Patent ID: 12243702

DESCRIPTION OF EMBODIMENT

An embodiment of a protection system and a protection method according to the present invention will now be described with reference to the drawings.

<Configuration of Embodiment>

FIG.1is a configuration diagram showing one embodiment of the protection system and the protection method according to the present invention. The protection system and protection method of the present invention are applicable to, for example, a photovoltaic system. Therefore, one embodiment will be described using the photovoltaic system1.

As shown inFIG.1, the photovoltaic system1includes a solar cell panel10, a PV fuse20, a DC switch30, a power converter40, and an electrical path (DC bus)50. The solar cell panel10, the PV fuse20, the DC switch30, and the power converter40are connected in series via an electrical path50. The photovoltaic system1is a system connected to a power system (not shown) on the right side in the drawing. Reference numeral60represents a signal line, and reference numeral70shows that a device short-circuit is occurring.

The solar cell panel10, which is also referred to as a solar panel, a solar cell module, a solar module, or simply a module, is made up of a plurality of solar cells combined into one panel.FIG.1shows one solar cell panel10for convenience, which, in reality, may form a solar cell string composed of a plurality of solar cell panels10combined in series or in parallel, or a solar cell array of solar cell strings combined. The solar cell panel10is an example of a DC power supply according to Claims, and supplies DC power to the power converter40.

When a current exceeding a predetermined current value flows through the electrical path50due to some abnormality in the photovoltaic system1, the PV fuse20protects the circuit in such a way that the alloy component contained therein is blown by Joule heat to open the circuit. However, the PV fuse20conforms to a standard in which it is not blown even if the DC power supplied from the solar cell panel10shown inFIG.1is the maximum. Hence, even if a device short-circuit occurs in the power converter40, the PV fuse20is not completely blown.

The DC switch30is series-connected to the electrical path50located between the PV fuse20and the power converter40, and connects or opens the electrical path50in response to a turn-on command or open command from the inverter control circuit42described later or an operator. When the DC switch30is opened, the DC current supplied from the solar cell panel10flowing into the power converter40is blocked. Note that the DC switch30is an example of the circuit breaker according to Claims.

The power converter40is connected to the solar cell panel10via the electrical path50, and converts the DC power generated by the solar cell panel10into AC power. The power converter40includes an inverter circuit41, an inverter control circuit42, a DC capacitor43, an ammeter44, and a voltmeter45.

The power converter40is configured to perform known maximum power point tracking (MPPT). Maximum power point tracking control (MPPT control) is a control function for extracting current at an output voltage that maximizes the power from the solar cell panel10. It is preferable that the power converter40also have an output limiter function. The power converter40is also referred to as an inverter unit, a power conditioner, and a power conditioning subsystem (PCS).

The inverter circuit41includes a plurality of switching devices such as IGBTs. The inverter control circuit42generates a pulse width modulation signal serving as a gate drive signal for the switching devices. The inverter control circuit42is connected to the inverter circuit41via a signal line60a, and controls the operation of the inverter circuit41.

The inverter control circuit42has the functions of the timer46and the protection determiner47, which will be described later. The inverter control circuit42is connected to the DC switch30, the inverter circuit41, the ammeter44, and the voltmeter45, via the signal line60. In the event of an accident such as a device short-circuit70in the inverter circuit41, the inverter control circuit42performs detection of and protection from overcurrent and overvoltage based on the value obtained through the ammeter44and the values obtained through other sensors. After detection of overcurrent and overvoltage for protection and, the inverter control circuit42halts the power converter40and an open operation command is given to the DC switch30via the signal line60b. Thus, the inverter control circuit42reduces the flow of short-circuit current into the inverter circuit41, thereby protecting the inverter circuit41.

The DC capacitor43is charged with DC power supplied from the solar cell panel10when the DC switch30is turned on during halt or gate block (GB) of the power converter40. Hence, in normal operation, upon the lapse of a predetermined length of time since the DC switch30is turned on, the value of the voltage of the DC capacitor43increases and the DC current value decreases. When the voltage of the DC capacitor43has increased and charging has been completed, discharge of the DC capacitor43triggers the operation of the power converter40and also cancels the gate block (GB).

However, in the event of an accident such as a device short-circuit70in the power converter40, even if the DC switch30is turned on, only the short-circuit current continues to flow and the DC capacitor43is not charged. Accordingly, in the event of the device short-circuit70in the power converter40, even after the lapse of a predetermined length of time since the DC switch30was turned on, the voltage of the DC capacitor43does not increase and the DC current value does not decrease. The present invention takes advantage of this nature of the DC capacitor43, the details of which will be described later.

The ammeter44is, for example, a current sensor such as a Hall CT, and measures the current flowing in the power converter40or the inverter circuit41. The ammeter44is connected to the inverter control circuit42via a signal line60c. Note that the ammeter44is not necessarily provided in the position shown inFIG.1, and may be provided in, for example, the inverter circuit41or the inverter control circuit42, instead. Note that the ammeter44is an example of the current detector according to Claims.

The voltmeter45is, for example, a DC voltage sensor, and measures the voltage of the DC capacitor43. The voltmeter45is connected to the inverter control circuit42via a signal line60d. The voltmeter45may be provided in any position where it can measure the voltage of the DC capacitor43, and is not necessarily be in the position shown inFIG.1. Note that the voltmeter45is an example of the voltage detector according to Claims.

The timer46is one of the functions of the inverter control circuit42and, at turn-on of the DC switch30during halt or gate block (GB) of the power converter40, detects turn-on of the DC switch30and counts a lapse of a predetermined length of time from the turn-on of the DC switch30. Note that the timer46may be provided separately from the inverter control circuit42.

The protection determiner47is one of the functions of the inverter control circuit42and detects whether or not an abnormality such as occurrence of the device short-circuit70by one or both of the following two methods. Upon detection of an abnormality such as the occurrence of a device short-circuit70, the protection determiner47opens the DC switch30via the signal line60bto protect the inverter circuit41. Note that the protection determiner47may be provided separately from the inverter control circuit42.

In a first method, when the timer46counts a lapse of a predetermined length of time from turn-on of the DC switch30, the protection determiner47determines whether the ammeter44has detected a decrease in current. When it is determined that the ammeter44has detected a decrease in current, it is determined that the device short-circuit70has not occurred, and the power converter40is made to continue its normal operation.

In contrast, upon determination that the ammeter44has not detected a decrease in current, the protection determiner47determines that an abnormality such as a device short-circuit70is occurring and issues an open operation command to the DC switch30via the signal line60b. When the device short-circuit70occurs, only the short-circuit current continues to flow and the DC capacitor43is not charged, so that the current value does not decrease. Hence, the protection determiner47detects an abnormality such as a device short-circuit70by taking advantage of the nature of the DC capacitor43and opens the DC switch30, thereby protecting the power converter40.

In a second method, when the timer46counts a lapse of a predetermined length of time from turn-on of the DC switch30, the protection determiner47determines whether the ammeter44has detected a current and the voltmeter45has detected a voltage. When it is determined that the ammeter44has detected a current and the voltmeter45has detected a voltage, it is determined that the device short-circuit70has not occurred, and the power converter40is made to continue its normal operation.

In contrast, upon determination that the ammeter44has detected a current and the voltmeter45has not detected a voltage, the protection determiner47determines that an abnormality such as a device short-circuit70is occurring and issues an open operation command to the DC switch30via the signal line60a. When the device short-circuit70occurs, only the short-circuit current continues to flow and the DC capacitor43is not charged, so that an increase in the voltage of the DC capacitor43is not observed. Hence, the protection determiner47detects an abnormality such as a device short-circuit70by taking advantage of the nature of the DC capacitor43and opens the DC switch30, thereby protecting the power converter40.

Note that the protection determiner47may detect an abnormality such as a device short-circuit70by using the first method or the second method or both. In other words, upon a lapse of a predetermined length of time, if it is determined that the ammeter44has detected a current but not detected a decrease in current and the voltmeter45has not detected a voltage, the protection determiner47may determine that an abnormality such as a device short-circuit70is occurring.

Next, the operation of one embodiment of the present invention will be described.

<Operation of Embodiment>

FIG.2is a flowchart showing an example of the operation of the protection system and the protection method shown inFIG.1. The flowchart ofFIG.2is started when an abnormality such as a device short-circuit70occurs in the inverter circuit41or the like.

In Step S1, the inverter control circuit42performs detection of and protection from overcurrent and overvoltage by a sensor or the like in the power converter40. In the event of an accident such as a device short-circuit70in the inverter circuit41, a sharp increase in current and voltage is observed and the inverter control circuit42therefore performs detection of and protection from the sharp increase in current and voltage.

In Step S2, the inverter control circuit42halts the operation of the power converter40and issues an open operation command to the DC switch30via the signal line60b. Accordingly, the power converter40goes into the gate block (GB) state. Thus, the inverter control circuit42prevents a short-circuit current from flowing into the inverter circuit41and to protect the power converter40.

In Step S3, the inverter control circuit42accepts the reset of the central processing unit (CPU) of the power converter40by the operator. In Step S3, the accident of the device short-circuit70has been correctly detected from a sharp increase in current and voltage in Step S2and the power converter40is in halt due to a serious failure. In this case, when the CPU of the power converter40is reset by the operator, neither current nor voltage exists in the inverter circuit41, and the operator cannot therefore determine whether or not an accident is occurring. Consequently, the power converter40and the inverter circuit41look normal to the operator, and the DC switch30may be erroneously turned on by the operator.

In Step S4, the inverter control circuit42accepts the re-turn on of the DC switch30by the operator. This is, for example, when the operator does not notice that an accident is occurring and the DC switch30is erroneously turned on by the operator. In this case, even if the DC switch30is turned on again, the current and voltage do not increase sharply in the inverter circuit41as in the event of an accident of the device short-circuit70. Consequently, as in Step S1, the inverter control circuit42does not perform detection of and protection from the overcurrent and overvoltage.

FIG.3is a configuration diagram showing an example of the state where the DC switch30has been turned on again. InFIG.3, the same components as those inFIG.1are denoted by the same reference numerals as those inFIG.1, and the detailed description will be omitted. As shown inFIG.3, during the device short-circuit70, a short-circuit current Isc90flows from the solar cell panel10toward the power converter40.

Here, a PV fuse20is provided in the position of the electrical path50a. However, since accidents supposed to be protected by the PV fuse20do not include an accident of a device short-circuit70in the inverter circuit41, the PV fuse20is not completely blown even if a device short-circuit70occurs in the inverter circuit41. Strictly speaking, also in this case, the PV fuse20may be blown partially but not completely. Therefore, the PV fuse20is not completely blown until Step S3, and the PV fuse20is not completely blown even in Step S4.

FIG.4is a configuration diagram showing an example of the case where a PV fuse20is completely blown. The photovoltaic system1′ shown inFIG.4differs from the photovoltaic system1shown inFIG.1in that three solar cell panels10A,10B, and10C are connected in parallel and are connected to the electrical path50at the connection point51. InFIG.4, the same components as those inFIG.1are denoted by the same reference numerals as those inFIG.1, and the detailed description will be omitted. Although three solar cell panels10A,10B, and10C are shown inFIG.4, these are not necessarily three and only have to be plural. Moreover, the plurality of solar cell panels10A,10B, and10C may be a plurality of solar cell strings or a plurality of solar cell arrays.

As shown inFIG.4, for example, when a ground-fault80occurs in the system of the solar cell panel10A, the ground-fault current85indicated by the arrow in the drawing flows into the system of the solar cell panel10A. In this case, a current many times higher than usual may flow into the electrical path50ain the system of the solar cell panel10A from the electrical paths50band50ctoward the ground-fault point. The PV fuse20conforms to a standard that is blown only when current flows from the system of the solar cell panels10B and10C, which is a system different from the system of the solar cell panel10A, toward the ground-fault point. Accordingly, since accidents in which the PV fuse20is supposed to be blown do not include an accident of a device short-circuit70in the power converter40, even in the event of an abnormality such as a device short-circuit70, the PV fuse20is not completely blown.

FIG.5is a graph showing an example of a solar cell current-voltage characteristic curve (I-V curve).FIG.5shows an optimum operating point Pmpp, a maximum output operating voltage Vmpp, a maximum output operating current Impp, an open circuit voltage Voc, and a short-circuit current Isc. The optimum operating point Pmpp is the maximum point at which the power characteristic curve S2, which is the product of the operating voltage and the operating current in the current-voltage characteristic curve S1, becomes the maximum.

The maximum output operating voltage Vmpp is the operating voltage at the MPP point which is the optimum operating point. The maximum output operating current Impp is the operating current at the MPP point which is the optimum operating point. The open circuit voltage Voc is a voltage obtained in the open state where no load or the like is connected to the output terminals of the solar cells. The short-circuit current Isc is the current that flows when the output terminals of the solar cells are short-circuited. The current-voltage characteristic curve S1is a generally rectangular curve having a bent portion near the MPP point which is the optimum operating point. The power characteristic curve S2rises straight from zero voltage toward the optimum operating point Pmpp, and falls sharply from the optimum operating point Pmpp.

As shown inFIG.5, the short-circuit current Isc is a current that can be assumed from the current-voltage characteristic (I-V characteristic) of the solar cell panel10, and is a halfway current slightly higher than the normal maximum output operating current Impp. Thus, as shown inFIG.3, even if the DC switch30is turned on again and the short-circuit current Isc90flows, the detection of and protection from overcurrent and overvoltage is not performed by the inverter control circuit42, and the PV fuse20is not completely blown. However, if the short-circuit current Isc90continues to flow, secondary damage such as breakage and burnout of the device occurs. Consequently, even in such a case, it is necessary to detect an abnormality such as a device short-circuit70as soon as possible.

Returning toFIG.2, in Step S5, the protection determiner47accepts the start of count of time elapsed since the DC switch30was turned on which is performed by the timer46.

In Step S6, the protection determiner47determines whether or not the timer46has counted a lapse of a predetermined length of time. Upon determination that the predetermined length of time has elapsed, the protection determiner47forces the process to proceed to Step S7. In contrast, upon determination that the predetermined length of time has not elapsed, the protection determiner47repeats the process up to Step S6until the predetermined length of time has elapsed.

In Step S7, the protection determiner47accepts the value of the current flowing in the power converter40measured by the ammeter44. Note that the ammeter44may constantly measure the value of the current flowing in the power converter40.

In Step S8, the protection determiner47accepts the value of the voltage of the DC capacitor43connected to the circuit in the power converter40measured by the voltmeter45. Note that the voltmeter45may constantly measure the value of the voltage of the DC capacitor43connected to the circuit in the power converter40.

In Step S9, the protection determiner47determines whether or not an abnormality such as a device short-circuit70is occurring in the inverter circuit41. The principle of the determination of whether or not an abnormality is occurring by the protection determiner47will now be explained.

FIG.6is a graph showing an example of a change in the DC current and a change in the voltage of the DC capacitor43after the DC switch30is turned on.

FIG.6Ais a graph showing an example of a change in the DC current after the DC switch30is turned on while no device short-circuit70is occurring. Referring toFIG.6A, the current increases after the DC switch30is turned on, but decreases after the predetermined time t1has elapsed. This is because the charging of the DC capacitor43is almost completed after the predetermined length of time t1has elapsed.

FIG.6Bis a graph showing an example of a change in the DC current after the DC switch30is turned on while a device short-circuit70is occurring. Referring toFIG.6B, the current sharply increases to the short-circuit current Isc after the DC switch30is turned on, and is maintained at the increased level even after the predetermined time t1has elapsed. This is because when the device short-circuit70is occurring, the DC capacitor43is not charged and the short-circuit current Isc continues to flow through the inverter circuit41.

FIG.6Cis a graph showing an example of a change in the voltage of the DC capacitor after the DC switch30is turned on while no device short-circuit70is occurring. Referring toFIG.6C, the current keeps increasing until the predetermined length of time t1has elapsed since the DC switch30is turned on and, after the lapse of the predetermined length of time t1, the voltage value is kept constant. This is because the charging of the DC capacitor43is almost completed after the predetermined length of time t1has elapsed.

FIG.6Dis a graph showing an example of a change in the voltage of the DC capacitor after the DC switch30is turned on while a device short-circuit70is occurring. Referring toFIG.6D, the voltage does not rise from 0 V even when the DC switch30is turned on, and remains 0 V even after the lapse of the predetermined length of time t1. This is because while the device short-circuit70is occurring, only the short-circuit current Isc continues to flow in the inverter circuit41, and the DC capacitor43is not charged.

As shown inFIG.6, the DC current measured after the lapse of the predetermined length of time t1from turn-on of the DC switch30, and the voltage of the DC capacitor43during the period when the device short-circuit70is occurring are different from those during the period when no device short-circuit70is occurring. This is because of the nature of the DC capacitor43. For this reason, the protection determiner47takes advantage of such a difference between the current values and voltage values to determine whether or not an abnormality such as a device short-circuit70is occurring in the inverter circuit41. The determination is made by one or both of the following two methods.

In other words, in Step S9, as the first method, the protection determiner47determines whether or not the ammeter44has detected a decrease in current. Upon determination that the ammeter44has detected a decrease in current (FIG.6A), the protection determiner47determines that no abnormality is occurring and ends the process of this flowchart. In contrast, upon determination that the ammeter44has not detected a decrease in current (FIG.6B), the protection determiner47determines that an abnormality is occurring and makes the process proceed to Step S10.

In Step S9, as the second method, the protection determiner47determines whether or not the voltmeter45has detected a voltage when the ammeter44detects a current. Upon determination that the ammeter44has detected a current and the voltmeter45has detected a voltage (FIG.6C), the protection determiner47determines that no abnormality is occurring and ends the process of this flowchart. In contrast, upon determination that the ammeter44has detected a current and the voltmeter45has not detected a voltage (FIG.6D), the protection determiner47determines that an abnormality is occurring and makes the process proceed to Step S10.

In Step S10, the protection determiner47issues an open operation command to the DC switch30and ends the process of this flowchart. Opening the DC switch30prevents the subsequent inflow of the short-circuit current Isc into the inverter circuit41.

<Effects of Embodiment>

As described above, in the first method in the embodiment shown inFIGS.1to6, when the ammeter44does not detect a decrease in current after a predetermined length of time has elapsed since the DC switch30is turned on, the protection determiner47issues an open operation command to the DC switch30. Consequently, even if the PV fuse20is not completely blown in the event of the device short-circuit70, the abnormality is detected more quickly upon restart of the power converter40, which contributes to a decrease in the expansion of secondary damage such as breakage and burnout of the power converter40.

In the second method, when the ammeter44detects a current but the voltmeter45does not detect a voltage after a predetermined length of time has elapsed since the DC switch30is turned on, the protection determiner47issues an open operation command to the DC switch30. Consequently, even if the PV fuse20is not completely blown in the event of the device short-circuit70, the abnormality is detected more quickly upon restart of the power converter40, which contributes to a decrease in the expansion of secondary damage such as breakage and burnout of the power converter40.

<Supplementary Information for Embodiment>

In the embodiment shown inFIGS.1to6, the voltmeter45is included in the configuration shown inFIG.1. However, if the first method is adopted, the voltmeter45is not necessarily included in the configuration. This is because the value of the voltage of the DC capacitor43is not taken into consideration when the first method is adopted in Step S9. In this case, the processing in Step S8inFIG.2may be omitted. This provides the same effect as that of the embodiment shown inFIGS.1to6.

In the embodiment shown inFIGS.1to6, the ammeter44is included in the configuration shown inFIG.1. However, when the second method is adopted, the ammeter44is not necessarily included in the configuration. This is because it is possible that, when the second method is adopted in Step S9, as a modification of the second method, the protection determiner47detects the occurrence of an abnormality, such as a device short-circuit70, depending only on whether or not the voltmeter45detects a voltage after a lapse of a predetermined length of time. In this case, the processing in Step S7inFIG.2may be omitted. This provides the same effect as that of the embodiment shown inFIGS.1to6.

The detailed description above will clarify the features and advantages of the embodiment. This is for the purpose of showing that the claims cover the features and advantages of the embodiment described above without departing from their spirit and scope. In addition, those skilled in the art should be able to easily conceive any improvements or changes. Therefore, the scope of the embodiment having novelty is not intended to be limited to the above description, and the claims can also be based on appropriate improvements and equivalents included in the scope disclosed in the embodiment.

REFERENCE SIGNS LIST

1,1′ Photovoltaic system10,10A,10B,10C Solar cell panel20PV fuse30DC switch40Power converter41Inverter circuit42Inverter control circuit43DC capacitor44Ammeter45Voltmeter46Timer47Protection determiner50,50a,50b,50cElectrical circuit (DC bus)51Connection point60,60a,60b,60c,60dSignal line70Device short-circuit80Ground-fault85Ground-fault current90Short-circuit current (Isc)