Switching under load The revolution by Dr.-Ing. Bernhard Jansen

The revolution by Dr.-Ing. Bernhard Jansen

Power grids must remain stable, even when the load changes. Transformers ensure this by adjusting the voltage so that devices function reliably. To do this, transformers can absorb and output different voltages.

To adjust this voltage during operation, so-called tap changers are used, which control different winding taps.

When switching the taps of a regulating transformer, high current peaks (inrush currents) occur briefly when the contacts change from one winding tap to the next.

Before the invention of the resistor-type tap-changer, transformers had to be switched off before switching and then switched on again afterwards. The result in a time of isolated networks: power outages.

The resistor-type tap-changer developed by Dr. Bernhard Jansen solves this problem with a two-stage switching principle.

Technical explanation

A resistor is connected before switching to the next stage.
The effect of the resistor:
- It limits the short-circuit current.
- It reduces sparking during contact changeover.
- It protects the switching contacts and the transformer.
The actual switchover takes place in a matter of milliseconds – hence the name “quick switch”.

As soon as the new stage is safely engaged, the resistor is short-circuited again. The current flows directly through the main contact again.
This allows the transformer to run in normal, low-loss operation again – the resistor is only active during switching.

The critical moment is the transition phase, during which high currents or sparks could otherwise occur. To prevent these hazards, the contact change takes place in milliseconds and with clearly defined resistance.

A resistor-type tap-changer essentially consists of:

Main contacts: carry the continuous current
Resistance contacts: briefly switch on the series resistor
Series resistors: limit the current during switching
Mechanics and drive: move the contacts very quickly and precisely

The contacts move in a precisely defined sequence (switching logic) to prevent any short circuits.

Enables continuous voltage regulation = no need to switch off the transformer
Prevents network feedback = greater stability and safety

This invention made voltage adjustment possible during operation for the first time. This was crucial for:

Security of supply in growing power grids
Integrating new consumers and generators without interrupting the grid
Paving the way for the development of modern on-load tap-changers, which are now standard worldwide.

Without the invention of the resistor-type tap-changer, the meshing and flexible grid control we know today would be inconceivable. Today, MR solutions control around half of the world's energy.

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