With advent in semi conductor technology conversion from AC to DC and again from DC to AC is very reliable, economical, efficient hence modern high power long distance transmission HVDC is in used, and found more economical and efficient...
Mainly it depends on the distance of transmission and the amount of power to be transmitted. Also depends of the application: Offshore transmission, interconnection between two asynchronous AC systems(50/60) Hz.. etc.
Offshore wind power plants maybe the only feasible transmission is HVDC due to the high charging currents produced in case of AC underwater transmission.
However, i think HVDC technology is moving faster that will replace most of HVAC systems.
It is mainly related to the capacitive effects in the transmission lines
If the line is long and/or the conductors are separate by a short distance (e.g. submarine cables), the equivalent line capacitance per phase is large. For instance if you assume a PI representation CLC of the line, the capacitance in each extreme of the PI representation is large and will draw a large steady state sinusoidal current for 50/60Hz operation.
However in a dc transmission system, the steady state current in the equivalent capacitances is not a problem.
This is a very simplified answer. There are other issues, for instance those already mentioned by Abouzeid.
There can not be a general answer to the question. Advantages vary depending being on applications. As on date both have to coexist even though with developments in semiconductor technologies HVDC will be slowly replacing HVAC but not eliminate.
HVDC is better than HVAC, in simple terms, HVDC does not have frequency component hence, synchronization of power will be very simple unlike in AC where it has to satisfy the conditions like V, F and phase sequence should be same.
Like this there are many advantages are there with respect to HVDC.
I'm a bit surprised that all your respondents are thinking of HVDC vs HVAC solely in terms bulk power transmission at EHV. I don't see any such bias in your original question. May I point out there's a fundamental difference in the topologies of HVDC and HVAC transmission/distribution networks? DC forms current loop(s); AC a set of voltage nodes. Consequently it's much easier to (a) to transform voltage levels in an AC system, and (b) connect/disconnect loads and other plant. Just think of the complexity of kit needed to plug a kettle into a DC loop, and the safety issues.
I would think the great benefit of no inductance reactance of HVDC in case of long distance above 80Km without the possibility of installing substation In the case of offshore wind farms, or to send power to very long distances from generation to consumers. Otherwise, I would stick of HVAC due to cost.
Nice to be participating in one of your discussions after considerable time!
It is true that the genesis of HVDC as an accepted technology is rooted in long distance transmission problems. There are some well known (I would say, almost classic !) aspects that are known to have contributed to the evolution - some of these are commonly cited in HVDC texts:
Long distance inter-area bulk power transactions, that in AC would "collect" excessive reactive power burden due to line capacitance, and be associated with consequent poor transmission efficiency is perhaps the major motivation to switch to DC ! Cases in point being the several high capacity links in the North America.
Almost as a follow-up of #1, we have the requirement of transmission across submarine cable routes, between several utilities in Europe. (Cable routes are almost always more capacitive than OHT's !!)
Another follow up of #1, which additionally brought forth possible stability issues with regard to AC over long distances ! A good example is the hydro generation in the Brazilian Andes that by plan and design, was expected to provide for industrial loads close to the Brazilian coast - leading to the classic Itaipu HVDC station (a project by ABB, if I am not wrong), and a volley of concepts related to unit connected converters.
But with all this, one must not overlook a genesis of HVDC that facilitates power transactions between networks functioning at different base frequencies. The so called back-to-back (or B2B) links need not involve long distance at all ! They simply uses the feature of AC-to-DC-to-AC conversion, which can operate with different frequencies at the two ends.
If I am not wrong, The B2B concept has origins in Japan, though it has thereafter been implemented successfully by many nations and utilities.
HVDC systems offer several operational and special technical advantages over HVAC transmission systems. HVDC power has no frequency, hence minimal problems of harmonics, oscillations or transients; they actually damp out transients. All angular stability problems disappear and even connection of systems at different frequency is feasible. The problems of cable charging current are eliminated. Thus cables can readily be used for underwater crossings. For weak AC grids, voltage source converter-based HVDC systems offer better controllability and grid flexibility for integrating intermittent renewable energy sources such as: wind and solar-power. Using power electronic switching devices or FACTS controllers, precise and fast power control in either direction is easier to achieve in HVDC than in HVAC systems as dc power is minimally affected by reactance. The ease of power flow control in HVDC lines makes it easy to achieve the maximum power and thermal capacity of transmission lines. It is also simple to control active power transfer at a predetermined level or even to modulate this to improve system damping. HVAC is suitable for short transmission lines.
I suggest folks are taking a very narrow focus to answering this question. A transmission link, whether AC or DC, is only part of the overall electricity supply system. The question is not which is "better", but which is more suited to the specific system requirements. AC or DC, each brings particular attributes to the system, and should be evaluated accordingly.
On a more fundamental basis, it is interesting to consider whether each leads to a radically network topology. If Edison had won the "war of the systems", would we now be operating in a current loop world, and what changes would we see? Would we be using domestic plugs that make/break a current loop, and appliances that have to be rated for the full loop current? Would that have stifled the expansion of domestic electric consumption? Would it have encouraged plasma generating sources? Something to mull over in the early hours when counting sheep doesn't work.