09-08-2012, 10:43 AM
Power Losses in Long String and Parallel-Connected Short Strings of Series-Connected Silicon-Based Photovoltaic Modules
1Power Losses.pdf (Size: 1.15 MB / Downloads: 29)
Abstract
Configuration of a photovoltaic (PV) power generator
has influence on the operation of the generator, especially if it
is prone to partial shading. In this paper, the mismatch losses and
the power losses due to failure in tracking of the global maximum
power point of a long string of 18 series-connected PV modules
and three short strings of six series-connected PV modules connected
in parallel are investigated under different partial shading
conditions by using a MATLAB Simulink simulation model. The
generators with parallel-connected short strings are studied in case
when they have the same operating voltage and when they operate
as separate strings. The results show that long series connection of
modules and parallel connections of strings via a single inverter to
the electrical grid should be minimized to avoid losses in case of
partial shading conditions. Under partial shading conditions, short
strings operating separately have the lowest power losses.
INTRODUCTION
GLOBAL warming and the limited resources of fossil fuels
have increased the need for renewable energy [1]–[3]. Solar
radiation is the largest source of renewable energy [4], [5] and
the only one by which the present primary energy consumption
can be replaced.
Photovoltaic (PV) power generators convert the energy of
solar radiation directly to electrical energy without any moving
parts. PV power generators can be classified into stand-alone
and grid-connected generators [6]. In stand-alone systems, the
energy storage has big influence on the design of the systems.
In grid-connected systems, the grid acts as an energy storage
into which the PV power generator can inject power whenever
power is available.
SHADING OF PV GENERATORS
Three different PV power generators composed of 18 PV
modules have been studied and are presented in Fig. 4. They are
(a) the Long string generator composed of 18 series-connected
PV modules, (b) the Parallel strings generator of three parallelconnected
strings of six PV modules and blocking diodes connected
in series, and © the Multi-string generator composed
of three separately operated strings of six series-connected PV
modules.
Characteristic values of NAPS NP190GKg PV modules have
been used as reference values in our simulations. PV modules
consists of 54 PV cells connected in series, which are protected
against hot spots by three bypass diodes, each of them connected
in antiparallel with 18 cells.
In simulations, shading of the generators has been done onethird
of a PV module (18 PV cells protected by one bypass
diode) at a time. The first five steps of the shading pattern in
the case of Long string generator are illustrated in Fig. 5.
RESULTS
In simulations, the main variables were the relative portion
of shaded PV cells in the PV power generator (system shading)
and the attenuation of irradiance due to the shading (shading
strength). Shading strength is the amount of lost irradiance due
to shading divided by the total irradiance without shading.
The simulated PV power generators were composed of 18 PV
modules. Because the shading of the PV power generators was
done one-third of a PV module at a time, the generators can be
divided into 54 blocks of 18 series-connected PV cells with a
bypass diode connected in antiparallel with the cells. In simulations,
the amount of shaded blocks of the PV power generators
(system shading) was varied from 0 to 54 (0 to 100%) yielding
55 different values for system shading.
CONCLUSION
The effects of partial shading on Long string, Parallel strings,
and Multi-string PV power generators have been investigated
by using an experimentally verified simulation model based on
the well-known one-diode model of a PV cell. Partial shading
was varied with respect to system shading and shading strength,
which represent the amount of shaded PV modules of the generator,
and the attenuation of the irradiance due to the shading,
respectively. The effects of partial shading was studied on the
power of the global MPP of the generators, on the mismatch
losses caused by operating at the global MPP, which differs
from the sum of the maximum powers of individual blocks of
PV cells with antiparallel-connected bypass diode, and on operating
at a local MPP instead of the global one in case of multiple
MPPs.