Is DSP really required for over current numerical relay. If we have processor with 16 bits ADC, frequency in MHz and some KB RAM, then also we can achieve over current protection with good accuracy for wide current range. So what is so special about DSP???
DSP (Digital Signal Processing) could emulate the response of an existing mechanical overload protector, and thus conform to pre-existing standards. It could also allow thermal modelling of the device being protected.
For example, this paper describes a TMS320F2812-based overcurrent protection that meets the IEC 255-3 standard.
The benefit is to avoid false tripping or failure to trip under operating conditions. The cost is that the calculations are complex and have to be completed in a short time, so that's where DSP comes in rather than a general purpose micro. Some MCUs have DSP functionality.