A SEMINAR ON Submitted by:- Rakesh Kumar Padhy 0901106221 Electrical engineering
CONTENTSPower Factor: An IntroductionTypes of circuit • Linear Circuit • Non-linear CircuitImportance of power factor in distribution systemsDisadvantages of low power factorPFC in linear loadsThe APFC (Automatic Power Factor Correction) unitParts of the APFC unitMaintenance of the APFC unitConclusion
Power factor: An introductionIt is the ratio of real power flowing through the load to the apparent power in the circuitIt is also defined as the cosine of the angle between the voltage and current phasesIt is a dimensionless no between 0 and 1Whether the current is leading or lagging the power factor is termed as leading or lagging PF correspondingly
Types of CircuitThere are two types of circuits based on the type of current waveform • Linear loads • Non-linear loadsThe type of loads which doesn’t change the current waveform shape are known as linear loadsThe loads on the others case which change the shape (distort) the current waveform are termed as non-linear loadsDistortion decreases average power transmitted to the load
Variation of PF in transmission systemsGenerally due to inductive loads the power factor never remains unity in realityA capacitive network gives reactive power and an inductive network takes the sameIn both the cases the power factor becomes less than 1 causing more current to be transferred for the same amount of real power
Disadvantages of low PFVoltage regulation becomes poorGreater amount of apparent power needed to get the same amount of real powerThe increase in reactive power increases the current flowing through the networkSize of the transformer, switch gears etc. increase thereby increasing the transmission cost
Power Factor Correction for linear loadsVarious methods of power factor correction (PFC)are adopted to minimize the apparent power as: Using capacitor banks in Automatic Power Factor Correction (APFC) units For high voltage rating the power electronic devices such as Static VAR compensator or STATCOM are used It’s also a good practice to use an unloaded synchronous motor
APFC unitIt’s an automatic system adjusting itself to control the power factor above a desired value by a bank of capacitors switched by means of contractorsContractors are controlled by a regulator that measures PF in the networkDepending upon the load PF the controller will adjust the PF by switching the necessary no of capacitors from the bank
Parts of the APFC unit1.Reactive power control relay Capable of measuring the reactive and active power of the network Works in conjunction with a capacitor bank Controls the programmed target PF by activating/deactivating the capacitors
The main functions of the reactive power control relay Automatic connection detection (Star or Delta) Automatic detection of capacitor stages Patented characteristics avoiding compensation of very low active power Reactive power requirement-switching delay time Alarm signals for • Failure to reach the target PF • Overcurrent in the capacitor • Defects at capacitor stages
2.Network connection pointsSince the power factor of the loads is to becompensated hence the connection of the APFC unit isalways done in parallel with the supply connection.Generally Y- connection of the capacitor bank ispreferred because the transmission distribution ofthree phase distribution system above 132 kv is Yconnected always
3.Slow-blow switches Slow blow switches are always used because of the following requirements • To sustain the inrush and starting current surges due to vehicular loads • It offers a time delay between overloading and fusing • They can withstand heavy currents (about ten times the rated)
4.Inrush current contactors Used for damping the inrush current arising due to • Remaining capacitor voltage due to fast switching • Short circuit power of supply • Fault level of supply networks The contractors also protect the welding of the main contacts of the contactors Prevent the capacitors from high stress
5.Capacitor BankThe capacitor bank is themain part of the APFCunits. Large capacitor banksare installed in the APFC’swhich switch in order tomeet the desired powerfactor requirement. Eachcapacitor is individuallyfused with appropriatelysized current limit fuse.
6.Control Transformer Monitors the switchboard load (or load to be corrected) Placed physically above (towards the supply) the connections for the PFC system In case more than one transformers are fitted they are always connected in series as they work as CT
Maintenance of the APFC unitThe major maintenance checks involved for the APFCunit are In “out of maximum demand hours” (eg. Late nights or weekends) switch off the system after internal and external checks Remove contactor tops and inspect for contact wear, check capacitance of each capacitor Check (and adjust if necessary) all connections to contactors, capacitors, fuses etc. Re-energize system and check current (true RMS) and harmonics drawn by each capacitor step. Check voltage level and fuses
Conclusion Hence the APFC units correct the power factor above a decided value by switching the appropriate no of capacitors from the capacitor bank In non-linear loads we use inductors, filters, valley-fill circuit etc. to suppress harmonics as they cause the current wave form to be distorted
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