# Power Factor Correction Capacitors Sizing Calculations – Part Four

Today, we will continue explaining the technical part for Power Factor Correction Capacitors Sizing Calculations. We will explain the following topics:

• How to make Power Factor Correction?
• Types of Power Factor Correction Capacitors

 1- How to make Power Factor Correction?

 Power factor correction can be made in two ways: Reduce the amount of reactive energy, Compensate artificially for the consumption of reactive energy.

 1.2 Compensate artificially for the consumption of reactive energy This can be done using one of the following equipment: Power Factor Capacitors, Rotary Machines.

 1.2.A Power Factor Capacitors By nature of its electrostatic field, the capacitor stores energy whenever the voltage applied across the capacitor is moving away from zero; it gives up energy after the voltage has crested. This sequence is opposite to that of the magnetic field, so the capacitor can be used to supply magnetizing current that would otherwise be drawn from the utility source. Power factor correction is achieved by the addition of capacitors in parallel with the connected motor circuits and can be applied at the starter, or applied at the switchboard or distribution panel. Capacitors connected at each starter and controlled by each starter is known as "Static Power Factor Correction" while capacitors connected at a distribution board and controlled independently from the individual starters are known as "Bulk Correction". Power capacitors serve as leading reactive current generators and counter the lagging reactive current in the system. By providing reactive current, they reduce the total amount of current your system must draw from the utility. Capacitors are generally the most economical source of reactive compensation. Other advantages include: Low losses (less than ¼ watt/KVAR), Essentially low maintenance, Light, compact units which can be combined as needed, make capacitors relatively easy to install and modify as reactive compensation need change.

 1.2.B Rotary Machines Such as phase advancers, synchronous machines and synchronous condensers. 1.2.B.1 Synchronous Machines Both synchronous motors and generators can provide reactive power by increasing the excitation field sufficiently. The KVAR available from fully loaded machine depends on the rated KW and power factor: KVAR = KW. Sin (acos (pf)) More KVAR is available if the machine is not fully loaded. For example, A 1.0 PF, 100 KW motor can provide 0 to 30 KVAR from full load down to no load by operating in a leading mode. Where auto-synchronous motors are employed, the power factor correction may be a secondary function. 1.2.B.2 Synchronous Condensers A synchronous condenser (see Fig.2) is essentially an unloaded motor whose sole task is to provide reactive power. Synchronous condensers are continuously variable within wide limits to generate or consume KVAR. Due to high initial costs, losses, and maintenance costs, synchronous condensers are not generally used for power factor correction unless their voltage stabilizing effects and influence on the short circuit capacity are needed. However, they do have the advantage that they do not cause harmonic resonance as capacitors sometimes. Therefore, they are used in certain difficult situations where the extra costs are justifiable. Fig.2

Table-1 shows a Comparison between Rotary machines and Power factor Capacitors.

 Rotary machines Power factor Capacitors High initial cost makes its use uneconomical, except where one is using rotating plant for a dual function: drive and power factor correction the initial cost is very low the wear and tear inherent in all rotating machines involves additional expense for upkeep and maintenance Upkeep and  maintenance costs are minimal They are used in certain difficult situations where the extra costs are justifiable. they can be used with the same high efficiency on all sizes of installation They are not generally used for power factor correction unless their voltage stabilizing effects and influence on the short circuit capacity are needed. They are compact, reliable, highly efficient & convenient to install and lend themselves to individual, group or automatic method of correction. They do not cause harmonic resonance. Sometimes cause harmonic resonance.
Table-1

 Notes for This Course In this course, we will concentrate on power factor correction using capacitors only. Other methods for power factor correction will be explained later in other courses. The static VAR compensators used for providing fast acting reactive power on high voltage transmission systems to regulate the transmission voltage or to improve power factor for large industrial loads are not included in this course.

 2- Types of Power Factor Correction Capacitors

 There are many types of Power Factor Correction Capacitors which can be categorized according to the following: According to complexity of control, According to location.

 2.1 According To Complexity of Control

 Two types of compensation shall be adopted depending on the complexity of control as follows: Single switching control, Selective switching control.

 2.1.A Single Switching Control

 In this case a fixed compensation is used by connection of a fixed-value capacitor bank; this arrangement uses one or more capacitor(s) to provide a constant level of compensation. Control may be: Manual: by circuit-breaker or load-break switch, Semi-automatic: by contactor, Direct connection to an appliance and switched with it. These capacitors are applied in the following: At the terminals of inductive loads (mainly motors), At bus bars supplying numerous small motors and inductive appliances for which individual compensation would be too costly, In cases where the load factor is reasonably constant.

 2.1.B Selective Switching Control

 Selective Switching Control can be categorized to two types: Automatic compensation, Dynamic compensation.

 2.1.B.1 Automatic Compensation

 In most installations there is not a constant absorption of reactive power due to working cycles of machines with different electrical characteristics, these fluctuating loads result in fluctuating power factor. In such installations, automatic power factor correction systems which use monitoring devices and power factor regulators to allow the automatic switching of different capacitor banks, thus following the variations of the absorbed reactive power and keeping the power factor of the installation constant. Automatic Compensation Main Parts An automatic compensation system is formed by (see Fig.3): Some sensors detecting current and voltage signals; An intelligent unit which compares the measured power factor with the desired one and operates the connection and disconnection of the capacitor banks with the necessary reactive power (power factor regulator); An electric power board comprising switching and protection devices; Some capacitor banks. Fig.3 Automatic compensation can be done by connection of different number of capacitor steps, allowing the adjustment of the reactive energy to the requested value. Automatic compensation is applied at points in an installation where the active-power and/or reactive power variations are relatively large, for example: At the busbars of a main distribution switch-board, At the terminals of a heavily-loaded feeder cable. Control of Automatic compensation is usually provided by contactors. For compensation of highly fluctuating loads, fast and highly repetitive connection of capacitors is necessary, and static switches must be used. Advantages of Automatic Compensation: Regulation of power Factor to the requested value. Better utilization of the capacitor compared to individual compensation. Eliminates the possibility of overcompensation. Possibility to extend existing banks without changes in the control equipment. Disadvantages of Automatic Compensation: Insensitivity to harmonics. General Notes for Automatic Compensation: The major important points in design of Automatic compensation are: Where the KVAR rating of the capacitors is less than, or equal to 15% of the supply transformer rating, a fixed value of compensation is appropriate. Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors. Choice of regulator characteristics (c/k and tolerance) and step size to avoid hunting. Correlation between minimum interval between switching and discharge device of capacitor to avoid energizing a charged capacitor. Disconnecting of all steps in case of a mains outage.

 2.1.B.2 Dynamic Compensation

 This kind of compensation is requested when highly fluctuating loads are present, and voltage fluctuations should be avoided. The principle of dynamic compensation is to associate a fixed capacitor bank, an electronic VAR compensator and thyristors switching modules to quickly connect or disconnect capacitors or inductors providing either leading or lagging reactive currents. The result is a continuously varying and fast compensation, perfectly suitable for loads such as lifts, crushers and spot welding.

 2.2 According To Location

 The location of low-voltage capacitors in an installation constitutes the mode of compensation; Individual, Group, Central or Hybrid compensation. There are no general rules applicable to every type of installation and, in theory, capacitors can be installed at any point, but it is necessary to evaluate the relevant practical and economic feasibility. In principle, the ideal compensation is applied at a point of consumption and at the level required at any instant. The successful operation of a power factor correction depends largely on the correct positioning of the capacitors in the network. The place for connection of capacitor banks in the electrical network is determined by: Global objective (avoid penalties on reactive energy, relieve of transformer or cables, avoid voltage drops and sags), Operating mode (stable or fluctuating loads), Foreseeable influence of capacitors on the network characteristics, Installation cost, Tariff in force, Metering point location; the physical location of the utility meter should be determined since all power capacitors must be installed “downstream” of the meter. Details of light, average and full load KVA, KW and power factor, Position of motors, welding equipment, transformers or other equipment causing bad power factor, Supply system problems such as harmonics. According to the location of the capacitors, the main methods of power factor correction are: Individual compensation, Group compensation, Central compensation, Hybrid compensation.

 2.2.A Individual Compensation

 2.2.A.1 Methods of wiring the Individual power factor correction to Motor Circuits

 Figure shows the common connection diagrams for the power factor correction of motors, which are (see Fig.5): Option#1: On the secondary of the overload relay, Option#2: Between the contactor and the overload relay, Option#3: Between the circuit breaker and the contactor. Fig.5

 Option#2: Between the contactor and the overload relay (see Fig.5) This installation method is normally preferred by motor control center and switchgear builders since the overload setting is simplified. Advantages of Option#2 The advantages are the same as Option#1. Disadvantages of Option#2 Disadvantages are the same as Option#1 except the overload relay can now be set to the full load amps as shown on the motor nameplate.

 Option#3: Between the circuit breaker and the contactor  (see Fig.5) The compensation bank is connected only after the motor has been started and disconnected in advance with respect to the switching off of the motor supply. Where there are multiple motors with low horsepower ratings, or motors which do not run continuously, the capacitors should be connected directly to feeders in the facility through an appropriate switching device to serve as a disconnect for servicing, or light loads. Locations should be as far downstream in the facility as possible for maximum benefit. Advantages of Option#3 Since the capacitor is not switched by the contactor, it can act as a central kvar source for several motors fed by the same circuit breaker. This location is recommended for jogging, plugging and reversing applications. Disadvantages of Option#3 There is a risk that the capacitor remains energized even when the motor or motors are not running, there exists the possibility of overcorrection and leading power factor during lightly loaded periods. Losses are higher than with Options#1&2 as the reactive current must be carried further.

 General Notes: Installations may be made at load centers when it is difficult to connect the capacitors directly across motor terminals or to feeders. Again, switching is a recommended practice. If only power bill penalties are to be offset, the total capacitor requirement can be installed on the load side of metering equipment. Such a location does not increase the capacity of the facility distribution system.

 Power Factor Correction Capacitor connection locations with different motor starter types (Auto-transformer, part-winding, wye-delta) and with multi-speed

 Fig.6 in Below show the wiring diagrams of the Power Factor Correction Capacitor connection locations with different motor starter types (Autotransformer, part-winding, wye-delta) and with multi-speed. Fig.6

In the next article, we will continue explaining other Types of Power Factor Correction Capacitors. Please, keep following.

The previous and related articles are listed in below table:
 Subject Of Previous Article Article Glossary of Power Factor Correction Capacitors Types of Loads, The Power Triangle, What is a power factor? Types of power factor Why utilities charge a power factor penalty? Billing Structure. What causes low power factor? Bad impacts of low power factor, Benefits of Power Factor correction.

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