Vol. 1 No. 3 Aug. 2018 Global Energy Interconnection DOI: 10.14171/j.2096-5117.gei.2018.03.007 www.geidco.org Full-length article Identification of system vulnerabilities in the Ethiopian electric power system Moges Alemu Tikuneh1, Getachew Biru Worku2 1. Department of Electrical and Computer Engineering, Debre Berhan University, Debre Berhan, Ethiopia 2. School of Electrical and Computer Engineering, Addis Ababa University, AAiT, Addis Ababa, Ethiopia Abstract: The Ethiopian Electric Power (EEP) has been operating and managing the national interconnected power system with dispersed and geographically isolated generators, a complex transmission system and loads. In recent years, with an increasing load demand due to 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, identifying the system vulnerabilities is the first step in this direction. In this paper, the vulnerability assessment is performed using indices called active power performance index (PIp) and voltage performance index (PIv). These indices provide a direct means of comparing the relative severity of the different line outages on the system loads and voltage profiles. Accordingly, it is found that the most severe line outages are those lines that interconnect the high load centered (Addis Ababa and Central regions) with the rest of the regional power systems. In addition, the most vulnerable buses of the network in respect of voltage limit violations are mainly found at the high load centers. Keywords: Power grids, power systems, vulnerability assessment, transmission system. 1 Introduction the power system, and thereby the national grid system’s vulnerability is increased from time to time. The past decade has witnessed a big growth in The main HV levels of the power transmission Ethiopia’s electricity sector. Between 2012 and 2016, systems are 400 kV, 230 kV, and 132 kV. The 500 kV the number of electricity customers of EEP reached over transmission system is on the way to come. Among four million from two million at the beginning of the the voltage levels, the 400 kV and 230 kV are the most period [1]. This steep growth in the electricity sector has important and are responsible for the bulk power flows led to significant constraints in EEP. Moreover, while the among the eight regional power systems: viz. Addis generation has been at the forefront of energy developments Ababa, Central, Western, Southwestern, Northwestern, in Ethiopia, the weaknesses in the transmission network Eastern, Northern and Northeastern regions. In addition, are not receiving their due attention. These weaknesses the power transmission network is extended to Djibouti affect the customers, the government, the economy and in the east and to the Republic of Sudan to the northwest. social welfare of the society. Due to these facts, fault(s) in The HV transmission network consists of 1071.76 km of the transmission system are threatening the operation of 400 kV circuits, 5895.54 km of 230 kV circuits and 4666.79 km of 132 kV circuits [1]. In Ethiopia, the power system’s vulnerability has Received: 20 November 2017/ Accepted: 12 December 2017/ Published: worsened from time to time because of the implicit and 25 August 2018 explicit reasons. However, these reasons have not been Moges Alemu Tikuneh studied in a concrete way which further exacerbate [email protected] the situation and as a result partial and total blackouts Getachew Biru Worku (PhD), frequently happen on the grid. Therefore, this paper [email protected] presents an approach for assessing vulnerability of the EEP 358 Moges Alemu Tikuneh et al. Identification of system vulnerabilities in the Ethiopian electric power system system by using two indices, called performance indices where (PIs) which reflect the health of the system. The indices Vi – The voltage magnitude corresponding to bus i SP allow assessing two different symptoms of system stress Vi – Specified voltage magnitude corresponding to such as voltage limit violations and overloads. bus i This process of vulnerability analysis involves studying n – A positive number and usually its value is 1 (n = 1, 2, … the effect of the removal of a system component on the etc...). system, particularly the power flows and the bus voltages. NB – Number of buses in the system whose voltage This component can be a generator, transmission line, magnitude is out of the specified ranges. transformer, and so on. This work however focuses on w – Real non-negative weighting factor (in general w = outages of the transmission network. The reason is that 1). most of the previous partial and total blackouts happened Thus, this index measures the severity of the out of limit as results of the faults on the transmission lines, as reported bus voltages and provides a direct means of comparing the by National Load Dispatch Center of EEP (NLDC) in [2]. relative severity of the different line outages on the system What follows is a qualitative and quantitative evaluation of voltage profile. which line outages lead to the most issues with regard to 2.2 Line Outage’s Severity Ranking voltage limit and line load violations. The method used to rank vulnerabilities is taken from Ranking is performed for all branch outages against [3], [4], [5], and [6]. Detailed explanation is given in their rated power carrying capacity (i.e., a load of 100%), Section II. and considering voltage violations outside the range of 0.9 pu to 1.1 pu. The most severe line outages and the 2 Vulnerability Assessment Methods performance indices for the two criteria are shown in descending order in Fig. 1 and 2. 2.1 Performance Indices The system performance index is a measure that can Line loading performance Index (PI) be used to evaluate the relative severity of a contingency 8 [3]. The most common form of system performance 7 indices gives a measure of the deviation from rated values 6 5 of system variables such as line flows, bus voltages, bus p 4 PI power injections, etc. The indices to quantify problems 3 related to loading and voltage limit violations are described 2 1 in equations (1) and (2). 0 e 1) Active power performance index (PIρ): This index is e I e e e used to measure the degree of line over loads and is given by [4]. 2n Jimma_Agaro 230kV line Kality I_Akaki I 230kV lin GG II_SekoruGG II_SebetaSebeta II 400kV II_Akaki II 400kV lin 400kV lin lin N Kality I_Akaki I 230kV line Gefersa_Sebeta II 230kV line 1 W Pl D/Markos _SulultaKality 400kVB/Dar I_Akaki line SebetaII_D/Markos I 230kV I_Mekanisa line400kV II 132kV line line Legetafo_Ayat GIS 132kV line PI = AlamataGefersa_Addis _CombolchaCombolcha_Legetafo North 230kV 132kV lin 230kV line line ° ∑ = max (1) Cotobie _Addis East II 132kV line i 1 2n p Cotobie _Weregenu TP 132kV line l Fig. 1 Line outages ranking in terms of PIρ where Pl – The MW flow of line l Fig. 1 shows the ranking of line outages in terms of max Pl – The MW capacity limit of line l the performance index calculated based on the branches Nl – The number of overloaded lines in the system overload. It can be seen that the most dangerous line outage W – Real power weighting factor (in general, W = 1) is the disconnection of Debre Markos_Sululta 400 kV line n – A positive number (n = 1, 2, 3… etc...). that leads to an increase in the path of the power supply to The summation is carried out on overloaded lines only the high load centers from Beles hydropower plant. to avoid the masking problem reported in [3], [4], [5]. On the other hand, Fig. 2 shows the line outages 2) Voltage performance index (PIv): The voltage level ranking based on the performance index calculated in terms performance index chosen to quantify system deficiency of bus voltage limit violations. It is found that the most due to out of limit bus voltages in defined by [4]. dangerous line outages from the voltage point of view are SP 2n the disconnections of lines of Alamata_Combolcha 230 kV, N Vi − Vi PIv = B ()w n (2) Akaki I_Kality I 230 kV line I & II, and Sebeta I_Kality I ∑i=1 2 sp Vi 230 kV lines. 359 Global Energy Interconnection Vol. 1 No. 3 Aug. 2018 data is taken from national load dispatch center (NLDC), power system planning department. At this specific time, 0.12 the system was loaded to 1539.74 MW from which 100 0.1 MW was exported to the Republic of Sudan and 50 MW 0.08 was exported to Djibouti. The full AC power flow is run 0.06 to get the steady state results, such as bus voltages and line 0.04 flows. Next, line outage analysis is done by considering 0.02 a load scaling factor of 1.33, as this factor is used by the 0 planning and operation department of NLDC for daily . I I e e peak load forecasting and generation dispatch scheduling. This data is fed to DIgSILENT PowerFactory software and computer simulation is then performed for each line outage. Gefersa_Sebeta I 230kV GG II_SekoruHurs 400kV o_PK12 line 230kV line Awash II_AselaHurs o_Akaki132kV line INazret 230kVAkaki II_Koka line I_Koka 132kV 230kV line line Simulation of the line outages begins with ascertaining Akaki_KalityAkaki_KalityKality I 230kVI_Sebeta I 230kV line I 230kVline II lin Sebeta II_Akaki 400kV line Akaki I_Koka 230kV line II Alaba_Shashemene 132kV line Alamata_Combolcha 230kV lin GondarGondar II_B/Dar II_B/Dar II 230kV II 230kV line lineI II Adama Wind II_Koka 230kV line Shashemene_Melkawakena yougo.. a steady state solution of the system at base case, and it is obtained that there is overloading of components.