Junction box: An
electrical junction box is a container for electrical
connections, usually intended to conceal them from sight and deter tampering/malfunctioning.
For a solar PV module junction box is used to bring out the terminal leads of
the module from the module’s internal cell connections. Also it protects the terminals
of the module from early degrading due to various reasons (mainly due to temperature).
It is mostly connected beneath the module.
Typically the junction box for solar PV
modules provides protection features such as current bypass, reverse current blocking
features which are most important for the proper functioning.
Internal structure of junction box:
The above figure shows the perfect conceal of the
conducting strips which avoids unnecessary exposure to atmosphere. The output
leads (cables) positive, negative used for further connections.
In detail functioning of Junction Box:
It is predominantly used for overcoming
the issues like shading effects of the panel, Reverse current blocking.
Shading effect on the panel:
As we know that module is the combination of the PV cells
arranged in series, where each cell has certain voltage and generates current,
however the current generation does depends on the area of the cell &
intensity of the sunlight. As the cells are arranged in series the current
through that (series connected) string is same and when ever shadow occurs on
some cells (portion of cells) then the shaded cells will not able to generate
current, all the un-shaded cells which generate current will force the shaded
cell to allow the current through it, the only way the
shaded cells can operate at a current higher than their short circuit current
is to operate in a region of negative voltage (reverse bias) that is to cause a
net voltage loss
to the system. If upon
allowing that current when there is no production from the cell the power
dissipation takes place as heat and cause “hotspots”.
As shaded cells with drag down the overall IV curve of the
group of cells the
performance of the system gets affected (low
power generation). The power loss of the overall system can go up to 50% if
there is 10% shading on the system (The effect of shading
is depends on how the module is shaded).
Solar modules/arrays rely
on bypass diodes to protect them from damage and minimize output power losses
when a section of an individual panel or a larger part of the array experiences
shading due to obstructions, clouds, snow, or other phenomena.
Consider the following
illustration of a PV module (60Cell, multi-crystalline silicon) to understand
the function of bypassing diode. Each cell has a generated voltage of 0.6V and
current depends on the sunlight intensity.
·
First picture shows the cell connections in a PV module.
· The second one (module without bypass diodes) determines that 16th cell of the module is shaded. Then the voltage across the shaded cell is -35.4V (Anode V = -44*0.6 = 26.4V, Cathode V = 15*0.6 = 9V). Voltage across shaded cell is greater than its reverse breakdown voltage (13.6V multi-crystalline silicon), there by the reverse conduction starts and power dissipation takes across the shaded cell.
·
The
third one (module with bypass diodes) shows the normal un-shaded operation.
·
Fourth
picture shows the conduction of bypass diode as the high negative voltage
generated by 44 cells (-26.4V) is at the cathode side, low positive voltage is
at anode side by which the diode gets forward biased and conducts ignoring the
shaded cell string. Here power generated by un-shaded portion is utilized;
however the generated power across the module is less.
Now
considering the practical situation where the shading happens on the scale of
modules/portion of modules, however the working is same as in the case of
Module (discussed above). The following figure shows the connection/functioning
of Bypass diodes in an array.
Whenever
some of the modules are shaded (happens mostly) due to some reasons, those
under the shade does not generate power are left out by the conduction of
bypass diodes and the generation process continues with other modules
unaffected by shading. Here, blocking diodes are mentioned and will be
explained in detail in reverse current section.
The four
module model displayed above has a bypass diode (green- schottky) each, considering
shadow on first-left module then the diode (green) across it conducts &
bypasses the current from other modules which are on string (series connected
modules) entering into the shaded module and the generation continues.
Reverse blocking current:
Reverse
current phenomena is very important as during the night time the panels do not
generate any power and acts as load, if DC system is considered then the
battery which remains in contact with the panels with some devices in between.
The panels actually draw the current from the source (battery in this case) and
these currents are higher in magnitude which may destroy the PV cells.
In order to
avoid this kind of function an appropriate diode is used as shown in the above
figure, as by its nature diode always allows current in one direction and
blocks other way. There by it blocks the reverse current entering in to the
Modules.
Active elements:
Several
companies have recently introduced an alternative to Schottky diodes in the
form of a new category of so-called “lossless”, or “active” diodes. In truth,
they are actually two-terminal FET-based switching circuits, designed as
pin-compatible replacements for conventional diodes. Several manufacturers
offer active bypass diodes for solar applications including Microsemi, STMicroelectronics, and Texas Instruments.
As with nearly any new technology, active bypass diodes have
a price disadvantage against the mature technology they are displacing. The
first generation of active diodes cost roughly 2 to 3 times what manufacturers
pay for high-quality Schottky diodes. However, the prices of some active
devices has declined by 10 to 20 percent over the past year, and smart
designers are beginning to understand that the devices can offer significant
dividends in terms of overall solution cost and added capabilities that repay
their higher cost several times over.
The most obvious advantage active bypass diodes offer is dramatically lower losses in both their bypass and “off” modes. A typical device has a 40 to 50 mV forward voltage versus the Schottky’s 0.4 V, which translates into roughly a 10 times reduction in power dissipation when running in bypass mode. This improves an array’s ability to operate efficiently when one or more panels in its string are subject to shading conditions (caused by neighboring buildings, trees, chimneys, etc.).
Power
dissipation Vs current comparison b/w schottky diode & active diode
Important Parameters of a Junction box:
Rated
current (A): It is the maximum current carrying
capacity of the diode.
Rated
voltage (V): It is the maximum voltage that can be
applied across the diode.
No.
of diodes/Model: Number of diodes used and model of the
diode used.
Operating/Maximum
temperature: Range of normal & maximum
withstanding temperatures.
Contact
material/contact resistance: Type of material used
for terminal contacts and its resistance.
Cable/Connectors:
Details of cable (length, area of cross-section) and connectors used.
Protection
mode (IPXX):
Classifies and rates the degree of
protection provided against intrusion (body parts such as hands and fingers),
dust, accidental contact, and water by mechanical casings and electrical
enclosures.
Flammability: It is the ability of a substance to burn or ignite, causing fire
or combustion. The degree of difficulty required to cause the combustion of a
substance is quantified through fire testing.
Datasheet:
The following links directs to technical datasheets of a
junction box
http://www.asteniksolar.com/products/materials/Astenik_jb/Product_sheet_jb_ast_1310B.pdf
http://www.kostal.com/industrie/download/1393594577_2013-11-21_Datenblatt_SAMKO-100-01_screen.EN.pdf
