Load estimate, supply features and sources

Importance of electrical load estimate

Load estimation finds the actual maximum demand of the installation from the connected load using demand and diversity factors (or ku and ks). It is important because:

• It fixes the size of transformer, generator, cables, busbars and switchgear. Over-estimate wastes money and runs equipment at low efficiency; under-estimate causes overheating and tripping.
• It sets the contract demand and tariff with the utility (NEA).
• It is needed for voltage drop, fault level and protection calculations.
• It allows planning for future expansion with a known margin.

Key features of the supply system to industry

1. Reliability and continuity: process loads need supply with few interruptions; alternative sources or ring/selective schemes are used.
2. Good voltage regulation: voltage within about ±5% of rated, so motors give rated torque.
3. Correct frequency (50 Hz in Nepal) and a sinusoidal waveform with low harmonics.
4. Adequate capacity for present load plus future expansion.
5. Safety: proper earthing, protection and isolation.
6. Flexibility: easy to extend and to isolate parts for maintenance.
7. Economy: low first cost, low losses, good power factor.
8. Supply voltage matched to the load size: 400/230 V for small plants, 11 kV or 33 kV for medium and large plants, 66 kV or 132 kV for very large plants.

Sources of supply to an industrial plant

1. Utility grid (NEA): the main source for most plants. Small industries get 400 V three-phase; larger ones take 11 kV or 33 kV and have their own substation. Cheapest and most reliable for normal running, but subject to load shedding and outages.
2. Captive power plant: the plant's own generator, such as a small hydro plant (common in Nepal for remote industries), a diesel generator set or a steam turbine. Used where grid supply is absent or unreliable.
3. Cogeneration: industries like sugar, paper and cement use process steam or waste heat (bagasse) to make electricity and heat together, and may sell surplus power to the grid.
4. Standby/emergency sources: diesel generator sets with automatic transfer switches, UPS and battery banks for critical loads (control systems, emergency lighting, computers).
5. Renewable sources: rooftop solar PV (grid-tied or with batteries), wind, biogas. They reduce the energy bill and dependence on the grid.
6. Dual utility feeders: two incoming feeders from different substations for high reliability.