You all know, What we will do to improve system power factor ..install the capacitors.
Consider below case:
Suppose you have a electric system on which 10 nos LT motors( Motor load = 10KW) are connected and when all motors are running, system power factor comes to 0.8 ,
You want to install the capacitors to improve the power factor from 0.8 to 0.95.
So capacitor required = Load X (Tan phi 1 – tan phi 2 )
System load will be 10 KW X 10 = 100 KW
Where phi 1 is 0.95 and phi 2 is 0.8
So capacitor required = 100 x 0.609 = 60.9 KVAR or 61 KVAR
So you installed one capacitor of 61 KVAR to improve the power factor.
Its good when all 10 motors are running.
But what if when only one motor of 10 KW is runing than if this capacitor of 61 KVAR is connected in the system ..it will have very bad effect on tbe system ..it will raise system voltage and due to over voltage, motor and system insulation will suffer
So what is solution ?
Solution is . Install capacitors in small sizes and switch on the capacitor according to the requirement.
So instead of installing one capacitor of 61 KVAR, we install 9 capacitors as below.
3 KVAR -1 Nos
5 KVAR – 2 Nos
7 KVAR – 3 Nos
9 KVAR – 3 Nos
So total of these capacitors are 61 KVAR.
Now the question is ? Who will monitor the system power factor and switch on the capacitors ..
Answer is ? APFC relay ..shown below..
APFC relay means Automatic Power Factor Correction relay ..
Means it automatically switch on the capacitors and switch off .. how much capacitor
Set the desired power factor in the realy you want to achieve ..
Give system current and voltage to realy.
Relay will do the rest.
Relay will calculate system power factor and turn on the capacitor according to requirements.
Suppose 2 motors are running and relay decides that only 2 nos 5 KVAR capacitors are required to improve the system power factor than relay will turn on only these two capacitors.
So in this way, we will improve the power factor but also save system from unnecessary overvoltage.
Wiring diagram of APFC relay:
So you see, APFC relay is connected to a 3 phase + Neutral 440 Volt system. The motors and capacitors are also connected to the same system.
Relay is given the current of one phase(L1) through CT and also given system voltage( L1-N).
Now further what you see ..6 No contacts ( 1 to 6) at the bottom of relay and 6 nos coils.
These coils are coils of contactor(K1 to K6) which is used to turn on the capacitors.
When relay closes No contact 1 and 2 than coil of contactor K1 and K2 pick up and it allows the capacitor connected to these contactor ( suppose C1 and C2 ) to switch on.
Contactors( K1 to K6 ) are just like a switch to turn on capacitors ( C1 to C6) .
So above relay is used to turn on 6 capacitors. This is called 6 step APFC ray or power factor controller.
APFC relay comes upto 12 steps generally.
Now come to APFC Panel.
When load is small hence capacitor required are also less. So capacitor and relay can be placed in same LT pabel which gives power to motors, light or any other load etc.
But when load is more than a dedicated panel is made which houses the APFC relay and capacitors ..this is called APFC panel.
APFC panel is charged by main LT panel through a cable.