A Study on Blackout Problems of Ethiopian National Grid and Its Mitigation Techniques Using Adaptive Under Frequency Load Shedding Scheme

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Addis Ababa University


An interconnected electric power system must balance load and generation in order to maintain frequency within acceptable range. The balance between generation and load within an interconnected electric power grid is expressed in the frequency of the system. Under frequency protection schemes are drastic measures employed if the system frequency falls below a specified value. Power system blackout is one of the worst events that power supply is totally interrupted so that industrial, commercial and domestic customers will be forced to use an alternate energy sources mainly diesel generators which are inefficient, expensive and environmentally not friendly. Under Frequency Load Shedding (UFLS) is a common technique to maintain power system stability by removing overload that might be created following system disturbance mainly during outage of power generating units. In this regard, the Ethiopian national grid under frequency load shedding scheme has been implemented many years before and it has not been revised despite the fact that a lot of big generating plants are connected in the grid. Due to this fact the system is unable to withstand major outages and system blackout is becoming frequent in our grid. The methodology used for the proposed load shedding algorithm is Adaptive Under Frequency Load Shedding (AUFLS) which considers both frequency and voltage as inputs for the design of AUFLS scheme. The disturbance magnitude is estimated using the rate of change of frequency and the location and the amount of load to be shed from each bus is decided using the voltage sensitivities. The national grid is properly modeled, and load flow and dynamic simulations are conducted using PSSE power system analyzing software for different disturbance cases that can affect the system frequency. The frequency and the voltage response for the simulated disturbance cases are analyzed and based on the results of the analysis new AUFLS scheme is proposed to improve performance of the existing UFLS scheme. The proposed AUFLS schemes are implemented on the modeled system and simulated again to see the performance of the new scheme. The study found out that the current under frequency load shedding protection is not supporting the grid for g



under frequency load shedding scheme, under frequency relays, dynamic simulation, outgo of generating units, speed governors, speed droop, national grid, adaptive under Frequency load shedding scheme, conventional under frequency scheme, tie line