Abstract:
Colombo being the commercial hub of Sri Lanka, maintaining high reliability of electricity
supply is critical. Thus, the electricity distribution network of Colombo city comprises of six,
11 kV ring networks formed around five grid substations and five primary substations.
However, the network is currently operated in open-ring topology.
In the present operation, under maintenance periods, at times of load shedding and “selfgeneration”
schemes, diesel generators are connected to the network. The generators, which
are connected under the self-generation schemes are synchronized with the network. The
generators, which are connected during maintenance and times of load shedding, are not
synchronized. Hence, it is required to split the satellite belts into several sub sections to match
the capacity of the generator to the load, so that the generator would not be tripped due to over
loading. With the government policies to encourage renewable generation, solar PV generation
has also been increasingly connected to the distribution network, which are not dispatched.
With increasing penetration of solar PV, proper planning is required to maintain system
stability and reliability under grid failures.
The substations in Colombo city do not follow the conventional diurnal load pattern of Sri
Lanka; instead, during day time, the load reaches a peak and during night time it drops. This
load pattern follows a curve similar to the diurnal generation curve of solar PV. Thus, by
introducing solar PV to such substations, the installed diesel generation capacity can be
reduced and the rings can be operated as microgrids when the grid fails.
In this research a case study is carried for the ring network based on primary substation-B of
the Colombo city network with the distributed generation of solar PV and diesel generators to
evaluate the ability of forming ring-connected microgrids for a commercial city with peak,
day-time loading. The ring is split into smaller rings to form microgrids with capacity below
10 MVA. The satellites are considered as open looped. Loads of the satellite loop and the
generation (solar PV/ diesel generators) are lumped to the respective radial/ ring substation.
The results indicate that when the network has three, 2 MVA generators fixed to the highest
loaded substations, the system is stable and when the network has 1 MVA generators fixed to
all the substations, the system becomes unstable at N-1 emergency operation. Instability occurs
due to small perturbations to diesel generators at some substations.
Research reveals that with increasing penetration of solar PV, the system can be planned to be
operated in ring-connected microgrids in stable operation to maintain the system reliability
under grid failures
Citation:
Herath, H.D.B.P. (2022). Stability analysis of an islanded microgrid with ring and radial distribution [Master's theses, University of Moratuwa]. Institutional Repository University of Moratuwa. http://dl.lib.uom.lk/handle/123/20090