1. Power Systems presentation from ch-3
Dated: 27th October inam ul haq

2. Variable load on Power stations
The load on power stations vary time to time due to uncertain demands of consumers and is known as "variable loads on power stations .
Variable load on power stations introduces many perplexities in its operation. some of the important effects of variable load on power stations are :
Need of additional equipment
For illustration consider a steam power plant ,air ,coal and water are the raw materials .if the load varies from time to time ,additional equipment will be required for the variable supply of the raw materials .

3. Increase in production cost
The alternators installed in the power stations operate at maximum efficiency neaf its rated output .
if a single alternator is used ,it will have poor efficiency during periods of light loads.this leads to increase in production cost of per KWh energy .
load curves
The curve showing the variation of load on power stations with respect to time is known as ' load curve'.

4. Importance of load curves
the daily load curve shows the variations of load on the power station during different hours of the day
The area under the daily load curve gives the number of units generated in the day . units generated/day=area under daily load curve (KWh)
The highest point in the curve represents the maximum demand of the day .
The rea under the daily load curve divided by the total number of hours gives the average load on the station on that day

5. The ratio of the area under the load curve to the total area of rectangle in which it is contained gives the load factor .
The load curve helps in selecting the size and number of generating units.
The load curve helps in preparing the operation schedule of the station.

6. Important terms and factors Connected load
It is the sum of continuous ratings of all the equipments connected to supply system .
Maximum demand
It is the greatest demand of load
on the power station during
a given period .
This factor helps in selecting
the capacity of the units
installed in a power system ,to meet
the maximum possible electrical
load.

7. Demand factor Average load
It is the ratio of 'maximum demand ' to 'the connected load' on the power station .
The value of demand factor is usually less than 1 .
The more is the value close to 1,more economicaly electrical energy is produced.
Generally,demand factor of a particular feeder or a sub panel is calculated.
Demand factor can be used to calculate the size of the sub main / sub panel.
Average load
The average loads occurirng on the power station in a given period is known as 'average load ' or ' average demand '.

9. Load factor
The ratio of ' average load' to 'the maximum demand ' during a given period is known as "load factor".
load factor may be daily,monthly ,or annual load factor . its value is always less than 1.
Load factor is a term that doesnt appear in the bill,but does affect the cost of electrical energy.
load factor is the measure how efficiently a consumers are utilizing the electrical energy

11. Diversity factor Plant capacity factor
It is the ratio of sum of individual maximum demands to the maximum demand on the power station.
The value of diversity factor is greater than 1.the greater the greater the diversity factor the lesser is the cost of call mi of power .
Diversity factor is applied to the group of feeders ,trasformers / submains.
Plant capacity factor
It is the ratio of 'actual energy produced ' to ' max. energy that could have been produced '

13. Plant use factor if we assume ,T=1 year, then
power stations are so designed to have some reserve capacity ,to meet the incremental load in the future .
Reserve capacity= plant capacity - max. demand.
Plant use factor
It is the ratio of ' KWh generated ' to ' the product of plant capacity and the number of operating hours'.

14. Units generated per annum
It is often required to find the KWh generated per annum from maximum demand and load factor,mathematically as follows

15. Load duration curve When the load elements of a
load curve are arranged in order
of descending magnitudes ,the
curve obtained is called
"load duration curve".

16. following are the characteristics of load duration curve
A load duration curve gives the data in more presentable form
The area under the load duration curve also gives KWh generated during that period.
The load duration curve can be extended to any period of time ,by laying out abscissa from 0 hour to 8760 hours.
Types of loads
Domestic load:
This load consists of lights ,fans ,refigerators,heaters,televisions etc
This type of load occurs only for some hors during the day,for this reason the load factor for this load is 10 to 12% and demand factor is .60 to 1.00

17. Commercial load Municipal load
This type of load consists of lighting for shops ,fans and electrical appliances used in restaurants.demand factor of this load is 0.60 to
Municipal load
This load consists of street lighting ,power required for water supply and drainage purposes.

18. Industrial load
This load includes load demanded by industries.small industry requires load upto 25KW,medium scale requires 25KW to 100KW, and large industries can require above 500 KW of supply.

19. Irrigation load
This load consists of electric power needed for pumps driven by motors to supply water to fields.

20. Traction load
This load includes tram cars ,trolley busses,railways etc, this load is at its peak during morning and evening.

21. Typical demand and diversity factors

24. Load curves and selection of generating units
The load on the power station is seldom constant ,it varies from time to time ,a single generating unit is not an economical proposition to meet this varying load as a single generating unit has poor efficiency during periods of light loads.
Therefore a number of generating units of different sizes are installed in a power station .The number and size of units are ddecided from load curve

25. Important points in selection of units
The number and sizes of the units should be so selected that they approximately fit the annual load curve .
The units should be preferably of different capacities to meet the load requirements.
The capacity of plant should be made 15% to 20% more than max.demand to meet the future load requirements.
There should be a spare generating unit so that repair and overhauling can be carried out .
Selecting large number of units of different capacities should be avoided ,because the investment cost per KW of capacity increases as the size of units decreases.

26. Base load Peak load The constant load which occurs
almost the whole day on station.
Peak load
The various peak demands of load over and
above base load is known as peak load.

27. Method of meeting the load
The total load on a power station consists of two parts viz., base load and peak load.The best method to meet the load efficiently is to connect two different power stations. The more efficient plant is used to supply base load and less efficient is used to supply varying peak load.
Illustration
The interconnection of steam and hydro plants is a beautiful way to meet the load.when water is available is sufficient quantity like in rainy seasons , base load is carried out by hydro plants and peak load is carried out by steam power plant.

29. Interconnected grid system
The connection of several generating stations in parallel is known as ' interconnected grid system' .
Although interconnection of generating stations require extra cost ,but considering the benefits ,cost can be compromised .some of the advantages of tis system are :
Exchange of peak loads
If the peak load of a power station exceeds the capacity of plant ,the excess load can be shared with other stations interconnected with it.
Use of older plants
Although old power plants are less efficient,but during periods of increased peak loads ,older plants can share the excess load of other generating stations interconnected with it ,hence their less efficiency can be compromised.

30. Ensures economical operation
Sharing if load is so arranged that more efficient stations work continuously throughout the year ,and less efficient plants work for peak hours only.
Reduces plant reserve capacity
Every power station is required to have a standby unit for emergencies .however when several generating stations are connected in parallel ,the reserve capacity of the system is much reduced.This increases the efficiency of the system.

31. Any questions ?

32. Thankyou for your kind attention
chapter # 3 End