Design and implementation of automatic power switch for grid- solar hybrid system with the power surge protection.

Abstract

In using more than one source of energy, a switching system either manual or automatic is necessary to change from one source of energy to another in case of any disruption in either sources. The current trend is to utilize automatically operated changeover switches, which do not require a person to monitor the available source of energy. However, the current available automatic changeover switches are faced with the switching transients while switching from one source to another, which can be destructive to the load and result into wear of switching contacts. Additionally, a US collaborator is interested in having locally made automatic changeover switches for deployment in health centers in Fort Portal aiming at local capacity development. Therefore, this project sought to locally design and build an automatic changeover switch with reduced switching transients. We simulated the circuit of the changeover switch using proteous simulation tool and Arduino integrated environment, built the prototype of the simulated circuit on the breadboard, and then measured the magnitude of switching transient in the MATLAB Simulink with and without resistor-capacitor (RC) snubber. We finally built a microcontroller based actual automatic changeover switch on the perforated board and tested it for switching using a 2 kW electric kettle. In order to effect the switching of only values in the desired range of 200 Vrms to 250 Vrms, we built a voltage sensing unit consisting of the step down transformer, the bridge rectifier circuit, the capacitor filter circuit and the voltage divider circuit. The voltage sensing units have their inputs connected to the two sources and their outputs are fed to the microcontroller input pins and depending on the signal provided to the microcontroller by the voltage sensing units, the microcontroller decides on which source to supply the load. The measured switching transient in MATLAB Simulink without an RC snubber connected across the switch was 23 V for the source of 240 Vrms. This reduced to 10 V when an RC snubber was connected across the switch, which is about 57% reduction in the magnitude of the switching transients. Similarly, the actual changeover switch built successfully switches from one source to another as desired. We recommend voltage sensors to be used together with the power modules capable of changing 240 Vac to 5 Vdc directly, and contactors for switching larger loads.