As an additional opportunistic component to the Southern African Mag- netotelluric Experiment (SAMTEX), audio-magnetotelluric (AMT) data were acquired during the most recent phase of the experiment (Phase IV) to inves- tigate the local-scale conductivity substructure in the Otjiwarongo and Ka- tima Mulilo regions (Namibia), as to aid in the installation of high-voltage direct current (HVDC) earth electrodes that has since taken place. Both of the AMT surveys are situated close to the edge of the orogenic Neo- Proterozoic Damara Mobile Belt (DMB). Previous studies all point to the existence of a highly conductive mid-crustal zone which correlates well with the spatial location of the DMB. Two-dimensional (2D) inverse modelling of the Otjiwarongo AMT data con rms the existence of the high conductive zone at mid-crustal depths (10-15 km). The high conductivity of the DMB is explained by the presence of interconnected graphite in the marble units present. The Katima Mulilo inversion results are characterized by a con- ductive upper crustal layer that does not form part of the DMB conductive belt. It is deduced that at the uppermost subsurface Kalahari sediments are responsible for the high conductivity observed while at greater depth it is due to ironstone within the Ghanzi Group. In contrast to the conductive DMB, the lithospheric structure of the neighbouring Archaean cratons, the Congo and Kalahari, are generally found to be electrically resistive. There- fore, it is hypothesized that ground return current, if present, will ow along a path between the Otjiwarongo and Katima Mulilo regions that lies either exclusively, or almost entirely within the DMB. The hypothesis is tested by inputting a three dimensional (3D) conductivity model (calculated using available magnetotelluric (MT) data and geological information) of the re- gion into a DC resistivity forward modelling code. Forward modelling shows that the return current is only con ned to, and follows regional trends char- acteristic of, the conductive DMB for approximately 200-300 km away from the injection point, after which there is no preferential flow.

It was the purpose of this investigation to study the effect of varying the direct to reverse time ratio of periodically reversed direct current from 1.0 to 20.0 on the electrolytic production of lead chromate at an average anode current density of 0.0059 amperes per square centimeter, and to study the effect of 60 and 502.3 ± 7.7 cycles per second from 0.00113 to 0.01546 amperes per square centimeter of peak superimposed alternating current on direct current on the yield of lead chromate prepared by the electrolysis of a bath containing potassium chromate and sodium nitrate between lead electrodes. Electrolysis of a bath containing 3.60 grams of potassium chromate, 11.62 grams of sodium nitrate, end 1000 grams of water with simple direct current yielded 6.07 grams of lead chromate per ampere-hour, the purity being 92.7 percent lead chromate. The anode current density was 0.0049 amperes per square centimeter, and the current efficiency was 98.1 percent. During the electrolysis, by maintaining the ph of the electrolyte at 6.0, by adding a solution to 2.0 weight percent chromic acid, the purity of the product was increased. Electrolysis of the same bath using periodically reversed direct current yielded 4.53 grams of lead chromate per ampere-hour, the purity being 93.9 percent lead chromate. The time ratio was 20.0, the anode current density was 0.0049 amperes per square centimeter, and the current efficiency was 66.75 percent. Decreasing the direct to reverse time ratio gave lower yields and purities. Apparently, there is no advantage in using periodically reversed direct current over the use of direct current for this reaction under the above conditions. Electrolysis of the same bath with alternating current superimposed on direct current yielded 5.49 grams of lead chromate per direct current ampere-hour, at a purity of 99.4 percent lead chromate, when using 494.7 cycle alternating current. The alternating and direct current densities were 0.0078 and 0.0048 amperes per square centimeter, respectively. This was the purest product obtained in this investigation. Use of 60 cycle alternating current yielded 3.83 grams of lead chromate per direct current ampere-hour, at a purity of 93.9 percent lead chromate. The alternating and direct current densities were 0.00141 and 0.00484 amperes per square centimeter. Further experiments were made using direct current and periodically reversed direct current on a bath containing 6.80 grams of potassium chromate, 8.14 grams of sodium nitrate, and 1000 grams of water. Results from these electrolytes were much poorer than those obtained with the previous bath, so no experiments with superimposed alternating current on direct current were carried out with this latter bath. / Master of Science

The management of parallel and series resonance conditions is important for ensuring that harmonic levels are managed on utility networks, and that shunt compensators are able to operate without constraints for various network conditions (states). For these and similar problems, harmonic impedance assessment of the ac network is required for the design of ac filter or shunt capacitor bank installations. This is particularly important for large installations connected to HV or EHV systems, because resonances at these voltage levels tend to be highly un-damped resulting in potentially damaging voltage and current amplification. The objective of this dissertation was to develop and demonstrate a design methodology which makes use of network impedance assessment methods to provide robust harmonic integration of large shunt compensators into a transmission and HVDC systems. The design methodology has two aspects. The first part considers network modeling, evaluation of different models and simulation of harmonic impedance. In the second part, methods of analyzing and assessing the simulated harmonic impedance are developed. A detailed step-by-step approach was taken in the development of the design methodology. The methodology was documented as a guideline and accompanied by the development of an Excel tool that can be used to assess the simulated harmonic impedance. The Excel tool permits a systematic assessment of the simulated network impedance where shunt compensators are integrated into transmission systems. The tool also ensures that the design of transmission and HVDC ac shunt compensation is optimally robust in terms of harmonic resonances. The theoretical and computational review has been tested and demonstrated on the existing Eskom Transmission system through several case studies. The results have shown the merits of the design methodology. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2013.