Evaluation Of Static Voltage Stability Of Ethiopian Grid

A power system is set to operate within its maximum operating limits for better utilization ofrnthe existing network facilities. The main factor causing voltage instability is the inability of arnpower system to meet the demand for reactive power. A system enters a state of voltagerninstability due to loss of loaded generation unit or transmission line, or a change in loadingrnconditions that causes progressive and uncontrollable decline or rise in voltage. This thesisrnattempts to evaluate voltage stability problems of Ethiopian high voltage transmission grid. rnLoad flow simulations using PSS/E software has been done for peak load (2,200MW) andrnlight-load (900MW) conditions. After conducting load flow simulations, ‘Voltage limitrnchecking’ results within tolerable range of +/- 0.05pu is applied to identify the buses/areas withrnunacceptable low and high voltage values. Low voltage profiles during peak hour arernregistered around Addis Ababa, and high voltage values are registered at North-western part ofrnEthiopia, i.e. around Bahirdar & Debre Markos. rnMitigation techniques applied to improve the low voltages for peak load conditions include;rninstallation of 15MVAr shunt capacitors each at Weregenu and Mekanissa substations,rnupgrading of transmission lines; i.e. Legetafo–Cottobie line from 132kV into 230kV,rnCottobie–Addis East-I line from 45kV into 132 kV double lines, and Sebeta1–Addis West linernfrom 45kV into 132kV, and upgrading of transformers; two 25MVA into two 50MVA at AddisrnNorth, two 12.5MVA into two 50MVA each at Addis East-I and Addis West, and each of tworn125MVA transformers are installed at Cottobie and Sebeta-1. Simulation results for peak loadrncase after mitigation show that, voltage at Addis West substation 15kV bus improved fromrn0.7647 to 0.9674pu and at Addis North substation 15kV bus from 0.8187 to 0.99pu, which isrn20.07% and 17.13% improvements respectively. rnMitigation techniques applied to improve the unacceptable voltages for light load conditionsrninclude; installation of shunt reactors with capacities of 45MVAr at Debre Markos, 30MVAr atrnGashena, and 15MVAr at Nifas Mewcha substations. Simulation results under light-loadrncondition after mitigation show that, voltage at Debre Markos 66 & 15kV buses are adjustedrnfrom 1.1428 to 0.962pu and 1.1407 to 0.964pu, which is -18.08% and -17.67% improvementsrnrespectively. This thesis assesses the blackouts that have occurred in Ethiopian power system during 2015rnand 2016. The observations from the blackouts show that, most of the faults are initiated by arnsudden tripping of generating units and transmission lines. These faults resulted in thernsurrounding area to be exposed to a lack of reactive power and several generators to be fieldrncurrent limited. rnThis project recommends voltage stability assessment to be investigated during each and everyrnchange on the transmission grid by including the sub-transmission and distribution sides. In thernfuture, dynamic voltage stability analysis should be investigated by including the commonrncontingencies, the distribution side, and the load behaviours.

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