I want to know the effects of a fuse in a phase (Say Y phase) of primary blown in a 3phase Dyn11 33kV/400V 250kVA distribution transformer. i am particularly interested in the effect of this on the secondary terminals (where the neutral is solidly earthed type). I understand that the primary voltages will be 33kV, 16.5kV, 16.5kV but have no technical idea about the following:

1. what will be the L-N voltages of secondary when unloaded?

2. will there be much difference in if it is loaded with balanced load?

3. will there be any big deviation if it is loaded by unbalanced load as in (2)

4. what about the neutral earth current in secondary?

5. will there be any damage to the transformer if this is left for long time?

6. much helpful if a complete phasor diagram can be provided

7.what are the effects of this on single phase loads and three phase loads connected to secondary?

I kindly expect a good answer!

 

Siva... I suggest you search the Control.Com Archives. Start with the following threads:

o 1026239176

o 1284199900

o 1307209459

Regards, Phil Corso

[<b>Moderator's note:</b> the url that goes before the number is: http://www.control.com/thread/ ]

 

Mr Corso,
thanks a lot

 

Mr Corso,

Now i have two doubts:

1. how can you say that all secondary voltages are 'in-phase'. i thought there are only two phases in-phase?

2. In the above transformer, i experimented the above but the results were not as expected.
The secondary voltages(phase-phase) were approx about 368V, 325V, and 58V as i remember.
that's why I have started to dig the theory?

3. your suggested threads (mine also was included) did not talk about neutral currents. is there a difference between solidly earthed neutral and unearthed one?

4. Is there an existing protection scheme to detect this phenomena(single-phase gone)?

Regards...

 

Siva... if you are unable to extract the information you need from the referenced threads please contact me directly at:

Phil Corso (cepsicon[at]AOL[dot]com

 

Siva... replying to your questions in the order presented:

1. The earlier topics cited revealed certain constraints to simplify the analysis. Please note that thread #102639176 suggested substitution of three (3) single-phase transformers, thus ignoring the effect of mutual impedances. The relevant paragraph is repeated here:

>B) Pre-Fault and Post-fault Primary Voltages.
The pre-fault connections for the three transformers are: Tr1 is connected to phases A-B; Tr2 to B-C; and Tr3 to C-A. Now, assume that the C-phase fuse operates. Then, post-fault voltages are: Tr1 is supplied with full voltage; Tr2 and Tr3 are placed in series so their primary terminal voltages are 50% each.<
I did inadvertently omit a very important criterion, which was: pre-fault and post-fault load-kVA are equal!

2) The secondary voltages presented in your "experiment" do not exhibit the same 100:50:50 ratio. Thus, they suggest evidence of residual magnetism or feedback voltage from the load if motors, or the transformer if 3-phase unit in one tank, or a combination of both, resulting in an asymmetrical collapse of the voltage triangle.
The values you cited yield a Voltage Unbalance Factor (VUF) of 76.8% and 82.5%, based on NEMA and IEC standards, respectively. Both of course are well above the recommended 5% maximum allowed in the standards. The corresponding VUF figures for Thread #102639176 are 100%, because there is a complete collapse of the equilateral voltage triangle.

3) Your question about neutral currents is a good one. If the neutral is solidly-earthed, and secondary-load balanced, then phase-neutral ratios are similar to the primary ratios, for a resistive load. If load is complex, results will vary.

If the neutral is unearthed and single-phase loads exist, they must be mathematically transformed into delta-loads, then paralleled with the 3-phase load, and the case treated as an unbalanced delta-load problem.

4. The simplest protection is to monitor the primary or secondary phase currents using a negative-sequence protective relay!

I also apologize for inserting your thread as the 3rd reference. It should have been Thread #1026230516, which was analyzed off-forum!

I would appreciate additional information about your experiment, on- or off-list.

Regards, Phil Corso