The continuous increase in energy demand and depletion of conventional
resources motivates the research towards the environment friendly renewable energy
sources like solar and wind energy. These sources are best suitable for rural, urban
and offshore locations, because of easy installation, less running cost and ample
resources (sun light and wind). The remote locations are mostly islanded in nature
and far away from technical expertise in case of troubleshooting. This motivates the
research on development of fault tolerant converters. These fault tolerant converters
increases the reliability, which provides the continuous power supply to critical
loads. From the last few decades, the integration of multilevel inverters with
renewable energy systems is also increasing because of advantages like, improved
power quality, total harmonic distortion (THD) and reduced output filter size
requirement. Employing conventional multilevel inverters for increasing the number
of voltage levels increases the device count and isolated DC sources. As a result
probability of semiconductor switch failure is more and energy balancing issue
between sources, which in-turn degrades the reliability and performance of the
inverter. The majority of conventional multilevel inverter topologies cannot address
energy balancing issues between multiple photovoltaic (PV) sources, which may
need because of partial shading, hotspots, uneven charging and discharging of
associated batteries etc. If energy sharing not addressed effectively, the batteries
which are connected to the shaded or faulty PV system will discharge faster which
may cause total system shutdown and leads to under-utilization of healthier part of
the system. To address these issues, fault tolerant multilevel inverter topologies with
energy balancing capability are presented in this thesis.
The major contributions of the proposed work are
Single phase and three phase fault tolerant multilevel inverter
topologies.
viii
Energy balancing between sources and dc off set minimization (or
batteries) due to uneven charging and discharging of batteries for
five-level inverter.
Extending the fault tolerance and energy balancing for higher number
of voltage levels.
The first work of this thesis is focused to develop fault tolerant single phase
and three phase multilevel inverter topologies for grid independent photovoltaic
systems. The topologies are formed by using three-level and two-level half bridge
inverters. The topology fed with multiple voltage sources formed by separate PV
strings with MPPT charge controllers and associated batteries. Here the topologies
are analyzed for different switch open circuit and/or source failures. The switching
redundancy of the proposed inverters is utilized during fault condition for supplying
power with lower voltage level so that critical loads are not affected.
In general, the power generation in the individual PV systems may not be
same at all the times, because of partial shading, local hotspots, wrong maximum
power point tracking, dirt accumulation, aging etc. To address this issue energy
balancing between individual sources is taken care with the help of redundant
switching combinations of proposed five-level inverter carried out in second work.
Because of partial shading the associated batteries with these panels will charge and
discharge unevenly, which results voltage difference between terminal voltages of
sources because of SOC difference. The energy balance between batteries is
achieved for all operating conditions by selecting appropriate switching
combination. For example during partial shading the associated battery with low
SOC is discharged at slower rate than the battery with more SOC until both SOC’s
are equal. This also helps in minimization of DC offset into the ac side output
voltage. The mathematical analysis is presented for possible percentage of energy
shared to load by both the sources during each voltage level.
The third work provides single phase multilevel inverter with improved fault
tolerance in terms of switch open circuit failures and energy balancing between
sources. Generally multilevel inverters for photovoltaic (PV) applications are fed
ix
with multiple voltage sources. For majority of the multilevel inverters the load
shared to individual voltage sources is not equal due to inverter structure and
switching combination. This leads to under-utilization of the voltage sources. To
address this issue optimal PV module distribution for multilevel inverters is
proposed. Mathematical analysis is carried out for optimal sharing of PV resources
for each voltage source. The proposed source distribution strategy ensures better
utilization of each voltage source, as well as minimizes the control complexity for
energy balancing issues. This topology requires four isolated DC-sources with a
voltage magnitude of Vdc/4 (where Vdc is the voltage requirement for the
conventional NPC multilevel inverter). These isolated DC voltage sources are
realized with multiple PV strings. The operation of proposed multilevel single phase
inverter is analyzed for different switch open-circuit failures.
All the presented topologies are simulated using MATLAB/Simulink and the
results are verified with laboratory prototyp
