What is difference between a current transformer and a power transformer?

A transformer is an electromagnetic device which often consists of two separate windings around the same iron core. The two windings are called the primary and the secondary and normally they are electrically insulated from each other. Current in the primary is normally driven by the circuit to which it is connected. Current in the secondary normally drives the circuit to which it is connected. Many variations of this theme exist and most of them aren't relevant here.
A power transformer is intended to transform large amounts of power at one voltage to a very similar amount of power at a different voltage. A little power will always be lost in the transformer core and windings, and good transformer design keeps that loss very small. You will most often come across 'step-down' transformers, where the voltage on the primary is high and the voltage on the secondary is low. For example if you have a model train set with electrically driven trains, for safety reasons the voltage on the rails has to be quite low - a few volts. Unfortunately for efficiency reasons the voltage of the power delivered to your home has to be quite high - one or two hundred volts. A power transformer is used to transform the voltage from the dangerous levels delivered to your home to a few volts, which is much safer. Nobody cares very much how accurately the transformer does this, so a cheap transformer with a turns ratio of the order of a hundred (on the primary) to one (on the secondary) will do. Note that in a power transformer, the voltage ratio from primary to secondary is approximately the same as the turns ratio of primary to secondary. The primary winding will usually be placed on the core first, then the secondary winding will be wound on top of that.
A current transformer is usually a measurement device. The emphasis here is on measurement, not on power transformation, and on accuracy, not on efficiency. If you work in a power station and you want to measure a current of tens of thousands of amperes you might not have a multimeter which has a range that high. So you can use a current transformer to make the measurement. The current transformer will (usually) mirror the flow of current in its primary with a current in its secondary which is exactly proportional to the flow in the primary but very much smaller. For example a current transformer with a turns ration of one (on the primary) to ten thousand (on the secondary) will produce a current in the secondary of one ten-thousandth of the current in the primary. Then your multimeter, connected directly across the secondary of the transformer and set to measure current, will register only 100 milliamps when there is a current in the primary of a thousand amps. Do not connect a multimeter on a current range to the secondary of a power transformer!
When a power transformer is not being used, the primary winding can be left connected to the power supply and the secondary winding can simply be disconnected. The transformer itself will consume very little power. With a current transformer, if it is left in circuit when it is not being used then the secondary winding must be short-circuited; otherwise dangerous voltages will be developed in the winding which may destroy insulation. Do not short-circuit the secondary of a power transformer!
http://en.wikipedia.org/wiki/Current_transformer -- GWH --

CURRENT TRANSFORMER: Think firstly of ac current indication on a distribution switchboard incomer - maybe on a power condition monitoring relay or an incomer protection relay. For a low voltage board ( 400V ) it may be several hundred amps or more. This current cannot flow directly through the relays, due to a number of reasons of practicality. The size of terminations & separation are not suitable for starters - consider the csa of bus bars carrying a few hundred amps.... A current transformer is an interface device which will provide a secondary winding current, proportional to that a) in its primary winding if wired or b) the conductor passing through its centre ( torroidal CT ), which can be used for metering, protection & indication purposes. CTs have standard ratios available, ie 100:5 & 2000:1 are common, & are also accuracy classed. Certain protection applications will require matched pairs of CTs to be used at higher accuracy levels. CTs are tested to demonstrate their kneepoint, which is the point at which they saturate, electromagnetically. A typical application of CTs would be on a large motor feeder circuit. At the MCC, the outgoing cable ends wound each pass through perhaps a 100 : 5 CT, & then all three cables would pass through a common CT for earth fault detection which maybe 2000:1 ratio. POWER TRANSFORMER: Changes voltage level up for distribution or down for consumption. Range from a few VA rated, to several hundred MVA. Can be tapped to control voltage. May be star or delta connected on each winding, and give phase shift depending on this connection & resultant vector group. eg Dyn11 May be air or oil, forced or natural cooled - ONAN, ONAF ( with fans ), OFAF ( fans and pumps ) Are protected for over temp, over pressure, over current, gas in oil etc. CTB.