This demo consists of the simulation of the WECC 10k-bus network in hybrid EMT-Phasor domain approach. The WECC system is divided into a 9k-bus using OPAL-RT's phasor-domain simulation tool at 10 ms, while the 555-bus is simulated at 50 us using OPAL-RT's EMT solution. For study purposes of inverter-based resource generators, three generic photovoltaic (PV) plants are added in the EMT portion. Interface nodes are represented as PQ loads in ePHASORSIM (phasor-domain tool) and voltage behind impedance in HYPERSIM (EMT tool). ePHASORSIM is embedded in HYPERSIM in Functional Mockup Interface (FMU), enabling EMT-phasor multi-rate (50 µs - 10 ms) co-simulation at scale. It is worth noting that ePHASORSIM computes and communicates the node Thevenin equivalent to HYPERSIM at each time step of the ePHASOMsim solver. An SEL-421 protection relay is added as a PMU to the simulator. The analog output of the OP5707XG simulator communicates instantaneous voltages to the SEL-421. Then, RMS voltage and current measurements from the SEL-421 are sent back to HYPERSIM via C37.118.1 protocol.