Power System Operation and Control

Load Variations and Load Characteristics, Load Curves, Load Duration Curves, Reserves, Load Forecasting and Unit Commitment

IntroductionPower Station delivers power to large number of

customersCustomer’s need (or load) is never constantElectric Power cannot be stored and must be

consumed upon productionAt the same time, alternators give maximum

efficiency at their rated capacityProper Load prediction and calculations are key

to efficient design of power Station

Topics CoveredLoad Curve.Daily Load Curve Interpretation.Few important terms and factors.Load Duration Curves.Base Loads, Peak loads.Reserves.Load Forecasting.Unit Commitment.

Variable LoadDefinition: The load on a power station varies from time to

time due to uncertain demands of customers. This is known as Variable load on power station

Effects of Variable Load:Need for additional Equipment

◦ Eg: Equipment to handle raw materials during varying load conditions in thermal power station.

Increase in Production Cost ◦ Eg: Alternators of different capacities must be installed to get

maximum efficiency by operating at rated capacity.

Load Curves

Definition: The curve showing variation of load on a power station w. ref. to time

Daily Load Curve Variation of load on a day plotted on

an hourly basis

Monthly Load Curve Variations of load in a month –

Plotted using daily load curves used to fix rates of

energy

Yearly Load Curve Extrapolated using monthly load

curves used to determine annual Load Factor 0 4 8 12 16 20 240

1

2

3

4

5

6

7

Load Curve

Hours

Load in M

W

Noon

Daily Load Curve Interpretation Shows variation of load on power

station on a day Units generated in a day = Area (in

kWh) under the daily load curve Highest point on curve Maximum

demand on the station for the day

Helps in

◦ Selecting Size and number of generating units

◦ Preparing operating schedule of the station

hrs

curvedailyunderkWhinAreastationtheonLoadAverage

24

)(

DemandMax

LoadAverageFactorLoad

Important Terms and Factors Connected Load: Sum of continuous rating of

all equipments connected to supply system Maximum Demand: Greatest demand of

Load on the power station during a given time

◦ usually <1◦ vital in determining capacity of plant equipment

Average load or Average Demand: Average of loads occurring on a power station in a given period

LoadConnected

DemandMaximumFactorDemand

8760

)(

)(

24

)(

yearaingeneratedkWhunitsofNoLoadavgYearly

monthainHours

monthaingeneratedkWhunitsofNoLoadavgMonthly

hrs

dayaingeneratedkWhunitsofNoLoadavgDaily

Important Terms and Factors – contd..

◦ always <1◦ Used for determining overall cost/unit

generation◦ Higher the load factor lesser the cost /

unit

◦ always > 1◦ Greater diversity factor lesser cost of generation of power

◦ Indicative of reserve capacity of the plant

TdemandMax

hrsTingeneratedunits

DemandMax

LoadAvgFactorLoad

stationpowerondemand

demandsindividualofSumFactorDiversity

max

max

8760

)(

'max

CapacityPlant

outputkWhAnnualFactorCapacityAnnual

hrsTinCapacityPlant

hrsTingeneratedUnitsDemandTotal

producedbeenvecouldthatenergy

producedEnergyActualFactorCapacity

Important Terms and Factors – contd..

Reserve Capacity = Plant Capacity – Max Demand.◦ Difference between L.F and C.F is indicative of reserve

capacity◦ If max demand = plant capacity, then L.F = C.F no reserve

capacity Unit Generated/Annum

useofhoursCapacityPlant

kWhinOutputStationFactorUsePlant

8760..

/

.

FLDemandMax

yearainhoursloadAvgannumgenunit

FLDemandMaxLoadAvg

DemandMax

LoadAvgFactorLoad

Load Duration CurveDefinition: When load elements of load curve are arranged in

order of descending magnitudes, the resulting curve is Load duration curve◦ Input is the load curve

◦ Max load is at left and decreasing loads are at right in descending order

◦ Area under load curve = Area under load duration curve

Base Load and Peak LoadThe unvarying load

which occurs almost the whole day Base Load

The varying peak demands of load over and above the base load Peak Load

Reserve Installed reserve is the amount of existing generation that is higher than that needed to

reliably meet a forecasted peak load

Spinning reserve is the unused capacity that can be activated by the system operator and

which is provided by generators synchronized to the network

Hot Reserve the reserve capacity that is synchronized to the grid system and should be

capable of meeting the demand within 10 minutes of a dispatch instruction. It is useful during

emergency conditions and unexpected load swings.

Cold or Non-Spinning Reserve capacity of generators not running at present that can be

ramped to capacity and synchronized to the grid within 10 minutes of a dispatch instruction

and should be capable of maintaining that output for at least two hours. Non-Spinning

Reserve is useful during emergency conditions

Unit CommitmentFdFd