Why did alternating current prevail?

Direct current vs alternating current

Historical context of electrification

At the beginning of electrification, in the second half of the 19th century, the electric current (ordered flow of electrical charges) used in urban networks was mainly direct current. The first installations of generators (conversion of mechanical energy into electrical energy) directly powered lamps and motors located nearby. Voltages were low, distances short and losses acceptable.

Limitations of direct current

With the rapid increase in demand for light, motive power and later household appliances, it became necessary to transport energy over tens, then hundreds of kilometers. Direct current then ran into a simple physical limit: to transport a lot of power without excessive losses, the voltage must be increased and the intensity reduced.

Synthesis between alternating current and direct current

Two complementary forms

The electric current (ordered flow of electrical charges) exists in two complementary forms: alternating current and direct current. The first varies periodically, while the second maintains a constant polarity. However, in modern systems, these two forms are not opposed: direct current is very often obtained by rectifying an alternating current.

Production and transport

Alternating current is generated directly by an alternator (electromechanical generator producing a periodic voltage), whose rotation creates a sinusoidal voltage. This form of energy is ideal for high-voltage transport, because it is easily transformed thanks to the transformer (electromagnetic device modifying an alternating voltage).

Rectification and power electronics

Direct current, historically produced by dynamos, today comes mainly from the rectification of an alternating current. Power electronics (set of components converting AC to DC) allows an alternating voltage to be converted into a stable direct voltage, used in batteries, electronics, storage systems and HVDC links (High Voltage Direct Current). Thus, modern direct current is not an independent form, but a transformation of alternating current.

Why high voltage is essential

Relationship between power, voltage and intensity

The electrical power transported is written \(P = U \times I\), where \(P\) is the power, \(U\) the voltage and \(I\) the intensity. For a given power, if the voltage is doubled, the intensity can be halved. However, the Joule losses in the lines are proportional to \(I^{2} \times R\), with \(R\) the resistance of the cables. Reducing the intensity is therefore the most effective way to limit losses over long distances.

Joule losses

In practice, this leads to using very high voltages for transport, then lowering these voltages as close as possible to the places of consumption. This is precisely what alternating current allows, thanks to a key device of electrical engineering: the transformer (electromagnetic device allowing an alternating voltage to be modified).

Realistic numerical example: power of a nuclear reactor

Let us consider an electrical power of \(P = 1\ \text{GW}\), an order of magnitude of the power supplied by a nuclear reactor. Suppose this power must be transported over \(100\ \text{km}\) by a line whose equivalent resistance (round trip) is \(R = 3\ \Omega\), which corresponds to about \(0{,}03\ \Omega/\text{km}\). Let us compare transport at \(50\ \text{kV}\) and at \(400\ \text{kV}\), which is part of current standards.

1) Transport at 50 kV
The required intensity is:
\(I = \frac{P}{U} = \frac{1\ \text{GW}}{50\ \text{kV}} = 20\ 000\ \text{A}\).
The Joule losses are then:
\(P_{\text{losses}} = I^{2} \times R = (20\ 000)^{2} \times 3 = 1{,}2 \times 10^{9}\ \text{W}\).
The losses reach about 1.2 GW in the form of heat, which means that much more than 1 GW would have to be supplied at the input of the line to deliver 1 GW at the other end. At such a low voltage, transport therefore becomes totally inefficient.

2) Transport at 400 kV
The intensity becomes:
\(I = \frac{1\ \text{GW}}{400\ \text{kV}} = 2\ 500\ \text{A}\).
The associated losses are:
\(P_{\text{losses}} = (2\ 500)^{2} \times 3 = 18{,}75 \times 10^{6}\ \text{W}\).
Only about 18.75 MW are lost, or less than 2% of the transported power.

Assessment

By going from \(50\ \text{kV}\) to \(400\ \text{kV}\), the voltage is multiplied by 8, the intensity is divided by 8 and the losses are divided by \(8^{2} = 64\). With a realistic line resistance, we see that very high voltage is essential to transport the power of a nuclear reactor with acceptable losses.

The decisive role of the transformer

Principle of operation

A transformer works only with an alternating current. It relies on the temporal variation of the magnetic flux in an iron circuit, which induces a voltage in a secondary winding when a primary winding is supplied. By adjusting the number of turns of each winding, the voltage is raised or lowered very efficiently.

Structuring of grids

This property made possible the level structure of modern grids: production at medium voltage, raising to very high voltage for transport, then progressive lowering down to domestic voltages. Without a transformer, each voltage level would have required specific machines and complex conversions. Alternating current therefore established itself as the simplest and most robust solution for an extended grid.

The War of the Currents

Edison against Tesla and Westinghouse

The famous "War of the Currents" pitted Thomas Edison (1847-1931), a supporter of direct current, against Nikola Tesla (1856-1943) and George Westinghouse (1846-1914), defenders of alternating current. Edison had already invested in direct current distribution networks and feared seeing his economic model called into question. Tesla, for his part, had designed polyphase systems particularly suited to alternating current.

Buffalo demonstration

The spectacular demonstrations of long-distance transport, such as the supply of the city of Buffalo from Niagara Falls at the end of the 19th century, showed the practical superiority of alternating current for large powers. Power plants could be installed where the primary energy was available, then electricity transported to urban centers with limited losses.

Technical advantages of alternating current: The problem of distance

Transformation and asynchronous motors

Alternating current has several major advantages for a large-scale grid. First, the possibility of easily transforming the voltage makes it possible to optimize each section of the grid according to the distance and the power to be transported. Then, rotating machines, such as asynchronous motors (machines converting electrical energy into mechanical energy), are simple, robust and inexpensive when supplied with alternating current.

Synchronization and reliability

Moreover, the synchronization of several generators on the same grid is facilitated by the periodic nature of alternating current. Large continental interconnections rely on this property of synchronism. Finally, the measurement and protection of lines are historically simpler with alternating quantities, which has contributed to the overall reliability of grids.

Why direct current is making a comeback

HVDC links

Direct current has not disappeared for all that. With the rise of power electronics, it has become possible to efficiently convert alternating current into direct current and vice versa. Very high voltage direct current links, called HVDC (High Voltage Direct Current), are now used to connect remote grids or to transport large powers under water.

Specialized niches

In these applications, direct current offers advantages such as the reduction of certain losses and the absence of synchronization problems between grids. However, these systems always rely on a predominantly alternating current environment, in which production, distribution and most uses remain alternating. The historical choice of alternating current therefore continues to structure the overall architecture of our grids.

A compromise between physics, economics and history

Legacy of standards

The success of alternating current is not only a question of physics. It also results from a compromise between the technologies available at the end of the 19th century, infrastructure costs and the industrial choices of the pioneers of electricity. Once the first large alternating grids were built, standardization effects reinforced this initial choice.

Current perspective

Even today, the 50 Hz frequency in Europe and 60 Hz in North America is a legacy of these historical decisions. Changing paradigm would involve modifying billions of devices and millions of kilometers of lines. Alternating current therefore remains the backbone of our electrical civilization, while direct current occupies specialized niches where its particular qualities are put to use.

Comparison between alternating current and direct current

The following table summarizes the main technical and historical differences between alternating current and direct current.