why the minimum frequency at power system chose 49.5 hz?

Answer 1

I am not sure but I think the standard frequency where you are at is 50 Hz. Am I correct? If so the power houses want to keep the frequency as close to the rated frequency as possible because all the electrical appliances are designed to work on that frequency.
Why 50Hz was chosen? Because most of Europe decided on using the lower frequency of 50 Hz as their standard when electricity was coming on line. i.e. it was first being insulted. And since at the time India was part of the British Empire 50Hz was used there.
As for the United States we use 60Hz as our standard with a lower voltage.

Answer 2

ok, you're referring to the countries that use 50 Hx rated power supply.
As a rule, a tolerance limit for the frequency is shered to.. + or - 2 %. so, 49 to 51 Hz. Let's explain why:
the load (ie, everything that's switched ON) on a power system consistes of two parts: one that's frequency-independent (ohmic loads like light-bulbs), the other that's dependent (all induction motors, fluorescent lights)
These freq-dependent loads have been manufactured such that they give the best possible performance at the rated frequency of the power system. ie, 50 Hz in our case. If the frequency goes too above or below 50 Hz, one or the other type of power loss (iron losses, hysteresis, vibrations, inductive reactance) would get out of control, and potentially damage the equipment in the long run.
That's why, to keep an agreed-upon optimum output, the guys in charge of the power system try their best to maintain frequency at 50 Hz.

In some countries like the US, the standard frequency is 60 Hz. this is more of an agreement-issue.. all equipment-designers follow a certain agreed-upon standard, and stick to it. Just compare it with the difference in driving lanes in many countries.. left in some, right in others. at the end of the day it's just a formality!