Transformer Safeties

You say it's a manufacturing company. Do you know the basics of how a transformer works, its calculations, and the per unit system? It would behoove you to read a book or two on the subject, especially since your question asks for information that is about the protection of a transformer, not how they're designed and manufactured. Good luck to you.

It may be a bit late to learn about transformer theory just before applying for a job at a transformer company.

Have you tried an internet search?

And you're worried because.....?

If they want you, they will TRAIN you!

I'm afraid this is going be the classic case of "please close the door quietly as you leave".

It's a bit late to start asking once the interview is pending. What possessed you to apply for the job?

If I were you, I'd hang onto the job you have now.

You don't have a chance in hell to land the new job.

The questions you have asked here are far too elementary to indicate any engineering competence, at all.

The questions you asked about the dredger were laughable.

You, apparently have no ability to search for answers on your own.

But, good luck. You'll need it.

What Lyn means is that CR4 is not a substitute for a proper education along with personal learning (internet, books, etc) and practical experience.

As I have told everyone who has ever worked for me, I'm happy for you if you can find a better job and I will not stand in your way.

I'll be glad to give you a truthful reference.

Same thing here. This is an engineering forum. We deal in facts.

Apart from RAMCONSULT, there are not many helpful replies so I will try "Transfo protection in a nutshell".

1. Transient over voltage. In open air rather than buried cable systems lightning strikes are likely and insulation needs protection. Simple stiff wires from line insulator and earth sides at poles and transfo make a basic spark gap. The system needs "automatic break and re-close" breakers feeding the line [unless manual reclose after hours is acceptable]. Better surge arrestors [on or close to transfo] have enclosed spark gap and voltage dependent resistors in series with the gaps that limit arc duration - so supply breakers do not trip.
2. Overheating/overloading: Oil temperature Indicators. Winding temperature indicators have a temperature sensor under the oil with a heater coil(s) which simulates the effect of load current on windings. Alarm and trip (shutdown) contacts may feed supervision and breaker systems. Actual temperature sensors in the windings have problems with insulation in HV transfos.
3. Primary Fuses - protect the system but cannot do much to protect the transfo except stopping a fire. Can clear quickly and limit energy and peak current of "arc welder" let loose by short circuit faults. Limited to small current ratings (roughly <100 amps) and about 20 kV. Common on voltage measuring transformers and smaller distribution transformers. Secondary fuses effective for short circuit but not overload protection. Thermal fuses useful inside small house voltage transfos, which can be scrapped if the fuse opens due to overheat.
   
4. Oil/gas relays (bucholz). These go in a sloping pipe between the winding/core tank and the oil reservoir above it. Gas bubbles rise into the relay body above the pipe and displace oil until a float drops and moves an alarm contact. Localised overheating and insulation electrical leaks cause gas generation. Gas/oil analysis can suggest cause and time to failure (say insulation versus core fault). A pivoting vane across the pipe swings round when hit by a surge of oil and gas from a heavy fault, moving a trip switch.
5. Overcurrent relays. These can give useful overload and short-circuit detection for bigger transfo above roughly 1 MVA. Current transformers - CTs - are needed to feed them (with problems of transient saturation errors from the usual iron core - modern electronic relays can use Hall effect or air cored "Rogowski" coils) and an expensive breaker is needed upstream to take action. "Inverse time" characteristic gives faster operation for bigger currents and matches the thermal withstand capacity of windings to avoid tripping except when essential.
6. Differential relays use CTs at inlet and outlet of transfo and compare the currents to sense current "lost" in the transfo due to fault. Also applied to primary or secondary winding leads alone as "restricted earth fault". This can act much faster than simple overcurrent. Inrush currents in the primary at voltage-on are asymmetric, may peak at currents similar to normal load and do NOT come out at secondary. False operation at Inrush has to be countered. Despite the problems, differential relays have been very successful.
7. Modern "digital" relays can apply a combination of techniques from available CT and voltage signals in one box.
8. Over-voltage protection and "voltage to frequency ratio" [normal voltage at half frequency = heavy iron saturation] protection are necessary on generator transformers, obviously protecting most downstream transformers.
9. Breakers and all this sensing kit are expensive and use depends on a) cost of transfo b) cost of transfo being out of service c) risk of personnel/public injury, damage to facilities or fire.
10. Fences, barriers and blast walls are also part of the protection, not entirely for "electrical" reasons. For equipment and personnel safety, earth rods, earth bonding and earth mats are essential, else earth faults may not be detected, could exist for long periods with dangerous foot to foot voltage difference on ground or ground to tank.

Good fortune with your interview.

67model

Not much to go on here. If you are making transformers why would you be that interested in the protection unless they are particularly bespoke.

10VA transformers, 100VA, 1MVA, 350MVA? Some of the punters have mentioned oil temp and bucholtz. Are your transformers wet or dry? What Voltage are you looking for? 50Hz, 400Hz? ONAN or ONAF and what does that mean? Your question tells us very little.

Tap changers?

Protecting a transformer is a different field altogether.

If you don't know the answers to these it will take the guy doing the interview seconds to suss you.

Probably good to Google Impedance as well.

There are many safeties involved for protecting power, distributed and other types of transformers, these safety measures are like wearing rubber gloves, helmet, and rubber shoes while repairing the transformer, make safety around your home, inspect the wire of equipment before use, use safe work practices every time when you use electrical appliances, limit the use of extension cords, use multi-plug adapters equipped with circuit breakers and fuse.

there are many ways to protect a transformer but back to the question from the punter - he wants to make transformers not protect them. Likely some of the kit would be on the transformer - possibly a Bucholtz, maybe an oil / core temp but from the question you can't determine the size, the voltage, the type and as such cannot come up with a recommendation.