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Topic Title: Sympathetic inrush
Topic Summary: PROTECTION MALFUNCTION
Created On: 12 April 2018 05:54 PM
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 12 April 2018 05:54 PM
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f13tun

Posts: 16
Joined: 14 March 2018

Could any one simply explain the Sympathetic inrush that can appear in one transformer T1 connected to system in parallel during energization of other in parallel transformer T2 because this energization caused the trip of T1
 12 April 2018 06:08 PM
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mapj1

Posts: 11566
Joined: 22 July 2004

Perhaps, but it would be good to know more about the exact set-up and sequence of events

Are T1 and T2 in // on the primary or the secondary, or both.
What sort of protection device tripped - primary current, secondary voltage, neutral earth bounce, other , i.e. what was being sensed and what got interrupted ?

What sort of transformers are these 3 phase or single, e.g. big substation 11kV to 230/400, building sites 230 to 110, or data centre star delta 400/230, or 12V ELV lights, I presume its safe to assume not redundant supplies for bell transformers for summoning the servants..

Was - T1 up and running,? Loaded or not ? When T2 was added primary first or secondary first.?

More details would help bound the problem.

-------------------------
regards Mike
 12 April 2018 06:49 PM
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f13tun

Posts: 16
Joined: 14 March 2018

1-T1 AND T2 in // on the PRY and SEC .150MVA ,220KV/66KV/21KV
2-Restricted earth fault tripped. I don t remember exactly primary or secondary or tertiairy.
3-Was - T1 up and running,? YES
4-Loaded or not ? YES LOADED connected to 66 kv busbar and the tertiary is connected to 21 kv reactor
5-we start always by closing the HV than LV .so 220 kv CB than 66 kv CB than 21 kv CB

I know it requires many informations specially from disturbance recorder My original question is about a simple explanation of Sympathetic inrush current because i could' nt find other causes in the internet but if you have other explanation please help me
 12 April 2018 07:39 PM
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Jaymack

Posts: 5510
Joined: 07 April 2004

Originally posted by: f13tun
2-Restricted earth fault tripped. I don t remember exactly primary or secondary or tertiairy.

Isn't the REF, simply 2 sets of individual unit protection, monitoring each transformer?

Regards
 12 April 2018 09:17 PM
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mapj1

Posts: 11566
Joined: 22 July 2004

OK, big HV stuff. More fun


Have I got the picture right in my head ?
circuit
star in star out, delta interwinding?
Or is it an autotransformer ?

So presumably we expect any out of balance neutral current or 3rd, 6th, 9th harmonic contribution to run round the interwinding and this see the reactor (external inductor), so this controls the maximum out of balance current, but does not affect a well balanced load. (as a well balanced 3 phase load gives a voltage that sums to zero on the interwinding.)

As the inner loop is closed by an external inductor this also will give some phase shift and energy storage - and here is part of the issue
And you have 2 of these, I presume with separate, not shared, reactors.

Closing HV side first is correct, as if you did it the other way could be trying to magnetise the 'empty' transformer with current that appears as a secondary load on the first.
So the only interconnection is at the primary side.
Depending on the phase of the incoming supply when the primary contactor closes the core will need to suddenly go from non magnetised to part way magnetised one way or another, and this is the normal inrush.
In the normal way of things the large inrush current rushing into the primary pulls down on the supply of primary voltage for the duration of the transient.
But you have energy stored as the magnetic field in the core of the transformer that is already on and the coil on the interwinding - and so as well as the inrush taking energy out of the incoming supply the voltage is also propped up by current flowing backwardsout of the transformer that is already energised, and this sudden current reversal is a large part of the problem.

Also presumably the outbound earths and neutrals are connected all the time,and the phases switched ?
There will also be interwinding capacitances that charge suddenly and may try to pull up the floating secondary as if it was one plate of a capacitor and the primary the other.

Edited because I find I can't link direct to a dropbox image.
side note.
At lower powers I am used to limiting inrush by switching at the zero voltage crossing for loads that are capacitors or resistors, and near top dead centre for inductive loads, but this only really works up to about 1kV or so due to limitations on what is sensible with semiconductors. ( three phases are then switched in sequence, 120 degrees apart)
My only experience of transformer inrush control on a system at higher voltage was very small fry compared to what you have here. There we had two sets of moving contacts, a 'burny' set that made first and broke last that connected power to the primary via some current limiting resistors, and a 'clean' set that closed to short out those resistors a cycle or so later. Should the second set fail to close the resistors would blow themselves up in short order, so it was not a fun thing if it went wrong.

-------------------------
regards Mike


Edited: 12 April 2018 at 10:19 PM by mapj1
 12 April 2018 10:27 PM
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alancapon

Posts: 7389
Joined: 27 December 2005

If the transformer tripped on REF, then it is likely that there is a real problem. Assuming the second transformer has been energised before, and therefore the CTs are the correct way round and the wiring is correct, there are few possibilities. It is possible that there has been a real fault "in zone" of the REF, perhaps a dirty bushing, a moisture issue on the bushing / cable termination, or a fault within the tank. It is possible that one of the REF CTs is damaged. It is also possible that the REF CTs are incorrectly specified, and one of them saturated due to the inrush current. Ideally you want to carry out testing of the REF protection and send an oil sample from the tank away for analysis before you attempt to re-energies the transformer.

Regards,

Alan.
 13 April 2018 12:07 AM
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mapj1

Posts: 11566
Joined: 22 July 2004

Isn't a Restricted Earth Fault trip the same as saying we have unexpected neutral current in or out of the star point in this context ?
If the star points of the 2 transformers are connected during energisation, even of the phases are not, then as the interwinding will probably magnetise assymetrically during power up this will force an out of balance current to flow (because the three phases will not come on quite together.)
I think it is quite possible to have perfectly balanced (neutral free) supply, and the same for exit, or even no real secondary load at all but by dint of equal and opposite circulation of interwinding currents, a neutral current in both secondaries.
Or is it like last time we discussed this and I have missed a trick?

-------------------------
regards Mike
 13 April 2018 07:57 AM
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alancapon

Posts: 7389
Joined: 27 December 2005

No, think of REF as the equivalent of a very sensitive RCD. It will be split into several bits, with a separate CT for each of the phases, and a further one on the neutral. These are either connected in parallel for a high impedance scheme and to a simple voltage moperated relay, or they could be taken into a numeric relay individually for the relay to do the maths. Typically, these are very sensitive fast acting trips.

Regards,

Alan.
 13 April 2018 09:59 AM
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Jaymack

Posts: 5510
Joined: 07 April 2004

Originally posted by: alancapon
No, think of REF as the equivalent of a very sensitive RCD.

To elaborate, and for the edification of the masses ..... it is known as Unit protection, in that it is localised, earth fault protection designed for the protection of a single Unit, e.g. a transformer, hence the term restricted.

I question whether the original installation had been commissioned properly.

Regards,
 15 April 2018 10:41 PM
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williamjohn

Posts: 196
Joined: 22 November 2010

Is it possible that the transformer neutrals are connected within the REF zone? The neutrals should be connected to earth outside the REF zone.

Edited: 15 April 2018 at 11:00 PM by williamjohn
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