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Improvement of Power FactorElectrical energy is generated, transmitted, distributed, and utilized as alternating current (AC). However, alternating current has several distinct disadvantages. One of these is the necessity of reactive power that needs to be supplied along with active power.Reactive power can be leading or lagging. While it is the active power that contributes to the energy consumed, or transmitted, reactive power does not contribute to the energy. Reactive power is an inherent part of the total power. Reactive power is either generated or consumed in almost every component of the system, generation, transmission, and distribution and eventually by the loads. The impedance of a branch of a circuit in an AC system consists of two components, resistance and reactance. Reactance can be either inductive or capacitive, which contribute to reactive power in the circuit. Most of the loads are inductive and causes lagging power factor.Need for Reactive Power CompensationPower capacitor is highly technical and complex deviceThin dielectric materials and high dielectric stresses are required.Highly sophisticated techniques are involved.In past most power capacitors were constructed by two sheets of thin aluminum foil separated by three or more layers of chemically impregnated Kraft paper.Power capacitors have been improved due toImprovements in dielectric materials and their more efficient utilizationImprovements in processing techniques.

Power CapacitorsSeries CapacitorsShunt CapacitorsPower FactorPower FactorPower TriangleDisadvantages of low pf (contd.)Disadvantages of low pf (contd.)A single phase motor connected to 400V, 50Hz supply takes 31.7A at pf of 0.7 lagging. Calculate the capacitance required in parallel with the motor to raise the pf to 0.9 lagging[94.3uF]Example 6.2Inductive LoadsLow power factor is caused by inductive loads such as transmission lines, transformers, induction motors, generators and certain lighting ballasts.Lightly loaded induction motors.Magnetizing current is dependent upon design, not on loadBest pf at full load (0.8 to 0.9).Low pf at light loads (0.2 to 0.3)Variable load on power systemLoad on power system is varying. During low load period, supply voltage is increased which increases the magnetization current. This results in decreased power factor.Causes of low power factorIf the circuit has a low pf it means the reactive power is too high. The reactive power is the energy required to build the magnetic field in an inductive circuit, or an electrostatic field in a capacitive circuitThe process of introducing reactive elements to bring the power factor closer to unity is called power-factor correction. Since most loads are inductive, the process normally involves introducing elements with capacitive characteristics. Pf can be improved by using following equipment:Static CapacitorSynchronous CondenserPhase AdvancerPower Factor CorrectionPower factor can be improved by connecting capacitors in parallel with the equipment operating at lagging pf.Capacitor draws leading current and neutralizes the lagging reactive component of current.Static capacitors are invariably used for pf improvement in factoriesAdvantages:Low Losses in static capacitorsNo moving part, therefore low maintenance requirements.Disadvantages:Limited age (8 10 years) and not easily repairableCapacitor switching with changing loads causes switching surges on the systemStatic CapacitorCalculation of pf CorrectionIndividual capacitors can be added in parallel to achieve desired KVar capacity.Capacitors can be connected in series to achieve required voltage.The whole assembly of capacitors is called capacitor bank.Fixed capacitors are sized to light or base load and connected permanently.Switched capacitors are installed for variable loads and switched according to requirement, but increases the cost of switchgear and control equipment.

Capacitor BankA 3 phase, 50 Hz, 3000 V motor develops 600hp (447.6kW), pf being 0.75 lagging and efficiency 0.93. A bank of capacitors is connected in delta across supply terminals and pf raised to 0.95 lagging. Each of capacitor unit is built of five similar 600 V capacitors. Determine the capacitance of each capacitor.[156uF]TP1-6.3:The load on an installation is 800 kW, 0.8pf lagging which works for 3000 hours per annum. The tariff is Rs 100 per kVA plus 20 paise per kWh. If the pf is improved to 0.9 lagging by means of loss free capacitors costing Rs 60 per kVA, calculate the annual saving effected. Allow 10% per annum for interest and depreciation on capacitors.[Rs 9836]Example 6.6Synchronous motor v curves:Synchronous Condenser

Synchronous Motor Phasor Diagram

Synchronous CondenserA station supplies 250 kVA at pf of 0.8 lagging. A synchronous motor is connected in parallel with the load. If combined load is 250kW with a pf of 0.9 lagging, determine:Leading kVAR taken by the motorkVA rating of the motorPf at which motor operates[28.9kVAR, 57.75kVA, 0.566 lead]TP1-6.5A generating station supplies the power to the following:

A lighting load of 100 kW.An induction motor of 800hp (596.8 kW) at pf of 0.8 lagging, efficiency 92%A rotary converter giving 150 A at 400 V at an efficiency of 0.95

What must be the pf of rotary convertor in order that power factor of the station may become unity

[0.128 leading]TP1-6.6A 3 phase synchronous motor is having a mechanical load of 122 kW, is connected in parallel with a load of 510 kW at 0.8pf lagging. The excitation of the motor is adjusted so that kVA input to the motor becomes 140 kVA. Determine the new pf of the whole system.[0.8956 lagging]TP1-6.9Stator winding of induction motor draws exciting current which lags behind supply voltage by 90 degrees.If exciting ampere turns are provided from some other AC source then stator winding will be relieved of exciting currents and the power factor of the motor can be improved.Phase advancer is simply an A.C exciter which is mounted on the shaft as the main motor and is connected in the rotor circuit of the motor. It provides exciting ampere turns to the rotor circuit.

Phase AdvancerFor industrial consumers: Industrial consumers has to pay electricity charges for maximum demand in kVA plus units consumed. Although pf improvement involve extra annual expenditure for pf correction equipment, yet improvement of pf to proper value results in net annual saving for consumer.Importance of pf improvementFor Generating Stations: Generators are rated in kVA but useful output is in kW. Greater the pf of the station, higher the kWh it delivers to the system. Thus pf improvement increases the earning capacity of the power station.Increasing pf will reduce kVA maximum demand charges, but increase the cost of pf correction equipment.The value to which pf should be improved so as to have maximum net annual saving.

x = Cost per KVA of MD per Annumy = Cost per kVar of pf correction equipment per annumMost economical pf depends only upon relative costs of supply and pf correction equipment.Most economical power factor

A factory has an average demand of 320 kW and an annual load factor of 50%. The tariff is 80 per annum per kVA of maximum demand plus 5 paise per kWh. Pf is 0.8 lagging. If the loss free capacitors costing 100 per kVAR are to be utilized, find the value of pf at which maximum saving will result. The annual interest and depreciation together amount to 12%. Also determine the annual saving effected by the improvement of pf to this value.[0.988, 3809]TP2-6.3Chapter 6 (VK Mehta)

Examples:1, 2, 4, 5, 6, 8, 9, 10, 11TP1 (p.116):1, 2, 3, 5, 6, 8, 9, 10TP2 (p.121):1, 2, 3

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