Benefits of Nanocrystalline Toroidal Core in Instrument Transformers

Benefits of Nanocrystalline Toroidal Core in Instrument Transformers

A nanocrystalline toroidal core is a doughnut-shaped magnetic core wound from nanocrystalline alloy ribbon. Its nanometer-scale grain structure gives very high permeability and very low core loss, which are exactly the properties that instrument transformers need to measure current and voltage accurately. This article explains the design, the unique properties and the practical benefits for instrument transformers such as current transformers (CTs) and voltage transformers (VTs).

How Instrument Transformers Use the Core

Instrument transformers are the measurement elements of power systems: they reproduce the primary current or voltage at a reduced, safe level for meters, relays and control equipment. A current transformer, for example, uses the magnetic core to couple the primary conductor to the secondary winding. The core must respond to small changes in the measured quantity, keep the ratio and phase error inside the accuracy class, and resist saturation during faults. Nanocrystalline toroidal cores meet these demands with a smaller cross-section than conventional materials.

Magnetic Properties That Drive Accuracy

High initial permeability: the core responds to even small magnetizing forces, so the excitation current is tiny and the ratio error stays low.

Low core loss: less energy is dissipated in the core, so more of the measured signal is transferred to the secondary side.

Stable magnetic fields: the flux is uniform in the toroid, giving consistent output over the working range.

High saturation flux density: up to 1.2-1.56 T, roughly three times ferrite, which improves behavior under DC offset and fault current.

Practical Benefits in Metering and Protection

Accuracy Classes

Metering CTs specified to IEC 61869-2, such as class 0.2S or 0.5S, need low excitation current over a wide current range. The high permeability of nanocrystalline material makes it easier to meet these classes with a compact core.

DC and Overcurrent Behavior

In residual current devices and DC-sensitive applications, the core must not saturate under DC bias. The high saturation flux density of the nanocrystalline ring keeps the measuring behavior linear, and the specified maximum DC overcurrent class can be verified during type tests.

Size and Weight

Because fewer turns and a smaller cross-section achieve the required magnetizing impedance, the transformer becomes smaller and lighter, which simplifies PCB mounting and reduces material cost.

Typical Core Data

Nanocrystalline ring cores for instrument transformers are available with inside diameters from about 14 mm to 25 mm, outside diameters from 19 mm to 32 mm, and heights from 6.5 mm to 10 mm. The effective cross-sectional area (Ae) ranges from about 12 mm2 to 59 mm2 and the magnetic path length from about 52 mm to 79 mm, with the maximum DC overcurrent class from 20 A to 120 A depending on the size. Protective boxes and specific dimensions are agreed per drawing.

Selection Notes

Match the bore size to the primary conductor and its insulation; for busbar applications, allow thermal clearance.

Confirm the accuracy class and burden required by the meter or relay specification.

Check the maximum DC overcurrent class when the CT operates in circuits with DC or asymmetrical fault currents.

Request the B-H curve and loss data to compare core material performance between suppliers.

Frequently Asked Questions

Q: Why are toroidal cores used in instrument transformers?
The closed toroid path gives high permeability with no air gap, which keeps the magnetizing current low and the measurement accurate.

Q: What accuracy class can nanocrystalline CTs reach?
With a proper design, classes such as 0.2S and 0.5S per IEC 61869-2 are achievable, along with protection classes like 5P10.

Q: How does DC bias affect a nanocrystalline core?
Thanks to the high saturation flux density, the core tolerates much larger DC components than ferrite before saturation, which is why it suits residual current and DC-sensitive measurement.

Q: Can the core be supplied with a protective box?
Yes, cores are commonly delivered with a protective box and can be wound or tested as a complete CT sub-assembly.

Q: Is the nanocrystalline ring suitable for high-frequency harmonics?
Yes, low core loss and high permeability keep the response linear up to tens of kilohertz, which improves accuracy in networks with harmonic content.

Q: Why does a nanocrystalline core improve metering accuracy compared with silicon steel?
Its high permeability means a much smaller excitation current is needed, and its low core loss keeps that current stable as the burden and frequency change. Both effects reduce the ratio error and phase displacement of a current or voltage transformer, so the instrument transformer stays inside its accuracy class over a wider current range, including the low currents where accuracy under load is hardest to hold.