Yes, CTs can get quite large depending on accuracy and burden requirements. What sort of environment are you working in? Low (<1000V AC), Medium (<30 kV AC) or high? How large are the primary currents? Are you measuring primary current in the switchgear or secondary current in the relay (1A / 5A nominal)?
For primary current over 100A, I would stick with CTs (or Rogowskis) because they are so robust. For secondary current, I guess Hall Effect sensors would offer better bandwidth and thus phase accuracy which is relevant for metering and precision protection.
There are a couple of gotchas with Hall Effect sensors, though:
- They can be sensitive to external fields (depends on make)
- They need temperature compensation (usually built in)
- For closed loop types: if input current is applied while the sensor is unpowered, the 'iron' core can be magnetised and pick up a DC offset.
- If high currents are applied (e.g. phase to phase fault), the 'iron' core can be magnetised and pick up a DC offset
For maximum accuracy, you could also look at placing a precision shunt and ADC (and digital isolators) on the reference potential. This will entail a certain about of screwing around with isolated supplies etc.
If you need the best possible accuracy, closed circuit fluxgate sensors seem to be the thing.
This one is specified in ppm, but hold on to your wallet!