Make your own free website on


Assessment of electrical energy requirements

Number & size of generating units

Base load & peak load plants





 Objective type questions

Objective type questions

From the economic point of view , a power system should have a load factor

  1. About 0.12
  2. About 0.5
  3. About1

Ans. c

From the economic point of view , a power system should have a diversity factor

  1. Much less than 1
  2. Nearly equal to but less than one
  3. Greater than one

Ans. c

Peak load hydro power plants compared to base load thermal plants have plant capacity factor

  1. Much lower
  2. Much higher
  3. Almost equal

Ans. a

Run-of-river plants are used for

  1. Base load operation
  2. Peak load operation
  3. Near base load

Ans. a

The two-part tariff is based on

  1. MW and MVar
  2. MW and MVA
  3. MWH and MVA

Ans. c


Assessment of electrical energy requirements

Past developmental patterns: Local forecasting (short term & long term) using historical data & future projections

Definition of terms used in Power System Planning:

Number & size of generating units

Factors in selection:

  1. Reliability- catering to planned & forced outages
  2. Efficiency and economy- Units should ideally operate near maximum efficiency loading. Variations in load dictate larger number of units. Reliability is better with larger number of units, but initial cost & maintenance cost goes up.

The thumb rule is; select a minimum number of units fitting the load curve as closely as possible

Definitions of some factors used in unit selection:

Plant capacity factor: Gives an idea of reserve available.

Plant use factor: considers actual period of operation.

For hydro plants, plant capacity and plant use factors are much lower than those for thermal plants. Due to water availability restrictions hydro units are shut down for prolonged periods. Loading also varies substantially for the same reason.

Base load & peak load plants

Base load exists for 100% of the time

Criterion for selecting units to operate in a portion of the load duration curve:

  1. Efficiency or incremental cost in case of thermal plants
  2. Type of plant /storage for hydro plants

Generally hydro plants are used for peak shaving replacing high incremental cost thermal generation. Gas turbine plants although very costly are also used during peak period due to quick start/stop and loading. Nuclear plants are used for base load due to difficulty in frequent load variation.

Run of river plants without pondage are obviously base loaded.


The costs can be broadly classified as

  1. Fixed costs

Fixed cost is the capital investment on all types of assets.

The cost per unit (kWh) chargeable to the consumer depends on

  1. Running cost or energy cost

This consists of

Single-part tariff- is based only on energy consumption

Two-part tariff- is based on energy consumption and on a component linked to maximum demand (kVA or MVA).

If a demand more than previous level remains sustained for say half an hour then this becomes the established maximum demand until some other demand crosses this limit during the month. The second component is sometimes the fixed cost. MD influences the installed capacity, and hence, tariffs on MD are justified. KVA or MVA is more appropriate than kW/MW due to thermal considerations for lines, transformers, etc. The consumer automatically pays more for poor power factor. There is thus an incentive to improve the power factor.

Sometimes penalty clauses are also built in to prevent excessive and unscheduled maximum demand. This is particularly important in developing countries with generation shortages