Terms Involved in Tariff
1. Variable load :
- The load on a power station varies from time to time due to uncertain demands of the consumers and is called as variable load.
- An ideal load is one which is always constant for a given duration of time. However, ideal load can be realized in practice.
- It is because consumers require small or large amount of power in accordance with their activities , need etc.
- Therefore, load demand of one consumer at a given time may be different from that of the other consumer.
- Due to such variations of load demand from consumer to consumer, the load on power station varies from time to time.
2. Effects of variable load :
- Due to variation of load on power station, some effects of variation of load on power stations are (i) Need of additional equipments. (ii) Increase in production cost.
3. Load curves :
- The curve which shows the variation of load on the power station with respect to time is known as load curve.
- We know that the load on power station is never constant. Hence, the variation in load of power station is recorded during the whole day (i.e. 24 hours) hourly or half hourly.
- The load of 24 hours recorded is plotted against time. This load curve is called daily load curve. Figure A. shows a typical daily load curve on power station.
- The curve shows that load is varying with time.
- On similar line, monthly load curve (load versus days in month), annual load curve can also be plotted. However, monthly load curve is obtained from daily load curve.
- In this case, average load of each day is calculated and a curve of average load versus day for a entire month is plotted. Such curve is called monthly load curve. Similarly load curve is obtained from monthly load curve.
 |
| Figure A |
4. Importance of load curve :
- The load curves, especially daily load curves, provide the following few important information about load on power station.
- The daily load curve shows variation of load on power station during 24 hours of the day.
- Daily load curve shows the highest load point (i.e. maximum demand) on the power station on a given day.
- The area under the curve gives the number of units generated in that day i.e. Units generated (in kWh) in a day equal to area of load curve.
- The daily load curve gives average load by using the following formula :
Average load = Area (in kWh) under
daily load curve / 24 hours - Load curve helps in selecting size and number of generating units.
- The load curve helps in preparing the operation schedule.
5. Important factors :
- For design of efficient and economic power stations for variable load, following terms must be studied.
Connected load :
- It is the sum of continuous ratings of all equipments (in watt or kilowatt) connected to supply system.
- We know that power station supplies load to thousands of consumers. Each consumer has/ his installed (connected) load.
- For example one consumer has 200 watts five bulbs, 80 watts five tans. 1000 watt heater, 100 watt T.V.connected to supply. Then his connected load is calculated as follows :
(i) Load of bulbs = 5 x 200 W
(ii) Load of fans = 5 x 80 = 400 W
(iii) Load of heater = 1 x 1000 = 1000 W
(iv) Load of T.V = 1 x 100 = 100 W / Total = 2500 W.
Hence, connected load of that consumer is 2500 W.
(ii) Maximum demand (or maximum load) :
- It is the highest demand of load on the power station during a given period.
- We know that load demand varies from time to time. The load curve shows the variation in the load.
- The highest load (i.e. greatest demand) on load curve for a given duration of time indicates maximum load. Following typical load curve in Figure B. shows maximum demand as 80 MW in 6 to 8 pm.
- The knowledge of maximum demand is important, since it gives the following information : (a) Maximum demand is generally less than connected load. (b) The installation capacity of generating stations (i.e. power stations) can be determined from maximum demand.
 |
| Figure B |
(iii) Demand factor :
- It is the ratio of maximum demand on the power station to its connected load i.e.
Demand factor
= maximum demand / Connected load
- The value of demand factor is generally less than 1. It 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 80 MW and the connected load is 100 MW, then demand factor = 80/100 = 0.8.
- The knowledge of demand factor is vital in determining the capacity of the plant equipment.
(iv) Average load :
- The average of loads occurring on the power station in a given period (day or month or year) is known as average load or average demand.
Daily average load = Number of
units (KWh) generated in a day / 24 hours
Monthly average load = Number of
units (KWh) generated in a month / Number of hours in a month
Yearly average load = Number of
units (KWh) Generated in a year / 8760 hours
(v) Load factor :
- The ratio of average to maximum demand during a given period is known as load factor.
Load factor
= Average load /
Maximum load
- If the plant is in operation for T hours,
Load factor
= Average load x T /
Maximum demand x T
= Units generated in T
hours / Maximum demand x T hours
- 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.
- Load factor is always less than 1 because average load is smaller than the maximum demand. The load factor is important in determining the overall cost per unit generated Higher the load factor of the power station, lesser will be the cost per unit generated.
- It is because higher load factor means lesser maximum demand. The station capacity is 50 selected that it must meet the maximum demand. Now, lower maximum demand means lower capacity of the plant which therefore, reduces the cost of the plant.
(vi) Diversity factor :
- The ratio of the sum of individual maximum demand to the maximum demand on power station is known as diversity factor i.e.
Diversity factor = Sum of
individual maximum demand / Maximum demand on power station
- 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.
(vii) Plant capacity factor :
- It is the ratio of actual energy produced to the maximum possible energy that could have been produced during a given period i.e.
Plant capacity factor
= Average
demand x T / Plant capacity x T
demand x T / Plant capacity x T
=
Average demand / Plant capacity
- Thus, if the considered period is one year,
Annual plant capacity factor =
Annual KWh output / Plant capacity x 8760
- The plant capacity factor is an indication of the reserve capacity of the plant. 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 generally greater than the maximum demand on the plant. Reverse capacity-Plant capacity-Maximum demand.
- It is interesting to note that difference between load factor and plant capacity factor is an indication of reserve capacity.
- If the maximum demand on the plant is equal to the plant capacity, then load factor and plant capacity factor will have the same value. In such a case, the plant will have no reserve capacity.
(viii) Plant use factor :
- It is the ratio of kWh generated to product of plant capacity and the number of hours for which the plant is Station output in kWh in operation i.e.
Plant use factor = Station output
in KWh / Plant capacity x Hours of use
- If a plant having installed capacity of 20 MW produces annual output of 80 x 109 kWh and remains in operation for 2190 hours in a year, then,
Plant use factor
= 80 x 10 / (20
x 10 ) x 2190
= 0.1826 = 18.26%
Units generated per year :
- It is often required to find the kWh generated per annum from maximum demand and load factor. The procedure is as follows :
Load factor = Average load / Maximum
demand
∴ Average
load = Maximum demand x L.F.
Units generated / annum
= Average load
(in KW) x Hours in a year
= Maximum demand (in KW) x L.F. x 8760
Load duration curve :
- When the load elements of a load curve are arranged in the order of descending magnitudes. the curve thus obtained is called a load duration.
- The load duration curve is obtained from the same data as the load curve but the ordinates are arranged in the order of descending magnitudes.
- In other words, the maximum load is represented to the left and decreasing loads are represented to the right in the descending order.
- Load duration curve gives the following information :
- The load duration curve gives the data in a more presentable form. In other words, it readily shows the number of hours during which the given load has prevailed.
- The area under the load duration curve is equal to that of the corresponding load curve. Obviously, area under daily load duration curve (in kWh) will give the units generated on that day.
- The load duration curve can be extended to include any period of time. By laying out the abscissa from O hour to 8760 hours, the variation and distribution demand for an entire year can be summarized in one curve. The curve thus obtained is called the annual load duration curve.
 |
| Figure C |
Types of Electrical loads :
A device or equipment which consumes electrical power is called load.
The load may be resistive (e.g. bulb, heater), inductive (e.g. induction motor), capacitive or combination of them. Depending upon various users, the various loads on power system are as follows :
(i) Domestic load : It refers to residential load. It consists of bulbs, heaters, fans, refrigerator, T.V., washing machine etc. Such load has low load factor (10 to 15%).
(ii) Commercial load : It consists of lighting , fans and electric appliances of shops, hotels, restaurants, exhibition etc. This load has seasonal variations.
(iii) Industrial load : It is a load demanded by industries. Industrial load has a major share in capacity of power station. This load is generally independent of weather conditions and has reasonably very good load factor (50 to 70%).
(iv) Irrigation load : It is a load required for running motor driven pumps to supply water for fields. Generally, this load exists during 12 hours of night.
(v) Railway ( Traction load ) : It is a load of railway, trams, trolley etc. It is also a variable load.
(vi) Municipal load : It consists of street lighting , drainage purpose and water supply.