Sympathetic Inrush Current in Transformers

When a power transformer is energized, it will result in a transient magnetizing inrush current flow in winding. (6-12 times full load current - times increasing with decreasing power MVA rating) This switching in operation of one transformer can result in an induced inrush current in a nearby parallel connected transformer (only primary in parallel) which is already in an energized condition. Secondary can be in parallel or not, can be in loaded or un-loaded condition.

This phenomenon in transformers was first analyzed 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 as '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. This sound is from the core saturation by DC current circulating between the units (B&C) connected to the same bus. Same increased noise level in an upstream power transformer (A) can also occur during the energization of a downstream transformer which is in parallel with a similar unit already in service.

(Courtesy: G M Kumbhar,2007)

2) Mal-tripping of differential relay. This will happen especially when one common differential relay is protecting two parallel transformers. In some rare cases, tripping can occur even with individual relays esp if the  connected grid is weak - The net line current, feeding the two transformers ( B&C) will be symmetrical (sum of asymmetrical inrush current in energized unit and asymmetrical sympathetic inrush current in the unit already in service)- devoid of any second harmonic components - will result in tripping of common differential relay.

3) Tripping of under voltage 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 energized 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 also, 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 transformers. The inrush current and consequent emanating noise will be alternating between transformers in each half cycle as per below current oscillograms.

(Courtesy: H S Bronzeado, 1995)

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 until 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 over current, bypassing the harmonic restraining function of relay.

For Further study:

1. C D Hayward, Prolonged Inrush currents with parallel transformers affect differential relaying, AIEE Transactions Vol60, 1941 Pages 1096-1101

2.Ake Carlson. Transferred Saturation, ABB Review 8/9 ,1990

3. S V Kulkarni, Influence of system operating conditions on magnetizing inrush phenomenon of transformers. Trafotech 1994, Session V1-Paper 4

4. H.S. Bronzeado, R.Yacamini, Phenomenon of sympathetic interaction between transformers caused by inrush transients, IEE Proc.Science.Mesaure.Technical Vol 142-4,1995

5. H S Bronzeado, P B Brogan, R Yacamini, Harmonic Analysis of transient currents, IEEE Transactions on Power systems Vol11 No4,1996 Pages2051-2056

6.J Pontt, J Rodriguez, J San Martin, R A Guileva, Mitigation of sympathetic interaction between power transformers fed by long overhead lines caused by inrush transient currents, IEEE 2007

7. G.B. Kumbhar, S V Kulkarni, Analysis of sympathetic inrush phenomena in Transformers using coupled field circuit application, IEEE 2007

8.JInsheng Peng &others, Influence of sympathetic inrush on voltage dips caused by transformer energization, IET Generation, Transmission and Distribution Vol 7 Issue11, 2013

9.H.S. Bronzeado &others, Sympathetic interaction phenomenon in series connected transformers, Confererencia Brasileira Qualidade deEnergia Electrica, 2019