The generating stations may be steam, hydro, nuclear, diesel or any other type. Steam power stations are best suited near the coal fields and also adopted where coal supply is available in plenty at reasonable rates, large amounts of power is required to be generated and financial, climatic and geographical conditions are not favorable to hydro and nuclear power stations. Hydropower stations are best suited in case water is available at certain height and nuclear power stations are best suited in area far away from collieries and where fuel costs are high and alternative cheap hydropower is not available.
Diesel power stations are installed where the supply of coal and water is not available in sufficient quantity or where power is to be generated in small quantity or where standby sets are required for continuity of supply such as in hospitals, telephone exchanges, radio stations, and cinemas.
The power stations should be as near as possible to the center of the load so that the transmission cost and losses are minimum. The other considerations for the design of the power station are reliability, minimum capital and operating costs. The layout of the station should be such that the maintenance and repairs can be carried out easily, the design must be compact and well planned ,equipment used must be standard one so that capital cost is reduced and replacement of worn-out parts becomes easy and equipment’s used should be simple so that it can be operated by semi-skilled workers.
The design of the power stations should be such that the station could be divided into a number of sections to avoid complete shutdown of the station when the fault occurs. And the other information to be kept is to
- estimate of probable load
- Future load conditions
- Location of loads
Types and rating of Generating Stations
The curve showing the variation of load on the power station with respect to(w.r.t) time is known as a load curve. The load on a power station is never constant, it varies from time to time. These load variations during the whole day(24 hours) are recorded half-hourly or hourly and are plotted against time on the graph. The curve thus obtained is known as daily load curve as it shows the variation of load w.r.t time during the day.
The load curves supply the following information’s:
- The variation of the load during different hours of the day.
- The area under the curve represents the total number of units generated in a day.
- The peak of the curve represents the maximum demand on the station on the particular day.
- The area under the load curve divided by the number of hours represents the average load on the power station.
- The ratio of the area under the load curve to the total area of the rectangle in which it is contained is gives the load factor.
- The load curve helps in selecting the size and number of generating stations.
- The load curve helps in preparing the operation schedule of the station.
The monthly load curve can be obtained from the daily load curves of that month. For this purpose, average values of power over a month at different times of the day are calculated and then plotted on the graph. The monthly load curve is generally used to fix the rates of energy.
The yearly load curve is obtained by considering the monthly load curves of that particular year. The yearly load curve is generally used to determine the annual load factor.
During the early morning hours the demand is always low. Around 5 AM the load starts increasing because of increasing industrial and traction load. Around 10 AM the load reaches a high value and remains almost constant till evening except for some dip during lunch hours. The load again starts increasing in the evening hours as the residential and traction load starts coming up. The peak occurs around the 6-8 PM and then the load starts decreasing.
The type of variations in load or demand on a plant depends on the domestic and industrial users. The load requirements of an individual consumers are different at different timings in the day. The load curves for winter and summer will also differ. The curves for rural, suburban and urban areas will also differ.
Load Duration curve
- This curve indicates the variation of load, but with the loads arranged in descending order of magnitude i.e. greatest load on the left ,lesser loads towards the right and the least load at the extreme right.
- The curve gives the number of hours for which a particular load lasts during the day.
- The area under this curve like load curve or chronological load curve gives the total number of units generated for the period considered.
- Load duration curve helps to find the load factor of the station .It also helps to predict the distribution of load between various generating units.
Integrated Load duration curve
- This curve gives the total number of units generated for the given demand. The ordinate represents the demand in kW or MW and the abscissa represents the units(kWh) generated at or below a given demand(in kW or MW).
- This type of curve can be obtained from the load duration curve keeping the abscissa corresponding to each ordinate equal to the area of the duration curve up to the value of that ordinate.
- This curve is plotted with units(kWh) as ordinate and time as abscissa.
- This curve gives the total energy consumed by the load up to a particular time in a day.
- This curve can be plotted from the chronological load curve by summing up the energy consumed up to different times starting at the zero time.
- This type of curve helps in the study of variation between the rate of water flow and the electrical load and the determination of the necessary storage.
- It is the sum of continuous ratings of all the equipment’s connected to supply system.
- For example: 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 equipment’s in the consumers premises is the “connected load” of the consumers. For instance, if a consumer has connections of five 100-watt lamps and a power point of 5oo watts, then connected load of the consumer is 5*100+500=1000 watts. The sum of the connected loads of all the consumers is the connected load to the power station.
- It is the greatest demand of load on the power station during a given period.
- The maximum of all the demands that have occurred during a given period is the maximum demand.
- It is also sometimes called the system peak.
- From above figure it is clear that the maximum demand on the power station during the day occurs at 6 PM.
- Maximum demand is generally less than the connected load because all the consumers do not switch on their connected load to the system as a time.
- Maximum demand helps in determining the installed capacity of the station. As the station must be capable of meeting the maximum demand.
- Maximum demand never means the greatest instantaneous maximum demand but the greatest short time average demand occurring during a long period of time under consideration.
- Maximum demand helps to determines the size and the cost of the installation.
- Demand factor is the ratio of maximum demand on the power station to its connected load i.e.
- It is always less than unity because the 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 100MW,then demand factor=80/100=0.8.
- Demand factor helps in determining the capacity of the plant equipment’s.
Average load or Demand
- The average of loads occurring on the power station in a given period(day or month or year) is known as the average load or average demand.
- It can also be stated as energy delivered in a given period divided by the number of hours in that period.
- The ratio of average load to the maximum demand during a given period is known as load factor
- If the plant is in operation for 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 always less than 1 because average load is smaller than the maximum demand.
- The load factor helps in determining the overall cost per unit generated.
- Higher the load factor of the power station lesser will be the cost per unit generated.
- The ratio of the sum of individual maximum demands to the maximum demand on a power station is known as the diversity factor i.e.
- Diversity factor always greater than unity. The greater the diversity factor the lesser is the cost of generation of power.
- The maximum demands of all the consumers supplied from an installation do not occur usually as the same time. Therefore ,the maximum demands on the power station is always less than the sum of individual maximum demands of the consumers.
- The reciprocal of the diversity factor is called the coincidence factor .i.e
- It is always less than unity.
Capacity factor or Plant 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 may also be defined as the actual energy generated divided by the maximum possible energy that the plant might have generated during a given period. i.e
- The plant factor is the 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. Hence the installed capacity of the plant is always somewhat greater than the maximum demand on the plant.
- The 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 case the plant will have no reserve capacity.
- utilization factor is a measure of the utility of the power plant capacity.
- It is defined as the ratio of maximum demand to the rated capacity of the power plant.
- It is always less than unity .
Plant operating factor or plant use factor
- It is the ratio of kWh generated to the product of plant capacity and the number of hours for which the plant was in operation i.e