Ecografía, principios físicos y generalidades

The entire PV system should be diagramed before final costing and installation. The diagram should include wiring of modules in the array or sub-arrays, regulators; AC and DC load centers, battery bank, inverter, grounding and circuit protection.

For a very simple system such as solar home systems, a simple wiring plan can be sketched as shown in figure 9.8.1.1.

The module capacity; length and size of wires; type and model of CR; type, capacity and voltage of the battery; load center details etc. are to be indicated in the diagram for easy installation process. However, actual location of the components of the system will depend upon the plan of the building where the system is to be installed. Selecting the

Module

CR

Battery Load

C

To loads Cable 1

Cable 2

Cable 3 Cable 4,

switches

Fig. 9.8.1.1 Installation diagram for simple PV system

best locations for mounting the module and installing the CR, battery and load center can minimize the required length of the cables.

The larger system requires more attention as simple errors can result in system malfunction or complete collapse of the system. Therefore for larger systems complete system diagram should be made. But rather than drawing all the components of the complete system at once, it will be helpful to sketch a simple system block diagram (fig.

9.8.1.2) and then add components and complexity.

The next step would be detailing each blocks of the simplified block diagram. The steps could be:

 Decided if a single regulator is available to handle the full array current. If not split the array into sub-arrays.

 If both AC and DC loads are to be used, decide the ratio of the respective powers.

For example if majority of the load is AC and DC load is just few DC lamps, then most probably separate module may be wired for DC load and array configured for AC loads.

 Decide the input voltage of the inverter and arrange modules in the sub-arrays to produce required DC system voltage.

 Wire the battery bank.

 Create AC/DC load centers.

 Decide on adding isolation/blocking diodes.

 Add disconnect switches, circuit breakers or fuses.

 Re-calculate required wire size and length for each sector.

 Ensure that the electrical system, the structures, junction boxes and load centers are grounded and protected from lightning.

Array

Regulator

Battery Bank Load

Center

Inverter

AC loads

DC l d

Fig. 9.8.1.2 Simple block diagram of larger system

 Number of CR and Sub-array

It is often easier and wise to divide a large array into smaller sub-arrays of required system voltage. These sub-arrays are then combined in parallel to produce full array current at system voltage. The first reason to break a large array into sub-arrays is that the charging current from the entire array may be too large for the charge regulators desired (selected CR with required facilities and options may not be suitable to handle full array current). Secondly, the wire size may be too high to carry the full array current; may be very expensive; stiff and difficult to work with. Finally, maintenance or repairs can be done on some modules or controllers without the entire system being shutdown.

Number of sub-arrays is primarily the function of current handling capacity of the selected CR. The number of sub-arrays and CR can be calculated using the simple formula:

N_CR or N _sub-array = (Np x Imp x 1.25)/ CR charge current rating (9.8.1.1) Where is:

NCR – Number of CR required;

Nsub-array – Number of sub-arrays required;

Np – Total Number of modules connected in parallel in the array;

Imp – Nominal current of the module.

In the above formula the factor 1.25 represents over-rating of CR current capacity for safety reasons.

As an example lets consider an array of 40 modules with 3.4 A nominal current connected in series (two modules in each string to produce 24 V system voltage) and 20 strings in parallel (fig. 9.8.1.3).

String 1 2 3 --- 19 20

Fig. 9.8.1.3 Example of an array with 40 modules at 24 V system voltage

CR

Suppose the selected CR has the current rating of 30 A and the system voltage of 24 V.

Then according to (6.8.1.1), the required number of CR or sub-arrays is:

NCR = (20 x 4.1 x 1.25)/30 = 3.42 rounds up to 4.

Thus each sub-array will now consist of strings of 5 parallel-connected modules. And a CR with 30A current rating controls each sub-array (fig. 9.8.1.4).

The number of sub-arrays calculated using above formula needs to be rounded up to next highest integer. As far as possible, each sub-array should contain equal number of strings.

In case if it is not possible, one sub-array may be slightly smaller than others.

 Module wiring on structures

Once the number of sub-arrays and controllers have been calculated, it would be wise to make a sketch of how the modules in the sub-array would be interconnect. In above example each sub-array consists of a total of 10 modules; two modules (one string) connected in series to produce 24 V system voltage and 5 strings connected in parallel to increase the current level at system voltage. Assuming that a single support structure can accommodate 10 modules (5 rows and two columns), the wiring diagram, with isolation diodes for each string, would look as shown in fig. 9.8.1.5.

In the figure it is assumed that the modules have two separate junction boxes each for positive and negative terminals. Some modules may have single junction box with separate terminals for positive and negative outputs. The weatherproof field diode box is used to house the isolation diodes and combining terminals (bus bars) to produce single outlet for each sub-array.

CR CR

Sub-array 1 Sub-array 4

Fig. 9.8.1.4 Dividing an array into sub-arrays

As the last stage of array installation design, complete diagram with individual sub-arrays as the input to the system may be sketched as follows (fig. 9.8.1.6a). This is the case when the number of CR is equal to number of sub-arrays.

+ - + -

+ - + -

+ - + -

+ - + -

+ - + -

+ -

Sub-array out

Isolation diodes Bus bar

Fig. 9.8.1.5 Schematic sketch of module wiring for a sub-array

In the case when a single CR is used but the array is grouped into sub-arrays, the final installation sketch would look like (fig. 9.8.1.6b):

 Fuse and Protection Design

Once the array wiring design is sketched and the wire size of each sector calculated, the next logical step would be to decide the number and rating of the fuses, circuit breakers

CR1 CRn

From sub-array 1 + -

From sub-array n + -

Battery bank

DC combining box with bus

To loads

Fig. 9.8.1.6a Sketch of final interconnection of sub-arrays with separate CR

CR To Load center

To battery bank

From sub-array 1

From sub-array n DC

combining

Fig. 9.8.1.6b Sketch of final interconnection of sub-arrays with single CR

and disconnects. It is usual practice to add fuses or circuit breakers of rated capacity at the output of each sub-array. Similarly PV disconnect circuit breaker is installed at the array input of CR. A separate battery disconnect circuit breaker of rated capacity is also required to isolate battery bank during installation and for maintenance purpose. Fused outlets from battery bank is also has to be designed for DC as well as AC loads. A suggestive sketch of complete PV layout with fuses and circuit breakers is shown in fig.

9.8.1.7 below.

The ratings of the fuses or the circuit breakers should be at least 1.5 times the maximum current flowing through the sector.