3.2
Design Parameters : Apart from the general considerations indicated above, the physical layout and the design of the sub-station will be governed by the following factors: i)
Number of incoming and outgoing feeders (existing and expected to come up in the future).
ii)
Expected load demand on the sub-station (existing and anticipated in the future).
iii)
Soil resistivity.
iv)
Expected fault levels at 11kV and 33kV.
v)
Climatic conditions (temperature, altitude, rainfall etc.
3.3
Standard Layout for 33/11kV Sub-stations.
3.3.1
REC has issued Construction Standards L-1 and L-2 indicating the standard layout for 33/11kV sub-stations with outdoor and indoor 11kV switchgear respectively. The standard layout should be followed for all 33/11kV sub-stations in the rural distribution systems as far as practicable.
3.3.2
The standard layout recommended by the REC is suitable for two power transformers and five outgoing 11kV feeders including two feeders for future. In case of indoor 11kV switchgear, provision has been made for four existing and two future 11kV panels and one bus-coupler panel has also been provided. The 33kV incoming arrangement will depend upon the requirement of each case, i.e. whether there are one or two incoming 33kV lines and whether any 33kV line and whether any 33kV line/lines are proposed to be taken out from the sub-station. The arrangement for the 33kV incoming and outgoing lines may be followed as per standard practice of the Board. However, an arrangement showing one incoming line and two outgoing lines has been suggested in the standard layout (shown dotted).
3.3.3
The standard layout (with out-door 11kV switchgear) shows only the general arrangement of the equipments, structures, bus bars etc., but the provision for internal roads/paths has not been shown. It is important that suitable provision is made in the switchyard depending upon the local conditions for roads/paths to facilitate transportation/shifting of the heavy equipment particularly the power transformers. It would be desirable if a truck can be taken to the site of the transformer for easy shifting of the transformer. In case of 5MVA transformers, it may be necessary to use a trailer for the purpose of transportation. It would be desirable to make provision accordingly.
4.
Land Development
4.1
The yard should be high enough to provide satisfactory drainage and safety against accumulation of water.
4.2
The sub-station yard should be suitably graded and sloped to prevent stagnation of water. The graded area should include the entire fenced area and 1 metre outside thereof.
4.3
Yard surface should be covered with granite metal, where considered desirable to prevent the growth of grass and weeds, the spread of dust and formation of mud and
slush during rainy season. Use of granite metal also helps in reducing step and touch potentials. 4.4
Fencing not less than 1.82 metres (6feet) in height should be provided all around the switchyard. Fencing should have a clearance of at least 5 meters from sub-station structures and live parts.
5.
Power Transformers
5.1
The capacity of the power transformers to be installed should take into account the load expected to come up on the sub-station at least during the next five years or so.
5.2
It would be desirable to have two transformers at the sub-stations for proper security of supply. However, if the load demand on the sub-station will remain well within the capacity of one transformer for some time, it may be desirable in certain areas to defer installation of the second transformer for economic considerations.
5.3
The standard ratings of 33/11kV transformers for use in rural areas as specified by the REC are 630 kVA, 1600 kVA, 3150 kVA and 5000 kVA and it is recommended that only these sizes should be used.
5.4
Transformers should be procured as per REC Specification7/1974, which lays down the main characteristics of 33/11kV power transformers. Where no specific provision has been made in the REC Specification, Indian Standards on Power Transformers (IS:2026) should be followed.
5.5
As the voltage conditions in the rural areas are generally very poor, it is strongly recommended that all transformers of 5000 kVA capacity should be provided with on load taps. Provision of on load taps is also considered necessary for 3150 kVA transformers in the case of those sub-stations where on load taps are not available on the power transformers at the main grid station feeding the 33/11kV sub-stations.
6.
Circuit Breakers
6.1
On 33kV side, one group control circuit breaker should be provided when there are two transformers, each having a capacity of 5 MVA or above. In other cases, the transformers should be protected by fuses.
6.2
33kV Circuit Breakers should be procurred as per REC Specification 12/1976, which lays down important technical characteristics of the circuit breakers as also of the instrument transformers.
6.3
The standard rated current for 33kV Circuit Breakers shall be 630A and the rated shortcircuit breaking current for these breakers shall be 12.5kA.
6.4
Oil or Vacuum Type Circuit Breakers should, necessarily, be provided on 11kV side of the individual 33/11kV transformer with capacities 1600 kVA and above.
6.5
11kV oil or vacuum type Circuit Breakers should be procured as per REC Specification 8/1975 or 22/1983 respectively, which lay down important technical characteristics of the circuit breakers as also of the current and voltage transformers.
6.6
The standard rated current for 11kV circuit breakers shall be 400 A and the bus-bar rating of these circuit breakers (indoor type) shall be 800A.
6.7
The rated short-circuit breaking current for 11kV breakers, as stipulated in the above REC Specification shall be 8kA or 12.5kA. However, at a number of locations the fault level may permit the use of 6kA circuit breakers, which are economical (REC Specification for such breakers/re-closures is under finalization).
6.8
The approximate fault level at 33/11kV sub-stations of various capacities are indicated below for guidance. These fault levels are based on the percentage impedance of 5% for 630kVA transformers 6.25% for each of 1600kVA and 3150kVA transformers and 7.15% for 5000kVA transformers as stipulated in the REC Specification 7/1974. Sub-station capacity
Fault Level
2x630kVA
25MVA ( 1.3kA)
2x1600kVA
53MVA ( 2.8kA)
2x3150kVA
90MVA ( 4.75kA)
2x5000kVA
143MVA ( 7.5kA)
It is recommended that breakers with 12.5kA breaking current may be used only when the proposed/anticipated transformer capacity is 2x5 MVA. For lower transformer capacity, breakers of 6kA and 8kA breaking current should be used. 6.9
The number and type of protective relays to be provided on various breaker panels installed in a 33/11kV substation shall be as shown in REC Construction Standard L-6.
6.10
In order to assess the system losses and to take remedial measures, it is essential that adequate metering arrangement is provided at the sub-stations. The meters/instruments on the incoming and outgoing circuit breaker panels should be provided in accordance with the REC Construction Standard L-3.
7.
Instrument transformers
7.1
Instrument transformers should be suitable for indoor or outdoor use as the case may be and should be of adequate VA capacity (burden). They should be of adequate accuracy class and should be able to withstand mechanical and thermal stress imposed during operation.
7.2
Instrument transformer circuits are vital in the operation any station and the following data should be obtained from the suppliers of the equipment and studied. i)
Ratio and phase angle errors of the transformers at various burdens.
ii)
Burden data of all connected instruments and devices.
iii)
Wiring of secondary circuits.
iv)
Resistance of interconnecting cables.
7.3
The following standard secondary ratings should be adopted: i)
Potential transformers
110 Volts.
ii)
Current transformers
5 Amp.
7.4
Voltage and current transformers for 33kV and 11kV circuit breakers should conform to REC Specification 12/1976 and 8/1975 respectively. These specifications lay down the rated VA burden and accuracy class for both voltage transformers and current transformers required at 33/11kV sub-stations.
8.
Isolating Switches
8.1
Double break type horizontal air-break switches should be used on the 33kV and 11kV sides of the power transformers as shown in the REC Standard Layout L-1 and L-2.
8.2
Isolators controlling the transformers should be provided with arcing horns.
9.
Lightning Arresters
9.1
Station type lightning arresters should be provided on 33kV bus bars as shown in REC Construction Standard L-1 and L-2.
9.2
Line type lightning arresters may be provided on the incoming and outgoing 33kV lines as per standard practice of the Electricity Boards.
9.3
Line type11kV lightning arresters should be installed on all outgoing 11kV feeders.
9.4
The standard voltage rating of 11kV lightning arresters should be 9kV and standard current rating 5kA.
10.
Out door Structures
10.1
Standard 8 metre 200Kg. PCC/RCC support or other suitable supports of equivalent height and strength should be used for stringing both 33kV and 11kV bus bars as per standard layout of the REC. In case of 33kV main bus two poles placed side by side should be used at each of the four corners and a stringing tension not exceeding 100150 Kg. may be applied on bus bar conductors which corresponds to a sag 150 to 100 mm in a span of 11.1 metres. In case RS joints are used, the minimum size recommended to be use for stringing 11kV bus-bars is 175x90 mm. For 33kV bus bars, heavier section of 200x140 mm shall be used. In case this size is not available, the next higher size may be used. Alternatively, two RS joints of 175x90mm placed side by side at each corner may be used for 33kV bus bars.
10.2
No guys should be provided for the supports in the switchyard.
10.3
Suitable cement concrete foundations should be provided for the circuit breakers and transformers so that the minimum height of the live parts should be maintained as per REC Construction Standards L-1 and L-2.
10.4
All steel structures should be suitably painted if ungalvanised, to avoid rusting and corrosion due to atmospheric conditions.
