Inverter system for photovoltaic power generation

Abstract

Provided is an inverter system capable of more economically and efficiently performing photovoltaic power generation by automatically switching an integrated operation and an independent operation of inverters according to voltage values and current values of photovoltaic panels without a separate communication function. The inverter system for photovoltaic power generation according to an exemplary embodiment of the present disclosure is an inverter system which changes direct current power output from a first photovoltaic panel and a second photovoltaic panel to alternating current power and includes: a first inverter and a second inverter, in which all of the outputs of the first and second photovoltaic panels are applied to the first inverter, or the output of the first photovoltaic panel is applied to the first inverter, and the output of the second photovoltaic panel is applied to the second inverter according to output values of the first and second photovoltaic panels.

Claims

1. An inverter system comprising: a first photovoltaic panel; a first inverter including a first switch and a first controller, wherein the first inverter is connected to the first photovoltaic panel; a second photovoltaic panel; and a second inverter including a second switch and a second controller, wherein the second inverter is connected to the second photovoltaic panel; wherein according to output values of the first and second photovoltaic panels, through on/off method of the first and second switches, all of the outputs of the first and second photovoltaic panels are applied to the first inverter, or the output of the first photovoltaic panel is applied to the first inverter and the output of the second photovoltaic panel is applied to the second inverter, wherein the first inverter and the second inverter are connected with a power cable, wherein the first and second switches are operated separately by the first and second controller, wherein the first inverter and the second inverter serve as a master inverter and a slave inverter, respectively, wherein in response to determining that the first inverter has failed, independent operation of the second inverter is performed by fixing a switch to a second inverter side, and wherein to determine whether the first inverter is in operation based on a switch state, a voltage of the second photovoltaic panel is temporarily increased to a value of a photovoltaic panel open voltage, wherein the first switch is located inside the first inverter and is connected to the second photovoltaic panel in order to additionally receive the output of the second photovoltaic panel, and the second switch is located inside the second inverter and is connected between the second photovoltaic panel and the first inverter in order to receive the output of the second photovoltaic panel or switch the output of the second photovoltaic panel to the first inverter to apply the output.

2. The inverter system of claim 1, wherein when the output value of the first photovoltaic panel is smaller than a predetermined power value, all of the outputs of the first and second photovoltaic panels are applied to the first inverter, and when the output value of the first photovoltaic panel is equal to or larger than the predetermined power value, the output of the first photovoltaic panel is applied to the first inverter, and the output of the second photovoltaic panel is applied to the second inverter.

3. The inverter system of claim 1, wherein when the output value of the second photovoltaic panel is smaller than a predetermined power value, all of the outputs of the first and second photovoltaic panels are applied to the first inverter, and when the output value of the second photovoltaic panel is equal to or larger than the predetermined power value, the output of the first photovoltaic panel is applied to the first inverter, and the output of the second photovoltaic panel is applied to the second inverter.

4. The inverter system of claim 1, wherein the first inverter and the second inverter are connected only with the power cable, which is transmitting electric power.

5. The inverter system of claim 1, wherein when the output value of the second photovoltaic panel is smaller than a predetermined power value, the first controller turns on the first switch, and the second controller switches the second switch to the first inverter side, and when the output value of the second photovoltaic panel is equal to or larger than the predetermined power value, the first controller turns off the first switch, and the second controller switches the second switch to the second inverter side.

6. The inverter system of claim 1, wherein the first and second controllers control so that the first and second inverters are operated at a maximum power point by monitoring voltages and current input from the first and second photovoltaic panels.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating a change in efficiency of a general photovoltaic inverter.

(2) FIG. 2 is a graph illustrating a change in an output voltage, an output current characteristic, and a maximum power point of a general photovoltaic panel.

(3) FIGS. 3 and 4 are configuration diagrams of an inverter system for photovoltaic power generation according to an exemplary embodiment of the present disclosure operated under different photovoltaic conditions.

DETAILED DESCRIPTION

(4) The aforementioned objects, characteristics, and advantages will be described below with reference to the accompanying drawings, and thus those skilled in the art to which the present disclosure pertains will easily implement the technical spirit of the present disclosure. In the following description, a detailed explanation of known related functions and constitutions may be omitted so as to avoid unnecessarily obscuring the subject manner of the present disclosure. Hereinafter, an exemplary embodiment according to the present disclosure will be described with reference to the accompanying drawings in detail.

(5) FIG. 2 illustrates a voltage-current output characteristic of a general photovoltaic panel. A density of photovoltaic energy is changed according to a time and a weather condition, and accordingly, an output of a photovoltaic panel is also changed. An inverter for photovoltaic power generation has a maximum power point tracking (MPPT) function in order to obtain maximum power, and adjusts a voltage-current output condition of the photovoltaic panel to a value at which maximum power may be generated according to a change in a density of photovoltaic energy.

(6) In this case, as illustrated in FIG. 1, when the inverter is operated in a section between W.sub.FS-min to W.sub.FS-max in which efficiency is high if possible, entire efficiency may be improved.

(7) FIGS. 3 and 4 are configuration diagrams of an inverter system for photovoltaic power generation according to an exemplary embodiment of the present disclosure operated under different photovoltaic conditions.

(8) Referring to FIGS. 3 and 4, the inverter system for photovoltaic power generation according to the exemplary embodiment of the present disclosure is an inverter system for changing direct current power output from a first photovoltaic panel 1 and a second photovoltaic panel 2 to alternating current power, and includes a first inverter 10 and a second inverter 20. In the inverter system according to the exemplary embodiment, all of the outputs of the first and second photovoltaic panels 1 and 2 are applied to the first inverter 10, or the output of the first photovoltaic panel 1 may be applied to the first inverter 10, and the output of the second photovoltaic panel 2 may be applied to the second inverter 20 according to output values of the first and second photovoltaic panels 1 and 2.

(9) In the present exemplary embodiment, the first inverter 10 and the second inverter 20 make one pair to serve as a master inverter and a slave inverter, respectively.

(10) The first and second inverters 10 and 20 include first and second controllers 1 and 2, respectively, and the first and second controllers 1 and 2 may control so that the respective inverters 10 and 20 are operated at a maximum power point by monitoring voltages and current input from the first and second photovoltaic panels 1 and 2.

(11) When the output value of the first photovoltaic panel 1 is smaller than a predetermined power value, the first and second inverters 10 and 20 may make the outputs of all of the first and second photovoltaic panels 1 and 2 be applied to the first inverter 10 (an integrated operation), and when the output value of the first photovoltaic panel 1 is equal to or higher than the predetermined power value, the first and second inverters 10 and 20 may make the output of the first photovoltaic panel 1 be applied to the first inverter 10, and the output of the second photovoltaic panel 2 be applied to the second inverter 20 (an independent operation).

(12) To this end, the first inverter 10 may include a first switch 11 adopting an on/off method connected to the second photovoltaic panel 2 in order to additionally receive the output of the second photovoltaic panel 2.

(13) The second inverter 20 may include a second switch 21 connected between the second photovoltaic panel 2 and the first inverter 10 in order to receive the output of the second photovoltaic panel 2, or switch the output of the second photovoltaic panel 2 to the first inverter 10 to apply the output.

(14) The first switch 11 may maintain an open state (off) as a basic value when a separate control is not performed by the first controller 1 of the first inverter 10, and the second switch 21 may maintain a state connected to the second inverter 20 as a basic value when a separate control is not performed by the second controller 2 of the second inverter 20.

(15) Hereinafter, for simplification of the description of the technique, it is assumed that performance of the first and second photovoltaic panels 1 and 2 is the same, performance of the first and second inverters 10 and 20 connected to the first and second photovoltaic panels 1 and 2 is the same, and a maximum input of the inverter is matched to a maximum output of the photovoltaic panel. A change in an output of the photovoltaic panel according to a temperature change, contamination of the panel, and aged deterioration is not considered. Symbols for describing a characteristic and an operation of the inverter will be described below.

(16) W.sub.FS-min: Minimum output power set for an efficient operation of the inverter

(17) W.sub.FS-max: Maximum output power set for an efficient operation of the inverter

(18) W.sub.in-max: Maximum input power enabling the inverter to be normally operated

(19) W.sub.in-min: Minimum input power enabling the inverter to normally continue an operation

(20) W.sub.in-start: Minimum input power enabling the inverter to normally start an operation in a stop state

(21) V.sub.M-max: Maximum input voltage of the first inverter 10

(22) I.sub.M-max: Maximum input current of the first inverter 10

(23) V.sub.M-in: Input voltage of the first inverter 10

(24) I.sub.M-in: Input current of the first inverter 10

(25) W.sub.M-in: Input power of the first inverter 10, V.sub.M-inI.sub.M-in

(26) V.sub.S-max: Maximum input voltage of the second inverter 20

(27) I.sub.S-max: Maximum input current of the second inverter 20

(28) V.sub.S-in: Input voltage of the second inverter 20

(29) I.sub.S-in: Input current of the second inverter 20

(30) W.sub.S-in: Input power of the second inverter 20, V.sub.S-inI.sub.S-in

(31) V.sup.i.sub.OC: Open voltage of the photovoltaic panel (when a insolation condition is i)

(32) I.sup.i.sub.SC: Short current of the photovoltaic panel (when a insolation condition is i)

(33) V.sup.i.sub.pmax: Output voltage of the photovoltaic panel at a maximum power point (when a insolation condition is i)

(34) V.sup.i.sub.pmax: Output of the photovoltaic panel at a maximum power point (when a insolation condition is i)

(35) V.sub.1: Output voltage of the first photovoltaic panel 1

(36) I.sub.1: Output current of the first photovoltaic panel 1

(37) W.sub.1: Output power of the first photovoltaic panel 1, V.sub.1I.sub.1

(38) V.sub.2: Output voltage of the second photovoltaic panel 2

(39) I.sub.2: Output current of the second photovoltaic panel 2

(40) W.sub.2: Output power of the second photovoltaic panel 2, V.sub.2I.sub.2

(41) At a time zone from immediately after a sunrise to the morning in which an altitude of the sun is not high, as illustrated in FIG. 3, the second switch 21 is switched to the first inverter 10 side, and the first switch 11 is short (on). Accordingly, the first and second photovoltaic panels 1 and 2 are connected in parallel, and the output power of the second photovoltaic panel 2 is combined with the output of the first photovoltaic panel 1 while passing through from a DC output terminal 23 at an upper end of the second switch 21 to a DC input terminal 13 at a lower end of the first switch 11 to be applied to the first inverter 10. In this case, the power W.sub.M-in supplied to the first inverter 10 is V.sub.1(I.sub.1+I.sub.2)=2(V.sub.1I.sub.1). Through this, the first inverter 10 may be operated when the output of each of the photovoltaic panels 1 and 2 is only equal to or larger than of the minimum value W.sub.in-min at which the operation of the inverter is possible, so that it is possible to advance a power generation start time, and the inverter may be operated in an operation efficiency range higher than that of each independent operation.

(42) When the outputs of the first and second photovoltaic panels 1 and 2 are increased to reach a range where the first inverter 10 and the second inverter 20 may be independently and efficiently operated according to the increase in the altitude of the sun, the first switch 11 is opened and the second switch 21 is switched to the second inverter 20 side as illustrated in FIG. 4. Accordingly, the first inverter 10 and the second inverter 20 may be separated and independently operated.

(43) A time point for switching to the independent operation is determined by comparing the power value V.sub.M-in input in the first inverter 10 with the minimum power value W.sub.FS-min or the maximum power value W.sub.FS-max set for the efficient operation of the inverter.

(44) That is, when the operation is switched to the independent operation in a case where W.sub.M-in>(2W.sub.FS-min) and WM-in>W.sub.FS-max, the operation is possible in a section in which efficiency of the first inverter 10 and the second inverter 20 is high.

(45) When the first switch 11 is opened for the independent operation of the first inverter 10, the voltage V.sub.2 of the second photovoltaic panel 2 is momentarily increased to the value of the open voltage V.sup.i.sub.OC. The second inverter 20 detects a value in the output voltage V.sub.2 of the second photovoltaic panel 2 and a variation of the value of the output voltage V.sub.2, and when it is determined that the first switch 11 is opened, the second inverter 20 switches the second switch 21 to the second inverter 20 side to start the independent operation.

(46) Separately, the second inverter 20 may independently determine the start of the independent operation. In the normally operated system, the output voltage of the photovoltaic panel is maintained with V.sup.i.sub.pmax by an MPPT function of the inverter. Accordingly, the output value W.sub.2=W.sup.i.sub.pmax of the second photovoltaic panel 2 may be recognized from V.sup.i.sub.pmax. Otherwise, the output value W.sub.2=V.sub.2I.sub.2 of the second photovoltaic panel 2 may also be obtained by simultaneously measuring the output current I.sub.2 of the second photovoltaic panel 2. In a case where the output W.sub.2 of the second photovoltaic panel 2 is W.sub.2>W.sub.FS-min and W.sub.2>W.sub.FS-max, the second switch 21 is switched to the second inverter 20 side to start the independent operation.

(47) When the altitude of the sun is decreased in the afternoon, as illustrated in FIG. 3, the outputs of the first and second photovoltaic panels 1 and 2 are integrated to the first inverter 10 again for the operation, thereby improving efficiency and extending a power generation time. When the input power W.sub.M-in of the first inverter 10 in the independent operation state is decreased to a range W.sub.M-in<W.sub.FS-min equal to or smaller than a range in which the independent operation may be efficiently performed, the integrated operation of the first inverter 10 is prepared by making the first switch 11 be short. When the input power W.sub.S-in of the second inverter 20 in the independent operation state is decreased to a range W.sub.S-in<W.sub.FS-min equal to or lower than a range in which the independent operation may be efficiently performed, the second switch 21 is switched to the first inverter 10 side.

(48) In this case, when the first switch 11 is not short yet, the voltage V.sub.2 of the second photovoltaic panel 2 is momentarily increased to the value of the open voltage V.sup.i.sub.OC, so that it is possible to identify whether the first switch 11 is short.

(49) Another method of determining whether the first switch 11 is short is a method of identifying a voltage of the output terminal 23 output to the first inverter 10. Since the second switch 21 is switched to the second inverter 20 side in the state of the independent operation, and the first switch 11 is also in the open state, a voltage is not applied to the output terminal 23 of the second switch 21. When the integrated operation of the first inverter 10 is prepared, the first switch 11 is short, so that the output voltage V.sub.1 of the photovoltaic panel 1 is detected in the output terminal 23 of the second switch 21. When it is detected whether the voltage exists, it is possible to identify whether the first switch 11 is opened.

(50) In a state where the first switch 11 is the open state, the second inverter 20 continues the independent operation in a case where the second switch 21 is connected to the second inverter 20 side.

(51) After it is identified that the first switch 11 is short by identifying the voltage of the output terminal 23 output to the first inverter 10, or a predetermined stand-by time elapses, the second switch 21 may be switched to the first inverter 10 side again. In a case where the first switch 11 is not short even though the switching operation of the second switch 21 is repeated by the predetermined number of times or more, or the input power W.sub.S-in is decreased to a value smaller than W.sub.FS-min by a predetermined level, it is determined that the first inverter 10 fails, and the independent operation may continue by fixing the second switch 21 to the second inverter 20 side.

(52) When the switching is normally performed, the second inverter 20 stops the independent operation.

(53) When the controllers 1 and 2 of the respective inverters 10 and 20 may automatically perform the aforementioned operation, it is possible to efficiently generate power even in a state where the amount of sunshine is smaller than that of an ordinary day, such as cloudy weather.

(54) From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.