Look attached document for more understanding. I have an incoming HV line Mufindi grid substation to the Mbeya Grid substation (Transmission tier). HV signal/connection travel to HV transformer (220KV/33KV) at the terminal of XFR-High. I created a Sub-transmission tier at the XFR-Low that ends with an MV circuit breaker at the Bus terminal. I made a Distribution tier at the MV Circuit breaker at the CB Line terminal.
My doubt about the Sub-transmission tier. And at what condition sub-transmission tier is used?