Power factor in Transamision What is normally the power factor in the transmssion.? Is there some standard or guidelines for that.?
Also, the low factor is not desirebale as it leads to more power loss. That makes sense. They say power factor can be improved adding a capacitor in parallel. Why in Parallel.? 
 A: Power factor is a control variable used by electric utility transmission system control software (Energy Management System) to provide voltage control in the transmission system.  Voltage sags on a node can be lifted up using added capacitor banks that are switched in or out of the circuit in real-time by EMS software.
Capacitors are shunts to ground so that is sometimes known as a parallel circuit however in my 40+ years working in this industry, I have never known a power systems engineer to call it a parallel connection.
Capacitance in a transmission grid also occurs in series and this is part of the transmission line lumped circuit model.  Capacitor shunts to ground though are reactive voltage power control devices and pose a much larger capacitance in the circuit when switched in at a node.
In the mathematics, the actual control variable is the phase angle between buses separated by transmission line.  The phase angle of course determines the instantaneous voltage difference between two nodes that are nominally at the same KV.
There are other control devices in the transmission grid too.  For example, phase angle transformers that are adjusted by software also affect the phase angle control variables.  And, Real Power is managed of course by traditional transformers that are used to bridge nominal voltage levels.
As to what is the normal power factor in the transmission grid, I don't know the answer to that and I have never thought it to be that important.  I am sure that there are those who monitor the actual values and it is even possible that some of the large regional electric utility (ISO or RTO) has web displays that report such things.  Just a guess though but it is possible.  There is usually a lot of good information found on web sites of such companies -- at least in the US but other countries likely as well.
A: 
What is normally the power factor in the transmission.? Is there some
  standard or guidelines for that?

Typical guidelines are around $90$-$95$%. If is easily achievable for residential power consumers, without any additional actions, since most devices used there have high power factors. Industrial power consumers, however, may need to implement power factor corrections, since some of high power AC loads may have poor power factors.

They say power factor can be improved adding a capacitor in parallel.
  Why in Parallel?

The power factor is low for the loads that have significant reactance, either of inductive or capacitive nature. For such loads, the phase difference between the voltage and the current may become significant, which means that some fraction of energy going into the load during each cycle, is going to be stored, in an inductor or a capacitor, and then (during another part of the cycle) send (reflected) back to the power source. 
This means that the current flowing in the wires all the way to the power source is higher than it needs to be, which is a waste. 
To correct that, we need to add a complimentary reactance local to the load, in which case, the energy stored in an excessive reactive component of a load will be transferred to the added complimentary reactive component instead of going back all the way to the power source.
So, if a load has an excessive inductance to start with (which is more common than excessive capacitance), adding appropriate amount of capacitance, will make the load, as a whole, less reactive (ideally, purely resistive), will reduce the phase difference between the voltage and the current and, therefore, will improve the power factor. The energy will be still bouncing back and forth between the reactive components, but it will be done locally and, therefore, the losses will be limited.
In theory, the correction could be achieved with both parallel and series capacitors, but some practical considerations make series capacitors less useful. For instance, a series capacitor would have to be rated for the same current as the load, which could be much higher than the current passing through a parallel capacitor. 
Another possible consideration is that, given the same inductive load, a series correction capacitor (due to resonant action), could increase the current flowing into the load, so some load adjustment could be needed to reduce the current to the nominal level. This does not happen with a parallel correction capacitor, since it does not affect the voltage applied to the original load (line voltage).     
