Allocation and Sizing of Distribution Transformers and Feeders
Distribution network planning is primarily identified by the allocation and sizing of distribution transformers. The location of transformers directly specifies the length and route of MV and LV feeders. Therefore, location and rating of transformers should be determined along with the length and size of MV and LV feeders.
For this purpose, an optimization procedure is required to minimize the investment cost of transformers and feeders; while, the loss cost is minimized and the system reliability is maximized. The voltage drop and the feeder current as constraints need to be maintained within their standard range.
Planning of LV network is to find the placement and rating of distribution transformers and LV feeders. This is implemented to minimize the investment cost of these devices along with the line loss.
Planning of MV network is to identify the location and size of distribution substations and MV feeders. The objective of MV network planning is to minimize the investment cost along with the line loss and reliability indices such as SAIDI (System Average Interruption Duration Index) and SAIFI (System Average Interruption Frequency Index).
There are several limitations which should be satisfied during the planning procedure.
The bus voltage as a constraint should be maintained within a standard range. The actual feeder current should be less than the rated current of the feeder. Improving the voltage profile, line loss and system reliability is a main concern planning of distribution networks particularly for semi-urban and rural areas.
Supporting the load growth and peak load level is another factor which should be considered in the planning procedure.
Installing capacitors is another way which highly increases the voltage level and reduces the line loss. The Voltage Regulators (VRs) are also common elements for covering these problems.
Reliability is another issue in planning of distribution networks. Long length of distribution lines increases the probability of occurrence of a failure in distribution lines which leads to a low system reliability. Installation of Cross-Connections (CC) is a useful way to lighten this difficulty.
Injecting the active and reactive powers, Distributed Generators (DG) decrease the reliability indices and improve the voltage profile. However, their high investment cost prevents the power engineers from wide use of these devices.
In practical distribution networks, the loads are growing gradually. Additionally, the load level is changing during a period. Conventionally, transformers and feeders need to be upgraded to support the load growth and peak load level. However, upgrading the transformer and feeder rating for peak load level, which is 1-2% of a year, may not be cost benefit.
That is why other supporters such as capacitors and DGs can be installed to avoid extra upgrades of transformers and feeders.
Regarding the discrete and nonlinear nature of the allocation and sizing problem, the resulting objective function has a number of local minima. This underlines the importance of selecting a proper optimization method.
Optimization methods are categorized into two main groups:
- Analytical-based methods and
- Heuristic-based methods.
The analytical methods have low computation time, but they do not deal appropriately with the local minima. For solving the local minima issue, the heuristic methods are extensively applied in the literature.
In this research, both analytical and heuristic methods will be implemented in Matlab, Discrete Nonlinear Programming (DNLP) as an analytical approach and Discrete Particle Swarm Optimization (DPSO) as a heuristic approach.
|Title:||Placement and rating planning of feeders & transformers for LV/MV distribution networks (Thesis) – Iman Ziari at School of Engineering Systems; Faculty of Built Environment and Engineering; Queensland University of Technology; Queensland, Australia|
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