Will DC current through a window/donut type CT damage the CT in anyway? 11

[edison123]

If DC current flows through a window/ring/donut type current transformer shown below, will the CT get damaged?

Will its magnetic properties get affected?

Can it be used again for AC current?

Thanks for inputs.

Muthu

http://www.edison.co.in

 

[FacEngrPE]

A current transformer is only sensitive to the changing (di/dt) portion of the current, so no it will not get damaged by high DC currents. A current transformer can get saturated from a large DC current which could result in inaccurate measurement of the AC component.

If you need to measure the DC current, this is a list of sensor types to investigate.

Source

https://dewesoft.com/products-news/dc-ct-current-transducer-technology-explained

I have no experience with this particular source.

(edit) There is some literature discussing demagnetizing transformer cores (

https://www.tdworld.com/substations/article/20964214/reliable-demagnetization-of-transformer-cores)

it is likely that if residual magnetism in a CT core creates signal distortion, it should be possible to demagnetize a CT with a similar tool.

 

[edison123]

Thanks, FacEngr. I will check with this method.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. edison123 (Electrical)(OP)6 Aug 22 04:58.
" #1. If DC current flows through a window/ring/donut type current transformer shown below, will the CT get damaged? ...#2. Will its magnetic properties get affected?... #3. Can it be used again for AC current?"
#1. The CT would NOT be damaged. Caution: when the primary is connected to dc supply, the conductor would be carrying heavy dc current. The conductor would over-heated due to heavy dc current; as there is no ac back emf. The dc current value would be Idc = Vdc/R, where R= conductor resistance + load resistance. The value of inductance is zero.
#2. I am of the opinion that its magnetic properties does NOT get affected.
#3. Yes. It can be used again for AC current.
Che Kuan Yau (Singapore)

 

[RRaghunath]

Sir, CT secondary resistance measurement is done by applying DC across its secondary terminals.
After the test is completed, it is the practice to demagnetise the core using hysteresis method.
The CT testing equipment available in market can also demagnetise the CTs.
Hope you are not meaning passing of large DC current through the CT! Large DC current can damage the CT core due to overfluxing and also cause thermal damage to the CT secondary winding (depending on the duration that the DC current was flowing, of course).

 

[edison123]

Thank you, che and Raghunath. The CT is rated for 1000/7.5 A. For drying out the generator stator winding, about 300 A DC was passed through the winding for about 8 hours (4 hours in one direction and then 4 hours in reverse direction). The secondaries were shorted during this time. Hence, my doubts whether the magnetic characteristics of CT got affected by this long time DC current.

Raghunath - Why would the secondary be damaged?

I will be testing the CT's with primary injection kit shortly and will let you know the results.

Muthu

http://www.edison.co.in

 

[scottf]

A relatively high amount of DC current in the primary over a period of time could certainly damage the CT...this is due to high heat generated in the core due to saturation.

Reading Edison123's last post, 8 hours at 300A DC might be enough to damage the CT. It would really depend on the CT's design. At the very least, it could have shortened the life of the CT.

 

[LionelHutz]

How does a constant magnetic field heat the core?

 

[RRaghunath]

Sir,
I misunderstood your initial post. If you passed the current in the primary bar conductor, that cannot affect secondary winding as there is no induction from primary to secondary (the current being DC).
But, I am not sure of the core. Even though the core may have got saturated / overfluxed,, there cannot be hysteresis and eddy current losses in the core (flux being constant - of DC).
We will know when the CT is tested for knee point voltage and magnetising current check. If demagnetising the core works to bring the parameters near to factory tested values, that means there is no damage to the core.

 

[che12345]

Dear Mr. edison123
@ all learned advisers
"....#1. The CT is rated for 1000/7.5 A. For drying out the generator stator winding, about 300 A DC was passed through the winding for about 8 hours (4 hours in one direction and then 4 hours in reverse direction)....#2. The secondaries were shorted during this time. Hence, my doubts whether the magnetic characteristics of CT got affected by this long time DC current....#3. .. Why would the secondary be damaged?...#4. I will be testing the CT's with primary injection kit shortly and will let you know the results...."
#1. The CT is rated 1000/7.5A. The DC current is 300A for 4h and reverse after 4h. I guess that the Idc = 300A, which is << 1000A. Therefore, the [primary single turn conductor or bar] would NOT be heated. I doubt the iron core would be heated up at all, for any reasons
#2. " The secondaries were shorted during this time... "
I am of the opinion that the [secondaries are shorted or open] is immaterial, as there is NO induction with DC. Therefore, there is NO induce voltage (that may cause high voltage insulation breakdown) or current (that may cause over-heating).
#3. I am of the opinion that there is NO reason of any damage; see #2 above.
#4. We look forward to your empirical test result.
Che Kuan Yau (Singapore)

 

[edison123]

Update:

We sent the 3 CT's to OEM for testing. Both in as it is condition and post demagnetization, the ratio test and ratio & phase angle errors etc. were the same as per original OEM specs. The CT's also passed 3 KV AC HV test to earth.

In conclusion, no magnetic and electrical damages happened to any of the 3 CT's by passing high DC current for a long time.

I thank you all once again for your valued tips.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. edison123 (Electrical)(OP)9 Aug 22 12:21
"...Update: We sent the 3 CT's to OEM for testing. Both in as it is condition and post demagnetization, the ratio test and ratio & phase angle errors etc. were the same as per original OEM specs. The CT's also passed 3 KV AC HV test to earth. ....In conclusion, no magnetic and electrical damages happened to any of the 3 CT's by passing high DC current for a long time."
Thank you for the update and conclusion.
Please clarify/advise whether any effect on the knee-point voltage ?
Che Kuan Yau (Singapore)

 

[edison123]

che - no change in any of the original parameters.

Muthu

http://www.edison.co.in

 

[iop95]

A risk for core overflux and an open secondary overvoltage was at switching on/off load with 300A.
I don't think that there was a variable source voltage and slowly increase / decrease load current...

 

[edison123]

iop - The DC current was raised and lowered slowly via a variac+bridge rectifier. No sudden DC.

Muthu

http://www.edison.co.in

 

[iop95]

edison123 - in this case very low risk.

 

[che12345]

@ iop95 (Electrical)10 Aug 22 08:25
" #1. A risk for core overflux and an open secondary overvoltage was at switching on/off load with 300A....#2. I don't think that there was a variable source voltage and slowly increase / decrease load current..."
#1. Can you kindly enlighten how a) a risk for core over-flux and an b) open secondary over-voltage was at switching on/off load with Idc 300A.
a) Consider that the CT iron core is rated for 1000/7.5 A. Where 300Adc is << lower than 1000Aac.
b) When the DC is switch on/off with a load, the rise/decay time is slowed down by the load. What can cause the risk of open secondary over-voltage ?.
#2. edison123 11 Aug 22 01:22
Advised that The DC current was raised and lowered slowly via a variac+bridge rectifier. No sudden DC.
Che Kuan Yau (Singapore)

 

[iop95]

@ che12345
a) It's a risk not a certainty.
Depends of power supply voltage, impedance, regulation (overshoot) and total stray capacity of setup (cables/load).
At sudden connection to a DC voltage source may expect a higher current spike than CT limit, but depend of above factors.
Same issue for overvoltage. A high di/dt may overpass CT voltage limit if secondary is open.
b) At slowly voltage increase / decrease there is a minimal risk for overvoltage at open CT secondary.
For CT, normal regime is shot-circuit so an open seconday may expect overvoltage even at minimal primary current variation as may be when variac mobile contact is interrupt or from any setup/test operation error that may lead to suddend cable / contact open / power grid off, etc.
Hard to belive that test/drying was performed from an online UPS to avoid any risk in the event of power outage.
A CT must always have a load (or seconday SC).

 

[prc]

1) High DC current flow will certainly create high residual flux in the core due to saturation but not any losses as current is not varying. Since it a bar primary CT , heating from DC current is not an issue.
2) A similar myth is, oil filled EHV CT may explode in case secondary circuit is broke open inside the CT. There may be arcing due to secondary opencircuit, but the arc energy is so low to create any CT explosion.

 

[RRaghunath]

I have seen a 400kV CT secondary terminal burnt due to loose connection and resultant sparking.
The sparking might have been there for quite sometime. Someone passing by in the switchyard noticed the noise and initiated action to shutdown and inspect.