CT burden, Knee point voltage, core saturation- Details of current transformer characteristics.

Application and choosing of current transformer for metering and protection is an important and critical task. There are some basic characteristics of conventional current transformer is to take in to consideration before choosing a current transformer in any application.

This characteristics are Current transformer burden, knee point voltage, saturation of current transformer. As the current transformer has two windings, in some cases it is confusing about the load of it. But the fact should be clear that-

The current of primary circuit is the load of a current transformer.

CT or current transformer burden:

CT burden is an impedance of secondary circuit expressed in ohms and power factor.This power factor is not the power factor of secondary load. CT Burden can be expressed in two terms- as example

  1. .5 ohm impedance
  2. 10 VA at 5A.(British method)- here 5A is rated secondary current and 2Volt across secondary.

Knee point voltage of Current transformer:

Knee point voltage of a current transformer is the magnitude of secondary of current transformer. After or beyond this voltage the linearity between primary and secondary circuit that is the desired property of Current transformer does not work any more.

In saturation zone -the error in transformation ratio is high, the secondary current is distorted by saturation. This is called core saturation of current transformer.

Significance of Magnetization or excitation curve for a Current transformer

A CT has a unique magnetization curve (for a given temperature and frequency). With the transformation ratio, this characterizes its operation. This magnetization curve (voltage Vo, magnetizing current function Im) can be divided into 3 zones:

  1. Non-saturated zone: Im is low and the voltage Vo (and therefore Is) increases virtually proportionately to the primary current.
  2. Intermediary zone: there is no real break in the curve and it is difficult to situate a precise point corresponding to the saturation voltage.
  3. Saturated zone: the curve becomes virtually horizontal; the error in transformation ratio is high, the secondary current is distorted by saturation.
Magnitization or excitation curve for a Current transformers
Magnitization or excitation curve for a Current transformers
Magnetization effect of Current transformer
Current transformer Schematic diagram
I1: primary current.I2 = Kn I1: secondary current for a perfect CT. Is: secondary current actually fl owing through the circuit. Im: magnetizing current. E: induced electromotive force. Vo: output voltage.
Lm: magnetization inductance (saturable) equivalent to the CT. Rct: resistance at the CT secondary. Rwir: resistance of the connection wiring. Rc: load resistance.

Current I2 is a perfect image of the primary current I1 in the transformation ratio. However, the actual output current (Is) is subject to an error due to the magnetization
current (Im).
I2 = Is + Im if the CT was perfect, we would have Im = 0 and Is = I2.

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