In Gujarat India, GETCO is known as transmission utility and PGVCL, MGVCL, UGVCL, DGVCL is known as distribution company. GETCO 66kV/11kV substation has different 11kV feeder called JGY (Jyotigram yojana), AG(Agricultural feeder), Express feeder, Urban feeder, Solar feeder.
GETCO give 24Hrs power to all feeder except AG feeder. AG feeder has agricultural load like motor pump. It is not possible to give 24Hr power to said feeder. In one 66kV/11kV Substation many AG feeder outgoing connected so GETCO give 8Hrs 3 Phase power to AG feeders.
For example if one substation has 5 AG feeder than 8Hrs power given to this 5 AG feeder but at different timing. This timing changes every week called power schedule.
As farming demand dedication so some farmer lives nearby their farms so they have lighting load like fan, light in their home. So if we give only 8Hrs power than other 16 Hrs they have to survive without electricity in their home. If we give 24Hrs 3 Phase power then they run their agricultural motor which result into Load>Generation leads to unbalance in grid.
As you can see above diagram it's shows normal 3 phase power distribution to AG load. Y phase and neutral taken for single phase load for household equipments.
So in AG feeder 3 Phase power given for 8Hrs duration and 1 Phase power given for remaining hours to cater household load like fan and light. As you can see above diagram if we give single phase to delta winding than current will not flow and secondary side voltage not produced.
To avoid this problem LSTC and changeover switch is installed between GETCO power transformer and PGVCL distribution transformer.
As you can see in diagram, One transformer is connected between GETCO power transformer and PGVCL distribution transformer it is LSTC. (Changeover switch not shown because it will complicate to understand through diagram)
LSTC is a single phase transformer, It has three terminal. When Y Phase of Power transformer applied to one winding of LSTC VY'N=6351 V, Mutually induced emf produced in second winding VB'N=4500V, Both have same phase angle, common point of both winding should be earthed. CT is connected in series with earthing it's secondary terminal feeded to earth fault IDMT relay.
Therefore Distribution transformer get Y phase directly from Power transformer, B Phase is coming from LSTC and R Phase being grounded.
Distribution transformer has VR'Y'=6351V, VY'B'=10.8KV(Applied voltage of LSTC and Mutually induced emf both have same phase angle therefore we can add both voltage directly) , VB'R'=4500V in its primary.
It has voltage ratio 11kV/415V.
Therefore secondary voltage will be,
Vrn=138V
Vyn=240V
Vbn=98V
As Three phase voltage is not enough to run motor so they cannot run motor if they try than motor will fail in low voltage. Here we get Vyn=240V which is enough to run single phase household load.
You can see image of LSTC which is looks alike distribution transformer but when you look carefully you will notice that there is only 3 terminal. Here Middle terminal is being grounded, White box attached with gantry, CT is installed in that box. On the top Changeover switch is mounted, which changeover supply from 3 phase to 1 phase. The most dangerous thing in LSTC is earthing, Because it is return path for current so if it's not proper than fatal voltage produced.
Hope you understand concept about LSTC. If you like the content than share with your electrical mates.
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