Browsing by Author "Mathebula, Vonani Clive."

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Application of bus transfer schemes to stabilise power supply in a coal fired power plant unit auxiliary reticulation.
(2019) Mathebula, Vonani Clive.; Saha, Akshay Kumar.
A multi-function bus transfer system comprising fast, in-phase and residual bus voltage transfer schemes is developed in the thesis. Bus residual voltage magnitude and phase angle are calculated by converting time domain components of a three phase system in an abc reference frame to dq0 components in a rotating reference frame using Park’s transformation equations. Residual bus voltage phase angle is then modelled by a Taylor’s series expansion to calculate the phasor angular position with reference to the alternate power supply ahead of time to enable synchronization of the two supplies. Simulations are performed to verify functionality and performance; and to deduce the characteristics of the respective schemes. The thesis then explores the feasibility of using the bus transfer system to stabilise power supply within a power generating plant auxiliary electrical reticulation when upstream electrical equipment failures occur; in particular focus is placed on the unit boiler furnace draught system which would normally result in reduction of up to half of unit generating capability if one set of the draught system is lost. Simulation results of case studies conducted provided practical understanding on the feasibility of using a bus transfer system, with fast bus transfer scheme being the most preferred method at 70 ms transfer time; which enables the forced draught fan motor to be transferred within 2 s before the unit begins to de-load. The thesis proposes a new reticulation configuration that allows the transfer of both forced and induced draught fan motors simultaneously while maintaining stable draught furnace pressure. The new configuration allows both fan motors to remain connected to the switchboard for up to 3 s before tripping the motors on under-voltage protection when upstream equipment failures occur, even though bus transfer can be executed in 70 ms or 520 ms using fast or in-phase transfer schemes respectively. The speed and minimum impact on the electrical system makes the fast transfer scheme the most preferred transfer method
Reliability of multi-channel IEC 61850 mission-critical substation communication networks based on Markov process incorporating linear dynamical systems and calculus inferences.
(2021) Mathebula, Vonani Clive.; Saha, Akshay Kumar.
IEC 61850 based Substation Communication Networks (SCN) enable substation processes to be digitalised to fulfil the most sought substation monitoring, protection and control of electrical systems. The standard enables peer-to-peer communication of mission critical messages, aided by onboard diagnostic capabilities to ease the identification of system faults. The implementation of Safety-Related Systems in industrial facilities comprising sensors, logic solvers and final elements in power distribution centres necessitate compliance to IEC 61508 standard, where circuit breakers act as final elements to isolate electrical machines. In recent times, combinatorial methods such as the Reliability Block Diagram have been used to evaluate the architecture of IEC 61850 based SCN reliability and availability due to the simplicity of the approach. These methods, however, assume that all system faults are identified and fully repaired, which is not the case in practice. In this thesis, the reliability of a repairable multi-channel IEC 61850 based SCN architecture is modelled using a structure function and the Markov process while Systems Thinking integrates imperfect repair factors into the model. Thereafter, a novel eigenvalue analysis method based on Markov partitions and symbolic dynamics in the context of linear dynamical systems is used to investigate the impact of imperfect repairs on the system's reliability based on the number of mean state transitions and dynamical behaviour. The eigenvalue method is then advanced by a complimentary analysis technique based on the absorbing Markov Chain process and matrix calculus methods to determine the system's responsiveness to repair factors. The case studies results demonstrate that imperfect repairs cannot be ignored for mission-critical applications because the simplifying assumptions of combinatorial analysis methods greatly over-state the system's reliability performance. The results also indicate that common causes of failure coupled with imperfect repairs significantly negatively impact the system's performance. Moreover, system performance is highly dependent on the diagnostic coverage of the individual subsystems than their repair efficiencies for high diagnostic coverages at 90% and 99% based on ISO 13849-1. Hence, the results demonstrate that emphasis should be more on the system diagnostic coverage for the fact that it is embedded in the system design itself that cannot easily be changed once the system is commissioned and operational.