The generation of electrical energy economically is not an ordinary matter, rather it requires a long experience to decide about the type, location and the rating of generating stations.
The generating stations may be steam, hydro, nuclear, diesel or any other type. This factor mainly depends upon the natural sources available in the areas. Steam power stations are best suited near the coal fields and also adopted where coal supply is available in plenty at reasonable rates, large amount of power is required to be generated and financial, climatic and geographical conditions are not favourable 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 as in Rajasthan. Diesel power stations are installed where 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 station should be as near as possible to the centre of the load so that the transmission cost and losses are minimum. This factor is important when dc supply system is adopted. However, in case of ac supply system where transformation of energy from lowers to higher voltage and vice versa is possible, power station can be located at places other than that of centre of load provided other conditions are favourable.
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The other considerations for the design of the power station are reliability, minimum capital and operating costs. For this it is necessary that the layout 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 used should be simple so that it can be operated by semi-skilled workers.
The design of power station 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. Another advantage of sectionalizing is that the erection can be completed and put into service in parts.
The equipment must be automatic whenever possible in order to save the labour cost provided the reliability of the station is not affected. The automatic equipment should be such that it could be operated manually in case it is required. The scheme employed should be such that extension could be made to meet with the increase in demand in future without incurring further heavy expenditure.
Before a power project is taken into hand, the project engineer should have the following information:
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(i) Estimate of probable load.
(ii) Future load conditions.
(iii) Location of the loads, especially in case of hydroelectric generating stations because the cost of transmission is also required to be considered.
For deciding the type and rating of generating plant it is necessary that engineer may be familiar with the following important terms:
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1. Load Curves:
The load on the power station is seldom constant; it varies from time to time. The daily variations in load on the power station from time to time—hourly or half hourly can be plotted on a graph taking load on Y-axis and time on X-axis. The curve so obtained is known as daily load curve.
From the daily load curves of a particular month, the monthly load curve can be plotted by calculating the average value of power at a particular time of the day. Similarly if we consider such monthly load curves of a particular year, and find the average value of the power at a particular time of the day, the annual load curve can be obtained. The load curve bears a great importance in generation.
The load curves supply the following information:
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(i) The variation of the load during different hours of the day.
(ii) The area under the curve represents the total number of units generated in a day.
(iii) The peak of the curve represents the maximum demand on the station on the particular day.
(iv) The area under the load curve divided by the number of hours represents the average load on the power station.
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(v) The ratio of the area under the load curve to the total area of the rectangle in which it is contained gives the load factor.
The type of variations in load or demand on a plant depends on the domestic and industrial users (their type and habits). The load requirements of an individual consumer are different at different timings in the day. The load curve for Sunday will be different from those for other days of the week. The load curves for winter and summer will also differ. The curves for rural, suburban and urban areas will also differ.
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 9 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 start coming up. The peak occurs around 7-9 PM and then the load starts decreasing. On Sundays the load demand is only due to residential consumers and so low as compared to those of other weekdays.
Thus, on the basis of above information’s the load curves help in deciding the size of the units to be installed and also in preparing the schedule of operation of the generating units.
2. Load Duration Curve:
This is another type of curve which indicates the variation of load, but with the loads arranged in descending order of magnitude i.e., the greatest load on the left, lesser loads towards the right and the least load at the extreme right. This 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. From this curve the load factor of the station can also be determined. From these curves the distribution of load between various generating units can also be predicted.
3. 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). Such a 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.
Such curves are useful in that with a given number of units (kWh) available, say from a river flow, the load (kW) that could be carried at the base or peak may be easily determined.
4. Mass Curve:
This curve is plotted with units (kWh) as ordinate and time as abscissa. Thus a mass curve gives the total energy consumed by the load up to a particular time in a day. This curve can be easily plotted from the chronological load curve by summing up the energy consumed up to different times starting at the zero time.
This curve is used in the study of variations between the rate of water flow and the electrical load and for the determination of the necessary storage.
5. Connected Load:
The sum of the continuous ratings of all electrical equipment connected to the supply system is known as connected load.
6. Maximum Demand:
It is not necessary that all the connected load be switched to a system at a time. The greatest of all “short time interval averaged” (15 minutes or 4 hour or ½ hour) during a given period (a day, a month or a year), on the power station is called the maximum demand. It is sometimes also called as system peak. It is to be clarified here that the 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.
Hence whenever the maximum demand is to be mentioned it is essential to indicate the short interval of time over which averaging has been carried out for the measurement of demand and the period of load under consideration. It is the maximum demand which determines the size and the cost of the installation.
7. Demand Factor:
The ratio of actual maximum demand on the system to the total rated load connected to the system is called the demand factor. It is always less than unity.
The idea of a demand factor was introduced due to the fact that all the equipment connected to the system will not be worked at a time in practice and the kW or kVA maximum demand of a group of electricity consuming devices will be always less than the sum of the kW or kVA ratings or capacities of these devices.
The minimum capacity of the generating plant must be such as to meet the maximum demand, the value of demand factor, therefore, determines the capacity and hence cost of the power equipment required to serve a given load.
8. Average Load or Demand:
The average load or demand on the power station is the average of loads occurring at the various events. It can also be stated as energy delivered in a given period divided by the number of hours in that period. Depending upon the duration of time period such as a day, a month or a year, we get daily, monthly or annual average load.
9. Load Factor:
The ratio of average load to the maximum demand during a certain period of time such as a day or a month or a year is called the load factor. Since average load is always less than the maximum demand, load factor is therefore, always less that, unity.
Load factor may also defined as the ratio of the number of units actually generated in a given period to the number of units which could have been generated had the load or demand remained at the maximum value throughout this period.
10. Diversity Factor:
The maximum demands of all the consumers supplied from an installation do not occur usually at the same time, maximum demand on the installation is, therefore, always less than the sum of individual maximum demands of all consumers connected to it.
The ratio of sum of the individual maximum demands of all the consumers supplied by it to the maximum demand of the power station is called the diversity factor.
It is always greater than unity.
11. Coincidence Factor:
The reciprocal of the diversity factor is called the coincidence factor.
It is always less than unity.
12. Capacity Factor or Plant Factor:
Every plant has to have a reserve capacity so as to take care of the future expansion and increase in load and, therefore, total installed capacity of the plant is usually greater than that actually required (maximum demand).
Capacity (or plant) factor is defined as the ratio of the average load to the rated capacity of the power plant.
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.
The capacity factor depicts the extent of the use of the power plant. It is different from load factor and the difference is an indication of the reserve capacity.
13. Utilization Factor:
It is a measure of the utility of the power plant capacity and is the ratio of maximum demand to the rated capacity of the power plant. It is always less than unity.
14. Plant Operating Factor or Plant Use Factor:
It is defined as the ratio of actual energy generated during a given period (say a year) to the product of capacity of the plant and the number of hours the plant has been actually in operation during the period.
15. Installed Capacity:
The total of station capacities available to supply the system load is called the installed capacity.
16. Firm Power:
It is the power intended to be always available (even under emergency conditions).
17. Prime Power:
It is the power (mechanical, hydraulic or thermal) that is always available for conversion into electrical power.
18. Dump Power:
It is power in excess of the load requirements and it is made available by surplus water. It refers to hydropower plants.
19. Cold Reserve:
It is that reserve generating capacity which is available for service but not normally ready for immediate loading.
20. Hot Reserve:
It usually refers to boiler excess capacity, which is kept hot and with steam pressure, ready for use.
21. Operating Reserve:
It refers to capacity in service in excess of peak load.
22. Spinning Reserve:
It is the generating capacity connected to the bus and ready to take load.
In order to take instantaneous peak loads, the system must have capacity in readiness to serve. For this purpose certain machines are kept running on no load i.e., floating on the system. This is called the spinning reserve of the system.
Significance of Load Factor and Diversity Factor:
Load factor and diversity factor play an important part in the cost of the supply of electrical energy. Higher the values of load factor and diversity factor, lower will be the overall cost per unit generated.
Higher load factor means greater average load, resulting in greater number of units generated for a given maximum demand. Thus, the standing charges, which are proportional to maximum demand and independent of number of units generated, can be distributed over a larger number of units supplied and therefore overall cost per unit of electrical energy generated will be reduced.
The capital cost of the power station depends upon the capacity of the power station. Lower the maximum demand of the power station, the lower is the capacity required and therefore lower is the capital cost of the plant. With a given number of consumers the higher the diversity factor of their loads, the smaller will be the capacity of the plant required and consequently the fixed charges due to capital investment will be much reduced.
The suppliers should always try to improve the load factor as well as diversity factor by inducing the consumers to use the electrical energy during off-peak hours and they may be charged at lower rates for such schemes.
Capacity Scheduling:
Capacity scheduling means the order in which the various generating units should be placed in operation in order to meet the total plant or system load. Certain units will be put in operation only during peak load demand hours. To ensure continuity of power supply, sufficient generating capacity should be in operation at all times so that if one unit fails, there is no interruption in power supply.
If the load on the system/plant is equal to or less than the capacity of one generating unit, then it is always advisable to run two units to supply the load so that non-interrupted power supply can be maintained. The order of putting the machines in operation depends upon their efficiencies. The machines are loaded in the descending order of the efficiency. The load is divided among the various units on the basis of incremental rates.
The capacity of the generating unit in active operation is known as the ‘spinning capacity’. Spinning capacity should always be more than the load in order to avoid interruption. “Spinning reserve” is the difference between the spinning capacity and the load on the system. The minimum magnitude of the spinning reserve is usually equal to the capacity of the largest unit.
Load Sharing between Base Load and Peak Load Plants:
Base load plants run throughout the year and have high load factors. The economic characteristics of base load plants should be such that they supply power at high capital costs but low cost of operation. Hydro and nuclear power plants are usually classified as base load plants. Peak load plants run for a few hours in the year and work at low load factors.
The economic characteristics of peak load plants should be such that they supply power at low capital costs, although at high cost of operation. Peak load power plants should be capable of starting quickly and should be inexpensive in starting and shutting down operations.
Let the operating costs of base load and peak load plants be Rs (a1kW + b1kWh) and Rs (a2kW + b2kWh) where a1 > a2 and b2 > b1.
Let P be the maximum demand on the power plant and x be the total number of units generated.
If P1 is the maximum demand on base load plant and x1 is the number of units generated by the base load plant.
i.e., for economics load sharing, peak-load power plant will operate for a1 – a2 / b2 – b1 hours per year.
Choice of Size and Number of Generating Units:
The selection of units and their operation play an important role in the working of a power station and on the economics of power generation. The number of units and the size of each unit are decided from the load curve.
The following factors should be considered while deciding the number of units and preparing the operating schedule:
1. The total capacity of the generating units must be capable of meeting the peak demand of the power station.
2. Since the machines operate with maximum efficiency at the three-fourth of the rated capacity, hence the number and size of units must be so selected that they operate at maximum efficiency and better overall efficiency and load factor of the power station is had.
3. Reliability of service is a very important factor. Cheaper power without reliability of supply is of no use. There should be a spare set of capacity of that largest unit in the power station so that maintenance and repairs or overhaul of the working units may be carried out without any disturbance in power supply.
4. The growth of the demand in near future should be kept in view.
5. The capacity of the power station should be 15 or 20% more than expected maximum demand.
The minimum number of generating units chosen could be one having a capacity equal the maximum demand on the power station. Large size units have lower capital cost per kW, need-less floor area, require less operating labour and have better efficiency. Thus large sized units are economical both from the point of view of initial investment and operating cost.
The size of thermal units, in India, has been progressively increasing for the past many years. Around 1955 only 30 MW units were popular while now 500 MW units are being installed. The number and size of units in hydropower stations are governed by the availability of head and water. The earliest hydro units in India installed before independence were of 5 MW capacity while the largest hydro units in operation now are 165 MW at Dehar power plant of Beas- Sutlej project in Himachal Pradesh.
However, selection of a single generating unit having a capacity equal to the maximum demand on the system or slightly more has got following drawbacks:
1. As the load on the system varies at different times of the day, there is a considerable period during the day when the load on the system would be much less than the peak load.
During this light load period the generating unit may be running at 50 per cent of the rated output or even practically on no load. Hence the unit would not be running at all times under conditions best suited for its operation to give maximum efficiency. So generating cost per unit would be high.
2. There will be complete failure of supply due to breakdown or maintenance and repairs to be carried out on the generating unit for weeks together.
If the plant is an isolated one and another unit of equal capacity is installed, as standby unit, so as to ensure continuity of supply, there will be substantial increase in the initial investment. In general, the system reserve requirements increase with increase in unit size.
Alternatively the size and number of generating units may be so chosen as to fit the load curve as closely as possible. Then each unit can be made to operate in such a way that it operates almost at full load or at a load to give maximum efficiency. The reserve required in this case would be only one unit of the largest size chosen and would be much smaller than the maximum reserve capacity needed in the former case. As the reserve capacity required would be lesser, it would result in better plant capacity factor and plant use factor.
If all the machines of same ratings and identical in operation are chosen, the initial cost would be less, space required would be less, requirements of spare parts would be less, the appearance would be symmetrical and good looking.
The drawbacks and difficulties involved in this alternative of selecting large number of smaller units are given below:
1. The floor area required and initial cost per kW would be increased.
2. There would be substantial increase in maintenance cost and in personnel’s for handling and operating the equipment.
From the above discussion it is concluded that the number of units in a power station should neither be very large nor very small. As such the size of units for a power station has to be compatible with the total capacity of the power station.