Centralized Protection In Distribution Substations - Testing Modular Relays With Conventional Ct, Vt Inputs and Process Bus Architecture

The increasing demand for electrical energy and the integration of distributed energy resources (DERs) are driving the need for new substation deployments and equipment upgrades to maintain safe and reliable service. Minimizing the physical footprint of distribution substations has become increasingly important and can be achieved through modular design in both primary switchgear and protection and control systems. Modular or centralized protection relays, capable of protecting multiple assets within a single device, represent a key element of this approach. This paper presents a case study on the implementation and testing of Centralized Protection and Control (CPC) relays for a distribution substation. The project was conducted in two phases. The first phase evaluated a CPC relay equipped with nine sets of three-phase CT inputs and one set of VT inputs, configured to protect a 69 kV transformer bank, a medium-voltage busbar, and two 12 kV feeders. The second phase tested relay with process bus capabilities, subscribing to IEC 61850-9-2LE sampled value streams from nine merging units for the same substation configuration. Extensive laboratory testing was performed to assess relay performance under a wide range of abnormal and fault conditions. Relay and Merging Unit equipment from different vendors was included to validate interoperability. Internal relay logic was engineered to comply with well-defined Southern California Edison (SCE) protection standards. Authors discuss a comparative performance assessment between relays with conventional CT/VT inputs and IED receiving IEC 61850-9-2LE SMVs from the Merging Units. The results show that while conventional inputs provide proven reliability, the process bus configuration significantly reduces wiring complexity, improves scalability, and deliver comparable protection performance when proper time synchronization is ensured. The paper discusses test results, including representative waveforms and relay performance, and summarizes key lessons learned from the evaluation.