Evaluation of Static Voltage Stability of Ethiopian Grid
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Date
2017-07
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Addis Ababa University
Abstract
A power system is set to operate within its maximum operating limits for better utilization of
the existing network facilities. The main factor causing voltage instability is the inability of a
power system to meet the demand for reactive power. A system enters a state of voltage
instability due to loss of loaded generation unit or transmission line, or a change in loading
conditions that causes progressive and uncontrollable decline or rise in voltage. This thesis
attempts to evaluate voltage stability problems of Ethiopian high voltage transmission grid.
Load flow simulations using PSS/E software has been done for peak load (2,200MW) and
light-load (900MW) conditions. After conducting load flow simulations, ‘Voltage limit
checking’ results within tolerable range of +/- 0.05pu is applied to identify the buses/areas with
unacceptable low and high voltage values. Low voltage profiles during peak hour are
registered around Addis Ababa, and high voltage values are registered at North-western part of
Ethiopia, i.e. around Bahirdar & Debre Markos.
Mitigation techniques applied to improve the low voltages for peak load conditions include;
installation of 15MVAr shunt capacitors each at Weregenu and Mekanissa substations,
upgrading of transmission lines; i.e. Legetafo–Cottobie line from 132kV into 230kV,
Cottobie–Addis East-I line from 45kV into 132 kV double lines, and Sebeta1–Addis West line
from 45kV into 132kV, and upgrading of transformers; two 25MVA into two 50MVA at Addis
North, two 12.5MVA into two 50MVA each at Addis East-I and Addis West, and each of two
125MVA transformers are installed at Cottobie and Sebeta-1. Simulation results for peak load
case after mitigation show that, voltage at Addis West substation 15kV bus improved from
0.7647 to 0.9674pu and at Addis North substation 15kV bus from 0.8187 to 0.99pu, which is
20.07% and 17.13% improvements respectively.
Mitigation techniques applied to improve the unacceptable voltages for light load conditions
include; installation of shunt reactors with capacities of 45MVAr at Debre Markos, 30MVAr at
Gashena, and 15MVAr at Nifas Mewcha substations. Simulation results under light-load
condition after mitigation show that, voltage at Debre Markos 66 & 15kV buses are adjusted
from 1.1428 to 0.962pu and 1.1407 to 0.964pu, which is -18.08% and -17.67% improvements
respectively. This thesis assesses the blackouts that have occurred in Ethiopian power system during 2015
and 2016. The observations from the blackouts show that, most of the faults are initiated by a
sudden tripping of generating units and transmission lines. These faults resulted in the
surrounding area to be exposed to a lack of reactive power and several generators to be field
current limited.
This project recommends voltage stability assessment to be investigated during each and every
change on the transmission grid by including the sub-transmission and distribution sides. In the
future, dynamic voltage stability analysis should be investigated by including the common
contingencies, the distribution side, and the load behaviours.
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Keywords
Static voltage stability, Ethiopian grid, reactive compensation, blackout, PSS/E