Why is it easier to raise AC current to high voltage than DC? In my country (and maybe all around the world I don't know) once electricity has been generated, it is then raised to 200k Volts for transportation.
I know this is to reduce the loss. Given $P=U.I$ and $P=I^2.R$, raising U will lower I and so limit the loss by joule effect.
From what I've read, one of the reason electricity is transported in AC is because this is easier/cheaper to raise AC to 200k Volts than if it was in DC.
Why?
 A: As with most engineering decisions, it all boils down to "what's the best trade off between conflicting constraints and requirements".

*

*AC allows very simple conversion between different voltages using passive transformers

*AC conversion scales nicely across a large range of voltages and powers

*AC is the historical standard and all appliances expect roughly 110V or 220V AC, even if most of them immediately convert this to something else.

*DC is more energy efficient, especially over long distances

As a result of this the majority of electrical power is transmitted using AC but there are plenty existing system that use DC. Some of the larger ones (in both terms of distance and power) are in China and Brazil. For a description of the technology, see https://en.wikipedia.org/wiki/High-voltage_direct_current.
It's a simple matter of choosing the right tool for the specific job.
A: Changing the Voltage of AC can be done with a simple iron core transformer. That's a simple device without moving parts that only consists of a magnetic core, copper wire and some isolation (optionally a cooling fluid). Almost nothing that can break. Good transformers can have amazing efficiency of way more than 95%.
There are other benefits to using AC over DC as well (and also downsides). With AC you have way less problems with arcing on switches because. If arcing starts with AC it will often stop the next zero crossing of the AC. With DC, the arc won't stop by itself. Also, with AC you have less problems with material starting to wander because of electrolytic effects. And running motors with (especially 3 phase) AC is close to trivial without the need for brushes. With DC you need brushes or some smart electronics (BLDC-Motors are basically AC motors with some smart electronics attached).
Also, a power grid with AC is self stabilizing (to some extent) via the frequency of the AC.
Downside of AC is losses due to capacitance (blind current also causes resistive losses). Phase shift is always an issue as soon as you work with AC.
Converting DC to another voltage takes more effort. One way is to drive a DC motor that is mechanically coupled with a DC generator. Such systems are big, have moving parts and have lower efficiency.
Today, we have the electronics to do that better. We basically chop the DC up into AC, put that trough a transformer and rectify the output of that again... voila, a DC to DC converter (this is all very simplified).
A: You are absolutely right, the higher voltage, the less energy loss due to Joule heating.
The main reason why electricity is generated and transported in AC is that the generated electricity from electromagnetic motor due to mechanisms of electric induction is in fact in AC. So it would require a rectifier for converting AC to DC and thanks to Joule heating, energy will be lost.
Moreover, many electrical appliances run on AC (e.g. hair dyer, electric vehicles...) due to the fact they convert AC into kinetic energy by electric motors. And if electricity supplied to our homes were DC, then we would need a MOSFET or IGBT to convert them back to AC which would entail additional loss of energy.
It is not necessarily easier to raise AC current to hight voltage, but because DC is not an option due to the need of several conversions.
To put it simple:

*

*Electricity generated is in AC

*Most electric appliances need to run on AC

Enjoy!
A: Because voltage is induced by the rate of change in the magnetic field.
If we tried to build a DC transformer, then to maintain the rate of change in the magnetic field the magnetic field would have to increase without bound, this is clearly impossible for two reasons.

*

*It would imply the input current increasing forever, this is clearly impossible.

*Ferromagnetic materials undergo a phenomenom known as saturation, when if the magnetic field gets to strong the relative permeability drops like a stone.

The result is we simply cannot build a DC voltage converter using static electromagnetic components alone. We need to resort to either moving parts or electronics.
A: You could run DC through a transformer if the magnetic core material had infinite saturation flux density.  But it doesn't, so you have to reverse the direction (polarity) now and then.  Hence, real transformers only work with AC.
