Some important terms and factors
1. Connected load:
It is the sum of continuous ratings of all the equipments connected to supply system. A power station supplies load to thousands of consumers. Each consumer has certain equipment installed in his premises. The sum of the continuous ratings of all the equipments in the consumer’s premises is the “connected load” of the consumer. For instance, if a consumer has connections of five 25 watts lamps and a power point of 750 watts, then connected load of the consumer is 5 × 25 + 750 = 875 watts. The sum of the connected loads of all the consumers is the connected load to the power station.
2. Demand:
The demand of an installation or system is the load that is drawn from the source of supply at the receiving terminals averaged over a suitable and specified interval of time (day or month or year). It is also known as average load or average demand. Demand is expressed in kilowatts (kW), kilovolt-amperes (kVA), amperes (A), or other suitable units.
3. Maximum demand:
It is the highest demand of load on the power station during a given period. The maximum of all the demands that have occurred during a given period (say a day) is the maximum demand. Thus referring back to the load curve of Figure, the maximum demand on the power station during the day is 35 kW. Maximum demand is generally less than the connected load because all the consumers do not switch on their connected load to the system at a time. The knowledge of maximum demand is very important as it helps in determining the installed capacity of the station. The station must be capable of meeting the maximum demand.
4. Demand factor:
It is the ratio of maximum demand on the power station to its connected load. The value of demand factor is usually less than 1, because is expected because maximum demand on the power station is generally less than the connected load.
If the maximum demand on the power station is 85 MW and the connected load is 100 MW, then demand factor = 85/100 = 0·85. The knowledge of demand factor is vital in determining the capacity of the plant equipment.
5. Load factor:
The ratio of average load to the maximum demand during a given period is known as load factor. The load factor may be daily load factor, monthly load factor or annual load factor if the time period considered is a day or month or year respectively. Load factor is always less than 1 because average load is smaller than the maximum demand. The load factor plays key role in determining the per unit generation cost. Higher the load factor of the power station, lesser will be the cost per unit generated.
5. Diversity factor:
The ratio of the sum of individual maximum demands to the maximum demand on power station is known as diversity factor. A power station supplies load to various types of consumers whose maximum demands generally do not occur at the same time. Therefore, the maximum demand on the power station is always less than the sum of individual maximum demands of the consumers. Obviously, diversity factor will always be greater than 1. The greater the diversity factor, the lesser is the cost of generation of power.
Example: Suppose We have four individual feeder-circuits with connected loads of 250 kVA, 200 kVA, 150 kVA and 400 kVA and demand factors of 90%, 80%, 75% and 85% respectively. Use a diversity factor of 1.5. Calculate transformer rating for the feeder-circuits.
Solution: Max demand of consumer-1: 250 kVA x 90% = 225 kVA
Max demand of consumer-2: 200 kVA x 80% = 160 kVA
Max demand of consumer-3: 150 kVA x 75% = 112.5 kVA
Max demand of consumer-4: 400 kVA x 85% = 340 kVA
Sum of individual maximum demands = 837.5 kVA
Max. demand on the feeder-circuits = 837.5 kVA ÷ 1.5 = 558 kVA .
So, the feeder-circuit would have to be supplied by a 600 kVA transformer.
However, If the main feeder-circuit were sized at unity diversity. Then, max. demand on the feeder-circuits = 837.5 kVA ÷ 1.00 = 837.5 kVA.
So, the feeder-circuit would have to be supplied by an 850 kVA transformer.
Thus, with a given number of consumers the higher the diversity factor of their loads, the smaller will be capacity of the power plant required and consequently the fixed charges due to capital investment will be much reduced. Hence the diversity factor has direct effect on fixed cost per unit generated.
6. Plant capacity factor:
It is the ratio of actual energy produced in kWh to the maximum possible energy that could have been produced during a given period. Expressing the definition mathematically,
A power station is so designed that it has some reserve capacity for meeting the increased load demand in future. Therefore, the installed capacity of the plant is always somewhat greater than the maximum demand on the plant.
Reserve capacity = Plant capacity − Maximum demand
If the maximum demand on the plant is equal to the plant capacity (or also called installed capacity), then load factor and plant capacity factor will have the same value. In such a case, the plant will have no reserve capacity.
7. Plant use factor:
It is ratio of kWh generated to the product of plant capacity and the number of hours for which the plant has been in operation.
Suppose a plant having installed capacity of 20 MW produces annual output of 7·8 × 10^{6} kWh and remains in operation for 2190 hours in a year. Then, plant use factor is (7·8 × 10^{9} / 20 x 10^{6} x 2190) = 0.178 i.e., 17.8 %.
8. Utilization factor:
The utilisation factor is defined as the ratio of the maximum demand to the plant capacity. Utilization Factor can be more than unity due to overloading of the plant.
9. Coincidence factor:
Coincidence factor is the ratio of the maximum demand by the sum of the individual maximum demands. It is always less than one.
10. Load diversity:
It is the difference between of sum of individual maximum demand and the maximum demand of the whole system.
LD = (Sum of the peaks of all individual loads) – (Peak of the combined load)
11. Loss factor:
It is the ratio of average power loss to power loss at peak load during the specified period of time.