LOAD CHARACTERISTICS AND LOAD FORECASTING

LOAD CHARACTERISTICS AND LOAD FORECASTING Load Electrical power needed in kW or kVA . Demand The power requirement (in kVA or kW) at the load averaged over a specified interval (15 min or 30 min). Sometimes it is given in amperes at a specified voltage level.Demand Intervals The time interval specified for demand (Di), usually 15 min or 30 min. This is obtained from daily demand curves or load duration curves.DEFINITIONSCONTD

Maximum Demand The maximum load (or the greatest if a unit or group of units) that occurred in a period of time as specified. This can be daily, weekly, seasonal or on annual basis (for billing purpose in India it is monthly and in kVA). Connected Load The sum total of the continuous rating of all the apparatus, equipment, etc., Connected to the system.

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Demand Factor The ratio of maximum demand to the total load connected to the systemUtilization Factor The ratio of maximum demand to the rated capacity of the system.Load Factor The ratio of average load in given interval of time to the peak during that interval.Annual Load Factor The ratio of total energy supplied in an year to annual peek load multiplied by 8760.

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Diversity Factor ( Df ) The ratio of sum of the individual maximum demands of various sub-divisions of the system to the maximum demand of the entire or complete system.Coincident Maximum Demand ( Dg ) Any demand that occurs simultaneously with any other demand and also the sum of any set of coincident demands.DEFINITIONSCONTD

Coincidence factor (Cf ) This is usually referred to a group of consumers or loads. It is defined as the ratio of coincident maximum demand Dg to sum total of maximum demands of individual or group of loads.Generally, it is taken as the reciprocal of the diversity factor.DEFINITIONSCONTD

Load Diversity The difference between the sum of peaks of two or more individual loads and the peak of combined load.Load diversity = Di DgDi = individual maximum demand Dg = coincident maximum demand

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Coincidence Factor This is a factor that is usually referred in distribution systems regarding the importance of weighted effect of a particular load.If C1 ,C2 , ..... Cn are the contribution factors of each of the n individual loads and D1, D2, D3 . Dn are their maximum demands.Dg = coincident maximum demand is taken as

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Loss Factors This is the ratio of average power loss in the system to power loss during peak load period and referred to the variable power losses, i.e., copper losses or power loss in conductors or windings but not to no load losses in transformers, etc.DEFINITIONSCONTD

A load or power requirement (also kVA) of a consumer varies widely. But in general the consumers can be grouped into a few categories as their needs and demands are the same.A broad classification of loads are(i) Domestic and residential loads(ii) Only lighting loads (such as for street lights etc.) (iii) Commercial loads (shops, business establishments, hospitals)(iv) Industrial loads(v) Agricultural loads and other rural loads

LOAD & LOAD CHARACTERISTICSNote : All these loads will have peak demands at different times and for different durations. Industrial and commercial loads may have two peak load periods. Agricultural loads are seasonal and vary very differently. Lighting loads such as street lighting etc. may have almost zero demand during day time and constant load during 10 p.m. to 6 a.m. and a slightly higher demand between 6p.m. to 10 p.m.Another classification of electrical loads is the billing categories used by the electrical distribution authorities or State electricity boards. This includes categories such as residential and domestic,industrial, commercial, rural, HT consumers and others.System Power Factor In ac systems, kVA demand is more appropriate than kW and load power factor is of importance. Typical p.f of residential, commercial and Industrial loads are as follows.Fluorescent lamps: 0.6Arc lamps and neon signs: 0.4 to 0.7Fans and small motors: 0.5 to 0.8Electronic gadgets: 0.6 to 0.8Domestic appliance : 0.6 to 0.7 (like washing machines ,vacuum cleaners etc.) F.H.p. motors (1 kW or less) : 0.4 to 0.75Water pumps (Large size 5 h.p) :0.65 to 0.8Chemical industries: 0.70 to 0.85Domestic and Residential LoadsThe important part in the distribution system is domestic and residential loads as they are highly variable and erotic. These consist of lighting loads, domestic appliances such as water heaters, washing machines , grinders and mixes ,TV and electronic gadgets etc. The duration of these loads will be few minutes to few hours in a day. The power factor of these loads in less and may vary between 0.5 to 0.7. In residential flats and bigger buildings, the diversity between each residence will be less typically between 1.1 to 1.15 . The load factor for domestic loads will be usually 0.5 to 0.6.Industrial LoadsIndustrial loads are of greater importance in distribution systems with demand factor 0.7 to 0.8 and load factor 0.6 to 0.7. For heavy industries demand factor may be 0.9 and load factor 0.7 to 0.8Typical power range for various loadsCottage and small-scale industries : 3 to 20 kW.Medium industries (like rice mills, oil mills, workshops, etc.) : 25 to 100 kWLarge industries connected to distribution feeders (33 kV and below): 100 to 500 kW.Water supply and Agricultural LoadsMost of the panchayats , small and medium municipalities have protected water system which usepumping stations. They normally operate in off peak time and use water pumps ranging from 10 h.p to 50 h.p or more, depending on the population and area.Agricultural and Irrigation LoadsMost of the rural irrigation in India depends on ground water pumping or lifting water from tanks or nearby canals. In most cases design and pump selection is very poor with efficiencies of the order of 25%. Single phase motors are used (up to 10 h.p.) for ground water level 15 m in depth or less with discharge of about 20 l/sec while multi stage submersible pumps with discharge of 800 to 1000 l/m may require motors of 15 to 20 h.p.Sensitive and important LoadsWith computer applications in every area, computer loads and computer controlled process loads are often non-linear and sensitive. They require close tolerance limits for voltage and frequency (voltage limit 5% and frequency 0.5 Hz with unbalance and wave form distortion less than 3%. This requires special attention while providing the distribution of electric power.Load Curves and Load Duration CurvesThe consumption of electrical power or energy by any utility varies from time to time in a day as well as during a week , month , season or year . For example in summer fans , AC units , cooler etc are used but not during winter or cold season. Industries working during day time will consume only lighting load during night( 10 pm to 6 am) . Hence knowledge of variation of loads and their nature is essential for distribution planning . The load characteristics are usually presented as load curves and load duration curvesLoad Curves The load ( power requirement ) of any concern or unit is tabulated as the amount of power required or consumed during a certain period in a day, week or a given season. (b) Load-Duration Curves This is a graph obtained from load curve showing the load in (kw) and duration over which it occurs in descending order of load magnitudes.Refer the following Load data on a Typical 11 kV Line and Answer the following questions Time Hour/Load kWStreet LightResidentialCommercialIndustrialAgricultural0.00 6.00 AM802003201006006.00 8.00 AM-7004001004008.00 9.00 AM-800400300-9.00 10.00 AM-600400400-10.00 5.00 PM-500700400-5.00 6.00 PM-600900400-6.00 7.00 PM808001200320-7.00 8.00 PM8010001200320-8.00 9.00 PM8010001200220-9.00 10.00 PM808001050170-10.00 12.00 PM80500320100400Q1 Draw Load Curve and Load Duration Curve .Q2 What are the peak ( maximum) demands of individual load , system maximum demand.Q3 What is the contribution factors for each of the loads ?Q4 What is the diversity factor and coincidence factor for the above loads ?Q5 For the feeder load given in the table , at a peak load of 1500 kW , the power loss recorded is 75 kW . If the annual loss factor is 0.2 , what is Annual average power loss , andTotal energy loss per year ?

Load Growth And Diversified DemandsAs the residential or commercial areas grow with increased population and new areas area added, it will be necessary to account for the new loads added and also to take in to account diversity between similar loads and non coincidence between peaks of different types of loads added. This will optimize the additional capacity to be added. For the purpose variation in the peaks of different kinds of loads is taken. LOAD GROWTH AND FORECASTING

Population growth and energy requirement do not grow linearly but follow non linear power law or exponential growth. The usual function that fits isy = ka x, where y is the new value after a growth period 'x', k is the initial value of y i.e, when x = 0 and 'a' the rate at which y increases logarithmically.

Power growth, i.e., increase in load demand after a period x with and annual growth rate of 'g' is usually expressed asPn =Po( 1 + g ) nPo = initial power demandg = growth raten = periodThis is also known as compound interest law (here 1 + g = a of the previous equation, y = kax)Future Demands are normally estimated knowing the growth rate factor gLOAD FORECASTINGBased on certain conditions and trends existing and assuming that they continue, load forecasting is a method by which future increase in loads are predicted. There is a great need for accurate forecasting of loads over a given period to meet with the power and energy requirements of the future and money to be spent. In our country there is a lot of pressure due to limited financial and energy resources and hence electrical load fore-casting is vital.The models adopted for load fore casting are statistical models based on Markov process, Time Series analysis and Sampling techniques. The method used is regression analysis.REGRESSION ANALYSIS

The mathematical modelling is done by taking the previous growth over a period and the future trend is extra polated . This is done by either fitting a linear or non linear curve for the growth to get least overall error or fitting a sequence of discontinuous non linear curves from the pervious data extra polating the results. The factors that are taken into account are basic trends,(ii) seasonal variations, and (iii) random and cyclic variations depending on weather conditions.The trends are fitted into either(i) Linear increase as P = a + bt(ii) Exponential or compound interest lawPn = Po(1 + g)n or(iii) other power laws like(a) P = A xb (exponential growth)(b) P =A + Bx + Cx2 (quadratic law)may also be used.These trends and estimates are checked with typical correlations from available records and actual values.To conclude, load forecasting and energy forecasting for future years is difficult but necessary process in order to plan for future power and energy requirements.