I don't have any personal experience, but the concern would be transformers in the same substation or quite close by. The dc in the inrush current can saturate the other transformer. For differential relaying using harmonic restraint, the problem is that this current in the energized transformer is low in harmonics, so the restraint is not effective.
I believe there is some discussion in Blackburn's book. There may be a little discussion in here:
A case and point would be two transformers protected by the same differential (I seen this). It dosen't work because the harmonics between the two transformers is never is seen by the CT's and will faulse trip.
Where you have two transformers with different differentials, you should mind the senisitivy of the harmonic restraint.
another time an utility require a study of inrush current to verify if our 230kV substation ,distant 110km from utility substation , could affect inrush magnetizing sympathetic current at utility substation.
I guess they had some experience about that in the past.
I lost that study.
I would recommended you find documents of Mr. Herivelto Bronzeado, He is user here too ( I think, you are from Brazil too).
Lot of information on the issue.
As dpc said, problem is muloperation of differential protection, but today is not problem
I found th study! I am sorry. I make a big mistake!
It was not sympatethic but normal magnetic inrush in a transformer distant 4.72 km from source:
Summary of study
AHE Stone Horse has two units of three-phase step-up transformers 13.8 / 230.0 kV - 90.0 MVA, Yn-d1.
In the initial phase, these transformers are energized by the secondary (230 kV) by TL 230 kV Governor Mangabeiras SE / SE Stone Horse (4.72 km), aiming to use the primary (13.8 kV) for powering loads of ancillary services.
The present work aims to analyze the transient electromagnetic energizing this through simulations with the "ATP - Alternative Transients Program"
slavag,
dpc said restraint is not effective. So, it could operate erroneously.
From experience, one inconvenience for the system, when simpáthetic magnetic inrush occurs in GSUT of power station(CCGT with big GSUTs) connected to fairly weak 400 kV system, huge voltage step change affecting the distribution network in the area, even with decent SVC connected in the area 2x -75/+150 MVAr.
We have to buy synch comps on to allow the power station to come on and run fault level studies every time they need to close the 400 kV CB to come online, depending on the outage pattern and what plant is already running on the day.
May you grow up to be righteous, may you grow up to be true...
When a power transformer is energised,it will result in a transient magnetising inrush current flow(6-8times full load current) in winding.This switching in operation can result in an induced inrush current in a parallel connected transformer( only primary paralled )which is already in an energised condition.Secondary can be either in parallel or not,can be loaded or not.
This phenomenon was first analysed and reported in 1941 by C D Hayward,the GE engineer who developed the harmonic restraining feature in transformer differential relay(1938).He called it 'prolonged in rush currents with parallel transformers' ABB termed it 'transferred saturation of core' (ABB review 1990 aug-sept) and some others (France) called it transmitted saturation of core.In mid 90's H S Bronzeado and other south American engineers started calling it sympathetic inrush current and it seems this term is gaining popularity,may be due to the emotional overtones associated with the term.
The operational consequence or disturbance effect of sympathetic inrush current is normally harmless,but sometimes can result in problems.These are
1)A prolonged humming sound in the already working transformer.Normally this will be the main complaint from user who will suspect some defect in the working unit- due to core saturation by DC current circulating between units.
Increased noise level in an upstream power transformer during energisation of a down stream distribution transformer with paralled similar unit alreay in service was also reported
2) Maltripping of differential relay. This will happen especially when one common differential relay is protecting both units.In some rare cases, tripping can occur even with individual relays esp if the connected grid is weak- The net line current,feeding the transformers will be symmetrical( sum of asymmetrical inrush current and asymmetrical sympatetic inrush current)- devoid of the second harmonic components - will result in tripping of common differential relay.
3) Tripping of undervoltage relay on transformer secondary- Inrush current produces a distorted voltage on primary/secondary bus.
Sympathetic inrush current can be explained as below:
When a transformer T2,connected to the same bus as an already energised transformer T1, is switched on, inrush current flows in T2.This asymmetrical current drawn from the generator causes a distorted voltage drop in the transmission line resistance which affects the terminal voltage of T1 unit already in service.Since the flux in the core is proportional to the area (integral)of the voltage waveform at the winding terminal,the core flux becomes asymmetrical,resulting in an inrush current in T1, a few cycles after switching in T2. Hence sympathetic inrush current depends on the resistance of the feeding line.Higher the resistance(ie weaker the system) higher the sympathetic inrush current.The magnitude of this sympathetic inrush current is much lower than a normal inrush current,but stays for much longer duration in both units.
Then DC component of the inrush currents start circulate in the loop formed by the primary windings of T1&T2.Since the direction of this Dc current flow in T2 is opposite to that of T1, the cores get saturated in opposite directions and hence the current peak in the two windings occur on alternate half cycles in opposite directions.Inrush current persists untill this DC component dies down. The current drawn from the line will be sum of the inrush currents of T1 &T2 (assuming T1 is under no-load)and the same will be almost a symmetrical overcurrent, bypassing the harmonic restraining of relay.
Gunnar, great! Our pioneers were doing exactly as you did and hence they knew what they were talking about. Hayward used 333 kva transformers for his lab tests.You may like to read the original paper of Hayward- C D Hayward_ Prolonged Inrush Currents with parallel Transformers affect differential Relays_AIEE Transactions Vol 60,1941.Later papers, including Prof Kulkarni's latest paper attached with earlier post shows the waveform of various currents.
Thank you all very much for the details. I had an additional question:
What would happen if before energizing T2( assuming T1 is already energized with load), a tie-in disconnect switch between secondary of T1 & T2 is closed first. These back charging currents from T1 to T2 - would these be the same as sympathetic currents? Can these cause any problems to differential relays.
Attached is a paper which although not directly related to sympathetic inrush has a simple explanation on its cause.
The paper describes how the sympathetic inrush distorts the transformers secondary voltage and results in neutral currents which can operate sensitive earth fault protection.
