On three winding station transformer,YNynyn, 400/11.5/11.5kV, rated 100/50/50MVA and above,is it recommended to have an additional delta winding on the secondary side? What are the advantages of having this additional delta winding?
2* The 400kV side is effectively (solidly) earthed and 11.5kV side is low/medium resistance earthed with E/F current limited to 300A.
> 4) What did you mean by "and above?" <
Whether additional delta winding on the secondary side is recommended for power transformers rated "100MVA and above?" In India, it is not a practice (nor is it mandated by legal bodies) to provide additional delta winding on secondary side of power transformers unless there is a requirement of auxiliary power for substation. (say a 33kV tertiary delta winding)
> Is the xfmr forced-air and/or forced-oilcooled? <
Cooling is OFAF: Oil-Forced and Air-Forced (Nothing to do with "and above"!)
Pramod... so far you have indicated at least two of the 3 windings are earthed. Furthermore, unless the tertiary-winding is also earthed, capacitive-coupling could elevate voltage of the entire tertiary-winding with respect to earth, so:
6) Is the tertiary wye-winding being used to carry load?
7) Is it (11.5kV) earthed, as is the secondary-winding?
8) Are you experiencing problems related to harmonics, or voltage-regulation? If so, more detail is required.
FYI, the 400kV wye-winding less costly then a delta one.
> 6) Is the tertiary wye-winding being used to carry load? <
- Yes
> 7) Is it (11.5kV) earthed, as is the secondary-winding? <
- Secondary and tertiary windings (both 11.5kV) are earthed through low resistance. Both of these windings are meant to carry load and are connected to different 11kV switchgears.
> 8) Are you experiencing problems related to harmonics, or voltage-regulation? <
- We apprehend the above problems related to harmonics and voltage balance. (system still in design stage)
9) Since 300A is the maximum earth fault current and immediate disconnection of supply is envisaged in case of an earth fault, I am not considering any major effects on the bus due to earth faults.
10) voltage drop considering 8% impedance , 0.8 PF lag and 90% loading will be close to 4%.
Can you specifically throw light on any potential problems like voltage unbalance/neutral shift/interference with communication networks/overheating etc
Pramod... The question you asked me should be addressed to the xfmr specifier and the xfmr vendors associated with your project. However, my answer follows:
The problems associated with 3-phase, 2-winding Y-Y core-type transformers is that that the zero-sequence component can't be readily eliminated. Thus if a line-to-earth occurs, then, thru "transformer-action" a like current will "flow" in the transformer tank. Installation of a delta-connected tertiary winding eliminates
this condition.
Assume the HV winding is the source of supply. And, as you stated, earth-fault current in the two MV systems are limited (even if you could attain instantaneous-tripping of the REF relay!) The question to ask is, "By how much?"
In the 3-winding Y-Y-Y case, total zero-sequence impedance consists of a series connection is the series sum of all three zero-sequences. Also, although the MV zero-sequence impedances are equal, that of the HV winding is greatly influenced by its return-path to the source of supply, meaning you can't rely on its value being zero!
Now, there are certain "fixes" the xfmr manufacture can apply to ameliorate the problem, but then the harder question to ask of your vendors is, "Can they guarantee the unit will be free of future problems?"
