What happens when a transformer is saturated?
The transformer core is made of ferromagnetic materials which gets saturated at a certain level of magnetic flux densities i.e., a further increase in the mmf does not lead to proportional increase in magnetic field flux(Φ).
Each type of core has it’s own capacity to accommodate flux density. i.e CRGO core has 1.8wb/m^2. also it has it’s own saturation characteristics flux vs. magnetizing current again flux depends on supply frequency, flux= voltage/frequency.
If frequency goes to zero value core goes into saturation region. i.e pure D.C. supply has zero frequency, so that’s why never try to give D.C. supply to the transformer.
Same way Im*sin(thita) component is directly proportional to flux so, as it increases up to certain level core will get saturated. where, thita is power factor angle.
What happens if current transformer gets saturated?
When the primary winding has excessive applied voltage, flux in the core may reach the saturation levels during peak moments of the AC sine wave cycle. And hence, voltage induced in the secondary will not remain sinusoidal anymore, creating harmonics in the secondary winding output. The presence of harmonics causes overheating, power loss, reduced efficiency and shortened lifespan of the devices.
When current transformer gets saturated they no longer supply secondary current proportional to supplied primary current. Depending on the level of saturation the measured current on the secondary is much smaller than the value present in the primary.
The non-proportional secondary current under saturation conditions is caused due to following reasons;
- Large primary current due to fault condition
- High burden (VA) on secondary side
- An open circuit condition occurred on the secondary side of CT.
How does exactly CT saturate?
When supplied current in primary side is multiple times of its nominal current value(In), then the magnetic core does not respond to further increase in magnetic flux as all magnetic domains on a ferromagnetic material are already aligned and thus does not respond to any further increase in the flux. Once the CT saturates the resultant secondary current is full of harmonics and it is highly distorted.
Current transformer saturation leads to malfunction of protection devices as the measured current does not correspond to the current on the system.
What happens if current transformer gets saturated?
CT are mainly used for Measurement/Metering & Protection Purpose. If the CT gets saturated, then secondary current will not be an exact replica of primary current.
A saturated CT will lead to following:
- Incorrect reading in Metering/Measurement equipment.
- Incorrect operation of protective equipment specially differential protection in Transformers, Reactors & Bus Bars.
- High Peaky voltage in secondary side.
- Excessive core loss & consequently overheating of CT. Chances of failure increases.
- Under/Over Reach of distance protection relay.
Magnetic inrush is a phenomenon that occurs when an unloaded transformer is energised. This happens during first 5-6 cycles. If transformer is energised at the instant when ac voltage passes its zero crossing point, the steady state value of flux demanded by the core is at its maxima. The core flux, which is initially zero, rises up to twice the maximum flux, called as doubling effect. The current drawn in to set up this flux is quite high, reaching up to 4-5 times the full load current.The flux contains a dc component and a steady state alternating component. The induced emf due to dc component of flux will be zero. So there is no doubling effect in voltage.
ReplyDeleteDue to mmf unbalance between primary and secondary during magnetic inrush, the windings are susceptible to large mechanical stresses developed and may get dislocated due to unbalanced axial forces along the core limb. Proper mechanical support ensures this does not happen.
If instant of energisation happens to be at maxima of applied voltage, doubling effect can be avoided but it is not a cakewalk to control the instant of energisation.
Such high currents drawn can lead to unnecessary tripping of circuit breaker as relay sees it as fault current level. To mitigate this, a small delay of corresponding to 6-7 cycles(~140ms considering 50Hz) can be allowed for relay operation. Thus unnecessary tripping of circuit breaker is avoided. Magnetic inrush current contains dominant second harmonic. Thus, duration of the transient as well as the magnitude of the second harmonic of the inrush current can be used to differentiate between short circuit current and inrush current.
Other causes of inrush could be recovery inrush (recovery after a fault) and sympathetic inrush (energisation of another parallel transformer) but magnitudes of these inrush currents is less than that of initial inrush described above.