Power Generation, Transmission, and Distribution Overview

Learning Objectives

By the end of this lesson you should be able to:

• Trace the path of electrical energy from a generator to a wall outlet, stage by stage.
• State the typical voltage band for each stage (generation, transmission, distribution, customer).
• Explain, using $P = I^2R$, why bulk power moves at high voltage and low current.
• Describe the role of step-up and step-down transformers in that chain.
• Name the main reasons HVDC is chosen for some links and not others.

Quick Review

You already have the two tools this whole lesson rests on. From Module 1: KVL and KCL let you analyze any network, and the power dissipated by a resistor is

$$ P_{\text{loss}} = I^2 R $$

From Module 2: a transformer is two coupled inductors, no electrical connection, and it trades voltage against current. Nothing else from earlier lessons is needed.

Theory

The three stages

A power system is a chain of conversions. Fuel (or wind, or water) turns a turbine, the turbine spins a synchronous generator, and from that point on the electricity is a product that has to be moved, not stored.

flowchart LR
  Gen["🏭<br/>Generation<br/>11-25 kV"] --> StepUp["⬆️<br/>Step-up transformer"]
  StepUp --> Trans["🗼<br/>Transmission<br/>115-765 kV"]
  Trans --> Sub["🔌<br/>Transmission substation"]
  Sub --> Dist["🏘️<br/>Distribution<br/>4-35 kV"]
  Dist --> DistTx["⬇️<br/>Distribution transformer"]
  DistTx --> Load["💡<br/>Customer<br/>230/400 V or 120/240 V"]

These numbers are conventions, not physics. The physics is entirely in the transformer ratio and in $I^2R$.

Why transmission is high voltage

Take a fixed power level $P$ delivered over a fixed line resistance $R$. At three-phase, line current is roughly $I = P/(\sqrt{3},V\cos\varphi)$. For a fixed $P$, current falls as voltage rises. Line loss is $3I^2R$, so loss falls with the square of the voltage ratio.

Raise the voltage 10 times, keep the power the same, and the loss drops 100 times. That is the entire economic argument for the transmission tower.

There is a second benefit. Conductor cross-section is set by current (thermal limit), so a higher transmission voltage delivers the same megawatts with thinner, cheaper, lighter conductors, which in turn needs shorter, cheaper towers.

At the same time, higher voltage forces taller towers, wider right-of-ways, larger insulators, and more expensive transformers and switchgear. Engineering here is a trade: pick the voltage that minimizes total lifetime cost for the distance and the power level.

[!NOTE] The grid has no meaningful storage. Generation is adjusted continuously to match demand, and the balance shows up as frequency: 50 Hz or 60 Hz. Too little generation for the load and frequency sags; too much and it rises. That is why frequency is the operator's headline number.

Step-up and step-down

The generator terminal voltage (11-25 kV) is too low for transmission miles away, so a step-up transformer at the plant raises it. At the far end, a transmission substation steps it down to subtransmission or distribution levels. Finally a distribution transformer near the customer steps it down to utilization voltage.

Transformers are why AC won the early grid wars: they are simple, efficient (98-99%+) and have no moving parts.

Distribution

Distribution is the part customers see. A distribution substation takes in, say, 138 kV and sends out several feeders at 11 kV or 33 kV. Feeders run along streets; distribution transformers on poles or in pads step down to the customer. Distribution is usually radial (one path from source to load) because it is cheap, at the price of longer outages after a fault. Urban networks sometimes add ties or loop schemes for reliability.

HVDC

High-voltage DC links convert AC to DC, transmit, and convert back. They win for:

• Very long overhead distances (no line reactance or charging current, so lower loss and no stability limit).
• Submarine cables beyond roughly 50 km, where AC cable charging current would eat the capacity.
• Connecting two grids of different frequency or phase, or unsynchronized regions.

They lose on cost and complexity for short links, because converter stations are expensive.

Worked Examples

Example 1: the loss ratio

A 100 MW load, unity power factor, is delivered over a line of $R = 0.1\ \Omega$ per phase.

Step 1. Current at 10 kV. $I = \dfrac{100\times10^6}{\sqrt{3}\cdot10\times10^3} \approx 5774\ \text{A}$

Step 2. Three-phase loss. $P_{\text{loss}} = 3I^2R = 3(5774)^2(0.1) \approx 10\ \text{MW}$ (10% of the load)

Step 3. Current at 400 kV. $I = \dfrac{100\times10^6}{\sqrt{3}\cdot400\times10^3} \approx 144\ \text{A}$

Step 4. Loss. $P_{\text{loss}} = 3(144)^2(0.1) \approx 6.2\ \text{kW}$

Step 5. Check the ratio. Voltage went up 40 times, so loss should drop $40^2 = 1600$ times: $10\ \text{MW}/1600 \approx 6.25\ \text{kW}$. It matches.

==Raise the voltage, shrink the current, and line loss falls with the square of that reduction.==

Example 2: transformer ratios along the chain

A plant generates at 20 kV, transmits at 400 kV, and feeds a district at 20 kV primary distribution.

Step 1. Step-up ratio at the plant: $400/20 = 20:1$. Current in the transmission line is 1/20 of the generator current (ignoring losses and magnetizing current).

Step 2. Transmission substation: 400 kV down to 20 kV, a 20:1 step-down.

Step 3. Distribution transformer: 20 kV down to 0.4 kV, a 50:1 step-down.

Step 4. A 400 kVA transformer at the last stage at 400 V line-to-line carries $I \approx 400\text{k}/ (\sqrt{3}\cdot400) \approx 577\ \text{A}$. Same kVA, 50 times the current: this is why service conductors are thick and transmission conductors are not.

Guided Practice

Work these on paper first, then check.

Exercise 1. A 60 MW load at unity power factor is sent over a line with $R = 0.5\ \Omega$/phase. Compute line current and loss at (a) 30 kV and (b) 300 kV.

Exercise 2. You double transmission voltage and keep power and line resistance fixed. By what factor does loss change? By what factor does the required conductor cross-section change for the same current density limit?

Exercise 3. A city is fed by two 138 kV distribution substations, each sending out four 11 kV feeders. Explain, in two sentences, which stage uses step-down transformers and how many voltage transformations a kilowatt undergoes from a 500 kV line to a lamp.

Exercise 4. For each case, say AC or HVDC and why: (a) a 700 km overhead link across a desert; (b) a 300 km submarine cable between two countries whose grids are not synchronized; (c) a 40 km link between two substations in the same city.

Key facts to lock in:

Generator terminal voltage is typically in the 11-25 kV range, so a step-up transformer is needed before transmission.
For fixed power, line current falls as voltage rises, and line loss falls with the square of the voltage ratio.
Transmission levels in practice run from about 115 kV up to 765 kV.
Distribution substations step down to primary feeder voltages of roughly 4-35 kV before the final distribution transformer.
HVDC is preferred for very long overhead links, long submarine cables, and connections between unsynchronized grids.
Grid frequency, 50 or 60 Hz, is the visible signal of the real-time balance between generation and demand.

Expert Corner

• Common mistake: assuming higher voltage is always better. Beyond a point, insulation, corona loss, tower height, and right-of-way cost more than the $I^2R$ savings. Voltage choice is an optimization, not a maximization.
• Common mistake: applying $P = VI$ to three-phase without the $\sqrt{3}$. For line-to-line quantities, $S = \sqrt{3},V_{LL}I$. A factor of 1.73 in current is a 3x error in loss.
• Rule of thumb: each stage steps voltage by roughly 10 to 20 times. If your answer implies a transformer ratio of 1000:1 in one unit, you have skipped a stage.
• Remember what determines loss: current density in the conductor. Voltage is just the lever that lets you carry the same power at lower current.
• Terminology: transmission moves bulk power between regions; distribution delivers to end users. Subtransmission sits in between, and utilities draw the line differently.

Summary and Next Steps

The chain is generation (11-25 kV) to step-up transformer to transmission (115-765 kV) to substations to distribution feeders (4-35 kV) to the distribution transformer to the customer. Everything about that chain follows from $P_{\text{loss}} = I^2R$ plus a transformer's ability to trade voltage for current. HVDC exists as a specialized alternative for distance, cables, and asynchronous ties.

Next lesson, AC Phasors and Power in Real Systems, picks up the $\cos\varphi$ we quietly set to 1 here. You will learn why real power is $VI\cos\varphi$, what reactive power physically does on a transmission line, and how to work with phasors instead of full time-domain sine waves.

Flashcard Review Deck

[
  { "front": "Typical generator terminal voltage?", "back": "About 11-25 kV, raised by a step-up transformer before transmission." },
  { "front": "Typical transmission voltage range in practice?", "back": "Roughly 115 kV to 765 kV line-to-line." },
  { "front": "Typical primary distribution voltage?", "back": "About 4-35 kV on the feeders, stepped down again at the customer." },
  { "front": "Why is bulk power transmitted at high voltage?", "back": "For fixed power, higher voltage means lower current, and line loss 3I^2R drops with the square of the voltage ratio." },
  { "front": "If transmission voltage rises 10x at constant power, what happens to line loss?", "back": "It falls by 100x, since loss scales with the square of the current." },
  { "front": "Three-phase power formula for line-to-line quantities?", "back": "S = sqrt(3) * V_LL * I (times cos phi for real power)." },
  { "front": "Role of the step-up transformer at a plant?", "back": "Raises generator voltage to transmission level while reducing current proportionally." },
  { "front": "Role of the distribution substation?", "back": "Steps transmission or subtransmission voltage down to primary feeder voltage." },
  { "front": "Role of the distribution transformer?", "back": "Steps feeder voltage down to utilization voltage such as 230/400 V or 120/240 V." },
  { "front": "Why is distribution usually radial?", "back": "It is cheap and simple, at the cost of longer outages after a fault." },
  { "front": "When is HVDC preferred?", "back": "Very long overhead links, long submarine cables, and ties between unsynchronized grids." },
  { "front": "What does grid frequency indicate?", "back": "The real-time balance between generation and demand; 50 or 60 Hz is the target." },
  { "front": "Why not raise transmission voltage indefinitely?", "back": "Insulation, corona, tower height, and right-of-way costs eventually outweigh the I^2R savings." }
]