Instrument transformers are used primarily where you are sampling a quantity such as voltage and you do not expect to have any load to speak of on the transformer. Some examples are:
Voltage measurement of a substation bus where the nominal voltage is 7200 volts to ground and the current levels may reach 2000 amps. The power transformer supplying this type of load will have a voltage drop internal to it as will bus sections, etc. To ensure you are measuring the actual voltage at any particular point you would attach a potential transformer. This instrument type transformer would only be installed to measure voltage, no intentional load would be placed on the transformer so that the losses in the transformer are at a minimum. With a ratio of say 60 to 1 if you are reading 120 volts then you know the point at which is attached to the bus is at 7200 volts. If however you added load to the instrument transformer a voltage drop would occur in the transformer and you might read 115 volts on it's secondary side and now you would say the buss voltage is at 6900 volts, which would be incorrect.
Instrument transformer are used for:
1) Potential measurement
2) Relay & Control
3) Phasing checks between sources
4) Synchronizing of sources (generators)
Power transformers are what they are termed. They are built to transformer power from one voltage to another at high load levels. At a power plant these may be 300 MV more or less. At regional power substations they may be 50 MV to 300MV. (MVA - million volt-amps). At distribution substations they are usually smaller between 40 MV down to 2.5 MV. They are generally used for:
1) Stepping voltage up from say 13 kV to 500 kV at a power plant.
2) Placed in parallel operation to support more load
3) Wound such that they are attached as Delta-Delta, Wye-Wye, Delta-Wye, Wye-Delta, Zig-Zag, etc. to match phase relationships between sources
4) Large power transformers often has Load Tap Changes, meaning they may changing "Taps" by changing the turns ratio and thus adjust the output voltage to a preset level while carrying full load.
5) They may be used as grounding banks
6) They may be used as balancing banks
In general they are subject to high load variation, voltage regulation, short circuits, lightning strikes, etc. and must be able to withstand all that and continue to work properly.
Hope this helps,