How transformers work
The physical principle of transformers has not changed in 130 years, but energy density, efficiency, costs, weight and dimensions have drastically improved
Transformers need an “amplifier” for the magnetic field so that the number of winding turns can be kept low. This “amplifier” is the so-called magnetic core. It consists of ferromagnetic iron, which contains microscopic elementary magnets that align to the transformer’s magnetic field as a compass needle aligns to the Earth’s magnetic field. The iron core is made of many thin ferromagnetic steel sheets that are electrically insulated against each other and stacked. This reduces classical eddy losses. The use of special alloys and manufacturing methods enables a minimum needed energy to change polarity of the elementary magnets.
This basic physical principle of transformers is still the same today as it was 130 years ago, but energy density, efficiency, costs, weight and dimensions have drastically improved. This can be compared to the history of cars and the internal combustion engine: Here too the basic principle has remained unchanged in 100 years, but technical progress has transformed the scope of possibilities almost beyond recognition. During the first decades of electrification, the main focus in transformer research and development was to increase power capacity (the power that can be transmitted by one unit). Furthermore, more and more effects concerning voltage transients became known that could endanger the transformer’s insulation. These include resonance effects in the coils that can be triggered by fast excitations such as the overvoltage impulse of a lightning strike. New coil designs mitigated these resonance effects.
Transformers are the main current-limiting element in case of short-circuit failures in the transmission system. The so called stray reactance, which represents the magnetic flux outside of the magnetic core limits the increase in current in such an event. If high currents flow through the coils uncontrolled, mechanical forces try to press the coils apart, and may cause damage if the construction is not sufficiently robust.
Due to the resistance and inductance of the power lines themselves, the voltage level may vary depending on load conditions. This means that less voltage “arrives” at the receiving end of a power line when the load is high. To keep the voltage level within an acceptable range, power transformers usually include an on-load tap changer to vary the number of active winding turns of coil by switching between different taps. In medium voltage (MV) distribution, this is usually done offline: This means the tap changers are adjusted once before the transformer is energized and then remain fixed.
The increasing importance in recent decades of UHV (ultra-high voltage) DC transmission lines for high power transmission over very long distances (greater than 1000 km) has made it necessary to develop UHV-DC converter transformers, which are a huge challenge especially for the electric insulation system. The 800kV UHV-DC Xiangjiaba-Shanghai line for example, has a capacity of up to 7200MVA, which is roughly comparable to the consumption of Switzerland.
Editor’s note: The original article was written by Max Claessens and was published in the special report on Transformers