Browsing by Author "Mizan, Welderufael"

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    Evaluation of Static Voltage Stability of Ethiopian Grid
    (Addis Ababa University, 2017-07) Mizan, Welderufael; Getachew, Biru (PhD)
    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.