supply system that satisfies the FRT requirements and achieves both uninterruptible power supply and load leveling functions using a common distributed power source.

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 that is provided on the power line closer to the commercial power system than the distributed power source and opens and closes 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 the system voltage on the commercial power system side of the changeover switch, and a non-fundamental current injection unit 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. The power supply device is characterized in that it is configured to include an input means and a non-fundamental voltage detection unit that detects a non-fundamental voltage, which is the voltage of the non-fundamental current on the commercial power system side of the changeover switch, to open the changeover switch when the detected system voltage falls below a predetermined first setpoint, to continue operating 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 to re-close the changeover switch when the detected system voltage becomes equal to or greater than the first setpoint and the detected non-fundamental voltage becomes equal to or less than a predetermined second setpoint after the changeover switch is opened.

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 becomes equal to or greater than the first set value and the non-fundamental wave voltage becomes equal to or less than the second set value. This makes it possible to prevent hunting, in which the changeover switch is repeatedly turned on and off, for example, when the system voltage rises due to LC series resonance during an open power outage, and to prevent adverse effects on important loads.

Here, when an integer harmonic is injected as the non-fundamental current, it is difficult to distinguish between the non-fundamental voltage and the system voltage. Therefore, in a specific embodiment of the power supply system, the non-fundamental current is preferably an interharmonic current.
In such a case, it is easy to distinguish between the system voltage and the non-fundamental wave voltage, and therefore there is no need to inject a large current as the non-fundamental wave current.

In this case, it is preferable that the non-fundamental current is a non-integer harmonic current between the second and third orders so as to reduce the inter-harmonic components absorbed by the power factor correction capacitor.

The present invention, configured in this way, can prevent open/close hunting of the changeover switch during an open power outage in a power supply system that satisfies the FRT requirements while simultaneously achieving uninterruptible power supply functionality and load leveling functionality using a common distributed power source.

1 is a diagram illustrating a schematic configuration of a power supply system according to an embodiment of the present invention; 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. 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. 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.

Below, a power supply system 100 according to one embodiment of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the power supply system 100 of this embodiment is provided between the commercial power system 10 and the important loads 30, and functions as an uninterruptible power supply system (uninterruptible power supply function) that supplies power to the important loads 30 when the commercial power system 10 is abnormal, and functions 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.

Here, the commercial power system 10 is the power supply network of a power company (electricity supplier), and includes power plants, a power transmission system, and a power distribution system. Furthermore, the important loads 30 are loads that should provide a stable supply of power even during system abnormalities such as power outages and voltage drops. Although there is only one important load in FIG. 1, there may be multiple important loads.

Specifically, the power supply system 100 includes a distributed power source 2, a changeover switch 3 that connects the distributed power source 2 and the important load 30 to the commercial power grid 10, an impedance element 4 connected in parallel to the changeover switch 3, a parallel-off switch 5 provided on the commercial power grid 10 side of the changeover switch 3, a grid-side voltage detection unit 61 that detects the voltage on the commercial power grid 10 side of the changeover switch 3, a grid abnormality detection unit 62 that detects at least momentary voltage drops and power outages from the detected voltage of the grid-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.

The distributed power source 2 is connected to a power line L1 for supplying power from the commercial power system 10 to the 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.

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 semiconductor switch or a changeover switch capable of high speed switching, such as a hybrid switch that combines 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. Furthermore, 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 opening and closing of this changeover switch 3 is controlled by the control unit 7.

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

The parallel-off switch 5 is provided on the power line L1 closer to the commercial power grid 10 than the distributed power source 2 (here, closer to the changeover switch 3), and is, for example, a mechanical switch. The opening and closing of the parallel-off switch 5 is controlled by the control unit 7.

The system-side voltage detection unit 61 detects the voltage on the commercial power system 10 side of the changeover switch 3 in the power line L1 via a voltage transformer. Specifically, the system-side voltage detection unit 61 is connected to the commercial power system 10 side of the parallel circuit consisting of the changeover switch 3 and the impedance element 4 via a voltage transformer.

The system abnormality detection unit 62 compares the detected voltage detected by the system side voltage detection unit 61 with a predetermined first set 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 set 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 set 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 set value is equal to or greater than a predetermined value.

The control unit 7 switches the open/closed state 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.

Specifically, when an instantaneous voltage drop is detected by the system anomaly detection unit 62, the control unit 7 opens the changeover switch 3. Then, with the commercial power system 10, the distributed power source 2, and the important load 30 connected via the impedance element 4, operation by the distributed power source 2 continues, charging and discharging the important load 30 so that the voltage and frequency are stabilized, and the power flow balance is adjusted.

When a power outage is detected by the system abnormality detection unit 62, the control unit 7 opens the changeover switch 3 and also opens the parallel-off switch 5, and causes the distributed power source 2 to operate independently. Specifically, the control unit 7 opens the parallel-off switch 5, and with the parallel-off switch 5 open, the distributed power source 2 enters the independent operation mode and supplies power to the important load 30.

The power supply system 100 of this embodiment is equipped with a non-fundamental current injection means 81 that injects a non-fundamental current, which is a current with a frequency different from the fundamental frequency of the commercial power system 10, into the power line L1 in order to prevent opening and closing hunting of the changeover switch 3 during an open power outage, 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.

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 constantly 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 constantly injects an interharmonic current into the power line L1 as a non-fundamental current. The interharmonic is a non-integer harmonics that exist between integer harmonics, for example, a harmonic between the second and third orders (for example, 2.6th order).

The non-fundamental voltage detection unit 82 constantly detects the 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, the non-fundamental voltage detection unit 82 is connected to the commercial power system 10 closer to the parallel circuit consisting of the changeover switch 3 and the impedance element 4 via a potential transformer. The non-fundamental voltage detection unit 82 may detect a harmonic voltage of the same order as the injected non-fundamental current, or may detect a harmonic voltage of a different order. For example, the non-fundamental current injection means 81 may inject a 2.6th order interharmonic current, and the non-fundamental voltage detection unit 82 may be configured to detect a harmonic component of a predetermined order in the range of 2.25th order to 2.75th order.

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 setpoint to determine whether the non-fundamental wave voltage is equal to or lower than the second setpoint, and outputs the determination result to the control unit 7.

In this embodiment, the control unit 7 is configured to re-close the changeover switch 3 based on the detection result of the system abnormality detection unit 62 and the judgment result of the non-fundamental wave voltage monitoring unit 83 after detecting an instantaneous voltage drop or power outage. Specifically, the control unit 7 is configured to re-close the changeover switch 3 only when the system abnormality detection unit 62 detects the recovery of 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 judges that the non-fundamental wave voltage is equal to or less than the second set value. In other words, even if the detected voltage by the system abnormality detection unit 62 is equal to or greater than the first set value, the control unit 7 does not re-close the changeover switch 3 if the non-fundamental wave voltage detected by the non-fundamental wave voltage monitoring unit 83 is greater than the second set value.

Next, the operation of the power supply system 100 of this embodiment (during continuous operation, during momentary voltage drops, and during power outages) will be described.

(1) During 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 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 cutting and peak off can be achieved by the reverse power flow from the distributed power source 2. Note that during this continuous operation, a non-fundamental current is injected into the power line L1 by the non-fundamental current injection means 81.

(2) When a momentary sag 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. The control unit 7 opens the changeover switch 3 when the detected voltage detected by the system-side voltage detection unit 61 is less than the first set value. As a result, the impedance element 4 connected in parallel to the changeover 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). After the changeover switch 3 is opened, when 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 closed again.

(3) Operation during and after disconnection due to grid voltage drop (occurring due to grid power outage, etc.) After detecting the above-mentioned momentary sag, if the voltage detected by 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. 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 opens without any overcurrent.

(4) Operation when the grid returns to normal after parallel-out (when power is restored) If the duration during which the voltage detected by the grid-side voltage detection unit 61 is equal to or greater than the first setpoint is equal to or greater than a predetermined value, and the non-fundamental voltage detected by the non-fundamental voltage detection unit 82 is equal to or less than the second setpoint, the control unit 7 turns on the change-over switch 3 again. After that, if 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-out switch 5. This resumes the operation during normal operation described above (1).

Next, the results of a simulation of the operation of the power supply system 100 of this embodiment during an open power outage are shown in Figures 2 and 3.

Fig. 2 shows the time changes of the open/closed state of the changeover switch 3 during an open power outage, the injection current [A] of the interharmonic (2.6th order), the instantaneous effective value [V] of the detected interharmonic (2.5th order), 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 also shows the time changes of the system voltage _Vr [V] detected by the system side voltage detection unit 61, the instantaneous effective value [V] of the system voltage _Vr , the open/closed state of the changeover switch 3, and the important load voltage [V] at the same time as in Fig. 2.

As shown in Figures 2 and 3, when the instantaneous effective value of the system voltage falls below the first set value, the changeover switch 3 is opened, and the instantaneous effective value of the interharmonic rises and exceeds the second set value (open power failure judgment level). When the duration during which the interharmonic 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 (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 momentary sag and the time of the recovery" and (Condition 2) "longer than the time during which the transient phenomenon of the instantaneous effective value of the interharmonic at the time 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, it can be seen that when an open power failure occurs, opening and closing hunting of the changeover switch 3 does not occur, and a transient waveform does not occur in the important load voltage.

According to the power supply system 100 of the present 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. When the voltage on the commercial power system 10 side falls below the first set value, the changeover switch 3 is opened, so that even during a 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 sag, so that it is possible to prevent a voltage drop to the important load 30 during a sag while satisfying the FRT requirements of the distributed power source 2. Then, after the changeover switch 3 is opened, if 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, the changeover switch 3 is turned on again. This prevents hunting, in which the changeover switch 3 is repeatedly turned on and off again, for example, when the system voltage rises due to LC series resonance during an open power outage, and prevents adverse effects on the important load 30. As a result, in a power supply system 100 that satisfies the FRT requirements and achieves both an uninterruptible power supply function and a load leveling function using a common distributed power source 2, opening and closing hunting of the changeover switch 3 during an open power outage can be prevented.

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

For example, in another embodiment of the power supply system 100, when the interharmonic injection method (see the grid interconnection regulations (JEAC9701-2016)) is used as the active method for the grid protection islanding detection function, the active signal may be configured to be shared.

The impedance element 4 may be a capacitor, or a combination of a reactor, resistor, or capacitor.

Furthermore, the system side voltage detection unit 61 of the above embodiment may be provided in the system interconnection protection device. Examples of system interconnection protection devices specified in the system interconnection regulations include an overvoltage relay (OVR), an undervoltage relay (UVR), a directional short circuit relay (DSR), a ground 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. In addition, the control unit 7 may also turn on the disconnection switch 5 when all the system interconnection protection devices are in an inoperative state and the detection voltage of the system 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 system side voltage detection unit, and the device configuration can be simplified.

It goes without saying that the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the invention.

REFERENCE SIGNS LIST 100: Power supply system 10: Commercial power system 30: Important load L1: Power line 2: Dispersed power source 3: Change-over switch 4: Impedance element 5: Disconnection switch 61: System side voltage detection unit 62: System abnormality detection unit 7: Control unit 81: Non-fundamental wave current injection means 82: Non-fundamental wave voltage detection unit 83: Non-fundamental wave voltage monitoring unit

Claims (3)

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 changeover 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;
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. The power supply system of claim 1, wherein the non-fundamental current is an interharmonic current. The power supply system according to claim 2, wherein the non-fundamental current is a non-integer harmonic current between second and third orders.