ABSTRACT:
This paper deals with a three-phase two-stage grid tied
SPV (solar photo-voltaic) system. The first stage is a boost converter, which
serves the purpose of MPPT (maximum power point tracking) and feeding the
extracted solar energy to the DC link of the PV inverter, whereas the second
stage is a two-level VSC (voltage source converter) serving as PV inverter
which feeds power from a boost converter into the grid. The proposed system uses
an adaptive DC link voltage which is made adaptive by adjusting reference DC
link voltage according to CPI (common point of interconnection) voltage. The
adaptive DC link voltage control helps in the reduction of switching power
losses. A feed forward term for solar contribution is used to improve the
dynamic response. The system is tested considering realistic grid voltage variations
for under voltage and over voltage. The performance improvement is verified
experimentally. The proposed system is advantageous not only in cases of
frequent and sustained under voltage (as in the cases of far radial ends of
Indian grid) but also in case of normal voltages at CPI. The THD (total
harmonics distortion) of grid current has been found well under the limit of an
IEEE-519 standard.
KEYWORDS:
1.
Adaptive DC
link
2.
MPPT
3.
Overvoltage
4.
Solar PV
5.
Two-stage
6.
Three phase
7.
Under voltage
SOFTWARE: MATLAB/SIMULINK
CIRCUIT DIAGRAM:
Fig.
1. System configuration.
CONTROL SYSTEM
Fig.
2. Block diagram for control approach.
EXPECTED SIMULATION RESULTS:
Fig.
3. Simulated performance for, (a) change in solar insolation without
feedforward for PV contribution,
(b)
change in solar insolation with feed forward for PV contribution,
(c)
normal to under voltage (415 V to 350 V),
(d)
CPI voltage variation from normal to over voltage (415 V to 480 V).
CONCLUSION:
A
two-stage system has been proposed for three-phase grid connected solar PV
generation. A composite InC based MPPT algorithm is used for control of the
boost converter. The performance of proposed system has been demonstrated for
wide range of CPI voltage variation. A simple and novel adaptive DC link
voltage control approach has been proposed for control of grid tied VSC. The DC
link voltage is made adaptive with respect to CPI voltage which helps in
reduction of losses in the system. Moreover, a PV array feed forward term is
used which helps in fast dynamic response. An approximate linear model of DC
link voltage control loop has been developed and analyzed considering feed
forward compensation. The PV array feed forward term is so selected that it is
to accommodate for change in PV power as well as for CPI voltage variation. A full
voltage and considerable power level prototype has verified the proposed
concept. The concept of adaptive DC link voltage has been proposed for grid tied VSC for PV application however,
the same concept can be extended for all shunt connected grid interfaced
devices such as, STATCOM, D-STATCOM etc. The proposed system yields increased
energy output using the same hardware resources just by virtue of difference in
DC link voltage control structure. The THDs of the grid currents and voltages
are found less than 5% (within IEEE-519 standard). The simulation and
experimental results have confirmed the feasibility of proposed control
algorithm.
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