Why we do not use transformer in DC powe?

Transformers operate on the principle that current in a wire generates a magnetic field. And more importantly that a change in magnetic flux creates a current in a wire within that field. The input side of a transformer consists of multiple wraps around an iron core. The outputs is a similar set of wraps around the same core (the ratio of number of input wraps to output directly determines how much the voltage is transformed.) Since electricity in the output is dependent on CHANGE in magnetic flux, a DC input will not create any output except at the time it is switched on or off. So AC current is used which will continuously provide changing magnetic flux and therefore power at the outputs. One thing more the core is saturated with DC current, the transformer acts like a capacitor resisting electrical current changes

if dc supply is given to transformer there will be no change in the flux with time.so there is no emf induced in the primary and secondary windings.and hence tranformer action will not take place.

The answer of Eng amr is very good with another reason which is z=R+JX    X=2*3.*F*L.    Frequency in DC is0 so Z=R.  R is very small so by ohm law I=V/R.  So the current is very high which make the transformer damge

for DC transformer acts as short circuit and the core does not demagnetize because of DC polarity. the hysteresis loop phenomena does not satisfied. for DC Buck and Boost converter is used.

The main reason for your question is that,

Transformers work on a principle of EMI (electromagnetic induction) as per Faraday's laws (not exact laws written but just an interpretation is mentioned) "when a AC alternating current will pass through a conductor, changing electromagnetic waves(flux) will form around it" and also that when a conductor is  placed inside an area of changing electromagnetic waves(flux) an AC alternating current will be  produced in it."

The working principles of a transformer utilizes both of  these laws, first law used by primary and second in secondary.

Therefore transformers don't  work on DC power.

However, we can utilize a transformer in a case that we are passing the DC power through an inverter circuit and then perform step up or step down and then again rectify the signal or use a voltage multiplier circuit to stay on the DC domain. I hope I have given you some insight. 

We can not use transformer in DC power. Because of  paradise law of mutual induction either conductor or flex have change for inducing EMF in another circuit. In transformer conductors are not movable and DC will produce steady flux only. So for the reason we are not using transformer in DC 

E=Nd(phi)/dt e.m.f is induced due to change of magnetic field AC power is having changing magnetic field that is why transformer can operate on AC and DC have zero frequency and no change in magnetic field that is why can not operate Transformer.

Transformers works on Magnetic effect of Current. Since DC doesn't have any magnetic field, they will not get step-up or down. So you will get0 voltage at the output. 

The basic principle of operation of a transformer is based on mutual induction. With a DC voltage supplied to the primary winding, it will remain steady. i.e it will not produce any flux which is required to produce voltage in the secondary winding of the transformer. The result, there will be voltage present in the primary winding, however, it will be blocked there. there will be no voltage in the secondary coil and hence no current as well. Therefore, DC transformers do not exist. However DC to DC conversion is possible through Choppers and DC to AC conversion is also possible with the help of inverters.

Transformer depend on magnatic flux that happend from AC current Other way DC current haven't magnatic flux

because most of the loads are inductive loads needs AC current to obtain the Q from the output power.