My comments in color below

A friend of mine who is an electrical engineer told me that creating extremely large stable power grids maybe an unsolvable problem. He then told me that there was an attempt to create a single power grid for the entire United States but the effort was abandoned because they could not make it stable. He described the problem being rooted in undamped oscillations forming within grid. DC power transmission systems have entirely different dynamical characteristics than AC systems. The reason is the need for phase sychronization in an AC grid. Every generator in an AC system must be in precise sychronization (to within a very few degrees) with every other generator. Generators tend to speed up when load is reduced, and slow down when load is increased. Normally, the steam inlet control automatically compensates for this and adjusts steam flow accordingly. However, if the load transient is too large, the rotor can not be speeded up or slowed down fast enough. As a result, it goes significantly out of phase. This then causes the main breakers to open, reducing the load on the generator to zero and dumping the generator's entire load onto the grid at large. This may, in turn, cause a load transient on some other part of the system, resulting in breakers popping all over the place and the entire grid, or at least major portions of it, goes dead. DC does not have any phase, so load changes are felt entirely as voltage changes. Slowing or speeding the generator affects the voltage only.

It is my understanding that AC power transmission is unavoidable due to the need for high voltage transmission. Supposably the transmitted power losses over distance go down as voltage increases. True. As power reaches the end consumer, the voltage needs to be stepped down to permit safe usage. True. This is easy to do if the power is AC (use ordinary transformers) but expensive if DC. It is not that simple. While it is true that DC to AC power conversion has a cost, there is a counteracting factor: DC transmission has much lower losses per unit distance. AC lines suffer from reactive losses (capacitative and inductive) which do not occur in DC transmission. In addition, AC is much more subject to corona losses. Furthermore, since the maximum voltage that AC uses is 1.414 times the average voltage, the same line can handle 1.414 times as much power as DC than it could as AC. This is a large increase in capacity for no cost (other than conversion)
If they could crack the problem of room temperature superconducting, that would change everything in terms of power transmission. Existing technology is sufficient, although superconductors would be dandy. There are several long-distance DC trunk lines that have been operating successfully for decades.