Several voltage collapses have been experienced by most practical power systems in recent times in Nigeria. These have been mostly attributed to island formation within the network as a result of switching out of critical buses within the network. This paper assesses the stability of a practical power network of the Nigerian National Grid (NNG) with reference to a critical transmission line which could cause island formation as a result of fault within the NNGnetwork.
The Matlab/ Simulink model for both the steady-state and dynamic behaviours of the critical bus within NNG network is presented. The simulations for various fault conditions are also presented. The results obtained from the simulations are discussed. From the simulation results, high flow of reactive power and current, as a result of various faults along the transmission lines, were observed.
Furthermore, non-sinusoidal waveforms generated from the simulation results show an indication that a high degree of compensation is required at the studied bus. The study uses Benin bus of NNG with respect to Benin-Onitsha-Alaoji 330-kV transmission network due to its location and sensitivity within the grid.
THE NIGERIAN GRID NETWORK
It shows the one-line diagram of a Nigerian 330KV grid network. The single circuit, Benin-nitsha-Alaoji 330KV transmission network, is the critical line within the NNG network. Thus, it has no level of redundancy due to it single nature circuit unlike the double circuit transmission lines that has some level of redundancy. The Nigerian 330KV transmission grid system is characterized by high power losses due to the very long transmission lines and its radial structure.
MODELLING AND PERFORMANCE ANALYSIS OF BENIN-ONITSHA-ALAOJI 330-KV TRANSMISSION NETWORK
Benin-Onitsha-Alaoji is a single circuit 330-kV transmission line. Therefore, faults along this line shall cause instability in the grid. In the case of a double circuit, load demand during fault could be served temporarily by the other circuit. A careful study of figure 1 shows that the Benin bus connects the Southern, Western, Eastern and Northern parts of NNG network.
This connectivity thus makes the bus to be sensitive, and therefore prone to faults and voltage collapse of NNG system. Hence, there is the need for the stability assessment of the grid with respect to Benin bus. Table 1 shows the connection of Benin bus to other buses within NNG and their associated line lengths, resistances and reactances in per unit.
SIMULATION RESULTS AND ANALYSIS
The steady state simulation is performed to determine the normal power-flow of the network and its operating characteristics such as voltages, currents and power at the various buses. The power generated by two generating plants is transmitted to the lines serving the loads connected to the three buses, at B1 136 MW and 84MVAR, B2 236MW and 146MVAR, and 248MW and 153MVAR.
The dynamic state simulation is carried out with the same model putting the fault breaker into On-mode to simulate different fault conditions at the three buses. The faults simulated are single line-to-ground fault, double line-to-ground fault, three-phase-to-ground fault, line-to-line short circuit fault, and sudden drop out of generating plant. The various faults conditions simulated are used to ascertain and analysed the system collapse.
DISCUSSION OF RESULTS
The simulation results of the steady state and dynamic faults analysis of Benin-Onitsha-Alaoji 330kV transmission line within NNG have been considered. Based on the steady-state simulation results, the highest fault current was observed in the three-phase-to-ground fault simulated in bus 1 while the single line-to-ground fault simulated into bus 1 has a voltage value close to the normal operating condition.
This paper has studied and analysed various dynamic faults along the transmission line of NNG with reference to Benin-Onitsha-Alaoji 330-kV transmission line. The simulation and analysis of the equivalent circuit for the network considered using Matlab/Simulink model has been presented.
Based on the results obtained, it is observed that the Benin bus is more prone to faults and hence, it can be concluded that the line should be compensated for the stability and hence security of the system. This paper researches on the needs to improve on the stability of the various buses in the NNG using the Benin bus as a reference bus. Matlab/Simulink model for the case study is presented. Both the steady-state and dynamic behaviour of the network of interest are analysed. Various types of fault for the system are simulated and analysed.
Source: University of Benin
Authors: Abel Ehimen Airoboman | Ignatius Kema Okakwu | Akintude Samson Alayande | Oluwasogo Emmanuel Seun