Thursday, September 16, 2010

Construction of 33KV substation

IE rule permits following limits of service voltage :-


LV........................ -6 to +6 %


Upto 33 KV......... -9 to +6 %


Above 33 KV....... -12.5 to +10 %


132 KV transformers have on load tap changers with -15 % to +5 % tap range. Tap changing is not provided in 33 KV distribution transformers.


Protection Scheme:-


132 KV:- Busbars ( Not protected ), Transformers ( Biased diff. with harmonic restraint for HV/LV windings, Inst. REF for LV windings, Overfluxing , IDMT OC and EF for backup,Buccholz, WT, OT), Feeders ( Dist with accelerated zone2, PLCC backup IDMT OC and EF).


33 KV:- Busbars (Not protected), Transformers 7.5 and 12.5MVA ( Biased diff., Inst REF, IDMT OC and EF), Feeders(IDMT OC and EF).

Then the system layout is designed in which ring main ( main and transfer bus) is decided for HV and LV side of 132 KV substation and single bus open ring for 33KV and double bus on 11 KV side ( indoor switchgear ) is decided for 33 KV substation. 11 KV line is erected on 9m PCC poles.


Service connection of 132 KV is given for 5MVA+ consumers. For 1.5 to 5 MVA consumers, 33 KV and for upto 1.5 MVA, 11 KV connection is given.


Fault level:- 11 KV:- S.C.Current ( 18KA), S.C.MVA ( 350MVA), 33 KV:- S.C.Current (18KA), S.C.MVA (1000MVA), 132 KV:- S.C.Current ( 32 KV ) , S.C. MVA ( 7200 MVA)


The transformer rating on 132/33 KV are 12.5, 20 and 25 MVA , on 132/11 KV are 6, 10, 12.5, 31.5 MVA and on 33/11 KV are 0.63, 1.6, 3.15, and 5 MVA .

The switchgear ratings for 132 KV are 1250 A or 2000 A with S.C. current rating of 12.5 KA to 31.5 KA ( i.e. rupturing capacity 2500 MVA to 7200 MVA). For 33 KV side, CBs are of rating 630 A to 1250 A with S.C. current rating of 12.5 KA to 18 KA.( i.e. rupuring capacity of 660 MVA and 750 MVA). 11 KV CBs are rated 400, 800 and 1250 A with rupturing capacity 250 MVA and 500 MVA.

The distribution transformers are of 500 KVA ( urban) and 40 KVA ( rural).

For conductors and cables, 125 sq.mm ACSR or 300 sq.mm XLPE cable is used for 33 KV. For 11 KV, 80 sq.mm. ACSR or 185 sq.mm XLPE cable is used. 50 sq.mm ACSR or 3 1/2 core PVC cable of 70 to 300 sq.mm is used on 415 V side.

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