Potential transformers

Potential transformers (german: Spannungswandler, engl. abbr. PT) are used to measure and monitor the voltage of high voltage lines by tranforming the line voltage to some easily measurable value, often 100V (nominally - see below). They are designed for relatively low power but high accuracy and high reliability. The iron core is much larger and the wire resistance lower than usually necessary for that power. This allows to draw 10 time the rated power and more, as long as the windings do not overheat (they are not designed for good heat exchange). The insulation is designed with an extremely high safety margin, as the device must not fail due to voltage transients, even of several times the rated voltage. This makes them virtually indestructible, except thermally.

Small potential transformer, built in 1955. (The ruler is 30cm long.) This one is rated , 180VA, 50Hz. Up to about 100Veff can be applied without saturation, which gives an output of 20kVeff. This PT was used to produce a nice climbing arc.

Click on image for larger version.

In Europe, where we have a three-phase power grid, PT ratings are usually given as phase-to-phase voltages. This phase-to-phase voltage is larger than the phase-to-ground voltage by a factor of . A single-phase PT rated 20,000:100 or therefore only sees 11.5kV under normal conditions, and the normal secondary voltage accordingly is 57.7V instead of 100V.

However, under fault conditions it may happen that the phase-to-ground voltage of one phase becomes the full phase-to-phase voltage, and the PT not only has to survive this condition for some time, but also has to give reliable voltage readings. Therefore, even though the normal secondary voltage is only 57.7V, the PT should handle 100V without saturation occuring (the accuracy as a measuring device might be less than specified, though).

A 100V potential transformer should not be connected to 120V (e.g. USA), not to mention 230V (e.g. Europe). Therefore, a variac is advisable. When the primary current suddenly goes up while turning up the voltage, the saturation limit has been exceeded. The transformer should not be operated above this limit. For 230V grids, an adaptor transformer for US devices (230V to 120V autotransformer) proves useful, provided it's power rating is satisfactory. I use a box containing a 230V/18A variac and a subsequent isolation transformer with double secondary, which can be configured for 230V to 230V/20A or 230V to 115V/40A.

The current you can draw from a potential transformer is lethal.

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