A81-Ghost Sounds from Transformers

Sometimes transformer users complain of ghost sounds (abnormal noise) from the transformers. The abnormal noise may be sporadic during service, or it may occur only during specific loading patterns on transformers e.g., overcurrent or overvoltage or with inrush current during energization.?Then the usual ‘music’ of the transformer changes with the pitch and rhythm of the notes starts varying.

Transformer sound level will go up when the flux density in the core goes up. The single parameter that affects maximum the transformer noise level is the core flux density. When the flux density goes up, magnetostriction (elongation and contraction of core laminations) goes up emanating increased noise. Flux density in the core varies as a function of V/f and it goes up when V (applied voltage on winding) goes up or f (frequency of the grid) comes down. So, with V/f higher than rated conditions, excessive howling noise will come out of the transformer due to the over-fluxing in the core.

When a transformer is switched on esp. when the applied voltage wave crosses zero and with the residual flux in the core, the excitation current will shoot up by 8,000 to 12,000 times (8-12 times full load rated current), and this momentary peaky inrush current generates excess core flux density and consequent higher noise. This will come down with the inrush current decaying out. In weak power grids (i.e., with high impedance or low fault levels) when a parallelly connected transformer is switched on with another nearby unit in service, both units may start a musical sound show, coming out alternately from these two paralleled units. This ding-dong sound is due to a phenomenon known as sympathetic inrush currents. The Inrush current apart from the switched-on unit also starts flowing in the running unit, with the inrush current peaks shifting between the two units. Both units give out high sound, the timing shifting between the units. This sympathetic inrush current flow may continue for a longer period than the time for the usual energization of a single unit.

Strange sounds can also come out when the varying amplitude surges (over-voltages) reach the transformer terminal. This can come from the arcing or defective contacts of the primary circuit breaker. ??The resulting overvoltage surges cause core saturation and increased noise level. Sometimes a phenomenon, known as Ferro-resonance can also contribute to high voltages on transformers and consequent higher sound. Please see Chapter 5.2 and Annexure B of CIGRE Technical Brochure No.569-2014 Resonance and Ferro-resonance in Power Networks. Ferro-resonance is the oscillation between the non-linear inductance of the transformer and the capacitance of the energised transmission or distribution lines. Ferro-resonance is a harmful low-frequency oscillation where a non-linear reactance from the transformer drives the iron core into saturation and oscillates with the circuit capacitance, giving rise to severe overvoltages. When enough energy is provided by the coupled source to compensate for the circuit losses, this oscillation can be sustained indefinitely. This can also happen in distribution transformers with particular winding connections and with energisation in one or two phases only.

Another type of abnormal noise is the ghost sounds that come out of the transformer which may scare the operating staff. This may remain for short periods usually during the switching on or off of the unit or during load-changing occasions. This may be a?vibrating metallic sound or a tuck-tuck hammering sound from a particular side or area of the transformer. In the initial period of my career, I had to handle such a ghost sound from a transformer and let me narrate it in some detail for the lessons that I learned. The Mumbai-Ahmedabad sector (Virar to Sabarmati)?of the Indian Railway was electrified around 1970 and all the trackside supply transformers were supplied by my company in the late 1960s. These were 10 and 12.5 MVA 132/25 kV single-phase units with DETC on the HV side. Today due to rising traffic, the rating of similar transformers in IR?is 40 MVA.

One of the 10 MVA units that we supplied to the Mahemdavad substation (near Ahmedabad) developed a unique problem. Whenever the transformer is switched on or off, a peculiar dig-dog sound was coming from the transformer, which continued for some more time. One of our senior engineers visited the site and recorded this sound using a portable tape recorder that he had bought during his visit to Japan. When we heard this recorded sound, we were convinced of the ghost sound and decided to bring back the unit to the factory for investigation and rectification as the unit was still under warranty period. As the roads were not well developed in those days, transformers of even that size had to be transported by low-bed rail wagons.?But our CEO who recently took charge of the company after retiring as chief engineer of the local utility overruled experts and asked the designer to go to the site and solve the problem. Since I designed the transformer, I had to leave for the site. Till then I?never had been to any electric substation and was really terrified of what to do. Fortunately, one of our assembly supervisors was also deputed to help me in the investigation.?

At the site we switched on and off the transformer several times. After some very close observations, we found that the sound is more from the LV side of the tank. Then we got a round wooden rod from the station and used this as a stethoscope, moving over the tank side surface. We could clearly locate the sound emanating point, the bottom tank side in between two limbs of windings. From the drawing, I found this is the bottom fixing point of the tie rod that we used to clamp the upper and lower core clamps. We used a 25 mm steel rod with nuts at the top and bottom to fix the core clamp. We drained the oil from the transformer and on inspection found the bottom nut (which was locked only by thread punching the screw thread on the tie rod) had fallen down after loosening out. The hole provided on the core clamp was a large diameter hole. When the nut fell down, the tie rod acted as a pendulum with a fixed top end. When the transformer was switched on or off, the resulting vibration caused the tie rod to swing as a pendulum and strike at the metal edges of the hole on the lower core clamp. From outside, we were experiencing this sound as if from all over the transformer!

The two weeks’ stay at the site taught me many things – the importance of proper locking of fasteners in transformers, the wooden stethoscope helping in amplifying sound, and the customer is more knowledgeable than you and always right, is not a cliché, but the absolute truth!!