Ok so there is advantages and disadvantages to HVDC. Let's start with the cons, in order to transmit DC any significant distance it needs to be a high voltage such as 500 kv. This requires two things , first a voltage rectifier to turn AC into DC and then an inverter to switch it back to AC. This is important because it is also one of the advantages, but this means that you require more speciality equipment which is drives up cost. As DC cannot be stepped up or down it has to first pass through a transformer as AC before becoming DC and again after it has been turned back into AC. This is important because while AC can change voltage the frequency remains the same 60hz here in the US but Canada uses 50hz. So that direct AC connection is impossible. However because DC to AC inverters can change frequency this is possible through a HVDC connection and because it's DC it only requires two conductors which reduces the cost. The other advantage is that power loss is actually less through the use of HVDC which makes extremely more efficient and cost effective but short distances too costly to implement.