Load scheduling and bus arrangement

Importance of load scheduling

Load scheduling means planning when each major load or process runs so that the plant's maximum demand stays low and the load curve is flat.

1. Lower demand (kVA) charges – the monthly demand charge is based on the highest demand; staggering large motors, furnaces and compressors lowers it.
2. Benefit from time-of-day tariff – flexible loads (pumping, batch processes, charging) moved to off-peak night hours use cheaper energy.
3. Higher load factor – better use of transformer, cables and DG set capacity; postpones new investment.
4. Avoids overloading and voltage dips – motors are not started all at once.
5. Lower losses – a flat load has lower I²R loss than a peaky load for the same kWh.
6. Helps the utility reduce system peak and load shedding.

Bus arrangements in industrial distribution

1. Single bus system – all incoming supplies and outgoing feeders on one bus.
   • Simple, cheap, easy to operate.
   • A bus fault or bus maintenance shuts down the whole plant. Least reliable.
2. Single bus with bus sectionaliser – bus split into two sections by a circuit breaker (bus coupler), each fed by its own transformer.
   • A fault on one section is isolated; the other continues. Loads can be shared between sections.
3. Main and transfer bus – a transfer bus with a transfer breaker allows any feeder breaker to be taken out for maintenance without interrupting that feeder.
4. Double bus (double breaker or double bus single breaker) – two main buses; each circuit can be connected to either bus through isolators/breakers.
   • Bus maintenance or bus fault: loads transferred to the other bus.
   • Most flexible and reliable, but costly.
5. Ring main bus – bus forms a closed ring with breakers between circuits; each circuit fed from two sides.

Single bus with sectionaliser
  T1          T2
  |           |
 =X===[CB]===X=   (bus coupler)
  | |       | |
  F1 F2     F3 F4

Double bus
 ==== Bus 1 ===========
 ==== Bus 2 ===========
    |  |   |   |
   CB  CB  CB  CB  (each can select bus 1 or 2)
    F1 F2  F3  F4

Which is more reliable and why

The double bus (especially double bus–double breaker) arrangement is the most reliable, because:

• any bus can be taken out for maintenance or isolated after a fault without loss of supply;
• loads can be divided between buses so a fault affects only part of the plant;
• every circuit has an alternate path.

For most medium industries, a sectionalised single bus fed from two transformers is chosen as a good compromise: it gives continuity for essential loads at much lower cost than a double bus. The single bus without sectionalising is the least reliable.