The best way I have found to explain this is to start with an open delta. Rather than a three phase transformer consider a three transformer bank for the purpose of explanation.
Remove the transformer from C phase. On the delta secondary there will be 6 kV volts generated by the open delta.
Now connect the low voltage winding only of the third transformer across the open delta. It is easy to see that the open delta will power the third transformer low voltage winding and the transformer will develop 110 kV on the open high voltage winding.
Now consider a fault at another location, on C phase of the distribution circuit.
C phase voltage drops to zero.
But we have shown that the C phase transformer is developing full voltage on the high voltage winding, powered by the healthy A and B phases of the circuit. Connected to C phase a heavy current will flow back into the fault.
The available current will be limited by three times the %impedance of the transformer bank.
The current will be further limited by the system impedance.
The current will still be high enough to take out a fuse.
Once one fuse clears the open delta no longer functions and the current drops to a very low value.
Second issue. Circulating currents.
Consider that the voltage on C phase is 10% low at 99 kV.
The healthy phases are back feeding the C phase transformer and it is trying to develop 110 kV while it is connected to a 99 kV source.
In this example the bank acts as a stabilizing force and may raise the voltage on the low phase.
The issue is that some or all of the transformer capacity may be used to correct the voltage on C phase with little or no capacity left to power the intended load.
Overheated transformers and blown fuses are common.
I hope that this explanation is clear.
Cultural issues.
Yes, cultural issues may have a direct influence on technical issues.
In the country where the wye:delta was common it was also common to see virtually all the wye:delta banks with one fused cut-out hanging open.
Part of the culture was kick-backs from the suppliers to any contractor or consultant bringing in a transformer order.
Transformers tended to be 200% to 300% oversized and most transformer banks could easily handle the load in open delta.
Then there would be an election and a change in regime.
The incoming Energy Minister would replace the head of the national power company with his favorite. The new head would replace a lot of department heads with his favorites. Someone would notice the open cut-outs all over town and issue orders that all fuses must be replaced. The fuses would be replaced with progressively larger fuses until the transformers started failing. This would continue for several months until the permanent workers were able to "train" the incoming political appointees.
Single phase switching.
In this area power outages on Sundays were common. Live line work was avoided and maintenance was regularly done on dead lines on Sundays.
Switching was generally done by closing fused cut-outs, one phase at a time.
On a residential circuit, when the first phase was energized, there was a single phase condition. With A phase energized, the B and C phase low voltage windings would be in series across the healthy A phase low voltage winding. On a balanced circuit each of B phase and C phase transformers would back-feed about 50% voltage into the circuit. After a 4 hour outage all the refrigerators and freezers on those circuits would try to start, but with 50% voltage most would stall and start heating.
When the second phase was energized, the residences on that phase would get about 95% of normal voltage applied to the stalled and heating refrigerator compressors.
Generally the thermal overload protection would save the motors but every weekend it was common for some refrigerators to fail.
There are some other conditions as well.
NickParker asked:
NickParker said:
Can anybody explain why this practice is been followed?
With respect to those suggesting fero-resonance issues, and an acknowledgement that at times ferro-resonance may be an important issue, I have seen and have read descriptions of wye point grounding when energizing wye:delta banks to avoid over-voltage transients.
This was at one time and in some areas a common practice.
This was a recommended practice to mitigate known issues.
As I said:
"Been there, done that, got the tee shirt". grin
Bill
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"Why not the best?"
Jimmy Carter