Variable Load on Power Station

 

 

The load on a power station varies from time to time due to uncertain demands of the consumers and we know it as variable load on the load factor station.

 

The load requirements of the consumers design to meet a power station .An ideal load on the station, from stand point of equipment needed and operating routine, would be one of constant magnitude and steady duration.

 

However, such a steady load on the station is never realized in actual practice. The consumers require their small or large block of power in accordance with the demands of their activities.

Thus the load demand of one consumer at any time may be different from that of the other consumer. The result is that load on the power station varies from time to time.

 

Effects of variable load:

The variable load on a power station introduces many perplexities in its operation. Some of the important effects of variable load on a power station are :

 

(i) Need of additional equipment:

The variable load on a power station necessitates to have additional equipment. By way of illustration, consider a steam power station. Air, coal and water are the raw materials for this plant. In order to produce variable power,We require the supply of these materials to vary correspondingly.

 

For instance, if the power demand on the plant increases, it must be follow by the increased flow of coal, air, and water to the boiler in order to meet the increased demand. Therefore, additional equipment has to be installed to accomplish this job.

As a matter of fact, in a modern power plant, there is much equipment devoted entirely to adjust the rates of supply of raw materials in accordance with the power demand made on the plant.

 

(ii) Increase in production cost:

The Variable Load on Power Station on the plant increases the cost of the production of electrical energy. An alternator operates at maximum efficiency near its rated capacity. If a single alternator is used, it will have poor efficiency during periods of light loads on the plant.

Therefore, in actual practice, a number of alternators of different capacities are installed so that most of the alternators can be operated at nearly full load capacity.

However, the use of a number of generating units increases the initial cost per kW of the plant capacity as well as floor area required. This leads to the increase in production cost of energy.

 

Load Curves

 

We know the curve showing the variation of load on the power station with respect to (w.r.t) time 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 (i.e., 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 variations of load w.r.t. time during the day.

 

Fig. 3.2 shows a typical daily load curve of a power station. It is clear that load on the power station is varying, being maximum at 6 P.M. in this case. It may be seen that load curve indicates at a glance the general character of the load that is being imposed on the plant. Such a clear representation cannot be obtained from tabulated figures.

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.

Importance:

The daily load curves have attained a great importance in generation as they supply the following information readily :

(i) The daily load curve shows the variations of load on the power station during different hours of the day.

(ii) The area under the daily load curve gives the number of units generated in the day.

Units generated/day = Area (in kWh) under daily load curve.

 

(iii) The highest point on the daily load curve represents the maximum demand on the station on that day.

(iv) The area under the daily load curve divided by the total number of hours gives the average load on the station in the day.

(v) The ratio of the area under the load curve to the total area of rectangle in which it is contained gives the load factor.

 

(vi) The load curve helps in selecting* the size and number of generating units.

(vii) The load curve helps in preparing the operation schedule of the station.

 

Important Terms and Factors

The variable load problem has introduced the following terms and factors in power plant engineering:

(i) Connect load: It is the sum of continuous ratings of all the equipments connect 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 100-watt lamps and a power point of 500 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.

 

(ii) Maximum demand:

It is the greatest demand of load on the power station during a given period.

The load on the power station varies from time to time. 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 Fig. 3.2, the maximum demand on the power station during the day is 6 MW and it occurs at 6 P.M.

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.

 

(iii) Demand factor:

It is the ratio of maximum demand on the power station to its connected load i.e.,

Demand factor =



The value of demand factor is usually less than 1. It is expect because maximum demand on the power station is generally less than the connect 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 know as average load or average demand.



Load factor: The ratio of average load to the maximum demand during a given period is known as load factor i.e.,

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 plays key role in determining the overall cost per unit generated.

Higher the load factor of the power station, lesser will be the cost per unit generated.

 

(vi) Diversity factor:

1. The ratio of the sum of individual maximum demands to the maximum demand on power station, we know it as diversity factor i.e.

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.

 

Plant capacity factor. It is the ratio of actual energy produce to the maximum possible energy that could have been produce during a given period i.e.,

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 always somewhat greater than the maximum demand on the plantReserve capacity = Plant capacity − Max. 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.

 

Plant use factor. It is ratio of kWh generate to the product of plant capacity and the number of hours for which the plant was in operation i.e.



Suppose a plant having installed capacity of 20 MW produces annual output of 7·35 ×10⁶ kWh and remains in operation for 2190 hours in a year. Then,

Types of Loads:

A device which taps electrical energy from the electric power system is call a load on the system. The load may be resistive (e.g., electric lamp), inductive (e.g., induction motor), capacitive or some combination of them. The various types of loads on the power system are :

 

(i) Domestic load: Domestic load consists of lights, fans, refrigerators, heaters, television, small motors for pumping water etc. Most of the residential load occurs only for some hours during the day (i.e., 24 hours) e.g., lighting load occurs during night time and domestic appliance load occurs for only a few hours. For this reason, the load factor is low (10% to 12%).

(ii) Commercial load: Commercial load consists of lighting for shops, fans and electric appliances used in restaurants etc. This class of load occurs for more hours during the day as compared to the domestic load. The commercial load has seasonal variations due to the extensive use of airconditioners and space heaters.

 

(iii) Industrial load: Industrial load consists of load demand by industries. The magnitude of industrial load depends upon the type of industry. Thus small scale industry requires load upto 25 kW, medium scale industry between 25kW and 100 kW and large-scale industry requires load above 500 kW. Industrial loads are generally not weather dependent.

 

(iv) Municipal load: Municipal load consists of street lighting, power required for water supply and drainage purposes. Street lighting load is practically constant throughout the hours of the night. For water supply, water is pump to overhead tanks by pumps driven by electric motors. During the off-peak period we  carry pumping, usually occurring during the night.

This helps to improve the load factor of the power system.

 

(v) Irrigation load: This type of load is the electric power need for pumps drive by motors to supply water to fields. Generally for 12 hours during night supply this type of load.

 

(vi) Traction load: This type of load includes tram cars, trolley buses, railways etc. This class of load has wide variation. During the morning hour, it reaches peak value because people have to go to their work place. After morning hours, the load starts decreasing and again rises during evening since the people start coming to their homes.

 

Typical Demand and Diversity Factors

The demand factor and diversity factor depend on the type of load and its magnitude.

 

Illustration:

For simplicity,in the figure shows only three consumers a, b, and c. The maximum demand of consumer a is the product of its connected load and the appropriate demand factor. Same is the case for consumers b and c.

The maximum demand on the transformer:

The sum of a, b and c’s maximum demands divided by the diversity factors between the consumers. Similarly, the maximum demand on the feeder is the sum of maximum demands on the distribution transformers connectto it divide by the diversity factor between transformers.

Likewise we recognise diversification between feeders when obtaining substation maximum demands and substation diversification when predicting maximum load on the power station.

Note:

that diversity factor is the sum of the individual maximum demands of the subdivisions of a system taken as they may occur during the daily cycle divided by the maximum simultaneous demand of the system.A certain transformer may serve a group of consumers by the “system” and a feeder etc.

Since individual variations have diminishing effect as one goes farther from the ultimate consumer in making measurements, one should expect decreasing numerical values of diversity factor as the power plant end of the system is approach. This is clear from the above table showing

diversity factors between different elements of the power system.



 

Important Points in the Selection of Units

While making the selection of number and sizes of the generating units, the following points should be kept in view :

(i)That they approximately fit the annual load curve of the station will selected the number and sizes of the units..

 

Note: It may be seen that the generating units can fit the load curve more closely if more units of smaller sizes are employed. However, using greater number of units increases the investment cost per kW of the capacity.

 

(ii) The units should be preferably of different capacities to meet the load requirements. Although use of identical units (i.e., having same capacity) ensures saving* in cost, they often do not meet the load requirement.

(iii) There capacity of the plant should make 15% to 20% more than the maximum demand to meet the future load requirements.

(iv) There should be a spare generating unit so that repairs and overhauling of the working units can be carried out.

(v) The tendency to select a large number of units of smaller capacity in order to fit the load curve very accurately should be avoided. Because of it , the investment cost per kW of capacity increases as the size of the units decreases.

 

Base Load and Peak Load on Power Station

The changing load on the power station makes its load curve of variable nature. Fig. 3.13 shows the typical load curve of a power station. It is clear that load on the power station varies from time to time. However,

a close look at the load curve reveals that load on the power station can be consider in two parts, namely;

(i) Base load

(ii) Peak load

 

(i)Base load  : The unvarying load which occurs almost the whole day on the station is know as base load.

Referring to the load curve of Fig. 3.13, it is clear that 20 MW of load has to be supplied by the station at all times of day and night i.e. throughout 24 hours. Therefore, 20 MW is the base load of the station. As base load on the station is almost of constant nature,

therefore, we can suitably supply it (as discussed in the next Article) without facing the problems of variable load.



 

(ii)load Peak :
Referring to the load curve of Fig. 3.13, it is clear that there are peak demands of load excluding base load. These peak demands of the

station generally form a small part of the total load and may occur throughout the day.

 

Method of Meeting the Load

The total load on a power station consists of two parts viz., base load and peak load. In order to achieve overall economy, the best method to meet load is to interconnect two different power stations. The base load uses to supply the more efficient

plant and we know as base load power station.

The peak loads use to supply:

The less efficient plant and we know it as peak load power station. There is no hard and fast rule for selection of base load and peak load stations as it would depend upon the particular situation.

For example,both hydro-electric and steam power stations are quite efficient and can be use as base load as well as peak load station to meet a particular load requirement.

 

 Grid System

We know the connection of several generating stations in parallel as an interconnect grid system. Interconnecting different power stations in parallel considerably reduce the various problems facing the power engineers.

Although the interconnection of station involves extra cost, yet considering the benefits derive from such an arrangement, it is gaining much favour these days.In below there some list of advantages of interconnect system :

 

(i) Exchange of peak loads : An important advantage of the interconnected system is that the peak load of the power station can be exchanged. If the load curve of a power station shows a peak demand that is greater than the rated capacity of the plant, then the excess load can be shared by other stations interconnected with it.

(ii) Use of older plants :

The interconnected system makes it possible to use the older and less efficient plants to carry peak loads of short duration.

When use alone may inadequate although such plants. Yet they have sufficient capacity to carry short peaks of loads when interconnects

with other modern plants. Therefore, the interconnected system gives a direct key to the use of obsolete plants.

 

(iii) Ensures economical operation: The interconnected system makes the operation of concerned power stations quite economical. It is because sharing in such a way arranges load among the stations that more efficient stations work continuously throughout the year

at a high load factor and the less efficient plants work for peak load hours only.

 

(iv) Increases diversity factor:

The load curves of different interconnected stations are generally different. The result is that the maximum demand on the system is much reduced as compared to the sum of individual maximum demands on different stations.

In other words, there improves the diversity factor of the system, thereby increasing the effective capacity of the system.

(v) Reduces plant reserve capacity :

Every Variable Load on Power station   is require to have a standby unit for emergencies. However, when we connect several power stations  in parallel, the reserve capacity of the system is much reduced. This increases the efficiency of the system.

 

(vi) Increases reliability of supply :

The interconnected system increases the reliability of supply. If a major breakdown occurs in one station, continuity of supply can be maintained by other healthy stations.