A few years ago, engineers designing split-core current transformers for the 50/60 Hz application 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.
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 revolutionized the characteristics of ferrites at these frequencies, bringing many advantages to a wide range of applications, including the fast growing energy efficiency market.
Particularly important to power and condition monitoring applications is the use of split-core current transformers to perform accurate measurement of AC signals in an extended frequency range that includes 50/60Hz. A new type of ferrite with dramatically improved magnetic permeability now enables such transformers, with high accuracy and excellent linearity even at very low current levels. Also the particularly low phase-shift between measured voltage and current is essential for accurate active power measurement systems. The hard and dense core allows for the minimization of air gaps and is virtually resistant to ageing and temperature changes, in contrast to other materials, such as FeSi or FeNi.
The new ferrite families’ principal advantage is significantly improved permeability, so much so that 50/60 Hz current transformers can now use this material 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 high-permeability ferrite offers the best of both worlds.
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
- an attractive price
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 smaller than 20 or 30 microns, and these are more sensitive to ageing and temperature changes. Add the small air gaps to the better linearity of the ferrite at low magnetic excitation (i.e. for low current), and the ferrite offers a better performance than FeNi-80%, and a lower cost.
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).
New ferrite core material applications
Energy conservation is a rapidly growing issue across the globe, for both cost and environmental considerations. 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 and is increasingly gaining importance for the public sector. Many countries are introducing campaigns and incentive budgets for reducing energy consumption. Taking advantage of such incentives requires organizations to develop accurate measurement capabilities. Solid-core transformers would be the ideal solution if it was not prohibitively expensive to shut down power, disconnect cables and retrofit solid-core transducers in all the places where they might be used.
Contactless split-core self-powered current transformers can simply be snapped over a cable, without the need to screw or weld on complex brackets, making installation and maintenance straightforward. Principal applications for split-core current transformers 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. Energy waste is generally caused by an appliance malfunction, bad power quality or inefficient equipment usage. In all these cases split-core current transformers enable an improvement in efficiency by monitoring device condition, power quality or electricity consumption respectively. Furthermore they can be installed in electrical control panels – thus avoiding complex wiring – to remotely monitor devices that sometimes operate in inaccessible or harsh environments. As mentioned above, the beauty of the split-core transformer is that it can be retrofitted into existing installations without shutting down operation, even in environments with limited space.
Split-core current transformers are not new, but conventional technologies used in these transformers have presented numerous shortcomings. Top among these were the fact that they were either implementing expensive materials, or providing poor performance. In this case, inaccuracy refers not to the readings themselves, but to the linearity, the sustainability of a reading over time and the accuracy of the current compared to the voltage (phase shift).
Many new installations are benefiting from solid-core transformers, and split-core technology is not designed to compete with these. However, existing machines and buildings equipment have no way to add solid core devices without a costly shut-down of the system. New ferrite split-core current transformers allow for immediate retrofitting of high-performance and cost-effective condition monitoring, energy metering and facility supervision systems.