Design of a low cost digital protection relay used as a teaching platform

The paper will present the design of a very low cost (under $25) experimental numerical relay. The relay has an impedance characteristic, which can be easily changed to either an offset mho, or a quadrilateral characteristic. First, a low cost, commercially available, microcontroller platform will be introduced. Then, a basic analog front end circuitry will be described. The free software environment, as well as the software kernel will be presented. Also, a freely available software tool will be described and used to produce the waveforms to be sampled by the relay. Finally, a discussion will highlight the strengths and weaknesses of the design. In the end, the reader will gain the knowledge needed to build and experiment with the relay using no other tool than a simple Personal Computer.