Application of bus transfer schemes to stabilise power supply in a coal fired power plant unit auxiliary reticulation.
Mathebula, Vonani Clive.
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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