Physics · Ch 4 — Electromagnetic Induction and Alternating Current
Advantages of AC in Long-Distance Power Transmission
Advantages of AC in Long-Distance Power Transmission
Electric power is generated on a large scale at power stations -- classified by fuel type as thermal, hydroelectric or nuclear -- which are typically located far from the towns and cities that actually consume the power, so the generated electricity must be transmitted over long distances through transmission lines, a process called power transmission. The fundamental difficulty is that a sizeable fraction of this transmitted power is unavoidably lost to Joule heating () in the transmission lines themselves, which can run for hundreds of kilometres; this loss can in principle be reduced either by lowering the current I or by lowering the line's resistance R (using thicker copper or aluminium wires), but thicker wires are far too costly to be economically viable at the scale of a national grid.
Because generated power is alternating (not direct) current, there is a much cheaper way out: since AC voltage can be stepped up and down efficiently using transformers, the current in the transmission line itself can be reduced -- at fixed power , raising the voltage V by some factor lowers the current I by the SAME factor, and since power LOSS scales as , the loss falls by the SQUARE of that factor, a dramatically more effective saving than the linear reduction achieved by cutting resistance alone. In practice, a step-up transformer raises the voltage (and lowers the current) at the generating (transmitting) end before the power travels down the transmission line; a matching step-down transformer at the receiving end near the consuming city then lowers the voltage (raising the current) back down to safe, usable levels before the power is finally supplied to consumers. This 'step up for transmission, step down for consumption' strategy -- possible ONLY because transformers work exclusively on alternating current -- is the central, practical advantag …
What this figure shows. A single-line diagram traces power all the way from a 'Power Station', through a 'Step-up transformer' that raises the generated voltage to a high value for the long 'Transmission line' run, to a 'City sub-station' where a matching 'Step-down transformer' lowers the voltage back down to safe, usable levels before it is finally distributed to consumers. The figure is the visual summary of the whole advantages-of-AC argument: generating power at a moderate voltage, stepping it UP for efficient (low-current, low-loss) transmission over long distances, and then stepping it back DOWN close to where it is actually consumed, is a strategy that is only practical because transformers work exclusive …
Worked out. An illustrative comparison transmits the same 2 MW of electric power over lines of total resistance 40 , once at a lower voltage of 10 kV and once at a higher voltage of 100 kV, to compare the resulting power losses. At 10 kV, the current is A, giving a power loss of W -- fully 80% of the 2 MW transmitted, an enormous and clearly unacceptable loss. At 100 kV, the current drops to A, giving a power loss of W, just 0.8% of the power transmitted. The hundred-fold voltage increase cuts the current by a factor of 10 and, because loss scales as the SQUARE of current, cuts the power loss by a factor of 100 -- from a crippling 80% down to a negligible 0.8% -- which is exactly why real transmission grids …
Worked out. An enrichment panel, explicitly marked 'not for examination', sketches the full multi-stage journey of electric power from generation to a household socket, organised into a transmission stage (primary and secondary transmission) followed by a distribution stage (primary and secondary distribution). A typical central generating station produces power at 11 kV, which is stepped UP to 132 kV for primary (high-voltage) transmission to a receiving station at a city's outskirts, then stepped DOWN to 33 kV for secondary transmission into sub-stations within the city. From there, primary distribution steps the voltage further down to 3.3 kV at local distribution sub-stations, and a final secondary distribution stage brings it down to the familiar 440 V (for factories) or 230 V (for homes) actually delivered through the distribution network to individual consumers -- illustrating that the single 'step-up then …