| Summary: | The main control objective of photovoltaic (PV) systems is usually to extract the max-
imum power. This is done to maximize the utilization of the solar plants due to their
high cost. Over the last decade, the PV power generation cost enjoyed a steady decline,
and it is projected to continue this trend in the coming years. The increasing penetra-
tion of solar-based energy systems has raised the need for an evolution in the control
associated with PV systems. As a result, disadvantages of the maximum power point
tracking (MPPT) operation of PV systems, e.g., reduced flexibility, are becoming more
prominent factors to consider in the control objectives. MPPT does not provide any free-
dom to improve the frequency response as operation is always at the maximum power
point (MPP), which may be problematic in power systems with high PV penetration.
Accordingly, standards for connecting the PV power plants to the grid are becoming
tougher and new grid codes are mandating additional functionalities for PV plants, like
frequency response and maintaining some power reserve.
These additional requirements can be met by using flexible power point tracking (FPPT)
in the PV systems. FPPT is a control technique that operates the PV at a power reference
that is lower than the MPP. Thus, leaving some PV power reserve that is the difference
between the maximum power and the operating power reference. This power reserve in
the PV system can be used to improve the frequency response and resilience of the grid.
The knowledge of available power reserve is also helpful in operation optimization in
energy management systems as the power reserve can be treated as an additional energy
storage. Determining the amount of power reserve depends on knowing the maximum
available power. Conventional FPPT methods cannot determine the maximum available
power since they operate at a power reference lower than the MPP. This research gap is
addressed to be filled in this thesis.
This research work introduces novel grid-connected PV system control methodologies
with a focus on determining available power reserve while injecting flexible power into
the grid. With constantly evolving grid codes and standards, modern PV systems are
compelled to offer sophisticated control mechanisms to enhance grid resilience. Among the pivotal grid requirements are the provision of flexible power injection, power reserve
control (PRC), and grid frequency regulation. These functionalities are embedded to
operate simultaneously in the novel control methodologies discussed in this thesis.
In this thesis, a two-stage grid connected PV system is considered. The proposed algorithms
are embedded in the dc-dc boost converter control. The contribution of each
algorithms introduced in this thesis is discussed below:
• Dynamic FPPT in grid-connected PV systems: In contrast to conventional static
FPPT methods, this algorithm offers the advantage of enhancing the power supplied
to the grid by guaranteeing the presence of a reserve power that can be
quickly utilized. This enables the consistent injection of power into the grid without
experiencing any fluctuations
• A PV system control methodology with dispatchable power reserve: This approach
maintains the average PV power by scanning a segment of the PV curve
containing the MPP. By doing so, it constantly updates the knowledge of the maximum
power accessible within the system without introducing fluctuations in the
power injected into the grid. This updated information serves to ascertain and
manage the available dispatchable power reserve.
• Dynamic RPPT for grid-connected PV systems: This methodology ensures availability
if the intended power reserve to provide grid support, even under challenging
conditions like partial shading. The average PV power output is regulated
by scanning the PV curve between two points for variable durations to meet the
FPPT requirement of injecting constant power into the grid, while simultaneously
tracking the available power reserve. Concurrently, as this operational approach
inherently follows the MPP, the reserve power point tracking (RPPT) achieves
both FPPT and MPPT functionalities simultaneously.
• Mitigating dc-link oscillations in dynamic PRC through interleaved PV strings:
This control methodology extends RPPT to apply for multiple PV strings. It reduces
the high dc-link oscillations caused by inherent and continuous sweeping of
the PV curve. By interleaving the PV strings, it notably reduces the ripple in the
dc-link voltage caused by RPPT.
Implementing these control methodology does not require any additional hardware or
curve fitting models. The effectiveness of these algorithms is demonstrated by simulation
and experimental results under changing solar irradiance, grid frequency deviation,and partial shading conditions. Through this multifaceted approach, this research contributes
to the advancement of grid-connected PV systems by addressing key operational
challenges and enhancing system performance under diverse operating conditions.
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