StepupTransformer function by stepdown transformer

[shkim2000]

There is a 440V/6.6kV transformer fed by 440V switchboard but 6.6kV/440V transformer needs for 6.6kV shore power. For the cost point of view, I'm very curious if 440V/6.6kV transformer can be used for both step up function and step down(6.6kV/440V) function. I don't know if it is implemented praticably. If possible, it is big advantage in cost point of view as one transformer is not needed.

 

[Skogsgurra]

Yes, it is possible. But you will have a little problem with the voltage level.

The 440/6,6 transformer is wound to compensate for the internal voltage drop (represented by the Uk percent number).

That means that you have more turns than theoretically needed on the 6,6 kV side.

When you connect the other way round, you will get a lower voltage than expected on the 440 V side. And when you load it, it will drop even more. The no-load voltage will be about Uk % too low and the full load voltage will be 2xUk % low.

Otherwise you should be OK.

 

[DanDel]

skogsgurra, I have never seen a transformer wound to compensate for its internal voltage drop. All transformers, per NETA spec, must test within 0.5% of the calculated ratio (and within 0.5% of each coil's ratio).

 

[Skogsgurra]

No understand Dan.

Are you talking VT?

This is - as I understand - a power transformer.

 

[PWR]

I believe DanDel is correct. All power and distribution class transformers I have ever tested or seen manufactured for that matter do not have winding compensation. Some current transformer designs do have such compensation but that is the only place I have seen it.

 

[Skogsgurra]

OK. So the answer would be yes. You can use the transformer the other way round.

Must have got that thing wrong. Long time ago. Have to find a text to be convinced.

 

[DanDel]

skogsgurra, check with any manufacturer. This is why the open -circuit voltage at a transformer secondary is usually correct when compared to the nameplate ratio, and the voltage under load is always less.

 

[digitrex]

Gents,

I think the shkim2000's main issue on reversing the power flow should be the neutral grounding.

A step up transformer should have a star windings at HV(6.6kV) side while the LV side is a delta. If this transformer is to be used a step-down, then there will be a problem to give the 440V system a grounded neutral.

Voltage drop should not be an issue as every transformer is provided with tap changing facility at the HV windings.

 

[Skogsgurra]

Yes, I did get that wrong. Know better now.

The great thing with this site is that if you give bad advice, it usually gets corrected so quickly that it doesn't do any harm. And the one that gives bad advice also learns in the process.

I like that!

 

[37pw56gy]

This thread relates to the issue presented in thread238-114844. Specifically, why would there be a discrepancy between the specified step-down vs. step-up voltage ratios? One manufacturer's specs indicate that the primary output voltage will be 15% low when the transformer is fed through its secondary. Compensation for leakage reactance and copper losses seem to be plausible explainations. Are there other considerations to be accounted for when back-driving a step-down transformer, e.g. short circuit impedance?

 

[Skogsgurra]

Thanks for finding that thread, 37. I was looking for it.

The difference is that that thread was about low-power transformers where the compensation exists. This thread is about larger transformers - and I have just learned that they do not have it.

 

[Warpspeed]

I have just learned the same lesson too.

If I order a 240v to 24v 100amp transformer, I expect to see 24.0 volts at the output when fully loaded. Voltage drops in both primary and secondary are compensated for by slight adjustment of the turns ratio by the original designer. I had no idea the biggies were any different.

 

[Luis1947]

Transformer is a bilateral circuit element at the nominal voltage value. That means if you excite with an input signal V1 at primary side and get an output V2 signal at the secondary side, then input V2 at the secondary you will give V1 at the primary.

The key issue for the application of the step up transformer as a step down is the three phase connection. For the application of the Power Transformers you have source side and load side and the fair design is to have Y with grounded neutral at source side in order to detect ground faults.

 

[mc5w]

Smaller American control transformers typically << 0.75 KVA usually have extra turns on the low voltage side to compensate for voltage drop during inrush of motor controllers. Usually, for a 240x480 volt primary "120" volt secondary control transformer the open circuit secondary voltage is usually 125 to 130 volts. I have seen a few applications where internal lighting for a control cabinet runs off of the control transformer and the light blackens inside and burns out prematurely.

For the 40 VA to 75 VA control transformers that have a nominal 24 volt secondary the open circuit voltage is usually 27 volts. Since these transformers supply small wire sizes and have higher impedance to limit short circuits, the extra voltage works out to about 24 volts at about 75% load.

A 6,600/440 volt transformer of any size is not going to have extra secondary turns. However, the primary has voltage adjustment taps that can be changed while the transformer is deenergized.

 

[tinfoil]

Its probably not a problem in this particular case, but beware of phase shifts when stepping down then up if there is a possibility of paralleling the higher voltages later.

This will not be an issue for wye-wye connected banks, but may be an issue for wye-delta or delta-wye banks which create a 30° shift, and could result in a 60° shift rather than 0° when two are in series!