FAQ
TL;DR: In real grids, a “400 kV” line still drops voltage under load; “the voltage is NOT the same at both ends.” [Elektroda, Olin Lathrop, post #21659690]
Why it matters: This FAQ helps students and practitioners quickly diagnose why current flows on long power lines and how phase, losses, and AC/DC choices affect reliability and cost.
Quick Facts
- Transmission lines have non‑zero resistance, so load current causes measurable voltage drop. [Elektroda, Olin Lathrop, post #21659690]
- Losses scale with current squared; raising voltage cuts I²R losses for the same power. [Elektroda, Olin Lathrop, post #21659690]
- At 60 Hz, even ~100 miles is a small fraction of a wavelength; model as R with some L. [Elektroda, Olin Lathrop, post #21659692]
- HVDC is typical for >400 miles overland or >30 miles submarine, or to bridge unsynchronized grids. [Elektroda, Rebeccaa Andrew, post #21659693]
- Think of line charging like filling an angled ice tray: distributed capacitance charges as waves move. [Elektroda, Cody Miller, post #21659689]
If the voltage is the same at both ends, how does current flow?
It doesn’t stay the same. Lines have resistance, so load current creates a voltage drop along the conductors. Treat the line as a long resistor at this level and apply Ohm’s law. That’s why the “400 kV” nameplate is nominal, not a guarantee at every tower span under load. “The voltage is NOT the same at both ends.” [Elektroda, Olin Lathrop, post #21659690]
Why do utilities transmit at very high voltage?
To reduce current for a given power, which slashes I²R losses and heating. Lower current also allows smaller conductors or longer spans before thermal limits. Operators balance conductor size, insulation, and corona or reactive effects against these savings. High voltage is therefore a core economic lever in grid design. [Elektroda, Olin Lathrop, post #21659690]
Does phase angle between voltage and current matter for power transfer?
Yes, but mostly between the source and the load. At power frequency and typical distances, you can model the line as a series resistor with some inductance. The phase relationship then stems from the generator and the connected load’s power factor more than from distributed line effects. [Elektroda, Olin Lathrop, post #21659692]
Are transmission lines lumped or distributed at 60 Hz?
For many grid spans, they behave close to lumped elements. At 60 Hz, even about 100 miles is a small fraction of a wavelength. A practical model is series resistance with some inductance; distributed effects become prominent as distances and frequencies rise. [Elektroda, Olin Lathrop, post #21659692]
When is HVDC preferable to AC for transmission?
Use HVDC for very long overland routes (typ. >400 miles or ~600 km), long submarine cables (typ. >30 miles or ~50 km), or to connect asynchronous AC systems. This reduces reactive losses and avoids synchronization issues. [Elektroda, Rebeccaa Andrew, post #21659693]
What is three‑phase AC in power grids?
Three‑phase AC uses three sinusoidal voltages, each 120° apart, to deliver near‑constant power. It enables efficient large‑scale transmission and simpler, robust motors. Most transmission networks use three‑phase AC; single‑phase appears in specific applications like some rail systems. [Elektroda, Rebeccaa Andrew, post #21659693]
Does DC behave differently on long lines?
For DC, think of current flowing along the conductor without alternating fields. The distributed capacitances charge once to the DC level, then steady‑state current depends mainly on resistance and load. “DC current will flow straight down the transmission line.” [Elektroda, Cody Miller, post #21659689]
How do utilities manage reactive power on lines?
They add compensation equipment such as shunt or series capacitors and related devices to correct power factor and support voltage. Field gear placement can make voltage behavior less obvious, but it stabilizes service under varying loads and helps during events like brownouts. [Elektroda, Scott Vickrey, post #21659694]
Can transmission lines ever have zero voltage drop?
Only in the ideal or superconducting limit. Conventional lines have non‑zero resistance, so any nonzero current causes drop and heating. Superconducting lines are specialized and uncommon; typical overhead lines warm under load because of I²R losses. [Elektroda, Olin Lathrop, post #21659690]
Is it wrong to say “current flows”?
Engineers often accept it as standard wording. One poster argues only charge flows, but another notes accepted usage matters: “Words can have multiple meanings and accepted usages.” Say “current flows” for clear, conventional communication. [Elektroda, Olin Lathrop, post #21659698]
Does a brownout prove voltage isn’t equal at both ends?
Yes. During heavy loading, end‑of‑line voltage sags more because line resistance and reactive effects drop voltage under current. Brownouts highlight that real networks do not maintain identical voltages across distant nodes during stress conditions. [Elektroda, Scott Vickrey, post #21659694]
What’s a quick way to estimate line voltage drop? (3 steps)
- Get the line’s approximate resistance per length and total current.
- Multiply I by R to estimate drop; compute I²R loss for heating.
- Check if the resulting voltage meets equipment limits at the load.
Use Ohm’s law at this level of detail. [Elektroda, Olin Lathrop, post #21659690]
Does frequency change how we model a power line?
Yes. At 60 Hz and moderate distances, a lumped R plus some L model works. At higher frequencies or much longer lines, distributed capacitance and propagation effects dominate, requiring full transmission‑line treatment. [Elektroda, Olin Lathrop, post #21659692]
What is a transmission line’s distributed capacitance in plain terms?
Visualize an ice tray tilted and filled from one end. Each small cup fills in sequence as the wave travels. That’s like the line’s many tiny capacitors charging as the voltage moves along the conductor. [Elektroda, Cody Miller, post #21659689]
Any edge cases where typical guidance fails?
Yes. Extremely long AC corridors or submarine routes push reactive and charging effects high, making AC inefficient. HVDC avoids these limits and connects grids that can’t synchronize cleanly. [Elektroda, Rebeccaa Andrew, post #21659693]
