Burden of Current Transformer

The impedance of the secondary circuit is expressed in ohms, power factor angle is called burden. But it is usually expressed in terms of apparent power in kVA and rated secondary current at specified power factor. This power factor is not the power factor of the secondary current of CT.
Rated 'Burdens' of CTs and VTs refer to the maximum load in volt-amperes (VA) which may be applied across the secondary terminals, without the ratio and phase angle errors exceeding the permissible limits. The burden depends upon the number of instruments or relays connected and their individual burdens' typical values.
CT Terminal Marking:
The terminal marking shall identify:
a) The primary and secondary windings.
b) The winding sections, if any.
c) The relative polarities of windings and winding sections; and
d) The intermediate tappings, if any.
CT Method of Marking:
1. The terminals shall be marked clearly and indelibly either on their surface or in
their immediate vicinity.
2. The marking shall consist of letters followed, or preceded, where necessary, by
numbers. The letters shall be in block capitals.
3. All the terminals marked P1, S1 and C1 shall have the same polarity at any
instant.
Magnetization curve: -
The magnetization curve of CT shows the excitation characteristic curve of typical oriented electrical steel. The excitation curve may be subdivided into four main regions (i) from origin to ankle point (ii) from ankle point to knee (iii) knee region (iv) saturation region. Knee point is defined as where a 10% increase in flux density causes 50% increase in exciting ampere-turns.
Protective current transformer generally operates an over-working range of flux density extending from the ankle-point to the knee region above, while the measuring current transformer has the flux density in the region of ankle-point only.
before saturation, the flux density in the core is proportional to ampere-turns. On reaching saturation, magnetizing inductance becomes low and the total primary current is utilized in exciting the core alone, therefore, the secondary output of CT disappears.
The saturation continues till the primary transient current is reduced below the saturation level. On energy in the saturation zone, the CT behaves an open circuited. It is difficult to avoid saturation during short-circuit conditions. The effect of saturation is the reduced output, hence the reduced speed of over-current relays. In differential relays, the saturation disturbs the balance, and stability of protection is affected.
The current transformer saturation curve is generally plotted in secondary volts vs. the exciting current measured in secondary. For the required magnitude of secondary voltage, the degree of saturation can be seen from the curve and is also indicated by the magnitude of the exciting current to produce this voltage.