power-market-trading-docs/primers/technical_power_grid_ac_fundamentals_en.md

Power Grid and AC Fundamentals

Overview

This primer starts with the basic structure of the electrical grid and then builds toward alternating current, impedance, inductors, capacitors, and Kirchhoff’s laws.

---

1. What Is an Electrical Power Grid?

An electrical power grid is a large interconnected system that moves electrical energy from power plants and other sources to homes, businesses, and factories.

The grid has four main parts:

1. Generation
2. Transmission
3. Substations and transformers
4. Distribution

1.1 Generation

Electricity is produced by sources such as:

• Natural-gas, coal, and nuclear plants
• Hydroelectric stations
• Wind turbines
• Solar farms
• Batteries and other energy-storage systems

Traditional generators usually produce alternating current, or AC. Solar panels and batteries naturally produce direct current, or DC, so power-electronic inverters are used to connect them to the AC grid.

1.2 Transmission

Transmission lines carry large amounts of electrical power over long distances.

Power is approximately:

P = VI

where:

• P is power
• V is voltage
• I is current

For the same amount of power, raising the voltage allows the current to be reduced.

Transmission losses are approximately:

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

Because losses depend on the square of current, high-voltage transmission greatly reduces heating losses.

1.3 Substations and Transformers

Substations connect different parts of the grid. They contain equipment that can:

• Raise or lower voltage
• Connect or disconnect lines
• Measure voltage and current
• Protect equipment during faults
• Control power flow

Transformers raise voltage for long-distance transmission and lower it again near customers.

1.4 Distribution

Distribution networks carry electricity from local substations to customers.

Additional transformers reduce the voltage to levels used by homes, offices, and industrial equipment.

---

2. Important Grid Quantities

2.1 Voltage

Voltage is the electrical potential difference between two points. It is the electrical “push” that drives current.

In an AC grid, voltage has both:

• A magnitude
• A phase angle

Different grid buses may have slightly different voltage magnitudes and phase angles.

2.2 Current

Current is the movement of electric charge through a conductor.

In the grid, line current depends on:

• Connected generation and loads
• Voltage differences
• Voltage phase-angle differences
• Line impedance
• Network topology

2.3 Real, Reactive, and Apparent Power

In AC systems, engineers distinguish between three types of power.

Real power P

Real power performs useful work and is measured in watts.

Examples include:

• Producing heat
• Turning a motor
• Producing light

Reactive power Q

Reactive power supports electric and magnetic fields. It is measured in vars.

Reactive power moves back and forth between sources and reactive components such as inductors and capacitors.

Apparent power S

Apparent power is measured in volt-amperes.

The three are related by:

S^2 = P^2 + Q^2

2.4 Frequency

Frequency is the number of AC cycles per second.

1\text{ Hz} = 1\text{ cycle per second}

Typical grid frequencies are:

• 60 Hz in the United States and some other countries
• 50 Hz in many other parts of the world

Frequency reflects the balance between generation and demand:

• If demand exceeds generation, frequency tends to fall.
• If generation exceeds demand, frequency tends to rise.

---

3. Buses, Branches, and Loads

Power engineers often represent the grid as a network.

Bus

A bus is a connection point where generators, loads, transformers, or transmission lines meet.

Branch

A branch is a transmission line or transformer connecting two buses.

Load

A load is equipment that consumes electrical power, such as:

• Motors
• Lighting
• Heating systems
• Computers and electronics

A simplified network is:

\text{Generator}
\rightarrow
\text{Bus 1}
\rightarrow
\text{Transmission line}
\rightarrow
\text{Bus 2}
\rightarrow
\text{Load}

Real power grids contain many parallel paths. Electricity divides among those paths according to the network’s electrical properties rather than following one assigned route.

---

4. Direct Current and Alternating Current

4.1 Direct Current

With direct current, or DC, charge flows mainly in one direction.

A battery is a common DC source.

4.2 Alternating Current

With alternating current, or AC, voltage and current repeatedly reverse direction.

An ideal sinusoidal voltage is:

v(t) = V_{\text{peak}}\sin(\omega t)

The voltage:

1. Rises positive
2. Falls to zero
3. Becomes negative
4. Returns to zero
5. Repeats

Negative voltage does not mean “less than no electricity.” It means that the polarity between two reference points has reversed.

4.3 How AC Is Generated

A generator converts mechanical rotation into electrical energy.

In a simplified generator:

1. A magnetic field rotates relative to coils of wire.
2. The magnetic flux through the coils changes.
3. A voltage is induced.
4. As the magnetic orientation reverses, the voltage reverses.

This produces AC.

Generators may be driven by:

• Steam turbines
• Gas turbines
• Water turbines
• Wind turbines
• Engines

4.4 RMS Voltage

Because AC voltage changes continuously, engineers often describe it using its root-mean-square value.

For a sine wave:

V_{\text{RMS}} = \frac{V_{\text{peak}}}{\sqrt{2}}

A 120 V household supply is approximately 120 V RMS, with a peak near 170 V.

RMS voltage is useful because it produces the same heating effect in a resistor as an equal DC voltage.

---

5. How Electrical Energy Travels in AC Systems

A wire already contains mobile electrons before it is energized.

The power plant does not send a specific group of electrons all the way to a customer. Instead:

1. The generator establishes an electromagnetic field.
2. That field propagates along the circuit.
3. Local electrons in the conductors begin moving.
4. Energy is transferred to the load.

In AC, electrons usually oscillate back and forth around approximately the same average position.

Two different speeds are involved:

• Electron drift speed: relatively slow
• Electromagnetic propagation speed: very fast, often a significant fraction of the speed of light

The electromagnetic energy flow is described by the Poynting vector:

\mathbf{S} = \mathbf{E} \times \mathbf{H}

where:

• \mathbf{E} is the electric field
• \mathbf{H} is the magnetic field
• \mathbf{S} describes the direction and rate of energy flow

The conductors guide the fields that carry energy toward the load.

---

6. Phase in AC Circuits

Two AC waveforms are in phase when their peaks and zero crossings occur at the same times.

They are out of phase when one waveform leads or lags the other.

Pure resistor

For an ideal resistor:

v(t) = i(t)R

Voltage and current are in phase.

Pure inductor

For an ideal inductor:

• Voltage leads current by 90^\circ
• Current lags voltage by 90^\circ

Pure capacitor

For an ideal capacitor:

• Current leads voltage by 90^\circ
• Voltage lags current by 90^\circ

---

7. Resistance, Reactance, and Impedance

7.1 Resistance

Resistance opposes current and converts electrical energy into heat.

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

Resistance is measured in ohms.

7.2 Reactance

Reactance is opposition to AC caused by energy storage in electric or magnetic fields.

It comes from:

• Inductance
• Capacitance

Unlike ideal resistance, ideal reactance does not permanently consume average real power. It stores energy temporarily and returns it to the circuit.

7.3 Impedance

Impedance is the total opposition to AC.

Z = R + jX

where:

• Z is impedance
• R is resistance
• X is reactance
• j represents a 90^\circ phase shift

For AC circuits, Ohm’s law becomes:

\underline{I} = \frac{\underline{V}}{Z}

The underlines indicate phasor quantities.

Example

If a line has:

Z = 2 + j8\ \Omega

then:

• Resistance is 2\ \Omega
• Inductive reactance is 8\ \Omega

Its magnitude is:

|Z| = \sqrt{2^2 + 8^2} \approx 8.25\ \Omega

The line is mostly inductive because its reactance is much larger than its resistance.

---

8. Inductors

An inductor is usually a coil of wire that stores energy in a magnetic field.

Its key behavior is:

An inductor resists changes in current.

8.1 How an Inductor Works

When current flows through a wire, it creates a magnetic field.

Winding the wire into a coil strengthens and concentrates that field.

If the current changes, the magnetic field changes. That changing field induces a voltage that opposes the original change. This is Lenz’s law.

The basic relationship is:

v_L = L\frac{di}{dt}

where:

• v_L is inductor voltage
• L is inductance in henries
• di/dt is the rate of change of current

8.2 DC Behavior

When a DC source is first connected, the inductor opposes the sudden rise in current.

In a series RL circuit:

i(t) = \frac{V}{R}\left(1-e^{-tR/L}\right)

The time constant is:

\tau = \frac{L}{R}

After the current becomes steady:

\frac{di}{dt}=0

so:

v_L = 0

An ideal inductor then behaves like a short circuit for steady DC.

8.3 Switching Off an Inductor

When current is interrupted suddenly, the collapsing magnetic field produces a voltage that tries to keep current flowing.

This can create:

• Sparks
• Arcing
• High-voltage transients
• Damage to switches or transistors

Protective devices such as flyback diodes provide a safe path for the current.

8.4 Stored Energy

An inductor stores energy in its magnetic field:

W_L = \frac{1}{2}LI^2

8.5 Inductive Reactance

For sinusoidal AC:

X_L = 2\pi fL

Inductive reactance increases when:

• Frequency increases
• Inductance increases

The impedance of an ideal inductor is:

Z_L = j\omega L

8.6 Inductors in the Grid

Grid equipment contains inductance even when there is no obvious coil.

Examples include:

• Transmission lines
• Transformers
• Motors
• Generators
• Reactors

Power-system reactors are large inductors used to:

• Limit fault current
• Control voltage
• Absorb reactive power
• Reduce transients

---

9. Capacitors

A capacitor stores energy in an electric field.

Its key behavior is:

A capacitor resists changes in voltage.

9.1 Construction

A basic capacitor has two conductive plates separated by an insulating dielectric.

\text{plate} \; | \; \text{dielectric} \; | \; \text{plate}

The dielectric may be air, ceramic, plastic film, oxide, or another insulating material.

9.2 Charge and Voltage

The stored charge is:

Q = CV

where:

• Q is charge
• C is capacitance in farads
• V is voltage

For ideal parallel plates:

C = \frac{\varepsilon A}{d}

Capacitance increases when:

• Plate area increases
• Plate spacing decreases
• Dielectric permittivity increases

9.3 Current-Voltage Relationship

The fundamental capacitor equation is:

i = C\frac{dv}{dt}

A capacitor carries current when its voltage is changing.

A finite current can only change capacitor voltage at a finite rate, so capacitor voltage cannot jump instantly in an ideal circuit.

9.4 DC Charging

When an uncharged capacitor is connected to a DC source through a resistor:

v_C(t) = V\left(1-e^{-t/RC}\right)

and:

i(t) = \frac{V}{R}e^{-t/RC}

The time constant is:

\tau = RC

After the capacitor is fully charged, its voltage becomes constant, so:

\frac{dv}{dt}=0

and therefore:

i=0

An ideal capacitor behaves like an open circuit for steady DC.

9.5 Stored Energy

A capacitor stores energy in its electric field:

W_C = \frac{1}{2}CV^2

9.6 Capacitive Reactance

For sinusoidal AC:

X_C = \frac{1}{2\pi fC}

Capacitive reactance decreases when:

• Frequency increases
• Capacitance increases

The impedance of an ideal capacitor is:

Z_C = \frac{1}{j\omega C} = -jX_C

9.7 Capacitors in the Grid

Grid capacitance appears in:

• Transmission lines
• Underground cables
• Capacitor banks
• Series compensation equipment
• Shunt compensation equipment

Capacitor banks are used to:

• Support voltage
• Supply reactive power locally
• Improve power factor
• Reduce line current
• Reduce I^2R losses

Series capacitors offset part of line inductive reactance:

X_{\text{net}} = X_L - X_C

Shunt capacitors are connected to a bus and help support bus voltage.

---

10. Inductors and Capacitors Compared

Property	Inductor	Capacitor
Stores energy in	Magnetic field	Electric field
Resists changes in	Current	Voltage
Basic equation	v=L\,di/dt	i=C\,dv/dt
Steady DC behavior	Short circuit ideally	Open circuit ideally
AC phase relationship	Current lags voltage	Current leads voltage
Reactance as frequency rises	Increases	Decreases
Ideal impedance	j\omega L	1/(j\omega C)

---

11. Why Current Can Reverse While Power Still Flows to a Load

For a pure resistor:

v(t)=V_{\text{peak}}\sin(\omega t)

i(t)=I_{\text{peak}}\sin(\omega t)

Instantaneous power is:

p(t)=v(t)i(t)

Therefore:

p(t)=V_{\text{peak}}I_{\text{peak}}\sin^2(\omega t)

Because \sin^2(\omega t) is never negative, the resistor receives energy during both halves of the cycle.

When voltage and current both reverse, their product remains positive.

For ideal inductors and capacitors, power alternates between positive and negative because energy is repeatedly stored and returned.

---

12. Kirchhoff’s Laws

Kirchhoff’s laws describe the basic conservation rules that electrical networks must obey.

12.1 Kirchhoff’s Current Law

At any node or bus:

\sum I_{\text{entering}} = \sum I_{\text{leaving}}

Current cannot accumulate at an ideal node.

In power-system form, this becomes a real- and reactive-power balance at each bus.

A simplified real-power balance is:

P_{\text{generation}} - P_{\text{load}}
=
P_{\text{exported}} + P_{\text{losses}}

A similar equation applies to reactive power.

12.2 Kirchhoff’s Voltage Law

Around any closed electrical loop:

\sum V = 0

The total voltage rises equal the total voltage drops.

For a transmission line:

\underline{V}_1 - \underline{V}_2 = \underline{I}Z

where:

• \underline{V}_1 and \underline{V}_2 are bus-voltage phasors
• \underline{I} is line current
• Z is line impedance

12.3 Role in Grid Analysis

Together, Kirchhoff’s laws allow engineers to calculate:

• Bus voltages
• Line currents
• Real-power flows
• Reactive-power flows
• Losses
• Fault currents

They are fundamental to:

• Load-flow studies
• Short-circuit studies
• Stability studies
• Protection design

---

13. How Power Flows Through an AC Grid

In an interconnected AC network, power divides among all available paths.

The distribution of power depends on:

• Line resistance
• Line reactance
• Bus-voltage magnitudes
• Voltage phase angles
• Network connections

A lower-impedance path generally carries more current than a higher-impedance path.

In high-voltage transmission systems, inductive reactance is often larger than resistance. As a result, real-power flow is strongly influenced by the phase-angle difference between buses.

Grid operators influence power flow by:

• Adjusting generator output
• Switching transmission lines
• Changing transformer taps
• Connecting or disconnecting capacitor banks or reactors
• Using power-electronic controllers

---

14. Core Mental Models

AC

\text{rotating generator}
\rightarrow
\text{alternating voltage}
\rightarrow
\text{alternating current}
\rightarrow
\text{energy transfer}

Inductor

\text{changing current}
\rightarrow
\text{changing magnetic field}
\rightarrow
\text{induced voltage}
\rightarrow
\text{opposition to current change}

Capacitor

\text{changing voltage}
\rightarrow
\text{movement of charge}
\rightarrow
\text{changing electric field}
\rightarrow
\text{current}

Grid analysis

\text{network model}
+
\text{impedances}
+
\text{Kirchhoff’s laws}
\rightarrow
\text{voltages, currents, and power flows}

---

15. Summary

The electrical grid is an interconnected AC network that transports energy from generators to loads.

The most important ideas are:

• High voltage reduces transmission current and resistive losses.
• AC voltage and current repeatedly reverse direction.
• Electrical energy is carried by electromagnetic fields guided by conductors.
• Resistance consumes real power.
• Inductors store energy in magnetic fields and resist changes in current.
• Capacitors store energy in electric fields and resist changes in voltage.
• Impedance combines resistance and reactance.
• Kirchhoff’s Current Law enforces balance at buses.
• Kirchhoff’s Voltage Law enforces voltage consistency around loops.
• These principles form the foundation of power-flow, fault, and stability analysis.