Photovoltaic system

1. Field of the Invention

Embodiments described herein relate generally to a photovoltaic system.

2. Description of the Related Art

In general, a photovoltaic system is known to convert direct-current (DC) power generated by a photovoltaic cell to alternating-current (AC) power and supply the AC power to an AC load. To convert DC power to AC power, an inverter is used.

In contrast, a power conversion system including a plurality of inverters in which the AC sides are connected in parallel is known. For example, it is disclosed that a plurality of inverters are controlled by separating them into a setting value less than a value obtained by dividing a power value required as the sum of the output power of each of the inverters by the number of inverters and a setting value greater than the above obtained value (see WO 2013/145262 A). For example, it is disclosed that the total output power of a plurality of inverters is controlled by adjusting the output power of each of the inverters in a control period shorter than a communication period between a controller and each inverter (see WO 2013/145263 A).

However, in a photovoltaic system, the electricity outputs of a plurality of photovoltaic cells may differ from each other. Therefore, if all of a plurality of inverters are controlled in the same manner at all times, the electricity output of the whole photovoltaic system may be reduced in comparison with the power which could be generated essentially because of a decrease in the electricity outputs of some of the photovoltaic cells.

Embodiments described herein aim to provide a photovoltaic system which can prevent a decrease in the electricity output of the whole system even when the electricity outputs of some photovoltaic cells are reduced.

In accordance with one embodiment, a photovoltaic system includes photovoltaic cells; inverters corresponding to the photovoltaic cells, respectively; a reduced electricity output state determiner which determines whether each of the photovoltaic cells corresponding to the inverters is in a reduced electricity output state where an electricity output is less than a predetermined standard; a first command value generator which generates a first command value for decreasing a first power factor of a first inverter determined as in the reduced electricity output state out of the inverters by the reduced electricity output state determiner; a second command value generator which generates a second command value for increasing a second power factor of at least one of second inverters so as to compensate for output power of the first inverter controlled based on the first command value generated by the first command value generator; the second inverters being not determined as in the reduced electricity output state out of the inverters by the reduced electricity output state determiner; and a controller which controls the inverters based on the first command value generated by the first command value generator and the second command value generated by the second command value generator.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a structural diagram showing the structure of a photovoltaic system according to the first embodiment of the present invention.

FIG. 2 is a structural diagram showing the structure of a controller according to the first embodiment.

FIG. 3 is a vector diagram showing a method of obtaining command values by a command value generation unit when a power factor is decreased according to the first embodiment.

FIG. 4 is a vector diagram showing a method of obtaining command values by the command value generation unit when a power factor is increased according to the first embodiment.

FIG. 5 is a vector diagram showing the output power of each inverter when no inverter is in a reduced electricity output state under control by the controller according to the first embodiment.

FIG. 6 is a vector diagram showing the output power of each inverter when one of the inverters is in a reduced electricity output state under control by the controller according to the first embodiment.

FIG. 7 is a structural diagram showing the structure of a controller according to the second embodiment of the present invention.

FIG. 8 is a vector diagram showing the output power of each inverter when one of the inverters is in a reduced electricity output state under control by the controller according to the second embodiment.

First Embodiment

FIG. 1 is a structural diagram showing the structure of a photovoltaic system 10 according to the first embodiment of the present invention. The same portions shown in the drawings are denoted by like reference numbers. Thus, the detailed explanation of such portions is omitted. Different portions are mainly explained.

The photovoltaic system 10 comprises a controller 1 , three photovoltaic cells 2 a , 2 b and 2 c , three inverters 3 a , 3 b and 3 c , three interconnection transformers 4 a , 4 b and 4 c , three AC current detectors 5 a , 5 b and 5 c , and an AC voltage detector 6 . Here, the photovoltaic system 10 comprises three inverters 3 a to 3 c . However, any number of inverters may be provided in the photovoltaic system 10 as long as the number of inverters is two or more.

The photovoltaic system 10 converts DC power generated by photovoltaic cells 2 a to 2 c to AC power which is used to interconnect a power system 9 . The photovoltaic system 10 supplies power to the power system 9 . For example, the photovoltaic system 10 is a mega solar system.

Photovoltaic cells 2 a to 2 c generate power from sunlight. Each of photovoltaic cells 2 a to 2 c may comprise any number of photovoltaic cell segments.

Inverters 3 a to 3 c convert DC power supplied from their respective photovoltaic cells 2 a to 2 c to AC power synchronized with the system voltage of the power system 9 . Inverters 3 a to 3 c supply AC power to the power system 9 via their respective interconnection transformers 4 a to 4 c . For example, inverters 3 a to 3 c are power conditioners.

Interconnection transformers 4 a to 4 c are provided on the output side (AC side) of inverters 3 a to 3 c , respectively. Interconnection transformers 4 a to 4 c changes the AC voltage input from inverters 3 a to 3 c , respectively, to the system voltage of the power system 9 .

AC current detectors 5 a to 5 c are provided on the output side of inverters 3 a to 3 c , respectively. AC current detectors 5 a to 5 c detect output currents I 1 , I 2 and I 3 of inverters 3 a to 3 c , respectively, and output the detected values to the controller 1 as detection signals.

The AC voltage detector 6 is provided on the output side of inverters 3 a to 3 c . The AC voltage detector 6 detects output voltage Vs of inverters 3 a to 3 c and outputs the detected value to the controller 1 as a detection signal. In place of only one AC voltage detector 6 , a plurality of AC voltage detectors may be provided on the output side of inverters 3 a to 3 c , respectively. In this case, a structure similar to that of the present embodiment can be obtained by using the output voltage detected by each AC voltage detector for the control of corresponding inverter 3 a , 3 b or 3 c in place of output voltage Vs detected by the AC voltage detector 6 of the present embodiment.

The controller 1 controls the output power of each of inverters 3 a to 3 c based on their respective output currents I 1 to I 3 detected by AC current detectors 5 a to 5 c , respectively, and output voltage Vs detected by the AC voltage detector 6 as the output voltage of inverters 3 a to 3 c.

FIG. 2 is a structural diagram showing the structure of the controller 1 according to the present embodiment.

The controller 1 comprises a reduced electricity output state determination unit 11 , a command value generation unit 12 and a command value output unit 13 .

The reduced electricity output state determination unit 11 determines whether the electricity output of photovoltaic cell 2 a , 2 b or 2 c connected to its inverter is less than that of the other photovoltaic cells (this state is referred to as a reduced electricity output state) based on output voltage Vs of inverters 3 a to 3 c and output currents I 1 to I 3 of inverters 3 a to 3 c , respectively. The reduced electricity output state determination unit 11 transmits data including output voltage Vs of inverters 3 a to 3 c and output currents I 1 to I 3 of inverters 3 a to 3 c , respectively, as information, to the command value generation unit 12 along with a result of determination. For example, the reduced electricity output state is produced when the electricity output of one or some of photovoltaic cells 2 a to 2 c is reduced in comparison with a normal state (for example, fine weather) because of weather. For example, when photovoltaic cells 2 a to 2 c are partially covered by clouds, corresponding inverters 3 a to 3 c are in a reduced electricity output state. Hereinafter, this specification explains a case where only one of inverters 3 a to 3 c is determined as in a reduced electricity output state. However, any number of inverters 3 a to 3 c may be determined as in a reduced electricity output state except for all of the inverters.

For example, whether inverters 3 a to 3 c are in a reduced electricity output state is determined as follows. This specification explains a method of determining the reduced electricity output state of the first inverter 3 a . However, this method is also applicable to the other inverters 3 b and 3 c . The reduced electricity output state determination unit 11 calculates the mean value of active elements (active currents) of all of output currents I 1 to I 3 , or the mean value of active elements of output currents I 2 and I 3 other than output current I 1 of the target inverter. When the active element of output current I 1 of inverter 3 a is less than the standard determined based on the calculated mean value, the reduced electricity output state determination unit 11 determines that inverter 3 a is in a reduced electricity output state. For example, the standard determined based on the mean value may be a value obtained by subtracting a predetermined value from the mean value, or may be a value obtained by multiplying a proportion less than one by the mean value.

The reduced electricity output state may be determined in any way as long as it is possible to determine whether the active elements of output currents I 1 to I 3 of inverters 3 a to 3 c , respectively, are reduced. For example, any statistical value may be used in place of the above mean values. The electricity outputs of photovoltaic cells 2 a to 2 c when the weather is unsuitable for electricity generation (for example, when it is cloudy) may be set as the standard in advance in the reduced electricity output state determination unit 11 . The reduced electricity output state determination unit 11 is capable of determining whether the active element of each of output currents I 1 to I 3 is reduced by comparing the active element of the output current with the standard set in advance.

The command value generation unit 12 generates command values R 1 , R 2 and R 3 for controlling the power factor of the output power of each of inverters 3 a to 3 c , respectively, based on the result of determination of the reduced electricity output state determination unit 11 , output currents I 1 to I 3 of inverters 3 a to 3 c , respectively, and output voltage Vs of inverters 3 a to 3 c . Command values R 1 to R 3 may include a command value for controlling the active power or reactive power output from the respective inverters 3 a to 3 c . The command value generation unit 12 outputs generated command values R 1 to R 3 of inverters 3 a to 3 c to the command value output unit 13 .

The command value output unit 13 outputs command values R 1 to R 3 to inverters 3 a to 3 c , respectively. In this manner, the output power of each of inverters 3 a to 3 c is controlled in accordance with their respective command values R 1 to R 3 .

Now, this specification explains how command values R 1 to R 3 are obtained by the command value generation unit 12 .

The explanation is begun with restrictions on command values R 1 to R 3 .

The active power [W] output from each of inverters 3 a to 3 c is defined as P 1 , P 2 and P 3 , respectively. The reactive power [Var] output from each of inverters 3 a to 3 c is defined as Q 1 , Q 2 and Q 3 , respectively. The maximum output power [VA] determined by the specification of each of inverters 3 a to 3 c , such as the capacity, is defined as VA 1 max, VA 2 max and VA 3 max, respectively. The electricity outputs [W] of photovoltaic cells 2 a to 2 c supplying power to inverters 3 a to 3 a are defined as P 1 max, P 2 max and P 3 max, respectively. In this case, each active power P 1 to P 3 and each reactive power Q 1 to Q 3 output from inverters 3 a to 3 c , respectively, must satisfy the following relationships.

P 1≤ P 1max  (1)

P 2≤ P 2max  (2)

P 3≤ P 3max  (3)

sqrt( P 1{circumflex over ( )}2+ Q 1{circumflex over ( )}2)≤ VA 1max  (4)

sqrt( P 2{circumflex over ( )}2+ Q 2{circumflex over ( )}2)≤ VA 2max  (5)

sqrt( P 3{circumflex over ( )}2+ Q 3{circumflex over ( )}2)≤ VA 3max  (6)

Here, “{circumflex over ( )}” is an operator indicating the power of the value, and “sqrt( )” is a function for calculating the square root of the value in parentheses.

Hereinafter, this specification assumes that all of relationships (1) to (6) are always satisfied unless otherwise stated.

FIG. 5 is a vector diagram showing output power VA 1 , VA 2 and VA 3 [VA] of each of inverters 3 a to 3 c , respectively, when no inverter is in a reduced electricity output state under control by the controller 1 according to the first embodiment. FIG. 6 is a vector diagram showing output power VA 1 , VA 2 and VA 3 [VA] of each of inverters 3 a to 3 c , respectively, when inverter 3 a is in a reduced electricity output state under control by the controller 1 according to the first embodiment. In the figures, power factors are shown by the angles of each output power VA 1 to VA 3 having vectors. When the power factor is 0, the angle is 0°. When the power factor is 1, the angle is 90°.

When no inverter is in a reduced electricity output state (in other words, when all of the inverters are in a normal state) as a result of determination of the reduced electricity output state determination unit 11 , each output power VA 1 to VA 3 of inverters 3 a to 3 c , respectively, is controlled with the same power factor. Thus, in this case, each output power VA 1 to VA 3 has the same angle of θ 0 . Thus, the sum of each output power VA 1 to VA 3 of each of inverters 3 a to 3 c is also the power factor shown by an angle of θ 0 . This power factor is a power factor determined in advance and requiring supply to the power system 9 .

When there is an inverter in a reduced electricity output state as a result of determination of the reduced electricity output state determination unit 11 , the power factors of inverters 3 a to 3 c are determined as follows. This specification assumes that the electricity output is supplied excessively to the inverters which are not in a reduced electricity output state such that each active power is increased by the increase in the power factors.

This specification explains a case where the reduced electricity output state determination unit 11 determines that the first inverter 3 a is in a reduced electricity output state. In this case, the command value generation unit 12 obtains command value R 1 such that the power factor of the first inverter 3 a is less than that in a normal state.

FIG. 3 is a vector diagram showing a method of obtaining command values R 1 to R 3 by the command value generation unit 12 when a power factor is decreased. FIG. 4 is a vector diagram showing a method of obtaining command values R 1 to R 3 by the command value generation unit 12 when a power factor is increased. The semicircular curve Cmax shows the set of operational points of the maximum output power [VA] of each of inverters 3 a to 3 c.

The output power of the first inverter 3 a in a normal state (when the first inverter 3 a is not in a reduced electricity output state) is at operational point S 0 in FIG. 3 . The electricity output of photovoltaic cell 2 a conforms to active power P 1 output from the first inverter 3 a . Assuming that the output from the first inverter 3 a is decreased from active power P 0 to active power Pa because of a decrease in electricity output of photovoltaic cell 2 a , the command value generation unit 12 obtains operational point Sa which is active power Pa on the curve Cmax. The command value generation unit 12 obtains a power factor such that the output power of the first inverter 3 a is at operational point Sa. At this time, the power factor is equivalent to angle θa shown in FIG. 3 . The command value generation unit 12 generates command value R 1 in conformity with the obtained power factor. By means of command value R 1 , the operational point of the output power of the first inverter 3 a is transferred from operational point S 0 to operational point Sa. The power factor of output power VA 1 of the first inverter 3 a is decreased from angle θ 0 to angle θa. In this manner, the reactive power output from the first inverter 3 a is increased by the decrease in the active power. The reactive power output from the first inverter 3 a is increased from reactive power Q 0 to reactive power Qa.

Each output power VA 2 and VA 3 of the second and third inverters 3 b and 3 c , respectively, which are not in a reduced electricity output state is controlled so as to compensate for the change in output power VA 1 of the first inverter 3 a . To perform this control, the command generation unit 12 obtains command values R 2 and R 3 such that the power factor of each of the second and third inverters 3 b and 3 c is greater than that in a normal state.

Each output power VA 2 and VA 3 of the second and third inverters 3 b and 3 c , respectively, in a normal state is at operational point S 0 in FIG. 4 . When each output power VA 2 and VA 3 of two inverters 3 b and 3 c , respectively, is controlled so as to be transferred from operational point S 0 to operational point Sb on the curve Cmax, the command value generation unit 12 obtains the power factor of operational point Sb. At this time, the power factor is equivalent to angle θb shown in FIG. 4 . The command value generation unit 12 generates command values R 2 and R 3 in conformity with the obtained power factor. By means of command values R 2 and R 3 , the operational points of two inverters 3 b and 3 c are transferred from operational point S 0 to operational point Sb. The power factors of two inverters 3 b and 3 c are increased from angle θ 0 to angle θb. In this manner, the active power output from each of two inverters 3 b and 3 c is increased by the decrease in the reactive power. The active power output from each of two inverters 3 b and 3 c is increased from active power P 0 to active power Pb.

Now, this specification explains the control applied to inverters 3 a to 3 c by the controller 1 when inverter 3 a is in a reduced electricity output state with reference to FIG. 6 .

The controller 1 performs control such that, even when inverter 3 a in a reduced electricity output state is detected, operational point Sf which is the sum of each output power VA 1 to VA 3 of inverters 3 a to 3 c , respectively, is the same as operational point Sf of FIG. 5 before detection.

The controller 1 increases reactive power Q 1 by the decrease in active power P 1 by decreasing the power factor of inverter 3 a in a reduced electricity output state from angle θ 0 to angle θ 1 .

Subsequently, the controller 1 generates command values R 2 and R 3 to be used to compensate for the change by the cooperation of two inverters 3 b and 3 c , based on the difference between operational point S 1 of output power VA 1 of inverter 3 a and operational point Sf, as the target, which is the sum of each output power VA 1 to VA 3 . As shown in FIG. 6 , when each power factor of output power VA 2 and VA 3 of two inverters 3 b and 3 c , respectively, is angle θ 2 , the sum of each output power VA 1 to VA 3 is at operational point Sf. The controller 1 generates command values R 2 and R 3 such that the power factors are set to angle θ 2 .

Command values R 2 and R 3 generated in the above manner are used to perform control such that each output power VA 2 and VA 3 of inverters 3 b and 3 c , respectively, which are not in a reduced electricity output state cooperates to compensate for output power VA 1 of inverter 3 a which is in a reduced electricity output state.

Specifically, reactive power Q 1 is increased by the decrease in active power P 1 by decreasing the power factor of output power VA 1 of inverter 3 a which is in a reduced electricity output state. In contrast, each active power P 2 and P 3 having redundant energy is increased by the decrease in each reactive power Q 2 and Q 3 by increasing the power factor of each output power VA 2 and VA 3 of inverters 3 b and 3 c , respectively, which are not in a reduced electricity output state.

In this manner, the controller 1 controls the active power element, reactive power element and power factor of the sum of each output power VA 1 to VA 3 of each of inverters 3 a to 3 c , respectively, so as to be a value (an operational point) which is substantially the same as that before the electricity output of photovoltaic cell 2 a is reduced.

According to the present embodiment, when the electricity output of a certain photovoltaic cell 2 a is reduced, it is possible to decrease output power VA 1 of inverter 3 a which receives power supply from photovoltaic cell 2 a and increase the power factors of each output power VA 2 and VA 3 of the other inverters 3 b and 3 c , respectively. In this manner, each output power VA 2 and VA 3 of the remaining inverters 3 b and 3 c , respectively, can cooperate to compensate for the change in output power VA 1 of inverter 3 a which is in a reduced electricity output state.

Even if the electricity output of a certain photovoltaic cell 2 a is reduced, the operational point (specifically, the active power, reactive power and power factor) of the sum of each output power VA 1 to VA 3 of inverters 3 a to 3 c , respectively, can be substantially the same as that before detection.

Second Embodiment

FIG. 7 is a structural diagram showing the structure of a controller 1 A according to the second embodiment of the present invention.

In the photovoltaic system of the present embodiment, the controller 1 A of the photovoltaic system 10 of the first embodiment shown in FIG. 1 is replaced by the controller 1 A shown in FIG. 7 . The other structures are the same as those of the photovoltaic system 10 of the first embodiment.

In the controller 1 A, the command value generation unit 12 of the controller 1 of the first embodiment shown in FIG. 2 is replaced by a command value generation unit 12 A. The other structures are the same as those of the controller 1 of the first embodiment. Here, portions different from those of control performed by the controller 1 of the first embodiment are mainly explained.

When none of inverters 3 a to 3 c is in a reduced electricity output state as a result of determination of a reduced electricity output state determination unit 11 , the controller 1 A controls all of inverters 3 a to 3 c with the same power factor in a manner similar to that of the first embodiment shown in FIG. 5 .

FIG. 8 is a vector diagram showing each output power VA 1 to VA 3 [VA] of inverters 3 a to 3 c , respectively, when inverter 3 a is in a reduced electricity output state under control by the controller 1 A according to the present embodiment.

The control for decreasing the power factor of the first inverter 3 a in reduced electricity output state is the same as that of the first embodiment.

The command value generation unit 12 A selects one of inverters 3 b and 3 c , which are not in a reduced electricity output state. Here, the second inverter 3 b is assumed to be selected. Output power VA 2 of the selected second inverter 3 b is controlled so as to compensate for output power VA 1 of inverter 3 a . Command value R 2 A of the second inverter 3 b is generated such that the sum of output power VA 1 of the first inverter 3 a in a reduced electricity output state and output power VA 2 of the second inverter 3 b selected for compensation is the same as that before the first inverter 3 a is transitioned to a reduced electricity output state.

Specifically, command value R 2 A of the second inverter 3 b is obtained as follows.

The power factor of the second inverter 3 b is obtained such that the sum of output power VA 1 and VA 2 of the first inverter 3 a in a reduced electricity output state and the second inverters 3 b for compensation is the same as that before the first inverter 3 a is transitioned to a reduced electricity output state. Thus, the operational point (specifically, the active power, reactive power and power factor) of the sum of output power VA 1 and VA 2 is controlled so as to be kept the same by increasing the power factor of the second inverter 3 b by the decrease in the power factor of the first inverter 3 a . The method of increasing the power factor is the same as that of the first embodiment shown in FIG. 4 . The command value generation unit 12 A generates command value R 2 A such that output power VA 2 of the second inverter 3 b is the obtained power factor.

The third inverter 3 c which is not selected for compensation is controlled with the power factor which is applied before the first inverter 3 a is transitioned to a reduced electricity output state. Thus, command value R 3 A which is generated for the third inverter 3 c by the command value generation unit 12 A is the same as that before the first inverter 3 a is transitioned to a reduced electricity output state. If active power P 2 which can be output from the second inverter 3 b is not enough to compensate for the decrease in active power P 1 output from the first inverter 3 a , the third inverter 3 c is also selected for compensation. In this case, command value R 3 A for the third inverter 3 c is obtained in a manner similar to that of command value R 2 A for the second inverter 3 b . By means of command value R 3 A, the power factor of output power VA 3 of the third inverter 3 c is increased so as to supplement the insufficient compensation of output power VA 2 of the second inverter 3 b.

According to the present embodiment, it is possible to obtain an effect similar to that of the first embodiment by decreasing the power factor of output power VA 1 in a reduced electricity output state and increasing the power factor of output power VA 2 of the second inverter 3 b selected for compensation.

There is no need to change the power factor of output power VA 3 of the third inverter 3 c which is not selected for compensation. Thus, the number of inverters 3 a to 3 c to which control change is applied can be reduced.

It is to be noted that the present invention is not restricted to the foregoing embodiments, and constituent elements can be modified and changed into shapes without departing from the scope of the invention at an embodying stage. Additionally, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the foregoing embodiments. For example, several constituent elements may be eliminated from all constituent elements disclosed in the embodiments. Furthermore, constituent elements in the different embodiments may be appropriately combined.