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Posted on 01 April 2019

New Ferrite Material Improves Split-Core Current Transformers for Power Measurement

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A few years ago, engineers designing for the 50/60 Hz domain were dreaming about “magic” materials for magnetic cores, which would provide the best performance at the best price. Unfortunately the available technologies offered either a good performance for a high price or much poorer performance for a low price.

By Pierre Turpin, Energy and Automation Project Manager, LEM

 

Although ferrite materials had been well known for years, their poor performance in terms of saturation level and magnetic permeability did not allow their use at frequencies as low as 50/60Hz. However, recent developments have revolutionised the use of ferrites at these frequencies, bringing many advantages to a wide range of applications, including the fast growing energy efficiency market. The use of a new type of ferrite with improved magnetic permeability allows splitcore current transformers to perform accurate measurement of AC signals in an extended frequency range that includes 50/60Hz. The ferrite core provides high accuracy and excellent linearity even at very low current levels, and the transformers have a particularly low phase-shift between measured voltage and current. The hard and dense core allows very small air gaps to be achieved and is virtually insensitive to ageing and temperature changes, in contrast to other materials, such as FeSi or FeNi.

The principal change in these new ferrite families is that the permeability has been improved, to such an extent that 50/60 Hz current transformers can now use it in the same way as FeSi or FeNi cores, despite the magnetic saturation level remaining low. This unlocks the other features of ferrites, which until now have been unavailable to 50/60Hz applications because of the permeability problem:

• Excellent linearity even at very low levels
• Hardness, allowing very small air gaps
• Large frequency range due to low loss level
• Low cost

Until now, the best materials for split-core current transformers have been FeNi (best performance but high price) and FeSi (best price but poor performance). The new highpermeability ferrite offers the best of both worlds, thanks to the above features.

The accurate measurement of true active power or energy introduces specific requirements:

• Low phase shift between measured voltage and current
• High linearity of the analogue sensing part, especially at low current levels
• Easy-to-install device such as split core current transformers

And all that at an attractive price, of course.

The high-permeability ferrite material does not give the best results in solid-core current transformers, so let us focus on split-core current transformers. The hardness of the solid material (consider ferrite as a ceramic) allows very fine machining, providing air gaps down to a few microns that are stable over many years. Laminated materials such as FeSi or FeNi do not allow air gaps better than 20 or 30 microns, and these are more sensitive to ageing and temperature changes. If this is added to the better linearity of the ferrite at low magnetic excitation (i.e. for low current), the ferrite offers a better performance than FeNi-80%, and a lower cost.

Figures 1, 2 and 3 are extracts from simulation comparing the phase shift behaviour of FeSi, FeNi and high-permeability ferrite in a 5A current transformer.

Ip = 20 percent of rated current, F = 50 Hz, Core FeSi, Air Gap = 25mm (one leg)

Ip = 20 percent of rated current, F = 50 Hz, Core FeNi-80 percent, Air Gap = 25mm (one leg)

Ip = 20 percent of rated current, F = 50 Hz, Core Ferrite, Air Gap = 5mm (one leg)

The phase shift for the ferrite is half that of the FeNi core, so FeNi is definitely out of the competition. The reduced air gap of the ferrite core also allows a better accuracy of transfer ratio (primary turns to secondary turns).

Let us look at some areas of application where the new ferrite core material brings strong benefits.

Saving energy, for both cost savings and environmental considerations, is a growing issue in many countries, but the key question is: how can a substantial and sustainable reduction of energy consumption be achieved? The most credible solution is to establish an understanding of how users consume their energy and make them responsible for it. Targeting this area remains an industrial concern but is also increasingly gaining importance for the public sector. Many countries are supporting this by introducing campaigns and incentive budgets for reducing energy consumption. Sub-metering is an extremely powerful tool and LEM has introduced the TT series of current transformers, which use the new ferrite material. These split-core current transformers dramatically improve the cost/performance ratio of electrical sub-metering solutions, especially for existing buildings. The TT series is also an ideal solution for many other applications that require accurate active power measurement, especially thanks to the particularly low phase shift between measured voltage and current and the high linearity, even at low current levels.

A TT transformer

Contactless split-core current transformers can simply be snapped over a cable, without the need to screw or weld on complex brackets, making installation and maintenance straightforward. Furthermore they can be installed in electrical control panels – thus avoiding complex wiring – to remotely monitor devices that often operate in harsh environments. In particular, since they are so easy to fit and put into operation, the new small, safe, self-powered, split core current transformers can be retro-fitted into existing installations without shutting down operation, even in environments with limited space.

Principal applications for current transformers based on the new ferrite material will be in the fields of energy sub-metering and cost allocation, dynamic consumption and peak analysis, energy waste or defective equipment detection, and power quality control.

 

 

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