Year

2015

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

The advent of inverter-interfaced, decentralised generation has incited a paradigm shift in distribution network design philosophy. Accordingly, the effects of decentralised gener- ation on distribution network power quality, stability and protection have become impor- tant topics within research. Whilst the focus of this Thesis is protection design philosophy amidst increasing decentralised generation, protection efficacy is indelibly linked to power quality and stability. Ergo, it is prudent that all three areas of research interest are examined.

The motivation for this Thesis is to overcome some of the technical constraints that pre- vent the continued proliferation of decentralised energy resources. Further, this Thesis provides simulation tools that will aid in understanding the effects of decentralised gen- eration on the protection adequacy of distribution networks. Generally speaking, there exists no consensus on how much embedded generation is permissible before traditional distribution network design philosophy becomes ineffective in maintaining protection, sta- bility and power quality adequacy. Hence, the work conducted in this Thesis will provide utilities with the knowledge and tools required to cope with the continued proliferation of decentralised generation.

Currently, the embedded generation penetration levels in Australia reportedly cause in- stances of over voltage compromising the power quality in distribution networks. This Thesis proposes an over voltage mitigation scheme, which utilises a controller capable of indirectly regulating the voltage magnitude at the terminals of an embedded generator through reactive power absorption and apparent power curtailment. The scheme max- imises the remuneration received by the embedded generator proprietor whilst adhering to voltage standards and does not require an off-line sensitivity analysis or communications infrastructure. The results indicate a proof of concept; however, there exists many con- cerns regarding how such a scheme could be incentivised { or, if necessary, enforced. The scheme operates best when all decentralised generators employ the proposed controller.

Instances of protection failure due to decentralised generation are non-existent in Aus- tralian distribution networks and requires a much more significant penetration of embed- ded generation than present. A small-signal analysis tool is required to determine whether the connection of an embedded generator will cause fault discrimination issues. Small- signal analysis is time-intensive and requires a large amount of information to accurately model the fault response of an embedded generator. This Thesis provides an alternative to small-signal analysis via an iterative solver that predicts the prefault, fault-instant and steady-state fault data. Utilisation of the proposed solver allows a thorough protection analysis study of a distribution network for every fault type and location. A report is then generated containing the specific information relevant to a protection engineer. The strengths and limitations of the proposed solver are detailed and compared with small- signal analysis. The proposed solver is capable of greatly improving the productivity of protection engineers when dealing with large penetrations of embedded generation.

This Thesis extends into the natural evolution of embedded generation proliferation, namely, the microgrid concept. Some alterations to contemporaneous microgrid design philosophies are proffered with the intention of improving the ability of islanded embed- ded generation protection devices to discern the presence of high impedance single-phase to earth fault. An iterative solver is also developed to calculate the fault response of islanded embedded generation when employing conventional droop control developed for microgrid applications. The solver is capable of identifying the prefault, fault-instant and steady-state fault data including the island frequency. A key contribution of this Thesis is the introduction of a voltage sequence protection scheme. Results indicate that the voltage sequence protection scheme possesses excellent fault discrimination capabilities for islanded networks rich in inverter-interfaced generators.

The findings of this Thesis contain conceptual significance with numerous possible ap- plications in future networks should the proliferation of inverter-interfaced generation be sustained. The ability to easily discern the impacts of increased decentralised genera- tion on protection adequacy is a major strength of this Thesis. Furthermore, the over voltage mitigation scheme and microgrid design philosophy, including voltage sequence protection, represent new techniques that address obstacles in perpetual energy resource decentralisation: over voltage in grid-connected applications and protection discrimination in intentional islanding applications.

All contributions within this Thesis have been simulated within the MATLAB environ- ment using an innovative simulation platform. Verification of simulation results are pre- sented where validation against other published results has been possible. However, in order to fully examine the performance of the contributions developed in this Thesis, fur- ther research is required to prototype and rigorously test the concepts that have been developed using real-world networks.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.