Distribution Substation of Industrial Plant

The transformer for a building or industry is sized on the maximum demand, not on the sum of connected loads, because loads rarely run at full rating and never all at the same time. Two factors (IEC/Schneider Electric method) are used:

• Factor of maximum utilization, Ku (≤ 1): the ratio of the power actually drawn by a load to its rated power. Typical: motors 0.75, lighting 1, heating 1, socket outlets 0.1–0.2 (depends on use), EV/AC as per duty.
• Factor of simultaneity (diversity), Ks (≤ 1): accounts for the fact that all loads of a group do not run together. Typical Ks for a distribution board (IEC 61439): 2–3 circuits 0.9, 4–5 circuits 0.8, 6–9 circuits 0.7, 10 or more circuits 0.6. For residential buildings Ks falls with number of consumers (e.g. 0.78 for 4 flats, about 0.4 for 50 flats).

Procedure

1. List every load with its rated power in kVA (kVA = kW / (η·cosφ)).
2. Multiply each load by Ku to get its utilized power.
3. Add the loads of each SDB and multiply by the Ks of that board.
4. Add the SDB demands at the MDB and multiply by the Ks of the MDB.
5. Add a margin for future expansion (about 20–25 %).
6. Choose the next higher standard transformer rating (25, 50, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000 kVA ...).

Example: a small factory

SDB-1: 54.0 × Ks(4 circuits = 0.8)   = 43.2 kVA
SDB-2: (12 + 3) × Ks(2 circuits = 0.9) = 13.5 kVA
SDB-3: 15 × Ks(3 circuits = 0.9)     = 13.5 kVA
Sum at MDB = 43.2 + 13.5 + 13.5      = 70.2 kVA
MDB demand = 70.2 × Ks(3 boards = 0.9) = 63.18 kVA
With 20 % future margin = 63.18 × 1.2 = 75.82 kVA

Selected transformer: 100 kVA, 11/0.4 kV (next standard size above 75.82 kVA).

Compare: the total connected load is 114 kVA, but the transformer is only 100 kVA, and it will run at about 63 % load at peak, near its best efficiency region.

Other applications

• Apartment building: flats have high connected load but low Ks (e.g. 50 flats × 8 kVA × Ks 0.4 = 160 kVA plus common services such as lifts and pumps).
• Office: lighting and AC Ku ≈ 1, computers and sockets lower; Ks of floors about 0.7–0.8.
• Industry: motor-dominated; Ku ≈ 0.75, Ks depends on shifts and process; large motors' starting must be checked.

A substation is an assembly of transformers, switchgear and protective devices that changes the voltage level and controls and distributes power. A distribution substation steps 11 kV down to 400/230 V for the consumers.

Types of substation

1. By service: step-up (generating) substation, primary grid (transmission) substation, secondary substation, distribution substation, industrial substation, converting substation (AC to DC, frequency change), switching substation.
2. By design / construction:
   • Indoor substation (equipment in a building; up to 11–33 kV, polluted or space-limited areas).
   • Outdoor substation: pole-mounted (single pole up to about 25 kVA, H-pole/double pole up to 250 kVA), plinth/foundation-mounted (above 250 kVA).
   • Underground substation (crowded city areas).
   • Package / compact (kiosk) substation with HT RMU, transformer and LT panel in one enclosure.
3. By voltage: HV, EHV, and distribution (11/0.4 kV) substations.

Single line diagram of 11 kV distribution substation

 11 kV overhead feeder (utility, NEA)
            |
        [LA]  lightning arresters (3 nos)
            |
        [AB]  air-break / gang-operated isolator
            |
        [DO]  drop-out fuse  (or HT VCB + relay)
            |
        [HT metering CT/PT]  (for HT consumers)
            |
        (TR)  11/0.4 kV, Dyn11 transformer
            |      neutral + body earthed
        [ACB / MCCB]  LT main breaker + LT meter
            |
 ======== MDB  400 V, 3-ph 4-wire busbar ========
     |            |            |            |
   MCCB         MCCB         MCCB         MCCB
     |            |            |            |
   SDB-1        SDB-2        SDB-3       APFC panel
  (motors)    (lighting)    (sockets)   (capacitors)
     |            |            |
   MCB/MPCB      MCB          MCB/RCCB
     |            |            |
   400 V M     230 V lamps   230 V outlets

Components: lightning arresters protect against surges; the AB switch isolates the substation; DO fuses (or VCB with relay for larger units) protect the transformer; the 11/0.4 kV Dyn11 transformer gives a 3-phase 4-wire output with earthed neutral; the LT breaker and MDB distribute power to SDBs, with an APFC panel for power factor correction.

The single line diagram (SLD) shows, with one line for all three phases, how power flows from the 11 kV utility line through the distribution substation to the 230 V end users.

Single line diagram

 11 kV overhead feeder (utility, NEA)
            |
        [LA]  lightning arresters (3 nos)
            |
        [AB]  air-break / gang-operated isolator
            |
        [DO]  drop-out fuse  (or HT VCB + relay)
            |
        [HT metering CT/PT]  (for HT consumers)
            |
        (TR)  11/0.4 kV, Dyn11 transformer
            |      neutral + body earthed
        [ACB / MCCB]  LT main breaker + LT meter
            |
 ======== MDB  400 V, 3-ph 4-wire busbar ========
     |            |            |            |
   MCCB         MCCB         MCCB         MCCB
     |            |            |            |
   SDB-1        SDB-2        SDB-3       APFC panel
  (motors)    (lighting)    (sockets)   (capacitors)
     |            |            |
   MCB/MPCB      MCB          MCB/RCCB
     |            |            |
   400 V M     230 V lamps   230 V outlets

Function of each component

1. 11 kV incoming feeder: overhead line (or HT XLPE cable) from the utility (NEA) substation.
2. Lightning arresters (LA): one per phase, connected between line and earth; divert lightning and switching surges to ground and protect the transformer insulation.
3. AB switch / isolator: gang-operated, off-load switch for isolating the substation for maintenance; operated only after the load is removed.
4. Drop-out fuse (DO) / HT VCB: protects the transformer and the 11 kV line from transformer faults. Large industrial substations use a VCB or SF₆ breaker with overcurrent and earth-fault relays.
5. HT metering (CT/PT): for HT consumers, measures energy in kWh, kVAh and maximum demand for billing.
6. Distribution transformer 11/0.4 kV, Dyn11: steps voltage down to 400 V line / 230 V phase; delta HV blocks triplen harmonics, star LV gives a neutral for single-phase loads. Neutral and tank are earthed separately.
7. LT main breaker (ACB/MCCB): main switch and overload/short-circuit protection on the secondary; LT energy meter, ammeter and voltmeter.
8. Main distribution board (MDB): copper busbars that feed outgoing circuits through MCCBs or switch-fuse units.
9. APFC panel: automatic capacitor bank that keeps power factor near 0.95–0.99, reducing current and penalty charges.
10. Sub-distribution boards (SDB): located near load centres (each shop or floor); they have MCCB/MCB incomers and MCB outgoings.
11. Final circuits: MCBs, RCCBs/ELCBs, motor starters (DOL, star-delta) for 400 V motors, and 230 V lighting and socket circuits between one phase and neutral.
12. Earthing system: separate earth electrodes for transformer neutral, body/equipment and lightning arresters; protects people and lets protection operate.

Single-phase end users are balanced across R, Y and B phases so the transformer is equally loaded.

The figure is not available, so the method is shown on an assumed industry with two sub-distribution boards; the same steps apply to any given layout.

Assumed loads

• SDB-A: 5 motors, each 11 kW, η = 0.88, pf = 0.85
• SDB-B: lighting 8 kVA, two heaters of 10 kVA each

Factors used (IEC / Schneider Electric guide)

• Ku (factor of maximum utilization): motors 0.75; lighting and heating 1.0.
• Ks (factor of simultaneity) for a board with n circuits: 2–3 → 0.9, 4–5 → 0.8, 6–9 → 0.7, 10 and above → 0.6.

Step 1: rated kVA of each motor

kVA = kW / (η × pf) = 11 / (0.88 × 0.85) = 14.71 kVA

Step 2: utilized power with Ku

Step 3: apply Ks at each board

SDB-A = 55.15 × Ks(5 circuits = 0.8)  = 44.12 kVA
SDB-B = (8 + 20) × Ks(3 circuits = 0.9) = 25.20 kVA
MDB   = (44.12 + 25.20) × Ks(2 = 0.9)  = 69.32 × 0.9
      = 62.39 kVA

Step 4: future expansion and selection

Required = 62.39 × 1.2 (20 % margin) = 74.86 kVA

Answer: select a 100 kVA, 11/0.4 kV transformer (next standard rating above 74.86 kVA; standard sizes 50, 100, 160, 200, 250 kVA ...).

Remarks

• Connected load is 101.5 kVA but maximum demand is only about 62 kVA; sizing on connected load would over-size the transformer and lower its efficiency.
• If the figure gives Ku and Ks values directly, use those instead of the typical values.
• Check that the largest motor's starting kVA does not cause an excessive dip (usually < 10–15 %) on the chosen transformer.

A distribution substation is the substation that receives power at primary distribution voltage (11 kV or 33 kV) and steps it down to utilization voltage (400/230 V) for the consumers, with switching, protection and metering equipment. In an industry it is the point where the plant takes supply from the utility and distributes it to all shops.

Classification according to service

1. Step-down (transformer) substation: 11/0.4 kV or 33/11 kV; the common type in industries.
2. Switching substation: no transformer; only switches and breakers that connect or sectionalise incoming and outgoing feeders (e.g. HT switchboard in a large plant with several transformers).
3. Industrial substation: dedicated to one large consumer; may have HT metering, multiple transformers and HT motor feeders.
4. Converting substation: AC to DC (electrolysis, traction, DC drives) or frequency conversion (induction furnaces).
5. Power factor correction substation: houses capacitor banks or synchronous condensers.

Classification according to design (construction)

Brief explanation

• Outdoor substations are cheaper, easy to extend and cool well, but need fencing and more space and are exposed to weather.
• Indoor substations are safe, compact, protected from dust and weather and suitable near the load centre, but cost more (building, ventilation, fire protection).
• Pole-mounted types are the cheapest; equipment is LA, AB switch, DO fuse, transformer and LT distribution box.
• Package substations are factory-tested and need little site work.

The choice depends on load (kVA), space, environment, safety, reliability and cost.

The transformer is sized on the maximum demand of the industry, found by applying demand factor to each block and then the diversity factor for the whole plant.

Step 1: maximum demand of each block

Max demand = connected load × demand factor
Block 1 = 250 × 0.90 = 225.0 kVA
Block 2 = 200 × 0.80 = 160.0 kVA
Block 3 = 150 × 0.75 = 112.5 kVA
Block 4 = 400 × 0.85 = 340.0 kVA
Sum of individual max demands = 837.5 kVA

Step 2: maximum demand of the industry

Diversity factor = sum of individual max demands
                   / max demand of whole system
Max demand = 837.5 / 1.5 = 558.33 kVA

Step 3: select the transformer

Available sizes: 50, 100, 250, 400, 600, 750 kVA. The smallest size ≥ 558.33 kVA is 600 kVA.

Loading at peak = 558.33 / 600 = 93 %, which is acceptable.

Answer: maximum demand = 558.33 kVA; select one 600 kVA transformer. (If a future expansion margin of 20 % were added, 670 kVA would be needed and the 750 kVA unit would be chosen.)

Site evaluation criteria for a substation

1. Near the load centre: reduces LT cable length, voltage drop, losses and copper cost.
2. Access to the HT line: close to the 11 kV feeder route so the HT tap is short.
3. Accessibility: road access for transporting the transformer and for maintenance and fire-fighting vehicles.
4. Ground conditions: firm, level, non-flooding land; low soil resistivity for good earthing.
5. Safety: away from fuel stores, chemicals, explosive areas, schools and crowded places; enough clearance from buildings and trees.
6. Space: enough area for the present equipment, future expansion, fencing and working clearance.
7. Environment: free from dust, corrosive fumes and salt spray (otherwise choose indoor type); good natural ventilation.
8. Cost and legal issues: land cost, right-of-way, approval of the utility and local bodies.
9. Aesthetics and noise: transformer noise and appearance in residential surroundings.

Bill of quantities: 11/0.4 kV, 50 kVA H-pole substation (typical)

Three separate earths are provided: one for the transformer neutral, one for the transformer body and metal parts, and one for the lightning arresters. Quantities are typical and vary with the utility's standard drawing.

A distribution substation receives power at 11 kV (or 33 kV) and steps it down to 400/230 V, while providing switching, protection and metering for the consumers it supplies.

Importance of distribution substation

1. Voltage transformation: utilization equipment works at 400/230 V; power is transmitted economically at 11 kV and stepped down near the loads.
2. Reduced losses: placing the substation near the load centre keeps the LT (high-current) cable short, reducing I²R loss and voltage drop.
3. Protection: LAs, fuses, breakers and relays isolate faults quickly and protect the transformer, lines and consumers.
4. Control and switching: individual feeders can be switched off for maintenance without disturbing others.
5. Metering: energy and maximum demand measurement for billing and energy audit.
6. Power quality: tap changing for voltage regulation and capacitor banks for power factor correction.
7. Reliability: in industries, duplicate transformers and bus couplers keep supply available during faults.
8. Neutral and earthing: the Dyn11 transformer gives an earthed neutral for 3-phase 4-wire supply and safe single-phase loads.

Single line diagram of 11 kV distribution substation

 11 kV overhead feeder (utility, NEA)
            |
        [LA]  lightning arresters (3 nos)
            |
        [AB]  air-break / gang-operated isolator
            |
        [DO]  drop-out fuse  (or HT VCB + relay)
            |
        [HT metering CT/PT]  (for HT consumers)
            |
        (TR)  11/0.4 kV, Dyn11 transformer
            |      neutral + body earthed
        [ACB / MCCB]  LT main breaker + LT meter
            |
 ======== MDB  400 V, 3-ph 4-wire busbar ========
     |            |            |            |
   MCCB         MCCB         MCCB         MCCB
     |            |            |            |
   SDB-1        SDB-2        SDB-3       APFC panel
  (motors)    (lighting)    (sockets)   (capacitors)
     |            |            |
   MCB/MPCB      MCB          MCB/RCCB
     |            |            |
   400 V M     230 V lamps   230 V outlets

Explanation: incoming 11 kV supply passes through lightning arresters (surge protection), an AB switch (isolation) and DO fuses or a VCB (protection) to the 11/0.4 kV transformer. The LT side has a main breaker and meter feeding the MDB, from which MCCB-protected feeders go to SDBs and an APFC panel. SDBs supply motors at 400 V and lighting and sockets at 230 V through MCBs and RCCBs.

A distribution substation of an industrial plant is the installation that receives power at 11 kV (or 33 kV) from the utility and, through transformers, switchgear and protection, supplies the plant at 400/230 V.

Factors for selecting the substation site

1. Load centre: the site should be near the centre of gravity of the loads, found as x̄ = Σ(Pᵢ·xᵢ) / ΣPᵢ, ȳ = Σ(Pᵢ·yᵢ) / ΣPᵢ. This minimises LT cable length, voltage drop, losses and cost.
2. Nearness to HT supply: short route for the incoming 11 kV line or cable.
3. Access: approach road for bringing in the transformer and for maintenance and fire vehicles.
4. Space: room for the equipment, safety clearances, fencing and future extension.
5. Ground and drainage: firm, flood-free, well drained land with low soil resistivity for earthing.
6. Safety: away from fire hazards (fuel, chemicals, explosives), vibration and people movement.
7. Environment: clean, ventilated and dust-free; indoor type if the air is dusty or corrosive.
8. Cost: land cost and cost of cables and civil works.
9. Utility rules: approval of the supply authority and statutory clearances.

Locating the distribution substation within the industry premises

The substation should be as close as practical to the electrical load centre, subject to safety and site limits.

1. Find the load centre: draw the plant layout on x-y axes, mark each load (or SDB) with its demand Pᵢ at (xᵢ, yᵢ) and compute

   x̄ = Σ Pᵢ·xᵢ / Σ Pᵢ
   ȳ = Σ Pᵢ·yᵢ / Σ Pᵢ
   

   The point (x̄, ȳ) gives the least product of load × distance, so LT cable cost, voltage drop and I²R loss are minimum.
2. Adjust for practical constraints: if the load centre falls inside a production hall or on a road, shift the substation to the nearest suitable free place, preferably towards the heaviest loads.
3. Check the HT supply route: the 11 kV line or cable should reach the site easily.
4. Access and space: crane/truck access for transformer replacement, space for fencing, clearances and future transformers.
5. Safety and environment: away from fire-prone stores, explosive or corrosive areas, flooding and vibration; good ventilation (or indoor/package type in dusty plants).
6. Large plants: if loads are spread over a large area, use more than one substation (unit substations), each near its own load group, connected by an HT ring.

Location of floor distribution board in a building

1. Load centre of the floor: so that final circuits are short and voltage drop is within limits.
2. Near the rising main / shaft: the DB should be close to the vertical rising main or electrical shaft so the tap-off is short.
3. Accessibility: easily reachable by authorised staff, in a corridor or electrical room, not inside private rooms or store rooms; at a convenient height (about 1.5–1.8 m to top).
4. Safety: away from water lines, toilets, kitchens, heat sources and fire escape routes; dry, ventilated place.
5. Space: enough wall space and front clearance (about 1 m) for operation and maintenance, and spare ways for future circuits.
6. Separation: separate DBs for lighting, power and emergency circuits where required; emergency DB in a fire-protected location.
7. Aesthetics: recessed or in a cupboard in offices and residences.
8. Cable routing: easy entry for cables and conduits from the riser and to the final circuits.

Classification of industrial distribution substations

According to service

• Step-down (transformer) substation: 11/0.4 kV or 33/11 kV.
• Switching substation: only switchgear, no transformer.
• Converting substation: AC to DC or frequency conversion.
• Power factor correction substation: capacitor banks.

According to design

• Outdoor: pole-mounted (single pole), H-pole (double pole, up to about 250 kVA), plinth-mounted (above 250 kVA).
• Indoor: equipment inside a building.
• Underground and packaged/compact (kiosk) substations.

Schematic diagram from 11 kV to end use

 11 kV overhead feeder (utility, NEA)
            |
        [LA]  lightning arresters (3 nos)
            |
        [AB]  air-break / gang-operated isolator
            |
        [DO]  drop-out fuse  (or HT VCB + relay)
            |
        [HT metering CT/PT]  (for HT consumers)
            |
        (TR)  11/0.4 kV, Dyn11 transformer
            |      neutral + body earthed
        [ACB / MCCB]  LT main breaker + LT meter
            |
 ======== MDB  400 V, 3-ph 4-wire busbar ========
     |            |            |            |
   MCCB         MCCB         MCCB         MCCB
     |            |            |            |
   SDB-1        SDB-2        SDB-3       APFC panel
  (motors)    (lighting)    (sockets)   (capacitors)
     |            |            |
   MCB/MPCB      MCB          MCB/RCCB
     |            |            |
   400 V M     230 V lamps   230 V outlets

Explanation

1. 11 kV incoming line from the utility feeder.
2. Lightning arresters protect the transformer from lightning and switching surges.
3. AB switch isolates the substation (off-load) for maintenance.
4. DO fuse (or VCB with overcurrent and earth fault relays in bigger plants) protects the transformer from internal faults and overloads.
5. HT metering for energy billing of HT consumers.
6. 11/0.4 kV Dyn11 transformer gives 400 V line and 230 V phase voltage with a solidly earthed neutral.
7. LT ACB/MCCB is the main LT switch and protection; LT meters show current, voltage and energy.
8. MDB busbars divide power into feeders with MCCBs; an APFC panel improves power factor.
9. SDBs near each shop feed motors (through starters and overload relays) at 400 V and lighting and sockets at 230 V through MCBs and RCCBs.
10. Earthing of neutral, equipment bodies and arresters completes the system.

Determining the proper transformer size

The transformer is sized on the maximum demand, not on connected load:

1. List all connected loads in kVA (kVA = kW / (η·pf)).
2. Apply demand factor (or Ku) to each load group to get its maximum demand.
3. Divide the sum of group maximum demands by the diversity factor (or multiply by Ks).
4. Add 20–25 % for future expansion.
5. Choose the next higher standard rating (50, 100, 160, 200, 250, 315, 400 kVA ...).

Example: an industry has motors 120 kVA (demand factor 0.7), lighting 20 kVA (DF 0.9) and other loads 40 kVA (DF 0.6); diversity factor 1.3.

Max demands: 120×0.7 = 84, 20×0.9 = 18, 40×0.6 = 24
Sum = 126 kVA
Plant max demand = 126 / 1.3 = 96.92 kVA
With 20 % margin = 96.92 × 1.2 = 116.31 kVA

Selected: 160 kVA, 11/0.4 kV transformer.

Single line diagram: 11 kV feeder to 230 V end user

 11 kV feeder
     |
   [LA]   lightning arrester
     |
   [AB]   isolator
     |
   [DO]   HT fuse
     |
   (TR)   11/0.4 kV, Dyn11
     |
   [MCCB] LT main + meter
     |
 === MDB 400 V ===
     |
   [MCCB] -> SDB (400/230 V)
     |
   [MCB + RCCB]
     |
   230 V end user (phase-neutral)

Purposes of a substation in an industrial electrical system

1. Step down the utility voltage (11 or 33 kV) to utilization voltage (400/230 V), or to 3.3/6.6 kV for large HT motors.
2. Switching: connect and disconnect feeders and transformers for operation and maintenance.
3. Protection: detect and isolate faults (overload, short circuit, earth fault, surges) to protect equipment and people.
4. Metering and monitoring: energy, maximum demand and power factor measurement for billing and energy management.
5. Power quality: voltage regulation (tap changers) and power factor correction (capacitor banks).
6. Reliability: duplicate transformers, bus coupler and standby generator changeover keep critical loads supplied.

Indoor substation

In an indoor substation all equipment (HT switchgear, transformer, LT switchboard, capacitor bank) is installed inside a building or room. It is used where the air is dusty, corrosive or saline, where space is limited, or where the substation must be placed in the load centre inside the plant.

 11 kV cable from utility / RMU
          |
  +---- HT panel room --------------+
  |  [Isolator]--[VCB/SF6]--[CT]    |
  |       |   O/C, E/F relay        |
  |  [HT metering CT/PT, LA]        |
  +-------|-------------------------+
          | HT cable
  +---- Transformer room -----------+
  |   (TR) 11/0.4 kV dry/oil type   |
  |   oil pit, louvres, fire wall   |
  +-------|-------------------------+
          | LT cable / bus duct
  +---- LT panel room --------------+
  |  [ACB]--MDB busbar--[MCCBs]     |
  |            |--[APFC panel]      |
  +------------|--------------------+
               v  feeders to SDBs

Functions of each part

• HT panel: incomer isolator and VCB/SF₆ breaker with overcurrent and earth fault relays; trips on transformer or cable faults. Surge arresters at cable termination.
• Transformer room: oil-filled transformer with oil soak pit and fire wall, or dry-type (cast resin) transformer for fire safety; natural or forced ventilation through louvres.
• LT panel: ACB as main incomer, busbars and outgoing MCCBs to SDBs, metering, APFC panel.
• Earthing: earth grid with separate electrodes for neutral and body; earth bus around rooms.
• Auxiliaries: battery and charger for breaker tripping, lighting, fire extinguishers, rubber mats, cable trench.

Operating characteristics

• Equipment is protected from rain, dust and pollution, so insulation stays clean and maintenance is lower.
• Safe operation: live parts are enclosed, access is limited to authorised persons.
• Can be placed at the load centre, reducing LT cable lengths and losses.
• Needs good ventilation since the transformer heat must be removed; temperature rise limits loading.
• Higher initial cost (building, fire protection), and extension is harder than for outdoor types.
• Usually limited to 11–33 kV due to clearance and building size.

Classification of industrial distribution substations

By service: step-down (11/0.4 kV) substation, switching substation, converting substation (AC/DC), power factor correction substation.

By design: outdoor (pole-mounted, H-pole, plinth-mounted), indoor, underground and packaged (compact/kiosk) substations.

Indoor substation: functions and operation

All equipment is inside a building, divided into HT room, transformer room and LT room.

 11 kV cable in
     |
 [Isolator]-[VCB + relay]   HT room
     |
   (TR) 11/0.4 kV           TR room
     |
 [ACB]== MDB ==[MCCBs]      LT room
               |-[APFC]
               v to SDBs

• HT room: isolator and VCB with overcurrent and earth-fault relays switch and protect the incoming 11 kV supply.
• Transformer room: steps down to 400/230 V; oil pit and fire wall for oil units, or dry-type for fire safety; ventilation louvres.
• LT room: ACB incomer, busbars, MCCB feeders, meters and capacitor bank distribute power to SDBs.

Operating characteristics: protected from weather and dust, safe and compact, can be placed at the load centre; but costlier, needs ventilation and fire protection, and is limited to about 33 kV.

The transformer for any building is chosen on its maximum demand, found from connected load reduced by utilization/demand factors and diversity (simultaneity) factors, plus a margin for growth, then rounded up to a standard rating.

Apartment building

• Find the load per flat (lights, sockets, kitchen, water heater, AC) e.g. 6–10 kVA.
• Apply a simultaneity factor that falls with the number of flats (e.g. about 0.78 for 4 flats, 0.53 for 15, 0.4 for 50 flats).
• Add common services: lifts, water pumps, corridor lighting, fire pumps (with their own Ku and Ks).
• Add 20–25 % margin; usually one transformer (or two for reliability) with standby generator for lifts and pumps.

Residential building (individual house / colony)

• Small loads, high diversity; demand per house ≈ 2–5 kVA after diversity.
• Colony transformer = number of houses × after-diversity demand + street lighting; usually pole-mounted 50–250 kVA.

Industry

• Mostly motor loads: kVA = kW/(η·pf), Ku ≈ 0.75, plus lighting and heating (Ku = 1).
• Apply Ks per SDB and MDB depending on number of circuits and shift pattern.
• Check starting of the largest motor (voltage dip) and harmonics from drives.
• Consider separate or duplicate transformers for critical processes, and future expansion.
• Choose next standard rating (e.g. 250, 315, 400, 500, 630 kVA) so normal loading is about 60–80 %.

The best site for a substation is at or near the load centre (centre of gravity of the loads), because it gives the least load × distance product, so LT cable cost, voltage drop and losses are minimum.

Load centre calculation

x̄ = Σ W·x / Σ W = 6050 / 244 = 24.80
ȳ = Σ W·y / Σ W = 5600 / 244 = 22.95

Selection and justification

• The load centre is at about (24.8, 23.0), very close to W1 (10, 20).
• W1 (200 kVA) is 82 % of the total load, so the substation should be placed next to W1, on the side facing the other loads.
• The heavy 200 kVA current then travels only a few metres, while only the small loads (10–12 kVA, low current) need long cables to the right side of the site, where voltage drop can be handled with modest cable sizes.
• Placing it at the geometric middle of the site (around x = 55) or near W2–W5 would make the large W1 feeder long and costly, with high losses.

Answer: locate the substation near W1, at about (25, 23) (or the option in the figure nearest to this point), subject to access, safety and HT line route.

A distribution substation in an industry receives power from the utility at 11 kV, steps it down to 400/230 V and distributes it to the shops through switchgear and protective devices. It is the heart of the plant's electrical system.

Single line diagram

 11 kV overhead line / cable (NEA)
            |
        [LA]   lightning arresters
            |
        [AB]   gang-operated isolator
            |
     [DO fuse] or [VCB + O/C, E/F relay]
            |
     [HT metering: CT/PT, kWh, kVA MD]
            |
        (TR)   11/0.4 kV, Dyn11
            |      neutral & body earthed
        [ACB/MCCB] LT incomer + meters
            |
 ====== MDB 400 V, 3-ph 4-wire busbar ======
    |          |          |          |
  MCCB       MCCB       MCCB      MCCB
    |          |          |          |
  SDB-1      SDB-2      SDB-3    APFC panel
  motors    lighting   sockets   (capacitors)
    |          |          |
 starters    MCB        MCB+RCCB
    |          |          |
  400 V M   230 V      230 V

Explanation of the parts

1. Incoming 11 kV supply: tapped from the utility feeder by overhead line or HT cable.
2. Lightning arresters: one per phase, protect the transformer and switchgear from lightning and switching surges by diverting them to earth.
3. AB switch / isolator: off-load switch to isolate the whole substation for maintenance.
4. HT protection: drop-out fuses for small (pole/H-pole) substations; VCB or SF₆ breaker with overcurrent and earth-fault relays for larger plants.
5. HT metering: CTs and PTs feed the trivector/kWh meter for energy and maximum demand billing.
6. Transformer: 11/0.4 kV, delta-star (Dyn11). The delta HV winding traps triplen harmonics; the star LV gives 400 V between lines and 230 V phase to neutral. Rating is chosen on maximum demand.
7. LT incomer (ACB/MCCB): main switch and protection against overload and short circuit on the LT side, with ammeter, voltmeter and kWh meter.
8. Main distribution board (MDB): copper/aluminium busbars with outgoing MCCBs or switch-fuse units to each SDB.
9. APFC panel: automatic capacitor bank to keep power factor near unity, reducing current, losses and demand charges.
10. Sub-distribution boards: near the load centres; feed motors through starters (DOL, star-delta, soft starter) and lighting/socket circuits through MCBs and RCCBs.
11. Earthing: separate electrodes for transformer neutral, equipment bodies and lightning arresters.
12. Standby supply: a diesel generator with changeover switch (ATS) at the MDB feeds essential loads during outages.

Types used: pole/H-pole mounted (up to about 250 kVA), plinth-mounted outdoor (above 250 kVA), indoor or packaged substations where space or environment demands.

An indoor substation houses all substation equipment inside a building. With a single transformer the layout is simple: an HT room, a transformer room and an LT room arranged in the order of power flow, with cable trenches between them.

General layout (plan)

          11 kV cable in (from RMU / pole)
                    |
 +------------------|----------------------------+
 |  HT ROOM         v                            |
 |  [Isolator]--[VCB]--[CT]--[relay panel]       |
 |  [HT meter]  [battery + charger]             |
 +------------------|----------------------------+
 |  TRANSFORMER     v           |  LT ROOM       |
 |  ROOM                        |                |
 |   +----------+               |  [ACB incomer] |
 |   |  (TR)    |-- LT cable -->|  == MDB ==     |
 |   | 11/0.4kV |   in trench   |  [MCCBs] out   |
 |   +----------+               |  [APFC panel]  |
 |   oil soak pit, fire wall    |                |
 |   louvres for air            |  feeders ->SDB |
 +------------------------------+----------------+
  door (wide, for TR)    earth pits outside

Description of the layout

1. HT room
   • Incoming 11 kV XLPE cable terminated with surge arresters.
   • HT panel: isolator/load-break switch, VCB or SF₆ breaker, CTs and PTs, overcurrent and earth-fault relays, HT metering.
   • DC battery and charger for breaker tripping and closing.
2. Transformer room
   • Single 11/0.4 kV transformer (oil-filled ONAN or dry-type cast resin).
   • Oil-filled units need an oil soak pit filled with pebbles, a fire-resistant wall and a fire door.
   • Low-level inlet and high-level outlet louvres (or exhaust fans) for cooling; clearance of about 0.75–1 m around the transformer.
   • Wide door and rails for moving the transformer in and out.
3. LT room
   • LT panel with ACB incomer, busbars, outgoing MCCBs to SDBs, meters and indication lamps.
   • APFC capacitor panel.
   • Changeover from DG set if standby supply is provided.
4. Cable trenches covered with chequered plates connect HT panel to transformer and transformer to LT panel; outgoing cables leave through trenches or ducts.
5. Earthing: earth bus inside the rooms connected to separate earth pits for neutral and body; LA earthing separate.
6. Safety items: rubber mats in front of panels, fire extinguishers (CO₂ / DCP), sand buckets, emergency lighting, danger boards, single-line diagram on the wall, adequate lighting and first-aid chart.

Features

• Safe and protected from dust, rain and pollution; good for chemical, textile and cement plants.
• Can be built at the load centre within the factory building.
• Higher cost and need for ventilation and fire protection; extension is limited since there is only one transformer (no redundancy). A second transformer bay may be kept for future.