Claims: We claim:
1. A non-isolated cell-string optimizer for optimization of output voltage of a standard photovoltaic module (401), comprising of at least three cell strings serially connected to each other, a junction box (403) connected across the serially connected three cell strings characterized in that an optimizer unit (405) is configured for optimizing the output across any two cell strings.
2. The optimizer unit (405) as claimed in claim 1, wherein the optimizer unit (405) comprises of a first converter circuit (402) for optimization of fraction input voltage of the first cell string.
3. The optimizer unit (405) as claimed in claim 1, wherein the optimizer unit (405) comprises of a second converter circuit (404) for optimization of fraction input voltage of the third cell string.
4. The optimizer unit (405) as claimed in claim 1, wherein the first converter circuit (402) and the second converter circuit (404) are electrically connected to the second cell string for obtaining direct input voltage of the second cell string.
, Description:A NON-ISOLATED CELL-STRING OPTIMIZER
FIELD OF INVENTION
The invention generally relates to the field of power optimization and particularly to a non-isolated cell-string optimizer.
BACKGROUND
Normally, a solar power generation system contains a plurality of photovoltaic modules. Each photovoltaic module is connected to a power optimizer for optimization of output power. Usually, two types of power optimizers are used to derive optimal output power from shaded photovoltaic modules. One type of power optimizer is a module level optimizer as shown in FIG 1. FIG. 1 shows a standard photovoltaic module 101 that consists of three cell strings denoted as CS1, CS2 and CS3 and are connected serially to each other. The output of these cell strings is derived from four terminals, viz. CS1-, junction of CS1+ and CS2-, junction of CS2+ and CS3-, and CS3+. The four terminals are connected to the four input terminals of a junction box 103. The module level optimizer 105 is connected to the junction box 103 through two input terminals (102, 104) and connected to string of photovoltaic modules through output terminal 106. The module level optimizer 105 optimizes the output voltage of whole photovoltaic module and reduces the power losses due to mismatches between different photovoltaic modules. The disadvantage of the module level optimizer 105 is that it is not useful when mismatches exist between the cell strings within a photovoltaic module. Another type of power optimizer is a cell-string optimizer as shown in the FIG.2. The cell-string optimizer 201 consists of three separate circuits 202, 204, and 206 for optimizing outputs of CS1, CS2 and CS3, respectively. The outputs of the three circuits 202, 204, and 206 are serially connected for generating total output of the cell-string optimizer. The cell-string optimizer 201 reduces the losses due to mismatches between the cell strings within the photovoltaic module and between the different photovoltaic modules. The disadvantage associated with the cell-string optimizer 201 is that each circuit requires two input terminals and the three circuits require total six input terminals (208, 210, 212, 214, 216, and 218), therefore, it is not possible to install on the standard photovoltaic module having four cell string terminals. Further, to connect the cell-string optimizer to the photovoltaic module, the cell strings need to be isolated from each other, which results in a need for change in the construction of the standard photovoltaic module. Hence, there is a need for an optimizer for optimization of the output voltage of cell strings without isolation of the cell strings, which is easily installable in a standard photovoltaic module.
BRIEF DESCRIPTION OF DRAWINGS
So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG. 1 shows a module level optimizer, according to a prior art of the invention.
FIG. 2 shows a cell-string optimizer, according to a prior art of the invention.
FIG.3 shows a block diagram of a non-isolated cell-string optimizer, according to an embodiment of the invention.
FIG.4 shows a circuit diagram of a non-isolated cell-string optimizer, according to an embodiment of the invention.
FIG.5 shows an actual circuit and an equivalent circuit for a non-isolated cell-string optimizer, according to an embodiment of the invention.
SUMMARY OF THE INVENTION
One aspect of the invention provides a non-isolated cell-string optimizer for optimization of output voltage of a standard photovoltaic module. The non-isolated cell-string optimizer includes at least three cell strings, serially connected to each other. A junction box is connected across the serially connected three cell strings characterised in that an optimizer unit is configured for optimising the output across any two pairs of cell strings. The optimizer unit comprises of a first converter circuit for optimization of fraction input voltage of the first pair of cell string and a second converter circuit for optimization of fraction input voltage of the third pair of cell string. Further, the first converter circuit and the second converter circuit are electrically connected to the second cell string for obtaining direct input voltage of the second cell string.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a non-isolated cell-string optimizer for optimization of output voltage of a standard photovoltaic module. The non-isolated cell-string optimizer includes at least three cell strings serially connected to each other. A junction box connected across the serially connected three cell strings characterised in that an optimizer unit is configured for optimising the output across any two pairs of cell strings. The optimizer described hereinabove shall be explained in detail. FIG.3 shows a block diagram of a non-isolated cell-string optimizer, according to an embodiment of the invention. The non-isolated cell-string optimizer includes a photovoltaic module 301, a junction box 303 and an optimizer unit 305. The optimizer unit 305 is connected to the junction box 303 through four terminals T1, T2, T3 and T4 for obtaining the input from three cell strings CS1, CS2 and CS3 of the photovoltaic module 301. The optimizer unit 305 generates output at 302 and 304 and is connected in series with the string of photovoltaic modules 306.
FIG.4 shows a circuit diagram of a non-isolated cell-string optimizer, according to an embodiment of the invention. The optimizer unit 405 comprises of a first converter circuit 402 for optimization of fraction input voltage of the first cell string CS1 and a second converter circuit 404 for optimization of fraction input voltage of the third cell string CS3 of the photovoltaic module 401. The first converter circuit 404 takes input from the first cell string CS1 through T1 and T2 terminal of the junction box 403 for generating output voltage VO12 that is a fraction of the input voltage VI12 and can be expressed as,
VO12 = F12 * VI12 ... (A)
The first converter circuit 402 manipulates the fraction F12 in order to derive maximum power from the first cell string CS1. The first converter circuit 402 includes a positive input terminal 406, a negative input terminal 408, a positive output terminal 410 and a negative output terminal 412. The positive output terminal 410 and the positive input terminal 406 are electrically connected within the circuit.
The second converter circuit 404 takes input from the third cell string CS3 through T3 and T4 terminal of the junction box 405 for generating output voltage VO34, that is a fraction of the input voltage VI34 and can be expressed as,
VO34 = F34 * VI34 … (B)
The second converter circuit 404 manipulates the fraction F34 in order to derive maximum power from the third cell string CS3. The second converter circuit 404 includes a positive input terminal 414, a negative input terminal 416, a positive output terminal 418, and a negative output terminal 420. The negative input terminal 416 and the negative output terminal 420 are electrically connected within the circuit.
The first converter circuit 402 and the second converter circuit 404 are electrically connected to the second cell string CS2 for obtaining direct input voltage of the second cell string. Hence, the output of the second cell string is expressed as, VO23 = VI23 …(C)
The total output voltage VO can be expressed as the sum of the voltages VO12, VO23 and VO34. Hence the total output voltage is expressed as,
VO = VO12 + VO23 + VO34
Substituting expressions for VO12, VO23, VO34 from the equations (A), (C), (B), respectively, and the total output voltage is,
VO = F12 * VI12 + VI23 + F34 * VI34
FIG.5 shows an actual circuit and equivalent circuit for the non-isolated cell-string optimizer, according to an embodiment of the invention. FIG.5a shows the actual circuit for the non-isolated cell-string optimizer and FIG.5b shows the equivalent circuit for the non-isolated cell-string optimizer. The functioning of the non-isolated cell-string optimizer can be better understood by considering FIG.5b. The junction of CS1+ and CS2- is shifted from a point 506 in FIG.5a to a point 508 in FIG.5b and the junction of CS2+ and CS3- is shifted from point 510 in FIG.5a to a point 512 in FIG.5b. From the FIG.5b, it can be seen that the total output voltage of the non isolated cell string optimizer, VO is equal to the sum of VO12, VI23 and VO34. In other words, the total output voltage VO is the sum of output of the first converter circuit 502, the voltage of the second cell string CS2, and the output of the second converter circuit 504.
Hence, the invention as described herein and as illustrated in the accompanying drawings provides a non-isolated cell-string optimizer for optimization of output voltage of the standard photovoltaic module. The non-isolated cell-string optimizer includes four output terminals to enable easy installation in the standard photovoltaic module. The non-isolated cell-string optimizer reduces power losses due to partial shading of any parts of the first or the third cell string, similar to the conventional cell-string optimizer. This suffices for a majority of real-life shadow conditions since shadows usually advance or recede from the edges of a photovoltaic module, impacting the first or third cell strings. Hence, optimizing power from these two cell strings is adequate for a majority of shading conditions. If the second cell string is shaded, the bypass diode for that cell string gets activated and enables the passage of current through the string. Thus, the non-isolated cell-string optimizer offers a cost-effective solution to reduce the power losses in a majority of shading conditions, without requiring any changes to be made to the existing solar photovoltaic modules.
The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the scope and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.