Transformer with Multiple Primary Windings 3

[whitbuzben]

Dear Sirs,

I was wondering if it would be possible to have a transformer with multiple primary windings that would allow two different voltages (lets say 11kV and 6.6kV) to be connected on the primary side. There would then be a single output on the secondary side of 33kV.

Presumably you would have two unconnected primary windings (one with 6.6kV across it and the other with 11kV across it). You would then series connect the two secondary windings with the turns ratio selected such that the voltages add to give 33kV. Does this sound logical.

Any advice would be much appreciated.

Many Thanks

 

[cranky108]

No. The Primary is the higest voltage.

It is possible to have multible secondaries, or tapable primaries.

 

[mgtrp]

The Primary is the higest voltage.

Not in my world (generating stations). The primary is the source side of the transformer, which could be the low-voltage winding of a generator step-up transformer, or the high-voltage winding of a station service step-down transformer.

It is quite possible to have a three-winding transformer, with separate 6.6 kV, 11 kV and 33 kV windings. You will only need one 33 kV winding per phase, so there is no need for a series connection.

 

[waross]

Not a good idea. If either primary voltages is a little high or low, reactive current will circulate in both windings. If the voltage error reaches three times the impedance voltage, the reactive current will be equal to full load current, plus the load current.
It could be worse. If there is a phase angle error real power will be transferred from one system to the other. It won't take much phase angle error to fry the transformer.
However, these effects will be the same with two transformers with with the same or different primary voltages with the secondaries connected to a common bus. In the event that the primaries are fed by transmission lines with different characteristics, tap changing may be used to compensate for line differences.
Less common is the use of a phase shifting transformer to vary the amount of real power supplied by each line.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

There are several options. When the transmission voltages were numerous, it was quite common to have multi-ratio transformers. When I started my career one of the first units to be made was a 10 MVA 110-66/33-22 kV Transformer. So the utility could use the same transformer with 4 voltage ratios by changing tap switches on HV and LV sides. Even today utilities procure such units to be used as spare in the system. Transformers with two primaries and one secondary is common in power stations. Two hydro generators feeding to a common step up transformer- in that case both primaries will have same MVA and kV ratings and HV will have double the individual primary MVA rating. Another scenario is in combined cycle thermal projects where the gas turbine and steam turbine generator transformers will be combined in to one transformer with two primaries having different voltages and different MVA ratings and the secondary (HV) with the combined MVA of primaries. In the first case the primaries will be made with less coupling, putting primaries one above the other and secondaries connected in parallel. So it becomes an axially split winding arrangement-two transformers one above the other, inside one tank with common core. In the second case it will be radially split secondaries ie P1-P2-S or P1-S-P2.

There are applications with one primary and multiple secondaries- with same variations as described above. Same MVA and voltage for secondaries when they are split to reduce short circuit MVA to meet breaker capacity. When sub transmission systems with different voltages are to be connected, MVA and kV of secondaries will be different.

 

[whitbuzben]

Hi guys

Thank you for your responses. PRC I think you have managed to identify what I was looking for. Your example of a combined cycle thermal power plant where the gas turbine and steam turbine generators feed one transformer with two primaries having different voltages and different MVA ratings is exactly the situation I have. Basically I have two generators with different voltages and MVA ratings and I am looking to feed them through a single transformer to produce 33 kV on the secondary side. However, although you have identified what I am looking for I'm afraid I don't follow your explanation of how this would actually be achieved (perhaps a diagram would help). You talk of " radially split secondaries ie P1-P2-S or P1-S-P2". I'm afraid I do not follow this perhaps you can elaborate for me.

Waross the point you make is certainly a concern. The primaries will be fed by cables (one rated at 6.6kV and one at 11kV). The cables will be of different lengths and have different characteristics. So, as you have highlighted, I presume tap changing will be necessary to prevent circulating current?? If anyone can elaborate on this it would be much appreciated.

Many Thanks

 

[prc]

P1-P2-S means the order in which the windings will be placed concentric to the core- innermost P1,next P2 etc.

I don't think the point raised by waross will be an issue in your case. The primaries will be connected to generators having impedance much more than the transformer and hence load sharing between the primaries will not be an issue. But testing (temperature rise test) at factory can be an issue. When you short the primaries and then energise the secondary to create full load current flow in windings it may not be possible to load two primaries according to their MVA unless you select the impedances between the circuits appropriately.ie X1 and X2 of the individual primaries shall be inversely proportional to their individual MVA ratings.

When two equal MVA secondaries are connected to common bus, there will be no circulating current when you adopt axially split arrangement ie non-coupled secondaries. In such a case it is also possible to load S1 (one winding alone ) keeping S2 (the other secondary) open still the terminal voltage of S2 will not change due to regulation of S1 with load.

 

[waross]

How long are your cables? Sharing reactive currents is more a concern with long transmission lines. The first effect is disproportionate sharing of reactive current, but generally no circulating current. With short cables the effect may be insignificant. This may be corrected by adjusting the voltages. Voltages are of more concern than the cable mismatches. The first effect of disproportionate voltages is disproportionate sharing of reactive current, but as the voltage disproportion increases the reactive current will drop to zero in one cable. Greater voltage errors will cause circulating currents where the reactive current in one cable changes direction and flows counter to the real current. At this point the reactive cable in the other cable will be the total reactive load plus the counter current in the second cable. This is more of a concern with long, dissimilar transmission lines feeding a common substation than generators feeding a common transformer. Note that this effect will be present even if you use two transformers tied together on a common 33 kV bus. It has to do with disproportionate voltages, not transformer choices. If you have control of the generation voltages you should have no serious issues.


Bill
--------------------
"Why not the best?"
Jimmy Carter