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Posted on 01 July 2019

Ferrite Material for Energy-Meter Current Transformer -T36

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External Current Transformer (CT) driven Energy-meters have only alternating component as current input. Unlike whole current meters these are not subject to DC superposition test.

By P. Mukherjee and S. Goswami, Epcos India

 

For current transformers used in these meters as sensing elements, the requirement of the core material is high permeability and low loss at power frequency (50 Hz). Ferrite materials have negligible loss at 50 Hz. If a ferrite core is used in current transformer for this application, the material must have permeability in the range of 7000 and a relatively flat permeability vs. temperature (m-T) curve from 0°C to 70°C without any pronounced Secondary Permeability Maximum (SPM) peak. The material T36 has been developed with this in view and also aimed at competitive pricing for the Energy meter market.

Application of Soft Ferrite Ring Cores in Electronic Energy-Meters

There has been wide use of soft ferrite ring cores in Electronic Energy meters in India. The ring cores are wound as Bar Primary current transformers and the load current (at 50 Hz) of the Energy meter is passed through the single turn primary. The secondary typically has few thousand turns of thin copper wire with dc resistance in the range of few hundred ohms and is terminated by a metal film burden resistor below 50 ohms. The voltage across the resistor is proportional to the load current of the Energy meter and is taken as a galvanically isolated current signal by the meter.

For uncoated ring cores an insulating tape is put on the surface and two leads are brought out from the winding. The wound transformer is put in a plastic casing and potted with resin. For coated ring cores the winding is done directly on the core without putting any insulation tape.

For accuracy requirement the Energy meter under test (with the wound ring core) is checked against a Standard Meter and the error is noted. The error can be due to the ratio error or the phase error of the ferrite wound current transformer. The ratio error depends mainly on the power loss in the core and for ferrites at 50 Hz this is negligible. The phase error depends on the inductance (AL) of the ring core and higher the AL lower is the phase error. Based on this the winders have specified minimum acceptance level for AL value of these ring cores which translates to a minimum value of the material permeability.

Additionally, as the wound current transformer is put in a plastic casing and potted with resin with thermal stresses involved during curing, the core material permeability must be as far as possible invariant with temperature so as not to allow shift in inductance after potting. The permeability vs. temperature curve should be relatively flat from 10°C to 70°C with no pronounced Secondary Permeability Maximum (SPM) peak in this temperature range.

Development of T36

T36 material has been developed with the view to keep the permeability close to 7000 with no pronounced SPM in permeability temperature curve from 10°C to 70°C. The magnetic and electrical parameters are given below in Table 1 for this new material.

Magnetic and Electrical Parameters

A typical calculation is shown below for ring core R22 (OD=22.1 mm, ID=13.7 mm, Height=10 mm). This dimension suits the manufacturers casing and also available EPCOS tooling. The metal film burden resistor is considered as Unity Power Factor at the operating frequency of 50Hz.

The following data is considered:

  • Effective magnetic cross section, Ae=41.2 mm2
  • Effective magnetic length, le=54.1 mm
  • Number of secondary turns, N=5000
  • Winding Resistance, RW=150 Ohms
  • Burden resistance, RL=22 Ohms
  • Burden power factor angle β=0 degrees (Unity Power Factor)
  • Primary current, Ip= 5 A
  • Frequency, f=50 Hz
  • The secondary current, Is=Ip/N=1 mA
  • The voltage across the secondary winding, Es=Is*(RL+RW)= 0.172 V

Permeability vs temperature curve of T36

The operating flux: 0φ=Es/(4.44*f*N)=0.155 μWebers
This gives the operating flux density: B00/Ae=3.76 mT, clearly a low signal application.
The measured relative permeability, μI=6800 (Table 1).
For single turn primary the inductance is the AL value of the ring core. AL is related to the core factor and μI by:

Equations 1-5

The above results compare well with the limits of pr<0.5 and θ<30 minutes for a class 0.5 CT. The permeability vs temperature (μ-T) curve for T36 is shown below and it can be seen that any pronounced SPM peak between 10°C and 70°C has been avoided. This material has been developed viewing at customer requirement for external CT driven energy meter and this is no replacement for any existing Epcos material.

 

 

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