Power Systems presentation from ch-3 Dated: 27th October 2015 inam ul haq

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 .

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'.

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

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.

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.

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 '.

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

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 '

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'.

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

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".

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

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 0.70 . Municipal load This load consists of street lighting ,power required for water supply and drainage purposes.

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.

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

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

Typical demand and diversity factors

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

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.

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.

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.

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.

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.

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Thankyou for your kind attention chapter # 3 End