AUTOMATIC POWER FACTOR CORRECTION

CONTENTS

Power factor: An introduction

It is the ratio of real power flowing through the load to the apparent power in the circuit

It is also defined as the cosine of the angle between the voltage and current phases

It 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 loads

The type of loads which doesn’t change the current waveform shape are known as linear loads

The loads on the others case which change the shape (distort) the current waveform are termed as non-linear loads

Distortion decreases average power transmitted to the load

Variation of PF in transmission systems

Generally due to inductive loads the power factor never remains unity in reality

A capacitive network gives reactive power and an inductive network takes the same

In 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 PF

Voltage 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) unitsFor high voltage rating the power electronic devices such as Static VAR compensator or STATCOM are usedIt’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 relayCapable of measuring the reactive and active power of the networkWorks in conjunction with a capacitor bankControls the programmed target PF by activating/deactivating the capacitors

The main functions of the reactive power control relayAutomatic connection detection (Star or Delta)Automatic detection of capacitor stagesPatented characteristics avoiding compensation of very low active powerReactive power requirement-switching delay timeAlarm signals for

• Failure to reach the target PF• Overcurrent in the capacitor• Defects at capacitor stages

2.Network connection points

Since the power factor of the loads is to be compensated hence the connection of the APFC unit is always done in parallel with the supply connection. Generally Y- connection of the capacitor bank is preferred because the transmission distribution of three phase distribution system above 132 kv is Y connected 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 the main part of the APFC units. Large capacitor banks are installed in the APFC’s which switch in order to meet the desired power factor requirement. Each capacitor is individually fused with appropriately sized 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 APFC

unit 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