Travelling-wave and time-domain-based protection functions provide significant response time improvements over conventional phasor-based protection functions in power systems. These types of protection functions require very high sampling rates in the order of several hundreds of kHz. This paper proposes a novel centralized substation protection architecture (CPC) based on distributed signal processing units (DSPU) that enables the deployment of these high sampling rate applications in digital substations utilizing an Ethernet-based process-level network. The design of the DSPU is elaborated, and its signal processing algorithms are discussed. Moreover, the performance of the DSPU is analysed through dynamic tests and verified through a numerical electromaanetic transient simulation.