Design of four-quadrant rectifier for cathodic protection systems

Metallic structures such as pipe lines near the direct current (DC) railway systems provide low resistance paths to stray currents. These stray currents adversely affect the operation of cathodic protection systems and leave the pipelines defenseless against corrosion. At the first stage of this thesis running rail, pipeline and the earth have been modelled as a resistive network to investigate the potential distribution created by the rail on the ground and over the pipeline. This model has been analyzed by using a converging method to investigate how the position of trains and running rail grounding schemes affect the potential over pipelines. Using the results of this, a novel method that diverts the stray currents away from the running rails by generating a potential which opposes the potential between the rail and pipeline. Twenty-nine different layouts of anodes and cathodes of the dc sources have been tested for two different train position scenarios to observe this reverse potential. Source current and voltages to minimize the potential difference between the pipeline and the ground have been obtained by using an optimization method for each case. The results show that the proposed method reduces the potential difference more than 80% with a proper anode-cathode layout. At the second part of the thesis a two-stage four-quadrant rectifier that can provide the required currents and voltages in a very short time was designed. The full-bridge converter at the second stage of the rectifier needs to respond fast both at continuous and discontinuous conduction mode, and therefore an adequate controller design is necessary. For that, the small signal models in each quadrant for both types of operation have been obtained. Transfer functions have been developed based on these models and parameters that affect the control have been analyzed. A novel method to design a cascade PI controller with load dependent coefficients has also been developed in the thesis. The proposed controller has been compared to the classical cascade PI controller in PLECS simulation environment. Finally, the proposed method has been implemented on a 100W prototype. The experimental results prove the effectiveness of the proposed method.