PMUs allow you to measure the physical quantity like voltage and its phase. You can lookup the concept of a phasor. These devices are equipped with a GPS to transmit synchronized time stamps that are used for a wide area monitoring of your system state (of course on transmission voltage level very usefull because of the long distances). This way you are able to anticipate changes towards system instability aswell as to support the rebuilding of the system after blackouts.
Phasor Measurement Units sense the instantaneous voltage and current phasor in the transmission lines and transmit those information to a central operator. The most important feature of PMUs is their ability to provide those measurements with accurate time stamps, so that data collected from large number of PMUs could give an accurate snapshot of the power system. Which is used to operate the power system in an optimum fashion.
Tie lines, are the line connected between the large load areas, states, zones or nabouering countries are just the weak link, connected mainly stability point view of system(to increase inertia M of system).Voltage and current phaser measured in tie line, hence is very important for knowing, the disturbance taken place in the load area, and immediately the precoasnery steps can be taken!!!!!!!!!!!
As Phillip and Manaz above have noted PM units determine the instantaneous values of voltage and current at either side of a tie line. However, as very long tie lines are considered as distributed parameter systems the voltage and current are travelling em waves and not continuous as in a lumped parameter system.
Thus if a fault occurs at one end (or somewhere in between) it will take time for the em waves to travel to other end or ends and as such this must be compensated for in the operation of the protection equipment and hence the use of PM units. In a sense it is no different to time zones but on a much smaller sub-second scale.
Long tie lines use distance protection where it is common practice to trip the side closest to the fault. Thus the transmission delay is key as it may result in incorrect tripping, which has happened and why PM was developed.
In the simple 2 machine model, power flow is proportional to sin(delta) where delta is the angle between the sending and receiving end voltage phasors. The PMU measures this angle. If the angle is too large the machine groups can lose synchronism.
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