Study of Blackouts on Ethiopian Electric Power Network and Identification of System Vulnerabilities
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Date
2017-10
Authors
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Journal ISSN
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Publisher
AAU
Abstract
The Ethiopian Electric Power (EEP) has been operating and managing the national
interconnected power system with dispersed and geographically isolated generators, complex
transmission system and loads. In recent years, with an increasing load demand for rural
electrification and industrialization, the Ethiopian power system has faced more frequent,
widely spread and long lasting blackouts. To slash the occurrence of such incidents, analysing
the reasons and the mechanisms of these blackouts is the first step in this direction.
In this study the causes, mechanism and extent of the past blackouts in the EEP grid are
investigated. This is accomplished by collecting the blackouts data from the National Load
Dispatch Centre (NLDC) archive. These data are characterized based on four significant indices,
namely- number of customers without service, loss of load (MW), duration of blackouts, and
their severity level. The investigation is made by considering the following sequential phases of
the blackout: system condition prior to the power failure, initiating events, cascading events and
the final state of the power system.
Based on analysis of the available data, it is found that the major initiating events for the
blackouts of the Ethiopian power grid are short circuits, overloads, loss of generations,
protection and communication system failures, switching and temporary transients. It is further
observed that 49 blackouts were happened on the national grid from 28
th
January 2013 to mid
of May 2016. It is also found that the highest load loss of 1401.04 MW, which occurred on 28
th
November 2015. The 27
th
July 2015 blackout duration was the longest and it lasted for about 22
hours. Furthermore, it is observed that 81.25 % of the blackouts (26/32) are having a severity
level of 10 to 100 system-minute. These are classified as “Severe” and the remaining 18.75 %
of the blackouts (6/32) are having a severity level of greater than 100 system-minute and are
classified as “Very Severe”.
Simulations studies are carried out using DIgSILENT PowerFactory software to further
investigate the initiating events resulting in cascaded tripping and subsequent blackouts. In the
first stage, power flow simulation studies are carried out to analyse the system performance under steady state conditions and to determine the voltage magnitude at critical buses and the
loadings of lines, transformers and generators prior to the disturbance. In the second stage, time
domain Root Mean Square (RMS) simulations are performed to analyse the system performance
under transient conditions for the specified initiating and cascading events. A vulnerability
analysis was performed using indices called active power performance index (PI
p
) and voltage
performance index (PI
v
). These indices provide a direct means of comparing the relative severity
of the different line outages on the system voltage profiles and loadings.
It is found that the line outage of Debre Markos_Sululta 400kV line is the most severe line
outage having a line loading performance index of 7.3238 pu. The outage of this line resulted
in the overloading of eight components. On the other hand, the outage of Alamata_Combolcha
230kV line is found to be the most severe line with respect to the voltage performance index
(PI
v
) of 0.1124 pu. Based on this analysis, it is found that the most vulnerable buses of the
network in respect of voltage limit violations are Sululta and Gefersa 132kV buses. It is further
observed that 11 different line outages led Sululta and Gefersa 132kV buses to have voltage
magnitudes of below 0.90 pu. The highest percentage voltage deviations for these buses are -14
% and -13 %, respectively. Furthermore, it is found that the network element’s most vulnerable
to overload is the transformer at Combolcha II 230/132kV substation, in which 12 different line
outages led this transformer to become overloaded.
Finally, from the simulation results and system vulnerability analysis, we identify the remedial
steps to coordinate and focus control actions that could mitigate the consequences of the
disturbances and reduce the risk of cascading events leading to system blackouts. It is
recommended that proper protection system coordination, high-speed fault clearing, proper
planned interruptions as well as adhering to the security rules and criterions during planning and
operation of the power system are the remedial measures that could be explored by EEP
Description
Keywords
Power system blackouts, Blackout indices, Cascading failures, Transient stability, System vulnerability, Contingency analysis