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Yes. That is what I had suggested from my manufacturing background.

The inrush current is maximum when the switching is done at zero of the voltage waveform. Simulations are done to achieve the maximum peak of inrush.

Oil's resistivity is less than that of air. The IR ratio in an air-to-oil-filled condition was 1:20. But with modern oils of high resistivity, this is now around 1:10.

If the tertiary is not loaded, one corner is kept earthed to avoid power frequency transferred overvoltages. An earthing transformer is used for extensive tertiary lines. Neutral earthing through the resistor is used to limit ground fault current, and the reactor is used to limit arc flash...

Most of the reported OLTC failures were in auto-transformers where the line end tap-changer is at a high voltage of 132 or 220 kV. OLTC is now being eliminated in India in 400 kV and 765 kV auto-transformers.

Theoretically, you can save material by going for an auto connection. However, the savings will decrease as the voltage ratio increases. It is almost 1:3 here(limit around 1:2), and you require both DETC and OLTC. Hence, I will suggest a two-winding transformer. We have already discussed...

Waross, thank you for the explanation. I have a question: Why is the primary neutral of T1 kept ungrounded? If it is grounded, is there no longer a need for a stabilizing delta winding?

Thank you, Scott. I was waiting for your response. What I meant was an oil-filled dead tank ( hairpin ) CT. When such CTs fail, there is normally an explosion with porcelain shattering. Many power engineers interpret that such failure can happen from open circuiting of the CT secondary. Post |...

It is well known that open-circuiting the secondary circuit of CT while carrying primary current will create overvoltage on secondary terminals. But will this lead to violent failure of CT with explosion? Has anyone noticed CTs in service with open-circuited secondary terminals?

The need for a tap changer depends on the voltage variations required on the secondary side, the voltage fluctuations expected on the primary side, and the characteristics of the grid. The purpose of a tap changer is to keep the voltage at the consumer (load) end constant. The secondary...

Answer to your question 2&3 Avoid a cable connection between the GIS and the transformer. Either directly connect the transformer to GIS by oil to SF-6 bushing or connect the transformer to GIS through a short overhead line with a surge arrester in between them.

Cranky, that tank current was 800 A !

You will find a current in the transformer's neutral connection to the substation's earth mat. When measured by a tong tester, this current was up to 100A in power and substations. There will also be current in the tank-to-ground earthing connection. The maximum current that came to my notice...

The OTI and WTI alarm limits are decided based on the maximum ambient temperature of the particular location plus actual top oil rise and winding hot spot temperature rise during the temperature rise tests. Maximum ambient air temperatures per IEC/IEE/IS are 40/40 and 50C. The average winding...

1)A temperature rating of 55/68 C was never used. From 1950 to 1980, transformers in the US were rated 55/65 C based on the average winding temperature rise of 55 C for transformers with ordinary paper for conductor insulation and 65 C for thermally upgraded paper. Later, only 65 C was used. 2)...

You can use the transformer at 12.47/7.2 kV condition without any tap changing. If situation is 13.8/7.98 kV , then also you can use as such but transformer will be over fluxed by 5 % . This is acceptable with extra iron losses and noise. But if you want 7.6 kV with 13.8 kV incoming, put the...

An ISO standard ISO12944 series covers various corrosive environmental conditions and recommends a paint system for each category. Your system will come under CX class (ISO 12944-2), and the paint usually used is two coats of two-pack epoxy primer + two coats of two-component epoxy intermediate...

When the insulation deteriorates, IR usually decreases, with an increase in the tan delta. Please check the measurements and insulation temperature at the time of measurement. IR decreases when the temperature goes up (halving for every 10 C), But the tan delta increases with the temperature.

Are you sure whether this 500 Hz component is from the core? Other vibrating fixtures on transformers also may give out noise, sometimes with a resonance frequency.

1) Thank you, bacon4life, for showing that the difference in loading between the HV and LV windings is significant. A star for you! But transformers are capable of handling such differences efficiently. Instead, the PF and reactance kVAR should be considered while selecting the kVA transformer...