Enquiry About Distribution Transformer

The incoming voltage could be fluctuating for any number of reasons causing the secondary to mimic, also you could have a loose tap or wiring connection...You do not state if this under load, what type of load, or no load...

Taps can be adjusted to match more closely the voltage desired if the output of transformer does not match the voltage present...

https://electrical-engineering-portal.com/using-tap-changers-to-match-the-system-voltage

What is the frequency of these fluctuations?

Is the load connected when you see these fluctuations?

Is field wire connected?Have you measured load current during these fluctuations?

Have you HI-Pot tested the transformer primary and secondary,as well as all connected wiring and loads?

Loose connections will obviously result in a spurious output,and not having the taps in the proper places will usually result in a different output voltage than desired or an overload situation.

Insulation breakdown can cause voltage fluctuations,but at these voltage levels,it is usually very evident;Follow the flash and smoke.

Check whether fluctuation in some other tap, if it is there, check incoming voltage, if not check tap changer contacts and it’s wiring for any loose connection

Are you measuring the output voltage with a true RMS meter or a moving iron meter?

A digital meter can be subject to external influences like RF etc so an old style moving iron meter is less subject to HF influences. Fluke sell an excellent snubber to connect between the meter terminals and to the device under test which supplies a load to deaden fluctuations.

As for the tap changer not having all phases connected to the same tap then you will have circulating currents in the windings heating the coils and likely failure will happen sooner. The only way a tap changer can get out of sync is if the shaft between the switches is broken and then you have BIG problems.

If the tap switch is not making then you will have arcing and the production of gas from the breakdown of oil but if you listen on the tank then you will certainly hear it burbling. The arcing will also generate RF.

I used to repair and test transformers for nearly 20 years back in the dark ages.

Distribution transformers are designed for a maximum load current, about 3700 amperes per phase [3 phases, 400V side] in your case.

The transformer is designed so voltage falls typically 4% between zero current and maximum current with uniform loading. The voltage at the high-voltage input of the transformer is closely regulated by the supply company.

Most load equipment can operate with +/- 10% voltage fluctuation without harm - fluctuations in light from lamps is most noticeable effect.

The users on your transformer turn their loads on-off when required - so the load is always fluctuating.

417volts/405 volts = 1.030 , say 3% variation.

Possibly the load is never zero or maximum within time-span of your measurement.

The fluctuation you see is what would be expected due to load variation.

Since you gave no indication of the interval between your readings or the load current or if they were one phase alone or maximum- minimum of three phases; no further comment is worthwhile.

Off-load tap changer will put all phases on same tap - since it is not operated on-load, it cannot cause variations.

A faulty mechanical linkage is unlikely - do you have reason to think taps have been changed since installation??

Loose electrical connections would cause overheating and operation of a gas warning relay on an oil-filled transfo of this size. You have not written if transfo is "dry" or oil-filled.

I agree with 67model.

This is possible since the voltage drop from no-load to full load will be 14.6 V

According to IEC 60076-5 Table 1 – Recognized minimum values of short-circuit impedance for transformers with two separate windings-for 2 501 to 6 300 kVA the minimum short-circuit impedance is 6%.

The rated current [low voltage] In=2600/0.4/sqrt(3)=3753 A

The voltage drop at rated current and 0.85 power factor will be:

DV=sqrt(3)*In*(R*cosfi+X*sinfi)

[usual X/R for this transformer is 10].

The transformer impedance Z=0.4^2/2.6*6/100=0.00369 ohm

X=0.003674 and R=0.000367 ohm.

DV=sqrt(3)*3753*(0.000367*0.85+0.003674*0.527)=14.6 V.

So, in order to produce 400 V [at tap 3] the no-load voltage has to be 414.6 V

On tap 4 it has to be 2.5% more: 400*(1+2.5/100)=410 V at full load and 424.6 at no load.

All this in case the high voltage will be 22 kV.

If the tap no. is now 4 -instead of 3 where it has to be 22/400 V- that means the high voltage was less.

Let's say at full load you have 405 V instead of 410 V and instead of 22 kV you have 405/410*22=21.73 kV

At no load the low voltage terminal voltage will be 405+14.6=419.6 V.

<...voltage is fluctuating between 405V -417V...>

Actually, it <...is fluctuating...> from -417V to +417V.

Of course, you're right. It happens 50 times in a second.

A <...fluctuating output voltage...> is to be expected, as this indicates the changes in load elsewhere on the distribution system as users switch onto and switch off it.

If <...the tap changer for each phase is not at the same tap...> then one might expect different voltages between phase and neutral on each phase. One might expect this with a range of single phase users on the distribution system anyway.