I am not clear on why DC cannot travel far distances. ?

Question

Is it because of the constant polarity and the build up of resistance?

Answer 1

Alternating polarity allows you to use coupled coils (a transformer) to step up the voltage and thus reduce the current.  The "line" losses are much less in terms of heat generation at low current.

This is why AC (Tesla) finally "beat out" DC (Edison), but eventually "we" will have a national DC network of electricity.  (See any of a number of articles on this in Science News, Scientific American, etc..), but that's the historical reason.

-Fred

Answer 2

DC *can* travel long distances. In fact, the very long distance transmission lines are all very high voltage DC, since DC has less losses than AC.

You won't see DC in medium or short length transmission lines, because it is difficult and expensive and wasteful to change voltages with DC, whereas with AC you can use simple and cheap transformers.

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Answer 3

Voltage of DC cannot travel very far because it begins to lose energy.

Hope you got ur answer..

good luck..

Answer 4

Advantages of HVDC over AC transmission

The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1000 km. High-voltage direct current transmission allows efficient use of energy sources remote from load centers.

In a number of applications HVDC is more effective than AC transmission. Examples include:

    * Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and Germany[9])

    * Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas

    * Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install

    * Power transmission between unsynchronised AC distribution systems

    * Connecting a remote generating plant to the distribution grid, for example Nelson River Bipole

    * Stabilizing a predominantly AC power-grid, without increasing maximum prospective short circuit current

    * Reducing line cost. HVDC needs fewer conductors as there is no need to support multiple phases. Also, thinner conductors can be used since HVDC does not suffer from the skin effect

    * Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies

    * Synchronize AC produced by renewable energy sources

Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses.

HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power consumption, which can lower costs.

Because HVDC allows power transmission between unsynchronised AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly affect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone.

[edit] Disadvantages

The required static inverters are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions in line construction cost and lower line loss.

In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line.

Answer 5

All conductors have some resistance to current flow, eventually DC current will decrease to zero if the conductor is long enough, DC has no rising and falling fields, it can't be amplified. AC, on the other hand can use transformers to boost voltage, that is why AC is used in long conductors.