Voltage transformer (VT) — accuracy class and burden in indirect voltage measurement, the mirror image of the current transformer

The guide on current-transformer accuracy class and burden in indirect kWh metering covers how a current transformer (CT) converts a large primary current into a measurable secondary current. For indirect measurement of higher voltage levels — for example on the medium-voltage side of a transformer substation — voltage is converted in a similar way by a voltage transformer (VT, also called PT, potential transformer). This article covers the VT's accuracy class and burden, and the crucial difference with the CT on the point of safe use.

Accuracy classes per IEC 61869-3

IEC 61869-3 (voltage transformers) specifies the following accuracy classes for measurement purposes:

Class 0.1 and 0.2: primarily intended for laboratory and reference applications, with the tightest error limits.
Class 0.5 and 1: the classes used in most industrial and billing-related measurement applications.
Class 3: a wider class, used mainly where a rough indication suffices.

Each class specifies the maximum ratio error and phase displacement error the VT may have at its rated voltage and rated burden.

Burden: two series, depending on the power factor

As with a current transformer, the burden of a VT is the load (expressed in VA) connected to the secondary winding — in practice the measuring instrument plus the wiring between them. IEC 61869-3 defines two standard series for the rated burden:

Burden range I (cos φ = 1): standard values 1.0 – 2.5 – 5.0 – 10 VA.
Burden range II (cos φ = 0.8 inductive): standard values 10 – 25 – 50 – 100 VA.

As with a CT, the specified accuracy class of a VT applies expressly at the rated burden; a burden that deviates significantly from the value for which the VT is characterized can push the actual measurement error outside the class designation.

The mirror image of the CT: never short-circuit, but leaving it open is fine

The most important practical difference between a VT and a CT lies in the opposite failure risk on the secondary side:

A current transformer (CT) must never be operated unloaded (open) while primary current is flowing through it: the entire primary passage then behaves as the magnetizing winding, producing a dangerously high secondary voltage spike.
A voltage transformer (VT), by contrast, must never be short-circuited on the secondary side: the VT behaves as a voltage source with a relatively low internal impedance, and a short circuit on the secondary side drives a very large current through the winding, resulting in overheating and possibly burning out the VT.

These two failure modes are opposite to each other, which can be a real source of confusion — and therefore of mistakes — when working with both CTs and VTs in the same measurement or protection installation.

Temporary overvoltage and the VT's role during an earth fault

During an earth fault in a network with an isolated neutral or a Petersen coil, the voltage on the healthy phases can temporarily rise (comparable in principle to the guide on temporary overvoltage due to an earth fault in the high-voltage network, albeit at a different voltage level and in a different context). A VT used for fault detection in such a network must continue to measure accurately without saturating during that temporary overvoltage — this capability is specified by the manufacturer as the VT's rated voltage factor (Vf). The exact Vf value and the associated permissible duration depend on the earthing system of the network in which the VT is applied; the full text of IEC 61869-3 and the grid operator's or manufacturer's specification are decisive here.

Practical relevance

When assessing a measurement or protection setup with VTs — for example on the medium-voltage side of a transformer substation — it is worth checking that the secondary wiring and the connected instrument pose no risk of an unintended short circuit of the VT secondary, and that the burden specified for the instrument matches the VT's rated burden. When working on VT secondary circuits it is essential, just as with the opposite rule for CTs, to know beforehand which action is actually dangerous for this transformer.

Common mistakes

Wrongly applying the CT rule ("never leave open") to a VT — for a VT the opposite risk applies: never short-circuit the secondary side.
Not comparing the burden of the connected instrument with the VT's rated burden, so that the actual measurement accuracy can fall outside the specified class.
Choosing class 0.1/0.2 for an ordinary billing or process measurement where class 0.5 or 1 amply suffices, resulting in unnecessary extra cost.
Not accounting for the rated voltage factor (Vf) when applying a VT for fault detection in a network with an isolated or Petersen-coil-earthed neutral, so that the VT can saturate and measure unreliably during a temporary overvoltage.

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