power factor correction techniques

In this topic we are going to discuss about the various power factor correction technique used in the substation and they mentions as well as protection of this equipments.

       Under normal operating conditions certain electrical loads draw not only active power from the supply (kilowatts KW) but also reactive power (reactive KVA, KVAR). This reactive power has no useful function, but is necessary for the equipment to operate correctly. Loads such as induction motors, welding equipment, arc furnaces and fluorescent lighting would fall into this category.

Definition

The Power Factor of a load is defined as being the ratio of active power to total demand. The uncorrected power factor of a load is cos Ø (where Ø is the phase angle between the uncorrected load and unity), and the corrected power factor is cos Ø2 (where Ø2 is the phase angle between the corrected load and unity). As cos Ø approaches to unity, reactive power drawn from the supply is minimized

Compensating Capacitor

A capacitor inside an op-amp that prevents oscillations is called compensating Capacitor. Also any capacitor that stabilizes an amplifier with a negative-feedback path. Without this capacitor, the amplifier will oscillate. The compensating capacitor produces a low critical frequency and decreases the voltage gain at a rate of 20 dB per decade above the mid-band. At the unity gain frequency, the phase shift is in the vicinity of 270°.

When the phase shift reaches 360°, the voltage gain is less than 1 and oscillations are impossible. The series capacitor is connected to compensate for the line inductance and thus decrease the line reactance so that more power can be transferred through the line thus the system stability can be increased.

The question is about connecting Capacitors in SERIES. Series connection is done for improving STABILITY of the network and for transferring more power (by reducing the resultant reactance) i.e to improve the power transfer capability but not for improving power factor. Power factor will be improved by connecting capacitors in parallel to the load.

Power factor correction

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive (VAR). The purpose is to counteract inductive loading from devices like electric motors and transmission lines to make the load appear to be mostly resistive.

Individual motor or lamp loads may have capacitors for power factor correction, or larger sets of capacitors (usually with automatic switching devices) may be installed at a load center within a building or in a large utility substation.

P.F Correction

When using power factor correction capacitors, the total KVAR on the load side of the motor controller should not exceed the value required to raise the no-load power factor to unity. Over corrective ness of this value may cause high transient voltages, currents, and torques that can increase safety hazards to personnel and possibly damage motor driven equipment.

Never connect power factor correction capacitors at motor terminals on elevator motors, plugging or jogging applications, multi-speed motors or open transition, wyedelta, auto-transformer starting and some part-winding start motors.

If possible, capacitors should be located at position 2. This does not change the current flowing through motor overload protectors. Connection of capacitors at position 3 requires a change of overload protectors. Capacitors should be located at position 1 for applications listed in paragraph 2 above. Be sure bus power factor is not increased above 95% under all loading conditions to avoid over excitation.

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