Layout plan and sections of 33 by 11 kV project. The availability of the substation plot area can be seen in the fence boundaries. The 33 kV incoming line direction and the orientation of the 11 kV outgoing line feeders, the approach road, etc., have a great bearing on the planning of the layout and location of the gantries and planning the transportation of materials to site. This particular substation has a very limited space along east-west direction to accommodate the transformer base and locate the transformers. Hence, the transformer base are turned by 90 degrees. Eleven kV outgoing line orientations are obtained from the utility or customer. Accordingly, double pole structures, six sets are positioned to mount 11 kV outdoor isolator and surge arrestors for each 11 kV circuit. Please observe the 33 kV incoming line orientation. North direction has been mapped for proper site orientation. The 33 kV line is coming from the west direction from the utility. The line has been terminated on a gantry inside the substation. Normally, the utility provides the first transmission line tower for stringing from the transmission line tower to the substation gantry. The associated conductors, shield wires, tension insulator strings, and the hardware are in the scope of the utility. The tap-off from this conductor and subsequent works inside the substation will be the part of substation contract. Please observe the facility provided for the removal of the transformers for which roads have been provided to bring the trailer inside. Please note that the supporting structure provided for the nine kV LA and the cable terminations, along with bus post insulators and the fence, have to be dismantled while removing the transformers. Please observe that authorized direct entry to the control building, housing the 11 kV switch gear, control and protection panels, tap changer panels, DC and AC boats, battery and associated charger, etc., is provided. There is no need to go through the switch yard. Free access to enter into the substation premises by unauthorized entry is prohibited considering safety and security aspects. Hence, the main gate is provided at the plot entry with fencing all around the substation area. Please also observe that authorized direct entry to switch yard without going to control building by means of fencing and separate gate has been envisaged. Co-ordinates for control building at any one corner, plot boundary corners, any one tower and transformer center lines, shall be marked. For this substation, all coordinates are marked for each 33 kV tower, substation plot corners at four corners, transformer center lines, substation control building, capacitor banks, 11 kV DP structures, etc. Lightning protection has been envisaged by means of spikes and towers. Firewall between the transformers is provided as per requirements. Firewall is rated for four hours. If any future bay extension is required, space provision shall be envisaged. In this case, no future extensions were planned due to limited space availability. Minimum electrical clearances adopted based on IE rules CBIP recommendations. Number 3, sectional clearance for 33 kV, it is 2,800 mm. For 11 kV, it is 2,600 mm. Number 4, ground clearance, elevation of live conductor about plinth level. For 33 kV, it is 3,700 mm. For 11 kV, it is 2,750 for double pole structure. Number 5, height of insulator bottom from ground level. For 33 kV, it is 2,440 mm. For 11 kV, it is 2,440 mm. Minimum height of insulator bottom out part had been kept at 2,440 mm above plinth level, since all dimensions are marked from top of plinth level. Distance between finished ground level and top of plinth level is maintained at 300 mm in this project. For 11 kV outdoor part double pore structure arrangement is provided where 2,440 mm for the insulator bottom has been maintained. It may be observed that the 33 kV vacuum circuit breaker and CT are mounted on a common structure. The breaker is a three-pole mechanically ganged and hence, the pole-to-pole spacing will be based on the manufacturer's type-tested design. It will be lower than the face-to-face distance being maintained in the layout. That is why the conductors are getting squeezed. Inside the control building, sufficient front clearance and rear clearance for 11 kV switch gear has been provided. The space between the 11 kV switch gear shall be either less than 200 mm or more than 750 mm in width, measured from the farthest outstanding part. A clear space of not less than 1,500 mm shall be provided in front of the 11 kV switch gear, to draw out the breaker trolley. If the space behind the 11 kV switch gear exceeds 750 mm in rate, there shall be passageway to a clear height of 1,800 mm to avoid any obstructions. Considerations of firewall between transformers for dimensioning aspects. Firewall between the 210-MVA transformers is required since the oil capacity in each transformer is more than 2,300 liters and the clear distance between the two transformers is approximately about four meters. The firewall shall extend beyond 600 mm on both sides from the transformer farthest point. The height of the wall shall also extend 600 mm above the transformer bushing height. Transformer RYB face orientation aspects. For transformers, the standard convention of phase bushing will be as follows. Looking from HV side towards low voltage side, left side bushing will be R phase, then Y phase, and then B phase. Accordingly, RYB phase sequences shall be maintained in the layout and coordinated with the 33 kV incoming line phase sequence. We will now review the layout considerations such as the ground clearance, bus bar height, and bay width, etc, for this project. Ground clearance at 3,800 mm. To ensure the safety of the personnel moving about within the switch yard, this is the height of the equipment interconnection height measured from top of plinth level to the center line of the conductor. It is kept at 3,800 mm above plinth level. Minimum height up to the bottom of the insulator, which is earth, shall be 2,440 mm. Phase to earth clearance is 320 mm minimum. Sectional clearance shall be 2,440 plus 320, which is equal to 2,760 mm. Working clearance of 1,000 mm to be maintained. Minimum requirement shall be 2,760 plus 1,000, which is equal to 3,760 mm. Hence, 3,800 mm is considered. Bus bar height at 5,500 mm. This is the height of the strong bus bar measured from top of plinth level to the center line of this trunk bus. Span length for the strong bus is 13 meters. Considering the worst sag of three percent, maximum sag will be 13,000 into 3 divided by 100, which is equal to 390 mm. Phase to phase clearance with the equipment connections below 3,800 mm is equal to 320 mm minimum. Considering a cushion of another 1,000 mm, total requirement shall be 3,800 plus 320 plus 390 plus 1,000, which is equal to 5,510 mm. The considered 5,500 mm is adequate. Incoming line-based termination at 6,500 mm. Span of the incoming line is usually higher hence, sag will be more. In addition, there is a suspension string also at the gantry, which occupies another 450 mm approximately. That is three into 145 mm. Center line of the beam to bottom of the beam will be another 300 mm, hence, the incoming line is terminated at a higher level of 6,500 mm instead of 5,500 mm. Lane bay width at 5,500 MM. The bay width is kept at 5,500 MM, as can be seen from the layout. Center line of structure to phase conductor is 1,250 MM. Phase to phase center line is 1,500 MM, hence, total bay width equals 1,250 plus 1,500 plus 1,500 plus 1,250, which is equal to 5,500 MM, span length for bus bars in transformer base at 13,000 MM. We have seen that the bay width of 5,500 MM is normally adopted for 33 KV, hence, we need a space of 5,500 into two, which is equal to 11,000 MM for two bays to plan. However, we have kept 13,000 MM between the two transformer base, considering the following reasons. To maintain a convenient distance between the two transformer base to have the firewall and mobile space around the transformers, to ensure sectional clearance between the two transform a base in case of maintenance of any one big. Statutory clearances. We will now study the clearance aspects which are to be kept in mind. These are required from safety considerations, and have to be approved by the electrical inspector prior to charging the substation. Please refer section A-A for 33 kV line bay and main bus, phase to earth clearance in section A-A marked 1. Corner of the beam to the quarter loops at incomer termination for connection to surge arrester and PT through the suspension string, similarly, the live terminals of the breaker to CT tank, section or clearance in section A-A marked 2. Between line side isolator live terminal to CT, it should have a minimum clearance, which is 2,800 MM here between bus side isolator live terminal to breaker minimum of 2,800 clearance to be maintained. Sectional clearance between B phase equipment interconnections in transformer B1 to R-phase equipment interconnections in transformer Beta-2 specifically between the surge arrestors and isolators of these two extreme phases. For clearance checks, one of the isolators is considered in closed position. This can be clearly checked in the plan. Phase to Phase clearance in section A-A marked 3. Between the strong main bus to the equipment interconnections below the bus bar, please refer section BB, 433 KV main bus and 33 KV side of the transformer B. Phase to earth clearance in section BB marked 1. The sectional clearance is applicable between live terminal of the breaker to CT tank. Sectional clearance in section BB marked 2, also the sectional clearance is applicable between transformer bay bus side isolator live terminal to break up. Note the following. If the transformer is to be taken out for any reason, the surge arrestors and the cable support structures are to be dismantled. No isolator is required in the transformer base after the CT since there is no back feed and researched from the 11 KV side. If 33 KV breaker trips downstream, the associated 11 KV transformer bay incomer breaker will trip. Eleven KV breaker can be closed only when upstream 33 KV breaker is closed. Please refer Section C-C, and D-D, for typical 11 KV outgoing line bay. Two polled structures are provided to mount the surge arrestors and the line isolator with earth switch. The operating mechanism box will be brought down to a convenient operating height of about 1,200 MM from plinth level. Phase to earth clearance shall be ensured between pole structure to light jumper for connection to LA, isolated base frame to Jumper for connection to cable termination. Please refer Section E-E and F-F for 11 bar 0.433 KV station transformer. Two poll structures are provided to mount the surge arrestors air brakes which dropout views and the transformer. The transformer base is mounted at a height above 2,440 MM, hence, no fence as such is required. In case of any maintenance, the shutdown has to be taken and the 11 KV switch gear associated breaker has to be tripped. Continued in next part.
