Your one line shows Molded Case Circuit Breakers (MCB) protecting the 110V secondary and an 11 kV fuse for clearing a faulted voltage transformer (VT). That is normal design for medimum voltage (15 kV Class) switchgear.
I don't understand your comment about the undervoltage tripping a breaker to protect the VT. If a VT has a problem, it is destroyed. There is no fuse or relay you can put on the circuit to clear the fault before the VT is destroyed. The fuse is there to prevent the fault from escalating and to isolate the fault from the system.
A fuse and VT disconnecting drawer assembly is about $2,500 including VT and fuses. An 11 kV breaker is about $25,000 not including the required CT's and protection relays. An 11 kV breaker will not be able to trip in time to save a VT since it is inherently slower than a fuse.
If you want to spend 10 times more money and 4 times the space to protect a potential transformer, you could. But engineers designing switchgear for the last 80 years haven't seen a need to do it.