There is a difference between the actual power harvested and power that can be harvested, the difference is more in the case of partial shading. More power can be harvested by improving the V-P characteristics and in turn the MPP of the PV array. This improvement can be done by altering the location of the shaded panels, varying interconnection schemes in accordance with the prevailing shading conditions.
Reconfigurations can be done either dynamically or it can be static.
1.2 Dynamic Reconfiguration
PV system is dynamically altered by
Switching the interconnection scheme that yields more power.
Distributing shade intensity to avoid mismatch.
Adjusting number of panels in series / parallel to equalize row current.
Electrical Array Reconfiguration (EAR)
EAR was initially utilized to optimize the performance of the volumetric pump. Switches are used to reconfigure the panels connected to them, reconfiguration occurs when an EAR controller senses the irradiation levels. The reconfiguration algorithm determines the interconnection and actuates appropriate switches in the switching matrix. EAR can be applied to S, P, SP, TCT, HC and BL.
Fig. 1.1 Electrical Array Reconfiguration
Advantages of EAR
Real time adaptation to the external condition (partial shading).
Self-capacitive for real time adaptation (no monitoring is required).
Drawbacks of EAR
The number of switches required to alter the interconnections increases with the array size.
Overall size, cost and complexity of the system increases.
1.2.2 Adaptive Array Reconfiguration (AAR)
In this type of reconfiguration there are two groups of PV arrays, fixed bank and adaptive bank. The fixed bank is configured in TCT and remains static. The adaptive bank is equally connected to each of the row in fixed bank under normal or uniform irradiation condition, through a switching matrix. Under non uniform irradiation conditions or partial shading the number of panels that are to be connected to each row of the fixed bank is determined and switched dynamically according to the prevailing shading condition. The rows which are more affected by the shading is given more share of the adaptive bank so that the output produced by each of the rows remains similar and current mismatch in each row can be avoided, therefore giving a single peak on VP curve. AAR requires an intelligent algorithm to dynamically control the switching matrix.
Fig. 1.2 Adaptive Array Reconfiguration
1.2.3 Irradiation equivalence by relocation of panels
This relocation technique is based on the principle of irradiation equivalence, that is, the PV panels are relocated to other rows such that there is no current limitation imposed by any serial string of parallel connected cells/panels. Each row contains same numbers of cell before and after the relocation, and the number of panels in series and in parallel also remains same.
But changing of physical location of panel/cell dynamically is a difficult task, so this is mimicked by altering the interconnections of the panels. Still the number of possible arrangements even for a very small array is very large and hence this configuration technique is not a popular choice.
Fig. 1.3 Arrangement in a Repositioning scheme
1.3 Static reconfiguration techniques
The interconnections are not altered dynamically but the physical location of the panel is planned strategically. The objective is to disperse the shading effect almost equally over the array. Since there no requirement of switches and associated auxiliary circuits, the implementation and control strategies are simpler as compared to that of the dynamic reconfiguration. The Sudoku puzzle pattern and magic square pattern are employed to decide the location of the solar cells within the array and the interconnection and wiring is done thereafter.
1.3.1 Sudoku puzzle pattern
In Sudoku pattern the panels / cells which belong to a particular row are physically placed at different locations in each of the other row. This arrangement results in the reduced mismatch losses. The panels are not arranged dynamically by sensing the prevailing shading conditions and does not require any intelligent algorithm, the arrangement of panels are in accordance with Sudoku puzzle patterns.
Suppose we have a M x M panels (m rows each having m number of cells), each physical row is arranged like a Sudoku, just like in Sudoku all the numbers put in a row are unique, each physical row will have a panel from all the internally connected row, there will be no two panels which are electrically connected to the same row put together in a physical row. In a TCT connection there will be mismatch in all the cases other than vertical and diagonal partial shading conditions, Sudoku pattern ensures that the mismatch is reduced in all the other partial shading cases.
For Sudoku arrangement all the cells/ panels are identified with two digits (row and column position). The left digit corresponds to the row and the right digit corresponds to the column.
Fig. 1.4 (a)Conventional TCT arrangement, (b)Sudoku arrangement, (c)Shade dispersion on Sudoku arrangement, (d) TCT interconnection
The arrangement of cells in a 3×3 array in conventional and Sudoku pattern are shown in figure 1.4, the Sudoku arrangement is subjected to partial shading conditions and following results are obtained.
1.3.2 Magic Square pattern
This pattern quite similar to Sudoku pattern, both are static reconfiguration techniques but for the fact that it uses a MxM magic square. All the panels are identified sequentially a magic and then rearranged to form a magic square. A magic square is a square grid filled with distinct numbers such that the sum of each row, column and diagonal is equal.
Fig. 1.5 (a) conventional TCT connection, (b) magic square arrangement, (c) shade dispersion on magic square pattern, (d) TCT interconnection
Improved Sudoku Arrangement
There are many algorithm that have been proposed to determine the position of cells/panels with in the array. These algorithms calculate the distance or the shifting displacement, d, between two adjacent panels. We have studied one such algorithm and implemented it.
This improved Sudoku arrangement relies on the fact that output is maximum when the row currents are matched in a TCT configuration.
To ensure that the shade is uniformly distributed the row that belong to a row should not be placed next to each other and should be separated by a minimum distance governed by the size of the array.
The location of panels in this proposed arrangement is determined by
Yij = Xkj where i=1,2,3….m
The row index k = (i+(j-1)*d),
where d=ceil (under root (m))