Dry transformers inrush currents

[lz5pl]

I have a problem with relay protection settings for two dry type transformers 6/0,4 kV, 2000 kVA.
Problem is in very high (according to me) inrush currents and very long period of the process. Transformers are protected by 7SJ62 relays (Siemens). No problems with O/C protection as 2-nd harmonic blocking is used. But we have several trips during the commissioning from E/F protection. Transformers were switched on off-load several times and sometimes they trip, sometimes not. You can see disturbance records in COMTRADE format. Earth fault current is measured via CT's connected in star (no cable-type transformer). E/F protection trips from unbalanced currents during the energizing process.

What makes me to worry is the high level of magnetizing currents and long periods of the process. On the records they are cut after approx. 300 ms, but due to E/F trip. Colleagues from site reported for processes longer than 1 s and phase currents over 1100 A, with unbalanced current in E/F input more than 150 A (E/F protection temporary disabled). It seems quite high values for me. From my experience the heaviest inrush I have found was on 40-years old Chinese oil-filled transformer, and it was not so heavy as these ones. I have no experience with dry transformers. I have listened that their magnetizing is harder than oil-filled trafos.
Any advice how to solve the problem will be very helpful.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[slavag]

Hi Plamen.
What is a setting of 51/50N function ( current and time)?
What is a CT size?
Are possible used in this type of relay internal ZC current calculation with 2-nd harmonic restraint?
Best Regards.
Slava

 

[lz5pl]

Hi Slava,

CT's are 200/5 A.

Settings of 51N function are (primary values):

1307 51N Pickup 20 A
1308 51N Time Dial 0,05 sec
1310 Drop-out characteristic Instantaneous
1311 IEC Curve Normal Inverse
1304 50N-1 Pickup 30 A
1305 50N-1 Time Delay 0,05 sec

What do you mean with ZC current calculation ?

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[slavag]

Hi Plamen.
Few points:
1. Phase inrush seems "O.K.", not longer and not high value.
2. I0 seems very starnge, we don't see phasor enough, but maybe your guys have mistake in one CT connection.
Ask them check polyriti again , but with all 3-phase in series connection and read data in Io. or connect trafo to load and check I0 and send to you DR.
3. From my point of view time setting 0.05sec is very low.
4. I meant option of I0 calculation internally into relay instead Holmogreen connection.

dry trafo have inrush from 200 up to 400ms, from my experience.
Best Regards.
Slava

 

[slavag]

BTW, additional Q.
What is grounding system of this trafos on the 6.6kV: solidly, resestive, ungrounded?
What is a vector group of trafo d/y, d/z or..?
Best Regards.
Slava

 

[AMBMI]

l5pl is saying that the E/F element is tripping in 300 ms.
It means that the 51N element is tripping and that the I0x3 is more than 20 A but less than 30 A. The peak above 30 A is lasting less than 0.05 s, otherwise the 50N element would trip....
Anyway I agree with Slavag: as general rule the E/F element should have at least a 0.1 s delay (sometimes a 0.2 s delay could be also OK)

 

[RRaghunath]

lz5pl,
"Earth fault current is measured via CT's connected in star (no cable-type transformer)"
What does the above mean - is the transformer primary star-connected!
Stability of earth fault protection during transformer energisation is an issue, especially when the protection is connected in the residual circuit of phase CTs. This is because during the magnetic inrush one or more of phase CTs are expected to saturate giving rise to unbalance current in residual output to the earth fault protection.
The solutions notrmally suggested are
1) Add a stabilising resistor (similar to REF protection calculation) in connection to the relay to ensure that the relay is stable even under CT saturation conditions.
2) Add a time delay, typically 0.1sec and if required even 0.2sec

 

[vima]

Try to use the poarised voltge for the earth fault relay. This can avoid tripping while energising.

 

[lz5pl]

Thank you all for your suggestions. I tried several ways to clear the problem. Final solution was to use 67N instead of 51N function, with "Uo AND Io" mode, so presence of Uo is used just as a start criterion for real earth fault.

The only remaining problem is why inrush current detectors set to 15% drop-out with 35% or more second harmonic present in the currents. But it is a question to relay manufacturer.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[SMB1]

Hi,
Actually, i have seen this. 7SJ62 51N relay operated due to long duration of inrush current.

As a matter of fact, the 2nd harmonics function in 7sj62 doesn't block the function operation but it blocks the trip signal. Unlike for example REL505 relay, which 2nd harmonics detection blocks the function from operation.

 

[slavag]

Plamen, thanks a lot for the feedback.
Regards.
Slava.
Hmmm, 67N, I start think, that this option is future option for many projects.