Browsing by Author "Oni, Oluwafemi Emmanuel."

Now showing 1 - 2 of 2

Stability of the grid incorporating multi terminal HVDC: case study of a south African network.
(2021) Oni, Oluwafemi Emmanuel.; Swanson, Andrew Graham.; Carpanen, Rudiren Pillay.
Transmission lines make one of the significant parts of power systems; faults or disturbances along any of the transmission medium often transcend to both the generating ends and the loads' end. Besides, the strength of any particular grid depends solely on the impedance of the tie-lines of that grid. Therefore, in this thesis, the line commutated converter (LCC) multiterminal high voltage direct current (MTDC) system is modelled and improved for the stability of an AC network. The converter control architecture and modelling are emphasized and explained. The effective short circuits ratio (ESCR) of the interconnecting AC lines is first described and analyzed as well. The existing CIGRE control techniques for a point-to-point LCC HVDC system have been enhanced and adapted for this study. The control and the filter parameters have also been calculated to generate a better and efficient result during a steady-state and dynamic analysis of the study. The work carried out in this study is divided into four sections, with each section focusing on each of the research objectives. In the first section, dynamic modelling and control of LCC MTDC systems were carried out with consideration to the ESCR of the inverter side of the AC substation. The impact of large-disturbance at the inverter is investigated. This analysis has been proposed to study the impact of AC short circuit fault on the three substations. The results from this study, which are shown on a subplot, show that the system experienced a large transient overcurrent and non-severe commutation failures. Also, a voltage dip at the faulted inverter station was recorded; however, the efficacy of the converter controller disallowed the transfer of such voltage dip to the other two converters. The second section of this study focuses on the application of MTDC system. We have carried out a comparative analysis of MTDC and AC transmission line on a single machine infinite bus (SMIB) network. The main focus of the investigation was on the transient and rotor angle stability of the SMIB network with or without MTDC link. The study also carried out a power-angle curve with the use of equal area criterion. The third section focuses on the interarea oscillation reduction in a power system. Kundur's two-area four-machine network was adapted to suit the scenarios of this study. Different fault analysis was carried out, and the response of the generator active power, frequencies, and DC-bus voltages are recorded. The results in this study show the better performance of the MTDC implemented in this study over the other well-known method of AC transmission medium. Also, the integration of the MTDC link is constrained by the variation of the current order of the overall power controller. The result is observed in the damping rate of the interarea oscillation of the network. The final section of this study carried out dynamic modelling of the South African grid, and detailed dynamic response to different stability studies was carried out. An auxiliary controller for the MTDC system capable of reducing the active power oscillation by generating a new current order is proposed. This secondary control for the MTDC system is based upon dynamic sensitivity analysis of the oscillations, and thereby generate a DC current compensation for the reduction of active power oscillations in the MTDC converters' station. Two network configurations were considered in this section. System disturbance during the first configuration shows a loss of synchronizing effect from both the AVR and PSS, which causes the generator to lose synchronism with subsequent oscillations. A negative damping torque for the rotor angle and negative synchronizing torque for the interarea oscillations was also observed. Meanwhile, the results during the second configuration recorded quick damping of the interarea oscillations with a significant improvement to the voltage profile. Among all of these benefits, the power carrying capacity at a reduced loss and cost stood out. The conclusion from this section is that the implementation of the MTDC link on the South African grid provided a better system performance. Therefore, the adoption of this research into South African transmission network will surely help enhance the stability margin of the grid. The proposed secondary controller also provided potential mitigation of excessive active power dip of the MTDC link during the system disturbance.
Technical performance and stability analysis of eskom power network using 600kv, 800kv, and 1000kv hvdc.
(2016) Oni, Oluwafemi Emmanuel.; Davidson, Innocent Ewean.
In designing electric power networks or implementing major expansions to existing networks, a number of the key issues regarding the technical performance of the network at both transmission and distribution level must be ascertained, namely: voltage regulation, voltage fluctuations, electrical losses, transmission/distribution plant loading and utilization, fault level, generation stability, harmonics, phase balancing, supply availability and system security. System studies and analysis conducted from time to time to ascertain the operating state of a network, taking into account, load growth projections for the future. Undue stresses on the system or anticipated problems are determined from power flow analysis or during operation and maintenance. Using a modified Eskom network (KwaZulu-Natal sub-grid) as a case study, the technical and stability analysis for different high voltage direct current (HVDC) transmission voltages: 600kV, 800kV and 1000kV were carried out using DIgSILENT PowerFactory engineering software tool, as an alternative for bulk power transfer using high voltage alternating current (HVAC) link along the major corridors. Static analysis using PV and QV curves; dynamic analysis using RMS time domain and electromagnetic EMT analysis were carried out. Dynamic analyses were performed to determine the system fault levels and critical fault clearing time. Results obtained from this investigation show that 600kV and 800kV HVDC transmission systems have greater power capacity than equivalent HVAC line. HVDC delivery systems were observed to have lower electrical losses, better voltage profile, increase fault clearing time, enabling robust protection schemes to be installed. Voltage distortion due to harmonic content and imperfect current waveform in Cahosa-Bassa LCC-HVDC link were also investigated, and re-engineering with the use of VSC-HVDC technology has been proposed. This option provides reduced harmonic content, excellent sinusoidal waveform and minimal vulnerability to commutation failure. A financial and economic analysis of a 500kV HVAC double circuit and ±600kV HVDC transmission network were compared. HVDC system was proposed the most suitable scheme for bulk transmission of electric power over long distances due to high efficiency and better economics.