DC can be transmitted longer distances than AC generally. The problem is that it requires more effort to change voltage levels for DC than simply using AC transformers. DC transmission tends to convert AC to DC on one side and then invert back to AC on the other. Then, all the voltage changes are handled in AC

https://www.hydro.mb.ca/corporate/operations/transmission/#bipole

The 'toba Bipole would like a word with you.

High-voltage DC seems ideal to me to cover the vast gap from Winnipeg to southern Ontario, Alberta to BC (this would still be challenging getting through the Rockies), and Quebec to the Maritimes. The advantages are a much lower power loss and cost over distance. The disadvantages of expensive transformer equipment is mitigated, because it's not intended for local distribution. The maintenance intensive and specialized parts at converter stations will be concentrated near select population centres, and it's through adjustment at these points that the intertie can be negotiated. The need for bespoke parts per province can be reduced since this would be a national energy project on a single, uniform standard. Existing HVDC lines can stay as they are.

https://en.wikipedia.org/wiki/High-voltage_direct_current

The highest scoring answer here does a lot to concisely explain the how's and why's

https://engineering.stackexchange.com/questions/19758/transmitting-power-over-long-distances-what-is-better-ac-or-dc