Active Power Flow Control in Ethiopian High Voltage Transmission Networks Using Phase Shifting Transformer to Enhance Utilization of Transmission Lines
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Date
2018-06
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Addis Ababa University
Abstract
The electricity supply industry of Ethiopia is undergoing a major transformation that requires a
redefined approach to increase the utilization of existing transmission line assets. Overloading of
transmission lines in a power system sometimes result stability issues, which may lead to
unwanted tripping or failure of equipments. The cause could be uneven loading of interconnectors
or parallel transmission lines in meshed networks due to different impedances caused by the tower
geometry, conductor sizing, number of sub-conductors and line length. Under these conditions, to
ensure economical and reliable operation of the grid, active power flow through the lines should be
controlled within their capability limits.
In view of above, the power flow needs to be controlled in order to enhance utilization of high
voltage transmission lines and secure the power system. Thus Control of power in AC network
requires special technology to be implemented on case to case basis. Operating efficiency of
electric transmission system can be improved by using appropriate Flexible Alternating Current
Transmission System (FACTS) devices. Phase shifting transformer is one of the FACTS families,
which can be used for power control in ac network.
This thesis presents a study on active power flow control within Ethiopian network for optimum
utilization of transmission lines using phase shifting transformer (PST). The study is performed
first by reviewing literatures on the use of phase shifting transformers how to redirect active power
flow in transmission networks throughout the world. To demonstrate the active power flow control
in the network, a 400/400 kV phase shifting transformer having a size of 685 MVA with a phase
shifting angle range of -200 to+200 and the high voltage transmission networks was modeled using
PSSE software(Power System Simulation for Engineers) for the peak load of 2040 MW in the year
2017. From the power flow studies/solution, various overloaded and under loaded transmission
lines are identified. By varying the phase angle of the phase shifting transformer, several
simulations are conducted to investigate the impact of PST on the active power flow distribution.
In this study, it has been demonstrated that the active power flow patterns which originally flow
via the low impedance and lower voltage system is fully controlled and restructured using phase
shifting transformer. By varying the phase shifting transformer angle, the active power flow in the
transmission lines can be redirected towards the alternate high voltage path. As the Phase shifting
transformer angle increased from -20° to +20°, the loading of Wolayta - Gibe II and Sebeta IIGibe
II 400kV transmission lines vary from 4% to 35% and 11% to 42% respectively. Similarly, Gelan - Wolayta400kV transmission line load increases from 15% to 43% as the Phase shifting
transformer angle decreases from +20° to -20°
Conventional ways of solving the network bottlenecks based on reinforcement and building new
transmission lines cannot be taken as sufficient and fast due to the problems of acquiring new
corridors and environmental limitations. Installation of Phase Shifting Transformer in the
transmission network is a better solution for controlling the active power flow and effective
utilization of existing high voltage transmission network assets.
Description
Keywords
PSSE, phase shifting transformer (PST), power flow control, FACTS devices, phase shifting angle