Variable load Demand in Power Station

Problems with Variable Load Demand

Various Load Curves

Importance of Load Curve

Load Characteristics Terms & Definitions

Types of Loads

Methods of Meeting the increasing load demand

PS delivers power to a large number of consumers

Power demands varies in accordance with consumers activities

PS load is never constant varies from time to time.

Complexities of modern power plant operation arise from the inherent variability of the load demanded by the users.

PS must produce electric power as and when demanded to meet the requirements as it cannot be stored

On one hand, the power engineer would like that the alternators in the PS should run at their rated capacity for \(\eta_{max}\) & on the other hand, the demands of the consumers have wide variations.

This makes the design of a power station highly complex.

Focus our attention on the problems of variable load on PS.

PS load varies from time to time due to uncertain demands of the consumers known as variable load on the station

Need of additional equipment:

\(\uparrow\) power demand must be followed by \(\uparrow\) flow of coal, air and water to the boiler

additional equipment has to be installed to accomplish this job.

Increase in production cost:

Alternator operates at \(\eta_{max}\) near its rated capacity

single alternator if used will have poor \(\eta\) during periods of light loads on the plant

In actual practice, a number of alternators of different capacities are installed operated at nearly F.L capacity

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

Curve showing the variation of load on the PS w.r.t time is known as a load curve

daily LC: whole day (i.e., 24 hours) are recorded half-hourly or hourly and are plotted against time

monthly LC: daily load curves of a month

average values of power over a month at different times of the day are calculated

generally used to fix the rates of energy

yearly LC: monthly load curves of that particular year

generally used to determine the annual load factor

shows variations of load on PS during different hours of the day

area under the curve gives number of units generated in the day \[\text{ Units generated/day} = \text{Area (in kWh) under the daily LC }\]

highest point on DLC \(\Rightarrow\) maximum demand on the station on that day

Area under DLC divided by the total number of hours gives the Average Load on the station in the day

\[\text{Average load} =\frac{\text { Area (in } \mathrm{kWh} \text { ) under daily load curve }}{24 \text { hours }}\]

ratio of the area under DLC to the total area of rectangle in which it is contained gives the load factor

\[\begin{aligned} \text { Load factor } &=\frac{\text { Average load }}{\text { Max. demand }}\\ &=\frac{\text { Average } \operatorname{load} \times 24}{\text { Max. demand } \times 24} \\ & =\frac{\text { Area (in kWh) under DLC }}{\text { Total area of rectangle LC is contained }} \end{aligned}\]

helps in selecting the size and number of generating units

helps in preparing the operation schedule of the station

Connected load: sum of continuous ratings of all the equipments connected to supply system.

Maximum demand: greatest demand of load on the power station during a given period.

Demand factor: ratio of maximum demand on the power station to its connected load \[\text{Demand factor} =\frac{\text { Maximum demand }}{\text { Connected load }} < 1\]

\(\Rightarrow\) average of loads occurring on the PS in a given period (day or month or year) \[\begin{aligned} \text{Daily AL} &=\frac{\text { No. of units }(\mathrm{kWh}) \text { generated in a day }}{24 \mathrm{hours}}\\ \text{ Monthly AL} &=\frac{\text { No. of units (kWh) generated in a month }}{\text { Number of hours in a month }}\\ \text{Yearly AL} & =\frac{\text { No. of units (kWh) generated in a year }}{8760 \text { hours }} \end{aligned}\]

\(\Rightarrow\) ratio of average load to MD during a given period

\[\text{Load factor}=\frac{\text { Average load }}{\text { Max. demand }} < 1\]

If the plant is in operation for T hours, \[\begin{aligned} \text { Load factor } &=\frac{\text { Average } \operatorname{load} \times \mathrm{T}}{\text { Max. demand } \times \mathrm{T}} \\ &=\frac{\text { Units generated (in T hrs.) }}{\text { Max. demand } \times \mathrm{T} \text { hrs. }} \end{aligned}\]

\(\uparrow\) LF \(\Rightarrow\) MD \(\downarrow\) \(\Rightarrow\) cost per unit generated \(\downarrow\)

Station capacity selected to meet the MD

\(\downarrow\) MD \(\Rightarrow\) \(\downarrow\) plant capacity \(\Rightarrow\) \(\downarrow\) cost of the plant.

\(\Rightarrow\) ratio of the sum of individual MDs to the MD on PS \[\text{Diversity factor} =\frac{\text { Sum of individual max. demands }}{\text { Max. demand on power station }} > 1\]

PS supplies load to various types of consumers whose MDs generally do not occur at the same time.

MD on the PS is always \(<\) sum of individual MDs of the consumers \(\Rightarrow\) DF \(>\) 1.

\(\uparrow\) DF \(\Rightarrow\) cost of generation of power \(\downarrow\).

\(\uparrow\) DF \(\Rightarrow\) MD \(\downarrow\) \(\Rightarrow\) plant capacity \(\downarrow\) \(\Rightarrow\) capital investment \(\downarrow\)

\(\Rightarrow\) ratio of actual energy produced to the maximum possible energy that could have been produced during a given period \[\begin{aligned} \text { Plant CF } &=\frac{\text { Actual energy produced }}{\text { Max. energy that could have been produced }} \\ &=\frac{\text { Average demand } \times \mathrm{T}}{\text { Plant capacity } \times \mathrm{T}} \\ &=\frac{\text { Average demand }}{\text { Plant capacity }} \end{aligned}\]

Plant CF is an indication of the reserve capacity of the plant

PS 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 MD on the plant \[\text{Reserve capacity} = \text{Plant capacity} - \text{Max. demand}\]

Difference between load factor and plant capacity factor is an indication of reserve capacity

If MD on the plant \(=\) to the plant capacity, then load factor and plant CF will have the same value

In such a case, the plant will have no reserve capacity.

\(\Rightarrow\) ratio of kWh generated to the product of plant capacity and the number of hours for which the plant was in operation \[\text{Plant use factor} =\frac{\text { Station output in } \mathrm{kWh}}{\text { Plant capacity } \times \text { Hours of use }}\] Suppose a plant having installed capacity of \(20 \mathrm{MW}\) produces annual output of \(7 \cdot 35 \times 10^{6} \mathrm{kWh}\) and remains in operation for 2190 hours in a year. Then, \[\text { Plant use factor }=\frac{7 \cdot 35 \times 10^{6}}{\left(20 \times 10^{3}\right) \times 2190}=0 \cdot 167=16 \cdot 7 \%\]

It is often required to find the \(\mathrm{kWh}\) generated per annum from maximum demand and load factor.

The procedure is as follows:

\[\begin{aligned} \text { Load factor } &=\frac{\text { Average load }}{\text { Max. demand }} \\ \therefore \quad \text { Average load } &=\text { Max. demand } \times \text { L.F. } \\ \text { Units generated/annum } &=\text { Average load (in kW) } \times \text { Hours in a year } \\ &=\text { Max. demand (in kW) } \times \text { L.F. } \times 8760 \end{aligned}\]

\(\Rightarrow\) Load elements of a LC are arranged in the order of descending magnitudes

LDC is obtained from the same data as the LC but the ordinates are arranged in the order of descending magnitudes

maximum load is represented to the left and decreasing loads are represented to the right in the descending order

LDC gives the data in a more presentable form

readily shows the number of hours during which the given load has prevailed

The area under LDC is equal to that of the corresponding LC

area under daily LDC (in kWh) will give the units generated on that day.

LDC can be extended to include any period of time

By laying out the abscissa from 0 hour to 8760 hours, the variation and distribution of demand for an entire year can be summarised in one curve.

The curve thus obtained is called the annual load duration curve

Domestic load

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%)

Commercial load

lighting for shops, fans and electric appliances used in restaurants etc.

occurs for more hours during the day as compared to the domestic load.

has seasonal variations due to the extensive use of air-conditioners and space heaters.

Industrial load

consists of load demand by industries

magnitude 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.

generally not weather dependent

Municipal load

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 pumped to overhead tanks by pumps driven by electric motors. Pumping is carried out during the off-peak period, usually occurring during the night.

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

Irrigation load

electric power needed for pumps driven by motors to supply water to fields.

supplied for 12 hours during night

Traction load

includes tram cars, trolley buses, railways etc

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.

Base load: The unvarying load which occurs almost the whole day on the station

Peak load: The various peak demands of load over and above the base load of the station

the best method interconnect two different power stations

The more efficient plant is used to supply the base load and is known as base load power station.

The less efficient plant is used to supply the peak loads and is known as peak load power station.

\(\Rightarrow\) connection of several generating stations in parallel

Exchange of peak loads

Use of older plants

Ensures economical operation

Increases diversity factor

Reduces plant reserve capacity

Increases reliability of supply