What is back charging of power transformer?

Do you mean "back-feeding"?

Normally a power transformer takes a primary voltage and steps it up or down (usually down) to a secondary voltage. However, Transformers are symmetrical; you can just as easily feed the secondary and generate the primary voltage.

This is particularly a problem with utility power distribution systems in a storm, such as a hurricane situation...

In a protracted power outage, some people are tempted to run backup generators to supply their inhouse loads while the utility company is effecting repairs. An intelligent user, one that reads the directions, will isolate the generator from the service distribution system so that it is impossible to back feed the service source.

Some people might ignore the directions and attempt to back feed their entire home. If they don't isolate the incoming mains breaker, this can back feed the distribution transformer and energize the primary line on the street, raising it to high voltage, such as 7620 volts.

This actually happened during Hurricane Gloria in 1985, and a utility worker was killed. Yes, the worker was At Fault for assuming that the line was dead, but the homeowner was also at fault for ignoring the directions that came with the generator.

Unless you have the proper equipment to synchronize, backfeed, and isolate your generator, DO NOT EVER DO THIS.