Abstract:
Large-scale photovoltaic (PV) plants are becoming more financially feasible and competitive
as their cost reduces; they are also being deployed worldwide at a rapid rate because of the increasing
interest in renewable energy as a way to mitigate climate change. Most of the growth
in global electricity demand is happening in developing countries as they undergo electrification
and their economies expand. In these countries, PV plants are increasingly being chosen
as a new-generation electricity source due to their low cost and environment-friendly nature.
However, ideal locations for PV plants, those with high solar potential, are often remote and
isolated and the grids there are commonly weak.
A weak grid is sensitive to voltage events as it has high resistance and low X/R ratio; thus,
intermittent and inertia-less PV plant integration could aggravate the operation of a weak grid.
Robust evaluation of the impact of a PV plant on a steady-state voltage of a weak grid is therefore
essential in the planning phase, as weak grids require different approaches in some aspects
from strong-grid PV plant integration studies. Most PV plant integration studies estimate an
expected generation profile of the PV plant using average solar radiation found from historical
radiation measurement data, but the expected generation profile does not represent the probable
maximum generation of the plant.
This thesis aims to find a method that can improve integration studies of large-scale PV
plants into weak grids so that the stable operation of a grid can be sustained after integration.
Prior to integrating new PV plants into weak grids, the impacts of such plants on the
steady-state voltages of the grid should be studied comprehensively, and accurate energy analysis
undertaken as part of the economic assessment of the plants. The thesis proposes a novel
framework that uses distinct generation profiles for integration analysis of large-scale PV plants
into weak grids. A probable maximum generation (PMG) profile based on clear-sky radiation
is suggested for voltage evaluation while an average generation (AG) profile based on average
radiation is used for energy analysis. The case study examined a large-scale PV plant integration
into one of the regional power systems in Mongolia. Load flow analysis was carried out
every hour for a full year on a simplified 15-bus case network for 5, 10, 15 and 20 MW PV plant
integration scenarios. Application of the PMG profile provided a more robust evaluation of the
PV plants’ impact on the voltage profile of a weak grid. However, aggregated PMG profiles
over days and months overestimated the actual electricity production of the PV plants, whereas
the AG profile showed accurate performance. The results show that the proposed framework
could help to improve the assessment of PV plant integration into weak grids by providing
robust voltage and accurate energy estimations.
Furthermore, a novel approach based on PMG profiles is proposed for sizing shunt VAR
compensator at the point of interconnection (POI) of a PV plant in a weak grid to reduce a
PV plant induced voltage rise. A shunt SVS size was estimated by the proposed approach for
various PV plant integration scenarios, and the SVS added to the POI of the PV plant in the
case network model. A simulation was carried out on the modified case network for every PV
integration scenario, and results were compared with the results from the same simulation with
the SVS sized by an existing approach. The SVS sized by the proposed approach presented
more robust performance and improved the voltage profile of the case network under different
PV plant integration scenarios.