Power transformer saturation 2

[MeinKaun11]

Hi All,

Practicing electrical engineering for the last 10 years this never came to my mind till my nephew who is studying electrical engineering starting discussing the difference between CT and power transformer concept. Let me start with what I know and may you can interject and kindly correct me.

Power Transformer: Voltage is applied to Primary winding and with no-load, winding generates magnetic field and then generates flux due to farady law and links with secondary winding. Due to varying magentic flux, V2 is generated in the secondary winding. When load is connected than
I2 is produced which generates counter flux and more current is produced in primary to maintain constant flux in core.

Saturation in power transformer will depict the same behavior as a CT that means, after saturation no change of flux and no voltage on secondary.
Is this correct ?

 

[veritas]

First thing to note is that both CT's and power transformers (PT's) are transformers and as such obey trfr theory. In generic terms they are electro-magnetic energy converters. However, their behaviour in the power system are radically different. With a PT the turns ratio N1/N2 is typically less than 20 whilst for a CT turns ratrios are 100's or 1000's.

With a PT the secondary terminal voltage (Vs) is more or less constant as this determines the secondary network voltage. Thus Vs does not fluctuate appreciably with secondary load. Thus the core flux remains more or less constant. In fact a PT cannot really go into saturation due to overloading. It will overheat if the loading is beyond it's rating for a sustained period but there will be no increase in flux as you correctly stated, due to the demagnetising effect on the core due to the secondary current being compensated by increased primary load current.

PT cores are usually saturated if the applied voltage is substantially higher than rated voltage. Should this be the case then, yes, the output I would think would also be distorted like that of a CT. Never really studies the output waveform of a saturated PT. I believe the waveforms are rich in 5th harmonics.

With CT's the (induced) secondary voltage is a function of the connected CT secondary load (burden). The secondary load has no effect on the primary current (except for little Imag). This is easily seen by considering a 1000/1 CT. Secondary load reflected to primary is RB/1000000 or 1 million times less! Here we do have primary load current possibly leading to saturation if the Vs is not enough to drive the secondary current through the secondary circuit.

There are volumes to be said on this topic. You can also search for the many posts here on CT performance, etc.


Regards.

 

[stevenal]

In my line of work I deal with power transformer ratios that commonly exceed 20 and CT ratios less than 100. Saturation is a voltage effect with both devices. Since voltage is related to current in a CT, current causes saturation in this device. The voltage on a power transformer is that of the system, and no amount of current will take it higher so saturation is rare. If the too low a voltage rated power transformer were selected, it would saturate and an over-current situation could result. The biggest difference is how the devices are connected to the system. Power transformer primaries are connected in parallel with source and other loads, CT primaries are connected in series.