Modelling of Transformer and Medium Voltage Powerline Channels for Data Communication on Single Wire Earth Return Distribution Grids
Tangible benefits such as remote metering and fault diagnosis may be accrued by the implementation of communication capability alongside distribution grids, especially those that supply power to sparsely populated rural communities. Keeping costs to a minimum is a major concern to the operators of rural grids, as evidenced by the development and regular improvements to specialized distribution systems such as the Single Wire Earth Return (SWER) system.
A possible low-cost option to implementing grid communications is Powerline Communication (PLC), which has a distinct advantage over other options, such as wireless, of the pre-existence of communication infrastructure in the form of power lines. This advantage over wireless becomes more apparent when geographical terrain makes the propagation of wireless signals without line-of-sight strategies difficult.
Previous work on PLC implementation on SWER grids was limited by the available methods for characterizing and modelling the earth return paths at PLC frequencies, which were mostly based on analytical approaches. Therefore, the first original and significant contribution to knowledge outlined in this thesis is the development of a hybrid empirical/analytical method for characterizing and modelling earth paths, inclusive of the earthing system and superfluous components, at narrowband PLC frequencies.
The method was empirically verified by field experiments on a replicated SWER Medium Voltage (MV) line consisting of an industry-standard conductor and earth rods, resulting in a hybrid empirical/analytical per-unit length SWER line model. A Two-wire MV line with conductor properties identical to that of the single conductor used in the SWER experiment was also analytically characterized at narrowband PLC frequencies, for comparison with the SWER MV line.
Data transfer on distribution grids through the exclusive use of PLC is limited by transformers, which create high impedance nodes at the MV/LV grid boundary that tend to attenuate the PLC signals to impractical levels. This high attenuation may be overcome by implementing a bypass channel, where the signal is made to go ‘around’ the transformer. However, this configuration has cost and safety-related disadvantages, which are addressed by implementing a ‘through’ transformer PLC configuration.
A major shortcoming of SWER systems is that they suffer from voltage regulation issues, resulting in voltage fluctuations with bulk loading conditions. Consequently, a study on the effects of energization levels on PLC signals passing through distribution transformers was also carried out. The study revealed that the mechanism of transformer insulation dielectric polarization causes the cyclic variation of high frequency signals passing through distribution transformers, due to the instantaneous energization levels of the transformer.
These cyclic variations were empirically determined via laboratory experiments that involved passing high frequency constant current signals through a range of single-phase 11 kV/230 V transformers. Models of the unenergized through-transformer channels of some of the transformers used in the experiments were also estimated for PLC simulation.
A general relationship between transformer insulation dielectric polarization and through-transformer PLC data throughput was established, from PLC simulations of the energized through-transformer channel models of one of the transformers. This general relationship forms the second original and significant contribution to knowledge, made through the research outlined in this thesis.
The method used in developing the simulation models of the energized through-transformer channels was based on a hybrid modelling approach. It involved running iterative scripts that directly scaled the outputs of the unenergized through-transformer models, to effect the cyclic high frequency signal magnitude and phase variations associated with the energized channels. The method has not been previously used in creating simulation models for energized through-transformer PLC simulation, therefore forming the third original and significant contribution to knowledge, made through the research outlined in this thesis.
These three original and significant contributions to knowledge demonstrate the feasibility of grid-wide PLC implementation on SWER grids, by considering the various MV/LV configurations that may be implemented across combinations of transformers and MV lines that are found on typical SWER grids, and distribution grids in general.