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The break even distance is the distance beyond which
DC transmission is economical
AC transmission is economical
Cost of the both systems are same
Both (b) and (c)
DC transmission is economical
The break-even distance is the transmission distance at which the total annual costs of DC and AC transmission systems are equal. Beyond this distance, the lower capital cost of DC terminals is offset by the reduced cost of lines and conductors, making DC more economical.
The break-even distance is the transmission distance at which the total annual costs of DC and AC transmission systems are equal. Beyond this distance, the lower capital cost of DC terminals is offset by the reduced cost of lines and conductors, making DC more economical.
CtotalтАЛ=CfixedтАЛ+CvariableтАЛ├ЧL тАФ Total cost as a function of line length L
CACтАЛ(L)=Csubs,ACтАЛ+Cline,ACтАЛ├ЧL тАФ Cost function for AC system
CDCтАЛ(L)=Cconv,DCтАЛ+Cline,DCтАЛ├ЧL тАФ Cost function for DC system
In DC transmission, costs consist of expensive converter station equipment and relatively cheaper line construction (only two conductors, no skin effect, no stability limits). In AC transmission, lines are cheaper to build initially due to simpler substation requirements (transformers vs. converters), but line costs increase rapidly with distance due to reactive power compensation, skin effect, and bundle conductor requirements. The intersection of the two cost curves represents the break-even distance.
The break-even distance typically ranges between 500 km to 800 km for overhead lines.
DC transmission is immune to the reactive power limitations that constrain AC transmission distance.
Higher converter station costs in DC are the primary barrier for short-distance applications.
Skin effect is absent in DC, allowing better utilization of the cross-sectional area of conductors.
Lower line losses over long distances.
Ability to interconnect asynchronous AC networks.
No reactive power compensation required for DC lines.
High cost of AC/DC converter terminals.
Complexity in DC circuit breaking/switching.
Generation of harmonics requiring expensive filters.
Long-distance bulk power transmission.
Undersea or underground cable links.
Interconnection of national grids.
| Feature | AC | DC |
|---|---|---|
Conductor Cost | Higher (3 phases) | Lower (2 poles) |
Terminal Cost | Low (Transformers) | High (Converters) |
For underground cables, the break-even distance is significantly lower (often 50-100 km) due to high capacitive charging currents in AC cables.
Option C is technically part of the definition but Option A more specifically identifies the regime where DC gains economic superiority.
A is correct тАФ The break-even distance is the point where the cost of DC transmission becomes lower than AC transmission due to superior long-distance economy.
Remember that while DC is economical for long distances, its high station costs mean it is never used for distribution, only for bulk transmission or specialized links.