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:
  1. Reduce the amount of reactive energy,
  2. Compensate artificially for the consumption of reactive energy.




1.1 Reduce the amount of reactive energy

  • Avoid supplying equipment with voltage in excess of the rated voltage;
  • Use the highest-speed motor that an application can accommodate. Two-pole (nominal 3600 rpm) motors have the highest power factors. Note that power factor decreases as the number of poles increases;
  • Eliminate unloaded motors and transformers. Size motors as close as possible to the horsepower demands of the load. A lightly loaded motor requires little real power. A heavily loaded motor requires more real power. Since the reactive power is almost constant, the ratio of real power to reactive power varies with induction motor load, and ranges from about 10 percent at no load to as high as 85 percent or more at full load (see Fig.1). An oversized motor, therefore, draws more reactive current at light load than does a smaller motor at full load;




Fig.1

  • Minimize the need for cycling loading of motors because the Low power factor results when motors are operated at less than full load. This often occurs in cyclic processes (such as circular saws, ball mills, conveyors, compressors, grinders, extruders, or punch presses) where motors are sized for the heaviest load. In these applications, power factor varies from moment to moment. Examples of situations include a surface grinder performing a light cut, an unloaded air compressor, and a circular saw spinning without cutting.






1.2 Compensate artificially for the consumption of reactive energy

This can be done using one of the following equipment:
  1. Power Factor Capacitors,
  2. 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:

  1. Low losses (less than ¼ watt/KVAR),
  2. Essentially low maintenance,
  3. 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:

  1. According to complexity of control,
  2. 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:

  1. Single switching control,
  2. 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:

  1. Manual: by circuit-breaker or load-break switch,
  2. Semi-automatic: by contactor,
  3. Direct connection to an appliance and switched with it.


These capacitors are applied in the following:

  1. At the terminals of inductive loads (mainly motors),
  2. At bus bars supplying numerous small motors and inductive appliances for which individual compensation would be too costly,
  3. In cases where the load factor is reasonably constant.




2.1.B Selective Switching Control



Selective Switching Control can be categorized to two types:

  1. Automatic compensation,
  2. 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:

  1. At the busbars of a main distribution switch-board,
  2. 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:

  1. Regulation of power Factor to the requested value.
  2. Better utilization of the capacitor compared to individual compensation.
  3. Eliminates the possibility of overcompensation.
  4. Possibility to extend existing banks without changes in the control equipment.


Disadvantages of Automatic Compensation:
  1. Insensitivity to harmonics.


General Notes for Automatic Compensation:

The major important points in design of Automatic compensation are:

  1. 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.
  2. Choice of regulator characteristics (c/k and tolerance) and step size to avoid hunting.
  3. Correlation between minimum interval between switching and discharge device of capacitor to avoid energizing a charged capacitor.
  4. 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:


  1. Global objective (avoid penalties on reactive energy, relieve of transformer or cables, avoid voltage drops and sags),
  2. Operating mode (stable or fluctuating loads),
  3. Foreseeable influence of capacitors on the network characteristics,
  4. Installation cost,
  5. Tariff in force,
  6. Metering point location; the physical location of the utility meter should be determined since all power capacitors must be installed “downstream” of the meter.
  7. Details of light, average and full load KVA, KW and power factor,
  8. Position of motors, welding equipment, transformers or other equipment causing bad power factor,
  9. Supply system problems such as harmonics.


According to the location of the capacitors, the main methods of power factor correction are:

  1. Individual compensation,
  2. Group compensation,
  3. Central compensation,
  4. Hybrid compensation.




2.2.A Individual Compensation



  • This type of compensation has many other names like Fixed, static, single or distributed compensation.
  • Individual compensation provides a constant amount of reactive power to compensate for the poor power factor at the level of each machine (see Fig.4). This is the technical ideal configuration, as the reactive energy is produced exactly where it is needed, and adjusted to the demand.
  • This configuration is well adapted when the load power is significant compared to the subscribed power (for example: motor load > 50kW).



Fig.4

  • Fixed capacitors are suitable for indoor or outdoor use. Fixed capacitors are available in low voltages (832 volt and below), from 0.5 KVAR up to 400 KVAR (If more than 400 KVAR is required, smaller units are paralleled together).


Applications of Individual compensation:

  • Individual power factor correction is advisable in the case of large electrical equipment with constant load and power and long connection times and it is generally used for motors and fluorescent lamps.
  • To compensate the no-load reactive power of transformers. Fixed compensation of transformers (+/-10 % of transformer rating).
  • For Individual compensation of large motors (>50kW = 70hp).
  • For drives in continuous operation.
  • For drives with long power supply cables or cables whose cross section allows no margin for error.


Notes:

  • Individual correction is commonly applied by using one contactor to control both the motor and the capacitors. It is better practice to use two contactors, one for the motor and one for the capacitors. Where one contactor is employed, it should be upsized for the capacitive load. The use of a second contactor eliminates the problems of resonance between the motor and the capacitors.
  • Where not to use fixed Power Factor Correction:

  1. Inverter: fixed Power factor correction must not be used when the motor is controlled by a variable speed drive or inverter.
  2. Solid State Soft Starter: fixed Power Factor correction capacitors must not be connected to the output of a solid state soft starter. When a solid state soft starter is used, the capacitors must be controlled by a separate contactor, and switched in when the soft starter output voltage has reached line voltage. Many soft starters provide a "top of ramp" or "bypass contactor control" which can be used to control the power factor correction capacitors.


Advantages of Individual Compensation

  • Complete control. Capacitors don’t cause problems on the line during light load conditions;
  • No need for separate switching. The motor always operates with its capacitor; capacitor and load can use the same protective devices against over currents and are connected and disconnected simultaneously.
  • Improved motor performance due to reduced voltage drops;
  • Motors and capacitors can be easily relocated together;
  • Easier to select the right capacitor for the load;
  • Increased system capacity.
  • Capacitors installed near the loads in a plant provide spot delivery of magnetizing current (KVAR) just at the load, which eliminates unnecessary reactive current in the feeder lines thereby reducing the line losses,
  • This is one of the most economical and efficient way of supplying KVARs, which relieves both you and your utility of the cost of carrying this extra KVARs load.
  • Low costs per KVAR


Disadvantages of Individual Compensation

  • Large number of capacitors may be needed for individual motor correction, increasing the installation costs ($ per KVAR compensation).
  • Also overload relay settings need to be changed to account for lower motor current draw. If the capacitors are installed between the contactor and the overload relay, the overload relay can be set for nameplate full load current of motor.
  • The capacitor is only utilized during the time that its associated consumer is in operation. A larger overall capacitor power rating is required as the coincidence factor cannot be taken into account,
  • It is not always easy to install the capacitors directly adjacent to the machines that they compensate (space constraints, installation costs).
  • The power factor correction system is distributed throughout the entire facility






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):

  1. Option#1: On the secondary of the overload relay,
  2. Option#2: Between the contactor and the overload relay,
  3. Option#3: Between the circuit breaker and the contactor.



Fig.5




Option#1: On the secondary of the overload relay

This method of installation commonly referred to as “at the load” or “motor switched” When the capacitors are installed directly at the induction motor terminals (on the secondary of the overload relay), the capacitors are turned on and off with the motors, eliminating the need for separate switching devices or over current protection. The capacitors are only energized when then motor is running (see Fig.5).


Advantages of Option#1:

  • The recommended location for new and existing motors; these capacitor ratings normally correct the motor no-load power factor to unity which in turn generally results in a full-load power factor of 94%-96%.
  • This is the most efficient location since the reactive power (KVAR) is produced at the same spot where it is consumed.
  • Line losses and voltage drop are minimized.
  • The capacitor is switched automatically by the motor starter, so it is only energized when the motor is running.
  • No separate switching device or overcurrent protection is required because of the presence of the motor starter components.


Disadvantages of Option#1:

  • Care must be taken in setting the overload relay since the capacitor will bring about a reduction in amps through the overload. Therefore, to give the same protection to the motor, the overload relay's trip setting should be readjusted or the heater elements should be resized.
  • After the disconnection from the supply, the motor will continue to rotate (residual kinetic energy) and self-excite with the reactive energy drawn from the capacitor bank, and may turn into an asynchronous generator. In this case, the voltage on the load side of the switching and control device is maintained, with the risk of dangerous over voltages (up to twice the rated voltage value).





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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